CA1054338A - Production of metal carbides and metals - Google Patents
Production of metal carbides and metalsInfo
- Publication number
- CA1054338A CA1054338A CA178,221A CA178221A CA1054338A CA 1054338 A CA1054338 A CA 1054338A CA 178221 A CA178221 A CA 178221A CA 1054338 A CA1054338 A CA 1054338A
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- Canada
- Prior art keywords
- chromium
- carbon
- sodium carbonate
- sodium
- reaction
- Prior art date
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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Abstract
PRODUCTION OF METAL CARBIDES AND METALS
ABSTRACT OF THE DISCLOSURE
Metals, particularly chromium, are recovered from low grade ore in substantially pure form using non-polluting procedures. Chromium in the ore is separated from gangue constituents including iron and is formed into sodium chromate.
The sodium chromate by solid state reaction is converted to the sponge metal. Metal carbides may be formed by solid state reduction of the corresponding oxides with carbon.
ABSTRACT OF THE DISCLOSURE
Metals, particularly chromium, are recovered from low grade ore in substantially pure form using non-polluting procedures. Chromium in the ore is separated from gangue constituents including iron and is formed into sodium chromate.
The sodium chromate by solid state reaction is converted to the sponge metal. Metal carbides may be formed by solid state reduction of the corresponding oxides with carbon.
Description
~OS~338 , The presen~ invention relates to the production of metal carbides and metals from ores and alloys, in particular from low grade ores.
. ' .
; In the productiQn of metals from ores, a smelting ;` procedure using co~e and a flux generally is practised, with liquid metal being trapped at intervalsO These pro-cedures require the use of high grade ores and high temp~ra- ;
: ~ -~, tures, and tend to emit noxious gases and particulate matter.
particular metal recovered in this manner is chromium. Present practice requires the use of an ore ;~
having a chromium to iron ratio of at least 3 to 1 and '~
1 attempts to use low grade chromite ares, of varying composi- ~1 ! . y 1 tion and having chromium to iron ratios typically 1 to 2 f more typically 1.2 to 1.5:1, in smelting procedur~ have been unsuccessful. Considerable effort has been expended in developing beneficiation techniques for use with such low grade ores to increase the chromium to iron ratio to at least I
3:1 prior to conventional smelting techniques. However, such procedures have not proved commercially successful.
' 20 In addition~ high carbon ferrochrome alloys areI used in steel making, containing typically55 to 66% Cr, 7 to 8%
f C and the balance Fe, and conventionally are formed from high grade chromite ore using carbon as a reducing agent Low carbon ferrochr:omium is produced from high grade ore and ~;
silicon chromium alloys as reducing agent. Low carbon con~
- taïning sponge chrome materials have not been prepared however.
The present invention seeks to overcome the above defects of the prior art, making it possible for the irst
. ' .
; In the productiQn of metals from ores, a smelting ;` procedure using co~e and a flux generally is practised, with liquid metal being trapped at intervalsO These pro-cedures require the use of high grade ores and high temp~ra- ;
: ~ -~, tures, and tend to emit noxious gases and particulate matter.
particular metal recovered in this manner is chromium. Present practice requires the use of an ore ;~
having a chromium to iron ratio of at least 3 to 1 and '~
1 attempts to use low grade chromite ares, of varying composi- ~1 ! . y 1 tion and having chromium to iron ratios typically 1 to 2 f more typically 1.2 to 1.5:1, in smelting procedur~ have been unsuccessful. Considerable effort has been expended in developing beneficiation techniques for use with such low grade ores to increase the chromium to iron ratio to at least I
3:1 prior to conventional smelting techniques. However, such procedures have not proved commercially successful.
' 20 In addition~ high carbon ferrochrome alloys areI used in steel making, containing typically55 to 66% Cr, 7 to 8%
f C and the balance Fe, and conventionally are formed from high grade chromite ore using carbon as a reducing agent Low carbon ferrochr:omium is produced from high grade ore and ~;
silicon chromium alloys as reducing agent. Low carbon con~
- taïning sponge chrome materials have not been prepared however.
The present invention seeks to overcome the above defects of the prior art, making it possible for the irst
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:~L05~L338 time to recover substantially pure chromium metal from chromite ores and conc~ntrates without smelting and in particular to recover pure chromium metal ~rom low grade chromite ores. The present invention also makes it possible for the first time to form a su~stantially pure chromium carbide for use as a steel additive from chromite ores.
In the process of the present invention, a sodium chromate free from iron and other ore metal values is formed from a chromite ore and the sodium chromate is formed into a chromium carbide or into chromïum metal by a solid state reaction. By the use of solid state reaction, lower temperatures are required and the process is essentially non-polluting.
:' In a further embodiment of the invention, there is provided a process fox the production of m~tal carbides by a solid state reduction of a corresponding metal oxide.
The production of a pure sodium chromate from low grade chromite ores has ~een suggested heretofore, such as in "U~ilization of Low Grade Domestic Chromate", by K. W.
Downes et al, Department of Mines and T~chnical Surveys, Canada, Mines Branch, Memorandum No. 116, published 1951, ~ ;
'Extraction of Chromium from Egyptian Chromium Ores" by M.K.Hussein et al, Canadian Metallurgical Quarterl~, Vol.II
. ~ ' ':, . ~ .
:-, -:, ' .. : :
35~33~3 No.3, (1972) p.4~1 to 4~9, and "Chemistry of Chromium and its Compounds", Marvin J. Udy, Reinhold Publishing Corporation, New ~ork, 1956, especially payes 265 to 273.
The chromite ore, typically a low grade chromite ore, or a concentrate thereof is roasted under oxidizing conditions with alkali salts, typically sodium carbonate.
.; Lime commonly also is lncorporated in the roasted mixture ~ ~
i to accelerate the oxidation procedure and to form insoluble ~ ~ :
compounds with part of the silica and alumina values of -the ore. :
In this way, the chromium values are converted to soluble -~ chromate. The iron vaIues of th-e ore are not solubilized in .~ this procedure and hence on leaching of the roasted material with water, the so.ubilized chromium values are separated from -l the undissolved solid which contains the iron values, other ~` unsolubilized values of the ore and insoluble compounds. . .
.' The alkali roast.causes solubilization of at least some of the silica and alumina values in the ore and hence -.
the aqueous solution resulting from the leach contains dis~
solved quantities of sodium silicate and sodi.um aluminate. '~. ; .
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,~: 20 The pH of the solution is adjusted to a more acid - value to cause preclpitation of alumina and s.ilica which are .~ .
' separated from the aqueous phase, which now is substantially ., free rom dissolved alumina and silica values. The pH may be ad~usted using sulphuric acid or carbon dioxide. It is preferred to use carbon dioxide since the alkali metal values ... .
. of the silicate and aluminate remaln as carbonate, which may be recovered later and recycled. When sulphuric acid is used, ~.
sodium sulphate remains in solut.ion, which may b~ separated and sold if desired.
: 30 _ ~ _ . -: ~ .
~S433 !3 -S-The aqueous so~ution remaining after separation of silica ~nd alumina contains dissol~ed quantities of a sodium chxomate. The form of the sodium chromate depends on the pH
resulting from the acidification. At pH's about 6, the sodium chromate is Na2CrO4 whereas at pH's about 3, the sodium chromate is sodium dichromate Na2Cr207. It is preferred to have the sodium chromate in the dichromate form since in this form it is more soluble than Na2CrO4.
The aqueous solution is evaporated to dryness to form a sodium chromate which is contaminated substantially only with sodium carbonate, where carbon dioxide has been used to adjust ~i ~he p~, or with sodium sulphate where `~ sulphuric acid is used to adjust the pH. Any solid material `~ contaminant precipitating from the solution during evaporation `~ may be separated as impurity.
The solid chromate is free from iron or other ore metal contaminants and is substantially pure except for the presence of excess alkali values.
The solid sodium chromate, preferably in the sodium dichromate form, is reacted in the solid state with carbon to form chromic oxide and sodium carbonate. The sodium carbonate formed in this way, together with sodium ; carbonate present in the sodium chromate, may be recycled to the solubilization.step. The reaction is represented by the following equation:
; Na2Cr27 + 2C > Cr203 ~ Na2C03 ~'C0 The sodium carbonate may be separated from the chromic oxide by water l~aching and crystallization. In addition, dilute acid may be used to remove traces o~ soda ash Tnore expe-diently.
Since sodiurn carbonate iB formed in this reaction ,;,~".Y, , ~L0~913313 it is preferred tha-t the sodium chrornate reacted is in ad-mixture with sodium carbonate, achieved by utilization of carbon dioxide to acidify the leach liquor, so that separation , of the materials leached from the chromic oxide is unnecessary prior to recycle of sodium carbonate.
The chromic oxlde is subjected to a solid state reduction to chromium carbide, in accordance with the equation:
2Cr23 ~ 7C ~ ~ Cr4C ~ 6CO -(2) The chromium carbide Cr4C is sometimes written, perhaps more accurately, as Cr23C6. In the pxesent application the former nomenclature will be used. This chromium carbide is the member of a family of chromium carbides, including Cr4C, Cr~C3 and Cr3Cz, which has the lowest carbon content and hence is the most acceptable product as a steel making ~, additive where low carbon additions are required. ' ;'' In order to promote interaction between the oxide and the carbon in the solid state and hence increase the ' ;
efficiency of the reaction, it is preferred that the components :, . , . , . , ~
;, are in intimately mixed ~inely divided form and agglomerated.
Preferably this is achieved by slurrying powders of the materials together, preferably in -300 mesh particle size, more pre~erably -400 mesh particle size, followed by dewarter-; ing of the slurry and partial drying of the mixture prior to ~ -agglomeration.
The temperature employed in the reduction reaction, . , .
(equation (2)), may vary over a wide range, typical~y from a~out 1025C to 1425C or higher. Over this temperature range, the partial pressure of carbon monoxide formed varies widely ~rom about 0.09 atmospheres at 1025C to about 21.0 atmospheres at 1~25C and dictates to some extent the tech-nique employed to remove the carbon monoxide. Removal o~
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- . ' l.C~5~3315 the carbon monoxide from the reaction vessel promotes the reduction reaction. At low partîal pressures of carbon monoxide below about 0.1 atmospheres, experienced at the low ; end of the temperature range, vacuum may be used to remove the carbon monoxîde. At higher partial pressures above about 0.1 atmospheres, the carbon monoxide may be flushed from the reaction vessel with an inert gas stream, such as argon.
It is preferred to flush the carbon monoxide from ` 10 the reaction vessel by an inert gas stream, and hence the : - . ., higher temperatures are used, generally above about 1200C.
- The reduction is continued until substantially ; -~
complete reaction is achieved, generalIy involving a time span of about 2 to 4 hours for quantities of the order of The precise form of the chromium ~arbide depends on the quantity of carbon used and the reaction temperature r ~ ~
since the lower temperatures promote ~he formation of lower ~ -carbon content carbides. Preferably, the amount of carbon employed corresponds to the stoichiometry of equation 2, . . . .
although other carbides, such as Cr3C2 and Cr7C3 may be formed at will by use of the appropriate quantity of carbon and temperature range.
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: iLQ5~338 ; ~, The ch omium carbide may be reacted in a solid state reaction with chromium oxide to form sponge chromium in substantially pure ~orm in accordance with t~e equation: -3Cr4C + Cr2O3 ~ 14Cr + 3CO
The chromic oxide used to achieve this reaction is ~-that formed from the sodium chromate and hence, in a pre-~erred embodiment of the invention, the chromic oxide, ;~
which is formed by solid sta~e reduction of the chromate, is divided into two portions, one portion being reduced to -~
chromium carbide while the other is mixed with the chromium carbide, the mixture being reacted to form chromium metal.
The diuision of the chromic oxide into these two portions preferably is made in accordance with the stoichiometry of `~
the above equation 4. `
The reaction of the chromium carbide with the ;
chromium oxide may be carried out under a wide range of con-, ditions. As in the case of the solid state reaction between j the oxide and the carbon to form the carbide 7 the reactants i; ~ preferably are finely divided and intimately mixed and ~ ~ -agglomerated, and the techni~ue described above to produce ;`
the intimate admixture of chromium oxide and carhon may ba used to achieve this intimate admixture.
- The temperature used for the solid state reaction :, . .
depends on a number of factors, such as the carbide involved and the technique to be used for the removal of the carbon ~ ~`
monoxide. ~ypically, a temperature in the range about 1300 `, to about 1750C or higher may be used.
, - In the temperature range where partial pressures of . ~
........... ... .
. .
43~8 carbon monoxide lower th~n 0.1 atmospheres result, a vacuum may be used to remove t~e carbon monoxide from the reaction vessel as it is Eormed Where/ however, the partial pressure of carbon monoxide exceeds about 0.1 atmospheres, then pre-ferably the carbon monoxide is removed using an inert gas stream, such as argon.
For the reaction of Cr4C with Cr2O3 in accordance with equation 4, the partial pxessure of carbon monoxide at 1525C is about 0.1 atmospheres. Hence, the reaction tempera-ture preferably exceeds about 1S25C and the carbon monoxidemay be removed from the vessel, conveniently a rotary kiln, using an inert gas stream.
Higher temperatures result in shorter reaction times, and hence are preferred. However, the evaporation of chromium metal may hecome significant, typically above about ~ ;
1700~C, and to reduce losses of product, a balance of reac-tion time with evaporative loss generally is chosen.
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''- , .' ' ' '' ': ' 5~33l3 The reaction time required to produce the metal depends to a large degree on the temperature employed as well as particle size and uniformi~y of mixing, and tends to be short.
The formation o Cr4C is preferred when chromium metal i~ to be formed in accordance with this invention since less carbon is required to form the carbide rom the oxide and ~
less carbon monoxide requires to be removed in the formation ~ -.. . . . .
of the metal than in the case when other chromium carkides ~ -are formed.
; In some instances, it may be possible to co~vert .-the chromium oxide directly to the metal by first carburizing a portion only of the chromium oxi~e with the stoichiometric amount-of carbon required to form completely a chromium metal, so that a mixture of chromic oxide and Cr4C is obtained.
This mixture then may be reacted further at higher tempera-ture to yield chromium metal.
In the latter procedure, therefore, the chromic ! - .
. ,; ' .
oxide is mixed with the required amoun~ of carbon and heated a~ ~he required elevated temperature to form the carbide-oxide mixture. The mixture then is heated to the required higher temperature to ~orm the chromium metal.
... ~ ~ .
`~ Another alternative procedure involves direct solid state xeduction of the oxide to the metal by using sufficient carbon to form carbon monoxide in accordance with the equation:
Cr23 + 3C~ 2Cr + 3CO
This reaction may be carried out at high temperatures, typical~y 1300 to 1700C, but the volume of carbon monoxide evolved per unit weight of chromium produced is greater than that evolved when the required carbon is supplied in the form ~` of Cr4C or another carbide of chromium.
.... . . .
~,''' , . ~ : ' ' 1~5~33~3 The invention is described ~urther, by way of illustration, with reference to the accompanying drawings, ~ -~ ,~
~; in which:
Figure 1 is a schematic flow sheet of one embodim~nt -of the present invention.
Referring to Figure 1, a chromite ore concentrate, which first may be pelletized, is fed by line 10 to a roaster 12. The chromite ore concentrate may be a concentrated low grade chromite ore, the low grade ore typically con~aining 15 10 to 26~ Cr2O3 and the concentrated ore typically about 38 to 45% Cr2O , with a Cr:Fe ratio in concentra~ed ore of about 1.5.
~,t The ore is roasted in the roaster 12 of any convenient.. . ~
'~! construction, such as, a rotary kiln, under any conveni2nt .
conditions in admixture with sodiu~ carbonate fed by line 14 to the roaster 12. The roasting is carried out in the ~
`i presence of molecular oxygen which may be provided by aix fed ' ~ -by line 16 The roasting is carxied out under conditions to oxidize substantially all the chromium values of the ore to sodium chromate, typically by roasting at ab~ut 850C for ~bout two hours.
., , The quantity of sodium carbonate added by line 14 to the roaster 12 is generally in excess of the quantity required to solubilize all the solubilizable materials of the ore concentrake, including the chromium, aluminum and ~ ~ -silicon values of the concentrate. This excess is used to ensure that the majority, and preferably substantially all, of the chromium values are converted to sodium chromate.
The roasted mixture, after cooling,quenching and crushing in any desired manner, is forwarded by line 18 to a leacher 20 wherein the roasted mixture is contacted with water '', , ; ' ,,, - 11 -! ~; "
. .
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''',, . " , " , ' . ' ~0S4338 fed by line 22 to clissolve soluble materials from the roasted mix-ture. The insoluble gangue cRmponents, including . :~
the iron oxide values of the original concentrate are . ..
separated from the resulting leach liquor solution ancl removed ~ - .
` from the leacher by line 2~
.; The leach liquor contains dissolved sodium salts, ` including.the chromate, aluminate, silicate and carbonate. ~:~
, :
The carbonate is present due to the use of excess sodium carbonate in the roasting step. However, since substantially . 10 no iron values are solubilized during the roasting step, .
j substantially cornplete separation of the chromium values ~-.q~ from the iron values of the ore concentrate is achieved.
:~ The leach liquor then is acidified in reactor 26 :l by bubbling carbon dioxide therethrough fed hy line 28.:, , : ,, .~¦ Preferably, the leach liquor has a temperature of about 5 to :
10C during the acidifying and the carbon dioxide is present j ~ as an atmosphere thereof at an elevated pressure. The ;~ ~
.~ . : ., ~ acidification of the leach liquor achieved in this way causes : ~ :
.. j precipitation of alumina and silica from the leach liquor .~l 20 with consequent formation of sodium carbonate. The precipitated ~ alumina and silica are separated from the mother liquor and ;~
.~ removed by line 30. The aci.dification may be continued to a pH :
of about 3 so that the chromate ions are converted to di-:, chromate ions, the solubility of sodium dichromate being ,.' ,...... . .
conslderably greater than that of sodium chromate. The leach - liquor may be maintained at substantially normal temperatures ; such as 20 to 25C during the bubbling of the carbon dioxide .. . .
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`, therethrough.
; The aqueous solution oE sodium carbonate and sodium :. .
. 30 - 12 -, . . .
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iL~5~33~
dichromate xesulting from the acidifier 26 is evaporated to dryness in an evaporator 32, tha evaporated water being collected by line 34. Prior to commencement of crystallization of sodium carbonate or sodium dichromate during the evaporation, any solid deposited material may be discarded as impurity.
The resulting solid, consisting of a mixture of sodium dichromate and sodium carbonate is recovered in line 36.
The solid mixture then is mixed in a kiln 38 with carbon f~d by line 40 and is heated at an elevated temperature to reduce the sodium dichromate to chromium sesguioxide and sodium carbonate in the solid state. If desired, the intimate admixture of sodium dichromate, sodium carbonate and carbon may be formed by dispersing the required amount of carbon in the a~ueous solution of sodium dichromate and sodium carbonate before crystallization thereof. The reduction of the sodium dichromate is carried out typically at a temperature of 850C
~, until all the chromium values are converted to chromic oxide, l typically in about 30 minutes. , In an alternative procedure, khe quantity of carbon added may be sufficient and the conditions such as to reduce the sodium dichromate to chromium carbide.
, The solid mixture of chromic oxide and sodium -- carbonate or chromium carbide and sodium carbonate, as the case may be, after cooling, if desired, is passed by line 42 to a leacher 44 wherein the sodlum carbonate values are ' leached by water fed by line 46 from the mixture to leave substantially pure chromium oxide or chromium carbide. Wash-! ing with dilute acid may be used to remove residual sodium ~ -carbonate and thereby fur~her purify the chromium oxide product. It has préviously been suggested that chromium~
formed from low grade ores in this manner, but the chromic oxide formed in this manner has been used to enrich low yrade .
.~-, , :
:~(3 5~338 ~ :
; ores in order to increase the chromium-to-iron ratio of such ores to a level of at least 3:1 for use in the formation of ~; ferrochromium alloys by smelting.
The aqueous solution of sodium carbonate resulting from the leaching in the leacher 44 is removed by line 48.
,. , The sodium carbonate values are recovered therefrom by crystallization and the solid sodium carbonate preferably is ~ recycled to the sodium carbonate feed in line 14 as at least ~-; part thereof. Under ideal conditions, the quantity of sodium carbonate recovered from the solution in line 48 lS substant-ially the same as that fed to the ore concentrate by line 14.
- However, losses of sodium carbonate values may occur and hence it may be necessary to supplement the recycled sodium ` carbonate material with additional sodium carbonate by line 50 to provide the raquired amount in line 14.
~, The chromic oxide resulting from the leacher 44 in ~` line 52 is split into two streams in lines 54 and 56. The ::-.
proportion of the chromic oxide in line 56 is fed to a kiln 58 wherein the oxide is carburized in intimate admixture with . . , ~, carbon fed by line 60 to a chromium carbide. Preferably, the quantity of carbon fed by line 60 is stoichiometrically .:
equivalent to that required to form Cr4C and th~ carbon and the chromium oxide are intimately mixed and agglomerated.
The resulting chromium carbide, or chromium carbide , resulting from reduction of sodium dichromate, if that pro-cedure is used, i5 fed by line 64 to a reactor 66 after crushing, i~ required, ~herein it is intimately mixed, ,agglomerated and reacted in admixture with the remainder of the chromium oxide in line 5~ to form sponge chromium having a low carbon content which is collected by line 68, the car-~ ~ 14 ~l~15~33~
bon monoxide formed in the process being removed by line 70by the application of a reduced pressure to the reactor 66 or by the passage of an inert gas, such as, argon, through the reactor 66. :
The division of the chromium oxide in line 54 is ~: .
, made on the basis of the stoichiometry requirements for the `` reaction in the reactor 66 to form the chromium metal without an excess of eith-er reac~ant l~eft.
The carbon monoxide form~d in the kiln 58 and the - .
reactor 66 and recovered therefrom by lines 62 and 70 may be , , i ::
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and preferablyis, used to provide a reducing atmosphere for ;~
the kiln 38.
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The invention is illustrated by the following Examples:
This example illustrates the recovery of chromium values as sodium chromate from low grade chromite ores.
Bird River chromite ore concentra-te analyzing 26.9% Cr (39.3% Cr203) and 22.4% total Fe was sized to 90% ;~ ;
-200 mesh and was mixed with soda ash and lime similarly ~ ~
sized to -200 mesh. The mixture was pelletized to approximat- ~ ~ ;
ely ~" diameter pellets and charged to a kiln through which air flowed. The pellets were formed from a to-tal weight of 388g. chromite concentrate, 311g. soda ash, 194.3g. lime i and 173.lg. water.
The pellets were heated for 2 hours up to the roasting temperature and were roasted at an average temperat-ure of about 855C for about 4 hours while about 18.4 SCFH ' -~
of air passed through the kiln.
The pellets were allowed to cool after roasting - and were quenched to room temperature. Thereafter, the `~
pellets were crushed and the fine material obtained was added to about 2 litres of distilled water maintained at about 95 to 100C~ The roast product was slurried in the distilled ~
water and was leached in three stages each of two hours ~-duration. After each of the first two stages, the slurry was filtered and the residue was placed in a fresh quantity of hot, distilled water and leaching commenced again. The -~ - 15 --- ,: , , , : .
. ..
33~
residue after the third stage was washed. ~ -` The resulting aqueous phase was concentrated and 354.6g of impure sodium chromate were recovered which was found to be substantially iron free.
The impure sodium chromate was dissolved in465 ml of distilled water and cooled to 10C. CO2 gas was passed through a copper cooling coil immersed in a water bath held at about 5C and then into the sodium chromate solution.
The pEl of the solution declined from about 13.2 to 8.9.
The solid deposited in this process was filtered and the filtrate concentrated to deposit a solid material.
The solid recovered from the filtrate was found to contain 72.7% of the Cr values of the impure chromate and `-27.9% Cr (86.9% ~a2CrO4). The major impurity was 8.6% NaHCO3.
.
This example illustrates the formation of chromic oxide from sodium chromate produced in a procedure similar to that outlined in Example 1.
5g. of sodium chromate crystals containing 23.9% Cr ;~
2~ (74.5% Na2CrO4) obtained from Bird River chromite ore were crushed to 100% -200 mesh and mixed with 0.29g. of graphite eized to 100% ~400 mesh and containing 95.0% fixed carbon.
The mixture was placed in a crucible in a Vycor tube which was flushed with 0.19 SCFM o~ nitrogen for 5 minutes and placed in a furnace preheated to 1000C. The charge was rapidly heated and held at 1000C for 30 minutes under a flow of nitrogen. The products were cooled quickly. AEter crush~
ing, the 4.5g. of products were washed in hot water at 60C
to remove sodium carbonate and any residual sodium chromate.
Residual sbdium carbonate was removed by washing the product wlth 1:10 hydroc}lloric acid.
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lOSD~3315~
Any excess carhon was removed by roas~ing the leached product in air at 1000C for one hour. The Einal weiyht of product obtained was 1.80g. This product analyzed 67.3% Cr, giving a Cr2O3 content of 98.4%.
EX~MPLE 3 This example illustrates the formation of chromium carbides from chromic oxide.
lOOg. of -400 mesh co~mercial grade chromic oxide (98.5% Cr2O3) was dry mixed with 72g. of -400 mesh graphite (95.0% fixed carbon). The mixture contained 2 times the stoichiometric carbon requirement based on the reaction:
3cr23 + 13C ~ 2Cr3C2 + 9CO
was pelletized with moisture and the pellets were dried.
The pellets were fired in alumina crucibles in the presence of argon at a flow rate of 0.05 CFM at about 1300C for 2 hours. After cooling, the pellets were crushed and the bulk of the excess carbon was removed by flotation in water. The product was subjected to X-ray diffraction analysis and Eound to ha~e the approximate composition:
80% Cr3C2, 15% Cr7C3 and 5% graphite. The product analysed 72.5% Cr and 14.5% C.
The procedure of Example 3 was repeated with a firing temperature of from 1300 to 1380C for a period of
, .:. , ,,, , , :
, '~: ' . .
:~L05~L338 time to recover substantially pure chromium metal from chromite ores and conc~ntrates without smelting and in particular to recover pure chromium metal ~rom low grade chromite ores. The present invention also makes it possible for the first time to form a su~stantially pure chromium carbide for use as a steel additive from chromite ores.
In the process of the present invention, a sodium chromate free from iron and other ore metal values is formed from a chromite ore and the sodium chromate is formed into a chromium carbide or into chromïum metal by a solid state reaction. By the use of solid state reaction, lower temperatures are required and the process is essentially non-polluting.
:' In a further embodiment of the invention, there is provided a process fox the production of m~tal carbides by a solid state reduction of a corresponding metal oxide.
The production of a pure sodium chromate from low grade chromite ores has ~een suggested heretofore, such as in "U~ilization of Low Grade Domestic Chromate", by K. W.
Downes et al, Department of Mines and T~chnical Surveys, Canada, Mines Branch, Memorandum No. 116, published 1951, ~ ;
'Extraction of Chromium from Egyptian Chromium Ores" by M.K.Hussein et al, Canadian Metallurgical Quarterl~, Vol.II
. ~ ' ':, . ~ .
:-, -:, ' .. : :
35~33~3 No.3, (1972) p.4~1 to 4~9, and "Chemistry of Chromium and its Compounds", Marvin J. Udy, Reinhold Publishing Corporation, New ~ork, 1956, especially payes 265 to 273.
The chromite ore, typically a low grade chromite ore, or a concentrate thereof is roasted under oxidizing conditions with alkali salts, typically sodium carbonate.
.; Lime commonly also is lncorporated in the roasted mixture ~ ~
i to accelerate the oxidation procedure and to form insoluble ~ ~ :
compounds with part of the silica and alumina values of -the ore. :
In this way, the chromium values are converted to soluble -~ chromate. The iron vaIues of th-e ore are not solubilized in .~ this procedure and hence on leaching of the roasted material with water, the so.ubilized chromium values are separated from -l the undissolved solid which contains the iron values, other ~` unsolubilized values of the ore and insoluble compounds. . .
.' The alkali roast.causes solubilization of at least some of the silica and alumina values in the ore and hence -.
the aqueous solution resulting from the leach contains dis~
solved quantities of sodium silicate and sodi.um aluminate. '~. ; .
. ` -~ . . j '-,, .
,~: 20 The pH of the solution is adjusted to a more acid - value to cause preclpitation of alumina and s.ilica which are .~ .
' separated from the aqueous phase, which now is substantially ., free rom dissolved alumina and silica values. The pH may be ad~usted using sulphuric acid or carbon dioxide. It is preferred to use carbon dioxide since the alkali metal values ... .
. of the silicate and aluminate remaln as carbonate, which may be recovered later and recycled. When sulphuric acid is used, ~.
sodium sulphate remains in solut.ion, which may b~ separated and sold if desired.
: 30 _ ~ _ . -: ~ .
~S433 !3 -S-The aqueous so~ution remaining after separation of silica ~nd alumina contains dissol~ed quantities of a sodium chxomate. The form of the sodium chromate depends on the pH
resulting from the acidification. At pH's about 6, the sodium chromate is Na2CrO4 whereas at pH's about 3, the sodium chromate is sodium dichromate Na2Cr207. It is preferred to have the sodium chromate in the dichromate form since in this form it is more soluble than Na2CrO4.
The aqueous solution is evaporated to dryness to form a sodium chromate which is contaminated substantially only with sodium carbonate, where carbon dioxide has been used to adjust ~i ~he p~, or with sodium sulphate where `~ sulphuric acid is used to adjust the pH. Any solid material `~ contaminant precipitating from the solution during evaporation `~ may be separated as impurity.
The solid chromate is free from iron or other ore metal contaminants and is substantially pure except for the presence of excess alkali values.
The solid sodium chromate, preferably in the sodium dichromate form, is reacted in the solid state with carbon to form chromic oxide and sodium carbonate. The sodium carbonate formed in this way, together with sodium ; carbonate present in the sodium chromate, may be recycled to the solubilization.step. The reaction is represented by the following equation:
; Na2Cr27 + 2C > Cr203 ~ Na2C03 ~'C0 The sodium carbonate may be separated from the chromic oxide by water l~aching and crystallization. In addition, dilute acid may be used to remove traces o~ soda ash Tnore expe-diently.
Since sodiurn carbonate iB formed in this reaction ,;,~".Y, , ~L0~913313 it is preferred tha-t the sodium chrornate reacted is in ad-mixture with sodium carbonate, achieved by utilization of carbon dioxide to acidify the leach liquor, so that separation , of the materials leached from the chromic oxide is unnecessary prior to recycle of sodium carbonate.
The chromic oxlde is subjected to a solid state reduction to chromium carbide, in accordance with the equation:
2Cr23 ~ 7C ~ ~ Cr4C ~ 6CO -(2) The chromium carbide Cr4C is sometimes written, perhaps more accurately, as Cr23C6. In the pxesent application the former nomenclature will be used. This chromium carbide is the member of a family of chromium carbides, including Cr4C, Cr~C3 and Cr3Cz, which has the lowest carbon content and hence is the most acceptable product as a steel making ~, additive where low carbon additions are required. ' ;'' In order to promote interaction between the oxide and the carbon in the solid state and hence increase the ' ;
efficiency of the reaction, it is preferred that the components :, . , . , . , ~
;, are in intimately mixed ~inely divided form and agglomerated.
Preferably this is achieved by slurrying powders of the materials together, preferably in -300 mesh particle size, more pre~erably -400 mesh particle size, followed by dewarter-; ing of the slurry and partial drying of the mixture prior to ~ -agglomeration.
The temperature employed in the reduction reaction, . , .
(equation (2)), may vary over a wide range, typical~y from a~out 1025C to 1425C or higher. Over this temperature range, the partial pressure of carbon monoxide formed varies widely ~rom about 0.09 atmospheres at 1025C to about 21.0 atmospheres at 1~25C and dictates to some extent the tech-nique employed to remove the carbon monoxide. Removal o~
'' . .
.: -.
- . ' l.C~5~3315 the carbon monoxide from the reaction vessel promotes the reduction reaction. At low partîal pressures of carbon monoxide below about 0.1 atmospheres, experienced at the low ; end of the temperature range, vacuum may be used to remove the carbon monoxîde. At higher partial pressures above about 0.1 atmospheres, the carbon monoxide may be flushed from the reaction vessel with an inert gas stream, such as argon.
It is preferred to flush the carbon monoxide from ` 10 the reaction vessel by an inert gas stream, and hence the : - . ., higher temperatures are used, generally above about 1200C.
- The reduction is continued until substantially ; -~
complete reaction is achieved, generalIy involving a time span of about 2 to 4 hours for quantities of the order of The precise form of the chromium ~arbide depends on the quantity of carbon used and the reaction temperature r ~ ~
since the lower temperatures promote ~he formation of lower ~ -carbon content carbides. Preferably, the amount of carbon employed corresponds to the stoichiometry of equation 2, . . . .
although other carbides, such as Cr3C2 and Cr7C3 may be formed at will by use of the appropriate quantity of carbon and temperature range.
- .
.',' '`' .,,, , . , ~ ,. .. .
: iLQ5~338 ; ~, The ch omium carbide may be reacted in a solid state reaction with chromium oxide to form sponge chromium in substantially pure ~orm in accordance with t~e equation: -3Cr4C + Cr2O3 ~ 14Cr + 3CO
The chromic oxide used to achieve this reaction is ~-that formed from the sodium chromate and hence, in a pre-~erred embodiment of the invention, the chromic oxide, ;~
which is formed by solid sta~e reduction of the chromate, is divided into two portions, one portion being reduced to -~
chromium carbide while the other is mixed with the chromium carbide, the mixture being reacted to form chromium metal.
The diuision of the chromic oxide into these two portions preferably is made in accordance with the stoichiometry of `~
the above equation 4. `
The reaction of the chromium carbide with the ;
chromium oxide may be carried out under a wide range of con-, ditions. As in the case of the solid state reaction between j the oxide and the carbon to form the carbide 7 the reactants i; ~ preferably are finely divided and intimately mixed and ~ ~ -agglomerated, and the techni~ue described above to produce ;`
the intimate admixture of chromium oxide and carhon may ba used to achieve this intimate admixture.
- The temperature used for the solid state reaction :, . .
depends on a number of factors, such as the carbide involved and the technique to be used for the removal of the carbon ~ ~`
monoxide. ~ypically, a temperature in the range about 1300 `, to about 1750C or higher may be used.
, - In the temperature range where partial pressures of . ~
........... ... .
. .
43~8 carbon monoxide lower th~n 0.1 atmospheres result, a vacuum may be used to remove t~e carbon monoxide from the reaction vessel as it is Eormed Where/ however, the partial pressure of carbon monoxide exceeds about 0.1 atmospheres, then pre-ferably the carbon monoxide is removed using an inert gas stream, such as argon.
For the reaction of Cr4C with Cr2O3 in accordance with equation 4, the partial pxessure of carbon monoxide at 1525C is about 0.1 atmospheres. Hence, the reaction tempera-ture preferably exceeds about 1S25C and the carbon monoxidemay be removed from the vessel, conveniently a rotary kiln, using an inert gas stream.
Higher temperatures result in shorter reaction times, and hence are preferred. However, the evaporation of chromium metal may hecome significant, typically above about ~ ;
1700~C, and to reduce losses of product, a balance of reac-tion time with evaporative loss generally is chosen.
' ~ '' ' ' `:'"
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.
:~ .
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.
''- , .' ' ' '' ': ' 5~33l3 The reaction time required to produce the metal depends to a large degree on the temperature employed as well as particle size and uniformi~y of mixing, and tends to be short.
The formation o Cr4C is preferred when chromium metal i~ to be formed in accordance with this invention since less carbon is required to form the carbide rom the oxide and ~
less carbon monoxide requires to be removed in the formation ~ -.. . . . .
of the metal than in the case when other chromium carkides ~ -are formed.
; In some instances, it may be possible to co~vert .-the chromium oxide directly to the metal by first carburizing a portion only of the chromium oxi~e with the stoichiometric amount-of carbon required to form completely a chromium metal, so that a mixture of chromic oxide and Cr4C is obtained.
This mixture then may be reacted further at higher tempera-ture to yield chromium metal.
In the latter procedure, therefore, the chromic ! - .
. ,; ' .
oxide is mixed with the required amoun~ of carbon and heated a~ ~he required elevated temperature to form the carbide-oxide mixture. The mixture then is heated to the required higher temperature to ~orm the chromium metal.
... ~ ~ .
`~ Another alternative procedure involves direct solid state xeduction of the oxide to the metal by using sufficient carbon to form carbon monoxide in accordance with the equation:
Cr23 + 3C~ 2Cr + 3CO
This reaction may be carried out at high temperatures, typical~y 1300 to 1700C, but the volume of carbon monoxide evolved per unit weight of chromium produced is greater than that evolved when the required carbon is supplied in the form ~` of Cr4C or another carbide of chromium.
.... . . .
~,''' , . ~ : ' ' 1~5~33~3 The invention is described ~urther, by way of illustration, with reference to the accompanying drawings, ~ -~ ,~
~; in which:
Figure 1 is a schematic flow sheet of one embodim~nt -of the present invention.
Referring to Figure 1, a chromite ore concentrate, which first may be pelletized, is fed by line 10 to a roaster 12. The chromite ore concentrate may be a concentrated low grade chromite ore, the low grade ore typically con~aining 15 10 to 26~ Cr2O3 and the concentrated ore typically about 38 to 45% Cr2O , with a Cr:Fe ratio in concentra~ed ore of about 1.5.
~,t The ore is roasted in the roaster 12 of any convenient.. . ~
'~! construction, such as, a rotary kiln, under any conveni2nt .
conditions in admixture with sodiu~ carbonate fed by line 14 to the roaster 12. The roasting is carried out in the ~
`i presence of molecular oxygen which may be provided by aix fed ' ~ -by line 16 The roasting is carxied out under conditions to oxidize substantially all the chromium values of the ore to sodium chromate, typically by roasting at ab~ut 850C for ~bout two hours.
., , The quantity of sodium carbonate added by line 14 to the roaster 12 is generally in excess of the quantity required to solubilize all the solubilizable materials of the ore concentrake, including the chromium, aluminum and ~ ~ -silicon values of the concentrate. This excess is used to ensure that the majority, and preferably substantially all, of the chromium values are converted to sodium chromate.
The roasted mixture, after cooling,quenching and crushing in any desired manner, is forwarded by line 18 to a leacher 20 wherein the roasted mixture is contacted with water '', , ; ' ,,, - 11 -! ~; "
. .
, ' ' , :' : ,. . .
''',, . " , " , ' . ' ~0S4338 fed by line 22 to clissolve soluble materials from the roasted mix-ture. The insoluble gangue cRmponents, including . :~
the iron oxide values of the original concentrate are . ..
separated from the resulting leach liquor solution ancl removed ~ - .
` from the leacher by line 2~
.; The leach liquor contains dissolved sodium salts, ` including.the chromate, aluminate, silicate and carbonate. ~:~
, :
The carbonate is present due to the use of excess sodium carbonate in the roasting step. However, since substantially . 10 no iron values are solubilized during the roasting step, .
j substantially cornplete separation of the chromium values ~-.q~ from the iron values of the ore concentrate is achieved.
:~ The leach liquor then is acidified in reactor 26 :l by bubbling carbon dioxide therethrough fed hy line 28.:, , : ,, .~¦ Preferably, the leach liquor has a temperature of about 5 to :
10C during the acidifying and the carbon dioxide is present j ~ as an atmosphere thereof at an elevated pressure. The ;~ ~
.~ . : ., ~ acidification of the leach liquor achieved in this way causes : ~ :
.. j precipitation of alumina and silica from the leach liquor .~l 20 with consequent formation of sodium carbonate. The precipitated ~ alumina and silica are separated from the mother liquor and ;~
.~ removed by line 30. The aci.dification may be continued to a pH :
of about 3 so that the chromate ions are converted to di-:, chromate ions, the solubility of sodium dichromate being ,.' ,...... . .
conslderably greater than that of sodium chromate. The leach - liquor may be maintained at substantially normal temperatures ; such as 20 to 25C during the bubbling of the carbon dioxide .. . .
::
`, therethrough.
; The aqueous solution oE sodium carbonate and sodium :. .
. 30 - 12 -, . . .
.~ .~ . . . .
. ~ , . '' ' ' ' .
... .
iL~5~33~
dichromate xesulting from the acidifier 26 is evaporated to dryness in an evaporator 32, tha evaporated water being collected by line 34. Prior to commencement of crystallization of sodium carbonate or sodium dichromate during the evaporation, any solid deposited material may be discarded as impurity.
The resulting solid, consisting of a mixture of sodium dichromate and sodium carbonate is recovered in line 36.
The solid mixture then is mixed in a kiln 38 with carbon f~d by line 40 and is heated at an elevated temperature to reduce the sodium dichromate to chromium sesguioxide and sodium carbonate in the solid state. If desired, the intimate admixture of sodium dichromate, sodium carbonate and carbon may be formed by dispersing the required amount of carbon in the a~ueous solution of sodium dichromate and sodium carbonate before crystallization thereof. The reduction of the sodium dichromate is carried out typically at a temperature of 850C
~, until all the chromium values are converted to chromic oxide, l typically in about 30 minutes. , In an alternative procedure, khe quantity of carbon added may be sufficient and the conditions such as to reduce the sodium dichromate to chromium carbide.
, The solid mixture of chromic oxide and sodium -- carbonate or chromium carbide and sodium carbonate, as the case may be, after cooling, if desired, is passed by line 42 to a leacher 44 wherein the sodlum carbonate values are ' leached by water fed by line 46 from the mixture to leave substantially pure chromium oxide or chromium carbide. Wash-! ing with dilute acid may be used to remove residual sodium ~ -carbonate and thereby fur~her purify the chromium oxide product. It has préviously been suggested that chromium~
formed from low grade ores in this manner, but the chromic oxide formed in this manner has been used to enrich low yrade .
.~-, , :
:~(3 5~338 ~ :
; ores in order to increase the chromium-to-iron ratio of such ores to a level of at least 3:1 for use in the formation of ~; ferrochromium alloys by smelting.
The aqueous solution of sodium carbonate resulting from the leaching in the leacher 44 is removed by line 48.
,. , The sodium carbonate values are recovered therefrom by crystallization and the solid sodium carbonate preferably is ~ recycled to the sodium carbonate feed in line 14 as at least ~-; part thereof. Under ideal conditions, the quantity of sodium carbonate recovered from the solution in line 48 lS substant-ially the same as that fed to the ore concentrate by line 14.
- However, losses of sodium carbonate values may occur and hence it may be necessary to supplement the recycled sodium ` carbonate material with additional sodium carbonate by line 50 to provide the raquired amount in line 14.
~, The chromic oxide resulting from the leacher 44 in ~` line 52 is split into two streams in lines 54 and 56. The ::-.
proportion of the chromic oxide in line 56 is fed to a kiln 58 wherein the oxide is carburized in intimate admixture with . . , ~, carbon fed by line 60 to a chromium carbide. Preferably, the quantity of carbon fed by line 60 is stoichiometrically .:
equivalent to that required to form Cr4C and th~ carbon and the chromium oxide are intimately mixed and agglomerated.
The resulting chromium carbide, or chromium carbide , resulting from reduction of sodium dichromate, if that pro-cedure is used, i5 fed by line 64 to a reactor 66 after crushing, i~ required, ~herein it is intimately mixed, ,agglomerated and reacted in admixture with the remainder of the chromium oxide in line 5~ to form sponge chromium having a low carbon content which is collected by line 68, the car-~ ~ 14 ~l~15~33~
bon monoxide formed in the process being removed by line 70by the application of a reduced pressure to the reactor 66 or by the passage of an inert gas, such as, argon, through the reactor 66. :
The division of the chromium oxide in line 54 is ~: .
, made on the basis of the stoichiometry requirements for the `` reaction in the reactor 66 to form the chromium metal without an excess of eith-er reac~ant l~eft.
The carbon monoxide form~d in the kiln 58 and the - .
reactor 66 and recovered therefrom by lines 62 and 70 may be , , i ::
, 1 1 .
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and preferablyis, used to provide a reducing atmosphere for ;~
the kiln 38.
; ~
The invention is illustrated by the following Examples:
This example illustrates the recovery of chromium values as sodium chromate from low grade chromite ores.
Bird River chromite ore concentra-te analyzing 26.9% Cr (39.3% Cr203) and 22.4% total Fe was sized to 90% ;~ ;
-200 mesh and was mixed with soda ash and lime similarly ~ ~
sized to -200 mesh. The mixture was pelletized to approximat- ~ ~ ;
ely ~" diameter pellets and charged to a kiln through which air flowed. The pellets were formed from a to-tal weight of 388g. chromite concentrate, 311g. soda ash, 194.3g. lime i and 173.lg. water.
The pellets were heated for 2 hours up to the roasting temperature and were roasted at an average temperat-ure of about 855C for about 4 hours while about 18.4 SCFH ' -~
of air passed through the kiln.
The pellets were allowed to cool after roasting - and were quenched to room temperature. Thereafter, the `~
pellets were crushed and the fine material obtained was added to about 2 litres of distilled water maintained at about 95 to 100C~ The roast product was slurried in the distilled ~
water and was leached in three stages each of two hours ~-duration. After each of the first two stages, the slurry was filtered and the residue was placed in a fresh quantity of hot, distilled water and leaching commenced again. The -~ - 15 --- ,: , , , : .
. ..
33~
residue after the third stage was washed. ~ -` The resulting aqueous phase was concentrated and 354.6g of impure sodium chromate were recovered which was found to be substantially iron free.
The impure sodium chromate was dissolved in465 ml of distilled water and cooled to 10C. CO2 gas was passed through a copper cooling coil immersed in a water bath held at about 5C and then into the sodium chromate solution.
The pEl of the solution declined from about 13.2 to 8.9.
The solid deposited in this process was filtered and the filtrate concentrated to deposit a solid material.
The solid recovered from the filtrate was found to contain 72.7% of the Cr values of the impure chromate and `-27.9% Cr (86.9% ~a2CrO4). The major impurity was 8.6% NaHCO3.
.
This example illustrates the formation of chromic oxide from sodium chromate produced in a procedure similar to that outlined in Example 1.
5g. of sodium chromate crystals containing 23.9% Cr ;~
2~ (74.5% Na2CrO4) obtained from Bird River chromite ore were crushed to 100% -200 mesh and mixed with 0.29g. of graphite eized to 100% ~400 mesh and containing 95.0% fixed carbon.
The mixture was placed in a crucible in a Vycor tube which was flushed with 0.19 SCFM o~ nitrogen for 5 minutes and placed in a furnace preheated to 1000C. The charge was rapidly heated and held at 1000C for 30 minutes under a flow of nitrogen. The products were cooled quickly. AEter crush~
ing, the 4.5g. of products were washed in hot water at 60C
to remove sodium carbonate and any residual sodium chromate.
Residual sbdium carbonate was removed by washing the product wlth 1:10 hydroc}lloric acid.
~ . , - . ~
.,., : - , . , ,, .: , . .
lOSD~3315~
Any excess carhon was removed by roas~ing the leached product in air at 1000C for one hour. The Einal weiyht of product obtained was 1.80g. This product analyzed 67.3% Cr, giving a Cr2O3 content of 98.4%.
EX~MPLE 3 This example illustrates the formation of chromium carbides from chromic oxide.
lOOg. of -400 mesh co~mercial grade chromic oxide (98.5% Cr2O3) was dry mixed with 72g. of -400 mesh graphite (95.0% fixed carbon). The mixture contained 2 times the stoichiometric carbon requirement based on the reaction:
3cr23 + 13C ~ 2Cr3C2 + 9CO
was pelletized with moisture and the pellets were dried.
The pellets were fired in alumina crucibles in the presence of argon at a flow rate of 0.05 CFM at about 1300C for 2 hours. After cooling, the pellets were crushed and the bulk of the excess carbon was removed by flotation in water. The product was subjected to X-ray diffraction analysis and Eound to ha~e the approximate composition:
80% Cr3C2, 15% Cr7C3 and 5% graphite. The product analysed 72.5% Cr and 14.5% C.
The procedure of Example 3 was repeated with a firing temperature of from 1300 to 1380C for a period of
3 3/4 hours 320g. of pellets were fired and 218.6g. of product were obtained.
The product was yround and screened on 100 and 400 mesh screens to separate excess carbon liberated d~ring the crushing. The quantity, composition and X-ray diffraction - 17 ~
- ~
: . :
' ,: . ~:
~5~33~ . :
analysis fox each Eraction :is reproduced in the following Table I:
TABL~ I
..
Fraction Wt Wt.% W~.% X-ray Dif~raction ~nalysis (Mesh) (g) Cr C Cr7C3 Cr3C2 Free C
Wt.% Wt.g Wt.% Wt~q ~t.% Wt.q ..
+10022.3 80.0 11.7 20.0 4.58 80.0 18.3 c1.0 0.2 -100 +49.7 73.3 23.1 10.0 4.97 g0.0 39.8 10.0 5.0 -400 145.0 50.0 47.7 5.0 7.25 75.0 10~.8 20.0 29.0 :~ ' ', ' Example 3 was repeated, with the pellets being for~ed by dry mixing the chromic oxide and graphite, slurrying the mix-ture in water and then vigorously stirring the mixture until it became uniform in colour. The excess water was evaporated slow-ly and the resulting mixture pelletized to form pellets 3/4" to 1" in diameter. The pellets were dried at 100C for 5 hours. - ~
The dried pellets were fired in argon for 7 hours and 50 minu-tes ~' at temperatures ranging from 1300 to 1450C. Only the stoiFhiometric requirement of graphite for the equation in Example 3 was used.
227.6g. of product in the form of very hard grey pellets were ohtained. The pellets were crushed and the powder analysed for Cr and C and subjected to X-ray diffract-ion analysis. The product contained 83.7% Cr and 11.65% C
with a Cr recovery of 97.3%. The X-ray diffraction analysis indicated that the product was composed of approximately 75% Cr3C2 and 25% Cr7C3.
.- :: ' .'-: - - :.: .. , .
. . . . . .
. . .:. . . . , . ,. . :
~5~331~ :
Example 5 was repeated with half the stoichiometric carbon require~ent for the equation in Example 3. The pellets were fired in argon for 6 hours at temperatures from 1300 to 1400C.
The crushed product was analysed for Cr and C
and subjected to X-ray dlffraction analysis, which revealed ~i -a product containing 75.0% Cr and 2.49% C and composed of approximately 70% Cr203 and 30% Cr23C6 (Cr4C).
This example shows the formation of sponge chromium from chromium carbides.
The -100 ~ 400 mesh fraction obtained in Example 4 was subjected to elutriation in a column of water flowing upwardly at a vertical linear speed of 4 inches per minute.
Heavy and light fractions were obtained and 20g. o~ the light fraction, containing 71.0% Cr and 25.3% C, were mixed with 19.23g. of commercial grade chromic oxide. The mixture was pressed to form a 1" diameter x 1~" high compact con-taining 90% of the stoichiometric oxygen required to formCO with the carbon content.
The charge was placed in a molybdenum foil crucible and heated via a molybdenum susceptor under a vacuum of 27 to 30 mm Hg. After 1 hour and 25 minutes, the temperature at the top of the compa~t had reached 2420F while that at the bottom had reached 2850F. At this point some vapour was ~, evolved from the compact and droplets were observed adhering to the inner surface of the molybdenum foil crucible, anA
hence the compact was adjudged partially molten.
.. J
:. ' '' ' ., .
:~5~338 ~ The power then was shut off and after cooliny, 31.85g of product were recovered. The product was crush~d and subjected to wet chemical and X-ray diffraction analysis.
The product analysed 94.6% Cr, 1.30% C and contained approxi~
mately 65% Cr metal, 20% Cr2 C6 and the balance unidentified constituents which were neither normal chromium carbides nor oxides. ;
This example shows the production of sponge chromium directly from chromic oxide.
Finely divided chromic oxide sized -400 mesh and of purity 98%+ was slurried with graphite sized -400 mesh (95% C) at high speed for about 20 minutes after which the slurry was partially dried and formed into a compact.
The ~uantity of graphite was stoichiometric to form carbon monoxide from the oxide in accordance with the equation:
Cr203 ~ 3C ______~7 2Cr + 3CO
A total weight of 21.7g of material was slurried of which 80~3% was chromic oxide.
The compact was placed in an alumina crucible in a chamber maintained under a vacuum of 70 cms of mercury.
The compact was heated at a temperature of about 1500 to 1700C until it exhibited a melted appearance after about 15 minutes. After cooling, it was found that a loss of about 10% of Cr values had occurred, probably due to vaporization, and the product wa~ found by X-ray diffraction analysis to be 95% sponge chromium and 5% chromic oxide.
The present invention therefore provides a process for the production of sponge chromium from low grade chromite iii".,~ . ' 5~338 ~ ~
ores in high yield and in a non-polluting manner. The invention clearly is applicable to the production oP other metals from low ~rade ores by the same procedures. ~ , Modifications are possible within the scope of the present invention.
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_~'PLEMENTP.PY DISCLOSURE~
In the principal disclosure, there is described a procedure for the formation of chromium carbide from low g~ade chromite ores. In the specific procedure described in the principal disclosure, a sodium chromate is recovered which contains the chromium values of the ore and the sodium chromate is subjected to a solid state reduction with carbon to form chromium oxide which in turn i5 subjected to a solid state reduction with carbon to form the chromium carbide.
The chromium carbide may be converted to chromium metal.
It has now been found, in accordance with this supplementary disclosure, that the sodium chromate may ~e converted directly in a single solid state reduction reaction ~ -to the chromium carbide or to the chromium metal. ~ ;
High ferrochrome alloys are used in steel making, containing typically 55 to 66% Cr, 7 to 8% C and the balance Fe, and conventionally are formed from high grade chromite ore using carbon as a reducing agent. Low carbon ferro~
chromium is produced from high grade ore and silicon-chromium , . .
alloys as reducing agent. In accordance with this supplemen- ~
tary disclosure, a high carbon ferrochrome alloy is formed ~ ;
from low ~rade chromite ores.
. .
In the principal dis~losure, it is indicaked that ;
in the acidification of the leach solution r~sulting from leachin~ the solubilized values of the ore, it is preferred to form sodium dichromate owing to its greater solubility ~ ' than sodium chromate. In this embodiment, sodium bicarbonate preferably is preferentially crystallized from the acidifi~d solution and recycled to the ore roasting step. After ; 30 removal of the bulk of the excess alkali values in this way, the chromate solution is slurried with powdered carhon and . ~ ., . :
evaporated to dryness to form an intimate admixture of sodium ;-~
- 2 2 - ~ :
B ~ -.
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~05~3313 chromate and carbo~. Sodium carbonate resulting from the reduction of sodium chromate and present in the intimate admixture may be leached from the products of the reduction and recycled to the ore roasting step.
When the acidification of the leach liquor i5 such as to provide the chromate, it is preferred to slurry the powdered carbon directly into the acidified solu~ion and evaporate the slurry to dryness to form an intimate admixture ~ of the components of the slurry. Following-the solid state reduction, sodium carbonate may be separated and recycled to the ore roasting step.
In accordance with this supplementary disclosure, the sodium chromate is subjected to a solid s~ate reduction ` ~ with carbon to form a chromium carbide or chromium metal.
The reduction of sodium dichromate to a chromium carbide may be represented by the reaction~
;~ 2Na2Cr2O7 + llC ~ -> Cr4C ~ 2Na2CO3 + 8CO - (5) ;~ As indicated in the principal disclosure the chromium carbide Cr4C is sometimes wrltten, perhaps more accuxately, as Cr23C6.
The temperature employed in the reduction reaction of equation (5) is the same as that employed ~or the reaction of equation (2) in the principal disclosure. Thus, the - temperature range may vary over a wide range, typically from about 1025C to 1425C or higher. Over this temperature range, i the partial pressure o~ carbon noxide formed varies widely from about 0.09 atmospheres at 1025C to about 21.0 atmospheres at 1425C and dictates to some extent the technique employed to remove the carbon monoxide. Removal of the carbon monoxid~
from the reaction vessel promotes ~he reduction reaction. ~,~
At low partial pressures of carbon monoxide below about 0.1 atmospheres, experienced at the low end of the temperature ~ ~'.
, ..... , .. ~ ,.. . .. . . . ..
: ~ , . ........................ .
~S4L33 51 ~
range, vacuum may be used to remove the carbon morcxide.
At higher partial pressures above about 0.1 atmo~pheres, the carbon monoxide may be flushed from the reaction vessel with an inert gas stream, such as argon.
It is preferred to flush the carbon monoxide from the reaction vessel by an inert gas stream, and hence the higher temperatures are used, generally above about 1200C. ~
The reduction is contlnued until substantially ~ i complete reaction is achieved, generally involving a time span of about 2 to 4 hours for quantities of the order of 1 lb.
The precise form of the chromium carbide depends on the quantity o~ carbon used and the reaction tempexature.
Preferably, the am~unt of carbon employed corresponds to the stoichiometry of the above equation, although other carbides, :: .
such as Cr3C2 and Cr7C3 may be formed at will by use of the , , .
appropriate quantity of carbon and temperature range.
This solid state reduction reaction may be used to form other metallic carbides~ The temperature ranges used .
for the formati~n of such other metallic aarbides may vary from the range used ~or the forma~ion of chromium carbides. i . ~ . .-For examplej the equivalent redu~tion of a permanganate to manganese carbide, Mn7C3 may be carried out conveniently in a rotary kiln at a temperature in the range of about 1125C to about 1325C, with the carbon monoxide being .. ..
removed using an inert gas stream since the partial pressure of carbon monoxide increases from about 0.1 atmospheres at 1125C to about 2.1 atmospheres at 1325C.
As described in the principal disclosure, the chromium carbide, i~ desired, may be reacted in a solid state reaction with chromium oxide to form sponge chromium B
.
i . . .. . , ' . . " . ' . . . .
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~a~s43;~3 ~
in a substantially pure form in accordance with the reaction of equation (4). Other metal carbides formed by the procedure of this supplementary disclosure may be converted to the ~ :
appropriate metal by a solid state reaction with the corresponding metal oxide, as described in the principal disclosure. ~-.
The invention is illustrated by the further ~ ;
accompanying drawing, in which:
Figure 2 is a flow sheet of an alternative embodiment of the invention to that illustrated in Figure 1 in ~he :~ :
principal disclosure.
., Referring to Figure 2, a chromite ore concentrate, which first may be pelletized~ i5 fed by line 110 to a roaster 112. The chromite ore concentrate may be a CQn- ~;
centrated low grade chromite ore, the low grade ore typically containing 15 to 26% Cr2O3 and the concentrated ore typically .
about 38 to 45% Cr2O3, with a Cr:Pe ratio ln concentrated . .
ore of about 1.5. The ore is roasted in the roaster 112 of any convenient constructiont such as, a rotary kiln, under any convenient condi~ions in admixture with sodium carbonate fed by line 114 to the roaster 112. The roasting is carried out in the presence of molecular oxygen which may be provided , ~
by air fed by line 116.
. The roasting is carried out under conditions to '.;~
oxidize substantially all the chromium values o the ore to sodium chromate, typically by roasting at about 850C for about - 25 ~
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2 hours.
The quantity of sodium carbonate added by line 114 to the roaster 112 is generally in excess of the quantity required to achieve the desired degree of solubilization of the solubilizable materials of the ore concentrate, including ~ -the chromium, aluminum and silicon values of the concentrates. ?
This excess is used to ensure tha~ the majority, and preferably substantially all, of the chromium values are converted to sodium chromate, with varying degrees of conversion of silica and alumina to their soluble form.
The roasted mixture, after cooling, quenching and crushing in any desired manner, is forwarded by line 118 to a ~-leacher 1?0 ~herein the roasted mixture is contacted with water fed by line 122 to dissolve soluble materials from the roasted mixture. The insoluble gangue components and the iron oxide values of the original concentrate are separated from the resulting leach li~uor 501utio~ and removad from he leacher by line ~24.
- The leach li~uor contains dissolved sodium salts 20 ~ including the chromate, aluminate, silicate and carbonate.
The carbonate is present due to the use of excess sodium carbonate in the roasting step. However, since substantially no iron values are solubilized during the roasting step, substantially complete eparation of the chromium values from ;
the iron value~ of the ore concentrate i achieved.
The leach liquor then is fed to a reactor 126 by ;~
line 128 and acidified therein by bubbling carbon dioxide `~
i therethrough fed by line 130. Preferably, the leach liquor has a temperature of about 5 to 10C during the acidifying and the carbon dioxide is present as an a$mosphere thereof at an elevated pressure. The acidification of the leach liquor B :; :
.
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... .. . ... . . . .. , . - .. .... . . ........... . . . .
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105~33~
achieved in this ~ay cause 5 precipitation of alumina and silica from the leach liquor. The precipitated alumina and silica are separated from the mother liquor and removed by line 132. The acidification may be continued to a pH at which the chromate ions are converted to dichromate ions, the solubility of sodium dichromate in aqueous solution being considerably greater than that of sodium chromate, whil the sodium carbonate is converted into sodium bicarbonate. The leach liquor should be maintained at temperatures such as 5 to 10C during the bubbling of the carbon dioxide therethrough.
The aqueous solution of sodium bicarbonate and sodium dichromate resulting from the acidifierl26 is evaporated in an evaporator 13~, the evaporated water being collected by line ~36. Differential crystallization of at least part of the sodium biczrbonate occurs and the sodium bicarbonate is removed by line 138 for recycle to the roaster 112 by line 140. Thereafter, powdered carbon is added to the mother liquor by line 142, together with part of the iron oxide recovered from the ore by line 124 if it is desired to form ~20 hi~h carbon ferrochrome, as discussed in more detail below, the iron oxide ~einy added by line 144. The mother liquor is slurried with the additives and evaporated to dryness to provide a solid intimate mixture of sodium dichromate, sodium bicarbonate, carbon and, possibly, iron oxide.
Where acidification only to a pH of about 6 occurs, the chromium values remain in the aqueous solution as sodium chromate while the bulk of the silica and alumina values present are precipitated and removed from the mother liquor.
Separate crystallization of sodiurn carbonate generally is omitted, although a limited amount of sodium carbonate may be removedl if desired, by crystallization. The mother liquor ': ~ , ~3 , .
... ,.,_,.,. . . . - . :
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5433~
is slurried with additives, as in the case of the sodium dichromate solu*ion, and evaporated to dryness to provide a similar intimate mixture, in this case containing sodium chromate, sodium carbonate, carbon and, possibly, iron oxide.
The solid mixture then is forwarded by line 146 to a kiln 148 wherein the mixture is heated at an elevated ternperature to effect a solid state reduction of the sodium dichromate to chromium carbide and sodium carbonate.
The solid mixture of chromic carbide and sodium carbonate, af~er cooling, if desired, is passed by line 150 to a leacher 152 wherein the sodium carbonate values are leached from the mixture by water fed by line 154 to leave substantially pure chromium carbide. Washing wi~h ailute acid may be used to remove residual sodiu~ carbonate and thereby ~ : :
further purify the chromium carbide productO
The aqueous solution of sodium carbonate resulting ~ ~ ~
from the leaching in the leacher 152 is removed by line 156. ~ ~:
The sodium carbonate values are recovered therefrom by -crystallizati~n and the solid sodium carbonate prefera~ly ~ :-is recycled to the sodium carbonate feed in line 114 as at least part thereof along with the sodium bicarbonate in line 140. Under ideal conditions, the quantity of sodium carbonate recovered from the solution in lines 138 and 156 is substantially the same as that fed to the ore concentrate by line 114. ~owever, losses of ~odium carbonate values may occur and hence it may be necessary to supplement the recyled sodium carbonate material with additional sodium carbonate by line 158 ~o provide the required amount in line 114.
The chromium carbide resulting from the leacher :
152 is removed theref~om by line 160. Preferably, the quantity of carbon fed by line 142 is stoichiometrically , - 28 -. . - , . . .
. .. . .
31(~54338 equivalent t~ that required to form Cr4C~
The chromium carbide, typically Cr4C, recovered in line 160 may be used as a high chromium additive in steel making.
In the il~ustrated emhodiment wherein iron oxide is included in the mixture fed to the kiln by addition by linel~4, appropriate proportioning of the components allows the production of a high ferrochrome alloy for use in steel making. Thus, in accordance with this procedure, there is produced, for the first time, a high carbon ferrochrome alloy from a low grade chromite ore without smPlting~
The invention is illustrated by the following :~
further Example:.
Example 9 :, .
This example illustrates ~he conversion of sodium chromate to chromium carbide. I
A kiln charg of 340.4g of sodium chromate and 12n.4g of -400 mesh graphite ~95% fixed C) equivalent to 220% of the stoichiometric C requirement o~ the equation~
.
4Na2CrO4 + 9C - ~ Cr4C ~ 4Na2CO3 + 4CO : `
was prepared and the kiln placed in a furnace preheated to .
1000C and nitrogen was passed through the kiln at a flow rate of about 50 ml/min. The kiln was rotated a~ about ~-S RPM and the ~harge temperature increased rapidly ~o 900C
in about 1 hour.
Thereafter, the furnace temperatre was increased ~ -.. . . .
slowly to about 1270C, increasing thP charge temperature to about 1200C. The nitrogen flow was maintained throughout the 6 1/2 hour test operation. At ~he end of the test, the kiln was removed from the furnace and rapidly cooled to room température, the nitrogen flow being maintained during cooling.
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~5~33~
The thermocouple sheath failed during the experiment and the sheath was obsexved to have a 1/4 inch thick coating of a metallic deposi~ plus a small quantity of non-metallic material adhering to it. The sheath analyzed 28.9 wt% Cr, 69.5% Fe and 4.6g C. The metallic portion bf the sheath coating analyzed 90% Cr7C3.
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The product was yround and screened on 100 and 400 mesh screens to separate excess carbon liberated d~ring the crushing. The quantity, composition and X-ray diffraction - 17 ~
- ~
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~5~33~ . :
analysis fox each Eraction :is reproduced in the following Table I:
TABL~ I
..
Fraction Wt Wt.% W~.% X-ray Dif~raction ~nalysis (Mesh) (g) Cr C Cr7C3 Cr3C2 Free C
Wt.% Wt.g Wt.% Wt~q ~t.% Wt.q ..
+10022.3 80.0 11.7 20.0 4.58 80.0 18.3 c1.0 0.2 -100 +49.7 73.3 23.1 10.0 4.97 g0.0 39.8 10.0 5.0 -400 145.0 50.0 47.7 5.0 7.25 75.0 10~.8 20.0 29.0 :~ ' ', ' Example 3 was repeated, with the pellets being for~ed by dry mixing the chromic oxide and graphite, slurrying the mix-ture in water and then vigorously stirring the mixture until it became uniform in colour. The excess water was evaporated slow-ly and the resulting mixture pelletized to form pellets 3/4" to 1" in diameter. The pellets were dried at 100C for 5 hours. - ~
The dried pellets were fired in argon for 7 hours and 50 minu-tes ~' at temperatures ranging from 1300 to 1450C. Only the stoiFhiometric requirement of graphite for the equation in Example 3 was used.
227.6g. of product in the form of very hard grey pellets were ohtained. The pellets were crushed and the powder analysed for Cr and C and subjected to X-ray diffract-ion analysis. The product contained 83.7% Cr and 11.65% C
with a Cr recovery of 97.3%. The X-ray diffraction analysis indicated that the product was composed of approximately 75% Cr3C2 and 25% Cr7C3.
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. . . . . .
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~5~331~ :
Example 5 was repeated with half the stoichiometric carbon require~ent for the equation in Example 3. The pellets were fired in argon for 6 hours at temperatures from 1300 to 1400C.
The crushed product was analysed for Cr and C
and subjected to X-ray dlffraction analysis, which revealed ~i -a product containing 75.0% Cr and 2.49% C and composed of approximately 70% Cr203 and 30% Cr23C6 (Cr4C).
This example shows the formation of sponge chromium from chromium carbides.
The -100 ~ 400 mesh fraction obtained in Example 4 was subjected to elutriation in a column of water flowing upwardly at a vertical linear speed of 4 inches per minute.
Heavy and light fractions were obtained and 20g. o~ the light fraction, containing 71.0% Cr and 25.3% C, were mixed with 19.23g. of commercial grade chromic oxide. The mixture was pressed to form a 1" diameter x 1~" high compact con-taining 90% of the stoichiometric oxygen required to formCO with the carbon content.
The charge was placed in a molybdenum foil crucible and heated via a molybdenum susceptor under a vacuum of 27 to 30 mm Hg. After 1 hour and 25 minutes, the temperature at the top of the compa~t had reached 2420F while that at the bottom had reached 2850F. At this point some vapour was ~, evolved from the compact and droplets were observed adhering to the inner surface of the molybdenum foil crucible, anA
hence the compact was adjudged partially molten.
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:~5~338 ~ The power then was shut off and after cooliny, 31.85g of product were recovered. The product was crush~d and subjected to wet chemical and X-ray diffraction analysis.
The product analysed 94.6% Cr, 1.30% C and contained approxi~
mately 65% Cr metal, 20% Cr2 C6 and the balance unidentified constituents which were neither normal chromium carbides nor oxides. ;
This example shows the production of sponge chromium directly from chromic oxide.
Finely divided chromic oxide sized -400 mesh and of purity 98%+ was slurried with graphite sized -400 mesh (95% C) at high speed for about 20 minutes after which the slurry was partially dried and formed into a compact.
The ~uantity of graphite was stoichiometric to form carbon monoxide from the oxide in accordance with the equation:
Cr203 ~ 3C ______~7 2Cr + 3CO
A total weight of 21.7g of material was slurried of which 80~3% was chromic oxide.
The compact was placed in an alumina crucible in a chamber maintained under a vacuum of 70 cms of mercury.
The compact was heated at a temperature of about 1500 to 1700C until it exhibited a melted appearance after about 15 minutes. After cooling, it was found that a loss of about 10% of Cr values had occurred, probably due to vaporization, and the product wa~ found by X-ray diffraction analysis to be 95% sponge chromium and 5% chromic oxide.
The present invention therefore provides a process for the production of sponge chromium from low grade chromite iii".,~ . ' 5~338 ~ ~
ores in high yield and in a non-polluting manner. The invention clearly is applicable to the production oP other metals from low ~rade ores by the same procedures. ~ , Modifications are possible within the scope of the present invention.
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_~'PLEMENTP.PY DISCLOSURE~
In the principal disclosure, there is described a procedure for the formation of chromium carbide from low g~ade chromite ores. In the specific procedure described in the principal disclosure, a sodium chromate is recovered which contains the chromium values of the ore and the sodium chromate is subjected to a solid state reduction with carbon to form chromium oxide which in turn i5 subjected to a solid state reduction with carbon to form the chromium carbide.
The chromium carbide may be converted to chromium metal.
It has now been found, in accordance with this supplementary disclosure, that the sodium chromate may ~e converted directly in a single solid state reduction reaction ~ -to the chromium carbide or to the chromium metal. ~ ;
High ferrochrome alloys are used in steel making, containing typically 55 to 66% Cr, 7 to 8% C and the balance Fe, and conventionally are formed from high grade chromite ore using carbon as a reducing agent. Low carbon ferro~
chromium is produced from high grade ore and silicon-chromium , . .
alloys as reducing agent. In accordance with this supplemen- ~
tary disclosure, a high carbon ferrochrome alloy is formed ~ ;
from low ~rade chromite ores.
. .
In the principal dis~losure, it is indicaked that ;
in the acidification of the leach solution r~sulting from leachin~ the solubilized values of the ore, it is preferred to form sodium dichromate owing to its greater solubility ~ ' than sodium chromate. In this embodiment, sodium bicarbonate preferably is preferentially crystallized from the acidifi~d solution and recycled to the ore roasting step. After ; 30 removal of the bulk of the excess alkali values in this way, the chromate solution is slurried with powdered carhon and . ~ ., . :
evaporated to dryness to form an intimate admixture of sodium ;-~
- 2 2 - ~ :
B ~ -.
.' i i . ' . :. ., .. '' ;, ' . i, , , , !
, . ! ' ' ' , , , . ' . . ' ' ' . ' ' ~ ~
~05~3313 chromate and carbo~. Sodium carbonate resulting from the reduction of sodium chromate and present in the intimate admixture may be leached from the products of the reduction and recycled to the ore roasting step.
When the acidification of the leach liquor i5 such as to provide the chromate, it is preferred to slurry the powdered carbon directly into the acidified solu~ion and evaporate the slurry to dryness to form an intimate admixture ~ of the components of the slurry. Following-the solid state reduction, sodium carbonate may be separated and recycled to the ore roasting step.
In accordance with this supplementary disclosure, the sodium chromate is subjected to a solid s~ate reduction ` ~ with carbon to form a chromium carbide or chromium metal.
The reduction of sodium dichromate to a chromium carbide may be represented by the reaction~
;~ 2Na2Cr2O7 + llC ~ -> Cr4C ~ 2Na2CO3 + 8CO - (5) ;~ As indicated in the principal disclosure the chromium carbide Cr4C is sometimes wrltten, perhaps more accuxately, as Cr23C6.
The temperature employed in the reduction reaction of equation (5) is the same as that employed ~or the reaction of equation (2) in the principal disclosure. Thus, the - temperature range may vary over a wide range, typically from about 1025C to 1425C or higher. Over this temperature range, i the partial pressure o~ carbon noxide formed varies widely from about 0.09 atmospheres at 1025C to about 21.0 atmospheres at 1425C and dictates to some extent the technique employed to remove the carbon monoxide. Removal of the carbon monoxid~
from the reaction vessel promotes ~he reduction reaction. ~,~
At low partial pressures of carbon monoxide below about 0.1 atmospheres, experienced at the low end of the temperature ~ ~'.
, ..... , .. ~ ,.. . .. . . . ..
: ~ , . ........................ .
~S4L33 51 ~
range, vacuum may be used to remove the carbon morcxide.
At higher partial pressures above about 0.1 atmo~pheres, the carbon monoxide may be flushed from the reaction vessel with an inert gas stream, such as argon.
It is preferred to flush the carbon monoxide from the reaction vessel by an inert gas stream, and hence the higher temperatures are used, generally above about 1200C. ~
The reduction is contlnued until substantially ~ i complete reaction is achieved, generally involving a time span of about 2 to 4 hours for quantities of the order of 1 lb.
The precise form of the chromium carbide depends on the quantity o~ carbon used and the reaction tempexature.
Preferably, the am~unt of carbon employed corresponds to the stoichiometry of the above equation, although other carbides, :: .
such as Cr3C2 and Cr7C3 may be formed at will by use of the , , .
appropriate quantity of carbon and temperature range.
This solid state reduction reaction may be used to form other metallic carbides~ The temperature ranges used .
for the formati~n of such other metallic aarbides may vary from the range used ~or the forma~ion of chromium carbides. i . ~ . .-For examplej the equivalent redu~tion of a permanganate to manganese carbide, Mn7C3 may be carried out conveniently in a rotary kiln at a temperature in the range of about 1125C to about 1325C, with the carbon monoxide being .. ..
removed using an inert gas stream since the partial pressure of carbon monoxide increases from about 0.1 atmospheres at 1125C to about 2.1 atmospheres at 1325C.
As described in the principal disclosure, the chromium carbide, i~ desired, may be reacted in a solid state reaction with chromium oxide to form sponge chromium B
.
i . . .. . , ' . . " . ' . . . .
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~a~s43;~3 ~
in a substantially pure form in accordance with the reaction of equation (4). Other metal carbides formed by the procedure of this supplementary disclosure may be converted to the ~ :
appropriate metal by a solid state reaction with the corresponding metal oxide, as described in the principal disclosure. ~-.
The invention is illustrated by the further ~ ;
accompanying drawing, in which:
Figure 2 is a flow sheet of an alternative embodiment of the invention to that illustrated in Figure 1 in ~he :~ :
principal disclosure.
., Referring to Figure 2, a chromite ore concentrate, which first may be pelletized~ i5 fed by line 110 to a roaster 112. The chromite ore concentrate may be a CQn- ~;
centrated low grade chromite ore, the low grade ore typically containing 15 to 26% Cr2O3 and the concentrated ore typically .
about 38 to 45% Cr2O3, with a Cr:Pe ratio ln concentrated . .
ore of about 1.5. The ore is roasted in the roaster 112 of any convenient constructiont such as, a rotary kiln, under any convenient condi~ions in admixture with sodium carbonate fed by line 114 to the roaster 112. The roasting is carried out in the presence of molecular oxygen which may be provided , ~
by air fed by line 116.
. The roasting is carried out under conditions to '.;~
oxidize substantially all the chromium values o the ore to sodium chromate, typically by roasting at about 850C for about - 25 ~
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2 hours.
The quantity of sodium carbonate added by line 114 to the roaster 112 is generally in excess of the quantity required to achieve the desired degree of solubilization of the solubilizable materials of the ore concentrate, including ~ -the chromium, aluminum and silicon values of the concentrates. ?
This excess is used to ensure tha~ the majority, and preferably substantially all, of the chromium values are converted to sodium chromate, with varying degrees of conversion of silica and alumina to their soluble form.
The roasted mixture, after cooling, quenching and crushing in any desired manner, is forwarded by line 118 to a ~-leacher 1?0 ~herein the roasted mixture is contacted with water fed by line 122 to dissolve soluble materials from the roasted mixture. The insoluble gangue components and the iron oxide values of the original concentrate are separated from the resulting leach li~uor 501utio~ and removad from he leacher by line ~24.
- The leach li~uor contains dissolved sodium salts 20 ~ including the chromate, aluminate, silicate and carbonate.
The carbonate is present due to the use of excess sodium carbonate in the roasting step. However, since substantially no iron values are solubilized during the roasting step, substantially complete eparation of the chromium values from ;
the iron value~ of the ore concentrate i achieved.
The leach liquor then is fed to a reactor 126 by ;~
line 128 and acidified therein by bubbling carbon dioxide `~
i therethrough fed by line 130. Preferably, the leach liquor has a temperature of about 5 to 10C during the acidifying and the carbon dioxide is present as an a$mosphere thereof at an elevated pressure. The acidification of the leach liquor B :; :
.
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-. ... . . . ... .. . .
... .. . ... . . . .. , . - .. .... . . ........... . . . .
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105~33~
achieved in this ~ay cause 5 precipitation of alumina and silica from the leach liquor. The precipitated alumina and silica are separated from the mother liquor and removed by line 132. The acidification may be continued to a pH at which the chromate ions are converted to dichromate ions, the solubility of sodium dichromate in aqueous solution being considerably greater than that of sodium chromate, whil the sodium carbonate is converted into sodium bicarbonate. The leach liquor should be maintained at temperatures such as 5 to 10C during the bubbling of the carbon dioxide therethrough.
The aqueous solution of sodium bicarbonate and sodium dichromate resulting from the acidifierl26 is evaporated in an evaporator 13~, the evaporated water being collected by line ~36. Differential crystallization of at least part of the sodium biczrbonate occurs and the sodium bicarbonate is removed by line 138 for recycle to the roaster 112 by line 140. Thereafter, powdered carbon is added to the mother liquor by line 142, together with part of the iron oxide recovered from the ore by line 124 if it is desired to form ~20 hi~h carbon ferrochrome, as discussed in more detail below, the iron oxide ~einy added by line 144. The mother liquor is slurried with the additives and evaporated to dryness to provide a solid intimate mixture of sodium dichromate, sodium bicarbonate, carbon and, possibly, iron oxide.
Where acidification only to a pH of about 6 occurs, the chromium values remain in the aqueous solution as sodium chromate while the bulk of the silica and alumina values present are precipitated and removed from the mother liquor.
Separate crystallization of sodiurn carbonate generally is omitted, although a limited amount of sodium carbonate may be removedl if desired, by crystallization. The mother liquor ': ~ , ~3 , .
... ,.,_,.,. . . . - . :
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5433~
is slurried with additives, as in the case of the sodium dichromate solu*ion, and evaporated to dryness to provide a similar intimate mixture, in this case containing sodium chromate, sodium carbonate, carbon and, possibly, iron oxide.
The solid mixture then is forwarded by line 146 to a kiln 148 wherein the mixture is heated at an elevated ternperature to effect a solid state reduction of the sodium dichromate to chromium carbide and sodium carbonate.
The solid mixture of chromic carbide and sodium carbonate, af~er cooling, if desired, is passed by line 150 to a leacher 152 wherein the sodium carbonate values are leached from the mixture by water fed by line 154 to leave substantially pure chromium carbide. Washing wi~h ailute acid may be used to remove residual sodiu~ carbonate and thereby ~ : :
further purify the chromium carbide productO
The aqueous solution of sodium carbonate resulting ~ ~ ~
from the leaching in the leacher 152 is removed by line 156. ~ ~:
The sodium carbonate values are recovered therefrom by -crystallizati~n and the solid sodium carbonate prefera~ly ~ :-is recycled to the sodium carbonate feed in line 114 as at least part thereof along with the sodium bicarbonate in line 140. Under ideal conditions, the quantity of sodium carbonate recovered from the solution in lines 138 and 156 is substantially the same as that fed to the ore concentrate by line 114. ~owever, losses of ~odium carbonate values may occur and hence it may be necessary to supplement the recyled sodium carbonate material with additional sodium carbonate by line 158 ~o provide the required amount in line 114.
The chromium carbide resulting from the leacher :
152 is removed theref~om by line 160. Preferably, the quantity of carbon fed by line 142 is stoichiometrically , - 28 -. . - , . . .
. .. . .
31(~54338 equivalent t~ that required to form Cr4C~
The chromium carbide, typically Cr4C, recovered in line 160 may be used as a high chromium additive in steel making.
In the il~ustrated emhodiment wherein iron oxide is included in the mixture fed to the kiln by addition by linel~4, appropriate proportioning of the components allows the production of a high ferrochrome alloy for use in steel making. Thus, in accordance with this procedure, there is produced, for the first time, a high carbon ferrochrome alloy from a low grade chromite ore without smPlting~
The invention is illustrated by the following :~
further Example:.
Example 9 :, .
This example illustrates ~he conversion of sodium chromate to chromium carbide. I
A kiln charg of 340.4g of sodium chromate and 12n.4g of -400 mesh graphite ~95% fixed C) equivalent to 220% of the stoichiometric C requirement o~ the equation~
.
4Na2CrO4 + 9C - ~ Cr4C ~ 4Na2CO3 + 4CO : `
was prepared and the kiln placed in a furnace preheated to .
1000C and nitrogen was passed through the kiln at a flow rate of about 50 ml/min. The kiln was rotated a~ about ~-S RPM and the ~harge temperature increased rapidly ~o 900C
in about 1 hour.
Thereafter, the furnace temperatre was increased ~ -.. . . .
slowly to about 1270C, increasing thP charge temperature to about 1200C. The nitrogen flow was maintained throughout the 6 1/2 hour test operation. At ~he end of the test, the kiln was removed from the furnace and rapidly cooled to room température, the nitrogen flow being maintained during cooling.
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.
~5~33~
The thermocouple sheath failed during the experiment and the sheath was obsexved to have a 1/4 inch thick coating of a metallic deposi~ plus a small quantity of non-metallic material adhering to it. The sheath analyzed 28.9 wt% Cr, 69.5% Fe and 4.6g C. The metallic portion bf the sheath coating analyzed 90% Cr7C3.
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Claims (25)
1. A process for the production of chromium carbide from low grade chromite ore containing chromium, iron, aluminum and silicon values, which comprises:
roasting said ore or concentrate thereof with sodium carbonate in the presence of oxygen to solubilize substantially all the chromium and at least part of the aluminum and silicon values of said ore while leaving said iron values substantially unaffected, leaching the roasted ore to separate soluble compounds from gangue constituents including said iron values and the remainder of said aluminum and silicon values, increasing the acidity of the leach solution with carbon dioxide to cause deposition of said soluble aluminum and silicon values, separating the deposited materials from the acidified solution to provide an aqueous solution of a sodium chromate and a sodium carbonate, concentrating the latter aqueous solution to provide a solid mixture of a sodium chromate and a sodium carbonate, providing said solid mixture in finely divided form and in admixture with finely divided carbon and reacting the components of said admixture in the solid state to form a product mixture containing sodium carbonate and chromium oxide, leaching said product mixture to form an aqueous solution of said sodium carbonate, separating said aqueous solution from said chromium oxide, recovering solid sodium carbonate from the aqueous solution, recycling said recovered solid sodium carbonate to form at least part of the sodium carbonate used in said roasting step, reducing at least part of said chromium oxide in intimate admixture with particulate carbon to form a chromium carbide at a temperature of at least 1025°C.
roasting said ore or concentrate thereof with sodium carbonate in the presence of oxygen to solubilize substantially all the chromium and at least part of the aluminum and silicon values of said ore while leaving said iron values substantially unaffected, leaching the roasted ore to separate soluble compounds from gangue constituents including said iron values and the remainder of said aluminum and silicon values, increasing the acidity of the leach solution with carbon dioxide to cause deposition of said soluble aluminum and silicon values, separating the deposited materials from the acidified solution to provide an aqueous solution of a sodium chromate and a sodium carbonate, concentrating the latter aqueous solution to provide a solid mixture of a sodium chromate and a sodium carbonate, providing said solid mixture in finely divided form and in admixture with finely divided carbon and reacting the components of said admixture in the solid state to form a product mixture containing sodium carbonate and chromium oxide, leaching said product mixture to form an aqueous solution of said sodium carbonate, separating said aqueous solution from said chromium oxide, recovering solid sodium carbonate from the aqueous solution, recycling said recovered solid sodium carbonate to form at least part of the sodium carbonate used in said roasting step, reducing at least part of said chromium oxide in intimate admixture with particulate carbon to form a chromium carbide at a temperature of at least 1025°C.
2, The process of claim 1, including intimately mixing said chromium carbide with chromium oxide, and subjecting the resulting mixture to a solid state reaction to form sponge chromium.
3. The process of claim 2 wherein said chromium oxide is divided into two portions, one of said portions being reduced with particulate carbon and the other of said portions constituting said chromic oxide intimately mixed with said chromium carbide.
4. The process of claim 1 wherein said ore or concentrate is roasted with sodium carbonate and lime whereby the bulk of said aluminum and silicon values are insolubilized by said lime and hence remain in said gangue constitutents.
5. The process of claim 1 wherein said intimate admixture of chromic oxide and particulate carbon is formed by slurrying the components sized to at least -300 mesh together in water, stirring the slurry to intermix thoroughly the particles, dewatering the slurry, and recovering a dried intimate admixture.
6. The process of claim 5 wherein the quantity of particulate carbon used is substantially stoichiometrically sufficient to form chromium carbide in accordance with the equation:
2Cr2O3 + 7C ? Cr4C + 6CO
and the carbon monoxide generated is removed from a reaction zone wherein said intimate admixture is reacted.
2Cr2O3 + 7C ? Cr4C + 6CO
and the carbon monoxide generated is removed from a reaction zone wherein said intimate admixture is reacted.
7. The process of claim 6 wherein said reduction is carried out at a temperature of about 1025° to 1425°C.
8. The Process of claim 6 wherein said reduction is carried out at a temperature of at least 1050°C and said carbon monoxide is removed by flowing a stream of an inert gas through said reaction zone.
9. The process of claim 2 wherein said intimate admixture of said other portion of chromic oxide and chromium carbide is formed by slurrying the components sized to at least -300 mesh together in water, stirring the slurry to intermix thoroughly the particles, dewatering the slurry, and recovering a dried intimate admixture.
10. The process of claim 9 wherein the amount of carbide and the amount of oxide reacted are stoichiometric with regard to carbon and oxygen to form carbon monoxide, and the carbon monoxide generated is removed from a reaction zone wherein said intimate admixture is reacted.
11. The process of claim 10 wherein said reaction is carried out at a temperature of at least 1300°C.
12. The process of claim 11 wherein said reaction is carried out at a temperature of about 1300° to 1750°C.
13. The process of claim 11 wherein said reaction is carried out at a temperature of at least 1525°C and said carbon monoxide is removed by flowing a stream of an inert gas through said reaction zone.
14. The process of claim 1 wherein said low grade chromite ore contains a chromium to iron ratio of about 1:1 to 2:1.
CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE
CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE
15. A process for the production of chromium carbide from low grade chromite ore containing chromium, iron, aluminum and silicon values, which comprises:
roasting said ore or concentrate thereof with sodium carbonate in the presence of oxygen to solubilize substantially all the chromium and at least part of the aluminum and silicon values of said ore while leaving said iron values substantially unaffected, leaching the roasted ore to separate soluble compounds from gangue constituents including said iron values and the remainder of said aluminum and silicon values, increasing the acidity of the leach solution with carbon dioxide to cause deposition of said soluble aluminum and silicon values, separating the deposited materials from the acidified solution to provide an aqueous solution of a sodium chromate and a sodium carbonate, concentrating the latter aqueous solution to provide a solid mixture of a sodium chromate and a sodium carbonate, providing said solid mixture in finely divided form and in admixture with finely divided carbon and reacting the components of said mixture in the solid state to form chromium carbide and sodium carbonate, separating said sodium carbonate, recycling said separated sodium carbonate to said roasting step, and recovering said chromium carbide.
roasting said ore or concentrate thereof with sodium carbonate in the presence of oxygen to solubilize substantially all the chromium and at least part of the aluminum and silicon values of said ore while leaving said iron values substantially unaffected, leaching the roasted ore to separate soluble compounds from gangue constituents including said iron values and the remainder of said aluminum and silicon values, increasing the acidity of the leach solution with carbon dioxide to cause deposition of said soluble aluminum and silicon values, separating the deposited materials from the acidified solution to provide an aqueous solution of a sodium chromate and a sodium carbonate, concentrating the latter aqueous solution to provide a solid mixture of a sodium chromate and a sodium carbonate, providing said solid mixture in finely divided form and in admixture with finely divided carbon and reacting the components of said mixture in the solid state to form chromium carbide and sodium carbonate, separating said sodium carbonate, recycling said separated sodium carbonate to said roasting step, and recovering said chromium carbide.
16 The process of claim 15 wherein said reaction of a sodium chromate and carbon produces said chromium carbide directly and said sodium carbonate is separated by leaching the resulting solid to form an aqueous solution of sodium carbonate and leave said chromium carbide which is recovered, and solid sodium carbonate for recycling is recovered from the aqueous solution thereof.
17. The process of claim 16 wherein said leach solution is acidified to provide sodium dichromate in said solution, said solution is evaporated to crystallize sodium bicarbonate therefrom, said deposited sodium bicarbonate is separated from the evaporated solution, the separated sodium bicarbonate is recycled as part of the sodium carbonate in the roasting step, and said reaction mixture is formed by slurrying said carbon in said evaporated solution and thereafter evaporating said slurry to dryness.
18. The process of claim 16 wherein said leach solution is acidified to provide sodium chromate in said solution and said reaction mixture is formed by slurrying said carbon in said acidified solution and thereafter evaporating the slurry to dryness.
19. The process of claim 15 or 16 including providing iron oxide in said reaction mixture.
20. The process of claim 17 or 18 including slurrying iron oxide from said gangue constituents along with said carbon and, by said solid state reaction, forming a ferro-chrome alloy containing 55 to 60% Cr, 7 to 8% C and the balance Fe.
21. The process of claim 16, 17 or 18, wherein said solid mixture of finely-divided carbon and a sodium chromate is reacted in a reaction zone to form a chromium carbide directly by an in situ solid state reaction at a temperature of about 1025°C to about 1425°C while carbon monoxide formed in said solid state reaction is removed from said reaction zone.
22. The process of claim 16, 17 or 18, wherein said solid mixture of finely-divided carbon and a sodium chromate is reacted in a reaction zone to form a chromium`
carbide directly by an in situ solid state reaction at a temperature of about 1200°C to about 1425°C while carbon monoxide is removed from said reaction zone by flushing an inert gas stream through said reaction zone.
carbide directly by an in situ solid state reaction at a temperature of about 1200°C to about 1425°C while carbon monoxide is removed from said reaction zone by flushing an inert gas stream through said reaction zone.
23. The process of claim 15 wherein said finely-divided carbon is graphite and has a particle size of -300 mesh.
24. The process of claim 15 wherein said graphite has a particle size of -400 mesh.
25. The process of claim 16, 17 or 18, wherein sufficient carbon is present in said solid mixture to produce Cr4C as said chromium carbide.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA178,221A CA1054338A (en) | 1973-08-07 | 1973-08-07 | Production of metal carbides and metals |
| CA306,953A CA1053910A (en) | 1973-08-07 | 1978-07-07 | Productions of metals |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA178,221A CA1054338A (en) | 1973-08-07 | 1973-08-07 | Production of metal carbides and metals |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1054338A true CA1054338A (en) | 1979-05-15 |
Family
ID=4097494
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA178,221A Expired CA1054338A (en) | 1973-08-07 | 1973-08-07 | Production of metal carbides and metals |
| CA306,953A Expired CA1053910A (en) | 1973-08-07 | 1978-07-07 | Productions of metals |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA306,953A Expired CA1053910A (en) | 1973-08-07 | 1978-07-07 | Productions of metals |
Country Status (1)
| Country | Link |
|---|---|
| CA (2) | CA1054338A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN107760863A (en) * | 2017-10-31 | 2018-03-06 | 攀钢集团攀枝花钢铁研究院有限公司 | With the baking mixed production method for carrying chromium of material containing chromium |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106591566B (en) * | 2016-12-12 | 2018-01-09 | 嵩县开拓者钼业有限公司 | A kind of method that tungsten associated minerals smelt W metallurgy |
| CN107245576B (en) * | 2017-06-30 | 2018-12-14 | 马鞍山华盛冶金科技发展有限公司 | A kind of preparation method of complex reducing agent and its method for smelting |
| CN109207728B (en) * | 2017-07-07 | 2020-08-14 | 中国科学院过程工程研究所 | Method for extracting chromium and vanadium from chromium-containing vanadium slag by high-temperature roasting method |
| CN109055724B (en) * | 2018-10-11 | 2020-09-01 | 攀钢集团钒钛资源股份有限公司 | Method for extracting vanadium and chromium from chrome vanadium ore/slag |
-
1973
- 1973-08-07 CA CA178,221A patent/CA1054338A/en not_active Expired
-
1978
- 1978-07-07 CA CA306,953A patent/CA1053910A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN107760863A (en) * | 2017-10-31 | 2018-03-06 | 攀钢集团攀枝花钢铁研究院有限公司 | With the baking mixed production method for carrying chromium of material containing chromium |
Also Published As
| Publication number | Publication date |
|---|---|
| CA1053910A (en) | 1979-05-08 |
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