CA1119383A - Recovery of cr.sub.2o.sub.3 from a chromium bearing solution - Google Patents

Abstract

RECOVERY OF Cr2O3 FROM A
CHROMIUM BEARING SOLUTION

Abstract of the Disclosure Method for the recovery of high purity Cr2O3 for use as a pigment or metallurgical additive from a Na2CrO4 or Na2Cr2O7 solution by producing (NH4)2CrO4 by solvent extraction, evaporating the aqueous (NH4)2CrO4 and igniting the resulting solids to provide high purity Cr2O3.

S P E C I F I C A T I O N

Description

~ 9383 9709-1 ~ he present inventlon is dlrected to the processing of Na2cr2o.7 or Na2CrO4 to produce a high purity Cr203. A particular embodiment of the present lnvention is directed to the processlng of chromium ores to obtain high purity Cr203 Chromium compounds are essential to the oil and gas industries for corrosion control and the preparation of catalysts, the food and beverage lndustries in refrigeration brlnes and cleansing compoundsJ the transportation industries ln diesel loco-motlves and automoblles, the iron and steel industries in stalnless 1~ steel metal and chromium plate, the alrcraft lndustry ~or anodizing aluminum and pickling magnesium, the copper industry for descaling brass and stripping copper, the electrical lndustry ln mercury-arc rectifiers and dry cells, the pyrotechnical industries in matches and ~ireworks, the photographic indus~ry in llthography and engraving.
However, the ma~or product ls chromic oxide or Cr203 which is used in metallurgy and as well as a plgment.
In the production of chromium metal from chromic oxlde, a relatively pure grade of Cr203 is desired. ~en the Cr203 material contains significant amounts o~ sodium a~ an impurity, sodium ~aporizes in the vacuum carbon reduction process of producing pure chromium.
mis sodium causes ~ire hazards when lt ls deposlted on the equlpment walls and later exposed to the atmosphere.
Also, it is important in many applications that a Cr23 product be essentially ~ree of aluminum and sulfur impurities.
It is therefore an ob~ect o~ the present lnventlon to provide a method for producing high purity Cr203 from Na2Cr207 or Na2CrO4 or mixtures thereof.
It ls a further ob~ect of the present lnvention to provide a method for producing high purity Cr2o3 from chromium ores.
Other ob~ects will be apparent from the following des-cription and claims taken in con~unction -;~ith the drawlng which sho~s a flowsheet of a method in accordance with the present invention.

,.~ ~

1~9383 Y'709-1 A method in accordance with the present lnventlon comprises:
(i) providing an aqueous solution of Na2Cr207 or Na2CrO4 or mixtures thereof:

(11) adding an acid to the solution of step (i) to provide a pH of about 1 to 2;
(lii) contacting the acidified solution of step (ii) with an o~ganic extractant to recover chromium values from the acidified solutlon in a chromium containing organic liquid phase; ~
(iv) mixing the chromium containing organic phase with a water solution of ~40H to obtain an aqueous solution of (NH4)2CrO4;

(v) sub~ecting the solution of (NH4)2CrO4 to evaporation to provide a solid chromium bearing material;
(vi) ignitlng the solid chromium bearing material to obtain Cr203 .

A p~rticular embodiment of a method in accordance with the present invention as applied to chromium ores comprises:
(i) roastlng in a gaseous oxidizing environment a mixture of chromium ore with Na2C03 and CaO at a~,temperature in the range o~
about 600C to 1100C for from about 0.5 to 5 hours, the amount of Na2C03 being that which provides from about 1.4 to 4.2 pounds o~
Na2C03 per pound of Cr203 in the ore and the amount of CaO being that which provides from about o.6 to 1 pound of CaO per pound o~ Cr203 ln the ore;
(ii) water leach~ng the roasted material obtained in step (i) at a temperature of from about 5C to the boiling point o~ water for from about 5 minutes to 5 hours;
(iii) adding an inorganic acid, e.g. HCl, HN03, H2S04 to the leach llquor obtained in step (ii) to provide a pH of from about 30 3 to 9.5 to cause precipltatlon of aluminum impurlties;

~9383 ~ i J~-l (iv) separating the precipitate frorn the leach liquor and thereafter adding an inorganic ac~d to the leach liquor to provide a p~ o~ about 1 to 2;
(v) Contacting the acldified leach liquor obtained in step (iv) with an or~anic extrac~ant to recover chromium values from the acidified leach liquor in a chromium containing organic llquid phase;
(vi) ~lixing the chromium conta~ning organic phase with a water solution of N~40~r. to obtain an aqueous solution of (~)2CrO4;
(vii) subjecting the solution of (NH4)2CrO4 to evaporation to provide a solid chromium-bearing material;
(viii) igniting the solid chromium-bearing material to obtain Cr203.
In the practice of a particular embodiment the present in-vention, and with reference to the drawing, a natural chromium ore, e. g. Transvaal ore (30 to 50~ Cr203J 15 to 25~ Fe, 2 to 10~ SiO2, 10 to 15/G~ Al~ less than l~o Na, 6-15~ MgO, 0.2-0.6~ Ca) is particulated to a sultable size, e. g. 200 mesh Tyler series and finer as indicated at 10 and mixed with Na2C03 and CaO (the CaO may be inltially present as CaC03) and subjecting the mixture to roasting as indicated at 20.
The amount of Na2C03 and CaO can be varied in the roast between the limits of 1.4 and 4.2 pounds of Na2C03 and o.6 to 1.0 pounds of CaO per pound of Cr203 in the ore.
The roasting temperature can be varied from 600C to 1100C
and the roasting time can be varied from 0.5 hours to 6.o hours.
m e preferred amounts of Na2C03 and CaO are 1.9 and o.6 pounds, respectively. m e preferred temperature and roasting time is 950C for 2 hours. The roasting is conducted in gaseous oxygen 11 19 ~ 3 9703-l environment; by providing an excess of oxygen by passing, e.g.
air, oxygen or a combustion g~s with a sufflcient excess of oxygen~ over the roast bed.
The following equation is representative of the roast reaction using Na2C03to solubilize the chromium 4 (Cr203-FeO) + 8Na2C3 + 72 -~ 2Fe203 ~ 8Na2CrO4 ~ 8co2~
The calcines obtained from the roast are conventionally water leached as shown at 30 to substantially solubilize and remove the water soluble chromium salts from the calcines. The leaching lG can be carried out at temperatures from 5C to the boiling point and require a time from 5 minutes to 5 hours depending on the chromium concentration in the leach and the temperature. Bubbling air through the leach liquor increased the amount of chromium extracted from the calcines.
The leach liquor, in addition to the chromium values, con-tains aluminum and s~dium as undesired impurities, e.g. as NaA102. Aluminum impurities are removed, as shown at 40 and ~O by adding an acid to the leach liquor until the pH of the liquor is reduced to the range of 3 to 9.5 preferably 8.o. The amount of chromium coprecipitated with the Al(OH)3 at a pH of 8 is typically about o.6% of the total chromium content in the liquor. However, as the pH is reduced further the coprecipitation of chromium increases. Acids other than H2S04, preferably HCl, should be used if a low sulfur product is desired.
The aluminum left in solution after the Al(OH)3 is filtered off is typically less than 0.7% A1203 based on the weight of equivalent Cr203 in solution.
In order to remove the remaining impurities, e.g. Na im-purities in the leach liquor obtained after the Al(OH)3 precip-3o itation step, a solvent extraction procedure is used as indicatedat 60. This procedure consists of treating a dilute solution of leach liquor with acid (other than H2S04 for low sulfur products) so that the final pH is in the range of l to 2, preferably 1.6 ~ 1119383 9709-l and the concentration of chromium as Cr203 is suitably in the range of l g/l to 25 g/l and preferably in the range of 5 g/l to 25 g/l with a particularly advant~geous concentration being 8.2 g/l. The organic solvent can be either benzene, xylene or toluene alone or mixed with an isoparafinic hydro-carbon such as Isopar H produced by Exxon. The e~tractant is a tertiary amine such as Alamine 336 produced by General Mills. A 0.1 molar solution of the extractant in the solvent was employed, however, lower or higher concentrations of C.02 to 0.3 molar can be employed. It has been found that with only

2 stages of extraction the organic phase can be loaded to 10 g/l of Cr203 and the aqueous phase (raffinate) reduced to only 0.04 g/l or 40 parts per million. No emulsions are formed and phase separations are rapid. The organic phase to aqueous phase ratio can be varied from 0.33 to 1Ø
The loaded organic phase is water washed to remove traces of aqueous leach liquor containing impurities of sodium and others. The number of stages of washing would depend on t~e type and efficiency of the washing step, however~ no more than l or 2 stages of washing would ordinarily be required.
The chromium values in the loaded organic phase are stripped, as indicated at 70, by mixing with NH40H solution at approximately 1.5 molar. The resulting phase separation is rapid ( ~-l minute).
m e aqueous phase contain (NH4)2CrO4 and the stripped organic contained r~0.04 g/l Cr203. The ratio by volume of aqueous NH40X to organic can range from 0.5 to 10. Thus, a relatively concentrated solution of (NH4)2CrO4 can be obtained. The stripped organic can be recycled as indicated at 65.
The (NH4)2CrO4 solution is evaporated to dryness as indi-cated at 80 and the resulting solid material is carefully ignitedto Cr203 in a gaseous oxygen environment~ e.g. air or oxygen, at about 500C, after drying as indicated at 90. me solid material ~ 383 ~709-1 consists essentially of a mixture oi (N~4)2CrO4 an~ 4)2CrO207 i.e. ammonium chromate compounds.
The foregoing description commencing with the description of the sol~ent extraction procedure is applicable to the method of the present invention as applied to aqueous chromium-bearing solutions prepared by dissolving, for example, a commercial grade Na2Cr207 or Na2CrO4 in water instead of obtaining an aqueous chromium-bearing solution of ~a2CrO4 by ore leaching.
With reference to the drawing~ as shown at 1, a commercial grade Na2Cr207 or Na2CrO4 or mixtures thereof is dissolved in water forming an aqueous solution having a concentravion of chromium as Cr203 suitably in the range of 1 g/l to 25 g/l and preferably 5 g/l to 25 g/l. If required, this solution may be filtered to remove undissolved solids. Similar to the foregoing, a solvent extraction procedure as shown at 60 is applied to the aqueous solution with the remaining steps of the method of the present invention applied as hereinbefore described.
An example of the method of the present invention as ap-plied to a Na2Cr207 aqueous solution is as follows:
EXAMPLE I
One liter of an aqueous Na2Cr207 solution was prepared by dissolving 11.5 grams of commercial grade Na2Cr207.2H O in water with 12 molar HCl being introduced into the solution so as to obtain a chromium concentration in solution based on Cr203 of 5.8 grams per liter at a pH of 1.6. The analysis of the Na2Cr207.
2H20 was as follows:
Na15.38~ (by chemical analysis) Cr34.63~ (by chemical analysis) S0.029~o (by chemical analysis) Alo.oo8-o.o8~0 (by spect. analysis) Ca0.001~0.01~ (by spect. analysis) Mg0.001-0.01% (by spect. analysis) V0.002-0.02~ (by spect. analysis) No other metallic elements were detected in the analysis.

1~9383 9709-1 A O.lM amine organic solution was prepared by dissolving 49 ml of Alamine 336 (tertiary amine) and 50 ml of isodecanol in benzene so that a total of 1 liter of organic solution ~ias made. ~hen 50 ml of the above aqueous solution and 50 ml organic were shaken it was found that emulsions formed.
However, when a tertiary amine was used with benzene or xylene or Aromatic 100 or 150 produced by Exxon or mixtures of these with an isoparafinic hydrocarbon such as Isopar H
produced by Ex~on without isodecanol no emulsion formed.

An example of this is as follows: ~hen 50 ml of the above aqueous solution is mixed and shaken with 50 ml of a O.lM organic solvent consisting of 49 ml Alamine 336 (tertiary amine) dissolved in benzene to provide 1 liter of solution, no emulsions formed and separation o~ the phases is rapid (~ 1 min.).
, The organic phase was water washed twice to remove traces of impurities.
The aqueous phase and washes were water white and contained negligible amounts of chrom~um. ~ess than O.l~o of the chromium ph Q~ ~
in the organic plaoc was lost in the washes.
The organic phase (50 ml) contained 5.79 grams per liter Cr203 and the aqueous phase (50 ml) contained 0.02 grams per liter Cr203. Thus, using an organic to aqueous ratio of 1.0, greater than 99~ of the chromium was extracted in a single stage extraction.
The 50 ml of loaded organic containing tertiary amine and benzene was stripped with 10 ml of 1.5 molar NH40H using an organic to aqueous ratio o~ 5.0 and produced about 10 ml of strip liquor. The stripped organic was colorless and contained 0,04 grams per liter Cr203. No emulsions were observed.

~ 9383 9709-1 The strip liquor was evaporated in a stainless steel dish and the resulting solids were ignited at 60~ C to Cr203. A typical analysis of the product Cr23 is Cr23 >99% (by dlfference) Al 0.0004-0.04% ~by spect. analysis) Ca o.oO8-0.08% (by spect. analysis) Mg 0.002-0.02% (by spect. analysis) Na o.o8 -o.8s~ (by spec~. analysis) S1 0.008-0.08~ (by spect. analysis) V 0.001-0.01~ (by spect. analysis) ;~
Through the practice of the present invention a high -~
purity (less than ~ o.o8 - o.8% Na) Cr203 product can be obtained ~rom Na2Cr207 or Na2cr4 The method of the present invention may be practiced on solutions of commercial grade Na2Cr207 or Na2CrO4 or mixtures o~ these two wherein the chromium concentration in so~ution based on Cr203 is suitably 1 to 25 grams per liter. If undissolved sollds are present in the solution, the solution may be first 23 filtered to remove them. -An example o~ the method of the present inventions asapplied to chromium ore is as follows:

EXAMPLE Il Transvaal chromium ore having the anal~sis shown below in the amount of 5 lbs. was subjected to grinding to finer than 200 mesh.
Chromium Ore Analysis (Oxides)(Chemical 'nalysis) Cr203 44.6% '~
Total Fe20327.5%
SiO2 3.5%
CaO 0.4%
A1203 26.9%
Na20 . 1%
MgO 10.1%
g - ~
. - . ,, , ~, 111~3 9703-1 100 g. of the ground ore was mixed with 25 grams of co~mercial grade caCG and 82.8 grams of powder reagent grade Na2C03. The mixture was placed in a Inconel X dish and roasted at 950C for 2 hours with air being forced over the mixture. After roasting, the calcined mi~ture was leached with 2 ml. of water per gram of calcine and then water washed with 2 ml. of water per gram of calcine to provide leach liquor and wash combined having the following analysis:
Cr20322.3 g/liter (by chem. anal.) 10 Al0.1-1 g/liter (by spect. anal.) Ga0.004-0.0~ g/liter ( " " "
Mg0.0004-0.004g/liter ( " " "
Na~ 20 g/liter ( " " "
Si0.004-0.04 g/liter ( " " "
12 molar HCl was introduced into the leach ]iquor to reduce the pH from 12.8to 8.o with resultant precipitation~of aluminum as Al(OH)3 After removal of the precipitate by filtration the acidified leach liquor had the following analysis:
Cr203 22.3 g/liter Al 0.13 g/liter Na ~ 20 g/liter Solvent extraction feed solution having a concentration of 5.8 g/l Cr203 was prepared by mixing 520 ml of concentrated leach liquor (pH 12.8) with 265 ml of 12 molar HCl and H20 to produce 2000 ml of solution having a pH of 1.6.
An organic solution was prepared by mixing isodecanol (5~ by volume~, Isopar H, and Alamine 336 (0.1 m) to form 2000 ml. of organic solution as suggested by General Mills Chemical Division Chromium-Liquid Ion Exchange~ ~ulletin CSDI-61. When 150 ml. of the above aqueous and 150 ml. of organic were shaken, emulsions formed and the organic turned brown.

~ hen the lsodecanol ~as omltted, emulsions sti'l ormed, but the organlc phase ~ras not brot~ but orange in color suOges-tir.g the prev~ous ~iscoloration ~.as cue to degradation of the alcohol.
/ihen the tertiary amine ;~as used ~ith benzene or xylene or ~arious aroma'ic diluents from Exxon, without isodecanol, no emulsion fol~ec. ~n example of this is as foilols~ .en 100 ml of the above 2queous ,ras mixeQ ~rith 100 ml of the organic solvent of the tertiary amine in ei4her benzene or xylene or ~raratic 100 or l~, no emulslon o~ed and separation OL the phases was rapid ( ~1 r~nute). The orOanic phase was water ;lashec three times to re-.o-te _r.lrurit~es. The aqueous rashes were water white and contained negl_~ble amoun~s o~ chromiur...
The amine ~id not degrace.
The or~anic phase contained ~.79 g/l Cr203 and the a~ueous phase contained 0.019 ~/l Cr203.
100 ml of the loaded or~anlc containing tertia~y amine and benzene ~ras stripped with 2C ml OL 1.5 molar l~'4G'T and produced about 20 ml of strlp liquor which after eva~oration and ignition of the resulting solids a~ 600C had the follo~inO
analysis:
2 3 (by di ference) Al .o8 - .8% (by spect. analysls) Ca 0.008-o.o~ (by spect. analysis) Mg 0.0004-0.004% (by spect. analy~is) ~a 0.04 -0.4% (by spect. analysis) Si 0.004 -0.04% (by spect. analysis) ~ 0.004 -0.04% (by spect. analysis) I~ e.~ulsions form on a lar~e scale extraction plant, the

3 process would not be practical, thus the choice o~ the proper dlluents or cor..blnations of the diluents for the amine lS extremely ~mportant.

`\~ ;~' .. .

1~9383 9709-1 The recovery o~ chromium was over 99% in the extraction step.
In the practice of the present invention a roasting temperature of 950C led to 94~ extraction of the chromium from the calcines, when air was bubbled into the leach liquor during the leaching step. Without air during leaching comparable con-ditions resulted in only 88~ extraction. Also, the ranges for the amounts of Na2C03 and CaO are important for economic reasons, 94~ extraction was obtained with 25 g. CaO and 82.8 g. ~a2C03 per 13 100 g. chrome ore containing 44.6~ Cr20~. The use of larger amounts 50 g. CaO and 82.8 g. Na2C03 lowered the extraction from 88~ to 83% when air was not used in the leaching step. The CaO
may be added as lime or limestone.
In the calcine leaching step the pH of ~-12 is adequate.
The pH range of 3 to 9.5 in the aluminum precipitation step is important, with an optimum pH at 8.o. At higher pH values more chromium is lost in the Al(OH)3 ppt. and rnore chromium is lost at the lower pH range by dissolution of the chrome hydrate.
Through the practice of the present invention a high purity Cr203 product, less than --0.2~ Ai and less than ~ 0.04 -0.4~g ~a, can be obtained from natural ch-omium ores such as Transvaal ore and other oxidic chromiu~ bearing materials.
In the practice of the present invention, the choice of the proper diluents or combinations of the diluents for the a.~ine i8 extremely important since if emulsions form in a large scale extraction plant- the process would not be practical.
The par~icular organic solvents used in the present invention are essential for the prevention of emulsions. Benzene, xylene, Aramatic 100 or 150 produced by Exxon or mixtures of the 3~ above or a mixture of one of the above with an isoparafinic hydro-carbon (such as Isopar H produced by Exxon) were found satisfactory.
It was found that the use of isodecanol as a solvent ex-traction modifier was undesirable due to its reaction with the chromlum in the solution which reaction causes the alcohol to brezk down and contribute to emulsion formation.
-12_ :

~1~9383 9709-1 In addition, by the practice of the present invention, an essentialiy sulfur-free product can be obtained due to the fact that inorganic acids other than H2S04 can be readily used in the practice of the invention and further that the solvent extraction purification step removes substantially all sulfur contamination which may be due to the use of H2S04 or which may be introduced as an impurity ~ith the materials invo]ved in the process. HCl would be a preferred acid to use in the practice of the present invention if a sulfur-free product is desired.

Claims (8)

WHAT IS CLAIMED IS:

1. A method for recovering high purity Cr2O3 from chromium ore which comprises:
(i) roasting in gaseous oxidizing environment a mixture of chromium ore with Na2CO3 and CaO at a temperature in the range of about 600°C to 1100°C for from about 0.5 to 6 hours, the amount of Na2CO3 being that which provides from about 1.4 to 4.2 pounds of Na2CO3 per pound of Cr2O3 in the ore and the amount of CaO being that which provides from about 0.6 to 1 pound of CaO per pound of Cr2O3 in the ore;
(ii) water leaching roasted material obtained in step (i) at a temperature of from about 5°C to the boiling point of water for from about 5 minutes to 5 hours;
(iii) adding acid to the leach liquor obtained in step (ii) to provide a pH of from about 3 to 9.5 to cause precipitation of aluminum impurities and separating said impurities from the leach liquor;
(iv) adding acid to the leach liquor to provide a pH of about 1 to 2;
(v) contacting the acidified leach liquor obtained in step (iv) with a tertiary amine dissolved in an organic solvent essentially free of isodecanol to inhibit the formation of emulsion, to recover chromium values from the acidified leach liquor in a chromium containing organic liquid phase;

(vi) treating the chromium containing organic phase with a water solution of NH4OH to obtain a solution of (NH4)2CrO4;
(vii) subjecting the solution of (NH4)2CrO4 to evaporation to provide a solid chromium-bearing material; and (viii) igniting the solid chromium-bearing material to obtain Cr2O3.

2. A method in accordance with claim 1 wherein the roasting temperature of step (i) is about 950°C and the roasting time is about 2 hours.

3. A method in accordance with claim 1 wherein an aggregate of about 1.9 pounds of Na2CO3 and 0.6 pound of CaO are used per pound of Cr2O3 in the ore.

4. A method in accordance with claim 1 wherein the pH provided in step (iii) is about 8.

5. A method in accordance with claim 1 wherein the pH provided in step (iv) is about 1.6.

6. A method in accordance with claim 1 wherein the acid used in steps (iii) and (iv) is an inorganic acid which does not contain sulfur when a low sulfur product is desired.

7. A method in accordance with claim 1 wherein acid used in steps (iii) and (iv) is HCl.

8. A method in accordance with claim 1 wherein in step (ii) air is bubbled through the water leaching liquid during the leaching step to increase the recovery of chromium values.