PH26656A - Thermal reduction of agglomerated metallurgical feed materials with metallic coatings - Google Patents

Description

—_— i 26656

FIELD OF THE INVENTION

This invention relates to the solid state thermal reduction of agglomerated metallurgical feed materials and, more particularly, relates to agglo- merated metallurgical feed materials in which the material to be reduced is sub-divided and mixed with a solid reductantg and optionally any required fluxes or other additives, prior to agglomeration.

More particularly, but not exclusively, the

Co 10 invention is concerned with the reduction of agglow= i co oo ' merated feed materials comprising chromium oxides, : SE i - | iron oxides such as hematitdy, and mixtures thereof, manganese oxide, vanadium oxide, or any other carw- . a bothermically reducible oxide, using a carbonaceous oo 15 redictant such as coal, preferably in the form of anthracite, char or other carbonaceous reductant in a step which is usually referred to as a pre~re= duction step preparatory to effecting smelting or melting and slag-metal separation in a suitable fur=- nace.

BACKGROUND TO THE INVENTION

In the solid state reduction of agglomerated finely sub-divided oxide material to be reduded, to- gether with a carbonaceous reductant, and any required = 2- oo fluxes or other additives, the agglomerated mate- rial is generally, after being prepared and cured, subjected to the action of off-gases from a combuse tion process in order to heat and/or reduce them.

Such off-gases contain, in various different pro portions, carbon-monoxide and carbon dioxide and can even contain free oxygen. The performance of the reduction reaction may be seriously affected by the gas composition, ;

In particular, oxidation of carbon reductant se pl wd vw Baste An the pellet can occur as-a result of reaction with =... von carbon dioxide contained in the process gases to Pros duce carbon monoxide by the so~called Boudouard re action, tee Clrertet) * 0% = 0 ee (process gases)

The consequences of this reaction are twow fold. Firstly removal of carbon from the pellet di=- minishes the amount of potential reductant availe able for prereduction and metallisation of, for insgw tance, chromium and iron species contained in the chromite or iron oxide present in the pellet and, secondly, an incompletely combusted gas (carbon mon

-_— ” 26656 oxide) is generated which must be either rerouted elsewhere for combustion to realise the available sensible and thermochemical energy, or he flared off to waste.

In addition, reoxidation of the metallised ferrochromium may occur ac the conditions become less reducing within the pellet as the carbon is oxidised. ~The oxidised ferrochromium is often identified micro- structurally by the presence of sesquioxide laths (11,0,.Cr 05) adjacent to residual metallic blebs that are presente. .

It is the object of this invention to provide a process whereby the adverse effects of such

Boudouard reaction between carbon and the co, in the process gases which are in close contact with the agglomerated material can, at least to some material oo extent, be diminished. | '

SUMMARY OF THE INVENTION

] In accordance with this invention there is provided a process for the solid state reduction of : agglomerated metallurgical feed material composed of a sub-divided material to be reduced in admixture ' oo Ce “ with a sub-divided solid reductant, the process be~ : ing characterized in that the units of agglomerated “bh -

material are coated with a layer of material chosen to at least inhibit contact between the components of the agglomerate and any component of heating gases employed for heating same which may react in a deleterious manner therewith, the material form- ing the said layer comprising a subdivided metal, alloy, metal carbide, alloy carbide, or mixtures thereof to render such layer essentially metallic in nature,

Further features of the invention provide for the reduction to be a prereduction step carried out at temperatures of from 1200 to 1500°¢, prefer~ ably about 1300°¢; for the agglomerate to be in any suitable form such as pellets, briquettes or the like; for the material to be reduced to be finely sub~divided oxides of chromium, iron or both, and in particular form such material to be a chromite . ore; for the reductant to be a carbonaceous re~ ductant, in particular anthracite, coal, coke, coal char, or charcola; for the agglomerate to pp- tionally include required flux additions; for the material coating the units of agglomerate to be . either purely metallic in nature or a mixture of at least 50% by mass metallic material with oxide coat- ing material; and for the heating to be carried out

Ce ———————————————— EEE rrr » i 26656 in process gases containing :~ 0 - 100% of a mixture of Co, and Co in the ratio of CO, : C0 of at least 1:1 ond op= tionally 3:1 or cven greater; 0 - 100% air, oxygen or a mixture thereof; and wherein the balance consists of other com- bustion products, nitrogen, and impurities,

It is to be noted that existing processes are : limited in this regard to rome extent. Generally,

Co 10 rotary furnaces are operated with a. 00,:C0 ratio of oo | about 2:1 whilst a shaft furnace is operated at a €0,1C0 ratio of about 1:10. The process of this ine- vention can, on the other hand, be operated at C0,:CO : ratios upwards of 3:1 and even up to 100:1. This “enables a process gas containing large amounts of CO, to be used, with or without excess oxygen (air) being present and allows substantially complete combustion of the heating fuel to take place whilst conserving reductant.

Freferably the metallic material comprises fer-— rochromium, a chromium-iron alloy or other ferro- alloys, or mixtures of ferrochromium or iron chromium alloy or other ferroalloys and oxide coating materials such as Andalusite. Hany of these materials are con-

veniently available in suitable inexpensive forms, for example ferrochromium scrap is produced as a normal product during smelting processes producing same.

The material used for forming the coating is usually finely sub-divided and can be mixed with a suitable binder prior to formation of the layer thereof on the outside of the units of agglomerate,

The formation of such a layer may conveniéntly be car~ ried out by, for example, pneumatically conveying

Cte ea. a the powdered material and spraying it onto the sur-

BE face of the agglomerate or by simply adding the coat- ing material to a conventional pelletizing apparatus together with preformed pellets where the agglomerate is in the form of pellets. Coatings may be obtained by any other convenient means for example dipping the agglomerate in a slurry or dusting the coating mate- rials onto the surface.

The thickness of the coating layer is prefer- ably of the order of up to 1,5mm and most preferably about 1 mm or less, In any event the quantity of coat~ ing material will usually not be more than 50% by mass of the total weight of the coated pellet and will usually be of the order of 15 to 20% and prefer- ably about 10%.

’ ' '

In the case of preformed agglomerates of » chromite together with a carbonaceous reductant (such as anthracite) and fluxes, preferred coating materials, based on research carried out to date, are ferrochromiun fines mixed with a suitable binder or a mixture thereof with an oxide material namely refractory grade and andnlusite employed together with a binder, which are nreferably o bentonite clay binder. Other oxide materials could also be em— ployed. Such materials iiclude tabular alumina, portland cement, used alumino-silicate brick, and refractory alumino-silicate cements. oe

Tests conducted to date have indicated that the mixture of ccating materials must be chosen carom fully as some materials eperate better than others ' and some are difficult tc form into an unbroken layer - on the agglomerate units withont excessive cracking or peeling, or otherwise without resulting in adverse effects during the processing of the agglomerate such to 20 ‘as, fusion for example. Purely metallic coatings such as ferrochromium, however, generally do not exhibit such difficulty.

In order to demonstrate the operation of this

Co - invention, the results of a few selected tests will now be described with reference to the accompanying

. drawings. BRIEF DESCRIPTION OF THE DRAWINGS

: In the drawings:ie=

FIGS. 1 & 2 are each graphs of the individual metallizaticn of iron (Fe) and chromium (Cr) after treatment for 180 minutes at 1300°¢ of a pelletized chromite feed mate- rial without a coating of this invention ’ in a gaseous atmosphere composed of

Co 10 .. various ratios of carbon dioxide and car- bon-monoxide Co oon

FIG. 3 is a similar graphical illustration wherein coated pellets according to the jnvention nre heated in a similar range of carbon monoxide to carbon dioxide ratios to tliose shown in Fig. 2} andy

FIG. 4 is a graph of temperature as % metal- © 1isation of pellets of hematite mixed with anthracite and coated with ferro=- chromium when heated for 180 minutes in 100% co, atrmospheress

DETAILED DESCRIPTION WIT! REFERENCE TO THE DRAWINGS : Initially tests were carried out on two some= -9 BAD ORIGINAL 9

L

— LL —_ what different recipes of a chromite ore with fine- ly sub-divided anthracite zs a reductant, the chro- mite, anthracite and fluxes having a particle size such that approximately 75% passed through a 74 pm (i.e. a 200 mesh) sieve. The two recipes are de- tailed in Table 1 and it will be noted that they in- ’ clude granite and fluorspar fluxes which have been found to give enhanced reduction of chromite and have been described fully in our Patent Ho. 87/5774 entitled "Process for enhanced reduction of Chromite".

However, these pellets do not necessarily have to contain the fluxes for the coatings to operate effectively.

The recipes were nixed by tumbling them in 15 . plastic drums with a light ball charge. Pellets were then formed in a disc pelletizer by the addition of a controlled amount of water followed by 8 - 12 hours air drying and oven drying at 110°C for approximate= ly 12 hours.

TABLT, 1

Pellet Compositions (expressed as % by mass and as recipe proportions) . Recipe 1 i Recipe 2 % (Proportions) % (Proportions

Mooinooi chromite 65,4 (100) 56,6 (100)

Grinaker anthracite 19,6 ( 30) 28,3 ( 50)

Granite 9,8 8,5 joao ( 20)

Fluorspar 3,3 2,8

B it y 8 entonite 1,9 3 + Moisture (%) 10,1 11,8

NB. Figures in brackets relate to proportions of an- - thracite and total fluxes to chromite (100)

The uncoated pellets were then heated isother- mally in various different gas mixtures of carbon monoxide and carbon dioxide nt 1300°¢ and the pro- cess was monitored with an on-line data logging facility. The various different gns mixtures that were used are as followsie

Vol % CO 100 95 85 75 50 25 0

Vol % COg 0 5 15 25 50 75 100 and the results are illustrated graphically in Figs.

I

— 1 and 2 for the two pellet compositions after heat- ing for 180 minutes in the shove gas atmospheres. It will be noted that, in both chases, the metallization of both iron and chromium vere extremely low at gas compositions in which therd was less than about 60 to 70% of carbon monoxide (ie. more than 30 to ho carbon dioxide) when compared to the cnse in which a concentration of 100% carbon monoxide was employed pellets of the Recipe type 3 described above : 10 were then coated with two different mnterials in order to test the operation of the present invention. , The following materials were tested as coating layers. - y (i) 98% by mass clean ferrochromium fines (75% =~ 7h pm) and 2/5 by mass bentonite binder; and, . (ii) A mixture of 56% by mass clean FeCr fines combinzd with 38% by mass andalusite and 6%

Behtonite clay binders. :

All the coated pellets were produced from one batch of standard recipe 1 pellets whichwere sized "20 to approximately 9mm in size in order to minimize i coating thickness variations.

The coating materials were premixed, where applicable, and gradually added to the sized pre= i | formed pellets in the rotating pelletizing disc. A small amount of water (or sodium silicate) was added to re-wet the pellet surfrces before adding the coat~ ing material,

The cneting additions were chosen to give a coating thickness of approximately 1 mm. The pellets were then dried at 110°¢ for approximately 12 hours.

In the case of the substautinlly pure ferrochromium coating layer (i) the costing amounted to about 30% by mass of dry coated pellets whilst in the case of ov the mixed coating layer (ii) the coating amounted to SL

Vn hee eg Bes sot 360 of the dry coated: pelletSecs vet ce EU same fon

The coated pellets were then heated, as in the case of the uncoated pellets, at 1300°¢ and the com- . positions determined after heating had taken place for 180 minutes. In addition the coated pellets were heated in air as wello

Both coating materials namely the ferrochromium and the ferrochromium and andalusite mixture gave suitable layers with respect to their physical in- tegrity and with respect to the metallization of chromium and iron as is shown in Fig. 3. (Calcula- tion of % metallisation for Cr and Fe taking cogni- sance of the Fe and Cr in the coating was carried out as follows:i=-

—— me

Metalligation and mass halance calculations (ct, % FM/LI) —= (IM X B)

Cr metallization (3) = —e—e——————eeeeeeen 100 (1)

In x A : (FL x FHALM) - (IM x D)

Fe metallization (3%) = ~—————m—mr———e— x 100 (F)

IM x C (CLXFI/LM) = ( IMxB) + (FLxFH/LM) = (IMxD) ‘ 5 Total metallization (%) = =————————————rr——erm———X. 100 (In x A) + (IM x C)

CR x RM x 0,01 x FM/LM

Cr recovery in residue (3) & ~——————————— x 100 (aq)

IM x A

IR x RM x 0,01 x FM/LM

Fe recovery in residue (3) = ———m—————" X 100 (1)

It x C

Deviation from closed mass balance:

Acr (5) = /F + G/ - 160

AFe (5) = /F + H/ - 100 where CL = mass of dissolved Cr (Metal) in leach solution (g)

FL = mass of dissolved Fe (Metal) in leach solution (g)

FM = final mass of sample

LM = mass of sample used for leaching

IM = initial mass of sample (g)

RH = mass of residue (g)

CR = % Cr in residue

IR = % Fe in residue cl

A = mass of Cr from chromite in base pellet/g ; / ’ . coated pellet (g)

B = mass of Cr from FeCr or mixed coating/g coated pellet (g)

C = mass of Fe from chromite in base pellet/g coated pellet (g)

D = mass of Fe from FeCr or mixed coating/g coated pellet (g)

It is to be noted this technique assumed that only a negligible amount of the FeCr coating is oxi=~ dised and reported to the residue. This assumption is justified even in 100% co, atmospheres from SEM (Scanning Electron Microscopy) analysis, which ihe oo dicated that only up to 50 =~ 100 pm of the Felr ' material in the outer coating surface is oxidised on a 1 mm thick continge :

It will be observed that, for a coating of ferro¢hromium approximately 90% of the iron and 73% of the chromium were metallized even in a 100% car-— . bon dioxide atmosphere. In comparison with results for the uncoated pellets, chromium metallization is only slightly less than in 100% carbon monoxide at- mosphere. Iron metallization was almost independent of gas composition. } . ; : | Furthermore, experimental work has indicated : oo that decreasing the nominal particle size of both

Co | 25 pase pellets and coating below 75% to pass a 74 pm

——— ee ooo, mmo mm msm screen does not significantly enhance the final

Jevels of metallization achievable,

In the case of the coating made from a mix— ture of ferrochromium and andnlnsite, it will be noted that the extent of metallization was some= what more dependent on the gas composition than in the case of the ferrochromium coated pellets. 1lron metallization is, as with the ferrochromium coated pellets, virtually independent of carbon monoxide to carbon dioxide ratice

Accordingly, it will be appreciated that ex- tremely advantageous rerults can be achieved using the present invention but that the coating material must be chosen carefully.

In general, tests carried out to date indi- cate that metallic coatings are more effective than oo N mixed metallic and oxide based coatings in prevent

Co ing carbon dioxide ingress into the pellet core.

Pellets coated with ferrochromium appear to rapidly oo Co 20 form a chromium oxide based outer film which, al- . oo though it is only about 25 = 50 pm thick, probably plays an important role in sealing the pellets from the carbon dioxide in the atmosphere. Once the . protective skin has formed, metallization can pro- ceed in the inner pellet core with 100% carbon di-

oxide gas compositions or even with uncombined oxygen present. The carhon monoxide gas which is a product of the carbothermnic reduction of the chromite ore can nevertheless escape through this protective layer and cnn itself be combusted to cao, by introducing air to rrovide additional oxygen thereby increasing the thermnal energy available to the nrecess (ie. decrensing the need to supply this amecunt of energy from aasother source). } 10 Further experiments have shown that the fer- oe | | rochromium coatings are effective even in the pretb i co sence of free oxygen in the furnace atmosphere. - : Lo When ferrochromium coated Recipe 1 pellets were : BE : heated for 2 hours in air at 1300°C and metalliza- ’ 15 tion levels of 92% iron and 70% chromium were achieved. The results after 180 minutes and 120 minutes (broken lines) are shown in Fig. 3 at the - | | left hand cide.

With regard to iron oxide reduction, experi- ments haverbeen conducted at temperatures of between 800°C and 1000°C where hematite and anthracite base : pellets, without fluxes, were coated with ferro chromium and heated in a 100% Co, atmosphere. The pellet composition is indicated in Table 2, while iron (Fe) metallisations are indicated in Figs Uo

—

The latter shows that 85% Fe metallisation was achieved after 180 minutes at 800°C, whilst 100%

Fe metallisation was achieved after 180 minutes at 1000°C.

TABLE 2

FeCr coated hematite pellet composition hematite 71,6 1) All material sized to 75% = anthracite 26,4 7h jm bentonite 2,0 2) Coating addition = 30% by mans of dry coated pelletoe

Coating -

FeCr fines 2a : bentcnite 2

The invention therefore provides a highly 14 offective exnedient for use in the solid state re= duction of pelletired feed materials mixed with a solid reductant and wherein contact between svch so- ; 1id reductant and the gases being used to heat same, in a solid state reduction process, 18 inhibited, at least to some measurable extente

Furthermore the useful sensible energy in hot ~ oxidising gases and the chemical energy in combugtie~ , ble gases which hitherto could not be used for heat-

ing and reducing oxide materials can now be em ployed and complete combustion of fuels (gaseous, liquid or solid) to the fully oxidised form can be carried out thereby utilizing all the available energy of combustion. Jven the presence of free oxyren with the combustion gases should not result in any significant loss of reduction with the pro- tection of such metallic or mixed —

A further advantrjje of such coatings is that they can inhibit pellets from sticking and enhance the physical strength of the agglomerate (eg. up to a six-fold increase in the pellet drop number using a standard drop test). ~19 - : [AD ORIGINAL 9

Claims (1)

1. — ee ,,,, — WHAT TS CLALHED I[S:= lo A process [or the solid state reduction of : agglomerated metallurgical feed material composed of a sub-divided mnterinl to be reduced in admixture with a sub-divided solid reductant, the process be- ing characterized in that the units of agglomerated material are cecated with =» layer of material chosen to at least inhibit contact between the components of the agglomerate and heating gases emnloyed for heating same which may react in a deleterious manner therewith, the material forming the said layer com- prising a subdivided metal, alloy, metal carbide, alloy carbide, or mixtures thereof to render such layer essentially metallic in natures 2e A process as claimed in Claim 1 in which the reduction js a prereduction step and heating is car- ried out at temperatures from 1200° - 1500° Co 3a A process as claimed in Claim 2 in which heat- ing is effected at about 1300°C. bo A process ac claimed in Claim 1 in which the heating is carried out in process gases containingi= : 0 - 100% of a mixture of co, and CO in the ratio of €0,:C0 of at least 1:1 ~nd optionally 3:1 and up to 100: vl — ee A’ BAD OT

   0 = 100% air, cxygen or a mixture thereof; and wherein the balance consists of other combustion products, nitrogen, and impurie ties, 5 A process as claimed in Claim 4 in which the process gases are obtained by the substantially com= plet.: cortustion of a heating fuel optionally with excess air (oxygen) therein, Co 6. A process as claimed in Claim 1 in which the fone 00 21000. units of agglomerated material are pellets, brie quettes or the like, 7o A process as claimed in Claim 1 in which the material to be reduced is formed, together with sub divided solid reductant and fluxes, into agglomerated units which are driedy followed by coating of the layer into the formed agglomerated units and drying of the coating layer prior to reduction being effected.

   8. A process as claimed in Claim 1 in which the sub-divided material to be reduced is a sub-divided oxide of ehromiumgy irony; or both and the sub=divided s0lid reductant is a carbonaceous reductant, 9e A process as claimed in Claim 8 in which the or ~ 21 = BAD ORIGINAL 9 bm —

   ee material to be reduced is a sub-divided chromite ore and the sub-divided reductant is a sub-divided coal, anthracite, coke, char or charcoal, 10, A process as claimed in Claim 1 in which the coating layer is substantially purely metallice

   11. A process as cla med in Claim 10 in which the coating layer is substantially ferrochromium, chromium iron alloy, or other ferrochromium alloy together with a suitable binders ! Cee J Feo tht, to 3 Co . Ce } - oo

   12. A process as claimed in Claim 1 in which the coating layer is at least 50% by mass (on a dry basis) metallic admixed with oxide coating materials. 13, A process AS claimed in Claim 12 in which the oxide coating material is Andalusites 1h, A process as claimed in Claim 1 in which the sub-divided material to be reduced, and the solid reductant has a particle size such that about 75% passed a 74 pm sieve.

   15. A process as claimed in Claim 1 in which fluxes are admixed with the material to be reduced and jt solid reductant. BAD ORIGINA' A / bee

   NN oN \ ey

   16. A process as claimed in Claim 1 in which the thickness of the coating layer is up to 1,5 mm.

   17. A process as claimed in Claim 16 in which the thickness of the coating layer is about 1 mm,

   18. A process as claimed in Claim 1 in which the material forming the coating constitutes less than 50% by mass of the coaled agglomerates 19, A process as claimed in Claim 18 in which the : material forming the coating constitutes 15 to 20% Geese ee eee My mass cof the coated agglomerates. " CT a ae NICHOLAS ADRIAN BARCZA __ROBERT CHRISTOPHER NUNNINGTON Inventors - 23 GR d \ Ro CT 0 0 Ww Co. oh re : \