US2206139A - Chromium reduction - Google Patents

Description

Patented July 2, f 'o UNITED STATESV PATENT OFFICE This invention relates to articial chromite materials, and it comprises as a new manufac ture a granular completely oxidized ferrochromium free or substantially free of carbon and unoxidized metal and useful in the production of low carbon chromium alloys. said granular material usuallyu containing a base such as lime, together with ferric oxide, chromium rsesquioxide vand chromium trioxide in some vform of chemical combination, and being available for various pur-- poses, and particularly for the direct production of molten lo'w carbon chromium alloys by exothermic reaction with admixed carbon-free reducing agents (silicon, metal silicides or silicon alloys, including silicon-chromium alloys, aluminum and alloys thereof, etc); it. further comprises an admixture of said material with one of said reducing agents in such proportions and so admixed as to enable, initiate and develop an exothermic action with production of metal containing chromium in a heated state; it further comprises a method of making such a material wherein a chromite ore, which may be either a natural or a beneiiciated ore, and Vadvantageously a low grade ore, is totaly reduced with an ampie amount of 'carbon in an electric furnace, non-metallic impurities being slagged voil?, the

ferrochromium so produced is lne ground to a powdered material and this ne ground metallic material is completely oxidized by roasting to free it of carbon, complete oxidation being usually accelerated by the-presenceof lime and a little soda in admixture and roasting being often carried `far enough to give a substantial content of CrOa in the roasted material; and it further comprises an advantageous. method of making low carbon chromium alloys in a molten .condition wherein such a roasted material is intimately admixed with a reducing agent to form 'a mixed body and the body is red to produce molten metal; all as more fully hereinafter'set forth and as claimed.l

By total reduction of any chromite ore in an electric furnace, slagging off the gangue and using plenty of carbon, a brittle metallic product can be obtained carrying all Ithe iron and most of the chromium. It may be regarded as a concentrate. It is, however, high in carbon and therefore unsuited for direct usein. chromium metallurgy where for the most part low carbon is required. Nor can it readily be roasted alone to reconstitute the original metal oxides and get rid of the carbon in a single roasting operation. In any given granule oxidation goes inward from the surface and stops while there is still unoxidized metal; theroasted product is magnetic.

' But by double roasting, that is roasting once, regrinding and reroasting, oxidation can be made complete; the product is composed Wholly of oxides and carries no carbon. Nor is it magnetic. The same result can .be attained in a single roasting by adding bases: lime or lime and soda. Such a double roasting or roasting in the presence of a base accelerating oxidation is con` templated in the present invention.

Most of Ithe stainless Vsteel and rustless iron made today are produced by utilizing low carbon ferrochromium containing' 70 percent or' more chromium 'as a source material for supplying chromium to a bath of molten iron. In most cases the il'nal alloy products are required to be extremely low in carbon, content and the ratio of carbon to chromium is low in this ferrochromium.

Not only is lowcarbon ferrochromium richvin chromium an expensive material, butit has an .linconveniently high melting point and requires temperatureshigher thanordinary steel melting temperatures in making alloys. .For this reason,

t the commercial llnished chrome-iron and chrome steel alloys are for the most part made in the electric furnace.

While the direct production of chrome yirons andchrome steels from chromite ore is practicable and in use, the employment of low` carbon ferrochromium is more favored in the art. The direct use otl chromite ores is attended with the disadvantage of a large slag volume; the slag being highly refractory because ofthe refractory nature o f the ore gangue.

In making commercial ferrochromium, it" is usually considered necessary to utilize onlyijqhe highest grade chromite ores; those which give upon reduction ferrochromium of a standard '70 per cent chromium content; a ratio of chromium to iron 2.3 to one or better. Thus the chromite ore deposits of the world have been high graded in order to obtain ferrochromium of high chro- `mium to iron ratios for making steel and iron alloys, which are mostly of a low chromium high iron ratio. These nished alloys may carry chromium in an amount ranging from a fraction of one per cent to upward of, perhaps, 30 per cent as a maximum.

1 It is a general object achieved in the invention to obviate the noted difficulties by providing a new source material in making chromium alloys; this material being free of carbon and of gangue and representing thechromium and present o iron of a chromite ore in a re-oxidized condition. M,

It may be here called an artificial or synthetic chromite ore, although it does not have the constitution of the mineral chromite; a spinel combination of FeO and CrnOs. The articial chromite of the present invention, which may advantageously be made from low grade, or high iron, chromite ores, is useful for various purposes and particularly in the production of molten chromium alloys by exothermic reaction with admixed silicon, etc. The reduced metal formed in small globules by the exothermic action may be carbon-free and is advantageous for alloying purposes in making commercial chromium alloys in which absence of carbon is desirable.

According to the present invention, a natural chromite ore, which may advantageously contain chromium and iron in a ratio of 1:1 or even less, is completely reduced with an ample amount of carbon in an electric furnace, non-metallic and gangue impurities being slagged off and eliminated. The high carbon ferrochromlum metal thus produced is ground to a line powder and roasted to completely reoxidize the chromium and iron of the metal and'to eliminate carbon. Complete oxidation of ground ferrochromiuin in one operation is, as stated, diiiicult but by roasting in the presence of lime and a little soda complete oxidation maybe obtained. In using lime there are, in any event, several advantages. In subsequent reaction with silica or ferrosilicon lime is available for slagging the produced silica and accelerating its production; Ordinarily, in roasting in the presence of admixed lime I use about equal -amounts of lime and of ferrochromium. This may or may not be enough in relation to the silica produced in subsequent reduction of the oxidized material with silicon, etc.

It, however, does give a composition not subject to change in the air and therefore advantageous for shipping. More lime can be physically admixed before the exothermic action if it be wanted. Another advantage-in roasting in the presence of lime is that oxidation may be readily carried beyond the CraOa point with production of CrOa. In reaction with silicon, CrOs produces considerably more heat than CrzOs.

If desired, the roasting may convert all4 the chromium to chromate. ABut it is usually better to have in the roasted product a considerable percentage of CrnOa. The reaction of CrOs with4 silicon is, as stated, more highly exothermic 'than that of .Cr203;l that is, .for equal quantities of chromium more heat is developed, the proportion of oxygen in CrOa being double that of CrzOa and the excess being, so to speak, loosely bound.

However, more slag is formed per unit of chromium as CrOa, more silicon being oxidized toA silica. The yield of chromium metal per unit of silicon used in reduction is greater from CrzOa than from Cros. It has been found advantageous in some cases to carry the roasting of ferrochromium so far as to give only a minor proportion of the chromium as chromate. However in other cases the proportion may be higher. Equal proportions of chromium in the two forms give a useful product. Any desired content of oxygen within limits can be put into the product in roastunder 1000 C. with somewhat less than 100 per cent by weight of lime with no soda ash or with less than one per cent of soda in the roasting charge converts the chromium mostly to chromate without complete oxidation and elimination of carbon from the ferrochrome metal in a reasonable length of time. AAbove 1000" the carbon is eliminated but the chromate breaks down to chromite. As the soda is increased the temperature can be raised to 1200 without breaking down the chromate and the carbon can be completely eliminated without raising the temperature above 1200*. Additions of soda in amounts above one per cent of the charge or two per cent of the metal permit regulation of the temperature to control the ratio of chromate to chromite (CrOs to CrzOs) in the oxidized product, and thus oi the oxygen content and exothermicity. I have found for example that addition to a roasting mix of ferrochromium and limeof soda ash (NanCOa) in the amount of 2 to 5 per cent by Weight of the ferrochromium metal results in 50 per cent conversion of the chromium to chromate in a short time with total elimination of carbon and oxidation of metal when the temperature of roasting is held around 1000 C.; the remaining chromiumA being oxidized to CrzOs appearing as calcium chromite. With more lime or more soda the proportion of chromate is increased. A 50-50 oxidation of chromium to chromite and chromate gives an oxidized material capable of a highly exothermic reaction with-sili- 45 con alloys. Such a compound of CaO, FezOa, CrzOa and CrOa containing a small amount of NanO from the soda ash and made from ferroehromium by roasting in air, is a highly advantageous material for exothermic production of chromium-iron metal. A fully chromated material containing CaO, NazO, FezOs and CrOa in chemical combination can readily be made. It contains less chromium but is more exothermic.

It is however possible, as stated, to oxidize high carbon ferrochromium completely by roasting it in finely divided form without the presence .of lime or other oxidation promoter; roasting being in two or more stages and the material reground between the roasting stages. This shortens the time and fuel required. 'Oxidation of ferrochromium is 'a vigorous exothermic action which starts at about 600 to 700' C., the temperature rising automatically to 1000" C. or higher. But complete elimination of metal and of carbon requires heating of the material for a time after the exothermic action has spent itself and this time is materially .shortened by .the presence of a base or other oxidation promoter with the ferrochromium. Double roasting has the advantage of facilitating grinding to a greater iineness than is usually possible for the unroasted metal. The lime and soda are omitted with advantage in the first roasting and added ln subsequent regrinding and reroasting.

When 'roasting in two stages, the temperature may well be carried considerably above 1000 C. in the first stage, 1350 for example, to completely eliminate carbon and oxidize the Cr to CraOa and after regrinding with lime and soda a second roasting at '100 to 1000 C. puts a good percentage of CrOz in the product, a proportion which can be above percent of the total contained chromium. `It is often advantageous in practice to roast to 100 per cent CrOa and mix the fully chromated product with chromite obtained by roasting at higher temperature for shorter time. So doing, the oxygen content of the mixture is readilyl adjusted a's desired for the subsequent exothermic action. t

Oxidation can be accelerated by raising the partial pressure of oxygen in wellunderstood ways, by the use of oxygen itself, or a compound. capable of releasing oxygen at the roasting temperature, such as sodium chlorate, sodium nitrate,fsodium bichromate, chromium trioxide, manganese dioxide,l or the like. The use o1 oxygen, by raising the oxygen content of the roasting atmosphere to a point above the ordinary concentration of oxygen in air, naturally facilitates oxidation. A small quantity of one of the oxygen-releasing compounds mentioned, present along with the lime, also shortens the roasting time, such compound serving when so used as an effective promoter of oxidation, while relying upon the air as the main source of oxygen. The roasting step may in fact be conducted, with or without lime, in the presence of a suffil cient quantity of one of the oxidizing agents or oxygen-releasing compounds mentioned hereinabove to furnish oxygen for oxidation of most orv all of the metal. When this is done, the oxidation becomes strongly exothermic yand is completed in a few minutes.

However, in the ordinary practice of the present invention, air and lime, with a, little soda, are relied on. Instead of lime, strontia or baryta may be used but their molecular weight is higher a'nd their cost greater.. Potash may be used instead of soda. The bases may be used in the form of carbonate. Magnesia or dolomitic lime may be used. V K

The roasting can be done in a roasting or calcining furnace of suitable type such as an open hearth or reverberatory furnace with mechanical rabbling or 'in a rotary kiln.I Stirrlng during roasting aids oxidation. Fine grinding of the mixture in a pebble or ball mill before roasting is good practice. l

The color of theroasted product is from black to gray to yellow, depending upon the CaO content and the CrOacontent. Complete oxidation of ferrochromium is attended with loss of magnetism and the fully roasted material is nonmagnetic. metal and of the carbon proceed together, with loss of magnetic power as the metal and carbon are eliminated with formation of FezOs and without formation of FeO. The magnetic test may be used to measure the elimination of metal, of FeO and of carbon, the roasting being stopped rwhen the material becomes non-magnetic.`

If the synthetic chromite Vis insufiiciently high in CrrFe ratio, it may be next beneflciated by replacement of iron with lime and preferential reduction of iron, in the manner described in my Patent No. 2,098,176. Or the ore may be beneciated before reduction `to high carbon ferrochromium. Whether or not such beneciation is practiced, the resultant chromite, which may` Ih the roasting, oxidation of the"4 be calcium ferrichromite or a' calciumchromite,

` or a chromated chromite, is then exothermically reducible with a` non-carbonaceous reducing` agent, such as ferrosilicon, ferrochrome silicon, aluminum, or the. like. 'I'his reduction results lin the production of an iron-chromium alloy low in carbon; e. g. chromium steel, low carbon ferrochromium, or even (where iron has been selectively reduced and removed) a chromium metal of high purity. low grade, high iron, materials are, however, advantageous in making chrome alloy steel and iron.

AAs a carbon-free reducing agent, any of the alloys of aluminum or silicon, or magnesium, `may be used. Calcium, magnesium and aluminum silicides are effective. Where nickel is wanted in the final alloy, nickel silicide may form va component of the reductant. Ferrochrome silicon is useful in adding chromium with the heat developed roasted ferrochrome by the silicon.

I n accordance with one aspect of my invention, roasted ferrochromium is mixed with 'ferrosilicon or ferrochrome silicon, both as iine powders, in such quantity as to supply suiiicient sili- Vcon to reduce the chromium and iron oxides to metal. To make a good exothermic mixture reacting completely and quickly, it is necessary that the mixing be exceptionally complete. It is Va useful expedient, after making the mixture to ball mill it for a time. The mixture is next introin exothermic reduction of` duced into a steel bath, as in the open hearth steel furnace, in such relative quantity as to supply the desired chromium content in the final steel product,4 This step maybe the final step in standard open-hearth steel manufacture; the step 'when exothermic action is initiated and 1 completed with productionof molten metal which enters the steel, while the Si02 formed, aswell as any SiOz present in the mixture added, combines with the CaO present in the chromite to form a non-refractory slag; a slag which is .freerunning at .the steel-making temperature, and Vnot objectionable either in amount or in character. In makirig this addition to molten iron or steel, there is no local chilling. As a matter of fact, with the usual exothermic. mixtures, there.

respondingly high. An oxygen-carrying compound such as sodium nitrate, sodium chlorate,

sodium bichromate, or the like may be added with the roasted ferrochromium, and the amount of silicony added in the reducing agent is made sumcient to supply silicon for oxidation by this added material and to give suiii'cient heat in being oxidized to melt the whole of the mix, in addition to that required forreducing the metal oxides of the chromite.

composition made by the exothermic reaction of iron oxide4 and ferrosilicon. As in the method previously described,- enough lime should ordinarily be present in the exothermic mixture to form, with the resultant silica, as well as any Iron ore may be used in` the Silico-thermic mixture and steel of a desired silica present as such in the chromite. a slag having a lime-silica ratio of approximately 1.5:1 by weight. Where somewhat higher or lower slag` ratios are desirable in the steel-making opera.- tion, the lime and silicon may be adjusted accordingly. i

`In using oxidized -ferrochromium as described for making chrome steel in the open hearth `furnace an advantageous procedure is to mix it in powdered form with finely vdivided ferrochrome silicon to form a silico-thermic mixture capable of converting itself by exothermic reaction into molten chromium-iron metal and lime silicate slag; then igniting the mixture inv an insulated furnace, allowing the reaction to complete itself and pouring the metal into the open hearth steel bath, with or Without the silicate slag. This procedure effects addition of molten low carbon ferrochromium to the open-hearth reiined steel. The amount of chromium thus put into the steel i is that required for the desired alloy composition.

In utilizing oxidized ferrochromium as the oxidant and ferrosilicon as the reductant in the exothermic mixtures, all Vof the silicon can be oxidized. With a small excess of silicon over this amount, some silicon will enter the metal.

Followlngare examples showing specific embodiments of my invention.

Example I A low grade chromite ore was reduced vwith carbon in a submerged arc electric furnace to obtain ferrochromium metal containing substantially all the chromium and iron of the ore. This metal was ground with lime in a ball mill to a iineness of 100 mesh androasted for about-one hour and a half at a maximum temperature of 1350 C. with stirring in a reverberatory furnace vto obtain complete oxidation of the metal and contained carbon. An artificial chromite, calcium ferrichromite, was obtained. It contained only a trace of carbon. This roasted product was ground together with ferrosilicon to a iineness of 100 mesh and the mixture was fed in packages to a bath of molten steel in an electric furnace. The mixture underwent a smoothv exothermic reaction delivering molten metal in the form of globules and the feeding was stopped when sumcient chromium was added to the steel. The alloy formed contained 18.7 percent chromium and carbon. under 0.1 per cent; this carbon coming partly from that in the steel and partly from the ferrosilicon.

In the above example the low grade chrome ore analysed:

The ferrochromium' obtained in smelting this ore was 42 per cent chromium, 48 per cent iron, 7 per cent carbon and 3 per cent silicon. Roasting this metal, Aground and mixed with 'about equal weight of lime, produced an artificial chromite of the following composition:

, Percent CraOa 26 Fea03 29 CaO 42 S102 2.7 Carbon- Trace In utilizing this low carbon' roasted product' (con-I taining 20 per cent available oxygen) for making 18 per cent' chromium steel, 236 parts of the chromlte (made from 100 parts of the ferrochromium metal) ground and mixed with 61 parts of ferro,-

silicon (50 per cent Si) gave in theexothermic reaction about 120 parts of metal containing 35 per cent chromium and this metal added to 94 parts of molten steel gave 213 parts stainless steel of 18.7 per cent chromium content with about 180 parts slag. 'I'he slag contained lime and silica in a ratio of about 1.5 to 1 (by Weight) and 2.2 per cent CrzOs. The chromium recovery from the ferrochromium metal was about 94 per cent.

Ewample II To make chromium steel from roasted ferrochromium and ferrosilicon alone, the 42:48 ferrochromium obtained from low grade chrome ore as in Example I was ground in a proportion of 100 parts with 434 parts lime and roasted in a. gas-fired rotary kiln at 1300 C. toform 563 parts of carbon-free calcium ferrichromite. This was ground with 264 parts ferrosilicon (50 per cent i CaO-SiOa ratio was around 1.5:,1. The slag contained less than 1 per cent CrzOa.

In this operation, the amount of ferrosilicon was suftlcient to reduce all the iron and chromium of the roasted chromite and to react with the NaClOa of the mixture, forming NaCl which was volatilized in the reaction. The silicon in the chrome steel product was less than 0.5 per cent` and the carbon less than 0.1 per cent.

Example III A high carbon ferrochromium made by total reduction of a substandard chrome ore, and containing 61 per cent Cr, 8 per cent C, 3 per cent Si and approximately 28 per cent Fe, was ground and mixed with lime and soda ash in a proportion of 132 parts lime and 5 parts soda per 100 parts metal and the mixture was roasted on an open hearth for about an hour at a temperature of 750 to 875 C. The roasted product was a chromated chromite. substantially carbon-free, containing 28 per cent CrOil 10 per cent CrzOa, 14 per cent FezOa, 44 per cent CaO, 1 per cent NazO and 2 per cent SiOz. This material contains 22 per cent chromium, about 10 per cent iron and 20 per cent available oxygen as CrOa, CraOa and FezOa. The roasting converted some 69 per cent of the chromium to chromate and 31 per cent to chromite. When this oxidized material is ground and mixed with 35 per cent by weight of 50 per cent ferrosilicon 'and the mixture ignited, it converts itself into a low-carbon metal, 44 per cent Cr and 55 per cent Fe and a lime silicate slag; one hundred parts of oxidized ferrochromium and 35 parts ferrosilicon becoming about 50 parts metal and 85 parts slag. In adequate quantity the mixture produces free running molten metal and slag.

'I'he exothermic mixture of Example 'i'i may Abei ignited by adding it to a bath of molten iron or steel in relative quantity such as to dilute the chromium content to that `wanted in the nished alloy. Or the mixture may be reacted in a separate furnace and the molten metal product run` into the steel furnace. In a particular instance 4oi. the chromite mixture, the steel was tapped with a chromium content of 1.2 per cent, representing a recovery of 85 percent of the chromium `contained in the silico-thermic mixture.

Any desired quantity of such mixture may be added without raising the silicon content `of the steel.

I have also utilized a material composed of roasted ferrochromium and ferrochrome silicon in intimate admixture for putting chromium into cast iron in a 50 poundfoundry ladle, at the same time diluting the carbon and silicon vcontents of the 4cupola metal and also raising the temperature of the metal somewhat, which facilitated the casting operation. Increasing the strength of the castiron by some 43 per cent was an important result, making it possible to vary the castings by ladle additions. The diluting action of the hot tion of the ironoxide formed in roasting ferro- `chromium as well `as otjthe iron in thesilicon alloy. Forlthis function, a roasted low vgrade ferrochromium high in iron is particularly adaptironoxide and ferrosilicon added to thejjexothermic mixture. v This application is in part a continuation of my prior copending application Serial No. 165,- 417, iiled Sept. 23, 1937.

In making the exothermic mixture of oxidized ferrochromium with a non-carbonaceous reducing agent, as described, the components of the composite reagent aref advantageously ground together to a iineness of 100 mesh. A grain size passing through a 200 mesh screen gives a desirable intimate admixture o1' the Vcomponents reacting to produce hot chromium alloy metal.

In the accompanying drawing, I have shown for purposes of illustration only, and not for purposes of limitation, a flow sheet illustrating several l of the possible chromium recovery processes which may be carried o ut in employing the principles tof the invention. Heavy lines have been employed to outline a complete chromium recoveryprocess commencing with the treatment of chromite ore initially and indicating the production ultimately' oi desirable chromium-bearing metal products.

In dotted lines, there is illustrated an alternativev oxidizing operation, and, in light lines, I have indicated the use of various oxidizing agents,

other than those produced directly in the oxidiz-' suitable for use in the production of chromium alloys which comprises oxidizing carbon-bearing ferrochromium in iinely divided condition at a temperature between 1000 C. and 1200 C. and

`forming an oxidized product low in carbon and containing iron and chromium in oxidized forms,

. and mixing the oxidized product in the-solid state with a solid, iinely divided, non-carbonaceous reducing agent lcapable of reducing the oxidizedl forms of iron and chromium to metallic .iron and metallic chromium.

2. The method of producing a reaction mixture suitable for use in the production of chromium alloys which comprises oxidizing carbon-bearing ferrochromium n nnely divided condition at a temperature between 1000 C. and 1200 C. and

forming an oxidized` product low in carbon and 'containing iron and chromium in oxidized forms, and mixing the oxidized product in the solid state with a solid, finely divided, silicon-containing reducing agent.

\ 3. 4The method of producing a reaction mixture suitable for use in the production of chromium alloys which comprises oxidizing carbon-bearing ferrochromium in iinely dividedI condition and in the presence of lime and soda ash at a temperature between 1000 C. and 1200 C. and forming an 'oxidized product low in carbon and containing iron and chromium in oxidized forms, and mixing the oxidized product in the solid state with a solid, finely divided, non-carbonaceous reducingagent capable of reducing the oxidized forms of iron and chromium to metallic iron andmetallic p l chromium.` 1 ed, further diluttion being eii'ected, if desired, by

4. The method or producing a reaction mixture suitable for use in the production of chromium alloys which comprises oxidizing carbon-bearing ferrochromium in iinely :divided condition and in the presence of lime and soda ash at a temperature between 1000 C. and 1200 C. and forming an oxidized product low in carbon and containing iron .and chromium in oxidized forms, and mixing the oxidized product in the solid state with a solid, iinely divided, silicon-containing reducing agent. V A

5. The method of producing a reaction mixture suitable for use in the production of chromium alloys which comprises oxidizing carbon-bearing ferrochromium in finely divided condition and inthepresence of lime and an amount of soda, ash equal to about 2 to 5 percent of the weight of the ferrochromium at a temperature between l1000 C. and 1200 C. and forming an oxidized product low in carbon and containing iron and chromium in oxidized forms, and mixing the oxidized product in the solid state with a solid, nely divided, non-carbonaceous reducing agent capable of reducing the oxidized forms of iron and chromium to metallic iron and metallic chromium.

l6. .The methodof producing a reaction mixture suitable for use in the production of chromium alloys which `comprises oxidizing carbon-bearing ferrochromium in iinely divided condition and in the presence of lime and an amount oi soda ash equal to about 2 to 5 percent4 of the Weight of the ierrochromium at a temperature between 1000 C. and 1200 C. and forming an oxidized product low in carbon and containing iron and chromium in oxidized forms, and mixing the oxidized product in the solid state with a solid, finely divided, silicon-containing reducing agent.

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