US2227287A - Chromium metallurgy - Google Patents

Description

Patented Dec. 31, 1940 UNITED STATES CHROMIUM METALLURGY Marvin J. Udy, Niagara Falls, N. Y.

No Drawing. Application September 24, 1937,

Serial No. 165,588

9 Claims. (CI. 75-52) This invention or discovery relates to-chromium metallurgy, and it comprises a new intermediate material useful in chromium metallurgy and having a composition generally like that of a chromium garnet, or uvarovite, but containing a substantially less amount of silica and more chromium, said material being substantially free of carbon, iron oxide and of other foreign metallic oxides; and it further comprises a process 10 of obtaining chromium from materials containing both chromium and iron wherein chromium metal is selectively oxidized in the presence of lime and suflicient silica to form a calcium silicochromite at a melting temperature and unoxidized iron metal is separated as a byproduct,

the silicochromite being an advantageous material for making chromium metal or, when mixed with a non-carbonaceous reducing agent such as silicon or a silicon alloy, for adding to molten ferrous metals to form chromium alloy steel; all as more fully hereinafter set forth and as claimed.

Metallic chromium-has become of great importance in the manufacture of non-corrodible or oxidationresistant metals. It is usually used in'the form of iron chromium alloys such as standard ferrochromium containing at least 66" per cent chromium, or a 2:1 ratio of chromium to iron. For most uses the ferrochromium must be nearly carbon free and this involves great expense in manufacture. In obtaining ferrochromium for use in making stainless steel and like alloys, the world is combed for high grade chromite ores, the mineral ferrous chromite being substantially the only source of chromium now known. Some of these ores are poor, containing much gangue and little reducible chromium, and most of them are low-grade in the sense that the ratio of iron to chromium is high.

' 40 Many ores are both poor and low-grade.

In making ferrochromium by reduction of chromite ores, it has been considered possible to use only native high chromium ores in which a portion of the FeO of the spinel mineral,

' FeCr204, is replaced by, another'oxide such as of chromite by lime or'similar base. The natural ore is smelted in an electric furnace with lime and a limited amount of reducing agent, the lime replacing the iron oxide in the chromite and the replaced iron oxide being reduced to metal and separated from an ore melt containing most of the chromic oxide of the chromite ore combined with lime as a chromite, together with the gangue of the ore.

In said prior application is disclosed and 10 claimed a specific modification of said beneficiating method wherein a portion of natural ore is treated with a ferrochromium, the metallicreduction products of chromite ore, in the presence of lime. The selective oxidation of the 15 chromium in the ferrochromium used produces an enrichment of the ore treated.

In this modification the ore is beneficiated ,both by replacement. and removal of iron and by addition of chromic oxide. In said process, iron 20 ore can be used instead of chromite ore for the purpose of selectively oxidizing the chromium of ferrochromium in the presence of lime forming a calcium chromite ore. These modifications of the invention of my prior acknowledged 25 application are described and claimedin a. copending application Serial No. 164,988, filed September 21, 1937 (Patent No. 2,127,074).

In the present invention, I selectively oxidize the chromium contained in ferrochromium or 30 4 in other metals and alloys in the presence of lime and silica and produce iron free compounds of lime, chromic oxide and silica, which are here designated silicochromites, and a byproduct metal which, if desired, may be made extremely 5 low in chromium. The iron and chromium in a a metal containing both are separated by selective oxidation at a melting temperature in the presence of lime (or similar base) combining with the chromium oxide formed, suflicient silica be- 40 ing added to the reaction mass to,form readily fusible calcium silicochromites which have a lower fusing point than that of a pure calcium chromite. These silicochromite compounds have a composition similar to that of 45 the relatively rare mineral uvarovite or chromium garnet," a mineral belonging to the garnet class. In nature it is not found pure or of ideal composition, there being usually A120: replacing part of its CraOa. It is a refractory material. When of ideal composition, it is Ca:Crz(SiO4) s. The silicochromites formed in the present invention are less rich in silica than the ideal min eral and are free of alumina, iron, etc. They are rich in easily reduced chromium and par- SI ticularly adapted for making chromium and its compounds. Ordinarily I make products containing not more than two-thirds as much silica as uvarovite and I may go down to a silica content of a third of that in the ideal mineral. As I have found, these calcium silicochromite compounds are formed in the molten state from ferrochromium at temperatures suitable for the selective oxidation of chromium metal and are readily separated from the molten iron metal remaining unoxidized. However, it is possible to use either more or less silica, and to separate molten compounds approaching the highly siliceous ideal uvarovite in composition on the one hand and on the other a calcium chromite, CaCrzO4 combined with one molecule or less of silica. The less silica in the compound, the higher is its melting point. While it is possible in an open arc furnace to make a calcium chromite free of silica, it is advantageous to add silica in moderate amount and tap oif a molten silicochromite. Uvarovite .of theoretical composition is relatively low in chromium and high in silica content. When the chromium is reduced,

'much slag is formed.

Calcium chromite has an extremely high melting point when not containing residual ferrous chromite and its direct production in the electric furnace by total reduction and removal of the iron of chromite ore requires inconveniently high temperature in the electric furnace. In the presence of a little silica, more convenient temperatures are practicable. Enough silica to approach uvarovite in composition is unnecessary and gives too much slag in later operations.

I have found these artificial silicochromites, which are substantially free of carbon and iron, to be of great utility for making chromium metal by reduction thereof with silicon and for making alloy steel by reduction of the CrzOa content with ferrosilicon serving as reducing agent. For example, a material corresponding approximately to the formula 3CaO.Cr2Oa.SiO2 is particularly useful by reason of its substantial chromic oxide content which is about 40 per cent and its ready fusibility at moderate electric furnace temperatures and in the open-hearth steel furnace.

Chromium alloy steels of the lower chromium contents are made in a basic open-hearth furnace and it is advantageous to have a source material for chromium fitting in with the requirements of an ordinary basic open-hearth furnace. This the present material does. It adds no alumina or other foreign oxides to the ordinary floating slag of calcium-iron silicate. The heat of reaction in reduction. of the CrzOa by silicon is carried directly into the steel bath, making it possible to produce in the open hearth furnace steels of higher chromium content than heretofore.

For use in making alloy steel, the silicochromite in granulated or finely ground form, is advantageously mixed or briquetted with fine-ground ferrosilicon and added to the steel bath in a relative quantity giving the desired chromium content in the finished steel. The iron of the ferrosilicon joins the bath and the silica from the silicon joins the slag. The chromite can be made as low in iron as desired and when mixed with, say 50 per cent ferrosilicon, which is readily made extremely low in carbon, not only has the advantages of a carbon-free ferro-chromium but also that of the exothermic reaction setting free chromium and adding heat to the bath: 2CrzOa+3Si=4Cr+3Si02+l00,200 calories.

The addition of a mixture of silicochromite and ferrosilicon to a steel melt has no appreciable chilling effect upon the heat.

A selective or differential oxidation of chromium in a ferro alloy can be effected according to the following reaction,

(1) Fe+2Cr+CaO+30=Ca0 C12O3+Fe in which oxidation of the Cr may be effected by the oxygen ofthe air or other source such as pure oxygen or by iron oxide or iron ores or other metallic oxides which are readily reduced. There is considerable heat evolved in this reaction and in a-properly designed furnace, very little power is required to keep up temperature after the oxidation begins until near the end when the CH0; has reached 40 per cent or more when power is necessary to complete the reaction.

The selective oxidation process is advantageously carried on in an electric furnace but may be carried out in a blast furnace or a bessemer converter.

In order to prevent oxidation of iron it is necessary to operate in an atmosphere non-oxidizing to iron. In the electric furnace I have found that oxidation of the chromium is very rapid and will take place if there be sufficient coke on the bathto prevent oxidation of the iron. Sufficient coke is carried on the bath to maintain a CO concentration preventing oxidation of the iron.

It is advantageous to use iron oxide in iron ores or chrome ores for selective oxidation of chromium as described in my Patent 2,098,176, and I have found in making high grade calcium silicochromite great advantage in the exothermic action of F6203 in oxidizing chromium metal. In

large heats this method is quite successful and is an important part of the process. In carrying out the process exothermetically it is usual to use sufiicient ferrosilicon or silicon metal and an oxidizer such as sodium nitrate to give additional heat and greater fluidity;

I have further found that chromium at high temperatures is oxidized by C02 and this oxidation is much more vigorous than the corresponding reaction between CO2 and Fe. In the case of chromium the reaction does not appear to be reversible. o The reaction for iron is given below (2) and the corresponding reaction for chromium (3).

Reaction 3 may be slightly reversible theoretically but from my experience it does not appear to be so to any material extent. It can readily be made to go to completion.

One method in which I make use of this retion is to smelt in an electric furnace ferrochrome of any Cr to Fe ratio or chromium bearing irons or alloy steels or chromium bearing scrap with calcium carbonate (lime rock) and silica to form a compound approximating 3CaOCI2O3S1O2 (or Coke is added to the bath in suflicient quantity to maintain a CO concentration in the CO: atmosphere preventing oxidation of the Fe during and after oxidation of the chromium. I have found that suflicient coke for this purpose can be carried on the bath, without affecting oxidation of the chromium.

The final products of this process whetherI begin with scrap metal containing chromium or with ferrochrome made from ores of any Cr to Fe ratio are iron having a low but substantial content of chromium and calcium silicochromites.

Refractory materials such as A120 and MgO are substantially absent in contrast to natural chromite ore. The calcium silicochromite may be readily converted to chromium compounds such as chromic acid, sulphate, oxide, etc. by known methods.

The following example illustrates the process and results obtained in making the new silicochromite product. In this case the melting was carried out in an open are electric furnace of the Heroult type.

The metal used analyzed .42 per cent Cr, 6 per cent C, 3 per cent Si, 49 per cent Fe. This metal to the amount of 2380* pounds was mixed with 3050 pounds of lime stone (53 per cent CaO) and 350 pounds of quartzite (96 per cent S102) and melted in the electric furnace using coke on the bath to insure sufiicient CO concentration to prevent oxidation of the Fe.

The calcium silicochromite produced was approximately 3564 pounds analyzing Cr2O3 38.5

per cent; CaO 47.4 per cent; SiOz 14 per cent.

As byproduct 1197 pounds of molten metal containing 8 'per cent Or were separated. The chromium in the byproduct metal can be controlled to almost any desired content. Usually I have made a sufiicient number of heats to give a sizable amount of metal to cast in moulds and then carried the oxidation to reduce the Cr in the metal under 3 per cent. Economic conditions usually determine how far the oxidation should be carried.

The following example shows a specific embodiment of my invention including the production of a finished alloy steel containing approximately 18 per cent chromium. The silicochromite produced as in the preceding example is ground and briquetted with ferrosilicon containing 50 per cent silicon, which has a low carbon content, in sufficient quantity to reduce the Cl'zOs present in the silicochromite. Thus I use 100 parts of silicochromite with 25.5 parts of 50 per cent ferrosilicon (10 per cent excess) and add as briquettes or as fine mix to a previously prepared steel bath containing 108.5 parts of low carbon steel. After the reaction and refining period there are produced 145 parts of steel containing 17.3 per cent chromium and less than .10 per cent carbon. The carbon content depends on the reduction obtained in the iron preparation previous to the addition of the silicochromite and ferrosilicon briquettes. The slag produced the slag as SiOz.

is 95.2 parts containing approximately 1.00 per cent CH: and has a lime-silica ratio of about The carbon free silicochromite used in the above example is iron freeand the cost of production is higher than when some iron is oxidized during the chromite formation. The iron free product however is highly useful in making high grade chromium metal for specific uses such as in the manufacture of alloys of the stellite and nichrome types.

Following is a specific example of my invention in the production of high grade chromium metal; One hundred pounds of the silicochromite of the above composition are mixed with 13.3 parts per cent excess) of 97 to 98 per cent silicon metal and smelted in an open arc furnace the Heroult type. The silicon reduces the Orzo: to chromium metal and the Si enters The slag is adjusted to a limesilica ratio of about 1.5 CaO to 1 SiOz by the additon of 12.7.. parts of CaO. The metal produced was 24.6 parts or about 90 per cent recovery and analyzed upwards of 98 per cent Cr. The

slag was approximately 104.5 parts and analyzed 2.4 per cent Crzoa.

It will be recognized that, when this product of silicochromite is used in making alloy steels, an iron free product is not required. Indeed it is much cheaper to make a product containing some iron as FeO. The process can be and is operated to produce such a product. The following are typical analyses of successive heats made to produce a silicochromite with some iron present but of a Cr/Fe ratio high enough to produce the standard 70 per cent ferrochrome,.or to be used to increase the ratio of Cr to Fe in ores having a Cr to Fe ratio too low to produce a '70 per cent ferrochrome or higher.

' Heat N 0.

CH0 FeO Gr/Fc ratio One skilled in the art can readily recognize the various possible uses for such products.

Another use of the silicochromite made as described is in the production of chromates, bichromates and chromic acid. The Cr203 of silicochromite is readily oxidized to C10 by roasting in the air at temperatures of-700 to 1000 0. Calcium chromate is formed 'and is readily leached from the insoluble calcium silicate. Chromic acid is advantageously produced by acidifying the calcium chromate in solution with ,sulfuric acid, precipitating calcium sulfate.

chromium in a molten condition with lime and silica and an oxidizing agent selectively reacting upon the chromium to form a fusible calcium silicochromite containing the greater part of the chromium of the ferrochromium as oxide and from 14 to 28 per cent silica and separating from said chromite a molten metal containing the greater part of the iron of the ferrochromium.

3. A process which comprises treating ferrochromium in a molten condition with lime and silica and iron oxide selectively reacting upon the chromium to form a fusible calcium silicochromite containing the greater part of the chromium of the ferrochromium as oxide and from 14 to 28 per cent silica and separating from said chromite a molten metal containing the greater part of the iron of the ferrochromium.

4. A process which comprises treating ferrochromium in a molten condition with calcium carbonate and silica selectively reacting upon the chromium to form a fusible calcium silicochromite in pure form containing the greater part of the chromium of the ferrochromium as oxide and from 14 to 28 per cent silica and separating from said chromite a molten metal containing the greater part of the iron of the ferro-chromium.

5. A process which comprises selectively oxiidizing the chromium of ferrochromium in the presence of silica and a base and at a melting temperature to obtain a basic silicochromite in a molten mass separatedfrom a molten metal containing most of the iron of/ the ferrochromium, mixing said silicochromite with ferrosilicon and adding the mixture 'to a molten steel bath.

6. A process which comprises selectively'oxi- Patent No. 2,227,287.

MARVIN J. may.

cEnrmcArr: F, comcrromf dizing. the chromium of ferrochromium in the presence of silica and a base and at a meltin temperature to obtain a basic silicochromite in a molten mass separated from a molten metal containing .most of the iron of the ferrochromium and reducing said silicochromite with silicon to obtain metallic chromium.

'7. A process which comprises selectively oxidizing the chromium of ferrochromium in the presence of silica and a base and at a melting temperature to obtain a basic silicochromite in a molten mass separated from a molten metal containing most of the iron of the ferrochromium and reducing said silicochromite with a silicon alloy.

8. In a process-for producing a product containing oxidized chromium involving the treatment of a molten alloy containing iron and chro- MARVIN J. UDY.

December 51, 191 0.

It is hereby certified that-error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2,- sec- 0nd column, line "58, for "exothermetically" read -:--exothermical1y--; line 58-59, for "retien" read "reaction" and line 72, for "exothermeti'cally" read --exothemically--; page 5, first column, .line 14., before "forms" insert fi'rst--- dter "furnace" insert --of--" line 5,-for "Ca read "60 second column, line 18,

, and that the-said Potters Patent should be read with this correction therein that the same may conform to the'reeord of the case in the Petentoffice.

signed and sealed this 17th day of March,

(Seal) Henry-Yen Arsdale, Acting-commissioner of Patents.

CERTIFICATE OF CORBECTION.

Patent No. 2,227,287. December 31, 19h0.

MARVIN J. UDY.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line "58, for "exothemetically" read-.--exothermically--; line 58-59Qfor "ration" read -reaction--; and line 72, for "exothermeigically" read -exothensically-; page 5, first cOlumn,.1 ine h, before Proms" insert -first-; line 5, for Ca read -C0 second column, line 18,

after "furnace" insert -of--; and that the said I etters Patent should be read with this correction therein that the same may conform to the-record of the case in the Patent Office.

Signed and sealed this 17th day or March, A. n. 1912.-

. flenry van Arsdale, (seal) Acting commissioner of Patents.