US2390970A - Metallurgy - Google Patents

Description

a w v Patented Dec. 11, 1945 UNITED STATES PATENT OFFICE No Drawing. Application April 20, 1942, Serial No. 439,721

1 Claim.

This invention relates to metallurgy and has for an object the provision of an improved metallurgical method or process and improved metallurgical products. More particularly, the invention contemplates the provision of an improved method or process for recovering chromium from chromite ore and improved chromium-bearing metallurgical products. The invention further contemplates the provision of an improved method or process for producing smelting charges and reaction mixtures suitable for use in the production of metallic chromium. A further object of the invention is to provide an improved method or process for obtaining from chromite ore by reduction metallic chromium-bearing products containing iron in which the ratios of chromium to iron are higher than the ratio of chromium to iron in the chromite ore. A specific object of the invention is to provide an improved method or process for utilizing so-called lowgrade chromite ore in the production of metallic chromium-bearing products in which the ratios of chromium to iron are higher than the ratio of chromium to iron in the chromite ore. Another object of the invention is to provide an improved method or process for producing reaction mixtures capable of reacting exothermically to produce metallic chromium.

Throughout the specification and in the claims, the term chromite ore is intended to include natural chromite ore, chromite ore concentrates, altered chromite ore and similar chromite minoral-bearing materials.

The invention contemplates the use of ironfree or substantially iron-free chromium compounds such as chromic oxide, calcium chromite and calcium chromate in the production of reduction charges and reaction mixtures which may be utilized in producing metallic products containing high ratios of chromium to iron.

According to some heretofore customary practices, chromite ores are subjected to oxidizing or roasting treatments in the presence of one or more alkali metal compounds such, for example, as sodium carbonate, sodium hydroxide and sodium sulphate under such conditions as to oxidize the trivalent chromium of the chromite ore to the hexavalent state and produce alkali metal chromates such, for example, as sodium chromate. Sodium chromate thus produced is separated from the product of the roasting or oxidizing treatment by leaching and crystallization and marketed as such or converted to sodium dichromate for marketing. Chromium compounds such as chromic oxide, calcium chromite and calcium chromate are produced by suitable reduction or conversion treatments of commercial sodium chromate or sodium dichromate. Usually, the production of such compounds is carried out independently of the chromate production operations and, consequently, the sodium oxide content of the chromate compound is wasted or, at any rate, is not recovered in a form in which it is readily available for re-use in the chromate production operations. Therefore, chromic oxide, calcium chromite and calcium chromate are relatively costly commercial products. Such compounds are highly desirable materials for metallurgical uses as well as for many other industrial uses, but their usefulness is not fully developed because of high costs.

The present invention provides for the production of such compounds under conditions and according to procedures such that the alkali metal oxide content of chromates employed in their production is utilized for the production of additional chromate. The invention thus provides for the production of such compounds at costs which permit their utilization effectively and economically in metallurgical operations.

A complete preferred process of the invention comprises (1) oxidation or roasting in air of chromite ore in the presence of one or more alkali metal compounds in accordance with well known oxidizing or roasting procedures with the production of alkali metal chromate, (2) separation of the chroma mm o er"inateria1s associated therewith in the oxidized or roasted charge as, for example, by leaching with an aqueous liquid followed by evaporation and crystallization, (3) reduction of the chromate directly or after conversion to dichromate with the production 01 chromic oxi e ds s'iiili of chromic oxide chemically combined with a basic oxide and one or more alkali metal compounds suitable for use in the oxidation or roasting treatment of chromite ore, (4) utilization of the one or more alkali metal compounds thus produced in an oxidizing or roasting treatment of additional chromite ore,

and (5) utilization of the chromic oxidein forming reduction charges and reaction mixtures which may be employed in the production of metallic chromium-bearing products. Through the recovery and re-use of the alkali metal oxide of the chromate, the costs of producing compounds such as chromic oxide, calcium chromite and calcium chromate are reduced materially and a wider field of usefulness for such compound is provided.

The invention also provides improved, more efiicient and less expensive methods or processes for utilizing commercial alkali metal chromates and. dichromates in the production of compounds such as chromic oxide, calcium chromite and calcium chromate.

Certain phases of my invention relating to the production of chromium compounds described but not claimed herein are described and claimed in my copending application Serial No. 428,207, filed January 26, 1942.

According to the invention, an alkali metal chromate, such as sodium chromate (Na2CrO4) or sodium dichromate (NazCrzOv), in finely divided form, is intimately mixed with finely divided solid carbonaceous reducing material and ignited. Finely divided charcoal, coke or coal, may be employed satisfactorily as the reducing material. Finely divided charcoal may be e ployed most advantageously for the production of pure products. Finely divided coke or coal may be employed When relatively impure products are satisfactory. When coal or coke is employed, the products resulting from reduction of the chromium of the chromate through elimination of a portion of the oxygen may be contaminated with coal or coke ash.

My investigations have indicated that reduction of alkali metal chromate and dichromate with carbon through ignition of intimate mixtures of the materials proceeds somewhat according to the following equations:

It will be noted that the above equations indicate the production of sodium chromite (NazO-CrzOs) and carbon monoxide or carbon dioxide in reducing sodium dichromate with carbon in different proportions relatively to sodium dichromate and the production of sodium chromite (NazO-CrzOs), sodium carbonate and carbon monoxide in reducing sodium chromate. When the solid reaction product resulting from the reduction of sodium dichromate with carbon is digested with water (hot or cold), a solution of sodium hydroxide substantially free of sodium carbonate and a solid residue consisting essentially of chromic hydroxide (Cr(OH)3) are produced. When the solid reaction product resulting from reduction of sodium chromate with carbon is digested with water, a solution of sodium hydroxide and sodium carbonate and a solid residue consisting essentially of chromic hydroxide (Cr(OH)a) are produced.

Digestion of the solid products resulting from reduction does not effect hydrolysis or decomposition of all of the sodium chromite (Nazo-Crzos) contained therein, and the solid residue consisting essentially of chromic hydroxide contains some sodium oxide probably chemically combined with chromic oxide as sodium chromite. The residual sodium oxide may be separated from the bulk of the chromic hydroxide by subjecting the residue resulting from digestion to an oxidizing treatment with air at an elevated temperature (above about 500 C.) and leaching the resulting product with 'water. The residual sodium oxide is converted to sodium chromate in the oxidizing treatment and the chromic hydroxide is converted to chromic oxide (CI2O3) Elimination of the residual sodium oxide is not essential to the eiTective use of the product of the oxidizing treatment in the production of reduction charges and reaction mixtures. In fact, in the production of exothermic reaction mixtures, retention of the residual sodium oxide may be desirable because of the capacity of the sodium oxide to promote oxidation of chromic oxide to calcium chromate in the presence of calcium oxide and because of beneficial slag-modifying effects of the sodium oxide.

Calcium chromite and sodium carbonate may be produced by forming and igniting an intimate mixture of finely divided sodium chromate or sodium dichromate, finely divided calcium oxide and finely divided carbon. It appears that the reactions proceed somewhat according to the following equations:

The sodium carbonate and calcium chromite produced in accordance with the reactions indicated by the above equations may be separated by digesting the solid reaction products with water, hot or cold. Calcium oxide may be employed as such or in chemical combination with carbon dioxide as calcium carbonate.

Equations 5 to 7 indicate the production of a compound approximating in chemical composition di-calcium chromite. Compounds approximating in chemical composition mono-calcium chromite or tri-calcium chromite may be produced by adjusting the proportions of chromate ordichromate and calcium oxide employed.

All of the reactions indicated by Equations 1 to 6 above are exothermic and usually proceed to completion without the application of heat from extraneous sources. When the calcium oxide is employed in the form of calcium carbonate and when the production of tri-calcium chromite is sought, it may be advisable to provide additional heat from extraneous sources to insure effective chemical combination of the calcium oxide with the chromic oxide.

Calcium chromate may be produced by roasting in air a solid residue obtained in the digestion of a solid reaction product resulting from the reduction of a chromate or dichromate in the presence of calcium oxide to produce calcium chromite. Substantially complete conversion of calcium chromate may be accomplished by roasting the di-calcium chromite and the tri-calcium chromite. Roasting 0f the mono-calcium chromite will result in the production of calcium chromate and chromic oxide. Complete conversion to chromate of the mono-calcium chromite may be accomplished by incorporating in the roasting or oxidizing charge at least one molecule of calcium oxide for each molecule of calcium chromite contained therein.

Calcium chromate may be produced also by roasting chromium hydroxide or chromic oxide produced in the process in air with calcium oxide (CaO) to a temperature in the range of about 750 C. to 1000 C. The amount of calcium chromate formed, relatively to the amount of chromium present, will be determined by the relative proportions of calcium oxide and chromic oxide present in the product subjected to treatment. Substantially complete conversion to calcium chromate may be accomplished by providing at least two molecules of calcium oxide for each atom of chromium, a mixture of calcium chromate r5. Mmuurer.

and chromic oxide may be produced by providing less than two molecules of calcium oxide for each atom of chromium.

In a complete preferred process of the invention involving the oxidation of chromite ore to produce sodium chromate, sodium carbonate, sodium hydroxide or mixtures of sodium carbonate and sodium hydroxide obtained by digesting the solid products resulting from reduction of sodium chromate and sodium dichromate may be employed in the oxidation treatment. The solutions obtained by digestion of the products of reduction may be evaporated partially or entirely to obtain solid sodium hydroxide or sodium carbonate or both, or to obtain sodium hydroxide, or sodium carbonate or both, partly in the solid state and partly in solution for re-use in the oxidation of chromite ore. Solutions containing sodium hydroxide may be passed in contact with gases, such as flue gases, containing carbon dioxide to carbonate the solution, or partly for the purpose of carbonating the solution and partly for the purpose of washing the gases, and produce, and convert the sodium hydroxide to, sodium carbonate or sodium bicarbonate or both.

The production of chromic oxide by reduction of sodium chromate with carbon is illustrated by the following example:

A reaction mixture was formed by grinding together 648 pounds of sodium chromate (NazCrol) and 46 pounds of carbon in the form of charcoal. The mixture was placed in a crucible, covered with finely divided charcoal and ignited by means of a gas flame. Upon ignition, the reaction proceeded quietly and smoothly.

The amount of carbon employed represents an excess of about thirty percent (30%) over that required to effect reduction of the chromate. It is desirable that sufiicient excess carbon be employed to prevent re-oxidation of the reaction product during cooling.

The solid reaction product was cooled under non-oxidizing conditions, digested by grinding in water to form a pulp to effect solution of sodium compounds and filtered. Hot or cold water may be employed for digestion. The filtrate obtained was an aqueous solution of sodium hydroxide and sodium carbonate. The solid residue consisting essentially of chromic hydroxide (Cr(OI-I)3) was roasted in air at an elevated temperature (1000 C.) to eliminate water and carbon and convert the chromic hydroxide to chromic oxide. The roasted product was 95 percent chromic oxide.

The chromic oxide-bearing product was leached with water to remove soluble compounds such as sodium chromate contained therein. The residue remaining after leaching was substantially pure chromic oxide. The sodium chromate thus obtained may be returned to the process. Conversion to chromic oxide of 97.5 percent of the chromium of the sodium chromate was effected.

Chromium-bearing reaction products and compounds roduced in accordance with the invention may be utilized in any suitable manner for producing reduction charges or reaction mixtures of the invention. They may be mixed singly or in combinations of two or more with carbonaceous or non-carbonaceous reducing materials and, if necessary or desirable, with slagforming materials or other reducible materials or both. Products and compounds containing chromium in the hexavalent state may be mixed advantageously with non-carbonaceous reducing materials such as aluminum or a silicon-containing material, for example ferrochrome silicon, to

form exothermic reaction mixtures. Exothermic reaction mixtures containing chromium in the trivalent state or the hexavalent state or both together with a reducing agent may contain, also, an oxidizing agent such as sodium nitrate. Products resulting from the oxidation of calcium chromite-bearing material may be employed with advantage in producing exothermic reaction mixtures. Such reaction mixtures may contain also calcium chromite or calcium chromite and one or more other chromium-containing compounds. Reaction mixtures comprising calcium chromate and calcium chromite may be produced by subjecting calcium chromite-bearing material to controlled oxidation to convert only a portion of the chromium to the hexavalent state or by oxidizing all or a large proportion of the chromium of one portion of calcium chromite to the hexavalent state and mixing the resulting product with another portion of calcium chromite. Exothermic reaction mixtures of the invention preferably are so formed as to contain non-carbonaceous materials and available oxygen in such amounts and proportions that the mixtures are capable upon ignition of reacting exothermically to produce molten chromium-bearing metal. When the reaction mixtures of the invention are to be ignited in contact with molten metal containing sensible heat for aiding in melting the reaction products, the amounts of reducing material and available oxygen need not be as great as when they are to be ignited in previously unheated environments.

Chromium-bearing reaction products and compounds of the invention may be employed advantageously in conjunction with chromite ore to produce, by direct reduction, ferro-chromium containing a higher ratio of chromium to iron than ferrochromium which might be produced by direct reduction of the ore alone. Thus, the invention provides for the production of high-grade ferrochromium from low-grade chromite ores. In accordance with this phase of the invention, one portion of low-grade ore may be treated in accordance with the invention to produce an ironfree product, for example in the form of chromic oxide, calcium chromite or calcium chromate, the product thus produced may be mixed with another portion of the low-grade ore, and the resulting mixture may be smelted with a suitable reducing agent, for example in a submerged are electric furnace, to produce ferrochromium. The smelting charge may contain low-grade ore and iron-free chromium-bearing material in proportions such as to produce ferrochromium of any suitable grade. 'ing products of the invention, such as chromic oxide, calcium chromite and calcium chromate may be utilized for producing substantially pure or substantially iron-free chromium-bearing metal products.

The invention may be utilized for producing high-grade low-carbon ferrochromium from lowgrade chromite ores by treating one portion of the ore in accordance with the invention to produce an iron-free product, for example in the form of chromic oxide, calcium chromite or calcium chromate, smelting another portion of the ore to produce high-carbon ferrochromium, incorporating silicon in the high-carbon ferrochromium to eliminate carbon with the production of high-silicon, low-carbon ferrochrome silicon, and utilizing the iron-free product or several of such products to eliminate all or a portion of the silicon. Elimination of the silicon The iron-free chromium-bearmay be accomplished in one or several stages. Thus, for example, high-silicon, low-carbon ferrochrome silicon may be mixed with one or more iron-free products, and with other oxidizing material such as sodium nitrate if desirable or necessary, to form a mixture capable of reacting exothermically upon ignition to produce (1) lowcarbon ferrochromium very low in or substantially free of silicion or (2) low-carbon, lowsilicon ferrochrome silicon. Low-carbon, lowsilicon ferrochrome silicon thus produced may be (1) refined with an additional quantity of the one or more iron-free products as, for example, in an open-arc electric furnace or (2) mixed with an additional quantity of the one or more ironfree products, and with other oxidizing material such as sodium nitrate if desirable or necessary, to form a mixture capable of reacting exothermically upon ignition to produce low-carbon ferrochromium very low in or substantially free of silicon. Such a reaction mixture may be ignited under such conditions that low-carbon ferrochromium may be produced and recovered as such or it may be ignited in contact with molten metal such as steel to produce low-carbon ferrochromium and incorporate the ferrochromium in the molten metal.

The components of exothermic reaction mixtures-of the invention preferably are intimately mixed in finely divided forms, for example, in the form of particles largely or entirely small enough to pass a IOU-mesh screen.

I claim:

The method of recovering chromium from low grade chromite ore which comprises subjecting a portion of the ore to an oxidizing treatment in the presence of an alkali metal compound to produce alkali metal chromate, treating the product of the oxidizing treatment to separate alkali metal chromate from other material associated therewith, treating the alkali metal chromate to produce an alkali metal compound and chromic oxide in chemical combination with calcium oxide, smelting another portion of the ore to produce high-carbon ferrochromium, incorporating silicon in the ferrochromium to eliminate carbon with the production of ferrochrome silicon, and utilizing the chromic oxide to eliminate silicon from the ferrochrome siliconwith the production of a metallic chromium-bearing product in which the ratio of chromium to iron is higher than the ratio of chromium to iron in the chromite ore.

MARVIN J. UDY.