US2601306A - Method for dissolution of chromium from chromite ores - Google Patents

Description

Patented June 24, 1952 METHOD FOR DISSOLUTION OF CHROIHIUM FROM CHROMITE ORES Rex R. Lloyd, Boulder City, Nev., and William T. Rawles, Pittsfield, Mass, assignors to the United States of America as represented by theSccretary of the Interior No Drawing.

2 Claims.

This invention relates to a method for the dissolution of chromium bearing ores, and it particularly relates to the dissolution of ores of the chromite series, chromium spinel ores, and similar .difiicultly soluble chrome ores. Still more particularly the invention relates to a method for the dissolution of such chromium ores using the spent anolyte from a process for the electrowinning of chromium.

Chromium ores such as those of the chromite series (Dana's The System of Mineralogy, 7th edition, vol. I, page 709), spinel series (page 689 of the aforementioned reference) and like ores can be dissolved only with great difficulty. It has been found possible to digest such ores in concentrated sulfuric acid (1. e., greater than 65% by weight H2804), and it is also necessary to employ strong oxidizing agents such as hexavalent chromium compounds, lead peroxide, manganese dioxide or the like in order to effect complete dissolution of the chromium values thereof. Whenever a solution containing ammonium ions is employed for dissolving the chrome ore, as in the case where the spent anolyte from processes for'the electrowinning of chromium is so used, a highly insoluble iron ammonium complex, probably a ferric ammonium complex forms during the digestion. This complex appears to contain or occlude a considerable portion of chromium and also causes chromium losses due to its tendency to form on and coat the ore particles, thereby preventing dissolution.

Accordingly, it is an objectof this invention to provide a simple and economical method for the dissolutionof certain diflicultly soluble chromium ores. 1

It is another object of the invention to provide a method for the dissolution of chrome ores when the solvent contains ammonium ions, without concurrently forming insoluble metallic ammonium complexes.

Other objects and advantages will be apparent or will appear hereinafter.

These objects and advantages are accomplished in accordance with this invention wherein the chromium ore is digested in a solvent comprising a dilute sulfuric acid solution containing an oxidizing agent at superatmospheric pressures.

Suitable chrome ores for treatment in accordance with this invention include those of the aforementioned chromite series such as magnesochromite, chromite, and the like; those of the spinel series such as chromium spinel, chro- Application February 24,1948, Serial No. 10,552.

(Granted under the act of March '3, 1883, as amended April 30, 1928; 3'70 0. G. 757) 2 mian hercynite and the like; and other chromium ores which exhibit some degree of solubility in sulfuric acid, in the presence of an oxidizing agent.

,In operation the chromium ore is preferably pulverized so that it will have a greater surface upon which the solvent can act. This apparently is not a critical factor to the operation of the invention, and satisfactory results have beenobtained bygrinding the ore or a chromium ore concentrate to sizes less than about mesh. The speed of the reaction, howeverjis a function of particle size. The groundore is charged into a chemically resistant reaction vessel capable of withstanding the pressures and temperatures involved. A stoichiometric equivalent of dilute sulfuric acid (1. e., acid not greater than about 50 per cent by weight and preferably stronger than about 20 per cent by weight) and containing an amount of chromic acid equal to about 2 to 7 per cent of the Weight of the sulfuric acid; or an equivalent amount of other oxidizing agents such as lead peroxide, manganese dioxide, and the like is mixed with the charge. The vessel is then sealed and the pressure is permitted to build up to between about 30 and 90 p. s. i. gage. This pressure will vary inversely with the acid concentration of the solvent. Since the reaction is exothermic, it will proceed without the application of external heat and pressure will depreferably between about to degrees centigrade. The completion of the digestion will be indicated by a drop in temperature of the reaction mass. The solution produced in this mannerwill be found to contain substantially all of the chromium in a soluble form. If ammonium ions are present in the solution, as might be as hereinafter described, no insoluble metallic ammonium complexes will be formed under the conditions outlined above.

In a preferred embodiment of the invention the spent anolyte from cells for the electrowinning of chromium is employed as the solvent for the chromite. The anolyte as discharged from such cells, generally contains about 300 grams per liter of sulfuric acid, 15 grams per liter of ammonia as ammonium sulfate, and 20grams of chromium as chromic acid. The acid-concentration of-the spent anolyte'is built up to about45 per cent sulfuric acid, using raw'sulfuric acid and,

3 if necessary, suiiicient chromic acid is added to bring the total chromic acid to about 2 to 7 per cent of the weight of the total sulfuric acid concentration. Batches of low grade chromite concentrates are wet ground to about minus 270 mesh and dried in any suitable manner. The ore is digested in batches using a stoichiometric quantity of the fortified anolyte. Such digestion has been carried out conveniently in an acid-resistant brick-lined digestor capable of withstanding pressures as great as 100 p. s. i. The temperature during digestion is maintained at between about 130 to 150 degrees centigrade, and

this control was achieved by maintaining the the raw digest liquor is removed as a normal metal sulfate solution and containing practical- 1y no excess sulfate radical.

The raw digest liquor contains various ionic constituents in addition to the chromium and sulfate; e. g., iron, aluminum, etc. The term normal metal sulfate solution is used to indicate that the amount of sulfate present is that stoichiometrically necessary to form normal sulfates of the various metals present in the raw digest liquor.

When it'is desired to recirculate this regenerated solution through the electrolytic cells, it is necessary to purify the raw digest liquor. This can be accomplished by diluting the raw digest liquor with spent mother liquor obtained in a subsequent crystallization process described hereinafter to produce a solution containing about 42 grams of chromium (calculated as elemental chromium) and 150 grams of ammonium sulfate per liter, appreciable quantities of iron, aluminum, magnesium, and sodium, and small quantities of several other metals. This diluted solution is filtered at 65 degrees centigrade to remove siliceous gangue and heated to so degrees centigrade for one hour. The liquor is sent immediately to a vacuum crystallizer, where all of the aluminum and most of the iron and magnesium are removed at 4 degrees centigrade.

These ammonium sulfate complex crystals contain most of the leached impurities. Chromium remains in solution because of the above-mentioned prolonged heat treatment. These impurity crystals are removed in rubber-lined basket centrifuges, and the purified mother liquor is clarified in a wooden filter press and sent to aging.

Aging takes place in a Koroseal-lined thickener at 30 degrees centigrade. Chromium reverts from the green to the violet modification and crystallizes as chromium ammonium alum, containing some ferric iron and small quantities of many other impurities. The chrome alum is removed by centrifuging; and the spent mother liquor containing about 22 grams chromium (calculated as elemental chromium) per liter, can be used to dilute the raw digest liquor as aforementioned.

One more crystallization of the chrome alum in a thickener at 30 degrees centigrade, using spent catholyte from the electrowinning cells as recrystallizing. liquor, reduces iron and other impurities below the critical concentrations. The iron in the crude alum is reduced to the ferrous state by the divalent chromium in the spent catholyte and does not recrystallize with the chromium. The pure chrome alum is mixed with glaubers salts, recovered from the impurity alums, and dissolved to produce cell feed containing grams of chromium (calculated as elemental chromium), grams of ammonium sulfate, and 60 grams of sodium sulfate per liter.

The following examples will show how the invention may be carried out, but the invention is not limited thereto.

Example I A sample of 500 grams of chromite was charged into a jacketed stainless steel autoclave and a solution was added which contained 1,250 grams of sulfuric acid, 2,325 grams of water, 45 grams of hexavalent chromium, and 34 grams of ammonia as ammonium sulfate. The chromite was digested in this 35 per cent acid for 6 /2 hours at 80 pounds pressure and 165-175 degrees centigrade. At the end of this period, 63 per cent of the chromium in the ore was rendered water soluble and no dehydrated alum was produced.

When this process was repeated, using a 65 per cent sulfuric acid solution, a large quantity of insoluble alum was produced which contained substantially all of the chromium.

Example II A brick-lined digestor was constructed with a pressure cover but without any means of applying external heat. This digestor was charged with 480 pounds of chromite ground to 98 per cent minus 325 mesh along with a solution containing 39.5 pounds of hexavalent chromium, 12.5 pounds of ammonia as ammonium sulfate and 210 pounds of sulfuric acid. The digestor was closed and an additional 960 pounds of acid was pumped in, making a 45 per cent sulfuric acid solution. The temperature began to rise and at the end of 30 minutes was 145 degrees centigrade, and the pressure was 38 pounds gage. Steam, and some inert gas, was bled off gradually to keep the temperature at to degrees centigrade. After 10 hours, the temperature dropped to 100 degrees centigrade, the digestor was opened and the solution tested. Analysis showed that 23 pounds of acid remained and 99 per cent of the chromium in the ore was water soluble. No dehydrated alums were present.

From the foregoing description and examples it is apparent that a simple process for the dissolution of chromite in the spent anolyte from electrolytic cells for the electrowinning of chromium has been developed.

While the invention has been particularly described for the digestion of chromite ores, it is not limited thereto; other chromium ores exhibiting solubility characteristics in sulfuric acid can also be dissolved by this process.

While the invention has been particularly described as employing hexavalent chromium as the oxidizing agent, it is not limited thereto and other oxidizing agents can be substituted therefor.

Since many widely differing embodiments of the invention will occur to one skilled in the art, the invention is not limited to the specific details illustrated and described, and various changes can be made therein without departing from the spirit and scope thereof.

What is claimed is:

1. A method for the dissolution of chrome ores in a solution containing ammonium ions without the formation of chrome ammonium alum which comprises dissolving said ores in a solution containing ammonium sulfate, chromic acid, and to per cent sulfuric acid by weight, said solution having a temperature within the range of 165 C. to C'., the temperature decreasing with the increase in percentage of sulfuric acid.

2. In a process for the electrowinning of chromium from an electrolyte which contains trivalent chromium in the presence of sulfate ion and ammonium ion, the improvement which comprises regenerating the chromium concentration of the spent electrolyte which contains chromic ion, ammonium ion and sulfate ion by increasing the sulfuric acid concentration to from 30 to 50 per cent by weight, and dissolving chrome ore in the thus formed solution at a temperature within the range of C. to 125 C., the temperature decreasing with the increase a in percentage of sulfuric acid.

REX R. LLOYD. WILLIAM T. RAWLES.

6 REFERENCES CITED- The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 743,668 Sucky et a1. Nov. 10, 1903 1,403,960 Hultman Jan. 17, 1922 1,502,035 Hasenclever July 22, 1924 1,723,536 Weise Aug. 6, 1929 1,749,443 Proctor Mar. 4, 1930 2,507,476 Lloyd May 9, 1950 FOREIGN PATENTS Number Country Date 187,636 Great Britain Nov. 2, 1922 OTHER REFERENCES Lloyd, Transactions of the Electrochemical Society, vol. 89, pages 443-454, 1946.

Claims (1)

1. A METHOD FOR THE DISSOLUTION OF CHROME ORES IN A SOLUTION CONTAINING AMMONIUM IONS WITHOUT THE FORMATION OF CHROME AMMONIUM ALUM WHICH COMPRISES DISSOLVING SAID ORES IN A SOLUTION CONTAINING AMMONIUM SULFATE, CHROMIC ACID, AND 30 TO 50 PER CENT SULFURIC ACID BY WEIGHT, SAID SOLUTION HAVING A TEMPERATURE WITHIN THE RANGE OF 165* C., THE TEMPERATURE DECREASING WITH THE INCREASE IN PERCENTAGE OF SULFURIC ACID.