US3105756A - Method of lowering the iron content of chromite ores or concentrates without appreciable lowering of the contained cr2o3 - Google Patents

Description

U ed at P e This invention relates to the mineral known as chromite,

.-.in any ores or concentrates, of any degree of fineness, that are termed chromite and that consist essentially of the oxides of chromium, iron, magnesium and aluminum in variable proportions, and has for its object an improved method for the extraction of part of the iron from the chromite mineral without appreciable lowering of the chromite content, thus in effect, raising the chromium: iron ratio to a higher factor than the ratio originally present.

The usual chemical formula used for chromite is FeO.Cr O and pure chromite should theoretically contain 68% chromic oxide and 32% ferrous oxide. Pure chromite is not found in nature, since in all natural deposits part of the chromium and part of the iron has been replaced by aluminum and magnesium. For metallurgical grade chromite, a chromite material containing a minimum of about 45% Cr O is required, and a chromium: iron ratio of at least 3:1 is a requisite. Metallurgical grade chromite of this character is the type required for the fabrication of alloys of chromium. The most highly industrialized countries have no commercial deposits of this grade. There are many deposits in many countries, wherein the chromiumriron ratio is 2:1 or 2.5 :1, and wherein the chromium content after mechanical concentration would be sufliciently high if a part of the iron could be removed without lowering the chromium content. Work in the past, to the bmt of my knowledge, has been devoted chiefly to attempts to leach out part of the iron by very fine grinding followed by leaching under pressure, at high temperatures for long periods of time with strong, expensive solvents. The costs of treatment by such methods have precluded favorable economic results, and the loss of chromite invariably experienced has not only meant a further charge against such processes but has also partly nullified the improvement in the chromiumziron ratio otherwise resultant. In my improved process for raising the chromiumfirbn ratio, I can make it possible to lower the iron to any economically desirable point without loss of more than 1% of the chromium content, using relatively small quantities of inexpensive reagents available everywhere, and since the iron leaching is done at air temperature, at atmospheric pressure, and in a few minutes time, the process can be continuous rather than a batch operation. Since chromium losses in my process are negligible, the total degree of iron removal accrues to the benefit of the improved chromiumziron ratio, and since the chromium, aluminum, and magnesium oxides are relatively unaffected in the process, all reagents consumed are almost 100% eflicient as regards iron removal.

The formula for chromite is'usually accepted to be FeO.Cr O The dot between the ferrous oxide compo nent-and the chromic oxide component indicates that there are two radicals present (a radical being a group of atoms which behave as an entity) which are somewhat loosely connected. FeO is usually soluble in common reagent acids without difiiculty, but since chromite can be ground very fine and agitated for long periods in varying strengths of sulphuric acid without appreciable decomposition or without solution of appreciable quantities of iron, the FeO is evidently protected in chromite by the Cr O radical. The FeO also appears to be protected against reduction since intense heating of chromite in a reducing atmosphere renders it only slightly more magnetic and only slightly more susceptible to attack by sulphuric acid of any strength. On the other hand, the FeO does not appear to have as much protection against oxidation, since samples of chromite of 10 mesh size which have been wetted and subsequently dried slowly turn from shiny black to dull brown. Such accidentally air-oxidized chromite can be caused to become noticeably magnetic by heating to redness in a reducing atmosphere.

I have found that chromite ores of any grade and of any chromiumziron ratio can be thoroughly oxidized by roasting at approximately 700 C. with access of air and constant rabbling. The time required for thorough oxidation depends upon the accessability of air, the efficiency of rabbling, and the maximum particle size. Onefourth inch material can be oxidized but I prefer to use minus 10 mesh chromite, sinceminus 10 mesh material gives me more rapid oxidation than coarser sizes, and less dusting than does finer sizes. With good air supply and good rabbling, on a thin bed, 30 minutes is suflicient time for oxidation, but in making tests with hand rabbling I prefer to use a three-hour oxidizing period when plotting variables other than length-of-oxidation period.

Since we start with FeO.Cr O= and oxidize at 700 C., I theorize the iron, after oxidation, must be in the form of Fe O This conclusion is fortified by the fact that the oxidized product is nonmagnetic, is only very slightly attacked by sulphuric acid at any strength, and shows the slight increase in weight necessary to support such assumption. Since there are now two atoms of iron in the Fe radical, the formula for the oxidized chromite must be written Fe O .2Cr O as the chromium to iron proportions have not been changed. I prefer to show the new arrangement thusly,

OrzOs The oxidation reaction formula is4(FeO.Cr O plus 0 equals 2Fe O .4Cr O I have found that the iron in the oxidized product can be readily preferentially reduced by exposure to a reducing atmosphere such as one high in carbon monoxide or hydrogenat a temperature of approximately 1200 C. For laboratory tests, I prefer to mix the minus 10 mesh oxidized product with about 15% of its own Weight of minus 10 mesh ordinary low-grade bituminous coal and reduce the charge for four hours at 1200-1260" C. in S-O-gram fireclay crucibles. Usingv 15% coal means theoretically a great excess, but the 30-gram crucibles are only about six inches tall, consequently much 00 1 8 expelled into the atmosphere. The efficiency of the reducing agent is dependent upon the opportunity it has for contact, therefore a long and narrow tube is much more eflicient. I have obtained an equal amount of reduction with much less coal when working with a fire-clay tube three inches in inside diameter by eighteen inches long. I have also attained very satisfactory reduction in iron pipes, one inch by fifteen inches long and three inches diameter by fifteen inches long. The iron pipe does not enter into any reaction and does not corrode on the inside but soon crystallizes and breaks up from the outside because of the direct heat. In a large'scale installation, reduction should be satisfactorily achieved in a rotary kiln and possibly in a roaster of the CFluo-Solids typefor sizes fine enough for iluidizaiton. If a kiln were used, the reducing agent could not be gained from coal fed in with the ore, but would have to be fed as a gas into the discharge end of the kiln so as to travel counter-currently 3, to the chromite flow. An advantage of using a rotary kiln which could be realized also with certain other types of apparatus, lies in the fact that the hot exhaust gases from the kiln can be used as the source of heat for the prior oxidation reaction, and the hot oxidized chromite can be charged directly into the reducing apparatus without substantial loss of heat. When coal or other hydrocarbons are used as the source of the reducing agent, and the reduction is performed in a tube-shaped vessel with at least one end having an aperture open so that there is no build-up in pressure, the iron in the chromite is preferentially reduced, in part, to an acid-soluble form. Such reduction takes place, to some extent, at all temperatures between 500 deg, C. and the fusing temperature of chromite (about 1500 deg. C.) but reductions at low temperature is slow and superficial. I prefer to use reducing temperatures of 1200-l260 deg. C. on minus 10 mesh material when using coal or other hydrocarbons, as the source of the reducing agent, in a relatively long, narrow vessel for a period of four hours total reduction time. i The formula cfor the reduction stage can be writt-en- '3Fe O .6Cr O plus 2C0 equals Fe O plus 3FeO.6Cr O plus 200 I prefer to show the reactions in a manner that better demonstrates the reasons why re-arrangement of the chromite molecule is possible. In FeO.Cr O any reduction of the ferrous radical can only result in the formation of metallic Fe. To change the iron to a readily soluble form requires oxidation to Fe O This cannot be done directly because direct oxidation results in the highest common state of oxidation, Fe O When the iron is oxidized to Fe O there must necessarily be two Cr O radicals to each one Fe O radical to make the number of atoms present of each be in balance. Instead of writing the formula as Fe O 2Or O I prefer to show i as V FezOaCmOa CrzOa Thus, while one chromium-bearing radical ordinarily protects its respective iron-bearing radical from attack, we now have a new molecular arrangement in which two chromium-bearing radicals are satisfied with sharing one iron bearing radical. Then, when the iron-hearing radical is reduced to a lower state of oxidation, only half of the iron is retained as FeO to satisfy the newly-acquired requirements of the molecule for a one-iron radical:twochromium radicals ratio. The entire set of equations may be written in balanced form as-- In oxidation 12(F60-CI'203) plus 30 equals 6F6203.12Cl'203 or to show the molecular arrangement I Then the reduction reaction can be shown as 6(FezOa.CrsOx) plus 400 equals 2Fe O4 plus 6(FeO.Cr2O3) plus 460,

Cl'zOa CrsOa in which each FeO radical is sufiicient to satisfy two Cr O radicals, the possibility of which is proved by the fact that the iron can be caused to go into a state of oxidation higher than the original FeO. I have found that it is possible,-with the use of sufiicient reducing agent, to cause more drastic re-arrangement on the same system by caus. ing one FeO radical to satisfy more than two Cr O radi cals, but when this is done, more drastic leaching procedures are necessary to extract all the available iron and recovery of chromite falls sharply.

'From chromite ore reduced in the above manner, about one-half of the original iron content is readily soluble without further grinding, in dilute sulphuric acid or in 4 a v sulphurous acid (a solution of S gas in water) while Cr O is attacked hardly at all, usually to the extent of less than one percent of the contained Cr O If ferric sulphate added to dilute sulphuric acid, no more iron is soluble than when dilute sulphuric acid alone is used, and the speed of iron-leaching is not increased. I take this as evidence that little of the iron has been reducedto metallic Fe, and the order of weight changes that take place during reduction further substantiate this conclusion. Since the reduced product is highly magnetic to; a

hand magnet, theresultant acid-soluble iron must be in the form of magnetite, Fe O v In most chromite ores, at the end of the reduction step, it is, found that ten or fifteen percent of the product is not as magnetic to a hand magnet as the balance of the product. less magnetic, proves by'assay to be lower in both iron and chromium than the bulk of the product, and is slight. ly less magnetic because of the difference in composition and not because of lack of reduction. Also, the chro miurnziron ratio is usually higher in this fraction than'it is in the more highly magnetic portion, which helps to compensate for the fact that the iron in the less-magnetic fraction is more resistant to the subsequent leaching.

A sample of chromite ore of unknown origin was crushed to minus mesh size, oxidized in air with handrabbling for three hours at 750 deg. (1., mixed with of its own weight of minus 10 mesh coal and reduced in an iron pipe three inches inside diameter by fifteen inches long which was plugged at one end and fitted with a 3 in.

, by 1 in. hell reducer on the other end. Reduction was at I 1200 deg. C. in a .butanefired furnace for three hours.

100 grams of the reduced product was separated with a hand-magnet into grams of highly magnetic grains and 15 grams of less magnetic grains. Assays made showed the following content of the respective fractions.

Product Grams Percent Percent Orzlie Or Fe Ratio Highly magnetic 85 24.98 I 12. 04 2. 07: Less magnetic 15 22. 74 8.85 2. 57:

the weight loss shown by the less magnetic product in which the ratio was raised only to 2.78 1.

Examination of the chromiteparticles at the end of the reduction step, under the microscope, reveals numerous straight brown lines which seemingly follow the exposed edges of parting planes, and a lesser number of small roughly circular brownish patches. After leaching of iron has beenaccomplished, examination of the particles under the microscope reveals that the brownish lines and patches have disappeared, leaving open crevices and craters; This migration of iron makes the subsequent leaching very rapid. I have found that chromite reduced in this manner and to this condition permits approximately one-half of its iron content to be extracted, at air temperatureand at atmospheric pressure and without any further grinding to smaller size, by leaching with sulphuric acid of 33 /s% strength (by volume) for one hour at atmospheric pressure and temperature, by leaching with sulphuric acid of 10% strength (by volume for reasonably longer periods, or :by leaching with sulphurous acid (a solution of 6% by weight S0 gas in Water) for periods of from 20 minutes to 4 hours, depending upon the percentage of iron to'be removed. Sulphurous acid-of 3% strength requires a slightly longer leaching time to be employed than when 7 iron in a shorter time than any strength of sulphuric acid solution when both are applied at air temperature.

Grinding of reduced chromite to a finer size, as to minus This ten or fifteen percent fraction, which is 100 mesh for example, and use of heated dilute sulphuric will hasten the leaching time to some degree, but I prefer to leach with sulphurous acid of 6% strength or nearly 6% strength at air temperature, without grinding finer than the size at which the chromite was reduced, for reasons of economy. With proper agitation during leaching, to insure constant contact of every particle with unsaturated leaching solution, the 20 minute leaching time is ample to put enough iron into solution to almost double the original chromiumziron ratio, and recovery of chrom ium in the unattacked residue is almost 100% usually well over 99%, when leaching is done with sulphurous acid at air temperature.

When dilute sulphuric acid is used with moderate heating such as at 100 deg. C., but little more chromiumis dissolved. At high concentrations of sulphuric acid applied at higher temperatures especially if done under pressure, greater amounts of chromium are lost as greater temperatures are applied and as pressure is increased, but in my improved process, such applications of heat and pressure, with resultant losses of chromium, are totally unnecessary. I prefer to use sulphurous acid, of 6% strength, or :as nearly 6% as practical to supply, as the iron-leaching solvent, applying the solvent counter-currently to the flow of ore, .so that the fresh solvent is applied on the oldest ore and the oldest solvent which has very little remaining S0 is on the newest ore. By this means, almost 100%. of the S0 has been made available for the solution of iron and there is no necessity for the regeneration of the solvent. The used-up solvent can be discarded without noxious vapors or free acidity and with no wastage charge against it except for the water cost. The S0 solution can be made by roasting pyrite (or other sulphides) and introducing the resultant S0 gas into water by conventional means. If the used solvent were to be regenerated with S0 gas, ferric sulphate and sulphuric acid wouldbe formedby the well-known auto-oxidation process, and since these have been shown to be inferior to sulphurous acid for my purpose, interests of economy dictate the counter-current flow as described with discard of used lixiviant. Theoretically, with most chromite ores, roasting of 3 tons of pyrites will furnish enough S0 to put 100 tons of chromite into the metallurgical grade class, as far as chromiumziron ratio is concerned. 7

The primary control over the degree of potential iron extraction desired lies in the quantity of reducing agent permitted to contact the chromite particles at reducing temperatures. Full'utilization of available carbonmonoxide'requires higher temperatures than does full utilization of hydrogen. Very satisfactory final chromiumziron ratios can be attained on any chromite ores with'a 99% plus chromite recovery if excessive reduction is avoided. Optimum reduction will vary with individual ores, but original chromiumziron ratios can be doubled in all instances without chromite losses exceeding 1%.

A l-kilo batch of minus 10 mesh chromite of unknown derivation which hadbeen concentrated by gravity to a product assaying 24.30% Cr and 12.05% Fewith a CrzFe ratio of 2.02:1 was roasted inan electric muffle furnace for 3 hours at approximately 700 deg. C. with'free access of air but no appreciable draft. The charge was handrabbled during the roast. At the end of theoxidizing period, the charge was removed, cooled, and mixed with 20% of its own weight of minus 10 mesh low-grade bitu minous coal. The mixture was poured into an iron pipe of one inch inside diameter and fourteen inches long. One end of the pipe was plugged and the other end was left open. This charge was. heated from a cold startto. approximately 1200 deg. C. in 30 minutes and held at ap-. proximately 1200 deg. C. for 2 /2 hours. At the end of the 3-hour reduction period the charge was quenched in water. Some coal not entirely consumed was washed out from the chromite. The chromite was all very magnetic to a hand magnet. Half of the reduced chromite product was ground to minus 100 mesh in a hand mortar and the 6 other half was left at minus mesh. The following leach tests were made on the reduced products:

calculated from chromium not found in filtrate.

RESULTS Percent Percent OrzFe Percent Leach Test Weight Gr Fe Ratio r Recovery RESULTS Percent Percent Gr: Fe Percent Leach Test Weight C'r Fe Ratio Cr Recovery With son. 9. 50 25. 72 7.75 3. 3211 99.26 With 1:2 H2SO4 9. 55 25.65 7. 53 3. :1 99. 25

Apparently these leaches were helped =less by the pressure app-lied than they were hurt by the fact that no agitation could be applied. I

1 kilo of minus 10 mesh chromite of unknown origin which had been concentrated by gravity to a product assaying 24.30% Cr, 12.05% Fe and 2.04% SiO' with a CrzFe ratio of 2.02:1 was oxidizedby roasting for 3 hours at a dull red heat on an iron plate on top of a butane-fired furnace with free access of air and constant hand-rabbling. At the end of the 3 hour oxidizing period, the charge was cooled and mixed with 20'% of its own weight of minus 1 0 mesh low-grade bituminous coal. The chromite-coal mixture was poured into a tube of refractory fire-clay 3 inches in inside diameter by 14 inches long. One end of the tube was plugged, and the other end was partially plugged with refractory cement, leaving a 1 inch opening for gas escape andsubsequent emptying. The charge in the tube was heated quickly-4n less than 15 minutes-4o 1260 deg. C. and held at that temchromium to establish the percentage of chromium recovery.

coal. Th1s mixture waspoured mto the 3-1nch tube and g Grams Mesh Solvent Tlme 3 39 reduced in the butane furnace for four hours at 1200 deg. C. At the end of the four hour reduction penod, the charge was quenched 1n water, washed free of un- 10 -10 1 2111804 100 50 consumed coal and timed. The reduced chromite was 50 0 6% $02---- 20 split into two halves, one-half pulverized to minus 100 50 1:2 H18 29 50 esh in a disc pulvcrizer, and the other half was left at g 188 28 plus 10 mesh-nnnus A1 lI'lCh. for leaching tests. All tests 50 20 were performed in rolhng bottles. 50 190 1:2 20 50 100 1:2 1O 50 100 6% 0 Leach Grams Mesh Solvent Time Temper- 50 100 1:2 20 Test ature, O. 10 -190 3% 20 10 1;5H:s0 20 grams of Fe(SO4) 25 plus 10 6% SOZ 20 111111.. 20 25 10 6% S02 replaced 20 mm. plus 20 25 plus 10 4 hours 20 with fresh S02 20 min. 25 plus 10 20 111111.. 20 after 20 min. 25 plus 10 4 hours.. 20 25 -100 20 111111.. e 20- 25 4hours 20 RESULTS 25 20 111111.- 20 25- 4 hours '20 25 4h0urs 20 Residue Percent Percent CrzFe Percent j 25 4 hours 20 Leach Test Weight Cr Fe Ratio r 20 Reeov.

9. 39 26.13 7.47 3.50:1 99.37 RESULTS 47. 80 25. 9o 7. 90 3. 28: 1 99. 68 47. 68 25. 93 7.72 3.26:1 99. 83 I 48.10 25. 79 8. 26 3.12:1 99-72 5 Leach Test Weight of Percent Percent Cr:Fe 47. 96 25. 79 s. 14 3. 17:1 99. 71 Residue 1- Fe Ratio 9. 09 26. 41 6. 62 3. 99:1 98. 79 47.31 26.10 7. 35 3. 55:1 99. 82 47. 12 26. 20 7. 25 3. 61: 1 99. 63 24, 71 23. 4g 9. 31 2, 52; 1 47. 36 26. 10 7. 47 3. 49:1 99. 69 41 23, 1 s, 84 2. 70:1 48. 66 26. 34 6. 93 3. 80:1 99.13 24.77 23. 35 9. '78 ,2. 39:1 48. 46 25. 65 8. 63 2. 97:1 99. 86 24. 50 23. 64 9. 05 2. 61:1 48. 43 25. 51 s. 87 2. 88:1 99. 77 24. 12 24. 25 8. 4o 2. 89:1 9.68 25.75 8.51 3. 11 99.62 24,14 24 2E 8,46 2,8711 23. 95 26.14 9.01 3. :1 99. 68 24.02 24.32 8.48 2.87:1 23. se 26. 07 8. 24 3. 16:1 99. 65 23. 97 24. 72 8. 2. 98:1 9 9 is 91 0 These tests show a Cr:Fe ratlo of better than 3 :1 in all instances except k and l in which leaching time was cut to 5 minutes. They show, that while hot 1:2 H 80 is the most effective solvent, cold leaching is as effective with S0 as 1:2 H 80 that ferric sulphate is of no benefit, and that, while minus 10 mesh material does not leach as fast as 100 mesh, leaching of 10 mesh is satisfactorily in leaching periods of as short as 20 minutes duration.

1 kilo of minus 10 mesh chromite of unknown origin which had been concentrated by gravity to a product assaying 22.87% Cr and 10.53% Fe with a chromiumziron ratio of 2.17: 1, without prior oxidation was mixed with 20% of its own weight of minus 10 mesh low-grade bituminous coal and poured into a refractory fire-clay tube of 3 inches inside diameter by 14 inches long. One end was plugged and one end was partially plugged leaving a 1-inch circular opening. This charge was heated to 1200 deg. C. and reduced for four hoursin a butanefired furnace. The charge was then quenched in water, cleaned of unconsumed coal, and dried. grams of the reduced chromite was leached for 20 minutes with 1:2 H 80 at 100 deg. C. The leached charge was filtered, dried, weighed and assayed. The leached residue weighed 48.81 grams, assayed 24.94% Cr and 9.18% Fe which showed a CrzFe ratio of 2.72:1. The conclusion is that prior oxidation is necessary and probably the only reason that some change in ratio was eifected here, was because of air-oxidation present in the old sample before treatment, and/ or because of some oxidation taking place in the charge in the early stages of the intended reduction period.

A sample of chromite of unknown origin consisting of particles as large as A inch down to 10 mesh size was screened to eliminate all plus A1 inch and all minus 10 mesh. The resultant minus inch, plush 10 mesh material which assayed 22.87% Cr and \l0.53% Fe with a CrzFe ratio of 2.17:1 was given an oxidizing roast at 700-750 deg. C. for 3 hours, hand-rabbled, on aniron plate on top of the butane-fired furnace. At the end of the 3-hour oxidation period, the change'was cooled and mixed with 2 0% of its own weight in minus 10 mesh Chromium was found in leach solutions in this series in trace amounts only. Although the altered ratios at tained here are short of the desired 3:1, the results are indicative of the surprising amount of oxidation and reduction that can be made to occur at relatively coarse sizes.

A S-kilo sample of lump chromite marked Dr. Itens High-Grade Specimens was crushed to minus 10 mesh size. The sample assayed'40.56% Cr and 10.28%-Fe showing a CrzFe ratio of 3.95:1. 1 kilo of the minus-10 mesh material was given an oxidizing roast for 3 hours at 750 deg. C. on an iron plate on top of a butane-fired furnace, with constant hand-rabbling. At the end of the 3-hour oxidation period, the charge was cooled and. mixed with 16% of its own weight in minus 10 mesh low-grade coal. This mixture was poured into two SO-gram fire-clay crucibles which were covered with loose-fitting porcelain covers. The charges were reduced-by heating to 1260 deg." C. for four'hours in the butane-fired furnace. At the end i hand mortar while the other half was left at minus IO mesh for leaching tests.

Leach Test Grams Mesh Solvent Time $9 161) 10 10 30 min; 25 -10 '20 minl... 20 25 -l0 20 mina 20 25 10 4 hours.... 20 10 100 25 100 20 25 100 26 25. 100 20 9 10 RESULTS RESULTS OF LEACHING ON 4-HOUR REDUCTION Residue Percent Percent Cr:Fe Percent Leeched Test Weight Cr Fe Ratio Cr Leach Test Residue Percent Percent CrzFe Percent Recov. 5 Weight .Or Fe Ratio Cr Recov.

9.30 49.02 7.92 5.43:1 98.83 24.10 42.34 3.29 5.11:1 99.13 10 I 2:53 i; g? g: 32 2: 2% 83:2? Apparently two hours reduct1on time was not suflic1ent under existlng cond1t1ons. 1:10 (byvolume) H 80 is as effective as 1:2 H 80 when leaching'times of 16 hours are employed. Also iron extraction is almost as satis- Thls test was runmore a quahtanve spmt than to factory at 10 mesh as it is at 100 mesh when a 16-hour determine exact quantitative results, and these results leaching time is used 3 would be much better if the vessels used in the reducing A large sample lump chromite ore marked step had been relatively longer and narrower than the Chromite was crushed to minus 10 mesh and concem short squat crucibles employed. Taking into considera- Hated by gravity to a Product assaying 2854% Cr tion the erratics allowable in assaying, it is evident that 36% Fe and 226% sioz Showing a CrzFe ratio Sulphurous acid (a solution of 6% S02 gas in water) is 20 2.41:1. One =kilo of this concentrate was given an oxidizequal to or superior to much stronger concentrations of mg roast on an iron plate on top of the butanefired mp H2504 leachmg 1S done at a1r temPeratIe l nace for 3 hours at a dull red heat, with constant handleaching time 1s confined to the short times allowable 1n rabblingl At the end f the 34mm. oxidizing period, the a commuous Processcharge was cooled and mixed with 16% of its own weight A la'rge P of chrmllte mark?d Lumnec of low-grade coal. The mixture was poured into two Chmmlte was gushed to mums 10 mes]? S126 and c911 -gram fire-clay crucibles and covered with loose-fitting centrated by gravlty a concentrate assaymg 28-34% porcelain covers. These were heated in the butane furnace 12387; Fe and 240% $02, Showing a CriFe ratio of to 1260 deg. C. At the end of two hours, one crucible 2.29:1. This minus 10 mesh concentrate was given an 30 was removed and its charge quenched in Water The OXidiZing roast for 3 hours at a dull Ted heat on an iron temperature was increased to 1370 deg. C. for the next Plate on p of the butane-fired furnace, with constant two hours. At the end of the second two-hour period, the hand-rabbling. At the end of the 3-hour oxidation period, second crucible was removed and its charge quenched in the charge was cooled and mixed with 16% of its own water. The two charges were washed free of remaining weight of minus 10 mesh low-grade coal. This mixture unconsumed coal and then dried. Each of the two prodwas poured into two 36-gram fire-clay crucibles and covucts was split into two halves, one half of which was ered with loose-fitting porcelain covers. The two crucibles ground to minus 100 h in a hand mortar while the were l d i h b fi d furna d heated to other half was left at minus 10 mesh for leaching tests. 1260 deg. C. One crucible was removed after two hours LEACHING TESTS ON 2-HOUR REDUCTION TIME reduction time and the charge quenched in water. The 40 other crucible was removed after four hours reduction Leach Test Grams Mesh Solvent Leach time and its charge quenched in water. Each charge was Tlmehts Washed free of remaining unconsumed coal, dried and split 25 16 into two halves. One half of each charge was ground to 25 -100 16 minus 100 mesh in a hand mortar and the other half left I 58 i2 at minus 10 mesh for leaching tests.

LEACHING TESTS ON 2-HOUR REDUCTION TIME RESULTS OF LEACHING ON 2.HOUR REDUCTION 5O Leach Test Grams Mesh Solvent Leach Leach Test Residue Percent Percent CrzFe Percent Time hrs. Weight 01' Fe Ratio Cr.Recov.

Leach Test Residue Percent Percent CrzFe Percent Weight Cr e Ratio Cr Recov.

LEACHING TESTS ON 4-HOUR REDUCTION TIME LEACHING TESTS 0N 4-HOUR REDUCTION TIME RESULTS OF LEACHING ON 4-HOUR REDUCTION Leach Test Grams Mesh Solvent Leach Leach Test Residue Percent Percent CrzFe Percent Time hrs. 7 0 Weight Cr Fe Ratio Cr.Recov.

Reduction time of two hours, under existing conditions was not enough. Four hour reduction time was sufficient, but evidently higher temperature employed (1370 deg. C.) during the latter two hours led to greater chromium losses and somewhat lesser iron solubility. I have observed partial fusion of chromite at temperatures of only 30 or 40 degrees above the peak reached here. A temperature of 1260 deg. C. should be the upper working limit.

What is claimed is:

1. A process of upgrading chromite ore having an initial Cr:Fe ratio of less than 3:1 to a metallurgical grade chromite ore having a final CrzFe ratio of more than 3:1 wherein FeO and Cr O are molecularly bonded in both the initial ore and in the metallurgical grade ore comprising the steps of comminuting the ore, heating the cornminuted ore at a preferred temperature range of about 700 to 750 centigrade under oxidizing conditions while controlling the deg-rec of comminution, the heating temperature and the oxidizing conditions so as to oxidize at least a portion of the FeO.Cr O therein to oxide.

References Cited in the tile or this patent, v

UNITED STATES PATENTS v I r 153,573 7 Kidwell July 28,1874 1,196,049 Raus'chenplat Aug. 29, 1916 1,403,237 -Eustis Jan. 10,1922

2,123,240 Ha'mmarberg July 12, 1938 while leaving the Cr O substantially unchanged, and exti acting said uncombined Fe Oi with a chemical reagent reactive therewith, whereby the ratioof Or to Fe is increased.

2. .A process as set tort-h in claim 1, wherein said reduction is conducted at a preferred temperature rangejoi about 1200 to 1260 degrees oentigrade. a i

3. A process as, set forth in claim 2, wherein said reduction is conducted in an atmosphere of carbon more

Claims (1)

1. A PROCESS OF UPGRADING CHRMITE ORE HAVING AN INITIAL CR:FE RATIO OF LESS THAN 3:1 TO A METALLURICAL GRADE CHRMITE ORE HAVING A FINAL CR:FE RATIO OF MORE THAN 3:1 WHEREIN FEO AND CR2O3 ARE MOLECULARLY GRADE ORE IN BOTH THE INTITIAL ORE AND IN THE METALLURGICAL RADE ORE COMPRISING THE STEPS OF COMMINUTING THE ORE, HEATING THE COMMINUTED ORE AT A PREFERRED TEMPERATURE RANGE OF ABOUT 700* TO 750* CENTIGRADE UNDER OXIDIZING CONDITIONS WHILE CONRLLING THE DEGREE OF COMMUNITION, THE HEATING THEMPERATURE AND THE OXIDIZING CONDITIONS SO AS TO OXIDIZE AT LEAST A PORTION OF THE FEO.CR2O3 THEREIN TO FE2O3ECR2O3, REDUCING AT LEAST A PORTION OF SAID FE2O32CRO3 AT A TEMPERATURE RANGE OF ABOUT 1200* TO 1370* CENTIGRADE TO FEO.2CREO3 AND UNCOMBINED FE3O4 WHILE LEAVING THE CR2O3 SUBSTANTIALLY UNCHANGED, AND EXTRACTING SAID UNOMBINED FE2O4 WITH A CHEMICAL REAGENT REACTIVE THEREWITH, WHEREBY THE RATIO OF CR TO FE IS INCREASED.