Bacterial Oxidation In Upgrading Sulfidic Ores And Coals

Abstract

Finely ground impure metal sulfides or pyritic ores, particularly coals, are upgraded by surface oxidation of the metal sulfide or pyritic material by selected bacteria to render the surface hydrophilic, followed by a particle separation step from an aqueous slurry e.g. selective agglomeration or flotation. A suitable bacteria is selected from the Thiobacillus-Ferrobacillus group.

Description

This invention is directed to beneficiating pyritic coals and other metal sulfide ores in which pyrite or other metal sulfide particles are finely disseminated. A limited bacterial oxidation of only the metal sulfide or pyrite particle surface or particle-particle interface is carried out, rendering the surface hydrophilic and enabling a good separation of the released and altered particles to be achieved.

Pyrites occur as mineral impurities in a number of ores often as finely disseminated inclusions which require fine grinding to obtain release. For example certain copper sulfide ores contain pyrite impurities and some low grade coals have unacceptably high ash and sulfur contents primarily due to pyrites. After fine grinding to release the pyrites, beneficiation of such systems has been attempted in a number of ways. Froth flotation has been investigated and in some cases has been satisfactory. However the surface properties of coal and pyrite, in particular, do not differ enough and the pyrite tends to float with the coal. Flotation depressants for the pyrite have not proved efficient. Chemical removal (reaction, extraction, etc.) has been tried but reagent costs, high reaction temperatures and other factors render most such operation uneconomical. Screening, gravity separation, and magnetic separations have also been attempted but ultimate grades and recoveries have usually been less than desired, particularly where very fine grinding has been necessary for release.

Bacterial oxidation or leaching has been developed where bacterial action directly or indirectly leads to solubilization of the value or impurity. Iron and/or sulfur oxidizing bacteria have been used before in pyrite-containing ores to convert ferrous to ferric iron and to oxidize sulfur including pyrite sulfur to sulphate. The iron and pyritic sulfur is thus rendered soluble in or leachable by acid media. In the past removal of oxidized iron or sulphate has always been in solution i.e. the bacterial action is continued until complete dissolution is attained. Acid leaching is usually used to remove the oxidation products. When using bacteria for removal of pyrite in this manner some of the difficulties encountered are (a) the handling of both the very finely divided materials and the acid mixtures and (b) the disposal of the oxidation products (sulfuric acid and ferric sulphate).

Decreasing the sulfur content of coal is becoming of great importance in view of the atmospheric pollution by sulfur dioxide (as well as ash and corrosion problems) on combustion. In coal the sulfur exists as sulphates, inorganic sulphides (primarily iron sulfides such as pyrite or marcasite) and bound organic sulfur (mercapto groups etc.). The sulphates are readily leached and the bound organic sulfur is by present techniques impossible to remove economically so that the main beneficiation achievable is with the inorganic sulphides.

Bacterial pyrite leaching in coals has been investigated recently. In one case about 25 percent of the pyrite in coal was removed by Thiobacillus ferrooxidans in 30 days incubation. In another case the bacterium Ferrobacillus ferrooxidans was able to remove appreciable quantities of pyrite from finely ground coal in 3 to 4 days. Removal was by dissolution in both cases.

Bacterial leaching of other sulfide-containing materials has been carried out but in many cases the rates of extraction were regarded as uneconomically slow e.g. 5 to 55 or more days required. Uranium, nickel and copper have been leached in this way.

In order to minimize liquid waste disposal problems, it would be desirable to remove most of the pyrites or metal sulfides as solids. It would also be desirable to obtain substantially complete metal sulfide or pyrite removal in shorter processing times. A decrease in grinding times or an accelerated release of components would be very advantageous.

We have now found that by allowing the oxidizing bacteria to act for a limited time to effect only a surface alteration of the metal compound or pyrite to hydrophilic form or to accelerate mineral interface separation, an efficient particle separation step can be carried out. Also by allowing the bacteria to act prior to or during wet grinding a more efficient pyrite or metal sulfide liberation has been achieved. That the bacteria is acting only to condition and/or separate the surfaces is shown by the fact that after the limited bacterial action the acid-insoluble sulfur content remains high but is reduced close to the organic-bound level after the particle separation.

The process of the invention comprises the following steps (on impure metal sulfide materials):

a. wet grinding to aid release of fine particles of the metal sulfide or pyritic material from the non-pyritic or non-sulfide solids;

b. subjecting an aqueous mixture containing the finely divided material to the action of inorganic sulfide-oxidizing bacteria selected from the Thiobacillus-Ferrobacillus group until only the surface of the sulfide or pyritic solids is rendered hydrophilic, and the pH is below about 5 when at about 30-50 wt. % solids in water;

c. adjusting the solids content to within about 2 to about 40 wt. % and providing that the pH of the resulting slurry is about about 5;

d. conducting a particle separation operation on the resulting slurry to separate non-sulfide including non-pyritic solids from the bacterial surface-oxidized sulfide particles, and

e. recovering the beneficiated values substantially free of the released impurities.

Steps (a) and (b) can be carried out in inverse order or concurrently.

For pyritic ores and coals a preferred procedure is

a. grinding to release fine particles of the pyritic material from the non-pyritic values;

b. subjecting an aqueous slurry of finely divided ore or coal to the action of an inorganic sulfide-oxidizing bacteria selected from the group Ferrobacillus ferrooxidans and Thiobacillus ferrooxidans until only the surface of the pyritic materials is rendered hydrophilic, and the pH is below about 4;

c. adjusting the solids content to within about 2 to about 40 wt. % and providing that the pH of the resulting slurry is above about 5;

d. conducting a particle separation operation on the resulting slurry to separate hydrophobic non-pyritic values, leaving the aqueous slurry containing the pyrites as tailings; and

e. recovering the desired values substantially free of pyritic materials.

Various sulfide or pyritic ores may be processed according to the invention. Of particular interest are the low grade coals where finely disseminated inorganic sulfides are present. Other ores which can be treated include pyritic copper sulfide ores (chalcopyrites, bornite etc.), nickel sulfide ores, zinc sulfide ores, arsenic sulfide ores and lead sulfide ores.

In some cases the desired metal values occur as sulfides and the bacteria can be used to attack the mineral interfaces and free the sulfide particles. The bacteria may preferentially attack certain metal sulfide interfaces or surfaces where two or more metal sulfides are present and this preferential action can be controlled to effect a separation. Thus the metal sulfide surfaces on which the bacteria have been working will be rendered relatively hydrophilic, so that different and less hydrophilic particles can be selectively removed. If there is not sufficient natural difference in surface properties, a selective conditioner may be necessary to alter the surface of one component. In the case of copper and nickel sulfides, promotors or conditioners are known which will be preferentially absorbed and bestow more hydrophobic properties to the surfaces. For instance selected xanthates or mercaptans can be used to condition copper sulfide, or nickel sulfide surfaces and permit a flotation or agglomeration separation to be achieved.

The bacteria are chosen from the group Thiobacillus-Ferrobacillus including Ferrobacillus ferrooxidans and Thiobacillus ferrooxidans and mixtures thereof. It is preferred that the bacteria be adapted to the particular ore or sulfide. Thus bacteria from mine tailings and naturally-weathered sources of the same ore have been found very effective and fast-acting. Active bacteria can be recovered and recycled after the particle separation step if desired.

A more detailed description of the bacteria-susceptible sulfide ores, and of the bacteria of the Thiobacillus-Ferrobacillus group, is given in M.P. Silverman and H.L. Ehrlich "Microbial Formation and Degradation of Minerals" Advances in Applied Microbiology Vol. 6 p. 170-197 (1964).

Conditions favoring rapid bacterial growth in the system will normally be used. Thus the temperature should be maintained within about 20.degree.-37.degree. C. Pulp solids content may range down to about 30 wt. %, suitably about 30-50%, although any wet system will do. Sufficient time for adequate bacterial action and surface or interface alteration should be allowed without significant leaching. Usually the time will be within about 12 to 72 hours. Careful selection of bacteria and optimization of growth conditions can reduce the time required.

It is preferred that the final stage of grinding to release size be carried out in the presence of water and the bacteria. It appears that the bacterial action aids cleavage or release at the mineral interfaces resulting in more effective release of pyritic materials for shorter grinding times. The ultimate particle size on grinding required for release will vary depending on the particular ore. The particle size is not critical for either the bacterial action or for the particle separation but usually will be -100 mesh or finer.

The operative particle separation steps are those that exploit the newly hydrophilic surfaces of the pyritic sulfide minerals after the limited bacterial action. Thus the more hydrophobic-surfaced particles can be agglomerated, floated, retained on selected hydrophobic surfaces, by adhesion selectively transferred to a water-immiscible phase etc. Combinations of these steps can be carried out for increased separation if necessary.

The preferred particle separation step is agglomeration by a water-immiscible organic (usually hydrocarbon) bridging liquid under good mixing or kneading conditions. For agglomeration the pulp solids content may range from about 2 to about 40 wt. % but is not sharply critical. The amount of bridging liquid will have some affect on the size of the agglomerates with a preferred concentration range being about 2-40 % by wt. of the agglomerated solids, preferably about 5 to 20%. It is usually desired to minimize the amount of bridging liquid so long as an acceptable separation is realized. Sufficient agitation and mixing can be achieved in the grinding mill, or other forms of agitation and mixing such as high speed blenders, bladed agitators, flotation cells without air addition, sump pumps etc. can be used. Modified turbine, disc or cone impellers may also be used. An apparatus in which a zone of high shear is produced in the annular space between a solid conically shaped rotor rapidly rotating inside another cone has been found to bring about the desired agglomeration quickly and also to serve as a blockage-resisting pump. After agglomeration, the values can be separated by gravity, cyclones, screening, water-washing etc. If desired the bridging liquid can be recovered e.g. by distillation or extraction and recycled.

With coal ores, the preferred bridging liquids are hydrocarbons such as kerosene, solvent mixtures such as Varsol (trademark), coal oil, and various light refined or semi-refined hydrocarbon liquids.

In a further stage the agglomerates may be fed to a balling device, such as a rotating balling disc, together with balling nuclei which comprise coarse (coal) particles usually having an average particle size of from 0.5 to 10 mm. (diameter). A binding oil capable of forming a balled product is also fed to the system and a balled product formed in which substantially each ball includes at least one balling nucleus in association with a number of the primary agglomerates. In effect the agglomerates are layered onto the balling nuclei. A wide variation of properties in this final balled product by appropriate control steps is possible (for further details see Charles E. Capes et al U.S. Pat. No. 3,665,066 May 23, 1972).

The bacterial action requires or will produce an acidic pH below about 4 but separation steps such as agglomeration and flotation require a higher pH for good results (at least above 5). Thus the pH should be raised with alkali (or the solids diluted or redispersed in a neutral or alkaline medium). With pyritic coal ores the usual pH for agglomeration is about 5-10 preferably about 5 to 8. Lime has been found very advantageous for raising the pH with these coal ores, and seems to aid in rejection of the pyrites from the agglomerates.

Froth flotation separations can work very satisfactorily in some cases. Frothing agents including hydrophobic collectors will normally be used, as is known in the art.

The following Examples are illustrative.

The coal used in this work was from the Minto N.B. coalfield. A two hundred pound sample of this run-of-mine material was first riffled into approximately 10-lb. samples which were subsequently reduced by manual crushing to minus 1/8 inch size. Small samples for use in the experiments were extracted from the 10-lb. sample by riffling. The approximate analysis of the coal was as follows:

% by wt. (dry basis) Ash 16-20 % Fe.sub.2 O.sub.3 8.3-9.0% Total sulphur 6.6-8.0% Sulphate Sulphur 0.1-0.3% Pyritic sulphur 6.0-6.4% Organic sulphur 1.4-1.7%

A second coal sample was used, referred to here as "Avon," and was a fine wash plant slurry with virtually the same analysis as that above, except that the material has been weathered, and approximately 50% of the pyritic sulphur had been oxidized to sulphate sulphur, at least partly by the action of Thiobacillus-Ferrobacillus bacteria.

The liberation size of the pyrites in these coals is extremely fine (believed to be about 1-2 microns) and further grinding was necessary during beneficiation. This was accomplished using stainless steel balls in water slurry in a procelain mill*(*(unless otherwise stated).). Although the grinding time varied from experiment to experiment, typically the coal was ground to 100% minus 50 microns and 70% minus 22 microns. Agglomeration of the fine coal was carried out in a high speed blender, usually with a loading of 500 cm.sup.3, using a 10 wt. % slurry*(*(unless other stated).). Varsol was used as a bridging liquid at the 20 to 40 wt. % level*(*(unless otherwise stated).). Mixing in the blender generally lasted 10 minutes, following which the load was dumped and washed on a 100 mesh screen to recover the beneficiated coal in the form of 0.5 to 2 mm. agglomerates *(*(unless otherwise stated).) Combustibles recovery was normally greater than 90%.

In evaluating these experiments it should be remembered that the organic sulphur in the NB coal is not accessible, so that 1.4-1.7% sulphur represents the ultimate beneficiation attainable. In addition, if generally the ash content of the coal is reduced to about 5% during agglomeration, then the organic sulphur content of 1.7% in the original coal becomes about 2.0% simply due to the concentrating effect of this ash reduction, so that 2.0% sulphur represents 100% pyrites removal.

EXAMPLE 1

One hundred and fifty grams of the coal ore was ground for 22 hours in water, with various chemicals added prior to the grinding (0.1 g chemical was added, corresponding to about 1.3 lb/ton of dry coal). The chemicals added were those reported to be depressants for pyrites during flotation (and also agglomeration) of the coals. The chemicals used included glycerine, chlorinated lime, ferric sulphate, pyrogallic acid, sodium cyanide, sodium carbonate, dextrine, manganese dioxide, potassium ferricyanide, potassium ferrocyanide and ferric ammonium sulphate. The slurry from the grinding mill was split into 3 equal parts, and the coal was agglomerated at the natural pH of the slurry (pH 7.5 to 8.5), and in the basic (pH 10 to 11) and acidic (pH 2 to 3) ranges for each depressant, using HNO.sub.3 and NaOH as pH adjusters.

The best results from these experiments may be summarized as:

pH during depressant % ash % S agglomeration used 8.4 sodium cyanide 5.8 4.9 9.2 sodium carbonate 5.5 4 to 5 8.5 to 12 potassium ferrocyanide 5.8 to 7.3 4 to 5 8.2 to 10.2 potassium ferricyanide 4.8 to 5.7 4 to 4.5

This example thus illustrates that by using common pyrite depressants, the best results obtainable are in the range of 4 to 5% sulphur. (See also blank run in example No. 7 below). It should also be emphasized that the sample of coal used in this example contained only 6.6% sulphur and 15.3% ash, compared with higher starting ash and sulphur levels in the examples below. This is due to the difficulty in obtaining a truly representative small sample from a larger quantity.

EXAMPLE 2

A Ferrobacillus ferrooxidans sample was obtained in an acidic concentrate from Dr. K.C. Ivarson, Canada Dept. of Agriculture. This bacteria had been isolated from a uranium ore.

One hundred and fifty grams of the run-of-mine coal was mixed with 300 cm.sup.3 of Silverman 9K nutrient solution, whose composition, per litre, was as follows:

(NH.sub.4).sub.2 SO.sub.4 3.0 g. KCl 0.1 g. K.sub.2 HPO.sub.4 0.5 g. MgSO.sub.4.sup.. 7H.sub.2 O 0.5 g. Ca(NO.sub. 3).sub.2 0.01 g. FeSO.sub.4.sup.. 7H.sub.2 O 44.2 g. 10N H.sub.2 SO.sub.4 0.1 ml pH.apprxeq.3.0

this mixture was ball-milled for 72 hours, after which the pH was 6.5. After dilution to 1,500 cm.sup.3 total volume with distilled water, and pH adjustment with H.sub.2 SO.sub.4 to pH 3.0, three c.c. of the bacteria concentrate were added. The mixture was stored in a large bottle at 30.degree. C while being sparged continuously with air.

After 16 days, samples of the mixture were agglomerated at both the pH of the bottle (pH 2.6) and at pH 7.1 obtained by adding NaOH solution. The following results were obtained:

In Agglomerates pH during agglomeration % ash % S % S(A.W.) 2.6 7.0 6.3 9.3 7.1 3.5 2.4 9.3

the % S(A.W.) values given above indicate the weight per cent sulphur remaining in the samples before agglomeration after being digested with hydrochloric acid. This procedure for determining sulphate sulphur in coals ("Standard Methods of Chemical Analysis," F.J. Welcher (Editor), Vol. 2, part A, p. 1,198, 6th ed., 1963, D. Van Nostrand Co., Inc.) was used to remove acid-soluble iron; hence, these anaylses indicate that a negligible amount of the original pyrite has been oxidized in the processing. In addition, an analysis of the tailings from some experiments showed that the major proportion of the iron rejected was in the form of pyritic iron, i.e. non-sulphate iron. Thus, this example shows that agglomeration following the bacterial treatment, is capable of rejecting quantities of pyrites considerably greater than that actually oxidized to sulphate by the bacteria.

EXAMPLE 3

The presence of bacteria of the Thiobacillus-Ferrobacillus group in the Avon coal samples mentioned above was confirmed. These bacteria were apparently responsible for oxidizing about 50% of the pyrites in this coal to sulphate, and this source of bacteria was selected in our work to treat the virgin coal. A series of experiments was done in which a proportion of Avon coal was mixed with the Minto coal, and the mixture was ground in a ball mill in water for 72 to 96 hours. Following agglomeration, the following results were obtained:

pH of 10% slurry % ash % S Mixture during agglomer- (avg) (avg) ation 20% Avon 6.8 to 7.0 7.0 2.7 (3 runs) 40% Avon 5.9 to 7.3 5.5 2.5 (5 runs)

Previous work had shown that Avon coal after grinding and agglomeration with Varsol, has a sulphur content of 2.7%, so it is readily seen that the virgin coal component of the 40% mixture has actually been beneficiated to about 2.3% sulphur. As noted above the organic sulphur content of the virgin coal was about 2.0% after correction for the ash removed in the agglomeration indicating that greater than 90% of the inorganic sulphur was removed in these experiments. These experiments also show that the weathered waste fine coals present at the mine site are a very suitable source of bacteria for use in our process.

EXAMPLE 4

A further series of four experiments were done in which Avon coal was first suspended (as a 15% slurry) in 9K nutrient solution described under Example 2 above. This suspension was stored at 30.degree. C with air sparge for 2 weeks, and was used to inoculate the coal. The amount of suspension used in inoculation was equivalent to 10% Avon coal in the mixture with virgin coal. After grinding for 72 hours, the mixture was agglomerated and the following results were obtained:

pH during % Ash % S agglomeration 6.2-6.7 4.7 2.6 7.3-8.0 4.1 2.3

These very good results show that a large proportion of Avon coal in the mixture was not essential in accomplishing sulphur removal. Although these examples show that good results may be obtained, even better results (in the sense of shorter treatment times, grinding times, etc.) using more active or concentrated bacteria sources and optimized conditions, should be possible.

EXAMPLE 5

This series of experiments was the same as that described for example 4, except that the coal was preground to about minus 100 mesh in a Wiley mill and the mixture of virgin and Avon coals (10% Avon) were subsequently ground for only 24 hours in a ball mill. The pH during agglomeration was adjusted to 7.5 to 7.8, and 2.8% total sulphur was obtained in the final product (duplicate experiments done). This sulphur level is not quite so low as that obtained with longer contact time in the ball mill, but the experiment does indicate that reasonably good results can be obtained at times shorter than 72 hours in the ball mill if some initial size reduction is done. As noted above, even shorter contact times should be possible under optimum incubation conditions.

EXAMPLE 6

About 150 g. virgin coal were smeared with 100 ml. of nutrient solution contaminated with Avon coal containing the bacteria. The system was ball-milled with flint balls for 2 hours. Then 40 ml. water was added and ball-milled for 26 hours. About 40 ml. water was added and the paste was ball-milled for 22 hours. The suspension was diluted with water filtered and used as a paste. The coal particles were ground to -400 mesh.

About 25 g. quantities of the paste containing about 12-14 g. coal were agglomerated using heptane and Varsol over a wide range of pH values.

The results obtained are shown below: ##SPC1##

It is noted that the sulphur left in the coal is virtually organic sulphur, after agglomeration at substantially neutral or higher pH values.

EXAMPLE 7

The virgin coal was ground for 72 hours in aqueous suspension in a ball mill. After filtering, about 150 g. of the coal paste was smeared with 20 ml. solution containing Ferrobacillus ferrooxidans by ball-milling for 1 hour using flint balls. The suspension was diluted with 100 ml. water and ball-milled for a further period of 20 hours. The suspension was diluted with water and filtered immediately.

About 25 g. paste was dispersed in 500 ml. water and 0.2 ml. of 1% K.sub.3 Fe(CN).sub.6 was added as a depressant for pyrites. Another experiment was conducted in the same fashion, but NaOH was used to increase the pH. The suspensions were agglomerated using 8 ml. heptane and 2 ml. Varsol.

The results are shown below: ##SPC2##

EXAMPLE 8

Avon coal which contained the bacteria was ball-milled to -400 mesh in aqueous suspension. About 20 parts of the ground Avon coal at pH 2.0 was mixed with 80 parts virgin coal ground to -400 mesh in water at pH 6.2 and ball-milled together for 19 hours. The slurry at pH 2.6 was filtered and used as a paste. On agglomeration the results were as follows (TABLE 1). ##SPC3##

EXAMPLE 9

a. In one case 100 g. of New Brunswick virgin coal containing about 9.0% total sulfur was moist milled with 10 ml. of mother liquor extracted from wet Avon coal, containing the bacteria. After milling for 11/4 hours, about 420 ml. of distilled water were added; the pH dropped from about 7 for the virgin coal to about 4.4. The pH dropped to 3.7 after 4 hours ball milling and to about 3.1 after 26 hours ball milling.

b. In another case 6.5 g. (30% moisture) of Avon coal and virgin coal from the same area in New Brunswick 93.5 g. (100 g. total) were ground together in a moist porcelain jar with flint balls for about 1 hr., then about 410 ml. distilled water were added. The suspension was ball milled for 29.5 hours. The pH of the suspension decreased from about 7 to 3.0 - 3.3 in the presence of the weathered coal.

Aliquot quantities of suspension from (a) and (b) containing about 10 g. coal were treated with NaOH solution to about pH 8.3 and the coal was reconstituted by agglomeration with 5 ml. heptane and 5 ml. Varsol. The dried reconstituted coal (agglomerates) contained about 3.3% ash and 3.8% total sulfur. The original coal contained about 19.3% ash and 9.0% total sulfur.

EXAMPLE 10

Flotation tests were done in a small experimental flotation cell, consisting basically of a 150 ml. Buchner funnel with a coarse fritted glass bottom. A number of "screening" experiments were first done to attain suitable operating conditions of air rate, solids concentration, frother concentration, etc. The following conditions were used in the comparative experiments as being reasonable conditions, although not necessarily optimum:

coal slurry volume: 150 cm.sup.3

frother: 0.1 lb/ton of a mixture containing equal parts of methyl isobutyl carbinol and kerosene

air rate: approximately 8 cm.sup.3 /min per cm.sup.2 of cell area

aeration time: 10 minutes.

pH = 5-7.

As in previous Examples, virgin N.B. coal was ground wet in a ball mill with stainless steel balls for 72 hours, one batch containing no added bacteria and a second containing the equivalent of about 10% Avon refuse coal which had been held for some time in a suspension under temperature and nutrient conditions to favor the growth of Ferrobacillus and Thiobacillus bacteria. Various concentrations of the ground suspension were then treated by flotation, with some also being done by oil agglomeration for comparison, giving the results in the accompanying Table 2. --------------------------------------------------------------------------- TABLE 2

Run g. solid % Ash in % Recovery Number per 100 g. Froth (or of Suspension Agglomerates) Combustibles __________________________________________________________________________ Bacteria Present (average ash in feed = 21% 1 2.0 19.9 78.4 2 4.1 18.8 78.6 3 8.8 17.7 74.4 *4 11.0 16.1 69.6 No Bacteria Present (average ash in feed = 24%) 5 3.3 22.9 81.9 6 5.8 22.0 75.5 *7 8.5 22.0 76.2 8 12.5 22.0 83.6 Bacteria Present and Treated by Agglomeration (approximately 25% Varsol used as bridging liquid and pH.apprxeq.6) 9 4.8 3.4 > 90 10 11.4 3.3 91.3 11 21.4 4.1 95.2 __________________________________________________________________________ * sulphur analysis done in these runs (see text).

One striking result is the much lower ash and higher recovery obtained by agglomeration compared with that in the froth from flotation. For very fine coal, oil agglomeration results are much superior to flotation.

Comparing flotation with bacteria (runs 1-4) and without (runs 5-8), there is a small improvement in ash rejection in the former case compared with the latter. This can be linked with improved pyrite rejection in the presence of the bacteria. Spot checks of total sulphur on runs 4 and 7 showed 6.0% and 6.7% total sulphur respectively, (with 6.8% total sulphur in the feed in each case) consistent with greater pyrite rejection in the presence of the bacteria. In the case of the agglomerated product (run 9), the total sulphur was 2.6%, again showing the superior results with agglomeration in the case of very fine coals. However conditions were probably near optimum for agglomeration but not for flotation.

These tests give an indication that the bacteria are able to depress pyrites in the case of flotation as well.

The examples illustrate a variety of ways of contacting the bacteria and coal. One which is not illustrated but which may prove the most practical is by storing the mixed coal in above-ground dumps, irrigated with water (mine drainage) containing the bacteria. This in fact is essentially the way in which the Avon coal, a waste, was stored, and the bacteria acted to reduce the pyritic iron.

Similar rejection of the surface-modified pyritic particles can be expected from other analogous particle separation techniques.

Advantages for the partial (surface only) bacterial action-plus-particle separation as against more complete dissolution due to bacterial action (leaching) include:

a. shorter processing time and decreased costs to attain substantially complete pyrite removal;

b. more efficient grinding or less grinding needed where bacterial action precedes or is concurrent with the final grinding; and

c. rejection of some siliceous or other acid-insoluble hydrophilic impurities as well.

Claims

1. A process for beneficiating impure metal sulfide-containing materials comprising:

a. wet grinding the impure material to aid release of fine particles of the metal sulfide from non-sulfide solids;

b. subjecting an aqueous mixture containing finely divided material to the action of inorganic sulfide-oxidizing bacteria selected from the Thiobacillus-Ferrobacillus group until only the surface of at least part of the sulfide solids is rendered hydrophilic and the pH, when at about 30-50 wt. % solids in water, is below about 5;

c. adjusting the solids content in water to within about 2 to about 40% wt. and providing that the pH of this slurry is above about 5;

d. conducting a particle separation operation selected from agglomeration, flotation, and retention on hydrophobic surfaces by adhesion on the resulting slurry to separate hydrophobic non-sulfide and non-oxidized sulfide solids from the hydrophilic and bacterial surface-oxidized sulfide particles; and

e. recovering the beneficiated values substantially free of the released

2. The process according to claim 1 for beneficiating pyritic ores and coals containing pyritic materials comprising:

a. wet grinding to aid release of fine particles of the pyritic material from the non-pyritic values;

b. subjecting an aqueous mixture of the finely divided ore or coal to the action of the selected bacteria until only the surface of the pyritic materials is rendered hydrophilic, and the pH is below about 4 to 5;

c. diluting with water and raising the pH of the slurry to above about 5, and near neutral;

d. conducting said particle separation operation on the resulting slurry to separate out hydrophobic or non-pyritic values, leaving the aqueous slurry containing the pyrites as tailings; and

e. recovering the desired values substantially free of pyritic materials.

3. The process of claim 1 wherein said bacteria is added and allowed to act

4. The process of claim 1 wherein (d) comprises an agglomeration with

6. The process of claim 1 wherein weathered pyritic coal or ore containing

7. The process of claim 1 wherein the incubation with the bacteria is carried out for not more than about 72 hours, with the final pH being

8. The process of claim 1 wherein low grade pyritic coal is beneficiated and in (d) agglomeration with hydrocarbon bridging liquid is carried out at a pH of about 5-10 with a pulp solids content of about 2-40 wt. % and coal agglomerates containing substantially reduced sulfur are recovered.

9. The process of claim 1 wherein the solids content during bacterial incubation is within about 30 to 50 wt. % and this is adjusted to within

10. The process of claim 1 wherein low grade pyritic coal is beneficiated and lime is added in (c) to raise the pH just before an agglomeration or flotation in step (d).