I
LOREN C. BRYNER and RALPH ANDERSON Brigham Young University, Provo, Utah
Microorganisms in Leaching Sulfide Minerals Here is a new idea for recovering molybdenum from low grade ores
'
THE
DISCOVERY of methods for reclaiming metals from low grade ore deposits has become an important problem due to the dwindling supply of high grade ores. The evidence is conclusive that sulfide minerals of iron and copper can be successfully oxidized and leached with autotrophic bacteria (2, 3, 7). The solubilized copper is recovered from the leachates by precipitation with iron. This article reports some laboratory experiments which show that autotrophic bacteria found in the leaching streams from the waste rock dumps in Bingham Canyon, Utah, can oxidize molybdenite (MoS~),and also show the effect of other sulfide minerals which occur with molybdenite in ore deposits on the oxidative process (7).
Biological Oxidation of Molybdenite
Materials and Methods. Molybdenite, MoS2,occurs as black, shiny-blue or dark gray hexagonal plates or sheets usually closely mixed with quartz or other siliceous material. (5). It is usually recovered from ores by flotation. The compositions of these various minerals are given in Table I. The apparatus used in the investigation consisted of a series of airlift percolators slightly modified from those previously described ( 3 ) . The percolators were made of 40-mm. glass tubing, approximately 400 mm. in length. The mineral samples were held in the large tube by a perforated porcelain disk covered with a layer of glass wool resting in the constricted bottom. An exterior 5mm. tube fitted to a source of compressed air served as the lifting channel. An opening in the bottom of the percolator, closed with a pinch clamp, served as a means of draining the apparatus ( A ) . The compressed air was bubbled through water and filtered through sterile cotton before use. The top of the percolator tube was plugged with cotton to allow the air to escape, keep out dust, and prevent contamination. The percolators were charged with a mixture of
Table 1. Sulfide Mineral Molybdenite (concentrate) Molybdenite ore Pyrite I1 Pyrite I11 Chalcopyrite I1
Analyses of Sulfide Minerals % Mo %S %cu % Fe
57.58 0.871 trace 0.04 trace
38.42 0.903 46.4 44.45 33.45
100 grams bf Ottawa sand (SiO2) and 5 grams of the desired sulfide mineral. The sand and mineral were first added to a 250-ml. beaker and mixed with a small amount of distilled water. This mixture was transferred to the percolator through the charging funnel ( B ) with a fine stream of distilled water from a wash bottle. If this precaution was not taken, the more dense mineral would gravitate to the bottom of the sand column. Quantitative analysis for iron was made by the dichromate method (6) and copper was obtained by use of the Beckman flame spectrophotbmeter (Model DU) (9). The oxidized molybdenum was determined colorimetrically (8). Stock cultures of the bacteria were grown in percolators which were charged with 100 grams of Ottawa sand, 5 grams of the desired sulfide mineral (FeS2, CuFeSz. MoS2), and 100 ml. of the nutrient solution (Table 11). After sterilization at 140' C. for 30 minutes, the percolators were inoculated with 5 ml. of fresh solution taken from the leaching streams of Bingham Canyon or the desired culture described in Table 111. These percolators were drained weekly and analyzed for the desired soluble products. When the rate of solution reached a steady state, requiring about 6 weeks, 5 ml. of the enriched culture was taken for the inoculation of other percolators used in this study. The activity of the microorganisms was found to increase with repeated transfers.
0.1 0.271 1.8 6.57 32.15
~
0.756 41.1 46.0 18.55
Table II.
% Insol. 3.9 97.199 8.4
0
0.08
15.85
Nutrient Solution
-L Wt., in Component G. Ammonium sulfate, (NH4)zSOd 1.0 Potassium hydrogen phosphate, KiHPOi 0.1 Aluminum sulfate, Alz(SO*)s.18He0 4.0 Magnesium sulfate, MgSO4.7Hz0 3.0 Calcium nitrate, Ca(N0&.4Hz0 0.1 Manganese sulfate, MnS04.Hz0 0.05 Sodium sulfate, NazSO4 0.05 Potassium chloride, KC1 0.05 Distilled water 1000 pH adjusted to 2.65 with HzSO4
LIFTING
CHARGING FUNNEL
NUTRIENT SOLUTION
SAND a MINERAL
AIR
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Experimental apparatus used this type of air-lift percolator (A) and charging funnel (B)
CLAMP
A VOL. 49, NO. 10
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Table 111.
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Bingham Canyon leaching streams Molybdenite concentrate medium Molybdenite concentrate and pyrite medium
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Bacterial Cultures
Culture
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Figure 1.
Biological oxidation of molybdenite
These bacteria are autotrophs which require carbon dioxide and oxygen from the air along with other inorganic substances for their growth. They obtain their energy for growth €rom the oxidation of the sulfide minerals. Their needs for carbon dioxide and oxygen are satisfied by the air supplied to the percolators (4, 70). Results
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Figure 2.
Effect of pyrite on biological oxidation of molybdenite
140
100
60
Biological Oxidation. MOLYBDENITE. Two percolators were charged with 100 grams of Ottawa sand and 5 grams of molybdenite. The nutrient solution of 100 ml. was added and the percolators were sterilized in the autoclave at 140 O C. for 30 minutes. These two percolators were placed in position and attached to the sterile air source. One percolator was inoculated with 5 ml. of culture I which was Bingham Canyon leaching solution; the other served as a sterile control. The nutrient solution (composition given in Table 11) for the refill of the percolators was also sterilized in the autoclave. The percolators were allowed to run at room temperature for 7 days and then they lvere drained and refilled Tvith fresh sterile nutrient. The leachate was analyzed for oxidation products of molybdenite. The results of this study (Figure 1.) show that approximately seven times more soluble molybdenum was formed by bacterial action than that under sterile conditions. The soluble molybdenum produced in the oxidation existed in both the penta- and hexavalent forms, in a ratio of 1 to 4. The molybdenite is oxidized to sulfuric acid and four fifths of the molybde,num to molybdic acid, H2hlo04. Measurements of the pH showed an increase in acidity. -4s molybdenite occurs with pyrite and chalcopyrite in the Bingham ore deposits, the following studies were undertaken to determine the effect of the pyrite and chalcopyrite on the oxidation of molybdenite.
MOLYBDFNTE AND PYRITE MIXTCRES. 20
0
2 0
I000
2000
3000
4000
FERROUS IRON CONCENTRATION
Figure 3.
1722
5000
6000
P.P.M.
Effect of ferrous ion concentration on biological oxidation of molybdenite
INDUSTRIAL
Three percolators A , 3: and C were charged with 5 grams of molybdenite and Ottawa sand. T o percolators R and C, 5 grams of pyrite I1 was also added. All three percolators and their contents were sterilized. Percolators A and B were inoculated with culture I. The results of this study are tabulated in
AND ENGINEERING CHEMISTRY
M I C R O O R G A N I S M S IN L E A C H I N G M I N E R A L S
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Figure 4. Biological oxidation of a molybdenite and chalcopyrite mixture
Table IV.
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22
WEEKS
Figure 5. Soluble copper and molybdenum simultaneously produced from molybdenite ore
Biological Oxidation of a Molybdenite-Pyrite Mixture Inoculated, Mg. Sterile, Mg.
Time, Days
Cumulative Mo
Cumulative
Cumulative
Cumulative
Fe
Mo
Fe
7 14 21 28 35 42 49 56 63 70 77
1.0 9.4 27.4 48.2 72.6 86.3 93.62 97.12 00.12 02.27 03.42
8 364 834 1304 1599 1757 1793 1873 1897 1905 1913
1.51 2.06 2.56 2.96 3.16 3.26 3.36 3.46 3.50 3.55 3.59
1.0 3.0 4.5 6.0 7.2 8.2 9.0 10.0 10.8 11.9 13.0
Table I V and the effect of the oxidation of pyrite with molybdenite is shown in Figure 2. These data show that the amount of soluble molybdenum formed was increased by approximately 350y0 when the pyrite and molybdenite were run together. There was 29 times more molybdenum and 163 times more iron solubilized under these conditions than in the sterile control. EFFECTOF FERROUSIRON. The results from the pyrite study indicate that the soluble iron may affect the increased activity on the biological oxidation of molybdenite. T o determine this, a series of percolators were set up, each with 5 grams of molybdenite and sand and 100 ml. of nutrient solution containing different concentrations of ferrous iron as ferrous sulfate. The ferrous iron concentration was varied from 0 to 6000 p.p.m. a t an initial p H of 2.65. The percolators were inoculated with culture 111, drained, and refilled weekly with fresh nutrient. The leachates were analyzed for oxidized molybdenum content. This investigation was run for a period of 5 weeks. These results (Figure 3) show a definite optimum ferrous iron concentration at 4000 p.p.m. MIXTURE OF MOLYBDENITE AND CHALCOPYRITE. Two percolators were pre-
IO
pared in the usual manner with 5 grams of molybdenite, 5 grams of chalcopyrite, and 100 grams of Ottawa sand. One percolator was inoculated with culture I while the other served as a control. The results of this study are shown in Figure 4. From the results of the preceding studies an increase in activity would be expected on a molybdenite-chalcopyrite mixture due to the presence of soluble iron. However, approximately the same amount of molybdenum solubilized as when molybdenite was oxidized alone. These data indicate that there may be a preferential oxidation of the copper sulfide mineral over the molybdenum sulfide. A series of runs were made on mixtures of molybdenite and iron pyrite (pyrite 111) which has a higher copper content than pyrite I1 and less than the molybdenite ore. The results, similar to the preceding ones, indicated a preferential oxidation of the copper sulfide over the molybdenite, and the cupric ion concentration in the leachates dropped to approximately 40 p,p.m. before significant oxidation of the molybdenite. Rate Study on Soluble Copper a n d Molybdenum from Molybdenite Ore. A percolator was charged with 100 grams of molybdenite ore crushed to 60 mesh or greater (Table I) to check the
possibility of preferential oxidation of the copper sulfides over molybdenite. This percolator was inoculated with culture 11, drained weekly, analyzed for soluble copper and molybdenum, and refilled with fresh nutrient solution. T h e results are shown in Figure 5. The molybdenite ore had an initial copper content of 0.55 %. The time required for the concentration of leached copper to reach 40 p.p.m. was about 8 weeks. At this time 81% of the copper in the original molybdenitt ore had been solubilized and oxidation of the molybdenite appeared to be significant. After running 17 weeks, 97.57, of the copper in the molybdenite ore had been solubilized. From this time until the completion of the run no more soluble copper was detected. The same general trend in all studies was found when the molybdenite was oxidized simultaneously with copper sulfide minerals. The cupric ion concentration u p to 75 p.p.m. on the oxidation of molybdenite had no observable effect. Effect of Particle Size. Four percolators were charged with molybdenite
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Figure 6. Effect of particle size on biological oxidation of molybdenite ore VOL. 49, NO. 10
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OCTOBER 1957
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A
INOCULATED (CULTURE N 0 . I ) TRANSFER N0.I
(CULTURE N0.2)
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Figure 7.
Effect of bacterial transfer on oxidation of molybdenite
ore (Table I ) : A 100 grams crushed and screened between the limits of 10 to 30 mesh; B 100 grams between the limits of 30 to 60 mesh; and C and D 100 grams of 60 mesh and greater. They were all sterilized in the usual manner. A , B, and C were inoculated with culture I11 while D was kept as a control. These results (Figure 6) show that the amount of leaching is a function of the particle size or surface area and that the molybdenite in the ore is oxidized biologically. Efficiency. Percolator D of the preceding study was allowed to run for 9 months and after this time elapsed, 36y0 of the molybdenum had been solubilized. Another percolator started a t the same time contained 5 grams of pyrite I1 in addition to the 100 grams of molybdenite ore. At the end of the 9-month run, 557, of the molybdenum
0 INOCULATED- CULTURE NO I ATRANSFER NO I TRANSFER NO 2
/
had been solubilized. The presence of the pyrite has increased the efficiency of the oxidative process from 36 to 55%. Hence, soluble iron seems to aid the oxidative process. Effect of Bacterial Transfer. Culture I1 was transferred to percolators set up the same as those described for the results in Figure 2. They were transferred the sixth week which was approximately their highest activity. These data (Figure 7) show an increase in the formation of soluble molybdenum by repeated transfer. The effect of transfers on the oxidation of a pyrite and molybdenite mixture was also studied. Culture I11 at the fifth week was transferred to a freshly prepared percolator similar to those described for results in Figure 3. These data for this transfer and a second transfer under the same conditions (Figure 8) show that results are similar to those obtained for molybdenite alone. Weekly draining of the percolators removed most of the bacterial cells that were in suspension. However, if the solutions were allowed to cycle for longer periods of time they became slightly turbid. Centrifuging and microscopic examination showed that bacterial cells were present. Most of the cells adhered to the surface of the sand and sulfide mineral. I t was not necessary to reinoculate the fresh solution that was added after draining. The sterile controls did not develop a turbidity or show appreciable oxidation of the sulfide minerals. Conclusions
TIME IN DAYS
b
Figure 8. Effect of bacterial transfer on oxidation of a pyrite and molybdenite mixture
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This study provides conclusive evidence that molybdenite can be oxidized by autotrophic bacteria. Soluble oxidized molybdenum was formed in the percolators from molybdenite and molybdenite ore that were inoculated with Bingham Canyon bacteria.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Samples of molybdenite inoculated with sulfide-oxidizing bacteria showed a sevenfold increase in the rate of solution over that in the sterile control, while inoculated samples of a mixture of molybdenite and pyrite oxidized 29 times more molybdenite than the sterile control. Inoculated samples of molybdenite mixed with chalcopyrite and samples of molybdenite ore containing copper sulfide showed a preferential oxidation of the copper sulfide. The cupric ion concentration in the nutrient up to 75 p.p.m. had no effect. The molybdenite underwent significant oxidation when the cupric ion concentration dropped to 40 p.p.m. or less. The results show an optimum near 4000 p.p.m. in the ferrous ion concentration for bacterial activity on molybdenite. The amount of leaching is a function of particle size and repeated bacterial transfers increased the rate of oxidation of molybdenite. The percentage of molybdenum solubilized from the ore in 9 months was 367, when the ore was oxidized alone, while 55y0was solubilized when the ore was oxidized along with pyrite.
Ae knowledgment The authors express their appreciation to the Western Mining Divisions of the Kennecott Copper Corp. for their financial assistance and to S. R. Zimmerly for supplying mineral samples. The assistance of Keith Jameson is also acknowledged. Literature Cited (1) Anderson, R., “Oxidation of Molybdenite with the ,4id of Microorganisms,” master’s thesis, Brigham Young University, 1956. (2) Beckwith, T. D., Gas 21, 47-8 (1945). (3) Bryner, L. C., Beck, J. V., Davis, D. B., Wilson, D. G., IND.ENG. CHEX.46,2587 (1954). (4) Davis, D. B., “Biological Oxidation of Copper Sulfide Minerals,” master’s thesis, Brigham Young University, 1953. (5) Fairhall, L. T., Dunn, R. C., Sharpless, N. E., Pritchard, E. A., “The Toxicity of Molybdenum,” U. S. Public Health Serv.? Public Health Bull. No. 293 5-7 (1945). ( 6 ) Kolthoff, I. M., Sandell, E. B., Textbook of Quantitative Inorganic Analysis,” 3rd ed., Macmillan, New York, 1953. (7) Leathen, W. W., Braky, S. A., McIntyre, L. D., ApFl. Microbial. 1, 61, 65 (1953). (8) Sandell, E. B., “Colorimetric Determination of Metals,” Interscience, New York, 1950. ( 9 ) Willard, H. H., Merritt, L. L., Jr., Dean, J. A , , “Instrumental Methods of Analysis,” Van Nostrand, New York, 1951. (10) Wilson, D. G., “Studies on the Biological Oxidation of Iron Pyrites,” master’s thesis, Brigham Young University, 1952.
RECEIVED for review December 7, 1956 ACCEPTED April 2, 1957
