Microorganisms in Leaching Sulfide Minerals LOREN C. BRYNER, JAY V. BECK, DELMAR B. DAVIS, AND DEAN G. WILSON Brigham Young University, Provo, Utah
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HE development of metallurgical methods for low grade ores is a problem of paramount importance today and will, no doubt, become more significant in the future years as our high grade ore deposits are depleted. The recovery of copper obtained in the open-pit mining operations in Bingham Canyon, Utah, by leaching waste rock with water has long been a profitable operation. The waste rock contains small amounts of iron, copper, and other sulfide minerals. The copper, which appears in the leaching water, is accompanied by soluble iron and sulfuric acid, and for a long time it has been thought that the copper appeared in solution as a result of the action of the sulfuric acid on the copper minerals. The sulfuric acid was considered to be produced by the atmospheric oxidation of the sulfuritic materials. Several observations revealed that perhaps the production of soluble iron, copper, and sulfuric acid in the leaching water was due not to a simple chemical procem ( I f ) but to the action of microorganisms. Laboratory studies ( 2 , 1 2 ) have confirmed this idea and have demonstrated that the production of acid and of soluble iron is indeed a microbial process. Similar studies on the drainage water from coal mines in the bituminous areas of Pennsylvania and West Virginia and on iron pyrites have shown that the soluble iron and sulfuric acid are produced largely as a result of an oxidative microbiological process (1, 8, 4, 6-10), The purpose of thjs paper is to report some laboratory experiments vhich show that the release of iron, copper, and sulfuric acid in the leaching n7ater is due largely to the action of microorganisms on sulfide minerals.
The Utah Copper Co. provided the sample of iron pyrites used in this investigation. Its color was a pale brassy-yellow varying to a darker gray. When received, the sample was in a finely divided state. The particles varied in size from those that were stopped by a standard 60-mesh screen to a fine powder that passed through a 200-mesh screen. iill particles were able to pass through a 20-mesh screen. An analysis of the sample showed Iron Pyiites Iron Sulfur Insoluble material Copper Molybdenite
% 42,25
49.90 6.80 0 95 Traces
The analysis of the sample agrees closely with the formula FeSs with regard to the ratio of iron to sulfur. The main apparatus used in the problem was an air-lift percolator. The percolators were made of 40-mm. glass tubing approximately 200 mm. long. The samples were held in the large tube by a perforated porcelain disk resting in the constricted bottom. An exterior 5-mm. tube fitted to a source of compressed
BIOLOGICAL OXIDATION OF IRON PYRITES
Materials and Methods. The organisms used in this study were obtained from the leaching stream issuing from the waste rock dumps in Bingham Canyon. The organisms apparently are autotrophic iron and sulfur oxidizing bacteria. The bacteria were grown in 125-ml. Erlenmeyer flasks containing 50 ml. of nutrient solution ( 5 ) (Table I ) to which was added 10 ml. of ferrous sulfate (10 grams FeS04.7HzO per 100 ml. HSO) solution per liter. The solutions were clear and colorless except for a slight white precipitate that settled to the bottom of the flask. After inoculation and incubation a t 20" to 25" C. for 3 to 5 days, a brownish-red precipitate of ferric salts settled from the solution. The cultures remained viable for several months without transfer to the fresh medium. T o obtain actively growing cultures, transfers were made to the fresh medium every 2 weeks. A sterile control sample remained clear over a 4-month period.
TABLIC I. COMPOSITION OF Ammonium sulfate Potassium chloride Magnesium sulfate Potassium hydrogen phosphate Calcium nitrate Distilled water
NUTRIENT SOLUTION Grams 0.15 0.06 0.50 0.05 0.01
1000.00 nil.
Figure 1. Portion of Percolator Battery 2587
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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 percolator. The compressed air was mashed and filtered through sterile cotton before use (Figure 1). The fresh air supply served as a source of oxygen and carbon dioxide.
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Figure 2. Oxidation of Iron Pyrites by Bacteria Found in Leaching Streams i n Bingham Canyon, Utah Sterilization effect is sh0w.n
Determination of Bacterial Activity. I n each of the percolators was placed a mixture of 200 grams of Ottama sand and 20 grams of finely divided pyrite. The charged percolators and the nutrient solution were sterilized in an autoclave a t 140" C. for 30 minutes. From then on, efforts were made t o keep all apparatus and contents sterile. In each percolator 50 ml. of the sterile nutrient solution was placed and allowed to circulat,e for several days. The circulating solutions were withdrawn each week for analysis. The contents of the percolators were washed with three 15-mL portions of fresh medium and the washings collected with the sample; 50 ml. of fresh medium was placed in the percolators after the washing. After leaching became constant as shown by analysis of the sample, the percolators were inoculated with the desired culture of bacteria. A sterile sample was maintained as a control. After 7 days the entire circulating solution was withdrawn for analysis and fresh sterile medium introduced. Part of the bacteria remained suspended on the surface of the pyrite and sand; therefore, it was not necessary to reinoculate the percolators. The results of thePe experiments are shown in Figure 2. The bacteria actively increase the rate a t which pyrite (FeSz) is osidized to soluble ferric iron and sulfuric acid. The amount of soluble iron produced in a 3-month period in the percolator inoculated with the iron and sulfur oxidizing bacteria from the Bingham mine water was twenty times as great as the soluble iron appearing in the sterile control samples. The progress of the oxidation could be roughly estimated b:observing the color of the nutrient solution. When first placed in the percolators, the solution was clear and colorless. After about 2 days, the nutrient turned slightly yellow, the color becoming darker as the oxidation continued. The solutions in the sterile controls remained rlear and colorless.
Vol. 46, No. 12
A gradual change was noticed in the appearance of the pyrite in the percolators. When the reaction was first started, it had a bright metallic luster, but as the oxidative process continued, it became dark and dull. After the bacteria had been allowed to gron- for 35 days, one of the percolators and its contents mere sterilized in the autoclave a t 15 Ib. for 30 minutes, and to the other was added a few milliliters of dilute (100 p.p.m.) mercuric chloride solution. Both treatments stopped the biological oxidation of the pyrite and reduced the rate of solution of iron to that of the sterile control samples (Figure 2 ) . A comparison between the amount of iron oxidized at room temperature and a t 0" C. shoved a decrease of 857, when the percolators were operated in the cold. When the percolators were returned to room temperature, the solubilization of iron returned to the normal rate. The operation of the percolators in complete darkness resulted in an appreciable increase in the amount of iron appearing in the circulating solution. The results of these experiments are shown in Figure 3. The effects of carbon dioxide and oxygen on the growth and oxidation activity of the bacteria were determined by altering the composition of the gas passing through the percolator. When pure carbon dioxide was passed through the percolator, the activity in connection with the pyrite oxidation ceased due to lack of oxygen. The amount of iron appearing in the percolating solution dropped to a minimum of 2.0y0 of the amount appearing in the solution when compressed air was used as the circulating mechanism. Next the air wa3 bubbled through potassium hydroxide solution to remove the carbon dioxide. During the 2 weeks immediately following the start of this absorption, the oxidative rate, as measured by the amount of iron in the solution, decreased 50.07,; during the third week, the rate decreased to 247, of normal. These data indicate that small amounts of carbon dioxide and oxygen are necessary in the solution for the normal growth and activity of the bacteria. These bacteria exist in acid surroundings which most other forms of bacteria are not able to tolerate. A s noted previously, the oxidation of pyrite produces sulfuric acid as one of the prod-
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aclr on packaging 1)rohlenis. h specific problem presented by fhe Savj- Bureau 01' Ordnance early in 1945 called for inhihiting the rusting of the interior of rocket bodies during the time belveen manufactuic in the United St,atesand the subsequent fiiling and use in the l'avific n-ar theater. The previous pracbicci of slushing the interior of the rocket body wit,h Grade 21 10 petroleum oil soon after fabricat,ion had proved inadequate in preventing rust \Then water condensed in the interior of the rocket body. The oil vias also very difficult to remove because of the construction of the rocket body. Studies were made of the effectiveiiess of the follo~vingmethods of inhibiting rusting ( 2 ) : (a)slushing the interior of the rocket body with Grade 2110 petroleum oil, ( 0 ) slushing the interior surfaces of the rocket body with Grade 2110 oil to \yhich an effcctive rust inhibitor had been added; (c) slushing tlie interior of the rocket with a typical comrncrcinl Eolvent cutback rust-iwcveiitive fluid; ( d ) the use of a desiccant, silica gel, under conditions which siinulated the breathing of a closed (but not scaled) container subjected to t,eniperature cycling in a humid atmo;.phere; and ( e ) the use inside the rocket body of a paper t u l l e which had previously been impregnated with diisopropylain-, moniuni nitrite. Only rockets treated by methods ( b ) , ( e ) , or ( e ) Kere fourid to be adequately protected against rusting during storage under humid conditions. Methods ( b ) and ( e ) entailcd tedious fieldcleaning operations, whereas the rocket i ~ o d yprotected bj- the impregnated paper tube was ready for use when opened. -1 specification mas therefore written for the procurement of the impregnated paper tubes (43). Work by two independent research groups (36, .45, 46, :i4) established that dicyclohexylaniinoriiuiii nilrite was less volatile and not so soluble in w a k r as the diisopropylarnniotiium nityitc and was in consequence a more suit,ahle paper impregnaiit. Boon after Il'orld War I1 ended a Saval Research report was made of the physical properties of these amine nitrites ( 2 ) and their effects on a wide variety of nonferrous metals. It was shown that these vapor rust inhibitors could be used in solving marry
