Table of Contents
Research in recent years has been directed toward seeking methods of metal extraction which are less labor and energy intensive, less costly in capital investment and are less insulting to the environment. Chemical metallurgy offers much toward meeting these criteria. Hydrometallurgy offers innovations in copper dump leaching, new developments in in-situ leaching of copper and heap-leaching uranium and improved schemes for leaching of metal concentrates. Although increased research effort has produced techniques which are now commonly used in extraction of copper and uranium, techniques for extraction of recalcitrant chalcopyrite have not been refined. Considerable research effort has been expended to discover methods of leaching chalcopyrite. Fine grinding and leaching with ferric chloride have been explored.
It was reported that a thermophilic bacterial strain enhanced the extraction of molybdenum from molybdenite and copper from chalcopyrite. These organisms, Sulfolobus acldocaldarius and a close relative, “ferrolobus” are ordinarily found in the acid thermal waters of Yellowstone National Park. They readily oxidize reduced sulfur and iron compounds in the acid pH range at temperatures ranging from 50 to 80°C. Morphologically unlike Thiobacillus ferrooxidans and Thiobacillus thiooxidans, the ubiquitous, rod-shaped inhabitants of copper leach dumps and pyritic uranium workings, Sulfolobus and “ferrolobus” are shperical and lack the usual cell wall structure of the thiobacilli. Flask studies indicated that the thermophilic bacteria more readily leached copper from porphyry copper minerals having chalcopyrite as the primary copper mineralization than T. ferrooxidans. Subsequent column leaching studies using low-grade chalcopyrite ores clearly indicated that Sulfolobus and “ferrolobus” could extract copper from several of these racalcitrant samples, but that one ore tested was resistant to leaching by these organisms. Favorably leached were -3 +100 mesh Duval Sierrita Corp. chalcopyrite ore and -½ inch + 48 mesh Pinto Valley Mine ore (Cities Service Corp.). Steward Mine ore from Butte Montana was not leached by the thermophilic bacteria, and microbiological analyses of the ore following the 189-day leach period indicated no colonization of the organisms in the ore column. The factor(s) which adversely affected microbial development and leaching of this ore was not defined.
Although the thermophilic bacteria, Sulfolobus and “ferrolobus” readily grow on most chalcopyrite ore when it is inoculated, a limited survey of leach dump environments, sulfide ore deposits, and pyritic tailings of the desert southwest showed that these organisms are not indigenous to sulfide areas. The strict temperature requirements of these organisms may definitely restrict their distribution.
Other research with the very thermophilic, acidophilic bacteria has primarily been of a physiological and biochemical nature with some studies on the distribution and ecology of these organisms in their natural habitat, the acid, thermal springs.
A more recent finding is the existence of thermophilic, rod-shaped bacteria with a strong resemblance to thiobacilli. This acido-thermophillc Thiobacillus oxidizes ferrous iron and pyrite at 30-50°C (22) and has subsequently been found to reside in copper leach dumps and a pilot leach facility.
The following report summarizes continued column leaching studies using Sulfolobus acidocaldarius and “ferrolobus” to extract copper and other metals from recalcitrant chalcopyrite ores from the southern U.S. including low-grade ores from Pinto Valley, San Manuel, and Cyprus Bagdad. The leaching of Pinto Valley ore using the thermophilic microbes is compared with the leaching of this ore using Thiobacillus ferrooxidans. The column leaching studies are divided into Series I and Series II tests to differentiate the two sets of studies conducted in the years 1977-78 and 1978-79. Also included in this report are the results of a flask leaching study of three low-grade chalcopyrite ores and the effect of eleven solvent extraction reagents at three concentrations on the respiratory activity of the thermophilic microbes, Sulfolobus and “ferrolobus.”
Materials and Methods
Leach Solution and Media
The leach solution for all studies was either 9k basal salts diluted with distilled water rather than ferrous iron solution or distilled water. The solutions were adjusted to an acid pH with sulfuric acid.
When elemental sulfur (flowers of sulfur) was used as an energy source, it was sterilized by intermittent steaming for 30 min on three consecutive days.
A solution of 25 g FeSO4 · 7H2O in 95 ml distilled water and 1 ml of concentrated H2SO4 was steriled by autoclaving. Four ml of this solution were added to 100 ml of medium to provide an iron energy source.
Batch cultures were supplemented with 0.02% or 0.2% yeast extract (Difco) for “ferrolobus” and acidocaldarius, respectively. The yeast extract was sterilized with the medium.
Inoculum for Column Leach Tests
Batch cultures of “ferrolobus” and S. acidocaldarius were prepared in Fernback flasks containing 1.5 liters of medium and 7.5 g elemental sulfur. The medium was supplemented with yeast extract. After 14 days of incubation at 60°C, the cells and sulfur were sedimented from solution at 27,000 g and resuspended in fresh medium. Cells were separated from sulfur by centrifuging the culture at 120 g for 10 min to sediment the sulfur. The number of cells was determined by the most probable number (MPN) method.
Ore
In the Series I tests conducted in 1977-78 all six leach columns contained a low-grade ore from the Pinto Valley Mine, Cities Services Corp., Arizona. Chalcopyrite was the primary copper mineral with some covellite and chalcocite present. Pyrite was abundant in this ore. This low-grade ore was sized and loaded into six leach columns:
Columns A and B — 90 lb, -2 inch + ½ inch
Columns C, D, E, and F — 110 lb, ½ inch + 50 mesh
Split samples of the two fractions were used to establish a size distribution and for chemical analyses of the ore. Weights of each mesh size were computed and weight percents calculated. All size fractions and splits were analyzed in triplicate for copper, total iron, and zinc. Additionally split fractions were assayed for lead, nickel, molybdenum, calcium as carbonate, calcium as silicate, magnesium as carbonate, and magnesium as silicate. The triplicate data for each assay were averaged, and the standard deviation and percent range of average for the three values were computed.
In the Series II column testing conducted in 1978-79 three low-grade, recalcitrant chalcopyrite ores were used for study. Pinto Valley ore and San Manuel ore were sized to -½ inch + 50 mesh; the ore from Cyprus Bagdad was initially sized to -2 inches + ½ inch. Each ore was thoroughly blended and split. Analyses for copper, iron, nickel, molybdenum, lead, zinc, calcium carbonate, clacium silicate, magnesium carbonate, and magnesium silicate were run in triplicate on each split sample. Table I, Appendix II summarizes the contents of each column. The Cyprus Bagdad ore was resized twice during the experimental period.
Flask Tests
The leachability of the three ores was tested in static flasks at 60 °C. Ten g of – ½ inch + 50 mesh were placed in a total volume of 90 distilled water. The pH of the leach solution with Pinto Valley ore was adjusted to 1.6, and the solutions with the other ores were adjusted to pH 2.2. One flask of each ore was inoculated with a dense suspension of Sulfolobus and “ferrolobus”; the second flask was sterilized with 50 ppm panacide, an anti-microbial agent. The pH values of the flasks were checked daily for one week, and H2SO4 was added to the Pinto Valley reactors to keep the pH at the desired range of about 1.6. After 16 and 30 days, filtered solutions were analyzed for pH, Eh, ferrous iron, total iron, and copper.
Column Leach Tests
Series I tests:
For Series I testing leach columns were designed and constructed as previously described. Four-foot lengths of ¼ inch PVC pipe were installed in each of two columns. The PVC pipes from top to bottom were perforated with 1/8 inch holes, and the pipes were encased in nylon mesh. The open ends of the pipes at the top of the columns were stoppered. Aeration was from the bottom of each column through the PVC pipes.
The temperatures of all leach columns were maintained between 60 and 80°C with heat tapes or nichrome wire. Temperatures were regulated with Yellow Springs Instrument thermister controllers. The insulated, 5 gallon, glass reservoirs, collecting the recycled leach solutions, were heated to 60°C with heat tapes controled by Fenwall switches.
The leach solution, 9K medium made up with distilled water rather than ferrous iron, was applied to the ore columns at a rate of 10 ml per min. Each leach unit contained a total volume of 18 liters of leach solution.
“Ferrolobus” and Sulfolobus, free of sulfur and growth medium, were suspended in fresh 9K medium to form a dense suspension. The numbers of these organisms were obtained by the 3-tube MPN method. Columns A, C, and E were inoculated and columns B, D, and F were initially sterilized with 200 ppm panacide (2,2-methylenebis-4-chlorophenol), an antimicrobial agent. Panacide was added at a concentration of 50 ppm weekly to the control columns to eliminate bacterial growth in the ore and leach solution.
Columns A and B were continuously leached by constant addition of 10 ml per min leach solution over the ore. Columns C and D were initially leached, but at day 32 a 4-day leach cycle and 3-day rest cycle were initiated. During the 3-day rest period, C and D were each aerated dally for 30 min. Columns E and F were continuously leached through day 92, when a rest/leach cycle identical to columns C and D was initiated. However, columns E and F were not aerated. Evaporative losses in all six columns were made up by weekly additions of distilled water.
Samples were collected weekly from the reservoirs of each column and at irregular intervals from the column effluents. Analyses performed on each sample included pH, Eh (during the initial phase of the study), ferrous iron, total iron, acidity, zinc, and copper. Periodic molybdenum assays were also completed. The acidity of the leach solution was maintained at 0.75 g H2SO4 per liter by addition of concentrated H2SO4. Samples were recurrently collected from the reservoirs and effluents of each unit for bacterial determination.
When the copper concentration of the leach solution reached approximately one g per liter, the copper was removed from the leach solution by cementation with iron filings. The columns were drained and from copper concentrations, derived from analyses, the amount of iron metal required for the cementation reaction was calculated. Iron filings were added to the reservoir solution, and the leach liquor was vigorously stirred for 30 min to enhance the reaction rate. The leach liquor was filtered through a layer of paper toweling to remove the copper metal and any unreacted iron filings. Analyses for ferrous and total iron immediately following cementation demonstrated the concentrations of ferrous iron added to the leach solution by the reaction. This concentration of ferrous iron was added to the control column as an acidified aqueous solution of FeSO4 · 7H2O.
Series II tests:
Seven glass columns, 3 feet by 5 inches, were wrapped with heat tapes and two-inch, aluminum-backed insulation. Temperature was controlled to about 70°C with Fenwal switches connected to a rheostat. The columns were designed for aeration at the bottom. Solutions were collected in 4.0l, Pyrex bottles which were heated by tapes and insulated. The leach solutions were recycled over the ore at a rate of 10 ml per minute using metering pumps. Columns D and F were sterilized with panacide. Aeration was initiated on day 58 for columns A through D and day 15 for columns E and F. Each column, except G, was aerated vigorously for 8 hours, 5 days per week; leaching proceeded the remainder of the time. Table 24 summarizes column conditions.
For the first month of operation the leach solutions were monitored daily for pH changes, and H2SO4 was added to attain the established pH range.
Sample collection for chemical and bacterial assays were performed as for Series I tests. No molybdenum analyses were done for these column experiments. Copper removal was by cementation as described previously.
Bacterial Assays
Samples collected from the leach units were subjected to qualitative and quantitative bacterial determinations.
For culturing of leach solution samples aliquots were diluted in 9K medium, if necessary, to obtain bacterial numbers in the range of the MPN tables, or the aliquots were added directly to culture tubes for the MPN test. For culturing of T. ferrooxidans, 0.2% ferrous iron was added, and the cultures were incubated at room temperature. Sulfolobus and “ferrolobus” were cultured in 9K medium with elemental sulfur and yeast extract, and the cultures were incubated at 60°C. The thermophilic thiobacilli were cultured in 9K medium and 0.2% ferrous iron supplemented with 0.02% yeast extract. When grab samples of the ore were cultured for thiobacilli and Sulfolobus/”ferrolobus”, 10-g samples were weighed into bottles containing 90 ml 9K. After vigorous shaking, aliquots were removed and cultured as described above. The ore particles in columns A and B (Series I) and columns E and F (Series II) were too large for easy manipulation, so a number of the ore pieces were coarsely crushed before weighing.
Chemistry
Ferrous and total iron concentrations were determined by titration with potassium dichromate. pH was measured with a Coming model 12 pH meter end Eh by a Fisher Accumet model 140 pH meter. Copper, zinc, total iron, nickel, lead, calcium, and magnesium were analyzed using a Perkin Elmer 303 atomic absorption spectrophotometer. Ores were digested and analyzed by techniques outlined by Brandvold. Molybdenum concentrations were, obtained using the thiocyanate assay. “Free” acidity was measured by titration with sodium carbonate using methyl orange indicator. Potassium iodide was added to reduce Ferric iron to the colorless, ferrous state and to precipitate copper as cuprous iodide.
Solvent Extraction (SX) Reagent Tests
Dense suspensions of Sulfolobus and “ferrolobus” were prepared as described for obtaining inoculum for the leach columns. The protein concentrations of these suspensions were determined by the Folin phenol method and cell numbers were established by the 3-tube MPN process.
Eleven solvent extraction reagents, KELEX 100, KELEX 120, LIX 64N, LIX 65N, LIX 54, di-2-ethyhexyl phosphoric acid, Alamine 336, Adogen 364, Isodecyl alcohol, N-butyl alcohol, and Warsol, were prepared in 0.001%, 0.01%, and 0.1% concentrations of 9K. The pH was adjusted to 2.5. The effect of these 11 reagents at these three concentrations on the oxygen uptake of Sulfolobus and “ferrolobus” was examined at 60°C using a Gilson Differential Respirometer according to standard manometric techniques. All oxygen uptake data were standarized to Q02 — microliters oxygen uptake per milligram protein per hour. Every experimental run included one control containing no solvent extraction reagent.
The Q02 value obtained for this control test was divided into the Q02 values observed for the experimental tests; the quotient was multiplied by 100 to obtain the percent activity. This manipulation normalized the, data so that all experimental runs could be compared directly. Each reagent concentration was tested at least three times for its affect on each of the two organisms, and the results of these replicate tests were averaged.
Leaching Results
Ore Analyses
Series I tests (1977-78):
Analyses for 8 elements including calcium and magnesium as carbonates and silicates are tabulated in Table 1 for the -2 inch + ½ inch head sample of Pinto Valley ore leached in the Series I tests. Analyses for copper, total iron, and zinc of the four size fractions of this ore are recorded in Table 2. From the size fractions the weight percent of each seive size, the pounds of each mesh size, and the pounds copper, iron and zinc were computed and recorded in Table 3. Tables 9, 10, and 11 contain analytical and weight fraction data for the – ½ inch + 50 mesh head sample of Pinto Valley ore.
Series II tests (1978-79):
The analyses of Pinto Valley, San Manuel, and Cypress Bagdad ores for copper, iron, nickel, molybdenum, lead, zinc, calcium (as carbonate), calcium (as silicate), magnesium (as carbonate) and magnesium (as silicate) are presented in Table 25. All results are the average of triplicate assays. The weight percent of size fractions in head samples of the three test ores used in Series II tests are recorded in Tables 26, 27, 28, and 29.
Flask Leach Study
Table 30 summarizes the results of the flask leach experiment on Pinto Valley, San Manuel, and Cyprus Bagdad ores. It was necessary to add H2SO4 to flasks 1 and 2 to maintain a pH of about 1.6. Acid was not added to any other flasks. The pH values at day 16 in all flasks were nearly the same as when the experiment was initiated; however, by day 30, the pH values of all flask contents had decreased. The Eh values for all inoculated reactors, except 1, were higher than for the corresponding controls. The percents copper leached from inoculated ores were uniformly higher than from sterile ores. The amount of total iron in solution was variable for different ores with no specific pattern observed. Soluble ferrous iron was also variable with reactors 3 and 5 having minimal ferrous Iron in solution after 30 days.
Column Leaching Studies
Series I tests (1977-78):
Columns A and B. Throughout the experimental period the analyses for copper, zinc, and total iron in the leach liquors of columns A and B were converted to percents of the metals extracted and recorded in Figures 1, 2, and 3. Figure 1 shows that after 267 days of leaching 3.4% of the copper was leached from the inoculated column A containing Pinto Valley ore sized to -2 inch -+ ½ inch. This value does represent a decrease from the 7.4% copper extraction noted on day 213. In the uninoculated control B after the same length of leaching time, 2.2% of the copper was extracted. A slight decrease in the percent copper extracted was observed following day 213. Between day 0 and day 213 the rate of copper extraction for column A was 2.3 mg per liter per day, and 0.7 mg per liter per day for B. Copper extraction rates for the entire 267 day period were 0.8 mg per liter per day for A and 0.5 mg per liter per day for B.
Figure 2 details the percent of zinc extracted from columns A and B during the 267-day leach period. From A 15.5% zinc was extracted for an overall rate of 0.3 mg per liter per day. A decrease in rate, and hence extraction, was observed between 213 and 267. From column B 25% zinc was removed at a rate of 0.4 mg per liter per day; at day 213 28% zinc had been extracted with a decline in rate noted after that time.
Since copper was not cemented from columns A and B and no iron was added, the iron extraction could be calculated. Figure 3 depicts percent iron extraction over the 267-day leaching period. Rates were variable. The final percent iron extracted from column A was 0.08% for an overall rate of 0.2 mg per liter per day. For column B, the control, 0.3% iron was extracted at a rate of 0.8 mg per liter per day. Figure 4 illustrates the variability in the concentrations of total iron and ferrous iron in the leach liquor of column A. The highest concentration of total iron observed during the 267-day experiment was 190 mg per liter. Ferrous iron concentrations were generally recorded to be half the value of the total iron concentration. The total iron values for column B (Figure 5) were much greater than those observed for column A with the concentration reaching nearly 450 mg per liter at day 198. Ferrous iron values were also higher for column B than A, and values were extremely variable throughout the leach period.
Acidity was maintained at 0.75 g H2SO4 per liter by addition of 88 ml H2SO4 to column A and 64 ml H2SO4 to column B during the 267 day period. Consequently the pH ranged between 2.2 and 2.4, but extremes of 1.7 and 2.7 were observed several times in both columns during the experiment.
Eh values were recorded during the first 113 days of the experiment. These values for column A ranged from +616 mv at the initiation of the experiment to +816 mv at day 50. Most values were between +750 and +800 mv. For column B the Eh started at +616 mv and increased to a high of +708 mv. The overall values for B were lower than A with the general range between +630 and +670 mv.
Table 22 summarizes the numbers and presence of T. ferrooxidans in all reservoirs and Sulfolobus arid “’ferrolobus” in the reservoirs and column effluents of all leach units. In reservoir A. T. ferrooxidans were observed to increase in number throughout the leach period, whereas the numbers of thermophilic organisms declined. This same trend was observed with the thermophiles in the effluent from column A. In leach unit B neither T. ferrooxidans nor thermophiles were observed in the reservoir or column effluent.
After removal of the ore from columns A and B, the material was re-sized and chemically analyzed. Table 1 lists the analyses for 8 elements in split samples from the tails of both columns. From the split tails data the percents copper extracted from A and B were 6.9% and 0%, respectively. These data indicate no zinc extraction from either column, and 36% and 38.5% iron extraction for A and B, respectively. Other percent extraction values for A include 0% lead, 33% nickel, 25% molybdenum, 16.7% total calcium and 11.8% total magnesium. For B percent extractions were 0% lead, 33% nickel, 75% molybdenum, 0% total calcium, and 26% total magnesium.
Table 4 details chemical analyses for copper, total iron, and zinc performed on four size fractions of the tails in column A; Table 5 supplements Table 4 with data on weight of fractions and the pounds of copper, iron, and zinc remaining in each fraction. When Tables 4 and 5 are compared with Tables 2 and 3, extraction of the three metals from the various size fractions can be made and the decrepitation of the ore can be determined. Over half of the six selected fractions in column A were broken down resulting in larger fractions in all of the other three size categories with the -1.5 inch + 0.75 inch fraction gaining the largest percent of the material. From the size fraction analyses the percents extraction from column A were 9.4% copper, 16.3% iron, and 5.5% zinc. In column B (Tables 3 and 7) no particular fraction size was noticeably degraded with each fraction being reduced several weight percent and the residual deposited in the -0.525 inch fraction. From the size fraction data supplied on Tables 2, 3, 6, and 7 the following percents of metals were extracted from column B: 6.4% copper, 15.8% iron, and 0% zinc.
Table 8 summarizes biological data collected from grab samples collected at the top, middle and bottom portions of columns A and B. In all three sample locations in column A. T. ferrooxidans were found in excess of 10 per g of ore. Also present in these same samples were the thermophilic bacteria Sulfolobus and “ferrolobus”; thermophilic Thiobacillus were observed in the sample collected at mid column. In column B, T. ferrooxidans were noted in samples collected at the column middle and bottom; no thermophiles were cultured from column B, and no culturing was done for the thermophilic Thiobacillus.
Columns C and D. Figure 6 graphically illustrates copper extraction from columns C (inoculated) and D (control) during the 286-day experimental period. After a rest/leach cycle with aeration was initiated on day 32, the rate of copper extraction in column C was seen to increase. The copper extraction rate prior to the rest/leach/aeration cycle was 3.2 mg per liter per day, and following initiation of the cycle, the rate was 14 mg per liter per day — a rate increase of 4.4 times. In column D the initial rate of copper extraction was 1.2 mg per liter per day, and after initiation of the special cycle, the rate was 3.7 mg per liter per day — an increase of 3 times. Copper was cemented from the liquor of leach unit C four times. The final percent copper extraction after 286 days from C was 45.5% according to data collected from leach solution analyses. From the control column D 12% copper was extracted.
Figure 7 shows the zinc extraction during the 286-day experimental period. The rest/leach/aeration cycle enhanced zinc extraction, but considerable-variation in the percents extracted were observed during the experiment. The overall rates observed for zinc extraction were 0.8 mg per liter per day for column C and 0.7 mg per liter per day for column D. The final percents of zinc extracted were 42.3% from C and 36.4% from D.
The percent iron extracted was not calculated because ferrous iron was added to the leach liquor of C during cementation and comparable quantities of iron were added to D at the same time. Figures 8 and 9 illustrate the ferrous and total iron concentrations observed during the experiment. Total and ferrous iron values increased greatly during cementation of copper in C (Figure 8), but both values rapidly decreased. Similar results were noted for D (Figure 9) when added ferrous iron concentrations rapidly diminished. In one instance, day 138, iron was not added to leach solution D during copper cementation from C.
The acidity values of leach liquors C and D were maintained at 0.75 g H2SO4 per liter by the addition of 92 ml concentrated H2SO4 to C and 95 ml to D. No acid was required for either column after the 150th day of leaching. The pH of leach liquor C varied from 1.68 to 3 but generally remained above 2.0. After day 214, the pH decreased to 1.8 and thereafter remained below pH 2. The pH of leach liquor D varied from 1.55 to 3.11, but the low pH phenomenon observed in C was not noted in D.
Eh values of leach solutions C and D were recorded through day 113. The values for C ranged from +366 mv, immediately following cementation, to +735 mv. Values were generally between +650 mv and +700 mv. For leach solution B the lowest Eh value recorded was +578 mv and the highest was +704 mv. In general Eh was between +600 mv and +650 mv.
Table 22 summarizes the numbers of bacteria observed in the reservoirs and effluents of columns C and D. T. ferrooxidans increased in numbers in reservoir C over the experimental period; Sulfolobus/”ferrolobus” numbers in reservoir C diminished during the 286 days as did their numbers in the effluent from column C. No organisms were obtained during culturing of the reservoir and effluent contents of column D.
During the removal of tails from columns C and D it was observed that the leached ore was yellow near the column top and gray toward the bottom. A yellow precipitate was noted where the heat tapes had been. Chemical analyses of the splits of the heads and tails of the material in columns C and D are found in Table 9. From these data the percents extraction for the elements tested in column C were 58.6% copper, 0% zinc, 0% lead, 66.7% nickel, 0% molybdenum, 0% total calcium, and 41% total magnesium. For column D the percents extraction were 34% copper, 50% zinc, 0% lead, 66% nickel, 70% molybdenum, 0% total calcium, and 33% total magnesium.
The decrepitation of the ore in column C was ascertained by comparing weight percents in Tables 11 and 13. Size fractions diminishing in weight percent over the leach period included -0.525 inch + 0.375 inch, -0.375 inch + 3 mesh, -4+6, -6+8, and -35+50. From the size fraction analyses (Tables 10, 11, 12, and 13) the percents of copper and zinc extracted from column C were 46.7% and 41.5%, respectively. Although iron values decreased during the leach period, the percent extraction was not computed, since iron was added during cementation. For column D decrepitation occurred in the same mesh sizes of ore as column C (Tables 11 and 15). Size fraction analyses (Tables 10, 11, 14, 15) showed the following percents extraction: 16.7% copper and 33% zinc. Iron was not calculated due to the addition of iron during the leach period.
Grab samples collected from column C at the time of removal of the tails and cultured (Table 16) showed that T. ferrooxidans were dispersed throughout the column in excess of 10³ per g. Sulfolobus/”ferrolobus” numbers exceeded 10 6 per g throughout the column. The thermophilic thiobacilli were found in the middle and bottom sections of column C. No organisms were found in column D.
Columns E and F. Figure 10 depicts the percents of copper extracted from columns E (inoculated) and column F (control) during a 288-day leaching experiment. A 4-day leach/3-day rest cycle was initiated on day 92. Copper was removed three times from column E by cementation. After 288 days, 39% of the copper was extracted from E at an overall rate of 10.9 mg per liter per day. Prior to the leach/rest cycle the rate was 7.5 mg per liter per day, and following initiation of the cycle, the leach rate was 12.5 mg per liter per day. For column F the rate of copper extraction before the leach/rest cycle was 1.0 mg per liter per day and 1.9 mg per liter per day after the cycle; the overall rate was 1.6 mg per liter per day. The percent copper extracted after 288 days was 5.7%.
The extraction of zinc is illustrated in Figure 11. The rates were variable over the 288-day leach period with 47% zinc extracted at an overall rate of 0.9 mg per liter per day from column E. From column F the control, 37% of the zinc was extracted at a rate of 0.7 mg per liter per day. Before initiation of the rest/leach cycle the rate of zinc extraction from column E was 2.3 mg per liter per day; after day 92 the rate was 0.25 mg per liter per day. For column F the zinc leach rate up to day 92 was 1.8 mg per liter per day, and following day 92 the rate was 0.2 mg per liter per day.
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The percents extraction of iron during the leach period were not plotted since ferrous iron was added to column E during cementation, and comparable quantities of iron were added to the control column. Figures 12 and 13 illustrate the quantities of ferrous and total iron in the leach liquors of columns E and F during the 288-day period. In column E (Figure 12) total iron values remained less than 0.4 g per liter until cementation when the iron concentration was nearly 3 g per liter. The concentration of ferrous iron remained low during the initial leach period.
The ferrous iron concentration decreased rapidly immediately following cementation. Similar results were observed immediately following cementation. Similar results were observed for the control, column F, with iron concentrations not exceeding 2.7 g per liter. The ferrous iron concentration was nearly equal to the total iron concentration during the entire experiment. Total and ferrous iron concentrations rapidly decreased immediately following addition of ferrous iron to the leach solution.
The acidity of columns E and F was maintained at 0.75 g per liter by the addition of 94 ml of concentrated H2SO4 to column E and 103.5 ml to column F. The pH was maintained between 2.2 and 2.5; however, the pH of both columns did reach a high of 2.8 and a low of 1.7. During the first 105 days of leaching when the Eh was measured, the Eh of column E reached a low of +594 mv and a high of +782 mv. The column F Eh was recorded between +574 mv and +702 mv. The Eh of column E leach solution was generally higher than that of column F.
Table 22 summarizes the bacterial counts obtained from cultures of the reservoir and effluent solutions of columns E and F. The numbers of T. ferrooxidans in the column E reservoir throughout the experimental period remained low while the numbers of Sulfolobus/”ferrolobus increased. The numbers of the thermophiles in the effluent solution of column. E were variable over the leach period with numbers ranging from 2.4 per ml to 1.2 x 10 4 per ml. No organisms were cultured from the reservoir and effluent solutions of column F leach unit excepting a positive indication of T. ferrooxidans development at day 207 in the reservoir.
When the ore was removed from columns E and F after the leach period, it was noted that the tails tended to be gray colored near the bottom while the tops of the column were more yellowish in color.
Grab samples of the ore from column E indicated the presence of ferrooxidans in excess of 10³ per g in samples from the top, middle, and bottom portions (Table 21). Sulfolobus/”ferrolobus” were present in large numbers in the top and middle portions but found in smaller numbers in the bottom of the column. Thermophilic thiobacilli were obtained in cultures from the top and middle samples but not from the bottom. No bacteria were cultured from any samples collected from column F.
Table 9 summarizes analytical data collected on split samples from the head and tails of ore from column E and F. These data show the following percents extraction for column E: 41.4% copper, 50% zinc, 0% lead, 67% nickel, 30% molybdenum, 0% calcium 26% magnesium. From Table 9 the following extraction data can be realized for column F by comparing the head and tails analyses: 21% copper, 0% zinc, 0% lead, 67% nickel, 30% molybdenum, 0% calcium, and 41% magnesium. Iron was not considered since it bad been extraneously added to both columns.
Tables 17 and 19 tabulate analytical data collected on the size fractions from the tails removed from columns E and F. Tables 18 and 20 present the weight percents of the size fractions by utilizing Tables 17 and 19 data to obtain quantities of the metals remaining in columns E and F tails. Comparing the size fraction data in Tables 17, 18, 19, and 20 with similar data for the head sample (Tables 10 and 11) the percents of the metals extracted can be obtained. The extraction of copper and zinc from column E was 37% and 35%, respectively. For column F 20% copper and 37% zinc were extracted.
Comparing Tables 11, 18, and 20 the decrepitation of the ore during the leach period can be noted. In both columns the fractions losing the largest percentages of particles were -0.575 inch + 0.375 inch, -4+6 mesh, -6+8 mesh, -35 + 50 mesh. Additionally column F lost particles from the -0.375 inch + 3 mesh fraction.
Series II tests (1978-79):
Table 24 defines the conditions established for each column, and Table 26, 27, 28 and 29 show the distribution of mesh sizes for the ores used.
Columns A and B. Initially the pH of the distilled water leach solution was maintained at pH 2.2, but on day 120 the pH was decreased to 1.6. Up to day 120, 26 ml of concentrated H2SO4 had been added. To maintain a lower pH an additional 48 ml of acid were added between day 120 and the completion of the experiment on day 228. Despite the continued additions of acid, the pH remained near 2.0 with a deviation to 1.6 between days 225 and 246, Column B was started at a pH of 1.6. Although 107 ml of H2SO4 were added during the 288-day experimental period, the pH stabilized near 2.0.
Figure 14 illustrates copper leach rates and percents extracted over the experimental period. Both columns were inoculated, but column A released the greater concentration of copper. The rate of copper extraction from A during the first 120 days when the pH was maintained at about 2.2 was 6.5 mg per liter per day; from day 120 through 288 the rate was 16.2 mg per liter per day. The overall copper extraction rate for column B was 8.0 mg per liter per day.
Figure 15 illustrates the total iron and ferrous iron contents of the leach solutions in columns A and B during the leach period. Iron concentrations were maximum during cementation of the copper from both columns. Ferrous iron values in column A remained lower than values in B. The total iron concentration in A was much higher than in B during the latter phase of the study.
Eh values were generally higher for column A than B, and these values were directly related to concentrations of ferrous and ferric iron in solution.
The zinc concentrations in columns A and B remained nearly the same throughout the experiment with approximately the same percents extracted at the termination of the experiment (Figure 16). The overall rates of extraction for A and B were 0.9 and 0.8 mg per liter per day, respectively.
A somewhat higher concentration of nickel was extracted from column A than from B (Figure 17), although a decrease was observed in A during the latter part of the experiment. An increase in nickel extraction was observed in A following the deliberate decrease of pH at day 120.
Table 31 tabulates the chemical analyses of the leached ore (tails) for columns A and B. Comparison of these data with analyses of the head sample (Table 25) shows the amounts of metals extracted based on assay of the tails. For metals initially present in low concentrations in the ore the data do not compare well with the results obtained from analyses of the leach solutions (Figures 16 and 17). Comparison of tails analyses (Table 32) with solution analyses (Figure 14) for copper compare well for column B. However, tails assay for A indicated 30% extracted whereby solution assay signified about 49% extraction.
Throughout the experiment bacterial analyses of the leach solution indicated low numbers of all microbes (Table 33), and analyses of the tails confirmed fairly low populations in the ore (Table 34). Much higher populations of both T. ferrooxidans and thermophiles were noted in column A solution and ore.
Size analysis of the tails (Table 26) showed that the larger ore particles were in less abundance in column A than B.
Columns C and D. Column conditions for columns C and D are tabulated in Table 24. Columns C and D were initially maintained at pH 2.2, but on day 121 the pH was decreased to 1.6 in each column. Total quantities of H2SO4 added were 143 ml for column C and 151 ml for column B. Despite continued acid additions the pH remained near 2.
Figure 18 illustrates the copper extracted from the San Manuel ore based on analysis of the leach solution. Copper extraction based on analysis of the tails (Table 32) compare well with the solution data. Decrease of the pH on day 121 increased the extraction rate (Figure 18). Rates between days 0 and 121 were 18 and 6 mg per liter per day for C and D, respectively. Between days 121 and 274 the rates were 69 and 17 mg per liter per day, respectively.
Figure 19 illustrates the iron concentrations in column C and D solutions. Increases in total iron and ferrous iron were observed during cementation with rapid decreases immediately following. Column D, the control, had a higher iron concentration overall. The concentration of ferrous iron in the inoculated column C was almost non-detectable by our assay during the course of the experiment. Eh values in column C were generally greater than +700 mv, whereas the Eh values in D were in the +600 mv range. Variations followed the soluble iron concentrations.
Figure 20 illustrates zinc extraction. Nearly the same percentages were extracted from both columns, and the rates were 0.2 mg per liter per day for both C and D.
Somewhat higher nickel concentrations were noted in the leach solution D than in C. The extraction pattern can be observed in Figure 21.
Results of analyses of the tails are reported in Table 31. These results can be compared with heads assays in Table 25. Analyses of elements originally in the ore in low concentrations did not compare well with analyses of the tails. Iron extractions cannot be computed due to the addition of iron during cementation.
Tables 33 and 34 summarize bacterial numbers found in the leach solutions and tails of columns C and D. Column C had both T. ferrooxidans and thermophilic bacteria present. Column D, the control, remained sterile.
Table 27 illustrates the size fractions of ore present in the head sample of Son Manuel ore and the tails in columns C and D. The ore did not demonstrate noticeable decrepitation.
Columns E and F. Leaching conditions in columns E and F are explained in Table 24. Figure 22 illustrates the extraction of copper from Cyprus Bagdad ore. This ore was originally sized to -2 inch + ½ inch, and during the course of the experiment the ore was removed twice and resized (Table 24). The size distribution data ate tabulated in Table 28. During the first 86 days when the ore was sized to -2 inch + ½ inch the copper extraction rates were 0.7 and 0.4 mg per liter per day for columns E and F, respectively. After the resizing, the extraction rate increased to 4.0 mg per liter per day for E and 1.6 mg per liter per day for F between days 86 and 183. After the final resizing the rates of copper extraction were 7.2 and 6 mg per liter per day. Table 32 lists the percent extraction of copper from these columns as ascertained by assay of the tails. There is absolutely no correlation between these data and the values obtained by analysis of the leach solutions.
The pH of the leach solutions was maintained at about pH 2.2 for the duration of the experiment. A total of 41 ml and 37.5 ml H2SO4 were added to columns E and F, respectively, to maintain the pH.
Figure 23 shows the pattern of iron concentrations in the leach solutions. There was a gradual increase in soluble iron with time, but both columns had approximately the same amount of ferrous and total iron in solution during the leach period.
Initially column E had higher Eh values than F, but after the first resizing of the ore, the Eh decreased to values of about +600 mv. These were comparable to values observed for F throughout the entire experimental period.
Figure 24 shows that zinc extractions for columns E and F were similar. Increases in rates were observed in both the inoculated and control units immediately following crushing.
The soluble nickel values for column E were variable with a very big decrease occurring at day 183. However, the final nickel extraction values were similar for E and F.
There was little correlation between results of analyses of heads and tails for the Cyprus Bagdad ore (Tables 25 and 31).
Culturing of the leach solutions of units E and F throughout the experimental period indicated that thermophiles were present at day 43 in the reservoirs, but the number declined. T. ferrooxidans numbers increased with time. Column F reservoir had T. ferrooxidans present (Table 33). Cultures of the tails at the time of completion of the experiment showed no organisms in column P and low numbers of T. ferrooxidans and Sulfolobus/”ferrolobus” in E (Table 34) .
Table 28 illustrates the distribution of particle sizes present in columns E and F during the experimental period. During the crushing procedure some ore was lost.
Column G. The conditions established for column G are described in Table 24. The pH was maintained at about 2.2 for 381 days by the addition of 8.5 ml H2SO4. The pH was then decreased to 1.6; to attain this an additional 43.5 ml H2SO4 was added during the final 161 days of experimentation.
Figure 26 illustrates the copper extraction. The final percent extracted, as determined by assay of the solution compares well with the amount extracted as noted from assay of the tails (Table 32). The rate of copper extraction during the first 381 days of leaching at pH 2.2 was 1.5 mg per liter per day. When the pH was decreased to 1.6 the extraction was 0.1 mg per liter per day.
Figure 27 shows the concentrations of ferrous and total iron in the leach solution of G. The total iron concentration steadily increased, but the amount of ferrous iron in solution was very small. The Eh of the solution stabilized at a little over +800 mv.
Analyses of the leach solution indicated that about 40% of the zinc had been extracted (Figure 28) but tails analyses (Tables 25 and 31) are not refined enough for direct comparison.
Periodic assay of the reservoir solution indicated a large population of T. ferrooxidans (Table 33). The tails also contained a large number of these organisms (Table 34).
There did not appear to be a substantial decrepitation of the ore during the leach period (Table 29).
Solvent Extraction (SX) Tests
Table 33 summarizes the results of the effect of eleven solvent extraction reagents at concentrations of 0.1%, 0.01%, and 0.001% on Sulfolobus acidocaldarius and “ferrolobus,” Qo2, the microliters of oxygen uptake per ml protein per hour, was converted to percent activity to enable data comparison. The respiratory activities of both Sulfolobus and “ferrolobus” were slightly inhibited at 0.1% concentrations of KELEX 100, LIX 64N, Alamine 336, and isodecyl alcohol. At 0.1% concentrations of Adogen 364 and Warsol the respiratory activities of both organisms were nearly completely inhibited. Di-2-ethylhexyl phosphoric acid was very inhibitory with a concentration of 0.001% reducing Sulfolobus respiratory activity by one-half and nearly totally inhibiting the respiratory activity of “ferrolobus.” LIX 64N at concentrations of 0.001% and 0.01% and LIX 54 at 0.1% increased the respiratory activity of Sulfolobus.
Discussion
Flask Study
The flask leach study was primarily designed to ascertain the leachability using distilled water of the San Manuel and Cyprus Bagdad ores and the problems of maintaining a pH of 1.6 with the Pinto Valley ore. No noticeable problems were observed with leaching of the previously untested ores, and the acid consumption noted in later column studies with the San Manuel ore was not seen in the flask studies. This was probably due to the low ratio of ore to liquid used in the flask reactors. A problem with stabilization of pH at 1.6 was observed with flask studies on the Pinto Valley ore, and this problem was borne out in the Series II column study. The use of distilled water presented no problem for the activity of the thermophilic microbes. Apparently, enough nutrients are leached from the ore to sustain their development.
Column Leach Studies
Series I tests (1977-78):
Copper. The rapid decline in copper concentration observed in the leach solution of column A (Figure 1) after day 200 was inexplicable. If the extraction value for copper obtained from leach solution A is compared with the values obtained when the column A tails were analyzed, it does appear that the 7.4% extraction value noted at day 213 was correct. It is likely that following day 213 precipitation of copper occurred in the leach solution of reservoir A. The copper concentration in the leach solution of column B was not observed to decline. Copper analyses of the tails from column B indicated low extraction, although these results did not directly compare with the result obtained by analysis of the leach solution. The extraction of copper was much less from column A than from columns C and E which were also inoculated, but contained particles of a smaller mesh size. It is well known that larger particles leach more slowly. In this particular study it may be that the available copper was readily leached from the surface and the remaining copper was inside the large particles and unavailable for extraction. Alternatively, the ferric iron which precipitated due to the high temperature may have coated the ore and resulted in a diminished extraction rate.
Copper was extracted from column C ore rapidly and steadily. The rest/leach cycle with recurrent aeration was initiated early in the experiment, so the increased rate change that was noted may not be entirely indicative of the increased leaching due to the novel cycling. Comparison among the percents extraction from the leach solution, the split fraction, and the sieve fractions compare well. The final amount of copper extracted from C was only slightly greater than that extracted from E, the unaerated column, indicating that the recurrent aeration did not greatly enhance copper extraction. When the extraction of copper from C was compared with results obtained in a 1975-76 experiment in which – ½ inch + 50 mesh Pinto Valley ore was column leached without a rest/leach cycle or recurrent aeration, it was noted that 49% of the copper was extracted in 202 days. This result compares favorably with the 45.5% copper extraction obtained from the present study. This further suggests that the rest/leach cycle with recurrent aeration does not enhance copper extraction. From column D, the aerated, control with a rest/leach cycle, 12% copper was extracted over the 288 day experiment. This figure compares well with the analyses of the size fractions, but doesn’t compare well with the split fraction analysis of the tails. Recurrent aeration coupled with a rest/leach cycle did not appear to enhance copper extraction from the uninoculated control, since similar low copper values were obtained from the unaerated column F (5.7%) and from a continuously leached control run in 1975-76. The initiation of the rest/leach cycle on day 32 did enhance the copper extraction rate by three times, but the initiation of the cycle early in the experimental period may not have given the column a chance to establish a regular pre-rest/leach cycle rate.
Columns E and F were subjected to a rest/leach cycle without aeration on day 92. Initiation of this cycle enhanced the copper extraction rate from E by one and a half times. The overall amount of copper extracted from column E was 39% a value relatively close to that extracted from column C and from the continously leached Pinto Valley ore run in 1975-76. The 39% extraction value compares closely with the extraction values obtained from analyses of the tails. From column F 5.7% copper was extracted. This value is lower than those values obtained by tails analyses and from column D, the recurrently aerated column, and from a continuously leached column. After initiation of the rest/leach cycle, the rate of copper extraction from F increased two-fold.
In summary the presence of Sulfolobus and “ferrolobus” in the low-grade Pinto Valley chalcopyrite ore enhanced copper extraction by four to six times. Particle size greatly affected leaching with large particles releasing less copper. A rest/leach cycle with or without recurrent aeration did not influence the overall extraction of copper.
Zinc. After the 267-day leach period, 10% more zinc was extracted from the control column B than from the inoculated column A; however, this was primarily due to the precipitation of zinc which occurred in A after day 213. It is not clear from the tails analyses whether this precipitation from the leach solution occurred in the reservoir or in the ore, since there was considerable discrepancy in the results. Zinc extraction was inhibited by the increased particle size as evidenced by the greater zinc extraction observed in columns C, D, E, and F and earlier experiments, all of which used – ½ inch + 50 mesh Pinto Valley ore.
Nearly equal percentages of zinc were extracted from columns C, inoculated, and D, control. Following initiation of the rest/leach cycle a profound increase on zinc extraction was noted; however, when Figure 7 is compared with Figure 11, the graphs exhibiting zinc extraction from columns E and F, the same increase is noted during the first 80 days of leaching. The initiation of a rest/leach cycle in columns E and F on day 92 did not enhance zinc extraction; in fact, this cycle diminished zinc solubilization. The final amounts of zinc extracted from columns E and F were 47% and 37%, respectively, representing values nearly identical to those from columns C and D.
The zinc extraction data obtained from analyses of the tails did not always correspond closely with that observed from analyses of the solutions. This probably resulted from the small quantities of zinc present and the consequent error in analyzing.
In general it can be stated that bacterial activity by the thermophiles does not enhance the extraction of zinc.
Iron. Since no Iron was added to columns A and B leach solutions, the percent iron extracted from the ore was calculated and depicted (Figure 3). Considerably more Iron was observed to be in solution in the control, B, than in the inoculated system A.
In A the ferrous iron concentration was approximately half of the total iron concentration (Figure 4), and in B the ferrous iron concentration was closer to the total iron concentration (Figure 5). This probably resulted from the bacterial population’s iron-oxidizing activity, and it is likely that the amount of iron in solution A was lower because the bacteria continuously oxidized iron which rapidly precipitated due to the decreased solubility of ferric iron at 60°C. The high ferrous iron value observed at day 30 in Figure 4 can only be attributed to experimental error.
In columns D and F ferrous iron was added in amounts equivalent to that which went into solution in columns C and D during copper cementation. The increases in iron concentration are readily observed in Figures 8, 9, 12, and 13. Ferrous iron concentrations were roughly half total iron concentrations throughout the entire experimental period. Immediately following cementation or ferrous iron addition, the iron concentration rapidly diminished due to oxidation and subsequent precipitation:
Other Metals. Nickel extraction from columns A and B was 33% each, and from the other four columns It was 67%. This lower extraction value for A and B again represents the diminished extraction rate from the larger particles. It is obvious that thermophilic bacterial activity does not contribute to nickel extraction, since these values were equal for experimental and control columns.
No lead was found to be extracted, however, this data may be deceptive since lead can be leached but is immediately redeposited in an acid environment as insoluble lead sulfate. Whether the bacteria contributed to solubilization of galena in this ore was not evident from these experiments.
The dissolution of molybdenum has been found to be dependent on the amount of iron in solution. It is likely that the values noted for molybdenum extraction from columns A and B (25% and 75%, respectively) were reflected by the high iron concentration in B (Figure 5) and somewhat lower concentration in A (Figure 4) at the time of experimental termination. Analyses of the tails indicated 0% molybdenum extraction from C and 70% from D. Examination of the iron in solution at the time the experiment was ended suggests that the molybdenum extracted should have been similar for both. More extensive analyses may have indicated this. The molybdenum data for columns E and F showed 30% extraction from both columns. These results are borne out by examination of iron concentrations on day 288 (Figures 12 and 13). Although it is known that the thermophilic bacteria enhance molybdenum dissolution from molybdenite (32), it is not possible from the present column experiments to measure the bacterial contribution to molybdenum dissolution. Iron concentration appears to be a more important factor toward molybdenum solubilization. It is likely that greater amounts of molybdenum were solubilized than noted from analyses, but it was precipitated onto the ore during changes in iron concentration on solution.
Acidity and pH. Columns C, D, E and F all required the same amounts of acid to maintain the acidity at approximately 0.75 g H2SO4 per liter. Column A did require slightly more acid than did B for maintenance of a stable acidity. The reason for this is unapparent. Acid consumption, or lack of, does not appear to be biologically mediated.
A constant pH was generally maintained in all columns throughout the leach period; however, a decrease in pH was observed in column C at day 214, and the pH did not return to normal during the remaining time of the experiment. This phenomenon can not be readily explained, since similar chemical and biological reactions were occurring in column E and similar pH circumstances were not observed.
Eh. Eh was not monitored during the entire experimental period. Values taken early on correlated well with ferrous/ferric ratios. Increasing Eh in columns A, C, and E represented high ferric values due to increasing bacterial activity.
Bacterial Numbers. Results of quantitative analyses of bacterial numbers in the reservoirs and column effluents during the experimental leach period indicated that microorganisms readily developed in the experimental columns and the columns treated with panacide remained free of microbes (Table 22). In the reservoirs of leach units A and C the numbers of ferrooxidans steadily increased over the experimental period. This suggests that the temperatures in these reservoirs were not kept high enough to avoid development of these organisms. Such was not the case with reservoir E which had a very low number of T. ferrooxidans throughout the leach period. The continuously diminishing numbers of thermophilic microbes observed in the reservoirs of the inoculated leach units (Table 22) reflect the decreasing temperature. The numbers of thermophiles cultured from the effluents of the three experimental units also declined over the leach period (Table 22). This probably resulted from the development of a stabilized, attached bacterial population within the ore. It is likely that initially a large microbial population, both attached and free-swimming developed, but after depletion of nutrients, the unattached population diminished.
Culturing of the tails after completion of the experiment revealed large populations of T. ferrooxidans and Sulfolobus/”ferrolobus” in-habiting the same portions of column A. Since these organisms do have distinct and different temperature requirements, these results suggest that niches of widely varying temperatures occurred within the column which offered a suitable environment for these different microbial species. This hypothesis is further substantiated by the presence of the thermophilic thiobacilli which require a temperature of approximately 50° C, intermediary between that required for T. ferrooxidans and Sulfolobus/”ferrolobus.” T. ferrooxidans were observed in the lower regions of column B, the panacide control. This may have resulted from inadequate penetration of the panacide to the bottom of the column and the resistance of ferrooxidans to the bacteriocide. This also suggests that column B was not maintained at a high enough temperature to avoid development of these organisms. Since the ore particles in columns A and B were too large for bacterial culturing purposes, the ore was slightly crushed to facilitate the process. This technique, however, does somewhat invalidate quantitative evaluation of the bacterial numbers, so the data collected must be viewed accordingly. Development of ferrooxidans, Sulfolobus/ “ferrolobus,” and the thermophilic thiobacilli was noted in column C and E suggesting temperature conditions similar to column A. No growth of any bacteria was observed in columns D and F indicating adequate sterilization of these columns.
Ore Decrepitation. The largest fractions of the -2 inch + ½ inch ore in columns A and B were found to decrepitate. Since this occurred in both columns the size reduction was probably chemically mediated.
As in columns A and B, the largest size fractions in columns C, D, E and F were observed to break-down. This decrepitation was probably chemically mediated and not the result of bacterial activity, since it occurred in experimental and control columns alike.
Series II tests (1978-79):
Copper. Results obtained from column A indicate that using acidified distilled water as a leaching medium does not diminish activity of the microorganisms. There is probably enough phosphate, nitrogen and trace elements solubilized to meet the requirements of the organisms. The approximate 49% copper extracted from column A compares well with the 45.5% extracted from a comparable column leach test run in the Series I test (Figure 6). The rate of increase in copper production in column A following decrease in pH to 1.6 suggests that the lower pH does enhance the extraction from this ore. The results of copper extraction from column B were disappointing. Throughout the entire experiment there was indication that the bacteria were not developing properly and results of culturing of the tails (Table 34) substantiated this. The reasons for this occurrence are unknown, but temperature control may be one cause. It was difficult to maintain the columns at pH 1.6. This had been substantiated by the earlier flask studies (Table 30).
The acid consumption of the San Manuel ore in columns C and D can be attributed to the relatively high carbonate concentration of this ore (Table 25). The extraction value for the inoculated San Manuel ore (column C) as compared to the control column D (Figure 18) clearly indicates that use of the thermophilic microbes enhances the extraction of the copper from this recalcitrant ore.
Leaching of copper from the -2 inch + ½ inch Cyprus Bagdad ore (Figure 22) proceeded slowly until the ore was removed and crushed to a smaller size. The increase in extraction rate was again observed when the ore was crushed a second time. However, the final extraction values were disappointingly low. The thermophilic microbes appeared to somewhat enhance the extraction of copper from this ore, but the enhancement was not great. The column leaching results did not correlate with those obtained from the flask leaching study (Table 30). The repeated crushing of the ore may have adversely affected the growth and activity of the organisms. The discrepancy between the analyses of the tails and the leach solution can be partially accounted for by the crushing process. Some material was lost each time the ore was crushed. Also, analysis of large particles is difficult because of the inability to obtain a representative sample.
The heated, inoculated columns have been repeatedly contaminated with T. ferrooxidans due to niches within the columns which were cool enough to allow development of organisms. A 528-day column leaching experiment using Pinto Valley ore (Figure 26) showed that the organisms do make about a 10% contribution to the extraction of copper from this recalcitrant ore. This amount of copper was leached out during the first 250 days of leaching, and continued leaching did not increase the amount of copper extracted even when the pH was decreased.
Iron and Eh. The patterns of total and ferrous iron in the solutions of the leach columns were similar to those observed in all previous leaching experiments. A low ferrous iron concentration was indicative of high microbial activity. Eh values correlated well with iron in solution. A high ferrous value immediately following cementation depressed the Eh. The Eh in column G was higher than in the heated columns. This was due to the higher solubility of oxygen at the lower temperature.
Zinc. As shown in earlier column experiments the extraction of zinc was a result of chemical rather than biological leaching. Sphalerite is quite soluble at the high temperature, acid conditions of these experiments. There was a disappointingly low Correlation between the results of solution assays and tails analyses. This is undoubtedly due to the low quantities of zinc in the ore and the insensitivity of the atomic absorption assay to these concentrations.