Table of Contents
Diminishing worldwide reserves of high-grade ores requires research be conducted to develop methods to recover metals from low-grade ores. Low-grade ore deposits are often unprofitable to mine and process because of the rising costs of energy and expense to protect our environment. Biological processing can be quite selective, is environmentally benign, and has the potential for being low-cost. As a result, biological methods are gaining increased attention for treating low-grade ores. Sulfide ores are bioleached commercially by oxidation using Thiobacilli bacteria (Flett, 1991). oxides and silicates.Ehrlich (1980), Holden and Madgwick (1983), Groudev (1987), and Bosecker (1989) have performed studies evaluating microbes capable of leaching
Several stirred bioreactor studies have shown that isolates cultured from soils and microorganisms indigenous to ores are both capable of solubilizing manganese from oxide ores. In 1962, the Bureau of Mines identified four heterotrophic bacterial species capable of solubilizing 71 to 93 pct of the manganese from a domestic oxide ore. Manganese was precipitated from solution with lime (Perkins and Novielli, 1962). Madgwick (1987) examined heterotrophic bacteria that could solubilize manganese from pyrolusite ore tailings located at the Groote Eylandt mine in the Northern Territory of Australia. Madgwick describes two mechanisms for the microbial reduction of manganese dioxide: (1) direct, whereby the bacteria must come in contact with the mineral surface to enzymatically reduce the manganese dioxide; and (2) indirect, in which the microorganisms produce manganese dioxide-reducing organic acids during fermentantive metabolism. Madgwick recovered manganese from solution by reoxidizing the manganese(II) with a mixed culture of bacteria and a unicellular alga. Other manganese bioleaching studies have been reported by Ehrlich (1987), Hart and Madgwick (1986), and Silverio and Madgwick (1985). Although manganese was recovered from the bioleach solutions, the feasibility of recycling the manganese-depleted solutions back to the bioleaching operation was not evaluated.
In 1991, the U.S. Bureau of Mines evaluated heap bioleaching as a means of extracting manganese from low-grade oxide ore from the Three Kids deposit in Southern Nevada (Noble, Baglin, Lampshire, Eisele, 1991). Laboratory studies were carried out using columns to simulate heap leaching conditions. This study, which used indigenous microorganisms fed with molasses nutrient medium, showed that 99 pct of the manganese could be biologically extracted from the Three Kids ore. However, methods for recovering the manganese from solution were not developed.
Objectives of the current study were to evaluate methods for recovering the manganese from sugar-depleted column bioleaching media and to develop an integrated bioleaching-recovery system for recovering the manganese and recycling the residual solution. Recycling media to the bioleaching stage is a necessary water conservation measure. This is essential if heap bioleaching is to become a useful process for extracting manganese from its ores.
The ore used in this study was from the Three Kids deposit in Clark County, NV. The deposit location, physical features of the district, and outline of the geology are described by Johnson and Trengrove (1956). The ore contained manganese oxides in a gangue of quartz, feldspar, gypsum, and clay. X-Ray diffraction showed the ore to be primarily amorphous with pyrolusite (βMnO2) as the only identifiable oxide. Chemical analysis of this ore is shown in table 1. The ore was crushed to minus 6.3 mm and agglomerated with a 15-pct factory molasses solution on a rotating disk pelletizer.
Diluted factory molasses media were used in the column tests as a source of nutrient for microorganisms resident to the ore. Factory molasses is an inexpensive by-product of sugarcane processing. Molasses contains the disaccharide sugar, sucrose, that is hydrolyzed during bioleaching into simple sugars, glucose and fructose, which are utilized by the microorganism. The media were prepared by diluting the molasses to a concentration of 5 pct (w/v) with distilled water. No additional nutrients were added. The initial pH of 5 pct molasses medium was about 5.2.
Ore was leached at ambient temperature (22°±3° C) in 50-mm ID glass columns capped with lids containing a small hole to which a 0.22 µm filter was attached. The filtered openings allowed some air into the column but prevented the entrance of airborne microbes. In each column test, 500 g of agglomerated ore was bioleached by circulating 5 L of 5-pct factory molasses medium through the ore at a rate of 4.8 mL/min (2.47 L/min-m³) . Manganese was extracted from the ore as long as an adequate amount of sugar was present in the nutrient medium. When sugar was depleted as a result of microbial metabolism, the solution was treated to remove the dissolved manganese.
After manganese removal, the sugar was replenished by the addition of 250 g of factory molasses to 5 L of solution. The replenished medium was reused in the bioleach stage. Molasses was added to ensure an ample nutrient supply for the microorganisms. This cycle of manganese solubilization, manganese removal from the media, replenishment of molassas to the media, and reuse of the media in the bioleach stage, was repeated until leaching ceased. Media samples were taken weekly and the pH, total carbon content, and metal concentration of the solutions were measured and recorded.
Media samples from the column tests, ore head and tail samples were analyzed for manganese and other metals by Inductively Coupled Plasma (ICP)/Optical Emission spectroscopy (OES). Total carbon content of the medium was determined by a high temperature combustion analyzer with infrared detection.
Precipitation: Various amounts of ammonium carbonate, oxalic acid, or lime were added to samples of bioleach medium to determine the quantities required to precipitate manganese from solution as MnCO3, MnC2O4, or hydrated manganese hydroxide, respectively.
Ion Exchange: Ion exchange tests were performed with many times excess of a weak cation exchange resin (Chelex 100). Adsorption of manganese from the molasses medium was performed by contacting the media with 250 g of ammonium form resin in a 2.5- and 4.5-cm-diam glass column. Manganese was stripped from the resin using 1N HCl and precipitated from the HCl solution as a carbonate salt using 2.5 times the stoichiometric amount of ammonium carbonate. The resin was regenerated with NH4OH, followed by water washing. The following conditions were varied in order to find the optimum conditions for this procedure: flow rate of medium through resin bed (1.5 to 7.9 ml/min- cm²), resin mesh size (50-100 and 100-200 mesh), cross sectional area of resin bed (4.91 and 15.9 cm²).
All manganese recovery experiments were conducted at ambient temperature (22°±3° C).
Microbial populations in the molasses media were counted on a weekly basis to determine whether the microorganisms survived the recovery methods used to remove the manganese from solution. Using a standard plate count procedure, 1 m of the medium from each test was serially diluted in a range between 1:10² to 1:10 9 with sterile water blanks. A 0.1 mL aliquot of each dilution was placed onto the surface of prepoured peptone-tryptone- glycerol-yeast extract agar in sterile petri dishes. The medium was spread over the agar with a sterile bent glass rod. The dish was incubated at 22° C for 7 days after which a colony count was recorded from the plate containing between 30-300 colonies. Populations in the original samples were calculated. Fresh 5-pct factory molasses media contained 10³ to 10 4 colony forming units per milliliter of media.
Results and Discussion
Four methods were screened using either precipitation or ion exchange for recovering manganese from column bioleach solutions. As shown in table 2, manganese recovery by precipitation techniques ranged from 73 to 99 pct. Quantities of precipitant in excess of the amounts required to precipitate the manganese were used because of the presence of other metals ions in the media such as calcium, magnesium, and potassium. Fresh molasses medium contained 1.3 g/L K, 210 ppm Ca, and 250 ppm Mg. Manganese products from each method are also shown in table 2.
Ion exchange tests using ammonium form Chelex 100 followed by stripping and precipitation with ammonium carbonate recovered 99 pct of the manganese from the pregnant media as manganese carbonate. Optimum conditions using ion exchange occurred at an adsorption capacity of 1.70 g of manganese per gram of resin and a medium flow rate of 3.4 mL/min-cm². Two and one-half times the stoichiometric amount of ammonium carbonate was required to remove 99 pct of the manganese from the strip solution.
These results indicated that ion exchange and direct precipitation using ammonium carbonate were the most promising methods for developing an integrated bioleaching-recovery system. Both systems removed manganese but not unreacted molasses from the media, maintained the near neutral pH essential for microbial activity, and did not add detrimental ions to the solutions. These conditions were necessary so the media could be reused in the bioleach stage after replenishment with fresh nutrient. Ammonium carbonate addition of 2.5 times stoichiometric to the medium increased the pH from 4.7 to 6.9 while the pH after ion exchange treatment varied only a few tenths from the starting pH. Both methods also introduced ammonium ions into the media which acted as a nitrogen source for the microbes. Nitrogen is needed for synthesis of cellular material.
Integrated Bioleaching-Recovery Systems
Four integrated column bioleaching-recovery systems were tested. Each system was composed of a bioleach column for solubilizing the manganese and a recovery system for removing the manganese from the medium. In each the test, manganese was solubilized from Three Kids ore using 5 L of 5-pct factory molasses medium as described in the Column Operation section of this report. For the first two tests, each cycle required that the bioleaching medium was circulated through the ore for 2 to 7 week periods. Each leaching cycle was stopped when the manganese concentration stopped increasing. Carbon availability was the measurable limiting factor for manganese biosolubilization. The carbon concentration of fresh molasses media was 10 g/L. When carbon concentration decreased below 4 g/L, manganese solubilization stopped. This point was reached between 2 to 7 weeks of bioleaching for each cycle.
In test 1, the manganese was removed from solution using ion exchange. In test 2, the manganese was directly precipitated from solution using 2.5 times the stoichiometric requirement of ammonium carbonate. After the completion of each leaching/recovery cycle, 250 g of fresh molasses was added to the solution and the medium was recycled to the bioleach column.
The extraction results of the two systems are shown in figure 1. In this figure, extraction of manganese is plotted as a function of cycles. Percent extraction was calculated by dividing the total amount of manganese solubilized after each cycle by the total amount of manganese in the ore. Each leaching/recovery cycle represents a molasses addition of 250 g. The system using ion exchange as the recovery method extracted 75 pct of manganese from the ore in seven cycles lasting a total of 24 weeks. The system using the precipitation recovery method, extracted 64 pct of manganese from the ore in five cycles lasting 23 weeks. Each system consumed 26-29 g of molasses per gram of manganese solubilized. The pH remained between 5 to 7 which is essential for effective microbial activity and manganese solubility.
Both bioleaching-recovery experiments showed that media could be replenished and recycled back to the bioleaching stage to solubilize more manganese. More than 95 pct of biosolubilized manganese was recovered in each experiment. A typical precipitate contained, in weight percent, 29 Mn, 5 Ca, 0.39 K, and 0.50 Mg.
Potassium from the molasses and ammonium ions from the ammonium carbonate increased in the media to concentrations as high as 5.0 g/L, but apparently did not inhibit microbial growth. This was confirmed by the count of microbes in the leaching medium. The microbial populations in the media increased from 10³ to 10 4 colony forming units per milliliter over the course of each experiment.
Since recoveries were similar in the first set of tests, direct precipitation of manganese from molasses media was chosen over ion exchange for further testing because of simplicity and economics. Ion exchange resins are expensive, require periodic replacement, and the stripping and regeneration steps require chemicals that increase reagent costs. Direct precipitation requires the same amount of ammonium carbonate as the ion exchange system but does not require the additional steps or reagents.
A second set of column tests was conducted to compare the integrated bioleaching- precipitation recovery system with a bioleaching experiment in which the medium was not recycled, but was replaced with fresh 5 pct molasses medium after each leaching cycle. The total leaching time for manganese extraction from the ore was decreased by running the bioleaching stage of each cycle for a 2 week interval versus the 2 to 7 week intervals used in tests 1 and 2. The ammonium ion buildup was also decreased-by using less ammonium carbonate precipitant (2 times versus 2.5 times the stoichiometric amount used previously). Leaching curves are presented in figure 2.
The bioleaching precipitation recovery test (3) used 2-week bioleaching stages for each cycle. Test 3 solubilized 98 pct of the manganese from the ore in 18 weeks compared to 64 pct of the manganese in 2 3 weeks in test 2 which used 2 to 7 week bioleaching stages for each cycle. However, as shown in figure 2, the use of recycled medium resulted in a lower rate of leaching and required more molasses than test 4 which used fresh medium every cycle. Test 4 used fresh medium every cycle and extracted 100 pct of the manganese from the ore in 14 weeks.
During the leaching stage of cycle 3, the rate of manganese extraction for the test using replenished media slowed down. A higher leaching rate resumed when the medium was replaced with fresh solution after the fifth cycle. This medium was then replenished and reused for subsequent cycles. The cause of the rate decrease is not known. However, observation of the column showed that a precipitate formed within the column and coated the ore particles starting at cycle 3. This is probably a direct result of an increase in solution pH. Normally, organic acids produced when sugars are metabolized by the microbial population moderate the pH in
the 5.2 to 6.5 range. During cycle 3 of test 3, the lowest pH attained was 7.1. Manganese salt precipitation has been consistently observed whenever pH excursions above 7 have occurred in bioleaching columns. After the medium was replaced at the fifth cycle, pH moderation improved, precipitate around the particles dissolved, and manganese extraction per cycle increased from 11 pct in the fifth cycle to 16 pct in cycle 6 (see table 3).
Biomass buildup on the ore particles, which may have prevented access of medium to ore minerals, and buildup of the ammonium and potassium ion concentrations in the medium, are other factors that may be responsible for the lower leaching rate in the experiment using replenished medium.
This investigation showed that manganese can be recovered from molasses medium used for column bioleaching of Three Kids ore and residual solutions can be replenished with fresh nutrient and recycled back to the bioleaching stage effectively extracting more manganese. Several methods of manganese removal from bioleach solutions were evaluated. High recoveries of manganese were achieved by precipitation using ammonium carbonate and ion exchange using Chelex 100. These recovery methods were evaluated using integrated column bioleaching/recovery systems. Up to 98 pct of the manganese was solubilized and more than 95 pet of the manganese was recovered from solution as a carbonate salt.
Column leaching experiments in which solutions were stripped of manganese, replenished with make-up molasses, and reused, required more molasses to leach the ore than experiments in which the medium was completely replaced when the molasses was depleted.
Nutrient availability and control of medium pH below 6.5 were the most important factors affecting manganese biosolubilization. However it is possible that biomass buildup on the ore particles in the column and ammonium ion and potassium ion buildup in the bioleach solutions may negatively impact manganese leaching rates.