Advanced Materials Research Vols. 20-21

Abstract: Traditionally, the kinetics of microbial ferrous iron oxidation have been studied in continuous culture or in batch. Both methods have drawbacks: in continuous culture experiments have to be repeated at a number of dilution rates to cover the entire spectrum of ferrous to ferric ratios, which is time-consuming. Furthermore, experiments at very low ferric to ferrous ratios generally fail due to microbial wash-out at the high dilution rates needed to achieve these. In batch experiments, on the other hand, the prevalent ferric to ferrous ratio rapidly changes due to substrate depletion while the microbial population continually grows, making determination of specific momentary rates difficult. The present paper describes initial work with a novel device, the Redostat™, which allows careful electrochemical control of ferric to ferrous ratio in a batch reactor. A culture of Leptospirillum ferriphilum was grown at 35°C and 5 g/L total iron by maintaining the ferric to ferrous ratio at 0.17, 0.51 and 1.65 (corresponding to redox potentials of 419, 452 and 482 mV vs. Ag/AgCl), respectively. The correlation of data obtained from off-gas and current measurements was excellent, and fitted Monod kinetics with ferric inhibition. A hitherto unobserved effect indicates the onset of ferric iron inhibition at the low redox potentials employed here.

Abstract: In this work the leaching of black shale ore and froth flotation concentrates produced from the black shale was studied. The complex black shale contained various sulphide minerals (alabandite, sphalerite, pyrrhotite, pentlandite, violarite, chalcopyrite, pyrite). Concentrations of base metals in the ore were Fe 13.2%, Mn 0.97%, Ni 0.3%, Zn 0.57%, Cu 0.23% and Co 0.03%. The base metal content was two to three times higher in the flotation concentrate than in the ore. Ore and concentrate were leached in shake flasks and stirred tank reactors at pulp density of 10 to 20%, pH 1.2 to 2.0 and 60 to 77°C. A thermophilic enrichment culture related to Sulfolobus metallicus was applied. The pulp was aerated with ambient air and dinitrogen gas was used in a control test. Experimental duration was from 12 to 49 days. Leaching of Mn, Fe, Zn, Ni, Cu, and Co was up to 96, 52, 99, 99, 97 and 76%, respectively. Mn and Zn were rapidly leached within the first 2 to 3 days. In test with the ore, metal recovery was negatively affected by precipitate formation towards the end of leaching period. The H2SO4 consumption was in the range of 177 to 346 g/kg ore and 11 to 122 g NaOH/kg ore was consumed respectively. In bioleaching tests with flotation concentrates the H2SO4 consumption was from 205 to 415 g/kg concentrate and 73 to 183 g NaOH/kg concentrate was consumed, respectively. The final redox potential varied between 423 and 710 mV vs. Ag/AgCl. In experiments with fine ground ore, -50 8m, the ferric iron was at best 64% of dissolved total iron. Compared with coarser material (250-355, 710-1000 8m) leaching proceeded best with fine ground ore -50 8m). In tests with coarse ore (710-1000 8m). dissolved ferric iron was up to 97% of the total dissolved iron. Mechanical stirring and fine particles caused increase of dead cell numbers during leaching. The study demonstrates that the thermophilic enrichment culture can leach complex black shale ore at high pulp density and temperature.

Abstract: Black schist ores in Finland are often enriched with sulfide minerals, containing a variety of base metals such as nickel, copper, zinc and cobalt. As these ores are low grade with respect to the metals contained and the sulfide minerals cannot be effectively concentrated from the schists, they are currently being studied with regard to their suitability for bioleaching [1]. As part of this investigation, a large-scale column measuring 3 x 3 x 9 m was built and filled with 110 tons of the crushed black schist ore. A solution was circulated in the column for 95 weeks; this solution was adjusted to 1.8 prior to entry in the column and averaged 2.7 when leaving the column. During this time, approximately 22% of Mn, 10% of Ni and 5% of Zn were leached from the ore. Iron was also leached, but precipitated in the column. Any soluble iron in the effluent was mainly Fe (II). During this same time period, total cell counts averaged 3.6 x 107 cells/ml of effluent. On three different occasions over nearly a one-year period, culturable cells were enumerated on a variety of solid media [2] and represented only about 1% of the total cell counts. Of the culturable cells, ironoxidizing acidophiles (namely Acidithiobacillus ferrooxidans) far outnumbered any other acidophile by at least a factor of ten. Changes in populations were also monitored by molecular means (T-RFLP and SSCP) on five different occasions during the same year; again, populations in early samples were dominated by Acidithiobacillus ferrooxidans (at least two strains/sub-species). As the temperature of the column was increased from ~20 to 35°C by heating both the recirculated liquor and the air used for column aeration, the relative abundance of At. ferrooxidans-like bacteria decreased while the abundance of unidentified bacteria increased. Some of these bacteria have also been detected in lab-scale column experiments using the same ore [3]. Total cell counts varied little as the temperature increased, nor was there any change in the rate of metal leaching. It was apparent that even though the leaching of metals from black schist ores was not greatly influenced by increases of temperature in the column, active microbial populations were present and were influenced by temperature.

Abstract: Both, the employment of leaching bacteria for metal winning as well as the mitigation of bioleaching processes at AMD/ARD sites, require reliable monitoring methods for assessing bacterial activities. Therefore, we have developed a robust and rapid test system combining two sensitive analytical techniques: quantification of heat evolution by microcalorimetry and determination of all relevant inorganic sulfur species by chromatographic methods (IC and HPLC). Generally, only about 1 g of sample is sufficient for a complete analysis. The combined test has been applied to various leaching biotopes such as bioreactors, columns, heaps and natural sites. The bacterial activity of diverse sulfidic materials such as lignite and coal wastes, pure metal sulfides and complex ores has been investigated. In our labs, microcalorimetry can be performed in the range of 5 to 80 °C, covering most of the temperature spectrum of leaching bacteria. Hence, the heat evolution values of samples as a direct measure for calculating leaching rates can be obtained at nearly all relevant in situ temperatures. The combination with sulfur species determination results in additional information on leaching mechanisms (thiosulfate or polysulfide pathway) and general leaching performance (e. g. accumulation of sulfur intermediates).

Abstract: During Zijinshan copper heap bioleaching, pyrite was leached in plenty resulting in high ferric concentration in solution. This affected bioleaching and extraction processing greatly. The paper studied the factors influencing in the course of leaching of pyrite and a mixture of chalcocite and pyrite respectively. It focuses on the effect of the redox potential for copper bioleaching so as to find key factors affecting the dissolution difference between chalcocite and pyrite. The experiment results showed that redox potential is an important influencing factor in bioleaching. Copper and pyrite extents of the leaching process were 95% and 11% respectively and there is great dissolution difference between them at redox potential 700 mV (SHE). It is indicated that copper can be selectively bioleached by controlling redox potential of bioleaching. The conclusion will provide theoretic foundation on balance of acid and iron during bioleaching for copper of higher S/Cu ratio. A simple sulfide, pyrite and chalcocite, were selected as test minerals. The samples were obtained from rich mineral in Zijin Mine. The concentrated samples were milled to obtain the size fraction of 320 mesh percent of 90. Bacterium culture: 9K culture medium contained the following composition in kg/m3 distilled water: (NH4)2SO4, 3.0; K2HPO4·3H2O, 0.5; KCl, 0.1; MgSO4·7H2O, 0.5; Ca(NO3)2·2H2O, 0.01. The initial ferrous concentration was 9.0g/L, the initial pH was adjusted to 1.6, and the temperature set at 30°C. The rotation speed of shaking bed was 150r/min. A three-day-old inoculum previously grown in medium was used in bioleaching process. A 5% inoculum was added to give an initial bacteria concentration in the medium was (3-6)×107 cells/m3. The experiments investigated factors influencing bioleaching including ferrous concentration and redox potential during bioleaching. The extent Cu leached was over 90%. Cu leached velocity increased as redox potential turned higher. Cu leached velocity rapid increased at redox potential 700mV. During chalcocite and pyrite bioleaching, redox potential is important factor to influence of bioleaching rate and the results showed that pyrite and chalcocite bioleaching rate was correlated with redox potential. It is indicated that copper can be selectively bioleached during copper bioleaching by controlling redox potential of bioleaching. It can be concluded that: (1) Pyrite and chalcocite bioleaching rate was correlated with redox potential. Redox potential was shown to be the key factor affecting the dissolution difference between chalcocite and pyrite. (2) Chalcocite and pyrite leaching efficiency were 95% and 11% respectively and there is great dissolution difference between them at redox potential 700 mV (SHE). (3) It is indicated that copper can be selectively bioleached during copper bioleaching by controlling redox potential of bioleaching.

Abstract: The release of contaminants from mining wastes containing sulphide minerals is addressed. The paper shows the application of a methodology, Performance Assessment (PA), to the long-term behaviour of waste deposits. The aim of the paper is to address the different processes that occur in deposits for mining waste. These processes are applied to one study case; drainage water from a waste rock dump at the Aitik mine in northern Sweden.

Abstract: A three tiered, iterative Environmental Risk Assessment methodology, including preliminary Qualitative Risk Assessment, Quantitative Hazard Assessment and Site Specific Quantitative Risk Assessment, was established to assess the environmental risk of point and diffuse pollution of mining origin at catchment scale [1]. The model site was an abandoned Pb and Zn sulphide ore mine in Gyöngyösoroszi, Toka-valley, NE Hungary [2]. The Integrated Risk Model considers the sources identified by the GIS-based (Geographical Information System) pollution map, the transport routes shown by the GIS-based flow accumulation model and the receptors of different land uses in the catchment. The site-specific quantitative risk was characterised by the Soil Testing Triad [3]. The three elements of the Triad are: physico-chemical analyses of the soil and the contaminants, the biological characterisation and ecotoxicity testing of the contaminated soil, measuring the response of single species in laboratory bioassays, the natural response of the soil microflora and plants or the dynamic response of the whole soil in microcosms. The Triad approach strongly supports the characterisation of the site specific risk as well as the selection and planning of the suitable remediation option.

Abstract: In a bench scale trial biological sulfate reduction was applied to convert anglesite (PbSO4) to galena (PbS). Anglesite is a main constituent of waste fractions such as the residue from an indirect leaching process or in lead paste from spent car batteries. The goal of this study was to develop a technology to decrease the lead (Pb) emissions by converting PbSO4 from a waste fraction into PbS, which can be recovered from the waste fraction using a flotation process or an electrochemical process. The conversion of anglesite to galena is based on the biological sulfate reduction process and a metal precipitation process. First sulfate is biologically reduced to sulfide. Secondly, the Pb2+ from the PbSO4 reacts chemically with the sulfide resulting from the first reaction. A bench-scale reactor was started up using sulfate- and sulfur-containing influent. The reactor was seeded with biocatalyst from several full-scale reactors. Anglesite-containing residue was added batch-wise when the formation of sulfide started. The residue contained mainly PbSO4 (51.7%), sulfate (SO4 2-, 19.9%) and elemental sulfur (S0, 15.1%). Galena precipitates in the bioreactor due to the near-neutral pH at which sulfate reduction is carried out. During the experiment a surplus of sulfide relative to Pb was maintained to prevent the formation of PbCO3 and the accompanying pH decrease that would unavoidable result in the inhibition of the biocatalyst. Both sulfate and sulfur present in the residue were biologically reduced. The formation of PbS was confirmed by the increased Pb:O ratio of the sludge (1:0.03) relative to the Pb:O ratio of the residue (1:0.3). A potential large-scale application is proposed.