History And Methods In Biohydrometallurgy

Transcription

1 Lecture 3 History And Methods In Biohydrometallurgy Keywords: Historical Perspectives, Methods, Biohydrometallurgy In lectures 3-5, historical perspectives in bioleaching, methods in biohydrometallurgy, microorganisms implicated in bioleaching and types of bioleaching of ores and concentrates are discussed [7-17]. Biohydrometallurgy as part of mineral or mining biotechnology incorporates several microbially-mediated processes, such as Bioleaching Biooxidation Bio-induced surface chemical changes and Bioremediation Bioleaching involves leaching (dissolution) of a metal from a solid mineral facilitated by activity of microorganisms (bacteria, archaea). On the otherhand, in biooxidation brought about by bacterial action, the valuable metal is just liberated from the mineral matrix and remains enriched in the leached solid residue. (Bioliberation of gold entrapped in pyrite and arsenopyrite matrix). Bioleaching is commercially used to process copper, uranium, zinc, copper and nickel-containing ores. Bio-induced surface chemical changes confer surface hydrophobicity or hydrophilicity on interacted minerals promoting their flotation, flocculation, dispersion or depression. Such microbially-induced beneficiation processes are essentially interfacial reactions without any mineral dissolution. Use of microbes in detoxification and decontamination of polluted soils and water is referred to as bioremediation. For example, acid mine drainage is caused by microbial activity and its remediation can be brought about using native organisms. 1

2 Historical perspectives Role of microbial processes in mineral dissolution was not recognized till late 1900s. However, natural leaching of metals from rocks was reported even as early as AD. Georgius Agricola ( ) referred to copper leaching from ores and leachates from mines. The history of bioleaching begins much earlier than one might ponder. In china and India, natural recovery of copper and other base metals such as zinc from solutions emanating from rocks was known almost 2000 years ago. The Rio Tinto in Spain owes its nomenclature to the presence of reddish brown water having higher concentrations of ferric ions. Natural iron and copper dissolution from minerals through activity of native microorganisms was later recognized at RioTinto. The contribution of sulfur and iron-oxidising Acidithiobacillus ferrooxidans in generating ferric-ion containing sulfuric acid which can dissolve minerals such as copper sulfides was scientifically established in Since the 1950s research activities on the use of Acidithiobacillus picked up and commercial applications of bioleaching particularly in copper dump and heap leaching began to emerge. The run-of-mine lean grade copper ores were stacked in waste dumps over 100m in height and leached using acidic solutions for economic copper extraction at the Bingham mines of Kennecott Copper Company during the late 1950s and early 1960s. Heap and in situ mining using native microorganisms were developed later. Heap bioleaching of uranium was reported in the 1960s, mainly in Canada. Commercial applications of biohydrometallurgy on a larger scale began in the 1980 s for heap bioleaching of secondary copper sulfides and oxidized ores. Numerous copper heap bioleach systems have been commissioned since the 1980 s. In 1986, bioreactor processing for refractory sulfidic gold-bearing concentrates was commercialized at Fairview mines in South Africa. Since then, several gold-bioreactor industrial operations have been initiated. Piloting and prototype stirred tank reactor systems for thermophilic bioleaching of base metal (copper, zinc, nickel) were then established. In 1999, commercial cobalt bioleaching from pyritic concentrates was established. 2

3 Some year-wise developments in metals biotechnology since 1947 are listed below: (Adapted from presentation at Mintek 75 Technical conference, June 2009 by P.J. van Staden) 1947: Acidithiobacillus ferrooxidans was identified (Colmer A.R., Hinkle M.E. (1947). Science 106: ) 1950 s: Leach-dumps for copper. Kennecott 1960 s: Dump / heap leaching for copper, uranium lean ores - insitu leaching. 1965: Iron and sulfur oxidizing archaea 1977: First international biohydrometallurgy meeting, Braunschweig, Germany. 1980: Lo Aguirre, first heap bioleaching plant. 1985: In-situ uranium heap leaching using intermittent flooding and forced aeration. 1986: Fairview S.Africa: first commercial refractory gold biooxidation plant (many gold bioreactors to follow). 1987: Paques anaerobic systems for effluent treatment. 1993: Forced aeration on heap bioleach systems. 1995: Bioleaching of chalcopyrite concentrate developed and evaluated on commercial scale : BioNic and BioZinc for nickel and zinc bioleaching. 1999: Cobalt bioreactor plant, Uganda. 2000: Commercial scale applications of archaea. 2002: Penoles, Mintek, BacTech Chalcopyrite concentrate bioleach pilot plant, Mexico, 2002: BHP Billiton / Alliance Copper commercial demonstration plant for copper- enargite concentrate 2002: GEOCOAT Thermophilic Bioleachng of Chalcopyrite Concentrates, Field Trials 2003: Demonstration. BioCop TM, BHP. Billiton (chalcopyrite) 2006: High temperature heap bioleaching, transitional primary / secondary copper ore (Mintek) 3

4 Major companies that developed biooxidation processes Newmont BIOPRO TM Heap bioleaching of sulfidic refractory gold ores ( g/t) BHP Billiton BIOCOP TM agitated tank bioleaching for copper sulfides. BIONIC TM agitated tank bioleaching for nickel sulfides. BIOZINC TM agitated tank bioleaching for zinc sulfides. High Temp. Heap bioleaching of chalcopyrite ore. Gold Fields BIOX TM Agitated tank biooxidation / leach for copper sulfides and refractory sulfide gold concentrates. Bac Tech Environment Geo Biotics Bac Tech / Mintek-agitated tank oxidation / leaching of copper sulfides and refractory sulfide gold concentrates. GEOCOAT TM Bioheap leaching of sulfide concentrates. Methods in biohydrometallurgy Bioleaching is critically dependent on factors such as bacterial concentration, activity and its growth, rate of biooxidation reactions, and iron oxidation ability of bacteria. Development of adapted cells play a vital role in the efficient and faster dissolution of minerals. Bacterial adaptation to toxic ions and ore minerals significantly enhances dissolution rates of sulphides, reduce the lag phase and decrease the solution ph. Bacterial tolerance limit can be increased by serial subculturing and adaptation to higher metal concentrations. Adapted strain of Acidithiobacillus ferrooxidans exhibited high copper and nickel extraction from a copper and nickel concentrate. Enhanced sphalerite dissolution has been observed by using a pyrite - adapted Acidithiobacillus ferrooxidans strain. In the simultaneous presence of zinc, copper is less toxic to Acidithiobacillus ferrooxidans and copper toxicity can be minimized. Major current commercial applications of bioleaching are extraction of copper and uranium from low grade ores and waste ore burden through heap bioleaching and insitu mining and 4

5 enhancement of gold recovery from sulphidic, refractory ores and concentrates, through stirred bioreactor processing. Bioreactor processing of pyritic cobalt concentrates has been commercialized in The relevance of bioleaching for the extraction of other base metals such as zinc, lead, cobalt, nickel and molybdenum need also to be stressed. Another significant application of biooxidation is in the processing of complex multisulphide ores and concentrates. Bioleaching of metal concentrates such as the ones containing copper and zinc in bioreactors is of great commercial interest. All these applications have direct relevance to India. Three methods have been commercially employed for extraction of metals from lean grade ores and waste overburden. In situ leaching or solution mining Dump leaching Heap leaching All the above methods have been practiced over the last four decades with advantages such as low cost, suitability for different mines and lean grade ores and environmental acceptability. In situ leaching Microbial in situ leaching (or solution mining) is a unique method of metal extraction from run of mine ores and uneconomical underground ore bodies. Underground mined stopes can be leached. Leach solutions are injected and percolated into rock cavities and the pregnant solution recovered by pumping. Selective blasting of rock can be done to gain access to regions of rich mineralization. Application of in situ leaching depends on the geological and topographical features of the ore body, and also on the hydrological and rock-mechanical properties of the host rock. Feasibility of in situ leaching need be established with respect to rock permeability and accessibility. 5

6 Dump bioleaching Mining operations over several years produce millions of tonnes of waste ore. Waste ore dumps remain at the mine site and cannot be economically processed for extraction of metals by any conventional methods. Bioleaching offers an economical alternative to process such overburden. Most of the dumps are generated without any planning and contain different sizes of ore particles. Dump geometry conducive for efficient leaching need be analyzed. Dumps positioned parallel to each other and greater in length than width and height are called finger dumps which have dimensions of the order of 800 m x 35 m x 200 m. Permeability of the fragmented rock mass in the dump is important. Too much of fines affect percolation of leaching solution. Dump leaching involves percolation of acid solution through the mineralised particles across the mass. Native microorganisms are allowed to proliferate by providing optimum growth conditions. Metals are biologically solubilized and the effluent collected at the bottom is enriched with the metal content. The leach solutions are treated to recover the dissolved metal. Several methods have been practised for efficient irrigation of the waste dumps. Formation of shallow square or rectangular ponds covering the top dump. The lixiviant (sulphuric acid solution at a ph of about 2) is pumped to the ponds through a network of distribution lines. Sections of such ponds can be flooded to permit continuous availability of percolating solution. Intermittent leaching with intervals of wetting and rest cycles is preferable. The leach solutions can be distributed on the upper portions and sides of the dump through sprinklers. Compressed air can be used and leach solutions are allowed to flow through the cross sections in the dump. Since the biooxidation of sulphides is exothermic, temperatures of the order of o C can be attained inside the dump. Microbial activity under different temperature conditions need be 6

7 understood for efficient leaching. Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans which are the most important acidophilic bacteria present in the dump have optimal activity at temperatures of the order of o C. With rise in temperature, mesophiles undergo thermal death. Presence of several thermophilic autotrophs which can tolerate higher temperatures has been reported in the dumps. Bioleaching with better reaction kinetics can thus be maintained even at higher temperatures. Synergistic activity between mesophiles and thermophiles bring about biooxidation of the sulphides in dump leaching operations. Heap bioleaching As different from dumps, heaps of low grade ores or waste materials could be prepared on impervious bottoms with control exercised with regard to amount of the ore materials, particle size, height and irrigation methodology. The heap bioleaching otherwise is similar to dump leaching. The following aspects need be considered. Topography, geological and mineralogical aspects Geography, environmental and ecological features Operational procedures including particle size distribution, dimensions and irrigation techniques. Heaps are arranged in the immediate vicinity of the mines. Ground impermeability is important. Nonporous pads are created on which heaps are arranged. Impermeable bottoms can be made of concrete, asphalt, pitch, coal-far or synthetic polymeric sheets. Slopes need be maintained to permit collection of percolated leach solution. Particle size distribution and porosity of the ore particles in the heap should be controlled to enable percolation of leach solution and oxygen diffusion. In many commercial heaps, run-of-mine ores containing about 4% mm and 25% - 20 mm particles are used. Excessive fines result in slime formation, impeding percolation. In case of excessive fines, prior agglomeration may become essential. The height of the heap need be so chosen to attain adequate oxygen diffusion through natural draft. Heap sizes of 6-10 m 7

8 height, 137 m length and 107 m width containing more than 50,000 tonnes of ore are common. Typical heap design is shown in fig.3.1. Acid solution Heap bioleach pad Leach solution recovery and metal extraction Oxidation and bacterial growth pond. Addition of energy source + nutrients Metal free acid solution Fig 3.1: Heap leaching set-up Commercial scale bioleaching started in the form of dump leaching during the 1950 s. Heap bioleaching of secondary copper sulfides has since taken over very effectively, especially in Chile. Bioleaching of refractory sulfidic gold ores in heaps was also tested on a demonstration scale and evaluated (Carlin, Nevada). In copper heap bioleaching crushed ores are blended and the fines agglomerated. Agglomerated and acid-preconditioned ore is then stacked (6-10 m high) on lined pads or top of a prior-leached ore. Air supply is provided through piping and irrigated with raffinate. Pregnant solutions are collected at the base of the heap. Copper is recovered through solvent extraction-electrowinning route and the solution recycled to the heap. Leach period could be days with upto 85% recovery. Unlike copper, heap bioleaching for enhanced gold extraction has not yet been widely commercialized. Data based on pilot trials are available. Even though, there are several 8

9 similarities between both copper and gold ore bioheap leaching, some essential differences do exist. The precious metal gets enriched in the bioleached residue and has to be lime-treated and cyanided to extract gold. Strict and frequent monitoring of bioheaps is essential. Pregnant leach liquors are analyzed for redox value, ph and metal concentrations. Similarly, ore samples from different depths of the heaps need be so assayed. Temperature and oxygen levels at different heap locations need be assessed. Bacterial counts are generally made in liquid samples. But role of solid-attached organisms also need be understood. Heap bioleaching microbiology has not been well understood. Many parameters influence microbiology, such as acid addition, nature and amount of sulfide minerals, solution chemistry, availability of oxygen and nutrients as well as temperature. Dominance of Acidithiobacillus group of organisms will be determined by ph, iron and sulfur content in the heap materials. Temperatures in different regions of the heap will be higher due to exothermic reactions and moderate and extreme thermophiles can play a role in mineral dissolution.[7-15] 9