Topic 3: Ore processing and metal recovery

Topic 3: Ore processing and metal recovery From a series of 5 lectures on Metals, minerals, mining and (some of) its problems prepared for London Mining Network by Mark Muller mmuller.earthsci@gmail.com 24 April 2009

Outline of Topic 3:• Mineral processing (beneficiation) to produce concentrate: grinding, milling, separation• Metallurgical extraction of metals: focus on hydrometallurgy (leaching)• Focus on heap leaching of gold using cyanide solutions Dump leaching Heap leaching methods Cyanidation wastes and risks Remediation of cyanidation wastes• Artisanal processing of gold ore with mercury

Mineral extraction: from mining to metalMining Mineral processing (beneficiation) Mineral concentrate Metallurgical extraction METAL EXTRACTION Metal Figure from Spitz and Trudinger, 2009.

Mineral processing and metallurgical extraction:These are the two activities of the mining industry that follow its first principal activity, mining, that liberates the orebody from the ground:Mineral processing (or beneficiation or ore-dressing) aims to physically separate and concentrate the ore mineral(s) from the ore-rock. Ore concentrate is often the final product delivered by mines.Metallurgical extraction aims to break-down the concentrated ore minerals in order to recover the desired metal or compound. Metallurgical extraction often takes place at localities separate or remote from mine sites.Heap leaching is an alternative approach that “short-circuits” more extended processing and metallurgical extraction routes by moving directly from coarse crushing of the orebody to hydrometallurgical (chemical) extraction of the target metal. It generally occurs on the mine site.

1. Ore crushingA wide range of crushing machines are used: for example jaw crushers,gyratory crushers, and vertical or horizontal shaft impact crushers.Crushing is highly energy intensive and is often the most expensive phaseof mineral beneficiation. Horizontal Shaft Impact Cone Crusher Schematic of a “jaw” Crusher crusher. Credit: Anatoly Verevkin http://en.wikipedia.org/wiki/File:Scheme_Ja http://www.rock-mining.com/5-Impact-Crusher.html http://www.rock-mining.com/8-Cone-Crusher.html

2. Grinding (milling):Grinding is done in grinding machinery in the presence of water and therefore generates tailings.The final particle size that emerges from grinding will depend on the requirements of the subsequent mineral separation stage (1 cm – 0.001 mm) Ball mill for grinding rock materials into fine powder. Rock fragments are loaded into the barrel that contains a grinding medium (e.g., steel balls). As the barrel rotates, the rock material is crushed by the grinding medium – producing a fine powder over a period of several hours. The longer the ball mill runs, the finer the powder will be. http://www.traderscity.com/board/products-1/offers-to-sell-and-export-1/ball-mill-grinder-crusher- pulverizer-sand-making-machine-28192/

3. Mineral separation and concentrationThe target mineral is separated from gangue and un-wanted metallic minerals using processes that take advantage of the target mineral’s unique physical characteristics (e.g., its density and magnetic properties). Distillation Ore Water Magnetic separation Crushing Grinding and sizing and milling Electrostatic separation Mineral concentrate Gravity separation Simplified flow-chart of a mineral Process chemicals processing operation. Flotation More than one mineral separation method may Tailings be used in succession Selective and mine Figure modified after dissolution waters in the processing route Ripley et al. (1996), Lottermoser (2007). if necessary.

Common flotation reagents, modifiers, flocculants, coagulants,hydrometallurgical reagents, and oxidants used in mineral separation. Table from Lottermoser, 2007, using references therein. (a terrible cocktail!)

4. Thickening. Thickening is achieved by allowing solids in the mineral concentrate slurries to settle at the bottom of cylindrical tanks (called “thickeners”), where they are scraped away to a discharge outlet by rotating “rakes”.5. Drying. Complete dewatering of the thickened mineral concentrate is in achieved in disk, drum or vacuum filters to produce a final, dry mineral concentrate product. Typical thickener tanks used to remove fluids from mineral- concentrate slurries. http://www.flsmidthminerals.com/Products/Sedimentation/Clarifiers+an d+Thickeners/Clarifiers+and+Thickeners.htm

Mineral extraction: from mining to metalMining Mineral processing (beneficiation) Mineral concentrate Metallurgical extraction METAL EXTRACTION Metal Figure from Spitz and Trudinger, 2009.

Metallurgical extraction:There are three metallurgical processing methods to liberate targetmetals.Pyrometallurgy: Breakdown of the mineral crystalline structure by heat infurnaces.Electrometallurgy: The electrochemical effect of an electric current is usedto extract metals from ore-concentrate (“electrowinning”).Hydrometallurgy: Solvents are used to dissolve minerals and produce aliquid with high concentrations of the target metal. Very often performed atthe mine-site, with accumulation of associated wastes on site.

Hydrometallurgy - vat leaching:Vat leaching is a high-production rate metal extraction process carried out in a system of closed vats or tanks using concentrated leaching solutions (solvents).Either Sulphuric acid or ammonium carbonate (an alkali) is used to extract metals from copper oxide and uranium oxide ores. Alkaline cyanide solutions are used to extract gold from ores.Because the ores are finely ground (unlike heap-leaching), large quantities of fine tailings are produced and require storage in tailings dams. The tailings will be acidic in the case of copper processing and alkaline in the case of gold processing.

Dump leaching:Most commonly used in the copper industry. The “dump” in dump leaching generally refers to old waste rock dumps that have been identified for reprocessing.There is therefore no lining present under the dump.Sulphuric acid is the main leach solution for recovering copper from copper ores. On some mines leachate from rainwater percolating through the dump is recovered (essentially recovered acid mine drainage!).Environmental problems: acidic groundwater and surface water.

Heap leaching:Heap leaching is a process commonly used for the recovery of precious metals (gold and silver), and less commonly for base metals and uranium, from amenable, oxidised low-grade ores, or occasionally from previously processed tailings.Amenable ores are oxidised. If not, oxidising bacteria may be used first to decompose sulphide minerals to facilitate the leaching process.No fine tailings are generated by heap leaching – probably its single most important advantage over conventional vat leaching.

Heap leaching – applied to gold recovery using cyanide“Expanded pad” heap configuration: old heaps are left in place, and new heaps are placed ahead. Sodium cyanide (NaCN) Gold, Au, plus lime (to increase recovered from NaAu(CN)2 alkalinity) Completed (barren) leach heaps Liner Liner Oxidised gold bearing ore Figure modified from Gold-cyanide complex NaAu(CN)2 Spitz and Trudinger, and caustic soda (lye) NaOH 2009.4 Au + 8 NaCN + O2 + H2O 4 NaAu(CN)2 + 4 NaOH Gold Sodium cyanide Oxygen Water Gold-cyanide complex Sodium-hydroxide(solid) (dissolved) (gas) (liquid) (dissolved) (dissolved)

Heap-leach piles www.airphotona.com Air-photo of a field of expanding heap pads, locality unknown (figure from Spitz and Trudinger, 2009).

Heap leaching – rinsing:After leaching is complete, barren heaps are rinsed with water, or may be allowed to rinse naturally in high rainfall areas. Generally eight pore volume displacements will remove all but the smallest trace of reagent (Hutchison and Ellison, 1992).Oxidising agents such as hypochlorite, peroxide, or specially bred strains of reagent-destroying bacteria may be added to the rinse solution.Oxidising agents are used to convert toxic cyanide complexes to significantly less harmful “cyanates”.

Heap-leach pad configurations“Expanded pad” heap configuration shown in a previous slide Valley pad system Barrick’s Pierina Mine, Peru uses heap leaching with a valley-pad configuration to extract gold and silver. Production costs in 1999 were US$ 50 per ounce of gold, making it the world’s lowest-cost major gold mine. Reusable pad system There is some risk of damaging the liner in the case of the reusable pad system, as spent heaps are recovered and new heaps are put in place.Figures from Spitz and Trudinger, 2009.

Heap leaching operations Large valley-pad heap- leach piles at the Yanacocha gold mine, Peru. The siliceous ore is so porous it can be leached without crushing.Photo: P. WilliamsCyanide heap-leach pile and plastic linedleachate collection ponds, Wirralee gold mine, Australia. Pictures from Lottermoser, 2007.

Heap-leach pad liner systems A geomembrane is normally a “plastic” liner made from polyethylene or polyvinyl chloride (PVC). A geoweb® is a flexible “framework” mesh, often made out polyethylene, and used to stabilize layers of granular material. From: Presto Geosystems www.prestogeo.comFigure from Hartman and Mutmansky, 2002.

Heap leaching – processing oxidised or sulphide ore:Cyanide solutions react with gold and silver.Cyanide solutions do not react with oxide minerals.Cyanide solutions do react with sulphide minerals.If ore in the leach heap is contains oxide minerals or is oxidised, the process produces:• gold and silver complexes (which is the target)• free cynide (CN-) and cyanide gas (HCN) by productsIf ore in the leach heap contains sulphide minerals, the process produces:• gold and silver complexes (which is the target)• free cynide (CN-) and cyanide gas (HCN) by products• a cocktail of other metallic cyanide complexes (bad news!)

Cyanide compounds and metal complexes LESS VERY TOXIC STABLE In remediation seek either to “WAD” cyanide (weak acid Move complexes up the chain to less dissociable) stable compounds and ultimately HCN gas or Move down the chain to precipitate stable strong complexes or thiocyanate and cyanate. MORE STABLE LESS TOXICTable from Lottermoser, 2007. See also Environment Australia, 1998.

Heap leaching – impacts during the leaching process:Potential serious risks include:• Leakage of pregnant cyanide solution through pad or pond liners - contaminates the underlying groundwater.• Discharge from over-topping of the solution ponds (due to excess water, pump failure, or physical damage to the ponds) - contaminates downstream surface water and/or groundwater.Heap leaching operations are less commonly carried out in high- rainfall areas because of problems in managing the large volumes of rainwater entering the system via leach heaps - exceptions Philippines and Indonesia.

Cyanidation wastes – remediation:Cyanide wastes are found in old heaps, tailings and mine waters.Cyanide and cyanide complexes will eventually break down naturally, at varying rates, that depend on water pH, temperature, salinity, concentration of the complexes, oxidant concentration and the intensity of UV radiation (Lottermoser, 2007).Remediation measures to “attenuate” (destroy) cyanide are based on• Accelerating natural processes,• Specifically “engineered” processes.

Cyanide attenuation and waste remediation (old heaps, tailings, waters) Treatment of cyanide waste is primarily about converting dissolved free cyanide and cyanide complexes into less harmful compounds or compounds that disperse more easily in nature. Photolitic degradation: In the presence of UV radiation from the sun, strong cyanide Oxidation to cyanate. Dissolved complexes break down to form free cyanide can be oxidised to free cyanide, which in turn less harmful cyanate by adding breaks down under UV radiation ozone, gaseous chlorine, to form the less harmful hypochlorite or hydrogenVolatilisation: Conversion of cyanate ion. peroxide. Cyanate in turn slowlyfree cyanide to hydrogen decomposes to form nitrate and SUNLIGHTcyanide gas (HCN). Reducing carbon dioxide or ammonia andpH of waters encourages ADD W bicarbonate A TER TSrelease of HCN. The gas AN IDdisperses or converts to D OX Formation of thiocyanate (SCN-). ADammonium and carbon dioxide. Oxidation of sulphide minerals in A tailings or heaps will yield sulphur RI bearing products. Free cyanide TE NTS AD RO C (I BA IE D N) D UTR reacts with these sulphur forms to M AD N ET OR produce less harmful thiocyanate. AL Biological oxidation. Bacteria S degrade cyanide into harmless by-products – dissolved formate, nitrate, ammonia, Precipitation: Conversion of cyanide bicarbonate, and sulphate. complexes to stable solids that settle Bacterial action encouraged by out of water – achieved by adding metals adding bacteria or nutrients to (often iron) to waters. waters.

Cyanide remediation using UV radiation < 1 mg/liter Total cyanide 300 mg/liter Cyanide-bearing seepage waters are collected at the base of a tailings dam, Red Dome gold mine, Australia. UV radiation causes the destruction of dissolved copper cyanide complexes and the precipitation of cyanate salts. Total cyanide is attenuated from 300 mg/l to less than 1 mg/l in successive ponds. From Lottermoser, 2007.

Mining-related cyanide accidents and spillages since 1990 Spillage of cyanide into the environment has generally occurred through: • accidents during transport of (solid) sodium cyanide (NaCN) to the mine site, or • release of tailings material from tailings dam that failed, or were “overtopped”, either through operational error and/or high rainfall.Table from Lottermoser, 2007.

Artisanal processing of alluvial gold deposits:Small scale artisanal mining (i.e., not using “modern” technology) has been estimated to account for 15 to 20% of the world’s non-fuel mineral production. The industry is highly labour intensive and employs 11.5 to 13 million people worldwide (Kafwembe and Veasey, 2001).Mercury is used to recover gold (and silver) from alluvial deposits using the processes of agglutination and amalgamation. The mercury process has been used since the 1970s in many developing countries.In Latin America, for example, over 1 million people are directly involved in artisanal gold mining, recovering between 115 – 190 tons/year of gold, while releasing more than 200 tons/year of mercury into the environment (Veiga, 1997).

Artisanal processing of alluvial gold deposits (continued):Mercury release into the Amazon. The Brazilian Amazon basin has become the site of a major gold-rush, starting in the early 1980s. Several hundred thousand men have recovered thousands of tons of alluvial gold from river banks and beds, subsequently processed using agglutination and amalgamation.Nearly 3,000 tons of mercury have been released into the Amazon environment in the last 15 years.Toxicity. Miners, gold-dealers, residents, fishermen are all exposed to the risk of direct exposure to toxic mercury concentrations, through vapour inhalation, or through contact with mercury films deposited on the insides and outsides of buildings, and on household utensils and foodstuffs.

Artisanal processing of alluvial gold depositsArtisanal gold mining, Manso Atwere Ore washing, Manso Atwere, Ghana.Ghana, 2007. (Photo: African Gold Group). (Photo: African Gold Group).http://www.africangoldgroup.com/i/photos/ghan http://www.africangoldgroup.com/i/photos/a/Manso-Atwere-Ghana,-2007.jpg ghana/Manso-Atwere-ore-washing.jpg AGGLUTINATION Carpet to concentrate gold (Photo: UNIDO, 2004). The figure caption in the original source is unclear, but the carpet is probably impregnated with mercury to concentrate gold by agglutination.

Artisanal processing of alluvial gold deposits An artisanal gold miner holds mercury amalgam BURN-OFF in her palm. Senegal. (Photo: Blacksmith Institute). http://www.worstpolluted.or g/projects_reports/display/56 AMALGAMATION Home-made retort, made of water pipes (Photo from UNIDO, 2004). Retorts allow the safe burn-off and capture of mercury from amalgam, but Typical burn-off of mercury their use is often met from amalgam, Thailand. with resistance from (Photo: Blacksmith Institute). miners. http://www.worstpolluted.org/pr ojects_reports/display/56Condensed mercury Amalgam placedemerges from here for inside cruciblecollection. here for burning.

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