Topic 3: Ore processing and metal recovery

From a series of 5 lectures onMetals, minerals, mining and (some of) its problems

prepared for London Mining Networkby

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 solutionsDump leachingHeap leaching methodsCyanidation wastes and risksRemediation of cyanidation wastes

• Artisanal processing of gold ore with mercury

Mineral extraction: from mining to metal

Figure from Spitz and Trudinger, 2009.

Mining

Mineralconcentrate

Metal

Mineral processing(beneficiation)

Metallurgical extraction METAL EXTRACTION

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 crushing

A 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 phase of mineral beneficiation.

http://en.wikipedia.org/wiki/File:Scheme_Jaw_Crusher.gif

Schematic of a “jaw” crusher. Credit: Anatoly Verevkin

http://www.rock-mining.com/8-Cone-Crusher.html

http://www.rock-mining.com/5-Impact-Crusher.html

Cone CrusherHorizontal Shaft Impact Crusher

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 concentration

The 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).

Ore

Crushing and sizing

Grinding and milling

Process chemicals

Water

Gravity separation

Magnetic separation

Flotation

Distillation

Mineral concentrate

Tailingsand mine waters

Simplified flow-chart of a mineral processing operation. More than one mineral separation method may be used in succession in the processing route if necessary.

Selective dissolutionFigure modified after

Ripley et al. (1996), Lottermoser (2007).

Electrostatic separation

Table from Lottermoser, 2007, using references therein.

Common flotation reagents, modifiers, flocculants, coagulants, hydrometallurgical reagents, and oxidants used in mineral separation.

(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.

http://www.flsmidthminerals.com/Products/Sedimentation/Clarifiers+and+Thickeners/Clarifiers+and+Thickeners.htm

Typical thickener tanks used to remove fluids from mineral-concentrate slurries.

Mineral extraction: from mining to metal

Figure from Spitz and Trudinger, 2009.

Mining

Mineralconcentrate

Metal

Mineral processing(beneficiation)

Metallurgical extraction METAL EXTRACTION

Metallurgical extraction:

There are three metallurgical processing methods to liberate target metals.

Pyrometallurgy: Breakdown of the mineral crystalline structure by heat in furnaces.

Electrometallurgy: The electrochemical effect of an electric current is used to extract metals from ore-concentrate (“electrowinning”).

Hydrometallurgy: Solvents are used to dissolve minerals and produce a liquid with high concentrations of the target metal. Very often performed at the 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

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)

Gold-cyanide complex NaAu(CN)2 and caustic soda (lye) NaOH

Gold, Au, recovered from NaAu(CN)2

“Expanded pad” heap configuration: old heaps are left in place, and new heaps are placed ahead.

Sodium cyanide (NaCN)plus lime (to increase alkalinity)

Oxidised gold bearing ore

Completed (barren) leach heaps

Liner Liner

Figure modified fromSpitz and Trudinger,

2009.

Heap-leach piles

Air-photo of a field of expanding heap pads, locality unknown (figure from Spitz and Trudinger, 2009).

www.airphotona.com

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”.

Figures from Spitz and Trudinger, 2009.

Heap-leach pad configurations

“Expanded pad” heap configuration shown in a previous slide

Valley pad system

Reusable 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.

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.

Heap leaching operations

Cyanide heap-leach pile and plastic lined leachate collection ponds, Wirralee gold

mine, Australia.

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. Williams

Pictures from Lottermoser, 2007.

Figure from Hartman and Mutmansky, 2002.

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.com

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 products

If 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

Table from Lottermoser, 2007. See also Environment Australia, 1998.

“WAD” cyanide(weak acid dissociable)

VERY TOXIC

LESS TOXIC

LESSSTABLE

MORE STABLE

In remediation seek either to

Move complexes up the chain to less stable compounds and ultimately HCN gas

or

Move down the chain to precipitate stable strong complexes or thiocyanate and cyanate.

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)

Volatilisation: Conversion of free cyanide to hydrogen cyanide gas (HCN). Reducing pH of waters encourages release of HCN. The gas disperses or converts to ammonium and carbon dioxide.

Precipitation: Conversion of cyanide complexes to stable solids that settle out of water – achieved by adding metals (often iron) to waters.

Biological oxidation. Bacteria degrade cyanide into harmless by-products – dissolved formate, nitrate, ammonia, bicarbonate, and sulphate. Bacterial action encouraged by adding bacteria or nutrients to 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.

Oxidation to cyanate. Dissolved free cyanide can be oxidised to less harmful cyanate by adding ozone, gaseous chlorine, hypochlorite or hydrogen peroxide. Cyanate in turn slowly decomposes to form nitrate and carbon dioxide or ammonia and bicarbonate

Photolitic degradation: In the presence of UV radiation from the sun, strong cyanide complexes break down to form free cyanide, which in turn breaks down under UV radiation to form the less harmful cyanate ion.

Formation of thiocyanate (SCN-). Oxidation of sulphide minerals in tailings or heaps will yield sulphur bearing products. Free cyanide reacts with these sulphur forms to produce less harmful thiocyanate.

ADD WATER

ADD BACTERIA

OR NUTRIENTS

ADD OXIDANTS

ADD METALS

(IRON)

SU

NL

IGH

T

Cyanide remediation using UV radiation

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.

Total cyanide300 mg/liter

< 1 mg/liter

Mining-related cyanide accidents and spillages since 1990

Table from Lottermoser, 2007.

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.

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 deposits

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.

Ore washing, Manso Atwere, Ghana.(Photo: African Gold Group). http://www.africangoldgroup.com/i/photos/ghana/Manso-Atwere-ore-washing.jpg

Artisanal gold mining, Manso Atwere Ghana, 2007. (Photo: African Gold Group). http://www.africangoldgroup.com/i/photos/ghana/Manso-Atwere-Ghana,-2007.jpg

AGGLUTINATION

Artisanal processing of alluvial gold deposits

Home-made retort, made of water pipes (Photo from UNIDO, 2004). Retorts allow the safe burn-off and capture of mercury from amalgam, but their use is often met with resistance from miners.

An artisanal gold miner holds mercury amalgam in her palm. Senegal. (Photo: Blacksmith Institute). http://www.worstpolluted.org/projects_reports/display/56

Typical burn-off of mercury from amalgam, Thailand. (Photo: Blacksmith Institute). http://www.worstpolluted.org/projects_reports/display/56

Amalgam placed inside crucible here for burning.

Condensed mercury emerges from here for collection.

AMALGAMATION

BURN-OFF