1. 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

2. 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

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

4. 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.

5. 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.
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

6. 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/

7. 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).
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.

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

9. 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

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

11. 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.

12. 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.

13. 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.

14. 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.

15. 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)

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

17. 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”.

18. 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.

19. 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. Williams
Cyanide heap-leach pile and plastic lined
leachate collection ponds, Wirralee gold
mine, Australia.
Pictures from Lottermoser, 2007.

20. 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
Figure from Hartman and Mutmansky, 2002.

21. 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!)

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

23. 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.

24. 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.

25. 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 hydrogen
Volatilisation: Conversion of cyanate ion. peroxide. Cyanate in turn slowly
free cyanide to hydrogen decomposes to form nitrate and
SUNLIGHT
cyanide gas (HCN). Reducing carbon dioxide or ammonia and
pH of waters encourages ADD W
bicarbonate
A TER TS
release of HCN. The gas AN
ID
disperses or converts to D OX Formation of thiocyanate (SCN-).
AD
ammonium 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.

26. 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.

27. 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.

28. 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).

29. 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.

30. Artisanal processing of alluvial gold deposits
Artisanal 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.

31. 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/56
Condensed mercury Amalgam placed
emerges from here for inside crucible
collection. here for burning.