GOLD EXTRACTION
requirements up to a maximum flow of 1,200
m³/h for a single reactor and, enable efficient
oxygen dispersion under high shear (velocities of
up to 10 m/s within the unit) and pressure (3 bar)
generated by the feed pump. Furthermore, the
shear exposure reduces boundary layer
resistances that influence most solid/liquid
reaction rates. This can enhance the reaction
kinetics tremendously and enable process
options not feasible otherwise. Essentially all of
the requirements of Elsner’s equation governing
gold leaching are met ensuring that rapid
dissolution of gold can take place.
A more recent application of the Aachen
reactors is within the well-known INCO cyanide
destruction process. Often cyanide destruction is
restricted due to oxygen being rate limiting.
However, Aachen reactors are able to solve this
through their very high oxygen transfer rates. A
successful trial was conducted on a West African
gold mine and this process is now being
implemented on a plant in South Africa.
In conclusion, it can be seen that whilst
Aachens are now established technology they are
continually finding new applications with further
development work now being directed towards
base metal leaching applications.
Cyanide-free gold
Approvals have been granted for implementation
of a full-scale Kell hydrometallurgical processing
plant at Sedibelo Platinum’s Pilanesberg
platinum mine in South Africa’s North West
province, and up to five Kell plants in Zimbabwe.
By partnering with Sedibelo Platinum Mines, the
South African Industrial Development Corp (IDC),
and the Zimbabwe Mining Development Corp
(ZMDC), industry and government buy-in to the
Kell initiative is demonstrated. The lack of power
capacity in these regions and high cost of
establishing platinum smelters and refineries is
taken care of by Kell, which uses less than a fifth
of the electricity required for smelting at a
fraction of the capital and operating costs, and
refines the PGMs and gold on site to high-purity
>99.95% sponge metals as part of the process.
Base metals are refined to LME grade cathode or
battery-grade salts, depending on the needs of
the client. By removi ng the constraints on
concentrate grade, chrome, MgO and other
impurities imposed by smelting, higher flotation
recoveries are achieved, and hence revenues
maximised. Other value elements such as cobalt,
which are currently lost in smelting, are
recovered in Kell processing.
The cyanide-free, low-emissions Kell Process is
capable of high recoveries of gold, silver, base
and rare metals from a range of feed materials,
including primary and upgraded PGM, refractory
gold, copper-gold and polymetallic concentrates.
Whilst the Kell Process was originally developed
44 International Mining | AUGUST 2017
for the extraction of PGMs and base metals from
UG2 chromitite flotation concentrates, the
process has been successfully tested at batch
and continuous pilot scale on various flotation
concentrates, including those from the Merensky
reef, the Platreef mafic/ultramafic layer, the MSZ
of the Great Dyke and various polymetallic ores
in North America. It has been shown to
consistently provide high (typically >95%) and
selective extraction efficiencies for key valuable
metals, eg Pt, Pd, Rh, Au, Ni, Co, and Cu and has
been subjected to several engineering studies all
showing favourable project economics. The Kell
Process consists of several commercially proven
unit operations and uses know-how gained from
extensive test work and piloting, as well as
applications in other sectors.
The process has been amenability tested
successfully on a range of refractory gold and
copper-gold concentrates and other gold-bearing
materials such as calcine-leach tailings where
cyanidation recoveries are low. Reagent
consumers and valuable by-products such as S,
Cu, Ni, Co, Zn are first selectively removed by use
of a pressure oxidation step during which the
dissolution of precious metals is minimised. High
recoveries of gold and silver are achieved by
subsequent chlorination, with typically low
reagent consumptions due to prior removal of
reagent consumers and the use of efficient
recycling. Subsequent metal recovery steps can
provide marketable high-purity primary and
secondary end products such as refined metal
sponges, bars or coins. In this way, third-party
smelting, treatment, transportation and refining
costs are removed from the value chain, with
saleable metals being produced on the mine site.
Residues have similar characteristics to flotation
tailings and may be co-disposed. Kell plant
design is modular, and hence readily scalable,
thereby significantly reducing capital risk via
fabrication to factory-level safety and quality
specifications. Kell plants can be located
relatively close to the mine site, reducing
The Kell Process is capable of high recoveries
of gold, silver, base and rare metals from a
range of feed materials
transportation and wharfage costs for shipping
concentrate.
The Kell Process generally consumes
significantly less energy, less electricity, and
requires lower capital and operating costs than
the equivalent conventional smelting and refining
facilities for the same duty. Unlike conventional
smelters or bioleaching plants, for example,
which lock up considerable gold and other
precious metals in circuit, due to very fast
reaction rates the Kell Process locks up
substantially less metal inventory and hence,
releases significant working capital early in the
project, sometimes enough to pay for most of the
plant capital costs. The costs of cyanide
detoxification and management are avoided, as
are the risks associated with cyanide
transportation and potential spill events. No toxic
gases or acids are emitted.
Capital costs for a gold concentrate processing
plant may be substantially reduced by co-
location of a KellGold plant at the same site as a
KellPGM plant, exploiting economies of scale,
with shared common processing areas, reagents
handling, administration, marketing and
operational management.
A range of refractory gold, copper-gold and
polymetallic concentrates have now undergone
proof-of-concept laboratory amenability test
work and preliminary engineering investigations.
For example, in ~2 kg tests, refractory gold-
copper-cobalt concentrate returned extractions of
96% Au, 98% Cu and 97% Co, while a refractory
polymetallic concentrate returned extractions of
98% Au, 97% Ag, 99% Zn, 97% Pb, 99% Cu, and
95% Sb. Several specific applications are
currently being assessed for pathways to
implementation.
The Kell Process has been described as a
“game changer” and by removing concentrate