Pressure Leaching of Zinc-Sulphide
Direct leaching of zinc-sulphide (ZnS) concentrates is a process that
zinc from zinc-sulphide. Zinc, the fourth most common metal
in use, is used to galvanize metals, and in batteries and paints, among
other uses. The process, developed in part by Alberta researchers and with
Alberta industry, has important ramifications for the provinces mining
Leaching is one of two commonly used methods of recovering zinc from zinc-sulphide
concentrates. The other, called roasting, has a serious disadvantage, in
that one of its end-products is sulphur dioxide, a toxic gas. Leaching
does not have this disadvantage, as the end-product is elemental sulphur,
which, while smelly, is relatively harmless. In fact, elemental sulphur is
an essential element in living organisms, and can be sold as fertilizer,
among other usages.
In the late 1950s, F.A. Forward, head of the Department of Mining and
Metallurgy at the University of British Columbia, and H. Veltman, a
research metallurgist for the research and development division of Sherritt Gordon Mines Limited, located in Fort Saskatchewan, Alberta,
started conducting experiments to see if a viable method of leaching zinc-sulphide
concentrates could be found. While previous work on this had been
conducted for decades, nothing satisfactory had been developed.
Forward and Veltman conducted experiments using a device called an
autoclave, a vessel used to create chemical reactions at high temperature
and high pressure. The two scientists found that zinc sulphide reacted
with hydrogen sulphate and oxygen to produce zinc sulphate and sulphur.
They proceeded to fine-tune certain variables such as the temperature,
pressure and time. In their findings, they stated that, given the right
conditions, 99 percent of the zinc could be extracted in two to four
This proved to be somewhat optimistic, and in 1968 Veltman and P.T. OKane,
also of Sherritt Gordon, concluded that somewhere near twice as much time
was needed, a fact that prevented commercial application of the process.
With further experimentation, they refined the process and reduced the
time to less than two hours.
Environmental restrictions on sulphur gas emissions were increasing,
adding urgency to make direct pressure leaching viable. Given the success
of the experiments, Sherritt Gordon conducted a large-scale pilot plant
campaign, in cooperation with another Canadian company, Cominco. Research
up to this point had taken place at the Sherritt Research Centre in Fort
Saskatchewan. The first large-scale pilot plant facility was built at
Trail, British Columbia in the late 1970s. The results were promising,
with the technical integrity of the process indicating that commercial
application of direct pressure leaching was likely.
Sherritt Gordon licensed the process, and other plants throughout Canada
and the world soon followed. In 1997, Dynatec Corporation took over the
Sherritt Gordon technology services and licensing, including the pressure
leaching patents. Dynatec continues to refine the process and apply it in
locations around the world.
The invention of pressure leaching is a process that spanned several
decades and represent Alberta innovation in mining and extraction. Dozens
of scientists and researchers worked on it, refining and developing it. In
the end they achieved a commercially viable process which
Achieved the complete elimination of atmospheric pollution due to sulfur
Recovers sulphur in its elemental form, readily storable and saleable
Has lower operating costs
Produces a high level of zinc (and other metals) recovery
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