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Pressure Leaching of Zinc-Sulphide

Stainless steel autoclaveDirect leaching of zinc-sulphide (ZnS) concentrates is a process  that recovers the  zinc from zinc-sulphide. Zinc, the fourth most common metal in use, is used to galvanize metals, and in batteries and paints, amongpatent record for this invention other uses. The process, developed in part by Alberta researchers and with Alberta industry, has important ramifications for the province’s mining sector.

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.

The larger processIn 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.

Sherritt's zinc pressure leach process 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 hours.

This proved to be somewhat optimistic, and in 1968 Veltman and P.T. O’Kane, 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 dioxide
• 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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