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MEI Online: Commodities: Metallic Ores: Gold: Latest News: December 17th 2009


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:: Multotec Pachuka Valves Solve Problem in Gold Reduction

In the past conventional aerator systems used in the operation of Pachuka vessels in gold leaching have not worked acceptably. Multotec developed and successfully field tested a multi aerator that addresses this 30-year-old problem.

“As more mines move away from mechanical and compressed air agitation systems because they are maintenance intensive, the Multotec Pachuka Valve has gained popularity. Compressed air is introduced at the bottom of the Pachuka tank to aerate the media. When air flow is stopped for whatever reason, slurry settles into the compressed air lines and causes severe blockage," Fritz Schoeman, engineer at Multotec Wear Linings, says. “This will prevent the system from being restarted and necessitates draining of the Pachuka vessel, which results in production losses and unscheduled maintenance costs."

Schoeman says that Multotec Pachuka Valves are used as aerators and/or agitators to keep material in suspension or to introduce air into media.

The design uses a composite of high alumina ceramics, polyurethane, stainless steels and PTFE components to withstand abrasion and corrosion and to provide mechanical strength and overall process efficiency.

Most modern gold recovery processes depend on gold’s ability to dissolve in dilute solutions of cyanide. Pachuka vessels are large tanks that typically hold between 450 and 600 m3 of a pulp comprising milled gold bearing ore and a diluted lime and cyanide solution and are continually agitated either by mechanical means or compressed air.

In the latter, air is injected into the pulp at the bottom of the Pachuka through the Multotec Pachuka Valve. This has a two-fold purpose, firstly to break up the air into extremely fine bubbles so that oxygen, which facilitates the gold leaching reaction, is absorbed by the slurry, and secondly to prevent passage of slurry into the air pipes in the vent of loss of air supply.

Slurry densities in the Pachuka tanks run to between 1.3 and 1.7, and if agitation stops a huge mass of corrosive gold bearing material settles out in the area of the aerator.

With conventional aeration systems, denser particles settle into the open air pipes. Air lines are blocked with settled material and tons of potentially highly profitable material has to be washed out of the large tanks manually. This means that the whole plant process has to be diverted, resulting in substantial losses through additional labour costs, wasted reagents and gold concentrate and slow startup times of an average of three days per event.

Essentially, the Multotec dual-action non-return air aerator, or Multotec Pachuka Valve, has eliminated the conventional systems of the past and adapted them into a most effective unit using to modern materials. The aerator closes automatically if air inlet pressure falls below a preset 1 bar air pressure, and reopens as soon as aeration is resumed. Air consumption is about one third of the amount used when operating without a Multotec Pachuka Valve.

Due to the layout of the aerator no slurry particles are able to enter the air line or the valve and blockages are avoided. There are no practical pressure limits either. Despite the high pressure of the settled slurry above the valve and the 6 bar air pressure on the inlet side, the aerator opens without any delay, even after a long settlement period, and agitates the slurry to full suspension within minutes after startup.

The design of the aerator imparts a dynamic, centrifugal movement of the rising air bubbles, promoting improved dispersion of bubbles and greater leaching efficiencies. “This has resulted in a tangible and recordable increase in the levels of productivity and profitability wherever the Multotec Pachuka Valves have been installed," Schoeman concludes.




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