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:: Gold Plant Pachuca Valve
The Multotec Pachuca Valve is an innovative, patented aerator which was developed by Multotec Wear Linings in answer to a major problem experienced over the last thirty years in operation of Pachuca vessels in gold reduction by leaching.
This is according to Ken Tuckey who spoke to Mining Weekly. He says that the design uses a composite of high alumina ceramics, polyurethane and stainless steel to overcome abrasion, corrosion and to provide mechanical strength and overall process efficiency.
Tuckey explains that conventional aerator designs in this particular application have not worked acceptably, and there was a need to develop an appropriate solution. With skillful adaptation of technology and incorporation of modern materials, Multotec Wear Linings produced a unit that will overcome the severe abrasion in this application and stand up to the extreme mechanical loads of the installation with resulting gains in production and profitability.
"Most modern gold recovery processes depend on gold's ability to dissolve in dilute solutions of cyanide. Pachucas 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 this latter, air is injected into the pulp at the bottom of the Pachuca through an aerator. 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.
Tuckey explains that slurry densities in the Pachuca tanks run to between 1.3 and 1.7, and if agitation stops a huge mass of corrosive gold-bearing silt settles out in the area of the aerator.
"Denser particles trapped in the aerator keep it open, allowing sediment in. Air lines clog and tons of potentially highly profitable material have to be washed out of the large tanks manually," he explains. "This means that the whole plant process has to be stopped resulting in substantial losses through additional labour costs, wasted reagents and gold concentrate and slow start-up times."
Essentially, the Multotec dual-action non-return air aerator, or Multi-Aerator, has taken the best of all the reciprocal activation designs of the past and adapted them to modern materials. The aerator closes automatically if air inlet pressure falls below a preset 1 bar, and reopens as soon as aeration is resumed.
"Due to the layout of the aerator no slurry particles are able to enter the air line and blockages are avoided. There are no practical pressure limits either. Despite the high pressure of the settled slurry above the valve and the 12 bar air pressure on the inlet side the aerator opens without hesitation, even after a long settlement period, and agitates the slurry to full suspension within two minutes of startup.
"All critical parts are manufactured from high-density alumina ceramic to minimise wear and corrosion in the highly aggressive environment of turbulent air/slurry particles and chemical reagents," Tuckey explains.
An initial production-model valve underwent extensive field trials at Vaal Reefs No 8 shaft, a reduction works on a large mine near Orkney and has proved capable of providing significant savings in plant downtime and maintenance related costs.
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