Pressing Onward: A Dewatering Blog

Filter Presses & the Merrill Crowe Process

By Michael Stock

 

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The days of going out into a river with a shovel and a pan to mine gold are long gone. Most gold that is mined today is in the form of microscopic particles in concentrations as little as 0.5 ppm (parts per million). As a result, simple tools have been replaced by complex equipment (like filter presses) and complex chemical processes to extract gold. Today, one of the most popular methods for gold extraction is the Merrill Crowe process.

 

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About the Process

While the Merrill Crowe process is a complex series of chemical reactions, it can be summed up in one sentence. Gold is leached from the ore by removing oxygen and adding Zinc powder to a cyanide solution.

If you like chemical equations, the Merrill Crowe process looks like this:

4 Au + 8(NaCN) +O2 + 2 H2O = 4 NaAu(CN)2 + 4 NaOH

If chemical equations aren't your thing, the process can be explained a little more like this:

  • The zinc, cyanide and gold ore are all mixed together.
  • The zinc has higher affinity to cyanide than gold, and bonds with it.
  • The gold has an affinity to the zinc and cyanide particles and forms deposits on the surface of the Zinc.
  • The zinc and gold particles are then filtered out using a filter press that has precoat of diatomaceous earth or perlite.

On a visual level, in terms of the types of equipment used and their relationship to each other, the Merrill Crowe process looks like the illustration above.

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About the Presses 

At Evoqua we have the J-Press filter press by JWI designed specifically for the Merrill Crowe process. These presses have different features and specifications than those used for other processes, even other mining processes.

First and foremost all of the components on Merrill Crowe presses must be made of materials that are resistant to cyanide corrosion. Bronze, brass and galvanized parts can't be used.

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The Merrill Crowe process requires and extremely high flow rate, much higher than the flow rate used than in many other applications. As a result, Merrill Crowe presses have oversized feed and filtrate ports in the filter plates. Our presses even use special Hi-Flow Plates with 8 stay bosses instead of 4.

Since most of the gold particles being dealt with are microscopic, the filter presses need a little extra help to capture them. One option is using polypropylene filter cloths covered by disposable filter paper. Another option is outfitting the press with recessed filter plates and then coating the filter cloths with a gold attracting precoat. As we mentioned before, most precoats are made of diatomaceous earth or perlite.

 

Designing a filter press that meets all of your mining facility's needs can be a challenge, especially if there is a lot of customization required. But never fear, we're here to help! Answer a few simple questions and our sales team can help put together a dewatering system that will meet all of your dewatering needs. Take our dewatering assessment today!

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