In the Merrill-Crowe process, a cyanide leach solution is mixed with the crushed ore to create gold and silver cyanide. The metals are then precipitated from the pregnant leach solution by adding zinc dust.

Attaining optimum results is highly dependent upon the design and operation of the conditioning steps preceding the zinc dust treatment, including clarification of the pregnant leach and the removal of dissolved oxygen. Following clarification and de-aeration, the solution and zinc dust are deposited uniformly on precipitate filters, where the zinc pulls the cyanide and leaves gold and silver as the precipitant. The barren solution is then recycled for reuse in the process.

The degree of clarification of the solution is a key factor in the effectiveness of the process. Virtually all impurities must be removed prior to its exposure to zinc dust in order for the pregnant leach to work most effectively.

“The solution must be clarified to create a very clean stream with essentially less than 1 ppm suspended solids,” explains Eldan Hill, project and service manager for Summit Valley Equipment & Engineering, Inc. (SVEE). SVEE designs and fabricates modular plants and equipment for gold and silver extraction worldwide. “The stream then goes through a vacuum tower that reduces dissolved oxygen down to less than 1 ppm. These two steps are necessary to make the process precipitate optimally and use less zinc.” Hill says the clarified effluent should be crystal clear. “Long-time operators emphasize that the stream should appear ‘brilliant,’ in order to provide optimal conditions for gold and silver recovery.”

Leaf Filters
Over the years, pressure leaf filters have emerged as the best option for clarification applications in Merrill-Crowe systems. In a typical system, the clarifying tanks are fitted with a compartment where the leaf filter elements are pre-coated with diatomaceous earth as a filter aid material. This pre-coat layer forms a dynamic filtration surface that achieves less than 1 ppm suspended solids and 1 micron quality.

Pre-coating the clarifier leaves with diatomaceous earth prevents scale-forming compounds—primarily carbonates and sulphates of lime—from moving downstream onto the precipitate filter. If these colloidal solids are not captured during the clarification stage, they encounter favorable conditions for scale formation in the precipitate filter cake. Scale deposits can cover the zinc surfaces, increasing zinc consumption while reducing zinc effectiveness in precipitating the silver and gold. Scale build up can also impair flow through the filter fabric, resulting in higher pressures and the need for excessive filter cloth changes.

The filters require frequent and thorough rinsing followed by a new layer of the pre-coat material. The fact that the filter cloth operates under the protective precoat layer helps preserve filter life and permeability. Filter cleaning is accomplished more quickly and completely because the pre-coat filter aid has trapped the harmful solids before they can foul the surface of the filter fabric. The diatomaceous earth material is actually the filtering medium that achieves fine filtration.

Minimizing Maintenance and Downtime
Apart from the objective of optimum clarification of the pregnant leach, operation and maintenance features often determine the type of filter selected. In a large plant, pressure leaf filters that are easy to service help save labor and costs.

“You can backwash a well-designed leaf filter in about five to 10 minutes, precoat it with diatomaceous earth, and put it back online,” says Hill. “Other filter types could be used. But, for example if you use a plate-and-frame press, you’d have to open up that press and physically clean it and then put it back online and then precoat it.” By contrast, the service time on a pressure leaf filter is typically very low.

Ease of automation also plays a role. When the operator observes that differential pressure on a filter is increasing, the necessary valves can be opened or closed electronically, and the pumps to backwash and pre-coat the filter can be operated remotely. “On a new project we’re carrying out in Turkey, operators will actually change the filters from the control room,” Hill added.

In addition to Merrill-Crowe systems, SVEE, based in West Bountiful, Utah, designs and fabricates electrowinning systems, mercury retort systems, carbon regeneration systems, and ADR plants for gold and silver operations worldwide. Hill says his firm typically designs Merrill-Crowe systems around the Auto-Jet pressure leaf filter from Whittier Filtration, based in Santa Fe Springs, California. “These filters offer high flux rates and can run continuously and clean every one to three days.” The filter is designed to remove fine clays down to the 0.5-1 micron size.

No “Can” Means Faster Cleaning
One of the advantages of the design of the Auto-Jet filter, according to Hill, is its sluicing system. As the filter leaves rotate, flat, knife-like jets of cleaning liquid efficiently remove heavy cake and scrub the entire surface area in three minutes or less. Hill says that some leaf filter designs have an enclosure, or a rolling “can,” over the actual filter area, which makes serving the units burdensome.

“A large space is required behind the filter for removing the can so that the filters can be exposed for service. To change filters out, re-cloth the plates or perform any other service, operators must roll this can off, which typically requires an overhead crane rail. Once the service is performed, it then has to be rolled back into position. The way the Auto-Jet is designed, a single operator can open its front door and pull all the filters out the front for service.”

Sizing Considerations
In one eastern European gold mine, two Auto-Jet clarifiers each handle up to 432 m3 of solution per hour. The filters are pre-coated with diatomaceous earth, supplied to the filters as a slurry at 2% solids. During filtration, a 5% slurry of diatomaceous earth is added to the clarifiers by high-pressure positive displacement pumps at a design rate of 0.07 kg/m3 to prevent filter media blinding and to maintain a high rate of filtration.

These filters are fully automated. When one filter is full, the feed is shut off and switched over to the standby filter. The internal high-pressure water jets automatically remove the filter cake and the cake slurry is pumped to a thickener and the clarified pregnant solution feeds directly to the Merrill-Crowe aeration tower. The newly cleaned filter is placed on standby for the next rotation.

“We often include three filters, depending on the size of the operation,” says Hill, “Two filters will be on line at one time, and when one loads up, it’s taken off line and the third filter is brought on. While one filter is backwashing and precoating, another filter is ready to be dropped in, so there is really no downtime.”

Clarifying filters for the Merrill-Crowe process are typically sized to run 0.75 to 0.8 gallons per minute (gpm) per square foot. “They sluice at half the flow rate,” Hill says. “For example, if a plant is to run 1,000 gallons per minute, it would require 1,250 square feet of filter area and the system would sluice at 500 gpm, at pressures between 55 and 60 psi,” said Hill. “The filter pre-coats at the same rate as the process (1,000 gpm in this example) and at about 20 psi.

“Typical dilution is about 5%-10% slurry, and the more dilute the better for the distribution of the pre-coat. The system is also body-fed (additional diatomaceous earth bled into the process stream) at a rate of 1.3 gpm at 1% concentration per 1,000 gpm of process,” said Hill. Ken Severing (ken.severing@veoliawater. com) is technical director for Whittier Filtration, Santa Fe Springs, California.