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The Merrill-Crowe gold recovery process removes precious metals from a cyanide solution by zinc precipitation.
The pregnant solution is first clarified through filters such as horizontal leaf type clarifiers. By using a precoat system (with diatomaceous earth) an extremely clear solution can be produced. The specific design criteria for these types of filters vary with the turbidity of the leach solutions. To avoid major upsets in the circuit, two filters should be online while another is being cleaned, precoated, and readied to go online as the next filter goes out of service. The solids removed by these filters are of no value and are backflushed to tails.
The vacuum system removes oxygen from the solution in a packed tower. Solution is percolated through a packing bed while under a vacuum. Flashing water vapor strips oxygen from the solution. Special attention must be paid to eliminating any air leaking into the tower, as this can decrease the efficiency of the vacuum system. Solution exits from the bottom of the tower and results in a very low net suction head on the precipitate feed pumps. A careful selection of these pumps will avoid cavitation and the tendency to pull air back into the solution. It should also be noted that these pumps will require liquid seal in the packing area so that air does not leak into back solution. Zinc feed addition utilizes a zinc feeder with an auger and moving side walls to avoid bridging of the material. Using a cone bottom tank with a steady head tank for zinc mixing solution (usually a cyanide solution) will assure that no air will leak into the system. With a rich pregnant solution, the zinc will be better utilized, and conversely, with a weak pregnant solution it will be less utilized.
Using lead nitrate in the zinc system can activate the zinc. It must be used in low dosages to prevent blinding off the surface of the zinc and preventing precipitation of precious metals. An excessive use of lead nitrate can form a lead hydroxide gel and blind the filters.
Zinc must be added at a constant rate. A variable speed feeder is recommended for this application. The zinc must be kept dry to keep it free flowing. The zinc solution is added to the line between the deaeration tower and the precipitate feed pumps.
Final filtration of the precious metals is accomplished with the precipitate filter feed pumps and filter presses. The feed pumps must have submerged liquid seals over the packing area or flushed double mechanical seals, and have a low NPSH requirement. Most filters are either the plate and frame type or the recessed plate type. Filter cake is collected in the chambers between the plates and can be air blown to remove a substantial amount of water.
This system will be fabricated and pre-assembled in our facility. We will do a hydrotest and in-service test.
Clarifier filters and precipitation filters will be set in place. A zinc, precoat and body feed module will be fabricated with both upper and lower levels. The upper level will be used for bag breakers, chemical storage, agitators, etc. The lower level will have pumps, tank, control panels, etc. The modules will be shipped in two pieces each, upper and lower, and will require re-assembly at the site. The pipe rack will be taken apart in three pieces (with pipes in place) and vertical supports will be shipped loose. Piping spool pieces between modules, clarifiers, precipitation filters, and the pipe rack will be prefabricated with valves and controls and shipped loose.
The procedure for re-assembly of the Merrill-Crowe System will be to set major pieces of equipment into place (modules and filters). Then set pipe rack vertical supports. The pipe rack modules can be set one at a time on top of the pipe rack supports. Cross bracing is then installed on the supports. Upper levels of modules and stairs are then set into place. All interconnecting piping can then be re-attached. All equipment and piping must be assembled loose and tightened down after final alignment has been achieved. Steel inbeds are recommended for easier field alignment of the system.
Process and utility lines can now be tied into the Merrill-Crowe System. Electrical and instruments can now be reconnected and the main power supply can be attached.
This entire re-assembly process will typically take approximately 500 man hours at site.
P.O.Box 3556
Littleton, CO 80161, USA
(303) 932-6280
Fax: (303) 932-6205
sales@denvermineral.com
