SUPPORT ROCK PREPARATION AND HANDLING

When barren support rock is used in the GEOCOAT® process, it is separated from the oxidized concentrate coating and returned to the support rock stockpile in preparation for recoating with fresh concentrate. The stockpile provides surge capacity to accommodate interruptions in either the screening process or in coating, and allows the use of different operating schedules for these activities.

During preparation for startup, sufficient support rock is crushed and screened to provide the initial inventory. Alternatively, crushing and screening rates may be matched to the demand as the heap is built. Once the heap is in full operation, only sufficient support rock is needed to make up the degradation losses which occur during each cycle of biooxidation. These relatively small quantities can be supplied by a portable crushing plant, operating one shift a week or two or three days each month. An alternative is to crush and stockpile sufficient make-up for a year or more using the services of a contractor with a portable crushing plant.

Support rock for coating is reclaimed from the stockpile by front-end loader and conveyed to the stacker on the heap. Oxidized material is unloaded from the heap with a loader or bucket wheel reclaimer and conveyed to the stripping screen. Stripped rock is returned to the stockpile by conveyor. For large operations, a support rock stockpile with a reclaim tunnel and feeders may be justified.

SUPPORT ROCK REQUIREMENTS

A support rock being considered for use in a commercial GEOCOAT® operation should have the following characteristics:

• Physical competency; i.e., resistance to degradation during mechanical handling
• Chemical resistance to the acidic bioleach solutions
• Absence of soluble constituents potentially toxic to the bacteria
• Low carbonate content
• Suitable particle shape and surface characteristics
• Local availability at low cost
• Ability to yield a clean, sized fraction by crushing and screening, without the need for scrubbing or washing (i.e., low levels of clays).

While these requirements may seem to be restrictive, a wide variety of rock types has been found to be suitable for use as GEOCOAT® support.

In addition, if ore is used as the support, the requirements for physical and chemical resistance may be relaxed somewhat, since degraded material can be separated by screening during the oxidized concentrate stripping process and routed to the mill or flotation plant for recovery of contained values.