PAD DESIGN

The area required for a GEOLEACH ™ pad depends on: processing rate, stacking height, operating system (On-Off vs Permanent), number of lifts, and the required biooxidation time. The sizing of a GEOLEACH ™ pad is conducted in a manner consistent with conventional acid leaching. These factors influence pad area as follows:

Processing Rate - Pad area increases in direct proportion to the processing rate for a given biooxidation time, stack height and number of lifts.

Stacking Height - Heat balance modeling and practical experience have suggested that stacking height should be of the order of 6m.

Biooxidation Time - The time required to complete oxidation to the desired extent can be predicted from the results of laboratory column testwork. However, continued adaptation of bacteria is expected to improve oxidation kinetics in a full-scale heap and oxidation time can be expected to decrease with successive cycles of operation. Pad area is directly proportional to biooxidation time.

Pad and Pond Construction

Pad design and construction is identical to that employed for conventional acid leaching. Biooxidation takes place on an impermeable pad, lined with either an HDPE membrane or coated with an asphalt system incorporating a rubberized barrier layer. The liner is protected by a one-meter layer of drain rock which incorporates solution collection piping and air distribution piping.

The pad is sloped to the solution collection ditch which runs the length of one or more sides of the heap. The collected solution is directed to the PLS or ILS solution pond. The solution ponds are excavated with sloping sides and lined with HDPE or equivalent membrane. Return solution from metal recovery (raffinate) is also stored in a pond equipped with organic recovery. An emergency overflow pond may be required in high rainfall areas. Pond and solution configurations are highly dependant on the pad design, ore type and leaching rates.

As well as containing the solution collection piping and the air distribution piping, the drainage layer protects the pad liner from damage if mobile equipment used to unload the oxidized material from the heap. The drainage rock layer may have the same specifications as the ore. However, a somewhat coarser rock may provide a better working surface for mobile equipment if this method is used for heap reclaim.

Solution collection piping, laid directly on the pad liner, is typically perforated HDPE. This material is widely used, readily available, and easily and quickly installed. Lycimeters are also installed within the heap base to facilitate panel solution sampling.

Air distribution piping is placed in the drain rock layer, above the solution collection piping. The perforated HDPE piping is connected to large-diameter headers running alongside the pad. Air is supplied by one or more low-pressure fans.