PAD DESIGN AND CONSTRUCTION

Pad Design

The area required for a GEOCOAT® pad depends on: coating ratio, processing rate, stacking height, and the required biooxidation time. These factors influence pad area as follows:

     Coating Ratio - The GEOCOAT® heap is typically designed to operate with a mass ratio of concentrate to support rock (coating ratio) in the range 1:5 to 1:10. Higher coating ratios represent higher concentrations of concentrate in the heap and smaller heap volumes for a given production rate. However, very high ratios of concentrate to support rock may result in reduced oxidation rates, extending time on the pad and increasing area requirements.

     Processing Rate - Pad area increases in direct proportion to the concentrate processing rate for a given biooxidation time and coating ratio.

     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

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 off-solution pond or sump from where it overflows to the on-solution pond. The on-solution pond is excavated with sloping sides and lined with HDPE or equivalent membrane. The off-solution pond serves only to collect undiluted solution for pumping to the neutralization circuit or, in the case of base metals, to recovery operations. It may be smaller than the on-solution pond or even replaced with an inground tank. An emergency overflow pond may be required in high rainfall areas.

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 support rock used in the heap itself. 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.

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.