Improved Heap Leach-pad Treatment

The traditional methods of heap leach-pad closure consist of fresh-water rinsing to attain compliance standards for heap solids and solutions. The resultant rinse solutions generally are treated chemically and discharged or evaporated.

Rinsing of heap leach pads generally requires three- to six-pore volumes to achieve weak-acid dissociable (WAD) cyanide concentrations of 0.2 milligrams per liter in draindown solutions. Rinsings can achieve cyanide-concentration regulatory requirements. However, the technique frequently fails to achieve standards for arsenic, mercury, selenium, sulfate, and nitrate.

As a cost-effective alternative, Golder Associates, in association with Times Limited, designed and constructed an in situ anaerobic biochemical system--termed the Biopass System--for passive treatment and discharge of residual drainage. The system was designed and constructed to collect, treat, and discharge residual gold-ore leach-pad drainage from a reclaimed cyanide heap leach at the Homestake Mining Company's Santa Fe Mine, in Mineral County, NV.

The approach was to convert one of the existing (in situ) solution ponds at the base of the pad into a biopass system. It would be capable of passively treating the drainage on an as-needed, long-term basis--allowing zero-maintenance facility closure. The significant design issues included

• characterizing expected draindown quantity and quality;
• establishing organic substrate mass and treatment retention times;
• devising and placing the substrate layer to minimize consolidation and maintain permeability;
• developing in situ field construction quality-assurance procedures;
• minimizing construction/installation difficulties; and
• providing a minimum 20-day retention time and a peak flow rate of 2.8 liters per second (45.1 GPM).

The biopass system was constructed in an emptied double geomembrane-lined solution pond and consisted of the following components, from the bottom upward:

• a seepage collection (influent) layer comprising gravel and perforated pipe;
• a substrate layer consisting of a spent ore gravel and composted cow manure mixture;
• an effluent-collection layer comprising gravel and perforated pipe; and
• a geotextile cushion, a geomembrane liner, and vegetative soil cover.

To the extent possible, existing facilities and on-site soil construction materials were used to reduce construction costs and minimize environmental disturbance.

Consequential draindown chemical constituents treated include WAD cyanide, sulfate, nitrate, mercury; and selenium. Treated solution gravity feeds through a buried pipeline from the effluent layer to a leach field where it is aerobically treated.

In its first year of operation, the biopass system reduced the concentration of: WAD cyanide by 83 percent, sulfate by 25 percent, nitrate by 91 percent, mercury by 98 percent, and selenium by 82 percent. The biopass system is operating at approximately a 20-day retention time and a flow rate of approximately 0.44 liters per second (7 GPM).

This case study is adapted from the presentation, "An In situ Anaerobic Biochemical System for Passively Treating Residual Cyanide Drainage," by Scott Miller and Ron Uhle, Golder Associates Corporation, 3730 Chamblee Tucker Road, Atlanta, GA 30341. Tel: 770.496.1893.