WATER BALANCE AND SUPPLY

Water Balance

Make-up water is required to replace losses by evaporation, and to compensate for water removed with the solution bleed stream and the oxidized concentrate thickener underflow. Inflows to a GEOLEACH ™ system include water added with the ore, heap rinse water (if employed), and precipitation on pad and pond surfaces. Deficiencies in the water balance are made up by the addition of fresh water, either to the ILS pond or as rinse water.

The typical GEOLEACH ™ solution application rate of 10L/m 2 .h corresponds to 240mm of solution applied to each square meter of heap daily. Short-duration precipitation events are, therefore, comparable in intensity to routine solution application operations and no adverse effects on the system are anticipated from such events. During periods of prolonged heavy rainfall, however, it may be necessary to stop the irrigation to maintain the heat balance.

Depending on climatic conditions at the site, an emergency storm water pond may be required to handle runoff resulting from storm events. A water balance analysis must be developed based on site-specific meteorological data to determine the surge capacity required. In drier climates where rainfall intensity is low, it may be possible to contain excess water from precipitation in the ILS pond. In this situation, sufficient freeboard must always be maintained in the pond to accommodate the runoff from the design storm event.

In any case, the water balance issues involved are the same as those which are routinely addressed in the design of heap operations for cyanide leaching of gold ores and acid leaching of copper ores. The principles are well understood, and provided that adequate meteorological data is available for the site, a solution management plan can be developed to ensure a safe and environmentally acceptable operation.

Water Supply

Tailings dam return water is not acceptable for use as make-up if it contains even traces of cyanide, thiocyanate, or other cyanide decomposition products. Cyanide and its decomposition products are extremely toxic to the microbes. Careful design of the water system is therefore required to ensure that contamination of the heap and solution circuit with cyanide cannot occur. Most GEOLEACH ™ systems will operate as a closed circuit with minimal to no effluents solutions. The lack of a bleed stream may result in high levels of many impurities developing in the solution system. Care needs to be taken to monitor toxicity levels of all recycled solutions particularly those that may contain organic.