Extended freezing conditions exist in many parts of the world in the winter because of latitude or high altitude or both. Such conditions, coupled with the sprinkling techniques commonly used in heap leaching, result in formation of layers of ice which inhibit leaching by tying up solution inventory on heap surfaces and shutting off solution percolation. Loss of leaching availability can be serious, cutting total available leaching time to as little as 8 months per year in many cases. To increase leaching time and precious metal production, several methods have been developed and others can be considered.
Some feel that heating of solutions might provide protection against glazing the heap surface with ice. A submerged combustion heater was tried because of its high thermal efficiency. This approach heated the circulating solutions economically but had the undesirable side effect of dissolving large amounts of carbon dioxide in alkaline liquors. This carbon dioxide in combination with the calcium ion already present made existing pipeline scaling problems much worse. A heat-exchanger can be installed in place of submerged combustion and has been used successfully ever since.
Nevada has pioneered the large scale application of buried drip irrigation for precious metal leaching. The installation was developed to insure continuity of leaching operations during the winter months.
Flexible solution application lines equipped with pressure regulated emitters are buried about 3 ft deep by unreeling the line behind a 3 ft long ripper on a bulldozer in a fashion similar to placement of telephone or television cable. This technique efficiently places the irrigation pipeline header at a depth well below the frost level.
Each emitter has a radius of influence of approximately 6 ft. Consequently, adjacent irrigation lines are placed on 12 ft centers. Capillary action causes emitted solution to spread outward in a circular pattern approximately level with elevation of the emitter. Sample excavations at various times have verified the original solution application concept. Soon after initiation of irrigation, leach solutions migrate outward from the emitter in a pattern which insures essentially complete internal coverage of the heap under leach.
The concept of buried irrigation has an irrigation system arranged on the heap surface and then carefully covered with enough additional ore to protect the irrigation assemblage from freezing. Upon advent of warmer weather, the ore cover is then sprinkled from above to provide leaching of the topmost material.