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Introduction Ore Processing Fire Assaying

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Heap and pregnant solution pond at Newmont Mining Corporation's Lone Tree Mine. In the background are the piles of gold ore with the sprinkler system spraying the leaching solution (sodium cyanide) onto the tops. In the foreground is the collection pond for the solution that contains the gold (pregnant solution) and is waiting for processing to recover the gold. The pond is lined with thick plastic and covered with netting to keepwildlife out.

This picture shows the heap and pregnant solution pond at Newmont Mining Corporation's Lone Tree Mine.

Leach Pad

The structure of the leach pad actually used in mining is designed for efficient leaching and environmental safety. In a mine, the construction of the leach pad begins with placement on the bottom of about 12 inches of a non-permeable layer of clay. On top of that is a system of pipes to detect any possible leaks in the pad. This is covered with a plastic sheet (80 mil). On top of the sheet is the system of pipes with holes to collect the solution as it comes through pile of rocks. A porous layer of rock covers this. Then, the ore is heaped on top. The leaching solution is introduced to the heap with a sprinkler system.

Recovery of the metal

The solution with the dissolved metal from the ore is collected and sent to a processing plant. The metal in the solution may be concentrated in one step, but eventually the metal ions in solution are reduced to the metal either by precipitation onto a more reactive metal as in our model, or by "electrowinning," a process in which an electric current passes through the solution and the metal deposits on the cathode of the electric cell.

The solution that no longer has any metal in it can be reused for more leaching on the heap.

Reclamation of the leach heap

Everything in mining must be returned to a condition as nearly natural as possible. This means the leach pad must be made environmentally benign by rinsing or bioprocessing so the water that flows off of the heap meets water quality standards. The slopes of the heap must be stabilized, and the hill must be planted with grasses and shrubs.

Gold Recovery

Although gold leaching is not as visible as copper leaching, the most common method of leaching gold uses cyanide, oxygen and water:

4Au + 8NaCN + O2 + 2H2O ® 4NaAu(CN)2 + 4NaOH

Copper Recovery

Some copper ores are processes with heap leaching. Sulfuric acid is used to dissolve copper carbonate ores. The general reaction is

Cu2CO3(OH)2 (s) + 2H2SO4 ® 2CuSO4 + H2CO3 (aq) + 2H2O

Copper sulfate is soluble in water, so can be readily processed. The carbonic acid (H2CO3) will probably break down to carbonate and water: H2CO3 ® H2O + CO2.

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