leaching

leaching, method of extraction in which a solvent is passed through a mixture to remove some desired substance from it. A simple example is the passage of boiling water through ground coffee to dissolve and carry out the chemicals necessary for producing the beverage. Another example is the removal of sugar from sugar beets using water as the solvent. Leaching is also used to remove metals from their ores. In one procedure certain crushed ores of copper are placed into a series of tanks. As a solvent, such as sulfuric acid, is pumped into the first tank, it dissolves the copper from the ore. Eventually overflowing the first tank, the solution passes into the second, where more copper is dissolved. When this tank overflows, the process is repeated in the third tank and so on. The copper is ultimately removed from the solution by chemical or other treatment.

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leaching

Loss of soluble substances and colloids from the top layer of soil by percolating precipitation. The materials are carried downward and are generally redeposited in a lower layer. This transport results in a porous and open top layer and a dense, compact lower layer. In areas of extensive leaching, the remaining quartz and hydroxides of iron, manganese, and aluminum form laterite. In such areas rapid bacterial action results in the absence of humus in the soil, because fallen plant material is oxidized and the products are leached away.