Leaching is  a process widely used in extractive metallurgy where ore is treated with chemicals to convert the valuable metals within the ore, into soluble salts while the impurity remains insoluble. These can then be washed out and processed to give the pure metal; the materials left over are commonly known as tailings. Compared to pyrometallurgy, leaching is easier to perform, requires less energy and is potentially less harmful as no gaseous pollution occurs. Drawbacks of leaching include its lower efficiency and the often significant quantities of waste effluent and tailings produced, which are usually either highly acidic or alkali as well as toxic (e.g. bauxite tailings). 
There are four types of leaching: 
# Cyanide leaching (e.g. gold ore)
# Ammonia leaching (e.g. crushed ore)  
# Alkali leaching (e.g. bauxite ore)  
# Acid leaching (e.g. sulfide ore)
Leaching is also notable in the extraction of rare earth elements, which consists of lanthanides, yttrium and scandium.