The ore usually obtained from the ground and therefore contained large amount unwanted impurities like sand, limestone etc.These impurities are know collectively as gangue or matrix.
The removal of these impurities from the ores is know as Concentration.The concentration id done by physical as well as chemical methods.
http://www.icbse.com/2009/387/froth-floatation-process-sulphide-ores.html