Ore is an impure metal containing large amount of sand and rocky material. The impurities like sand, rocky materials, limestone, mica etc present in the ore is called gangue or matrix. These impurities must be removed from the ore before the extraction of the metal. The substance added to the ore to remove the matrix called flux results in the formation of a fusible compound called slag.
The processes used for removing the gangue from the ore are based on the differences between the physical or chemical properties of the gangue and the ore. At first the ore is crushed to powder.
The pulverized ore is separated by physical processes like hydraulic washing, froth-floatation, and magnetic separation or by chemical processes, depending on the nature of the ore and its impurities. Concentration of the ore is also known as 'dressing' or 'enrichment' of ore.