Removal of the unwanted materials (e.g., sand, clays, etc.) from the ore is known as concentration, dressing or benefaction. Before proceeding for concentration, ores are graded and crushed to reasonable size. Concentration of ores involves several steps and selection of these steps depends upon the differences in physical properties of the compound of the metal present and that of the gangue. The type of the metal, the available facilities and the environmental factors are also taken into consideration. Some of the important procedures for concentration of ore are:

(i) Hydraulic washing

(ii) Magnetic separat

(iii)Froth-floatation process

(iv) Leaching

Which of the following ores cannot be concentrated by hydraulic washing?

Pyrolusite

The ore particles must be heavier than its gangue particles in order to concentrate by hydraulic washing. Haematite and tinstone, two oxide ores of iron and tin, as well as native ores of silver and gold, can all be cleaned using hydraulics. By using magnetic separation, pyrolusite is concentrated.