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Four main concentration methods are used depending on the ore type. Gravity (hydraulic) separation relies on density difference between ore and gangue — heavier ore settles on a vibrating inclined table while lighter gangue washes away. Best for oxide ores like cassiterite (SnO2) and haematite (Fe2O3).
Magnetic separation uses an electromagnetic roller to separate magnetic from non-magnetic materials. Two scenarios: (1) ore is magnetic — chromite attracted, siliceous gangue falls; (2) impurity is magnetic — wolframite (FeWO4, magnetic) removed from cassiterite (SnO2, non-magnetic).
Froth flotation is specifically for sulphide ores (PbS, ZnS, CuFeS2). The ore is mixed with water, collector (pine oil/xanthates — makes sulphide hydrophobic), and frother (cresol — stabilises froth). Air bubbles carry hydrophobic sulphide particles as froth; hydrophilic gangue sinks. Selective flotation uses depressants: NaCN depresses ZnS by forming Na2[Zn(CN)4] surface layer, allowing PbS to float first. CuSO4 later activates ZnS.
Leaching is chemical dissolution: Bayer's process for bauxite (NaOH dissolves Al2O3, rejects Fe2O3) and cyanide process for gold (NaCN + O2 + H2O dissolves Au as Na[Au(CN)2]).