Specifically for sulphide ores (PbS, ZnS, CuFeS2). Powdered ore mixed with water + collector (pine oil, xanthates — makes sulphide surface hydrophobic) + frother (cresol — stabilises froth). Air blown through → sulphide particles attach to oil-coated bubbles, float as froth; siliceous gangue (hydrophilic) sinks. Selective flotation: Separating mixed sulphide ores (PbS + ZnS). NaCN added as depressant — forms Na2[Zn(CN)4] on ZnS surface, making it hydrophilic (sinks). PbS floats first. ZnS recovered separately. CuSO4 can later activate ZnS for flotation. This technique is critical for lead-zinc ores (Rajasthan mines).