Size reduction is required to liberate the minerals of interest to recover them selectively Ore Textures Require Different Grinding Strategies / Stages to liberate minerals Coarse Grind Req’d
Medium Grind Req’d
Massive Sulphide
Net Texture
Pyrrhotite (Waste) Copper Mineral
Fine Grind Req’d
Disseminated Texture
Ni Mineral Serpentine (Waste)
Scale of pictures: each square represents rock 3.5mm x 3.5mm
Particle Liberation where economic mineral is coloured red
Barren
Locked
Middling
Liberated
Scale: particle images are from +53 or +75um size fractions
A typical Mineral Processing Plant will be designed to maximize the recovery of minerals of interest / limited by the constraints of capital and operating costs and required concentrate grade 8
The fine art of milling – “Grinding Mills” Wet grinding Material ground as a slurry Percent solids by weight (60 to 80%) Dependent on subsequent process Requires less power compared to dry Media and liners consumption higher than dry Eliminates formation of dust
The fine art of milling – “Grinding Mills” Dry grinding Ground in a dry state Material less than 1% H20 Dependent on subsequent process Requires 1.3 times power as wet Media and liners consumption less than wet Requires dust collection system
The fine art of milling – “Grinding Mills” Open circuit One pass through mill Average 1.4 to 1.5 more power over closed circuit Broad product distribution Less control compared to closed circuit
The fine art of milling – “Grinding Mills” Closed circuit Mill discharge is fed to a sizing device with oversize returning to
the mill Oversize returning is called circulating load Less kW per unit ground vs. open circuit Product top size control Used in both wet and dry milling
Grinds product from primary grinding step, which is normally; Rod Mill, AG-Mill or SAG Mill Grinding
Grinding media (Pebbles) consists of screened out fractions of the ore itself, synthetic pebbles or manufactured Al2O3. 50-90 mm in size, sometimes larger
Grate discharge
Lower charge and pulp density than Ball Mill Grinding
Pebble Mills are larger than Ball Mills running at the same horse power draw
Pebble consumption normally between 5-20% of the ore quantity
