Question 1 of 7: Mineral Processing (20 marks)

Question 1 — Mineral Processing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The ten terms trace the same path as any beneficiation flowsheet: break the ore small enough to free the valuable mineral (a–c), separate value from waste by flotation (d, e, g, h, and the intermediate case f), then aggregate and dewater the resulting streams (i, j).

(a) Liberation. Liberation is the physical freeing of a valuable mineral grain from the gangue matrix that encloses it, achieved by comminution (crushing and grinding) to a particle size fine enough that individual particles are either pure value or pure gangue rather than locked composites. The liberation size is set by the grain size of the mineral within the host rock; grinding finer than that size only wastes energy without improving liberation.

(b) Concentration. Concentration is any physical or physicochemical separation process (flotation, gravity, magnetic, electrostatic) that raises the proportion of valuable mineral in a stream relative to the feed, producing a concentrate enriched in value and a tailing depleted of it. It presumes the feed has already been liberated; concentration cannot separate what comminution has not first freed.

(c) Comminution. Comminution is the size-reduction of ore by crushing (coarse, dry, mechanical compression/impact) followed by grinding (fine, wet, in a rotating rod or ball mill), performed to achieve liberation and to produce a particle size distribution suited to the downstream concentration process. It is the single largest energy consumer in a mineral-processing plant.

(d) Gangue. Gangue is the valueless mineral matter associated with an ore — the host-rock silicates, carbonates or oxides that carry no payable metal and must be rejected as tailings. The ore grade is, by definition, the mass fraction of value against a background of gangue.

(e) Tailing. Tailing (or tailings) is the final waste stream of a concentration process: the gangue-rich, value-depleted product discharged after all economically recoverable mineral has been removed to the concentrate. Tailings still carry some residual value (the process is never perfectly efficient), which is what percentage copper loss in the tailings quantifies in Question 2.

(f) Middlings. Middlings are the intermediate, imperfectly liberated particles — composite grains of value locked with gangue — that report neither cleanly to the concentrate nor cleanly to the tailings. They are usually recycled to regrinding to improve liberation and recovered on a subsequent pass, rather than accepted or rejected outright.

(g) Direct flotation. Direct flotation renders the valuable mineral hydrophobic with a collector so it attaches to rising air bubbles and reports to the froth, while the gangue stays wetted and leaves in the cell underflow as tailings. It is the default arrangement because the value is usually the minor component of the feed, so floating the smaller stream needs less reagent and less air per tonne treated — the copper sulphide flotation of Question 2 is a direct-flotation circuit.

(h) Reverse flotation. Reverse flotation inverts that logic: the gangue mineral is floated into the froth and the value is recovered in the cell underflow. It is used when the gangue is the more amenable species to collect selectively, or when the value is too abundant to float economically — silica flotation from a magnetite/haematite iron-ore concentrate with an amine collector is the standard industrial example.

(i) Flocculation. Flocculation is the aggregation of fine slurry particles by a long-chain polymer (a flocculant) that adsorbs on several particles at once and bridges them into large, open, fast-settling flocs. It is dosed at the feed well of a thickener to accelerate the gravity settling described in (j).

(j) Thickening. Thickening is gravity sedimentation of a dilute slurry in a large, shallow tank, producing a concentrated underflow of solids (raised in % solids, as in the pulp-density calculation of Question 2(e)) and a clarified overflow of water recycled to the plant. It is the standard first dewatering stage ahead of filtration for both concentrates and tailings.