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 span the whole mineral-processing flowsheet, from the size reduction that liberates value from gangue, through the physical (density, conductivity) and chemical (surface) methods used to separate the two, to the vocabulary used to describe how well a plant is doing its job. They are taken in flowsheet order rather than alphabetically.

(a) Gravity concentration. Gravity concentration separates particles by the difference between their densities, using a fluid (normally water) as the separating medium rather than any chemical reagent. Jigs, shaking tables, spirals and Reichert cones all exploit differential settling velocity or differential response to pulsation and flowing-film motion; how separable two minerals are is summarised by the concentration criterion $CC=(\rho_h-\rho_f)/(\rho_l-\rho_f)$, where $\rho_h$, $\rho_l$ and $\rho_f$ are the heavy-mineral, light-mineral and fluid densities. A value above about 2.5 (as for gold in quartz) makes gravity separation easy at almost any size; below about 1.25 it becomes impractical below a few hundred microns, which is why fine, low-density-contrast ores are handed off to flotation instead.

(b) Dense media separation. Dense media separation (DMS, also called heavy-media separation) is a static-density gravity method: crushed feed is immersed in a suspension of finely ground ferrosilicon or magnetite in water, whose bulk density is set between that of the valuable mineral and the gangue. Particles denser than the medium sink, particles lighter than it float, and the cut is essentially a sharp sink–float split independent of particle size over a wide range — a genuine advantage over jigs and tables, whose efficiency is size-dependent. The medium is recovered magnetically and recirculated. DMS is the standard pre-concentration step for coal (rejecting shale on a ferrosilicon medium) and for kimberlite diamond ore, where it is normally the very first unit operation after crushing, ahead of any flotation or chemical step.

(c) High tension (electrostatic) separation. High tension separation sorts particles by electrical conductivity rather than density. A thin monolayer of dry, charged feed is fed onto a grounded, rotating drum electrode inside a high-voltage field (20–30 kV); conductive particles acquire and lose an induced charge quickly, are held only weakly to the drum, and are thrown off ballistically, while non-conducting particles retain their charge and cling to the drum until mechanically brushed off further round. It is the workhorse of heavy-mineral-sand processing, separating conductive rutile and ilmenite from non-conductive zircon and silica, and it requires the feed to be dry and closely sized, since moisture and coarse fines both defeat the charge transfer the method depends on.

(d) Recovery. Recovery is the fraction of a valuable component reporting to the concentrate, expressed as a percentage of that component present in the feed:

where $f$ and $c$ are the assay grades of feed and concentrate and $F$ and $C$ are their respective mass flows. Recovery and grade trade against each other in every real circuit — pushing recovery higher by floating longer or coarser inevitably drags in more gangue and lowers concentrate grade — so recovery is never quoted alone in a metallurgical balance; it is always read together with the grade it was bought at.

(e) Comminution. Comminution is the staged size reduction of run-of-mine ore — primary and secondary crushing followed by rod, ball or SAG (semi-autogenous) milling — carried out to liberate the valuable mineral grains from the gangue that encloses them. It is deliberately staged because the specific energy to break rock rises sharply as particle size falls (Bond's law, $W=10\,W_i\left(1/\sqrt{P_{80}}-1/\sqrt{F_{80}}\right)$), so a plant grinds only as fine as the liberation size actually requires and no finer; over-grinding wastes energy and can generate slimes that are difficult to float or settle. Comminution is consistently the largest single energy consumer in a concentrator, commonly 35–50 % of total site power.

(f) Tailing. Tailing is the discard, low-value stream leaving a concentration step — what is left after the wanted mineral has been removed. In flotation it is the pulp that never attached to a bubble; in gravity or DMS circuits it is the light or low-density fraction. Tailings are normally thickened and pumped to an impoundment; because they still carry whatever reagents and residual value the circuit did not recover, tailings-facility design and closure are now as much a part of mine economics and permitting as the plant itself.

(g) Middlings. Middlings are the intermediate-grade fraction reporting between clean concentrate and true tailing — composite particles in which valuable mineral and gangue are still physically locked together (incomplete liberation), or particles of borderline density/conductivity that a separator cannot confidently assign to either product. Rather than being discarded with the tailing or accepted into the concentrate, middlings are almost always recirculated: reground to improve liberation and re-fed to the head of the separation stage, or recleaned through a scavenger circuit. A middlings stream is therefore a normal, expected part of circuit design, not evidence of a fault.

(h) Froth flotation. Froth flotation is a surface-chemistry separation: finely ground pulp is aerated in a cell, a collector reagent renders the target mineral's surface hydrophobic, and those particles attach to rising air bubbles and are carried into a froth layer that overflows into a launder, while hydrophilic gangue stays wetted and reports to the cell underflow. Because it discriminates by surface chemistry rather than by density or conductivity, flotation reaches particle sizes and mineral pairs that gravity and electrostatic methods cannot, and it is consequently the dominant concentration method for base-metal sulphide ores worldwide.

(i) Selective flocculation. Selective flocculation uses a polymeric flocculant that adsorbs preferentially on the surface chemistry of one fine mineral, bridging those particles into large, fast-settling flocs while leaving the other minerals in the slurry individually dispersed (often with a dispersant added deliberately to keep them that way). It is aimed squarely at the size range — typically below about 20 µm — where flotation loses efficiency and gravity methods fail outright because Stokes settling becomes too slow to be practical; the classic industrial example is flocculating fine haematite out of a dispersed silica slime in iron-ore fines processing.

(j) Hydrophobicity. Hydrophobicity is the property of a surface that is not readily wetted by water, expressed quantitatively as a finite contact angle (Young's equation, $\gamma_{SW}=\gamma_{SA}+\gamma_{AW}\cos\theta$) and, equivalently, a non-zero work of adhesion to an air bubble. A handful of minerals — graphite, molybdenite, talc, native sulphur, coal — are naturally hydrophobic because their surfaces present no broken ionic bonds to water; almost every sulphide, oxide and silicate of economic interest is naturally hydrophilic and must be made hydrophobic on purpose by adsorbing a collector. Hydrophobicity is therefore the single property froth flotation is built to create and exploit, term (h) above.