Question 1 of 7: Design and Operational Knowledge – True/False with Explanation

Question 1: Design and Operational Knowledge – True/False with Explanation (20%)

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.

i) Grizzly aperture vs. gape

Answer: True. Standard primary-crusher design practice (Mular, Halbe & Barratt, Mineral Processing Plant Design, Ch. 5) sets the run-of-mine grizzly (scalping screen ahead of the primary crusher) at roughly 80–85% of the crusher's gape – the gape being the maximum feed opening. Scalping out material that already passes this size before it ever enters the crusher reduces the load on the crusher and prevents fines from being needlessly re-broken, directly increasing crushing-station throughput for a given installed power.

ii) Jaw crusher above 600 t/h

Answer: False. A jaw crusher's reciprocating, single-toggle action crushes only on the forward stroke, which caps its practical continuous throughput; above roughly 600–800 t/h a primary gyratory crusher is the standard selection instead, because its full-circle gyrating mantle crushes continuously through every part of the cycle and scales far more economically to high, continuous tonnage. Jaw crushers remain the better choice for lower/moderate tonnage, portable and semi-mobile plants, and duties where the smaller footprint and lower capital cost outweigh the throughput ceiling.

iii) Autogenous mill diameter:length ratio

Answer: True. Because an AG/SAG mill uses the ore itself (plus, for SAG, a small ball charge) as grinding media rather than a dense rod/ball charge, it needs a large mill diameter relative to its length (typical D:L on the order of 2:1 to 3:1) to develop enough cascading/impact energy per revolution – the opposite proportion to a conventional rod or ball mill, whose D:L is close to 1:1 to 1.5:1 because the dense rod/ball charge itself supplies the grinding energy and a longer barrel simply adds residence time.

iv) Rod mill L:D ratio and 30-foot rods

Answer: False. Rod mills are indeed held to a narrow L:D of about 1.4:1 to 1.6:1 (a ratio below about 1.4:1 lets rods tangle and bend, ruining selective coarse-particle grinding), so the first clause is broadly correct. The statement is false overall, though, because rod length is limited in practice to about 6–6.4 m (20–21 ft) by rod straightness and bending under their own weight – a 30-foot (≈9.1 m) rod is well beyond what mills are actually built and loaded with, regardless of the L:D target.

v) Dry grinding energy allowance

Answer: False. The standard Bond/Rowland efficiency correction for dry (vs. wet) grinding is a factor of about 1.3, i.e. roughly 30% extra energy, not 15% – dry grinding loses efficiency to poorer particle transport, cushioning by fines, and the absence of the lubricating/dispersing action water provides between grinding media and particles, and design practice allows for the larger figure.

vi) Slimes and wet gravity separation

Answer: True. Wet gravity devices (jigs, spirals, shaking tables, dense- medium units) separate by settling-rate/density differences in a viscous carrier fluid; fine slimes (minus 400 mesh, ≈37 μm) raise the effective pulp viscosity and coat coarser particles, degrading the density-based separation. In excess of about 10% slimes, gravity circuits routinely lose efficiency badly enough that desliming (typically by hydrocyclone) ahead of the gravity stage becomes standard practice.

vii) Cyanidation vs. flotation slurry density

Answer: True. Cyanide leach (CIL/CIP) circuits are normally run thick – typically 40–50% solids by weight – to maximise retention time and reagent/tank utilisation over the many hours a leach needs, whereas flotation cells need a thinner, more mobile pulp (typically 25–40% solids) for good bubble–particle contacting, adequate air dispersion and froth mobility. Gold cyanidation plants are therefore designed for meaningfully greater slurry density than flotation plants.

viii) Rougher retention-time scale-up factor

Answer: True. Bench flotation cells are small, well-mixed and close to ideal plug behaviour compared with a full-size mechanical cell bank, which suffers short-circuiting and non-ideal mixing; industry design practice therefore scales bench rougher retention time up by a factor of about two when sizing plant rougher cell volume, to reproduce the recovery actually achieved at bench scale.

ix) Water addition to the grinding-circuit pump box

Answer: False. Water addition at the pump box is in fact a standard, deliberate density-control tool, used to hold cyclone feed density (and hence classification sharpness and circulating load) at its design point. It is true that adding water raises the volumetric flow the cyclone cluster must handle (a real capacity/number-of-units trade-off, exactly the correction factor $$C_1=\left[\frac{53-V}{53}\right]^{-1.43}$$ used later in Question 6 quantifies, where a lower percent-solids-by-volume V actually sharpens/fines the D_50c cut) – but that trade-off is managed by circuit design, not avoided by withholding water, so the blanket "should be avoided" claim is incorrect.

x) Hydrophobicity in sink-float separation

Answer: False. Sink-float (dense-medium/heavy-liquid) separation sorts particles purely on specific-gravity difference relative to the suspension or medium density – cassiterite (SnO₂, SG≈7.0) is recovered from silicate gangue (SG≈2.7) precisely because of this large density contrast. Hydrophobicity (the tendency of a mineral surface to attach to an air bubble) is the controlling parameter in flotation, not in a purely gravity-driven sink-float process.