21-Mat-B1 Hydrometallurgy and Electrometallurgy · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Professional Examinations, December 2014 — 10-Met-B1, Mineral Processing. Three hours, closed book, approved Casio/Sharp calculator only. Six numbered Problems (all compulsory except Problem 5) plus a two-mark Bonus Question. Problem 5's rubric asks for any SIX of eleven sketch-and-describe topics; all eleven topics are answered below.
Nothing on the paper is a hydrometallurgy (leaching, solvent extraction, electrowinning) or electrometallurgy question; the syllabus actually examined is comminution and grinding-circuit mass balance, particle settling, flotation kinetics, and mineral-processing equipment/terminology — the physical/mechanical beneficiation stage that precedes hydro- or pyro-metallurgical extraction.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
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 exam rubric asks for any six of the eleven topics below; all eleven are sketched and described here so this set works as a complete study reference.
A primary crusher consisting of a conical crushing head (mantle) mounted on a shaft that gyrates eccentrically inside a fixed conical crushing surface (the concave). Ore fed from the top drops into the annular gap between mantle and concave; as the mantle gyrates, the gap on each side alternately narrows (crushing rock by compression) and opens (allowing crushed product to fall through), giving continuous rather than intermittent crushing. Gyratory crushers handle very large, high-tonnage primary-crushing duties — exactly the -7 in, in-pit duty at Highland Valley in Problem 1 — at typical reduction ratios of 4:1 to 7:1.
A gravity settling/thickening unit fitted with a stack of closely spaced, inclined parallel plates inside the settling zone. Because each plate provides its own shallow settling surface, a particle only has to fall a short perpendicular distance to a plate before sliding down into the sludge zone, rather than the full tank depth. The total effective settling area scales with the sum of the plate areas (projected horizontally), so a lamella unit can match the clarification/thickening capacity of a conventional thickener many times its plan area — valuable wherever floor space is limited.
A static (no moving parts) classifier: slurry is injected tangentially into a cylindrical-conical vessel, generating a strong swirling flow. An outer, downward-spiralling vortex carries the coarser, denser particles to the conical apex (underflow); an inner, upward-spiralling vortex carries the finer particles up through a central vortex finder to the overflow. The separation (cut) size, $d_{50}$, is set by the vortex-finder and apex diameters, feed pressure, and feed pulp density — exactly the unit classifying the ball mill discharge in Problem 2.
The addition of a long-chain, high-molecular-weight polymer (flocculant) to a suspension of fine particles. Segments of a single polymer molecule adsorb onto several particle surfaces at once, physically bridging particles together into loose, open aggregates (flocs) far larger than any individual particle. Because Stokes' settling velocity scales with the square of particle diameter, flocculated material settles dramatically faster than the un-flocculated fines, which is why flocculants are dosed ahead of thickeners to speed clarification and raise underflow density; this is distinct from coagulation, which neutralises surface charge to form smaller, denser aggregates rather than bridged flocs.
A gravity-concentration device in which a bed of particles rests on a perforated screen and is subjected to a pulsating (alternating up-and-down) water flow generated by a diaphragm or plunger. On the pulsion (upward) stroke the bed dilates; on the following suction stroke, particles resettle under near-hindered-settling conditions, and because dense (valuable mineral) particles settle faster than lighter gangue of similar size, the bed progressively stratifies with heavy mineral concentrating near the screen. Concentrate is periodically drawn off through the screen into a hutch beneath the bed.
A surface-active reagent (e.g. pine oil, MIBC, or a polyglycol ether such as Dowfroth 250, as used in Problem 1's bulk circuit) added to a flotation pulp to lower the surface tension of water and generate and stabilize a persistent froth. Each frother molecule has a polar (hydrophilic) head that remains in the water film and a non-polar (hydrophobic) tail that projects into the air phase of a bubble; this orientation stiffens the bubble's liquid film, producing small, tough bubbles that can carry hydrophobic mineral particles up to the concentrate launder without collapsing prematurely.
The electrical potential measured at the shear (slipping) plane of the diffuse ion layer that surrounds a charged particle suspended in an electrolyte, located just outside the tightly bound Stern layer of counter-ions. Zeta potential governs the electrostatic component of particle-particle interaction (repulsion when like-signed and large in magnitude, promoting dispersion; attraction/coagulation as it approaches zero at the point of zero charge). Because a mineral surface's charge, and therefore its zeta potential, is strongly pH-dependent, zeta-potential measurements are used to select the pH and reagent suite (collectors, activators, depressants) that will selectively render one mineral hydrophobic while leaving another hydrophilic.
A synthetic anionic sulphydryl (thio) collector — e.g. potassium amyl xanthate (PAX), used in Problem 1's bulk copper-molybdenum circuit — used almost universally to float sulphide minerals. Its polar dithiocarbonate head chemisorbs onto exposed metal-sulphide surface sites, while its hydrocarbon tail projects outward into the pulp, converting an otherwise water-wetted (hydrophilic) mineral surface into a hydrophobic one that will attach to rising air bubbles. Selectivity between different sulphide minerals (e.g. floating chalcopyrite while depressing pyrite with lime, or copper minerals while depressing them with NaHS ahead of molybdenum flotation) is achieved by controlling pH, xanthate dosage, and companion depressant reagents.
A tall, mechanically quiescent flotation vessel with no impellers: feed slurry enters partway up the column and falls countercurrent against a swarm of fine bubbles generated by a sparger near the base. Hydrophobic particles attach to bubbles and rise into a deep froth layer at the top, which is typically washed with a small addition of clean water to displace and reject entrained (mechanically carried, non-selectively floated) gangue before the froth overflows to the concentrate launder; barren pulp exits the bottom as tailings. The long, quiescent countercurrent contact and froth wash give columns markedly higher selectivity (lower entrainment) than conventional mechanical cells, particularly in cleaner duty such as the molybdenum cleaner column in Problem 1.
An engineered embankment, raised in stages (by the upstream, downstream, or centerline method as the impoundment fills), that retains the fine-grained slurry tailings rejected from the flotation circuit (the ultimate destination of the tailings stream in Problem 1's flow sheet). Solids settle within the impoundment, forming a clarified supernatant pond that is reclaimed for process water, while seepage-collection systems beneath and around the embankment manage the water balance and support geotechnical stability. Tailings dams carry significant long-term environmental and public-safety liability, so their design, staged construction, and monitoring are governed by strict geotechnical and regulatory standards.
Laser diffraction particle-size analysis passes a collimated laser beam through a dilute particle suspension (or a dry dispersed aerosol); each particle scatters/diffracts the light at an angle that is inversely related to its size — larger particles scatter at small angles, fine particles scatter more widely. A ring array of photodetectors records the resulting angular intensity pattern, and Mie scattering theory (or the simpler Fraunhofer approximation for coarser, opaque particles) is used to numerically invert that pattern into a full particle-size distribution. The method covers roughly 0.1 to 3,000 microns in a single, rapid (seconds), representative measurement with no physical screens, making it well suited to the fine fractions (e.g. a 200-micron cyclone overflow) that are difficult to size accurately by sieving.
