Question 5 of 6: Mineral Processing Terminology, With Sketches

Question 5 — Mineral Processing Terminology, With Sketches (30 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 exam rubric asks for any six of the ten topics; all ten are described below as a complete study resource.

5.1 — Gravitational dense medium separator (DMS)

A dense-medium separator concentrates ore by immersing it in a fluid whose density is set BETWEEN the density of the valuable mineral and the gangue, so that the valuable (or the gangue, depending on which is denser) floats while the other sinks — a direct physical application of Archimedes' principle rather than of a settling-rate difference. The medium is usually a suspension of finely-ground ferrosilicon or magnetite in water, whose bulk density is tuned by controlling the solids concentration of the suspension; because the medium is a true (or near-true) fluid, separation is essentially independent of particle size over a wide range, which is DMS's main advantage over gravity methods that rely on differential settling.

Industrially the medium is continuously recovered from both products (by magnetic separation of the ferrosilicon) and recirculated, and the bath is very often replaced by a dense-medium CYCLONE for finer feed, which adds a centrifugal force field to sharpen the density cut at small particle sizes. DMS is the standard preparation method for coal (rejecting shale/rock) and for pre-concentrating diamond ore and some base-metal ores ahead of finer downstream processing.

5.2 — Tailings dams

A tailings dam (impoundment) is the engineered structure that stores the fine-grained slurry rejected from the flotation circuit — the final-tailings stream of Question 1's flow sheet, at 0.052% Cu here but still the overwhelming majority of the original ore mass. The retaining embankment is raised in stages as the impoundment fills, using one of three constructions: the UPSTREAM method (each new raise built on the crest of settled tailings beach, cheapest but weakest, prone to liquefaction under seismic loading), the DOWNSTREAM method (each raise built outward over compacted fill or the original starter dyke, strongest, needs the most borrow material), or the CENTRELINE method (a middle ground, raised vertically on a mix of tailings and fill).

Solids in the impounded slurry settle out, and the resulting clarified supernatant pond is reclaimed as process water — reducing the mill's fresh-water demand and the volume of contaminated water needing treatment before discharge. Because a tailings dam is a permanent, large-volume structure that must remain stable for decades after the mine closes, its design, staged construction, and instrumentation (piezometers, survey monuments, seepage flow monitoring) are governed by strict geotechnical and regulatory standards; a number of major mining-industry failures worldwide have made tailings-dam design and independent review one of the highest-scrutiny disciplines in mineral processing engineering.

5.3 — Flocculation

Flocculation is the process of aggregating fine (typically sub-10 µm) suspended particles into larger, loosely-bound floccules using a high-molecular-weight polymer (a flocculant) that bridges between particles — long polymer chains adsorb onto multiple particle surfaces simultaneously (often via charged functional groups attracted to oppositely-charged sites on the mineral), physically bridging particles together into an open, settleable network. This is fundamentally different from coagulation, which instead compresses the electrical double layer (e.g. by adding salt or shifting pH) to let short-range attractive forces dominate, producing denser but generally smaller aggregates.

Flocculation is essential wherever very fine mineral slurries (mill overflow, tailings, thickener feed) must be settled or clarified in a reasonable time — unflocculated fines can take days to settle under gravity alone, while a well-selected/well-dosed flocculant can reduce settling times to minutes, which is why every tailings thickener and most concentrate thickeners in a modern mill dose a flocculant continuously ahead of the thickener feedwell.

5.4 — Hydrocyclone classifier

A hydrocyclone is a classifier with no moving parts that separates particles by SIZE (strictly, by settling rate) using a centrifugal field generated by tangential feed entry into a cylindrical-conical vessel. The feed slurry enters tangentially near the top of the cylindrical section, spiralling downward along the cone wall; coarse, fast-settling particles are thrown outward and report to the underflow (apex/spigot, the "sands"), while fine, slow-settling particles are carried inward and upward in an internal vortex to the overflow through the vortex finder (the "overflow"). It is the standard classifier paired with a closed-circuit ball mill, exactly as in Question 2's circuit.

The size at which a particle has an equal (50%) chance of reporting to either product is the corrected cut size, $d_{50c}$ (see 5.10), which for a given cyclone geometry is controlled mainly by feed pressure/flow rate, vortex-finder and apex diameters, and slurry density.

5.5 — Frother

A frother is a flotation reagent (typically a weak, heteropolar surfactant such as methyl isobutyl carbinol, MIBC, or a polyglycol ether) whose job is to stabilize the air bubbles generated in a flotation cell, producing a persistent but not overly stable froth of the right bubble size and texture to carry hydrophobic mineral particles to the cell lip without immediately coalescing or bursting. Frothers act purely at the air–water interface and are not intended to change mineral surface chemistry (that is the collector's job); the two reagent classes are complementary and are normally dosed together.

Frother TYPE and dosage strongly influence bubble size (finer bubbles generally give better fine-particle recovery but a less selective froth), froth stability/mobility across the cell, and froth drainage — getting the frother wrong is one of the fastest ways to lose either recovery (froth collapses, entrained particles fall back) or grade (an overly stable froth entrains too much unwanted gangue).

5.6 — Tabling

Tabling is a gravity concentration technique performed on a slightly inclined, riffled deck (a shaking table, e.g. the classical Wilfley table) that is driven with a rapid, asymmetric reciprocating (forward-slow, back-fast) motion along its long axis, while a thin film of wash water flows continuously across the deck from the feed side toward the far edge. The combination of the riffle-trapped, longitudinal shaking motion and the transverse wash-water film causes particles to stratify and migrate diagonally across the deck: dense, well-liberated particles are thrown along the riffles toward the discharge end near the feed corner (concentrate), while light gangue is washed transversely off the riffles and down the deck's open slope to the opposite corner (tailings), with a middlings band recovered between the two.

Tabling gives one of the sharpest separations of any gravity method — a single deck can produce distinct concentrate, several middlings, and tailings streams from one pass — but throughput per deck is low (tonnes/hour, not tens of tonnes/hour), so tables are used mainly as a final cleaning or scavenging stage (e.g. gold, tin, tungsten, and heavy-mineral-sand circuits) or for small-tonnage high-value ores, rather than as a primary high-throughput concentrator like a spiral or a hydrocyclone.

5.7 — Xanthate

Xanthates (dialkyl dithiocarbonates, general formula $\text{ROCS}_2^-$, e.g. potassium ethyl xanthate, KEX, or sodium isopropyl xanthate, SIPX) are the most widely used class of SULPHIDE-mineral COLLECTOR in froth flotation. The polar dithiocarbonate head group chemisorbs onto sulphide mineral surfaces (chalcopyrite, galena, sphalerite, pyrite, etc.), while the non-polar hydrocarbon tail projects into the pulp water, rendering the treated surface hydrophobic so it can attach to an air bubble.

Because most common sulphides respond to xanthate collectors, SELECTIVITY in a polymetallic ore (such as the chalcopyrite ore of Question 1, which also carries pyrite gangue) is achieved not by choosing a xanthate that binds only copper minerals, but by pairing xanthate with DEPRESSANTS (e.g. lime or cyanide to depress pyrite, per Question 6(i)) and ACTIVATORS/pH control that selectively suppress or enhance xanthate's chemisorption on the minerals that must be rejected or promoted at each flotation stage.

5.8 — Zeta potential

Zeta potential is the electrical potential at the SHEAR PLANE of a charged mineral particle suspended in an aqueous pulp — the plane that separates the thin layer of ions that moves rigidly with the particle (the Stern layer of tightly-bound counter-ions) from the more loosely-associated diffuse layer of counter-ions that is left behind when the particle moves relative to the bulk solution. Most mineral surfaces acquire a native surface charge in water (from broken bonds, preferential ion dissolution, or specific ion adsorption), which attracts a compensating "electrical double layer" of oppositely-charged ions from solution.

Zeta potential (not the surface potential itself, which is not directly measurable) governs whether two particles, or a particle and a bubble, will approach closely enough for short-range van der Waals/hydrophobic forces to dominate and cause attachment or aggregation, or whether electrostatic repulsion keeps them apart (DLVO theory). It is central to controlling selective flocculation, dispersion, and collector adsorption in flotation, and is why pulp pH is one of the most powerful "reagents" available in a flotation circuit — it directly shifts most oxide/silicate mineral surfaces' zeta potential, including through its isoelectric point (where zeta potential is zero).

5.9 — Thickener

A thickener is a large-diameter, shallow gravity settling tank used to increase the solids concentration of a dilute slurry (raising %solids) while producing a clarified overflow water for reuse — the unit operation named directly in Question 1's flow sheet, immediately ahead of filtering and drying of the final concentrate, and used identically on the tailings stream ahead of the tailings dam of Topic 5.2. Feed slurry (typically pre-dosed with a flocculant to speed settling) enters a central feed well, which dissipates the incoming stream's momentum so it does not disturb the quiescent settling zone. Solids settle by gravity toward the sloped tank floor, are consolidated by a slowly rotating rake mechanism that also drags the thickened solids toward a central discharge cone, and leave as underflow; the clarified water rises to the top and overflows a peripheral launder around the tank rim.

Sizing a thickener is governed by the solids settling FLUX (mass of solids settling per unit area per unit time), which for a flocculated pulp is normally determined from a batch settling test and Coe–Clevenger or Kynch analysis — the required tank AREA scales with tonnage and inversely with the limiting flux, while tank DEPTH is set mainly by the required underflow residence time and rake torque/clearance, not by the area calculation. Undersizing a thickener produces a dilute, high-torque underflow and a turbid (poor-quality) overflow; oversizing wastes capital on unnecessary tank area.

5.10 — Flotation column

A flotation column is a tall (typically 10–15 m), quiescent vessel that achieves froth flotation without the mechanically agitated impeller of a conventional flotation cell. Feed slurry enters partway up the column; fine air bubbles are generated at the base by spargers and rise counter-currently through a deep collection (pulp) zone, where hydrophobic particles attach and are carried up into a froth zone at the top. Wash water is added at the very top of the froth, percolating downward through the froth and displacing entrained (non-selectively attached) fine gangue back down into the pulp — a "counter-current washing" effect a mechanical cell cannot reproduce.

Columns achieve much higher selectivity (grade) than mechanical cells for a given recovery, at the cost of a larger footprint and slower throughput per unit volume, and are typically used as a cleaning stage (e.g. final cleaner) rather than as roughers.