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 — 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.

5.3 — Spiral (spiral concentrator)

A spiral concentrator is a gravity separator consisting of a helical trough wrapped around a vertical axis, down which a dilute slurry flows under gravity. As the slurry flows around each turn, the combination of gravity, centrifugal force and differential drag causes the denser, heavy-mineral particles to migrate toward the inner edge of the trough (where the flow is thinnest and slowest) while the lighter gangue is carried by the faster outer-edge flow toward the outer edge; adjustable splitters at the bottom of the spiral cut the cross-section into concentrate, middlings and tailings streams.

Spirals need no external power beyond the initial pumping/feed head, have a low capital and operating cost per tonne, and are widely used for coarse-to-medium (roughly 3 mm down to about 75 µm) heavy-mineral sands (ilmenite, rutile, zircon), iron ore, and chromite concentration.

5.4 — 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).

5.5 — 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.

5.6 — Rod mill

A rod mill is a horizontal, rotating cylindrical drum (length somewhat greater than diameter) partly filled with long steel rods that act as the grinding media. As the drum rotates, the rods tumble and roll (rather than cascade freely, as balls do), giving a preferential LINE-CONTACT grinding action that crushes coarse particles selectively while passing fines with comparatively little further breakage — producing a narrower, more uniform product size distribution than a ball mill fed the same material.

Rod mills are almost always used as the FIRST stage of fine grinding, taking the relatively coarse (centimetre-scale) crusher product down to a few millimetres, exactly as in Question 2's circuit, before a ball mill reduces the product further to the sub-hundred-micron range needed for flotation liberation.

5.7 — 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.

5.8 — 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).

5.9 — 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.

5.10 — $d_{50c}$ (corrected cut size)

$d_{50c}$ is the particle size at which a classifier (typically a hydrocyclone) reports EXACTLY 50% of that size fraction to the underflow (coarse product) and 50% to the overflow (fine product), after correcting the raw (actual) partition curve for the fraction of fine slurry that simply bypasses classification and reports mechanically to the underflow with the water split (the "fishhook"/bypass effect) — the "c" specifically denotes this bypass-corrected value, as opposed to the uncorrected raw $d_{50}$.