Question 3 of 7: Flotation (20 marks)

Question 3 — Flotation (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.

Froth flotation separates finely ground minerals by exploiting differences in how readily their surfaces attach to air bubbles rather than by any difference in density or size. The whole technique rests on a single interfacial idea, so it is worth setting that out before taking the eight terms in turn.

At the three-phase contact line the solid–water, solid–air and air–water interfacial tensions must balance along the solid surface, which gives Young's equation, $\gamma_{SW}=\gamma_{SA}+\gamma_{AW}\cos\theta$. The contact angle θ, measured through the water, is therefore not an arbitrary property of the mineral but the observable consequence of those three tensions. A mineral with θ near zero is completely wetted and cannot hold a bubble; one with a large θ sheds water at the contact and attaches firmly. Flotation chemistry is, in essence, the deliberate manipulation of θ on one mineral without disturbing it on the others.

(a) Direct flotation. Direct flotation is the arrangement in which the valuable mineral is made hydrophobic and reports to the froth, while the gangue stays wetted and is drawn off as tailings from the bottom of the cell. It is the default configuration because the valuable species is normally the minor component: making the smaller stream float means less air, less reagent and a smaller froth-handling duty per tonne of feed. Sulphide flotation — chalcopyrite, galena, sphalerite floated away from a silicate gangue — is the classic example, and it is the arrangement used at every porphyry copper concentrator in British Columbia.

(b) Reverse flotation. Reverse flotation inverts that logic: the gangue is floated into the froth and the valuable mineral is recovered from the cell underflow. It is adopted when the gangue is more amenable to selective collection than the value, or when the value is so abundant that floating it would mean lifting most of the feed. The two standard cases are iron ore, where silica is floated off a haematite or magnetite concentrate with an amine collector, and potash, where clay slimes and insoluble gangue are floated away from sylvite in saturated brine — the practice at the Saskatchewan potash operations. Reverse flotation is also common as a cleaning stage, used to strip a specific contaminant such as talc or carbonate out of an otherwise finished concentrate.

(c) Work of adhesion. The work of adhesion, $W_a$, is the reversible work per unit area needed to pull an attached bubble off the mineral surface and restore separate solid–water and air–water interfaces. Combining the interfacial energy change with Young's equation gives the Dupré–Young result $$W_a=\gamma_{AW}\left(1-\cos\theta\right)$$ where $\gamma_{AW}$ is the air–water surface tension. The expression is the quantitative statement of what the contact angle means: at $\theta=0$ the work of adhesion vanishes and no bubble will stay attached, whereas at $\theta=90^\circ$ it equals $\gamma_{AW}$ and the attachment can survive a good deal of turbulence. Because $W_a$ measures the energy holding the particle to the bubble, it also sets the largest particle a bubble can lift against gravity and against the shear in the pulp, which is why flotation feeds are ground fine and why coarse middlings are the first thing lost when a cell is over-agitated.

(d) Hydrophobicity. Hydrophobicity is the property of a surface that water does not readily wet — equivalently, a surface with a finite contact angle and therefore a non-zero work of adhesion to air. A few minerals are naturally hydrophobic because their surfaces present no broken ionic or covalent bonds to the water: graphite, molybdenite, talc, elemental sulphur and coal all float with nothing but a frother. Almost every other mineral of economic interest is naturally hydrophilic, its freshly fractured surface carrying charged sites that hydrogen-bond to water, and must be made hydrophobic on purpose by adsorbing a collector. The distinction matters in practice because the naturally hydrophobic minerals are the ones that float when they are not wanted — talc reporting to a nickel concentrate is a standing nuisance — and they must then be suppressed with a depressant.

(e) Collector. A collector is a surfactant that adsorbs selectively on the target mineral and renders it hydrophobic. The molecule is deliberately two-faced: a polar head group that bonds to the mineral surface, and a hydrocarbon tail, typically C₂ to C₁₈, that projects into the water and presents an oily, non-wetting outer surface. Xanthates are the workhorse for sulphides, chemisorbing on the metal sites through the sulphur of the head group; fatty acids and their soaps collect oxides, carbonates and phosphates; amines collect silicates and soluble salts, and are the collector used in the reverse flotation of iron ore. Selectivity is controlled through dosage, pH and choice of head group, since the same xanthate that floats galena at pH 9 will also float pyrite if the pH is allowed to drift down. Too much collector is as damaging as too little: it begins to coat the gangue as well and the concentrate grade collapses.

(f) Frother. A frother is a surface-active reagent that adsorbs at the air–water interface and stabilises the bubbles and the froth layer above the pulp. Its function is physical rather than chemical: by lowering the air–water surface tension and resisting local thinning of the liquid films, it prevents small bubbles from coalescing into large ones, so the cell keeps a high bubble surface area for a given air rate, and it holds the loaded froth together long enough to be scraped over the lip into the launder. Methyl isobutyl carbinol (MIBC), pine oil and the polypropylene glycols are the common choices. A good frother is chosen for a froth that is stable in the cell but breaks readily in the downstream launder and thickener — an over-persistent froth simply moves the separation problem to the next unit. Frothers should have little collecting power of their own, so that bubble stability and mineral selectivity can be tuned independently.

(g) Activator. An activator is a reagent that makes a mineral respond to a collector it would otherwise ignore, by chemically altering the surface to provide sites the collector can bond to. The textbook case is copper sulphate on sphalerite: zinc sulphide adsorbs xanthate only weakly, but Cu²⁺ exchanges for Zn²⁺ in the surface lattice to leave a thin covellite-like layer that takes xanthate readily, so the sphalerite floats. Sodium sulphide performs the analogous service for oxidised lead and copper minerals, sulphidising the surface so that a sulphide collector will work on an oxide ore. Activation is also the source of a classic metallurgical problem: dissolved copper released by an oxidising ore can unintentionally activate both sphalerite and pyrite in the lead circuit, so the copper is often deliberately complexed with cyanide to prevent it.

(h) Depressant. A depressant does the opposite of an activator: it prevents a mineral from floating, either by rendering its surface hydrophilic or by blocking collector adsorption. It is the tool that makes differential flotation of a polymetallic ore possible, since the minerals must be floated one at a time. Lime, the most-used depressant of all, raises the pH and precipitates hydrophilic hydroxide on pyrite, keeping it out of the copper concentrate; sodium cyanide depresses pyrite and sphalerite in the lead circuit by complexing surface metal ions; sodium silicate and dextrin depress silicate gangue and talc respectively; and dichromate depresses galena in a lead–copper separation. Depressants are usually reversible — the depressed mineral can be reactivated in a later stage, which is precisely how a bulk concentrate is subsequently split into separate lead, copper and zinc products.