Question 1 of 7: Mineral Processing (20 marks)

Question 1 — Mineral Processing (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.

The first four terms describe the two operating modes of froth flotation and the surface-chemistry ideas that make either mode possible; the last four are the reagent classes a flotation circuit is built around, each acting at a different point in the bubble-particle-collector system.

(a) Direct flotation. In direct flotation the valuable mineral is the one rendered hydrophobic by the collector; it attaches to rising air bubbles and reports to the froth as concentrate, while the gangue is left hydrophilic and remains in the pulp, leaving in the cell underflow as tailings. Direct flotation is the default mode whenever the value mineral is the minority component of the feed, because floating a small mass fraction to the froth costs far less reagent and cell volume than floating the bulk gangue — floating chalcopyrite away from a silicate gangue with a xanthate collector is the textbook example.

(b) Reverse flotation. Reverse flotation inverts the roles: the gangue mineral is made hydrophobic and floated away, while the valuable mineral is depressed and recovered from the cell underflow (sink product). It is chosen when the gangue is the smaller, more selectively floatable fraction, or when floating the value mineral directly would drag too much gangue with it. The standard example is iron-ore beneficiation: quartz is floated with an amine collector at high pH while haematite/magnetite is depressed with starch, because it is easier to selectively float the small silica fraction than to selectively float the bulk iron oxide.

(c) Work of adhesion. The work of adhesion, $W_a$, is the energy released when a solid–liquid interface and a liquid–vapour interface are replaced by a single solid–vapour interface — physically, the energy released when an air bubble displaces the water film and attaches to a mineral surface. Combining the Dupré equation with Young's equation for the contact angle $\theta$ gives

where $\gamma_{LV}$ is the liquid–vapour (water–air) surface tension. A larger contact angle — a more hydrophobic surface — gives a larger work of adhesion: more energy must be supplied to strip the bubble back off the particle, so $W_a$ is really a measure of how firmly, not merely whether, a floating particle is held to its bubble.

(d) Hydrophobicity. Hydrophobicity is the property of a surface not readily wetted by water, expressed quantitatively as a finite contact angle $\theta>0$ through Young's equation, $\gamma_{SV}=\gamma_{SL}+\gamma_{LV}\cos\theta$. Almost every sulphide, oxide and silicate mineral of economic interest is naturally hydrophilic ($\theta\approx0$) and must be made hydrophobic on purpose by adsorbing a collector; a handful of minerals — graphite, molybdenite, talc, native sulphur, coal — are naturally hydrophobic and can float with little or no collector at all. Creating hydrophobicity on the target mineral (direct flotation) or on the gangue (reverse flotation) is precisely what froth flotation is built to exploit.

(e) Collector. A collector is a heteropolar organic reagent — a xanthate or dithiophosphate for sulphides, a fatty acid or amine for oxides and silicates — whose polar head chemisorbs selectively onto the target mineral's surface while its hydrocarbon tail points outward into the pulp, rendering that surface hydrophobic so it will attach to a rising bubble. Collector selectivity for one mineral over another, set by reagent chemistry and pulp pH, is the single most important choice in designing a flotation flowsheet.

(f) Frother. A frother — methyl isobutyl carbinol (MIBC), pine oil, or a polyglycol ether — is a surface-active reagent that lowers the surface tension of the pulp and generates small, stable air bubbles that persist long enough at the pulp surface to build a mineral-laden froth layer that can be skimmed off as concentrate. Frothers act at the air–water interface rather than on the mineral surface, and are deliberately chosen to be only weakly collecting so they do not interfere with the collector's own selectivity.

(g) Activator. An activator is a reagent that conditions a mineral surface, typically by an ion-exchange or surface-precipitation reaction, so that a collector which would not otherwise adsorb on it now can. The classic case is copper sulphate activating sphalerite (ZnS) for xanthate collection: $\mathrm{Cu^{2+}}$ ions exchange onto the sphalerite surface and form a thin copper-sulphide-like layer that xanthate collects readily, even though sphalerite alone floats poorly with xanthate.

(h) Depressant. A depressant is a reagent that selectively prevents one mineral from floating — by keeping its surface hydrophilic or by consuming/blocking the collector that would otherwise adsorb on it — so that a bulk ore can be separated mineral by mineral rather than all at once. Lime (raising pulp pH) and cyanide are classic pyrite depressants in copper flotation; zinc sulphate depresses sphalerite in a sequential copper/zinc separation, allowing copper to be floated first and zinc recovered afterward from the same pulp.