Question 5 of 7: Hydrometallurgy (20 marks)

Question 5 — Hydrometallurgy (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.

Hydrometallurgy dissolves value out of an ore or concentrate with an aqueous reagent rather than by heat, so its vocabulary is drawn from solution chemistry (parts a–d, h) and from the mechanical equipment that contacts solid and liquid (parts e, f, g).

(a) Amphoteric substance. An amphoteric substance is one that can react as either an acid or a base depending on what it is reacted with — it will donate a proton (or accept OH⁻) in a strongly basic environment and accept a proton (or donate OH⁻) in a strongly acidic one. Most metal hydroxides that show this behaviour are amphoteric because the metal–oxygen bond can be polarised either way depending on the pH of the surrounding solution.

(b) Example. Aluminium hydroxide, Al(OH)₃, is the textbook case and the one this syllabus turns into an industrial process (Question 4a): it dissolves in excess strong base as aluminate,

and it also dissolves in excess strong acid as the aquo cation, $\mathrm{Al(OH)_3+3\,H^+\rightarrow Al^{3+}+3\,H_2O}$. Zinc hydroxide, Zn(OH)₂, and chromium(III) hydroxide, Cr(OH)₃, behave the same way and are the reason zinc and chromium are troublesome to precipitate cleanly from a leach liquor — overshoot the pH window on the high side and the metal simply redissolves as the hydroxo-complex.

(c) Buffer solution. A buffer solution is one that resists a change in pH when a small amount of acid or base is added, because it contains a conjugate acid–base pair (a weak acid and its salt, or a weak base and its salt) present in comparable concentration. Added acid is consumed by the conjugate base component and added base by the conjugate acid component, so the pH shifts only slightly, following the Henderson–Hasselbalch relation $\mathrm{pH=pK_a+\log([A^-]/[HA])}$. Buffering matters in hydrometallurgy because many leach and precipitation steps must be held in a narrow pH band to control which metal hydroxide precipitates and which stays in solution.

(d) Neutralization. Neutralization is the reaction of an acid with a base to produce a salt and water (in the aqueous case), driving the solution pH toward 7 (or, more generally in process terms, to whatever target pH the following step needs): $\mathrm{H^++OH^-\rightarrow H_2O}$. In hydrometallurgical practice, neutralization with lime or limestone is the standard method for raising a spent, acidic leach liquor's pH to precipitate impurities (iron, aluminium) as hydroxides before the pregnant liquor is sent to metal recovery, and again at the end of the flowsheet to condition raffinate and tailings water for discharge.

(e) VAT leaching. VAT leaching contacts a static bed of relatively coarse, crushed (but not finely ground) ore, loaded into a large open tank ("vat"), with lixiviant that percolates down through the bed by gravity or is pumped through and recirculated. Because there is no agitation, mass transfer relies on the lixiviant diffusing into and reacting through the pore structure of the ore particles, which makes VAT leaching slow — cycles run days to weeks — but cheap, since it needs no grinding beyond crushing and no mechanical agitation. It suits ores where the value is reasonably accessible without fine grinding, and remains common for lower-grade oxide ores where capital cost per tonne treated must stay low.

(f) Pulp leaching. Pulp (or agitation) leaching finely grinds the ore into a pumpable slurry and contacts it with lixiviant in a mechanically or air-agitated tank, so that fresh reagent is continuously brought to every particle surface rather than having to diffuse through a static bed. This gives much faster kinetics — hours rather than days — and much higher recovery from a given ore, at the cost of the extra grinding energy and the agitation and solid–liquid separation equipment it requires. It is the standard route wherever the economics justify fine grinding, and it is the form of leaching carried out under pressure in an autoclave, part (g).

(g) Autoclave. An autoclave is a sealed, pressure-rated vessel used to leach (or otherwise react) a pulp at temperatures above the atmospheric boiling point of water, typically 150–230 °C at pressures of several MPa, often with oxygen or air injected directly into the pulp. Running above atmospheric boiling dramatically accelerates reaction kinetics and lets oxygen partial pressure be pushed high enough to oxidise refractory sulphides that will not leach at atmospheric pressure — pressure oxidation of refractory gold and nickel-sulphide concentrates being the classic application. The trade-off is high capital cost and demanding materials of construction (titanium or acid-resistant brick linings) to survive the combination of heat, pressure and acid.

(h) Hydrolysis. Hydrolysis is the reaction of a dissolved species with water that splits a water molecule and typically produces a precipitate plus free acid, for example the hydrolytic precipitation of ferric iron from a sulphate leach liquor:

Because hydrolysis of this kind releases acid, it is self-limiting unless the acid is neutralised as it forms, and it is exploited deliberately in hydrometallurgy as an impurity-removal step — precipitating iron, aluminium or other unwanted metals as hydroxides or basic salts (jarosite, goethite) ahead of the main metal-recovery circuit, controlled by holding pH and temperature in the band that hydrolyses the impurity without hydrolysing the value metal.