21-Mat-B2 Pyrometallurgy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2015 — 10-Met-B2, Hydrometallurgy and Electrometallurgy. Three hours, closed book, approved Casio/Sharp calculator only. Six numbered Problems, each worth 20 marks: Problems 1 and 2 are compulsory; the rubric asks for any 3 of the remaining 4 (Problems 3-6). All six Problems are answered below.
Nothing on the paper is a pyrometallurgy (roasting, smelting) question; the syllabus actually examined is aqueous leaching, solubility/Eh-pH (Pourbaix) diagrams, electrowinning thermodynamics, and hydrometallurgical flowsheeting.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
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.
A hydrometallurgical flow sheet is built from five generic unit-operation blocks, in series, with two recycle loops layered back onto them (Fig. 1).
1. Comminution (crushing/grinding). The ore, containing valuable metal A locked in a gangue matrix, is crushed and ground to liberate metal-A-bearing mineral grains and to expose surface area for leaching. Product is a solid pulp or dry ground ore, sized to the leaching method chosen (fine grind for agitated tank leaching, run-of-mine or crushed lumps for heap leaching).
2. Leaching. The ground ore contacts a lixiviant (an acid, base, or complexing/oxidizing solution — e.g. H2SO4, NaOH/NaCN, or an ammoniacal solution, chosen for metal A's chemistry) that selectively dissolves metal A into solution while leaving most of the gangue as an insoluble solid residue. This is where recycled barren solution (raffinate, see below) is normally reused as makeup lixiviant, reducing fresh reagent consumption.
3. Solid/liquid separation (S/L). The leach pulp is a slurry of leach residue solids suspended in the pregnant leach solution (PLS). Counter-current decantation (CCD) thickeners, or filtration, split this into (a) a solid leach residue stream (washed to recover entrained metal A, then sent to a tailings facility) and (b) a clarified liquid PLS stream carrying the dissolved metal A plus dissolved impurities.
4. Purification. The PLS is not pure — it carries co-dissolved impurity ions. Purification (solvent extraction, ion exchange, selective precipitation, or cementation, depending on the impurity/metal-A chemistry) removes those impurities (as a solid impurity residue or a liquid effluent stream to treatment) and/or upgrades and concentrates metal A into a purified, concentrated liquor.
5. Metal recovery. The purified liquor is converted to the finished product: electrowinning (metallic A deposited at the cathode, with a gas such as O2 or H2 evolved at the anode) for a metal product, or chemical precipitation/crystallization for a pure compound product (e.g. a salt or oxide of A).
Streams (per the question's own checklist). Solid streams: ground ore feed, leach residue, impurity residue, final metal/compound product. Liquid streams: lixiviant feed, pregnant leach solution, purified liquor. Recycle: the barren solution leaving metal recovery is recycled back to leaching as makeup lixiviant — this is what keeps fresh-reagent consumption low. Bleed: a small fraction of the recycle loop is continuously bled off (rather than recycled) to prevent the slow build-up of unwanted dissolved impurities and by-product ions that would otherwise accumulate indefinitely in a fully closed loop; the bleed is treated as an effluent or sent for separate impurity recovery. Effluents/residues: the leach residue (solid tailings), the impurity residue/effluent from purification, and the bleed stream.
