21-Mat-B2 Pyrometallurgy · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2018 — 12-Mtl-B2, Hydrometallurgy and Electrometallurgy. Three hours, closed book, approved Sharp/Casio 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 here, since this set is a study resource rather than an exam script. Given constants: R = 8.314 J/(mol K); F = 96,485 C/g-eq; for all aqueous species, activities are taken equal to concentrations.
Nothing on the paper is a pyrometallurgy (roasting, smelting) question — the syllabus actually examined is aqueous flow-sheeting terminology, cyanide-complex electrochemistry (Eh-pH diagram reading), metal-hydroxide speciation/solubility, sulfide precipitation, and electrowinning energetics.
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 generic hydrometallurgical flowsheet is built from five core unit-operation blocks in series — comminution, leaching, solid/liquid separation, purification and metal recovery — closed by a recycle loop and a mandatory bleed (Fig. 1). An optional oxidative-roasting pretreatment step is shown ahead of leaching for refractory sulfide feeds, since "as detailed as possible" invites noting when a purely hydrometallurgical route is not sufficient on its own.
1. Comminution (crushing/grinding). The freshly excavated ore is crushed and ground to liberate the valuable-metal-bearing mineral grains from the gangue and to expose surface area for leaching. Product sizing is chosen for the leaching method (fine grind for agitated tank leaching, coarse/run-of-mine for heap leaching).
2. Roasting (optional pretreatment). Refractory ores — e.g. sulfides that encapsulate the valuable metal, or carbonaceous/preg-robbing gold ores — are oxidatively roasted before leaching to convert sulfide to oxide/sulfate and break the encapsulation, releasing SO2 off-gas (routed to acid-plant or scrubbing). Free-milling oxide/oxidized ores skip straight to leaching.
3. Leaching. The ground (or calcined) ore contacts a lixiviant — an acid, base, or complexing/oxidizing solution (H2SO4, NaOH/NaCN, ammoniacal liquor, etc., chosen for the target metal's chemistry) — that selectively dissolves the valuable metal while leaving most gangue as an insoluble residue. Recycled barren solution is normally reused here as makeup lixiviant.
4. Solid/liquid separation (S/L). Counter-current decantation (CCD) thickeners or filtration split the leach pulp into (a) a washed leach-residue solid (to tailings) and (b) a clarified pregnant leach solution (PLS) carrying the dissolved valuable metal plus co-dissolved impurities.
5. Purification. Solvent extraction, ion exchange, selective precipitation or cementation removes co-dissolved impurities (as a solid impurity residue or an effluent to treatment) and concentrates/upgrades the valuable-metal liquor.
6. Metal recovery. The purified liquor is converted to the finished product — electrowinning (metal deposited at the cathode, a gas such as O2 or H2 evolved at the anode) for a metallic product, or chemical precipitation/crystallization for a pure compound product (a salt or oxide), matching the question's own "metal or pure compound" framing.
Streams (per the question's own checklist). Solid streams: ground/calcined ore feed, leach residue, impurity residue, final product. Liquid streams: lixiviant feed, pregnant leach solution, purified liquor. Recycle: barren solution leaving metal recovery returns to leaching as makeup lixiviant, cutting fresh-reagent consumption. Bleed: a small, continuous purge of the recycle loop prevents unwanted dissolved species (sulfate, chloride, slowly-accumulating impurities) from building up indefinitely in an otherwise closed loop. Effluents/residues: leach residue (solid tailings), roaster off-gas, the purification impurity stream, and the bleed.
