Question 4 of 7: Copper Production (20 marks)

Question 4 — Copper Production (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.

Copper is produced by two fundamentally different routes depending on the ore's mineralogy: sulphide ores (chalcopyrite, chalcocite, bornite) go by pyrometallurgy — concentration, smelting, converting and refining — while oxide/carbonate ores (malachite, azurite, chrysocolla) that would not smelt economically go by hydrometallurgy — leach, solvent extraction, electrowinning (SX-EW).

(a) Flow sheet — copper from sulphide ores.

(b) Description — sulphide route. Run-of-mine sulphide ore (typically 0.5–2 % Cu, chiefly chalcopyrite $\mathrm{CuFeS_2}$) is crushed, ground and concentrated by froth flotation to roughly 20–30 % Cu (Question 1's direct-flotation chemistry). The concentrate is smelted with silica flux and oxygen-enriched air (flash or reverberatory smelting) to produce a molten copper-iron sulphide matte and a discard slag:

Matte (45–70 % Cu) is transferred molten to a converter, where blowing air/oxygen through it in two stages first slags off the remaining iron as $\mathrm{FeO}$ (with silica flux, skimmed as slag) and then oxidizes copper sulphide directly to metal:

giving blister copper at 98.5–99.5 % Cu (named for the blistered surface left by escaping $\mathrm{SO_2}$ as it solidifies). $\mathrm{SO_2}$ from both smelting and converting is captured and converted to sulphuric acid — a major byproduct revenue stream and an environmental necessity at any modern smelter. Blister is fire-refined in an anode furnace (oxidized to remove remaining sulphur, then "poled" to reduce dissolved oxygen) and cast as anodes at ∼99.5 % Cu. Anodes are finally electro-refined in an acidic $\mathrm{CuSO_4}$/$\mathrm{H_2SO_4}$ electrolyte: copper dissolves anodically and deposits as 99.99 % pure cathode copper, while gold, silver, selenium and tellurium — too noble to dissolve — fall to the cell bottom as anode slime, itself a valuable byproduct recovered separately.

(c) Flow sheet — copper from oxide ores.

(d) Description — oxide route (SX-EW). Oxide and carbonate copper minerals — malachite $\mathrm{Cu_2CO_3(OH)_2}$, azurite, chrysocolla — dissolve readily in dilute sulphuric acid, so they need no smelter at all. Crushed, often low-grade ore is stacked and irrigated with dilute $\mathrm{H_2SO_4}$ (heap leaching) or agitated in tanks (vat leaching):

giving a dilute pregnant leach solution (PLS) of $\mathrm{Cu^{2+}}$. Solvent extraction then does the concentration and purification job a flotation/smelter combination would otherwise do: an organic extractant (an oxime-type reagent) selectively complexes copper from the dilute, impure PLS into an organic phase, which is then stripped by concentrated spent electrolyte (strong $\mathrm{H_2SO_4}$) into a small volume of clean, concentrated copper electrolyte; the barren aqueous raffinate is recycled back to leaching. Electrowinning plates copper directly from that electrolyte onto stainless-steel starter cathodes, using inert lead-alloy anodes:

The sulphuric acid regenerated at the cathode is returned to leaching, closing the acid loop. The whole route needs no matte smelting, no converting and no $\mathrm{SO_2}$ capture, which is what makes it economical for the very large tonnages of low-grade oxide and mixed ore that would never smelt profitably; cathode purity (99.99 % Cu) is essentially identical to the sulphide route's electro-refined product.