Copper concentrate grade, roasting, and the zinc penalty
A copper smelter receives a low-grade sulphide ore assaying 1.8% Cu, present mainly as chalcopyrite (CuFeS2), with silica gangue and about 3% sphalerite (ZnS). The current flowsheet crushes the ore, concentrates it by froth flotation, roasts the concentrate in excess air at about 750 K, and smelts the calcine with silica flux. A trial gives a concentrate assaying 24% Cu but only 71% Cu recovery; the smelter also applies a penalty if zinc in the concentrate exceeds 4%. The metallurgist proposes adding sodium cyanide to the flotation pulp and raising the roast temperature, while the plant chemist suggests abandoning roasting and simply leaching the ground ore with dilute sulphuric acid. The roast gas contains SO2 and must meet emission limits. Management wants a decision within a week, using the existing flotation and roasting equipment.
1. Explain the chemical role of NaCN in the flotation pulp and predict the effect on copper grade and recovery if the dose is excessive.
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NaCN is a depressant, not a collector. CN− adsorbs on sphalerite (ZnS) and pyrite (FeS2) surfaces and also forms the soluble complex [Zn(CN)4]2−, so these particles become hydrophilic and stay in the tailings. Chalcopyrite remains hydrophobic and is floated by the frother. The copper grade therefore rises because zinc and iron sulphides are removed from the concentrate, and the zinc penalty is avoided.
If the dose is too high, CN− also begins to depress chalcopyrite: copper recovery falls below the current 71%, and the concentrate grade may rise while copper lost to tailings increases. Excess cyanide is also toxic, so tailings require alkaline chlorination or H2O2 treatment before discharge. The correct judgement is to add the minimum NaCN that keeps Zn below 4%, and to monitor copper recovery as the control variable.
2. Write the roasting reaction for CuFeS2 in excess air and justify roasting rather than calcination. State where zinc reports in the smelt.
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In excess air at about 750 K the overall roasting reaction can be written as:
4CuFeS2 + 13O2 → 4CuO + 2Fe2O3 + 8SO2
The sulphide sulphur is oxidised to SO2 and the metals form oxides. This is roasting. Calcination is heating in a limited supply of air and is used for carbonates and hydrated oxides, for example CaCO3 → CaO + CO2. Calcination cannot remove sulphur from a sulphide because there is not enough oxygen to oxidise S to SO2. Roasting is therefore the correct pretreatment for a sulphide concentrate.
Zinc reports as ZnO in the calcine. During smelting with silica flux it enters the slag as zinc silicate. If zinc exceeds the penalty limit, slag viscosity rises and copper loss to slag increases, so control must begin in flotation rather than at the furnace.
3. Why is direct dilute H2SO4 leaching of chalcopyrite a poor substitute for the roast–smelt route? Give one process condition that would improve leaching.
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Chalcopyrite is refractory. Its copper is present largely as Cu(I) and the sulphur as sulphide; dilute H2SO4 without an oxidant cannot oxidise sulphide sulphur, so copper extraction is slow and low. The acid may dissolve any oxide already present, but it does not break the sulphide lattice.
A useful leach needs an oxidant such as Fe3+ or dissolved O2, often with bacteria (bioleaching), or elevated pressure and temperature, together with fine grinding. The roast–smelt route is chosen because the existing roaster oxidises the sulphide and makes copper recoverable in the subsequent smelt. Direct acid leaching would need a new oxidant circuit and would not by itself solve the SO2 emission problem.