Question 4 of 7: Pyrometallurgical Processes (20 marks)

Question 4 — Pyrometallurgical Processes (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.

(a) What is roasting? Roasting is a gas–solid pyrometallurgical treatment carried out below the melting point of the charge, in which a mineral concentrate is heated in a controlled atmosphere — usually air or oxygen-enriched air — to convert it into a chemical form better suited to the extraction step that follows. It does not itself produce metal. Its purpose is preparation: converting a sulphide to an oxide that can be reduced with carbon, or to a sulphate that can be leached, driving off volatile impurities such as arsenic and antimony, and delivering the sulphur as a concentrated SO₂ stream that a modern plant sends to an acid plant rather than to atmosphere. Because the charge stays solid, roasting is done in fluidised-bed, multiple-hearth or flash roasters, all of which are designed to keep gas and fine solids in intimate contact.

(b) Two examples with balanced reactions. The two workhorse types are oxidising roasting and sulphating roasting.

Oxidising (dead) roasting of a zinc concentrate, which converts sphalerite to calcine for subsequent reduction or leaching:

Oxidising roasting of molybdenite, the step that produces technical molybdic oxide:

Sulphating roasting is the other important variant, run at a lower temperature and higher oxygen potential so that the sulphur is retained as a water-soluble sulphate ready for leaching:

(c) What is dead roasting? Dead roasting — also called complete or total oxidising roasting — is a roast driven to completion so that essentially all of the sulphur is removed and the product is a fully oxidised calcine containing no residual sulphide. It is carried out at the higher end of the roasting temperature range and with ample excess air, and its endpoint is judged by the residual sulphur in the calcine, typically below a few tenths of one percent. The term contrasts with partial or matte roasting, in which the roast is deliberately stopped short so that a controlled amount of sulphide remains — as in copper practice, where enough sulphur must survive to form a matte in the smelting furnace. Dead roasting is what is wanted before carbothermic reduction or before an acid leach, where residual sulphide would consume reductant or leave the value locked up. For copper sulphide, for instance:

(d) What is chloridizing roasting? Chloridizing roasting is a roast conducted in the presence of a chloride source — normally common salt, sometimes calcium chloride or chlorine gas — so that the metal is converted into a chloride rather than an oxide. It is used when the resulting chloride offers a handling advantage the oxide does not: many metal chlorides are water-soluble and can be leached directly, while others are volatile at roasting temperature and can be separated by condensation from the gas stream. Historically it was the route to silver and gold recovery from complex sulphide ores, and it remains relevant for treating refractory materials and for recovering non-ferrous values from residues and flue dusts. Its drawbacks are the corrosivity of the off-gas and the tendency of chlorides to attack refractory and ductwork, which is why oxidising roasting is preferred wherever it will serve.

(e) A balanced chloridizing-roast reaction. The classical salt roast of a silver sulphide, which converts the value to leachable silver chloride and fixes the sulphur as sodium sulphate:

The same stoichiometry applies to copper sulphide, $\mathrm{CuS + 2\,NaCl + 2\,O_2 \longrightarrow CuCl_2 + Na_2SO_4}$, the soluble cupric chloride then being recovered by water leach.

(f) Roast-gas composition for ZnS with 25 % excess air.

Given. Sphalerite concentrate is dead-roasted in air supplied 25 % in excess of the stoichiometric requirement; the roast goes to completion, producing zinc oxide calcine and sulphur dioxide.

Find. The volumetric (equivalently molar) composition of the roast gas leaving the roaster, in percent.

Approach. Take one mole of ZnS as the basis, get the stoichiometric oxygen from the balanced roast, scale it up by the excess factor, bring in the nitrogen that accompanies that oxygen, then tally only the species that leave as gas — sulphur dioxide, unreacted oxygen and nitrogen — and express each as a fraction of the total.

1. Fix the stoichiometric oxygen demand. The balanced roast consumes three moles of oxygen for every two of sphalerite, so on a basis of $n_{ZnS}=1$ mol, $$n_{O_2,\text{theo}}=\tfrac{3}{2}\,n_{ZnS}=1.500\ \text{mol}$$
2. Apply the 25 % excess. Excess air is quoted relative to that theoretical demand, so the oxygen actually supplied is $$n_{O_2,\text{sup}}=1.25\times1.500=1.875\ \text{mol}$$
3. Bring in the nitrogen that accompanies it. Air is 21 % oxygen and 79 % nitrogen by volume, so each mole of oxygen carries $79/21=3.762$ mol of nitrogen: $$n_{N_2}=1.875\times\frac{79}{21}=7.054\ \text{mol}$$ The total air blown is therefore $1.875+7.054=8.929$ mol, of which more than three quarters is inert.
4. Identify what actually leaves in the gas. The zinc oxide is a solid and reports to the calcine, so it takes no part in a volumetric gas analysis. The gas carries the sulphur dioxide produced, the oxygen that was supplied but not consumed, and all of the nitrogen: $$n_{SO_2}=1.000\ \text{mol},\qquad n_{O_2,\text{out}}=1.875-1.500=0.375\ \text{mol},\qquad n_{N_2}=7.054\ \text{mol}$$
5. Total the roast gas. Summing the three species, $$n_{\text{gas}}=1.000+0.375+7.054=8.429\ \text{mol}$$ Because all three are gases at roaster temperature and behave ideally at atmospheric pressure, mole fraction and volume fraction are the same number, so the composition follows directly by division.
6. Express each species as a volume percent. Dividing each term by the total and multiplying by 100, $$\boxed{\ \mathrm{SO_2}=11.86\ \%,\qquad \mathrm{O_2}=4.45\ \%,\qquad \mathrm{N_2}=83.69\ \%\ }$$ The three sum to 100.00 %, as they must.
7. Check the balance on oxygen atoms. Oxygen enters as $2\times1.875=3.750$ atom-mol. It leaves as $2\times1.000=2.000$ in the SO₂, $1\times1.000=1.000$ locked in the ZnO calcine, and $2\times0.375=0.750$ as free oxygen — a total of 3.750 atom-mol. The balance closes exactly, which confirms the split between calcine and gas.