Question 3 of 7: Pyrometallurgical Processes (20 marks)

Question 3 — 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.

These seven terms describe the thermal pre-treatments that convert a mined or concentrated feed into a form suited to smelting: driving off moisture and volatiles (c, a, b), decomposing carbonates and hydrates or oxidizing sulphides (d, e), agglomerating fines (f), and a specific high-temperature volatilization technique for recovering zinc from a slag (g).

(a) Low temperature coking. Low-temperature coking (carbonization) heats coal to roughly 500–700 °C in the absence of air, driving off a large fraction of the volatile matter as tar and gas while leaving a soft, reactive, high-volatile "char" coke. Because much volatile matter remains, low-temperature coke is friable and not strong enough for blast-furnace use; it is used mainly as a smokeless domestic/industrial fuel and as a source of low-temperature tar.

(b) High temperature coking. High-temperature coking heats coal to about 900–1100 °C, also in the absence of air, driving off nearly all volatile matter and leaving a hard, strong, porous coke with high fixed-carbon content. This is metallurgical (blast-furnace) coke: its mechanical strength and porosity let it support the burden and remain permeable to gas flow deep in a blast furnace, which low-temperature coke cannot do.

(c) Drying. Drying removes free (surface and pore) moisture from an ore or concentrate by simple evaporative heating, typically below 100–200 °C, without decomposing any mineral phase. It is the first and mildest thermal stage a wet flotation concentrate undergoes before roasting or smelting, reducing transport weight and preventing steam explosions in a hot furnace.

(d) Calcination. Calcination is thermal decomposition of a carbonate or hydrate mineral (e.g. limestone $\text{CaCO}_3\rightarrow\text{CaO}+\text{CO}_2$, or bauxite's hydrated alumina) by heating in air or an inert atmosphere, driving off $\text{CO}_2$ or combined water without oxidizing the metal itself. It differs from roasting in that no sulphide-to-oxide chemistry occurs; the metal's oxidation state is essentially unchanged.

(e) Roasting. Roasting is controlled oxidation of a sulphide concentrate in air (or oxygen-enriched air) at elevated temperature, below the melting point of the charge, converting metal sulphides to oxides or sulphates and releasing $\text{SO}_2$ — e.g. $2\text{ZnS}+3\text{O}_2\rightarrow2\text{ZnO}+2\text{SO}_2$. It is the essential pre-treatment before reduction smelting of a sulphide ore (Question 6) and the $\text{SO}_2$ evolved is normally captured and converted to sulphuric acid.

(f) Briquetting. Briquetting agglomerates fine ore, concentrate or coke breeze — usually with a binder, under pressure — into dense, sized lumps suitable for feeding a shaft furnace or blast furnace, whose permeability would otherwise be destroyed by fines. It is one of several agglomeration routes alongside sintering and pelletizing, chosen when the fine material is not itself well suited to sintering.

(g) Zinc fuming. Zinc fuming recovers residual zinc from a molten slag (typically a lead blast-furnace or copper smelting slag still carrying several percent ZnO) by injecting pulverized coal or natural gas with air into the slag bath. The reducing conditions volatilize zinc as metal vapour, which immediately reoxidizes on contact with air above the bath to a fine $\text{ZnO}$ fume that is collected in a baghouse as a saleable zinc-oxide product — it is a slag-cleaning operation, not a primary zinc-production route.