Study of Heavy Metals

Introduction

This unit studies the occurrence, extraction and properties of five industrially important heavy metals: copper, zinc, mercury, iron and silver. Each metal is extracted from its principal ore by a metallurgical process suited to the chemical nature of that ore (sulphide, oxide, or other), then purified to the required grade.

7.1 Copper (Cu)

Occurrence. Copper occurs mainly as the sulphide ore copper pyrites (chalcopyrite), $CuFeS_2$, along with smaller deposits of cuprite ($Cu_2O$) and malachite ($CuCO_3\cdot Cu(OH)_2$).

Extraction from copper pyrites.

1. Concentration: the crushed ore is concentrated by the froth flotation process, which separates the sulphide mineral from the gangue (waste rock).
2. Roasting: the concentrated ore is heated strongly in excess air. This removes sulphur, arsenic and antimony as volatile oxides and converts part of the ore to a mixture of copper(I) sulphide and iron(II) sulphide: $$2CuFeS_2 + O_2 \rightarrow Cu_2S + 2FeS + SO_2$$
3. Smelting: the roasted ore is mixed with silica ($SiO_2$) and coke and heated in a reverberatory furnace. Iron(II) oxide formed during roasting combines with silica to form a fusible slag, which is removed, leaving a molten mixture of $Cu_2S$ and $FeS$ called copper matte: $$FeO + SiO_2 \rightarrow FeSiO_3 \ (\text{slag})$$
4. Bessemerisation: the molten matte is transferred to a Bessemer converter and a blast of air mixed with sand is blown through it. Any remaining $FeS$ is oxidised and slagged off, and the $Cu_2S$ is partly oxidised to $Cu_2O$, which then reacts with the remaining $Cu_2S$ to give metallic copper: $$ \begin{aligned} 2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2 \ 2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2 \end{aligned} $$

The copper obtained (about 98% pure) is called blister copper, because escaping $SO_2$ gas leaves it covered with blister-like marks as it solidifies. 5. Refining: blister copper is refined electrolytically. Impure copper is made the anode and a thin sheet of pure copper the cathode, in a bath of acidified copper sulphate solution. $$ \begin{aligned} \text{Anode: } Cu \rightarrow Cu^{2+} + 2e^- \ \text{Cathode: } Cu^{2+} + 2e^- \rightarrow Cu \end{aligned} $$

Pure copper is deposited on the cathode; the more valuable impurities (silver, gold) collect below the anode as anode mud.

Physical properties. Reddish-brown, lustrous, highly malleable and ductile metal; an excellent conductor of heat and electricity, second only to silver.

Chemical properties.

• Does not react with dry air at ordinary temperature, but on prolonged exposure to moist air containing $CO_2$, it develops a green coating of basic copper carbonate: $2Cu + O_2 + CO_2 + H_2O \rightarrow CuCO_3\cdot Cu(OH)_2$.
• Reacts with dilute nitric acid but not with dilute HCl or dilute $H_2SO_4$, since copper lies below hydrogen in the reactivity series; with hot concentrated $H_2SO_4$ it liberates $SO_2$: $Cu + 2H_2SO_4 \rightarrow CuSO_4 + SO_2 + 2H_2O$.
• Dissolves in aqueous ammonia in the presence of air to form a deep blue complex: $Cu + 4NH_3 + \tfrac{1}{2}O_2 + H_2O \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-$.
• Displaces less reactive metal ions from solution, e.g. copper displaces silver from silver nitrate solution.

Uses. Electrical wiring and cables, alloys such as brass (Cu-Zn) and bronze (Cu-Sn), utensils, and roofing.

Compounds of copper.

• Copper sulphate (blue vitriol), $CuSO_4\cdot 5H_2O$: prepared by dissolving copper(II) oxide or basic copper carbonate in dilute sulphuric acid and crystallising; used as a fungicide (Bordeaux mixture), in electroplating, and in the laboratory.
• Copper(I) oxide (red oxide/cuprous oxide), $Cu_2O$ and copper(II) oxide (black oxide/cupric oxide), $CuO$: used as pigments and in Fehling's/Benedict's tests for reducing sugars.