Study of Heavy Metals: Copper

In moist air containing carbon dioxide, copper slowly forms a green layer of basic copper carbonate (verdigris or patina):

The layer sticks to the metal and protects it from further corrosion.

Nickel lies above copper in the electrochemical series (\(E^\circ_{\mathrm{Ni^{2+}/Ni}}=-0.25\ \mathrm{V}\) and \(E^\circ_{\mathrm{Cu^{2+}/Cu}}=+0.34\ \mathrm{V}\)). Nickel therefore displaces copper from the solution, and the vessel is eaten away:

The same reason holds for zinc and iron containers.

Concentrated nitric acid is a strong oxidising agent. It oxidises copper to copper(II) nitrate and is itself reduced to nitrogen dioxide, a brown gas:

Copper does not react with dilute sulphuric acid alone, so it is dissolved in the hot acid with a current of air (or after being heated in air to CuO):

The solution is concentrated and cooled, and blue crystals of blue vitriol separate:

Use: it is used as a fungicide and insecticide, for example in Bordeaux mixture (blue vitriol, slaked lime and water) sprayed on grape vines and potato crops.

First, a bluish-white precipitate of copper(II) hydroxide forms:

With excess ammonia the precipitate dissolves and a deep blue (ink blue) solution of tetraamminecopper(II) sulphate forms:

This solution is called Schweitzer’s reagent. It dissolves cellulose and is used to make rayon (artificial silk).

\(\mathrm{CuSO_4\cdot5H_2O}\) (copper(II) sulphate pentahydrate).

\(\mathrm{CuCO_3\cdot Cu(OH)_2}\) (green basic copper carbonate).

\(\mathrm{Cu_2O}\) (cuprous oxide, also called cuprite).

\(\mathrm{CuO}\) (cupric oxide).

Formula: \(\mathrm{CuO}\). Use: it gives blue or green colour to glass and ceramics, and it is used as an oxidising agent (to estimate carbon and hydrogen in organic compounds).

Formula: \(\mathrm{Cu_2O}\). Use: it is used in anti-rust and antifouling paints and to make red glass.

Formula: \(\mathrm{CuSO_4\cdot5H_2O}\). Use: it is used as a fungicide and insecticide (Bordeaux mixture), and in electroplating.

Dissolve copper in hot dilute sulphuric acid while air is blown through it:

Filter, concentrate the solution by heating and cool it. Blue crystals of \(\mathrm{CuSO_4\cdot5H_2O}\) separate. (With hot concentrated sulphuric acid the reaction is \(\mathrm{Cu+2H_2SO_4\longrightarrow CuSO_4+SO_2\uparrow+2H_2O}\).)

The blue crystals lose their water of crystallisation in steps and turn white, and the anhydrous salt decomposes at a high temperature:

Copper(II) oxidises iodide to iodine and is reduced to white cuprous iodide. The solution turns brown because of the iodine that is set free:

“Vitriol” is the old name (from Latin vitreus, glassy) for the glassy, crystalline hydrated sulphates of metals. Sulphuric acid itself was called “oil of vitriol”. Blue vitriol is the glassy blue hydrated sulphate of copper, so it is a vitriol. Other examples are green vitriol (\(\mathrm{FeSO_4\cdot7H_2O}\)) and white vitriol (\(\mathrm{ZnSO_4\cdot7H_2O}\)).

The concentrated ore (chalcopyrite) is heated strongly in excess air in a reverberatory furnace:

Part of the sulphides is further oxidised:

The impurities of phosphorus, sulphur and arsenic leave as volatile oxides: \(\mathrm{4P+5O_2\rightarrow2P_2O_5}\), \(\mathrm{S+O_2\rightarrow SO_2}\), \(\mathrm{4As+3O_2\rightarrow2As_2O_3}\).

In moist air containing carbon dioxide, a green coating forms:

Introduction: Blue vitriol is copper(II) sulphate pentahydrate, \(\mathrm{CuSO_4\cdot5H_2O}\). It is called nilo tutho in Nepali and is the most important compound of copper.

Preparation: by the action of dilute sulphuric acid on copper (with air), CuO, \(\mathrm{Cu(OH)_2}\) or \(\mathrm{CuCO_3}\), followed by crystallisation:

Physical properties: it is a blue crystalline solid, soluble in water and insoluble in alcohol. Its anhydrous form is white.

Chemical properties:

Uses:

The chief ore of copper is copper pyrite (chalcopyrite), \(\mathrm{CuFeS_2}\). Copper is extracted from it by pyrometallurgy in the following steps.

1. Crushing and pulverisation. Big lumps of the ore are broken into small pieces by a jaw crusher or hammer and then ground to a powder in a ball mill.

2. Concentration (froth flotation). The powdered ore is mixed with water and pine oil in a tank, and compressed air is blown through the mixture. The sulphide particles are wetted by the oil and rise to the surface in a froth, while the earthy impurities (gangue) settle at the bottom.

3. Roasting. The concentrated ore is strongly heated in excess air in a reverberatory furnace. Copper pyrite changes into a mixture of cuprous sulphide and ferrous sulphide, and part of the sulphides is oxidised:

Impurities of P, S and As escape as volatile oxides \(\mathrm{P_2O_5}\), \(\mathrm{SO_2}\) and \(\mathrm{As_2O_3}\).

4. Smelting. The roasted ore is mixed with coke and sand (silica) and heated to about 1000 °C in a blast furnace with hot air from the tuyères. Remaining FeS is oxidised to FeO, and cuprous oxide reacts with unreacted FeS to give cuprous sulphide again. The FeO combines with silica to form a fusible slag:

The slag floats on top and the heavier molten mass below it is matte, a mixture of copper and iron sulphides with about 50% copper. All the FeS must be removed, otherwise iron is carried into the copper.

5. Bessemerisation. The molten matte is mixed with silica and charged into a Bessemer converter, a pear-shaped furnace with a lining of basic material, and hot air is blown through the tuyères. The remaining iron is removed as slag, then cuprous sulphide is partly oxidised to cuprous oxide, which reacts with the rest of the cuprous sulphide to give copper (auto-reduction):

On cooling slowly, the dissolved \(\mathrm{SO_2}\) escapes and leaves blisters on the surface. This is blister copper, 98 to 99% pure.

Figure: Bessemer converter. Air blown through the molten matte removes iron as slag and gives blister copper

6. Purification. Blister copper contains oxides and sulphides of copper and iron and traces of Ag, Au, Zn and Ni. It is purified by either of two methods.

(i) Poling (old method). The molten copper is stirred with a green pole of bamboo. The hydrocarbons in the pole reduce the cuprous oxide to copper, \(\mathrm{Cu_2O+C\rightarrow2Cu+CO\uparrow}\), and 99.5% pure copper is obtained.

(ii) Electrolytic refining (modern method). Impure copper is the anode and a thin sheet of pure copper is the cathode in acidified copper sulphate solution. When current passes, copper dissolves from the anode and deposits on the cathode, and a muddy mass of silver, gold and platinum falls below the anode as anode mud.

The copper obtained is 99.9% pure. An impure rod is used as the anode because it must supply the Cu²⁺ ions that deposit on the cathode. If a graphite rod were used as the anode, no copper would dissolve, the solution would lose its copper ions, and oxygen would be liberated at the anode instead.

Figure: Electrolytic refining of copper. Impure copper dissolves at the anode and pure copper deposits on the cathode; the insoluble impurities fall as anode mud

Blister copper is the impure copper (98 to 99% pure) obtained from the Bessemer converter. When the molten copper cools slowly, the dissolved sulphur dioxide gas escapes and forms blister-like bubbles on the surface, which gives it its name.

Extraction from chalcopyrite (\(\mathrm{CuFeS_2}\)):

The metal M is copper (Cu), atomic number 29, a coinage metal of Group 11.

The chief ore is copper pyrite (chalcopyrite), \(\mathrm{CuFeS_2}\) (also written \(\mathrm{Cu_2S\cdot Fe_2S_3}\)).

(The diagrams are given with Short Answer Question 9.)

Copper lies below hydrogen in the electrochemical series, so it does not liberate hydrogen from dilute acid. In the presence of air (oxygen), however, it dissolves:

When the blue solution is concentrated and cooled, blue crystals of blue vitriol, \(\mathrm{CuSO_4\cdot5H_2O}\), separate.

Below 1100 °C copper gives black CuO, and above 1100 °C CuO loses oxygen and the red \(\mathrm{Cu_2O}\) is obtained.

\(\mathrm{CuSO_4\cdot5H_2O}\), copper(II) sulphate pentahydrate.

Black copper(II) oxide is dissolved in warm dilute sulphuric acid:

The blue solution is filtered to remove any unreacted CuO, concentrated by heating and cooled. Blue crystals of \(\mathrm{CuSO_4\cdot5H_2O}\) separate out.

It loses its water of crystallisation step by step and turns from blue to white; the anhydrous salt decomposes at a still higher temperature:

A bluish-white precipitate of copper(II) hydroxide forms first. It dissolves in excess ammonia and gives a deep blue solution of tetraamminecopper(II) sulphate (Schweitzer’s reagent):

Overall: \(\mathrm{CuSO_4+4NH_3\rightarrow[Cu(NH_3)_4]SO_4}\).