CHAPTER XXXIII.

COPPER, SILVER, AND GOLD.

455. Relation of Copper, etc., to the Alkali Metals. Copper, silver, and gold are in most respects different from the other members of the first periodic group, but are related to these elements - the alkali metals much as zinc, cadmium, and mercury are related to the calcium group.

They are not changed by water or by pure air; hence they occur native as well as combined with other elements. Because they occur native all three have been known for thousands of years, while none of the alkali metals was known until 1807.

It

456. Occurrence and Metallurgy of Copper. Copper is abundant and widely distributed. occurs native, especially in the Lake Superior region, and in combination as ruby copper, Cu2O; malachite, CuCO3. Cu(OH)2; chalcocite, Cu2S; and copper pyrites, Cu2S. Fe2S3. Bornite is (Cu2S)3. Fe2S3; azurite, Cu(OH)2. 2 CuCO3.

Of the world's supply of copper, the United States produces more than half: 504,000 tons in 1910.

Native copper is obtained by crushing the ore, washing away the lighter particles, and smelting and refining the concentrated "mineral." Lake Superior ore has from 0.75% to 5% of copper.

OCCURRENCE AND METALLURGY.

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Oxide, hydroxide, and carbonate ores are smelted with coke in blast-furnaces. One fusion often yields a 96% copper.

Sulphide ores usually contain iron, as well as lead, arsenic, etc. Except for ores of very low grade, the processes are as follows:

1. The ores are concentrated by crushing and washing, and are then roasted (cf. § 252), to remove most of the sulphur. The sulphur dioxide produced may be used for sulphuric acid. Part of the iron becomes iron oxide, and most of the copper, copper oxide.

2. The roasted ore is then mixed with coke, to reduce the copper oxide, and with acid silicates, to unite with the iron oxide. The mixture is smelted in a blast-furnace (cf. § 482; Fig. 88). Iron silicate is formed as a light slag that can be drawn off, while the heavy liquid below it consists largely of the sulphides of iron and copper. It contains from 30% to 60% copper, and is called "matte."

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3. The third operation is the formation of "blister copper," by the oxidation of the matte in a Bessemer "converter," like the one used in making steel (cf. § 487; Fig. 90). In the 99 converter a blast of hot air, sometimes containing sand, is forced up through the melted matte, and removes most of the remaining sulphur, arsenic, antimony, lead, and iron, as oxides. The volatile oxides escape, while the non-volatile ones form a scum on the surface, or unite with the silica, with which the converter is lined, to form silicates, or slags. Some of the copper oxide formed reacts with cuprous sulphide:

Occluded sulphur dioxide (cf. §§ 53 and 463), which escapes. when the copper solidifies, gives the metal a characteristic blistered appearance.

4. The air-blast of the converter oxidizes some of the copper. The process called "poling" consists in reducing the copper

oxide by stirring the melted blister copper in a reverberatory furnace (cf. § 462; Fig. 86) with long poles of " green" wood. Finally, the product is cast in the form of thick plates called "anodes," and refined by electrolysis.

5. Refining of Copper. The refining process (Fig. 85) consists in making the thick plates of impure copper the anodes of electrolytic cells containing cupric sulphate solution; while a thin plate of pure copper forms the cathode. The anode wastes away, and its copper is deposited, in pure form, upon the cathode. Many copper ores contain small amounts of gold and silver,

and these accompany the copper up to the refining process. In the electrolysis, however, the precious metals, along with some cuprous sulphide, lead, and bismuth, are left in the refining tanks mud." This is collected, and the gold and silver are extracted from it.

as

457. Properties. Copper has a red color, and is ductile and malleable. It melts at about 1080° C., while silver melts at 954° C. and gold at 1060° C. Copper is the best conductor of electricity known, except silver; iron is the only metal having greater tensile strength. The specific gravity of copper is 8.9.

COPPER COMPOUNDS.

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In the atmosphere copper becomes coated with the basic carbonate (cf. malachite, § 456). The action of nitric acid and sulphuric acid upon copper has been discussed in §§ 227 and 265, respectively. Hot, concentrated hydrochloric acid reacts slightly with it, giving cuprous chloride and hydrogen. Dilute acids act on it slowly, in the presence of air, giving cupric salts. The sulphate is made in this way.

Cu+O+H2SO4→→→→ CuSO4+H2O.

Copper may be separated from solutions of its salts by zinc, iron, etc., and by electrolysis.

458. Uses. Copper is used as an electric conductor, as ships' sheathing and bolts, and for electrical apparatus, utensils, coins, boilers, stills, etc. It is used, also, in copper-plating and electrotyping, and as a part of many alloys.

Brass usually contains 28% to 34% zinc and the remainder

copper.

Bronze contains copper, zinc, and tin.

Gun-metal is about 90% copper and 10% tin.

Bell-metal consists of copper, about 75%, and tin, 25%. German silver is, approximately, 50% copper, 30% zinc, and 20% nickel.

Aluminum bronze is copper with 5% to 10% of aluminum. It has the color of gold, is hard and elastic, and does not tarnish easily. Aluminum containing 3% of copper is whiter than pure aluminum.

459. Copper Compounds. Copper, like mercury, iron, etc., forms two series of compounds: cuprous and cupric compounds (cf. §§ 111 and 169). Examples

are:

Cuprous chloride, CuCl or Cu2Cl2.

Cuprous oxide, Cu2O.

Cuprous sulphide, Cu,S.

Cupric chloride, CuCl2.

Cupric oxide, CuO.
Cupric sulphide, CuS.

In the cupric compounds copper is evidently bivalent. If copper is bivalent in the cuprous compounds of the halogens, e. g., in cuprous chloride, the simpler formula must be doubled.

The constitutional formula will then be

Cu - Cl.

Cu - Cl.

Cuprous oxide occurs naturally as "ruby copper." It is formed as a black scale when the metal is heated in the air, and as a red precipitate when a solution of a cupric salt is heated with the solution of an alkali in the presence of a suitable reducing agent, e. g., grape-sugar, C6H12O6.

Cuprous sulphide is made by heating copper with sulphur. Cupric oxide is formed by treating a boiling solution of a cupric salt with the solution of an alkali. The blue cupric hydroxide, Cu(OH)2, which is formed if the solution is cold, cannot exist in the boiling solution.

Cupric sulphate pentahydrate is blue vitriol (cf. § 88). The anhydrous salt is white. It dissolves in hot, concentrated sulphuric acid, and separates, when cold, in white crystals. Blue vitriol is used in making blue and green pigments, in copperplating, in preserving wood, and for gravity batteries. It is also used extensively in agriculture as a germicide and fungicide. The Bordeaux mixture consists of calcium hydroxide and cupric sulphate. A minute amount of cupric sulphate, too small to kill larger water animals, is able to destroy the algae of ponds and water reservoirs, and so to purify the water.