COBALT SULFIDES 251

COBALT SULFIDES

Occurrence and Uses

Cobalt forms four sulfides: (1) cobalt(II) sulfide or cobaltous sulfide, CoS, MW 91.00, CAS [1317-42-6]. (2) cobalt(III) sulfide or cobaltic sulfide, or cobalt sesquisulfide, Co2S3, MW 214.06, CAS [1332-71-4] (3) cobalt disulfide, CoS2, MW 123.05. (4) tricobalt tetrasulfide, Co3S4, MW 305.04

Among these sulfides, only the ordinary cobalt(II) sulfide, CoS has commercial applications. It is used as a catalyst for hydrogenation or hydrodesulfurization reactions. Cobalt(II) sulfide is found in nature as the mineral sycoporite. The mineral linneite is made up of Co3S4, tricobalt tetrasulfide.

Physical Properties

Cobalt(II) sulfide is reddish brown to black octahedral crystal; density 5.45 g/cm3; melts above 1,100°C; practically insoluble in water (3.8 mg/L); slightly soluble in acids.

Cobalt(III) sulfide is a grayish-black crystalline substance; density 4.80 g/cm3; insoluble in water; decomposes in acids.

Cobalt disulfide is a black cubic crystal; density 4.27 g/cm3; insoluble in water; soluble in nitric acid.

Tricobalt tetrasulfide has a reddish color; density 4.86 g/cm3; decomposes at 480°C; insoluble in water.

Preparation

Cobalt sulfides are found in minerals, sycoporite and linneite, in different forms. Also, they may be readily prepared in the laboratory. A black precipitate of CoS is obtained by passing hydrogen sulfide through an alkaline solution of Co(II) salt, such as CoCl2. Also, the compound is produced by heating cobalt metal with H2S at 700°C. Heating CoS with molten sulfur for a prolonged period yields cobalt difulfide, CoS2 as a black powder. The disulfide may decompose and lose sulfur if heated at elevated temperatures.

Heating cobalt metal at 400°C with H2S yields tricobalt tetrasulfide, Co3S4. The temperatures must be well controlled in these preparative processes to obtain a specific sulfide. As the temperatures near 700°C the metal yields CoS, and above 700°C, it produces a sulfide that probably has the composition Co9S8.

Analysis

The stoichiometric compositions may be determined from cobalt analysis of nitric acid extract of the solid material by AA, ICP, or other instruments. The structural form of sulfides and their composition may be analysed by x-ray diffraction or fluorescence methods.

252 TRICOBALT TETROXIDE

TRICOBALT TETROXIDE

[1308-06-1]

Formula: Co3O4; MW 240.80

Synonyms: cobaltic cobaltous oxide; cobalto cobaltic oxide; cobaltosic oxide; tricobalt tetraoxide

Uses

Tricobalt tetroxide is a minor component of commercial cobalt oxides. It is used in ceramics, pigments, and enamels. Other applications are in grinding wheels, in semiconductors, and for preparing cobalt metal.

Physical Properties

Black cubic crystal; density 6.11 g/cm3; decomposes above 900°C, losing oxygen; insoluble in water; soluble in acids and alkalis

Thermochemical Properties

Preparation

Tricobalt tetroxide is obtained when cobalt(II) carbonate, cobalt(II) or cobalt(III) oxide, or cobalt hydroxide oxide, CoO(OH) is heated in air at temperatures above 265°C. The temperature must not exceed 800°C (see decomposition temperature above).

Reactions

Heating above 900°C expels oxygen out of the molecule forming cobalt(II) oxide:

2Co O >900oCt 6CoO + O

3 4 → 2

Tricobalt tetroxide absorbs oxygen at lower temperatures, but there is no change in the crystal structure.)

The oxide is reduced to its metal by hydrogen, carbon or carbon monoxide.

Analysis

Elemental composition: Co 73.42%, O 26.58%. The nitric acid extract of the oxide may be analyzed for cobalt by various instrumental methods (see Cobalt). Additionally, the solid crystalline product may be characterized by x- ray techniques.

COPPER 253

COPPER

[7440-50-8]

Symbol Cu; atomic number 29; atomic weight 63.546; a Group IB (Group 11) metal; electron configuration [Ar]3d104s1; (electron configuration of Cu+, [Ar]3d10 and Cu2+ [Ar]3d9); most common valence states +1, +2; two natural isotopes, Cu-63 (69.09%), Cu-65 (30.91%).

Occurrence and Uses

The use of copper dates back to prehistoric times. The metal, its compounds, and alloys have numerous applications in every sphere of life–mak- ing it one of the most important metals. Practically all coinages in the world are made out of copper or its alloys. Its alloys, bronze and brass, date from ancient times. More modern alloys such as monel, gun metals, and berylliumcopper also have wide applications. The metal is an excellent conductor of electricity and heat and is used in electric wiring, switches and electrodes. Other applications are in plumbing, piping, roofing, cooking utensils, construction materials, and electroplated protective coatings. Its compounds, namely the oxides, sulfates, and chlorides, have numerous of commercial applications.

Copper is distributed widely in nature as sulfides, oxides, arsenides, arsenosulfides, and carbonates. It occurs in the minerals cuprite, chalcopyrite, azurite, chalcocite, malachite and bornite. Most copper minerals are sulfides or oxides. Native copper contains the metal in uncombined form. The principal copper minerals with their chemical compositions and percentage of copper are listed below:

Physical Properties

Reddish brown metal; face-centered cubic crystal; density 8.92 g/cm3; Mohs hardness 2.5 to 3.0; Brinnel hardness 43 (annealed); electrical resistivity 1.71 microhm-cm at 25°C; Poisson’s ratio 0.33; melts at 1,083°C; vaporizes at 2,567°C; insoluble in water; dissolves in nitric acid and hot sulfuric acid; slightly soluble in hydrochloric acid; also soluble in ammonium hydroxide, ammonium carbonate and potassium cyanide solutions.

254 COPPER

Production

In general, copper metal is extracted from its ores by various wet processes. These include leaching with dilute sulfuric acid or complexing with ligands (e.g., salicylaldoximes), followed by solvent extraction. The solution is then electrolyzed to refine copper.

In most industrial processes, copper is produced from the ore chalcopyrite, a mixed copper-iron sulfide mineral, or from the carbonate ores azurite and malachite. The extraction process depends on the chemical compositions of the ore. The ore is crushed and copper is separated by flotation. It then is roasted at high temperatures to remove volatile impurities. In air, chalcopyrite is oxidized to iron(II) oxide and copper(II) oxide:

2CuFeS2 + 3O2 → 2FeO + 2CuS + 2SO2

Then the roasted ore is combined with sand, powdered limestone, and some unroasted ore (containing copper(II) sulfide), and heated at 1,100°C in a reverberatory furnace. Copper(II) sulfide is reduced to copper(I) sulfide. Calcium carbonate and silica react at this temperature to form calcium silicate, CaSiO3 The liquid melt of CaSiO3 dissolves iron(II) oxide forming a molten slag of mixed silicate:

CaSiO3 (l) + FeO (s) + SiO2 (s) 1100oC → CaSiO3•FeSiO3 (l)

Lighter mixed silicate slag floats over the denser, molten copper(I) sulfide. Slag is drained off from time to time. Molten Cu2S is transferred to a Bessmer converter where it is air oxidized at elevated temperatures producing metallic copper and sulfur dioxide:

Metallic copper obtained above is purified by electrolytic refining. The electrolytic cell consists of a cathode made of thin sheets of very pure copper connected to the negative terminal of a direct-current generator, and a lump of extracted impure copper from the ore serving as an anode. A solution of copper(II) sulfate in sulfuric acid is used as electrolyte. Electrolysis causes trans-

COPPER 255

fer of copper from the anode to the electrolyte solution, and from there to the cathode. Pure copper is deposited on the cathode which grows longer and larger in size. The impure copper anode correspondingly becomes smaller and smaller in size. Also, a sludge, known as anode mud, collects under the anode. The mud contains ore impurities, such as silver, gold, and tellurium, which are more difficult to oxidize than copper. Copper-plating on other metals is done by similar methods.

Reactions

Copper forms practically all its stable compounds in +1 and +2 valence states. The metal oxidizes readily to +1 state in the presence of various complexing or precipitating reactants. However, in aqueous solutions +2 state is more stable than +1. Only in the presence of ammonia, cyanide ion, chloride ion, or some other complexing group in aqueous solution, is the +1 valence state (cuprous form) more stable then the +2 (cupric form). Water-soluble copper compounds are, therefore, mostly cupric unless complexing ions or molecules are present in the system. The conversion of cuprous to cupric state and metallic copper in aqueous media (ionic reaction, 2Cu+ → Cu° + Cu2+) has a K value of 1.2x106 at 25°C.

Heating the metal in dry air or oxygen yields black copper(II) oxide which on further heating at high temperatures converts to the red cuprous form, Cu2O.

Copper combines with chlorine on heating forming copper(II) chloride. This dissociates into copper(I) chloride and chlorine when heated to elevated temperatures.

Cu + Cl heat CuCl

2 → 2

elevated

2CuCl2 temperatures → Cu2Cl2 + Cl2

A similar reaction occurs with bromine; at first copper(II) bromide is formed which at red heat converts to copper(I) bromide. Fluorination yields CuF2. Heating the metal with iodine and concentrated hydriodic acid produces copper(I) iodide. When copper is heated in an atmosphere of hydrogen sulfide and hydrogen, the product is copper(I) sulfide, Cu2S.

The standard electrode potentials, E° for the half-reactions are:

The metal is not strong enough to reduce H+ from acids to H2. Therefore, under ordinary conditions, copper metal does not liberate hydrogen from mineral acids. Copper can reduce Ag+, Au3+, and Hg2+ ions that have greater positive E° values for reduction half reactions, thus displacing these metals from their aqueous solutions.