Cobalt oxides and hydroxides
1. Cobalt oxide
Cobalt can form three oxides: CoO, Co 3 O 4 , Co 2 O 3 . The first two are stable, the latter can only exist below 3oO °C. CoO 2 can only be produced in an anodic oxidation process, often in the form of aqueous hydroxides.
(a) CoO: It is the final product of cobalt carbonate or other oxide of cobalt or Co(OH) 3 calcined in a neutral or microreducing atmosphere. Pure CoO readily absorbs oxygen at room temperature to form high-priced oxides Co 2 O 3 , Co 3 O 4 , and the higher the calcination temperature, the less oxygen is absorbed. In order to obtain a relatively pure CoO, the calcination temperature must be higher than 1050 ° C, and it must be cooled in an inert atmosphere or a weakly reducing atmosphere after calcination. CoO is stable above 850 ° C, and the decompression pressure at 1000 ° C is 3.36 × 10 -12 atm. Preparation with different methods, CoO grayish green to dark gray, CoO molecular weight of 94.97, theoretically containing 78.65% cobalt, for multi-metallurgy and chemistry gray CoO, generally contain Co76%, often contain a small amount of Co 3 O 4 .
The CoO crystal is a face-centered cube with a lattice parameter of 4.2 sA, a specific gravity of 6.2 to 6.6, a heat of formation of 55.6 to 57.5 kcal/mol, and a melting point of 1810 °C. The free enthalpy change of cobalt oxidation to CoO in different temperature ranges is:
When the temperature is between 120 and 200 ° C, high-priced cobalt oxide begins to be reduced by H 2 and CO. The equilibrium constant of the CoO reduction reaction varies with temperature as follows:
The CoO hydrate has a molecular formula of Co(OH) 2 and a solubility product of about 1.6×10 -18 , which is highly soluble in hot acid.
(2) Co 2 O 3 : The molecular weight is 165.88, and the theoretical cobalt content is 71.03%. Many people oxidize CoO at an oxygen pressure of 100 atm or low temperature from Co(N 3 O) 3 , CoCl 3 to produce an oxygen content close to or equal to the oxygen content in the Co 2 O 3 meter. Co 2 O 3 . However, aqueous Co 2 O 3 is produced only in the anodization process, and dehydrated to obtain Co 2 O 3 at less than 200 °C.
(3) Co 3 O 4 : The theoretical cobalt content is 73.43%, the molecular weight is 240.82, black. Co 3 O 4 is formed when OO is oxidized in air at 400 to 900 ° C or in oxygen at 300 to 400 ° C. Co 3 O 4 becomes Co 2 O 3 due to continuous oxidation or may be chemically adsorbed in oxygen at 250 to 400 ° C, but still maintains the spinel structure of Co 3 O 4 . When dissociated or desorbed above 450 ° C, the composition of the oxide can recover or Co 3 O 4 .
When CoCO 3 or aqueous cobalt sulphate is heated in air to above 265 ° C without exceeding 800 ° C, Co 3 O 4 is formed.
Since cobalt oxides tend to form solid solutions with each other, it is difficult to measure the respective decompression pressures and stable temperature ranges, and it is considered that Co 2 O 3 ·nH 2 O is completely decomposed into Co 3 O 4 at 250 to 280 ° C. The decompression pressure of Co 3 O 4 can be lg Po 2 =- Calculated at +13.3636, it is known that Co 3 O 4 in the air dissociates mostly into CoO at 910-920 ° C, and can be completely dissociated by 980 ° C. The resulting CoO still has the spinel structure of the original Co 3 O 4 .
Co 3 O 4 is extremely difficult to dissolve in dry sulfuric acid.
Figure 1 shows the solubility of oxygen in solid metal cobalt between 600 °C and 1490 °C. The sharp decrease in the solubility of oxygen at 875 ° C is due to the crystal form change of cobalt. When the O 2 was dissolved at 0.26% (corresponding to CoO 1%), eutectic appeared, and the temperature was 1446 °C. The solidification temperatures corresponding to CoO 3.3% and CoO 14.6% were 1600 ° C and 1700 ° C.
Figure 1 Co-O system state diagram
Second, cobalt hydroxide
(1) Co(OH) 2 : It is a weak amphoteric compound which is highly soluble in acid and insoluble in water. The solubility product is 1.6 × 10 -18 . When NaOH is added to the cobalt salt solution, Co(OH) 2 is formed, which may be blue, green, and red depending on factors such as particles, adsorbed ions, time, temperature, and alkalinity. When the pH was 6 to 7 and room temperature, the blue precipitate initially precipitated was α-Co(OH) 2 . The aging becomes stable rosy β-Co(OH) 2 , and the solubility products of both are about 10 -12.8 .
Co(OH) 2 is easily oxidized to Co(OH) 3 by oxygen in the air at normal temperature:
Co(OH) 2 dissolves well in inorganic acids and organic acids and produces corresponding salts. A plurality of oxidizing agents are capable of oxidizing a solution of Co(OH) 2 and a divalent cobalt salt to Co(OH) 3 in the presence of a base.
(2) Co(OH) 3 : This is an unstable compound which is easily absorbed by water and is hardly soluble in water. The solubility product is 2.5×10 -43 . It is more soluble in hydrochloric acid and sulfurous acid and hardly soluble in sulfuric acid.
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