2nh3 H2s04 Nh42s04

After conversion and adjustment of the ammonia concentration, this solution is suitable to be fed to the reduction autoclaves. The objective of the reduction process is the chemical reduction of cobaltous ammine to cobalt powder. The reduction of cobalt is a batch operation that occurs in a reduction autoclave under hydrogen atmosphere at elevated temperature and pressure.

Cobalt powder is produced in a cycle comprising one nucleation reduction, in which fine cobalt powder of sufficient surface area to function as catalyst is prepared, and up to 60 densifications. Nucleation reduction refers to the hydrogen reduction process by which fine cobalt seed powder is produced with the aid of a sodium sulfide/sodium cyanide catalyst. At the completion of the nucleation reduction, the solution is decanted from the seed powder, which remains in the autoclave to act as a catalyst during subsequent densification operations. In the course of a cycle, the originally fine seed powder coarsens or becomes densified.

The purpose of the densification reduction is to deposit reduced cobalt onto a seed particle until desired powder density, chemical composition, and screen fraction have been achieved. Each densification is a hydrogen reduction operation in which the autoclave is filled with the purified cobaltous solution, and hydrogen is added under pressure. Metallic cobalt then precipitates from solution depositing on the seed powder, by the following reaction:

The rate of the reduction reaction and its completeness depend on several factors including the ammonium sulfate concentration, the ratio of ammonia to cobalt, temperature, the hydrogen partial pressure, and the amount of seed material present. At the end of each densification, the powder is settled, the spent solution is decanted, and a fresh charge of cobaltous solution is introduced.

The slurry is then discharged from the autoclave to a flash tank where the initial separation of cobalt powder from the end solution is accomplished. The cobalt powder is transferred from the flash tank to a vacuum pan filter. After the cobalt is washed to remove ammonium sulfate, and dried, the product is packaged as powder or compacted into briquettes and sintered. Typical analysis of the Sherritt cobalt powder is given in Table 1.

Table 1 Typical properties of the Sherritt cobalt powder

Chemical composition, %













Mesh screen size, %



-100 + 150


-150 + 200


-200 + 250


-250 + 325




Physical property

Apparent density, g/cm3


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