Powder Production

The major producers of electrolytic copper powder generally follow the procedures used in electrolytic refining. However, as has been mentioned, changes in operating conditions are required to obtain a deposit of powder. Typical conditions for the production of copper powder are illustrated in Fig. 6 and can be summarized as:

Condition

Quantity

Copper

5-15 g/L (0.6-2 oz/gal)

Sulfuric acid

150-175 g/L (20-23 oz/gal)

Temperature

25-60 °C (77-140 °F)

Anode current density

430-550 A/m2 (39-50 A/ft2)

Cathode current density

700-1100 A/m2 (64-100 A/ft2)

Cell potential

1.0-1.5 V

Generally, the anodes are electrolytically refined copper, and the cathodes are lead alloy sheet. In a typical installation, the cathodes are 61 by 86 by 0.95 cm (24 by 34 by 0.37 in.) in dimension (Ref 7). Both anodes and cathodes are short to allow enough space at the bottom of the tank for the collection of the powder. The electrodes are arranged parallel to one another in lead-lined, rubber-lined, or plastic tanks, typically 3.4 m long by 1.1 m wide by 1.2 m high (11 ft long by 3.6 ft wide by 3.9 ft high). Each cell contains 18 cathodes spaced at 16 cm (6.4 in.) intervals and 19 anodes that are hung alternately. To ensure uniform current density and to eliminate short circuits in the cells, the electrodes are inspected frequently for prevention of excessive nodule buildup.

Typically, the electrolyte is pumped to an elevated storage tank, from which it flows by gravity into the tops of the cells and out through the bottoms. Thus, circulation is from top to bottom of the tanks, a type of circulation that yields a more homogeneous powder than that obtained by bottom-to-top circulation. The overflow returns to basement storage tanks to be recirculated.

The copper deposits on the cathode in the form of dendritic particles. To prevent short circuits between anode and cathode and heavy accumulation of powder on the cathode, both of which would decrease the cathode current density, the deposit is removed periodically by brushing.

After operating the cell for several days, the power is turned off. Most of the electrolyte is drained from the cell, leaving enough solution to cover the powder. The anodes and cathodes are washed down and removed, the remaining electrolyte is drained from the cell, and the powder is removed.

Thorough washing of the powder is essential. All traces of the electrolyte must be removed to prevent the powder from becoming oxidized. In addition, any remaining sulfate damages the heating elements if an electric furnace is used for subsequent drying and treatment of the powder. Various methods are employed to wash the powder. Although centrifuging to remove the electrolyte and wash the powder yields a clean product, the particles are compacted, and production of a low-density powder by this procedure is difficult.

In another method, the powder is transferred into a large tank, and water is added to produce a slurry that is pumped into a filter. In the filter, the powder is dewatered, washed several times, and again dewatered. Because the wet powder oxidizes readily due to its finely divided state and active surface, addition of a stabilizer is desirable. Treatment with an aqueous solution of gelatin protects the powder from oxidation in the intervals between successive operations (Ref 8). The addition of surface-active agents during washing or subsequent powder treatment also protects the powder from oxidation. Next, the powder must be subjected to a furnace operation to obtain an acceptable grade.

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