Anodes, cast

Impressed-current CP uses an outside power supply such as a rectifier to control the voltage between the pipe and an anode (cast iron, graphite, platinum clad, mixed metal oxide) in such a manner that the pipe becomes the cathode in the circuit and corrosion is mitigated. 

Because of a strong reduction atmosphere in the furnace, some lead metal forms a layer in the bottom of the furnace. The metal is tapped every two or three days through a hole surrounded by a copper block. Molten metal is tapped into kettles through a castable-lined launder. The production rate is 10 -12 tons per each tapping. It is sent to the present anode casting plant to be used as an antimony additive. Typical analysis of the metal is shown in Table III and the minor elements in the metal are ... 

Lead-tin (Pb-Sn) [L54D00 - L55099] Usual 5-10 wt.% Sn Historically 4 wt.% Sn Tin increases mechanical strength, forms corrosion-resistant intermetallics, and improves melt fluidity during anode casting Cobalt electrowinning (500 A.m b... 

Anodes are lifted from the cells at the end of their nominated life, which may be between six and eight days. The slime deposit is removed by brushing using steel brushes and the residual scrap lead is recycled back to anode casting. Residual lead consists of that part of the anode above solution line as well as the remaining sheet below the solution line, which is needed to structurally hold the sUme deposit. The amount of umefined lead recycled in this way is about 25 to 40 per cent of input lead. 

Minor impurities can be controlled by a bleed of electrolyte and often the bleed resulting from entrainment in anode slimes is sufficient for this purpose. As indicated above, elements that will dissolve are mainly tin, nickel, iron, cadmium and zinc. Tin should be removed prior to anode casting if it is significant, otherwise if present in small amounts the electrolyte can be passed through a column of granulated lead for removal by cementation. This is suitable for controlling the bismuth concentration in electrolyte to less than 25 mg/L, which is equivalent to around 10 ppm in cathode lead. 

As detailed in Chapter 13 the electrical energy required for electrolysis is around 150 kWh/t of product lead. Some additional power for solution handling and fuels is required for the anode casting operation and for melting cathode lead in preparation for casting and starter sheet preparation. Estimates are provided in Table 16.4. 

The converter slag is typically higher in copper content, and requires slag treatment (Table II) [6], The flue gases from this step require processing in the sulfuric acid plant. The copper melt is sent to anode casting and then on to electrolysis. 

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