On the other hand, the disadvantages of the system, except under special conditions, are very serious.
(a) More scrap is produced and requires re-treating, owing to the difficulty of separating the new deposit from the remaining portions of old anode, which often adhere very firmly.
(b) Higher voltage through the tanks is required, owing to the large number of electrodes in series in the bath. Hence the danger of current-leakage and short circuiting is greatly increased, especially when impure anodes are used, since they tend to produce conducting layers of mud on the bottom of the vat.
(c) The anode plates have to be made particularly smooth and even on the surface, since in order to lessen the voltage required, the plates are brought as close together as possible, in consequence of which, any excrescences upon the surface greatly increase the danger of short circuiting. Preliminary furnace-refining and special straightening of the anodes are therefore essential in connection with the series method.
(d) Special tanks are required, as the protecting lead liner cannot be employed, since the danger of current-leakage through it is increased, owing to the higher voltages required. Hence special acid-resisting material, such as slate, is necessary, the expense of which is considerable.
(e) The cost of stripping the cathodes is high, and the operation is often difficult.
(f) The cost of maintaining the plant is greater.
The special advantages of the multiple system are that—
- (1) It is applicable to all grades of metal;
- (2) It permits of either high or low current-density being employed;
- (3) It permits the use of cheaper lead-lined wooden tanks for working;
- (4) Anode plates may be used without previous refining, if so desired.
It is, however, general to carry out this preliminary refining, which yields sounder anodes, keeps the electrolyte purer, and promotes the more regular working of the electrodes and electrolyte—although some smelters still cast the anodes direct from converter metal.