Wednesday, April 30, 2008

Wire Mesh Anode

With the introduction of perforated and wire mesh titanium anode baskets, the use of primarily nickel has become the predominant nickel anode material. Small squares of electrolytic nickel, usually about 2.6 x 2.6 cm or small squares or buttons of sulfur depolarized electrolyte nickel (designated SD nickel or S nickel), or Mond nickel pellets are the forms used.

Electrolytic nickel is the least expensive and purest form of anode nickel and about 90% of all the nickel anodes now used in the U.S. are these electrolytic squares, the SD type amounting to about 50% of the total. The SD electrolytic nickel, like regular electrolytic nickel, has very low (about 0.01%) iron and copper impurities, but contains about 0.02% sulfur dispersed through its structure, which makes the nickel anode material with respect to pH, anode current density, and very low chloride concentration. Regular electrolytic nickel contains no depolarizer and is the least active nickel anode material.

The SD nickel can be used for chloride-free and high pH baths of sulfate and sulfamate type. There is no metallic sludge with SD nickel, but the nickel sulfide formed, while not voluminous and voluminous, nevertheless can be carried out into the bath if the anode bags are not kept above the solution level, which should be the general anyway. The best bag for regular electrolytic nickel are not necessarily the best bags for SD nickel, for which a tighter weave is preferred. Double bagging is especially preferred for titanium baskets over 1 m long, the outside bag being made of approved plastic fibers and of a more open weave. Cotton bags, when used without an auxiliary outer plastic bag, must in general be changed about every six to eight weeks of service due to deterioration which may lead to breaks in the bag. Great care should be exercised in changing bags to keep anode sludge from falling into the tank. it is for these reasons that plastic bags which do not rot are tested to find combinations that are as good in holding back the sludge as the best cotton bags.

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