Friday, April 25, 2008

Removal Organic and Inorganic Contaminant

Remove Organic Contaminants
In plant processing steel, iron is the usual contaminant and with constant good filtration. Iron usually presents few problems. High pH treatments in an auxiliary tank with moist nickel carbonate dispersed with good agitation of the bath are used to remove excessive concentrations of iron, as well as aluminum, trivalent chromium, or phosphate ions. The addition of about 0.5 to 1 ml of 30 % hydrogen peroxide with the high pH treatment removes all the iron as ferric hydroxide, provided at least 2 hr of agitation is used and the temperature of the bath is kept at around 60 to 65oC. Also carbon dioxide evolved from the use of nickel carbonate is removed by agitation and heating.

In plants processing copper, brass or zinc die casting, the contaminants copper and zinc are best removed by low-current density purification, preferable continuously. Zinc, copper and lead may all be removed at the same time by electrolyzing at about 0.2 A/dm2. The faster agitation, the higher is the current density that can be used to remove these metallic impurities.

Removal of Organic Contaminants
Organic contaminants may be introduced by improperly chosen tank linings, rack coatings, filter hose, patching rubber, by airborne contamination from thick dust lint, or oil spray or drippings from loose welding scale on work pieces, from inferior cleaning methods, and by breakdown at the anode or cathode of certain class II addition agents.

If the bath is excessively contaminated with organic material, the best method of removal is by adsorption on activated carbon in the auxiliary tank, after correcting as far as possible the sources of the organic contamination. Addition of hydrogen peroxide as previously described, and agitation of the bath for several hours with 1 to 3 g/l of the activated carbonate, will help the purification. After settling with filter aid and testing a sample of the treated solution in the laboratory for the condition of the plate, the solution if approved is filtered back into the plating tank.

The use of small amounts of activated carbon in the filters, such as 2 to 4 g/100 lt of solution and adding similar amounts to the filter once or twice a week is an excellent method of avoiding or minimizing costly bath treatment of plain, semibright, or bright nickel plating baths. A check must be made at regular intervals to be sure that the wetting agent or other brighteners are not excessively absorbed.

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