Reduction of soluble chromate in wood ash by formaldehyde

The wood-working industry produces large amounts of wood residues. A considerable part of these is subjected to thermal utilization, and the wood ash obtained is usually disposed of. Owing to the severe regulations in several countries, disposal of wood ash has become problematic. These regulations restrict especially the water-solubility of heavy metals and organic compounds of wastes. Leachates of wood ash samples in many cases exceed the limit value for Cr-VI. To allow disposal of wood ash in future, the content of soluble Cr-VI must be lowered. In addition to a previously described procedure for the treatment of leachates containing up to 2 mg/l Cr-VI, this study presents a method of treating leachates with higher Cr-VI concentrations up to 10 mg/l. The treatment includes mixing the wood ash with aqueous formaldehyde solutions of appropriate concentration.