Self-propagating Reactions for Environmental Protection:  Remarks on the Treatment and Recycling of Zinc Hydrometallurgical Wastes

Abstract

A new method of treatment of a highly toxic solid waste from electrolytic zinc plants, i.e., goethite waste, based on self-propagating thermite reactions is proposed. This technique consists of blending this waste with a suitable amount of reducing agents (aluminum, silicon, magnesium, and calcium) used either individually or combined and ferric oxide and igniting the resulting mixture so that a self-propagating front reaction rapidly travels through the mixture without requiring additional energy. Reactants are converted into two solid products (P 1 and P 2 ) with different mass, composition, and structure and a gas constituted by SO 2 . Because of magnesium's relatively higher reducing character, its use greatly increases the reactivity of the mixture, thus allowing the treatment of a larger amount of waste. However, the combination aluminum/silicon was preferred as the reducing agent, because the reaction product P 1, which represents about 85% of the total mass of the original mixture, is constituted by an amorphous glassy structure of alumino silicates which embodies heavy metals, such as Pb and Zn. Leaching tests of reaction product P 1 demonstrate the fulfillment of the environmental regulations for heavy metals. On the other hand, the solid product P 2 can be recycled in the roasting unit of the industrial zinc production plant. A waste treatment process is also proposed.