Removal of Mercury from the Sludge Produced by the Chlor-Alkali Industry by Using the Electro-Leaching Process

摘要

In different regions of the planet, such as Asia, Europe and South America, the chlor-alkali industry, which produces caustic soda, chlorine, hydrogen and sodium hy-pochlorite from concentrated brine solution (NaCl), generates effluents and residues loaded with Mercury. During the production process re-circulated brine is submitted to a chemical treatment that precipitates cations, among them the Hg~(2+) species, as hydroxides. This precipitate ends up encapsulating both the mercury droplets and mercury sulphide (HgS), constituting the so-called chemical mud. The electro-leaching of such mud is accomplished by hypochlorite ions, as a result of the chemical reaction of chlorine, being produced by the oxidation of chloride ions from the brine solution, electrolyte of the cells, with water. This technical contribution aimed at providing a feasible technological process for treating those high mercury content residues (frequently > =6.1g of Hg/kg of residue), far beyond the limits established by CONAMA (National Environment Commission). During the electro-leaching process the hypochlorite ions, generated in the bulk of the residue suspension, are oxidant enough to dissolve the aforementioned mercury species, Hg~0 and HgS, generating quite stable soluble species (HgCl~(2-)_4) that are simultaneously reduced at the cathode surface. The results obtained so far indicate how promising is the electro-leaching process for remediating such high-mercury content, and, by choosing the suitable experimental conditions, it has been possible to remove nearly whole mercury content (> 99 pencent).