Stabilisation of chromium-containing electroplating sludge by cementation

Electroplating processes and other industrial methods often produce wastes containing heavy metals, such as chromium. If recovery of these wastes is not feasible, a technically possible remedy can be their immobilisation in cement and practical use of the hardened product, provided the matrix ensures safe stabilisation/solidification. At the high pH value of hydrated cements (typically 13-13.5) most heavy metals precipitate in the form of sparingly soluble hydroxides. As the basic character of the matrix hardly decreases in time, conditions for immobilisation prevail in the long term. Confinement of chromium waste, however, is more difficult. Depending on its oxidation state, it can either be present as readily soluble CrO₄ ^2- or precipitate as Cr(OH)₃. Ordinary Portland cement, by virtue of its 200-300 mV positive electrode potential, is unsuitable for immobilising chromium. After being complemented by reducing character additives (such as blast furnace slag, BFS), however, it may be satisfactory. The quantity of safety confinable Cr-load is assessed in this paper by poreload expression and quasi-dynamic leaching techniques as function of matrix composition. Mass ratios of OPC:BFS studied were 1:1, 1:3 and 1:9, whereas chromium levels (as potassium dichromate) in the mixing water were 2, 4, 8 and 40 g/l.