Comparative column elution of jarosite waste and its autoclaved product-evidence for the immobilization of deleterious elements in jarosite

Jarosite waste, originating from the zinc industry using hydrometallurgical processing routes, is considered hazardous. Metals such as zinc, cadmium and copper are present in an environmentally mobile form, thus the jarosite waste is usually stored in lined ponds. Autoclaving of this material with a cellulose type waste material, such as sewage sludge, with the addition of a neutralizing agent will transform most of the metals into oxides, which are immobile under neutral conditions. Jarosite waste and its treated product were tested in standard column tests, as prescribed by Dutch legislation if the material is to be used as construction material. In three weeks time an amount of simulated rainwater, equal to ten times the weight of the solid, was percolated through the samples. The moisture content of jarosite waste is about 40 wt%, reflecting the leach liquor from the zinc refining process; its solutes are easily leached out. This leach liquor initially contained high concentrations of Zn, Cu and Cd, but when a liquid/solid ratio of two (L/S=2) was reached, most of these metals were removed from the jarosite waste. Co-treatment of jarosite with sewage sludge yields a product which is far more acceptable. Even so, the leached amounts of zinc and cadmium do not match the Dutch criteria for second-grade construction material. Counter current washing of the residue improves its properties as a construction material, although the best environmental option for the solid residue is further (thermal) treatment (Rage et al., 1996).