Abscheidung von Nickel und anderen Schwermetallen bei der Reinigung von Zinksulfatlösungen durch Zementieren mit Zinkstaub
The industrial production of zinc sulfate employs aqueous ZnSO4 solutions. These are made from leaching zinc-containing secondary materials with sulfuric acid, with the result that they are contaminated with considerable quantities of heavy metals. These metals are removed from the solution by the process of zinc lye purification, which is performed similarly to electrolytic zinc refinement. During the cementation, other heavy metals which are nobler than zinc, e.g. lead, cadmium, cobalt, copper and nickel, are separated in the form of solid phases by adding zinc dust. Numerous experiments – both at the laboratory scale and large scale – have been performed to optimise cementation as part of zinc sulfate production, by studying the effects of parameters such as temperature, the type and concentration of impurities and the quantity of zinc dust used. These studies have employed both technical zinc sulphate solutions (working solutions) and solutions with much lower concentrations of heavy metals (pharmaceutical solutions). The present study primarily concentrated on investigating the effect of metals on the cementation of nickel, which is almost always contained in the starting solution in the large scale production of zinc sulfate. In accordance with the literature, the results showed that the cementation of nickel can be described as a first order reaction. Moreover, there was significant correlation between the rate constants for nickel cementation and the stochiometric proportions in the solution. This made it possible for the first time to eliminate two stoichiometric factors from the rate constants which unambiguously describe the concentrations in the solution. In practice, this signifies that the initial conditions in the solution are of decisive importance for the course of nickel cementation. Thus the course of nickel separation is only complete and adequately rapid when the concentrations of lead, cadmium and copper in the starting solution, at the beginning of the cementation process, are high and their ratios to the nickel concentration are also high (about 10). If these conditions are fulfilled, and other parameters (such as temperature, pH and stirring rate) are kept constant, the cementation is only influenced by the zinc dust/metal ratio in the starting solution, which must always be greater than 2. Evaluation of the temperature dependence of the rate constants found that the activation energy for nickel cementation was 49.41 kJ/mol. This indicates that the rate of nickel separation from zinc sulfate solutions in the technical cementation process is not determined by diffusion, but is determined, or strongly influenced, by a chemically or electrochemically controlled reaction. In addition, practical possibilities are listed for reliable quantitative removal of nickel and for avoiding overdosage with zinc dust. The cementation product produced during zinc lye purification is a sludge, in which analysis of the structure and composition of the individual substance phases is not possible. For this reason, an attempt was made to simulate the cementation process by using thermodynamic calculations. This was intended to describe the basic behaviour of the metals and the possible composition of the cementation products. In this manner, it was possible to describe the effect of the formation of intermetallic phases and the coupled sequence of the separation of the metals during cementation.
info:eu-repo/classification/ddc/620, Zink, Elektrochemie, Zementation <Metallurgie>, Zinksulfat, Laugung, Ingenieurwissenschaften, Zinkstaub, Grillo, Nickel, cementation, leaching, zinc sulphate, zinc dust
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