MATERIALS RECYCLING PROCESSES AND PRODUCTS

In this study, active zinc oxide containing over 98 wt. % ZnO was synthesized by hydrometallurgical process starting from waste zinc powder (blue zinc oxide). In the first step of the study, 95 wt. % Zn containing waste zinc powder was reacted with diluted sulphuric acid solution and then Fe content was removed from the system by addition of hydrogen peroxide. In the second step, zinc sulphate solution was mixed with different stoichiometric sodium carbonate solution. After solid / liquid separation, obtained zinc carbonate was dried over 12 hours. In the last step, calcination of zinc carbonate was carried out at different temperatures and time to set the alternative quality of active zinc oxide. Elemental analysis of the waste zinc powder and active zinc oxide was performed by using XRD, XRF, BET and AAS.

Crude zinc oxide, the collected fume from a Waelz kiln, is calcined in a rotary kiln to remove volatile salts (such as lead chloride and potassium sulfate). Accretions develop in the calcination kiln. The work reported here examined the fundamental cause. It is found that a small amount of liquid forms at the calcination temperature, and binds together the zinc oxide, similar to the mechanism by which moisture increases the strength of wet sand. Both potassium sulfate and calcium silicate cause liquid formation. Laboratory trials tested whether the composition of the binding liquid responds to additions of silica and alumina. The calcium silicate appears to contain all of the calcium oxide and silica from the feed material (crude zinc oxide); It is possible to manipulate the composition and amount of the calcium silicate by silica additions to the feed, but the calcium silicate does not respond to alumina additions. While this work has clarified the likely causes of accretion formation, it does not lead to any obvious solutions. It does not appear possible to eliminate all liquid in this process, since potassium sulfate (with a melting point of 1069°C) is one of the binder liquids.