Process for recovery of Zn and Mn from Bunker Hill Water Treatment Plant sludge: Part I - Leaching and solution purification

A hydrometallurgical process for recovery of Zn and Mn from Bunker Hill Water Treatment Plant sludge was developed. The process is unique because it treats an unusual combination of metals, Zn and Mn. Another non-standard feature of the process is that leaching takes place with H2SO4 in the presence of a reductant (SO2), Zn and Mn are recovered simultaneously by electrowinning, Zn on a cathode and Mn on an anode, as MnO2. The major purpose of the process is to detoxify the sludge by removing toxic heavy metals. The recovered metal values could be marketed to offset the overall costs of water and sludge processing. This work is presented in two separate parts, I. Leaching and Solution Purification, and II. Electrowinning. This paper covers only the first part of the hydrometallurgical process: Leaching and Solution Purification. Initially, the paper briefly discusses the history of the pollution problem at the Bunker Hill Superfund Site, the origin of heavy metals in the mine waters, and the development and use of the water treatment plant. The leaching of sludge was examined as a function of the following parameters: constant vs, initial py initial vs. delayed addition of SO2 temperature, and pulp density. It was found that leaching with H2SO4 at pH 2.0 for 15 minutes (at 60 degrees C), followed by SO2 addition for another 15 minutes, could completely dissolve Zn and Mn. Solution purification was done in three stages. In the first stage iron was removed by adjustment of pH (to pH 5.0) and oxidation of ferrous to ferric iron (with H2O2). In the second stage, Cu and Cd were removed by Zn dust cementation. Finally, Co and Ni were precipitated with the organic reagents 1-2 nitroso-naphthol and nitroso, respectively. Electrowinning of Zn and Mn is discussed separately in Part II of this paper.