Preparation of an As(V) solution for scorodite synthesis and a proposal for an integrated As fixation process in a Zn refinery

A method for producing an As(V) solution from an As-bearing material obtained from a nonferrous hydrometallurgical process was investigated. Preparation of the As(V) solution included oxidative leaching of As with a NaOH solution, elimination of As as a calcium arsenate precipitate (johnbaumite: Ca5(AsO4)3(OH)) with a Ca(OH)2 secondary salt, washing the Ca–As compounds, and reaction of the Ca–As precipitate with H2SO4. This process was shown to be industrially applicable. The As ions and Cu ions were effectively separated by oxidative leaching with O2 gas injection under strongly basic conditions. In this system, As dissolved in the NaOH solution and Cu precipitated with the residue. The dissolved As in this highly concentrated NaOH solution was then effectively precipitated from the solution by addition of a surplus amount of CaO, which allowed recycling of the NaOH solution. The addition of surplus Ca precipitated Ca5(AsO4)3(OH) and Ca(OH)2, which inhibited the leaching of As but did leach Ca and Na. When the Ca–As compounds were dissolved with H2SO4, Ca ions precipitated in the form of gypsum from the As-bearing solution. The gypsum produced by this process is likely to give rise to a number of As-related issues and the As level, therefore, needs to be reduced. This process is advantageous for the treatment of As since it is stabilized as scorodite. The production of an As(V) solution could be applied to hydrometallurgical operations as it necessary for the removal of As. This process is shown to be practically useful to As removal in Zn refining and a closed flow circuit is proposed for integration of this process into a Zn hydrometallurgical operation.