Recovery of aluminum alum from waste anode-oxidizing solution

Recovery of aluminum alum (aluminum ammonium sulfate) by crystallization from waste anode-oxidizing solution in the aluminum surface finishing industry was investigated in this study. Effects of various operating conditions including the mole ratio of ammonium hydroxide and aluminum ion, temperature and seed alum dosage on the aluminum alum formation, acid recovery and aluminum ion removal were examined. Both one- and two-step processes of crystallization were employed in synthesizing the aluminum alum and in the meantime in reducing the aluminum ion concentration in the waste anode-oxidizing solution. Based on the test results, optimum operating conditions were recommended for efficient operation of the crystallization process. The residual acid solution after crystallization was found suitable for reuse in the anode-oxidizing process.