Continuous Foam Separation of Metals Enhanced by Down-Flowing Surfactant Solution from Column Top

A novel operational mode of foam separation was suggested in this study. In a conventional operation, the separation of a target solute has its own limit because non-target ones exist in the interstitial water between foam and get mixed in the recovered foamate solution. As a countermeasure for this inherent drawback in foam separation, an aqueous surfactant solution was injected into the rising foam bed to enhance a downflow of the interstitial water containing unfavourable solutes. The flow characteristics of the system with and without the injection of a washing solution were investigated in terms of the liquid holdup, ϕ and the drainage velocity, ULD, under various operating conditions. Their correlation was obtained as ULD (cm s−1) = 1.85ϕ2 + 0.0005. The application to metal separation was also performed using a nonionic surfactant, polyoxyethylene nonylphenyl ether (PONPE 20), with an affinity to Au(III). The non-specific metal, Cu(II), was effectively washed out from the foam bed, and satisfactory separation of Au(III) was attained.