Technical Paper: Metal Removal And Recovery From Process Solutions Using Short Bed Ion Exchange

Paul Pajunen, P.ENG., and Michael Sheedy, P.ENG., Eco-Tec Inc., Pickering, Ontario

Metal removal by ion exchange is not one of the common topics of discussion at the International Water Conference, but does have a significant history of industrial application. IX resins have been used for purification and recovery in metal surface finishing operations such as the electroplating of nickel and chrome, steel galvanizing, the pickling of mild and stainless steels, the anodizing, brightening and etching of Al, and recovery of Cu from printed circuit board production.

They have also been used in hydrometallurgical processes in the primary metals industries. Here they have been used to remove Sb, Bi, and Fe from copper refinery electrolytes, Au from CN solutions, the recovery and separation of Ni and Co, and Cu and Zn removal from both sulphate and chloride solutions. Ion exchange resins have also been useful for the removal of toxic heavy metals, such as As, from groundwater.

The solution chemistries and the objectives of the processes in which these resins are used are significantly different from those associated with water demineralization. In many cases metals must be removed selectively from solutions containing the following: very high TDS, strong oxidizing chemicals like chromic or nitric acid, high acid concentrations, and a high temperature.

While SAC and SBA resins are widely used, the challenging environments and the need for selective metal removal often requires the use of more expensive specialty chelating resins. Beyond the functional groups, consideration must also be given to what resin matrix should be used and with what degree of cross-linkage.

Other significant differences relate to overall process considerations and objectives. For example, in some cases temperature reduction is necessary to avoid potentially explosive conditions or rapid resin oxidation. Routinely the metal must be recycled or recovered to other plant processes, thus making the spent regenerant stream the product. This can raise concerns related to concentration, pH and purity of the recovered metal.

This paper discusses the short bed (Recoflo) ion exchange process and its application to metal recovery and separation. Short bed technology addresses many of the limitations of conventional ion exchange technology and is particularly suited to use with expensive chelating resins.

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