Metals Recycling from Waste Sludges by Ammoniacal Leaching Followed by Solvent Extraction

The Phase I objective is to demonstrate the feasibility of applying recently developed enhanced ammoniacal leaching techniques and solvent extraction reagents to recycle nickel, copper, cobalt, zinc, and cadmium from hydroxide sludges such as those produced by electroplating shops, metal finishers, treatment of acid mine drainage, and industrial wastewater in general. Electroplating sludges, classified as F006 hazardous waste, are expensive to dispose. Recycling of the metal values in these sludges represents a potential method of offsetting disposal costs and an opportunity to recycle a nonrenewable resource. These materials currently are recycled to some degree, and the U.S. Environmental Protection Agency is taking measures to encourage further recycling. Current recycling techniques involve shipping to a copper or nickel smelter, or to other recyclers employing hydrometallurgical techniques that typically involve nonselective acid or alkaline leaching steps. In addition, numerous mining companies are obligated to undertake long-term treatment of acid mine drainage to minimize impacts to receiving streams. These sludges typically are lower grade than electroplating sludges and produced at much higher annual volumes. Virtually no recycling of these mine waste-derived sludges currently occurs, and large volumes of sludges now are placed in lagoons or repositories. Ammoniacal leaching of hydroxide sludges was evaluated more than 15 - 20 years ago, and although it was found to be highly selective for specific metals, recoveries were disappointing. MSE Technology Applications, Inc., will reevaluate ammoniacal leaching of hydroxide sludges using recently developed techniques to significantly increase leaching efficiency. The leaching component is followed by solvent extraction to separate the leached metals, using solvent extraction reagents developed in the last 5 - 10 years, particularly for selective nickel and copper extraction from ammoniacal solutions. Project objectives are to determine optimum leaching conditions for the target metals, determine suitable solvent extraction reagents and conditions, estimate ammonia recycle efficiencies, prepare a system conceptual design, and perform economic analyses of the proposed process compared with current alternatives. The anticipated results are that the process will efficiently leach, separate, and recover metals from hydroxide sludges, and the economic analyses ultimately will determine the attractiveness or unattractiveness of the process. If the economics appear to be promising, the process would represent a less expensive alternative to F006 generators than current choices, as well as an opportunity for mining companies faced with long-term treatment of acid mine drainage to recover at least a portion of their treatment costs. The process will be developed further in a Phase II effort including development and operation of a pilot plant and obtaining buy-in from a commercial entity.