Ammoniacal Leaching for the Extraction of Valuable Metals from Secondary Resources: A Review

There is a growing interest in extracting valuable metals from secondary resources due to the depletion of primary metal resources and environmental concerns associated with solid wastes. As a result of the challenges associated with traditional extraction lixiviants, non-conventional lixiviants, particularly ammoniacal reagents, have gained a lot of attention in hydrometallurgical processes for metal extraction or recycling techniques. The present article reviews the use of ammonia and ammonia-ammonium salt leaching systems to extract valuable metals from secondary resources. This paper highlights the use of ammoniacal leaching for successful extraction of metals like nickel, cobalt, copper, zinc, vanadium, and molybdenum from diverse sources such as spent batteries, printed circuit boards, spent catalyst, sludge, dust, slag, and fly ash. Moreover, this paper summarizes and discusses research on the comparative outcomes of several ammonia leaching systems from these secondary resources. Furthermore, the potential benefits and challenges of using the ammonia leaching method for metal extraction from these sources are highlighted. Key advantages of ammoniacal leaching are low toxicity, cheap cost, high metal selectivity, closed-loop ammonia regeneration and lower operational costs. Overall, this article has demonstrated that ammonia leaching is a viable technique for extracting valuable metals from secondary resources.