Recovery of valuable metals from manganese nodules by ammonia leaching

The selective leaching of valuable metals such as Co, Ni, Cu etc. from manganese nodules was investigated by using ammonia. When ammonium carbonate solution was used as a leaching reagent, valuable metals were selectively leached with a yield of 77 approx. 83%. However, this method required a large amount of the leaching reagent and the leaching rate was not so satisfactory. With liquid ammonia as a leaching reagent, both Ni and Cu were selectively leached with a yield ranging from 62 to 80% by using high leaching temperature and long leaching time compared to the case with ammonia solution, although the leaching rate of Co was relatively small. In order to increase the leaching rate of Co, it was necessary to add a large amount of ammonium chloride as a leaching reagent, which also increased the leaching rate of Mn. Moreover, in order to diminish loss of valuable metals by adsorption in the leaching with ammonia solution, the following two treatments were investigated. (1) The leaching residue was treated with weak acid solution. (2) The leaching solution heated in the autoclave was filtered during the hot condition.