Selective pressure leaching of Fe (II)-rich limonitic laterite ores from Indonesia using nitric acid

Abstract

The selective extraction of nickel and cobalt over iron from an Indonesian limonitic laterite was investigated using nitric acid pressure leaching (NAPL). The mineralogical analysis showed that the major minerals were goethite and magnetite, and the content of the divalent iron was as high as 7.06%. Nickel and cobalt were mainly distributed in these two minerals; however, the distribution was non-uniform. A series experiments were conducted to examine the basic parameters and propose the optimal conditions for the extraction. When the ore was treated via HPAL under the optimal condition, the extracted nickel and cobalt were less than 75%, and the iron concentration in the leach liquor was over 12.5g/L. By contrast, over 85% of nickel and cobalt were extracted and about 1.8g/L iron was achieved using NAPL. The loss of nickel and cobalt can be mainly attributed to the undissolved magnetite and manganese minerals. The leaching process of NAPL is a dissolution–oxidation–precipitation mechanism, and in this process nitric acid acts as both a lixiviant and an oxidant. The formation of hematite results in a low iron concentration in the leach liquor without oxygen injected. Meanwhile, the oxidation and the precipitation of dissolved divalent iron results in a calculated savings in acid consumption of about 120kg nitric acid per ton of ore can be obtained, which is equal to over 93kg of sulfuric acid per ton of ore. Moreover, lower residual acid (20g/L nitric acid) is a significant advantage of NAPL. The iron residues had a high iron content (>56wt%) with no sulfur, making it suitable as raw materials for ironmaking.