Managing Chromium in Nickel Hydrometallurgy Operations

Indoterra Metallurgy Solutions – Literature Study

Chromium in nickel laterite ores occurs in different mineralogical forms, which determine its reactivity and associated environmental risks. Limonitic ores contain amorphous Fe-Cr oxides and ferrit-chromite submicron particles, which are highly reactive in acid and represent the main sources of Cr(VI) mobilization. In contrast, saprolitic ores are dominated by crystalline chromite, a refractory phase with very low solubility under leaching conditions.

Chromium is released into solution as both Cr(III) and Cr(VI), each stabilizing under different redox conditions. Cr(VI) is highly soluble across a wide pH range (0–14) and under varying oxidation potentials. The Cr Pourbaix diagram can be used to indicate the stability of chromium species under prevailing Eh–pH conditions. The formation of Cr(VI) in HPAL operations is notable for two reasons: (1) the limonitic feed inherently contains reactive chromium minerals, and (2) the operating conditions favor the stability of Cr(VI).

The risk of Cr(VI) generation arises at several stages of hydrometallurgical processing. Inside the HPAL autoclave, partial dissolution of Fe-Cr oxides can release chromium, some of which may persist as Cr(VI). In tailings and stockpiles, exposure to oxygen and fluctuating pH conditions can drive the oxidation of Cr(III) into Cr(VI), which may accumulate in porewater and be flushed into seepage or effluent streams by rainfall.

Two predictive indices can be used to assess and manage these risks. The Chromate Generation Index (CGI) measures the tendency of an ore to generate Cr(VI), while the Chromate Mobility Index (CMI) evaluates the likelihood of mobilized Cr(VI) being transported into effluents. Together, these indices guide ore domain classification, blending strategies, predictive control of reductant dosing in the plant, and construction designs that mitigate Cr(VI) release from stockpiles, dry-tail landfills, and TSFs.

Mitigation strategies fall into two complementary approaches. Anticipative measures include ore blending to lower CGI, controlling Eh–pH conditions during processing, predictive reductant dosing, and designing stockpiles and TSFs with compaction, impermeable covers, drainage systems, and permanent water covers to minimize oxygen ingress. Responsive measures focus on treatment and monitoring, such as ensuring Cr(OH)₃ co-precipitation during iron neutralization, dosing reductants like FeSO₄ or NaHSO₃ in effluent treatment, and managing seepage through continuous monitoring and corrective action.

Ultimately, effective chromium management requires the integration of both anticipative geometallurgy and responsive treatment strategies. This combined approach ensures regulatory compliance, protects the environment, and enhances the sustainability of nickel hydrometallurgical operations.