Pressure acid leaching of nickel laterite ores: an in situ diffraction study of the mechanism and rate of reaction

This paper outlines the use of an innovative system for the in situ investigation of hydrothermal reactions by X-ray diffraction. The key features are the use of: (i) a purpose-built capillary reaction vessel which allows close emulation of the conditions present in mineral processing plants; (ii) Mo Kα radiation, to ensure that the X-ray beam penetrates through the capillary, and (iii) an Inel CPS120 position-sensitive detector, to enable simultaneous collection of a wide range (120° 2θ) of diffraction data. The pressure acid leaching (PAL) of nickel laterite ores is used to illustrate the capabilities of this system, with a particular focus on the PAL of saprolite in strong H₂SO₄ at 493 K. Saprolitic ore, which largely consists of serpentine mineral phases [(Mg,Fe,Ni,Al)₃(Si,Al)₂O₅(OH)₄], undergoes a number of mineralogical changes during both the acid leaching reaction and subsequent cooling, thus making it difficult to examine accurately using traditional post-reaction ex situ techniques. In particular, kieserite (MgSO₄.­H₂O), which forms during leaching, has a negative temperature coefficient of solubility, causing it to dissolve on cooling. The in situ technique described in this paper allows the direct observation of kieserite formation during the saprolite PAL at 493 K and its dissolution upon cooling to ambient temperature.