Pressure acid leaching of nickel laterite ores: anin situdiffraction study of the mechanism and rate of reaction
This paper outlines the use of an innovative system for thein situinvestigation 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) MoKα 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 H2SO4at 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-reactionex situtechniques. In particular, kieserite (MgSO4.H2O), which forms during leaching, has a negative temperature coefficient of solubility, causing it to dissolve on cooling. Thein situtechnique 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.