Hydrometallurgical Processing of a Nigerian Sphalerite Ore in Nitric Acid: Characterization and Dissolution Kinetics

The results of experimental investigation on the dissolution kinetics of a Nigerian sphalerite ore in nitric acid solution were discussed. The effects of acid concentration, temperature, particle size, stirring speed and soild/liquid ratio on the leaching of sphalerite was examined. The results of the elemental analysis of sphalerite by X-ray fluorescence (XRF)  showed that the sphalerite mineral exist mainly as ZnS with metals  such as Fe, Na, Mg, Al, Ca, Mn, P, Ti, and Cr occurring as minor or trace elements. The XRD result confirmed the originality of the sphalerite sample as it revealed the presence of sphalerite (ZnS) as the dominant mineral and cerium germanium sulphide (Ce₂GeS₂) as an associated mineral. The scanning electron micrograph (SEM) result revealed a high level of crystallinity of the sample, indicating a high level of purity. The FTIR analysis also confirmed the presence of sulphur in the sphalerite ore. The leaching experiments showed that sphalerite dissolution in nitric acid increases with increase in acid concentration, temperature and stirring speed and decreases with increase in solid/liquid ratio and particle diameter. In 10M HNO₃ at a temperature of 363K using 75µm particle diameter with solid/liquid ratio of 20g/L and stirring speed of 540 rpm, about 86.6% of sphalerite was dissolved. The values of activation energy, reaction order, Arrhenius constant and reaction constant calculated at the conditions above were 26.42KJ/mol, 0.91, 23.06s^-1 and 5.23 ×   respectively. The kinetic data analysis suggested that the shrinking core model for spherical particles is applicable and thus, the leaching reaction follow the diffusion controlled mechanism, with the surface chemical reaction as the rate determining step. The XRD analysis of the post-leaching residue revealed the presence of sulphur, quartz and sylvite.