Oguh, Ukachukwu. I
(2012).
Multiphase contacting in PGM hydrometallurgy.
University of Birmingham.
Eng.D.
Abstract
This thesis describes hydrodynamic studies of the leach and solvent extraction stages of a Platinum Group Metal (PGM) hydrometallurgical flowsheet. The studies were motivated by the need to increase PGM throughput in Johnson Matthey’s PGM refining business. In the leach stage, key components in the feed are selectively dissolved using acids in a stirred tank before they are recovered by liquid-liquid (L-L) solvent extraction and finally purified. The work described in this thesis tackles four main areas: hydrodynamic studies of L-L PGM solvent extraction in both mixer and settler stages, whilst for the leach stage, studies of particle behaviour in gas evolving solid-liquid (S-L) reactions and gas-liquid-solid (GLS) characterisation by a novel Electrical Resistance Tomography (ERT) technique are performed.
In the mixer-settler, the effects of impeller diameter, D, to vessel diameter, T, ratio (D/T), the phase flow ratio, cφ/dφ; (where cφis the continuous phase flow fraction and dφ is the dispersed phase flow fraction) and the specific power input,Tε, upon the droplet size distribution in a L-L system and their phase separation were investigated. Changing a smaller D/T impeller for a larger D/T impeller at constant P/V and cφ/dφincreased droplet size because the maximum shear rate decreased as a result of increasing ratio of impeller pumping capacity (Q) with tip speed (Utip). Changing a larger cφ/dφfor smaller cφ/dφat a fixed P/V and D/T impeller increased droplet size because turbulent dampening increased since the average density, ρ ∝ dφ. Meanwhile, Kolmogoroff-Hinze’s theory was shown to apply for the measured relationship between Tε and droplet size.
A settler design criterion, which relates the dispersed phase concentration (Ca) in the dispersion band to the dispersed phase throughput (Qd/A) agreed with the model by Ryon et al. (1959). Ca was significantly dependent on P/V and Qd/A, whilst the effects of Qc/Qd (where Qc is the continuous phase flowrate and Qd is the dispersed phase flowrate) and D/T were minimal. Droplet size analysis of the sedimenting region of the dispersion band and dense packed layer revealed a transitional distribution of droplet sizes due to the counteracting effects of droplet sedimentation, hindered settling and droplet-droplet coalescence.
Particle behaviour in gas evolving S-L systems were quantified using the Zwietering ‘just-suspended’ impeller speed (Njs) condition in a sponge nickel® and sodium hypochlorite system. The presence of gas caused Njs to increase, however a coherent relationship between Njs in an ungassed and gassed system
3
could not be easily ascertained. Further work with Positron Emission Particle Tracking (PEPT) was advised to quantify the relationship.
A well-known electrical concept called skin effect, which describes how the effective resistance of an electrical conductor varies as the frequency of an alternating current (AC) increases and decreases, was used to investigate GLS behaviour via a novel ERT spectroscopic technique. The process relies on the change in effective resistance of conducting objects with changing AC frequency to selectively detect different phases. The concept was initially validated with static phantoms of a stainless steel and plume of gas before being applied to dispersible stainless steel particles and gas. ERT spectroscopy showed that two AC frequencies (0.3 kHz and 9.6 kHz) could successfully isolate and simultaneously detect the gas and solid phases at a fixed current. By subtracting solids and gas conductivity, the change in solids and gas holdup were obtained.
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