A Study on the Electrochemical Dissolution of Ilmenite, Zircon and Lead Sulphate

Abstract

Ilmenite and zircon used in the investigation were collected in a single lot from Bangladesh Atomic Energy Commission's Pilot Plant for separation of heavy minerals from beach sands at Cox's Bazar. The lead sulphate was collected on dismantling of locally available waste lead-acid batteries. The ilmenite, zircon and lead sulphate were characterized by XRD and EDAX analyses. The electrochemical dissolution technique was used to investigate on the dissolutions of ilmenite, zircon and lead sulphate in sulfuric acid solutions, sodium hydroxide solutions and nitric acid solutions, respectively, at various conditions. Cyclic voltammetry was used as a reliable source to determine the dissolution potentials of ilmenite, zircon and lead sulphate. It was seen from the cyclic voltammetric studies that the dissolution of ilmenite was very difficult without the addition of graphite powder in ilmenite. The effects of ilmenite-graphite ratio, acid concentration and temperature on cyclic voltammograms were investigated to understand the dissolution process of ilmenite. The investigated results showed that the dissolution rate of ilmenite (FeTiO3) was low at low applied reduction potentials (˂-0.40 V) and temperatures (˂60 C). However, the dissolution rate was increased at more negative applied reduction potentials and higher temperatures. The dissolution rate was also increased on increasing acid concentration up to 1 mol dm-3; and at more acid concentration and higher reduction potential, it was decreased due to the starting of H2 gas evolution which eventually decreased the active surface area of pellet by adsorption. The activation energy (Ea) was estimated as 50±10 kJ mol-1 in the higher temperature region (htr) and 15±5 kJ mol-1 in the lower temperature region (ltr). The value of activation energy suggested the process to be diffusion controlled at ltr and chemically controlled at htr. The dissolution of zircon fraction could not be obtained in NaOH solution without the addition of graphite in zircon fraction due to low conductivity of zircon fraction. The effects of zircon-graphite ratio, acid concentration and temperature on cyclic voltammograms were investigated to understand the dissolution process of zircon. The dissolution rate of zircon (ZrSiO4) was low at low applied reduction potentials (<-0.30 V) and temperatures (<75 C). At more negative potentials and higher temperatures, the dissolution rate of zircon was increased through increasing rate of the reduction of Zr4+. The dissolution rate was also increased on increasing alkali concentration up to 3 mol dm-3; and at higher alkali concentration and higher reduction potentials, it was decreased due to the starting of H2 gas evolution which eventually decreased the active surface area of zircon pellet by adsorption. The values of Ea in the higher (~ 85 C), intermediate (~70 C) and lower (~30 C) temperature regions are 23±1 kJ mol-1, 45±10 kJ mol-1 and 20±5 kJ mol-1, respectively. The values of Ea, at low temperature region suggested that the process was diffusion controlled; and with the rise of temperature, the diffusion layer/film was anyhow destroyed to convert the process to be chemical controlling at ~70 C. Low value of activation energy at higher temperature was an indicative to two parallel reactions. The dissolution of storage battery waste product (PbSO4) could not be obtained in HNO3 solution without the addition of graphite in battery waste due to low conductivity of battery waste product (PbSO4). The dissolution rate of lead from storage battery waste product (PbSO4) was low at applied reduction potentials up to -0.52 V and temperatures below 60 C. At higher temperatures, the dissolution rate of lead from battery waste product (PbSO4) was increased through increasing rate of the reduction of PbO2 and PbSO4. The dissolution rate of lead was also increased on increasing HNO3 acid concentration up to 1 mol dm-3; and at more higher HNO3 concentration (˃1.0 mol dm-3), the slow increasing rate of the dissolution of lead from storage battery waste was observed due to the starting of H2 gas evolution resulting in the decrease of the active surface area of storage battery waste pellet by adsorption. The values of Ea in the higher (˃60 C)) and lower temperature regions (˂60 C) were estimated to be 45±5 kJ mol-1 and 2±1 kJ mol-1, respectively. The values of Ea suggested the existence of a diffusion film at lower temperature which was believed to be destroyed at higher temperature. At lower temperature region, the process was diffusion controlled and with the rise of temperature, the process became chemically controlling.