Development of a continuous metal recovery contactor using ion-exchange fibres

Abstract

Wastewater from industrial processes usually contain heavy metals such as copper (Cu2+ ions) which are harmful to the environment and human health. Therefore, it is imperative that these effluent streams are treated prior to being discharged. In recent past, ion exchange fibres (IEFs) have been suggested for the treatment of such waste streams because they have higher adsorption/desorption rate due to their larger surface areas and shorter diffusion paths compared to granular ion exchangers. In this study, the development of an IEF contactor for the removal of Cu2+ ions from synthetic dilute solutions was carried out. The table pan filter model was proposed for a contactor for continuous and simultaneous Cu2+ ions loading, elution and rinsing of Cu2+ ions. In order to generate the equilibrium and kinetic data required for the design of a contactor, IEFs were contacted with dilute synthetic solutions of Cu2+ ions in batch systems. The Temkin and pseudo-second order model were found to best describe the equilibrium and kinetics of the system, respectively. The batch tests conducted for multi-component system showed that the selected IEFs have a higher selectivity for Cu2+ ions. The effects of bed packing density and flow rate on loading and elution of Cu2+ ions were investigated using columns at lab scale. The results showed that effective loading of Cu2+ ions can be conducted at low IEFs bed packing density and low feed flow rate that allowed sufficient residence time for the ion exchange to occur. The results also showed that the elution rate of Cu2+ ions can be increased by increasing acid flow rate provided the acid is sufficiently concentrated for elution purposes. In this study, the Yoon-Nelson model was the most suitable model for predicting how effluent concentration during loading varies with time. Other models fitted were the bed depth service time (BDST) model and the Thomas model. However, the Yoon-Nelson model had parameters which were comparable to those obtained experimentally for all tests. Following the column test results, a segment of a rotating contactor utilising IEFs was proposed, designed and fabricated from clear polyvinyl chloride (PVC). The contactor was then tested for loading and elution duties. The study showed that the contactor was able to remove 45 mg of Cu2+ ions /g of fibre from the feed solution as compared to 54 mg Cu2+ ions /g of fibre obtained in column tests. Loading of Cu2+ ions onto the designed contactor pan was found to be low due to the higher cross sectional area which consequently resulted in uneven distribution of feed solution. The IEFs loaded with Cu2+ ions were successfully regenerated with 3 bed volumes of 2 M HCl which removed 99.84% of the loaded Cu2+ ions as compared to the 7 bed volumes which was required to elute the entire loaded Cu2+ ions. Although the loading of Cu2+ ions was low, the designed contactor is promising as it was able to remove Cu2+ ions from feed solution and the IEFs could be completely eluted for reuse. In contrast to column Cu2+ ions breakthrough tests, the Yoon-Nelson model was unable to predict the breakthrough of Cu2+ ions in the designed IEF contactor pan. It is recommended that mathematical and/or linear driving force models be fitted into the breakthrough profile of the contactor for further development. This will allow simulations of breakthrough profile and consequently optimization of the IEFs contactor.