Keywords: diatomaceous earth material, metal recovery, packed-bed reactor, wastewater purification, affinity separation, process parameters, flow rate, Nigeria, heavy metals
Development of a packed-bed reactor for the recovery of metals from industrial wastewaters
Affinity separation is becoming well established for the purification and recovery of metals from industrial wastewaters. Although the method has been successfully applied on a bench-scale, there is a need for reaction engineering studies to scale-up the process for use in large-scale wastewater purification and recovery of valuable metals. In this study, a pilot-scale packed-bed reactor has been developed in order to investigate influences of process parameters and their interactions on the recovery of metals from process wastewaters using diatomaceous material. The material was formed into a packed configuration to provide a high adsorbent area-to-aqueous solution volume ratio. Activated carbon and ion-exchange resin were tested for comparison. The results obtained show that the flow-rate, residence time, metal concentration loading and packed-bed geometry significantly influenced the efficiency of the reactor system metal recovery and have to be considered in the design of large-scale system. An optimum residence time of 2 h was found to give metal recovery capacities of 60%, 76% and 80% for the ion-exchange resin, diatomaceous material and activated carbon, respectively, which confirmed the bench-scale studies. The sequestered metals were efficiently eluted from the diatomaceous material using different elution solutions for different metals. The selectivity of the adsorbent for specific metals can be manipulated on the uptake and elution sides of the process. The study also revealed that diatomaceous material can be regenerated and used many times over, just like conventional adsorbents or ion-exchange resins in process wastewater purification as well as in metal recovery processes. The application of the packed-bed reactor using the locally available diatomaceous material is therefore recommended for economical and cost-effective small-scale purification and recovery of metals from industrial wastewaters loaded with metals in Nigeria.