Development of an iron electrowinning process using anion exchange membranes
Van der Spoel Badenhorst, W.D.
MetadataShow full item record
Traditionally, spent leaching solutions (SLS) originating from base metal refineries are treated using (i) tailings dams, (ii) anoxic limestone drains (ALDs), or (iii) jarosite, hematite or goethite precipitation. While these methods provide adequate treatment of SLSs from various mining processes, they do not yield marketable products. As such, these treatment methods have a significant financial impact, while often having a negative impact on the environment. As an alternative to these traditional methods, anion exchange membrane-based electrowinning (EW) is proposed in this study for the treatment of SLSs. The use of specifically anion exchange membranes (AEMs) during EW (AEM-EW) enables the recovery of both electrolytic iron and the regeneration of the leaching acid for re-use with the upstream leaching process. However, as iron is a base metal with a low intrinsic value, the total cost of an AEM-EW process for the treatment of SLSs should ideally be minimized to allow for widespread application within the industry. Therefore, the effect of various parameters, namely (i) boric acid concentration, (ii) catholyte pH, (iii) electrolyte temperature, (iv) sodium sulphate concentration, (iv) AEM type and composition, and (v) iron concentration on the AEM-EW process were investigated in this study. The addition of boric acid was found to not have any significant effect on the AEM-EW process performance and was therefore not used in any further experiments. In contrast to this, the addition of sulphuric acid to the starting catholyte, which simulated the presence of unspent acid in the SLS, led to a substantially reduced current efficiency of 9 % (12.5 g/L H2SO4) compared to the current efficiency of 91 % obtained when no H2SO4 had been added to the starting catholyte. The decrease in current efficiency after the addition of sulphuric acid to the catholyte was attributed to an increase in hydrogen evolution. Similarly, increasing both the electrolyte temperature (up to 70 °C) and sodium sulphate content (up to 100 g/L sodium sulphate) led to a significant decrease in the specific energy consumption (SEC) of the process. Preliminary results indicated that the FAB-PK-130 (Fumatech GmbH) membrane outperformed all the membranes that were initially tested. The last variable investigated was the effect of the iron concentration of the starting catholyte solution on the AEM-EW process, where it was found that low initial iron concentrations correlated with low current efficiencies and high SEC values when sodium sulphate was not added. The low current efficiency and high SEC could therefore be attributed to the low electrolyte conductivity under such conditions. In addition to the various parameters that were tested (Chapter 3), the stability of various commercial and novel AEMs was determined using both the Fenton test and membrane durability studies performed over a period of three weeks. Some of the novel, blended AEMs that were prepared were impregnated with Ceria (Ce2O3), which is known to increase the operational lifetime of membranes in an oxidative/radical environment. The Fenton test results confirmed this, as the novel 2408-2 and BM-5 membranes that were impregnated with 5 wt % Ceria showed increased Fenton stability. During AEM-EW, the novel AEMs and VM-FAPQ-8130-PK, a novel non-commercial membrane from Fumatech, confirmed the improved stability by outperforming the FAB-PK-130 membrane used in Chapter 3. The improved stability of the VM-FAPQ-8130-PK membrane compared to the FAB-PK-130 membrane was further highlighted during the membrane durability test, where the VM-FAPQ-8130-PK membrane was able to operate at an SEC of 5.83 kWh/kg iron, whereas the FAB-PK-130 membrane operated at an SEC of 8.60 kWh/kg iron after three weeks of continued operation. Using the data obtained from this study (Chapter 3 & 4) as well as information supplied by Fumatech GmbH and Tharisa PLC, the capital expense (CAPEX) and operating expense (OPEX) of a set of scaled AEM-EW units operating in a two-stage configuration with an iron treatment capacity of 45 kg/h were estimated (Chapter 5). The CAPEX was calculated as R 1,327,360 and the levelized OPEX as R 17 per kg iron treated. It was also estimated that the power consumption of the two-stage AEM-EW process would constitute 72 % of the OPEX, while membrane replacement costs would contribute 25 % of the total OPEX, with the remainder being attributed to maintenance. This OPEX breakdown confirms the relevance of this study which aimed to reduce the power consumption of the AEM-EW process while minimising the membrane replacement cost.
- Engineering