Resumen de Integration of nanofiltration and diffusion dialysis for the sustainable management of acidic liquid wastes
Nowadays mining and hydrometallurgical industries generate a considerable amount of acidic liquid wastes (ALWs), which are characterised by a low pH and the presence of metals (e.g. Fe, Cu, Zn and Rare Earth Elements (REEs) among others) and non-metals (e.g. As, Se). These effluents are usually treated by the addition of lime to neutralise the acidity and to remove the metals as hydroxides and hydroxyl-sulphates, and the non-metals as calcium-based electrolytes and the resultant effluent is discharged into the natural water bodies. However, the high cost associated with this treatment makes necessary to explore other more sustainable alternatives. Nowadays, the European Union (EU) is promoting circular schemes to valorise effluents for the recovery of valuable elements. Additionally, it is worthy of mention initiatives such as the Critical Raw Material List, which promotes the recovery of valuable compounds from secondary resources. Therefore, ALWs from mining, hydrometallurgical and metallurgical industries can be a source of acid and metal recovery. Among the alternatives to the established neutralisation by lime addition, membrane technologies are being studied. In this thesis, two membrane technologies such as nanofiltration (NF) and diffusion dialysis (DD) will be used for the valorisation of ALWs.Acid Mine Drainage (AMD), which is a by-product of the mining industry, was the first model system treated with NF for acid recovery and concentrating the metals in solution. AMDs are characterised by a pH "menor que"3 and the presence of metals (Al, Fe, Cu and Zn, among others). REEs, which are identified as Critical Raw Materials by the EU, can be found as minor components. Different membranes, including acid-resistant, were tested under different feed composition to evaluate their influence in the membrane performance. NF membranes were able to treat AMD, especially those based on an active layer made of polyamide, with high metal rejections ("mayor que" 98%) and favouring the acid transport. Nevertheless, their low stability in acid media makes these membranes susceptible to use at long- term operation. Changes in the chemical and physical structure of the active layer were evaluated by using FSEM-EDAX, FTIR and XPS. Solution speciation was found to have a significant impact on membrane rejection. Moreover, a numerical approach based on the Solution-Electro-Diffusion model (SEDM) coupled with reactive transport, was developed to characterise the transport of species through membrane permeances. Furthermore, the prediction capability of the developed model was studied and tested to determine its potential application for process design.Besides, an ALW from a metallurgical industry composed by a mixture of H2SO4 and HCl (0.3"menor que" pH "menor que" 0.7) containing non-metallic (As) and metallic (Fe, Cu, Zn) impurities was treated with a polyamide-based membrane (NF270). The membrane was able to reject metals by more than 80%, whereas the transport of arsenic was below 50%. This lower As rejection was related to the presence of As(V) as a non-charged species (i.e. H3AsO4), which is not rejected by the electric charge of the membrane.Finally, a highly acidic (220 g/L H2SO4) ALW from a copper smelter containing arsenic as the main impurity (3.4 g/L) was treated with a DD module. The speciation of arsenic as As(III) or As(V) was studied. Acid recovery and ion leakage were studied at different conditions. The speciation of arsenic does not influence the recovery of the acid. Arsenic was not rejected (ion leakage below 50%) by the membrane due to its presence as non-charged species, such as H3AsO3 for As(III) and H3AsO4 for As(V). However, the membrane can reject more As(III) than As(V) due to the presence of the former as a cation (H2AsO +). DD at the optimum evaluated conditions was able to recover the 69±2% of the acid (146.3 g/L) with arsenic as the main impurity (1.3 g/L and passage of 39±1%), while the total content of metals was below 0.1 g/L
