Production of acidic and alkaline solutions from concentrated seawater brines from saltworks by electrodialysis in combination with bipolar membranes

Abstract

The scarcity of raw materials in the European Union has made necessary to obtain them from secondary resources. Within this context, sea mining has been postulated as one of the most promising alternatives. In this context, SEArcularMINE, a €6 million research and innovation project funded by EU Horizon 2020 comes into play. This project is working to guarantee a sustainable supply of critical raw materials (CRMs) in Europe. To avoid the need for external reagents to carry out the recovery processes, the concentrated brines will be used to generate the required alkaline and acidic reactants using electrodialysis with bipolar membranes (EDBM), promoting a fully circular process. On-site generation of reactants is main subject of the master thesis. Bipolar membrane electrodialysis (EDBM) is a membrane technology able to generate alkalis and acids from their corresponding salts. At the SEArcularMINE project, the corresponding salt is seawater brines from saltworks. For the first time a study is focused on valorizing brines from saltworks; SEArcularMINE introduces this concept to be validated experimentally. Being a part of the SEArcularMINE project, the aim behind this thesis is to focus especially on evaluating several operational parameters such as brine content, initial volume ratio, and current density. For this purpose, experiments were carried out using an EDBM lab-unit. Solutions obtained as a result of electrodialysis were analyzed by various analytical methods including Acid-Base Titration, high performance ionic chromatography (IC), and inductively coupled plasma (ICP). Based on the analyses results, the optimum operation conditions area external current density of 300 A/m2, initial volume ratio of 1:1:5 for synthetic brine solution according to assessment of concentration profile of target final solutions (acid and base), specific energy consumption (SEC) and current efficiency (CE). Moreover, this study proposes new technique to enhace the NaOH production using EDBM by boosting product concentration and yields. This technique involves adding a saturated NaCl supply solution. The addition of a NaCl-saturated solution resulted in a satisfactory improvement in NaOH production efficiency and yield. In summary, it can be concluded that the EDBM is an efficient process for generating acid and alkalis from saltworks' seawater brines. A more extended study should be done for improving the EDBM process reducing the SEC and enhancing the final concentration of the products.