Lithium recovery from diluted brines by means of a flow-through electrodes electrochemical reactor

Abstract

Lithium has become an important raw material in various sectors because of the continuously growing market of its derivative products, in particular of rechargeable batteries. Its demand is expected to grow hugely in the near future, due to the development and spread of hybrid and fully electric vehicles and of lithium-ion batteries for stationary energy storage. Currently, lithium is mainly extracted from brine by means of the lime-soda evaporation process, consisting in a solar evaporation to increase its initial concentration and other following chemical treatments to remove unwanted cations. This process has several disadvantages, such as the unreliability due to the weather conditions, the water consumption, the slowness, and the large amount of chemical wastage. Therefore, a more efficient, faster and environmental friendly lithium recovery technology is urgently needed. In the last decade, many efforts have been done in order to find better alternatives for the extraction of lithium from brine. In particular, La Mantia et al. have firstly introduced a new technique, known as electrochemical ion pumping. The electrochemical ion pumping technique consists in applying a current to extract lithium cations from brine, driving them in a lithium-selective electrode by means of the intercalation mechanism. The cations are then released in a recovery solution applying the current in the opposite direction. The lighter environmental impact and the higher speed of this technique compared to the lime-soda evaporation process make the electrochemical ion pumping a good alternative for the lithium production. The other problem related to the production of lithium regards its global sources. The brine sources with a relative high lithium concentration to be exploited through the evaporation-based process are located mostly in South America. Together with the increase in the demand, the economic monopoly of lithium production is the reason of lithium increasing price. On this grounds, the diversification of the lithium supply will play an important role in the spread and competitiveness of lithium based technologies. The possibility to exploit other more diluted lithium sources (10-50 mg/l of lithium concentration), such as geothermal waters, brines produced in salt-works, waste waters from gas and oil extraction wells, is of worldwide interest. The aim of this PhD thesis has been to design and test a suitable electrochemical reactor for the extraction of lithium through electrochemical ion pumping from diluted brines, down to a lithium concentration of 7 mg/l (1 mM). The extraction from low concentrated solution is challenging due to the mass transport limitations in the liquid phase that reduce the process efficiency. Therefore a flow-through electrodes reactor has been designed, in order to improve the mass transport by adding a convective flux of the electrolyte. Firstly, a preliminary study of the thermodynamic behavior of the materials used in the reactor was carried out. Then the capturing process has been implemented and investigated at various conditions. The capture efficiency was tested at various lithium concentrations and flow rates, finding that the amount of captured lithium increases with the flow rate up to a maximum value that decreases with the concentrations. The efficiency at low concentrations has been optimized by improving the active material distribution on the electrode. Further investigations on the capturing process have been carried out at various currents. The results show that, by decreasing the current, the amount of captured lithium increases, while the flow rate to be applied reduces, thus sparing hydraulic energy. A mathematical model has been developed to explain and support the experimental results. The model is based on a simplified description of the electrode porous distribution and it reproduces the experimental behavior at various flow rates, concentrations, porous distributions and currents. The results show that the model can be used to investigate the optimal process parameters and to size the cell components. Finally the total process (capturing and release) has been performed. More than one litre of brine at 1 mM LiCl was treated, extracting lithium with a capturing yield of 60 %. The lithium cations were released back in 5 ml of solution reaching a concentration of 100 mM (700 mg/l) and a purity of 94 %, with a release efficiency of 75 %. The high achieved concentration and purity of the final solution demonstrate that the developed reactor can extract efficiently lithium from diluted brine and it represents a valid response to the envisaged lithium market demand.