Optimization of the COOL+ Process for the Battery-Grade Li2CO3 production

The goal of the COOL+ Process is the selective recovery of lithium. Read in this article about the process and the work of METALLICO partner TU Bergakademie Freiberg (TUBAF) during the last months.

The COOL+ Process represents an innovative advancement toward sustainable lithium extraction from primary sources, particularly spodumene, achieving a recovery rate exceeding 92% for lithium. For large-scale implementation, process optimization is essential, and considerable research has been conducted at TUBAF to date. Several parameters influence process efficiency, including sodium carbonate (Na₂CO₃) dosage and mixing, calcination temperature and duration, liquid-to-solid ratio, agitation speed, and cooling temperature. Each of these factors has been systematically addressed and refined to enhance overall performance.

The liquid-to-solid ratio is an important parameter in this process. A higher ratio results in a smaller amount of spodumene being leached in a larger volume of water, leading to lithium concentrations in the solution that are 8-9 times lower compared to processes with lower liquid-to-solid ratios, although the percentage of lithium recovery is only slightly reduced. The downstream stage, after filtration and digestion, requires significant energy consumption during evaporation. To address this, initial tests were performed using nanofiltration. Ideally, nanofiltration would enable the separation of lithium from sodium, the main impurity, by concentrating the solution with lithium ions; however, due to the unavailability of lithium-selective membranes and considerable lithium loss  (more than 25 %) with alternative membranes, this method was not pursued further. Subsequently, reverse osmosis membranes were utilized, successfully concentrating the solutions in the retentate and maximizing water removal prior to precipitation and lithium carbonate (Li₂CO₃) isolation. In the case of reverse osmosis, the lithium loss is lower than 1.5 %.

To ensure equipment longevity and effective solution concentration, it is essential to eliminate silicon, as its presence can lead to scaling and precipitation on membranes, resulting in malfunctions or reduced membrane lifespan. Magnesium (II) oxide (MgO) is employed for silicon removal from the solution. At a pH of 9 and a moderately elevated temperature of 40 °C, over 75% of silicon is removed without any visible silicon precipitate forming in the solution or at the membrane. Following reverse osmosis, the remaining water is evaporated, and crude Li₂CO₃ is subsequently separated by filtration. To achieve battery-grade purity, residual impurities are eliminated by washing the Li₂CO₃ with water.

It was also investigated that the activation of leaching residue, rich in nepheline, was unsuccessful with sulfuric acid (H₂SO₄) due to the high crystallinity of nepheline. Alternative methods should be aimed for in research and in the final application in order to achieve the zero-waste concept. One conceivable usage is in the cement and ceramic industries.

The next steps will include optimization of the silicon removal process to maximize the silicon removal and the washing step for Battery-Grade Li₂CO₃ isolation to mitigate lithium loss.