LITHIUM HYDROXIDE FORMATION BY MEMBRANE ELECTROLYSIS

The production of high-purity lithium hydroxide (LiOH) solution by electrochemical conversion of soluble lithium salts (membrane electrolysis) was tested on semi-industrial scale. Stainless steel (cathode) and lead (anode) were used as electrode materials. Lithium sulphate solution was utilised as anolyte and water - as catholyte. During membrane electrolysis, water oxidation takes place in the anode compartment with the formation of gaseous oxygen and protons. Lithium ions permeate through the cationic membrane into the cathode compartment, where gaseous hydrogen and hydroxide ions are formed as the products of water decomposition on the cathode, so lithium hydroxide gets concentrated up to 33-36 g/dm3 with respect to lithium oxide. Electrolysis efficiency is 50-55 % for one process cycle. Sulphuric acid formed in the anode compartment may be then neutralised by adding lithium carbonate, and the formed lithium sulphate may be re-used in membrane electrolysis, which means anolyte recycling. Five successive electrolysis cycles with recycled anolyte allow an increase in the degree of lithium ion transfer from the anode to cathode compartment to 95-98 %. It is established that, in addition to lithium, other metal ions (sodium, potassium, calcium, etc.) and sulphate ions migrate through the cation-exchange membrane into the cathode compartment from anolyte solution. To evaluate the quality of the obtained lithium hydroxide monohydrate (LiOH•H2O), lithium hydroxide solutions obtained both by membrane electrolysis and by the traditional lime causticisation process were evaporated. Comparison between the concentrations of impurity ions in lithium hydroxide monohydrate samples obtained using different methods shows that the product of better purity may be synthesized from lithium sulphate solution by membrane electrolysis.