Automated on-line monitoring of a lithium hydroxide production process using micro-discharge OES†

The fast, precise and continuous on-line analysis of highly saline process solutions is challenging for conventional laboratory techniques like ICP-OES or ICP-MS due to the necessity of high plasma gas flow rates (Ar and He), a high power consumption and the limited resistance of the sensitive spectrometer technique to harsh on-site conditions like dust, vibrations or temperature fluctuations. Therefore, an on-site and on-line method with comparable performance despite such conditions would be preferable. In this study we used the Micro-Discharge Optical Emission Spectroscopy (μDOES) for the given challenge of monitoring fully automated on-site and on-line production of lithium hydroxide, which is an essential precursor for the battery industry. The technology is based on creating a micro-plasma directly inside the aqueous sample without any carrier gas by using electrodes and high voltage pulses and thus enabling optical emission spectroscopy on-site. After optimisation of several parameters like sample conductivity, signal integration settings or selection of emission lines, measurements were carried out at an industrial pilot plant. The entire process chain was monitored, starting with the leaching of the calcined lithium-containing ore, through several intermediate products to the end product lithium hydroxide monohydrate of battery grade. The individual process steps were measured continuously (10–20 h), simplifying the monitoring of the process and allowing trends in the concentrations of the elements Li, Na, K, Ca, Mg and Rb to be identified. Reference measurements were performed using laboratory ICP-OES and/or ion chromatography to verify the results. Micro-discharge OES proved to be useful for a fast and precise on-site and on-line analysis of saline solutions with good long-term stability and a high agreement with the used reference methods, resulting in deviations below 10% for the most important components Li, Na and K.