The Importance of pH and ORP Sensors and Probes in Lithium Extraction Yields

Lithium’s role in Lithium-Ion batteries makes it a key enabler of vehicle electrification and large-scale energy storage making it an important resource in reducing carbon emissions. It is the lightest metal and when used in a battery, the lithium ion moves with relative ease from anode to cathode thereby enabling lithium ion batteries to achieve the highest energy density of any battery type. Although concerns exist with the environmental impact of extracting and refining this metal, there is broad recognition that it will play a pivotal role in the transition to a low carbon economy. This has led to initiatives to ensure a sustainable approach to the consumption and supply of lithium such as the recycling of end-of-life batteries. The extraction of lithium, whether from one of its natural sources or through recycling, requires the measurement and control of pH and ORP to ensure conditions favourable for its liberation.

The word lithium comes from the Greek word ‘lithos’ which means stone. And indeed, lithium is found in hard rock and is one of the main natural sources of the metal, however, vast reserves are also found in continental brines extracted beneath large evaporated lakes (aka ‘salars’). There are other sources such as underground geothermal waters or oilfield brines but hard rock (e.g. Australia, Canada, China, USA) and salars (e.g. Argentina, Chile, China, USA) are currently the two largest sources. As one might expect, the extraction processes for hard rock are quite different from those for brines. In either case, however, pH and ORP measurement and control are important to ensure the highest lithium yields.

Lithium found in minerals such as spodumene can be stubborn to extract. Following mineral separation and flotation, further processing of spodumene concentrate may target changing the crystal structure to make it amenable to sulphuric acid leaching. Further refining may include processes such as precipitation.  Brines, in contrast, start with varying levels of dissolved lithium in an aqueous solution that need to be separated from other elements. Extraction stages may include precipitation, solvent extraction or stripping, to target either the impurities or the lithium directly. Moreover, whether extracting lithium from brine or hard rock, many of the extraction steps require control of pH and/ or ORP to ensure the required thermodynamic conditions.

The stages of extraction described quite often involve aggressive chemicals and high levels of impurities – pH and ORP sensors (Example: Model 648) must survive in these environments in order to provide the necessary feedback for control. Froth flotation, for example, is used to concentrate lithium-bearing minerals and is inherently dirty. It contains high levels of fine particulate as well as reagents to promote froth and floatability. Sulphuric acid used for leaching, and extraction agents used for solvent extraction, are aggressive chemicals that cause degradation of process equipment. To perform in these types of applications, pH and ORP sensors must be selected for chemical compatibility, temperature resistance and mechanical robustness. There are many processes for lithium extraction and refining and the industry is evolving quickly. For all of these reasons, a detailed review of physical and chemical conditions for a lithium extraction process should be conducted to ensure i) accurate pH and ORP control, ii) adequate sensor life, and iii) minimal maintenance intervals.