Process Challenges in Extracting Lithium

Lithium found in minerals such as spodumene or in brines such as those in salt deserts (aka “salars”) can be difficult to extract. In spodumene, after separation and flotation, further processing of the  concentrate may target changing the crystal structure through heat treatment to make it amenable to roasting or leaching with sulphuric acid. Further refining may include processes such as purification or precipitation at temperatures approaching 100 °C.  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 removing impurities or recovering lithium directly. Moreover, whether extracting lithium from brine or minerals, most extraction stages require control of pH and/ or ORP to ensure the required thermodynamic conditions to pull the desired metal into solution.

pH and ORP Sensors that Survive in Aggressive Chemicals and High Levels of Impurities

The stages of extraction described quite often involve aggressive chemicals and high levels of impurities – pH and ORP sensors (Example: Model 648 pH Sensor) 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 and sulfides, as well as reagents which are added 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.

Sensor Features for Lithium Processes – Some Key Examples

pH Sensor coated (eg. froth flotation) or scale (eg. precipitation).

Lithium Processing pH Sensor coated (eg. froth flotation) or scale (eg. precipitation).

Simple Sensor Geometry – for processes that tend to coat (e.g. froth flotation) or scale (e.g. precipitation), sensor tip geometry is simplified for ease of cleaning and to reduce the surfaces upon which scale or coatings can develop.

IC Controls Model 607 pH Sensor

IC Controls Model 607 Hot Tap pH Sensor or ORP Sensor which is ball valve retractable and has a replaceable cartridge.

Connector Provision on Cable – some sensors have connectors incorporated on the body of the sensor. While this saves cable, it makes the connection vulnerable to contamination since it places the connection point very close to the process. Lithium processes tend to have high salt concentrations and are quite conductive which can lead to signal integrity issues. Connectors should be provided with a suitable length of cable to allow connection away from the process.

Reference Junction Configuration – because of the diversity in processes to extract lithium, a one-size-fits-all approach to sensor construction can pose a major compromise for sensor performance and life. In some processing stages such as froth flotation, dirt and particulate can be the major challenge. Here, a diffuse-porous wood species for the reference junction can be most effective in resisting plugging.  In contrast, a lithium stripping process at low pH and high temperature should be measured using a sensor with a synthetic reference junction to prevent failure.

 

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