1. Introduction
Lithium, driven by the exponential growth in demand in recent years, has emerged as a pivotal element in enabling the transition towards cleaner and more sustainable energy alternatives, primarily through the advancement of lithium-ion batteries (LIBs) (Aaldering,2019). Lithium is obtained by extracting it from lithium minerals through mining, crushing, and chemical processing, as well as from lithium-rich brine through evaporation and subsequent chemical processes (Flexer et al., 2018). lithium can be sustainably sourced from produced water in oil and gas operations through the application of recovery methods like adsorption, membrane processes, and electrolysis-based systems (Kumar et al.,2018). Lithium is a vital component in various sectors, including glass, ceramics, chemicals, pharmaceuticals, lubricants, and batteries for hybrid and electric vehicles (Talens et al.,2013). The USA currently relies entirely on imported rare earth elements, essential for numerous commercial and industrial uses (Massari et al., 2013). Hence, considering the importance of resource security, there's a growing focus on extracting lithium from produced water in oil fields, both in the United States and worldwide. Produced water generated by hydraulic fracturing is considered a large stream, causing brine spills (North Dakota Department of Health, 2015). Approximately 77 million cubic meters of produced water were generated in the year 2018, and this volume is projected to grow by 143% by the year 2035 (Almousa et al ,2023 & Waisi et al., 2015). The main method for disposing of produced water is through deep-well injection, involving transportation to injection wells and pumping into underground formations (Shrestha et al., 2017). This approach is costly and poses risks like saltwater spills with lasting environmental effects, concerning well owners (Torres et al., 2016). The oil industry and environmental agencies are constantly challenged to minimize the amount of freshwater required for hydraulic fracturing operations (Gregory et al., 2011). Thus, another way to handle produced water is by extracting valuable elements like critical minerals, essential for the economic prosperity of major global economies. Critical metals such as lithium can be recovered from the produced water, providing an environmentally and economically beneficial solution.
While the United States currently relies on lithium imports for Li batteries from South American countries, including Argentina, Bolivia, and Chile, there has been a growing emphasis on the recovery of lithium from produced water in oilfields, both in the United States and worldwide (Mauk et al., 2021). Although high concentrations of Li in oilfield brines, lack of research about exploiting the oilfield brines as a Li resource. For example, North Dakota Devonian formations contain (100-288) mg/l of Li concentration and Smackover brines in the United States exhibit lithium concentrations exceeding 500 mg/L (Disu et al.,2023 & Daitch 2018). This has led to the start of various projects to evaluate the brines as a viable lithium resource.
Beyond the presence of lithium in produced water, other essential elements (K, Sr, Mg, Mn) contribute significantly to the economic and national security of the United States. Li is the most expensive metal compared with other significant metals found in the produced water (K,
$12.1–
$13.6/kg), and the only production of Li in the U.S. is in the state of Nevada (USGS,2020). This indicates that the produced water from the Bakken oilfield could serve as a crucial domestic source of lithium.
Figure 1 shows the seawater comparison and produced water of the Bakken oilfield (Frank et al, 2022).
There have been applied different technologies on Li recovery from brine using different technologies such as ion exchange, adsorption, solvent extraction, and chemical precipitation (Aljarrah et al. 2023, Zhong et al., Shi et al.,2017, 2021, wang et al., 2018, and Ji et al.,2017). Adsorption and ion exchange are widely recognized as conventional approaches for the recovery of metals and they are considered efficient for lithium extraction. However, Adsorbents and ion exchange resins can deteriorate over time due to exposure to harsh chemical environments, leading to a decrease in their effectiveness (Zhong et al., 2021). Additionally, it's notably more challenging to selectively capture lithium compared to other metals like copper as the brine contains high concentrations of metals such as sodium, potassium, magnesium, and calcium which have higher molar concentrations than Li, making the process insufficient (Kumar et al., 2019).
The lithium precipitation process is among the viable techniques used in industrial facilities due to its ease of use and affordability (Zhang et al., 2018). Zhang also emphasizes that the effectiveness of the process is influenced by factors such as precipitant dosage, pH level, temperature, and the size of precipitate particles. Li is usually removed by the precipitation technique as carbonate compounds, however, the 13.3 g/l solubility of lithium carbonate makes insufficient precipitation, which has recently been replaced by phosphate precipitation which is called the POSCO technique (POSCO,2018 & Jamasmie and POSCO, 2018). It's worth noting that in the lithium extraction process from brines, the precipitation method efficiently separates lithium from magnesium when the mass ratio of Mg to Li (Mg/Li) is less than 6 (Zhao et al. 2013). When the ratio is below 6, there's no need for prior treatment to eliminate magnesium. In this scenario, impurity metal ions in the solution can be precipitated by hydroxide ions (OH-) over a range of pH levels from 4 to 12. Still, lithium remains in the solution due to its greater solubility compared to other metals (Song & Zhao, 2018). Alsabbagh et al., 2021 worked on lithium removal from the end brine of the Dead Sea water [30-40 mg/l] using phosphorous compounds, and the removal efficiency yielded up to 55%.
In this study, three types of sodium phosphate salts were employed as lithium-precipitating reagents from the produced water. Among them, the most promising candidate underwent testing at various operating conditions, including stirring temperature and concentration of the precipitating reagent, to determine the optimal conditions for the pre-concentration stage. Based on these findings, the most effective reagent was selected for lithium removal after the evaporation process. However, extracting lithium directly from the produced water without crystallization proved to be inefficient not as proved previously on seawater end brine [Alsabbagh et al., 2021 and Tandy and Caniy, 1993].