This report discusses the rising market demand for lithium and the land reserves of lithium is going to be depleted by the end of 2080. That’s why there is a need to find some alternative. As rejected brine seawater contains a lot amount of lithium, there is a need to extract that lithium from the seawater. The process description and working of the electrochemical cell are explained in this report. A comparison of different techniques to extract lithium is also provided and the best possible techniques so far are discussed which were practically performed by the King Abdullah University of Science and Technology (KAUST) and some results of this experiment have also been shared.
Lithium is a critical component of electric car batteries, but rising demand is predicted to deplete land-based lithium deposits by 2080 (according to an academic article from 2018).1 However, extracting lithium from rejected brine seawater efficiently poses a challenge: how do you isolate one type of ion (Lithium-ion) from a dilute solution (seawater) containing numerous chemically identical but significantly more plentiful ions (like sodium, magnesium and calcium ions)?
Market demand for Lithium
Lithium minerals are used by manufacturers worldwide for more than 160,000 metric tons per year. According to data from the US Geological Survey (USGS), 71 per cent of that material was utilized to create batteries in 2020. This is up from 23% in 2010 when lithium’s major applications were glass and ceramics.
And the amount of lithium consumed around the world is going to rise very sharply. According to the International Energy Agency, the number of electric passenger cars on the road in 2020 will have surpassed 10 million, approximately double the figure indicated just two years ago. This number of electric cars is fast increasing as governments adopt climate change measures that involve phasing out petroleum-powered vehicles. According to industry estimates, the majority of new light-duty cars will be battery-powered within the next 20 years. Batteries used by electric utility systems to store solar and wind energy are another use pushing lithium demand.
Figure 1: Uses of Lithium
Lithium harvesting from leftovers of desalination plants
The oceans have 5,000 times more amount of lithium as compared to land, e.g., let’s suppose that if the land has 100 kg of lithium, then as compared to land, the sea will have 500000 kg (100kg*5000) of lithium. Although the concentration of lithium in the seawater is extremely low i.e., 0.2 parts per million (ppm). It means that if we divide the seawater into 1 million parts then 0.2 parts out of 1 million parts will contain lithium, while the rest of the 999999.8 parts (106-0.2 parts) will have no lithium. Separation is also costly due to the low lithium concentration of about 0.2ppm. Larger ions such as sodium, magnesium, and potassium are all present in much higher amounts in seawater as compared to lithium.
Figure 2: Lithium reserves
Comparison of Lithium composition in feed seawater and rejected brine As compared to the feed seawater for the desalination plant, the rejected brine contains a relatively higher concentration of these ions i.e., Li+, Na+, Mg2+, Ca2+, Cl- etc. The samples of rejected brine were collected from two seawater reverse osmosis desalination plants. The reverse osmosis desalination units at Desalination Research Plant (DRP) Shuwaikh and Doha have production capacities of 136,000 and 300 m3/d, respectively. The total dissolved solids (TDS) in the rejected brine of Shuwaikh and Desalination Research plant (DRP) are 78,000 and 58,000 ppm, respectively. The lithium concentration in the feed seawater is 0.2 mg/L and in the rejected brine is 1.7 mg/L as shown in Table-1. It will be easier to separate lithium from the rejected brine as compared to the feed seawater.
DRP RO plant has a total recovery of 40%, but the Shuwaikh SWRO plant has a total recovery of 50%–60%. Tables-1 illustrates the average composition of seawater feed and SWRO brine discharged from DRP and Shuwaikh desalination facilities. According to the results, the TDS of rejected brine from the DRP SWRO (seawater reverse osmosis) desalination plant is much lesser than that of the DRP SWRO desalination plant. This is due to the reason of difference in the intake of the feed system, and the water recovery ratio percentage. The DRP SWRO plant’s feed intake system is a beach well, whereas the Shuwaikh desalination plant gets its feed directly from the sea. As the main waste of the desalination plant is brine. It contains higher concentrations of ions as compared to the feed seawater. Brine has environmental effects as well, it may damage plants and aquatic life because of the high concentration of salts. There is a need to nullify these effects. That’s why methods i.e., electrochemical cells, adsorption and precipitation of metals are preferred to lower the concentration of salts. The removal of lithium from the rejected brine stream will be a profitable option.
Comparison of technologies
The lack of proper lithium extraction technologies has hampered lithium mining from the ocean. Adsorption and electrochemical strategies have been studied so far. However, both techniques have drawbacks. The use of the adsorption method was limited because of poor (low) adsorption rates and adsorbent dissolution. Furthermore, electrochemical tests were either limited to lithium-enriched brine or carried out in a static electrochemical cell, which necessitated electrode replacement6. But the researchers at KAUST (King Abdullah University of Science and Technology) have devised a cost-effective technique for extracting high-purity lithium from seawater. By enriching water from the Red Sea, researchers generated lithium phosphate with a purity of 99.94 per cent.
An electrochemical cell having a ceramic membrane which was built of lithium lanthanum titanium oxide (LLTO) was developed by the KAUST team to overcome the challenge of how to harvest lithium from seawater as its concentration is only 0.2ppm. Its crystal structure has small pores that allow lithium ions to pass through while inhibiting bigger metal ions like Na+, K+, Mg2+, Ca2+ etc. According to Postdoc Zhen Li:
“Lithium ions have never been extracted and concentrated using lithium lanthanum titanium oxide (LLTO) membranes before.”
As the electrochemical cell is being used to harvest from the leftover (rejected seawater containing a large amount of salt as compared to the feed seawater) of the desalination plant, that’s why the basics of the electrochemical cell will be discussed for a better explanation of harvesting of lithium from the seawater.
An electrochemical cell is a device that contains two electrodes (conductor usually made of metals e.g., zinc or copper etc.) dipped in a solution of an electrolyte (the substances that have the ability to conduct electricity in their molten states or aqueous solutions are called electrolytes e.g., solutions of acid, salt and base are good electrolytes). This cell may either create electrical energy from chemical processes taking place inside it or use electrical energy supplied to it to aid chemical reactions taking place inside it. Chemical energy can be converted to electrical energy or vice versa using these devices. A conventional 1.5-volt cell, which is used to power numerous electrical items such as TV remotes and clocks, is an example of an electrochemical cell. There are two types of electrochemical cells:
a. Electrolytic cell
The cell in which a non-spontaneous chemical reaction is carried out because of the passage of the electric current through the solution is called an electrolytic cell. The process that takes place in an electrolytic cell is called electrolysis. It is defined as the chemical decomposition of a compound into its components by passing a current through the solution of the compound or in the molten state of the compound. Examples of these cells are Downs cell, and Nelson’s cell.
Figure 3: Electrolytic cell
b. Galvanic cell
The cell in which a spontaneous reaction is carried out and an electric current is generated as a result of the chemical reaction is called a galvanic cell. A Daniel cell is an example of this type of cell.
Figure 4: Galvanic cell
Now let’s discuss the main topic.
An electrochemical cell is used to harvest lithium from the leftover (rejected seawater) of the desalination plant. There are three compartments in the cell as shown in figure 6.
a. Feed compartment
The feed compartment contains the continuous feed of the leftover seawater.
b. Cathode compartment
The cathode compartment contains a buffer solution (electrolyte) and a cathode made of copper metal. The hollow fibre (HF) metallic copper cathode is also coated with ruthenium and platinum. This coating is applied in order to facilitate the reaction of hydrogen evolution.
The hollow fibre copper cathode has a standard finger-like porous structure, which allows CO2 to blow out through the porous structure, and eventually release uniformly into the cathode compartment. As previously reported, the emitted CO2 creates an acidic environment near the cathode, which improves faradaic efficiency at high current densities. Concentrated H3PO4 is also employed as an auxiliary solution to control the pH. The CO2 and H3PO4 form a buffer solution to keep the pH of the cathode compartment between 4.5 and 5.5 to prevent alkaline corrosion of the LLTO membrane.
When a voltage of 3.5V is applied, the following reactions8 are carried out at the cathode in the cathode compartment:
c. Anode compartment
The anode compartment contains a saturated sodium chloride solution (electrolyte) and an anode made of platinum-ruthenium. When a voltage of 3.5V is applied, the following reactions are carried out at anode in the anode compartment:
Lithium lanthanum titanium oxide (LLTO) membrane
Li0.33La0.56TiO3 (LLTO) membrane is a dense glass-type membrane that is used to separate the cathode and feed compartments. The diameter and thickness of the membrane are 20 mm and 55μm respectively. This membrane is made by sintering (coalescing into a solid/porous substance upon heating) the powder of LLTO to a molten state to form a dense glass-type membrane. The membrane surface appeared smooth with no grain boundaries in the high magnification SEM image. The membrane has a stress of 110 MPa and a ductility of 0.066 per cent in the mechanical test, which is a typical ceramic mechanical property: hard but brittle.
The thickness of the membrane is crucial. One of the aspects important to getting high Li+ permeance was optimizing the membrane fabrication procedure to generate a thickness ten times thinner than those described in the literature. The membrane, on the other hand, is sufficiently strong to form a stable membrane during the test.
LLTO is one of the best solid-state lithium-ion superconductors. Its crystal structure explains its excellent lithium-ion conductivity and high selectivity with respect to other ions. The crystal structure of LLTO is perovskite-like, as shown in figure 5(a). XRD validated the crystal structure of the LLTO membrane. The LLTO lattice is made up of interconnected TiO6 octahedra that create cubic cages that hold Li+ and La3+. The massive La3+ ions stabilize the crystal structure by acting as support pillars.
Figure 5: (a) the crystal structure of LLTO in ball-and-stick mode (b) illustration of the percolation of lithium ions in the LLTO lattice
The high valency of La3+ induces an alternate arrangement of La-rich and La-poor layers along the c-axis, resulting in a large number of vacancies in the structure that allow Li+ to intercalate. Li+ must pass through a square window of 1.07 Å defined by four nearby TiO6 tetrahedra for it to be transported from one cage to the others. The size of Li+ (1.18 Å) is somewhat larger, necessitating a minor distortion in the framework to enlarge the windows as shown in figure 5(b) and it is possible due to thermal vibrations of the TiO6 octahedra.
Other ions in seawater (such as Na+, K+, Mg2+, Ca2+, and so on) are significantly larger than lithium, necessitating a much larger distortion and hence a much higher energy barrier to transport. As a result of the characteristics of the LLTO membrane, we expect the LLTO membrane to allow fast Li+ transit while blocking all other significant ions present in seawater.
Working of the cell
After the introduction of seawater into the central feed chamber, the electric current is passed through the cell and the substances and salts (NaCl, KCl, CaCO3, MgCl2 etc.) dissociate into its ions e.g., Na+, Ca2+, Mg2+, Li+, Cl-, CO32− etc. As the opposite charges attract each other, so positive ions will move towards the cathode (negatively charged) and negative ions will move toward the anode (Positively charged). Lithium lanthanum titanium oxide membrane has a crystal structure containing small pores which allow only the lithium ions. Positive lithium ions, present in the feed compartment, move through the lithium lanthanum titanium oxide (LLTO) membrane into a side compartment (cathode compartment). In this way, lithium is separated from the seawater and accumulates in the cathode compartment.
Figure 6: Layout of working cell
During the electrical pumping membrane process, hydrogen is continually created from the cathode via reactions (discussed above), allowing continuous transportation of lithium from the feed compartment to the cathode compartment via the LLTO membrane.
The water enriched with lithium is then used as a feedstock for four additional processing cycles. Eventually, it reaches a concentration of over 9,000 parts per million. By adjusting the pH of this solution, solid lithium phosphate with just minor residues of other metal ions is produced, which is pure enough to suit the demands of battery makers.
From the feed compartment, the positive lithium ions are attracted by the cathode (negatively charged), meanwhile, in the same way, the anode (positively charged), present in the anode compartment, attracts the negative ions. The negative ions move from the feed to the anode compartment by passing through the standard anion exchange membrane (AEM). AEM only allows anions (negative ions) to pass through.
Chlorine gas is produced from the anode compartment via reaction (discussed above). As chlorine gas does not dissolve in a saturated NaCl solution, it causes Cl- and other anions (i.e., 𝐻𝐶𝑂3−, 𝐻2𝑃𝑂4−, 𝐻𝑃𝑂42−) to be transported from the feed to the anode compartment over the AEM membrane.
Advantages of using an electrolytic cell
Monovalent ions, such as sodium and potassium ions, are not a problem in the traditional precipitation process because their salts are extremely soluble. Instead, the low concentration of lithium and its ratio to other multivalent ions like Mg2+ and Ca2+ are the most important factors to consider.
The membrane process is one of the most energy-efficient separation technologies, having the potential of saving up to 90%8 of energy in several industrially essential separation processes. Furthermore, this membrane process is not a batch process it runs continuously and is very simple to scale up so that it can be easily used on an industrial level.
Unlike traditional/conventional membrane processes, where a transport proceeds down the concentration gradient i.e., the substance transports from the area of higher concentration towards the area of lower concentration until the substance becomes evenly distributed, the electrically-driven membrane process has the ability to upgrade the concentration i.e., transport of the substance from the area of higher concentration towards the area of lower concentration. This technology has also been commercialized for the purification of hydrogen.
Results and conclusion
According to the researchers, the cell would only require $5 of electricity to extract 1 kg of lithium from seawater. The value of the hydrogen and chlorine produced by the cell would be enough to overcome the cost of $5.3
The lithium lanthanum titanium oxide membrane showed ‘negligible’ degradation after 2,000 hours of use. The membrane concentrates lithium ions in a separate solution to over 9,000 ppm by using electricity. Researchers then used approximately 76.3kWh of electricity to electrolyze this solution to produce 1kg of lithium phosphate. This method also produced hydrogen and chlorine byproducts, each with its own resale value.
The paper’s economic analysis used an electricity price of $65 per MWh to calculate its profitability. Using $5 of power, the process created $6.9-$11.7 of hydrogen and chlorine as byproducts. Lithium prices have varied significantly over the past year, and different concentrations demand greatly different prices. However, a conservative estimate would give a return of at least as much as the byproducts.
Depending on the above results, it can be concluded that this project will be a great revolutionary in the field of lithium batteries and electric cars and it will help us to control pollution. Separation of lithium from the brine is even more profitable for the desalination plants and it will help to overcome the environmental impacts of the brine as well.
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- Average Seawater Composition of the Arabian Gulf at Al-Jubail | Download Table. Accessed October 23, 2021. https://www.researchgate.net/figure/Average-Seawater-Composition-of-the-Arabian-Gulf-at-Al-Jubail_tbl1_242152295
- Can seawater give us the lithium to meet our battery needs? Accessed October 22, 2021. https://cen.acs.org/materials/inorganic-chemistry/Can-seawater-give-us-lithium-to-meet-our-battery-needs/99/i36
- KAUST Team Unveils Method to Extract Lithium from Seawater – Batteries News. Accessed October 23, 2021. https://batteriesnews.com/kaust-method-extract-lithium-seawater/
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