Fig. 1. Installation of continuous pumping of seawater through an electric diaphragm pump: (a) three-chamber cell for lithium enrichment with simultaneous generation of H2 and Cl2 at the cathode and anode, respectively; (b) photograph of the installation; (c) crystal structure LLTO; (d) percolation of lithium ions through the LLTO lattice; (e) glass type LLTO membrane (≈20 mm in diameter); (f) a hollow copper cathode coated on one side with a Pt / Ru catalyst (dark in the photo).
Lithium is a key element in modern batteries that power almost all technology, from smartphones to cars. But the reserves of this metal on land are very limited. If we extrapolate the growing demand, then the proven reserves will only last until 2080.
But we all know where the largest reservoir of gold, platinum, lithium and other rare elements is located – this is the World Ocean. Chemists have not yet found a cost-effective way to extract gold from seawater, but with lithium it turned out better. A group of scientists from the University of Science and Technology in Saudi Arabia showed a ready-made solution with energy costs of only $ 5 per kilogram of orthophosphate (Li3POfour).
The problem with mining lithium on the ground is also that it causes great damage to the environment, while wasting a huge amount of water – about 1900 tons of water per 1 ton of lithium mined.
The marine reserves of lithium are 5,000 times greater than those on land. But there it is in low concentrations: only 0.2 ppm (parts per million). All previous attempts to extract lithium from the ocean have been ineffective. But now the team led by Zhiping Lai has taken a unique approach. They created an electrochemical cell with a ceramic membrane made of lithium lanthanum titanium oxide (LLTO). Basically, it’s a simple mesh that filters out lithium.
In the crystal structure of the cell, tiny pores are so large that lithium ions pass through the membrane, but ions of larger metals do not.
There are three chambers inside the cell. First, seawater enters the loading bay, where positive and negative ions are separated. Here, positive ions escape through the LLTO membrane into a side chamber with an intermediate solution and a copper cathode coated with platinum and ruthenium on one side. Meanwhile, negative ions leave the chamber through a standard anion exchange membrane (AEM) into the third compartment with NaCl solution and platinum-ruthenium anode.
Researchers tested the system in the water of the Red Sea. At 3.25 V, the cell generates hydrogen gas at the cathode and chlorine gas at the anode. This leads to the transport of lithium across the LLTO membrane, where it accumulates in the side chamber. The lithium-rich water then becomes the feedstock for four more processing cycles, eventually reaching a concentration of 9000 ppm.
Lithium concentration in seawater at different stages of the process
Fig. 2. Extraction of lithium from seawater using an electric membrane pump (effective membrane area 2.01 cm², voltage 3.25 V): (a) chronoamperometric curve at each stage; the area under the curve shows the total charge passed through the membrane in pendants for each stage; (b) constant current and concentration of lithium at different stages; the number of different ions passing through the membrane at each stage; (d) the fraction of the Faraday efficiency of the various ions for each stage
This saturated solution readily separates solid lithium phosphate containing only traces of other metal ions – pure enough to meet the industrial requirements of battery manufacturers.
Fig. 3. Product Li3POfourprecipitated from the fifth enriched solution: (a) photo of the collected powder; (b) X-ray diffraction analysis of the powder. All diffraction peaks coincide with the standard Li X-ray diffraction pattern3POfour
Researchers estimate that a cell requires only five dollars of electricity to extract a kilogram of phosphate from seawater (at 6.5 cents per kilowatt-hour). The cost of hydrogen and chlorine produced by the cell more than compensates for these costs, and the remaining seawater can be used in desalination plants, the authors say. Indeed, pure hydrogen as a by-product is very nice.
It seems that the power consumption of $ 5 is not too much. For comparison, world prices for industrial lithium carbonate (Li2CO3) in March 2021 increased to $ 23 per kg, for LioH hydroxide – up to $ 25… Orthophosphate Li3POfour small wholesale costs from $ 22 per kg (unfortunately, it was not possible to find it on commodity exchanges).
Another question is, what is the wear of the filters, how long are there enough catalysts made of platinum and a membrane made of lithium-lanthanum-titanium oxide, this is expensive material? And in general, what is the total cost of production. Only industrial operations will answer these questions. Scientists expect production to be profitable. Let’s hope that Saudi Arabia will have no problem funding this experiment.
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