LITHIUM STOCKS AND CHARTS

[TradingCharts]

Clay Mineralogy and Ion Transport

Clay minerals consist of microscopic framework layers composed of Li, Na, K, Al, Si, Mg, Ca, Fe, O, and/or OH, and inter-layer spaces through which cations like Li, Na, K, and Mg may be conducted in water or other electrolytes (like “books on a bookshelf”, Tesla’s metaphor for lithium in cathode materials). The position of the lithium atom in this mineral structure makes all the difference for how it can be extracted, e.g. if the lithium is found within the framework layer or floating in the interlayer.


We believe the mechanism for how a saline extraction could work would fall into one of two categories. First, the mechanism could be a chemical reaction between NaCl (or a product of NaCl) and the sedimentary clay mineral, which could degrade or modify the framework layer structure to liberate lithium. Second, the mechanism could be an ion exchange-type process that swaps the lithium for a sodium ion, ending with a LiCl solution and a sodiated clay with minimal modifications to the framework layer.

Below is a list of five theoretical mechanistic contributions that we believe are plausible pieces of the puzzle for explaining how a saline leach process could work. It is possible that not one of them in isolation is sufficient to explain a saline leach, but combined, they could constitute a viable mechanism to explain how Tesla’s saline extraction could work.

1. Octahedral Layer Site Exchange

The octahedral layer within the framework layer (such as in hectorite) is restrictive to ion diffusion and exchange. The reason being is that a monovalent cation would need to be sufficiently small to diffuse through the silicate layer, or through the edges of the layer, while also adopting a favorable octahedral geometry. A LiO6 unit may exist in the octahedral layer because the radius ratio of Li:O is sufficiently small. However, the larger Na:O radius ratio is slightly above the boundary between an octahedron, but does form NaO6 in several other mineral cases (such as wulffite and meieranite). A more favorable Na:O radius ratio in clays could occur at an elevated temperature where O gets larger, decreasing the radius ratio to allow NaO6 into the octahedral layer. (4) In conjunction with high sodium concentration to get over the Donnan Potential barrier, sodium could then exchange into the octahedral layer, liberating lithium in the process. Upon cooling, the NaO6 site would be unstable. The octahedral site's instability could present a situation for lithium diffusion back into its original site. This has process design implications that could compromise the relevance of "cook and look" bench-scale extraction experiments.

2. Interlayer Site Exchange

If lithium resides in the interlayer spaces of the clay and it is mobile, then mixing it with a very high concentration NaCl solution could provide an entropic driving force to imbue the same ratio of Li/Na in the solution and in the clay interlayer. For example, if the interlayer spaces were in fluid communication with the bulk solution, then with enough time, the Li/Na ratio in each could end up being the same both within the interlayer and in the bulk fluid. It is possible to control that ratio, meaning the lithium could be cajoled out of the interlayer. There could be steric and other surface chemistry factors that could affect how this diffusive process would work both thermodynamically and kinetically. Considering that much more aggressive leaching techniques have been advanced by most sediment projects in development, we think it’s unlikely that a significant fraction of the lithium is mobile in the interlayer, however it is not impossible that Tesla could have identified a unique mineralogy in which this is the case.

3. Differences Between Enthalpies of Hydration

Lithium has a higher enthalpy of hydration than sodium, meaning it holds onto water more strongly under common conditions as chlorides. This is the basis for sodium chloride and potassium chloride crystallization in evaporation ponds. Lithium chloride holds onto water more tightly than the other monovalent chlorides, so when water is removed by evaporation, the other metal chloride salts crystallize first. Tesla could leverage this effect in a saline leach. If the clay mineral was modified, or made less stable using heat or reagents to a certain point that it was possible to extract lithium (from either the framework layers or interlayers, in exchange for sodium or otherwise), lithium’s enthalpic driving force to complex water molecules could be a driver for it to enter solution, i.e. to be extracted from the mineral into a leachate. This driving force would be reduced if the total dissolved solids of the leach (extractant fluid) was too high, and the lithium ion had to compete with too many sodium ions for water molecules to complex. The Gibbs free energy change of extracting lithium from the octahedral layer may be positive, but the Gibbs free energy change of solvation or whatever else happens in solution must also be considered in the total Gibbs free energy change, i.e. to judge if the process would occur spontaneously or not. (5)

4. An Inverse Hofmann-Klemen Process

If the mineral is hectorite, then negative structural charge of the clay ensures that the concentration of sodium at the framework layer surface is much higher than that of chlorine if in solution. This would suggest that it is more likely that sodium is the active agent in an aqueous sodium chloride extraction. However, sodium would not dissolve the aluminosilicate octahedral layer like an acid would. A possibility for sodium to liberate lithium from the octahedral sheet could be the use of an inverse Hofmann-Klemen process, in which sodium replaces magnesium in the octahedral sheet. This could destabilize the mineral structure and result in a simultaneous expulsion of lithium. A similar possibility is that sodium could enter a vacancy in the octahedral layer, expelling lithium in order to maintain charge balance. This means that sodium doesn’t necessarily need to ion exchange with lithium directly in order to liberate lithium, but if other cations were exchanged first, it could mean those other cations could constitute more impurities in the leachate, similar to an acid leach. (6)

5. Chlorination by Calcination

The melting point of sodium chloride is 801°C. Tesla could heat a dry mixture of salt and clay to near or above this temperature, causing the sodium chloride to melt and/or potentially becoming much more reactive. This could result in chloride ions disintegrating the framework layer of the clay mineral, liberating lithium in the process, and allowing it to be washed out with water. This could work similarly to the “sulfation” extraction process which is being pushed forward by the Sonora sedimentary clay project in Mexico, in which CaCO3 and CaSO4?2H2O is calcined with the clay to liberate lithium. This process was originally developed by the US Department of the Interior, and was also the chosen flowsheet for the Thacker Pass project until it was switched to an acid extraction process. (7) The equipment required for a calcination process could look similar to a cement kiln. If Tesla is already working on high temperature processes for cathode manufacturing, then they might be able to leverage some of their learnings from that processing to develop a high temperature salt roast process for sedimentary lithium extraction. (8)

Though we do not claim that Tesla’s saline extraction process works, applies to any particular sedimentary clay material, or works economically, we believe that some of the mechanisms described above could be useful for explaining how it could work. We believe it is highly likely that their process includes high temperature processing and/or other reagents not mentioned at Battery Day.

https://www.jadecove.com/research/teslasaltclay