Thanks Neil for pointing out the change to the Process Flowsheet that now includes CO2 as an input and Okeido for providing links to the Limestone Cycle. I missed that update and it would be a smart move to recapture one of their waste streams to reintroduce it into the process. This might offset some of the CO2 emissions, but I am skeptical that it would be any more than a modest offset to overall CO2 emisisons. I have copied (below) a post I made from earlier this year detailing CO2 generation from the proposed process at Thacker Pass and underlined "sodium carbonate" because this is where it would make sense to me to substitute CO2 in the process. However I don't know what the consequences of this would be with respect to pH of the lithium solution and resulting solubility of lithium carbonate with respect to crystallization. Elsewhere upstream in the process calcium carbonate and quicklime are used to neutralize the lithium containing sulfuric acid leachate and remove magnesium (pg 82 of PFS copied below). CO2 cannot be substituted at these steps because in the absence of the cation (Ca2+ in CaCO3 or Na+ in NaCO3) CO2 becomes carbonic acid in solution and would drop the pH. Quicklime and limestone neutralize the sulfuric acid by raising the pH. 

I see the largest source of carbon emissions as being from the quicklime that is being used to remove Magnesium from the sulfuric acid leachate. There is so much magnesium that after phase 2 is complete over 1.2 Million tons of magnesium sulfate will be produced as a byproduct. That is, for every ton of lithium carbonate equivalent there will be 20 tons of magnesium sulfate produced. This is also the same reason why the process consumes so much sulfur/sulfuric acid. One can't remove the lithium from the ore using sulfuric acid leaching without also removing magnesium which is present at a much higher concentration.

Regarding limestone, when limestone (a.k.a CaCO3 or calcium carbonate) is added to a sulfuric acid solution, the carbonate spontaneously changes to carbonic acid, carbonic acid interconverts to carbon dioxide, and if the concentration of carbon dioxide exceeds the solubility of the gas in solution it will fizz out. This is akin to mixing vinegar (acetic acid) and baking soda (sodium bicarbonate) which is done to make volcanos for science projects. This is where carbon dioxide could be captured for sequestration off site or recycled back into the process, but I have yet to see this included in the process flow sheet. Hopefully this will be addressed in the DFS.

Because quicklime is sourced and not produced on site from limestone, much of the CO2 will be generated offsite. If CO2 emissions come at a cost, I would guess that would be passed through LAC and on to the consumer, or distributed throughout the supply chain. The bigger issue I see is who gets the blame for these CO2 emissions. It would not be unreasonable to hold LAC accountable for all of the CO2 emissions generated directly by their process as well as those upstream in making the input reagents as they designed the process and have the power to change it. However, I do not know what a reasonable alterantives to quicklime and calcium carbonate would be. This is one potential criticism I  can't defend and an aspect where direct lithium extraction appears to hold an advantage.

Pg 82 of the PFS for Thacker Pass

Three-Step Purification

The acid recycles “mother liquor” (see Section 13.2.1.4) was used to test stepwise crystallization and precipitation for lithium purification (Lithium Americas Corp. 2017f). The experimental work flow is provided in Figure 13-5 and can be considered a three-step purification. The first is neutralization (CaSO4 removal), the second is crystallization (MgSO4 removal), and the third is precipitation (MgOH2 and CaSO4). Table 13-2 shows the lithium recoveries of each successive step of the process and the overall lithium recovery.

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In figure 17-4 of the PFS on pg 137 describes the process flow sheet. The last step describes removing Li+ in the form of lithium carbonate and leaving behind Na+ and K+ in solution. This is done by adding sodium carbonate to the solution. The way this works is that the sodium (Na+) and carbonate (CO3-) dissociate in solution and are free to react with other species. For reference, the solubility of sodium carbonate is ~35% weight per volume of water at room temperature and the solubility of lithium carbonate is ~1%. I presume the concentration of lithium at this step will be below 1%, lets presume 0.3% which was measured in the starting sulfuric acid leachate. When adding sodium carbonate, nothing should precipitate out if lithium carbonate remains below its limit of solubility. When this liquid is sent to the evaporator, heat (and maybe vacuum pressure) is used to remove water and increase the concentration of the lithium in solution. Once  the concentration exceeds the limit of solubility ~1%, lithium carbonate (Li2CO3) will spontaneously crystallize, precipitate out and can be removed by filtration. After lithium crystalls are filtered out the solution still contains ~1% lithium. They will cycle this stream back into the process per: "The zero-liquid discharge crystallizer will purge a small amount of barren lithium brine back to the liming system (Section 17.2.2) to maintain a predetermined lithium concentration in the crystallizer that will optimize lithium recovery."