This is a a tweet that got too long. When you are looking at lithium brine companies before they have a flowsheet what contaminants do you need to be concerned about? I'm not an expert in this area, but here's what I've gleaned after some reading. If this is inaccurate please let me know so I can correct it.
Here's an Albermarle spec sheet we can look at for clues.
Stuff I care a LOT About
Li, K, Na, Mg, Be, & Ca are all clustered around eachother on the periodic table in column 1 ions with a +1 charge or column 2 ions with a +2 charge. This makes them a lot like Lithium, which we want to keep. One method of removal is to add slaked lime. The problem is that it's not a perfect process and you lose lithium along with the magnesium, as the amount of magenisum you have to remove goes up so does the amount of lithium you lose.
The Li:Mg ratio is usually what makes a brine work or not work. The traditional rule of thumb is that Li:Mg ratio must be higher than 1:9 or 1:10.
Stuff I Care About
Calcium seems to make it through most conventional evaporation ponds into the LiCl concentrate that gets sent to the Lithium Chloride plant. It tends to be removed in a similar way as Mg (ie slaked lime), but being heavier it seems to be easier to remove without losing too much lithium.
Stuff I don't understand
Fe is both magnetic and conductive, which is an issue in batteries. It's also quite a bit heavier than Li. I honestly don't know how big of an issue it is in source brine or how difficult it is to remove, but my guess would be it's not too much of a problem.
It should be in the list with +2 charge, but maybe crustal abundance is just naturally low.
Stuff I don't really care that much about
Cl is typically bound to Li and Na, it's not a problem at all in the source brine, just in the spec sheet to let you know you have to process your lithium from the form it came out of the ground.
SO4 is typically introduced in the chemical process with evaporation ponds, I don't expect it to be an issue with source brine.
In evaporation ponds you can add halite and polyhalite, wait for some H2O evaporation and Na precipitates and 99% of Li moves on to the next pond.
After precipitating Na you add Halite & Sylvinite, wait for some H20 evaporation and K precipitates and you recover 97% of Li.