ASN General Discussion, page-12528

Also IMHO the Resin is "not" the key to the process it's the "simulated moving bed" SMB that's the key , my understanding is that Lithium Boron and Magnesium ions are charged one way either +ve or -ve hence they get drawn to the resin but it's the size of the molecule which allows the SMB to to allow the lithium mostly to pass through . It basically acts as a type of sieve and when it moves I assume that this causes boron and Magnesium to to move on the sieve not blocking it for the Lithium . Ultimately although it's a thorough process it's not perfect and "some" boron and magnesium gets through requiring further finite filtration.
FYI I am not a chemist this just what I learned from researching and my own general knowledge. Regards TT