There are multiple routes to THCA purification and subsequent crystallization.

One of the main issues to be aware of is to avoid heat to avoid decarboxylation of THCA into THC. For example if you use soxhlet it will require heat and you will convert THCA into THC. THC doesn't crystallize so the material will have a more oily consistency if too much THCA is converted.

So to remove solvents without heat you basically need vacuum (hence a rotovap).