Nathan Cowieson:
I have found that ion exchange is a good way to concentrate protein. If there is no reason that you can't pass the whole 450 mls through the column then you should do that.
Another way is to load dilute protein onto small hydroxylapatite column. Provided that the buffer does not have phosphates, almost all proteins bind to hydroxylapatite regardless of the salt (at least up to 0.5 M). The obvious advantages is that one does not need to dilute and that for most proteins elution can be done with only 50-100 mM Pi. The disadvantage in comparison to ion exchangers is about 4-fold lower - usually, around 20-30 mg/ml of the sorbent.
Dima
