That makes more sense seeing that photo!
Are they actually building a concrete tower around it? Theres no reason for it other than aesthetics - the old tower is a concrete structure - there is no steel frame inside it, the sheave wheels are supported by the concrete. Unless Boulby is changing its vent strategy, from a ventilation viewpoint it is better to leave it like it is - the rock shaft is exhaust upcast - the fans are on the manshaft, which is airlocked. Putting a concrete column around that will restrict ventilation needlessly.