A nonlinear geometric feedback controller with and without state estimation (the extended continuous-discrete Kalman filter) is developed and applied to a 0.027 m(3) potash alum batch crystallizer. The manipulated variable isthetemperature of the inlet cooling water supplied to the jacket of the crystallizer, and the controlled variable is the supersaturation. It is shown that the controller eliminates the large initial peak in the supersaturation (which results in excessive nucleation) and maintains the supersaturation at its set-point, provided that themanipulated variable does not reach its constraints. The controller performs well with only two measured states (the crystallizer temperature and the soluteconcentration) and results in larger terminal crystal mean size in com-parison with natural cooling and linear cooling policies with fines dissolution.