A novel experimental device for measurement of agglomerative crystallization forces

A novel experimental apparatus has been developed which enables the estimation of agglomeration forces between two crystals. The set-up consists of an inverted optical microscope, fitted with an adapted stage on which two micromanipulators are employed to hold a pair of micropipettes. The two crystals in close proximity within the supersaturated solution are then allowed to agglomerate over a fixed time period. The force necessary to break the agglomerate (e.g., the adhesion force between two agglomerated crystals) is measured by using two linear variable differential transducers, one of which detects the movement of the moving manipulator and the other one the flexion of a calibrated flexible blade. The geometry of crystal surfaces at the contact points and the dynamic development of the bond are captured on video camera and characterized using an image analysis technique. The experimental apparatus has been designed to allow control of supersaturation, orientation of crystal faces, distance between crystals, relative movement of crystals and contact time. Agglomerate adhesion forces ranging from 1 to 9 x 10(4) N per m(2) of contact area were measured for agglomerates of potash alum in aqueous supersaturated solutions. Several measurements were performed at varying levels of supersaturation. The experimental results show that the agglomeration force per unit contact area increases with increasing supersaturation. This evidence constitutes a fundamental validation of the experimental finding that crystal agglomerates become more difficult to disrupt at high supersaturation. The experimental results were used in combination with theoretical expressions for mechanical stresses on suspended particles in an agitated system to estimate the size of fragments to be expected in a crystallizer as a function of particle size and agitation conditions. These estimates proved to be within the range of values of fragment sizes measured in agitated suspensions of agglomerated crystals.