Pratola, Federica; (2004) Micro-mechanics of agglomerative crystallization processes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The broad aim of this dissertation is to give a fundamental contribution towards the understanding of the agglomeration mechanisms in crystallization processes. This is achieved through direct measurement of agglomerative forces in crystal agglomerates. A novel device, consisting of a micro-force balance and a crystal growth cell mounted on a specifically designed microscope stage, was developed for the procurement of agglomerative force data under varying conditions of agglomeration (in particular supersaturation level, contact time and type of faces in contact), and using different types of crystals. The micro-mechanics of organic and inorganic crystal agglomerates was investigated using succinic acid and potash alum. Agglomerative strengths in the range of 1-9*10^4 and 0.7-3.5*10^4 N per m2 of contact area were measured for potash alum and succinic acid respectively. Agglomerative strength was found to depend on the type of crystal faces in contact during agglomeration, and to increase at higher supersaturation. This evidence supports experimental literature data obtained in continuous crystallizers, which show that crystal agglomerates are more difficult to disrupt at high supersaturation. Measured agglomerate adhesion forces were related to the mechanical stresses acting on suspended particles in a crystallizer to calculate the size of fragments obtained from parental crystals of different sizes. These estimates were in close agreement with literature data from laboratory scale crystallizers. On the basis of morphological observations on contact surfaces in crystal agglomerates and evidence collected on the micro-mechanics of crystal agglomerates, the effect of supersaturation on agglomerative strength (i.e. agglomerative strength increases with increasing supersaturation) is attributed to higher crystal growth rate, rather than to changes in the nature of bonds between agglomerating crystals. This hypothesis on agglomeration mechanism is consistent with all experimental results on agglomerative strength obtained using different crystals, conditions of agglomeration, and faces in contact.

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