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Title: High resolution imaging of crystalline agglomerates
Author: Collier, A. P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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A novel method of forming pure calcite seed materials by nucleation from an alkaline solution was developed. Batch experiments were conducted to investigate the growth and agglomeration rates of the calcite seed particles under varying agitation rates. Stirrer speeds between 700 and 2000 rpm were studied. The experiments were all carried out in a saline solution of ionic strength 0.153 M, with an initial supersaturation of approximately 4.0. The data from the experiments were analysed using the program Batch (Hounslow, 1990). This program extracts growth and agglomeration rates from the changes observed in particle size distributions with time. It is found that the growth rate is independent of the crystal size and the agitation rate. Growth rates from all experiments can be expressed by a single equation with a second order dependence of the growth rate on the supersaturation. This dependence, and the magnitude of the growth rate constant are consistent with other reports in the literature. The agglomeration rate is best described by a size-independent agglomeration kernel. At all except the lowest stirrer speed, the agglomeration rate is found to be proportional to the growth rate. The agglomeration rate is found to decrease as the stirrer speed increases. Correlation of the agglomeration rate with the stirrer speed shows that the agglomeration rate is proportional to the stirrer speed raised to the (-1.5) power. A comparison with estimates for the average shear rate within a stirred vessel suggest that the agglomeration rate is inversely proportional to the average shear rate within the crystalliser. Microscopic examination of many calcite samples was undertaken with SEM systems. Several hundred images of calcium carbonate particles were obtained. High resolution images were used to identify the phases present in preliminary trials, and to confirm that the calcite was the main product of the batch growth experiments. Images of samples taken from the batch growth experiments were used to confirm the decrease in the agglomeration rate at higher stirrer speeds, and the constancy of the growth rate. No evidence was found for any preferential alignment of the crystallites within the agglomerates produced by this method.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available