The design, development and testing of a non-invasive continuous crystalliser system for the study of calcium oxalate crystallisation processes

Doctoral Thesis


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University of Cape Town

The MSMPR was designed according to chemical engineering principles and tests showed that it conforms to the assumptions necessary for conventional MSMPR analysis. Various crystalliser offtakes and six system designs were developed and tested to allow measurements using the thermostatted flow cell. The flow cell allows for continuous measurement of the sample dispersion using a noninvasive monitoring instrument - a Malvern Particle Size Analyser. In this technique, measurement of the steady state crystal size distribution is based on a diffraction pattern produced by the particles in a parallel beam of monochromatic light. Comparative measurements were obtained by sampling directly from the crystalliser and analysing the product in a Coulter Counter. In this technique, the steady state crystal size distribution is measured by the change in resistance as particles, in an electrically conductive liquid, pass through the Coulter aperture. Due to the large volumes required by the crystalliser, it was necessary to pool urines. Aliquots of volume 1. 5 dm3 from the 24 hour urines of each of 8 male controls were pooled. The first series of experiments were performed to investigate the effect on calcium oxalate crystallisation of varying the concentration of sodium oxalate solutions used to initiate crystallisation, and also to investigate the effect of varying the temperature. A further series of experiments was performed to determine the effect of adding inhibitors individually (citrate, magnesium, chondroitin sulphate A) and in combination (citrate and magnesium). The inhibitor experiments were performed using two different approaches. In the first, the pooled urine was pre-treated with the inhibitor prior to being introduced into the crystalliser. In the second set of experiments, the inhibitor was introduced into the crystalliser via a separate, independent feed. It is suggested that these approaches represent crude models respectively of (a) the endogenous presence of inhibitors and (b) the oral administration of such inhibitors. Each individual experiment was performed using three different urine pools. The steady state crystal size distributions obtained from both the Coulter and the Malvern instruments were used to determine nucleation and growth rates in each urine. Scanning electron microscopy was used to obtain qualitative and semi-quantitative data related to crystal morphology, number, size and degree of aggregation. Both nucleation and growth rates increased as the concentrations of the initiating sodium oxalate solutions increased. Nucleation and growth rates also increased with increasing temperatures. In the latter series of experiments, different calcium oxalate crystal phases precipitated at the various temperatures. Citrate inhibited nucleation and growth of calcium oxalate, irrespective of how it was admitted to the crystalliser. Magnesium was found to inhibit growth only and, like citrate, this role was independent of how it was introduced into the crystalliser. Although both inhibited growth, the effect of the citrate was greater. When both components were added in combination, inhibition was observed to be additive. In all cases, inhibitory effects increased with increasing concentration. Chondroitin sulphate A was found to inhibit nucleation; inhibition of growth was minimal at the concentrations tested. Aggregation of calcium oxalate crystals was found to be inhibited by chondroitin sulphate A. The results of the calcium oxalate crystallisation kinetic studies, obtained by using Malvern particle sizing and Coulter Counter techniques independently of each other, showed general agreement. These results, in turn, were in agreement with the published results of others, thereby lending confidence to the findings and highlighting the potential application of the non-invasive continuous crystalliser in urolithiasis research.

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