Understanding the fine grinding of calcium carbonate

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Rimmer, Sophie (2022). Understanding the fine grinding of calcium carbonate. University of Birmingham. Ph.D.

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Abstract

Stirred media mills are commonly used in industry for the fine grinding of many materials, including calcium carbonate. Since stirred media mills are composed of a vessel equipped with a stirrer, they can be considered analogous to stirred tanks. This thesis looks at key differences between stirred tanks and stirred media mills and considers how established literature theories for stirred tanks can be adapted for successful application to stirred media mills. A focus is placed on assessing different methods of predicting power draw based on the dimensionless Power number and Reynolds number. The most successful method proposed utilises the vane rheometer to measure a combined viscosity of the slurry and grinding media. However, some experimental data does not fit well to the proposed correlation models.

Using Positron Emission Particle Tracking (PEPT), it was found that the cases that do not fit well to the power prediction correlations exhibit a significantly wider vortex than the cases that are a good fit to the correlations. The vortex shape is challenging to predict, but it was observed that cases with both a low fluid viscosity and low grinding media amount showed the most significant vortexing which lead to power draw being over predicted. To develop an enhanced understanding of flow patterns inside stirred media mills and compliment PEPT findings, high-speed imaging was utilised to observe grinding media. Although only the grinding media at the walls of the vessel can be observed, this technique provides the advantage that close-to-instantaneous flow patterns can be obtained.

To consider the most important scale-up parameters and compare them with those important for standard mixing applications, experiments were conducted at three different equipment scales. Since lab-scale milling is frequently operated as a batch whilst larger scale milling is operated continuously, a semi-continuous lab-scale system was developed and compared to the batch system. Lab-scale residence time distributions are presented and suitable methods of determining pilot-scale residence time distributions experimentally are proposed and suggested as further study. At lab-scale, it was found that the solid and liquid phases have similar residence time distributions, close to that of a continuous stirred tank reactor (CSTR).

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):