Control of size and morphology of calcium carbonate crystals made by the reaction of CO2 gas with Calcium Chloride solution.

Calcium carbonate has many uses in industry and consumer products. The highest value material tends to be in the micron size range which is usually produced through grinding of larger material, which is very energy inefficient as well as producing a wide size distribution. Since users want a narrow size range a lot of material is recycled or lost. This project aims at controlling the size and morphology by controlling its growth from the bottom up and, at the same time investigating what sort of shapes can be produced.
The method to be used will be bubbling CO2 gas through a solution of Calcium chloride solution to form the carbonate. An additional benefit is that this is a means of capturing CO2 and turning it from a negative environmental material, due to greenhouse gas, to a valuable material with lots of uses.
There is a lot of publications for this reaction reporting a wide variety of results, often appearing to contradict each other. This project takes the view that a broader, more in depth study will help understand why different results are obtained and how it can be more predictable. So far no one has studied it from an engineering perspective with the view point of turning it into an industrial process and so there are no information available on the impact of bubble size, raw material concentrations nor temperatures and additives. Work has been qualitative rather than quantitative. Thus a great deal needs to be learnt around mechanisms and control variables to turn this from academic papers into a process whereby consistent product quality can be produced.
The project will be aided by the use of well-defined membranes to control bubble size and well controlled flowrates and will build on work done in previous bubble reactors, ending up with a model of the process.
Simple models will elucidate what is happening in the process, through mass balances over a bubble as it rises and reacts to the interactions of many bubbles inside a reactor. From early data, as an MSc project it appears that the hydrodynamics is playing a role in control of the shape and size of the crystal and that amine based additives can have a significant impact on the rate of reaction as well as the crystal morphology.
It is hoped that this mechanistic understanding will provide the basis for further work in producing other speciality crystals whose properties are highly dependent on size and shape, such as zinc carbonate.
The project will investigate many of the characterisation tools available across the university from cryo slicing and Xray diffraction ( to build up an understanding as to how the larger crystals are formed) to testing novel ultrasonic techniques to track reaction rates and assess if onset of crystallisation can be controlled to the benefit of tailoring size and morphology.
As part of the modelling work we are working closely with an expert in modelling bubble reactors. Mariano Martin is an assistant professor at the University of Salamanca, Spain and he has already visited once on an Erasmus scholarship.
Currently the process is a batch one, whereby the Calcium chloride is depleted over time. Once the understanding of the main reactions are understood in a way that can be controlled the next step of the project will be to develop this into a continuous process.