Experimental and theoretical studies on the nucleation, growth and habit modification of some inorganic carbonates, phosphates and sulphates
A detailed study has been conducted on the crystallisation kinetics and growth of some inorganic sulphates and carbonates, in view of their growing importance as enviromentally friendly high surface area powders in detergent applications. Predictions of the crystal morphology based on lattice geometry laws have been obtained and compared to the growth morphology. A mechanism for habit modification has been postulated and compared to experimental studies of the effects of impurities on the crystallisation of sodium carbonate monohydrate and sodium carbonate decahydrate. Intermolecular force potentials have been empirically derived for β potassium sulphate which accurately predict its solid state properties. The intermolecular force potentials are successfully transferred and applied to a simulation of the low temperature phase of sodium sulphate. The phase diagram of the Na₂CO₃-Na₂SO₄-H₂0 system has been redetermined using powder diffraction, chemical analysis and Scanning Electron Microscopy as analysis tools. Evidence for burkeite supercell formation has been noted at a variety of solution compositions and crystallisation temperatures. The degree of agglomeration present in the system has been determined to be a function of the carbonate content of the crystallising solution. Burkeite is noted to act as an agglomeration nucleator, extensively binding to the other phases present. A reduction in precipitation temperature leads to a corresponding increase in the particle size distribution. The application of nucleation theory to inorganic systems is examined through the use of an automated crystallisation cell employing solution turbidometry, conductivity and temperature, to probe nuclei formation in the cases of potash alum (AIK(SO₄)₂.12H₂0), ammonium dihydrogen orthophosphate (NH₄H₂PO₄), sodium carbonate decahydrate (Na₂CO₃.10H₂0) and sodium sulphate decahydrate (Na₂SO₄.10H₂0). Turbidometry is determined to be the most reliable means of detecting the onset of nucleation. Reasonable correlation is noted for the systems studied, between measured saturation curves as determined by this instrument and previous observations. Interfacial energies based on induction time measurements are determined to be significantly smaller than those determined in previous kinetic and dissolution enthalpy studies. The preparation of highly porous, high surface area powders for carrying liquids in detergent powders has been examined. It was determined that a significant proportion of the desirable powder porosity for a burkeite powder occurred during the initial batch crystallisation stage. No evidence for a burkeite powder to tend towards a monosize particle distribution was noted. Milling and spray drying of slurry lead to a significant increase in powder porosity. Increasing the drying time during spray drying led to a corresponding increase in the powder porosity. It was observed that further studies of the spray drying conditions employed for burkeite production are required, for optimisation of the liquid uptake of the resultant powder.