Grinding or What? A Proposal for Alternative Pigment Manufacture
The author proposes to replace grinding by a crystallization process that tightly controls the morphology of the pigment crystals and their size distribution. Control of size and morphology will yield ultimate control of the color and hue of the pigments and will allow adjusting these parameters at will. Crystallization might preferentially be controlled in the chemical process of dye formation. Alternatively, controlled precipitations may be performed after the dye has been purified, or during a recrystallization procedure. A difficulty for this procedure is the control of the nucleation and growth processes during precipitation of the materials.
Precipitation of insoluble materials is defined by a relatively short time in which the crystals are formed (nucleation) followed by growth. Of these, nucleation is more difficult to control and is more important for the control the final crystal size and product yield.
Significant experimental and conceptual contributions have been made to model the control of the nucleation process, both for batch and for continuous precipitations. These concepts and models are generally applicable for controlled precipitation of organic and inorganic materials. Of particular interest for the production pigments and dyes is the application of these concepts to the controlled manufacture of these materials. The controlled precipitation process can provide greater control of crystal size and morphology, and savings in the manufacturing process.
An overview of modern techniques and models for the control of crystallization will be presented. This review concentrates on the progress in modeling the nucleation process of particles by a balanced nucleation-growth (BNG) process. The BNG model will be compared with other models that try to predict material nucleation. Compared to other models, the BNG model allows one to quantify the nucleation rate, maximum growth rate, and supersaturation during the nucleation period as a function of nucleation efficiency and maximum growth rate of the crystals. From this model, equations are derived that correlate the number of stable crystals formed with molar addition rate of reactants, solubility of the crystals, and temperature. The BNG model predicts the experimental result that many crystallization processes result in a limited number of crystals followed by growth. The model also predicts that factors like diffusion and kinetically controlled growth processes, Ostwald ripening agents, and growth restrainers control the crystal number. Equations are given for each of the variables that agree with experiment. The BNG model predicts the conditions for renucleation (formation of new crystals during precipitation). It leads to new equations for the prediction of crystal number and crystal size during controlled continuous precipitation in the continuous stirred tank reactor (CSTR) as a function of precipitation conditions.
Document Type: Research Article
Publication date: 2000-01-01
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