The formation mechanism of precipitated calcium carbonate from aqueous solution under industrial conditions (high supersaturation) was investigated in a semi-batch and in a continuous mixing nozzle process. The influence of potassium hydroxide and magnesium chloride on crystal modification and particle size were investigated in both processes. Rheological properties of the magnesium stabilized amorphous gel structure have been measured. A similarity between the formation process of calcium carbonate and precipitated silica has been proven.
Precipitated silica (SiO 2 ) is industrially produced by mixing a silicate solution with acid in a semi-batch process. Polycondensation of monomeric silica leads to the formation of particles which aggregate and eventually form a particulate gel. However, this is instantly fragmented by the mechanical energy input caused by the stirrer. Shrinkage and compaction of these fragments lead to the final product aggregates. It is the aim of this study to enable tailoring of the structure and size of these product particles via the process parameters. The present paper describes the influence of the parameters temperature, ionic strength, and composition, which affect the pH, on the properties of unstirred gels regarding their elasticity and thus their fragmentation behavior. Gelation time may be qualitatively estimated solely from the concentration of the feed materials.
Precipitated silica (SiO 2 ) is industrially produced by mixing a silicate solution with acid in a semi-batch process. Polycondensation of monomeric silica leads to the formation of particles which aggregate and eventually form a particulate gel. However, this is instantly fragmented by the mechanical energy input caused by the stirrer. Shrinkage and compaction of these fragments lead to the final product aggregates. It is the aim of this study to enable tailoring of the structure and size of these product particles via the process parameters. In the present paper this is achieved by varying the stirrer speed. It can be shown that there is an analogy between the behavior of the gel and that of highly viscous liquids. Unexpectedly, however, the fragment size can not be reversibly controlled by the power input. The influence of the process parameters on the strength of the gel has been described in the first part of this series [1]. IntroductionAmorphous precipitated silica is a widely used filler and is industrially produced in large quantities. The results presented here contribute to the control of aggregate size and structure of precipitated silica via the process parameters. The findings are presented in a series of two papers, the first of which is dedicated to gelation [1], while the present one investigates the fragmentation of the gel, which constitutes an intermediate stage in the industrial semi-batch process. This series is an expansion of a previous study on mechanical fragmentation of silica gels [2].In a stirred tank, fragmentation of the gel takes place during gelation and essentially influences the properties of the product. Since primary particles in the gel interact strongly with each other but are not rigidly connected, it is assumed that the gel's breakup behavior is similar to that of a highly viscous liquid. The concept of the Weber number [4], in which the surface tension of the liquid counteracts the deformation and, finally, the rupture of a droplet in turbulent flow, was expanded by Arai et al. [3] for highly viscous liquids. The resistance of the liquid against deformation due to surface tension and viscosity was described by a spring and dashpot model. Deformation in a turbulent shear flow was approximated by periodic pressure fluctuations. A relationship between the maximum droplet size x max and the stirring power input e was found for the two extreme cases of very high interfacial tension and very high viscosity. While this again results in the Weber number for viscosity values which are negligible compared to the surface tension, high viscosity combined with negligible surface tension gives the following dependence:(1)This correlation should also be valid for the fragmentation of silica gel. An interfacial tension between the gel and the surrounding liquid is not existent because the gel largely consists of just that liquid, meaning that there is no real interface.It is furthermore probable that the fragment size is defined by an equilibrium between breakage and reaggregation and ...
In the past, the importance of the industrial mass production of high-quality products and specialty chemicals by precipitation increased rapidly. In the industry, usually larger aggregates are produced and in order to produce colloidal systems with the desired properties, a more or less intense dispersion of the precipitated particles is necessary. The micromechanical properties such as the maximum indentation force, the plastic and elastic deformation energy, and the strength can give information on the product and the efficiency of the dispersion process. Generally, the temperature, as one of the most important parameters of the semibatch precipitation process of silica, was varied in order to change the structure, the primary particle size, the aggregate size, and the primary particle interactions in the aggregates. Dispersion of the precipitated silica in a stirred media mill and a dissolver show that the higher the precipitation temperature, the higher is the product fineness and, thus, the smaller is the strength of the aggregates. The reason for this effect is the increase of the primary particle size and the decrease of the solid bonds with increasing precipitation temperature. Because of higher stress intensities, the product fineness in a stirred media mill is considerably higher than in the dissolver. In principal, the maximum achievable product fineness and the energy efficiency of the dispersion process increase with decreasing particle strength and maximum indentation force. Besides the maximum indentation force, the maximum achievable product fineness and the energy efficiency of the dispersion process increase with increasing quotient of plastic and elastic deformation energy.
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