Agglomeration of precipitates

The term polyethylene separator is somewhat misleading, since this separator consists mainly of agglomerates of precipitated silica, held within a network of extremely long-chained, ultrahigh-molecular weight polyethylene molecules. The raw materials, precipitated silica (Si02 — about 60 percent), ultrahigh-... 

Tubular Precipitator. This type of continuous operation may be employed to reduce polydispersity of precipitates (Raphael et al. 1997 Raphael and Rohani 1999). The tubular precipitator may operate either under the turbulent flow or laminar flow regime. The reactants are added into the inlet section equipped with static mixers and may also enter as a multi-port feed along the length of the tubular precipitator. If the reactant feeding streams are too concentrated or if too excessive formation of precipitate occurs in the inlet section of the precipitator, a third stream of solvent is also fed to dilute the flowing suspension. The latter may contain a protective colloid or surfactant that prevent agglomeration of precipitate. 

Kerosene has been used as a binding agent for the production of spherical agglomerates of precipitated inorganic substances, e.g. calcium carbonate (Bos and Zuiderweg, 1985 Kawashima et al., 1986). 

The battery separators currently used by most flooded-cell-type lead acid battery manufacturers are of the microporous PE type. It was invented in the late 1 %0s by W. R. Grace Co. [193]. The term polyethylene separators is somewhat misleading, since such a separator consists mainly of agglomerates of precipitated sUica, being held within a network of extremely long-chain UHMWPE [194]. A typical PE... 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

The toxic nature of mercury and its compounds has caused concern over environmental pollution, and governmental agencies have imposed severe restrictions on release of mercury compounds to waterways and the air (see Mercury). Methods of precipitation and agglomeration of mercurial wastes from process water have been developed. These methods generally depend on the formation of relatively insoluble compounds such as mercury sulfides, oxides, and thiocarbamates. MetaUic mercury is invariably formed as a by-product. The use of coprecipitants, which adsorb mercury on their surfaces facihtating removal, is frequent. 

Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstmetures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, foUowed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

Figure 8.25 Monte Carlo simulation of distribution of primary particle residence times (oo size) within MSMPR precipitated agglomerates of 5 and 20 crystals (Hostomsky and Jones, 1993a)...

Budz, J., Jones, A.G. and Mullin, J.W., 1987b. Agglomeration of potassium sulphate in an MSMPR crystallizer. In Fundamental aspects of crystallization and precipitation processes, American Institute of Chemical Engineers. Symposium Series, No. 253, 83, New York American Institute of Chemical Engineers, pp. 78-84. 

Hostomsky, J. and Jones, A.G., 1993a. Modelling and analysis of agglomeration during precipitation from solution. In Industrial Crystallization 93. Ed. Z. Rojkowski, University of Warsaw, 1993, pp. 2037-2041. 

Wachi, S. and Jones, A.G., 1992. Dynamic modelling of particle size distribution and degree of agglomeration during precipitation. Chemical Engineering Science, 47, 3145-3148. 

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

Carboxypolymethylene exhibits a stabilizing effect against separation and viscosity change of a cetyl alcohol-stearic acid-sodium lauryl sulfate system. Samples 1 and 2 are identical, except that the former contains carboxypolymethylene (adjusted to pH 7). Sample 1 shows no separation, while sample 2 shows 13% separation after 6 months. Figure 8 shows the viscosity characteristics of the above samples. The broken line represents the viscosities of sample 1 at different time intervals, and the unbroken line, the viscosities at the same time intervals for sample 2. Apparently the carboxypolymethylene prevents the agglomeration and precipitation of stearic acid, which would result in separation and loss of viscosity. 

In the second kind of explanation, it was postulated that particles of certain sizes are particularly abundant in the size distribution The occurrence of magic agglomeration numbers was explainai by the following mechanism of precipitation ... 

The polymer/additive system in combination with the proposed extraction technique determines the preferred solvent. In ASE the solvent must swell but not dissolve the polymer, whereas MAE requires a high dielectric solvent or solvent component. This makes solvent selection for MAE more problematical than for ASE . Therefore, MAE may be the preferred method for a plant laboratory analysing large numbers of similar samples (e.g. nonpolar or polar additives in polyolefins [210]). At variance to ASE , in MAE dissolution of the polymer will not block any transfer lines. Complete dissolution of the sample leads to rapid extractions, the polymer precipitating when the solvent cools. However, partial dissolution and softening of the polymer will result in agglomeration of particles and a reduction in extraction rate. 

After the addition of precipitating chemicals, the precipitation reaction commences to form very small particles called precipitation nuclei. The flocculating agents allow these particles to agglomerate. 

---