Particle Formation

EXECUTIVE SUMMARY

Agglomeration is an inherent problem in almost all industrially relevent powder processing techniques. Succesful dispersion of powders often requires the total elimination of agglomerates. To achieve this, it is important to understand the nature of, and to ascertain the properties of, these agglomerates. Of particular importance is the strength of the interparticle bonds, (between primary particle units forming the agglomerate), in relation to the powder processing procedure used for their generation. The unit operation which is most closely associated with the formation of hard agglomerates during powder processing is drying. However, the various mechanisms which lead to the formation of hard agglomerates during drying have not been previously systematically studied.

The effect of capillary pressure during drying on the strength of fine powder agglomerates was investigated. Silica and titania slurries were dried at different drying rates, using both spray drying and tray drying, and the strength and strength distribution of the dry agglomerates was quantitatively measured using a calibrated ultrasonic field. The slurry surface tension was varied by using aprotic and protic solvents of different surface tension and by using mixtures of water and n-propanol or n-butanol of varying composition. Particles of four different diameter (20 nm, 28 nm, 60 nm, and 500 nm) were used to vary the effective radius of curvature between the particles.

The average agglomerate strength was found to increase with increasing surface tension and/or decreasing particle radius. Alcohol washed samples had similar agglomerate strength to samples washed with aprotic solvents of similar surface tension. Based on FTIR, TGA, and 1% NMR results, it was concluded that the role of alcohol washing, which is commonly employed in fine ceramic powder preparation to produce softer agglomerates, is that of primarily surface tension reduction and not particle surface esterification as previously presumed.

The effect of particle solubility and dissolution rate on agglomerate was studied by drying silica and titania particles from aqueous slurries with pH ranging from 2 to 12. The agglomerate strength and strength distribution was measured by a calibrated ultrasonic force and the strength was found to increase with increasing solubility and dissolution rate. Two different particle size silica powders (60 nm and 500 nm) were studied and smaller sized particles were found to form stronger agglomerates. The drying rate of the powders was varied by using both spray drying and tray drying and it was shown that slower drying leads to higher agglomerate strength.

The agglomerate strength of titania powder (insoluble in water) was found to be independent of the pH while the agglomerate strength of silica was found to depend on the PH. It was concluded that the solubility and dissolution rate are important parameters governing the strength of agglomerates.

EXECUTIVE SUMMARY

An experimental and theoretical study of the agglomeration phenomenon which causes destabilization of certain low and high temperature fluidized beds was performed. A theoretical model was proposed to determine the conditions under which defluidization occurs in fluidized beds in which cohesion forces between granules arise due to the presence of sticky fluids and/or high temperatures. Bonding mechanisms between particles such as solid-liquid bridges, viscoelastic flattening and high temperature sintering were all considered. The model, which predicts breakup of aggregates by bubble motion, was compared to limiting fluidization-defluidization (quenching) experiments performed by the authors and others. An experimental method to measure surface softening of small particles heated to high temperatures was developed by using a dilatometer to measure the surface viscosity of the particles from rate of deformation data. Experimental methods to determine the minimum sintering temperatures of a variety of granules were also presented. Lastly, experiments were performed to study the dynamic strength of a liquid bridge between two spheres coated with a liquid and moving away from one another, It was shown that the strength of the dynamic bridge was at least one order of magnitude larger than the corresponding strength of the static bridge between the two spheres. This result accounts for the relatively high gas velocities necessary to keep a bed of sticky particles in continuous fluidization.

An art review on the measurement of various agglomerate strengths in Japan, as the request of IFPRI, has been already reported on 20th April, 1982. The present paper deals with the experimental results of some agglomerate strengths for polymeric materials such as polypropylene of polyethylene particles. These agglomerates of polymeric particles were produced by hot fluidized beds at The City College of The City University of New York.

As indicated in the preliminary IFPRI report (Gabriel Tardos, Dominick Mazzone and Robert Pfeffer: Agglomeration of Particle Systems in Fluidized Beds - Phase 31, Tardos, G. et al. made a detailed investigation into the minimum sintering fluidization velocity necessary to keep a bed of sticky or agglomerated particles in the fluidized state. In this connection, three samples of agglomerated polymeric particles were sent from New York to Tokyo in order to obtain agglomerate strength data at several temperatures between 80 and 170°C.

Unfortunately, the usual strength testing procedure regarding a cylindrical agglomerate with the diameter/length ratio about half does not apply to three samples of agglomerated polymeric particles because of irregular shape. An advantage of agglomerate strength measurements in this work, for this reason, is that the presumed tensile strengths of the agglomerates can be calculated from experimental results of interparticle forces at temperatures ranging from 80 to 170°C. In addition to this measurement, a new method for measuring the breakage strengths of three samples in streams of hot air was introduced into this work.

This is the third phase of the research performed on agglomeration of particles in fluidized bed systems including both theoretical and experimental work on aggregating fluidized beds. The report contains an updated review on high temperature agglomerating fluidized beds with emphasis on fluidized bed conbustors. The report also includes a detailed comparison between predicted and measured minimum gas velocities necessary to keep a bed of sticky (wet) or sintered granules in the fluidized state.

The experimental procedure using a dilatometer to measure the apparent surface viscosities of sintering particles is described together with measurements to determine agglomerate strength. These quantities must be known before any theoretical model which predicts defluidization in fluidized beds can be applied. Furthermore, a theoretical model based on the growth of vertical channels in a defluidized bed to predict refluidization of a defluidized (packed bed) is described. Future theoretical and experimental work on the project is also outlined.

Summary

This is the fourth phase of a research program to study the agglomeration of particles in fluidized bed systems. The report contains an updated review of the literature on high temperature agglomerating fluidized beds as well as equilibrium shapes of liquid bridges between particles. It also includes a comparison between predicted and measured minimum gas velocities necessary to keep a bed of sintered granules in the fluidized state.

The use of a dilatometer to measure the minimum sintering temperature of potentially agglomerating particles is described together with measurements to determine agglomerate strength. Different materials such as glass, polymers, coal and inorganic salts were used during the experiments.

The correlation between the sintering temperature and other thermodynamic properties of these materials, as observed from a differential scanning calorimeter test, was determined experimentally. A new original theory concerning the strengthening of a liquid bridge between two particles due to relative motion (viscous dissipation) is proposed. Future theoretical and experimental work on the project including a means to verify the above theory are also outlined.

Summary

This is the fifth phase of a research program to study the agglomeration of particles in fluidized bed systems. The report contains an updated review of the literature on theoretical models of fluidized beds containing fine cohesive and/or moist particles as well as a review of research on forces and pressures on objects immersed in fluidized beds.

Liquid bridges between stationary and moving spheres were studied experimentally and theoretically. A new, original model was developed to study the shape and strength of a stationary bridge between two spheres which accounts for the weight of the liquid between the particles and this model was verified by liquid bridge strength measurements using strain gauges. Furthermore, moving liquid bridges between a stationary and a free falling sphere were studied using high speed photography.

The forces and pressures on free and attached agglomerates in a fluidized bed were measured using a direct experimental method employing a set of strain gauges mounted on the agglomerate. The measured values were analyzed as to peak distribution, power spectra and other characteristics, using advanced computer data processing.

Work performed on agglomeration of particle systems in fluidized beds at both low temperatures (granulation) and high temperatures (sintaring) is described in Part I and Part II of this report, respectively.

Part I

Part I includes a detailed review of the available literature on low temperature granulation as well as a description of original work performed on the behaviour of liquid bridges between two relatively moving particles. Experimental results are given for bridge strength measurements using two small spheres in a vibrational motion. These results are then compared to a modified form of a theoretical model developed from the well-known lubrication approximation. From both, the reviewed literature as well as from the experimental and theoretical work performed on moving liquid bridges, it is clearly concluded that the fluid (binder) viscosity and its rate of change with time are two of the most important binder characteristics which ultimately determine agglomerate growth in a granulator. It is also concluded that the instrument at CCNY used to measure bridge strength as a function of particle velocity (frequency, amplitude) and binder viscosity may be an efficient tool to characterize industrial binders. Based on these findings, future work is proposed to actually correlate bridge strength measurements with agglomerate growth rates in an experimental granulator.

Part II

Part II of the report includes the description of a large number of dilatometer and defluidization experiments performed at high temperatures with a great variety of amorphous and crystalline materials. Sometimes these experiments were complemented by differential scanning calorimeter (DSC) experiments to determine characteristic temperatures of recrystallization and/or phase change. The defluidization experiments were performed on the newly constructed fluidized bed agglomerator capable of operating at temperatures up to about 1150°C. Among the materials characterized during these experiments were different polymers and glass powders, sodium chloride, sodium bromide, sodium citrate and ferrous chloride crystals and a large number of more complex materials such as a titanium dioxide ore, FCC catalyst and fly ash samples. It was clearly established that the minimum sintering temperature (and other phase transition temperatures) can be determined using dilatometry. It was also shown that fluid bed defluidization (high temperature agglomeration) always occurs at temperatures somewhat higher than the minimum sintering measured in the dilatometer. Furthermore, the behaviour of the powder during high temperature fluidization can be reliably determined from the dilatometer experiments. Future work on this project will include upgrading both the dilatometer and the fluid bed agglomerator to withstand temperatures as high as 1500°C. The new dilatometer will also enable tests to be performed under a controlled atmosphere thereby allowing study of agglomeration due to chemical reactions.

Executive Summary

A literature review of precipitation from the liquid state has been undertaken. Two areas of concern to precipitation technologies were highlighted i) factors controlling precipitate size, morphology and agglomeration, and ii) parameters influencing the crystalline phase precipitated.

WHAT IS KNOWN

A wide variety of materials have been precipitated as particles of uniform size in the micron and submicron size range. No limitations on preparing narrow size distribution precipitates appears to be imposed by the chemical nature of the material of interest. The importance of colloidal interactions in the control of size distribution cannot be over emphasized.

Equilibrium thermodynamic factors as well as solution and surface reaction rates are important in determining the phase and morphology of precipitates. An understanding of these factors at small length and short time scales is required for morphological control. Impurities and additives strongly affect particle growth rates and the phase precipitated. Today, shape and phase control are more art than science.

Many of the technologies currently available for the preparation of uniform precipitates have been developed at the bench scale and have yields in the milligram to gram per liter range. With the exception of silver halides and emulsion polymers, no work has been reported in the literature describing scale-up to industrial quantities. Poor understanding of surface reactivity of freshly prepared particles and their colloidal nature currently limits development of separation schemes when unagglomerated particles are required. Due to the lack of published information on large scale processes, no attempt was made to quantify limitations on post precipitaton processing steps.

Agglomeration of nanometer sixed particle nuclei appears to be a critical step in the formation of many uniform precipitates. Heterogeneous size distributions arise when aggregation/agglomeration is not halted at the required size.

Little information was found on the cost of generating uniform particles in the sub-ten micron size range. Much of this information resides in industry and not academia.

WHAT IS UNKNOWN

No general rules for controlling the precipitate phase or morphology have been developed despite the industrial need.

Growth mechanisms of precipitates are poorly understood for most materials. The chemistry occurring in supersaturated solutions far from equilibrium impacts strongly on precipitate phase and morphology but is not well characterized. This poor understanding limits attempts to scale-up current precipitation recipes.

Mechanisms governing nucleation and growth of uniform particles have seen surprisingly little study. In the early stages of precipitation, growth mechanisms are difficult to assign. Often, even the size of the growth unit is poorly characterized.

Scale-up of precipitation chemistries resulting in uniform particles has not been extensively studied. As a result little is known about the magnitude and size of the scale-up difficulties.

WHAT IS PROPOSED

Critical review of the current state of the precipitation of uniform particles has led to the formulation of two key areas of opportunity where fundamental research could lead to enhanced understanding and thus to better precipitation technologies. These opportunities are discussed on pages 26-29 of the review and are briefly described below.

Particles Fundamental Studies of Nucleation and Growth During Precipitation of Uniform Particles

While the chemistry of precipitation reactions is very complex, broad themes important to preparing uniform precipitates may be more general. This program would search for these themes. Three areas have been identified as being of critical importance. First, prenucleation chemistry and what species form nuclei would be explored. Secondly, the colloidal interaction of nuclei would be studied in order to establish factors which act to give colloidal stability to growing precipitate particles. Thirdly, growth mechanisms of the particles would be determined to establish the importance of aggregation on final particle morphology. The goal of this work would be to develop an understanding of the physical chemistry leading to uniform particles and to determine the generality of the mechanisms such that they can be applied to new systems.

Fundamental Studies in the Scale-up of Precipitation of Narrow Size Distribution Particles

Few attempts have been published detailing the scale-up of precipitation technologies leading to uniform particles. As a result, little is known about the difficulties in increasing the size or the yield of current precipitaton technique. The thrust of these studies would be: i) to explore various scale-up scherncs, ii) develop quantitative models from a basic understanding of the reaction pathways, and iii) provide a means of assessing the difficulties in scale-up operations. The general goal of the research would be to determine the difficulties in scaling up precipitation reactions that result in uniformily sized particles.

This review was undertaken to evaluate existing information about the fundamentals and engineering aspects related to particle formation from the gas phase.

What is Known:

• Particle production mechanisms in the gas phase are fairly well understood for dilute systems, but not for concentrated systems.
• The three major mechanisms are nucleation, condensation and coagulation.
• Coagulation is the most detrimental mechanism as it can lead to increased polydispersity and undesired aggregation. Control of coagulation is essential for powder manufacture.
• In dilute systems, the average primary particle size can be controlled by the correct choice of available vapor and the number of nuclei produced.
• The width of the particle size distribution is determined by the residence time and temperature distribution of both the reactants and the condensing vapor.
• Coagulation rate is a function of the particle concentration, relative particle motion and turbulence. Coagulation leads to broader size distributions.
• Particle shape depends on the temperature and the environment.
• Primary particle shape can be spherical or facetted depending on the precursors. Continued gas reactions can produce porous particles, the porosity being eliminated by high temperature.
• Aggregates and agglomerates have differing morphologies depending on such factors as field gradients and particle dielectric constants.
• Gas produced particles are very pure, and purity can be ensured by the use of inert gas sheaths to prevent wall contact.
• Powder yields can be high - depending on the reactor type - because particle production can be continuous.
• The product quality and economics are often a trade-off between reactor throughput and the acceptable degree of agglomeration.

What is Unknown

• The relative importance of reactor design, growth process competition and coagulation suppression are not well understood for concentrated systems.
• The importance of additives, applied fields and reactor design criteria have not been widely studied.
• Dilution systems are not widely applied because of the additional gas volume involved in the final gas solid separation process, but strategic dilution, i.e. dilution at key reactor locations or at key residence times has not been explored and might provide a solution with minimal extra gas volume.
• Sensors for monitoring real time size and concentration information are not available for high solids gas loadings and hostile environment.
• Methods of producing pure non-aggregated powders of size distribution greater than 0.5 micrometer on high concentration gas systems are not available.

What is Needed

A fundamental research program which will:

• study the relative importance and interplay of the three major mechanisms using a simple design of laboratory reactor.
• study the effect of reactor geometry and design on particle size distribution, aggregation, purity and yield.
• develop a suitable reactor to study the role of additives and strategic dilution. With the overall objective of extending the useful size range of gas/solid formation processed to coarser sizes while maintaining high yield, purity, morphology and no aggregation.