ARR - Annual Report

SUMMARY

This report consists of an account of work done under IFPRI contract in Cambridge University during the year to November 1984. The principal achievements during that time are:

1. The construction and commissioning of an aerated flow rig.
2. The performance of some preliminary experiments in the rig, whose results are evaluated in this report.
3. The completion of the theoretical work undertaken previously.

Also included is a proposal for the continuation of the work.

During the second year systematic measurements were performed on sterically stabilised suspensions of extreme particle sizes and covering a wide concentration range. A scaling law was found to hold up to moderate concentrations. The dependence of one of the scaling parameters on particle size was established. The scaling law was tested for 3 temperatures and two suspending fluids. Deviations from hard sphere behaviour were found, which are This effect was characterized by means of oscillatory measurements. attributed to stabiliser layer deformability. Under some conditions anomalous behaviour was encountered. For weakly flocculated systems time-dependent behaviour (i.e. thixotropy) was investigated. From experiments involving stepwise changes in shear rate some conclusions about the kinetics were obtained. The kinetics of recovery small amplitude oscillatory measurements. in effect of the after cessation of flow were investigated using preceding shear rate was found and information about the nature of the developing structure could be derived.

Summary

An experimental rig to characterise the flow of a powder when aerated has been designed, built and commissioned.

Preliminary results on two powders supplied by IFPRI members have shown interesting results which could be very significant in improving the understanding of the flow of aerated powders. Powders behave as powders at low strain rates, even at high aeration, but become liquid like at aeration rates less than that required for minimum fluidisation when the strain rate is higher. Even a little aeration has a large effect on the magnitude of the transmitted stress.

The objectives of our project are to establish an industrial estimation method for any powder yield locus and to develop a design procedure for industrial powder processes by use of the yield locus. The complete powder properties at any possible porosity under given conditions may be necessary to design particulate processes.

However, it is very difficult or almost impossible in time, technically or economically. We can point out the importance of a new problem "how to estimate powder property from the design standpoint of particulate process". Then, we will discuss a historical review on the birth of this concept of powder property estimation.

Next, the problem of "how the estimation method will be developed to be effective and inevitable in the design procedure, will be explained mainly by use of our method. And we will outline the design method of particulate process on the basis of the estimation method.

"THD makes no warranty or representation with respect to the application of any result or technique described in this report, and TBD shall not be responsible in any way for any claims or liabilities arising out of the application by any party or any result or technique described herein. Any application, authorised or unauthorised by THD or IFPRI, shall be made at the users own risk."

This disclaimer is in accordance with the standard policy of the T.H. Delft.

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.

According to the IFPRI-feasibility study of May 1982 on the evaluation of an electrostatic classifier, it has been shown, that it should be possible to build and operate a classifier in the size range between approximately 0.05 um and 2 um. This statement was valid, however, only if the simplifying assumptions made in the evaluation could be transfered to the actual system.

Approximately 18 months have now been spent on a preliminary system rainly consisting of the feeding and dispersion zones for the extremely fine feed material. and a separation zone following them to determine for instance the ion density available, and the charges transfered to the particles under varying conditions. A china clay with all particles below approximately 0.8 pm and a median particle size of the cumulation distribution by weight of approximately 0.27 pm has been chosen as feed material. The handling. feeding and the dispersion or deayqlomeration of this powder has turned out to be extremely difficult. Even measure- ments of the particle size distribution with a centrifugal sedimentation technique (ALpine SZF 400, Auqsburyl were not as reliable as anticipated. We consequently had to start an investigation of the centrifuge and we are still busy improving it. It might be necessary to additionally analyse the distri- butions obtained by image analysis. The new Quantimet 900, which we received only recently, using REM images could possibly overcome our problems, but will undoubtedly cause new ones. Full financial support to purchase the necessary equipment, high voltage supplies etc. had unfortunately not be granted for the first year. We received, however full funding several months ago and we are now able to finish the final set up and the production of the ccmplete classifier. Work should proceed now at a faster rate.

Summary

Suspensions of submicron particles in a liquid display a broad range of rheological behavior dependent on the size, concentration, and shape of the particles and the nature and magnitude of the interparticle potentials. The most dramatic phenomena occur when one of the interparticle forces dominates, e.g., strong van der Waals forces for flocculated carbon black dispersions, long range electrostatic repulsions for colloidal crystals, and the interactions between adsorbed layers in sterically stabilized suspensions near closest packing. Even with hard sphere interactions though, the rheology is significantly non- Newtonian at moderate concentrations.

The goal of our work is a quantitative theoretical understanding of the rheology of colloidal suspensions of industrial relevance. To this end we focus on well-defined systems of the type studied experimentally by Professor Mewis and seek to predict the observed viscosities and elastic moduli for the highly concentrated stable dispersions and the elasticity and plastic flow of the weakly flocculated carbon black dispersions.

Many powders used in practice as pigments, fillers, etc. contain primary particles that have dimensions less than 1 micrometer and hence high specific surface area. AS particle size decreases the cohesiveness of the powder (the strength of the forces t&t cause agglomeration) increases, and this affects the operation of various processes such as the dispersion of powder in liquids, granulation. fluidization and handling of powders.

Powders normally contain some aggregates of primary particles which are attached to other aggregates and/or primary particles to form agglomerates. Aggregates are groups of primary particles joined such that the total surface area is reduced over that of the individual units. Both manufacturing procedure and storage conditions are responsible for the particular state a powder achieves.

To aid in handling, powders are often further agglomerated which is generally referred to as size enlargement. This process encompasses many different operations and can be subdivided into the following three classes which may overlap:

1. those where particles are brought into close contact, usually in the presence of a liquid, so that short range interparticle forces take effect. Examples are pelletizing of fertilizers and clustering of colloidal particles in a loose fashion so that they may be subsequently readily disperse
2. those that involve mechanical compaction, such as tabletting of pharmaceuticals and briquetting of soft metals
3. those that depend on heating the powder, as in the sintering of ores and ceramics

We are interested in the dispersion of powders containing submicron particles in liquid media. This process involves a number of stages:

1. incorporation of the powder in the liquid
2. wetting the surface of the primary particles
3. breakdown of agglomerates and aggregates into primary particles
4. stabilization of the dispersed primary particles

In practice these stages overlap. The efficiency of the first three collectively is termed the dispersibility of a powder/liquid system. With an increase in powder cohesiveness comes a lowering of the efficiency of this process.

In the dispersion of powders in liquids two types of wetting occur: in the incorporation stage only the external surface of the agglomerates needs to be wetted, but for disagglomeration the liquid must penetrate the internal structure of the agglomerates. Theoretical analysis of these wetting phenomena suggest that the requirements in terms of surface tension and contact angle differ for each stage. This work aims at defining the critical values of these parameters for carbon black in aqueous media.