R. Greenwood

Selection of Suitable Dispersants for Aqueous Suspensions of Zirconia and Titania Powders using Acoustophoresis

Acoustophoresis was used to study the effect of adding various commercially available dispersants onto aqueous suspensions of two zirconia and two titania powders. These powders were characterised for elemental composition by X-ray fluorescence (XRF) spectroscopy and for surface area by BET single point nitrogen adsorption. From the maximum value of the zeta potential, it was possible to select the most promising dispersants. From the shape of the curve the minimum amount of dispersant required to stabilise the powder particles was noted. The iso electric point (i.e.p) of the powders was also identified. Several dispersants can be recommended for the first titania powder, whilst none can be recommended for the second titania as the final zeta potentials on addition of the dispersants were low. The two powders had different surface chemistries which was reflected in a large difference in their i.e.p; the first at pH 7·5 and the second at pH 6·1. This was due to different coatings on the powder surfaces; alumina and an organic material respectively. Removal of this organic coating by calcinatian then enabled the dispersants to fully adsorb. Similarly dispersants for the first zirconia powder could be identified and the i.e.p identified at pH 5·4. However, no dispersants can be recommended for the second zirconia powder as yttria dissolves out of the powder under the naturally occurring weakly acidic conditions. The i.e.p was estimated to be pH i.e.p 7–7·5.

Review of the measurement of zeta potentials in concentrated aqueous suspensions using electroacoustics

This paper reviews the technique of electroacoustics as it has been applied to aqueous suspensions of inorganic particles. It starts by charting the development of the technique from its earliest beginnings in the 1930s to the present day. The technique has become well established in the last decade with the advent of the Acoustosizer and the Acoustosizer II. Illustrations of how the technique can be used are given based on the authors own experience, especially the measurement of iso electric points, the adsorption of polyelectrolytes, the effect of ionic strength, the effect of powder surface area and the dissolution of material from powders. Some new data on (a) the adsorption of different molecular weights of polyacrylic acid onto alumina and (b) titanium dioxide suspensions are also presented.

Electroacoustic and rheological properties of aqueous Ce-ZrO2(Ce-TZP) suspensions

Abstract Ce-ZrO2 (13 mol% CeO2) powder was found to be slightly soluble under acidic conditions and the isoelectric point was determined to be pHiep = 7.9 using electroacoustics in concentrated suspensions. The adsorption isotherms of two well-characterized polyelectrolyte dispersants, Dispex A40 and Duramax D3007, were related to the molecular structure. The fully ionized polyacrylic acid (Dispex A40) adsorbed in a flat conformation on the Ce-ZrO2 powder, while the Duramax D3007 displayed higher adsorbed amounts which was attributed to possible multilayer adsorption caused by the zwitterionic nature and the existence of hydrophobic parts of the Duramax D3007. It was found that the viscosity is at a minimum at monolayer coverage of the dispersant when the polyelectrolyte-covered particles are highly negatively charged. The study illustrates how rheology and electroacoustics can be utilized in the investigation and optimization of the properties of concentrated ceramic suspensions.

Preparation of concentrated aqueous alumina suspensions for tape casting

Abstract Aqueous suspensions suitable for the tape casting of alumina were produced, such that ceramic bodies with very high final sintered densities were obtained (100% of theoretical). This enables high purity, smooth, substrates to be produced without using environmentally damaging solvents. To prevent competitive adsorption of the binder with the dispersant, which leads to an increase in the viscosity and in the elastic modulus of the suspension, the binder was mixed very slowly for a short time in a ball mill. In the presence of the binder the optimum amount of dispersant was 0.3 mg m −2 (dry weight basis). These suspensions were not thixotropic. The minimum amount of binder required to produce a flexible, robust tape was established from tape casting experiments, as was the minimum amount of binder required to produce a tape that did not crack on drying. All suspensions were cast with both blades set at a gap of 0.4 mm and a Mylar tape speed of 20 cm min −1 ; the removal of air bubbles was achieved by exposing the slip to a vacuum for a few minutes. The final sintered density could be correlated directly with the volume fraction of powder (at a fixed binder content) and directly with the binder content (at fixed volume fraction). Sintering the discs revealed a greater shrinkage in the thickness direction than laterally owing to the non-spherical nature of the particles. However no correlation could be found between the thickness of the green body after casting/drying and the suspension viscosity.

Minimising the viscosity of concentrated dispersions by using bimodal particle size distributions

Abstract The effect of composition (i.e. the volume of small particles compared to the volume of large particles) on the rheological properties of a bimodal dispersion has been studied in detail using two monodisperse polystyrene lattices. The diameter ratio of these lattices was 4.76. The relative high shear rate viscosity and dynamic viscosity of these bimodal dispersions were measured using the Bohlin VOR. Firstly, the rheology of the two monomodal dispersions was measured as a function of volume fraction, then 10 compositions ranging from 10% to 35% small particles by volume were prepared and again the rheology followed as a function of volume fraction. A minimum in the relative high shear rate limiting viscosity was found in the range 15–20% by volume of small particles and another minimum was also found at 20% by volume of small particles for the dynamic viscosity measurements. Thus, it can be concluded that reductions in viscosity can be achieved with a bimodal dispersion with the small particles occupying 20% of the total volume fraction. This is in agreement with other studies. However, if the adsorbed layer thickness is taken into account then the effective composition works out to be 27–36%; the lower limit of which is in excellent agreement with theory. The relative high shear rate viscosities of the bimodal dispersions were then compared to the theoretical viscosities calculated from an effective medium model. The results are in reasonable agreement with the measured data, but a rigorous analysis could not be performed due to the inability to calculate accurately the effective volume fraction, due to the presence of an adsorbed polymer layer on the particles.

Particle separation from gases using cross-flow filtration

Abstract Most filters used in solid/gas separation are applied in the “dead-end” mode, where the aerosol particles arrive at the filter on trajectories that are approximately perpendicular to it. This study concerns the alternative method of cross-flow filtration, which is in common use for filtering liquids. In this mode, previously studied by Menard et al. [Powder Technol. 71 (1992) 263], Thomas et al. [Powder Technol. 76 (1993) 79] and Ferrer et al. [Powder Technol. 113 (200) 197], some of the flow passes through the filter, depositing particles on the surface, while some sweeps past the surface, causing shear on the deposited cake. Depending on the operating conditions, particle aggregation can occur on the surface and the resulting aggregates can be removed by the through-flow and collected in a downstream device. Laboratory scale equipment has been set up in which a cross-flow filter module is coupled to a cyclone postseparator to separate the aggregates leaving the filter. The filters used were ceramic tubes of 6 cm outside diameter, 4 cm inside diameter and variable lengths. An aerosol of limestone dust particles (mmd=5.0 μm) in ambient air enters the inside of the filter along the axis; due to the aerodynamic conditions within the filter some of the particles are deflected towards the filter and are captured. The results show that the cake is detached in the form of loose agglomerates rather than individual particles, which are easily collected by means of the cyclone. The collection efficiency shown by the cross-flow filter/cyclone combination is over 99% for 5.0 μm (mmd) particles under optimum conditions compared to ≈90% shown by the stand-alone cyclone. This separation efficiency is comparable to that of a surface filter and the pressure loss savings with this system make it an attractive gas cleaning option.

A new method for determining the optimum dispersant concentration in aqueous grinding

The use of electroacoustics as a method for determining the optimum dispersant dosage for the ultrafine grinding of limestone is presented. The technique measures the zeta potential of concentrated suspensions, hence it was possible to study the adsorption of three commercially available polyelectrolyte dispersants onto the limestone. The optimum dosage of the dispersants was determined and the most suitable one chosen for the grinding experiments. By utilising this optimum dispersant dosage, the productivity and throughput of a stirred vertical mill was greatly enhanced.

A method for making alumina fibres by co-extrusion of an alumina and starch paste

A method of producing alumina fibres has been developed. The process used a water-based alumina and rice starch paste. An initial feed-rod was made from a central cylindrical rod of alumina paste surrounded by a square shaped section of starch paste. This feed rod was subsequently extruded so that it is reduced in size by a determined ratio. The extrudate was cut into short lengths and re-assembled to the exact dimensions of the initial feed rod so that it contained many smaller alumina rods. This latter specimen was re-extruded and the steps repeated as many times as required to obtain a large amount of small diameter fibres. By selecting the die diameter and the number of extrusions the diameter of the fibres was easily controlled. Since all the fibres were in one easily handleable bunch, the bottleneck associated with sintering could be avoided. Hence an economic method of fibre production can be envisaged.

Effect of ionic strength on the adsorption of cationic polyelectrolytes onto alumina studied using electroacoustic measurements

The adsorption of numerous cationic polyelectrolytes onto alumina in aqueous suspensions was investigated using electroacoustic measurements. Initially, the isoelectric point (IEP) of the Sumitomo AA05 alumina was identified at several different electrolyte (KCl) strengths. The IEP was found to depend on the presence of the electrolyte. With no electrolyte present, the IEP was 9.4; but with electrolyte present, there was a slight shift to higher pH, possibly due to adsorption of potassium ions on the alumina surface. The natural pH of the aqueous suspensions was 4.9; at this pH value, the particles were highly positively charged and none of the polyelectrolytes adsorbed. The pH of the suspensions was then increased to 11.4; at this value, the particles were highly negatively charged and all the polyelectrolytes adsorbed, causing charge reversal. The amounts of polyelectrolyte then required to obtain both zero zeta potential and the maximum plateau zeta potential were found to depend on the electrolyte strength of the suspension. This was attributed to the different configurations taken up by the polyelectrolyte on adsorption.

The zeta potential of kaolin suspensions measured by electrophoresis and electroacoustics

The zeta potentials of kaolin dilute and concentrated suspensions were monitored using the techniques of electrophoresis and electroacoustics, respectively. The effect of addition of salt (KCl), a polymer material (Triton X-100), and an anionic surfactant (sodium dodecyl sulphate, SDS) on the suspension properties was investigated by electrophoresis. Electroacoustics was employed for the measurement of zeta potentials for the highest possible kaolin content in suspension and the effect of dilution. The effect of aging of a freshly prepared sample and kaolin isoelectric point was also studied. Using both techniques it was noted that there was no isoelectric point, just a maximum value in the magnitude of the kaolin suspension zeta potential. These maxima were observed also in the presence of Triton X-100 and SDS. An increase of the concentration of KCl and SDS in suspension shifted the maxima towards more acidic values, while in the presence of Triton X-100 the position of the zeta potential maxima remained constant. Electroacoustic techniques revealed that a freshly prepared concentrated suspension requires about six hours to equilibrate to achieve a steady zeta potential. Diluting the concentrated suspensions led to decrease of the zeta potential as ions bound to the surface desorbed and screened the surface charge. The zeta potential maxima remained unchanged even after heating the powder in an oven at 200°C (to remove any organic material) thereby suggesting that the most likely explanation for the maxima is isomorphic substitution.