Electricity: measuring and testing – Determining nonelectric properties by measuring electric... – Particle counting
Patent
1994-09-16
1996-11-19
Wieder, Kenneth A.
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
Particle counting
324439, 137 5, 73 6141, 73 6108, G01N 1502, G01N 2700
Patent
active
055766170
DESCRIPTION:
BRIEF SUMMARY
The invention relates to apparatus and a method for measuring the average aspect ratio of non-spherical, e.g. laminar or ellipsoidal, particles in a suspension. In particular, although not exclusively, the invention relates to the measurement of the average aspect ratio of kaolin particles in suspension in a fluid.
In many applications of particulate solid materials the aspect ratio of the particles of the material is a parameter which profoundly affects the performance of the material. For example, if the particulate material is used in a composition for coating paper, the surface finish of the paper is determined to a large degree by the average aspect ratio of the particles. If it is required to produce a coated paper which has a smooth, glossy finish the particulate material will need to have a different average aspect ratio from that required if the coated paper is to have a matt surface with greater ink absorbency.
FIG. 1 illustrates the meaning of the expression "aspect ratio" used in this application. The expression "aspect ratio" means "the diameter of the circle of area equivalent to that of a face of the particle divided by the mean thickness of that particle". In FIG. 1 a kaolin particle P is shown with a superimposed circle having an area equivalent to that of the face of the particle. The diameter of that circle is d, the thickness of the particle is t and the aspect ratio of the particle is d/t.
It has previously been found that the average aspect ratio of particles in a suspension may be calculated from the measurement of the conductivity of the suspension. In the present applicant's previous British Patent Application No. 9101291.4 (Publication No. 2240398) a method and an apparatus are described for obtaining a measurement indicative of the aspect ratio of non-spherical particles in suspension. The conductivity of the suspension is measured between points for two different orientations of the particles in suspension and the difference between the two measured conductivities is used as an indication of the particle aspect ratio. The particle orientation may be aligned for the first conductivity measurement and may be aligned transverse to the first orientation direction, or have random alignment, for the second conductivity measurement. The specification also discloses a further alternative in which there is a single aligned orientation but transversely directed conductivity measurements.
However, problems have been discovered with this prior art apparatus and method, in particular with the obtaining of consistent correlation of aspect ratio measurement with change of conductivity. It has been determined that variations in temperature of the particle suspension can result in significantly erroneous readings for the comparative conductivity measurements. In particular this may occur in the case where measurement is effected after a flow of suspension is stopped to allow the asymmetric particles to revert to a random orientation through the action of Brownian forces. This may take several minutes for large colloidal particles and during this period the static suspension may undergo a small temperature change as it comes into equilibrium with its surroundings. However, the electrical conductivity of solutions may vary significantly with temperature. This variation may typically be as much as 2% per .degree.C. and, in the case of kaolinitc suspensions, a temperature coefficient of 0.77% per .degree.C. has been measured. As a result a relatively small change in temperature occurring over several minutes can lead to errors in the value for the conductivity and thus lead to errors in the shape factor computed from it.
To overcome this problem, control of the ambient temperature surrounding the conductivity cell or direct control of the suspension temperature has been considered. However to achieve this results in an increase in the complexity of the apparatus so increasing its size and cost. In addition, such an approach does not lend itself to continuous control in a production environment, e.g.
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Gate Leonard F.
Webb Terence W.
Do Diep
ECC International Limited
Wieder Kenneth A.
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