Physico-Chemical Studies of Silica of Small Particle Size

Massie, William H (1951) Physico-Chemical Studies of Silica of Small Particle Size. PhD thesis, University of Glasgow.

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Aim of Research: to develop and evaluate an apparatus for the investigation of the air-settling characteristics of silica dusts of size dangerous to health, to examine its use for the investigation of certain factors connected with dust suppression and to examine other appropriate applications of the apparatus. 1. A brief account is given of the supersedure of the 'Mechanical Theory' by the 'Solubility Theory' of silicosis, and mention is made of some practical problems associated with the measurement of dust suppression and health hazard. 2. The preparation of graded silica particles and fine mineral dusts, including sizes dangerous to health, by sedimentation from water or ethanol is described. 3. An apparatus for the examination of the air-settling characteristics of fine silica, consisting of a dust chamber into which the dust is injected and dispersed and incorporating a pair of compensated photo-electric cells connected in opposition and to a mirror galvanometer, is described. A beam of light traverses the dust cloud and is picked up by one cell; the other cell picks up the beam uninterrupted by dust. The presence of dust in the chamber therefore produces a differential current which is measured by the galvanometer. 4. Using the prepared silica dusts in the apparatus, a technique has been developed which gives the following information from settling curves obtained by plotting galvanometer readings (converted to a percentage basis) against time; 1. Settling Factors, which for the majority of dusts, are approximately proportional to specific surface and to rates of settliig of the dust clouds. Settling Factors may be used to compare different dusts or the effect of various factors on any particular dust. 2. A measure of the average terminal velocity of a dust under the turbulent conditions obtaining in the dust chamber. The texminal velocity, if the average particle size of the dust is known, gives an indication of the extent of aggregation of the particles in the dust cloud, modified by the fact that the measured terminal velocity, being affected by convection and diffusion, is lower than that which would be observed under calm conditions. Particles of the size ca. 1 and 1-3 microns show considerable aggregation in the chamber but the larger grades (3-5 and 5-10 microns) show a much lesser degree of aggregation. 5. An account is given of the preparation, measurement and use in the dust apparatus, of aqueous sprays of known droplet size as dust suppressing agents and the following conclusions drawn: 1. Suppressing efficiency as measured by the Settling Factors does not change very markedly over the droplet size range 30-110 microns; the smaller droplets are slightly more effective but dust reduction is less than 50%. Dusts with particle size ca. 1 micron undergo the greatest reduction. 2. Under similar conditions the use of aqueous sprays containing various commercial wetting agents brings about a greater degree of reduction (60-70%). 3. With water sprays only, little or no actual wetting occurs, and reduction is presumably due mainly to the 'sweeping' action of the descending spray droplets. With sprays containing wetting agents, the effect is augmented by increased wetting of the dust particles. 6. Two other factors connected with dust suppression, namely, the effect of increased relative humidity, and of mixing mineral dusts with silica, were investigated. Increased relative humidity has no effect on the rates of settling of fine silica. The results obtained by mixing silica with certain mineral dusts of known specific surface do not confirm reports which state that the rates of sedimentation of certain mineral-silica mixtures are greater than the rates for the separate constituents of the mixtures. 7. The apparatus has also been applied to the measurement of the specific surface of small amounts (ca. 0.05 g.) of dust. By plotting Settling Factors against the known specific surfaces of the prepared dusts a calibration curve is prepared by means of which the specific surface of an unknown dust may be determined (accuracy, ca.+/-5%). The method is independent of the known decreased light extinction by particles under ca. 1 micron in size - a defect inherent in the usual light extinction methods of surface measurement. Certain dusts, however, settle unexpectedly slowly, leading to overestimation of specific surface. Such anomalous dusts contained less than 50% by weight of fine particles, the balance consisting of particles with a diameter at least 5 microns greater. It is suggested that the effect is due to deaggregation of the aggregated small particles by the larger. From the work completed it is suggested that two types of dispersion are related to every dust. One which depends on the method of dispersion and one which depends on the size and distribution of the particles and is inherent to the dust. 8. The South Wales method for the evaluation of health hazard is discussed and figures given by this method compared with Settling Factors for dusts of similar specific surface. The South Wales figures and the Settling Factors are almost identical, but the Settling Factors for anomalous dusts are considerably higher, and it is suggested that the Settling Factors give an equally effective and, in some cases, better measure of the health hazard, since they take account of dispersion in air. 9. An account is given of the methods of specific surface and particle size measurement used in the research. 10. Suggestions are made in a general discussion for future work.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Analytical chemistry, Physical chemistry, Occupational safety
Date of Award: 1951
Depositing User: Enlighten Team
Unique ID: glathesis:1951-78879
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:54
Last Modified: 30 Jan 2020 14:54

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