Physico-chemical studies on silica of small particle size

Aim of Research. (i) To develop and evaluate an apparatus for the investigation of the air-settling characteristics of silica dust of a particle-size dangerous to health; (ii) to examine its use for the investigation of the effect of aqueous sprays on the dust in suspension in the chamber. 1. A brief account is given of the causation of silicosis, and the methods employed in mines to measure dust concentrations, and to suppress the fine dust particles, are mentioned. 2. The apparatus used for the preparation of a dust cloud of very fine dust particles of approximately reproducible size distribution and concentration is described. 3. An apparatus is described for the examination of the air-settling characteristics of fine silica, consisting of a dust chamber into which the dust is injected and dispersed, incorporating three pairs of compensated photoelectric cells connected in opposition and to a mirror galvanometer. A beam of light traverses the dust cloud and is picked up by one cell of each pair; the other cell picks up the beam uninterrupted by dust. The presence of dust in the chamber produces a differential current which is measured by the galvanometer. 4. A thermal precipitator (T.P.) is built into the apparatus to enable samples of the dust to be withdrawn from different depths in the chamber. The concentration (particles per cubic centimeter (p.p.c.c.)) and size distribution of each sample is estimated. 5. Two types of water spray (i) the hydraulic or water pressure; (ii) the compressed-air-operated are developed and calibrated, but only the former is used in the dust chamber. 6. Using the above apparatus the following conclusions are drawn. A. - The atomisation of the water-pressure spray increases when (i) the pressure of the water increases; (ii) the orifice diameter increases over the range investigated; and (iii) a surface active (S/A) agent is added to the spray solution. (This effect is more pronounced at lower pressures. All the S/A agents investigated, with the exception of Fixanol C, are found to be equally effective in increasing the atomisation.) The compressed-air-operated spray produces smaller droplets than the water pressure spray, but is not suitable for use in the chamber because of the large volume of air involved in the atomisation process. B. - The light extinction method was used to estimate the dust concentration and the following results obtained:- (i) The rate of sedimentation is less at the lowest concentration of dust than at the three higher concentrations, suggesting the effect of aggregation at the latter. (ii) The same efficiency of dust removal calculated as reduction in surface area, is achieved by spraying downwards or upwards through the dust. (iii) The plot of reduction of dust concentration against duration of spraying does not conform to an exponential curve, as would be expected if the only factor influencing the removal of the dust was the concentration. It is suggested that the larger particles are preferentially removed by spraying, resulting in a proportionally larger decrease in surface area. (iv) The inclusion of S/A agents in the spray solution does not result in an improvement in spray efficiency when the water pressure spray was used at a pressure of 62 lb./in.2 A 0.5% solution of Lissapol NDB is as effective as water alone in suppressing the dust while a 0.5% solution of Alcohol 0 in alcohol is less effective than water or Lissapol NDB solution. The same efficiency of dust removal is found for a wide range of concentration of Lissapol NDB. The reason for the lower efficiency of Alcopol 0 solution is thought to be the increase in rate of evaporation of the spray droplets due to the alcohol in the spray solution. C. - Application of Smoluchowski's law to the rate of decrease of concentration (p.p.c.c.) with time permits the calculation of X (the Smoluchowski constant). MSC and LAS are found to have different values of K, both greater than the theoretical. The reasons are thought to be :- (i) the non-uniform size-distribution of the dust cloud; and (ii) the electrostatic charges which exist on the particles and which have been shown to influence the aggregation (Section 5). (Abstract shortened by ProQuest.)