Physico-chemical studies on dusts: The effect of aqueous sprays on airborne dusts

The aim of the research was to investigate the suppression of fine airborne dusts by aqueous sprays under controlled conditions and to develop a quantitative airborne dust sampler. The mechanism and causes of dust diseases are discussed and a brief summary given of the history of silicosis. The production of dust in mines, its sampling and the available methods of removal are mentioned. The swirl atomiser was employed as it was the simplest type of nozzle for use under mining conditions. It was developed and calibrated for use in the apparatus described below. Methods were developed to allow collection and accurate evaluation of the spray droplets of various liquids. A description is given of the apparatus used for the preparation of a dust cloud of very fine dust particles of approximately reproducible size distribution and concentration. The apparatus in which the suspended clouds were sprayed is described. It consisted of a cylindrical dust chamber of large volume into which the dust was injected and dispersed. A Thermal Precipitator was built into the apparatus to enable samples of the dust to be withdrawn from different heights in the chamber. The concentration and size distribution of each sample was estimated. The following conelumions were drawn from the work. It was found that the spray variables could be connected by an empirical formula by which droplet size could be predicted at any pressure, provided two relevant points were determined experimentally. A relationship was also obtained between the energy applied to the spray and the energy used in the stomisation process. In general, stomisation was found to increase with increase in spray pressure and cone angle and decrease with increase in orifice diameter and with the distance from the spray nozzle. The efficiency of atomisation was found to be much lower than expected, indicating that a high proportion of the energy imparted to the spray was not utilised in the atomisation process. The use of solution of low surface tension was found to have little effect on the atomisation achieved. Only below 30 lb./in2. was any improvement obtained. The two methods of droplet sampling employed were found to give reproducible and comparable results if sufficient samples were taken. It was thought that the energy required to effect equivalent atomisation of the solution of low surface tension would be less than that required for water. This was not so and it is concluded that at the moment of droplet formation the surface tension is not the equilibrium value but some dynamic value, possibly higher than the equilibrium value. Change of surface-active agent concentration was only found to affect atomisation at low pressure owing to the effect on the surface tension of the solution. At higher pressures concentration of surface-active agent would only affect atomisation through viscosity or density increases. Spraying test showed that the dust clouds (coal dust, silica dust and fly ash) were affected by aqueous sprays and that the amount of dust removed was in each case increased when (a) the duration of spraying was increased (b) the average droplet size was increased over the range investigated (33-96 microns) (c) the relative velocity of the droplets was increased (d) cycloHexane was used at low spray pressures. Estimation of the size distribution of the dust clouds under different conditions led to the following conclusions: (a) the size distribution of the clouds alters very slowly with time after the first 90 minutes (b) the small particles aggregate to follow large particles (c) the large aggregates settle out (d) spraying removes the larger particles and breaks up the aggregates (e) cycloHexane appears to remove more of dangerous size fraction than the other liquids tried. It would appear that the most effective droplet size is greater than 96 microns for the clouds sprayed. A dust sampler was developed capable of collecting a representative sample of the dust in suspension (coal dust or silica dust) and is described. It is capable of collecting, in a reasonable time, a sample sufficiently large to allow chemical analysis to be carried out. Only 100 mgm. Of the total weight of dust collected (on 5 gm.) is retained in the pores of the collecting thimble. It has also been shown to compare favourably with another bulk sampler now being produced commercially