Physico-Chemical Studies on Dusts: The Suppression of Airborne Dust by Aqueous Sprays

This study is concerned with the capture of airborne dust particles by sprayed liquid droplets, with particular reference to the airborne coal dust encountered in mining practice. The particle size range 0.5-5.0 microns, of physiological importance in causing occupational health hazard, has been investigated. The significance of duets and their harmful particle-size and concentration are discussed and the incidences of coal-miners' pneumoconiosis and existing methods of dust suppression in mines are surveyed. The general mechanism of liquid spray formation from nozzles is investigated and the theoretical probability of capture of dust particles by spray droplets is discussed. Experiments on dust suppression were carried out on moving dust clouds in a wind-tunnel of 45.72 cm. (18 in.) diameter and 20 metres (65 ft.) long, under controlled conditions. Dust clouds, of concentrations in the range 300 and 3000 p.p.c.c., were produced using Hattersley'a laboratory type dust generator. A three-throw reciprocating pump provided spray pressures up to about 210 kg./sq. cm. (3,000 p s i.g.) and the spray nozzles were operated at the axis of the tunnel against the flow of dust-laden air. Simultaneous dust sampling was done by two thermal precipitators located ahead of and beyond the spray nozzle. Distribution of air velocity in the tunnel was studied and modifications in the tunnel were made to straighten out the air flow. The effect on air flow pattern of baffle plates suitably placed in the tunnel is illustrated in the form of iso-velocity curves. Distribution of dust concentration In the tunnel was studied with salicylic acid filters and the rate of decay of dust concentration with distance along the tunnel was found to be constant . The probable mechanisms of dust fall-out are discussed. A system for counting the thermal precipitator dust slides using a five-channel Automatic Particle Counter and Sizer was developed and an analysis of variance for the machine was made. Simultaneous dust sampling by thermal precipitator and salicylic acid filter was carried out in the tunnel and a correlation factor was derived on the basis of proportional number percentage size frequencies in both the samples. Measurment of Average Droplet Size was carried out for water sprays in the range 35.15-190.16 kg./sq. cm. (500-2,750 p.s.i.g) using a solid cone spray nozzle and the relationship A.D.S a p(-0.28) was obtained. A functional non-dimensional relationship was also derived between the characteristics of the spray and the Average Droplet Size. The dust suppression work was mainly concerned with small high pressure spray nozzles and the following issues were brought to focus: The effects on the efficiency of dust suppression of, (a) hollow cone and solid cone spraying, (b) high spray pressures, (c) surface tension of sprayed liquid, (d) counterflow air velocity, and (e) a tandem arrangement of spray nozzles. The effect of spray throughput on dust-water ratio was also studied. Solid cone spraying was found to be about 5 per cent more efficient than the hollow cone spray for the same nozzle. Maximum dust suppression was effected at 140.6 kg./sq. cm. in both the cases. (53.58% and 48.5%). Surface active agent solutions were found to give only a small increase in dust suppression efficiency, E. The wetting power of different types of surface active agents is discussed. A relationship was obtained between the efficiency, spray pressure and the surface tension of the sprayed liquid. Tandem spraying was found to give the best dust suppression efficiencies without much increase in water throughput. The rate of increase in E with number of spray nozzles in tandem was found to be maximum (about 6.5%) at 140.6 kg./sq. cm. An increase in air velocity resulted in a rapid decrease of E. The rapid deceleration of droplets on discharge into the gaseous medium, its lowered efficacy of impact on dust particles and the consequent reduction of the "capture cross-section" are discussed. The dust-water ratio was calculated for all spray pressures and nozzles employed and was found to generally decrease with increase in throughput. The sprayed droplets did not appear to be selective in suppressing any particular size range of dust particles. An improved type of dust feeding machine was put into operation and is discussed in the Appendix