Dust precipitation

Silane, see Silicon tetrahydride Silica, Amorphous Total inhalable dust Respirable dust Precipitated... 

Over the polar regions of the north, snow, fog and dust precipitated out the lead. Unknown to mankind, the lead lay there, encased in Greenland s glacial ice cap, for 2500 years. 

Rice and Getz J made a very complete study of the matter from this point of view. At 65° C. they found k = 0-286, compared with the value 0-292 of Daniels and Johnston. In order to test the possibility that the reaction might depend on catalysis by dust, they compared the velocity constants for filtered and unfiltered nitrogen pentoxide and for gas which had been passed through an electrical dust precipitator. In some experiments the gas was dried with phosphorus pentoxide, in others not. In some it was prepared by the dehydration of nitric acid with phosphorus pentoxide and in others by the action of chlorine on silver nitrate. Nitric acid was found to have no catalytic effect on the decomposition. Some of the principal results are summarized below, all data referring to 65° C. 

The first step in treating metallurgical offgas is cooling in preparation for electrostatic dust precipitation. Electrostatic precipitators operate at about 600 K. Above this temperature, their steel structure weakens. Below this temperature, sulfuric acid forms from small amounts of S03 and HzO(g) in the furnace offgas - causing corrosion of the precipitator. 

Electric Furnace Baghouse Dust, BOF Dust, OH Dust, Coke Fines, Raw Materials, Iron Ore Pelletizing Baghouse Dust, Mold Sand Fines Silicon, Ferrosilicon, Ferromanganese, Ferrochrome Concentrates, Smelter Dust, Precipitates Concentrates, Sinter Mix, Flue Dust, Drosses Tungsten, Molybdenum, Antimony, Brass, Tin,... 

Spray-dry scrubbers are an alternative to conventional wet scrubbers. In this type of scrubber, an alkaline slurry or solution is sprayed in fine droplets into a reaction vessel, along with the flue gas. The droplefs rapidly react with the sulfur dioxide while drying to a fine powder of sulfite salts. This powder is entrained in the gas stream, and is carried to a dust precipitator where it is collected, as shown in Fig. 7. Most of the sulfur dioxide is collected in liquid-phase reactions while the droplets are drying, but 10-15 /o additional sulfur dioxide can be absorbed in gas/solid reactions, as the absorbent powder is swept through the ductwork and particulate collector. These are cocurrent devices, and so the limestone utilization and sulfur removal efficiency are inherently lower than those of countercurrent devices such as wet scrubbers. Partial recycle of the sorbent is often used to improve the sorbent utilization. 

Spray-dryers are simpler and more compact than conventional wet scrubbers and have a lower capital and operating cost. Also, they do not produce large quantities of wastewater, and the spent absorbent is dry, thereby eliminating the need for thickening and filtration of the sludge. However, if the same dust precipitator is used for both the fly-ash and fhe spray-dryer product, the mixture of fly-ash and spent absorbent that they produce is unmarketable, and must be disposed of. Also, they require more expensive absorbents than conventional wet scrubbers. They are most suitable for retrofitting small plants that burn medium-sulfur coals, where capital costs and space restrictions are more of a consideration. ... 

The rate of cyanidation reactions depends on the cyanide concentration as well as the alkalinity of the leach solution. The optimal pH of the solution is 10.3, typically controlled by caustic (NaOH). Gold is recovered either by adsorption on activated carbon or by zinc-dust precipitation. 

McLEAN One of your slides showed a decrease in the calcium amount. Could that be caused by a decrease in dust precipitation. 

Figure /1.8 Fuel gas cleaning with dry dust precipitation and wet RME scrubbing In gassing. ("Source Reproduced from Ref [36 )...

The resistivity of a layer of dust precipitated in an electrofilter is determined not only by the inherent resistivity of the material at the given temperature, but also by the particle size, density of the dust layer, and surface conductivity of the particles, which may vary as a result of adsorption of gases or vapors. It was found in [316] that the resistivity of copper oxide powder decreases with increasing particle size and with increasing layer density, the layer density being the more important factor. The resistivity of a dust layer is usually 2-3 orders of magnitude greater than the resistivity of the material itself, if the comparison is made under the same ambient conditions. 

Filtration and Adhesion. In the cleanup of dust-containing process gases or air, the particles suspended in the gas stream pass through a porous filter medium and stick to the surfaces of this material. The dust precipitate thus accumulated becomes, in turn, the filter medium for the subsequent particles. As the deposit builds up, the porosity of the medium decreases, hindering the free flow of gas. At some point this deposit must be removed. 

The relationship between the amount of dust precipitated from a stream and the flow velocity can be expressed by the equation [333] ... 

The dust carried out of the hole with this flushing air can be trapped in a dust collector, which is mounted on the drilling machine. It comprises a cyclone in which the coarser particles are precipitated, while the finer ones are retained in special filters. The suction extractor is connected to a flexible tube which terminates in a plastic sleeve forming an airtight closure over the mouth of the borehole, so that all the dust can be collected. For testing the samples it is important not only to analyse the dust precipitated in the cyclone, but also to include the fine particles trapped in the filter equipment. 

Planted areas of this kind do not themselves produce dust, and much of the dust carried into them by wind is trapped. More particularly, the slowing down of the air currents by the foliage cause them to discharge much of their dust burden. This result is more effectively achieved if the dust-laden air can penetrate into the belt of trees, so that dust precipitation takes place in or just behind it. Eddy formation in front of dense forest also results in a certain smount of precipitation, but a lot of the dust remains airborne and is carried along in the wind that sweeps over the top of such forest instead of penetrating into it. 

Roughly speaking, forest with 40% penetrability achieves the best dust precipitating effect. In winter, when the trees and bushes have shed their leaves, their effectiveness is reduced to about 60% of that attained in summer. 

The sampling station has a dust collection system. The dust precipitated in the latter is returned to the sample splitter, so that no dust losses occur. 

Dust precipitation in textile filter media is accomplished by the following processes ... 

A conditioning unit (water spray tower) installed upstream of the precipitator can serve to lower the temperature of the incoming gas to 130°— 150° C and also raise the dew-point of the gas, so as to enable better dust precipitation to be achieved. At the same time, the gas entering the conditioning unit should still have a sufficiently high entry temperature, uniform distribution and sufficiently long residence time to enable all the sprayed water to be completely evaporated (Fig. 16). 

In the analysis of dust precipitation in a duct, attention has so far been paid to a single particle only. In a precipitator, as the gas enters and flows horizontally, particles exist at all locations of the cross section. One is then interested in finding out the density of particles as a function of two coordinates z, in the direction of bulk gas flow, andy, normal to it. To that end, for particles of size tp, a more general analysis may be carried out to determine the fraction of particles collected. Consider laminar gas flow using the particle diffusion equation (6.2.55) in the gas in the presence of an external force, no particle generation or agglomeration, dilute suspension and a quasi-stationary state ... 

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