Entrainment of dust

Strontium is widely distributed in the earth s crust and oceans. It is released into the atmosphere as a result of natural processes such as entrainment of dust particles, resuspension of soil by wind, and sea spray. Strontium is released into surface water and groundwater from the natural weathering of rocks and soils. Human activities, including milling and processing of strontium compounds, burning of coal, land application of phosphate fertilizers, and use of pyrotechnic devices, release strontium into the atmosphere. Discharges of industrial waste water and runoff from land treated with phosphate fertilizers are human-related processes that release strontium into streams and aquifers. 

Strontium is widely distributed in the earth s crust and oceans. Strontium is released into the atmosphere primarily as a result of natural sources, such as entrainment of dust particles and resuspension of soil. Radioactive strontium is released into the environment as a direct result of anthropogenic activities. Stable strontium can be neither created nor destroyed. However, strontium compounds may transform into other chemical compounds. Radioactive strontium is formed by nuclear reactions. Radioactive decay is the only mechanism for decreasing the concentration of radiostrontium. The half-life of 90Sr is 29 years. 

Other potential sources of atmospheric releases include incineration of scrap wood treated with the mixture and re-entrainment of dust and soils contaminated with components of the mixture in the vicinity of hazardous waste sites. Creosote was not detected in any air samples of the 46 current or former NPL sites where creosote was identified in some environmental medium (HazDat 2002). 

Spillages should be cleaned up to prevent entrainment of dust. Use of vacuum equipment connected to a filter is generally preferable to shovelling or sweeping. 

The only safe way of avoiding formation of dust clouds is to use air velocities at which no entrainment of dust takes place. In band and tray dryers, the control of velocity is easier, but this is difficult to fulfill in dryers that involve suspended-particle processing. In general, the nature of the drying operation is such that dust entrainment is not easy or feasible for various reasons. 

The first term in the right-hand side of Eq. (X.77) characterizes the entrainment of dust particles by the air stream, as given by Eq. (X.68), and the second characterizes the entrainment due to the airborne dust particles. Under the conditions existing in iron mines, with = 1-8 m/sec, we find that a = 1.85, b = 2, and/ = 0.083. 

The outlet air temperature and pressure from the compressors gives a good indication of the state of downstream process plant, where additional pressure drop may be experienced due to the entrainment of dust in the drying bed, breakdown of the catalyst or malfunction of the control valves. The gas discharge pressure and preferably also the temperature should be recorded. In addition, for safety reasons and to protect the compressor, a high-pressure switch/alarm combination should be used to stop the compressor in the event of overpressure. The sulphur pumping system must be interlocked with compressor operation, to stop the pump when no process air is available. 

Above the top limit the total efficiency no longer increases with increasing pressure drop and it may actually decline due to re-entrainment of dust from the dust outlet orifice. It is, therefore, wasteful of energy to operate cyclones above the limit. At pressure drops below the bottom limit, the cyclone represents... 

Shao, Y, Raupach, M.R., and Findlater, P.A. 1993. The effect of saltation bombardment on the entrainment of dust by wind. Journal of Geophysical Research 98(D7) 12719-12726. 

In general, this equipment offers an economical heat-transfer area for first cost as well as operating cost. Capacity is hmited primarily by the air velocity which can be used without excessive dust entrainment. Table 12-32 shows hmiting air velocities suitable for various sohds particles. Usually, the equipment is satisfactory for particles larger than 100 mesh in size. [The use of indirect-heated conveyors eliminates the problem of dust entrainment, but capacity is limited by the heat-transfer coefficients obtainable on the deck (see Sec. 11)]. 

As with any hood system, design methods are used to obtain required exhaust rates and hood dimensions.The main mechanisms of dust generation are air induction, material splash, air displacement, and air entrainment. 

These operations may sometimes be better kno Ti as mist entrainment, decantation, dust collection, filtration, centrifugation, sedimentation, screening, classification, scrubbing, etc. They often involve handling relatively large quantities of one phase in order to collect or separate the other. Therefore the size of the equipment may become very large. For the sake of space and cost it is important that the equipment be specified and rated to Operate as efficiently as possible [9]. This subject will be limited here to the removal or separation of liquid or solid particles from a vapor or gas carrier stream (1. and 3. above) or separation of solid particles from a liquid (item 4j. Reference [56] is a helpful review. 

Having completed the survey, the next stage is to draw the system showing the sources of dust and the duct runs. An assessment must then be made on the air volumes and velocities required giving control. This is largely a matter of experience, as air-entrainment rates are derived empirically. It is possible to calculate the rates but is unusual in general engineering. There are published lists for air rates and many companies have their own standards. 

It is clear that both atmospheric and surface dusts are complex materials and not all that easy to describe. A summary is given in Fig. 2 of the sources of atmospheric and surface dusts and their inter-connection. Both natural and anthropogenic sources contribute to both dusts. The inter-connection between the two dusts is wet and dry deposition from the atmosphere to the ground, and the re-entrainment of surface dust through wind and human activity into the atmosphere. Dust is an important global component of our earth, and impinges on the wellbeing of people. 

The cometary coma The coma and the nucleus form the head of the comet the streams of dust and gas released by the comet form a very large, extremely tenuous atmosphere called the coma, which can have a spread up to around 104—105 km. The coma is not developed when the comet is a long way from the sun, but when it comes closer (at around 5 AU), the ice mixture begins to sublime and is ejected as a gas stream. Dust particles are entrained at a velocity of around one kilometre per second. 

Mechanical Scrubbers Mechanical scrubbers comprise those devices in which a power-driven rotor produces the fine spray and the contacting of gas and liquid. As in other types of scrubbers, it is the droplets that are the principal collecting bodies for the dust particles. The rotor acts as a turbulence producer. An entrainment separator must be used to prevent carry-over of spray. Among potential maintenance problems are unbalancing of the rotor by buildup of dust deposits and abrasion by coarse particles. 

There is no evidence to suggest that coarse aggregate shape or geological origin affect the amount of air entrainment obtained. The only exception is where crushed rock aggregates contain an appreciable quantity of dust which could influence the fine aggregate gradings considered below. 

One particular type of source that should be studied carefully Is entrained soil. As shown above, this Is often the greatest contributor of TSP In urban areas. As there Is so much of It present, we need to know concentrations of all measured elements quite well to make an accurate determination of the residual amounts left to be accounted for by other sources. The composition of selved soil Is often used for the soil component, but there may be considerable fractionation Imposed by entrainment, e.g., preferential selection of very fine clay mineral particles. Such fractionation has been demonstrated In the very limited studies of entrainment of particles from soil of known composition (e.g.. Refs. 21, 49). These studies can probably best be done In controlled environments such as wind tunnels. One cannot simply collect ambient particles In the countryside and consider it to be soil, as there are anthropogenic contributions even at great distances from cities ( ). There Is further confusion betwen clean, "continental" dust and "urban" dust. The latter, which Is usually collected near city streets (21, 50), typically has a composition of soil contaminated by anthropogenic emissions, especially from motor vehicles. 

For abandoned opencast sites, and for large spoil heaps associated with underground coal mines, landscaping and revegetation are clearly the key to long-term prevention of dust entrainment by the wind (see below). 

Dust dispersion—Aerosols of mineral particles and a few other materials may be prepared by dust dispersal techniques. A number of dust dispersal devices have been constructed. A commonly used device is the Wright Dust Feeder. In this device the dust is compressed into a cylindrical compact, which is placed in an electronically driven mechanism that rotates it and drives it against a fixed knife blade at a fixed rate. Material is scraped off the compact by the blade, entrained in an air stream, and blown against a metal plate to break up particle aggregates. 

The design of a suitable dust control system for a particular application requires knowledge of dust generation and air entrainment. The interaction of these two parameters with the process (e.g. solids throughput, powder properties, temperature, drop height, velocity) also must be considered for this purpose. Unfortunately, the existing techniques to... 

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