Cyclone collectors, pollutants

The dry product is primarily collected in cyclone collectors (a few bag houses still remain), sieved, and finally packaged in moisture barrier containers. The exit air from the dryer often has to be treated to meet local pollution control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer installations use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained solids and gaseous volatile flavors. 

Dust control systems usually include hoods and enclosures connected to a local exhaust ventilation system that discharges to cyclone collectors and scrubbers or a baghouse in series. The most common type of control equipment is the baghouse these are increasing in use as more stringent pollution control codes are adopted. These filters provide excellent collection efficiency with little or no visible emissions and the collected dry fines are sometimes usable in concrete mixes. However, if the exhaust gas temperature is at or near the dew point of the gas, condensation of moisture on the fabric filters is always a possibility and poses a serious fire hazard. The other type of the commonly used control equipment is the venturi wet... 

The principal technological developments in the control of air pollution by engineering during the nineteenth century were the stoker for mechanical firing of coal, the scrubber for removing acid gases from effluent gas streams, cyclone and bag house dust collectors, and the introduction of physical and chemical principles into process design. 

The emission of volatile trace elements from roasting, smelting, and converting processes is undesirable from both an air pollution and an economic standpoint. Gravity collectors, cyclones, and ESPs are used to attain collection efficiencies of up to 99.7% for dust and fumes. 

When the pollutant loading is exeeptionally high or consists of relatively large particles (> 2 /tm), venturi scrubbers or spray chambers may be used to reduce the load on the ESP. Much larger particles (> 10 /tm) are controlled with mechanical collectors such as cyclones. Gas conditioning equipment to reduce both inlet concentration and gas temperature is occasionally used as part of the original design of wet ESPs (AWMA, 1992 Flynn, 1999). 

The second major wastewater discharge (about 1.2m /min or 318 gpm) is from the bottom of the scrubbers used after a dry dust collector cyclone to reduce the dust concentration in the effluent air stream from the phosphate dryer. The underflow of the scrubber contains a high concentration of dust and mud, laden with tiny phosphate organic and inorganic silica and clay particles, and is disposed of in a nearby stream. The main pollutant in the flow to the stream is P2O5 at a concentration of nearly 1200 mg/L and it remains mostly as suspended particles. 

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