Dust recovery

Based on dryer cost alone, indirect-heat dryers are more expensive to build and install than direct-heat dryers designed for the same duty. As environmental concerns and resulting restrictions on process emissions increase, however, indirect-heat dryers are more attractive because they employ purge gas only to remove vapor and not to transport heat as well. Dust and vapor recovery systems for indirect-heat dryers are smaller and less cosdy to supply heat for drying, gas throughput in direct-heat dryers is 3—10 kg/kg of water evaporated indirect-heat dryers require only 1—1.5 kg/kg of vapor removed. System costs vary directly with size, so whereas more money may be spent for the dryer, much more is saved in recovery costs. Wet scmbbers ate employed for dust recovery on indirect-heat dryers because dryer exit gas usually is close to saturation. Where dry systems are employed, all external surfaces must be insulated and traced to prevent vapor condensation inside. 

In most catalytic-reactor systems, no sohds removal is necessary as the catalyst is retained in the system and sohds loss is in the form of fines that are not collected by the dust-recovery system. 

Define dust recovery, as percentage below a certain particle size. 

Sulfide ores usually contain small amounts of mercury, arsenic, selenium, and tellurium, and these impurities volatilize during the ore treatment. All the volatilized impurities, with the exception of mercury, are collected in the dust recovery systems. On account of its being present in low concentrations, mercury is not removed by such a system and passes out with the exit gases. The problem of mercury contamination is particularly pertinent to zinc plants since the sulfidic ores of zinc contain traces of mercury (20-300 ppm). The mercury traces in zinc sulfide concentrates volatilize during roasting and contaminate the sulfuric acid that is made from the sulfur dioxide produced. If the acid is then used to produce phosphatic fertilizers, this may lead to mercury entering the food chain as a contaminant. Several processes have been developed for the removal of mercury, but these are not yet widely adopted. 

Most processes emit large amounts of dust, which must be recovered to abate pollution or because the dust itself contains valuable metals equipment for dust recovery is bulky and expensive. 

Because large amounts of gas are required to supply all the heat for drying, dust-recovery equipment may be very large and expensive when drying very small particles. 

Dust recovery and dusty materials can be handled more satisfactorily in indirect dryers than in direct dryers. 

Gravity vessels are suitable for low-, medium-, and high-temperature operation in the last case, the housing will be lined completely with refractory brick. Dust-recovery equipment is minimized in this type of operation since the bed actually performs as a dust collector itself, and dust in the bed will not, in a successful application, exist in large quantities. 

Catalyst and metal dust recovery, soot filtration... 

Dust recovery from calcination processes e.g., magnesium oxide production. 

Solids Recovery Equipment. Gas take-off from the surface of the fluid bed is accompanied by entrainment of solids, particularly of the fine and intermediate size range created in part by attrition. A hindered settling or disengaging zone above the bed is provided and the fine particles with low free-fall velocities are carried over into dust-recovery equipment consisting of one or a combination of dry or wet cyclones, multiclones, and electrostatic precipitators. Bag filters are prone to plug but must sometimes be used where expensive solids are involved. Cyclones may be placed within the reactor shell (Fig. 4-4a) or external to it (Fig. 4-4E>), depending on the freedom of the cyclone operation from mechanical troubles. 

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