Drying equipment direct rotary dryers

For handling large quantities of granular solids, a cascading rotary dryer is often the equipment of choice. If the material is not naturally free-flowing, recycling of a portion of the final dry product may be used to precondition the feed, either in an external mixer or directly inside the drum. Hanging link chains and/or scrapper chains are also used for sticky feed materials. 

After beneficiation, the washed rock may contain 7%-20% moisture, which is reduced to between 1% and 2% moisture using direct-fired rotary dryers. Emissions expected from the dryer consist primarily of fine rock dust. Some sulfur dioxide may also be present in the dryer exhaust from the combustion of sulfur in the fuel. Phosphate rock dryers are usually equipped with dry cyclones, followed by wet scrubber systems for control of rock dust. The scrubber systems will operate at typical particulate collection efficiencies of 97%-98% with emissions ranging from 0.1 to 0.3 kg/t of phosphoric anhydride (P2O5) [77]. 

Equipment commonly employed for the drying of solids is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Dryer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum dryers, furnaces, steam-tube dryers, and rotary calciners. 

To roughly estimate the total direct cost of the drying installation, it is recommended to multiply the equipment FOB cost by a factor of 2.25 if the equipment is made of carbon steel or by 2.75 if stainless-steel equipment is used [29]. These figures include the cost of piping, instrumentation, electrical connections, insulation, building space, and engineering when applied to spray, flash, rotary, or fluid-bed continuous dryers. In specific cases, however, different factors may have to be used. For instance, the costs of instrumentation and piping may be included in the purchase price of a fully equipped dryer [30]. 

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