Drying in rotary driers

Boric acid crystals are usually separated from aqueous slurries by centrifugation and dried in rotary driers heated indirecdy by warm air. To avoid overdrying, the product temperature should not exceed 50°C. Powdered and impalpable bode acid are produced by milling the crystalline matedal. 

The crystalline sodium chlorate is usually dried in rotary driers to less than 0.2 wt % moisture content and is loaded into shipping containers or stored in moisture-free bins or silos prior to packaging. For conventional chlorine dioxide generators, sodium chlorate is shipped as a solution containing ca 200 g/L (15 wt %, 3.4 M) sodium chloride ca 350 g/L (26 wt %, 3.3 M) sodium chlorate and 130 ppm chromium. Alternatively, for newer chlorine dioxide generators, 600 g/L sodium chlorate 30 g/L sodium chloride and less than 30 ppm chromium is used. 

The study of drying in rotary driers is based upon the following relationships derived from basic theory ... 

The pulp, leaving the diffuser after the extraction of sugar from the beet cossettes, is pressed in screw-t5q>e presses to remove water. The pressed pulp is enriched by the addition of molasses or concentrated Steffen filtrate and is dried in rotary driers. Molasses-dried beet pulp... 

High-frequency insonation has been shown to increase the rate of drying in rotary driers substantially [13]. 

Borax decahydrate is produced from borate ores, primarily colemanite and also from dry lake brines. When produced from its ore, the ore is crushed and then blended with B2O3. The blend is mixed with hot recycle liquor in a dissolving plant. Rock and clay particles from the liquor are removed over vibrating screens. The hquor is then fed to thickeners for settling of insolubles after which the underflow mud is washed with water. The strong liquor of borax hydrates is then pumped into continuous vacuum crystallizers for the separation of the pentahydrate first and then the decahydrate. The products are dried in rotary or fluid bed driers. 

In practice, the excess activated sludge is conditioned with 3-6% pcClj and tillered on vacuum filters. This reduces the moisture to about 80% and produces a filter cake which is dried in rotary or spray driers to a moisture content of less than 5%. It is bagged and sold direct as a fertilizer, or to fertilizer manufacturers who use it in mixed fertilizer. 

Section V deals with the drying of porous solids in continuous operations. The study of drying in rotary and tunnel dryers is presented based on the relationships derived from basic theory. The effect of the operating variables on drier performance is discussed. A suitable procedure is developed for sizing rotary and tunnel driers. 

The most extensive work on holdup and retention time in rotary driers has been conducted by Friedman and Marshall (F2). Unfortunately, much of their work w as done on dry materials with no air flow. From an experimental standpoint, their results have a disadvantage because one of the conditions requires that the test be conducted at zero air velocity. With many materials the handling characteristics at zero air velocity are different from those with air flow in that the material may change considerably during drying. 

The resulting suspension of PCC is screened at about 50 pm and is either used as an aqueous suspension containing 20 to 50 % of solids, or de-watered using rotary vacuum filters, pressure filters or centrifuges. The resulting moist cake is dried in rotary, spray or flash driers. 

The floated mica concentrate is dewatered. After dewatering (qv), the mica is either dried in a fluid-bed rotary drier, flash dried in a fluid energy mill, or sold "drip-dry" to other mica grinders. The dry mica is then ground and screened to a si2e gradation dictated by the customer. 

The drying of the curd is completed by passing through a rotary drier such as the Pillet-Bordeaux. In this machine the curd passes down a chute with hot air running counter to the direction of flow of material. Strict temperature control is essential, the maximum discharge temperature being below 65°C. If such control is not made then the resin will be dark in colour. 

Fluidized-bed driers are also widely used due to their large heat- and mass-transfer coefficients. However, materials of even moderate adherence and cohesiveness cannot be dried in a fluid bed. The same applies to materials that are sensitive to oxygen, especially at elevated temperatures. Vacuum drying is often necessary for oxygen sensitive materials and this is not easy to realize in fluid-bed driers, although there are systems to deal with this problem. Fluid-bed driers are not as easy to clean as shelf driers or rotary driers. 

Sodium hydrosulfite is produced through the Formate process where sodium formate solution, sodium hydroxide, and liquid sulfur dioxide reacted in the presence of a recycled stream of methanol solvent. Other products are sodium sulfite, sodium bicarbonate, and carbon monoxide. In the reactor, sodium hydrosulfite is precipitated to form a slurry of sodium hydrosulfite in the solution of methanol, methyl formate, and other coproducts. The mixture is sent to a pressurized filter system to recover sodium hydrosulfite crystals that are dried in a steam-heated rotary drier before being packaged. Heat supply in this process is highly monitored in order not to decompose sodium hydrosulfite to sulfite. Purging is periodically carried out on the recycle stream, particularly those involving methanol, to avoid excessive buildup of impurities. Also, vaporized methanol from the drying process and liquors from the filtration process are recycled to the solvent recovery system to improve the efficiency of the plant. 

Rotary driers may also be used for combined drying roasting and calcining. In this case they are known as rotary kilns. 

Typical CAN granulators include drums and pugmills. The calcium carbonate may be mixed with the AN solution before granulation or in the granulator itself. Granules from this process will normally require drying in a fluidized bed or rotary drier.103... 

A flow diagram for the production of aspirin is shown in Fig. 10.6 [9]. A mixture of salicylic acid powder, acetic anhydride, and a small amount of sulfuric acid is charged into a batch reactor. The mixture is stirred for 2 to 3 hours and then transferred to a crystallizing kettle. A portion of the mother liquor is recycled in the next run, and the other part of the slurry is centrifuged. The resulting crystalline material is dried in a rotary drier to yield acetylsalicylic acid (aspirin). Excess mother liquor is distilled and the excess of acetic anhydride is recycled. 

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