Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 6The heated kiln surface area needed to produce 1 tonne HF/d is approximately 5 to 10 m 2 withthis arrangement. Dried fluorspar is not easily mixed with the sulphuric acid due to the flotationchemicals. Table 6.3 shows the options to achieve a higher HF production capacity for a givenkiln and Figure 6.2 illustrates the different option including energy supply and heat recovery.Energy consumption is reduced by heat recovery in most plants, by preheating the air suppliedto the main burner in a recuperator using the rotary kiln flue-gas as a heat source. Theproduction of HF solution causes higher fuel consumption, because of the higher level of H 2 Opresent in the kiln.PreheatingH 2 SO 4Pre-reactor(kneader)CalcinationMixing fluorspar and feed sulphuric acid in an indirectly heated pre-reactor beforefeeding the partially reacted mass to the rotary kiln. In the pre-reactor, the reactionmixture changes from liquid to a pasty and sticky consistency and is extremely corrosiveand abrasive. It needs special knowledge regarding the materials of construction toreduce corrosion. The use of pre-reactors can significantly reduce the heated kiln surfacearea typically by 30 %Calcination of the dried fluorspar, by heating it directly with flue-gases containingoxygen to 400 – 450 ºC. The calcined spar is free of organics; 95 % are burned to CO 2and H 2 O and 5 % are emitted as crack-products. This can be easily fed with thesulphuric acid. SO 2 formation is avoided and the heated kiln surface necessary toproduce 1 tonne HF/day is only 2.5 – 3 m 2Table 6.3: Options to achieve a higher production capacity for a given kilnProcess gases may be discharged either at the feed end or at the anhydrite discharge end of thekiln. In the first case, the temperature of the process gases leaving the reactor system isapproximately 150 °C and approximately 220 °C in the second case where the temperature ofthe anhydrite is in the range of 200 – 220 °C.VentVentSpar calcinationEnergy supplyBurner IIAirorHeatrecoveryPreheated airto burner IDry fluorsparorProcessgasesFeed H 2SO 4Pre-reactorKneaderMain reactorRotary kilnPreheatingorAnhydriteand/orEnergy supplyBurner IFigure 6.2: Increasing the production capacity for a given kiln and energy supply/recovery[22, CEFIC, 2000]260 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers