Drum, sulfur-coating

The sulfur-coating drum is about 4 ft in diameter and 6 ft long. The internals of the drum and the associated sulfur piping are completely different depending on whether the sulfur is atomized pneumatically or hydraulically. A schematic of sulfur-coating drum operation with the two types of sulfur spraying is shown in Figure 2. 

USING PNEUMATIC SULFUR SPRAYING USING HYDRAULIC SULFUR SPRAYING Figure 2. Sulfur-coating drum operation... 

Position of sulfur nozzles. For best results, the spray nozzle header is located parallel to the axis of the sulfur-coating drum. The nozzles in the header are 8 in. apart on centerline. The header is located so each nozzle is spraying vertically down 4% in. onto the fastest moving section of the rolling bed. If the nozzles are placed farther away, the sulfur dust loading in the drum is increased and the quality of the product is decreased. Increasing the spray distance beyond about 8 in. from the bed is both impractical and hazardous as sulfur explosions may occur if... 

AVERAGE UREA TEMPERATURE IN SULFUR COATING DRUM, °F... 

Figure 8. Effects of urea temperature in sulfur-coating drum on dissolution of sulfur-coated urea with no seal coating using the pneumatic sulfur-spraying process...

Figure 8 shows the effect of the substrate temperature on the quality of the final product. This figure also shows that if the temperature of the material is allowed to exceed 204°F in the sulfur-coating drum, then quick cooling, such as in the fluid bed cooler, will prevent a complete degradation of the protective coating. It has also been shown that some quick cooling is helpful on products where the temperature does not exceed 204°F. Therefore, a small cooler is recommended. 

Safety. The minimum explosive concentration for sulfur is 35 g/cu m of air. The heaviest dust loading measured in the sulfur-coating drum while using hydraulic atomizing nozzles was 3.2 g/cu m of air, well below the explosive range. 

Sulfur-coating drum build-ups. Such build-ups appear to have been eliminated. A related problem of agglomerates caused by sulfur particles coalescing on the hot header piping with subsequent dripping of molten sulfur into the bed of urea has also been substantially reduced. 

Sulfur-coating drum temperature. The sulfur-coating drum temperature should not exceed about 200°F. Best results were obtained when the exit temperature of the sulfur-coated urea was maintained at 160-185 °F. 

Process temperatures in the hydraulic system are much lower than those in the pneumatic system. The urea should be preheated to at least 135 °F for best results, but the temperature in the sulfur-coating drum should not exceed 175 °F. At operating temperatures on either side of these values, the product quality falls off rapidly. Quick cooling of the final product to about 130 °F is necessary to prevent some product degradation. 

Fig. 10.21 Flow sheet of a fluid drum granulating (FDG) process for the coating or fattening of granules with sulfur and/or purge solution (courtesy Kaltenbach-Thuring, Beauvais, France).

Fig. 6.6-30 shows cross sections of broken melt-coated fertilizer granules [B.97]. In this case, the cores of conventionally granulated (by tumble/growth agglomeration. Section 6.6.1) TSP (triple super phosphate) were melt-coated with sulfur to provide an additional nutrient for sulfur-deficient soils and/or obtain a slow-release fertilizer. Fig. 6.6-31 is the flow diagram that is used for this process. The centerpiece of the system is the so-called fluid drum granulator (FDG). 

---