Dust Control Handbook for Industrial Minerals Mining and Processing

For valve-type bags, three major dust sources need to be addressed for effective dust control.The first dust source is from product blowback, which occurs during bag filling and results fromproduct spewing out of the bag valve. Product blowback occurs as excess pressure builds insidethe bag during bag filling and is then relieved by air and product flowing out of the bag valvearound the fill nozzle. The second major dust source is product spewing from the fill nozzle andbag valve as the bag is ejected from the filling machine. This is typically called a "rooster tail"because of the spray pattern of the escaping product. Both product blowback and product"rooster tail" occur because of the air pressure injected into the bag with product during filling.Both release dust into the air and contaminate the outside surface of the bag. The contaminatedbags then become the third significant source of dust exposure for the bag stackers, or for anyother individuals handling the bags, including the end user of the product. Figure 6.1 showsproduct blowback during bag filling, the product "rooster tail" as the bag is ejected from the fillnozzle, and dust contamination on the outside of the bag after loading is completed. If bags areundersized, they create a greater amount of product blowback and "rooster tail" during thebagging process, and this needs to be considered when evaluating these dust sources.Figure 6.1. Product "rooster tail" shown on left, product blowbackshown on top right, and dust-soiled bag shown on bottom right.With 50- to 100-pound bags, another area that impacts the amount of dust generated is the bagvalve. The valve is an internal sleeve in the bag through which the fill nozzle is inserted prior tothe bag being filled with product (Figure 6.2). The type of bag valve used impacts the amount ofproduct blowback and product "rooster tail." Once the bag falls from the fill station, the weightand volume of the product inside the bag is supposed to force the valve closed, sealing the areaand keeping product from leaking from it during the conveying, stacking, and the transportationprocess. However, many times the bag valve is lined with product and does not seal properly,causing product and dust to leak from the valve during movement/transportation. This sealingability also varies for different types of bag valves.One last dust source at bagging operations is from broken, torn, or ruptured bags. When a bagbreaks, a significant amount of product and dust is released into the mill atmosphere. Defects158 Bagging