Dust Control Handbook for Industrial Minerals Mining and Processing

Figure 6.5. Open-top bag being loaded with product(normal whole-grain material, which is less dusty).The second and most common bag type used in mineral processing operations is the pasted valvebag. The choice of valve type from among different commercially available bag valves can be asignificant factor in the ability to effectively seal 50- to 100-pound bags of product and tominimize the amount of dust liberated during the bag filling, conveying, and stacking process.From a study performed a number of years ago, five different valves were tested to compare theireffectiveness at sealing the bag and minimizing dust liberation. The valves were: standardpaper, polyethylene, extended polyethylene, double trap, and foam [USBM 1986b; Cecala andMuldoon 1986]. By far the most effective valve was the extended polyethylene (Figure 6.6).This is simply a plastic valve approximately two inches longer than the standard paper orpolyethylene valve.In the study, two factors determined the effectiveness of the bag valve: valve length and valvematerial. The longer valves were more effective in reducing the amount of product blowbackand bag-generated dust. However, it is speculated that when the valve length became too muchlonger than the fill nozzle, it began to negatively impact the bag filling performance. For thesecond factor, which is valve material, foam was the most effective material tested. The foamvalve was the shortest valve tested for the evaluation at four inches in length, while the extendedpolyethylene valve was six inches long. The rankings of valve types from the most to leasteffective were as follows: extended polyethylene, foam, standard paper, polyethylene, anddouble trap. Figure 6.7 shows a comparison of the extended polyethylene and the foam valvecompared to the standard paper valve. Respirable dust levels ranged from approximately 45 to65 percent lower with the extended polyethylene as compared to the standard paper valve[USBM 1986b; Cecala and Muldoon 1986].Bagging 163