The first subsystem was designed to provide passive dust control based on the existing Benetech^® “MaxZone^®” product line, comprising an enclosure system for settling dust and containment of fugitive dust emissions. The system includes baffles, shrouding, and a unique interface that seals onto the moving belt. Ease of maintenance and elimination of confined space entry requirements are integral to the system.

At their facilities in Chicago, IL, USA, Benetech^® designed and fabricated a dust containment system tailored to the cone crushers and conveyor system at the CPM field site. The system is based on their existing technology in which dust containment is achieved by modular sections of shrouding mounted onto the conveyor frame, with internal baffles that contain the dust inside the enclosed space, and external access panels for ease of inspection. The design incorporates an interface between the shrouds and the belt to prevent escape of dust, which includes an internal liner that can be externally adjusted and serviced. This thereby eliminates confined space entry requirements and enables the ability to achieve fine clearance adjustments and alignment. The interface also includes a dual rubber seal that is adjustable with quick-adjust clamps, ensuring that it rides smoothly on the belt, to prevent spillage and escape of fugitive dust (Fig. 1).

The second subsystem was designed to augment the passive dust settling by active collection and filtration of any residual airborne dust inside the shroud enclosure before the conveyed material exits the enclosed system.

At their facilities in Spokane, NIOSH designed and tested various portable dust filtration prototypes that would be appropriate for inclusion in the hybrid system [15]. The work included laboratory evaluation of pleated filter media, combined with a pulse-jet cleaning akin to a system described previously [16]. The design of the active dust control system includes a compressed air tank and air delivery system located within the filter housing that allows air-jet blast cleaning of the clogged filters (Fig. 2). The system was designed to incorporate off-the-shelf parts whenever possible to make the end product more commercially viable. The prototype has been dubbed the “Dustinator.”

Design trade-offs included minimizing size and weight while maximizing flow rate and thus filtering capacity. The final prototype was therefore limited to a three-horsepower blower mounted atop the unit (Model Compact GI-4V-126, NY Blower Co., Willowbrook, IL, USA). The blower has a maximum flow rate of 1200 ft³/min and can maintain 1000 ft3/min when drawing air through four seasoned filters. To provide an adequate filter surface area to maintain low face velocity, the system was designed with four 8-inch diameter filters that are 21-inches long.

The protype is approximately 2′ × 2′ × 3′ in size and consists of an upper and lower section. The blower evacuates the upper section which in turn draws air up from the lower section through the four MERV-rated pleated filter cartridges. Filter cleaning is achieved by pneumatic jet-pulse valves mounted directly above the filters, actuated by a Pentair/Goyen ECS controller (Goyen Controls LTD, NSW, Australia) that monitors differential pressure between the upper and lower sections and activates jet pulses when the differential reaches a preset threshold. In a previous study, tests were conducted to optimize the size and outlet geometry of the valves to maximize the potential for complete filter cleaning while minimizing compressed air usage [15]. Through testing, it was determined that optimum parameters for this system were a source pressure of at least 60 psi, pulse time of 150 ms, and valves with a 1″ exit port.

The third subsystem includes a surfactant spray bar placed at the exit that was designed to quell any residual dust that might exit the enclosed system. This was achieved by synchronizing the spray of surfactant to the pulse-jet cleaning of subsystem 2. Synchronizing the spray with the pulse-jet cleaning ensures that the surfactant and associated water are only consumed when needed due to the dust generated by the pulse-jet cleaning which would otherwise be contained by subsystem 2.

To achieve this, Benetech^® engineers devised a modified version of their current portable dust suppression system, called the “Mini-Pak,” that mixes a proprietary surfactant with water to feed a spray-bar suppression system (Fig. 3). The Mini-Pak contains a surfactant-metering pump that is driven by the flow in the water supply line. For this application, the Mini-Pak was modified by adding a solenoid valve downstream from the metering pump to affect the previously mentioned synchronization. Additionally, due to the cold climate at the CPM field site, the system was insulated and fitted with a heater to prevent freezing of the pump and surfactant tank.

Before installation of the combined subsystems into a hybrid system at the field site, the prototype active filtering system and the spray suppression system were tested using a NIOSH-conceived test setup described previously [15]. For the current work, the prototype “Dustinator” (Fig. 2) was mounted onto the test bed, and the dust generator was adjusted to emulate the conditions expected inside an enclosed transfer point by mixing rock dust into a large chamber (Fig. 4) at concentrations of approximately 300 mg/m³. The modified Mini-Pak was mounted on a rack nearby, and the activation of the spray surfactant system was integrated into the jet-pulse controller. The Mini-Pak system was designed to deliver spray surfactant during each jet-pulse sequence to settle the potential dust cloud created by the pulsing.

Testing of the active filtration system focused on two things: verifying that the filter surface area was adequate for the flow rates expected from the chosen blower, and effectiveness of filter cleaning. To evaluate the match between the flow rate and filter surface area, the differential pressure across the filters was measured continually using a Dwyer Magnehelic gage while dust was injected into the test chamber during operation. Hubercarb Q100 Mine Safety Dust (JM Huber Corporation) was used for this purpose and the concentration monitored using a calibrated nephelometer (pDR-1500, Thermo Fisher Scientific, Waltham, MA, USA) to ensure that the dust generation remained approximately constant. As dust built up on the filters, the differential pressure increased until it reached a preset threshold, at which time a series of jet pulses cleaned the filters. During testing, the flow rate was monitored to ensure adequate flow as the dust cake built up on the filters. The initial flow rate for this system was approximately 1000 ft³/min, and the desired lowest flow rate (when filters require cleaning) was targeted as 600 ft³/min. Results showed that for dust concentrations of approximately 300 mg/m³, this resulted in differential pressures in the range of approximately 2″ to 4″ (inches of water column, IWC) and cycle times on the order of 35 min (Fig. 5). The cycle time indicates how long it takes for enough dust to build up on filters that the pressure differential reaches the upper threshold, and the jet pulses are triggered.

The effectiveness of the jet-pulse cleaning system was evaluated by observing the sudden drop in differential pressure across the filters when the pulses were initiated. This can be seen in Fig. 5. At the run time of 33 min, the differential pressure reached the pre-set upper threshold, and pulsing was initiated at each filter in turn, until the lower threshold was reached, which typically took 3–5 pulses. Optimization of the system was achieved by monitoring the air flow and differential pressure and adjusting the controller to provide air pulses to clean the filters at appropriate set points. During testing, particulate concentrations were measured before and after the filters, and (mass based) filtration efficiencies of approximately 99% were calculated from nephelometer data for all tests. The validity of this approach is supported by the fact that the mineral content and small size range (mainly respirable sized particles) are similar to the “Arizona road dust” that was used to calibrate the nephelometer [17].

Once the subsystems were individually tested, they were installed at the CPM field site. The aim of the field deployment was to demonstrate and evaluate the effectiveness of the hybrid system in real on-site conditions. Installation of the hybrid system was customized to fit the geometry of and reduce dust generated by two cone crushers and a horizontal conveyor.

At the site, the #15 horizontal conveyor carries material from #1 and #2 cone crushers (Nordberg HP300 crushers) and dumps the material onto the #13 conveyor to return material to the shaker deck (Fig. 6). The purpose of the installation was to enclose the #15 conveyor, the crusher chutes feeding it, and the transfer point between the #15 and #13 conveyors.

The installation included retrofitting the crusher and conveyor system in four steps:

Retrofitting the #15 conveyor with a passive dust control system based on the MaxZone^® technology (shown in Fig. 1)

Installing the passive dust control system entailed retrofitting shrouding onto the #15 conveyor (as in Fig. 1). The shrouding was mounted to the conveyor frame in short sections with baffling placed strategically between the sections. The baffles are made of wear-resistant rubber and hang downward to the expected level of the material on the belt. The sealing system includes an internal liner that can be adjusted and serviced externally (eliminating confined space entry requirements) and enables the ability to accomplish fine clearance adjustments and alignment. To prevent dust from escaping between the shrouding and the moving belt, a dual-rubber seal runs the length of the shrouding. The seal is adjustable with quick-adjust clamps, so that it rides smoothly on the belt, preventing both spillage and escape of fugitive dust.

While adjustment of the sealing interface against the belt proved to be extremely effective, special attention was required to seal the areas between the crusher chutes and the newly installed shrouding. To seal those areas, a soft low-durometer rubber was used and attached to the conveyor framing to prevent vibration of the chutes from being transmitted to the shrouding and conveyor frame.

To prevent dust leakage around the transfer point, a system of shrouding was custom built to enclose the entire transfer point, with removable access panels for inspection and maintenance (Fig. 7). The enclosure was attached to the frame of the #13 conveyor and rubber flaps were positioned to cover gaps as needed. Prevention of leakage around the transfer point was improved by the nearby dust collector, which created a slight vacuum inside the transfer point, reducing the escape of fugitive dust. To prevent the escape of dust from the exit, where conveyor #13 carries material to the shaker deck, a 3-nozzle spray bar was installed inside the shrouding near the exit, with a standoff of approximately 16″ from the belt, and a rubber flap was hung to cover the exit and ride on top of the conveyed material. The spray nozzles were fan-type nozzles with an angular pattern that provided slight overlap of the applied spray. The individual nozzle flow rate was chosen as 0.5 gal/min which proved adequate to wet the material while minimizing water and surfactant usage.

Once the passive system was installed, the Dustinator was mounted downstream of the second crusher, using a customized flange-mount built into the MaxZone^® conveyor shrouding. The location of the dust collector (Fig. 7) was strategically chosen to remove unsettled dust from within the shrouding downstream of the crushers, and to draw air and dust from within the transfer point enclosure to reduce leakage from the transfer point.

Power for the blower, air compressor, and Goyen controller were pulled from a nearby junction box. The system was wired so that when the conveyor is running, the blower, air compressor, and controller are also activated. The compressor tank stores enough air for multiple pulses and refills the internal Dustinator tank to 80 psi within 15 s after each pulse. The controller monitors the differential pressure across the filters, and when it reaches the upper threshold, it initiates a pulsing sequence until the pressure drops to a lower threshold.

During normal operation, the passive and active systems work together to control dust in the system, but when the jets pulse to clean the filters a burst of pressure and dust is created, challenging the system. To quell that burst, during each pulsing sequence the controller sends power to the water solenoid to activate the spray suppression system.

The final step of the field installation was to incorporate the spray suppression system. The modified Mini-Pak (Fig. 3) was placed inside a steel cabinet mounted on the crusher frame near the Dustinator. The inlet was supplied with 80-psi water pressure, and the outlet was connected by hose to the spray bar system. The water solenoid was wired to the Goyen controller, and a separate power line was provided for the cabinet heater.

To evaluate for reduction of fugitive emissions and RCS achieved by the hybrid system, NIOSH researchers conducted air sampling around the crushers before, during, and after the installation of the hybrid dust control systems. Air sampling was conducted one day before the installation, one day when the “passive portion” of the system had been installed, and a third day after the complete hybrid dust control system was installed and functioning.

Three days of air sampling were conducted using 14 samplers each day for a total of 42 area air samples. The air sampling was designed to investigate differences in concentrations of area airborne respirable dust concentrations in all directions around the conveyor transfer point. Samplers were positioned approximately 5–10 ft from the primary point sources of dust generation (i.e., rock crushers and conveyors) and 5–10 ft apart and at a height of approximately 5 ft above the ground, as shown in Fig. 8. The precision of placing the air samplers in the exact location was ensured by referring to a sampler map drawn over an aerial satellite view.

Area air samples were collected according to NIOSH Method 600 [18]. Additionally, the airborne respirable quartz concentrations were analyzed according to NIOSH 7500 by an American Industrial Hygiene Association accredited laboratory. Specifically, personal air sampling pumps (SKC AirChek TOUCH, SKC In) calibrated to 1.7 L per minute flow rate were used with 10-mm (mm) Dorr Oliver size selective nylon cyclones to collect the respirable fraction of the area airborne dust samples. The Dorr Oliver cyclones used 3-piece cassettes containing 37-mm diameter 5-μm pore size GLA-5000 PVC filters. Sample filters were pre- and post-weighed following the NIOSH Method 600 to yield the mass of dust on each filter sample. The dust mass, pump flow rate, and collection time determined each sample’s time-weighted average (TWA) respirable dust concentration. While the particle concentration and deposition rate varied, the TWA approach captures the average deposition over the time span of sampling period and is considered a standard approach when taking dust samples. The sampling times varied slightly for each day, depending on the duty cycle of the crusher, but all samples were collected for at least 2 h while the crushers were run without interruption, specifically the sampling duration ranged between 120 and 406 min. The sampling duration was dictated by the period of time for which the crusher was operated on a given sampling day.

All area respirable dust samples were above the limit of detection (LOD) of the NIOSH Method 600, except for one collected on the final post-install sampling day. This sample was thus estimated using a method of estimation commonly used for nondetectable values. Specifically, since the data possessed a moderate skewness, with a geometric standard deviation of 2 or less for each sampling day, the analytical LOD divided by √2 was used to estimate this below detect value [19]. The minimum detectable concentration for the one nondetectable sample was 49 μg/m³.

Due to the impact of wind variations and weather on air samples, measurements were taken on days with similar prevailing wind direction and meteorological conditions (Table 1).