Efficient conveyor discharge without carryback

Figure 1 - The placement of the cleaners is based on the type of cleaner, material properties, head pulley size, and belt speed.

Figure 2 - Discharge from the belt no longer needs to be a dirty and dusty process.

Figure 3 – The cost of cleanup and improved safety justify the expense of a well-designed discharge solution.

Figure 4 - Innovative cleaner designs require less monitoring and no tensioning.

Figure 5 - The V-plow diverts fugitive carryback to either side of the system, ensuring tail pulley health.

Figure 6 - The direction of the cannon shot matches the stream trajectory to encourage constant flow.

Figure 7 – Potential Dust Generated at discharge zone.

Figure 8 - The roller is encapsulated by spillage at the discharge zone.

Figure 9 - Cleaning the belt of dust and fines with few telltale streaks on the belt denoting carryback.

Dan Marshall, Process Engineer, Martin Engineering

In the harsh and demanding environment of bulk material handling, carryback at the conveyor discharge zone is a common occurrence. Whether in raw form or calcined, cargo clings to the belt and drops along the return, piling under systems, emitting dust, and cluttering walkways. It doesn’t end there. The gritty dust and spillage foul rolling components and gum up oiled machine parts, causing unscheduled downtime for digging the system out, replacing equipment, and readjusting cleaner tension…again. It doesn’t have to be this way.

So, installing a high-quality belt cleaning system solves the problem and achieves efficiency, right? Not necessarily.

Following the installation of modern belt cleaning technology, some bulk handlers realize that the volume of material entering the transfer chute grows exponentially. Rather than piling around the discharge zone, the sudden surge in volume leads to blockages in the transfer chute. This is followed by downtime and labor to unclog it, which can be a safety issue in and of itself.

However, designers can take a holistic approach and engineer an efficient discharge transfer point with components that work together. This approach strives to make equipment last between scheduled closures, improves safety by minimizing maintenance, and addresses the causes of inefficiency. [Fig. 1]

Discharge Zone Inefficiencies
The discharge zone starts at the last troughed idler before the conveyor belt flattens and encounters the head pulley. Cargo falls from the conveyor into a transfer “drop” chute that can lead to several places, including another conveyor, a storage silo/pile, a transport vehicle, etc. The primary cleaner is located after the discharge stream to clear any adhered material caused by the weight or characteristics of the cargo (moisture, cohesion, heat, etc.). A secondary cleaner removes dust and fines from divots and cracks in the belt. Material cleared from the secondary cleaner is generally directed to a sloped surface connected to the transfer chute.

Obvious signs of discharge inefficiency are spillage, carryback, chute clogging, and dust. Alone, each can lead to a workplace safety violation, but together, they result in unscheduled downtime and an increased cost of operation. From an operational standpoint, three of the most expensive consequences are workplace injuries, belt damage from friction, and fouled equipment replacement. [Fig. 2]

Spillage and Safety
Primary cleaners or “scrapers” can fail in several ways, causing adhered coarse aggregate and caked fines to pass by the blade and spill around the discharge area. This fugitive material can build up quickly and encapsulate the belt, fouling rolling components and causing the belt to ride on top of the course pile. This can lead to serious belt damage and increased belt temperatures from friction.

Fugitive material spills into walkways, obstructs maintenance access, and creates a trip and fall hazard. When course grit fouls rollers, it causes them to freeze, leading to friction and high-heat damage to the vulnerable return side of the belt, lowering the equipment’s life. To avoid belt fires and dust explosions, seized idlers/rollers should be maintained and changed immediately, making clear access to the system imperative.

Cleaning spillage can be costly, divert staff from other essential duties, and become a workplace safety issue if workers clear material around a running belt.[Fig. 3] It may seem like a routine job in the beginning, clearing spillage by shoveling it back into the cargo stream or into bins, requires more labor as time goes on and the problem worsens. Clearing material using machinery (front loaders, industrial vacuums, etc.) can result in accidental contact with the stringer or supports, potentially leading to belt mistracking.

Mistracking can be a significant cause of spillage, not just along the belt path but also at the discharge point. The blade is centered on the head pulley, but if the belt is not, adhered material becomes spillage.

Recommendation: Install a belt tracker a distance of 3 to 4 times the width of the belt before the head pulley the head pulley as the trough angle flattens to ensure the belt hits the head pulley in the center.

Over-/under-tensioning and/or extending blade changes for too long can also cause spillage. Over-tensioning causes rapid wear on the belt/splice and lower blade life. Under-tensioning allows the material to pass without being removed. Allowing primary cleaners to wear excessively can result in pull-through, where the force of the belt causes the blade to face the opposite direction and, in some cases, break off.

Recommendation: Enter a service agreement with the blade manufacturer to regularly monitor, tension, and change the blades as needed. Consider installing a modern assembly that allows workers to slide units from the stringer for fast and easy one-person blade changes. There is also the option of innovative cleaner technology with 4x the life of the standard primary blade and needs no tensioning for the life of the blade. [Fig. 4]

Reducing Carryback
Anything that clings onto the return side of the belt and travels with it is considered carryback, which seriously damages a system. Not only is it a significant source of fugitive dust and fines, but it can migrate easily into return rollers and takeup pulleys, fouling the bearings, mechanical drives, and the face of the roller. [Fig. 5] The grit grinds down roller bearings and can lead to excessive friction heat, causing them to misshapen and seize.

Like spillage, carryback can migrate to the non-carrying underside of the belt. These chunks travel all the way to the tail pulley. The intense pressure between the pulley and the belt causes the hard, sharp mass to damage the vulnerable side of the belt and the pulley face, cycling over and over, delivering more damage as it does. Along with lowering the belt's life, dust and fines fill these blemishes and foul the pulley face.

When a roller or pulley face becomes fouled, it is caked with abrasive grit that degrades and damages the belting over time. In some cases, fouling causes slippage, which can disrupt the smooth operation of the belt and promote mistracking.

Recommendation – If there is adequate space, install secondary and tertiary cleaners to ensure the belt is clear on the return. To improve safety, consider units that allow a single worker to pull them away from the stringer for faster external servicing. Consider a diagonal or V-shaped plow placed underneath the loading zone right before the tail pulley that rides on the underside of the belt, removing any loose traveling material. For more effective cleaning and reduced friction damage, consider a plow with torsion arms rather than one held in place by chains. To ensure alignment, install belt trackers or crown rollers along the upper and low belt path.

Safely Addressing Bulk Handling Clogs
A clogged transfer chute or hopper is one of the most dangerous situations in bulk handling. Untrained and uncertified (enclosed chute entry certification) personnel should never enter a clogged chute or bin under any circumstances. A sudden discharge can be deadly as unknown voids engulf and crush a worker. Material adhered vertically to the sides can loosen and send a sheet of debris falling on anyone occupying the vessel.

Buildup points in chutes include:
• Rockboxes – Shelves, even if they’re sloped, can experience buildup.
• Exit gates or doors – As these help control flow, they are also prone to clogging.
• Sloped points – Under the secondary cleaner, chute grades, or located at choke points.
• Metal surface grain – The metal grain of chute plating should match the cargo flow.
• Exposed surfaces – Surfaces where moisture can collect and cause buildup.
• Damaged surfaces – Surfaces with scratching, denting, creasing, or divots.

Misguided practices for addressing buildup are banging on the sides of the hopper with a mallet or loosening the obstruction by poking at it from below. In some operations, clogs are so frequent that spots for pounding are marked, and mallets are left in the area for convenience. This is hazardous because it reduces the vessel's structure or chute, causing it to buckle. Ripple damage from pounding can make it easier for material to build, shortening periods between clogs and leading to more unscheduled downtime. Poking from below is even more dangerous since a sudden discharge sends tons of material in a surge that injures anyone in the vicinity and breaks equipment below.

Recommendation – Air cannons strategically installed around the chute have nozzles pointed toward the material flow. Powerful shots of air are distributed across the surface inside the vessel, dislodging material and preventing buildup. The air cannons are supported by vibration units that ensure gates and narrow spouts on hoppers and chutes retain proper flow before bridging starts. In many cases, vibration alone can handle most dry material flow, but changes in humidity that raise the stickiness of cargo and chute surfaces, along with fluctuations in production volumes, are much better handled by air cannons. [Fig. 6]

Discharge Dust
Emissions at the discharge zone can be found billowing out of the chute against the direction of the cargo stream or exiting the sides and bottom as it loosens from the belt’s return side. Dust has become a highly regulated workplace and environmental concern, which leads to stiff fines and potential forced downtime if high volumes of respirable crystalline silica (RCS) are detected. RCS is found in nearly every substance pulled from the earth but is prevalent in limestone, coal, clay, etc. Regulators measure fugitive particulate matter (PM) at the size of <10 microns mass (μm) in volumes of >50 micrograms (μg) per cubic meter (m3) over an 8-hour time weighted average (TWA). This is the volume and size determined to cause serious chronic lung issues in workers and it doesn’t just apply to RCS, it is any PM.

Dust emissions returning back from the chute can be from uncontrolled airflow at the exit point. The emissions can also be caused by hitting rock boxes meant to slow the flow of material or an unobstructed impact causing turbulence.

Dust from carryback permeates the area and spreads emissions down the entire length of the belt return. If the belt reaches into a tower or is exposed to the outdoors, this can cause dust to be carried long distances on air currents into nearby communities leading to possible violations. Studies have shown that dust can be controlled by adequate cleaning at the discharge using Levels 1-3. One is a primary cleaner, two is a secondary cleaner, and level three is a tertiary cleaner. [Fig. 7]

Recommendation - By reconfiguring the chute’s exit into a sloping scoop, the material can be slowed and loaded onto the next belt in a controlled and centered manner with less turbulence. Air cannons installed along the chute are pointed with the material stream and can help direct airflow.

Case Study – Carryback in Ukrainian Cement Plant
The ArcelorMittal steel plant in Fos-sur-Mer, France, experienced issues with carryback on its raw iron ore conveyor, which leads from the loading dock to the plant. The conveyor is 1800mm (70 in.) wide with a belt speed of 4.5m/s. It runs continuously as cargo ships offload. Material adhered to the belt and easily passed under the primary cleaner, dropping along the return belt path, resulting in significant product loss. Crews cleaned walkways and around the discharge zone, raising operational costs. Workers cleaning near a moving conveyor also posed a potential workplace hazard. To reduce the volume of carryback and the number of man-hours spent cleaning, managers decided to seek a more heavy-duty solution for belt cleaning. [Fig. 8]

Technicians from Martin Engineering France examined the system and recommended a CleanScrape® Primary Cleaner HD (heavy-duty) and a CleanScrape® Secondary Cleaner. Installed at an angle across the discharge pulley, the primary cleaner is designed for the harshest bulk handling environments. It is equipped with tungsten carbide tips while remaining safe for use on mechanical splices by applying minimal pressure to the belt. After initial tests and readjustments, it requires no re-tensioning and lasts up to four times longer than standard polyurethane blades. The CleanScrape Secondary Cleaner features an all-stainless steel assembly and six-inch wide blades with carbide tips. The unit eliminates any remaining fines on the belt to reduce dust. [Fig. 9]

Following installation, the results were immediately clear as the CleanScrape primary and secondary cleaner system eliminated the vast majority of adhered cargo, leaving the return side of the belt free of carryback. The cleaning schedule was reduced to only routine sessions as needed. Operators report that the solution has drastically lowered product loss due to carryback. With less labor required for cleaning, operational costs have decreased, further improving the return on investment (ROI) of the equipment. Managers are pleased with the outcome and continue to collaborate with Martin Engineering on solutions in other parts of the operation.

Conclusion
Modern cement plants are changing and growing every day because the demand for raw and processed materials for construction and manufacturing keeps rising. Production increases can change throughput volumes and belt speeds, which directly affect spillage, carryback, clogging, and dust.

Retroactively installing equipment that improves both safety and efficiency should be a priority of any operator. Although the initial capital investment might be slightly higher, the return on investment (ROI) and benefits are not just in fewer injuries, but reduced labor costs for maintenance, less equipment replacements, greater compliance and an overall lower cost of operation.