Minimizing Air Velocity & Enhancing Material Cohesiveness
Airborne dust is a common byproduct whenever dry materials are moved, manipulated, or exposed to strong air currents that can lift or redirect small particles. This often occurs at conveyor transfer points during loading, unloading, or material transit, where air currents carry dust away from the handling system. To mitigate dust emissions during conveying, several measures can be employed: engineered transfer systems, effective sealing, dust-suppression systems, and efficient dust-collection systems.
The primary goal in dust control is to minimize dust creation. While complete elimination is challenging, optimizing system design and production techniques can significantly reduce dust generation. For example, designing conveyor systems with minimal material-drop distances helps lessen the impact energy at drop points, thereby reducing dust dispersal.
These engineering improvements can be implemented during system retrofitting or at the initial plant-design phase.
Techniques include:
- Shortening drops between conveyors
- Loading material in the same direction as the receiving belt is moving
- Avoiding drastic changes in material trajectory
Achieving these methods involves thoughtful conveyor layout and innovative transfer design. For instance, using an engineered chute with a "hood and spoon" design can significantly aid in reducing dust. Other engineering enhancements are also available to effectively decrease airborne dust creation and release. Successful implementation of these improvements can potentially eliminate the need for, or substantially reduce the scale and costs associated with, dust-collection and suppression systems.
Minimizing Air Velocity
The most straightforward and effective approach to dust control involves minimizing air velocity. Dust particles, being heavier than air, settle out if conditions are calm and given enough time. By reducing air movement, particles can settle back into the material stream. Since dust travels with air currents, managing airflow logically leads to dust management.
One of the earliest and simplest dust-control methods is to enclose either the airborne dust or the location where dust is generated. This enclosure allows dust particles to settle before they can be carried out of the area by air currents. Increasing the volume of the enclosure decreases air velocity, facilitating the settling of airborne particles.
An engineered transfer chute is another effective tool for reducing air velocity. It achieves this by minimizing the intake of surrounding air at the transfer point and sealing leaks that might allow dust-laden air to escape, thus giving dust particles time to settle. Traditional steel chutework is commonly used for this purpose due to its rigidity and ability to completely enclose transfer points. Even simple measures, like installing dust curtains at the chute's exit, can significantly slow down air movement.
While effective enclosures can theoretically be applied to any transfer point, some operational constraints may prevent their use. For instance, in mobile operations such as sand and gravel handling, fixed, completely enclosed transfer chutes are impractical. In other cases, visual monitoring requirements may also necessitate non-enclosed equipment.
Increasing Particle Size
Dust collection, involving vacuum extraction of air and dust from material-handling systems, is another method to manage airborne dust. Dust is collected on filters or centrally, or it can be redeposited on belts using local collectors. However, these systems require enclosed transfer points and considerable overhead space. While effective in capturing airborne dust, they do not prevent dust creation during material handling, necessitating ongoing management.
Dust-suppression systems leverage this principle by augmenting dust particle weight through water or a water-chemical mixture. This process involves combining airborne dust with water droplets, increasing their mass so they settle back into the main material stream rather than dispersing into the atmosphere.
Increasing the size of dust particles can also aid dust control by making them heavier and more prone to settling out of airflow. This method reduces dust dispersal and minimizes its re-entrainment into the atmosphere.
Increasing Material Cohesiveness
Enhancing material cohesiveness is another effective strategy. By modifying material properties to encourage particles to stick together, dust generation can be reduced. For instance, adding water or a binding agent can increase material cohesiveness, preventing dust release during handling and reducing airborne dust.
However, applying moisture effectively is critical. Surface wetting alone may not suffice, as dry surfaces exposed during material handling can still release dust. Ideally, moisture should be applied when the material is in free fall, allowing it to penetrate deeper and cover more surface area.
While using water for dust suppression has residual benefits, such as maintaining cohesive properties and reducing dust generation, it comes with challenges. Applying sufficient water can be resource-intensive and may lead to operational issues like screen blinding, chute blockages, and reduced equipment efficiency.
Moreover, moisture can affect materials that require heating or burning, potentially incurring thermal penalties that outweigh the benefits of dust suppression. Some materials, such as cement, are incompatible with water exposure, necessitating careful consideration before implementing suppression systems.
To minimize water usage while effectively suppressing dust, surfactants can be added to enhance water's ability to wet material surfaces. However, this approach increases operational costs.
In Closing...
Effective dust control requires a thorough understanding of materials and processes. By implementing appropriate strategies—from minimizing air velocity and enhancing particle size to improving material cohesiveness—dust emissions can be significantly reduced or managed, ensuring operational efficiency and product quality without compromising safety or environmental standards.