When dust becomes a recurring issue or valuable cargo is being lost during the handling process, the question is not simply how to contain it. The more important question is: where is it coming from in the first place?
For terminals handling materials such as fertilizers, phosphates, rock phosphate, iron and alumina, there is rarely one single cause. Dust and spillage are influenced by the characteristics of the material, the way it is handled and the equipment used throughout the process. The consequences can range from additional cleaning and material loss to stricter environmental requirements and pressure to reduce emissions.
For cargo owners, the impact is more direct: material lost during handling is product that never reaches its intended destination. And when that cargo has already gone through a long and costly production process, even relatively small losses can have a commercial impact.
Understanding where dust and material loss originate, and how the different factors interact, is therefore the first step toward reducing them. In this blog, we look at the main causes, how to recognize where the problem occurs and what role the grab can play in improving the handling process.
What causes dust and material loss during dry bulk handling?
Dust forms when fine particles become airborne. This can happen at different points in the handling cycle, including when cargo is discharged from a grab into a hopper. As the material falls, particles collide with each other and the receiving surface. Brittle pieces can break into smaller particles, while the falling stream displaces air. This airflow carries fine particles into the surrounding environment, where wind can spread them even further.
The amount of dust depends on several factors, including particle size, moisture content, falling speed and wind conditions. Both fine, low-density powders and denser materials can generate significant dust when they are dry.
A useful starting point is to look at when and where the dust or spillage occurs. Does it start when the cargo is exposed? Does material escape after the grab is lifted? Or is most of the dust created when the material is discharged into the hopper? Identifying that moment helps narrow down the likely causes.
1. Material characteristics
Not all bulk materials behave in the same way. Fine, dry powders are particularly prone to becoming airborne, but dry, high-density materials can also generate dust during handling, especially when brittle pieces break apart on impact. Flow behavior also plays a role, influencing how the material fills the grab and how it behaves when the grab opens. As a result, a handling setup that works well for one material may perform very differently with another.
2. Overfilling the grab
The way the grab fills and closes can directly affect how well the material remains contained. Filling the grab with as much material as possible may appear to increase productivity. But if material becomes trapped between the closing edges, the grab may not close properly. This creates openings through which fine material can escape once the grab is lifted.
The grab therefore needs to be matched to both the material and the application. The aim is to achieve the required capacity while still allowing the grab to close properly and keep the cargo contained.
3. Hoisting and traveling
Once the grab is lifted, movement can expose problems that were not visible while filling. If the material is not fully contained, fine particles can escape during hoisting and traveling. Wind can then carry this material further away from the handling area.
The timing of the problem can provide a useful clue. If material starts escaping immediately after lifting, it is worth looking closely at filling, closing and sealing. This can help determine whether the issue is related to the grab itself, its configuration or the way it is being operated.
4. Drop height during unloading
The discharge into the hopper is another important part of the handling cycle. When material falls over a greater distance, fine particles have more opportunity to spread and become airborne.
Keeping the grab as low as practically possible before opening can help reduce this effect. A lower drop height reduces the impact between particles and the receiving surface, while also limiting the amount of air displaced by the falling material. Releasing the cargo in a controlled way can further reduce dust formation. Where available, dust extraction around the hopper can provide an additional measure.
5. Wind and operating conditions
The surrounding conditions can have a strong influence on how dust develops. When a ship's cargo hold is opened, wind can already carry dust away from exposed, dry material. Once handling starts, the airflow generated by falling cargo can add to this effect. The speed of discharge, the way the grab is filled and the height at which it opens all influence how much dust enters the air.
The role of the grab in reducing dust and material loss
Not every factor in the handling process can be controlled. You cannot change the wind, and the characteristics of the cargo are often a given. The grab, however, can be selected and configured specifically for the material and application.
For dusty bulk materials, we recommend using a fully closing clamshell grab. Its closing principle makes it possible to contain the material between two shells throughout the handling cycle. The exact configuration can then be adapted to the characteristics of the material and the requirements of the operation.
When water mist is not always an option
Water mist is one way to suppress dust during bulk handling, but it is not suitable for every material. Some cargoes must remain dry because additional moisture can degrade the product or interfere with the process in which it will be used.
Biomass is a clear example where keeping the cargo dry is essential. Wood pellets can become unusable as fuel when exposed to water, while excess moisture in wood chips can contribute to self-heating. This process generates heat through oxidation and can eventually lead to spontaneous combustion.
This means that water-based dust suppression cannot always be relied upon. Where adding moisture is not an option, limiting dust at its source becomes particularly important. Appropriate grab selection, effective closing and controlled discharge can all contribute to reducing dust and material loss without introducing additional moisture into the cargo.
Why clamshell grabs are well suited to fine bulk materials
Containment is particularly important when handling materials that can escape through small openings. A clamshell grab encloses the cargo between its shells, helping prevent material from escaping during the transfer from vessel to hopper. Its effectiveness depends on the grab filling correctly and the shells closing completely. To achieve this, the grab must be configured for the specific material and application.
Matching the clamshell grab to the material
The right configuration starts with understanding what is being handled. Relevant factors include:
- Particle size
- Moisture content
- Bulk density
- Flow behavior
- Required grab capacity
- Crane configuration
- Lifting capacity
These factors determine how the grab should be sized and configured to handle the material effectively. For fine or dust-sensitive materials, containment should be considered alongside capacity and productivity when selecting a grab.
Closing and sealing for better material containment
A fully closing clamshell provides the basis for good containment, but the details of the closing system also matter. If material becomes trapped between the closing edges, the shells may not close completely. Fine particles can then escape through the remaining openings during hoisting and traveling. The design of the closing edges can help limit this risk. Depending on the material and application, knife-edge sealing and additional rubber sealing can further reduce leakage between the shells.
Reducing dust starts with identifying the cause
As this blog shows, dust and material loss can come from several points in the handling process. Material characteristics, overfilling, sealing, wind and discharge conditions can all play a role. The most effective approach is to identify where the problem occurs and determine which factors can be controlled.
The right grab configuration is an important part of that assessment, particularly when the cargo must remain dry. Nemag's experts in dry bulk handling can help you evaluate the material, crane and grab configuration to identify where improvements may be possible.
Want to explore clamshell configurations first?
