Adding Chemicals to Water
It is a common practice to “enhance” the dust-suppression performance of water by adding surfactants—surface-acting agents. The addition of these chemicals will improve the wetting characteristics of water, reducing overall water usage and minimizing the drawbacks associated with excessive moisture addition.
If dust from coal, petroleum coke, or a similar material falls onto a puddle of water, the dust particles can, if undisturbed, lie on top of the pool for hours. This phenomenon takes place because these materials are hydrophobic—they do not mix well with water. Since it is not practical to alter the nature of the dust particles to give them greater affinity for water, chemicals are added to alter the water particles, so they attract, or at least join with, the dust particles more readily.
By adding surfactants, the surface tension of water is reduced, allowing dust particles to become wet. Surfactants are substances that, when added to water, improve the water’s ability to wet surfaces and form fine droplets. Surfactants lower the water’s surface tension and overcome the internal attraction between the molecules of water, ultimately resulting in improved droplet formation.
To understand surface tension, imagine a drop of water lying on a smooth, flat surface. It will usually form a liquid bubble with well-defined sides. It is the surface tension of the water that prevents the droplet walls from collapsing. A drop of water that has been mixed with a surfactant—such as dishwashing soap—will not form a liquid bubble, because its surface tension has been drastically reduced. The “walls” of the droplet cannot support the weight of the droplet, because the forces holding the walls together have been altered. This is the reason surfactant technology is applied to dust control. If the water droplets no longer have a surface that is a barrier to contact with the dust fines, then random collisions between droplets of water and dust will result in the wetting and enlargement of the fines to the point they will drop out of suspension in the air.
Choosing a Surfactant
The number of surfactants and surfactant blends currently in use is quite extensive. A number of specialty chemical companies have products formulated to address specific dust-control needs. Choosing the correct product and addition rate for a given application requires material testing as well as an understanding of the process and the method of application.
Objections to chemical-additive-enhanced water suppression systems include the ongoing costs of purchasing chemical additive. Costs can be higher, particularly when considering amortization and depreciation of the equipment. In addition, these systems require regular maintenance, which adds labor expense to the continuing operating costs.
As contamination of the materials, or the process, can be a concern in some industries, the additive chemical must be reviewed in this light. It is important that chemical additives are compatible with the process, with the bulk materials, and with system equipment, including the conveyor belting. Although the use of a surfactant reduces the amount of water added to the dusting material, water/surfactant sprays may still add more water than is acceptable. It is common practice for a chemical supplier to provide samples to the customer for testing the effects on the end product.
Application by Spray or Foam
Once an efficient wetting agent has been selected, the decision must be made whether to apply the material as a wet spray, as discussed above, or as foam. Both systems offer advantages. Generally speaking, the moisture-addition rate of a wet-spray system is higher than that of a foam-generating system. Although the dilution rate is lower for the foam suppression system, the expansion of the foam allows it to provide effective suppression with less moisture added to the materials. Recent developments have improved surfactant technology to the point that some mixtures can be applied as a spray at the lower moisture levels of a foam system while providing good dust suppression. This provides the benefit of limited moisture addition with minimal chemical cost, due to the higher dilution rates with the spray-applied surfactants.
|Maximum Typical Moisture-addition Levels
|Water with Surfactant
|Nominal Rate of Moisture Addition
|Chemical Usage Rate