Why is particle size distribution a critical factor in fly ash disposal for landfilling?

Particle size distribution is a critical factor in fly ash disposal for landfilling because it significantly affects the ash's physical properties, including its permeability, compaction characteristics, and potential for leaching. Fly ash consists of fine particles collected from flue gases during combustion. Permeability refers to how easily liquids can pass through a material; a high permeability allows water to flow through the ash more readily. Ash with a wide range of particle sizes, meaning a mixture of both fine and coarse particles, tends to have lower permeability because the finer particles fill the spaces between the larger particles, reducing the interconnected pore space. Lower permeability reduces the rate at which rainwater can infiltrate the landfill, decreasing the amount of leachate generated. Leachate is the liquid that has percolated through solid waste and contains dissolved and suspended materials, which can contaminate groundwater if not properly managed. Compaction characteristics describe how effectively the ash can be compressed. A well-graded particle size distribution allows for better compaction, resulting in a denser material with higher stability. This minimizes settling within the landfill, reducing the risk of structural problems and maintaining the integrity of landfill covers. Furthermore, the particle size affects the surface area available for chemical reactions. Finer particles have a larger surface area, potentially increasing the rate at which contaminants can leach out of the ash. Controlling the particle size distribution through various techniques like blending or sieving can improve the ash's handling properties and reduce the environmental risks associated with landfill disposal by minimizing leachate generation and enhancing stability.