Factors affecting emissions that are common to most operations include moisture content in the rock, type of rock processed, type of equipment , and operating practices employed. These factors apply to both process (primary) sources and fugitive (secondary) sources in quarry and plant operations.
Depending on geographic and climatic conditions, the inherent moisture content or wetness of quarried rock may range from nearly zero percent to several percent. The effect of moisture content is especially important during quarrying, material handling, and initial plant operations such as primary crushing.
Surface wetness causes fine particles to agglomerate or adhere to the faces of larger stones, resulting in a dust suppression effect. However, as new fine particles are created by crushing and attrition and since moisture content is reduced by evaporation, this suppressive effect diminishes in later stages.
The type of rock processed is also important. Soft rock produces a higher percentage of fines than do hard rocks because they are more friable. Therefore, processing of soft rocks has greater potential for emissions.
The type of equipment and operating practices employed also affect uncontrolled emissions. Equipment selection is based on a variety of parameters, including quarry characteristics, rock type processed, and desired end products. Emissions from process equipment such as crushers , screens and conveyors are generally a function of the size distribution of the material, and the amount of mechanically induced velocity imparted to it.