Typically, the phosphate rock used for manufacturing superphosphate is a blend of multiple types of phosphate rock.
The phosphate rocks are collected from storage by front-end loader and blended volumetrically. Belt conveyors transport the blended rock to the raw rock bins on each of the two grinding mills.
Two Bradley air swept ring roller mills are used to grind the phosphate rock so that typically 80-90% is smaller than 75 micrometres, although this varies depending upon rock type. A stream of air carries the ground rock out of the mill where it is separated in a cyclone while the air is returned to the mill base.
To control humidity a proportion of the air is withdrawn continuously, passed through a bag filter to remove any remaining fine rock dust and discharged to atmosphere. The removal efficiency is 100% for particles larger than 5 microns and typically 95 - 99+% for particles smaller than 5 microns. A continuous monitor is employed to detect any leakage of dust caused by a failure of a bag in the bag filter. Should this occur, the rock grinding process will automatically shut down.
Screw conveyors transport the ground rock from the cyclone and bag filter to the fine rock storage bins.
The activity is totally enclosed within a building and any dust generated is contained within the building.