A major asphalt company that operates several batch and drum plants converted one of their set-ups to a rotary recycle mixer plant. The conversion included screening and weigh bridge, recycle bins, new total plant controls an upgraded dryer shell and a new 3-silo system that included slat conveyors.
After the conversion, the producer was getting used to the new set-up but had an occasional issue of segregation in the material. There was not a specific pattern, it was just highly irregular when the segregation occurred.
Because there has been segregation of material in the past linked to poor silo system control and trickle feeding of material into the silo, the operator assumed the segregation was from the silos. Various technicians examined the situation and verified the batcher was open only long enough to allow the mix to fall out of the batcher and not let material trickle feed from the slat conveyor all the way through the silo without accumulating in mass.
As the problem continued, the producer devised an idea to build inverted cones inside the silo, this solution did not help. After many attempts to resolve the issue, a veteran in the industry was called in to investigate the situation. After being on site about 30 minutes, and not intending to be rude to anyone, the veteran took the general manager and equipment superintendent to the back of the yard. He asked them if they noticed anything different about their stockpiles? They said “no, it’s the same we’ve been using for 20 years.” “We brought the material in, we even brought some in my railcar.”
The facility had limited space which forced them to pile stone very high. The extreme height of these piles would cause roll over into other piles. There were approximately four different materials, but probably 12 to 15 different combinations of materials in some piles. Obviously, when they were using their batch plant, the screens on the tower sorted it all out. This does not happen with the drum plant. One must be very accurate in the sizing of material in each of your cold feed bins. You can’t accidentally put something else in or have the man on the loader contaminate one bin with another size of material. It took a long time and a great deal of trial and error, but the answer was simple – back to fundamentals.
