After being crushed to an optimum size for processing, the raw slate is fed into Stalite’s patented pre-heater. The pre-heater conditions the rock before entering the rotary kiln. The pre-heater slowly heats the rock as it comes down to the kiln. The kiln is fueled by coal that is pulverized and blown into the kiln. In the kiln, the slate is heated to approximately 2200 degrees Fahrenheit. As the slate approaches this temperature it becomes almost molten, like lava. In this molten state, the rock is plastic enough to allow gases to evolve, forming small, unconnected voids uniformly throughout its mass. The expanded rock then falls to the cooler, where it is ari-cooled. When the expanded slate is cooled, the cells remain allowing for its low unit weight.
Expanded slate aggregate is produced by the rotary kiln method. This discussion describes one specific lightweight aggregate manufacturing plant. Other rotary kiln process facilities are similar, but may have variations from the process described herein.
The rotary kiln is a long tube that rotates on large bearings. The kiln is lined with insulation and refractory materials. Raw slate is fed from the storage silos into patented pre-heaters that allow the rock to heat up at a moderate rate. It then enters the upper end of the kiln where it slowly revolves and moves toward the “burn zone” near the lower end of the kiln. The “burn zone” reaches temperatures in excess of 2200 degrees F (1200 degrees C)
Stalite uses high BTU, low sulfur coal for its heat source. Some lightweight aggregate production facilities use natural gas, while others supplement these traditional energy sources by burning hazardous wastes. In the “burn zone” of the rotary kiln, the slate becomes sufficiently plastic to allow expanding gases to form masses of small, unconnected cells. As the expanded slate cools, these cells remain, giving the aggregate its low unit weight and low absorption. The expanded material, called clinker at this point, leaves the lower end of the kiln and enters a forced-air cooling system. This cooling process reduces the chance that the aggregate will crystallize as can happen in water-cooled systems where the very hot expanded material is dropped directly into a pit filled with cold water.
From the cooler, the clinker is conveyed to a classification area. Here the material is crushed and screened to various size fractions. These different size fractions are kept separate until, by means of an automatically controlled blending system, specified gradations are produced for various applications. After blending, actual moisture content is automatically adjusted to a predetermined level. The expanded slate aggregate is then tested for proper gradation, moisture content, specific gravity and unit weight. After testing is completed, the expanded slate aggregate is stored or conveyed directly to trucks or railcars for shipment. Two procedures are used to minimize segregation during storage: 1) Coarse grades are stored in low-elevation stockpiles that feature a moisture control system 2) Fine grades are stored in low-height silos that feature a perimeter port feeding design to minimize segregation. Prior to loading, trucks and railcars must be inspected for cleanliness and washed when needed. A particular procedure is carefully followed when loading material from either the storage silos or the stockpiles to ensure that material consistency is maintained. A rigid quality control and testing program confirms compliance with the customer's specified needs.
Environmentally Conscious – Stalite uses high BTU, low sulfur coal for its heat source. Fly ash from burning the coal is collected in high efficiency bag houses supplied by the StaClean Corporation. Stack gases are monitored by a state-of-the-art Controlled Emissions Monitoring (CEM) system.