Figure 1 shows the flow sheet of an artificial lightweight aggregate manufacturing plant. This plant is comprised of silos, feeders, conveyors, a mixer, a pan-type pelletizer, a sinterstrand, a dust collector, and fans.

Figure 1: Flow sheet of artificial lightweight aggregate manufacturing process.

[Manufacturing Process]

The fly ash extracted from the silo in a given volume is fed to the mixer. If the unburnt coal content in the fly ash is low, a small amount of pulverized coal is added. The fly ash mixed with a proper volume of water by the mixer is fed to the pan-type pelletizer and pelletized into 5-15 mm green pellets.

The green pellets are fed to the sinterstrand and indurated. The characteristics of the pellets are controlled by promoting sintering reaction at the subsequent sintering zone. After sintering, the pellets are slowly cooled by air, and then are screened into 5-10 mm and 10-15 mm piece sizes for use as commercial pellets.

[Features of the Process and Plant]

- A large volume of fly ash can be processed.

- For sintering, heat generated by combustion of unburnt coal remaining in green 
  pellets is used.  Compared to the external heating system, the fuel consumption 
  is small.

- High-temperature exhaust gas generated at the sintering zone is recycled to 
  save energy.

- The adoption of a closed system in which all the generated dust is recovered 
  and used as part of the material has eliminated the need for dust disposal.

- This process can be applied for the treatment of other wastes.

[Product Properties]

Concerning the properties which artificial lightweight aggregate must comprise, the Japanese Industrial Standard (A5002, Lightweight Aggregate for Structural Concrete) specifies, for instance, specific gravity in absolute dry condition, and particle size distribution. Table 1 shows the product properties obtained in the plant, all of which satisfy the specified requirements.

Table 1: Properties of artificial lightweight aggregate

The artificial lightweight aggregate manufactured at the plant was approved by the Ministry of Construction in 1985 and has been put on the market. Presently, about 45,000 tons of commercial aggregate are manufactured and marketed yearly, and applied for the construction of various buildings.