The process of feeding material into rotary kilns is one of the key points in stabilizing the process and optimizing costs. The systems available for feeding raw meal have, in the past, proven to have several disadvantages, such as high electricity consumption for material transport, infiltration of air into the heat exchanger, and inaccurate measuring. With the development of the rotor weighfeeder, which was first used to feed furners with coal dust and subsequently for feeding cement mills, the possibility arose for them for feeding raw meal into rotary furnaces (Photo 1).

Until 1990, Industria Nacional de Cemento S.A. (INCSA) in Cartago, Costa Rica, used two feed systems with regulated drives for the two existing heat exchangers. The material was transported from the two storage bins for raw meal into two preliminary containers with a stabilized level, and then finally guided into the furnace via a weighing belt and pneumatic conveyor.

Because this was an older system, no provision was made at the time for carrying out a weight check. The only possible method of checking was a weight comparison using calibrated chains. In addition to the problems already mentioned, the amount of dust produced was very high, due to the open belt system.

As for the electricity consumption, the average value for each dosage supply route was 65 kW. In 1990, a new system was installed, although the existing system was kept operational. A new transport system was installed for the job, with a bucket conveyor and a nuclear throughput measuring device which was built into the screw conveyor. INCSA noted that a considerable improvement was achieved in the electricity consumption and the amount of air infiltrating into the heat exchanger was decreased.

The silo used was divided into two separate volumes, and accordingly had two outlets, so that material flows could be discharged separately. Problems arose with stabilization of the material flow along the conveyor systems, which in turn had a negative effect on the flow measurement. Caking of the material along the walls of the screw conveyor caused major difficulties in taring of the nuclear weighing machine and consequently with the reliability of the display (the dosage).

In 1997, the nuclear weighing system was replaced with a Pfister rotor weighfeeder (Figure 1) operating at a nominal feed rate of 75 tph.

The present system transports raw meal on air conveyor chutes from the storage bin to the bucket conveyor, which in turn transports it to the existing dosing feed silo. Two separate outlets from this dosing silo feed the corresponding dosing devices along air conveyor chutes. Feeding of the dosing devices is kept at a constant level by controlled dosing rollers. For safety reasons, two closing rollers also were installed, which are monitored by the Pfister control family.

The filling level in the dosing devices is generally maintained at 90%, which maintains a high and constant raw meal dosage flow. Any deviations are already regulated by the individual control system before the material is fed into the process. For this reason, the system is able to correct feed changes in advance. An air conveyor chute was installed downstream from the dosing device.

Pfister fitted internal flow-out systems for the event of material blockages occurring. To guarantee that the system functions properly, it is important to ensure that the raw meal is free of foreign particles (scraps of cloth, screws, metal parts, etc.) which may cause blockages. A wide-meshed sieve at the entrance to the silo was an adequate solution to problems of this kind at INCSA.

By using Pfister weighfeeders, INCSA found that this system offered no problems with dosage measurements, stabilization of furnaces and a reduction in energy consumption. Each feed line now only used an average of 30 kW, a cost reduction of approximately 70 kW and an energy savings of US$27,000 per year in this area, according to Pfister.

This includes a reduction in maintenance costs since no work has been required on the system since it was put into operation. The manufacturer also notes a potential cost saving for furnaces, since downtime caused by the raw meal feeding process can be reduced or eliminated entirely.