Cement mill feeding section facilitates a reliable feeding of clinker, gypsum and other additive like limestone, slag, fly ash etc. Mill feeding consists of material hoppers, weighing, conveying to mill.
Mill Feeding Hoppers. Hoppers for clinker/Gypsum/ Additives serve the purpose of providing a buffer storage for mill feed and a convenient arrangement for feeding to weigh feeders for proportioning purpose. Hoppers used to be designed to hold the requirements of one shift. However, for large capacity cement mill it is no longer practical to do so. Further with reclaimer belts running round the clock it is as well no longer necessary to do so. Thus hoppers may be designed to hold only 4 hours' requirements. On the other hand, requirements of gypsum and additives are very low that hoppers to hold 4 hours' needs are too small. Therefore, for practical reasons hoppers for them would be designed to hold about 8-24 hours' needs; but seldom more because of sluggish flow materials like gypsum in hoppers. Whatever capacity and geometry you may be having in existing plant or planning to have in proposed plant, we need to be careful to check that the material segregation is minimum and that the disturbances by virtue of this phenomena are negligible.
Hoppers can be in steel or in R.C.C. Hoppers can either be rectangular or circular in shape. A circular cone is preferred as it eliminates valley angles and consequently improves material flow. Having rectangular or square shape at the top and changing to conical at discharge creates shelves at junction, where material accumulates, which causes problems of flow of material in extraction. Cone surfaces can also be lined with special liners with smooth surface. Bag filters are generally installed at the top to vent air when material is fed to a hopper.
Mill Feeders: Feeders will be directly installed under hoppers with rod gate in between. The feeders are generally 2 to 3 m long and discharge on to a common conveyor which will take feed to mill. Feeders for mills meter feed rates and also totalize tons fed. For cement mill feeding, table feeders, belt feeders, and weigh feeders have been used. However, weigh feeders are the most commonly preferred feeders.
Metering on Feeders. Metering can be either direct (gravimetric) or indirect (Volumetric/impact). In direct method of metering the material passes over a load cell installed in weigh feeder/apron feeder and the travel speed is monitored with installed tachometer. Weight and speed together determines the mass flow rate of material in metric tons per hour (t/hr). Feed rate is generally controlled with the speed of weigh feeder belt, which is driven by a variable speed drive. Feed rate is monitored and controlled by a control panel generally supplied with weigh feeder. Set points are passed to control panel from CCR by Operator. In indirect metering system feed rate is determined by measuring cross section of material and rate at which it flows and taking into account bulk density of material.
Calibration and Drop Test Facilities. Provisions for drop test for calibration of weigh feeders are commonly available in cement plants to validate production figures. Although the weigh feeder calibration is not required frequently unless there is a disturbance in mechanical system due to various reason including maintenance. However, it is a common practice to validate feed rate through drop test periodically. Weigh feeders generally come with self-calibration devices. A simple way of 'self-calibration' is to have the hopper mounted on load cells, so that a weight loss for predefined time will be used for calibration purpose, and in this case calibration of hopper load cells needs to done at least once a year with standard weights.
Metal Detector and Magnetic Separator. Arrangement of metal detector and magnetic separator is integral part of feeding system in vertical roller mills and roller presses. Both are installed on mill feeding belt conveyor. Magnetic separator, separates out small metallic impurities from mill feed. While as metal detector signals the presence of metallic debris, which can damage the grinding path and give rise vibrations issues.