RAW MATERIALS
In our cement manufacturing process, the main raw materials are limestone, kunkur, and bauxite and iron ore. The primary materials are limestone and Kunkur (low carbonate limestone.) Limestone is supplied to the factory by Kabini and Bissel quarry, Kajiado District while Kunkur is supplied by kunkur quarry situated in Athi River. All these quarries are owned by the company.
The tertiary materials, iron ore and bauxite are supplied by contracted supplies and are mined from Tanzania and Western part of Kenya.
RAW MATERIAL PREPARATION
Raw materials can be mechanically reduced by impact, attrition or compression method. All crushers may employ one or a combination of these methods.
Mining of limestone requires the use of drilling and blasting techniques. The blasting techniques use the latest technology to ensure vibration, dust, and noise emissions are kept at a minimum. Blasting produces materials in a wide range of sizes from approximately 1.5 meters in diameter to small particles less than a few millimeters in diameter.
Material is loaded at the blasting face into trucks for transportation to the crushing plant. Through a series of crushers and screens, the limestone is reduced to a size less than 100 mm and stored until required.
Depending on size, the other materials (kunkur, bauxite and iron ore) may or may not be crushed and pre-blended before being stored in separate areas until required.
RAW MEAL GRINDING
The mill that this company uses is a vertical mill grinding system (Atox Mill 32.5). The Atox mill is very sensitive to even minor changes, reacting quickly and therefore automatic controls are specifically suited to operate it to optimize mill output.
Each raw material i.e. limestone, kunkur, bauxite and iron ore is dosed by means of weigh feeders according to the set ratio. The raw mix is fed into the mill through an airtight sluicing device (rotary feeder). Before the raw material mix enter the mill foreign materials like ferrous iron pieces are sorted by means of a magnetic separator while non ferrous metallic objects are sorted out by metal detector.
The raw materials are dried with waste process gases and ground to a size where the majority of the materials are less than 90 microns, with 10% to 12% being the allowed residue. The dry materials exiting the raw mill are called "raw meal" which is the kiln feed. The kiln feed is pneumatically blended to ensure the chemical composition of the kiln feed is well homogenized and then stored in silos until required.
PYROPROCESSING
Once raw materials have been selected and blended, and ground and homogenized into a fine and uniform kiln feed, they must then be subjected to enough heat to allow the clinkering reactions to proceed.
RAW MATERIAL PREPARATION
The process we use is the dry process, where chemical reactions take place. Basic chemical reactions are: evaporating all moisture, calcining the limestone to produce free calcium oxide, and reacting the calcium oxide with the minor materials (kunkur, bauxite and iron ore). This process takes place at about 1450°C in the kiln and results are in a final black, nodular product known as "clinker" which has the desired hydraulic properties.
In the dry process, kiln feed is fed to a 4 cyclone preheater tower. Material from the preheater tower is discharged to a rotary kiln with a diameter of 3.7m and length of approximately 54.0 m. The preheater tower and rotary kiln are made of steel and lined with special refractory materials to protect it from the high process temperatures.
The rotary kiln is fired with an intense flame, produced by burning furnace oil or coal. Preheater towers are equipped with firing as well.
The rotary kiln discharges the red-hot clinker under the intense flame into the clinker cooler. The clinker cooler recovers heat from the clinker and returns the heat to the pyroprocessing system thus reducing fuel consumption and improving energy efficiency. Clinker leaving the clinker cooler is at a temperature of about 100°C conducive to being handled on standard conveying equipment.
FINISH GRINDING AND DISTRIBUTION
The black, nodular clinker is stored on site in sheds until needed for cement production. Clinker, gypsum, and pozzolana are ground together in ball mills to form the final cement products. Fineness of the final products, amount of gypsum added, and the amount of process additions added are all varied to develop a desired performance in each of the final cement products.
The finished grinding of cement can be achieved by closed circuit grinding system or open circuit grinding system. In the closed circuit mill, the mill itself is shorter than that found in open circuit systems. The mill product is conveyed either by elevator or suspended in an air stream to the separator where it is split into fines and course fraction. In open circuit mill, the mill is longer and has a length of five to six times its diameter. The material is fed into the mill trunion and the finished product is screwed out through the discharge hood.
Each cement product is stored in an individual bulk silo until needed by the customer. Bulk cement can be distributed in bulk by truck or rail depending on the customer's needs. Cement is distributed by truck or rail.
ENVIRONMENTAL IMPACT
In all the plant unit operations, special attention is given in environmental protection by utilizing bag and electrostatic filters in capturing fugitive dust. East African Portland has invested heavily to ensure that the environment is not affected by its operations.
