Raw Mix Proportioning and Raw Mix Design of cement
The raw materials mixture is called raw mix or raw meal or kiln feed, The continuous production of high-quality cement is possible only if the raw mix possesses optimum composition, The purpose of calculating the composition of the raw mix is to determine the quantitative proportions of the components of the raw materials to give clinker the desired chemical and mineralogical composition and for smooth kiln operation.
Main Parameters for Raw Mix Design,
The raw mix composition is usually characterized by certain ratios called moduli. They are proportioning formulas in which percentages of various oxides as determined by chemical analysis are included., The three important moduli are lime saturation factor (LSF), silica modulus (SM), and alumina modulus (AM). Lime Saturation Factor (LSF), LSF represents the ratio of the actual amount of lime (CaO) to the theoretical lime required by other major oxides in raw mix/clinker., It is the ratio of CaO to the other three main oxides.
A clinker with a higher LSF will have a higher proportion of elite than a clinker with a low LSF., Limiting the range of LSF in clinker is 0.66 – 1.02 and the preferable range is 0.92 – 0.96. Silica Modulus (SM), SM represents the proportion of SiO2 to the total of Al2O3 and Fe2O3., It is the ratio of SiO2 to the sum of Al2O3 and Fe2O3. SiO2 SM = Al2O3 + Fe2O3,
A high SM means that more calcium silicates (C3S + C2S) and less aluminate (C3A) and ferrite (C4AF) are present in the clinker., In addition, it characterizes the ratio of solid/liquid and the amount of liquid phase in the clinker., Kiln process (coatings, rings, dusty clinker) is sensitive to SM changes., The limiting range of SM in clinker is 1.9 – 3.2 and the preferable range is 2.1 – 2.7. Alumina Modulus (AM), AM characterizes the raw meal/clinker by the proportion of alumina to iron oxide., It is the ratio of Al2O3 to Fe2O3. Al2O3 AM = Fe2O3, AM determines the potential proportions of aluminate (C3A) and ferrite phase (C4AF) in the clinker., In addition, it characterizes the composition of the liquid phase in the clinker,
Alumina and iron oxide have flux effect., The liquid phase promotes the C3S formation., Higher iron oxide decreases the viscosity of the melt, increasing the speed of reaction between CaO and SiO2., Low alumina modulus = Easier burning due to low viscosity., Limiting the range of AM in clinker is 1.2 – 2.5 and a preferable range is 1.3 – 1.6 Raw Mix Design Calculation Prerequisites for Raw Mix Design Calculation, Raw Materials 1. Chemistry 2. Materials cost,
There are many methods of calculation: from the simplest to more complicated ones., The basis for calculation is the chemical composition of the raw materials. Generally, data of chemical analysis should be accurate to two places of decimals. Results of the analysis are more than 100%, each constituent being proportionally reduced. If on the other hand, the total of constituents is less than 100 the constituents are not proportionally increased to 100. In this case, the difference from 100 is denoted as ‘rest’, so that the total of all constituents is then 100.
Ingredients and properties of cement compounds
1. Lime (CaO):, This is the important ingredient of cement and its proportion is to be carefully maintained.,A sufficient quantity of lime forms tricalcium silicate (C3S) and dicalcium silicate (C2S),
The lime in excess makes the cement unsound and causes the expansion and disintegration of the cement. , On the other hand, if the lime is deficient, it will decrease the strength of the cement and will cause it to set quickly.
2. Silica (SiO2):, It imparts strength to the cement due to the formation of dicalcium silicate and tricalcium silicate.,If silica is present in excess quantity setting time of cement is prolonged.
3. Alumina (Al2O3):, Alumina imparts quick setting property to the cement.,It acts as a flux and it lowers the clinker temperature., Alumina in excess reduces the strength of cement.
4. Iron Oxide (Fe2O3):, Iron oxide imparts color, hardness, and strength to cement.
5. Magnesia (MgO):, Magnesia, if present in a small amount, imparts hardness and color to the cement.,The high content of magnesia makes the cement unsound.
6. Calcium Sulphate (CaSO4):, Calcium sulfate is in the form of gypsum and its function is to increase the initial setting time of cement.
7. Sulphur (S):, A very small amount of sulfur is useful in making sound cement. If it is in excess, it causes the cement to become unsound.
8. Alkalies: Most of the alkalies present in raw materials are carried away by the flue gases during heating and the cement contains only a small amount of alkalies. ,
If they are in excess in cement, they cause several troubles such as alkali-aggregate reaction, efflorescence, and staining when used in concrete, brickwork, or masonry mortar.
Properties of Cement Compounds
Tricalcium Silicate (C3S), Hydrates and hardens rapidly. , Largely responsible for initial set and early strength(up to 7 days), Portland cements with higher percentages of C3S will exhibit higher early strength. , Produces more heat of hydration.
Dicalcium Silicate (C2S), Hydrates and hardens slowly. , Largely responsible for strength increase beyond 7 days. , Produces much of ultimate strength.,Produces less heat of hydration.
Tricalcium Aluminate (C3A), Hydrates and hardens the quickest. , Liberates a large amount of heat almost immediately. , Contributes somewhat to early strength, especially 1day strength. ,
Gypsum is added to retard C3A hydration. Without gypsum, C3A hydration would cause the cement to set almost immediately after adding water.
(Flash set) , Cement with Low C3A is more resistant to sulfate attack.
Tetra calcium Alumino Ferrite (C4AF), Hydrates rapidly but contributes very little to strength.,The presence of iron oxide lowers clinker temperature.,Colour effects (grey color) are due to C4AF.
Cement manufacturing in steps
The cement manufacturing process involves 8 steps starting from the mining of limestone to the packing of cement. We will discuss one by one all the 8 steps.
1. Mining of Limestone: • Cement manufacturing process starts with the mining of limestone, which is the main raw material for the production of cement. • Limestone is excavated from the limestone mines by blasting and blasted limestone is transferred to the crusher as boulders by dumpers.
2. Crushing of Limestone: • Limestone as mined is fed to the limestone crusher (impact crusher). • The limestone crusher crushes the limestone to -60 to -80 mm size (VRM) / – 25 mm size (ball mill) and discharges the material into a belt conveyor which takes it to the stacker/reclaimer.
3. Stacking and Reclaiming of Limestone: • Limestone is stacked in longitudinal stockpiles. • Limestone is extracted transversely from the stockpiles by reclaimers and conveyed to the raw mill hoppers for grinding of limestone.
4. Grinding of Limestone and Additives: • Limestone and additives are mixed in required proportions so that the specified composition of the final product is maintained properly and ground in the raw mill (VRM / ball mill) to a fine powder. • The fine powder is known as raw meal
5. Storage of Raw Meal: • Raw meal is stored in a continuous flow silo (CF silo) where it is blended and stored and kept ready to be fed into the rotary kiln. • The blended material is known as kiln feed. Kiln feed is a homogenized raw meal. • Burning of Kiln Feed: • Kiln feed is fed to a four to six-stage preheater / precalciner system and then to a rotary kiln, where the temperature goes up to 1450oC. • For burning the kiln feed, pulverized coal is fed through the lower end of the kiln. • Raw coal is ground in a coal mill (VRM/ball mill) to fine coal or pulverized coal. • The hot materials coming out of the kiln are known as clinker. • Clinker is in the form of nodules of various sizes, usually 1-25 mm in diameter. • The hot clinker nodules are cooled to about 100 – 120oC in the cooler attached to the kiln. In the cooler, cold air is blown to effect heat exchange between hot clinker and cold air. The cooled clinker is stored in a clinker silo.
6. Grinding of Clinker: • Cooled clinker is ground along with 3 -5 % gypsum in a cement grinding mill (ball mill / VRM/ Roll press combined with a ball mill) to a fine powder, known as cement.
7. Storage of Cement: • The ground cement is stored in cement silos (having different compartments) from where it is fed to packer machines.
8. Packing of Cement: • The cement is packed in 50 kg cement bags by electronic packer machines and despatched by truck/rail.