Raw Mix Proportioning and Raw Mix Design | Cement Chemistry

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Lecture-8: Raw Mix Proportioning and Raw Mix Design by Dr. K. Mohan.

Dr K Mohan is an internationally renowned and an acclaimed cement scientist. He is a former Director General of National Council for Cement and Building Materials (NCCBM).
He holds a Ph.D. degree in Cement Chemistry from University of Aberdeen, Scotland, U.K.

• 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 raw materials components in order to give clinker the desired chemical and mineralogical composition and for smooth kiln operation.

Main Parameters for Raw Mix Design (Modulii):

• The raw mix composition is usually characterised by certain ratios called modulii.
• They are proportioning formulas in which percentages of various oxides as determined by chemical analysis are included.
• The three important modulii are lime saturation factor (LSF), silica modulus (SM) and alumina modulus (AM).

Lime Saturation Factor (LSF)

• LSF represents the ratio of 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 other three main oxides.

CaO
LSF = --------------------------------------------------------------
2.8 SiO2 + 1.18 Al2O3 + 0.65 Fe2O3

CaO - 0.7 SO3
LSF = ---------------------------------------------------------------
2.8 SiO2 + 1.18 Al2O3 + 0.65 Fe2O3

• Often it is referred to as percentage and hence multiplied by 100.
• A clinker with a higher LSF will have a higher proportion of alite to belite than a clinker with a low LSF.
• Limiting range of LSF in clinker is 0.66 - 1.02 and 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 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 characterises 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.
• Limiting range of SM in clinker is 1.9 - 3.2 and preferable range is 2.1 - 2.7.

Alumina Modulus (AM)

• AM characterises 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 characterises the composition of liquid phase in the clinker.
• Alumina and iron oxide have flux effect.
• 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 range of AM in clinker is 1.2 - 2.5 and preferable range is 1.3 - 1.6

Raw Mix Design Calculation:

Prerequisites for Raw Mix Design Calculation:

• Raw Materials
1. Chemistry
2. Materials cost
• Fuel Data
1. Type of fuel and Proportioning
2. Ash Chemistry
3. Proximate Analysis (Sulphur, CV, Ash)
4. Kiln heat consumption (kcal/ kg clinker)
• General
1. Target free lime
2. Constraints for control parameter (LSF, SM, AM, C3S)
3. Chemistry and Quantity of kiln dust

Calculation of Raw Mix:

• 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 analysis is in excess of 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.

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