Clinker formation-concepts
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This document discusses clinker burning and the formation of Portland cement clinker. It begins with an introduction on why studying clinker burning is important. It then covers normal and abnormal kiln operation, and discusses the analogy to metamorphic rock formation. The document outlines the two principal steps of raw material disintegration and structural rearrangement during heating. It also discusses features of clinker formation like being a complex system that requires energy and produces minerals that are not stable at normal temperatures. The document provides details on reaction pathways, mechanisms, kinetics, thermodynamics, and intermediate products involved in clinker formation. It closes with sections on controlling the burning process and assessing raw meal burnability.
Clinker formation-concepts
- 1. Introduction: Why study clinker burning? “To understand the influence of changes in kiln operation conditions” Normal kiln operation Influence of chemistry, fineness, mineralogy changes Influence of new mix components (pozzolan, AFR, sand, etc.) Abnormal kiln operation know causes of badly burnt clinker understand why rings and deposits form be able to suggest counter measures Synthetic Hydraulic Minerals Analogy to transformation of igneous and sedimentary rocks into metamorphic rocks Difference in T, p, t
- 2. ============================================= ============================================= = Two principal steps during transformation into clinker Disintegration of original structure Mechanical crushing and grinding Thermal decomposition Structural rearrangement on heating (e.g. polymorphism Formation of new structures Occurrence of intermediate products Genesis and growth of final clinker minerals Crystallization of liquid phase ============================================= ======================================== Features characterising the clinker formation process -Complex system (series of diverse mechanisms)! -Requires mechanical, thermal and electrical energy -Reaction rate is slow (necessity of high temperatures, finely dispersed material) -Clinker minerals are not stable at normal temperature! -Quality of product is determined by: Clinker chemistry Clinker microstructure ============================================= =======================================
- 3. Control of burning process Material technological aspects -Raw meal burning behaviour Burnability Dust formation Coating behaviour Granulation of clinker etc. -Quantity and properties of liquid phase Process technological aspects -Temperature profile -Kiln atmosphere -Fuel type -Flame characteristics ============================================= ======================================
- 4. Reaction pathway indicates the intermediate products occurring between reactants and products Reaction mechanism type(s) and reaction(s) taking place Reaction kinetics indicates rate at which the final products are produced Reaction thermodynamics dictates whether reaction will be at all possible, and what t he heat and temperature requirements will be =================================================================== ===================================================== Now to my linkedin colleges I finished the first part of my lecture if you have any question contact me here or in my linkedin account Or contact me by Email : honestgroupcoltd@gmail.com =========================================== =========================================== =========== Reaction Pathways Encountered During Clinker Formation Basic Sequence of Reactions Mineralogical and Chemical Characteristics of Raw Mixes Intermediate Products Liquid Phase
- 5. The Overall Reaction Sequence To fully describe the pathway of clinkering, it is necessary to consider the following aspects: -the chemical and mineralogical content of the raw mix -the overall sequence of reactions -the chemical and mineralogical nature of the intermediate products Heating (°C) 20 – 100 Evaporation of H2O 100 – 300 Loss of physically adsorbed water 400 – 900 Removal of structural H2O (H2O and OH groups) from clay minerals >500 Structural changes in silicate minerals 600 – 900 Dissociation of carbonates >800 Formation of belite, intermediate products, aluminate and ferrite >1250 Formation of liquid phase (aluminate and ferrite melt) ~1450 Completion of reaction and re-crystallization of alite and belite
- 6. Cooling (°C) 1300 – 1240Crystallization of liquid phase into mainly aluminate and ferrite ================================================= ================================================= ========================== Mineralogical characteristics of raw mixes Carbonates MgCO3, ankerite CaCO3 (Mg,Fe)CO3 magnesite MgCO3, siderite FeCO3calcite CaCO3, dolomite CaCO3 Simple Oxides quartz SiO2, hematite Fe2O3, magnetite Fe3O4 Feldspars potassium feldspars (Na,K)Si3O8 and plagioclase series (Na,Ca)(Si,Al)Al2Si2O8 Sheet silicates minerals of the mica and chlorite groups (e.g. biotite, muscovite, chlorite), clay minerals (e.g. kaolinite, montomorillonite, illite, palygorskite) Hydroxides Al-hydroxides (e.g. boehmite), Fe-hydroxides (e.g. goethite, limonite) Sulfides / sulfates H2Opyrites FeS2, anhydrite CaSO4, gypsum CaSO4 Fluorides fluorspar CaF2
- 7. ================================================= =========================================== ================================================= ============================================ Intermediate products encountered during clinkerproduction T ype M ineral Name Formula Simple Sulfates anhydrite C aSO4
- 8. arcanite K2SO4 C a SO4Compound Sulfates ”sulfate”-spurrite 2(C2S) ”calcium”-langbeinite K2Ca2(SO4)3 C aCO3Compound Carbonates spurrite 2(C2S) Simple Chlorides sylvite KC l A l2O3 7 Al2O3CaO Calcium Aluminates mayenite 12 CaO C alcium Ferrites Fe2O3 2 C aO SiO2 Al2O3Calcium Alumino-Silicates gehlenite 2 C aO ================================================================================================== ======================================= Reasons for the formation of intermediate products -Intermediate products are preferentially formed by kinetically faster reaction rates -Intermediate products are the reaction products of localised zones in the meal charge, i.e. local equilibrium but not overall equilibrium was reached (e.g. gehlenite formation) -Intermediate products are really the equilibrium products at the given temperature and gas atmosphere, but not at the final clinkering temperature (e.g. spurrite formation) ================================================= ================================================= ===== Liquid Phase basically created by early melting compounds such as Fe2O3 and Al2O3 and some minor compounds such as MgO and Alkalis
- 9. The composition of the raw mix determines -temperature at which liquid will first be formed -amount of liquid formed at any given temperature -the physical properties of the liquid at any particular temperature, especially its viscosity -Although most raw mixes show about the same minimum temperature of liquid formation (eutectic point), the quantity of liquid formed at this and progressively higher temperatures varies according to the raw mix chemistry. -In the Portland cement relevant parts of the system C – S – A – F, in which melting begins at 1338 °C, the composition of the liquid is: CaO - 55 % SiO2 - 6 % Alumina ratio Al2O3 - 23 % (AR) = 1.38 Fe2O3 - 16 % Quantity calculation formulae acc. to LEA, considering different temperature: 1338 o C = 6.1 Fe2O3 + MgO + Na2O + K2O if AR 1.38 8.2 Al2O3 – 5.22 Fe2O3 + MgO + Na2O + K2O if AR 1.38 1400 o C = 2.95 Al2O3 + 2.2 Fe2O3 + MgO + Na2O + K2O for MgO 2 % 1450 o C = 3.0 Al2O3 + 2.25 Fe2O3 + MgO + Na2O + K2O for MgO 2 % Quantitative change of liquid phase with temperature in several group plants (influence of MgO, Na2O and K2O included)
- 10. Influence of Al2O3 and Fe2O3 alone on the quantity of liquid formed at 1338 °C. The most effective use of Al2O3 and Fe2O3 – with respect to liquid formation at 1338 °C – occurs when the two are used in the weight ratio of 1.38
- 11. Viscosity of liquid phase -The viscosity of the liquid phase diminishes exponentially with increasing temperature and at 1400 °C is reduced by addition of fluxing components in the following order: Na2O < CaO < MgO < Fe2O3 < MnO -With increasing SiO2 content of the melt and to a lesser extent with increasing Al2O3, appreciable increases in viscosity occur. ================================================= ================================================= ========================================= The overall reaction sequence, displayed based on qualitative change of minerals from samples taken from an operational kiln Minerals identified at different locations (long wet kiln) Sequence of compound formation according to chemical composition
- 12. ============================================== ============================================== ============================================== ======== Now to my linkedin colleges I finished the second part of my lecture if you have any question contact me here or in my linkedin account
- 13. Or contact me by Email : honestgroupcoltd@gmail.com =========================================== =========================================== =============================== Reaction Mechanisms Definitions State of matter solid: definitive volume and definite shape liquid: definitive volume, assumes shape of container gaseous: neither definitive volume nor definite shape Classification of reactions 1. according to their type: low quartzstructural change high quartz CaO + CO2decomposition CaCO3 C2Scombination 2CaO + SiO2 2. according to the state of matter: belitesolid – solid quartz and free CaO crystallisation of aluminate + ferritesolid – liquid liquid phase CaO + CO2solid – gas CaCO3 liquid – liquid - liquid – gas drying process, volatilisation of alkalis CO2gas – gas CO + 1/2 O2 3. according to rate controlling step (kinetics of reaction)
- 14. diffusion formation of alite belite (initial reaction)phase boundary quartz + free CaO nucleation liquid phase crystallisation of aluminate + ferrite; alite formation Examples Structural changes: Arrangement of the atoms in low and high quartz Structural changes: Calcite – Aragonite transition Decomposition reactions (during clinker production) -solid / gas type
- 15. De-hydroxylation of the clay minerals (kaolinite, etc.) De-carbonation of the carbonate minerals (magnesite, dolomite, calcite, spurrite) -solid / solid type decomposition of alite Characteristic of this reaction type is that the single reactant is transformed into two products. ================================================= ================================================= ======================================= Decomposition reaction: Equilibrium dissociation pressure of calcite and spurrite with temperature Decomposition reaction: Decomposition of C3S at 1175 °C In the case of impure C3S, i.e. clinker alite, the rate of decomposition is appreciably accelerated by: the presence of lime and C2S nuclei the presence of Fe2+ , H2O and K2SO4 / CaSO4 melts
- 16. Combination reaction: Formation of Belite Belite formation is the result of a combination between the calcite and silica components of the raw mix. The rate limiting mechanism by which belite is formed (after an initial phase boundary controlled reaction) depends on the diffusion of ions through the solid state. The rate of this reaction is thus dependent on: the path distance that the diffusing species have to travel defects in the reactant’s crystal lattices. Combination reaction: Formation of Alite Formation of alite only at T > 1250 °C (lower stability limit). At that temperature, the liquid phase is also starting to form: The formation of alite is a liquid – solid reaction
- 17. The formation of alite and its stabilisation depends on the presence of the liquid phase. The rate of reaction is dependent on: the path distance that the diffusing species have to travel quantity and viscosity of liquid phase ========================== ========================== ========================== ========================== ========================== ================ Now to my linkedin colleges I finished the third part of my lecture if you have any question contact me here or in my linkedin account Or contact me by Email : honestgroupcoltd@gmail.com ======================== ======================== ======================== ======================== ===================== 4. Kinetics of Clinker Burning
- 18. Theoretical consequences: Rate increases with higher temperature (but also costs!) Rate decreases with higher activation energy (different raw mix mineralogy) Rate increases with higher frequency factor (larger contact surface, i.e. finer mix) The rate of reaction increases with temperature and contact surface between raw mix components (frequency factor A) decreases with higher activation energy Ea for raw mix components. To compensate for the slow reactivity of the less reactive minerals, a higher burning temperature and / or longer burning period (longer clinkering zone) is required. Practical considerations:
- 19. development of suspension preheater In practice, the most convenient method of following the reaction is by measuring the rate of decrease of non-combined lime (i.e. free lime). This technique is illustrated in the following figures that show two raw mixes, I and II, of identical chemistry (LS = 95, SR = 3.2 and AR = 2.2) and similar fineness (R200m = 0.5 %, R90m = 7 % and R60m = 15 %). It is evident that the difference in mineralogy and actual particle size of the individual crystals influence both the mechanism and rate of reaction, especially at start of the clinker formation.
- 20. Limestone Calcite 97 % Dolomite ~ 2 % Quartz traces Chlorite - Illite and Micas - Pyrite traces Feldspars - Shale A Calcite ~ 40 % Dolomite - Quartz ~ 25 % Chlorite ~ 20 % Illite and Micas ~ 10 % Pyrite ~ 2 % Feldspars ~ 2 % Shale S Calcite ~ 10 % Dolomite - Quartz ~ 55 % Chlorite ~ 10 % Illite and Micas ~ 20 %
- 22. ============================================== ============================================== ===== Assessment of Raw Meal Burnability In practice, simple methods are mostly applied to asses the “burnability” of a mix, i.e. the ease of formation of the clinker minerals. Three distinct methods are practiced at HGRS: Statistical burning model – in which ten material parameters influence the rate of clinker formation. The non-combined CaO value, of any raw mix, relative to that of a standard raw mix is calculated. Physicochemical burning model – requires no standard raw mix. Only 4 parameters need to be considered. ================================================= ================================================= =============================================== Statistical Burnability Model Quantitative evaluation of the data obtained by the Mark burnability test o The 1350, 1400 and 1450 free lime values of other raw mixes from the same raw material components can be determined based on one single burnability test of one mix o Chemical Parameters: lime saturation, silica ratio, alumina ratio, K2O + Na2O, MgO o Physical Parameters: residue on 200 m and 90 m sieves, quantity of mica, quartz and iron minerals NOTE : The burnability model can be used as an instrument for optimization of raw mixes ================================================= ================================================= ================================================= =
- 23. Physiochemical Burnability Model the amount of uncombined lime depends on o Specific reaction area (area of contact between grains) o Local oversaturation (grain size of individual minerals) o Ambient conditions (pressure, temperature, burning time) o Diffusion coefficient of CaO through the liquid phase (composition of the liquid phase) o Amount of liquid phase formed during burning o Supply and demand of CaO all these influencing factors may be incorporated in four parameters: SR, LS, amount of oversized quartz grains, amount of oversized calcite grains. (Pressure, temperature and burning time are considered to be constant.) Silica ratio (SR) and lime saturation (LS) The formation of C3S from C2S and CaO is governed by the diffusion of CaO through the melt. The silica modules and lime saturation are sufficient to describe this chemical reaction quantitatively. The amount of CaO which can be accommodated within the liquid phase and in which it can diffuse and thus react, is inversely proportional to the silica ratio. A linear relationship exists between max. lime saturation and silica ratio values at which no free lime can be observed. Quartz and calcite grains Whether a grain of material reacts fully under given burning conditions depends on its diameter, structure and chemical composition. Too large calcite grains result in CaO not being completely combined as also results from grains whose lime saturation is over 100 %.
- 24. For the “Holderbank” burnability test conditions the following grain diameters were found to be critical limits: o quartz 32 m calcite 90 m ========================== ========================== ========================== ========================== ========================== ================ Now to my linkedin colleges I finished the Fourth part of my lecture if you have any question contact me here or in my linkedin account Or contact me by Email : honestgroupcoltd@gmail.com ======================== ======================== ========================
- 25. ======================== ===================== Thermodynamics of Clinker Formation During clinker production, heat is both absorbed (endothermic heat changes) and produced (exothermic heat changes) Temp. (°C) Type of Reaction Heat Change 20 – 100 Evaporation of free H2 O Endothermic 100 – 300 Loss of physically adsorbed H2O Endothermic 400 – 900 Removal of structural H2O (H2O, OH groups from clay minerals) Endothermic 600 – 900 Dissociation of CO2 from carbonate Endothermic > 800 Formation of intermediate products, belite, aluminate and ferrite Exothermic > 1250 Formation of liquid phase (aluminate and ferrite melt) EndothermicFormation of alite Exothermic 1300 – 1240 Crystallization of liquid phase into mainly (cooling cycle) aluminate and ferrite Exothermic Examples for exothermic reactions (heat liberated) o Coal (C) + O2 CO2 o Lime (CaO) + H2O Ca(OH)2 o Cement + H2O Cement Hydrates o Liquid K2SO4 Solid K2SO4 Examples for endothermic reactions (heat absorbed) o H2O (liquid) H2O (steam) o CaCO3 CaO + CO2 ================================================= ================================================= ================================================= ========
- 26. DTA curves of typical cement raw meals The greatest heat requirement occurs between 850 – 900 °C, i.e. for the decomposition of the carbonate minerals.The total heat requirements for dehydration, decarbonisation and melting exceed the heat liberated by the formation of belite and the intermediate and final products. Endothermic processes kJ/kg clinker
- 27. 170dehydration of clays 1990decarbonisation of calcite 105heat of melting 2050heating of raw materials 0 – 1450 °C 4315Total endothermic Exothermic processes kJ/kg clinker -40crystallization of dehydrated clay -420heat of formation of clinker minerals -105crystallization of melt -1400cooling of clinker -500cooling of CO2 (ex calcite) -85cooling of H2O (ex clays) -2550Total exothermic Net theoretical heat of clinker formation + 1765 ============================================== ============================================== ====================== Heat balance of wet and dry kiln, kJ/kg clinker ( HFW Taylor: Cement Chemistry, 1998 )
- 28. Dry kiln Wet kiln Evaporation of H2O 13 (0.4%) 2,364 (41.5%) Heat of reaction 1,807 (54.6%) 1,741 (30.5%) Heat losses through 711 (21.5%) 812 (12.3%) gas, clinker, dust, etc. Heat lost in air from cooler 427 (13.0%) 100 (1.7%) Heat losses by radiation 348 (10.5%) 682 (12.0%) and convection 3,306 kJ/kg 5,699 kJ/kg