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(1)CemNet Training - Course CKPC01. Module 1 – Session 7. Cement Kiln Process Chemistry Module 1. Cement kiln energy efficiency and productivity. 1.7 Alkali cycles.. Cement Kiln Chemistry. 1.7 Alkali cycles. In the preceding sessions of the course we have been focusing on the reactions of the major oxides, CaO, SiO2, Al2O3 and Fe2O3 to form the clinker minerals C3S, C2S, C3A and C4AF. We have seen that minor components such as MgO, K2O, Na2O, SO3 and F- affect the heat of clinker formation, amount of flux formed at different temperatures in the kiln, properties of that flux and can inhibit or mineralise clinker formation. In this module 1 we are concerned with the thermal energy efficiency and productivity of the cement kiln….….the reliability of the kiln operation has a major role to play in determining this thermal energy efficiency and productivity.. Cement Kiln Chemistry. 1.7 Alkali cycles. In the preceding sessions of the course we have been focusing on the reactions of the major oxides, CaO, SiO2, Al2O3 and Fe2O3 to form the clinker minerals C3S, C2S, C3A and C4AF. We have seen that minor components such as MgO, K2O, Na2O, SO3 and F- affect the heat of clinker formation, amount of flux formed at different temperatures in the kiln, properties of that flux and can inhibit or mineralise clinker formation. Every time the kiln stops the heat capacity must be re-established to reach the temperatures required for C3S formation. …. ….unplanned kiln stops can arise for many reasons, mechanical or electrical equipment failures, mistakes in operations……. …….these are not our concern in this course, we are focusing on the process chemistry and its potential effects on the reliability of kiln operations.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(2) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. In the preceding sessions of the course we have been focusing on the reactions of the major oxides, CaO, SiO2, Al2O3 and Fe2O3 to form the clinker minerals C3S, C2S, C3A and C4AF. We have seen that minor components such as MgO, K2O, Na2O, SO3 and F- affect the heat of clinker formation, amount of flux formed at different temperatures in the kiln, properties of that flux and can inhibit or mineralise clinker formation. Every time the kiln stops the heat capacity must be re-established to reach the temperatures required for C3S formation. ….….the behaviour of the minor components in the kiln feed (and fuels) have more affect on the reliability of kiln operations than do the major oxides. In particular the recirculation of volatile species which is known as the alkali cycle.. Cement Kiln Chemistry. 1.7 Alkali cycles. In the preceding sessions of the course we have been focusing on the reactions of the major oxides, CaO, SiO2, Al2O3 and Fe2O3 to form the clinker minerals C3S, C2S, C3A and C4AF. We have seen that minor components such as MgO, K2O, Na2O, SO3 and F- affect the heat of clinker formation, amount of flux formed at different temperatures in the kiln, properties of that flux and can inhibit or mineralise clinker formation. The volatile species that participate in these alkali cycles are the carbonates, sulphates and chlorides of the alkali metals, K+ and Na+. The alkali metals are introduced into the kiln with the clay minerals where they occupy positions between the alumino-silicate layers.. Cement Kiln Chemistry. 1.7 Alkali cycles. In the preceding sessions of the course we have been focusing on the reactions of the major oxides, CaO, SiO2, Al2O3 and Fe2O3 to form the clinker minerals C3S, C2S, C3A and C4AF. We have seen that minor components such as MgO, K2O, Na2O, SO3 and F- affect the heat of clinker formation, amount of flux formed at different temperatures in the kiln, properties of that flux and can inhibit or mineralise clinker formation. They may also be present as alkali carbonates, or carbonates may be formed in the preheater…. ….sulphates or sulphur can be introduced in the raw materials or fuels…… ……chlorides may also be introduced in the raw materials or fuels, and can also be introduced as salt, NaCl, in the process water introduced in conditioning towers etc.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(3) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatile species in the kiln feed pass down the preheater and into the rotary kiln.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatile species in the kiln feed pass down the preheater and into the rotary kiln. Some of the volatiles pass out of the kiln in the clinker.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatile species in the kiln feed pass down the preheater and into the rotary kiln. Some of the volatiles pass out of the kiln in the clinker. Some evaporate in the burning zone of the kiln and are carried away with the combustion product gases.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(4) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatile species in the kiln feed pass down the preheater and into the rotary kiln. Some of the volatiles pass out of the kiln in the clinker. This evaporation of volatiles in the burning zone of the kiln absorbs thermal energy which is carried away with the combustion product gases.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatilised material from the feed is joined by any volatile material from the fuel.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatilised material from the feed is joined by any volatile material from the fuel. The combined volatilised material from the feed and fuel are carried to the preheater in the exhaust gases where they condense on the incoming feed and are carried back into the kiln.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(5) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatilised material from the feed is joined by any volatile material from the fuel. Condensation of the volatiles on the feed in the preheater releases the thermal energy absorbed by their evaporation in the burning zone. The alkali cycle transfers thermal energy from the burning zone to the preheater, in a similar way to recarbonation transferring thermal energy up the preheater, or combustible materials in the feed.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Volatilised material from the feed is joined by any volatile material from the fuel. Condensation of the volatiles on the feed in the preheater releases the thermal energy absorbed in their evaporation in the burning zone. The thermal energy from the burning zone transferred back to the preheater is not used efficiently and increases preheater exhaust gas temperature and increases overall thermal energy consumption.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The cycle of volatile material builds up…….. ….until the amount of material exiting in the clinker equals the amount of material entering in the feed & fuel.. Concentration of recirculating volatile material is significantly higher than in either the feed, fuel or clinker.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(6) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Measurement of the input levels in feed and fuel……. …….output levels in the clinker…... …….and the levels in the hot meal. Allows the volatility of each species in the particular kiln to be calculatated: Volatility = 1 – Volatile %Clinker / Volatile %Hot Meal The concentration in hot meal, Volatile %Hot Meal, is on a loss-free basis.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. The hot meal can stick to the walls of the preheater ducts causing restrictions and in the worst cases cause blockage of the preheater. This is one of potential causes of unplanned kiln stops that …………..even without can arise due to the alkali cycle….. blocking the preheater the restriction to venting the exhaust gases is increased, reducing kiln output.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. The hot meal can stick to the walls of the preheater ducts causing restrictions and in the worst cases cause blockage of the preheater. The preheater blockage usually results from a piece of coating falling from one of the ducts and bridging the bottom of the cyclone outlets.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(7) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 1. Steps can be taken to reduce the intensity of the volatile cycles……. …………..these strategies depend to a great extent on the volatile species that are building up in the alkali cycle…... …..we will consider these factors in the next two sessions of the course.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Distribution plate Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..meal distribution plates in the preheater ensure that the feed is evenly distributed through the rising gas stream in the preheater…... …..promoting condensation of volatiles on the descending feed rather than the duct walls.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Distribution plate Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..meal distribution plates in the preheater ensure that the feed is evenly distributed through the rising gas stream in the preheater…... …..also cooling the rising gases and preventing volatiles migrating higher up the preheater.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(8) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: Coating resistant refractory The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..the gas riser from the kiln inlet and the ducts in the lower part of the preheater can be lined with coating resistant refractories.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..air cannons can be fitted to prevent coating forming in known areas of problems…... …..local knowledge of the particular kiln is critical to correctly siting the air cannons.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..air cannons can be fitted to prevent coating forming in known areas of problems…... ….. air cannons must fire in the direction of material flow, rather than opposing the material flow.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(9) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..air cannons can be fitted to prevent coating forming in known areas of problems…... ….. air cannons have limited range of no more than 1.5m and therefore multiple cannons are likely to be needed.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..air cannons can be fitted to prevent coating forming in known areas of problems…... …..acoustic horns are an alternative to air cannons.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..dead areas with low gas velocity need to be avoided to prevent accumulation of “sticky” dust.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(10) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..feed flaps need to be fitted to cyclone exit chutes to prevent gas shortcircuiting up into the cyclones and preventing free flow of the material out of the cyclones.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. Various strategies can be adopted to reduce or eliminate these problems: 2. Steps can be taken to prevent the material sticking to the ducts in the preheater……. ……..angle of incline of cyclone exit chutes need to be at least 55° to ensure feed falls rapidly through the chutes.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. The hot meal can stick to the walls of the preheater ducts causing restrictions and in the worst cases cause blockage of the preheater. The concentration of volatile species in the hot meal entering the kiln also affects the properties of the flux and the “clinkering range” of the hot meal in the kiln.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(11) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. The hot meal can stick to the walls of the preheater ducts causing restrictions and in the worst cases cause blockage of the preheater. This can be the cause of ball and ring formation in the kiln, which can again lead to unplanned stops of the kiln…. ….again, more of this in succeeding sessions.. Cement Kiln Chemistry. 1.7 Alkali cycles. The mechanism of the alkali cycle will be well known to most cement manufacturers: This cycle between the rotary kiln and the preheater is the classical “internal” alkali cycle. There is also an “external” alkali cycle, which must be considered.. Cement Kiln Chemistry. 1.7 Alkali cycles. The volatilised fumes exit the rotary kiln to the preheater together with the combustion product gases from the main burner….. …..and encounter the The volatilised fumes feed descending the condense and are preheater. collected in the cyclone where the temperature is below the melting point of the volatile species….. …..chlorides will condense higher up the preheater than sulphates.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(12) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. The volatilised fumes exit the rotary kiln to the preheater together with the combustion product gases from the main burner….. …..and encounter the The volatilised fumes feed descending the condense and are preheater. collected in the cyclone where the Melting Point temperature is below K2SO4 1069°C the melting point of Na2SO4 884°C the volatile species….. NaCl 801°C KCl 776°C …..chlorides will condense higher up the preheater than sulphates.. Cement Kiln Chemistry. 1.7 Alkali cycles. The volatilised fumes exit the rotary kiln to the preheater together with the combustion product gases from the main burner….. …..and encounter the feed descending the We have already preheater. seen in session 1.3 that the preheater cyclones do not have 100% collection efficiency….. …..the lower cyclones may only have 80~85% collection efficiency. Some of the volatile species are therefore carried up the preheater with the uncollected feed….. …..and some exits the top cyclone in the dust.. Cement Kiln Chemistry. 1.7 Alkali cycles. The volatilised fumes exit the rotary kiln to the preheater together with the combustion product gases from the main burner….. …..and encounter the feed descending the preheater. There is therefore an “external” cycle between the preheater and the precipitator in the dust. Some of the volatile species are therefore carried up the preheater with the uncollected feed….. …..and some exits the top cyclone in the dust.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(13) CemNet Training - Course CKPC01. Cement Kiln Chemistry. Module 1 – Session 7. 1.7 Alkali cycles. Some components, such as sulphides, in the kiln feed will oxidise …..and are lost as in the upper stages of the preheater….. gases (SO2) in the preheater exhaust gas. Some will be absorbed onto the raw mix in the raw mill…... …...and is returned to the kiln in the kiln feed. Another external cycle between the preheater and the raw mill.. Cement Kiln Chemistry. 1.7 Alkali cycles. Other highly volatile species, such as ammonium compounds, will evaporate in the upper stages of the preheater and be partially absorbed in the raw mill. Yet another external cycle between the preheater and the raw mill. We are going to return to these external cycles in Module 3 when we consider emissions from cement kilns.. Cement Kiln Chemistry. 1.7 Alkali cycles. The melting points of volatile species we introduced earlier are of the discrete species….. Melting Point In practice there will K2SO4 1069°C be a mixture of these Na2SO4 884°C volatile species salts in NaCl 801°C the hot meal….. KCl 776°C …..softening and melting points may be even lower due to the formation of low temperature eutectics between the salts. We will talk about the cycles of the individual species and the mixtures in the next sessions of the course.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

(14) CemNet Training - Course CKPC01. Module 1 – Session 7. Cement Kiln Process Chemistry Module 1. Cement kiln energy efficiency and productivity. 1.7 Alkali cycles.. This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com.

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