Monolithic refractories for cement kiln system

Castable technology is gaining more importance in cement rotary kiln system, feels Deepak Banawalikar of Dalmia Refractories Ltd.

The Indian cement industry – since its de-regularization in 1982 – has attracted huge investments. India from importing cement in 80s now has became the second largest producer of cement in the world, next to China. S

Modern kilns with modern pyro-processing technologies have increased productivity substantially but at the cost of severe abuses on the refractory materials. The higher capacities in turn higher thermal load clubbed with costly downtime has enhanced the criticality of the refractory materials. Chemical abuses due to alternate fuel further aggravated the operational parameters. To address these challenges refractory producers have come up with new refractory products with higher hot properties.

Dalmia Group has played an important duel role in this growth story by raising installed cement capacity to 24 mn t in cement Production while Dalmia Refractory Limited (DRL) executed major turn-key projects with new business strategy. DRL?s presence in shaped alumina refractories for cement industry was always at the top with leading brands of Lofal, Dalsint, Dalkar of 90s were further reinforced with Dalburn, Dalsupreme & DAL-CR range of products with evolving cement technology. If we closely study the usage of monolithic refractory in cement segment, it is gaining importance & being used in critical application areas. DRL has come up with many ranges of monolithic refractories from conventional (dense & insulating), low & ultra-low cement castables, speciality castables like non-wetting to no-cement along with gunning a variety for diversified applications in the cement industry. Dalmia Institute of Scientific & Industrial Research?s (DISIR) contribution in technical support is also commendable. With in-depth study in development & application of these monolithic products an attempt is made to review the monolithic refractories scenario in modern cement plants.

Indian cement scenario & technological challenges
The present per capita cement consumption in India is around 190 kg against world average of 500 kg. Domestic cement consumption is given in fig. 1. It is growing at CAGR of 15.7 per cent and expected to touch 398 mn t by 2017. Thus cement manufacturing capacity, which is presently at 366 mn t is going to touch 421 mn t by 2017. In this capacity enhancement, DRL has played a major share. Table-1 is the cement projects executed by DRL in last few years in India.

With these modern projects it was observed that in cement industry monolithic refractories have made impacts in a wide variety of applications in line with the global trend. Conventionally insulating & dense castables were applied widely in pre-heaters & kiln hood, etc. With the advent of low cement castable application areas got extended to cooler, tip casting, bull-nose areas. Gunning technology further helped in lowering the down time in wide variety of applications including TAD bends, pre-heaters and coolers.

Alternate fuel concept brought in many challenges. Cement industry is one of the largest consumers of the fuel. Fuel cost economy is very crucial. In recent past cement rotary kiln is being considered as incinerators & a wide variety of waste materials including used tyres & other chemicals like waste oil, etc. created differential combustion behaviour abusing refractory substantially. Pet-coke the bye-product of refineries became one of the major alternate fuels. Pet-coke being high sulphur fuel has further disturbed the balance of alkali/sulphur vapour ratio creating build up problems in the smoke chamber, lower cyclones and riser ducts. Thus non-wetting castables have been developed. In recent past no-cement castables are developed with niche properties and are making strong inroads in variety of applications from bull-nose, tip casting and for burners. Thus importance of monolithic refractories in cement industry have enhanced substantially over a period of time.

Clinkerisation process & volatile cycle
Raw meal is the mix of limestone, quartz, clay with corrective lateritic materials. Briefly clinkerisation process can be simplified in following manner:

• The decomposition of limestone into CaO and dehydration of clay leading to release of quartz
• This triggers the reaction sequence of formation of C2S & C12A7 with some amount of C3A
• As temperature rises above 14000 C in the main burning zone liquid concentration rises to around 24-30 per cent
• On cooling C3S & C4AF phases recrystallises out in final clinker with very abrasive nature

The silica & alumina moduli, lime saturation factor (LSF) along with liquid content decide the burnability of the raw meal. This burnability factor has bearing on refractory performance throughout the kiln system. Finally operational parameters, in conjunction with above indices decide the mineral phase assemblage, which define the quality of cement.

The raw meal & fuel brings in very small quantities of salts of sodium, potassium, sulphur & chlorine in the kiln burning environment. Alkali oxides are very low melting materials with 400 & 1,100o C meting points for K2O & Na2O respectively. They tend to form sulphates and chlorides of sodium / potassium. Even these salts are having very low melting points in the range of 800 to 1,070o C. These salts tend to get partially volatalised as they enter the kiln burning zone and rest escapes with clinker. These vapours travel along with flue gases towards kiln inlet areas and condesnses on the raw meal in the colder regions of the up-stream. Now raw meal rich in alkali salts enters the kiln thus setting up the volatalization-condensation cycle.

This volatalisation & subsequent condensation of semi-fused mass of alkali salts on the incoming raw meal or refractory surafce developes stickyness. Incoming raw meal keeps on depositing on alkali coated refractory. This slowely leads to build ups.

The nature of compounds formed in the kiln environment is function of Alkali Sulphur Ratio or ASR.

This phenomenon disturbs the smooth flow of raw meal de-stabilising the kiln system & productivity. This is very common in lower riser ducts, kiln inlets, smoke chambers affecting the production.

Operating conditions of cement kiln system
Nowadays the output of the average kiln is around 4,500 to 5,500 tpd. With technological advent, new projects are coming up with 8,000-10,000 tpd. This enhances thermal loads substantially above 5.5 Gw/m2h. Due to thermo-chemical constraints, monolithic refractories have not made inroads inside the rotary kiln. Attempts are being made for development of Basic Castable for its obvious advantages. Monolithic refractories have presently limited to tip casting area of the rotary kiln. Beyond rotary kiln diversified applications have been developed for monolithic refractories. Thus relevant operating conditions are being discussed herewith.

Prevailing operating conditions of modern cement kiln systems are given in Table 2. From the point of criticality of the operating parameters, refractory can be broadly classified in following categories in isolation OR in combination.

• Thermal shock resistant
• Coating/chemical resistant
• Abrasion resistant
• Application friendly

Thermal shock resistant castable: Areas of cement kiln system observe very high thermal fluctuations are tip casting, grate cooler & burners. This thermal fluctuation is the main cause of developing stresses in the refractory body. The stress intensity is the function of thermal expansion. The low coefficient of thermal expansion & low MOE helps refractories withstand these thermo-mechanical stresses. Andalucite based Low Cement castables are preferred due to intrinsic lower thermal expansion of these materials.

In last few decades the low cement technology with use of lower high alumina cement (HAC) in combination with micro-fines has totally changed the concept. It helped in development of high strength castables. High strength in turn helped in improving thermal shock resistance in combination with spall resistant materials.

The behaviour of these materials is dependent on

• Suitable use of micro-fines
• Effective use of quality de-flocculating agents

With captive manufacture of high alumina cement under controlled conditions at Katni Dalmia Refractories could develop consistent vast range of engineered products. In practice usage of one property cannot be considered in isolation. Thus we need to have a combination of desired properties to make product effective.

Coating repellent castable: Porosity is the inherent part of general refractories but for fuse-cast refractories like AZS. The extent of porosity can be defined while designing the product. As mentioned above, alkali salt vapor carried by flue gases starts condensing on cooler parts of the refractories. Alkali vapours first infiltrate into these pores and react with alumino-silicate refractories forming expansive low melting feldspathic compounds. Infiltration of these materials first densifies & then initiates cracks due to heavy expansion on cooling thus making refractory failure eminent. This phenomenon is known as alkali bursting. At the kiln outlet also atmosphere are normally alkaline due to liquid in the clinker.

Thus new class of refractory has been developed to prevent the deposition & infiltration of the alkali salts. The coating repellent refractories are modified on surface of silicon carbide making it non-wetting.

SiC (s) + 3/2 O2 (g) ? SiO2 (s) + CO (g)
SiO2 (s) + Na2CO3 (l) ) ?
Na2SiO3 (glass) + CO2 (g)
SiO2 (s) Na2SO4 (l) + 2C + 3/2 O2
(g) ? Na2SiO3 (glass) + SO2 (g) + 2CO2 (g)

SiC is converted to vitreous silicate glass on the surface making smooth surface minimising any adherence of semi-fused sticky mass. Thus build up can definitely be minimised. Same property has been used to develop the alkali resistance also in other parts of the kiln system.

DRL has come up with range of SiC-based castables under the name of Dalresicoat class of castables with varying range of SiC with the severity of alkali attack. The quality of SiC is very crucial for this development. The microstructure of the SiC used for this is attached in Fig. 2. Microstructure clubbed with purity defines the quality. This class of refractories performed successfully in host of the cement plants applications. The non-wetting characteristic of clearly seen in compatibility test carried out with alkali test as shown in fig 3 a & b.

Considering alkali attack by environment special quality burner castable was developed having combination of alkali resistance with thermal shock resistance. This product Dalresicoat-B (BP) has performed excellently in Central Indian plant. Fig: 4 shows the very good condition of the burner after four months usage and continued further.

Abrasion resistant castable: The clinker formed in the burning zone is gushing out of the discharge zone. Abrasion is the major mechanical abuse clubbed with thermal cycling and alkali environment. Thus tip casting material also needs very high abrasion resistant with very high thermal shock resistance. Making the product alkali resistant further improves the performance. Conventionally corundum based LC castable or andalucite-based LC castables were used in kiln outlet/tip casting area with satisfactory performance. In modern kilns with environmental conditions, Andalucite & corundum-based LC Castables started showing performance limitations. To overcome these limitations, DRL has now developed Dalresicoat-B (TC), which is specially developed highly abrasion resistant and can withstand both thermal shock as well as an alkali attack. Fig 5 shows the very good condition of tip casting after six months in Central Indian cement kiln with 6,000 tpd capacity.

In new development 90 per cent alumina corundum based no-cement castable is gaining importance both in tip casting and bull nose areas. The advantages with no cement castable are as follows:

• Very extended shelf life as no binder in dry mix
• Setting mechanism is based on condensation reaction forming Si-O-Si bond system
• Moisture is free & can be driven out fast. Dry-out thus not by dehydration
• Si-O-Si bond forming reaction accelerates with dry-out resulting in very high strength
• No calcium aluminate phases in matrix this enhances hot properties
• High sulphur & alkali resistance
• Do not require high shear mixing as in LC castable
• Installation & dry-out can be very fast lowering turn-around time substantially

Dalmia Refractories has developed this no-cement castable under the name Dalultima and is being promoted. This castable is having excellent properties. DRL has applied it in critical use of burner in North East cement plant & running satisfactorily. Expected life is more than eight months.

Take off duct & tertiary aid Ducts are other areas, which demand very high abrasion resistance materials. This is mainly due to dust laden gases. Depending upon the plant specific severity various grades of low cement castables are used in these applications. Commonly 45 per cent Al2O3 LC castables are used. For very severe application, DRL has developed Rapidense-60, which with proven performance. This is specially engineered with high purity, low porosity alumino-silicates with excellent abrasion resistance.

Application-friendly castables: With high capacities, modern Indian kilns is desirable to have scheduled maintenance and shut down. Down-time being a very costly affair it is always better to have application friendly materials which can help in lowering of turn-around time.

Different application techniques and product technologies may be thought over for this purpose. These products can be looked from different perspectives like

1.Installation technology like gunning
2.Product technologybIn the first case gunning/shotcreting products will be of use.

These materials can be used in faster installations in maintenance for critical areas like cyclone pre-heaters, TAD ducts, coolers, etc. DRL has a range of products in gunning variety. Dalresistal-80 Gun is being successfully used in many plants around Katni for cooler and TAD damper locations. Fig. 6 shows the installation of Dalresistal-80 Gun in Central Indian plant. This product is having very low rebound loss & excellent adherence.

In second case products like newly developed no-cement castable can be considered. In this case although application can be made using vibro-casting but as mentioned earlier material characteristics helps in simplified faster application with efficient dry-out. Gunning products are being developed in developed using no-cement technology. Dalmia Refractories range of speciality castables are given in Table 3

Conclusion
In short castable technology is gaining more importance in cement rotary kiln system. Major usage of castable is outside the main rotary kiln except for tip casting & discharge zone. Development of high strength LC Castables paved way for use of castable in many critical areas. The alternate fuel added woes to refractory materials. Development of non-wetting castables followed by gunning materials further increased the castable consumption in the cement industry. If we analyse the monolithic consumption in projects it ranges from 0.6 to 0.8 kg per mn t of clinker and will vary for different OEMs. Also in running plants, monolithic refractory consumption hovers around 0.08 kg per mn t of clinker, which is significant.

References
1)Cement Industry in India -August 2015 by Indian Brand Equity Foundation
2)Swapan Kumar Das & Co. – Indian Ceramic Society-2012 (Bangalore Chapter) (71-78)
3)I. N. Chakraborty – IREFCON-2012 (48-53)
4)Reni von der Heyde & Co. – IREFCON-2014 (152-157)

(SO3/80)
Q (ASR) = –
{(Na2O/62) + (K2O/94)
+ (Cl/71)}
When Q = 1 Alkalis Salts are in balanced condition in environment
When Q > 1 Environment contains Alkali salts with Free SO3
When Q < 1 Environment contains Salts with Free Alkali Oxides

Table 1: Turn-key projects handled by Dalmia Refractories

Table 2: Operating Conditions 120oC