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IMPROVED GRANULAR PROCESSING: TOWARDS ENERGY EFFICIENCY IN CEMENT MANUFACTURE

A PROJECT PROPOSAL PREPARED BY THE EXPERT GROUP

FOR NEW MILLENIUM INDIAN TECHNOLOGY LEADERSHIP INITIATIVE

CO-ORDINATED BY COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH NEW DELHI 2002

1 1. EXECUTIVE SUMMARY

Considerable international research effort has focussed on developing a better understanding of granular matter, that is dry, particulate systems, and progress has been made on many fronts. This new understanding has the potential to contribute to improved efficiencies in industrial processes involving particulate systems. Granular processing is widespread in India, and there is also credible technical expertise in this area across a number of institutes and industries. It is proposed that we consider granular processing as a niche area for a new technology initiative. Our proposal is to initiate a research and development programme to address the main issues of energy efficiency in cement manufacture based on improved granular processing. Two energy intensive unit operations of cement manufacture (clinker formation and grinding) are chosen for study with a focus on: (i) cement clinker formation in rotary kilns and (ii) formation of blended cements using mechano-chemical techniques. The emphasis of the project will be on developing novel designs and optimization of operating parameters based on recent scientific advances to achieve technological leadership on some niche areas of cement manufacture. The project will carried out in a collaboration between industry partners (ACC, L&T, …) institutes (IITB, …) and national laboratories (NML, NCL, …).

2 2. TITLE OF THE PROJECT

Improved Granular Processing: Towards Energy Efficiency in Cement Manufacture

3. INTRODUCTION

It has been recognized for some time now that granular processing (the terminology used to refer to the unit operations involving particulate systems) is poorly understood in spite of its widespread industrial importance. Examples of industries in which granular processing is significant include agriculture and food processing, fertilizer, ceramic, chemical, construction, cement and pharmaceutical among others. In the chemical industry approximately one-half of the products and at least three-quarters of the raw materials are in granular form. It was estimated in 1994 that $61 billion of turnover in the US chemical industry was linked to particle technology [1]. Design of processes in most cases is empirical and most likely suboptimal. This recognition has resulted in an international research effort into the science and technology of granular processing spanning a range of disciplines [2]. Results obtained over the last ten years promise to lead to improvements in efficiencies.

Granular processing is widespread in India, given the large scales of operations of the industries. There has also developed over the last few years considerable research expertise in this area over a number of institutes and industries (IIT Bombay, IISc Bangalore, SNBNCBS, JNU, ISI Calcutta, IIT Kanpur, IIT Delhi, ACC, NML, L&T, Grasim, TRDDC Pune, and many more). Translation of this expertise into improvements in efficiencies could be of considerable benefit, and this is the motivation for considering granular processing as a niche area for a new technology initiative. In this proposal our focus is on cement manufacture.

There are several challenging issues related to granular processing in cement manufacture. Furthermore, there are large capacities of manufacture in India, and usage is projected to increase with the anticipated infrastructure and housing projects. India is the world’s second largest cement producer after China. However, per capita consumption of cement in India is 99 kg as against world average of 260 kg. India has an installed capacity of 116 million tonnes. Cement production by the end of year 2000 was 96 million tonnes with a production growth of approximately 4.6%. Estimated production of cement by 2004-2005 would be 125 million tonnes. Most recent data indicates that world wide consumption of

3 cement has climbed to 1.62 billion tonnes. Given a continuing annual growth rate of 3%, global cement consumption would reach 2 billion tonnes by 2008 which would boost the requirement for new and replacement production units in many regions (present world cement capacity is 1.75 billion tonnes).

Cement manufacture requires a large input of energy and it has a significant environmental impact with 1 ton of CO2 produced for each ton of cement. Cement is manufactured using limestone (75-80%) and additives like clay, shale, laterite, and bauxite (20-25%). The raw materials are quarried and crushed and then ground in correct proportions in raw mill to give raw mix of desired quality. This is subsequently burnt in a rotary kiln at a temperature around 1450oC. Pulverized Coal / Oil / Gas is used as fuel for burning in rotary kiln. Coal is ground in coal mill. The raw mix undergoes a number of chemical reactions in the burning phase and leaves the kiln as cement clinker. Finally, clinker (size 0 to 35 mm) is ground to a fine powder-cement, to desired fineness in a cement mill together with 3-5% gypsum.

Crushing of raw materials and grinding of raw mix, coal and cement involves comminution process in cement plant adds up to approximately 70% of total electrical energy consumption of plant and clinker grinding alone accounts for almost 40%. Another energy intensive operation is clinker formation with an energy requirement of 700-800 kcal/kg of cement [3]. This amounts to 70-80 percent of the total energy used, and accounts for 50% in terms of cost of energy used. Even relatively small improvements in energy efficiency of the process could thus produce a significant overall economic impact for the process.

Innovations in cement manufacture

The manufacture of cement has been the focus of considerable attention worldwide because of the high energy usage and high environmental impact of the process. As a result, there have been several recent innovations in cement process technology, and some of the most important ones are listed here. A recent advance in grinding technology is high pressure grinding which can result in 40\% lower energy requirements for brittle materials. This translates to savings of 7 kWh/ton for raw meal grinding and 10 kWh/ton for finish grinding. Improved process control and burning have led to improvements in efficiency leading to savings of 2-10% of fuel. Blended cements are also being proposed to reduce the environmental impact of cement manufacture, and this also has a significant impact on

4 energy utilization (up to 1.41 GJ/ton). Recent patents have been proposed to improve cross- sectional mixing by introducing wave-like internals. A detailed analysis of the economic impact of various innovations is reported by Worrel et al. [4].

4. OBJECTIVES

The primary objective of the present project is to develop new technologies to improve the energy efficiency of the cement manufacturing process. The focus will be on the two major energy intensive processes: (i) clinker formation in rotary kilns and (ii) grinding and mechano-chemical activation. In (i) the energy efficiency improvements will be based on design and process improvements, while in (ii) they will be based on using cements based on clinker blended with wastes (fly ash and blast furnace slag) as well as improved grinder designs. Several different elements of these processes will be considered in parallel, and these include: novel kiln internals for promoting heat transfer, optimal kiln temperature profiles, improved burner designs, mechano-chemical activation of blended cements, and improved grinding machines. Specific objectives are listed below.

Rotary kiln design and optimization

The overall objectives of this part of the project relate to clinker production in a rotary kiln are:

 Reducing energy consumption.

 Controlling the clinker size and morphology.

 Increasing capacity.

These objectives will be achieved by using novel kiln internals for improving heat transfer and by process optimization. Some of the issues considered will be the effect of size distribution of raw meal, temperature profile in the kiln, degree of filling and transverse mixing on the energy efficiency, the product quality and the size of clinker produced. The approach will include batch experiments, CFD modelling and detailed product and process characterization.

5 Grinding and mechano-chemical activation

The objectives of the proposed study is to focus on the combined effect of chemical and mechanical activation:

 To increase utilization of the fly ash and slags in the cements without degradation in cement properties.

 To develop high performance cements with current levels of utilization of fly ash and blast furnace slag.

 To explore the prospects of making high performance clinkerless cements from slags and other industrial wastes.

 To develop improved grinders.

The work would involve study of effect of mechanical activation on the mechanical activation of individual constituents used in the blended cements, studies on the combined effect of chemical and mechanical activation of cement constituents, studies on mechanical activation and dry blending of cement, studies on mechanical activation and blending of cement with waste (Fly ash and slag) activated in wet condition and studies on clinkerless slag cements

5. NOVELTY OF THE PROPOSAL

The proposal is based on the application of newly developed scientific understanding in different areas to the improvement of the cement manufacturing process. The design and optimization of the rotary kiln will be based on new knowledge of how to generate chaotic motion in the cross section rotary kiln and thus to improve mixing and heat transfer. Optimization of designs will be carried out using CFD. Such tools have not been previously applied for rotary kiln design. Recent results from mechano-chemistry show how efficient activation of fine particles can be achieved. This has promise for making blended cements using wastes as well as high strength cements. Simultaneous mechanical and chemical activation has not been previously reported.

6 6. DELIVERABLES AND IPR

The specific deliverables and IPR generated from the project are given below for both of the project components.

Rotary kiln design and optimization  Increasing throughput by 25% over the existing typical capacities.  Reduction in fuel consumption by 5% based on existing usage.  Controlled clinker granulometry with a size variation of 5 mm compared to the current variation of 15 mm.  Stabilization of reactive polymorphs of clinker phases to yield clinker with improved grindability and cement of improved quality.  CFD models to simulate performance of burner, radiative heat transfer to kiln wall and bed, and fluid dynamics, heat transfer and reactions in kiln.

The IPR resulting from the project is anticipated to be patents for new kiln internals and new burner designs, and process modifications. Validated models for the kiln would also be of commercial value.

Grinding and mechano-chemical activation  10-20% increase in the current level of utilization of industrial wastes, namely slags and fly ash without degradation in cement properties  High performance cements with current level of waste usage in cement industry  Clinkerless cements based on industrial wastes, BF and other slags and fly ash  Evaluation of alternative strategies in cement processing based on the mechanical activation of slag and fly ash in wet condition.  Improved grinding machine.

The IPR resulting from the project is anticipated to be patents for new additives for mechano- chemical activation, new high strength cements and new grinding technologies.

7. REFERENCES

[1] B. J. Ennis, J. Green and R. Davis, Granular processing: Legacy of neglect in the United States, Chem. Eng. Prog. 90, 32-43 (1994).

7 [2] J. M. Ottino and D. V. Khakhar, Fundamental research in heaping, mixing and segregation of granular materials: Perspectives and challenges, Powder Technol., 121, 117-122 (2001). [3] C. M. Kutty Sankaran, Cement 2005, published by Context Date Services, pp. 3 (1998). [4] E. Worrell, N. Martin and L. Price, Potentials for energy efficiency in the US cement industry, Energy, 25, 1189-1214 (2000).

8 Improved Rotary Kiln Design and Operation: Detailed Proposal

1. CLINKER FORMATION IN A ROTARY KILN

Clinker formation in modern kilns involves feeding precalcined (up to 96%) and preheated raw materials at one end of a rotating cylinder and heating by means of burning powdered coal at the other end. A conventional rotary kiln is a simple large diameter cylindrical shell (more than 45 m length, 2.5-6 m diameter), lined with refractory bricks, installed at an incline and rotated slowly about its axis (1-3 rpm). The product is discharged from an outlet at the lower end. Typical residence times are 1-3 h. Low fill fractions (7-15%) are used to ensure rapid cross- sectional mixing.

The heat source introduced into the kiln from the outlet side is a direct flame formed by burning coal or oil. Different modes of heat transfer include

- Radiative heat transfer from the hot gases to the materials.

- Convective heat transfer between the hot gases and the materials.

- Radiative heat transfer between the refractory lining and the materials.

- Conductive heat transfer from the refractory lining to the materials.

Approximately ninety percent of the heat transferred to the materials by radiation produced by the hot gases, and the remaining ten percent of the heat is transferred from the refractory lining to the material by conduction. Heat transfer by convection in the system is small. Heat transfer between the flame and the particles is dominated by radiation and the clinkerization process is controlled by heat transfer, since it is highly endothermic. Thus, uniform exposure of all particles to radiation, which is determined by transverse mixing, is important for the process.

As materials travel from inlet to outlet section of the kiln they are first calcined and then pass through a high temperature zone (the so called sintering zone). Here molecules of raw material break up and recombine to form new components referred to as “clinker'”. The clinker formation in a rotary kiln comprises of various complex phenomena. Temperature, residence time, mixing and particle size are of importance in the burning of raw materials. For example, if particle sizes are too small then they will be entrained with exhaust gases. Though smaller

9 particle sizes provide more surface area for heat transfer and also prevent de-mixing, yet larger sizes are preferred in order to decrease the stack load. Lack of uniform heating of the bed may result due to insufficient mixing within the bed, causing the surface layers to become overburned or overheated and other portions to be underburned.

The final microstructure of the clinker is heterogeneous comprising different compounds and depends on processing. Uniform chemical reaction requires effective heat transfer and mixture homogeneity, and this eventually determines the properties of the cement produced.

The theoretical heat of formation of cement clinker is 420 kcal/kg. Both dry and wet process kilns consume far greater energy, approximately 700-850 kcal/kg of clinker and 1300- 1600 kcal/kg of clinker, respectively [1]. The low heat efficiency results from surplus heat being dissipated in the stack gases lost by radiative heat transfer through the shell of the kiln and also being dissipated with the product itself. There appears to be considerable scope for further improvements in energy efficiency.

Scope

Quality of clinker for a given raw material mixture depends mainly on heating rate and homogeneity of the mixture. Efficient utilization of heat sources saves energy cost and proper mixing maintains quality. The present work focuses on development of proper internals that not only increase heat transfer but also improve homogeneity within the mixture, which ultimately results in a good quality product. Previous studies related to this subject are briefly reviewed; the objectives of the proposed work, the approach to be followed and the work elements are given.

2. BACKGROUND

Elements of granular mixing

Mixing of granular materials in rotary systems has some special characteristics and we briefly discuss these here as the base knowledge for the proposed project.

The flow in rotary systems without internals is limited to a thin surface flowing layer while the remaining material rotates as a packed bed [2]. Mixing thus occurs only in the surface layer where particles can move relative to each other. An important parameter for mixing in rotating cylinders is the degree of filling: the rate of mixing is slowest for 50% filling and

10 increases with reduced filling [2]. Since the residence time is nearly independent of the degree of filling [3], better mixing is accompanied a reduction in capacity.

A second important feature of granular mixing is spontaneous segregation of particles due to size differences and/or density differences between particles in the mixture [4]. In rotary cylinders, the smaller particles or the more dense particles form a central core due to segregation [4]. These particles are shielded by the larger or the less dense particles from the heating medium in the freeboard. They also have a slightly longer residence time in the kiln. Both these effects can lead to non-uniformity of the product.

Our recent work has shown how to generate chaotic motion in rotary systems. This leads to improved mixing [5]. The implications for segregating systems have also been considered (Hill et al., 1999).

The above elements have not been previously addressed in the design of rotary kilns and rotary dryers in a systematic manner. These elements provide the foundation for improving the design of rotary systems with regard to mechanical mixing and thus for heat transfer efficiency enhancement.

Kiln internals – patent survey

A patent search showed that there were 1101 US patents related to rotary kilns registered in the period 1996-2001, indicating that this is an active area of research. We summarize below some of the developments related to the internal design of rotary kilns to improve the heat transfer and mixing quality.

Thomas in 1913 [7] suggested an improvement of internals modification of rotary kiln. His invention consisted on introduction of projected ledges, shoulders or rib on the periphery of the rotary kiln. Material from the bottom of the load was carried up the sides and while falling from the top, material would come in direct contact with the flame.

All the earlier proposed lifter or projections were knife-edges but Tomlinson in 1925 [8] modified the projections. His projections were of rectangular or quadrilateral pyramid structure. His lining arrangement was in such a fashion that all the material would come in contact with lining during passage of the material from the upper to the lower end of the kiln. The projection was in the direction of rotation and the projection was placed in such a way so that flow of heating gases would not be disturbed.

11 Burke in 1933 [9] designed the kiln lining in such a fashion where bricks of the lining projected inwardly to catch materials and lifted the same, instead off permitting the materials to remain on the bottom of the kiln as it rotated.

Olsen et al. in 1969 [10] also improved the rotary kiln lining. They proposed that kiln lining must be possess at least one lifter element that comprises a monolithic refractive mass of substantial size but of diminutive length compared with the total length of the kiln. Also such type of element was disposed generally longitudinally in the kiln or generally parallel to the longitudinal axis of the kiln. The leading and trailing faces of such lifter element taper toward each other and in one embodiment the trailing face is elongated or exhibits a smaller angle to the base line of the element as compared with the leading face. Lifters were spaced as desired along the internal circumference of the shell. A plurality of lifters placed symmetrically about the periphery of the lining provides best heat balance in the kiln.

Sunnergren et al. in 1979 [11] suggested the introduction of mixer block in rotary kiln to improve the mixing and heating rate on clinker formation. The mixer block made of refractory material or coated with refractory was triangular cross-section was attached on the lining of the kiln. The use of mixer blocks of this invention in the refractory lining of the kiln, gave a more uniformly calcined product with little formation of dust and small particles due to breakage of the material being calcined. Calcination takes less time than normally required to calcine the same amount of material to the same degree, thereby resulting in an energy saving.

Though lifters in rotary kiln have been known for several years yet Tyler [12] made some improvement. One of the problems which occurs with such refractory lifters is that they disintegrate prematurely, both by spilling away of the inner face of the lifter and also by cracking of the lifter at the inner surface of the surrounding refractory lining. According to this invention, an improved refractory lifter with embedded metal anchor having a plurality of branches at its outer end was introduced. Some of the branches extended to the inner face of the lifter and the point at which the branches commence was closer to the metal shell of the hot face of the refractory lining adjacent to the lifter. His invention concerned mainly the improvement of the structure and location of the anchors used to reinforce refractory lifters and with this invention he was able to solve the problem of premature disintegration of the lifters.

Applying the principle of regenerative heat transfer Holopainen [13] introduced a plurality of rods within the inner shell of the kiln. Circumferentially placed rods help alternate contact of

12 hot gases and materials while kiln rotates. The effect of regenerative heat transfer is primarily dependent on the mass of the rods. Rod length is up to 30% of the diameter of the kiln. Rods of various cross sections are suggested, like square or droplet shaped. The orientation of the rods is in a direction which facilitates the movement of the particles and also provide a good contact of materials and rods.

Mosci [14] improved the heat transfer within the kiln by introducing the polygonal lining. The lining was polygonal cross-section, more specifically hexagonal configuration. He claimed that the polygonal lining improved the heat transfer between the high temperature gases and the materials within the kiln. On this type of lining, materials were more exposed on the hot surfaces. The polygonal lining was formed by installing pre-shaped bricks or by casting an appropriate heat and abrasion resistant refractory or ceramic material onto the inner wall of the shell such that when viewed along its longitudinal axis, the lining will be polygonal cross- section. Conventional rotary kilns are lining with refractories or bricks that protect the shells of rotary kilns against heat and abrasion. Generally, tapered bricks are placed in a ring manner along the circumference of the steel shell of the kiln. In addition to protecting the steel shell, the refractory bricks reduce the heat loss through the steel shell.

Mosci [15] improved the fabrication of the polygonal cross sectional lining by casting an appropriate heat and abrasion resistant refractory or ceramic material onto the inner wall of the shell. The particular configuration may be achieved by the use of forms and appropriate spacers, which define the volume, which is to be filled or cast with the refractory material.

Doerksen [16] replaced the conventional lifters by steel lifters. These steel lifters attached with the kiln shell and are configured in such a way to provide good mixing along with longer life.

Mosci [17] introduced wavy or sinusoidal wave shaped (predetermined amplitude and wavelength) inner lining (Fig. 1). The inner face of the lining contains a plurality of peaks and troughs of equal height. With the incorporation wavy shape lining, he was able to improve the efficiency of heat transfer. Homogeneity, turbulence, and also residence time of the materials inside the kiln increased to a significant extent.

13 Fig. 1. Rotary kiln wavy internal (Mosci, 1999)

Modelling

Performance analysis of the rotary kiln requires a detailed understanding of the heat transfer, flow and chemical reactions in the kiln. The performance of rotary kilns is usually measured in terms of (1) through put or capacity (2) fuel consumption per unit weight of clinker and (3) uniformity of clinker size. Several attempts have been made to develop computational models to represent processes occurring in cement kilns and to simulate its performance. Most of the attempts carried out before 1990 assumed that the bed is well mixed in the transverse plane and used one-dimensional plug flow model (see for example, Barr et al., [17]). Such models ignore the motion of the bed in the transverse plane, which re-distributes the energy absorbed at the interface. Attempts are made to model the granular motion of particles in rotary kilns (Boateng, [18]; Khakhar et al. [2]). Boateng and Barr [19] have incorporated such a granular flow model into a thermal model of rotary kiln. They did not however modeled combustion (and radiation) in the free board region and chemical reactions in the bed. Recently, Mastorakos et al. [20] developed a computational fluid dynamics (CFD) based model, which included combustion, radiative heat transfer, conduction in the bed/walls and chemical reactions.

14 They have, however, ignored motion of particles within the bed. Combustion (flame) was modeled by considering an axis-symmetric two-dimensional solution domain. The melting and sintering of particles was also ignored. Although the clinker burning process has been studied in individual pellets [21], this information has not been integrated into kiln design. Thus, none of the available models are adequate to relate or to estimate the above mentioned performance measures of the cement rotary kiln.

3. PROPOSED WORK

The survey presented above clearly shows that clinker formation in cement manufacture consumes large amounts of energy and accounts for a significant fraction of the cost of the cement. The efficiency of the process is strongly influenced by granular mixing and segregation, but these processes have not been analyzed in any systematic way for the design of kilns. Technologies to improve mixing at higher loadings in rotary should improve efficiencies and capacity of systems, which promise to have a significant positive effect on the economics of the process as well as on environmental impact. Recent works from our laboratory (described briefly above) and elsewhere have led to a better understanding of flow and mixing in rotating cylinders. These provide a starting point for the detailed analysis and design of rotary kilns.

Our proposal is to initiate a research and development programme to address the main issues of rotary kiln design and operation in cement manufacture. The emphasis will be on developing new designs, particularly with respect to kiln internals, and optimization of operating parameters. Modelling of the process would be carried out in parallel to aid in optimization. The programme should be multi-institutional and in collaboration with industry. The programme would involve model experiments in small scale systems and by measurements in existing pilot scale systems. It would address issues of scaleup and implementation in existing plants in the form of retrofits.

Objectives

The specific objectives of the project include

* Reducing energy consumption

* Controlling the clinker size and morphology

* Increasing capacity

15 in clinker production in a rotary kiln. These will be achieved by using novel kiln internals for improving heat transfer and by process optimization using CFD. Some of the issues considered will be the effect of size distribution of raw meal, temperature profile in the kiln, degree of filling and transverse mixing on the energy efficiency, the product quality and the size of clinker produced.

Approach

Batch experiments in cold and hot systems will be carried out to test the efficacy of different kiln internals. Cold experiments will first be carried out to evaluate different internals in terms of cross-sectional mixing. Hot experiments in a batch system will be carried out using internals selected based on the cold experiment results. The temperature will be varied with time to simulate the temperature experienced by material as it travels through the kiln with a given temperature profile along the axis. The material produced will be characterized in terms of size, chemistry and morphology. This will include studies using SEM-EDACS, XRD and optical microscopy. Grindability tests will be carried out and in selected cases cement will be made and tested using standard tests. Temperature distribution within the kiln will be monitored by infra red imaging and thermocouples, and heat transfer rates will be inferred.

A multi-layer modeling framework including development of a model for coal-fired burner and subsequent heat transfer via radiation, convection and conduction to reacting bed will be developed. Synthesis of results obtained from these three modeling layers will allow one to understand and to optimize performance characteristics of cement rotary kiln. New ideas of kiln internals can be evaluated to identify couple of most promising designs for further testing and pilot plant studies. Burners of cement rotary kiln need to produce short, narrow and strongly radiant flame with minimum primary air to ensure efficient heat transfer, correctly developed clinker phases and long refractory life. A detailed CFD based model to simulate coal-fired burners will be developed. Complicated geometry of burner will be accurately modeled since it controls the generated swirl and stability of the flame. Particle laden turbulent flow with combustion and heat transfer will be modeled. Ability to handle different fuels without jeopardizing key characteristics (short, narrow and strongly radiant) can then be evaluated to explore possible performance enhancement of burners. Burner model, heat transfer (convection, conduction and radiation), regenerative heat transfer due to rotating kiln walls and heat sinks due to chemical reactions need to be coupled together to formulate overall model to simulate

16 performance of rotary kiln. At this stage, the CFD model for the burner needs to be coupled with clinker formation model and radial mixing model. Iterative algorithm will be developed to handle such a strong coupling between different component models. This comprehensive model will be used to evolve and to evaluate new concepts and ideas for performance enhancement cement rotary kilns. Availability of the relevant kinetics may pose a serious challenge for carrying our realistic simulations. In absence of such kinetics, some data either from the plant or laboratory/ pilot kilns is essential to calibrate the computational models. The model and batch experiments with programmed temperature variation will be used to optimize the temperature profile. Finally, the best design will be implemented at the pilot scale to prove the concept.

Work elements

A list of work elements required for the project are listed below.

 Design and fabrication of high temperature laboratory scale batch kiln. (L&T, ACC)

 Design and fabrication of instrumentation of batch kiln. (L&T, ACC)

 Design and fabrication of high temperature resistant internals for batch kiln. (ACC, L&T)

 Mixing experiments on cold batch system with different internals (existing). (IITB)

 Experiments on hot batch systems. (IITB)

 Characterization of product from hot batch experiments. (ACC, IITB)

 CFD modelling of kiln. (NCL)

 Validation of kiln model (NCL, ACC, IITB)

 Optimization of operating conditions by means of model and experiments.

 (NCL, IITB)

The above work elements will be distributed in different institutions and industries based on expertise available as indicated above.

4. REFERENCES

[1] C. M. Kutty Sankaran, Cement 2005, published by Context Date Services, pp. 3 (1998).

17 [2] D. V. Khakhar, J. J. McCarthy, T. Shinbrot and J. M. Ottino, Transverse flow and mixing of granular materials in a rotating cylinder, Phys. Fluids, 9, 31-43 (1997).

[3] S. Das Gupta, S. K. Bhatia and D. V. Khakhar, Axial transport of granular solids in horizontal rotating cylinders. Part 1: Theory, Powder Technol., 67, 145-151 (1991).

[4] D. V. Khakhar, J. J. McCarthy and J. M. Ottino, Radial segregation of granular mixtures in a rotating cylinder, Phys. Fluids, 9, 3600-3614 (1997).

[5] D. V. Khakhar, J. J. McCarthy, J. F. Gilchrist and J. M. Ottino, Chaotic Mixing of Granular Materials in 2D Tumbling Mixers, CHAOS, 9, 195-205 (1999).

[6] K. M. Hill, D. V. Khakhar, J. F. Gilchrist and J. M. Ottino, Segregation driven organization in chaotic granular flows, Proc. Nat. Acad. Sci., 96, 11701-11706 (1999).

[7] A. Thomas, US Patent No. 1065597, (1913).

[8] C. J. Tomlinson, US Patent No. 1544504, (1925).

[9] R. W. Burke, US Patent No. 1920677, (1933).

[10] G. F. Olsen, Colton and D. L. Mcleod, US Patent No. 3445099 (1969).

[11] C. E. Sunnergren, J. K. Simms and D. W. Brinker, US Patent No. 4136965 (1979).

[12] G. A. Tyler, US Patent No. 4475886 (1984) \bibitemhol88 O. Holopainen, US Patent No. 4753019 (1988).

[13] R. A. Mosci, US Patent No. 5299933 (1994).

[14] R. A. Mosci, US Patent No. 5460518 (1995).

[15] B. J. Doerksen , US Patent No. 5975752 (1999).

[16] R. A. Mosci, US Patent No. 5873714 (1999).

[17] Barr, P.V., J.K. Brimacombe and A.P. Watkinson (1989), A heat transfer model for the

rotary kiln, Met. Trans., 20B, 403-419.

[18] Boateng, A.A. (1993), Rotary kiln transport phenomena: study of the bed motion and heat

transfer, Ph.D. Dissertation, The University of British Columbia, Vancouver.

18 [19] Boateng, A.A. and P.V. Barr (1996), A thermal model for rotary kiln including heat

transfer within the bed, Int. J. Heat Mass Transfer, 39, 2131-2147.

[20] Mastorakas, E., A. Massias, C.D. Tsakiroglou, D.A. Goussis, V.N. Burganos and A.C.

Payatakes (1999), CFD predictions for cement kiln including flame modeling heat transfer

and clinker chemistry, Appl. Math. Modeling, 23, 55-76.

[21] I.A. Altun, Influence of heating rate on the burning of cement clinker, Cement Concrete Research, 29, 599 (1999).

19 Mechanochemical Activation in Cement Processing: Detailed Proposal

1. INTRODUCTION

Mechanochemistry/Mechanical Activation

Mechanochemistry is the branch of chemistry that has primarily evolved in the twentieth century [1]. It deals with the field of reactions caused by mechanical energy, often referred to as Mechanical or Mechanochemical Activation [1-8]. The process of activation depends on the breakage process and the rate at which energy is supplied to the system [2-7]. In contrast to coarse grinding, where the objective is size reduction, mechanical activation is concerned with structural changes that are brought about by application of mechanical energy. Fine grinding is an intermediate case between coarse grinding and mechanical activation [2,8]. The solid state reaction during mechanical activation are generally believed to be favoured due to increase in surface area, stresses and defects induced in the solid structures, phase transformations, localised and overall thermal effects, repeated welding of interfaces and fracture leading to dynamic creation of fresh surfaces for reactions etc. A new dimension is added to the discipline of mechanical activation with the introduction of the concept of Soft Mechanochemistry. Soft Mechanochemical Synthesis/ Activation refers to the solid state reactions between chemically reactive oxyhydroxide/hydroxide species or compounds resulting in the formation of water [9,10]. The chemical affinity between the solid species and lower hardness (as a rule 3-4 times lower than the than corresponding anhydrous oxide species) permit lowering of the level of mechanical loads and milder conditions of activation. Due to presence of water some of the important phenomena that may take place during soft mechanochemical reactions are stress induced polarisation of surface water and OH- species, role of surface water in material transport, surface conditions similar to hydrothermal reactions etc [9,10].

Recently there has been a spurt of activities in the applications of mechanical activation for the development of new materials and metallurgical processes [e.g. 1-20]. Early studies on mechanochemical activation of cements date back to late 1950's [21,22]. In fact, much of the development in fine grinding and mechanical activation can be ascribed to researchers working in the area of cement and silicate technology [2]. Granulometric composition of the cement

20 powder for optimal preparation of a composite has been one of the chief concerns of these studies. Research and Development in the area upto 1990 have been documented in the Monograph on 'Mechanical Activation of Minerals by Grinding: Pulverising and Morphology of Particles' [2] and summarised in Table 1. Subsequent development in the field has been recently reviewed [23]. Current research is directed towards improvement in the mineral composition, hydration and durability of cement materials, energy savings and maximisation of waste materials as additives in the cement, for example fly ash and blast furnace slags [23-45]. Important developments in the recent past has taken place with regard to combined effect of chemical and mechanical activation, utilisation of industrial wastes such as slag and fly ash, comparative study of mechanical activation devices, etc.

Table 1. Summary of research and development upto 1990 in the area of mechanical activation (MA) with respect to the processing of cement (compiled from Ref. 2)

 Grindability of clinker minerals and clinker Mechanical activation of quartz, silicates, limestone etc  Mechanical activation (MA) of cement  Mechanical activation of clinker minerals  Mechanical activation of belite Elemental Substitutions in clinker minerals and effect on mechanical activation  Effect of surfactant on MA of cement minerals  Mechanical activation BF slag cement  wide coverage  good  poor  scanty

Indian Cement Industry and Waste Utilisation

India is one of the largest producers of the cement in the world and current production level is of the tune of 100 million tones. Ordinary Portland Cement (OPC) (popularly known as grey cement and comprising of 95% clinker and 5% of gypsum and other materials) accounts for about 70 percent of the production. Blended cements, Portland Pozolona Cement (PPC) [Typical constitution: 80% clinker, 15-20% pozolona (fly ash/burnt clay/coal waste) and 5% gypsum] and Portland Blast Furnace Slag Cement (PBFSC) (Typical constitution: 45% clinker, 50% blast furnace slag and 5% gypsum) account for 10 and 18 % of the total cement consumed in the

21 country. Overall production of Portland cement clinker in the country amounts to ~ 90 %. The quantity of fly ash used in the cement is 2-3% of the total generation and about 30% of the blast furnace is utilized in the cement production [46]. Steel making and other slag are yet to find any place in the cement production. Published data indicate that the ratio of clinker produced and total cement production in India is ~ 0.9 as compared to 0.7 for China [47]. There is a need to maximize utilization of industrial waste in the cement industry. Chemical and mechanical activation of blended cement constituents, in conjunction with suitable admixtures, offers immense potential to increase utilization of industrial wastes in cement industry, besides preparation of clinker less cements purely based on slag and other wastes.

2. POTENTIAL APPLCATION OF MECHANICAL ACTIVATION IN CEMENT PROCESSING AND PROSPECTS

The process of cement making involves grinding, firstly during raw material preparation for clinker formation and secondly during conversion of clinker into final cement product. The process of grinding in general can be divided in three stages [2]:

Stage I corresponds the reduction in size and a proportionality relationship is followed between energy input and new surface area produced (this stage is also known as Rittinger stage since the proportionality relationship is known as Rittinger’s Law). There is no interaction between particles during this stage.

Stage II leads to decrease in size but the particles start to interact with each other through weak and reversible van der Waals type adhesion forces (aggregation stage) and the proportionality law is not followed.

Stage III refers to agglomeration stage and during this stage first decrease in size drops to negligible value or stops and this may ultimately give rise to even increase in size because of intense particle interaction. Agglomeration involves very compact irreversible interaction of particles (co-crystallisation, fusion, mechanochemical reactions etc) in which bonding may also play an important role. Energy input to the particles during this stage takes place through plastic deformation.

The general motive of grinding studies on cement raw materials has been to accelerate the process of sintering and moderation in sintering conditions. Studies have also been carried out to prepare the cement minerals/cement directly through mechanochemical

22 activation and altogether eliminate the sintering process. Grindability of phases present in clinker and influence of clinker microstructure on grindability have received maximum attention. Important development has taken place in the area of grinding aids to minimise the energy consumption during grinding. Both bulk and surface reactivity of mineral phases can play an important role in affecting the hydration characteristics of the cement. Mechanical activation, alone or in conjunction with chemical activators/additives can play an important role in altering the nature of interface(s) and bulk phases in cement to tailor the microstructure and improve cement strength, durability and other physicochemical properties. In connection with the technological relevance of fine grinding and mechanical activation, it should be mentioned that the strength of cement (and other hydraulic binding materials) is the result of several factors and it does not always follow that high fineness and the associated high initial activity are the best features for high final strength (problems of "overgrinding" of cement). Optimum dispersion and activity have to be adjusted to the technology of the application or new technologies for concrete preparation have to be developed which can fully exploit high dispersion and high activity. In addition to the application in traditional cement making, mechanical activation of belite has received considerable attention to improve its hydration properties and develop energy saving cements. Mechanical activation of belite led to some very promising results but long grinding time hindered further developments.

Much of the results on mechanical activation of cement are based on fragmented studies. Over the years, our understanding of mechanical activation and the devices used for activation have increased considerably. Also, parallel developments have taken place in the understanding of cement and cement minerals, hydration process, role of admixtures and additives, etc. The concepts of mechanically activation have been successfully exploited in the development of 'Energetically Modified Cements' [25-30]. Recently, interesting results have been obtained by the researchers abroad on the mechanical activation of slags and fly ash (materials that have inferior or latent hydraulic activity) and increase utilisation of these waste materials in blended cements or to develop cements based entirely on these waste materials. In many of these developments, the concepts of chemical and mechanical activation are exploited along with novel additives [37,38,44]. Another very important development that has taken place recently is low temperature synthesis of reactive belite (C2S) and other very promising phases with 2:1 stoichiometry (termed as phases for the future cements) using mechanochemical approach [45]. A base paper prepared by National Metallurgical Laboratory on ‘Mechanochemistry in Cement

23 Research’ after extensive survey of world-wide efforts have identified several areas of research where mechanical activation holds promises in cement research [23]. Studies on the combined effect of mechanical and chemical activation holds maximum promise in terms of increased utilisation of industrial wastes, such as slag and fly ash, in cement processing, development of high performance and even clinkerless cements based purely based on BF slag. Fine grinding and mechanical activation requires much less energy when carried out in wet condition. Alternate strategies in cement processing, outsides the domain of conventional cements processing also need to be looked into considering this aspect.

Mechanical activation devices are not the same as the grinding devices and require to be used judiciously with feed of proper size. Also, mechanical activation applications in cement processing should not be viewed in isolation and a multidisciplinary approach is called for to address the issues of cement structure and properties, energy savings, innovative cements, waste material utilisation and environmental concerns.

3. STATE OF THE ART

Production of Portland cement involves the grinding of raw materials, the calcination of raw materials at ~ 1500°C and the grinding of cement clinker and gypsum. It is an energy- intensive process and requires approximately 4000 MJ/t of cement [31]. Utilisation of industrial waste, such as slags and fly ash, in cement industry is not only beneficial from the point of view of resource conservation, energy savings and CO2 emissions but also help in gainful utilisation of the wastes and solve problems associated with the disposal of the wastes. Typically, 15-35% fly ash and 37-70% ground granulated blast furnace slag (GGBFS) [as a mass of cementious material] are used as a portland cement replacement in Portland fly ash cement and Portland blast furnace slag cement, respectively. Fly ash and blast furnace slag usage in blended cements is restricted by the hydraulic activity of these materials. Mechanical activation offers opportunities for the increased utilisation of certain industrial wastes, such as slags and fly ash, in the cement industry and for the development of waste-free technologies. The basic idea of mechanical activation is to increase the reactivity of these waste materials that are hydraulically inert (have latent hydraulic activity) or insufficiently active. The aim is to develop blended cements that uses increasing amount of these materials without degradation in properties or cements that are entirely based on the waste materials.

24 Table 2. Advantages of using Fly ash and BF slag when used as replacement for Portland Cement Fly Ash  Increased early and late compressive strengths  Increased resistance to alkali silica reaction (ASR) when >15% is added  Less heat generation during hydration  Increased pore refinement  Decreased permeability  Decreased water demand  Increased workability  Decreased cost

Ground Granulated Blast Furnace Slag

 Increased sulfate resistance  Increased alkali silica reaction resistance  Increased pore refinement  Decreased water demand  Decreased permeability  Increased long-term strength  Less heat generated during hydration  Produces white cement

Fly Ash Based Cements

There have been some interesting recent studies on mechanical activation of fly ash and its use in cement industry [32-39]. In a typical study, mechanical activation of ordinary Portland cement with 20% fly ash in a vibro mill with rings for three minutes resulted in 58% increase in the compressive strength (after 28 days) [36]. Chemical activation of fly ash is also possible using alkali activation and sulphate activation [39]. Alkali activation involves the breaking down of glass phase in the alkali environment and accelerate the reaction; sulphate (CaSO4. 2H2O,

CaSO4, Na2SO4 etc) activation is based on ability of sulphate to react with aluminium oxide in the glass phase of fly ash to produce ettringite (AFt) that contributes strengths at early stages

25 [39]. Studies on lime–fly ash, lime–fly ash–slag and Portland fly ash cements have indicated that combined effect of chemical and mechanical activation results in much higher strength than chemical activation (with Na2SO4) or mechanical activation separately [37,38]. The literature on combined effect of chemical and mechanical activation is in general scanty.

Slag Cements

Blended cements that incorporate pozzolans or granulated blast furnace slags usually develop mechanical properties more slowly than comparable Portland cements. It was found that for the production of cement of high slag content, separate grinding is more advantageous [40]. A comparative study of mechanical activation of slag blended cements in a ball mill, vibro mill with balls and vibro mill with rings have indicated that vibro-mill with balls required the shortest activation time [41]. A novel approach has been suggested to overcome the problem of slow development of mechanical properties of BF slag blended cements [42,43], This consists of grinding the Portland cements to very high specific areas and then using these cements in the blended systems. When so used, the controlled particle size distribution (CPSD) cements compensate for the relatively slow reactivity of fly ash and granulated slags. These CPSD blended cements have shown properties approximately equal to or superior to those made with normally ground cements of same compositions [42,43]. In the recent past, important developments have taken place in the area of chemical activation of slags of different origins and these have led to the development of cements with very high strength (High Performance Cements) [31]. There are indications that chemical activation when used in conjunction with mechanical activation will lead to novel cements of very high strengths. The main idea of the HP cement technology is based on mechanical-and-chemical activation of the cement grinding process for increasing dispersion and reaction ability, as well as modifying of material surface by chemical admixtures [44]. The technology involves intergrinding of a complex admixture of clinker, gypsum, and selected mineral admixtures like granulated blast furnace slag (35- 50%) in the composition to increase chemical and thermal resistance. According to the present experience, high performance (HP) cement and mortars with compressive strength up to 95 and 145 MPa, respectively can be produced. High performance cement based mortars possess low permeability, high resistance to chemical attack, thermal resistance, and excellent freezing and thawing resistance [44].

26 No literature seems to be available on the mechanochemical activation of waste material (slag and fly ash) in wet condition and their use in cement industry.

4. OBJECTIVES

The objectives of the proposed study is to focus on the combined effect of chemical and mechanical activation are:

(a) To increase utilisation of the fly ash and slags in the cements without degradation in cement properties

(b) To develop high performance cements with current levels of utilisation of fly ash and BF slag

(c) To explore the prospects of making high performance clinkerless cements from slags and other industrial wastes

5. METHODOLOGY

The methodology to be followed in the study is as follows:

(i) Effect of mechanical activation on the mechanical activation of individual constituents used in the blended cements

Major emphasis on creating base information including following aspects

- Complete characterization of base starting materials [cement(s), slag(s) and fly ash(es)] in terms of chemistry, size distribution, specific surface area, phases, glassy constituents, etc.

- Comparative study of mechanical activation in dry and wet condition

- Morphology changes (SEM-EDS), phase changes (XRD, DTA/TG), crystallite size, strain (XRD)

- Reactivity of material as a whole and individual phases

- Effective coupling of grinding with mechanical activation

(ii) Studies on the combined effect of chemical and mechanical activation of cement constituents

27 Major emphasis in these studies will be on

- Morphology changes (SEM-EDS), phase changes (XRD, DTA/TG), crystallite size, strain (XRD)

- Reactivity of material as a whole and individual phases

- Sequencing/coupling of chemical and mechanical activation

(iii) Studies on mechanical activation and dry blending of cement

Major emphasis on the following

- Systematic studies on the blending of cement constituents

- Testing of different cement formulation

- Monitoring of heat evolution with time to monitor and its correlation with evolution of phases and strength development

- Detailed studies on select formulations

- Role of admixtures

(iv) Studies on mechanical activation and blending of cement with waste (Fly ash and slag) activated in wet condition

Major emphasis on the following

- Systematic studies on the wet blending of cement constituents

- Testing of different cement formulation

- Monitoring of heat evolution with time to monitor and its correlation with evolution of phases and strength development

- Detailed studies on select formulations

- Role of admixtures

(v) Studies on clinkerless slag cements

- Role of chemical activation in strength development

- Combined effect of chemical and mechanical activation

- Testing of different cement formulation

28 - Monitoring of heat evolution with time to monitor and its correlation with evolution of phases and strength development

- Role of cement admixtures

6. PROPOSED TARGETS/DELIVERABLES

 10-20% increase in the current level of utilization of industrial wastes, namely slags and fly ash without degradation in cement properties

 High performance cements with current level of waste usage in cement industry

 Clinkerless cements based on industrial wastes, BF and other slags and fly ash

 Evaluation of alternative strategies in cement processing based on the mechanical activation of slag and fly ash in wet condition.

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