A

PROJECT REPORT

On

“ROLE OF KILN IN CEMENT

MANUFACTURING PROCESS”

For the Partial fulfillment

of

B-TECH MECH ENGG. 3rd year

During Session 2009 - 10

MAHARISHI ARVIND INSTITUTE OF ENGINEERING AND TECHNOLOGY, JAIPUR

Guided By: Submitted By :

Mr. R. P. Singh MAYANK AGRAWALG M -RTC & HRD B-TECH MECH EN. 3rd year

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As per the requirement of ………course J K Cement WORKS has been kind enough to permit me to complete my project on “………………………

PREFACE

The objective of this project is to study the cement manufacturing process of JK cement Ltd.

This report prepared during the practical training which is student’sfirst and greatest treasure as it full of experience,observation and knowledge.

The summer training was very interesting and gainful as it close to real what have been studied is all the years through was seen implemented in a modified and practical form.

The only fault was that the time available was short and there was much to learn ,yet the things learned shall never be oblivion and areof great aid in the near future.

MAYANK AGRAWALB-TECH MECH ENGG.3rd yearring the practical training. Which is student’s first and greatest treasure as it is full of experience, observation and knowledge.

The summer training was very interesting and gainful as it is close to real what have been studied is all the years through was seen implemented in a

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ACKNOWLEDGEMENT

It gives to me tremendous pleasure in acknowledge the invaluable assistance to me by various personalities in successful completion of this report.

I express my gratitude towards Mr.D.Ravishankar, President and the entire management of JK cement works for giving me a chance to work as a management trainee in their esteemed organisation.

I wish to acknowledge my profound gratitude towards my training department head Mr R.P SINGH, GM-HRD & RTC, Mr SHAILENDAR SHARMA, ASST. MGR-RTC for giving me opportunity in this field.

I would be a thankless child if I don’t maintian the name of continous source of energy and inspiration that is my parents who always encourage and support me at right step in my life.

MAYANK AGRAWALB-TECH MECH ENGG.3rd year

)

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It gives to me tremendous ple asure in acknowledge the invaluable

An

Introduction

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C O N T E N T S

1. Overview of J K Cement WORKS

1.1 History & Milestones

1.2 Production Capacity and Financial Performance

2. Management Set- up.

3. Organisational Chart.

4. Regional Training Centre - North

5. Corporate Social Responsibility

6. Cement Manufacturing Process :

6.1. Process Flow Chart

6.1. 1 History of Cement

6.1.2 Introduction of Cement

6.1.3 Hydration

6.1.4 Types and Uses of Cement

6.1.5 Types of Processes

6.1.6 Process Overview

6.2 Mining

6.3 Crushing of Limestone, Stacker and Reclaimers

6.4 Grinding of Raw Material

6.5 Pyro-processing

6.6 Cement Grinding

6.7 Packing Plant

6.8 Quality Characteristics of Cement

7. ROLE OF KILN IN CEMENT MANUFACTURING PROCESS

1. OVERVIEW OF J K CEMENT WORKS

1.1 HISTORY BEHIND J K CEMENT

The initial "J.K." stands for a father- son team, namely: JuggilalKamlapathSinghania5

J .K. organization started in the year 1884 at Calcutta. J .K. started their business as a Financier, Investor, Trading Supplier of cotton belts and manufacturer of small machinery parts like ‘V' belts, etc. They established few small cotton textile industries also.

In the year 1914 they shifted their business from Calcutta to Kanpur where they established many big industries like J.K. cotton Mills, Straw product Co, Lohia Mach, J.K. Pulp and Raymond’s Woolen, etc.

In the year 1934 J.K. organization started one more division, as J.K. Synthetics Ltd. They established various big plants of Nylon, Acrylic fiber, etc. at Kota and Tyre Cord, Chemical and Pesticides at Jhalawar.

In the year 1974 under the same division one more unit was started for manufacturing of Grey Cement at Nimbahera.

The present cement factory was commissioned in the year 1974. The plant started its production from 27th Dec 1974.

Ist plant / kiln was commissioned in 1974 and the capacity of this plant was 900 tonne per day and 3 lakh tonne per year. After modification in Preheater, its present capacity is 1200 TPD.

Expansion of this plant took place in the year 1979, when 2nd kiln was commissioned with a capacity of 1200 tonne per day and 7 lakh tonne per year. After modification in Preheater its present capacity is 1800 TPD.

Again in the third phase, a kiln was erected in the year 1982 and production of this kiln was 1350 tonne per day.

In the year 1988 a new technology was introduced in this 3rd Kiln that consisted of precalcination process, which raised the capacity of this plant to 3400 tonne per day, which was earlier 1350 tonne per day. In Aug.-2003 after again some modification in Preheater and Folex cooler its capacity is increased to 5000 TPD.

Besides, J.K. cement plant is having its own diesel generator sets, producing power to meet the power energy requirements.

Main raw material forcement is LIMESTONE, for limestone we have our own open cast mines adjoining to the plant. Besides we have developed few more mines at Maliakhera, Karoonda and Tilakhera for producing 10,000 tonnes limestone per day as needed.

J .K. Cement erected one more plant from Jan. 2001 with the capacity of 1400 tonne per day at village Mangrol. In Nov.-2003 after modification in Preheater and installation of Mechanical elevator its capacity increased to 2200 TPD.

Due to power shortage as imposed by Ajmer electricity supply board J.K. established its own Thermal Power Plant at village Bamania, near Shambhupura, which is generating 15 M.W. power every day, which is consumed by J.K. Cement Plant.

J K Cement also has a plant of 400TPD installed capacity of White Cement at Gotan, Nagour (Raj).

Cement Plant at Karnataka of over 5500 TPD and Thermal Power Plant of capacity 30 MW.

Thermal Power Plant at Nimbahera of 22 MW.

Waste Heat Recovery Plant at Nimbahera of 15 MW capacity.

J.K. Cement has started the following projects:

J K Cement Project at Fuzairah, UAE

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J.K. cement is one of the most productive, cost efficient cement producing plant in the country, a company, believing in corporate responsibility to society, integrity and fairness. The company’s cement is sold under the J.K. SarveShaktiman brand name, enjoys good brand image and a price premium.

The following types of cements are produced by J K Cement Works.

(a) Ordinary Portland Cement (OPC)

(b) PortlandPozzolana Cement (PPC)

(c) Super Silicate Cement (SSC)

(d) Masonry Cement (MC)

J. K. Cement manufactures and markets cement and clinker for both domestic as well as exports markets.

1.2 PRESENT CAPACITY AND PERFORMANCE

1.2.1 CLINKER PRODUCTION – Nimbahera and Mangrol Plant

Ist Plant / Kiln 1200 Tonne Per Day (TPD)IInd Plant / Kiln 1800 TPDIIIrd Plant / Kiln 5000 TPDIVth Plant at Mangrol 2200 TPD

Total Capacity 10200 TPD

1.2.2 PRODUCTION ANALYSIS TABLE: IN TONS – Nimbahera and Mangrol Plant

Year Clinker Cement

2005-06 3170268 3511022

2006-07 2907196 3638786

2007-08 2917045 3690726

2008-09 3024091 3646220

2009-10 3096463 3594387

1.2.3 FINANCIAL ANALYSIS: IN Million - J K Cement Ltd.

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2.1 Corporate Level- Kanpur

Chairman - Dr Gaur HariSinghania

Managing Director - Shri Y P Singhania

Group Executive President - Shri R G Bagla

2.2 Unit Heads:

President Shri D Ravisankar J K Cement, Nimbahera, Mangrol and Mudhol, Karnataka Plants

President Shri B K Arora J K Cement- White and Grey Plants at Gotan

2.3 Other Services:President Shri M P Rawal Management & Technical Services

& Marketing –SouthPresident Shri R C Shukla Grey Cement Marketing - NorthPresident Shri V P Singh White Cement Marketing

2.4 J K Organizations:

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J K Organization

J K White, Gotan (Raj.)White Cement –1000 TPD Grey Cement – 1200TPDCPP – 7.5MWJK Wall Putty

J K Cement Works,

Nimbahera (Raj.)

Grey Cement–

8000TPD

C P P–22MW

W H R – 15 MW

J K Thermal Power, Bamania (Raj.)

CPP – 15MW

J K Cement

Works, Mangrol

(Raj.)

Grey Cement–2200TPD

J K Cement, Mudhol, Karnataka

Grey Cement – 3500 TPDCPP– 30MW

Project J K Cement Fuzairah, UAE

Year Turnover PBT

2005-06 11087 522

2006-07 15297 2720

2007-08 18128 3466

2008-09 18765 2340

4 . REGIONAL TRAINING CENTRE- NORTH

SECOND DECADE OF COMPETENCY ENHANCEMENT AND SKILL DEVELOPMENT IN

CEMENT INDUSTRY

The Regional Training Centre - North is a premier training centre of North India promoted with

assistance from World Bank, DANIDA and Govt. of India as a unique HRD project in Cement

Industry. It is equipped with modern training aids and caters to the skill enhancement and

competency developmental needs of more than 20 cement and other plants. It has trained over

8000 technical and managerial personnel during the last 14 years. The centre has conducted

many tailor-made in-house programs for cement and other industries:

ABROAD:

Oman Cement, Oman Star Cement, Dubai Hama Cement, Syria / EHDASSE Sanat Corp. Iran.

INDIA:

J & K Cement, Srinagar Thermax Ltd., Pune Jindal Steel & Power Ltd., Raigarh, etc.

RTC has specialized packages / modules in Mining, Process,Maintenance disciplines

likeOperation & Maintenance of HEMM / Gear-boxes / Pumps / Compressors / Electrical &

Electronics Equipments / Energy Conservation / Environment Management and Machinery

Alignment, etc. designed and developed by renowned International / National agencies like FLS

Denmark, NCCBM, TATA Interactive Systems, VEC, NITTTR, etc. More than 100 senior line

mangers from ten plants have been trained at Denmark, NITTTR, Bhopal and Chennai, who act

as resource persons for these programs. Besides, OEM’s and management experts of national

repute are invited for various technical and management programs to make them effective and

gainful experience for the participants.

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5. CORPORATE SOCIAL RESPONSIBILITY

J K Cement has contributed significantly to the development of various services in and around its

offices and plants. Some of such activities can be enumerated as under :

Education Schools and University: Dr Gaur HariSinghania Institute of Management & Research, Kanpur Sir PadampatSinghania Technical University at Bhatewar, Udaipur. J K Institute of Technology – JKIT at Nimbahera PadamVidyaVihar – Primary School at Nimbahera KailashVidyaVihar – Sr Secondary School at Nimbahera Sr Secondary School, Gotan

Educational services: Construction of rooms in Govt.College at Nimbahera. Running JK Institute of Technology, ITI in five trades affiliated to NCVT. Running 10+2 CBSE affiliated school Running Regional Training Centre for Cement technocrat’s aided by WB & DANIDA. Various constructions in nearby govt. Schools of Chittorgarh district. We are involved in girls school (under construction) and committed reasonable financial

contribution for above

Medical services Rs. 36 lacks contribution for the construction of govt.Hospital at Nimbahera. Ambulance to govt.Hospital. Free facility of pathological laboratory for the persons of surrounding area. Financial contribution to various NGOS for medical camps in the district. Financial contribution for construction of dispensary & health centre in nearby villages. Free Homeopathic consultancy/medicines for the patients of nearby area.

Religious services Radhakrishna temple at colony premises. prayer hall in hanuman temple in Nimbahera. Bheemkeshwar temple in staff colony. Dharmashala at Bhanwarmata (tourist/ religious place). 8 rooms for Dharamshala at PashupatiNath temple in Mandsaur (M.P.). Various temples in number of nearby villages.

Sports services Sports infrastructure like wooden badminton court, table tennis court, billiard room, and

cricket ground, volleyball ground in colony campus. Sponsoring all India youth football, volley ball and badminton tournaments. Sponsoring inter-district tournaments. Arranging summer camps for various sports.

Other social services

Construction of approach roads in and around villages of mining area. Digging of tube wells. Supply of tube well pumps. Construction of water tanks.10

Supply of drinking water in tankers in nearby needy places during summer. Regular plantation in plant, colony and nearby villages. Direct and indirect employment to thousands of persons of surrounding area. Financial helps to NGOS. Financial aid to organize religious festivals by municipal board.

6. CEMENT MANUFACTURING:

Ordinary Portland cement is produced by grinding cement clinker in association with gypsum (3-5%) to specified fineness depending on the requirements of the cement consumers. Cement clinker is produced on large scale by heating finely pulverised Calcareous and Argillaceous materials at very high temperature upto 1450oC in rotary kilns. The Calcarious and Argillaceous materials obtained from the earth are properly proportioned to get a suitable ratio of lime (CaO), Silica (SiO2), Alumina (Al2O3) and Iron (Fe2O3) present in the mixture. As the raw materials are obtained directly from limestone and clay mines, minor constituents like Magnesia (MgO), Sodium, Potassium, Sulphur, Chlorine compounds etc., may also be present in the raw materials upto limited extent which do not adversely affect either the manufacturing process or the quality of cement produced.

Limestone is the major raw material used for manufacture of cement and about 35% of raw materials are lost in the atmosphere in the form of gaseous compounds of which carbon dioxide is the major one. Therefore cement units are necessarily located near the cement grade limestone deposit. The major steps or unit operations involved in cement manufacturing process include:

Mining, Crushing, Pre-homogenisation, Grinding and Final Blending of raw materials for preparation of kiln feed.

Pyroprocessing of kiln feed in presence of combustion gas/ flame generated from combustion of pulverized coal, mineral oil or natural gas.

Grinding of cement clinker along with

Gypsum for production of OPC

Gypsum and other additive / blending components for production of cement other than OPC.

Packing and dispatch of cement.

The various unit operations from mining of raw materials to cement dispatch are discussed as under.

6.1.1 HISTORY OF CEMENT

The history of cement is the story of civilization from primitive caves of pre-historic times to the skyscrapers of the modern age. It is said that the use of cement is form the period of use of fire. Egyptians utilized gypsum plaster as cementing material as early as 3000 BC building their monuments.

However, It was in 1824, sixty-eighty years after the discovery of hydraulic properties of lime Joseph Aspdin patented his product, which was called "Portland Cement" The plants manufacturing portland cement outside England were commissioned in Belgium and Germany in 1855. The interest that is evoked in the technology o f cement resulted in the development of Rotary kilns in 1886.

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Modern cement is the outcome of the combined research and development efforts of chemists, technologists and architects. The cement technology is an offshoot of the overall development in other industries, technology, constructional activities and knowledge and the availability of raw material.

6.1.2 INTRODUCTION OF CEMENT

Cement can be defined as any substance, which can join unite two or more pieces of some other substance together to from a unit mass. Cement, as used in construction industries, is a fine powder which when mixed with water and allowed to set and harden can join different components or members together to give a mechanically storng structure. Thus cement can be used as bonding material for bricks or for bonding solid particles of different sizes (rubber masonry) to form a monolith.

6.1.2.1 Ordinary Portland Cement

The most common type of cement used by concrete manufacturers is Portland cement, which is prepared by igniting a mixture of raw materials mainly composed of calcium carbonate or aluminum silicates. According to ASTM standard specification C 150, Portland cement is defined as “a hydraulic cement produced by pulverizing clinker consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an interground addition” . The phase compositions in Portland Cement are shown below and they are denoted as tricalcium silicate (C3S), dicalciumsilicate (C2S), tricalcium aluminate (C3A), and tetracalciumaluminoferrite (C4AF).

6.1.2.1.1 Components of Portland Cement

Description Formula % Composition

Tricalcium Silicate (C3S) 3CaO SiO2 49

Dicalcium Silicate (C2S) 2CaO SiO2 25

Tricalcium Aluminate (C3A) 3CaO Al2O3 12

TetracalciumAluminoferrite (C4AF) 4CaO Al2O3 Fe2O3 8

6.1.2.2 COMPOSITION OF ORDINARY PORTLAND CEMENT:

Ordinary Portland Cement is the basic cement and it has three grades namely 33, 43 and 53 respectively. Limestone is the principal raw material for the manufacturing of cement. Our country has enough reserve of raw material needed in the cement industry. Cement consumption growth is highly correlated to the GDP growth and serves as a leading indicator. More industrial activity and greater purchasing power means more asset formation and construction and thus more consumption of cement.

Ingredient Percentage Range

lime 64 64-68

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Silica 22 17-25

Alumina 5 3-6

Calcium sulphate 4 3-5

Iron Oxide 3 3-4

Magnesia 2 0.1-3.0

Sulphur 1 1-3

Alkalise 1 0.2-1.06.1.3 HYDRATION

In this experiment, the hydration rate for each type of tricalcium silicate component was measured. Hydration is the reaction that takes place between cement and water that leads to setting and hardening. All compounds present in Portland cement clinker are anhydrous, but when brought into contact with water, they are all attacked or decomposed, forming hydrated compounds. When the tri- or di- calcium silicates react with water a calcium-silicate-hydrate gel is formed. This calcium-silicatehydrate (C-S-H) is the principal hydration product and primary binding phase in Portland cement.

The chemical reaction that take place during hydration are summarized below:-

Tricalcium Silicate (C3S)

2(3CaO-SiO2 ) + 6H2O→3CaO - 2SiO2 –3H2O + 3Ca (OH)2

Dicalcium Silicate (C2S)

2(2CaO-SiO2 ) + 4H2O→ 3CaO - 2SiO2 –3H2O + Ca (OH)2

Tricalcium Aluminate (C3A)

3CaO-A12O3 + 6H2O→ 3CaO – A12O3 –6H2O

TetracalciumAluminoferrite (C4AF)

4CaO-A12 O3 – Fe2O3 + 2Ca(OH)2 + 10H2O →3CaO – A12O3 –6H2O+3CaO-Fe2O3-6H2O

6.1.4 TYPES AND USES OF CEMENT:

(a) Type of Cement:

Grey Cement

White Cement

(b) Uses of Cement:

Types of Cement Application

Ordinary Portland Cement (OPC)

General construction

Portland Slag Cement General construction and marine works.

PortlandPozzolona Cement (PPC)

General construction, hydraulic construction & marine.

White Portland Cement Architectural purposes, decorative work and in manufacturing of titles.

Oil Well Cement Connecting the steel casing to the walls of gas oil wells at high

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temperature and to seal porous formations in petroleum industry.

Low Heat Portland Cement Where low heat on hydration is required as in mass concrete for dams.

Super Sulphated Cement In a varity of aggressive conditions like marine works, concrete sewers carrying industrial effluents.

High Alumina Cement Mainly as refractory cement and as structural material giving high early strength development in cold regions.

6.1.5 TYPES OF PROCESSES: Basically there are two types of process for cement manufacturing that is -

Hydro Processing (Wet Process) Pyro Processing (Dry Process)

We are using Pyro Process in JK Cement Works at all our plants.

PROCESS OVERVIEW :

6.2 MINING OF LIMESTONE

The major quantity of limestone is obtained from the captive limestone mines.The methods of mining are discussed as under.

6.2.1 MINING METHODS

Limestone mining operation traditionally is carried out generally as Opencast mining in India. However, newly developed Eco-friendly Surface Miners also find use in limestone mines today in J K Cement plants.

Planning and executing a systematic exploration programme.

Draw scope of drilling campaign. How to carry out survey and perform drilling activities for exploration purpose.

Establish system for computerized mine-planning in order to ensure supply of limestone with consistent quality.

Planning and executing drilling and blasting programme in normal course at site to take optimum output from blasting as well as achieving economy in explosive consumption.

(Approximate boulder size: 1.0 M * 1.4 M * 1.1M)

Loading and transportation of lime stone boulders to crusher site.

Implementing statutory requirement for safety and environment

Resources:

(a) Explosives

(b) Equipment:

Drilling Machines

Excavators / Shovels

Dumpers

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Dozers

Loaders

(C) Operations:

Drilling and Blasting

Loading and transportation

6.2.2 DRILLING / BLASTING METHODS OF MINING

Out of the eight cement plants of J K Cements Limited, seven plants are raising the limestone from their captive limestone mines by using drilling and blasting techniques. Sequential Timer Blasting Machines are used for blasting operations. This timer machine reduces the problem of noise, ground vibrations and improves fragmentation by introducing inter row delays. Noise, dust and illumination surveys are conducted at regular intervals in our mines to keep the same under control. Instruments used for noise and surveys include Noise Dosimeter and Gravimetric Dust Sampler respectively. Equipments used in this method of Open cast Mining includes HM-25 dumpers, Poclain&Demag shovels, Hindustan and Beml loaders, Beml dozers etc. The limestone produced by this method are in the form of big boulders having their physical dimensions may be from 1.5 meter to small fragment size upto few millimeters. Therefore before taking the limestone to grinding machines boulder crushing in one or two stages is required for reducing the size of limestone for preparing suitable feed size to grinding mills.

6.2.3 MINING WITH SURFACE MINER

Surface miners were firstly developed for road making and are now used for limestone mining operation also. Being ecofriendly machine, Surface Miners eliminate the problems associated with drilling, blasting, crushing & loading and the complaints associated with their activities. There are three surface miner machines working in Adanakurichi Limestone Mine of ICL’s Dalavoi works. The capacity of each machine is 200 tonnes per hour limestone.

The surface miner is a track-mounted machine with a powerful diesel engine and hydraulic pumps for delivering the power to the cutting drum. The cutting drum is made up of a special alloy steel with replaceable Tungsten Carbide cutting tools (spikes), which can be quickly detached or fixed. The drum can be lowered or lifted by hydraulic system with powerful hydraulic motors thereby varying the depth of cut. Dust is suppressed at the source itself by water spraying on to the milling drum thereby making it an environment friendly machine. As the cut materials are of uniform size and the impact on the crusher while crushing is reduced. Whenever, there is an interstitial reject band encountered, the same is eliminated by loading and dumping it separately in the specified dump thereby facilitating Selective Mining with this machine.

6.3 CRUSHING OF LIMESTONE

The big boulders produced during drilling and blasting methods of limestone mining are crushed in suitable type of crushers. The crushing is carried out either in single or double stages by using Primary crusher and Secondary crusher, or in a single stage crushing machine depending upon the size of the boulder produced while mining. This also depends on the type of grinding mills used for grinding of raw materials for preparation of finally pulverized raw meal. Jaw crushers as

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well as impact crushers / hammer crushers of different capacities are employed in J K Cements Ltd., for reduction of size of limestone boulders to a suitable feed size acceptable to the different types of grinding machines installed in plants. The crushers are mainly installed at the plant site. The limestone produced in the mine is transported to crusher site with the help of dumpers and tippers of different capacities. In some mines the crushers are installed at mine site and crushed limestone is transported to plant stackpile with the help of Belt conveyor / Ropeway. Figure 6.3 below illustrates the raw limestone mining and crushing at a limestone mine of a cement plant.

Purpose:

Size reduction from 1.0M * 1.4M * 1.1M boulder to 25mm size limestone pieces.

Common type of Crushers:

Double Toggle Jaw Crusher (Capacity : 400 TPH): Used as primary crusher. Swing Hammer Crusher (Capacity : 200 TPH) used as secondary crusher. Compound Impactor (Capacity : 800 TPH) combined unit of primary and secondary crusher.

6.3.1 PREHOMOGENISATION

The crushed limestone is then transported to stacker reclaimer site with the help of belt conveyor / rope ways installed at different plant site. Finally the crushed limestone is pre-blended with the help of stacker and reclaimer systems. The crushed limestone travelling on the belt conveyors is stacked in layers with the help of stacker machine, which moves to and fro along the side of stacking yard. The stacked materials is then cut in slices with the help of a reclaiming machine which mixes the layers of stacked limestone which reduces the variation in quality of limestone as compared to the large variations obtained in the limestone obtained from mines. The full fledge stack pile ready for reclaiming and feeding to raw mill hoppers can be seen in the above figure.

STACKER & RECLAIMER AT J K CEMENT WORKS

Purpose:

Homogenization of crushed limestone.

Stacker:

Type of Pile: Longitudinal.16

Fig. 6.3Fig. 6.3

Details of Piles: 20000 – 30000 tonnes per pile. Height of pile upto 11.00 meters. Rated capacity: 1000 tonnes per hour. It varies from plant to plant depending upon the

production requirement. The stacker moves on longitudinal rails.

Reclaimer

Type: Bridge Scrapper Type.

Rated Capacity : 600 tonnes per hour It will vary from plant to plant depending on the production requirement(in TPD).

Working Principle: Cuts Stack Pile in slice from parallel to face of pile.Shifting material (limestone) to belt with the help of scrapper.

6.4 GRINDING OF RAW MATERIALS

The pre-blended limestone from stack pile is transported to raw mill hoppers. More than one hoppers are used for proportioning of raw mix incase the limestone is obtained from more than one sources or purchased sweetner or additive materials are required to be mixed with captive mines limestone. Presently Raw mill hoppers are provided with continuous weighing machines known as weigh feeders in order to produce a suitable raw meal proportioned appropriately for production of desired good quality of cement clinker. Vertical Roller Mill and Tube Mill Grinding machines are used for production of pulverized raw meal at J K Cements Plants. Figure presented below is a typical representation of raw mill grinding section in a cement plant where raw mill hoppers, vertical roller mill alongwith dust collecting arrangement can be seen.

Materials used for grinding

Limestone and additives (Raw Mix) Coal and other fuels Clinker

Feed size of incoming materials

Limestone 25mm Size for Ball Mills 35mm Size for VRM (Segregated)

(Secondary Crusher is used to feed Ball Mills) Coal – 25mm size Clinker – 25 to 35 mm size.

Fineness of output materials

Raw Mix: 15 – 17 % Residue On 90 #1.8 – 2.2 % Residue On 200 #

Coal Powder: 15 – 17% Residue On 90 #18-22 % Residue On 90 #(For use in Pre-Calciner, there is separate arrangement to grind coal)

Cement: 33 Grade – 2600 To 2800 Blaine43 Grade – 2850 To 3000 Blaine

53 Grade – 3200 To 3400 Blaine

Grinding Systems

Raw Mix:- Ball Mill

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- Vertical Roller Mill- Combination of Roller Press and Ball Mill

(Generally open circuit is used in wet process and closed circuit is used in dry process. In closed circuit systems, fixed and dynamic separators are used.)

Coal and fuel: Ball Mill Vertical Roller Mill

(Closed circuit used)

Clinker Grinding: Ball Mill Combination of Roller Press and Ball Mill(Open circuit and closed circuit used. In closed circuit systems, fixed and dynamic

separators are used.)Materials used for grinding

Limestone and additives (Raw Mix) Coal and other fuels Clinker

6.4.1 HOMOGENISATION

The raw meal ground in the raw mill is thoroughly blended in vertically tall blending silos of capacity upto 10,000-15,000 tonnes or more. The blending is performed pneumatically by introducing the compressed air in the bed of fine raw meal fed to the blending silo or mechanically by distributing the raw meal at different cross section of the silo with the help of airslides. The blended raw meal is taken out of the silo with the help of air slides and fed in a central discharge bin, which is continuously aerated for accomplishing final blending of raw materials. The characteristics of blending raw meal should satisfy the requirement of standard deviation variation in the range of (+/-) 0.2% CaO of raw meal. The moisture content of raw meal powder is less than 1%. The properly blended raw meal is now ready for burning the same to produce cement clinker in cement rotary kiln.

6.5 PYROPROCESSING

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The modern pyroprocessing system comprises of three important sections namely preheating and precalcining, clinkerisation and cooling. The preheating section is 90-100 meters tall and comprises of battery of cyclones arranged one over the other in series. The modern preheaters are comprising of 5-6 stage of low pressure cyclone as compared to conventional 4 stages of cyclone preheaters. The riser ducts of top stage cyclones are connected with powerful induced draft fan also known as preheater fan, smoke gas fan etc. Precalcining of raw meal is carried out in separate vessel vertically held and placed in between preheating and clinkerisation section. The clinkerisation reaction is carried out in a rotary kiln. The rotary kiln is a long cylindrical shell provided with refractory bricks from inside which prevents the heat loss from the kiln and protects the steel shell from any damage due to persistent high temperature maintained inside the kiln. The kiln is inclined at an angle of about 3-5o from horizontal from preheating to the cooling end. The kilns are mounted on tyres and rotated at a speed of 2.5-4 rpm. The dry and properly blended raw meal is lifted either by Air lift or mechanically by Bucket Elevator from the bottom of raw meal blending / storage silo to the top of the preheater, and fed at the top stage of cyclone inlet duct with the help of screw conveyor and rotary air lock. Raw meal weigh-feeders are installed for continuous weighment of raw meal for feeding the same to preheater at a constant rate.

To transform Raw Mix into CLINKER through PYRO-PROCRESSING.

Sections of a Typical Kiln System:KILN feed system.Pre-heater (Four stage to Six stage)Pre-Calciner (ILC, SLC)Kiln Cooler (including Clinker Hammer) Planetary Cooler Grate Cooler

In order to manufacture cement from the raw mix, it is required to heat raw meal to a temperature of 1450OC, thus carrying out SINTERING OR CLINKERISATION. The burning process requires an oxidising atmosphere in the kiln, as in the opposite case a clinker of brown colour (contrary to the normal greenish –gray) will be formed and the resulting cement will be quicker setting and with lower strength.

CHEMICAL TRANSFORMATIONS

During heating of the raw meal to the burning temperature 1450oC (clinkerization or sintering) certain physio-chemical processes take place.

These include:

Dehydration of the argillaceous minerals; decomposition of the carbonates (decarbonisation or expulsion of CO2 commonly known as calcination); reactions in solid phase and reactions with the participation of one liquid phase and crystallizations.

These processes are influenced by chemical factors in the raw meal (such as its chemical composition), by mineralogical factors (its mineralogical composition), by physical factors (fineness or particle size in the raw meal), homogeneity and other factors. The complete course of these endothermic reactions plays a decisive role in quality of the resulting cement.

In per-heater kiln, the first five transformations shows in figure 4.1 will take place in pre-heater tower. The decomposition of limestone and other carbonates will primarily take place in the calciner vessel where the calcination temperature is maintained by injection of fuel. The last two transformations will take place in the rotary kiln.

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The carbonate Ca CO3 decomposes between 600 – 800OC to form CaO. Quartz and clay will have started decomposing slightly before that to liberate free reactive Al2O3 and SiO2

The CaO being formed at this stage, now reacts with SiO2 to form C2S and later with more CaO to form C3S. Some CaO will also react with Al2O3 and Fe2O3 to form various intermediate components such as CA, C12A7 and others, which will decompose at higher temperature at later stage.

C2S content is seen to grow steadily during the heating and reach maximum content at approx. 1300OC which is a point where liquid phase appears. The major part of C2S is then transformed to C3S in the liquid phase and the final content of C2S in the clinker is less than the content of C3S.

6.5.1 COAL GRINDING SYSTEM

Combustible fuels eg., coal, liquid fuel oil, or natural gas are used for providing heat for converting raw meal into clinker.

 Indian cement plants use coal obtained from Indian Coalmines or imported from South Africa, china,Australia and Indonesia. The coal obtained in the form of lump containing upto 10% moisture is ground to suitable fineness in vertical roller mills or closed circuit tube mills at different cement manufacturing facilities. The cooler exhaust/ part of preheater gases are used for driving away the moisture from coal while grinding the same in the airswept VRM or tube mills. The figure above 6.5.1 represents a typical pyroprocessing and coal preparation section in a dry process 4-stage suspension preheater kiln equipped with folax cooler.

Conversion of raw meal into cement clinker is accomplished in steps in various zones of kiln circuit. The pulverized fuel (about 35-40% of total fuel to be fed to kiln system) is pushed into the burning zone of rotary kiln through a specially designed burner pipe along with the carrier air known as Primary air. The high temperature persisting in burning zone makes the fine coal to burn near the tip of burner pipe and helps in flame propogation.

The combustion gases generated from burning of purlverized coal in clinkerisation zone of the kiln flow towards the inlet of PH fan under the influence of the induced draft created in the kiln circuit. While flowing from burning zone towards the inlet of fan after passing through Kiln Precalciner–Preheater circuit, the high temperature combustion gas transfer its heat to the finally derived raw meal which is fed to the inlet duct of 1st stage twin cyclone and falls towards the bottom end of preheater after passing through all stages of cyclones under the influence of hot gases flowing in the circuit. The moisture and other volatile contents present are completely 20

Fig. 6.5.1

driven away and thus raw meal attains a precalcination of about 35-40% before reaching the precalcining vessel installed in between preheater and kiln. The precalciner is fired with 40-55% of total pulverized fuel for increasing the precalcination degree of raw meal upto 90-92% before the same is fed to kiln for accomplishing the clinkerisation reaction. The remaining 8-10% degree of calcinations of raw meal is performed in the kiln before the meal enters in to the burning zone. Thus burning zone in rotary kiln receives complete decarbonated material, the part of which is transformed into liquid after achieving appropriate melting temperature of some of the raw meal components and powdery form of raw meal gets converted into nodulized clinker form. The final clinkerisation of raw meal is achieved between the temperature range of 1250-1450oC depending upon the raw meal characteristics. The high temperature clinker nodules varying in size then fall out of the kiln and enter the cooler. Most of the Kilns in J K Cements Ltd are provided with modern folax grate coolers except in Sankari wet process plant where planetary coolers are provided for performing clinker cooling operation.

The modern Folax grate coolers are provided with fixed and moving grate plates. Below the grate are provided number of air chambers which receive atmospheric cold air with the help of number of high pressure discharge fans in different compartments. The pressurized air flows through the holes provided in the grate plates and cools the clinker which is travelling in the form of granules on the grate plates. The clinker is cooled down to a temperature of 100-150oC while leaving the outlet end of the cooler. The cold clinker is crushed continuously in a suitable clinker crusher provided at the outlet end of the cooler before the same is discharged on the clinker transportation system for transporting the same to clinker storage Silo stock/ Pile.

6.6 CEMENT GRINDING

In a modern cement plant, the total power consumption is about 100 Kwh / tonne whereas cement grinding process accounts about 40%. The quality of final cement product depends on operational mode and parameters of cement grinding plant.

Cement has to be ground fine enough to meet the requirements for strength properties specified in current standards. As it takes quite long time to determine especially the late strength, the hour –to-hour and day –to-day control of cement grinding has to be based on cement fineness.

Strength development of concrete is the result of hydration of the particles. Smaller the particles, larger the specific surface and faster the hydration. Particles coarser than 30–50 microns hydrate very slowly and will only affect late strengths. On the other hand, superfine particle with 2-3 micron size may hydrate before the concrete has been cast and will have limited influence on strength development.

In order to achieve the objectives of energy conservation, the clinker produced in rotary kiln cooled in cooler is usually stored for few days before it is ground in cement grinding mills along with appropriate quantity of gypsum and other additive materials for production of finely pulverized cement with desired fineness.

The Ball / Tube mills (in open circuit or closed circuit mode) are used for clinker grinding in cement plant worldwide. However, the energy requirement of Ball Mills . Being comparatively higher towards reducing energy consumption in cement grinding newer types of energy efficient equipments like Vertical Roller Mills, Roll Presses / Roller Presses in combination with Ball Mill / Closed Circuit Ball Mills, Horro Mills etc., are now being used for grinding cement clinker. Most of the plants in J K cements Limited are using tube mill grinding machines for cement grinding.

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The cement grinding unit along with dust collector, cement storage silo and the dispatch section are illustrated in the figure 6.6 shown.

 

6.7 CEMENT STORAGE PACKING & DESPATCH

The pulverized different types of cements are stored in different silos installed with different capacities. Depending upon the market requirements the cement is loaded in bulk or packed in 50KG bags with the help of conventional rotary packers or electronic packers, loaded on to trucks and finally dispatched to the required destinations. Purpose:

To pack cement in appropriate packages suitable for consumption at site.

Common packs available: Grey Cement:

50 Kg bags Bulk handling of cement has started at selected places e.g. bulk-handling

project near Mumbai by ACC. White Cement:

50 Kg and small size packs as per market demandPackers:

Mechanical Packers Electronic Packers

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Fig. 6.6.

6.8 QUALITY CHARACTERISTICS OF CEMENT

The survival and well being of the cement plants / companies in the market depends

upon the quality of product and its cost. Quality and cost together define the value of the

product (i.e. cement in this case). The concept of quality has undergone a sea change

from mere quality control of the product to total quality management (T.Q.M.) with

emphasis on quality defined as “totality of features and characteristics of product /

services that bears on its ability to satisfy the stated and implied needs”.

The quality of the product depends on variety of factors such as technology, quality of

raw materials and fuels, operation and quality control procedures to produced consistent

product.

The broad quality parameters of cement relate to chemical and physical properties as

per IS Code are as mentioned below:

CHEMICAL PROPERTIES:

Loss on ignition [LOI]

Insoluble residue [IR]

Sulphur trioxide [SO3]

Magnesium oxide [MgO]

Total chloride [Cl]

Lime saturation factor [LSF]

Alumina modulus [AM]

PHYSICAL PROPERTIES

Fineness

Consistency

Setting time – initial and final

Soundness

Compressive strengths (3 days, 7 days and 28 days)

Heat of hydration

Drying shrinkage [for PPC]

7 Cement kiln

Hot end of medium sized modern cement kiln, showing tyres, rollers and drive gear

Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually define the capacity of the cement plant. As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of their efficiency has been the central concern of cement manufacturing technology.

Contents

7.1 The manufacture of cement clinker7.2Early history7.3The rotary kiln7.4Kiln fuels7.5Kiln control7.6Cement kiln emissions

7.6.1Carbon dioxide7.6.2Dust7.6.3Nitrogen oxides (NOx)7.6.4Sulfur dioxide (SO2)7.6.5Carbon monoxide (CO) and total carbon7.6.6Dioxins and furans (PCDD/F)7.6.7Polychlorinated biphenyls (PCB)7.6.8Polycyclic aromatic hydrocarbons (PAH)7.6.9Benzene, toluene, ethylbenzene, xylene (BTEX)7.6.10Gaseous inorganic chlorine compounds (HCl)

7.1 The manufacture of cement clinker

A precalciner tower, rawmix silo and exhaust stack. Bottom left: rawmill. Bottom right: rotary kiln with tertiary air duct above. The U-shaped duct leading from the kiln inlet is an "alkali bleed".

A typical process of manufacture consists of three stages:

grinding a mixture of limestone and clay or shale to make a fine "rawmix" (see Rawmill);

heating the rawmix to sintering temperature (up to 1450 °C) in a cement kiln;

grinding the resulting clinker to make cement (see Cement mill).

In the second stage, the rawmix is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel. Successive chemical reactions take place as the temperature of the rawmix rises:

70 to 110 °C - Free water is evaporated. 400 to 600 °C - clay-like minerals are decomposed into their constituent

oxides; principally SiO2 and Al2O3. Dolomite (CaMg(CO3)2) decomposes to calcium carbonate, MgO and CO2.

650 to 900 °C - calcium carbonate reacts with SiO2 to form belite (Ca2SiO4).

900 to 1050 °C - the remaining calcium carbonate decomposes to calcium oxide and CO2.

1300 to 1450 °C - partial (20–30%) melting takes place, and belite reacts with calcium oxide to form alite (Ca3O·SiO4).

Typical clinker nodules

Alite is the characteristic constituent of Portland cement. Typically, a peak temperature of 1400–1450 °C is required to complete the reaction. The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1–10 mm. This is called clinker. The hot clinker next falls into a cooler which recovers most of its heat, and cools the clinker to around 100 °C, at which temperature it can be conveniently conveyed to storage. The cement kiln system is designed to accomplish these processes efficiently.

7.2 Early history

Portland cement clinker was first made (in 1842) in a modified form of the traditional static lime kiln. The basic, egg-cup shaped lime kiln was provided with a conical or beehive shaped extension to increase draught and thus obtain the higher temperature needed to make cement clinker. For nearly half a century, this design, and minor modifications, remained the only method of manufacture. The kiln was restricted in size by the strength of the chunks of rawmix: if the charge in the kiln collapsed under its own weight, the kiln would be extinguished. For this reason, beehive kilns never made more than 30 tonnes of clinker per batch. A batch took one week to turn around: a day to fill the kiln, three days to burn off, two days to cool, and a day to unload. Thus, a kiln would produce about 1500 tonnes per year.

A kiln is basically an industrial oven, and although the term is generic, several quite distinctive designs have been used over the years. Although perhaps more normally associated with pottery making, both ‘Bottle’ and their very close relatives ‘Beehive’ kilns, were also the central feature of any cement works. Early designs tended to be updraft kilns, which were often built as a straight sided cone into which the flame was introduced at, or below, floor level. Reaching heights of up to 70 ft, the dome or bottle shape of the kiln, known as the ‘hovel’, would be quite a prominent landmark. As well as protecting the inner kiln or ‘crown’, the opening at the top of the hovel also acted as a flue, to remove the smoke and exhaust gases that were produced during the production process. There was a three to four foot gap between the outer wall of the hovel and inner shell of the crown. Due to the fact that the one foot thick crown wall would expand and contract during firing, it was strengthened with a number of iron bands, known as ‘bonts’. These were set twelve inches apart and ran right around the circular oven. The development of downdraft kilns in the early 20th Century proved to be much more fuel efficient and were designed to force the heated air to circulate more around the kiln. The design incorporated a gentle curve at the 'shoulders' of the kiln, which served to reflect the rising heat from the fire at the bottom of the kiln, back down again over the material. The smoke and exhaust was then sucked out through holes at the bottom of the kiln via a flue, which was connected to a nearby chimney. The chimney would also serve a number of neighbouring kilns as well. The kiln would be fired for several days to achieve the high temperatures required to produce cement clinker, and although the above

methods were successful, the problem with any batch kiln was that it was intermittent and once the product had been produced, the fire had to be extinguished and the contents allowed to cool. This not only wasted a lot of the heat, but also added to the expense of the finished product.

In order to save money on fuel, a kiln was required that could run almost continuously, whilst the raw material was somehow fed through it. It was this scenario that lead to the development of the ‘Chamber’ kiln in the late 1850s. This particular kiln comprised a number of individual chambers, which were arranged so that the hot flue gases from one chamber, were drawn off and used to pre-heat the material in the following chambers, before they were drawn up the chimney. Once the first chamber had been filled with raw material, coal was added through the roof holes of the chamber and was then set alight. At the same time, the second chamber was being filled with raw material. The airflow from the first chamber was then adjusted, using a number of dampers, to funnel the hot air through to the second chamber to pre-heat the material. More coal was then poured into the second chamber and ignited, as the third chamber was being filled and so on. This process continued along the length of the kiln, so that by the time the last chamber had been fired, the first chamber had already been cleared and re-filled with more raw material so that the process could start again. Although such chamber kilns were still being installed as late as 1900, the development of the rotary kiln was already starting to have a major impact. The rotary kiln was a major advancement for the industry as it provided the continuous production of a much more uniform product in larger quantities.

Around 1885, experiments began on design of continuous kilns. One design was the shaft kiln, similar in design to a blast furnace. Rawmix in the form of lumps and fuel were continuously added at the top, and clinker was continually withdrawn at the bottom. Air was blown through under pressure from the base to combust the fuel. The shaft kiln had a brief period of use before it was eclipsed by the rotary kiln, but it had a limited renaissance from 1970 onward in China and elsewhere, when it was used for small-scale, low-tech plants in rural areas away from transport routes. Several thousand such kilns were constructed in China. A typical shaft kiln produces 100-200 tonnes per day.

From 1885, trials began on the development of the rotary kiln, which today accounts for more than 95% of world production.

7.3 The rotary kiln

General layout of a rotary kiln

The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.

The earliest successful rotary kilns were developed in Pennsylvania around 1890, and were about 1.5 m in diameter and 15 m in length. Such a kiln made about 20 tonnes of clinker per day. The fuel, initially, was oil, which was readily available in Pennsylvania at the time. It was particularly easy to get a good flame with this fuel. Within the next 10 years, the technique of firing by blowing in pulverized coal was developed, allowing the use of the cheapest available fuel. By 1905, the largest kilns were 2.7 x 60 m in size, and made 190 tonnes per day. At that date, after only 15 years of development, rotary kilns accounted for half of world production. Since then, the capacity of kilns has increased steadily, and the largest kilns today produce around 10,000 tonnes per day. In contrast to static kilns, the material passes through quickly: it takes from 3 hours (in some old wet process kilns) to as little as 10 minutes (in short precalciner kilns). Rotary kilns run 24 hours a day, and are typically stopped only for a few days once or twice a

year for essential maintenance. This is an important discipline, because heating up and cooling down are long, wasteful and damaging processes. Uninterrupted runs as long as 18 months have been achieved.

Ancillary equipment

Essential equipment in addition to the kiln tube and the preheater are:

Cooler Fuel mills Fans Exhaust gas cleaning equipment.

Coolers

A pair of kilns with satellite coolers in Ashaka, Nigeria Sysy

Early systems used rotary coolers, which were rotating cylinders similar to the kiln, into which the hot clinker dropped[8]. The combustion air was drawn up through the cooler as the clinker moved down, cascading through the air stream. In the 1920s, satellite coolers became common and remained in use until recently. These consist of a set (typically 7–9) of tubes attached to the kiln tube. They have the advantage that they are sealed to the kiln, and require no separate drive. From about 1930, the grate cooler was developed.

7.4 Kiln fuels

Used tires being fed mid-kiln to a pair of long kilns

Fuels that have been used for primary firing include coal, petroleum coke, heavy fuel oil, natural gas, landfill off-gas and oil refinery flare gas.High carbon fuels such as coal are preferred for kiln firing, because they yield a luminous flame. The clinker is brought to its peak temperature mainly by radiant heat transfer, and a bright (i.e. high emissivity) and hot flame is essential for this. In favorable circumstances, high-rank bituminous coal can produce a flame at 2050 °C. Natural gas can only produce a flame of, at best 1950 °C, and this is also less luminous, so it tends to result in lower kiln output.

In addition to these primary fuels, various combustible waste materials have been fed to kilns, notably used tires, which are very difficult to dispose of by other means. In theory, cement kilns are an attractive way of disposing of hazardous materials, because of:

the temperatures in the kiln, which are much higher than in other combustion systems (e.g. incinerators),

the alkaline conditions in the kiln, afforded by the high-calcium rawmix, which can absorb acidic combustion products,

the ability of the clinker to absorb heavy metals into its structure.

Whole tires are commonly introduced in the kiln, by rolling them into the upper end of a preheater kiln, or by dropping them through a slot midway along a long wet kiln. In either case, the high gas temperatures (1000–1200 °C) cause almost instantaneous, complete and smokeless combustion of the tire. Alternatively, tires are chopped into 5–10 mm chips, in which form they can be injected into a precalciner combustion chamber.

The steel and zinc in the tires become chemically incorporated into the clinker.Other wastes have included solvents and clinical wastes. A very high level of monitoring of both the fuel and its combustion products is necessary to maintain safe operation.For maximum kiln efficiency, high quality conventional fuels are the best choice.

When using waste materials, in order to avoid prohibited emissions (e.g. of dioxins) it is necessary to control the kiln system in a manner that is non-optimal for efficiency and output, and coarse combustibles such as tires can cause major product quality problems.

7.5 Kiln control

Online X-ray diffraction with automatic sample feed for free calcium oxide measurement

The objective of kiln operation is to make clinker with the required chemical and physical properties, at the maximum rate that the size of kiln will allow, while meeting environmental standards, at the lowest possible operating cost. The kiln is very sensitive to control strategies, and a poorly run kiln can easily double cement plant operating costs.

Formation of the desired clinker minerals involves heating the rawmix through the temperature stages mentioned above. The finishing transformation that takes place in the hottest part of the kiln, under the flame, is the reaction of belite (Ca2SiO4) with calcium oxide to form alite (Ca3O·SiO4):

Ca2SiO4 + CaO → Ca3SiO5

Also abbreviated in the cement chemist notation (CCN) as:

C2S + C → C3STricalcium silicate is thermodynamically unstable below 1250°C, but can be preserved in a metastable state at room temperature by fast cooling: on slow cooling it tends to revert to belite (Ca2SiO4) and CaO.

If the reaction is incomplete, excessive amounts of free calcium oxide remain in the clinker. Regular measurement of the free CaO content is used as a means of tracking the clinker quality. As a parameter in kiln control, free CaO data is somewhat ineffective because, even with fast automated sampling and analysis, the data, when it arrives, may be 10 minutes "out of date", and more immediate data must be used for minute-to-minute control.

However, for efficient operation, steady conditions need to be maintained throughout the whole kiln system. The feed at each stage must be at a temperature such that it is "ready" for processing in the next stage. To ensure this, the temperature of both feed and gas must be optimized and maintained at every point. The external controls available to achieve this are few:

Feed rate: this defines the kiln output Rotary kiln speed: this controls the rate at which the feed moves through

the kiln tube Fuel injection rate: this controls the rate at which the "hot end" of the

system is heated Exhaust fan speed or power: this controls gas flow, and the rate at which

heat is drawn from the "hot end" of the system to the "cold end"

In the case of precalciner kilns, further controls are available:

Independent control of fuel to kiln and calciner Independent fan controls where there are multiple preheater strings.

As an exercise in process control, kiln control is extremely challenging, because of multiple inter-related variables, non-linear responses, and variable process lags. Computer control systems were first tried in the early 1960s, initially with poor results due mainly to poor process measurements. Since 1990, complex high level supervisory control systems have been standard on new installations. These operate using expert systemstrategies, that maintain a "just sufficient" burning zone temperature, below which the kiln's operating condition will deteriorate catastrophically, thus requiring rapid-response, "knife-edge" control.

7.6 Cement kiln emissions

Emissions from cement works are determined both by continuous and discontinuous measuring methods, which are described in corresponding national guidelines and standards. Continuous measurement is primarily used for dust, NOx and SO2, while the remaining parameters relevant pursuant to ambient pollution legislation are usually determined discontinuously by individual measurements.

The following descriptions of emissions refer to modern kiln plants based on dry process technology.

7.6.1 Carbon dioxide

During the clinker burning process CO2 is emitted. CO2 accounts for the main share of these gases. CO2 emissions are both raw material-related and energy-related. Raw material-related emissions are produced during limestonedecarbonation (CaCO3) and account for about 60 % of total CO2 emissions.

7.6.2 Dust

To manufacture 1 t of Portland cement, about 1.5 to 1.7 t raw materials, 0.1 t coal and 1 t clinker (besides other cement constituents and sulfate agents) must be ground to dust fineness during production. In this process, the steps of raw material processing, fuel preparation, clinker burning and cement grinding constitute major emission sources for particulate components.

7.6.3 Nitrogen oxides (NOx)

The clinker burning process is a high-temperature process resulting in the formation of nitrogen oxides (NOx). The amount formed is directly related to the main flame temperature (typically 1850 – 2000 °C). Nitrogen monoxide (NO) accounts for about 95 %, and nitrogen dioxide (NO2) for about 5 % of this compound present in the exhaust gas of rotary kiln plants. As most of the NO is converted to NO2 in the atmosphere, emissions are given as NO2 per cubic metre exhaust gas.

7.6.4 Sulfur dioxide (SO2)

Sulfur is input into the clinker burning process via raw materials and fuels. Depending on their origin, the raw materials may contain sulfur bound as sulfide or sulfate. Higher SO2 emissions by rotary kiln systems in the cement industry are often attributable to the sulfides contained in the raw material, which become oxidised to form SO2 at the temperatures between 370 °C and 420 °C prevailing in the kiln preheater. Most of the sulfides are pyrite or marcasite contained in the raw materials. Given the sulfide concentrations found e.g. in German raw material deposits, SO2 emission concentrations can total up to 1.2 g/m3depending on the site location. In some cases, injected calcium hydroxide is used to lower SO2 emissions.

7.6.5 Carbon monoxide (CO) and total carbon

The exhaust gas concentrations of CO and organically bound carbon are a yardstick for the burn-out rate of the fuels utilised in energy conversion plants, such as power stations. By contrast, the clinker burning process is a material conversion process that must always be operated with excess air for reasons of clinker quality. In concert with long residence times in the high-temperature range, this leads to complete fuel burn-up.

7.6.6 Dioxins and furans (PCDD/F)

Thus, temperature distribution and residence time in rotary kilns afford particularly favourable conditions for organic compounds. For that reason, only very low concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (colloquially "dioxins and furans") can be found in the exhaust gas from cement rotary kilns.

7.6.7 Polychlorinated biphenyls (PCB)

The emission behaviour of PCB is comparable to that of dioxins and furans. PCB may be introduced into the process via alternative raw materials and fuels. The rotary kiln systems of the cement industry destroy these trace components virtually completely.

7.6.8 Polycyclic aromatic hydrocarbons (PAH)

PAHs (according to EPA 610) in the exhaust gas of rotary kilns usually appear at a distribution dominated by naphthalene, which accounts for a share of more than 90 % by mass. The rotary kiln systems of the cement industry destroy

virtually completely the PAHs input via fuels. Emissions are generated from organic constituents in the raw material.

7.6.9 Benzene, toluene, ethylbenzene, xylene (BTEX)

As a rule benzene, toluene, ethylbenzene and xylene are present in the exhaust gas of rotary kilns in a characteristic ratio. BTEX is formed during the thermal decomposition of organic raw material constituents in the preheater.

7.6.10 Gaseous inorganic chlorine compounds (HCl)

Chlorides are minor additional constituents contained in the raw materials and fuels of the clinker burning process. They are released when the fuels are burnt or the kiln feed is heated, and primarily react with the alkalis from the kiln feed to form alkali chlorides. These compounds, which are initially vaporous, condense on the kiln feed or the kiln dust, at temperatures between 700 °C and 900 °C, subsequently re-enter the rotary kiln system and evaporate again.

BIBLOGRAPHY

1. RG Blezard, the history of calcareous cement in PC Hewlett.

2. A C Davis , A hundred years of Portland cement.

3.G R Redgrave & C Spackman , calcareous cement.

4.K E peray, The Rotary cement kiln.

WEB SITES

1. www.wikipedia.org

2.www.jkcement.com

3.www.rtcnorthindia.org