CEMENT MANUFACTURING PROCESS

CEMENT MANUFACTURING PROCESS

Definition: Defined as a product material obtained by calcination of calcareous (a material containing lime) and argillaceous (a material which contain silica) materials. According to (specification stand as) ASTM (American standard testing material) Cement is defined as a product obtained by pulverizing (crushing followed by grinding) clinkers to which no addition has been made except H2O.

Composition Chemically cement is a mixture of Silicates, Aluminates like aluminoferrites with some amount of MgO and gypsum. Various complexes which are contained in cement are (2CaO.SiO 2 ) Di Calcium Silicate (C 2 S) 22-24% (3CaO.SiO 2 ) Tri Calcium Silicate (C 3 S) 50-58% (3CaO.Al 2 O 3 ) Tri Calcium Aluminate (C 3 A) 10-11% (4CaO.Al 2 O 3.Fe 2 O 3 ) Tetra Calcium Aluminoferrite (C 4 AF) 8-9% (6CaO.Al 2 O 3.Fe 2 O 3 ) Hexa Calcium Aluminoferrite (C 6 AF) traces MgO <5% Gypsum (CaSO 4.2H 2 O) 4-5%

Various oxides Cement contains various oxides as follows 1) CaO 60-66% 2) Total silica 20-15% a) Combined silica/active silica b) Uncombined or inert silica 3) Alumina (Al 2 O 3 ) 3-8% 4) Fe 2 O 3 1-5% 5) MgO <5% 6)Alkalies <5%

Color The gray color in cement is due to presence of complexes of Ferric ions in O.P.C (ordinary Portland cement). White cement is produced from those raw materials which contain extremely low conc. of Iron & even like low conc. of Iron is converted from Fe +3 to Fe +2 (which can be done by reducing or maintaining atmosphere in the kiln) & eventually the product appears to be white.

Raw materials used in the production of O.P.C: 1) Lime stone 2) Clay 3) Gypsum

Lime stone: Lime stone on calcinations gives CaO which is an essential component of all the complexes (silicates, aluminates and aluminoferrites) present in cement. Lime stone is available in two lime forms. A) High lime: High lime is that lime stone which on calcination gives CaO of greater than 90% purity. The high lime contain Magnesia (MgO) less than 5% and rest of the impurities may be alumina, Fe 2 O 3,SiO 2. High lime is available in Pakistan in the area of Banu District Kohat,D.G khan, D.I khan,salt area of Mianwali.

Cont.. B) Low lime Low lime means the limestone which on decomposition yields CaO of less than 90% purity. Alumina & MgO is present to an extent of greater than10%. Dolomite rich in MgCO 3 should not be used in the manufacture of Portland cement.

Clay The complexes such as silicates, aluminates & aluminoferrites all contain SiO 2, & Al 2 O 3.These are added into O.P.C by clay. Ordinary clay found in the area close to the mines of lime stone have high quality clay. The clay need for all production of O.P.C should meet the following requirements. 1) It must be high in silica contents 2) Comparatively low in the amounts of Al 2 O 3 3) Low in the amount of alkalis In case if clay doesn t conform to the required specifications of oxides then the clay has to be beneficiated before use.

Gypsum When H 2 O is added into the cement & it is not thoroughly mixed then tricalciumaluminate picks up H 2 O and rapidly hardens. This phenomenon is called as flash setting. The flash setting in cement is retarded by the addition of Gypsum which forms the following complex (3CaOAl 2 O 3. 3CaSO 4.31H 2 O) The time of hardening of cement is increased. Thus the Gypsum containing cement gets hardened in comparatively longer period of lime.

Fuel Prior to 1971 the cement industry used furnace oil as fuel in cement industry. As an higher energy consuming industry in which energy is required for crushing, grinding, mixing & running of various units. Prior to 1971 most of the industries in Pakistan used furnace oil as a fuel. However after 1971 the majority of the cement industries was switched over to natural gas. In the past few years the importance of natural gas was realized for the production of petrochemicals and the cement plants were converted into furnace oil fired units now most of the cement industry is being switched over to using coal as a fuel.coal is available in Pakistan in Baluchistan, area of Makarwal (Mianwali district)

The indegeneous coal contains high % of organic as well as inorganic sulphur which in some cases is as high as 6.8%. High sulphur contents make the Pakistan coal inferior as the S compound lower the calorific value of coal. If coal contains high contents of S is burnt, proportional higher quantity of SO 2 or SO 3 will be added into the exhaust of flue gasses.

Water If cement is produced by wet process large quantity of H 2 O is needed. The H 2 O should not be saline in nature.

Various steps involved in the Production of cement: Cement production involves following steps 1) Crushing & grinding of clay and lime stone. 2) Mixing of ground raw material to obtain an almost homogeneous mass. 3) Transportation of material ground raw materials to rotary kilns & burning of raw materials to clinkers. 4) Grinding of clinkers in the presence of CaSO 4.2H 2 O

Step no 3 & 4 are as like on all the cement plants however step no 1 & 2 may be carried out in two ways. a) The crushed materials is ground & mixed in the presence of H 2 O to obtain slurry containing 40%H 2 O &60% solids. This method is called as wet method. b) If the crushed material is ground & mixed in absence of H 2 O & dry homogenized mass is fed into the kiln for burning. The process is called as dry process.

Comparison between wet and dry processes: Wet process 1. Soft raw material is used 2. The crushed material is ground in the presence of H 2 O to form a slurry paste.the slurry is than fed in to the rotary kiln for the manufacture of cement 3. Grinding is done in rotary ball mills in the presence of H 2 O to obtain slurry of ground lime stone & clay. Dry process 1. Hard or soft any type of raw material is used for the production of cement. 2. The crushed material is ground in dry &the feed given to the kiln is in the form of dry powder. 3. The grinding is conducted in the roller mill which combines grinding drying as well as classification. In that roller mills particles of dried size are collected through a classification.

4. Fuel consumption is comparatively more as the slurry added in the rotary kiln is 30-40%H 2 O and 60-70% solids. Substantial amount of heat is used in evaporation of H 2 O 5. Exposure to the effect of heat is for a longer period and hence the operating life of the lining of the kiln is less. 6. The length of the rotary kiln is more which may be between 300-400 ft usually and it may be sometime even as long as 500 ft. 4. Since water has not to be evaporated so less fuel consumption. 5. Exposure to the effect of heat is less & hence the operating life of the lining of the kiln is more. 6. The length of kiln is usually about 150 ft which may in some cases be 250 ft.

7. The cost of insulation is high. 8. Thermal efficiency is higher as the flue gases leave the kiln at about 100 0 C. 7. The cost of insulation of kiln is less. 8. Thermal efficiency in the kiln is less as the gases leaving the rotary kiln are about 300 o C. 9. The loss of raw material in the form of dust contained in the flue gas coming out of the kiln is less. It amounts to about 1.5% of the clinker produced. 9. The loss of the raw material in the form of dust is much higher and about 15% raw material is lost with the flue gas coming out of the kiln.

10. The dust particles leaving with the flue gas are recovered by installing scrubbers, cyclone separators and by installing high density electrical precipitators. 11. The recovery of heat from the flue gases is less. 10. Dust contained in the flue gas is recovered in the same way as in wet process. 11. The heat recovery in dry process is 20% more in this process.

Crushing & grinding of clay and lime stone The limestone and the clay needed for the production of cement as excavated from the quarries are crushed to approximately a particle size of 100mm using crushers which are called as the primary crushers. The secondary crusher installed in the same area crushes the limestone and the clay to obtain a particle size of about 10mm. In the wet process, the crushed lime and clay is fed into a rotary grinding ball mill in the presence of water. The grinding is carried out using the technique of close circuit.

Mixing of ground raw material to obtain an almost homogeneous mass The lime and clay obtained from the classifier is received in a correcting tank where the composition w.r.t. limestone and clay is adjusted. The slurry should contain 65-70% limestone and 30-35% clay in it. In case any of the two is less in quantity, the difficulty is made up by adding required quantity of deficient material. Then slurry containing corrected limestone and clay is pumped to a dark thickener where excess of water of any is removed which in turn is reused for grinding in the ball mill. The slurry obtained from the dark thickener should contain 30-40% water and 60-70% of the solids. This slurry is then introduced into the kiln for pyro processing (heat treatment).

Transportation of ground raw materials to rotary kilns & burning of raw materials to clinkers Construction and working of the rotary kiln: The pyro-processing of slurry obtained in the wet method and the dry ground raw materials in the dry process is carried out in a rotary kiln. It is made from sheets of plain carbon steel riveted together to obtain a shell. In the old versions the rotary kiln tapered towards the top. Now a days large no. of cement industry is using rotary kiln of uniform internal diameter. In the old versions the rotary kiln was placed in a way that it made an angle of 15 degrees with the horizontal which facilitates the movement of material from the feed end to the discharge end.

Construction and working of the rotary kiln: Now a days the inclination of shell with the horizontal has been reduced to about 5-6 degrees. To facilitate the movement of the material from feed end to the discharge end, chains are installed inside the rotary kiln. The kiln rotates in the clockwise direction by standing at the discharge end. The revolutions of the kiln about its axis ranges between half to one rpm in some cases, while in the others it may be 1-3 rpm. This depends upon the size of the kiln which may be 300-400 ft in case of wet process and up to 150 ft in dry process. From the feed end to the discharge end the total time taken for the movement of material is between 2-3 hours.

Cont.. The temperature maintained inside the rotary kiln ranges between 300-1400 degree Celsius in dry process and 100-1400 degree Celsius in wet process. At elevated temperature, calcium oxide behaves as a very strong base and it can attach the material of construction of the shell of rotary kiln. It is therefore necessary to take appropriate measure to increase the working life of rotary kiln. For this purpose, lining is provided on the interior of the shell of rotary kiln.

Merits of Lining in rotary kiln A lining provided in the rotary kiln has the following merits: 1. The chemical attack of calcium oxide leading to corrosion of material of construction of shell be eliminated. 2. The lining used is of a refractory material which processes very low thermal conductivity hence heat losses from rotary kiln to atmosphere would be reduced.

Cont.. The ordinary fire bricks which contain a very high percentage of silica cannot be used for the purpose of lining as silica contained in it may react with CaO to form CaSiO 2 which may solidify inside the kiln forming rings which may obstruct the movement of material inside the kiln from the feed end to discharge end. Usually high aluminium bricks produced from bauxite are used for the purpose of lining. In the old version of the rotary kiln the lining of 9 inches were provided from first 100 ft staring from discharge end where as the next 110ft was lined with 7 inches thick brick of bauxite. Now a days the kiln usually processes a uniform lining.

Temperature Zones of Rotary kiln Temperature Range Working 1. 100-300 0 C 2. 300-600 0 C 3. 400-800/900 0 C 1. Evaporation of water. This zone shall not be existed in dry process. 2. (a)evaporation of combined water means chemically bound water. (b)decomposition of carbonate starts. 3. Burning of organic matter.

Cont.. 4. 600-900 o C 5. 900-1200 0 C 4. Volatilization of alkalis. Volatilization of chlorides. Evolution of CO 2 from CaCO 3 and the existence of free CaO start. 5. In this zone, main reaction between lime and clay takes place which is further subdivided into the following: a) 900-1100 0 C (Formation of tricalcium aluminate and tetracalcium aluminoferrite.) b) 1100-1200 0 C (Max. formation of tricalcium aluminate and dicalcium silicate.)

Cont.. 6. 1250-1280 0 C 7. 1280-1400 0 C 6. Commencement of liquid formation. 7. Further formation of molten mass and the formation of main constituent of cement i.e. tricalcium silicate. In this range most of CaO from the decomposition of lime stone gets associated to form various complexes.

Clinker Formation The product obtained due to high melting temperature gets converted into the form of circular balls called as clinkers which vary in this size between 1/8 to 3/8 inches, the clinkers are collected in a cooler where they are cooled using a draft of cold air. Wet air gets heated to 700-800 0 C and used in the burning of fuel in the rotary kiln. The flue gases leaving at the feed end pass through an integrated system of scrubbers, cyclone separator and electrical precipitators to recover the dust particles of the raw material.

CLINKER GYPSUM

Grinding of the clinkers: The clinkers are ground in the presence of 4-5% gypsum to obtain fine powder which may confirm to the following specification: ASTM The 78% of the ground material should pass through 200 mesh sieve.

THE CEMENT MANUFACTURING PROCESS quarry loader dumper Quarry face 1. BLASTING 2. TRANSPORT crushing conveyor storage at the plant 3. CRUSHING & TRANSPORTATION 1. BLASTING : The raw materials that are used to manufacture cement (mainly limestone and clay) are blasted from the quarry. 2. TRANSPORT : The raw materials are loaded into a dumper. 3. CRUSHING AND TRANSPORTATION : The raw materials, after crushing, are transported to the plant by conveyor. The plant stores the materials before they are homogenized. Next

THE CEMENT MANUFACTURING PROCESS Raw grinding and burning storage at the plant conveyor Raw mill Raw mix preheating 1. RAW GRINDING kiln cooling clinker 2. BURNING 1. RAW GRINDING : The raw materials are very finely ground in order to produce the raw mix. 2. BURNING : The raw mix is preheated before it goes into the kiln, which is heated by a flame that can be as hot as 2000 C. The raw mix burns at 1500 C producing clinker which, when it leaves the kiln, is rapidly cooled with air fans. So, the raw mix is burnt to produce clinker : the basic material needed to make cement. Back Next

THE CEMENT MANUFACTURING PROCESS Grinding, storage, packing, dispatch clinker storage Gypsum and the secondary additives are added to the clinker. Finish grinding silos 1. GRINDING dispatch bags 2. STORAGE, PACKING, DISPATCH 1.GRINDING : The clinker and the gypsum are very finely ground giving a pure cement. Other secondary additives and cementitious materials can also be added to make a blended cement. 2. STORAGE, PACKING, DISPATCH :The cement is stored in silos before being dispatched either in bulk or in bags to its final destination. Back Home page