Page 1 : ENGINEERING CHEMISTRY, SILICA INDUSTRIES, 784, 785, breaks. By so doing breaking speed is known., 6 Truth, It is ascertained that the wheel has got the dimensione, as desired by means of caliper and further that when it rotates, an upright spindle, it remains at strictly 90 degrees., Raw Materials for Cement Manufacture. There are two, kinds of raw materials for the manufacture of Portland cements., 1. Calcareous Materials. They supply lime. The following, kinds of calcareous materials are used:, (i) Lime stone (containing 65 to 80%, CaCO3), 14.20. Availability of Natural Abrasives in India,, Diamonds. The production of diamonds in India is very small, (ii) Marl., (iii) Chalk., These are mined from Panna Mines., Corundum. Good quality corundum is found in Assam, Rewa., Madras (Salem) and Mysore., Garnet. Garnet sands suitable for abrasive purposes occur in, Bihar and in the Trivunelvelly district, Madras., Siliceous Abrasives. Many of these are found in India in abun-, dance. Quartz and quartz sand, and flint which is used for making, millstones and whetstones are available. High grade talc is widely, distributed. Felspar is available as a waste product from mica, (iv) Alkali waste which contains precipitated CaCO3. It is, obtained from the manufacture of caustic soda., Calcareous materials should be such that they contain less, than 3.3% of MgO and 3 to 4% of SiO2, Fe2O3 and Al203 combined., 2. Argillaceous Materials. They supply silica, iron oxide and, alumina. Agrillaceous materials used are the following:, (i) Clay., (ii) Shale., (iii) Slate., (iv) Blast Furnace Slag., All these máterials should have 2.5 to 4 times more of silica, mines., than alumina., Diatomitc, tripolite, pumice are not found in workable quan-, tities., 14.21. Manufacture of Cement., Synthetic abrasives are not manufactured as yet in India., Portland Cement, Concrete & Mortar, Lime, Gypsum, and Plaster of Paris, It is a class of hydraulic cements, the essential constituents of, which are two calcium silicates, 3CAO.SiO2 and 2CAO.SiO2., Aluminium and iron which are found in raw materials are in the, form of 3CAO.Al203 and 4CAO.Al203 Fe2O3. These two compounds, are in the form of solid solution or under cooled liquid (that is,, glass). In minor quantity there are other impurities also in the, Portland cement., Outline of Manufacture. Manufacture of cement lies in, mixing calcareous and argillaceous materials together very, thoroughly in such a ratio that the required composition is ob-, tained. The mixture is heated to near fusion points, that is, 1400°C to 1600°C. The clinker obtained is cooled and mixed with 2, to 3% gypsum and then finely pulverised., There are two methods of manufacture :, 1. Dry process and, 2. Wet process., 1. Dry Process. Dry process is used for the manufacture of, cement when raw material is either cement rock or blast furnace, slag. Raw materials are separately crushed to 50 mm or smaller, pieces and then dried and stored separately. Crushing is done in, gyratory crushers and drying is done by means of rotary driers., Following is the general composition of Portland cement:, CaO, 60 to 66%, SiO2, Al203, 17 to 25%, 3 to 8%, The raw materials are then mixed in proper ratio by automatic, weighing machines. The mixture is finely ground and then burnt., Dry process is quite similar to wet process with the only, difference that no water is added to the material in grinding and, thus no slurry is made., 2 to 6% [sometimes as low as 0.5%, in white cements], Fe202, MgO, NagO and K20, 0.1 to 5.5%, 0.5 to 1.5%, 1 to 3%

Page 2 : ENGINEERING CHEMISTRY, SILICA INDUSTRIES, 787, 786, Wet Process. Wet process is quite common and is universally, emploved for the manufacture of Portland cement. The process.co, be split up into the following steps., @) Crushing. Raw materials are crushed by gyratory crushers, receives impacts of the pebbles and finer the discharged product., (Fig. 14.4)., and are stored., (ii) Grinding. The grinding of raw materials is carried out in, two stages, first by ball and then by tube mill. Material from bal, mill to the tube mill is conveyed by means of a screw driver. Before, grinding, the material is mixed with 30 to 40% water,, Inlet, Outlet, Ball Mill Grinding. It consists of cast iron drum containing, chilled iron or steel balls of different sizes. Dust passes through the, perforated plate and falls on the fine sieve. The coarse particles are, retained by the screens and return to the interior of the drum, through an opening. (Fig. 14.3), Tube Mill, Fig. 14.4., After grinding 95 to 98% of the material passes through a, 100-mesh sieve. The finer the particles the better is the cement, produced as the reacting particles when finer have got more surface, contact., Cast Iron, Drum, (ii) Storage of Ground Materials. Ground raw materials, containing 30 to 40% of water are stored in separate basins or tanks, equipped with agitators., (iv) Correction Vats. From raw material tanks the materials, are sent in fixed proportion to correction vats. When the vat is full,, a sample is drawn and analysed. If the slurry is not of the required, composition, it is corrected by adding more of the deficient raw, material., Inlet, Balls, Perforated Plate, Fine Sieve, (1) Kiln Feed Basins. From correction vats the material in the, form of slurry goes to the kiln feed basins. These basins are also, equipped with agitators. Now the material is ready for feeding into, the rotary kiln., (iv) Rotary Kiln. Rotary kiln is a steel cylinder lined with, refractory bricks. The kiln is set at an angle from the horizontal., Generally inclination is 0.5 to 0.75 inch in one foot. The kiln has, several tiers which run on rollers. The kiln generally rotates at a, speed of 0.5 to one revolution per minute. The diameter of the kiln, of 6 ft. to 12 ft. Length of the kiln is 60 to 350 ft. and sometimes it, may go to 500 ft. The upper end of the kiln ends in a circular opening, into a concrete chamber. This concrete chamber is lined with, refractory bricks and is connected with the kiln feed basins., Fig. 14.3 Ball Mill., The principle used in the ball mill is impact and shear produced, by sliding, tumbling and rolling of a large number of steel or flint, balls or pebbles., Tube Mill Grinding. From the ball mill the material goes to, the tube mill. In construction the tube mill is just like the ball mill, but has a different shape. It is conical at the discharge end., In principle tube mill is the same as ball mill but it is continual-, ly operative. On one side the material is fed and from the other side, it comes out. The size of the grinding depends upon the speed of the, feeding. The slower the speed of feeding, the longer the material, As the lower end of the kiln there is a small chamber or hood, made of fire-clay bricks. This hood has two openings, one for the, fuel burning operation and the other for the observation of the, whole operation and recording the temperature etc. Through the, hole sometimes bar etc, is also inserted to break the blocks formed

Page 3 : ENGINEERING CHEMISTRY, SILICA INDUSTRIES, 789, 788, by the clinker, otherwise the passage will be choked. The lower no, of the hood is kept open for the clinker to fall down. [Fig. 14.51, (a) Up to 100°C there is evaporation of free water. The, reaction is endothermic., Fire brick, lining, (b) At 500°C and above, there is evolution of combined water, from clay. The reaction is endothermic., Feed pipe, Drying zone, Fuel hopper, Caleining zone, (c) At 900°C and above, there is evolution of CO2 from, CACO3. The reaction is endothermic., Burning zone, (d) Between 900°C and 1200°C there takes place the main, reaction between the lime and clay. The reaction is ex-, othermic., Exhaust fan for, discharging gases Chain system, from the kiln, Fuel, Fuel, pipe, (e) Between 1250 and 1280°C, there is commencement of, liquid formation. The reaction is endothermic., for facilitating, heat transfer from, Clinker, blower, gases to slurry, Fig. 14.5 Flow Sheet of Rotary Kiln., () At 1280°C and above, there is further formation of liquid, and compounds. The reaction may be endothermic., Heating of the Kiln. Heating of kiln is done by burning, pulverised coal which is injected through special burners., Temperature maintained is 1400°C to 1500°C. Coal is pulverised, by first crushing in jaw crusher and then grinding in ball mills. Size, of coal is such that 90% passes through 100-mesh sieve., (vii) Cooling of the clinker. The clinker through the clinker, conveyor falls into the coolers. In the coolers the atmospheric air, for burner passes over this hot clinker, cooling the clinker and itself, getting heated. The common type of coolers are rotary coolers. The, rotary cooler consists of a shell fitted with baffle plates which lift, the clinker and drop it through a draft of air., The quality of cement produced depends greatly on the rate of, cooling. The following are the effects of rate of cooling on the, properties of cement :-, (a) Rate of cooling very slow. If the rate of cooling is so slow, that no glass is formed, (as glass is formed by super cooling) there, will occur a phenomenon known as dusting. Dusting means conv, sion of dicalcium silicate to powder form and not crystalline. This, is caused by the conversion of beta form into gamma form of, dicalcium silicate. But the gamma form is not desired as it has no, binding property and it hydrates very slowly., (b) Rate of cooling quite high. When rate of cooling is high, then all the melted alumina and iron solidifies into glass and there, is no formation of any crystalline compounds of alumina and iron., This non-formation of any crystalline compounds is advantageous, as the crystalline compound 3CAO.Al2O3 has some unfavourable, properties and the crystalline compound 4CAO.Al2O3.Fe2O3 has no, hydraulic properties., of the length of the kiln is, 4th, Reactions in Kiln. 1. The first, the drying zone. In this zone water is driven out of the slurry by, means of hot gases., 2. The second zone is the calcining zone. Here organic matter, burns away. Calcium carbonate decomposes into calcium oxide and, carbon dioxide which escapes., CaCO3→, CaO + CO2, Temperature reached in this zone is 900°C. When the reaction, is complete the material has travelled of the length of the kiln., 4., This decarbonation is an endothermic reaction., 3. After decarbonation the material goes into the hottest zone., The temperature in the zone is about 1400 - 1500°C. Here 20 to, 30% of the material is converted into liquid, that is, it melts. This, melted liquid reacts with surrounding particles. Sintering takes, place in this zone. Clinker is formed and clinker falls out from the, lower end of this kiln., (c) If the rate of cooling is such (medium) that the melted liquid, in the clinker crystallises there is formed a large quantity of, 3Ca0.SiO2 which gives high strength to cement., All these reactions in the rotary kiln can be described as given, below :

Page 4 : Raw Material, Storage, Stone From, Query, Shale Sand, Scale, Hammer, Gyratory, Crusher, Scale, Ore, Iron Ore, -Sand Stone, Mill, -Shale, Hoppers W Lime Stone, Belt Conveyor., -Bin, Primary Mill, For Grinding, Thickened Slurry, Coarse Particles, Kiln Feed, Basins, Sieving, Secondary Mill For Grinding, Thickener, Coarse Particles, Pump, Sieving, Sump Or Storage, For Ground Materials, Sump Or, Correction Vats, O-Pump, MLMISTRY, 790, SILICA INDUSTRIES, 791, excessive expansion of cement in after years., melted clinker., other materials are mixed with the cement as additives ·, H2O. The quantity, CaSO4. 2H20, or plaster of Paris, CaSO4., added is 2.5 to 3%., (b) Dispersing Agent. Small quantities of dispersing agents, such as sodium salts of polymers of condensed naphthalene or, sulphonic acid are added to the cement. These dispersing agents, prevent the formation of lumps or cakes in the cement. The, cost of grinding is reduced by 30 to 50%., (c) Additives which cause air entrainment. Air entrain-, ment into cement improves the durability of concrete structures, especially when repeatedly attacked by cycles of freezing and, thawing. For entrainment or air certain materials such as Vinsol, resin or Darex may be added. These agents have the property of, infusing air into the cement paste., (d) Water-proofing agents are also added., (ix) Grinding of Clinker. The final product which is clinker, power, consists of granular masses having size from - inch to, 1, inch. It is, 4., 88, pulverized and then it is further ground in tube mills. During, grinding additives are mixed. The ground fine powder is packed 0y, automatic machines into packages., Fineness of cement. Portland cement is so fine that 98 to 30, passes through a 200-mesh sieve and 90% through a 325-mesu, sieve., Fig. 14.6 Flow sheet diagram for cement manufacture., clinker turns into glass and if is also in the clinker, (d) If the rate of is so high that the in the, that will also be into glass and will not out as, be to a of of the, cooling the and there will not be, It is very and that of should, does not to a high degree when exposed to So by, cement as the very slowly, cement after some years. When is in the form of it, when it is to and this of the, periclase This will give effect to the of