1. Silicate industry for under graduate student.pptx

Industrial chemistry for under graduate students

1. Silicate Industry The industry processing the natural compounds of silicon is called the silicate industry. It embraces the pro­duction of cement, glass, and ceramics . About 95% of the earth’s crust is largely composed of silica and silicate minerals, which form the principal constituents of all rocks and of the sands, clays and soils that result from degradation of rocks. 1 . Ceramics Ceramics are defined as a class of inorganic, nonmetallic solids that are subjected to high temperature in manufacture and/or use . The most common ceramics are composed of oxides, carbides, and nitrides

2. Cont ….. Ceramic products are frequently classified into two groups on the basis of their chemical composition Clay ceramic materials with more than 20% by weight of clay minerals in the raw mixture Specialty ceramic materials which have a lower clay mineral content or are clay mineral free.

3. Cont … The general classifications of traditional ceramics are: Pottery is sometimes used as a generic term for ceramics that contain clay and are not used for structural, technical, or refractory purposes. White-ware refers to ceramic ware that is white, ivory, or light gray in color after firing. It is further classified as earthenware, stoneware, chinaware, porcelain, and technical ceramics .

4. Cont …

5. Cont … Characteristics of ceramics are high temperature stability high hardness Brittleness low elongation under application of stress low thermal and electrical conductivities

6. Process Description for production of Ceramics 1. Raw Material Procurement To begin the process, raw materials are transported and stored at the manufacturing facility. Naturally occurring raw materials used to manufacture ceramics include silica, sand, quartz, flint, silicates, and aluminosilicates (e. g., clays and feldspar ). 2. Beneficiation : The basic beneficiation processes include :- Comminution entails reducing the particle size of the raw material by crushing, grinding, and milling or fine grinding. Its purpose is to liberate impurities , break up aggregates, modify particle morphology and size distribution , facilitate mixing and forming , and produce a more reactive material for firing

7. Cont … purification, sizing, classification Calcining : consists of heating a ceramic material to a temperature well below its melting point to liberate undesirable gases or other material and to bring about structural transformation to produce the desired composition and phase product

8. 3. Mixing purpose of mixing is to combine the constituents of a ceramic powder to produce a more chemically and physically homogenous material for forming . Binders and plasticizers are used in dry powder and plastic forming; in slurry processing, deflocculants , surfactants, and antifoaming agents are added to improve processing . 4. Forming In the forming step, dry powders, plastic bodies, pastes, or slurries are consolidated and molded to produce a cohesive body of the desired shape and size .

9. 5. Green Machining the ceramic shape often is machined to eliminate rough surfaces and seams or to modify the shape. The methods used to machine green ceramics include surface grinding to smooth surfaces, blanking and punching to cut the shape and create holes or cavities, and laminating for multilayer ceramics . 6. Drying Drying must be carefully controlled to strike a balance between minimizing drying time and avoiding differential shrinkage, warping, and distortion. The most commonly used method of drying ceramics is by convection, in which heated air is circulated around the ceramics.

10. 7. Presinter Thermal Processing Prior to firing, ceramics often are heat-treated at temperatures well below firing temperatures. The purpose of this thermal processing is to provide additional drying, to vaporize or decompose organic additives and other impurities, and to remove residual, crystalline, and chemically bound water. Presinter thermal processing can be applied as a separate step, which is referred to as bisque firing , or by gradually raising and holding the temperature in several stage

11. 8.Glazing Glazing refers to a vitreous coating to a ceramic material whose primary purposes are decoration or protection Some of the more commonly used raw materials for glazes are quartz, feldspars, carbonates, borates, and zircon. 9. Firing Firing is the process by which ceramics are thermally consolidated into a dense, cohesive body comprised of fine, uniform grains . This process also is referred to as sintering or densification . Conventional firing is accomplished by heating the green ceramic to approximately two-thirds of the melting point of the material at ambient pressure and holding it for a specified time in a periodic or tunnel kiln

12. 10. Final Processing Following firing, some ceramic products are processed further to enhance their characteristics or to meet dimensional tolerances.

13. 2 . Glass and Its Properties Like less viscous liquids, glass has an amorphous, noncrystalline physical structure. Its properties depend on how different atoms and molecules interact within that structure. The primary components of glass are silica, lime, soda ash and alumina . However, most glass contains small amounts of many other components “ Glass is an amorphous, hard, brittle, transparent or translucent, super-cooled liquid , obtained by fusing a mixture of a number of metallic silicates, most commonly Na, K,Ca and Pb ”.

14. Cont … PROPERTIES OF GLASS The properties of glass are mainly governed by factors like composition of the constituents, state of surface and thermal treatment conditions. It has no definite crystalline structure . It has no sharp melting point . It is affected by alkalis . It is extremely brittle. It is not usually affected by air and water .

15. Manufacture of Glasses Raw Materials 1 . Sand Good glass sand is over 99.5% silica (quartz). Silica , or silicon dioxide, is the most common glass-forming oxide and the base for most glasses. The next most common glass-forming oxides are the oxides of boron and phosphorus.

16. Cont … 2. Lime Lime can be magnesium oxide, calcium oxide or a mixture of the two, depending on the type of glass being made. Most lime for glassmaking is obtained from mined dolomite or calcite. Lime is the most common glass modifier , used to make the glass easier to heat and melt. 3. Soda Ash Soda ash is sodium carbonate which is serving as the source of sodium oxide (Na 2 O), a common flux added to glass . Fluxes act on glass at relatively low temperatures and improve its forming characteristics. Soda ash can actually make glass soluble in water if enough stabilizers are not added.

17. 4. Feldspar Feldspar is a source of alumina (Al 2 O 3 ), which is one of the most important stabilizers added to glass. Glass stabilizers improve the resistance of the glass to chemicals , including water.

18. Cont … 5. Other Ingredients Elements commonly added to glass include arsenic, antimony, barium, fluorine, iron and lead . Compounds commonly added to glass include various oxides and sulfates . Some of these ingredients make glass easier to prepare and process. Some improve its consistency, strength, flexibility, optical characteristics, opacity, heat resistance, insulating ability, chemical resistance, resistance to thermal shock and many other properties. Metal oxides give glass color , e.g., selenium for red, chromium for green and cobalt for blue

19. Steps of manufacturing Glass Mixing Melting Glass melting furnaces melt the mixed raw materials to enable working the molten glass into a finished product 3. Conditioning Glass conditioning furnaces ( forehearths ) are usually separate compartments attached to the melting furnace. These furnaces have their own temperature control system. Conditioning makes the smaller amount of glass a uniform temperature throughout and helps avoid imperfections called blisters, seeds and stones.

20. 4. Forming The methods of forming, or shaping, depend on the type of glass being made Figure 1. Manufacturing of bottles

21. 5. Flat Glass Flat glass is drawn, rolled or floated. Two glass drawing processes are the Fourcault process and the Colburn process. In the Fourcault process, drawing rolls pull a vertical ribbon of glass through a kiln . The Colburn process is similar to the Fourcault process except that the glass is bent to the horizontal right after drawing Float glass is the newest type of flat glass. In this process, molten glass floats over a bath of liquid tin in a nitrogen atmosphere with some hydrogen

22. 6. Annealing Glass is annealed in furnaces called lehrs . Annealing removes the strain glass undergoes when solidifying . If this strain is not removed, the glass products will break very easily . Flat glass is annealed after it is drawn, rolled or floated and before it is cut. Glass containers are annealed after they are formed. Most annealing lehrs are fuel fired.

23. 7. Tempering Glass is greatly strengthened by being heated in a tempering furnace, and then cooled in a specified manner. The tempering process introduces compressive forces at the glass surface that counterbalance the tension in the central layers of the glass . This balancing of forces within the glass makes it very strong.

24. Types of Glasses The most common glasses based on their constituents are the soda-lime glasses , borosilicate , aluminosilicate glass phosphate glass and lead glass . 1. Soda-lime Silica Glasses By far the greatest number of industrially produced glasses Composed of 71%-75% sand, 12%-16% soda and 10%-15% lime Magnesia ( MgO ) replaces some CaO in float process . Used for making windows, food and beverage containers like bottles, jars, drinking glasses; and lamp envelops Composition Na 2 O . CaO . 6SiO 2

25. 2. Borosilicate Glasses Higher percentage of SiO 2 than other glass Composed of 70%-80% SiO 2 , 7%-13% B 2 O 3 , 4%-8% Na 2 O and K 2 O and 2%-7% Al 2 O 3 Posses a high resistance to chemical and heat as they have low coefficient of thermal expansion. Used to make laboratory glassware, sight glass within industrial equipment, transformer bushings, explosion-proof lighting, and exterior aircraft lenses.

26. 3. Aluminosilicate Glass Aluminosilicate glass contains 20% aluminium oxide (alumina-Al 2 O 3 ) often including calcium oxide, magnesium oxide and boric oxide in relatively small amounts but with only very small amounts of soda or potash. It is able to withstand high temperatures and thermal shock. Typically used in combustion tubes, gauge glasses for high-pressure steam boilers, and in halogen-tungsten lamps capable of operating at temperature as high as 750°C.

27. 4. Lead Glass It is made by using lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide Lead Oxide replaces much of the lime resulting in a glass type known as lead crystal Composed of 54%-65% SiO 2 , 18%-38% PbO , and 13%-15% Soda Ash or potash Exhibit very high refractive index, suited for decorative purposes Lead glass is used to make a wide variety of decorative glass objects Composition K 2 O . PbO . 6SiO 2

28. 6. Phosphate Glasses None silicate glass. These glasses contain 20~40 mol % P 2 O 5 and are modified by high index oxides to produce the desired optical properties. Mainly composed of the network former P 2 O 5 where the previously mentioned glasses use SiO 2 or B 2 O 3 as the primary glass network formers. These glasses have low thermal and chemical resistance except their high resistance to hydrofluoric acid. The compositions can be made more resistant to the chemicals and the environment by engineering the composition for the specific applications

29. 3.Bonding agent Bonding agents in the construction industry are inorganic substances which harden in the presence of water or air which bond pieces of stone H ydraulic bonding agents: harden in air and s water (e.g. cement) A ir bonding agents: harden only in air (e.g . calcined gypsum)

30. 3. Cement It is a fine grey powder which when reacted with water hardens to form a rigid chemical mineral structure which gives concrete its high strengths A material with adhesive and cohesive properties which make it capable of bonding minerals fragments into a compact whole. Classification of Cement Portland cement Natural cement Expansive cement High-alumina cement

31. Portland Cement 1 . Ordinary Portland cements (OPC) Limestone Clay San d Gypsum 2. Portland pozzolana cement (PPC) Limestone Clay Sand Gypsum and Pumice

32. Manufacture of Portland cement Portland cement is made by heating raw materials rich in oxides of silicon, calcium, aluminum and iron to temperatures of around 1200 - 1 450 o C The cement manufacturing process involves four distinct stages, and these are outlined below. Step 1 – Quarrying (Digging out the Raw materials) The raw material for cement manufacture is a rock mixture which is about 80% limestone (which is rich in CaCO 3 ) and 20% clay or shale (a source of silica, alumina and Fe 2 O 3 ). The lime and silica provide the main strength of the cement, while the iron reduces the reaction temperature and gives the cement its characteristic grey color

33. Step 2 - Raw material preparation There are two main cement manufacturing processes currently used: dry process In this process the starting materials are dry ground and mixed to “raw powder ”. The quarried clay and limestone are crushed separately to get 2-5 cm size pieces The clay and limestone are then fed together into a mill where the rock is ground until more than 85% of the material is less than 90µm in diameter.

34. Cont … wet process This process is only economic where the starting materials have high water contents (> 20%) The clay is washed with water in wash mills to remove adhering organic matter. The washed clay is stored separately. Raw materials are crushed, powdered and stored in silos. Powdered lime stone and wet clay are allowed to flow in channel and transfer to grinding mills where they are intimately mixed and paste is formed known as slurry .

35. Figure: Manufacturing of Cement by Wet Process

36. Comparison between wet and dry process

37. Cont... Step 3 - Clinkering The finely ground material is dried, heated (to enable the sintering reactions to take place) and then cooled down again The powder from the dry process does not contain much moisture, so can be dried in a pre-heater tower. As it falls through the tower it is heated from 70 to 800 o C. The moisture evaporates, up to 20% of the decarbonation (loss of CO 2 ) occurs and some intermediate phases such as CaO•Al 2 O 3 begin to appear. The mixture is then fed into the kiln.

38. Cont... The slurry from the wet process contains too much moisture to be successfully dried in a preheater tower. Instead , the slurry is fed directly into the kiln where it is formed into dry balls by the heat and rotation of the kiln. Because of this extra role of the kiln, wet process kilns are generally longer than dry process kilns

39. Main Reaction in Kiln The basic reactions that take place in production of Portland cement can be summarized in four zones : Zone 1: 800 - 1100 o C Decarbonation - CaCO 3 CaO + CO 2 : Formation of 3CaO•Al 2 O 3 above 900 o C. Melting of fluxing compounds Al 2 O 3 and Fe 2 O 3 . Zone 2: 1100 - 1300 o C Exothermic reactions and the formation of secondary silicate phases as follows 2CaO + SiO 2 2CaO•SiO 2  

40. Cont... Zone 3:1300 - 1450 - 1300 C Sintering and reaction within the melt to form ternary silicates and tetracalcium alumino - ferrates: 2CaO•SiO 2 + CaO 3CaO•SiO 2 3CaO•Al 2 O 3 + CaO + Fe 2 O 3 4CaO•Al 2 O 3 •Fe 2 O 3 Zone 4 : 1300 - 1000 o C Cooling and crystallization of the various mineral phases formed in the kiln aluminates and silicates of calcium fuse to gather to form small and hard stones are known as clinkers. The size of the clinker is varies from 5-10mm.  

41. Cont … Table 1. Main constituents of clinker

42. Step 4 - Clinker milling To produce the final product the clinker is mixed with 2-3% powdered gypsum (CaSO 4 •2H 2 O), which used as retarding agent , So that, resulting cement does not settle quickly, when comes in contact with water. The cement flows from the inlet to the outlet of the mill (a rotating chamber), being first ground with 60 mm then 30 mm diameter steel balls. The first grinding breaks up the material and the second grinds it to a fine powder.

43. Setting and hardening of Portland cement The time from the addition of the water to the initial and final set are known as the setting times The setting and hardening of concrete are the result of chemical and physical processes that take place between Portland cement and water, i.e., hydration . Upon mixing cement powder with water, part of the tricalcium aluminate C 3 A and gypsum dissolve and react together forming ettringite : 3CaO.A1 2 O 3 + 3CaSO 4 + 32H 2 O 3CaO.A1 2 O 3 .3CaSO 4 .31H 2 O . This is the basis of the delay in solidification upon adding gypsum to cement.

44. Cont... Portland cement (containing gypsum) solidifies after 1 to 3 hours. The hydration of tricalcium silicate C 3 S and dicalcium silicate C 2 S are responsible for the further solidification of Portland cement. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate . 3C 3 S + 6H 2 O → 3Ca(OH) 2 + 3CaO.2SiO 2 .3H 2 O 2C 2 S + 4H 2 O →Ca(OH) 2 + 3CaO.2SiO 2 .3H 2 O

45. Pumice Mainly of natural origin(volcanic materials ) , which in themselves possess little or no cementitious value It contain large amount of SiO 2 and Al 2 O 3 in a suitable reactive form, so that it can react with Ca (OH) 2 at ordinary temperature in finely divided form and in the presence of moisture, to form compounds possessing cementitious properties