Refractories are ceramic materials that are characterized by high strength and fracture toughness. They are also inert to most chemical attacks, are abrasion resistant and can withstand high temperatures. Generally, refractory materials can withstand temperatures above 1100 °C without softening, and this property is sometimes used to differentiate refractory materials from non-refractory materials.
The strength of refractory materials is due in part to the relatively reduced number and distribution of microcracks in thefinished product. They are used in applica- tions where heat resistance, hardness, fracture toughness and strength are essential.
Specific applications of refractory materials are in incinerators, furnace and kilns where they are used as linings, in engine bearings, engine and turbine parts, cru- cibles and cutting tools. They are also used in electronic and structural applications.
Table 5.15 Compressive strengths of different cements at different durations
Curing period OPC RHC LHC PBFSC PPC MC SRC Compressive strength kg/cm2
Day 1 160 – – – – – –
Day 3 160 275 100 160 – – 150
Day 7 220 – 160 220 220 25 220
Day 28 – – 350 – 310 50 300
OPC = Ordinary Portland cement; RHC = Rapid hardening cement; LHC = Low heat cement; PBFSC = Portland blast furnace slag cement; SRC = Sulfate-resistant cement;
PPC = Portland pozzolana cement; MC = Masonry cement
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5.9.1 Types of Refractory Materials
Refractories may be classified based on their behavior in a chemical environment, their composition or simply as clay and non-clay. Based on their behavior in a chemical environment, they are classified as acidic, basic or neutral. Acidic refractories cannot be used in a basic environment as they would be corroded.
Examples of such refractories include zircon,fireclay and silica. Similarly, basic refractories cannot be used in an acidic environment. Dolomite, magnesite are examples of basic refractories. About 95% of refractories manufactured are non-basic. Among the neutral refractories are alumina, chromite, silicon carbide, carbon and mullite. These latter ones can be used in both acidic and basic environments.
Based on their composition, refractories may be grouped as oxides, nitrides, borides or carbides. The oxides are compounds of oxygen. Among the oxides is aluminum oxide or alumina (Al2O3), beryllium oxide or beryllia (BeO), zirconium oxide or zirconia (ZrO2), and mullite, which is a mixture of alumina and silica (SiO2/Al2O3)—71.8% alumina and 28.2% silica. The nitrides invariably have nitrogen as a prominent element in their compound. Examples are aluminum nitride (AlN), silica nitride (Si3N4)) and two forms of boron nitride (BN)—the cubic and hexagonal forms. Prominent among the carbides used in the preparation of refractories are silicon carbides. They are two distinct forms of silicon carbides: the beta silicon carbide (a cubic form) and alpha silicon carbide, which is a generalized term for the non-cubic form of silicon carbide.
Clay refractories are produced from fireclay (hydrous silicates of alumina) and alumina (57–87.5%). Kaolin, bentonite, ball clay and common clay are some of the other clay minerals used in the production of refractories. The non-clay refractories are made from a composition of alumina (<87.5%), mullite, chromite, magnesite, silica, silicon carbide, zircon and other non-clays.
5.9.2 Manufacture of Refractory Materials
Refractories are produced in two basic forms, namely formed objects and unformed granulated or plastic compositions. The formed objects are called bricks and shapes.
These are used to construct the walls, arches and floor tiles of high-temperature process equipment. The unformed compositions include mortars, gunning mixes, castables (refractory concretes), ramming mixes and plastics. These latter products are cured in place to form a monolithic, internal structure after application. The single-most important property to produce during refractory manufacture is high bulk density. The density affects such other properties as strength, volume stability, slag and spalling resistance, and heat capacity. For insulating refractories, a porous structure is required, which translates into a low-density material. Refractory products are manufactured by one of three approaches. These are the mold and burn process, electric fusion process and ceramicfiber formation process.
5.9 Refractory Materials 165
5.9.2.1 Mold and Burn Process
The four major unit operations involved in this process are raw material processing, product forming,firing andfinal processing. A generalizedflowchart for the mold and burn process is shown in Fig.5.3.
Thefirst step in this process is the preparation of the raw material. The object of this step is to reduce the size of the lumps and the moisture content in the raw materials to the level that will facilitate the subsequent operations of forming and firing. This step involves crushing and grinding of the crude clay or ore followed by
Fig. 5.3 A generalizedflowchart for ceramic manufacture by the mold and burn method
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size classification and calcining or drying. The size of the particles in a batch is one of the important factors during refractory formation. It is known that a mixture in which the proportion of coarse andfine particles is about 55:45, with only a few intermediate particles gives the densest mixtures. Achieving this requires careful screening, separation and recycling. The method works well on highly crystalline materials but is difficult to obtain in mixes of high plasticity. The processed raw material may be dry-mixed with other minerals and chemical compounds, packaged and shipped as a product. The mixing is done with the sole aim to distribute the plastic materials so as to coat thoroughly the non-plastic constituents. This serves the purpose of lubricating the materials during molding and allows the bonding of the mass with a minimum number of voids.
Forming consists of mixing the raw materials and molding them into desired shapes. The operation occurs under wet or moist conditions. The molding is a dry-press process with mechanically separated presses. This method is particularly well suited for batches that consist mostly of non-plastic constituents. It has been found necessary to de-air the bricks during pressing. This avoids laminations and cracking when the pressure is released. When present, the air is absorbed by the clay or condensed during pressing. De-airing is achieved by applying vacuum through vents in the molds box. The mold is dried before firing. The drying removes the moisture added before molding to develop plasticity. The elimination of water leaves voids in the mold and causes extensive shrinkage and internal strains. To avoid this in some cases, drying is completely skipped and the mold fired immediately after forming.
Firinginvolves heating the formed shapes to very high temperatures in a batch or continuous kiln to form the ceramic bond that gives the product its refractory properties. Two important things happen duringfiring. First, permanent bonds are developed in the mixture by partial vitrification. Secondly, stable mineral forms are developed. The changes that take place are the removal of water of hydration, followed by the calcination of carbonates and the oxidation of ferrous iron. The final volume of the product may be as much as 30% less than the non-fired material.
The reduction in volume causes severe strains in thefinal product. The shrinkage may be avoided by pre-stabilization of the materials used. The final processing stage involvesmilling, grindingandsandblastingthe product from the kiln to give it the correct shape, size and aesthetic appeal. Sometimes, during this stage, the product is impregnated with tar and pitch andfinally packaged.
5.9.2.2 Electric Fusion Process
In this process, the raw material, often a mixture of diaspore clays of high alumina (to furnish a 3Al2O3/2SiO2), is introduced into the top of an electric arc furnace.
Molten alumina silicate at 1800 °C is subsequently tapped from the furnace at intervals and run into molds built from sand slabs. The molds containing the blocks are annealed for about 6–10 days before the blocks are usable. The products from this process have a vitreous, non-porous body that has a linear coefficient of expansion of one and half times that of goodfirebrick. The voids content is only
5.9 Refractory Materials 167
about 0.5% in contrast to the usual 17–29% forfire-clay blocks. Refractory blocks cannot be cut or molded. They can, however, be ground on Alundum wheels. The products have a long life and minimum wear but have a high initial cost. They are used in glass furnaces, in linings of hot zones of rotary kilns, in modern boiler furnaces exposed to severe duty and in metallurgical equipment such as in forging furnaces.
5.9.2.3 Ceramic Fiber Formation
This process requires that the feed material, for instance, the calcined kaolin, isfirst melted in an electric arc furnace. Subsequently, the molten clay is made intofiber in a blow chamber with a centrifuge device. An alternative route to fiberizing the molten clay is by dropping the melt into an air jet and immediately blowing it into fine strands. The last stage of this process is curing thefiber in an oven. The curing adds structural rigidity to thefibers. Oils are added to lubricate both thefibers and the machinery used to handle and form thefibers during the curing process. This process is very similar to that used for the production of mineral wool. Table5.16 lists some varieties of refractory materials, their application and composition.
5.9.3 The Cement Industry in Ghana
As of the end of 2018, 11 cement plants operated in Ghana. Out of these, 10 were grinding plants. The Savannah Cement Company plant at Buipe in northern Ghana is an integrated plant that has the capacity to produce clinker. However, for a long while the plant only operated as a grinding plant. As of 2018, no plant in Ghana produced clinker. What this means is that the clinker used by cement plants in Ghana are all imported into the country. The 11 plants and their location in Ghana are listed in Table5.17. These plants have an installed capacity of 12 million tons as of the end of 2018. The annual demand for cement in Ghana stood at about 8 million tons in 2018. Of this amount, 500,000 tons are bagged cement imported into the country annually.
5.9.3.1 Sources of Raw Materials in Ghana for the Production of Cement
There are sufficient deposits of lime and clay, key ingredients for the production of cement clinker, to support the production of cement in the country for several years.
However, as of the end of 2018, with the exception of the Buipe plant, no other clinker plant existed in the country.
Limestone deposits in the country are estimated to be over 500 million tons. The four major limestone deposits in Ghana are found in Buipe in the Savannah region, Nauli in the western region, Oterkpolu in the eastern region and Bongo-Da in the northeastern region. Smaller deposits can also be found at Wenchi, Abeasi and Kintampo (all in Bono East Region), Daboya in the northern region, and Salega-Yeji in the Savannah region, Sadan-Abetifiin Ashanti region, Anyaboni in the eastern region, and Du area in the upper eastern region.
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The limestone deposits in Oterkpolu and Nauli are already being mined by Ghacem to supplement its limestone import into the country. Ghacem uses lime- stone as a clinkerfiller for the production of cement in Ghana. About 24% of the clinker weight in cement is replaced with limestone by the company.
The distribution of notable limestone deposits in the country is as follows: Nauli in the western region has over 400 million tons which could be used for the production of cement clinker to substitute all imports of clinker. Buipe in the northern region has limestone and mudstone deposits estimated at over 6.03 million tons. It also has limestone and dolomite deposits estimated at over 44 million tons.
Limestone deposits at Bango-Da are estimated to be over 15 million tons in addition to 30 million tons of dolomite deposits. In Daboya over 162,000 tons of limestone and over 500,000 tons of dolomite can be found.
Like limestone, the other major raw material in clinker production, clay is also abundant in Ghana. It is estimated that clay deposits in Ghana amount to over 600 Table 5.16 Varieties of refractory materials
Refractory Application Composition
Fire-clay brick
Most widely used refractory material used in linings furnace, foundries kilns, boilers, gas-generating sets
SiO2/Al2O3mixtures of varied compositions
Silica brick Used for making arches in large furnace, are also used in coke ovens and in gas retorts
95–96% SiO2and 2% line
High-alumina Used in lining of glass furnace, oil fired furnaces, high pressure oil stills, roof of lead softening furnaces and in regenerator checkers of blast furnaces
Made from clay rich in bauxite and diaspore
Magnesia Used in open-hearth and
electric-furnace walls, in the burning zones of cement kilns, and in roofs of non-ferrous reverberatory furnaces
Made from domestic magnesites or magnesia extracted from brines
Insulating brick
Used as backing for refractory bricks or in place of regular refractory bricks depending on the type. Fire-clay ground, mixed and molded
There are two types. They are made from natural porous diatomaceous earth or from waste cork Silicon
carbide
Used for making rocket nozzles, furnace and radiant heater tubes, combustion chambers for
ceramic-based gas turbine engines, and in iron-making blast furnace
Made from a mixture of crude silicon carbide clay orfinely ground silicon carbide
Electrocast refractory
Used in glass furnace, in linings of hot zones of rotary kilns, in boiler furnaces exposed to severe duty, and in metallurgical equipment such as forging furnace
Made from an electrically fused mixture of 2SiO2/3Al2O3ratio
Pure oxide refractory
They have been developed for light refractory products
They are made from alumina, magnesia, beryllia, thoria and zirconia
5.9 Refractory Materials 169
million tons and can be found in all the regions in Ghana. However, there is no known deposit of gypsum, the other key component of clinker making in Ghana.
Further Readings
Alsop, P. A. (2019).The cement plant operations handbook(7th ed.). Tradeship Publications Ltd.
Bye, G. C. (1999).Portland cement composition, production and properties. Thomas Telford.
Bhatty, J. I., Miller, F. M., & Boahn, R. P. (2011). Innovations in Portland cement manufacturing.
Skokie, IL: Portland Cement Association.
Chatterjee, A. K. (2018).Cement production technology: Principles and practice. CRC Press.
Deolalkar, S. P. (2009).Handbook for designing cement plants. CRC Press.
Deolalkar, S. P., Shah, A., & Davergave, N. (2013). Cement designing green plants. Butterworth-Heinemann.
Johansen, V., Taylor, P. C., & Tennis, P. D. (2006). Effects of cement characteristics on concrete properties. Skokie, IL: Portland Cement Association.
Kohlbaas, B., & Labahn, O. (eds.). (1982). Cement engineer’s handbook. Intl Public Service.
Locher, F. W. (2005).Cement: Principles of production and use. Verlag Bau + Technik.
Taylor, H. F. W. (1997).Cement chemistry. Thomas Telford.
Table 5.17 Cement manufacturers in Ghana as of 2019 Name of company Location Type of
plant
Type of cement
Plant capacity 106 tons/yr
Wan Heng Ghana Ltd Accra – – 0.5
Xin An Safe Cement Konongo – – 0.5
Ciments de L’Afrique Ghana Ltd
Tema Grinding – 1.0
CBI Ghana Ltd Tema Grinding 32.5R 0.8
Dangote Cement Ghana Ltd
Tema Bagging Portland 1.5
Dangote Cement Ghana Ltd
Takoradi Grinding Portland 1.5
Ghacem Ltd Tema Grinding 32.5R/42.4R 2.2
Ghacem Ltd Takoradi Grinding 32.5R/42.5R 2.2
Diamond Cement Ghana Ltd
Aflao Grinding 42.5R/32.5R 1.8 Savannah Cement
Company Ltd
Buipe Integrated 42.5R/32.5R 0.44 Western Diamond Cement Takoradi Grinding 42.5R/32.5R 1.0
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6
Pulp and Paper Technology
Abstract
Paper has many uses such as in writing, printing, packaging, paper money, cheques voucher, cleaning, as construction materials, etc. Based on its applications, paper may be characterized as bank paper, book paper, bond paper, construction paper, cotton paper, electronic paper, photo paper, wallpaper, etc. The popular raw materials used to make paper are esparto-grass, straw, wood, flax, hemp, jute and rags-cotton and linen. The paper manufacturing process consists of two main stages: pulp making and conversion of pulp into paper. The process of producing pulp may be done by chemical or mechanical means, or by a combination of both depending on the nature of raw material used and the end application of the produced paper. Three operations are required to manufacture paper that are beating, conversion of pulp to paper andfinishing.
This chapter comprehensively covers the types of papers, their uses, raw materials and their manufacturing processes. Pulp and paper mills have adverse effects on the environment such as depletion of forest covers, and chemical pollution caused by liquid effluent discharges, solid wastes and air emissions from particulates, H2S, NOx, SOx. These have been covered in detail including means to reduce their emissions and treatment methods.