What refractory materials are used in metal works

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1 Montanuniversität Leoben - University of Leoben Department of Metallurgy - Department of Metallurgy Non-ferrous Metallurgy - Nonferrous Metallurgy DIPLOMA THESIS Topic: Determination of selection criteria for refractory building materials in aluminum melting furnaces Creator: Thomas Hauer Supervisor: Dipl.-Ing. Michael Potesser Assessment: Ao.Univ.Prof.Dipl.-Ing.Dr.mont. Helmut Antrekowitsch Leoben, December 06

2 Affidavit I declare in lieu of an oath that I have written this work independently and without outside help, that I have not used any sources or aids other than those specified, and that I have marked passages that have been taken literally and in terms of content as such. Thomas Hauer Leoben, December 2006

3 Acknowledgments This work was carried out in the 2006 academic year at AMAG rolling GmbH in Ranshofen and at the non-ferrous metallurgy department of the Montanuniversität Leoben. Ao.Univ.-Prof. Dipl.-Ing.Dr.mont. Helmut Antrekowitsch, Head of Non-Ferrous Metallurgy and Dipl-Ing. Thank you Helmut Suppan from AMAG rolling GmbH for transferring this thesis. Many thanks also go to Dipl.-Ing. Michael Potesser and Mr. Erich Troger for their valuable assistance in carrying out the tests, as well as the employees of AMAG rolling GmbH for their support. The biggest thanks go to my family, who made my university education possible and always supported me. Leoben, November 2006 Thomas Hauer

4 Non-ferrous metallurgy Montanuniversität A-8700 Leoben cand. Ing. Thomas Hauer December 2006 Determination of selection criteria for refractory building materials in aluminum melting furnaces AMAG rolling GmbH in Ranshofen is a global manufacturer of high-quality rolled aluminum products. 80% of the production is exported to the main markets of Europe and overseas. Today, AMAG rolling is the market leader in gloss qualities for the lighting industry and in tread plates for use in the transport and mechanical engineering industries. As part of the diploma thesis, selection criteria for refractory building materials in aluminum melting furnaces are developed. The theoretical basic knowledge and the state of the art from a literature search were the basis for the tests on a laboratory scale and led to recommendations for a production and test stand. General principles of aluminum furnaces and their refractory lining as well as the various loads are part of the theoretical section. The experimental part comprises an elaborate test procedure in order to test refractory materials under different types of stress as they occur in the secondary aluminum industry. The aim of the work is a recommendation for the optimization of the test stand and the test set-up so that a clear determination of requirement profiles is made possible. Leoben, December 2006

5 Abstract In the present diploma thesis, selection criteria for refractory building materials in aluminum melting furnaces are developed. In the first part of the thesis, there is a brief assessment of the materials used so far in the aluminum melting furnace of AMAG rolling GmbH and a description of the different types of loads for refractory masses and stones. The practical area includes an elaborate test procedure to test different refractory materials under different types of stress. Finally, a recommendation is given for optimizing the test stand and test set-up so that a clear determination of requirement profiles is made possible. Abstract In the diploma thesis criteria for refractory materials in a melting furnace for the secondary aluminum industry were developed. The first part gives a short summary of the used materials in the closed well furnace of Austria metal AG. Furthermore a description of the different types of load for refractory materials in the mentioned furnace was done. The practical part of the thesis includes the design, manufacture and start-up of a test facility for determination of behavior of the lining system. Due of these investigations concerning the different load and refractory materials recommendations regarding the optimization of the testing equipment, experimental set up and the refractory materials can be given.

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7 4.5.2 Resistance to thermal shock Chemical resistance Strength properties General problems with refractory concretes Flame temperatures of the burners Anti-wetting agents Selection of the suitable refractory lining Construction features Lining of selected melting systems Hearth melting furnace Rotary drum furnace Crucible induction furnaces EXPERIMENTAL SETUP Matrix Preliminary tests Matrix main tests Test materials Alugard HS Didurit 120 AL Didurit 130 5 formulas / Al Carath 1650 ULC / AZS Alu-Cast 90 HS Refracast LC C Sample preparation Sieving Mixing and shaping Firing Change in the body weight distribution Selection of the shapes EXERCISE PERFORMANCE Heat and infiltration (fingertip test) Mechanical abrasion Assessment fingertip test Classification (overall evaluation) Observation criteria Adhesion Infiltration Cracking Crumbled PREVERSE ... 60 page II

8 7.1 Aluminum baths covering salt Results of preliminary tests MAIN TESTS Evaluation of the main tests Result of main tests SUMMARY LITERATURE LIST OF ABBREVIATIONS FROM A-Z LIST OF TABLES AND FIGURES APPENDIX ... I Page III

9 Introduction 1 Introduction AMAG rolling GmbH is a leading supplier of high quality rolled products, special profiles and cast alloys in the form of ingots and round bars. The products are made from 100% secondary aluminum (e.g. scrap, shavings, etc.). AMAG casting uses hearth furnaces of various sizes and designs for remelting the secondary raw materials. AMAG has been a technology leader in the aluminum industry for more than 60 years. The flexible, medium-sized group of companies based in Ranshofen Austria. Today it is one of the largest aluminum recyclers in Europe at one location, approx. Tons of high-quality aluminum alloys leave its production facilities every year. From the scrap available on the market, high-quality cast aluminum alloys are produced through appropriate sorting and processing and using the currently best melting and casting technology. 80% of the production is exported to the main markets of Europe and overseas. AMAG is represented there with its own sales companies or representatives. In the past, problems arose with the refractory lining in the ramp area of ​​a furnace, and the service life of the lining did not reach the specified times. Due to various loads (temperature, chemical attack, etc.), the brick lining on the bridge is so severely attacked that no shelf life of more than 12 months was achieved. The aim of this work is to test different refractory products for their usability and properties. The theoretical basic knowledge and the state of the art from a literature research were the basis for the tests on a laboratory scale and led to recommendations for a production and test stand. The practical tests include the fingertip test, i.e. the refractory samples were immersed in liquid aluminum for 96 hours and then the mechanical load took place, with a metal plate scraping the stone surface 150 times at 700, 900 and 1000 C. The abrasion test simulated the scraping unit used by Austria Metall AG. The evaluation of the results consisted of five different sub-areas, the adhesion and the infiltration of the aluminum melt, the crack formation, the friability on the cut surface and the material removal by the scraping unit. page 1

10 Problem 2 Problem In the past, durability problems arose in the ramp area of ​​furnace 3 (see Figure 1) in the foundry. The frequent repairs and relining resulted in long downtimes and high costs. The desired shelf life of the refractory material should be 12 months, but was not achieved by any of the materials used (see Figure 2). So far this has only been exceeded by the Formula 5A from Stellar Materials Incorporated. The short operating times are caused by chemical attack by the dross, the burners aimed directly at the ramp, and mechanical loads on the dross unit. Figure 1: Sketch of the furnace on page 2

11 Problem Figure 2: Durability of the refractory products used up to now The materials used up to now (see Table 1) are non-basic, unshaped refractory products that have a high Al 2 O 3 content; due to their high chemical resistance and high strength, these are often used in Aluminum ovens used. In the past, the best results were achieved with Formula 5A from Stellar. The larger problem in terms of wear and tear is the scraper unit. A steel structure weighing 5075 kg is pulled over the refractory lining to pull the aluminum scraps away from the molten metal. The burner, which is aimed directly at the ramp, creates high temperatures that have a negative effect on the mechanical abrasion resistance and the corrosion protection of the refractory products. High temperatures lead to the decomposition of the anti-wetting agents, gases are formed which lead to an increase in porosity and an increase in the formation of corundum. Furthermore, the resulting BaO and CaO act as strong fluxing agents, so that there are structural changes and flaking in the refractory lining. For these reasons, the low durability of the refractory masses result. Page 3

12 Problem table 1: Ramp masses in the furnace 3 Furnace repair material Al 2 O 3 [%] SiO 2 [%] Fe 2 O 3 [%] Supplier Ramp renewal Door lintel renewal AK 85P1 81.0 10.0 1.4 Gouda furnace bridge renew Rapidobloc LC, 0-95.0 1.5-3.0 0.2-0.3 Lafarge Hot repair furnace ramp (right) Hot repair furnace ramp (center) Ramp renewal Aloset 9000 ??? Lafarge Aloset 9000 ??? Lafarge Vibron 150 GR (prefabricated parts) 73.0 11.0 1.2 Gouda ramp repair Vibron 150 GR 73.0 11.0 1.2 Gouda ramp repair Vibron 150 GR 73.0 11.0 1.2 Gouda ramp renovation Alugard RBG 37, 0 32.0 0.6 Gasser hot repair Aloset 9000 ??? Lafarge ramp repair Formula 5A 85.6 2.1 1.0 Stellar ramp repair Formula 5A 85.6 2.1 1.0 Stellar ramp repair Formula 5A 85.6 2.1 1.0 Stellar ramp repair Formula 5A 86.3 2.6 0 , 9 Stellar ramp replacement Formula 4 86.3 2.6 0.9 Stellar ramp repair Formula 5A 85.6 2.1 1.0 Stellar ramp replacement Ceramite BRC-B 50.0-70.0 5.0-15.0 - Elkem Ramp renewal Alugard HS85 85.8 6.4 1.2 Gasser page 4

13 Aluminum 3 Aluminum The general principles of aluminum, such as production, occurrence and properties, can be found in the relevant specialist literature. 3.1 Terminology in the aluminum industry Ultra-pure aluminum is metallurgically obtained directly from aluminum or return aluminum and then cast into molds by the smelters [1]. Primary aluminum is aluminum (primary aluminum) produced from alumina by fused-salt electrolysis. Recycled aluminum is made from secondary raw materials, i.e. old and / or new scrap, chips and dross (secondary aluminum). Refiners are companies that manufacture cast alloys and deoxidized aluminum from old and new scrap and deliver them in the form of ingots and liquid aluminum. Remelters are those companies that produce wrought alloys from mostly pure wrought alloy scrap and deliver them in the form of rolling bars and extrusion billets. Old scrap is aluminum products that, after being used as a secondary raw material, end up in the recycling systems. Production scrap (new scrap) is sorted by type in the processing of semi-finished products in the form of processing residues, e.g. Chips, stamping residues, sections, sprues, feeders and rejects. Circular scrap is the production scrap that arises from the processing of aluminum in a company and is sorted, melted down again and processed in this [2]. page 5

14 Aluminum 3.2 Secondary aluminum Aluminum is an excellent recycling material, for several reasons: The loss of value is extremely low thanks to the energy stored in the aluminum. This is shown by the high revenues that can be achieved with aluminum scrap. Under certain conditions, secondary processing requires up to 95% less energy than primary generation. In many areas of application, aluminum already remains in a closed material cycle; real recycling takes place. That means: A cast part becomes a cast part again, and an aluminum beverage can becomes liquid metal again for the production of new beverage cans. If the same product cannot be produced, the new application is just as valuable as the previous one, an advantage that not all materials have by a long way. Metals are in this context other materials, such as clearly superior to polymers. Among them, aluminum occupies a leading position, as mature technologies are already available. The high material value of the aluminum scrap helps to cover the logistics and processing costs, so that collection and recycling systems work without subsidies. There is no quantitative limit for the recycling of aluminum scrap. 100% of them can be returned to the material cycle again and again. Effective metal filtration systems ensure that the secondary aluminum is qualitatively equivalent to the primary aluminum. In the aluminum industry, a distinction is made between cast and wrought alloys: Cast alloys can only be cast and are then no longer deformed. The desired alloying elements in the scrap are mainly magnesium, silicon and copper. Wrought alloys, on the other hand, are intended for forming, for example for extrusion, rolling or forging, and must therefore be easily formable. Important elements are magnesium, silicon, copper, manganese, zinc and iron [2]. The global production of aluminum was 31.2 million tons in 2005, the share of secondary aluminum comprises 25% of the total annual production, and the trend is increasing in most industrialized countries (see Figure 3). Primary aluminum production in Germany peaked in 1984. It remained largely stable from 1985 to 1988, only to then decline rapidly. Only after 1994 did a slight recovery appear, with energy policy leading to electrolyses having to close in 2005 and 2006. The trend in the consumption of primary aluminum is increasing. In secondary aluminum production, there has been a slowdown in production growth since 1988. There were declines in production between 1991 and 1993 as well as page 6

15 aluminum was recorded in 1994 and 1995. The demand also decreased between 1991 and In addition, there were economic slumps in both branches of industry in connection with the oil price shocks of the 1970s. Economic influences on production declines were relevant at the beginning of the 1990s. Of particular importance for the aluminum industry is the development of the transport sectors, in particular automotive, construction, mechanical engineering, metals, electrical engineering and packaging. Above all, the rising energy prices are cited as the reason that primary generation is being relocated abroad; this also applies to Europe in general. Outsourcing countries include Australia, Brazil and Canada. Other countries with a strong increase in aluminum production were Venezuela, Bahrain, India and Norway. Since technical developments meanwhile allow secondary alloys to be produced in the quality of the primary alloys, it is assumed that secondary aluminum will penetrate more and more into areas of application that were previously reserved for primary aluminum. However, it is not possible to completely replace primary aluminum with secondary aluminum. Since secondary aluminum is significantly more favorable in terms of quantity than primary aluminum in its environmental balance in terms of the indicators of energy and water consumption, waste generation and emissions, it also helps to conserve resources, reduces interference with nature through ore mining and relieves the disposal side (landfill), the restructuring towards secondary industry can are rated at least as relatively environmentally friendly. Production demand 800 tonnes 10³ Figure 3: Production and demand of secondary aluminum in Germany page 7

16 Aluminum Aluminum scrap are secondary raw materials that are used in the recycling of aluminum-containing products such as e.g. from automobiles, aircraft, from the construction industry or from the production and processing of aluminum. According to their nature and their chemical composition, the aluminum-containing secondary raw materials can be divided into the following categories [2]. Scrap from: Unalloyed aluminum Wire and cable A single wrought alloy Two or more wrought alloys of the same alloy group Two or more wrought alloys Castings Non-ferrous metals from shredders for aluminum separation processes Aluminum separation processes from shredded non-ferrous materials Used aluminum beverage cans Aluminum / copper coolers Chips from a single alloy or mixed chips more Alloys Scrap from used aluminum packaging Aluminum scrap from used aluminum packaging that has been freed of coating Scrap from dross A perfect processing of scrap for wrought alloys can only take place in specially equipped smelters. As a rule, modern shredder technology with subsequent wet separation technology is used today.After coarse sorting or shredding and any float-sink separation, the aluminum scrap is melted down. Adhering metals such as Iron, nickel or copper, whose melting points are higher than those of aluminum, can be removed by melting the aluminum in a furnace on the melting bridge. Melting in a rotary drum furnace (see 3.4.1) under a salt cover is also widespread at the moment. The production of cast alloys from scrap is generally limited to the removal of disruptive high magnesium contents by introducing chlorine gas into the molten metal. The flushing gas treatment with chlorine and nitrogen is also used to clean the melt and remove interfering gases such as hydrogen. page 8

17 Aluminum However, it is general practice to reduce undesirably high foreign metal contents by diluting the melt with pure aluminum [7]. The use of master alloys facilitates or enables the introduction of alloy elements such as e.g. Oxides or borides, especially those with a high melting point. The use of these alloys enables exact dosing and distribution of the additives. Aluminum master alloys are standardized according to DIN EN 575. Delivery forms are ingots, wires, notched plates and fragments. 3.3 Secondary aluminum alloys Secondary aluminum alloys (see Table 27) have the same quality as similar alloys made from primary aluminum. The mixtures belong to the Al-Si group and Al-Mg group, the concentration range for the main alloy element is up to 20%. These are therefore in the hypoeutectic, eutectic and hypereutectic range of the Al-Si binary system. 3.4 Melting furnaces in the secondary aluminum industry Criteria for choosing the most suitable melting furnace are the contamination of the scrap by oxides, adhering iron and organic foreign matter, the magnesium content, the ratio of surface to the mass of the scrap particles and the thinnest wall thickness of the scrap particles. When selecting the melting furnace, technical and economic aspects must also be taken into account. The melting unit must have a high level of thermal efficiency, as its economic efficiency depends on it. When designing the furnace, attention must also be paid to the loading, because the service openings must be arranged in such a way that this can be done in the shortest possible time. Based on a qualitative assignment, Figure 4 derives quantifiable criteria for the assignment of scrap or waste / residual materials to the individual melting units. Page 9

18 Aluminum Figure 4: Secondary aluminum production for processing all scrap categories In principle, it is possible to melt down scrap with or without salt. In normal smelting operations without a salt cover, metal is lost through oxidation of the surface of the hot aluminum melt by means of oxygen. The salt cover of an aluminum melt fulfills various functions. These are the prevention of the oxidation of the aluminum by the atmospheric oxygen and the guarantee of a good heat transfer between the burner and the melt. In addition, the molten salt serves to absorb impurities by absorbing the impurities brought in with the scrap. The salt-powered drum furnaces, which are rigid or tiltable, salt-free hearth furnaces or induction furnaces are used as melting units. The higher the oxide content in a scrap, the more likely it is to be melted down in a rotary drum furnace. This also leads to an increased accumulation of salt slag. Since the dumping of salt slag, which was practiced in the past in Austria and Germany, is no longer permitted for reasons of environmental protection, the salt is recovered from the salt slag. Two of the most important processes are REKAL and SEGL processing. Page 10

19 Aluminum rotary drum furnace In drum furnaces (Figure 5), aluminum waste of all types and aluminum ingots can be melted down. Due to the special way the furnace works, it is particularly suitable for the use of chips and small pieces of material. In rotary drum furnaces, as the name suggests, the salt bath, mainly sodium and potassium chloride, is mixed with the molten metal by rotation. When melting down, the resulting Al 2 O 3 layer is absorbed by the salt. The rotation speed can be up to 8 revolutions per minute. Between the two substances, impurities from the molten metal pass into the salt bath, which is then drained from the furnace as salt slag. Extracting one ton of aluminum can produce up to 500 kg of salt slag. The heating is done by gaseous or liquid fuels. The burner is located on the front side; if the guide is reversed, the flame and the exhaust pipe are on the same side. With this design, the furnace lengths can be kept shorter. The furnace can be charged via the burner or exhaust side, the tap openings for metal and slag can be attached to the front wall or to the furnace shell. Usually the slag is removed from the front and the metal from the furnace shell [10]. The heat transfer to the melting material takes place through radiation, less through convection. For scrap with high levels of contamination Rotates slowly around the horizontal axis Up to 60 t capacity Produces top quality aluminum Under a salt cover (NaCl / KCl mixture) Works against oxidation Cleans melt per ton of Al approx. 500 kg salt slag [3] page 11

20 Aluminum Figure 5: Rotary drum furnace [3] Induction furnace The induction crucible furnace is a transformer whose primary coil is connected to the mains frequency and the material to be melted forms a short-circuited secondary coil. Eddy currents are induced and the melted material is heated as a result. Electrodynamic forces cause a bath movement, which immediately flushes the solid components of the scrap with liquid metal, which reduces the burn-off loss. In addition, the movement of the bath homogenizes the melt and enables the addition of alloying elements. The mains frequency induction crucible furnace is particularly suitable for melting down aluminum scrap. However, the furnace must be started up with lumpy metal [10]. At a frequency of 50 Hz, the scrap particles can have a minimum size of 100 mm in order to ensure appropriate heating. An induction channel furnace is shown in Figure 6 [4]. External alternating current Heated up by eddy current Good mixing Up to 12 tons capacity Low oxidation High melting capacity Page 12

21 Aluminum Figure 6: Induction furnace [5] Hearth furnaces In hearth furnaces (Figure 7) scrap is predominantly used; the furnace is fired by oil or gas burners. Tank furnaces are built for a capacity of 0.5-80 t. Because of their size, many of these ovens are not fixed, but tiltable to facilitate emptying via a pouring chute [7]. Flat tubs (also tub furnace) Often tiltable Large area heated with flame from above and side Large oxidation High throughput Page 13

22 Aluminum Figure 7: Tiltable melting / holding furnace [5] Two-chamber process As can be seen from the sketch shown in Figure 8, the tub attached to the side of the furnace allows the use of scrap of any type. The metal is heated up in the main hearth, the melt circulates through the forehearth at high speed by pumping, whereby the solid material in the forehearth is liquefied with high melting capacity under exclusion of air or lack of air and the organic substances are decomposed [11]. Suitable for lacquered, oily and thin-walled scrap With forehearth and melting furnace (main hearth) Possible use of the energy content of the carbonization gases for the melting process and batch preheating Low metal losses, since melting is almost completely sealed off Complete collection and cleaning of all gases occurring in the system [3] page 14

23 Aluminum Figure 8: Two-chamber furnace [3] Pyrolysis / Bright Annealing / Melting Pyrolysis (Figure 9) is a process in which organic material is thermally decomposed in the absence of air or a lack of air - here between 500 and 600 C - with pyrolysis gases and pyrolysis coke being produced and the aluminum is produced in a bare form. An indirectly heated rotary kiln serves as the pyrolysis reactor. The heat required to heat the rotary kiln is generated internally by burning the pyrolysis gases. A mix consisting of around 60% of the flexible packaging turned out to be the optimal system input. For this material, which consists mainly of coated aluminum foils (paper, cardboard, PE), there is no other processing method that leads directly to liquid metal. The pyrolytically separated aluminum, sieved from the pyrocoke, then annealed and freed from interfering metals, is stirred into special "Vortex" melting furnaces. From this, the metal either goes directly to the foundry in liquid form or is poured into ingots. The secondary aluminum produced is of excellent quality and is used, among other things. as a raw material for innovative high-quality light metal alloys [8]. Page 15

24 Aluminum Ideal for composite materials Pyrolysis and incineration are separate Use of the energy content of the pyrolysis gases Low metal losses, since in the absence of air Complete collection and cleaning of all gases occurring in the system 10% more expensive than conventional processes [3] Figure 9: Pyrolysis / bright annealing / melting [3] Shaft furnaces In the past, a number of shaft furnaces that are particularly suitable for melting down scrap have been developed. Fundamental considerations for the use of shaft furnaces as smelting furnaces are aimed at utilizing the exhaust gas enthalpy to preheat the feedstock in the furnace shaft. Due to their robust and simple construction without complex heat recovery technology, they are often found in foundries, where they are used to melt foundry returns. These hearth shaft furnaces are used as a combined melting, holding and potting furnace and can be designed to be tiltable. Stationary designs with tapping are also in use. In this case, lumpy scrap is charged into the melting shaft located on the side of the oval hearth (Figure 10). Even fine-grained materials such as dry chips to be charged. When passing through page 16

25 aluminum-hot gases through the shaft, the scrap is dried and preheated. The furnace is completely emptied by tilting the entire furnace around the pouring spout or by tapping. Two burners are used for heating. The scrap column standing on the melting bridge is melted by a melting burner that is inclined in the side wall and drains into the hearth. The weld pool in the stove is kept at the same temperature by the holding burner [21]. Figure 10: Shaft melting and holding furnace / type STRIKO [21] page 17

26 Refractory materials 4 Refractory materials According to international stipulations, refractory products are non-metallic ceramic materials, including those that contain metals that have a fire resistance, i.e. a cone drop point (temperature) of 1500 C. The cone drop point is determined on small, slightly inclined pyramid-shaped test specimens, which are heated together with standard ceramic cones with a known drop temperature. The refractory materials usually do not have a clear melting point, but instead melt or soften within a more or less narrow temperature range. Instead of the melting point, a point that can be determined during softening is therefore determined by checking the falling point of the cone. Some products of the refractory industry do not meet the definition of refractory due to their cone drop point, but they meet other special characteristics of refractory materials and are included in classifications of refractory materials and are usually referred to as refractory products. In common parlance, fire-resistant products are those that are used at high temperatures (~ 600 C to 2000 C), especially in plants in the primary industry [9]. A distinction is made between four types of refractory products: Shaped products (bricks) Unshaped products (building and repair compounds, jointing materials, ...) Functional products (construction elements) Thermal insulation products Page 18

27 Refractory materials 4.1 Refractory basic materials The refractory materials in Table 28 (see appendix) are based on the 6 basic oxides SiO 2, Al 2 O 3, MgO, CaO, Cr 2 O 3 and ZrO 2 or on compounds between them, increasingly in combination with Carbon. Table 29 (see appendix) shows the properties of the basic level. In addition to carbon and silicon carbide, small amounts of boron carbide (B 4 C) and nitrides (Si 3 N 4, BN) are used for special applications. These basic materials and compounds (see Figure 11) are characterized by the fact that they only melt at high temperatures [9]. Figure 11: Base material pyramid for refractory products [9] page 19

28 Refractory materials 4.2 Dense molded refractory products The total porosity of the dense molded refractory products is less than 45%. The most important products are listed below. Non-basic molded products This group includes the materials of the SiO 2 -Al 2 O 3 series and other materials that cannot be classified according to their chemical reaction behavior, such as SiC and carbon products. % from SiO 2 (Table 30 in the appendix). As a rule, the SiO 2 content is 95 to 97% by weight. The following limit values ​​are specified according to ASTM C: Al 2 O <53% Fe 2 O 3 <2.5% TiO 2 <0.2% CaO <4% The further subdivision is very much based on the respective purpose. Of the physical properties (Table 31 in the appendix), the thermal expansion behavior of silica bricks is primarily important for practical use. The course of the reversible transformation is shaped by the three SiO 2 modifications. When heated, the stones expand greatly; at 800 to 1000 C, the maximum thermal expansion is reached at around 12 to 15 mm / m. Above approx. 600 C, well-converted silica bricks have an extremely good thermal shock resistance (TWB) due to the small change in thermal expansion. However, below 500 C they are extremely sensitive to temperature changes because the thermal expansion gradient is highest in this temperature range. High-quality stones can be used under favorable conditions in the temperature range up to almost 1700 C. The corrosion resistance to acidic melts is good. Alkali-containing gases and vapors can cause intensive corrosion below 1470 C through the formation of alkali silicate melts. Important areas of application for silica bricks are: coke and gas furnaces, superstructures of glass melting furnaces, wind heaters and lids of electric arc furnaces [9]. Page 20

29 Refractory materials Fireclay bricks Fireclay bricks essentially consist of the oxide components SiO 2 and Al 2 O 3. The SiO 2 content is between 50 and 80% and the Al 2 O 3 content between 10 and 45% (Table 32 in the appendix) . These stones are differentiated according to their Al 2 O 3 content. Further subdivision characteristics are the shaping behavior and the purpose of use with specific property limit values. Fireclay bricks contain: Fe 2 O 3 <3% (low-iron types down to less than 1%) TiO 2 <3% CaO + MgO <4% Na + K 2 O <3.5% By using low-flux, especially low-alkali raw materials, a Firestone with 40% Al 2 O 3 the proportion of the glass phase can be reduced to 20%, with about 55% mullite (Al 9 Si 3 O 19) and% cristobalite (SiO 2). The proportion of the glass phase and its chemical composition and the formation of the mullite determine the softening behavior. The glass phase softens at just under 1000 C, the softening interval being large due to the high viscosity of the glass. The glass phase thus essentially determines the property values. The main areas of application are: furnace construction, blast furnace / wind heater, steel works foundries, furnace systems in the non-ferrous metal industry, coke and gas furnaces, glass industry, float glass production and in the cement industry [9] Alumina-rich stones Alumina-rich stones have an Al 2 O 3 content of at least 45% to over 99% (Table 34 in the appendix). As with the firebricks, they are divided into groups according to the Al 2 O 3 content. The naming is based on the main raw material used in the manufacture of the stones. In addition to the products made from just one raw material rich in aluminum with a binding agent, predominantly 5-15% binding clay, various combinations of raw materials, especially with chamotte, are used. The addition of fused corundum, for example, increases the abrasion resistance (Table 35 in the Appendix) [9]. Page 21

30 Refractory materials Zirconium-containing stones The zirconium-containing materials are divided into the pure zirconium silicate materials, some of which also contain free zirconium oxide, and the sintered alumina-zirconia-silica (AZS) materials, a mixture of zircon, mullite and corundum (Table 36 in the appendix). The basic raw material is the natural zirconium silicate (ZrSiO 4). The theoretical composition of zirconium is 67.2% ZrO 2 and 32.8% SiO 2. Zircon decomposes into ZrO 2 and SiO 2 at temperatures above 1676 C. In the presence of impurities or admixtures, the decomposition temperature can drop to ~ 1500 C. . For example, contents of> 0.2% Al 2 O 3 or> 0.1% Fe 2 O 3 are mentioned here. Zirconium-containing stones are characterized by a high level of corrosion resistance to a large number of aggressive media, especially acidic melts (Table 37 in the appendix).They are mainly used in the glass and steel industries. Dense AZS materials are installed in lead glass tubs and in the base of soda-lime glass tubs. Porous AZS stones are used in soda-lime glass tubs in the front hearth area and in the working tub. The dense materials are to be handled very carefully because of their brittleness. Due to their comparatively low TWB, slow tempering and tempering is unavoidable. With prolonged use in contact with alkali-rich melts and under high temperature stress, zirconium decomposes and low-melting silicates are formed in addition to baddeleyite, a modification of zirconium dioxide [9] Carbon and graphite stones The chemical element carbon comes as amorphous carbon with different degrees of crystalline order and in four modifications as chain-shaped carbon, as fullerene with a spherical cage structure, as graphite with a layered lattice and as a cubic diamond (Table 38 in the appendix). The amorphous carbon and the graphitic modification are used on an industrial scale as refractory materials due to their excellent high-temperature properties. Carbon and graphite have no melting point, a constant or slightly increasing strength with increasing temperature, good to very good electrical conductivity and low expansion coefficients. Refractory bricks made of carbon can be divided into three groups, amorphous, partially or semi-graphite and graphite bricks. The bulk densities and strengths of carbon and graphite bricks are low compared to most other refractory materials. The thermal expansion is low, see page 22

31 Refractory materials High thermal conductivity and very high for graphite. The thermal shock resistance (TWB) is usually sufficient for the usual applications (Table 39). Applications are limited by susceptibility to oxygen, water vapor and CO 2 above 400 C. Under a reducing atmosphere, their application limit temperature is approx. C, for other uses up to 3000 C and higher. In the manufacture of aluminum, the lining of the electrolysis tank is made of carbon bricks. This tub is not only used as a refractory lining but mainly as a cathodic current discharge [9]. 4.3 Unshaped refractory materials This term is the correct name for materials that are still often referred to today as refractory masses or generally as ramming masses. The ramming masses represent only a small part of the unshaped refractory products anyway. The adjective unshaped expresses the main difference to the refractory stone, which has a certain format, is usually homogeneously pre-fired and has to be laid dry or with a suitable mortar. In contrast, unshaped refractory products can usually be placed behind formwork in larger fields at the place of use and form the furnace lining after hardening [9]. The general definition used in the ISO 1927 and DIN ENV standards for unshaped refractory products is: Mixtures consisting of aggregates and one or more binders, prepared for direct use, either in the delivery condition or after the addition of one or more suitable liquids, and the requirement with regard to fire resistance according to ISO R 836. They can contain metallic, organic or ceramic fibers. These mixtures are either dense or insulating. Insulating mixtures are those that have a total porosity of> 45%, determined in accordance with EN on test specimens fired under specified conditions. Page 23

32 Refractory materials Classification according to type of use The unshaped refractory products are divided into three groups according to their intended use: Materials for monolithic constructions Materials for repairs Materials for laying and grouting While the last group can clearly be assigned to the refractory mortars, the first-named ones are Overlap. Partial repairs are often carried out with the same types of material as monolithic constructions Classification according to the type of bond A distinction is made between four types of bond: Hydraulic bond with solidification and hydraulic hardening at room temperature Ceramic bond with hardening through sintering during fire Chemical bond (inorganic or organic-inorganic) with hardening through chemical, but not hydraulic, reaction at room temperature or at a temperature below the ceramic bond.Organic bond with hardening or hardening at room temperature or at higher temperatures. Of course, mixed bonds often occur, for example in a phosphate-bonded ramming compound (= chemical-ceramic) or in a hydraulically hardening mending and repair compound which, in addition to high-alumina cement, contains significant amounts of binding clay (= hydraulic-ceramic). In such cases, the type of bond that plays the main role in hardening must be specified. Page 24

33 Refractory materials Classification according to product types The extensive range of unshaped refractory products is classified as follows: Refractory concretes Mouldable refractory materials Plastic masses Ramming masses (ramming masses) Tap hole masses Gunning mixes Refractory mortar 4.4 Refractory concretes Concrete is an artificial stone made from a mixture of cement, concrete aggregate ( ie granulate) and water, possibly also from concrete additives, are created by the hardening of the cement paste. The standardized hydraulic binders are regarded as cements. To put it more concretely, concrete is a composite or composite material that is initially mixed as a malleable mass, fresh concrete, of a certain consistency from the mixture components granulate and additives, microfillers, i.e. grain mixtures of the most varied types and sizes, binders and water or other mixing or additional liquids and over the course of time, due to hydraulic hardening of the cement paste or specific hardening processes of non-hydraulic binders or special hydrothermal hardening, it hardens to form an artificial stone. Most of the information in the specialist literature specifies cement with hydraulic hardening as the only binding binding agent for concretes. The use of cements is not typical for all refractory concretes, although in many countries only refractory concretes bound with alumina cement are generally recognized as such and are also described as such in the literature. In the case of concretes and concrete-like refractory materials, however, in addition to hydraulic binders, a large number of binders with different binding mechanisms are now used and linked to the process of concrete binding. According to the relevant building material literature and practical customs, this is evidenced by the following terms for concrete derived from the binding agent [12]: Page 25

34 Refractory Materials Gypsum Gypsum Concrete Hydrothermally Generated Phases Silica Concrete Waterglass Waterglass Concrete Bitumen Bitumen Concrete Synthetic Resin Synthetic Resin or Polymer Concrete Clay Clay Concrete The term concrete is not generally linked to the use of hydraulic cements, but results from the material formation process described above. Refractory concrete is a concrete-typical technology from a mixture of generally heat- and heat-resistant or refractory granulates, possibly microfillers, additives, usually inorganic-chemical, hydraulic, but also organic or mixed binders and water or other mixing liquids through hardening Dense, porous, mechanically strong material produced from room temperature to moderate temperatures Raw materials and components According to the definition given in Section 4.4, refractory concretes and masses, mixtures, are produced from heat and heat-resistant as well as refractory dense or porous components, possibly additives, binders and mixing liquids. The mixture changes its phase composition more or less clearly on solidification and on heating. The most important raw materials are characterized from the point of view of chemical / mineralogical composition, physical and technical properties [12] Granulates as additives. Refractory granulates are all natural and artificial raw materials with a grain size> 0.5 used in the refractory industry mm suitable. Fine material (<0.1 mm) serves as a micro-filler and is generally not referred to as an aggregate. Page 26

35 Refractory materials Aluminum silicate granulates The most important representatives of this group of granulate materials are chamotte and sintered products with high alumina content. These are used in a variety of forms and as the most frequently used ceramic materials for refractory concretes. The raw chamotte is used according to the appropriate classification. Fireclay is produced by firing kaolitic, refractory clay raw materials (clays, kaolins) and sintering, e.g. Refractory material artificially enriched with Al 2 O 3 (corundum) with varying composition [12]. acidic quartz fireclay (10 to 30% Al 2 O 3) normal fireclay (30 to 45% Al 2 O 3) high alumina corundum chamotte (46 to 56% Al 2 O 3) high alumina corundum chamotte (56 to> 75%) Al 2 O 3) The structure consists of mullite, quartz, cristobalite, corundum and glass phase in varying amounts. The composition and some characteristic values ​​are given in Table 40 (see appendix). The fireclay materials are based on the SiO 2 -Al 2 O 3 system. The only compound in this thermal system is mullite, a mixed crystal with the boundary compositions A 3 S 2 and A 2 S. Sillimanite, andalusite and cyanite of the formula Al 2 O 3 -SiO 2 only occur in nature. In the acidic quartz chamottes, 68 to 85% SiO 2, part of the free SiO 2 is present as unconverted quartz. Subsequent transformation of the cristobalite at operating temperatures can lead to irreversible waxes and damage. Enriching with corundum increases the total Al 2 O 3 content by up to 75% and higher, which results in better thermomechanical properties. These chamottes are typical composite materials and therefore cannot be classified in the SiO 2 -Al 2 O 3 phase equilibrium [12]. Page 27

36 Refractory materials Corundum An important granulate material for highly refractory concretes is corundum (-Al 2 O 3), due to its high melting point of 2050 C and its high mechanical resistance. Three typical basic types are used [12]: fused or electrocorundum tabular alumina hollow spherical corundum The properties of corundum and zirconium dioxide granulate materials are summarized in table 41 (see appendix). In addition to -Al 2 O 3, silicate glass phases and other crystalline phases such as mullite, aluminates, spinels and -Al 2 O 3 also occur in the fused corundum. The pressure softening occurs at around 1900 C. The creep speed of sintered corundum at 1715 C is about 10-3 h -1. Under certain conditions, a reaction with CaO can take place to form CA 6, which leads to irreversible volume expansion. Tabular alumina is a highly sintered, very pure Al 2 O 3 with a tabular crystal habit> 0.1 mm, it consists of up to 95% corundum. For heat-insulating concrete, hollow spherical corundum is an excellent material that is easy to process. The balls are thin-walled with a maximum diameter of 5 mm and consist of -Al 2 O [12] zirconium oxide (baddeleyite) and -silicate (zirconium) Technical zirconium dioxide contains> 90% ZrO 2. It is a hard, highly refractory material, its chemical properties Composition can be seen in Table 41. At RT it occurs in the monoclinic form, which reversibly converts to the tetragonal form between 1000 and 1200 C. Zirconium silicate ZrSiO 4 is a natural mineral product. It decomposes at 1700 C into ZrO 2 and SiO 2 with a fusion bond [12]. Page 28

37 Refractory materials Basic oxide and silicate granulates The basic materials include: Periclase (> 80% MgO) Sintered dolomite (30 38% MgO) Periclase chromite (55 80% MgO) Chromite periclase (25 55% MgO) Chromite (> 25 % Cr 2 O 3, <25% MgO) Forsterite (% MgO) Binder With regard to refractory concretes, binders (binding building materials, binders, binders, putty materials) are reactive substances that are either mixed with water or in aqueous, partly Colloidal solution present in a mixture with the granules and the additives result in a processable mass with different consistencies. The hardening by cementing the granules and additives leads to a compact and volume-stable body. The various hardening and binding mechanisms differ, of course, from the type of binder. Binders are reactive substances that either harden independently with the help of water or from and in air or in combination with finely divided components (binders in the broadest sense) lead to hardening. The following classification appears appropriate [12]. Powdery compound that hardens hydraulically with water, which may react with the fine fraction of the granules or with an added micro-filler. Reactive solutions, alkali silicate or phosphoric acid, in connection with a micro filler. Finely divided oxides in connection with sulfate, chloride or phosphate solutions. Finely divided oxides in connection with alkaline solutions (hydrothermal hardening). Finely divided activated SiO 2 or aluminosilicate suspensions (Keramobinder). Liquid or solid organic substances. Page 29

38 Refractory materials Additives Additives are all additives that do not fall into the main groups of granulates and binders. These include, above all, certain fine additives (micro-fillers) to optimize the binding agent and additives to influence the setting and fresh concrete as well as to create certain hardened concrete characteristics. Micro-filler To achieve optimal concrete properties, it is necessary to add finest-grain refractory materials to the existing mix. Their tasks are: binding the free CaO in the PZ concrete increasing the viscosity of the melt phase acting as a cavity filler improving the binding capacity by simultaneously acting as a binder component increasing strength by improving the ceramic bond reducing the shrinkage due to the formation of connections [12] These fillers are based on SiO 2, Clays, magnesia and chrome ore Reinforcing components Various property improvements, such as increasing strength and compressive strength, increasing the TWB, are brought about by incorporating reinforcing components. Inorganic-non-metallic materials which can be used technically and economically are primarily aluminum-silicate fibers. The compositions of various fibers are summarized in Table 42. These are generally glassy to partially crystalline and completely devitrify above 1250 C, where they shrink and lose their strength. The application temperature can be increased to over 1500 C by adding Cr 2 O 3, TiO 2 or ZrO 2 [12]. Page 30

39 Refractory materials 4.5 Requirements for refractory materials Various mechanisms must be taken into account with regard to the correct use of refractory materials: The operating temperature, the chemical attack of the molten medium and the mechanical stress wear the refractory material in different ways Understood substance to withstand high temperatures according to a certain Segerkegelfallpunktes. Materials that can withstand temperatures of 1500 C (Segerkegel 17 = SK 17) are referred to as fire-resistant. Fabrics with a Seger cone 37 are marked as highly refractory (see Table 2). Table 2: Characteristic Seger cone values ​​[14] Seger cone small 150 [C / h] old Seger cone small 150 [C / h] ISO temperature [C] Remarks Fireproof, highly refractory Seger cone with highest falling temperature A fireproof building material consists of a granular, porous pile of different high-melting crystals that are connected to one another by substances that have already melted. The so-called binders (see 4.4.3) have the vitreous state, which changes the physical properties only gradually during the transition from the solid to the liquid state (see Figure 12). Page 31

40 Refractory materials Properties Crystalline body Glass-like body Melting point Temperature T 0 T 0 Figure 12: The properties of crystalline and glass-like substances as a function of temperature A similar sudden change can be determined when the atoms leave their originally occupied positions and adopt a new arrangement. This phenomenon occurs both in metals and in compounds and is called polymorphism. In the case of refractory materials which contain compounds with two or more modifications, this leads to typical properties and characteristic behavior when heated. Since the individual modifications of the connections also have different properties, e.g. Density, the volume can change abruptly or discontinuously during the conversion. In the case of refractory masses and bricks, this can lead to cracking and, in particularly severe cases, to the bricks falling apart. For these reasons, the range of use of materials with many modifications can be severely restricted. In this context, the sintering process should be pointed out: As the temperature rises, the mobility of the atoms increases and due to changes in place, self-diffusion and collective crystallization, reactions also take place below the solidus line [14]. Table 3 below shows some application temperatures of various refractory products. Table 3: Examples for different application temperatures Product Maximum application temperature [C] Silica 1650 Chamotte 1400 Magnesia 1700 Page 32

41 Refractory materials Resistance to temperature changes Another important property of refractory materials is their ability to withstand temperature fluctuations; this is known as thermal shock resistance (TWB). During the tapping of a melting furnace, which takes a few minutes, the temperature of the furnace lining drops. The temperature drops further when it is used in cold conditions; it rises when it is heated up and melted down. The masonry of a furnace has to withstand large temperature fluctuations without breaking. A number of important phenomena are linked to the heat state of a body: Change in volume Change in physical state Change in color Change in electrical resistance All these properties can also be used to measure the heat state of a body. The test of the TWB is carried out according to the water quenching method (DIN sheet 1). In the so-called normal stone method, a stone is heated on one side to 950 C and then the heated end is immersed in cold C water and thus quenched. This process is repeated (see Table 4) until the stone is destroyed or until at least 50% of the surface has flaked off [14]. Table 4: Quenching numbers of important stone qualities [14] Product Quenching number [-] Fireclay 15 Mullite 25 Corundum 15 Silicon carbide 25 The resistance to TWB increases with the same amount of coarse grain, with a higher firing temperature the resistance decreases [14]. Page 33

42 Refractory materials Chemical resistance The chemical behavior of refractory materials essentially relates to the interaction with foreign substances at high temperatures, such as melts, dusts, gases, etc. Ingress of gases and slagging of all kinds from the furnace atmosphere. The behavior of the refractory material and its resistance to chemical corrosion depend on many factors [12]: chemical relationship of the components; an acidic slag and a basic stone result in a chemical reaction. Temperature: the higher the temperature, the greater the reaction rate Viscosity: the greater the liquidity of the attacking substance, the stronger the chemical attack on the refractory lining is as a rule. Porosity (reactive surface): the higher the porosity, the stronger the chemical attack. Presence of certain third components. The temperature plays a decisive role here, since the wetting and reactivity increases sharply with the temperature and the viscosities decrease. The chemical nature (acidic, basic, neutral) must be observed in particular, materials with chrome ore, chromite periclase or periclase have favorable properties compared to basic melts. Products based on corundum or sillimanite are particularly resistant to both acidic and basic melts [12]. In general, the smaller the reaction gradient between slag or another flux and the refractory product, the more resistant it is to chemical attack. An important group of chemical compounds are the oxides, i.e. compounds of elements with oxygen, which e.g. arise during combustion or can be formed during corrosion. They can be broken down according to their ability to implement them (see Table 5) [14]: page 34

43 Refractory materials Basic oxides have a relatively strong reactivity with acidic oxides. Acid oxides naturally have a strong reactivity with basic oxides. Amphoteric oxides can react with both acidic and basic oxides, their reactivity depends on the environment and in particular on the strength of the acidic or basic reactant Table 5: Refractory oxides with chemical formula, melting temperature and chem. Character [14] Compound Formula Melting temperature [C] Chemical character Calcium oxide CaO 2600 basic magnesium oxide MgO 2800 basic barium oxide BaO 1920 basic aluminum oxide Al 2 O amphoteric chromium oxide Cr 2 O amphoteric silica SiO acid titanium dioxide TiO acid zirconium dioxide ZrO acid Independent of conversions between refractory materials and gas - and slag flows can also react with a simple contact of two different composite refractory materials at high temperatures (over 1600 C) in the furnace wall. A practical example of a chemical attack is described in the following paragraph: If alkali oxides, preferably sodium and potassium oxide, are carried along in the flue gases, these bases react with the acidic components of the lining to form low-melting compounds and eutectics. This leads to sintering and thus also to post-shrinkage and a reduction in fire resistance. Fireclay, sillimanite and corundum materials can absorb alkali oxides in the edge zones: the reactions lead to the formation of nepheline (NaAlSiO 3 = Na 2 O Al 2 O 3 2SiO 2) and of potassium philite (KAlSiO 3 = K 2 O Al 2 O 3 2SiO 2) . The volume expansions associated with the formation of these minerals lead to thin shells popping off [14]. Page 35

44 Refractory materials Strength properties Mechanical strength encompasses a whole complex of resistance to a wide variety of external mechanical loads. Of these, the compressive and flexural strengths are of greater technical importance than the tensile, split tensile and shear strengths with regard to their determination and use. When looking at strength models, a distinction must be made between theoretical and practical strength. The practical strengths are considerably lower, they are in the order of magnitude of about 1% of the theoretical. This large difference is due to the presence of voids, volume and surface cracks. The refractory material is subjected to mechanical wear, abrasion and erosion in various production plants. A good resistance to such influences ensures a longer service life. Corundum concretes with high alumina cement were described by American authors as particularly resistant in the 1950s, and they played a certain role alongside the phosphate-bound corundum compounds. However, a comparison published at the time by Venable (1959) of the relative erosion resistance of phosphate concrete (code number 90 to 100) and refractory concrete (13 to 35) and the comparison with SiC stone (80 to 100) and corundum stone (30 to 50) showed that Conventional refractory concretes, which are roughly equivalent to fireclay bricks, cannot compete with the best materials. The abrasion resistance is favorably influenced by the high purity of the components, few additives and dense grain packing, but it is unfavorably influenced by a high proportion of mixing water [12]. The abrasion resistance increases as the strength of the refractory products increases. Page 36

45 Refractory materials 4.6 General problems with refractory concretes There are some problems associated with the use of refractory compounds in aluminum smelting furnaces. The following chapter provides an overview of this. Flame temperatures of the burners The use of burners with high flame temperatures of up to 1900 C makes the use of refractory concretes with high application temperatures necessary. Figure 13 shows the temperature profile of a regenerative burner flame in an aluminum melting furnace. Figure 13: Temperature profile in the aluminum melting furnace Anti-wetting agent At temperatures above 900 C, the anti-wetting agents BaSO 4 or CaF 2. BaSO 4 BaO + SO 3 CaF 2 CaO + F 2 BaO and CaO act as strong fluxes in refractory concrete, the resulting structural change leads to flaking on the surface. This means premature wear and tear and shorter operating times for the refractory lining. SO 3 and F 2 escape from the refractory concrete as a gas, which is associated with an increase in porosity and an increase in the formation of corundum. Page 37

46 Refractory materials 4.7 Selection of the suitable refractory lining Which of the numerous stones or masses should be used for a new lining or repair differs from case to case and must be tailored to the special applications and requirements. The advantages and disadvantages of refractory concretes in relation to bricks are summarized in Table 6. In many cases, the casting and injection molding compounds have the advantage of requiring little work during installation. If the lining is exposed to particular chemical loads, ceramic or chemically and ceramic-setting compounds are used. In the case of masses, the thermal insulation is generally better than that of stones. The layer thicknesses can be better adapted to the requirements of the masses, since there is no connection to the stone dimensions. Working with refractory masses has the advantage of faster delivery times compared to the classic method with stones. Large amounts of mass can be introduced in a short time by centrifuging, vibrating or spraying. As described above, refractory concretes have better thermal insulation. Due to the formation of different layers with a different sintered state, they have better resistance to temperature changes. There are also advantages of masses in technical handling, since the number of permeable joints is lower, and the mechanical resistance to vibration and impact is higher than with the corresponding structural part made of stones. Of course, refractory concretes also have disadvantages. can be seen in the improper selection of varieties. Of course, the construction must also be geared towards working with masses. Possible errors in the processing of the masses often lead to shorter downtimes and higher wear. It should be particularly emphasized that the drying, the shaping process and the burning in the manufacture of bricks are carried out under the most favorable conditions in the refractory plant. While in the construction with masses their final properties are only obtained in operation and the supplier can only guarantee the correct composition. Page 38

47Some points must be observed here. Oven geometry The base of an oven can be rectangular or round. This means that the sole profile is either cylindrical or defined with a dome-shaped curvature [21]. Reinforcement In round furnaces, a closed armor armor forms the reinforcement, which has to absorb the forces arising from the thermal expansion of the brickwork and the static pressure of the melt. In the case of rectangular base areas, a combination of plating and vertical reinforcement columns, which are connected at the top and bottom, is used to absorb the forces [21]. Page 39

48 Refractory materials Heat transfer In principle, the position of the isotherms in the masonry can be changed significantly via the wall structure. The location of the isotherms in the individual masonry areas essentially determines how deep material can penetrate the stone before it solidifies. By forced cooling of the jacket or the floor, e.g. by using cooling segments, the temperature gradient in the infeed can be increased [21]. Type of delivery In order to achieve maximum stability and durability of masonry lining, dimensionally accurate stones with a flat contact surface must be used. For reasons of stability, both round and rectangular oven soles should be laid in a longitudinal structure. In order to achieve a dense masonry, the narrowest stone joints must be used [21]. Installation of expansion joints For ovens of large dimensions, e.g. In hearth furnaces where it is not possible to carry out a complete lining without expansion joints, so-called sliding joint effects must be observed in the area of ​​the expansion inserts, i.e. setting mortar joints in the direction of the specified expansion path must be avoided [21]. Monolithic lining The lining made from refractory masses has the advantage of being adaptable to the furnace geometry and the fact that wall thicknesses are independent of format. This has particular advantages when setting predetermined heat transfer rates. Although the expression monolithic lining suggests a seamless refractory lining, joints are also necessary for assemblies that are lined in this way. These construction joints arise because fields of any size cannot be introduced and joints are created at the field boundaries. The following processes play an essential role in monolithic lining [21]: Shrinkage during drying due to setting reactions, shrinkage or waxing when heated for the first time, and reversible changes in length due to the effect of temperature. Thermal expansion The chemical composition of a fire-resistant building material is decisive for its expansion behavior. In addition, temperature, furnace size and free expansion space are factors for calculating the thermal expansion [21]. Page 40

49 Refractory materials Temperature fluctuations from around 200 C cause a change in the dimensions of the building. Masonry that is not elastically clamped is therefore subject to greater pressure fluctuations, which have a disadvantageous effect. When cooling, this leads to unpressurized masonry with shrinkage cracks, which experience has shown when reheating, even if they are not contaminated, can leave open joints, into which liquid metal can penetrate. In the case of elastically clamped masonry, springs at least partially equalize the elongation and shrinkage behavior of the stones [21]. 4.8 Infeed of selected melting systems In the following chapter, infeed concepts for some selected aluminum melting furnaces are described. Hearth melting furnace For large hearth melting furnaces, dense, high-quality firebricks are used in the bath area or, for higher temperature loads and aggressive melts, high-alumina, chemically bonded, fired or heat-treated materials based on corundum with infiltration-resistant additives are used that are laid with appropriate fire putties. In addition to the molded products, infiltration-resistant masses are used in the bathroom area and as safety lining. Thermally insulating products that are used as permanent lining for backing are usually lightweight refractory bricks or lightweight refractory concrete. In the lower bath area of ​​aluminum melting and casting furnaces, the first insulating layer must also serve as a safety lining [21]. Figure 14 shows a possible infeed variant with refractory bricks and the masses of a hearth furnace. The different types of stone or mass are marked with A, B, C and D, their chemical composition and classification are shown in Table 7. Page 41

50 Refractory materials Burner D Furnace door CAB Aluminum melt E Figure 14: Sketch of a possible furnace lining Table 7: Summary of a possible furnace lining Abbreviation Classification Raw material Al 2 O 3 [%] SiO 2 [%] CaO [%] ZrO 2 [%] Fe 2 O 3 [%] MgO [%] AB Low-cement refractory concrete high-alumina stone bauxite bauxite C firebrick fireclay D refractory concrete synthetic raw materials E lightweight refractory concrete light fireclay page 42

51