PL148248B1 - Method of melting glass furnace charges - Google Patents

Abstract

Batch material for glass or ceramic is mixed with solid or liq. fuel which is combusted in pre-heating zone; ash and unmelted batch are passed to 2nd stage to melt batch, melting is then completed under oxidising conditions. Powdered material is heated so that source of O2 is supplied to combustion zone; gas and solid flows are co-current. Constituents not melting below 900 deg.C are mixed with fuel which is combusted to give 900 deg.C; mixt. of batch and ash is passed to 2nd zone and mixed with lower melting components and melting then completed. Batch for 1st zone includes alkaline earth carbonates which become calcined.

Description

The invention relates to a method of melting glass batches using carbonaceous fuels (such as coal) and the use of mixtures of solid or liquid fuels with raw materials for these processes. In regions where coal is abundant, coal is usually the cheapest source of energy compared to other traditional energy sources such as natural gas, fuel oil and electricity. Therefore, there have been proposals to use coal as a fuel in glass melting and similar processes. Examples of such proposals are given in U.S. Patent Nos. 3,969,068 and 4,006,003. However, the use of coal as a fuel in directly fired furnaces has certain disadvantages which have hindered the spread of this method. The main one is the presence of ash in the coal. When burning coal in the top burner in an open hearth furnace, which is usually used in In glass melting, the exhaust gases carry away a significant amount of ash, which can clog the generators, so it is necessary to remove the ash from the exhaust gases before they are released into the atmosphere. Some of the ash is deposited on the walls of the melting chamber, melts, and flows down the walls of the tank with the liquid slag into the molten glass. The flow of the molten slag adversely affects the refractory lining of the furnace, and it combines with the molten glass, changing its composition and causing inhomogeneity in the product. The slag often contains more iron than the glass, and this flow causes colored streaks to appear in the glass. These are significant obstacles to the use of coal as a fuel for direct heating in melting processes, where the product uniformity. This is particularly true for window glass, where compositional variations cause optical distortions. Another disadvantage of using coal or other carbonaceous fuels in admixture with raw materials, especially in the melting of clear glass, is that carbon in quantities sufficient to produce the necessary energy in the melting process has a reducing effect on the molten product, and the reduced iron and sulfur turn the glass brown. Furthermore, some of the iron and sulfur from the coal passes into the molten glass. Small amounts of powdered coal (usually less than 0.1% by weight) were added to the charge for clear glass to facilitate melting, but such amounts were not significant as an energy source, and adding larger amounts was considered harmful. Even in the production of brown glass, the amount of coal used was of no energy significance. United States Patent No. 3,294,505 describes the smelting of glass in a bed of briquetted charge and coke. This process is limited to a narrow class of low-viscosity and low-quality glass. It would also be desirable to avoid the costs of agglomerating the charge. A parallel United States patent application, W.fftl.879, describes a technique for wetting the charge for glass melting with fuel oil. The oil provides only a small portion of the energy necessary for smelting. This problem is caused by the relatively volatile hydrocarbon fractions contained in coal and other carbonaceous fuels, which are If they are heated before ignition, they escape with the exhaust gases. The problem occurs especially if it is necessary to preheat the mixture of the charge with coal fuel. Feeding coal fuels in unground form to the combustion zone leads to the emission of smoke, which is undesirable from the environmental protection point of view. Post-combustion or other preservation of the exhaust gas or pre-coking of the fuel are expensive operations. The method according to the invention for melting a glass or similar batch, in which the combustion of fuel containing ash is used as a heat source for the process, consists in mixing a liquid or solid fuel containing ash in direct contact with the glass batch, causing combustion of this fuel and heating of the batch, wherein the amount of fuel contained in the batch is increased. The amount of fuel in the mixture is maintained at a level sufficient to preheat the batch to a temperature slightly below its fusion temperature before the final liquefaction stage, or the amount of fuel is maintained at a level sufficient to provide most of the heat required in the liquefaction state after the batch has reached a temperature slightly below its fusion temperature and ash from the burned fuel is allowed to remain in the finished glass product. In the method of the invention, an ash-containing fuel (e.g., coal) is used as the primary energy source for the smelting process, while avoiding the problems associated with the presence of ash. The ash-containing fuel is burned in a batchwise process of preheating the batch in the smelting process, the ash being combined with the batch, the batch and the fuel. should be fed mixed to the preheating stage to ensure uniform distribution of ash in the charge and intimate contact between the fuel and the charge during combustion. The combustion process in the preheating stage is maintained by feeding an oxidizer, preferably substantially pure oxygen, to the combustion zone. In this stage, optionally, the material is mixed to improve contact between the charge and the burning fuel. The heated mixture of charge and ash, preferably still in powdered form, passes to the next stage, in which the mixture becomes a liquid. A significant portion of the total energy requirement for the smelting process can be met by a cheap fuel, such as coal, by heating the charge to a temperature slightly below the temperature at which fusion occurs. By feeding a nearly homogeneous mixture of charge and ash to the melting stage, a liquid substance with a specific homogeneous composition despite the introduction of a significant amount of ash. This solves one of the problems associated with using ash-containing fuels, such as coal. By burning the fuel in contact with the charge, which avoids ash entrainment with the exhaust gas and the formation of slag on the inner walls of the tank, the problems of environmental pollution and furnace damage, which are significant in smelting processes, are also avoided. The method according to the invention is particularly advantageous in smelting processes and similar ones, where compositional homogeneity is important, by avoiding slag flowing into the molten mixture. This method can even produce glasses that are difficult to homogenize, such as soda-lime-silica window glass, which meet the requirements of High optical quality standards. Another advantage is that it is not necessary to agglomerate the charge. In the method according to the invention, the oxygen supply and temperature can be controlled in the preheating stage so as to produce significant amounts of carbon monoxide as a product of fuel combustion. The carbon monoxide is transferred to the next stage, i.e., the fusion stage, where it constitutes at least part of the combusted fuel. Alternatively, combustion in the first stage may be incomplete, as a result of which some fuel may pass with the charge to the next stage, where it constitutes at least part of the fuel required there. In order to be able to use higher temperatures in the preheating stage and increase the amount of carbon monoxide produced, the easily melted components of the charge can be omitted in the first stage and introduced in the second or By removing the sodium source (e.g., sodium carbonate) from the soda-lime-silica glass feed, temperatures high enough to calcine carbonates such as limestone and dolomite can be used in the first stage and produce carbon monoxide. By processing only the silicon source (sand) in the first stage, very high temperatures can be used and large quantities of carbon monoxide can be obtained. 45 During the solid-to-liquid phase, a subsequent, third stage occurs, in which fusion processes can also occur. When the feed is mixed with fuel, incomplete contact of the fuel with the oxidant during the fusion stage is possible, or the presence of excess fuel is possible, as a result of which the molten substance can leave the second stage in a reduced form. In this case, the third stage involves the use of re-oxidation agents. molten substances, for example, subsurface combustion with an oxygen-rich flame or bubbling an oxidizing agent (preferably oxygen) through the molten substance. Reoxidation prevents, in particular, discoloration of transparent glass. Undesirable discoloration of transparent glass by iron and sulfur ions can be avoided by reoxidizing the molten substance in a third step. The chemical components of coal ash essentially correspond to those of most glasses, hence glasses can absorb certain quantities of ash without harming the product, provided the ash is homogeneously melted in the molten mass. However, in the case of glasses with high optical requirements, difficulties may arise in uniformly distributing the resulting ash throughout the glass, as coal is the primary fuel in a conventional melting process. The advantage of the method according to the invention in this situation is the use of coal as a fuel in the first, periodic stage of the entire melting process, which allows the charge to mix with the ash before liquefying. In the periodic preheating stage, the energy requirement is lower than the total requirement, so less coal can be used and less ash is produced. The overall efficiency of the staged melting process is due to the lower energy requirement of the entire melting process, which in turn reduces coal consumption. Therefore, coal can be the most important energy source for heating the glass batch to near fusion temperature. In some cases, coal may be the most important or sole energy source for the entire fusion process. The new method of using fuel according to the invention can apply to the entire amount of fuel or supplement conventional heat sources. A portion of the total thermal energy requirement in the preheating stage is significant, i.e., 3% greater than in the case of previously known methods of adding carbonaceous substances as fluxes, dyes, or binders. It is believed that the previous methods of adding carbonaceous substances covered at most 5% of the energy requirement of the process. For economic reasons, it is desirable that the new method of using the fuel according to the invention should provide most of the energy required in the preheating stage, preferably all of this energy. Another feature of the preferred embodiment of the invention is the reduction of incomplete combustion products, such as smoke, soot, or oxidizable substances from the fuel. As the feedstock mixed with liquid or solid fuel moves toward the heated zone, the fuel temperature gradually increases and it begins to release volatile components and smolder before ignition. The released volatile components are eliminated in the process according to the invention because during the preheating stage they flow in the same direction as the feedstock-fuel mixture. In a co-current flow, volatile components from the initial heating stages reach the combustion zone, where the combustible components are incinerated. This not only removes the emitted pollutants, but their combustion heat also contributes to the heating of the feedstock. The exhaust gas from the preheating stage can be further combusted by being sent to the next combustion zone, for example, in a smelting stage. The invention also offers other environmental benefits. The staged process allows the use of oxygen instead of air to support combustion. Eliminating or reducing the amount of nitrogen in the exhaust gas reduces the amount of nitrogen oxides (NOx) produced. The use of oxygen significantly reduces the volume of the exhaust gas, and therefore its velocity, allowing the gas to entrain fewer particles of the particulate feedstock. The absence of nitrogen results in a higher flame temperature. The use of pure oxygen and the complete elimination of air enhances these benefits, but they can only be partially achieved, depending on the extent to which the oxygen concentration exceeds that of air. Other environmental benefits result from reduced sulfur emissions from the combustion of sulfur-containing fuels such as coal. Contact between the combustion gases and the feedstock (particularly glass feedstock containing limestone and similar substances) can remove sulfur oxides from the gas stream. Alternatively, in the process of the invention, the ash-containing fuel is burned during the pre-melting stage of the feedstock, so that any hearth glass resulting from the presence of ash is homogeneously combined with the material being fused. Because the hearth glass is fused with the product in the initial melting stage, the product can be homogenized during this and subsequent melting stages. Preferred methods for implementing the melting stage involve the use of inclined melting surfaces surrounding the cavity, so that a significant portion of the vessel's internal surface is melted material that absorbs ash or slag. As the molten mass flows down the inclined surfaces to the outlet, the charge is fed onto these surfaces. In the next stage, the melting process continues. The small area of refractory lining exposed to the slag during melting reduces the risk of tank erosion and intensive slag flow into the molten mass. The coal or other solid or liquid fuel should be in contact with each other during fuel combustion in the melting stage. The fuel and charge can be fed separately, but it is preferable to mix them before charging. Once the melting zone has heated to the fuel's ignition temperature, the combustion process is maintained by supplying the combustion zone with an oxidizer, preferably pure oxygen. An alternative method of implementing the method of the invention involves using a burner that burns powdered solid fuels, such as coal, in the melting stage. The ash entrained with the gases collects on the surrounding melting surfaces and combines with the molten batch. The invention is more fully understood by reference to the drawings and the following description of an embodiment of the method. Figure 1 shows a preferred apparatus for carrying out the method according to the invention, comprising a preheating stage rotary kiln, partially in cross-section, a second stage rotary melting chamber, and a third stage purification chamber, and Figure 2 shows an enlarged cross-section of the second and third stage apparatuses, the third stage utilizing a subsurface combustion device. The method according to the invention is described in detail below with reference to the example of glass melting, for which it is particularly useful. The method according to the invention can be used to produce all types of glass, including window glass, bottle glass, fiberglass, and water glass. This means that the invention can be used in the melting of other, similar substances, and in particular in the melting of mineral substances. Other examples include: the melting of glass and ceramic substances, the melting of frits, and the smelting of metals from ores. In the first stage, any device for contacting gases with solids can be used, but preferably the rotary kiln 1 shown in Fig. 1 is used. Alternative devices include a fluidized bed or cyclone distributor/contactor, as known in the art. The rotary kiln comprises a cylindrical shell 2 supported for rotation on rollers 3 and forming a small angle with the horizontal. A single-walled metal cylinder such as the one shown may be suitable, although better thermal efficiency may be achieved by using a refractory lining or a double-walled metal cylinder with insulation between the walls. A fixed inlet housing 4 closes the inlet to the rotary kiln. A feed channel 5 passes through the housing wall to direct the crushed charge from the metering device 6 to the rotary kiln. The charge B may be mixed with the fuel before being charged to the kiln, or the fuel and charge may be fed separately to the rotary kiln, the mixing in which case will take place in the kiln itself. Oxidizing gas fop. Air (preferably oxygen) is supplied to the furnace through a pipe 7 passing through the wall of the inlet housing 4. The pipe 7 should enter the furnace to such a depth that the combustion zone is formed at a certain distance from the feed point. Feed substances containing oxygen-containing compounds, such as carbonates, can provide some of the oxygen to support combustion. This is advantageous because it leads to the removal of carbon dioxide before the feed melts. After melting the charge, the release of carbon dioxide would lead to the formation of bubbles in the melt that would be difficult to remove. According to a preferred embodiment, the combustion products flow co-currently with the charge into the second-stage melting device 8 through an outlet closure 9 connecting the devices of both stages. The second-stage melting device 8 consists of a tank 10 closed by a refractory cover 11 having at least one opening. At least one line 12 passes through this opening. The next processing phase takes place in the tank 13, into which the molten charge flows so that the melting can take place. The ignition in the combustion zone can be initiated by an auxiliary heating device, such as a burner introduced into the furnace for a certain period. Once the fuel in contact with the charge has ignited, the combustion zone can be maintained within a specific area of the rotary kiln by controlling the oxygen supply and the speed at which the charge and fuel travel through the kiln. Solids and gases travel through the kiln in co-current flow, allowing volatile substances initially stripped from the fuel to enter the combustion zone and be incinerated. Although not preferred, the gas and charge in the rotary kiln or other preheater can flow countercurrently. In this case, it may be necessary to use exhaust gas cleaning measures that meet environmental requirements, such as baghouse dust collectors. A portion of the exhaust gases can be recycled to the combustion zone or to the preheating stage or to the melting stage to avoid combustible particles. Another method of treating the exhaust gases and recovering heat is to contact the exhaust gases with the feedstock in an additional preheating stage. A feedstock mixture containing carbonates (e.g., limestone) is useful for removing sulfur oxides from the exhaust gases. A particularly advantageous method of carrying out the second stage is shown in Figure 2 and is consistent with the teachings of U.S. Patent No. 4,381,934 and U.S. Patent Application No. 661,267. In the second stage, the feedstock melts on inclined melting surfaces. The charge placed on them melts and flows quickly down the inclined surface as a thin film. The melting device 8 shown in Figure 2 is a preferred device for carrying out the method according to the above patent descriptions. In the device 8 there are inclined surfaces surrounding a central recess, and the entire tank 10 rotates around a vertical axis. The rotating tank "id" provides certain advantages for the method according to the invention and the overall efficiency of the melting processes, but it should be borne in mind that the invention in a broader sense is not limited to a rotary melting device. Separating the melting stage from the other process stages ensures higher energy efficiency of each of the process stages by optimizing the conditions at each stage and adapting them to the specific requirements of each stage. Additional advantages are achieved by surrounding the heating zone with the feedstock and thermally insulating the melting zone with an insulating layer of the feedstock or other suitable substance. Due to the energy efficiency of the stage process and the use of only part of the energy necessary for the melting process in the melting stage, the amount of energy used in this stage is relatively small. The heat source is small, which allows for efficient use of various heat sources. Heating with coal is recommended, especially when using oxygen; electrical energy in the form of an electric arc or plasma torch can also be used. Coal or other solid fuel may constitute all or part of the fuel used in the second stage, and it may be part of the fuel unused in the first stage. If carbon monoxide is produced in the first stage, the exhaust gases from the first stage are sent to the second stage, where they can cover a significant part of the energy requirement. Referring to Fig. 2, the second stage melting device 8 comprises a cylindrical tank 10, which may be a steel drum. The tank 10 is suspended on a rotating frame 14, which is rotated about a vertical axis corresponding to the tank's axis of symmetry by a number of supporting and adjusting rollers 15. 16. The lower part 17 of the vessel is an axially mounted annular sleeve 18, which is the main outlet for the molten charge 19. The sleeve 18 is composed of a plurality of ceramic fragments, and the lower part 17 is releasably attached to the remainder of the vessel 10, which facilitates replacement of the annular sleeve 18. A refractory cover 11, preferably in the form of a dome, is supported by a fixed, surrounding frame 20. The cover 11 has at least one opening through which passes at least one conduit 12, which either supplies cooled gas, or constitutes a burner, or may be a conduit for supplying oxygen or another oxidizing agent to support combustion of the fuel supplied to the fusion chamber. If the fuel is supplied from the first stage, the conduit 12 may be used to supply oxygen. or similar gas into the vessel after the ignition temperature has been reached. In addition to the energy supplied by the fuel from the first stage, some heat for the fusion stage may optionally be provided by a conventional burner or other heat source. Conduit 12 may be, as shown, centrally located to fill the entire vessel with oxygen, or may be angled off-center to direct oxygen and/or fuel onto the molten bed. Cover 11 has an opening 21 for feeding the charge into the fusion stage, and as shown in Fig. 2, the end of rotary kiln 1 may be provided with an outlet closure with a discharge chute to direct the charge into the fusion stage. An adjustable deflector 22 at the end of the discharge chute may direct the charge flow against the side walls of vessel 10. 10 is covered by a solid layer of particulate mass 23 which serves to insulate the vessel 10 from the high internal temperature. In applications where contamination of the product must be avoided, layer 23 should have the same composition as the charge. Before the melting process begins, layer 23 is placed in the furnace by feeding a loose powdered mass, for example the charge material, into it and rotating the vessel 10. The loose material is arranged on a paraboloidal surface, as shown in Fig. 2. The powdered material can be moistened, for example with water, in the first step of lining formation to facilitate adhesion of layer 23 to the walls. When layer 23 is the charge material, it need not contain fuel which is later introduced into the charge. Other minor differences between the layer insert 23 and the charge are permissible and depend on the requirements of the particular process. During the melting operation, the charge stream 10 fed to the melting stage 8 is distributed over the surface of the solid lining 23, heated by the combustion heat in the tank 10 and melted, forming a transition layer 24 which flows to the bottom 5 of the tank and flows through the opening 19 in the sleeve 18. The molten substance 25 flows from the second stage melting device 8 into the tank 13 where further processing takes place. In this way, the initial melting stage of the charge can be carried out efficiently, because the charge immediately after melting flows away from the vicinity of the heat source and is continuously replenished with fresh material. charge, which results in a large temperature difference and a high heat transfer rate in the 2biomix. Solid replenishment with relatively cool, fresh charge and the insulating properties of the lining protect the tank from damage despite the lack of external, forced cooling of the tank. Solid layer 23 constitutes thermal insulation and serves as a non-contaminating contact surface for the transitional, molten layer 24. It should preferably contain one or more components of the charge. The thermal conductivity of the lining should be relatively low to apply a layer of reasonable thickness and avoid uneconomical forced cooling of the external side of the tank. Usually, granular or powdered charge is a good thermal insulator, but in In some cases it will be possible to use intermediates or products of the melting process as non-contaminating, durable layers. For example, in a glass-making process, powdered cullet (defects) can form a durable layer, although the thickness of layer 38 will have to be greater due to the higher thermal conductivity of glass compared to the glass batch. In metallurgical processes, using a metallic product to form a durable layer would require forming a layer 40 of considerable thickness to provide thermal protection to the vessel, but some ores may be well suited as insulating layers. The preferred method of carrying out the melting step described above involves rotation of the liner about the central bore, but it should be understood that the method of the invention also includes Applications in which a lining surrounds a rotating, heated opening. The method of the invention additionally encompasses applications in which the lining does not rest on inclined walls but does not surround a heat source (i.e., a flame). Examples of such processes are explained in the patents mentioned above. Air is used as the oxidant, but oxygen (i.e., a gas with a higher oxygen concentration than in air) is preferably used, which reduces the volume of gases in the process. As a result, the first and second stage devices can be more compact because the exhaust gas stream is of relatively low intensity and high temperature. Removal of nitrogen from the system also increases the flame emissivity and thus accelerates heat transfer. The intense combustion heat flux associated with the use of oxygen is consistent with the preferred method. * the second stage is necessary for thermal protection and efficient heat exchange with the surrounding lining. The temperature reached in the preheating stage depends on the combustion intensity, which in turn depends on the amount of fuel and oxygen supplied. Even a moderate combustion is beneficial in terms of the heat transferred to the charge. Advantageously, the amount of heat associated with combustion in the first stage raises the charge temperature almost to the fusion temperature of the charge components, where the charge ceases to be free-flowing. For example, a typical window glass melting charge containing significant amounts of sodium carbonate may be heated to a temperature slightly below the melting point of sodium carbonate (851°C), and preferably even slightly lower, to prevent One possibility is to omit the relatively easily melted components of the charge fed to the first stage and feed them directly to the second stage, which allows higher temperatures in the first stage. It is advantageous that at temperatures above 870°C, calcium and magnesium carbonates, typical components of glass charge, undergo calcination, i.e., decomposition, releasing carbon dioxide. Removing the carbon dioxide while the charge substances are still in an old state is advantageous because it prevents the formation of carbon dioxide bubbles in the molten mass. Even higher temperatures can prevail in the preheater if the substance heated in this stage consists of the most difficult-to-heat substances. The remaining components are added in a subsequent stage. For example, heating the sand alone allows the temperature to exceed 1000°C during preheating. Separate preheating facilities can be provided for any component not entering the first stage. Some of the feed components, e.g., sodium carbonate and sodium hydroxide, can be fed to the second stage in molten form. It may also be advisable to feed cullet directly to the second stage when relatively high temperatures are used in the first stage. In this case, the cullet can be preheated with exhaust gases. At most operating temperatures, combustion in the first stage leads to the formation of some carbon monoxide if the oxygen supply is insufficient for complete combustion of the fuel. The exhaust gases from the first stage can then be sent to the second stage, where the carbon monoxide present, combusted with additional oxygen, provides some or all of the fuel required for the second stage. The carbon monoxide content of the combustion products increases, and the carbon dioxide content decreases, with increasing temperature. Therefore, to produce significant amounts of carbon monoxide as fuel for the second stage, temperatures exceeding about 9000°C must be used in the first stage. If there is sufficient fuel and insufficient oxygen in the first stage, the entire fuel requirement for the second stage can be met with carbon monoxide from the first stage. Combustion of the fuel to carbon monoxide releases about one-third of the heat of combustion, and the rest is released by combustion of the carbon monoxide to dioxide. Therefore, when selecting the amount of carbon monoxide to be produced in the first stage, the relative energy requirements of the first and second stages must be taken into account. For example, a glass batch may consume twice as much energy in the first preheating stage as in the melting stage, so producing only carbon dioxide in the first stage may not be an energy-efficient procedure. By heating the entire batch for window glass production, an appropriate distribution of the fuel thermal efficiency can be achieved, producing in the first stage a gas containing approximately 50 mole percent carbon monoxide and 50 mole percent carbon dioxide. The advantage of using the method according to the invention is the possibility of using coal, which is abundant in some regions and relatively cheap. However, other fuels can also be used to advantage, for example, fuel oil, coke dust, petroleum coke, peat, brown coal, shale oil, sawdust, bagasse, and waste paper. Liquid petroleum products, such as fuel oil, wet the charge and prevent dust formation and entrainment in the exhaust gases. For economic reasons, coal, especially hard coal, is the best fuel. The calorific value of typical Pennsylvania hard coal is 25.5 to 34.8 MJ/kg with an ash content of 3 to 9% by weight, depending on the source. An efficient glass melting process, burning natural gas or fuel oil in the upper burners of a regenerative furnace, consumes at least about 7 to 8 MJ/kg of glass produced. Taking Pennsylvanian coal, for example, with a calorific value of about 32 MJ/kg and an ash content of about 7% by weight, it can be calculated that burning such coal in a conventional glass furnace to meet the energy requirements of the melt would produce too much ash. The smelting process described above consumes about 2.3 to 3.5 MJ/kg of product. At this level of energy consumption, significantly less coal is needed to meet energy needs, so the amount of ash introduced into the melt does not exceed the level permitted even for the production of high-quality window glass. The amount of coal used depends on the temperature desired during preheating and on the calorific value of the given coal, which in turn depends on its carbon content. Because combustion may be incomplete because some of the coal may not reach oxygen, it is recommended to use a slight excess of coal compared to the theoretical amount required. For example, about 26% to about 3% by weight of the Pennsylvanian coal described above, when mixed with a charge for window glass production, preheats the charge to a temperature of about 550°C to about 650°C when burned with excess oxygen. In this case, little carbon monoxide is produced. Another example is a feedstock for window glass production, in which sodium carbonate has been omitted in the first stage (it therefore consists mainly of sand, limestone and dolomite), and which, after the addition of about (P/t to about 10% by weight of carbon), is preheated by fuel combustion to a temperature of about 1100°C to about 1300°C. A significant amount of the limestone and dolomite is calcined and if a limited amount of oxygen is supplied, the exhaust gas stream contains more carbon monoxide than carbon dioxide. The carbon can be replaced by other carbonaceous fuels in quantities determined by their calorific values. It should be noted that although the fuel in contact with the feedstock can provide most, and preferably all, of the energy required in the preheating stage, Partial benefits are also achieved by using smaller amounts of fuel fed with the feed. In such a case, some of the energy can be supplied by a conventional burner in the preheating stage. In plants where gas flows countercurrently to the feed during preheating, exhaust gases from the melting stage can provide some of the energy for preheating. Solid fuels, such as coal, should be finely divided before being mixed with the feed. The coal grains should pass through a 0.250 mm sieve, and preferably through a 0.074 mm sieve. The ignition temperature of different coal grades varies somewhat, but oxidation of typical hard coal can begin at about 170°C, and combustion, when fed with clean fuel, can occur at about 170°C. oxygen, maintains itself at temperatures above 250°C. A typical composition of ash from 25 parts by weight of coal is as follows: parts by weight SiO* 1.2 AlgO, 0.6 Fe2Oa 0.27 CaO 0.1 Na, 0.5 It can be seen that the ash components are consistent with those of soda-lime-silica window glass, which may have the following composition: •% by weight SiO2 72—74 AlgO, 0—2 Na2O 12—15 K2O 0—1 MgO 3—5 CaO 8—10 v Fe2Ot 0—0.2 SO, -" 0—0.5 Soda-lime-silica glass of the above type usually has a viscosity at least 10 Pa»s at 1425°C. The temperature at which the batch is melted in the second stage depends on the composition of the batch, and in particular on the amount and melting point of the most easily melted components. In glass batches, the most common easily melted component 14 is sodium carbonate with a melting point of 851°C. In practice, it is known that industrial glass batches melt at a slightly higher temperature, around 1090°C to about 1150°C. The heat of the melting stage can raise the melt temperature slightly higher, so that the melted batch leaving the melting device has a temperature of the order of 1260°C, but it usually does not exceed 1320°C. This temperature The temperature and short residence time in the melting tank are rarely sufficient to complete the complex chemical reactions and physical transformations of the melting process. Therefore, the molten substance passes to the third stage, the so-called refining stage, which is a further stage of the melting process. In the case of glass, treatment in the refining zone requires raising the temperature of the molten substance to facilitate the fusion of the remaining sand grains and to remove gas inclusions from the molten mass. Refining window glass requires a temperature of about 1370°C to about 1510°C. Another desirable operation that can be carried out at this stage is mixing the molten substance to homogenize it. If the batch has otherwise melted under reducing conditions and passes to the refining stage, In the case of refining in a reduced state, it is sometimes necessary to reoxidize the melt. The function of the refining step carried out by method 80 of the invention is to introduce an oxidizing agent into the melt. All of these objectives are achieved by the preferred method shown in Fig. 2. Vigorous mixing during the refining step promotes not only the change in the oxidation state of the melt but also the addition of dyes, cullet, or relatively easily melted additives that change the composition. This allows for the production of a wide variety of products. The preferred method of carrying out the refining step shown in Fig. 2 utilizes a subsurface combustion process in both chambers. For some applications, single-chamber refining is sufficient, but the process for manufacturing window glass must be carried out in two chambers. The chambers are provided with oxygen delivery pipes 30 and 31 and water-cooled burners 32 and 33 located below the melt level. A subsurface constricted opening 34 allows the substance to flow from chamber 26 into chamber 27. An opening 35 in the upper part of chamber 26 allows the molten substance to flow from the melting stage into chamber 26. Exhaust gases from the melting stage 8 and first stage 1 may flow through opening 35 into the refining stage. An outlet opening (not shown in Fig. 2) may be provided in the upper part of chamber 26. Chamber 27 has an outlet opening in its upper part for the gases. 36. 60 A fuel, such as natural gas, and an oxidizer, preferably oxygen, are fed to burners 32 and 33, and combustion occurs when the gas stream contacts the melt reservoirs 28 and 29. Another fuel that can be used in subsurface burners 85 is hydrogen, because its combustion product is water, which is highly soluble in molten glass. The use of oxygen as oxidizer is advantageous because it avoids the introduction of nitrogen from the air, which is poorly soluble in molten glass, into the melt. The use of dilute oxygen also improves the contact between the oxygen and the reduced components of the melt. An excess of oxidizer, compared to the amount of fuel, can be fed to the burners to improve The oxidation state of the melt entering the refining stage. Alternatively, if the melt entering the lower refining stage contains sufficient unconsumed carbon, the melt temperature can be raised, and if raising the melt temperature is not necessary, only oxygen can be pumped into the melt tanks 28 and 29 for the sole purpose of semi-oxidizing the charge. The oxidizer can be fed without the subsurface burners, for example, through the pump tubes 30 and 31. It is advantageous to use oxygen bubbling and subsurface combustion simultaneously. The burners can be adapted to pump a stream of oxidizer bubbles into the melt, which increases the surface area of the melt with the oxidizer, and the vigorous mixing caused by the subsurface combustion promotes the distribution of the oxidizer bubbles. in the melt. Subsurface combustion also homogenizes the melt. The excess oxidant supplied to the refining stage depends on the specific conditions, i.e., the degree of reduction of the substance entering this stage and the required degree of oxidation of the final product. Important factors in the reoxidation process are the intensity of mixing, the size and shape of the tank, the effectiveness of gas-liquid contact, and the residence time in the refining stage. To achieve homogeneous oxidation, as required for window glass, it is advisable to carry out the reoxidation in two successive chambers, as shown in the figure, to ensure that each portion of the melt is subjected to the oxidation process for the appropriate residence time. In the case of glass, the reduced state manifests itself as a brown color due to the presence of sulfur in the form of sulfides and iron. If glass is produced colorless, this re-oxidation leads to an increased degree of oxidation of the coloring ions, expressed by the Fe+a/Fe+2 ratio. For standard commercial colorless glass, this ratio is from about 1.5 to 3.0, with a transmittance of at least 70% (preferably at least 80%) for light at a wavelength of 380 nm at a thickness of 6 mm. This glass is sometimes characterized by 60% transmittance at a wavelength of 1000 nm (6 mm thick). Much higher Fe+3/Fe+2 ratios have been achieved by bubbling oxygen through molten glass initially colored dark brown. The loss of color during oxidation is readily visible, and therefore the degree of oxidation can be easily determined by observation. Although carbon can To increase the iron content in the melt, re-oxidation allows for obtaining colorless glass. However, precise spectroscopic measurements by comparison with standard glass transmittance may indicate the need to reduce the amount of iron, usually added intentionally (in the form of polishing rouge) to obtain color. The re-oxidation chambers may be followed by a conditioning chamber in which the product remains for a certain time, necessary for the escape of gaseous inclusions from the molten mass M and for cooling the melt to a temperature suitable for further processing. The molten substance flows into the conditioning chamber 37 through a subsurface, narrowed opening 38. In the arrangement shown, the residence time in chamber 37 is further extended A subsurface barrier 39 and an upper barrier 40, which force a complex flow of the melt stream, can then leave the refining stage through a channel 41 leading to a forming stage or the like, where, in the case of glass, it is formed into panes, fibers, bottles, and the like by known methods. In an alternative embodiment of the method of the invention, the ash-containing fuel is burned in the melting stage. A preheating stage is not necessary. By separating the melting stage from the rest of the melting process, an environment is created in which most, or practically all, of the ash contained in the fuel 30 can enter the product without compromising the product's uniformity. The rapid outflow of the molten substance from the melting stage contributes to its good mixing, and the processing in the subsequent stages further homogenizes the molten charge and molten slag. Furthermore, since the fusion takes place in a relatively thin layer, the fuel mixed with the charge has easy access to oxygen, so that combustion is essentially complete. As with other implementation methods, the fuel can be fed separately to the combustion zone, but it is preferable to feed the fusion step with a mixture of fuel and charge. It is also advisable to use oxygen feed. Incomplete combustion of the fuel in the fusion step leads to partially reduced molten matter being fed to the refining step, which may require correction. Thus, in this implementation method, the function of the refining step is to introduce an oxidizing agent into the molten mass. The amount of coal consumed in The melting zone depends, of course, on the calorific value of the given type of coal, which in turn depends on its elemental carbon content. When using the above-mentioned Pennsylvanian coal, adding coal in an amount of about 0% by weight of the charge theoretically provides all the energy necessary in the melting stage of the charge for window glass production. However, due to the incomplete combustion resulting from the lack of oxygen access to some of the coal, it is advisable to use slightly more than the theoretically required amount of coal if the coal is to provide all the energy for the melting stage. For example, it is advisable to add coal in an amount of about 10% by weight of the charge. Other than coal Carbonaceous fuels can be added in amounts determined by their calorific values. The method according to the invention also allows the energy requirement of the melting stage to be partially met by the coal contained in the charge. In such a case, part of the energy is supplied by the coal in the charge, and the remaining part by a conventional burner or other heating device in the melting chamber. In an example appropriate for an alternative implementation method in the system of Fig. 2 without a preheating stage, a standard industrial glass batch (without sulfur-containing fluxes such as technical sodium sulfate or gypsum) was mixed with 5% to 6% by weight of coal and melted at a rate of about 6.8 kg per hour. The coal was the sole source of energy for the melting stage, and the melted charge at the entrance to the refining stage was It was foamy and brown in color. Each reoxidation chamber was equipped with a single subsurface burner and a delivery tube. Each subsurface burner was fed with 7 m³/h of hydrogen and 3.6 m³/h of oxygen. Each delivery tube was fed with 0.56 m³/h of oxygen. The volume of melt in each chamber ranged from 0.28 to 0.56 m³, and the average arrival time of the melt portion in the refining stage was estimated at about 30 minutes. The temperature in the first chamber was about 1290°C, and in the second about 1370°C. An additional burner (not shown in Fig. 2) in the upper part of chamber 47 was used to extinguish the foam. The glass The flow-off from the refining stage was colorless, almost bubble-free, and more oxidized than commercial glass. The batch used should conventionally produce a glass with an iron content, expressed as Fe2O3, of about 0.11% by weight. By transferring some iron from the carbon, the glass in the example contained 0.16% by weight of carbon. The sulfur from the carbon caused the glass to contain 0.063% by weight of SO3 without reoxidation and less than 0.01% SO3 with reoxidation. Claims 1. A method of melting a glass batch or the like, wherein the combustion of an ash-containing fuel is used as a heat source for the process, characterized in that the liquid or solid fuel containing ash is mixed in direct contact with the glass batch, the fuel is allowed to combust and the batch is heated, the fuel being kept in the mixture at a level sufficient to preheat the batch to a temperature slightly above its fusion temperature before the final liquefaction step, or the fuel being kept at a level sufficient to provide most of the heat required in the liquefaction step after the batch has reached a temperature slightly below its fusion temperature and leaving ash from the combusted fuel in the finished glass product. 2. A method according to claim 1, characterized in that the ash-containing fuel is combusted in a preheating step in which the batch is heated to a temperature slightly above its fusion temperature. 3. A method as claimed in claim 2, wherein all the energy required for the preheating step is supplied with the ash-containing fuel. 4. A method as claimed in claim 2 or 3, wherein in the preheating step the feedstock is mixed in the rotating tank as the feedstock moves from one end of the rotating tank to the other. 5. A method as claimed in claim 2 or 3, wherein a gas with an oxygen concentration higher than that of air is supplied to the preheating step to maintain combustion. 6. A method as claimed in claim 5, wherein in that oxygen is used as the oxygen source. 7. A method as claimed in claim 2 or 6, wherein part of the oxygen for combustion of the fuel in the preheating stage is supplied in oxygen-containing feedstock compounds. 8. A method as claimed in claim 2, wherein in the preheating stage the feedstock is transported in co-current and the combustion gases in this stage are passed in the opposite direction. 9. A method as claimed in claim 8, wherein the combustion products from the preheating stage are passed to the fusion stage. 10. A method as claimed in claim 8 or 9, wherein the gaseous oxygen source is introduced into the preheating vessel at an intermediate location remote from both the inlet and the outlet. 11. A method as claimed in claim 2, wherein in that the feedstock is fed to the preheating stage in a loose and powdered form. 12. A method as claimed in claim 1, wherein the temperature of the combustion zone in the preheating stage is maintained at a temperature of at least 900°C. 13. A method as claimed in claim 1 or 12, wherein the feedstock comprises alkaline earth metal carbonates and a temperature sufficient to calcine a substantial portion of the carbonates is maintained in this stage. 14. A method as claimed in claim 13, wherein the feedstock does not contain components melting at temperatures lower than the calcination temperature and the feedstock melting at temperatures lower than the calcination temperature is fed to the liquefaction stage. 15. A method as claimed in claim 1, wherein the major part of the energy required to melt the feedstock is supplied by burning ash-containing fuel in the liquefaction zone. 16. A method as claimed in claim 15, wherein the molten material is withdrawn from the second zone in a reduced state and transferred to a subsequent zone where conditions for secondary oxidation prevail. 17. A method as claimed in claim 16, wherein oxidizing conditions are maintained in the subsequent zone by introducing oxygen gas into the interior of the molten material. 18. A method as claimed in claim 17, wherein the material in the subsequent zone is mixed by introducing combustion gases into the interior of the molten material. 19. A method as claimed in claim 17, wherein in a further zone divided into two chambers the molten substance moves from one chamber to the other, with subsurface combustion occurring in each chamber. 20. A method as claimed in claim 15, wherein the total amount of energy required to melt the feed substance is supplied by combustion of ash-containing fuel in the liquefaction zone. 21. A method as claimed in claim 15 or 20, wherein the substance to be melted is withdrawn from the second zone in a partially unmelted state. 22. A method as claimed in claim 21, wherein the fuel is mixed with the feedstock just before the feedstock is fed to the liquefaction zone. 23. A method as claimed in claim 2, wherein the feedstock in the liquefaction zone melts on an inclined surface from which it flows freely. 24. A method as claimed in claim 23, wherein the feedstock entering the liquefaction zone is deposited on an inclined surface surrounding the central recess. 26. The method of claim 1, wherein the fuel is coal. 27. The method of claim 1, wherein the fuel is a liquid petroleum product. 148 248 ¦P/K 32 34 31 33 38 39 40 Fig. 2 PL PL PL PL PL PL PL PL PL PL PL PL

Claims (1)

1.