WO2016118013A1

WO2016118013A1 - A method for producing glass

Info

Publication number: WO2016118013A1
Application number: PCT/NL2016/050056
Authority: WO
Inventors: Joseph Dominicus Joannes Maria BECKERS
Original assignee: Beckers Joseph Dominicus Joannes Maria
Priority date: 2015-01-23
Filing date: 2016-01-25
Publication date: 2016-07-28
Also published as: NL2014180A; NL2014180B1

Abstract

The present invention relates to a method for producing glass in a furnace comprising a step of feeding raw materials to said furnace, maintaining said furnace under process conditions for producing said glass and withdrawing a liquid glass product from said furnace, wherein said method further comprises several steps: contacting said hot liquid glass product with a heat transfer medium in a down stream heat exchanger; transferring the thus heated heat transfer medium to an upstream heat exchanger; contacting said heated heat transfer medium with said raw materials in said upstream heat exchanger, wherein said raw materials are preheated before entering said furnace.

Description

A method for producing glass

Description The present invention relates to a method for producing glass in a furnace comprising a step of feeding raw materials to the furnace, maintaining the furnace under process conditions for producing glass and withdrawing a liquid glass product from the furnace.

Glass is an amorphous (non-crystalline) solid material which is often transparent and has widespread practical, technological, and decorative usage in things like window panes, tableware, optoelectronics etc. Most types of glass are based on the chemical compound silica (silicon dioxide), the primary constituent of sand. Of the many silica-based glasses that exist, ordinary glazing and container glass is formed from a specific type called soda-lime glass, composed of approximately 75% silicon dioxide (Si0₂), sodium oxide (Na₂0) from sodium carbonate (Na₂C0₃), calcium oxide, also called lime (CaO), and several minor additives. A very clear and durable quartz glass can be made from pure silica.

European patent application EP 0 363 197 relates to a method of preparing water glass (aqueous sodium silicate) by heating Si0₂, NaOH and water under heightened pressure to a temperature above 150 °C, wherein in this hydrothermal process sand, cristobalite or tridymite is used as the Si0₂ source. This document further discloses that water glass can be prepared in two ways, namely by the furnace process and the hydrothermal process. Both methods of working are amply described in the literature.

Thus in German patent 3012073 a method is given for the preparation of water glass by fusing silicon dioxide in a furnace with, for example, alkali-metal hydroxides at a temperature between 900 and 1600 °C. By this method one obtains water glass with a relatively high molar ratio of about 3.0 to about 3.5 Si0₂/Na₂0.

The hydrothermal process is described in the French patent

1 1 12807. The substances used as the starting point of this process, NaOH and Si0₂, are used in a ratio of 1 :0.5 - 1 :2: finely divided Si0₂ with a grain size of about 0.1 - 1500 micron are brought into reaction with each other at a temperature of 150 to 320 °C and a pressure varying from 9 to 1 15 atm. German Offenlegungsschrift DE 30 12 073 relates to a method for manufacturing of water-soluble alkali metal silicates from Si0₂ and suitable alkali metal compounds by a melt process, wherein one of the reagents is preheated using the heat energy in the process waste gases and/or from an outside source, and after homogeneously mixing the reagents are transferred to the melt process.

US Patent Application publication number 201 1 -179829 relates to the production of a mineral melt by burning combustible material in the presence of inorganic particulate material and thereby forming a melt, wherein the melt can then be fiberised to form mineral fibres or used in other industrial processes. Such a method of making a mineral melt comprises the steps of: providing a circulating combustion chamber; injecting fuel, preheated mineral material and combustion gas into the circulating combustion chamber; combusting the fuel in the circulating combustion chamber thereby melting the mineral material to form a mineral melt and generating exhaust gases; separating the exhaust gases from the mineral melt, collecting the mineral melt (9) and passing the exhaust gases to a heat exchange system, wherein the heat exchange system comprises a first cyclone preheater and a first conduit from the circulating combustion chamber to the first cyclone preheater through which the exhaust gases pass.

US Patent Application publication number 2013-199245 relates to an apparatus for the production of a mineral melt by burning combustible material in the presence of inorganic particulate material and thereby forming a melt, said apparatus comprising: a circulating combustion chamber for receiving a supply of a fuel, pre-heated mineral material and a combustion gas and combusting the fuel in the circulating combustion chamber thereby melting the mineral material to form a mineral melt and generating exhaust gases, wherein the apparatus is configured and operative so that the exhaust gases are separated from the mineral melt and the exhaust gases are passed upwards through an exhaust pipe to a conduit of a heat exchange system, wherein the heat exchange system for pre-heating mineral material comprises a first pre-heater cyclone, a conduit for transporting exhaust gases from the circulating combustion chamber to the first pre-heater cyclone, a material inlet for injecting the mineral material into the conduit, and a flow communication from the first pre-heater cyclone to the circulating combustion chamber for feeding pre-heated mineral material to said combustion chamber. US Patent Number 4,752,314 relates to a method for pretreating glass-forming raw materials including particulate refractory materials and lower- melting flux materials comprising melting the flux materials, transporting the particulate refractory material with a primary gas through a nozzle, and delivering the molten flux into the nozzle whereby to provide high-sheer mixing, contact and adherence of the refractory particles with the molten flux, wherein the preheating of particulate refractory material upstream of the nozzle further comprises countercurrently flowing the particulate refractory material and high-temperature exhaust gases from the glas-melting furnace in convective heat exchange relationship to preheat the particulate refractory materials.

US Patent Application publication number 2010-037804 relates to a process for the production of cement, the process comprising: calcinating calcium carbonate-containing raw meal in a calciner heated by combustion of a carbon- containing fuel with a gas comprising oxygen and from 0 to 80% by volume of carbon dioxide, and substantially free of nitrogen, and isolating the gas produced by combustion and calcination in the calciner, wherein the raw meal is ground and then fed to the top of a preheater, wherein the preheater comprising a series of cyclones and connection ducts receives hot gas from the kiln and the raw meal is heated as the gases are cooled in the risers and the cyclones capture the meal so that it can be fed to the next lower stage.

From US Patent Application publication number 2014/0196503 is known a float glass furnace, comprising: a melting furnace, which heats raw materials to form a molten glass batch.

In a furnace process the attack on the sand is so fierce that silicates with high molar ratios can be obtained. An aspect of the furnace method is that it is relatively expensive because of the high temperature. The process conditions are so aggressive that any impurities in the sand such as iron, titanium and aluminum are dissolved in the water glass so obtained and it is less pure than water glass produced by the hydrothermal method. In addition the furnace method produces a solid material, whereas the direct product of the hydrothermal process is an aqueous solution.

By producing glass, such as container-glass, flat glass and frites- glass, and water glass etc. huge amounts of energy are necessary to produce the desired products. A lot of research and development work has been done and is still ongoing to reduce the energy demand per ton of product. Most of the easy to realize actions are done and now the industry has reached the 'asymptotic' region. In this region a lot of work is necessary to achieve small improvements, 1 -2 %. For instance by using mathematical modeling techniques it is possible to do parameter sensitivity analysis to find out where the best possibilities for improvement are. But even with this sophisticated approach it is difficult to find opportunities for improving the energy efficiency significantly.

Nowadays one of the last big steps for reducing the energy demand in a method for producing glass is using the heat in the flue gas for preheating the raw materials before these raw materials are entering the furnace. Some technical institutes have calculated that by producing container-glass an energy reduction of about 18 % can be achieved by preheating the raw materials up to 300 °C. Given this positive outcome it is expected that many glass producers would be using this raw material preheat approach. However, there are some disadvantages in this approach. One of them is that preheating may be difficult due to the preliminary reactions in the raw material mixture before entering the furnace and this makes it difficult to handle. In addition, there are many blockages etc. and as a result the calculated savings are seldom realized. The pay out time of the investment increases significantly and goes over 3 years. This makes it an unattractive investment in the current market.

For the production of water glass these savings are even more difficult to realize since the mixture of raw materials is more reactive due to the high content of soda ash. Moreover the relatively small scale of operation and the lower gas consumption per ton makes a short payback time almost impossible. In addition, the step of preheating the raw materials to an acceptable level is not possible because the flue gas doesn't contain enough energy to reach such a level.

The present inventor found that by making the furnace more efficient the flue gas flow will drop significantly and there will be less heat/energy available for preheating the mixture of raw materials. It will become almost impossible to preheat the raw materials to reach a temperature that gives a relevant payback time of 3 years or shorter.

Another aspect of this approach is that it will not anymore be attractive to invest in other efficiency improvements because the overall efficiency will not significantly improve. The better the heat-transfer in the furnace, e.g. better burners, the less flue gas there will be and the less heat can be recovered from the flue gas. As a consequence the temperature of the pre-heated raw materials will come down and more fuel will be needed in the furnace. The net result will be limited savings on fuel. The more efficient a furnace is, the less interesting it will be to preheat the raw materials with hot flue gas. Pre-heating in this way will only increase the complexity of the operation for making glass on a commercial scale.

An object of the present invention is to provide an energy efficient method for producing glass.

Another object of the present invention is to provide a method for producing glass wherein the amount of energy recovered is independent of the energy efficiency of the furnace(s) used.

Another object of the present invention is to provide a method for producing glass with a higher ratio of Si0₂/Na₂0 than 3.5.

Another object of the present invention is to provide a method for producing glass wherein the water glass thus produced can be easily reduced into small particles.

The present invention relates to a method for producing glass in a furnace comprising a step of feeding raw materials to the furnace, maintaining the furnace under process conditions for producing the glass and withdrawing a liquid glass product from the furnace, wherein the method further comprises:

contacting said hot liquid glass product with a heat transfer medium in a down stream heat exchanger;

transferring the thus heated heat transfer medium to an upstream heat exchanger;

contacting said heated heat transfer medium with said raw materials in said upstream heat exchanger, wherein said raw materials are preheated before entering said furnace.

The present inventor found that by using the above inventive concept in a method for producing glass the heat can be taken directly from the products that are leaving the furnace and it is possible to extract the energy from the product stream and to bring this energy to the in-going raw materials. According to the present invention the amount of flue gas will not have an impact on the preheating step as mentioned above. Transfer of energy can be done by using air to absorb the energy from the hot melt leaving the furnace and to preheat the raw materials. The hot melt leaving the furnace can be cooled from temperatures above 900 °C to a temperature below 100 °C by the use of air. Thus, in a preferred embodiment the heat transfer medium comprises air. In another embodiment steam, especially dry steam, can be used as a heat transfer medium. An advantage of using air as a transport medium for heat is that air is inert. However it might also be possible to use other media like dry- steam. The higher heat capacity of dry-steam might be beneficial to keep the investment low and have a more efficient recovery of heat from the product. The usage of steam might also be helpful to get a better control over the cooling down process.

The present inventor also found that in an embodiment it might be beneficial to use another way of using the recovered heat. The recovered heat might be used in micro turbines that are designed to handle very high temperatures. For example, the outlet temperature of a micro turbine can be about 500 °C. This is a temperature that is within the design range of the current batch-preheaters, i.e. the heaters that are used for preheating the raw materials in the present method. The electricity produced by those micro turbines can be used for electrical heating of the furnace. An aspect of (micro) steam turbines and ORC (organic Rankine cycles) is that they reduce the quality of a major part of the recovered heat.

The term "furnace" as used herein may comprise several heating units, also sometimes identified as unit(s) melter. The furnace is primarily used to convert the raw materials into the desired product, namely glass.

The terms "downstream and upstream" both relate to its specific position with regard to the furnace and the flow of direction of the raw materials.

The term "raw materials" relates to the materials to be used in making the final product and also includes all the recycle materials (cullet in the container glass industry).

The present inventor found that the temperature of the glass leaving the furnace is much higher than the temperature of the flue gas after the regenerator or the recuperator / waste heat boiler (1050 °C vs. 300 °C) of a standard furnace for making glass. By using the present inventive concept it is now possible to preheat the raw materials to a much higher temperature by using the heat in the glass instead by using the heat in the flue gas according to the prior art techniques. The limiting factor will be the temperature at which handling the raw materials will become problematic, for example the melting point of one of the ingredients of the raw material mixture. It is preferred when the effluent (product) leaves the present process at a temperature << 200 °C. In an embodiment of the production of products like bottles, tableware etc. , a part of the heat is to be recovered downstream in the process as well. Thus, in a specific embodiment the present method further comprises at least one step of molding said liquid glass product into final products, wherein the thus moulded final products are contacted with a heat transfer medium.

In the present method glass is made at a high temperature in the furnace. However, to process the glass, it has to be cooled down to the right temperature. It is preferred to re-use the heat that is released during this cool down phase. In such an embodiment the heat thus recovered is used for preheating the raw materials. Thus, hot air from the forming and the cooling section is used to recover the heat in a counter-current process in the most efficient way.

Pre heating of raw materials can be done in batch-preheaters that are currently available on the market e.g. Zippe, Sorg, Nienhuis concept, Vibrated Plate Technology etc. However the temperature of the process air might be significantly higher than the current temperature of the flue gas after the regenerator. Redesigning of the equipment or designing an adapted way of working might be necessary.

In a furnace, for example in a furnace working according to the Siemens Martenoven principle, there will be batch consisting of the raw materials floating on top of the already formed liquid glass. The heat from the flames is partly transported by means of conduction through this layer. The heat conductivity of this layer is negatively influenced by:

The air that is sucked into the furnace with the raw materials;

The C0₂ that is released due to the dissociation of soda ash (Na₂C0₃ -> Na₂0 + C0₂) will form bubbles.

Forming of water-vapor bubbles due to the evaporation of water. The present inventor found that by removing the water from the raw materials before entering the furnace and due to the fact that the dissociation of soda will partly take place in the preheating section at elevated temperatures (500 K - 900 K), the floating layer on top of the liquid glass in the furnace will encounter fewer bubbles. The presence of bubbles has a negative influence on the heat conductivity of the layer and thus it is preferred to create conditions in the furnace wherein the formation of these bubbles is highly prevented. The effect of preventing small bubbles in this layer is a much better heat conductivity of this layer and due to this an additional saving on energy and a higher melting capacity of the furnace are realized.

Moreover air, C0₂ and water-vapor will extract a lot of heat from the furnace since these gaseous streams leave the furnace at a high temperature. From this point of view it is preferred to remove the water from the raw materials and to feed the furnace with moisture free raw materials, i.e. raw materials having a minimum amount of crystal water thereby improving the efficiency of the furnace- operation. Also the dissociation of soda, (Na₂C0₃ -> Na₂0 + C0₂), should take place during the preheating as much as possible.

The present inventor found that after entering the furnace the unmelted raw materials will form a layer which is floating on top of the glass. Due to the heat from the flames the floating sodium carbonate will dissociate into sodium- oxide and carbon dioxide. In general the carbon dioxide formed in this layer will create small bubbles in the partly molten top layer. These bubbles have a very negative effect on the heat transfer of the raw-material mixture floating on the glass surface. Due to the preheating this dissociation might partly take place in the preheater. As a result the number of bubbles in the top layer will be reduced significantly and the heat transfer in the raw material mix floating on the glass furnace will increase. This will result in a higher melting capacity of the furnace.

Another advantage of preheating the raw materials to very high temperatures is that the layer of raw materials floating on top of the glass will decrease significantly. Due to this there will be much more clean glass surface. Because heat transfer to a clean glass surface is much more effective than to a layer of batch, the overall heat transfer will increase significantly. This will also result in a saving on energy.

According to a preferred embodiment of the present method the upstream heat exchanger comprises a series of cyclones. Such a series of cyclones provides an effective way of transferring the energy contained in the heated air to the raw materials. The raw materials are principally in a solid form, i.e. solid particles, and can be directly heated by close contact with the heat transfer medium, especially air. In addition, the use of a series of cyclones enables the removal of moisture from the raw materials.

It is preferred to have the flow direction of raw materials through the series of cyclones in counter-current with the flow direction of the transfer medium through the series of cyclones, wherein the outlet temperature in the ultimate cyclone of the series of cyclones is the highest temperature of all cyclones. The ultimate cyclone is in fact the "last step" before the thus heated raw materials enter the glass furnace. By applying the highest temperature of all cyclones in that specific ultimate cyclone the temperature drop between the thus heated raw materials and the temperature prevailing at the entrance of the furnace will be low.

For preventing an unwanted reaction between the raw materials in the series of cyclones it is preferred that the temperature of the thus preheated raw materials is lower than the temperature prevailing in the furnace.

In a specific embodiment the temperature difference between the outlet of the ultimate cyclone and the temperature prevailing in the furnace is 500 °C at maximum.

The present inventor found that small traces of moisture, i.e. up to about 2-10 wt. %, will have a negative influence on the free flowing properties of the raw materials. Thus it is preferred to remove moisture from the mixture of raw materials, especially by operating the first cyclone in the series of cyclones operated under an influx of flue gasses coming from a regenerator or recuperator, wherein the temperature prevailing in the first cyclone is preferably less than 125 °C. The present inventor found that the use of flue gasses coming from a regenerator or recuperator has a beneficial effect, especially in connection with the hydration effect. Thus hydration effect occurs when soda reacts with water present in sand thereby forming lumps resulting in clogging and blockages.

From energy efficiency point of view it is preferred to use flue gasses exiting the furnace for preheating the combustion air entering the furnace. In this embodiment the heat capacity of the hot flue gasses is used for preheating the combustion air entering the furnace. According to another embodiment flue gasses exiting the recuperator or regenerator can be used for additionally preheating the raw materials entering the furnace as well.

The present inventor found that an additional step of pre cleaning of the flue gas might be an issue when maximizing the heat recovery. In order to recover as much heat as possible it will be beneficial to cool down the flue gas to the dew point (ca 45 °C). However, sulfuric acid will start condensing at temperatures below 200 °C. Sulfuric acid is formed out of the sulfur in the fuel. The concentration of sulfur (mol/m³) is much higher in the natural gas at high pressure. Also in the liquid fuel. Pre-cleaning of the fuel might be easier to remove the sulfur. In case there are other disturbing chemicals in the fuel it is preferred to remove them before entering the furnace-system.

The apparatus to be used as the down stream heat exchanger is not restricted to a specific cooler but in some embodiments it is preferred to use as the down stream heat exchanger a heat exchanger chosen from the group of grate coolers and planetary cooler. These types of coolers can withstand high temperatures and provide high energy efficiency. The products coming out of a grate cooler are not lumps but particles.

According to a preferred embodiment the present method further comprises feeding the thus cooled glass stream coming from the down stream heat exchanger to a hammer mill. Such a hammer mill provided a flow of small glass particles, i.e. particles having size less than 1500 micron. Water glass-lumps are dissolved in a next process step, the dissolver section. A way of operating is using an excess of lumps in the dissolver because the lumps are difficult to solve. Due to the exothermic reaction and the excess of lumps, this process step can be quite dangerous. According to a preferred embodiment of the present process there will be granules instead of lumps. These will dissolve much easier and therefore it will not be necessary to use an excess of materials. This way of working is much safer because the exothermic dissolving process can be managed much easier. The dissolution rate will be much better defined and unexpected run-away situations can be avoided by the right design. Also the risk for blocking of the dissolvers because of the forming of poly-silicate or the forming of stickers will be significantly lower. In addition, due to savings on raw materials, the production of products at a higher ratio of Si0₂ / Na₂0 (3.3 -> 3.5 or even higher) will be economically possible. This will be possible since the extra energy demand of the furnace, due to this higher glass temperature because of the higher viscosity of the melt, will be recovered afterwards by taking out the heat out of the glass.

In the present description the term "glass" is not restricted to a specific type of glass. The invention is based on the concept that the energy present in the hot liquid glass product leaving the furnace is used for preheating the raw materials entering the furnace, wherein said transfer of energy is carried out by the use of an intermediate medium, namely a heat transfer medium, and contacting the hot liquid glass product with a heat transfer medium in a down stream heat exchanger. Hot liquid glass products are obtained when manufacturing for example frit glass and water glass. Thus the present method is particularly suitable for the manufacturing of water glass wherein the raw materials comprise alkaline and quartz sand. The term "glass" as used here encompasses thus, inter alia, tableware, bottles, container glass, flat glass etc.

Furthermore, according to a preferred embodiment of the present method the hot liquid glass product leaving the furnace is directly sent to the downstream heat exchanger. This means that in such an embodiment no additional intermediate process steps, such as a moulding step, cutting step or fracturing step, are carried out. The energy present in the hot liquid glass product leaving the furnace is directly recovered in the downstream heat exchanger. But the remaining energy present in the stream originating from that downstream heat exchanger may be recovered as well. In another embodiment the hot liquid glass coming from the furnace is first molded and then cooled. During the moulding step and the cool down step a heat transfer medium, or several heat transfer media, can be used to recover the heat and to use the heat thus recovered for preheating the raw materials, the combustion air, or a combination thereof.

Techniques from other industries like calcining and pre-calcining could be interesting techniques to combine with this way of working as described in here. They are developed for preheating the raw materials to high temperatures and removing the C0₂ in other industries. C0₂ will be formed due to the reaction: Na₂C0₃ -> Na₂0 + C0₂.

Another type of glass that can be manufactured according to the present method is frit glass. Specific raw materials for manufacturing glass are such as trona, sand, feldspar, sodium carbonate (soda), lime, dolomite, silicon dioxide (silica), aluminum oxide (alumina), sodium sulfates, dolomite, limestone and sodium chloride. Soda includes bot synthetic and natural soda. Raw materials also include recycle glass streams.

The present inventor found that the present method, especially the concept of recovering of the heat from the product and to re-use that heat in the process, is useful in other industries, such as the production of enamel, 3D-products, synthetic cement (synthetic binders), cooling down of products in the liquid state to the solid state (metals like zinc, aluminum, iron etc.), porcelain, stone wool, glass wool. In other words, the present concepts is applicable on products and processes where partly or complete sintering and/or melting at elevated temperatures takes place and where cooling down to ambient temperature is necessary. Another application is in llong-afterglow as in Galileo mirrors, or cooling of slag in the steel industry: no intermediate additives needed such as steel balls, or solidifying of metals and alloys.

Specific description of the invention.

The invention will now be described in more detail with reference to the following examples and Tables.

Figure 1 is a basic flow scheme of the present invention.

Figure 2 is another embodiment of a flow scheme of the present invention.

Figure 3 is a preferred embodiment of the present invention.

Figure 1 depicts a basic flow scheme according to the present invention, identified as reference number 50, namely a method for producing glass in a furnace 8. Raw materials 1 , 2, 4 are mixed in a mixer apparatus 3 and the mixed raw materials feed 5 is sent to an upstream heat exchanger 6. Although only three raw materials 1 , 2, 4 have shown, the present invention is not restricted to a specific number of raw materials. In addition, in specific embodiments some type of raw materials may also be sent directly to furnace 8. The thus preheated raw materials 7 are sent to furnace 8. In furnace 8 glass is maintained under such process conditions that glass is formed. From viewpoint of legibility furnace 8 is depicted as a standard glass furnace without any additional and necessary equipment such as heaters, pumps, fans, conduits etc. Liquid glass product 9 is withdrawn from furnace 8 and passed to a down stream heat exchanger 10. In down stream heat exchanger 10 the hot liquid glass product 9 is contacted with a heat transfer medium 1 1 . Due to this heat transfer a heated heat transfer medium 12 is formed and forwarded to the upstream heat exchanger 6 for preheating the mixed raw materials feed 5. In upstream heat exchanger 6 the heat from heated heat transfer medium 12 is transferred to the mixed raw materials feed 5. The thus cooled down heat transfer medium 20 leaves the upstream heat exchanger 6. In an embodiment the heat capacity of stream 20 is such that stream 20 can be used as a source for the combustion air, i.e. stream 18. Stream 20 will probably have a higher temperature than the fresh air and therefore there is still energy in stream 20 that can be used. The effluent 13 from down stream heat exchanger 10 is preferably sent to a unit for further processing, such as a hammer mill station 14 and reduced in size thereby forming a glass particle stream 15. In a preferred embodiment hot flue gasses 16 coming from furnace 8 are passed to a combustion air heat exchanger 17 for preheating air stream 18 resulting in a preheated combustion air stream 19. Although not shown, hot flue gasses 16 coming from furnace 8 can also be directly passed to upstream heat exchanger 6 for preheating mixed raw materials feed 5. In another preferred embodiment a part of stream 16 is passed to combustion air heat exchanger 17 and another part is sent to upstream heat exchanger 6. In addition both down stream heat exchanger 10 and upstream heat exchanger 6 may comprise several heat exchanging units. As already shown here in Figure 1 , hot flue gas coming out of heat exchanger 17, namely stream 32, can be used to support the preheating of the raw materials as well. Thus, in a preferred embodiment of the present invention, preheating of the raw materials can be further supported by stream 32 and stream 16, besides the heated heat transfer medium 12.

Figure 2 is another embodiment of a flow scheme of the present invention. In this embodiment of a flow scheme of the present invention the same reference numbers apply as the one used in the above discussed Figure 1 . An essential difference between the flow scheme according to Figure 1 and the flow scheme according to Figure 2 is that in Figure 2 the effluent 13 from down stream heat exchanger 10 is sent to another heat exchanger section 36. This heat exchanger section 36 receives a heat transfer medium 35 and the thus heated heat transfer medium 38 is sent to upstream heat exchanger 6. The reason for this additional heat exchanger section 36 is that the after a step of moulding the liquid glass product into final products these final products still have a high temperature. By contacting the still hot final products with a heat transfer medium the energy from these final products can be used for preheating the raw materials as shown in Figure 2. The effluent 37 from heat exchanger section 36 can be used for further processing. Please note that unit 14 is not restricted to a hammer mill unit. In another embodiment the energy present in heat transfer medium 38 can be used for preheating air stream 18 as well (not shown here). Figure 3 is a preferred embodiment of the present invention, identified as reference number 40, namely a preferred embodiment of upstream heat exchanger 6 now comprising a series of cyclones 21 , 22, 23, and 24. Heated heat transfer medium 12 enters the first cyclone 24, in which cyclone 24 the temperature is the highest temperature of all cyclones 21 , 22, 23, and 24. The heat transfer medium 28 leaves cyclone 24 and enters the subsequent cyclone 23. Heat transfer medium 29 leaving cyclone 23 enters subsequent cyclone 22, and heat transfer medium 30 leaving cyclone 22 enters the ultimate cyclone 21 . The effluent of cyclone 21 , i.e. heat transfer medium 20, may be sent to the environment.

In cyclone 21 there is also the mixed raw materials feed 5 (see

Figure 1 ), which mixed raw materials feed 5 is heated in cyclone 21 and its effluent, i.e. a feed of raw materials 25 is now passed to cyclone 22. Outlet feed of raw materials 26 is passed to cyclone 23 and heated by contact with heat transfer medium 28. The thus heated raw materials 27 are passed to cyclone 24 and the thus preheated raw materials 7 are sent to furnace 8 (see Figure 1 ). In a specific embodiment air stream 31 enters cyclone 21 for moisture removing purposes. In such an embodiment a part of stream 30 coming from the previous cyclone, i.e. cyclone 22, may bypass cyclone 21 and may be sent to the environment. According to the preferred embodiment in Figure 2 the temperature in cyclone 24 is the highest, whereas the temperature in cyclone 21 is the lowest. It is to be noted that the present invention is not restricted to a specific number of cyclones, but it is preferred to have at least three cyclones as upstream heat exchanger 6. The direction of flows through cyclones 21 , 22, 23, and 24 is such that the flow direction of raw materials through cyclones 21 , 22, 23, and 24 is in counter-current with the flow direction of the heat transfer medium through cyclones 21 , 22, 23, and 24. In Figure 2 only heated heat transfer medium 12 has been shown here, but, as discussed in Figure 1 , other streams, such as stream 32 and stream 16, may also be used for heating purposes. In a specific embodiment one or more cyclones 21 , 22, 23, and 24 are provided with any one of streams 12, 16 and 32, or a combination thereof.

In Table 1 and 2 below an indication of process parameters for the manufacture of water glass is provided. These parameters are only intended to illustrate the present invention but are in no way a limitation on the scope of protection conferred by the attached claims. Both Table 1 and Table 2 read on the above discussed Figure 1 and Figure 3. Table 1

In Table 1 one must realize that the efficiency of a recuperator and a regenerator may differ substantially resulting in broad temperature ranges.

In the heat exchangers there is a significant temperature difference since these units are operated with cold (ambient) and hot streams.

Table 2

Reference Description Temperature Remarks number

5 Mixed raw materials feed Ambient

7 Preheated raw materials leaving 500 - 1000 °C

fourth cyclone 24

12 Heated heat transfer medium 500 - 1 100 °C

20 Cooled down heat transfer medium < 125 °C Stream 20 can leaving upstream heat exchanger or be sent to unit preheater 6 1 1

21 First cyclone Ambient-150 °C

22 Second cyclone 125 - 500 °C

23 Third cyclone 300 - 700 °C

24 Fourth cyclone 500 - 1000 °C

25 Heated raw materials leaving first 75 - 150 °C

cyclone 21

26 Heated raw materials leaving 100 - 500 °C

second cyclone 22

27 Heated raw materials leaving third 300 - 700 °C

cyclone 23

28 Heat transfer medium leaving fourth 300 - 800 °C

cyclone 24

29 Heat transfer medium leaving third 200 - 500 °C

cyclone 23

30 Heat transfer medium leaving 150 - 300 °C

second cyclone 22

31 Air stream Ambient

Claims

1 . A method for producing glass in a furnace comprising a step of feeding raw materials to said furnace, maintaining said furnace under process conditions for producing said glass and withdrawing a liquid glass product from said furnace, wherein said method further comprises several steps:

2. The method according to claim 1 , wherein said method further comprises at least one step of molding said liquid glass product into final products, wherein the thus moulded final products are contacted with a heat transfer medium.

3. The method according to claim 2, wherein the thus heated heat transfer medium is used for preheating said raw materials.

4. The method according to claim 2, wherein the thus heated heat transfer medium is used for preheating the combustion air entering said furnace.

5. The method according to any one or more of the claims 1 -4, wherein said heat transfer medium comprises air.

6. The method according to any one or more of the claims 1 -4, wherein said heat transfer medium comprises steam, especially dry steam.

7. The method according to any one of the claims 1 -6, wherein said upstream heat exchanger comprises a series of cyclones.

8. The method according to claim 7, wherein the flow direction of raw materials through said series of cyclones is in counter-current with the flow direction of said heat transfer medium through said series of cyclones, wherein the outlet temperature in the ultimate cyclone of said series of cyclones is the highest temperature of all cyclones.

9. The method according to any one of the claims 1 -8, wherein the temperature of the thus preheated raw materials is lower than the temperature prevailing in said furnace.

10. The method according to claim 9, wherein said temperature difference is 500 °C at maximum.

1 1 . The method according to any one of the claims 7-10, wherein the first cyclone in said series of cyclones is operated under in flux of flue gasses, wherein the temperature prevailing in said first cyclone is less than 125 °C.

12. The method according to any one of the claims 1 -1 1 , wherein flue gasses from said furnace are used for preheating the combustion air entering said furnace.

13. The method according to any one of the claims 1 -12, wherein said down stream heat exchanger is chosen from the group of grate coolers and planetary cooler.

14. The method according to any one or more of the claims 1 -13, further comprising feeding the thus cooled glass stream coming from said down stream heat exchanger to a hammer mill.

15. The method according to any one or more of the claims 1 -14, wherein said raw materials comprise alkaline and quartz sand for producing water glass.