CA2673495A1 - Process and facility for producing soluble glass using heat recovery - Google Patents

Abstract

The method for the production of a product by means of melting a supplied material and letting it harden, preferably a method for the production of types of glass, in particular soluble glass, in a furnace, preferably in a tank furnace, using heat recovery, is characterized in that at least one part of the heat given off by the product produced is used for preheating the supplied material. Thereby energy is saved, the capacity of the tank furnace is increased, the occupational safety is improved, and less water for cooling the conveyor belt is used for the melted glass produced.

Description

Process and facility for producing soluble glass using heat recovery The invention relates to a process for producing a product by melting a supplied material and letting it solidify, preferably a process for producing glasses, in particular soluble glass, in a furnace, preferably in a tank furnace, using heat recovery.
A general overview of the production of soluble glass can be found in "Henkel-Referaten [Henkel Presentations]" 34, 1998, pages 7 to 13.

Three processes are used to produce soluble glass on an industrial scale; these are the conventional melting process in a tank furnace, the melting process in a rotary tubular kiln and the hydrothermal process.

Most of the alkali metal silicates which are common in industry are produced using the conventional melting process. The soda process for producing solid soda soluble glasses is a high-temperature process in which a mixture (batch) of sand and soda is subjected to alkaline disintegration to form soluble glass at temperatures of 1300 - 1500 C in tank furnaces of the Siemens-Martin regenerative furnace type or in a rotary tubular kiln. The air for combustion is fed to the regenerative chambers via fans and reversing elements and preheated to approximately 1200 C.

At these high temperatures, the alkaline soda reacts with the quartz sand to form sodium silicate. The molten soluble glass is continuously removed from the furnace, cooled and supplied to the storage area or directly to the dissolution units. Figure 1 shows the production of soluble glass in a tank furnace.

In the case of production in a rotary tubular kiln, the prepared batch is fed in at the higher kiln side and transported from the cold region to the hot region by virtue of the cylindrical kiln being rotated. This continuously forms new surfaces. The kiln, which is tilted by 3 - 7 , is rotated about its axis very slowly by a toothed drive, roller drive or worm wheel drive.
Heating is carried out from the bottom end using oil or gas. The glass melt is removed at the lower end and supplied to the plant for further processing.

Rather than being used as a solid material, solid soluble glass from the tank furnace or rotary tubular kiln is used almost exclusively as an aqueous solution, mostly in a 3501 strength concentration. It is produced by dissolving the solid glass lumps obtained from the melting process and cooled down to 300 C in water at temperatures between 100 C at atmospheric pressure and 150 C in a pressure vessel. Depending on requirements, the solution is processed further, that is to say filtered, concentrated and, if appropriate, modified with inorganic or organic additives.

Finally, in the case of the production of soluble glass using the hydrothermal process, the disintegration and dissolution processes are performed in one operation.
In this process, alkali metal silicates are obtained directly from sand and soda lye as liquid soluble glasses at approximately 200 C and at a high pressure of approximately 20 bar in an autoclave without involving the high-temperature process.

In addition to soda soluble glasses, potash soluble glasses are also used to a lesser degree. As more expensive products, potassium silicates are used only where sodium would cause problems.

The process for melting, by way of example, glasses and metals in industrial furnaces takes place at very high temperatures and therefore consumes a large amount of energy. The heat is usually dissipated from the melt by means of a so-called cooling belt downstream of the furnace outlet. This heat dissipation is usually necessary for the subsequent process steps. This heat content of the melt occurs as heat loss.

In the case of the conventional production method, the waste heat from the crystallization which frequently follows is not used at all or is not used efficiently in some other way. It is not yet known to use the heat of fusion from the furnaces.

In a conventional facility, the free heat from the melt heats the space at the cooling belt. The hot ambient temperature therefore makes it harder to work in the vicinity of melting furnaces and impairs the performance of the facility operators.

A report by the German Federal Environment Agency in June 2001, entitled "Large Volume Solid Inorganic Chemicals, Natriumsilikat [Large Volume Solid Inorganic Chemicals, Sodium Silicate]" describes in detail, inter alia, the heat recovery in the production of sodium silicate according to the prior art. In the case of production in a rotary tubular kiln, the heat can be recovered using two process variants. Firstly, the material which enters at the top end of the rotary tube is preheated by the waste gas, that is to say the hot waste air. This is possible since sand and soda are conducted in counter-current flow to the conduction of the waste gas while the furnace is simultaneously rotated. Secondly, the residual heat from the waste gas, after the latter has emerged from the rotary tubular kiln, is supplied to a recuperator in order to heat the required combustion air, and the waste gas is cooled down from approximately 600 C to 200 to 250 C in this recuperator. External air is simultaneously heated to 350 to 400 C and then passes to the burner at the bottom end of the rotary tubular kiln.

On account of the different procedure during the production process in a tank furnace, it is not possible to conduct charge materials and waste gas in counter-current flow, as expressly stated at the bottom of page 10 in the report mentioned. Preheating of the supplied material as in the case of a rotary tubular kiln is therefore not performed in the prior art. All that is known in the case of this process is to preheat the required combustion air by alternately using a plurality of flues. In this process, the hot flue gas is led away via a brickwork flue and this heats the brickwork. After a certain time, the flue gas is led away via a different flue. The still-cold combustion air then flows through the heated flue and is heated.
Quasi-continuous operation is achieved by regularly switching between the flues.
The invention is based on the object of simultaneously saving energy, increasing the capacity of the tank furnace, improving occupational safety and consuming less cooling water for cooling the conveyor belt for the molten glass produced in the process of the type mentioned in the introduction. The water is sprayed against the cooling belt during cooling and is evaporated there.

In the process of the type mentioned in the introduction, this object is achieved according to the invention in that at least some of the heat emitted by the product produced, in particular when it solidifies, is used to preheat the supplied material.
According to the invention, the heat content of the still-molten soluble glass which has just been finished, that is to say in particular its solidification heat, is at least partially returned and used again. The use of this heat is not disclosed anywhere in the prior art, not even in the rotary tube process. Only the recovery of the heat from the waste gas or the waste air was known to date.
Advantageous refinements of the invention are specified in the subclaims.

The invention also relates to a corresponding facility as claimed in claims 8 and 9.

The first heat exchanger is arranged around the cooling belt, which moves continuously obliquely upward, and above the cooling belt onto which the molten soluble glass from the tank furnace drips and/or flows in order to solidify and cool there. When the cooling belt returns obliquely downward, it is sprayed with water and cooled in this way. The hood, which is likewise arranged obliquely and above the cooling belt, intensifies the air flow between the hood and the cooling belt, and this increases the convective portion of the heat transfer from the molten glass or the hot cooling belt to the hood, which is simultaneously the first heat exchanger. Tubes which run parallel to the cooling belt and in which the heat-transfer medium, in particular water, flows at elevated pressure are preferably arranged in the hood. This surprisingly heats the water which is still cold on entry (20 -C) to at least approximately 1400C.
The heat is primarily transferred in this case by radiation. In one advantageous refinement of the invention, however, an additional waste air chimney which leads vertically upward and causes a stronger chimney effect may also be provided in the hood, and therefore the air velocity is increased from approximately 1 m/s to approximately 2 m/s. This results in a temperature increase of approximately 10a on account of the considerable improvement in the heat transfer.

In the exemplary embodiment described below, the first heat exchanger, that is to say the hood, operates in co-current flow with the molten soluble glass, which is likewise transported from the bottom upward. However, it is also possible and possibly even particularly advantageous to perform cooling in counter-current flow.

By way of example, the invention leads to the production of steam by recovering the heat capacity of the hot melt. The steam produced via melt transport may be used, for example, for heating the supplied material. By heating the supplied material, it is firstly possible to save energy. Secondly, the preheating of the supplied material increases the capacity of the furnaces, since more can be produced per unit of time. In addition, occupational safety is increased by using heat exchangers which are to be fitted to screen the hot product.

The invention therefore results in the following advantages:
= Saving energy by preheating the supplied material = Saving energy by using the hot condensate for the dissolution process = Further use of the energy, for example for producing steam = Increasing the capacity by means of the preheated supplied material = Improving work conditions during operation by reducing the room temperature = Increasing occupational safety.

In particular, the invention consists in recovering the quantity of heat to be dissipated, which currently occurs as heat loss, over the cooling belt by means of a newly fitted heat exchanger and returning it into the process for further use. This quantity of heat can be used, for example, to produce superheated or saturated steam.

All or the majority of the steam produced from the heat exchanger can be sold or used in some other way. Some of the steam produced in the first heat exchanger may, for example, be supplied to another suitable heat exchanger in order to preheat the supplied material.
This saves energy and increases the capacity of the furnace.

The flow of steam, which leaves the second heat exchanger as a condensate, may be used for another process. If the condensate from the second heat exchanger is used directly for another process, the required energy consumption can accordingly be reduced.
An exemplary embodiment of the invention is described in more detail below with reference to drawings, the prior art also being illustrated with reference to a drawing. In the drawings:
Figure 1 shows a schematic illustration of the production of soluble glass according to the prior art, Figure 2 shows a schematic overview of the process according to the invention and the facility according to the invention according to an exemplary embodiment (without the region around the hood 16 being illustrated precisely), and Figure 3 shows the region around the hood 16.

In all of the drawings, the same reference symbols have the same meaning and are therefore explained only once, if appropriate.

Figure 1 schematically illustrates the production according to the prior art. Sand and soda are supplied to a furnace 3 via a belt weigher 1 and a mixing screw 2, said furnace being heated using an oil or gas burner 4. As an alternative, the heating may take place electrically or by means of a combination of the heating types mentioned. Fresh air is fed into the furnace 3 via a blower 5 and a regenerative chamber 6.
The waste gases 7 leave the furnace via a second regenerative chamber 8, a waste-gas cooler 9 and an electrostatic filter 10.

The molten soluble glass drips onto a cooling belt 11, where it solidifies and from which it is discharged as glass lumps 12. If the glass lumps 12 are not stored and sold on, they pass into the so-called solutizer 13.
The glass lumps are dissolved in this tank with supply of water and under pressure, and therefore liquid glass 14 is finally obtained.

An example of the process according to the invention and the facility according to the invention is illustrated in figures 2 and 3. The material to be melted, that is to say the molten soluble glass, flows out of the melting furnace 3 onto the cooling belt 11, which moves upward in the manner of an escalator. The molten soluble glass 15 rests on the "steps" of this "escalator". When it reaches the top of the escalator, the solidified soluble glass is thrown off the escalator and collected as so-called glass lumps 12.
When the escalator returns from the top to the bottom, the "steps" are cooled by being sprayed with water on the underside.

According to the invention, the cooling belt 11 is surrounded by a hood 16 which is open at the bottom and is equipped on the inside with tubes 17 running parallel to the cooling belt. Fresh water is fed in at excess pressure (approximately 20 bar) at the bottom end of these tubes via a pump 18 and is heated in the tubes on account of the high temperature of the cooling belt and the melt of approximately 1000 C in the lower region of the cooling belt, that is to say at the point where the molten soluble glass is fed in. At the outlet, that is to say at the top of the cooling belt, the soluble glass and the cooling belt only have a temperature of approximately 300 C. The water is recirculated under excess pressure via a condenser 19.
4-bar steam is produced by reducing pressure at the valve 25 down to 4 bar. Saturated steam at 4 bar and 163 C is obtained at the top end of the tubes 17. Some of the steam is emitted via the line 20 as external steam for purposes other than the production of soluble glass. The rest of the steam produced is supplied via the line 21 to a second heat exchanger 22, namely a plate heat exchanger for bulk material, and this second heat exchanger preheats the supplied material, namely the mixture of sand and soda, to a temperature of approximately 125 C. The preheating of the supplied material permits a higher throughput in the melting furnace 3, into which the preheated supplied material is fed. After the heat is emitted, the steam flows into a condenser 23. The hot condensate is fed via a pump 24 into the solutizer 13, where it is used to save externally supplied 4-bar steam.

The proposed concept is very suitable, for example, for efficiently using the heat content of the melt in the production of glasses (cf. figures 2 and 3) . According to the invention, the heat content of the melt may be used to produce steam using a suitable tubular heat exchanger. All technical heat exchangers known to a person skilled in the art may be used as the heat exchanger in this case. In addition, the heat transfer can be improved by technical measures such as, for example, the fitting of a hood, blower etc. In addition, the heat transfer can be increased by optimizing the surface properties (e.g. color, coating, roughness) of the tubes or of the heat exchanger.

Given suitable transportation and operating conditions in a heat exchanger, 0.4 metric ton of 4-bar steam per metric ton of product per hour can be produced over the cooling belt. The heat exchanger comprises tube bundles with a hood (cf. figures 2 and 3) in order to increase the air velocity by means of a chimney effect. This structure results in the following advantages:
= The convective mass transfer is improved = The ambient temperature is reduced as a result of the hot air being removed = The occupational safety is increased as a result of the hot melt being encased.
Most of the steam produced from the first heat exchanger (hood 16) can be sold or used within the plant. At least some of the steam produced in the first heat exchanger (hood 16) is supplied, for example, to another suitable heat exchanger in order, according to the invention, to preheat the supplied material of sand and soda. The mixture is heated to approximately 125 C
in the second heat exchanger. In principle, all customary types of heat exchangers may be used here. In particular, plate heat exchangers, and most particularly vibrating heat exchangers, are suitable for preheating solids such as, for example, the sand or sand and soda used. Given suitable operating conditions, the moisture of the supplied material is irrelevant for the process.

The capacity of the furnace is increased and energy can be saved by means of the preheated supplied material.
The flow of steam, which leaves the second heat exchanger 22 as condensate, may be used for another process. If, according to the invention, the condensate from the second heat exchanger 22 is used directly for the dissolution process, the required energy consumption may accordingly be reduced.

However, the energy obtained can also be used differently for this or else any other desired process.
The abovementioned novel concept results in the following advantages for this application:

- producing 4-bar steam - batch preheating - saving 4-bar steam during the dissolution process - increasing the capacity of the furnaces - increasing the occupational safety and providing more acceptable work conditions - reducing the energy consumption for the dissolution process.

According to the invention, the cooling belt 11 is primarily cooled by means of the first heat exchanger 16. As previously, the rest of the cooling is carried out with water, which is sprayed from below onto the top side of the belt which is running back from top to bottom. The water which does not evaporate may likewise be used for the solutizer.

The hood 16 is designed in such a way that the tubes can be effectively cleaned from the outside.

List of reference symbols 1 Belt weigher 2 Mixing screw 3 Furnace 4 Burner oil/gas 5 Blower 6 Regenerative chamber 7 Waste gases 8 Regenerative chamber 9 Waste-gas cooler 10 Electrostatic filter 11 Cooling belt 12 Glass lumps 13 Solutizer 14 Liquid glass, aqueous 15 Molten soluble glass 16 Hood, first heat exchanger 17 Tubes 18 Pump 19 Condenser 20 Tube line 21 Tube line 22 Second heat exchanger, plate heat exchanger 23 Condenser 24 Pump 25 Valve

Claims (9)

1. A process for producing a product by melting a supplied material and letting it solidify, preferably a process for producing glasses, in particular soluble glass, in a furnace, preferably in a tank furnace, using heat recovery, characterized in that at least some of the heat emitted by the product produced, in particular when it solidifies, is used to preheat the supplied material.

2. The process as claimed in claim 1, characterized in that the glass produced emits its heat to a first heat exchanger (16), which passes the heat to a second heat exchanger (22), which preheats the supplied material.

3. The process as claimed in claim 2, characterized in that the first heat exchanger (16) is embodied as a hood (16), which is arranged around, and in particular above, a cooling belt (11) laden with the glass produced.

4. The process as claimed in claim 2 or 3, characterized in that the second heat exchanger (22) is embodied as a plate heat exchanger for bulk material.

5. The process as claimed in one of the preceding claims, characterized in that at least some of the heat emitted by the glass produced when it solidifies is used to supply heat when the solidified glass (solid glass) (12) is dissolved in water.

6. The process as claimed in claim 5, characterized in that water is used as the heat-transfer medium for taking up the heat emitted by the solid glass (12) produced and at least some of said heated heat-transfer medium is fed into the tank (solutizer) (13) used to dissolve the solid glass.

7. The process as claimed in claim 6, characterized in that the condensate emitted by the second heat exchanger (22) is fed into the solutizer (13).

8. A facility for producing glasses, in particular soluble glass, comprising a furnace, in particular a tank furnace (3), with a cooling device (11) for the still-molten glass produced, characterized in that a hood (16) embodied as a first heat exchanger is arranged around, and preferably only above, the cooling device (11), in that a second heat exchanger (22) is provided for preheating the supplied material and the second heat exchanger (22) is fed by the heated heat-transfer medium emitted by the first heat exchanger (16).

9. The facility as claimed in claim 8, characterized in that a tank (solutizer) (13) for dissolving the solid glass (12) in water is provided, and in that an inlet of the solutizer (13) is connected to the condensate outlet of the second heat exchanger (22).