Energy Savings in Batch Melting

Stanislav Kasa, František Novotný Institute of Chemical Technology in Prague, Department of Glass and

Technology, the Czech Republic

The paper deals with the effect of the reaction path leading from the raw materials to the intermediate products on the character of the technological process. It is the specific mechanism of the chemical reactions occurring at this stage that decides whether the melting will create conditions for the required manner of batch transformation into the melt or whether uncalled-for structures causing production problems and reducing the efficiency of the glass melting process will arise. The paper describes some of such uncalled-for structures that may occur in the batch in result of an unsuitable process control and assesses their technological importance. The possibility of preventing them from arising is also discussed. The conclusions are corroborated by published data about the energy savings achieved by conducting correctly the processes of batch transformation into the glass melt. Introduction

The fact that this paper on energy savings is conceived as a contribution dealing with the preliminary reactions of raw materials may be quite surprising. The first thought coming to everybody’s mind would namely be: Doesn’t the pre-reacting mean that we have to heat the raw materials twice? Some chemical reactions take place during the first heating but, then, the material is left to cool down to be reheated again during the melting process. Therefore, how the energy would be saved? Because the energy consumption should be larger and not smaller!? The aim of the present paper is to disprove this objection and to refute the doubts about the correctness of this approach. The paper should demonstrate that the use of the energy in processes prior to the glass melting itself, ie even for the repeated heating of raw materials will pay off not only as regards the very glass melting process but it can also result in substantial benefits as energy savings. However, only a proper, correct way of raw material pre-reacting or any other suitable methods of their treatment would be accompanied by energy savings. Energy consumption in the individual stages of the heat-treatment process

The energy consumption in the glass melting process is distributed in a quite non-uniform way. Let’s begin with the energy distribution in time and space. Of course, the largest amount of energy is consumed in the first stage of glass melting. According to Beerkens [8] about 75 to 90 percent of the total amount of heat needed for melting is absorbed by the batch during the first 45 to 60 minutes, which is about 5 % of the total time required for melting. However, we should be aware of the fact that the molten glass is maintained at a high temperature for 24 … 60 hours, but only 5 % of this period of time involves the heat transfer. Therefore, the heat transfer to the batch represents the decisive process that controls the glass melting rate. Within 15 … 25 minutes the batch is transformed into a silicate melt with individual non-dissolved grains of sand or other minerals. The dissolution of these grains does not

require much heat any more. Then, the glass melt must be heated a little to allow for the decomposition of the fining agents but the amount of energy needed for this process is not of great importance from the viewpoint of the total consumption of energy. Therefore, the glass melting process lasts for too long a time from the viewpoint of the heat input and it is quite disproportionate to the amount of heat that must be introduced into the process. Even the space occupied by melting is not proportionate to the reactions taking place there. The heat is consumed in a rather small area between the glass melt and the batch. The heat generated by the combustion gases or the heating electrodes must be conveyed to this reaction zone, which is always a difficult process accompanied by substantial losses. Therefore, the above mentioned 5 % of time corresponding to the stage of the primary melting decide about the final state of the molten glass, which is in agreement with the data published by Beerkens [8]. Hence, the first stage of the batch melting is very important from the viewpoint of the possible formation of defects as it can affect a variety of phenomena associated with the batch melting in furnaces. Reaction diagram. Problems associated with the formation of intermediate products. Structure formation.

Chemical reactions affecting technological parameters will now be discussed by using an example of a batch composed of sand, soda ash and limestone. There is a great number of possible reactions because any of the above raw materials can react with any other raw materials as well as with the intermediate products; furthermore, the intermediate products themselves may react mutually. Some reactions leading to a problem-free formation of the melt are desirable but those giving rise – even temporarily – to some specific structures are not. Before we start dealing with the formation of such structures we should say a few words about the reactions among the individual raw materials and about the technological properties of intermediate products resulting from such reactions. Reaction of the soda ash with silica sand

The reaction of sand with soda ash represents the basic reaction taking place during the glass melting process. This quite complicated reaction is accompanied by the formation of complex structures. The physical properties of sodium silicates occurring during this reaction are the main reason of the complexity. Because of the properties of such silicates the reaction does not run in a simple manner but complex structures of the intermediate products are formed in dependence on the process conditions. Such structures may affect the melting process in a very substantial way.

In the beginning, a thin layer of solid metasilicate (Na2O.SiO2) is coating sand grains when the reaction of sand with soda ash is initiated. Metasilicate forms a solid shell on the sand grain. This chemical compound is characterized by a high melting temperature (1 088 oC) and the subequent reactions are slowed down by the diffusion across this layer. Manring [6] published the findings he obtained by observing this reaction in a microscope. The sand grains subjected to a temperature of 750 oC at a heating rate of 10 oC/minute were covered with a bright coating that was shifting on the grain surface and looked like humidity. He assumed that this was

a low-melting eutectics of a disilicate and silicon oxide. This coating was then transformed into metasilicate under the action of soda ash. Molten soda ash spread very quickly over the whole surface of the sand grains and reacted readily by giving rise to metasilicate. The grains coated by metasilicate remained apparently without any change for some time but, then, their surface liquefied suddenly when the metasilicate layer reacted through from inside. As soon as such a “moist” grain came in contact with a grain of soda ash, it liquefied instantaneously and the liquid spread over the surface of the sand grain. The metasilicate crust was formed once more and the process was repeated until the temperature of 1 070 oC was reached when metasilicate melted down and the cycle was stopped.

In a similar experiment, Cable [3] observed a quartz rod immersed into molten soda ash; also in this case metasilicate formed a layered structure on the rod.

The picture (Fig. 1) of the layer formed in the described way and the sketch (Fig. 2) of the typical shape of the layer contribute well to the explanation of the above mentioned character of the reaction, ie the primarily formed metasilicate SiO2 + Na2CO3 = Na2SiO3 + CO2 has a high melting temperature whereas the disilicate formed subsequently is characterized by a low melting temperature Na2SiO3 + SiO2 = Na2Si2O5 . Therefore, intermediate silicate layers are formed during the reaction of soda with quartz according to a diagram published by Vidal [4].

Fig. 1 A part of a white deposit formed in result of the exposure of a quartz rod to

soda ash melt at a temperature of 950 oC. The picture shows a step-like structure of the underlying surface as well as the orientation of the crystals in the region of the

deposit (the crystals are arranged in the direction towards the quartz rod) [3]

Fig. 4 Phase diagram of the Na2O-SiO2 system [6]

Fig. 2 Diagram representing the structure of the solid deposit and showing the overall conical shape

with the step-like underlying surface [3]

Fig. 3 As the diffusion rate of Na2O is faster than that of SiO2 , first Na2SiO3 is formed as the main product (B). The modification transformation of

quartz at 575 oC activates the SiO2 surface thus favoring the reaction between SiO2 and Na2SiO3 that yields Na2Si2O5 (C). The disilicate structure changes at a temperature of about 678 oC (for

instance, at 707 oC), an intense reaction with soda ash takes place (D) and the formation rate of

metasilicate increases [4].

The reaction has a step-like character as shown by Manring [6] with a zigzag line in the phase diagram. This character is explained by the alternating mutual transformation of metasilicate into disilicate and vice versa.

The above reaction mechanism is of paramount technological importance. If the quartz surface is small (coarse-grained silica sand) as compared to the amount of the present soda ash, the formation rate of metasilicate on the grain surface is fast and the grains are coated permanently with a hard-melting layer . If, on the contrary, the quartz surface is large (fine-grained silica sand), the quick reaction results in the consumption of the soda ash and, consequently, in the transformation of metasilicate into molten disilicate; eventually the silica grains will stick together. A relatively mobile molten interlayer composed of metasilicate and quartz arises and – because of the incomplete dissolution of easily wettable metasilicate crystals – a white transitory solid melt is formed which is spreading very slowly. The inhomogeneous batch characterized by the presence of many silica grains one close to another reacts eventually without giving rise to metasilicate and the supply of alkalis is controlled only by diffusion. This process results in a quick formation of a seedy melt with a high silicate content with sand grains being “embedded” mutually. In such a way, an uncalled-for structure is obtained that brings about operational problems. Reaction of soda ash with limestone

Another reaction that can give rise to structures causing problems during the glass melting is the reaction between soda ash and limestone:

Na2CO3 + CaCO3 = Na2Ca(CO3)2 . The reaction of soda ash with limestone gives rise to a melt of double

carbonate Na2Ca(CO3)2 that melts easily at a temperature of 813 oC. The viscosity of this ionic carbonate melt (log η = 2) is lower by several orders of magnitude than that of simple silicates (log η = 5 to 6). Therefore, it can flow very readily and the flotation of sand grains in this type of melt is easy so that the segregation can take place thus giving rise to large regions containing only quartz grains or their aggregates. These conditions result in a substantial slowdown of the glass melting process because the dissolution of silica grains in the melt is controlled by diffusion and the duration of this process is proportional to the second power of the diffusion path. The deceleration of the melting process is considerable because segregated regions having the size of several centimeters are thus formed. Reaction of limestone with silica grains

The reaction of limestone with sand grains gives rise to hard-melting minerals the homogenization of which with the surrounding glass melt is very difficult. Some of these minerals do not melt completely under the conditions existing in the glass melting furnace – their dissolution in the surrounding melt is gradual. The surrounding melt does not possess a sufficient ability to dissolve them if the present sodium oxide had enough time to reach a sufficient degree of dilution in the dissolved silicon oxide, and the glass melt may remain inhomogeneous. An alleged influence of this phenomenon on the spontaneous cracking of glass products was described by Novotny [2]. The inhomogeneous region of glass melt formed in such a way can be characterized as an “uncalled-for structure”.

Fig. 7 The crust with batch residues

Crust occurrence in the batch Of course, all the above structures may occur even in the batch. It is important

to pay attention to the crust which forms on the glass bath in the all-electric tank furnace below the floating batch. The following pictures show a crust observed in a commercial furnace.

The top surface of the crust was in contact with the loose batch. Therefore, a part of the batch adhered to the crust can be removed by shaking it off so that only a small amount of baked-on batch sticks to the crust surface. The baked-on batch falls off the crust after washing in water and a relatively smooth continuous surface can be obtained.

The bottom surface of the crust is arched because there was a gas pocket below the crust in that place causing its deformation.

There were also a few cullet fragments adhered to the crust. The cullet pieces

fused with the crust and they obviously participated in the heat transfer to the batch because of a substantial extent of baked-on batch to the cullet surface. The detailed view of a large piece of cullet shows how the heat penetrated through the cullet into the batch and how the crust was arching in the zone close to the cullet.

Substantial differences between the top surface of the crust and the bottom one are revealed by observing the crust in a microscope.

Fig. 8 A detailed picture showing a piece of cullet on the crust

Fig. 6 Bottom surface of the crust Fig. 5 Top surface of the crust

Fig. 9 Top surface of the crust

Fig. 10 Bottom surface of the crust Because of its contact with the loose batch the top surface of the crust is

formed by large grains bonded by melt. The bottom surface was in contact with molten glass and it is covered by very fine seeds having the size of 100 micrometers. These seeds disappear in the proximity of large bubbles (r = 0.25 µm). The transition

between the molten glass and the crust is very sharp, which is demonstrated by the seed arrangement in one line. Fine seeds obviously form directly on this boundary and are absorbed gradually by large bubbles. The large bubbles can travel along the bottom surface of the crust so that they shift their places and coalesce until they concentrate in a place where they give rise to a gas escape chimney. Suppression of the formation of uncalled-for structures

All the above mentioned structures are undesirable because they affect adversely the glass melting process. Therefore, there is the tendency to suppress them. In total, six different ways to suppress the formation of uncalled-for structures are surveyed below.

a) The first method of the above approach is to ensure that the reactions among all the above mentioned raw materials occurred simultaneously in such a way that any of the above described individual (partial) reactions did not take place before the remaining two reactions. This goal can be achieved by selecting correctly the grain size of raw materials. Fine grains of the respective raw materials are consumed quickly if one of the basic reactions is too fast, and the process slows down.

b) The method used for charging the batch into the glass melting furnace represents another factor that can prevent the uncalled-for structures from forming. If the batch blanket is too thin, it heats up quickly and the low-viscosity melt does not have any place to flow; a thick layer of foam can not form either.

c) A simultaneous occurrence of all three basic reactions can also be achieved if the raw materials are pelletized or briquetted . In such a way every pellet represents a separate, self-enclosed unit in which the raw materials react mutually whereas the reaction with another unit is limited. Of course, this process can take place only until the escaping carbon dioxide degrades the system.

d) The so-called “selective batching” represents a very interesting method. In this case, two types of pellets are prepared: A pellets with all soda ash and a stoichiometric quantity of sand and B pellets with all limestone and a stoichiometric quantity of sand. The remaining quantity of sand is introduced into the batch in the loose form (un-pelletized). As the batch is heated, first the reactions in the pellets will take place while the rest of the material does not react. As soon as the decarbonization reactions are finished, the A pellets melt down and the melt formed in such a way begins to react preferentially and quickly with the product of B pellets. Eventually, a silicate melt with incompletely solved sand grains is obtained. This melt does not give rise to any uncalled-for structures any more.

e) The control of the content of individual grainsize fractions of silica grains is based on the similar approach as the “selective batching”. The sand is supplied in two very different grainsize fractions – coarse-grained and fine-grained. The content of the fine-grained fraction is just enough to allow for the reaction of the present soda ash to sodium metasilicate. This reaction is very quick because of the large surface of fine-grained sand. Metasilicate formed in this way is not in contact with any sub-surface SiO2 layers because the fine-

grained sand grains reacted to the full; therefore, the further reaction is practically stopped. As the temperature continues further to increase, sodium oxide remains fixed (bonded) in the metasilicate so that only the limestone decarbonization can take place. The sodium metasilicate melts down at a temperature of 1088 oC, the melt spreads down and reacts intensely with the present calcium oxide. Similarly as mentioned in the previous case, a decarbonated viscous melt is obtained; this melts surrounds incompletely solved sand grains which are then slowly transformed into molten glass. In contrast to the a) case aimed at ensuring the simultaneous occurrence of all the reactions by controlling the grainsize, the goal pursued in this case is, on the contrary, the separation of the reactions into separate time periods.

f) There is also another method that can be applied: man-made silicates prepared usually by solid-state reactions in a separate process [7] are used as starting raw materials. It is true that the material must be heated twice but only about 17 % of sand is heated to a relatively low temperature at a high efficiency. Waste heat can be used for this purpose so that this process is not as disadvantageous as it may seem. The main benefit is the possibility to control perfectly the whole process and to prevent the formation of uncalled-for structures. It is difficult to assess the heat savings obtained by adopting the above

measures. More data would be needed for such an assessment. Quantitative data published by W.M. Carty et al. [5] may be used for the comparison of the laboratory melting processes carried out with treated and untreated batches. The reduction in the melting time is substantial indeed (Fig. 11).

The above authors estimate at 15 to 30 % the energy savings that could be achieved by using pretreated batch.

The assumed heat saving should be regarded with caution even if it is quite understandable that any investment into the “intelligent process control” will pay off. A tendency towards the specialization of various compartments of the glass melting furnace can already be observed in the furnace design for some time now. There is a transition from the furnaces with just one compartment to those in which every part of the process takes place separately. A similar tendency can now also be observed in the chemistry of the reactions taking place during the glass melting process. The traditional process based on mixing all raw materials together, feeding the batch into the furnace and melting it there would preferentially be replaced by a selective implementation of the individual steps. These new challenges should be taken in consideration seriously by the glass industry and more attention should be paid to new approaches to the glass melting process.

Fig. 11 Reduction in the melting time [5] References [1] Novotny F., Kasa S.: Processes occurring during the batch melting in all-

electric furnaces. Sklar a keramik 56 (2006), No. 3-4, p. 41-48 [2] Novotny F.: Batch sintering. Conference “Heat saving in the glass industry”,

CSS, Prague, May 2006 [3] Cable M., Martlew D.: Formation of solid reaction products in the dissolution of

silica in molten sodium carbonate. Glass Technology 25 (1984), No. 1, p. 24-30

[4] Vidal A.: Research and cinematographic investigation into the primary reactions of batch components by means of a high-temperature hot-stage microscope. Glastechn. Ber. 36 (1963), No. 8, p. 305-323

[5] Carty W.M. et al.: Selective batching for improved commercial glassmelting. American Ceramic Society Bulletin 83 (2004), No. 10, p. 28-32

[6] Manring W.H., Bauer W.C.: Influence of batch preparation process on the melting and fining of glass. Glass Industry 45 (1964), No. 7, p. 354, No. 8, p. 413

[7] Montoya B.G. et al.: Alternative batch compositions in the glass-forming region of the Na2O-CaO-SiO2 system. Journal of Non-crystalline Solids 329 (2003), p. 22-26

[8] Beerkens R.: Modular melting (Industrial glassmelting process analysis). American Ceramic Society Bulletin 83 (2004), No. 7, p. 35-38

[9] Pita-Czesniewski A.: EP 1 547 982 A2

Assoc. Prof., Ing. Stanislav Kasa, CSc., ICT Prague, Department of Glass and Technology, Technická 5, 166 28 Prague 6, the Czech Republic, e-mail: stanislav.kasa@vscht.cz Ing. František Novotný, CSc. ICT Prague, Department of Glass and Technology, Technická 5, 166 28 Prague 6, the Czech Republic, e-mail: frantisek.novotny@vscht.cz