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Cullet as a substitute for soda
414
Journal of Non-Crystalline Solids 84 (1986) 414-420 North-Holland, Amsterdam
CULLET AS A SUBSTITUTE FOR S O D A J.R. LOREDO, A. M A R T I N E Z and B. B E C E R R I L Gerencia de Materiales, Vitro Tec, PO Box 2867, Monterrey, NL, Mexico
This work reviews the main contributions to the understanding of the phenomena involved in the recyclabilityof glass. Emphasizing the expected benefits in the melting of glass batches and the avoided consumption of soda ash and energy, it is suggested that multi-variable, statistically designed experiments can be a tool for an overall assessment of variables and effects. A basic outline is proposed, as well as some specific areas for further research.
1. Introduction Two of the main factors for the final cost of glass articles are the raw materials, e.g. soda ash, and the energy required to melt them. Among the different alternatives for reducing cost, increased usage of cullet appears to offer both less energy and raw materials consumption. In this paper, a review is presented about the potential impact of increasing the percentage of cullet in the batch. Thermodynamic calculations result in linear relationships between cullet usage and the resultant decrease in energy and soda ash, but as some authors have shown, care should be taken when trying to extrapolate beyond 50-60% because a departure from linearity could be found, due to a series of factors that are not yet fully understood.
2. Purpose, environment and scope The specific reason to focus the attention on soda is that the cost of its introduction in the batch is the single one most important of all manufacturing costs. Coupled to this is the fact that the energy consumption per unit of mass for the production of soda is very similar to that of the glass itself. Hence, the subject is found to be important both for short-term concerns on manufacturing costs as well as long-term overall energy conservation strategic planning. The physics and chemistry oi some of the specific processes occurring in the melting of different levels of cullet/batch ratio have been covered only to a limited extent. Specific answers have been developed for many of the posed questions as for instance the size of the cullet particles and other important parameters [1,2]. More basic approaches [3-5] provide information about some of the evolution of the reactions and dissolution processes during melting, but our knowledge about the subject is far from complete. 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J.R. Loredo et aL / Cullet as substitute for soda
415
In this paper a review is presented about the highlights detected in the literature, concerning specifically the soda and energy consumption as dependent variables on the cullet/batch ratio. Additionally, a discussion and slight modifications are proposed for the calculus of some of the physical parameters involved. All of the above is used to propose a statistical design of an experiment to be conducted in order to generate a clearer picture of the energy and soda consumption as functions of the cullet/batch ratio. For the purposes of this paper, it has been assumed that cullet is available in unlimited quantities, and of a manageable chemical composition and particle size. 3. Cullet in the melting process It is now widely recognized that the melting process is modified by the use of cullet in different ways: (1) The consumption of raw materials per mass of produced glass is reduced, specially those of sand and sodium carbonate; (2) The energy usage per unit mass of glass produced is expected to decrease; (3) The melting time is modified; (4) The furnace temperature and its distribution are changed (or forced to change); and (5) The quality of the glass produced can be affected. The impact of the above-mentioned effects has been the subject of the publications of many authors (for example, see refs. [2,3]). Due to the obvious difficulty involved in modeling the melting of batches in industrial glass furnaces, alternative analysis of each one of the above effects has been conducted separately. Keeping in mind that it is generally agreed that as high as possible cullet ratios are desired, a complete assessment should include all of the possible effects for that as well as the proposed solutions to the potential problems encountered. It has been proposed that modifications to the design or operation of the furnaces [2] and raw materials handling systems are necessary in order to take full advantage of the potential benefits of high cullet ratios. Valuable and interesting findings and suggestions on the practicality of acquiring, processing and handling the cullet in the glass plants and furnaces are found in the literature, a presentation of the corresponding survey is postponed as part of an amplified version of the present work in order to go now directly to the subject matter. 4. Sodium contribution from cullet
A general equation to evaluate the savings in soda ash is: Total charge = batch + cullet + soda compensation,
(1)
where soda compensation means: a) soda to compensate the composition
J.R. Loredo et al. / Cullet as substitute for soda
416
difference between the final glass and cullet, and b) soda to compensate the N a 2 0 volatilization. Usually, in the glass plants routine analyses are applied to the cullet, so the calculation of the corresponding compensation is straightforward. Furthermore, empirical equations have been used for the management of the chemical reactions occurring as a result of difference in redox behavior between the foreign cullet used and the regular batch [2]. The soda compensation due to N a 2 0 volatilization is given by 1.71xS where x represent the losses due to volatilization per unit area in the mean residence time t and S the free surface of the melter and the refiner. Investigations about volatilization of alkali from glass remelting are reported in the literature. For instance. Felemer [6,7] found that if cullet was melted alone, alkali compensations were needed up to a level of about 3% for laboratory conditions. Salmang [8] finds that most of the losses of alkali occur when they are still not combined in the form of silicates; once it is accomplished, the losses are given by log
A -2x A
-
kt,
(2)
where A = initial concentration of alkali, and k = temperature-dependent parameter, according to the Arrhenius equation k = C e x p ( - B / T ) . Cable [9] considers that the convection currents in the melt invalidate the character of diffusion as the mechanism controlling the volatilization and recommends several equations to explain the kinetics of volatilization. However, the total soda compensation is not found significant in practice [10,18] and it seems reasonable to simplify for this purpose eq. (1) as Total charge = batch + cullet.
(3)
When the cullet is of the same composition as the desired glass, the actual need of soda could be established as follows: W = W0(1 - b/100),
(4)
where W0 is the soda amount to produce 1 kg from the plain batch. For a particular glass [11] with the following chemical analysis: 72% SiO 2, 11,2% CaO2, 2.1% MgO, 13.8% Na20, W0 = 0.232 kg (b = % cullet).
5. Saved energy as a function of cullet ratio
For the purpose of this work, it suffices to analyze the energy involved in the melting process in the way shown in fig. 1. If the energy saving is defined relative to the fuel consumption, it will be hidden by the fixed losses of heat. However, if we refer to savings as the energy not required, the value is independent of the furnace. Considering
J,R. Loredo et a L / Cullet as substitute for soda
FIXED
HEAT LOSSES
\
/
ENERGYSOURCE~ ( F U R N A C E )
/
417
HEATS OF REACTION TRANSFORMATION, MIXING AND DILUTION
---
G L A S S SENSIBLE HEAT
\
RECOVEREDHEAT
HEAT LOSSES DUE TO REACTION GASES
Fig. l. Energy inputs and outputsin a Nassfurnace.
Kri3ger's [12] approach to calculate the theoretical energy to melt a unit mass of glass it is possible to propose the next equation for energy savings: A H = [ HR25 + (1 -- ~ ) m p ~ p m A T ] • b / ] 0 0 ,
(5)
where HR2 5 = heat of transformation, mixing, dilution and reaction at 25°C (kJ/kg), and ~/= efficiency of the regenerator, m e = fraction of gases evolved from reactions (kg/kg), Cppm= average specific heat of the evolved gases, and AT = temperature of the gases before the regenerator, - 2 5 ° C . The main assumption of Kr~3ger's approach is that HR2 5 is evaluated at 25°C and under equilibrium conditions. It is now possible to know more about the temperature of the reactions due to published results of thermal analysis [13-15], but the total comprehension of the melting process is far from complete. So far, KriSger's approach is still a valid approximation as Cooper [16] mentioned. Using eq. (5) for the composition assumed by Badger and Lehr and assuming ~ = 0.45, A H = 867b/100
( k J / k g glass).
Trier et al. [1] obtained, for different conditions, and following a different procedure: A H = 951b/100
( k J / k g glass)
Equation 5 is, in general, not found to be valid in practice throughout the whole range. For instance, Moore [19] refers to experiences that showed that the benefits diminish beyond 50-70% of cullet, giving as some of the reasons for that the loss of the bubbling and stirring action inherent to the melting of the raw materials as well as their role of avoiding the bonding of the particles of cullet. Other industrial experiences [18] point out that a lower energy consumption was still met at the level of 80% cullet.
418
J.R. Loredo et al. / Cullet as substitute for soda
6. The melting time Former references [8] on melting time mentioned that the melting rate of soda-glass doubled with a 50°C increment in temperature. The melting rate is proportional to the surface of the quartz grains. The reactions of the fusion process require 10% of the total time and the rest is consumed diluting the quartz [8]. Manring and Conroy [17] conducted experiments varying the cullet ratio from 0 to 40%, finding that the melting time increased and that the way of distributing the cullet in the charge was determinant. Following Manring's classical results that showed the selectivity of soda to react with culler, the apparent solution was to distribute the cullet in a layer below a corresponding layer of batch, opposed to the heat source. A particular consideration was given to the formation of the metasilicate as a controlling phenomenon for the melting rate. The addition of cullet to the batch can then alterate the sequence of the reactions. On the other hand, a trial of the same authors in an industrial furnace showed the reverse (and normally expected) behaviour: the melting was accelerated by the addition of culler, in a non-linear fashion. A basic contribution to this subject is that of Hrma [3] who mentioned that the dissolution rate of silica grains during the decarbonization stage is much faster than in subsequent stages. The dissolution time of a grain of initial radius r0 is: t d = ( r 0 -- rl)//- e + rl/i" s, where r I = r(tl) radius at decarbonization time, /'e = dissolution rate during decarbonization time, and ?s = dissolution rate after decarbonization time. The silica dissolution process is considered as a diffusion controlled one, by Cable and Martlew [4,5] and the rate of dissolution of the silica grains in molten sodium carbonate is found to be slowed down following an exponential relationship with the original amount of silica in the batch. An "effective diffusivity" was derived in their work and is given by:
Do,.r = 3.269 × 109 e x p ( - 0 . 1 0 1 8 W ) e x p ( - 1 7 1 9 9 / ( T + 273)). The units of D are/~m2-s -1, and this parameter characterizes the diffusivity of silica dissolving in molten sodium silicate containing W% (weight basis) at a temperature T (°C). Even though these last results cannot be used directly in the present work, it is possible to anticipate that the time for the dissolution of a given grain of silica will be longer in a batch of remelted glass than in one of sodium carbonate. However, it is possible to reduce the melting time alternating the cullet and raw materials. 7. Discussion
A review of the references included in this work shows that the benefits of the use of high levels of cullet are not completely clear at this time. It appears
J.R. Loredo et al. / C u l l e t as substitute for soda
419
to be no question that there are savings in raw materials, but the evidence is confusing about energy. When operational problems are found while using high levels of cullet, additional modifications in the raw materials might be needed. However small, they should be considered at the level of industrial overall economic evaluations and at the fundamental level of the mechanisms of the reactions involved. It appears that both raw materials and energy should be included in new studies. It seems clear that the savings in soda are straightforward, and concern the operational level. However, the compensation for possible losses is not clear at the fundamental level, and more research seems to be needed in that area, although important contributions are and have been made on some of the mechanisms occurring during melting. More work, considering the cullet itself, might be of considerable help to clear the picture: the rate of dissolution of the silica, the sequence and rate of the reactions, as well as more specific subjects as the encapsulation of air by the pieces of cullet are some of the topics. The importance and necessity, as well as the way of approaching new basic research is being stressed by the questions posed at the industrial level. Appreciating the gap existing between the two levels of knowledge, it appears to the authors of this paper that a more complete and organized knowledge of the factors at the industrial level could be useful for two purposes: a) short cuts can be provided to asses and modify operations with
INPUT VARIABLES
OUTPUT
CULLET LEVEL
FURNACE TEMPERATURE
CULLET SIZE
CULLET ADITION WAY
SODA ASH CONSUMPTION ( OR RAW MATERIALS )
FINING
ENERGY SAVINGS
AGENT
GLASS WORKABILITY
ENERGY CONSUMPTION FEEDING SYSTEM
VARIABLES
GLASS BRITTLENESS
--~,
THERMAL EFFICIENCY
COLOUR
UTILIZATION FACTOR
BUBBLES BATCH FREE TIME
tt
t
CULLET COMPOSITION NOISE : CULLET
Fig. 2. Variables involved in the use of cullet.
CONTAMINATION
420
J.R. Loredo et al. / Culler as substitute for soda
high cullet ratios, a n d b) valuable i n f o r m a t i o n can be retrofitted to existing basic approaches, helping further to orient their objectives. According to the above considerations, what is proposed here is the c o n d u c t i o n of a statistically designed experiment. A b r o a d picture of such an experiment is f o u n d in fig. 2 where a list is proposed for the c o r r e s p o n d i n g variables, classifying them according to their assigned role i n the experiment. The reason for i n c l u d i n g additional variables besides cuUet ratio is that it is n o t expected that acceptable operating c o n d i t i o n s can be o b t a i n e d u n d e r all of the desired levels of e x p e r i m e n t a t i o n : some c o m p e n s a t i o n will have to be i n t r o d u c e d for potential adverse effects. Before designing the experiment, it will be necessary to establish the definite list of variables to be considered, as well as their levels in the experiment.
References [1] W. Trier, J. Lauter and L. Shumacher, The use of waste glass for melting of glass. Fachaussch. Ber. Dt. Glastech. Ges. (1978) 71. [2] T.J. Roberts, Proc. 45th Conf. on Glass Problems, Nov. 1984 (Ohio State Univ.) p. 190. [3] P. HRMA, J. Am. Ceram. Soc. 68 (1985) 337. [4] M. Cable and D. Martlew, Glass Technol. 25 (1984) 139. [5] M. Cable and D. Martlew, Glass Technol. 25 (1984) 270. [6] E. Felemer, Keram, Rundschau 34 (1926) 293. [7] E. Felemer, J. Soc Glass Technol. 11 [41] 7. [8] H. Salmang, Fundamentos fisico-quimicos de la fabrication del vidrio (Ed. Aguilar, Madrid, (1962) p. 113. [9] M. Cable, Principles of Glass Science and Technology, eds. D.R. Uhlmann and N.J. Kreidl, Vol. 2 (Academic Press, New York, 1980) p. 38. [10] M.J. Hilson, Proc. 45th Conf. on Glass Problems, Nov. 1984 (Ohio State Univ.) p. 179. [11] A.E. Badger and G.J. Lehr, Glass Ind. 49 (1968) 29. [12] C. Kr~3ger,Glastechn. Ber. 26 (1953) 202. [13] T.D. Taylor and K.C. Rowan, J. Amer. Ceram. Soc. 66 (1983) 277. [14] H.H. Russell III and W.R. Ott, Glass Technol. 21 (1980) 226. [15] J.A. Williams, Proc. 39th Ann. Conf. on Glass Problems, Nov. 1978 (Ohio State Univ.) p. 67. [16] A.R. Cooper, private communication. [17] W.H. Manring and A.R. Conroy, The Melting Process in the Glass Industry, ed. A.G. Pincus (Books for Industry, New York, 1980) p. 72. [18] F. Villarreal, private communication. [19] H. Moore, Glass Ind. 64 (1983) 14.
Journal of Non-Crystalline Solids 84 (1986) 414-420 North-Holland, Amsterdam
CULLET AS A SUBSTITUTE FOR S O D A J.R. LOREDO, A. M A R T I N E Z and B. B E C E R R I L Gerencia de Materiales, Vitro Tec, PO Box 2867, Monterrey, NL, Mexico
This work reviews the main contributions to the understanding of the phenomena involved in the recyclabilityof glass. Emphasizing the expected benefits in the melting of glass batches and the avoided consumption of soda ash and energy, it is suggested that multi-variable, statistically designed experiments can be a tool for an overall assessment of variables and effects. A basic outline is proposed, as well as some specific areas for further research.
1. Introduction Two of the main factors for the final cost of glass articles are the raw materials, e.g. soda ash, and the energy required to melt them. Among the different alternatives for reducing cost, increased usage of cullet appears to offer both less energy and raw materials consumption. In this paper, a review is presented about the potential impact of increasing the percentage of cullet in the batch. Thermodynamic calculations result in linear relationships between cullet usage and the resultant decrease in energy and soda ash, but as some authors have shown, care should be taken when trying to extrapolate beyond 50-60% because a departure from linearity could be found, due to a series of factors that are not yet fully understood.
2. Purpose, environment and scope The specific reason to focus the attention on soda is that the cost of its introduction in the batch is the single one most important of all manufacturing costs. Coupled to this is the fact that the energy consumption per unit of mass for the production of soda is very similar to that of the glass itself. Hence, the subject is found to be important both for short-term concerns on manufacturing costs as well as long-term overall energy conservation strategic planning. The physics and chemistry oi some of the specific processes occurring in the melting of different levels of cullet/batch ratio have been covered only to a limited extent. Specific answers have been developed for many of the posed questions as for instance the size of the cullet particles and other important parameters [1,2]. More basic approaches [3-5] provide information about some of the evolution of the reactions and dissolution processes during melting, but our knowledge about the subject is far from complete. 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J.R. Loredo et aL / Cullet as substitute for soda
415
In this paper a review is presented about the highlights detected in the literature, concerning specifically the soda and energy consumption as dependent variables on the cullet/batch ratio. Additionally, a discussion and slight modifications are proposed for the calculus of some of the physical parameters involved. All of the above is used to propose a statistical design of an experiment to be conducted in order to generate a clearer picture of the energy and soda consumption as functions of the cullet/batch ratio. For the purposes of this paper, it has been assumed that cullet is available in unlimited quantities, and of a manageable chemical composition and particle size. 3. Cullet in the melting process It is now widely recognized that the melting process is modified by the use of cullet in different ways: (1) The consumption of raw materials per mass of produced glass is reduced, specially those of sand and sodium carbonate; (2) The energy usage per unit mass of glass produced is expected to decrease; (3) The melting time is modified; (4) The furnace temperature and its distribution are changed (or forced to change); and (5) The quality of the glass produced can be affected. The impact of the above-mentioned effects has been the subject of the publications of many authors (for example, see refs. [2,3]). Due to the obvious difficulty involved in modeling the melting of batches in industrial glass furnaces, alternative analysis of each one of the above effects has been conducted separately. Keeping in mind that it is generally agreed that as high as possible cullet ratios are desired, a complete assessment should include all of the possible effects for that as well as the proposed solutions to the potential problems encountered. It has been proposed that modifications to the design or operation of the furnaces [2] and raw materials handling systems are necessary in order to take full advantage of the potential benefits of high cullet ratios. Valuable and interesting findings and suggestions on the practicality of acquiring, processing and handling the cullet in the glass plants and furnaces are found in the literature, a presentation of the corresponding survey is postponed as part of an amplified version of the present work in order to go now directly to the subject matter. 4. Sodium contribution from cullet
A general equation to evaluate the savings in soda ash is: Total charge = batch + cullet + soda compensation,
(1)
where soda compensation means: a) soda to compensate the composition
J.R. Loredo et al. / Cullet as substitute for soda
416
difference between the final glass and cullet, and b) soda to compensate the N a 2 0 volatilization. Usually, in the glass plants routine analyses are applied to the cullet, so the calculation of the corresponding compensation is straightforward. Furthermore, empirical equations have been used for the management of the chemical reactions occurring as a result of difference in redox behavior between the foreign cullet used and the regular batch [2]. The soda compensation due to N a 2 0 volatilization is given by 1.71xS where x represent the losses due to volatilization per unit area in the mean residence time t and S the free surface of the melter and the refiner. Investigations about volatilization of alkali from glass remelting are reported in the literature. For instance. Felemer [6,7] found that if cullet was melted alone, alkali compensations were needed up to a level of about 3% for laboratory conditions. Salmang [8] finds that most of the losses of alkali occur when they are still not combined in the form of silicates; once it is accomplished, the losses are given by log
A -2x A
-
kt,
(2)
where A = initial concentration of alkali, and k = temperature-dependent parameter, according to the Arrhenius equation k = C e x p ( - B / T ) . Cable [9] considers that the convection currents in the melt invalidate the character of diffusion as the mechanism controlling the volatilization and recommends several equations to explain the kinetics of volatilization. However, the total soda compensation is not found significant in practice [10,18] and it seems reasonable to simplify for this purpose eq. (1) as Total charge = batch + cullet.
(3)
When the cullet is of the same composition as the desired glass, the actual need of soda could be established as follows: W = W0(1 - b/100),
(4)
where W0 is the soda amount to produce 1 kg from the plain batch. For a particular glass [11] with the following chemical analysis: 72% SiO 2, 11,2% CaO2, 2.1% MgO, 13.8% Na20, W0 = 0.232 kg (b = % cullet).
5. Saved energy as a function of cullet ratio
For the purpose of this work, it suffices to analyze the energy involved in the melting process in the way shown in fig. 1. If the energy saving is defined relative to the fuel consumption, it will be hidden by the fixed losses of heat. However, if we refer to savings as the energy not required, the value is independent of the furnace. Considering
J,R. Loredo et a L / Cullet as substitute for soda
FIXED
HEAT LOSSES
\
/
ENERGYSOURCE~ ( F U R N A C E )
/
417
HEATS OF REACTION TRANSFORMATION, MIXING AND DILUTION
---
G L A S S SENSIBLE HEAT
\
RECOVEREDHEAT
HEAT LOSSES DUE TO REACTION GASES
Fig. l. Energy inputs and outputsin a Nassfurnace.
Kri3ger's [12] approach to calculate the theoretical energy to melt a unit mass of glass it is possible to propose the next equation for energy savings: A H = [ HR25 + (1 -- ~ ) m p ~ p m A T ] • b / ] 0 0 ,
(5)
where HR2 5 = heat of transformation, mixing, dilution and reaction at 25°C (kJ/kg), and ~/= efficiency of the regenerator, m e = fraction of gases evolved from reactions (kg/kg), Cppm= average specific heat of the evolved gases, and AT = temperature of the gases before the regenerator, - 2 5 ° C . The main assumption of Kr~3ger's approach is that HR2 5 is evaluated at 25°C and under equilibrium conditions. It is now possible to know more about the temperature of the reactions due to published results of thermal analysis [13-15], but the total comprehension of the melting process is far from complete. So far, KriSger's approach is still a valid approximation as Cooper [16] mentioned. Using eq. (5) for the composition assumed by Badger and Lehr and assuming ~ = 0.45, A H = 867b/100
( k J / k g glass).
Trier et al. [1] obtained, for different conditions, and following a different procedure: A H = 951b/100
( k J / k g glass)
Equation 5 is, in general, not found to be valid in practice throughout the whole range. For instance, Moore [19] refers to experiences that showed that the benefits diminish beyond 50-70% of cullet, giving as some of the reasons for that the loss of the bubbling and stirring action inherent to the melting of the raw materials as well as their role of avoiding the bonding of the particles of cullet. Other industrial experiences [18] point out that a lower energy consumption was still met at the level of 80% cullet.
418
J.R. Loredo et al. / Cullet as substitute for soda
6. The melting time Former references [8] on melting time mentioned that the melting rate of soda-glass doubled with a 50°C increment in temperature. The melting rate is proportional to the surface of the quartz grains. The reactions of the fusion process require 10% of the total time and the rest is consumed diluting the quartz [8]. Manring and Conroy [17] conducted experiments varying the cullet ratio from 0 to 40%, finding that the melting time increased and that the way of distributing the cullet in the charge was determinant. Following Manring's classical results that showed the selectivity of soda to react with culler, the apparent solution was to distribute the cullet in a layer below a corresponding layer of batch, opposed to the heat source. A particular consideration was given to the formation of the metasilicate as a controlling phenomenon for the melting rate. The addition of cullet to the batch can then alterate the sequence of the reactions. On the other hand, a trial of the same authors in an industrial furnace showed the reverse (and normally expected) behaviour: the melting was accelerated by the addition of culler, in a non-linear fashion. A basic contribution to this subject is that of Hrma [3] who mentioned that the dissolution rate of silica grains during the decarbonization stage is much faster than in subsequent stages. The dissolution time of a grain of initial radius r0 is: t d = ( r 0 -- rl)//- e + rl/i" s, where r I = r(tl) radius at decarbonization time, /'e = dissolution rate during decarbonization time, and ?s = dissolution rate after decarbonization time. The silica dissolution process is considered as a diffusion controlled one, by Cable and Martlew [4,5] and the rate of dissolution of the silica grains in molten sodium carbonate is found to be slowed down following an exponential relationship with the original amount of silica in the batch. An "effective diffusivity" was derived in their work and is given by:
Do,.r = 3.269 × 109 e x p ( - 0 . 1 0 1 8 W ) e x p ( - 1 7 1 9 9 / ( T + 273)). The units of D are/~m2-s -1, and this parameter characterizes the diffusivity of silica dissolving in molten sodium silicate containing W% (weight basis) at a temperature T (°C). Even though these last results cannot be used directly in the present work, it is possible to anticipate that the time for the dissolution of a given grain of silica will be longer in a batch of remelted glass than in one of sodium carbonate. However, it is possible to reduce the melting time alternating the cullet and raw materials. 7. Discussion
A review of the references included in this work shows that the benefits of the use of high levels of cullet are not completely clear at this time. It appears
J.R. Loredo et al. / C u l l e t as substitute for soda
419
to be no question that there are savings in raw materials, but the evidence is confusing about energy. When operational problems are found while using high levels of cullet, additional modifications in the raw materials might be needed. However small, they should be considered at the level of industrial overall economic evaluations and at the fundamental level of the mechanisms of the reactions involved. It appears that both raw materials and energy should be included in new studies. It seems clear that the savings in soda are straightforward, and concern the operational level. However, the compensation for possible losses is not clear at the fundamental level, and more research seems to be needed in that area, although important contributions are and have been made on some of the mechanisms occurring during melting. More work, considering the cullet itself, might be of considerable help to clear the picture: the rate of dissolution of the silica, the sequence and rate of the reactions, as well as more specific subjects as the encapsulation of air by the pieces of cullet are some of the topics. The importance and necessity, as well as the way of approaching new basic research is being stressed by the questions posed at the industrial level. Appreciating the gap existing between the two levels of knowledge, it appears to the authors of this paper that a more complete and organized knowledge of the factors at the industrial level could be useful for two purposes: a) short cuts can be provided to asses and modify operations with
INPUT VARIABLES
OUTPUT
CULLET LEVEL
FURNACE TEMPERATURE
CULLET SIZE
CULLET ADITION WAY
SODA ASH CONSUMPTION ( OR RAW MATERIALS )
FINING
ENERGY SAVINGS
AGENT
GLASS WORKABILITY
ENERGY CONSUMPTION FEEDING SYSTEM
VARIABLES
GLASS BRITTLENESS
--~,
THERMAL EFFICIENCY
COLOUR
UTILIZATION FACTOR
BUBBLES BATCH FREE TIME
tt
t
CULLET COMPOSITION NOISE : CULLET
Fig. 2. Variables involved in the use of cullet.
CONTAMINATION
420
J.R. Loredo et al. / Culler as substitute for soda
high cullet ratios, a n d b) valuable i n f o r m a t i o n can be retrofitted to existing basic approaches, helping further to orient their objectives. According to the above considerations, what is proposed here is the c o n d u c t i o n of a statistically designed experiment. A b r o a d picture of such an experiment is f o u n d in fig. 2 where a list is proposed for the c o r r e s p o n d i n g variables, classifying them according to their assigned role i n the experiment. The reason for i n c l u d i n g additional variables besides cuUet ratio is that it is n o t expected that acceptable operating c o n d i t i o n s can be o b t a i n e d u n d e r all of the desired levels of e x p e r i m e n t a t i o n : some c o m p e n s a t i o n will have to be i n t r o d u c e d for potential adverse effects. Before designing the experiment, it will be necessary to establish the definite list of variables to be considered, as well as their levels in the experiment.
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