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A practical approach to evaluating redox phenomena involved in the melting-fining of soda-lime glasses
Journal of N o n - C r y s t a l l i n e Solids 38 & 39 © North-Holland Publishing Company
(1980)
813-818
A P R A C T I C A L A P P R O A C H TO E V A L U A T I N G REDOX P H E N O M E N A INVOLVED IN THE M E L T I N G - F I N I N G OF SODA-LIME (;LASSES W.H. M a n r i n g & G.M. Diken FMC C o r p o r a t i o n Industrial Chemical Group Princeton, New Jersey
U,S.A.
Increased furnace efficiency through increased furnace production or pull rate can be realized with o p t i m i z a t i o n and control of ti~e redox conditions during the m e l t i n g and fining of glass. An empirical model and related redox factors, n e c e s s a r y for o p t i m i z a t i o n of the m e l t i n g / f i n i n g p r o c e s s e s in sulfur-sulfate refined glasse~, are described. In addition to examining the redox conditions necessary to produce very stable reduced flint and amber glasses, discussed is a new fining redox number which relates the glass redox conditions to the observed refining ability of the glass. Hot stage p h o t o g r a p h y and w a t e r cooled p e r i s c o p e films taken in p r o d u c t i o n furnaces illustrate the importance of SO 2 e v o l u t i o n and control in the glass m e l t i n g and fining process.
INTRODUCTION Much has been w r i t t e n about the influence of furnace a t m o s p h e r e on tile melting/ fining rate of soda-lime glasses. A d v a n t a g e s which have been realized with increased control of glass m e l t i n g and fining include reductions in m e l t i n g temperature, p o l l u t i o n emissions, and seed count in addition co increases in production throughput and r e f r a c t o r y life. The intent of this paper is to outline an empirical approach to q u a n t i t a t i v e l y evaluate and to s u b s e q u e n t l y control the batch and glass redox conditions of reduced flint fining systems in terms ut th~ s u l f a t e / s u l f u r dioxide e q u i l i b r i u m during the m e l t i n g / f i n i n g process. Since a complete detailed d i s c u s s i o n of this approach would be ]engtby and has been documented, in part, (1,2) only a cursory treatment illustrated by several examples will be provided. One of the most important p a r a m e t e r s in c o n t r o l l i n g glass redox or glass fining ability is the sulfur dioxide (S0o) content present in the furnace atmosphere.. Shaw and Jones (3) showed "the presence of S0o in the atmosphere signit ieantly improves the fining of all glasses" provided [ certain m i n i m u m concentr~iti~ll is present. Similar results were obtained from hot stage m i c r o s c o p e studies coodurted at FHC as will be shown later in a f~]m presentation. Small amounts e| S(]~ were shown to have a remarkable influence on the surface tension of glass res~lting in increased fining rates and in sore{~ cases reduced surface _~oaming. These results have ~l]so been confirmed in full scale glass iurn;ice operations. Sulfates enhance tile m e l t i n g and fining of glass because of the dual sulriactnot and int<,rfacial turbulence effects they provide during [he m e l t i n g operatios, especially in the presence of reducing agents as listed in Table I. Their eff
813
81~
W.H. Manring,
TABLE i
-
E~IRICAL
G.M. Diken / Melting-Fining
OXIDATION-REDUCTION
1 ib Salt Cake (Na~SO.) 1 ib Gypsum (CaSO~2H%0) 1 ib Barytes (BaSOA) ~ 1 ib Nitre (NAN03)1% H_O in Batch Cull~t Caustic Soda (NaOH) Air/Fuel Ratio *
= = = = = = = =
of Soda-Lime Glasses
FACTORS*
+i.0 +0.9 +0.6 +3.0 +4.0 ? ? ?
1 ib 1 Ib iib i ib i ib i ib i ib i ib
Revised 1979 value ss, original reference
Pure Carbon Ground Coal Sulfur Iron Pyrite (FeSp) Flourspar (CaF2)Salt (NaCI) Ferrous Oxide (FeO) Arsenic __(As203)
= = = = = = = =
-23.7 -16.0 -13.3 - 6.5 - 1.8 - 1.0 - 1.0 ?
(I),
in the presence of metal forms chromophores. The high diffusion rate of these decomposition products across the particle interfaces disrupt the interfacial surface tension releasing considerable energy which results in a vigorous stirring of the melt. It is the development of insoluble sulfur phases which is necessary for optimization of the glass fining process in reduced flint glasses. i.
SULFUR SOLUBILITY AND GLASS REDOX
Past studies by Budd (5), who investigated the relationship between sulfur solubility and the oxidation state of glass, showed that the solubility of sulfur in glass decreases as the glass becomes more reduced until highly reduced glasses are formed, at which point the sulfur solubility rapidly increases with the formation of amber chromophores. Budd's solubility data are replotted in Figure 1 on a linear scale as a function of glass redox number, which represents the chemical oxygen demand (COD value) of the glass batch as modified by oxidizing and reducing potential of the glass batch constituents listed in Table i. Glass compositions to the left in Figure I, having lower negative values are considered oxidized systems~while those toward the right are more reduced. The optimum fining region for flint glasses occurs over the range of glass redox numbers -20 to -50 where the glasses have low sulfur content. The reduced sulfur solubility permits more rapid expulsion of the SOp, thereby increasing fining rate and reducing the potential of reboil. Althoug~ reduced flint fining systems are preferable, care must be taken not to form highly reduced flints, since this could result in generation of amber streaks and~in extremely reducing conditions~development of blisters and bublos. A review of the sulfur compounds and fining aids which can be used for fining soda-lime glasses is described in a prior publication by Manring, et al (6).
SULFUR SOLUBILITY VS CALCULATED OXIDATION STATE OF GLASS
FIGURE 1
I
0.30
~
I OPTIMUM
/
C 0.20
0.I0
0.00
)XIDIZED FLINT SO 3 ,
-i0
REDUCED FLINT
~ I
I -20
STABLE AMBER
SO 2 ,
-310 -40
I -50
UNSTABLE
! ~'~'~
"~'S=INa2S
~ i -610 -70
-8;
CALCULATED GLASS REDOX NUMBER
AMBER ~;
,I -910 -i00
W.H. Manring, G.M. Diken / Melting-Fining
2.
PHOTOGRAPHIC
I N V E S T I G A T I O N OF A T M O S P H E R I C
of Soda-Lime Glasses
81~
EFFECTS
The film p r e s e n t a t i o n of which the scenes are d e s c r i b e d [n the following ILst shows the effect of atmosphere on glass m e l t i n g and fining, Included are results obtained from hot stage m i c r o s c o p e studies and from w a t e r - c o o l e d p e r i s c o p e studies taken in full p r o d u c t i o n glass furnaces. Film Scenes Scene #i - Hot stage microscope, air atmosphere, seeds, blisters being generated by reacting a small sample of oxidized flint glass with reduced amber glass. Scene #2 - SO reboil generated by r e - h e a t i n g an oxidized glass @ 2700°F; note resorption, solubility of SO 3. Scene #3 - Low sulfur content gIass held at 2700°F in air atmosphere; no reboil, no surfactant action, entrapped air. Scene #4 - Sulfate fined glass in a high n i t r o g e n atmosphere. Scene #5 - Sulfate fined glass in an enriched 09 atmosphere. Scene #6 - Sulfate fined glass in a high SO 2 atmosphere. Scene #7 Sulfate fined glass, SO 2 + O. atmosphere. Scene #8 W a t e r - c o o l e d periscope, batc~ m e l t i n g - cascading. Scene #9 - W a t e r - c o o l e d periscope, batch m e l t i n g - foaming controlled. Scene #I0- W a t e r - c o o l e d periscope, flame geometry in o p e r a t i n g furnace, wet furnace vs. dry furnace conditions. 3.
GLASS R E D O X C A L C U L A T I O N
The c a l c u l a t i o n of glass redox n u m b e r - (GRN) is illustrated in Tables 2, 3, 4, and 5 for several typical glass batch compositions. Upon d e t e r m i n a t i o n of the chemical oxygen demand (COD) of the r e s p e c t i v e batch constituents, the carbon equivalent or total reducing p o t e n t i a l is c a l c u l a t e d as shown. The total reducing potential is then corrected for the o x i d i z i n g p o t e n t i a l of the components listed
TABLE 2
CARBON-SALT
CAKE, FLINT GLASS REDOX C A L C U L A T I O N S
Batch Sand Soda Ash Limestone Feldspar Salt Cake Ground Coal Cullet Decolorizers Dry Batch;
TABLE 3
COD (ppm C) 2000 ib 150 680 75 600 500 200 260 26 -2.5 650000 i000 -4 -4512.5 Glass Yield = 2.0 tons
SLAG-GYPSUM,
Batch Sand Soda Ash Limestone Slag Feldspar Gypsum Cullet l)ecolorizers
Total COD 0.3 (i) 0.i (2) 0.3 (3) 0.i (4) -(5) 1,7 (6) -(7) -2.5
-23.7 x 2.5 =-59.3 Sulfate c o r r e c t i o n = +26.0 Batch Redox =-33.3 Glass Redox No. =-16.6 ibs SO_ from batch = 13.0 lhs SO 2 per ton glass = 6.5 2 Refining No. = 6.5/-16.6 = -0.4
FLINT GLASS REDOX C A L C U L A T I O N S
COD (ppm C) 2000 ib ]50 700 75 500 500 i00 ii000 140 260 22 -i000 -5 -4467 Dry Batch; Glass Yield = 2.0 tons
Total COD 0.3 (i) 0.I (2) 0.3 (3) i.] (4) 0.[ (5) -(6) -(7) (8) -1.9 (9)
-23.7 x 1.9 = -45.0 Sulfate correction = +19.8 Batch Redox =-25.2 Glass Redox No. - -12.6 SO 9 from slag = 1,8 ~b SO- from gypsum = 8.5 ib To2tal Batch SO_ = 1O.3 ib z SO 9 per ton glass = 5.]5 ]b Re{ining No. = 5.15/-12.6 = -0.4
W.H. Manring, G.M. Diken / Melting-Fining of Soda-Lime Glasses
816
TABLE
4
-
CARBON-SALT
Batch
CAKE,
Total COD 0.3 (i) 0.I (2) Limestone~ 170 4200 0.7 (3) Limestone 480 400 0.2 (4) Salt Cake 70 --(5) Cullet 2000 --(6) Ground Coal 6 650000 3.9 (7) (8) 3 wt% H20 5326 5.2 W e t Batch; Glass Yield = 2.36 ton; High Cullet Ratio a) Sea-Water Limestone; b) Dolomitic Limestone Sand Soda Ash
TABLE
5
2000 ib 600
AMBER CONTAINER
Batch Sand Soda Ash Limestone Neph. Syenite Salt Cake Ground Coal Gullet Iron Pyrites 3 wt% H20 W e t Batch;
COD
FLOAT GLASS REDOX CALCULATIONS (ppm C) 150 75
GLASS REDOX CALCULATIONS
COD 2000 ib 700 600 220 16 6 900 8
-23.7 x 5.2 =-123.2 H O Correction = +12.0 2 Sulfate Correction = +70.0 Batch Redox = -31.2 Glass Redox No. = -13.2 SO 2 from batch = 30.1 ib SO 2 per ton glass = 12.8 Ib Refining No. = 12.8/-13.2 = -i.0
(ppm C) 150 75 500 250 -650000 -275000
4450 Glass Yield = 1.96 ton
Total COD 0.3 (I) 0.I (2) 0.3 (3) 0.i (4) -(5) 3.9 (6) -(7) 2.2 (8) (9) 6.9 (i0)
-23.7 x 6.9 =-163.5 H20 Correction = +12.0 Sulfate Correction = +16.0 Batch Redox = -135.5 Glass Redox No. = -69.1 SOn from FeS~ = 7.8 ib SO~ from N a ~ O . = 6.9 ib To~al batchZSO~ = 14.7 ib SO 2 per ton glass = 7.5 Ib Refining No. = 7.5/-69.1 = -0.i
in Table i. For the glass described in Table 2, only the salt cake, cullet, and p o s s i b l y the deeolorizers contribute to the calculation of the oxidizing potential Since the oxidizing potential of the cullet and decolorizers have not yet been established, they have been excluded from the calculation. The batch redox is, thus, given by the summation of the total reducing and total oxidizing potential of the constituents. Division of the batch redox value by the final glass yield in tons yields the glass redox number. A new expression introduced to characterize the fining ability of glass is 'refining number' which is the ratio of SO evolution to glass redox number. The 2 SOp value is described by the pounds of SOp produced per ton of glass. The sign i f i c a n c e of the refining number is best iIlustrated in Figure 2, which shows the o b s e r v e d shift in sulfur solubility as a function of the total SO2 available per ton of glass and the glass redox number. As the amount of availaSle SO 2 per ton of glass increases, the point of minimum solubility as determined by the developm e n t of amber streaks in flint glass shifts toward increasingly negative glass r e d o x numbers. This shift results from the increased amount of reducing material w h i c h must be present for complete conversion of the oxidized sulfur batch constituents to SO . The minima shown in Figure 2 were determined by observation 2 d u r i n g a number of plant tests and correspond to a refining number of about 0.07. T h e actual shape of the curves may vary from that shown~ however, based on the efforts of Budd (5) and Fincham, et al (7), one would expect the general shape to be similar to that shown in Figure i. For convenience the refining numbers corr e s p o n d i n g to the SO content of the glass for each decade change in redox number are listed below the2graph. Each of the curves in Figure 2 describe the glass r e d o x conditions as a function of available SO 2 similar to Figure i. Consider the first curve where the available SO 2 per ton of glass is 2 ibs. As the calculated glass redox number increases negatively, the glasses go from an oxidized condition through a region characterized as reduced flint, where glass redox num-
W.H. Manring, G.M. Diken / Melting-Fining of Soda-Lime Glasses
FIGURE
2
8]7
R E L A T I O N S H I P BETWEEN SO~ (ibs/ton glass) AND R E F I N I N G Z N U M B E R
0.35' ,#
0.30
\
k
0.25
/
2 ibs SO 2 j ' 4
/
/
/
/ 6
/
/
C u~
0.20,
•
0.15
i
0 -lO -2o\-3o
-40"
- 6 0 ] ~ . . . -80 ~'l',,.~-loo /
0.1~ ~ F O P ~ A T 10N
0.05. 0.00~
Sal t Cake
R E F I N I N G NUMBERS
J~
(ibs/~on)
+
-i0
-20
-30
-40
-50 - 6 0
-70
-80 - 9 0
2.0
+
-0.2 - , i 0 - , 0 7
-.05
4.0
+
-0.4 - , 2 0 - . 1 3
-,i0 - , 0 8 - . 0 7 - . 0 6 -.05
'i00 -ii0 E q u i v 9
6.0
+
-0.6 -,30 -,20 -,15 - . 1 2 - . 1 0
8 0
+
-0 8
18
-.08 -,07 -.076.06 -. 05
27
-. 401- 27 -. 201 - 1 6 - . 1 3 -.11 - 1 0 -. 0 ~ . o8 -.07
36
l
bet is b e t w e e n -5 and -20, to a region of m i n i m u m sulfur solubility, and fina]ly to a region of amber formation. The reduced flint region of these glasses, wher~ o p t i m u m fining occurs, is very small and is c h a r a c t e r i z e d by a curve with relatively steep slope. Consequently, minor changes in the batch redox or furnace redox conditions will s i g n i f i c a n t l y i n f l u e n c e the fining process in the glass. In the c~isc of carbon-salt cake fined glass, a 15% error Jn the coal addition or insufficient mixing and d i s p e r s i o n of the coal could yield an oxidized seedy glass if th<~ batch has i n s u f f i c i e n t r e d u c t i o n p o t e n t i a l or amber streaks if excessive reducing conditions occur. Similar effects could occur w i t h even smaller w l r i a t i o n s in thc~ slag added to glass batches. A l t h o u g h flint glasses baying low SO contents can 2 be readily produced, a m i n i m u m of 4-5 ibs of SO o per ton of glass ~s n e c e s s a r y t<) realize the refLning benefits From sulfate fining systems and m a x i m i z e furnace efficiency. In c o m p a r i s o n to the first curve, the curve for glasses c o n t a i n i n g 6 Ibs ot SO? per ton has a m i n i m u m sulfur solubility or amber forming region for glass redo~ numbers near -80. The reduced flint region for these glasses is much greater, from abont -15 to -70 GRN, compared to that for 2 Ibs of SO 2 per ton of glass and th~ slope is more shallow. Consequently, glass typified by this curve is much less sensitive to changes in batch redox or changing furnace conditions. The m a x i m u m SO 2 content a l l o w a b l e is essentially dictated by local air pollution standards. In most areas of the U.S., this eurresponds to an SO,~ content of about ]0 Ibs llcr ton of glass in fossil fuel fired furnaces. Providing a rather wide r e f i n i n g redox zone is very important when raw material sources vary, their redox p o t e n t i a l fluctuates over a large range, or even wilton the cuilet ratio varies daily~ all of which are typical situations faced by mo~t manufacturers. Without control of the glass redox conditions minor fluctuations of the above could result in reduced glass quality, higher furnace temperaturc~s, or amber streaking. Although greater SO 2 content per ton of glass provides mor~'
8]8
W.H. Manring,
G.M. Diken / Melting-Fining of Soda-Lime Glasses
latitude and flexibility for the batch materials, the economic tradeoffs between the addition of sulfur compounds, optimum furnace operation, and final glass quality must also be considered. In fossil fuel fired furnaces another factor which influences the glass redox conditions is the sulfur content of the gas and oil. Before the enactment of the new stringent air pollution standards, the effect of the fuel sulfur content on glass quality was very noticeable. Today, however, with the recent trend toward the use of cleaner fuels, the effect of fuel sulfur content has been observed to have only minor effects relative to the final glass redox conditions.
SUMMARY As a result of increased scarcity, high cost, and more stringent air pollution requirements fining agents as fluorspar, salt, arsenic, etc. are seeing very limited application. Only sulfur-containing materials remain viable fining agents for soda-lime glasses and even these materials cannot be used in excess. Optimization of the melting-fining process in the manufacture of flint glasses requires control of the sulfur redox to promote maximum evolution of SO 2 in the glass. Insufficient SO 2 content results in seed problems, higher furnace temperatures, and wet furnace conditions, which reduce furnace life. An optimum desired range of SO 2 content per ton of flint glass is between 6 and i0 ibs Within this range the calculated glass redox number should lie between -25 and 175 which corresponds to refining numbers between 0.08 and 0.40. Close control of glass redox parameters is essential for realization of maximum pull rates of high quality glass at minimum furnace temperatures. Application of increased redox control can result in lowering of furnace temperatures as much as 50°F or more.
ACKNOWLEDGEMENT The authors wish to express their appreciation to FMC Corporation for permission to publish this work and to W.C.Bauer and R.E.Davis for their invaluable contributions.
REFERENCES (i) (2) (3) (4) (5) (6) (7)
Manring, W.H. and Hopkins, R.W., Use of Sulfates in Glass, Glass Ind. 39 (3) (1958) 139-142, 170. Manring, W.H. and Davis, R.E., Controlling Redox Conditions in Glass Melting, Glass Ind. May (1978). Shaw, F. and Jones, S.P., Effect of Sodium Sulfate and Furnmce Atmosphere on Fining Container-Type Glass, Ceramic Bull, 45 (1966) 1004-1008. Conroy, A.R., Manring, W.H., et al~ The Role of Sulfate in the Melting and Fining of Glass Batch, 47 (2,3) (1966). Budd, S.M., Oxidation-Reduction Equilibrium in Glass With Special Reference to Sulfur, presented ACS Symposium: Gases in Glass, May (1965). Manring, W.H., Billings, D.D., et al~ Reduced Sulfur Compounds, Glass Ind. July (1967). Fincham, C.J. and Richardson, F.D., The Behavior of Sulphur in Silicate and Aluminate Melts, Proc. Roy, Soc. 223(A) (1954) 40-62.
(1980)
813-818
A P R A C T I C A L A P P R O A C H TO E V A L U A T I N G REDOX P H E N O M E N A INVOLVED IN THE M E L T I N G - F I N I N G OF SODA-LIME (;LASSES W.H. M a n r i n g & G.M. Diken FMC C o r p o r a t i o n Industrial Chemical Group Princeton, New Jersey
U,S.A.
Increased furnace efficiency through increased furnace production or pull rate can be realized with o p t i m i z a t i o n and control of ti~e redox conditions during the m e l t i n g and fining of glass. An empirical model and related redox factors, n e c e s s a r y for o p t i m i z a t i o n of the m e l t i n g / f i n i n g p r o c e s s e s in sulfur-sulfate refined glasse~, are described. In addition to examining the redox conditions necessary to produce very stable reduced flint and amber glasses, discussed is a new fining redox number which relates the glass redox conditions to the observed refining ability of the glass. Hot stage p h o t o g r a p h y and w a t e r cooled p e r i s c o p e films taken in p r o d u c t i o n furnaces illustrate the importance of SO 2 e v o l u t i o n and control in the glass m e l t i n g and fining process.
INTRODUCTION Much has been w r i t t e n about the influence of furnace a t m o s p h e r e on tile melting/ fining rate of soda-lime glasses. A d v a n t a g e s which have been realized with increased control of glass m e l t i n g and fining include reductions in m e l t i n g temperature, p o l l u t i o n emissions, and seed count in addition co increases in production throughput and r e f r a c t o r y life. The intent of this paper is to outline an empirical approach to q u a n t i t a t i v e l y evaluate and to s u b s e q u e n t l y control the batch and glass redox conditions of reduced flint fining systems in terms ut th~ s u l f a t e / s u l f u r dioxide e q u i l i b r i u m during the m e l t i n g / f i n i n g process. Since a complete detailed d i s c u s s i o n of this approach would be ]engtby and has been documented, in part, (1,2) only a cursory treatment illustrated by several examples will be provided. One of the most important p a r a m e t e r s in c o n t r o l l i n g glass redox or glass fining ability is the sulfur dioxide (S0o) content present in the furnace atmosphere.. Shaw and Jones (3) showed "the presence of S0o in the atmosphere signit ieantly improves the fining of all glasses" provided [ certain m i n i m u m concentr~iti~ll is present. Similar results were obtained from hot stage m i c r o s c o p e studies coodurted at FHC as will be shown later in a f~]m presentation. Small amounts e| S(]~ were shown to have a remarkable influence on the surface tension of glass res~lting in increased fining rates and in sore{~ cases reduced surface _~oaming. These results have ~l]so been confirmed in full scale glass iurn;ice operations. Sulfates enhance tile m e l t i n g and fining of glass because of the dual sulriactnot and int<,rfacial turbulence effects they provide during [he m e l t i n g operatios, especially in the presence of reducing agents as listed in Table I. Their eff
813
81~
W.H. Manring,
TABLE i
-
E~IRICAL
G.M. Diken / Melting-Fining
OXIDATION-REDUCTION
1 ib Salt Cake (Na~SO.) 1 ib Gypsum (CaSO~2H%0) 1 ib Barytes (BaSOA) ~ 1 ib Nitre (NAN03)1% H_O in Batch Cull~t Caustic Soda (NaOH) Air/Fuel Ratio *
= = = = = = = =
of Soda-Lime Glasses
FACTORS*
+i.0 +0.9 +0.6 +3.0 +4.0 ? ? ?
1 ib 1 Ib iib i ib i ib i ib i ib i ib
Revised 1979 value ss, original reference
Pure Carbon Ground Coal Sulfur Iron Pyrite (FeSp) Flourspar (CaF2)Salt (NaCI) Ferrous Oxide (FeO) Arsenic __(As203)
= = = = = = = =
-23.7 -16.0 -13.3 - 6.5 - 1.8 - 1.0 - 1.0 ?
(I),
in the presence of metal forms chromophores. The high diffusion rate of these decomposition products across the particle interfaces disrupt the interfacial surface tension releasing considerable energy which results in a vigorous stirring of the melt. It is the development of insoluble sulfur phases which is necessary for optimization of the glass fining process in reduced flint glasses. i.
SULFUR SOLUBILITY AND GLASS REDOX
Past studies by Budd (5), who investigated the relationship between sulfur solubility and the oxidation state of glass, showed that the solubility of sulfur in glass decreases as the glass becomes more reduced until highly reduced glasses are formed, at which point the sulfur solubility rapidly increases with the formation of amber chromophores. Budd's solubility data are replotted in Figure 1 on a linear scale as a function of glass redox number, which represents the chemical oxygen demand (COD value) of the glass batch as modified by oxidizing and reducing potential of the glass batch constituents listed in Table i. Glass compositions to the left in Figure I, having lower negative values are considered oxidized systems~while those toward the right are more reduced. The optimum fining region for flint glasses occurs over the range of glass redox numbers -20 to -50 where the glasses have low sulfur content. The reduced sulfur solubility permits more rapid expulsion of the SOp, thereby increasing fining rate and reducing the potential of reboil. Althoug~ reduced flint fining systems are preferable, care must be taken not to form highly reduced flints, since this could result in generation of amber streaks and~in extremely reducing conditions~development of blisters and bublos. A review of the sulfur compounds and fining aids which can be used for fining soda-lime glasses is described in a prior publication by Manring, et al (6).
SULFUR SOLUBILITY VS CALCULATED OXIDATION STATE OF GLASS
FIGURE 1
I
0.30
~
I OPTIMUM
/
C 0.20
0.I0
0.00
)XIDIZED FLINT SO 3 ,
-i0
REDUCED FLINT
~ I
I -20
STABLE AMBER
SO 2 ,
-310 -40
I -50
UNSTABLE
! ~'~'~
"~'S=INa2S
~ i -610 -70
-8;
CALCULATED GLASS REDOX NUMBER
AMBER ~;
,I -910 -i00
W.H. Manring, G.M. Diken / Melting-Fining
2.
PHOTOGRAPHIC
I N V E S T I G A T I O N OF A T M O S P H E R I C
of Soda-Lime Glasses
81~
EFFECTS
The film p r e s e n t a t i o n of which the scenes are d e s c r i b e d [n the following ILst shows the effect of atmosphere on glass m e l t i n g and fining, Included are results obtained from hot stage m i c r o s c o p e studies and from w a t e r - c o o l e d p e r i s c o p e studies taken in full p r o d u c t i o n glass furnaces. Film Scenes Scene #i - Hot stage microscope, air atmosphere, seeds, blisters being generated by reacting a small sample of oxidized flint glass with reduced amber glass. Scene #2 - SO reboil generated by r e - h e a t i n g an oxidized glass @ 2700°F; note resorption, solubility of SO 3. Scene #3 - Low sulfur content gIass held at 2700°F in air atmosphere; no reboil, no surfactant action, entrapped air. Scene #4 - Sulfate fined glass in a high n i t r o g e n atmosphere. Scene #5 - Sulfate fined glass in an enriched 09 atmosphere. Scene #6 - Sulfate fined glass in a high SO 2 atmosphere. Scene #7 Sulfate fined glass, SO 2 + O. atmosphere. Scene #8 W a t e r - c o o l e d periscope, batc~ m e l t i n g - cascading. Scene #9 - W a t e r - c o o l e d periscope, batch m e l t i n g - foaming controlled. Scene #I0- W a t e r - c o o l e d periscope, flame geometry in o p e r a t i n g furnace, wet furnace vs. dry furnace conditions. 3.
GLASS R E D O X C A L C U L A T I O N
The c a l c u l a t i o n of glass redox n u m b e r - (GRN) is illustrated in Tables 2, 3, 4, and 5 for several typical glass batch compositions. Upon d e t e r m i n a t i o n of the chemical oxygen demand (COD) of the r e s p e c t i v e batch constituents, the carbon equivalent or total reducing p o t e n t i a l is c a l c u l a t e d as shown. The total reducing potential is then corrected for the o x i d i z i n g p o t e n t i a l of the components listed
TABLE 2
CARBON-SALT
CAKE, FLINT GLASS REDOX C A L C U L A T I O N S
Batch Sand Soda Ash Limestone Feldspar Salt Cake Ground Coal Cullet Decolorizers Dry Batch;
TABLE 3
COD (ppm C) 2000 ib 150 680 75 600 500 200 260 26 -2.5 650000 i000 -4 -4512.5 Glass Yield = 2.0 tons
SLAG-GYPSUM,
Batch Sand Soda Ash Limestone Slag Feldspar Gypsum Cullet l)ecolorizers
Total COD 0.3 (i) 0.i (2) 0.3 (3) 0.i (4) -(5) 1,7 (6) -(7) -2.5
-23.7 x 2.5 =-59.3 Sulfate c o r r e c t i o n = +26.0 Batch Redox =-33.3 Glass Redox No. =-16.6 ibs SO_ from batch = 13.0 lhs SO 2 per ton glass = 6.5 2 Refining No. = 6.5/-16.6 = -0.4
FLINT GLASS REDOX C A L C U L A T I O N S
COD (ppm C) 2000 ib ]50 700 75 500 500 i00 ii000 140 260 22 -i000 -5 -4467 Dry Batch; Glass Yield = 2.0 tons
Total COD 0.3 (i) 0.I (2) 0.3 (3) i.] (4) 0.[ (5) -(6) -(7) (8) -1.9 (9)
-23.7 x 1.9 = -45.0 Sulfate correction = +19.8 Batch Redox =-25.2 Glass Redox No. - -12.6 SO 9 from slag = 1,8 ~b SO- from gypsum = 8.5 ib To2tal Batch SO_ = 1O.3 ib z SO 9 per ton glass = 5.]5 ]b Re{ining No. = 5.15/-12.6 = -0.4
W.H. Manring, G.M. Diken / Melting-Fining of Soda-Lime Glasses
816
TABLE
4
-
CARBON-SALT
Batch
CAKE,
Total COD 0.3 (i) 0.I (2) Limestone~ 170 4200 0.7 (3) Limestone 480 400 0.2 (4) Salt Cake 70 --(5) Cullet 2000 --(6) Ground Coal 6 650000 3.9 (7) (8) 3 wt% H20 5326 5.2 W e t Batch; Glass Yield = 2.36 ton; High Cullet Ratio a) Sea-Water Limestone; b) Dolomitic Limestone Sand Soda Ash
TABLE
5
2000 ib 600
AMBER CONTAINER
Batch Sand Soda Ash Limestone Neph. Syenite Salt Cake Ground Coal Gullet Iron Pyrites 3 wt% H20 W e t Batch;
COD
FLOAT GLASS REDOX CALCULATIONS (ppm C) 150 75
GLASS REDOX CALCULATIONS
COD 2000 ib 700 600 220 16 6 900 8
-23.7 x 5.2 =-123.2 H O Correction = +12.0 2 Sulfate Correction = +70.0 Batch Redox = -31.2 Glass Redox No. = -13.2 SO 2 from batch = 30.1 ib SO 2 per ton glass = 12.8 Ib Refining No. = 12.8/-13.2 = -i.0
(ppm C) 150 75 500 250 -650000 -275000
4450 Glass Yield = 1.96 ton
Total COD 0.3 (I) 0.I (2) 0.3 (3) 0.i (4) -(5) 3.9 (6) -(7) 2.2 (8) (9) 6.9 (i0)
-23.7 x 6.9 =-163.5 H20 Correction = +12.0 Sulfate Correction = +16.0 Batch Redox = -135.5 Glass Redox No. = -69.1 SOn from FeS~ = 7.8 ib SO~ from N a ~ O . = 6.9 ib To~al batchZSO~ = 14.7 ib SO 2 per ton glass = 7.5 Ib Refining No. = 7.5/-69.1 = -0.i
in Table i. For the glass described in Table 2, only the salt cake, cullet, and p o s s i b l y the deeolorizers contribute to the calculation of the oxidizing potential Since the oxidizing potential of the cullet and decolorizers have not yet been established, they have been excluded from the calculation. The batch redox is, thus, given by the summation of the total reducing and total oxidizing potential of the constituents. Division of the batch redox value by the final glass yield in tons yields the glass redox number. A new expression introduced to characterize the fining ability of glass is 'refining number' which is the ratio of SO evolution to glass redox number. The 2 SOp value is described by the pounds of SOp produced per ton of glass. The sign i f i c a n c e of the refining number is best iIlustrated in Figure 2, which shows the o b s e r v e d shift in sulfur solubility as a function of the total SO2 available per ton of glass and the glass redox number. As the amount of availaSle SO 2 per ton of glass increases, the point of minimum solubility as determined by the developm e n t of amber streaks in flint glass shifts toward increasingly negative glass r e d o x numbers. This shift results from the increased amount of reducing material w h i c h must be present for complete conversion of the oxidized sulfur batch constituents to SO . The minima shown in Figure 2 were determined by observation 2 d u r i n g a number of plant tests and correspond to a refining number of about 0.07. T h e actual shape of the curves may vary from that shown~ however, based on the efforts of Budd (5) and Fincham, et al (7), one would expect the general shape to be similar to that shown in Figure i. For convenience the refining numbers corr e s p o n d i n g to the SO content of the glass for each decade change in redox number are listed below the2graph. Each of the curves in Figure 2 describe the glass r e d o x conditions as a function of available SO 2 similar to Figure i. Consider the first curve where the available SO 2 per ton of glass is 2 ibs. As the calculated glass redox number increases negatively, the glasses go from an oxidized condition through a region characterized as reduced flint, where glass redox num-
W.H. Manring, G.M. Diken / Melting-Fining of Soda-Lime Glasses
FIGURE
2
8]7
R E L A T I O N S H I P BETWEEN SO~ (ibs/ton glass) AND R E F I N I N G Z N U M B E R
0.35' ,#
0.30
\
k
0.25
/
2 ibs SO 2 j ' 4
/
/
/
/ 6
/
/
C u~
0.20,
•
0.15
i
0 -lO -2o\-3o
-40"
- 6 0 ] ~ . . . -80 ~'l',,.~-loo /
0.1~ ~ F O P ~ A T 10N
0.05. 0.00~
Sal t Cake
R E F I N I N G NUMBERS
J~
(ibs/~on)
+
-i0
-20
-30
-40
-50 - 6 0
-70
-80 - 9 0
2.0
+
-0.2 - , i 0 - , 0 7
-.05
4.0
+
-0.4 - , 2 0 - . 1 3
-,i0 - , 0 8 - . 0 7 - . 0 6 -.05
'i00 -ii0 E q u i v 9
6.0
+
-0.6 -,30 -,20 -,15 - . 1 2 - . 1 0
8 0
+
-0 8
18
-.08 -,07 -.076.06 -. 05
27
-. 401- 27 -. 201 - 1 6 - . 1 3 -.11 - 1 0 -. 0 ~ . o8 -.07
36
l
bet is b e t w e e n -5 and -20, to a region of m i n i m u m sulfur solubility, and fina]ly to a region of amber formation. The reduced flint region of these glasses, wher~ o p t i m u m fining occurs, is very small and is c h a r a c t e r i z e d by a curve with relatively steep slope. Consequently, minor changes in the batch redox or furnace redox conditions will s i g n i f i c a n t l y i n f l u e n c e the fining process in the glass. In the c~isc of carbon-salt cake fined glass, a 15% error Jn the coal addition or insufficient mixing and d i s p e r s i o n of the coal could yield an oxidized seedy glass if th<~ batch has i n s u f f i c i e n t r e d u c t i o n p o t e n t i a l or amber streaks if excessive reducing conditions occur. Similar effects could occur w i t h even smaller w l r i a t i o n s in thc~ slag added to glass batches. A l t h o u g h flint glasses baying low SO contents can 2 be readily produced, a m i n i m u m of 4-5 ibs of SO o per ton of glass ~s n e c e s s a r y t<) realize the refLning benefits From sulfate fining systems and m a x i m i z e furnace efficiency. In c o m p a r i s o n to the first curve, the curve for glasses c o n t a i n i n g 6 Ibs ot SO? per ton has a m i n i m u m sulfur solubility or amber forming region for glass redo~ numbers near -80. The reduced flint region for these glasses is much greater, from abont -15 to -70 GRN, compared to that for 2 Ibs of SO 2 per ton of glass and th~ slope is more shallow. Consequently, glass typified by this curve is much less sensitive to changes in batch redox or changing furnace conditions. The m a x i m u m SO 2 content a l l o w a b l e is essentially dictated by local air pollution standards. In most areas of the U.S., this eurresponds to an SO,~ content of about ]0 Ibs llcr ton of glass in fossil fuel fired furnaces. Providing a rather wide r e f i n i n g redox zone is very important when raw material sources vary, their redox p o t e n t i a l fluctuates over a large range, or even wilton the cuilet ratio varies daily~ all of which are typical situations faced by mo~t manufacturers. Without control of the glass redox conditions minor fluctuations of the above could result in reduced glass quality, higher furnace temperaturc~s, or amber streaking. Although greater SO 2 content per ton of glass provides mor~'
8]8
W.H. Manring,
G.M. Diken / Melting-Fining of Soda-Lime Glasses
latitude and flexibility for the batch materials, the economic tradeoffs between the addition of sulfur compounds, optimum furnace operation, and final glass quality must also be considered. In fossil fuel fired furnaces another factor which influences the glass redox conditions is the sulfur content of the gas and oil. Before the enactment of the new stringent air pollution standards, the effect of the fuel sulfur content on glass quality was very noticeable. Today, however, with the recent trend toward the use of cleaner fuels, the effect of fuel sulfur content has been observed to have only minor effects relative to the final glass redox conditions.
SUMMARY As a result of increased scarcity, high cost, and more stringent air pollution requirements fining agents as fluorspar, salt, arsenic, etc. are seeing very limited application. Only sulfur-containing materials remain viable fining agents for soda-lime glasses and even these materials cannot be used in excess. Optimization of the melting-fining process in the manufacture of flint glasses requires control of the sulfur redox to promote maximum evolution of SO 2 in the glass. Insufficient SO 2 content results in seed problems, higher furnace temperatures, and wet furnace conditions, which reduce furnace life. An optimum desired range of SO 2 content per ton of flint glass is between 6 and i0 ibs Within this range the calculated glass redox number should lie between -25 and 175 which corresponds to refining numbers between 0.08 and 0.40. Close control of glass redox parameters is essential for realization of maximum pull rates of high quality glass at minimum furnace temperatures. Application of increased redox control can result in lowering of furnace temperatures as much as 50°F or more.
ACKNOWLEDGEMENT The authors wish to express their appreciation to FMC Corporation for permission to publish this work and to W.C.Bauer and R.E.Davis for their invaluable contributions.
REFERENCES (i) (2) (3) (4) (5) (6) (7)
Manring, W.H. and Hopkins, R.W., Use of Sulfates in Glass, Glass Ind. 39 (3) (1958) 139-142, 170. Manring, W.H. and Davis, R.E., Controlling Redox Conditions in Glass Melting, Glass Ind. May (1978). Shaw, F. and Jones, S.P., Effect of Sodium Sulfate and Furnmce Atmosphere on Fining Container-Type Glass, Ceramic Bull, 45 (1966) 1004-1008. Conroy, A.R., Manring, W.H., et al~ The Role of Sulfate in the Melting and Fining of Glass Batch, 47 (2,3) (1966). Budd, S.M., Oxidation-Reduction Equilibrium in Glass With Special Reference to Sulfur, presented ACS Symposium: Gases in Glass, May (1965). Manring, W.H., Billings, D.D., et al~ Reduced Sulfur Compounds, Glass Ind. July (1967). Fincham, C.J. and Richardson, F.D., The Behavior of Sulphur in Silicate and Aluminate Melts, Proc. Roy, Soc. 223(A) (1954) 40-62.