Reformulation of Container Glasses for Environmental

8/11/2019 Reformulation of Container Glasses for Environmental

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Gl ass Technol ogy Vol . 46 N o. 1 Febr uar y 2005 www.sgt .org 11

Glass Technol., 2005,46(1), 1119

The chemical composit ion of container glass has evolved

over many years to become almost gener ic, providi ng a

cost effecti ve mater ial which fi ts it s purpose very well .

Cont ainer glass composit ions are essenti all y t he cheap-

est acceptable formulati ons in t erms of raw materi als,

but increasingly str ingent environmental legislat ion may

soon cause changes in the economic balance. A large

body of work has been published regarding melt ing aidsand new raw materials in order to i ncrease melt ing effi -

ciency. H owever there is far less publi shed work deali ng

wit h the specif ic aim of modify ing the chemical compo-

siti on of container glass to reduce it s melt ing tempera-

ture, whilst at the same time maintaining i ts desir able

characteristi cs. Environmental benefi ts result ing f rom re-

formulati on may i nclude reducti ons in melt ing energy

and lower emissions of CO2, NOx, SOx, part iculates and

heavy metals.

This paper discusses the development of a number of

reformulat ed container glass composit ions, wi th the main

emphasis on the physical propert ies of the new glasses

and estimated reductions in thermal NOxgenerati on.

Composit ional changes ranged from single substit uti ons

to complex alterat ions involving several components. Com-

puter calculati on of the high temperature viscosit y of the

new composit ions was carr ied out and the result s agreed

very well with measured values. H igh t emperat ure viscos-

it y, liquidus temperature, chemical durabili ty, t hermal

expansion coef fi cient, densit y, dil atometr ic soft ening

point, glass tr ansit ion temperature and flux li ne corrosion

were measured on a selecti on of the new glasses. The re-

sults were analysed and evaluated against a set of prop-

erty cri teri a based on a benchmark glass composit ion

representat ive of current UK container glass compositi ons.

Reformulated glass composit ions exhibi ted reductions

in melt ing t emperature, defined as the log 2 viscosit y, of

up to 115C. Several of the new glasses also ful f i l led the

benchmark cr it eri a for chemical durabil it y and liquidus

temperature. I t was estimated that the reduced fur nace

temperat ures which these composit ions would enable

could cut generat ion of thermal NOxby approximately

2040%. Reductions in melt ing energy and emissions ofCO2, SOx, part iculates and heavy metals may also re-

sult , however t hey were not quant if ied in this study as

their accurate estimation requires consideration of the

effects of raw materi als and cull et. Further work is cur-

rently underway t o study t hese ef fects and the behav-

iour of the glasses on a larger industr ial scale.

Recent publications discussed the effects and benefitsof replacing a small amount of SiO2by P2O5in con-tainer glass,(1)and gave a brief overview of a project toreformulate container glass with the aim of reducingits melting temperature and thereby decreasing melt-ing energy and emissions.(2)That project, from whichthe work discussed in this paper was also derived, wasthe first stage of a series of studies and therefore itsscope was not comprehensive. For example, the effectsof raw materials and cullet additions have not yet beenincluded in development of the new compositions, yetthey are important to the glass manufacturing proc-ess. I t is intended that further stages of the project willstudy these issues. The reader is referred to the firststage project report, available in electronic form fromthe authors, for full details of the work to date.(3)Whilstthe choice of raw materials and cullet levels are majorfactors influencing melting energy, particulates andcertain other emissions, it is the oxide composition of

the final glass which determines its high temperature

Reformulation of container glasses for environmentalbenefit through lower meltingtemperaturesP. A. Bingham

Immobili sation Science L aboratory, Department of Engineeri ng M aterials,

Universit y of Sheff ield, Sheff ield, S1 3JD, U K

M . M arshall1

Glass Technology Services Ltd, 9 Churchil l Way, Chapelt own, Sheff ield, S35 2PY, U K

Manuscript received 5 August 2004Revised version received 15 November 2004Accepted 7 December 2004

1Corresponding author. Email address: [email protected]


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12 Glass Technol ogy Vol . 46 No. 1 Februar y 2005 www.sgt .or g

viscosity. Viscosity is the main parameter governingthe potential reduction in furnace temperature whichcan be achieved. Reduction in high temperature vis-cosity was therefore the focus of the first stage of thework.

The melting of container glass is an energy inten-sive process which produces a number of side effects

that are environmentally undesirable. Carbon diox-ide results from burning gas or oil and generating elec-tricity, and also from decomposition of some rawmaterials. The melting process also causes the emis-sion of thermally created nitrogen oxides (NOx),which are pollutants. Thermal NOxgeneration is verytemperature sensitive, particularly at temperaturesabove approximately 1400C, such as are found inglass furnaces.(35)For example it is estimated that ina typical glass melting furnace, a reduction in cham-ber temperature from 1600C to 1500C could reducethermal NOxgeneration by up to half.

(35)This be-haviour means that even small decreases in furnacetemperature from their current levels could lead tosubstantial decreases in thermal NOxgeneration. Inaddition the melting process also generates emissionsof oxides of sulphur (SOx), particulates, and heavymetals, specifically selenium.

UK and European legislation is tougher than everbefore in terms of emissions limits for CO2, NOx, SOx,particulates and heavy metals.(6)This legislation willaffect the economic balance which has hitherto existedfor manufacturers breaches of emission limits willincur harsh penalties. Glass manufacturers must there-fore look to new and novel technologies in order tomeet not only the current requirements but also thoseprojected for the future. The current best available tech-

nologies (BATs) for emissions reduction, as describedby the integrated pollution prevention and control(IPPC) guidance document,(7)consist of secondary(abatement) and primary methods. Electrostaticprecipitators (EPs), bag filters and acid gas scrubbingoperations are technologies offered for the reductionof particulates and SOx, but these methods have sub-stantial electricity requirements and therefore gener-ate CO2. They also create waste materials which mustbe disposed of, although some manufacturers now re-cycle their filter dusts. I n addition the installation andoperating costs of abatement technology can be high.The 3R process developed by Pilkington, which sacri-ficially burns methane to reduce NOxemissions, isamongst the secondary technologies. However it car-ries a 65% energy penalty and generates additionalCO2. Primary methods of emissions reduction includea number of combustion technologies such as low-NOxburners and oxy-fuel melting and each carry theirown advantages and disadvantages. Other primarymethods not discussed in the IPPC guidance docu-ment include chemical reformulation and the use ofalternative raw materials. A sizeable body of workhas been published dealing with the use of new rawmaterials to improve the properties or melting behav-iour of glass.(814)These have included materials suchas spodumene, borax and blast furnace slag, the lat-

ter of which is now widely used in UK container and

flat glass manufacture.Sodalimesilica container glass compositions have

evolved over many years to reach their current form.They are used extensively today because they strike abalance between the often competing factors of rawmaterials availability, cost, melting and fining charac-teristics, machine productivity, chemical durability,

optical properties and thermal shock resistance.(15,16)

By1932, container glass compositions showed a reason-able resemblance to those found today.(1720)However,average SiO2contents declined steadily from 741 wt%in 1932 to 717% in 1960. Over the same period, theintermediate and stabilising oxide content (Al2O3+MgO+CaO+SrO+BaO) rose steadily from 91 to 135wt% and alkalis and minor fluxes (Na2O+K2O+B2O3+F2+SO3) decreased from 168 to 149 wt%. Dif-ferences between these glasses and modern composi-tions are relatively small, however they are notnegligible. Deliberate additions of boric oxide (B2O3)at up to 1 wt%, barium oxide (BaO) at up to 07 wt%,and fluorine (F2) at up to 02 wt% were made through-out the period 19321960. By 1977, B2O3and BaO hadbeen removed from US glass compositions, althoughthey were still used in some European container glassesin the late 1980s.(20)Over the period 1948 to 1960, glassproperties changed slightly, reflecting the changes incomposition. Gob temperatures corresponding to log3 viscosity fell by about 10C, whilst softening tem-peratures increased by 5C. These small changes incomposition coincided with greatly improved furnaceefficiencies which were the focus of much effort in the1960s and 1970s.

Current UK and European environmental legisla-tion controls the release of CO2, NOx, SOx, Cl, F, Cd,

Tl, As, Co, Ni, Se, Sb, Pb, Cr, Cu, Mn, V, Sn, and NH4into the atmosphere as described in the relevant IPPCguidance document.(7)Release of Pb, Cd, Cr6+and Hginto solution is strictly controlled. Clearly the use ofhalogens such as fluorides and chlorides, as well as leadand several other heavy metals and transition metals,are prohibited by legislation from deliberate additionat any meaningful level in container glass, and theywere therefore rejected as potential candidates for thereformulation process.

The physical, chemical and mechanical propertiesof container glass are fundamentally important to itsapplications. These include:Viscositytemperature relationshipGlass stabilityThermal propertiesOptical propertiesRefractory corrosionChemical durabilityDensity.

Volf(21)discussed a large proportion of the workcarried out on individual glass components in termsof their effects on physical properties and behaviourin sodalimesilica type glasses. The effects of chang-ing the levels of the main glass components and otheradditions can now be accurately predicted based onthe work of Lakatos et al .(2225)This was utilised dur-

ing early formulation stages to model prospective can-

P. A. BINGHAM & M. MARSHALL: REFORMULATION OF CONTAINER GLASSES FOR ENVIRONMENTAL BENEFIT


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didate compositions and review their expected perform-ances in terms of viscosity reduction prior to labora-tory melting trials.

Liquidus temperatures in container glasses are gen-erally 1020C below the working point (log 4 viscos-ity), and hence the likelihood of crystallisation duringforming is kept to a minimum. The difference between

forming and liquidus temperatures is an importantcriterion in the development of new glass formulations,and it is defined here as DTFL. For example, using thephase relations in the relevant part of the SiO2Na2OCaO system, the replacement of 1% SiO2by Na2Owould be expected to have only a small effect onliquidus temperature within the Na2O.3CaO.6SiO2(devitrite) phase field.(26)Viscosity would decrease soDTFLwould decrease, possibly becoming negative, hencethere would be an increased risk of devitrification dur-ing forming. Whether this would actually cause prob-lems in reality is dependent largely on the particularconditions in a given furnace and forming operation.We have received anecdotal evidence that some manu-facturers operate with negativeDTFLquite safely. How-ever this could not be assessed in a laboratory, so theDTFL criterion was introduced to assess the potentiallikelihood of devitrification problems. This criterionstates that if DTFLfor a given glass is significantly lessthan DTFL for glass B, the representative benchmarkcomposition, then the likelihood of crystallisation prob-lems would increase. This would make such a composi-tion less desirable in production but does not necessarilypreclude it from consideration.

At SiO2contents below approximately 68 wt%, thechemical durability of flat glass decreases rapidly withfurther decreases in SiO2content, although this can be

mitigated by increasing Al2O3levels.(27)

That situationis compounded if one refers to the relevant part of theSiO2Na2OCaO phase diagram.

(26)At SiO2contentslower than about 6869 wt%, the Na2O.3CaO.6SiO2phase field gives way to the Na2O.2CaO.3SiO2phasefield. In this phase field, a small reduction in SiO2con-tent strongly increases the liquidus temperature. Thecombined effects of decreasing SiO2below 68 wt% ondurability and liquidus temperature mean that SiO2contents below approximately 68 wt% are likely to beundesirable for container glass. Reformulations in thisstudy were therefore limited to SiO2contents above thislevel. The effect of composition on durability in suchglasses has been investigated.(15, 28)

Density is a factor requiring consideration becauseit affects the number of articles which can be producedfrom a given weight of glass. I t was thus considereddesirable for the density of candidate glasses to not besignificantly greater than the benchmark glass, glass B.

Refractory corrosion testing is regularly carried outin the glass industry as a means of assessing both theresistance of refractories and the aggressiveness of glassunder melting conditions. Several workers have dis-cussed various factors affecting refractory corrosion,(29)

the effects of temperature,(30)and the effects of glasscomposition.(31)Pavlovskii & Sobolev(30)noted a ten-dency for increasing activation energy with increasing

corrosion resistance of a melt cast refractory. The cor-

rosion rate displayed a linear relationship when log(corrosion rate) was plotted against reciprocal tempera-ture. Therefore at high temperatures, small decreasesin the melting temperature can cause large decreasesin corrosion rate. These effects were quantified.(31)Cor-rosion rates, R, were found to increase with tempera-ture roughly by factors of R1500C/R1400C=3 andR1550C/

R1500C=2. Hence any increase in refractory corrosionresulting from a more aggressive glass composition maybe offset by the decreased furnace temperatures.

The linear coefficient of thermal expansion of con-tainer glass is 859010-7/C in the range 20300C.It is a rule of thumb that additions which decreaseviscosity also increase thermal expansion and decreasethermal shock resistance. This does not apply in allcases, for example B2O3, but it can be used as a generalrule. Several workers have discussed mathematical re-lationships to describe the effect of different constitu-ents on the thermal expansion coefficient. These workswere discussed in depth and summarised by Volf.(32)

The main optical property considerations for con-tainer glass tend to be issues of colour rather than con-sideration of refractive index or dispersion. The maincomponents involved in container glass colour are ironand selenium (colourless flint glass), iron and chro-mium (green glass) and iron, carbon and sulphur (am-ber glass). These colours and their mechanisms andformulations are well established, and the colourantsused are present only at low levels. Colour in glass wascovered fully by Weyl.(33)Significant changes in the lev-els of colourants would dramatically alter the opticalproperties of container glass, so the addition ofcolourants such as most transition metals and lantha-nide elements was eliminated from this study.

A survey of current UK container glass composi-tions was carried out to provide a representative bench-mark composition against which candidate glassescould be assessed. The survey showed differences be-tween glasses from different manufacturers, but a rep-resentative average was chosen, namely SiO2720, Al2O314, Na2O 134, K2O 04, MgO 17, CaO 109, SO301,Fe2O301 (wt%). Slight differences between industrialand laboratory compositions in terms of levels of SO3and trace contaminants were ignored on the basis thatthe expected effects on the important properties wouldbe negligible.

Experimental

Batches were weighed to produce 500 g of glass using acalibrated two decimal point balance, and mixed for fiveminutes in a Turbula mixer, then added to the crucibleand placed in the furnace. Glasses were melted in anelectric furnace in DPH platinum crucibles at 1450Cfor 1 h. They were then poured into water, dried, crushedand remelted at 1450C for a further 5 h to ensure ho-mogeneity. Glasses were then poured into preheatedmoulds and held at 560C in an oven for 1 h to relieveinternal stresses, then cooled at 08C/min to room tem-perature. Visual inspection with polarised lightmicroscopy ensured the final glass samples were ho-mogeneous. Raw materials were analytical grade silica

sand, sodium carbonate, sodium sulphate, potassium



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carbonate, magnesium carbonate, calcium carbonate,lithium carbonate, and boric acid. Nominal glass com-positions based on batch calculations and measuredcompositions are shown in Table 1.

Elemental analyses were carried out on the candi-date glasses using a JEOL JSM6400 scanning electronmicroscope with energy dispersive spectroscopy (EDS)facility. All samples were measured at an accelerationvoltage of 20 kV. Several EDS measurements weremade at different points on each sample, and these re-sults were averaged. By comparing modelled viscositybased on batch compositions with measured viscosityfor each glass, steady underestimation by approximately10C in log two viscosity and 5C in log 4 viscosityresulted in each case. These differences were attributedto volatilisation losses during melting. It was consid-ered that these small but consistent losses may influ-ence glass composition enough to slightly affectphysical properties, however these factors were takeninto account when evaluating the errors associated witheach property measurement. Owing to limitations of

the EDS system it was not possible to measure the con-tent of light elements (Li, B, Na). Sodium content wasmeasured using x-ray fluorescence (XRF) spectroscopywith a Uniquant analysis program. Both EDS andXRF Uniquant are semi-quantitative methods and theerrors associated with these techniques are estimatedto be 1 wt% for SiO2, 05 wt% for other major con-stituent oxides (CaO and Na2O) and 03 wt% forminor constituent oxides (Al2O3, MgO, K2O, SO3).Within the limits of error of these techniques, glass

compositions were generally the same as compositionsbased on batch calculations. Volatilisation of smallamounts of alkali and boron could be expected to oc-cur during melting, but within the limits of error ofthe analysis techniques used it was not possible to quan-tify losses. In depth fully quantitative analysis wouldbe necessary during optimisation of any of these com-positions in future work in order to understand thevolatilisation losses and to help minimise them. I tshould be noted that sample C60A was a powderedsample whereas all others were solid glass, and thismay explain a slightly higher measured CaO contentin this sample, since EDS can be influenced by samplegeometry.

High temperature viscosity was measured by a ro-tating viscometer fitted with a platinum spindle in anelectric furnace. A calibrated thermocouple was usedto measure the glass temperature. M easurements werecarried using the method of ASTM Standard C965-81; Standard practices for measuring viscosity of glassabove the softening point. Modelled and measured

viscosities are shown in Table 2.Liquidus temperature was measured using a tem-

perature gradient furnace. Pieces of sample glass wereplaced in a platinum boat and held at temperature for24 h in a known temperature gradient measured witha calibrated thermocouple. Liquidus temperature wasmeasured by observing the sample using a polarisedlight microscope and deriving the corresponding tem-perature above which no crystals existed. Results areshown in Table 2.


Table 2. Measured properties of benchmark and candidate glasses

Property Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass

B C44 C3 C112A C60A C16B C39C C39B C112C C60B C46B

Modelled Tlog 2 (C) 1446 1430 1403 1406 1405 1397 1372 1375 1376 1365 1347Modelled Tlog 3 (C) 1178 1163 1139 1145 1137 1126 1108 1110 1112 1099 1083Modelled Tlog 4 (C) 1027 1013 996 1001 995 986 968 969 972 962 949Measured Tlog 2 (C) 1458 1430 1411 1417 1408 1405 1380 1385 1384 1376 1343Measured Tlog 3 (C) 1188 1175 1148 1158 1155 1134 1116 1136 1120 1129 1103Measured Tlog 4 (C) 1021 1010 990 994 996 980 962 974 966 970 951Liquidus 5 (C) 1015 1032 1010 990 955 942 975 985 965 964 950DTFL 5 (C) +6 22 20 +4 +41 +38 -13 -11 +1 +6 +1Density (gcm-3) 2507 2508 2524 2519 2518 2493 2570 2500 2524 2561 2508Density factor - 1000 1007 1005 1004 0994 1025 0997 1007 1022 1000a10-7(C) 897 913 969 1028 980 954 995 1038 1063 1033 1116Td(C) 624 614 625 612 628 637 601 599 600 600 591USP 25 acid (ml) 669 - 1010 799 758 - - 924 1035 644 878USP classification III - NP III II I - - NP NP III NPStatic corrosion 3 (%) 173 - 160 173 190 - - - - - -Tmreduction (C) - 28 47 41 50 53 78 73 74 82 115Estimated thermal - 20 30 25 30 30 35 35 35 38 43

NOxreduction 5 (%)

Table 1. Compositions in wt% based on batch calculations. Bracketed numbers are normalised combinedEDS and XRF analyses

Wt % Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass

B C44 C3 C112A C60A C16B C39C C39B C112C C60B C46B

SiO2 720 (725) 7175 (719) 704 (711) 694 (702) 700 (694) 700 (705) 698 (705) 692 (704) 695 (703) 690 (701) 6890 (707)Na2O 134 (133) 134 (134) 15 (146) 134 (133) 134 (134) 134 (135) 146 (141) 146 (142) 145 (147) 140 (137) 145 (140)K2O 04 (04) 04 (03) 04 (04) 30 (25) 24 (21) 24 (20) 20 (17) 20 (18) 24 (19) 25 (21) 20 (16)M gO 17 (17) 17 (19) 17 (16) 17 (17) 07 (06) 0 (0) 05 (07) 05 (05) 05 (06) 05 (05) 03 (03)

CaO 109 (106) 109 (107) 109 (108) 109 (107) 109 (118) 109 (108) 111 (110) 112 (107) 111 (107) 109 (108) 112 (103)Al 2O3 14 (12) 14 (12) 14 (12) 14 (11) 14 (13) 14 (12) 14 (13) 19 (17) 15 (13) 15 (12) 14 (11)Fe2O3 01 (0) 01 (0) 01 (0) 01 (0) 01 (0) 01 (0) 01 (0) 01 (0) 01 (0) 01 (0) 01 (0)SO3 01 (03) 01 (03) 01 (03) 01 (03) 01 (04) 01 (03) 01 (03) 01 (03) 01 (03) 01 (03) 01 (03)Li 2O 0 025 0 0 0 0 04 04 03 04 05B2O3 0 0 0 0 10 17 0 0 0 10 10


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NOxgeneration was decreased by approximately 20%.These results demonstrate the reasons why lithium haslong been under discussion as an ingredient of con-tainer glasses. Objections have thus far been basedlargely on economic grounds but with this situationpotentially changing, the use of lithium may becomemore acceptable in future.

Substituting 16% SiO2by Na2O in glass C3 broughtthe expected changes in measured properties. The USPdurability deteriorated from 669 to 1010 ml of 002Nacid required for neutralisation, hence the durabilityclass changed from Type II I to Type NP. The log 2viscosity was reduced by 47C, and, as with the addi-tion of lithium, this may cause concerns due to a valueof DTFLof -20C. This study found a reduction ofonly 5C in liquidus temperature between glasses Band C3, whereas a reduction of about 45C was sug-gested in studies on the substitution of similar levelsof SiO2by Na2O in similar glasses.

(35)Observation ofthe phase diagram for the Na2OCaOSiO2glass sys-tem(26)does not support a decrease of 45C in liquidustemperature if the primary phase field isNa2O.3CaO.6SiO2, devitrite. I t may be that the higherlevels of Al2O3in the other glasses

(35)caused the pri-mary phase field to shift to SiO2, tridymite, and thismay explain such large shifts in liquidus temperaturefor small replacements of SiO2by Na2O. Glass C3 gavean estimated 30% reduction in thermal NOxgenera-tion. This glass would probably not be considered forproduction without further modifications owing to itsrelatively poor durability and negative DTFL.

Glass 112A completes the sub-series of glassesstudying the partial replacement of SiO2by alkali ox-ides. This glass is particularly promising because it ful-

fils the criteria set out earlier for container glass. Thebenefits include reductions of 41C in log two viscos-ity, corresponding to a reduction of approximately 25%in thermal NOxgeneration. In addition the liquidustemperature decreased to 990C, and this gives a valueof DTFL of +4C, matching that of glass B. Therefore,the likelihood of devitrification problems is no greaterthan for glass B. The chemical durability is only slightlylower than glass B, but still USP 25 Type III . This isevidence of the mixed alkali effect, which has receivedconsiderable discussion. Early work by Peddle(41)dem-onstrated the strength of this effect on the durabilityof sodaleadsilica glasses containing Na2O and K2O.A glass with 3 mol% K2O and 12 mol% Na2O wastwice as durable as one containing 15 mol% Na2O. Thisexplains the relatively high durability of glass C112Aconsidering its higher total alkali content. Fluxline cor-rosion was unaffected by this increase, within the er-rors of measurement. Thermal expansion increased to102810-7/C however it is unlikely that this repre-sents a significant change in thermal shock resistance.Glass C112A demonstrates the potential benefits ofadding K2O to container glass, and utilising the mixedalkali effect to maintain durability.

The action of the alkaline earth elements in sodalimesilica type glasses is well understood(15,16,21,26)so itis possible to accurately predict their effects on prop-

erties such as viscosity,(2225,32)

thermal expansion(32)

and

density.(32)The phase relations have also been carefullystudied in these systems(15,21,26,42)and qualitative pre-dictions of liquidus temperature and crystallisation rateare possible based on phase diagrams.(26)I t is widelyheld that the addition of MgO decreases the liquidustemperature of sodalimesilica glass. The phase rela-tions in sodalimesilica glasses where some of the

CaO is replaced by M gO were discussed by Morey.(42)

For glasses with compositions in the tridymite phasefield, such changes can dramatically reduce liquidustemperature, with a minimum at 25 wt% MgO.(21,42)

However, the compositions of modern container glassare such that the primary phase field is devitrite, andthe addition of more MgO to these glasses would notproduce significant reductions in liquidus temperature.Morey(42)indicated that when devitrite is the primaryphase field, the addition of MgO to the parent glasswould increase liquidus temperature. The effects ofAl2O3were found to be similar to M gO,

(42)in that whenthe primary phase field is tridymite, the addition ofAl2O3strongly decreases liquidus temperature, butwhen the primary phase field shifted to devitrite, theliquidus temperature increased with increasing Al2O3levels. Increased levels of MgO or Al2O3could there-fore not be justified in many of the glasses in this study.In addition to the lack of beneficial effects on glassstability in the devitrite phase region they increase hightemperature viscosity, contrary to the objectives of thisstudy. The beneficial effects of Al2O3on chemical du-rability are well known(21,26,42)so in general the level ofAl2O3was maintained, however the replacement ofMgO proved very effective in reducing liquidus tem-perature and high temperature viscosity, as shown inTable 2.

Boron is one of the most efficient melting accelera-tors in silicate glasses when added at low levels.(9,10)Bo-ron compounds in the batch initiate glass formation atan early stage and boron can therefore be useful as asubsidiary flux in sodalimesilica glass batches inamounts up to about 15% B2O3.

(9)Boron lowers sur-face tension, further increasing the speed of meltingand refining, and it reduces thermal expansion. A l-though it has not been quantified in this study, it isclear that such fluxing action would contribute to lowermelting energies. In the 1920s and 1930s a great numberof container glass manufacturers in the UK and USAincorporated small levels (0315%) of B2O3in theirglass.(10,17,43)Simplification of glass batches in the in-tervening years led to the removal of B2O3from mostwestern container glasses. Other workers have indicatedthat low levels of B2O3can reduce melting and refin-ing times and decrease the liquidus temperature andcrystallisation rate.(10,4245)Morey(42)described the effectsof B2O3on crystallisation of sodalimesilica glass.Addition of B2O3shifted the phase field boundary be-tween Na2O.3CaO.6SiO2 (devitrite) andNa2O.2CaO.3SiO2to lower SiO2contents and acted toreduce liquidus temperatures. The beneficial effects ofB2O3on viscosity and chemical durability are well-documented.(21,2325,28,42)The addition of B2O3and moreK2O to glasses C60A and C16B reduced both viscos-

ity and liquidus temperature. This effect was probably



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composition and raw materials so quantification at thisstage would also be limited in its usefulness. Based ontemperature factors alone and assuming all other fac-tors remain unchanged, we estimated that reductionsin particulates of up to 50% may be possible for a dropof 100C in melting temperature.(3)However, anychemical reformulation of container glass will require

different levels and/or types of raw materials and culletadditions. This will in turn affect melting behaviourand particulate generation. I t was therefore consideredthat discussion in this paper of the potential reduc-tions in melting energy and particulate generationwould be premature, and should only be carried outwhen an investigation of the effects of raw materialsand cullet has been carried out. L ikewise the potentialreductions in SOxand heavy metal (selenium) emis-sions, as these are also dependent upon the final glassbatch. We believe it is likely that reductions in SOxandselenium would occur using the reformulated compo-sitions, as volatilisation of these components is stronglytemperature dependent.(46,50)SOxemissions are alsogoverned by glass redox, sulphate solubility, meltingrate and raw materials so these will vary from furnaceto furnace. As with melting energy and particulates,further work will be required to quantify these reduc-tions. Nevertheless, we feel that there is a strong likeli-hood of achieving reductions in melting energy,particulates, SOxand heavy metals by implementingour reformulated compositions.

In addition to its direct effect upon melting energy,the use of cullet could also affect other aspects of theproduction of reformulated glasses. Since the culletcurrently available would no longer be of the samecomposition as the final target glass, it would be nec-

essary to make batch alterations to take account ofthis fact. Over time the average cullet compositionwould gradually change as the new glass entered therecycling chain. This cullet would then begin to moreclosely resemble the final glass in its composition. Thisopens up a number of logistical and political issueswhich would need to be addressed, and it depends uponuptake of the technology and cullet and raw materialsourcing.

Although some efforts have been made towards as-sessing the logistical and economic factors associatedwith container glass reformulation,(3)further studiesare required in order to link all the pertinent factorsand direct development. Crystal growth rate was notmeasured, however changing the glass compositionmay affect this parameter.(21,26,45)It is suggested thatfuture work should feature analysis of crystal growthrate, volatilisation losses, furnace crown refractory cor-rosion and the melting and refining behaviour of newbatches. Quantification of reductions in melting en-ergy and emissions of CO2, NOx, SOx, particulates andheavy metals could take place in tandem with thisoptimisation process.

Conclusions

A number of reformulated container glass composi-tions have been developed with reductions of up to

115C in melting temperature and an estimated 20

40% in thermal NOxgeneration. Reductions in melt-ing energy and emissions of SOx, particulates and heavymetals are also expected to occur but have notbeendiscussed here, as the effects of raw materials must firstbe assessed.

Several candidate glass compositions met the prop-erty criteria set out for the benchmark composition,

glass B. These criteria included high temperature vis-cosity, liquidus temperature, DTFL , and USP Type 25chemical durability. Other important measured prop-erties were density, dilatometric softening point, staticrefractory fluxline corrosion and thermal expansioncoefficient. The new compositions offer a range ofoptions for optimisation through further development,from relatively simple substitutions to multiple com-ponent changes. However, it is estimated that even rela-tively small compositional changes can result insignificant reductions in thermal NOxgeneration.

A full optimisation process must take account ofwider issues impacting upon the industrial containerglass production process, including logistical and eco-nomic concerns, furnace lifetimes, volatilisation andparticulate generation, raw materials and cullet selec-tion and availability, and melting reactions. This willallow full development of the most suitable, most ac-ceptable new glass composition. We believe the tech-nological foundations for such a process have been laidin this paper.

Acknowledgements

The majority of the work described in this paper wascarried out at Glass Technology Services Ltd. P. A .Bingham is now employed by the University of Shef-field and wishes to thank both organisations for their

support. The authors acknowledge the funding sup-port of the Carbon Trust for the work described inthis paper, which constitutes part of a larger project.The statements made and views expressed are those ofthe authors and should not be attributed to the Car-bon Trust which accepts no responsibility for the con-tents of the paper. The Carbon Trusts funding isprovided by grant from Defra (the Department for En-vironment, Food and Rural A ffairs), the Scottish Ex-ecutive, the National Assembly for Wales and InvestNorthern Ireland. Glass Technology Services have nowsecured further funding from the Carbon Trust to con-tinue the project. This work will include industrial tri-als involving the UK container glass industry. Enquiriesconcerning the ongoing project should be directed toM. Marshall.

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Reformulation of Container Glasses for Environmental