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Glass International July/August 2008

Batch plant

The economic significance of batch

calculationAs in many other branches of industry, raw material costs havea decisive share in the manufacturing costs of glass production.This applies especially to mass produced products such asbottles or float glass. Henning Katte* of Ilis, Germany explainsthe economic potentials that exist when using modern batchcalculation programmes such as Ilis’ BatchMaker Suite.

The objective of batchcalculation in glass productionis compliance with a defined

chemical glass composition, whichdetermines the properties of thefinal product but also has aninfluence on melting properties andprocessability. Thus batchcalculation also has a substantialinfluence on the energy costsinvolved in glass manufacture.

Batch calculationFFiigg 11 shows the basic batchcalculation process. A pre-condition is knowledge ofthe chemical compositionof the raw materials usedand the types of cullet. Theraw material analyses areeither maintained directlyin BatchMaker or importedfrom a separate laboratory

glass recipe. The batch recipecontaining the weights of theindividual raw materials and thetheoretical glass compositioninversely calculated from these(known as the synthesis) is calculatedautomatically from the glass recipeand the raw material analyses. Thesynthesis should correspond to thedesired glass composition.

In calculation it must be takeninto account that, for example, SiO2

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information management system(LIMS). The data originates from therelevant raw materials supplier and/orare based on own analyses.

The desired chemical compositionof glass (nominal analysis) as well asthe statement of the main carriers foreach component (for example glasssand for SiO2, soda for Na2O,limestone for CaO, feldspar for Al2O3)are the essential components of the

Fig 1. Typical batch calculation process. Thebatch recipe, containing raw material weights, the theoretical glass composition as well as theglass properties resulting from this, is calculated from the raw material analyses andthe glass recipe. �

is included not only by theassigned main carrier but alsooriginates from carrier rawmaterials of other oxides such asfeldspar. Furthermore, not all rawmaterials can be calculatedautomatically since certainprerequisites must be fulfilled thatcannot be derived from the glasscomposition, for example fixedglass percentages for cullet orfixed additions of reducing,refining or colouring agents.

The calculated weights are thencommunicated to the batch house.The batch recipe contains theassociated batch and glassproperties as well as the rawmaterial costs and the glass price(€/tonne of glass).

FFiigg 22 shows a typical glass recipefor container flint glass including a

statement of thedesired glasscomposition andallocation of thecarrier raw materials.FFiigg 33 shows the batchrecipe calculated fromthese. It is notablethat raw materialssuch as Calumite orrecycled cullet bringmany different

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components into the glass, whichwould make manual batchcalculation very time-consumingand susceptible to error.

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Glass International July/August 2008

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Batch plant

Three specific examples follow,for which saving potentials existwhen using modern batchcalculation programmes such asBatchMaker.

Example 1Let the current chemicalcomposition of a container glass be72% SiO2, 14% Na2O, 10% CaO,1.5% Al2O3 and 2.5% MgO (all datain mass-%). With assumed rawmaterial costs of €30/t glass sand,€200/t soda, €15/t limestone,€20/t feldspar and €30/t dolomite,raw materials costs amount toapproximately €22,000/day at anaverage daily production of 300 tonnes glass (see TTaabbllee 11).

By reducing the sodium oxideshare by 1% (in exchange for CaOand SiO2), the raw material costscan be reduced to approximately€21,000/day in this scenario. Thesavings potential therefore isapproximately €350,000/year. Sincethe glass is now ‘shorter’ andtherefore reaches its strength faster,an increase in cutting rate of 2-3%can also be expected.

Example 2To avoid unwanted greencolouration, the iron percentage inflint glass should not be greaterthan 0.15%. For this purpose glasssand with a relatively low ironcontent is used, which is €5/t moreexpensive than the normal quality.The actual iron percentage in the

produced glass lies in this example atapproximately 0.1%, therefore isclearly below the limit.

By definition of a nominal value of0.15% for Fe2O3 and stating thestandard sand as ‘carrier raw material’for iron, BatchMaker can calculateautomatically how much of the low-iron sand can be replaced by thenormal quality without the limitbeing exceeded. For a productionamount of 300tpd and 40% cullet thesavings potential is approximately€500/day or €180,000/year.

Example 3When fixed weights are used evensmall changes in the raw materialchemistry inevitably lead to smaller orlarger fluctuations in glasscomposition. This is not desirable,especially for components that are

brought into the glass by expensivesynthetic raw materials such as soda.The fluctuation range can beminimised by regular recalculation ofthe weights on the basis of the currentraw material and glass analyses.Assuming a soda price of €200/t and adaily tonnage of 300 tonnes the rawmaterial costs of glass can be reducedby approximately €100/day or€36,500/year by reducing the size ofthe Na2O fluctuation range forinstance from 0.2% to 0.1%.

Prediction of propertiesGreat economic importance isattached to the prediction of glassproperties, since the number of trialmelts and thus the expense of timeand costs can be reduced drasticallyby further developments of glasses.

Fig 2. Glass recipe for a typical bottle flint glass in BatchMaker.Fig 3. Batch recipe for the glass recipe from Fig 2. The right side of the windowshows which components the raw material marked on the left brings into theglass and at which percentages.

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Table 1. Saving potential by changing the glass chemistry for a container glass tank with adaily production of 300 tonnes glass.

Before change After change

SiO2 72% 72.5% (+0.5%)

Na2O 14% 13% (-1%)

CaO 10% 10.5% (+0.5%)

Al2O3 1.5% 1.5%

MgO 2.5% 2.5%

Thermal expansion 9.32 10-6/K 9.01 10-6/K

Density 2.504 g/cm3 2.504 g/cm3

Rel. machine speed 110.2% 113.3% (+3%)

Batch costs (per day) €21,938.60 €20,986.62 (€-951.98)

Batch costs (per year) €8,007,589 €7,660,116 (€-347,473)

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In addition, many glass properties,such as viscosity, can only bedetermined chemically with greatmeasuring effort. The measuringequipment required for this is notavailable in many glass plants. Also,authorising an external laboratorymeans a time delay, which in thecase of a fault can cause a loss ofseveral days’ production.

However, even during normalproduction it is importantfor maximising the yieldand avoiding complaints to know whether changesin glass chemistry, whether intentional or not,have an effect on theprocessing ability and theproduct propertiespromised to the customer.The calculation of therelevant glass properties on

the basis of the current glassanalysis is therefore an importantbuilding block in the sense ofprocess stability.

For a long time there has been alarge number of simplemathematical models for calculatingglass properties from chemicalcomposition. It is important in thisconnection that each model is validonly for certain glasses.

Unfortunately, the applicationlimits are not clearly defined formany literature models and an errorconsideration is often not possible.

Therefore, modern calculationmodels that are based on thestatistical analysis of hundreds ofsingle analyses are used in Ilis’BatchMaker. FFiigg 44 shows, as anexample, the temperature-dependent viscosity curve as well

as important viscosityfixed points for threedifferent glasses. Theconfidence interval isstated in each case afterthe calculatedtemperature. The redbackground in the ‘leadglass’ column means thatthe associated glasscomposition lies outsidethe application limits. �

* Henning Katte, Ilis GmbH, Germany.Email: info@ilis.deWebsite: www.ilis.de

Fig 4. Calculated viscosity curve forthree different glass compositions.Apart from the calculated temperaturevalues, the confidence intervals arestated in the table.