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76 GLASS INTERNATIONAL SEPTEMBER/OCTOBER 2004

Creating an optimal glass batch quality is not only a matter of mixer performance, says Fons Rikken*.

Batch mixers, which belong tothe group of mechanicalintensive or non-intensiveagitators, cover a capacityrange from 1 litre to 5m3.

Types like rotating pan mixers; ribbonmixers; orbiting screw mixers;ploughshare mixers and ring trough mix-ers are very common in glassmaking.Improvement of the agitator tools is stillgoing on in combination with a more effi-cient way of supplying energy into thebatch. Due to an increasingvariety of materials to bemixed and an increasing“batch quality” demand,many mixers have beenadapted to more than oneapplication. Like otherprocess equipment, thedevelopment of mixers hasbeen empirical. The lack ofreliable up-scaling rules is aproblem in choosing the bestmixer for each application.Of course mixer manufacturers possessmuch empirical data from their own testruns. However it is regrettable that mostmixer manufacturers publish their mixerperformance data in a rather confusingway. This often covers their commercialinterest, while the mixer performancedata is often missing the correct informa-tion to enable an objective comparison,and is mainly presented in terms of“degree of variation”, whatever thatmeans. It would appear that all mixer

types perform at top class with a degreeof variation below 0.5%.

Philips in mixingPhilips glass industry has met this diffusedata problem several times during thepast 25 years, in trying to make a mixerchoice for its glass and non-glass batchplants and shows an arbitrary variety ofmixers for the same application.Worldwide Philips has some 40 mixers inoperation for its powder batching activi-

ties with a total bulk amountof about 600,000 tonnes peryear. Some 15 years agoPhilips glass industry took thedecision to eliminate the lessefficient premix groups formicro ingredients(<500ppm). New types ofintensive mixers were able toperform “in line mixing” formicro ingredients down to100ppm. To investigate thisnew phenomena and check

the factual mixer performance data, areliable batch homogeneity measurementwas needed. The “variance analyses”method had already proven reliable indefining the “variation coefficient” (VC)as an index for the batch homogeneity. Inmicro ingredients “in line mixing”, VC-results showed that the performance ofintensive mixers was indeed comparablewith the traditional pre-mix process or evenbetter. Consequently,

Philips has sinceeliminated allthe premixgroups in itsbatch houses. The variation analysismethod proved to be sustainable and isstill used for checking the performance ofbatch mixers and batch quality.

Defining VC mixer performanceAlthough some mixer manufacturersdoubt the reliability of defining the batchhomogeneity in the mixer itself (prefer-ring sampling downstream), Philips doesso because the reproducibility has showngood results with the chosen method. Asignificant measurable tracer as represen-tative of the total batch is important for areliable result and an amount of 100ppmis added to the batch. For tracer analysisin the batch, X-ray fluorescence is com-monly applied. Also photo spectral analy-sis and acid soluble methods are in use.For the purpose of analysis, five (20ppm)samples are taken at different positions inthe mixer (a series of ten samples aretested as well, however, the results are notsignificantly different). Samples are nottaken from the batch surface layer, whichmeans that a sample spoon with remov-able cover must be applied (see picture).

Focus on mixer performanceand glass batch quality

� Sample spoon withcover.

� Sampling the mixer.

� Example of calculation chart of one series with 5x4 samples.

� Performance of four non-intensive mixer types (~1-2kW/100kg).

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Sample size is limited to 30ppm for a reli-able application of the variance analysismethod. The samples are taken out of thebatch in the mixer, by stopping the mixernormally at one-minute intervals.

Each sample is divided into four sub-samples, giving 20 samples to beanalysed. The reason for the sub-sam-pling is the correct interpretation of thevariance analyses by excluding the mea-suring failure in defining the so called “inbetween variance” instead of the “totalvariance”. The total variance outcomealways shows a better result and is there-fore more often used. The correct inter-pretation of the variance analyses ischecked by the F factor, a statistical checkfor the variance analysis application:“F factor” = s2 (between)/s2 (within) to be > 3.The formula for the “variance between” is:s2 (between) = {(sum of squares total – sum of

squares within)} / {(degrees of freedom total) – (degrees of freedom within)}

The formula for the variation coefficient (homogeneity index):

VC (between) = s (between)/total average * 100%

Performance of mixer typesWith the application of the “varianceanalyses method” Philips has checked theperformance of almost all its batch mix-ers. For each mixer type, a series of 5x4samples were taken in one-minute inter-vals up to a total mixing time of 6 min-utes. For checking the reproducibility theprocedure was repeated twice. Each pointin the VC curve is the outcome of 20analyses, so in total 120 analyses arerequired to present a VC curve for onemixer. Although the analysis method isexpensive, it can be compensated by anefficient selection of the tracer material.Costs must always be in coherence withexpected results.

The first series of VC measurementsshows the performance of four “non-intensive” mixers, processing dry “light-

ing” glass batch withoutcullet, factoring in: thescrew orbit mixer, rib-bon mixer, ring troughmixer and rotating panmixer. The batchenergy consumptioncomes to 1kW/100kg fororbit screw and ribbonmixers and 2kW/100kgfor the ring pan androtating pan mixers.

The rotating pan mixer shows from oneminute mixing time onwards the bestresult. The performance of the orbit screwmixer after 30 minutes showed a VC levelof about 35%.

The second series of VC measurementsshows the performance of four “intensivemixers” processing dry “lighting” glassbatch without cullet, factoring in: the ringtrough mixer with intensive rotor, ploughshare mixer with intensive rotor, rotatingpan mixer (horizontal type) with intensiverotor and rotating pan mixer (20ºinclined type) with intensive rotor. Energyconsumption was 5kW/100kg.

In this series the inclined rotating panmixer is superior to the other ones.However it is essential to know if thehighest created batch homogeneity levelis really required. It mattered whenPhilips tried to optimise the productivityof a specific glass plant. In trying toincrease the melting capacity of a tubelight furnace, Philips performed someinvestigations in the relation betweenbatch homogeneity and the specific glass-melting rate (SMR). Results showed thatfor keeping the 90% furnace productivity,the SMR could only be raised from 60 to75 (kg/m2.hr), by increasing the batchhomogeneity at a level of VC <10. Philipsnow prefers to use inclined rotating pan

mixers for specific appli-cations. In Philips TVglass business, batchhomogeneity require-ment are even morestrict down to VC <5.

What about batch quality?In the glass industry,batch quality is morethan mixer performance,it is the outcome of a

chosen compromise between homogene-ity, consistency and flow behaviour condi-tions, knowing that batch consistency andflow behaviour are always in conflict witheach other. The maximum attainablebatch homogeneity is only related to themixer performance under the givenmaterial grain size conditions. It is com-mon in batch processing that “homo-geneity decrease” starts at the moment ofmixer discharge and continues down-stream by transport manipulation. Torestrict the homogeneity decrease istherefore a matter of correct batch con-sistency and flow conditions.

Batch consistency can be improved byselecting the batch ingredients in a smallgrain size, small grain size distributionand batch moisturising (mainly water oroil). Batch flow behaviour can beimproved by selecting the batch ingredi-ents in larger grain size and preferablydry batch. Because of the compromise it isimportant that the batch mixing andtransport equipment are chosen correctly.

The third series of VC measurementsshows the decrease of batch homogeneitydownstream from mixer to furnace batchcharging in coherence with the batchmoisture content.

ConclusionBatch quality requires an optimal choiceof batch consistency to restrict segrega-tion in downstream transport while keep-ing the optimal batch flow conditions.Batch quality also requires a consideredselection of transport equipment and“mass flow” silo design for storage. Whilerealising that the required batch homo-geneity will be different for each glassapplication. Personally I think the inten-sive mixer is a must in the glass industry,because the attainable homogeneityincreases favour productivity and furnaceenergy consumption.

GLASS INTERNATIONAL SEPTEMBER/OCTOBER 2004 77

* Fons Rikken, Philips Lighting Components, Eindhoven, The Netherlands. Tel: +31 4027 88492.Email: fons.rikken@philips.com

� Performance of four intensivemixer types (~5kW/100kg).

� Decrease of batch homogeneity in downstreamtransport from intensivemixer to furnace.

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