Microsilica containing refractory castables - Elkem

More documents

Recommendations

Info

$ageing of refractory castables; which interactions exist and ... - Elkem$

chains and that longer side chains consequentlyretard more than shorter 2 .Deflocculants and flow:To check quality of microsilica or cement or othercomponents of low cement castables, Elkem hassince the early 1990’ies, used a model LCC basedon white fused alumina with 6% CAC and 8%microsilica. The castable is composed to follow anAdreassen type PSD with a q-value close to 0.25giving flow properties close to self-flowing at4.15wt% water (=13 volume %). The castablecomposition is given in Table 1.Table 1 Standard LCC castable composition.[wt%]Microsilica (18 or 24 m 2 /g) 8CA-14 [wt%] 6White Fused Alumina. 3-5mm 10White Fused Alumina. 0,5-3 mm 32White Fused Alumina. 0-0,5mm 16White Fused Alumina. -74 micron 12Calcined alumina: CT 9FG 16Deflocculant: Castament FS20 0,01-0,1*Water [wt%] 4.15* Standard dosage, 0.05%To check the effect of the Castament FS20 on flowof a castable, the standard LCC mix (Table 1) wasused with the amount of deflocculant as thevariable. Two different microsilica qualities withsurface area of 18 and 24m 2 /g were used. In Table1, the composition of this mix is given. In Figure 7,the resultant self-flow values are shown as afunction of deflocculant addition per microsilicasurface area (mg/m 2 ).The two microsilica qualities gives very similarresults when expressed as a function of surfacearea, only the maximum self flow differssomewhat. Although lower self flow was obtainedwith the microsilica with the higher surface area,and conclusions in the direction of lower flow athigher surface area would be tempting, we do nothave data proving that this actually is the case.Self-flow (%)10090807060504030201024 m 2 /g00 0.2 0.4 0.6 0.8FS20 dosage (mg/m2)18 m 2 /gFigure 5. Self flow of white fused alumina basedLCC as a function of deflocculant addition.Effect of dispersantAbove, the good correlation between microsilicasurface area, flow and amount of additive wasshown. Those findings were based on flowmeasurements of LCC based on fused alumina, butthe same correlations exist for bauxite basedsystems.Table 2: Bauxite based LCC compositions.(%) 1 2Chinese bauxite 1-30 304mmChinese bauxite 0-30 301mmFused Al. -74 mic 18 18Secar 71 6 6Calcined alumina 8 8Microsilica (971U) 8 8FS20 0.01-0.15Calgon 0.01-0.40Water 5.50 5.50In Figure 6, self-flow of castable 1 and 2 (Table 2)is presented as a function of addition of dispersantin mg per square meter microsilica surface. Inpractical terms, an addition of 1 mg/m 2 equals to anaddition level of approximately 0.2%.Earlier results on white fused aluminasystems(Table 1) have shown similar dependenciesfor FS20 and Calgon, as shown in Figure 7.
120100FS20Calgon120100FS20CalgonDarvan 811DSelf-flow (%)806040Self-flow (%)806040202000.00 0.50 1.00 1.50 2.00 2.50 3.00Dosage (mg/m 2 )Figure 6: Effect of dosage of dispersants on selfflowof bauxite-based LCC (number 1&2 in Table2).If Figure 6 and 7 are compared, it is seen that forFS20 the appearance is quite similar, while forCalgon it is in principle similar, but somewhatskewed. There can be several reasons for theslightly distorted shape of the curve for Calgon, butthe first that come into mind is that the phosphatereacts with impurities of the bauxite, thusnecessitating higher dosages than in the purer whitefused alumina system. Further, as also Darvan811D is included, it should be commented.Darvan being a poly-acrylate as opposed to FS20being a carboxylate-ether differs by having apronounced optimum addition level as opposed tothe more flat plateau level that is seen for FS20.This is probably connected to the mode ofstabilisation of colloidal slurries of the twodifferent additives. Darvan is by electrostaticrepulsion, FS20 by a combined steric andelectrostatic mechanism.00 0.5 1 1.5Dosage (mg/m 2 )Figure 7: Effect of dispersant addition on self flowof white fused alumina based castables with 6%cement and 8% microsilica, 4.15% water.If we look at the Zeta-potential for the combinationof microsilica and the additives in question thefollowing is found, Figure 8.Zeta-potential (mV)0-10-20-30-40-50-60-70-80-900 0.5 1 1.5 2Surfactant content (mg/m2)FS20Darvan 811DCalgonFigure8: Zeta potential of 10% aqueous slurry ofmicrosilica 971 as a function of dispersant additionexpressed as mg/m 2 microsilica surface.Figure 8 shows the zeta-potential of a microsilicaslurry as a function of dispersant addition. Here,Darvan 811D, Calgon and FS20 have been tested.Two phenomena may be noticed, firstly thelowering (or lack of it) of zeta-potential caused byFS20 indicates that the stabilisation mechanism isprimarily of steric (uncharged) nature. The secondobservation is that there is a good correlationbetween the zeta-potential and the flow. This issuch that the maximum flow appears at, or close to,the point of the relevant zeta-potential curve whereit either attains minimum or, in the case of Darvan,the slope changes abruptly.
Page 1: MICROSILICA CONTANINGREFRACTORY CAS
Page 4 and 5: Based on theoretical and experiment
Page 8 and 9: Set of microsilica containing casta
Page 10 and 11: FS20 then becomes relatively stable

Microsilica containing refractory castables - Elkem

Create successful ePaper yourself

Delete template?

Save as template?