Rapport nr. 52 - Elkem

MgO – SiO 2 – H 2 Obonded MgO based castablesPart 2: Effect of pumping and wet shotcreting on cold and hotproperties and slag resistanceNingsheng Zhou, Sanhua Zhang, Shuhe Hu, Zhenyong Bi, Luoyang Institute of Refractories ResearchZhiqiang Chen*, Cecilie Ødegård, Bjørn Myhre, Bjørn Sandberg, Elkem ASA MaterialsPresented at UNITECR ’03, 8th Biennal Worldwide Conference on Refractoriesin Osaka, Japan, October 19-22, 2003.The fine art of

MgO – SiO 2 – H 2 O bonded castablesPart 2: Effect of pumping and wet shotcreting on cold and hotproperties and slag resistanceNingsheng Zhou, Sanhua Zhang, Shuhe Hu, Zhenyong Bi, Luoyang Institute of Refractories ResearchZhiqiang Chen*, Cecilie Ødegård, Bjørn Myhre, Bjørn Sandberg, Elkem ASA MaterialsAbstractTrial installations of MgO refractory castables inmoulds by pumping and wet sprayed onto woodenboards were performed using an Allentown Pump RP-10. Based on previous work, the self-flowing castablewith microsilica contents of 6 wt% for both selfflowingand vibration castable was chosen out of anumber of test castables. The given two recipes are suitablefor pumping due to their flowability andsufficient working time. The pumped and sprayed sampleswere compared to the samples prepared in thelaboratory in terms of bulk density, apparent porosity,cold modulus of rupture, hot modulus of rupture at1200-1500°C and refractoriness under load. Samplesfrom the lab-prepared, pumped and shotcreted, weresubjected to slag attack by rotary slag test to comparetheir slag resistance. Two kinds of steel-making slagwith high and low CaO/SiO 2 ratios were used.Differences have been found and discussed.1. IntroductionWith good resistance to high basicity slag in modernsteel-making processes, MgO clinker is an indispensableraw material for basic refractories, both bricks andmonolithics. High-performance basic monolithicrefractories have attracted strong interest and attentionfor state-of-the-art steel-making processes and forclean steel production, due to many unique advantages.MgO-based castables have become a hot topic recently.Studies have shown that microsilica added to anMgO-based castable mix can react with water andmagnesia fines to give strong bonding1, 2, 3). This reactionis believed to be formation of a magnesia-silicatehydrategel4), which facilitates the setting and binderstrength development. It has also been found thatmicrosilica in such a system suppresses the slakingtendency of MgO.Castables with the MgO-SiO 2 -H 2 O bond have beenused in some countries for steel ladles and tundishweirs or dams. Another key role of microsilica is toimprove flow and setting properties. Otherwise themixes would have poor flow characteristics, inhibitingthe use of highly efficient installation techniques suchas pumping and shotcreting. With proper combinationof particle size distribution, microsilica and additives,MgO-based mixes can be modified to allow forpumping and wet spraying.As the castable is heated, microsilica and magnesiaform forsterite. The MgO-SiO 2 system mainly reliesupon the forsterite for high temperature bonding4).Forsterite is a refractory phase with a melting pointabout 1890°C, and the eutectic in the system of2MgO·SiO 2 –MgO is at 1850°C. As alumina isintroduced in the system, a low melting phase is likelyto form, rather than a strong spinel-forsterite bond. Ithas been suggested that formation of some in-situspinel may counteract the volume shrinkage.Based on our previously published work as Part 1,“Effect on flow, set and hot properties when substitutingmicrosilica with alumina in pumpable MgO-basedcastables”, one recipe was selected from the selfflowingcastables, with MS/RA=6/7.2 (wt.), labelled asEL-3; and one from the vibration castables, withMS/RA=6/0 (wt.), labeled as EL-5. The El-3 has a freeflowvalue of 120%, while the EL-5 about 60%. Bothcastables have a working time of no less than 2h,allowing them to be good candidates for trial shooting5).When a castable is installed by spraying, its propertieswill be affected, mainly the increased porosity,impurity from the accelerator and inhomogeneity,having influence on the performance and refractorinessof the lining. Another influence is the lack ofexperience in operating the machine, in terms of airpressure, accelerator type and addition, distance,moving speed, etc. The aim of this work was toinvestigate what effects this new installation method ofwet shotcreting or, in other words, wet spraying, mayhave on the hot properties (hot modulus of rupture,refractoriness under load behavior and slag resistance)of such MgO-based castables.2

2. Procedure2.1 Samples preparationIn the laboratory, the ingredients were dry-mixed in aHobart mixer for 4 min., water was added, and then wetmixed for an additional 4 min. to make EL-3 and EL-5samples. Same mixing time and water addition wereused in the case of larger on-site mixer with a batch of500kg dry mixes one time. 1000kg, in two batches,mixed-fresh castables, were transported by a crane tothe hopper of the pumping and spraying machine.The EL-3 and EL-5 castable mixes were pumped andsprayed by an Allentown Pump RP-10 machine, undersuch operation parameters as pumping capacity of 8-10tons per hour, pumping pressure 14MPa, hose diameter75mm, hose length 15m, nozzle tip 38mm, air pressure0.7MPa and air flow 6m3/min. Al 2 (SO 4 ) 3 saturatedsolution was used as an instant accelerator to the wetmixes, with an average calculated addition of 0.12%solution, based on the dry-mix mass. The distancebetween nozzle tip and the sprayed surface was0.5-1m. The ambient temperature in the workshop was5-10ºC. The shoebox-like pumped blocks with adimension of 230mm in length, 160mm in width and140mm in height, and the sprayed blocks, 900 mm inlength, 700 mm in width and approximately 150 mm inheight, were properly cured and then subjected to400ºC treatment prior to being cut into different shapesand sizes for different tests. The pumped specimen waslabelled with P, i.e. PEL-3 and PEL-5, and the sprayedas S, i.e. SEL-3 and SEL-5.2.2 Cold modulus of rupture (CMOR)Samples were then fired at 600ºC, 1000ºC, 1200ºC,1400ºC and 1600ºC for 5h. CMOR was measured onsamples of 40x40x160mm according to ASTM C348.2.3 Hot modulus of rupture (HMOR)HMOR measurement was performed on samples of25x25x150mm in accordance with the standard ofPRE/R18. Samples were pre-fired for 24h at the testtemperatures and allowed to cool before being loadedinto the test furnace. The heating rate was 300ºC/h forboth pre-firing and testing, and the samples wereallowed to soak at the test temperature for 30 min priorto loading.2.4 Refractoriness under load (RUL)RUL was measured on cylinders of 50mm in diameterand 50mm in height with a 12mm central bore, asdescribed in ISO R1893 (1970). Loading was 0.2MPa.The samples had been pre-fired for 24h at 1500ºC,unless otherwise stated. Heating rate was 300ºC/h forboth pre-firing and testing. The equipment wasdesigned in accordance with descriptions of ISOR1893 (1970); the maximum accessible temperaturewas 1800ºC for testing under rising temperature.2.5 Rotary slag testTwo batches of rotary slag test were conducted, eachusing 12 different specimens, with a longitudinalsection of approximately 230mm in length and 60mmin height, installed to make a drum in the furnace, to besubjected to the attack of in-plant picked BOF slagwith CaO/SiO2=4.58 and EAF slag withCaO/SiO2=1.26, respectively. The furnace rotating at 6rpm was heated to about 1500ºC and first batch of 1kgslag was fed; the temperature was raised to 1600-1650ºC and another 1kg slag was fed, then soaked at1650ºC ±50ºC for 130 min, during which 4kg slag wasfed at a rate of about 1kg per 30-35min. The furnacewas naturally cooled down. The slag-attackedspecimens were sectioned longitudinally. Visualexamination and measurement on local eroded depthwere pursued. Averaged eroded depth (H) was used torepresent slag resistance, calculated by the givenformula: H=Ho-h, where Ho is the averaged originalheight, using 3 measurements at a 50mm intervalseparating from the central symmetrical line, andh=∑hi/11, (i=1~11) is the averaged remainingunattacked height, using 11 local data at a measuringstep of 10mm symmetrically to the central symmetricalline.2.6 RecipesChinese-fused magnesia (MgO>98%) was adopted asthe main raw material. Reactive Alumina CTC50 wassupplied from Alcoa and 971U Elkem Microsilicafrom Elkem Materials. Table 1 provides the recipeswith q=0.25 for self-flowing castables, while q = 0.28for vibration castables5). The water addition was keptat 15.2 vol%.3. Results and Discussion3.1 Bulk density (BD) and apparent porosity (AP)Not surprisingly, samples cast in lab show the highestBD and lowest AP, see Fig. 1 and Fig. 2. Comparisonof properties of the lab samples to the pumped and thesprayed will show how much difference is introducedby the placement method. In Fig. 2 it is seen that thedifference in AP of the 1200-1600°C fired castables isobvious, with the sprayed being the highest, the labpreparedthe lowest and the pumped inbetween. Toolow of an ambient temperature that the trials were subjectedto could be a partial reason leading to reducedflowability and setting, hence less density and higherporosity.3

Table 1 Composition of the two kinds of castablesLabel EL-3 EL-5Castable type Self-flow Vibrationq-value of Andreassen model 0.25 0.28MS/RA volume ratio 60/40 100/03-5 mm 10 10Fused MgO1-3 mm 27 270-1 mm 25 27-44 micron 24.8 30Microsilica 971U (MS) 6 6Reactive Al 2 O 3 CTC50 (RA) 7.2 0Dispersant Vanisperse CB 0.25 0.25Water (15.2 vol.%), wt% 5.16 5.19Fig. 3 PLC as a function of temperature3.3 Cold Modulus of Rupture (CMOR)There is a strength valley for all the samples fired at1000°C, as shown in Fig. 4. This is in accordance withthe finding that forsterite bonding in this system canonly form when temperature goes above 1100°C4).Al 2 O 3 free EL-5, PEL-5 and SEL-5 samples havehigher bending strength than Al 2 O 3 , containing EL-3,PEL-3 and SEL-3 ones from above 1000°C, more significantat higher temperature. For both series, thesprayed samples show lower CMOR than the in-labprepared and pumped ones.Fig. 1 Bulk density of the samples vs temperatureFig. 2 Apparent porosity of the samples vs temperature3.2 Permanent Linear Change (PLC)PLC of the different samples, as shown in Fig. 3,doesn’t show much difference, except for SEL-3.These castables showed negative PLC at high temperaturesdue to sintering, but why SEL-3 showed largeexpansion from 1000°C up and much delayed turndownbehavior in PLC (very exceptional whencompared to the others) is beyond understanding forthe time being, probably connected to the accelerator.This phenomenon is worth further investigation.Fig. 4 CMOR as a function temperature3.4 Hot Modulus of Rupture (HMOR)From Fig. 5 and Fig. 6, it can be seen that HMORreaches a maximum at 1300°C, and then declines astemperature increases. HMOR of Al 2 O 3 free series ishigher than Al 2 O 3 -containing ones, indicating thatforsterite bonding is favorable to hot strength. Placementmethod-induced HMOR gap becomes narrowerat higher temperature end; nevertheless the lab samplesdemonstrate higher hot strength than the pumped andthe sprayed samples.4

Fig. 5 HMOR of EL-3, PEL-3 and SEL-3 vs temperature3.5 Refractoriness under load (RUL)RUL behavior of the samples pre-fired at 1500°C ineach of the two series is similar, with the sprayedstarting to subside at a slightly lower temperature,compared with the pumped and the lab-prepared.Fig. 6 HMOR of EL-5, PEL-5 and SEL-5 vs temperatureComparing Fig. 8 with Fig. 7 shows some difference inRUL curves that MgO-SiO 2 matrix imparts with higherRUL than MgO-Al 2 O 3 -SiO 2 does; at 1500°C the EL-5group shows a 2% expansion, while EL-3 group shows1.5%. It is well in accordance with HMOR.Fig. 7 RUL curves for EL-3, PEL-3 and SEL-3 samplesFig. 8 RUL curves for EL-5, PEL-5 and SEL-5 samples5

3.6 Rotary slag testChemical composition of the adopted slag is given inTable 2, with distinguished CaO/SiO 2 ratio and ferricoxide content between the two kinds of steel-makingslag. The photographed sections of the samples afterslag test are shown in Fig. 9 and Fig. 10.For comparison purpose in each run of the slag test, atypical CA cement-bonded Al 2 O 3 -spinel ladlecastable, marked as AM (using Tabular alumina, whitefusedalumina and pre-synthesized MA spinel as rawmaterials, MgO content 5% and water addition5.4 wt%), was also tested as a reference.Table 2 Chemical composition of the slagBOF slag EAF slagCaO, % 41.69 19.32SiO 2 , % 9.10 15.28Fe 2 O 3 , % 31.23 52.66Al 2 O 3 , % 2.14 2.40MgO, % 10.80 5.81MnO, % 2.47 4.38CaO/SiO 2 (wt) 4.58 1.26As basic slag always contains CaO, the MgO-CaO-SiO 2 system should be considered. Real operatingtemperatures in a ladle exposed to basic slag may be upto 1700°C. It is easy to understand that MgO-basedbasic refractories have better resistance to basic slagthan Al 2 O 3 -based ones, as proved in Fig. 9, that EL-3and EL-5 show better slag resistance than AM’s. Whythe pumped and sprayed samples performed so poorlyin slag resistance, especially PEL-3 and PEL-5 (beingeven worse than SEL-3 and SEL-5), remains unclear,but is believed to be connected to higher porosity,segregation and the accelerator.The poor slag penetration resistance to model EAFslag and ladle slag has been reported by N. Li et al6).Corrosion of MgO castable by EAF slag was related tothe dissolution of both MgO and M 2 S into C 3 MS 2 andCMS, both being low melting phases and easilycorroded and washed away by moving slag. Butpericlase can take in Fe 2 O 3 . In this work it was foundthat MgO castables EL-3 and EL-5 had comparable,better resistance to EAF (low CaO/SiO 2 ) slag attackthan AM castable, see Fig 10.Fig.9 Rotary slag test results, BOF slagThe interesting finding from the slag test is that thepumped and sprayed castables have shown poor resistanceto the basic slag. Higher porosity led byinexperience in adopting adequate additives andoperating the shotcreting may account for the poorperformance.Surprisingly, the pumped samples show a pooreresistance to the EAF slag than the sprayed ones, inparticular for PEL-3 with 53.2 mm eroded depthagainst 11.2 mm for SEL-3. Same tendency has alsobeen observed in the case of BOF slag, 33.1mm ineroded depth for SEL-5, in comparison to 42.8mm forPEL-5. At the present stage, this can hardly be wellexplained,and deserves further investigation, withparticular attention to the texture in relation toplacement method.6

Fig. 11 Slag-refractories interface observed by SEM in EL-5attacked by BOF slag. C 4 AF (white light part), MgO(dark round/granular), C 2 S (granule in network) and MA(grey network).Fig. 10 Rotary slag test results, EAF slag3.7 Microstructure after slag testAfter both BOF and EAF slag tests, the samples preparedby different placement methods had similarphase composition and distribution, crossing the hotface to cold face; only the extent was different. Fig. 11shows the SEM observation of EL-5 attacked by BOFslag. In the slag-refractories interface, C 4 AF, MgO,C 2 S and MA coexist. Periclase and spinel haveabsorbed Fe 2 O 3 to form a solid solution. C 2 S is auseful phase to resist slag. Down to the reacted layer,concentration of CaO and Fe 2 O 3 is lowered; thenC 3 MS 2 and MgO coexist; further on, CMS and M 2 Sare found; finally, in the unattacked layer, only MgOand M 2 S are found dominant.Fig.12 Slag-refractories interface observed by SEM in EL-5attacked by EAF slag. MgO (dark), Fe 2 O 3 rich complex spinel(white ring around MgO and precipitated from MgO), C 3 MS 2 andCMS (continuous grey part).Fig. 12 is the SEM micrograph of EL-5 attacked byEAF slag. In the slag-refractories interface, M, MA,C 3 MS 2 and CMS. Complex spinel formed around periclaseas well as precipitated inside periclase, proved byas much as 54.8% of Fe 2 O 3 has been detected byEDAX in the bright ring part, and as a result of that theFe 2 O 3 in the slag has reacted with magnesia in thecastable. From hot face down to the reacted layer, theconcentration of CaO and Fe 2 O 3 becomes reduced;then M and CMS and little MA are found to coexist; Mand M 2 S are found further down, and the unattackedlayer is composed of M and M 2 S.7

4. Concluding remarksThrough this work, it has been found that for the MgO-SiO 2 -H 2 O bonded MgO-based castables, placementmethod and conditions do have significant influenceon their cold and hot properties as well as slag resistance.The developed MgO based castables of both selfflowingtype and vibration type are pumpable andsprayable. From an installation point of view they aresuccessful, while from the point of view of propertiesor service performance, further efforts are needed toreduce porosity, increase strength, enhance refractorinessunder load and to improve slag resistance, whenpumping and shotcreting techniques, especially thelatter, are applied.So far the achieved properties of the MgO-based castablesplaced by pumping and spraying are not competitiveto the in-lab prepared ones under favorable conditions.Optimisation work on additives and ambientconditions, as well as operating parameters, should beattached importance.Some interesting but inexplicable findings, e.g., theexceptional PLC curve of SEL-3 and pumped samplesshowing poorer slag resistance than the sprayed in-slagresistance, need to be understood and are the basis forfurther investigations inspired by this work.References1) B. Myhre: “Cement-free castables in the system MgO-SiO 2 :The effect of bond-phase modifiers on strength”, Presented atthe 93rd Annual Meeting of The American Ceramic Society,Cincinnatti, 1991.2) B. Sandberg, B. Myhre and J. L. Holm: “Castables in thesystem MgO-Al2O3-SiO2”, Proc. of UNITECR´95, Kyoto,Japan, Nov. 19-22, 1995, Vol. II, p. 173-80.3) B. Myhre, B. Sandberg and A. M. Hundere: “Castables withMgO-SiO 2 -Al 2 O 3 as bond phase”, Proc. of XXVI ALAFARCongress, San Juan, Puerto Rico, Oct. 29-Nov. 1, 1997,p. I/1-I/10.4) Nan Li, Yaowu Wei, Hongpeng Wu, B. Myhre and C.Ødegård: “Properties of MgO castables and effect of reactionin microsilica-MgO bond system”, Proc. of UNITECR ’99,Sep. 6-9, 1999, Berlin, Germany, p.97-102.5) C. Odegaard, Z. Chen, B. Myhre et al: “MgO-SiO 2 -H 2 Obonded MgO castables, Part 1: Effect on flow, set and hotproperties when substituting microsilica with alumina inpumpable MgO based castables”, Proc. of the 4thInternational Symposium on Refractories, Dalian, China, Mar.24-28, 2003, p. 148-156.6) N. Li, S. Zhang and W. E. Lee: “Penetration and corrosion ofmagnesia castables by silicate slags”. Proc. of UNITECR ’01,Cancun, Mexico, Nov. 4-7, 2001, p.65-79.AcknowledgmentThis work was supported by Henan Gengsheng Refractories Co.,Ltd., China, allowing use of its shotcreting equipment and rotaryslag test furnace. The authors wish to express their sincere thanksfor the support and convenience provided by Henan GengshengRefractories Co., Ltd.8

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