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MgO SiO 2 H 2Obonded MgO based castables

Part 2: Effect of pumping and wet shotcreting on cold and hotproperties and slag resistance

Ningsheng Zhou, Sanhua Zhang, Shuhe Hu, Zhenyong Bi, Luoyang Institute of Refractories ResearchZhiqiang Chen*, Cecilie degrd, Bjrn Myhre, Bjrn Sandberg, Elkem ASA Materials

Presented at UNITECR 03, 8th Biennal Worldwide Conference on Refractoriesin Osaka, Japan, October 19-22, 2003.

The fine art of

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MgO SiO 2 H 2O bonded castablesPart 2: Effect of pumping and wet shotcreting on cold and hot

properties and slag resistance

Ningsheng Zhou, Sanhua Zhang, Shuhe Hu, Zhenyong Bi, Luoyang Institute of Refractories ResearchZhiqiang Chen*, Cecilie degrd, Bjrn Myhre, Bjrn Sandberg, Elkem ASA Materials

AbstractTrial installations of MgO refractory castables inmoulds by pumping and wet sprayed onto wooden

boards were performed using an Allentown Pump RP-

10. Based on previous work, the self-flowing castablewith microsilica contents of 6 wt% for both self-flowing and vibration castable was chosen out of anumber of test castables. The given two recipes are sui-table for pumping due to their flowability and sufficient working time. The pumped and sprayed sam-

ples were compared to the samples prepared in thelaboratory in terms of bulk density, apparent porosity,cold modulus of rupture, hot modulus of rupture at1200-1500C 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 indispensa-

ble raw material for basic refractories, both bricks and monolithics. High-performance basic monolithic

refractories have attracted strong interest and attentionfor state-of-the-art steel-making processes and for clean steel production, due to many unique advantages.MgO-based castables have become a hot topic recent-ly. Studies have shown that microsilica added to anMgO-based castable mix can react with water and magnesia fines to give strong bonding 1, 2, 3) . This reac-tion is believed to be formation of a magnesia-silicate-hydrate gel 4), which facilitates the setting and binder strength development. It has also been found thatmicrosilica in such a system suppresses the slakingtendency of MgO.Castables with the MgO-SiO 2-H2O bond have beenused in some countries for steel ladles and tundish

weirs 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 combination

of particle size distribution, microsilica and additives,MgO-based mixes can be modified to allow for

pumping and wet spraying.As the castable is heated, microsilica and magnesiaform forsterite. The MgO-SiO 2 system mainly reliesupon the forsterite for high temperature bonding 4).Forsterite is a refractory phase with a melting pointabout 1890C, and the eutectic in the system of 2MgOSiO 2 MgO is at 1850C. As alumina isintroduced in the system, a low melting phase is likelyto form, rather than a strong spinel-forsterite bond. It

has 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-based castables, one recipe was selected from the self-flowing castables, 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 free-flow value 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 shooting 5).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 of experience in operating the machine, in terms of air

pressure, accelerator type and addition, distance,moving speed, etc. The aim of this work was toinvestigate what effects this new installation method of wet shotcreting or, in other words, wet spraying, may

have on the hot properties (hot modulus of rupture,refractoriness under load behavior and slag resistance)of such MgO-based castables.

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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 of 500kg 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 and sprayed by an Allentown Pump RP-10 machine, under such operation parameters as pumping capacity of 8-10tons per hour, pumping pressure 14MPa, hose diameter 75mm, hose length 15m, nozzle tip 38mm, air pressure0.7MPa and air flow 6m 3/min. Al 2(SO 4)3 saturated solution 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 distance

between nozzle tip and the sprayed surface was0.5-1m. The ambient temperature in the workshop was5-10C. The shoebox-like pumped blocks with adimension of 230mm in length, 160mm in width and 140mm in height, and the sprayed blocks, 900 mm inlength, 700 mm in width and approximately 150 mm inheight, were properly cured and then subjected to400C 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 sprayed as S, i.e. SEL-3 and SEL-5.

2.2 Cold modulus of rupture (CMOR)Samples were then fired at 600C, 1000C, 1200C,1400C and 1600C 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 of 25x25x150mm in accordance with the standard of PRE/R18. Samples were pre-fired for 24h at the testtemperatures and allowed to cool before being loaded into the test furnace. The heating rate was 300C/h for

both pre-firing and testing, and the samples wereallowed to soak at the test temperature for 30 min prior to loading.

2.4 Refractoriness under load (RUL)RUL was measured on cylinders of 50mm in diameter and 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 1500C,unless otherwise stated. Heating rate was 300C/h for

both pre-firing and testing. The equipment was

designed in accordance with descriptions of ISOR1893 (1970); the maximum accessible temperaturewas 1800C 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 1500C and first batch of 1kgslag was fed; the temperature was raised to 1600-1650C and another 1kg slag was fed, then soaked at1650C 50C for 130 min, during which 4kg slag wasfed at a rate of about 1kg per 30-35min. The furnacewas naturally cooled down. The slag-attacked specimens were sectioned longitudinally. Visualexamination and measurement on local eroded depthwere pursued. Averaged eroded depth (H) was used torepresent slag resistance, calculated by the given

formula: H=Ho-h, where Ho is the averaged originalheight, using 3 measurements at a 50mm intervalseparating from the central symmetrical line, and h=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 was

supplied 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 castables 5). 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 introduced

by the placement method. In Fig. 2 it is seen that thedifference in AP of the 1200-1600C fired castables isobvious, with the sprayed being the highest, the lab-

prepared the lowest and the pumped inbetween. Toolow of an ambient temperature that the trials were sub-

jected to could be a partial reason leading to reduced flowability and setting, hence less density and higher

porosity.

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Table 1 Composition of the two kinds of castables

Label EL-3 EL-5

Castable type Self-flow Vibration

q-value of Andreassen model 0.25 0.28

MS/RA volume ratio 60/40 100/0

3-5 mm 10 10

Fused MgO1-3 mm 27 27

0-1 mm 25 27-44 micron 24.8 30

Microsilica 971U (MS) 6 6

Reactive Al 2O3 CTC50 (RA) 7.2 0

Dispersant Vanisperse CB 0.25 0.25

Water (15.2 vol.%), wt% 5.16 5.19

Fig. 1 Bulk density of the samples vs temperature

Fig. 2 Apparent porosity of the samples vs temperature

3.2 Permanent Linear Change (PLC)PLC of the different samples, as shown in Fig. 3 ,doesnt show much difference, except for SEL-3.These castables showed negative PLC at high tempera-tures due to sintering, but why SEL-3 showed largeexpansion from 1000C up and much delayed turn-down behavior in PLC (very exceptional when

compared to the others) is beyond understanding for the time being, probably connected to the accelerator.This phenomenon is worth further investigation.

Fig. 3 PLC as a function of temperature

Fig. 4 CMOR as a function temperature

3.3 Cold Modulus of Rupture (CMOR)There is a strength valley for all the samples fired at1000C, as shown in Fig. 4. This is in accordance withthe finding that forsterite bonding in this system canonly form when temperature goes above 1100C 4).Al2O3 free EL-5, PEL-5 and SEL-5 samples havehigher bending strength than Al 2O3, containing EL-3,PEL-3 and SEL-3 ones from above 1000C, more sig-nificant at higher temperature. For both series, thesprayed samples show lower CMOR than the in-lab

prepared and pumped ones.

3.4 Hot Modulus of Rupture (HMOR)From Fig. 5 and Fig. 6 , it can be seen that HMOR reaches a maximum at 1300C, and then declines astemperature increases. HMOR of Al 2O3 free series ishigher than Al 2O3-containing ones, indicating thatforsterite bonding is favorable to hot strength. Place-ment method-induced HMOR gap becomes narrower

at higher temperature end; nevertheless the lab samplesdemonstrate higher hot strength than the pumped and the sprayed samples.

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Fig. 5 HMOR of EL-3, PEL-3 and SEL-3 vs temperature Fig. 6 HMOR of EL-5, PEL-5 and SEL-5 vs temperature

3.5 Refractoriness under load (RUL)RUL behavior of the samples pre-fired at 1500C ineach of the two series is similar, with the sprayed starting to subside at a slightly lower temperature,compared with the pumped and the lab-prepared.

Comparing Fig. 8 with Fig. 7 shows some difference inRUL curves that MgO-SiO 2 matrix imparts with higher RUL than MgO-Al 2O3-SiO 2 does; at 1500C 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 samples

Fig. 8 RUL curves for EL-5, PEL-5 and SEL-5 samples

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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 after slag test are shown in Fig. 9 and Fig. 10 .For comparison purpose in each run of the slag test, atypical CA cement-bonded Al 2O3-spinel ladlecastable, marked as AM (using Tabular alumina, white-fused alumina 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 slag

BOF slag EAF slag

CaO, % 41.69 19.32

SiO2, % 9.10 15.28

Fe2O3, % 31.23 52.66

Al2O3, % 2.14 2.40MgO, % 10.80 5.81

MnO, % 2.47 4.38

CaO/SiO 2 (wt) 4.58 1.26

As basic slag always contains CaO, the MgO-CaO-SiO2 system should be considered. Real operatingtemperatures in a ladle exposed to basic slag may be upto 1700C. It is easy to understand that MgO-based

basic refractories have better resistance to basic slagthan Al 2O3-based ones, as proved in Fig. 9 , that EL-3and EL-5 show better slag resistance than AMs. 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 al 6).Corrosion of MgO castable by EAF slag was related tothe dissolution of both MgO and M 2S into C 3MS2 and

CMS, both being low melting phases and easilycorroded and washed away by moving slag. But

periclase can take in Fe 2O3. In this work it was found that MgO castables EL-3 and EL-5 had comparable,

better resistance to EAF (low CaO/SiO 2) slag attack than AM castable, see Fig 10 .

The interesting finding from the slag test is that the pumped and sprayed castables have shown poor resis-tance to the basic slag. Higher porosity led byinexperience in adopting adequate additives and operating the shotcreting may account for the poor

performance.Surprisingly, the pumped samples show a poorer

esistance to the EAF slag than the sprayed ones, in particular for PEL-3 with 53.2 mm eroded depthagainst 11.2 mm for SEL-3. Same tendency has also

been observed in the case of BOF slag, 33.1mm ineroded depth for SEL-5, in comparison to 42.8mm for PEL-5. At the present stage, this can hardly be well-explained, and deserves further investigation, with

particular attention to the texture in relation to placement method.

Fig.9 Rotary slag test results, BOF slag

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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 UNITECR95, Kyoto,

Japan, Nov. 19-22, 1995, Vol. II, p. 173-80.3) B. Myhre, B. Sandberg and A. M. Hundere: Castables with

MgO-SiO 2-Al2O3 as bond phase, Proc. of XXVI ALAFAR Congress, 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.degrd: 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-H2O bonded MgO castables, Part 1: Effect on flow, set and hot

properties when substituting microsilica with alumina in pumpable 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 of magnesia castables by silicate slags. Proc. of UNITECR 01,Cancun, Mexico, Nov. 4-7, 2001, p.65-79.

Acknowledgment

This 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.

4. Concluding remarksThrough this work, it has been found that for the MgO-SiO2-H2O bonded MgO-based castables, placementmethod and conditions do have significant influenceon their cold and hot properties as well as slag resis-tance.The developed MgO based castables of both self-flowing type and vibration type are pumpable and sprayable. 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 refractor-iness under load and to improve slag resistance, when

pumping and shotcreting techniques, especially thelatter, are applied.So far the achieved properties of the MgO-based casta-

bles placed by pumping and spraying are not competi-tive to the in-lab prepared ones under favorable condi-tions. Optimisation work on additives and ambient

conditions, 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 for further investigations inspired by this work.

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