Flow and properties of MgO based castables

Transcription

1 Flow and properties of MgO based castables Presented at XXXII ALAFAR Congress Antigua, Guatemala Nov, 7-1, 24 Cecilie Ødegård, Bjørn Myhre, Elkem ASA Materials Ningsheng Zhou, Sanhua Zhang, Luoyang Institute of Refractories Research The fine art of

2 FLOW AND PROPERTIES OF MgO BASED CASTABLES Cecilie Ødegård, Bjorn Myhre, Elkem ASA Materials. Ningsheng Zhou, Sanhua Zhang, Luoyang Institute of Refractories Research. ABSTRACT Cement free MgO castables with MgO-SiO 2 -H 2 O bond have been studied with respect to flowability, flow decay and hot properties. In the superfine fraction, mixtures of microsilica and/or reactive alumina were used as the main variable. Two particle size distributions were tested, one with a q-value of.25 (self flowing) and one with a q-value of.28 (vibratable). It was found that the R.U.L tests gave T.5 above 165 o C for the MgO-SiO 2 and MgO-Al 2 O 3 systems. From HMOR testing, the MgO-Al 2 O 3 -SiO 2 bonding phase shows evidence of liquid formation at around 15 o C. All compositions were subjected to rotary slag testing to compare their slag resistance, with an alumina-spinel as reference. Both BOF and EAF slag were tested. Many of the, MgO based castables showed better slag resistance to both slags than the reference castable. INTRODUCTION Basic castables based on MgO-SiO 2 -hydrate bonding. The development of basic self flowing castables is an ongoing quest. Today, use MgO in hydrauliclly bond castables has no widespread use due to some fundamental problems. The tendency of magnesia to hydrate, lack of a good binder and the thermal expansion of magnesia are some of the problems. A proposed binder is microsilica and fine magnesia that is allowed to react with water. This gives strong bodies 1,2,3. The use of microsilica prevents hydration (slaking) which else causes the castables to crack and disintegrate. Because of the setting reaction between MgO and microsilica, no calcium aluminate cement is needed. The set reaction is considered to be the formation of magnesia-silicate-hydrate phases, but it is not fully understood yet. Improved flowability is achieved by using more than 4 wt% microsilica in these cement-free castables together with an appropriate dispersing and/or retarding additive. Above approximately 1 C, forsterite (2 MgO. SiO 2 ) forms from MgO and microsilica. In one study it was found that forsterite existed in all samples fired at 11 C and 16 C, and in a 9 wt % microsilica containing castable close to 6 % forsterite was present in the matrix 4. Forsterite has a melting point of approximately 189 C, eutectic temperature in the system 2 MgO. SiO 2 MgO is 186 C. Addition of alumina to this system has a couple of implications. Substituting microsilica with alumina, usually lowers flow values assuming particle size distribution overlap between microsilica and superfine alumina. 5 Further, some volume expansion may be experienced due to spinel formation. This can counteract the shrinkage from forsterite formation. However, the combination of SiO 2 with Al 2 O 3 and MgO in the bond phase often give low melting phases. Eutectic temperature in the compatibility triangle forsterite-periclase-spinel is approximately 171 C with a composition close to 5%MgO, 3%SiO 2 and 2% Al 2 O 3. Since both alumina and silica is in the bond-phase, it can be postulated that mixtures with microsilica/reactive alumina ratios close to 3/2 will experience rapid softening of the bond at temperatures approaching 17 C. This work reviews parts of a comprehensive study of the MgO-Al 2 O 3 -SiO 2 system as candidate for basic self-flowing castables. The investigation comprises castables changing from MgO-SiO 2 bond system over MgO-SiO 2 -Al 2 O 3 and finally to a MgO-Al 2 O 3 bond system. The effect on placement properties like flow and set time is investigated as are high-temperature properties like Refractoriness Under Load (RUL) Hot-MOR and Rotary slag testing. Manipulation of particle size distribution is known to influence flow of castables significantly. A predictive flow can thus be tailor made. E.g. by choosing q-values of.25 and.28 from the Andreassen Model 8, potentially free flowing

3 or vibratable castables can be modelled. Therefore castables with these two PSD were modelled for further investigations. The experimental set up was to replace some of the microsilica with equivalent volumes of reactive alumina and to test the effect this had on properties. The substitution was based on volume in order to maintain a constant PSD, but for simplicity, with some exceptions, the ratios will be presented as the actual amounts in the castable recipes. E.g. MS/RA=4/3.6 indicates that the castable contains 4wt% microsilica (MS) and 3.6wt% reactive alumina (RA). The volume based equivalent would be MS/RA=67/33. PROCEDURE` Castable mixing After the ingredients were dry-mixed for 4 minutes, water was added, and the castable wet mixed for 4 minutes. Flow measurements Flow measurements were performed using a flow cone (height 6 mm, top diameter 7 mm and lower diameter 1 mm). The testing was performed on a vibration table set at 5 Hz at a double amplitude of.75mm. Immediately after mixing, the castable was filled into a dozen such cones, cured at the ambient environment, temperature around 2 C, and relative humidity around 7%. After a predetermined time, one flow cone was removed and the castable allowed to spread by the action of gravity alone. When the spreading stopped, the percentage increase in the diameter of the castable was taken as the free-flow value. The spread castable was then subjected to 15 seconds of vibration, and the resultant spreading taken as the vibra-flow value. This was repeated a number of times at least until the vibration flow value dropped to below 7%. Hot modulus of rupture Hot M.O.R. testing was performed on samples of 25x25x15 mm in accordance with PRE/R18. The samples were cured for 24 hours before drying at 11 C. Samples were then prefired for 24 hours at test temperature and allowed to cool before being loaded into the test machine. The heating rate was 3 C/hour for both the prefiring and the testing, and the samples were allowed to equilibrate at test temperature for 3 minutes prior to testing. Refractoriness under load, R.U.L R.U.L was measured on cylinders 5 mm high with central bore 12.5 mm as described in ISO R1893 (197). The constant load on the test samples was.2 MPa. The samples had been prefired for 24 hours at 15 C. Heating rate was 3 C/hour for both testing and prefiring. The equipment was designed in accordance with descriptions of ISO R1893 (197); maximum accessible temperature was 18 C for testing under rising temperature. Rotary slag test Tests were performed on samples (12) with length approximately 23mm and 6mm thickness. These were installed to make a drum-like rotating shell, fired with a gas torch. The furnace rotating at 6 rpm was heated to about 15ºC and a first batch of 1kg slag was fed. Then, the temperature was raised to ºC and another 1kg slag was added. Subsequently, at a temperature of 165 ±5 C, 4kg slag was fed at a rate of approximately 1kg per 3-35min. Total duration 13min. After the testing, the furnace was allowed to cool. After cooling and disassembling, the slag attacked samples were longitudinally sectioned. Visual examination and measurement on eroded depth were undertaken. The averaged eroded depth (H) was used to represent slag resistance, and was calculated by the following formula: H=H o -h, where H o is the average original height, using 3 measurements at a 5mm interval separating from the central symmetrical line, and h is the averaged remained unattacked height, using 11 local data at a measuring step of 1mm symmetrically to the central symmetrical line. RAW MATERIALS The castables were based on chinese fused magnesia with MgO content higher than 98%. The other materials were reactive alumina CTC 5 from Almatis and Elkem Microsilica Grade 971 U. As additive, Vanisperse CB (an oxylignine) from Borregaard was used. Table 1 shows the compositions with q=.25 for self-flowing castables, while Table 2 shows those with q=.28 for vibration castables.

4 Table 1: Composition of the self-flowing castables (wt.%), q=.25 MS/RA volume ratio 1/ 8/2 6/4 4/6 3-5 mm Fused MgO 1-3 mm mm micron Microsilica (MS) 971U Reactive Al 2 O 3 (RA) CTC Dispersant Vanisperse CB Water (15.2 vol.%) Table 2: Composition of the vibration castables (wt.%), q =.28 MS/RA volume ratio 1/ 67/33 33/67 /1 3-5 mm Fused MgO 1-3 mm mm micron Microsilica (MS) 971U Reactive Al 2 O 3 (RA) CTC Dispersant Vanisperse CB Water (15.2 vol.%) RESULTS AND DISCUSSION Flow value Flow value, % q=.25 self flow vibra-flow Flow value, % q=.28 self flow vibra-flow 3 3 1/ 8/3.6 6/7.2 4/1.8 MS/RA ratio 6/ 4/3.6 2/7.2 /1.8 MS/RA ratio Figure 1: MS/RA ratio(weight) and flow It is seen that for both particle size distributions there is a maximum flow value for those with 6% microsilica.

5 Further, the effect of the PSD on flow is demonstrated. The castable with a q-value of.25 has a higher self-flow value than the castables with q=.28. This is in line with the expected flow, at a q-value of.25 self-flow is expected to be much higher than at a q-value of.28. For the self-flowing composition (q=.25), somewhat unexpectedly, the flow increases when microsilica is replaced by reactive alumina, with a maximum at 6% microsilica and 7.2% alumina. The reason for this is not understood, but unpublished results, indicate that higher microsilica contents promote setting, i.e. flow loss. For q=.28, self-flow and vibration flow decreases with increasing alumina content. These mixes have to be installed by vibration. Only the 6 wt% microsilica containing castable has free flow behaviour. Free-flow decay For q=.25, the recipes with MS/RA =6/7.2 and MS/RA 4/1.8 gave the longest working times. With 8wt% microsilica, the initial free flow of almost 9 % is lost after only 45 minutes, Figure 2 left. For the vibra-flow mixes with q=.28 there is no initial free flow for recipes with 2wt % and wt % microsilica. There is not sufficient working time with low microsilica contents. With 6 wt % microsilica however there was more than 12 minutes working time, Figure 2 right Free flow value,% MS/RA 1/ MS/RA 8/3.6 MS/RA 6/7.2 MS/RA 4/1.8 Free flow value,% MS/RA 6/ MS/RA 4/3.6 Figure 2: Time, min Time, min Decay of free flow, left q =.25, right q =.28. MS/RA based on weight. Explosion resistance This test is performed on 5x5x5mm cubes. After curing for 24 hours, two samples were put into a hot furnace, in which temperature had been raised to the desired level in advance. After being soaked at the given temperature for 2 minutes, the samples were checked. If the sample had exploded or cracked, X was marked in Table 3. If the sample was intact, O was recorded. Table 3: Explosion resistance MS/RA wt. q =.25 q =.28 Ratio 1/ 8/3.6 6/7.2 4/1.8 6/ 4/3.6 2/7.2 /1.8 35ºC O O O X 55ºC O O O O O O O O O O O O O O ºC O O O O O O O O O O O O O X ºC O O O O O O O O O O O O ºC O O O O O O O O O O O O ºC O X O X O O O O O O O O O: no explosion; X: explosion or cracked

6 Very good explosion resistance was found for the free flowing mixtures (q=.25). Only one cracked sample when heated to 11 o C for the mixtures with the highest microsilica content (1 wt% and 8 wt%). Using 2 wt% or wt% in the vibra flow mixtures (q=.28) gave exploded samples at lower temperatures (65 o C and 35 o C).

7 Permanent linear change PLC reflects the phase formation and sintering taking place in the castables bond phase. The microsilica free sample with 1.8 wt% alumina shows the characteristic expansion when spinel is being formed. A permanent expansion of 2.3% is seen for the MS/RA= /1.8 sample at 12 o C (Figure 4). Also the alumina-free mixture MS/RA =1/ expands at 12 o C but only.6 %. At 16 o C the castable has a negative PLC of.6%. The samples with alumina have a higher shrinkage, up to 1.% for the castables MS/RA =8/3.6 and MS/RA= 6/7.2 (Figure 3) MS/RA=1/ MS/RA=6/7.2 MS/RA=8/3.6 MS/RA=4/1.8 PLC, % Figure 3: Temperature, [ºC] PLC of MgO castables with q-value.25 as a function of temperature. PLC, % MS/RA=6/ MS/RA=2/7.2 MS/RA=4/3.6 MS/RA=/ Temperature, [ºC] Figure 4: PLC of MgO castables with q-value.28 as a function of temperature. Hot modulus of rupture In Figure 5 it is seen that the sample MS/RA=8/3.6 has the highest HMOR from 12ºC to 14ºC, but at 15 o C this sample have the lowest HMOR. At 15 o C the MgO-SiO 2 sample MS/RA=1/ gives 1 MPa. There is a trend of decreasing HMOR with increasing alumina content from 3.6 wt % to 1.8 % in the temperature range o C, which can be explained by the entropy driven phase changes in such systems described in reference 2. For q =.28 there is a similar pattern. MS/RA=4/3.6 have the highest HMOR at 12 and 13 o C and decreasing as more alumina is introduced. At 15 o C the alumina free sample have a HMOR of 5.5 MPa. At 14 and 15 o C, not unexpectedly, the microsilica free MgO-Al 2 O 3 sample has a higher HMOR than the MS/RA = 2/7.2 sample. The hot strength of the alumina-free MgO-SiO 2 sample with q-value.25 is higher than the q-value of.28. This may be connected to the amount of Forsterite that forms, but is more likely to be an effect of the coarser texture of the q=.28 castable.

8 HMOR, MPa MS/RA = 6/ q =,28 MS/RA = 2/7.2 MS/RA = / Temperature, [ºC] MS/RA = 4/3.6 MS/RA = 1/ q =,25 MS/RA = 8/3.6 HMOR, MPa MS/RA = 6/7.2 MS/RA = 4/ Temperature, [ºC] Figure 5: HMOR of MgO castables with q =.25 (left) and q=.28 (right) at o C. Refractoriness under load, R.U.L The sample with q=.25 and MS/RA = 1/ in Fig 6 showed a thermal expansion of 2. % reached at 154 o C. The T.5 of this sample was measured to 165 o C. The other mixes with alumina had T.5 of approx 161 o C. For q =.28 in Fig 7, the SiO 2 -MgO sample starts subsiding at 148 o C after expanding 1.9 %. The 2/7.2 and 4/3.6 samples have similar patterns, with T.5 of about 163 o C. The microsilica free sample of MS/RA = /1.8 has the slowest creep rate above 16 o C. This is due to the in-situ formed magnesium aluminate spinel. Slower creep is also seen for the other binary mixes, the 6/ (Fig. 7) and 1/ (Fig. 6) castable. 3 2 MS/RA=8/3.6 MS/RA=1/ Expansion [%] 1-1 MS/RA=4/1.8 MS/RA=6/ Figure 6: Temperature [ C] R.U.L. test of MgO castables with q=.25. Samples prefired at 15 C. The abrupt drop seen for castables with both microsilica and reactive alumina around 165 C can be explained by the formation of a eutectic liquid, which according to the phase diagram should be formed at approximately 171 C. Presence of impurities may very well explain this somewhat lower temperature experienced.

9 3 MS/RA=/1.8 2 Expansion [%] 1-1 MS/RA=2/7.2 MS/RA=4/3.6-2 MS/RA=6/ Temperature [ C] Figure 7: R.U.L. test of MgO castables with q=.28 Rotary slag test Chemical composition of the adopted slag is given in Table 4, with different CaO/SiO 2 ratios and ferric oxide content for the two kinds of steel-making slag. Photographed sections of the samples after slag test are shown in Figure 8 and 9. Figure 8 shows the results with BOF slag, both as picture and graphically as eroded depth of the tested samples. Please note that the pictures, both in Figure 8 and 9 have been slightly manipulated/adjusted to include the reference AM. It should be stressed that all samples, also the AM sample was part of the same run, but that from the picture some samples has been removed (originally 12). Also, note that the labeling in the figures are as MS/RA expressed as volume%. For conversion to MS/RA(wt%) please confer the attached legend. For comparison, also a typical CA-cement bonded Al 2 O 3 -spinel ladle castable, was tested (marked as AM). This was based on Tabular alumina, white fused alumina and pre-synthesized MA spinel as raw materials, with a MgO content 5% and water addition 5.4 wt%. Not so clearly visible from the photographs, this castable had a somewhat coarser structure than the other. The castable mixture without microsilica q=.28 /1.8(wt%) that performed so well in the RUL testing was omitted in the slag testing because of very bad casting properties, both set and flow, but also the disruptive expansion from spinel formation (Fig.4 ). Table 4: Chemical composition of the slags used in the rotary slag testing. BOF slag EAF slag CaO, % SiO 2, % Fe 2 O 3, % Al 2 O 3, % MgO, % MnO, % CaO/SiO 2 (wt)

10 Legend: MS/RA(vol%) = MS/RA(wt%) Q=.25 1/(vol%)=1/(wt%) 8/2(vol%)=8/3.6(wt%) 6/4(vol%)=6/7.2wt% 4/6(vol%)=4/1.8(wt%) Q=.25 1/ Q=.25 8/2 Q=.25 6/4 Q=.25 4/6 Q=.28 1/ Q=.28 67/33 Q=.28 33/67 AM Q=.28 1/(vol%)=6/(wt%) 67/33(vol%)=4/3.6(wt%) 33/67(vol%)=2/7.2(wt%) 3 BOF Slag CaO/SiO2: 4.58 Erode Depth, mm Q=.25 1/ Q=.25 8/2 Q=.25 6/4 Q=.25 4/6 Q=.28 1/ Q=.28 67/33 Q=.28 33/67 AM Figure 8: Rotary slag test results, BOF slag MS/RA ratios based on vol%. Not unexpected, the sample with the highest amount of microsilica, 1% (q=.25 1/) shows the highest erosion. The lowest erosion is seen with 6% microsilica. When evaluating the results, each PSD should be jugded separately because of the influence the amount of bond phase may have on the results. For q=.25, addition of reactive alumina seems to improve slag resistance, while for q=.28 alumina reduces erosion resistance. Even though alumina as such giving spinel may have a positive effect, the main influence here probably is the casting characteristics. Figure 2 shows the decay of the free-flow and it is seen that the best castables, i.e. those with the highest initial flow and sufficient set time, are those giving the lowest erosion towards BOF slag. It should also be noted that most of the experimental castables have results similar or superior to the industrial AM castable.

11 Legend: MS/RA(vol%) = MS/RA(wt%) Q=.25 1/(vol%)=1/(wt%) 8/2(vol%)=8/3.6(wt%) 6/4(vol%)=6/7.2wt% 4/6(vol%)=4/1.8(wt%) Q=.25 1/ Q=.25 8/2 Q=.25 6/4 Q=.25 4/6 Q=.28 1/ Q=.28 67/33 Q=.28 33/67 AM Q=.28 1/(vol%)=6/(wt%) 67/33(vol%)=4/3.6(wt%) 33/67(vol%)=2/7.2(wt%) 3 Erode Depth, mm 2 1 EAF Slag CaO/SiO 2 : Q=.25 1/ Q=.25 8/2 Q=.25 6/4 Q=.25 4/6 Q=.28 1/ Q=.28 67/33 Q=.28 33/67 AM Figure 9: Rotary slag testing. Results of castables using EAF slag. Towards EAF slag, all experimental castables performed better than the reference AM. The influence of casting characteristics seems not so pronounced, if any. There might be a positive contribution from spinel formation, although the indications are uncertain.

12 CONCLUSION It has been demonstrated that it is possible to utilise the MgO-SiO 2 bond to make castables that both exhibit good flow, controlled setting characteristics, and attractive hot properties. The slag resistance was found surprisingly good both for BOF and for EAF slag, results that are contradictory to some commonly accepted perceptions of the refractory industry. Approximately 6% microsilica seems to be essential in the makeup of these castables giving good placement properties and also slag resistance. REFERENCES 1. B. Myhre:"Cement-free castables in the system MgO-SiO 2 ; The effect of bond-phase modifiers on strength". Presented at The American Ceramic Society, 93rd Annual Meeting, Cincinnatti, B. Sandberg, B. Myhre and J. L Holm: " Castables in the system MgO-Al 2 O 3 -SiO 2 " in Proc. Of UNITECR 95, The Techn. Ass. of Refr., Japan. Kyoto, Japan Nov , vol. II, p B. Myhre, B. Sandberg and A. M. Hundere: Castables with MgO-SiO2-Al2O3 as bond phase In Proc. Of XXVI ALAFAR Congress, San Juan, Puerto Rico, Oct Nov. 1, 1997, p I/1-I/1 4. Nan Li, Yaowu Wei, Hongpeng Wu, B. Myhre and C. Ødegård: Properties of MgO Castables and Effect of Reaction in Microsilica-MgO bond System In Proc. Of UNITECR 99, The German Refractory Assosiation, Sept. 6-9, 1999, Berlin, Germany, p B. Myhre, A.M. Hundere: "Substitution of Reactive Alumina with Microsilica in Low Cement and Ultra Low Cement Castables. Part I: Properties related to Installation and Demoulding " in Proc. of UNITECR 97 in New Orleans, USA, Nov , vol. I, p F. N. Cunha, R. C. Bradt: " Reactions of constituents for in-situ bonds of MgAl2O4, Mg2SiO4 & 3 Al2O3. SiO2 in refractories" in Proc. of the 57 th Electric Furnace Conference, Pittsburgh PA, Nov , 1999, p G.W.Kriechbaum et al.: The Matrix Advantage System, a new approach to low moisture LC selfleveling alumina and alumina spinel castables. In Proc. XXXIX Int. Feuerfest-Koll, Aachen, Sept , p A. H. M. Andreassen and J. Andersen, Kolloid Z.5 (193) p