Effect of Mild-calcined Coal Gangue Addition on Hot Strength and Refractoriness Under Load of Ultra

China's RefractoriesFeng Sisi gained her B. Sc. and M. Sc. from Henan University of Science and Technology in 2007 and2011, respectively, majored in Materials Science. Her research interest is in monolithic refractoriesBEffect of Mild-calcined Coal Gangue additionon Hot Strength and refractoriness under loadof Ultra Low Cement Castablesin ALO3-Sio2 SystemFENG Sisi, ZHOU NingshengHigh Temperature Materials Institute, Henan University of Science and Technology,Luoyang 471003, ChinaAbstractThis work explored the way to improve hot modulofrupture( HMOR ) and refractoriness under loads the mainstream of monolithic refractoriesruL) by adding mild-calcined coal gangue(MCG) Al2O,SiO, castables are widely used, for whichin AL20, -Sio2 ultra low cement ULC )castables, amounts of raw materials are consumed, including fire-making use of the in-situ effect of the MCG during heat- clay, bauxite, mullite, alumina, etc. Most of the rawing-up. The influence of respective additions of 5%materials are of naturally occurred minerals and need to10% and 15% of the MCG powders calcined at 700c be calcined, sintered or fused before use.Howeverwas investigated on HMOR at 1 400 C and rul of the there are some challenges in raw materials, e.g. lackcastables.With increased addition of the MCG, HMOR of some cost-effective raw materials, large heat con-and RUL become significantly enhanced. At 10% of the sumption in calcination of raw minerals and how toMCG addition, HMOR reaches 3 MPa, as compared to make full use of some beneficial in-situ effect of some0. 3 MPa in the case of no MCG addition. RUL of the natural minerals during heating-up. Direct addition ofspecimens dried at 110 C for 24 h can be increased by raw bauxite and raw coal gangue without any pre-calci-some 270C with 10% of the MCG addition. RUL of nation in A2 -SiO2 castables has shown positive re-e specimens preheated at 1 500 C for 3 h maintains sultHot strength, thermal shock resistance andhe growth trend with the MCG addition increasing. The explosion resistance during drying-out can even be im-microstructure of the heated castable samples was investi- proved by adding them. However, the harsh removal ofgated by means of SEM. The in-situ formed needle-like structural water or decomposition of such minerals durand interlaced mullite in the matrix is contributive to the ing heating may have negative effect on structural stability of the castablesKey words: Coal gangue, Hot modulus of ruptureCalcination ofRefractoriness under load, Ultra low cement castable, temperature to remor中国煤化 rely low-out oc-lumina-silicacurrence of mullitizatCNMHGhat18 China's Refractories Vol 20, No3, 2011Effect of Mild-calcined Coal Gangue Addition on Hot Strength and RefractorinessUnder Load of Ultra Low Cement Castables in Alo -Sio SystemCHNPcall"mild calcination",deserves to be a new way to 2. 2 Mild Calcination of the Raw Coal Ganguerealize energy saving on one hand, and on the otherFrom the tga curve of the raw coal gangue shownhand, to make use of the mullitization in favor of prop in Fig. 1, by 700C the removal of the structural watererties. In this work, mild-calcined coal gangue is almost finished. The mild calcination was accordinMCG )is added in ULC AL, O,-Sio, castables and its ly set at 700C for 6 heffect on hot properties in terms of hot modulus of rup.Heat release TH 3.0ture( HMOR)and refractoriness under load(RUL) isResidual mass:2 Experimental Preeedurowiatsdiasks8232%09y1505员2.1 Raw MaterialsThe adopted raw materials include white fused alu-100200300400500600700800900mina( WFA, w(Al,03)299%, in size fractions of8-5 mm. 5-3 mm. 3-1 mm. 1-0 mm and <44Fig. 1 TG-DSC curves of the raw coal gangueum), fused mullite(<74 um), ultrafine alumina 2.3 Specimen Preparation(uf-Al2O3,≤5μm,v(Al2O3)≥99%), silica fumeAs shown in Table 2, the MCG addition is 0%(u( Si02 )296%), CA cement( Secar 71, <74 5%, 10% and 15%, marked as CG-0, CG-5, CG-10um), raw coal gangue($I mm)and MCG(<74 and CG-15, respectively, to replace firstly fused mulm). Chemical compositions of the mullite and thelite powders and finally also WFA powderscoal gangue are given in Table 1The mixes, at the water additions to reach a vibration nlow-value over 180 mm, were cast under vibrationTable 1 Chemical compositions of the mullite and raw coal into 40 mm x40 mm x 160 mm bars for HMOR test andgangue/wt%oALO,sio, Fe,0, To,"N, f d50 mm X 50 mm for RUL test, following a regularcastable specimen making procedure. After demoulding,Fused mullite72.6025.570.010.010.01the specimens were dried at 110 C for 24 h and parts ofRaw coal gangue45.7452.700.300.410.07them were heated at 1 500C for 3 h in a muffle fumaceTable 2 Recipes of the castables /wt%Raw materialCG-0CG-10CG-15WFA(8-5 mm, 5-3 mm, 3-1 mm, and 1-0 mm)0Silica fume +uf-Al, O,+Secar 71+3+35+3+35+3+35+3+3WFA(≤44μm)99Fused mullite(≤74pm)MCG(≤74μm)04.24.44.52. 4 Properties Testingcastables at 1 400C is shown in Fig. 2. HMOR isHMOR was tested at 1 400 C for 1 h according to significantly improved when an appropriate amount ofthe standard GB/T 3002-2004. RUL was tested in ac- MCG is added. The measured value reaches 3 MPa atcordance with the standard YB/T 2203-1998. The 10% MCG addition, 10 times higher than the referencespecimens for the tests were dried at 110 C for 24 h without MCG additionand preheated at 1 500C for 3 hThe resistance to fracture or defoMicrostructure observation was carried out by fractory at high temperatures depends primarily on itsmeans of SEMmicrostructure characteristics, presented by"crystallization effect", including the growth and distribution ofO Pesutttand Discussionneswscrystals, and"glass effect", i. e. the quantity and vis-cosity of the glassy phase at high temperatures. In the3. 1 Hot Modulus of Rupturelated system中国煤化工 guid phaseThe effect of MCG addition on HMOR of the at high temperatuCNMH Gof the rawChina's Refractories Vol 20. No. 3. 2011 19FENG Sisi, ZHOU Ningshengmaterials. Therefore the high temperature propertiesIt is worth of attention that the beneficial effectsdepend mainly on the crystallization effectby the in-situ mullitization leading to favorable micro-structure and the accompanied expansion usually occurin the first heating-up. It is necessary to investigatewhether the effect of hot strength enhancement can bemaintained when subjecting to reheating. Positive answer to this question has been obtained by110℃24h-1500℃3hof the HMOR of the specimens after drying at 110 Cand those after preheating at 1 500 C. As shown inFig. 2, the two curves match each other very well.w(MCG addition)/%3. 2 Refractoriness Under LoadFig. 2 Effect of MCG addition on HMOR at 1 400C forFigure 5 shows the effect of MCG addition on RULI h of the castablesof the castables dried at 110 C for 24 h. The expanAs known, meta-kaolinite is the dominated miner- sion of CG-0 is the largest, while its RUL representedal phase of the adopted MCG. It can be converted to by the temperature at 0. 6% deformation is the lowestmullite and silica rich glassy phase, the latter can fur- The To.6% of CG-10 is the highest, over 1 700C.Thether react with Al, 0, in the matrix to form secondary curves of CG-5, CG-10 and CG-15 are fluctuatedmullite. As seen in Fig. 3, this in-situ formed mullite implying reactions occurred in the specimens. Thesetends to growthree curves all show an expansion peak aroundcastables, which can enhance the bonding of the crys- 1.C, connecting to the maximum primarytals in the matrix and also the bonding between the mullitizationaggregates and the matrix, as shown in Fig. 4. Higherhot strength can therefore be obtainedGIO20040060080010001200140016001800Fig. 5 Effect of MCG addition on RUL of the castablesdried at 110℃for24hzekU(x3, eae 4453 BESAgain it is unclear whether the RUL enhancementcan be maintained when subjecting to reheating. RULFig. 3 SEM photograph of the matrix of CG-10 sampletest of the specimens preheated at 1 500 C for 3 h was(110℃24h) after the hmor test at1400℃e reslt is presented in Fig.6G10RsIO um20040060080010001200140016001800Fig.4 SEM photograph of CG-10 sample heated at 1 500C Fig. 6 Effect of MCG中国煤化工 les pre-for 3 hheated at 1 500HCNMHG20 China's Refractories Vol 20. No. 3. 2011Effect of Mild-calcined Coal Gangue Addition on Hot Strength and RefractorinessUnder Load of Ultra Low Cement Castables in Al-o -sio SystemCHNAThe four curves are much smoother, compared this work has tried to add coal gangue in Al203-Siowith Fig. 5, implying the mullitization had happened ULC castable after mild calcination to remove its struc-during preheating at 1 500 C. Nevertheless the To.6% tural water. Based on the results from this work, thevalues are not as high as that in the case of the specifollowing conclusions can be obtainedmens after drying at 110C, as compared in Table 3(1)Adding mild-calcined coal gangue in thethe RUL enhancement by MCG addition maintains the Al203-Sio, ULC castables can enhance the HMOR atsame tendency, i.e. RUL of the preheated castables is 1 400C. The mild-calcined coal gangue at 10% addialso significantly improved by the addition of the coal tion leads to the maximum improvement of HMOR atgangue. It is believed the interlaced mullite intensive 1 400 C. The enhancing effect can be maintainedmatrix and the mullite formation around boundary of when subjecting to reheatingcorundum grains are beneficial to the RUL enhance-(2)Refractoriness under load of the castables canment, see Fig. 4. The in-situ effect of the MCG during also be improved by the mild-caleined coal gangue ad-heating-up is very positive.dition, preferably 10% to 15%Table 3 Refractoriness under load in relationship with the3)The matrix of the coal gangue addedMCG additioncastables is featured by the in-situ formed needle-likeTemprareat0.6%dfomtionr℃"andinedacedmulile,andsuchfavorahemicrotrucDried at 110℃Preheated atture is contributive to the improvement in hot propertiesfor 24 h1500℃for3hof the castables1411.91625.9Rern●saGIO>17001644.4C151595.31579.9[1] Hou Wanguo, Zhou Ningsheng. Effect of raw bauxite addi-tion on thermal behaviour of ultra-low cement Al, O3 -SiO2castables. Chinas Refractories, 2009, 18(3): 15-17.[2] Hou Wanguo. Effects of in-situ Reactions of Raw Bauxiteand Raw Coal Gangue on Properties of Castables in Al,O,Concerning the considerable structural water inSiO, System Masters degree thesis]. Luoyang: Henacoal ganguhichUniversity of Science and Technology, 2009structure due to harsh decomposition during heatingReceived in Mar. 2011LIRR's Project of "Study on Spalling Mechanism, of DenseRefractory Castables by Rapid Heating-up "Being obtainedthe Support from National Natural ScienceFoundation of chinas LIRR's project of"Research on Spalling Mechanism Due to Rapid Heating-up of DenseRefractory Castables"declared by Wang Zhanmin, vice-president of LIRR, will gain the fundsupport from National Natural Science Foundation of China in 2011The project will analyze the spalling mechanism of ULC corundum based castables byinvestigating the change of technical parameters during heating-up The project will dothermodynamic testing for air permeability and intemal vapor pressure of castables, study the.role of thermal stress in castables spalling discuss the spalling mechanism, and offer thetheoretical support for the development of new dense refractory castables中国煤化工CNMHGChinas Refractories Vol 20, No 3, 2011 21