Technical Paper NEW STRONGER BONDS FOR MONOLITHIC CASTABLES THROUGH SURFACE REACTIVE CALCIUM ALUMINATE AGGREGATES

Transcription

1 Page : 1/9 NEW STRONGER BONDS FOR MONOLITHIC CASTABLES THROUGH SURFACE REACTIVE CALCIUM ALUMINATE AGGREGATES C. Wöhrmeyer, C. Parr, H. Fryda, E. Frier Kerneos GmbH, Oberhausen, Germany, Presented at The International Colloqium on Refractories, AACHEN, Germany, September 8 th and 9 th, 21.

2 Page : 2/9 ABSTRACT This investigation studies the influence of calcium aluminate aggregates on refractory castable properties in comparison to bauxite and fireclay aggregates. Calcium aluminate aggregates with a mineralogical composition similar to CAC are able to develop stronger bonding forces with the cement than aggregates from the alumina-silica system. The chemical affinity between the calcium aluminate aggregates and the matrix enables direct chemical linkages via calcium aluminate hydrate bridges. During the de-hydration period and the re-crystallisation of anhydrous calcium aluminate phases, the linkages between the calcium aluminate matrix and the surface of the calcium aluminate aggregates remain stronger since the aggregates are of a similar mineralogical nature as the anhydrous cement. This effect results in higher strength compared to bauxite even though the density is lower with the fused calcium aluminate aggregate R5. For a given furnace design, 1% less material is needed to fill the forms, and better insulation properties and energy savings are expected. This makes R5 interesting for many applications where good heat containment, abrasion resistance and low liquid penetration in combination with low accretion build-up are required. In aluminium contact areas, it can even be used with less or without antiwetting additives which makes it more temperature resistant. The sintered calcium aluminate aggregate R6 has a higher intrinsic open porosity than R5. It makes castables even lighter than with fireclay but without negative effects with respect to bending strength. Both R5 and R6 are calcium aluminate aggregates which suit applications up to 135 C- 14 C, for example, in secondary aluminium furnaces, back-up linings in many industrial furnaces, DRI-production, boilers and re-heating furnaces.

3 Page : 3/9 1 Introduction In recent years, the refractory raw material situation has become a critical and even political subject mainly driven by the high demand for refractory aggregates as consequence of the booming steel industry in China. In 29, China counted for 47% of the global crude steel production (568 Mio t out of global steel output of 122 Mio t) vs. only 15% just 1 years back when China produced only 127 Mio t of the total of 848 Mio t [1]. On the other hand, China is one of the very few countries with very important geological reserves for refractory grade bauxite [2]. Only 1 years ago, a large amount of this bauxite was available for the global refractory industry at low prices. But now, China needs a large portion of this bauxite for its own aluminium and steel production. China s steel production has more than quadrupled during the last 1 years. This situation has even become a political issue. The Commission of the European Communities launched a position paper in 28 with the title The raw materials initiative Meeting our critical needs for growth and jobs in Europe [3]. However, political processes need their time and can only try to improve the trade rules and stimulate research in new raw materials. The European refractory and raw materials industries need to develop strategies to adjust to the dramatically changed global situation and need to turn it into new opportunities. An example for the use of new calcium aluminate aggregate based products was given by [4] which proposed calcium aluminate aggregates for tin bath bottom bricks to overcome problems with alkalis when aggregates from alumina-silica systems are used. Other publications discussed the application of calcium aluminate aggregates in bricks [5] and castables [6, 7] for non-ferrous and other industries. This present paper will specifically study the calcium aluminate aggregates and their surface reaction with the binder phase. When calcium aluminate aggregates consist of hydraulic phases like CA and CA2, they can interact chemically during hydration and de-hydration with the calcium aluminate binder, unlike those calcium aluminate aggregates which consist of CA6, corundum or mullite. Specifically, the effect of this stronger bond on the physical properties of castables will be investigated in this paper, in comparison to bauxite and fireclay in similar formulations. Calcium Aluminate Aggregates Calcium aluminate cement is well-known for many years as the binder for refractory castables. Also, calcium aluminate aggregates are known from peri-refractory applications (ALAG ) and from fluxing applications for metallurgical slags (LDSF ). Both are fused materials and start to melt below 14 C, ALAG due to its relatively high iron oxide content combined in different aluminate phases and LDSF due to its high C12A7 mineral phase content (Tab. 1, 2). ALAG is, for example, used in the back-up zones of blast furnace cast houses and the blast furnace slag granulation station. Other fields of application are the ramps in coke oven plants and industrial floors in the aluminium industry where high abrasion resistance is required. A low iron oxide, fused calcium aluminate aggregate (R5) for castables has been presented by [6]. Compared to ALAG the R5 product contains more Al 2 O 3. This results, unlike what is stated in [8], in a very low C12A7 phase content (<1%) and a high amount of calcium monoaluminate (CA) in the fused R5. This study will further investigate the effect of the surface reaction of calcium aluminate aggregates on the properties of different castable systems. The fused, almost pore free aggregate, R5, will be compared mainly with bauxite in order to investigate the potential as an aggregate in areas where high abrasion resistance, low liquid penetration and/or low accretion formation is required. Furthermore a sintered and more porous 6% alumina aggregate (R6) will be investigated. R6 is derived from a calcium aluminate aggregate which is normally used in the steel industry as a metallurgical slag modifier. This aggregate will be compared to fireclay in order to study its ability to reduce the castable density without penalising the strength. This is one of the key requirements for future furnace linings to save more energy through better heat containment inside the furnace.

4 Page : 4/9 Tab.1: Chemistry and mineralogy of aggregates Calcium aluminate Fireclay Bauxite R6 R5 ALAG LDSF FC B Sintered Fused Fused Fused Sintered Sintered Chemistry Al 2O ,5 39, ,6 89,5 SiO 2 6 5,5 4,5 3,5 51,5 4 CaO ,5 5,5,1 Fe 2O 3 2,5 2 15,5 2,9 1,5 TiO 2 2, ,7 3,7 Mineralogy (A= Al 2O 3, S=SiO 2, C=CaO, F=Fe 2O 3) A XXX A3S2 S CA2 CA C12A7 C3A C2S C2AS C4AF XXX X XX X XXX X XXX XXX X XXX X X - - XX XX X X XXX= main phase, XX=secondary phase, X= minor phase, - = below 1% 2 Test Methods Classical castable test methods such as vibration flow, porosity by water immersion and strength measurements have been used to study the basic castable properties. Quantitative chemical and mineralogical analyses have been conducted using XRF and XRD methods. Model regular castables (Tab. 2, 3) and deflocculated medium cement castables (MCC) have been investigated (Tab. 4). Tab. 2: Model regular castable recipes CC-FC and mm CC-FC R6 3-6 % - 28 R6 1-3 % - 25 R6-1 % - 22 Fireclay 3-6 % 28 - Fireclay 1-3 % 25 - Fireclay -1 % 22 - Secar 51 % H 2O % 8,5 1,5 Working time min Test Results Calcium aluminate aggregate (R6) in regular castable The R6 aggregate is a sintered calcium aluminate with about 3% open porosity, higher than for bauxite or dense fireclay aggregates. Fig. 1 compares the rheology of the fireclay-based castable CC-FC with the R6-based. Both need the same amount of water to reach identical initial flow properties. While fluidity under vibration slightly increases during the first 3 minutes and then stays almost stable when fireclay is used, the flow decays in the case of due to the higher pore volume into which the water can migrate over time. Then, less water is available to fluidise the cement matrix and the castable starts to stiffen. It is assumed that the drop in flow is not caused by the mineralogy of R6 since it contains mineral phases which have low hydraulic properties at 2 C (Tab. 1). The observed stiffening effect with R6 could be of interest for gunning mixes but has not been further studied here. After firing at 8, 11 and 135 C, the bulk density of is about 5% lower than CC-FC (Fig. 2) and the open porosity is almost 4% higher (Fig. 3). With classical aggregates from the

5 Page : 5/9 alumina-silica system, one would expect a drop in strength [9]. But in this case, despite the lower bulk density and higher porosity, flexural strength is the same as CC-FC (Fig. 4). Vibration flow (mm) CC-FC Time (min) Fig. 1: Vibration flow of regular castables CC-FC and at 2 C Bulk density (g/cm3) 2,5 2,3 2,1 1,9 1,7 1,5 CC-FC Fig. 2: Bulk density of regular castables CC-FC and Open porosity (Vol.%) CC-FC Fig. 3: App. porosity of regular castables CC-FC and CMOR (MPa) CC-FC Fig. 4: CMOR of regular castables CC-FC and CC- R6 R5 in regular castables The fused and dense R5-aggregate has been compared with Chinese bauxite aggregate in a model recipe as shown in Tab. 3. As can be seen in Fig. 5, both casables exhibit a very similar flow profile with the same amount of mixing water. Due to the dense R5-grains, no flow decay has been observed. The dense grains, together with the mineralogy of R5, with its high calcium monoaluminate and low C12A7-content (<1%), prevent early stiffening. The working time is equivalent to that of the bauxite based castable. But, compared to bauxite, the R5 reduces the castable density by 1% (Fig. 6). Despite the lower density, the cold modulus of rupture increases by almost 5% after firing at 8 and 12 C (Fig. 7). For R5, the refractoriness under load test indicates that the bauxite mix softens earlier than R5 but can be used in this type of recipe with a matrix consisting of 5% alumina cement up to ca. 14 C while for R5 the application limit would be at ca. 135 C (Fig. 8).

6 Page : 6/9 Tab. 3: Model regular castable recipes CC-R5 and Vibration flow (mm) mm CC-R5 R5 3-5 % 31 - R5 1-3 % 2 - R5-1 % 29 - Bauxite 3-6 % - 31 Bauxite 1-3 % - 2 Bauxite -1 % - 29 Secar 51 % 2 2 H 2O % Working time min CC-R5 T T3 T6 T9 T12 Time (min) Fig. 5: Vibration flow of regular castables CC-R5 and at 2 C Bulk density (g/cm3) 2,8 2,7 2,6 2,5 2,4 2,3 2,2 CC-R5 CMOR (MPa) CC-R5 2 (6 h) 2 (24h) Fig. 7: CMOR of regular castables CC-R5 and CC- BX delta L (%) CC-R Fig. 8: Refractoriness under load (.2 MPa) of regular castables CC-R5 and R5 in deflocculated medium cement castables (MCC) The effect of R5 in a medium cement containing model recipe (MCC) with different amounts of barium sulphate (as an anti-wetting material for aluminium contact areas) is shown in Tab. 4. Whilst the barium sulphate is absolutely necessary to get the required anti-wetting effect for the bauxite mix, the amount can be reduced or even completely eliminated when R5 is used [6]. 2, Fig. 6: Bulk density of regular castables CC-R5 and

7 Page : 7/9 Tab. 4: Model MCC recipes with bauxite (B1-B3) and R5 (R1-R3) with different barium sulphate contents. Bauxite R5 B1 B2 B3 R1 R2 R3 R5 3-5 mm % - 33 R5 1-3 mm % - 12 R5-1 mm % - 2 SECAR 51 % Bauxite 3-6 mm % 21 - Bauxite 1-3 mm % 22 - Bauxite -1 mm % 22 - Bauxite -.9 % Calc. Alumina % BaSO 4 % React. Alumina % 1 1 SECAR 71 % PP fibres %.5.5 Peramin AL 2 % H 2O % containing mix B1 with 1% BaSO 4 (which can be seen as a reference for this type of application). In the case of R5, less barium sulphate can be used (R2, R3) which has a positive effect in that the castable can be heated to higher temperatures without drastic volume changes (caused by the disintegration of the barium sulphate as in case of B1). Even R5 with 1% barium sulphate (mix R1) shows only moderate volume expansion at 14 C, whilst the mix with bauxite (mix B1) expands strongly. Heated to 14 C, the MCC with R5 keeps a significant lower porosity level than the bauxite based mixes (Fig. 13). It offers an additional safety measure should unexpectedly high temperatures occur in the course of aluminium production. Vibration flow (mm) Time (min) Fig. 9: Flow of MCC with R5 (R1-R3) and bauxite (B1-B3) R1 R3 R2 B1 B3 B2 A microsilica-free MCC has been chosen to minimise the potential reactions between the aluminium melt and the castable matrix. The polycarboxylate ether-based deflocculant (Peramin AL 2) fluidifies the mixes very efficiently with 5% water in case of bauxite and only 4.5% in case of R5. With R5, the dosage of the superplasticizer could even be reduced. Despite the lower water addition and lower deflocculant content, better flow properties have been achieved for all R5 containing MCC s (R1- R3) as can be seen in Fig. 9. Here, open porosity after heating at 8 and 12 C, the critical temperature range for the aluminium application, is, in all cases, significantly lower with R5 despite the lower bulk density (Fig. 1, 11). The cold crushing strength is, independently of the barium sulphate content, higher than with the bauxite App. Porosity (Vol.%) C 12 C B1 B2 B3 R1 R2 R3 Castable Fig. 1: App. Porosity of MCC with R5 (R1-R3) and bauxite (B1-B3)

8 Page : 8/9 Bulk density (Vol.%) 3 2,95 2,9 2,85 2,8 2,75 2,7 2,65 2,6 2,55 2,5 8 C 12 C B1 B2 B3 R1 R2 R3 Castable Fig. 11: Bulk density of MCC with R5 (R1-R3) and bauxite (B1-B3) CCS (MPa) C 12 C B1 B2 B3 R1 R2 R3 Castable Fig. 12: Cold crushing strength of MCC with R5 (R1- R3) and bauxite (B1-B3) App. Porosity (Vol. %) B3 R3 R2 B2 R1-2, -1,, 1, 2, 3, 4, 5, Permanent linear change 11 C --> 14 C (%) Fig. 13: Permanent linear change and open porosity after firing at 14 C of MCC with R5 (R1-R3) and bauxite (B1-B3) B1 4 Summary Calcium aluminate aggregates show very particular bonding properties due to their surface reaction with the cement. Despite the lower density and higher porosity of the sintered R6 aggregate compared to fireclay, equivalent strengths have been found with a regular castable. The fused R5 aggregate has very little open porosity but reduced density when compared to bauxite due to its calcium aluminate nature. In addition, strength is even higher with R5 despite the lower density, due to the better bonding between the matrix and calcium aluminate aggregate. Thus a given furnace lining design could be cast with less material without sacrificing the strength and/or abrasion resistance. It is expected that the thermal heat containment will also be improved, which would, in turn, save energy and costs. R5 used in microsilica-free medium cement castables for aluminium applications offers new ways to formulate the castable with better temperature resistance. R5 reduces the pore structure of the castable, with very low open porosity recorded even after firing at 14 C. Barium sulphate starts to decompose at 12 C which leads to a massive volume expansion in bauxite containing castables at 14 C. Volume stability is much better when R5 is used, even in combination with barium sulphate. However, due to the improved matrix/aggregate bonding properties and the low wettability of calcium aluminates with aluminium, the barium sulphate content can be reduced when R5 is used. This further improves the temperature stability. Both R5 and R6 calcium aluminate aggregates can be used up to 135 C to 14 C depending on the castable type and the calcium aluminate cement and filler used.

9 Page : 9/9 5 References [1] World Steel Association ( [2] G. Bardossy, P.J. Combes: Karst bauxites: Interfingering of deposition and paleoweathering. Spec. Publication No. 27 of Int. Ass. Sedimentologists, p , [3] Commission of the European Communities. Communication from the commission to the European parliament and council: The raw materials initiative meeting our critical needs for growth and jobs in Europe. Brussels, COM (28)699, SEC(28)2741, 28. [4] Patent US54287A (Didier-Werke AG), [5] K. Santowski, K. Gamweger, B. Schmalenbach, T. Weichert: Ceramic bonded refractories based on calcium aluminate minerals for the use in glass, aluminium and lime furnaces. Unitecr congress, p , Dresden, 27. [6] C. Wöhrmeyer, N.Kreuels, C.Parr, M.Vialle: Calcium aluminate: Zemente und neue Zuschlagstoffe für den Einsatz in der Aluminiumindustrie. 41st Int. Colloquium on Refractories, p. 8-86, Aachen, [7] C. Wöhrmeyer, N.Kreuels, C.Parr, T.Bier: The use of calcium aluminate solutions in the aluminium industry. Unitecr congress, Berlin, [8] Patent WO A2 (MCGOWAN), 25. [9] G. Routschka: Feuerfeste Werkstoffe, Vulkan Verlag, 1996.