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TECHNICAL PAPER


5 and 6. The dry castable components were mixed in a Hobart mixer type A 200 for one minute and another four minutes after the addition of water. The ready mixed castables were cast into the moulds under vibration. The amplitude was varied during the casting and densification process within the range from 0.2 to 0.5 mm depending on the specimen size and the densification behaviour of the castable. A special focus was given on the stability against separation of the different CA6


aggregates due to the difference in density. Observations made are discussed below.


Thermal conductivity testing up to 1200°C was performed by DIFK, Bonn, Germany according to the hot wire method (parallel wire) DIN EN 993-15 using standard sized bricks pre-fired at 1000°C/5h.


Wynn et al. [8] highlighted the importance of microstructure for the thermal conductivity. In their study insulating firebricks produced by casting provided the lowest thermal conductivity when compared to bricks of similar chemical composition which were produced by the slinger or the extrusion process. The cast material is said to be advantageous because of the micro-porous structure.


VIB 1.1


Component SLA-92


VIB 2.2 - 2.6: Bonite LD


VIB 2.7: Bonite T60/T64


Reactive Alumina Cement


Additives H2 O


Castable properties C MoR


CCS


Bulk density PLC


Thermal conductivity (prefired 1000°C/5h)


20°C / 24h 110°C / 24h 1000°C / 5h 1500°C / 5h 20°C / 24h 110°C / 24h 1000°C / 5h 1500°C / 5h 110°C / 24h 1000°C / 5h 1500°C / 5h 110°C / 24h 1000°C / 5h 1500°C / 5h 20°C


300°C 600°C


1000°C 1200°C


3 - 6 mm 1 - 3 mm 0 - 1 mm 3 - 6 mm 1 - 3 mm


0.5 - 1 mm 0 - 0.5 mm -45 MY -20 MY -20 MY CL 370


CT 10 SG CA-25 R CA-14 M CA-270 ADS/W


Carboxymethyl- cellulose


% 25 % 20 % 25 % % % % % % % % %


% 30 % % %


15 27 21


30 25 15 7


15 25 15 7


10 6


7 4 6


20


% 0.06 0.04 % 60


10 3


10 12


VIB 1.3


VIB 2.2


Castable Development The target for the development of the CA6


21


test castables was a


homogeneous blend of lightweight SLA-92 and dense Bonite in different ratios in order to adjust the bulk density in the range from 1.1 to 2.7 g/cm³. In addition, different strength and insulating properties should be achieved.


A low castable density of 1.1 or 1.3 g/cm³ is represented by pure SLA-92 castables which require high water addition because of the high porosity of SLA-92. For these castables small amounts of Carboxymethyl-cellulose (CMC) were added to the mix to improve the homogeneity and avoid bleeding of the mix. The CMC was either added as a powder to the dry mixed castable or together with the mixing water as Blanose sol. Because of the high water demand, these castables require a smooth vibration during casting. In order to reduce the water demand of VIB 1.1 and to improve the strength properties, the castable matrix was optimised by adding calcined and reactive aluminas as partial replacement of the cement and also by adding dispersing aluminas. This resulted in VIB 1.3.


VIB 2.4


VIB 2.6


VIB 2.7


20 20 15 15 5 7


13 5 10 1.5 1111 43 16.5 12.5 6.6 9.8


MPa 112324 MPa 1358 10


MPa 124578 MPa MPa MPa MPa MPa


3 4 5 5 6


6 6


15 12 23


14 10 27 23 62


21 16 59 42


38 18 60 55


18


37 31


112 59


116 202 186


g/cm³ 1.17 1.39 2.31 2.51 2.67 2.77 g/cm³ 1.11 1.28 2.23 2.44 2.60 2.72 g/cm³ 1.11 1.27 2.18 2.39 2.58 2.68 % - 0.11 - 0.06 - 0.07 - 0.04 - 0.06 - 0.04 % - 0.08 - 0.11 - 0.10 - 0.07 - 0.05 - 0.05 % - 0.72 - 0.19 + 0.66 + 0.45 + 0.07 + 0.37 W/mK 0.46 W/mK 0.33 W/mK 0.27 W/mK 0.29 W/mK 0.38


1.50 1.29 1.18 1.07 1.19


Table 2: Vibration castable formulations based on SLA-92 and Bonite 2.50


1.50 2.00 1.30 1.80 1.30 1.70 1.50 2.00


20 20 15 15 5 7


13 5


The castables containing both SLA-92 and Bonite achieve densities of 2.2 and 2.4 g/cm³. SLA-92 is used as size 0 – 1 mm only to ensure a better and more homogeneous distribution of the lightweight aggregate in the castable mixture. The cement content in the intermediate density formulations was lowered to 10 %, being partly substituted by reactive alumina CL 370. Water demand for these mixes is in the range of 12 to 17 %.


Castable compositions which contain SLA-92 in the coarse fraction, have also been tested. All these tests resulted in separation of SLA-92 and Bonite as well as separation of water after a certain period of time. The use of commercially available stabilisers only slightly improved that behaviour but resulted in higher plasticity of the castable, which hampered the flow properties. This approach was therefore disregarded.


The final mixes in the test series are VIB 2.6 and VIB 2.7, low cement castables based on Bonite LD and Bonite which require 9.8 resp. 6.6 % water addition to achieve vibration flow consistency.


In addition to the vibration castables, two SLA-92 based patch mixes have also been developed. The formulations shown in table 3, for example, can be used as spray coating for submerged nozzle insulation (PATCH SLA 1) or as plastic filler (PATCH SLA 2). Both recipes contain a considerable amount of SLA- 92 0 – 1 mm. In PATCH SLA 1 cement is the binder and Bentonite works as a plasticiser. The SLA-92 content in


NOVEMBER 2014 ISSUE


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