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Technical Paper


2: Experimental 2.1: Mix design


To understand the drying behaviour, two series of bauxite based NCCs were designed. Table 1 shows the mix designs. SioxX-Zero is tailored for microsilica-gel bonded NCCs. The NCC-1 series was used to investigate the effect of sample dimensions and heat-up profiles on drying behaviour of industrial-scale samples. A modified recipe NCC-2 (modified due to availability of raw materials in the lab) was used for further explosion resistance tests. Since SioxX-Zero contains both some alumina and microsilica as carrier material, the compositions were adjusted accordingly. Three types of drying agent/anti-explosion agent were used in the tests; commercial available fibres, P1 and P2, and EMSIL-DRY from Elkem, Norway.


NCC-1 A


Bauxite, 0-6mm Kyanite, 0-0.16mm


Sintered alumina, 0-0.5mm Alumina Fines


Elkem Microsilica 971U 70% CAC


SioxX-Zero Fiber-P1 Fiber-P2


EMSIL-DRY Water content 4.5 5 Table 1: Mix design of bauxite based NCCs (wt%) A


2.2: Experimental procedures Self-flow and vibration-flow were measured after four minutes wet-mixing using the flow-cone described in ASTM C230 (height 50mm, not the 80mm self-flow cone described in EN 1402-4:2003). The self-flow value is the percentage increase of the diameter measured 90 seconds after removing the cone.


Lab-scale explosion resistance testing per Chinese Standard YB/T4117-2003 were carried out for all mixes. 50mm cubes are placed into a hot furnace at a pre-set temperature. The cubes are inspected after 30-minute exposure. The temperature at which cracks start to form or explosive spalling occurs is reported as the explosion resistance.


For industrial-scale blocks, the castables were cast into larger moulds with dimensions of: i) 300x300x300mm (~80kg), ii) 800x600x200mm (~300kg) and iii) 600mmx600mmx350mm (~400kg). The castables were de-moulded after one day and put into the oven for further drying behaviour studies and/or explosion resistance tests. For industrial-scale explosion resistance tests, both 75kg and 400kg blocks were used.


3: Results and discussion 3.1: Flowability


Self- and vibration-flow measurements are summarised in Figure 2. The addition of Fiber-P1 has a strong negative impact on the self-flow value.


Figure 3: Effect of samples size and holding temperature on removal of free water


CMOR (MPa) CCS (MPa) 3.4 17.1 B 3.2 15.4 C 3.2 15.4 Table 2: Green strength of bauxite based NCCs (MPa) D 3.1 14.2 E 3.2 15.7 4.4


65.5 2.5 10


12.5 6


0.5 3


B


65.5 2.5 10


12.5 6


0.5 3


0.1 0.1 0.1 NCC-1 NCC-2 C 52


30.5 10 6


0.5 3


NCC-2 D 52


30.5 10 6


0.5 3


E 52


30.5 10 6


0.5 3


When the dosage is 0.1%, the self-flow value drops from 66% to 16% at a water content of 5.0%. When 0.1% EMSIL-DRY and Fiber-P2 were introduced, the flowability was still fine, approximately 40% at a water content of 4.4%. At 5% water, the self-flow was around 88%. This indicates that the type of drying agent has a strong impact on flowability.


Table 2 shows the green crushing strength (CCS) and modulus of rupture (CMOR) (24hrs at >90%RH and 20°C). The microsilica-gel bonded NCCs have adequate green strengths, with CMOR around 3.0 to 4.0 MPa. The type of drying agent mainly influences the self-flow value, and does not interfere with the strength development.


Figure 2: Flowability of microsilica-gel bonded NCCs with different drying agents


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18


ENGINEER THE REFRACTORIES


September 2018 Issue


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