DRY-OUT BEHAVIOUR & EXPLOSION RESISTANCE OF REFRACTORY CASTABLES
Hong Peng* and Bjørn Myhre
Elkem Silicon Materials, Kristiansand, Norway *Email: hong.pe
Drying industrial-scale specimens are not straight forward and always more complicated than lab-scale samples. For cement bonded refractory castables
explosive spalling due to removal of free water and/or
dehydration of cement hydrate have always been challenging. Naturally, cement free castables are very interesting due to fast dry-out and excellent hot properties. However, their green strength is often so low that even the removal of free water becomes a challenge, particularly in larger pieces.
In this paper, the dry-out behaviour and explosion resistance of microsilica- gel bonded no-cement refractories (NCCs) have been evaluated based on testing of large industrial-scale specimens. Heat-up profiles, physical dimensions and types of drying agents have all strong impact on water loss and explosion resistance. The results give new insight into dry-out behaviour and explosion resistance of refractory castables. Replacing cement bond with microsilica-gel bond seems to be an alternative to realise true fast dry-out and to improve explosion resistance. The explosion resistance can further be significantly improved by using a specialty drying agent (EMSIL- DRY™
); as demonstrated by a perfect 400kg block of microsilica-gel bonded NCCs that was produced with no problems using a fast firing schedule (20°C to 850°C at a heating rate of 50°C/hr).
Key words: Dry-out behaviour, fast heat-up, no-cement castable, NCC, microsilica-gel bond, microsilica.
It is well known that the vapour pressure increases potentially with temperature in a closed liquid/vapor aqueous system, as described by Antoine’s equation (1)
Figure 1: Vapour pressure (Pv) increases exponentially with the temperature according to Antoine’s equation (2
The dry-out process of cement bonded castables involves three stages: i) evaporation from room temperature to 100°C, ii) ebullition from 100 to ~300°C, iii) hydrate decomposition at a temperature above 250 - 350°C.
When the temperature in the sample reaches 100°C, the ebullition starts and leads to massive water loss, and the water removal is ruled by vapour pressure. This is the most critical dewatering step and spalling and/or explosion most likely takes place in this stage. Vapour pressure is dependent on the heating profile and the permeability and thickness of the refractory body. It is not easy to obtain a balance between the generated vapour inside the body and the water withdrawal at the surface. Hence, optimisation of dry-out schedules and improvement in permeability of hydraulic bonded refractories have been given special attention to reduce the risk of spalling and/or explosion. The permeability of hydraulic bonded refractory castables can be increased by i) adding polymeric fibers or Al- metal powders (2) hydrate formation (3)
; and ii) replacing cement by colloidal silica to inhibit .
Naturally, cement free castables such as colloidal silica bonded refractory castables are very interesting due to their fast dry-out and excellent hot properties. However, their green strength is often so low that demoulding and handling after curing become a challenge, particularly for larger pieces as opposed to small laboratory test specimens 4-6
Recent work by Elkem demonstrates that microsilica-gel bonded NCCs show improved green strength compared to colloidal silica bonded, excellent hot properties and exhibit fast dry-out performance compared to LCCs7-12
. In microsilica-gel bonded no-cement castables, only small
amounts of the mixing water remain after drying at 110°C, hence, most of the free water can be removed by simple drying. In LCC, a high fraction of the water is chemically bound with the cement and must be fired at temperatures up to 600°C to remove it.
Drying industrial-scale specimens are not easy and always more Figure 1 (2) shows the evolution of vapour pressure (Pv) as a function of
temperature according to Antoine’s equations and the typical green tensile strength for refractory castables are indicated.
complicated than lab-scale samples. In this paper, bauxite based refractory castables have been chosen to investigate the dry-out behaviour and explosion resistance of refractory castables based on both lab-scale and industrial-scale trials. The following aspects are covered: i) effects of drying agent/anti-explosion agent on flowability, ii) fast dry-out of microsilica-gel bonded NCC, and iii) improvement in explosion resistance of microsilica- gel bonded NCC by introducing anti-explosion agent, EMSIL-DRY.
September 2018 Issue
ENGINEER THE REFRACTORIES
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