Technical Paper
Recent development in microsilica-gel bonded no-cement castable technology
Hong Peng* and Bjørn Myhre
Elkem Silicon Materials, Kristiansand, Norway *Email:
hong.peng@
elkem.no
ABSTRACT
Silica-sol bonded no-cement castables (NCCs) have been used in the refractory industry for many years due to their fast dry-out and excellent high temperature performance compared to low cement castables (LCCs). Nevertheless, their use has been limited due to challenges such as long set-time/complex set-behaviour and handling problems due to inadequate green strength. This paper focuses on a technology with a “dry-version” silica binder, using microsilica powder as a valuable alternative to silica-sol. A microsilica-gel bond system has been designed using microsilica powder and a tailor-made speciality product (SioxX®
-
Zero) as dispersant package to give excellent flow and to control the setting behaviour of the NCCs. The flow, set-behaviour and mechanical properties have been studied in comparison with silica-sol bonded NCCs and cement-bonded ultra-low and low cement castables (ULCC and LCC). The microsilica-gel bonded NCCs with SioxX-Zero not only exhibit higher flow, better workability and adequate green strength compared to the silica-sol bond system, but also entail easier handling, storage and transportation thanks to the all-in-the-bag solution. The microsilica-gel bond system contains only a minor amount of cement as coagulating agent; hence, the hot properties are much better than that of ULCCs and LCCs. Phase transformations and bond mechanism were investigated by SEM and XRD techniques after pre-firing at 1400 and 1500°C. Furthermore, the explosion resistance was studied. The drying behaviour and explosion resistance of microsilica-gel bonded NCCs was further significantly improved by using a speciality drying agent (EMSIL- DRY™); a perfect 400kgs block was produced with no problems using a fast firing program (20°C to 850°C at a heating rate of 50°C).
1. INTRODUCTION
Silica-sol bonded no-cement castables (NCCs) have been used in the refractory industry for many years. Compared to low-cement castables, they exhibit fast dry-out and excellent high temperature properties. However, the use has been limited due to long set-time/complex set- behaviour and inadequate development of green strength. Handling, storage and use of liquid silica-sol are logistic factors that must be dealt with, especially at lower temperatures1-5
.
Naturally, a technology of a “dry-version” silica binder, using microsilica powder, is of great interest to further develop NCCs. Microsilica is spherical, amorphous silicon dioxide (SiO2
) particles with an average
particle size of 150nm. Recent reports disclose that a genuine bond based on microsilica coagulation is created, and that the setting of microsilica- gel bond castables is caused by cations6-8
, a similar set mechanism to
colloidal silica. The cations not only contribute to the reduction of the net repulsion effect of microsilica, but also react with the negatively charged microsilica particles. If, e.g. calcium aluminate cement is used as coagulating agent, Ca2+
(and/or other polyvalent cations) released
during dissolution of the cement will react at the negative sites on the microsilica surface to form a three-dimensional network of linked microsilica particles.
Self-flow and vibration-flow of the fresh mix were measured using the flow-cone described in ASTM C230 (height of 50mm, not the more recent cone of 80mm described in EN 1402-4:2003). The self-flow value
REFRACTORIES ENGINEER
THE
Microsilica-gel bond not only provides similar advantages as silica-sol but also eliminates many of the drawbacks of a two-component system. Furthermore, due to the spherical shape of microsilica, closer packing and consequently enhanced flowability/reduction in water demand are also important factors6
. Despite this, refractories using microsilica-gel as
binder are not currently widely used, even though the potential has been known for nearly 20 years. One of the major challenges is the lack of a suitable additive package to overcome the intrinsic weakness of relatively long set time and moderate/low green strength at demoulding.
Based on Elkem’s experience and understanding of the characteristics of microsilica and its performance in refractory castables for the last thirty years, a new speciality product (SioxX® microsilica-gel bonded NCCs9-10
. For ease of application and improved
functionality, high-grade microsilica is used as carrier in the product; and the recommended dosage is approximately 3wt%. SioxX-Zero was compared to two other commercial available dispersants in bauxite based microsilica-gel bonded NCCs. It turned out that the type of dispersant has a strong impact on set-behaviour and green mechanical properties. The use of SioxX-Zero not only provides “well-defined” set and short time to “final” strength, but also give high self-flow and improved mechanical properties. By using SioxX-Zero in combination with polyvalent cations, the set time can be controlled.
Tabular alumina aggregate based NCCs exhibit outstanding physico- mechanical and refractory properties after firing at 1500°C due to the presence of mullite in the bond phase with very little CaO. This enables their use in a variety of applications such as in steel, aluminium, copper, glass, cement, chemical and ceramics production. In this paper, tabular alumina based microsilica-gel bonded NCCs were chosen as examples to present our recent progress in NCC technology. The paper covers the following aspects: i) the effects of dosage of microsilica on workability and hot properties of microsilica-gel bonded NCCs, ii) the advantages of using microsilica-gel binder in comparison with silica-sol and cement binders; iii) further improvement of explosion resistance and drying behaviour by using SioxX-Zero in combination with a speciality drying agent (EMSIL-DRY™).
2. EXPERIMENTAL 2.1 Composition design
Table 1 shows the overall compositions of the castables. In order to understand the mullite formation of microsilica-gel bonded NCCs and the benefits of using microsilica-gel binder, two series of recipes were purposely designed. I) Microsilica gel-bonded NCCs where microsilica (MS) content varied from 3% to 7% (labelled MSZ-3, MSZ-5 and MSZ-7 with the number giving the total content of silica in the full mix); II) Silica- sol bonded NCC, ULCC and LCC, all containing 7% of total silica (labelled SOL-7, ULCC-7 and LCC-7). SioxX-Zero and SioxX-Flow were chosen as dispersants for microsilica-gel and silica-sol bonded NCCs, respectively, while SioxX was used for both ULCC-7 and LCC-7. To improve the setting-behaviour of ULCC-7, SioxX-Set was used as accelerator.
The
SioxX product family is produced at Elkem Silicon Materials, Norway. Since SioxX-range products use some alumina and microsilica as carrier materials, the compositions of the mixes were adjusted accordingly. The silica-sol used was Bindzil 40/130 (40 mass% solids loading, surface area 130m2
-Zero) has been developed for
/g, produced by Akzo Nobel, Sweden). The water content was 4.5 % for all mixes. Castable MSZ-7, both with and without the anti- explosion/fast drying agent EMSIL-DRY (Elkem, Norway) was used for both lab- and industrial-scale explosion resistance testing.
2.2 Properties and characterization
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September 2019 Issue
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