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REFRACTORY CASTABLES VIA USE OF PERMEABILITY ENHANCING ACTIVE COMPOUND


Christoph Wöhrmeyer*, Jean-Michel Auvray, Carl Zetterström Kerneos S.A., Paris-La Defense, France


Abstract


The removal of 'free' and 'bonded' water is a critical step in the preparation of dense refractory castables for use in modern furnaces, with the risk of premature failure due to an incompatible dry-out schedule commonly referred to as 'explosive steam spalling'. The risk of this mode of failure increasing with highly deflocculated castables with very low permeability is well documented and creates a significant safety concern during the installation process as well as potential financial implications. This paper discusses the impact of REFPAC®


MIPORE 20 (MP20), an innovative


permeability enhancing active compound (PEAC) for calcium aluminate cement bonded castables. Investigations show how the MP20-powder interacts with the cement hydration path creating a new microstructure that significantly improves gas permeability at temperatures above 100°C while for example the traditionally used Polypropylene-fibers are effective only above 150°C. Test results show that the new microstructure allows more water removal at lower temperatures and lower pressure build-up. A more permeable microstructure is formed and investigations show how it can effectively release moisture prior to critical temperatures being reached which avoids high steam pressure build-up in the pores. This paper also explores how formulation types, curing conditions, and dimensions of the monolith impact the water release.


Introduction


The development of highly densified, low porous castables during the last decades have resulted in high performing products with long service life that have enabled replacement of bricks in many furnaces by joint-free monolithic linings [1]. However, with the reduction of water demand through the introduction of very efficient deflocculants and sophisticated micro- fillers, the safe water removal process has become increasingly critical. This is due to the low gas permeability of modern dense deflocculated castables as consequence of low open porosity and small pore diameters. A common method in castable technology is to add a small amount of Polypropylene fibers (PP-fibers) to the castable which after shrinkage and melting above 150ºC leave some pore channels through which the steam can be more easily transported to the surface of the material [2-4]. However, below 150ºC, the PP-fibers are not functional and up to that temperature it remains difficult to transport water to the surface of the monolith so that steam pressure build-up can occur. Above 150ºC, when the fibers have created the pore channels the free water can then more easily be transported, but at the same time already additional water from the de-hydration process of the calcium aluminate hydrates starts to be liberated into the pores. That


November 2017 Issue


adds to the steam pressure build-up. Explosive spalling can occur if the heating schedule is not well adapted to the type of castable and size of monolith that has to be dried [5, 6].


Zetterström et al. [7] and Auvray et al. [8] reported about a new concept of creating gas permeability for cement bonded castables. They presented a permeability enhancing active compound (PEAC) that creates permeability at lower temperatures than PP-fibers (Figure 1). As consequence a significant reduction in steam pressure was observed for a MCC and LCC castable (Figure 2). Furthermore, they commented that compared to PP-fibers the PEAC-powder can be precisely dosed into the dry-mix with automated powder feeding systems or added as one bag per 1t of dry-mix. The dosage of PP-fibers often requires manual operations and is a potential source of dosage errors. Furthermore, homogenisation of a small quantity of fibers in a castable dry-mix remains a challenge.


DRY OUT OF DENSE


Technical Paper


Figure 1: Gas permeability of LCC after different heat treatments [7]


This paper further investigates the underlying mechanisms of this permeability


commercialized under the name REFPAC®


enhancing active compound (PEAC) that has been MIPORE 20 (MP20). Different


parameters are studied which could potentially have an influence on the dry-out process like curing time, curing temperature, and the size of the monolith. But also the impact of the enhanced permeability on other castable properties like porosity, pore size distribution and strength are discussed.


ENGINEER THE REFRACTORIES


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