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41. INTERNATIONALE FEUERFESTKOLLOQIUM, AACHEN 1998

Dewatering Refractory Castable Monoliths

Ing. Molin Adam, Deputy Director, R&D, Refrasil,s.r.o. Sznapkova Petra, student, VSB Ostrava Josiek Bogdan, R&D, Refrasil,s.r.o.

Czech Republic

Czech Republic

Czech Republic

Abstract:

This paper deals with the investigations carried out on the field of dewatering of refractory castable precast shapes. Several grades of refractory castables have been prepared with diffrent cement content (CC,LCC,ULCC,NCC) including a selflevelling grade.Based on permeability measurements, DTA curves, apperent porosity values and strength of the investigated specimens depending on temperature in the range between 100-600 °C the most critical dewatering areas for particular types of refractory castables have been found. The paper provides us with a clue how to dry out and heat up refractory castables to make the process effective and safer.

1. Introduction.

Since the early days of 80th we have been following the falling interest in using shaped refractory materials , a part of which has been gradualy replaced by precast shapes, mostly made of refractory castables. This trend towards monoliths has been brought about by both changes of user technologies and by the advantage itself of newly developped and sophisticated refractory castable formulations. These are based on a very dense structure with extremely low porosity due to the employment of continuous aggregate grading schemes by introducing particles finer than the cementitous components. Water demand has fallen down, in some cases down to a half of the value of conventional castables, previously used. In spite of the fact that water content has been decreased significantly, explosive spalling tendency has even increased. Judging from experience it is just the fear of explosive spalling that in many cases and unfortunately causes that bricks are used for linings where monoliths are expected to perform better.

Lot of work has been already done in the field of dewatering of refractory castable monoliths, heaps of paper have been typed. We seem to have solved it, because key factors have been described, computer modelling has been introduced, lot of laboratory and on-sitetrials have been carried out, but still much to our consternation we come accrosss situations in practice where we are almost helpless and we in fact do not know how to go about dewatering. Despite all these work already done there is still a long way to go. The aim of this paper is to contribute a bit to better understanding of dewatering process of refractory castable monoliths in view of cement contents, flowability and fiber addition effect.

2. Dewatering of Refractory Castable Precast Shapes

In the coarse of drying and heating of monoliths cast of refractory castables a physically bonded water and consequently a chemically bonded one is dehydrated from the material. The water content and the ratio between the two types of water bonded in the material depends also on cement content ( type ) of refractory castable. Dewatering process is usually finished around the temperature of 600°C. There is a number of variables that influence the process (1) : Texture Mix Constitution Permeability Strength Thermal Conductivity Moisture Content Casting and Curing Practice Binder Level and Type Dryout Practice and Schedule Installation Geometry

For the purpose of this work permeability measurements, cold crushing strength developments and weight losses avaluations have been selected as they directly relate to how a certain amount of steam can be led out throgh refractory body with a certain permeability and strength. If permeability and strength is high, water or steam can be driven through refractory body almost irrespective of how quick the material is dried. It goes without saying that there are limits. There is a narrow relationship between permeability (porosity) and corrossion media penetration. We are looking for a solution to make the refractory body both permeable for moisture and resistant against corrossion penetration. But this is hehind the frame of this paper. As organic fiber addition increases permeability and thus the explosive spalling resistance, fiber additions have been investigated.

3. Experimental Procedure

Laboratory tests were carried out on(with?) cylinders, dimensions of which were 50 x 50 mm. CC, LCC, ULCC and NCC bauxite based mixes were vibration cast, SFLC cylindrical specimens were cast without vibration. Specimens were dried at 110°C and subsequently tempered at 200, 300, 400, 500, 600°C/4hours. One cylindrical specimen was used for permeability study, another one was used for porosity testing and one more for cold crushing strength tests. Cylinders weight was recorded so weight losses could be measured after every stage (heating at a temperature). Tests were suplemented by DTA and GTA for all the types of castable used. These tests were conducted with samples of crushed and ground cylinders having been previously cured at ambient temperature. A study on organic fiber adition to castable were carried out using polypropylene fibers to SFLC as a baseline formulation. Since fibers incorporation into a castable causes a change in casting behavior of the material, an aditional water had to be involved to obtain the

same flowability. A simple flow test was carried out to determine the apropriate water content for each fiber loading such that the flow degree remained constant regardless of fiber concentration. Cylindrical specimens cast with different fiber contents ( 0; 0.03; 0.06; 0.1; 0.2 wt%) were heated at 300°C/4hours, then the same procedure was used as described previously.

4, Results and Discussion Results on cold crushing strength after heat treatment at temperatures 100 600°C for

studied castables with differennt cement contents are ilustarated in Figure 1. It was evident that cold crushing strength development after heating indicated no significant difference in the range of 100 600°C. CC formulations displayed obviously lower strength values after

drying at 100°C and moreover this strength decreased slightly with the temperature growing due to decomposition of high alumina cements. NCC formulations followed the similar run with just strength values being higher. SFLC formulation showed very high strength values after drying at 110°C and further heat treatments up to 600°C did not indicate any significant change. LCC and ULCC specimens showed slight strength increase upon heating while the ULCC values were lower. Results on permeability testing versus temperature of heat treatment,coupled by weight losses, are ilustrated in Figures 2,3. There is a huge difference between CC permeability values and those cast of the others formulations. Permeability data of CC specimens were five fold higher and moreover these values increase as a function of heat treatment was more noticable. The higher the density of the castable the lower permeability of the monolith As far as weight losses of CC monoliths are concerned the most significant change occured at around 300°C. With cement content dropping in monoliths the value of weight losses was decreasing and the temperature of a maximum loss mooved from 300°C to

200°C, whereas NCC mix displayed again the maximum at about 300°C like CC mix but with permeability values being considerably lower. Figure4 ilustrates the relation between cumulative weight losses and temperature. During the heat treatment 100 600°C weight

values were dropping, obviously the most significant change showed CC material (around 300°C.) Other mixes showed a slight weight decrease at temperatures up to 500°C, values of these changes were dropping with cement content going down. Above 500°C there was no significant weight loss for all the types of castable. DTA and GTA curves are not listed in the paper due to limited space. CC mix showed two endothermic reactions at 150 and 300°C. LCC showed a flat endothermic reaction at 200°C, ULCC showed no significant change. DTA curve of NCC displayed slow endothermic reactions at 150 and 540°C. Polypropylene fibers addition tests showed generally the decrease in properties once polypropylene fibers were added to castable. Figure5 indicates that the addition rate of 0,03wt.% caused water demand increase by 0,5wt.%. Fiber content 0,1wt.% seemed to be a limited value from selfflowability standpoint of the castable under investigation. Castable with 0,2wt.% fibers loading was vibration cast, because no selfflowability was observed. Cold crushing strength values also decreased with fibers addition. The most significant drop in CCS was observed when 0,1wt.% of fibers was added. Figure6 showes porosity and permeability development when polypropylene fibers were incorporated to selfflowing bauxite based castable. It was evident that fiber addition to castable resulted in conduits being created within refractory body, these conduits connected particular pores making the body permeable. Fiber adition of 0,2wt.% caused 1,7 fold porosity increase, whereas permeability values raised by almost 50 times. The most noticeable permeability increase was indicated when 0,3 0,6wt.% of fibers were added.

5. Summary

Results of this work on dewatering of refractory castable precast shapes showed on some of the existing relations between explosive spalling tendency and permeability, porosity, weight losses, strength. Investigations were conducted with refractory castables based on bauxite aggregate with different cement content (CC, LCC, ULCC, NCC) and workability (SFLC). Polypropylene fiber addition to selfflowing bauxite based castable was also investigated. There was a huge difference between CC properties and those of low cement and moisture content. This difference resulted from the principle itself of low moisture formulations,i.e. from the dense structure and low permeability. Apparent porosity values seemed to be of a less importance unlike permeability values that indicate how particular pores in the refractory body are connected and how monoliths are permeable for steam going through. With cement dropping in castables under investigation the explosive spalling tendency during dewatering increases, critical temoerature moves from300 to 200°C (CCLCC- ULCC), whereas NCC critical temperatureseems being again around 300°C. In view of the fact that organic fiber aditions result in property decrease, particularly as far as selfflowing mixes are concerned , an optimum adition of 0,06wt.% to SFLC has been found out. For limited space of the paper and for the subject itself that is more complex it was impossible to deal with some of the aspects that would contribute to better understanding of dewatering processes.

References: 1)Moore,R.E. Severinn N. : Dewatering Monolithic Refractory Castables: Experimental and Practical Experience . University of Missouri-Rolla, Department of Ceramic

Engineering,p.573-582

2)Jason M.Canon, Todd P. Sander: Effect of Organic Fiber Additions on Permeability of Refractory Concrete , UNITECR Proceedings1997, p.583

FIG. 1. COLD CRUSHING STRENGTH AS A FUNCTION OF HEAT TREATMENT TEMPERATURE FIG. 2. PERMEABILITY AND WEIGHT LOSSES VERSUS HEAT TREATMENT TEMPERATURE OF CC SPECIMENS FIG. 3. PERMEABILITY AND WEIGHT LOSSES VERSUS HEAT TREATMENT TEMPERATURE FIG. 4. CUMULATIVE WEIGHT LOSSES VERSUS TEMPERATURE FIG. 5. WATER DEMAND AND COLD CRUSHING STRENGTH AS A FUNCTION OF POLYPROPYLENE FIBER ADDITION FIG. 6. APPARENT POROSITY AND PERMEABILITY AS A FUNCTION OF POLYPROPYLENE FIBER ADDITION

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