dewatering refractory castable monoliths aachen 1999

8/10/2019 Dewatering Refractory Castable Monoliths Aachen 1999

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41. INTERNATIONALE FEUERFESTKOLLOQIUM, AACHEN 1998

Dewatering Refractory Castable Monoliths

Ing. Molin Adam, Deputy Director, R&D, Refrasil,s.r.o. Czech Republic

Sznapkova Petra, student, VSB Ostrava Czech Republic

Josiek Bogdan, R&D, Refrasil,s.r.o. Czech Republic
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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.
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Lot of work has been already done in the field of dewatering of refractory castable

monoliths, heaps of paper have beentyped. We seem to have solved it, because key factors

have been described, computer modelling has been introduced, lot of laboratory and on-site-

trials 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 600C. There is a number of variables that

influence the process (1) :

Texture Moisture Content

Mix Constitution Casting and Curing Practice

Permeability Binder Level and Type

Strength Dryout Practice and Schedule

Thermal Conductivity Installation Geometry


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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 110C and

subsequently tempered at 200, 300, 400, 500, 600C/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


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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 300C/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 600C 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 600C. CC formulations displayed obviously lower strength values after

drying at 100C 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 110C and further heat treatments up to 600C 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 300C. With cement content dropping in monoliths the value of

weight losses was decreasing and the temperature of a maximum loss mooved from 300C to


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200C, whereas NCC mix displayed again the maximum at about 300C 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 600C weight

values were dropping, obviously the most significant change showed CC material (around

300C.) Other mixes showed a slight weight decrease at temperatures up to 500C, values of

these changes were dropping with cement content going down. Above 500C 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 300C. LCC showed a flat endothermic reaction at

200C, ULCC showed no significant change. DTA curve of NCC displayed slow endothermic

reactions at 150 and 540C.

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.


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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 200C (CC-

LCC- ULCC), whereas NCC critical temperatureseems being again around 300C.

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.


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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


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FIG. 1. COLD CRUSHING STRENGTH ASA 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 FIBERADDITION


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dewatering refractory castable monoliths aachen 1999

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