CA1189214A - Bonding agent for refractory materials and the application of such agent - Google Patents

Abstract

ABSTRACT
This invention relates to a hydrocarbon-based bonding agent, or binder, for producing carbon bodies, other refractory bodies or adhesives and tamping pastes/
ramming compounds for high temperature use. The binder consists of two components, A and B, and a catalyst.
A is thermo-hardening and/or catalyst-hardening in the temperature range 10 -250°C. After hardening compon-ent A, the solid matrix is impregnated with component B
with a tar which becomes solid at a temperature at least 200°C higher than the temperature required for harden-ing A.

Description

1 BONDING AGENT FOR RE~R~CTORY MATERIALS AND THE
APPLICATION OF SUCH AGENT.

By the term bonding agent, also called bonding system or simply binder, is understood here a mixture of substances, or simply one substance, which is mixed with solid parti-cles and which can be hardened, whereby the mixture with the solid particles forms a solid body.
The inventio.n relates to a hydrocarbon-based binder for bonding refractory materials, in the form of bodies or powder, at low tempPratures (10 - 30C) and in which the bonding phase, after resultant joiningr must have low permeability and high compressive strength, also,in the event, after high temperature hardening.

The binder is liquid at room temperature and consists of two components of which one component A is thermo-harde-ning and/or catalyst -hardening within the temperature range 10-250C, and after hardening is impregnated with the other component B.
Component B is a tar which becomes solid at at least 200C over thehardening temperature for component A.

More specifically, the present disclosure concerns a binder (bonding agent) which is liquid at room temperature and which consists of two components A and B and a catalyst, characterized in that one o the components, A, is thermo-~s~ r~/en~9 hardening and/or catalyst-~a~h~ in the temperature range 10-250C and that after hardening the solid matrix is .

%~

impregnated with the second com~onen-t, s, and in which the second component, B, is a tar which becomes solid at a temperature at least 200 higher than the temperature required for hardening component A.

In another aspect there is provided application oi the binder as described above characterized in that the binder is mixed with powdered carbon, carbides, silicides, borides, nitrides, metal oxides, silicates, or other minerals, either alone or mixed, for producing bodies, tamping pastes or adhesives which may be wormed at room temperature (20C), and which can be hardened at a temperature below 250C.

Refractory binders for use in reducing atmosphere must usually fulfil the following requirements:
- High melting point - Inert to chemical attack - Low permea~ility/high coke yield - High compressive strength/tensile strength at low and high temperatures - High resistance to the effects of temperature changes - Not be harmful to the environment - Low price .~

l A number of different binders have been introduced for use in inert/reducing atmospheres at high temperatures, ana are in service today. For example, for carbon it is cus-tomary to use products based on tar/pitch. The viscosity of such a bindex can be made sufficiently low at room temp-erature to permit application/forming, by selecting special tar fractions or by adding a viscosity-lowering agent.
US patent 4032653 describes the use of aromatic hydrocarbons, methylnaphthalenes, for reducing the viscosity of low-temperature pitch. The amount of methylnaphthalenes isstated as 8 - 16~ of the quantity of bincler. Nethylnapht-,halene is undesirable, because it has a low coke yield andat the same time a strong, penetrating smell and a consider-able va,or pressure at a temperature as low as 25C.

If this viscosity-reducing agent is used in quantities corresponding to the upper limit, the evaporation is in-creased whilst at the same time the coke remnant in the binder is reduced because of the low coke yield in methylnaphthalenes. If the amoun-t of this viscosity-lowering agent is reduced down to the minimum, the vis-cosity is so high that a good compression/tamping of the paste becomes difficult. The permeability ox the finished, tamped paste can therefore easily be too high.
where has been disclosed a hinder which is a mixture of molasses ( max 15% with max 5% water and up to 15% of a high-temperature softening agent.
The necessary binder content in the finished paste is given as 20 - 30%; partly as molasses and partly,as high-temperature pitch (6-15%~.
3~ Large binder quantities require that the binder has a high coke~yield. This is satisfied by high-temperature pitch, J f l but not by molasses which has a coke yield of around 30%.
A further disadvantage of the system described is that the quantity of liquid binder (molasses can be too low, with the result that the paste easily becomes too dry. A
large total volume of binder to which is added only small quantities of liquid binder with low coke yield will easily lead to the'finished paste having high permeabili-ty.
Trials have also been conducted using tar as a binder for ~0 forming at room temperature. In spite of the lack of correlation between viscosity and coke yield! the follow-ing figures can serve to illustrate the posit,ion:

Viscosity~_mPas, 20C Maximum coke yield %
Tar 1 80-100000 40-45 " 2 5-800~ 30-~5 " 3 2000 27 Factors such as the C/H ratio , degree of cross linkage between the molecules and the ratio between chain compounds and ring compounds will have a great influence on the coke yield''.' -'' One particular tar can, for example, have a viscos:Lty of 200,000 mPas, and a coke.~'yleld of only 20%.
Traditional tamping pastes used as jointing medla between carbon bodies forming the linings of electrothermic ancl electrochemical furnaces, usually contain binders with a softening point 50-120C and a coke yield from 52-53~
to about 60%. These products are usually acceptable on the best of quality criteria.
However, the operation of ,~pplyin~ the ho-t tamping paste has negative'aspects for the opera-tor and also for the environ-ment. One of the above-mentioned tar binders with a vis-cosity of 5-8000 mPas would be advantageous from the point of view of the opera-tor applying i-t and the environment, 32:1~

l but would not be satisfactory qualitywise, at all events not for applications where the finished product has to have low permeability. Synthetic binders with a high coke yield (50-60%) have been known for a number of years, and then primarily phenolformaldehyde and furan derivates.
Several circumstances help to limit the use of these.
formaldehyde, furfural and furfurylalcohol are, for health reasons, permitted in only low concentrations in the working atmosphere according to Swedish environmental legislation of Jan 1,1979. Fenol 1 ppm 4 my/m3 Formaldehyde 1 " 1,3 "
Furfural 5 20 Furfurylalcohol 5 " 20 "
For high temperature purposes, it is furan derivates which in recent years, have been most to the forefront as syn-thetic binders. French patent No. 2255395 describes a paste with a furan derivate and a high temperature pitch in powder form plus a catalyst. I-ts shelf life is short (must be used soon af-ter making up and the quality pro-blems and environmental problems are considerable.
Furthermore, furan resins are very costly compared with tar/pitch 3-6 times the price of thesej. In addi-tion furan resins usually form brittle, glassy carbon when coked.
US patent (1944) No. 2.345-966 describes the use of a pure :Euran resin (mixture of furfurylalcohol and furfural, for use alone or together with various fillers, and -to-gether with a latent catalyst. The objections to this will, in general be the same as those raised against French patent No. 2.255.395, with the exception of the short shelf life.
Norwegian patent application document No. 139601, laid open for public inspection, describes the use of a furan resin (also based on base-catalyzed furfurylalcohol and furfural~ as a binder, together with anthracite recovered from aluminium reduction cells. The same objections as those raised against the last two systems referred to, also apply here. In the Norwegian document the assumption is that the catalyst must be a base, and be present in the dry material.
Common to most binder systems is that the hardening of the binder phase produces stresses as a result of expansion/contraction.
For binders based on pyrolysis, the driving off of volatile components causes expansion, whilst the subsequent dehydrogenation and re-orientation of the macromolecules results in contraction of the binder phase and the formation of pores.
When density, low permeability and high compressive and tensile strength are important, carbon materials are usually impregnated under subsequent heat treatment to 500-750 C once or several times. The binder system according to the present invention will fulfill the most important demands made of binders for high temperature use in inert or reducing atmospheres.
The binder is liquid at room temperature and consists of two components, A
and B, and a catalyst.
Component A is thermo-hardening and/or catalyst-hardening within the temperature range 10-250 C, and after hardening the solid matrix is impregnated with the other component B, which is a tar which is transformed into a solid state at a temperature which is at least 200C higher than the temperature required for hardening component A.
The characteristic properties of the system are that low temperature hardening provides joints or elements oE stable ~8~ 4 l shape at temperatures under 250C, and that the remaining part of the binder impregnates the resulting solid matrix before this undergoes separate pyrolysis to solid state at a considerably higher temperature.
This binder system is particularly suitable for produc-ing adhesives and tamping pastes at room temperature (10-30C) and for use where high compressive and/or high tensile strength is important in view of the strains and stresses to which the matexial is subjected in the case, for example, of rapid heating up and in which the car-bonaceous binder phase is favourable for achieving a chemically inert product and high electrical and/or thermal conductivity between elements or in granular materials.
In the binder 5ystem `uncler discussion, component A consists of partly polymerized furfuryl resin with a maximum of 50~
by weight(wt~) monomers, preferahly 10-30 wt~ monomers and maximum viscosity 500 mPas, the polymer part of the fur-furylalcohol being produced by self-condensation.

Component B is a tar with a softening point lower than 60 R & B and a coke remnant preferably over 40 wt%.

Preferably, 92-99 wt% of the binder should consist of a mixture of component A plus component B, with component B
accounting for between 40 and 90 wt~ of this component mix, and the catalyst 8-1 wt~ of the sum, A~B.

In a preferred embodiment, compcnent A contains the thermo hardening or catalyst-hardening component, 10-60 wt%
f~Lran resin and component B with a softening point lower than 60 R B accounting for up to 90 wt~ of the binder.
Compared with traditional binders it is further a very important point that the shape stability avhieved by hardening at a temperature below 150C is sufficient to permit the rapid heating up of the elements in the exist-l ing process without prior baking.
The liberation of furan resin in the form oE vapour isgreatly reduced as a result of the smaller quantity of this resin and the content oE monomer in it.
Mixing into the synthetic binder A a highly aromatic fraction B, gives the following advantages:
1. The evaporation oE the furan resinS both in storage and during hardening~is reduced.
A mixture of 25% furan resin and 75% oE a tar with a 47% coke yield had thus, after 4B hrs hardening at 120C , a coke yield of 50%. Under the same conditions, the coke yield was 40.5% without the addition oE tar.

2. Increased elasticity in the bindex phase on account of less pronounced glass carbon formation in the binder phase.

3. Binder system is lower cost.
When selecting furan resin it is important to look at the degree of pre~polymerisatiOn~The higher the number of OH
groups, the more water there will be during the hardening baking process and more porous binder cove. For the pre-sent invention,furan resins with a maximum 10~ total water content are preferred, calculated on the basis of Owl and H20, and with a maximum content of furfurylalcohol o 50~.
Provided the catalyst is Einely dispersed throughout the paste, the shelf life of the paste will be long, 1-4 months depending upon quantity and type of catalyst. The catalyst can also be added to the tar component in ad-vance.
The binder can be produced by the hardening process being catalyzed with an acid or a base7 either added or being formed by the hardening process, whereby the water-free acid or basic catalyst accounts for 8 wt% of thy total . . . ,, -l weight of component A + component B.
For example, in a preferred method of producing the bin-der, component B, consisting of tar with coke yield preferably o'er 40 wt%, is mixed with component A, which consists of partly polymerized furfurylalcohol with a maximum monomer content o-f 50 wt%, preferably 10-30% monomer, and maximum viscosity 500 mPas7and in which the polymer part of the furfurylalcohol is pro-duced by self-condensation.
The reaction between furfurylalcohol and the catalyst is exothermic to a certain degree. A large temperature rise during mixing reduces the lon~-term stability of the paste.
This can be avoided in two ways:
1. By reducing the quantity of active reactants in relation to the other components. This is achieved by the tar/
pitch mixing referred to above.
2. By adding sufficient quantlties of the solid components before adding the catalyst~to ensure that the quantity of binder then accounts for some 30 50~ of the quan-tity of the mixture. The reaction heat will then have a larger mass to clissipate through, with the result that the temperature rise becomes lnsignificant.
If long shelf life is required, 2-6 months, latent catalyst is to be preferred to free acid. A latent catalyst can, if desired, be added direct to the binder.
The following examples illustrate the invention:
1. Paste for tamping cathode bars in an aluminium re-duction cell:
Dry material: Electrically calcined anthracite with maximum grain size 3 mm, 50% larger than 0.2 mm, 30% smaller than 0.1 mm.
I' . . .

Binder : 17.5~ total, consisting of 25% furan resin with max 40~ furfurylalcohol, viscosity approx. 100 mPas at 20C, and 75% of a tar with viscosity approx. 100,000 mPas at 20C
and coke yield approx. 47~
Catalyst : 2.5% paratoluene-sulphonic acid, 70% solu--tion in water.
After mixing the resin and the tar, approxO one half of the anthracite was added (preferably the coarsest part.
lo The binder content was then about 35% which was suffici-ent to produce a paste-like compound. The catalyst was then added and mixed for about 5 min. The rest of the anthra`cite was added and the final mixing continued for 15 min.,all at room temperature.
150x150xl50 mm blocks were vibxated~ corresponding to a compressive force of around 15 MPa.
Density, green 1.53 g.cm baked l.41 " (8 hrs. of heating up to 950C, natural cooling) Coke yield 50~ 2 Sp.el. resistance 70 ohm.mm .m Compressive strength 20 MPa Y modulus 5000 MPa Permeability 0.5 centidarcy 2. Adhesive/paste for applications requiring high electrical conductivity, low wetting, :Low perme-ability; for example, bonding cathode bars in alu-minium electrolytic reduction cell to the cathode.
Dry material : Compact iron particles.
max grain size 0.5 mm 25% smaller than 0.1 mm.
Binder : 10-20% type as example 1 Catalyst : 2-4% paratoluene-sul~honic acid 70% solution in water.

1 Sp.el resistance : 2.5 ohm.mm2.m 1 Permeability : less than 0.1 centidarcy.

3. Carbon bound fosterite aggregate Dry material : Max grain size 3.5 mm 45% larger than 0.8 mm 25~ smaller than 0.1 mm Binder: 4~ of type mentioned in examples 1 and 2 Catalyst: 6% paratoluene-sulphonic acid lo 70~ solution in water Compressive strength after hardening fox 48 hrs. at 100C: 20 Mpa. Compressive strength aster heating in reducing atmosphere, for 8 hrs to 950C, natural cooling: 20 Mpa.
The binder system in accordance with the present invent tion can also be used for a number of other purposes:
The binder calm be mixed with powder of carbon, carbides, silicides, borides, nitrides, metal, metal oxides, silicates, or other minerals, either alone or in mix-tures, for making bodies, tamping pastes or adhesives,which can be formed at room temperature (20C) and which can be hardened at temperatures less than 250C~
Example of application of binder system in adheslves:
An adhesive consisting of 45% binder and 55~ anthracite with a maximum grain si2e 0.4 mm and 50% - O.1 mm :is used for bonding lining elements of pre-baked anthraclte products in tapping :Ladles for the fexro-alloys in-dustries.
Trials with less porous dry material have shown tha-t it is possible to reduce the binder content to 15-20%, which gives high strength, low shrinkage and low permeability in the bonded seam.

.

2i~

1 The binder can also be used to impregnate bodies to achieve lower permeability, higher resistance to chemi-cals and/or higher mechanical strength compared with other known binders.
The binder system to which only latent catalyst has been added can also be used as a wetting agent with a particu-larly high coke remnant for tamping hot and cold tamping pastes with ceramic material and~or anthracite as dry material.

The values stated for coke yield in -this patent refer to ASTM D ~416-73 method "modified Conradson".
Principle: The volatiles are driven off the sample which is pyrolyzed for a stipulated period ~30 min) at a stipu-lated temperature (900C) in a special apparatus which limits the oxygen supply. The % yield is considered to be coke value (coke yield ).

Claims (10)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS
   CLAIMED ARE DEFINED AS FOLLOWS:
   
   l. A binder which is liquid at room temperature in combination with an aggregate material uniformly dispersed therein, said binder consisting essentially of two components A and B in combination with a catalyst, wherein said component A is a mixture of a partially polymerized furfuryl resin, based on the self-condensation of furfuryl alcohol, said component A having a maximum monomer content of 50% by weight of uncondensed furfuryl alcohol and a maximum viscosity of 500 mPas and which is hardened by said catalyst in the temperature range of 0-250°C and wherein component B is a tar which becomes solid at a temperature of at least 200° above the temperature required for hardeningcomponent A, said binder being made up of about 92-99 wt% of components A + B, with component B accounting for between 40 and 90 wt% of this mixture, and wherein the catalyst is present in an amount of 1-8 wt% of the total mixture and wherein the aggregate material is selected from the group consisting of powdered carbon, carbides, silicides, borides, nitrides, metals, metal oxides, silicates or mixtures thereof; said binder being present in an amount of 4-20 wt%, based upon the total amount of the binder and aggregate material.
2. 2. A binder according to claim 1 in which the tar has a softening point lower than 60°C of A and B.
3. 3. A binder according to claim 2 in which component A has a maximum viscosity of 500 mPas.
4. 4. A binder according to claim 3 characterized in that the partly polymerized furfuryl resin has a monomer content of 10-30 wt%.
5. 5. A binder according to claim 4 in which the monomer content made up of furfuryl alcohol which is condensed upon hardening to produce the completely polymerized furfuryl resin.
6. 6. A binder according to claim 5 in which the tar has a coke yield of over 40 wt%.
7. 7. A binder according to claim 6 which contains the binder and powdered carbon uniformly dispersed throughout the binder.
8. 8. A binder acccrding to claim 7 in which the binder contains iron particles uniformly dispersed throughout.
9. 9. A method of bonding refractory bodies, which comprises applying the binder of claim 1 to the refractory body to be bonded and then hardening the binder so as to produce a bond possessing excellent permeability, chemical resistance and mechanical strength.
10. 10. A solid body produced according to claim 9 which comprises intimately mixing one or more of the components recited therein with the binder and then subjecting the mixture to a temperature at which the binder phase hardens with one or more of the aggregates of claim 9 intimately mixed therein.