FR2481263A1 - Refractory silica-alumina fibres contg. mullite crystals - used as thermal insulation material with high resistance to attack by acids - Google Patents

Abstract

The fibres contain by wt 30-80%SiO2, 20-70%Al203, 0-12% Cr203, and less than 4% of alkali(ne earth)metal oxides; the total of these constituents forms min 97% of the compsn of the fibres, which possess a degree of crystallisation (Dc) of min 0.3, pref min 0.4, and esp min 0.5. The term Dc is ratio of the intensity of the principal ray of 3.38 angstroms obtd from mullite to the intensity of the same ray after the fibres have been heated for 100 hours at 1200 deg C. These rays are obtd in crystallographic analysis using X-rays. The fibres pref contain less than 1% of alkali(ne earth)metal oxides; and are pref used in bulk or as a cloth. The fibres are pref mixed or treated, before use, with a mineral binder, esp kaoline and/or micronised silica. Used as a thermally insulating material resisting attack by acids in all types of plant, e.g. in steel-, glass- or petrochemical- works, to reduce heat losses, thus saving energy.

Description

The invention relates to the use of silica and alumina-based refractory fibers containing mullite crystals as acid-resistant thermal insulation material
The use of fibrous materials as insulators for iron and steel, glass, petrochemical and incineration equipment. The rising cost of energy is also leading to a growing need for insulation In many installations, the cold side is now working at temperatures below the dew point of the water vapor or acids that may form with fuel impurities
Among these acids, there are essentially three: the sulfuric and hydrochloric acids directly from the impurities of the fuels and the nitric acid from the NO formed in the flames at high temperature
Sulfuric and hydrochloric acids are mainly found in petrochemicals (sulfur-rich fuels) and in incineration furnaces for household and industrial waste (combustion of plastics). Nitric acids are found mainly in flame furnaces. very hot (stoves, glass furnaces, blacksmoke ovens, etc.).

 In the first apparatuses, for reasons of cost price and ignorance of the problems of the acids, the condensing zone of these furnaces was isolated with wools of rocks, wools of slag or fibers of glass. It soon became clear that these mineral wools containing alkaline and alkaline-earth oxides were often completely dissolved after a few months of service. It was therefore decided to replace them with refractory fibers containing only silica and alumina. The most common fibers ranged between 45 and 50% Al2O3, and 53 to 48% SiO2. The sums of the other constituents Tir2, Fie203, CaO, MgO, Na2O reaching a maximum of 2 to 5%. They were generally used in the form of flexible sheets with densities of 96 kg / m3 to 140 kg / m3. These fibers obtained by electrical fusion, put in fiber form and cooling, are in a vitreous form, absolutely X-ray amorphous, and an average diameter of 2 to 3 microns.

However, it was soon obvious that these refractory fibers, while having a much higher resistance to the current insulating fibers containing alkaline and alkaline-earth elements, still show considerable attack by normal solutions of acids and are practically dissolved at higher concentrations and temperatures of 1000 C.

 The attack of these fibers is by formation of alumina salts leaving an amorphous residue without any consistency. In this case, not only the insulating effect is destroyed, but the void left by the disappearance of the fibers creates many possibilities of hot spots due to the passage of flames.

JF Mac Dowell and GH Beall described, in the Journal of American Ceramic Society Vol.52,
January 1969, No. 1, pages 17-25, the devitrification of a glass of alumina-silica type by heat treatment, with formation of a crystalline mullite phase.

There is no question, however, in this article of either fiber or acid resistance properties.

 French Patent No. 2,272,968 describes crystalline ceramic fine grain refractory fibers of the alumina-silica type obtained by heating vitreous fibers above their devitrification temperature for a long enough time, to convert these glass fibers to a crystalline state. fine grain, but short enough to prevent excessive grain growth. These fibers have an excellent ability to elastically recover, in use, their original configuration after having undergone compression deformation. It should be noted that these fibers should have a relatively low degree of crystallization because, otherwise, their elastic recovery properties would be seriously affected. In any case, no indication concerning the acid resistance of these crystalline fibers is given.

The object of the invention is to provide an acid-resistant fibrous insulating material and more specifically relates to the use of silica and alumina-based refractory fibers containing mullite crystals as heat-resistant thermal insulation material. acids, the chemical composition, by weight, on the oxide basis, of these fibers being the following 80 to 30% of silica, 20 to 70% of Al2O3, 0 to 12% of
Cr 2 O 3 and less than 4% alkali and alkaline earth metal oxides, the sum of these constituents being at least 97% of the total composition and these fibers having a degree of crystallization DC (hereinafter defined) of at least 0.30. Preferably, the degree of crystallization DC is at least 0.40, and advantageously at least 0.50.

 According to the applicant, at least the fibers having an I> C of at least 0.50 constitute a new material.

 According to the present invention, it has surprisingly been found that a silica-alumina fiber containing a high proportion of mullite crystals is practically insensitive to sulfuric acid, hydrochloric acid attack. and nitric even at concentrations exceeding 40%, and particularly suitable as insulating material to be in contact with acidic atmospheres.

 the acid-resistant silica and alumina refractory fibers are prepared by heating at a temperature above 95 ° C. of substantially amorphous refractory fibers whose chemical composition, by weight, on the basis of the oxides is as follows: 80 to 30 % silica, 20 to 70% Al 2 O 2, 0 to 12% Cr 2 O 3 and less than 4% alkali and alkaline earth oxides, the sum of these constituents being at least 91740 of the total composition, while sufficient time for the degree of crystallization DC of the fibers obtained to be at least 0.30, the degree of crystallization DC being defined as being equal to the ratio of the intensity It of the main line at 3.38 i of the mullite said fibers at the intensity 1max of said fiber line of the same type having been heated at 12000C for 100 hours.

 Preferably, the heat treatment is carried out at a temperature of at least 10000C, advantageously at least 11000C. The duration of the heat treatment can vary widely, depending on the temperature, but is, in most cases, at least 2 hours.

 Preferably, finally, the content of alkali and alkaline earth metal oxides is less than 1% by weight.

 According to one embodiment of the invention, refractory fibers which have been heated in bulk or in sheets at a temperature greater than 950 ° C. are used. The fibrous materials obtained can be used directly in this form as insulation of the cold face. equipment or installations to be isolated. The products thus obtained retain some flexibility, but their density does not generally exceed 200 kg / m3. By compressing them further, the fibers tend to crush and become pulverulent because the heat treatment of crystallization has made them lose a significant part of their elasticity.

A preferred embodiment of the invention is to use a product obtained by a process consisting of dispersing bulk fibers in water, adding to it a mineral binder (for example kaolin, and / or micronized silica), to suck the mixture on a form, to dry and to cook the whole of the part thus obtained at a temperature greater than 9500.degree. C., preferably greater than 1000.degree. C., advantageously greater than 1100.degree. conditions of semi-rigid blocks with a density ranging from 200 kg / m3 to 400 kg / m3 which, thanks to their higher density and the presence of the mineral binder, resist acid attacks even more effectively than loose fibers or During their firing, these blocks undergo a shrinkage of 2% to 4% due to both the shrinkage of the fibers during crystallization and the beginning of sintering between the fibers and the inorganic binder.
These molded parts can be solid or hollow. Hollow parts can be used as building modules, one of the faces of which can be at the internal temperature of the oven and the other face of the temperature of the cold wall. To improve the insulating power of these modules or hollow bricks, they can be filled with bulk fibers. Advantageously, these hollow bricks will be filled with bulk fibers having been calcined at more than 950 ° C., preferably above 10,000 ° C. and advantageously above 1100 ° C.

If you want to reduce the shrinkage to less than 2% during cooking, use instead of ordinary bulk fibers, fibers that have been heat-treated at high temperature, which will then be dispersed in water with addition of a mineral binder0
After forming the piece and drying, however, the whole is cured at 950 ° C. and higher, both to crystallize the binder and to sinter the fibers and the binder together. As before, we will obtain a semi-rigid block of density between 200 and 400 kg / m3.

 The refractory fibers used in the invention can be contacted for 100 hours at 1000C with sulfuric acid, hydrochloric acid and / or nitric acid at 40% or more without losing, after washing, more than 100% of their weight, and while maintaining their shape and initial appearance.

 In the form of semi-rigid blocks with a density of 200 kg / m3 to 400 kg / m3, this loss of weight after 100 hours of treatment at 1000C will be less than 5%. The blocks after treatment, washing and drying, will have kept their initial appearance and most of their mechanical properties.

 On the other hand, fibers which are not heat-treated and therefore not recrystallized, whether in bulk or in compacted blocks at 200 or 400 kg / m 3, when subjected to the same chemical treatment, namely 100 h at 1000 ° C. in a solution 40% acid, will disintegrate easily and are therefore unusable as heat insulants.

 To delay and further reduce the danger of attack by the acids, it is possible to fix, for example by gluing, on the hot surface of crystallized fiber sheets or crystalline semi-rigid blocks, aluminum foils, stainless steel sheets or any other vapor barrier material to reduce acid penetration and condensation in cold areas.

 The fibers containing mullite crystals can be used according to the invention in any thermal installation where there are problems of condensation of acids. Non-limiting examples include reforming furnaces, primary petrochemical furnaces for the synthesis of ammonia or the direct reduction of iron ore, incineration furnaces for household waste or industrial waste, in particular those intended for the combustion of chlorine-rich plastic waste, steel-working stoves, as well as glass-making furnaces and their regenerators.

The following nonlimiting examples will make the invention well understood
Acid attack test
In this test, the fiber sample is weighed, then introduced into a flask containing an acid solution and provided with a reflux condenser.

Its crystalline state is defined by the It / Imax ratio of the 3.38 mullite line.

 The acid solution is boiled and the attack is thus maintained for 100 hours.

 After the attack, the sample is rinsed on a buchner funnel, then parboiled and weighed. We note the percentage of weight loss and the appearance of the fibers.

 The examination is eventually supplemented by a chemical analysis that makes it possible to identify the compound (s) attacked.

EXAMPLE 1
Aluminosilicate fiber with 47% alumina
The characteristics of this fiber used in the form of a 25 mm thick and 0.96 density needled sheet are as follows:
- Chemical analysis: Loss on fire 0,2%
SiO2 51.55
Al2O3 46.95
Fe203 0.15 TiCl2 0.15
CaO 0.54
MgO 0.06
Na2O 0.17
K2O 0.12
Li20 (0.01
Cr2O3 <0.05
MnO <~ 0.05
100.00% - Crystallographic analysis: amorphous X-ray
(DC = 0) - Mechanical characteristic t tensile strength *
0.35 daN / cm2 * The tensile strengths indicated in this
example and in the following ones are measured on the
needled plies in the longitudinal direction and
not the tensile strengths of fibers
unit.

 With this fiber, the attack test with a 40% by weight solution of sulfuric acid gives a weight loss of 34%.

- the chemical analysis after attack is:
Loss of fire 6.8%
SiO2 77.9
A1205 14.44
Fe2O3 0.07
TiO2 0.06
CaO 0.16
0.04 MgO
Na2O 0.17
K20 0.06
Li2O 0
SO3 0.3
Cr203 O
MnO o
100.00%
As for the mechanical characteristics, they are zero, the sample no longer has any mechanical strength.

 This same fiber was heated in air for 100 h at 1000 C, 1100 C, 12000C, as well as for 2 hours at 8000C, 900 C, 1000 C, 110000, 1200 C.

The results obtained are summarized in the table below

<tb><SEP> STATE <SEP> INITIAL <SEP> AFTER <SEP> ATTACK.
<tb><SEP> Treatment <SEP> Resistance <SEP> to <SEP> 100 <SEP> h <SEP> H2SO4 <SEP> to
<tb><SEP> thermal <SEP> the <SEP> tensile, <SEP> 40%
<tb><SEP> daN / cm2 <SEP> Loss <SEP> of <SEP> weight, <SEP>%
<tb><SEP> 0 <SEP> 0 <SEP> 0.35 <SEP> 34
<tb><SEP> 2 <SEP> h <SEP> - <SEP> 800 C <SEP> 0 <SEP> 0.26 <SEP> 34
<tb><SEP> 2 <SEP> 2 <SEP> h <SEP> - <SEP> 900 C <SEP> O <SEP> 0.25 <SEP> 40
<tb><SEP> 2 <SEP> h <SEP> - <SEP> 1000G <SEP> 0.42 <SEP> 0.16 <SEP> 3.4 <SEP>
<tb><SEP> 2 <SEP> h <SEP> - <SEP> 1100 C <SEP> 0.51 <SEP> 0.15 <SEP> 2.4
<tb><SEP> 2 <SEP> h <SEP> - <SEP> 1200 C <SEP> 0.65 <SEP> 0.14 <SEP> 2.3
<tb> 100 <SEP> h <SEP> - <SEP> 1000 C <SEP> 0.43 <SEP> 0.14 <SEP> 1.2
<tb> 70Q <SEP> h <SEP> - <SEP> 11000C <SEP> 0.70 <SEP> 0.14 <SEP> 1.5
<tb> 100 <SEP> h <SEP> - <SEP> 1200 C <SEP> 1.00 <SEP> 0.19 <SEP> 1.4
<Tb>

These results show, in particular, that for a product heated only for 2 hours at 100,000 the attack is considerably reduced since it goes from a weight loss of 34% to a weight loss of 3.4%
The chemical analysis of the products under attack is little different from the analysis of the starting product since
SiO2 53%
Al2O3 45%
EXAMPLE 2
Aluminosilicate fiber with 60% alumina
This fiber was used in the form of a 25 mm thick needled sheet with a density of 0.128.

The chemical analysis of this fiber was
Loss on fire 0.3%
SiO2 38.7
Al2O3 60.06
Fe2C3 0.13
TiO2 0.08
CaO 0.16
Na2O 0.34
K20 0.16
LiO2 <0.01
MgO 0.06
100.00%
The attack test with a 40% solution of sulfuric acid causes a weight loss of 58%.

The chemical analysis after attack gives, for the main elements
SiO2 93.2%
Al2O3 2.4%
This same fiber was heated in air for 10 h at 12000C to obtain a DC equal to 0.90. The loss of weight of this fiber treated according to the invention in the attack test mentioned above is only 2%, compared with the weight loss of 58% for the untreated fiber.

EXAMPLE 3
Aluminosilicate fiber with 37% alumina
This fiber was used in the form of a needlepunched web having a thickness of 25 mm and a density of 0.96. The chemical analysis of this fiber was
Loss on fire O%
SiO2 59.8
Al2O3 36.91
Fe2O3 1.45
TiO2 0.75
CaO 0.62
MgO 0.21
Na2O 0.06
K20 0.15
LiO2 0.05
MnO O
100.00% - X-ray amorphous crystallographic analysis
(DC = 0) - Mechanical characteristics: tensile strength
0.16 daN / cm 2.

 With this fiber, the attack test conducted under the same conditions as for Examples 1 and 2 gives a weight loss of 31%.

Chemical analysis after attack is for the main elements:
SiO2 74.1%
Al2O3 19.8% the sample after attack no longer has any mechanical resistance.

If this fiber is heated for 2 hours at 900 ° C., it crystallizes to a degree of crystallization.
DC of 0.24, the acid attack causes a weight loss of 18%.

We then note: SiO2 64.3%
Al2O3 30.6% again, the sample no longer has mechanical strength.

 On the other hand, if the fiber is heated for 2 hours at 1000 ° C., it crystallizes at a degree of DC crystallization of 0.32 and the loss in acid attack weight is reduced to 1%. The chemical analysis is very little different from that of the product before attack.

The sample retains its shape and mechanical strength.

EXAMPLE 4
Silico-aluminous fiber containing chromium oxide
This product is used in the form of a non-needled web with a thickness of 8 mm and a density of 0.96. His chemical analysis is
Loss of fire 3.7%
SiO2 51.4
Al2O3 40.9
Fe2O3 0.35
TiO2 0.015
CaO 0.14
0.04 MgO
Na2O 0.22
K20 0.01
Li2O <0.01
Cr2O3 3.25
100.00%
It is also X-ray amorphous (DC = 0).

 Its tensile strength is 0.9 daN / cm 2.

 The acid attack test conducted as in the previous examples causes a weight loss of 20%.

Chemical analysis after attack is for the main elements:
Loss of fire 9.0%
SiO2 63.1
Al2O3 24.5
Cr2O3 1.9
It should be noted that the slightly greenish sample before attack becomes ocher.

If this fiber is heated for 2 hours at 1200 ° C., it crystallizes to a degree of crystallization.
DC of 0.73. Mullite is the only crystalline phase. The acid attack test only causes a weight loss of 0.7% on the fiber treated according to the invention.

EXAMPLE 5
Similar effects to those described in Examples 1, 2, 3, 4 were observed when the 4% SO 4 H 2 solution was replaced with nitric or hydrochloric acid solutions at 5 to 60% acid concentrations.

The results are gathered in the
Table I which shows the weight loss in TABLE I

% <SEP> of <SEP> loss <SEP> of <SEP> weight <SEP> for <SEP> a <SEP> concentrates
Nature <SEP> Nature <SEP> Status <SEP> initial <SEP> of
SEP <SEP><SEP><SEP><SEP><SEP> solution of
<tb>
<tb> of <SEP> the <SEP> fiber
<tb> acid <SEP><SEP> fiber <SEP> 5% <SEP> 10% <SEP> 20% <SEP> 40% <SEP> 60%
<tb> Fiber <SEP> to <SEP> 47% <SEP> Al2O3 <SEP> H2SO4 <SEP> 28 <SEP> 31 <SEP> 36 <SEP> 37 <SEP> 38
<tb> amorphous
<tb> HCl <SEP> 28 <SEP> 31 <SEP> 37 <SEP> 41 <SEP> 42
<tb> (piece <SEP> of <SEP> tablecloth)
<tb> HNO3 <SEP> DC <SEP> = <SEP> 0 <SEP> 20 <SEP> 28 <SEP> 35 <SEP> 39 <SEP> 40
<tb> Fiber <SEP> to <SEP> 60% <SEP> Al2O3 <SEP> H2SO4 <SEP> 46 <SEP> 53 <SEP> 58 <SEP> 58 <SEP> 62
<tb> amorphous
<tb> HCl <SEP> 22 <SEP> 49 <SEP> 62 <SEP> 63 <SEP> 62
<tb> (piece <SEP> of <SEP> tablecloth) <SEP> Dc <SEP> = <SEP> 0
<tb> NHO3 <SEP> 21 <SEP> 40 <SEP> 59 <SEP> 61 <SEP> 61
<tb> H2SO4 <SEP> 3 <SEP> 3 <SEP> 4 <SEP> 2 <SEP> 3,5
<tb> crystallized HCl <SEP><SEP> 1 <SEP> 1 <SEP> 1 <SEP> 1 <SEP> 1
<tb> Fiber <SEP> to <SEP> 47% <SEP> of Al2O3
<tb> HNO3 <SEP> 1 <SEP> 1 <SEP> 1 <SEP> 1 <SEP> 1
<tb> Dc <SEP> = <SEP> 0.9
<tb> H2SO4 <SEP> 2 <SEP> 1 <SEP> 1 <SEP> 2 <SEP> 2
<tb> crystallized HCl <SEP><SEP> 0.5 <SEP> 0.5 <SEP> 1 <SEP> 2 <SEP> 2
<tb> Fiber <SEP> to <SEP> 60% <SEP> of Al2O3
<tb> HNO3 <SEP> 1 <SEP> 1.5 <SEP> 1.5 <SEP> 2 <SEP> 2
<tb> DC <SEP> = <SEP> 0.9
<Tb>
EXAMPLE 6
It is, in this example, a fibrous product transformed with a mineral binder formed of kaolin.

Its density is 0.25 and its chemical analysis is
Loss of fire 0,25%
SiO2 51.9
Al2O3 46.9
Fe2O3 0.40
TiO2 0.25
CaO 0.13
0.04 MgO
Na20 0.09
K20 0.07
Li2O 0.01 NnC O
100.00% - Crystallographic analysis: this product being commercial
cooked at 900 ° C, a crystalline start
mullite
DC = 0.1 - Mechanical characteristic: RC mechanical strength
at the compression in daN / cm2
for a deformation of 5% RC = 0.83
10% 1.02
20% 1.34
30% 1.74.

40% 2.46
50% 3.78
For this sample, the acid etch test with a 40% sulfuric acid solution results in a weight loss of 35%.

- Chemical analysis after attack is
Loss of fire 4.3 *
SiO2 77.0
A1203 17.8
Fe2O3 0.10
TiO2 0.35
CaO 0.25
MgO 0.06
Na20 0.10
K2O 0.06
Li2O 0
MnO O
100.00%
The product after attack is destroyed under the pressure of the hand.

The resulting fibrous product was crystallized from the air by heating
10) for 12 hours at 11000C
20) for 12 hours at 12000C
30) for 100 h at 12000C (to have a treated product having a 1max and used to calculate the DC).

1 / Case of the sample treated for 12 hours at 1100 C
Before attack: degree of crystallization
DC = 0.43
mechanical resistance RC at the
compression
for a deformation of 5% RC = 0.93 daN / cm2
10% 1.06
20% 1.28
30% 1.60
40% 2.05
50% 2.91
After attack with 40% H2SO4: the product remains intact,
the weight loss to acid attack is 2.4%.

The chemical analysis is little different from that of the starting material9 ie for the main elements
SiO2 53.9%
Al2O3 44.2%
The product retains a good mechanical resistance to compression, indeed
for a deformation of 5% RC = 0.83 daN / cm2
10% 0.96
20% 1.22
30% 1.57
40% 2.11
50% 3.2 2 / Case of 11 sample treated for 12 hours at 120000
Before attack - degree of crystallization DC = 0.7
RC mechanical resistance to compression
for a deformation of 5% RC = 0.93 daN / cm2
10% 1.06
20% 1.25
30% 1.44
40% 1.76
50% 2.40
After attack: the product is intact
The weight loss to acid attack is 1.4%.

 The analysis of the product after the attack is practically identical to that of the starting product, only a loss of Al.sub.2 O.sub.3 of less than 1% is noted.

Claims (8)

 1. The use of silica and alumina refractory fibers containing mullite crystals as acid-resistant thermal insulation material, the chemical composition, by weight, based on the oxides, of these fibers being as follows : 80 to 30% of silica, 20 to 70% of Al 2 O 3, 0 to 12% of Cr 2 O 3 and less than 4% of alkali and alkaline earth metal oxides, the sum of these constituents representing at least 97% of the total composition, and these fibers having a DC degree of crystallization of at least 0.30.  2. The use according to claim 1, characterized in that the fibers have a degree of crystallization of at least 0.40.  3. The use according to claim 1, characterized in that the fibers have a degree of crystallization of at least 0.50.  4. The use according to any one of claims 1 to 3, characterized in that the fiber content of alkali metal and alkaline earth metal oxides is less than 1% by weight.  5. The use according to any one of claims 1 to 4, characterized in that one uses fibers in bulk or in the form of a web.  6. The use according to any one of claims 1 to 4, characterized in that fibers which have been mixed or pretreated with a mineral binder are used.  7. The use according to claim 6, characterized in that the mineral binder is kaolin and / or micronized silica.  8. As novel products, the crystalline refractory fibers defined in any one of claims 3 to 7.