JP2000319073A - Monolithic refractory material and waste melting furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a (fused zirconia)-spinel-magnesia monolithic refractory material which is appropriately used for a waste melting furnace or the like, capable of coping with chromium pollution problems, and excellent in corrosion resistance and spalling resistance. SOLUTION: This monolithic refractory material consists of 93-99 wt.% of refractory particles and 1-7 wt.% of a binder, wherein: the content of fused zirconia particles in the refractory particles is 5-35 wt.%; the total content of the spinel particles and magnesia particles in the refractory particles is 65-95 wt.%; and desirably, the content of alumina cement in the binder is 50-100 wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various furnaces for steel, non-ferrous, cement kiln, glass, etc.
The present invention relates to an amorphous refractory suitable for incinerators that does not contain chromium and that responds to environmental issues.

[0002]

2. Description of the Related Art In recent years, the amount of waste generated has been steadily increasing, and its disposal has become a major social problem. For example, in order to treat a huge amount of waste, it is necessary to secure landfill sites. In addition, depending on the disposal and disposal of incinerated ash and fly ash generated by incineration of waste, sewage sludge, and the like, secondary pollution may be caused.

[0003] As a countermeasure, it is desired to reduce the volume, detoxify or recycle the waste, and a melting method is attracting attention as one of the measures. The melting method is a method in which inorganic substances in waste are taken out as molten slag and the volume is significantly reduced. As for the method of melting waste, solid waste (garbage) is directly pyrolyzed and melted, or waste is primarily incinerated in an incinerator and the resulting incinerated ash, fly ash, and sewage sludge are secondarily melted. There is a way to do it.

The chemical composition of incinerated ash, fly ash, sewage sludge, etc. is generally SiO ₂ : 15 to 45% by weight, Al ₂ O ₃ : 1
0-20 wt%, CaO: 5 to 45 wt%, Na ₂ O:
1 to 15% by weight. In addition, incineration ash and fly ash include C
Many harmful metals such as d, Pb, Zn, Cu, As, Cr, and Hg are also contained. The sewage sludge, metal is small but contains P ₂ O ₅ is 5 to 15 wt%. Further, S, Cl, and the like are contained in large amounts as volatile components.

[0005] The erosion of the refractory used in the melting furnace largely depends on the melting slag components such as incineration ash, fly ash, sewage sludge and the like which are charged into the furnace and the melting temperature. The components of the molten slag vary depending on the type of waste and the like, but in general, the weight ratio of CaO to SiO ₂ is CaO / SiO ₂ = 0.
It tends to be as low as 1 to 1.5. Further, the temperature inside the melting furnace must be as high as 1400 to 1650 ° C.

Refractory materials such as alumina, magnesia, and silica-alumina are known as refractories that can withstand such processing conditions. However, these materials tend to react with molten slag components and thus tend to be eroded. is there. Also,
Refractories containing carbon have low reactivity with molten slag components, but tend to be oxidized and consumed when used in a high temperature range.

For this reason, refractories containing chromium oxide are now widely used as refractories exhibiting high corrosion resistance. For example, JP-A-63-30363 and JP-A-6-32162.
8, JP-A-8-48574, JP-A-10-81672, and the like.

[0008] The refractory containing chromium oxide has a higher corrosion resistance as the content of chromium oxide is higher. However, when chromium oxide in the refractory is used at high temperature and in an atmosphere such as alkali, it is harmful hexavalent. The conversion to chromium may cause environmental pollution. In addition, the following refractories that do not contain chromium oxide and that have been developed for the purpose of improving corrosion resistance include the following refractories. A refractory containing electrofused zirconia as a main component and containing silicon carbide (JP-A-7-2938)
51), spinel-based refractories (JP-A-7-25622)
9), containing 90% by weight of silicon carbide, with the balance being Al, S
Refractory material which is an oxide of i and Fe (Japanese Patent No. 28082)
No. 93). However, none of the refractories are stable and exhibit high corrosion resistance.

[0009] steel, nonferrous, in various kilns of cement kiln, a glass or the like, and the slag line portion and regenerator major production furnace, is still used often refractory MgO-Cr ₂ O ₃ system. Recently, especially in kilns such as steel and cement kilns, MgO.Al ₂ O ₃ —TiO ₂ , MgO.Al ₂ O ₃
-ZrO ₂ system and, fired refractory of the MgO · CaO-ZrO ₂ system is beginning to be used. However, these refractories are
In particular, cement kilns currently do not exhibit sufficient durability against changes in furnace operating conditions, such as intermittent operation, utilization of waste as fuel, and production of small quantities of various types.

[0010]

SUMMARY OF THE INVENTION The present invention provides a low cost refractory having excellent corrosion resistance and thermal shock resistance which can be used in kilns, waste incinerators and melting furnaces such as steel, non-ferrous, cement kilns and glass. It is intended to provide a refractory material that is easy to construct with a refractory and to eliminate the problem of environmental pollution caused by hexavalent chromium.

[0011]

SUMMARY OF THE INVENTION The present invention provides an amorphous refractory comprising 93 to 99% by weight of refractory particles mainly composed of fused zirconia particles, spinel particles and magnesia particles, and 1 to 7% by weight of a binder. Wherein the content of the molten zirconia particles in the refractory particles is 5 to 35% by weight, and the total content of the spinel particles and the magnesia particles is 65 to 95% by weight. I will provide a.

Further, the present invention provides a waste melting furnace using an irregular refractory construction body formed from the above irregular refractory for at least a part of a furnace wall.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the present specification, an amorphous refractory refers to the whole powder before water is added, and a construction formed from the irregular refractory is referred to as an irregular refractory construction. Say. The amorphous refractory of the present invention contains 93 to 99% by weight of refractory particles mainly composed of molten zirconia particles, spinel particles and magnesia particles.

Here, the molten zirconia particles are obtained by melting a zirconia raw material by a method such as electromelting and re-solidifying, and contain a glass phase. As the molten zirconia particles, those containing a monoclinic ZrO ₂ crystal phase and a glass phase are preferable, and specific examples include the following. For example, desiliconized molten zirconia obtained by desiliconizing zircon, monoclinic molten zirconia containing a glass phase,
Fused zirconia-fused alumina containing a glass phase.

The desiliconized molten zirconia is made of ZrO._TwoIs about 95
Wt%, monoclinic ZrO_TwoAnd a crystalline phase containing
O_Two, Fe_TwoO_Three, TiO_TwoAnd Al_TwoO_ThreeGala consisting of etc.
Phase. Monoclinic fused zirconia containing a glass phase
A is ZrO_TwoIs about 94% by weight, and monoclinic ZrO_TwoTo
Containing crystal phase and P_TwoO_Five, Na_TwoO, Al_TwoO_ThreeAnd Si
O _TwoAnd the like. This glass phase is monoclinic
Crystal ZrO_TwoIt covers around the crystal.

Further, a molten zirconia containing a glass phase
Fused alumina is composed of about 40% by weight of monoclinic ZrO ₂ and about 4%.
0% by weight corundum (Al ₂ O ₃ ) and about 20% by weight
And a glass phase composed of SiO ₂ , Al ₂ O ₃ , Na ₂ O or the like. Here, as the molten zirconia particles,
Those containing 3 to 25% by weight, particularly 5 to 20% by weight of a glass phase are preferred.

When the content of the glass phase is less than 3% by weight, the change in volume at the transition temperature of zirconia is not sufficiently absorbed, and becomes weak due to the generation of residual stress.
If it is larger, the corrosion resistance to molten slag, metal and glass and the high-temperature strength decrease. Thus, monoclinic ZrO
_The glass phase present around the _two crystals absorbs the volume change at the transition temperature of zirconia and prevents the refractory particles from collapsing.

On the other hand, when molten zirconia stabilized with MgO, CaO, Y ₂ O _3, or the like is used as refractory particles, the stabilizers MgO, CaO, and Y ₂ O ₃ become molten metal, molten slag, or glass. And destabilization. For this reason, refractory particles made of stabilized molten zirconia are:
Because it collapses at the transition temperature and cannot maintain its shape,
It is not preferable as an aggregate of irregular refractories. Therefore, the content of stabilized molten zirconia is preferably kept as small as possible.

The molten zirconia particles in the present invention are obtained, for example, as follows. A raw material obtained by mixing a predetermined amount of a molten zirconia obtained by desiliconizing zircon sand and a raw material having a component capable of forming a desired glass phase, or S which forms alumina and a glass phase with desiliconized molten zirconia
A raw material obtained by mixing a predetermined amount of iO ₂ , Al ₂ O ₃ , and Na ₂ O,
After being melted by electric arc melting, the obtained melt is blown off to be granulated or cast into a carbon mold, and then cooled. This is pulverized and sized to use as refractory particles.

In the present invention, spinel (MgO.Al)
_{The 2} O ₃ ) particles can be produced, for example, by the following method. Seawater magnesia and alumina, whose chemical composition is MgO: 28.3
% By weight, and a raw material mixture mixed to be Al ₂ O ₃ : 71.7% by weight, is baked in a rotary kiln, or the raw material mixture is melted by an electric melting method, and cooled.
After pulverization, a method of sizing and the like are used.

The spinel particles in the present invention have a chemical composition
Is MgO: 25 to 30% by weight, Al _TwoO_Three: 69-74
% Can be used. That is, the theoretical chemical composition ratio
With a high MgO composition, periclase crystals
Can be used, and A is calculated from the theoretical chemical composition ratio.
l_TwoO_ThreeWith a high content of corundum crystals
You can also use the one that is out.

However, in the case where corundum crystals are precipitated, Al ₂ O ₃ reacts with MgO and turns into spinel when used, and expands, so that the formed amorphous refractory swells,
Care must be taken because cracks may occur. However, for the purpose of preventing the refractory from shrinking, it is effective to use spinel particles containing an appropriate amount of corundum crystals.

In the present invention, as the magnesia particles, fired magnesia obtained by firing magnesium hydroxide obtained from seawater at a high temperature, electrofused magnesia obtained by melting and solidifying the fired magnesia by electromelting, and the like can be preferably used. It should be noted that spinel particles and magnesia particles having a large magnesia component are easily digested when used with water. Therefore, it is preferable to use those that have been subjected to an anti-digestion treatment.

In the amorphous refractory of the present invention, the fused zirconia particles, spinel particles and magnesia particles mainly constitute an aggregate portion, and are characterized by properties as a refractory. Therefore, in order to demonstrate the characteristics of these particles,
It is desirable that the content of these components is as large as possible. Specifically, the total amount of the fused zirconia particles, spinel particles, and magnesia particles in the refractory particles is preferably set to 90% by weight or more.

The content of the molten zirconia particles in the refractory particles is 5 to 35% by weight, and the total content of the spinel particles and the magnesia particles is 65 to 95% by weight. If the content of the molten zirconia particles is less than 5% by weight, the corrosion resistance and thermal shock resistance characteristic of the molten zirconia will not be sufficiently exhibited, and if it is more than 35% by weight, the formed amorphous refractory becomes porous. Poor corrosion resistance.

The spinel particles in the refractory particles react with the molten slag and become highly viscous, and have an effect of suppressing penetration of the molten slag and improving corrosion resistance. Also,
The magnesia particles have the effect of densifying the matrix and increasing the strength of the resulting construction. Therefore, the content of magnesia particles in the refractory particles is 39-7.
Preferably, the content of the magnesia particles in the refractory particles is 40-70% by weight, and the content of the spinel particles is 15-35%. % By weight.

All or most of the refractory particles in the present invention constitute an aggregate portion as an amorphous refractory.
The refractory particles may have various particle sizes, but it is preferable to select an appropriate particle size in the range of 10 μm to 20 mm. In particular, the fused zirconia particles are mainly composed of fine particles (less than 0.105 mm), and the spinel particles are composed of coarse particles (1.19 mm to 5 mm).
mm) and medium grains (0.105 mm or more, 1.19)
magnesia particles are mainly composed of coarse particles (1.19 mm or more and less than 5 mm), medium particles (0.105 mm or more and less than 1.19 mm) and fine particles (less than 0.105 mm). It is preferable to use the following.

Further, as the amorphous refractory of the present invention, it is preferable to use the one in which the content of alumina cement in the binder is 50 to 100% by weight. Thereby, the applied amorphous refractory can maintain sufficient dry strength and high temperature strength. As the alumina cement, generally, various alumina cements containing calcium aluminate as a main component can be used.

[0029] The amorphous refractory of the present invention may be used in a binder.
In addition, those containing aluminum lactates are preferred. here
And, aluminum lactates are Al (OCOCH (O
H) CH _Three)_Three, Al (OH) (OCOCH (OH) CH
_Three)_Two, Al (OH)_Two(OCOCH (OH) CH_Three) And
And hydrates thereof. Part of the alumina cement
By replacing it with aluminum lactate
Densified amorphous refractories to improve hot strength and corrosion resistance.
Can be improved.

Further, it is preferable to use a binder containing SiO ₂ in addition to aluminum lactates. For example, Al ₂ O ₃ : 24% by weight, SiO ₂ : 1
A white powder consisting of 1.5% by weight and lactic acid: 31.0% by weight, with the balance being water of crystallization, is preferably used because it produces α-alumina and a small amount of mullite at high temperatures.

The amorphous refractory of the present invention contains 9 particles of refractory.
It contains 1 to 7% by weight of the binder while containing 3 to 99% by weight. In particular, those containing 94 to 98% by weight of the refractory particles and 2 to 6% by weight of the binder are preferred.

The amorphous refractory of the present invention is constructed by adding a predetermined amount of water. However, in order to more effectively exert the function of the refractory particles, an antidigestion agent, a dispersant or a curing regulator is required. Is preferably used. This antidigestion agent is an antidigestion agent for spinel particles and magnesia particles having a high magnesia content.

When the content of the antidigestion agent is less than 0.1% by weight, the effect of preventing digestion of spinel particles or magnesia particles having a high magnesia content is small, and when the content exceeds 2% by weight, the applied amorphous refractory is porous. It is not preferable because it tends to be liquefied.

The amorphous refractory of the present invention preferably contains a mixed salt composed of aluminum hydroxide, citric acid and lactic acid. The mixed salt acts as an antidigestion agent. As the mixed salt, for example, the chemical composition is Al ₂ O ₃ :
17.5% by weight, lactic acid: 46.5% by weight, citric acid: 3
It is preferred to use white powder which is 3.0% by weight. The digestion inhibitor is preferably contained in the amorphous refractory in an amount of 0.1 to 2% by weight.

In the present invention, the dispersant and the curing regulator are
It is added to reduce the effects of workability and construction temperature, and any one can be used. As the dispersant, sodium tripolyphosphate, β-naphthalene sulfonate and the like can be preferably used. The dispersant is preferably contained in the amorphous refractory in an amount of 0.02 to 0.2% by weight.

The curing regulator includes a curing accelerator and a curing retarder. As the curing accelerator, quick lime, lithium carbonate and the like can be preferably used, and as the curing retarder, oxalic acid and boric acid are preferably used. it can. In addition, since the hardening of the alumina cement is slow at a low temperature of 15 ° C. or less and the hardening is fast at a temperature of 30 ° C. or more, it is necessary to change the addition amount of the hardening regulator depending on the temperature at the time of construction. The curing regulator is
The content is preferably 0.05 to 0.2% by weight in the amorphous refractory.

The dispersant and the curing regulator may be added to the mixture of the refractory particles and the binder in advance, or may be dissolved or suspended in water added during kneading.

Since the amorphous refractory of the present invention contains fused zirconia particles containing a glass phase, spinel particles and magnesia particles, it is easy to apply and the applied amorphous refractory becomes dense. . In addition, the amorphous refractory of the present invention contains molten zirconia having high corrosion resistance and low thermal conductivity, contains spinel particles that suppress molten slag infiltration, and densifies the matrix. Since it contains magnesia particles that enhance the strength of the material, it has excellent corrosion resistance, low penetration of molten slag, and is unlikely to cause structural spalling.

In the present invention, it is preferable to use the irregularly shaped refractory construction body formed from the irregularly shaped refractory of the present invention for at least a part of the furnace wall of a waste melting furnace, and particularly to be in contact with molten slag. It is preferably used for a part of the furnace wall. In this case, Si contained in the molten slag of incinerated ash and fly ash
Components such as O ₂ , CaO, Al ₂ O ₃ , Fe ₂ O ₃ , and Na ₂ O react with molten zirconia particles, spinel particles, and magnesia particles, but the reaction product of the molten zirconia particles becomes a high-melting substance, Since the spinel particles become highly viscous substances, it is considered that the corrosion resistance does not easily decrease. Therefore, the construction body of the amorphous refractory of the present invention has excellent resistance to penetration of molten slag and the like, and as a result, exhibits high corrosion resistance and thermal shock resistance.

[0040]

EXAMPLES Examples and comparative examples of the present invention will be described below. Examples 1 to 10 are examples of the present invention.
Examples 1 to 19 are comparative examples. Each raw material was weighed and mixed with a universal mixer so that the mixing ratio shown in Tables 1 and 2 was obtained, and water having the amount of water shown in the table was added to obtain a kneaded product. The kneaded material was cast into a mold of 40 mm × 40 mm × 160 mm while vibrating with a vibrator, cured for a predetermined time, demolded, and dried at 110 ° C. for 24 hours to obtain a test specimen.

As a comparative example, stabilized zirconia D
Is a refractory particle, an amorphous refractory containing a binder containing alumina cement (Examples 15, 16, 17), Al ₂ O ₃
Specimens of an alumina-based amorphous refractory having a content of 96% by weight (Example 18) and an alumina-chromia-based amorphous refractory containing 10% by weight of Cr ₂ O ₃ (Example 19) were also prepared. Tables 1 and 2 show the mixing ratios of the raw materials and the evaluation results of the obtained specimens. In addition, the unit of each raw material in Table 1 and Table 2 is part by weight.

[0042]

[Table 1]

[0043]

[Table 2]

In Tables 1 and 2, the molten zirconia A forms molten zirconia particles (desiliconized zirconia) obtained by desiliconization of zircon, and the molten zirconia B forms desiliconized zirconia and a glass phase. Mix the ingredients,
The fused zirconia particles and the fused zirconia C obtained by arc melting are fused zirconia-corundum particles obtained by mixing desiliconized zirconia, alumina, and a component that forms a desired glass, and then performing arc melting. The stabilized zirconia D is obtained by adding MgO to desiliconized zirconia to stabilize zirconia, and the content ratio of MgO is 4% by weight. Table 3 shows the chemical composition, crystal morphology, and glass phase ratio of molten zirconia A to C and stabilized zirconia D.

[0045]

[Table 3]

In Examples 1 to 16, the molten zirconia A
To C and stabilized zirconia D, as fine particles (0.1
(Less than 05 mm). The stabilized zirconia D of Example 17 was sized as coarse grains (1.19 mm or more and less than 5 mm), medium grains (0.105 mm or more and less than 1.19 mm), and fine grains (less than 0.105 mm). Was used.

As the spinel particles, the chemical composition is Mg
O: 28.3% by weight, Al ₂ O ₃ : 71.7% by weight, sized as coarse particles (1.19 mm or more and less than 5 mm) and medium particles (0.105 mm or more and less than 1.19 mm) Melted spinel particles were used. The magnesia particles have a purity of 99% by weight, a coarse particle (1.19 mm or more and less than 5 mm), a medium particle (0.105 mm or more and 1.1% or less).
Those having been sized as fine particles (less than 0.105 mm) were used.

As the alumina cement as the binder,
It is mainly composed of CaO and Al ₂ O ₃ ,
_{The 2} O ₃ content is 75% by weight and the specific surface area is 6000 cm ² /
g. Also, as a binder,
Aluminum lactates (Taki Chemical Co., Ltd. product name: Taxelam M)
-2500) was used.

In Examples 17 to 19, alumina particles having a particle size of 5 μm or less or silica particles having a particle size of 5 μm or less were used as a binder in addition to the alumina cement. As an antidigestion agent, a mixed salt composed of aluminum hydroxide, citric acid and lactic acid (Takiserum AS-300, product name of Taki Kagaku Co., Ltd.) was used.
In addition, sodium tripolyphosphate was used as a dispersant.

[Evaluation Results] Using the specimens obtained in Examples 1 to 19, the physical properties and properties shown in Tables 1 and 2 were evaluated. Evaluation items and measurement methods are as follows. The bulk specific gravity was measured by a general refractory test method (based on JIS R2205). Bending strength A is a three-point bending strength after heat treatment at 110 ° C. for 24 hours, and bending strength B is 15
The three-point bending strength after firing in an electric furnace at 00 ° C. for 3 hours. The thermal shock resistance was obtained by repeating a cycle in which a fired product of a specimen fired at 1300 ° C. for 3 hours was held in an electric furnace at 1300 ° C. for 15 minutes, and then rapidly cooled to room temperature.
The number of times until peeling was measured. The number of cycles was limited to 25. The thermal shock resistance is better when the number of times until peeling is larger. In addition, the thing which does not have peeling at the time of repeating 25 times was represented as 25+.

The corrosion resistance test was performed by the following method. Test
Cut out and polish multiple trapezoidal column-shaped test pieces from the body
To the specified dimensions and line it in a rotating drum.
Was. Next, while rotating the rotating drum,
Inject oxygen propane flame in axial direction and heat to 1600 ° C
did. While maintaining the temperature at 1600 ° C.,
Pour synthetic slag of incineration ash and fly ash into a rotating drum,
Rotated for 6 hours. Chemical composition of synthetic slag is based on weight
And Al_TwoO_Three: 16%, CaO: 32%, SiO _Two: 3
2%, Fe_TwoO_Three: 8%, K_TwoO: 2%, Na_TwoO: 2%,
MgO: 2%, P_TwoO_Five: Synthesis using 6%
The slag was freshly introduced every 30 minutes and tested.

After cooling the rotating drum, the test piece was taken out and cut, and the erosion amount (mm) and the slag penetration depth (m
m) was measured at each part of the test piece, and the average value was determined. In addition, the ratio of the amount of erosion in each example when the amount of erosion in Example 19 was set to 100 was calculated as the corrosion resistance index. A small corrosion resistance index indicates that the corrosion resistance is good.

The digestion resistance test was performed according to the method of Gakushin method 7 “Drug digestion test of dolomite clinker” at 134 ° C.
And the rate of weight increase (%) after holding for 2 hours in an autoclave at 3 atm was measured. The digestion resistance indicates that the smaller the value, the better.

[0054]

The amorphous refractory of the present invention is easy to construct at low cost, and has excellent corrosion resistance, penetration resistance and thermal shock resistance to molten metal, molten slag, glass and the like after construction. Furthermore, since it does not contain chromium oxide, there is no possibility of causing environmental pollution. Therefore, the amorphous refractory of the present invention is a zirconia-chromium-based, alumina-chromium-based, magnesia-chromium-based, which is currently used in industrial furnaces for waste incinerators and melt processing furnaces, such as steel, cement, and non-ferrous metals. It can be replaced with refractories, and has great industrial value.

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Claims (10)

[Claims] (1) 93 to 99% by weight of refractory particles mainly composed of fused zirconia particles, spinel particles and magnesia particles.
And 1 to 7% by weight of a binder, wherein the content of the molten zirconia particles in the refractory particles is 5 to 5.
An amorphous refractory comprising 35% by weight and a total content of spinel particles and magnesia particles of 65 to 95% by weight. 2. The method according to claim 1, wherein the content of alumina cement in the binder is 5%.
The amorphous refractory according to claim 1, wherein the amount is 0 to 100% by weight. 3. The molten zirconia particles have a glass phase of 3 to 25.
The amorphous refractory according to claim 1, wherein the refractory is contained by weight. 4. The refractory according to claim 1, wherein the molten zirconia particles contain a monoclinic ZrO ₂ crystal and a glass phase. 5. The amorphous refractory according to claim 1, wherein the total amount of the fused zirconia particles, spinel particles and magnesia particles in the refractory particles is 90% by weight or more. 6. The content of magnesia particles in the refractory particles is 39 to 76% by weight, and the content of spinel particles is 13 to 38.
The amorphous refractory according to any one of claims 1 to 5, which is in terms of% by weight. 7. The amorphous refractory according to claim 1, wherein the binder contains aluminum lactates. 8. An antidigestion agent is added to an amorphous refractory in an amount of 0.1 to 0.1%.
The irregular-shaped refractory according to claim 7, which contains 2% by weight. 9. The dispersant may be incorporated in an amorphous refractory in an amount of from 0.02 to 0.02.
The amorphous refractory according to any one of claims 1 to 8, comprising 0.3% by weight. 10. A waste melting furnace using an amorphous refractory construction formed from the amorphous refractory according to claim 1 for at least a part of a furnace wall.