WO2013032065A1

WO2013032065A1 - Low cement corrosion-resistive unshaped refractories

Info

Publication number: WO2013032065A1
Application number: PCT/KR2011/008188
Authority: WO
Inventors: Kyoung Ran Han; Sang Whan Park; Chang Sam Kim; Hak Man Lee
Original assignee: Korea Institute Of Science And Technology
Priority date: 2011-08-31
Filing date: 2011-10-31
Publication date: 2013-03-07
Also published as: KR20130024562A; KR101262077B1

Abstract

Disclosed is an unshaped refractory containing a refractory mixture of alumina (Al₂O₃)<sub/>and silicon carbide (SiC) to which alumina cement, zircon and an alumina binder are added. The refractory exhibits high strength and density as well as significantly improved corrosion resistance, which is due to the reaction of zircon with calcium oxide (CaO), greatly suppressing corrosion by slag. Use of the alumina binder provides fast drying and strength sufficient for handling. Since shrinkage hardly occurs after heat treatment and low porosity and high strength can be obtained, the refractory is very suitable for use, in particular, a gasifier.

Description

LOW CEMENT CORROSION-RESISTIVE UNSHAPED REFRACTORIES

The present invention relates to an unshaped refractory containing zircon, cement and an alumina binder that can be used in the lining of a gasifier.

Unshaped refractories are preferred to refractory bricks having the problem of gaps between the bricks, and improvements are made in terms of lifespan, material and construction method. Although calcium aluminate cement has been predominantly used as inorganic binder, low-calcium aluminate cement is preferred as the operation environment becomes harsher (e.g., higher temperature and pressure). Recently, hydratable alumina with a trace amount of calcium is used in unshaped refractories in order to improve resistance to thermal wear and corrosion. However, this refractory has low strength (1.2-2.0 MPa) at 800-1200 ºC where dehydration occurs and binding between ceramics disappear.

A gasifier used in the integrated gasification combined cycle (IGCC) technology, which is valued highly as an ecofriendly power generation system recently, is operated under harsh environments. Since a stainless steel pipe through which cooling water flows is attached to an unshaped refractory used in the lining of the gasifier, the unshaped refractory should allow good attachment of the stainless steel pipe, have good thermal conductivity, and allow easy formation of a slag layer initially. In general, the unshaped refractory is made of oxide, carbide or a mixture thereof as a refractory as well as alumina cement as a binder. When the cement is used in a small amount, the as-pressed refractory has a low density and is difficult to handle and the control of drying time is difficult, although resistance to slag corrosion and penetration is improved. It is because calcium oxide (CaO) included in the cement reacts at high temperature with the slag, allowing the slag easily penetrate into the refractory by decreasing viscosity. Further, since evaporation of water from the cement occurs slowly, sufficient drying is necessary at room temperature. As an alternative to the cement-based refractories, phosphate-based unshaped refractories are used. For example, one using monoaluminum phosphate as a binder and magnesium oxide (MgO) as a hardening agent is used for a combustion furnace. But, it is not suited for the gasifier where oxygen is insufficient. Since low melting point compounds are formed in the P₂O₅-MgO system and the water-soluble monoaluminum phosphate migrates to the surface, the strength may become nonuniform. Further, P₂O₅ evaporates under high-temperature reducing environment, resulting in decreased and nonuniform strength. Recently, hydratable alumina is used as an inorganic binder. Commercially available ones include AlphaBond 300 and 500 of Alcoa Industrial Chemicals. The refractory including the hydratable alumina binder requires a long mixing time. Also, although use of finely particulate silica and colloidal alumina with alumina cement is reported, bending strength at room temperature is only 5-17 MPa. Further, the unshaped refractory includes 7-10 wt% of ultrafine reactive alumina and at least 20 wt% of fine particulate powder including silica in addition to aggregate in order to improve flowability.

As described, the existing refractories are unsuited for use in the high-temperature, high-pressure environment of the gasifier because of the problems of adhesivity, thermal conductivity, drying time, corrosion resistance, etc. , as well as strength.

The inventors of the present invention have studied for a refractory that can be used under the harsh environment of high temperature and pressure of a gasifier, and developed a refractory having superior adhesivity, strength, thermal conductivity and corrosion resistance.

The present invention is directed to providing an unshaped refractory including a refractory mixture of alumina (Al₂O₃)and silicon carbide (SiC) to which one or more of alumina cement, zircon and an alumina binder is added.

In one general aspect, the present invention provides an unshaped refractory including a refractory mixture of alumina (Al₂O₃)and silicon carbide (SiC) to which one or more of alumina cement, zircon and an alumina binder is added.

Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.

In the refractory of the present invention, a small amount of the cement and alumina binder provides working flowability to the refractory mixture, and a small amount of the zircon significantly prevents corrosion by the calcium oxide (CaO) included in the cement in an amount of about 20%. Since the refractory of the present invention maintains shape after drying, has sufficient strength for handling, exhibits little shrinkage after heat treatment and shows low porosity, it is particularly suited for use in the gasifier. The alumina binder is mixed with the fine particulate alumina and coats the surface of the refractory like glue, improving contact between the refractories and facilitating evaporation of water as the sol transforms into gel. Further, the alumina gel film formed from the small amount of alumina binder prevents cracking during drying even when the heating speed is fast and facilitates sintering of the fine particulate alumina powder, thereby proving high strength resulting from ceramic binding between the refractories.

Fig. 1 shows a cross-sectional photographic image of a refractory sample of Comparative Example 1 comprising only an alumina binder after slag corrosion test.

Fig. 2 shows a cross-sectional photographic image of a refractory sample of Example 6 comprising 2 wt% of zircon, 3 wt% of cement and an alumina binder after slag corrosion test.

Fig. 3 shows a cross-sectional photographic image of a refractory sample of Comparative Example 2 comprising 3 wt% of alumina cement after slag corrosion test.

Hereinafter, the embodiments of the present invention will be described in detail with reference to accompanying drawings.

The present invention provides an unshaped refractory comprising a refractory mixture of alumina (Al₂O₃)and silicon carbide (SiC) to which alumina cement, zircon, an alumina binder or a mixture thereof is added.

In an exemplary embodiment of the present invention, magnesium oxide, spinel, zirconia, chromia, hafnium oxide or a mixture thereof is further added to the refractory mixture of alumina (Al₂O₃)and silicon carbide (SiC).

In an exemplary embodiment of the present invention, the alumina binder is included in an amount of 0.5-4 parts by weight based on alumina per 100 parts by weight of the total unshaped refractory comprising alumina (Al₂O₃)and silicon carbide (SiC).

In an exemplary embodiment of the present invention, the alumina cement comprises 10-30 wt% of calcium oxide (CaO).

In an exemplary embodiment of the present invention, the alumina cement is added in an amount of 0.5-5 wt% based on the total weight of the unshaped refractory.

In an exemplary embodiment of the present invention, the zircon is added in an amount of 1-5 wt% based on the total weight of the unshaped refractory.

Since cement is associated with severe slag corrosion, a hydratable alumina binder is used for the refractory lining which cannot endure even a small amount of calcium oxide (CaO). Calcium oxide (CaO) is included in an amount less than 0.1 wt% in Alphabond 300 and in an amount of about 0.6 wt% in Alphabond 500, which are the products of Almatis. And, since it is included in an amount of about 3 wt% in castables, the calcium oxide (CaO) is present in the refractory as impurity levels. In US Patent No. 5,972,102 (1999), fine particulate dead-burned magnesia is used instead of calcium aluminate cement to improve resistance to slag corrosion. By adding a compound that can improve corrosion resistance by reacting with calcium oxide (CaO), it will be possible to reduce the use amount of cement. In the pressurized pulverized coal combustion (PPCC) combined cycle, a flue gas is passed through a column packed with ceramic balls before entering the gas turbine and the steam cycle and a purification process of slag separation and removal of alkali metals follows. At present, chromium oxide (Cr₂O₃) ceramic, which is stable against slag by forming the (Cr,Al)₂O₃ corundum phases, is used. It is reported that HfO₂ or ZrSiO₄(zircon) has superior corrosion resistance although inferior to the Cr₂O₃ ceramic (J. Eur. Ceram. Soc. 29 (2009) 2721-2725). By using a small amount of the zircon together with the alumina cement, resistance to slag corrosion could be improved. Fig. 1 shows a result of corrosion test for a refractory prepared by adding only an alumina binder to the refractory mixture composition D, using Usibelli slag with low viscosity under a reducing condition of 20% CO/N₂ at 1550 ºC/4 hr. It can be seen that penetration hardly occurred. In contrast, complete penetration was observed for a refractory prepared by adding 3 wt% of alumina cement (Fig. 3). The penetration was about 2 mm for a refractory prepared by adding 3 wt% of alumina cement and 2 wt% of zircon (Fig. 2). This confirms that zircon greatly improves corrosion resistance.

The fine particulate powder is added to improve the flowability of the unshaped refractory. The powder is sintered at heat treatment and thus improves strength. In general, ultrafine particulate α-alumina powder is used in an amount of 10 wt%. The possibility of replacing the expensive ultrafine particulate alumina powder with cement was investigated. The refractory composition B to which a small amount (3~5 wt%) of cement was added without the ultrafine particulate powder showed strength of about 25 MPa and density. In contrast, the composition M wherein only part of the ultrafine particulate powder was replaced with cement showed a high bending strength of about 40 MPa.

The addition amount of cement is specifically 5 wt% or less, more specifically 3 wt%.

For the dispersing agent, polycarboxylate ether (PCE)-based SF60 and VP65 (BASF) were used.

It was confirmed that a small amount of alumina cement [3 wt% of cement comprising 0.6 wt% of calcium oxide (CaO)] can be used instead of the expensive ultrafine particulate alumina for a refractory which can endure a small amount of calcium oxide (CaO). And, it was also confirmed that, for a refractory which cannot endure even a small amount of calcium oxide (CaO), slag penetration can be greatly suppressed by adding a small amount of zircon.

The examples and experiments will now be described. The following examples and experiments are for illustrative purposes only and not intended to limit the scope of the present invention.

The composition of a cement comprising about 80 wt% of alumina used according to the present invention is described in Table 1, and the compositions of refractory mixtures according to the present invention are described in Table 2.

CaO	Al₂O₃	SiO₂	TiO₂	Fe₂O₃	Na₂O	MgO	> 38 μm
17-18.5	79-81	0.1-0.3	-	0.02-0.1	0.2-0.4	0.1-0.4	18-22

Composition (wt%)		A	B	D	M	G
Al₂O₃	3-5 mm	7	7	0	0	15
SiC (average particle size)	0.3 μm	7.6	0	7.6	4	0
	1.25 mm	38	38	45	40	30
	750 μm	23	23	23	20	15
	90 μm	3	3	3	10	7.4
	20 μm	20	20	20	20	20
	5 μm	1.4	1.4	1.4	6	5
Sum (%)		100	92.4	100	100	92.4
Boehmite (%)		0	0	3	3	3

Example 1

The refractory mixture composition A was mixed with cement and zircon with various amounts, put in a 120 x (~15) x 20 mm stainless steel mold, and formed into an unshaped refractory by tapping on an electric vibration table. After drying at room temperature followed by drying at 60 ºC and then at 100 ºC for over 6 hours, respectively, the refractory was heated in the air at 5 ºC/min to 1350 ºC, kept at that temperature for 3 hours, and then cooled down at a rate of 5 ºC/min. The sample was subjected to 3-point bending strength measurement with an outer span of 35 mm. Density was measured by the Archimedes’ method in boiling water. The characteristics of the refractories depending on the additives are shown in Table 3.

Table 3 shows the characteristics of the refractories prepared from the refractory mixture composition A.

Sample	Additives (wt%; 10 wt% B)*	Dispersing agent (SF60, wt%)	Water (wt%)	Density (g/cm³)	Bending strength (MPa)
A-1	0Z0CA3B	0.3	5.4	2.61	22
A-2	0Z0CA6B	0	0	2.63	32
A-3	0Z0CA8.3B	0	0	2.64	33
A-4	0Z1CA1.8B	0	4.3	2.64	28
A-5	1Z1CA5.3B	0	3.6	2.63	29
A-6	3CA 0B	0.2	6	2.76	17
A-7	3CA6.4B	0	3.6	2.68	41
A-8	5Z3CA6.2B	0	3.9	2.63	35
A-9	5Z10CA0B	VP0.2	7.3	2.67	24

* Z: zircon, CA: alumina cement, B: alumina sol (10 wt%)

For example, 0Z0CA3B means 0 wt% of zircon/0 wt% of cement/3 wt% of alumina sol (10wt%).

Example 2

Table 4 shows the characteristics of the refractories prepared from the refractory mixture composition B in the same manner as in Example 1.

Sample	Additives (wt%; 10 wt% B)*	Dispersing agent (SF60, wt%)	Water (wt%)	Density (g/cm³)	Bending strength (MPa)
B-1	0CA3.8B	0	0	2.53	6
B-2	1CA1.8B	0	6.3	2.51	14
B-3	3CA9.8B	0	0	2.49	23
B-4	5CA9.8B	0	0	2.52	27

* Z: zircon, CA: alumina cement, B: alumina sol (10 wt%)

For example, 0Z0CA3B means 0 wt% zircon/0 wt% cement/3 wt% alumina sol (10wt%).

Example 3

Table 5 shows the characteristics of the refractories prepared from the refractory mixture composition D in the same manner as in Example 1.

Sample	Additives (wt%; 10 wt% B)*	Dispersing agent (SF60, wt%)	Water (wt%)	Density (g/cm³)	Bending strength (MPa)
D-1	0CA4.2B	0	4.1	2.56	19
D-2	0CA7.6B	0	0	2.54	38
D-2	1CA1.9B	0	6.3	2.56	30
D-3	3CA1.9B	0	6.7	2.54	35
D-4	2Z3CA	0	7.5	2.56	37
D-5	3CA0B	0	7.1	2.57	36

* Z: zircon, CA: alumina cement, B: alumina sol (10 wt%)

Example 4

Table 6 shows the characteristics of the refractories prepared from the refractory mixture composition M in the same manner as in Example 1.

Sample	Additives (wt%; 10 wt% B)*	Dispersing agent (SF60, wt%)	Water (wt%)	Density (g/cm³)	Bending strength (MPa)
M-1	0CA0.85B	0	7.8	2.50	27
M-2	1CA 0B	0	9.0	2.45	41
M-3	1Z1CA0.8B	0	8.5	2.43	40.5
M-4	3CA0B	0	10.04	2.48	41.5
M-5	1Z2CA0.8B	0	9.5	2.51	41.5
M-6	2Z1CA0.8B	0	8.5	2.49	40

* Z: zircon, CA: alumina cement, B: alumina sol (10 wt%)

Example 5

Table 7 shows the characteristics of the refractories prepared from the refractory mixture composition G in the same manner as in Example 1.

Sample	Additives (wt%; 10 wt% B)*	Dispersing agent (SF60, wt%)	Water (wt%)	Density (g/cm³)	Bending strength (MPa)
G-1	2Z1CA0B	0.3	7.7	2.56	27.8
G-2	2Z1CA0B	0.2	7.7	2.54	23.2
G-3	2Z1CA0.83B	0.3	6.7	2.54	25.6
G-4	1CA0B	0.2	8.6	2.59	23
G-5	3CA0B	0.2	8.4	2.62	25
G-6	5CA0B	0.2	8.1	2.59	32
G-7	5CA1.8B	0	6.7	2.58	31

* Z: zircon, CA: alumina cement, B: alumina sol (10 wt%)

Example 6

Slag corrosion test was performed using Usibelli slag under the condition of 20% CO/N₂ atmosphere at 1550 ºC/4 hr. A castable comprising the refractory mixture composition D as well as 2 wt% of zircon and 3 wt% of cement was formed into a shape of a cup with an outer diameter of 60 mm and an inner diameter of 30 mm. Then, heat treatment was performed as described in Example 1 to prepare a refractory sample D-4. After adding 35 g of slag powder over 2 hours while keeping temperature at 1550 ºC and maintaining the temperature for additional 2 hours, the sample was furnace cooled and the cross-section of the sample was observed (Fig. 2).

Comparative Example 1

A refractory sample D-2 was prepared in the same manner as Example 7 by adding only alumina sol without addition of zircon or cement to the refractory mixture composition D. Slag corrosion test result is shown in Fig. 1.

Comparative Example 2

A refractory sample D-5 was prepared in the same manner as Example 7 by adding only 3 wt% of cement to the refractory mixture composition D. Slag corrosion test result is shown in Fig. 3.

The present application contains subject matter related to Korean Patent Application No. 10-2011-0088085, filed in the Korean Intellectual Property Office on August 31, 2011, the entire contents of which is incorporated herein by reference.

Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.

Claims

An unshaped refractory comprising a refractory mixture of alumina (Al₂O₃)and silicon carbide (SiC) to which alumina cement, zircon and an alumina binder are added.
The unshaped refractory according to Claim 1, wherein magnesium oxide, spinel, zirconia, chromia, hafnium oxide or a mixture thereof is further added to the refractory mixture of alumina (Al₂O₃)and silicon carbide (SiC).
The unshaped refractory according to Claim 1, wherein the alumina binder is included in an amount of 0.5-4 parts by weight based on alumina per 100 parts by weight of the total unshaped refractory comprising alumina (Al₂O₃)and silicon carbide (SiC).
The unshaped refractory according to Claim 1, wherein the alumina cement comprises 10-30 wt% of calcium oxide (CaO).
The unshaped refractory according to Claim 1, wherein the alumina cement is added in an amount of 0.5-5 wt% based on the total weight of the unshaped refractory.
The unshaped refractory according to Claim 1, wherein the zircon is added in an amount of 1-5 wt% based on the total weight of the unshaped refractory.