Porous sintered body

Details Porous sintered body Porous sintered body

2001-07-31

2004-03-23

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Composed of waste material

C501S080000, C501S039000, C501S032000, C588S011000, C588S256000

Reexamination Certificate

active

06710001

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a porous sintered body to be used for pottery, a sound absorber, a filter, a filtration medium, a thermal insulator, a heat insulator, a heat exchange medium, an absorber, a catalyst, a water-retaining medium or the like.
In addition, the present invention relates to a porous sintered body containing various heavy metals contaminated wastes, besides vitreous wastes.
BACKGROUND OF THE INVENTION
For recycling wastes, it is required to prevent leaching of heavy metal contaminants. Various techniques for preventing the leaching such as solidifying the wastes by using cements and sintering the wastes after adding a sintering additive have been proposed.
Japanese Patent No. 2796243 discloses a method of sintering wastes after adding a sintering additive. In this method, 100 parts by weight of furnace ash is mixed with 5-30 parts by weight of an additive which consists of a silicate or mixture of silicate and Na
2
O containing material and which has a weight ratio of (SiO
2
+Al
2
O
3
)/Na
2
O of 9.0 to 1.0, molded, and sintered.
Japanese patent publication 5-58707A discloses a method of sintering a mixture of wastes and at least one of feldspar, glass powder, bentonite and frit. Japanese patent publication 7-17757A discloses to sinter a mixture of wastes and at least one of fly ash, pumice, obsidian, perlite, shirasn, tuff and waste glass.
However, heavy metals eluate at a high rate from a sintered body sintered according to either of the above conventional methods.
Vitrification is widely employed to solidify wastes such as furnace ash to produce a solidified body with little heavy metals to be leached. According to this vitrification, the solidified body should be a compact body. Such a compact body can not be used in a wider field and, for example, used as a roadbed material.
It is well known that fused slag of furnace ash of municipal solid waste is mixed with ceramics raw material such as clay, and is fired to produce ceramics such as tiles. However, heavy metals are leached at a high rate from the ceramics thus prepared.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a porous sintered body from which heavy metals are hard to be leached and which has high safety and high availability.
A porous sintered body of the present invention contains heavy metals and 0.5-15 weight % of B
2
O
3
, wherein
the leaching amount of Cd is 0.01 mg/l or less;
the leaching amount of Pb is 0.01 mg/l or less;
the leaching amount of As is 0.01 mg/l or less;
the leaching amount of Se is 0.01 mg/l or less;
the leaching amount of Cr
+6
is 0.05 mg/l or less
the leaching amount of Cd is 0.01 mg/l or less; and
the leaching amount of total Hg is 0.0005 mg/l or less.
It should be noted that the leaching amounts of heavy metals are measured by the following method according to Environmental Quality Standards (EQS) for soil pollution issued by Japanese Ministry of the Environment. Any one of the above-listed low leaching amounts of the heavy metals meets with the EQS of Japanese Ministry of the Environment.
Method of Measuring Leaching Amounts of Heavy Metals (EQS)
(1) Preparation of sample solution: A sample (gram) and a solvent (hydrochloric acid is added into pure water to make a solvent having hydrogen ion concentration index from 5.8 to 6.3) (milliliter) are mixed at a weight/volume ratio of 10% in such a manner as to form a mixture 500 ml or more.
(2) Leaching: The prepared sample solution is shaken for 6 hours at an ordinary temperature (approximately 20° C.) and an atmospheric pressure (approximately 1 atm) by using a shaker (the shaking ratio is previously set at 200 per minute and the shaking width is previously set from 4 cm to 5 cm).
(3) Preparation of test liquid: The sample solution prepared in (1) and processed in (2) is statically placed on a shelf for 10 minutes to 30 minutes and is then centrifuged at approximately 3,000 revolutions per minutes for 20 minutes. After that, supernatant is took from the sample solution and is filtered by a membrane filter of which pore size is 0.45 &mgr;m to collect filtrate. A predetermined amount of the filtrate is accurately measured and took as a test liquid.
(4) Measurement of leaching amounts: the leaching amounts are measured according to the JIS (Japanese Industrial Standard) K 0102.
In the present invention, the porous sintered body preferably contains 20-60 weight % of Al
2
O
3
, 18-60 weight % of SiO
2
, 1-30 weight % of the sum of CaO, SrO, BaO, and MgO, and 0.5-15 weight % of B
2
O
3
. It is also preferable that it further contains 1-12 weight % of the sum of Na
2
O, K
2
O, Li
2
O, and P
2
O
5
.
B
2
O
3
cooperates together with SiO
2
or the like in the porous sintered body to form a small amount of glass. In this glass, since B
2
O
3
is in the quadrivalent form i.e. to be BO
4
, excess charge is generated around BO
4
. This excess charge captures heavy metals. Therefore, although the sintered body is porous, the leaching of concealed heavy metal can be prevented. B
2
O
3
prevents the leaching of heavy metals, and also improves chemical durability and water resistance of the resultant sintered body so that the sintered body is suitable for various kinds of porous materials such as a filter, a water-retaining medium, or a ceramic tile. However, when B
2
O
3
is contained in an amount exceeding the aforementioned range, the B
2
O
3
may be in the trivalent form i.e. to be BO
3
providing poor chemical durability, thus easily allowing the leaching of heavy metals.
For example, the porous sintered body of the present invention is prepared by mixing glass, a ceramic material such as clay to be dehydrated when fired, and a raw material containing B
2
O
3
to form a mixture in such a manner that chemical components of a resultant sintered body are 20-60 weight % of Al
2
O
3
, 18-60 weight % of SiO
2
, 1-12 weight % of the sum of Na
2
O, K
2
O, Li
2
O, and P
2
O
5
, 1-30 weight % of the sum of CaO, SrO, BaO, and MgO, and 0.5-15 weight % of B
2
O
3
; molding the mixture; and firing the molded mixture. In this case, it is preferable that the glass contains B
2
O
3
. It is further preferable that the glass has a chemical composition ratio of B
2
O
3
/SiO
2
being 0.01-0.40.
Dehydrated materials of the raw material to be dehydrated due to firing such as clay has excellent high activity so as to easily react with glass during the sintering process, thereby forming crystalline phases of SiO
2
—Al
2
O
3
—RO wherein RO is at least one selected from a group consisting of Na
2
O, K
2
O, CaO, SrO, BaO, and MgO.
The crystallization of SiO
2
—Al
2
O
3
—RO makes the strength of the sintered body higher than a case where glass in the raw materials still leaves as glass in the sintered body. In addition to the fact that the strength of crystals is higher than that of glass, significant distortion takes place in glass during cooling process after sintering process and microcrak takes place in this glass portion, thus reducing the strength of the sintered body. On the other hand, it is considered that the crystallization of glass significantly reduces such distortion.
A melted glass phase disappears by crystallization of the glass, so that pores of the sintered body is prevented from being packed with melted glass having low viscosity. Since the crystalline phases are relatively uniformly formed to have a size from 0.1 to 10 &mgr;m, pores sized in a range from 0.1 to 10 &mgr;m are formed between the fine crystals, thereby providing a quality suitable for a separator filter or a water-retaining medium.
The raw material of the sintered body of the invention contains B
2
O
3
or glass including B
2
O
3
, so that the sintered body contains a small amount of glass including boron in the quadrivalent form i.e. to be BO
4
, whereby heavy metals are fixed and thus prevented from being leached.
Aluminum hydroxide is also dehydrated by firing. Dehydrated aluminum hydroxide has higher activity than that of clay so as to easily react with glass, whereby heavy metals are leached

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