Chemical apparatus and process disinfecting – deodorizing – preser – Process disinfecting – preserving – deodorizing – or sterilizing – Maintaining environment nondestructive to metal
Reexamination Certificate
1999-01-29
2001-08-21
McKane, Elizabeth (Department: 1744)
Chemical apparatus and process disinfecting, deodorizing, preser
Process disinfecting, preserving, deodorizing, or sterilizing
Maintaining environment nondestructive to metal
C422S019000, C106S014050, C148S243000, C148S276000, C148S286000, C148S287000
Reexamination Certificate
active
06277327
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method for the anticorrosive treatment of waste plastics treating equipment in which waste plastics are treated by supercritical water.
It is conventionally known that natural and synthetic polymeric compounds can be decomposed by means of supercritical or subcritical water.
Meanwhile, in equipment for the treatment of chlorine-containing waste plastics such as vinyl chloride resin and polychlorotrifluoroethylene, chlorine is released from such resins, and the equipment material (e.g., stainless steel) is heavily corroded by the chlorine. Although the degree of corrosion may vary according to the chemical composition of the equipment material and the Cl concentration in the aqueous solution, the treatment especially with supercritical water is carried out under high-temperature and high-pressure conditions. Consequently, when stainless steel is used as the equipment material, it is highly susceptible to pitting, crevice corrosion, stress corrosion cracking and other corrosion due to chlorine ion. As a countermeasure against it, nickel alloys having special chemical compositions have been used.
However, such nickel alloys are so expensive that the construction cost of the equipment is raised to an undue extent. Accordingly, in the case of equipment for the treatment of chlorine-containing waste plastics such as vinyl chloride resin and polychlorotrifluoroethylene, there has been developed a process which avoids the direct treatment of such resins with supercritical water. Specifically, according to this waste plastics treating process, such resins are first dechlorinated by pyrolysis at 300-350° C. in a separate dechlorination apparatus, and the remaining dechlorinated resins are then treated with supercritical water.
As a result of dechlorination, this process permits relatively inexpensive stainless steel to be used as the equipment material. However, this process is more complicated and requires a greater number of system components, as compared with the process for treating waste plastics directly. As a result, this process still involves the problem that the construction cost of the system is very high.
SUMMARY OF THE INVENTION
In view of the above-described existing state of the art, the present invention relates to equipment for treating the aforesaid waste plastics directly with supercritical water, and an object thereof is to provide a method for the anticorrosive treatment of waste plastics treating equipment which makes it possible to use inexpensive stainless steel.
According to the present invention, there is provided a method for the anticorrosive treatment of waste plastics treating equipment which comprises the steps of providing waste plastics treating equipment made of stainless steel, charging the equipment with a predetermined amount of an aqueous solution containing at least one alkali metal salt, deaerating the aqueous solution, raising the temperature and pressure of the equipment until supercritical conditions are established, and maintaining the temperature and pressure for a predetermined period of time.
According to the anticorrosive treatment method of the present invention, a dense black film having a thickness of several micrometers is formed on the surface of the stainless steel. This film has excellent corrosion resistance, scratch resistance and abrasion resistance, and is never corroded under supercritical water conditions including a saturated aqueous solution of NaCl. The black film formed in the present invention is an oxide film composed of Fe
3
O
4
and FeCr
2
O
4
, and its composition is equal to that of a film formed by conventional passivation treatment which will be described later. However, as contrasted with the conventional passivation treatment film which has a thickness of several tens of nanometers, the black film formed in the present invention has a very large thickness of several micrometers and is much more excellent in corrosion resistance, scratch resistance and abrasion resistance. This film can be thickened by extending the anticorrosive treatment time.
As is evident from the above description, the present invention relates to equipment for treating the aforesaid waste plastics directly with supercritical water, and provides a method for the anticorrosive treatment of waste plastics treating equipment which makes it possible to use inexpensive stainless steel.
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patent: 5741372 (1998-04-01), Gugel
patent: 5911841 (1999-06-01), Ohmi
patent: 4312417 A1 (1994-10-01), None
patent: 4329461 A1 (1995-03-01), None
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Austrian Patent Office Search Report, Date of Mailing: Feb. 29, 2000 for Singapore Patent Application No. 9900216-4.
European Search Report dated Jun. 8, 1999 for European Patent Application No. 99101085.1-2307.
Hasegawa Shigeo
Iwasaki Kenji
Kaneko Masato
Kobayashi Kazuto
Matsubara Wataru
McKane Elizabeth
Myers Bigel Sibley & Sajovec P.A.
Tohoku Electric Power Company Inc.
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