Method of decomposing polychlorobiphenyls

A method of decomposing a polychlorobiphenyl compound-containing material into an unharmful state, wherein the material is treated with a supercritical water containing an alkaline substance at a temperature higher than the critical temperature of water and a pressure higher than the critical pressure of water.

BACKGROUND OF THE INVENTION 
Polychlorobiphenyls (hereinafter referred to as PCBs) were used in a wide 
variety of applications such as insulating oil, machine oil, plasticizers 
and paints, because of excellent insulating properties, good solubility in 
oils and low vaporizability thereof. In 1966, PCBs, which are harmful to 
human bodies and chemically extremely stable, were found to cause 
environmental pollution and to be stored in a high concentration within 
living beings through food chains. Thus, early in 1970s, the manufacture 
of PCBs was prohibited. As a result, a total of about 10.sup.6 tons of 
PCBs have been stored in the world. There is a great demand for the 
establishment of technology for decomposing PCBs. 
Known methods hitherto proposed for the decomposition of PCBs include (a) 
combustion at a temperature of at least 1,100.degree. C., (b) catalytic 
hydrogenation, (c) photochemical decomposition with UV rays, and (d) 
biological decomposition using microorganisms. These methods are not fully 
satisfactory with respect to energy consumption, treatment time, equipment 
cost or formation by-products. 
JP-B-2590421 discloses a method of decomposing PCBs, wherein PCBs are 
contacted with an aqueous solution containing an oxidizing agent at a 
temperature of 623 K-823 K and a pressure of 20-60 MPa. The presence of 
the oxidizing agent is described as being essential in order to 
effectively decompose PCBs. This method is promising because PCBs can be 
effectively decomposed within a relatively short period of time but has a 
problem because the oxidizing agent cause corrosion of the treatment 
apparatus and decomposition of other useful materials such as oil in which 
PCBs are contained. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an 
industrially suitably applicable method which can effectively decompose 
PCBs and which is devoid of the drawbacks of the conventional methods. 
It is a special object of the present invention to provide a method of 
decomposing PCBs contained in an oil while permitting the recovery of the 
oil. 
In accomplishing the foregoing objects, there is provided in accordance 
with the present invention a method of decomposing a polychlorobiphenyl 
compound-containing material, comprising treating said material with 
supercritical water containing an alkaline substance at a temperature 
higher than the critical temperature of water and a pressure higher than 
the critical pressure of water. 
It has been found that PCBs, when treated with supercritical water 
containing an alkaline substance, can be decomposed into, biphenyl, 
phenol, carbon dioxide, etc. without using an oxidizing agent. Since 
hydrogen chloride formed in situ by the decomposition of PCBs is 
neutralized with the alkaline substance, no corrosion of the reactor is 
caused. Further, when the PCB-containing material is a PCB-containing oil, 
the oil can be recovered after the decomposition of PCBs. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Any PCB-containing material, either in the form of a liquid or a solid, may 
be treated in accordance with the present invention. The concentration of 
PCBs in the material to be treated generally ranges from 0.001% by weight 
up to 100% by weight. PCBs are often contained in oils such as insulating 
oils composed of mineral oils and alkylbenzenes. Such PCB-containing oils 
are suitably treated by the method of the present invention, while 
recovering the oils. 
The PCB-containing material is fed to a reactor and is heated together with 
water in the presence of an alkaline substance at a temperature higher 
than the critical temperature of water, i.e. at a temperature higher than 
647.3 K, preferably 653-873 K, more preferably 653-773 K, and at a 
pressure higher than the critical pressure of water, i.e. at a pressure 
higher than 22.12 MPa, preferably 22.12-40 MPa, more preferably 25-35 MPa. 
The treatment time is generally 3-120 minutes, preferably 5-60 minutes. 
The alkaline substance is preferably a weak acid salt or hydroxide of an 
alkali metal or an alkaline earth metal. Illustrative of preferable 
alkaline substances are sodium hydroxide, potassium hydroxide, sodium 
carbonate and calcium hydroxide. The alkaline substance is generally used 
in an amount of 1-15 parts by weight per part by weight of PCBs. For 
reasons of costs, sodium hydroxide is preferably used as the alkaline 
substance in an amount of 2-10 equivalents per equivalent of the chlorine 
atoms contained in PCBs. 
The amount of water present in the reactor is generally 50-150 parts by 
weight per part by weight of PCBs contained in the PCB-containing material 
to be treated. 
The following examples will further illustrate the present invention.

EXAMPLES 1-3 
In an autoclave were charged water, PCBs and sodium hydroxide in amounts as 
shown in Table 1. The mixture was then treated at a temperature of 723 K 
and a pressure of 30 MPa for 20 minutes. The mixture after the treatment 
was analyzed for the amount of residual PCBs. The PCB decomposition rate Z 
(%) was calculated according to the following formula: 
EQU Z=(W.sub.0 -W.sub.1)/W.sub.0 .times.100 
where W.sub.0 represents the amount of PCBs charged in the autoclave and 
W.sub.1 is the amount of PCBs remaining in the reaction mixture after the 
treatment. The results are also shown in Table 1. 
TABLE 1 
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Example 
Amount of Amount of Amount of 
Decomposition 
No. PCBs (mg) Water (g) NaOH* Rate Z (%) 
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1 10.05 1.332 3.2 99.99 
2 10.64 1.331 5.0 99.93 
3 11.04 1.328 9.7 99.99 
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*Equivalent (mole) of NaOH per one equivalent (one mole) of the chlorine 
atoms of PCBs 
EXAMPLES 4-6 
In an autoclave were charged water, a PCB-containing insulating oil 
(content of PCBs: about 2% by weight) and sodium hydroxide in amounts as 
shown in Table 2. The mixture was then treated at a temperature of 723 K 
and a pressure of 30 MPa for 20 minutes in Examples 4 and 5. The reaction 
conditions in Example 6 involved a temperature of 653 K, a pressure of 30 
MPa and a time of 20 minutes. The PCB decomposition rate is shown in Table 
2. The insulating oil remaining in the mixture after the treatment was 
found not to be deteriorated. 
TABLE 2 
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Example No. 4 5 6 
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Amount of PCB- 49.93 49.73 50.04 
Containing Oil (mg) 
Amount of PCBs (mg) 1.03 1.02 1.03 
Amount of Water (g) 1.336 1.332 4.780 
Amount of NaOH* 4.9 9.7 9.7 
Decomposition Rate Z (%) 99.45 99.79 99.63 
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*Equivalent (mole) of NaOH per one equivalent (one mole) of the chlorine 
atoms of PCBs 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all the 
changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embraced therein.