Method of decomposing polychlorobiphenyls

Abstract: 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.

Description: Polychlorobiphenyls (hereinafter referred to as PCBs) were once 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 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°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 623K-823K 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 causes corrosion of the treatment apparatus and decomposition of other useful materials such as oil in which PCBs are contained. A paper by Thomason et al entitled the MODAR Supercritical Water Oxidation Process" in the Innovative Waste Treatment Technology Series, Vol.1, 1999, Lancaster, PA, USA discloses a supercritical water oxidation process for disposing of hazardous waste materials, for example, PCBs-containing wastes such as transformer oil, in which the complete oxidation of the wastes is achieved by means of added oxygen. Caustic is added to neutralise the acids which are formed when the wastes contain heteroatoms which produce mineral acids. In accordance with the present invention there is provided a method of decomposing a polychlorobiphenyl compound-containing material, comprising treating said material in the absence of added oxidizing agent 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. Thus, it has been found in accordance with this invention 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 PCB-containing material is a PCB containing oil, the oil can be recovered after the decomposition of PCBs. 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 preferred 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 723K 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: Z = (W 0 - W 1 )/W 0 x 100 where W 0 represents the amount of PCBs charged in the autoclave and W 1 is the amount of PCBs remaining in the reaction mixture after the treatment. The results are also shown in Table 1. Example No. Amount of PCBs (mg) Amount of Water (g) Amount of NaOH Decomposition Rate Z (%) 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 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 723K and a pressure of 30 MPa for 20 minutes in Examples 4 and 5. The reaction conditions in Example 6 involved a temperature of 653K, 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. Example No. 4 5 6 Amount of PCB-Containing Oil (mg) 49.93 49.73 50.04 Amount of PCBs (mg) 1.03 1.02 1.03 Amount of Water (g) 1.336 1.332 4.780 Amount of HaOH 4.9 9.7 9.7 Decomposition Rate Z (%) 99.45 99.79 99.63

Claims: A method of decomposing a polychlorobiphenyl compound-containing material, comprising treating said material in the absence of added oxidizing agent 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. A method as claimed in claim 1, wherein said treatment is performed at a temperature 653-873 K and a pressure of 22.12-40 MPa. A method as claimed in claim 1 or claim 2, wherein said alkaline substance is present in an amount of 1-15 parts by weight per part by weight of said polychlorobiphenyl compound. A method as claimed in any preceding claim, wherein said alkaline substance is a weak acid salt or hydroxide of an alkali metal or an alkaline earth metal. A method as claimed in claim 4, wherein said alkaline substance is sodium hydroxide. A method as claimed in claim 5, wherein said sodium hydroxide is present in an amount of 2-10 equivalents per equivalent of the chlorine atoms contained in said polychlorobiphenyl compound. A method as claimed in any preceding claim, wherein said water is present in an amount of 50-150 parts by weight per part by weight of said polychlorobiphenyl compound. A method as claimed in any preceding claim, wherein said polychlorobiphenyl compound-containing material consists essentially of polychlorobiphenyl compounds. A method as claimed in any one of claims 1-7, wherein said polychlorobiphenyl compound-containing material is a mixture containing polychlorobiphenyl compounds and an oil. A method as claimed in claim 9, wherein said oil is an insulating oil.

Inventors: OKANO MAKOTO, OTAKE KATSUTO, SAKO TAKESHI, SATO MASAHITO, SUGETA TSUTOMU, OKANO, MAKOTO, OTAKE, KATSUTO, SAKO, TAKESHI, SATO, MASAHITO, SUGETA, TSUTOMU
Applicants: AGENCY IND SCIENCE TECHN, DIRECTOR-GENERAL OF THE AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY
Application year: 1997
Main classification: A62D3
IPC codes: A62D3, A62D101