JP2003231886A - Method for pyrolysis treatment of waste plastic and apparatus and fuel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for pyrolysis treatment of waste plastics by which a low-viscosity pyrolysis oil of good quality can be obtained and an apparatus therefor. <P>SOLUTION: The method for pyrolysis treatment is to pyrolyze the waste plastics in an oil. In the method, contained chlorine is evaporated and removed and pyrolysis is advanced so as to suppress the production of organic volatiles by using the density of the resultant melt oil produced by the pyrolysis as an index. The pyrolysis oil is recovered without separating light organic components by distillation. The density of the melt oil is preferably controlled to ≥300 kg/m<SP>3</SP>to ≤700 kg/m<SP>3</SP>to carry out the pyrolysis. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention efficiently removes chlorine and solid foreign substances such as metal and glass contained in waste plastics, and the waste plastics after dechlorination are fluid fuels of relatively low temperature and low viscosity. The present invention relates to a method and an apparatus for decomposing waste plastic, which can be converted into a solid fuel or can be treated as a solid fuel having good crushability at room temperature. The processing method or apparatus of the present invention is a multifunctional waste plastic processing system capable of adjusting the properties of the processed material according to the needs of the user.

[0002]

2. Description of the Related Art Conventionally, a method for decomposing waste plastics is an oilification process mainly for recovering oil from waste plastics, and a solid fuel used as a raw material for a blast furnace in a steel mill, a kiln for cement, gasification, etc. It is known that the process is mainly based on manufacturing. The oilification process produces low-boiling hydrocarbon oils from plastics, and waste plastics are melted at a relatively low temperature and desalted.
By heating at a high temperature of 0 ° C to 450 ° C for a long time,
This is a method in which thermal decomposition of molten plastic is promoted, light pyrolysis products are separated by distillation to recover light oil, and residual heavy oil and char are returned to the pyrolysis step and pyrolyzed again. On the other hand, in the process for producing a solid fuel, waste plastic is heated in a rotary kiln system or a screw through a heating medium, and is retained at a temperature of about 280 to 350 ° C. for 20 to 40 minutes for dechlorination and then cooled. Use solid fuel.

[0003]

In the conventional solid fuel manufacturing process, in the method using the rotary kiln, the waste plastic is directly heated under a gas phase atmosphere, and in many cases, under reduced pressure. Therefore, there is a problem that the proportion of gas or char in which the decomposition products are secondarily decomposed by local overheating increases, and the amount of liquid oil recovered decreases. Further, since the amount of gas to be evaporated is large, the viscosity of the liquid oil becomes high, so that it is difficult to separate solid foreign substances such as metal and glass mixed in the liquid oil, and these solid foreign substances are likely to remain in the solid fuel. There's a problem. In addition,
When the liquid oil containing such solid foreign matter is solidified, it becomes a hard mass and the pulverizability is poor. Further, a method of decomposing waste plastic by adding an organic solvent is also known (Japanese Patent Application No. 10-22874, etc.), but this method complicates the process such as recovery of an organic solvent and addition of a recycling step, There is a problem that energy efficiency is also reduced. Further, there is a problem that the use of solid fuel is limited and it is not possible to flexibly respond to changes in usage trends of waste plastic.

On the other hand, in the conventional oiling process, it is known to add 2 parts by weight or more of distilled heavy oil to 1 part by weight of waste plastic in order to reduce the viscosity of cracked oil (special characteristics). (Kaihei 11-310659), a large amount of liquid oil is circulated and added from the distillation process to the thermal decomposition process, which complicates the equipment and lowers the treatment efficiency. Moreover, a large amount of heat is required in the thermal decomposition process and the distillation process. Further, the conventional method controls the thermal decomposition by using the viscosity of the cracked oil as an index, but the melted liquid oil generated by the thermal decomposition often shows a foaming phenomenon when the thermal decomposition temperature rises, and thus depends on the viscosity. And thermal decomposition may not be well controlled.

[0005]

The present invention solves the above-mentioned problems in the conventional method for decomposing waste plastics, and has a low viscosity and clean decomposition products without adding an organic solvent or distilled heavy oil. Provided is a decomposition treatment method capable of recovering a substance in either a liquid state or a solid state.

The present invention (1) uses a density of molten oil produced by thermal decomposition as an index in a treatment method of thermally decomposing waste plastic in oil, and pyrolyzes and removes contained chlorine to remove organic volatilization. The present invention relates to a method for decomposing waste plastics, characterized by advancing thermal decomposition so as to suppress the generation of a component, and recovering decomposed oil without distilling and separating light organic components.

The decomposition treatment method of the present invention comprises: (2) a melting and decomposition step of thermally decomposing waste plastic to volatilize and remove contained chlorine and produce a molten liquid oil in which generation of organic volatile components is suppressed; This is a treatment method in which a reforming adjustment step of thermally decomposing and reforming into cracked oil is performed in one step, and the density of molten oil is controlled to 300 kg / m ³ or more to 700 kg / m ³ or less and light cracking is performed. In the method for decomposing waste plastic of (1) above for recovering oil, (3) in the one-step treatment of (2) above, the heating temperature is 320 to 420 ° C., and the heating time is 30 minutes or less,
It includes a method for decomposing waste plastics, in which the density of the melted liquid oil is controlled to 300 kg / m ³ or more and 700 kg / m ³ or less and the viscosity of the melted liquid oil is controlled to 300 cp or less.

The decomposition treatment method of the present invention is a treatment method in which (4) the melt decomposition step and the modification adjusting step are carried out in two stages,
In the first-stage melt cracking process, the density of the molten liquid oil was adjusted to 40
0 kg / m ³ or more ~700kg / m ³ was controlled to below the control to the density of the cracked oil below 300 kg / m ³ or more ~600kg / m ³ for recovering the light cracked oil in the reforming adjusting process of the second stage The method for decomposing waste plastic as described in (1) above,
(5) In the two-step treatment of (4) above, the heating temperature in the melt decomposition step is set to 250 to 350 ° C., the heating time is set to 60 minutes or less, and the density of the molten oil is set to 400 kg / m ³ or more and 700 kg / m ³ or less. In addition to controlling the viscosity to 600 c.p or less, the heating temperature in the reforming adjustment process is 350 to 420 ° C., and the heating time is 3
0 minutes or less, the density of cracked oil is 300kg / m ³ or more to 60
Includes a method for decomposing waste plastics, which controls the viscosity to 0 kg / m ³ or less and the viscosity to 300 c.p or less.

The present invention also provides (6) 200 ° C. after cooling the cracked oil obtained by the treatment method according to any one of (1) to (5) above.
It includes a method for decomposing waste plastics, which is maintained at 300 ° C as a liquid raw material or liquid fuel or cooled to room temperature as a solid raw material or solid fuel.

Further, according to the present invention, (7) a melting and decomposing tank for thermally decomposing waste plastic in oil, a reforming adjusting tank for promoting liquefaction by heating a decomposition product generated in the thermal decomposition step, and each tank is independent. (8) A decomposition processing apparatus for waste plastic, characterized by having a heating means for controlling the temperature, a pipeline connecting each tank, a liquid feeding means, and a means for monitoring the density and viscosity of a mixture of waste plastic and oil, (8) The present invention relates to a solid fuel having a pencil hardness of 3B or less, which is obtained by the method according to any one of (1) to (6) above.

In the decomposition treatment method of the present invention, when waste plastic is thermally decomposed in liquid oil, the density of the molten liquid oil and the decomposed oil produced by thermal decomposition is used as an index, and the chlorine contained is thermally decomposed and volatilized and separated. On the other hand, it is a thermal cracking method in which thermal cracking is promoted so as to suppress the formation of organic volatiles as much as possible and converted into light cracked oil. The method of the present invention comprises a melt decomposition step of thermally decomposing waste plastic to volatilize and remove contained chlorine and to produce a molten liquid oil in which the production of organic volatiles is suppressed, and the molten liquid oil is further thermally decomposed into decomposed oil. And a reforming adjusting step of reforming. The melt decomposition step and the modification adjusting step may be performed in one step using a single processing tank, or the melt decomposition step and the modification adjusting step may be performed in two steps using each processing tank. .

Chlorine contained in the waste plastic is substantially volatilized in the melting and decomposition step and removed to the outside of the system. Further, through the melt decomposition step and the reforming adjustment step, the production of organic volatiles is suppressed as much as possible to proceed with thermal decomposition, and the cracked oil is recovered without separating the light organic components by distillation. Therefore, according to the method of the present invention, the waste plastic is pyrolyzed without the conventional distillative separation of light organic components and without the addition of heavy oil, etc. Pyrolysis can be promoted in it, and high-quality, low-viscosity cracked oil with a low chlorine content can be recovered. Further, it can be recovered in either form of liquid fuel or solid fuel from which impurities are removed, and it can be used in various fields.

As previously pointed out, in the conventional thermal decomposition of waste plastic using a rotary kiln or the like, the conversion rate to gas and char is high, and the recovery rate of decomposed oil is low.
Also, in the conventional oilification process, a distillation step is provided after the pyrolysis step to separate light organic components by distillation, and the remaining middle and heavy oils are returned to the pyrolysis step, so the equipment is complicated and the processing efficiency is high. Is low. On the other hand, in the treatment method of the present invention, the production of organic volatiles is suppressed and the cracked oil is recovered without distilling and separating the light organic components, so that the recovery rate of the cracked oil is high and the viscosity of the cracked oil is low.

Further, in the treatment method of the present invention, the thermal decomposition is controlled by using the densities of the melt oil and the decomposed oil generated by the thermal decomposition of the waste plastic as an index, so that the volatilization separation of chlorine contained and the suppression of organic volatile components can be effectively performed. You can do it. By controlling the thermal decomposition using the density as an index, excessive heating and foaming of the liquid oil are controlled as little as possible. The density of the melt oil or cracked oil is 300 kg / m ³ or more to 700 kg / when the melt cracking step and the reforming adjusting step are performed in one step.
and m ³ or less, in the case of performing the melt decomposition process and reforming adjusting step in two stages, the density of the melt oil and 400 kg / m ³ or more ～700Kg / m ³ or less in the thermal decomposition step of the first stage, the In the two-stage reforming adjustment process, the density of cracked oil was 300 kg / m ³
It is preferable to control the above to 600 kg / m ³ or less.

The viscosity may be controlled together with the density.
In the case of the one-step treatment, the viscosity of the melt oil is preferably controlled to 300 c.p or less. Further, in the case of the two-stage treatment, it is preferable to control the viscosity of the melt oil in the melt cracking step to 600 c.p or less and the viscosity of the cracked oil in the reforming adjusting step to 300 c.p or less.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below based on Examples. An example of an apparatus configuration for carrying out the processing method of the present invention is shown in FIGS. The apparatus example of FIG. 1 is an example in which the melt decomposition step and the reforming adjustment step are performed in one step, and in the melt decomposition tank 10, thermal decomposition of waste plastic and reforming of molten liquid oil are performed. The apparatus configuration of FIG. 2 is an example in which the thermal decomposition of the waste plastic by the melt decomposition tank 10 and the reforming of the molten liquid oil by the reforming adjustment tank 11 are performed in two stages, and the apparatus configuration of FIG. This is an example having a quality adjusting tank 11 and further having a system in which a part of the cracked oil recovered from the reforming adjusting tank 11 is circulated in the thermal decomposition step. In the apparatus example of FIG. 1, the melting decomposition tank 10
2 is provided with a density meter 12a and a viscometer 12b.
In the apparatus example of FIG. 3, the density meter 12a is provided in the reforming adjustment tank 11.
And a viscometer 12b are provided. In addition, the melting decomposition tank 1
0 and the reforming adjusting tank 11 are provided with a heating system 13 having a temperature adjusting means. Further melting and decomposition tank 10
A liquid feed pump 14 and a filtering unit 15 are provided between the tank bottom and the reforming adjustment tank 11.

In the treatment system of FIG. 1 in which the step of melting and decomposing waste plastic and the step of modifying and adjusting the molten liquid oil are performed in one step, the density of the molten liquid oil generated by the thermal decomposition of the waste plastic is 300 kg / m ³ or more. The cracked oil is collected without controlling the light organic components by distillation to 700 kg / m ³ or less. In addition, the heating temperature is preferably 320 to 420 ° C., preferably 360 to 400 ° C., the heating time is 30 minutes or less to control the density of the molten oil within the above range and the viscosity of the molten oil to 300 c.p or less. Control. In the one-step treatment, since the melted liquid oil and the cracked oil are produced in the same tank, the density of the melted oil in the tank may be used as an index or the density of the extracted cracked oil may be used as an index. The density of the molten oil includes both cases.

In the above-mentioned one-step processing system, specifically, for example, the heating temperature and time of the melting and cracking tank are adjusted to the above range, and the density of the molten liquid oil generated by thermal decomposition is maintained in the above range. Then, the production of organic volatile matter is suppressed and the thermal decomposition of waste plastic is promoted to produce a molten liquid oil.
Chlorine contained in the waste plastic is volatilized by thermal decomposition, so it is taken out of the system and removed. The thermal decomposition is further promoted to promote the liquefaction of the melted oil, the generated decomposed oil is extracted from the bottom of the tank, guided by the pump 13 to the filter section 15, and solid foreign matters such as metal and glass are removed through the filter to recover the decomposed oil. . The pore size of the filter is preferably 0.1 mm to 10 mm, for example, depending on the viscosity of the cracked oil. Since this decomposed oil contains a light organic component and has a low viscosity, solid foreign matters such as metal and glass can be easily separated. The recovered cracked oil is passed through the cooler 16 to about 200 ° C.
It is cooled to about 300 ° C and taken out of the system.

In the treatment system shown in FIG. 2 in which the melting and decomposing process of the waste plastic and the reforming and adjusting process of the molten liquid oil are carried out in two steps, the melting and decomposing step of the first step is carried out at a temperature of 250 to 350 ° C.
It is preferably carried out at 300 to 340 ° C. for 60 minutes or less, the density of the molten oil is 500 kg / m ³ or more and the viscosity is 600 c.p or less, and the second stage reforming adjustment is carried out at a temperature of 350 to 420 ° C., preferably 360 to 400 ° C. 30 minutes or less at ℃, density 300
Convert to cracked oil with kg / m ³ or more and viscosity of 300 c.p or less. The heating temperature may be determined according to the type and mixing ratio of the waste plastic.

2 and 3, the waste plastic charged into the melting and decomposing tank 10 is heated and decomposed at the above temperature to form a molten liquid oil. Further, chlorine contained in the waste plastic is volatilized and desalted in the melting decomposition tank, so that it is led to the outside of the system. On the other hand, the melted oil is withdrawn from the bottom of the tank and sent to the filtering section 15, where solid foreign matters such as metal and glass are removed and then sent to the reforming adjustment tank 11. In the reforming adjustment tank 11, the density and viscosity of the melted liquid oil are controlled within the above range to promote liquefaction. The optimum temperature of the reforming adjusting tank 11 is the density meter 12a and the viscometer 12 provided in the reforming adjusting tank 11.
It may be determined based on the density and viscosity of the cracked oil measured by b. By adjusting the density within the above range, it is possible to prevent excessive decomposition or foaming phenomenon in the reforming adjustment tank. Further, the viscosity can be adjusted within the above range to obtain cracked oil with low viscosity. Since this temperature varies depending on the type and mixing ratio of the waste plastic, it is advisable to adjust it based on this type and ratio.

By holding the molten plastic at a temperature of 350 ° C. to 420 ° C. for a short time, for example, about 5 minutes to 60 minutes in the reforming adjustment tank 11, liquefaction of the plastic is promoted and decomposed oil having a low viscosity. Can be obtained, and the loss of organic components due to volatilization can be reduced. Modification adjustment tank 1
The cracked oil generated in 1 is taken out from the bottom of the tank, sent to the cooler 16, and cooled to about 200 ° C to 300 ° C. This cooling suppresses the progress of thermal decomposition and prevents the loss of volatile components.

The thermal decomposition in the reforming adjustment tank 11 may be controlled by using a density meter 12a and a viscometer 12b installed in the reforming adjustment tank. The molten liquid oil in the tank has a thermal decomposition rate that increases with an increase in temperature, and a foaming phenomenon occurs due to the generation of thermal decomposition gas. Since this foaming phenomenon appears as a change in apparent density or viscosity, this change is measured and the temperature of the reforming adjustment tank is controlled. Although the foaming phenomenon changes depending on the component ratio of the molten plastic and the like, it generally occurs in the temperature range of 350 ° C to 420 ° C. By using this foaming temperature as a guide and shortening the heating time, it is possible to obtain a cracked oil having low viscosity while preventing loss of organic volatile components. In the conventional treatment method, since the residence time for pyrolysis or liquefaction promotion is long, organic volatile components escape with the progress of pyrolysis, so cracked oil becomes a product with high carbon content and high viscosity. When this is pyrolyzed, it becomes gas and coke.

In the processing apparatus shown in FIG. 3, the reforming adjustment tank 11 is used.
A part of the cracked oil retained for a certain period of time is returned to the melt cracking tank 10 and mixed with the oil in the tank. By this mixing, the pyrolyzed oil is cooled to a tank temperature of 250 ° C to 350 ° C. The cracked oil returned to the melting and cracking tank 10 differs from the distilled heavy oil in the conventional thermal cracking in that it is thermally cracked without distilling and separating organic volatile components, so it contains light organic components and melts and decomposes. It also acts as a solvent that reduces the viscosity of the oil in the tank. The amount of cracked oil circulated in the melting and cracking tank 10 is preferably 0.1 to 10 times the amount of waste plastic input.

[0024]

As described above, according to the treatment method of the present invention,
Based on the residence time, temperature, circulation amount, etc. in the one-step treatment by the melt decomposition tank or the two-step treatment by the melt decomposition tank and the reforming adjustment tank, the density in the tank is used as a parameter, and preferably the viscosity is used as a parameter together with the density. As a result, the properties of the cracked oil obtained from the melt cracking tank and the reforming adjusting tank can be adjusted to a desired state by controlling the thermal cracking step and the liquefaction promoting step. Specifically, density 30
By setting the viscosity to 0 kg / m ³ or more and the viscosity to 300 c.p or less, it is possible to obtain clean cracked oil having sufficient fluidity, containing no solid foreign matter such as metal or glass, and having an extremely small chlorine content. . Further, the cracked oil obtained by the treatment method of the present invention becomes a solid with good grindability when cooled to room temperature, and therefore can be used as a solid raw material or a solid fuel.

[0025]

EXAMPLES The present invention will be specifically described below with reference to examples. [Examples 1 to 3] The heating temperature, the residence time in the tank, the circulating amount, the density of cracked oil, etc. were controlled to the values shown in Table 1 by the apparatus constitution of the cracking treatment method shown in FIGS. 1, 2 and 3. The waste plastic was pyrolyzed. Waste plastic is PV
A material containing C, PE, PP, PS was used. The viscosity, yield and chlorine residual rate of the recovered cracked oil are shown in Table 1. The relationship between the temperature and the viscosity of the cracked oil is shown in FIG. As shown in the results of Table 1, according to the treatment method of the present invention, the viscosity of 10 c.p ~ is obtained by the one-step treatment (apparatus example of FIG. 1: Example 1).
830 c.p low viscosity cracked oil can be obtained at a high rate of 91.8-96.4%. Further, according to the two-step treatment (apparatus example of FIG. 2: Example 2), a low-viscosity cracked oil having a viscosity of 23 c.p to 530 c.p can be obtained at a high rate of 93.1 to 96.2%. . Furthermore, according to the two-stage treatment in which cracked oil is circulated (apparatus example in FIG. 3: Example 3), an extremely low-viscosity cracked oil having a viscosity of 35 c.p to 75 c.p has a high rate of 94.1 to 95.2%. Can be obtained at Further, the residual chlorine rate of the cracked oil according to the treatment method of the present invention is extremely low.

[Comparative Examples 1 and 2] In the processing constitutions of FIGS. 1 and 3, decomposition treatment of waste plastic is carried out within the processing conditions of the present invention except that the heating temperature of the melting decomposition tank or the reforming adjusting tank is 500 ° C. I went. The results are shown in comparison with Table 1. All of the comparative examples having a high heating temperature were excessively decomposed and the yield was lowered.

[0027]

[Table 1]

[Example 4 and Comparative Example 3] A solid substance (Example 4) made of the cracked oil obtained in Example 1 and a solid substance obtained by thermally decomposing waste plastic using a conventional small rotary kiln.
Regarding (Comparative Example 3), the pulverizability was examined using the pulverization tester shown in FIG. As a test method, a solid substance was put into a rotor speed mill 51 (20000 rpm) inside the tester, the pulverized product was passed through a sieve 52 (80 mesh), the residue was returned, and the pulverization was repeated. The result (residual passage rate with respect to the number of pulverizations) is shown in the graph of FIG. As shown in the figure, the solid matter according to the embodiment of the present invention has good pulverizability, and the residual rate of 20 is obtained by pulverizing about 10 times.
Although it is about wt%, the solid matter of the comparative example has poor pulverizability, and about 80 wt% remains even if the pulverization is performed 12 times. The pencil hardness of the solid of Comparative Example 3 was about 2H, but the pencil hardness of the solid of Example 4 was about 3B or less. This solid is suitable as a solid fuel because it has good pulverizability, and is suitable for various applications.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a device configuration example of one-step processing of the present invention.

FIG. 2 is a conceptual diagram showing a device configuration example of the second floor processing of the present invention.

FIG. 3 is a conceptual diagram showing a configuration example of an apparatus having a two-step processing and a circulation system of the present invention.

FIG. 4 is a graph showing the relationship between cracked oil (spilled oil) temperature and viscosity.

FIG. 5 is a schematic diagram of a grinding tester.

FIG. 6 is a graph showing the results of a grinding test.

[Explanation of symbols]

10-melt decomposition tank, 11-reforming adjustment tank, 12a density meter,
12b-viscosimeter, 13-heating system, 14-liquid transfer pump, 15-filtration part, 16-cooler. 51-rotor speed mill, 52-sieving.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Naoki Teramae             1002-1 Mukaiyama, Naka-machi, Naka-gun, Ibaraki Prefecture             Materials Research Laboratories Naka Research Center             In the center (72) Inventor Kenji Nishimura             1002-1 Mukaiyama, Naka-machi, Naka-gun, Ibaraki Prefecture             Materials Research Laboratories Naka Research Center             In the center (72) Inventor Ryohei Mori             1002-1 Mukaiyama, Naka-machi, Naka-gun, Ibaraki Prefecture             Material Co., Ltd. System Business Center             Within (72) Inventor Katsunori Shinohara             1-3-25 Koishikawa, Bunkyo-ku, Tokyo Koishikawa Univ.             Kuni Building Mitsubishi Materials Corporation System             Within the business center F-term (reference) 4F301 CA09 CA27 CA41 CA53 CA71                       CA72                 4H015 AA17 BA01 BB03 CB01                 4H029 CA15

Claims (8)

[Claims] 1. In a treatment method of thermally decomposing waste plastics in oil, the density of a molten liquid oil produced by thermal decomposition is used as an index to thermally decompose the chlorine contained and volatilize and remove it, while suppressing the formation of organic volatile components. As described above, the method for decomposing waste plastics is characterized by promoting thermal decomposition and recovering decomposed oil without distilling and separating light organic components. 2. A melt-decomposition step of thermally decomposing waste plastic to volatilize and remove chlorine contained therein and to produce a molten liquid oil in which generation of organic volatile matter is suppressed, and to further decompose the molten liquid oil into a decomposed oil. It is a treatment method in which a reforming and adjusting step for improving quality is performed in one step, and the density of the melt oil is 300 kg / m ³ or more to 70 or more.
The light cracked oil is recovered by controlling to 0 kg / m ³ or less.
Disposal of waste plastics. 3. The one-step treatment according to claim 2, wherein the heating temperature is 320 to 420 ° C., the heating time is 30 minutes or less, and the density of the molten oil is controlled to 300 kg / m ³ or more and 700 kg / m ³ or less. Along with this, a method for decomposing waste plastics that controls the viscosity of the molten oil to 300 c.p or less. 4. A treatment method in which a melt cracking step and a reforming adjusting step are carried out in two steps, wherein the density of the molten liquid oil is 400 kg / m ³ or more to 700 kg / m ³ or less in the first step melt cracking step. controlled, decomposition treatment method of waste plastics according to claim 1, the density of the cracked oil in the reforming adjusting process of the second stage 300 kg / m ³ or more ~600kg / m ³ by controlling the following recovering light cracked oil . 5. The two-stage treatment according to claim 4, wherein the heating temperature in the melt decomposition step is 250 to 350 ° C., the heating time is 60 minutes or less, and the density of the molten oil is 400 kg / m ^{3 to} 700 k.
The viscosity is controlled to g / m ³ or less and the viscosity is controlled to 600 c.p or less, the heating temperature in the reforming adjustment step is 350 to 420 ° C., the heating time is 30 minutes or less, and the density of cracked oil is 300 kg / m ³ or more to A method for decomposing waste plastics, controlling the viscosity to 300 kg / m ³ or less and the viscosity to 300 c.p or less. 6. After cooling the cracked oil obtained by the treatment method according to any one of claims 1 to 5, it is maintained at 200 to 300 ° C. to be a liquid raw material or liquid fuel, or cooled to room temperature to be a solid raw material or liquid fuel. A method for decomposing waste plastics used as solid fuels. 7. A melting and decomposing tank for thermally decomposing waste plastic in oil, a reforming adjusting tank for promoting liquefaction by heating a decomposition product generated in the thermal decomposition step, and heating means for controlling each tank to an independent temperature. An apparatus for decomposing waste plastic, comprising: a pipe communicating with each tank, a liquid feeding means, and a means for monitoring the density and viscosity of a mixture of waste plastic and oil. 8. A solid fuel having a pencil hardness of 3B or less, which is obtained by the method according to claim 1.