JP2000219884A - Conversion of plastic waste to fuel oil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently cover fuel oil by using a specific clay mineral as a catalyst and pyrolyzing waste plastics. SOLUTION: A clay mineral classified in 1:1 layer type clay mineral (for example, kaolinite, dickite, nacrite, halloysite, chrysotile, antigorite, pecorite, greenalite, (KAPUOPIRAITO), Al lizardite or the like) is used as a catalyst to pyrolyze plastic waste and recover fuel oil. In addition, 2:1 layer type clay mineral (for example, pyrophillite, minnesotaite, cerolite, allite, sericite, glauconite, chlorite, donbassite, vermiculite, montmorillonite, beidellite, palygorskite or the like) also may be used as a catalyst. According to this process, the pyrolytic temperature of the plastic waste can be lowered from 400-500 deg.C in the conventional process down to about 400 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for obtaining fuel oil by decomposing waste plastic.

[0002]

2. Description of the Related Art Disposal of used waste plastic (hereinafter referred to as waste plastic) is a present-day problem. In the old days, there were only treatment means for burying and disposing in the soil or burning it to recover thermal energy. Needless to say, these treatment means may cause environmental problems. Therefore, instead of this, at present, processing means for thermally decomposing it to obtain an oily product as a decomposition product has been proposed, and in particular, has been attracting attention as an effective processing means for thermoplastic polymers such as polyethylene, polypropylene, and polystyrene. I have.

[0003] In the treatment of waste plastics, it goes without saying that economics must be considered. In the above-mentioned treatment means for thermal decomposition and conversion to oil, fuel oil and the like can be recovered as a decomposition product and recycled, but on the other hand, a large amount of heat energy must be used for thermal decomposition. It is pointed out.

[0004] Therefore, it is desired to reduce the reaction temperature to save thermal energy for the decomposition reaction and to efficiently recover useful decomposition products. Instead of oiling, there has been known a technique of performing thermal decomposition or hydrogenation using a catalyst. As the catalyst, zeolite, silica / alumina, and those obtained by adding metals such as iron and aluminum to them have been known.

[0005]

However, the thermal decomposition treatment using the above-mentioned catalyst is not yet complete from the viewpoint of economy, and waste plastics are produced at a relatively low temperature and in a short time using a less expensive catalyst. There has been a keen need for a technology that can efficiently decompose the oil and obtain fuel oil of high utility value.

The present invention has been made in view of the above-mentioned problems of the prior art, and uses a catalyst for waste plastics, mainly thermoplastic polymers such as polyethylene, polypropylene, and polystyrene, to obtain a conventional catalyst-free decomposition or catalyst. It is an object of the present invention to provide a catalyst that decomposes in a shorter time and with lower thermal energy compared to cracking and recovers it as usable cracked oil (mainly fuel oil usable as fuel) or styrene monomer.

[0007]

That is, the method for converting waste plastics into fuel oil according to the present invention is characterized in that clay minerals are used as catalysts when recovering fuel oil by thermally decomposing waste plastics.

In this case, in the present invention, in addition to using natural clay minerals as a catalyst as they are, those obtained by treating them with an acid to make them acidic, or utilizing the cation exchange property of the clay minerals, Exchange-modified with ions of
Or clay minerals added with metals and metal oxides such as nickel, zinc, copper and chromium oxide, or zirconia, copper, titania, vanadia, etc. inserted between layers based on the layered structure of clay minerals, or these Also disclosed is the use of a combination which has improved activity.

It is also disclosed that the pyrolysis is performed in a stream of nitrogen or a mixed gas of nitrogen and hydrogen.

In the method for converting waste plastics into fuel oil according to the present invention having the above-mentioned structure, the waste plastics are discharged at a temperature of about 400 ° C. which is lower than the decomposition reaction temperature of 400 ° C. to 500 ° C. when a conventional catalyst is used. An effect is obtained that enables the plastic to be decomposed, and an effect that a fuel oil having high utility value can be obtained.

[0011]

DETAILED DESCRIPTION OF THE INVENTION (Clay Mineral) First, a natural clay mineral will be outlined as follows.

The composition of a naturally occurring clay mineral is mainly composed of silica (SiO ₂ ), alumina (Al ₂ O ₃ ) and water, iron oxide, magnesium oxide, calcium oxide,
Contains oxides such as sodium and potassium. The component containing the most is silica, and silica forms a tetrahedron of SiO ₄ in which four O ²⁻ are surrounded by Si ⁴⁺ (oxygen ions are coordinated at each vertex centered on Si). And this S
A tetrahedral sheet composed of i-O bonds is two-dimensionally continuous and connected in a net-like fashion to form one tetrahedral sheet. In addition, there is a network connection of octahedrons composed of Al—O bonds combined with this tetrahedral sheet (oxygen ions are coordinated at each vertex of the octahedron with Al as the center). That is, it is an octahedral sheet.

Clay minerals can be broadly classified into 1: 1 clay in which both sheets are combined in a ratio of 1: 1 and 2: 1 clay in which both sheets are combined in a ratio of 2: 1. Kaolinite, halloysite, serpentine and the like are known as 1: 1 clay. There are many types of 2: 1 clay such as mica, montmorillonite, terra alba (montmorillonite-beiderite type fine clay mainly composed of smectite), saponite, sepiolite, and palygorskite.
A mixture of several kinds of the above clay minerals exists in nature. The details are summarized in the table below.

(Classification of Clay Minerals) 1: 1 clay clay kaolinite, deckite, nacrite, halloysite, chrysotile, antigolite,
Pecolite, Greenerite, Capiopyrite, Al lizardite 2: 1 clay clay Pyrophyllite, Minnesotaite,
Kerolite, alite, sericite, chlorite, chlorite,
Donbasite, vermiculite, montmorillonite,
Hyderite, nontronite, saponite, hectorite, sepiolite, palygorskite, and other mixed layer minerals include a mixture of 1: 1 clay and 2: 1 clay. Further, there are allophane and imogolite as amorphous or low crystalline minerals.

In the clay mineral, as described above:
One layer or 2: 1 layer is a main constituent layer to form a layered material. The central element of the tetrahedron or octahedron is F
When the layer is replaced by e and Mg ions, the layer itself has a negative charge. Therefore, cations of Na, K, and Ca enter between the layers in order to supplement the charge, and become exchangeable ions. Therefore, a novel clay mineral in which those cations are ion-exchanged with other cations can be synthesized. Alternatively, a compound in which another compound is inserted between layers (referred to as intercalation) can also be synthesized.

(Preparation of Catalyst) Next, a method of preparing a catalyst for carrying out the present invention will be described. As explained in the section on clay minerals, natural clay minerals often have alkali and alkaline earth metals incorporated between layers. Therefore, in order to integrate these ions into single ions, a clay mineral is added to an aqueous solution of a water-soluble alkali and an alkaline earth metal, and the mixture is stirred at room temperature for about half a day for ion exchange. After the exchange, wash well with water until the anions disappear, and dry at 100 ° C. In this way, a clay in which only an alkali metal or only an alkaline earth metal is inserted between layers can be prepared. For example, a clay mineral in which ions between layers are replaced with protons (H ⁺ ) is prepared as follows.

The clay mineral is made up of 5 to 20% (vol%, hereinafter the same) aqueous sulfuric acid or aqueous hydrochloric acid, or
10% nitric acid aqueous solution, or 10-20% acetic acid aqueous solution,
Alternatively, it is placed in an aqueous phosphoric acid solution and stirred at room temperature for 1 to 2 hours. The clay mineral thus treated is washed with water,
And dried to obtain a catalyst. The solution prepared by inserting protons between the layers prepared above was placed in an aqueous solution of a salt of a rare earth element such as aluminum and lanthanum (1-2 mol / l), stirred at room temperature for several hours, and exchanged with ions. create. Wash thoroughly with water after replacement 1
It is dried at 00 ° C. to obtain a catalyst. A clay mineral to which nickel, copper, zinc, chromium, etc. are added is prepared as follows.

The clay mineral treated as above is placed in an aqueous solution of nitrate such as nickel, copper, zinc or the like, chloride or chromium trioxide, left to stand at room temperature for 1 day after stirring, filtered, washed with water and dried at 100 ° C. . Then, it is calcined at 500 ° C. for several hours under air flow. It is used as a catalyst as it is, or in the case of a reaction in a hydrogen atmosphere, is reduced in advance at 500 ° C. for several hours under a hydrogen flow before the reaction. Nickel, copper, zinc,
The addition amount of chromium is 2 to 20% by weight as a metal. A clay mineral obtained by intercalating titania, copper, zirconia, vanadium oxide and the like is prepared as follows.

An aqueous acetic acid solution containing 80% by weight of titanium tetraisopropoxide or copper ethoxide or zirconium oxychloride or methyl orthovanadium is mixed with a suspension consisting of clay and water. The above compound was inserted between the layers. After washing with water, it is dried at 100 ° C. and calcined in air at 300 ° C. for 2 to 3 hours to obtain a catalyst.

[0020]

EXAMPLES Examples of the method for converting waste plastics into fuel oil according to the present invention will be described below based on experimental results.

FIG. 1 and FIG. 2 are views showing a reaction apparatus used in the experiment, and both are mainly composed of glass tubes. The reaction apparatus 1 shown in FIG.
And a trap T2 of liquid nitrogen / toluene refrigerant for collecting products, a flow meter F2 for measuring generated gas, and a Tedlar bag TB for collecting gas products. In the figure, reference symbol C denotes a plastic and a catalyst, E denotes an electric furnace for heating the reaction tube R, TC denotes a thermocouple for measuring temperature, and F1 denotes a flow meter for measuring gas to be supplied.

In the reactor 2 shown in FIG. 2, a reaction tube B for hydrogenation is connected in addition to a reaction tube A for decomposing waste plastic, and a trap for collecting products is also cooled with ice water. A trap T1 and a trap T2 for liquid nitrogen / toluene refrigerant are provided. In the figure, C1 is a plastic decomposition catalyst, C2 is a hydrogenation catalyst or a skeletal isomerization catalyst, F
2 is a flow meter for measuring product gas, TB is a Tedlar bag for collecting gas products, E1 is an electric furnace for heating the reaction tube A, E2 is an electric furnace for heating the reaction tube B, T
C is a thermocouple for measuring temperature, F1 is a flow meter for measuring gas to be supplied, and K is a cock for supplying gas. In the figure, reference symbol S denotes a plastic feeder driven by a motor M, and P denotes a plastic inlet.

When a reaction is carried out in a hydrogen atmosphere by introducing hydrogen, in a reaction apparatus 1 shown in FIG. 1, a plastic cracking catalyst and a hydrogenation catalyst are mixed, and the catalyst is reduced with hydrogen at a predetermined temperature in advance, and then, Plastic disassembly was performed.
When hydrogenation is carried out using another reactor, a reactor 2 shown in FIG. 2 is used, a plastic cracking catalyst C1 is placed in a reaction tube A, and a hydrogenation catalyst C2 is placed in a reaction tube B, respectively.
Plastic decomposition and hydrogenation reactions were performed at different temperatures. That is, only nitrogen gas was allowed to flow through the reaction tube A at a predetermined temperature (400 to 500 ° C.), and only hydrogen was allowed to flow through the reaction tube B at a predetermined temperature (400 to 500 ° C.) for 3 to 5 hours for reduction. After the reduction, while flowing hydrogen, the reaction tube B
The temperature was lowered to 250 ° C., the reaction tube A and the reaction tube B were connected, and waste plastic was slowly supplied by a plastic feeder S (made of metal) to cause a reaction. In each case of performing the hydrogenation reaction, a mixed gas of hydrogen and nitrogen was used.

In each of the reactions, the reaction tube was maintained for 0.5 hour after the white smoke accompanying the decomposition of the plastic did not come out of the reaction tube, and then the reaction was terminated. Analysis of liquid and gaseous products was by gas chromatography. The weight of the liquid and gaseous products was determined by measuring the weight of the liquid collected in the trap and of the gaseous products collected in the Tedlar bag. When the weight of the gaseous product was measured, it was obtained by correcting the result of analysis by gas chromatography. The weight of the residue in the reaction tube was determined by directly measuring the weight of the reaction tube and subtracting the weight of the reaction tube before the reaction.

The separation and quantitative analysis of olefins, paraffins, and aromatic compounds is described in JIS K2536 (1973) and K2835 (19
73) Performed by the method described.

(Comparative Example) Polyethylene (hereinafter abbreviated as PE) was added without a catalyst to 400 before a decomposition reaction using a catalyst was carried out.
When it was decomposed at 400 ° C. and 450 ° C., in the case of PE, only a waxy product was obtained, and no liquid oil was obtained.
Similar reaction behavior was observed in the case of decomposition of polypropylene (hereinafter abbreviated as PP) at 400 ° C. On the other hand, by using an appropriate catalyst, 400
Liquid cracked oil is obtained by a cracking reaction at a relatively low temperature of 450C or 450C.

Example 1 In the reactor 1 shown in FIG. 1, only the reaction tube R was used, and the waste plastic was decomposed under the following reaction conditions.

Reaction conditions: Catalyst amount 1 g PE (hard) and PP 20 g Reaction temperature 400 ° C. to 500 ° C. Nitrogen flow rate 60 ml / min (standard condition) Nitrogen is preheated and heated to 350 ° C. or more and put in a reaction tube. . Therefore, the nitrogen flow rate is 2.3 to 2.8 times and enters the reaction tube. PE and PP are supplied from the feeder at 0.5-0.
It was fed continuously at a rate of 7 g / min. When supplying a mixed gas of hydrogen and nitrogen, the mixing ratio of hydrogen to nitrogen is 1 to
2: 1 to 5 for a total flow rate of 60 to 100 ml / min.

Montmorillonite (natural 2: 1 clay)
Was used as a catalyst to decompose PE at 400 ° C. Nitrogen gas was flowed to remove gaseous products. The liquid product was 53% of the feed fed, the gaseous product 5% and the balance was residual oil (mainly wax) and carbon. The liquid product was a hydrocarbon of from 4 to 16 to 17 carbon atoms, consisting of paraffins and olefins of 64:36. Almost no aromatic compounds are contained. The gaseous product was a hydrocarbon having from 1 to 4 carbon atoms and consisted of paraffins and olefins. The paraffin to olefin ratio was almost identical to that of the liquid product. The generation of hydrogen was also recognized, and the amount was 2% by volume of the total gas amount.

(Example 2) The same reaction apparatus as in Example 1
Under reaction conditions, palygorskite (natural 2: 1 layer clay)
Was used as a catalyst to decompose PE at 450 ° C. Liquid generation
68% gaseous products, 6% gaseous products, the balance being residual oil and carbon
there were. C5 hydrocarbons in the liquid product, C_FiveIs
13%, below C₆7%, C₇4%, C₈8%, C₉34
%, C_Ten~ 33%, hydrogen 2% of gaseous product, C₁4
1%, C_Two30%, C_Three16%, C _FourIt was 11%.

(Example 3) The same reactor as in Example 1
Under the reaction conditions, using montmorillonite treated with acid as a catalyst,
The PE was decomposed at 400 ° C. 68% liquid product, 7% gaseous product, balance oil and carbon. 13% of C ₅ , 7% of C ₆ , 8% of C _7, 8% of C ₈ ,
C ₉ 30%, C ₁₀ -33%, hydrogen 3 among gaseous products
%, C ₁ 38%, C ₂ 29%, C ₃ 19%, C ₄ 11%
It became.

(Example 4) The same reactor as in Example 1
Under the reaction conditions, PE was decomposed at 450 ° C. using an acid-treated palygorskite as a catalyst. 81 liquid products
%, 10% gaseous products, the balance being residual oil and carbon.

(Example 5) The same reaction apparatus as in Example 1
Under the reaction conditions, PP was decomposed at 450 ° C. using acid-treated sepiolite (natural 2: 1 layer clay) as a catalyst. The liquid product was 85%, the gaseous product was 8%, the balance was residual oil and carbon.

(Example 6) The same reactor as in Example 1
Under the reaction conditions, PE was decomposed at 450 ° C. using a catalyst in which zinc oxide was added to palygorskite. The liquid product was 87%, the gaseous product was 11%, and the remainder was residual oil and carbon, producing less residual oil and carbon. A paraffin to olefin volume ratio of 74:26 yielded a paraffin-rich oil.

(Example 7) The same reactor as in Example 1
Under the reaction conditions, PP was decomposed at 400 ° C. using a catalyst in which cations in palygorskite were exchanged with lanthanum ions. The liquid product was 65%, the gaseous product was 5%, the balance was residual oil and carbon.

Example 8 The same reaction apparatus as in Example 1
Under the reaction conditions, PE was decomposed at 500 ° C. using a catalyst obtained by adding chromium oxide to palygorskite. The liquid product was 87%, the gaseous product was 10%, the balance was residual oil and carbon.

(Example 9) The same reactor as in Example 1
Under the reaction conditions, PE was decomposed at 450 ° C. using a catalyst in which copper (oxide) was added to palygorskite. The liquid product was 90%, the gaseous product was 8%, the balance was residual oil and carbon.

In the above embodiments, the use of an acid-treated clay mineral and a catalyst obtained by adding a metal oxide (zinc oxide, chromium oxide, copper oxide, etc.) to the clay mineral were the same. Both liquid and gaseous product amounts were higher than the mineral catalyst. That is, it is understood that the clay mineral to which the acid treatment and the metal oxide are added is more effective as a catalyst for decomposing the waste plastic. In addition, the function as a catalyst did not change even when treated with either a strong acid or a weak acid.

Example 10 In the reactor 2 shown in FIG.
Using the reaction tube A and the reaction tube B, waste plastic was decomposed under the following reaction conditions.

Reaction conditions: amount of catalyst 1 g of reaction tube A 0.5 g of reaction tube B Reaction temperature Reaction tube A 400 to 450 ° C. Reaction tube B 200 to 300 ° C. Nitrogen flow rate 30 to 60 ml / min (standard condition) Hydrogen flow rate 30 to 45 ml / min (standard state) PE (hard) and 20 g of PP A mixture of acid-treated palygorskite and zinc oxide was added as a catalyst, and PE was decomposed at 450 ° C. Only nitrogen was supplied as a carrier gas. 88% liquid product, 7% gaseous product, 5% residual oil and carbon
%. The paraffin to olefin ratio is 76:24
Became.

Example 11 Hydrogen was added to the reaction system of Example 10 and reacted under the same reaction conditions. The mixed catalyst was previously subjected to hydrogen reduction at 450 ° C. for 3 hours. 88% liquid product, 9% gaseous product, 3% residual oil and carbon
%. Since the unreacted hydrogen is contained in the gaseous product, the value of the gaseous product amount is expected to be slightly smaller. No waxy substance was found in the residual oil, and it is considered that only carbon was present, but the amount thereof was reduced in the presence of hydrogen. The paraffin to olefin ratio was 98: 2 and the paraffin ratio was increased.

Example 12 A mixture of an acid-treated palygorskite and a nickel oxide / silica catalyst was placed in a reaction tube under the same reactor and reaction conditions as in Example 10, and PP was decomposed at 450 ° C. . Catalyst 4 in advance
Hydrogen reduction was performed at 50 ° C. for 3 hours. 83 liquid products
%, Gaseous products 15%, residual oil and carbon 2%. The product consisted of almost 100% paraffin.
The product oil fraction of about C ₁₁ from C ₆ to produce many gasoline component (branched paraffinic hydrocarbons).

The decomposition of PE and PP by such a method may be due to the accelerated decomposition by the hydrogenation catalyst, possibly resulting in a decrease in the amount of the liquid product and an increase in the amount of the gas product. This method is suitable for producing many gaseous products having a low molecular weight.

(Example 13) Under the same reaction apparatus and reaction conditions as in Example 10, a reaction tube A was filled with an acid-treated palygorskite, and a reaction tube B was filled with a catalyst obtained by adding zinc oxide to palygorskite. Hydrogen at 450 ° C, 3 ~
The solution was reduced by flowing for 5 hours. Nitrogen gas was supplied to the reaction tube A, and hydrogen was supplied to the reaction tube B to decompose PE at 450 ° C. The temperature of the reaction tube B was kept at 300 ° C. 88% liquid product,
The gaseous product was 11% and the residual oil and carbon were 1%.

(Example 14) Under the same reactor and reaction conditions as in Example 10, the reaction tube A was filled with an acid-treated palygorskite, and the reaction tube B was filled with a catalyst obtained by adding nickel oxide to palygorskite. 450 ° C hydrogen
The solution was reduced by flowing for 3 to 5 hours. Nitrogen gas was supplied to the reaction tube A, and hydrogen was supplied to the reaction tube B to decompose PE at 450 ° C.
The temperature of the reaction tube B was kept at 300 ° C. The liquid product is 91
%, Gaseous products 8%, residual oil and carbon 1%. Composition of liquid product _{C 5 9%, C 6 11} %,
_{C 7 11%, C 8 9} %, C 9 8%, C 10 8%, C 11 ~4
It was 4%. Also, 29% of C ₁ , C ₂
63%, C ₃ 4% and C ₄ 4%.

(Example 15) Under the same reaction apparatus and reaction conditions as in Example 10, acid-treated sepiolite was filled in reaction tube A, and nickel oxide-calcia catalyst supported on silica was filled in reaction tube B. At 450 ° C. for 3 to 5 hours for reduction. The reaction temperature in the reaction tube A was 500 ° C., and the reaction temperature in the reaction tube B was 200 ° C. The liquid product is 90
%, Gaseous products 10%, residual oil and carbon 0%. The paraffin to olefin ratio in the liquid product is approximately 1
It became 0, and became 100% paraffin-based fuel oil.

When polyethylene and polypropylene are subjected to catalytic cracking by the above method using the reactor 1 shown in FIG. 1 and the reactor 2 shown in FIG. 2, the obtained oil is mainly light oil (linear hydrocarbon). In the latter case, a gasoline fraction (a branched hydrocarbon) is also included. Increasing the gasoline fraction in the oil obtained by catalytic cracking of polyethylene and the gasoline fraction in the catalytic cracking oil of polypropylene will increase the added value as a fuel.

(Example 16) 0.5 to 1 g of a catalyst having a skeletal isomerization action was placed in a reaction tube B of the reactor shown in FIG.
The reaction was performed at 150 to 250 ° C. while flowing a mixed gas of nitrogen and hydrogen. An acid-treated palygorskite catalyst was placed in the reaction tube A, and polyethylene was decomposed at 400 to 450 ° C. A gasoline fraction of 30 to 45% was found in the obtained oil.

[0049]

According to the fuel oil conversion method of the present invention having the above-mentioned structure, a natural clay mineral is used as a catalyst as it is, and the clay mineral is treated with an acid to make it acidic or a clay mineral. Exchange-modified with various ions utilizing the cation exchange property of the clay mineral or nickel, zinc,
Copper, chromium oxide and other metals and metal oxides added, or zirconia, copper, titania, vanadia, etc. inserted between layers based on the layered structure of clay minerals, or those with improved activity by combining these By using, the thermal decomposition of waste plastic can be performed in a shorter time than conventional non-catalytic decomposition with less energy consumption, and a highly useful fuel oil can be obtained.

On the other hand, since clay minerals are inexpensive as compared with conventionally known catalysts, it becomes possible to convert fuel oil into fuel oil at low cost in combination with the above-mentioned energy saving effect. Since it is oil, it is highly recyclable, and is a useful practical invention that can be a guide to the practical use of waste plastic treatment means that is thermally decomposed into oil.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a reactor for carrying out the present invention.

FIG. 2 is a sectional view of a reaction apparatus according to another embodiment of the present invention;

[Explanation of symbols]

 1 reactor 2 reactor

 ──────────────────────────────────────────────────続 き Continuing on the front page F term (reference) 4F301 AA13 AA14 AA15 AD02 BA21 BE35 BE44 BF20 BF31 BG50 4G069 AA01 AA08 AA15 BA10A BA10B BB02A BB04A BB10A BB10B BB14B38BBC BCBC BC BC BC EC22Y FA01 FB30 FB77 FB79 4H029 CA01 CA09

Claims (9)

[Claims] 1. A method for converting waste plastic to fuel oil, comprising using, as a catalyst, a clay mineral classified as a 1: 1 clay mineral when recovering fuel oil by pyrolyzing the waste plastic. 2. A method for converting waste plastics to fuel oil, comprising using a clay mineral classified as a 2: 1 clay mineral as a catalyst in recovering the fuel oil by pyrolyzing the waste plastics. 3. The process for converting waste plastics into fuel oil according to claim 1, wherein a clay mineral treated with an acid to impart acidity is used as a catalyst. 4. The method according to claim 3, wherein a strong acid such as sulfuric acid, nitric acid or hydrochloric acid is used as an acid and acetic acid or phosphoric acid is used as a weak acid. 5. The method according to claim 1, wherein a cation present between the layers of the clay mineral is exchanged with another cation for use as a catalyst. 6. The method according to claim 5, wherein the cation to be exchanged is an ion of a rare earth element. 7. The waste plastic according to claim 3, wherein a clay mineral obtained by adding a metal or a metal oxide to a clay mineral having acidity, or a mixture of a clay mineral having acidity and a clay mineral having acidity is used as a catalyst. Fuel oil conversion method. 8. The method according to claim 7, wherein the metal is nickel, copper or zinc, and the metal oxide is chromium oxide. 9. A method for converting waste plastics into a fuel oil by recovering oil rich in paraffinic hydrocarbons by performing the fuel oil conversion method according to claim 3 or 4 or 7 or 8 in a hydrogen atmosphere.