Method and apparatus for recovering oil from oil-containing waste liquid

Abstract

An oil recovery method for recovering oil from an oil-containing waste liquid, characterized by: an extraction step of transferring oil in the oil-containing waste liquid to a solvent by subjecting the oil-containing waste liquid to an extraction treatment with the solvent; a separation step of separating the solvent containing the oil extracted from the waste liquid; and an oil recovery step of separating and recovering oil from the solvent containing the oil separated, the solvent being n-hexane, the oil concentration in the oil-containing waste liquid supplied to the extraction step being 1000 to 5000 mg / L, the volume ratio of the solvent to the oil-containing waste liquid (solvent / waste liquid ratio) being 0.01 to 0.05, and the oil concentration in the solvent containing the oil separated in the separation step being 4% by mass or more. An oil recovery method for recovering oil from an oil-containing waste liquid, which can efficiently separate and recover oil from an oil-containing waste liquid by a solvent extraction method.

Description

Technical Field

[0001] This invention relates to an oil recovery method and apparatus for recovering oil from oil-containing waste liquid, and particularly to an oil recovery method and apparatus for recovering oil from oil-containing waste liquid using solvent extraction. Background Technology

[0002] Water treatment for separating and recovering oil from oil-containing wastewater is typically based on separation using a separation weir (oil separator) that allows oil to float in water, or flotation treatment using microbubbles or microbubbles and reagents (e.g., Patent Document 1). Furthermore, methods can also be considered for solvent extraction using solvents and mixed clarifying agents, distributing the oil to the solvent side, and then recovering the oil by allowing the solvent to evaporate.

[0003] The oil separated in this way contains a significant amount of energy, which has practical value. Furthermore, using the separated and recovered oil as an energy source is also related to waste reduction. Therefore, waste energy recovery, including power generation from waste containing oil, is expected to receive increasing attention in the future in order to form a circular society.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent document 1: Japanese Patent Application Publication No. 2021-171748. Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] In conventional oil-water separation technology, during flotation treatment, components other than oil are mixed in with the components separated by flotation, such as suspended solids (SS) components other than oil contained in the wastewater, components from chemicals, and water. Therefore, in order to separate only the oil for use as fuel, it is necessary to remove these impurities through refining processes such as centrifugation.

[0009] In the separation and recovery of oils through solvent extraction, since the solvent used for extraction is non-polar, impurities such as hydrophilic substances like ions are not introduced, making it a suitable method for utilizing the recovered oil. However, there are almost no examples of applying solvent extraction to wastewater treatment, and currently, there is a lack of insight into the treatment conditions for such applications.

[0010] The objective of this invention is to provide an oil recovery method and apparatus for recovering oil from oil-containing waste liquid, which can efficiently separate and recover oil from oil-containing waste liquid by solvent extraction.

[0011] Methods for solving problems

[0012] In order to solve the above-mentioned problems, the inventors have repeatedly conducted research and found that by controlling the oil concentration of the oil-containing waste liquid supplied to the solvent for extraction and the volume ratio of the solvent used for extraction to the oil-containing waste liquid to an appropriate value, oil can be efficiently separated and recovered from the oil-containing waste liquid by solvent extraction.

[0013] The main points of this invention are as follows.

[0014] [1] A method for recovering oil from oil-containing waste liquid, characterized in that it includes: The extraction process involves using a solvent to extract oil-containing waste liquid, thereby transferring the oil in the waste liquid to the solvent. The separation process separates the extracted solvent containing the oil from the waste liquid; and The oil recovery process separates and recovers the oil from the solvent containing the separated oil. The solvent is n-hexane. The oil concentration in the oil-containing wastewater supplied to the extraction process is 1000–5000 mg / L. The volume ratio of the solvent to the oil-containing waste liquid (solvent / waste liquid ratio) is 0.01 to 0.05, and The concentration of oil in the solvent containing the oil separated in the separation process is 4% by mass or more.

[0015] [2] The oil recovery method for recovering oil from oil-containing waste liquid according to [1], wherein in the oil recovery step, the oil is separated from the solvent containing the oil by heating distillation and the solvent is recovered, and the recovered solvent is reused in the extraction step.

[0016] [3] The oil recovery method for recovering oil from oil-containing waste liquid according to [2], wherein, in the separation based on heating distillation, the heat of the separated solvent or the oil is used to heat the solvent containing the oil using a heat exchanger or a heat pump.

[0017] [4] An oil recovery method for recovering oil from oil-containing waste liquid according to any one of [1] to [3], wherein, before the extraction step, the method further includes a concentration step of concentrating the oil-containing waste liquid or a dilution step of diluting the oil-containing waste liquid.

[0018] [5] An oil recovery method for recovering oil from oil-containing waste liquid according to any one of [1] to [4], wherein the oil-containing waste liquid is any one of the following: waste liquid from automobile and other parts manufacturing plants containing mineral oil, water-based cutting waste liquid, waste liquid from paper or pulp mills containing turpentine, waste liquid from oil mills containing animal or vegetable oils, food and beverage factories or food service businesses.

[0019] [6] An oil recovery device for recovering oil from oil-containing waste liquid, characterized in that it comprises: An extraction unit that transfers oil from an oil-containing waste liquid to a solvent by extracting the waste liquid with a solvent; A separation unit that separates the extracted oil-containing solvent from the waste liquid; and An oil recovery unit separates and recovers oil from the solvent containing the separated oil. The solvent is n-hexane. The oil concentration in the oil-containing waste liquid supplied to the extraction unit is 1000-5000 mg / L, the volume ratio of the solvent to the oil-containing waste liquid (solvent / waste liquid ratio) is 0.01-0.05, and the oil concentration in the solvent containing the oil separated by the separation unit is 4% by mass or more.

[0020] [7] The apparatus for recovering oil from oil-containing waste liquid according to [6], wherein the oil recovery unit is a unit that separates the oil from the solvent containing the oil by heating and distillation, and the oil recovery apparatus has a liquid supply unit that supplies the solvent obtained by separating the oil from the oil in the oil recovery unit to the extraction unit.

[0021] [8] An oil recovery device for recovering oil from oil-containing waste liquid according to [7] has a heat exchanger or heat pump for heating the solvent containing the oil using the heat of the solvent or the oil that has passed through the heated distillation separation.

[0022] [9] An oil recovery apparatus for recovering oil from oil-containing waste liquid according to any one of [6] to [8], wherein a concentration unit or a dilution unit is further provided in front of the extraction unit, the concentration unit concentrating the oil-containing waste liquid and the dilution unit diluting the oil-containing waste liquid.

[0023]

[10] An oil recovery apparatus for recovering oil from oil-containing waste liquid according to any one of [6] to [9], wherein the oil-containing waste liquid is any one of the following: waste liquid from automobile or other parts manufacturing plants containing mineral oil, water-based cutting waste liquid, waste liquid from paper or pulp mills containing turpentine, waste liquid from oil mills containing animal or vegetable oils, food and beverage factories or food service businesses.

[0024] Invention Effects

[0025] According to the present invention, oil can be efficiently separated and recovered from oil-containing waste liquid by solvent extraction.

[0026] According to the present invention, the amount of oil in oily wastewater that hinders drainage treatment can be significantly reduced, and high-purity oil can be recovered and efficiently reused as an energy source in various fields. Furthermore, according to the present invention, the amount of waste such as sludge containing large amounts of oil that has been discarded to date can be significantly reduced.

[0027] Therefore, the present invention has great industrial value. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating an oil recovery method and apparatus for recovering oil from oil-containing waste liquid according to an embodiment of the present invention. Detailed Implementation

[0029] Before describing the embodiments of the oil recovery method and oil recovery apparatus for recovering oil from oil-containing waste liquid of the present invention, the mechanism of solvent extraction in the present invention will be explained first.

[0030] In oil-containing waste liquids, where the oil is typically separated from the solvent by evaporation of the solvent containing the oil during the oil recovery process after solvent extraction, the oil contains enough energy to supply the energy required for the evaporation.

[0031] On the other hand, in a system that uses n-hexane as an extraction solvent to extract oil from oil-containing waste liquid through solvent extraction, separates and recovers the residual oil by evaporating the n-hexane used in the extraction, and recovers the evaporated n-hexane by natural cooling (air cooling) and reuses it for extraction, and uses the recovered oil as fuel (energy source), it is considered that the energy used to evaporate n-hexane accounts for the majority of the energy (of which, in addition to the energy consumed by evaporation as a solvent, there are pump power for continuous processing, solvent loss, etc.).

[0032] Therefore, from the perspective of energy balance, the following equation must hold.

[0033] Energy consumed (heating hexane containing oil to its boiling point, further evaporating all the hexane) < Energy gained (heat obtained from burning the recovered oil)

[0034] Based on the basic calorific information of n-hexane and oil shown in Table 1 below, and assuming that the energy consumed is set to electricity (efficiency 100%) and the energy obtained is also set to electricity (efficiency of about 25% for generating electricity from oil), as shown in Table 2 below, the oil concentration in the extracted n-hexane containing oil needs to be above 4% by mass.

[0035] [Table 1]

[0036] Table 1. Basic information on various heat sources

[0037] [Table 2]

[0038] Table 2 Energy Balance

[0039] On the other hand, if a large amount of oil is dissolved in the solvent during solvent extraction, the specific gravity and viscosity of the resulting mixture of solvent and oil will increase. As a result, solid components and water are easily mixed into the mixture, making it difficult to extract and separate pure oil from the oil-containing waste liquid.

[0040] Therefore, limiting the range of oil concentration in the solvent from which oil is extracted is effective in efficiently extracting oil from oil-containing waste liquid to the solvent side.

[0041] Based on the above, by performing pretreatments such as concentration and dilution as needed, the oil concentration range in the oil-containing waste liquid supplied for extraction is set within a specified range, and the amount of solvent used is limited. This ensures that the oil concentration in the extracted oil-containing solvent is above a specified value, thereby improving the separation of solvent and water and minimizing solvent loss to the water side. Furthermore, by determining an appropriate size for the oil extraction equipment corresponding to the amount of oil to be extracted, efficient extraction of oil from the solvent can be achieved.

[0042] Based on this mechanism, according to the present invention, by setting the oil concentration supplied to the extracted oil-containing waste liquid and the volume ratio of the solvent at that oil concentration to the oil-containing waste liquid (hereinafter, sometimes simply referred to as "solvent / waste liquid ratio") within a specified range, and by setting the oil concentration in the extracted oil-containing solvent to a specified value or above, efficient solvent extraction can be performed.

[0043] Furthermore, the separated and recovered oil can be efficiently and effectively utilized as an energy source.

[0044] In this invention, the oil concentration in the oil-containing waste liquid supplied to the solvent for extraction is set to 1000-5000 mg / L.

[0045] When the oil concentration in the waste liquid containing oil is less than 1000 mg / L, the recovered oil is insufficient as an energy source for the heating distillation of the extracted solvent (n-hexane).

[0046] On the other hand, if the oil concentration exceeds 5000 mg / L, as mentioned above, due to the increase in the specific gravity and viscosity of the mixture of solvent and oil from which the oil was extracted, it is impossible to recover the high-purity oil, and the separation of the extracted oil becomes difficult, further increasing the loss of solvent.

[0047] From this perspective, the oil concentration in the oil-containing waste liquid supplied for solvent extraction is preferably 1000–3000 mg / L, more preferably 1200–2500 mg / L.

[0048] In addition, in this invention, the solvent (n-hexane) and the oil-containing waste liquid are supplied to the solvent extractor at a solvent / waste liquid ratio of 0.01 to 0.05.

[0049] When the solvent / waste liquid ratio is less than 0.01, the amount of solvent used for extraction is too small to achieve efficient oil extraction, and the oil remains on the side containing the oil waste liquid.

[0050] On the other hand, when the solvent / waste liquid ratio exceeds 0.05, the amount of solvent used for extraction is excessive and uneconomical, and the oil concentration in n-hexane also becomes low, which is not preferable from the viewpoint of large-scale extraction equipment.

[0051] From this perspective, the solvent / waste liquid ratio is preferably 0.02 to 0.04, and more preferably 0.025 to 0.03.

[0052] In this invention, the concentration of oil in the solvent after extracting oil from oil-containing waste liquid is set to be 4% by mass or more.

[0053] Therefore, when n-hexane is used for extraction according to the present invention, from the viewpoints of oil separation efficiency and thermal energy efficiency in solvent regeneration, the oil concentration of the extracted n-hexane is 4% by mass or more, preferably 4 to 12% by mass, and more preferably 4 to 7% by mass.

[0054] The following is for reference Figure 1 This invention describes an oil recovery method and apparatus for recovering oil from oil-containing wastewater by controlling the oil concentration and solvent / wastewater ratio of such oil-containing wastewater.

[0055] However, the present invention is not limited to any of the following methods, and various modifications can be made within the scope of the spirit of the present invention.

[0056] Figure 1 This is a flowchart illustrating an oil recovery method and apparatus for recovering oil from oil-containing waste liquid according to an embodiment of the present invention.

[0057] The oil-containing wastewater that is the object of treatment in this invention is a water-based wastewater with a water content of 85% or more by mass. Examples include thick oil-containing wastewater generated from petroleum processes, steel rolling processes, paper or pulp manufacturing, metal cutting processes, food manufacturing, leather manufacturing, kitchens, and sewage systems. Specifically, examples include water-based cutting wastewater; turpentine-containing wastewater discharged from paper and pulp mills; and food wastewater, but these are not limited to these.

[0058] The oil in the oily waste liquid can be any of mineral oil, animal or vegetable oil.

[0059] The oil concentration of these oil-containing waste liquids is typically around 0.05–1% by mass (500–10000 mg / L). Therefore, when the oil concentration of the oil-containing waste liquid treated in this invention deviates from the range of 1000–5000 mg / L, a concentration step or a dilution step is set up before the extraction step to concentrate the oil-containing waste liquid.

[0060] More specifically, examples of oil-containing waste liquids include waste liquids from automobile and other parts manufacturing plants containing mineral oils, waste liquids from pulp mills containing turpentine, waste liquids from oil refining plants containing animal and vegetable oils, and waste liquids from food and beverage factories or food service businesses.

[0061] Oil-containing waste liquid can be organic waste liquid that has been extracted into a non-polar solvent.

[0062] exist Figure 1 In this process, the oil-containing wastewater is introduced into the membrane separation unit 2 via pipe 1, where it is concentrated to the aforementioned oil concentration through membrane separation. A portion of the permeate water that has passed through the membrane is returned to the membrane separation unit 2 via pipe 4A. The remaining permeate water is sent to a wastewater treatment facility (not shown) via pipe 4B for treatment.

[0063] The concentrated water and extraction solvent of the membrane separation device 2 are supplied to the mixing section 6a of the extraction device 6 via pipes 5 and 7, respectively, at the aforementioned solvent / waste liquid ratio.

[0064] Concentrated water and extraction solvent supplied to mixing section 6a are mixed in mixing section 6a, causing the oil in the concentrated water to enter the solvent. That is, the oil in the concentrated water dissolves in the extraction solvent. The solvent containing the dissolved oil and the concentrated water (residue) after oil extraction are naturally separated (by gravity) in separation section 6b, and the solvent containing the dissolved oil is supplied to the hot and cold source flow path of heat exchanger 9 via pipe 8.

[0065] The solvent containing the oil undergoes heat exchange with the solvent vapor flowing in the cooled fluid path, and after being heated, it is introduced into the distillation column 11 from the piping 10.

[0066] In distillation column 11, solvents with boiling points lower than that of oil (n-hexane) flow towards the top of the column, while oils with high boiling points flow towards the bottom of the column.

[0067] Solvent vapor is supplied from distillation column 11 to the cooled fluid flow path of heat exchanger 9 via piping 12, where it exchanges heat with the extractant effluent (low-temperature liquid) flowing in the hot / cold source flow path and is cooled to become a liquid solvent. This liquid solvent is then transported via piping 7 to the mixing section 6a of extraction device 6 and reused as an extraction solvent.

[0068] It should be noted that solvent components that are transferred to the treated water described later or lost due to evaporation from the device shall be replenished from outside the system.

[0069] The separated oil is taken out from the bottom of distillation column 11 via pipe 14. This oil is used as recovered oil for power generation, or as a liquid fuel or combustion improver, and for various other purposes. Further processing of the recovered oil may be carried out as needed.

[0070] The residual liquid after oil extraction is taken out from the separation section 6b of the extraction device 6 via the piping 20. The residual liquid is then transported to a membrane separation device 21, such as a reverse osmosis membrane separation device, where the solvent component (residual solvent) in the residual liquid is concentrated through membrane separation.

[0071] The permeate water through the membrane 21a of the membrane separation unit 21 is taken out as treated water via the piping 22. The concentrated water of the membrane separation unit 21 is discharged via the piping 23 and transported to the wastewater treatment process (not shown) for concentration treatment or drainage treatment, etc.

[0072] In the above embodiments, although the membrane separation device 2 is used to perform membrane separation treatment on the oil-containing waste liquid to increase the oil concentration, for oil that can be easily separated from water, separation treatment can also be carried out by using the specific gravity difference or by using an oil-water separator with microbubbles. After separation treatment, such as membrane separation treatment, the oil is then transported to the extraction device 6.

[0073] Extraction device 6 can be either continuous or intermittent. Continuous liquid-liquid extraction devices used on an industrial scale include mixing and clarification tanks, spray towers, countercurrent contact towers, and perforated plate towers (sieve plate towers). The extraction device can be any of these, or it can be a device other than these.

[0074] In this invention, since hexane (boiling point 67°C), which has a lower boiling point than the oil to be separated (around 250-600°C), is used as the extraction solvent, even when separating the oil from the solvent containing the oil through extraction by heating distillation, it is more energy-efficient than directly distilling the oil-containing waste liquid.

[0075] exist Figure 1 In this process, oil is separated from the solvent containing oil by heating distillation, thereby regenerating the solvent. Solvent regeneration is preferably achieved by heating distillation utilizing the boiling point difference between the oil and the solvent. As a method of heating distillation, either batch distillation or continuous distillation can be used, and methods such as flash distillation, plate columns, and packed columns can be employed.

[0076] In addition to heating distillation, vacuum distillation can also be used as a method to separate oil from a solvent containing oil.

[0077] As described above, in this invention, the oil concentration in the oil-containing wastewater supplied for extraction is 1000–5000 mg / L. Therefore, when the oil concentration in the oil-containing wastewater is lower than the above range, the oil-containing wastewater is concentrated by gravity separation, pressurized flotation, membrane separation, or other concentration methods before extraction.

[0078] On the other hand, if the oil concentration in the oily waste liquid supplied for extraction is higher than the above range, a dilution mixing tank or similar device is installed to dilute the oily waste liquid by using industrial water, tap water, drainage from a different system than the target oily wastewater, or condensate.

[0079] To facilitate the separation of oil from oil-containing waste liquid, salts, alkalis, or acids such as sodium chloride can be added to the oil-containing waste liquid before extraction treatment.

[0080] For example, if it is sodium chloride, the oil separation efficiency can be improved by adding about 0.1% to 30% by mass to oil-containing waste liquid containing about 0.05% to 1% by mass, or by lowering the pH to about 2.

[0081] As described above, the oil concentration of the solvent after oil extraction from the oil-containing waste liquid is 4% by mass or more. Therefore, when using n-hexane for extraction according to the present invention, by setting the oil concentration of the extracted n-hexane to 4% by mass or more, preferably 4 to 12% by mass, and more preferably 4 to 7% by mass, the oil separation efficiency and the thermal efficiency in solvent regeneration can be improved.

[0082] exist Figure 1 In this process, although the residual substances in the waste liquid (residual liquid) after oil separation are separated by membrane separation device 21 using membrane separation treatment such as reverse osmosis membrane, the solvent components in the waste liquid (residual liquid) after oil separation can also be separated by adsorption treatment such as activated carbon. In addition, residual solvents can be decomposed by adding oxidants or through biological treatment.

[0083] Example

[0084] The following are experimental examples, which are alternative embodiments, to illustrate the effects of the present invention.

[0085] [Experimental Example 1]

[0086] As an example of wastewater containing oil, the washing and drainage of dairy products was considered. Milk (milk fat content 3.5% by mass, non-fat milk solids content 8.4% by mass, and water content 88.1% by mass) was diluted with pure water to prepare a simulated wastewater containing oil. An oil extraction experiment using n-hexane was then carried out.

[0087] Milk, pure water, and n-hexane were added to a 2000 mL capped flask. The mixture was shaken horizontally at 180 rpm for 30 minutes to extract the phase. The extract was then transferred to a separatory funnel and allowed to stand for 30 minutes to separate the n-hexane and aqueous phases. The transparent portion of the n-hexane phase was collected and evaporated in an 80°C constant temperature bath. The weight of the remaining residue was then measured.

[0088] It should be noted that there are also cases where the n-hexane phase is almost an emulsion with no transparent portion (poor separation). In such cases, centrifugation (3000 rpm, 10 minutes) is performed to collect the transparent portion. The results are shown in Table 3.

[0089] The maximum extraction concentration in Table 3 is the concentration of all oil (milk fat) in the sample (a substance obtained by diluting milk with pure water) that is extracted using n-hexane. The extraction rate is the value of this concentration as the denominator and the numerator as the actual extraction concentration.

[0090] In addition, the criteria for judging separability are as follows.

[0091] <Separability>

[0092] ○: Good separation properties.

[0093] △: Able to separate.

[0094] ×: Cannot be separated.

[0095] [Table 3]

[0096] Table 3 Experimental Results

[0097] The following information can be obtained from Table 3.

[0098] The smaller the solvent / waste liquid ratio, the higher the oil concentration in the sample, and the more likely the separation performance will deteriorate. When the solvent / waste liquid ratio is 0.01, separation is impossible when the oil concentration in the sample exceeds 5000 mg / L and reaches 8750 mg / L.

[0099] On the other hand, it can be seen that the oil concentration (extraction concentration) in n-hexane cannot meet the concentration of 4 by mass calculated from the above energy balance when the solvent / waste liquid ratio is large. However, if the solvent / waste liquid ratio is reduced, the condition is met.

[0100] That is, it can be known that when using n-hexane to extract oil from oil-containing waste liquid, the oil-containing waste liquid has an appropriate oil concentration, solvent / waste liquid ratio, and extraction concentration.

[0101] Although the invention has been described in detail using specific methods, it will be apparent to those skilled in the art that various modifications can be made without departing from the intent and scope of the invention.

[0102] This application is based on Japanese Patent Application No. 2023-197548, filed on November 21, 2023, the entire contents of which are incorporated herein by reference.

[0103] Explanation of reference numerals in the attached figures

[0104] 2,21: Membrane separation device.

[0105] 6: Extraction device.

[0106] 11: Distillation column.

Claims

1. A method for recovering oil from oil-containing waste liquid, characterized in that, include: The extraction process involves using a solvent to extract oil-containing waste liquid, thereby transferring the oil in the waste liquid to the solvent. The separation process separates the extracted solvent containing the oil from the waste liquid; and The oil recovery process separates and recovers the oil from the solvent containing the separated oil. The solvent is n-hexane. The oil concentration in the oil-containing wastewater supplied to the extraction process is 1000–5000 mg / L. The volume ratio of the solvent to the oil-containing waste liquid is 0.01 to 0.05, which is the solvent / waste liquid ratio. The concentration of oil in the solvent containing the oil separated in the separation process is 4% by mass or more.

2. The oil recovery method for recovering oil from oil-containing waste liquid according to claim 1, wherein, In the oil recovery process, the oil is separated from the solvent containing the oil by heating and distillation, and the solvent is recovered. The recovered solvent is then reused in the extraction process.

3. The oil recovery method for recovering oil from oil-containing waste liquid according to claim 2, wherein, In the separation based on heating distillation, the heat of the separated solvent or oil fraction is used to heat the solvent containing the oil fraction using a heat exchanger or heat pump.

4. The oil recovery method for recovering oil from oil-containing waste liquid according to claim 1 or 2, wherein, Prior to the extraction step, there is also a concentration step to concentrate the oil-containing waste liquid or a dilution step to dilute the oil-containing waste liquid.

5. The oil recovery method for recovering oil from oil-containing waste liquid according to claim 1 or 2, wherein, The oil-containing waste liquid is any one of the following: waste liquid from automobile and other parts manufacturing plants containing mineral oil, water-based cutting waste liquid, waste liquid from paper or pulp mills containing turpentine, or waste liquid from oil mills, food and beverage factories, or food and beverage industries containing animal or vegetable oils.

6. An oil recovery device for recovering oil from oil-containing waste liquid, characterized in that, have: An extraction unit that transfers oil from an oil-containing waste liquid to a solvent by extracting the waste liquid with a solvent; A separation unit that separates the extracted solvent containing the oil from the waste liquid; and An oil recovery unit separates and recovers oil from the solvent containing the separated oil. The solvent is n-hexane. The oil concentration in the oil-containing wastewater supplied to the extraction unit is 1000–5000 mg / L. The volume ratio of the solvent to the oil-containing waste liquid is 0.01 to 0.05, which is the solvent / waste liquid ratio. The oil concentration in the solvent containing the oil separated by the separation unit is 4% by mass or more.

7. The oil recovery device for recovering oil from oil-containing waste liquid according to claim 6, wherein, The oil recovery unit is a unit that separates the oil from the solvent containing the oil by heating and distillation. The oil recovery device has a liquid supply unit, which supplies the solvent obtained from the oil separation by the oil recovery unit to the extraction unit.

8. The oil recovery apparatus for recovering oil from oil-containing waste liquid according to claim 7, wherein, The oil recovery device has a heat exchanger or heat pump for heating the solvent containing the oil using the heat of the solvent or oil separated by the heating distillation.

9. The oil recovery apparatus for recovering oil from oil-containing wastewater according to claim 6 or 7, wherein, The extraction unit is further provided with a concentration unit or a dilution unit in front of it. The concentration unit concentrates the oil-containing waste liquid, and the dilution unit dilutes the oil-containing waste liquid.

10. The oil recovery apparatus for recovering oil from oil-containing wastewater according to claim 6 or 7, wherein, The oil-containing waste liquid is any one of the following: waste liquid from automobile and other parts manufacturing plants containing mineral oil, water-based cutting waste liquid, waste liquid from paper or pulp mills containing turpentine, or waste liquid from oil mills, food and beverage factories, or food and beverage industries containing animal or vegetable oils.

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