METHOD FOR RECYCLING COLD-ROLLED OIL SLUDGE

Description

CROSS REFERENCE TO RELATED APPLICATION

This patent application claims the benefit and priority of Chinese Patent Application No. 202411463582.7 filed with the China National Intellectual Property Administration on Oct. 21, 2024, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.

TECHNICAL FIELD

The present disclosure relates to the technical field of harmless disposal and recycling of hazardous wastes, and in particular to a method for recycling a cold-rolled oil sludge.

BACKGROUND

Cold-rolled oil sludge is a complex water-in-oil (W/O) emulsified mixture, which is formed by a three-phase mixture of iron filing particles generated by friction between rollers and rolled pieces during the cold rolling of a crude steel, waste rolling oil and water in the emulsion. Such an oil sludge shows a black and viscous state, is seriously emulsified and has bad odor, and is difficult to biodegrade. Due to high toxicity and foul odor, the cold-rolled oil sludge is listed as a hazardous waste in the category of waste mineral oil and mineral oil-containing waste (HW08) in the “National List of Hazardous Wastes” (2021 edition). It is estimated that about 1.8 million tons of the cold-rolled oil sludge is generated in China per year, and has attracted much attention from the government and the public. In addition, the cold-rolled oil sludge includes oil and fat accounting for 5 wt % to 60 wt %, main components being palm oil, oleic acid, stearic acid and other C14 to C18 high-grade fatty acids, as well as iron filings (mainly Fe and Fe₃O₄) accounting for 20 wt % to 90 wt %. The direct discharge of the cold-rolled oil sludge not only pollutes the air, groundwater, and soil, causing serious ecological problems and endangering people's health, but also leads to serious waste of oil and iron resources.

At present, main methods for disposal of the cold-rolled oil sludge include incineration and pyrolysis at high-temperature, distillation at medium-temperature, as well as organic solvent extraction, acid-alkali cleaning, and microwave radiation at room-temperature. However, the above methods all have certain shortages in industrial applications. The incineration is prone to secondary pollution, the pyrolysis has high energy consumption, the distillation requires pre-dehydration, the organic solvent extraction has high operating costs, the acid-alkali cleaning requires equipment to have strong corrosion resistance, and the microwave radiation has high investment costs in equipment, thus seriously restricting recycling of the cold-rolled oil sludge. In addition, some researchers directly add the cold-rolled oil sludge into an iron ore sintering process or mix the cold-rolled oil sludge with coal powder into a blast furnace for injection, but there is a relatively low compounding ratio due to process limitations.

CN109679676A discloses a method in which water in rolled oil sludge is separated by using an alkaline conditioning agent, and then an obtained water-free oil sludge reacts with an alkaline methanol solution to synthesize biodiesel. The method requires the alkaline conditioning agent to pre-treat the cold-rolled oil sludge by dehydration, and requires a large amount of alkaline reagent added. Zhou et al. has disclosed a method for preparing biodiesel by extracting and recovering oil and fat from the cold-rolled oil sludge using a mixed solvent of 1,2-dichloroethane and isopropanol, and then promoting esterification and transesterification using H₂SO₄ as a homogeneous catalyst. The method requires the separation of oil and fat in cold-rolled oil sludge, which requires a large amount of organic solvents. CN109797002A discloses that in the presence of a catalyst and a solvent, scrap steel rolling oil is subjected to hydrogenation in a hydrogen atmosphere, a mixture obtained after the hydrogenation is subjected to solid-liquid separation, and an obtained separated liquid phase is distilled to obtain gasoline and diesel fractions. In the method, hydrogen is used as a reaction atmosphere, which results in high energy consumption and high risk, while it is difficult to ensure that the hydrogen is fully reacted during the hydrogenation, causing a waste of resources. Therefore, it is of great significance to provide a method for recycling the cold-rolled oil sludge using a small amount of chemical reagents safely and efficiently.

SUMMARY

The present disclosure is to provide a method for recycling a cold-rolled oil sludge. In the method according to the present disclosure, methanol is used as a raw material, with just small amount of chemical reagents and a simple disposal process, which could achieve harmless disposal and recycling of the cold-rolled oil sludge.

To achieve the above objects, the present disclosure provides the following technical solutions:

The present disclosure provides a method for recycling a cold-rolled oil sludge, including the following steps:

• • (1) mixing methanol with the cold-rolled oil sludge to obtain a mixture, and subjecting the mixture to esterification under supercritical conditions to obtain a solid-liquid mixture;
  • (2) subjecting the solid-liquid mixture obtained in step (1) to solid-liquid separation to obtain an iron filing and a liquid phase; and
  • (3) subjecting the liquid phase obtained in step (2) to methanol removal and extraction in sequence to obtain an oil phase product and an aqueous phase product.

In some embodiments of the present disclosure, in step (1) a mass ratio of the methanol to the cold-rolled oil sludge is in a range of 4:1 to 19:1.

In some embodiments of the present disclosure, the supercritical conditions in step (1) refer to heating a mixture of the methanol and the cold-rolled oil sludge in a closed reactor and then subjecting the mixture to heat preservation.

In some embodiments of the present disclosure, the heat preservation is conducted at a temperature of 250° C. to 325° C. for 15 minutes to 60 minutes.

In some embodiments of the present disclosure, the heating is conducted at a heating rate of 2° C./min to 4° C./min.

In some embodiments of the present disclosure, the esterification in step (1) is conducted under an inert atmosphere.

In some embodiments of the present disclosure, the solid-liquid separation in step (2) is conducted by vacuum filtration separation.

In some embodiments of the present disclosure, after the solid-liquid separation in step (2), the method further comprises subjecting a resulting solid phase to washing and drying to obtain the iron filing.

In some embodiments of the present disclosure, the methanol removal in step (3) is conducted by rotary evaporation.

In some embodiments of the present disclosure, an extractant for the extraction in step (3) is dichloromethane (DCM).

The present disclosure provides a method for recycling the cold-rolled oil sludge as described above, including the following steps: (1) mixing methanol with the cold-rolled oil sludge to obtain a mixture, and subjecting the mixture to esterification under supercritical conditions to obtain a solid-liquid mixture; (2) subjecting the solid-liquid mixture obtained in step (1) to solid-liquid separation to obtain an iron filing and a liquid phase; and (3) subjecting the liquid phase obtained in step (2) to methanol removal and extraction in sequence to obtain an oil phase product and an aqueous phase product. In the present disclosure, fatty acids at the interfaces of oil-water, oil-solid, and oil-water-solid in the cold-rolled oil sludge are subjected to esterification with methanol to generate fatty acid methyl ester due to poor hydrogen bonding degree, low dielectric constant, and strong reactivity of methanol in the supercritical state. Further, the strong solubility of methanol in the supercritical state makes the fatty acid methyl ester dissolve in methanol. Due to hydrophobicity and FeO_x repellency of the fatty acid methyl ester, interfacial tension of the oil-water, oil-solid, and oil-water-solid in the cold-rolled oil sludge are reduced, leading to the collapse of the interfacial structure and the aggregation of droplets. After simple solid-liquid separation, high-grade iron filings and a liquid phase consisting of oil-water can be obtained. Then, oil-water separation is achieved through extraction, such that oil, water, and solid phases in the cold-rolled oil sludge are separated. The results of examples show that the method for recycling the cold-rolled oil sludge allows for separation of water, oil, and solid phases of the cold-rolled oil sludge, where the oil phase has a yield of 47.23% and a calorific value of 38.14 MJ/kg, and can be used as a fuel; the solid phase is an iron filing with a grade of 59.89%, and can be reused as iron ore; and the aqueous phase does not need to be further processed and can be directly discharged as industrial wastewater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE shows a schematic flow chart of the method for recycling the cold-rolled oil sludge in an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

There is no particular limitation on sources of all the raw materials, which can be purchased from the market or prepared according to conventional methods known to those skilled in the art.

There is no particular limitation on the purity of all the raw materials. In some embodiments, industrial pure raw materials are used.

The present disclosure provides a method for recycling a cold-rolled oil sludge, including the following steps:

• • (1) mixing methanol with the cold-rolled oil sludge to obtain a mixture, and subjecting the mixture to esterification under supercritical conditions to obtain a solid-liquid mixture;
  • (2) subjecting the solid-liquid mixture obtained in step (1) to solid-liquid separation to obtain an iron filing and a liquid phase; and
  • (3) subjecting the liquid phase obtained in step (2) to methanol removal and extraction in sequence to obtain an oil phase product and an aqueous phase product.

In the present disclosure, methanol is mixed with the cold-rolled oil sludge and then subjected to esterification under supercritical conditions to obtain a solid-liquid mixture.

There is no special requirement for the source of the cold-rolled oil sludge, and the oil sludge generated in the steel cold rolling may be used. In an embodiment, the cold-rolled oil sludge comes from the cold rolling of a large domestic steel enterprise. The cold-rolled oil sludge is deep black and viscous. Microscopic observation shows that its interior has an oil-in-water (W/O) structure as a whole. A continuous oil phase wraps around solid phase iron filing particles and emulsified water droplets. The oil, water, and solid phases are intertwined and highly emulsified, and the emulsified water droplets are almost spherical. The oil, water, and solid phases in the cold-rolled oil sludge have mass fractions of 47.28%, 27.15%, and 25.57%, respectively. The oil phase in the cold-rolled oil sludge is mainly composed of long-chain fatty acids such as palmitic acid, oleic acid, and stearic acid, accounting for as high as 82.772%, and a sum of mass fractions of C, H and O elements in the oil phase is 99.82%, showing an extremely high calorific value; and the solid phase in cold-rolled oil sludge includes mainly Fe₃O₄ and metallic Fe, accounting for 95.782%, which can be reused as concentrated iron ore.

In the present disclosure, methanol is added into the cold-rolled oil sludge, and the characteristics of methanol in a supercritical state can promote esterification with the oil in the cold-rolled oil sludge to produce fatty acid methyl ester, which is dissolved in supercritical methanol. Due to the hydrophobicity and FeO_x repellency of fatty acid methyl ester, the interfacial tension of oil-water, oil-solid, and oil-water-solid in the cold-rolled oil sludge are reduced, and the droplets aggregate. After simple solid-liquid separation, high-grade iron filings and a liquid phase composed of oil and water can be obtained, and methanol in the liquid phase can be recycled.

In some embodiments of the present disclosure, a mass ratio of the methanol to the cold-rolled oil sludge is in a range of (4-19): 1, particularly (8-19): 1, and even more particularly (12-19): 1. In the present disclosure, the larger mass ratio of the methanol to the cold-rolled oil sludge, more prone the esterification of the oil phase in the cold-rolled oil sludge with methanol, and more oil phase can be dissolved in methanol, promoting solid-liquid separation. However, too much methanol may cause excessive energy consumption in subsequent methanol recovery. Limiting the mass ratio of the methanol to the cold-rolled oil sludge within the above range can not only realize the recycling of the oil phase in the cold-rolled oil sludge, but also take into account the energy consumption of methanol recovery.

In some embodiments of the present disclosure, the supercritical conditions refer to heating a mixture of methanol and the cold-rolled oil sludge in a closed reactor and then subjecting the mixture to heat preservation. There are no special requirements for the closed reactor, as long as it meets the supercritical conditions, such as an autoclave.

In some embodiments of the present disclosure, the heat preservation is conducted at a temperature of 250° C. to 325° C., particularly 275° C. to 300° C. Heating methanol in a closed reactor to make it reach a supercritical state is beneficial for methanol to reach a supercritical state within the above temperature range without causing excessive energy consumption.

In some embodiments of the present disclosure, the heat preservation is conducted for 15 min to 60 min, particularly 30 min to 45 min. Within the above time range, the oil phase in the cold-rolled oil sludge undergoes full esterification with methanol, which is beneficial to the improvement of the yield of the oil phase.

In some embodiments of the present disclosure, the heating is conducted at a heating rate of 2° C./min to 4° C./min. The above heating rate range takes both the reaction rate and the reaction stability of the esterification into consideration.

In some embodiments of the present disclosure, the esterification is conducted under an inert atmosphere. The inert atmosphere is nitrogen atmosphere. In some embodiments of the present disclosure, the mixture of methanol and the cold-rolled oil sludge is added into the autoclave, and air in the autoclave is purged and discharged by introducing nitrogen into the autoclave, such that the nitrogen atmosphere is formed in the autoclave. In some embodiments, the nitrogen atmosphere is at a normal pressure.

In some embodiments of the present disclosure, the esterification is conducted under stirring, and the stirring is conducted at 60 r/min to 300 r/min with a stirring paddle. The stirring can allow the cold-rolled oil sludge to fully contact and react with methanol, which is beneficial to the improvement of the yield of the oil phase.

Supercritical methanol has special physical and chemical properties such as low dielectric constant, weak hydrogen bonding, high reactivity, and strong solubility, and can quickly react with high-grade fatty acids in the oil phase of the cold-rolled oil sludge to produce high-value biodiesel—fatty acid methyl ester. In addition, the supercritical methanol can be used as a hydrogen donor to help stabilize free radicals to reduce the repolymerization of intermediate products, and its strong solubility allows to dissolve the product fatty acid methyl ester into a liquid product to prevent coke formation. In addition, supercritical methanol has a low dielectric constant and strong solubility, can promote the dissolution of organic matter and reaction products in steel rolling oil sludge into methanol, thus avoiding the formation of a new emulsified structure among the product and water and solid phases to destroy the stable emulsified structure of the steel rolling oil sludge. Moreover, Fe3d orbital Lewis acid iron sites of iron oxide (FeO_x) in the steel rolling sludge and water are easily ionized under subcritical conditions. The hydronium ions (H₃O⁺) produced by dissociation can be used as a catalyst for the esterification of supercritical methanol and high-grade fatty acids, thereby further improving a reaction rate of supercritical methanol and high-grade fatty acids in the oil phase of steel rolling oil sludge.

In some embodiments of the present disclosure, after the esterification reaction, methanol is added to clean the autoclave and the stirring paddle, and an obtained cleaning liquid is combined with a resulting product from the esterification as the solid-liquid mixture.

In the present disclosure, after the solid-liquid mixture is obtained, the solid-liquid mixture is subjected to solid-liquid separation to obtain an iron filing and a liquid phase.

In some embodiments of the present disclosure, the solid-liquid separation is conducted by vacuum filtration separation. There is no particular requirement for specific conditions of the vacuum filtration separation, as long as the solid phase and the liquid phase could be separated well. The vacuum filtration separation requires simple equipment and is easy to operate, making it suitable for industrial-scale use.

In some embodiments of the present disclosure, the solid phase obtained by the solid-liquid separation is washed and dried to obtain the iron filing. There is no particular requirement for a specific drying method, as long as methanol on the surface of the iron filing could be removed. In some embodiments, the drying is conducted by an oven drying, and the oven drying is conducted at 100° C. to 110° C.

In the present disclosure, the liquid phase obtained is subjected to methanol removal and extraction in sequence to obtain an oil phase product and an aqueous phase product.

In some embodiments of the present disclosure, the methanol removal is conducted by rotary evaporation. There is no particular requirement for specific conditions of the rotary evaporation, as long as methanol in the liquid phase could be separated and recovered.

In some embodiments of the present disclosure, an extractant for the extraction is DCM. The liquid phase obtained by the solid-liquid separation is extracted with the DCM, such that oil phase substances such as fatty acid methyl ester are dissolved in the DCM while water-soluble substances such as inorganic salts are dissolved in water, thereby achieving separation of the oil phase and the aqueous phase.

In some embodiments of the present disclosure, a volume ratio of the extractant to the liquid phase obtained after the methanol removal is in a range of (1-3): 1. The higher dosage of the extractant, the better the separation effect of the oil phase and the aqueous phase, but a subsequent recovery cost of the extractant may increase. Limiting the dosage of the extractant within the above range can achieve a better extraction effect while taking into account the recovery cost of the extractant.

In some embodiments of the present disclosure, after the extraction, a resulting product is subjected to separation to obtain an extraction solution and an aqueous phase product, and the extractant is removed from the extraction solution to obtain the oil phase product.

In the present disclosure, under the condition that the extractant is DCM, the obtained extraction solution is a DCM solution; and the DCM solution is subjected to rotary evaporation to remove DCM, obtaining the oil phase product. There is no particular requirement for specific conditions of the rotary evaporation, as long as the DCM could be separated and recovered.

In the present disclosure, fatty acids at the interfaces of oil-water, oil-solid, and oil-water-solid in the cold-rolled oil sludge are subjected to esterification with methanol to generate fatty acid methyl ester due to poor hydrogen bonding degree, low dielectric constant, and strong reactivity of methanol in the supercritical state. Further, a strong solubility of the supercritical methanol makes the fatty acid methyl ester dissolve in methanol. Due to hydrophobicity and FeO_x repellency of the fatty acid methyl ester, interfacial tensions of the oil-water, oil-solid, and oil-water-solid in the cold-rolled oil sludge are reduced, leading to the collapse of the interfacial structure and the aggregation of droplets. After simple solid-liquid separation, high-grade iron filings and a liquid phase consisting of oil and water can be obtained. Then, oil-water separation is achieved through extraction, such that oil, water, and solid phases in the cold-rolled oil sludge are separated to realize the recycling of the cold-rolled oil sludge.

The technical solutions of the present disclosure will be clearly and completely described below with reference to the examples of the present disclosure. Apparently, the described examples are merely a part rather than all of the examples of the present disclosure. All other examples obtained by those skilled in the art based on the examples of the present disclosure without creative efforts shall fall within the protection scope of the present disclosure.

Example 1

A method for recycling a cold-rolled oil sludge was illustrated in a flow chart shown in FIGURE, and was performed by the following steps:

• • (1) 6.97 g of the cold-rolled oil sludge was weighed and added into a beaker, and methanol was then added thereto in a mass ratio of the methanol to the cold-rolled oil sludge of 5.66:1 to obtain a mixture;
  • (2) The mixture was added into an autoclave and stirred at 300 rpm. Air in the autoclave was exhausted with nitrogen. The mixture was subjected to esterification by heating to 275° C. at 3° C./min and holding for 30 min. After the esterification was completed, the autoclave was cooled to room temperature along with furnace. A resulting cooled mixture was poured into a beaker, 15 mL of methanol was added to clean the interior of the autoclave and the stirring paddle, and a resulting cleaning liquid was also poured into the beaker.
  • (3) The mixed solution in the beaker was vacuum-filtered through filter paper, and a resulting residue was washed with 5 mL of methanol to obtain a filtrate and a filter residue. The filter residue was dried at 105° C. for 5 h to obtain 2.40 g of residue product, which was iron filings.
  • (4) The filtrate was subjected to rotary evaporation at 50° C. for 30 min to remove methanol, obtaining a water-containing oil phase.
  • (5) 100 mL of DCM was added into the water-containing oil phase, and a resulting system was subjected to separation in a separatory funnel to obtain an upper aqueous phase product and a lower DCM solution.
  • (6) The lower DCM solution was subjected to rotary evaporation at 50° C. for 30 min to remove DCM, thereby obtaining 3.29 g of an oil phase product.

Example 2

This example was performed according to Example 1 except that the esterification was conducted at 250° C.

Example 3

This example was performed according to Example 1 except that the esterification was conducted at 300° C.

Example 4

This example was performed according to Example 1 except that the esterification was conducted at 325° C.

Comparative Example 1

This example was performed according to Example 1 except that the esterification was conducted at 200° C.

Example 5

This example was performed according to Example 1 except that a mass ratio of the methanol to the cold-rolled oil sludge was 19:1.

Example 6

This example was performed according to Example 1 except that a mass ratio of the methanol to the cold-rolled oil sludge was 9:1.

Example 7

This example was performed according to Example 1 except that a mass ratio of the methanol to the cold-rolled oil sludge was 4:1.

Example 8

This example was performed according to Example 1 except that a mass ratio of the methanol to the cold-rolled oil sludge was 3:1.

Example 9

This example was performed according to Example 1 except that the esterification was conducted for 15 min.

Example 10

This example was performed according to Example 1 except that the esterification was conducted for 45 min.

Example 11

This example was performed according to Example 1 except that the esterification was conducted for 5 min.

The iron filings obtained in the examples and comparative example were subjected to a grade test (Iron ores—Determination of total iron content—Titanium (III) chloride reduction method, GB/T 6730.5-2007), and the oil phase product was subjected to a calorific value test (Separated two-stage hydrothermal liquefaction of livestock manure for high-quality bio-oil with low-nitrogen content: Insights on nitrogen migration and evolution, Chemical Engineering Journal 477 (2023) 146999). The test data were recorded in Table 1.

As shown in Examples 1 to 4 and Comparative Example 1, when the system temperature is not enough to make methanol reach a supercritical state, the oil phase yield and the oil phase calorific value are low; when the system temperature allows methanol to reach a supercritical state, the higher the temperature, the higher the oil phase yield and the oil phase calorific value; but after the temperature reaches 275° C., the oil phase yield and the oil phase calorific value are slightly reduced if the temperature continues to rise.

As shown in Examples 5 to 8, the higher the mass ratio of the methanol to the cold-rolled oil sludge, the higher oil phase yield and the oil phase calorific value; but when the mass ratio of the methanol to the cold-rolled oil sludge reaches 5.66:1, continuing to increase the proportion of methanol could increase the oil phase yield and the oil phase calorific value, while a too low cold-rolled oil sludge proportion might lead to a decrease in disposal efficiency.

As shown in Examples 1, 9 to 11, a too short esterification time and incomplete esterification lead to low oil phase yield and low oil phase calorific value; when the esterification time reaches 30 min, further extending the esterification time results in extremely small changes in the oil phase yield and oil phase calorific value.

The above examples and comparative example show that supercritical methanol has the characteristics of low hydrogen bonding degree, poor dielectric constant, and strong reactivity, enabling the separation of oil, water and solid phases in the cold-rolled oil sludge. The oil phase with high yield and calorific value can be used as a fuel; the solid phase is high-grade iron filings, which can be reused as iron ore; the aqueous phase does not need to be further processed and can be directly discharged as industrial wastewater, thereby realizing recycling of the cold-rolled oil sludge.

The above descriptions are merely preferred embodiments of the present disclosure. It should be noted that a person of ordinary skill in the art may further make several improvements and modifications without departing from the principle of the present disclosure, but such improvements and modifications should be deemed as falling within the scope of the present disclosure.