An oil-containing metal solid waste treatment method based on pressurized dissolution and vacuum enhanced separation

Abstract

The application discloses a kind of oil-containing metal solid waste processing methods based on pressurized dissolution and vacuum strengthening separation, belong to metallurgical industry solid waste processing and resource recovery technical field, including the following steps: liquid organic solvent is mixed with dry sludge in inert gas under heating and pressurization, solvent-sludge mixture is obtained;And solvent-sludge mixture is gradient temperature distillation in vacuum environment, wherein gradient temperature distillation includes: first stage distillation is carried out, and organic solvent is recovered;Second stage distillation is carried out at 280~320 DEG C, and recovered oil and solid residue are obtained;Wherein the organic solvent used is saturated hydrocarbon.The process flow of the processing method is simple, not only the characteristic component obtained by recovery lubrication is well maintained;And, the oil content of the obtained solid residue can be greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment and resource recycling technology in the metallurgical industry, and more specifically, to a method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation. Background Technology

[0002] Oily metal solid waste, especially steel rolling sludge, is not only complex in composition, consisting of oil, water, iron powder and other impurities, but also has a high oil content, usually reaching more than 40%.

[0003] Among existing methods for treating steel rolling sludge, solvent extraction is a commonly used method. For example, Chinese patent application CN116062954A discloses a process for multi-stage extraction and separation using a composite solvent. The process includes homogenization, extraction, filtration, distillation, and other steps, and the recovered oil obtained after separation also requires further filtration and purification. While this method can achieve a high oil removal rate, the process is relatively complex.

[0004] Another common method is to use supercritical fluids for treatment. For example, Chinese patent application CN119263567A discloses a method for the resource recovery of rolling mill sludge. This method utilizes the strong reactivity of supercritical methanol to react with fatty acids in the sludge through esterification, generating fatty acid methyl esters, thereby disrupting the emulsion system and achieving three-phase separation. This application alters the composition of the original lubricating oil, making it difficult to preserve its original properties. Therefore, the recovered oil cannot be directly reused in the original rolling process. Furthermore, most rolling mill lubricating oils are based on mineral oil, with paraffinic or cycloalkyl hydrocarbons as their main components, and limited fatty acid content. Therefore, the application scenarios for generating fatty acid methyl esters through esterification are relatively limited. In addition, some methods utilize supercritical solvents that do not chemically react with the lubricating oil, such as CO2, for extraction. However, these supercritical solvents require higher reaction pressures or temperatures, and the process parameters are more stringent. Moreover, as the reaction temperature increases, the composition of the recovered oil also changes. Summary of the Invention

[0005] To address the technical problems of the need for further simplification in existing treatment processes for oily metal solid waste and the alteration of the composition of recovered oil, this invention provides a method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation. This scheme optimizes the treatment process for oily metal solid waste, simplifying the recovery of lubricating oil and the organic solvents used in the treatment process. Simultaneously, the characteristic components of the recovered lubricant are well preserved, allowing it to be directly reused as lubricating oil in the steel rolling process. Furthermore, compared to a single degreasing treatment method, it significantly reduces the oil content of the resulting solid residue.

[0006] To achieve the above objectives, the technical solution provided by this invention is as follows:

[0007] This invention provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation, comprising the following steps: mixing liquid organic solvent with dried oil sludge under heating and pressure in an inert gas atmosphere to fully dissolve the oil in the oil sludge, obtaining a solvent-oil sludge mixture; and performing gradient temperature distillation of the solvent-oil sludge mixture under vacuum, the gradient temperature distillation comprising: performing a first-stage distillation, by heating to vaporize and condense the organic solvent for recovery; and performing a second-stage distillation at 280~320℃ to obtain recovered oil and solid residue; wherein the organic solvent used is a saturated hydrocarbon.

[0008] Furthermore, the oily metal solid waste is steel rolling sludge. Because oil, water, iron powder, and other impurities are intertwined and tightly coexist in steel rolling sludge, it differs from typical oily copper scrap or petroleum industry sludge. Chinese Patent Publication No. CN115193076A mentions a method of direct stepped heating distillation, which is used to treat oily copper scrap. The inventors have attempted to separate steel rolling sludge using this method (see the comparative examples below), but the oil removal rate was limited, and the resulting solid residue had a high oil content. Under the same distillation conditions, even with further extension of the distillation time, the subsequent oil removal rate hardly improved. Extraction of the sludge using organic solvents at room temperature or below 100°C, as described in the background section, requires a combination of multiple treatment methods such as vacuum centrifugation, filtration, and distillation; a single treatment method is unlikely to achieve a good oil removal rate. Existing deoiling methods generally consider the following: on the one hand, minimizing the oil content of the resulting solid residue to recover the iron powder contained therein; on the other hand, maximizing the deoiling rate to recover the lubricating oil. However, they do not focus on the changes in the recovered oil components relative to the original lubricating oil components. This invention uses the characteristic component retention rate to describe the characteristic components of the lubricating oil, characterizing its similarity to the original lubricating oil. The characteristic component retention rate = (total characteristic peak area of ​​the recovered oil phase / total characteristic peak area of ​​the original lubricating oil) × 100%.

[0009] It should be noted that choosing saturated hydrocarbons as solvents effectively avoids chemical reactions between them and the components in the steel rolling sludge; the two only undergo physical dissolution. Specifically, saturated hydrocarbons have a strong selective dissolution ability for mineral oils and greases in the steel rolling sludge, but will not dissolve solid impurities such as iron powder and silt in the sludge, facilitating subsequent solid-liquid separation and ensuring a high retention rate of characteristic components. Generally speaking, temperature and pressure are key factors determining the physical state and mass transfer efficiency of solvents. Increasing the temperature enhances the penetration and dissolution capacity of organic solvents, thereby improving the subsequent vacuum distillation deoiling effect. However, as the temperature rises, the boiling point of saturated hydrocarbons is reached. By applying pressure, not only can the diffusion and dissolution performance of organic solvents be improved simultaneously, but the boiling point of organic solvents at the corresponding pressure can also be increased, thereby further increasing the upper limit of the heating temperature that organic solvents can withstand, in order to maintain their liquid state.

[0010] By applying pressure and heat, the organic solvent can effectively penetrate the internal pores and oil-solid interface of the sludge particles, pre-detaching and dissolving most of the adsorbed and encapsulated stubborn oil components, forming a homogeneous oil-solvent solution that floats on the upper layer of the mixture. This process effectively disrupts the oil-solid bond system and creates crucial reaction conditions, facilitating subsequent vacuum distillation. Vacuum treatment alone lacks the necessary pre-dissolution and mass transfer stages, resulting in ineffective oil-solid bond disruption and incomplete deoiling. Furthermore, without pre-dissolving the sludge with organic solvents, subsequent distillation will not allow some components to volatilize, significantly reducing the retention rate of characteristic components. Mixing the organic solvent with the steel rolling sludge under heating and pressure also improves the deoiling rate.

[0011] This invention designs a processing flow based on maximizing the retention rate of characteristic components in the recovered oil and simultaneously reducing the oil removal rate of the resulting solid residue. The process requires only mixing an organic solvent with the sludge and performing vacuum gradient temperature distillation on the solvent-sludge mixture. During vacuum distillation, the organic solvent, recovered oil, and solid residue are collected. Typical extraction processes include centrifugation after dissolution to obtain an upper oil phase and a lower oil-containing solid phase, which are then processed separately. Compared to existing steel rolling sludge processing flows, this invention significantly simplifies the process.

[0012] More importantly, in traditional extraction processes, centrifugation is required after mixing the organic solvent and sludge. Because the organic solvent has varying solubility for different components in the original sludge, the proportion of each component carried away by the upper oil layer differs relative to the original lubricating oil. After distillation of the organic solvent, the remaining oil phase obtained from the upper oil layer differs significantly from the composition of the original lubricating oil. This also leads to a significant difference in composition between the recovered oil obtained from the subsequent distillation of the lower oil-containing solid phase and the original lubricating oil, thus reducing the retention rate of characteristic components. Furthermore, even if the two recovered lubricating oils are mixed, the retention rate of characteristic components, while increasing somewhat, remains low. The differences between the two recovered oils are due to the different processing steps, and the entire process involves irreversible thermodynamic imbalances in component proportions and differences in the chemical potential of component structures. Mixing can only change the spatial distribution of components; it cannot repair the deviation from thermodynamic equilibrium and the irreversible damage to the structure. Therefore, the retention rate of characteristic components can never reach the level of this invention; that is, the degree of approximation to the original lubricating oil state after mixing is still lower than the technical solution of this invention.

[0013] In this invention, the solid-liquid mixed phase obtained after solvent dissolution is not separated, thereby solving the technical problem of low retention rate of characteristic components in the traditional steel rolling sludge treatment process.

[0014] Furthermore, the liquid organic solvent and the dried sludge are mixed under heating and pressure in an inert gas atmosphere. Specifically, the liquid organic solvent and the dried sludge are mixed at 100-150°C and 2-4 MPa. These process parameters are crucial for activating the dissolving properties of the organic solvent and breaking down the viscous emulsion structure of the rolling mill sludge. The specific mechanism is as follows: Within this temperature range, the interfacial tension between the organic solvent and the grease is effectively reduced, enhancing the solvent's penetration into the pores of the sludge, thereby efficiently stripping away adsorbed and encapsulated stubborn grease from the sludge. Increasing the temperature significantly reduces the saturated vapor pressure of the organic solvent, easily leading to its volatilization and loss. Therefore, it is necessary to simultaneously increase the system pressure to 2-4 MPa. By controlling the pressure, the boiling point of the organic solvent is raised, ensuring it remains in a stable, flowing liquid state. This guarantees sufficient contact between the organic solvent and the grease and allows for optimal dissolution. Therefore, it is essential to carefully control the temperature and pressure parameters. However, in a positive pressure reaction system, further increasing the temperature of the mixing system will significantly increase the activation energy of organic solvents and oil molecules, exceeding the molecular bond stability threshold, thereby initiating structural reactions such as polymerization and condensation between the organic solvents and oils. Simultaneously, further increasing the pressure of the mixing system will stimulate the catalytic activity of iron oxides such as FeO in the sludge, further inducing irreversible reactions between the organic solvents and oils, ultimately leading to severe damage to the characteristic components in the oils and rendering the recovered oil unusable for steel rolling. Therefore, the pressure and temperature should be selected within the aforementioned range.

[0015] Furthermore, the organic solvent is selected from any one or a combination of more than one of cycloalkanes, C6-C10 straight-chain alkanes, or C6-C10 branched-chain alkanes.

[0016] Furthermore, the organic solvent used is cyclohexane.

[0017] Furthermore, the ratio of the organic solvent to the dried sludge is 2-4:1, and the mixing time is 30-60 minutes.

[0018] Furthermore, the step of performing gradient temperature distillation on the solvent-sludge mixture under vacuum is specifically: performing gradient temperature distillation on the solvent-sludge mixture at 500~10000Pa.

[0019] Furthermore, the temperature of the first-stage distillation is 100-150℃, and the distillation time is 15-20 minutes, while the time of the second-stage distillation is 20-40 minutes. The specific temperature setting of the first-stage distillation accelerates the rate at which the organic solvent is distilled from the mixed solution, thus shortening the distillation time. Simultaneously, because the organic solvent is already fully dissolved in the oil, a higher temperature is required to drive the organic solvent to evaporate and separate from the solvent-oil sludge mixture.

[0020] Furthermore, the characteristic component retention rate of the recovered oil is >95%, and the deoiling rate of the obtained solid residue is >82%.

[0021] Furthermore, it also includes a pretreatment step for the sludge, specifically as follows: the sludge is dried at 100℃~120℃ for 12~24h to obtain dried sludge.

[0022] Compared with the prior art, the technical solution provided by this invention has the following advantages: (1) This invention optimizes the treatment process for oily metal solid waste. Specifically, the oily metal solid waste is first thoroughly mixed with a liquid organic solvent. The organic solvent can effectively penetrate into the internal pores of the sludge particles and the oil-solid interface, pre-stripping and dissolving most of the adsorbed and encapsulated stubborn oil components to form a homogeneous oil-solvent solution that floats on the upper layer of the mixed system. This process effectively disrupts the oil-solid bond system and creates crucial reaction conditions, laying the foundation for subsequent vacuum distillation. Then, gradient temperature vacuum distillation is performed. The organic solvent is obtained through the first stage of vacuum distillation, and the recovered oil and solid residue are obtained through the second stage of vacuum distillation. The characteristic components of the recovered oil are retained at a high rate. Furthermore, when the oily metal solid waste is selected as steel rolling sludge, the lubricating oil obtained by this method can be directly reused in the rolling process. In addition, the process of sludge treatment is greatly simplified.

[0023] (2) This invention further optimizes the mixing conditions of the organic solvent and the sludge. Specifically, the liquid organic solvent and the dried sludge are mixed at 100-150°C and 2-4 MPa. These process parameters are key to activating the dissolving properties of the organic solvent and breaking down the viscous emulsion structure of the rolling mill sludge. By raising the boiling point of the organic solvent, it is kept in a stable, flowing liquid state, ensuring sufficient contact between the organic solvent and the grease and achieving better dissolving effect. Simultaneously, within this temperature and pressure range, the interfacial tension between the organic solvent and the grease can be effectively reduced, enhancing the solvent's penetration into the pores of the sludge, thereby efficiently removing adsorbed and encapsulated stubborn grease from the sludge. In the subsequent distillation process, this effectively ensures that the lubricating oil can be extracted from the oil-containing solid, simultaneously improving the deoiling rate and the retention rate of characteristic components. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of the oil-containing metal solid waste treatment method according to an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of the process of the oil-containing metal solid waste treatment method according to an embodiment of the present invention. Detailed Implementation

[0026] To further understand the content of this invention, the invention will be described in detail with reference to the embodiments.

[0027] It should be noted that the organic solvent used in the following embodiments of the present invention is cyclohexane, which is only used to illustrate the treatment method and should not be construed as a limitation on the types of organic solvents used in the present invention. It can also be a saturated hydrocarbon, especially any one or more of cycloalkanes, C6-C10 straight-chain alkanes or C6-C10 branched-chain alkanes.

[0028] The steel rolling sludge used in the following embodiments of the present invention all comes from the cold rolling process and is only for illustrating the treatment method. Sludge from the hot rolling process can also be selected. The use of steel rolling sludge as an example to illustrate oily metal solid waste should not be construed as a limitation on the types of oily metal solid waste used in the present invention.

[0029] It should be noted that this invention not only focuses on the deoiling rate of steel rolling sludge, but also emphasizes the recovery of lubricating oil from the sludge. Therefore, the cold rolling sludge used is sludge that has not undergone air oxidation, and preferably fresh sludge.

[0030] Experimental materials and equipment Experimental materials: steel rolling sludge (taken from the cold rolling process of a steel plant, with an oil content of 58.31%), cyclohexane, nitrogen, etc.

[0031] Experimental equipment: stirred reactor, oven, stirrer, high-pressure reactor, vacuum pump, rotary evaporator, electronic balance, Soxhlet extraction apparatus, etc.

[0032] The oil content determination method referred to in this invention refers to the oil phase determination method in USEPA 3540C-1196 standard, which involves extraction with organic solvents and calculation of oil content based on mass loss.

[0033]

[0034] In Formula 1 above The solid oil content is expressed as a percentage, where m0 is the mass of the solid phase before extraction (i.e., the mass of the dried sludge), and m1 is the mass of the solid phase after treatment (i.e., the mass of the resulting solid residue). The initial solid oil content is denoted as... 0, the solid oil content after two-stage vacuum distillation is recorded as 1.

[0035] Example 1 This embodiment provides a method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation, as detailed below: Step 1: Pre-treatment of oil sludge The specific pretreatment process is as follows: Take 100g of cold-rolled sludge from a steel plant, place it in a mixer and stir at a rate of 60r / min for 30min to ensure thorough homogenization. Then, spread the homogenized sludge evenly on a tray and place it in an oven to dry at 110℃ for 24 hours to obtain dried sludge.

[0036] The second step involves mixing the organic solvent with the sludge under pressure and heating. The specific operation process is as follows: Weigh 20g of pretreated steel rolling sludge and mix it with 40g of cyclohexane, i.e., mix the sludge and organic solvent at a mass ratio of 1:2; add the above mixture to a high-pressure reactor, then seal the high-pressure reactor and heat the system to 150℃ at a heating rate of 5℃ / min. Subsequently, nitrogen gas is injected into the reactor to stabilize the absolute pressure of the reactor at 3.6MPa; maintain this temperature and pressure, turn on the stirring function of the reactor and stir continuously for 60min at a stirring speed of 200r / min to ensure that the steel rolling sludge and organic solvent are fully mixed to obtain a solvent-sludge mixture.

[0037] The third step is to perform gradient temperature vacuum distillation on the solvent-sludge mixture. The specific steps of vacuum distillation are as follows: After the pressure dissolution is completed, keep the reactor sealed and turn on the vacuum pump connected to the exhaust pipe of the reactor to reduce the internal system pressure to 4000 Pa. Maintain this vacuum condition, start the heating system, and raise the system temperature to 150°C at a heating rate of 5°C / min. Hold this temperature for 20 min to perform vaporization and condensation recovery of cyclohexane. Subsequently, under the same pressure conditions, raise the system temperature from 150°C to 300°C at a heating rate of 5°C / min, and hold this temperature for 30 min to perform volatilization and condensation recovery of the oil. After distillation, wait for the high-pressure reactor to cool to room temperature, open the lid, remove the residual solid phase in the inner cavity, i.e., the solid residue containing iron powder, and use Soxhlet extraction to determine its oil content.

[0038] Oil removal rate in the embodiments of the present invention The following formula is used for statistical analysis, as detailed below:

[0039] In the above formula 2 This indicates the oil content in the initial solid state, that is, the oil content of the sludge before pretreatment. The oil content of the solid residue obtained from the second distillation is shown in Table 1.

[0040] Retention rate detection of characteristic components in the oil phase: Gas chromatography was used, with n-hexadecane and cyclohexane as standards, to detect the sum of characteristic peak areas of C16~C30 straight-chain alkanes and cycloalkanes in the raw material sludge and recovered oil phase, and the retention rate was calculated according to the following formula:

[0041] As can be seen from the above description, the second step of mixing and dissolving the organic solvent with the sludge and the third step of vacuum distilling the solvent-sludge mixture are both carried out sequentially inside the same reaction vessel without the need to transfer the reaction vessel, thereby reducing the loss of raw materials caused by the transfer.

[0042] Example 2 This embodiment provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Embodiment 1 lies in the mass ratio of oil sludge to organic solvent and the pressure inside the reaction vessel during the step of mixing the organic solvent with the oil sludge under pressurized heating. Specifically, the oil sludge and organic solvent are mixed at a mass ratio of 1:3, i.e., 60g of cyclohexane is used, and the dissolution pressure is 3.8 MPa. Other operational procedures are basically the same. The experimental results are detailed in Table 1.

[0043] Example 3 This embodiment provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Embodiment 1 is that the temperature and pressure inside the reactor are slightly different in the step of mixing the organic solvent and oil sludge under pressurized heating conditions. Specifically, the temperature is 150°C and the dissolution pressure is 4 MPa. Other operational procedures are basically the same. The experimental results are detailed in Table 1.

[0044] Example 4 This embodiment provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Embodiment 1 is that the temperature and pressure inside the reactor are slightly different in the step of mixing the organic solvent and oil sludge under pressurized heating conditions. Specifically, the temperature is 140℃ and the dissolution pressure is 2.3 MPa. Other operational procedures are basically the same. The experimental results are detailed in Table 1.

[0045] Example 5 This embodiment provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Embodiment 1 is that the pressure inside the reactor is slightly different in the step of gradient-temperature vacuum distillation of the solvent-oil sludge mixture. Specifically, the distillation pressure is 1000 Pa. Other operational procedures are basically the same. The experimental results are detailed in Table 1.

[0046] Example 6 This embodiment provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Embodiment 1 lies in the following: In the step of mixing the organic solvent and oil sludge under pressurized heating, the temperature inside the reactor is different, specifically 100°C. In the step of performing gradient-heating vacuum distillation of the solvent-oil sludge mixture, the pressure and temperature inside the reactor are slightly different, specifically: the distillation pressure is 10000 Pa, and the second-stage distillation is heated to 320°C. Other operational procedures are basically the same. The experimental results are detailed in Table 1.

[0047] Comparative Example 1 This comparative example provides a method for treating steel rolling sludge, which differs from Example 1 in that the mixing process of solvent and sludge in the second step is omitted. Instead, the sludge is directly placed inside the reactor for gradient heating vacuum distillation. In the vacuum distillation, the temperature is directly increased to 300°C in one step at a rate of 5°C / min and held for 30min. Other operations are basically the same. The experimental results are detailed in Table 1.

[0048] Comparative Example 2 This comparative example provides a method for treating steel rolling sludge, which differs from Comparative Example 1 in that the sludge is directly placed inside the reactor for vacuum distillation. During vacuum distillation, the temperature is directly raised to 300℃ in one step at a heating rate of 5℃ / min, and then held at that temperature for 300min. Other operational procedures are basically the same. The experimental results are detailed in Table 1.

[0049] Comparative Example 3 This comparative example provides a method for treating steel rolling sludge, which differs from Example 1 in that the mixing process of solvent and sludge in the second step is omitted. Instead, the sludge is directly placed inside the reactor for vacuum distillation. In the vacuum distillation, the temperature is directly raised to 400°C at a rate of 5°C / min and held for 60min. Other operations are basically the same. The experimental results are detailed in Table 1.

[0050] Comparative Example 4 This comparative example provides a method for treating steel rolling sludge, which differs from Example 1 in that the mixing conditions of the organic solvent and sludge inside the reactor are different. Specifically, the mixing temperature is ambient temperature and pressure, followed by centrifugation. The upper oil phase obtained by centrifugation is then vacuum distilled at 4000 Pa and 150 °C to recover the organic solvent and obtain recovered oil. The characteristic component retention rate of this recovered oil is 85.34%. The lower oil-containing solid phase is then distilled at 4000 Pa and 300 °C to obtain recovered oil, with a characteristic component retention rate of 88.32%. After mixing the two recovered oils, the overall recovered oil has a characteristic component retention rate of 88.57%, as detailed in Table 1.

[0051] Comparative Example 5 This comparative example provides a method for treating steel rolling sludge, which differs from Example 1 in that: centrifugation is performed between the second and third steps, and the upper oil phase obtained by centrifugation is vacuum distilled at 4000 Pa and 150 °C to recover the organic solvent and obtain recovered oil. The characteristic component retention rate of this recovered oil is 93.83%. The lower oil-containing solid phase is then subjected to vacuum distillation at 4000 Pa and 300 °C to obtain recovered oil. The characteristic component retention rate of this recovered oil is 75.94%. After mixing the two recovered oils, the overall recovered oil has a characteristic component retention rate of 92.16%, as detailed in Table 1.

[0052] Comparative Example 6 This comparative example provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Example 1 is that the pressure inside the reactor is slightly different when mixing the organic solvent and oil sludge under pressurized heating conditions. Specifically, the dissolution pressure is 4.2 MPa, while the other steps remain unchanged. The data results are shown in Table 1.

[0053] Comparative Example 7 This comparative example provides a method for treating oily metal solid waste based on pressurized dissolution and vacuum-enhanced separation. The difference between this method and Example 1 is that the pressure inside the reactor is slightly different when mixing the organic solvent and oil sludge under pressurized heating conditions. Specifically, the dissolution pressure is 1.8 MPa, while the other steps remain unchanged. The data results are shown in Table 1.

[0054] Comparative Example 8 This comparative example provides a method for treating oily metal solid waste based on pressure dissolution and vacuum enhanced separation. The difference between this method and Example 1 is that the temperature inside the reactor is slightly different when the organic solvent and oil sludge are mixed under pressure and heating conditions. Specifically, the temperature is 200°C. The other steps remain the same. The data results are shown in Table 1.

[0055] Comparative Example 9 This comparative example provides a method for treating oily metal solid waste based on pressure dissolution and vacuum enhanced separation. The difference between this method and Example 1 is that the temperature inside the reactor is slightly different when the organic solvent and oil sludge are mixed under pressure and heating conditions. Specifically, the temperature is 80°C. The other steps remain the same. The data results are shown in Table 1.

[0056] Table 1. Experimental data for each embodiment and comparative example.

[0057] The following conclusions can be drawn from Table 1: (1) The parameter comparison of Examples 1-6 further illustrates that, as can be seen from the comparison of Examples 1 and 3, appropriately increasing the pressure can enhance the penetration and dissolution capacity of cyclohexane at low temperature; as can be seen from the comparison of Examples 1 and 2, increasing the amount of organic solvent can enhance the penetration and dissolution capacity of cyclohexane on sludge, thereby improving the deoiling effect; as can be seen from the comparison of Examples 1 and 5, optimizing the vacuum conditions of vacuum distillation is beneficial to further reduce the residual oil content.

[0058] (2) The oil removal rate of Example 1 and Comparative Examples 1-2 is much higher than that of vacuum distillation / reduced pressure distillation alone. If only vacuum distillation is performed, the preceding dissolution and mass transfer stages are lacking, resulting in the oil-solid bond not being effectively destroyed, thus the oil removal is incomplete. The reason is as follows: The internal structure of steel rolling sludge is highly mixed and complexly entangled. It is difficult to break the bond between oil and iron powder by relying solely on the molecular diffusion force generated by vacuum. Even if the heat preservation time is extended, a good oil removal effect cannot be obtained by simply relying on vacuum separation. At the same time, the residual oil rate of the solid residue obtained in Example 1 of this invention is much lower than that of the existing single solvent extraction method, which proves the synergistic performance of the process route of "first pressurized dissolution, then reduced pressure separation". It has a good technical effect in ensuring the oil removal rate and improving the retention rate of characteristic components.

[0059] (3) The comparison between Comparative Example 3 and Example 1 shows that increasing the temperature during distillation can force the separation of some oils. Although the residual oil rate is improved and seems close to Example 1, the recovered oil obtained at this time has been severely cracked, and its characteristic component retention rate is only 50.36%, which verifies that its composition has changed significantly. Therefore, it cannot be directly used in the rolling process.

[0060] (4) Compared with Example 1, Comparative Examples 4-5 added a centrifugal separation step after dissolving in the organic solvent, followed by vacuum separation. During the centrifugal separation process, the organic solvent exhibited different solubility selectivity for each characteristic component of the lubricating oil, resulting in "component splitting" of the system: the upper oil phase was enriched with light components, while the lower oil-containing solid phase retained heavy components. If the two parts of the oil were recovered separately, both would have incomplete component recovery, with the retention rate of characteristic components significantly lower than that of the Example. The reason is that the upper oil phase can achieve complete component recovery through gentle distillation; however, the heavy components in the lower solid phase have undergone irreversible component loss due to adsorption and dissolution differentiation. Subsequently, even if the retention rate of characteristic components increases after mixing the two parts of the recovered oil, the original ratio cannot be restored. This thermodynamically irreversible imbalance causes a systematic deviation in the component characteristics of the recovered oil and the original lubricating oil. Therefore, the retention rate of the comparative process is significantly lower than that of the present invention, confirming the rationality of the overall distillation process design.

[0061] (5) Iron in steel rolling sludge exists in the form of elemental Fe and FeOx, and has certain catalytic activity. Comparing Comparative Example 6 with Example 1, it can be seen that increasing the system pressure for organic solvent dissolution significantly promotes the polymerization and condensation reaction of cyclohexane and oil due to the activation of the catalytic effect of FeOx. At this time, the retention rate of characteristic components is only 75.42%. Comparing Comparative Example 8 with Example 1, it can be seen that increasing the temperature for organic solvent dissolution triggers a strong reaction due to the catalytic performance of iron powder at high temperature, and the characteristic components of oil are severely destroyed, with the retention rate of characteristic components being only 69.15%.

[0062] (6) Comparison of Comparative Example 7 and Example 1 shows that at 150°C, the pressure is only 1.8 MPa, and cyclohexane is in a gaseous state, which cannot fully dissolve and penetrate the sludge and destroy its emulsion structure. Therefore, the residual oil rate increases and the deoiling rate decreases. It can be seen that at the same temperature, liquid organic solvents have a better solvent effect on sludge. Comparison of Comparative Example 9 and Example 1 shows that when the temperature is lower than the optimal value, the diffusion ability of cyclohexane decreases and it cannot effectively destroy the emulsion structure of the sludge. Therefore, the residual oil rate increases and the deoiling rate decreases.

[0063] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation, characterized in that, Includes the following steps: Liquid organic solvent is mixed with dried sludge in an inert gas atmosphere and under heating and pressure to allow the organic solvent to fully dissolve the grease in the sludge, resulting in a solvent-sludge mixture. The solvent-sludge mixture is subjected to gradient temperature distillation under vacuum, wherein the gradient temperature distillation includes: performing a first-stage distillation, in which the organic solvent is vaporized and condensed for recovery by heating; and performing a second-stage distillation at 280~320℃ to obtain recovered oil and solid residue. The organic solvent used is a saturated hydrocarbon.

2. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 1, characterized in that, The oily metal solid waste is steel rolling sludge.

3. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 1, characterized in that, The process of mixing the liquid organic solvent with the dried sludge under inert gas and under heating and pressure specifically involves: The liquid organic solvent is mixed with the dried sludge at 100-150°C and 2-4 MPa.

4. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 1, characterized in that, The organic solvent is selected from any one or a combination of more than one of cycloalkanes, C6-C10 straight-chain alkanes, or C6-C10 branched-chain alkanes.

5. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 1, characterized in that, The organic solvent used is cyclohexane.

6. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to any one of claims 1-5, characterized in that, The mass ratio of the organic solvent to the dried sludge is 2-4:1, and the mixing time is 30-60 min.

7. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 1, characterized in that, The process of distilling the solvent-sludge mixture under a vacuum with a gradient temperature increase is specifically as follows: The solvent-sludge mixture was distilled under a gradient temperature increase at 500~10000Pa.

8. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 7, characterized in that, The temperature of the first stage distillation is 100~150℃ and the distillation time is 15~20min, while the time of the second stage distillation is 20~40min.

9. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to claim 8, characterized in that, The characteristic component retention rate of the recovered oil is >95%, and the oil removal rate of the obtained solid residue is >82%.

10. The method for treating oily metal solid waste based on pressure dissolution and vacuum-enhanced separation according to any one of claims 1-5 and 7-9, characterized in that, It also includes a pretreatment step for the sludge, as detailed below: The sludge was dried at 100℃~120℃ for 12~24h to obtain the dried sludge.

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