A method for pre-drying treatment of magnesium oxide powder for mineral insulated cables

Abstract

This invention relates to the field of wire and cable materials technology, and particularly to a method for pre-drying magnesium oxide powder for mineral-insulated cables. The method includes: pretreatment, transferring the magnesium oxide powder to be treated to a closed fluidized bed system and purging with dry nitrogen for preheating; rapid drying, switching the inlet air of the closed fluidized bed system to superheated steam for fluidized drying of the magnesium oxide powder; nitrogen switching dehumidification, switching the inlet air of the closed fluidized bed system to dry nitrogen; in-situ gas-phase hydrophobication, injecting a gaseous silane coupling agent into the nitrogen flow; nitrogen purging cooling, shutting off the silane coupling agent supply and purging with dry nitrogen to cool the magnesium oxide powder; and closed-loop conveying and filling, conveying the magnesium oxide powder to a filling station under dry nitrogen for filling. This invention avoids the risk of re-hygroscopic absorption of magnesium oxide powder through a two-stage atmosphere switching process of rapid superheated steam drying and nitrogen switching dehumidification, as well as a closed-loop filling process.

Description

Technical Field

[0001] This invention relates to the field of wire and cable materials technology, and in particular to a method for pre-drying magnesium oxide powder for mineral-insulated cables. Background Technology

[0002] Mineral insulated cable (MI cable) is a special type of cable with a copper or copper alloy sheath and highly compressed magnesium oxide (MgO) powder as insulation. However, magnesium oxide powder is a typical highly hygroscopic inorganic material with extremely high surface activity. It readily captures water molecules from the environment through physical and chemical adsorption. The presence of moisture significantly reduces the insulation resistance of magnesium oxide and causes it to ionize and conduct electricity at high temperatures, leading to deterioration or even breakdown of the cable's insulation performance. Therefore, during the manufacturing process of mineral insulated cables, the magnesium oxide powder must undergo strict pre-drying treatment before being filled into the metal sheath, ensuring its moisture content is controlled below 0.05%.

[0003] In existing technologies, the pre-drying treatment of magnesium oxide powder mainly employs methods such as high-temperature calcination, hot air rotary drying, or static vacuum drying. High-temperature calcination typically requires prolonged treatment at 850℃~950℃. Although it can remove moisture, it consumes extremely high energy. Furthermore, high temperatures can easily cause powder particles to sinter and blunt their edges, reducing the filling density in the subsequent metal sheath. Moreover, it is prone to re-dilution during subsequent storage and filling. Existing technologies usually employ liquid-phase moisture-proof treatment methods for secondary drying; however, magnesium hydroxide may be generated during the secondary drying process, which can also lead to a sharp decline in insulation performance.

[0004] Therefore, there is a need for a pre-drying treatment method that can efficiently dehydrate, prevent hydration and re-moisture absorption, and avoid re-moisture absorption during transportation and exposure. Summary of the Invention

[0005] The main objective of this invention is to provide a method for pre-drying magnesium oxide powder for mineral-insulated cables, aiming to solve the problems of difficulty in ensuring the dehydration effect of magnesium oxide and re-hygroscopicity during transportation and storage in existing magnesium oxide powder drying methods.

[0006] To achieve the above objectives, this invention proposes a method for pre-drying magnesium oxide powder for mineral-insulated cables, comprising the following steps:

[0007] Pretreatment involves transferring the magnesium oxide powder to be treated into a closed fluidized bed system, purging and preheating it with dry nitrogen gas to reduce the oxygen content in the closed fluidized bed system to below 0.5%, and preheating the powder to 120~150℃.

[0008] Rapid drying: The intake air of the closed fluidized bed system is switched to superheated steam at 400℃~450℃ to fluidize and dry the magnesium oxide powder until the moisture content of the magnesium oxide powder drops to below 0.3%.

[0009] Nitrogen gas switching dehumidification: The inlet gas of the closed fluidized bed system is switched to dry nitrogen gas at a temperature of 120℃~150℃ for 10~15 minutes to replace residual water vapor in the bed and reduce the powder temperature to the hydrophobic reaction temperature window.

[0010] In-situ gas-phase hydrophobicization was carried out by controlling the temperature of the closed fluidized bed system at 100℃~130℃ and injecting a gas-phase silane coupling agent into the nitrogen gas flow, so that magnesium oxide powder formed a monomolecular hydrophobic layer.

[0011] Nitrogen gas was used for purging and cooling. The delivery of the silane coupling agent was turned off. Dry nitrogen gas was introduced to cool the magnesium oxide powder to below 40°C, which carried out the unreacted silane coupling agent and reaction byproducts.

[0012] The magnesium oxide powder, after being purged and cooled, is transported through a closed pipeline to the filling station of the cable's metal sheath under a dry nitrogen atmosphere to complete the filling process.

[0013] Preferably, in the pretreatment step, the dew point of the dry nitrogen is ≤-50℃, the oxygen content is ≤10ppm, the purging gas velocity is 0.8~1.5m / s, and the purging time is 15~20min.

[0014] Preferably, in the rapid drying step, the apparent gas velocity of the fluidized drying is controlled to be 0.4 to 0.8 times the carry-out velocity of the magnesium oxide particles.

[0015] Preferably, the median diameter of the magnesium oxide powder to be treated is 40~100 μm, and the apparent gas velocity of the fluidized drying is 0.12~0.48 m / s.

[0016] Preferably, the silane coupling agent is one or more of vinyltrimethoxysilane, γ-aminopropyltriethoxysilane, or γ-glycidoxypropyltrimethoxysilane; the volume concentration of the silane coupling agent in the nitrogen-coupling agent mixed gas is 0.5-1.5%, the nitrogen carrier gas flow rate is 2-5 L / min, and the treatment time is 30-50 min.

[0017] Preferably, in the nitrogen purging and cooling step, the gas velocity of the dry nitrogen is 0.5~1.0 m / s, the purging time is 15~20 min, and the oxygen content in the closed fluidized bed system is always maintained below 0.5% during the cooling process.

[0018] Preferably, in the nitrogen switching dehumidification step, the moisture content of magnesium oxide powder is monitored in real time by an online infrared moisture meter. When the moisture content is detected to drop below 0.3%, the atmosphere is switched from superheated steam to dry high-purity nitrogen.

[0019] This invention avoids the risk of rehydration caused by low-temperature water vapor contacting magnesium oxide during the cooling process by using a two-stage atmosphere switching process of rapid drying with superheated steam and dehumidification with nitrogen. At the same time, it integrates drying, in-situ vapor phase hydrophobication, cooling and closed filling processes into the same closed system, so that the treated magnesium oxide powder can be directly transported to the cable filling station through a closed pipeline, avoiding the risk of magnesium oxide powder reabsorbing moisture. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the processes shown in these drawings without creative effort.

[0021] Figure 1 This is a schematic flowchart of a method for pre-drying magnesium oxide powder for mineral-insulated cables according to an embodiment of the present invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0024] Furthermore, the use of terms such as "first" and "second" in this invention is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. When the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed by this invention.

[0025] Currently, the pre-drying treatment of magnesium oxide powder mainly employs methods such as high-temperature calcination, hot air rotary drying, or static vacuum drying. High-temperature calcination typically requires prolonged calcination at 850℃~950℃. While this method effectively removes moisture, it is extremely energy-intensive, and the high temperature can easily cause sintering and passivation of the magnesium oxide powder, reducing its packing density in the subsequent metal sheath. Hot air drying typically operates at temperatures between 300℃ and 600℃, with long drying times. Direct contact between hot air and magnesium oxide powder can easily introduce dust impurities, and there is also a risk of dust explosion. Although static vacuum drying can avoid oxidation, the migration rate of moisture in deep pores is slow, resulting in low drying efficiency. Furthermore, after drying, the powder needs to be transported to the filling station, where it is exposed to air, posing a risk of re-hygroscopicity.

[0026] Based on this, such as Figure 1 As shown, the present invention provides a method for pre-drying magnesium oxide powder for mineral-insulated cables, comprising the following steps:

[0027] Step S10, pretreatment: transfer the magnesium oxide powder to be treated to a closed fluidized bed system, purge and preheat it with dry nitrogen gas to reduce the oxygen content in the closed fluidized bed system to below 0.5%, and preheat the powder to 120~150℃.

[0028] Step S20: Rapid drying. Switch the inlet air of the closed fluidized bed system to superheated steam at 400℃~450℃ to fluidize and dry the magnesium oxide powder until the moisture content of the magnesium oxide powder drops to below 0.3%.

[0029] Step S30: Nitrogen gas switching and dehumidification. The air inlet of the closed fluidized bed system is switched to dry nitrogen gas at a temperature of 120℃~150℃ for 10~15 minutes to replace the residual water vapor in the bed and reduce the powder temperature to the hydrophobic reaction temperature window.

[0030] Step S40: In-situ gas-phase hydrophobication. The temperature of the closed fluidized bed system is controlled at 100℃~130℃. A gas-phase silane coupling agent is injected into the nitrogen gas flow to form a monomolecular hydrophobic layer of magnesium oxide powder.

[0031] Step S50: Nitrogen purging and cooling, shut off the silane coupling agent supply, and introduce dry nitrogen to cool the magnesium oxide powder to below 40°C, thereby removing unreacted silane coupling agent and reaction byproducts.

[0032] Step S60: Closed-loop conveying and filling. The purged and cooled magnesium oxide powder is conveyed through a closed pipeline to the filling station of the cable metal sheath under a dry nitrogen atmosphere to complete the filling.

[0033] The closed fluidized bed system involved in this invention includes: a fluidized bed drying chamber (with a built-in porous distribution plate, an opening rate of 5%, and a pore size of 1.0 mm), a superheated steam generating unit (including a steam boiler and a superheater), a nitrogen gas source, a vapor phase hydrophobic agent supply assembly (including a silane coupling agent storage tank, a nitrogen carrier gas atomizing gas, and a mass flow controller), a cyclone separator, a condensation recovery assembly, and a closed nitrogen gas delivery pipeline connecting the fluidized bed outlet to the filling hopper.

[0034] The present invention also discloses the following three sets of embodiments and three sets of comparative embodiments, and the specific implementation of each embodiment is as follows:

[0035] Example 1:

[0036] Raw material: Magnesium oxide powder (median diameter 65 μm, initial moisture content 0.85%, volume resistivity 1.2 × 10⁻⁶ at 20℃) 14 Ω*cm)

[0037] Step S1: Purge the closed fluidized bed system with dry nitrogen (dew point -52℃, oxygen content 5 ppm) at a gas velocity of 1.2 m / s for 18 min until the oxygen content in the closed fluidized bed system drops to 0.3%. Start the steam heat exchanger in the bed to raise the nitrogen temperature to 180℃ and preheat the magnesium oxide powder to be treated to 140℃.

[0038] Step S2: Switch the inlet air to 420℃ superheated steam, adjust the apparent gas velocity to 0.25 m / s (0.55 times the velocity carried out by the particle terminal), form stable turbulent fluidization, continuously feed through a two-stage airlock rotary valve, control the magnesium oxide powder to stay in the bed for an average of 5 minutes, the superheated steam carrying evaporated moisture is removed by a cyclone separator, and 35%±1% of the steam enters the condenser for condensation and discharge, the remaining 65%±1% of the steam is reheated to 420℃ by a steam superheater and then recycled;

[0039] Step S3: Close the superheated steam inlet valve and simultaneously open the dry nitrogen inlet valve to introduce dry nitrogen gas (dew point -55℃) at a temperature of 140℃ into the closed fluidized bed. The fluidization gas velocity is 0.8 m / s, and the purging is continued for 12 min to reduce the partial pressure of residual water vapor in the closed fluidized bed to 85 Pa, while the temperature of magnesium oxide powder drops from 420℃ to 125℃.

[0040] Step S4: Adjust the temperature in the closed fluidized bed to 115℃ and the fluidizing gas velocity to 0.4m / s. Add KH-550 (γ-aminopropyltriethoxysilane) to the nitrogen gas flow through the gas phase hydrophobic agent supply unit to form a nitrogen-coupling agent mixed gas and treat it for 40min. The volume concentration of the coupling agent is 1.0% and the nitrogen carrier gas flow rate is 3L / min.

[0041] Step S5: Stop supplying KH-550 and continue to purge with dry nitrogen at a gas velocity of 0.8 m / s for 18 minutes to cool the magnesium oxide powder to 38°C. During this process, the nitrogen temperature gradually decreases to room temperature, and the oxygen content in the closed fluidized bed system is maintained at 0.3%.

[0042] Step S6: Open the discharge valve of the closed fluidized bed system to allow magnesium oxide powder to fall into the closed screw conveyor protected by nitrogen (dew point -52℃), and be directly conveyed to the filling hopper of the mineral insulated cable at a flow rate of 10L / min, and filling is completed under nitrogen atmosphere.

[0043] Example 2:

[0044] Raw material: Magnesium oxide powder (median diameter 65 μm, initial moisture content 0.85%, volume resistivity 1.2 × 10⁻⁶ at 20℃) 14 Ω*cm)

[0045] Step S1: Purge the closed fluidized bed system with dry nitrogen (dew point -52℃, oxygen content 5 ppm) at a gas velocity of 1.2 m / s for 18 min until the oxygen content in the closed fluidized bed system drops to 0.3%. Start the steam heat exchanger in the bed to raise the nitrogen temperature to 180℃ and preheat the magnesium oxide powder to be treated to 140℃.

[0046] Step S2: Switch the inlet air to 450℃ superheated steam, adjust the apparent gas velocity to 0.35 m / s (0.7 times the velocity carried out by the particle terminal), form a stable turbulent fluidization, continuously feed through a two-stage airlock rotary valve, control the magnesium oxide powder to stay in the bed for an average of 3.5 min, the superheated steam carrying evaporated moisture is dedusted by a cyclone separator, and 40%±1% of the steam enters the condenser for condensation and discharge, the remaining 60%±1% of the steam is reheated to 420℃ by a steam superheater and then recycled;

[0047] Step S3: Close the superheated steam inlet valve and simultaneously open the dry nitrogen inlet valve to introduce dry nitrogen gas (dew point -55℃) at a temperature of 145℃ into the closed fluidized bed. The fluidization gas velocity is 0.9m / s, and the purging is continued for 10 min to reduce the partial pressure of residual water vapor in the closed fluidized bed to 85 Pa, while the temperature of magnesium oxide powder drops from 420℃ to 125℃.

[0048] Step S4: Adjust the temperature in the closed fluidized bed to 125℃, adjust the fluidizing gas velocity to 0.4m / s, and add vinyltrimethoxysilane (A-171) to the nitrogen gas flow through the gas phase hydrophobic agent supply unit to form a nitrogen-coupling agent mixed gas and treat it for 35min. The volume concentration of the coupling agent is 0.8%, and the nitrogen carrier gas flow rate is 3L / min.

[0049] Step S5: Stop supplying A-171, continue to purge with dry nitrogen at a gas velocity of 0.8 m / s for 18 min to cool the magnesium oxide powder to 38°C. During this process, the nitrogen temperature gradually decreases to room temperature, and the oxygen content in the closed fluidized bed system is maintained at 0.3%.

[0050] Step S6: Open the discharge valve of the closed fluidized bed system to allow magnesium oxide powder to fall into the closed screw conveyor protected by nitrogen (dew point -52℃), and be directly conveyed to the filling hopper of the mineral insulated cable at a flow rate of 10L / min, and filling is completed under nitrogen atmosphere.

[0051] It can be seen that, compared with Example 1, Example 2 adjusted the temperature of the superheated steam to a higher temperature, shortened the residence time, and adjusted the selection of the hydrophobic agent.

[0052] Example 3:

[0053] Raw material: Magnesium oxide powder (median diameter 65 μm, initial moisture content 0.85%, volume resistivity 1.2 × 10⁻⁶ at 20℃) 14 Ω*cm)

[0054] Step S1: Purge the closed fluidized bed system with dry nitrogen (dew point -52℃, oxygen content 5 ppm) at a gas velocity of 1.2 m / s for 18 min until the oxygen content in the closed fluidized bed system drops to 0.3%. Start the steam heat exchanger in the bed to raise the nitrogen temperature to 180℃ and preheat the magnesium oxide powder to be treated to 140℃.

[0055] Step S2: Switch the inlet air to 405℃ superheated steam, adjust the apparent gas velocity to 0.22 m / s (0.50 times the velocity carried out by the particle terminal), form stable turbulent fluidization, continuously feed through a two-stage airlock rotary valve, control the magnesium oxide powder to stay in the bed for an average of 6 minutes, the superheated steam carrying evaporated moisture is removed by a cyclone separator, and 35%±1% of the steam enters the condenser for condensation and discharge, the remaining 65%±1% of the steam is reheated to 420℃ by a steam superheater and then recycled;

[0056] Step S3: Close the superheated steam inlet valve and simultaneously open the dry nitrogen inlet valve to introduce dry nitrogen gas at a temperature of 135℃ (dew point -55℃) into the closed fluidized bed. The fluidization gas velocity is 0.7 m / s, and the purging is continued for 14 min to reduce the partial pressure of residual water vapor in the closed fluidized bed to 85 Pa, while the temperature of magnesium oxide powder drops from 420℃ to 125℃.

[0057] Step S4: Adjust the temperature in the closed fluidized bed to 115℃ and the fluidizing gas velocity to 0.4m / s. Add KH-560 (γ-glycidyl etheroxypropyltrimethoxysilane) to the nitrogen gas stream through the gas phase hydrophobic agent supply unit to form a nitrogen-coupling agent mixed gas and treat it for 45min. In the last 10min of the purging stage, a small amount of ammonia (0.1 vol%) is added to the nitrogen gas stream as a condensation reaction catalyst. The volume concentration of the coupling agent is 1.2%, and the nitrogen carrier gas flow rate is 2.5L / min.

[0058] Step S5: Stop supplying KH-560 and continue to purge with dry nitrogen at a gas velocity of 0.7 m / s for 20 minutes to cool the magnesium oxide powder to 35°C. During this process, the nitrogen temperature gradually decreases to room temperature, and the oxygen content in the closed fluidized bed system is maintained at 0.3%.

[0059] Step S6: Open the discharge valve of the closed fluidized bed system to allow magnesium oxide powder to fall into the closed screw conveyor protected by nitrogen (dew point -52℃), and be directly conveyed to the filling hopper of the mineral insulated cable at a flow rate of 10L / min, and filling is completed under nitrogen atmosphere.

[0060] It can be seen that Example 3 uses a different silane coupling agent and process parameters than Example 1, and introduces a trace amount of ammonia gas to catalyze the condensation reaction during the purging stage to obtain a denser hydrophobic layer.

[0061] Comparative Example 1:

[0062] Raw material: Magnesium oxide powder (median diameter 65 μm, initial moisture content 0.85%, volume resistivity 1.2 × 10⁻⁶ at 20℃) 14 Ω*cm)

[0063] Step S1: Place magnesium oxide powder in an open fluidized bed, introduce superheated steam at 350℃ (gauge pressure 0.1 MPa) at a gas velocity of 0.3 m / s, and dry for 15 min;

[0064] Step S2: After drying, the material is discharged and allowed to cool naturally to room temperature in the air without any hydrophobic treatment. Then, it is transferred to the filling station for filling.

[0065] It is understandable that Comparative Example 1 uses a conventional superheated steam drying process and does not take protective measures against magnesium oxide hydration.

[0066] Comparative Example 2:

[0067] Raw material: Magnesium oxide powder (median diameter 65 μm, initial moisture content 0.85%, volume resistivity 1.2 × 10⁻⁶ at 20℃) 14 Ω*cm)

[0068] Step S1: Place magnesium oxide powder in a rotary drying oven, introduce drying nitrogen gas at 180°C, rotate at 8 rpm, and dry for 60 min.

[0069] Step S2: After drying is complete, allow the material to be discharged and cooled.

[0070] Step S3: Mix the dried powder with an organosilicon oil-toluene solution (mass ratio of organosilicon oil to toluene is 100:5) and stir for 30 min. Centrifuge to remove the solvent and then vacuum dry at 120℃ for 2 h to remove residual toluene.

[0071] Step S4: The material is directly conveyed to the filling hopper of the mineral-insulated cable and filled in an air atmosphere.

[0072] It is understandable that Comparative Example 2 uses a traditional nitrogen hot air drying process combined with liquid phase silicone oil moisture-proof treatment.

[0073] Comparative Example 3:

[0074] Raw material: Magnesium oxide powder (median diameter 65 μm, initial moisture content 0.85%, volume resistivity 1.2 × 10⁻⁶ at 20℃) 14 Ω*cm)

[0075] Step S1: Place magnesium oxide powder in a vacuum drying oven, evacuate to -0.095 MPa, heat to 200℃, and statically dry for 120 min.

[0076] Step S2: After drying, the vacuum is broken and the material is discharged, then transported to the filling station in the air without any hydrophobic treatment.

[0077] It is understandable that, in Comparative Example 3, vacuum drying was used and the drying temperature was set at 200°C.

[0078] This invention also discloses the performance tests of Examples 1-3 and Comparative Examples 1-3. The products prepared in Examples 1-3 and Comparative Examples 1-3 were used as standard samples, and the specific test methods are as follows:

[0079] (1) Moisture content test:

[0080] Experimental equipment: Karl Fischer moisture analyzer;

[0081] Experimental procedure: Magnesium oxide powder treated in each group of examples / control examples was sampled under nitrogen protection and quickly transferred to a Karl Fischer titration cell for determination according to GB / T 6283; at the same time, the powder of each group was exposed to air at 25°C and 60% relative humidity for 24 h, and the moisture content was measured again to calculate the re-hygroscopic increment.

[0082] (2) Insulation resistance test:

[0083] Experimental equipment: High resistance meter (range 10) 16 Ω);

[0084] Experimental procedure: The treated magnesium oxide powder from each group was filled into copper sheathed tubes with a diameter of 5 mm and a length of 500 mm according to GB / T 13033 standard, and the compaction density was controlled at 3.0 g / cm³. 3 After standing for 24 hours at 20℃ and 50% relative humidity, the insulation resistance was measured.

[0085] (3) Filling density test:

[0086] Experimental equipment: High resistance meter (range 10) 16 Ω);

[0087] Experimental procedure: Fill each group of powders into a standard graduated cylinder, vibrate at a frequency of 2000 times / min for 5 minutes, measure the compaction density, and calculate the ratio of the compaction density to the theoretical density as the filling density.

[0088] The specific performance test results are shown in the table below:

[0089] Table 1. Performance test results of samples prepared in Examples 1-3 and Control Examples 1-3

[0090]

[0091] Experimental conclusion:

[0092] Based on the experimental data from Examples 1-3 and Comparative Examples 1-3 above, it can be seen that:

[0093] The final moisture content of Examples 1-3 was ≤0.05%, and the drying time was only 3~8 min (superheated steam section) + 10~15 min (nitrogen switching section), with a total processing time of no more than 30 min; while the control examples 2 and 3 (static vacuum) took longer, and the final moisture content of the three control examples was higher than that of the examples. After in-situ vapor-phase hydrophobication, the re-hygroscopic increment of Examples 1-3 after 24 hours of exposure to air was ≤0.015%, significantly lower than that of Control Example 1 (0.42%), Control Example 2 (0.28%), and Control Example 3 (0.55%), demonstrating that the in-situ hydrophobication and closed-loop transportation technology effectively blocked the re-hygroscopic path. The insulation resistance of Examples 1-3 was ≥6500 MΩ·km, which was about 2.2 to 3.4 times higher than that of Control Example 1, about 1.4 to 1.5 times higher than that of Control Example 2, and about 1.9 to 2.1 times higher than that of Control Example 3. The filling density of Examples 1-3 was ≥93.5%, significantly better than that of Control Example 1 (hydration and agglomeration resulted in a density of only 81.5%).

[0094] This invention avoids the risk of rehydration caused by low-temperature water vapor contacting magnesium oxide during the cooling process by using a two-stage atmosphere switching process of rapid drying with superheated steam and dehumidification with nitrogen. It also integrates drying, in-situ vapor-phase hydrophobication, cooling and closed filling processes into the same closed system, allowing the treated magnesium oxide powder to be directly transported to the cable filling station through a closed pipeline, avoiding the risk of re-hygroscopic absorption of magnesium oxide powder. Furthermore, it exhibits excellent insulation performance, indicating that the hydrophobic layer formed in situ has excellent thermal stability and long-term durability.

[0095] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for pre-drying magnesium oxide powder for mineral-insulated cables, characterized in that, Includes the following steps: Pretreatment involves transferring the magnesium oxide powder to be treated into a closed fluidized bed system, purging and preheating it with dry nitrogen gas to reduce the oxygen content in the closed fluidized bed system to below 0.5%, and preheating the powder to 120~150℃. Rapid drying: The intake air of the closed fluidized bed system is switched to superheated steam at 400℃~450℃ to fluidize and dry the magnesium oxide powder until the moisture content of the magnesium oxide powder drops to below 0.3%. Nitrogen gas switching dehumidification: The inlet gas of the closed fluidized bed system is switched to dry nitrogen gas at a temperature of 120℃~150℃ for 10~15 minutes to replace residual water vapor in the bed and reduce the powder temperature to the hydrophobic reaction temperature window. In-situ gas-phase hydrophobicization was carried out by controlling the temperature of the closed fluidized bed system at 100℃~130℃ and injecting a gas-phase silane coupling agent into the nitrogen gas flow, so that magnesium oxide powder formed a monomolecular hydrophobic layer. Nitrogen gas was used for purging and cooling. The delivery of the silane coupling agent was turned off. Dry nitrogen gas was introduced to cool the magnesium oxide powder to below 40°C, which carried out the unreacted silane coupling agent and reaction byproducts. The magnesium oxide powder, after being purged and cooled, is transported through a closed pipeline to the filling station of the cable's metal sheath under a dry nitrogen atmosphere to complete the filling process.

2. The method for pre-drying magnesium oxide powder for mineral-insulated cables as described in claim 1, characterized in that, In the pretreatment step, the dew point of the dry nitrogen is ≤-50℃, the oxygen content is ≤10ppm, the purging gas velocity is 0.8~1.5m / s, and the purging time is 15~20min.

3. The method for pre-drying magnesium oxide powder for mineral-insulated cables as described in claim 1, characterized in that, In the rapid drying step, the apparent gas velocity of the fluidized drying is controlled to be 0.4 to 0.8 times the carry-out velocity of the magnesium oxide particles.

4. The method for pre-drying magnesium oxide powder for mineral-insulated cables as described in claim 3, characterized in that, The median diameter of the magnesium oxide powder to be treated is 40~100 μm, and the apparent gas velocity of the fluidized drying is 0.12~0.48 m / s.

5. The method for pre-drying magnesium oxide powder for mineral-insulated cables as described in claim 1, characterized in that, The silane coupling agent is one or more of vinyltrimethoxysilane, γ-aminopropyltriethoxysilane, or γ-glycidoxypropyltrimethoxysilane; the volume concentration of the silane coupling agent in the nitrogen-coupling agent mixed gas is 0.5-1.5%, the nitrogen carrier gas flow rate is 2-5 L / min, and the treatment time is 30-50 min.

6. The method for pre-drying magnesium oxide powder for mineral-insulated cables as described in claim 1, characterized in that, In the nitrogen purging and cooling step, the gas velocity of the dry nitrogen is 0.5~1.0 m / s, the purging time is 15~20 min, and the oxygen content in the closed fluidized bed system is always maintained below 0.5% during the cooling process.

7. The method for pre-drying magnesium oxide powder for mineral-insulated cables as described in claim 1, characterized in that, In the nitrogen-switching dehumidification step, the moisture content of magnesium oxide powder is monitored in real time by an online infrared moisture meter. When the moisture content is detected to drop below 0.3%, the atmosphere is switched from superheated steam to dry high-purity nitrogen.

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