Production method, additive and application of electrical-grade magnesium oxide powder for inhibiting blackening

Abstract

The invention discloses a production method of electrical grade magnesium oxide powder for inhibiting blackening, an additive and application. The production method comprises the following steps: S1, taking iron oxide and quartz powder as raw materials, preparing materials according to a molar ratio of Fe to Si of 2: 3, and performing high-temperature synthesis to obtain an iron and ferrous silicate compound; s2, the compound is smashed and screened, and particles with the particle size ranging from 100 meshes to 300 meshes are selected; s3, firing the particles at 900-1000 DEG C for 2-3 hours, and converting low-valence iron into high-valence iron in a high-temperature oxidation environment to obtain a blackening inhibiting additive mainly containing iron silicate; and step S4, uniformly mixing the additive into the fused magnesium oxide base powder according to the weight ratio of 0.3-1.0%. The high-temperature oxidizing property of Fe < 3 + > in high-iron silicate is utilized, carbon generated by carbonization of organic matter can be effectively oxidized into gas to be discharged, aluminum separated out by a heating wire is oxidized into insulating aluminum oxide, and reaction byproducts ferrous silicate and silicon dioxide are insulators. The magnesia powder can obviously solve the problem of organic and inorganic blackening of the electric heating element, improve the insulating property and prolong the service life.

Description

Technical Field

[0001] This invention belongs to the field of electrical insulation materials technology, and more specifically relates to a method for producing electrical grade magnesium oxide powder, additives, and applications for inhibiting blackening. Background Technology

[0002] Electrical-grade magnesium oxide powder is the core functional material of tubular electric heating elements. It fills the space between the metal tube shell and the internal spiral heating wire, undertaking a highly contradictory dual task: on the one hand, it must have extremely high insulation and dielectric strength to ensure the safety of electrical use; on the other hand, it must have excellent thermal conductivity to quickly transfer the heat generated by the heating wire to the surface of the tube shell.

[0003] With the rapid development of the electric heating field, especially the small household appliance industry, the requirements for the performance of electric heating elements, particularly their electrical performance, are becoming increasingly stringent. Under this trend, "magnesium oxide blackening" has become a long-standing technical problem plaguing the industry. According to research, the blackening phenomenon can be mainly attributed to two distinct mechanisms: 1. Organic products turn black: This is the most common form of blackening. To improve moisture resistance, electrical-grade magnesium oxide powder typically undergoes varying degrees of hydrophobic treatment on the particle surface using organosilicon, usually at a dosage of 50 ppm to 0.4 wt%. Furthermore, due to control factors, varying degrees of organic contaminants are introduced during the manufacturing process of heating elements, such as from tubes, wires, lead rods, powder filling machines, and even the MgO itself. When the heating element operates under heat in a relatively enclosed and oxygen-deficient environment, these organic macromolecular chains undergo thermal decomposition, dehydrogenating to form elemental carbon (C).

[0004] Carbon is an excellent conductor. Even trace amounts of carbon deposition can form conductive pathways on the surface of magnesium oxide particles, leading to an exponential decrease in insulation resistance and a sharp increase in the loss tangent, ultimately causing leakage or breakdown accidents. Visually, this blackening manifests as the powder appearing grayish-black overall or in localized areas.

[0005] 2. Non-organic (inorganic) blackening: This blackening often occurs in high-temperature, high-load electric heating tubes that use iron-chromium-aluminum (0Cr25Al5, etc.) heating wires. Although iron-chromium-aluminum alloys are resistant to high temperatures, at high temperatures, the aluminum element (Al) in them has a very high activity and vapor pressure, and tends to precipitate from the crystal lattice and diffuse into the surrounding magnesium oxide medium.

[0006] The precipitated aluminum is a strong reducing agent; it may undergo a displacement reaction with magnesium oxide or react with trace impurities in the powder (such as iron oxide or silicon oxide) to form black, non-stoichiometric compounds or low-valence metal oxides. This blackening typically spreads axially around the heating filament and expands until it covers the entire filament. The formation of this conductive material also severely degrades the electrical performance of the component.

[0007] Existing technologies for addressing blackening issues primarily focus on prevention, such as reducing silicone oil usage, improving the quality of cleaning tubes, wires, and lead rods, and enhancing on-site control, or using higher-grade high-temperature resistant alloys. However, simply reducing silicone oil sacrifices moisture resistance; increasing purity significantly increases costs. Adding oxidants, such as magnesium peroxide and manganese dioxide, has also been attempted, but these oxidants either have poor thermal stability, produce harmful byproducts, or have low insulating strength themselves and their decomposition products, limiting their application and effectiveness.

[0008] Therefore, developing an inhibitor that can continuously exert an oxidizing effect at high temperatures to eliminate carbon deposits and aluminum precipitation, without introducing harmful impurities, and whose product has excellent insulation properties is a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] The primary objective of this invention is to provide a method for producing electrical-grade magnesium oxide powder that can significantly suppress blackening. This method is based on redox chemistry principles and innovatively introduces high-valent iron (Fe²⁺). 3+ It serves as an oxidation core and stabilizes it in the silicate lattice structure, thereby achieving dual suppression of organic carbonization and inorganic metal precipitation.

[0010] Another object of the present invention is to provide a blackening inhibitor prepared by the above method, which has the characteristics of high temperature resistance and insulating reaction products.

[0011] A third objective of this invention is to provide an electrical-grade magnesium oxide powder containing the above-mentioned additives and its application in an electric heating element.

[0012] To achieve the above objectives, the present invention adopts the following technical solution: A method for producing electrical grade magnesium oxide powder that inhibits blackening, the method comprising the following steps: Step S1: Precursor formulation and synthesis Iron oxide (Fe2O3) and quartz powder (SiO2) were selected as the main raw materials and precisely measured and mixed according to a molar ratio of iron (Fe) to silicon (Si) of 2:3. The mixture was then processed using a high-temperature synthesis process to prepare and purify the iron and ferrous silicate composite. The key to this step is the formation of a Fe-Si-O framework structure. Because iron undergoes multiple valence states at high temperatures, the products at this stage may include ferric silicate (Fe2(SiO3)3), ferrous silicate (FeSiO3), and intermediate iron-silicon oxides.

[0013] Step S2: Mechanical activation and grading The iron and ferrous silicate composite obtained in step S1 is mechanically pulverized in a pulverizing device, and the pulverized powder is sieved to select particles with a particle size distribution between 100 mesh and 300 mesh as intermediates. To prevent the introduction of metallic iron impurities (which could lead to short circuit hazards), a ceramic liner and ceramic grinding media must be used. After pulverization, the powder is poured out and strictly sieved to select particles with a particle size between 100 mesh and 300 mesh (approximately 150 μm to 48 μm) for later use.

[0014] Particle size control is crucial: oversized particles (<100 mesh) will result in uneven distribution in magnesium oxide, forming localized spots; overly fine particles (>300 mesh) are prone to agglomeration, and excessively high reactivity may lead to rapid consumption in the early stages.

[0015] Step S3: High-temperature oxidation crystal form reconstruction The intermediate particles obtained in step S2 are placed in a high-temperature furnace and calcined in an oxidizing atmosphere. The calcination temperature is controlled between 900°C and 1000°C, and the holding time is 2 to 3 hours. After cooling to room temperature in the furnace, the particles are removed to obtain an iron silicate compound (Fe2(SiO3)3) mainly composed of ferrous iron, which is the blackening inhibitor. This step is the key step of this invention. Through long-term calcination in a high-temperature, oxygen-rich environment, the ferrous iron (Fe2(SiO3)3) that may be present in the composite is eliminated. 2+ Forced oxidation to trivalent iron (Fe) 3+ This process involves the formation of thermodynamically stable iron silicate compounds. This high-valence iron silicate acts as a blackening inhibitor with "chemical potential energy."

[0016] Step S4: Compound Modification The blackening-inhibiting additive prepared in step S3 is added to the fused magnesium oxide-based powder at a weight percentage of 0.3% to 1.0%, and mechanically mixed until homogeneous to obtain the blackening-inhibiting electrical grade magnesium oxide powder.

[0017] In step S1, the high-temperature synthesis process is selected from the high-temperature solid-state reaction method or the molten salt synthesis method; When using the high-temperature solid-state reaction method, the sintering temperature is 1100℃-1300℃, and the holding time is 3-6 hours. When using the molten salt synthesis method, the molten salt medium used is sodium chloride-potassium chloride eutectic salt or sodium sulfate molten salt, and the reaction temperature is 800℃-1000℃.

[0018] In step S2, the pulverizing equipment is a ceramic ball mill or a high-energy hammer mill, and the grinding media used in the pulverizing process are zirconia balls or corundum balls to avoid introducing exogenous metal impurities; the sieving process uses a standard Taylor screen for grading.

[0019] In step S3, the oxidizing atmosphere is flowing air or oxygen-enriched air; the furnace cooling rate is controlled at 2°C / min to 5°C / min to maintain the stability of the iron silicate crystal structure.

[0020] In step S4, the fused magnesium oxide base powder is selected from medium-temperature moisture-proof electrical grade magnesium oxide powder or high-temperature electrical grade magnesium oxide powder; the uniform mixing means that the coefficient of variation (CV value) of iron element in the mixed material is less than 5%.

[0021] The medium-temperature moisture-proof electrical grade magnesium oxide powder is coated with organosilicon oil, and the content of the organosilicon oil is 0.25wt%-0.40wt%.

[0022] The present invention also provides an additive to inhibit blackening, which is prepared by the above steps S1 to S3; The main crystalline phase of this additive is iron silicate (Fe2(SiO3)3), and it appears as a reddish-brown or dark brown powder. This additive exhibits high-temperature oxidizing activity and can undergo the following chemical reaction in high-temperature environments above 700℃: For carbon (C): C + Fe2(SiO3)3 = CO↑ + 2FeSiO3 + SiO2; For aluminum (Al): 2Al + 3Fe2(SiO3)3 = Al2O3 + 6FeSiO3 + 3SiO2.

[0023] The present invention also provides an electrical grade magnesium oxide powder that inhibits blackening, which is prepared by the production method of steps S1 to S4 described above; The magnesium oxide powder comprises a matrix component and a modifying component. The matrix component is electrofused crystalline magnesium oxide particles, and the modifying component is a blackening inhibitor dispersed in the matrix component. The content of the blackening inhibitor is 0.3wt% to 1.0wt% of the total weight of magnesium oxide powder.

[0024] This magnesium oxide powder has the following performance characteristics: (a) After the heating tube was heat-treated at 900°C for 30 minutes, there were no visible black carbonized spots on the inner insulation layer of the dissected tube. (b) At a pipe load of 10 W / cm 2 After 100 hours of dry burning with electricity under the specified conditions, no black discoloration area caused by metal precipitation appeared in the powder at the interface with the heating wire.

[0025] The volume resistivity of this magnesium oxide powder at room temperature is greater than 10. 12 Ω·cm, and the hot volume resistivity is greater than 10 at an operating temperature of 800℃. 8 Ω·cm.

[0026] The present invention also provides an electric heating element, comprising a metal shell, a spiral heating wire coaxially disposed inside the metal shell, and an insulating filler densely filling the space between the metal shell and the heating wire; The insulating filler is the aforementioned electrical grade magnesium oxide powder that inhibits blackening.

[0027] The heating wire is made of iron-chromium-aluminum alloy (Fe-Cr-Al) or nickel-chromium alloy (Ni-Cr); the metal shell is made of stainless steel, Ingleshire alloy or copper.

[0028] The heating element is an air dry-burning heating tube, a mold heating tube, or a liquid heating tube.

[0029] The theoretical basis of this invention lies in utilizing the high-valence iron ions (Fe) in ferric silicate. 3+ The strong oxidizing properties of the redox system allow for the construction of a microscopic "self-cleaning" redox system in a high-temperature insulating environment.

[0030] The principle of this invention is as follows: 1. Inhibition mechanism for blackening of organic compounds: When organic matter (such as silicone oil or lubricating oil) in magnesium oxide powder decomposes to produce carbon (C) at high temperatures, the dispersed ferrosilicon particles act as oxidants. Above 900℃, the following solid-solid or gas-solid reactions occur: C+Fe2(SiO3)3=CO↑+2FeSiO3+SiO2 Product Analysis: CO: Carbon monoxide gas. When generated in trace amounts, it will escape through the pipe or be further oxidized into CO2 and discharged, thus eliminating the black carbon source.

[0031] FeSiO3: Ferrous silicate, a light-colored silicate mineral with good insulation properties.

[0032] SiO2: Silicon dioxide, which is itself an excellent insulator.

[0033] Thus, the black conductive carbon is transformed into insulating silicates and gas, and the powder regains its original color.

[0034] 2. Inhibition mechanism for inorganic blackening: When active aluminum (Al) atoms are released from the iron-chromium-aluminum heating wire, aluminum, as a strong reducing agent, will preferentially react with nearby iron silicate: 2Al+3Fe2(SiO3)3=Al2O3+6FeSiO3+3SiO2 Product Analysis: Al2O3: Alumina, is a top-grade high-temperature resistant insulating ceramic material, and its color is white.

[0035] FeSiO3 and SiO2: Same as above, both are insulators.

[0036] This reaction not only eliminates the black aluminum precipitates, but the resulting aluminum oxide layer may even deposit on the surface of the wire, forming a passivation protective layer that further hinders the diffusion of aluminum.

[0037] The beneficial effects of this invention are: Significant de-blackening effect: Experiments show that after the magnesium oxide powder with the inhibitor of this invention is heat-treated at 900℃ or dry-burned for 100 hours, the MgO insulation layer of the electric heating tube made of it can maintain a good normal state, eliminating the blackening phenomenon of organic carbonization and inorganic precipitation.

[0038] Electrical safety: Unlike traditional additives, the reaction products of this invention (ferrous silicate, alumina, and silicon dioxide) are all high-temperature insulating materials. This means that the process of inhibiting blackening does not come at the expense of insulation resistance or dielectric strength, ensuring the long-term electrical reliability of the heating element.

[0039] High process adaptability: This additive is not only suitable for ordinary medium-temperature moisture-proof powder containing organosilicon, but also for high-temperature modified powder. Moreover, the preparation process is simple, the raw materials (iron oxide, quartz powder) are widely available and inexpensive, making it suitable for large-scale industrial production. Attached Figure Description

[0040] Figure 1 This is a comparison diagram of the effect of Example 3 of the present invention and its control group A on inhibiting the blackening of organic compounds.

[0041] Figure 2 This is an image showing the effect of inhibiting blackening of non-organic (inorganic) substances in Embodiment 4 of the present invention.

[0042] Figure 3 This is an image showing the effect of the control group in Example 4 of the present invention in inhibiting the blackening of non-organic (inorganic) substances. Detailed Implementation

[0043] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. 1. Raw material preparation and specifications

[0044] The selection of raw materials is crucial to ensuring the purity and reactivity of the final additive.

[0045] Iron oxide (Fe2O3): Select red iron oxide powder of analytical grade (AR) or chemical grade (CP) with a particle size D50 < 10 μm to ensure the contact area for solid-phase reaction.

[0046] Quartz powder (SiO2): High-purity micro powder with a purity greater than 99.5% should be selected. The content of alkali metal oxides (K2O, Na2O) in the impurity components should be extremely low to avoid the formation of a low-melting-point glass phase at high temperatures. 2. Preparation process examples of blackening inhibitors

[0047] Example 1: High-temperature solid-state preparation (1) Ingredients: Weigh 160g (about 1mol) of chemically pure iron oxide and 180g (about 3mol) of quartz powder to meet the molar ratio requirement of Fe:Si=2:3. Put both into a high-speed mixer and premix for 30 minutes.

[0048] (2) Synthesis: The mixed powder was loaded into a high-purity alumina crucible and slightly compacted. It was then placed in a high-temperature furnace with silicon molybdenum rods. The heating program was set to increase the temperature to 1250℃ at a rate of 5℃ / min and hold for 4 hours. At this temperature, iron oxide and silicon dioxide undergo a solid-phase diffusion reaction to form a dense iron-silicate composite.

[0049] (3) Crushing and sieving: After natural cooling, the sintered blocks are removed and appear as hard, dark brown lumps. They are first coarsely crushed using a jaw crusher, and then transferred to a ball mill lined with zirconia. Zirconia balls with a diameter of 5-10 mm are added at a ball-to-material ratio of 5:1, and the mill is run for 6 hours. The powder is then poured out and classified using a vibrating screen. The components that pass through a 100-mesh screen but remain between the 300-mesh screen are collected.

[0050] (4) Oxidation activation: The selected particles were spread evenly in a shallow corundum dish, with a thickness not exceeding 2 cm, to facilitate oxygen penetration. The dish was placed in a box-type resistance furnace and heated to 950°C under a flowing air atmosphere, and calcined at a constant temperature for 2.5 hours. After cooling, a reddish-brown final additive was obtained.

[0051] Example 2: Preparation by molten salt method (1) Ingredients: In addition to iron oxide and quartz powder in a molar ratio of Fe:Si=2:3, an additional molten salt (NaCl:KCl=1:1 molar ratio) accounting for 50% of the total raw material weight is added as a reaction medium.

[0052] (2) Synthesis: The mixture was reacted at 900℃ for 3 hours. The molten salt formed a liquid phase environment, which greatly accelerated the diffusion of ions and the growth of crystals, and helped to reduce the synthesis temperature and improve the uniformity of the product.

[0053] (3) Post-treatment: After the reaction product is cooled, it is repeatedly washed with deionized water to remove soluble salts (NaCl / KCl) until no precipitate is detected in the washing solution by silver nitrate test. After drying, it is pulverized and sieved (100-300 mesh).

[0054] (4) Oxidative activation: Same as in Example 1, calcined at 900°C for 3 hours to ensure complete conversion of residual trace amounts of low-valent iron. 3. Preparation Examples of Electrical Grade Magnesium Oxide Powder

[0055] Example 3: Modification of medium-temperature moisture-proof powder Base material: 1000 kg of commercially available D-900 type medium-temperature electrical grade magnesium oxide powder was selected. The powder was sprayed with 0.35% methyl silicone oil for moisture protection before leaving the factory.

[0056] Addition: Weigh 5 kg of the additive prepared in Example 1.

[0057] Mixing: A large double-cone rotary vacuum dryer (without vacuum) was used at a speed of 15 r / min for 45 minutes. Samples were taken after mixing; the powder color was uniform with no visible color difference.

[0058] Effect verification test: Prepare heating element samples. Use SUS304 tubing with a diameter of 8.0 mm and a length of 500 mm. Fill the tubing with the modified magnesium powder and reduce the diameter to 6.6 mm using a tubing shrinker.

[0059] The heating element was placed in a high-temperature furnace at 900℃ for simulated heat treatment (annealing) for 30 minutes. During this time, the silicone oil inside the element decomposed violently.

[0060] After cooling, dissect the tube for observation, such as... Figure 1 As shown: Control group A (no inhibitor added): The magnesium powder was dark gray with obvious carbon black streaks.

[0061] In this example, the magnesium powder is grayish-white to white overall, with only very faint traces of beige, indicating that most of the carbon black has been oxidized and eliminated.

[0062] Example 4: Modification of high-temperature powder Base material: 1000 kg of high-temperature electrical grade magnesium oxide powder.

[0063] Addition: Weigh 4 kg of the additive prepared in Example 1.

[0064] Mixing: Same as above.

[0065] Effect verification test: Manufacture a high-load single-ended electric heating element. The heating wire is made of 0Cr25Al5 iron-chromium-aluminum alloy wire. The surface load of the element is designed to be 10W / cm². 2 .

[0066] After the heating element is sealed, it is dry-burned by applying electricity in still air.

[0067] Dissection after 100 hours of continuous power supply followed by power cut-off: Control group B (no inhibitor added): The heating wire spiral coil was surrounded by a distinct black hard shell, which was confirmed by analysis to be aluminate and non-stoichiometric oxides.

[0068] In this embodiment: the area around the heating wire is clean, the magnesium powder retains its original color, and the interface is clear. 4. Summary Table of Experimental Data

[0069] The table below summarizes the performance comparison under different addition ratios and process conditions: Table 1 Experiment number Additive preparation temperature Additive particle size Add ratio (wt%) Organic blackening at 900℃ for 30 minutes <![CDATA[10W / cm 2 Inorganic blackening after 100 hours]]> Insulation strength / Dielectric strength Control group A - - 0 Severely blackened (dark gray) - Insulation resistance < 100MΩ, withstand voltage < 1000V / 0.5mA Example 3 950℃ 100-300 mesh 0.5 No obvious blackening (white) - Insulation resistance > 1000MΩ, withstand voltage > 2000V / 0.5mA Example 4 950℃ 100-300 mesh 0.4 - No obvious blackening (white) Insulation resistance > 1000MΩ, withstand voltage > 2000V / 0.5mA Control group B - - 0 - Severely darkened (black ring around the silk threads) Insulation > 1000MΩ, withstand voltage < 1000V / 0.5mA, blackened area with MgO leakage current. As shown in Table 1, the results of the examples and the control group indicate that without the addition of a blackening inhibitor, electrical grade magnesium oxide powder inevitably exhibits severe blackening under high-temperature treatment or high-load dry-firing conditions, accompanied by a significant decrease in insulation resistance and withstand voltage. However, in Examples 3 and 4, by introducing only 0.4wt%–0.5wt% of a blackening inhibitor with a particle size of 100–300 mesh into the magnesium oxide powder, it was possible to achieve blackening under organic blackening conditions at 900℃ / 30min and with a resistance of 10 W / cm². 2 Under conditions of heavy load and 100 hours of dry burning, this invention effectively suppresses both types of blackening phenomena simultaneously. Furthermore, while inhibiting blackening, the magnesium oxide powder's insulation resistance and withstand voltage performance not only do not decrease due to the introduction of additives, but remain at a high level. These results demonstrate that this invention achieves simultaneous suppression of both organic and inorganic blackening with extremely low additive amounts, overcoming the existing technical understanding that "blackening suppression inevitably introduces a conductive phase or sacrifices insulation performance." The synergistic improvement in both suppression effect and electrical performance stability yields technical effects that are difficult for those skilled in the art to anticipate.

[0070] This invention synthesizes an iron silicate additive with specific redox properties through rigorous process control. This technical solution ingeniously solves the long-standing blackening problem in the electric heating element industry, without introducing harmful impurities, and possesses extremely high industrial application value and market prospects.

Claims

1. A method for producing electrical grade magnesium oxide powder that inhibits blackening, characterized in that, The method includes the following steps: Step S1: Precursor formulation and synthesis Iron oxide (Fe2O3) and quartz powder (SiO2) were selected as the main raw materials, and the iron (Fe) and silicon (Si) elements were precisely measured and mixed according to a molar ratio of 2:

3. The mixture was treated by a high-temperature synthesis process to prepare and purify the iron and ferrous silicate composite. Step S2: Mechanical activation and grading The iron and ferrous silicate complex obtained in step S1 is placed in a pulverizing device for mechanical pulverization, and the pulverized powder is sieved to select particles with a particle size distribution between 100 mesh and 300 mesh as intermediates. Step S3: High-temperature oxidation crystal form reconstruction The intermediate particles obtained in step S2 are placed in a high-temperature furnace and calcined in an oxidizing atmosphere. The calcination temperature is controlled between 900°C and 1000°C, and the holding time is 2 to 3 hours. After cooling to room temperature in the furnace, the particles are removed to obtain iron silicate compound (Fe2(SiO3)3) mainly composed of high-valence iron, which is the blackening inhibitor. Step S4: Compound Modification The blackening-inhibiting additive prepared in step S3 is added to the fused magnesium oxide-based powder at a weight percentage of 0.3% to 1.0%, and mechanically mixed until homogeneous to obtain the blackening-inhibiting electrical grade magnesium oxide powder.

2. The production method according to claim 1, characterized in that, In step S1, the high-temperature synthesis process is selected from the high-temperature solid-state reaction method or the molten salt synthesis method; When using the high-temperature solid-state reaction method, the sintering temperature is 1100℃-1300℃, and the holding time is 3-6 hours. When using the molten salt synthesis method, the molten salt medium used is sodium chloride-potassium chloride eutectic salt or sodium sulfate molten salt, and the reaction temperature is 800℃-1000℃.

3. The production method according to claim 1, characterized in that, In step S2, the pulverizing equipment is a ceramic ball mill or a high-energy hammer mill, and the grinding media used in the pulverizing process are zirconia balls or corundum balls to avoid introducing exogenous metal impurities; the sieving process uses a standard Taylor screen for grading.

4. The production method according to claim 1, characterized in that, In step S3, the oxidizing atmosphere is flowing air or oxygen-enriched air; the furnace cooling rate is controlled at 2°C / min to 5°C / min to maintain the stability of the iron silicate crystal structure.

5. The production method according to claim 1, characterized in that, In step S4, the fused magnesium oxide base powder is selected from medium-temperature moisture-proof electrical grade magnesium oxide powder or high-temperature electrical grade magnesium oxide powder; the uniform mixing means that the coefficient of variation (CV value) of iron element in the mixed material is less than 5%.

6. The production method according to claim 5, characterized in that, The medium-temperature moisture-proof electrical grade magnesium oxide powder is coated with organosilicon oil, and the content of the organosilicon oil is 0.25wt%-0.40wt%.

7. A blackening-inhibiting additive, characterized in that, The additive is prepared by any one of steps S1 to S3 according to claims 1 to 6; The main crystalline phase of this additive is iron silicate (Fe2(SiO3)3), and it appears as a reddish-brown or dark brown powder. This additive exhibits high-temperature oxidizing activity and can undergo the following chemical reaction in high-temperature environments above 700℃: For carbon (C): C + Fe2(SiO3)3 = CO↑ + 2FeSiO3 + SiO2; For aluminum (Al): 2Al + 3Fe2(SiO3)3 = Al2O3 + 6FeSiO3 + 3SiO2.

8. An electrical grade magnesium oxide powder that inhibits blackening, characterized in that, The magnesium oxide powder is prepared by the production method according to any one of claims 1 to 6; The magnesium oxide powder comprises a matrix component and a modifying component. The matrix component is electrofused crystalline magnesium oxide particles, and the modifying component is a blackening inhibitor dispersed in the matrix component. The content of the blackening inhibitor is 0.3wt% to 1.0wt% of the total weight of magnesium oxide powder.

9. The electrical grade magnesium oxide powder according to claim 8, characterized in that, This magnesium oxide powder has the following performance characteristics: (a) After the heating tube was heat-treated at 900°C for 30 minutes, there were no visible black carbonized spots on the inner insulation layer of the dissected tube. (b) At a pipe load of 10 W / cm 2 After 100 hours of dry burning with electricity under the specified conditions, no black discoloration area caused by metal precipitation appeared in the powder at the interface with the heating wire.

10. The electrical grade magnesium oxide powder according to claim 8, characterized in that, The magnesium oxide powder has a volume resistivity greater than 10 at room temperature. 12 Ω·cm, and the hot volume resistivity is greater than 10 at an operating temperature of 800℃. 8 Ω·cm.

11. A heating element, characterized in that, It includes a metal tube shell, a spiral heating wire coaxially disposed inside the metal tube shell, and an insulating filler densely filling the space between the metal tube shell and the heating wire; The insulating filler is the electrical grade magnesium oxide powder for inhibiting blackening as described in any one of claims 8 to 10.

12. The heating element according to claim 11, characterized in that, The heating wire is made of iron-chromium-aluminum alloy (Fe-Cr-Al) or nickel-chromium alloy (Ni-Cr); the metal shell is made of stainless steel, Ingleshire alloy or copper.

13. The heating element according to claim 11, characterized in that, The heating element is an air dry-burning heating tube, a mold heating tube, or a liquid heating tube.

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