Hydromagnesite fractional precipitation and double-type compound silicon steel magnesium oxide and preparation method

Abstract

The invention discloses hydromagnesite fractional precipitation and double-type compound silicon steel magnesium oxide and a preparation method thereof, and the preparation method comprises the following steps: mixing gradient-calcined light-boiled hydromagnesite with a hydrochloric acid solution to obtain a magnesium chloride solution; a-type magnesium oxide with the CAA activity of 20-50 s and B-type magnesium oxide with the CAA activity of 80-120 s are prepared by adding a precipitator ammonium bicarbonate or sodium hydroxide into a magnesium chloride solution through fractional precipitation, and the A-type magnesium oxide and the B-type magnesium oxide are regulated and controlled according to the physical compounding proportion to obtain the double-type compounded silicon steel magnesium oxide with the average particle size smaller than or equal to 3 microns, the purity larger than or equal to 99% and the CAA activity of 50-80 s. The method synergistically solves the contradiction between the silicon steel magnesium oxide hydration rate and the suspension stability, and is suitable for preparing the high-magnetic-induction oriented silicon steel annealing isolation coating.

Description

Technical Field

[0001] This invention belongs to the technical field of silicon steel annealing release agent materials, specifically relating to a silicon steel magnesium oxide material based on stepwise precipitation of hydromagnesia and dual-type compounding and its preparation method. Background Technology

[0002] As a high-temperature annealing isolating agent and insulating film forming agent for grain-oriented silicon steel, silicon steel grade magnesium oxide must simultaneously achieve three major functions during the high-temperature annealing process of silicon steel sheets: isolating and preventing sintering, desulfurizing and dephosphorizing, and forming a dense magnesium silicate underlayer. Achieving these functions places stringent requirements on the purity, hydration rate, suspension properties, and adhesion of magnesium oxide. The quality of magnesium oxide directly determines the density, adhesion, and magnetic domain refinement effect of the magnesium silicate insulating underlayer.

[0003] Currently, commonly used industrial preparation methods include the brine-ammonium bicarbonate method, the brine-ammonia method, and the ore carbonization method. However, these traditional processes generally suffer from low efficiency, high energy consumption, and significant environmental pollution. For example, the brine-ammonium bicarbonate method has drawbacks such as difficult byproduct treatment, low raw material utilization, or unstable product performance. The ore carbonization method is affected by raw material impurities, resulting in large fluctuations in product purity, and energy consumption accounts for a high proportion of production costs. Furthermore, traditional processes struggle to balance the conflict between activity and hydration rate; excessively high activity can easily lead to hydration agglomeration, while high-temperature calcination, although reducing activity, causes deterioration in suspension properties. Summary of the Invention

[0004] To address the aforementioned deficiencies in existing technologies, the present invention aims to provide a silicon steel magnesium oxide material based on stepwise precipitation of magnesite and dual-type compounding, and its preparation method. A high-purity magnesium source is obtained through a gradient calcination-acid leaching purification process. A stepwise precipitation technique is employed to prepare magnesium oxide precursors with differentiated activities. Furthermore, physical compounding achieves precise matching of particle morphology and reactivity, ultimately yielding a high-purity silicon steel magnesium oxide material with appropriate hydration rate and excellent suspension stability.

[0005] The present invention is achieved through the following technical solution.

[0006] One aspect of the present invention provides a method for preparing magnesium oxide from hydromagnesite through stepwise precipitation and dual-type composite silicon steel, comprising the following steps: (a) Gradient calcination of hydromagnesite ore powder to obtain lightly calcined hydromagnesite; (b) Mix lightly calcined magnesite with hydrochloric acid solution at a solid-liquid ratio of 1:(3-5), control the pH value and reaction temperature, and filter to obtain magnesium chloride solution; (c) Stepwise precipitation preparation of type A and type B magnesium oxide: Preparation of type A magnesium oxide: Add precipitant to magnesium chloride solution according to the molar ratio of precipitant to magnesium chloride of (1.05-1.15):1, control the pH at the reaction endpoint, generate basic magnesium carbonate precipitate, and obtain type A magnesium oxide with CAA activity of 20-50s after washing, drying and calcination. Preparation of type B magnesium oxide: Add precipitant to magnesium chloride solution according to the molar ratio of precipitant to magnesium chloride of (2.1-2.2):1, control the pH at the reaction endpoint, generate magnesium hydroxide precipitate, and obtain type B magnesium oxide with CAA activity of 80-120s after washing, drying and calcination. (d) After mixing type A and type B magnesium oxide at a mass ratio of 1:(4-1) at high speed, a dual-type composite silicon steel magnesium oxide is obtained.

[0007] Preferably, in step (a), the gradient calcination is carried out in three stages of temperature control: the first stage is held at 500-550℃ for 0.5 hours, the second stage is held at 600-650℃ for 1 hour, and the third stage is held at 680-700℃ for 0.5 hours.

[0008] Preferably, in step (b), the concentration of the hydrochloric acid solution is 2.0-3.0 mol / L, and the SiO2 content and Fe2O3 content in the solution after acid leaching are ≤ 20 mg / L and ≤ 20 mg / L, respectively.

[0009] Preferably, in step (b), the pH is controlled at 2.5-3.5, and the reaction is carried out at 60-80°C for 2-4 hours.

[0010] Preferably, in step (c), the preparation of type A magnesium oxide involves adding a precipitant of carbon dioxide or ammonium bicarbonate solution to the magnesium chloride solution. In the preparation of type B magnesium oxide, ammonia or sodium hydroxide is added as a precipitant to the magnesium chloride solution. The precipitant is added at a rate of 0.5-1.0% of the solution volume per minute.

[0011] Preferably, in step (c), when preparing type A magnesium oxide, ammonia is used to adjust the pH of the reaction system to 8.0-9.0, the reaction temperature is 40-60℃, and the precipitation time is 1-2 hours; the drying temperature is 110-140℃ and the time is 2-4 hours; the calcination temperature is 750-950℃ and the time is 1-3 hours.

[0012] Preferably, in step (c), during the preparation of type B magnesium oxide, the pH is controlled at 9.5-11.0, the reaction temperature at 40-60℃, and the precipitation time at 1-2 hours; the drying temperature is 110-140℃ and the time is 2-4 hours; and the calcination temperature is 950-1150℃ and the time is 1-3 hours.

[0013] Preferably, in step (d), the mixing time is 0.5-1 hour.

[0014] In another aspect, the present invention provides a stepwise precipitation of hydromagnesia and a dual-type composite silicon steel magnesium oxide prepared by the method described above.

[0015] The present invention, by adopting the above technical solution, has the following beneficial effects: 1. This invention utilizes the high magnesium content, loose structure, and fewer impurities of hydromagnesia, which can effectively reduce the impact of impurities on the quality of magnesium oxide for silicon steel during the preparation process, reduce the cost of impurity removal and process complexity, thereby providing better initial raw material conditions for the preparation of high-purity magnesium oxide for silicon steel.

[0016] 2. The process adopts a three-stage temperature gradient calcination of hydromagnesite. By optimizing the calcination decomposition path, high-purity magnesium solution is selectively leached, achieving selective decomposition of magnesium components. This avoids the structural densification caused by traditional single-stage calcination, significantly improving the efficiency of subsequent acid leaching and ensuring the preparation of high-purity magnesium chloride precursor solution.

[0017] 3. In the preparation of type A magnesium oxide, carbon dioxide or ammonium bicarbonate is selected as a precipitant to achieve stepwise precipitation control. After further calcining basic magnesium carbonate, highly active type A magnesium oxide with CAA activity of 20-50s is prepared. This component ensures the reactivity of magnesium oxide with the silicon steel surface during annealing.

[0018] 4. In the preparation of type B magnesium oxide, ammonia or sodium hydroxide is selected as a precipitant to achieve stepwise precipitation control. After further calcination, the obtained magnesium hydroxide is used to prepare low-activity type B magnesium oxide with CAA activity of 80-120s. This component ensures the suspension stability of the magnesium oxide isolator.

[0019] 5. By physically blending type A and type B magnesium oxide at a specific mass ratio, a precise match between particle morphology and reactivity is achieved, resolving the contradiction between the hydration rate and suspension stability of magnesium oxide in traditional silicon steel. The stepwise precipitation of hydromagnesia and the dual-type blended magnesium oxide in silicon steel prepared by this invention reduce impurity interference from the source, solving the problems of poor suspension and high energy consumption in traditional methods. It can be applied in the preparation of annealed isolation coatings for high-magnetic-induction oriented silicon steel. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a process flow diagram for preparing magnesium oxide silicon steel according to the present invention; Figure 2 The particle size distribution curve of the silicon steel magnesium oxide material prepared by the process in Example 1; Figure 3 The image shows a scanning electron microscope (SEM) image of the magnesium oxide silicon steel prepared by the process in Example 1. Detailed Implementation

[0021] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0022] like Figure 1 As shown, the present invention provides a method for preparing magnesium oxide for silicon steel based on stepwise precipitation of magnesite and dual-type compounding, comprising the following steps: S1. Raw material pretreatment: Hydromagnesite powder is calcined at 500-700℃ for 1-3 hours to obtain lightly calcined hydromagnesite. The gradient calcination is carried out in three stages of temperature control: the first stage is held at 500-550℃ for 0.5 hours, the second stage is held at 600-650℃ for 1 hour, and the third stage is held at 680-700℃ for 0.5 hours.

[0023] The gradient calcination design is based on the thermal decomposition kinetics of magnesite. The first stage, controlled at 500-550℃, achieves controlled decomposition of hydroxide ions and initial carbonates, maintaining the high porosity of the precursor. The second stage, controlled at 600-650℃, initiates carbonate decomposition while preserving the porous framework. The third stage, controlled at 680-700℃, completes decomposition while inhibiting excessive grain growth. This three-stage temperature control avoids the densification phenomenon caused by traditional single-stage calcination, preserving the original loose structure and significantly increasing the specific surface area. This provides a highly reactive surface for subsequent acid leaching. Simultaneously, leveraging the low impurity content of magnesite, it lays the foundation for preparing high-purity magnesium oxide for silicon steel production.

[0024] S2. Acid leaching purification: Lightly calcined magnesite is mixed with a 2.0-3.0 mol / L hydrochloric acid solution at a solid-liquid ratio of 1:(3-5), and the pH is controlled at 2.5-3.5. The mixture is reacted at 60-80℃ for 2-4 hours, and then filtered to obtain a magnesium chloride solution. After acid leaching, the SiO2 content in the solution is ≤ 20 mg / L, and the Fe2O3 content is ≤ 20 mg / L.

[0025] Hydrochloric acid concentration and pH control are based on Mg 2+ Unlike the dissolution kinetics of impurity ions, the Mg component completely dissolves within the pH range of 2.5-3.5. By controlling the pH value, the SiO2 colloid is dehydrated, and some Fe is dissolved. 3+ Hydroxide or phosphate particles are formed and removed using solid-liquid separation. A solid-liquid ratio of 1:(3-5) ensures sufficient H+. +Concentration and diffusion space, along with a temperature of 60-80℃, optimized the reaction rate and selectivity. Under these conditions, the magnesium dissolution rate reached over 98%, while coexisting impurities such as silica and iron were effectively separated by forming slightly soluble substances. This avoided the impact of impurity ions on the purity of the final product in traditional processes and solved the surface abnormalities caused by impurities during the annealing of silicon steel.

[0026] S3. Stepwise sedimentation: Preparation of Type A magnesium oxide: The molar ratio of precipitant to magnesium chloride is (1.05-1.15):1. Carbon dioxide is bubbled into the magnesium chloride solution or ammonium bicarbonate solution is added as a precipitant at a rate of 0.5-2.0% of the solution volume per minute. The pH of the reaction system is adjusted to 8.0-9.0 using ammonia water. The reaction temperature is 40-60℃, and the precipitation time is 1-2 hours, producing basic magnesium carbonate precipitate. After washing and drying at 110-140℃ for 2-4 hours, and calcination at 750-950℃ for 1-3 hours, Type A magnesium oxide with a CAA activity of 20-50s is obtained.

[0027] The formation of basic magnesium carbonate [Mg5(CO3)4(OH)2·4H2O] is precisely controlled within the pH range of 8.0-9.0, rather than ordinary magnesium carbonate. This special structure, after calcination at 750-950℃, forms nanocrystalline magnesium oxide rich in lattice defects, with a specific surface area of ​​35-45 m². 2 / g. The precipitant is added slowly (0.5-1.0% / min) to avoid impurity encapsulation caused by localized supersaturation. A reaction temperature of 40-60℃ optimizes the balance between crystal nucleation and growth. Medium-temperature calcination retains high surface energy and abundant oxygen vacancies, keeping CAA activity within the 20-50s range, ensuring high reactivity during silicon steel annealing and promoting the formation of the Mg2SiO4 underlayer and the desulfurization reaction.

[0028] CAA activity refers to the number of seconds required for magnesium oxide to react with acid to a specific pH endpoint (usually pH=4.5) when 2.0g of magnesium oxide sample is added to 100mL of 0.4N citric acid aqueous solution at 30℃.

[0029] Preparation of Type B magnesium oxide: Ammonia or sodium hydroxide solution is added to another portion of magnesium chloride solution as a precipitant at a molar ratio of precipitant to magnesium chloride of (2.1-2.2):1. The precipitant is added at a rate of 0.5-1.0% of the solution volume per minute. The final pH of the reaction is controlled at 9.5-11.0, the reaction temperature at 40-60℃, and the precipitation time at 1-2 hours to generate magnesium hydroxide precipitate. After washing and drying at 110-140℃ for 2-4 hours, and calcining at 950-1150℃ for 1-3 hours, Type B magnesium oxide with a CAA activity of 80-120s is obtained.

[0030] Type B magnesium oxide forms a dense structure with low hydration activity through high-temperature calcination, which inhibits excessive hydration and agglomeration during the solution preparation process. This, in turn, works synergistically with Type A magnesium oxide to maintain the long-term physicochemical stability of the coating solution.

[0031] S4. Physical compounding: Type A and Type B magnesium oxides are mixed in a high-speed mixer at a mass ratio of 1:(4-1) for 0.5-1 hours until they are evenly mixed to obtain the finished product of dual-type compound silicon steel magnesium oxide.

[0032] Controlling the compounding ratio to 1:(4-1) achieves a precise balance between the dual functional requirements of magnesium oxide in silicon steel: Type A high-activity magnesium oxide (20-50s) provides chemical reactivity with the silicon steel surface, promoting the formation of the Mg2SiO4 insulating underlayer and achieving desulfurization; Type B low-activity magnesium oxide (80-120s) provides physical isolation performance and suspension stability. At this ratio, the hydration rate (≤2.5%) and suspension stability (≥8h) reach optimal balance, resolving the technical contradiction that traditional single-activity magnesium oxide cannot simultaneously satisfy high reactivity and high stability. High-speed mixing ensures uniform distribution at the microscale, enabling the two types of magnesium oxide to form a synergistic network in the coating. This ensures both the integrity of the magnesium silicate underlayer formed during annealing and maintains the uniform coverage of the coating, significantly improving the magnetic properties and surface quality of the oriented silicon steel.

[0033] The performance indicators of the finished silicon steel magnesium oxide product are: MgO purity ≥ 99%, average particle size ≤ 3μm, Cl - Content ≤0.03%, CAA activity 50-80s, hydration rate ≤2.5%, suspension stability ≥8h.

[0034] The obtained silicon steel magnesium oxide has a bimodal particle distribution, in which type A magnesium oxide with a primary particle size of 0.5-1μm accounts for 20-50%, and type B magnesium oxide with a particle size of 1.0-3.0μm accounts for 50-80%, and the CAA activity values ​​of type A and type B magnesium oxide are 20-50s and 80-120s, respectively.

[0035] The present invention will now be described in detail with reference to specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0036] Example 1 Raw material pretreatment: Hydromagnesite ore powder is subjected to gradient calcination. The first stage is held at 520℃ for 0.5 hours, the second stage is held at 630℃ for 1 hour, and the third stage is held at 690℃ for 0.5 hours to obtain lightly calcined hydromagnesite powder.

[0037] Acid leaching purification: Lightly calcined magnesite powder was mixed with 2.8 mol / L hydrochloric acid at a solid-liquid ratio of 1:4. The reaction temperature was controlled at 65℃ and pH=3.0. The mixture was stirred continuously for 3 hours. After filtration, a magnesium chloride solution was obtained (Mg... 2+ Concentration 18.9 g / L, SiO2 19 mg / L, Fe2O3 17 mg / L).

[0038] Step-by-step sedimentation: Preparation of type A magnesium oxide: The magnesium chloride solution was heated to 45℃, and ammonium bicarbonate solution was added dropwise to the magnesium chloride solution at a rate of 1.0 mL / min. The molar ratio of ammonium bicarbonate to magnesium chloride was 1.08:1. The pH of the system was controlled at 8.5. The precipitation reaction was carried out at 50℃ for 1.5 hours. The obtained basic magnesium carbonate was washed three times with deionized water, dried at 120℃ for 3 hours, and then calcined in a rotary kiln at 750℃ for 2 hours to obtain type A magnesium oxide (D50=0.5μm, CAA activity 35s).

[0039] Preparation of type B magnesium oxide: The remaining filtrate was kept at 50°C, and 10% ammonia was added to the magnesium chloride solution at a rate of 0.7% / min. The molar ratio of ammonia to magnesium chloride was 2.15:1. The final pH was 10.8. After aging at 50°C for 2 hours, magnesium hydroxide slurry was obtained. After plate and frame filtration and washing with ethanol, it was dried at 120°C for 3 hours and calcined at 950°C in a tunnel kiln for 3 hours to obtain type B magnesium oxide (D50=1.6μm, CAA activity 98s).

[0040] Physical compounding: Add type A and type B to a V-type mixer at a mass ratio of 1:4 and mix at a speed of 28 r / min for 50 minutes.

[0041] Finished product testing: MgO 99.3%, Cl - 0.025%, CAA activity 68s, hydration rate 2.1%, suspension stability 9.2h.

[0042] The particle size distribution curve of the prepared silicon steel magnesium oxide material is shown in the figure. Figure 2 As shown, Figure 3 This is a scanning electron microscope image of magnesium oxide in silicon steel.

[0043] Example 2 Raw material pretreatment: Hydromagnesite ore powder is subjected to gradient calcination. The first stage is held at 500℃ for 0.5 hours, the second stage is held at 650℃ for 1 hour, and the third stage is held at 700℃ for 0.5 hours to obtain lightly calcined hydromagnesite powder.

[0044] Acid leaching purification: Lightly calcined magnesite powder was reacted with 3.0 mol / L hydrochloric acid at a solid-liquid ratio of 1:3 and pH=3.5 at 80℃ for 2 hours. The filtrate contained Mg. 2+ 19.3g / L, SiO219mg / L, Fe2O319mg / L.

[0045] Step-by-step sedimentation: Preparation of Type A: The magnesium chloride solution was heated to 45℃, and CO2 gas (purity 99.5%) was introduced into the magnesium chloride solution. The molar ratio of carbon dioxide to magnesium chloride was 1.05:1, the gas flow rate was controlled at 2.5 L / min, the reaction temperature was 60℃, and precipitation was carried out for 1 hour. The pH of the system was controlled at 8.0. After the precipitate was spray-dried at 110℃ for 4 hours, it was treated in a dynamic calcination furnace at 750℃ for 3 hours to obtain Type A magnesium oxide (D50=1.4μm, activity 28s).

[0046] Preparation of Type B: Magnesium chloride solution was precipitated with 5% NaOH solution at a dropping rate of 0.9 mL / min·L. The molar ratio of NaOH solution to magnesium chloride was 2.2:1, the endpoint pH was 11.0, and the slurry was aged at 60℃ for 1.5 hours to obtain magnesium hydroxide slurry. After plate and frame filtration, washing with ethanol, drying at 110℃ for 4 hours, and calcining at 1000℃ for 2 hours in a tunnel kiln, type B magnesium oxide (D50=2.8μm, activity 110s) was obtained.

[0047] Physical compounding: A:B = 1:2.5, mix and process in a V-type mixer for 60 minutes.

[0048] Finished product properties: MgO 98.9%, CAA activity 72s, hydration rate 2.4%, suspension stability 8.5h, Cl - Content: 0.028%.

[0049] Example 3 Raw material pretreatment: High-grade hydromagnesite is used for gradient calcination. The first stage is held at 550℃ for 0.5 hours, the second stage is held at 600℃ for 1 hour, and the third stage is held at 680℃ for 0.5 hours to obtain lightly calcined hydromagnesite.

[0050] Acid leaching purification: Lightly calcined magnesite powder was mixed with 2.0 mol / L hydrochloric acid at a solid-liquid ratio of 1:5, pH=2.5, and the reaction temperature was controlled at 60℃. The mixture was stirred continuously for 4 hours, and the solution of magnesium chloride (Mg²⁺) was obtained after filtration. + Concentration 18.9 g / L, SiO2 content 16 mg / L, Fe2O3 content 18 mg / L).

[0051] Step-by-step sedimentation: Preparation of Type A: The magnesium chloride solution was heated to 45°C, and ammonium bicarbonate solution was added to the magnesium chloride solution. The molar ratio of ammonium bicarbonate to magnesium chloride was 1.15:1. The pH of the system was controlled at 9.0. After the precipitate was dried at 140°C for 2 hours, it was calcined at 950°C in a dynamic calcination furnace for 1 hour to obtain Type A magnesium oxide (D50=0.6μm).

[0052] Preparation of Type B: Magnesium chloride solution was precipitated with 25% NaOH solution at a dropping rate of 0.5 mL / min·L. The molar ratio of NaOH solution to magnesium chloride was 2.1:1, and the endpoint pH was 9.5. After aging at 40℃ for 2 hours, magnesium hydroxide slurry was obtained. After plate and frame filtration, washing with ethanol, drying at 140℃ for 2 hours, and calcining at 1150℃ for 1 hour in a tunnel kiln, type B magnesium oxide (D50=3μm, activity 110s) was obtained.

[0053] Physical compounding: Mix A:B=1:1 and process in a V-type mixer for 30 minutes.

[0054] Product properties: MgO 99.1%, CAA activity 75s, hydration rate 2.3%, suspension stability 8.9h, Cl - Content: 0.025%.

[0055] Comparative Example Magnesium oxide for silicon steel production was prepared using the traditional brine-ammonium bicarbonate method. Industrial-grade brine (Mg...) was used. 2+ Concentration 20.5 g / L, Ca 2+ Content 0.12wt%, Fe 3+ With a content of 0.03wt%, add 20% ammonium bicarbonate solution (molar ratio 1.1:1), control pH=8.5, react at 45℃ for 2 hours, filter to obtain basic magnesium carbonate precipitate, dry at 120℃ for 2 hours, calcine at 900℃ for 2 hours, grind to a particle size D50=1.8μm, and obtain silicon steel magnesium oxide product.

[0056] Table 1 shows a comparison of the performance indicators of the dual-type composite silicon steel magnesium oxide prepared in this invention with those of the comparative examples.

[0057] Table 1. Comparison of industry performance indicators of magnesium oxide prepared by the examples and magnesium oxide in silicon steel.

[0058] As can be seen from the above examples, the stepwise precipitation process effectively controls the physical compounding of the activity ranges of type A (20-50s) and type B (80-120s). The activity of the finished product is stable at 50-80s, with an average particle size ≤3μm. The contradiction between the hydration rate ≤2.5% and the suspension stability ≥8h is synergistically resolved. Through the selection of raw materials and the optimization of the process, the purity of MgO is ≥99%.

[0059] The magnesium oxide material for silicon steel prepared by the stepwise precipitation and dual-type compounding of hydromagnesia in this invention significantly improves the purity, activity, hydration rate, and suspension stability of magnesium oxide, solving the problems of poor suspension and high energy consumption of traditional methods. The magnesium oxide material for silicon steel prepared by this invention can be widely used in the preparation of annealed isolation coatings for high magnetic orientation silicon steel.

[0060] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A method for preparing magnesium oxide from hydromagnesite through stepwise precipitation and dual-type composite silicon steel, characterized in that, Includes the following steps: (a) Gradient calcination of hydromagnesite ore powder to obtain lightly calcined hydromagnesite; (b) Mix lightly calcined magnesite with hydrochloric acid solution at a solid-liquid ratio of 1:(3-5), and control the pH and reaction temperature to obtain magnesium chloride solution; (c) Stepwise precipitation preparation of type A and type B magnesium oxide: Preparation of type A magnesium oxide: Add precipitant to magnesium chloride solution according to the molar ratio of precipitant to magnesium chloride of (1.05-1.15):1, control the pH at the reaction endpoint, generate basic magnesium carbonate precipitate, and obtain type A magnesium oxide with CAA activity of 20-50s after washing, drying and calcination. Preparation of type B magnesium oxide: Add precipitant to magnesium chloride solution according to the molar ratio of precipitant to magnesium chloride of (2.1-2.2):1, control the pH at the reaction endpoint, generate magnesium hydroxide precipitate, and obtain type B magnesium oxide with CAA activity of 80-120s after washing, drying and calcination. (d) After mixing type A and type B magnesium oxide at a mass ratio of 1:(4-1) at high speed, a dual-type composite silicon steel magnesium oxide is obtained.

2. The method for preparing magnesium oxide from hydromagnesite through stepwise precipitation and dual-type composite silicon steel according to claim 1, characterized in that, In step (a), the gradient calcination is carried out in three stages of temperature control: the first stage is held at 500-550℃ for 0.5 hours, the second stage is held at 600-650℃ for 1 hour, and the third stage is held at 680-700℃ for 0.5 hours.

3. The method for preparing magnesium oxide from hydromagnesite through stepwise precipitation and dual-type composite silicon steel according to claim 1, characterized in that, In step (b), the concentration of the hydrochloric acid solution is 2.0-3.0 mol / L, and the SiO2 content in the solution after acid leaching is ≤20 mg / L and the Fe2O3 content is ≤20 mg / L.

4. The method for preparing magnesium oxide from hydromagnesite through stepwise precipitation and dual-type composite silicon steel according to claim 1, characterized in that, In step (b), the pH is controlled at 2.5-3.5, and the reaction is carried out at 60-80℃ for 2-4 hours.

5. The method for preparing hydromagnesite stepwise precipitation and dual-type composite silicon steel magnesium oxide according to claim 1, characterized in that: In step (c), for the preparation of type A magnesium oxide, a precipitant, either carbon dioxide or ammonium bicarbonate solution, is added to the magnesium chloride solution; To prepare type B magnesium oxide, ammonia or sodium hydroxide is introduced as a precipitant into a magnesium chloride solution. The precipitant is added at a rate of 0.5-1.0% of the solution volume per minute.

6. The method for preparing hydromagnesite stepwise precipitation and dual-type composite silicon steel magnesium oxide according to claim 1, characterized in that: In step (c), when preparing type A magnesium oxide, ammonia water is used to adjust the pH of the reaction system to 8.0-9.0, the reaction temperature is 40-60℃, and the precipitation time is 1-2 hours; the drying temperature is 110-140℃ and the time is 2-4 hours; the calcination temperature is 750-950℃ and the time is 1-3 hours.

7. The method for preparing hydromagnesite stepwise precipitation and dual-type composite silicon steel magnesium oxide according to claim 1, characterized in that, In step (c), when preparing type B magnesium oxide, the pH is controlled at 9.5-11.0, the reaction temperature is 40-60℃, and the precipitation time is 1-2 hours; the drying temperature is 110-140℃ and the time is 2-4 hours; the calcination temperature is 950-1150℃ and the time is 1-3 hours.

8. The method for preparing magnesium oxide from hydromagnesite through stepwise precipitation and dual-type composite silicon steel according to claim 1, characterized in that, In step (d), the mixing time is 0.5-1 hour.

9. A stepwise precipitation of hydromagnesia and a dual-type composite silicon steel magnesium oxide prepared by the method according to any one of claims 1-8.

10. The application of the stepwise precipitation of hydromagnesia and the dual-type compounded silicon steel magnesium oxide as described in claim 9 in the preparation of an annealing isolation coating for high magnetic induction oriented silicon steel.

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