Hot state magnesia spinel brick surface coating and its preparation method

Abstract

The application discloses a hot-state magnesia spinel brick surface coating and a preparation method thereof. The surface coating raw material comprises the following components in percentage by weight: aggregate system 40-70%, bonding system 28-50%, sintering aid 1-5%, and additive 1-5%; wherein the aggregate system comprises the following components in parts by weight: magnesia spinel brick regenerated powder 30-50 parts and functional fine powder 10-20 parts; and the bonding system comprises the following components in parts by weight: active alumina micro powder 10-20 parts, light-burned magnesium oxide powder 5-15 parts, and silicon micro powder 3-8 parts. The coating forms a firm chemical bonding interface, the bonding strength is significantly higher than that of a traditional normal-temperature coating, and the anti-peeling capacity is greatly improved; the waste brick is used as the raw material, the cost is reduced, the recycling economic concept is met, the brick life is prolonged, the replacement frequency is reduced, and energy and cost are saved.

Description

Technical Field

[0001] This application relates to the field of magnesium spinel brick coating technology, and in particular to a hot-state magnesium spinel brick surface coating and its preparation method. Background Technology

[0002] Magnesia spinel bricks are widely used in critical areas such as RH furnace vacuum chambers, cement rotary kiln transition zones, and glass melting furnace regenerators due to their excellent resistance to alkali corrosion and high-temperature performance. However, in actual use, temperature fluctuations within the kiln and the penetration and corrosion of harmful components (such as alkali metals, sulfur, and chlorine) can gradually damage the surface of magnesia spinel bricks, causing issues such as peeling and loosening of the structure. This necessitates kiln shutdowns for replacement, severely impacting production efficiency and economic benefits.

[0003] Hot-state coatings can act as "active armor," forming a highly dense and erosion-resistant barrier in situ on the brick surface, specifically compensating for the performance degradation of the base material under extreme conditions. Most existing refractory coatings are applied at room temperature or require the refractory bricks to be cooled to a lower temperature before treatment, resulting in poor bonding strength, thermal compatibility, and impermeability between the coating and the hot substrate. The bonding between the coating and the hot substrate relies mainly on anchoring and limited physicochemical bonding; the interfacial strength decreases significantly at high temperatures, thermomechanical properties are mismatched, and the difference in thermal expansion coefficients between the coating and the substrate leads to thermal stress concentration. Furthermore, the protective effect of the coating is limited, and it is prone to peeling or failure under complex kiln conditions, failing to achieve effective online repair and protection of hot-state magnesia spinel bricks. Summary of the Invention

[0004] This invention provides a surface coating for hot-state magnesia spinel bricks and its preparation method, in order to solve the technical problem that refractory material coatings in the prior art are prone to peeling or failure under complex working conditions and cannot achieve online repair or preventive strengthening of hot-state magnesia spinel bricks.

[0005] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0006] In a first aspect, the present invention provides a surface coating for hot-state magnesia spinel bricks, the raw materials of which comprise the following components by weight percentage: 40%–70% aggregate system, 28%–50% binder system, 1%–5% sintering aid, and 1%–5% additives; wherein the aggregate system comprises the following components by weight: 30–50 parts recycled magnesia spinel brick powder and 10–20 parts functional fine powder; the binder system comprises the following components by weight: 10–20 parts activated alumina micropowder, 5–15 parts lightly calcined magnesia powder, and 3–8 parts silica micropowder.

[0007] Furthermore, the particle size of the recycled magnesium spinel brick powder is less than or equal to 0.088 mm, and the particle size of the functional fine powder is less than or equal to 0.044 mm.

[0008] Furthermore, the d of the activated alumina micro powder 50 ≤2.5μm; the particle size of the lightly calcined magnesium oxide powder is less than or equal to 0.044mm; the particle size of the silicon micro powder is less than or equal to 1μm.

[0009] Furthermore, the additive includes a dispersant and a thickener, wherein the weight ratio of the dispersant to the thickener is 1:0.1 to 2.

[0010] Furthermore, the sintering aid is selected from one or more of boric acid, anhydrous borax, lithium carbonate, calcium fluoride, zinc oxide, and barium salts.

[0011] Furthermore, the functional fine powder is selected from one or more of fused magnesia powder, sintered magnesia powder, zirconium oxide powder, and silicon carbide powder.

[0012] Furthermore, its raw materials include the following components by weight: 30 to 50 parts of recycled magnesium spinel brick powder, 10 to 20 parts of functional fine powder, 10 to 20 parts of activated alumina micro powder, 5 to 15 parts of lightly calcined magnesium oxide powder, 3 to 8 parts of silicon micro powder, 2 to 5 parts of sintering aid, and 2 to 5 parts of additives.

[0013] A second aspect of the present invention provides a method for preparing the above-mentioned surface coating of hot-state magnesia spinel bricks, comprising the following steps: pretreating the surface of the hot-state magnesia spinel bricks; weighing the aggregate system, the bonding system, and the sintering aid according to the specified ratio and dry mixing them to obtain a dry mixture; weighing the additives according to the specified ratio and adding the additives to a liquid carrier for mixing, and then adding the dry mixture to obtain a suspension slurry; coating the suspension slurry onto the surface of the pretreated hot-state magnesia spinel bricks to obtain the surface coating of the hot-state magnesia spinel bricks.

[0014] Furthermore, the surface temperature of the hot-state magnesium spinel brick is kept not lower than 800°C during pretreatment.

[0015] Furthermore, after the coating process is completed, the steps also include raising the temperature to above 1000°C and maintaining it for 10 to 30 minutes.

[0016] Further, the steps specifically include: preparing a base layer slurry, a middle layer slurry, and a surface layer slurry respectively, and sequentially spraying the base layer slurry, the middle layer slurry, and the surface layer slurry using a multi-fluid spray gun or sequentially brushing them with a brush to form a gradient coating on the hot magnesium spinel surface; wherein, the weight percentage of each component in the base layer slurry is: 40%–50% aggregate system, 45%–50% bonding system, 3%–5% sintering aid, and 2%–5% additives; the weight percentage of each component in the middle layer slurry is: 30%–45% aggregate system, 45%–50% bonding system, 8%–15% sintering aid, and 2%–5% additives; the weight percentage of each component in the surface layer slurry is: 20%–40% aggregate system, 45%–60% bonding system, 8%–10% sintering aid, and 7%–10% additives.

[0017] Furthermore, in the above steps, the coating temperature is 800℃~1200℃, the distance between the spray gun nozzle and the surface of the hot magnesium spinel brick is 200mm~400mm, and the thickness of the coating is 1.0mm~3.0mm.

[0018] The hot-state magnesia spinel brick surface coating provided by this invention uses recycled magnesia spinel brick powder as the main aggregate, combined with active powder and sintering aids. This allows the coating to undergo an in-situ spinelization reaction at the interface with the substrate under hot conditions, achieving a chemical metallurgical bond between the coating and the substrate, rather than simple physical adhesion. This forms a strong chemical bond interface, resulting in a significantly higher coating bonding strength than traditional room-temperature coatings and a substantial improvement in resistance to peeling. Furthermore, the coating utilizes waste bricks as raw materials, reducing costs and aligning with the principles of a circular economy. This extends the lifespan of the bricks, reduces replacement frequency, and saves energy and costs.

[0019] The above-mentioned coating preparation method does not require kiln shutdown for cooling, can be quickly applied online, and forms an effective protective layer on the surface of magnesium spinel, greatly reducing production downtime and achieving true online maintenance. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0021] A first aspect of this application provides a surface coating for hot-state magnesia spinel bricks, the raw materials of which include the following components by weight percentage: 40%–70% aggregate system, 28%–50% binder system, 1%–5% sintering aid, and 1%–5% additives; wherein the aggregate system includes the following components by weight: 30–50 parts recycled magnesia spinel brick powder and 10–20 parts functional fine powder; the binder system includes the following components by weight: 10–20 parts activated alumina micropowder, 5–15 parts lightly calcined magnesia powder, and 3–8 parts silicon micropowder.

[0022] Currently, magnesia spinel bricks in high-temperature industrial kilns suffer surface damage due to kiln temperature fluctuations, penetration, and erosion by harmful components. The conventional approach is to replace the entire magnesia spinel brick after kiln shutdown and cooling, which is energy-intensive, costly, and causes production interruptions, resulting in significant output and economic losses. Most existing refractory brick coatings are applied at room temperature or require cooling the refractory brick to a lower temperature before treatment. These coatings often have poor bonding strength, thermal compatibility, and impermeability with the refractory brick substrate, making them prone to peeling or failure under complex kiln conditions. Therefore, they cannot effectively repair and protect hot magnesia spinel bricks online.

[0023] The hot-state magnesia spinel brick surface coating of this application uses recycled magnesia spinel brick powder as the main aggregate, combined with active powder and sintering aids. This allows the coating to undergo an in-situ spinelization reaction at the interface with the substrate under hot conditions, achieving a chemical metallurgical bond between the coating and the substrate, rather than simple physical adhesion. This forms a strong chemical bond interface, resulting in a significantly higher coating bonding strength than traditional room-temperature coatings and a substantial improvement in resistance to peeling. Furthermore, the above coating utilizes waste bricks as raw materials, reducing costs and conforming to the concept of a circular economy. This extends the lifespan of the bricks, reduces replacement frequency, and saves energy and costs.

[0024] In this embodiment, the recycled magnesium spinel brick powder is preferably recycled powder that has undergone iron removal, homogenization, and two-stage microwave activation treatment at 500℃ and 950℃. Specifically, the particle size of the recycled magnesium spinel brick powder is less than or equal to 0.088 mm. The particle size of the functional fine powder is less than or equal to 0.044 mm. The d of the activated alumina micro powder... 50 ≤2.5μm. The particle size of lightly calcined magnesia powder is less than or equal to 0.044mm. The particle size of silica powder is less than or equal to 1μm. Functional fine powders are selected from one or more of fused magnesia powder, sintered magnesia powder, zirconium oxide powder, and silicon carbide powder.

[0025] In some embodiments, the raw material for the surface coating of hot-state magnesia spinel bricks includes the following components by weight: 30 to 50 parts of recycled magnesia spinel brick powder, 10 to 20 parts of functional fine powder, 10 to 20 parts of activated alumina micro powder, 5 to 15 parts of lightly calcined magnesia powder, 3 to 8 parts of silicon micro powder, 2 to 5 parts of sintering aid, and 2 to 5 parts of additives.

[0026] A second aspect of this application provides a method for preparing the surface coating of the above-mentioned hot-state magnesia spinel brick, comprising the following steps: pretreating the surface of the hot-state magnesia spinel brick; weighing the aggregate system, bonding system and sintering aid according to the proportions and dry mixing them to obtain a dry mixture; weighing the additives according to the proportions and adding the additives to a liquid carrier for mixing, and then adding the dry mixture to obtain a suspension slurry; coating the suspension slurry onto the surface of the pretreated hot-state magnesia spinel brick to obtain the surface coating of the hot-state magnesia spinel brick.

[0027] The method for preparing the surface coating of hot-state magnesia spinel bricks in this application embodiment eliminates the need for kiln shutdown and cooling, enabling rapid online coating and forming an effective protective layer on the magnesia spinel surface. This significantly reduces production downtime and achieves true online maintenance. The viscosity of the aforementioned suspension slurry is 800 mPa·s to 1500 mPa·s (25°C).

[0028] In the above preparation method, the surface pretreatment mainly includes cleaning the surface of the hot magnesia spinel brick while the kiln is in operation, using compressed air to blow away loose deposits, slag and dust on the surface, and exposing a relatively solid and complete brick base surface.

[0029] In some embodiments, the additive includes a dispersant and a thickener, with a weight ratio of dispersant to thickener of 1:0.1 to 2. In the embodiments of this application, during the preparation of the suspension slurry, the additive is dissolved in a liquid carrier, which can be water or ethanol. The dispersant accounts for 0.1% to 0.5% of the weight of the liquid carrier, and the thickener accounts for 0.05% to 0.2% of the weight of the liquid carrier.

[0030] In some embodiments, the surface temperature of the hot magnesia spinel brick is maintained at no less than 800°C during pretreatment. In the embodiments of this application, the coating operation is directly carried out under the condition that the surface temperature of the magnesia spinel brick is greater than or equal to 800°C, and the high temperature of the substrate promotes the interfacial reaction and sintering of the coating, thereby achieving a strong bond between the substrate and the coating.

[0031] In other embodiments, the process after coating includes raising the temperature to above 1000°C and maintaining it for 10 to 30 minutes. In the embodiments of this application, after coating, the coating dries rapidly and undergoes a high-temperature physicochemical reaction due to the heat storage of the hot substrate and the thermal radiation of the kiln environment. At temperatures above 1000°C, the active alumina and magnesium oxide in the coating undergo an in-situ solid-state reaction to generate magnesium aluminum spinel (MgAl2O4), while sintering aids promote the densification of the particles at a lower temperature. The above process is completed within 10 to 30 minutes, ultimately forming a protective coating that is chemically bonded to the substrate and has a dense structure on the substrate surface.

[0032] In some embodiments, the steps specifically include: preparing a base layer slurry, a middle layer slurry, and a surface layer slurry respectively, and sequentially spraying the base layer slurry, the middle layer slurry, and the surface layer slurry using a multi-fluid spray gun or sequentially brushing them with a brush to form a gradient coating on the surface of hot magnesium spinel; wherein, the weight percentage of each component in the base layer slurry is: 40%–50% aggregate system, 45%–50% binder system, 3%–5% sintering aid, and 2%–5% additives; the weight percentage of each component in the middle layer slurry is: 30%–45% aggregate system, 45%–50% binder system, 8%–15% sintering aid, and 2%–5% additives; the weight percentage of each component in the surface layer slurry is: 20%–40% aggregate system, 45%–60% binder system, 8%–10% sintering aid, and 7%–10% additives.

[0033] In this embodiment, a higher weight percentage of aggregate system in the bottom slurry results in a higher content of magnesium spinel brick recycled powder; an increase in the weight percentage of the bonding system in the middle slurry results in an increase in the proportion of activated alumina micro powder; and a further increase in the weight percentage of the bonding system in the surface slurry results in the highest content of activated alumina micro powder.

[0034] The aforementioned gradient coating method can construct a material system with a continuous transition in chemical composition and thermophysical properties from the substrate to the coating surface. The gradient structure effectively buffers the thermal stress caused by the difference in thermal expansion coefficients between the coating and the substrate, allowing the coating to remain intact even when the kiln temperature fluctuates drastically. Furthermore, the dense gradient coating can effectively block the penetration and erosion of harmful substances such as molten slag and alkaline vapor, providing targeted protection for damaged areas.

[0035] Specifically, in the above steps, the coating temperature is 800℃~1200℃, the distance between the spray gun nozzle and the surface of the hot magnesium spinel brick is 200mm~400mm, and the coating thickness is 1.0mm~3.0mm.

[0036] This application relates to a method for directly preparing a protective coating on the surface of hot magnesia spinel bricks after use in high-temperature industrial kilns. It is applicable to the rapid hot repair of linings in steel smelting and industrial thermal furnaces, aiming to solve the problem of weakened interfacial bonding between the coating and the substrate under high-temperature conditions. A continuous gradient design of the coating composition is achieved through gradient coating, thereby optimizing the distribution of thermal stress. Simultaneously, by combining the in-situ formation mechanism of spinel and utilizing the sintering-promoting effect of activated oxides, the density of the coating is enhanced. The preparation method of this application not only avoids the risk of peeling associated with traditional coatings but also improves the chemical stability of the coating in complex slag environments through a non-radical activation mechanism.

[0037] The above technical solution will be further illustrated by specific embodiments below. All reagents involved in the following embodiments are commercially available.

[0038] Example 1

[0039] Case study of hot repair of magnesium spinel bricks in the transition zone of a cement rotary kiln: Magnesium spinel bricks in the transition zone of a cement plant were used. The surface temperature was about 950℃ during use, and there was local peeling and erosion.

[0040] The raw materials for the above-mentioned hot-state magnesia spinel brick surface coating for repair include: 60% aggregate system, 30% binder system, 5% sintering aid, and 5% additives. Specifically, it contains 30 parts recycled magnesia spinel brick powder (0.088mm), 20 parts fused magnesia powder (0.044mm), and activated alumina micro powder (d... 50 =2μm) 10 parts, lightly calcined magnesium oxide powder 15 parts, silica powder 8 parts, boric acid 3 parts, lithium carbonate 2 parts, polycarboxylate dispersant 3 parts, hydroxypropyl methylcellulose 2 parts. The liquid carrier is water, the percentage content of polycarboxylate dispersant is 0.3%, and the percentage content of hydroxypropyl methylcellulose is 0.1%.

[0041] The above-mentioned method for preparing the surface coating of hot-state magnesia spinel bricks for repair includes the following steps:

[0042] 1. During the operation of the transition zone of the cement rotary kiln, the surface of the hot magnesia spinel bricks to be repaired should be cleaned. Compressed air should be used to blow away loose adhering materials, slag, and dust to expose a relatively solid and intact brick base. During the pretreatment process, the surface temperature of the base should be maintained at 950℃.

[0043] 2. Weigh out the aggregate system, bonding system, and sintering aid powders according to the proportions, and dry mix for 15 minutes to obtain a dry mixture; weigh out the polycarboxylate dispersant and hydroxypropyl methylcellulose according to the proportions and add them to the water, then slowly add the above dry mixture, controlling the feeding speed, adjusting the stirring speed and time to obtain a uniform, stable suspension slurry suitable for spraying.

[0044] The bottom layer slurry, middle layer slurry, and surface layer slurry were prepared according to the above method. Specifically, the bottom layer slurry contained 40% aggregate, 50% binder, 5% sintering aid, and 5% additives; the middle layer slurry contained 45% aggregate, 45% binder, 8% sintering aid, and 2% additives; and the surface layer slurry contained 20% aggregate, 60% binder, 10% sintering aid, and 10% additives.

[0045] Using a multi-fluid spray gun, sequentially spray the base coat, intermediate coat, and top coat of the above-mentioned slurry, each with a thickness of 0.7 mm, for a total thickness of 2.1 mm. The spraying distance is 300 mm, and the operating temperature is 1000℃. After spraying, raise the temperature to 1200℃ and maintain it for 15 minutes.

[0046] Tests showed that the coating exhibited a flexural bond strength of ≥5.0 MPa at room temperature and showed no peeling after five cycles of water-cooled thermal shock at 1100℃.

[0047] After running online for 3 months following the repair, the repaired area remained intact, with no signs of further erosion.

[0048] Example 2

[0049] Case study of preventive strengthening of magnesium spinel bricks in the vacuum chamber of an RH furnace: Magnesium spinel bricks in the upper part of the vacuum chamber of an RH furnace in a steel plant, with a surface temperature of approximately 1050℃ during operation.

[0050] The raw materials for the surface coating of the aforementioned hot-state magnesia spinel brick after reinforcement include: 50% aggregate system, 40% binder system, 5% sintering aid, and 5% additives. Specifically, it comprises 10 parts recycled magnesia spinel brick powder, 10 parts activated alumina micro powder, 20 parts lightly calcined magnesia powder, 3 parts silica micro powder, 2 parts calcium fluoride, 3 parts polycarboxylate dispersant, and 2 parts hydroxypropyl methylcellulose. The liquid carrier is water, and the percentage content of polycarboxylate dispersant is 0.3%, and the percentage content of hydroxypropyl methylcellulose is 0.1%.

[0051] The preparation method of the above-mentioned hot magnesium spinel brick surface coating is basically the same as that in Example 1, except that the substrate surface temperature should be kept at 1050℃ during the pretreatment process; the spraying thickness of the bottom layer slurry, the middle layer slurry, and the top layer slurry is 0.5mm, and the total coating thickness is about 1.5mm.

[0052] Tests showed that the flexural bonding strength of the coating at 1400℃ was ≥8.0MPa; slag penetration tests showed that the penetration depth was reduced by more than 60% compared to uncoated bricks.

[0053] Example 3

[0054] This embodiment is basically the same as embodiment 1, except that a regular spray gun is used for a single uniform spraying without a gradient structure.

[0055] Testing revealed that after thermal shock testing (water cooling at 1100℃, 3 times), the coating exhibited edge lifting and localized peeling, with obvious cracks visible at the interface.

[0056] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a surface coating on hot-state magnesia spinel bricks, characterized in that, Includes the following steps: Surface pretreatment of hot-state magnesia spinel bricks; Weigh out the aggregate system, bonding system and sintering aid according to the proportions and dry mix them to obtain a dry mixture; weigh out the additives according to the proportions and add them to the liquid carrier and mix them, and then add them to the dry mixture to obtain a suspension slurry; The suspension slurry is applied to the surface of the pretreated hot-state magnesia spinel brick to obtain the surface coating of the hot-state magnesia spinel brick. The steps specifically include: preparing a base layer slurry, a middle layer slurry, and a surface layer slurry respectively, and sequentially spraying the base layer slurry, the middle layer slurry, and the surface layer slurry using a multi-fluid spray gun or sequentially brushing them with a brush to form a gradient coating on the hot magnesium spinel surface; The weight percentages of each component in the bottom layer slurry are as follows: aggregate system 40%–50%, bonding system 45%–50%, sintering aid 3%–5%, and additives 2%–5%; the weight percentages of each component in the middle layer slurry are as follows: aggregate system 30%–45%, bonding system 45%–50%, sintering aid 8%–15%, and additives 2%–5%; and the weight percentages of each component in the surface layer slurry are as follows: aggregate system 20%–40%, bonding system 45%–60%, sintering aid 8%–10%, and additives 7%–10%. The aggregate system comprises the following components by weight: 30 to 50 parts of recycled magnesium spinel brick powder and 10 to 20 parts of functional fine powder; the bonding system comprises the following components by weight: 10 to 20 parts of activated alumina micro powder, 5 to 15 parts of lightly calcined magnesium oxide powder, and 3 to 8 parts of silicon micro powder.

2. The preparation method according to claim 1, characterized in that, The particle size of the recycled magnesium spinel brick powder is less than or equal to 0.088 mm, and the particle size of the functional fine powder is less than or equal to 0.044 mm.

3. The preparation method according to claim 1, characterized in that, The activated alumina micro powder d 50 ≤2.5μm; the particle size of the lightly calcined magnesium oxide powder is less than or equal to 0.044mm; the particle size of the silicon micro powder is less than or equal to 1μm.

4. The preparation method according to any one of claims 1 to 3, characterized in that, The additives include dispersants and thickeners, and the weight ratio of the dispersant to the thickener is 1:0.1 to 2.

5. The preparation method according to any one of claims 1 to 3, characterized in that, The sintering aid is selected from one or more of boric acid, anhydrous borax, lithium carbonate, calcium fluoride, zinc oxide, and barium salts; and / or, The functional fine powder is selected from one or more of fused magnesia powder, sintered magnesia powder, zirconium oxide powder, and silicon carbide powder.

6. The preparation method according to any one of claims 1 to 3, characterized in that, Its raw materials include the following components by weight: 30 to 50 parts of recycled magnesium spinel brick powder, 10 to 20 parts of functional fine powder, 10 to 20 parts of activated alumina micro powder, 5 to 15 parts of lightly calcined magnesium oxide powder, 3 to 8 parts of silicon micro powder, 2 to 5 parts of sintering aid, and 2 to 5 parts of additives.

7. The preparation method according to claim 1, characterized in that, When pretreating the hot-state magnesium spinel brick, its surface temperature should be kept not lower than 800°C.

8. The preparation method according to claim 1, characterized in that, After the coating process is completed, the steps also include raising the temperature to above 1000°C and maintaining it for 10 to 30 minutes.

9. The preparation method according to claim 1, characterized in that, In the above steps, the coating temperature is 800℃~1200℃, the distance between the spray gun nozzle and the surface of the hot magnesium spinel brick is 200mm~400mm, and the thickness of the coating is 1.0mm~3.0mm.

10. A surface coating for hot-state magnesia spinel bricks, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.