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Two common recycling methods of waste magnesia carbon bricks and their key technical issues and application prospects

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Recycling brick making technology of waste magnesia carbon brick refractory materials

Steel production requires a large amount of magnesia-carbon bricks, which provides an outlet for the recycling of used magnesia-carbon refractory materials. Therefore, the recycling brick-making technology of used magnesia-carbon refractory materials has become the focus of development for domestic and foreign steel mills and refractory material manufacturers. Domestic Baosteel, Jinan Iron and Steel, etc. have developed brick-making technology from recycled magnesia carbon bricks and conducted on-site trials.

magnesia carbon brick

Baosteel’s post-consumer magnesia carbon refractory material recycling brick making technology

Baosteel obtains recycled granular materials after removing the slag layer, sorting, magnetic separation, crushing, pulverizing, screening and other processing of used magnesia carbon bricks. It uses high-temperature chemical reaction to hydrate and remove Al4C3 in the recycled materials. The performance of Ransheng magnesia carbon bricks made of 80% recycled materials and 20% new raw materials exceeds the corresponding national magnesia carbon brick A-level level. It was trialled in batches at the slag line of a 300t ladle in the steelmaking plant of Baosteel Branch. The service life reached 82 furnaces, reaching the level of magnesia carbon bricks normally used in Baosteel Steelmaking Plant and exceeding the national industry standard.

Brick-making technology of recycled magnesia-carbon refractory materials in Jinan Iron and Steel Co., Ltd.

After Jinan Iron and Steel Co., Ltd. removes the alteration layer from the used magnesia carbon bricks, they first coarsely break them into particles ≤50mm, and then put them into a sand mixer for stirring. At the same time, they add composite additives to reduce the bonding strength of the resin and promote the hydration of Al4C3. After the material is trapped for 24 hours, it is crushed and crushed. After screening, particles ≤1mm are then ground, and the fine powder size after grinding is controlled within 0.1mm. In the obtained recycled material, the mass fraction of particles >1mm is 40%~45%, those of ≤1mm are 25%~30%, and fine powder is 30%~35%.

When making recycled MgO-C bricks, in addition to recycled materials, other raw materials are fused magnesia, graphite and thermosetting resin used in the normal production of carbon-containing products in the workshop. After experimental adjustments, two relatively mature formulas were formed, in which the introduction amounts of recycled materials were 68% and 50% respectively (see Table 1).

 Recycled material/mmFused magnesia/mmGraphiteAntioxidant (additional)Thermoplastic resin (additional)
raw material>1≤1≤0.1>1≤1≤0.088
Recipe 1(68%)508105126933.2
Recipe 2(50%)2015153553733.4

The recycled bricks are used together with new bricks for the working lining of 160t ladle. The LF refining ratio is 73% and the refining time is 42 minutes on average. When furnace 32 was used, the 3-layer slag line was slightly repaired. After normal use of furnace 66, it was offline. It was found that the thickness of the remaining bricks in the most severely eroded part of the recycled bricks was 84mm, while the new bricks were 87mm. The thickness of the recycled magnesia carbon residual bricks in other parts is 123mm, while the new bricks are 125mm, indicating that the recycled bricks can fully meet the usage requirements.

Recycling of waste magnesia carbon bricks into ladle gunning material

Baosteel Co., Ltd. Meishan Iron and Steel Co., Ltd. processed the MgO-C bricks disassembled from the ladle slag line and used them to make ladle gunning materials. Field applications have proven that they can replace the currently used gunning materials.

Regeneration treatment process of used magnesia carbon bricks

The regeneration process of used MgO-C bricks adopted by Meishan Iron and Steel Co., Ltd. is: used magnesia carbon bricks → removal of surface slag and iron crushing → iron removal → screening → qualified raw materials. The chemical composition of used MgO-C bricks is shown in Table 2. Since the graphite added during the production of bricks is fine powder, the C content in the 1~0 particles after crushing is relatively high.

MgOT.C  Al₂O₃    SiO₂
3~1mm79.7511    3.5    1.97
1-0mm71.1418.3    4.45    2.44

Field application results of gunning materials

Because it is hot gunning, the water in the sprayed layer will evaporate quickly. Even if there is Al4C3 in it, it will not have time to hydrate, so the regenerated gunning material is not hydrated. 5t of recycled gunning material was produced, and the amount of recycled material introduced was 20%. Twenty-five hot ladle gunning tests were conducted, and the results showed that the regenerated gunning material has good adhesion rate, dense spray coating, fast curing speed, and durability of up to 6 furnaces, while the new gunning material can only be used for 4 to 5 furnaces. It shows that regenerated gunning material can completely replace new gunning material.

Key technical issues in the recycling of used magnesium carbon refractory materials

If magnesium carbon refractory materials can successfully realize high value-added recycling, the problems of Al4C3 and false particles must be solved.

Hydration removal of Al4C3

MgO-C bricks are refractory products made of magnesia, graphite, carbon binders and added antioxidants, which are mixed, formed and heat treated at around 220°C. Metallic Al powder is one of the commonly used antioxidants in magnesia-carbon bricks. AI easily reacts with C at high temperatures to form Al4C3, which can improve the high-temperature performance and oxidation resistance of refractory materials. However, AI4C3 is extremely easy to hydrate and will react with the water produced by the binding agent as follows at high temperatures:

Al4C3+12H2O=4AI(0H)3+3CH4↑

This reaction produces CH4 gas, and the volume of the solid produced by the reaction also increases by 1.65 times. When regenerated into bricks, it will cause the product to expand, pulverize and crack, affecting its performance. Therefore, when recycling magnesia carbon bricks, they must be hydrated to remove the Al4C3 component in advance. Commonly used hydration methods include water washing method and high temperature steam method. When using water washing method at room temperature, the water immersion time is usually ≥24 hours.

Since metal AI powder is added to MgO-C bricks in the form of fine powder, Al4c3 is mainly concentrated in fine powder <1mm. Liaoning University of Science and Technology conducted research on the optimal hydration technology of this part of the material at different temperatures. Laboratory research has found that when hydrating Al4c3 at room temperature (25°C), the water amount (mass) added is 2 times the material and the hydration time is 6 hours for the best effect. At 60°C, adding water (mass) 1.5 times the material and hydration time 4 hours has the best effect; at 90°C, adding water the same amount (mass) as the material and hydration time 6 hours has the best effect.

Removal of false particles

Recycling of magnesium carbon refractory materials usually goes through processes such as sorting, slag removal, crushing, and screening. There are a large number of false particles coexisting with aggregate and matrix in the initially crushed particles. The false particles have low volume density and many defects such as pores and cracks. When reused, the volume density of recycled products will be significantly reduced, and secondary crushing is prone to occur during molding, which affects product quality. To achieve high value-added utilization of post-used magnesium carbon refractory materials, false particles must be removed. The methods for removing false particles include grinding, dipping, flotation, and loss-on-ignition. The grinding method is usually used. The grinding method is relatively simple to operate. Through grinding, the fine powder on the surface of the granular material can be peeled off to achieve the separation of aggregate and matrix.

Wuhan University of Science and Technology conducted research on false particle removal technology. It was found that the large particles obtained by directly crushing the residual bricks after use are mainly false particles formed by fine powder wrapped on the surface of the original particles. The use of mixers and wheel mills to process the granular materials can achieve the goal of removing “fake particles” Effect. However, the roller mill has the shortest processing time and the highest efficiency. The optimal time for rolling magnesium carbon pellets is 7 minutes. If the time is too long, the pellets will be over-comminuted. The bulk density of the pellets decreases as the rolling time increases.

Analysis on the prospects of recycling waste magnesia carbon bricks

As the country continues to improve its steel production capacity, production capacity replacement has become an important development path for steel companies. In the next few years, it is expected that steel plants will have a large amount of waste refractory materials removed from thermal equipment such as converters, electric furnaces, refining furnaces, and ladles. Among these waste refractory materials, magnesium carbon accounts for nearly 60%. If it is recycled, it can not only reduce solid waste emissions, but also reduce the cost of refractory materials and save mineral resources such as magnesia and graphite. It has multiple environmental significance, economic benefits and social significance. This article takes magnesium carbon waste refractory materials as an example to briefly explain the current recycling technology of waste refractory materials.

From the perspective of circular economy, the recycling of used magnesium carbon refractory materials is the recycling process of solid waste in steel plants and an effective means of 3R in steel plants.

From an environmental protection perspective, due to years of extensive development, the country’s environment and resources have been greatly damaged. The central government has made up its mind to improve environmental protection. All walks of life must develop in an orderly manner within the framework of meeting the new environmental protection policies. . Foreign developed countries have put forward requirements for the recycling of used refractory materials since the end of the 20th century. The current recycling rate has reached 80%, while in my country it is only 20%~30%. In the future, the emission reduction of used refractory materials will surely It is listed as the focus of solid waste emission reduction, so the recycling of post-consumer refractory materials will be the only way to go.

In terms of economic benefits, the current steel mill operations have entered an era of low or no profit. To survive, they must transform and upgrade, refine their operations, and build a new profit point under the new normal. As far as Anshan Iron and Steel Co., Ltd. is concerned, 50,000 tons of waste refractory materials are generated every year. If they can be recycled, considerable profits will be made, transforming environmental protection from an investment project of steel mills to a profitable project. Taking magnesia carbon refractory materials as an example, recycled magnesia carbon materials are secondary resources after processing magnesia carbon bricks used in steel plants. Using them to replace part of fused magnesia can reduce the raw material cost of magnesia carbon bricks by nearly 1,000 yuan/t. , significant economic benefits.

There are many ways to recycle used magnesia carbon bricks, which can be used to make magnesia carbon bricks, ladle gunning materials, and tundish vibrating materials. Judging from mature experience at home and abroad, by taking corresponding technical measures to remove harmful components such as Al4C3 and fake particles in recycled materials, high value-added utilization can be achieved without reducing product quality. Steel plants should pay attention to the recycling of used magnesia carbon bricks and open up new ways for environmental protection and efficiency.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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