Differential Effects Of Binders On The Properties Of Magnesia Refractories

Approximately 85% of magnesia refractories produced domestically are applied in the iron and steel industry. During steelmaking, alkaline magnesia refractories can not only resist the corrosion of basic molten slag, but also absorb impurities in molten steel to purify the molten steel. Accordingly, higher standards are imposed on the quality and performance of magnesia refractories in modern industrial production. To a certain extent, binders for magnesia refractories determine the output, microstructure and high-temperature properties of magnesia refractory products. For this reason, the selection of single-phase or composite additives in the manufacturing process of magnesia refractories has become a key research focus. This paper reviews the basic classification of binders for magnesia refractories as well as their applications in monolithic magnesia refractories and shaped magnesia refractory products.

 Binders for magnesia refractories refer to substances that bond refractory aggregates and powders with specific particle sizes together and endow the formed bodies with sufficient room-temperature strength or high-temperature bonding strength, also known as cementing agents or adhesives. According to their chemical properties, they are divided into two major categories: inorganic binders (silicates, aluminates, phosphates, sulfates, chlorides, etc.) and organic binders (pitch, starch, lignosulfonate, phenolic resin, epoxy resin, and so on).

 Binders for magnesia refractory castables are mainly composed of oxide micropowders combined with magnesia cement, sodium polyphosphate and other components.

 Xu Yong adopted aluminium oxychloride as the binder for magnesia-based basic castables. The test results show that aluminium oxychloride can prevent strength degradation of magnesia-based basic castables at medium temperatures and improve the structural stability inside the castables after calcination at 1500 ℃.

 Li Yousheng et al. separately investigated the effects of binders including magnesium chloride hexahydrate, SiO₂ micropowder and ρ-Al₂O₃ on the properties of magnesia castables. The results indicated that specimens prepared with SiO₂ micropowder and magnesium chloride hexahydrate as binders exhibited superior cold compressive strength and hydration resistance compared with those using ρ-Al₂O₃ as the binder. For magnesia castables bonded with magnesium chloride hexahydrate and ρ-Al₂O₃, their compressive strength decreased significantly after heat treatment at 1100 ℃ for 3 hours.

 Cao Renfeng et al. used magnesium oxychloride cement as the binder for magnesia castables. This binder not only has advantages such as no impurity introduction and excellent slag resistance, but also avoids reactions with raw materials at high temperatures to form low-melting-point phases, which would otherwise destroy the internal structure of the material and impair its high-temperature service performance. In addition, magnesium oxychloride cement can significantly improve the cold modulus of rupture of magnesia castables.

 Zhao Zhongxuan et al. investigated the effects of brown fused alumina fine powder as an additive on the physical properties and slag resistance of magnesia castables. The experimental results revealed that the specimens achieved optimal strength and superior slag resistance when the addition amount of brown fused alumina fine powder was 5 wt%.

 Traditional magnesia dry vibratable mixes adopt phenolic resin as the binder. However, substances such as NH₃ will be released during baking and hardening, causing serious environmental pollution. In addition, carbonization of the resin will lead to carbon pickup in molten steel. To address the above problems, Gu Huazhi et al. adopted metasilicate as the binder and added reinforcing agents to fuse with the metasilicate. With the increase of calcination temperature, the bonding mechanism transforms from adhesive bonding to ceramic bonding. This method solves the problems of environmental pollution and molten steel carbon pickup while maintaining excellent bonding strength of the vibratable mixes.

 Gu Lifang et al. selected inorganic salts as binders for magnesia dry vibratable mixes to meet the production demands of low-carbon steel grades. Comparative tests showed that composite additives composed of metasilicates, phosphates and borates deliver outstanding corrosion resistance, and the corresponding samples possess sufficient strength and service life to meet practical application requirements.

 Li Yousheng et al. prepared samples by mixing fused magnesia (particle size ≤0.088 mm) with five types of low-temperature binders and six types of medium-temperature binders separately, followed by experimental characterization. The results demonstrate that magnesia dry mixes bonded with phenolic resin, pitch and rosin exhibit higher low-temperature strength. Binders including boric acid, borax decahydrate and borate glass can significantly promote high-temperature sintering of magnesia materials and improve their high-temperature strength. Magnesia dry mixes bonded with rosin-borate glass possess excellent room-temperature physical properties and favorable environmental performance.

 Wu Feng et al. used glucose to replace phenolic resin, and combined it with water glass as a composite binder for magnesia dry mixes. Experiments show that the flue gas generated by this binder is significantly less than that produced by phenolic resin, which meets the requirements of environmental friendliness.

 Magnesia refractory sprays, also known as magnesia refractory gunning mixes, feature high refractoriness and excellent resistance to basic slag erosion. Their commonly used binders include sodium silicate, phosphates, resins, pitch and so on.

 Song Yunjie adopted sintered magnesia and fused magnesia as main raw materials, and sodium hexametaphosphate as both binder and dispersant to prepare magnesia gunning mixes. Sodium hexametaphosphate can promote sintering and reduce water content. Test results show that the gunning mixes exhibit optimal high-temperature properties and corrosion resistance when the addition amount of sodium hexametaphosphate is 2 wt%.

 Zhang Zhaoxia et al. selected high-quality fused magnesia and sintered magnesia, optimized particle size distribution, and added a composite binder dominated by silicates with a small amount of phosphate as auxiliary component. They developed a slag line gunning mix that achieves a maximum service life of 16 heats for VOD ladles.

 Zhang Jingyu et al. used waste magnesia-chrome bricks from RH furnaces as raw materials, sodium hexametaphosphate as binder, together with slaked lime and silica fume, to develop a new type of magnesia gunning mix. The service performance of the magnesia gunning mix reaches the best level when the dosage of sodium hexametaphosphate is 0.2 wt%.

 To produce high-quality low-phosphorus steel, Jin Jiao abandoned conventional phosphate binders and adopted a slaked lime-silicate composite binder to prepare phosphorus-free magnesia electric furnace sprays. Studies reveal that the gunning mix has the most suitable hardening time, outstanding adhesion, workability, corrosion resistance and maximum strength when the binder content is 3 wt%, satisfying the production requirements of clean steel.

 Despite encouraging research achievements on magnesia refractory sprays, they still suffer from poor corrosion resistance and short service life. Magnesia-carbon refractory sprays can well overcome these drawbacks. Yao Yashuang et al. prepared magnesia-carbon refractory sprays with good flowability and extended service life by using magnesia as aggregate, fine magnesia powder as matrix, and 5–7 wt% pitch as binder, which achieved a 30% increase in service life. Ren Bing used sintered magnesia as aggregate, fused magnesia as matrix, and phenolic resin-pitch composite binder to produce magnesia-carbon gunning mixes free of cracking and spalling after service, with an average service life exceeding 45 hours. Ouyang Degang et al. successfully fabricated magnesia gunning mixes using medium-grade magnesia as raw material and phosphate-silica fume composite binder. When the phosphate dosage ranges from 4 wt% to 5 wt%, the magnesia repair mixes possess excellent penetration resistance, which can effectively prolong the service life of induction furnace crucibles and be applied in large-scale industrial production.

 Magnesia refractory coating mixes feature excellent wear resistance. The residual coating after service can be easily stripped from the permanent lining, and they have a long service life to support continuous casting for multiple heats. They are widely applied in various thermal equipment, with industrial-grade sodium tripolyphosphate commonly adopted as the binder.

 Tian Shouxin et al. adopted sodium tripolyphosphate as the binder for tundish coating mixes. The results show that the prepared ultra-lightweight magnesia coating mixes perform remarkably in tundish heat preservation and stable continuous casting, while reducing material consumption and production costs. Ding Zhongshan et al. replaced part of sintered magnesia with waste magnesia-chrome bricks as raw materials, and added CA-70 cement and silica fume as composite binders. CA-70 cement endows the coating mixes with rapid hardening and high strength, whereas silica fume improves slurry flowability and workability, thus producing coating mixes that meet practical application requirements.

 Sui Jun prepared erosion and scouring resistant magnesia tundish coating mixes using sodium polyphosphate as the binder. After hardening, sodium polyphosphate does not undergo severe dehydration. During its reaction with MgO, harmful Na₂O gas is released, which alleviates damage to the high-temperature performance of refractories. Moreover, the bonded magnesia coating mixes exhibit outstanding spalling resistance.

 Zhang Zhian et al. adopted high-purity magnesia as aggregate and fused magnesia powder as the matrix, and combined low-temperature and medium-temperature binders to fabricate magnesia-calcium coating mixes with superior comprehensive properties.

 Chen Changping et al. adopted silica micropowder-resin composite binders to replace traditional clay and cement. Such binders not only satisfy the strength requirements of lightweight coating mixes, but also increase the viscosity and yield stress of the mixes for convenient construction.

 Binders commonly used for magnesia refractory ramming mixes include brine, phenolic resin, pitch and other substances. Zhou Zhuogang adopted brine as the binder, and the prepared magnesia ramming mixes were used to replace silica ramming mixes. During construction, it was found that the mixes can achieve rapid shaping under natural curing with excellent compactness. Xu Zhendong et al. took fused magnesia as raw material and solid phenolic resin as binder to successfully manufacture high-performance dry magnesia ramming mixes for tundishes. Compared with magnesia coating mixes, these products reduce heat loss and prevent creep of the steel shell.

 Magnesia bricks are basic refractory products with magnesium oxide as the main chemical component and periclase as the primary crystalline phase. Sulfite pulp waste liquor and brine are widely used as binders during production. Yang Jian et al. successfully fabricated sintered magnesia bricks with excellent thermal shock resistance using pulp waste liquor as the binder. Bonded by sulfite pulp waste liquor, the magnesia bricks possess outstanding strength and thermal stability.

 Magnesia-calcium bricks are basic refractories mainly composed of MgO and CaO, which can purify molten steel. However, they suffer from poor hydration resistance. Therefore, binders with high carbon content and zero water content should be preferentially selected during production, and anhydrous resins or paraffin wax are commonly adopted.

 Zhang Qingshan used magnesia-calcium clinker and sintered magnesia as raw materials, and uniformly mixed K-96 waterproofing agent, anhydrous resin and sodium hexametaphosphate in a certain proportion as a novel composite binder to prepare magnesia-calcium bricks with satisfactory overall properties.

 Li Yanping added paraffin wax and 3 wt% magnesium titanate as binders into sintered magnesia-calcium clinker and high-calcium fused magnesia to produce low-cost magnesia-calcium bricks. The increase of Ca/Si ratio facilitates the formation of high-melting-point compounds, thereby improving the high-temperature performance and corrosion resistance of the bricks.

 Wang Qingxian et al. adopted dolomite clinker, fused magnesia-calcium clinker and fused magnesia as raw materials, and anhydrous resin with 84 wt% solid content (carbon content ≤42 wt%) as the binder. They fabricated unburned magnesia-calcium bricks with excellent properties and 25–35 wt% CaO. The bricks perform well when applied to ladle bottoms, effectively preventing crack generation and steel penetration.

 Magnesia-chrome bricks are conventionally applied in the transition zones of cement rotary kilns and arch sections of sleeve kilns. Nevertheless, chromium-containing refractories exhibit poor corrosion resistance, while magnesia-alumina spinel bricks possess outstanding corrosion resistance and thus serve as ideal alternatives to magnesia-chrome bricks. Pulp liquor and brine are commonly used binders for such bricks.

 Yang Zhongde et al. adopted high-purity magnesia, fused magnesia and fused spinel as main raw materials, and maltodextrin as the binder to fabricate periclase-magnesia alumina spinel bricks. Bonded with maltodextrin, spinel phases are uniformly distributed inside the bricks, which helps improve the high-temperature performance of the products.

 Zhang Daoyun et al. used fused spinel, fused magnesia, medium-grade magnesia and α-Al₂O₃ as raw materials, and sulfite pulp waste liquor as the binder. After forming and calcination, they developed a new type of magnesia-alumina spinel brick for sleeve lime kilns.

 Forsterite bricks are refractories produced from calcined or uncalcined peridotite or calcined serpentinite. They feature low thermal conductivity and excellent chemical stability, showing promising prospects for manufacturing lightweight thermal insulation refractories. Accordingly, selecting appropriate binders is critical during the production of forsterite bricks.To develop basic shaped refractories for glass kilns, Zheng Lianying et al. used forsterite clinker and magnesia as main raw materials, and compared the effects of three binders including MgCl₂·6H₂O, sulfite pulp liquor and QH on product properties. The results reveal that lightweight forsterite bricks with outstanding performance for glass kilns can be obtained when the addition dosage of QH binder is 7.5 wt%.Yuan Guangliang et al. compared the influences of three binders, namely carboxymethyl cellulose, silica sol and MgCl₂·6H₂O, on the properties of lightweight forsterite bricks. When MgCl₂·6H₂O was adopted as the binder, the prepared lightweight forsterite bricks achieved a bulk density of 1.15 g·cm⁻³ and a compressive strength of 3.58 MPa.

 Magnesia-carbon bricks are widely used in the iron and steel metallurgy industry due to their excellent service performance. However, the properties of binders exert a significant influence on their high-temperature performance. Therefore, modification research on phenolic resin, the conventional binder for magnesia-carbon bricks, has become a research hotspot in recent years.

 You Jiegang et al. conducted experiments on the dosage of phenolic resin for recycled magnesia-carbon bricks from ladle slag lines. The results indicate that the optimal addition amount of phenolic resin in recycled magnesia-carbon bricks ranges from 3.5 wt% to 4 wt%. Zhang Xuesong et al. adopted a mixture of phenolic resin and pitch as the composite binder for magnesia-carbon bricks, and investigated the effect of the mixing ratio of phenolic resin to pitch on the residual carbon rate of binders through experiments. The residual carbon content rises with the increase of pitch dosage, yet pitch will cause certain environmental pollution. To solve this problem, researchers have attempted to improve the residual carbon rate by modifying phenolic resins.Liao Qingling et al. developed a novel nano-SiO₂ modified phenolic resin via the sol-gel method and in-situ growth method, and applied it to magnesia-carbon bricks. The results show that adding organosilicon to phenolic resin forms a cross-linked network structure with the resin, endowing magnesia-carbon bricks with excellent high-temperature service performance. Li Yawei et al. directly incorporated nickel oxide into phenolic resin. During resin decomposition, nickel oxide is reduced to metallic nickel, which slowly precipitates to form whisker-like and sheet-like structures, optimizing the performance and microstructure of pyrolytic carbon. Nevertheless, its low dispersion efficiency impairs the overall service performance of the bricks.

 Li Yawei et al. further blended phenolic resin with sol precursors of transition metals including iron, cobalt and nickel to enhance dispersion performance. Magnesia-carbon bricks prepared with this modified phenolic resin binder exhibit remarkably improved oxidation resistance, thermal shock resistance and compressive strength.

With the continuous advancement of industrial production in China, the demand for magnesia refractories keeps growing year by year. A wide variety of binders are available for magnesia refractories, yet the ultimate development goals lie in realizing long service life, environmental friendliness and low production cost of magnesia refractory products. In view of the above, several suggestions on the application of binders for magnesia refractories are put forward as follows:

 (1) Most inorganic binders contain harmful impurities such as CaO, Na₂O and K₂O, which deteriorate the high-temperature service performance of magnesia refractories. To tackle this problem, binders with higher purity and melting point should be adopted to minimize impurity content. The wettability and adhesion between binders and magnesia raw materials shall also be taken into consideration during selection, and intense quick-setting reactions between the two sides should be avoided.

 (2) Composite binders shall be widely adopted and novel binders shall be developed to transform the bonding mode of unshaped magnesia refractories from hydration bonding and chemical bonding to coagulation bonding, so that the magnesia refractories can maintain uniform strength across all temperature ranges.

 (3) For organic binders, bonding strength, production cost and flue gas pollution restrict their further development, especially under the background of low-carbon refractories. Therefore, novel eco-friendly binders other than pitch and phenolic resin need to be continuously researched and developed.