CHENG Jiuifa ¹, WANG Yerui ², YANG Wenhua ², REN Chao ², WAN Longgang ³, BAO Kun ⁴

(1. School of Ceramics of Shanxi College of Technology, Shuozhou 036000, Shanxi, China; 2. School of Materials Engineering of Shanxi College of Technology, Shuozhou 036000, Shanxi, China; 3. Luoyang Refractory Research Institute Co., Ltd., Luoyang 471039, Henan, China; 4. The Olefins Branch of Ningxia Coal Industry Company, National Energy Investment Group, Yinchuan 750411, Ningxia, China)

Extended abstract:[Background and purposes] Ferrosilicon-manganese slag is a solid waste generated during the smelting process of ferrous alloys. Its accumulation not only occupies land resources, but also the elements such as manganese in the ferrosilicon-manganese slag will cause environmental pollution. Therefore, the resource utilization of ferrosilicon-manganese slag is of great significance for environmental protection and sustainable development. The water-quenched ferrosilicon-manganese slag mainly presents an amorphous structure, with characteristics of high hardness, high brittleness and low density. It is mainly composed of silicates and manganates. Its high chemical activity and good stability make it a research hotspot in the field of building decoration materials, such as glass ceramics and artificial cast stone. However, the properties of water-quenched ferrosilicon-manganese slag, such as porous structure, uneven particle size and poor grindability, have affected potential for high-value utilization. The traditional resistor furnace heating and sintering have problems, such as low sintering efficiency, uneven heating and high energy consumption. Sol-gel method can be used to prepare nanoscale precursors. Their high specific surface area and activity require low activation energy for sintering, thus helping improve the density and mechanical properties of the materials. Microwave sintering has the advantages of fast heating, short sintering time, low sintering temperature and suppression of abnormal grain growth. Therefore, in this study, water-quenched ferrosilicon-manganese slag was used as the main raw material and a sol-microwave synergistic method was adopted to prepare a new type of ferrosilicon-manganese slag-based composite ceramics. By comparing with traditional heating and sintering, the effects of alumina sol, nanoscale alumina ceramic particles and microwave sintering on the phase composition, microstructure, mechanical properties and wear resistance of ceramics were studied.[Methods] Firstly, the water-quenched ferrosilicon-manganese slag was dried in an oven at 110 ℃ for 24 h to remove moisture. Then, it was fully ground for 15 min with a laboratory sample-making pulverizer by using the quartering method until 100% of the slag completely passed through a 200-mesh sieve. Alumina (boehmite) sol was prepared as follows. The molar ratio of distilled water to Al(OC₃H₇)₃ was 100∶1. Under the reflux condition at 90 ℃, the mixture was hydrolyzed with intense stirring for 2 h. Then, nitric acid was added to adjust the pH value to 3.5, followed by another 10-hour intense stirring. Nanoscale alumina powder was added according to the mass ratios of alumina to ferrosilicon - manganese slag of 3∶100, 7∶100, 9∶100 and 11∶100. The mixtures were thoroughly blended in a laboratory mixer for 5 h before use. Furthermore, under the action of a powerful electric stirrer, 15 wt.% of boehmite sol was slowly dropped into the premixes. After stirring for 2 h, the mixtures were aged for 48 h. Finally, the aged mixtures were filled into graphite grooves and compacted and then sintered in a microwave muffle furnace at 950 ℃ and 1150 ℃ for 1 h. Chemical elements of the ferrosilicon - manganese slag were analyzed by using a PANalytical Zetium X-ray fluorescence spectrometer for semi-quantitative analysis. Thermogravimetric curve (TDA) and heat-flow curve (DSC) of the ferrosilicon-manganese slag were recorded by using a TA-SDT650 synchronous thermal analyzer. The test conditions were protected by N₂ gas, at heating rate of 10 ℃·min⁻¹, over test temperature range from 20 ℃ to 1200 ℃. Phase composition of the ferrosilicon - manganese slag and ceramic samples was analyzed by using a Rigaku MiniFlex-600 X-ray diffractometer (Japan). The test conditions include working voltage of 40 kV, working current of 15 mA, scanning rate of 2·min⁻¹ and scanning range of 10°–80°. Microstructure of the ceramic samples was observed by using a Zeiss field-emission scanning electron microscope (FE-SEM) sigma360. Porosity and bulk density of the samples were measured by using an XQK-02 apparent porosity and bulk density tester from Sinosteel Luoyang Institute of Refractories Research. Compressive strength and flexural strength of the samples were measured by using an electronic universal testing machine DKZ5000. Friction and wear amount and friction coefficient of the samples were measured by using a material friction and wear testing machine HT1000.[Results] According to the FE-SEM images, it can be seen that, in the samples without the addition of boehmite sol, there are high-density "pinhole" defects. However, when the sol is introduced into the system, the density of the internal "pinhole" defects is significantly reduced. This observation strongly indicates that the amorphous or nanocrystalline structure derived from the sol can effectively promote the densification process of the bulk materials. In addition, by comparing the micro-morphologies after resistance wire heating and microwave heating, it can be found that the latter has a lower porosity. This means that the uniform component distribution of the sol can effectively suppress the formation of defects, such as cracking caused by local overheating during microwave heating. Meanwhile, the uniform heating of microwaves can inhibit the abnormal growth of nanoscale particles derived from the sol, thus ultimately leading to materials with finer and more uniform microstructure. According to the XRD results, the main crystalline phases of the samples sintered with microwaves are gehlenite and anorthite, and the secondary crystalline phases to be alumina and hausmannite. The characteristic peaks corresponding to Al₂O₃ were detected in all samples sintered at 950 ℃, which is related to the alumina sol and nanoscale alumina particles added in the raw materials. When the temperature was increased to 1150 ℃, the main crystalline phases of the samples were gehlenite and gehlenite-aluminosilicate. With the increasing content of the alumina sol and nanoscale alumina particles, the open porosity of the samples shows an obvious downward trend, decreasing from 24.96% of M1 to 17.73% of M4, with a decrease of 28.96%. With decreasing open porosity, the bulk density of the samples with different ratios also increases, from 2.35 g·cm⁻³ of M1 to 2.58 g·cm⁻³ of M4. Regardless of resistance heating or microwave heating, with gradual increasing content of nanoscale Al₂O₃, the compressive strength of the samples shows a gradually rising trend. When the holding temperature is 950 ℃, the maximum compressive strength under resistance heating is 36.36 MPa, while the maximum compressive strength under microwave heating reaches 61.11 MPa, showing an increase of 68.07%. When the holding temperature is 1150 ℃, the maximum compressive strength under resistance heating is 40.50 MPa, while the maximum compressive strength under microwave heating reaches 85.87 MPa, with an increase of 1.12 times. In addition, when the mass ratio of alumina to ferrosilicon-manganese slag is 9∶100, the SS-based ceramics have the highest compressive strength and flexural strength. This is because there are a large number of fibrous crystals inside the sample, which are interwoven one another to form a staggered network structure, effectively improving the compressive strength and flexural strength of the SS-based ceramics. When the sintering temperature is increased from 900 ℃ to 950 ℃, the wear amount decreases significantly. When the microwave sintering temperature is increased to 1150 ℃, the sample shows the lowest wear amount. When the sintering temperature is 1150 ℃, the friction coefficient of the sample is between 0.01 and 0.08. This indicates that the increase in the content of nanoscale Al₂O₃ and the microwave sintering temperature has an obvious improvement in the wear resistance of the SS-based ceramics.[Conclusions] Using ferrosilicon-manganese slag as the main raw material, and alumina sol and nanoscale alumina ceramic particles as auxiliary materials, ferrosilicon-manganese slag-based ceramic materials with smooth surfaces and dense interiors were successfully obtained by using microwave sintering at 950 ℃ and 1150 ℃ for 1 h. The content of nanoscale Al₂O₃, sintering temperature and sintering method had strong effects on the phase composition, microstructure, mechanical properties and wear resistance of SS-based ceramics were studied. After microwave sintering at 950 ℃ for 1 h, the phases of the sample were composed of gehlenite, secondary crystal of anorthite, alumina and hausmannite. After microwave sintering at 1150 ℃ for 1 h, the sample was only composed of gehlenite, secondary crystal of anorthite and hausmannite. When the microwave sintering temperature was 1150 ℃, the bulk density of the M4 sample reached the maximum value of 3.20 g·cm⁻³ and its open porosity was only 4.23%. When the mass ratio of alumina to ferrosilicon-manganese slag was 9∶100, the SS-based ceramics had the highest compressive strength, flexural strength and wear resistance. Alumina sol, nanoscale Al₂O₃ particles and microwave sintering could be employed to greatly promote the phase formation, reduce the sintering temperature of ceramics, enhance their comprehensive properties and exert a synergistic effect.

Key words: sol-gel; microwave sintering; silicon-manganese slag-based (SS) composite ceramics; mechanical properties; coefficient of friction