The mineral heat furnace is the main smelting equipment used in the production of ferroalloy products. It utilizes the energy generated by electric arcs and electrical resistance within the furnace to melt metals. The smelting process is typically carried out in an alkaline environment, and magnesium bricks are commonly used as the furnace lining.
In the complex environment of high temperature, erosion, and heat dissipation, damage to the furnace lining increases energy consumption and production costs. Therefore, selecting a reasonable refractory configuration and optimizing the furnace lining structure are important approaches to improving energy efficiency in mineral heat furnaces.
In recent years, with the decline in the iron and steel industry, the non-ferrous metal industry has remained stable. In particular, the demand for ferroalloys has been rising year by year, which has led to a continuous increase in the demand for magnesium refractory materials used in mineral heat furnaces.
In this study, mid-grade magnesium bricks with 95% magnesium content, high-purity magnesium bricks with 97% content, and electrofused magnesium bricks with 98% content were selected as the research objects. Their raw materials and products were studied in terms of microstructure, thermal conductivity, and linear expansion. Additionally, static slag resistance experiments were conducted using nickel-iron slag to evaluate their performance.
Microstructure of raw materials of different grades of magnesium sand
The SEM images of mid-grade magnesium sand (95% Mg), high-purity magnesium sand (97% Mg), and electrofused magnesium sand (98% Mg) are shown in Fig. 1.
Fig. 1 SEM Photo of Magnesia Sand
From Fig. 1, it can be clearly seen that 95% mid-grade magnesia sand has more pores, with large pore sizes distributed in the intercrystalline regions. The crystal size is mostly between 50–100 μm, and the magnesite crystals are bonded by silicates, with a higher presence of silicate phases.
The 97% high-purity magnesia sand also has significant porosity, but the pore diameter is smaller and distributed in both intra-crystalline and inter-crystalline regions, often forming closed loops. The crystal size ranges from 25–80 μm. There are fewer acidic phases, and the magnesite crystals are either directly bonded or bonded by silicates.
In contrast, the 98% electrofused magnesia has fewer pores, with smaller pore sizes and lower overall porosity. The magnesite crystals are well-developed, with crystal sizes exceeding 500 μm-some even reaching the millimetre scale. The silicate phase between crystals is distributed in a thin-film form, with clear and relatively straight grain boundaries. The microstructure of the electrofused magnesia sand fully reflects the development of magnesite crystals.
Thermal Conductivity and Linear Expansion of Different Grades of Magnesia Bricks
The thermal conductivity and linear expansion of different grades of magnesia bricks are shown in Fig. 2.
Fig. 2 Thermal Conductivity and Linear Expansion of Magnesia Bricks of Different Grades
As shown in Fig. 2-1, with increasing temperature, the thermal conductivity of magnesia bricks decreases. At the same temperature, the higher the purity of the magnesia content, the greater the thermal conductivity coefficient, which is consistent with theoretical expectations.
As shown in Fig. 2-2, linear expansion increases with temperature. The linear expansion of 97% high-purity magnesia bricks is greater at the same temperature. This is because the magnesite crystals in high-purity magnesia sand are small and tend to undergo secondary development and growth at high temperatures, leading to greater expansion of the product.
In mineral heat furnaces using magnesia linings, differences in the brick material and its thermal parameters in the furnace's working layer affect the expansion and heat transfer through the furnace shell steel plate. This can cause expansion, leading to upward warping of the furnace bottom and deformation-or even cracking-of the lower part of the furnace shell. Therefore, it is essential to select suitable magnesia bricks for the working layer that match the furnace structure, smelting duration, and smelting temperature, in order to achieve the desired service life.
Static Slag Resistance Experiment
Using slag from nickel laterite ore as the slag source, a static slag resistance test was conducted to compare the performance of the three types of magnesia bricks. After testing, all magnesia brick specimens exhibited vertical cracking.
3.1 Macro photographs
The specimen was cut after the slag resistance test, and a photo of the cross-section is shown in Fig. 3. It can be seen that, after slag melting and erosion, a slag layer formed around the upper part of the brick, leaving behind grooves with a depth of approximately 3 mm. Below the slag surface, although no obvious metamorphic layer is visible macroscopically, slag penetration into the brick is clearly evident, with a penetration depth of about 10 mm.
Fig. 3 Experimental Profiles of Slag Resistance of Magnesia Bricks of Different Grades
3.2 Microanalysis
For magnesia bricks of the same material after slag infiltration, SEM analysis was conducted at different locations. Figure 4 shows the micrographs of 95% mid-grade magnesia bricks after the slag resistance experiment.
Fig. 4 Micrographs of 95% Magnesia Bricks After Slag Resistance Test
Figure 4-a shows a micrograph of the working layer at 50× magnification, where the slag layer and the reaction layer are clearly visible. Figure 4-b is a micrograph of the slag–brick interface at 200× magnification. The magnesia particles have been eroded by the slag, resulting in a smoother surface. The upper portion has experienced significant fusion loss.
Fig. 5 Micrographs of 97% High-Purity Magnesia Bricks After Slag Resistance Test
Figure 5-a shows a micrograph of the working layer at 50× magnification, where the slag layer and the reaction layer are clearly visible. Figure 5-b is a 400× magnified image of point A from Figure 5-a. The magnesia particles have been eroded by the slag, and the edges of the particles show signs of melting damage, although to a lesser extent.
Fig. 6 Micrographs of 98% Electrofused Magnesia Bricks After Slag Resistance Test
Figure 6 shows the micrographs of 98% electrofused magnesia bricks after the slag resistance test.
Figure 6-a is a photograph of the working layer at 50× magnification, clearly showing the slag layer and the reaction layer.
Figure 6-b is a 400× magnified image of point A from Figure 6-a. The electrofused magnesia particles show only a small amount of silicate penetration at the grain boundaries, and the particles are almost unaffected by slag erosion.
The erosion of 98% electrofused magnesia bricks by the slag is less significant than that observed in the other two types of bricks. The presence of large electrofused magnesia particles effectively blocks slag infiltration.
Conclusion
The microstructures of magnesia produced by different manufacturing processes vary significantly. The crystal size of electrofused magnesia is the largest, followed by that of mid-grade magnesia, while high-purity magnesia has the smallest crystal size.
Based on the results of the slag resistance tests for the three types of magnesia bricks, 95% mid-grade magnesia bricks exhibit the poorest resistance to erosion and slag penetration. The performance of 97% high-purity magnesia bricks is slightly better, while 98% electrofused magnesia bricks demonstrate the best slag resistance.
A comparison of the thermal expansion and thermal conductivity data of the three types of magnesia bricks shows that, from a cost-performance perspective, 95% mid-grade magnesia bricks can meet the operational requirements for the working layer of a mineral heat furnace.