The requirements for refractory materials in non-ferrous metal smelting are relatively complex. They must have sufficient high-temperature resistance, certain high-temperature strength, and good resistance to slag erosion and slag and flue gas scouring. Therefore, there are strict requirements for the selection of refractory materials in the furnace. At the same time, the smelting of various non-ferrous metals has its characteristics, and refractory materials need to be selected selectively.
At present, the refractory materials used in the non-ferrous metal smelting industry in China are roughly divided into two categories: acidic refractory materials and alkaline refractory materials. Acidic refractory materials are mainly trivalent oxides (Al₂O₃-SiO₂ series), mainly including high alumina bricks, mullite bricks, zirconium corundum bricks, etc.; while alkaline refractory materials are mainly divalent oxides (MgO-Al₂O₃, MgO-Cr₂O₃ series), including magnesia chrome bricks, magnesia aluminum bricks, magnesia aluminum spinel bricks, etc.
Design and application practice of refractory materials in the lead metallurgical industry:
1. Furnace bottom design
After years of actual production experience, for lead smelting, the metallurgical furnaces used include dozens of metallurgical furnaces that process various lead materials, but the refractory linings of metallurgical furnaces mainly use magnesia-chrome bricks, high-alumina bricks, high-alumina refractory ramming materials, etc.
Furnace bottom permanent layer area: In the design of the furnace lining, the selection of refractory materials varies accordingly at different positions in the furnace body. Taking the fixed horizontal metallurgical furnace body as an example, the furnace bottom generally uses magnesia-chrome bricks, high-alumina bricks, aluminum-chrome spinel, high-alumina ramming materials, magnesium ramming materials, etc., and some use high-strength anti-seepage ramming materials, whose composition also belongs to the Al₂O₃-SiO₂ system, and the content of Al₂O₃>75%. The specific gravity of liquid lead is 10.6g/cm³, and its permeability is extremely strong. Therefore, the furnace bottom refractory materials must have both the function of heat dissipation and the ability to prevent lead seepage.
At present, the widely used practice is to first lay high-aluminum bricks on the furnace bottom steel plate. High-aluminum bricks have high compressive strength (compressive strength at room temperature 40~60MPa), and it is more reasonable to use them as a cushion layer at the bottom of the furnace; a layer of refractory materials with resistance to lead penetration should be set on the upper part of the furnace bottom cushion layer. At present, magnesium ramming materials or high-strength anti-seepage ramming materials (high-aluminum) are used. Both can play the role of interlayer. The ratio of magnesium ramming materials is: magnesia sand: magnesium powder = 7:3, with brine, magnesia sand particle size: 0.2~0.5mm70%, 1.5~3.0mm 30%; the composition of high-strength anti-seepage ramming materials is: high-aluminum aggregates and bone powder of various particle sizes. After high-temperature baking, the aggregates of various particle sizes expand and combine closely to achieve the ideal purpose of anti-seepage lead. It must be noted that after the ramming of magnesium and magnesium-chromium ramming materials is completed, they need to be baked at low temperature. After baking out the free water, the expansion joints are filled with fine magnesium powder to ensure the strength and density of the ramming layer. The thickness of the ramming material is recommended to be 150~300mm, which is convenient for one-time ramming and can be baked more evenly to form an overall layer with good anti-seepage effect.
Furnace bottom working layer area: Magnesia-chrome bricks are widely used for the selection of refractory materials for the safety layer and working layer of the furnace bottom. The safety layer can be made of direct-bonded magnesia-chrome bricks, and the working layer can be made of semi-rebonded magnesia-chrome bricks. As the lead liquid level fluctuates, the temperature of the furnace bottom fluctuates significantly, so semi-rebonded magnesia-chrome bricks with good thermal shock resistance should be selected. High-alumina bricks are also used for the safety layer and working layer of the furnace bottom. Generally, this type of furnace will have a bottom lead layer of ~400mm high, so the furnace bottom will not be corroded by the slag. High-alumina bricks with an Al₂O₃ content of not less than 75% can be selected as lining bricks for the safety layer and working layer of the furnace bottom.
2. Working area in the furnace
The selection of refractory materials in the working area (furnace wall, furnace top) in the furnace is divided into two areas, one is the refractory bricks in the molten pool area (especially the slag line area) and the other is the refractory bricks in the meteorological area.
Molten pool area in the furnace: The refractory bricks in the molten pool area (especially the slag line area) will be corroded and washed by the slag. The composition of lead smelting slag is relatively complex. High-aluminum refractory materials will participate in the slag-making reaction. Therefore, it is inappropriate to use high-aluminum refractory bricks. Magnesia-chrome refractory bricks should be used. At the same time, considering the slag erosion and scouring resistance of refractory bricks, electric melting recombination magnesia-chrome bricks should be used.
Bricks of this material are better than semi-recombined magnesia-chrome bricks in slag erosion resistance. Increasing the content of Cr₂O₃ can improve the slag erosion resistance of bricks, so try to choose magnesia-chrome refractory bricks with higher Cr₂O₃ content.
Meteorological zone in the furnace: Refractory bricks in the meteorological zone will not be corroded by slag, but only by a small amount of slag splashing and scouring of dusty flue gas. Therefore, magnesia-chrome refractory bricks with lower Cr₂O₃ content can be selected. The magnesia-chrome bricks used in the lead reduction furnace of a domestic factory used direct-bonded magnesia-chrome bricks with a high content of Cr2O₃ in the early stage of production. The magnesia-chrome bricks in the meteorological zone had no metal and slag on the surface, the bricks were broken into two sections, and the structure was loose. According to the analysis results, it was judged that the Fe³﹢ and Fe²﹢ in the refractory bricks were reduced to elemental Fe in large quantities, resulting in a loose brick structure.
Therefore, during maintenance, fused rebonded magnesia-chrome bricks with a low content of Cr2O₃ (Cr2O₃ content of 12%) were used. The main reason for this improvement is that the apparent porosity of fused rebonded magnesia-chrome bricks is low, and at the same time, the content of Fe³﹢ and Fe²﹢ in the refractory bricks is reduced, so that it is more suitable for the strong reducing atmosphere in the meteorological zone and prolongs the service life. After switching to this type of fused rebonded magnesia-chrome bricks, the service life was greatly extended and good results were achieved.
There are many types of furnaces used in the domestic lead smelting industry, and cooling devices are also used in various metallurgical furnaces, which have a good effect on extending the service life of metallurgical furnaces. However, from the perspective of use, for lead smelting, which has a large degree of superheat, complex raw materials, and lead matte that easily corrodes the cooling device, there are still certain safety hazards in the operation of slag hanging by the cooling device. Therefore, it is still indispensable to line the cooling device with refractory materials. The correct use of refractory materials and cooling devices complement each other and can protect each other.
Based on the characteristics of the smelting process, the characteristics of the smelting materials, the correct selection and use of refractory materials, in order to ensure the normal operation of the metallurgical furnace, ensure a reasonable furnace life, and enable the enterprise to obtain economic benefits, there must also be a correct and reasonable refractory design, including structural design, expansion calculation, and masonry heating and baking, which all affect the normal use of refractory materials.