According to the needs of comprehensive utilization of vanadium-titanium magnetite, Panzhihua Iron and Steel began to study the process of direct reduction of vanadium-titanium magnetite in the 1980s. Based on previous studies, this process boldly proposes a new process route of direct reduction melting and deep reduction-vanadium extraction from hot metal-high titanium slag based on the characteristics of vanadium-titanium magnetite, which not only realizes the realization of vanadium-titanium magnetite smelting hot metal It also creatively realizes the one-time separation and comprehensive recycling of iron, vanadium and titanium. However, since the new process smelting electric furnace has been put into production, there have been many production and operation problems. The excessively high open arc temperature caused serious deformation of the furnace cover, and the mismatch of the furnace lining and the acid titanium slag caused the rapid corrosion of the furnace lining, which had the greatest impact on production and test operation.
At present, magnesia carbon bricks (C: 14% to 16%) are used as refractory materials in the melting furnace, which has a low service life and cannot meet the needs of field tests. For this reason, it is necessary to choose a kind of refractory material suitable for smelting in electric melting furnace. By carrying out the dynamic slag erosion resistance comparison test, calculation, analysis and other methods of various refractory bricks, we can find refractory materials that can meet the working conditions of the melting furnace and can be produced for a long time. In particular, the refractory material needs to have good corrosion resistance to ferrous oxide and titanium dioxide acid slag, excellent erosion resistance to low-carbon molten iron, and high refractoriness, suitable for high temperature and ultra-high temperature smelting in electric furnaces.
Overview of refractory materials for melting furnace
1. Overview of melting furnace
The melting furnace of the pilot line is mainly composed of furnace base, furnace body, furnace wall (furnace door), furnace cover, three-phase graphite electrode (A phase, B phase, C phase), refractory materials, iron tap, and slag tap , Operating platform, electrode lifting device, feeding system, smoke exhaust system and other parts, power 12500kV.A, furnace diameter 7.4m, furnace diameter 5.2m, depth 2.1m, molten pool diameter 3.8m, depth 0.8m. The slag outlet is perpendicular to the tap hole at 90°, and the tap hole is directly aligned with the phase A electrode. After the slag and iron are separated in the final stage of smelting in the electric furnace, the slag will be discharged from the slag opening, and then the iron can be tapped after the slag is clean. The slag is acidic slag with a basicity of about 0.2.
2. The use of refractory materials in the melting furnace
2.1 The initial design of refractory masonry structure
The main material of the working layer of the refractory masonry structure in the initial design of the electric furnace is medium carbon magnesia carbon bricks, the permanent layer of the furnace lining is made of magnesia bricks, and the permanent layer of the furnace bottom is made of high alumina bricks and magnesia bricks. The iron tap and the slag tap are made of magnesium-carbon preforms.
2.2 The masonry situation of refractory materials in 1 ~ 5 furnaces
After the smelting electric furnace was put into production, due to rapid erosion, refractory materials were overhauled and replaced, and 2 to 5 furnace campaigns were implemented before and after. The situation is as follows: The second furnace service refractory masonry structure is based on the first furnace service masonry structure foundation Improvements and optimizations have been made on the above, the thickness of the refractory material in the molten pool is reduced, and the converter is used in the tap hole and the slag tap hole. At present, tap hole bricks are used instead of prefabricated parts. The refractory masonry structure of the third service was further optimized, reducing the thickness of the permanent lining of magnesia bricks, increasing the thickness of the magnesia carbon brick lining, and increasing the thickness of the refractories above the slag line of the molten pool on the basis of the second service. The tap hole and slag tap hole are still made of tap hole bricks, and the tap hole is reduced by about 50mm. The refractory masonry structure of the fourth furnace is similar to that of the third furnace, with little change, mainly because the refractory material in the molten pool is slightly thickened and the diameter of the molten pool becomes smaller. The refractory masonry structure in the early stage of the fifth campaign was the same as that of the fourth campaign. At the end of the test, the upper steel shell of the melting furnace was red, and the upper refractory material of the melting furnace was improved, and the thickness of the upper magnesia carbon brick was thickened. , And masonry magnesia bricks are used to protect the steel shell.
2.3 Analysis of corrosion of refractory materials in 1 ~ 5 furnaces of electric melting furnace
The refractory linings of the 1~5 furnaces of the electric melting furnace were severely corroded and damaged in the middle and later stages of the test. The serious corrosion of the lining was mainly near the slag line, especially the area behind the three-phase electrode, showing a clear three-phase plum petal-like contour. The corrosion and damage of the refractory materials on the iron wire and below and the upper refractory on the slag line are not serious, indicating that the slag has the most significant corrosion impact on the refractory. According to the working conditions and smelting conditions of the electric melting furnace, there are four main reasons for the analysis of the corrosion and damage of the furnace lining:
(1) Acidic molten slag (FeO, SiO2 and TiO2) corrodes refractory materials. The carbon in the magnesia carbon brick is oxidized and lost to form holes. After the molten slag enters, it reacts with magnesium oxide to form magnesium ferrite, magnesium silicate, and titanic acid. Low melting point substances such as magnesium.
(2) Ultra-high temperature smelting conditions and arc irradiation and erosion cause corrosion and damage to refractory materials.
(3) The intermittent operation causes the refractory to be subjected to the thermal shock of alternating cold and heat, causing cracks and falling blocks, which reduces the performance of the refractory.
(4) The slag and low-carbon molten iron are stirred by the electric arc, which strongly scours refractory materials, causing mechanical erosion and damage to the furnace lining. (Picture 1)
Refractory corrosion mechanism analysis
- Changes in the properties of refractory materials after use
The physical properties of the remaining magnesia-carbon bricks after use are analyzed: the porosity of the post-refractory bricks is higher than that of the original bricks, 2~4 times more than the original bricks, the bulk density is slightly lower than the original bricks, and the strength index is less than 1/of the strength performance of the original bricks. 2. It shows that the performance of refractory bricks is significantly reduced after use. The performance of the working layer of the same brick is the worst. The permanent layer of the transition layer is slightly better; the carbon content of the original brick is about 15%. The content gradually decreases, indicating that the carbon of the refractory brick near the working layer reacts with the molten slag and is oxidized and lost, making the brick matrix loose.
- Microstructure analysis of refractory materials after use
After taking the residual magnesia carbon brick (sticky slag) to analyze the phase composition of the slag layer, the XRD (X-Ray Diffraction, X-ray diffractometer) diagram is shown in the figure below. The phase analysis of post-magnesia-carbon bricks showed that the main phases after the reaction of magnesia-carbon refractories and slag were forsterite (Mg2SiO4), magnesia-titanium spinel (MgTi2O5) and magnesia-aluminum spinel (MgAI204). The refractory properties of forsterite, magnesia-titanium spinel and magnesia-aluminum spinel are far lower than magnesia or magnesia-carbon refractory materials, and form a melt at high or ultra-high temperatures, which will seriously corrode refractory materials. (Figure II)
Selection of refractory materials for melting furnace
- Comparative analysis of conventional physical properties
Choose magnesia carbon bricks (high carbon, medium carbon, low carbon), magnesia silicon carbide carbon bricks, magnesia bricks, aluminum silicon carbide carbon bricks, carbon bricks, Aoer bricks (corundum composite bricks), magnesia chrome bricks, silicon carbide bricks, etc. Perform conventional physical performance analysis. The bulk density, porosity, normal temperature compressive strength, high temperature flexural strength and other properties of magnesia carbon brick series decrease with the increase of carbon content, which is similar to magnesia carbon brick series, magnesia brick, carbon brick , Aoer bricks, magnesia-chrome bricks, and silicon carbide have high porosity, indicating poor compactness. In addition, these types of bricks are all fired bricks, indicating that the high temperature strength properties of this type of bricks are relatively poor. Combined with the smelting conditions of the electric melting furnace, it can be preliminarily determined that the series of magnesia carbon bricks are superior to other types of refractory bricks.
- Analysis of dynamic slag erosion resistance
After 3 groups of dynamic anti-slag erosion tests, the first group is magnesia-carbon series bricks (high, medium and low magnesia carbon bricks), carbon bricks, magnesia bricks, aluminum carbon bricks, Orr bricks (corundum composite bricks), magnesia chrome bricks , Silicon carbide bricks; the second group is magnesia carbon bricks (high, medium and low magnesium carbon bricks), magnesia silicon carbide carbon bricks, aluminum carbon bricks, aluminum silicon carbide carbon bricks and aluminum magnesia carbon bricks; the third group is high The three groups of magnesia carbon brick, aluminum silicon carbide carbon brick series and magnesium silicon carbide carbon brick series are compared with each other to find the refractory material with the best resistance to corrosion by the slag of the melting sub-electric furnace.
(1) Group 1: Magnesia-carbon brick series are superior to aluminum-carbon bricks. Carbon bricks, magnesia bricks, silicon carbide bricks, magnesia chrome bricks, and corundum bricks have all been eroded away by the slag, indicating that carbon bricks, Magnesium bricks, silicon carbide bricks, magnesia chrome bricks, and corundum bricks have poor resistance to melting and slag erosion and cannot meet on-site requirements. The corrosion resistance of aluminum silicon carbide carbon brick is obviously worse than that of magnesia carbon brick series. Moreover, the magnesia-carbon brick series showed that with the increase of carbon content, the slag corrosion resistance was improved, indicating that the increase of carbon content was in line with the corrosion resistance. Comparing the degree of erosion of refractory bricks by molten slag and molten iron, slag is the first factor in the erosion of refractory materials.
Through the comparison of the first group of experiments, it is concluded that the dynamic resistance of magnesia-carbon brick series refractories is significantly better than that of aluminum-carbon bricks, carbon bricks, magnesia bricks, silicon carbide bricks, magnesia-chrome bricks and corundum bricks.
(2) The second group: high carbon magnesia carbon bricks, aluminum silicon carbide carbon bricks and magnesia silicon carbide carbon bricks have the least depth of erosion and the best corrosion resistance. Low-carbon magnesia-carbon bricks and medium-carbon magnesia-carbon bricks have relatively heavy slag line erosion, and aluminum-magnesia-carbon bricks and aluminum-carbon bricks have the deepest erosion depth and the worst corrosion resistance.
Through the comparison of the second group of experiments, it is concluded that high-carbon magnesia carbon bricks, magnesia silicon carbide carbon bricks and aluminum silicon carbide carbon bricks are better than other refractory bricks in dynamic resistance to slag erosion.
(3) Group 3: The corrosion resistance of magnesia silicon carbide carbon bricks is better than that of aluminum silicon carbide carbon bricks and high carbon magnesia carbon bricks, and with the increase of silicon carbide content, the corrosion resistance has a trend of further improvement It shows that adding silicon carbide to magnesia carbon bricks can improve the slag erosion resistance of the bricks.
The table shows high carbon magnesia carbon bricks (GMT), medium carbon magnesia carbon bricks (MT), low carbon magnesia carbon bricks (DMT), magnesia silicon carbide carbon bricks (MST), aluminum silicon carbide carbon bricks (LST), The thickness of the decarburization reaction layer of aluminum-carbon bricks (LT) and aluminum-magnesium-carbon bricks (LMT), among which GMT, MST and LST samples have the smallest erosion depth, and LT samples have the largest erosion depth. (Picture 3)
To sum up:
Through conventional physical performance analysis and dynamic slag erosion test analysis, the following conclusions are drawn:
(1) Magnesia-carbon brick series refractories have excellent high temperature resistance, compact matrix, low porosity, good dynamic resistance to slag erosion, and with the increase of carbon content, the performance is better.
(2) Adding silicon carbide to magnesia-carbon bricks can further improve the brick’s resistance to slag erosion and improve the brick’s oxidation resistance at high temperatures.
(3) High carbon magnesia carbon bricks and magnesia silicon carbide carbon bricks are more suitable for the smelting requirements of the melting furnace.
(4) Put forward the suggestion to use carbon magnesia carbon bricks instead of medium carbon magnesia carbon bricks to carry out application tests on the melting furnace.