Chinese large reserves of niobium, has proven Jiangxi, Guangdong, Guangxi, Inner Mongolia and other provinces have more than a dozen niobium mine, but low-grade ore of niobium. Nb in steel can play grain refinement strengthening the role of solid-dissolved to make ferrite, NbC, NbN precipitation strengthening effect of Nb steel to increase the strength, niobium steels having improved grain coarsening temperature, reduced The superheat sensitivity and temper brittleness of steel improve the toughness of steel and improve the intergranular corrosion of austenitic stainless steel. The application of Handan Iron and Steel is more and more extensive, which is of great significance for the development of China's resources and the expansion of the application.
I. Thermodynamic Analysis Figure 1 is a graph showing the relationship between the free energy of formation and the temperature of various oxides of Nb. It can be seen from Figure 1 that NbO is the most difficult to be reduced in all oxides of Nb; the oxygen potential of CO The temperature corresponding to the intersection of the line and the oxygen potential line of NbO is 1489 ° C, indicating that the minimum theoretical temperature for direct reduction of Nb by C is 1489 ° C under standard conditions; but under the same conditions, the minimum theory of direct reduction of Fe by C The temperature (705 ° C) is much lower than 1489 ° C. Therefore, the iron in the tailings can be first reduced between 705 and 1489 ° C, and then the iron and the cerium oxide are separated to further enrich the cerium. Further, at 1489 ° C or higher, the cerium oxide is directly reduced by C, thereby obtaining low-grade ferroniobium, thereby achieving the purpose of extracting hydrazine.
Second, the melting point test direct reduction of iron ore fines is a solid state reduction. Therefore, the tank type direct reduction test of different charging systems must be carried out on the basis of determining the melting property of the ore powder. Therefore, the melting property test of the ore powder is carried out in advance in order to determine the reduction temperature of the tank type reduction. A schematic diagram of the mineral powder melting performance test, as shown in Figure 2.
3. Tank type reduction test (I) Test method 1. The chemical composition of the raw materials used in the raw material condition test is shown in Table 1 and Table 2. Table 1 Chemical composition (mass fraction) of mineral powder
| TFe | CaO | SiO 2 | Nb 2 O 5 | other | |
| Tailings | 16.26 | 31.99 | 15.47 | 0.14 | 29.96 |
| Iron concentrate | 65.89 | <0.1 | 5.6 | - | 3.37 |
TABLE 2 Composition and properties of coal powder
| Sample | Ash /% | Volatile matter /% | Total sulfur /% | Bonding index /% | Colloid layer/mm | Fixed carbon /% |
| A coal | 8.67 | 29.63 | 0.23 | 78 | 11 | 59.54 |
| B coal | 10.22 | 10.22 | 0.30 | - | - | 79.56 |
2. Test conditions The test conditions for tank type reduction are shown in Table 3. Table 3 Tank type reduction test conditions
| time | Columnar tank method (850, 950 ° C) | Pie pot method (950 ° C) | With carbon block type method (850, 950 ° C) |
| 2, 4, 6, 8h | 2, 4, 6, 8h | 2, 4, 6, 8h | |
| Surface area / volume / mm -1 | 0.145 | 0.147 | 0.285 |
Test procedure: 1) Place the pressed sample on the pad of the stage; 2) Send the sample into the constant temperature zone of the furnace, and the heating furnace starts to heat up; 3) When the temperature of the furnace rises to 600 °C, start An image of the sample is taken. During the test, the computer automatically recognizes the shrinkage rate of the sample at different temperatures, and automatically collects and stores the corresponding image; 4) The sample shrinks to more than 80%, and the test is completed. The temperature was stopped and the sample was naturally cooled in the furnace. During the test, the camera (CCD) can monitor the melting process of the sample at all times, and collect images of the sample at different temperatures and different shrinkage rates by computer. According to the definition of the characteristic melting temperature and melting time, the required images are collected accordingly. Figure 3 is the result of the melting performance test. It can be seen from Figure 3 that the sample begins to shrink to 5% at a temperature of 1020 ° C, 20% at 1200 ° C, 50% at 1230 ° C, and shrink at 1320 ° C. The rate is 70%. Therefore, the reduction temperature of the tank test should not exceed 1000 ° C. In this study, 850 ° C and 950 ° C were selected. 3. Filling method The tank type reduction test is carried out in a heat-resistant steel crucible with a height of 135 mm, an inner diameter of 75 mm and an outer diameter of 85 mm. The ore powder is packed in the form of a column, a cake and a carbon block (with A coal). Into the center of the heat-resistant steel crucible, fill the remaining space in the crucible with B coal, and then put the crucible into the muffle furnace which has been raised to the predetermined temperature for reduction. (The charging method of the tank type reduction is shown in Fig. 4 Show). In the columnar tank method, a paper cylinder with a height of 85 mm and an inner diameter of 30 mm is prepared, placed in the center of a heat-resistant steel crucible, filled with tailings powder in the paper cylinder, and then filled with other space in the crucible with B coal; In the method, a half-tank high pulverized coal is placed in the crucible, and a circular paper sheet (for the purpose of supporting the ore powder) is placed on the coal powder, and the diameter thereof is about 75 mm, and the crucible powder is placed on the paper sheet, The volume of the ore powder is the same as that of the columnar charge. After filling, the thickness is about 13 mm, and then the B space is filled in the remaining space in the tank. With carbon compact, the coal powder and iron-containing powder are bonded into a block by the coal bonding property. The pulverized coal and iron ore powder are mixed and mixed according to a certain ratio, heated at a constant temperature, and then hot pressed into a block to obtain a carbon compacted block. The heating temperature is such that the coal is made to have good thermoplastic properties. This process does not add a binder, and makes full use of the thermoplasticity of coal. When the coal is softened and melted, it is bonded to the iron-containing raw material to form a block. The carbon in the molten coal intrudes into the voids of the iron-containing raw material particles or covers the surface of the particles, and the iron-containing raw material is in full contact with the coal powder particles. The formation process of the colloid is shown in Fig. 5.
The specific production method of the carbon pressure block is as follows: l) After mixing the tailings powder and the coal powder, 7 g is weighed, put into a mold, placed in a muffle furnace at 500 ° C, and heated at a constant temperature for 12 min; 2) the mold is taken out. Put it on the pressurizer and pressurize it to 30MPa (gauge pressure), constant pressure for lmin, then demoulding to remove the carbon-filled briquettes, and the finished carbon-filled briquettes are 21mm×19mm×13mm ellipsoidal briquettes.
(II) Test results and analysis 1. Column reduction test The columnar tank method reduction test results of the tailings and iron concentrates are shown in Fig. 6. l) 850 ℃ niobium metal tailings final reduction rate of about 55%, 950 ℃ final metallization rate of about 73%. This is because the high temperature is favorable for the diffusion of gaseous reactants and gaseous products between the ore powder and the coal powder, that is, changing the temperature also correspondingly changes the thermodynamic and kinetic conditions of the reaction, thereby making the metallization rate high. 2) Under the conditions of 850 and 950 °C, the metallization rate reached the maximum after about 4 hours of reduction of the tailings and iron concentrate. The reduction time was prolonged and the metallization rate increased. 3) At 950 °C, after a certain period of reduction, the metallization rate of iron concentrate is about 90%, and the metallization rate of tailings is about 73%, which is due to the high-grade iron essence under the same reducing atmosphere. The contact condition of iron oxide and reducing gas in the ore (omegaTFe is 66%) is favorable for the metallization rate. 2. Piece reduction test The results of the cake-like reduction test of the tailings and iron concentrates are shown in Fig. 7. It can be seen from Fig. 7 that the metallization rate reaches a maximum after the reduction of the tailings and iron concentrates at 950 °C for 4 hours (the former is about 84%, the latter is about 90%), and the metallization rate is increased. Not obvious. Due to the low iron grade (ωTFe is 16%), the iron ore tailings have poor contact conditions with iron oxides and reducing agents, resulting in a lower metallization rate than iron concentrates (omegaTFe is 66%).3, with carbon compaction reduction test 10% and 20% coal blending with iron tailings with carbon compaction test results shown in Figure 8.
l) The reduction temperature has an important influence on the metallization rate of the carbon block. Regardless of the carbon content of 10% or 20%, the metallization rate at 950 °C is significantly higher than 850 °C (about 14% higher). This is because C direct reduction is an endothermic reaction. Increasing the temperature improves the thermodynamic conditions of the reaction, and at the same time speeds up the kinetic conditions such as the diffusion reaction rate of the gaseous reactants and the gaseous reaction product. Therefore, increasing the temperature is beneficial to increase the carbon loading pressure. The metallization rate of the block. 2) The metallization rate of the carbon compacted block increases with the prolongation of the reduction time, but the increase is slower after 2 hours. This is because as the reaction progresses, most of the iron oxide is reduced, the contact area of ​​the remaining iron oxide with the reducing agent is gradually reduced, and the reaction kinetic conditions are deteriorated, resulting in a decrease in the reaction rate. 3) The effect of coal blending on the metallization rate of carbon fittings. The metallization rate of 20% of the 850 and 950 °C coal blending is 2% to 3% higher than 10%. It can be seen that compared with the temperature, the effect of coal blending on the metallization rate of the compact is not obvious. This is because the raw material used in this test is iron-containing tailings with low iron content (about 16% of ωTFe). The increase of coal blending has little effect on the contact conditions of iron oxides and carbon, thus metallizing the carbon fittings. The rate is not significantly affected by the amount of coal blended. 4. Comparative analysis of different charging systems Based on the above research, this paper investigates the effects of different charging systems on the metallization rate of tailings under the same reduction conditions (Figure 9).
1) For any charging system, the metallization rate of the ore powder increases with the increase of the reduction time, but after 2 hours, the increase rate is obviously slowed down. Under the test conditions, the reduction time was extended from 2h to 8h, and the metallization rate was only increased by 7% to 8%. 2) For the columnar and cake-like charging methods, as the surface area to volume ratio increases, the metallization rate of the ore powder also increases, increasing by about 10%. 3) Under the same reduction conditions, the metallization rate of the carbon compacted block (10% or 20% coal blending) and the cake-like charging method are not much different (less than 4%), but they are obviously superior to the column charging system. 4) For the column, cake and carbon briquetting charging methods, the enrichment degree of Nb2O5 changes with the change of charging mode and reduction temperature. The specific changes are shown in Table 4. (The enrichment rate is the ratio of the Nb2O5 content after separation of metallic iron after reduction to the Nb2O5 content before reduction.) As shown in Table 4, under the same reduction conditions, Nb2O5 increases with the metallization rate of the ore powder. The degree of enrichment also increases. However, for carbon-filled briquettes, as the amount of carbon is increased, the ash that is brought in is also increased, resulting in the enrichment rate of Nb2O5 decreasing with the increase of carbon content at the same reduction temperature. small. In the experimental case of this study, the enrichment rate of the cake-like method Nb2O5 was the highest. For the cake-like charge, the metallization rate is about 10% higher than that of the columnar charge under the same reduction conditions. The reason may be that the volume of the ore powders of the two charging methods is equal, but the surface area is not equal; the reaction area of ​​the cake-like charge At oh, it is larger than the columnar charge; as the reaction progresses, the reaction area of ​​the cake charge is constant, and the reaction area of ​​the column charge is gradually reduced, that is, the reaction area of ​​the cake charge and the column charge The difference gradually increases (the surface area to volume ratio gradually increases), and the above causes the metallization rate of the cake-like charge to be higher than that of the columnar charge.
Table 4 Enrichment of Nb2O5 after reduction
| parameter | Columnar tank method | Pie pot method | With carbon block | ||||
| 850 ° C | 950 ° C | 950 ° C | With carbon 10% 850 ° C | With carbon 20% 850 ° C | With carbon 10% 950 ° C | With carbon 20% 950 ° C | |
| Metallization rate /% | 55 | 73 | 84 | 69.42 | 73.90 | 83.77 | 88.37 |
| Enrichment rate | 1.146 | 1.204 | 1.242 | 1.206 | 1.176 | 1.227 | 1.225 |
For the carbon-filled briquettes, the reason for the above results is that the surface area to volume ratio of the carbon-carrying briquettes is greater than that of the cake-like and column-shaped charges; the main reason is that the carbon-containing briquettes are carbon-bound and have a good microstructure, coal The ore particles are in close contact, and the carbonization and carbon oxide reduction reactions simultaneously and promote each other, providing good thermodynamic and kinetic conditions for the reduction reaction. However, when carbon is contained in the carbon compact, a certain amount of ash is introduced to reduce the Nb content in the slag phase, which weakens its advantage to some extent. And with the carbon compact in the briquetting stage, the material needs to be heated and pressed into a block, which requires a lot of energy. Therefore, under the conditions of this experiment, considering the factors affecting the reduction characteristics of the tailings, this paper considers that the best charging method should be the cake-like method, the reduction temperature is 950 ° C, and the time is 4 h.
IV. Conclusions (1) Under the same conditions of other reduction conditions, the reduction temperature will be increased and the metallization rate of the ore powder will increase. (2) Regardless of the tailings or iron concentrates, after a certain period of reduction, the reduction of the reduction time is continued, and the metallization rate of the ore powder is not significantly increased. (3) For the three charging methods of columnar, cake and carbon packing, as the surface area to volume ratio increases, the metallization rate of the ore powder also increases. (4) For the carbon-filled briquettes, under the same reduction conditions, the metallization rate increases with the increase of the coal blending amount. Under the test conditions, the best charging method is the cake-like method, the reduction temperature is 950 ° C, and the time is 4 h.Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece. This may be done varying direction on one or several axes, cutter head speed, and pressure. Milling covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes for machining custom parts to precise tolerances.
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