Sinter and coke samples with different zinc contents were prepared by soaking in zinc acetate solution and adding zinc. The reduction pulverization rate and reduction index of sintered samples at low temperature were tested, and the CO2 reactivity and post-reaction strength of coke samples were tested. The results show that with the increase of zinc content, RDI+3. 15 and RDI+6.3 of sinter decrease, RDI-0.5 increases obviously, indirect reduction rate and RI decrease, CRI of coke increases and CSR decreases. The increase of zinc content in sinter makes its low-temperature reduction pulverization and reducibility worse, and the increase of zinc content in coke makes its thermal performance worse. Compared with the method of spraying ZnSO4 _ 4 aqueous solution and adding zinc, the method of dipping zinc in zinc acetate aqueous solution can more accurately determine the influence degree of ZnO on the thermal properties of coke.
Keywords: papers on steel materials
Zinc in blast furnace mainly comes from ironmaking raw materials, including iron ore, coke and recycled products [1-3]. At the same time, zinc will be continuously enriched in the blast furnace, so that the zinc content in the blast furnace burden far exceeds that when it is added from the top of the furnace [4-5]. Therefore, researchers have done a lot of research on the distribution of zinc in blast furnace, the suitable operating system of blast furnace under high zinc load, and the influence mechanism of zinc on blast furnace refractories and smelting process [6-9]. In the existing research, there are many ways to add zinc to iron ore and coke. Yin Huichao et al. [10] introduced zinc to the surface of iron ore by fumigation, and studied the effect of zinc on the reduction pulverization of iron ore at low temperature. Kang Zepeng et al. [1 1] studied the influence of zinc on low-temperature reduction pulverization of iron ore, coke reactivity and strength after reaction by spraying ZnSO4 solution on the surface of the sample, but on the one hand, ZnSO4 began to decompose at about 650℃ and would not decompose to produce ZnO at the test temperature of low-temperature reduction pulverization rate of iron ore (500℃), so spraying ZnSO4 was not suitable for zinc pair. On the other hand, ZnSO4 _ 4 can be decomposed violently at 720℃, so when the thermal properties of coke are tested at 1 100℃, SO3 produced by its decomposition can catalyze the coke reaction [12], which obviously hinders the correct judgment of the internal relationship between the thermal properties of coke and zinc content. In addition, the effect of zinc on reducibility of iron ore has not been reported in the literature. Therefore, in order to better simulate the phenomenon that the blast furnace charge absorbs ZnO powder in the blocky zone, this paper adds ZnO to the sample by soaking zinc acetate solution, and studies the influence of ZnO content on the metallurgical properties of blast furnace charge, including reducibility of iron ore.
1 test
1. 1 sample preparation
Sinter and coke used in the test were taken from the production site of No.5 blast furnace of Wuhan Iron and Steel (Group) Company. See table 1 for the chemical composition of the sinter. The industrial analysis results of coke are shown in Table 2. 2H2O) is analytically pure. Zinc acetate dihydrate is soluble in water, and crystal water can be removed below 200℃. Anhydrous zinc acetate melts at 242℃ and completely decomposes into ZnO at 370℃. According to these characteristics of zinc acetate, this paper designs a method of soaking sinter with zinc acetate aqueous solution and adding zinc to coke: firstly, a certain mass percentage concentration of zinc acetate aqueous solution is prepared as required, the sample is soaked and boiled for a period of time, and then the quality of zinc acetate dihydrate added to the sample is obtained by taking out, filtering, drying and weighing. In the subsequent metallurgical performance test of iron smelting furnace materials, the added zinc acetate dihydrate will remove crystal water and decompose into solid ZnO. The mass percentage of ZnO in the non-soaked sample is the ZnO increment of the sample. By adjusting the concentration and boiling time of zinc acetate aqueous solution, the quality of zinc addition can be accurately controlled. 500 grams of sinter with particle size of 10 ~ 12.5 mm and 200 grams of coke with particle size of 2 1 ~ 25 mm are soaked and added with zinc. The addition scheme of zinc is shown in Table 3.
1.2 test method
The determination of low-temperature reduction pulverization performance of iron ore is carried out according to the method specified in GB13242-92-92. The upper conditions of the simulated blast furnace are: temperature 500℃, reaction time 60min, gas composition: N2, CO, CO2 volume fraction 60%, 20%, 20% respectively, gas flow rate 65438±05L/min, total drum revolution 300r, rotation speed 30r/min. The reducibility of sinter is tested according to the detection method specified in GB13241-91. The experimental conditions were as follows: temperature 900℃, reaction time 65438±080min, gas composition: N2, CO volume fraction 70% and 30% respectively, and gas flow rate 65438 05L/. The reactivity and post-reaction strength of coke were determined according to GB/T 4000-2008. The experimental conditions were as follows: temperature 1 100℃, reaction time 120min, pure CO2 gas, gas flow rate of 5 l/min, total rotating speed of drum of 600r and rotating speed of 20 r/min.
2 Results and analysis
2. 1 Effect of zinc addition on low-temperature reduction and pulverization of sinter
The low-temperature reduction pulverization index rdi+3. 15, reduction strength index rdi+6.3 and wear index rdi-0.5 of the sintered ore samples before and after adding zinc are shown in figure 1. As can be seen from the figure 1, with the increase of ZnO content in the sinter, both RDI+3. 15 and RDI+6.3 show a downward trend, while the wear index RDI-0.5 shows an upward trend, indicating that with the increase of ZnO content, the low-temperature reduction pulverization performance of the sinter becomes worse. The initial temperature of the reaction between ZnO and Fe2O3 to produce zinc ferrite is 500℃, and the reaction speed increases with the increase of temperature [13]. The temperature of the low-temperature reduction pulverization rate test is just 500℃, which suggests that part of zinc oxide can react with hematite in sintered ore to generate zinc ferrite. Because of the low temperature, the generated zinc ferrite is not easy to be reduced and decomposed by CO and remains stable. Zinc ferrite belongs to spinel type mineral, which is equiaxed with a density of 5.20 g/cm3, while hematite belongs to hexagonal system with a density of 4.9 ~ 5.3 g/cm3. The crystal form and density of the two minerals are obviously different, which means that the newly generated zinc ferrite will be stripped from massive hematite to form powder, which may reduce the strength of hematite. This may be the internal reason for the deterioration of sinter reduction pulverization performance at low temperature, especially the sharp increase of wear index rdi-0.5.
2.2 Effect of zinc addition on sinter reducibility
The reduction experiment of zinc-added sinter was carried out, and the curves of weight loss (including sinter weight loss and zinc oxide weight loss) with reduction time were obtained, as shown in Figure 2. By analyzing the weight loss curve in Figure 2, it can be seen that when the reduction time is less than 60min, the weight loss rate of sinter with different ZnO content is large, and the weight loss values are similar. The reason is that at the initial stage of reduction, ZnO has no obvious inhibitory effect on the reduction process of sinter, which is mainly due to the weight loss caused by the reduction of ZnO and iron oxide on the ore surface by CO; When the reaction time is 60 ~ 120 min, the reaction occurs inside the ore particles. For ores with high ZnO COntent, the open pores are more likely to be blocked by ZnO powder, which reduces the chance of co contacting with iron oxides, and the amount of zinc ferrite is also large, so with the increase of ZnO content, the weight loss rate of samples gradually decreases. When the reaction time is 120 ~ 180 min, the reduction rate of sinter with four kinds of ZnO content is close to zero, which indicates that the reduction reaction at this stage has basically ended. SEM and EDS analysis of the sintered samples after the reduction test show that there is almost no residual Zn element, so it can be assumed that there is no residual ZnO in the samples at the end of the test, and the reduction degree RI of each sintered sample can be calculated from the weight loss of 180 minutes, as shown in Table 4. It can be seen from Table 4 that with the increase of zinc content in sinter, the reducibility of sinter becomes worse, and the effect of ZnO increment on RI value is basically linear, with the increase of-7.13% (ri)/1%(w (ZnO)). The indirect reduction of sinter is blocked, which means that the coke ratio of blast furnace may be increased. There may be two reasons why ZnO hinders the reduction reaction of sinter: first, ZnO powder attached to the surface of sinter particles and the wall of pores hinders the contact between iron oxide and CO; Second, the reaction between ZnO and Fe2O3 will generate zinc ferrite, and the reduction and decomposition of zinc ferrite requires high kinetic conditions, which hinders the reduction of iron ore [13].
2.3 Effect of zinc addition on thermal properties of coke
The test results of reactivity (CRI) and post-reaction strength (CSR) of coke samples with different zinc additions are shown in Table 5. As can be seen from Table 5, with the increase of ZnO content, the CRI value of coke has an increasing trend, while the CSR value has a corresponding decreasing trend, indicating that ZnO has a negative impact on the thermal properties of coke. The factors affecting coke reactivity are mainly divided into two categories: first, the microstructure of coke, in which the graphitization degree of coke and the coal type of coking coal have the greatest influence; The second is the influence of external factors, mainly including the porosity, pore structure and internal minerals of coke. The greater the porosity of coke, the more uniform the pore distribution and the higher the reactivity of coke; Alkali metals in minerals have the greatest influence on the gasification reaction of coke, followed by alkaline earth metals and transition elements [14], while group Ⅱ b elements (zinc, cadmium and mercury) are often classified as transition elements because of their similar ability to form stable coordination compounds. In this study, ZnO was added to coke. Under the experimental conditions of coke reactivity, ZnO was easily reduced to zinc vapor by carbon, which increased the porosity of coke and promoted gasification reaction to a certain extent, thus improving CRI value. On the other hand, similar to alkaline earth metals, the transformation between zinc and ZnO accords with the conditions of electron migration theory and oxygen migration theory [15], so zinc also plays a catalytic role in gasification reaction. With the increase of porosity and catalytic gasification reaction, the addition of ZnO increases the CRI of coke, while CSR decreases due to the increase of porosity and gasification reaction. Literature [1 1] reported that when w(ZnO) in coke increased from 0.06% to 3.09%, CRI increased from 20.77% to 25.53%, which was nearly 5 percentage points higher. CSR decreased from 70% to 60%, a decrease of about 10 percentage point. In this study, when the increment of ZnO increased from 0 to 3.45%, CRI increased from 25.44% to 28.89%, an increase of 3.45 percentage points, and CSR decreased from 6 1.62% to 57.42%, a decrease of 4.2 percentage points. It is found that the influence range of ZnO in this paper is only about 70% of the literature [1 1], and the influence range of ZnO on CSR is only about 40% of the literature [1 1] when the increment of ZnO in coke is basically the same. This may be due to the different ways of adding zinc. Document [1 1] adopts the method of spraying aqueous solution of ZnSO4, and ZnSO4 decomposes at 1 100℃ to generate SO3, which also has obvious catalytic effect on coke gasification. Therefore, ZnO has a great influence on the thermal properties of coke.
3 Conclusion
With the increase of ZnO content in sinter, the low-temperature reduction pulverization index rdi+3. 15, reduction strength index rdi+6.3 and wear index rdi-0.5 of sinter decreased obviously. The increase of zinc content in sinter makes the low temperature reduction pulverization of sinter worse. The reason for the poor pulverization performance of low-temperature reduction may be that zinc ferrite and hematite produced by low-temperature reduction sintering with ZnO are quite different in crystal form and density.
(2) The increase of zinc content in sinter makes the reduction of sinter worse, and the decrease of reduction degree RI of sinter is basically linear with the increase of ZnO. On the one hand, the opening of sinter is blocked by ZnO, on the other hand, it may be because the generated zinc ferrite is difficult to be reduced and decomposed by CO, which hinders the reduction of Fe3+.
(3) With the increase of ZnO content in coke, CRI of sinter increases and CSR decreases. The increase of zinc content in coke makes the thermal properties of coke worse. On the one hand, the deterioration of thermal properties of coke is caused by the reaction between ZnO and C, and on the other hand, Zn plays a catalytic role in coke gasification.
(4) Compared with the method of spraying ZnSO4 _ 4 solution and adding zinc, the method of dipping zinc in zinc acetate solution can more accurately determine the influence degree of ZnO on the thermal properties of coke.
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