Experimental study on recovery of iron from blast furnace gas ash

Modern steel production process generates a lot of dust and mud, have a greater harm to the production site and the surrounding environment, it must be rendered harmless. Among these dusts, the valuable element Fe and the harmful impurities S, P, K, etc. often coexist, so generally referred to as iron dust, which includes blast furnace gas ash (mud), converter red dust, electric (transfer) furnace dust ash, Cold (hot) rolling sludge, rolled iron oxide scale, sintered dust, iron collecting dust, oily iron filings, etc. As the country pays more and more attention to resource and environmental issues, research on the comprehensive utilization of pollution-free iron dust will produce good economic and social benefits.

As the main varieties of iron-containing dust and sludge, blast furnace dust (sludge) from dust coal blast furnace ironmaking process with the gas discharged together. It has obvious differences with the nature of natural ore. Fine-grained minerals melt together under the action of high temperature, which is easy to wrap gangue minerals. Its composition is more complicated, and the recovery rate of valuable elements is lower. At present, blast furnaces are treated at home and abroad. There are three methods for gas ash: 1 direct efflux storage, easy to cause environmental pollution, large steel companies have basically eliminated the method; 2 direct use, return to sintering or pellets, used by many domestic steel companies, but gas Gray harmful impurities such as K, Na, Zn, S, P, etc. are generally higher, equipped with sintering or pellets, reducing the utilization coefficient of the blast furnace, thereby affecting the economic and technical indicators of ironmaking; 3 comprehensive recovery, extracting valuable elements. At present, the extraction of valuable elements such as iron and carbon from gas ash is an important development direction.

In nature, there is a large class of weakly magnetic minerals, such as hematite, limonite, titanium ore, is difficult by conventional magnetic separation of these minerals, typically use high intensity magnetic separation, flotation, magnetic roasting - weakening Selecting process technology to extract iron concentrate. Gas ash contains considerable weak magnetic hematite and coke , so it can directly carry out magnetization roasting and recover iron concentrate. There are few reports on this. This experimental study analyzed the process mineralogy characteristics of Baosteel gas ash. Based on this, a variety of magnetic separation process was used to recover iron. The corresponding process parameters were explored and the process flow was compared.

First, the mineralogical characteristics of gas ash process

(1) Analysis of chemical composition and iron phase of gas ash

The gas ash raw material is taken from the Baotou Steel Ironmaking Plant. The chemical analysis results of gas ash multi-element are shown in Table 1. The XRD diffraction analysis results are shown in Figure 1.

Table 1 Results of chemical analysis of gas gray multi-element

Figure 1 XRD diffraction pattern of gas ash

â–²-Fe ₂ O ₃ ;●-Fe ₃ O ₄ ;â– -C

From Table 1, TFe 31. 00%, the carbonaceous _{33. 60%, SiO 2 .87%} , CaO 4.35%, the harmful elements S, Zn, Pb and other content is higher. As can be seen from Figure 1, the main phases are hematite, magnetite and C.

(2) Gas ash particle size analysis

The shape of the gas ash is gray-black powder, the particle size is not uniform, the large particles are honeycomb, block, sheet, etc., and the surface has voids. The results of particle size analysis of gas gray iron minerals are shown in Table 2.

Table 2 Gas ash particle size analysis results

It can be seen from Table 2 that most of the iron is distributed at -50 +200 mesh and -325 mesh, and the distribution rate accounts for 88.66% of the total. Wherein --50 + 200 Japan metallic iron distribution rate of 52.18%, further --325 mesh metallic iron distribution rate reached 34.68%, so that the two size fractions of iron is an important mineral to be recovered.

Second, the beneficiation test program

(1) Test equipment

The test equipment used φXPZ-175 disc crusher , Urumqi Jinxiangrui Mining Equipment Co., Ltd.; QM-SB planetary ball mill , Nanjing University Instrument Factory; φXCGS-50 magnetic separator, Tangshan Hongda mining machinery and equipment research High gradient magnetic separator; XTLZ multi-purpose vacuum filter, Sichuan Provincial Bureau of Geology and Minerals 102; KTF-1700 vacuum tube resistance furnace, Yixing Qianjin Furnace Equipment Co., Ltd.; DY-20 desktop electric tablet press Tianjin Science and Technology High-tech Company.

(2) Test procedure

The weak magnetic separation-strong magnetic separation and magnetization roasting-weak magnetic separation test flow are shown in Fig. 2 and Fig. 3.

Figure 2 Grinding - weak magnetic separation - strong magnetic separation test process

Figure 3 Magnetization roasting - weak magnetic separation test procedure

Third, test results and analysis

(1) Weak magnetic separation-high gradient strong magnetic separation test

1. Influence of magnetic induction on weak magnetic separation

The effect of magnetic induction tube magnetic induction on weak magnetic separation is shown in Fig. 4.

Figure 4 Effect of magnetic induction on weak magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 4 that as the magnetic induction intensity increases, the grade of the iron concentrate decreases slightly, and the recovery rate increases rapidly. When the magnetic induction intensity is 0.10T and 0.12T, the grade of iron concentrate is unchanged, which is 58.70%, and the recovery rate is increased from 50.47% to 56.12%; when the magnetic induction intensity reaches 0.14 T, the grade of iron concentrate It was reduced by 0.8 percentage points and the recovery rate reached 58.10%. The weak magnetic separation is mainly to recover large granular magnetic minerals, and the -325 mesh fine-grain magnetic minerals and weak magnetic iron minerals are not effectively recovered. Therefore, the recovery rate is not high enough, indicating that a considerable number of weak magnetic and fine-grained magnetic minerals enter the tailings, so high-gradient strong magnetic separation must be performed on the weakly magnetically selected tailings.

2. Influence of magnetic induction on strong magnetic separation

Test conditions: slurry flow rate of 4.2 cm / s, pulp concentration of 10%, magnetic medium filling rate of 8%. The test results of the influence of magnetic induction on strong magnetic separation are shown in Fig. 5.

Figure 5 Effect of magnetic induction on strong magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 5 that as the magnetic induction intensity increases, the recovery rate of iron concentrate increases and the grade decreases. When the magnetic induction intensity increased from 0.4T to 0.5T, the recovery rate increased by 5 percentage points, reaching more than 29%, while the grade was 44.47%, which was not much. Continue to increase the magnetic induction, the recovery rate is not obvious, but the grade drops sharply. Because, when the magnetic induction intensity is relatively strong, the magnetic adsorption force is also large, resulting in many weak magnetic continuous minerals and gangues entering the strong magnetic separation concentrate.

3. Influence of slurry concentration on strong magnetic separation

The test results of the influence of slurry concentration on strong magnetic separation concentrate are shown in Fig. 6. Test conditions: gas ash - 200 mesh accounted for 70%, pulp flow rate 4.2 cm / s, magnetic medium filling rate 8%, strong magnetic selection magnetic induction strength 0.5T.

Figure 6 Effect of slurry concentration on strong magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 6 that when the concentration of the slurry changes from 10% to 15%, the grade of the iron concentrate does not change much, but the recovery rate has been greatly improved, from 76.79% to 82.83%: when the slurry concentration reaches 20 %, although the concentrate recovery rate reached more than 90%, but the grade dropped to 47.53%. This is because the high viscosity feed slurry concentration was increased mineral fraction, serious mechanical inclusions, easily lead gangue minerals sandwiched magnetic product, it reduces the purification effect of the magnetic separator, reduction concentrate grade If the concentration of the pulp is too small, it will cause waste of water resources, and the processing capacity of production equipment will be relatively reduced.

4. Influence of slurry flow rate on strong magnetic separation

Test conditions: pulp particle size - 200 mesh accounted for 70%, pulp concentration 15%, magnetic media fill rate 8%, strong magnetic selective magnetic induction strength 0.5T. The test results of the influence of slurry flow rate on strong magnetic separation concentrate are shown in Fig. 7.

Figure 7 Effect of slurry flow rate on strong magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 7 that as the slurry flow rate increases, the grade gradually increases and the recovery rate decreases. When the volume flow rate is 4.2 cm/s, the grade rises to 52.87%, because the larger the volume flow rate, the shorter the retention time of the mineral mixture in the magnetic separator, and some weakly magnetic substances are washed out. The recovery rate is low and the grade is increased.

Through the above experiments, the best process conditions are that the weak magnetic separation magnetic induction intensity is 0.12T, the strong magnetic separation magnetic induction intensity is 0.5T, the slurry flow rate is 4.2 cm/s, the slurry concentration is 15%, and the grinding fineness is -200 mesh. The magnetic separation indicators are shown in Table 3.

Table 3 Weak magnetic separation - strong magnetic magnetic separation test results %

It can be seen from Table 3 that the recovery rate of iron reaches 79.48% and the grade is increased to 55.42%, which can be used as ingredients in blast furnace iron making. In addition, the content of carbon, zinc and magnesium in the tailings is relatively increased, which lays a foundation for the recovery of these substances. Due to the magnetic separation process of the high gradient magnetic separator, mechanical inclusions and magnetic agglomeration are likely to occur, so that some impurities also enter the concentrate, which affects the concentrate grade. Therefore, the concentrate obtained by the grinding, weak magnetic separation-strong magnetic separation process must be processed by other mineral processing methods such as re-election, flotation, etc. to obtain qualified iron concentrate.

(2) Magnetization roasting-weak magnetic separation test

1. Effect of calcination temperature on the reduction degree of magnetization roasting

Gas ash contains considerable hematite, and the effect of calcination temperature on the reduction of gas ash is studied. The test results are shown in Fig. 8 under the condition that the gas ash particle size - 200 mesh accounts for 40% and the reducing agent is gas ash itself with carbonaceous material.

Figure 8 Effect of calcination temperature on reduction degree

By definition, the degree of reduction = FeO content / TFe content × 100%, in the case of ideal calcination, when Fe ₂ O _{3 is} all reduced to Fe ₃ O ₄ , the degree of reduction of the theoretical calcined ore is 42.8%. It can be seen from Fig. 8 that when the temperature is between 700 and 850 ° C, the degree of reduction of the iron mineral increases as the magnetization calcination temperature increases. When the calcination temperature is 700-750 °C, the reduction degree of iron minerals of gas ash is not much increased, and the degree of reduction is 39.1% and 40.2%, respectively. 8%。 The degree of reduction is close to 42.8% at 800 ° C. When the temperature reached 850 ° C, over-reduction occurred, and the test 800 ° C was the optimum temperature for the magnetization calcination reaction.

2. Effect of calcination temperature on weak magnetic separation

Test conditions: roasting time 60 min, ore sample size - 200 mesh accounted for 70%, magnetic tube magnetic induction strength 0.12 T, gas ash particle size - 200 mesh accounted for 40%. Figure 9 shows the magnetic separation results of magnetized roasting ore obtained at different calcination temperatures.

Figure 9 Effect of calcination temperature on magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 9 that as the calcination temperature increases, the grade of iron concentrate gradually increases, and the recovery rate decreases. At 700,750 °C, the grade of iron concentrate is 58.20%, 58.80%, the change is not large, the recovery rate is reduced from 78.80% at 700 °C to 73.53% at 750 °C; At 800,850 °C, the grade of iron concentrate has increased to 60.80%, 61.90%, the recovery rate of iron concentrate is still above 70% at 800 °C, and the recovery rate at 850 °C is only 40.09%; This is mainly because under the conditions of high temperature and excessive reducing agent, over-reduction phenomenon occurs, and weak magnetic rich body or weak magnetic iron silicate is formed.

3. Effect of roasting time on weak magnetic separation

Test conditions: calcination temperature 800 ° C, mineral sample size - 200 mesh accounted for 70%, magnetic induction strength 0.12 T, gas ash particle size - 200 mesh accounted for 40%. Figure 10 shows the magnetic separation results of magnetized roasting ore obtained at different firing times.

Figure 10 Effect of calcination time on magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 9 that with the increase of the magnetization roasting time, the grade of the obtained iron concentrate does not change much, and it remains above 60.70%, and the iron recovery rate increases significantly when roasting for 30 min to 60 min. From 64. 22010 when calcined at 30 min, it quickly increased to 70.61% at 60 min. When the calcination time was increased to 90 min, the recovery rate of concentrate was 71.99%, which was only increased by 1.31 percentage points. This indicates that the weak magnetic iron mineral in the gas ash has not been fully reduced to a ferromagnetic mineral at 30 min after calcination. After the calcination time is increased to 60 min, the weak magnetic minerals are basically reduced to ferromagnetic iron minerals.

4. Influence of grinding fineness on weak magnetic separation

The test conditions were a calcination temperature of 800 ° C, a calcination time of 60 min, and a magnetic induction of 0.12 T. The effect of grinding fineness on the weak magnetic separation effect is shown in Figure 11.

Figure 11 Effect of grinding fineness on magnetic separation

●-grade; ▲-recovery rate

It can be seen from Fig. 11 that as the fineness of the grinding becomes fine, the grade of the iron concentrate is slightly increased, and the recovery rate is rapidly decreased. - 200 mesh accounted for 50%, 70%, 90% of the roasting ore, the magnetically-selected iron concentrate grades were 59.90%, 60.80%, 61.10%, and the recovery rates were 75.72%, 70.61%, 62. twenty three%. Because, as the ore sample is finer, the magnetic mineral particle size is reduced and the magnetic force is reduced. In addition, after the ore sample is ground, the slurry is easily mixed due to agglomeration, which affects the iron recovery rate. A good grinding fineness of -200 mesh accounts for 700/0.

Through the above tests, the optimal process conditions for gas gray roasting-weak magnetic separation were determined: calcination temperature 800 ° C, roasting time 60 mm, ore grinding fineness - 200 mesh accounted for 70%, reducing agent gas ash particle size - 200 mesh 40%, weak magnetic selection magnetic induction strength 0.12 T. Under this condition, an iron concentrate with a grade greater than 60.70% and a recovery rate greater than 70% can be obtained, wherein the sulfur and phosphorus contents are only 0.17% and 0.021%, respectively, which basically meets the requirements of the blast furnace ironmaking level.

Fourth, the conclusion

(1) Through the process mineralogy of chemical composition, main mineral composition, and iron minerals embedded in the ash of Baotou Steel, it is determined that the iron minerals in the gas ash are mainly hematite and magnetite, most of the iron ore. The total iron distribution rate is 86.86% in the -50 +200 mesh and -325 mesh, and the metal distribution rate of iron in the 325 mesh reaches 34.88%. Because the steel ash of Baotou Steel is affected by the Baiyun Obo ore, it is more difficult to recover valuable elements.

(2) Weak magnetic separation-strong magnetic separation process test shows that magnetic induction intensity, slurry concentration, slurry flow rate, etc. have an effect on the test, in the weak magnetic separation 0.12 T, strong magnetic separation 0.5 T, grinding fineness - 200 mesh 70%, the pulp concentration is 15%, the slurry flow rate is 4.2 cm/s, and the magnetic medium filling rate is 8%. The mixed iron concentrate with the grade of 55.42% and the recovery rate of 79.48% is obtained.

(3) The magnetization roasting-weak magnetic separation process test shows that the calcination temperature, roasting time, magnetic induction intensity, grinding fineness, etc. all have an effect on the test, at the calcination temperature of 800 ° C, the roasting time of 60 min, the grinding fineness - 200 70%, the reducing agent gas ash particle size - 200 mesh accounted for 40%, and the weak magnetic magnetic induction intensity of 0.12 T, obtained an iron concentrate with a grade of 60.70% and a recovery rate of 70% or more.

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