Steel industry is a pillar industry of the national economy. However, the steel production process produces a large amount of dust, and its types include blast furnace gas ash (mud), converter red dust, electric (transfer) furnace dust ash, cold (hot) rolling sludge, rolled iron oxide scale, sintered dust, Dust collection, oily iron filings, etc. These dusts generally contain 30% to 60% of iron, accounting for 800 to 12% of steel production. According to statistics, the average domestic metallurgical enterprises occurred about the amount of 100kg / t of steel. According to China's current steel production calculation, the iron and steel dust discharged by the steel industry is as high as 40 million tons per year. At present, there are three methods for dealing with iron-containing dust at home and abroad: 1 direct discharge storage, easy to cause environmental pollution, large iron and steel enterprises have been basically eliminated; 2 direct use, containing iron dust back to sintering or pellets, such as Angang , Bengang. But particle size is too fine dust and sludge, impurities and harmful high, with the sintered ore, the sintering machine may cause clogging of the grate bars, blast S, P and alkali metals enrichment cycle, coal in blast furnace gas recovery, lining life and yield; â‘¢ Comprehensive Recovery , extract valuable elements.
Magnetized roasting is an effective means of recovering weak magnetic minerals such as hematite. Zhou Jianjun used magnetic roasting-weak magnetic separation to treat the stellite red limonite magnet in a certain area of ​​Yunnan. The iron grade in the mine increased to 60.18%, and the recovery rate reached 85.91%. Li Chao et al. used magnetized roasting to treat iron ore tailings. An iron concentrate with an iron grade of 61.3% and a recovery rate of 88.2% was obtained. However, research reports on magnetized roasting of iron dust have not been reported. In this paper, the mixed magnetization roasting-weakening process of Baosteel blast furnace gas ash and converter red dust is studied, and an effective way to recover iron from it is explored.
I. Experimental results and analysis
(1) Single weak magnetic separation test
Under different excitation currents, ie different magnetic field strengths, weak magnetic separation of gas ash and converter red dust was carried out. The experimental results are shown in Figure 1. Nearly 60% of the iron concentrate can be obtained in both mines, but the corresponding iron recovery rate is low because the weak magnetic hematite is the majority of the two types of dust. In comparison, the recovery rate of gas grey iron concentrate can reach 50% to 60%, which is higher than 15% of red dust in converter, indicating that the former has higher magnetite content than the latter. However, these recovery rate indicators are not attractive from an industrial point of view, so a single weak magnetic selection is not desirable.
(2) Mixing material magnetization roasting-weak magnetic separation test
In general, the main factors affecting the magnetization roasting-weak magnetic separation process are calcination temperature and time, grinding fineness, and magnetic separation current intensity. Experiment and discuss one by one below.
The effect of calcination temperature on the grade and recovery of iron concentrate. Figure 2 shows the grade and recovery of iron concentrate obtained by roasting the mixture at 650-800 °C. Other test conditions are: grinding particle size - 200 mesh accounted for 90%, calcination time 60 min, magnetic separation excitation current 1.OA. In Figure 2, the grade and recovery curve of the iron concentrate first rises and reaches a peak at 750 °C (TFe is 60.6%, the recovery is 1%) and then begins to decline. This is mainly because the temperature of the reaction system is lower than the carbon gasification temperature (700-800 ° C, depending on the carbon type), which is a solid-solid reaction, mainly by direct reduction, and the reaction rate is slow; when the temperature of the system exceeds carbon At the initial temperature of the gasification reaction, a large amount of CO starts to be produced in the reaction system. At this time, the reduction reaction is mainly indirect reduction, and the reduction reaction rate is greatly increased. The magnetic separation concentrate grade and recovery rate reach a certain peak at 700-800 °C. When the reduction temperature continues to rise, over-reduction occurs, so that the mineral is reduced to weak magnetic ferrite or iron-containing silicate in the reduction zone, and the quality of the ferromagnetic ore is reduced, and the concentrate grade and recovery rate are lowered.
Considering the concentrate grade and recovery rate, the magnetization roasting temperature is preferably 750 °C.
The effect of calcination time on the grade and recovery of iron concentrate. The effect of roasting holding time on the magnetic separation index of iron concentrate was investigated by fixed calcination at 750 °C. Other test conditions were the same as above, and the test results are shown in Fig. 3. As the calcination time increases, the iron grade and recovery rate also increase. After 60 min of calcination, the TFe reaches 60.4%, and the recovery rate is 88.6%. Then the iron grade and recovery curve begin to decrease. Although an endothermic reaction mainly occurs during the entire calcination process, when the temperature of the system exceeds the initial temperature of the carbon gasification reaction, the reaction rate in the reaction system is greatly increased. If the calcination time is too long at a temperature of 750 ° C, the phenomenon of excessive calcination is liable to occur, which directly leads to the formation of weak magnetic ferrite or iron-containing silicate during the calcination process, thereby seriously affecting the calcination effect. Taking into account the cost and selection criteria, we chose 60min for the ideal roasting time.
The effect of grinding fineness on the grade and recovery of iron concentrate. After the mixture is magnetized and calcined, most of the hematite is converted into magnetite, and the iron concentrate can be obtained by weak magnetic separation. There are two important influencing factors in the magnetic separation of roasting ore: grinding grain size and excitation current. The specific magnetic susceptibility of magnetite decreases with the decrease of mineral particles, that is, the magnetic properties are weakened. The more fine-grained content, the more the loss of magnetic iron in the sorting process, resulting in a decrease in beneficiation .
First, the effect of grinding size on the magnetic separation index of roasting ore was investigated. Figure 4 shows that the magnetic separation concentrate grade increases as the mineral size decreases. At -300 mesh, the magnetic separation iron concentrate grade reached 61.5%, but the recovery rate was only 57.5%; the recovery curve showed the opposite trend and grade. This is because the magnetic force received by the magnetite during the magnetic separation process is proportional to its volume, and the particle size of a magnetite particle is reduced by a factor of two, and accordingly, the magnetic force is reduced by eight times. The decrease in magnetic force inevitably leads to a decrease in the selected magnetite. The more the fine fraction is, the more the loss of magnetite in the sorting process is, which leads to a decrease in beneficiation efficiency and a serious iron loss. According to the test results, it is determined that the appropriate grinding fineness is -200 mesh and accounts for 90%.
The effect of magnetic separation current on the grade and recovery of iron concentrate. The larger the excitation current of the magnetic separation tube, the larger the magnetic separation magnetic field, and the larger the magnetic force acting on the magnetite particles, and vice versa. However, if the magnetic field is too large, it will increase the power consumption cost of the beneficiation. Figure 5 shows the magnetic separation index of iron concentrate obtained under different magnetic separation currents. It can be seen that as the excitation current increases, the recovery rate of concentrate iron increases, and the grade decreases. The reason is that when the excitation current or magnetic field is large, some weak magnetic minerals such as iron-containing silicates, or magnetites that are not separated by monomers and their encapsulating minerals, are magnetically absorbed into the concentrate. This naturally increases the recovery rate but reduces the concentrate grade. Figure 5 shows that the preferred magnetic separation current is 1.0A.
In summary, the optimal conditions for the magnetization roasting-magnetic separation process are: calcination temperature 750 ° C; calcination time 60 min; grinding particle size - 200 mesh accounted for 90%; magnetic separation current 1.0 A. Through this process, an iron concentrate having a grade of 60.4% and a recovery rate of 85% was obtained. The impurity content of iron concentrate, especially Si0 2 , S, P and F, is lower than that of Baosteel's current self-produced iron concentrate, which basically reaches the blast furnace furnace conditions.
Second, the conclusion
(I) XRD analysis shows that the gas ash of converter blast furnace blast furnace and the red dust ore phase of converter are mainly in the form of hematite. It is difficult to recover iron effectively by single weak magnetic separation. Mixed magnetization roasting-weak magnetic separation is a reasonable alternative process.
(2) The roasting temperature is between 650 and 750 °C, and the iron concentrate grade and recovery rate increase with increasing temperature. Because the reaction rate is increased, the magnetic susceptibility is increasing and the baking effect is getting better and better. At 750 ° C, the concentrate grade and recovery rate reached a peak; after the calcination temperature was 750 ° C, the reduction reaction began to occur, resulting in a certain weak magnetic floating body or iron-containing silicate, so the concentrate grade, recovery rate It decreases with increasing temperature.
(3) Through the study, the best experimental conditions for the hybrid magnetization roasting-weak magnetic separation process are as follows: calcination temperature 750 ° C, calcination time 60 min, grinding particle size -200 mesh, magnetic excitation excitation current 1.0 A. Using this process, a magnetite concentrate having a grade of 60.4% and a recovery of 88.6% was obtained.
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