(2) In the coagulation and sintering mixture of moisture , the temperature of the exhaust gas after passing through the drying zone is lowered from 1100 to 1500 °C to 200 to 300 ° C. At this time, P' H2O drops a lot, and exhaust gas containing many water vapor is formed in the lower mixed layer. Condensed water, the surface of the sintered material begins to get too wet. At this time, P H2O > P' H2O . This is the mechanism by which the wet zone is created in the lower layer. The performance of the sinter layer being too wet begins generally two seconds after ignition until the drying belt moves to the furnace. This can be seen from the exhaust gas temperature profile.
It can be seen from Fig. 3 that the exhaust gas temperature jumps from 15 to 20 ° C to 40 to 50 ° C after two minutes of sintering, and remains unchanged until the end of sintering. This jump is caused by the exothermic reaction of moisture condensation in the exhaust gas. Figure 4 records the temperature changes after sintering of four thermocouples placed in different positions in the sinter layer. They start to jump after about 1 minute and reach 52-53 ° C in 2 minutes. This test also proves that the layer is too wet. The situation of formation.
Figure 5 shows the distribution of moisture measured after dissection of the sintering cup. One minute after ignition, a 20 mm drying belt appeared. At this time, the exhaust gas discharged little water, and moisture condensation began at 80 mm from the material surface. The wet belt thickness was 60 mm. After 2 minutes, the water discharge increased, and the moisture was increased. 130mm, the highest moisture is 8.9%; after 3 minutes, the over-wet zone is 80mm, the wet moisture is still as high as 8.9%; after 5 minutes, the wet zone is reduced to 30mm, and the wet moisture is 8.6%. The whole process is wet. It is 1.3~1.4% higher than the original water; that is, it increases by 17%, and the maximum over-humidity value is .2~3 minutes. [next]
(3) Calculation of sintering dew point and measures to prevent over-wetting are analyzed from the theory of moisture condensation to prevent over-humidity in sintering. The temperature of the material should be kept above the dew point, ie P H2O ≤ P' H2O , thus fundamentally Prevent condensation of moisture on the surface. Therefore, the temperature of the exhaust gas does not fall below the dew point temperature after passing through the layer, so that the precipitation of moisture in the exhaust gas can be prevented. This temperature can be calculated as follows:
The mixture contains 7% water, the mass of the mixture is 8 kg, the sintering time is 10 minutes, and the exhaust gas volume is 450 liters/min.
This item is the vapor pressure P H2O of the water vapor in the exhaust gas.
If P' H2O = P H2O, it will not condense. If the temperature of the exhaust gas at the saturated vapor pressure of the water is found to be t=52.50 °C, the moisture condensation will not occur if the exhaust gas temperature is greater than 52.5 °C.
Ways to reduce excessive humidity:
(1) Reducing the water content in the exhaust gas actually reduces the partial pressure of the moisture in the exhaust gas. The addition of appropriate moisture to the sinter mix results in better layer breathability to ensure maximum sinter productivity. However, the original moisture permeability is not guaranteed to achieve the highest productivity, that is, the highest productivity water value is smaller than the original moisture permeability. This is because when the fine concentrate is sintered, a large amount of moisture is condensed to form a super-wet layer, and the under-layer material may even become a muddy state to deteriorate the sintering conditions. Appropriately reducing the original moisture of the sintered mixture, although it may make the sintering into a less spherical shape, the initial gas permeability is reduced, but the average gas permeability is increased due to the decrease of water, and the moisture condensation is less, the drying time is shortened, and the whole sintering speed is accelerated. . Therefore, the initial moisture of the sintered mixture is usually about 2% lower than the optimum moisture of the initial gas permeability.
(2) Increase the temperature of the mixture. Increasing the temperature of the mixture to above the dew point of the water in the exhaust gas prevents condensation of moisture in the exhaust gas. It is the most economical method to use hot returning to preheat the mixture, but this preheating method can only preheat the mixture to 40~50 °C, which is less than the dew point temperature. In addition, the amount of hot return mine feed is not easy to control and the working conditions are also poor. Despite this, the elimination of the over-wet phenomenon still plays a part.
The steam is preheated, that is, steam is added to the secondary mixer, and the material is heated by the steam condensation heat (when the material is heated to 60 ° C, the moisture of the material is increased by 1% due to steam condensation), according to tests conducted by Shougang in the 1962 machine at 0.4 machine. : When the material temperature is raised from 57.4 ° C (without steam preheating) to 87.7 ° C (preheated with steam), the vertical speed of sintering increases from 27.1 mm / min to 35.7 mm / min, increasing production by 31.9%, Shougang No. 1 in July 1981 The steam test was stopped, and only the hot returning mine was preheated in the mixture. The material temperature dropped from 70 °C to 52 °C, and the sintering utilization coefficient decreased from 1.55 t/m 2 · h to 1.456/m 2 · h, which decreased by 10%. The disadvantage of the steam preheating method is that the thermal efficiency is too low. Recently, the improved structure of Shougang has enabled the steam nozzle to be inserted into the material to improve its heat utilization rate, and the steam cost is rich due to the compensation of the yield increase.
The sintering waste preheating sintering mixture is in the experimental research stage. The Taigang laboratory uses the hot exhaust gas of 250-350 °C to preheat the mixture without increasing the suction area. The sintering increases the yield by 4% and the fuel consumption drops by 10kg/t.
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