Experiment on the influence of MgO on the smelting performance and process parameters of sinter and blast furnace slag

Domestic and international research and practice shows, blast furnace slag contains a certain amount of MgO in the slag viscosity may be reduced, to improve slag fluidity, and desulfurization performance, thereby improving the metallurgical properties; different materials with different conditions of blast furnace slag which has a suitable content of MgO The range is not the higher the better or the lower the better. Regarding the influence of MgO on the quality of sinter, the conclusions drawn by domestic and foreign research institutes are inconsistent, especially the influence of MgO on the normal temperature strength of sinter. Some studies believe that the strength of MgO increases after rising, while others have the opposite, the same granulated MgO content. There should also be a suitable range.

In the past, Panzhihua Iron and Steel has carried out several industrial tests to improve the MgO content of sinter and blast furnace slag, and achieved certain results. However, with the change of Pangang's sintering and blast furnace production conditions, the sinter MgO decreased, and the slag MgO mass fraction decreased from 7.9% in 2000 to 7.2% in 2007. Under the current conditions, in order to explore the influence of MgO on sintering and blast furnace smelting and its suitable range, an industrial test to improve the MgO content of sinter and blast furnace slag was carried out.

I. Improving the industrial test of sinter MgO

(1) Test overview

The test period is June 11-16, 2008, and the base period is June 3, 6-10. In order to ensure the balance of the feeding system, the tests are carried out simultaneously on No. 3, No. 6 and No. 4 blast furnaces. The test raw fuel conditions are subject to the production site. The addition of dolomite instead of part of limestone improves the sintered ore MgO, and the binary alkalinity remains basically unchanged. The physical and chemical properties of dolomite and limestone are listed in Tables 1 and 2.

To ensure comparability with the reference test period and the ratio of iron ore is not bigger variety adjustment test, it is omitted. The ratio of dolomite is controlled at about 2.5%, and the coke powder is slightly improved.

Table 1 Physical and chemical properties of dolomite and limestone (1)

Table 2 Physical and chemical properties of dolomite and limestone (2)

(II) Test results and analysis

1. Analysis of influence of MgO on sintering process parameters

Table 3 Process parameters change before and after adding dolomite to No. 3 and No. 6 sintering machines, (1)

Table 4 Process parameters change before and after adding dolomite to No. 3 and No. 6 sintering machines, (2)

l) Mixture moisture and particle size. It can be seen from Tables 3 and 4 and Figure 1 that the moisture content of the mixture during the test period is improved to a certain extent, indicating that it is necessary to increase the moisture after adding dolomite to ensure the granulation effect. It can be seen from Fig. 1 that the particle size of the No. 3 machine mixture fluctuates greatly, the moisture fluctuation of No. 6 is large, and the grain size of No. 3 machine is better than that of No. 6 machine.

2) The mixture is mixed with carbon and material temperature. It can be seen from Tables 3 and 4 and Figure 2 that due to the formation of high melting point compounds after decomposition of dolomite, the carbon in the test period is higher than the reference period; the ratio of material temperature to dolomite is not significant, mainly related to the amount of returned ore, and the ratio of quicklime. related.

Since the dolomite decomposes during the sintering process:

It can be seen that the decomposition of dolomite can be completed in two steps. The reaction is an endothermic process. When the binary alkalinity is constant, the total amount of sintering flux increases, and the heat consumption needs to be increased. Under the condition of the same amount of carbon, the increase of decomposition heat absorption due to the increase of dolomite ratio will undoubtedly affect the heat balance in the sintered layer, and the amount of carbon should be appropriately increased.

3) Sintering negative pressure and end temperature. It can be seen from Tables 3 and 4 and Fig. 3 that the negative pressure of the No. 3 machine rises during the test period, and that the No. 6 machine has a severe negative pressure due to air leakage. The temperature rise of the sintering end point of No. 3 machine has a good influence on the improvement of quality; the temperature of the end point of No. 6 machine drops, which is unfavorable for the quality of production. Dolomite has an effect on increasing the negative pressure and reducing the vertical sintering speed, but is advantageous for improving the strength and the yield.

4) Thickness of the layer and vertical sintering speed. It can be seen from Tables 3, 4 and 4 that during the test period, the thickness of the No. 3 machine layer changes little and the fluctuation is small; while in the test period of the No. 6 machine, the air leakage occurs several times, and the material layer rises and fluctuates greatly. For the vertical sintering speed affecting the quality of production, the No. 3 machine dropped from 19.09 mm/min in the reference period to 18.67 mm/min, while the No. 6 machine remained at 19.12 mm/min. MgO acts refractory phase, the liquidus temperature rises during the sintering process, due to the formation of magnesium-containing high melting point, high salinity MgO sintering temperature requires a high temperature and a long holding time, so there is a certain vertical sintering speed Reduced.

Table 5 Sintering output and solid fuel consumption during the test period and the reference period

2. Effect of MgO on sintering output and solid fuel consumption

It can be seen from Table 5 that during the test period, the No. 3 machine showed an increase in the production of MgO in the background, while the No. 6 was the opposite. The main reason is that during the test period, the No. 3 machine increased due to the increase in negative air pressure (fan characteristics), and the output at the Taiwan time increased. On the 6th machine, the leakage of the exhaust pipe was severe, the negative pressure decreased, and the output decreased. During the test period, the vertical sintering speed of No. 3 machine decreased, and the output should decrease, but the increase in yield led to an increase in the comprehensive effect of production.

Solid fuel consumption and production generally change in a counter-trend, while increased production reduces solid fuel consumption, and vice versa. During the test period, the solid fuel consumption of the No. 3 machine decreased, and the solid fuel consumption of the No. 6 machine increased. The increase in solid fuel consumption of Unit 6 is also related to air leakage.

Table 6 Analysis of Sinter Strength and Ditch Grading Level in the Test Period and the Base Period (I)

Table 7 Analysis of Sinter Strength and Ditch Grading Level in the Test Period and the Base Period (II)

3. Effect of MgO on the strength and grain size of sinter

It can be seen from Tables 6 and 7 that the increase of MgO does not have much influence on the chemical index of sinter. During the test period, MgO increases by 0.20%, the binary alkalinity does not change much, but the ternary alkalinity increases by 0.034; and the total proportion of MgO+CaO+SiO ₂ is from the benchmark. The period of 19.99% increased to 20.12%, indicating an increase in the total amount of bonded phase, which is beneficial to improve strength and yield.

It can be seen from Tables 6 and 7 that as the MgO content increases, the drum index of the sinter increases and the yield increases. The main reason is that the stable magnesium-containing magnetite in the sinter increases, and the magnesium silicate mineral (magnesium silicon) The content of calcium stone and forsterite increased. From the analysis of mineral phase (Tables 8 and 9), it can be seen that the total amount of silicate in the test period increases, which is beneficial to improve the cold strength. Secondly, MgO is easy to generate multiple layers during the sintering process. The melt increases the bonding phase and increases the crystallization ability, and the glassy content decreases. Third, the limestone ratio decreases, and the CaO white point in the sinter decreases accordingly. Fourth, the vertical sintering speed decreases, which is favorable for liquid phase crystallization. After the MgO content increases, the overall trend shows that MgO has an adverse effect on the size of the sintered ore, although the strength is improved, and the particle size tends to be worse.

4. On-site sinter mineral phase analysis and metallurgical performance testing

1) Sinter mineral structure. It is seen from Tables 8 and 9, of the test site titanium magnetite sinter increases, reducing the titanium content of hematite, garnet and increasing titanium silicate mineral binder, little change in the content of calcium and iron perovskite. In addition, magnesium-containing minerals such as calcium forsterite (CaO·MgO·SiO ₂ ) and magmatic travertine (3CaO·MgO·2SiO ₂ ) are formed in a small amount, and the total binder phase minerals increase, while the vitreous binder phase decreases. Mineral composition and structure tend to be complex.

Table 8 Phase composition (volume fraction) % of on-site sintered ore (1)

Table 9 Phase composition (volume fraction)% of on-site sintered ore (2)

From the perspective of microstructure, the structure of the sinter in the base period and the test period is basically the same, and the main ones are titanium hematite, titanomagnetite and ferrite. The ferrite crystal form is mostly plate-shaped, it is shaped like a crystal and a small number of fine needles. The main difference is that the base ferrite crystal form is not complete, most of them are short columnar, a small part is granular structure, rhombohedral titanium hematite is high, and silicate is filled between titanium magnetite and ferrite gap. The glass content is high (Fig. 5); while the magnesium-bearing magnetite is increased during the test period, the mineral crystallinity is high, the grain is refined, the crystal form is more complete, the mineral distribution is more uniform, and the titanomagnetite and ferrite are melted. The degree of corrosion is also high, and the rhombohedral hematite and vitreous content are reduced (Fig. 6), which is one of the main reasons for the improvement of the strength of the sinter during the test period.

2) Sintering metallurgical properties. It can be seen from Tables 10 and 11 that the low-temperature reduction pulverization rate of the sintered ore during the test period is greatly reduced, and the degree of reduction is slightly decreased. After MgO is increased, MgO is dissolved in magnetite to promote the stable existence of difficult-reducing magnetite, which reduces the formation of coarse triangular and incomplete quadrilateral titanium hematite (Tables 8 and 9). At the same time, more Minerals such as calcium-magnesium olivine, which are difficult to reduce, form an aerobic interweaving structure with magnetite and calcium ferrite, which makes the sinter more dense. Although this structure is beneficial to the improvement of strength, it is more difficult to reduce.

Table 10 Determination of sinter ore reduction and soft-melt dripping performance (1)

Table 11 Determination of sinter ore reduction and soft-melt dripping performance (2)

Note: Ta—softening start temperature, °C; Tb—softening end temperature, °C; ΔT _ba —softening interval, °C; Ts—starting melting temperature, °C; Td—starting dripping temperature, °C; Tds—dropping interval, °C, Tds=Td—Ts; △Pm—the highest differential pressure of the column, Pa; displacement at A—Ts, mm; displacement at B—Tm, mm; H—drop thickness, mm, H=B— A; S—droplet performance characteristic value, , Pm - the pressure difference at the start of melting, Pa.

With the increase of MgO content, due to the presence of high-melting minerals such as magnesium magnetite and calcium forsterite, the softening temperature and melting temperature of the sintered ore increase, the soft melt temperature zone and the droplet interval become thinner, and the gas permeability of the column changes. Ok, the highest pressure difference is reduced. In addition, the slag drip smoothly and the drip time interval is shortened, indicating that the slag creep degree is lowered and the fluidity is improved. Obviously, this is good for blast furnace operation.

Second, improve the furnace MgO smelting industry test

(1) Test overview

The test was carried out on Panzhihua Steel's No. 4 blast furnace. The test period was June 11-16, and the base period was June 3, 6-10. On June 4th, 4BF broke the wind for 405min due to the main belt, which had an impact on the production index on the 5th. Therefore, the two-day index did not enter the baseline analysis. The coke performance and particle size composition during the test period and the reference period are shown in Tables 12 and 13.

Table 12 Analysis of performance and particle size of blast furnace coke in 4BF test period and reference period (1)

Table 13 Analysis of performance and particle size of blast furnace coke in 4BF test period and reference period (2)

(2) Test results and analysis

1. Effect of slag MgO on production

It can be seen from Tables 14, 15 and 7 that the 4BF slag MgO is increased from 7.50% of the reference period to 7.85% of the test period, and the average daily output of the blast furnace is increased from 3327t/d to 3337t/d, and the yield increase is only 0.3%. It shows that MgO is in this range, and the yield increases slightly as MgO rises. The main reason is that after the sinter MgO is increased, the low-temperature reduction pulverization rate is reduced, the powder produced by the reduction of the upper part of the blast furnace in the range of 400-600 ° C is reduced, the gas permeability of the furnace material is improved, and the smelting is improved smoothly; the second is that as the slag MgO rises, The slag viscosity is reduced, the fluidity is improved, the slag iron separation is good, and the iron loss is reduced. Third, the slag fluidity is improved, the furnace operation and the tapping slag are smooth, which is beneficial to the stability and activity of the hearth.

Table 14 Technical and economic indicators of smelting during 4BF test period and base period (1)

Table 15 Technical and economic indicators of smelting during 4BF test period and base period (2)

However, due to the influence of other factors and the increase of slag MgO is not large, it has not shown much influence on the increase of production, and the reduction of sinter has a certain decrease after the increase of MgO, which has a certain impact on pig iron production.

2, the impact of slag MgO focus ratio

It can be seen from Tables 14, 15 and 7 that the coke ratio in the test period decreased by 7.38 kg/t, and the coal injection ratio decreased by 3.38 kg/t, which caused the overall coke ratio to decrease by 10.08 kg/t and the MgO to increase by 0.1%. The comprehensive coke ratio decreased by 2.97kg/t, and the improvement of slag MgO has a very obvious effect on reducing the overall coke ratio. The main reason is that the slag opening degree is reduced, the slag iron fluidity is improved, the slag iron separation is good, the iron loss is reduced, and the heat loss is reduced. Second, the low-pressure reduction pulverization rate is lowered, the upper part of the blast furnace is improved in gas permeability, and the antegrade improvement is improved; The quality of coke is slightly improved; the fourth is that some parameter changes are beneficial to the coke.

3. Effect of MgO on slag performance

It can be seen from Table 13 that with the increase of MgO, the binary change has little alkalinity, and the ternary and quaternary alkalinity increase to different degrees, which is the main reason for the improvement of slag fluidity. At the same time, after the increase of MgO, the decrease of TiO _{2 and the} decrease of TiO ₂ /SiO ₂ also created conditions for the slag fluidity.

It can be seen from Fig. 8 that the melting temperature and viscosity of the slag are lowered during the test period, and the fluidity is improved. Compared with the reference period, the slag melting temperature decreased from 1372 ° C to 1358 ° C, the viscosity decreased from 0.58 Pa·S to 0.62 Pa·S, and the slag fluidity increased from 115 mm to 170 mm. Due to the greater improvement in liquidity, the iron loss decreased from 7.09% to 5.15%.

After the MgO content of the slag to improve the titanium and titanium-rich diopside diopside low melting point minerals to increase, decrease, and magnesium aluminum spinel perovskite content, thereby reducing the melting temperature. Studies have shown that the selection of suitable MgO, appropriate reduction of CaO / SiO ₂ , increase (CaO + MgO) / SiO ₂ , so that the slag system has a melting temperature suitable for smelting, which is beneficial to inhibit the slag thickening and desulfurization.

4. Analysis of process parameters change of blast furnace during test period

The relevant chart of the blast furnace process parameters during the test period is omitted.

1) Air volume and air temperature. Compared with the reference period, the air volume increased during the test period, and the wind temperature decreased, but the amplitude was not large, indicating that the improvement of the permeability of the upper part of the blast furnace after the improvement of MgO, the wind pressure is reduced, and the antegrade improvement is beneficial to increase the air volume, thereby increasing the production of coke. The wind temperature drops but the magnitude is not large, basically it belongs to high wind temperature smelting. During the test period, the effect of increasing the production coke was still obtained in the case of reduced oxygen enrichment, indicating that after the slag MgO is properly increased, the oxygen-enriched strengthening conditions required for the blast furnace can be weakened and the same effect is obtained.

2) Smelting strength and load. Compared with the baseline period, the smelting strength and coke load decreased during the test period to a certain extent, indicating that although the smelting strength and load decreased after the slag MgO was increased, the formula “utilization coefficient=coke smelting strength/coke ratio (pure coke smelting)) Or = comprehensive smelting strength / fuel ratio (with injection), it can be seen that under the condition of maintaining the same output, reducing the smelting strength will inevitably reduce the coke ratio. This test reflects this law, the yield change is not large, and the coke ratio is reduced. obvious.

3) Top temperature and pressure. Compared with the reference period, the top pressure of the test period decreased, the pressure difference in the furnace increased, the top temperature decreased, the gas utilization rate increased, the amplitude did not change much, and the gas permeability index did not change much, but the overall effect was an increase in air volume, indicating that MgO Appropriate improvement should be beneficial to the output. The lower top temperature is beneficial to the coke and extend the life of the top equipment to ensure the safety of smelting.

4) The effect of MgO on the desulfurization effect of pig iron. Compared with the reference period, the Ti and Si contents of pig iron increased during the test period, but the desulfurization capacity decreased or decreased, mainly due to the increase of binary and ternary alkalinity of slag, which easily formed sulfides and affected the desulfurization capacity. The sulfur load and the increase in furnace temperature also affect the desulfurization effect. The increase in furnace temperature is also conducive to the recovery of vanadium resources, and the amount of vanadium in pig iron rises, so the increase in MgO has a greater beneficial effect on the composition of pig iron.

(III) Test yield and coke ratio correction

Due to the numerous factors affecting the output and coke ratio of the blast furnace, many factors have changed before and after the test, which has an impact on the output and coke ratio. Therefore, the test results must be corrected. The calibration basis is based on the empirical data of the Blast Furnace Ironmaking Production Technical Manual. .

During the test, 11 factors including furnace grade, furnace powder, coke ash, coke strength, wind temperature, oxygen enrichment rate, pig iron containing silicon, slag alkalinity, top pressure, smelting strength and slag amount were changed. These factors were corrected, and the effect was that the coke ratio decreased by 3.51 kg/t and the output increased by 28 t/d. The corrected results are shown in Table 14. The actual effect of 4BF to improve the slag MgO test is that the output decreases by 18t/d, the coke ratio decreases by 3.87kg/t, the coal ratio decreases by 3.38kg/t, and the comprehensive coke ratio decreases by 6.57kg/t. Although the increase of MgO content in the test slag is not large, the optimum MgO content of the current blast furnace slag of Panzhihua Iron and Steel Co., Ltd. has been explored.

Table 14 Results of the 4BF test period yield and coke ratio correction

Third, the conclusion

(1) At present, the appropriate increase of MgO by 0.2% of vanadium-titanium sinter can improve the strength of the sintered ore, and the strength improvement effect is about 0.10%, but the size of the sintered ore deteriorates.

(2) Adding dolomite to improve the sinter MgO has little effect on the sintering process parameters. The test shows that the output of the No. 3 machine increases, the energy consumption decreases, but the amplitude is not large; the No. 6 machine has the opposite result due to serious air leakage.

(3) Increasing the sinter MgO can reduce the low-temperature reduction pulverization rate, but the reduction degree is slightly decreased, while the blast furnace permeability is improved and the air volume is increased; at the same time, the sinter softening performance can be improved, the reflow temperature is increased, the reflow and melting are The droplet interval becomes thinner, the gas permeability of the material column becomes better, and the highest pressure difference is lowered, which is beneficial to the stable antegrade of the blast furnace.

(4) The test showed that the slag MgO content increased by 0.35%, reaching 7.8%-8.0%, the slag melting temperature decreased from 1372 °C to 1368 °C, the viscosity decreased from 0.68 Pa·s to 0.62 Pa·S, and the fluidity increased by 55 mm. Iron loss was reduced by 1.94%. Although the corrected output has a certain decline, the impact is not significant, but the comprehensive coke ratio drops by 6.57 kg/t, and the coke effect is obvious.

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