1. Introduction
In the process of large-scale blast furnace, safe and long-life smelting of blast furnace is becoming more and more important. Safe and long-life smelting is also one of the key technologies for low-carbon metallurgy to realize carbon neutrality strategy in iron and steel industry. At the end of the blast furnace campaign, the carbon brick will be eroded by molten iron and form a brittle layer on the surface to increase its porosity. The carbon brick is prone to fracture under the thermal stress in the iron infiltration section and the non-iron infiltration section, which will lead to the damage of the hearth sidewall in severe cases.1) To eliminate the abnormal erosion of the carbon brick on the sidewall of the hearth, it is necessary to take furnace maintenance measures. Titanium ore furnace protection is an important way to ensure the safe production of blast furnaces at the end of service.2,3,4,5)
Contemporary blast furnaces often adopt pouring repair methods, but the thermal conductivity of the hearth sidewall structure is low. Whether an effective titanium protective layer can be formed during the repair process is worth exploring, and the amount of titanium ore that can guarantee effective furnace protection and a reasonable range of titanium content in molten iron need to be continuously studied. Through the study of the formation mechanism of the protective layer of the hearth carbon brick, the authors6) found that when the concentration product of [Ti], [C], and [N] in the molten iron reaches saturation, Ti(C,N) crystals will precipitate and migrate to the low-temperature zone of the hearth to form a protective layer. Similarly, the studies7,8) pointed out that Ti(C, N) solid solution precipitates in molten iron and agglomerates in slag by grain boundary migration. The existence of slag phase plays a certain role in the formation of protective layer. Li et al.9) found that titanium compounds with high melting point will increase the viscosity of molten iron, make the protective layer easier to form and play the role of solid fulcrum, which is conducive to improving the strength of the protective layer. The author10,11) also established an economical and efficient furnace protection model and a titanium content control model by calculating the carbon unsaturation of molten iron. Zhang12) found that when the TiO2 content in the furnace charge is controlled at 10–12 kg/t and the iron [Ti] content is 0.11–0.16%, which can achieve the maintenance effect. The effect is more obvious when the TiO2 content in the charge is increased to 18–20 kg/t, and the furnace maintenance cycle is 7 to 10 days. Chang et al.13) point out that properly increasing slag basicity, Si content in hot metal, and decreasing S content can better realize titanium ore furnace protection while increasing titanium ore charging load. Mo et al.14) found that controlling the [Ti] content in the range of 0.100–0.151% to make the corresponding titanium load greater than 3.5 kg/t can play the role of repairing the hearth. Wang et al.15) and Wu et al.16) found that the active condition of the hearth should also be considered in furnace protection operation. Controlling the furnace temperature can improve the yield of titanium, and can also activate the hearth to ensure the effect of furnace protection. Ma et al.17) found that the titanium load should be controlled at 6–10 kg/t in the study of the law of titanium ore furnace protection. Wu et al.18) found that when the titanium load was 7–8 kg/t, the addition of titanium pellets had a significant effect on reducing and stabilizing the hearth temperature. Shi et al.19) analyzed the titanium balance of titanium ore furnace protection and concluded that increasing the basicity of slag, reducing the circulation of molten iron, and maintaining appropriate smelting intensity are all conducive to increasing the amount of titanium deposition, and low titanium furnace protection is also an important direction. In addition, most carbon bricks also contain titanium,20,21,22) which has a good effect on the furnace.
The safety of the blast furnace side wall is the most important factor to achieve high efficiency and long life of blast furnace.23,24,25,26,27,28) There are many theoretical studies and industrial practices on furnace maintenance with titanium ore. In terms of the casting blast furnace hearth, however, the low thermal conductivity of casting materials, the amount of titanium deposited in the hearth, and the deposition form and process all have a great influence on the titanium-ore-based furnace protection effect. This paper focuses on the analysis of the titanium equilibrium when titanium ore is used to protect the furnace, to study the influencing factors of titanium deposition and the unsteady process, and to investigate the effect of this kind of maintenance on the fuel consumption of the blast furnace, thus providing better guidance for the operation of maintenance with titanium ore.
3. Results and Discussion
3.1. Titanium Ore Furnace Protection Full Cycle Production Practice
As shown in Fig. 1(a), compared with the coke ratio and fuel ratio of the blast furnace, the change trend of the coke ratio and fuel ratio is the same, showing a downward trend in the base period, continuing to decline after adding titanium ore in the maintenance period, and an upward trend in the middle. However, in general, the coke ratio and fuel ratio in the maintenance period are still lower than in the base period. Under normal circumstances, the addition of titanium ore will increase the consumption of slag and carbon generated by the blast furnace, increase the heat consumed, and increase the coke ratio and fuel ratio. However, the results of industrial tests show that the actual fuel ratio is reduced by about 5 kg/t.
Figure 1(b) is the change of blast furnace gas utilization rate and utilization coefficient. The gas utilization rate in the blast furnace is maintained in the range of 45.7–47.8%, and the utilization coefficient of the blast furnace is on the rise. On the whole, the utilization coefficient of the maintenance period is higher than that of the reference period, indicating that the addition of titanium ore at this stage plays a positive role in the smelting process. It not only has a good furnace protection effect but also helps to reduce fuel consumption.
As shown in Fig. 1(c), the temperature monitoring of two thermocouples TE3136 and TE3155 on the side wall of the hearth found that after adding titanium ore during the maintenance period, the temperature of the hearthside wall first increased, and the turning point began to appear after 6 days, and the temperature of the side wall point began to decrease. After the maintenance period, the thermocouple temperature began to gradually rise.
The bottom center thermocouples TE3010 and TE3041 temperature monitoring found that after entering the maintenance period, the temperature first increased and then decreased, after stopping titanium gradually returned to the original temperature conditions. The change of thermocouple temperature at the bottom of the furnace is not particularly obvious, indicating that the effect of titanium ore furnace protection is mainly reflected in the side wall of the hearth. Although it has an impact on the titanium deposition at the bottom of the furnace, the effect is not obvious.
Figure 1(d) is the change of water temperature difference. Through the monitoring of the water temperature difference, it is found that although the fluctuation of water temperature difference is more frequent, it can be seen that the water temperature difference in the base period shows a slight upward trend. After entering the maintenance period, the water temperature difference begins to decrease slowly, indicating that the titanium ore added in the maintenance period is deposited in the blast furnace hearth, which plays a protective role in the blast furnace hearth.
The influence mechanism of titanium ore on the interior of blast furnace hearth is shown in Fig. 2. A small amount of TiO2 brought into the blast furnace by titanium ore can improve the fluidity of the slag, thereby improving the activity of the hearth. At the same time, the increase of TiO2 content in the slag causes the liquid phase temperature of the slag to decrease. Therefore, the furnace temperature is slightly reduced under the same superheat condition, and the sensible heat carried by the slag iron is reduced. In addition, the heat storage capacity of the slag itself is better, resulting in a reduction in fuel consumption. In addition, the formation of the titanium-containing protective layer increases the thermal resistance of the hearth, thereby increasing the heat flux, cooling water temperature difference and hearthside wall temperature decreased, to a certain extent, reducing the heat loss, and contributing to the reduction of fuel ratio.
Figure 3 shows the change in titanium content in molten iron and TiO2 content in slag. The titanium content in molten iron in the base period is maintained at 0.022–0.036%. In the maintenance period, titanium content has a clear upward trend and began to decline at the end of strengthening. The total titanium content in the stop titanium addition period is still higher than that in the reference period, indicating that the titanium deposition in the furnace is a slow process, and there is a lag in time, about 7 days, which is basically consistent with the time delay of the thermocouple temperature drop. At the end of the stop titanium addition period, the titanium deposition and chemical reaction in the furnace reached a stable state, and the titanium content returned to the same level as the reference period. The content of TiO2 in the slag remained at 0.83–1.14% in the base period. After entering the maintenance period, it showed an increasing trend and began to decrease in the period of stopping titanium addition, and finally stabilized at 0.69–1.01%.
To better explore the non-steady-state changes of the titanium ore furnace protection process, the maintenance period of the furnace protection process was analyzed separately. Figure 4 is the change in titanium content in the two periods of the maintenance period. After adding titanium ore, the titanium content increased from 0.029% to 0.08% during the period from 6 o’clock to 10 o’clock, and then the content decreased for a short time and continued to rise at 18 o’clock. The process lasted 12 hours, which was basically consistent with the smelting cycle of the blast furnace. At the end of maintenance, the titanium content in molten iron gradually decreased from 0.08% to 0.036% from 0 o’clock, and the decline process took two days. The previous analysis pointed out that in the early stage of stopping titanium addition, the titanium content in molten iron was higher than that in the reference period. With the extension of time, the titanium content showed a downward trend and eventually stabilized. At this stage, Ti in molten iron gradually deposited and formed a Ti (C, N) protective layer. The whole unsteady period was about 10 days.
To explore the effect of titanium deposition, the titanium load and titanium deposition in the hearth were analyzed. The deposition amount of titanium is the difference between the amount of TiO2 in total income and the amount of TiO2 in total expenditure, and the deposition rate is the ratio of the amount of deposition to the amount of TiO2 in total income. Titanium load is the amount of titanium brought into the blast furnace per ton of titanium ore. Figure 5 shows the change in titanium deposition in each stage of the blast furnace. There is almost no titanium deposition in the furnace during the base period and the stop titanium period. In the conventional period, the addition of titanium ore increases the income of TiO2 in the furnace. At this time, the content of Ti in the reduced molten iron increases, and the deposition of titanium will be formed. The effect of titanium ore furnace protection is the most obvious in the strengthening period. As shown in Fig. 6, the analysis of the titanium deposition rate and titanium load shows that the blast furnace hearth is eroded by molten iron in the reference period, and the deposition rate is negative. At this stage, the titanium load is 2.5–3.5 kg/t. After entering the maintenance period, the titanium load of the blast furnace increased to more than 7.6 kg/t, the titanium deposition rate was 5–30%, and the titanium load in the stop-adding titanium period returned to the base period level again, maintained at about 2.3 kg/t. The increase of titanium load will increase the activity of TiO2 in slag, so the amount of TiO2 reduced to Ti (C, N) will increase. However, high content may lead to sticky slag and difficulty to operate. Therefore, in the process of furnace protection, the titanium load should be controlled within an appropriate range. According to the actual production, the titanium load is maintained in the range of 6–8 kg/t, which can play a better furnace protection effect.
When the titanium and carbon in the molten iron reaches a certain concentration, the dissolved [Ti] and [C] in the molten iron will react chemically to precipitate TiC, and the N2 contained in the furnace will dissolve in the molten iron. The [N] and [Ti] will form a refractory material TiN, which will then deposit to form a Ti(C,N) protective layer, and the chemical reaction to form TiC and TiN is:
|
[Ti]+[C]=
TiC
(s)
Δ
G
θ
=-176 934+100.11 T
| (1) |
|
[Ti]+[N]=
TiN
(s)
Δ
G
θ
=-314 800+114.35 T
| (2) |
When the chemical reaction of Eqs. (1) and (2) reaches equilibrium, ΔG=0, there is:
|
Δ
G
θ
+RTln
a
[TiC]
ω
[C]
⋅
f
C
⋅
f
Ti
⋅ω[Ti]
=0
| (3) |
|
Δ
G
θ
+RTln
a
[TiN]
ω
[N]
⋅
f
N
⋅
f
Ti
⋅ω[Ti]
=0
| (4) |
Where α[TiC] and α[TiN] are the activities of TiC and TiN, respectively, both of which are solids, so the activity is 1, fTi, fC and fN are the activity coefficients of dissolved titanium, carbon, and nitrogen in molten iron, which can be solved by the interaction coefficients between the elements listed in Table 2,29) and ω[N] is the mass fraction of N2 dissolved in molten iron.
Table 2. Interaction coefficient between elements.
| Element | C | Si | Mn | P | S | Ti | V |
|---|
| eiTi | −0.165 | 0.05 | 0.0043 | −0.064 | −0.11 | 0.013 | – |
| eiC | 0.14 | 0.08 | −0.012 | 0.051 | 0.046 | −0.165 | −16.1 |
| eiN | 0.13 | 0.047 | −0.021 | 0.045 | 0.007 | −0.53 | −0.093 |
The dissolution of N2 in molten iron can be expressed as:
|
1/2
N
2
=[N]
K
N
θ
=
a[N]
p
N
2
1/2
| (5) |
|
log
K
N
θ
=-
518
T
-1.063
| (6) |
The relationship between ω[N] in molten iron and temperature and N2 pressure can be calculated:
|
ω
[N]
=
K
N
θ
p
N
2
1/2
f
[N]
=
10
-518/T-1.063
p
N
2
1/2
10
0.3302×
10
-3
T+0.0543
| (7) |
The expression of the relationship between ω[Ti] and temperature in the equilibrium of TiC and TiN in molten iron can be solved by Eqs. (3), (4), and (7):
|
logω
[Ti]
=
Δ
G
θ
2.303RT
-log
f
C
-log ω
[C]
-log
f
Ti
| (8) |
|
log ω
[Ti]
=
16 959
T
+7.1127+0.3348×
10
-3
T
-log
p
N
2
1/2
+0.53×
10
-518/T-1.063
p
N
2
1/2
10
0.3302×
10
-3
T+0.0543
| (9) |
From Fig. 7, it can be seen that at 1773 K, the content of ω[Ti] in equilibrium with TiC is 0.2953%. Although it is much higher than the measured value of titanium in molten iron represented by the Grey horizontal line in the figure, considering the strong cooling intensity of the hearth, the temperature of the hearthside wall area is low, so TiC can be precipitated in the low-temperature area near the hearthside wall or the bottom of the hearth. The above thermocouple temperature change also shows that titanium deposition mainly occurs in the hearth sidewall area. The temperature corresponding to the intersection of the Grey horizontal line and the curve is the critical temperature of TiC precipitation in molten iron. When the temperature is lower than this value, TiC can be generated and precipitated from molten iron. For the precipitation of TiN, the horizontal line is always above the curve, so the titanium content in the molten iron during the industrial test meets the precipitation conditions of TiN.
To reasonably regulate the content of titanium in molten iron, the distribution ratio of titanium during the test was calculated. The reduction reaction of TiO2 between slag and iron can be expressed as:
|
(
TiO
2
)+2[C]=[Ti]+2CO Δ
G
1
θ
=634 720-307.22 T
| (10) |
The reaction equilibrium constant is:
|
K
1
=
f[Ti]⋅w[Ti]⋅
(
P
CO
P
θ
)
2
γ(
TiO
2
)⋅x(
TiO
2
)⋅
(f[C]⋅w[C])
2
| (11) |
where
w[Ti] and
w[C] are the mass fractions of titanium and carbon in molten iron;
f[Ti],
f[C] and
γ(TiO
2) are the activity coefficients of titanium, carbon in molten iron and TiO
2 in slag, respectively;
x(TiO
2) is the mole fraction of TiO
2 in slag;
PCO is the partial pressure of CO at equilibrium;
Pφ is the standard pressure.
The titanium distribution ratio is defined as:
|
L(Ti)=
w(Ti)
x(
TiO
2
)
| (12) |
From Eqs. (11) and (12):
|
L(Ti)=
w[Ti]
x(
TiO
2
)
=
exp
-Δ
G
1
θ
RT
⋅(γ(
TiO
2
)⋅
(f[C]⋅w[C])
2
)
f[Ti]⋅
(
P
CO
P
θ
)
2
| (13) |
Figure 8 shows the comparison between the titanium distribution ratio and the theoretical calculation value during the industrial test. It can be seen that the overall trend is in good agreement. The titanium distribution ratio in the reference period is in the range of 0.247–0.398. After entering the conventional maintenance period, the titanium distribution ratio increases. With the extension of time, the content of TiO2 in the slag increases continuously and titanium deposition occurs in the furnace. The titanium distribution ratio shows a downward trend, and the titanium distribution ratio remains in the range of 0.231–0.452 during the entire maintenance period. After entering the period of stopping titanium addition, the reduction of slag amount reduces the heat consumption. In the case of good temperature conditions in the blast furnace, it is beneficial to promote the migration of titanium to molten iron. Therefore, the distribution ratio of titanium tends to increase and gradually decreases to the baseline level during the deposition process.
The actual titanium content in molten iron is not only related to the smelting temperature in the furnace but also related to the content of elements in molten iron. According to the analysis of the change of [Ti] content and [C] content in molten iron during the reference period and maintenance period, with the increase of [C] content in molten iron during the reference period, the titanium content remained in the range of 0.02–0.04%. During the maintenance period, with the increase of [C] content, the reduction reaction is promoted, and the [Ti] content in the molten iron also increases, indicating that increasing the [C] content in an appropriate range is beneficial to promote the deposition of titanium in the molten iron, and the [Ti] content in the molten iron increases, the corresponding [C] content decreases, inhibiting the carburizing of the molten iron, which is beneficial to reduce the carbon consumption. By analyzing the relationship between [Ti] content and [Si] content, it can be seen that the influence of [Si] content in molten iron on [Ti] content increases significantly during the maintenance period, so the [Si] content in molten iron can be used as the judgment basis during the furnace protection period. According to the above production practice and thermodynamic calculation, the [Ti] content in molten iron should be more than 0.074%, and the corresponding Si content should be more than 0.42%. The analysis of the relationship between [Ti] content and [S] content shows that with the increase of [S] content in molten iron, [Ti] in molten iron shows a decreasing trend. In general, the [S] content of molten iron should be lower than 0.061% to ensure the [Ti] content of molten iron. If the content of [S] in molten iron is high, the corrosion of high sulfur molten iron to refractory can also be weakened by adding titanium.
