Titanium metallurgy usually uses chlorination metallurgy to extract titanium from ilmenite or titanium-rich materials in the form of titanium tetrachloride, and then selects reduction or oxidation according to the downstream products to produce metallic titanium or titanium oxide. The chlorination metallurgy of titanium is mainly divided into molten salt chlorination and boiling chlorination according to its reaction system.

Boiling chlorination has been the mainstream process of titanium tetrachloride production in recent years due to its advantages such as large production capacity and no waste salt. In particular, the internationally advanced chlorination titanium dioxide production technology adopts the boiling chlorination production process. The boiling chlorination of the titanium metallurgical process is mainly carried out in the boiling chlorination furnace, and the lining material is the most important component of the boiling chlorination furnace, which is the key to its normal use and operation.

1. Introduction to the lining structure of the chlorination furnace

The main function of the entire boiling chlorination furnace lining is to form the airflow boundary and the constraint of the impact flow, so that the material is reasonably fluidized. And it plays a role of heat insulation and heat insulation under high temperature reaction. During the entire operation process, the composition of the chlorination furnace is very complex, and the main components are: high-titanium slag particles, TiCl4, Cl₂, O₂, CO, CO₂, N₂, etc. The boiling chlorination furnace is mainly divided into the following parts:

(I) Furnace bottom and reaction section

Due to the special structure of the boiling chlorination furnace, the chlorine gas inlet is mainly installed in this section and evenly distributed around. After the gas is introduced, an impact flow is formed, and under the constraints of the furnace bottom and the surrounding lining, an upward fluidized reaction bed is formed. The normal working temperature of the furnace bottom and the reaction section lining is 900-1000℃. This area mainly bears the direct impact and friction of the furnace charge under the action of the impact flow. This part is prone to damage to the lining material due to the impact of the material, the intrusion of the rising wear gas, and the reaction of carbon deposition. The main reasons for damage are:

1. Thermal effect: thermal load and thermal shock;

2. Chemical effect: due to the intrusion of chemical elements such as chlorine, alkali metals, slag, the lining material is slagified and damaged, and the chemical effect of chlorine and refractory materials;

3. Mechanical effect: due to the friction of material particles and the wear of dusty flue gas, the lining material is damaged.

(II) Feeding section

In this section, feed is periodically made (depending on the process), and special materials are used in the feed port to reduce the wear of solid particles. However, due to the operation of the process, the rise and fall of the reaction section, the scouring of the lining of this part by a large amount of fluidized high-temperature flow is the main destructive factor, especially the lining near the feed port is the most severely scour part.

1. Transition section. The transition section plays a transition role between the reaction section and the furnace bottom. The temperature of this part is about 800℃. The lining of the transition section is mainly in contact with the unreacted material, and the erosion effect of the material is not great.

2. Expansion section. The upper part of the boiling chlorination furnace is used to reduce the air flow velocity, so that the unreacted materials return to the reaction section under the action of gravity to continue the reaction, effectively utilizing the materials.

2. Analysis of the damage mechanism of the chlorination furnace lining

(I) Composition and mineral composition of the lining material

The lining material of the boiling chlorination furnace is mainly Al₂O₃ and SiO₂, and the phase composition is mainly mullite. The main chemical composition and physical property test are shown in Table 1.

Table 1 Requirements and test analysis results of chlorination furnace lining materials

(II) Study on the damage process of chlorination furnace lining materials

From a thermodynamic point of view, in the chlorination reaction, the phases of Al₂O₃-SiO₂ system materials [4] are: mullite phase (A₃S₂) is the most unstable, its composition is shown in Table 2, quartz phase (SiO₂) is the most stable, and corundum phase (Al₂O₃) is in the middle; in carbon thermal reduction: quartz phase is the most unstable, corundum phase is the most stable, and mullite phase is in the middle. In practice, how the lining material is damaged and how the process occurs can be understood from the microstructure analysis of the residual bricks.

Table 2 Mullite composition

Take the residual bricks in the chlorination furnace, slice them into original brick layer, metamorphic layer, and reaction layer, and use 6300SEM electron microscope and OXFORD silicon lithium detector for energy spectrum to analyze and study the different layers of the bricks.

From the microstructure of the residual bricks and the results of energy spectrum analysis, the Al2O3/SiO2 ratio in the furnace lining material (mainly mullite phase and quartz phase) is constantly increasing from the original layer, metamorphic layer to the reaction layer, indicating that the SiO2 in the furnace lining material is constantly "losing" outward. From a kinetic point of view, it is very difficult for SiO2 in refractory materials to migrate outward in the form of solid phase. From a thermodynamic point of view, in the absence of C participating in the reaction inside the brick (metamorphic layer), Cl2 reacts with SiO2 to form SiCl4, and it is impossible to migrate outward in the form of SiCl4 gas. Therefore, under the operating conditions of the chlorination furnace, the "loss" phenomenon of internal SiO2 in the furnace lining material can only migrate outward in the form of SiO gas, which may be caused by the carbon thermal reduction reaction of SiO2. The carbon thermal reduction reaction process of SiO2 is as follows:

(1) SiO₂+2C=Si+2CO(g)

(2) Si+C=SiC

(3) SiO₂+3C=SiC+2CO(g)

(4) SiO₂+C=SiO(g)+CO(g)

(5) SiO(g)+2C=SiC+CO(g)

(6) SiO(g)+C+2Cl₂(g)=SiCl4(g)+CO(g)

(7) SiO₂+CO(g)=SiO(g)+CO₂(g)△G=△G0+RTln(PSiOPCO₂/PCO)

According to the phase law, there can be more than 5 phases at the same time in the Si-C-O system at the invariant point: P=3+2=5. That is, under the temperature conditions for forming SiC, the reaction starts at about 1350°C, and the Si-C-O system may contain the following individual substances: C, Si, SiO, SiO₂, SiC, CO and CO₂.

When C is in excess and directly participates in the reaction, CO₂ does not exist in the reaction system. From a kinetic point of view, most elementary reactions are unimolecular reactions and bimolecular reactions, with few trimolecular reactions and even fewer tetramolecular reactions. Therefore, except for a small number of tightly bound and mutually surrounded SiO₂ and C directly generating SiC nuclei through the solid phase reaction shown in formula (3), the probability of SiO₂ and three C molecules simultaneously meeting and reacting is small, that is, the degree of occurrence of the reaction shown in formula (3) is very limited, and it should be regarded as the total reaction formula of several addition reactions.

In the Si-C-O system reaction, C and SiO₂ are the reaction raw materials, Si and SiO are the intermediate products of the reaction, and SiC and CO are the final products. The melting point of Si is 1410℃ and the boiling point is 2355℃, while SiO is unstable at any temperature below 1180℃ and exists in gaseous state at high temperature, so there are only several possible phases in the system, such as SiO₂, C, CO, SiO, and SiC.

It can be determined that the elementary reaction steps of carbon thermal reduction of SiO₂ are:

SiO₂+C=SiO(g)+CO(g); SiO(g)+2C=SiC+CO(g)

But in fact, with the presence of Cl₂, the second step of the elementary reaction of carbon thermal reduction of SiO₂ may not be carried out according to formula (6), but according to formula (7), resulting in the continuous "loss" of SiO₂ in the furnace lining material. In the surface reaction layer of the residual brick, the "loss" of SiO₂ may be destroyed according to this process, which is consistent with the fact that SiC is not visible in the microstructure of the residual brick reaction layer.

When C is in excess but does not directly participate in the reaction (i.e., the PCO/PCO₂ ratio is large), there is no C in the reaction system, and the carbon thermal reduction reaction of SiO₂ will proceed according to formula (7). After thermodynamic calculation, the starting temperature of the reaction of formula (7) is about 1400℃. The residual bricks of the furnace lining form a metamorphic layer under abnormal furnace conditions and overheating, and the "loss" of SiO₂ in it may be damaged according to this process.

III. Conclusion

From the previous analysis, it can be seen that the damage rate of the chlorination furnace lining material is mainly controlled by the carbon thermal reduction rate in the Al₂O₃-SiO₂ material. The damage process is:

Inside the brick, the external CO gas diffuses into the material through the pores of the material, undergoes carbon thermal reduction reaction with SiO₂ to generate SiO and CO₂ gas, which diffuses to the outside. When it reaches the surface of the brick, the SiO gas reacts with the surface C and Cl₂ to generate SiCl4 gas, and the SiCl4 gas diffuses into the furnace. The carbon thermal reduction reaction of CO gas and SiO₂ causes the material structure to loosen, further forming a channel for external CO to penetrate into the interior of the brick, and this process repeats until the CO pressure inside the brick is too low to react with SiO₂ by carbon thermal reduction or the reaction temperature cannot be reached.

On the surface of the brick, SiO₂ directly reacts with C to form SiO and CO, and the generated SiO reacts with C and Cl₂ to form SiCl4, which diffuses directly into the furnace.

The TiCl4 gas mixed with fine TiO₂ particles on the outside continuously penetrates into the interior of the brick through the channel formed by the carbon thermal reduction reaction of SiO₂, forming an Al₂O₃-SiO₂-TiO₂ liquid phase (there is no data on the effect of TiCl4 on this phase diagram), thereby accelerating the structural disintegration of the material and causing a sharp decline in wear resistance.

Whether inside or on the surface of the brick, the mullite particles basically maintain their original particle morphology, which once again proves that their damage rate is controlled by the carbon thermal reduction rate and has nothing to do with the chlorination reaction rate (although mullite is most susceptible to chlorination reaction from a thermodynamic point of view).

IV. Optimization suggestions for chlorination furnace lining

(I) Material selection

1. It is very necessary to use Al₂O₃-SiO₂ series bricks with high alumina content for lining materials.

2. The use of high-purity fused mullite and fused corundum as the main raw materials for lining materials will effectively increase its life and tolerate fluctuations in operating processes within a wider range.

3. On the basis of ensuring the thermal shock performance of bricks, it is very important to reduce the porosity of bricks and improve the hot strength (wear resistance).

4. Selecting reasonable brick grading and binder is the technical core to ensure the long-term use of corundum-mullite bricks in chlorination furnaces.

(II) Structural optimization design

Renovation is carried out according to the damage of different parts. The structure of the furnace after the transformation is: the more wear-resistant fused mullite and fused corundum are used to build the feed port. In other parts, the first layer uses high-alumina bricks with a slightly lower alumina content, and the second and third layers use high-alumina bricks or clay bricks with even lower alumina content.

Such a structure can enhance the strength of the furnace body, improve the airtightness of the furnace body, prevent chlorine from leaking from the masonry joints, and reduce the cost of use. Its maintenance is also simple. Each maintenance only requires replacing the damaged part. After each maintenance, it can be used continuously for about one year, and each maintenance time is about one week, which effectively improves production efficiency.

The transformation of the above structure is a theoretical optimization design based on the damage mechanism of the furnace lining and the working conditions of chlorination metallurgy. It also needs to be continuously improved in practice, and comprehensive consideration should be given to the service life and cost.