Titanium metal obtained from raw ore is called sponge titanium because of its porous and spongy appearance. Titanium is very abundant as a chemical element. Among the most abundant metal elements in the earth's crust, titanium ranks fourth (after Al, Fe and Mg). The first mineral used to produce titanium is rutile ( TiO2) or ilmenite (FeTiO3), the preparation of metallic titanium from these ore minerals is divided into the following 5 different steps or procedures, namely:
(1) Minerals are chlorinated to form TiCl4;
(2) the distillation purification of TiCl;
(3) Reduction of TiCl4 to produce metallic titanium [Kroll process];
(4) Remove the by-products of the reduction process to purify titanium metal (sponge titanium);
(5) Crushing and grading of metallic titanium in order to obtain products suitable for the next step of commercial pure titanium (CP titanium) and titanium alloy smelting.
The chlorination process does not require high purity of rutile. If ilmenite is used instead of rutile, the raw material is titanium slag rich in TiO2, which is a by-product of smelting ilmenite with carbon in an electric furnace to produce iron. The chlorination reaction takes place in a boiling furnace containing TiO2, impurities and carbon (coke) that enter the chlorinator together with rutile, see Figure 3.1. In contact with carbon, the reaction products are metal chloride (MClx), CO2, CO and gaseous TiCl4 (the boiling point of TiCl4 is 136 ° C), these reaction products are discharged from the top conduit of the reactor and directly enter the fractionation unit (see Figure 3.2).
The basic chlorination reaction formula is as follows:
TiO2+2Cl2+C→TiCl4+CO2
and
TiO2+2Cl2+2C→TiCl4+2CO
The second step in the production process is the distillation step, because the primary TiCl4 from the chlorination step needs to be further purified. Purification is accomplished by fractional distillation of TiCl4 as shown in Figure 3.2, which shows a two-step distillation purification process. The first step is to remove low-boiling impurities, such as CO and CO2, and the second step is to remove high-boiling impurities, such as SiCl4 and SnCl4. The purified TiC4 has been stored under the protection of inert gas until use.
The next step in the production process is the reduction of TiCl4, the Kroll process. The purified TiCl4 is added to the reactor filled with magnesium metal and filled with inert gas. When heated to 800~850℃, the following general reduction reaction occurs:
TiCl4+2Mg→Ti+2MgCl2
The reaction is actually completed by the following two steps:
TiCl4+Mg→TiCl2+MgCl2
followed by
TiCl2+Mg→Ti+MgCl2
The schematic diagram of Kroll reduction reactor is shown in Figure 3.3. The reduction reactor on the left is coupled with the vacuum distiller on the right. The reduction reaction was first studied by Kroll in the late 1930s, and the process of reducing TiCl4 with Mg is still called the Kroll process. The final product metal titanium reduced by the above reaction formula itself is quite pure, but pure metal titanium will mix with MgCl2. With the progress of Kroll reduction process, most of MgCl2 is continuously removed, but there are Certain residual amounts, their removal will be discussed in the subsequent titanium metal purification stage.
Since the reduction reaction is an exothermic reaction, the rate of adding TiCl4 to the Mg-containing reactor should be under a controllable temperature, which is necessary to prevent the formation of dense solid reactants and hinder the volatilization of other products. The product of this reaction is a mixture of metallic titanium and MgCl2, called "sponge titanium block", which is the product of the Kroll process.
As early as 1910, Hunter confirmed that TiCl4 can be reduced by molten Na, and this method of preparing sponge titanium is called the Hunter method. Between 1960 and 1995, a large amount of sponge titanium was produced using this method. At present, there are no factories for large-scale production of titanium sponge using this method, mainly because the use of magnesium as a reducing agent is more attractive than the use of sodium from an economic point of view.
The next step in the production process is the purification of metallic titanium, that is, the removal of residual MgCl2 from the sponge titanium block. The MgCl2 can be separated by one of the following methods: acid leaching, inert gas purging or vacuum distillation. The first method exploits the preferential solubility of MgCl2 in acidic solutions, and MgCl2 can be removed from the fragmented titanium sponge by a separation leaching method that is no longer widely used. Other methods have the advantage of removing MgCl directly in the Kroll reactor. These methods take advantage of the high vapor pressure of MgCl to selectively remove MgCl by evaporation followed by condensation to recover Mg and Cl from sponge titanium , and the inert gas rule is to use argon as a carrier to transport MgCl2 vapor.
Figure 3.3 is a schematic diagram of the vacuum distillation process (VDP). In this process, the sponge titanium block is heated under vacuum in the Kroll reactor on the left. At this time, volatile MgCl2 and excess metal Mg are caused by vapour pressure and is condensed in another vessel (see right vessel in Figure 3.3) which, after fresh addition of Mg, serves as the Kroll reactor for the next reduction period, while The container with the titanium sponge block on the left in Figure 3.3 is replaced with an empty tank, which is a semi-continuous process with economical advantages. Among the three purification processes of titanium sponge, the titanium sponge block treated by vacuum distillation process (VDP) has the lowest content of volatile substances. Due to the mass transfer in the reactor under vacuum distillation process (VDP) at high temperature (700~850°C), that is, titanium sponge will indeed absorb a small amount of Fe and Ni from the stainless steel reactor. Among superalloys, Ni especially Undesirable because Ni content above the limit reduces its creep strength, which is also true in the sintering of sponge titanium blocks.
In both processes (inert gas purge and VDP), Mg and Cl2 are recovered and recycled. At present, the production of titanium sponge by Mg reduction has basically achieved batch closed-loop production, but it is necessary to "mix" an appropriate amount of Mg and Cl2 between batches.
The final step in the production process is the crushing and grading of the titanium sponge. After removing excess Mg and MgCl2, the bulk titanium sponge was broken into granular metallic titanium. After crushing and classification, the coarser grades of titanium sponge are sheared to further reduce their size. The crushing and shearing operations are carried out in the air, but care should be taken because titanium is a potential pyrophoric substance, and any ignition source that occurs during the operation will produce nitrogen-rich areas and contaminate the titanium sponge, resulting in subsequent smelting defects. The higher operating temperature of the VDP process makes it difficult to segment the titanium sponge block. Unless there is a special request, sponge titanium manufacturers will not pursue the production of products with an actual average particle size of less than 3~5cm, which not only eliminates the operation cost of further crushing and shearing, but also avoids the risk of fire in the sponge titanium during these operations. . The desired or specific titanium sponge particle size depends on the final product to be produced. Coarse-grained grades (up to 2.5 cm) of titanium sponge can be used to produce commercially pure titanium (CP titanium) and most standard grades of titanium alloys. In high-performance fields, such as aircraft engine blades, a smaller particle size (maximum 1cm) of titanium sponge is required, which is mainly based on the consideration of gap stability defects in the application of blade-grade materials. The particle size of such sponge titanium is such as As shown in Figure 3.4.
For the production process of other titanium metals, research has been carried out for many years, and most of the researches are devoted to reducing the production cost of sponge titanium, but they are generally unsuccessful. Electrolytic (also called electrowinning) production of titanium is an attractive example, and Dow-Howmet successfully built a pilot-scale demonstration plant in the United States between 1975 and 1985 [3.3 ], Due to the downturn in the titanium market at that time, large-scale production could not be carried out. Therefore, it can be said that, in fact, a system that is reliable enough to undertake large-scale electrolytic reduction has not been realized, and the problem to be verified is to seal the large electrolytic reduction. The ability of the cell to maintain a clean operating environment and long-term stability of the electrode.
In addition, recent efforts to produce high-purity titanium through electrorefining have been very successful both technically and economically. Electrolytic refining first dissolves impure titanium in an electrolyte, and then redeposits it as high-purity titanium. By carefully controlling the deposition conditions and the purity of the electrolyte, a high-purity product can be obtained, and this high-purity metal can be made into a sputtering target for the production of electronic devices. The economic feasibility of electrolytic refining of titanium is that users who use high-purity titanium materials use a relatively small amount of this high-value-added product, which is completely different from the application of structural materials in terms of economy.
At present, a new process for preparing sponge titanium is being studied in depth, which is called Electro-Deoxidation (EDO)TM. The EDO process uses a molten CaCl2 molten pool and a graphite electrode to separate oxygen from titanium oxide-containing ions through electrolysis, thereby converting the compacted or sintered TiO2 cathode into titanium, and the porous metal titanium is precipitated on the original cathode after the reaction. In principle, if the oxygen content of the desired alloying element is mixed with cathode oxygen and electrolytically reduced with TiO2, then this process also has the ability to prepare pre-alloyed titanium sponge, but the effect achieved by this process is very Limited, and the possibility of large-scale production still needs to be analyzed and justified, the process is exciting nonetheless for several reasons. First, it can prepare pre-alloyed titanium sponge, which will omit the steps of titanium sponge preparation, alloying element mixing, mechanical compaction, etc., all of which are for the preparation of initial melting electrodes for melting metal ingots, which will greatly reduce the Manufacturing cost; Second, the process has the ability to add alloying elements (such as W, Cu, etc.) to titanium, which is difficult to practice for traditional metal ingots, which will be discussed later. The new process opens up the possibility of simultaneously selecting multiple alloying elements, which was previously impossible to envisage due to the limitations of smelting. The technical feasibility of the EDO process has been confirmed, but many details after scale-up, from reproducibility to production costs, still require in-depth research and analysis. Although it is unclear whether the EDO process will be commercially available in the future, it is mentioned here because of its revolutionary changes.
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Nicole
Company: Baoji Jimiyun Dynamic Co., Ltd
Cuntry:China
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