PROCESS FOR PREPARING TITANIUM TETRACHLORIDE USING OFF-GASES FROM A SILICA AND ZIRCON CARBO-CHLORINATION

PROCESS CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 60/692,792 filed August 1 , 2007, incorporated herein by reference in its entirety.

Field of the Disclosure The present disclosure relates to a process for preparing titanium tetrachloride, and in particular to a process for preparing titanium tetrachloride that utilizes off gases from the carbo-chlorination process to prepare silicon and zircon tetrachloride.

Background of the Disclosure The direct chlorination of zircon ore particles or silica particles in a carbo-chlorination process in which a fluidized bed reactor in the presence of high concentrations of carbon particles, and at a temperature within the range at which zircon will react with chlorine to form zirconium and silicon chlorides is known. The bed temperatures run well in excess of 1000 ^° C, and this presents a number of issues. First, high levels of CO are generated in place of CO ₂ . CO uses more coke in the carbo-chlorination process leading to higher operating costs. Next, the high temperature leads to accelerated brick wear in the chlohnator. Additionally, operating at lower temperatures to minimize brick loss, results in un-reacted chlorine in the chlorinator off gas. This un-reacted chlorine would be a chlorine yield loss and require additional treatment steps downstream.

A need exists for a process that recovers the CO generated in the direct chlorination of zircon ore particles, and a process for economically preparing titanium tetrachloride. A need also exists for a process that increases the life of the bricks in the chlorinator, and utilizes the un- reacted chlorine that results from running the carbon chlorination process at a lower temperature.

SUMMARY OF THE DISCLOSURE

The present disclosure provides an improved process for preparing titanium tetrachloride comprising:

(a) reacting an ore comprising an oxide with a chlorine and a carbon compound at a temperature of about

900 ^° C to about 1300 ^° C to form the corresponding chlorides and off-gas comprising carbon monoxide, wherein the oxide is selected from the group consisting of silica, zircon and mixtures thereof, and wherein the carbon compound is selected from the group consisting of coke, charcoal, silicon carbide and mixtures thereof; and

(b) reacting titanium dioxide with a stream comprising the off gas formed in step (a) under conditions effective for forming titanium tetrachloride and carbon dioxide.

The disclosure solves the problem caused by CO generation in the chlorination of oxide particles, such as zircon and silica and mixtures thereof, especially the high levels formed at high temperatures as well as the problem of unreacted chlorine that results when low temperatures are used to avoid brick wear by feeding the CO-containing off gas to a carbo- chlorination process for forming titanium tetrachloride.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified schematic flow diagram of the process for preparing titanium tetrachloride by utilizing the off-gases from the process for preparing zirconium tetrachloride and silicon tetrachloride.

Figure 2 is a simplified schematic flow diagram of the process for preparing titanium tetrachloride integrated with the oxidation of titanium tetrachloride to produce titanium dioxide.

DETAILED DESCRIPTION OF THE DISCLOSURE The ore which can be used as a feed source for the oxide is a oxide-bearing ore. The metal oxide-bearing ore is selected from the group consisting of zircon and silica. Sources for the ore include zircon sand and

silica. The term "metal oxide" may be used herein to include silica as well as zircon.

Titanium tetrachloride can be produced by a two stage process shown in Figure 1. In a specific embodiment, in the first stage, the disclosure comprises chlorinating within a fluidized bed reactor a granular zircon ore in the presence of carbon by continuously passing a chlorine- containing gas through the ore and carbon mixture to maintain said bed in a fluidized state, throughout the reaction maintaining a high ratio on the order of about 1 :2, typically about 1 :3 to about 3:4 by weight of unreacted carbon to total solids in the bed, maintaining the temperature in the reaction bed within the range of about 900 to about 1300 ^° C, more typically in the range of about 1000 to about 1150 ^° C, even up to 1200 ^° C, even more typically from about 1025 to about 1150 ^° C while continuously withdrawing the reaction product vapors, and separating and recovering the zirconium tetrachloride and silicon tetrachloride from the withdrawn products.

In general, the process comprises a first carbo-chlorination process and a second carbon chlorination process. In the first carbon chlorination process a metal oxide-bearing ore can be reacted with chlorine and a carbon source to provide the corresponding metal chloride and gas comprising CO and CO2. In the second carbon chlorination process a titanium-bearing ore is reacted with chlorine and a carbon source in the presence of the gas from the first carbon chlorination process to form titanium tetrachloride. The titanium tetrachloride can be used in a process for making titanium dioxide in an oxidation process.

Referring to Figure 1 , there is shown a fluidized bed reaction chamber 14, more particularly, a first fluidized bed chlorinator, suitably insulated to provide the desired reaction temperature and low corrosion of the reactor's refractory walls. Inlets are disposed as shown through which solid reactants such as granular zircon ore 11 and other carbon source 13, such as coke, can be continuously or intermittently fed. An inlet situated at the bottom of the reaction chamber 14 provides a means for introducing vapor reactants, such as chlorine 12, and, if desired, inert gas to maintain

the solids in fluidized state in the bed which is contained within the lower portion of the reaction chamber 14. A space is maintained above the solids in the fluidized bed for separating solids from the reaction product vapors. An outlet is provided positioned in the upper portion of the reactor through which the exit gases from the reactor can optionally pass to a small cyclone separator (not shown), having a discharge outlet for separation and removal of the finest bed particles blown from the bed. The shape of the reactor and number of inlets for fluidizing gas and for solids feed can be varied as desired. Typically, materials of construction utilized are resistant to the corrosive conditions of the reaction, a suitable refractory for the shell being high silica bricks. The free space above the fluidized bed reactor is typically operated slightly above atmospheric pressure (in the range of 2-10 p.s.i.) to avoid leakage of moisture containing air into the reactor, and to simplify control. If desired, however, pressures higher or lower can be utilized.

The carbon used as the reducing agent can be in the form of relatively coarse granular material, the 8 to +100 mesh range being typical. The solids feed to the reactor can be adjusted to maintain a high ratio of unreacted carbon to total solids in the bed, and this is expected to improve yield. A ratio of about 1 :2 by weight is typical, and acceptable results can be achieved if the ratio is maintained within the range of 1 : 3 to 3:4. Chlorine gas can be introduced into the reactor at a sufficient velocity and pressure to fluidize the bed and expand it to about twice its static volume. The velocity can be dependant on the size and materials in the bed. The reaction temperature can, preferably, be maintained within the range of about 1025 to about 1150 ^° C. Since the reaction is exothermic, it can not be necessary to supply heat, except in small scale reactors having very high heat losses. To increase the reaction temperature for rate control, oxygen can be supplied along with the chlorine gas. The oxygen can be supplied in any desired volume ratio compared to chlorine up to as high as about 3:2, as long as a high CO/CO2 ratio in the exit gases is maintained. As a person skilled in the art of carbon chlorination would realize, this high CO/CO2 ratio in the exit gases can be maintained by

controlling the proportion of carbon and oxygen in the reactor, by way of an example, the ratio can be maintained by the use of high ratios of carbon to zircon ore in the feed. However, the difference can vary depending upon the carbon and oxygen content of the starting materials. Once the gases exiting the chlorinator are cooled in recovery device

15, the zirconium tetrachloride 16a can be collected as a powder and silicon tetrachloride 16b can be condensed and collected as a liquid by known methods to those skilled in the art. As an example, a suitable recovery device can be a condenser. A stream of non-condensable gases comprising CO and CO ₂ , typically comprising a major proportion of CO and CO ₂ with a minor proportion of other gaseous materials from the reaction, such as unreacted chlorine, is withdrawn from the recovery device 15 via line 17. This stream of non-condensable gases can be referred to as an off-gas, more particularly, a first off-gas.

In other embodiments of the first carbon chlorination process the metal oxide can be SiO ₂ which is free of ZrO ₂ . Various technologies to chlorinate SiO ₂ can include but are not limited to 1 ) SiO ₂ carbon chlorination with external heating as described in U.S. patent 3,010,793, 2) Co-chlohnation of SiO ₂ and silicon metal as described in U.S. patent

3,197,283, 3) Co-chlorination of Silicon Carbide and SiO ₂ as described in U.S. patent 2,843,458, or 4) Chlorination of silica and carbonized biomass as described in U.S. patent 4,847,059. Typically, reaction temperatures are maintained in the range of about 1000 ^° C to about 1300 ^° C, more typically in the range of about 1100 ^° C to about 1200 ^° C.

Referring to Figure 1 , in the second carbon chlorination process, the non-condensable gas 17, is mixed with a stream of chlorine 18a, and introduced into a second fluidized bed chlorinator 19 through an inlet positioned in the lower part of the reactor. A TiO ₂ containing feedstock 20, typically ilmenite, rutile, or titanoferrous slag or a mixture thereof, and optionally a finely divided carbonaceous material, such as finely divided coke, coal or anthracite contained respectively in feed hoppers (not shown) can be charged by gravity into the fluidized bed chlorinator 19

wherein the TiO ₂ containing feedstock in the presence of the carbonaceous material and added chlorine undergoes carbon chlorination. The conditions effective for the carbon chlorination comprise temperatures ranging from about 900 ^° C to about 1100 ^° C and the atmosphere can be substantially free of nitrogen as described in U.S. Patent No. 4,854,972. Substantially pure oxygen can optionally be introduced into the fluidized bed chlohnator 19, along with the mixture of CO ₂ and CO and chlorine. In the case of equipment breakdown or failure, additional CO ₂ can be introduced as an emergency purging gas to maintain the fluidization of solids in the absence of chlorine.

The process additionally can be incorporated into a process for oxidizing titanium tetrachloride to make titanium dioxide. In this manner the process can use the first off-gas comprising carbon monoxide, shown as the non-condensable gas 17 in Figure 2, in the process for oxidizing titanium tetrachloride to make titanium dioxide. The preparation of titanium dioxide by oxidizing titanium tetrachloride in the vapor state with an oxygen-containing gas is well known and such processes have been described in U.S. Patent Nos. 2,488,439; 2,488,440; 2,559,638; and 2,833,627. An alternate embodiment for recovering the chlorine gas from the zirconia and/or silica chlorination stream can be to feed the off-gas containing CO and Cl ₂ as a gaseous feed to the oxidation reactor used in the process for oxidizing titanium tetrachloride to make titanium dioxide, as discussed above. Referring to Figure 2, the off-gas 18b from the titanium tetrachloride oxidation process 26 which comprises carbon monoxide, and chlorides, can then in turn be fed to the second fluidized bed chlorinator 19 to form the gaseous material containing titanium tetrachloride. The off- gas 18b from the oxidation, typically containing recycle Cl ₂ , can also be fed back to the first carbochlorination reactor 14 where the Cl ₂ can be recovered (not shown). The advantages of this embodiment include that any silicon tetrachloride entrained in the off-gas can react in the oxidation process to provide a boost in carbon black undertone (CBU), reduce agglomeration or provide a silica treatment for the TiO ₂ , usually by way of

a silica coating on the TiO ₂ . For a description of CBU, reference is made to U.S. Patent Nos. 2,488,440 and 5,201 ,949 which are incorporated by reference in their entireties. This embodiment can also prevent the silicon tetrachloride from carrying through reaction which can cause pluggage in the fume disposal section. Also the chlorides that do not react in the chlorination to form silicon tetrachloride can be fully recovered. Typically, the recovery mostly takes place in the second fluidized bed chlohnator.

Thus, the disclosure additionally relates to a process for preparing titanium tetrachloride comprising: (a) reacting ore comprising a metal oxide with a chlorine and a carbon compound at a temperature of about 900 ^° C to about 1300 ^° C to form the corresponding chloride and first off-gas comprising carbon monoxide, wherein the metal oxide is selected from the group consisting of silica, zircon and mixtures thereof, and wherein the carbon compound is selected from the group consisting of coke, charcoal, silicon carbide and mixtures thereof; and

(b) reacting titanium tetrachloride and an oxygen containing gas in the vapor phase and in the presence of the first off-gas to form titanium dioxide and a second off-gas comprising carbon monoxide;

(c) reacting a titanium dioxide-containing feedstock with the second off-gas under conditions effective for forming titanium tetrachloride and carbon dioxide.

In another alternative embodiment, the exit gas from the first fluidized bed chlohnator containing silicon tetrachloride can be incorporated into the titanium tetrachloride oxidation process to treat the titanium dioxide product with silicon tetrachloride to form a silica-treated titanium dioxide product. In this embodiment the silicon tetrachloride can be fed along with the TiCI ₄ feed to the oxidation reactor, to the titanium dioxide-containing discharge of the TiCI ₄ oxidation reactor, or to the flue downstream of the oxidation reactor as described in U.S. Patent Nos.

5,562,764; 6,852,306; and 7,029,648. By adding the exit gas containing silicon tetrachloride at these locations, the silicon tetrachloride can react to form SiO2 on the titanium dioxide and the excess CI2 can be recovered by feeding a CI ₂ -containing product stream from the oxidation reactor to the second fluidized bed chlohnator via line 18b.

Titanium tetrachloride (TiCI ₄ ), ferrous chloride (FeCI ₂ ), ferric chloride (FeCIs) and chlorides of other contaminating metals and impurities are simultaneously formed, volatilized and removed as a gaseous mixture along with CO and Cθ ₂ from the chlorinator 19 through line 21 , which gaseous mixture is then delivered to a cyclone/condenser unit 22 to remove precipitating and condensing compounds which are discharged through line 23. The gaseous phase which is withdrawn and contains TiCI ₄ , CO, CO ₂ , HCI, Cl ₂ , COS and entrained fine solids can be sent to a TiCI ₄ purification unit 24 in order to separate the TiCI ₄ vapor from the other gaseous components and thus obtain pure TiCI ₄ vapor which is removed through line 25. The ratio of CO:CO ₂ exiting the TiO ₂ chlorination stage is much lower than the CO:CO2 ratio exiting the ZrO ₂ or SiO ₂ chlorination process.

An economically useful operation would be to chlorinate high titania zircon ores. Typically, high TiO ₂ -containing zircon ores contain about 0.15 wt. % to about 45 wt. % TiO2, more typically from about 2 wt. % to about 10 wt. % TiO ₂ , based on the total amount of the ore. The high TiO ₂ zircon ores can be used in the first fluidized bed chlorinator. Such ores are available at a lower price than low TiO ₂ ores. The titanium tetrachloride and silicon tetrachloride that is condensed can be separated by means of distillation or condensed separately with the titanium tetrachloride fed to the second fluidized bed chlorinator for chlorinating titanium dioxide- containing feedstock.