Abstract:
The present disclosure relates to reacting tin metal with crude TiCl 4  containing arsenic to produce pure TiCl 4 , SnCl 4 , and an arsenic solid co-product. In some embodiments, the contaminant vanadium is removed as well. The reaction is preferably done in a continuous fashion in two stages for maximum through-put and utility at an elevated temperature. Distillation can be used to purify the TiCl 4  produced and simultaneously yield a purified SnCl 4  product. The synthesis of SnCl 4  in this method utilizes waste chloride to save virgin chlorine which would otherwise be used.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/445,801, filed Feb. 23, 2011, which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a process for purifying TiCl 4  produced via a chloride process. 
     BACKGROUND OF THE INVENTION 
     Pigmentary TiO 2  is commercially produced through the sulfate or the chloride process. The chloride process is also used to produce TiCl 4  for titanium metal production. In the chloride process, titanoferrous ore is carbochlorinated to produce TiCl 4  and a range of other metal chlorides from the ore impurities. The crude TiCl 4  produced in the carbochlorination is processed with a series of physical separation steps to produce a usable TiCl 4  product. One contaminating element found in titanoferrous ore is arsenic. The chlorination of the arsenic species present in the ore produces AsCl 3 . AsCl 3  has a boiling point very similar to that of TiCl 4 , making removal more problematic. 
     Different ores can contain significantly different levels of arsenic ranging from non-detectible to greater than 100 ppm. Standard purification methods for the chloride process involve first removing solids chlorides and then removing vanadium in a separate step. AsCl 3  is a liquid, so it is not removed by the solids removal steps. Known vanadium removal steps such as organic treating agents, like plant and animal oils, soaps, fats and waxes, do not react with AsCl 3 . Another known commercial process is using elemental copper to remove vanadium from crude TiCl 4 . Copper also shows no reactivity to AsCl 3 . As a result, all of the AsCl 3  that forms from chlorination is present in the pure TiCl 4  sent to oxidation and can end up in the TiO 2  product. High levels of arsenic are undesirable in TiO 2  pigment. Pigmentary TiO 2  used in FDA products such as cosmetics require &lt;1 ppm arsenic by the FDA method. Low levels are also desired in other pigmentary application such as some plastics and coatings products. Arsenic levels in TiCl 4  used to produce titanium metal must also be kept low to avoid deformations in the final metal pieces. Typical levels for TiCl 4  for titanium metal are &lt;10 ppm arsenic. 
     Since AsCl 3  passes through all the known vanadium removal processes, such as organic treatment or copper metal, all the AsCl 3  will end up in the purified TiCl 4 . If high concentrations of arsenic were present in the ore, elevated levels of AsCl 3  will also be present. Two technologies are known to remove AsCl 3  from pure TiCl 4 . If a partial reduction of the concentration from, for example, 100 ppm to 10 ppm is all that is required, distillation can be used with effective production of the desired product, but a significant yield loss of TiCl 4  is also required. Lower concentrations can also be achieved at greater penalties for energy consumption and equipment sizing. AsCl 3  has little commercial value. Arsenic is currently only used in a few specific applications, and each of these requires a high purity level, such as gallium arsenide production. As a result, using distillation of produce a highly concentrated AsCl 3  product would reduce the yield loss of TiCl 4  but would not yield a useful product. The AsCl 3 /TiCl 4  stream would need disposal in a proper manner. Since the boiling points of AsCl 3  and TiCl 4  are so close, only 6° C. apart, a large amount of energy would be required to produce this waste stream. 
     Another potential method for removing AsCl 3  from purified TiCl 4  is to use carbon adsorption. This method does not work on crude TiCl 4 . Carbon adsorption can remove the AsCl 3  to very low levels that would be suitable for all applications including cosmetics. However, the carbon adsorption is not selective for only AsCl 3 . Many other species are present in the pure TiCl 4  such as the sulfur gases produced from the carbochlorination, like SO 2 , COS, and CS 2 . These species will adsorb competitively on to the carbon, limiting the capacity. As a result, this method is not commercially viable for large scale production such as pigmentary TiO 2  for large markets like plastics and coatings. 
     Thus, the problem to be solved is removal of AsCl 3  from TiCl 4  produced via the chloride process in an economical, efficient, and safe manner. 
     SUMMARY OF THE INVENTION 
     Applicants have solved the aforementioned problems by using tin metal to remove arsenic from crude TiCl 4  produced via the chloride process. 
     One aspect is for a process for the purification of TiCl 4  comprising contacting arsenic-containing crude TiCl 4  with tin to produce purified TiCl 4 , SnCl 4 , and solid arsenic and separating the solid arsenic from the purified TiCl 4  and SnCl 4 . In some aspects, the contacting and separating steps are performed by a two stage process comprising reducing the arsenic content in the arsenic-containing crude TiCl 4  by contacting the arsenic-containing crude TiCl 4  with a less than excess amount of tin to produce partially purified TiCl 4 , SnCl 4 , and solid arsenic; separating the solid arsenic from the partially purified TiCl 4  and SnCl 4 ; further reducing the arsenic content in the partially purified TiCl 4  by contacting the partially purified TiCl 4  with an excess of tin to produce purified TiCl 4 , SnCl 4 , solid arsenic, and excess tin; and separating the solid arsenic and excess tin from the purified TiCl 4  and SnCl 4 . 
     Another aspect is for a process for the purification of TiCl 4  comprising contacting arsenic- and vanadium-containing crude TiCl 4  with tin to produce purified TiCl 4 , SnCl 4 , solid arsenic, and solid vanadium and separating the solid arsenic and solid vanadium from the purified TiCl 4  and SnCl 4 . In some aspects, the contacting and separating steps are performed by a two stage process comprising reducing the arsenic and vanadium content in the arsenic- and vanadium-containing crude TiCl 4  by contacting the arsenic- and vanadium-containing crude TiCl 4  with a less than excess amount of tin to produce partially purified TiCl 4 , SnCl 4 , solid arsenic, and solid vanadium; separating the solid arsenic and the solid vanadium from the partially purified TiCl 4  and SnCl 4 ; further reducing the arsenic and vanadium content in the partially purified TiCl 4  by contacting the partially purified TiCl 4  with an excess of tin to produce purified TiCl 4 , SnCl 4 , solid arsenic, solid vanadium, and excess tin; and separating the solid arsenic and the solid vanadium and excess tin from the purified TiCl 4  and SnCl 4 . 
     Other objects and advantages will become apparent to those skilled in the art upon reference to the detailed description that hereinafter follows. 
    
    
     DETAILED DESCRIPTION 
     Applicants specifically incorporate the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. 
     When tin metal is reacted with the arsenic in the crude TiCl 4  (i.e., titanium tetrachloride produced by a chloride process, which has been subjected to partial purification procedures to remove some metal chlorides), a solid arsenic product is produced along with SnCl 4 . This treatment process works with all ranges of arsenic seen in the variety of ores available with levels from 10 ppm to 100 ppm arsenic but has not been seen to have any limitations either with lower or higher concentrations. SnCl 4  is a liquid, not a solid like copper chloride. As a result, the SnCl 4  does not contaminate the arsenic solid. Tin metal, being a milder reducing agent, also does not appear to react with TiCl 4 , unlike copper metal. As a result, a simple two stage reactor system can be used with tin powder with essentially no extra yield loss of TiCl 4  or tin through reaction with the purified TiCl 4 . By the term “purified TiCl 4 ” it is meant that the concentration of the arsenic in the TiCl 4  is at least significantly lowered if not reduced to a level below that which can be detected by known analytical techniques. The product TiCl 4  has arsenic removed to a level suitable for use in the production of TiO 2  or titanium metal. The TiO 2  may be suitable for use in applications where lower arsenic concentrations are desired. 
     In the step of contacting the crude TiCl 4  with the tin material, the tin can be added to the TiCl 4  by any suitable addition or mixing method. The tin can be added as a fine powder using known engineering methods such as a star valve or screw feeder with appropriate consideration made for controlling TiCl 4  vapors back flowing into the system. Mixing of the tin powder with the crude TiCl 4  may be done with agitation such as paddle mixer, sparging, or other engineering methods appropriate for the difficulties associated with handling TiCl 4 . In some embodiments, the amount of tin added to the crude TiCl 4  is an excess amount. For a given equipment size and temperature, the rate of the reaction will be adjusted by the amount of excess tin added. When a single stage configuration is used, excess amounts could be very high, such as 20 times excess. A two stage configuration allows less excess to be used in the final stage, and lower amounts such as eight times excess can be used. The excess used in the final stage is also utilized later in the first stage. 
     SnCl 4  can be separated from the resulting pure TiCl 4  through, for example, distillation. SnCl 4  is a valuable product used as a catalyst and the starting material for the production of organometallic tin compounds that are used in a wide variety of applications. So, in this process, a valuable co-product is produced, and many other technical problems are eliminated. 
     First, by converting liquid AsCl 3  into a solid, disposal of the arsenic atoms becomes much easier. Liquid AsCl 3  is a water reactive, corrosive material that releases HCl upon contact with atmospheric moisture. As such, it cannot be disposed of directly. If it was removed from the product TiCl 4  stream through distillation, it would be mixed with larger concentrations of TiCl 4 , which is also a water reactive, corrosive material that releases HCl upon contact with atmospheric moisture, and both liquids would need to be converted into a different product before disposal. By converting the AsCl 3  into a solid as part of the removal process and then separating all of the TiCl 4  from the solid, a less hazardous material is produced. The residual solid is not contaminated with treating agent such as copper chloride or organic residue that must be separated since SnCl 4  was formed and already separated. This separation also produces a much small stream to handle. This stream might be much easier to convert into an acceptable form for landfill or other appropriate disposal. 
     Second, while a distillation step would still be required to recover SnCl 4 , the energy intensity would be lower to produce the TiCl 4  product. For TiO 2  production in the chloride process, significantly higher concentrations of SnCl 4  are allowed in the TiCl 4  since the Sn does not end up incorporated into the final TiO 2  product. So, the initial separation where a lower SnCl 4 - and much lower AsCl 3 -containing TiCl 4  product is produced from the bottom of the distillation column, would experience two benefits: (1) an increase in the separation of the boiling points of the two species being separated and (2) an increase in the amount of tolerated contamination in the product TiCl 4 . So, for example, a starting crude TiCl 4  with 100 ppm As might have to be distilled to reduce the arsenic level to 10 ppm As. With a 6° C. difference in the boiling points, 130° C. for AsCl 3  and 136° C. for TiCl 4 , a large column with many trays and considerable energy input would be required. If this process were used on the crude TiCl 4  to reduce the AsCl 3  from 100 ppm to 10 ppm As, then the only extra energy input required would be to reduce the SnCl 4  concentration. Some SnCl 4  is present in crude TiCl 4  to start due to the ore composition. As a result, the SnCl 4  concentration might need to be reduced from 2000 ppm to 1000 ppm in the product TiCl 4 ; however, that reduction is much easier to achieve, plus a 22° C. difference in the boiling points exists between the 114° C. for SnCl 4  and 136° C. for TiCl 4 . Now the extra energy can be applied to converting the high SnCl 4  material into a suitable product. 
     Third, a valuable product is produced in the reaction instead of material with disposal issues. SnCl 4  is typically made through the reaction of tin metal and chlorine at elevated temperatures. In this reaction, instead of using virgin chlorine, the chloride ligand is obtained in the purification process. These chlorine ligands would be lost, for example through the copper chloride disposal in other systems. In this case, the chloride, an expensive and energy intensive reagent, is conserved instead of lost. 
     Fourth, no opportunity for undesirable production of Persistent Bio-accumulative and Toxic (PBT) organic compounds exists because no carbon is introduced into the system. When organic treating agents are used, the combination of heat, chlorine and carbon can under some conditions produce PBTs such as chlorinated dioxins and furans. 
     Finally, tin provides an opportunity to simultaneously remove both vanadium and arsenic in one unit operation. Using carbon adsorption to remove the arsenic would first require a traditional purification step such as organic treating agents, followed by a separate unit operation for the arsenic removal. If very low AsCl 3  levels were required, such as &lt;1 ppm As, and low levels of SnCl 4  were also required, a distillation column might also be required to meet the final product specifications. 
     In some embodiments, the SnCl 4  is subsequently recovered from the TiCl 4 . This separation can be accomplished through, for example, distillation. All of the SnCl 4  does not need to be removed from the TiCl 4  for the TiCl 4  to be used for TiO 2  production. Most of the SnCl 4  could be recovered in this process and recycled to produce a more concentrated SnCl 4  stream. The concentration of SnCl 4  does not impact the rate of the arsenic removal step One example of the separation of TiCl 4  and SnCl 4  would involve two separate distillation columns. The first column would be fed the product from the vanadium removal stage to the upper portion of the column. TiCl 4  suitable for commercial use would be collected from the bottom of the first column. The purity requirements for TiCl 4  used for TiO 2  or titanium metal manufacture would determine the configuration of this column, typically set using Aspen modeling conditions or similar engineering principles. The stream collected from the top of the first column would provide the reflux flow to the first column and feed a second column. The second column would be used to produce a finished SnCl 4  product from the top of the column. The material from the bottom of the second column would be high in TiCl 4  and lower in SnCl 4 . The bottom material would be recycled to the tank used to provide the reflux to the first column. In this manner, no TiCl 4  would be lost while conserving energy. The size of the columns and number of trays would be related to the total purification strategy for the crude TiCl 4  since that will determine the amount of SnCl 4  present. SnCl 4  can also be present in crude TiCl 4  due to tin oxide in the ores. The SnCl 4  from the crude TiCl 4  will also be accounted for in the distillation. 
     One embodiment is for crude TiCl 4  to be purified in two stages. In the first stage, the arsenic concentration is only partially reduced so that the tin metal reaction can be driven to completion. The solid arsenic product is separated from this stage and a liquid (or vapor) TiCl 4  stream containing arsenic is transferred to a second stage. This step preferably occurs at least at 100° C. More preferably, this step occurs under pressure at temperatures elevated above the boiling point of TiCl 4  (about 150° C. to about 200° C. range). The arsenic solids can be collected in a drying chamber, for example a drying chamber found after a purge separation (see, e.g., U.S. Pat. No. 7,368,096, incorporated herein by reference). Alternatively, they may be collected by other known engineering methods such as, for example, filtration. 
     In the second stage, the arsenic is removed to the desired low levels and excess tin metal is present. The excess tin metal stream (containing some arsenic solid) is removed and can be sent to the first stage for further reaction. The TiCl 4 /SnCl 4  with no arsenic is then separated, in one embodiment in a distillation column. 
     Distillation may be operated in different methods depending on the end use of the TiCl 4 . In one embodiment, the initial TiCl 4 /SnCl 4  mixture is sent to a rough distillation column where a stream containing low enough amounts of SnCl 4  in TiCl 4  is produced from the bottom of the column and a high SnCl 4  stream is produced from the top of the column. The bottom stream of TiCl 4  can be used to produce TiO 2 . The top stream can be sent to a polishing distillation column which is used to produce a pure SnCl 4  stream from the top and a rough TiCl 4 /SnCl 4  stream from the bottom. The bottom stream from this column can be recycled back to the start of the first distillation column. Through the use of multiple distillation columns, essentially no TiCl 4  yield loss occurs and both a TiCl 4  product and SnCl 4  product can be produced. A third distillation column (or batch operation of the second distillation column) can be used in some embodiments to produce a TiCl 4  product ideal for titanium metal production. The benefit of using elemental tin compared to organic treating agents is no organic residue is present in the TiCl 4 , which is highly detrimental to the titanium metal. 
     In some embodiments, the contaminant vanadium is also removed by a process described herein. The vanadium chlorination products, VOCl 3  or VCl 4 , have boiling points close to that of TiCl 4 , which makes removal problematic. When tin metal is reacted with the vanadium in the crude TiCl 4 , a solid vanadium product is produced along with SnCl 4 . This treatment process works with all ranges of vanadium seen in the variety of ores available with levels from 100 ppm V to 3000 ppm V but has not been seen to have any limitations either with lower or higher concentrations. As with arsenic noted above, the SnCl 4  does not contaminate the vanadium solid. 
     The product TiCl 4  has vanadium removed to a level suitable for use in the production of TiO 2  or titanium metal Additionally, vanadium can be lowered to an operator specified concentration. 
     Vanadium can be removed using either the one stage or two stage process described above for arsenic. The solid vanadium product that is produced by a process described herein is suitable to become a feedstock into other processes such as the production of steel. 
     The TiCl 4  product of the process described herein can be used in any application for which titanium tetrachloride is useful. The TiCl 4  can be used as a starting material for making titanium dioxide and derivatives thereof especially as a feedstream for the well-known chlorination and oxidation processes for making titanium dioxide. 
     Titanium dioxide can be suitable for use as a pigment. The majority of TiO 2  produced is used for this property. Common applications are in paints, paper and plastics. The TiCl 4  produced in this process is suitable for use in production of TiO 2  for all of these applications. 
     Titanium dioxide is useful in, for example, compounding; extrusion of sheets, films and shapes; pultrusion; coextrusion; ram extrusion; spinning; blown film; injection molding; insert molding; isostatic molding; compression molding; rotomolding; thermoforming; sputter coating; lamination; wire coating; calendaring; welding; powder coating; sintering; cosmetics; and catalysts. 
     Alternatively, titanium dioxide can be in the nano-size range (average particle diameter less than 100 nm), which is usually translucent or transparent. TiO 2  of this particle size range is typically used for non-optical properties such as photo-protection. 
     The TiCl 4  from this process is also suitable for use to produce titanium metal through any of the known commercial pathways such as the Kroll and Hunter processes. The TiCl 4  is also suitable for use in the production of titanium based catalysts such as organo-titanates or Ziegler-Natta type catalysts. 
     EXAMPLES 
     The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the preferred features of this invention, and without departing from the spirit and scope thereof, can make various changes and modification of the invention to adapt it to various uses and conditions. 
     Example 1 
     Crude TiCl 4  and One Stage Removal with Elemental Sn 
     A 100 mL aliquot of commercial crude TiCl 4  was added into a 250 mL reaction flask equipped with a magnetic stirrer, heating mantle, powder addition funnel and Dean Stark trap for condensate collection. The crude TiCl 4  contained a range of impurities including vanadium, iron and other elements including SnCl 4  plus 36 ppm arsenic as AsCl 3 . The dark yellow TiCl 4  was mixed with 2.0 g of powdered elemental Sn (&lt;45 micron size, Aldrich, 98.8%) and the heated to reflux. The TiCl 4  and Sn were refluxed together for 3 hours. All of the color was removed from the distillate. The TiCl 4  was then distilled from the solids. The overheads were measured to contain &lt;1 ppm V and &lt;5 ppm As. They also contained 2000 ppm of Sn which includes the SnCl 4  which was present in the crude TiCl 4 . 
     Example 2 
     Crude TiCl 4  and Two Stage Removal with Elemental Sn 
     A 100 mL aliquot of commercial crude TiCl 4  was added into a 250 mL reaction flask equipped with a magnetic stirrer, heating mantle, powder addition funnel and Dean Stark trap for condensate collection. The crude TiCl 4  contained a range of impurities including vanadium, iron and other elements including SnCl 4  plus 40 ppm arsenic as AsCl 3 . The dark yellow TiCl 4  was heated to 100° C. and mixed with 1.2 g of powdered elemental Sn. The TiCl 4  and Sn were refluxed together for 12 hours to ensure that an endpoint had been achieved. The distillate was still a strong yellow color indicating that only a portion of the vanadium was removed. Another 1.1 g of Sn was then added. The slurry was refluxed for 1 more hour. All of the color was removed from the distillate. The TiCl 4  was then distilled from the solids. The overheads were measured to contain &lt;1 ppm V and &lt;5 ppm As. They also contained 2000 ppm of Sn which includes the SnCl 4  which was present in the crude TiCl 4 .