Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
  • G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/10—Obtaining titanium, zirconium or hafnium
  • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
  • C22B34/1218—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by dry processes
  • C22B34/1222—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by dry processes using a halogen containing agent
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G23/00—Compounds of titanium
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G23/00—Compounds of titanium
  • C01G23/02—Halides of titanium
  • C01G23/028—Titanium fluoride
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B1/00—Preliminary treatment of ores or scrap
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/10—Obtaining titanium, zirconium or hafnium
  • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
  • C22B34/1236—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by wet processes, e.g. by leaching
  • C22B34/124—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by wet processes, e.g. by leaching using acidic solutions or liquors
  • C22B34/1245—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by wet processes, e.g. by leaching using acidic solutions or liquors containing a halogen ion as active agent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/10—Obtaining titanium, zirconium or hafnium
  • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
  • C22B34/1236—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by wet processes, e.g. by leaching
  • C22B34/1259—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining titanium or titanium compounds from ores or scrap by wet processes, e.g. by leaching treatment or purification of titanium containing solutions or liquors or slurries
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/10—Obtaining titanium, zirconium or hafnium
  • C22B34/14—Obtaining zirconium or hafnium

Definitions

• the present invention is directed to a method for the preparation and purification of Group IVb transition metal-alkali metal-fluoride salts, such as sodium fluotitanate and sodium fluozirconate, from Group IVb transition metal oxide ores, such as ilmenite and zircon, by fluorinating the ore with an alkali metal fluoride salt, such as sodium silicofluoride and the like, in a kiln reactor at elevated temperature between about 650°C and 1000°C.
• Group IVb transition metal-alkali metal-fluoride salts such as sodium fluotitanate and sodium fluozirconate
• Titanium metal has been essential to the aerospace industry since the early 1950's because it combines a high strength-to-weight ratio with the ability to perform at much higher temperatures than aluminum or magnesium. Titanium also has experienced growing usage in the chemical processing industry because of its excellent resistance to chloride corrosion.
• titanium metal In the United States, primary titanium is imported from Japan, Europe and the USSR.
• the bulk of titanium metal is made by the "Kroll" Process, which involves the magnesium reduction of titanium tetrachloride, which is made from rutile (TiO 2 ) or synthetic rutile. Titanium metal is also made from titanium tetrachloride by sodium reduction and electrowinning.
• the product of the Kroll Process is metallic sponge, which is consolidated by a high temperature arc melting process.
• the most important consideration in titanium production is the prevention of contamination of the titanium with metallic and non-metallic impurities because the presence of impurities, even in very small amounts, can make the product brittle and unworkable.
• steps are taken to prevent the contamination of the titanium metal with oxygen, nitrogen, alkali metals, alkaline earth metals, iron, manganese, halides, carbon, silica and the like.
• controlled amounts of oxygen, nitrogen and carbon can be added to titanium to yield strengthened titanium alloys.
• U.S. Patent No.2, 550, 447 teaches a process for preparing titanium metal from titanium oxide ores, such as rutile, anatase and ilmenite, which comprises reduction of the ore by aluminum metal followed by iodination of the product obtained from such reduction. The iodinated product is then reacted with potassium iodine to yield titanium tetraiodide. The titanium tetraiodide, after removal of the potassium iodide, is converted to titanium metal by either heat decomposition or reduction. This is a very expensive method for preparing titanium metal.
• U.S. Patent No. 2,781,261 discloses the process for converting titanium dioxide to titanium by fluorinating the titanium dioxide, neutralizing the fluotitanic acid product and reducing the neutralized fluotitanic acid product with aluminum.
• U.S. Patent No. 2,837,426 discloses a process for converting ilmenite to an alkali metal fluotitanate by reacting ilmenite with sulphuric acid to form titanium sulfate, removing a portion of the iron included with the titanium sulfate by reduction and precipitation of the reduced iron compound, and converting the titanic sulfate filtrate to an insoluble fluotitanate with ammonium and/or alkali fluoride solution.
• U.S. No. Patent 2, 857, 264 teaches a process for preparing an alkali metal chlorotitanate by digesting ilmenite ore in a mixture of sulphuric acid and hydrochloric acid.
• the iron present is precipitated out as ferrous sulfate and then further recovered by the addition of hydrochloric acid to precipitate a ferrous chloride.
• potassium chloride is added to the titanium-containing liquor to salt out potassium chlorotitanate, which may be reduced with a Group I metal to titanium.
• U.S. Patent No.3, 012, 878 discloses a process for reducing titanium halides to titanium metal by the use of sodium metal.
• U.S. Patent No. 3,825,415 discloses a process similar to the process disclosed in U.S. Patent No. 3,012,878, except that the process is carried out in the vapor phase.
• U.S. Patent Nos.. 4,127,409 and 4,072,506 are related to the recovery of zirconium and hafnium by the reduction of the corresponding potassium chlorozirconates and hafniates by means of an alloy of aluminum and zinc.
• U.S. Patent No. 4,390,365 is directed to a process for making titanium metal from ilmenite ore by fluorinating the ore with an alkali metal fluosilicate at a temperature of from about 600oC to about 1000°C to form titanium fluorides optionally in the presence of carbon and/or silica tetrafluoride; leaching the titanium fluoride to provide an aqueous solution of titanium fluoride, crystallizing the titanium fluoride out of solution and reducing titanium fluoride to titanium metal with a reducing alloy of aluminum and zinc.
• the leach solution optionally contains hydrogen fluoride and, optionally, the leach solution is oxidized and hydrolyzed to form insoluble iron hydroxide, which is separated from the leach solution before crystallization of the titanium fluoride.
• U.S. Patent No. 4,468,248 is directed to a process for reducing titanium halides to titanium metal by reducing the titanium halides with a molten alloy of aluminum and zinc at elevated temperatures to produce a titanium and zinc alloy.
• the titanium metal is recovered by distilling off the zinc.
• U.S. Patent No. 4,470,847 is directed to a process for making titanium, zirconium and hafnium metal and alloy particles suitable for powder metallurgy from the corresponding Group IVb transition metal-zinc alloy by distilling off the zinc to form a corresponding sponge, sintering the sponge, hydriding the sponge, comminuting the hydrided sponge, dehydriding the resulting hydrided particles and pacifying the dehydrided particles.
• the present invention is directed to a process for the preparation of +4 valent Group IVb transition metal-alkali metal-fluoride salt (also referred to as "+4 valent salt") from a Group IVb transition metal oxide ore which comprises the steps of fluorinating a Group IVb transition metal oxide ore by contacting said ore in an unsealed reactor with an alkali metal fluosilicate (also known as "alkali metal silicofluoride”) and alkali metal fluoride salt at a temperature of from about 600°C to about 1000°C to convert the Group IVb transition metal oxides in the ore to +3 and
• the ore is contacted with at least about 0.8 molar equivalent of an alkali metal fluosilicate and with at least about 1.6 molar equivalents of an alkali metal fluoride salt to minimize the loss of fluorine as silicon tetrafluoride gas from the reactor. It has been found that, in a kiln reactor operation, one mole of silicon tetrafluoride gas escapes in the exhaust gases for each two moles of alkali metal fluosilicate introduced into the kiln. This gas, which has a residence time of less than 30 seconds in the kiln, is exited with the exhaust gas from the kiln and very little reacts with the ore.
• one mole of a Group IVb transition metal oxide should react with one mole of an alkali metal fluosilicate to form one mole of +4 valent Group IVb transition metal-alkali metal-fluoride salt and one mole of silicon dioxide.
• one mole of a Group IVb transition metal oxide is introduced with one mole of an alkali metal fluosilicate into a kiln reactor, only one-half of the silicon tetrafluoride generated from the decomposition of the fluosilicate reacts to form Group IVb transition metal fluoride salts.
• alkali metal non-fluoride salts may be employed in place of alkali metal fluoride salts to obtain substantially complete conversion of the metal oxide to the corresponding metal fluoride salt with little, if any, production of silicon tetrafluoride gas.
• an alkali metal basic salt is present, an additional amount of an alkali metal silicofluoride salt sufficient to convert the alkali metal non-fluoride salt to an alkali metal fluoride salt is required. This improved process for the preparation of
• Group IVb transition metal-alkali metal-fluoride values from Group IVb transition metal oxide ores comprises fluorinating a Group IVb transition metal oxide ore by contacting the ore in an unsealed fluorination zone with an alkali metal fluosilicate at a temperature of from about 600oC to about 1000°C to convert the metal oxide to +3 and +4 valent
• the ore is contacted with at least 1.07 molar equivalents of an alkali metal fluosilicate and with at least 1.07 molar equivalents of an alkali metal in the form of an alkali metal basic salt.
• the alkali metal basic salt is a carbonate salt, a bicarbonate salt, or oxide salt, such as sodium hydroxide.
• This process has the added advantage that alkali metal basic salts may be employed rather than the more costly alkali metal fluoride salts.
• the alkali metal basic salts are converted to alkali metal fluoride salts, which may react with the metal oxide in the ore to fluorinate the metal oxide.
• the ore can be fluorinated with at least 1.6 molar equivalents of an alkali metal fluosilicate to fluorinate substantially all the ore and produce approximately 0.8 molar equivalent of silicon tetrafluoride gas.
• the gas exits from the kiln with the exhaust gas.
• the exhaust gas can be scrubbed with water to capture the silicon tetrafluoride and convert it to fluosilicic acid and silicon dioxide. It is difficult to use fluosilicic acid as an additional fluorinating agent for the ore fluorination because fluosilicic acid at elevated temperature decomposes to silicon tetrafluoride and hydrofluoric acid.
• the fluosilicic acid can be contacted with an alkali metal halide salt, such as sodium chloride, to produce an alkali metal fluosilicate and a hydrogen halide, such as sodium fluosilicate and hydrochloric acid.
• an alkali metal halide salt such as sodium chloride
• a hydrogen halide such as sodium fluosilicate and hydrochloric acid.
• the alkali metal fluosilicate is separated from the acid and employed in the ore fluorination reaction.
• This alternative process for preparation of +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values from a Group IVb transition metal oxide ore comprises the steps of: a) fluorinating a Group IVb transition metal oxide ore by contacting the ore in an unsealed fluorination zone with an alkali metal fluosilicate at a temperature of from about 600oC to about 1000oC to produce a fluorinated ore containing +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values, the ore is contacted with at least 1.6 molar equivalents of the alkali metal fluosilicate for each mole of Group IVb transition metal oxide present in said ore, 0.5 mole of silicon tetrafluoride gas being formed and liberated for each mole of alkali metal fluosilicate introduced into the fluorination zone.
• Plus 4 (+4) valent Group IVb metal-alkali metal-fluoride values can be recovered by the additional steps of: b) removing the fluorinated ore from the fluorination zone and introducing it into a leaching zone wherein the fluorinated ore is leached with an aqueous hydrochloric acid solution to convert the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values to +4 valent
• the silicon tetrafluoride gas formed and liberated in the fluorination zone can be recovered to produce alkali metal fluosilicate and hydrochloric acid, which can be recycled, by the additional steps of: f) introducing the silicon tetrafluoride gas from the fluorination zone into a hydrolysis zone wherein the silicon tetrafluoride is contacted with water to convert the silicon tetrafluoride to fluosilicic acid and silicon dioxide; g) introducing the fluosilicic acid into an alkali metal fluosilicate production zone wherein the fluosilicic acid is contacted with an alkali metal chloride to produce an alkali metal fluosilicate and aqueous hydrochloric acid; h) separating the aqueous hydrochloric acid solution from the alkali metal fluosilicate; i) introducing the separated hydrochloric acid solution into the leaching zone; and j) introducing the separated alkali fluosilicate into the fluorin
• This process has the advantage that it does not require the use of alkali metal fluoride salts or alkali metal basic salts in the fluorination step, and it avoids the loss of fluoride from the overall reaction scheme as silicon tetrafluoride.
• the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values produced from the above-described fluorination processes are a mixture comprising +3 transition metal fluoride value and +4 transition metal fluoride value, such as Na 3 TiF 6 and Na 2 TiOF 4 . These values can be converted to a +4 valent Group IVb transition metal-alkali metal-fluoride salt, such as sodium fluotitanate or sodium fluozirconate.
• This conversion is carried out in the leaching step in the presence of aqueous hydrochloric acid.
• This conversion requires the presence of ferric iron, which is converted to ferrous chloride.
• the conversion requires at least about 1.75 moles of hydrogen chloride per mole of Group IVb metal present as +3 and +4 valent values.
• time, temperature and agitation at least 95% by weight of the +3 and +4 valent values are solubilized.
• the leachant is separated from the insoluble component of the leaching step and cooled to a- temperature of at least about 60oC to precipitate out silicon impurities. Silicon impurities have a very low solubility in the leachant at temperatures of 60oC or lower, and substantially all of the silicon impurities drop out.
• the +4 valent salt depending on the HCl concentration and generally less than 10% by weight, also drops out with the silicon impurities.
• hydrochloric acid concentrations favor a better split by maximizing the leaching of the +4 valent salt.
• strong acid concentrations favor a back reaction between silica and the +4 valent salt.
• the recycled solid silicon impurities cannot cause a build-up of silicon impurities in the leachant.
• the +4 valent salt is dropped out of the leachant by cooling the leachant to a temperature no greater than 30°C.
• sodium chloride (solid) and/or hydrogen fluoride is added to the leachant to help salt out the +4 valent salt.
• the +4 valent salt crystallizes out in substantially pure form; however, it does contain minor amounts of dissolved iron and manganese values. Unexpectedly, the iron and manganese values or impurities can be easily removed from the product crystals by washing the crystals one or more time(s) with dilute aqueous hydrochloric acid.
• the +4 valent Group IVb transition metal-alkali metal-fluoride salt isto be used to produce a Group IVb transition metal by reducing the +4 valent salt as described herein.
• the preferred means of reducing the +4 valent salt is by adding the salt to a molten aluminum-zinc alloy and agitating the same to obtain intimate contact between the alloy and the salt.
• the +4 valent salt is reduced to a Group IVb metal which is soluble in the zinc and the aluminum is oxidized to an aluminum fluoride salt.
• This type of reduction can also be utilized to purify the Group IVb transition metal.
• Impurities that are not soluble in the molten zinc or the molten Group IVb transition metal-zinc alloy report to the top of the molten alloy or drop to the bottom of the molten alloy depending upon respective densities of the alloy and the impurities.
• Group IVb metal scrap such as titanium metal scrap, containing insoluble impurities such as halide salts, alkali metal salts, alkaline earth metal salts, tungsten carbide and nitrides.
• Such metal scrap can be purified by adding it to molten zinc, which will dissolve the Group IVb transition metal but not the insoluble impurities.
• the insoluble impurities that is, for example, halide salts, alkali metal salts, alkaline earth metal salts, and nitrides, will report because of their insolubility and density to the top of the molten Group IVb transition metal-zinc alloy (referred to as the "molten alloy") to form a separate phase and/or slag.
• molten alloy molten Group IVb transition metal-zinc alloy
• Other insoluble impurities which are denser than the molten alloy such as tungsten carbide, will drop to the bottom of the molten alloy.
• Group IVb transition metal-zinc alloy contains unacceptable amounts of impurities that are soluble in the molten alloy ("soluble impurities")
• a high purity Group IVb transition metal can be added to the molten alloy to dilute the soluble impurities concentration to yield a Group IVb transition metal-zinc alloy which has an acceptable impurity level.
• a +4 valent Group IVb transition metal-alkali metal-fluoride salt and a molar equivalent of aluminum can be added to the molten alloy wherein the +4 valent salt will be reduced to a Group IVb transition metal to dilute the soluble impurity content.
• the insoluble impurities on the top of the molten alloy are skimmed off, and the molten alloy is removed from the heavier insoluble impurities that drop to the bottom of the molten alloy to yield a molten alloy substantially free of insoluble impurities.
• the zinc is removed by sublimation either under a vacuum or with the aid of an inert gas purge to yield a Group IVb transition metal sponge that can be converted to a Group IVb transition metal powder suitable for powder metallurgical usage as described in U.S. Patent No. 4,470,847.
• FIG. 1 is a schematic illustration of the process of the present invention for the production of sodium fluotitanate
• FIG. 2 is a schematic representation of the purification steps of the present invention for the removal of silicon, iron and magnesium impurities from a sodium fluotitanate product
• FIG. 3 is a schematic illustration of a purification process for the present invention for purifying Group IVb transition metal scrap to produce a metal sponge;
• FIG. 4 is a graph showing the effect of washing sodium fluotitanate crystals with 1% by weight of HC1 washes.
• the fluorination zone 10 is in an unsealed kiln wherein the ore and, optionally, additives are added into the feed end 12. Hot gas is introduced into the product end 14 counter-current to the flow of the ore.
• the kiln is heated by indirect means, such as kiln wall heaters.
• the ore is ground, mixed with the other feed materials and formed into agglomerates of less than 2-inch diameter.
• the salt, carbon and iron are compounded with the ore to form an integral component of the feed agglomerates.
• the ore is compounded with at least 1.6 molar equivalents of alkali metal fluoride, such as sodium fluoride, or alkali metal basic salt, such as sodium carbonate, for each mole of Group IVb transition metal in the ore.
• the feed agglomerates are also compounded with the fluorination agent, an alkali metal fluosilicate, preferably sodium fluosilicate. If the fluosilicate is the sole fluorinating agent, at least 1.6 molar equivalents of fluosilicate are compounded with the ore for each mole of
• the feed agglomerates are compounded with at least 0.8 molar equivalent of an alkali metal fluosilicate and at least 1.6 molar equivalents of an alkali metal fluoride salt for each mole of Group IVb transition metal in the ore.
• the feed agglomerates are compounded with an alkali metal basic salt, the feed agglomerates are compounded with at least about 1.07 molar equivalents of an alkali metal fluosilicate and with at least 1.07 molar equivalents of an alkali metal as an alkali metal basic salt for each mole of Group IVb transition metal in the ore.
• the additional alkali metal fluosilicate is required to fluorinate the akali metal basic salt.
• the ore, iron (if any), carbon, fluorination agent and alkali metal basic salt or fluoride salt (if any) are ground to a particle size from about 30 to about 400 mesh, preferably from 100 to 400 mesh, and thoroughly mixed and then formed into agglomerates for feeding into the fluorination zone kiln.
• the feed agglomerates are heated in the fluorination zone to a temperature between about 600°C and about 1000°C, preferably from about 750°C to about 950°C to convert the Group IVb transition metal oxide to +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values.
• the fluorination is believed to proceed by the series of reactions which are set forth below. The reactions are illustrated employing sodium salts and Group IVb transition metal values (M +3 and M +4 ). However, the same reactions occur with other alkali metal salts.
• Reaction (1) occurs at elevated temperatures, such as
• Reactions (2) through (4) are reactions between silicon tetrafluoride with alkali metal basic salts, which yield an alkali metal fluoride salt, carbon dioxide, silicon dioxide and water as the case may be.
• Reaction (5) is the reaction between a Group IVb transition metal oxide (MO 2 ) with 1 molar equivalent of an alkali metal fluosilicate and 2 molar equivalent of an alkali metal as an alkali metal basic salt, such as sodium carbonate. In this fluorination reaction, the Group IVb transition metal oxide is fluorinated to form a +4 valent Group IVb transitionmetal-alkali metal-oxide-fluoride value Na 2 MOF 4 .
• Reaction (6) shows that if Reaction (5) is conducted in the presence of at least 2 molar equivalents of an alkali metal fluoride salt rather than a basic salt, 2 molar equivalents of the metal oxide are fluorinated.
• Reaction (7) is a reaction that occurs between the carbon source and iron oxide in the ore to produce ferrous oxide and carbon monoxide.
• the ferrous oxide reacts with the alkali metal fluosilicate to yield an alkali metal ferrous fluoride as shown in Reaction (8).
• the alkali metal ferrous fluoride reacts with the alkali metal fluosilicate and the Group
• IVb transition metal oxide MO 2 to form a +3 valent Group IVb transition metal-alkali metal-fluoride value, ferric oxide, silica tetrafluoride and silica as shown in Reaction
• the silicon tetrafluoride will react with the salt to form an alkali metal fluoride salt [see Reactions (2) and (4)].
• Some of the Group IVb transition metal oxide (MO 2 ) reacts directly with the alkaline metal fluosilicate as shown in Reaction (10) to form a +4 valent Group IVb transition metal-alkali metal-oxide-fluoride value, silicon tetrafluoride and silica. If an alkali metal basic salt is present, the silicon tetrafluoride will react with the salt to form an alkali metal fluoride salt.
• Reaction (12) is a compilation of Reactions (8) through (11) wherein, for each mole of Group IVb transition metal oxide in the ore, the ore is contacted with 1.6 molar equivalents of an alkali metal fluosilicate and 0.48 molar equivalents of ferrous oxide.
• the carbon and the ferric oxide compounded in the feed agglomerates react to form ferrous oxide and carbon monoxide.
• the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values comprise a mixture of a +3 valent Group IVb transition metal-alkali metal-fluoride value (Na 3 MFg) and a +4 valent Group IVb transition metal-alkali metal-oxide-fluoride value (Na 2 MOF 4 ).
• a portion of the ferrous oxide is convertedto an alkali metal-ferrous-fluoride salt (Na 3 FeF 6 ).
• Approximately 1 molar equivalent of silicon tetrafluoride is produced for each mole of alkali metal fluosilicate present in the feed agglomerates.
• the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride value products (Na 3 MF 6 andNa 2 MOF 4 ) of Reaction (12) are converted to the desired +4 valent Group IVb transition metal-alkali metal-fluoride salt, namely alkali metal fluotitanates and alkali metal fluozirconates.
• the conversion is accomplished by contacting the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values with an aqueous hydrogen chloride solution in the presence of ferric oxide and an alkali metal ferric fluoride salt as shown.
• the feed mixture should contain at least 0.24 molar equivalent of iron per mole of Group IVb transition metal oxide, preferably as iron oxide, to supply sufficient fluoride as an alkali metal ferric fluoride salt (Na 3 FeF 6 ) to form Na 2 MF 6 during the leach step.
• the feed mixture can also contain more than 1.6 molar equivalents of alkali metal fluosilicate per mole of Group IVb transition metal oxide. However, only 1.6 molar equivalents of the fluosilicates are necessary when at least 0.24 molar equivalent of iron is present.
• fluorinated ore 16 comprising the solid products of Reaction (12) and other solid impurities is passed from the product end 14 of the kiln to a leach zone 18, wherein the fluorinated ore is leached with aqueous hydrochloric acid.
• the leach solution has an HCl concentration of less than 10% HCl by weight, preferably from about 1% to about 3% by weight.
• Conventional leaching equipment can be employed in the leached zone to solubilize the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values (Na 3 MF 6 and Na 2 MF 4 ) identified in Reaction (12).
• the solubilized salts are converted to the desired +4 valent Group IVb transition metal-alkali metal-fluoride salt (Na 2 MF 6 ), namely, alkali metal fluotitanates and alkali metal fluozirconates. It is believed that the chemical reactions responsible for this conversion are those set forth in Reaction (13).
• Reactions (12) and (13) illustrate the importance of iron values to the fluorination and conversion reactions to produce the desired +4 valent Group IVb transition metal-alkali metal-fluoride salt.
• the Group IVb transition metal values of Reaction (12) report to the aqueous leachant in the leach zone.
• the leachant is separated from the insoluble solids, following the leaching, which comprise iron-containing solids, silicon impurities (primarily silicon dioxide), clays and the like.
• the leaching operation is normally conducted at a temperature between about 60°C to the boiling point temperature of the leach solution, preferably between about 100°C and 110oC.
• the leaching is normally done in a continuous mode, rather than a batch mode, with a residence time between about 1 minute and about 3 hours, preferably for about 4 to about 10 minutes.
• the hot leachant solution 20 containing the desired +4 valent Group IVb transition metal-alkali metal-fluoride salt is passed to a crystallization zone 22 wherein the leachant is cooled to a temperature from about 60°C to about 25°C, preferably between about 27oC and 33oC, wherein a portion, generally less than 10% by weight, of the desired +4 valent Group IVb transition metal-alkali metalfluoride salt is crystallized out and substantially all of the silicon impurities are precipitated out.
• the exhaust gases are passed through a conventional gas scrubber (H 2 SiF 6 Production Zone) 32 wherein the gases are washed with water to capture the silicon tetrafluoride to produce fluosilicic acid and silicon dioxide as shown in Reaction (14). Silicon dioxide solids are separated from the fluosilicic acid.
• the fluosilicic acid 34 is passed to a sodium fluosilicate production zone 36, wherein the fluosilicic acid is reacted with sodium chloride 38 or other alkali metal chloride to form sodium or other alkali metal fluosilicate and an aqueous hydrochloric acid solution, as shown in Reaction (15).
• the hot exhaust gas containing the silicon tetrafluoride is rapidly cooled by the scrubber water, thus eliminating the need to cool the exhaust gas before scrubbing.
• the exhaust gas can be pre ⁇ ooled before scrubbing, if desired.
• the amount of fluosilicic acid produced in the fluosilicic acid production zone is insufficient to produce the required amount of alkali metal fluosilicate needed in the fluorination zone. Accordingly, and preferably, additional fluosilicic acid 40 is added to the alkali metal fluosilicate production zone to produce at least 1.6 moles of alkali metal fluosilicate for each mole of Group IVb transition metal oxide introduced into the fluorination zone.
• the alkali metal fluosilicate produced in the alkali fluosilicate production zone which is sparingly soluble in the aqueous hydrochloric acid solution, is separated from the hydrochloric acid solution, preferably after the entire mixture is cooled to a temperature of at least 30°C.
• the separated alkali metal fluosilicate is dried and mixed with the ore and additives as described above, to form the feed mixture for the fluorination zone.
• the hydrochloric acid solution 42 after separation, is introduced into the leach zone to leach and convert the +3 and +4 valent Group IVb transition metal-alkali metal-fluoride values as described above.
• the leached solution contains less than 10% by weight HCl, preferably from about 1% to about
• Fig. 2 shows a preferred embodiment of the present process to yield a +4 valent Group IVb transitionmetal-alkali metal-fluoride salt of enhanced purity with a very high salt recovery.
• the leached zone 18 illustrated in Fig. 2 is operated in the same manner as the leached zone illustrated in Fig. 1.
• the leachant 20 is passed to a first crystallization zone 50 wherein the leachant is cooled to a temperature between about 25oC and about 60oC, preferably between about 27°C and about 33oC.
• a portion of the salt product and substantially all the silicon impurities are dropped out as insoluble solids 52 and separated from the cooled leachant.
• the separated solids are recycled back to the leach zone to recover the +4 valent Group IVb transition metal-alkali metal-fluoride salt, which is solubilized in the leachant.
• Some of the silicon impurities are also solubilized by the leachant; however, since the leachant in the leach zone is substantially saturated with silicon impurities, most of the silicon impurities from the first crystallization zone drop out with the other insolubles 54 in the leach zone. Thus, recycling of the silicon impurities from the first crystallization zone back to the leach zone does not result in a concentration or build-up of silicon impurities in the leachant.
• the leachant 54 from the first crystallization zone is passed to a second crystallization zone 56 where the leachant is cooled to a temperature of between about -10o C and about 30oC, preferably from about -10°C to about 0°C, to substantially drop out all the +4 valent Group IVb transition metal-alkali metal-fluoride salt.
• the rate of crystallization can be enhanced by salting the mother liquor in the second crystallization zone with sodium chloride 58.
• Small amounts of hydrofluoric acid can also be added to improve yields.
• hydrofluoric acid is relatively expensive and quite corrosive. Its use can be minimized or omitted by operating the leach step under optimum conditions.
• the salting out is carried out by adding between about 50 and about 150 grams of sodium chloride per liter of leachant. From 0 to about 10 grams of hydrofluoric acid (49% solution) per liter of leachant can be added to improve titanium yield.
• the crystallized solids 58, containing the desired fluoride salt product and substantially free of silicon impurities, are separated from the leachant of the second crystallization zone and passed to multi-stage counter-current wash zone 60.
• the spent leachant 62 can be disposed of in an environmentally acceptable manner or can be recycled as the salting-out medium for the sodium silicofluoride crystallization zone.
• the +4 valent Group IVb transition metal-alkali metal-fluoride salts are washed in a counter-current manner in one or more stage (s) with dilute hydrogen chloride solution to remove substantially all the iron and manganese and other minor impurities soluble in dilute hydrochloric acid.
• the crystals are washed with at least an equal weight amount of dilute HCl.
• the dilute hydrochloric acid contains less than 5% by weight HCl, preferably less than 2% by weight.
• the washing is done at a temperature of less than 30°C, preferably at ambient temperature.
• a single washing of the salt with the dilute hydrochloric acid solution reduces the iron content and manganese content in the salt product to less than 100 parts per million (ppm).
• the wash liquid with the soluble impurities is disposed of by conventional environmentally sound procedures.
• the salt is dried to remove moisture.
• the salt can then be reduced by conventional methods to produce titanium metal or advantageously reduced with an aluminum and zinc molten alloy, as taught in U.S. Patent Nos. 4,390,365 and 4,468,248, to produce a Group IVb metal-zinc molten alloy.
• the molten alloy after the separation of the insoluble impurities, can be converted to a pure Group IVb metal by subliming off the zinc, preferably under a vacuum or with the sweep of an inert gas stream, such as an argon gas stream.
• the dried crystals of the fluorides of the Group IVb metal can be reduced in a reducing zone where they are contacted in a molten state with a molten zinc-aluminum alloy.
• the alloy may comprise from 1:99 to 20:80 parts by weight of aluminum to zinc.
• the molten salt and the alloy are mutually immiscible; therefore, agitation of the molten salt and molten alloy to insure intimate contact is preferred.
• Reduction takes place at a temperature from at least 650oC to about 1000°C, preferably from about 850oC to about 950°C.
• the time of contacting the molten alloy and the molten salt can be varied to assure that the Group IVb metal present in the salt is converted into a Group IVb metal-zinc alloy.
• the aluminum present in the aluminumzinc alloy during the course of the reduction is converted into the corresponding fluoride salt and may be isolated as the pseudocryolite.
• the reduced mixture separates with the molten salt rising to the top, where it maybe decanted from the molten alloy.
• the molten Group IVb metal-zinc alloy may be separated from the bottom of the vessel and passed to a reducing zone wherein the zinc is separated from the Group IVb metal.
• the removal or reduction of the zinc must take place under inert conditions because of the Group IVb metal's propensity to pick up oxygen and nitrogen.
• an argon atmosphere is present during the sublimation step or the sublimation is carried out under a vacuum.
• a suitable vessel for carrying out the sublimation, as well as other of the various high temperature operations described herein, is a graphite-lined reactor. It has been found that during the salt reduction step, it is advisable that both the salt and alloy phases be maintained above the licruidous temperature to avoid solids formation, which can be abrasive to both the agitator and the reactor walls.
• the salt reduction step can take place under an elevated pressure to raise the boiling point temperature of the Group IVb metal-zinc molten alloy, which enhances the solubility of the Group IVb metal into molten zinc.
• a titanium-zinc alloy boils at approximately 915oC. At that temperature, only about 15% by weight titanium metal can be dissolved into molten zinc before solids formation.
• the molten titanium-zinc alloy boils at about 950oC and approximately 25% by weight titanium metal can be dissolved in the zinc before the onset of solids formation.
• the principal limitation of the reduction temperature is dictated by the materials of construction of the reactant vessel. For example, at temperatures approaching 1000oC, there is a substantial increase in contamination of the Group IVb metal by carbon from the graphite reactor wall. The degree of carbon contamination becomes severe above 1100°C.
• the sublimation is carried out under vacuum or inert gas atmosphere, such as argon gas atmosphere, to prevent oxygen and nitrogen contamination of the Group IVb metal.
• the zinc is distilled off at a temperature from about 800°C to about 1000°C to leave a
• the Group IVb metal sponge may be sintered to reduce its surface area. After sintering and cooling, the sponge can be pacified by exposure to a controlled amount of oxygen-containing gas to give a thin (monomolecular) protective coating of the Group IVb transition metal oxide thereon before the sponge is exposed to the air or other non-inert atmosphere.
• the sublimed zinc can be recovered and recycled for use in subsequent salt reduction steps.
• Group IVb metal scrap contaminated with impurities soluble in molten Group IVb metal and molten zinc is added to a molten aluminum-zinc alloy in a zinc alloy zone 70 to dissolve the metal scrap.
• a Group IVb transition metal fluoride salt is added to the zinc alloy zone 70, which is reduced by the molten aluminum metal to a Group IVb metal which reports to the zinc to dilute the soluble impurities in the molten Group IVb metal-zinc alloy.
• the aluminum is reduced to form an aluminum fluoride salt insoluble in the molten Group IVb metal-zinc alloy. The reduction is preferably conducted with agitation to insure intimate contact between the molten metal salt and the molten aluminum-zinc alloy.
• the zinc and aluminum separately, or as an aluminum-zinc alloy, and the metal salt may be co-melted in the zinc alloying zone.
• the purity of the scrap metal can be increased by diluting the soluble impurities in the metal with pure Group IVb transition metal produced from the metal fluoride in the presence of the scrap so that the resulting metal can meet industrial and/or military specifications.
• Group IVb metal scrap can be added to the zinc alloying zone, wherein the zinc is not alloyed with aluminum, to remove insoluble contaminants, such as carbon, halide salts, alkali metal salts, alkaline earth metal salts, aluminum salts, silica and nitrogen, which are insoluble in molten zinc and molten Group IVb transition metal. These latter contaminants will float to the surface of the Group IVb metal-zinc molten alloy and can be decanted off.
• the metal scrap contains dense insoluble contaminants, such as tungsten carbide, tungsten molybdenum or the like, these contaminants will drop to the bottom of the zinc alloying zone after agitation is complete, and the molten Group IVb metal-zinc molten alloy can be decanted from such dense insoluble contaminants. If the metal scrap contains undesirable amounts of soluble impurities, high purity Group IVb transition metal can be added to the zinc alloying zone to dilute the soluble contaminants in the metal scrap to yield Group IVb transition metal in the metal-zinc molten alloy which will pass industrial and/or military specifications.
• dense insoluble contaminants such as tungsten carbide, tungsten molybdenum or the like
• the Group IVb metal-zinc alloy produced in the zinc alloying zone can be alloyed with alloying agents, such as vanadium, aluminum, nickel and the like, to form a Group IVb transition metal alloy after the removal of the zinc.
• alloying agents such as vanadium, aluminum, nickel and the like
• the metal scrap, the alloying agents, the high purity metal and/or the metal salt can be co-melted with zinc and/or aluminum and zinc.
• the molten alloy is passed to a sublimation zone wherein the zinc is removed by sublimation to yield a Group IVb transition metal sponge.
• the sublimation if conducted under an inert gas atmosphere, can be conducted at a temperature between about 800oC and about 1000°C. If the sublimation is carried out in a vacuum, sublimation can be conducted at a temperature of less than 800°C. To prevent oxygen and nitrogen contamination of the Group IVb metal during sublimation, the sublimation is conducted under an inert gas atmosphere unless it is conducted in a vacuum.
• the metal sponge may be sintered to reduce its surface area and then passified with an oxygen-containing gas in order that the sintered Group IVb metal sponge can be further treated to produce Group IVb metal powder usable for metallurgical powder purposes in accordance with the procedures taught in U.S. Patent Nos. 4,470,847 and 4,595,413.
• the present invention will be further described with reference to the following examples, which are presented for the purposes of illustrating the invention. These examples are not intended as a limitation of the invention.
• Sodium silicofluoride is combined with ilmenite ore containing 1 mole of titanium dioxide and heated in a fluorination zone, opened in kiln 10 to a temperature between about 700oC and 800oC. The sodium silicofluoride decomposes in the kiln to form sodium fluoride and silicon tetrafluoride.
• silicon tetrafluoride 0.75 mole reacts with the titanium dioxide in the ore to form +3 and +4 valent titanium fluoride salts and silicon dioxide.
• the remaining half of the silicon tetrafluoride exits the reactor as silicon tetrafluoride gas in the exhaust gases 30.
• the silicon tetrafluoride in the exhaust gas is passed to the silicon tetrafluoride production zone, where it is scrubbed with water to form 0.25 mole of silicon dioxide and 0.5 mole of fluosilicic acid 34 (approximately a 23% by weight aqueous solution).
• the fluosilicic acid is recycled to the sodium fluosilicate production zone.
• the fluorinated ore 16 is ground and then passed to leach zone
• the fluorinated ore is leached with aqueous hydrochloric acid to convert the +3 and +4 valent titanium fluoride salts to titanium fluoride which is solubilized by the leachate.
• the leachate is separated from the insoluble solids and passed to the crystallization zone 22. In the crystallization zone, the leachate is cooled to form crystals of titanium fluoride and a spent leachate containing soluble impurities of iron, hydrochloric acid, sodium fluoride and the like.
• Example 2 The process of Example 1 can be conducted in a manner to eliminate the loss of silicon tetrafluoride gas in the exhaust gas.
• the ore containing 1 mole of titanium dioxide is compounded with 1.6 moles of sodium fluoride and 0.8 mole of sodium silicofluoride and introduced into the fluorination zone to produce fluorinated ore containing the +3 and +4 valent titanium fluoride salts, sodium fluoride, silicon dioxide impurities, iron oxide and iron salts, and unreacted gangue.
• the fluorinated ore is passed to a leach zone where it is leached with a hydrochloric acid solution obtained from an external source to produce a leachate containing the desired +4 valent sodium fluotitanate to solubilize in the leachate and in the soluble solids which are separated from the leachate.
• the leachate is passed to the crystallization zone to crystallize out the desired sodium fluotitanate product, leaving a spent leachate containing soluble sodium chloride, soluble iron salts, other impurities and excess hydrochloric acid.
• Example 3 The process of Example 1 can be conducted in such a manner as to prevent the loss of silicon tetrafluoride in the exhaust gases by compounding the ilmenite ore which contains 1 mole of titanium dioxide with 1.07 moles of sodium silicofluoride and 1.07 moles of a basic sodium salt, such as sodium carbonate, sodium bicarbonate or sodium hydroxide.
• the feed material is passed through fluorination zone 10, wherein the feed material is heated to form +3 and +4 valent titanium fluoride salts, sodium fluoride, silicon dioxide impurities, iron salts and carbon dioxide or water.
• Fluorinated ore is passed to leach zone 18, wherein the fluorinated ore is leached with hydrochloric acid solution obtained from an external source to solubilize titanium fluoride salts and produce the soluble sodium fluotitanate.
• the leachate is separated from the insoluble solids and passed to a crystallization zone 22, wherein it is cooled to crystallize out the desired sodium fluotitanate.
• EXAMPLE 4 A mixture of finely ground ilmenite ore (36.8 parts by weight), sodium silicofluoride (61.5 parts by weight) and calcined petroleum coke (1.7 parts by weight) was intimately mixed dry. The mixture contained 1.36 molar equivalents of sodium fluosilicate per mole of titanium dioxide. Water (9.4% by weight) was blended into the mixture, and balls having a diameter between +3/8 and -5/8 inch prepared from the mixture in a balling pan. The balls were dried in a tray drier.
• the dried balls (2501.5 grams) were placed in a pot, heated by an indirect heater to a temperature between 750o C and 788° C for a total of 4 hours to insure that the charge was fully reacted with silicon tetrafluoride, and 256 grams of silicon tetrafluoride were metered into the pot over the last two hours of the reaction after the evolution of silicon tetrafluoride from the reaction had essentially ceased. Only a small amount (about 35 grams) of this later-added silicon tetrafluoride was consumed by the charge.
• the fluorinated ore from the reaction was in its original shape, that is, in the shape of balls, but the fluorinated ore had less bulk density because of the weight loss occurring during the reaction.
• the balls were fryable and were ground in a hammer mill to 90% -60 mesh. A small sample of the ground fluorinated ore was leached in two stages with hydrochloric acid (3.6 wt%). Seventy percent of the titanium in the ore was solubilized.
• the remainder of the fluorinated ore was leached in a single stage with an aqueous hydrogen fluoride-hydrogen chloride solution (3 wt% HF and 7 wt% HCl) in a ratio of 1 part by weight fluorinated ore to 5.1 parts by weight of a leaching solution.
• the titanium was solubilized and sodium fluotitanate crystals were recovered from the leachate (63.8 wt%).
• a new ball formulation comprising 33.1 parts by weight ilmenite ore, 65.3 parts by weight sodium fluosilicate and 1.6 parts by weight calcined petroleum coke was prepared in the same manner as in Example 4.
• the mixture was finely ground and formed into balls and heated in accordance with the process of Example 4.
• the fluorinated ore after being ground, was leached with a hydrochloric acid-hydrofluoric acid solution (6 wt% HCl and 5 wt% HF).
• a hydrochloric acid-hydrofluoric acid solution (6 wt% HCl and 5 wt% HF).
• EXAMPLE 6 Another ball formulation was prepared comprising 28.7 parts by weight ilmenite ore, 69.9 parts by weight sodium fluosilicate and 1.4 parts by weight calcined petroleum coke. The mixture was finely ground and formed into balls as described in Example 4, and then heated in accordance with the process of Example 4. The fluorinated ore was ground and leached with an aqueous hydrochloric acid solution (9.5 wt% HCl) and 95% of the titanium was solubilized in the leachate. Nearly 96% of the titanium in the leachate was recovered as sodium fluotitanate crystals by cooling the leachate to -7°C and adding 100 grams of sodium chloride per liter of leachate to salt out the sodium fluotitanate.
• aqueous hydrochloric acid solution 9.5 wt% HCl
• EXAMPLE 7 Wet sodium fluotitanate crystals recovered fr ⁇ mleachates, such as the leachate in Example 6, were washed with successive batches of a 1% by weight hydrochloric acid solution. The iron and magnesium impurities in the sodium fluotitanate crystals were reduced to low levels in the first few washings. The residual level of iron and manganese in the sodium fluotitanate crystals is dependent upon the quality and size of crystals grown which, in turn, depends on the quality of the crystallizer design and operating parameters. To aid in the removal of these impurities, the crystals are preferably grown in small size, which is accomplished by cooling the leachate down quickly to a temperature below 0°C.
• Silicon impurities are not reduced by washing. Since some of the sodium fluotitanate crystals are dissolved during the washing operation, the silicon impurity level increases because the sodium silicofluoride is not dissolved by the wash liquor.
• Fig. 4 The effectiveness of washing the sodium fluotitanate crystals crystallized from leachates, containing about 7% to about 10% by weight hydrogen chloride, with a 1% by weight aqueous hydrochloric acid wash, is shown in Fig. 4.
• the first wash effectively lowers the concentration of iron to below 100 parts per million (Curve A).
• each washing with the 1% by weight aqueous hydrogen chloride wash gradually increases the concentration of sodium silicofluoride impurities in the crystals (Curve B).
• the leachate obtained from leached fluorinated balls of Example 6 was cooled from the leach temperature of 95oC to 35°C in a crystallizer.
• the titanium content in the leachate was reduced from 20.8 to 15.0 grams per liter.
• the silicon content in the leachate was reduced from 0.23 to 0.001 gram per liter.
• the leachate was filtered to remove the precipitated solids containing silicon and titanium.
• the filtered leachate was then cooled to -7°C in a crystallizer, while slowly adding 100 grams of sodium chloride for each liter of leachate and 5.0 grams of hydrofluoric acid (49 wt%) for each liter of leachate.
• Sodium fluotitanate crystals dropped out and the titanium content of the leachate was reduced to 0.3 gram titanium per liter of leachate.
• the sodium fluotitanate crystals were separated from the leachate by filtration and washed with 1% by weight aqueous hydrochloric acid wash as disclosed in Example 7 to remove iron, manganese and other minor impurities soluble in the leachate.
• EXAMPLE 9 The solids recovered from the leachate in Example 8, where the leachate is cooled to 35°C, can be recycled to the initial step of leaching the ore.
• the sodium fluotitanate is solubilized in the leachate with some of the silicon impurities. However, the bulk of the silicon impurities are not solubilized and are removed from the leachate by filtration. Since the leachate, at this stage, is relatively saturated with respect to silicon impurities, this recycling step does not result in a build-up of silicon impurity concentration in the leachate.

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• 1988
  • 1988-03-11 EP EP19880903534 patent/EP0305502A4/de not_active Withdrawn
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