PROCESSING AQUEOUS TITANIUM CHLORIDE SOLUTIONS TO ULTRAFINE TITANIUM DIOXIDE

Description

[0001] The present invention relates to a process for producing ultrafine or "nano-sized" titanium dioxide, parts of the process, and the product of the process. In particular, the present invention relates to the processing of aqueous titanium solutions to ultrafine titanium dioxide. The process includes a novel combination of operational steps to economically produce a high quality ultrafine titanium dioxide.

BACKGROUND OF THE INVENTION

[0002] Titanium dioxide (TiO₂) for pigment use may be present in either of two predominant forms, anatase or rutile, and is used in paints, paper, plastics, ceramics, inks, etc. The titanium dioxide for pigment use generally has an average particle size of 150 to 250 nanometer and is considered the principal while pigment of commerce. It has an exceptionally high refractive index, negligible color and is quite inert. Titanium dioxide having a smaller average particle size, for instance in the 10 to 100 nanometer median particle size range, is used commercially in cosmetics and personal care products, plastics, surface coatings, self-cleaning surfaces, and photovoltaic applications. This titanium dioxide is referred to as ultrafine or nano-sized titanium dioxide.

[0003] There are several processes for making ultrafine titanium dioxide, some in commercial use and some in development. Some use anhydrous titanium tetrachloride as a feedstock and burn it in an oxygen-hydrogen flame or in a plasma arc.

[0004] GB-481,892 describes a process for the production of titanium dioxide having the rutile crystalline structure comprising hydrolysing an aqueous solution of titanium tetrachloride and calcining the precipitate wherein the hydrolytic precipitation is effected in the presence of negative divalent ions derived from a substance added to the solution in a proportion not exceeding O-1 mol to each mol of titanium reckoned as TiO₂ present in the hydrolysis solution, and wherein the hydrolytic precipitation is effected in the absence of any added insoluble material suspended in the solution.

[0005] Another process uses a titanyl sulphate solution as the feedstock. In this process, titanium dioxide is precipitated from the feedstock in a controlled manner, followed sequentially by calcination and intense steam micronization to break up agglomerates formed during the calcination step.

[0006] Both types of processes, however, suffer from a lack of control over the product particle size distribution, as well as the product mineralogy. In other words although the average particle size of the titanium dioxide may be from 10 to 100 nanometer, the particle size distribution may vary greatly. Furthermore, the titanyl sulfate process produces an anatase form whereas the anhydrous chloride oxidation produces a rutile crystallographic modification. Therefore, there is a need for a process to economically make ultrafine titanium dioxide having a narrow particle size distribution and having a specific crystallographic modification made by an easily controlled, lower cost, environmentally sound route.

SUMMARY OF THE INVENTION

[0007] The present invention provides an economical hydrometallurgical process for producing ultrafine TiO₂ from aqueous titanium chloride solutions. The solutions may be derived from any of several sources, but are practically derived from processing mineral ores and, in particular, ilmenite ore or an itmenite mineral concentrate. The processing to produce the solutions can be, for instance, a leaching or dissolution process, followed by any of several means for solution purification. The solution could also be produced by hydration of anhydrous titanium tetrachloride. The solution may also be derived from the process described in U.S. 6,375,923.

[0008] The aqueous titanium chloride solution is generally comprised of water, hydrochloric acid, titanium oxychlorides and titanium chlorides. The solutions may vary widely in composition with respect to the hydrochloric acid content and the titanium content.

[0009] Minor quantities of any of several chemical control agents may be introduced into the solution. The solution is further converted to a titanium oxide solid in a process involving total, controlled evaporation of the solution and the formation of a thin film of titanium dioxide. This process is conducted above the boiling point of the solution and below the temperature where there is significant crystal growth. The water and hydrochloric acid are vaporized and the hydrochloric acid may be recovered.

[0010] The titanium oxide is next calcined at an elevated temperature to induce and control crystallization. The concentration and type of chemical control agent as well as the calcination conditions determine the desired crystalline form and crystal size of the ultrafine titanium dioxide.

[0011] Following calcination, the titanium dioxide is milled or dispersed to yield a final nano-sized or ultrafine titanium dioxide having a narrow particle size distribution.

[0012] The advantages of the process according to the present invention include a superior high quality ultrafine titanium dioxide due to the narrow particle size distribution and readily controlled physical and chemical characteristics, and low cost processing.

[0013] Thus, the present invention provides a process for producing nano-sized titanium dioxide according to claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] 

FIG. 1 is a flow sheet of the general aspect of the process according to the present invention.

FIG. 2 is a flow sheet of one embodiment of the process according to the present invention, including vacuum evaporation and recycling of gases.

FIG. 3 is a scanning electron micrograph of a spherical particle of unmilled nano-sized titanium dioxide produced according to the method of the present invention and which is magnified 70,000 fold. An amount of 0.16% phosphoric acid was added to the solution as a chemical control.

FIG. 4 is a scanning electron micrograph showing the cross-section of a shell of the material depicted in FIG. 3. It is magnified 140,000 fold.

FIG. 5 is a scanning electron micrograph of unmilled nano-sized titanium dioxide produced according to the method of the present invention and which is magnified 100,000 fold. An amount of 0.32 % phosphoric acid was added as a chemical control.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is a hydrometallurgical process for producing ultra-fine or "nano-sized" grade TiO₂ from an aqueous titanium chloride solution. Such solutions may be derived from any of several sources including anhydrous titanium chloride, but are preferably derived from the processing of titaniferous ores using hydrochloric acid. Referring now to FIG. 1, the general process according to the present invention is shown.

Titanium Chloride Solutions

[0016] The aqueous titanium chloride solution is comprised of water, hydrochloric acid, titanium oxychlorides and titanium chlorides. The solution may vary widely in composition with the respect to the hydrochloric acid content and the titanium content. For example, the content of the feed solution may vary from about 3 wt. percent hydrochloric acid to as high as 40 wt. percent HCl and the titanium content may vary from as low as 30 grams of titanium per liter to as high as 200 grams titanium per liter.

[0017] The source of the titanium chloride solution may be from any known process in which chloride is used during the processing of titanium containing material. For example, the titanium chloride solution may be derived from the processing of titaniferous ores using hydrochloric acid. The solution may also be produced by hydration of anhydrous TiCl₄. Additionally, the source of the titanium chloride may be from those described in U.S. 6,375,923 U.S. 6,440,383.

[0018] Minor quantities of chemical control agents may be introduced into the titanium chloride solution to control the physical and mineralogical characteristics of the solid titanium dioxide product resulting from the conversion of the solutions. These chemical control and seeding agents can be, but are not limited to, the chloride salts of sodium, potassium, aluminum, tin, and zinc. Carbonate, fluoride, sulfate and other suitable salts of the same aforementioned elements may be used. Additionally, phosphoric acid and phosphate salts of the aforementioned elements or of a metal may be used. Furthermore, the chemical control agent may be selected from a number of organic additives that may be used alone or in conjunction with chemical control agents described above. The organic additives include, but are not limited to: organic acids such oxalic, citric, stearic, etc.; salts from these organic acids and inorganic compounds; other organic additives, such as polyacrylates, glycols, siloxane and their compounds.

Vacuum Evaporation

[0019] Optionally, the titanium chloride feed solutions to the process, before or after the chemical control and seeding agents are added, may be concentrated with respect to titanium content by any of several methods. A preferred method is by vacuum evaporation, as is shown in FIG. 2. The solutions can be vacuum evaporated under conditions to inhibit formation of titanium dioxide while removing excess water and HCl as vapors.

Hydrolysis

[0020] The titanium chloride solutions containing the chemical control agents are converted to a titanium oxide solid in a process involving total evaporation of the solution and the formation of a thin film of titanium dioxide. This process is conducted at a temperature above the boiling point of the solution and below the temperature where there is significant crystallization. The process is also conducted at a temperature above the boiling point of the solution and below the calcinations temperature of the product. The hydrolysis successively evaporates part of the solution, hydrolyzes the titanium in solution to form titanium dioxide, and evaporates the remainder of the solution. Water and hydrochloric acid gas are vaporized and the hydrochloric acid may be recovered by any known process.

[0021] The evaporation, hydrolysis and drying are performed in a manner to control the physical form of the product oxide. Preferably, hydrolysis is accomplished by spraying the solution while it is heated at a temperature in the range from 120°C. to 350°C., and most preferably in the range from 200°C. to 250°C. This process is called spray hydrolysis. Spray hydrolysis may be effected in a spray dryer.

[0022] Through control of the operating parameters, including temperature and chemical composition of the feed solutions to the spray hydrolysis operation, the resulting physical and chemical characteristics of the solid titanium product can be reliably controlled within a fairly narrow range. For example, the titanium oxide resulting from spray hydrolysis in a spray dryer will be composed of hollow, thin-filmed spheres or parts of spheres. The dimensions of the spheres may vary over a wide range, from less than 1 µm to 100 µm or more in diameter, and a shell thickness in the range from 30 nanometer to 1000 nanometer or more.

[0023] The structure of the shell consists of an amorphous, hydrated, polymeric titanium oxide containing some residual chloride.

[0024] Without being bound by any theory, it is believed that spray hydrolysis in the given conditions yields spheres composed of a thin film of an amorphous solid that can readily be converted to nano-sized rutile, nano-sized anatase titanium dioxide, or mixtures thereof, depending on the types of chemical controls introduced in the titanium chloride feed solutions and on the physical parameters used in the spray hydrolysis operation. Spray hydrolysis also has the advantage of direct processing of the solution so that the formation of titanium dioxide and evaporation of water and HCl are simultaneously accomplished.

Calcination

[0025] The titanium oxide product resulting from the spray hydrolysis operation is calcined at a temperature and for a length of time sufficient to convert the amorphous oxide to titanium dioxide of the desired crystal structure, namely ultrafine titanium dioxide. The temperature is low enough and the time is sufficiently short to limit excessive crystal growth. Calcination temperatures can range between 450°C to over 1100°C. Preferably, the calcination is conducted at temperatures ranging from 600° C to 900° C. The calcination time also varies over a wide range, from about 20 minutes to as long as 80 hours. Preferably, the calcination time is in the range from about 30 minutes to about 8 hours. Lower temperatures will require longer calcination times. The product of calcination is a thin film showing a structure of individual units that can be broken up by milling into particles of the desired median size and size distribution.

[0026] During calcination, any residual chloride in the amorphous oxide feed is expelled as HCl gas, which can be recovered.

[0027] Additionally and optionally, chemical control agents may be added to the amorphous oxide just prior to calcination to promote and control conversion of the oxide to the desired crystal structure and other physical characteristics such as crystal size and millability. These chemical control agents can be but are not limited to the chloride salts of sodium, potassium, aluminum, tin, and zinc. Carbonate, fluoride, sulfate and other suitable salts of the same aforementioned elements may be used. Additionally, phosphoric acid and phosphate salts of the aforementioned elements or of a metal may be used.

[0028] Alternatively, phosphoric acid and phosphate salts of the aforementioned elements or of a metal may be used. Furthermore, the chemical control agent may be selected from a number of organic additives that may be used alone or in conjunction with chemical control agents described above. The organic additives include, but are not limited to: organic acids such oxalic, citric, stearic, etc.; salts from these organic acids and inorganic compounds; other organic additives, such as polyacrylates, glycols, siloxane and their compounds.

Milling and Finishing

[0029] After calcination, the titanium dioxide consists of more than 99.5% of either ultrafine ("nano-sized") anatase or ultrafine ("nano-sized") rutile, depending on the conditions and chemical control agents used in spray hydrolysis and calcination. After calcination, the titanium dioxide is milled and dispersed to break up the thin film into individual particles. Finishing steps may be added as required.

[0030] The following examples illustrate, but do not limit, the present invention. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

[0031] A solution containing 30 g/l Ti and 210 g/l Cl was made according to the process described in U.S. Patent Application Serial No. 60/141,114. Phosphoric acid was added in an amount equal to 0.08 wt-% of the amount of solution. The solution was fed to a spray dryer, consisting of a reaction chamber followed by bag filters and an HCl absorption system. The solution was injected at a rate of 2.25 liters/min through an atomizing disk. Gases from the combustion of natural gas, diluted with air to 550°C were also injected around the disk. The outlet temperature was 250°C, and the total gas flow rate was about 800 scfm. Reactor off gasses were sent to a bag filter to collect the TiO₂ product. The recovered product was further calcined at 920°C for 1 hour.

[0032] Scanning electron micrographs of the product after calcination show spheres with an elemental particle size of the order of 50 nanometer and a film thickness of the order of 1000 nanometer.

EXAMPLE 2

[0033] A solution containing 30 g/l Ti and 136 g/l Cl was made by dissolving anhydrous liquid TiCl₄ in 3 M HCl and further diluting with water to reach to required titanium concentration. Phosphoric acid was added in an amount equal to 0.16% of the weight of solution. The solution was fed to a spray dryer under the same conditions as those mentioned in Example 1. The product recovered on the bag filter was calcined at 800°C for 6 hours.

[0034] FIG. 3 is a scanning electron micrograph of the calcined product magnified 70,000 fold. The diameter of the particle shown is about 1.2 µm. Fig. 4 is a scanning electron micrograph of the same product, showing the edge of a shell. The shell thickness is about 350 nm and elemental particles have a size of about 40 nm. The product is subsequently milled to liberate the individual particles. The milled product has a specific surface area measured by the BET method of 33 m²/gram.

EXAMPLE 3

[0035] In the same solution used in Example 2, phosphoric acid was added in an amount equal to 0.32% of the weight of solution. The solution was fed to a spray dryer and the product was calcined under the same conditions as those mentioned in Example 1.

[0036] FIG. 5 is a scanning electron micrograph, magnified 100,000 fold, of the section of a shell after calcination. Individual particles are about 30 nanometer in diameter. The shell thickness is about 400 nm. The product is subsequently milled to liberate the individual particles. The specific surface area of the milled product as measured by the BET method is 43 m²/gram.

EXAMPLE 4

[0037] A weight of oxalic acid equal to 1 % of the amount of solution was added to a solution of Ti chloride in hydrochloric acid containing 50 g/l Ti and 230 g/l total chloride. The solution was fed to a spray hydrolyzer under the same conditions as those given in Example 3. While the product after spray hydrolysis had a brown-gray-buff color, due to the presence of some organic residue, the subsequent calcination step reduced the residual carbon content to very low levels as the calcined product had a pure white appearance. The conditions of calcination were the same as those mentioned in Example 3. The calcined product showed a structure of relatively large rutile crystals that were more easily millable than those of the previous example. After milling in a media mill, a product with a median particle size of 50 nm was obtained.

Claims

1. A process for producing nano-sized titanium dioxide from a titanium chloride solution comprising:

a. adding a chemical control agent to the titanium chloride solution to form a feed solution;

b. hydrolyzing the feed solution to form titanium oxide particles in a controlled temperature, evaporation process at a temperature above the boiling point of the solution and up to 350°C, wherein the hydrolysing is effected by spray drying;

c. calcining the hydrolyzed product to form titanium dioxide; and

d. milling the calcined product to liberate elemental nano-sized particles.

2. The process of claim 1 wherein the hydrolyzing step successively evaporates part of the solution, hydrolyzes the titanium in solution to form TiO₂, and evaporates the remainder of the solution.

3. The process of claim 1 wherein during the hydrolyzing step, hydrochloric acid is formed and water is removed.

4. The process of claim 3 wherein the hydrochloric acid and water are recovered.

5. The process of claim 1 wherein the spray dryer temperature is between 120°C and 350°C.

6. The process of claim 5 wherein the spray dryer temperature is between 200°C and 250°C.

7. The process of claim 1 wherein the chemical control agent is a salt wherein the cation is selected from sodium, potassium, aluminum, tin and zinc and wherein the anion is selected from chloride, carbonate, fluoride, sulfate, and phosphate.

8. The process of claim 1 wherein the chemical control agent is selected from the group consisting of polyacrylates, glycols, siloxanes, organic acids, lithium, sodium, potassium, and ammonium salts of the organic acids comprising oxalic acid, citric acid and stearic acid and mixture thereof.

9. The process of claim 1 wherein the chemical control agent is phosphoric acid.

10. The process of claim 1 where a thin film of amorphous titanium oxide is formed during the hydrolysis process.

11. The process of claim 10 wherein the amorphous titanium oxide comprises hollow spheres or parts of spheres.

12. The process of claim 10 wherein the thin film has a thickness of 30 nanometer to 1000 nanometer.

13. The process of claim 10 wherein the amorphous titanium oxide comprises thin film membrane hollow spheres or parts of spheres having a diameter of between 1 and 100 µm and a membrane thickness of between 30 nanometer and 1000 nanometer.

14. The process of claim 1 wherein the feed solution is first concentrated by vacuum evaporation under conditions to avoid hydrolysis of the titanium chloride.

15. The process of claim 1 wherein the calcining is conducted at a temperature between 450°C and 1100°C.

16. The process of claim 1 wherein the calcining is conducted at a temperature between 600°C and 900°C.

17. The process of claim 1 wherein the titanium dioxide particles have a particle size less than 100 nanometer.

Ansprüche

1. Verfahren zur Herstellung von Titandioxid im Nanogrößenbereich aus einer Titanchloridlösung, umfassend:

a) Zufügen eines chemischen Kontrollmittels zur Titanchloridlösung, um eine Zugabelösung zu bilden;

b) Hydrolysieren der Zugabelösung, um Titanoxidteilchen in einer kontrollierten Temperatur zu bilden, Verdampfungsverfahren bei einer Temperatur oberhalb des Siedepunkts der Lösung und bis zu 350°C. worin die Hydrolyse durch Sprühtrocknen veranlasst wird:

c) Calcinieren des hydrolysierten Produktes, um Titandioxid zu bilden; und

d) Mahlen des calcinierten Produktes, um elementare Teilchen im Nanogrößenbereich freizusetzen.

2. Verfahren nach Anspruch 1, worin der Hydrolyseschritt sukzessiv Teile der Lösung verdampft, das Titan in Lösung hydrolysiert, um TiO₂ zu bilden, und den Rest der Lösung verdampft.

3. Verfahren nach Anspruch 1, worin sich während des Hydrolyseschrittes Salzsäure bildet und Wasser entfernt wird.

4. Verfahren nach Anspruch 3, worin die Salzsäure und Wasser zurückgewonnen werden.

5. Verfahren nach Anspruch 1, worin die Temperatur des Sprühtrockners zwischen 120°C und 350°C liegt.

6. Verfahren nach Anspruch 5, worin die Temperatur des Sprühtrockners zwischen 200°C und 250°C liegt

7. Verfahren nach Anspruch 1, worin das chemische Kontrollmittel ein Salz ist, worin das Kation ausgewählt ist aus Natrium, Kalium, Aluminium, Zinn und Zink und worin das Anion ausgewählt ist aus Chlorid. Carbonat, Fluorid, Sulfat und Phosphat

8. Verfahren nach Anspruch 1. worin das chemische Kontrollmittel ausgewählt ist aus der Gruppe bestehend aus Polyacrylaten, Glykolen, Siloxanen, organischen Säuren, Lithium-, Natrium-, Kalium- und Ammoniumsalzen der organischen Säuren, umfassend Oxalsäure, Citronensäure und Stearinsäure, und Mischungen daraus.

9. Verfahren nach Anspruch 1, worin das chemische Kontrollmittel Phosphorsäure ist.

10. Verfahren nach Anspruch 1, worin ein dünner Film aus amorphen Titanoxid während des Hydrolyseverfahrens gebildet wird.

11. Verfahren nach Anspruch 10, worin das amorphe Titandioxid hohle Kugeln oder Teile von Kugeln umfasst.

12. Verfahren nach Anspruch 10, worin der dünne Film eine Dicke von 30 Nanometer bis 1000 Nanometer hat.

13. Verfahren nach Anspruch 10, worin das amorphe Titanoxid hohle Kugeln oder Teile von Kugeln mit einer dünnen Filmmembran mit einem Durchmesser zwischen 1 und 100 µm und einer Membrandicke zwischen 30 Nanometer und 1000 Nanometer umfasst.

14. Verfahren nach Anspruch 1, worin die Zugabelösung zunächst mittels Vakuumverdampfung unter Bedingungen konzentriert wird, bei denen die Hydrolyse des Titanchlorids vermieden wird.

15. Verfahren nach Anspruch 1, worin das Calcinieren bei einer Temperatur zwischen 450°C und 1100°C durchgeführt wird.

16. Verfahren nach Anspruch 1, worin das Calcinieren bei einer Temperatur zwischen 600°C und 900°C durchgeführt wird.

17. Verfahren nach Anspruch 1, worin die Titandioxidteilchen eine Teilchengröße von weniger als 100 Nanometer aufweisen.

Revendications

1. Procédé de production de dioxyde de titane ayant une taille de l'ordre du nanomètre à partir d'une solution de chlorure de titane comprenant :

a. l'addition d'un agent de contrôle chimique à une solution de chlorure de titane pour former une solution d'alimentation ;

b. l'hydrolyse de la solution d'alimentation pour former des particules d'oxyde de titane dans un procédé d'évaporation à une température contrôlée, à une température supérieure au point d'ébullition de la solution et jusqu'à 350°C, où l'hydrolyse est réalisée par séchage par atomisation ;

c. la calcination du produit hydrolysé pour former du dioxyde de titane ; et

d. le broyage du produit calciné pour libérer des particules élémentaires ayant une taille de l'ordre du nanomètre.

2. Procédé selon la revendication 1, dans lequel l'étape d'hydrolyse évapore successivement une partie de la solution, hydrolyse le titane dans la solution pour former du TiO₂ et évapore le reste de la solution.

3. Procédé selon la revendication 1, dans lequel pendant l'étape d'hydrolyse, de l'acide chlorhydrique est formé et de l'eau est éliminée.

4. Procédé selon la revendication 3, dans lequel l'acide chlorhydrique et l'eau sont éliminés.

5. Procédé selon la revendication 1, dans lequel la température du sécheur par atomisation est comprise entre 120°C et 350°C.

6. Procédé selon la revendication 5, dans lequel la température du sécheur par atomisation est comprise entre 200°C et 250°C.

7. Procédé selon la revendication 1, dans lequel l'agent de contrôle chimique est un sel dans lequel le cation est choisi parmi le sodium, le potassium, l'aluminium, l'étain et le zinc, et dans lequel l'anion est choisi parmi le chlorure, le carbonate, le fluorure, le sulfate et le phosphate.

8. Procédé selon la revendication 1, dans lequel l'agent de contrôle chimique est choisi dans le groupe constitué par les polyacrylates, les glycols, les siloxanes, les acides organiques, les sels de lithium, de sodium, de potassium et d'ammonium des acides organiques comprenant l'acide oxalique, l'acide citrique et l'acide stéarique et un mélange de ceux-ci.

9. Procédé selon la revendication 1, dans lequel l'agent de contrôle chimique est l'acide phosphorique.

10. Procédé selon la revendication 1, dans lequel un film mince d'oxyde de titane amorphe est formé pendant le procédé d'hydrolyse.

11. Procédé selon la revendication 10, dans lequel l'oxyde de titane amorphe comprend des sphères creuses ou des parties de sphères.

12. Procédé selon la revendication 10, dans lequel le film mince a une épaisseur de 30 nanomètres à 1000 nanomètres.

13. Procédé selon la revendication 10, dans lequel l'oxyde de titane amorphe comprend des sphères creuses à membrane en film mince ou des parties de sphères ayant un diamètre compris entre 1 et 100 µm et une épaisseur de membrane comprise entre 30 nanomètres et 1000 nanomètres.

14. Procédé selon la revendication 1, dans lequel la solution d'alimentation est d'abord concentrée par évaporation sous vide dans des conditions permettant d'éviter l'hydrolyse du chlorure de titane.

15. Procédé selon la revendication 1, dans lequel la calcination est réalisée à une température comprise entre 450°C et 1100°C.

16. Procédé selon la revendication 1, dans lequel la calcination est réalisée à une température comprise entre 600°C et 900°C.

17. Procédé selon la revendication 1, dans lequel les particules de dioxyde de titane ont une taille de particule inférieure à 100 nanomètres.

Drawing