EP0674025B1 - Electrochemical process for producing aqueous titanylnitrate solutions with a low chloride content - Google Patents

Abstract

Prodn of aq TiO(NO3)2 solns contg little chloride comprises electrolysis of TiCl4 or TiOCl2 in the presence HNO3 at an anode potential of 1.1-1.7 V, giving a prod contg less than 200 ppm residual chloride.

Description

The application relates to an electrochemical process for the production of low-chloride aqueous solutions of titanyl nitrate (titanium oxide nitrate, TiO (NO ₃ ) ₂ ) which have a chloride content of less than 200 ppm.

Soluble titanium compounds enjoy a high level of interest in chemistry and related technical fields. For example, they can be used as versatile reagents in chemical synthesis and analysis. Of particular importance is also the use of these compounds for the deposition of titanium dioxide in or from solutions, for example in the form of sols and gels, of finely divided powders, as thin coatings on any substrates, such as on glass for optical or decorative purposes or in Production of pearlescent pigments based on mica coated with TiO ₂ . In the field of high-performance ceramics, the element titanium plays a widespread and often essential role in functional ceramics, especially in electrical and piezo ceramics.

For the applications mentioned, titanyl nitrate or aqueous titanyl nitrate solution would be the titanium-supplying component of choice.

Other soluble or liquid titanium compounds such as titanyl sulfate (TiOSO ₄ ), titanium tetrachloride and titanyl chloride (TiOCl ₂ ) are already unsuitable as such for ceramic purposes. Organic titanium compounds, such as titanium orthoesters, are expensive. All of these have in common that they are very unstable and problematic to handle due to their high sensitivity to hydrolysis.

The key substance for practically all essential titanium compounds is titanium tetrachloride, which is made from titanium dioxide. The latter is obtained from naturally occurring minerals.

Surprisingly, the relevant specialist literature offers hardly any indications for a practical and possibly even technically feasible production of titanyl nitrate or its aqueous solution.

oder $${\text{TiOCl}}_{\text{2}}{\text{+ 2 HNO}}_{\text{3}}{\text{→ TiO(NO}}_{\text{3}}{\text{)}}_{\text{2}}\text{+ 2 HCl}$$

in wäßriger Lösung herzustellen.Theoretically, it should be possible to produce titanyl nitrate from titanium tetrachloride or its partial hydrolysis product titanyl chloride by reaction with nitric acid according to the formula $${\text{TiCl}}_{\text{4th}}{\text{+ 2 ENT}}_{\text{3rd}}{\text{+ H}}_{\text{2nd}}{\text{O → TiO (NO}}_{\text{3rd}}{\text{)}}_{\text{2nd}}\text{+ 4 HCl}$$

or $${\text{TiOCl}}_{\text{2nd}}{\text{+ 2 ENT}}_{\text{3rd}}{\text{→ TiO (NO}}_{\text{3rd}}{\text{)}}_{\text{2nd}}\text{+ 2 HCl}$$

to produce in aqueous solution.

In practice, however, reactions based on these reactions do not lead to the goal, since at least partial hydrolysis begins, usually already during the reaction. It is practically impossible to completely dissolve a titanium dioxide or the TiO ₂ hydrogel once it has precipitated. Regardless of this, it is not possible to completely remove the hydrochloric acid formed in the reaction from the reaction solution. The attempt to drive them out, for example by heating or passing inert gas through, remains incomplete and likewise leads to TiO ₂ precipitations. In principle, the conceivable precipitation as silver chloride is impractical for economic reasons, even for lower residual chloride contents.

A chloride content is extremely undesirable in high-temperature solid-state reactions, such as the sintering of ceramics or the calcination of TiO ₂ coatings. Metal chlorides are known to be extremely volatile at high temperatures. Even very small amounts of chloride in ceramic compositions for high-performance ceramics therefore have the effect that changes in composition occur during sintering and, for example, the contents of dopants change dramatically.

About 200 ppm, based on titanyl nitrate, can be regarded as an acceptable limit for a residual chloride content which can still be tolerated here.

DE 41 10 685 A1 describes a process for preparing low-chloride aqueous solutions of titanyl nitrate by reacting titanium tetrachloride or titanyl chloride with nitric acid, in which reaction is carried out in the presence of excess nitric acid and / or hydrogen peroxide, as a result of which the chloride content is oxidized to chlorine and to obtain a product with a residual chloride content of less than 200 ppm.

However, this method, which in itself leads to an excellent result, has some disadvantages which are unpleasantly noticeable in practical and in particular technical implementation. On the one hand, this process requires the handling of concentrated, especially fuming nitric acid and highly concentrated hydrogen peroxide. These chemicals are known to be extremely dangerous. Transport, storage and use require the strictest security measures. On the other hand, in addition to chlorine gas, the reaction also produces large amounts of nitrous gases that have to be trapped and rendered harmless. Furthermore, the end point of the reaction at which the desired low chloride content is reached is difficult to determine if one does not want to work with large excesses of nitric acid or hydrogen peroxide.

It has now been found that low-chloride aqueous solutions of titanyl nitrate can be obtained by subjecting titanium tetrachloride or titanyl chloride to electrolysis in the presence of nitric acid, in which the potential of the anode is between 1.1 and 1.7 volts, based on the saturated Ag / AgCl reference electrode.

The invention thus relates to a process for the preparation of low-chloride aqueous solutions of titanyl nitrate, in which titanium tetrachloride or titanyl chloride is subjected to electrolysis in the presence of nitric acid at electrode potentials between 1.1 and 1.7 volts, based on the saturated Ag / AgCl reference electrode to obtain a product with a residual chloride content of less than 200 ppm.

$${\text{Anodenreaktion: 4 Cl}}^{\text{-}}{\text{→ 2 Cl}}_{\text{2}}{\text{↑ + 4e}}^{\text{-}}$$

$${\text{Kathodenreaktion: 4 H}}^{\text{+}}{\text{+ 4 e}}^{\text{-}}{\text{→ 2 H}}_{\text{2}}\text{↑}$$

$${\text{Anodenreaktion: 2Cl}}^{\text{-}}{\text{→ Cl}}_{\text{2}}{\text{↑ + 2e}}^{\text{-}}$$

$${\text{Kathodenreaktion: 2 H}}^{\text{+}}{\text{+ 2e}}^{\text{-}}{\text{→ H}}_{\text{2}}\text{↑}$$

Depending on the titanium compounds used, the process according to the invention is based on the following reactions:

$${\text{Anode reaction: 4 Cl}}^{\text{-}}{\text{→ 2 cl}}_{\text{2nd}}{\text{↑ + 4e}}^{\text{-}}$$

$${\text{Cathode reaction: 4H}}^{\text{+}}{\text{+ 4 e}}^{\text{-}}{\text{→ 2 H}}_{\text{2nd}}\text{↑}$$

$${\text{Anode reaction: 2Cl}}^{\text{-}}{\text{→ Cl}}_{\text{2nd}}{\text{↑ + 2e}}^{\text{-}}$$

$${\text{Cathode reaction: 2 H}}^{\text{+}}{\text{+ 2e}}^{\text{-}}{\text{→ H}}_{\text{2nd}}\text{↑}$$

The reaction equilibria (Ia) or (IIa) of the actual chemical reaction are shifted to the right by the anode reaction (Ib) or (IIb) and the cathode reaction (Ic) or (IIc), that is to say to the formation of titanyl nitrate.

Only chlorine and hydrogen gas occur as further reaction products, which are comparatively easy to manage and dispose of. It has been found that the above reactions take place without problems and without the formation of undesired products or by-products such as, in particular, nitrous gases, if the potential of the anode is set potentiostatically to values in the range between 1.1 and 1.7 volts during electrolysis. It is preferred to work in the potential range between 1.2 and 1.6 volts, with maintaining a constant potential of 1.4 volts having proven particularly favorable.

A particular operational advantage of the method according to the invention is that it can be worked with dilute or at most moderately concentrated nitric acid. Hydrogen peroxide is completely unnecessary.

Because of the development of chlorine gas at the anode and hydrogen gas at the cathode, which together can form explosive mixtures, it is advisable to separate the anode compartment and the cathode compartment by means of a diaphragm and to ensure that the two gases are removed separately. The anode compartment is filled with an aqueous mixture of titanium tetrachloride or titanyl chloride and nitric acid; the cathode compartment expediently contains an aqueous nitric acid solution. In the mixed solution of the anode compartment, the molar mixing ratio of titanium tetrachloride or titanyl chloride and nitric acid can be between 1: 2 and 1: 5. Titanium tetrachloride or titanyl chloride are expediently used in the form of 20 to 50% strength aqueous solutions. The use of a 30% aqueous solution is particularly preferred. The nitric acid to be mixed can have a concentration of 30 to 70% by weight, preferably 65% by weight. The cathode compartment is filled with 5-25%, preferably 10%, aqueous nitric acid.

The method according to the invention can be carried out extremely simply and without major expenditure on equipment. The process can also be carried out without problems, especially on a pilot plant and production scale. Known and conventional electrolysis apparatus and techniques can be used here. In principle, electrolytic cells with the desired production volume and volume of inert material, such as glass, ceramic or resistant plastics such as polytetrafluoroethylene, are suitable. The apparatuses are equipped with devices for removing the reaction gases and can expediently be provided with inlet and outlet devices for the solutions and with stirring or mixing devices. Materials are considered for the electrodes, compared to the solutions used and the electrolysis conditions are inert. For example, electrodes made of platinum or titanium are well suited. They can be dimensioned and shaped in any way and suitably depend on the size and design of the electrolysis cells. Porous glass or ceramic materials or permeable plastic membranes, for example made of polytetrafluoroethylene, can serve as diaphragms for separating the electrode spaces.

The process is carried out in such a way that, after the electrolysis apparatus has been filled with the appropriate solutions, a voltage in the range mentioned is applied and the electrolysis is continued until it is completely implemented, roughly ascertainable by the end of gas evolution. The potential of the anode can expediently be adjusted to a fixed value, approximately 1.4 volts, using a conventional potentiostat. The progress of the reaction can be followed by conventional measurement technology using the current-voltage curve. For appropriate measurements, the apparatus can be equipped with the reference electrodes and measuring devices required for this. If the flowing current reaches a minimum value, the end of the reaction is reached. The chloride ion concentration in the reaction solution of the anode compartment can be determined by sampling and ion chromatography of the sample. The electrolysis time is essentially dependent on the amount of the reaction solution, the size and performance of the apparatus and on the regulated current flow.

In typical semi-technical experiments, a final current of 0.09 amperes was measured after an initial voltage of 1.2 volts and an initial current of 1 ampere after about 13 hours at a final voltage of 1.4 volts. The residual chloride ion concentration in the reaction solution was 0.0009 mol / l.

With the electrochemical method according to the invention, reaction solutions with a residual chloride content of less than 200 ppm, based on the content of titanyl nitrate, can be obtained in any case. As a rule, residual contents of 100 to 10 ppm or less are reached. The process is therefore particularly suitable for the preparation of low-chloride aqueous solutions of titanyl nitrate.

example 1

A divided cell was used as the electrolysis apparatus. It consisted of 2 cylindrical half-cells in double jacket design with an outer diameter of 12 cm, which were separated from each other by a Teflon membrane. The electrodes in the anode and cathode compartments consisted of circular coated titanium expanded metal disks with a diameter of 7 cm and an area of 35 cm ² .

The reaction was carried out under potentiostatic conditions. For this purpose, the potential of the titanium working electrode was tapped in the anode compartment with an Ag / AgCl, KCl (total) reference electrode, designed as a Haber-Luggin capillary.

At the beginning of the tests, the anode compartment of the measuring cell was filled with electrolyte with the composition 36 ml 65% HNO ₃ + 50 ml 30% TiCl ₄ . This corresponds to a molar ratio of TiCl ₄ : HNO ₃ of 1: 5.

10% HNO ₃ solution was used for the cathode compartment. The gases Cl ₂ and H ₂ formed during the reaction in the anode and cathode compartments were derived separately. Cl ₂ was absorbed in NaOH. H _{2 was released} outdoors via ventilation.

The reactions were carried out at room temperature.

The potential of the working electrode was set potentiostatically to an initial value of 1.2 V. The initial current of 1 A decreased over the course of 13 h with decreasing Cl ^- concentration to 0.09 A. The potential was adjusted discontinuously by hand until a maximum final value of 1.4 V was reached.

After a test period of 13 h, a residual chloride content of 0.0009 mol / l was found in the anode compartment, which corresponds to a proportion of 158 ppm based on TiO (NO ₃ ) ₂ .

Claims (5)

1. Process for preparing low-chloride aqueous solutions of titanyl nitrate by reaction of titanium tetrachloride or titanyl chloride with nitric acid, characterized in that titanium tetrachloride or titanyl chloride is subjected to electrolysis in the presence of nitric acid at an anode potential between 1.1 and 1.7 volts, giving a product having a residual chloride content of less than 200 ppm.
2. Process according to Claim 1, characterized in that the electrolysis is carried out in an electrolysis apparatus in which the anode space and the cathode space are separated by a membrane, with the anode space containing an aqueous mixture of titanium tetrachloride or titanyl chloride and nitric acid and the cathode space containing an aqueous solution of nitric acid.
3. Process according to Claim 2, characterized in that the anode space contains titanium tetrachloride or titanyl chloride and nitric acid in a molar ratio of from 1:2 to 1:5.
4. Process according to Claim 2 or 3, characterized in that the solution used for the anode space is a mixture of 20-50% aqueous titanium tetrachloride or titanyl chloride solution and 30-70% aqueous nitric acid and the solution used for the cathode space is 5-25% aqueous nitric acid.
5. Process according to any of Claims 1 to 4, characterized in that the electrolysis is carried out at anode potentials between 1.2 and 1.6 volts, preferably at about 1.4 volts.