EP0291416A1 - Process for the reduction of a solution containing titanium and iron - Google Patents

Abstract

The invention relates to a cell for the electrolysis of a solution produced by the sulphuric digestion of ilmenite. …<??>The solution to be treated passes through the cathode compartment of one or two cells which are equipped with an ion exchange membrane of cationic type.

Description

The present invention relates to an electrolysis cell and a method for the reduction of a solution comprising titanium and iron and in particular of a solution resulting from a sulfuric attack of ilmenite.

It is known that the production of titanium dioxide involves an attack by a sulfuric acid solution of a titaniferous ore of the ilmenite, anatase or rutile type. After this attack, a solution is obtained which contains titanyl sulfate and especially ferric and ferrous iron sulfates.

However, this solution must be reduced in order to transform the ferric ions into ferrous ions, the presence of the ferric ions having to be avoided during the subsequent step of hydrolysis of titanyl sulfate.

Several methods are known for this reduction of ferric iron. Industrially it is produced by scrap iron.

This method has various drawbacks. In particular, it is discontinuous. On the other hand, it requires a subsequent separation of large quantities of iron giving in particular ferrous sulphate waste.

Electrochemical reductions have been proposed. One of these methods is described in particular in French patent No. 2,363,642.

However, the different types of electrolysers studied so far do not allow good energy yields to be obtained at high current densities, that is to say at least 10 A / dm2.

The main object of the invention is therefore an electrolysis cell making it possible to work with a high current density and efficiency.

A second object of the invention is a method usable with such a cell.

According to the invention, the electrolysis cell for the reduction of a solution comprising titanium and iron ions is of the type comprising an anode compartment, a cathode compartment and an ion exchange membrane separating the two compartments and it is characterized in what the membrane is a cationic membrane.

The method according to the invention is characterized in that said solution is circulated in the cathode compartment of the cell described above.

Other characteristics and advantages of the invention will be better understood on reading the description which follows and the appended drawing in which the single figure is a schematic representation of an implementation of cells according to the invention.

The cell of the invention will now be described more precisely.

This cell has two compartments, an anode, a cathode separated by an ion exchange membrane.

According to the main characteristic of the invention, this membrane is of the cationic type, in particular with strong acid groups of the sulfonic type, for example. As a membrane of this species, mention may be made, for example, of those sold under the brands NAFION and SELEMION.

The use of a cationic membrane brings about a certain number of advantages linked to the very qualities of this type of membrane. In fact, their superior strength to that of anionics makes the cell less fragile. It is also possible to operate with higher current intensities.

With regard to the electrodes, the cathode can be based on different materials.

According to a preferred embodiment of the invention, a copper-based cathode is used, this type of cathode offering the highest faradaic yields thanks to the excellent mass transfer obtained on this material.

However, it is also possible to use a cathode based on at least one material chosen from the group comprising lead, titanium, special steels.

In the latter case, more particularly one can use either lead or titanium cathodes alone, or lead on a suitable substrate, for example lead on titanium or lead on copper, or even titanium coated with at least one precious metal.

As precious metals, mention may be made of platinum, iridium, palladium and, for example, a 0.2% palladium titanium cathode can be used.

As special steels, mention may be made of those of type Uranus B 6 and Incoloy 825, that is to say, steels comprising chromium, nickel and molybdenum, the molybdenum content, however, generally not having to exceed approximately 15%.

As regards the anode, the nature of this is not critical insofar as it exhibits sufficient chemical resistance during the oxidation of water in an acid medium. In general, titanium coated with precious metals or with oxides of precious metals as defined above is used.

The electrodes can be in different forms, for example planar, perforated, deployed.

The membrane can be placed in abutment on the anode. Turbulence promoters can be placed in the compartments of the cell.

We will now describe in more detail the process for the implementation of the electrolysis cell.

This process essentially consists in circulating in the cathode compartment of the cell which has just been described the solution to be treated.

This solution includes titanium and iron ions. Titanium is essentially present in the form of titanium IV, the FeII / FeIII ratio can be variable.

This solution may also contain H⁺ ions and anions of the sulfate type.

It will be recalled that the process for preparing titanium dioxide essentially comprises the following stages.

The first stage consists of an attack on the titaniferous ore with a sulfuric acid solution. The etching solution thus obtained is reduced in a second step and then clarified in a third, steps 2 and 3 can be reversed. A fourth step consists in crystallizing and then in separating part of the ferrous sulfate in solution. The solution thus obtained undergoes a concentration in a fifth step then, in a last and sixth step, the titanyl sulfate is hydrolyzed and the titanium hydroxide is separated, which will then be calcined.

The cell and the method of the invention apply very particularly to the reduction of the solution from the first aforementioned step, that is to say of the sulfuric attack on the titaniferous ore of the ilmenite type in particular.

In such a case, of course, the process reduction step (second step) is carried out entirely by electrolytic means.

However, it is also possible to carry out the reduction at any point in the process for preparing TiO₂ between the attack and the hydrolysis and in particular immediately before the hydrolysis.

In the anode compartment, it is possible to circulate either acidified water, for example a 0.5 N solution of H₂SO₄, or a solution of ferrous salt.

Of course, the solution circulating in the cathode compartment can be recycled there at the outlet thereof.

It is also possible to circulate the solution in the cathode compartments of two cells mounted in parallel. Such an installation ensures a constant running of the production unit even in the event of failure of one of the cells.

According to a particular embodiment of the invention, the solution to be treated is separated into a first and a second part, the second part is treated by passing through the cathode compartment of the aforementioned cell, the solution thus treated is stored in a reserve. and the solution resulting from this reservation is combined with the aforementioned first part.

The figure illustrates this embodiment.

The solution to be treated arrives at 1, a first main part 2 continues in the process while a second part 3 will undergo the electrolytic treatment.

The stream 3 is divided into two parts 4 and 5 and feeds the cathode compartments of the two cells 6 and 7 according to the invention mounted in parallel. The two parts of this same flow are combined at the outlet at 8 and open into a reserve 9.

Via a line 10, flow 2 is joined.

Lines 12 and 11 allow at least part of the solution from the reserve 9 to be recycled into the cathode compartment (s) of at least one of cells 6 and 7.

Such a system with reserve and two cells makes it possible to have greater stability of operation of the cells even in the event instability of the FeII / FeIII ratio of the main flux. It is also possible, thanks to this system, to treat only part of the main flow insofar as the reduction of titanium has been carried out quite far, for example of the order of 100 g / l.

Concrete examples will now be given.

 EXAMPLE 1

An electrolysis cell having the characteristics and under the conditions given below is used:
- cationic membrane: NAFION 423.
- anode: expanded titanium coated with platinum-iridium.
- cathode: expanded copper.
- current density: 30 A / dm².

In addition, the following media are circulated there:
- anolyte H₂SO₄ 0.5 N
- catholyte at the entrance: Ti⁴⁺ 120 g / l, Fe²⁺ 45 g / l, Fe³⁺ 3 g / l, H₂SO₄270 g / l.

For a circulation speed of the catholyte of 10 cm / s and of the anolyte of 0.5 cm / s with a cell temperature of 65 ° C., a catholyte of the following composition is obtained at the exit from the cathode compartment:
Ti⁴⁺ 104 g / l, Fe²⁺ 48 g / l, Ti³⁺ 16 g / l

The cathodic faradic yield is 99%.

EXAMPLE 2

The operating conditions are as follows:

An electrolysis cell having the characteristics and under the conditions below is used:
- cationic membrane: NAFION 423.
- anode: expanded titanium coated with iridium platinum,
- cathode: perforated palladium titanium,
- current density: 20 A / dm².

In addition, the following media are circulated there:
- anolyte H₂SO₄ O, 5 N
- catholyte at the inlet Ti⁴⁺ 120 g / l, Fe²⁺ 47 g / l, Fe³⁺ 4 g / l, H₂SO₄ 270 g / l.

For a circulation speed of the anolyte of 0.5 cm / s and of the catholyte of 10 cm / s at a cell temperature of 65 ° C., a catholyte of composition is obtained at the outlet of the cathode compartment:
Ti⁴⁺ 113 g / l, Fe²⁺ 51 g / l, Ti³⁺ 7 g / l with a faradic cathodic yield of 99%.

EXAMPLE 3

In this example, different types of cathodes are used according to tests 1, 2 and 3.

The operating conditions of the cell are as follows:
- inlet catholyte Ti⁴⁺ 120 g / l Fe²⁺ 46 g / l Fe³⁺ 3 g / l, H₂SO₄ 270 g / l.
- catholyte circulation speed: 30 cm / s.
- cell temperature: 65 ° C,
- cationic membrane: NAFION 423.
- current density: 30 A / dm².
- anolyte H₂SO₄ O, 5 N for tests 1 and 2, solution of a ferrous salt: Fe²⁺ 40 g / l for test 3.
- anode: expanded titanium coated with iridium platinum for tests 1 and 2.
- graphite for test 3.

The results are given below.

EXAMPLE 4

This example shows the possibility of obtaining with the cell of the invention solutions highly concentrated in Ti³⁺.

The operating conditions of the cell are as follows:
- anolyte: H₂SO₄ O, 5 N.
- inlet catholyte Ti⁴⁺ 120 g / l Fe²⁺ 45.7 g / l Fe³⁺ 3.4 g / l, H₂SO₄ 270 g / l.
- catholyte circulation speed: 60 cm / s,
- anolyte circulation speed: 0.5 cm / s,
- cell temperature: 65 ° C,
- cationic membrane: NAFION 423.
- anode: expanded titanium coated with platinum-iridium,
- cathode: perforated copper,
- current density: 17 A / dm2.

A catholyte of the following composition is obtained at the outlet:
Ti⁴⁺ 46.4 g / l Fe²⁺ 49.1 g / l Ti ³⁺ 73.6 g / l.

The cathodic faradic yield is 97.5%.

EXAMPLE 5

An electrolysis cell having the characteristics and under the conditions given below is used:
- cationic membrane: NAFION 423,
- anode: expanded titanium coated with platinum-iridium,
- cathode: lead,
- current density: 20 A / dm2.

In addition, we circulate the media below:
- anolyte H₂SO₄ 0.5 N
- catholyte at the entrance: Ti⁴⁺ 120 g / l, Fe²⁺ 45 g / l, Ti³⁺ 1 g / l H₂SO₄ 270 g / l.

For a circulation speed of the catholyte of 10 cm / s and of the anolyte of 0.5 cm / s with a cell temperature of 65 ° C., a catholyte of the following composition is obtained at the exit from the cathode compartment:
Ti⁴ 104 g / l, Fe²⁺ 48 g / l, Ti³⁺ 8 g / l.

The cathodic faradic yield is 80%.

EXAMPLE 6

An electrolysis cell having the characteristics and under the conditions given below is used:
- cationic membrane: NAFION 423,
- anode: expanded titanium coated with platinum-iridium,
- cathode: expanded titanium + lead,
- current density: 30 A / dm2.

In addition, the following media are circulated there:
- anolyte H₂SO₄ 0.5 N
- catholyte at the entrance: Ti⁴⁺ 120 g / l, Fe²⁺ 45 g / l, Ti³⁺ 1 g / l H₂SO₄ 270 g / l.

For a circulation speed of the catholyte of 10 cm / s and of the anolyte of 0.5 cm / s with a cell temperature of 65 ° C., a catholyte of the following composition is obtained at the exit from the cathode compartment:
Ti⁴⁺ 120 g / l, Fe²⁺ 48 g / l, Ti³⁺ 9 g / l.

The faradaic yield is 90%.

Claims (13)

1. Electrolysis cell for the reduction of a solution comprising titanium and iron ions, of the type comprising an anode compartment, a cathode compartment and an ion exchange membrane separating the two compartments, characterized in that the membrane is a cationic membrane. 2. Cell according to claim 1, characterized in that the cathode compartment comprises a copper-based cathode. 3. Cell according to claim 1, characterized in that the cathode compartment comprises a cathode based on at least one material chosen from the group comprising lead, titanium, special steels. 4. Cell according to claim 3 characterized in that the cathode is made of lead deposited on a substrate, in particular lead on titanium or lead on copper, or in titanium coated with at least one precious metal. 5. Method for the electrolytic reduction of a solution comprising titanium and iron ions, characterized in that said solution is circulated in the cathode compartment of a cell according to one of claims 1 to 4. 6. Method according to claim 5, characterized in that the aforementioned solution comes from the sulfuric attack of a titaniferous mineral of the ilmenite type in particular. 7. Method according to claim 5 or 6, characterized in that circulates in the anode compartment of the aforementioned cell acidified water or a solution of ferrous salts. 8. Method according to claim 6 or 7 characterized in that the aforementioned solution is circulated in the cathode compartment immediately before the hydrolysis step in a process for the preparation of titanium dioxide. 9. Method according to one of claims 5 to 8 characterized in that the solution circulating in the cathode compartment is partly recycled there at the outlet thereof. 10. Method according to one of claims 5 to 9, characterized in that the solution is circulated in the cathode compartments of two cells mounted in parallel. 11. Method according to one of claims 5 to 10, characterized in what the above solution is separated into a first and a second part, the second part is treated by passage through the cathode compartment of the aforementioned cell, the solution thus treated is stored in a reserve and the solution from this reserve is combined to the first part above. 12. Method according to claim 11 characterized in that at least part of the solution from the above-mentioned reserve is recycled to the cathode compartment of the cell. 13. Method according to claim 11 or 12 characterized in that the aforementioned second part is circulated in the cathode compartments of two cells mounted in parallel.

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