EP0064154B1 - Method of preparing blue iron-hexacyanoferrate-iii pigments - Google Patents

Description

The invention relates to a process for the preparation of blue iron hexacyanoferrate III pigments.

Blue iron hexacyanoferrate III pigments (I) (C.I. Pigment Blue 27; C.I. No. 77510) are commercially available under various names such as Prussian blue, Berlin blue, Milori blue or iron blue.

These blue pigments are formed by oxidizing complex iron (II) hexacyanoferrate (II) compounds, also known as Berlin's white or white dough (II), with oxidizing agents in dilute acids such as chlorate / hydrochloric acid, dichromate or air in dilute sulfuric acid (pH <0 , 5) received.

und die Blaupigmente durch

wiedergegeben, worin Me ein Alkalimetallkation, vorzugweise Kalium- oder Natriumion, ein Ammonium oder ein Gemisch dieser Kationen ist.The chemical composition of the white dough (II) and the blue iron hexacyanoferrate III pigments (I) is complex and, within certain limits, also depends on the manufacturing process. In the following we 11 by the (simplified) formula

and the blue pigments through

reproduced, wherein Me is an alkali metal cation, preferably potassium or sodium ion, an ammonium or a mixture of these cations.

For the composition, reference is made to H. Kittel, “Pigments”, Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart, 1960, pp. 341/343 and the literature cited there.

According to the prior art processes, the iron-II-cyanoferrate-II (II) is prepared by precipitating iron-II salts with complex alkali metal hexacyanoferrates-II in aqueous solution. In this process, approximately 2 parts by weight of alkali metal salts are obtained per part by weight of “white dough” (II), which are a considerable burden on the waste water. Another disadvantage of the prior art method is that before oxidation to I, the salts contained in (II) have to be washed out, which is very time consuming.

This disadvantage also exists if the white dough is produced by reacting freshly precipitated iron (II) hydroxide with hydrogen cyanide in an alkaline medium or by reacting iron (II) salts with hydrogen cyanide with the addition of alkali metal hydroxide or ammonia at pH> 4. The object of the present invention was to develop a technically inexpensive and environmentally friendly process for the production of blue iron hexacyanoferrate III pigments.

It has been found that blue iron hexacyanoferrate III pigments can be obtained by oxidation of complex iron II cyan compounds of the formula II if the complex iron II hexacyanoferrate II compound is obtained by anodic oxidation of metallic iron in hydrogen cyanide as the reaction medium or in a reaction medium containing hydrogen cyanide at pH <7 and an anode potential - measured against the hydrogen normal electrode - of <0.76 V.

The complex iron-II-cyan compound (II) is obtained in high yield and high purity by the process according to the present invention. The process according to the invention is very environmentally friendly since there are practically no by-products or by-products due to the reaction. It was surprising that the anodic oxidation, i. H. by electrochemical reaction of iron with hydrogen cyanide in an acidic reaction medium (electrolyte) the complex iron-II-cyano compound (II) is formed practically quantitatively. In contrast, the complexes (II) are obtained by the process of the prior art only in an alkaline medium at pH> 8.

The process is generally carried out in such a way that the reaction medium - also referred to as an electrolyte - is filled into an electrolysis cell with an iron anode and an iron - or chromium-nickel-steel cathode and the electrolysis is measured at the desired temperature at an anode potential the normal hydrogen electrode - of ≤ 0.76 V.

The process can be carried out batchwise or continuously. In the case of technical implementation, the electrolyte will be circulated in the cell.

In the former case, electrolysis is carried out until sufficient (II) has formed. The suspension is then filtered and the filter material is washed with water largely to practically salt-free.

The filtrate can be used again as an electrolyte after the used parts have been added.

If the process is carried out continuously, the electrolyte will be circulated in the cells and the iron II-cyano compound (II) formed is continuously separated from part of the electrolyte. After the used components have been added, the filtrate is continuously returned to the system.

The isolated (II) can then be oxidized to MeFe [Fe (CN) _6] pigments (I) in a manner known per se. However, the iron-II complex can also be oxidized directly in the electrolyte, which offers advantages in terms of technical implementation.

Hydrogen cyanide and its mixtures with C ₁ - bis come as the reaction medium for the electrolysis C ₄ -alkanols with primary, secondary or tertiary hydroxyl group, C ₂ - to C ₆ -alkanediols, diethylene glycol, triethylene glycol, dipropylene glycol, C ₃ - to C _s -alkane polyols, water or with mixtures of these liquids.

The reaction medium (electrolyte) is preferably a mixture of hydrogen cyanide and water.

The hydrogen cyanide content in the electrolyte can be between 100 and 0.001% by weight. A mixture of 99.9 to 95% by weight of water and 0.1 to 5% by weight of hydrogen cyanide, based on the electrolyte, is preferably used as the electrolyte.

In order to improve the conductivity and to dope the iron (II) cyano compounds (11), conductive salts are advantageously added to the electrolyte. As the conductive salts, there are those of the prior art which are soluble in at least the required concentration in the electrolytes mentioned.

The amount of the conductive salts is generally 0.1 to 10% by weight, based on the electrolyte. As conductive salts come z. B. into consideration: salts of alkali metals, alkaline earth metals, earth metals and rare earths, such as salts of lithium, sodium, potassium, rubidium, magnesium, calcium, strontium, aluminum, cerium, as well as salts of metals of the iron group and ammonium

Suitable anions are e.g. B. chloride, sulfate, hydrogen sulfate, mono- and dihydrogen phosphate, hydrogen sulfite, cyanide, the hexacyanoferrates, hydrogen oxalate, oxalate, maleate and fumarate.

Ammonium and potassium salts are preferred as conductive salts. Ammonium chloride, ammonium hydrogen oxalate, potassium chloride, potassium hydrogen sulfate, potassium hydrogen sulfite and potassium hydrogen oxalate are particularly preferred as conductive salts, since iron-II-cyano complexes (11) are obtained in the presence of these conductive salts, which give oxidation forms to (I) which are particularly strongly colored pigment forms which have a reddish blue color give high gloss.

The electrochemical reaction, in which iron is anodically oxidized and hydrogen is deposited on the cathode, can be carried out at temperatures of from -20 ° C. to 150 ° C., if appropriate under pressure. The reaction is preferably carried out without pressure at temperatures from -5 to + 20 ° C. If you work at temperatures> 20 ° C, the reaction must be carried out under pressure because of the low boiling point of the hydrogen cyanide.

Since the conductivity of the electrolyte depends on the temperature and the conductivity increases with the temperature, the electrolysis will be carried out at normal pressure, in particular at temperatures from 10 to 20 ° C.

For electrolysis in the pH range ≤ 7, an anode potential of ≤ + 0.76 V (measured against the normal hydrogen electrode) should be maintained. At higher anode potential, dicyan and cyanate are formed, which reduces the yield.

The electrolysis is preferably carried out in the pH range from 1 to 6, in particular in the pH range from 2 to 5.

The current density is generally 30 to 5000 A / m ^2. It is advantageous to work at current densities of 200 to 2,000 A / m ^{2 in} order to ensure current yields of approximately 95% and above. At current densities> 2,000 A / m ² , a very good mass transfer must be ensured so that there is no depletion of cyanide ions in the anode boundary layer and therefore a drop in the current efficiency. Current densities of <30 A / m ² require a higher concentration of hydrogen cyanide in order to ensure the complexation of the anodized iron.

As anodes on which the conversion to (11) takes place, come fluidized bed or fluidized bed anodes, which, for. B. from pieces of iron, iron granules or iron filings on an electrically conductive base or compact iron anodes in the form of rods, blocks or sheets or iron oxide anodes. In the case of fluidized bed and fluidized bed anodes, the anode material (iron) is separated from the cathode compartment by diaphragms or anodically resistant screens or nets made of metal or plastic. The electrical contacting of fluidized bed and fluidized bed anodes with the power source can take place when using metal sieves via the sieve or via anodic, stable metal or carbon rods inserted into the filling.

When using compact iron anodes it is advisable if the distance between anode and cathode can be adjusted, otherwise the cell voltage has to be increased with increasing consumption of the anode (through the formation of (II)) in order to keep the anode current constant. This regulation can take place automatically or by hand, depending on the cell voltage.

Above all, iron, stainless steels or other conductive materials with low hydrogen overvoltage come into consideration as cathode material. Advantageously, the cathode to reduce the hydrogen overvoltage on the surface, for. B. with nickel-aluminum-zinc alloys, with nickel, cobalt, molybdenum, molybdenum-iron alloys, with tungsten, with tungsten-iron-nickel alloys or iron-cobalt alloys (iron content 65 to 95 wt.%; DE -OS 30 03 819 (P 30 03 819.8)) coated with vanadium, vanadium alloys or sulfides of molybdenum, tungsten, nickel or cobalt.

The electrolysis cells are preferably cells with compact iron anodes in the form of rods or plates. The distance between the anode and cathode is preferably 2 to 10, in particular 3 to 6 mm.

Process product (11) can be separated and isolated by filtering, centrifuging or decanting electrolytes. In the case of separation by filtration, the reaction mixture is added Advantageously, filter aids beforehand, whereby the filtration time can be shortened considerably. After the missing components have been added, the filtrate can be used again as an electrolyte.

The oxidation of the complex iron (II) cyano compound (II) is carried out in a manner known per se, e.g. B. in aqueous suspension at pH <6 with chlorate, chlorine or hydrogen peroxide.

The white dough II obtained by the process according to the invention is preferably oxidized in aqueous sulfuric acid suspension at pH 0 to 3 and at temperatures between 70 and 95 ° C. with air or oxygen. Under these conditions, very strong, grain-soft and reddish pigments I are obtained which are very easily dispersible and give very brilliant colorations.

The oxidation is preferably carried out at temperatures between 75 and 85 ° C. For this purpose, the air or oxygen is stirred into the suspension and finely distributed or injected via a jet nozzle. The oxidation can also take place in a column into which air or oxygen is injected in a fine distribution below. The redox potential of the suspension is advantageously checked during the oxidation of II to avoid overoxidation. The oxidation can be regarded as complete when 95 to 99% of the iron (II) cyano compound I have been oxidized.

When carrying out the process industrially, it is also possible to dispense with the separation of the iron-II complex and to carry out the oxidation of the iron-II complex directly in the electrolyte and then isolate (I) in the customary manner.

In the case of oxidation with atmospheric oxygen or hydrogen peroxide, the filtrate from (I) can be reused as an electrolyte after the missing components have been added.

Very fine-particle pigments I, which are well dispersible in water, are obtained by oxidation of II with air or oxygen at pH> 8 and temperatures of 20 to 50 ° C. The oxidation can be followed by measuring the redox potential. After the oxidation has ended, the reaction mixture is acidified and the pigment is isolated. To improve dispersibility in water, a small amount (i.e. 0.01 to 0.2% by weight, based on (I)) of polyols, such as diethylene glycol, triethylene glycol or glycerol, is added to the reaction mixture.

The iron-II complex (II) can be anodically oxidized both in the acidic and in the alkaline range.

The invention is illustrated by the following examples. The percentages relate to the weight. The specified potentials were measured against the hydrogen standard electrode.

example 1

In a tubular cell, the stainless steel wall of which is the cathode and which contains a round, rod-shaped compact iron anode (distance anode-cathode: approx. 4 mm; length of the cell: 600 mm), the electrolyte is a solution of 97% water, 2% hydrogen cyanide and 1% potassium hydrogen sulfate. At a current density of 1 000 A / m ² , cell voltage ≈ 2.5 V, a flow rate of the electrolyte of 1.2 m / sec and an anode potential of - 0.2 V (measured against the normal hydrogen electrode), electrolysis is carried out until the hydrogen cyanide concentration in the electrolyte is 0.05%. The white dough suspension obtained is then adjusted to pH 1.5 with Swedish acid and oxidized by gassing with atmospheric oxygen at 88 ° C. (duration: approx. 3 h). The MeFe [Fe (CN) _6] (Berlin blue) formed during the oxidation is filtered off and washed neutral with water. The filter cake is then dried at 120 ° C. The yield is ≈ 97% pigment, based on the hydrogen cyanide used.

The pigment gives purer, redder and glossier colorations in the lacquer than products which are obtained by the process of the prior art from iron (II) salt and sodium or potassium cyanoferrate (II).

Example 2

In the electrolysis cell specified in Example 1, a solution of 93% water, 5% hydrogen cyanide and 2% potassium chloride is introduced as the electrolyte. With a current density of 1,500 A / m ² , a cell voltage of 2.0 V, a flow rate of 1.5 m / sec and an anode potential of = - 0.2 V (measured against the hydrogen normal electrode), electrolysis is carried out until the hydrogen cyanide concentration in the electrolyte is 0.04%. The white dough suspension obtained is then added 5 mg of Fe ⁺⁺ in the form of FeSO ₄ per liter of suspension and the pH is adjusted to 1.0 using sulfuric acid. The oxidation to the pigment and the work-up are carried out as in Example 1.

A very granular pigment is obtained which, in comparison with the pigments of the prior art in the lacquer and in the printing ink, gives purer and redder colorations which are superior in gloss. The colorings are approx. 19% stronger than those obtained with the strongest colored corresponding pigments on the market.

Example 3

In the electrolysis cell specified in Example 1, a solution of 88% methanol, 1.5% water, 10% hydrogen cyanide and 0.5% potassium chloride. At a current density of 1 200 A / m ² , a cell voltage of 4.8 V, a flow rate of 1.8 m / sec and an anode potential of ≈ - 0.2 V (measured against the hydrogen normal electrode), electrolysis is carried out until the hydrogen cyanide concentration in the electrolyte is 0.02%. This suspension is separated off on a suction filter under nitrogen and the isolated white dough is introduced into so much water that an 8% suspension is formed. The aqueous suspension is adjusted to pH 1.0 with dilute sulfuric acid and, after addition of 0.15% potassium chlorate (based on the suspension), oxidized at 80 ° C. for 1 hour. The MeFe [Fe (CN) _6] pigment (Berlin blue) resulting from the oxidation is worked up as in Example 1.

Example 4

It is electrolyzed as in Example 2, but the suspension obtained is filtered under nitrogen and the filter material is introduced into sufficient water to form a 5% suspension. The suspension is adjusted to pH 12 with aqueous 25% potassium hydroxide solution and oxidized with atmospheric oxygen at 30 ° C. (duration about 1 h). After the oxidation, the suspension is acidified to pH 1 with dilute sulfuric acid and the Berlin blue is worked up as in Example 1. A finely divided pigment is obtained which can be very easily dispersed in water after the addition of a little triethylene glycol.

Example 5

• 5.1 93 % Wasser, 5 % Cyanwasserstoff, 2 % MgCI₂
• 5.2 93 % Wasser, 5 % Cyanwasserstoff, 2 % CaC1₂
• 5.3 93 % Wasser, 5 % Cyanwasserstoff, 2 % Al(H₂O)₆Cl₃
• 5.4 93 % Wasser, 5 % Cyanwasserstoff, 2 % Ce₂(SO₄)₃
• 5.5 91 % Wasser, 5 % Cyanwasserstoff, 2 % NiS0₄ + 2 % KHS0₄

The procedure is as in Example 1, but the following solutions are used as the electrolyte:

• 5.1 93% water, 5% hydrogen cyanide, 2% MgCl ₂
• 5.2 93% water, 5% hydrogen cyanide, 2% CaC1 ₂
• 5.3 93% water, 5% hydrogen cyanide, 2% Al (H ₂ O) ₆ Cl ₃
• 5.4 93% water, 5% hydrogen cyanide, 2% Ce ₂ (SO ₄ ) ₃
• 5.5 91% water, 5% hydrogen cyanide, 2% NiS0 ₄ + 2% KHS0 ₄

The electrolysis is stopped at a hydrogen cyanide content of 0.05%. The suspensions are oxidized at pH 1.5 and 80 ° C by gassing air (duration: approx. 3 h). After separation, washing and drying, pigments are obtained which give the paint good coverage. Depending on the alkaline earth or earth metal used, pigments are obtained which provide more or less greenish blue tints.

Claims (11)

1. A process for the preparation of blue iron hexacyanoferrate-III pigments by oxidizing complex iron-II hexacyanoferrate-II compounds of the formula Me₂Fe[Fe(CN)_6], wherein the complex iron-II hexacyanoferrate-II compound is preparad by anodic oxidation of metallic iron in hydrogen cyanide as the reaction medium or in a reaction medium containing hydrogen cyanide, at pH < 7 and at an anode potential of ≤ 0.76 V, measured against the standard hydrogen electrode.

2. A process as claimed in claim 1, wherein a mixture of hydrogen cyanide and water or of hydrogen cyanide and C₁-C₄-alkanols and/or C₁-C₄-alkanediols is used as the reaction medium.

3. A process as claimed in claim 1 or 2, wherein the anodic oxidation of the iron is carried out at a pH of from 2 to 5.

4. A process as claimed in claim 1, 2 or 3, wherein the reaction medium contains one or more conductive salts.

5. A process as claimed in claim 4, wherein the conductive salts used are potassium or ammonium salts or mixtures thereof.

6. A process as claimed in claim 1, 2, 3, 4 or 5, wherein the anodic oxidation of the iron is carried out at from - 20 to 100 °C at atmospheric or superatmospheric pressure.

7. A process as claimed in claims 1 to 5, wherein the anodic oxidation is carried out at from - 5 to 25 °C.

8. A process as claimed in claims 1 to 7, wherein the complex iron-II hexacyanoferrate-II compound is oxidized to MeFe[Fe(CN)_6] at pH > 8.

9. A process as claimed in claim 8, wherein the oxidation is carried out with air or oxygen.

10. A process as claimed in claims 1 to 7, wherein the oxidation of the complex iron-II hexacyanoferrate-II compound is carried out with air or oxygen at from 70 to 95 °C and a pH of from 0 to 4.

11. A process as claimed in claim 8, 9 or 10, wherein the oxidation is carried out in the presence of potassium and/or ammonium ions.