WO2008028947A1

WO2008028947A1 - Process for preparing ferrates (vi)

Info

Publication number: WO2008028947A1
Application number: PCT/EP2007/059357
Authority: WO
Inventors: Leon Ninane; Claude Criado
Original assignee: Solvay (Societe Anonyme)
Priority date: 2006-09-08
Filing date: 2007-09-07
Publication date: 2008-03-13
Also published as: FR2905692B1; FR2905692A1

Abstract

Process for preparing ferrate (VI) granules continuously in a fluidized-bed reactor, comprising the continuous injection of an aqueous solution or dispersion of at least one salt or oxide of Fe(II) and/or Fe(III) into the reactor, the continuous injection of alkali and/or alkaline-earth metal hypohalite and alkali and/or alkaline-earth metal hydroxide, in the form of one or more aqueous solutions or dispersions, into the reactor, the control of the flow rate, pressure and temperature of the fluidization gas so as to evaporate the water introduced by the aqueous solutions and/or dispersions, and the removal of ferrate (VI) granules from the reactor.

Description

Process for preparing ferrates (VI)

The present invention relates to a process for preparing ferrates (VI) from salts and oxides of iron (II) and iron (III).

Alkali and alkaline-earth metal ferrates, for example Na₂FeO₄, K₂FeO₄, CaFeO₄, BaFeO₄, etc., are powerful oxidizers that can especially be used for bleaching textiles, wastewater treatment and in organic or mineral chemistry processes.

The many methods of synthesis which have been proposed to date may be subdivided into three types: electrochemical processes, oxidation processes in a wet medium and oxidation processes in a dry medium. Preparation of ferrate (VI) by electrochemical means involves the oxidation of an iron anode in an electrochemical cell, which was carried out for the first time in 1841 by Poggendorf, and remains one of the oldest electrochemical preparation methods. This method makes it possible to obtain liquid sodium ferrate without impurities, but it generates high costs. The problems associated with preparations in aqueous medium are, amongst others, the more or less rapid degradation of the ferrates obtained and the more or less laborious separation of the ferrate from the other components of the mixture, for example by precipitation, starting from aqueous solutions.

Patent US 5217584 describes, for example, a process for the aqueous-phase preparation of ferrate (VI) in a hypohalite solution in the presence of an alkali or alkaline-earth metal hydroxide exclusively from the B-Fe₂O₃ oxide. However, the process proposed requires, in addition, very pure reactants and it is very sensitive to impurities. Among these harmful impurities, mention is especially made of ferrous [Fe(II)] ions. The use of a fluidized bed is only mentioned therein for drying the precipitated ferrates.

Synthesis by the dry route is increasingly used because it enables, to a large extent, the reactions with water to be prevented and makes it possible to obtain a more stable product. In some known processes, the conversion of ferric oxide is carried out at high temperature, often above 400⁰C, or even 800⁰C, for example in the presence of potassium or sodium hydroxide and oxygen or else in the presence of sodium peroxide and oxygen. However, these laboratory-scale methods involve expensive reactants and are difficult to scale up. Patent EP 0 354 843 discloses a relatively complex process for the solid- phase production of alkali or alkaline-earth metal ferrates by reaction of a hypochlorite with an at most trivalent iron compound in the presence of alkali or alkaline-earth metal hydroxides, in which an iron (II) or iron (III) salt and an alkali or alkaline-earth metal hypochlorite are mixed to a powder that passes through a first mesh. Next, a first layer of this powdered mixture is formed in a flat container connected to a mechanical vibration generator, then deposited onto this first layer is a second layer formed of alkali or alkaline-earth metal hydroxide granules containing between 10 and 30% by weight of water, the granules being rejected by a second mesh that is significantly larger than the first. The container is then subjected to vibration for 10 to 60 minutes at less than 40⁰C, so that the ferrate is formed within the granules. The powder residues are removed by screening through a mesh that is intermediate between the first and second meshes and the excess of hydroxide is removed by washing with a substantially anhydrous organic solvent.

In summary, the large-scale preparation of ferrates (VI) according to the known processes, although technically possible, is still marred by one, or even several, more or less serious disadvantages mentioned above. Indeed, generally, the limiting factors are the constraints concerning the nature and purity of the reactants, too large a number or too great a complexity of the steps in the process, the production and therefore treatment of too many intermediate products, high energy expenditure or the impossibility of scaling-up a process that is advantageous on a laboratory scale. Object of the invention The object of the present invention is consequently to provide a simple and effective process for preparing ferrate (VI) from salts and oxides of iron (II) and iron (III), which makes it possible to produce ferrates (VI) on a larger scale. General description of the invention

In accordance with the invention, this objective is achieved by a process for preparing ferrate (VI) granules continuously in a fluidized-bed reactor, comprising the following steps:

^■ continuous injection of an aqueous solution or dispersion of at least one salt or oxide of Fe(II) and/or Fe(III) into the reactor;

^■ continuous injection of at least one oxidizer and alkali and/or alkaline-earth metal hydroxide, in the form of one or more aqueous solutions or dispersions, into the reactor; ^■ control of the flow rate, pressure and temperature of the fluidization gas so as to evaporate the water introduced by the aqueous solutions and/or dispersions; and

^■ removal of ferrate (VI) granules from the reactor. The advantages of the present process compared to the known processes are numerous.

The main advantage of the present process is, on the one hand, that it is operated continuously and that its design is relatively simple, and this despite the high ferrate (VI) yields observed. On the other hand, as the sizing of such a process does not pose any particular problems, unlike the known processes, it is possible to apply it on an industrial scale. Indeed, the industrial scaling-up of a fluidized-bed reactor is easier than for other types of reactors.

Another major advantage of the invention is its favourable energy balance. This is because, although the almost complete evaporation of the water introduced by the solutions or dispersions is energy consuming (endothermic process), the conversion reaction of the ferrous and/or ferric salts and oxides to ferrate (VI) is exothermic. Consequently, depending on the operating conditions chosen, it is possible to operate the present process very economically from an energy point of view. Nevertheless, it may be advantageous to provide the process with a certain amount of energy to prevent runaway of the reaction: this is because a controlled supply of energy makes it possible to more effectively and more easily manage the reactions that take place inside the reactor. Consequently, an additional advantage of the process according to the invention is the excellent control of the temperature and synthesis conditions.

The ferrate (VI) granules are preferably removed continuously from the reactor and they are preferably essentially dry, that is to say they generally contain at most 2% by weight of free water, preferably at most 1% by weight and particularly preferably at most 0.5% by weight. The expression "free water" is understood to mean any water which is not found in the form of water of crystallization or solvation of the constituents of the granules. The granules have a diameter D50 between 0.1 and 2 mm, preferably between 0.5 and 1 mm. The diameters are defined as the diameters of spheres having an identical volume to that of the particle. Consequently, the above process makes it possible to avoid a drying operation following the oxidation reaction, as drying is carried out at the same time as the ferrate (VI) synthesis reaction. That said, it can also be envisaged to subject the granules to an additional drying operation after their removal from the reactor in a conventional drying device, for example in a fluidized-bed dryer.

On the other hand, adjusting the conditions for continuous injection of each of the solutions or dispersions into the reactor, for example by control of the air spraying pressure and the flow rate of the solutions in each nozzle or sprayer makes it possible to finely disperse these solutions within the fluidized bed. Consequently, the precise dosages that such a fluidized-bed process makes possible by action over the injection conditions (e.g. the air spraying pressure and the flow rate through the sprayer) allow excellent control of both the particle size and composition of the finished product.

Moreover, the use of this type of reactor allows not only very close contact between the reactants sprayed over the solid layer, but also excellent exchange of heat and matter in order to carry out the reaction between two or more solutions on a solid support followed by instantaneous drying, which is an indicator of stability as soon as the ferrate (VI) granules are dry.

Finally, unlike most of the known processes, the process according to the invention makes it possible to prepare ferrate (VI) starting from salts and/or oxides of iron (II) or (III) of very different natures and even from mixtures. In this case, the Fe (II) and/or Fe (III) salts and/or oxides may preferably be chosen from ferrous or ferric sulphates, nitrates, hydroxides or chlorides, ferrous or ferric oxides and also mixtures thereof.

Consequently, the present invention is suitable not only for organic or inorganic chemistry processes requiring relatively pure and expensive reactants, but it may advantageously be used in practice for reconverting wastes from certain industries, such as for example metallurgy or titanium dioxide production.

The oxidizer may be chlorine (in gaseous form), potassium permanganate or potassium or sodium persulphate. Alkali and/or alkaline-earth metal hypohalites are recommended, in particular sodium, potassium, calcium or magnesium hypochlorite or hypobromite, and also mixtures thereof. The hypohalite can advantageously be produced in situ. For instance, sodium hypochlorite can be produced in situ by reaction of chlorine with sodium hydroxide. The alkali and/or alkaline-earth metal hydroxide or hydroxides are preferably chosen from sodium, potassium, calcium or magnesium hydroxides, and also mixtures thereof. In one particularly advantageous embodiment, the iron salt is ferrous and/or ferric sulphate, the alkali and/or alkaline-earth metal hypohalite is sodium hypochlorite and the alkali and/or alkaline-earth metal hydroxide is potassium hydroxide. Generally, the oxidizer and the alkali and/or alkaline-earth metal hydroxide are injected separately into the fluidized bed. However, in one recommended variant of the invention, at least one oxidizer and at least one alkali and/or alkaline-earth metal hydroxide are both injected in the form of a aqueous solution or dispersion. These reactants are then preferably mixed before being injected, and thus injected in the form of a single aqueous solution or dispersion.

The fluidization gas used in a process according to the invention may be any suitable gas or gas mixture, but for practical and economic reasons it is generally air and the flow rate, pressure and temperature of this fluidization gas are controlled so as to obtain a fluidized-bed temperature between 30 and 100⁰C, preferably between 50 and 80⁰C and particularly preferably between 60 and 70⁰C.

In another recommended variant of the invention, the oxidiser is in a gaseous state and is used, pure or dilute, as fluidization gas. In this variant, it is recommended to use chlorine gas as oxidiser. The chlorine gas can be diluted, for instance with air, for a easier control of the energetics of the reaction. Detailed description of the invention

Other particularities and features of the invention will appear from the detailed description of some advantageous embodiments given below, by way of illustration, with reference to the appended drawings. Fig.1 : Diagram of the principle of a fluidized-bed reactor for the continuous preparation of ferrates (VI). Fig.2: Granules of ferrate (VI) obtained according to Example 1.

Fig. 1 shows the principle of a fluidized-bed reactor 1 for implementing a process according to the invention. The reactor 1 comprises a bed 2 on top of a perforated plate 3, the bed 2 being fluidized by the introduction of a fluidization gas 6, generally air, through the lower part of the reactor 1.

The solution or dispersion 4 of the salt or oxide of Fe(II) and/or Fe(III) is injected continuously into the fluidized bed 2 through a nozzle 4A, for example a sprayer. Similarly, the hydroxide and hypohalite solution or dispersion 5 is also injected continuously into the fluidized bed through a nozzle 5 A, preferably also a sprayer. It should be noted that the reactor may comprise a higher number of nozzles depending on the number of solutions or dispersions chosen.

The solutions or dispersions 4 and 5 are deposited on the fluidized particles and are dried by the fluidization gas 6, which passes through the fluidized bed 2, by evaporation of the water introduced by the solutions or dispersions 4 and 5. The water-laden fluidization gas 7 then leaves the reactor 1 through its upper part.

The ferrate (VI) oxidation of the iron salts to ferrate by reaction with the hypohalite in the presence of hydroxide takes place continuously and the size of the particles increases through the continuous provision of additional reactants 4 and 5.

The ferrate (VI) granules 8 of sufficient size may be removed continuously from the reactor 1.

In practice, the reactor 1 as described schematically above comprises, if necessary, other components that are not shown, such as for example a device for heating and controlling the flow of the fluidization gas, metering pumps for the injected solutions, a device for heating and/or thermally insulating the reactor, temperature sensors, a device for separating and/or recycling the fines carried by the fluidization gas, etc. Fig. 2 shows ferrate (VI) granules obtained by a process according to the invention conforming to the operating conditions of Example 1 below. Examples Material and reactants used

The fluidized bed was composed of: 1. a glass column having a height of 100 mm and a diameter of 100 mm;

2. an extension produced using a 100/200 mm reduction made of glass having a height of 200 mm that stops fine particles from blowing away; and

3. a fluidization grid.

The temperature of the installation was maintained by a controlled heating tape. The assembly was thermally insulated.

The compressed air feeding the bed was first heated by passing through an exchanger.

The heating power was adjusted thanks to the temperature reading supplied by a probe located at the inlet to the bed. The air flow rate was measured by a float rotameter calibrated to the working pressure of the installation. The solutions were injected using calibrated pumps via sprayers. The spraying pressure was controlled by pressure-reducing valves installed in the air circuit. The sprayers were installed at the mid-height of the bed. A calibrated temperature probe was installed at the same level. The temperatures of the bed and the compressed air were monitored and recorded.

The iron solutions were prepared by dissolving an iron (II) and/or (III) salt in demineralized water at ambient temperature.

Mixed MOH-NaClO solutions were prepared in an ice bath by mixing a fresh solution of NaClO, demineralized water and NaOH or KOH pellets.

The hydroxide was introduced slowly so that the temperature of the mixture did not exceed 30⁰C in order to prevent the formation of chlorates.

The solution was cooled below 10⁰C to precipitate as much as possible of the NaCl contained in the starting hypochlorite solution, then filtered over a filter having a porosity of 0.45 μm.

The filtrate was analysed for a possible rectification of the solution. It was kept at a temperature below 10⁰C to prevent decomposition of the ClO^". Example 1: synthesis from NaClO, KOH and Fe₂(SO₄)₃

The reaction and the introduction flow rates used were the following: Fe₂(SOJ₃ + 3NaClO + 10NaOH > 2K₂FeO₄ + 3K₂SO₄ + 3NaCl + 5H₂O

Solution 1 : 98.9 g/h of Fe₂(SO₄)₃ and 296.8 g/h of H₂O Solution 2: 55.3 g/h of NaClO, 29.4 g/h of NaCl, 138.8 g/h of KOH and 426.6 g/h OfH₂O

Compressed air for fluidization: 22 m³/h at a temperature varying from 121 to 123°C.

Which corresponded to:

^• a theoretical flow rate of solid of 300.1 g/h; and

^• a maximum iron content of 92.1 g/h.

For a programmed trial of 6 h, the theoretical turnover number of the starting charge is 2.3 when the flow rates of the reactants remain at 100% of the nominal.

The introduction of the 250 g/kg Fe₂(SO₄)S solution was carried out at a nominal flow rate of 0.396 kg/h, that of the mixed 85.0 g/kg NaClO and 213.5 g/kg KOH solution at a nominal flow rate of 0.65 kg/h. The solutions were introduced under a spraying pressure of 2 bar. The violet coloration of the grains appeared after a few minutes. The temperature of the bed was quickly stabilized between 65 and 67°C. The flow rate of compressed air required was 22 m³/h for a temperature between 121 and 123⁰C and a pressure difference in the installation of 0.24 bar. Very few fine particles were produced during the trial and the grains matured rapidly (see Fig. 2).

The analyses were carried out on the fraction below 1.6 mm to avoid analysing agglomerates OfFe³⁺ and underestimating [Fe⁶⁺].

The maximum Fe⁶⁺ content obtained in the samples withdrawn was 52 g/kg for a total Fe of 71 g/kg.

The [Fe⁶⁺]/[total Fe] degrees of conversion were high and stable, between 70 and 80% throughout the whole trial.

The [total Fe]/[maximum theoretical Fe] turnover rate was 77%.

The heating power was sufficient to steadily evaporate all the water. The amount of product recovered in the final charge (1042 g relative to the initial 799 g) and the particle size, measured by vibrating sieves (Retsch A200 Digit) confirm the significant enlargement of the grains formed in the bed:

^• Di₀: 738 μm

^• D₅₀: 879 μm • D₉₀: 981 μm

^• Span: 0.28

The synthesis of ferrate (VI) from NaClO, KOH and Fe₂(SO₄)S gave very good yields OfFe⁶⁺. The process was very stable both in operation and in the reaction itself. The bed was easily kept between 65 and 67°C due to the 22 m³/h compressed air heated between 121 and 123°C.

The degrees of conversion were steady from the start of the trial (70 to 80%), which could give an Fe⁶⁺ concentration between 64 and 74 g/kg in the case of a total turnover of the initial charge. Example 2: synthesis from NaClO, KOH and FeSO₄ The reaction and the introduction flow rates used were the following:

FeSO₄ + 2NaClO + 4K0H >K₂Fe0₄ + K₂SO₄ + 2NaCl + 2H₂O

Even though the stoichiometries of the reactions differ, the flow rates for introducing the solutions are calculated in order to have the same amounts of water to be evaporated as in the preceding trial. Solution 1 : 58.5 g/h Of FeSO₄ and 234.1 g/h of H₂O Solution 2: 57.4 g/h of NaClO, 45.0 g/h of NaCl, 86.5 g/h of KOH and 485.9 g/h OfH₂O

Compressed air for fluidization: 22 m³/h at 120 to 121°C. Which corresponded to: • a theoretical flow rate of solid of 233.5 g/h; and • a maximum iron content of 92.1 g/h.

For a programmed trial of 6 h, the theoretical turnover number of the starting charge is 1.8 when the flow rates of the reactants remain at 100% of the nominal. The introduction of the 200 g/kg FeSO₄ solution at a nominal flow rate of

0.293 kg/h and of the mixed 85.0 g/kg NaClO and 128 g/kg KOH solution at a nominal flow rate of 0.675 kg/h were carried out by two pumps.

The solutions were introduced after 10 minutes of the trial at 100% of the nominal flow rate under a pressure of 2.5 bar. The pressure difference of the installation was 0.22 bar.

The violet coloration of the grains appeared after a few minutes and the grains matured rapidly. The temperature of the bed was stabilized between 65 and 70⁰C.

The flow rate of compressed air required was 22 m /h for a temperature between 118 and 12 FC.

The trial was stopped after 6 h and the analyses were carried out on the fraction below 1.6 mm to avoid analysing too many agglomerates OfFe²⁺ and underestimating [Fe⁶⁺].

The maximum Fe⁶⁺ content obtained in the samples withdrawn was 42 g/kg for a total Fe of 64 g/kg, namely a 65% [Fe⁶⁺]/[total Fe] degree of conversion.

The heating power was sufficient to steadily evaporate all the water.

The synthesis of ferrate (VI) from NaClO, KOH and FeSO₄ gave very good yields OfFe⁶⁺. The [Fe⁶⁺]/[total Fe] degrees of conversion of around 65% gave Fe⁶⁺ concentrations equal to 60 g/kg when the initial charge was completely turned over.

Example 3: synthesis from NaClO, KOH and FeCl₃

The reaction and the introduction flow rates used were the following: 2FeCl₃ + 3NaClO + 1 OKOH > 2K₂FeO₄ + 6KCl + 3NaCl + 5H₂O

Solution 1 : 80.3 g/h Of FeCl₃ and 315.5 g/h of H₂O Solution 2: 55.3 g/h of NaClO, 39.1 g/h of NaCl, 138.8 g/h of KOH and 416.9 g/h OfH₂O

Compressed air for fluidization: 22 m³/h at 123 to 125°C. Which corresponded to: • a theoretical flow rate of solid of 291.1 g/h; and • a maximum iron content of 94.9 g/h.

For a programmed trial of 6 h, the theoretical turnover number of the starting charge is 2.2 when the flow rates of the reactants remain at 100% of the nominal. The introduction of the 202.8 g/kg FeCl₃ solution at a nominal flow rate of

0.396 kg/h was carried out by the Gilson pump, that of the mixed 85.0 g/kg NaClO and 213.5g/kg KOH solution at a nominal flow rate of 0.65 kg/h by the pump.

The solutions were introduced after 20 minutes of the trial at 100% of the nominal flow rate under a pressure of 2.5 bar.

The pressure difference of the installation was 0.22 bar. The violet coloration of the grains appeared after a few minutes. The temperature of the bed was stabilized in less than an hour between 65 and 67°C. The flow rate of compressed air required was 22 m³/h for a temperature between 123 and 125°C. In view of the blowing away of fines despite the enlarged extension located on the bed, the spraying pressure of the solutions was therefore lowered slightly (1.5 to 2 bar).

The analyses were carried out on the fraction below 1.6 mm to avoid analysing agglomerates OfFe³⁺ and underestimating [Fe⁶⁺] too much. The maximum Fe⁶⁺ content obtained in the samples withdrawn was

22 g/kg for a total Fe of 30 g/kg.

The [Fe⁶⁺]/[total Fe] degree of conversion was high: 75%.

The heating power was sufficient to steadily evaporate all the water.

The synthesis of ferrate (VI) from NaClO, KOH and FeCl₃ gave very good [Fe⁶⁺]/[total Fe] degrees of conversion: 75%.

When the trial was maintained until complete turnover of the initial charge, [Fe⁶⁺] reached 71 g/kg. Examples 4-6

Syntheses of ferrate (VI) from other iron salts (Fe(NO₃)3, FeCl₂) and/or hydroxides (NaOH) were carried out by following, to a large extent, the operating procedures presented in Examples 1 to 3 above. Generally, in the above tests it was observed that:

^■ the rate of fluidization of the gases preferably lies between 0.1 and 1.5 m per second;

^■ the optimum reaction temperature generally lies between 60 and 80⁰C and more particularly around 70⁰C;

^■ spraying the reactants in a fluidized solid may be carried out by a two-fluid sprayer using, on the one hand, an iron solution and, on the other hand, a combined hypohalite and hydroxide solution;

^■ use of NaClO could be replaced by chlorine gas, acting as fluidization gas; ^■ metering of the reactants and control of the flow rate, pressure and temperature of the fluidization gas may be carried out very accurately;

^■ it is possible to insert a tube bundle into the fluidized bed in order to supply the heat contribution required for the reaction;

^■ it may be necessary or advantageous to ensure the capture of fines carried by the fluidization gases, for example by cyclone capture and recycling in the reactor;

^■ the equilibrium particle size in the reactor is a dynamic equilibrium between the particles which enlarge through successive layers and the new particles generated within the bed, especially by the action of breaking particles resulting from the action of the spraying air which exits the nozzles (sprayers) and which breaks the particles; and

^■ all the soluble iron salts may a priori be suitable; one disadvantage of ferric salts is the rapid corrosion of stainless steel sprayers; the use of suitable materials, such as titanium is consequently necessary in certain cases in order to guarantee good operation of the process.

Claims

C L A I M S

1. Process for preparing ferrate (VI) granules continuously in a fluidized- bed reactor, comprising the following steps:

^■ continuous injection of at least one oxidizer and at least one alkali and/or alkaline-earth metal hydroxide, at least the latter being in the form of one or more aqueous solutions or dispersions, into the reactor;

^■ control of the flow rate, pressure and temperature of the fluidization gas so as to evaporate the water introduced by the aqueous solutions and/or dispersions; and

^■ removal of ferrate (VI) granules from the reactor.

2. Process according to Claim 1, in which the Fe (II) and/or Fe (III) salts are chosen from ferrous or ferric sulphates, nitrates, hydroxides or chlorides, and also mixtures thereof.

3. Process according to Claim 1 or 2, in which the oxidizer is an alkali and/or alkaline-earth metal hypohalite.

4. Process according to Claim 3, in which the alkali and/or alkaline- earth metal hypohalite is chosen from sodium, potassium, calcium or magnesium hypochlorite or hypobromite, and also mixtures thereof.

5. Process according to any one of the preceding claims, in which the alkali and/or alkaline-earth metal hydroxide is chosen from sodium, potassium, calcium or magnesium hydroxides, and also mixtures thereof.

6. Process according to any one of the preceding claims, in which the iron salt is ferrous and/or ferric sulphate, the alkali and/or alkaline-earth metal hypohalite is sodium hypochlorite and the alkali and/or alkaline-earth metal hydroxide is potassium hydroxide.

7. Process according to any one of the preceding claims, in which the fluidization gas is air and the flow rate, pressure and temperature of the fluidization gas are controlled so as to obtain a fluidized -bed temperature between 30 and 100⁰C, preferably between 50 and 80⁰C, and particularly preferably between 60 and 70⁰C.

8. Process according to any one of the preceding claims, in which the ferrate (VI) granules removed from the reactor contain at most 2% by weight of free water, preferably at most 1% by weight, and particularly preferably at most 0.5% by weight.

9. Process according to any one of the preceding claims, in which the ferrate (VI) granules removed from the reactor have an average diameter D50 between 0.1 and 2 mm, preferably between 0.5 and 1 mm.

10. Process according to any one of the preceding claims, in which at least one oxidizer and at least one alkali and/or alkaline-earth metal hydroxide are injected in the form of a single aqueous solution or dispersion.