FR2693125A1 - Treating waste gas contg. sulphur and nitrogen oxide(s) - with cerium-contg. oxidising soln. to recover nitric and sulphuric acids - Google Patents

Abstract

Treatment of residual gas contg. SO2 and NOx involves a main stage of passing the gas into a gas/liq. contactor in which an oxidising soln. is circulated, esp. in countercurrent, to obtain nitric and sulphuric acids by absorption and oxidn. in the soln., the novelty being that the oxidising soln. is an aq. esp. acidic Ce(IV) contg. soln. circulating in a closed circuit. The oxidising soln. is a sulphuric acid soln. circulated in a circuit which includes an electrochemical cell, at the anode of which the Ce(III), obtained at the contactor outlet, is re-oxidised to Ce(IV). ADVANTAGE - The process allows efficient removal of pollutants from waste gas and produces separate nitric acid and sulphuric acid which are free from catalyst traces and are thus marketable.

Description

"Process for the treatment of waste gases containing
sulfur dioxide and nitrogen oxides
The present invention relates to a process for treating waste gases containing sulfur dioxide and nitrogen oxides by which, in a main step, the waste gases to be treated are passed through a gas / liquid contactor in which they are circulated, in particular against the current, an oxidizing solution so as to obtain nitric acid and sulfuric acid by absorption and oxidation in this solution.

 It is well known that nitrogen oxides and sulfur dioxide are an important source of air pollution.

 However, many industries generate more or less significant releases of these oxides. Given their importance and their harmful nature, many researchers have tried to develop methods for treating these releases rich in sulfur dioxide and nitrogen oxides.

 Among the processes of this type which have already been proposed, there may be mentioned, by way of example, absorption-washing processes with alkaline solutions or with dilute sulfuric acid and hydrogen peroxide, processes absorption on molecular sieves or catalytic oxidation processes.

 The processes of absorption of oxides by alkaline solutions, which are currently used in many types of installation, present a certain number of economic and ecological problems, and the disadvantage of not making it possible to recover nitrogen in a simple manner and sulfur in a usable form. For this reason, there is currently an increasing trend towards the use of processes for the oxidation of sulfur and nitrogen oxides by air or pure oxygen in an aqueous medium.

 This oxidation has, however, the disadvantage of being particularly slow and of requiring the addition of catalysts to the oxidation medium to allow its realization with kinetics compatible with the constraints of industrial exploitation. Among the numerous catalysts proposed to date, mention may be made of activated carbon, platinum, manganese, or metal salts such as for example the vanadium oxides V2051 of chromium Cr203 or of iron Fe2O3. Such catalysts can be used either in agitated reactors in the case where the solution containing the oxides to be eliminated has been previously prepared, or in packed columns or trays which make it possible to absorb the gas to be treated during its oxidation.

 However, and despite their advantages, these methods have not made it possible to obtain complete satisfaction, in particular given the inconvenient nature of their implementation, their cost price, the risk of finding traces of catalysts in the sulfuric acid and nitric acid obtained and, above all, the incomplete nature of the oxidation of sulfur dioxide and the nitrogen oxides present.

 The object of the present invention is to remedy these drawbacks by proposing a new process for treating waste gases containing sulfur dioxide and nitrogen oxides by absorption and oxidation in an aqueous oxidizing solution according to a clean process allowing effective elimination. polluting species and the production of separate solutions of nitric acid and sulfuric acid devoid of traces of catalysts and therefore recoverable.

 To this end, the invention relates to a process of the aforementioned type characterized in that the oxidizing solution is an aqueous solution, in particular an acid solution containing tetravalent cerium Ce (IV) circulating in closed circuit.

 According to the invention, the oxidizing solution is preferably a solution of sulfuric acid.

Given the high value of the potential
Redox of the Ce4 + / Ce3 + pair which is equal to 1.44 V / NHE in a sulfuric medium, the tetravalent cerium is a very powerful oxidizing agent, such as to be advantageously used for the complete oxidation of sulfur dioxide and nitrogen contained in waste gases, in accordance with the invention.

 The gas / liquid contactor which can be used in this main stage of the invention can be of the conventional type, and, by way of example, be constituted by columns with packing or with plates operating in counter-current, columns with bubbles, agitated tanks ...

According to another characteristic of the invention, the circuit for circulating the oxidizing solution contains an electrochemical cell at the anode of which is re-oxidized to tetravalent cerium
Ce (IV), the reduced trivalent cerium Ce (III) obtained at the outlet of the gas / liquid contactor following the oxidation of waste gases.

The electrolytic cell used can, of course, be any without departing from the scope of the invention; it is possible, in particular, to use a stirred reactor with concentric electrodes or else with parallel electrodes (cells of the press filter type). It should, however, be noted that it is particularly advantageous to carry out the electrochemical oxidation of cerium in compartmentalized cells, that is to say provided with membranes, for reasons of productivity and yield: indeed , in the case of a non-compartmentalized cell, the reaction which occurs at the cathode corresponds to the reduction of part of the
This (IV) formed at the anode, and therefore a reduction in the productivity of the electrolytic cell.

 In all cases, the regenerated oxidizing solution, that is to say enriched in Ce (IV) obtained at the outlet of the electrochemical cell is returned to the inlet of the gas / liquid contactor; the oxidizing solution containing cerium therefore circulates in a closed loop through, on the one hand, the gas / liquid contactor and, on the other hand, the electrochemical cell.

 It is clear, given its circulation in a closed circuit, that the oxidizing solution is constantly enriched with sulfuric and nitric acid over time, depending on the contents of polluting species to be treated. The recovery of these acids therefore implies an intermittent withdrawal of this solution.

 To this end, and according to another characteristic of the invention, when the oxidizing solution enriched in sulfuric acid and in nitric acid has reached a determined concentration of sulfuric acid, in particular between 10 and 55%, this is withdrawn and replaces it with a sulfuric acid solution of reduced concentration, preferably of the order of 5%.

 At this stage of the process, it is necessary to separate the nitric acid and the sulfuric acid; However, it is well known that sulfuric acid has a boiling point much higher than that of water or nitric acid: the separation of these acids in a distillation column therefore constitutes an easily achievable operation whose implementation can be envisaged in the context of the process according to the invention.

 Consequently, according to another characteristic of the invention, the withdrawn solution enriched in sulfuric acid and in nitric acid is subjected to a distillation, in particular at ambient pressure and at a temperature of the order of 900C so as to recover, on the one hand, at the top of the column, a solution of nitric acid, and on the other hand, at the bottom of the column, a solution of recovered sulfuric acid.

 According to the invention, the operating parameters of this distillation step are advantageously chosen so as to obtain a recovery sulfuric acid solution having a sulfuric acid concentration greater than or equal to 45%.

 This distillation makes it possible to obtain an essentially pure nitric acid solution, but the recovery sulfuric acid solution is polluted by the presence of the tetravalent cerium Ce (IV) which is complexed by the sulfate. This solution cannot, therefore, be used without precautions and must be subjected to a subsequent treatment to allow its recovery.

 To this end, and according to another characteristic of the invention, in a first extraction step, the recovery sulfuric acid solution is treated in a first liquid / liquid contactor, in particular a column with trays, in which one makes also circulate an organic extraction solution, so as to pass the tetravalent cerium Ce (IV) into the organic phase.

 At the outlet of this first liquid / liquid contactor, an organic extraction solution containing Ce (IV) and a recovery sulfuric acid solution preferably having a concentration greater than or equal to 45% is therefore obtained: this solution is essential pure and can be reused as is.

 Independently of this recovery of sulfuric acid, it is also proposed, in accordance with the invention, to re-extract the cerium from the organic phase in order to be able to use it again as an oxidizing agent in the main stage of the process, for the oxidation of waste gases containing sulfur dioxide and nitrogen oxides to be treated.

According to the invention, in this second extraction step, the organic extraction solution containing the tetravalent cerium is treated
Ce (IV) in a second liquid / liquid contactor, in particular a second column with trays, in which an aqueous extraction solution is also circulated against the current, so as to again pass the tetravalent cerium Ce (IV) in the aqueous phase and to obtain, on the one hand, an organic extraction solution essentially free of cerium and, on the other hand, an aqueous extraction solution containing the tetravalent cerium Ce (IV).

 According to the invention, this aqueous extraction solution can advantageously consist of a solution of sulfuric acid at a concentration between 10 and 40%; this can be obtained by simple dilution of part of the essentially pure recovery sulfuric acid solution obtained at the end of the first extraction step.

 The sulfuric acid solution containing the cerium is suitable, after dilution, to be used as an oxidizing solution in the main stage of the process according to the invention.

 According to a particularly advantageous characteristic of the invention, the organic extraction solution is circulated in a closed extraction circuit containing the first and the second liquid / liquid contactors.

 According to the invention, this organic extraction solution is preferably a solution of di-2-ethyl-hexyl-phosphoric acid (HDEHP) in kerosene.

 In view of the above, it is clear that the process according to the invention operates in a closed loop of cerium.

The characteristics of the process which is the subject of the invention will be described in more detail with reference to the accompanying drawings, in which - FIG. 1 is a diagram showing a
installation for the implementation of the stage
main process corresponding to the oxidation of
sulfur dioxide and nitrogen oxides contained
in the waste gases to be treated, - Figure 2 is a diagram showing a
installation for the implementation of different
oxidative solution treatment steps after
its racking.

 According to Figure 1, is circulated, according to the arrows, an oxidizing solution consisting of a dilute sulfuric acid solution containing tetravalent cerium Ce (IV) in a closed circuit 1 comprising, on the one hand, a gas / liquid contactor 2 and, on the other hand, an electrolysis cell 3. Pumps 4 and 5 allow this circulation.

 The gases to be treated are admitted according to arrow b at the lower part of the contactor 2 and come out purified according to arrow c at the upper part thereof; the oxidizing solution which circulates against the current in the contactor (arrows a) allows efficient absorption and oxidation of nitrogen oxides and sulfur dioxide to nitric acid and sulfuric acid thanks to the presence of tetravalent cerium which is an agent powerful oxidation. Of course, this is reduced in parallel to trivalent cerium Ce (III). It should be noted that the elimination of polluting species is all the more effective the faster the gas / liquid transfer.

 The pump 4 makes it possible in particular to continuously send the oxidizing solution enriched in nitric acid and sulfuric acid to the electrochemical reactor 3.

 According to the figure, this reactor is constituted by an electrolysis cell provided with a membrane 8 which separates the anode compartment 6 from the cathode compartment 7.

 The solution leaving the contactor 2 is introduced at the anode 6 into which the trivalent cerium Ce (III) is reoxidized to tetravalent cerium Ce (IV), that is to say regenerated so that it can be sent again towards the contactor 2. The anode compartment 6 is, at the same time, the seat of a slight release of oxygen, the importance of which can be reduced by judicious control of the current applied to the cell 3.

 The cathode compartment 7 is, for its part, the site of a release of hydrogen in a dilute sulfuric acid solution which is supplied by means of a pump 9.

 It is clear, in view of the above, that the oxidizing solution containing the cerium circulates in a closed loop in circuit 1. As a result, the concentration of sulfuric and nitric acid increases over time as a function of the levels of polluting species treat.

 When this oxidizing solution has reached a determined concentration of sulfuric acid, this is drawn off and replaced by a sulfuric acid solution of reduced concentration of the order of 5%.

 According to FIG. 2, the withdrawn solution enriched in sulfuric acid and in nitric acid is introduced according to arrow d into a distillation column 10 at the level of which it is subjected to a distillation carried out at ambient pressure and at a temperature of the order of 900C so as to recover, on the one hand, at the top of the column, an essentially pure nitric acid solution (arrow e) and on the other hand, at the bottom of the column a recovery sulfuric acid solution containing the cerium tetravalent Ce (IV) (arrow f).

 The sulfuric acid recovery solution leaving the distillation column 10 according to arrow f is subjected to a first extraction step in a first liquid / liquid contactor 11 at which the tetravalent cerium Ce (IV) is extracted from the sulfuric solution by means of a solution of di-2-ethylhexyl-phosphoric acid (HDEHP) in solution in kerosene circulating in closed loop according to the arrows g in an extraction circuit 12. A pump 13 controls this circulation.

 At the outlet of the first contactor 11, there is therefore obtained, on the one hand, an essentially pure recovery sulfuric acid solution (arrow h) and on the other hand, an organic extraction solution containing the cerium (arrow g) .

 According to the figure, the extraction circuit 12 also contains a second liquid / liquid contactor 14 essentially similar to the contactor 11 at which the cerium is re-extracted from the organic phase.

 For this purpose, part i of the recovery sulfuric acid solution leaving the first contactor 11 according to arrow h, is diluted with water arriving according to arrow j, so as to obtain an aqueous solution of sulfuric acid k concentration of 10 to 40%; this is carried out at the inlet of the second contactor 14 parallel to the organic extraction solution containing the tetravalent cerium (arrow g). An aqueous solution 1 containing the cerium which is diluted (arrow m) with water in an amount adjusted so as to reform (arrow n) is thus recovered at the outlet of the second contactor 14 a solution which can be used again in the main stage of absorption and oxidation of waste gases (arrow a - figure 1).

 In summary, the overall system shown in Figures 1 and 2 operates in a closed loop of cerium. The sulfur dioxide is transformed into an essentially pure sulfuric acid solution while the nitrogen oxides are converted into a separate solution of nitric acid.

 The feasibility of the process which is the subject of the invention is confirmed by four significant examples reported below. These examples relate to preliminary absorption / oxidation work in gas / liquid laboratory reactors (examples 1 and 2), tests for the regeneration of tetravalent cerium on the surface of an anode (example 3) and the extraction conditions. and counter-extraction of tetravalent cerium (example 4).

Example 1
NO2 absorption / oxidation in a plane interface reactor.

A gas flow rate equal to 2 liters
TPN per minute and containing 830 ppm of nitrogen dioxide (NO2) is continuously sent to a cylindrical laboratory reactor with a volume of approximately 2.5 liters. Nitrogen dioxide is absorbed at the plane interface of the reactor and oxidized in a 5% sulfuric solution containing 8.1 10-2 M of
This (IV). The gas and liquid phases are stirred by slow mechanical stirring. Despite the low transfer flow provided by this laboratory device, the presence of the oxidizing agent allows a reduction in NO2 close to 40%. This reduction rate is practically zero in the case of a simple absorption in a dilute sulfuric acid solution.

 Furthermore, the quantity of oxidizing agent consumed during the duration of the experiment (approximately two hours) corresponds perfectly to the quantity of nitrogen dioxide absorbed and oxidized, which attests to the selective nature of the oxidizing agent chosen.

Example 2
SO2 absorption / oxidant in a centrifugal gas / liquid reactor.

 This second laboratory reactor is open to gas and liquid. It allows particularly important gas-liquid transfer flows under the action of a rotor which plates the liquid to the cylindrical wall of the reactor. A gas stream with a flow rate equal to 1.6 liters per minute and containing 1058 ppm of sulfur dioxide feeds the gas / liquid reactor.

The gases are absorbed in a 5% sulfuric solution containing 10-3 M of Ce (IV) and the sulfur dioxide content at the outlet of the reactor varies between 95 and 160 ppm, depending on the speed of rotation of the rotor.

The use of a more concentrated solution
This (IV), 0.01 M for example, should allow almost total purification of the gas.

Example 3
Regeneration of Ce (IV) by anodic oxidation
A 5% sulfuric acid solution containing 0.02 M trivalent cerium sulfate
This (III), circulates in a closed loop through the anode compartment of a press type filter cell at a speed of 0.24 ms-1. The anode consists of a stack of expanded platinum titanium plates and has an area of approximately 120 cm2. The cathode, of the same kind, is the seat of a release of hydrogen. The anodic production of tetravalent cerium can be carried out at a conversion greater than 98% for currents close to the diffusional limit current with current yields greater than 80%: the initial current density is of the order of 50 Am-2, taking into account the speed of the solution and the Ce (III) concentration. The current density decreases regularly as the Ce (IV) is converted. The corresponding cell voltage is of the order of 2.3 volts during the entire operation.

Example 4
Separation of the sulfuric acid produced from the Ce (IV) solution
The example is presented for the treatment of 100 liters of 55% sulfuric acid solution containing 0.03 M of Ce (IV).

The extraction is carried out using an organic phase consisting of 1 M extractant (HDEHP) (325 g / liter) in kerosene. The partition coefficient of Ce (IV) between the organic and aqueous phases is 13.5. The extraction of Ce (IV) is envisaged in a column with plates operating against the current using a volume of 30 liters of organic phase; given the value of the partition coefficient and the volume of the two phases, it is possible to obtain a 55% sulfuric acid solution containing less than 1 ppm (7 10-6 M) of
This (IV) at the exit of the column comprising less than ten theoretical stages.

 Counter-extraction is also envisaged in a column with plates operating at counter-current. Twenty liters of 30% sulfuric solution (5 M) are prepared by diluting in water a small part of the 55% sulfuric solution obtained above. The partition coefficient of Ce (IV) between the organic phase obtained by extraction and the 30% sulfuric acid solution is 0.15. Given the value of this coefficient and the volume of the two phases, it is possible to obtain a 30% sulfuric acid solution containing 0.15 M of Ce (IV) at the outlet of the column comprising less than one ten theoretical stages; the organic phase leaves the column with a Ce (IV) content of less than 10 ppm and can be reused. The Ce (IV) solution obtained is diluted five times in water to obtain a solution of sulfuric acid at around 10% and containing 0.03 M in oxidizing agent: this solution will again be used in the closed loop d 'absorption / oxidation and electrochemical regeneration shown schematically in Figure 1.

Claims (3)

 1) Method for treating waste gases containing sulfur dioxide and nitrogen oxides by which, in a main step, the waste gases to be treated are passed through a gas / liquid contactor in which one circulates, in particular against -current, an oxidizing solution so as to obtain nitric acid and sulfuric acid by absorption and oxidation in this solution, process characterized in that the oxidizing solution is an aqueous solution, in particular acid containing tetravalent cerium Ce ( IV) circulating in a closed circuit.  2) Method according to claim 1, characterized in that the oxidizing solution is a sulfuric acid solution.  3) Method according to any one of claims 1 and 2, characterized in that the circulation circuit of the oxidizing solution contains an electrochemical cell at the anode of which is re-oxidized to tetravalent cerium Ce (IV), the trivalent cerium Ce (III) obtained at the outlet of the gas / liquid contactor.  *) Method according to any one of claims 1 to 3, characterized in that, when the oxidizing solution enriched in sulfuric acid and in nitric acid has reached a determined concentration of sulfuric acid, in particular between 10 and 55%, is withdrawn this and is replaced by a sulfuric acid solution of reduced concentration, preferably of the order of 5%.  5) Process according to claim 4, characterized in that the withdrawn solution enriched in sulfuric acid and in nitric acid is subjected to a distillation, in particular at ambient pressure and at a temperature of the order of 900 C so as to recover , on the one hand, at the top of the column, a solution of pure essential nitric acid, and on the other hand, at the bottom of the column, a solution of recovered sulfuric acid containing the tetravalent cerium Ce (IV).  6) Method according to claim 5, characterized in that the recovery sulfuric acid solution obtained at the bottom of the distillation column has a sulfuric acid concentration greater than or equal to 45%.  7) A method according to any one of claims 5 and 6, characterized in that, in a first extraction step, the sulfuric acid recovery solution is treated in a first liquid / liquid contactor in which a organic extraction solution, so as to pass the tetravalent cerium Ce (IV) into the organic phase and to obtain, on the one hand, an essentially pure recovery sulfuric acid solution and, on the other hand, a solution of organic extraction containing Ce (IV).  8) Process according to claim 7, characterized in that, in a second extraction step, the organic extraction solution containing the tetravalent cerium Ce (IV) is treated in a second liquid / liquid contactor in which a aqueous extraction solution consisting of a sulfuric acid solution at a concentration of between 10 and 40%, in particular obtained by diluting part of the essentially pure recovery sulfuric acid solution so as to pass the tetravalent cerium Ce (IV) in the aqueous phase and to obtain, on the one hand, an organic extraction solution essentially free of cerium and, on the other hand, a sulfuric acid solution containing tetravalent cerium Ce (IV) , suitable, after dilution, to be used in the main stage of the process.  9) Method according to any one of claims 7 and 8, characterized in that the organic extraction solution is a solution of di-2-ethyl-hexyl-phosphoric acid (HDEHP) in kerosene.  10) Method according to any one of claims 8 and 9, characterized in that the organic extraction solution circulates in a closed circuit in an extraction circuit containing the first and the second liquid / liquid contactors.