FR3072396A1 - METHOD FOR TREATING ELECTROFILTER ASHES FROM THE RECOVERY BOILER IN A KRAFT PROCESS - Google Patents

Abstract

The invention relates to a method for treating the ashes of electrofilters from the boiler of a circuit for recovering liquors in a process for producing kraft-type paper pulp. . According to the method of the invention, carbon dioxide is added to the aqueous streams used for the dissolution of the ash, the acidification of the ash being able to be completed by the use of a mineral acid. Conventional, for example sulfuric acid and / or acid spent acid.

Description

METHOD FOR PROCESSING ASH OF ELECTROFILTERS FROM THE RECOVERY BOILER IN A KRAFT PROCESS

The invention relates to a method for treating electrofilter ash from the boiler of a liquor regeneration circuit in a Kraft process for the production of paper pulp, comprising a step of dissolving or supersaturated solution (slurry) of the ash in the presence of acid, a step of separation of two phases: a liquid phase containing chlorides and potassium and a solid phase containing sodium ions, sulfides, sulfates and carbonates, as well as a step of recycling the solid phase in the liquor regeneration circuit and a step of recycling part of the liquid phase to the step of dissolving the ash and purging the second part of said liquid phase.

The production of pulp from wood using the process known as the Kraft process is a chemical process involving the use of a cooking agent - a solution of sodium hydroxide and sodium sulfide - which is impregnated with wood chips; this step is called cooking, the solution used is called white liquor. The Kraft process can be briefly described as follows:

“White liquor” - an aqueous solution containing sodium hydroxide (NaOH) and sodium sulfide (Na2S) - permeates the wood chips. During this stage, the cellulosic fibers are not degraded, while the hemicelluloses, lignin and the other non-cellulosic constituents of the wood are degraded and go into solution, then forming a liquor called "black liquor" because of its color. All of the products resulting from cooking form a paste made of cellulosic fibers and black liquor which contains the degraded compounds.

The cellulose fibers are separated from the black liquor, they will form the paper pulp; The Kraft process also includes the treatment of the black liquor including a combustion step which allows to recover most of the chemicals composing it and allows to use the calories obtained by combustion of the various organic compounds from the wood dissolved in the liquor during cooking.

The recovery of useful products is part of the regeneration cycle of the liquor, the operating principle of which is as follows.

The black liquor separated from the cellulose pulp is concentrated in evaporators, it is then pulverized and burned in a recovery boiler. During combustion, the main reaction is the reduction of sodium sulphate to sodium sulphide by the action of the carbon contained in the black liquor according to the global reaction:

Na ₂ SO ₄ + 2C - »Na ₂ S + 2CO ₂ producing a saline compound of sodium carbonate and molten sodium sulfide. The salt flows out at the bottom of the boiler, is collected in a tank where it is dissolved in water. The "green liquor" (Na ₂ CO ₃ + Na ₂ S) is obtained.

The next stage in the treatment of the liquor is the caustification stage, which consists in adding lime to the green liquor, leading to the following reaction:

Na ₂ CO ₃ + Ca (OH) ₂ -> 2NaOH + CaCO ₃ i

The calcium carbonate is recovered and calcined to regenerate the lime: CaCO ₃ -> CaO + CO ₂

Na ₂ S remains present in the solution but does not participate in the causticization reaction; NaOH and Na ₂ S form as indicated above the white liquor, it will be used during the cooking of wood shavings.

The principle of the cycle of use / treatment of liquors - we will call it in a simplified way the cycle of liquors - which aims to recycle the useful compounds can thus be schematized:

- "white liquor" + "black liquor" shavings + cellulose pulp;

- “black liquor” via combustion -> “saline”;

- "saline" via dissolution ^ "green liquor";

- "green liquor" + lime "white liquor" + CaCO ₃ .

The compounds which participate in the liquor cycle are useful, but useless compounds, even harmful under certain conditions, gradually accumulate during the successive cycles of use / treatment of the liquors. Derived in particular from the process for producing cellulose pulp, these elements which are not consumed in the process for producing paper pulp are called NPE elements - by reference to the English expression "non process elements" (npe). The elements chloride (CI) and potassium (K ⁺ ) form the essential of the elements NPE. In the rest of the text, and depending on the context, the expression NPE elements may be used for the sake of simplicity.

Chloride ions (CI) come partly from wood, - in particular when this comes from or transits through coastal areas, - and / or from the reuse of bleaching plant effluents which results from the increasing closure of circuits necessary in a paper-intensive paper industry. The accumulation of the chlorine element is harmful because it forms salts which corrode in particular the heat exchanger tubes of the recovery boiler.

The potassium element is also harmful since potassium salts form deposits in particular in the tubes of the recovery boiler, thus reducing the capacity of the tubes to exchange heat.

The CI 'and K ⁺ ions are found in the black liquor and therefore in the recovery boiler.

During the combustion of the black liquor in the recovery boiler, fumes are emitted, containing the combustion gases - mainly water vapor and CO2, but also very fine particles containing combustion products; because of their very small size they were entrained with the combustion gases.

These particles, or ashes, are entrained in the fumes, they circulate in the heat exchanger tubes and are then recovered in the electrostatic precipitators before the fumes are sent to the atmosphere.

The different chemical elements present before combustion in the black liquor are after combustion present in the saline and also present in the ashes, the composition of the ashes differs however from that of the saline, the electrofilter ash (also called electrofilter dust ) are enriched in CL and K ⁺ compared to other mineral elements due to their greater volatility; they therefore constitute an excellent location for preferentially eliminating all or part of the CL and K ⁺ .

Solutions have therefore been implemented by the paper industry for several decades to reduce the amounts of chloride and potassium ions, by using ash from electrostatic precipitators.

A first solution adopted consisted in the (partial) landfill of the electrofilter ash, the additional ash being dissolved in water to be recycled in the liquor circuit. However, since the ashes contain, in addition to harmful elements, elements useful to the process - even if they are in a lower proportion there than in the saline - the papermaker has no interest in putting these elements in indiscriminate discharge.

First, they must be replaced; the papermaker therefore has a certain economic interest in recovering these products as much as possible, and therefore in operating as much as possible a selective separation of the harmful elements in the ashes of the electrostatic filters so as to recycle the elements useful to the process and to eliminate the elements useless.

Then they increase the volume of waste, but during these last decades, environmental restrictions have been added to the economic concerns, all of these considerations inciting to increase the recirculation of the different flows and in particular the aqous flows, in the different circuits linked to the pulp production process.

However, this recirculation itself has consequences, thus, we observe both:

- increasingly large quantities of materials which previously evacuated from pulp mills with discharged water are now recirculated with these same waters,

- an increasing closure of the aqueous circuits which leads to a significant presence of calcium carbonates which, if they are useful compounds of the papermaking process, are also compounds which have the disadvantage of having - depending on the pH - variable solubility in the water which will disturb the separation selectivity of chlorine or potassium compounds.

More efficient methods have therefore been sought in order to sufficiently decrease the contents of chloride and potassium ions in the circuits recirculating the chemical reagents so as to prevent them from accumulating therein, while at the same time retaining the useful compounds as well as possible.

An improved solution for controlling the increase in the content of CI 'and K ⁺ elements has been proposed. It logically uses the same location, that is to say the ashes of the electrostatic precipitators since these ashes are enriched in Chlorides and Potassium, and it provides for the treatment of the ashes to selectively remove K ⁺ and Cl '.

This improved solution is the preferred solution for the paper industry. It is based on the difference in solubility in water of the salts present in the ashes of the electrostatic precipitators. It is first of all the difference in solubility between the alkali sulfates (Na ₂ SO ₄ and K2SO4) and the alkali chlorides (NaCl and KCI) which allows the separation of Cf and K ⁺ .

Three main technologies are used industrially for the treatment of electrostatic ash, all three apply this solution because they are all based on the difference in the solubilities in water of the salts coming from the ashes, and all three eliminate Cf and K ⁺ ions in a purge stream and recycle the purified chemical compounds in the liquor regeneration cycle.

1. leaching - or leaching; the electrofilter ash is partially dissolved in hot water, at a controlled ash / water ratio to obtain a supersaturated solution (liquid mud or slurry in English). The higher solubilities of NaCl and KCI than that of Na ₂ SO ₄ make it possible to dissolve the chlorides while Na ₂ SO ₄ is maintained in the solid phase. This dissolution step is carried out in a mixing reactor. Subsequently, the solid phase is separated from the liquid phase, either by centrifugation or by filtration.

2. evaporation / crystallization; the ash is completely dissolved, in an excess of water in a mixing reactor. The water is removed via one or more evaporators under vacuum (the condensates will be reused to put the ash in solution). The Na ₂ SO4 crystals are formed first, they are separated by centrifugation.

3. cold crystallization (freeze crystallization in English); the ashes are completely dissolved, in an excess of water in a mixing reactor. Cold crystallization is then carried out at a temperature below 20 ° C., a temperature below which the Na ₂ SO ₄ , 10H ₂ O salts (Glauber salts) precipitate. The crystals thus formed, larger than the crystals of Na ₂ SO _4, are easily separated.

These three technologies therefore exploit in the first place the difference in solubility between on the one hand the alkali sulfates (Na ₂ SO ₄ and K ₂ SO ₄ ) and on the other the alkali chlorides (NaCI and KCI) which allows to separate the ions K ⁺ and CI; they remove the K ⁺ and CL ions in a purge stream and recycle the purified chemical compounds in the liquor regeneration cycle. To be effective, they must however take into account the great complexity of the aqueous solutions containing mixtures of salts, and take into account in particular that the differences in solubility between the salts are disturbed when these salts are present in mixture in water .

The presence of NaCI decreases the solubility of Na ₂ SO ₄ due to the common ion effect; leaching and crystallization operations use this common ion effect to their advantage.

However, other salts present in the medium to be treated complicate the overall functioning of the system; thus, the conditions under which NaCI is separated from NazSO ₄ are also favorable for the elimination of Na ₂ CO ₃ , so that a greater quantity of sodium - useful element - will unfortunately be eliminated from the regeneration cycle of the liquors.

It should also be noted that eliminating K ⁺ from the regeneration cycle of liquors is more complex due to the variety of compounds that potassium ions can form. K ⁺ may for example be present in the form of potassium sulfate K2SO4, but also in the form of the double glaserite salt (3K2SO4, Na2SO4) after solubilization and recrystallization. These compounds containing K ⁺ ions tend to behave like NazSO4, so that the elimination of potassium is less.

In practice, it is impossible to carry out a complete separation of chlorides and potassium ions, we therefore seek to optimize the efficiency of the elimination of K ⁺ and CL ions and of the recovery of Na +, SO4 ² ′ ions, and CO3 ² '.

In order to achieve the selective elimination of chloride and potassium ions, the ashes recovered from the electrostatic precipitators must first be put in the form of an aqueous solution or of a supersaturated solution (slurry) by addition of supplemented water. by a part of the recycled filtrates) from the separation step; the solution or the saturated solution is then subjected to the process of selective separation of the K + and Cl · ions which it contains.

The initial pH of the ashes is typically of the order of 11.4 to 11.7; during dissolution, so that the carbonate ions contained in the solution are transformed into hydrogen carbonate ions, thereby improving the elimination of K ⁺ and Cl 'ions, - the pH must be lowered to a value of l '' from 8 to 10; in this way, the carbonate ions present are transformed into bicarbonates, which will promote the elimination of K ⁺ and CL.

This operation is conventionally carried out by adding sulfuric acid or a mixture of used acids recycled from the papermaking process, known as "spent acid" - according to its English name. This mixture of acids essentially consists of a mixture sulfuric acid, sodium sulfate, water with lesser amounts of chlorine dioxide and chlorine.

US 3866462 A describes a process for the removal of sodium chloride accumulated in the ash of electrostatic precipitators from the process for regenerating liquors without excessive loss of sodium sulfate and other useful chemical compounds. The process includes, in a simplified manner, a leaching of the electrofilter ash which contains sodium sulphate, sodium carbonate, sodium chloride as well as organic matter, with sufficient sulfuric acid to obtain a leached slurry of ph 3- 6, 15 thus converting the sodium carbonate into sodium sulfate and recovering sodium chloride.

WO 2014076361 describes a method which can be used in connection with a one or two step process for leaching electrostatic ash from the recovery boiler of a pulp factory, particularly when the ash contains a significant amount of carbonate . According to the method, calcium compounds, preferably calcium oxide CaO and / or calcium hydroxide Ca (OH) ₂ are used as additives in at least one leaching step. The liquid fraction formed is used outside the main recovery cycle, preferably in substitution for Na (OH) purchased in the pulp bleaching line. The fraction containing the solids can be added to the black liquor stream of the installation or subjected to an additional treatment to separate the calcium compounds for recycling.

WO 196012847 A describes a process for purifying the ashes of CL and K ⁺ ions by leaching, filtration and washing of the cake obtained, the leaching uses water as well as the recirculated filtrates from a previous washing. The method is characterized in that filtration and washing are carried out in a pressure filter in a single receptacle.

In view of the prior art, there is a need for a method for treating the ash from electrostatic precipitators which allows satisfactory recovery of the useful compounds, together with sufficient removal of chlorides and potassium ions, all avoiding the problems of the prior art, in particular linked to excessive use of sulfuric acid and / or spent acid; these problems being the increase in sulphidity in the recovery cycle of the liquors and / or the loss by landfill of part of the compounds which should have been recovered, with therefore a loss of useful products which must be compensated for, but also the emission of unwanted odors.

An object of the present invention is to provide a solution which makes it possible to solve this problem of increasing the sulfidity linked to the use of sulfuric acid and of used acids in significant quantity.

Another object of the invention is to provide a solution which does not provide additional pollution, which is easy to implement and which is economical.

The solution of the invention aims to replace all or part of the sulfuric acid and / or used acids used during the step of dissolving the ashes with carbon dioxide which will be injected upstream of the step of dissolving the ashes, this injection must ensure satisfactory dispersion of carbon dioxide within the liquid so that its efficiency is optimized in the mixing reactor.

For this, the solution of the invention is a method for treating the ash from electrostatic precipitators coming from the boiler of a liquor regeneration circuit in a Kraft process for the production of paper pulp, method comprising at least:

a step of dissolving the ashes in an aqueous medium, in a mixing reactor, with stirring so as to form a solution or a supersaturated solution, the supply of water being preferably provided by recycling the filtrate from the separation stage, supplemented with make-up water,

- an acidification step reduction of the pH of the solution - saturated or unsaturated,

a step for separating a liquid phase containing chloride and potassium ions from a solid phase containing sodium, sulphide and carbonate ions,

a step of recycling e all or part of said liquid phase at the step of dissolving, a second part of the liquid phase being able to be purged,

a step of recycling all or part of the solid phase in the liquor regeneration circuit, and in which the step of reducing the pH lowers the pH of the solution to a pH value of between 8.0 and 10 and is ensured, at least in part, by the injection of carbon dioxide into one of the aqueous streams ensuring the supply of water to the dissolving step. |

The ash dissolving and acidification steps are thus simultaneous thanks to the prior injection of carbon dioxide into one of the aqueous streams used for dissolving the ashes, thus ensuring the optimization of the dispersion of the acid in the dilution medium as well as the optimization of the ash solution which takes place at a favorable pH. There is therefore both an optimization of the use of carbon dioxide and an optimization of the quality of the acidification.

The invention uses CO2, pure or impure, in total or partial substitution of the aforementioned acids. The solution of the invention is applicable to all of the processes which have the same objective (leaching, evaporation-crystallization, or crystallization by cold) since for each of them, one of the steps prior to the separation of the compounds is the aqueous solution of the ashes or the formation of a supersaturated solution (or liquid mud or slurry) from the same ashes, each of the technologies using the difference in solubility in water of the salts present to selectively separate the K ions ⁺ and CI ', in particular in the presence of carbonates, and finally separating two phases, one aqueous and liquid, the other essentially solid and containing compounds to be recycled.

In the text, the term solution may designate a solution or a supersaturated solution - unless the context clearly shows that it is otherwise.

Carbon dioxide can be used in any solid, liquid, gaseous form. It can be used pure or impure, the main thing being to achieve a satisfactory dispersion within the medium.

The method of the invention may include all or part of the following variants, alone or in combination:

- the carbon dioxide can be pure or impure, and can be gaseous, liquid or solid,

- carbon dioxide can be injected into make-up water,

- carbon dioxide can be injected into the first recycled part of the liquid phase from the separation step,

- the carbon dioxide can be injected partly into make-up water and partly into said first recycled part of the liquid phase resulting from the separation step,

- carbon dioxide can be added directly to the pipe or pipes feeding the mixing reactor and in which the liquid circulates at a speed of the order of 1.5 to 3 ms ¹ , via a gas-liquid contactor or a static mixer with a residence time of carbon dioxide in the liquid sufficient for the mixing to take place, and at least equal to 10 seconds, preferably between 10 and 30 seconds,

- the carbon dioxide is added to the aqueous stream via a bypass loop formed on the pipe supplying the mixing reactor or on an intermediate buffer tank, in which the dissolution pressure and the residence time can be adjusted,

- the carbon dioxide injected being in the form of an impure gas, with a carbon dioxide content of less than 90%, preferably between 50% and 90%, an open reactor is used for dissolving the ashes so as to allow the evacuation of unconsumed gases,

- the carbon dioxide injected being in the form of an impure gas, with a carbon dioxide content of less than 90%, preferably between 50% and 90%, a closed reactor is used for the dissolution of the ashes which a gaseous sky is provided, the gas contained in the sky being recycled via a recirculation loop so as to maximize the consumption of residual carbon dioxide, a purge of the inert gases being carried out in the sky of the reactor,

the injection of the liquid formed by the mixture of carbon dioxide and water into the mixing reactor is carried out in a zone of maximized mixing within the reactor, preferably near the stirring means,

- a mineral acid is used to complement or partially replace carbon dioxide. The acid will preferably be added upstream of the injection of CO ₂ .

- the mineral acid contains sulfuric acid, or spent acid or a mixture of these two acids.

The invention will be better understood on reading the following description and in view of the figures in which:

Figure 1 shows schematically an example of implementation of the solution of the invention applied to the case of leaching of electrostatic filter ash.

FIG. 2 shows an example of CO ₂ injection according to a first preferred mode,

Figure 3 shows an example of CO ₂ injection according to an alternative preferential mode.

The method for treating electrofilter ash according to the invention as presented in a simplified manner in FIG. 1 operates in the following manner. The diagram presented corresponds to a one-stage leaching method: a leaching step followed by a separation step.

The ashes 1 of electrostatic precipitators are introduced via line 2 into the ash leaching reactor A - also called mixing reactor -, as well as make-up water 3 via the pipe 4, and the recycled part 11 of the filtrate 9 resulting from the separation of the supersaturated solution carried out at B. The mixture is dispersed with stirring, the resulting supersaturated solution 5 is pumped at 6, the supersaturated solution under pressure 7 being sent for separation into a centrifuge B (or any other means of adequate separation). The separation thus makes it possible to separate a solid phase 8 which is recycled in the liquor recovery circuit, and a liquid phase (or filtrate 9, part of which 10 is returned as an aqueous stream to the mixing reactor A for setting up ash solution, while a second part 11 is purged.

The ash treatment method according to the invention completely or partially replaces sulfuric acid and / or spent acid (added in the ash solubilization step to acidify the ash) with dioxide. carbon which is injected into at least one of the aqueous streams added to the mixing reactor. Thus, carbon dioxide can be injected at 12, that is to say in the added filtrate, it can also be added at 13, that is to say in make-up water.

Acidifying the ash solution (supersaturated or not) via carbon dioxide in total or partial replacement of the acids previously used by the papermakers in the liquor circuit makes it possible to optimize this step, in fact substituting CO ₂ for sulfuric acid or "spent acid" which is a byproduct of the chlorine dioxide generator (used for bleaching) is particularly advisable at the desired pH for this step (of the order of 8 to 10 approximately), the substitution is beneficial since the dioxide carbon is transformed in water into hydrogen carbonates. There is therefore little or no carbon dioxide CO ₂ in dissolved or free form which would be released and therefore lost on neutralization.

This injection can be done in different ways:

CO ₂ can be injected in one of three forms, that is to say gas (preferred), liquid but also solid (dry ice).

The addition of carbon dioxide according to the invention is not made in the reactor itself (in the suspension or in the gaseous sky above the suspension), or even in a recirculation loop of the suspension. This solution is possible, but is difficult and / or expensive to implement, due to the more liquid solid nature of the medium in which the addition is to be carried out.

The solution of the invention which injects carbon dioxide into aqueous streams entering the reactor free of solids or freed from a large part of the suspended solids, that is to say in:

the dilution water, stream 4 of FIG. 1, the recycled part of the filtrate 9 leaving the separation step B, stream 10 of FIG. 1, thereby avoids these problems.

Depending on the hydraulic conditions of these flows, in particular pressure and speed in particular, and in order to obtain a maximum transfer of CO ₂ into the aqueous flow, provision may be made, as the diagrams in Figures 2 and 3 show:

According to the diagram in Figure 2, a simple way to inject carbon dioxide into an aqueous stream (water or filtrate!) Can consist, as long as the hydraulic conditions) are good, i.e. turbulence and average speed ideally between 1 , 5 and 3 m / s, to inject carbon dioxide directly into the pipe, as long as the pipe length is sufficient to guarantee a residence time of carbon dioxide in the liquid from 10 to 30 s minimum before the introduction into the reactor. The injection of CO ₂ can be done via a static mixer or any other equivalent device, for example a gas-liquid contactor of the “CS nozzle” type).

If the hydraulic conditions do not allow direct injection into the aqueous stream (insufficient conditions, too slow speed for example), an alternative solution according to the invention consists in adding a loop fitted with a pump taking up a greater or lesser part of the flow, so as to have the hydraulic conditions favorable to the dissolution of the gas in the liquid and increase the pressure in the loop to inject CO ₂ more effectively; a back-pressure valve is preferably added at the end of the loop, it makes it possible to adjust the pressure, to the desired value, often between 2 and 10 b, preferably between 2 and 5 b, pressure which allows the optimization of the dissolution some gas. The residence time must also be sufficient there after injection, preferably greater than 5 s and more preferably between 10 and 30 s.

Installing a buffer tank can improve the installation of a loop.

Example of conditions for injecting carbon dioxide via an injection loop:

For a CO ₂ injection of 600 kg / h, with a solubility in the liquid of 1.5 g CO ₂ / I at atmospheric pressure and working at a pressure of 10 bar abs, a loop will preferably be installed mainly constituted:

a pump of 40 m3 / h with a total head (HMT) of 100 m minimum, connected to a CS nozzle type injector or static mixer of 80 or 100 mm in diameter, itself connected to a a length of 25 m minimum for a diameter 100 mm, fitted at the end of the loop with a back pressure valve allowing the pressure to be adjusted to the design pressure, i.e. 10 bar, the assembly emerging in the area of strong turbulence inside the mixing reactor.

The CO ₂ injected can be more or less pure, that is to say have a CO ₂ concentration of between 0 and 100%, preferably between 50 and 100% (therefore pure).

In the case of an impure gas, two solutions are possible:

The first consists in injecting the gas with reduced injection performance compared to a pure gas (less partial pressure). Thus, the residual CO ₂ in the outlet gas after being brought into contact with the substrate / suspension can be returned to the atmosphere. In this case, the reactor is opened.

| The second consists in closing the reactor and recycling the gas [after being brought into contact with the suspension according to the methods already described, namely a self-aspirating turbine or an external loop so as to consume the maximum of residual CO ₂ . In this case, a purge of the inert materials (excluding CO ₂ ) must be carried out on the reactor atmosphere. This purge can be easily determined from the concentration of

CO ₂ and the flow rate of the incoming gas on the one hand and the CO _{2 requirement} of the suspension.

Claims (12)

1. Method for treating electrofilter ash from the boiler of a liquor recovery circuit in a Kraft process for the production of paper pulp, method comprising at least: a step of dissolving the ash in an aqueous medium, in a mixing reactor, with stirring so as to form a solution or a supersaturated solution, the supply of water being ensured by recycling the filtrate from the step of separation, supplemented by make-up water, a step of acidification of the solution - saturated or unsaturated, a step of separation of a liquid phase containing chloride and potassium ions, of a solid phase containing ions sodium, sulphides and carbonates, a step of recycling all or part of said liquid phase at the level of the dissolution step, a second part of said liquid phase can be purged, a step of recycling all or part of the solid phase in the liquor regeneration / recovery circuit, characterized in that the acidification step lowering the pH of the solution to a pH value between 8 and 10 is carried out, at least in part, by l 'inj ection of carbon dioxide in one of the aqueous streams ensuring the supply of water during the dissolving step. 2. Method according to claim 1 in which the carbon dioxide can be pure or impure, and can be gaseous, liquid or solid, 3. Method according to claim 1 or claim 2 wherein the carbon dioxide is injected into the make-up water. 4. Method according to one of claimsl to 3 wherein the carbon dioxide is injected into said first recycled part of the liquid phase from the separation step. 5. Method according to one of claims 1 to 3 wherein the carbon dioxide is injected into make-up water and into said first recycled part of the liquid phase resulting from the separation step 6. Method according to one of claims 1 to 5 wherein the carbon dioxide is added directly into the pipe or pipes feeding the mixing reactor and in which the liquid circulates at a speed of the order of 1.5 to 3 ms ' ¹ , via a gas-liquid contactor or a static mixer with a residence time of carbon dioxide in the liquid sufficient for the mixing to take place, and at least equal to 405 seconds, preferably between 10 and 30 seconds. 7. Method according to one of claims 1 to 5 wherein the carbon dioxide is added to the aqueous stream via a bypass loop formed on the pipe supplying the mixing reactor, or on an intermediate buffer tank, in which the pressure of dissolution and residence time can be adjusted. 8. Method according to one of claims 1 to 7 wherein the carbon dioxide injected being in the form of an impure gas, with a carbon dioxide content of less than 90%, is used for the dissolution of the ashes reactor open so as to allow the evacuation of unconsumed gases. 9. Method according to one of claims 1 to 7 wherein the carbon dioxide injected being in the form of an impure gas, with a carbon dioxide content of less than 90%, is used for the dissolution of the ashes closed reactor in which an overhead gas is provided, the gas contained in the overhead being recycled via a recirculation loop so as to maximize the consumption of residual carbon dioxide, a purge of the inert gases being carried out in the overhead of the reactor. 10. Method according to one of claims 1 to 9 wherein the injection of the carbon dioxide-water mixture into the mixing reactor is carried out in a zone of maximized mixing within the reactor, preferably near the stirring means. 11. Method according to one of claims 1 to 10 according to which a mineral acid is used, in addition to or in partial replacement of carbon dioxide. 12. Method according to claim 11 according to which the mineral acid contains sulfuric acid, or spent acid "spent acid" or a mixture of these two acids.