EP3802439A1 - <sup2/><sub2/> 2 method for treating an industrial effluent charged with aluminium using co - Google Patents

Abstract

The invention proposes a method for treating an industrial effluent which is charged with aluminium in order to eliminate all or some of the dissolved aluminium therefrom, comprising the implementation of the following steps: - the effluent to be treated is conveyed to a first zone, which is constituted, for example, by a tank, and in which a pH lower than 9.5 is maintained, and more preferably a pH between 6.5 and 8.5, and even more preferably between 7 and 8, so as to promote the precipitation of the aluminium in the form of aluminium hydroxide and thus make it easier to eliminate it; - a second zone is provided and a portion of the medium which is located in zone 1 is recirculated to zone 2, then the medium is returned from there to zone 1, and gaseous CO2 is injected into the recirculated medium; and - the solid particles which are formed in zone 1 are separated and discharged. The method is characterised in that, taking into account the recirculation of said medium into which CO2 has been injected, the quantity of dissolved CO2 available in zone 1 is 0.5 to 3 times greater, preferably between 1 and 1.5 times greater, than is necessary to precipitate the incoming effluent.

Description

Method of treatment of an industrial effluent loaded with aluminum using CO ₂

The present invention relates to the field of treatment of highly alkalinized effluents and loaded with metals, including alkaline earths with a view to neutralize them and significantly eliminate the metals present in the effluent. This problem is found particularly in the iron and steel industry, but other industries may be concerned and in particular we can cite the case of the treatment of water from the production of aluminum based on natural raw materials such as bauxite (called "sludge"). red ", very heavily loaded with aluminum, among others).

With the addition of carbon dioxide, it is possible to neutralize this type of effluent and to precipitate the metals in order to eliminate them. But in the vast majority of cases, a direct injection poses the problem of complete or partial blockage of the injection systems and losses of performance of the installation. Indeed, the injection is intended to neutralize but also to precipitate mineral compounds, the formation of solids in the vicinity of the injection points, where the CO2 required for precipitation is introduced, is mandatory. It is understandable that a very large amount of solid, up to several tons per hour, forming in the vicinity of the CO2 injection points, they can clog easily and adversely affect the performance of the installation.

The problem with this method is that the direct addition of the acid (CO2) in a very basic water and loaded with metals causes an instantaneous precipitation that is difficult to control. And many industrial cases reported where injection sites and mixing as pipes are clogged very quickly.

Let us now consider in the following the case of the treatment of water from the production of aluminum. It is then a question of neutralizing an aqueous liquid flow which contains a lot of dissolved aluminum, which it is necessary to eliminate before rejection of the waters. In general, this effluent is very basic and the aluminum is dissolved in the form of aluminate (Al (OH) ₄ ). This is due to the processing of ore made with soda ("leaching").

 The removal of aluminum can be achieved by adding CO2 to convert the solubilized aluminate in basic medium aluminum hydroxide AI (OH) 3 which, poorly soluble, will tend to precipitate. The pH of the effluent is reduced, or more exactly neutralized or partially neutralized and thus freed of aluminum, it will be purified. The working pH range for this operation is generally:

 - low, therefore lower than that of the incoming effluent to be treated,

- but the pH can not be too low (<5) because then the aluminum solubilizes in Al ³⁺ form.

In summary, for the case of aluminum, an optimum working pH of typically between 5 and 8.5 is recommended.

 Nevertheless, the dissolution of CO2 and its contact with the effluent loaded with electrolytes is always difficult. Indeed, CO2, as for any strong acid, causes a high concentration of acid at the injection site. In this highly acidic zone, the formation of solid aluminum hydroxides is very important and the risk of clogging of the injection point is then very high.

In other words, in general, the effluent contains a lot of aluminate which effectively requires a lot of acid to cause precipitation and neutralization. Unfortunately, this operation proves difficult to achieve because:

 • The homogenization or mixing of the effluent (liquid) and the gas (CO2) is not easy, that is to say not instantaneous. Also, the injection zone (whether it is an injector or a perforated tube for example), which forms the interface between effluent and gas turns out to produce very quickly a lot of precipitates which cause blockages. difficult to eliminate: stopping and stripping with strong acid for example.

• Furthermore, the particles formed are fairly strong and can lead, if they accumulate, clogging in the downstream portion of the injection site. • Finally, the various stages (injection / transfer of the gas and contact of the dissolved CO2 with the effluent containing the aluminates) are often carried out in the same zone, the same apparatus or piping, while the conditions required for each step are different. .

Thus, in summary, given the elements mentioned above, the CO2 injection operation combined with the precipitation can be very difficult to drive or almost impossible and its application therefore abandoned by the skilled person.

An example of treatment provided in this industry is described below:

 an encrusting / scaling effluent containing a lot of dissolved aluminum to be removed. The effluent with a high pH (pHi close to 12) is sent to a basin which is kept at a lower pH (pht close to 7-8.5) by CO2 injection. This value is considered ideal for precipitating aluminum hydroxides.

 - the effluent neutralized with its solid leaves by the bottom of the basin (pumped) to be subsequently decanted. At this stage, it is also possible to use the addition of products (such as viscosity, electrolyte, coagulant or surfactant, for example) to enlarge the size of the solid particles initially formed (by agglomeration, acceleration of the crystallization, etc.) in order to improve their "decantability" or filtration.

- a variant is often encountered: the injection is alternatively directly carried out not in a basin but in line, in a pipe. In the effluent, after pumping, the gaseous (or liquid) CO2 is injected, via a means that goes from the simplest (for example a simple opening pipe) to the most refined (many are the gas-liquid contactors commercially available to improve dissolution, such as static mixers, venturis or the like ...). The particles are then formed directly in the flow, a flux that normally remains only to separate from the liquid (filtration, settling tank ...). But unfortunately, as has been pointed out, the direct addition of the acid (CO2) in a very basic water and highly charged with aluminate causes an instantaneous precipitation that is difficult to control. In a generalized way, when the water is very charged with dissolved aluminum, the acidification causes massive blockages very frequent, so frequent that:

 - Because of this, often, for online treatments, the pipes are doubled to allow to continue operating on one line while the other is cleaned with strong acids, possibly under high pressure. Running costs and maintenance are prohibitive.

 - and for treatments in agitated tanks, we often operate in "batch" mode (also called "batch"), that is to say that the diet and treatment are limited in duration, again, for allow cleaning.

It can be emphasized that the deposits thus made are difficult or very difficult to eliminate and that it often takes both a chemical action (strong concentrated acid) and a mechanical action (scraping, jet high pressure ...) to eliminate the deposition layer. This is even more difficult in closed processes such as pipes.

In order to overcome the drawbacks mentioned above, it is possible to think of a first solution consisting of dispersing the CO2 at best at the inlet of the clarifier-decanter, for example by injecting the flow of CO2 at the center of the clarifier at one or more points. injection.

Nevertheless, this solution has several disadvantages:

 - Locally around each injector, one finds oneself in the most difficult situation since one directly adds the pure CO2 in the effluent, which causes the precipitation, even at the point of injection. Optionally, it is necessary to renew the maximum liquid around the injectors but it remains delicate in a stirred tank.

the transfer rate, that is to say the dissolution of CO2, can be limited. Indeed, it is difficult to have a stirred tank that keeps the bubbles, whatever happens, in the liquid, enough time for the dissolution to be almost complete. Bubbles can coalesce, rise quickly, regardless of the desired liquid circulation, which makes them stand out without having had time to be consumed. The phenomenon will be more marked if the stirring means are not efficient and it will be critical when the apparent density of the solution will increase. Indeed, in concentrated solution, the aluminates can polymerize (inorganic polymerization), trapping particles ... which will cause a sharp increase in viscosity. It will then be very difficult to maintain good agitation, very difficult to have a gas-liquid interface, which will harm the dissolution rate (therefore a total interface related to the fine size of the bubbles and therefore to their large number) of same as to prevent the clogging (the holes, being blocked, will deform the flows injected ...).

finally, a large quantity of CO ₂ will not be dissolved and will be released in the sky from the stirred reactor. The CO ₂ being toxic, it will be necessary to manage it: addition of a cowling, addition of detector, purge and thus loss of some of the CO ₂ introduced ....

A second solution was then proposed in the literature consisting in avoiding direct contact, this using a water, often clean (industrial network on site for example), previously carbonated. Thus, beforehand, the CO ₂ is injected into a water, clean industrial (not an effluent). This "diverted" way of adding CO ₂ is similar to the manufacture of seltz water.

A large quantity of CO ₂ is dissolved. Only then, this "driving" water containing a desired quantity of dissolved CO ₂ is mixed with the effluent to be treated in a zone of pH such that aluminum hydroxide, which is sparingly soluble, precipitates in the stirred tank (associated with a downstream separation means) or directly within a clarifier or decanter.

Nevertheless, this solution has the following disadvantage: water is consumed with an associated cost and an unfavorable environmental footprint because it increases the total liquid flow that must be treated on the decanter by diluting the effluent. As will be seen in more detail in what follows, the present invention then seeks to propose a new solution for treating such effluents rich in aluminum, allowing the optimal implementation of CO2.

For this, acidification using a weak diacid such as CO2 (and not a strong acid such as sulfuric acid for example) has several advantages:

 - a better control of the target pH to carry out the precipitation.

- the consumption of CO2 that can be recovered as a by-product of manufacturing or combustion (this is called "fatal" CO2).

 solid formation such as aluminum hydroxide. This is indeed very slightly soluble which guarantees a good efficiency in the precipitation process.

The proposed solution according to the present invention is then based on the separation of the process into two distinct phases, into two distinct zones:

 - The first phase: during which we seek to achieve the generation of crystals in the form of aluminum hydroxide mainly. The conditions which prevail there, in particular a low pH (preferentially of 5 to

8.5), are favorable there and in no case a direct injection of CO2 will be realized in this zone because it would cause the malfunctions of the prior art as previously mentioned.

 This zone "1" may be a basin, an area in which the highly alkaline and aluminum-rich effluent arrives (for example at a pH of the order of

12.5).

It is then ensured, as will be explained in more detail below, to have sufficient dissolved CO2 in this zone so that the effluent in this zone changes from an alkaline pH to a pH that is preferably less than 9. 5 and more preferably between 6.5 and 8.5 in all cases. Indeed, if the flow rate of effluent to be treated is variable, the amount of compounds to be neutralized is variable and it is therefore necessary to avoid, during a peak quantity (concentration and / or flow) to be in a situation of not have enough dissolved CO2 to neutralize it. By this reduction in pH the dissolved form of the aluminum (aluminate) will pass in the form of aluminum hydroxide and precipitate.

 In summary the formation of the solid that can be incrustant takes place in zone 1 and so if the solid is incrustant at the time of its formation the problem appears in this first zone which must contain enough dissolved CO2 to "neutralize" the raw effluent incoming.

- A second phase (second zone): in which one seeks to realize the injection and the maximized, maximized dissolution of the CO2 _, by limiting very strongly, or even by canceling any phenomenon of precipitation, more exactly of generation of hydroxide crystals aluminum.

To do this, it is proposed according to the present invention that the second zone, which can be called "CO2 dissolution zone" can:

 - Allow to take a portion of the effluent already treated, that is to say, which contains more or almost no more electrolyte to precipitate, so aluminate (aluminum in dissolved form). This flow is therefore derived from a part of the effluent leaving this zone 2 and therefore from the complete process. In the absence of dissolved aluminum (or in very small quantities), CO2 can be injected into this diversion / recirculation without causing significant precipitation since there is no more or almost no more.

 - CO2 is injected into this sample by maximizing the mass transfer of the gas to the liquid. This is guaranteed by the hydraulic regime (turbulent regime if possible), the lowest temperature if possible (preferably 5 ° to 45 ° C, and even more preferably between 15 and 30 ° C), the highest possible pressure (preferably between 1, 5 bar and 20 bar, and even more preferably between 2 and 4 bar, but which is reasonable for reasons of cost implementation, it is therefore preferred a pressure of less than 10 bar) and a mixing time during which the gas and the liquid remain in contact for a long time (preference is therefore given between 3 and 30 seconds, preferably between 5 and 15 seconds of contact).

in other words, zone 2 serves to dissolve the CO2 in a fluid flow that is pumped from zone 1 and that is returned to zone 1. This zone is calculated to dissolve enough CO2 to lower the pH of zone 1 of the arrival value of the alkaline effluent (for example 12.5) at the desired setpoint (for example 8).

 In this zone 2, there is no reduction of a strongly alkaline pH (such as 12.5) to a neutral or acidic value, but zone 1 fluid, thus close to neutral, is pumped for the first time. acidify even more. There is therefore no formation of solid in this zone 1 (or in an anecdotal manner) and thus the risk of clogging is eliminated or reduced very significantly.

 - So the pH of the pond / zone 1 is calculated by calculating for a given flow rate and a given initial pH, the amount of CO2 that must be dissolved in zone 2 (recirculation loop) to reach the desired pH in the zone 1, with obviously a margin of safety.

the sizing is based on the combination of a gas-liquid ratio (generally between 0.1 and 5 Nm ³ of gas per m ³ of liquid, preferably 0.1 and 1 Nm ³ of gas per m ³ of liquid ), the pressure in this zone and a flow of effluent taken that will dissolve all the CO2 necessary to maintain the desired operating conditions, in particular the pH in the zone 1 in which the initial effluent arrives, in general continuously.

 the solution will include, in particular, a pump, a gas-liquid contactor (a static mixer for example), a pipe of good size to remain in a turbulent regime and of sufficient length to guarantee the desired residence time (and therefore the contact time ), for example a contact time close to 10 seconds.

- Dissolution being made largely in this zone 2, the carbonated effluent can be returned to the first zone. It therefore contains substantially CO ₂ in dissolved form, namely CO ₂ and bicarbonate HCO3 and therefore hardly gas if the operating conditions have been met (transfer rate higher than 80% or even 90%). The operating conditions are thus maintained to guarantee these dissolved forms, namely in general a low pH (less than 8 to 9), preferably not falling below 5.

In summary, Zone 1 must allow: - First of all, the most intimate mixture possible between the fresh incoming effluent (charged with aluminate) and the by-product stream which was carbonated in zone 2 and which is returned to zone 1 (charged with enough dissolved CO ₂ , in HCO3 form and especially CO ₂ , to guarantee the target pH of precipitation in zone 1). This will maximize the production of solids / precipitates. The mixture will then consist of liquid freed of a large part of its aluminum and solid particles.

 - Similarly, to control well and ensure precipitation in all cases, that is to say, even if the incoming effluent changes quality (more or less alkalii resulting from the product flow by alkali concentration) changing), it is preferable to provide 1, 5 to 2 times more CO2 dissolved in zone 1 than necessary to neutralize all the average incoming effluent (stoichiometric need in dissolved CO2 to reach the pH target value to precipitate the maximum dissolved aluminum). Thus, any peak demand (thus the alkalinity peak of the incoming effluent) will be well neutralized by the dissolved CO2 present in zone 1. In summary, zone 1 is sized to ensure a desired target volume / pH pair.

These zones 1 and 2 will therefore consist for example of:

 for zone 1, a tank which contains a stirred part allowing the good mixing of the liquid streams (fresh effluent to be treated and return from zone 2) and good precipitation (no strong shear by the agitator (s), sufficient residence time if it operates in "batch", semi-continuous or continuous ...), even a partial decantation (via a zone protected by a deflector for example) to be able to extract a part of the treated effluent in this tank and send it to zone 2 (recirculation pipe). After treatment, the liquid and solid mixture can be decanted directly into the tank ("batch" operation on a stirred tank) or can be decanted downstream.

for zone 2, for example a recirculation loop mounted on the equipment of zone 1, namely a pump which sends the flow to a gas-liquid contactor (static mixer) which makes it possible to inject and to mix the gas-CO2 and the liquid to promote the dissolution of the CO2 and finally enough length of piping to guarantee a residence time sufficient to favor, here too, the dissolution before the return of the water thus carbonated in zone 1. Its sizing is based essentially on the gas-liquid ratio, the quantity of CO2 necessary to precipitate what is wanted (almost all in general) the aluminum contained in the effluent to be treated fresh and the solubility of CO2 in the effluent. The total volume of zone 2 is therefore not critical here.

It is the whole of this optimized implementation (in particular without motive water), in particular for effluents or aqueous solutions very highly concentrated in electrolyte (mainly in aluminum), which is remarkable in the present technical proposal.

Let us illustrate in the following the invention by an implementation example and by the appended FIG. 1, for a better understanding of the approach of the invention.

Consider an effluent, at a flow rate of 30 m ³ / h with an initial pH of 12, which one wants to lower to 8.2 to be able to reject it in a network while having precipitated beforehand the aluminum salts that 'it contains.

It is therefore necessary to implement in terms of CO2 of the order of 600 g / lx 30 = 18 kg / h of C0 _2.

 In the proposed implementation, the effluent arrives in the center of the neutralization basin where it ensures a good homogeneity, basin equipped with a stirring system, possibly supplemented by an additional agitator if what is already in place is not enough.

 The recirculation loop must therefore provide in dissolved form at least 18 kg / h of CO2.

The temperature of the effluent being around 25 ° C, the solubility is of the order of 1, 4 kg CO2 / 1T1 ³ to 1 bar abs.

The loop working at an absolute pressure of 2 bar, the enrichment flow must be close to 6.5 m ³ / h.

Those skilled in the art understand that they will have to take a margin and then retain a loop rate of the order of 10 m ³ / h.

It is therefore a question of treating an encrusting / scaling effluent, containing a lot of dissolved aluminum to be eliminated. The initial effluent at high pH (pHi neighbor of 12) is sent to a basin that is kept at a lower pH (next to 8-8.4). It is the target value chosen according to the invention in this case for precipitating the aluminum hydroxides.

 The neutralized effluent with its solid exits the bottom (pumped) to be subsequently decanted (filtration).

 Part of the contents of the tank is pumped by the external loop (zone 2) in which the CO2 is injected via an injector (for example a static mixer), this effluent was removed in the basin of the the vast majority of its dissolved minerals, it is therefore accordingly, much more weakly scaling.

 A turbulent regime is maintained throughout the loop. Then, under pressure generated by the pump, a pH (phb) even closer to the neutrality or acidity (pH3 <pH2 <pHi) pH which guarantees the major formation of hydrogencarbonate and the presence of dissolved CO 2 is reached. .

 The gas-liquid mixture is then sent into a coil whose length ensures a sufficient contact time to maximize the amount of CO2 transferred into the stream.

 Finally, this flux acidified with pFb is returned to the basin where, as close as possible to the incoming flow at pHi, it will mix to guarantee the phh that prevails in the basin, phh optimum for the formation of crystals of aluminum hydroxide .

It may be noted that in this case, the lower the pH, the more the precipitation is promoted, to a pH limit of 5.

 In summary, it can be noted that:

- The incoming effluent, at high pH (10-12), is mainly composed of dissolved aluminum in AI (OH) _{4 form} .

 - To rid it of a maximum of dissolved aluminum, it is advantageous to work between 5.5 and 8 approximately, where a large part of the aluminum is transformed into AI (OH) 3 which is not very soluble .

- It is almost impossible for CO2 to have a pH lower than 5-5.5 and to resolubilize the particles of aluminum hydroxide that have been formed. CO2 is a weak diacid whose first pKa does not allow to fall below about 5. - The effluent, after being treated and thus rid of a large amount of dissolved aluminum, will be able to absorb CO ₂ (part of the flux derived between zones 1 and 2). It is nevertheless preferred to work in the upper part of the targeted zone (rather to 8 than to 5) in order to have more dissolved CO ₂ (better solubility of CO ₂ at high pH than low because the hydrogen carbonate form HCO3 is favored rather than CO ₂ free).

The appended FIG. 1 recognizes the following elements, already mentioned several times in the description above:

 a basin 1 constituting the zone 1, equipped with a stirring system 3 and fed with the initial effluent to be treated 4;

 - A recirculation loop 10 constituting the zone 2, adapted to take (5) a portion of the medium present in the basin 1 through a pump 2, loop which receives a CO2 injection and which is provided with a coil whose length ensures sufficient contact time to maximize the amount of CO2 transferred to the flow;

 at the end of the loop, the flow thus treated is returned to the pond 1, thus contributing, by mixing the initial effluent (4) and the medium treated with CO 2 in the loop 10, to achieve the target pH prevailing in the basin 1;

 the basin is provided with means (6) for extracting the treated effluent.

The advantages of this solution are:

 - To avoid the consumption of industrial type water to produce a soda water as in the conventional solution of the prior art.

- Always avoid any precipitation in the area where the CO2 will be injected. The precipitation zone 1 (reactor, decanter, etc.) must always be maintained at a pH lower than that of the incoming flow to be treated in order to allow the precipitation of aluminum in the form of aluminum hydroxide (around pH 8 per minute). example). The effluent to be treated arriving is then diluted in zone 1, which should lead to a slight increase in pH, which will in fact be offset by the injection of CO2 into zone 2, namely the recirculation loop. We try to ensure a quantity of dissolved CO2 higher than the average requirement (1, 5 to 2 for example) by the choice of the right pair of zone 1 volume and target pH. - To guarantee a maximized CO2 transfer rate, which results from the choice of operating conditions and technologies in the recirculation loop (turbulence). This will result in a consumption of the system's needs (no overconsumption).

Zone 2 only serves to dissolve the CO2 in a water flow (effluent) pumped from zone 1 and return to zone 1. This zone is calculated to dissolve enough CO2 to lowering the pH of zone 1 of the arrival value of the alkaline effluent example 12.5 to the example set point 8. It is also possible to use this zone to supply a portion of the CO2 in the form of gas (fine bubbles) from zone 1 to 2.

 In this zone 2, there is no reduction of a strongly alkaline pH (12.5) to a neutral or acidic value, but the effluent from zone 1 is pumped, thus neutral or acid, to acidify it further. more. So there is no solid formation in this zone 1 (or anecdotally) and thus reduces the risk of clogging. It is even possible if even more CO2 is injected and this zone is further acidified, the aluminum solids formed at neutral pH are dissolved and the zone 1 can be decoloured even if in theory it is not necessary.

As mentioned above, it is attached according to the present invention that, given the recirculation of the medium in which CO2 has been injected, the amount of dissolved CO2 available in zone 1 is 0.5 to 3 times greater, preferably between 1 and 1.5 times greater than the need necessary for the precipitation of the incoming effluent.

 Let's explain this in more detail in the following.

Let's explain how to determine the CO2 requirement of a pond (zone 1) and an example of calculation for a quantity of dissolved CO2 available that is 0.5 to 3 times higher than the CO2 needed for precipitation in zone 1 .

The effluent entering zone 1 is very alkaline (thus pH 1 and high dissolved aluminum concentration). This is where he will be put in contact with the effluent, the pH was lowered to 3 ^rd pH containing dissolved CO2 necessary from zone 2 (pH 2 low since the effluent will contain at least all the CO2 necessary for the precipitation). In contact with the two, the dissolved CO2 will neutralize the alkalis of the incoming effluent and thus reduce the pH to allow precipitation and thus rid the effluent of its dissolved aluminum. The resulting pH pH 3 will be between the two previous pHs, adjusted to promote precipitation.

 In continuous operation the necessary amount of CO2 supplied (thus of weak diacid) compensates as well as the alkalinity of the incoming effluent (thus stoichiometric ratio between acid and alkali or base). Nevertheless, if there is a sudden change in operating conditions and the amount of alkali increases, an imbalance in the acid-base ratio is created which must be compensated. We can then reach the fact that the effluent which circulates continuously in zone 2 may have a supplement of alkali which will cause precipitation in this zone. This can even block the complete system by plugging, often very fast in many applications because of the high alkalinity of the effluent to be treated. Of course, a control-regulation system could adjust the amount of CO2 to the amount of alkali (which will have to lower the pH of the precipitation zone 1) but this is still difficult. In fact, the quantities involved (size of zone 1) can induce a slow evolution of the operating parameters: with a sudden and strong influx of alkali, the pH of zone 1 will only move slowly if it is of significant size. (and therefore with a significant residence time). Thus the amount of CO2 will not respond immediately or even too late, which can cause a rapid and strong undesirable precipitation in zone 2, a phenomenon that must be absolutely avoided at the risk of stopping everything by a clogging too massive.

 The approach of the present invention is therefore to keep a larger quantity than required by the effluent entering free CO2, dissolved and available in zone 1 where precipitation takes place.

Thus, if there is a variation of the operating conditions (quantity of alkali entering for example), this supplement of dissolved CO2 compared to the quantity of CO2 necessary for the precipitation (thus stoichiometric) will allow to "neutralize" this supplement in a given time. This will avoid in any state The reason is to send dissolved aluminum or alkali to Zone 2 and allow time for the control system to adjust the amount of CO2 to be injected to compensate for this supplement.

 It is estimated according to the present invention that an excess of dissolved CO2, therefore available, of the order of 0.5 to 3 times the amount necessary for the neutralization and therefore the optimized precipitation of the incoming effluent is required.

The example below makes it possible to illustrate more clearly the proposal of the invention.

The data below were obtained using commercially available software to study equilibria in water. The simulation was carried out in several stages: from a water composition (standard), the addition of soda allowed to rise to pH 12. This allows to have a "synthetic" effluent. For this, it was necessary to add 0.56 kg / m ³ of soda.

Then, the addition of CO2 allowed to neutralize it at pH 8 first and then 7.5 It is the effluent neutralized at 8 which was then preserved for example. To neutralize the effluent from 12.0 to 8.0, it was necessary to add 0.60 g / m ³ of CO2, leaving 18 g / m ³ of free CO2 in the effluent.

Also, for an effluent flow rate of 100 m ³ / h, it will take 1 kg / min of CO2 to neutralize it.

In addition, if the basin (or zone 1) measures 55 m ³ , it will contain only 1 kg of free CO2, therefore available to compensate for excessively sudden excess of alkalinity. This would compensate for one minute the arrival of effluent, in case of stopping the injection of CO2 for example, or to compensate for a rise in the amount of alkali (for example, 1 kg / min of need at 1, 5 for example). In the latter case, in 40 seconds, the free CO2 will be consumed and the pH will then go up, the reactor or zone 1 will no longer precipitate all the incoming alkali and all the process will destabilize. Eventually, dissolved alkali will enter Zone 2, which could lead to precipitation and clogging. It is therefore preferred to increase the volume of zone 1 so as to have more free CO2 available to "erase" or neutralize the fluctuations or disturbances of the incoming effluent, among others.

Still in the above case, with a zone 1 of 166 m ³ , we will have 3 kg of free CO2 to neutralize a supplement of incoming alkalinity. If this excess causes a CO2 requirement of 1.5 kg / min, the system will drift only after 2 minutes. Thus, again in this example, the reaction time necessary to compensate for the sudden excess of alkalinity will have been multiplied by 5. There is therefore much more flexibility and robustness for the process since it allows the complete system time to control and regulate itself.

 In summary, the determination of zone 1 is carried out so that this zone contains at least 1.5 times to 3 times the amount of CO2 necessary to neutralize the alkali that arrive every minute in the zone and thus allow the precipitation of almost all of the aluminum (its oxides) in this zone 1. The amount of free CO2 in the area can be increased beyond 3, but not exceeding 10 or 15 for economic reasons.

In our example, the free CO2 is 18 g / m ³ for a target pH of 8 and 57 g / m ³ for a set pH of 7.5

 To determine the volume of zone 1 and calculate the amount of free CO2 available, divide the quantity of free CO2 in kg required by the free CO2 concentration in the effluent at the target pH.

 Thus: Volume of zone 1 = amount of CO2 in kg that has been determined / concentration of free CO2 at the target pH.

Claims

A method of treating an industrial effluent charged with aluminum to remove all or part of the aluminum, comprising the following measures:

 the effluent to be treated is conducted in a first zone, first zone for example consisting of a basin, the first zone where a pH of less than 9.5 is maintained, and more preferably between 6.5 and 8.5, and even more preferably between 7 and 8, so as to promote the precipitation of aluminum in the form of aluminum hydroxide and thus facilitate its removal;

 a second zone is arranged and the recirculation of a portion of the medium located in zone 1 to zone 2 and then its return to zone 1 and the injection of gaseous CO2 in the medium are organized recirculated;

 - the separation and evacuation of the solid particles formed in zone 1,

 characterized in that, in view of the recirculation of said medium in which CO2 has been injected, the quantity of dissolved CO2 available in zone 1 is 0.5 to 3 times greater, preferably between 1 and 1.5 times greater than the necessary need to the precipitation of the incoming effluent.

2. Method according to claim 1, characterized in that the regime prevailing in zone 2 is a turbulent regime.