Treatment of industrial wastewater containing aluminum using co2

Abstract

To provide a method for treating industrial wastewater containing aluminum in order to remove all or part of dissolved aluminum.SOLUTION: A method is presented comprising the implementation of the following steps of: conveying the wastewater to be treated to a first zone, constituted for example by a tank, in which a pH of less than 9.5 is maintained, in order to promote the precipitation of aluminum in the form of aluminum hydroxide and to facilitate its elimination; providing a second zone, recirculating a part of the media located in the zone 1 to the zone 2, then returning the media from there to the zone 1 and injecting gaseous CO2 into the recycled media; and separating and discharging the solid particles formed in the zone 1. In view of the recirculation of the media injected with CO2, the amount of dissolved CO2 available in the zone 1 is 0.5 to 3 times higher, preferably 1 to 1.5 times higher than the amount necessary for the sedimentation of the incoming wastewater.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to the field of the treatment of highly basified wastewaters containing metals, including alkaline earth metals, for example, with a view to neutralizing them and to removing significantly the metals present in the wastewaters. This problem is particularly found in the steel industry, but can concern other industries as well, and in particular in the case of the treatment of aqueous liquors resulting from the manufacture of aluminium based on natural raw materials such as bauxite (known as "red mud" with a very high aluminium content). [Background technology]

[0002] By adding carbon dioxide gas, it is possible to neutralize this type of wastewater and to precipitate the metals, removing them. However, very often, direct injection presents problems of total or partial blocking of the injection system and loss of the performance quality of the equipment. This is because, since the injection is aimed at neutralization, but also at the precipitation of inorganic compounds, the formation of solids is inevitable near the injection points, where the CO2 necessary for precipitation is introduced. It is easy to see that these injection points can easily become blocked and can impair the performance quality of the equipment, since very large amounts of solids, which can reach several tons per hour, can be formed near the injection points of CO2.

[0003] The problem with this method is that the direct addition of acid (CO2) to a very basic water body containing metals results in instantaneous precipitation that is difficult to control. There are numerous reported industrial cases where injection and mixing sites such as conduits become blocked very quickly.

[0004] Consider the case of treating aqueous liquor resulting from the manufacture of aluminum. There is the problem of neutralizing the aqueous liquid stream containing large amounts of dissolved aluminum which must be removed before the aqueous liquor can be discharged. Generally, the wastewater is highly basic, and aluminum is converted to the aluminate ion (Al(OH)4 -) due to the treatment of the ore ("leaching") carried out with sodium hydroxide.

[0005] It is possible to remove aluminum by adding CO2 to convert the dissolved aluminate ions in a basic medium to aluminum hydroxide Al(OH)3, which is not very soluble and will have a tendency to precipitate. The pH of the wastewater will be lowered or properly neutralized or partially neutralized, and thus the aluminum will be removed and it will thus be purified. The working pH range for this operation is generally as follows: - low and therefore lower than that of the incoming wastewater to be treated, -However, aluminum is Al 3+ Since it dissolves in the form of , the pH cannot be too low (<5).

[0006] In summary, for aluminum, an optimal working pH of 5 to 8.5 is typically recommended.

[0007] Nevertheless, dissolving CO2 and contacting it with wastewater containing electrolytes is still difficult because CO2, like any strong acid, leads to high concentrations of acid at the injection point. In this highly acidic zone, the formation of solid aluminum hydroxide is very large and the risk of plugging the injection point is very high.

[0008] In other words, wastewater generally contains a large amount of aluminate ions that effectively require a large amount of acid to effect precipitation and neutralization. Unfortunately, this operation proves difficult to carry out for the following reasons: Homogenization or mixing of wastewater (liquid) and gas (CO2) is not easy, i.e. not instantaneous. Thus, injection zones that create an interface between wastewater and gas (whether or not they involve an injection device or perforated tubes for example) have been found to very quickly create large amounts of precipitate, which causes blocking that is difficult to remove: for example, by shutting down and stripping with strong acids. Furthermore, the particles formed prove to be very resistant and, if they accumulate, can lead to blocking downstream of the injection location. Finally, the various steps (gas injection / transfer and contact of dissolved CO2 with wastewater containing aluminate ions) are often carried out in exactly the same zone, in exactly the same equipment or ducts, but the conditions required for each step are different.

[0009] In summary, therefore, taking into account the factors listed above, the operation of CO injection combined with precipitation can be very difficult to implement, and in fact is virtually impossible, and therefore its application is abandoned by those skilled in the art.

[0010] Some examples of the processes offered in this industry include: -Furring / scaling of wastewater containing large amounts of dissolved aluminum to be removed. High pH wastewater (pH1 around 12) is sent to a tank maintained at a lower pH (pH2 around 7-8.5) by injection of CO2. This value is considered ideal for precipitating aluminum hydroxide. - The neutralized wastewater leaves its solids through the bottom of the tank (through pumping) and is then separated by settling. At this stage it is also possible to use the addition of products (e.g. thickeners, electrolytes, coagulants, surfactants, etc.) to increase the size of the initially formed solid particles (by flocculation, promoting crystallization, etc.) in order to improve their "decantability" or filterability. -Other configurations are often used: in other configurations, the injection is not performed directly into the tank, but in-line into a conduit. After pumping, gaseous (or liquid) CO2 is injected into the wastewater by means ranging from the simplest (e.g. simple pipe emergence) to the most sophisticated (static mixers, Venturi tubes, many commercially available gas-liquid contactors to improve dissolution), particles are formed directly in the stream and are usually only separated from the liquid (filters, settlers, etc.) leaving the stream.

[0011] Unfortunately, however, as noted above, the direct addition of acid (CO2) to a very basic water body highly loaded with aluminate ions results in instantaneous precipitation that is difficult to control. In fact, generally, when a water body is highly loaded with dissolved aluminum, acidification very often causes significant blocking, and often -For this reason, often with in-line treatment, pipelines are duplicated to allow continuing to operate on one line while the other is being cleaned, optionally with strong acids under high pressure. Operating costs and maintenance are inhibiting. And for treatment in stirred vessels, the operating mode is often batchwise, ie the feed and treatment are limited in time, also to allow cleaning.

[0012] It can therefore be emphasised that removing the resulting deposits is difficult, indeed very difficult, and that chemical (strong concentrated acids) and mechanical (scraping, high pressure jets, etc.) actions are often necessary to successfully remove the deposit layer. This is even more difficult in closure processes such as conduits.

[0013] To overcome the above mentioned disadvantages, it may be possible to consider a first solution consisting of dispersing as much CO2 as possible at the inlet of the refinery-precipitator, for example by injecting a CO2 stream in the center of the refinery at one or more injection points.

[0014] Nevertheless, this solution presents some disadvantages: - The most difficult situation is localized around each injector, since pure CO2 is added directly to the wastewater, which also leads to precipitation at the injection point. Optionally, it is necessary to displace as much liquid as possible around the injector, but this remains problematic in the stirred vessel. The rate of transfer of -CO2, i.e. dissolution, can be limited. This is because it is difficult to make the stirring vessel hold the bubbles in the liquid long enough for the dissolution to be substantially complete, whatever happens. The bubbles can coalesce and rise quickly, independent of the desired liquid circulation, which makes them appear without time to dissipate. The phenomenon is more pronounced if the stirring means are not of high performance, and it is critical if the volume density of the solution increases. This is because in concentrated solutions the aluminate ions can polymerize (inorganic polymerization), trapping particles and thereby resulting in a strong increase in viscosity. It is very difficult to maintain good stirring and to have gas-liquid interfaces (and therefore the total interfaces, and the large number of them, related to the final diameter of the bubbles), which would impair the level of dissolution, as well as to prevent blocking (blocked holes distort the injection flow). - Finally, a large amount of CO2 does not dissolve and is released into the headspace of the stirred reactor. CO2 is toxic and needs to be managed: adding capping, adding detectors, seepage and loss of some of the introduced CO2, etc.

[0015] A second solution is presented in the literature that consists in preventing direct contact, which is often achieved by using clean (e.g. on-site industrial networks), pre-carbonated water bodies. Thus, beforehand, CO2 is injected into the industrial clean water (not wastewater). This "circumlocutionary" means of adding CO2 is similar to the production of seltzer.

[0016] A large amount of CO2 is dissolved. Only then is this "drive" water, containing the desired amount of dissolved CO2, mixed with wastewater and treated in a pH zone such that aluminum hydroxide is preferably formed and, since it is not very soluble, either precipitates in a stirred vessel (in combination with downstream separation means) or directly in a purification or precipitator.

[0017] Nevertheless, this solution presents the following disadvantages: water is consumed and, by diluting the wastewater, the total liquid flow that has to be treated in the sedimentation device increases, leading to associated costs and an unfavorable environmental footprint. Summary of the Invention [Means for solving the problem]

[0018] As will be shown in more detail below, the present invention seeks to provide a novel solution for the treatment of such aluminum-rich wastewaters, which allows optimal use of CO2.

[0019] In this regard, acidification using a weak diprotic acid such as CO2 (rather than a strong acid such as sulfuric acid) shows several advantages: - Better control of the target pH to carry out the precipitation. -Consumption of CO2 that can be captured as a production or combustion by-product (the term "unavoidable" CO2 is used). - the formation of solids such as aluminium hydroxide, since the latter is practically insoluble, which ensures a good effect on the precipitation process. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 illustrates an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The solution provided by the present invention is based on the separation of the process into two different stages in two different zones. - First stage: the objective during this stage is to carry out the formation of crystals, mainly in the form of aluminium hydroxide. The conditions prevailing therein are favourable, in particular a low pH (preferably between 5 and 8.5), and no direct injection of CO2 is carried out in this zone, since this would result in the disadvantages of the prior art, as mentioned above.

[0022] This zone "1" can be a tank that is very alkaline, the zone where the aluminum-rich wastewater reaches (eg, with a pH of the order of 12.5).

[0023] As explained in more detail below, it is arranged to have enough dissolved CO2 available in this zone for the wastewater in this zone to change from an alkaline pH to a pH of preferably less than 9.5, more preferably 6.5 to 8.5 in all deployments. This is because when the flow rate of the wastewater being treated changes, the amount of compounds being neutralized changes and therefore it is necessary to avoid being in a situation during peaks in amount (concentration and / or flow rate) where there is not enough dissolved CO2 to neutralize it. This drop in pH will cause the dissolved form of aluminum (aluminate) to change to the form of aluminum hydroxide and precipitate.

[0024] In summary, the formation of solids, which may be furring, occurs in Zone 1, therefore if the solids are to be furring at the time of their formation, the problem appears in this first zone, which must contain sufficient dissolved CO2 to "neutralize" the arriving raw wastewater. - Second stage (second zone): In this stage, the objective is to carry out the injection of CO2 and an optimized maximum dissolution while very largely limiting, practically nullifying, the phenomenon of precipitation, or more precisely the formation of aluminum oxide crystals.

[0025] To do this, it is proposed according to the invention that a second zone, which can be called "zone of dissolution of CO2", is capable of: - makes it possible to recover a portion of the pretreated wastewater, i.e. a portion of the wastewater that no longer contains, or substantially no longer contains, the electrolytes to precipitate and therefore aluminates (aluminium in dissolved form). This stream is therefore diverted from the portion of the wastewater leaving this zone 2 and therefore from the complete process. It is then possible to inject CO2 in this bypass / recycle without significant precipitation, since, in the absence (or very little) of dissolved aluminium, there is no longer any of it, or substantially none of it. -CO2 is thus injected into this collected sample, maximizing the large scale transfer of gas to the liquid. This is ensured by hydraulic conditions (turbulent conditions if possible), the lowest possible temperature (preferably between 5 and 45°C, even more preferably between 15 and 30°C), the highest possible pressure (preferably between 1.5 and 20 bar, more preferably between 2 and 4 bar, but as reasonable for operational cost reasons, pressures below 10 bar are therefore preferred), and a mixing time in which the gas and liquid are kept in contact for a sufficiently long time (thus a contact of 3 to 30 seconds, preferably between 5 and 15 seconds is preferred). - In other words, Zone 2 is used to dissolve CO2 in the liquid stream pumped from Zone 1 and returned to Zone 1. This zone is calculated to dissolve enough CO2 to lower the pH of Zone 1 from the incoming alkaline wastewater value (e.g. 12.5) to the desired set point (e.g. 8).

[0026] In this zone 2, there is no drop from a highly alkaline pH (such as 12.5) to a neutral or acidic value, but the fluid is pumped from zone 1 in order to make it more acidic, and therefore close to neutral. There is therefore (or is trivial) no formation of solids in this zone 1, and therefore the risk of blockage is eliminated or very significantly reduced. -Thus, for a given flow rate and a given initial pH, the pH of the tank / zone 1 is ensured by calculating the amount of CO2 that must be dissolved in zone 2 (recirculation loop) to achieve the desired pH in zone 1, quite obviously with a margin of safety. -The balance is the gas-liquid ratio (typically 1 m of liquid) 3 0.1~5Sm per 3 of gas, preferably liquid, 1 m 3 0.1~1Sm per 3 The focus is on a combination of the amount of CO2 (gas), pressure in this zone, and the flow rate of the recovered wastewater that will allow dissolving all the CO2 required to maintain the desired operating conditions, and in particular the pH of zone 1, where the first wastewater stream typically reaches in succession. - This solution comprises in particular pumps, gas-liquid contactors (e.g. static mixers), conduits of the correct size to maintain under turbulent conditions and with sufficient length to guarantee the desired residence time (and therefore contact time), e.g. a contact time of the order of 10 seconds. - Dissolution is mainly carried out in this zone 2, allowing the carbonated wastewater to be returned to the first zone. When the operating conditions are observed, essentially CO2 is in dissolved form, i.e. CO2 and bicarbonate ions HCO3 - and therefore without any gas present (transfer levels higher than 80%, in fact higher than 90%). Operating conditions are therefore maintained to ensure the presence of these dissolved forms, i.e. a low pH (less than 8-9), preferably not lower than 5.

[0027] In summary, Zone 1 must be able to: First, incoming fresh wastewater (containing aluminates) is carbonated in zone 2 and returned to zone 1 (adding HCO3 to ensure the target precipitation pH in zone 1). -and the most intimate mixing possible with the converted stream (including sufficient dissolved CO2, especially in the form of CO2). This allows for maximizing the production of solids / precipitates. The mixture will then consist of liquid with most of its aluminum and solid particles removed. -Similarly, in all deployments, i.e. even if the quality of the incoming wastewater varies (high or low alkalinity resulting from product flow rates that vary with the concentration of alkali), to accurately control and guarantee precipitation, it is preferred to provide 1.5 to 2 times more dissolved CO2 in zone 1 than is required to neutralize all of the incoming wastewater (the stoichiometric need for dissolved CO2 to achieve the target pH value to precipitate the maximum of dissolved aluminum). Thus, any peaks in demand (and therefore in the alkalinity of the incoming wastewater) are accurately neutralized by the dissolved CO2 present in zone 1. In summary, zone 1 is matched to guarantee the desired target pH / volume pair.

[0028] These zones 1 and 2 thus consist, for example, of: - for zone 1, a vessel that contains an agitated section, allowing good mixing of the liquid streams (fresh wastewater to be treated and that returning from zone 2) and good settling (no strong shear by the agitator, sufficient residence time if it is operated in batch, semi-continuous or continuous mode), partial separation by settling (for example by a zone protected by a deflector), in order to be able to extract part of the wastewater to be treated in this vessel and send it to zone 2 (recirculation pipeline). After treatment, the liquid and solid mixture can be separated by settling directly in the vessel (batch operation for agitated vessels) or by settling downstream. -For example, for zone 2 of the recirculation loop attached to the plant of zone 1, a gas-liquid contactor (static mixer) capable of injecting and strongly stirring gaseous CO2 and liquid to promote dissolution of CO2, and finally a pump sending the flow to a conduit of sufficient length to ensure sufficient residence time to promote dissolution again before returning the carbonated water to zone 1. The match is essentially based on the gas-liquid ratio, the amount of CO2 required to precipitate what is desired (generally substantially all of it), the aluminum contained in the new wastewater to be treated and the solubility of CO2 in the wastewater. The total volume of zone 2 is therefore not critical here.

[0029] It is worth noting in this proposal that, especially for aqueous wastewaters or solutions in which the electrolyte (mainly aluminum) is very highly concentrated, it is this optimized embodiment (particularly without driving water) that is the whole point.

[0030] For a better understanding of the inventive approach, the invention is illustrated below by way of examples and by way of the accompanying FIG.

[0031] Considering wastewater, with an initial pH of 12, 30 m 3 At a flow rate of 100 / hr, it is preferable to lower it to 8.2, since the aluminum salts contained therein can be released into the network while precipitating them.

[0032] Therefore, in terms of CO2, a use of the order of 600g / l x 30 = 18kg / hour of CO2 is required.

[0033] In the proposed embodiment, the wastewater reaches the center of the neutralization tank, where good uniformity is provided. The tank is equipped with an agitation system, optionally supplemented with further agitators, if the agitators already in place are not sufficient.

[0034] Therefore, the recirculation loop must provide at least 18 kg / hr of CO2 in dissolved form.

[0035] Since the temperature of the wastewater is around 25 °C, the solubility is 1.4 kg of CO2 / m at 1 absolute bar. 3 in terms of order of magnitude.

[0036] Since the loop is operated at an absolute pressure of 2 bar, the concentration flow rate must be around 6.5 m 3 / hour.

[0037] One skilled in the art will understand that a margin must be incorporated and instead will maintain a loop flow rate on the order of 10 m 3 / hour.

[0038] Therefore, it is a problem of treating furring / scaling wastewater containing a large amount of dissolved aluminum to be removed. The initial high pH wastewater (pH1 around 12) is sent to a tank maintained at a lower pH (pH2 around 8 - 8.4). This is the target value selected by the present invention in this case to precipitate aluminum hydroxide.

[0039] The wastewater neutralized by its solids is then discharged (pumped) through the bottom for separation by precipitation (filtration).

[0040] A portion of the contents of the pH2 tank is pumped by an external loop (zone 2) into which CO2 is injected by an injection device (e.g., a static mixer); this wastewater does not contain a large amount of its dissolved minerals in the tank; thus, it is less scaling.

[0041] Turbulent conditions are maintained through the loop. Then, under the pressure generated by the pump, a pH closer to neutral or acidic (pH3) is achieved (pH3 < pH2 < pH1). This pH ensures the predominant formation of bicarbonate and the presence of dissolved CO2.

[0042] The gas-liquid mixture is then transferred to a coil of a length sufficient to ensure sufficient contact time to maximize the amount of CO2 transferred into the stream.

[0043] Finally, this acidified stream, with a pH of 3, is returned to the tank, where it is closest to the incoming stream, with a pH of 1, and is mixed to ensure that a pH of 2 prevails in the tank, which is optimal for aluminum hydroxide crystal formation.

[0044] Note that in this case, the lower the pH, up to a limiting pH of 5, the more favorable the precipitation.

[0045] So, in summary, please note the following: - The incoming wastewater is treated with Al(OH)4 at high pH (10-12). - It is primarily composed of molten aluminum in the form of - In order to make it contain as little dissolved aluminum as possible, it is advantageous to operate at about 5.5-8, where most of the aluminum is converted into the less soluble form of Al(OH)3. -CO2 makes it virtually impossible to have a pH lower than 5-5.5 and redissolve the aluminum hydroxide particles formed. CO2 is a weak dibasic acid, the first of which has a pKa of about 5 or lower. - The wastewater can absorb CO2 (part of the flow diverted between zones 1 and 2) after treatment and therefore after the removal of the bulk of the dissolved aluminum. Nevertheless, it is preferable to operate at the top of the target zone (thus towards 8 and not towards 5) in order to dissolve more CO2 (more hydrogen carbonate ions HCO3 than free CO2). - (Since this form is preferred, CO2 has better solubility at higher pH than at lower pH.)

[0046] The following elements have already been mentioned several times in the above description and can be seen in the attached Figure 1: a tank 1 comprising a zone 1, equipped with an agitation system 3 and supplied with the initial wastewater 4 to be treated; a recirculation loop 10 constituting a zone 2 capable of recovering (5) a portion of the medium present in the tank 1 by means of a pump 2, receiving an injection of CO2, equipped with a coil and whose length is capable of ensuring a contact time sufficient to maximize the amount of CO2 transferred to the flow; - after the end of the loop 10, the flow so treated is returned to tank 1, thus contributing to the generation of the target pH prevailing in tank 1 by mixing in the loop 10 the initial wastewater (4) and the medium treated with CO2; The tank is equipped with means (6) for extraction of treated wastewater.

[0047] The advantages of this solution are: The consumption of industrial-type water for producing seltzer water, as is the case with conventional solutions of the prior art, is avoided. - Any precipitation is always prevented in the zone where CO2 is injected. The precipitation zone 1 (reactor, precipitator, etc.) must always be maintained at a lower pH than that of the incoming stream to be treated (for example around pH 8) in order to allow the precipitation of aluminum in the form of aluminum hydroxide. The incoming wastewater to be treated should then be diluted in zone 1, which results in a slight increase in pH, which is in fact compensated by the injection of CO2 in zone 2, i.e. the recirculation loop. By choosing a combination of a reliable volume and a target pH in zone 1, an attempt is made to provide a larger amount of dissolved CO2 than the average required amount (for example 1.5-2). - Maximized CO2 transfer rate is guaranteed, which is obtained from the choice of operating conditions and technology (turbulence) in the recirculation loop, resulting in a consumption as close as possible to the system requirements (not over-consumption).

[0048] Zone 2 is used solely for dissolving CO2 in the water (wastewater) stream pumped from Zone 1 and returned to Zone 1. This zone is calculated to dissolve enough CO2 to lower the pH of Zone 1 from the incoming value of the alkaline wastewater Example 12.5 to the setpoint Example 8. It is also possible to use this zone to introduce some of the CO2 in gas form (fine bubbles) from Zone 1 to 2.

[0049] In this zone 2, there is no drop from a highly alkaline pH (12.5) to neutral or acidic values, but the wastewater is pumped from zone 1 to be further acidified and is therefore neutral or acidic. Thus, there is no (or trivial) formation of solids in this zone 1 and therefore the risk of blockage is reduced. If more CO2 is injected and this zone is made more acidic, it is also possible to dissolve the aluminum solids formed at the mediocre pH and optionally declog this zone 1, even though in theory this is not necessary.

[0050] As indicated above, according to the present invention, particular attention is paid to the amount of dissolved CO2 available in zone 1 being 0.5 to 3 times higher, preferably 1 to 1.5 times higher, than the requirements necessary for the precipitation of the incoming wastewater, taking into account the recirculation of the CO2-injected medium.

[0051] This is explained in more detail below.

[0052] In particular, we explain how we determined the CO2 requirement for the tank (zone 1) and provide an example of the calculation of the amount of available dissolved CO2 that is 0.5 to 3 times higher than the CO2 requirement necessary for precipitation in zone 1.

[0053] The wastewater entering zone 1 is very alkaline (hence high pH1 and high concentration of dissolved aluminium). There it comes into contact with wastewater containing the necessary dissolved CO2 coming from zone 2 (low pH2 because the wastewater contains at least all the CO2 necessary for precipitation) whose pH has been lowered to a third pH. At the contact of the two, the dissolved CO2 makes it possible to neutralise the alkalinity of the incoming wastewater and thus lower the pH in order to allow precipitation and to remove the dissolved aluminium from the wastewater. The resulting pH (pH3) is between the two previous pH values ​​and is adjusted to favour precipitation.

[0054] In continuous operation, the required amount of CO2 (of weak dibasic acid) introduced compensates as far as possible the alkalinity (stoichiometric ratio of acid to alkali or base) of the incoming wastewater. Nevertheless, if there is an abrupt change in the operating conditions and if the amount of alkali increases, an imbalance in the acid-base ratio occurs, which must be compensated. A phenomenon can then be achieved in which the wastewater circulating continuously through zone 2 may have excess alkali that leads to precipitation in this zone. This can even disturb the complete system by blocking, often in many very rapid applications, giving a high alkalinity of the treated wastewater. Of course, a control regulation system can adjust the amount of CO2 relative to the amount of alkali (which must cause a drop in the pH of the precipitation zone 1), but this remains problematic. This is because the quantity involved (diameter of zone 1) can lead to slow changes in the operating parameters: with a high and abrupt incoming amount of alkali, the pH of zone 1 will only change slowly if the amount is large in diameter (and therefore at high residence times). Therefore, the amount of CO2 does not react immediately, in fact extremely slowly, which could lead to undesirable rapid and strong precipitation in zone 2, a phenomenon that must be absolutely avoided at the risk of stopping everything due to excessive blocking.

[0055] Thus, the rationale for the present invention is to retain a larger amount of liberated dissolved CO2 available in zone 1 where precipitation occurs than would be required by the incoming wastewater.

[0056] Thus, when there is a variation in the operating conditions (e.g. amount of incoming alkali), this extra dissolved CO2 makes it possible to "neutralize" this extra amount in a certain period of time with respect to the amount of CO2 required for precipitation (hence stoichiometric). This prevents dissolved aluminum or alkali from being sent to zone 2 in any case, and gives the control regulation system time to adjust the flow rate of CO2 injected to compensate for this additional amount.

[0057] It is estimated by the present invention that an excess amount of dissolved and therefore available CO2 is required on the order of 0.5 to 3 times the amount required for optimized neutralization and therefore optimized precipitation of the incoming wastewater.

[0058] The following examples make it possible to illustrate the proposal of the invention more clearly. EXAMPLES

[0059] The following data were obtained by using commercially available software that makes it possible to study equilibria in water bodies. The simulation was carried out in several stages: starting from a (standard) water composition, it was possible to raise the pH to 12 by adding sodium hydroxide. This makes it possible to make available a "synthetic" wastewater. For this, a concentration of 0.56 kg / m 3 It was necessary to add sodium hydroxide.

[0060] It was then possible to neutralise it by the addition of CO2, first to a pH of 8, then to 7.5. The wastewater was neutralised to 8, for example, and then stored. To neutralise the wastewater from 12.0 to 8.0, 0.60 g / m 3It is necessary to add 18 g / m 3 of liberated CO2 remains in the wastewater.

[0061] Therefore, 100m 3 For a wastewater flow rate of 1 kg / min of CO2 is needed to neutralize it.

[0062] Furthermore, the tank (or zone 1) is 55 m 3 If the reactor is of dimension 1, it will contain only 1 kg of free CO2. It is therefore possible to compensate for a sudden excess of alkalinity. This makes it possible, for example, to compensate for 1 minute for the arrival of wastewater in case of a stop in the CO2 injection, or to compensate for an increase in the amount of alkali (for example a requirement of 1 to 1.5 kg / min). In the latter case, in 40 seconds the liberated CO2 will be consumed and the pH will then rise, the reactor or zone 1 will no longer precipitate all the incoming alkali and the whole process will become unstable. Eventually the dissolved alkali will enter zone 2 and may result in precipitation and blockages.

[0063] It is therefore preferable to increase the volume of Zone 1, particularly so that it has more liberated CO2 available to "quench" or neutralize any fluctuations or upsets in the incoming wastewater.

[0064] In the above case, 166m 3 With Zone 1 of 3 kg of liberated CO2 is available to neutralize the excess incoming alkalinity. If this excess results in a CO2 requirement of 1.5 kg / min, the system drifts only after 2 minutes. Thus, still in this example, the reaction time required to compensate for the rapid excess of alkalinity was 5 times. Thus, there is much more flexibility and robustness of the process, since the whole process allows time control and definition.

[0065] In summary, zone 1 is established so that this zone contains a minimum of 1.5 to 3 times the amount of CO2 required to neutralize the alkali arriving at each instantaneous zone, thus allowing the precipitation of substantially all the aluminum (its oxides) in this zone 1. The amount of free CO2 in the zone can be higher than 3, but for economic reasons will not exceed 10 or 15.

[0066] In our example, the free CO2 is 18 g / m for a set pH of 8. 3 , 57 g / m at a set pH of 7.5 3 It is.

[0067] To determine the volume of zone 1 and calculate the amount of available free CO2, the amount of free CO2 required in kg is divided by the concentration of free CO2 in the wastewater at the set pH.

[0068] Therefore: Volume of Zone 1 = determined amount of CO2 in kg / concentration of free CO2 at the set pH.

Claims

1. A method for treating industrial wastewater containing aluminum, for the purpose of removing all or part of the aluminum therefrom, comprising the steps of: conveying said wastewater to be treated to a first zone, for example consisting of a tank, in which a pH of less than 9.5, more preferably between 6.5 and 8.5, and even more preferably between 7 and 8, is maintained, in order to promote the precipitation of said aluminium in the form of aluminium hydroxide and thus facilitate its removal; - a second zone is available and recycling of part of the medium located in zones 1 and 2, then returning it from there to said zone 1, as well as gaseous CO2 to said recycled medium. ２ An injection of is placed; - separating and discharging the solid particles formed in said zone 1; The method comprises carrying out ２ Taking into account the recirculation of the medium into which CO was injected, the dissolved CO available in zone 1 ２ is 0.5 to 3 times higher, preferably 1 to 1.5 times higher than the requirements necessary for precipitation of the incoming wastewater.

2. 2. The method of claim 1, wherein the conditions prevailing in zone 2 are turbulent conditions.

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