Method for treating industrial wastewater containing aluminum using CO2

Abstract

The present invention provides a method for treating industrial wastewater containing aluminum in order to remove all or some of the dissolved aluminum therefrom, comprising the following steps: conveying the wastewater to be treated to a first zone, consisting for example 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 aluminum in the form of aluminum hydroxide and thus facilitate its removal; a second zone is provided, in which part of the medium located in zone 1 is recycled to zone 2, from which the medium is then returned to zone 1 and gaseous CO2 is injected into the recycled medium; and separating and releasing the solid particles formed in zone 1. The method is characterized in that the amount of dissolved CO2 available in zone 1, taking into account the recycling of the CO2-injected medium, is 0.5 to 3 times higher, preferably 1 to 1.5 times higher, than the amount required for the precipitation of the incoming wastewater. [Selection diagram] None

Description

[Technical Field]

[0001] The present invention relates to the field of treatment of highly basicized wastewater containing metals, including alkaline earth metals, for purposes such as neutralization and significant removal of metals present in the wastewater. This problem is particularly seen in the steel industry but may be relevant to other industries, and in particular to the treatment of aqueous liquor resulting from the production of aluminum based on natural raw materials such as bauxite (known as "red mud" that contains a very high amount of aluminum). [Background technology]

[0002] The addition of carbon dioxide gas can neutralize this type of wastewater and precipitate and remove metals. However, very often, direct injection exhibits problems of complete or partial blocking of the injection system and loss of equipment performance quality. This is because, although the injection aims at neutralization, it also aims at precipitation of inorganic compounds, and therefore, the formation of solids near the injection point where the CO2 necessary for precipitation is introduced is inevitable. It is easy to understand that these injection points can easily become blocked and the performance quality of the equipment can be impaired, as very large amounts of solid, potentially reaching several tons per hour, can form near the CO2 injection point.

[0003] The problem with this method is that the direct addition of an acid (CO2) to a highly basic aqueous substance containing metal results in instantaneous precipitation that is difficult to control. Many industrial cases have been reported where injection and mixing points, such as conduits, become blocked very rapidly.

[0004] Let's consider the treatment of aqueous liquor resulting from aluminum production. This involves the problem of neutralizing the aqueous liquid stream containing a large amount of dissolved aluminum, which must be removed before the aqueous liquor is discharged. Generally, this wastewater is highly alkaline, and aluminum contains aluminate ions (Al(OH)4). -It is dissolved in the form of ). This is due to the treatment of the ore with sodium hydroxide ("leaching").

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

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

[0007] Nevertheless, the dissolution of CO2 and the process of bringing it into contact with wastewater containing electrolytes remain difficult. This is because CO2, like any strong acid, results in a high concentration of acid at the injection site. In this highly acidic zone, the formation of solid aluminum hydroxide is very large, and the risk of blockage at the injection point is very high.

[0008] In other words, wastewater generally contains large amounts of aluminate ions, which require large amounts of acid to effectively bring about 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. Therefore, injection zones that create an interface between wastewater and gas (whether, for example, an injection device or a perforated tube) are known to produce large amounts of precipitate very quickly, which causes blocking that is difficult to remove: e.g., shutdown and stripping with strong acid. Furthermore, the formed particles have been found to be highly resistant, and if they accumulate, they could cause blocking in the downstream portion of the injection site. Finally, although the various stages (gas injection / transfer and contact of dissolved CO2 with wastewater containing aluminate ions) are often carried out in the exact same zone, with the exact same equipment or conduits, the conditions required for each stage differ.

[0009] Therefore, in summary, considering the factors mentioned above, the operation of injecting CO2 in combination with precipitation can be extremely difficult to carry out, and in fact practically impossible, and thus its application is abandoned by those skilled in the art.

[0010] An example of the processing offered in this industry is as follows: - Furring / scaling of wastewater containing large amounts of dissolved aluminum to be removed. High pH wastewater (pH 1 around 12) is transferred to a tank maintained at a lower pH (pH 2 around 7-8.5) by injecting CO2. This value is considered ideal for precipitating aluminum hydroxide. -The neutralized wastewater is allowed to pass through the bottom of the tank (which is pumped) and its solids are then separated by sedimentation. 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 solid particles that are initially formed (by means of flocculation, crystallization promotion, etc.) in order to improve their "decantation" or filtration. -Other forms are also frequently used: In other forms, injection is not performed directly into the tank but in line into the conduit. By means ranging from the simplest (e.g., simple pipe appearance) to the most sophisticated (static mixers, many commercially available gas-liquid contactors to improve dissolution such as venturi tubes), after pumping, gaseous (or liquid) CO2 is injected into the wastewater. Particles are formed directly in the flow and are usually separated from the liquid while leaving the flow as is (filtration, sedimentation tank, etc.).

[0011] However, unfortunately, as mentioned above, the direct addition of an acid (CO2) to a highly basic aqueous substance containing aluminate ions results in instantaneous precipitation that is difficult to control. In fact, generally speaking, when an aqueous substance contains a high concentration of dissolved aluminum, acidification very often causes significant blocking, and often, For this reason, pipelines are often duplicated in relation to inline processing to allow operations to continue on one line while other lines are being selectively cleaned with strong acids under high pressure. Operating costs and maintenance are kept to a minimum. - And regarding processing in a stirring vessel, the operating mode is often batch-based, meaning that supply and processing are time-limited in order to allow for cleaning.

[0012] Therefore, it can be emphasized that removing the resulting sediments is difficult, in fact very difficult, and that chemical (high-concentration acids) and mechanical (scraping, high-pressure jets, etc.) actions are often necessary to successfully remove the sediment layers. This is even more difficult in closure processes such as those for conduits.

[0013] To overcome the above inconveniences, a first solution can be considered, for example, which involves dispersing as much CO2 as possible at the inlet of the purification-precipitation unit by injecting a CO2 stream into the center of the purification unit at one or more injection points.

[0014] Nevertheless, this solution exhibits several drawbacks: -The most challenging situation is localized around each injection device, as pure CO2 is added directly to the wastewater, causing precipitation even at the injection point. Optionally, it is necessary to replace as much liquid as possible around the injection device, but this still involves problems within the agitated vessel. - The transfer, i.e., the rate of dissolution, of CO2 can be limited. This is because, no matter what happens, it is difficult to keep the stirring vessel holding bubbles in the liquid for a sufficient time for substantially complete dissolution. Bubbles can coalesce and rise rapidly, independently of the desired liquid circulation, which gives rise to them without time to consume them. This phenomenon is more pronounced if the stirring means is not high-performance, and it is significant if the volume density of the solution increases. This is because, in concentrated solutions, aluminate ions can polymerize (inorganic polymerization), trapping particles and thereby resulting in a strong increase in viscosity. Maintaining good stirring is extremely difficult, as are having gas-liquid interfaces (and thus all interfaces related to the final diameter of bubbles, and many of them), which will impair the level of dissolution, as is preventing blocking (blocked holes distort the injection flow). - Finally, large amounts of CO2 do not dissolve and are released into the headspace of the agitated reactor. CO2 is toxic and must be managed: by adding capping, adding detectors, and allowing some of the introduced CO2 to leach out and be lost.

[0015] A second solution, described in the literature, involves preventing direct contact, which is often achieved by using clean (e.g., on-site industrial networks) pre-carbonated water. Thus, CO2 is pre-injected into clean industrial water (not wastewater). This "indirect" method for adding CO2 is similar to the production of seltzer carbonated water.

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

[0017] Nevertheless, this solution exhibits the following disadvantages: water is consumed, and diluting the wastewater increases the total liquid flow rate that must be treated in the precipitation apparatus, resulting in associated costs and an unfavorable environmental footprint. Summary of the Invention Means for Solving the Problems

[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 wastewater that enables optimal use of CO₂.

[0019] In this regard, acidification using a weak dibasic acid such as CO₂ (and not a strong acid such as sulfuric acid, for example) offers several advantages: - Better control of the target pH for carrying out precipitation. - Consumption of CO₂ that can be recovered as a manufacturing or combustion by-product (the term "unavoidable CO₂" is used). - Formation of a solid such as aluminum hydroxide. This is because the latter is substantially insoluble, which ensures a good effect for the precipitation process. Brief Description of the Drawings

[0020] [Figure 1] Figure showing an embodiment of the present invention. Mode for Carrying Out the Invention

[0021] The solution provided by the present invention is based on separating 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, primarily in the form of aluminum hydroxide. The conditions that prevail therein, particularly a low pH (preferably 5-8.5), are preferred, and as mentioned above, direct injection of CO2 is not carried out in this zone because it would result in the drawbacks of the prior art.

[0022] Zone "1" can be a tank that is highly alkaline and where aluminum-rich wastewater reaches (for example, at a pH of 12.5).

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

[0024] In summary, the formation of a solid that can be furring occurs in zone 1, and therefore, if the solid is furring at the time of its formation, the problem arises in this first zone, and it must contain enough dissolved CO2 to "neutralize" the crude wastewater that reaches it. -Second stage (second zone): In this stage, the objective is to perform CO2 injection and optimized maximum dissolution while greatly limiting, and effectively neutralizing, the phenomenon of precipitation, or more precisely, the formation of aluminum oxide crystals.

[0025] To accomplish this, it is proposed that a second zone (which it may call the "CO2 dissolution zone") may be possible according to the present invention: - This allows for the recovery of pre-treated wastewater, i.e., a portion of wastewater that no longer contains, or substantially no longer contains, the electrolyte to be precipitated, and therefore aluminates (aluminum in dissolved form). Thus, this flow is diverted from a portion of the wastewater coming out of this Zone 2, and therefore from the entire process. If dissolved aluminum is absent (or present in very small amounts), then it is possible to inject CO2 in this bypass / recirculation without causing significant precipitation, since there is no longer any of it, or substantially none of it. Therefore, CO2 is injected into this recovered sample while maximizing the large-scale transfer of gas into liquid. This is ensured by hydrostatic conditions (turbulent conditions if possible), the lowest possible temperature (preferably 5-45°C, more preferably 15-30°C), the highest possible pressure (preferably 1.5-20 bar, more preferably 2-4 bar, but a pressure of less than 10 bar is preferable as is reasonable for reasons of operational cost), and a mixing time in which the gas and liquid are in contact and maintained for a sufficiently long time (therefore, 3-30 seconds, preferably 5-15 seconds of contact is preferable). In other words, Zone 2 is used to dissolve CO2 in the liquid flow that is 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 a desired set value (e.g., 8).

[0026] In Zone 2, there is no decrease from a highly alkaline pH (e.g., 12.5) to neutral or acidic values, but to further acidify it, the fluid is pumped from Zone 1 and is therefore close to neutral. Consequently, there is no (or obvious) solid formation in Zone 1, and therefore the risk of blockage is eliminated or significantly reduced. Therefore, given a given flow rate and a given initial pH, the pH of tank / zone 1 is ensured, with a very clearly safe margin, by calculating the amount of CO2 that must be dissolved in zone 2 (recirculation loop) to achieve the desired pH of zone 1. - Harmony is the gas-liquid ratio (generally, 1 ml of liquid). 3 Approximately 0.1-5 Sm 3 A gas, preferably a liquid, 1 m 3 Approximately 0.1 to 1 Sm 3 The focus is on the gaseous state, the pressure in this zone, and the flow rate of recovered wastewater that allows all the CO2 necessary to dissolve to maintain the desired operating conditions, particularly the pH combination of Zone 1, where the initial wastewater generally arrives sequentially. -This solution comprises a conduit of the correct size, having sufficient length to maintain in a pump, gas-liquid contactor (e.g., static mixer), and to ensure a desirable residence time (and therefore contact time), for example, a contact time of about 10 seconds. - Dissolution is mainly carried out in Zone 2, and the carbonated wastewater can be returned to Zone 1. If the operating conditions are observed, the CO2 is essentially in a dissolved form, i.e., CO2 and bicarbonate ions HCO3. - It is contained in such a way that no gases are present (transfer levels higher than 80%, actually higher than 90%). Therefore, the operating conditions are maintained to ensure these dissolved forms, i.e., a low pH (less than 8-9), preferably not below 5.

[0027] In summary, Zone 1 must be able to do the following: - Firstly, incoming new wastewater (containing aluminates) is carbonated in Zone 2 and returned to Zone 1 (HCO3 to ensure target precipitation pH in Zone 1). -And the most close possible mixing with the converted flow (including CO2 that is sufficiently dissolved, especially in the form of CO2). This makes it possible to maximize the formation of solids / precipitates. The mixture will then consist of the liquid from which most of its aluminum and solid particles have been removed. Similarly, in all deployments, i.e., even if the quality of the incoming wastewater changes (whether it is highly or lowly alkaline, which results from the product flow rate which changes with the alkali concentration), it is preferable to provide 1.5 to 2 times more dissolved CO2 in Zone 1 than what is needed to neutralize all of the incoming wastewater (the stoichiometric need for dissolved CO2 to achieve the target pH value for precipitation of the maximum amount of dissolved aluminum), in order to precisely control and guarantee precipitation. Therefore, any peak in demand (and thus the peak in the alkalinity of the incoming wastewater) is precisely neutralized by the dissolved CO2 present in Zone 1. In summary, Zone 1 is harmonized to guarantee the desired target pH / volume pair.

[0028] Therefore, these zones 1 and 2 consist of, for example, the following: -Regarding Zone 1, the container contains an agitation section and allows for good mixing and good sedimentation of the liquid flow (new wastewater to be treated and that returned from Zone 2) (without strong shearing by the agitator, sufficient residence time if it is operated in batch, semi-continuous, or continuous mode), and partial separation by sedimentation (e.g., by a zone protected by a deflector). After treatment, the liquid and solid mixture can be separated directly by sedimentation in the container (batch operation in the case of an agitated container) or by sedimentation downstream. - For example, regarding zone 2 of the recirculation loop installed in the plant of zone 1, a gas-liquid contactor (static mixer) capable of injecting gaseous CO₂ and liquid and performing intense stirring to promote the dissolution of CO₂, and finally a pump that delivers the flow to a conduit of sufficient length to ensure sufficient residence time to further promote dissolution before returning the carbonated water to zone 1. The balance is essentially based on the gas-liquid ratio, the amount of CO₂ required to precipitate the desired proportion (generally substantially all), the aluminum contained in the fresh wastewater to be treated and the solubility of CO₂ in the wastewater. Therefore, the total volume of zone 2 is not critical here.

[0029] Particularly for aqueous wastewater or solutions with a very high concentration of electrolytes (mainly aluminum), this is the entirety of this optimized embodiment (particularly without driving water), which is noteworthy in the present proposal.

[0030] For a better understanding of the approach of the present invention, the invention is illustrated hereinafter by way of example and with reference to the accompanying Figure 1.

[0031] When considering wastewater, at an initial pH of 12, 30 m 3 / hour flow rate, it is preferable to lower the pH to 8.2, since this allows the aluminum salts contained to be precipitated in advance while the wastewater can be discharged into the network.

[0032] Accordingly, for CO₂, the use of CO₂ on the order of 600 g / l×30=18 kg / hour is required.

[0033] In the proposed example, the wastewater reaches the center of a neutralization tank where good uniformity is provided. The tank is equipped with a stirring system optionally supplemented with an additional stirrer if the already arranged stirrer is not sufficient.

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

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

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

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

[0038] Therefore, this is a problem of treating furring / scaling wastewater containing a large amount of dissolved aluminum to be removed. Initial high-pH wastewater (pH 1 around 12) is sent to a tank maintained at a lower pH (pH 2 around 8 to 8.4), which is the target value selected according to the present invention in this case for precipitating aluminum hydroxide.

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

[0040] A portion of the content in the pH 2 tank is pumped through an external loop (zone 2) where CO₂ is injected by an injection device (e.g., a static mixer); this wastewater does not contain a large amount of the dissolved mineral in the tank; therefore, it causes weaker scaling.

[0041] Turbulent flow conditions are maintained throughout the loop. Then, under the pressure generated by the pump, a pH closer to neutral or acidic (pH 3) is achieved (pH 3 < pH 2 < pH 1). This pH ensures the predominant formation of bicarbonate and the presence of dissolved CO₂.

[0042] Next, the gas-liquid mixture is transferred to a coil, whose length is such that it ensures sufficient contact time to maximize the amount of CO2 transferred into the flow.

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

[0044] In this case, note that the lower the pH, the more precipitation is promoted, up to the limiting pH of 5.

[0045] Therefore, in summary, please note the following: - The incoming wastewater contains Al(OH)4 at high pH (10-12). - It is mainly composed of molten aluminum in that form. - To ensure that it contains as little dissolved aluminum as possible, it is advantageous to operate at around 5.5–8, where most of the aluminum is converted to the less soluble form Al(OH)3. -CO2 makes it virtually impossible to have a pH lower than 5-5.5 and to redissolve the formed aluminum hydroxide particles. CO2 is a weak dibasic acid, and its primary pKa cannot be below approximately 5. - The wastewater, after treatment, and therefore after a large amount of dissolved aluminum has been removed, can absorb CO2 (part of the flow whose course is changed between zones 1 and 2). Nevertheless, in order to dissolve more CO2, it is preferable to operate in the upper part of the target zone (and therefore towards 8 rather than towards 5) (bicarbonate ions HCO3 rather than free CO2). - (Since this form is preferable, the solubility of CO2 is better at higher pH levels than at lower pH levels.)

[0046] The following elements have already been mentioned several times in the above explanation, but can be seen in the attached Figure 1: - A tank 1 constituting Zone 1, equipped with a stirring system 3 and supplied with initial wastewater 4 to be treated; - A recirculation loop 10 comprising a zone 2 in which a portion of the medium present in tank 1 can be recovered (5) by pump 2, which receives CO2 injection, is equipped with a coil, and its length is such that it can ensure a contact time sufficient to maximize the amount of CO2 transferred into the flow; -After the completion of loop 10, the treated flow is returned to tank 1, and thus, by mixing the initial wastewater (4) and the CO2-treated medium in loop 10, contributes to producing a target pH that is dominant in tank 1; - The tank is equipped with means (6) for extracting treated wastewater.

[0047] The benefits of this solution are as follows: - The consumption of industrial-grade water for producing seltzer carbonated water is avoided, as is the case with conventional solutions of prior art. - In the zone where CO2 is injected, any precipitation must always be prevented. Precipitation zone 1 (reactor, precipitation unit, etc.) must always be maintained at a pH lower than the pH of the incoming flow being treated (e.g., around pH 8) to allow precipitation of aluminum in the form of aluminum hydroxide. The incoming wastewater being treated should then be diluted in zone 1, thereby resulting in a slight increase in pH. This is actually compensated for by the injection of CO2 in zone 2, i.e., the recirculation loop. An attempt is made to provide more dissolved CO2 than the average requirement (e.g., 1.5-2) by selecting a reliable volume and target pH combination for zone 1. - Ensuring the maximum CO2 transfer rate. This is achieved through the selection of operating conditions and techniques (turbulence) in the recirculation loop. This results in consumption that is as close as possible to the system's requirements (not excessive consumption).

[0048] Zone 2 is used solely for dissolving CO2 in the water (wastewater) flow that is 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 alkaline wastewater example 12.5 to the set value example 8. It is also possible to use this zone to introduce some of the CO2 from Zone 1 to Zone 2 in gaseous form (microbubbles).

[0049] In Zone 2, there is no decrease from the highly alkaline pH (12.5) to neutral or acidic values, but the wastewater is pumped from Zone 1 to be further acidified and therefore neutral or acidic. Therefore, there is no (or obvious) solid formation in Zone 1, and thus the risk of blockage is reduced. If more CO2 is injected and this zone is further acidified, it is possible to dissolve the aluminum solids formed at neutral pH, even though this is not theoretically necessary, and optionally declog Zone 1.

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

[0051] This will be explained in more detail below.

[0052] In particular, this section explains how the CO2 requirements for tank (zone 1) were determined, and provides an example of calculating the amount of available dissolved CO2 that is 0.5 to 3 times higher than the CO2 requirements needed for precipitation in zone 1.

[0053] The wastewater entering Zone 1 is highly alkaline (hence high pH 1 and high concentration of dissolved aluminum). It then comes into contact with wastewater from Zone 2 (low pH 2, as the wastewater contains at least all the CO2 necessary for precipitation), which has been reduced to a third pH and contains the necessary dissolved CO2. At the contact between the two, the dissolved CO2 neutralizes the alkalinity of the incoming wastewater, thus lowering its pH, in order to enable precipitation and remove the dissolved aluminum from the wastewater. The resulting pH (pH 3) is adjusted to be between the previous two pH values, favorably for precipitation.

[0054] In continuous operation, the required amount of CO2 (of a weak dibasic acid) introduced compensates as much as possible for the alkalinity (stoichiometric ratio of acid to alkali or base) of the incoming wastewater. Nevertheless, if there are abrupt changes in operating conditions and the amount of alkali increases, an imbalance in the acid-base ratio will occur, which must be compensated for. Subsequently, a phenomenon can occur where wastewater circulating continuously through zone 2 may have excess alkali that leads to precipitation in this zone. This can even disrupt the complete system by blocking, often in many very rapid applications, resulting in high alkalinity of the wastewater being treated. Of course, the control system can adjust the amount of CO2 in relation to the amount of alkali (which must cause a decrease in the pH of precipitation zone 1), but this remains problematic. This is because the relevant amount (diameter of zone 1) can lead to slow changes in the operating parameters: with a high and sudden influx of alkali, if the amount is large in diameter (and therefore with a high residence time), the pH of zone 1 will only change slowly. Therefore, the amount of CO2 does not react immediately, but rather extremely slowly, which could lead to undesirable rapid and strong precipitation in zone 2, a phenomenon that must be absolutely avoided in the event of the danger of stopping everything due to extremely large blocking.

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

[0056] Therefore, if there are variations in operating conditions (e.g., the amount of incoming alkali), this excess dissolved CO2 allows for the "neutralization" of this excess amount over a certain period of time with respect to the amount of CO2 required for precipitation (and thus stoichiometrically). This prevents the dissolved aluminum or alkali from being sent to Zone 2 in any case, and gives the control system time to adjust the flow rate of CO2 injected to compensate for this excess.

[0057] The present invention estimates that an excess amount of dissolved, and therefore available, is required for optimized neutralization, and therefore for optimized precipitation of incoming wastewater, in orders of 0.5 to 3 times the amount needed.

[0058] The following embodiments will allow for a clearer illustration of the proposed features of the present invention. [Examples]

[0059] The following data were obtained by using commercially available software that allows for the study of equilibrium in aqueous substances. The simulation was performed in several stages: starting with a (standard) aqueous composition, it was possible to raise the pH up to 12 by adding sodium hydroxide. This makes it possible to make "synthetic" wastewater usable. For this purpose, 0.56 kg / m³ 3 It was necessary to add sodium hydroxide.

[0060] Subsequently, by adding CO2, it was possible to neutralize it first to pH 8, and then to 7.5. The wastewater was neutralized to pH 8 and then stored, for example. To neutralize the wastewater from 12.0 to 8.0, 0.60 g / m³ was used. 3It is necessary to add CO2, which will result in a concentration of 18g / m³. 3 The released CO2 remains in the wastewater.

[0061] Therefore, 100m 3 Regarding the wastewater flow rate per hour, 1 kg / min of CO2 is needed to neutralize it.

[0062] Furthermore, the tank (or Zone 1) is 55m 3 If the dimensions are such that it contains only 1 kg of free CO2, it is possible to compensate for any sudden excess alkalinity. This allows for, for example, compensation for 1 minute for the arrival of wastewater in the event of a cessation of CO2 injection, or compensation for an increase in the amount of alkali (e.g., a requirement of 1-1.5 kg / min). In the latter case, the freed CO2 will be consumed in 40 seconds, and the pH will subsequently rise, the reactor or zone 1 will no longer precipitate all incoming alkali, and the entire process will become unstable. Eventually, the dissolved alkali will enter zone 2, which may result in precipitation and interference.

[0063] Therefore, it is preferable to increase the volume of Zone 1 in particular to have more free CO2 available to "eliminate" or neutralize fluctuations or disturbances in incoming wastewater.

[0064] In the above case, 166m 3 In Zone 1, 3 kg of freed CO2 is available to neutralize the excess incoming alkalinity. If this excess results in a CO2 requirement of 1.5 kg / min, the system will drift after only 2 minutes. Therefore, in this embodiment, the reaction time required to compensate for the rapid excess alkalinity was five times longer. Thus, the entire process allows for time control and regulation, resulting in greater process flexibility and robustness.

[0065] In summary, Zone 1 is established to contain at least 1.5 to 3 times the amount of CO2 necessary to neutralize the alkali arriving in each instantaneous zone, and thus to allow for the precipitation of substantially all aluminum (its oxides) in Zone 1. The amount of free CO2 in the zone can be higher than 3, but for economic reasons, it does not exceed 10 or 15.

[0066] In our example, the free CO2 was 18 g / m³ at a set pH of 8. 3 , 57 g / m² for a pH of 7.5 3 That is the case.

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

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

Claims

1. A method for treating industrial wastewater containing aluminum, with the aim of removing all aluminum from it, comprising the following steps: - To promote the precipitation of aluminum in the form of aluminum hydroxide and thus facilitate its removal, the wastewater to be treated is transported to a first zone where a pH of 6.5 to 8.5 is maintained; - The second zone is available, and a portion of the medium located in the first zone goes to the second zone and then returns from there to the first zone for recirculation, and gaseous CO2 is added to the medium being recirculated. ２ An injection is performed; - Separating and releasing the solid particles formed in the first zone; This includes the implementation of a CO2 return from the second zone to the first zone. ２ Considering the recirculation of the medium into which CO has been injected, dissolved CO is available in the first zone. ２ The method is characterized in that the amount is 1 to 3 times higher than the necessary conditions for the sedimentation of wastewater at a predetermined alkalinity and flow rate, in order to compensate for the sudden arrival of excess alkalinity.

2. The method according to claim 1, characterized in that the first zone consists of a tank.

3. The method according to claim 1, characterized in that the dominant condition in the second zone is a turbulent condition.

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