METHOD FOR TREATING AN INDUSTRIAL EFFLUENT CHARGED WITH ALUMINIUM USING CO2

Abstract

A method for treatment of an industrial effluent with aluminum, comprising: the effluent to be treated is carried to a first zone constituted by a tank having a pH of less than 9.5, so as to promote precipitation of the aluminum in aluminum hydroxide form and to facilitate its removal; a second zone is available and the recirculation of a part of a medium located in the zone 1 to the zone 2 and then return to the zone 1, and the injection of gaseous CO.sub.2 into the recirculated medium, are arranged; the solid particles formed in the zone 1 are separated and discharged; wherein in view of the recirculation of the medium where CO.sub.2 has been injected, the amount of dissolved CO.sub.2 available in the zone 1 is 0.5 to 3 times greater than the requirement necessary for the precipitation of the incoming effluent.

Claims

1. A process for the treatment of an industrial effluent with aluminum for the purpose of removing all or part of the aluminum therefrom, comprising: carrying the effluent to be treated to a first zone, where a pH of less than 9.5 is maintained in the first zone, so as to promote precipitation of the aluminum in an aluminum hydroxide form and to thus facilitate its removal; providing a second zone, a circulation of a part of a medium located in the first zone to the second zone and then, from there, returned to the first zone, and an injection of gaseous CO.sub.2 into the recirculated medium, are arranged; separating and discharging solid particles formed in the first zone; wherein, in view of the recirculation of said medium where CO.sub.2 has been injected, the amount of dissolved CO.sub.2 available in the first zone is 0.5 to 3 times greater than the requirement necessary for the precipitation of a incoming effluent.

2. The process as claimed in claim 1, wherein the conditions prevailing in the zone 2 are turbulent conditions.

3. The process of claim 1, wherein the first zone is a tank.

4. The process of claim 1, wherein a pH of between 6.5 and 8.5 is maintained in the first zone.

5. The process of claim 1, wherein a pH of between 7 and 8 is maintained in the first zone.

6. The process of claim 1, wherein the amount of dissolved CO.sub.2 available in the first zone is between 1 and 1.5 times greater than the requirement necessary for the precipitation of the incoming effluent.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0059] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0060] FIG. 1 illustrates a system for treating an industrial effluent charged with aluminium using CO.sub.2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0061] Let us illustrate the invention in what follows by an implementational example and by the appended FIG. 1, for a better understanding of the approach of the invention.

[0062] Let us consider an effluent, at a flow rate of 30 m.sup.3/h with an initial pH of 12, which it is desired to lower to 8.2 in order to be able to discharge it to a network while having precipitated beforehand the aluminum salts which it contains.

[0063] It is thus necessary to employ, in terms of CO.sub.2, of the order of 600 g/l×30=18 kg/h de CO.sub.2.

[0064] In the implementation proposed, the effluent arrives in the center of the neutralization tank where good homogeneity is provided, which tank is equipped with a stirring system, optionally supplemented by an additional stirrer if that which is already in place is not sufficient.

[0065] The recirculation loop thus has to contribute, in dissolved form, at least 18 kg/h of CO.sub.2.

[0066] As the temperature of the effluent is in the vicinity of 25° C., the solubility is of the order of 1.4 kg CO.sub.2/m.sup.3 at 1 bar abs.

[0067] As the loop operates at an absolute pressure of 2 bar, the enriching flow rate has to be in the vicinity of 6.5 m.sup.3/h.

[0068] A person skilled in the art understands that he will have to incorporate a margin and will then retain instead a loop flow rate of the order of 10 m.sup.3/h.

[0069] It is thus a matter of treating a furring/scaling effluent, containing a great deal of dissolved aluminum to be removed The initial high-pH effluent (pH.sub.1 in the vicinity of 12) is sent to a tank which is maintained at a lower pH (pH.sub.2 in the vicinity of 8-8.4). This is the target value chosen according to the invention in this case for precipitating the aluminum hydroxides.

[0070] The neutralized effluent with its solid exits via the bottom (pumped) in order to be subsequently separated by settling (filtration).

[0071] A part of the contents of the tank at pH2 is pumped by the external loop (zone 2) into which the CO.sub.2 is injected via an injector (for example a static mixer); this effluent was freed in the tank from the great majority of its dissolved minerals; it is thus consequently much more weakly scaling.

[0072] Turbulent conditions are maintained throughout the loop. There is then achieved, under pressure generated by the pump, a pH (pH3) even closer to neutrality or to acidity (pH.sub.3<pH.sub.2<pH.sub.1), which pH guarantees the predominant formation of hydrogencarbonate and the presence of dissolved CO.sub.2.

[0073] The gas-liquid mixture is then sent to a coil, the length of which makes it possible to guarantee a sufficient contact time to maximize the amount of CO.sub.2 transferred into the stream.

[0074] Finally, this acidified stream at pH.sub.3 is returned to the tank where, at the closest to the incoming stream at pH.sub.1, it will be mixed to guarantee the pH.sub.2 which prevails in the tank, which pH.sub.2 is optimum for the formation of aluminum hydroxide crystals.

[0075] It should be noted that, in this case, the more the pH falls, the more the precipitation is promoted, down to a limiting pH of 5.

[0076] It should thus be noted, to sum up, that: [0077] The incoming effluent, at high pH (10-12), is predominantly composed of dissolved aluminum in the Al(OH), form. [0078] In order to free it of as much dissolved aluminum as possible, it is advantageous to operate between 5.5 and 8 approximately, there where a large part of the aluminum is transformed into the Al(OH).sub.3 form, which is not very soluble. [0079] It is virtually impossible, with CO.sub.2, to have a lower pH than 5-5.5 and to redissolve the aluminum hydroxide particles which would have been formed. CO.sub.2 is a weak diacid, the first pKa of which does not make it possible to fall below approximately 5. [0080] The effluent, after having been treated and thus freed from a large amount of dissolved aluminum, can absorb CO.sub.2 (part of the stream diverted between the zones 1 and 2). Nevertheless, it is preferable to operate in the top part of the targeted zone (rather towards 8 than towards 5 thus) in order to have more dissolved CO.sub.2 (better solubility of the CO.sub.2 at high pH than at low pH because the hydrogencarbonate HCO.sub.3.sup.− form rather than free CO.sub.2 is favored).

[0081] The following elements, already touched on several occasions in the above description, are recognized in the appended FIG. 1: [0082] a tank 1 constituting the zone 1, equipped with a stirring system 3 and fed with initial effluent to be treated 4; [0083] a recirculation loop 10 constituting the zone 2, capable of withdrawing (5) a part of the medium present in the tank 1 by virtue of a pump 2, which loop receives an injection of CO.sub.2 and which is equipped with a coil, the length of which makes it possible to guarantee a contact time sufficient to maximize the amount of CO.sub.2 transferred into the stream; [0084] at the end of the loop 10, the stream thus treated is returned to the tank 1, thus contributing, by the mixing between the initial effluent (4) and the medium treated with CO.sub.2 in the loop 10, to producing the target pH prevailing in the tank 1; [0085] the tank is equipped with means (6) for extraction of the treated effluent.

[0086] The advantages of the present solution are as follows: [0087] Of avoiding the consumption of water of industrial type to produce a seltzer water as in the conventional solution of the prior art. [0088] Of always preventing any precipitation in the zone where the CO.sub.2 will be injected. The precipitation zone 1 (reactor, settler, and the like) always has to be maintained at a lower pH than that of the incoming stream to be treated in order to make possible the precipitation of the aluminum in the aluminum hydroxide form (in the vicinity of pH 8, for example). The incoming effluent to be treated is then diluted in the zone 1, which should result in a slight increase in the pH, which will in fact be compensated for by the injection of CO.sub.2 in zone 2, namely the recirculation loop. An attempt is made to provide an amount of dissolved CO.sub.2 greater than the mean requirement (1.5 to 2, for example) by the choice of the reliable volume of zone 1 and target pH pair. [0089] Of guaranteeing a rate of transfer of the CO.sub.2 which is maximized, which results from the choice of the operating conditions and of the technologies in the recirculation loop (turbulence). This will result in a consumption which is as close as possible to the requirement of the system (no overconsumption).

[0090] The zone 2 is used only to dissolve the CO.sub.2 in a stream of water (effluent) which is pumped from the zone 1 and which is returned to this zone 1. This zone is calculated in order to dissolve sufficient CO.sub.2 to lower the pH of the zone 1 from the incoming value of the alkaline effluent example 12.5 to the set value example 8. It is also possible to use this zone to introduce a part of the CO.sub.2 in the gas form (fine bubbles) from the zone 1 to 2.

[0091] In this zone 2, there is no reduction from a highly alkaline pH (12.5) to a neutral or acid value but the effluent is pumped from the zone 1, thus neutral or acid, in order to acidify it even more. There is thus no formation of solid in this zone 1 (or trivially) and thus the risk of plugging is thus reduced. It is even possible, if even more CO.sub.2 is injected and this zone is acidified even more, to dissolve the aluminum solids formed at neutral pH and optionally to declog this zone 1 even if in theory this is not necessary.

[0092] As indicated above, particular attention is paid, according to the present invention, in view of the recirculation of the medium where CO.sub.2 has been injected, to the amount of dissolved CO.sub.2 available in the zone 1 being 0.5 to 3 times greater, preferentially between 1 and 1.5 times greater, than the requirement necessary for the precipitation of the incoming effluent.

[0093] Let us explain this in more detail in what follows.

[0094] Let us explain in particular how to determine the CO.sub.2 requirement of a tank (zone 1) and an example of calculation for an amount of available dissolved CO.sub.2 which is from 0.5 to 3 times greater than the CO.sub.2 requirement necessary for the precipitation in the zone 1.

[0095] The effluent entering the zone 1 is very alkaline (thus high pH 1 and high concentration of dissolved aluminum). That is where it will be brought into contact with the effluent, the pH of which has been lowered to a 3′ pH, containing the necessary dissolved CO.sub.2 coming from the zone 2 (low pH 2 since the effluent will contain at least all the CO.sub.2 necessary for the precipitation). On contact of the two, the dissolved CO.sub.2 will make it possible to neutralize the alkalis of the incoming effluent and thus to reduce the pH in order to make possible the precipitation and thus to free this effluent from its dissolved aluminum. The resulting pH, pH 3, will be between the two preceding pH values, and is adjusted so as to favor the precipitation.

[0096] In continuous running, the necessary amount of CO.sub.2 introduced (thus of weak diacid) compensates as much as possible for the alkalinity of the incoming effluent (thus stoichiometric ratio of acid to alkali or base). Nevertheless, if there is a sudden modification to operating conditions and if the amount of alkali increases, an imbalance is created in the acid-base ratio which has to be compensated for. The phenomenon may then be achieved that the effluent which circulates continuously in the zone 2 can have extra alkali which will bring about the precipitation in this zone. This can even obstruct the complete system by blocking, often very rapid in numerous applications, given the high alkalinity of the effluent to be treated. Of course, a control-regulation system might adjust the amount of CO.sub.2 to the amount of alkali (which must cause the pH of the precipitation zone 1 to fall) but this remains problematic. This is because the amounts involved (size of the zone 1) can bring about a slow change in the operating parameters: with a high and sudden incoming amount of alkali, the pH of the zone 1 will change only slowly if the amount is large in size (and thus with a high residence time). Thus, the amount of CO.sub.2 will not respond immediately, indeed even excessively late, which can bring about an undesirable rapid and strong precipitation in the zone 2, which phenomenon absolutely has to be avoided at the risk of stopping everything by an excessively massive blocking.

[0097] The reasoning of the present invention is thus to retain a greater amount than rendered necessary by the incoming effluent of free dissolved CO.sub.2 available in the zone 1 where the precipitation takes place.

[0098] Thus, if there is a variation in the operating conditions (amount of incoming alkali, for example), this extra dissolved CO.sub.2, with respect to the amount of CO.sub.2 necessary for the precipitation (thus stoichiometric), will make it possible to “neutralize” this extra amount in a given period of time. This will, in any case, prevent dissolved aluminum or alkali being sent into the zone 2 and will allow the control-regulation system the time to adjust the flow rate of CO.sub.2 to be injected in order to compensate for this additional amount.

[0099] It is estimated, according to the present invention, that an excess of dissolved, and thus available, CO.sub.2 of the order of 0.5 to 3 times the amount necessary for the optimized neutralization and thus for the optimized precipitation of the incoming effluent is necessary.

[0100] The example below makes it possible to more clearly illustrate the proposal of the invention.

[0101] The data below were obtained by using commercially available software, making it possible to study the equilibria in a body of water. The simulation was carried out in several stages: starting from a (standard) water composition, the addition of sodium hydroxide made it possible to rise to pH 12. This makes it possible to have available a “synthetic” effluent. For this, it was necessary to add 0.56 kg/m.sup.3 of sodium hydroxide.

[0102] Subsequently, the addition of CO.sub.2 made it possible to neutralize it to pH 8 first and then 7.5. It is the effluent neutralized to 8 which was subsequently stored for example. In order to neutralize the effluent from 12.0 to 8.0, it was necessary to add 0.60 g/m.sup.3 of CO.sub.2, which leaves 18 g/m.sup.3 of free CO.sub.2 in the effluent.

[0103] Consequently, for an effluent flow rate of 100 m.sup.3/h, 1 kg/min of CO.sub.2 will be necessary in order to neutralize it.

[0104] Furthermore, if the tank (or zone 1) measures 55 m.sup.3, it will contain only 1 kg of free CO.sub.2, thus available to compensate for an excess of alkalinity suddenly arriving. This would make it possible to compensate for one minute for the arrival of effluent, in the event of shutdown of the injection of CO.sub.2, for example, or else to compensate for a rise in the amount of alkali (for example, from 1 kg/min of requirement to 1.5, for example). In the latter case, in 40 seconds, the free CO.sub.2 will be consumed and the pH will subsequently rise, the reactor or zone 1 will no longer precipitate all the incoming alkali and the entire process will be destabilized. Eventually, dissolved alkali will enter the zone 2, which might result in precipitation and in blockage.

[0105] It is thus preferable to increase the volume of the zone 1 so as to have more free CO.sub.2 available for “erasing” or neutralizing the fluctuations or disruptions in the incoming effluent, inter alia.

[0106] Still in the above case, with a zone 1 of 166 m.sup.3, 3 kg of free CO.sub.2 will be available to neutralize an extra incoming alkalinity. If this excess brings about a requirement for CO.sub.2 of 1.5 kg/min, the system will drift only after 2 minutes. Thus, still in this example, the reaction time necessary to compensate for the sudden excess in alkalinity will have been multiplied by 5. There thus exists much more flexibility and robustness for the process since the complete system is allowed the time to be controlled and to be regulated.

[0107] To sum up, the zone 1 is established in order for this zone to contain a minimum of 1.5 times to 3 times the amount of CO.sub.2 necessary to neutralize the alkalis which arrive each minute in the zone and thus to make possible the precipitation of virtually all of the aluminum (its oxides) in this zone 1. The amount of free CO.sub.2 in the zone can be brought above 3, but without exceeding 10 or 15 for economic reasons.

[0108] In our example, the free CO.sub.2 is 18 g/m.sup.3 for a set pH of 8 and 57 g/m.sup.3 for a set pH of 7.5.

[0109] In order to determine the volume of the zone 1 and to calculate the amount of free CO.sub.2 available, the amount in kg of free CO.sub.2 needed is divided by the concentration of free CO.sub.2 in the effluent at the set pH.

[0110] Thus: Volume of the zone 1=amount of CO.sub.2 in kg which has been determined/concentration of free CO.sub.2 at the set pH.

[0111] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.