FR2989285A1 - Method for purification of flue gases by wet process in household refuse incineration factory, involves purifying flow of residual liquids by water treatment, and recycling remainder of flow of liquids towards acid and alkaline washers - Google Patents

Abstract

The method involves sending smoke (1) to be purified to an acid washer (101) e.g. limestone, and to an alkaline washer (102) e.g. soda. The acid washer is purged (4) and sent to a precipitation-crystallization unit (301) after being purified by water treatment. The alkaline washer is purged (7) and sent in a separate manner to the precipitation-crystallization unit. A flow of residual liquids (12) is purified by water treatment to produce a brine (15). A remainder (14) of flow of residual liquids is recycled towards the acid washer and the alkaline washer. An independent claim is also included for a purifying installation.

Description

The present invention relates to a method and a plant for cleaning wet combustion fumes. In this type of process, a washing liquid is brought into contact with fumes to be purified in a scrubber so that the dust suspended in these fumes is transferred into the washing liquid together with the gaseous pollutants, and in particular hydrochloric acid and sulfur dioxide. The flow rate of liquid required to ensure good purification is generally high, for example of the order of several tens to a few hundred m3 / h, so that the liquid purge of the scrubber is recycled, only a small fraction of that is required. ci being withdrawn for deconcentration. For example, in the case of a household waste incineration plant, sulfur dioxide, hydrochloric acid and heavy metals are captured in the wash purge and these pollutants must find an outlet. It can then be useful to separate the heavy metals using a water treatment. However, this water treatment will generate a sludge cake which, in addition to the precipitated metal hydroxides, also often contains gypsum.

Moreover, with wet scrubbers, it is often necessary to work with two stages of washing, that is to say with two scrubbers succeeding to treat combustion fumes, namely an acid scrubber, operating at low pH, typically less than 4, in particular for capturing hydrochloric acid, and an alkaline scrubber, operating at a higher pH, typically greater than 5, to capture in particular sulfur dioxide. In order to avoid having too large respective purges at the outlet of these two washers, a hydraulic cascade is often carried out in which the purge of one of the washers, in general the alkaline scrubber, is fed back into the other scrubber. occurrence the acidic scrubber. In addition, different neutralization reagents can be used in each of the two scrubbers, in particular limestone or lime in the acid scrubber and sodium hydroxide in the alkaline scrubber. In all cases, we end up, in fine, mixing the respective liquid purges of the acidic and alkaline washers, while these purges have very different respective compositions, which can contribute to uncontrolled precipitation that are sources of fouling of scrubbers and downstream equipment. This is particularly the case when limestone or lime is used in the acidic scrubber and sodium hydroxide in the alkaline scrubber since the mixture of calcium chloride, which results from the neutralization of hydrochloric acid, and of Sodium sulphate, which results from the neutralization of sulfur dioxide, will give gypsum, poorly soluble and precipitates. Even in the case where the precipitation of this gypsum is done in a manner almost controlled, that is to say, not anarchic, the gypsum finishes the treatment of water mentioned above: thus this gypsum is found mixed with sludge. metal hydroxides and thereby increases their volume. However, as these sludge must generally be put in controlled landfills, this increases the cost substantially. In addition, the precipitation occurs under conditions where the heavy metals are still in solution, so that the gypsum itself can only be of poor quality, in the sense that it contains too many heavy metals, which the makes it unsuitable for any valuation. Finally, there are situations where, on the same industrial site, several scrubber trains are present, using different reagents, for example because fumes respectively from different processes must be purified. Again, at a given moment, the various purges from the scrubbers present are mixed and are therefore likely to generate precipitation that is both uneconomic and sources of trouble for washing and water treatment. The object of the present invention is to provide a process and a purification plant, which are more efficient, particularly as regards the fouling of purification equipment, and more economical, especially by potential upstream recovery. For this purpose, the subject of the invention is a process for the purification of humid combustion fumes, in which the fumes to be purified are sent to an acidic scrubber then to an alkaline scrubber, the purge of the acid scrubber is sent to a precipitation-crystallization unit after having been purified by a first water treatment, - the purging of the alkaline scrubber is sent, separately from the purge of the acid scrubber, to the precipitation-crystallization unit, - the outlet of the precipitation-crystallization unit, containing precipitated gypsum, is admitted into a liquid-solid separator to produce a gypsum stream, containing less than 15% by weight of free water and intended to be recovered, and a flow of residual liquids, and - 30 to 100% in flow rate, preferably 50 to 80% in flow, of the flow of residual liquids are purified by a second treatment of water to produce a brine intended for recovery, the remainder of the flow of liquid The residuals are recycled to one and / or the other of the acid and alkaline scrubbers.

The subject of the invention is also a plant for the purification of humid combustion fumes, comprising: an acid scrubber fed with fumes to be purified; an alkaline scrubber fed with fumes discharging from the acid scrubber; a first unit; water treatment unit, fed by the purge of the acid scrubber, - a precipitation-crystallization unit, supplied, separately, by the purge of the acid scrubber, purified by the first water treatment unit, and by purging of the alkaline scrubber, - a liquid-solid separator, supplied by the outlet of the precipitation-crystallization unit, containing precipitated gypsum, and adapted to produce a gypsum flow, containing less than 15% of free water and intended for to be upgraded, and a residual liquid flow, - a second water treatment unit, fed with 30 to 100% flow rate, preferably 50 to 80% flow rate, of the residual liquid flow, and adapted to produce a brine. re intended to be upgraded, and - means for recycling to one and / or other of the acidic and alkaline washers the remainder of the flow of residual liquids, untreated by the second water treatment unit. Thus, the invention makes it possible to carry out controlled gypsum precipitation and under clean conditions so as to obtain separately: a fairly pure gypsum, intended to be recovered, a brine substantially free of heavy metals, which can also be recovered, and a small amount of metal hydroxide cakes, also recoverable by recovery of the metals they contain, including zinc.

The invention makes it possible to generate separate and recoverable streams for each outgoing product, as well as to ensure operating conditions for the equipment, particularly the washers, protecting them from potential rainfall. According to advantageous additional features of the process and the installation according to the invention, taken alone or in any technically possible combination: at least one stream containing sulphates, other than the purging of the alkaline scrubber, is also admitted into the precipitation-crystallization unit; the fumes treated by the acidic and alkaline scrubbers are waste incineration fumes, while incineration fumes from the water treatment plant sludge are treated at least by another alkaline scrubber whose purge constitutes said at least one stream containing sulfates; the second water treatment is more intensive than the first water treatment; the residual liquid stream is sent to the acidic scrubber; an acid stream, in particular hydrochloric acid or sulfuric acid, is also introduced into the precipitation-crystallization unit; the precipitation-crystallization unit comprises a crystallizer or a stirred tank; the precipitation-crystallization unit comprises hydrocyclones; the liquid-solid separator is constituted by a band filter or a centrifuge. The invention will be better understood on reading the description which will follow, given solely by way of example and with reference to the drawings in which: FIG. 1 is a diagram of an installation according to the invention; implementing a method according to the invention; and - Figure 2 is a view similar to Figure 1, illustrating a variant of the installation and the method according to the invention.

In the installation shown in FIG. 1, the fumes to be purified 1 pass through an acid scrubber 101, using, for example, as a neutralization reagent for lime or limestone, and then a second alkaline scrubber 102, using, for example, sodium hydroxide as an agent. neutralizer. The fumes exiting the acid washer 101 and entering the alkaline scrubber are referenced 2. Each scrubber 101, 102 is generally provided with a supply of fresh water, not shown, which makes it possible, among other things, to compensate for evaporation of water in the washers. The purified fumes 3, leaving the downstream alkaline scrubber 102, are evacuated, for example via a chimney and a draft fan, or are subjected to a complementary treatment, which is not the subject of the invention. which is not shown in FIG. 1. The liquid purge 4 of the acid scrubber 101 is sent to a water treatment unit 201 in which the purge 4 is freed of at least 90% by weight of the heavy metals, such as lead, zinc or cadmium. The water treatment implemented in the unit 201 is known in itself and is not the subject of the invention. A stream 5, consisting of hydroxide sludge or hydroxide cake, is derived from the water treatment unit 201: this product stream 5 is valuable because it is rich in metals. The main outlet of the water treatment unit 201 is a liquid stream 6 consisting of the purge of the acid scrubber 101, freed of the heavy metals by the water treatment used in the unit 201. This stream 6 is sent to a precipitation-crystallization unit 301 detailed below.

The unit 301 also receives the purge 7 of the alkaline scrubber 102: as shown in FIG. 1, this purge 7 is sent from the alkaline scrubber 102 to the precipitation-crystallization unit 301 separately from the purge flow from the acid scrubber 101, and this directly, that is to say without treatment significantly modifying the composition of this purge 7. The precipitation-crystallization unit 301 makes it possible to achieve a controlled precipitation by crystallization of the gypsum which will form there. More specifically, insofar as, in this unit 301, the treated purge 6 from the acid scrubber 101, which contains especially calcium chloride in solution produced by the neutralization reaction between the hydrochloric acid and the neutralization reagent used, for example limestone or lime, is mixed with the purge 7 of the alkaline scrubber 101 containing sulphates, which are produced by the neutralization reaction between the sulfur dioxide and the neutralizing agent used, for example soda, gypsum is formed by the reaction: Cal brought by 4) + SO4 - (provided by 7) Ca S041 (in hydrated form) Optionally, an acid flow 8, which may be sulfuric acid or hydrochloric acid, is also introduced into the precipitation-crystallization unit 301: it may indeed be beneficial to carry out the precipitation-crystallization reaction of the gypsum under slightly acidic pH conditions. The precipitation-crystallization unit 301 is of a known technology. In particular, this unit 301 may consist of a stirred tank or a crystallizer and auxiliary units such as pumps and hydrocyclones intended to maintain nucleation nuclei in this unit 301. The main outlet 9 of the unit precipitation-crystallization 301, which contains the precipitated gypsum produced, is directed to a liquid-solid separator 302. This liquid-solid separator 302, which is for example a belt filter or a centrifuge, is also supplied with water 11 which, in the separator 302, washes the gypsum to ensure its characteristics making it valuable. This separator 302 then makes it possible to produce a flow of gypsum 10, which contains less than 15% by weight of free water and whose gypsum is purified and white. By way of example, the purity of the gypsum contained in this stream 10 is such that this stream contains at least 95% by weight of pure gypsum, as well as, at the most, 0.1% by weight of magnesium, 0.01%. by weight of chlorine, 0.5% by weight of calcium sulphite (CaSO 3), 0.15% by weight of ferric oxide (Fe 2 O 3), 2.5% by weight of silica (SiO 2) and 0.3% by weight of weight of alumina (A1203). Residual liquids leaving the liquid-solid separator 302 form a stream 12 at least a portion or all of which forms a stream 13 sent to a water treatment unit 401, which is different from the unit 201 and which falls under of a technology known per se. According to the invention, the flow 13 sent to the water treatment unit 401 represents between 30 and 100% flow rate of the flow 12, preferably between 50 and 80% flow rate of this flow 12. Under the action of water treatment implemented in this unit 401, the stream 13 is purified to produce a brine 15 potentially recoverable. In particular, this brine consists essentially of sodium chloride in aqueous solution. The residues, resulting from the water treatment implemented in the unit 401, form sludge or cake 16, which are removed from the installation. It will be noted that, according to an advantageous arrangement of the invention, the water treatment, implemented in the unit 201, need not be as intensive as that used in the unit 401, in the sense that where, unlike the brine 15 to be upgraded, the stream 6 leaving the unit 201 necessarily undergoes, downstream of this unit 201, complementary processing, as described above. In practice, this means that the unit 401 is provided to implement a complete water treatment while the unit 201 is provided only to implement a partial water treatment. In the case where only a part of the residual liquid flow 12 is sent to the water treatment unit 401, that is to say in the case where the flow 13 is not equal to 100% of the flow 12, the remainder 14 of the residual liquid stream 12 is recycled into the installation, by being returned to one and / or the other of the washers 101 and 102, preferably to the acid washer 101 as shown in FIG. Recycling the remainder 14 reduces the amount of fresh water generally consumed for flue gas cleaning. Thus, as will be understood from reading the above, the essence of the invention is to separate the purges 4 and 7 from the washers 101 and 102, to extract the heavy metals from the purge 4 of the acid washer 101 and to then mixing the latter with the purge 7 of the alkaline scrubber 102, containing sulfates, under controlled conditions to precipitate-crystallize gypsum. In this way and according to the invention, there is obtained: a stream of gypsum 10 intended to be recovered, a brine 15 consisting essentially of sodium chloride in water, potentially recoverable, and sludges or cakes. and 16, which are in limited quantity since they no longer contain gypsum and are rich in metals which makes them also likely to be valued. As will also be understood, by separating the purges 4 and 7 from one another, any problem of uncontrolled precipitation in the washers 101 and 102, as well as immediately downstream of these washers, is avoided.

It will also be noted that, according to an advantageous characteristic of the invention, the precipitation-crystallization which takes place in the unit 301, is made from solutions which contain only very few heavy metals, in particular because the treatment water used in the unit 201 has treated the purge 4 which is the main source of these metals.

This ensures a high purity gypsum which is formed in the unit precipitation-crystallization 301. The invention is also of particular interest in cases where, on the same industrial site, there are several streams containing sulfates. This is particularly the case for incineration sites on which one or more lines are dedicated to the incineration of waste or garbage, while one or more other lines are dedicated to the incineration of sludge of waste stations. purification of water: this typical case is illustrated by FIG. 2. Indeed, in FIG. 2, the materials 101, 102, 201, 301, 302 and 401 are respectively identical to those described above with respect to FIG. 1 and provide the same respective functions. Thus, these materials 101, 102, 201, 301, 302 and 401 treat the fumes to be purified 1 which are for example waste incineration fumes. At the same time, the fumes to be purified 1 'constituted of fumes incineration sludge of a water treatment plant, are treated by a dedicated line which, as shown in Figure 2, is separated from the washers 101 and 102. This dedicated line produces an effluent 17 rich in sulfates. Typically, this dedicated line comprises at least two scrubbers 501 and 502, the scrubber 502 being an alkaline scrubber having captured the sulfur dioxide from the flue gases 1 'and producing the stream 17. According to an advantageous characteristic of the invention, the flow 17 rich in sulphates feeds the precipitation-crystallization unit 301, which makes it possible to produce more gypsum. Thus, an additional benefit is related to the fact that, without this provision of the invention, the stream 17 should be treated differently before being discharged to the environment.

Claims (10)

CLAIMS 1. A process for the purification of humid combustion fumes, in which: the fumes to be purified (1) are sent to an acid scrubber (101) then to an alkaline scrubber (102), - the purge (4) the acid scrubber (101) is sent to a precipitation-crystallization unit (301) after having been purified by a first water treatment, - the purge (7) of the alkaline scrubber (102) is sent, separately from the purge of the acidic scrubber, to the precipitation-crystallization unit (301), - the outlet (9) of the precipitation-crystallization unit (301), containing precipitated gypsum, is admitted into a liquid-solid separator (302). ) to produce a gypsum stream (10), containing less than 15% by weight of free water and intended to be recovered, and a residual liquid stream (12), and - 30 to 100% in flow, preferably 50 at a flow rate of 80% of the residual liquid stream (12) are purified by a second water treatment to produce a brine (1). 5) intended to be recovered, the remainder (14) of the residual liquid stream being recycled to one and / or the other acid (101) and alkaline (102) washers. The process according to claim 1, wherein at least one sulphate-containing stream (17), other than the purge (7) of the alkaline scrubber (102), is also admitted to the precipitation-crystallization unit (301). . 3. The process according to claim 2, wherein the fumes (1) treated with the acid (101) and alkaline (102) scrubbers are waste incineration fumes, while fumes (1 ') for incineration of sludge of water treatment plant are treated at least by another alkaline scrubber (502), the purge constitutes said at least one stream (17) containing sulfates. 4. A process according to any one of the preceding claims, wherein the second water treatment is more intensive than the first water treatment. 5. A process as claimed in any one of the preceding claims wherein the balance (14) of the residual liquid stream (12) is fed to the acidic scrubber (101). 6. A process according to any one of the preceding claims, wherein an acid stream (8), in particular hydrochloric acid or sulfuric acid, is also introduced into the precipitation-crystallization unit (301). 7.- Installation for cleaning flue gases by wet process, comprising: - an acid scrubber (101) fed with fumes to be purified (1), - an alkaline scrubber (102) fed by fumes (2) leaving the acid scrubber (101), - a first water treatment unit (201), fed by the purge (4) of the acid scrubber (101), - a precipitation-crystallization unit (301), fed separately, by purging the acid scrubber, purified by the first water treatment unit (201), and the purge (7) of the alkaline scrubber (102), - a liquid-solid separator (302), fed by the outlet ( 9) of the precipitation-crystallization unit (301), containing precipitated gypsum, and adapted to produce a stream of gypsum (10), containing less than 15% of free water and intended to be recovered, and a flow of residual liquids (12), - a second water treatment unit (401), supplied with 30 to 100% flow rate, preferably 50 to 80% flow rate, of the flow of e residual liquids (12), and adapted to produce a brine (15) for recovery, and - means for recycling to the one and / or the other of the acid washers (101) and alkaline (102) the remainder (14) the residual liquid stream (12), untreated by the second water treatment unit (401). 8. Installation according to claim 7, wherein the precipitation-crystallization unit (301) comprises a crystallizer or a stirred tank. 9.- Installation according to one of claims 7 or 8, wherein the precipitation-crystallization unit (301) comprises hydrocyclones. 10.- Installation according to any one of claims 7 to 9, wherein the liquid-solid separator (302) is constituted by a band filter or a centrifuge.