WO1997047362A1

WO1997047362A1 - Removal of heavy metals from waste incineration or glass furnace residues

Info

Publication number: WO1997047362A1
Application number: PCT/FR1997/001065
Authority: WO
Inventors: Fabrice Fourcot
Original assignee: Novacarb
Priority date: 1996-06-14
Filing date: 1997-06-13
Publication date: 1997-12-18
Also published as: FR2749772B1; FR2749772A1; AU3347897A

Abstract

A method for removing heavy and/or toxic metals from the solids produced by the dry, semi-dry or semi-wet processing of waste incineration gases or glass furnace gases, is disclosed. The method comprises the steps of preferably selectively dissolving the sodium chlorides and/or sulphates present in said solid residues; separating the remaining solids in suspension in the solution; processing the resulting solution with at least one carbonate or hydroxycarbonate, at least one silicate or aluminosilicate and at least one sulphur compound and/or at least one phosphate, as the purification agents; and separating the resulting solids.

Description

REMOVAL OF HEAVY METALS FROM WASTE INCINERATION WASTE OR GLASS OVEN

The subject of the present invention is a process for removing heavy and / or toxic metals present in the solids resulting from the treatment, by dry, semi-dry or semi-wet process, of waste incineration gases or of gases from glass furnaces. In order to quickly summarize the situation in terms of gaseous releases to be treated, two types of releases are known, the first is characterized by the fact that it contains mainly, in addition to heavy metals and other organic pollutants, acids of formula HX, with X put for a halogen; the second is characterized by the fact that it mainly contains, in addition to heavy metals and other organic pollutants, compounds of the SOx type. Among the gas-generating processes mainly comprising HX-type compounds, one can cite, among other things, household waste incinerators, hospital waste and special industrial waste incinerators.

As for the processes rejecting gases mainly comprising sulfur oxides, thermal power stations, glass furnaces, depending on the quality of the fuel and raw materials used, are primarily concerned, it should be noted that by

SOx means SO ₂ , S0 ₃ species. These fumes can also include sulfites and / or bisulfites.

Due to the increasing pressure on environmental regulations concerning installations emitting such discharges, the processes for treating acid fumes are experiencing significant development.

Conventional lime neutralization treatments, whether dry, semi-dry or semi-wet, generate a significant amount of solid waste, consisting mainly of gypsum polluted by metals considered to be toxic, such as lead, cadmium, aluminum, for example. Given the composition of this waste and the highly soluble nature of polluting metallic elements in water, the legislation requires its deposit in a controlled landfill, after immobilization.

This is a very significant drawback since not only are such treatments expensive, but also the tonnages of waste produced are large in quantity. In addition, there is no way to easily revalue one or more products resulting from such treatments.

The object of the present invention is therefore to propose an effective method for removing the heavy metals contained in the solid waste resulting from the treatment. by dry, semi-dry or semi-wet route, of gas mainly comprising compounds of the HX or SOx type.

The aim of the process is to propose a method aimed at considerably reducing the ultimate waste that must be landfilled. Finally, the invention makes it possible to obtain solutions of sodium sulphates or chlorides, depending on the nature of the gases initially treated, which are pure and easily recoverable.

These and other objectives are achieved by the present invention which therefore relates to a process for removing heavy metals present in the solid residues resulting from the dry, semi-dry or semi-wet treatment of gases mainly comprising HX type compounds, with X representing a halogen, and / or mainly SOx type compounds, with at least sodium carbonate and / or bicarbonate, characterized in that the following steps are carried out:

-sulfates and / or sodium chlorides preferably present in the aforementioned solid residues are preferably dissolved selectively,

- the solids remaining in suspension in the solution are separated, - the resulting solution is treated with at least one carbonate or a hydroxycarbonate, at least one silicate or an aluminosilicate, and at least one sulfur-containing compound and / or at least one compound chosen from phosphates, as purifying agents,

- The solids formed are separated.

It has been found quite surprisingly that the combination of a gas treatment by dry, semi-dry or semi-wet route with at least sodium carbonate or sodium bicarbonate, followed by the steps of treatment of solids resulting as just explained, allows to obtain on the one hand ultimate waste in less quantity. On the other hand, such a combination makes it possible to reach levels of purity of very high purity solutions of sodium chlorides and / or sulphates, and this for a wide range of pH.

However, other characteristics and advantages of the present invention will appear more clearly on reading the description and the examples which follow.

First of all, it is indicated that by specifying that a gas mainly comprises compounds of the HX or SOx type, we define gases whose concentration in one of the two types of compounds is predominant compared to the others constituents of said gas, without however excluding the presence of the other type of compound. Furthermore, it is understood that these treated gases (or fumes) also include heavy metals, which may also be qualified as metals considered toxic or polyvalent. This name covers more particularly but without intending to be limited, mercury, cadmium, lead, arsenic, selenium, silver, tin, antimony, zinc, aluminum, silicon, calcium, titanium, chromium, thallium, manganese, iron, nickel, copper, molybdenum, bismuth, vanadium, cobalt.

Optionally, but to a lesser extent, the treated fumes may include organic type pollutants such as dioxins, furans, or even mineral compounds such as NOx.

In addition to the pollutants which have just been mentioned, the fumes treated according to the invention may also include dust or fly ash.

For clarity, the gas treatment process will be described.

As mentioned previously, the treatment is carried out in the dry, semi-dry or semi-wet route.

The term “dry” and “semi-dry” means more particularly a treatment in which the neutralizing agent is introduced in the solid state into the flue gases.

In this case, two types of implementation can be distinguished, the first variant is free of water (dry process) and the second variant is carried out in the presence of water (semi-dry process).

According to the first variant of the dry method, anhydrous reagents are used more particularly.

According to the second variant of the dry method, the fumes are brought into contact with an aforementioned reagent, in the presence of water but with an amount such that the reagent retains its powdery appearance. The reagent used can be in anhydrous form or comprising water of crystallization.

Water can be brought into the process in several ways. This can in particular be present intrinsically in the reagent or else be brought into use temporarily, before the treatment of the fumes or else, during the treatment of these fumes. Of course, the combination of these possibilities is possible.

In the case where the reagent comprises water of crystallization, it may be possible not to supply the water extemporaneously and to carry out the treatment in the presence only of the water of crystallization of the reagent.

However, according to a preferred embodiment of the invention, the water is supplied extemporaneously, whether or not the reagent comprises water of crystallization. In this case, the water content is between 2 and 15% by weight relative to the reagent.

By semi-wet, we define a process in which the reagent is introduced in the presence of water in an amount such that the products obtained after the treatment are in solid form. In this case, the reagent is used in the form of a suspension or a solution.

The water content necessary to obtain a solution or a suspension of the reagent can be determined by a person skilled in the art depending on the technological constraints of the process. More particularly, this content depends on the smoke treatment temperature, the evaporation capacity of the equipment and the fact that it is desired to obtain, after said treatment, a solid product.

In the case where the reagent is in solid form, it may be preferable to use a product of fine particle size and controlled so as to improve the kinetics of the neutralization treatment.

As an indication, it is preferred to use a reagent whose average particle size is less than 50 μm. According to a more particular embodiment, a reagent is used whose average particle size is between 5 and 30 μm.

If a reagent with a greater particle size or of the order of 50 μm is used, it may be advantageous to carry out, prior to the treatment of the fumes, a step of grinding said reagent to achieve a particle size of the order of 5 at 30 μm. Such a step can be implemented by any conventional means known to those skilled in the art.

The carbonates and bicarbonates present on the market are suitable for carrying out the present invention.

The sodium carbonate used in the process is more particularly light sodium carbonate. The latter may or may not include water of hydration. By way of example, anhydrous sodium carbonate, monohydrate, heptahydrate or decahydrate are suitable for carrying out the invention, these compounds being able to be used alone or as a mixture.

It is likewise conceivable to implement the method according to the invention with a reagent such as sodium sesquicarbonate, which corresponds to an equimolar mixture of bicarbonate and sodium carbonate comprising two molecules of water. According to a preferred variant of the invention, use is made of a reagent chosen from sodium bicarbonate, or a sodium bicarbonate / sodium carbonate mixture in which the molar proportion of bicarbonate is greater than that of sodium carbonate. As an indication, the treatment is carried out with a reagent comprising at least 70% sodium bicarbonate. According to a particularly advantageous variant, it is possible to use a mixture of sodium carbonate and bicarbonate, obtained by a heat treatment of ammoniacal bicarbonate, at a temperature varying between 20 and 150 ° C, preferably between 20 and 130 ° C, under a controlled atmosphere, optionally comprising carbon dioxide and / or water. Thermally treated ammonia bicarbonate is the intermediate product obtained by implementing the process for the preparation of sodium carbonate by the technique of ammonia soda, also known as the "Solvay process". More precisely, the product obtained after dissolving gaseous ammonia in an aqueous solution of sodium chloride so as to produce an ammoniacal brine is called ammoniacal bicarbonate for the purposes of the present invention.

This is then treated with a gas containing carbon dioxide to give ammoniacal bicarbonate crystals.

Ammoniacal bicarbonate in fact designates a composition mainly comprising sodium bicarbonate, sodium carbonate, and to a lesser extent, ammonia in the form of ammonium salts such as bicarbonate, carbonate and / or carbamate, and water. . As an indication, this composition comprises: 70 to 90% by weight of sodium bicarbonate (NaHC03), 2 to 20% by weight of sodium carbonate (Na2CO3), 0.2 to 1.0% by weight of expressed ammonia in the form of ammonium ion, 0.3 to 2.0% by weight of ammonium carbamate (NH4O2CNH2), and 5 to 20% by weight of water; the sum of the constituents being equal to 100%. More particularly, the heat treatment is carried out in the presence of an atmosphere comprising water vapor in a content of between 0 and 10 mol%.

Furthermore, the heat treatment is carried out in the presence of an atmosphere comprising a carbon dioxide content of between 0 and 100 mol%. The complement to reach the desired total pressure is ensured, if necessary, and advantageously by air, or any gas, an inert gas such as rare gases, or nitrogen.

It should be noted that the heat treatment of the ammoniacal bicarbonate can be carried out under any pressure. The heat treatment is preferably carried out in the presence of water vapor and carbon dioxide.

The treatment according to the invention is preferably carried out under a gas flow, and with stirring.

It has been found that the treatment temperature of the ammoniacal bicarbonate can be adapted as a function of the quantity of water vapor and carbon dioxide present.

Thus, in the case where the atmosphere is a nitrogen / oxygen mixture or air, the temperature range for the heat treatment will preferably be less than 80 ° C. Furthermore, if the treatment is carried out under a mixture of carbon dioxide and water vapor, it is then possible to carry out the latter at a temperature close to 120-130 ° C., although lower temperatures remain possible. . For the above, the temperature values are expressed at atmospheric pressure. Those skilled in the art are able to adapt the temperature values as a function of the pressure under which the treatment is carried out.

Usually, the duration of the operation is at most a few hours. Ammoniacal bicarbonate has in particular been described in international application WO 97/16377.

The quantity of reagent used in the smoke treatment process according to the invention depends on the quantity of compounds to be neutralized present. By compounds to be neutralized, we mainly refer to the main compounds, that is to say to SOx and / or acids of formula HX.

According to a particular embodiment of the invention, the smoke treatment is carried out so that the stoichiometric ratio between the reagent and the compounds of the SOx type, and / or acids of formula HX, is between 1.02 and 2. Preferably, said stoichiometric ratio is between 1.02 and 1.5.

If the fumes to be treated include organic pollutants as well as mercury, it may be advantageous to inject an adsorbent into the gas stream, along with the reagent and possibly water. The adsorbent comprising the organic pollutants can then be recovered with the solids from the smoke treatment. The adsorbent is generally activated carbon and / or lignite coke.

It should be noted that if the fumes include dust or fly ash, these can be recovered with the solids produced during neutralization, or else prior to treatment according to the invention, by conventional methods. The contacting of said fumes with the reagent, optionally water and the adsorbent, is carried out in a known manner.

Thus, it can take place co-current or counter-current. The treatment can likewise be carried out in any type of reactor promoting the dispersion of the solid, the suspension or the solution in fumes, such as reactors comprising Venturi type systems, or fluidized bed reactors.

If the second variant of the dry method is used, water can be provided before contacting the fumes. In this case, all known methods are suitable. In general, the water in spray form is brought into contact with the reagent in a humidifying drum. Still within the framework of this same variant, water can be brought in when the reagent is brought into contact with the fumes. In this case, water is then injected in the required quantities directly into the gas stream. The solids produced by the process are then separated using conventional techniques such as electrostatic precipitators, or bag filters.

According to an advantageous embodiment of the present invention, at least part of the solids resulting from the treatment of the fumes is recycled to a new stage of treatment of the fumes.

The smoke treatment temperature depends on various technological constraints.

It depends in particular on the temperature of the fumes to be treated, on the water content, on the presence of a catalytic treatment of NOx, on the thermal resistance of the equipment.

As an indication, the smoke treatment temperature is between 70 and 500 ° C and more particularly between 130 and 300 ° C.

In the case where a treatment is carried out in the presence of water brought in temporarily, said solids can be recycled in the zone for preparing the reactive / water mixture.

If water is brought into contact in the neutralization zone, the solids are introduced into this zone, separately or with the introduction of fresh reagent.

Such an embodiment is advantageous for further increasing the efficiency of the process, in particular in the event of a short residence time in the neutralization zone. This mode also makes it possible to lower the smoke purification thresholds.

With regard to the treatment of fumes comprising mainly compounds of the HX or SOx type, reference may be made to patent applications WO 97/16377 and WO 97/16236.

Once the solid residues resulting from the treatment of the fumes are collected, these are dissolved in water so as to preferably dissolve selectively, the sodium sulphates and / or chlorides present in the aforementioned solid residues.

The dissolution step is generally carried out in conventional equipment such as for example those allowing dissolution in one or more successive stages, against the current. Conventionally, the dissolution is carried out in a reactor which is preferably stirred, then the resulting sludge is sent to a decanter or any other apparatus allowing the separation of the solid and the liquid, such as filter presses for example.

It should be noted that the dissolution can take place in one or more successive stages, in the case where there remain soluble salts of sodium chloride or of sodium sulphate in the sludges obtained. In this case, the process consists in juxtaposing several stages of dissolution • separation.

Thus, and if necessary, the solid residues are introduced into a first reactor, preferably agitated, supplied with water. The suspension obtained in this first reactor θ is then sent to a decanter, the overflow of which will be treated according to the method of the invention which will be described in detail below. The sludge recovered in this first decanter is introduced into a second reactor supplied with water, then introduced into a second decanter. According to a particularly advantageous embodiment, the overflow of this second settling tank is used to supply water to the first reactor.

This sequence is continued until a minimum content of soluble salts in the solids is obtained.

The dissolution operation is more particularly carried out by controlling the pH of the medium to be in a range of minimum solubility of metals, which are in particular in the form of hydroxide and / or carbonates.

It is clear that the range depends on the nature of the metals present and a compromise between each of them may be necessary. As a general rule, the dissolution is carried out in a pH range of between 3 and 10, these two limits being included.

According to a first variant, and if aluminum is not present in a high quantity compared to the other metals, a pH range between 7 and 10, the two limits being included, represents a good compromise. More particularly, the dissolution can be carried out in a pH range between 8 and 9.5, these two limits being included.

According to a second variant and in the case where the amount of aluminum is high, it may be desirable to carry out the dissolution at a pH of between 3 and 7, these two limits being understood.

It would not go beyond the scope of the present invention to implement the two aforementioned variants successively. Thus, the dissolution step can correspond to two successive steps with an intermediate separation step, one carried out at a pH between 7 and 10, the second at a pH between 3 and 7, the order depending on the pH value of the initial solution.

This type of method has in particular been described in patent application WO 97/16376, specific to the treatment of fumes comprising mainly SOx. Obviously, this method is also applicable to fumes comprising mainly HX type compounds.

In the case where the residues originate from the treatment of fumes mainly comprising compounds of the HX type, the dissolution step is preferably carried out while maintaining a solid / liquid weight ratio of between 0.5 and 10. It should be noted that it is particularly advantageous to maintain such a weight ratio so as to avoid the formation of complexes of said metals with the halides present in the medium, which would result in redissolving all or part of the heavy metals.

In the case where the residues originate from the treatment of fumes comprising mainly compounds of the SOx type, the risk of redissolving the heavy metals does not arise. Consequently, it is possible to deal with solutions close to saturation.

The pH value is checked by adding either an acid or a base, chosen so as not to pollute the mixture. Thus, it is preferred to use sulfuric acid or sodium carbonate or bicarbonate. In the particular case of solid residues originating from the treatment of fumes comprising mainly SOx, the step of selective dissolution of the sulfates is carried out, according to a particular embodiment of the invention, at a temperature of between 15 and 80 ° C., more particularly between 20 and 60 ° C. An advantageous embodiment consists in carrying out this dissolution at a temperature in the region of 30-40 ° C.

It should be noted that after dissolution and in the case where the treated fumes contain sulfites or bisulfites, it is possible to oxidize these species to sulfates. This operation is generally carried out by adding at least one oxidizing compound to the mixture, possibly in combination with heating of the latter. By way of example of oxidizing compounds, mention may be made of air, hydrogen peroxide, persulfates or even the salt of Caro acid.

In the case where the solid residues come from the treatment of fumes mainly comprising compounds of the HX type, this step is generally carried out at ambient temperature. At the end of the dissolution step or steps, the recovered sludge is preferably dehydrated, more particularly by means of a filter press and can be landfilled. At this stage, the quantity of solid residues is considerably reduced compared to the quantity of residues that would have to be removed without such treatment. Furthermore, the landfill conditions are less expensive because only the insoluble metals are kept in these residues. Thus, the leachability of the residues is reduced.

The solution is then subjected to a purification treatment. By solution is meant the saline solution (s) coming from the settling tank (s), as well as the filtrate obtained after dewatering the sludge after the dissolution step. The purification step consists in treating said solution with at least one carbonate or hydroxycarbonate, at least one silicate or an aluminosilicate, and at least one sulfur compound and / or at least one compound chosen from phosphates. The first constituent agent used in the purification treatment is therefore chosen, either from carbonates, hydroxycarbonates.

The alkali metal, preferably sodium, carbonates are used more particularly. With regard to hydroxycarbonates, more particularly compounds such as hydrotalcite or dawsonite are used. It is recalled that the hydrotalcite is a basic carbonate of magnesium and aluminum. Dawsonite is a basic aluminum and sodium carbonate.

The second constituent agent used in the purification treatment is therefore at least one silicate or at least one aluminosilicate, which are more particularly alkali metal silicate and aluminosilicate, in particular sodium or potassium.

The second agent is advantageously a sodium silicate, according to a preferred embodiment, said sodium silicate then has a Siθ2 / Na2θ molar ratio of between 0.5 and 3.8, for example equal to approximately 2.

The third agent is chosen from sulfur compounds, phosphates, alone or as a mixture.

The sulfur compounds can be mineral or, preferably, organic.

As an inorganic sulfur compound, it is possible to use an inorganic sulfide, in particular a hydrogen sulfide, a barium sulfide (BaS) or strontium (SrS), or iron (FeS) or, preferably, a (poly ) mineral thiocarboπate, in particular an (poly) thiocarbonate of alkali metal, for example potassium or sodium. It is thus possible to use any salt of thiocarbonic acid, such as potassium thiocarbonate (K ₂ CS ₃ ).

As organic sulfur compound suitable for carrying out the invention, an organothiophosphate or an organodithiophosphate, in particular a dialkyl- or diaryl-dithi-dithiophosphate of alkali metal, for example of sodium, can be used.

The alkali metal dialkyl- or diaryl-dithiophosphates which can be used correspond in particular to the following formula:

in which X is an alkali metal, for example sodium, R is an aryl or, preferably, alkyl radical, for example, methyl, ethyl, n-propyl, isopropyl, 1-methyl propyl, isobutyl.

As organic sulfur compound, it is possible to use, preferably, a (poly) mercapto compound, in particular a mercapto, dimercapto or trimercapto compound. This organic sulfur compound can then be a triazine (for example a triazine-as or, preferably, a triazine-S), substituted by 1, 2 or 3 monovalent radicals -SH.

More particularly, this organic sulfur compound is trimercapto triazine-S, which corresponds to the following formula:

It is especially marketed under the names TMT 15®. It is also possible to use compounds sold under the names of Metalsorb L®, Metalsorb DTL®, Metalsorb ZT®, Prosedim Met 07®.

With regard to phosphates, the phosphoric acid, phosphates, hydrogenophosphates, pyrophosphates or also polyphosphates of alkali or alkaline-earth metals are more particularly suitable for the embodiment of the present invention.

According to a particularly advantageous embodiment of the invention, the purification treatment is carried out by introducing the three types of agents mentioned above successively.

The order of introduction is indifferent, but preferably, and in particular to limit the amount of agents added, the procedure is carried out in the order indicated above, that is to say the (hydrogen) carbonates, the (alumino) silicates and sulfur compounds and / or phosphates. According to a first variant of this first embodiment, these compounds are introduced successively.

The amounts of the various agents used depend first of all on the composition of the solution to be treated. The various contents can be determined by a person skilled in the art by applying his general knowledge of the field. According to a second variant of this first embodiment, the agents are introduced at least in the form of a solid comprising all or part of said agents. According to this particularly advantageous variant, the agents are shaped in the presence of a support. In fact, according to this variant, the solid is preferably made up of several successive layers; the agent to be used last being closest to the support or the core of said solid, so as to delay its dissolution in the medium.

It should also be noted that the solid may contain all of the above agents or only part of them. In this last hypothesis, we will add simultaneously or successively said solid and the agent or agents from which the solid is free.

The support used is preferably a clay, which can be of natural or synthetic origin. The clay used advantageously has a high content by weight of

AI2O3; this content is for example between 20 and 40%.

It is possible to use, according to the invention, a clay of lamellar or phyllosilicatθ structure. Thus one can use a clay chosen from the group comprising kaolinites, serpentines. The clay can also be chosen from the group comprising montmorillonites, bentonites (in particular alkaline), talc, mica.

Preferably, the clay chosen belongs to one of these two groups and preferably it is chosen from montmorillonites or bentonites. It is also possible to use a clay of chlorite type structure.

The content of support, in particular clay, usually represents 5 to 90% by weight, for example 10 to 35% by weight, relative to the total weight of said product.

If a composite product is used as described above, it is usually in the form of a granule or a powder, the grain size of which is preferably between 0.1 and 2, 0 mm, in particular between 0.2 and 1.6 mm.

Generally, the solid product has a weight content of free water (or humidity) of between 10 and 30%.

The quantities of the various agents used again depend on the solution to be treated. However, reference may be made to international application WO 97/16248 as regards the contents of the three types of agent.

With regard to the preparation of the aforementioned solids used in the context of this second variant, international application WO 97/16248 gives the details.

It would not be departing from the scope of the present invention to use the three types of purifying agents simultaneously, for example using them in the form of a liquid mixture. The process for purifying the solution is carried out at a controlled temperature which depends on that used during the preceding dissolution step.

Thus, in the case where the dissolved solid residues come from fumes comprising predominantly sulfur oxides, the purification temperature is between 20 and 50 ° C. In the case where these residues come from fumes mainly comprising HX type compounds, then this temperature is close to ambient temperature.

The contacting takes place preferably with stirring. Once the first, second and third agents, used separately or in the form of a composite product, brought into contact with the solution to be treated, the solid resulting from the supernatant solution is separated by any known means, such as decantation. or filtration, or both. The solid is then dehydrated and then landfilled or optionally recycled in any type of appropriate process, for example in the preparation of glasses, subject, if necessary, to an adjustment of the composition of said solid.

The resulting solution which mainly contains sodium sulphate or sodium chloride, depending on the fumes originally treated, is very pure and can be revalued, advantageously, as it is, without further purification being necessary.

Thus, if the solution essentially comprises sodium sulphate, one can envisage crystallizing said sulphate.

This operation can be carried out by any means known to those skilled in the art.

According to a first variant, the filtrate is cooled until a temperature difference of 15 to 20 ° C. is obtained between the dissolution and crystallization temperatures.

In this case, the crystals obtained are in decahydrate form.

According to a second variant, the sodium sulfate is crystallized by evaporation of at least part of the water from the solution.

The resulting crystals are in anhydrous form.

The purity of these crystals is such that they can be recovered directly or after drying or granulation, in particular be revalued in glass furnaces for example. The aqueous solutions of sodium chloride can be used in various industrial processes, such as for example the synthesis of sodium carbonate or bicarbonate (Solvay process), or in the preparation of sodium hydroxide solutions by electrolysis.

Concrete but nonlimiting examples of the invention will now be presented.

EXAMPLE 1

Household waste incinerator fumes are treated with sodium bicarbonate previously ground to a particle size of less than 10 μm.

The ratio of the mass of bicarbonate used on the mass to stoichiometry is 1, 1. The removal efficiency of acid gases exceeds 90%. The solids from the neutralization of the fumes are recovered on a bag filter.

These solids are dissolved so as to have a 90 g / l solution of sodium chloride. The solid / liquid weight ratio is 3.

The dissolution is carried out at a pH of 9.5 and at room temperature.

The characteristics of the solution are collated in the first column of Table 1 below.

This solution is treated by adding a sodium silicate solution having a concentration of 0.2 mol / l with a ratio to the stoichiometry of the metals to be treated of 1.5, and a solution of trisodium phosphate under stoichiometric conditions relative to the metals to be treated.

3 ml of TMT 15® are then added per liter of solution to be treated and a sodium phosphate solution.

The resulting solution has the characteristics collected in the second column of the following table:

It is noted that the sodium chloride solution obtained is very pure and can be used in an electrolysis process to give soda or even be used in the synthesis of sodium carbonate. EXAMPLE 2

Fumes from a heavy fuel oil melting installation are treated with sodium bicarbonate with a stoichiometry of 0.84, which makes it possible to neutralize up to 70% of the sulfur oxides present. The solids are collected in a bag filter.

Due to the raw materials used, the resulting solids contain different metals including 0.01% by weight of chromium, 0.18% of vanadium, 0.1% of lead, 0.2% of iron.

These solids of which dissolved in a solution of sodium sulphate coming from a subsequent stage of the dissolution process, carried out at 35 ° C., so as to be very close to the saturation in sodium sulphate. The pH is fixed at 9 and the potential is maintained so that the chromium is at the oxidation state III. This solution is then separated from the solids by filtration. The solids are intended for landfill and meet the landfill leaching tests without inerting.

3 ml of TMT 15® are then added per liter of solution to be treated.

The liquid is cooled to 15 ° C. and crystals of sodium sulphate decahydrate are recovered which are then dried. The sodium sulfate obtained has a purity corresponding to the sum of the metals of less than 3 ppm.

Claims

1 - Process for eliminating heavy metals present in solid residues resulting from the treatment by dry, semi-dry or semi-wet route, of gas mainly comprising compounds of the HX type, with X representing a halogen, and / or mainly SOx type compounds, with at least sodium carbonate and / or bicarbonate, characterized in that the following steps are carried out:

-sulfates and / or sodium chlorides preferably present in the aforementioned solid residues are preferably dissolved selectively, -the solids remaining in suspension in the solution are separated,

- the resulting solution is treated with at least one carbonate or hydroxycarbonate, at least one silicate or aluminosilicate, and at least one sulfur compound and / or at least one compound chosen from phosphates, as purifying agents, - the solids formed are separated.

2- A method according to the preceding claim, characterized in that the sulfates and / or chlorides are selectively dissolved by controlling the pH of the medium so that it is in a minimum solubility range of the metals present.

3- Method according to the preceding claim, characterized in that the sulfates and / or chlorides are selectively dissolved at a pH between 7 and 10, if the aluminum is not present in high quantity.

4- A method according to claim 2, in that the sulfates and / or chlorides are selectively dissolved at a pH between 3 and 7, if the aluminum is present in high quantity.

5- Method according to any one of the preceding claim, characterized in that the sulfates and / or the chlorides are selectively dissolved in two successive stages with an intermediate separation stage, one carried out at a pH between 7 and 10, the second at a pH between 3 and 7, the order depending on the pH value of the initial solution.

6- Method according to any one of the preceding claims, characterized in that the dissolution step is carried out while maintaining a solid / liquid weight ratio of between 0.5 and 10, in the case where the residues originate from the treatment. smoke mainly comprising HX type compounds. 7- Method according to any one of claims 1 to 5, characterized in that one carries out the step of dissolution at a temperature between 15 and 80 ° C, in the case where the solid residues come from the treatment of fumes mainly comprising SOx.

8- Method according to any one of the preceding claims, characterized in that the solution is treated with at least one carbonate or a hydroxycarbonate chosen from alkali metal carbonates, hydrotalcite, or dawsonite.

9- Process according to any one of the preceding claims, characterized in that the solution is treated with at least one silicate or at least one aluminosilicate, chosen from silicates and aluminosilicates.

10- A method according to any one of the preceding claims, characterized in that the solution is treated with at least one sulfur compound, mineral or preferably organic, at least one phosphate, these agents being used alone or as a mixture.

11- A method according to claim 10, characterized in that the mineral sulfur compound is chosen from hydrogen sulfides, barium, strontium, iron, or mineral (poly) thiocarbonates such as (poly) alkali metal thiocarbonates or any salt of thiocarbonic acid, such as potassium thiocarbonate.

12- Process according to claim 10, characterized in that the organic sulfur compound is chosen from organothiophosphates or organodithiophosphates, such as the dialkyl- or diaryl-dithiophosphates of alkali metal corresponding in particular to the following formula:

^RO v ^

P RO ^ SX

in which X is an alkali metal, for example sodium, R is an aryl or, preferably, alkyl radical, for example, methyl, ethyl, n-propyl, isopropyl, 1-methyl propyl, isobutyl. 13- A method according to claim 10, characterized in that the organic sulfur compound is a compound (poly) mercapto, in particular a mercapto, dimercapto or trimercapto compound, such as for example triazines substituted by 1, 2 or 3 monovalent radicals -SH.

14- Method according to claim 13, characterized in that the sulfur compound is trimercapto triazine-S, which corresponds to the following formula:

15- Process according to claim 10, characterized in that the phosphates are chosen from phosphoric acid, phosphates, hydrogenophosphates, pyrophosphates or also polyphosphates of alkali or alkaline earth metals.

16- Process according to any one of the preceding claims, characterized in that the solution is treated by successively introducing the abovementioned purification agents, preferably in the following order: (hydrogen) carbonates, (alumino) silicates and the sulfur compounds and / or phosphates.

17- Method according to any one of the preceding claims, characterized in that the aforementioned agents are introduced into the solution to be treated at least in the form of a solid comprising all or part of said agents, the solid being optionally shaped into presence of a support.

18- Method according to the preceding claim, characterized in that the solid consists of several successive layers, the agent to be used last being located closest to the support or the core of said solid.