FR2749772A1 - ELIMINATION OF HEAVY METALS PRESENT IN REJECTS OF INCINERATION OF WASTE OR GLAZED FURNACES - Google Patents

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

ELIMINATION OF HEAVY METALS IN DISCHARGES
INCINERATION OF WASTE OR GLASS FURNACES
The subject of the present invention is a process for eliminating heavy and / or toxic metals present in the solids resulting from the treatment, by dry, semi-dry or semi-wet process, of waste incineration gas or of gas from glass furnaces.

 In order to quickly summarize the situation with regard to the gaseous discharges to be treated, two types of discharges are mainly known, the first one being characterized by the fact that, in addition to heavy metals and other organic pollutants, it mainly contains acids of formula HX, with X set 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 comprising mainly HX-type compounds, there may be mentioned, among other things, household waste incinerators, hospital waste incinerators and special industrial waste incinerators.

As for the processes releasing gases mainly comprising sulfur oxides, thermal power plants, glass furnaces, depending on the quality of the fuel and raw materials used, are primarily concerned. it should be noted that
SOx means the species S02, SQ3. These fumes may further include sulfites and / or bisulfites.

 Due to the growing pressure on environmental regulations for facilities emitting such releases, processes for the treatment of acid fumes are booming.

 Conventional lime neutralization treatments, whether dry, semi-dry or semi-wet, generate a large quantity of solid waste, most of which is composed of gypsum polluted by metals considered to be toxic, such as lead, cadmium, aluminum, for example.

 Given the composition of these wastes and the highly water-soluble nature of the polluting metallic elements, the legislation imposes their deposit in controlled landfill, after immobilization.

 This is a very important disadvantage because not only such treatments are expensive but the tonnages of waste products are important in quantity. In addition, there is no way to easily revaluate one or more products resulting from such treatments.

 The present invention therefore aims to provide an effective process for the removal of heavy metals contained in the solid waste resulting from the treatment by dry, semi-dry or semi-wet, gas comprising predominantly HX or SOx type compounds.

 The aim of the method is to propose a method for considerably reducing the ultimate waste that must be disposed of in a landfill.

 Finally, the invention makes it possible to obtain solutions of sulphates or sodium chlorides, depending on the nature of the gases initially treated, which are pure and easily revalorizable.

These objectives and others are achieved by the present invention which therefore relates to a process for the removal of heavy metals present in the solid residues resulting from the dry, semi-dry or semi-wet treatment, of gas comprising mainly compounds of the HX type, with X representing a halogen, or mainly of the SOx type compounds, with at least carbonate and / or sodium bicarbonate, characterized in that the following steps are carried out:
the sulphates and / or sodium chlorides present in the aforementioned solid residues are preferably dissolved,
the remaining solids suspended 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 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 dry, semi-dry or semi-wet gas treatment with at least sodium carbonate or sodium bicarbonate, followed by solid processing steps. resulting as it has just been explained, makes it possible firstly to obtain ultimate waste in a smaller quantity. On the other hand, such a combination makes it possible to reach purity levels of solutions of chlorides and / or sodium sulphates of a very high purity, and this for a wide pH range.

 But other features and advantages of the present invention will appear more clearly on reading the description and examples which follow.

First of all, it is indicated that by specifying that a gas mainly comprises compounds of the HX or SOx type, gases are defined whose concentration in one of the two types of compounds is predominant over the others. constituents of said gas, without however excluding the presence of the other type of compound. Moreover, it is understood that these gases (or fumes) treated further include heavy metals, which may also be called metals considered toxic or polyvalent. This name covers more particularly but without intention 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 pollutants such as dioxins, furans, or inorganic compounds of the NOx type.

 In addition to the pollutants just 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 above, the treatment is carried out in the dry, semi-dry or semi-wet manner.

 Dry, and semi-dry means more particularly a treatment in which the neutralizing agent is introduced in the solid state into the fumes.

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

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

 According to the second variant of the dry method, the fumes are brought into contact with a reagent mentioned above, in the presence of water but with a quantity such that the reagent retains its powder appearance. The reagent used may be in anhydrous form or comprising water of crystallization.

 Water can be provided in the process in a number of ways. This may in particular be present intrinsically in the reagent or be brought extemporaneously, before the treatment of fumes or even during the treatment of these fumes. Of course, the combination of these possibilities is conceivable.

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

 However, according to a preferred embodiment of the invention, water is brought 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.

 Semi-wet means a process in which the reagent is introduced in the presence of water in such a quantity that the products obtained after the treatment are in a 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 those skilled in the art depending on the technological constraints of the process. More particularly, this content depends on the flue gas 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 microns. According to a more particular embodiment, a reagent whose mean particle size is between 5 and 30 clam is used.

 If a reagent of greater particle size or of the order of 50 clam is used, it may be advantageous to carry out before the treatment of the fumes, a grinding step of said reagent to reach a particle size of about 5 at 30 clam.

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 also conceivable to use the process according to the invention with a reagent such as sodium sesquicarbonate, which corresponds to an equimolar mixture of bicarbonate and sodium carbonate comprising two water molecules.

 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% of sodium bicarbonate.

According to a particularly advantageous variant, it is possible to use a mixture of carbonate and sodium bicarbonate, obtained by a thermal treatment of ammoniacal bicarbonate, at a temperature ranging between 20 and 1500C, preferably between 20 and 1 30 C. under a controlled atmosphere optionally comprising carbon dioxide and / or water,
The thermally treated ammoniacal bicarbonate is the intermediate product obtained by carrying out the process for the preparation of sodium carbonate by the technique of ammonia soda, also known under the name of the Solvay® process.

 More specifically, ammonia bicarbonate is used for the purposes of the present invention, the product obtained after dissolution of ammonia gas in an aqueous solution of sodium chloride, so as to produce an ammoniacal brine.

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

 Ammoniacal bicarbonate refers to a mixture comprising mainly sodium bicarbonate, and to a lesser extent ammonium bicarbonate, sodium carbamate, water and sodium chloride.

 As an indication, the composition of this mixture is as follows: 75 to 80% by weight of sodium bicarbonate, 1 to 2% of ammonium bicarbonate, 2 to 3% of sodium carbamate (NaNH 2 CO 2), 15 to 20% less than 0.1% sodium chloride.

 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 achieve the desired total pressure is provided, 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 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 gaseous flow, and with stirring.

 It has been found that the ammonia bicarbonate treatment temperature can be adapted according to the amount 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 perform the latter at a temperature close to 120-130 ° C, although lower temperatures are possible .

 For the above, the values of the temperatures are expressed at atmospheric pressure. Those skilled in the art are able to adapt the values of the temperatures according to the pressure under which the treatment is carried out.

 Usually, the duration of the operation is at most a few hours.

The amount of reagent used in the flue gas treatment method according to the invention depends on the quantity of compounds to be neutralized present. By compounds to be neutralized, reference is made mainly to the main compounds, that is to say to the
SOx and possibly with HX acids.

 According to one particular embodiment of the invention, the flue gas treatment is carried out in such a way that the stoichiometric ratio between the reagent and the compounds of the SOx type, and optionally the acids of formulas 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 and mercury, it may be advantageous to inject into the gas stream an adsorbent, with the reagent and optionally with water. The adsorbent comprising the organic pollutants can then be recovered with the solids from the flue gas treatment. The adsorbent is generally activated carbon and / or lignite coke.

 It should be noted that if the fumes comprise dusts or fly ash, they can be recovered with the solids produced during the neutralization, or prior to the treatment according to the invention, by conventional methods.

 The said fumes are brought into contact with the reagent, optionally the water and the adsorbent, in a known manner.

 Thus, it can take place in co-current or countercurrent.

 The treatment can likewise be carried out in any type of reactor that promotes the dispersion of the solid, the suspension or the solution in fumes, such as the reactors comprising Venturi type systems, or the fluidized bed reactors.

 If the second variant of the dry method is used, the water can be brought before contact with the fumes. In this case, all known methods are suitable. In general, the water in spray form and the reagent are contacted in a humidifying drum.

 Still within the context of this same variant, it is possible to supply the water when the reagent is brought into contact with the fumes. In this case, the water is then injected in the required amounts directly into the gas stream.

 The solids produced by the process are then separated using conventional techniques such as electrofilters, or bag filters.

 According to an advantageous embodiment of the present invention, at least a portion of the solids from the flue gas treatment is recycled to a new flue gas treatment step.

 The flue gas treatment temperature depends on various technological constraints.

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

 As an indication, the flue gas treatment temperature is between 70 and 500 ° C and more particularly between 130 and 3000 ° C.

 In the case where a treatment is carried out in the presence of extemporaneously supplied water, it is possible to recycle said solids in the preparation zone of the reactive mixture / water.

 If the 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 interesting to further increase the efficiency of the process, especially in case of short residence time in the neutralization zone. This mode also makes it possible to lower the smoke purification thresholds.

It will be possible, with regard to the treatment of fumes mainly comprising compounds of the HX or SOx type, to refer to the patent applications
FR 95 12972 and FR 9512973.

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

 The dissolution step is generally carried out in conventional equipment such as for example those allowing a dissolution in one or more successive stages, against the current.

 Conventionally, the dissolution is carried out in a reactor which is preferably agitated, then the resulting sludge is sent to a decanter or other apparatus allowing 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 sodium sulfate in the sludge obtained. In this case, the method consists in juxtaposing several dissolution - separation steps.

 Thus, and if necessary, the solid residues are introduced into a first reactor, preferably stirred, supplied with water. The suspension obtained in this first reactor is then sent to a decanter, the overflow 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, and then introduced into a second decanter.

 According to a particularly advantageous embodiment, the overflow of this second decanter serves to supply water to the first reactor.

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

 The dissolution process 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 rule, the dissolution is carried out in a pH range of between 3 and 10, these two limits being included.

 In a first variant, and if the aluminum is not present in a high amount relative to other metals, a pH range between 7 and 10, the two terminals being included, represents a good compromise. More particularly, the dissolution can be carried out in a pH range of 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 terminals being included.

It is not beyond the scope of the present invention by implementing the two aforementioned variants successively. Thus, the dissolution step may 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 depends on the pH value of the initial solution.

This type of method has in particular been described in the patent application
FR 95 12975, specific for the treatment of fumes mainly comprising SOx.

Of course, this method is also applicable to fumes mainly comprising compounds of HX type.

 In the case where the residues come from the treatment of fumes mainly comprising compounds of the HX type, the dissolution step is preferably carried out while maintaining a liquid solid-liquid 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 have the effect of redissolving all or part of the heavy metals.

 In the case where the residues come from the treatment of fumes mainly comprising compounds of the SOx type, the risk of redissolving the heavy metals does not occur. Therefore, it is possible to treat solutions close to saturation.

 The control of the pH value is carried out 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 resulting from the treatment of fumes mainly comprising SOx, the step of selective dissolution of the sulphates is carried out, according to one particular embodiment of the invention, at a temperature of between 15 and 80.degree. 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 sulphites or bisulphites, these species can be oxidized to sulphates. This operation is generally carried out by adding at least one oxidizing compound to the mixture, optionally in combination with a heating thereof. By way of example of oxidizing compounds, mention may be made of air, hydrogen peroxide, persulfates or the salt of the 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 (s), the recovered sludge is preferably dehydrated, more particularly by means of a filter press and can be discharged. At this stage, the amount of solid residues is considerably reduced relative to the amount of residues that would have to be discarded without such treatment. In addition, the landfill conditions are less expensive because only the insoluble metals are retained in these residues. Thus, the leachable nature of the residues is diminished.

 The solution is then subjected to a purification treatment. By solution is meant the saline solution (s) from the decanter (s) and the filtrate obtained after dehydration of the sludge after the dissolution step.

 The purification step comprises treating said solution with at least one carbonate or a 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 constitutive agent entering the purification treatment is therefore chosen from among carbonates and hydroxycarbonates.

 Alkali metal carbonates, preferably sodium carbonates, are more particularly used. As regards the hydroxycarbonates, compounds such as hydrotalcite or dawsonite are more particularly used. It is recalled that hydrotalcite is a basic carbonate of magnesium and aluminum. Dawsonite is a basic carbonate of aluminum and sodium.

 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 an SiO 2 / Na 2 O molar ratio of between 0.5 and 3.8, for example equal to approximately 2.

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

 The sulfur compounds may be inorganic or, preferably, organic.

 As the inorganic sulfur compound, it is possible to employ a mineral sulphide, in particular hydrogen sulphide, barium sulphide (BaS) or strontium (SrS), or iron (FeS) or, preferably, (poly ) inorganic thiocarbonate, especially an alkali metal (poly) thiocarbonate, for example potassium or sodium. It is thus possible to use any salt of thiocarbonic acid, such as potassium thiocarbonate (K2CS3).

 As organic sulfur compound suitable for carrying out the invention, it is possible to use an organothiophosphate or an organodithiophosphate, in particular an alkali metal dialkyl- or diaryl-dithiophosphate, for example sodium.

dans laquelle X est un métal alcalin, par exemple le sodium, R est un radical aryle ou, de préférence, alkyle, par exemple, méthyle, éthyle, n-propyle, isopropyle, méthyl-1 propyle, isobutyle.The alkali metal dialkyl- or diaryl-dithiophosphates that can be used correspond, in particular, to the following formula:

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

 As an organic sulfur compound, it is preferable to employ a (poly) mercapto compound, especially a mercapto, dimercapto or trimercapto compound.

This organic sulfur compound may then be a triazine (for example a triazine-as or, preferably, an S-triazine), substituted with 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 1 5'b '.

It is also possible to use compounds sold under the names of
Metalsorb L, Metalsorb DTL, Metalsorb ZUT9, Prosedim Met 07e.

 With regard to phosphates, phosphoric acid, phosphates, hydrogenphosphates, pyrophosphates or else polyphosphates of alkaline 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 aforementioned types of agents successively.

 The order of introduction is indifferent, but preferably, and in particular to limit the amount of added agents, one proceeds in the order indicated above, that is to say the (hydrogen) carbonates, (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 depends primarily on the composition of the solution to be treated. The various contents can be determined by those 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 composed of several successive layers; The agent to be used last is closest to the support or the core of said solid, so as to delay its dissolution in the medium.

 It should further be noted that the solid may contain all or a portion of all the aforementioned agents. In this

 The clay employed advantageously has a high weight content of Au 2 O 3; this content is for example between 20 and 40%.

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

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

 The content of support, particularly clay, is usually 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.

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

 The amounts of the various agents used depends again on the solution to be treated. However, it will be possible to refer to the French patent application FR 95 12974 with regard to the contents of the three types of agents.

 With regard to the preparation of the aforementioned solids implemented in the context of this second variant, the French patent application FR 95 12974 gives the details.

 It is not beyond the scope of the present invention to use the three types of purifying agents simultaneously, for example by using them in the form of a liquid mixture.

 The purification process of the solution is carried out at a controlled temperature which depends on that used during the previous dissolution step.

 Thus, in the case where the dissolved solid residues come from fumes mainly comprising sulfur oxides, the purification temperature is between 20 and 50 ° C.

 In the case where these residues come from fumes comprising predominantly type HX compounds, then this temperature is close to room temperature.

 The contacting preferably takes place 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 resulting solid is separated from the supernatant solution, by any known means, such as decantation. or filtration, or both. The solid is then dehydrated and then discharged or optionally recycled in any appropriate type of process, for example in the preparation of the glasses, with, if necessary, an adjustment of the composition of said solid.

 The resulting solution which contains mainly sodium sulphate or sodium chloride, depending on the fumes originally treated, is very pure and can be upgraded, advantageously, as such, without the need for additional purification.

 Thus, if the solution essentially comprises sodium sulphate, it is possible to crystallize said sulphate.

 This operation can be implemented 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 sulphate is crystallized by evaporation of at least part of the water of the solution.

 The resulting crystals are in anhydrous form.

 The purity of these crystals is such that they can be recoverable directly or after drying or granulation, especially be upgraded in glass furnaces for example.

 The aqueous solutions of sodium chloride can be recoverable in various industrial processes, for example the synthesis of carbonate or sodium bicarbonate (Solvay process), or in the preparation of sodium hydroxide solutions by electrolysis.

 Concrete but non-limiting 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 less than 10 .mu.m.

 The ratio of the mass of bicarbonate used on the mass to stoichiometry is 1.1. The acid gas removal efficiency exceeds 90%.

 The solids resulting from the neutralization of the fumes are recovered on a bag filter.

 These solids are dissolved in order to have a solution of 90 g / l 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 solution of sodium silicate 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 with respect to metals to be treated.

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

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

<tb><SEP> Element <SEP> solution <SEP> after <SEP> dissolution <SEP> solution <SEP> after <SEP> purification
<tb><SEP> (ms / l) <SEP> (m <SEP>
<tb> Zn <SEP> 0.15 <SEP><<SEP> 0.02
<tb> Cu <SEP> 0.15 <SEP><<SEP> 0.01
<tb> Ni <SEP><<SEP> 0.09 <SEP><SEP> 0.02
<tb> Cd <SEP><0.09<SEP><<SEP> 0.02
<tb> Pb <SEP> 0.2 <SEP><<SEP> 0.02
<tb> Sn <SEP><<SEP> 0.09 <SEP><0.02<SEP>
<tb> Hg <SEP> 0.44 <SEP><<SEP> 0.01 <SEP>
<tb> Ag <SEP> 4 <SEP><0.1
<tb> Fe <SEP> no <SEP> assayed <SEP><<SEP> 0.1
<tb> Sb <SEP> 0.3 <SEP><<SEP> 0.05
<tb> Al <SEP> 23 <SEP><<SEP> 0.1
<Tb>
It is found that the sodium chloride solution obtained is very pure and can be used in an electrolysis process to give soda or be used in the synthesis of sodium carbonate.

EXAMPLE 2
Fumes from a fusion plant operating with heavy fuel are treated with sodium bicarbonate with a stoichiometry of 0.84, which can neutralize up to 70% of the sulfur oxides present.

 The solids are recovered in a bag filter.

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

 These solids dissolved in a solution of sodium sulfate from a subsequent step of the dissolution process, conducted at 350C, so as to be very close to saturation with sodium sulfate. The pH is set at 9 and the potential is maintained so that chromium is at oxidation state III.

 This solution is then separated from the solids by filtration. The solids are intended for landfilling and meet landfill leaching tests without inerting.

 This solution is treated by adding a solution of sodium silicate 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 with respect to metals to be treated.

 3 ml of TMT 15'8 'per liter of solution to be treated are then added.

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

Claims (18)

A process for the elimination of heavy metals present in the solid residues resulting from the dry, semi-dry or semi-wet treatment of gases comprising predominantly HX-type compounds, with X representing a halogen, or mainly compounds of type SOx, with at least carbonate and / or sodium bicarbonate, characterized in that the following steps are carried out:  the sulphates and / or sodium chlorides present in the aforementioned solid residues are preferably dissolved,  the remaining solids suspended 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 compound and / or at least one compound chosen from phosphates, as purifying agents,  the solids formed are separated. 2. Process according to the preceding claim, characterized in that the sulphates and / or chlorides are selectively dissolved by controlling the pH of the medium so that it is within a minimum solubility range of the metals present. 3. Process according to the preceding claim, characterized in that the sulphates and / or chlorides are selectively dissolved at a pH of between 7 and 10, if the aluminum is not present in a high quantity.  4. Process according to claim 2, in that the sulphates and / or chlorides are selectively dissolved at a pH of between 3 and 7, if the aluminum is present in a high quantity. 5. Process according to any one of the preceding claims, characterized in that the sulphates and / or chlorides are selectively dissolved in two successive stages with an intermediate separation step, one carried out at a pH of 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. Process according to any one of the preceding claims, characterized in that the dissolution step is carried out maintaining a solid / liquid weight ratio of between 0.5 and 10, in the case where the residues come from the treatment. of fumes mainly comprising compounds of the HX type. 7- Process according to any one of claims 1 to 5, characterized in that one carries out the dissolution step at a temperature between 15 and 800C, in the case where the solid residues come from the treatment of fumes comprising predominantly SOx. 8- Process according to any one of the preceding claims, characterized in that the solution is treated with at least one carbonate or hydroxycarbonate selected 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, selected from silicates and aluminosilicates. 10- Process according to any one of the preceding claims, characterized in that the solution is treated with at least one sulfur compound, inorganic or preferably organic, at least one phosphate, these agents are used alone or in mixture. 11- The method of claim 10, characterized in that the inorganic sulfur compound is selected from hydrogen sulfides, barium, strontium, iron, or the (poly) thiocarbonates minerals such as (poly) thiocarbonates of alkali metal or any salt of thiocarbonic acid, such as potassium thiocarbonate. 12- The method of claim 10, characterized in that the organic sulfur compound is selected from organothiophosphates or organodithiophosphates, such as alkali metal dialkyl- or diaryl-dithiophosphates corresponding in particular to the following formula:

 wherein X is an alkali metal, for example sodium, R is an aryl radical or, preferably, alkyl, for example, methyl, ethyl, n-propyl, isopropyl, methyl propyl, isobutyl. 13- The method of claim 10, characterized in that the organic sulfur compound is a compound (poly) mercapto, especially a mercapto compound, dimercapto or trimercapto, such as triazines substituted with 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, hydrogenphosphates, pyrophosphates or else 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 aforementioned 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 is closest to the support or the core of said solid.