WO2000048710A1

WO2000048710A1 - Gas treating agent, method for preparing same, and gas treating method using said agent

Info

Publication number: WO2000048710A1
Application number: PCT/BE1999/000022
Authority: WO
Inventors: Paul Hubert; Georges Poncelet; Stéphane Bartiaux; Michel Bultreys
Original assignee: Carmeuse S.A.
Priority date: 1999-02-17
Filing date: 1999-02-17
Publication date: 2000-08-24

Abstract

The invention concerns a sorbent for treating gas, having an active phase essentially consisting of hydrated calcium silicate, present in the form of a pre-tobermorite phase, a method for preparing said sorbent starting from a cristobalite and/or amorphous silica and a gas treatment method using such a sorbent. The method can be a dry process, whereby the gases are directly contacted with the sorbent. The treated gases can pass through at least one fixed bed containing said sorbent. The invention also concerns a device for treating gases comprising one or several fixed bed(s) containing said sorbent.

Description

GAS TREATMENT AGENT, PROCESS FOR PREPARING SAME, AND GAS TREATMENT METHOD THEREOF

AGENT.

The present invention relates to a solid capture agent which can be used in the treatment of gases, in particular flue gas, as well as a process for the preparation of such an agent. The present invention also relates to a method of treating gas by means of said capture agent as well as a device for treating gas.

"Gas treatment" is understood to mean a process for removing acid compounds present in the gas. The acid compounds which it is sought to eliminate from the gases are more particularly: S0 ₂ , S0 ₃ , HC1, HBr and HF.

Many industrial processes emit as unwanted side products gases containing one or more of the above-mentioned acidic compounds. It is desirable to avoid as much as possible these acidic compounds being released into the atmosphere, these compounds being generally perceived as being harmful, and at least some of these compounds participating in the formation of acid rain.

Considerable efforts have already been made to develop and improve gas treatment processes.

Among the known treatment methods, several use a solid agent, called capture agent. So that it captures the acid compounds present in the gases, this agent is brought into contact with the gases to be purified, either in the form of powder or of particles or in liquid suspension.

In a first type of treatment process, called the wet process, the gases are washed in an absorber at using an aqueous suspension of a capture agent. The captured acid compounds are recovered in the suspension at the outlet of the absorber in the form of reaction products, combined with the capture agent. For example, the SO ₂ and SO 3 captured are recovered in this suspension in the form of sulphites and / or sulphates.

According to a second type of treatment process, known as a semi-wet process, the aqueous suspension of a capture agent is injected into the absorber in the form of droplets. The flow rate of the suspension, the concentration of capture agent in said suspension and the temperature of the gases to be treated are such that the water present in the suspension is evaporated and entrained by the gases. The captured acid compounds are recovered in the form of reaction products in solid residues.

In a third type of treatment process, known as the dry process, the gases are brought into direct contact with a solid capture agent, either by dry injection of said agent into the absorber or in a entrained bed, or by holding the agent in a fluidized bed. It is also possible to pass the gases through a fixed bed of a capture agent. The captured compounds are present in the form of reaction products in the solid residue.

Traditionally, compounds containing calcium in a form capable of reacting with acidic compounds have been used as solid capture agents.

Among the aforementioned acidic compounds, SO ₂ is generally the most difficult to capture by chemical reaction because of its lower acidity. Thus, a basic capture agent which effectively captures S0 ₂ a fortiori captures more acidic compounds such as HC1, HBr, HF and S0 ₃ .

Therefore, capture agents can be assessed by their ability to capture S0 ₂ being understood that they also capture the other acidic compounds mentioned above. This approach is also adopted in the present description.

A first example of a known solid capture agent is Ca (OH) ₂ or calcium hydroxide.

The reaction between Ca (OH) ₂ and SO ₂ present in the gases requires high humidity, such as that encountered, for example, in wet processes or in semi-wet processes. In order to arrive at an acceptable capture of S0 ₂ during the implementation of a so-called dry process, it is generally accepted that the injection of water either in the gases or with Ca (OH) ₂ improves the performances of the process.

A significant disadvantage of Ca (OH) ₂ is its pasty and / or sticky consistency at high relative humidity, in particular when the gases to be treated contain HC1, which is frequently the case. This results in the formation of solid deposits in the installations and the risk of clogging thereof or the obligation to treat the gases at low relative humidity and therefore under non-optimal gas treatment conditions. In the case of a fixed Ca (OH) ₂ bed, this pasty consistency causes the bed to gradually lose its porosity and / or its integrity, which are essential to the efficiency of the treatment process.

Another disadvantage of Ca (OH) ₂ used in the dry process is its lack of selectivity (significant capture of CO ₂ ) and its limited reactivity with respect to SO ₂ . It has in fact been found that during gas treatment, the reactivity of a Ca (OH) _2- based agent present in the form of granules drops to a very low level and therefore uninteresting although said agent always contains a significant amount of Ca (OH) ₂ which has not reacted with the gas compounds. It has thus been found experimentally that under advantageous treatment conditions using a Ca (OH) ₂ with a large specific surface (56 m ² / g measured by the BET method) and with a large pore volume (0.27 cm ³ / g measured by the BJH method in the hypothesis of cylindrical pores, a pore volume of 0.135 cm ³ / g corresponding to pores having a diameter between 100 and 400 Å) with a gas treated at 65 ° C and 66% relative humidity , the saturated residue of Ca (OH) ₂ contained less than 10% by mass of sulfur. Less than one in three calcium atoms had reacted with SO ₂ , an almost equivalent quantity having reacted with CO ₂ present in the gas.

In practice, it is therefore found that the Ca (OH) ₂ must be used in a significant excess for the treatment of gas, which also results in a high quantity of waste to be landfilled.

Other known solid capture agents are hydrated calcium silicates of formula (CaO) _x (Si0 ₂ ) _y (H ₂ 0) _z containing a variable amount of free water. In DE-0S-36 11 769, it is proposed to use as capture agent a granulate of hydrated calcium silicate rich in lime, as resulting from the concrete manufacturing process, this agent preferably having a high porosity. In the semi-wet process described in US

4,804,521, a hydrated calcium silicate or a hydrated calcium aluminate is used as capture agent, prepared by reaction of an aqueous suspension containing an alkaline calcium compound (CaO or Ca (OH) ₂ ) with a silica or an alumina.

According to the dry process described in US 5,100,643, a semi-dry fluid powder containing such a calcium silicate is injected into the gas.

A process for the preparation of such a powder semi-dry is described in US-5, 401, 81.

With the known capture agents based on hydrated calcium silicates, it is observed that the residues of these agents after reaction may contain a significant fraction of calcium which has not reacted during the treatment of the gases, so that usually an excess of capture agent, which again leads to an excess of solid waste.

In order to remedy this problem, it is proposed in US 4,804,521, US 5,100,643 and US 5,401,481 to recycle, at least partially, the solid residues from the treatment process, residues which may also include fly ash containing silica. Thus these solid residues are added to the aqueous suspension in which the hydrated calcium silicate is prepared.

An object of the present invention is to remedy the disadvantages of known capture agents containing calcium.

Another object of the invention is to provide an effective treatment method using a solid capture agent, which produces a minimum of solid waste.

The subject of the present invention is a capture agent having an active phase containing hydrated calcium silicate. This active phase more particularly consists essentially of a pretobermoritic phase.

A large number of hydrated calcium silicates of different compositions and crystal structures are known. A detailed study of different hydrated calcium silicates, their structures and their preparation processes can be found in chapter 5 "The Calcium Silicate Hydrates" of the book "The CHEMISTRY of CEMENTS" edited by HFW Taylor and published by Académie Press in 1964.

Among the hydrated calcium silicates, there are crystalline compounds such as in particular tobermorite, xonotlite, foshagite, afwillite, hillebrandite, and poorly or poorly crystallized compounds, such as in particular CSH (I) and CSH (II).

In the context of the present invention, the term “pretobermoritic phase” means a phase consisting of

"tobermorite gel", the pretobermoritic phase optionally also containing an amount of Ca (OH) ₂ present as such.

Such pretobermoritic phases are described in chapter 7. II of "The Chemistry of Cements" as intermediates in the hydration of tricalcium silicate or dicalcium β-silicate at low temperature. The nature of the "tobermorite gel" is described in more detail in chapter 5.IV.D of the said work (pages 199 and 200).

The representation of a typical X-ray diffraction spectrum of "tobermorite gel" as defined above is given in FIG. 1. This spectrum corresponds to that given by Taylor, in the work mentioned above, on page

183 figure 9.

FIG. 2 represents an X-ray diffraction spectrum typical of a pretobermoritic phase containing Ca (OH) ₂ as such.

In FIGS. 1 and 2, the peaks which correspond to the "tobermorite gel" are indicated by T, those which correspond to Ca (OH) ₂ are indicated by C.

The pretobermoritic phase of the capture agent according to the invention may have a Si / Ca molar ratio of between 0.2 and 1, or between 0.2 and 0.65 or even between 0.4 and 0.65.

It should be noted that in this molar ratio, the amount of calcium in the pretobermoritic phase consists on the one hand of calcium present in the "tobermorite gel", and on the other hand of calcium present in the form of

Ca (OH) ₂ as such (see Chapter 7. II.A of "The

Chemistry of Cements "). To avoid any confusion, the reader's attention is drawn to the fact that, in general, the CaO / Si0 ₂ molar ratios cited in chapter 7. II of

"The Chemistry of Cernent" correspond to the molar ratios in the "tobermorite gel" only. In the pretobermoritic phase of the capture agent according to the invention, the H ₂ O / Ca molar ratio may be between 0.1 and 2, between 0.1 and 1 or even between 0.25 and 1, H ₂ 0 being the water remaining in a sample of the pretobermoritic phase after drying in an oven at 105 ° C, until reaching a constant weight.

In practice, the pretobermoritic phase also contains water absorbed in different forms.

The amount of water present in the capture agent can also vary during the treatment of the gases. You can optionally inject water on the capture agent before or during its use.

For the implementation of certain treatment methods, for example treatment in a fixed bed, the capture agent is advantageously present in the form of granules.

According to a preferred embodiment, said granules have in substance a diameter which is between 0.5 and 30 mm, preferably between 1 and 16 mm or between 1 and 8 mm. It is conceivable to use as a capture agent a support covered with the active phase or impregnated with it. The support is preferably porous. It can be inert but can also have activity in the treatment of gases. According to a preferred embodiment of the invention, the capture agent however consists essentially of the active phase, that is to say the pretobermoritic phase as defined above. In this case, the agent can consist of a set of granules, or agglomerates.

The invention also relates to a process for the preparation of a capture agent as described above. The process comprises the following stages: 1 ° an aqueous suspension of silica and lime is prepared, preferably in an Si / Ca molar ratio of between 0.2 and 1, the suspension preferably comprising from 5 to 60% of dry matter total, and more preferably between 20 and 35% of dry matter. 2. the suspension is heated with stirring to a temperature preferably between 70 and 100 ° C. for approximately 1 to 20 hours,

3 ° after cooling, the suspension can be dried to obtain a product whose residual humidity (as measured after drying in an oven at 105 ° C to constant weight) is preferably less than 10% and moreover preferred less than 5%.

This third step can advantageously be carried out in the case where the capture agent is intended to be used in a dry gas treatment process. In the case of wet or semi-wet treatment processes, this third drying step is not necessary.

It is known that amorphous materials are more reactive than crystalline materials. Among the silicas, one can distinguish amorphous silicas, crystalline silicas, such as quartz and cristobalite. It has now surprisingly been discovered that an aqueous suspension of silica can be prepared from cristobalite for the preparation of a effective capture agent according to the invention.

According to a first method of preparation, the starting silica can be amorphous silica.

According to another particularly preferred mode of preparation, the starting silica is cristobalite or a mixture consisting mainly of cristobalite and amorphous silica.

Advantageously, the cristobalite is ground to obtain particles with a diameter d ₅ o less than 30 µm and more preferably less than

10 μm. By d ₅₀ is meant the diameter for which 50% by volume of the particles have a smaller diameter.

The subject of the invention is also a method of treating gas using the agent according to the invention, or an agent prepared according to the invention.

The gas treatment process according to the invention can be a wet process or a semi-wet process, in which the capture agent is injected into the gases to be treated in the form of an aqueous suspension. The gas treatment process can also be a dry process, in which the gases to be treated are brought into direct contact with the capture agent of the invention. In particular, the gas treatment process according to the invention can be a dry process in which the gases are brought into contact with the capture agent in a fluidized bed or in a entrained bed of the latter.

According to a specific embodiment of the invention, the gas treatment process is a dry process in which the gases to be treated are passed through at least one fixed bed containing the capture agent according to the invention. In this case, this capture agent is preferably present in the form of granules or agglomerates, such as for example described above.

In all forms of execution of the gas treatment, it is particularly advantageous to use the capture agent according to the invention which consists, in substance, of a pretobermoritic phase.

In order to promote the reaction between the capture agent and the acid compounds present in the gases, the temperature of the gases to be treated is between 40 ° C and 450 ° C.

Preferably, the temperature of the gases entering the bed is between 60 and 300 ° C, more preferably between 60 and 150 ° C or between 65 and 85 ° C. Also preferably, the relative humidity of the gases at the inlet of the bed is between 3 and 100%, or more preferably between 30 and 100%.

The efficiency of the fixed bed can be monitored by detecting the presence of acidic compounds in the gases leaving this fixed bed. It is advisable to measure the concentration of at least one acidic compound called "indicator compound", in the gases leaving the fixed bed.

When the concentration of indicator compound in the gases leaving the fixed bed exceeds a fixed limit value, it is advisable to replace the used capture agent with a new batch of capture agent. Since S0 ₂ is the most difficult acid compound to capture, it is advisable to test the efficiency of the bed by measuring the concentration of S0 ₂ in the gases leaving the bed. There are a large number of systems for detecting SO ₂ in gases which can be used in the present process.

The limit value for the concentration of S0 ₂ in the gases leaving the fixed bed may vary, in particular according to the legal restrictions for gaseous effluents.

To ensure the continuous operation of the gas treatment process, provision may be made, among other applications, for example two fixed beds in parallel. We passes the gases to be treated through one of the two fixed beds until the concentration of indicator compound reaches the limit value. Then, the gases to be treated are directed to the second fixed bed and the spent capture agent of the first bed is replaced. When the concentration of indicator compound at the outlet of the second bed reaches the limit value, the gases to be treated are again directed to the first fixed bed and the spent capture agent of the second fixed bed is replaced, and so on. One can also consider a treatment process with an installation comprising two fixed beds in series. When the content of the indicator compound for the effluent from the downstream bed is too high, the agent for capturing the fixed bed upstream is discharged. The capture agent from the downstream fixed bed is moved to the upstream fixed bed and the downstream fixed bed is filled with a batch of fresh capture agent.

According to a preferred embodiment, the treatment installation is a series-parallel system in which each parallel system comprises two fixed beds in series as described above. In a first stage, the gases to be treated are directed to the two beds in series of the first system in parallel.

When the concentration of indicator compound in the gases leaving the fixed downstream bed exceeds the limit value, the gases to be treated are directed to the two fixed beds in series of the second system in parallel. Thus, the gas treatment continues while the fixed beds of the first system in parallel are replaced, for example as described above.

The invention also relates to a device for the treatment of gas by means of the capture agent according to the invention or by means of the capture agent prepared according to the invention. Said device comprises one or several fixed beds containing the capture agent. The device is further adapted to be inserted into a gas passage to be treated, so as to force the gases to pass through said fixed bed or through at least one of the fixed beds.

Preferably, the device also includes a detection system capable of detecting the presence of one or more of the acid compounds S0 ₂ , S0 ₃ , HCl, HBr and HF in gases. This detection system is incorporated in the device so that it detects the presence of this or these acid compounds in the gases after their passage through the fixed bed or beds.

Such a device can advantageously be used, for example, for the treatment of gases originating from individual heaters or from assemblies such as hospitals or schools.

By way of example, there is described below a process for preparing a capture agent according to the invention. In a first step, an aqueous stirred suspension of silica having 40% dry matter is prepared at a temperature of 25 ° C.

In a second step, ground aqueous quicklime is added to this aqueous suspension of silica, the particles of which have a substance <100 μm in diameter. The suspension is diluted with water until it contains no more than 30% dry matter. After 15 minutes of homogenization, during which the lime hydrates, the stirred mixture is heated to 90 ° C. and kept at this temperature for 5 hours. Finally, the product is dried at 225 ° C in an oven for 5 to 6 hours. This essentially gives a pretobermoritic phase having a residual humidity of less than 5%. The pretobermoritic structure of the product can be checked by X-ray diffraction (see Figures 1 and 2).

It may be necessary to granulate the dried product. It may also be advisable to compact said product. There are a large number of variants of the preparation process described above. We can thus

- from a more dilute, or more concentrated silica suspension, - from a higher or lower temperature than 25 ° C,

- use coarser lime, consisting, for example, of particles having a diameter less than 5mm,

- use hydrated lime or milk of lime, - synthesize with lower dry matter contents (for example 5%) or higher (for example 50%),

- reach a temperature of 90 ° C before adding lime,

- modify the synthesis temperature by 90 ° C up or down (for example, a temperature of 70 ° C also makes it possible to obtain a good product),

- drying at various temperatures, for example, 100 ° C and even less, or even 400 ° C,

- use any type of dryer, - do not dry the product.

Since the Si / Ca molar proportions in the final product correspond to the molar proportions in the starting mixture, the methods described above easily make it possible to vary the molar proportions of Si / Ca in the capture agent.

The capture agent, its preparation process and the gas treatment processes according to the invention have a large number of advantages.

Unlike solid capture agents known, the capture agent according to the invention has the advantage of maintaining an effective activity until in substance all the calcium present in the active phase has reacted, even with an active phase having for example a thickness of 8 mm.

Thus, thanks to the invention, it has become unnecessary to provide a step of recycling of capture agent as proposed in US 4,804,521, US 5,100,643 and US 5,401,481. Such a step considerably increased the cost of implementing the gas treatment process.

A pretobermoritic phase easily lends itself to a granulation operation.

An advantage of the capture agent according to the invention is that even if it entirely consists of a pretobermoritic phase, the particles or granules do not become pasty and do not stick together even with gases having a relative humidity of 70%, and even after the capture, for example, of SO ₂ and / or HCl until saturation. The particles or granules can thus be easily arranged in a fixed bed, easily removed from the bed after the treatment of the gases and also easily removed from any type of filter, such as for example a bag filter.

In the fixed bed process, the bed retains its integrity and porosity, that is, the bed does not become more compact during the treatment of gases. Even after deactivation of the capture agent, the pressure drop does not increase or in any case does not increase significantly. The advantages of the capture agent according to the invention mentioned above allow its use in a greater variety of applications than the capture agents known from the prior art.

Indeed, wet gas treatments known, if they allow to react almost all of the calcium present in the agent, are in practice only used for installations with large gas flow rates. The treatment of gases by the semi-wet route does not make it possible to completely react the calcium present in the agent with the acid gas or gases to be captured. Consequently, reliable gas treatment can only be ensured by using a known surplus of capture agent.

The treatment of gas by semi-wet route according to the state of the art is also ill-suited for the treatment of gaseous effluents at reduced or variable flow.

Given the activity of the capture agent according to the invention (almost total reaction of calcium) and its usefulness for treatment in a fixed bed, the present invention makes it possible in a simple and reliable manner to treat gases containing acid compounds, even when these gases or these compounds have a reduced and / or variable flow rate.

The invention makes it possible to carry out the gas treatment at optimal capture conditions (high relative humidity) without the risk of obtaining a pasty and / or sticky solid product.

The invention makes it possible to treat gases with a minimum of capture agent. Thanks to the high content of neutralized acid compounds in the exhausted capture agent, the invention makes it possible to treat the gases with a small quantity of solids to be landfilled, particularly when the acid gases to be treated essentially contain S0 ₂ to be captured. .

It is particularly remarkable that with the capture agents according to the invention which consist entirely of the active phase, and in particular when the Si / Ca molar ratio is below 0.65, it is possible to obtain a almost total calcium reaction

(even when the main gas to be captured is S0 ₂ ) with the acid compounds present in the gases, while having a reduced mass of solid residues after reaction.

It is known, inter alia from the work "The CHEMISTRY of CEMENTS", page 172, that hydrated calcium silicates, and particularly in humid conditions, are capable of reacting with C0 ₂ present in gases with formation of CaC0 ₃ and amorphous silica. Such a reaction between calcium and C0 ₂ is disadvantageous in the treatment of gases because it limits, by its consumption of calcium from the active phase, the capacity of the latter to capture acidic compounds, such as S0 ₂ , S0 ₃ , HCl, HBr and HF.

It has now been surprisingly discovered that the capture agent according to the present invention captures SO ₂ and the other acid compounds mentioned above, selectively with respect to CO ₂ .

A decrease in the temperature of the gases to be treated without modifying the absolute humidity leads to an increase in the capture of S0 ₂ and of the other acid compounds without the capture of C0 ₂ increasing.

The capture agents according to the invention which consist entirely of the active phase, and in particular when the Si / Ca molar ratio equals 1, maintain appreciable activity until all the calcium has reacted.

In the case of a capture agent consisting entirely of a pretobermoritic phase, it is thus possible to find almost 15% by weight of sulfur, theoretical maximum, in the final residue, with gases to be desulfurized at 65 ° C. and a relative humidity of 70%. It has been observed that, under the same conditions, by lowering the Si / Ca molar ratio, the capture of SO ₂ remains as good up to Si / Ca molar ratios as low as 0.5 and even 0.2. An important aspect of any process that results in the formation of solid waste is the mass to be landfilled. Thanks to the activity and the high selectivity of the capture agent according to the invention in combination with its limited silicon content, the treatment methods according to the invention entail a minimum of solids to be landfilled, in particular when the capture agent consists of the active phase and that the treatment process is dry. This constitutes a particularly important aspect of the invention, both economic and environmental.

Another advantage of the present invention is to provide an inexpensive process for the preparation of a capture agent. In fact, cristobalite constitutes a source of silica which has proved to be entirely suitable for the preparation of a capture agent according to the invention. This source of silica is inexpensive and can be obtained, for example, by calcining sand. Some advantages of the present invention are illustrated by the following application examples.

A fixed bed of capture agent is prepared by pouring the granular capture agent into a cylindrical tube having a diameter of 24 mm. In the tube, the fixed bed is preceded and followed by an inert layer, which ensures a uniform distribution of gases at the inlet of the fixed bed and avoids the loss of capture agent at the outlet of the bed. The flow rate, temperature and composition of the gases entering the fixed bed (including humidity) are adjusted and the composition of the gases leaving the fixed bed is measured. Three different capture agents are tested.

1) Ca (OH) ₂ having a BET specific surface of 57 m ² / g, and a pore volume of 0.27 cm ³ / g measured by the BJH method in the hypothesis of cylindrical pores, 50% of said pore volume being occupied by pores having a diameter between 100 and 400 Å.

2) Xonotlite (hydrated crystalline calcium silicate) of formula (CaO) _b . (Si0 ₂ ) 6.H ₂ 0 with a BET specific surface of 25 m ² / g and a pore volume of 0.10 cm ³ / g.

3) The capture agent according to the invention, consisting entirely of a pretobermorite phase having a BET specific surface of 120 m ² / g and a pore volume of 1.1 cm ³ / g, the molar ratio Si / Ca in the pretobermoritic phase being 0.52.

In each test, the capture agent is present in the form of granules having a diameter of between 0.5 and 1 mm.

The gas at the inlet of the fixed beds is a mixture of dry air, S0 ₂ (0.15% V), C0 ₂ (8% V) and H ₂ 0. When the temperature of the gas at the inlet bed temperature is 65 ° C, its relative humidity is 66%. When the temperature of the gases entering the bed is 85 ° C, its relative humidity is

29%.

The gases are passed through the respective fixed beds until the bed is inactive, that is to say until almost identical between the composition of the gases at the inlet and at the outlet of the fixed bed.

The fixed bed is then emptied and the residue of the capture agent is analyzed. The results at 65 ° C and 85 ° C are presented in Tables 1 and 2 respectively. Table 1

Table 2

Figures 3 and 4 show the evolution in the time of the relative concentration of S0 ₂ ([S0 ₂ ] _RE ι in the gas at the outlet of a fixed bed of the 3 abovementioned capture agents, the relative concentration being the ratio between the concentration of S0 measured at the outlet of the fixed bed and the S0 ₂ concentration of gases at the inlet of the fixed bed.

FIG. 3 represents the results obtained when the gas temperature at the inlet of the fixed bed is 65 ° C., FIG. 4 those obtained with a gas temperature of 85 ° C.

The materials, experimental conditions and installation were as described above.

The amount of sorbent in the fixed bed of this agent was 10 g each time.

The curves "a" represent the results obtained with a fixed bed of Ca (OH) ₂ . The curves "b" represent the results obtained with a fixed bed of xonotlite.

The curves "c" represent the results obtained with a fixed bed of capture agent according to the invention.

It can be seen (curves a) that the Ca (OH) ₂ allows a total reduction, that is to say a complete elimination of the

S0 ₂ gas for a limited period up to a time a time ta.

Xonotlite (curves b) allows a so-called total gas treatment only for a period up to time tb (<ta).

The pretobermoritic phase according to the invention (curves c) allows a total reduction of the gas for a period extending up to time te (> ta> tb).

It is observed that the capture agent according to the invention at any time captures more S0 ₂ than the two capture agents according to the state of the art

The elimination of S0 ₂ is more important at all times than with the state capture agents. technical.

The aim of gas treatment is generally to limit to a minimum lower than the standards in force, (for example, 25% of the initial concentration of acid compounds) the concentration of acid compounds in the gaseous effluents. It should be noted that the capture agent according to the invention is particularly advantageous in such circumstances.

The present invention can advantageously be used for the treatment of gaseous effluents from incineration plants for household refuse, industrial waste and waste of any other nature, thermal power stations (coal, oil, lignite or operating with any other fuel likely to generate gases containing acid compounds), glass furnaces, any installation in the ceramic industry likely to generate gases containing acid compounds such as the oven of a brickyard and any other installation likely to generate fumes containing acid compounds (HCl, S0 ₂ , S0 ₃ , HF, HBr).

Claims

1.- Capture agent for gas treatment, having an active phase which consists essentially of a hydrated calcium silicate characterized in that the hydrated calcium silicate is present in the form of a pretobermoritic phase.

2.- Agent according to claim 1, characterized in that the Si / Ca molar ratio in the pretobermoritic phase is 0.2 <Si / Ca <1.

3.- Agent according to any one of the preceding claims, characterized in that the H ₂ O / Ca molar ratio in the pretobermorite phase is

_{0 ≤} i____ _{≤ 2 l} Ca

H ₂ 0 being the water remaining in the pretobermoritic phase after drying in an oven at 105 ° C to constant weight.

4.- Agent according to any one of the preceding claims, characterized in that it is present in the form of granules having in substance a diameter between 0.5 and 30 mm.

5.- Agent according to any one of the preceding claims, characterized in that the agent consists of the active phase.

6.- Process for the preparation of a capture agent according to any one of the preceding claims, characterized in that the pretobermorite phase of the hydrated calcium silicate is obtained by:

1 ° preparation of an aqueous suspension of silica and lime, starting from cristobalite or a mixture mainly consisting of cristobalite and amorphous silica,

2 ° heating with stirring of this suspension.

7.- Preparation process according to the preceding claim, characterized in that the cristobalite is ground until particles with a diameter d ₅ o less than 30 microns are obtained.

8.- Preparation process according to any one of claims 6 and 7, characterized in that the starting suspension comprises between 5 and 60% of dry matter.

9. A method of treating gas by means of a capture agent, characterized in that one uses a capture agent according to any one of claims 1 to 5 or a capture agent prepared according to any one of claims 6 to 8.

10.- Treatment process according to claim 9, characterized in that it consists of a dry process, in which the gases are brought into direct contact with the capture agent.

11. A treatment method according to claim 10, characterized in that the gases to be treated are passed through at least one fixed bed containing the capture agent.

12. A treatment method according to claim 11, characterized in that the capture agent is a capture agent according to claim 4

13. A treatment method according to any one of claims 9 to 12, characterized in that the gases have a temperature between 40 and 450 ° C.

14.- Treatment process according to claim 13, characterized in that the relative humidity of the gases is between 3 and 100%.

15.- Treatment method according to one any one of claims 11 to 14, characterized in that the concentration of at least one acidic compound, called "indicator compound", is measured in the gases leaving the fixed bed and that the agent for capturing the fixed bed when the concentration exceeds a fixed limit value.

16.- Treatment method according to claim 15, characterized in that the indicator compound is SO ₂ .

17. A treatment method according to any one of claims 11 to 16, characterized in that two fixed beds are used, containing the capture agent, in parallel, the gases to be treated passing through one of the two fixed beds until the concentration of indicator compound in the gases leaving the fixed bed reaches the limit value, after which the gases to be treated are directed to the second fixed bed and the capture agent of the first fixed bed is replaced by a new charge of capture agent.

18. A treatment method according to any one of claims 11 to 17, characterized in that two fixed beds containing the capture agent are used in series, and that the concentration of indicator compound in the gases to be measured is measured. the output of the downstream bed, that the agent for capturing the bed upstream by the agent for the capture of the downstream bed, and that the agent for the capture of the bed downstream is replaced by a new charge of capture agent when the concentration of indicator compound reaches the limit value.

19.- Device for the treatment of gas, comprising one or more fixed beds containing a capture agent according to any one of claims 1 to 5 or a capture agent prepared according to any one of claims 6 to 8, the device being able to be inserted into a gas passage so as to force the gases to pass through the fixed bed or through at least one of the fixed beds.

20.- Device according to claim 19, characterized in that it comprises a detection system capable of detecting the presence of one or more acid compounds chosen from S0 ₂ , S0 ₃ , HCl, HBr and HF in the gases after passage through the fixed bed (s).