FR2516908A1 - PROCESS FOR MANUFACTURING VERY FINALLY CRYSTALLIZED SYNTHETIC ANHYDRITE - Google Patents

Abstract

Process for the preparation of synthetic anhydrite in very finely crystalline form from a very finely crystalline mixture of an intermediate containing calcium sulphite hemihydrate and calcium sulphate dihydrate from flue gas desulphurisation, in which the portion of the intermediate containing calcium sulphite hemihydrate is first dehydrated in the presence of oxygen by a heat treatment in the temperature range from 200 to 900 DEG C to give calcium sulphite, which is then oxidised to calcium sulphate, and the portion of the intermediate containing calcium sulphate dihydrate is dehydrated.

Description

 The present invention relates to a process for manufacturing synthetic anhydrite in a very finely crystallized form from a very finely crystallized mixture of an intermediate product of a flue gas desulfurization containing calcium sulphite hemihydrate and calcium sulfate dihydrate .

The manufacture of anhydrite can be carried out in different ways. Depending on the manufacturing process, a difference is made between natural anhydrite and synthetic anhydrite. Natural anhydrite can be made by: a) slaughtering and preparing natural anhydrite
existing or b) by thermal dehydration of calcium sulphate
natural dihydrate (gypsum).

 The anhydrites obtained by these processes are not usable, because of their morphology, in the building materials industry due to their reaction inertia but, for an application in the building materials industry, they must be further conditioned, in which case it is necessary to use, in addition to a very fine grinding, other additives, for example in the form of reaction accelerators.

 Synthetic anhydrite has been obtained so far, in a known manner, from the manufacture of hydrofluoric acid. Its use in the building materials industry is also possible only when it has been finely ground and when it has been provided with additives to improve reactivity. Synthetic anhydrite can also be made by a dry-burning process of chemical gypsum. Synthetic anhydrite can also be obtained from a flue gas desulphurization by initially performing, by wet oxidation and in a known manner, a transformation of the calcium sulphite containing fractions and residues of the agent. absorption of calcium sulfate dihydrate and then dehydration of the latter to form anhydrite.

 The possibilities of application of natural anhydrite and synthetic anhydrite in the building materials industry mainly concern the use in industry of cement in the form of anhydrite-based binders in accordance with the Norm German DIN 4208, or in the form of binder components for building materials, which are currently used in different applications (for example to support waxing, as gallery supports).

It has been found that in various processes, for example flue gas desulfurization, intermediate products containing calcium sulphite hemihydrate and calcium sulphate dihydrate are formed which may be another raw material for the manufacture of anhydrite. synthetic
It is therefore an object of the present invention to provide a process by which, using fumed desulfurization products containing calcium sulphite hemihydrate and calcium sulfate dihydrate, and by conditioning treatment, it is possible to obtain synthetic anhydrite in a physical form that allows its application in the building materials industry virtually without the intervention of reaction activation additives.

 To solve this problem, it is proposed according to the invention a method which is characterized in that, in the presence of oxygen and by a treatment in a temperature range between 200 and 9000C, the fraction of the intermediate product containing sulphite of Calcium hemihydrate is initially dehydrated to calcium sulphite and is then oxidized to calcium sulphate and the calcium sulphate intermediate product fraction dihydrate is dehydrated.

 Provided that the intermediate product is obtained from a wet desulfurization of fumes, it is proposed, in accordance with an advantageous embodiment of the invention, that the mixture is at least partially mechanically and / or thermally dehydrated before the heat treatment.

 The advantages obtained with the process according to the invention consist in that the product formed by flue gas desulfurization already has a very finely crystallized structure, which is established during the heat treatment process the chemical reactions thus triggered.

 The very finely crystalline anhydrite thus obtained can therefore be used immediately afterwards and only requires additives for specific applications.

 In addition, it can be thinned, like any known anhydrite so far, using filler, for example sand. It is also possible to obtain mixtures with calcined gypsum hemihydrates in a known manner for the manufacture of polyphase gypsums.

 Another major area of application is its potential for use as a setting regulator in the manufacture of cement. For this purpose, the anhydrite obtained synthetically can serve alone to ensure the setting regulation, but it is also possible to grind with dihydrate type, in which case the gypsum dihydrate can again be natural or synthetic.

 Advantageously, it is also possible to form a mixture of synthetic anhydrite and dried gypsum obtained from fumes so as to be able to involve in the cement plant a "finishing mix".

 Compared to the final product currently used and corresponding to "gypsum from fumes", a higher quality building material is produced which, thanks to its greater market value, improves the overall profitability of the desulfurization process. fumes.

 In the manufacture of anhydrite from a flue gas desulphurization product, calcium sulphite hemihydrate is conventionally oxidized to an aqueous suspension at a low pH value by adding air to the sulphate. calcium dihydrate. Since in most cases the desulphurization product still contains unreacted absorption agent, it is necessary to add, in order to reach the low pH value necessary for the oxidation, significant amounts of sulfuric acid. As a result of the oxidation, mechanical and thermal dewatering occurs.

 Compared to this method of manufacturing anhydrite from flue gas desulphurization products, the proposed process has the advantage of not requiring any addition of sulfuric acid and, in addition, the heat of oxidation released during heating. oxidation of calcium sulphite to calcium sulphate can be exploited for thermal dehydration.

 With the aid of the exemplary embodiments given below, the technical field of application of the synthetic anhydrite obtained by the process according to the invention will be described.

Example 1
In FIG. 1, there is shown by way of example how one could envisage the realization of an apparatus for the implementation of the method according to the invention.

 The fumes of a boiler are introduced, after being removed from the boiler, being, if necessary, necessarily dusted, in a washing installation for the purpose of separating the sulfur oxides. For this purpose, the fumes are brought into washing devices in contact with a suspension containing lime.

This washing suspension is circulated.

In a partial stream, the reaction products are extracted and dehydrated mechanically. In this example, hydrocyclones and drum filters are used for this purpose. In the tank of the washing device, air can be blown in order to produce in this zone a significant degree of oxidation of calcium sulphite hemihydrate to calcium sulphite dihydrate.

This oxidation carried out in the tank of the washing device is advantageous for the mechanical dehydration taking place subsequently. On the other hand, there is thus released in the heat treatment less oxidation heat, so that the economic optimum should not correspond to a high degree of oxidation in the scrubber.

 The product which has been mechanically pre-dehydrated is subjected to a heat treatment where the additional dehydration of the product and the oxidation of the still existing calcium sulphite are carried out. In this respect, the heat requirements are completely or partially covered by the heat of oxidation. The additional energy requirements may be covered completely or partially by introduction of hot fumes, or by combustion air from the thermal power plant and / or by combustion of combustible materials.

 In the example shown, the suspension is circulated in the washing device with a solids content of about 7 to 10% by weight. Mechanical dehydration is carried out to a residual moisture of 10 to 15% by weight. In the washing device, more than 95% of the calcium sulphite hemihydrate is oxidized by air blowing into calcium sulphate dihydrate. In a two-stage heat treatment involving a maximum temperature of 515 ° C., synthetic anhydrite is finally obtained.

At this stage, the presence of calcium sulphite can no longer be demonstrated.

In the following, some properties of the anhydrite obtained in this way will be given. Anhydrite obtained by the process according to the invention
without additives.

The test carried out in accordance with DIN 4208 has given the following results:

<tb><SEP> Controlled <SEP> Parameter <SEP> RESULTS
<tb><SEP> value of <SEP> pH <SEP> 7.0
<tb> Rigidity <SEP> normalized <SEP> / <SEP>% <SEP> H2O <SEP> 37
<tb> Start <SEP> of <SEP> solidification <SEP> / <SEP> min <SEP> 110
<tb> End <SEP> of <SEP> solidification <SEP> / <SEP> min <SEP> 330
<tb> Constancy <SEP> of <SEP> satisfied <SEP> volume
<Tb>
Proposed applications: DIN 4208 anhydrite binder
Cement additive 1.2 Anhydrite obtained by the process according to the invention,
for fast solidification with additive.

The control in accordance with DIN 4208 gave the
following results:

<tb><SEP> Controlled <SEP> Parameter <SEP> RESULTS
<tb><SEP> value of <SEP> pH <SEP> 10
<tb> Rigidity <SEP> normalized <SEP> + <SEP>% <SEP> HzO <SEP> 44
<tb> Start <SEP> of <SEP> solidification <SEP> / <SEP> min <SEP> 8
<tb> End <SEP> of <SEP> solidification <SEP> / <SEP> min <SEP> 13
<tb> Constant <SEP> of <SEP> satisfied <SEP> volume
<tb> Resistance <SEP> a <SEP><SEP><SEP> Compression <SEP> / <SEP> N <SEP> ~ <SEP> mm2
<tb> at <SEP> last <SEP> of <SEP> 2 <SEP> days <SEP> 17
<tb> at <SEP> last <SEP> of <SEP> 7 <SEP> days <SEP> 25
<tb> at <SEP> last <SEP> of <SEP> 14 <SEP> days <SEP> 27
<tb> - Coating mass for
underground work at
because of a solidifica
fast.

Example 2
In FIG. 2, there is shown another example of application of the present invention. The treatment of fumes ~ is identical to that of Example 1.

 The partial stream from the wash suspension circuit was only partially mechanically pre-dehydrated, which makes it possible, for example, to use hydrocyclones or vacuum filters.

Oxidation in the scrubber is performed only to the extent permitted by the oxygen content of the flue gas. The slurry / sludge which has been mechanically pre-dehydrated is thermally dried in a drying apparatus, in which case it is possible, depending on the possibilities of the waste heat, to use little heat, for example of the heat contained in the fumes before their introduction into the stage. desulfurization. The pre-dried mixture is then subjected to another heat treatment, which makes it possible to extract the residual water and to oxidize calcium sulphite to calcium sulphate.

In comparison with the first embodiment, a.

A significant fraction of the heat required for the second heat treatment is provided by the oxidation heat. As long as it is possible to carry out a drying with lost heat or a low cost, this embodiment is energetically more favorable than the first example.

Example 3
In FIG. 3, there is shown another example.

The fumes come from a boiler, they pass, if necessary, in a pre-dusting stage and they are dusted in order to meet the requirements for the final product.

 Then, the fumes are introduced into a spray absorption stage where the absorption agent is sprayed into the fumes in the form of a suspension.

This suspension is thus dried by the heat contained in the fumes. In parallel, there is a separation of sulfur oxides.

 In a dust collector following the spray absorber, the reaction products are separated at the same time as the residual flying and non-reactive flying dusts. Part of this mixture can be returned to the spray absorber to improve the degree of utilization of the absorption agent, while the rest is subjected to a repetition of the heat treatment according to the invention.

 The intermediate product resulting from the flue gas desulfurization is heated, for thermal treatment, in a rotary tube furnace, for example at 4500C and for a residence time of about 1 hour.

The product comes out after this treatment in a dry and fine grain condition. The compositions before and after the heat treatment are given in the following table:
Table: Intermediate before, and anhydrite after treatment
thermal (no complete analysis).

<Tb>

<SEP> Components of <SEP> Fraction /% <SEP> in <SEP> Weight <SEP> Components <SEP> Fraction /% Weight <SEP>
<tb> product <SEP> inter <SEP> of <SEP> product <SEP> of anhydrite <SEP> I <SEP><SEP> of anhydrite
<tb><SEP> intermediate <SEP> intermediate
<tb> CaS03 <SEP><SEP> ~ <SEP> 1/2 <SEP> H20 <SEP> 25 <SEP> CaS03 <SEP><SEP> 0.35
<tb> CaS04 <SEP><SEP> * <SEP><SEP> 2 <SEP> H20 <SEP> 35 <SEP> CaS04 <SEP><SEP> 62.16
<tb> Ca <SEP> (OH) 2 <SEP><SEP> 0.3 <SEP> CaO <SEP> 0.31
<tb> Ash <SEP> Flying <SEP> 30.0 <SEP> Ashes Fly
<Tb>
This anhydrite sample was subjected to cement manufacturing tests.

 In this regard, 2700 g of cement clinker was milled with 287 g of anhydrite in a laboratory ball mill until a surface area of 4000 cm 2 / g was obtained according to Blaine.

 The quantity of anhydrite added to the cement clinker then corresponds to an addition of 3.5% by weight of SO 3, as specified in the DIN 1164 standard, sheet 1, point 2.2.4.

Portland cement manufactured in this manner has the following properties:
Table: Portland cement manufactured with anhydrite according to
the invention.

<Tb>

<SEP> Parameters <SEP> Controlled <SEP> Results <SEP> Imperative <SEP> DIN <SEP> 1164
<tb><SEP> Addition <SEP> SO3 <SEP> /% <SEP> in <SEP> Weight <SEP>. <SEP> 3.5 <SEP><SEP> 3.5
<tb><SEP> Surface <SEP> specific / cm.g-1 <SEP> 4000 <SEP>
<tb> Rigidity <SEP> normalized <SEP>#<SEP> water
<tb> make-up <SEP> / <SEP>% <SEP> in <SEP> weight <SEP> ... 25.8
<tb> Start <SEP> of <SEP> solidification /
<tb> min <SEP> ...................... <SEP> 120 <SEP>><SEP> 60
<tb> End <SEP> of <SEP> solidification /
<tb> min <SEP> ...................... <SEP> 230 <SEP><720
<tb> Constancy <SEP> of <SEP> volume <SEP> by
<tb> Test <SEP> of <SEP> cooking <SEP> ......... <SEP> confirmed <SEP> confirmed
<Tb>

Claims (3)

 1. A process for producing synthetic anhydrite in a very finely crystallized form from a very finely crystallized mixture of an intermediate product of a flue gas desulfurization containing calcium sulphite hemihydrate and calcium sulfate dihydrate, characterized in that, in the presence of oxygen and by a heat treatment in a temperature range between 200 and 9000C, the fraction of the intermediate product containing calcium sulphite hemihydrate is initially dehydrated to calcium sulphite and is then oxidized to sulphate of calcium and that the fraction of the intermediate product containing calcium sulfate dihydrate is dehydrated.  2. A process according to claim 1, with the existence of a mixture from a wet desulfurization of fumes, characterized in that the mixture is at least partially mechanically and / or thermally dehydrated before the heat treatment.  3. A process according to one of claims 1 or 2, characterized in that the thermal energy necessary for the heat treatment is provided by introducing heat from outside and / or by the exothermic oxidation heat when of the transformation reaction of calcium sulphite into calcium sulphate.