Chemistry: physical processes – Physical processes – Crystallization
Reexamination Certificate
2001-10-25
2004-04-20
Silverman, Stanley S. (Department: 1754)
Chemistry: physical processes
Physical processes
Crystallization
C423S490000, C423S497000
Reexamination Certificate
active
06723139
ABSTRACT:
FIELD OF THE INVENTION
The subject-matter of the present invention is a process for recycling pulverulent calcium fluoride in the form of micrometric particles. It also relates to a process for manufacturing hydrogen fluoride employing the said process.
BACKGROUND OF THE INVENTION
compositions comprising fluoride ions are often obtained as manufacturing waste in various industrial plants, such as plants for the production of hydrofluoric acid (HF) or compounds of the chlorofluorocarbon, hydrochlorofluorocarbon or hydrofluorocarbon type, which can be used in particular as refrigerants.
These compositions are generally neutralized with lime or with calcium carbonate. The precipitate formed is separated by flocculation, then settling of the suspensions which result from the neutralization, and finally filtration by appropriate means, such as drum filters. These compositions exist in practice in the form of sludges or cakes and are distinguished, on the one hand, in that the calcium fluoride is present in the form of extremely fine micrometric particles of the order of 0.5 to 10 &mgr;m and, on the other hand, in that they comprise a very high water content, approximately from 20 to 60%, depending on the filtration method used.
Given the size of the amounts of HF or of compounds usable in particular as refrigerants produced in an industrial plant, the abovementioned compositions are themselves produced at the rate of several thousand tonnes per year and are generally stored in landfill sites.
Now, calcium fluoride, also known as fluorospar (from the name of the ore in which it is present), is precisely one of the starting materials of the industrial process widely used for the manufacture of hydrogen fluoride (HF).
This is because this process employs fluorospar and sulphuric acid to produce HF and calcium sulphate according to the reaction:
CaF
2
+H
2
SO
4
→2HF+CaSO
4
(1).
In this process, the fluorospar is brought into contact, in solid pulverulent form, with sulphuric acid in a rotary furnace at a temperature of approximately 500° C.
A rotary furnace is a horizontal cylinder driven with a rotational movement about its axis. The reactants are introduced at one end of this cylinder, the HF produced in the gaseous form being collected at this same end. The calcium sulphate is recovered at the other end in the form of a solid which is also pulverulent. The reaction must take place in the absence of water in order to avoid corrosion of the furnace.
It would therefore be highly desirable to use compositions based on micrometric CaF
2
particles, such as those mentioned above, as starting material for the production of HF. However, such a use raises difficulties in carrying out the reaction (1) in a rotary furnace which are related to the particle size of the said compositions and to the drying of the said compositions to the level required for the said use.
Patent Application EP 460,524 discloses a process for the manufacture of HF employing the reaction (1) which comprises a stage of purification of the gas produced by the reaction, on conclusion of which an aqueous solution is obtained comprising from 30 to 50% of fluorosilic acid (H
2
SiF
6
) and 5 to 20% of HF. The treatment with lime of such a solution results in a mixture of calcium fluoride (CaF
2
) and of silica (SiO
2
) which is unsuitable for any use and must be stored as waste. This document teaches a treatment of this solution with calcium carbonate and then with an alkaline substance which makes it possible to separate the calcium chloride from the silica. This document consequently recommends the use of the calcium fluoride thus separated as starting material for the manufacture of HF. It also envisages, for this same use and with the aim of reducing the content of silica still present in the form of an impurity, the mixture of the said fluoride with fluorospar. However, it gives no detailed information on the effective use of such a product under the industrial operating conditions of a furnace for the manufacture of HF.
Now, as set out above, the use of solid pulverulent calcium fluoride as starting material for the industrial manufacture of HF requires that it is essential to satisfy the criteria of size of the particles. The use of CaF
2
in the form of fine particles (of the order of a micrometre) is thus ruled out, due to problems of these particles being carried away in the gaseous HF produced in the furnace and a rise in viscosity of the mixture in the reaction region of such a nature as to cause it to adhere to the internal walls of the furnace, these various problems jeopardizing the smooth running of a continuous operation.
These difficulties, related to the use in an HF furnace of finely divided fluorospar, are well recognized, on the other hand, by Japanese Patent JP 52-10893. This document recommends, in order to solve this problem, carrying out a granulation by mixing the said powder in a granulator-mixer with, as additional agent, dilute hydrofluoric acid, dilute sulphuric acid and a concentrated solution of gypsum or a solution in which it is present. A drying stage at a temperature of between 100 and 450° C. is also required. However, this process requires an additional shaping operation and industrial equipment suited to its implementation.
DESCRIPTION OF THE INVENTION
One aim of the present invention is to avoid the discharging, which is always harmful to the environment, of a composition comprising calcium fluoride in the form of micrometric particles.
Another aim of the invention is to provide a process for recycling such a composition which allows it to be recovered in value, without requiring the installation of new industrial equipment.
Another aim of the invention is to provide a process for recycling such a composition which makes possible the industrial manufacture of HF under improved conditions.
It has now been found that these aims can be achieved, in whole or in part, by means of the recycling process which is disclosed below.
The subject-matter of the invention is therefore, firstly, a process for recycling a composition comprising pulverulent calcium fluoride in the form of micrometric particles comprising:
(i) a stage of mixing, in the presence of water, the said composition with pulverulent calcium fluoride (also known as natural spar), the particle size of which is suitable for feeding an HF furnace, in a ratio r of less than 75% by weight, preferably of less than 30%, r being the weight of calcium fluoride of the said composition divided by the total weight of CaF
2
in the mixture, then
(ii) a stage of drying the mixture resulting from (i) until a residual water content is obtained which is also suitable for feeding an HF furnace;
the stage (ii) optionally being carried out simultaneously with the stage (i).
This process makes it possible to re-use, under satisfactory conditions of continuous operation of a rotary furnace for the manufacture of HF according to the reaction (1), a synthetic spar composition (as defined below) which should otherwise be stored as a waste. Furthermore, it has been found that the calcium fluoride as obtained by this process contributes various advantages as relates to the production of HF, as explained later.
The percentages indicated in the present text are, in the absence of a contrary indication, percentages by weight.
The composition to be recycled generally comprises calcium fluoride in the form of particles with a mean nominal diameter of between 0.5 and 10 &mgr;m, preferably between 1 and 5 &mgr;m.
The mean nominal diameter is the median of the distribution of the sizes of the calcium fluoride particles.
The sizes are determined, depending on their order of magnitude, either by a technique involving a measurement of rate of sedimentation or by a laser diffraction technique or by sieving with sieves of increasing mesh size.
In addition to CaF
2
, the composition to be recycled comprises water at a content of between 20 and 60%, preferably between 30 and 55%. Such a composition is also denoted in the present text by the a
Biragnet Jean-Marc
Kouache Henri
Murphy John-David
Atofina
Johnson Edward M.
Silverman Stanley S.
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