RU2339444C2 - Method of sorbent production - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods of obtaining sorbents for catching gaseous fluorides: hydrogen fluoride, molybdenum, tungsten, rhenium, uranium, etc. hexafluorides. Method of sorbet production includes preparation of charge from fluorides of alkaline or alkali-earth metals and porephore, formation of granules, their drying at temperature 60-150°C and sintering at temperature 350-550°C, as porephore ammonium carbonate in amount 15÷50% of fluoride weight is used. Invention allows to reduce corrosive impact of charge components on equipment materials and eliminate of equipment blocking with ammonium fluoride.

EFFECT: obtaining cyclically stable granules which have acceptable specific surface and mechanical strength.

2 cl, 3 ex, 1 tbl

Description

The invention relates to methods for producing sorbents for trapping gaseous fluorides.

To extract volatile fluorides — hydrogen fluoride, molybdenum hexafluoride, tungsten, rhenium, uranium, etc. hexafluorides from gas streams — sorbents based on alkali fluorides (sodium, lithium), alkaline earth (calcium, barium), and other metals are usually used.

Known powder sorbent for the extraction of hydrogen fluoride from gases based on alkali metal fluorides (lithium, sodium) with the addition of barium fluoride powder in an amount of 5.6-35.5% (AS No. 1549581, IPC B01J 20/02, B01D 53 / 02, publ. 15.03.90, BI No. 1).

The disadvantage of powdered sorbents is dusting during loading and unloading from the sorption column, high hydraulic resistance to the passage of the gas stream, the possibility of caking of the powder, etc. Therefore, they strive to obtain a sorbent in the form of granules (tablets), the bulk layer of which easily passes the gas stream.

The granules must have sufficient mechanical strength, satisfactory porosity and the size of the inner surface of the pores, as well as cyclic stability, i.e. maintain physico-chemical properties in cyclic sorption-desorption processes.

A known method for producing a granular sorbent based on sodium fluoride and lithium fluoride used for the selective extraction of hydrogen fluoride from gas mixtures, which uses the pressing of dry powders of acid sodium or lithium bifluorides into tablets, followed by calcination and sintering at a temperature of 300-500 ° C ( Galkin N.P., Zaitsev V.A., Seregin M.B. Capture and processing of fluorine-containing gases (Moscow: Atomizdat, 1975, p. 70, 89-90, 117). The resulting granules have a low specific surface area of internal pores of 1-3 m ² / g and low cyclic stability.

A known method of producing a sorbent of granular sodium fluoride from sodium fluoride moistened with water by sintering tablets (granules) in the temperature range 650-930 ° C (US Patent No. 3372004, publ. 1968). The method includes preparing a charge based on sodium fluoride, forming tablets, drying the tablets and sintering them at a temperature of 650-930 ° C, as a result of which the tablets have very low porosity - about 10% and specific surface area - about 0.1 m ² / g.

A known method of producing a granular sorbent based on lithium fluoride, according to which, when preparing the charge, potassium or magnesium or ammonium fluorides are added to the lithium fluoride, the mixture is moistened to 5-7% water content and molded into granules that are sintered at a temperature of 250-500 ° C and treated with hydrofluoric acid (RF Patent No. 2211726, IPC B01J 20/02, B01D 53/68, publ. 09/10/2003). Sorbent is used to extract hydrogen fluoride from gases.

An addition of magnesium or calcium fluoride is introduced into the mixture to harden the sorbent granules by creating a “skeleton” of the granule structure from the additives inactive during sorption. Ammonium fluoride additive is used to form pores in the structure of the sorbent granules, which are formed when the mixture is heated as a result of evolution of gases (ammonia and hydrogen fluoride) during the decomposition of ammonium fluoride. The method allows to obtain a sorbent containing 75-90% of lithium fluoride and having a specific surface area of 3.8-8.5 m ² / g This method is selected for the prototype.

The method has the following disadvantages. Hydrogen fluoride formed during the decomposition of ammonium fluoride, entering the ventilation ducts, causes intense corrosion of duct materials. At temperatures below 140 ° C, the recombination of gas components (ammonia and hydrogen fluoride) ammonium fluoride, which settles on the surface of the ducts and causes them to clog. Further, being a hygroscopic substance, ammonium fluoride gains moisture and forms corrosive smudges on the surface of air ducts.

The objective of the invention is to develop a method for producing a sorbent from inorganic fluorides, which reduces the corrosive effects of charge components on equipment materials and eliminates clogging of equipment with ammonium fluoride.

The problem is solved by the fact that in the method for producing a sorbent for trapping volatile fluorides, which includes preparing a mixture of alkali and / or alkaline earth metal fluorides and a blowing agent, forming granules, drying and sintering, ammonium carbonate is added to the mixture as a blowing agent.

Ammonium carbonate is added in an amount of 15-50% by weight of fluorides.

The granules are dried at a temperature of 60-150 ° C, and sintering at a temperature of 350-550 ° C.

The sorbent is prepared as follows.

To a metal fluoride powder, or to a mixture of fluorides of alkali and / or alkaline earth metals, which are the basis of the sorbent, add 15-50 wt.% Ammonium carbonate, and the resulting mixture is thoroughly mixed. Water is introduced after thoroughly mixing the dry components. The amount of water is determined experimentally by the quality of pressing, but usually it is about 7%. After molding granules with a diameter of 6 mm, they are dried at a temperature of 60-150 ° C. At a temperature of 60 ° C, decomposition of ammonium carbonate begins, and at 150 ° C, decomposition ends by the reaction:

The decomposition process proceeds irreversibly with the formation of a large number of gaseous products, the release of which during the drying process promotes pore formation in the sorbent granules. The decomposition products are corrosion inactive and do not recombine back to ammonium carbonate. Decomposition products are removed without leaving traces in the flues.

When sintering porous preforms of granules in the temperature range 350-550 ° C, sufficiently strong granules are formed.

The following are examples of the method.

Example 1. In a mixture of barium and lithium fluorides, taken in the ratio BaF ₂ / LiF = 80% / 20%, ammonium carbonate (NH ₄ ) ₂ CO ₃ powder was added in an amount of 30% by weight of fluorides. After thoroughly mixing the dry components and adding water in an amount of 7% of the dry mass, the granules were formed and dried at a temperature of 60-150 ° C for 1 hour, and then the granules were sintered at a temperature of 450 ° C for 2 hours. The porosity of the obtained granules was 50%, the strength was 10 kgf / cm ² .

Example 2. Lithium fluoride powder was mixed with ammonium carbonate powder, while ammonium carbonate was taken in an amount of 40% by weight of lithium fluoride. According to the technology described in example 1, sorbent granules were obtained. The porosity of the obtained granules was 60-80%, strength 10-15 kgf / cm ² .

Example 3. Made three batches of granules with different contents in the mixture of ammonium carbonate. For this, lithium fluoride powder was mixed with ammonium carbonate powder taken from the mass of lithium fluoride in an amount of 15% (first batch), 30% (second batch) and 50% (third batch). After thoroughly mixing the dry components and adding water in an amount of 7% of the dry mass, the granules were formed and dried at a temperature of 60-150 ° C for 1 hour, and then the granules were sintered at a temperature of 450 ° C for 2 hours.

The obtained granules were subjected to repeated tests in the sorption-desorption cycles of hydrogen fluoride, after which part of the granules went to study their physicochemical characteristics. The porosity was determined by the pycnometric method, the specific surface was determined by the Brunauer-Emmett-Teller method (BET method), and the mechanical strength was determined in accordance with GOST 21560.2-82.

The results of studies of the properties of granules of sorbents are given in the table.

Table Party number Porosity,% Specific surface, m ² / g Mechanical strength, kgf / cm ² source after the 1st cycle source after the 1st cycle after the 3rd cycle after the 5th cycle source after the 3rd cycle after the 5th cycle one 48.0 64.0 1,62 9.88 10.71 17.5 16,2 13,2 14.7 2 58.7 76.0 1.01 11.6 15.04 16.9 15.6 14.3 14.0 3 64.9 76.0 0.78 13.6 17.3 18.03 11.3 11.3 12.7

As can be seen from the table, the granules obtained by the proposed technology are cyclically stable. With an increase in the sorption-desorption cycles, the specific surface of the granules accessible to the sorbate increases and stabilizes due to the opening of internal pores, and the mechanical strength of the granules is practically unchanged.

The static capacity of the obtained sorbents was 0.6 mol HF / mol LiF, which corresponds to 85% of the theoretically possible capacity of powdered lithium fluoride (0.77 mol HF / mol LiF).

The addition of ammonium carbonate to the mixture in an amount less than 15 wt.% Did not provide the required porosity of the obtained granules (less than 40%), and the addition in an amount exceeding 50 wt.% Did not provide the necessary strength of the granules.

The inventive method of producing a sorbent in comparison with the prototype method can reduce the corrosive effects of the charge components on the equipment materials and eliminate clogging of the equipment with ammonium fluoride.

Claims (1)

1. A method of producing a sorbent for trapping volatile fluorides, including the preparation of a mixture by mixing dry components, including fluorides of alkali and / or alkaline earth metals and a blowing agent, forming a moistened mixture into granules, drying and sintering, characterized in that the blowing agent is introduced into the charge into ammonium carbonate the amount of 15 ÷ 50% by weight of fluorides. 2 The method according to claim 1, characterized in that the granules are dried at a temperature of 60-150 ° C, and sintering at a temperature of 350-550 ° C.