RU2363777C1 - Method of preparing raw material from sodium or caesium iodides for growing crystals - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the technology of preparing free running salts of metal halides and can be used in chemical industry, particularly for preparing initial salts of sodium or caesium iodides for growing monocrystals - Nal(Tl), Csl, CsI(Tl), CsI(Na). The method involves loading initial salt into an ampoule, evacuation of the ampoule, heating the salt to 420-450°C with constant evacuation, letting in dry air with subsequent holding, repeated air inlet, holding and pumping in the given modes and repeated evacuation, after which carbon dioxide gas is let in until attaining pressure of 200-400 mm Hg and keeping the salt in a carbon dioxide gas atmosphere for 15-20 minutes.

EFFECT: invention allows for obtaining raw material, more specifically iodides of alkali metals, without impurities of hydroxyl groups, which provides for good scintillation properties of crystals, as well as photochemical stability of crystals; also, the initial salt has a homogeneous powdered structure, ie retains free-running properties.

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Description

The invention relates to a technology for the preparation of bulk salts of metal halides for growing crystals and can be used in the chemical industry in preparing the starting salts of sodium and cesium iodides for growing single crystals based on them.

Modern science and technology has a significant need for single crystals based on sodium and cesium iodides - NaI (Tl), CsI, CsI (Tl), CsI (Na). High-quality scintillation characteristics and photochemical stability of single crystals largely depend on the conditions for the preparation of crystals and the purity of the starting salts [Goriletsky V.I., Grinev B.V., Zaslavsky B.G., Smirnov N.N., Suzdal V.S. Crystal growth. Kharkov, Act, 2002, 535 pp.]. The indicated characteristics of single crystals based on sodium and cesium iodides are significantly deteriorated in the presence of oxygen-containing impurities, which are formed when the salt interacts with water vapor and air oxygen. Among the impurities with oxygen content, hydroxyl groups should be especially distinguished, which themselves reduce the scintillation efficiency of crystals, and after partial conversion to carbonate ions lead to photochemical instability of crystals [Grinev BV, Shpilinskaya LV, Kovaleva LV , Kudin A.M., Mitichkin A.I., Charkina T.A. Photo- and radiation-chemical transformations of carbonate ions in CsI and CsI (Tl) crystals. Optics and Spectroscopy, 2000, vol. 89, No. 1, p. 57-62]. Hydroxyl groups in significant quantities appear during the hydrolysis of salt both during its storage (especially in the light) and as a result of high-temperature drying of the salt in preparation for its cultivation [Sofronov DS, Voloshko A.Yu., Kisil EM , Kudin K.A., Shishkin O.V. Investigation of the causes of high-temperature water evolution during dehydration of sodium iodide in a vacuum. Dielectrics and semiconductors in radiation detectors. Kharkiv. Institute of Single Crystals, 2006, p. 346-357].

To clean salts from impurities with an oxygen content, there are a number of methods, namely:

- vacuum distillation for purification from hydroxide, sulfate and phosphations, which are concentrated in the residue [Bannik V.V., Demirskaya O.V., Chervonnaya L.N., Pulyaeva I.V., Eksperiandrova L.P. Fractional separation of oxygen-containing impurities in sodium iodide during vacuum distillation. Physics and chemistry of solids. Collection of scientific papers. Kharkov, 1983, p. 87-90];

- treatment of iodides with hydrogen iodide to remove carbonate ions. The content of hydroxides of ions practically does not change [Bannik V.V., Demirskaya O.V., Chervonnaya L.I. et al. Purification of cesium iodide from oxygen-containing impurities by gaseous hydrogen iodide. Single crystals and technology. Collection of scientific papers. Kharkiv. VNII Monocrystals. 1983, No. 11, pp. 90-93];

- treatment of the melt with spectrally pure carbon followed by filtration of the melt can reduce the amount of impurities with oxygen content, but such a decrease is not enough [Antoniv IP, Garapin IV, Matviychuk DI Obtaining single crystals of cesium iodide with a low concentration of oxygen-containing impurities. High purity substances. 1990, No. 5, p. 175-177].

In practice, in the conditions of industrial crystal growth, it is not always possible to realize additional purification of salt from impurities with an oxygen content by the indicated methods. The use of hydrogen iodides leads to corrosion of metal components and parts of growth plants, and the purification of the melt by carbon or vacuum distillation requires additional equipment. In addition, these methods cannot be used on an industrial scale, since it is not possible to pour purified raw materials into the feed hopper of growth plants to obtain large crystals.

A known method of dehydration of sodium iodide in a microwave field [application No. 2006 06594 from 06/13/2006 for a patent of Ukraine for an invention, cl. F26B 3/347, C01D 3/12], which prevents the hydrolysis of sodium iodide, does not lead to an increase in the concentration of hydroxyl groups, but does not reduce the amount of hydroxyl groups already contained in the salt. In addition, this method requires the use of complex and expensive microwave equipment and currently has not found wide industrial use.

A known method of producing scintillation material [and.with. USSR No. 1429601, class СВВ 11/02], which includes evacuating the ampoule with salt to a residual pressure of 1 · 10 ^-1 mm Hg, heating the salt from room temperature to 600 ° C for 4 hours with constant evacuation, holding under these conditions for 20 hours for salt dehydration. Then the ampoule is placed in the chamber of a growth furnace, the temperature in which is set at 750 ° C, the ampoule is filled with oxygen to a pressure of 150-530 mm Hg, it is kept until the salt is completely melted and the ampoule with the melt is re-vacuum to a residual pressure of 1 · 10 ^-1 mm Hg, after which they grow.

The above method does not provide cleaning salts from hydroxyl groups. In addition, the process of preparing the feedstock by this method proceeds at high temperatures, which leads to salt sintering. This method of preparing raw materials is suitable only for growing single crystals by the Stockbarger method, according to which the preparation of raw materials and growing are carried out in the same ampoule.

For growing large-sized single crystals (with a diameter of more than 500 mm), constant feeding of the melt with raw materials is necessary, for which the raw materials prepared in the ampoule must be poured into the hopper of a growth furnace.

A known method of preparing raw materials for growing single crystals [Laboratory technological regulation No. R 14-01 for obtaining dehydrated sodium salts of iodide and cesium iodide for growing single crystals by vacuum drying, followed by heat treatment and burning of organic impurities. Research department of alkali halide crystals with research production of the Scientific and Production Institute "Monocrystals" of the National Academy of Sciences of Ukraine, Kharkov, 2001], which includes loading the initial salt into the ampoule, evacuating the ampoule for 0.5-1.0 hours at a residual pressure of 6 -3 · 10 ^-3 mm Hg, heating the salt to 550-580 ° C with constant evacuation for 2-3 hours (the raw material is pyrolyzed), introducing a mixture with argon and oxygen content (oxygen content 20-30 %) to a pressure of 400 mm Hg, holding at the specified temperature for 1 hour pumping for 2 hours and cooling raw "rastrusku" clinker raw manually.

The process of oxidation of organic impurities by this method is accompanied by the formation of impurities with an oxygen content, and at the indicated temperatures salt sintering is observed, although to a lesser extent, but manual “splashing” of salt in ampoules is an obligatory operation for the subsequent transfer of raw materials into growth hoppers installations.

A known method of preparing raw materials based on sodium iodide for growing single crystals [p. Ukraine on the invention No. 80305, s. No. a 200505421 dated 06.06.05, class СВВ 11/02], which includes loading the initial salt of sodium iodide into an ampoule, evacuating the ampoule, heating the salt to a temperature of 420-450 ° С with constant evacuation, introducing dry air with subsequent exposure, re-admitting dry air, holding and pumping out under these conditions and re-evacuation.

According to this method, when the declared parameters exclude sintering of salt and an additional operation of “crushing” the salt in ampoules, however, this method does not ensure the purification of raw materials from hydroxyl groups (OH ^- ).

The basis of this invention is the task of developing a method of preparing raw materials for growing single crystals, namely sodium or cesium iodides, which provides the source salt without hydroxyl groups while maintaining the flowability of the salt.

As a prototype, we have chosen the last of the analogues.

The solution to the problem is based on the fact that when interacting with carbon dioxide, hydroxyl groups form carbonates in accordance with the reaction:

According to reaction (1), volatile products are formed that are easily removed from the volume by pumping. The compound Na ₂ CO ₃ is thermally less stable compared to NaOH and decomposes in the melt by the following reaction:

As a result of this reaction, sodium oxide is formed. The Na ₂ O compound, unlike NaOH, does not significantly affect the characteristics of scintillators based on NaI crystals. It is known that NaI: TI crystals that contain O ² - ions do not differ from the purest crystals (the content of O ² - ions is less than 5 · 10 ^-4 wt.%) In such indicators as light yield, energy resolution and scintillation decay time, even when the concentration of О ^2- - ions is equal to the number of TI ⁺ cations, namely ~ 5 · 10 ^-2 wt.%.

Similarly to sodium iodide, it is possible to purify cesium iodide hydroxide groups. As a result of the carbonization reaction of cesium hydroxide, a carbonate is formed, which decomposes upon heating to oxide, which does not significantly affect the characteristics of scintillators based on CsI crystals.

Thus, the solution of the problem is ensured by the fact that in the method of preparing raw materials from sodium or cesium iodides for growing crystals based on them, which includes loading the original salt into an ampoule, its evacuation, heating the salt with constant evacuation to a temperature of 420-450 ° C, the introduction of dry air with subsequent exposure, re-admission of dry air, soaking and pumping under these conditions, re-evacuation, according to the invention after re-evacuation carry out the inlet of carbon dioxide and to a pressure of 200-400 mmHg and extracting salt in an atmosphere of carbon dioxide for 15-20 minutes.

The inlet of carbon dioxide is carried out after the oxidation of organic impurities. As our studies have shown, beyond the boundaries of the declared parameters of technological operations determined experimentally, the task is not solved. A decrease in pressure and exposure time leads to incomplete reaction (1), and an increase in these parameters leads to an excess of carbon dioxide. Under the stated conditions, the feedstock for growing single crystals, namely sodium or cesium iodides, do not contain hydroxyl groups, which ensures both high-quality scintillation characteristics of crystals based on them (light output, energy distinguishability) and photochemical stability of crystals. In addition, the obtained salt has a uniform powder structure, which makes it easy to pour the prepared raw materials from the ampoule into the feed hopper of the growth furnace for growing single crystals.

Example 1

9 kg of sodium iodide are loaded into an ampoule with a diameter of 250 mm and a height of 800 mm. The ampoule is pumped out to a residual pressure of 2 · 10 ^-2 mm Hg. for 12 hours at room temperature. The ampoule is placed in a furnace heated to 420 ° C and the salt is kept under constant evacuation for 6 hours. Under these conditions, gaseous hydrocarbons, CO and CO ₂ are already formed, which are freely removed from the ampoule upon evacuation to a pressure of 1 · 10 ^-2 mm Hg. even before the dry air inlet. Then the ampoule is filled with dry air up to 600 mm Hg. The raw materials are kept in an atmosphere of dry air for 20 minutes, after which the volume of the ampoule is evacuated to a residual pressure of 1 · 10 ^-2 mm Hg, while removing gaseous oxidation products together with the remaining air. Dry air is then poured again up to 600 mm Hg. and re-burn for 20 minutes, followed by evacuation to 1 · 10 ^-2 mm Hg, carbon dioxide is injected to a pressure of 400 mm Hg. and maintain the salt in an atmosphere of carbon dioxide for 20 minutes, vacuum to a pressure of 1 · 10 ^-2 mm RT.article, re-inject carbon dioxide to a pressure of 400 mm RT.article, maintain the salt in an atmosphere of carbon dioxide for 20 minutes and vacuum until 2 · 10 ^-2 mm RT.article The ampoule with salt is discharged from the furnace and cooled to room temperature. The prepared salt does not contain impurities of hydroxyl groups and is easily poured into the hopper of a growth plant for growing single crystals.

Example 2

9 kg of cesium iodide are loaded into an ampoule with a diameter of 250 mm and a height of 800 mm. The ampoule is pumped out to a residual pressure of 2 · 10 ^-2 mm Hg. for 12 hours at room temperature. The ampoule is placed in a furnace heated to 420 ° C and the salt is kept under constant evacuation for 6 hours. Carbon dioxide is injected to a pressure of 300 mmHg. and maintain the salt in an atmosphere of carbon dioxide for 15 minutes, vacuum to a pressure of 1 · 10 ^-2 mm RT.article, re-inject carbon dioxide to a pressure of 300 mm RT.article, maintain the salt in an atmosphere of carbon dioxide for 15 minutes and vacuum to 2 · 10 ^-2 mm RT.article The ampoule with salt is discharged from the furnace and cooled to room temperature. The prepared salt does not contain impurities of hydroxyl groups and is easily poured into the feed hopper of a growth plant for growing single crystals.

Claims (1)

A method of preparing raw materials from sodium or cesium iodides for growing crystals based on them, which includes loading the source salt into an ampoule, evacuating the ampoule, heating the salt with constant evacuation to a temperature of 420-450 ° C, letting in dry air, followed by holding it, re-letting in dry air , exposure and pumping under these modes and re-evacuation, characterized in that after re-evacuation is carried out the inlet of carbon dioxide to a pressure of 200-400 mm RT.article and the exposure of salt in an atmosphere of carbon dioxide for 15-20 minutes