RU2236375C2 - Method for preparing sodium hexafluoroscandiate - Google Patents

Abstract

FIELD: inorganic chemistry, chemical technology.

SUBSTANCE: invention relates to technology for preparing chelate compounds of scandium. The parent scandium carbonate is dissolved in hydrochloric or nitric acid to pH = 1-2. Then ammonium fluoride and scandium-containing chloride or nitrate solution in the ratio (mole) sodium fluoride : ammonium fluoride : scandium = (3.1-3.2):(3.0-3.1):1, respectively, are added sequentially to the saturated and heated to 80-95^oC solution of sodium fluoride. Chelate scandium salt is precipitated. Pulp is kept at stirring for 20-40 min. Precipitated deposit of sodium hexafluoroscandiate is filtered off, washed out with acetone and dried at temperature 100-150^oC for 2-3 h. Mother liquor obtained after filtration of sodium hexafluoroscandiate is treated with hydrochloric or nitric acid up to the ratio (mol) fluoride ions : hydrogen ions = 1:(0.9-0.95). The obtained solution is used for treatment of the parent scandium carbonate, filtrate is separated and discarded and precipitate containing scandium carbonate and fluoride is fed to the process head. Invention provides preparing crystalline precipitates that promotes to formation more pure compounds and elevating filtration rate, diminishing amount of formed fluoro-containing sewage wherein the residual concentration of fluoride ions in discharge solutions doesn't exceed 3-5 mg/l.

EFFECT: improved preparing method.

2 tbl, 1 ex

Description

The present invention relates to the technology of rare earth compounds, more specifically to the technology of compounds of scandium. Scandium belongs to the group of rare-earth elements and is of great interest to it due to a number of unique properties of scandium. In particular, scandium-aluminum alloys are used to produce high-strength aluminum alloys used in modern advanced technology. In the manufacture of aluminum scandium ligatures, the use of sodium hexafluoroscandiate instead of metallic scandium seems promising. The literature describes methods for producing fluorine-containing compounds of rare-earth elements and yttrium [1] by reacting the initial REE with ammonium fluoride, separating the precipitate, and calcining at a temperature of 450-600 ° С. A number of works are devoted to the production of complex REE and lithium fluorides. To obtain double lithium and yttrium fluoride, the yttrium salt is mixed with lithium carbonate and hydrofluoric acid. The complex salt is washed from impurities and calcined at 400-500 ° C [2]. In the patent [3], the authors recommend cesium tetrafluorochlorate to obtain REE fluorine complexes and alkali metals. The closest in technical solution and the achieved result is a method for producing sodium fluoroscandiate by the interaction of fluoride solutions of sodium and ammonium with scandium fluoride [4]. The disadvantage of this technical solution is the formation of slowly filtered precipitates of complex fluorides, significant contamination of wastewater with fluorine ion and the need for additional high-temperature calcining operations. Fluoride mother liquors removed from the process can contain from 0.5 to 10 g / l of fluorine ion. In order to eliminate these shortcomings, we propose the following technical solution.

A preliminary solution of scandium chloride or nitrate is prepared by dissolving the starting scandium carbonate in hydrochloric or nitric acid to pH 1-2. Then, in a saturated and heated to a temperature of 80-95 ° С sodium fluoride solution, ammonium fluoride and a scandium-containing chloride or nitrate solution are successively introduced with a precipitate of scandium fluoride in the ratio (weight): sodium fluoride: ammonium fluoride: scandium = 3.1-3, 2: 3.0-3.1: 1. The complex salt of scandium precipitates. The pulp is kept under stirring for 20-40 minutes, the precipitated precipitate of sodium hexafluoroscandiate is filtered off, washed with acetone and dried at a temperature of 100-150 ° C for 2-3 hours. The mother liquor from the filtration of sodium hexafluoroscandiate is treated with hydrochloric or nitric acid to the ratio (mol.): Fluorine ion: hydrogen ion = 1: 0.9-0.95. The resulting solution is treated with the starting scandium carbonate, the filtrate is separated and discarded, and the precipitate is sent to the head of the process for dissolution in hydrochloric or nitric acid.

The recommendation to maintain a pH of 1-2 at the end of the dissolution of the starting scandium carbonate is associated with the need to have an excessive amount of hydrogen ions in order to avoid the formation of hydroxofluorides. The replacement of part of sodium fluoride with ammonium fluoride is recommended by the authors to reduce the volume of aqueous solutions, because ammonium fluoride, unlike sodium fluoride, is highly soluble in water.

The recommended range of the ratio of sodium fluoride, ammonium fluoride and scandium is explained by the fact that when the ratio is reduced to 2.5-2.8, the extraction of scandium decreases, and when the ratio increases more than 3.2-3.3, the amount of fluoride ions in the mother liquors increases ( table 1a). The temperature range and the order of flow of the solutions during the precipitation of sodium hexafluoroscandiate is explained by the fact that in this case a crystalline, well-filtered precipitate is obtained. The precipitate becomes more amorphous when the temperature drops to 70 ° C and lower, a temperature increase of more than 95 ° C does not improve the result (Table 1b). The temperature interval during drying of the complex salt is explained by the fact that when the temperature drops below 100 ° C, the drying time is unacceptably longer , when the temperature rises above 150 ° C, the formation of oxyfluoride impurities is possible. The range of the ratio of the concentration of fluorine ions to hydrogen ions is due to the fact that with an increase in the ratio of fluorine ion: hydrogen ion more than 1.0-1.05, the content of fluorine ions in waste solutions increases (up to 0.1 g / l), and with the ratio less than 0.9 increases the content of scandium in waste solutions (up to 0.05-0.15 g / l). The dissolution of scandium oxide can be carried out both in hydrochloric and in nitric acids. After dissolution, it is advisable to have some excess acid (2-4 g / l) in order to avoid the formation of hydroxo compounds.

Experience

The synthesis of sodium hexafluoroscandiate was carried out several times under various conditions. Changed the sequence of operations and individual parameters.

A weighed portion of scandium carbonate weighing 2 g was dissolved in hydrochloric acid to an excess acidity of 0.01-0.05 M. Then, a variable amount of sodium fluoride (1.5 to 2.1 g) was dissolved in 100 ml of water, and the solution was heated to 85 ° C. and the calculated amount of ammonium fluoride was introduced. After dissolving the salt, a scandium-containing solution was added. The pulp was mixed for 20-30 minutes, filtered and the precipitate was washed with water. The scandium extraction results are shown in Tables 1a, 1b. To the combined filtrate and wash solution containing 0.5-1 g / L (0.02-0.05 M) fluoride ion, hydrochloric acid or alkali was added in an amount in which the concentration of free hydrogen ions was 0.9 of molar the content of fluorine ion in the mother liquor (i.e., 0.018-0.045 M). The resulting solution was mixed with the original scandium carbonate weighing 2 g. The pulp was filtered. The content of fluorine ion in waste solutions ranges from 3 to 5 mg / l. Scandium carbonate mixed with scandium fluoride was sent to the head of the process for dissolution in acid. To remove water from the fluoroscandiate precipitate, it is inappropriate to use calcination or heating in a muffle because of the danger of hydrolysis and destruction of the complex compound. The authors recommend flushing the fluoroscandiate precipitate directly on the filter with acetone after removing the bulk of the water by filtration. The washed precipitate dries quickly and takes the form of a light friable powder. To remove traces of acetone, the washed precipitate is dried in a muffle at a temperature of 100-150 ° C for 2-3 hours.

The X-ray phase analysis showed that in the powders obtained by the proposed method, the content of the complex salt of sodium hexafluoroscandiate forms the basis. As impurities, up to 1% sodium tetrafluoroscandiate and 0.5-0.7% scandium fluoride are contained. In the samples obtained by calcining the starting components at temperatures of 400-500 ° C, there is an increased content of scandium oxofluoride (up to 1%).

If the task of sodium fluoride content in fluoroscandiate is not so urgent, then it is possible to carry out the deposition not in a solution of scandium fluoride, but in a system with sodium fluoride pulp. The authors found that part of the sodium fluoride can be used in the form of crystals (when dissolved, sodium fluoride forms solutions similar to colloids). This allows you to further reduce the initial volume of solutions, which significantly increases the productivity of the process.

The advantage of the method is that there is no need to initially obtain scandium fluoride. This is a rather complicated and environmentally harmful operation. The bulk of scandium is introduced into the system in the form of scandium carbonate. In the process of precipitation, due to an increase in temperature and a certain sequence of input of solutions, crystalline precipitates are obtained, which contributes to the formation of cleaner compounds and an increase in the filtration rate. The amount of fluorine-containing wastewater generated is significantly reduced by at least 2 times due to the use of ammonium fluoride. The residual concentration of fluorine ion in the effluent after processing the initial scandium carbonate with mother liquors does not exceed 3-5 mg / l.

Literature

1. Barashnikov N.V. et al. A method for producing anhydrous rare earth and yttrium oxyfluorides. Auth. testimonial. USSR No. 1527163, 10.26.1987, class. C 01 F 17/00.

2. Melnichenko E.I. et al. A method for producing powdered double lithium and yttrium fluoride. Auth. testimonial. USSR No. 1551653, 12/30/1987, class. C 01 F 17/00.

3. Popov A.I. etc. A method for producing fluorocomplexes of rare-earth elements and alkali metals. Auth. testimonial. USSR No. 1581695, 03/28/1989, class. C 01 F 7/00.

4. RF patent No. 2040471, 07.25.1995, cl. C 01 F 17/00.

Claims (1)

The method of producing sodium hexafluoroscandiate by the interaction of fluorine-containing sodium and ammonium compounds with scandium compounds, characterized in that scandium carbonate is used as the starting compound, which is treated with hydrochloric or nitric acids to achieve a pH of 1-2, the resulting solution is added to a temperature of 80-95 ° With a sodium fluoride solution into which ammonium fluoride is preliminarily added in the ratio (mol) sodium fluoride: ammonium fluoride: scandium = 3.1-3.2: 3.0-3.1: 1, a precipitate of the complex salt of hexafluoroscandiate sodium is separated by filtration, washed with acetone and dried at a temperature of 100-150 ° C, the mother liquor from the filtration is treated with hydrochloric or nitric acid to the ratio (mol) of fluorine ion: hydrogen ion = 1: 0.9-0.95 and the resulting solution is treated with the original scandium carbonate, the filtrate is separated and discharged, and the precipitate is sent to the head of the process for dissolution in hydrochloric or nitric acids.