CN113336255B - Purification method of rare earth halide molten salt - Google Patents
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Abstract
The invention provides a method for purifying rare earth halide molten salt, which comprises the following steps: s1: a heating step: heating anhydrous rare earth halide containing water and oxygen impurities to exceed the melting point of the anhydrous rare earth halide, and fully melting the anhydrous rare earth halide into molten salt; s2: constant temperature step: directly contacting molten salt with a graphite filter, wherein the molten salt flows into a collecting container after being filtered and purified by graphite; s3: and (3) cooling: reducing the temperature of the melt below the corresponding melting point and condensing the melt into a purified solid crystal block; wherein, the graphite filter is made of materials with the purity higher than 99.9 percent and the density higher than 1.4 g.cm‑3The homogeneous graphite of (2). The purification method has simple equipment structure, does not need to introduce a new chemical protective agent, can use the cheap anhydrous rare earth halide raw material containing water and oxygen impurities to refine the high-purity anhydrous rare earth halide for crystal growth through high-temperature heat treatment, and has higher economic value.
Description
Technical Field
The invention belongs to the field of rare earth halide purification methods, and particularly relates to a purification method of rare earth halide molten salt.
Background
Anhydrous and oxygen-free high-purity rare earth halide in the metal halide is an important raw material for preparing scintillation crystal, and anhydrous rare earth bromide and anhydrous rare earth iodide in the anhydrous rare earth halide have a large specific gravity.
Due to the active chemical property of the anhydrous rare earth halide, the anhydrous rare earth halide is often combined with water in the environment to generate hydrated rare earth halide in the preparation process, and if the anhydrous rare earth halide is directly heated for dehydration, hydrolysis reaction is generated to form impurities such as rare earth oxyhalide, valence-variable rare earth halide and the like. The anhydrous rare earth halide containing the impurities has the quality problems of crystal cracking, crystal inclusion flaw, poor crystal transparency and the like in the application occasions with requirements on indexes such as crystal orientation, purity and the like, such as crystal growth and the like, and the yield is seriously influenced. The anhydrous rare earth bromide is widely applied to the preparation of scintillation crystals due to the special optical characteristics of crystals thereof, particularly cerium-doped lanthanum bromide scintillation crystals, and can be prepared into the gamma ray scintillator detector with the most excellent comprehensive performance at present. The anhydrous rare earth iodide is widely applied to iodide scintillation crystals due to the larger density and the unique electronic structure of the anhydrous rare earth iodide. Is often used as a host or dopant component for scintillation single crystals. But because of the chemical characteristics of easy moisture absorption, easy decomposition and the like of the rare earth halide, the popularization of the product is restricted.
The existing common anhydrous rare earth halide purification methods include a direct dehydration method, an ammonium halide protection method, a solvent protection method, a chemical vapor transport method, a direct halogenation method and the like. The direct dehydration method is that the hydrous rare earth halide is directly heated and dehydrated in protective atmosphere, vacuum environment or air atmosphere, the method generates oxyhalide with the content of about 5 percent under the condition of atmosphere protection, the hydrous rare earth bromide is directly heated in the air to generate pure bromine oxide and bromine, and the hydrous rare earth iodide is directly heated in the air to be oxidized by oxygen to generate iodate and purple iodine steam.
In patent CN201110344581.7 "PROGRAM HEATING METHOD FOR PREPARING HIGH-PURITY ANHYDRIUM BROMIDE OR CERIUM BROMIDE", a process for mass production of ANHYDRIUM BROMIDE or CERIUM BROMIDE using rare earth carbonate is introduced; patent CN201910343336.0 "a method for preparing anhydrous rare earth bromide" describes a process for purification by adding ammonium bromide to a bromide solution prepared from rare earth oxide and then raising the temperature by program. The method belongs to an ammonium bromide protection method, namely, by adding excessive ammonium bromide, the rare earth bromide and the ammonium bromide form complex salt with a more stable structure in the temperature rise process, the rare earth bromide is heated and dehydrated, and meanwhile, the ammonium bromide is decomposed to generate hydrogen bromide and ammonia gas, an acidic protective atmosphere is formed, and finally, the ammonium bromide is heated and removed to obtain relatively purified rare earth bromide. The product of the method usually has less than 1 percent of ammonium bromide remained, which is difficult to remove and contains about 2 percent of bromine oxide. And the produced excessive ammonium bromide is easy to block the pipeline of the reactor and corrode metal equipment. Similarly, the ammonium iodide protection method, i.e., the method of synthesizing rare earth iodide by adding excess ammonium iodide and co-heating with rare earth oxide, has the biggest problem of introducing ammonium halide which is not easy to be removed and impurities carried in the ammonium halide.
The solvent protection method is to combine the rare earth bromide with the solvent such as ethanol to form organic chelate by replacing water with the solvent, and then to remove the organic solvent by heating. Because the organic solvent is difficult to remove, the final product often has carbon deposition generated by the thermal decomposition of the organic solvent, and the product cannot be used as a scintillation crystal raw material.
The chemical vapor transport method forms double salt by co-heating the alkali metal bromide and the rare earth bromide, and then removes water at the hot end of the double salt and separates the alkali metal bromide by sublimation through temperature zone control, but the yield is low, the collection is difficult, and the industrial production is not facilitated.
The direct bromination method obtains pure rare earth bromide by reacting bromine simple substance or hydrogen bromide gas with rare earth metal or rare earth oxide. Because bromine reacts violently with rare earth metal, the reaction process is difficult to control, and industrial application is not formed, and the corrosivity of hydrogen bromide to equipment can be enhanced by water vapor generated by the reaction of hydrogen bromide and rare earth oxide. In order to complete the reaction, the process must be continuously charged with a bromine source, and both hydrogen bromide and bromine have toxicity, large process difficulty and high equipment requirement, and have potential risks to the surrounding environment and operators. The direct iodination method obtains pure rare earth iodide by reacting iodine simple substance or dry hydrogen iodide gas with rare earth metal or rare earth oxide. Because the reaction of iodine and rare earth metal is incomplete, the reaction process is difficult to control, a large amount of iodine is easily wasted, and industrial application is not formed, and the corrosivity of hydrogen iodide on equipment can be enhanced by water vapor generated by the reaction of hydrogen iodide and rare earth oxide. In order to complete the reaction, the process must be continuously charged with an iodine source, and both hydrogen iodide and iodine vapor have toxicity, large process difficulty and high equipment requirement, and have potential risks to the surrounding environment and operators. Patent CN201510782891.5 "preparation method of glass and glass film containing rare earth ion doped lutetium iodide microcrystal" introduces a method for preparing scintillator glass film by treating lutetium iodide sol gel with hydrogen iodide gas. But is not suitable for the manufacture of single crystal scintillators.
Therefore, there is a need for a process that can rapidly remove residual moisture and oxyhalides from anhydrous rare earth halides without introducing new impurities.
Disclosure of Invention
In view of the above, the present invention provides a method for purifying rare earth halide molten salt, which aims to overcome the defects in the prior art.
In order to achieve the purpose, the technical scheme of the invention is realized as follows:
a method for purifying rare earth halide molten salt comprises the following steps:
s1: heating step
Heating the anhydrous rare earth halide containing impurities to 800-900 ℃, preserving the heat for more than 0.5h, and fully melting the anhydrous rare earth halide containing impurities into molten salt; below this time, an anhydrous rare earth halide molten salt having good fluidity cannot be obtained.
S2: step of constant temperature
Directly contacting molten salt with a graphite filter, wherein the molten salt flows into a collecting container after being filtered and purified by the graphite filter;
s3: step of Cooling
Reducing the temperature of the melt below the corresponding melting point and condensing the melt into a purified solid crystal block;
wherein, the graphite filter is made of materials with the purity higher than 99.9 percent and the density higher than 1.4 g.cm-3The homogeneous graphite of (a); the rare earth halide refers to an ionic compound LnaX(3a+b)Ln includes yttrium, scandium and othersA cation of one or more rare earth elements including lanthanide rare earth elements, X is bromine or iodine; according to the different types of the rare earth halides, a is more than or equal to 1, b can be 0, positive number and negative number, but 3a + b is more than or equal to 2.
Preferably, the graphite filter is provided with an open-topped cavity for containing molten salt. Including but not limited to crucibles, petri dishes, trays, and the like, in conventional shapes.
Preferably, the collection vessel is disposed at the bottom of the graphite filter for receiving the molten salt filtered through the bottom of the graphite filter.
Preferably, when the rare earth halide is rare earth iodide, the material of the collecting container is silicon carbide.
Preferably, when the rare earth halide is rare earth bromide, the material of the collecting container is quartz, pyrolytic graphite, silicon carbide or platinum.
Preferably, the content of oxyhalide in the impurity-containing anhydrous rare earth halide is 1-20%.
Preferably, the molten salt penetrates through the graphite filter into the collection vessel by means of a traction force, which is gravity, more preferably, the traction force is provided by mechanical pressing, gas pressurization.
Preferably, the heating step is performed under an inert atmosphere protection and/or a vacuum degassing environment with an internal pressure of 100Pa or less, which prevents further deterioration of the molten salt.
More preferably, the heating step is 1.333X 10 provided after the argon purge using an oil-free vacuum pump-6Pa-1.333×10-1And (4) carrying out the reaction in a Pa vacuum environment.
Preferably, the heat preservation time of the constant temperature step is 6-8 h. Sufficient time is required during the molten salt filtration process to allow the molten salt to float the solid oxyhalide impurities above the melt by thermal convection.
Experiments prove that the rare earth halide fused salt can generate characteristic osmosis when being contained in a homogenizing and purifying graphite container, and solid solution is generated by forming graphite interlayer compounds to cause fused salt leakage and cannot be used as a stable container. But therefore, the homogeneous purified graphite has the potential of being used as a rare earth halide molten salt filter material. Since the homogeneously purified graphite has a microscopically layered molecular structure with a large gap in the direction perpendicular to the plane of the carbon layers, part of the substance can be allowed to pass through. The rare earth halide fused salt and graphite form a conductive interlayer compound, fused salt ions penetrate through graphite layer gaps under the influence of traction force to permeate, solid oxyhalide in the melt cannot pass through the graphite layer gaps by utilizing graphite layer interlayer channels, and the rare earth oxyhalide floats above the melt due to the fact that the rare earth oxyhalide is lower in density than the rare earth halide, and cannot block the melt from filtering downwards.
Compared with the prior art, the invention has the following advantages:
the invention creates the filtration purification method and the purification equipment, and provides a new idea for purifying and refining the anhydrous rare earth halide. By finding out the permeability of the rare earth metal halide molten salt to the solid graphite and utilizing the permeability, the rare earth halide molten salt permeates a homogeneous purification graphite filter under the action of traction forces such as attraction and the like, and simultaneously, the solid impurities in the molten salt are separated by utilizing the difference of the melting points of the rare earth oxyhalide and the rare earth halide (generally, the melting point of the rare earth oxybromide is hundreds of ℃ higher than that of the pure halide). Thereby playing a role in purifying the molten salt. The method has simple equipment structure, does not need to introduce a new chemical protective agent, can use the cheap rare earth halide raw material to refine the high-purity anhydrous rare earth halide for crystal growth through high-temperature heat treatment, and has higher economic value.
Drawings
FIG. 1 is a schematic cross-sectional front view of a cylindrical graphite filter and a molten salt collecting container according to an embodiment;
FIG. 2 is a schematic cross-sectional front view of the cylindrical graphite filter and molten salt collecting vessel according to comparative examples 1 to 3;
FIG. 3 is a photograph of the bottom of the molten salt filtration tank 2 of comparative example 3 after use;
FIG. 4 is a photograph of the residue inside the molten salt filtration tank 2 in comparative example 3;
FIG. 5 is a photograph showing the broken residue inside the molten salt filtration tank 2 in comparative example 3 (a large amount of the entrapped flaky cerium bromooxide crystals can be seen);
FIG. 6 is a partial close-up of the lamellar structure of the residue inside the molten salt filtering tank 2 of comparative example 3 with anhydrous cerium bromide sandwiched
Description of reference numerals:
1. an upper cover; 2. homogenizing and purifying a graphite cylinder filter; 3. a cylindrical fused silica collection crucible or a cylindrical silicon carbide collection crucible; 4. a cylindrical quartz collection crucible.
Detailed Description
Unless defined otherwise, technical terms used in the following examples have the same meanings as commonly understood by one of ordinary skill in the art to which the present invention belongs. The test reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the experimental methods are conventional methods unless otherwise specified.
The invention will be described in detail with reference to the following examples.
Example 1: purification of anhydrous lanthanum bromide
As shown in FIG. 1, in which 1 is a top lid, 2 is a homogeneous purified graphite cylinder filter of 5mm bottom, 10mm wall thickness, 200mm height, and 3 is a cylindrical fused silica collection crucible. 1kg of powdery anhydrous LaBr containing impurities and with the granularity of 1mm is added into a covered cylindrical graphite filter3A homogeneous purified graphite cylinder filter was inserted above a cylindrical fused silica collection crucible. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-6Pa-1.333×10-1Pa vacuum environment) to 800 ℃, keeping the temperature for more than 0.5h, directly melting the raw materials in a homogeneous purified graphite cylinder filter, dripping the raw materials into a quartz collection crucible through the bottom of the filter, standing for 8 hours at constant temperature, uniformly cooling at 5 ℃/min, and collecting colorless transparent crystals in the crucible.
Example 2: purification of anhydrous cerium bromide
As shown in FIG. 1, wherein 1 is a top lid, 2 is a bottom 5mm, wall thickness is 10mm, and height is 200mm, and 3 is a cylindrical fused silica collection crucible. 1kg of powdery anhydrous CeBr containing impurities and with the granularity of 1mm is added into a covered cylindrical graphite filter3Inserting a filter intoAbove the cylindrical fused silica collection crucible. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-6Pa-1.333×10-1Pa vacuum environment) to 850 ℃, keeping the temperature for more than 0.5h, directly melting the raw materials in the homogeneous purified graphite cylinder filter, dripping the raw materials into a cylindrical fused quartz collecting crucible through the bottom of the homogeneous purified graphite cylinder filter, standing for 8 hours at constant temperature, then uniformly cooling at 5 ℃/min, and obtaining colorless transparent crystals in the cylindrical fused quartz collecting crucible.
Example 3: purification of anhydrous lanthanum iodide
As shown in FIG. 1, in which 1 is a top lid, 2 is a bottom 5mm, wall thickness is 10mm, and height is 200mm, a homogeneous purified graphite cylinder filter, and 3 is a cylindrical silicon carbide collecting crucible. 1kg of powdery anhydrous LaI containing impurities and with the granularity of 1mm is added into a covered cylindrical graphite filter3The filter was inserted above a cylindrical silicon carbide collection crucible. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-6Pa-1.333×10-1Pa vacuum environment) to 810 ℃, and preserving heat for more than 0.5h, directly melting the raw materials in the homogeneous purified graphite cylindrical filter, dripping the raw materials into a cylindrical silicon carbide collecting crucible through the bottom of the filter, standing at constant temperature for 8 hours, then uniformly cooling at 5 ℃/min, and obtaining colorless transparent crystals in the cylindrical silicon carbide collecting crucible.
Example 4: purification of anhydrous cerium iodide
As shown in FIG. 1, in which 1 is a top lid, 2 is a bottom 5mm, wall thickness is 10mm, and height is 200mm, a homogeneous purified graphite cylinder filter, and 3 is a cylindrical silicon carbide collecting crucible. 1kg of powdery anhydrous CeI containing impurities and with the granularity of 1mm is added into a covered homogeneous purified graphite cylinder filter3And embedding the homogeneous purified graphite cylinder filter above the cylindrical silicon carbide collecting crucible. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-6Pa-1.333×10-1Pa vacuum environment) to 800 deg.C, maintaining for more than 0.5 hr, directly melting the raw materials in the filter, and dripping into cylindrical silicon carbide through homogenizing and purifying graphite cylindrical filterAnd standing the mixture in a crucible at constant temperature for 8 hours, then uniformly cooling the mixture at a speed of 5 ℃/minute, and collecting the cylindrical silicon carbide in the crucible to obtain colorless transparent crystals.
Comparative example 1: purification of anhydrous cerium bromide (sublimation, top collection)
As shown in FIG. 2, 2 is the same homogeneous purified graphite cylindrical filter as in the above example without a lid, 5mm in the bottom, 10mm in the wall thickness, and 200mm in height, and 4 is a cylindrical quartz collection crucible. 1kg of powdery anhydrous CeBr containing impurities and with the granularity of 1mm is added into a cylinder filter of homogenized and purified graphite3And embedding a cylindrical quartz collecting crucible above the homogeneous purified graphite cylindrical filter. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-1Pa-1.333×10-6Pa vacuum environment) to 850 deg.C, keeping the temperature for more than 0.5h, melting the raw materials in the homogeneous purified graphite cylindrical filter, standing at constant temperature for 8h, cooling at constant speed of 5 deg.C/min, collecting the melt in the furnace through the bottom of the homogeneous purified graphite cylindrical filter, and collecting a small amount of milky opaque solid in the top cylindrical quartz collecting crucible. A white hard shell remained inside the homogeneous purified graphite cylinder filter. The bottom of the cylinder has white traces. The graphite permeation rate of the melt at the temperature is proved to be obviously higher than the sublimation purification rate, and the production efficiency of the filtration process is proved to be higher than that of the sublimation purification process.
Comparative example 2: purification of anhydrous cerium iodide (sublimation, top collection)
As shown in FIG. 2, 2 is the same homogeneous purified graphite cylindrical filter as in the above example without a lid, 5mm in the bottom, 10mm in the wall thickness, and 200mm in height, and 4 is a cylindrical quartz collection crucible. 1kg of powdery anhydrous CeI containing impurities and with the granularity of 1mm is added into a cylinder filter of homogenized and purified graphite3A cylindrical quartz collection crucible was inserted over the filter. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-6Pa-1.333×10-1Pa vacuum environment) to 800 deg.C, maintaining for 0.5 hr or more, directly melting the raw materials in a homogenizing and purifying graphite cylindrical filter, standing at constant temperature for 8 hr, cooling at 5 deg.C/min, homogenizing the melt, and extractingThe pure graphite cylinder filter was not efficiently collected by dripping from the bottom into the furnace, and a small amount of milky opaque solid was collected by the top cylindrical quartz collection crucible. A white hard shell remained inside the homogeneous purified graphite cylinder filter. The bottom of the cylinder has white traces. The graphite permeation rate of the melt at the temperature is proved to be obviously higher than the sublimation purification rate, and the production efficiency of the filtration process is proved to be higher than that of the sublimation purification process.
Comparative example 3
As shown in FIG. 1, wherein 1 is a top lid, 2 is a bottom 5mm, wall thickness is 10mm, and height is 200mm, and 3 is a cylindrical fused silica collection crucible. 1kg of powdery anhydrous CeBr containing impurities and having a particle size of 1mm was added to a covered cylinder filter of homogenized and purified graphite3A homogeneous purified graphite cylinder filter was inserted above a cylindrical fused silica collection crucible. The whole body is vertically arranged in a vacuum furnace in a high vacuum environment (the pressure is between 1.333 and 10)-6Pa-1.333×10-1Pa vacuum environment) to 910 deg.C, keeping the temperature for more than 0.5h, melting the raw materials in the filter, dripping the raw materials into a cylindrical fused quartz collecting crucible through the bottom of the homogeneous purified graphite cylindrical filter, standing at constant temperature for 8 hours, and then cooling at constant speed of 5 deg.C/min, wherein only a small amount of transparent crystals are obtained in the cylindrical fused quartz collecting crucible. The anhydrous cerium bromide which is remained with a large amount of wrapping and clamping sheet-shaped crystallisates in the cylinder filter for homogenizing and purifying the graphite is shown in figures 3-6, and due to overhigh heating temperature, the fused salt is boiled, so that the bromine oxide shell which should float on the surface of the fused salt is crushed and mixed into the melt, the fused salt is physically isolated from the cylinder filter for homogenizing and purifying the graphite, and the permeation efficiency is reduced. The fact that the filtering effect is reduced by the fact that the filtering temperature of the molten salt is too high is proved, so that the optimal filtering temperature of the invention is between 800 and 900 ℃ corresponding to the theoretical melting point of the rare earth halide.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and should not be taken as limiting the invention, so that any modifications, equivalents, improvements and the like, which are within the spirit and principle of the present invention, should be included in the scope of the present invention.
Claims (8)
1. A method for purifying rare earth halide molten salt is characterized by comprising the following steps: the method comprises the following steps:
s1: heating step
Heating the anhydrous rare earth halide containing impurities to 800-900 ℃, preserving the heat for more than 0.5h, and fully melting the anhydrous rare earth halide containing impurities into molten salt;
s2: step of constant temperature
Directly contacting molten salt with a graphite filter, wherein the molten salt flows into a collecting container after being filtered and purified by the graphite filter; the graphite filter is provided with a cavity with an opening at the top, and the cavity is used for containing molten salt; the collecting container is arranged at the bottom of the graphite filter and used for receiving the molten salt filtered by permeating through the bottom of the graphite filter;
s3: step of Cooling
Reducing the temperature of the melt below the corresponding melting point and condensing the melt into a purified solid crystal block;
wherein, the graphite filter is made of materials with the purity higher than 99.9 percent and the density higher than 1.4 g.cm-3The homogeneous graphite of (a);
the rare earth halide refers to an ionic compound LnaX(3a+b)Ln comprises cations of one or more rare earth elements including yttrium, scandium and other lanthanide rare earth elements, and X is bromide ion or iodide ion; according to the different types of the rare earth halides, a is more than or equal to 1, b can be 0, positive number and negative number, but 3a + b is more than or equal to 2.
2. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: when the rare earth halide is rare earth iodide, the collection container is made of silicon carbide.
3. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: when the rare earth halide is rare earth bromide, the collecting container is made of quartz, pyrolytic graphite, silicon carbide or platinum.
4. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: the content of oxyhalide in the anhydrous rare earth halide containing impurities is 1-20%.
5. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: the molten salt permeates the graphite filter to the collection vessel by a traction force, which is gravity.
6. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: the heating step is carried out in an inert atmosphere protection and/or a vacuum degassing environment with an internal pressure of less than 100 Pa.
7. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: the heating step was 1.333X 10 provided after argon purge using an oil-free vacuum pump-6Pa-1.333×10-1And (4) carrying out the reaction in a Pa vacuum environment.
8. The method of purifying a rare earth halide molten salt according to claim 1, characterized in that: the heat preservation time of the constant temperature step is 6-8 h.
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