Method for preparing low-chlorine rare earth oxide by utilizing pyrolysis of rare earth chloride

Abstract

The invention relates to a method for preparing low-chlorine rare earth oxide through pyrolysis of rare earth chloride, and belongs to the field of nonferrous metallurgy. According to the method, rare earth chloride and alkali are used as raw materials, the rare earth chloride is pyrolyzed into rare earth oxide in a high-temperature and high-pressure atmosphere, and a chloride byproduct is obtained. The method provided by the invention abandons the problems of long process flow and large amount of high-salinity wastewater of traditional wet precipitation, promotes deep pyrolysis of the rare earth chloride by using a high-pressure closed environment, realizes resource utilization of chlorine, solves the defects of incomplete pyrolysis and high pyrolysis temperature of the rare earth chloride in a conventional pyrolysis process, and has a good industrial application prospect.

Description

Technical Field

[0001] The invention relates to a method for preparing low-chlorine rare earth oxide by pyrolysis of rare earth chloride, and belongs to the field of nonferrous metal metallurgy. Technical Background

[0002] Rare earth oxides are the most important application form of rare earths, and more than 85% of the world's rare earths are consumed in the form of rare earth oxides. Purity or impurity content is the most important technical indicator for measuring the quality of rare earth oxides, among which chlorine content is the most prominent. Therefore, the research on the preparation of low-chlorine rare earth oxides has always been highly valued and concerned by academia and industry.

[0003] At present, the main methods for preparing rare earth oxides include precipitation method, sol-gel method, hydrothermal method, microemulsion method, spray pyrolysis method and other processes. Among them, the precipitation method is the most commonly studied and widely used. According to the difference in precipitants, the precipitation method can be roughly divided into two categories. The first category uses alkaline solutions such as sodium carbonate, ammonium bicarbonate, sodium hydroxide, and ammonia water as precipitants. By controlling the precipitation process parameters and combining the washing-drying-roasting process, rare earth oxides are obtained; the second category uses oxalic acid as a precipitant. Under appropriate process conditions, rare earth oxalates are first obtained, and then rare earth oxide products are obtained by roasting. Compared with the alkaline precipitation process, the oxalic acid precipitation process is easy to control, and the resulting rare earth oxides are of higher purity, but the price of oxalic acid is high and the cost of wastewater treatment is high, which makes the oxalic acid precipitation method have a higher production cost and is usually used to prepare rare earth oxides with higher value and higher purity requirements. Based on cost control, most rare earth oxides mainly use the alkaline precipitation process, which is simple, easy to operate, and has low equipment requirements. It is the preferred method for preparing rare earth oxides. However, the precipitation method also has some obvious shortcomings. For example, the preparation of rare earth oxides by the precipitation method requires precipitation, washing, drying, calcination and grading, resulting in a lengthy production process, large rare earth losses, large amounts of high-salt wastewater and high environmental load. Secondly, the precipitation method requires the consumption of a large amount of precipitant, especially the oxalic acid precipitation method, which requires large investment and high cost. In addition, the precipitation method introduces a large amount of impurities, a high chlorine content and a coarse particle size and severe agglomeration of the resulting rare earth oxides, resulting in low product quality and added value.

[0004] In response to the shortcomings of the precipitation method for preparing rare earth oxides, researchers have carried out a lot of work and made some progress. The more representative processes are combustion and spray pyrolysis. However, the combustion method uses rare earth nitrates as raw materials, heats them to 250-400°C, and then burns and decomposes them into corresponding rare earth oxides. The combustion method has a short process and relatively mild conditions, but the price of the rare earth nitrates used is relatively high, which makes the production cost high. At the same time, the treatment of the ammonia nitrogen tail gas produced is difficult and costly. Therefore, it is still in the research stage. The spray pyrolysis method uses rare earth chloride solution as raw material, sprays it into a high-temperature thermal field, causes it to undergo a pyrolysis reaction, and obtains rare earth oxides. This process is relatively simple, but rare earth chlorides are difficult to completely pyrolyze, resulting in a high chlorine content in the product. At the same time, the amount of water evaporation is large, resulting in high energy consumption and serious equipment corrosion.

[0005] In short, technicians have carried out a lot of work on the preparation of rare earth oxides and have made some progress, especially the pyrolysis process, which has distinct characteristics. However, this method has the disadvantages of low pyrolysis depth and high chlorine content in the product, resulting in low product quality. In addition, it has severe corrosion and high energy consumption, which leads to high requirements for process equipment. It still needs to be further improved. The industry urgently needs to develop a simple and low-cost method for producing low-chlorine rare earth oxides. Summary of the invention

[0006] Based on the above findings, a method for producing low-chlorine rare earth oxides with simple process, low cost and high efficiency is provided to address the shortcomings of the existing pyrolysis process for preparing rare earth oxides.

[0007] The pyrolysis process of rare earth chloride is a typical in-situ gas-solid reaction, that is, after water vapor and oxygen contact the rare earth chloride particles, diffusion and decomposition reactions occur. As the reaction proceeds, a coating layer formed by the product is formed on the surface of the particles, and the subsequent pyrolysis reaction will diffuse into the interior of the particles through water vapor and oxygen. Undoubtedly, this coating will hinder the contact of water vapor and oxygen with the rare earth chloride inside the particles, resulting in reduced efficiency or even stagnation of the pyrolysis reaction. Therefore, to achieve complete pyrolysis of rare earth chloride, it is necessary to increase the diffusion rate of water vapor and oxygen in the product layer. Based on this analysis, combined with a large number of experiments, the inventors found that the use of high-pressure and high-temperature reaction atmosphere can significantly improve the pyrolysis effect of rare earth chloride. Under high temperature and high pressure conditions, the diffusion rate of water vapor and oxygen in the product layer is accelerated, and the pyrolysis of rare earth chloride inside the particles is no longer affected by the product layer, so the pyrolysis effect is improved.

[0008] To this end, the present invention provides the following technical solutions:

[0009] A method for preparing low-chlorine rare earth oxides by pyrolysis of rare earth chlorides, using rare earth chlorides and alkali as raw materials and a high-pressure pyrolysis furnace as a reactor, pyrolyzing the rare earth chlorides under high temperature and high pressure conditions to obtain low-chlorine rare earth oxides and chloride byproducts, wherein during the pyrolysis process, the rare earth chlorides and the alkali are not in direct contact.

[0010] Preferably, the rare earth chloride is one or more crystalline salts of cerium, gadolinium, terbium, dysprosium, holmium, ytterbium, lutetium, scandium, yttrium, RECl 3 ·nH 2 O, n is less than or equal to 2, and greater than or equal to 0.1. When n is greater than 2, the rare earth chloride needs to be dehydrated.

[0011] Preferably, the alkali is an oxide or hydroxide of alkali metals Na, K and alkaline earth metals Ca, Mg.

[0012] Preferably, the molar ratio of the alkali to the rare earth chloride is 1.5 to 3.5:1.

[0013] Preferably, the high temperature and high pressure are a temperature greater than 350° C. and less than 600° C., and a pressure greater than 2.5 MPa and less than 3.5 MPa.

[0014] The chlorine content of the rare earth oxide described in the present invention is less than 50ppm.

[0015] Principles and advantages

[0016] The present invention accelerates the diffusion rate of water vapor and oxygen, eliminates the obstruction of the product layer to the pyrolysis reaction, and improves the pyrolysis efficiency and pyrolysis depth of rare earth chlorides by constructing a high-temperature and high-pressure reaction system, thereby realizing the preparation of low-chlorine rare earth oxides by pyrolysis, and outputting chlorine as a by-product of chloride, thereby realizing the resource utilization of rare earths and chlorine.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] (1) The present invention can significantly improve the pyrolysis efficiency and pyrolysis depth of rare earth chlorides by constructing a high-temperature and high-pressure reaction system.

[0019] (2) The process of the present invention is short, easy to operate, has low production cost, and is easy to realize industrialization.

[0020] (3) The rare earth oxide product obtained by the present invention has low chlorine content and high added value.

[0021] (4) The present invention is environmentally friendly, chlorine is produced in the form of chloride, and no waste gas, waste water or waste residue is generated. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1The XRD patterns of the pyrolysis products obtained in Example 1 and Comparative Example 1 of the present invention are shown in FIG.

[0023] Figure 2 (a) is the SEM image of the pyrolysis product of Example 2 and (b) is the result of the chlorine distribution energy spectrum scanning. Figure 3 (a) is the SEM image of the pyrolysis product of Comparative Example 2 and (b) is the result of the chlorine distribution energy spectrum scanning. Figure 4 (a) is the SEM image of the pyrolysis product of Comparative Example 3 and (b) is the result of the surface scanning of the chlorine distribution energy spectrum. DETAILED DESCRIPTION

[0024] The following examples are intended to further illustrate the present invention, rather than to limit the scope of the claims of the present invention.

[0025] Embodiment 1:

[0026] First, add 0.5 kg of cerium chloride (CeCl 3 1.2H 2 O), 0.18kg CaO. Then close the high-pressure pyrolysis furnace and raise the temperature to 360℃. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 2.5-2.6MPa, and keep the temperature and pressure for 80min. After the reaction is completed, cool to 80℃, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1# is the pyrolysis product of cerium chloride, and crucible 2# is the byproduct of calcium chloride. Sampling analysis shows that the sample in crucible 1# is pure CeO 2 , TREO>99.9%, chlorine content is only 21ppm.

[0027] Comparative Example 1:

[0028] First, add 0.5 kg of cerium chloride (CeCl 3 1.2H 2 O), 0.05kg CaO. Then close the high-pressure pyrolysis furnace and raise the temperature to 360℃. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 2.5-2.6MPa, and keep the temperature and pressure for 80min. After the reaction is completed, cool to 80℃, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1# is the pyrolysis product of cerium chloride, and crucible 2# is the by-product of calcium chloride. Sampling analysis shows that the sample in crucible 1# is CeCl 3 and CeO 2 The mixture has a chlorine content of up to 17.2%.

[0029] Figure 1 The XRD patterns of the pyrolysis products obtained in Example 1 and Comparative Example 1 of the present invention are shown in FIG. Figure 1 It can be seen that the product obtained in Example 1 is a single phase CeO2 , with sharp peaks and no impurities; the product obtained in Comparative Example 1 is CeO 2 and CeCl 3 ·H 2 O mixture, indicating that cerium chloride was not completely thermally decomposed under this condition.

[0030] It can be seen from Example 1 and Comparative Example 1 that a sufficient amount of alkali must be introduced into the reaction system to ensure the smooth progress of the pyrolysis reaction.

[0031] Embodiment 2:

[0032] First, add 0.5 kg of yttrium chloride (YCl 3 1.6H 2 O), 0.30kg Ca(OH) 2 . Then close the high-pressure pyrolysis furnace and raise the temperature to 500°C. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 3.1-3.2MPa, and keep the temperature and pressure for 70 minutes. After the reaction is completed, cool to 80°C, release the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1 is the pyrolysis product of yttrium chloride, and crucible 2 is the by-product of calcium chloride. Sampling analysis shows that the sample in crucible 1 is pure Y 2 O 3 , TREO>99.9%, chlorine content is only 29ppm.

[0033] Comparative Example 2:

[0034] First, add 0.5 kg of yttrium chloride (YCl 3 0.04H 2 O), 0.30kg Ca(OH) 2 . Then close the high-pressure pyrolysis furnace and raise the temperature to 500°C. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 3.1-3.2MPa, and keep the temperature and pressure for 70 minutes. After the reaction is completed, cool to 80°C, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1 is the pyrolysis product of yttrium chloride, and crucible 2 is the by-product of calcium chloride. Sampling analysis shows that the sample in crucible 1 is YCl 3 and Y 2 O 3 The mixture has a chlorine content of up to 0.78%.

[0035] Comparative Example 3:

[0036] First, add 0.5 kg of yttrium chloride (YCl 3 4.5H 2 O), 0.30kg Ca(OH) 2. Then close the high-pressure pyrolysis furnace and raise the temperature to 500°C. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 3.1-3.2MPa, and keep the temperature and pressure for 70 minutes. After the reaction is completed, cool to 80°C, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1 is the pyrolysis product of yttrium chloride, and crucible 2 is the by-product of calcium chloride. Sampling analysis shows that the sample in crucible 1 is YCl 3 and Y 2 O 3 The mixture has a chlorine content of up to 0.37%.

[0037] Figure 2 The SEM image (a) and the chlorine distribution spectrum scanning result image (b) of the pyrolysis product of Example 2 are shown. Figure 2 It can be seen from a that the pyrolysis products are spherical particles with abundant pores on the surface. Figure 2 b It can be seen that no chlorine is detected in the obtained pyrolysis product, indicating that yttrium chloride has been completely pyrolyzed under this condition and converted into yttrium oxide.

[0038] Figure 3 The SEM image (a) and the chlorine distribution spectrum scanning result image (b) of the pyrolysis product of comparative example 2 are shown. Figure 3 It can be seen from a that the pyrolysis products are spherical particles with a dense surface and no obvious holes. Figure 3 b It can be seen that a large amount of chlorine can still be scanned in the obtained pyrolysis product, indicating that yttrium chloride is not completely pyrolyzed under this condition, and yttrium chloride or yttrium oxychloride is still present in the product.

[0039] Figure 4 The SEM image (a) and the chlorine distribution spectrum scanning result image (b) of the pyrolysis product of Comparative Example 3 are shown. Figure 4 From a, we can see that the pyrolysis products are spherical particles with a small amount of holes. Figure 4 b It can be seen that a large amount of chlorine can still be detected in the obtained pyrolysis product, indicating that yttrium chloride is not completely pyrolyzed under this condition, and yttrium chloride or yttrium oxychloride is still present in the product.

[0040] It can be seen from Example 2 and Comparative Examples 2 and 3 that when the crystal water content of the rare earth chloride is too low or too high, deep pyrolysis of the rare earth chloride cannot be achieved.

[0041] Embodiment 3:

[0042] First, add 0.5 kg of ytterbium chloride (YbCl) into the 1# and 2# corundum crucibles respectively. 3 0.8H 2O), 0.23kg NaOH. Then close the high-pressure pyrolysis furnace and raise the temperature to 480℃. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 3.0-3.1MPa, and keep the temperature and pressure for 100min. After the reaction is completed, cool to 80℃, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1# is the pyrolysis product of ytterbium chloride, and crucible 2# is the by-product of sodium chloride. Sampling analysis shows that the sample in crucible 1# is pure Yb 2 O 3 , TREO>99.9%, chlorine content is only 33ppm.

[0043] Embodiment 4:

[0044] First, add 0.5 kg of cerium chloride (CeCl 3 0.3H 2 O), 0.23kg KOH. Then close the high-pressure pyrolysis furnace and raise the temperature to 380℃. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 2.6-2.7MPa, and keep the temperature and pressure for 110min. After the reaction is completed, cool to 80℃, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1# is the pyrolysis product of cerium chloride, and crucible 2# is the by-product of potassium chloride. Sampling analysis shows that the sample in crucible 1# is pure CeO 2 , TREO>99.9%, chlorine content is only 36ppm.

[0045] Embodiment 5:

[0046] First, add 0.5 kg of gadolinium chloride (GdCl) into the 1# and 2# corundum crucibles respectively. 3 1.1H 2 O), 0.12kg MgO. Then close the high-pressure pyrolysis furnace and raise the temperature to 450℃. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 2.9-3.0MPa, and keep the temperature and pressure for 90min. After the reaction is completed, cool to 80℃, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible, of which crucible 1# is the pyrolysis product of gadolinium chloride, and crucible 2# is the by-product of magnesium chloride. Sampling analysis shows that the sample in crucible 1# is pure Gd 2 O 3 , TREO>99.9%, chlorine content is only 34ppm.

[0047] Embodiment 6:

[0048] First, add 0.5 kg of yttrium chloride (YCl 3 0.7H 2O), 0.25kg CaO. Then close the high-pressure pyrolysis furnace and raise the temperature to 540℃. When the temperature reaches the target value, introduce oxygen to make the reaction pressure reach 3.3-3.4MPa, and keep the temperature and pressure for 60min. After the reaction is over, cool to 80℃, unload the pressure to normal pressure, open the pyrolysis furnace, and take out the crucible. Crucible 1# is the pyrolysis product of yttrium chloride, and crucible 2# is the byproduct of calcium chloride. Sampling analysis shows that the sample in crucible 1# is pure Y 2 O 3 , TREO>99.9%, chlorine content is only 39ppm.

Claims

1. A method for preparing low-chlorine rare earth oxides by pyrolysis of rare earth chlorides, characterized in that: With rare earth chloride and alkali as raw materials and a high-pressure pyrolysis furnace as a reactor, the rare earth chloride is pyrolyzed under high temperature and high pressure conditions to obtain low-chlorine rare earth oxides and chloride by-products, wherein the rare earth chloride and the alkali are not in direct contact during the pyrolysis process.

2. The method according to claim 1, characterized in that: The rare earth chloride is one or more crystalline salts of cerium, gadolinium, terbium, dysprosium, holmium, ytterbium, lutetium, scandium and yttrium, RECl3·nH2O, where n is less than or equal to 2 and greater than or equal to 0.1; when n is greater than 2, the rare earth chloride needs to be dehydrated.

3. The method according to claim 1, characterized in that: The alkali is an oxide or hydroxide of an alkali metal Na, K and an alkaline earth metal Ca, Mg.

4. The method according to claim 1, characterized in that: The molar ratio of the alkali to the rare earth chloride is 1.5 to 3.5:

1.

5. The method according to claim 1, characterized in that: The high temperature and high pressure means that the temperature is greater than 350° C. and less than 600° C.; the pressure is greater than 2.5 MPa and less than 3.5 MPa.

6. The method according to claim 1, characterized in that: The rare earth oxide has a chlorine content of less than 50 ppm.

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