Method for preparing mixed acid of hydrofluoric acid and fluosilicic acid through desulfurization and concentration of rare earth mineral sulfuric acid slurrying decomposition tail gas

Abstract

The invention relates to the technical field of rare earth mineral wet-process smelting, in particular to a method for preparing mixed acid of hydrofluoric acid and fluosilicic acid through desulfurization and concentration of rare earth mineral sulfuric acid slurrying decomposition tail gas. The method comprises the following steps: enabling tail gas generated by a rare earth mineral sulfuric acid slurrying decomposition reaction to sequentially pass through a carbon fiber demister, a condenser and an alkali spray tower to obtain mixed acid of hydrofluoric acid and fluosilicic acid; and the condensing temperature of the condenser is less than or equal to 10 DEG C. Sulfuric acid mist and water mist in the tail gas generated in the rare earth mineral sulfuric acid slurrying decomposition reaction are separated preferentially and flow back to a reaction system, purification of the mixed acid of hydrofluoric acid and fluosilicic acid can be achieved, the content of sulfuric acid in the mixed acid is reduced, the concentration of the mixed acid of hydrofluoric acid and fluosilicic acid is increased to be larger than or equal to 20%, and the fluorine recovery rate is larger than or equal to 95%; reflux of sulfuric acid mist and water mist can also reduce sulfuric acid consumption of the reaction. The acid mist and water mist trapping efficiency of the carbon fiber demister can be larger than or equal to 99% by limiting the diameter of the carbon fibers to be smaller than or equal to 1 micrometer.

Description

Technical Field

[0001] The invention relates to the technical field of wet smelting of rare earth minerals, and in particular to a method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate slurry and desulfurizing and concentrating it. Background Art

[0002] The mixed rare earth concentrate in Bayan Obo is composed of fluorocarbon cerium ore and monazite ore. The decomposition process of the mixed rare earth concentrate mainly adopts the concentrated sulfuric acid high temperature roasting decomposition process and the concentrated alkali normal pressure decomposition process. The concentrated sulfuric acid high temperature roasting process evenly mixes the mixed rare earth concentrate with concentrated sulfuric acid and iron powder, and then roasts it in a rotary kiln at a high temperature of about 500-700°C. The kiln head produces a mixed tail gas containing SO2, SO3, HF, SiF4, sulfuric acid mist, mineral dust, etc., with a large gas volume, high temperature, and complex composition. Generally, multi-stage spraying is used to form a mixed acid of sulfuric acid, hydrofluoric acid and fluorosilicic acid, and then heated and steamed to recover sulfuric acid and a mixed acid of hydrofluoric acid and fluorosilicic acid. The tail gas treatment process is long and energy-intensive, and the output fluorosilicic acid concentration is low, generally less than 12%, and contains a small amount of sulfuric acid impurities.

[0003] Patent CN109022838A discloses a method for treating fluorine-containing rare earth mineral particles, wherein the fluorine-containing rare earth mineral particles are mixed with a sulfuric acid solution in a certain proportion, and then heated and insulated for liquid-solid reaction, and the generated tail gas is condensed and absorbed by the tail gas system to obtain a mixed acid product of hydrofluoric acid and fluosilicic acid. This method can achieve rapid decomposition of fluorine-containing rare earth mineral particles, the reaction is easy to control, and the mixed acid product of hydrofluoric acid and fluosilicic acid can be recovered, but at a reaction temperature of 100-180°C, sulfuric acid mist will enter the tail gas system, so that the content of sulfate in the mixed acid product of hydrofluoric acid and fluosilicic acid obtained by direct condensation of the tail gas is high and the concentration is low, which affects the downstream application of the product. Summary of the invention

[0004] Based on the above content, the present invention provides a method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate pulp and desulfurizing and concentrating. The mass concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid prepared by the method of the present invention is ≥20%, and the concentration of sulfuric acid in the mixed acid of hydrofluoric acid and fluorosilicic acid is ≤0.05%.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] The invention provides a method for preparing a mixed acid of hydrofluoric acid and fluosilicic acid by desulfurizing and concentrating tail gas from sulfuric acid pulping decomposition of rare earth minerals. The method comprises the following steps: mixing rare earth concentrate with sulfuric acid solution, carrying out a pulping decomposition reaction, wherein the reaction temperature is between 100°C and 180°C, wherein the concentration of the sulfuric acid solution is 40-85wt%, and the tail gas generated by the sulfuric acid pulping decomposition reaction of the rare earth minerals is sequentially passed through a carbon fiber demister with a fiber diameter of ≤1 micron, a condenser and an alkali spray tower, and then discharged in compliance with the standards; sulfuric acid mist and water mist removed by the carbon fiber demister are refluxed to the pulping reaction system, and condensed to obtain a mixed acid of hydrofluoric acid and fluosilicic acid with a concentration of ≥20wt%, wherein the sulfuric acid content is ≤0.05wt%; and the condensation temperature of the condenser is ≤10°C.

[0007] Carbon fiber demisters are generally used to remove water mist. The present invention finds that limiting the diameter of demister fibers to ≤1 micron can effectively improve the removal efficiency of sulfuric acid mist. The sulfuric acid mist and water mist removed by the carbon fiber demister are returned to the pulping reaction system.

[0008] In some embodiments of the present invention, the tail gas includes HF, SiF4, water and sulfuric acid mist.

[0009] In some embodiments of the present invention, the content of SiF4 in the tail gas is 1wt% to 5wt%; the content of F in the tail gas is 5wt% to 15wt%.

[0010] In some embodiments of the present invention, the condenser is a graphite tube condenser or a silicon carbide tube condenser.

[0011] In some embodiments of the present invention, the alkali solution used in the alkali spray tower is sodium hydroxide or calcium hydroxide.

[0012] The mixed acid of hydrofluoric acid and fluorosilicic acid can be used to produce ammonium bifluoride or rare earth fluoride. The above applications generally require that the concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid is ≥20%. If the concentration is low, the process of producing ammonium bifluoride or rare earth fluoride is difficult to control, the wastewater output is large, and the mass concentration of sulfuric acid in the mixed acid of hydrofluoric acid and fluorosilicic acid is ≤0.05%. Therefore, the direct preparation of the mixed acid of hydrofluoric acid and fluorosilicic acid that meets the requirements of downstream applications is of great significance for practical production applications.

[0013] The present invention preferentially separates sulfuric acid mist and water mist in the tail gas generated by the sulfuric acid pulp decomposition reaction of rare earth minerals and refluxes them to the reaction system, which can achieve the purification of the mixed acid of hydrofluoric acid and fluosilicic acid, reduce the sulfuric acid content therein, and increase the concentration of the mixed acid of hydrofluoric acid and fluosilicic acid by ≥20%, and the fluorine recovery rate is ≥95%. The reflux of sulfuric acid mist and water mist can also reduce the sulfuric acid consumption of the reaction. The carbon fiber demister has high strength and corrosion resistance at 100°C to 180°C, and can be effectively applied to the treatment of tail gas from the sulfuric acid pulp decomposition of rare earth minerals. In the present invention, by limiting the carbon fiber diameter to ≤1 micron, the carbon fiber demister can achieve a capture efficiency of ≥99% for acid mist and water mist.

[0014] The invention allows tail gas containing HF, SiF4, water and sulfuric acid mist at 100° C. to 180° C. to be passed through a carbon fiber demister to preferentially remove sulfuric acid mist and water mist, and then condenses HF, SiF4 and water vapor into a mixed acid of hydrofluoric acid and fluosilicic acid with a concentration of ≥20%, thereby achieving impurity removal and concentration of the mixed acid of hydrofluoric acid and fluosilicic acid.

[0015] The present invention discloses the following technical effects:

[0016] The present invention allows the tail gas to pass through a carbon fiber demister first, thereby achieving impurity removal and purification of a mixed acid of hydrofluoric acid and fluorosilicic acid, reducing the sulfuric acid content therein, reducing the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid from 1% to 5% to ≤0.05%, and increasing the concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid to ≥20%. The sulfuric acid mist and water mist separated by the carbon fiber demister can flow back to the reaction system, thereby reducing the sulfuric acid consumption of the reaction.

[0017] The condensation temperature in the condenser is ≤10°C, which is lower than the boiling point of the mixed acid of hydrofluoric acid and fluosilicic acid, so that the mixed acid of hydrofluoric acid and fluosilicic acid can be quickly condensed from gaseous state to liquid state, thereby improving the condensation efficiency and recovery rate of the mixed acid of hydrofluoric acid and fluosilicic acid, the fluorine recovery rate is ≥95%, and the obtained mixed acid of hydrofluoric acid and fluosilicic acid is not easy to volatilize again.

[0018] Graphite tube condensers or silicon carbide tube condensers have the characteristics of large heat exchange area and high heat exchange efficiency, which helps to improve the condensation efficiency of the mixed acid of hydrofluoric acid and fluosilicic acid.

[0019] The alkali spray tower can absorb a small amount of uncondensed acidic gas in the tail gas, thus achieving standard tail gas emission. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 The present invention is a process flow chart for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate slurry and desulfurizing and concentrating the tail gas. DETAILED DESCRIPTION

[0022] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0023] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0024] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0025] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0026] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0027] The "%" described in the present invention, unless otherwise specified, refers to mass percentage.

[0028] The present invention provides a method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate pulp and desulfurizing and concentrating. The process flow chart is as follows: Figure 1 shown.

[0029] The technical solutions described in the present invention, unless otherwise specified, are all conventional solutions in the art, and the reagents or raw materials used, unless otherwise specified, are purchased from commercial channels or have been disclosed.

[0030] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0031] Example 1

[0032] 10kg of rare earth concentrate is mixed with 40kg of sulfuric acid solution to carry out pulping decomposition reaction, the reaction temperature is 100℃, the concentration of sulfuric acid solution is 40wt%, and the tail gas produced is a mixture of HF, SiF4, water and sulfuric acid mist, wherein the sulfuric acid mist content is 1% and the F content is 5%. The tail gas is sequentially treated by a carbon fiber demister with a fiber diameter of 1 micron, a graphite tube condenser and a sodium hydroxide spray tower and then reaches the discharge standard. In this embodiment, the capture rate of acid mist and water mist by the carbon fiber demister is ≥99%, and the removed sulfuric acid mist and water mist are refluxed to the pulping reaction system. The condensation temperature of the graphite tube condenser is 10℃, the concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid obtained is 20%, the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid is 0.048%, and the fluorine recovery rate is 95%. The gas that is not condensed by the graphite tube condenser is treated by a sodium hydroxide spray tower and then reaches the discharge standard.

[0033] Example 2

[0034] 10kg of rare earth concentrate is mixed with 40kg of sulfuric acid solution to carry out pulping decomposition reaction. The reaction temperature is 140℃, wherein the concentration of sulfuric acid solution is 70wt%, and the tail gas produced is a mixture of HF, SiF4, water, and sulfuric acid mist, wherein the sulfuric acid mist content is 3% and the F content is 9%. The tail gas is sequentially treated by a carbon fiber demister with a fiber diameter of 1 micron, a silicon carbide tube condenser, and a sodium hydroxide spray tower and then discharged in compliance with the standards. In this embodiment, the carbon fiber demister has a capture efficiency of ≥99% for acid mist and water mist, and the removed sulfuric acid mist and water mist are refluxed to the pulping reaction system. The condensation temperature of the graphite condenser is 8℃, and the concentration of the obtained mixed acid of hydrofluoric acid and fluorosilicic acid is 22%, the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid is 0.05%, and the fluorine recovery rate is 96.1%. The gas that is not condensed by the graphite tube condenser is treated by a sodium hydroxide spray tower and then discharged in compliance with the standards.

[0035] Example 3

[0036] 10kg of rare earth concentrate is mixed with 40kg of sulfuric acid solution to carry out pulping decomposition reaction, the reaction temperature is 180℃, the concentration of sulfuric acid solution is 85wt%, and the tail gas produced is a mixture of HF, SiF4, water and sulfuric acid mist, wherein the sulfuric acid mist content is 5% and the F content is 15%. The tail gas is sequentially treated by a carbon fiber demister with a fiber diameter of 1 micron, a graphite tube condenser and a calcium hydroxide spray tower and then discharged in compliance with the standards. In this embodiment, the carbon fiber demister has a capture efficiency of ≥99% for acid mist and water mist, and the removed sulfuric acid mist and water mist are refluxed to the pulping reaction system. The condensation temperature of the graphite condenser is 5℃, the concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid obtained is 28%, the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid is 0.038%, and the fluorine recovery rate is 96.9%. The gas that is not condensed by the graphite tube condenser is treated by a sodium hydroxide spray tower and then discharged in compliance with the standards.

[0037] Comparative Example 1

[0038] 10kg of rare earth concentrate was mixed with 40kg of sulfuric acid solution for pulping and decomposition reaction. The reaction temperature was between 180℃, the concentration of sulfuric acid solution was 85wt%, and the tail gas produced was a mixture of HF, SiF4, water and sulfuric acid mist, wherein the sulfuric acid mist content was 5% and the F content was 15%. The tail gas was sequentially treated by a carbon fiber demister with a fiber diameter of 2 microns, a graphite tube condenser and a calcium hydroxide spray tower and then discharged in compliance with the standards. In this comparative example, the carbon fiber demister had a capture efficiency of 60% for acid mist and water mist, and the removed sulfuric acid mist and water mist were refluxed to the pulping reaction system. The condensation temperature of the graphite condenser was 5℃, the concentration of the obtained mixed acid of hydrofluoric acid and fluorosilicic acid was 18%, the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid was 1.95%, and the fluorine recovery rate was 95.2%. The gas that was not condensed by the graphite tube condenser was treated by a sodium hydroxide spray tower and then discharged in compliance with the standards.

[0039] Comparative Example 2

[0040] 10kg of rare earth concentrate was mixed with 40kg of sulfuric acid solution for pulping and decomposition reaction. The reaction temperature was between 180℃, the concentration of sulfuric acid solution was 85wt%, and the tail gas produced was a mixture of HF, SiF4, water and sulfuric acid mist, wherein the sulfuric acid mist content was 5% and the F content was 15%. The tail gas was sequentially treated by a carbon fiber demister with a fiber diameter of 2 microns, a graphite tube condenser and a calcium hydroxide spray tower and then discharged in compliance with the standards. In this comparative example, the carbon fiber demister had a capture efficiency of 60% for acid mist and water mist, and the removed sulfuric acid mist and water mist were refluxed to the pulping reaction system. The condensation temperature of the graphite condenser was 15℃, the concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid obtained was 16%, the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid was 1.84%, and the fluorine recovery rate was 79%. The gas that was not condensed by the graphite tube condenser was treated by a sodium hydroxide spray tower and then discharged in compliance with the standards.

[0041] By comparing Example 3 with Comparative Examples 1 and 2, it can be seen that when the fiber diameter of the carbon fiber demister is greater than 1 micron, the removal rate of sulfuric acid mist and water mist will be significantly reduced. The reduction in the removal rate of sulfuric acid mist will lead to an increase in the sulfuric acid content in the mixed acid of hydrofluoric acid and fluorosilicic acid, and the reduction in the water mist removal rate will lead to a decrease in the concentration of the mixed acid of hydrofluoric acid and fluorosilicic acid, affecting the quality and application of the mixed acid of hydrofluoric acid and fluorosilicic acid. When the condensation temperature of the condenser is greater than 10°C, the condensation effect of the condenser is significantly reduced, and part of the hydrogen fluoride cannot be condensed, resulting in a significant reduction in the recovery rate of fluorine.

[0042] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas by desulfurization and concentration of rare earth mineral slurry, wherein rare earth concentrate is mixed with sulfuric acid solution, and slurry decomposition reaction is carried out, the reaction temperature is between 100°C and 180°C, wherein the concentration of sulfuric acid solution is 40-85wt%, characterized in that: The following steps are involved: The tail gas generated by the sulfuric acid pulping decomposition reaction of rare earth minerals passes through a carbon fiber demister with a fiber diameter of ≤1 micron, a condenser and an alkali spray tower in sequence before being discharged in compliance with the emission standards. The sulfuric acid mist and water mist removed by the carbon fiber demister are returned to the pulping reaction system and condensed to obtain a mixed acid of hydrofluoric acid and fluorosilicic acid with a concentration of ≥20wt%, wherein the sulfuric acid content is ≤0.05wt%; the condensation temperature of the condenser is ≤10°C.

2. The method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate pulp according to claim 1, characterized in that: The tail gas includes HF, SiF4, water and sulfuric acid mist.

3. The method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate pulp according to claim 2, characterized in that: The content of SiF4 in the tail gas is 1wt% to 5wt%; the content of F in the tail gas is 5wt% to 15wt%.

4. The method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate pulp according to claim 1, characterized in that: The condenser is a graphite tube condenser or a silicon carbide tube condenser.

5. The method for preparing a mixed acid of hydrofluoric acid and fluorosilicic acid by decomposing tail gas from rare earth mineral sulfate pulp according to claim 1, characterized in that: The alkali solution used in the alkali spray tower is sodium hydroxide or calcium hydroxide.

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