Process for preparing sodium carbonate by defluorinating natural alkali

Abstract

The invention provides a process for preparing sodium carbonate by defluorinating natural alkali, and relates to the technical field of alkali products. According to the method disclosed by the invention, the calcium fluoride is enriched by taking concentrated first brine from raw brine of solution mining or mechanically mined trona ore as a raw material and carrying out two-stage causticization and calcination on fluorine-containing first discharge liquid which is separated and discharged by alkali decahydrate crystallization; when it is detected that the content of solids with the particle size larger than 3 mm in the lime milk obtained through primary causticization exceeds 5 wt.%, causticized mud obtained through secondary causticization is subjected to acid pickling and separation, and a calcium fluoride product is discharged. According to the method, new elements do not need to be introduced, the fluorine element in the raw material feed can be efficiently enriched and separated while the high-quality sodium carbonate product is obtained, the calcium fluoride product and the caustic soda solution with economic values are produced as byproducts, and the calcium fluoride product and the caustic soda solution with economic values are produced as byproducts. The raw materials are wide in applicability and have important popularization and application values.

Description

Technical Field

[0001] This invention relates to the field of alkali product technology, specifically to a process for preparing soda ash by defluorinating natural alkali. Background Technology

[0002] Natural soda ash is a mineral primarily derived from natural soda ash mines or evaporative salts from natural soda ash lakes. It is hydrated sodium bicarbonate and is commonly used to produce soda ash or baking soda. Common methods for mining soda ash mines include water-based mining and mechanical tunneling. Water-based mining yields sodium carbonate and sodium bicarbonate solutions, which are then filtered, stripped, evaporated, and concentrated to obtain concentrated brine. Alternatively, mechanical tunneling of natural soda ash mines, followed by crushing, calcining, dissolving, filtering, and clarification, yields brine primarily containing sodium carbonate. Natural soda ash lake water can be spread and dried in salt fields to obtain natural soda ash brine. The brine obtained through these processes can be used to produce soda ash or baking soda through one or more of the following operations: cooling crystallization, evaporation crystallization, and carbonation crystallization.

[0003] In the actual mining and utilization of natural soda ash, the extracted soda ash may contain impurities originally associated with the soda ash ore, such as NaCl, Na₂SO₄, and NaF. While most natural soda ash has a low fluoride content, there are still cases where the sodium fluoride content in low-grade natural soda ash ore or lake water ranges from 0.5 wt.% to 3 wt.%. When the sodium fluoride content in the natural soda ash ore exceeds 0.3 wt.% (dry basis), it will affect the quality of the final heavy soda ash and requires special treatment. Since sodium fluoride decomposes at 700℃ to produce hydrogen fluoride gas, using soda ash with high fluoride content in certain high-temperature applications can negatively impact the environment and human safety. Therefore, fluoride removal should be considered during the natural soda ash production process.

[0004] Patent US3980754 describes dissolving alkali metal carbonates in a magnesium bicarbonate solution or suspension to generate magnesium fluoride precipitate for removing fluoride ions from the carbonates. However, this method not only introduces new magnesium ions into the system, but the added magnesium also reacts with carbonate ions to form magnesium carbonate precipitate, resulting in carbonate loss.

[0005] Patent GB1489781A uses the method of evaporating and crystallizing a sodium carbonate solution containing sodium fluoride to generate sodium carbonate monohydrate crystals, and then separating the sodium carbonate monohydrate crystals and the mother liquor containing sodium fluoride. Because the solubility of sodium fluoride does not change significantly with temperature, sodium fluoride may also precipitate during the evaporation and concentration process, leading to poor crystallization of sodium carbonate monohydrate crystals. Therefore, this patent cannot effectively remove sodium fluoride in actual processes.

[0006] Patent CN104445286A uses a process of calcining, dissolving, and low-temperature crystallizing sodium carbonate decahydrate, followed by alkali evaporation and crystallization of sodium carbonate monohydrate to obtain heavy soda ash. Although cooling crystallization in this process can effectively ensure that the amount of sodium fluoride contained in the crystallized sodium carbonate decahydrate is low, the crystallization mother liquor rich in NaF will be recycled back to the dissolution section, resulting in the enrichment of fluoride ions. Therefore, in the actual process, it is necessary to remove some of the crystallization mother liquor and further process it in subsequent stages to remove fluoride ions. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention discloses a process for preparing soda ash by defluorinating natural alkali. This process does not require the introduction of new metal ions, does not discharge harmful pollutants such as sodium fluoride, and can efficiently remove fluoride from sodium carbonate brine to obtain high-quality soda ash with a sodium fluoride content of ≤0.05wt.% and produce calcium fluoride as a byproduct.

[0008] To achieve the above technical objectives, this invention proposes a process for preparing soda ash by defluorinating natural alkali, comprising:

[0009] The raw brine obtained from leaching or mechanically mined natural alkali ore is pretreated to obtain concentrated first brine; the first brine undergoes decahydrate alkali crystallization and separation processes to discharge a first effluent containing fluoride, which is then subjected to the following operations:

[0010] First-stage causticizing process: A portion of the first discharge liquid comes into contact with quicklime to obtain lime slurry; the lime slurry is screened to discharge solid particles with a particle size ≥1cm; lime residue with a particle size ≥3mm is screened out, and the lime residue is crushed and sent back to the lime slurry;

[0011] Secondary causticizing process: The ash slurry is contacted with another portion of the first discharge liquid, and after clarification and separation, a dilute caustic soda solution and causticized mud are obtained;

[0012] Calcination process: The solid particles and the causticized mud are calcined to obtain quicklime containing calcium fluoride, which is then fed into the first-stage causticization process;

[0013] The process further includes: when the sum of the mass of the solid particles and the ash exceeds 5 wt.% of the mass of the first effluent, the causticized mud is acid-washed and separated to output calcium fluoride product.

[0014] The above technical solution can efficiently enrich and remove fluorine from raw materials. Specifically: 1) The first discharge liquid from the decahydrate alkali crystallization section is a fluorine-containing solution. In the first-stage causticizing process, fluorine reacts with quicklime to obtain insoluble calcium fluoride. The solid particles discharged after sieving mainly include CaF2 and CaCO3. These solid particles will be calcined to obtain quicklime. The ash residue obtained from sieving mainly includes CaO, CaF2, and Ca(OH)2, etc., which will be crushed and then enter the second-stage causticizing process. In the second-stage causticizing process, the causticizing reaction is further promoted. The resulting causticized mud is calcined to obtain quicklime containing calcium fluoride. By using this part of the causticizing mud to produce quicklime, the fluorine can be further processed into quicklime. Quicklime is fed into the first-stage causticizing process, thereby converting fluorine in the first effluent into calcium fluoride, which is then continuously enriched in the first and second-stage causticizing processes. 2) Through extensive process practice, the research team of this invention discovered that when the solid content (total mass of solid particles and ash residue) in the slurry obtained from the first-stage causticizing process exceeds 5 wt.% of the slurry (the slurry initially obtained from the first-stage causticizing process, i.e., the slurry before solid particle removal and ash residue screening), it indicates that the calcium fluoride contained in the slurry has been enriched to a high content. At this point, the causticized mud obtained from the second-stage causticizing process can be acid-washed to remove calcium oxide and separate solid calcium fluoride products. This achieves the enrichment and removal of fluorine in the overall process route, while also producing economically valuable calcium fluoride products and caustic soda solution as byproducts.

[0015] Understandably, the technical solution of the present invention can enrich the fluorine element contained in the raw materials and finally discharge it from the process system. Therefore, the process of preparing soda ash by defluorination of natural alkali of the present invention has a wide range of raw material applicability. It is not only applicable to raw materials with high sodium fluoride content (such as sodium fluoride content greater than 0.3 wt% in the first brine), but also applicable to the process of preparing soda ash from raw materials containing a small amount of fluorine.

[0016] In a further example of the present invention, the causticized sludge may be acid-washed and separated when the sum of the masses of the solid particles and the ash exceeds 10 wt.% of the mass of the first effluent, and calcium fluoride product may be output.

[0017] In a further example of the present invention, the amount of the first effluent used in the primary and secondary causticizing processes was explored and optimized. Optionally, the mass ratio of the first effluent used in the primary and secondary causticizing processes is 1:(2-6), thereby using a relatively small amount of the first effluent in the primary causticizing process for the slaking of quicklime and the formation of calcium fluoride and the discharge of solid particles; and carrying out a complete causticizing process in the further secondary causticizing process to obtain causticized mud and a clarified dilute caustic soda solution. In an optional example of the present invention, the mass ratio of the first effluent used in the primary and secondary causticizing processes is 1:(3-4).

[0018] In a further example of the present invention, the crystallization and separation of the decahydrate alkali further includes dissolving the decahydrate alkali, evaporating and crystallizing the monohydrate alkali and separating it, and drying the monohydrate alkali to obtain the soda ash product. The technical solution of the present invention can obtain high-quality natural soda ash products. The first brine obtained after pretreatment of the raw brine from leaching or mechanically mined natural soda ash ore is a brine nearly saturated with sodium carbonate. The decahydrate alkali crystals precipitated from this first brine through the decahydrate alkali crystallization section do not contain sodium fluoride. Subsequently, after separation, dissolution, and crystallization and drying of the monohydrate alkali, high-quality heavy soda ash is obtained, with the sodium fluoride content in the soda ash ≤0.05wt.%.

[0019] In a further example of the invention, a diluent is added to the primary causticizing step. Process practice has confirmed that adding a diluent to the first effluent to dilute its sodium carbonate concentration for primary causticizing, followed by a secondary causticizing step using another portion of the effluent (with a relatively higher sodium carbonate concentration), can increase the settling velocity of the subsequent causticized sludge and improve efficiency. Optionally, the diluent may be derived from process condensate and / or industrial water. Co-utilization of process condensate (such as the condensate provided by the primary alkali evaporation, crystallization, and separation step) can reduce process energy consumption and improve resource utilization efficiency.

[0020] In a further example of the present invention, the primary causticizing process further includes pre-washing the ash residue before crushing it, thereby improving operability. Optionally, the wash water is input into the primary causticizing process to fully recover and utilize the effective components in the first effluent. Optionally, the wash water comes from process condensate and / or industrial water, which can reduce the fluoride content in the obtained soda ash product, reduce process energy consumption, and improve resource utilization.

[0021] Optionally, the input of the first-stage causticized quicklime may also include purchased quicklime products, which may contain a small amount of silicon dioxide. Therefore, in some optional examples of the present invention, the solid particles may also contain a small amount of SiO2.

[0022] In a further example of the present invention, the concentration of the dilute caustic soda solution obtained in the secondary causticizing process is 5 wt.% to 13 wt.%, and this portion of the dilute caustic soda solution can be applied to other process steps as needed.

[0023] In a further example of the present invention, the dilute caustic soda solution is concentrated by evaporation to remove sodium chloride and / or sodium sulfate, yielding a concentrated caustic soda solution. Optionally, the concentration of the concentrated caustic soda is 20 wt.% to 30 wt.%, and preferably 28 wt.% to 30 wt.% according to market needs. Optionally, the concentrated caustic soda solution is obtained by multi-effect evaporation of the dilute caustic soda solution. When soda ash is prepared from the extracted brine, the first brine contains some sodium bicarbonate, which needs to be neutralized before the crystallization and separation of decahydrate soda; optionally, the concentrated caustic soda is used to neutralize the first brine.

[0024] It should be noted that the present invention does not limit the equipment used for the primary and secondary causticization processes. Any equipment capable of performing quicklime slaking and causticization operations is acceptable, such as a quicklime slaking machine. This does not limit the scope of protection of the present invention.

[0025] In a further example of the present invention, the calcination process further includes pre-washing the causticized mud to remove soluble substances adhering to its surface. Optionally, the wash water comes from process condensate and / or industrial water. Optionally, the wash water is input into the primary causticization process to fully recover the washed-off soluble components. Optionally, when soda ash is prepared from natural soda ash ore, the wash water can be used as a dissolving solution in the pretreatment process.

[0026] In a further example of the present invention, the temperature of the calcination process is 800-1100°C. At this calcination temperature, it is beneficial to fully decompose calcium carbonate to obtain quicklime while avoiding the high-temperature decomposition of calcium fluoride, thus obtaining quicklime containing calcium fluoride.

[0027] Based on the above technical solution, there are no restrictions on the equipment used for the calcination process; any equipment capable of high-temperature decomposition of calcium carbonate to prepare quicklime is acceptable. For example, the calcination process can be carried out in a rotary kiln.

[0028] In a further example of the present invention, the acid washing operation uses hydrochloric acid to acid wash the causticized mud. The main components of the causticized mud are calcium carbonate and calcium fluoride. The calcium carbonate in the causticized mud is dissolved by acid washing with hydrochloric acid to obtain calcium fluoride solid and calcium chloride mother liquor.

[0029] In a further example of the invention, the separated calcium fluoride is washed with wash water derived from process condensate and / or industrial water, thereby improving the purity of the resulting calcium fluoride product. Recycling the process condensate reduces the fluoride content in the resulting soda ash product, lowering process energy consumption and improving resource utilization efficiency. Optionally, the wash water is input into the primary causticizing process, returning calcium chloride to the causticizing process to generate calcium carbonate and sodium chloride. This sodium chloride impurity may exist in the dilute caustic soda solution and be discharged during subsequent concentration treatment.

[0030] In a further example of the present invention, the Na2CO3 concentration in the first brine is 23wt.% to 33wt.%. First brine meeting this concentration condition can be input into the decahydrate alkali crystallization and separation process to obtain decahydrate alkali through crystallization. Optionally, the decahydrate alkali crystallization is low-temperature crystallization; further optionally, an ice machine system is used to provide cooling for the decahydrate alkali crystallization; further optionally, the compressor outlet condenser in the ice machine system is a two-stage series condenser, wherein the first-stage condenser utilizes a decahydrate alkali solution containing decahydrate alkali crystals as a cooling source.

[0031] In a further example of the present invention, the decahydrate alkali is dissolved to obtain a second brine. The total alkalinity of the second brine, calculated as Na2CO3, is 23wt.% to 30wt.%. The second brine that meets this concentration range can be evaporated and crystallized to obtain monohydrate alkali. Subsequently, monohydrate alkali crystals and monohydrate alkali crystallization mother liquor are obtained by separation. Then, the monohydrate alkali product is obtained by fluidized bed drying, crushing, sieving and other operations.

[0032] In a further example of the present invention, the sodium fluoride content in the soda ash product is no more than 0.05 wt.%.

[0033] In a further example of the present invention, the specific operations of the preprocessing operation were explored and optimized.

[0034] Optionally, when using the extracted raw brine as a raw material to prepare soda ash, the pretreatment operation includes: filtering the raw brine and performing at least two stages of stripping to obtain the first brine. Further optionally, the condensate from the stripping operation can be used in other stages of the process.

[0035] Optionally, when preparing soda ash using natural soda ash ore as raw material, the pretreatment operation includes: crushing and calcining the natural soda ash ore, followed by dissolution and clarification to obtain the first brine. Further, the calcination temperature of the natural soda ash ore is 180–200°C, and the calcination time is 0.5–1 hour, yielding crude soda ash. By optimizing the calcination conditions of the natural soda ash ore, the sodium bicarbonate in the ore is completely decomposed into sodium carbonate and carbon dioxide. Optionally, the carbon dioxide tail gas generated in the calcination furnace can be recovered as a carbon dioxide product after washing. Further, the calcined product is dissolved in a solution at 30–70°C to obtain a nearly saturated sodium carbonate solution. This sodium carbonate solution can be optionally input into a clarification tank for clarification, and the overflowing first brine is used for subsequent treatment processes. The impurities obtained from clarification can be treated as solid waste and discharged periodically. Further, the dissolving liquid used to dissolve the calcined product is at least one of process condensate, industrial water, or the first discharge liquid.

[0036] In a further example of the present invention, a second effluent is discharged during the evaporation and crystallization of the monohydrate alkali, thereby reducing the sodium chloride and / or sodium sulfate content in the mother liquor. Optionally, the second effluent is input into the decahydrate alkali crystallization and separation process. Optionally, when soda ash is prepared from natural alkali ore, the second effluent is input as a dissolving solution into the pretreatment process.

[0037] In a further example of the present invention, in the decahydrate alkali dissolution process, the decahydrate alkali crystals are mixed with a dissolving solution to dissolve the crystals and obtain a sodium carbonate solution with very low fluorine content. The dissolving solution used is at least one of process condensate and monohydrate alkali crystallization mother liquor.

[0038] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention uses concentrated first brine from leached brine or mechanically mined natural soda ash as raw material to obtain soda ash with a sodium fluoride content ≤0.05 wt.%; furthermore, this invention performs primary and secondary causticization on the fluoride-containing first discharge liquid discharged from the crystallization of decahydrate soda ash and quicklime, and then converts the causticized mud into quicklime containing calcium fluoride through calcination and uses it in the primary causticization process, thereby achieving the purpose of enriching fluoride in the first discharge liquid; this invention also detects the content of solids with a particle size greater than or equal to 3 mm in the lime slurry obtained from the primary causticization process. When the content of such solids exceeds 5 wt.% of the mass of the lime slurry, calcium fluoride product is output through acid washing and separation of the causticized mud. Therefore, the process of this invention does not require the introduction of new elements, and while obtaining high-quality soda ash product, it can efficiently enrich and separate fluoride in the raw material feed, and by-products of economically valuable calcium fluoride product and caustic soda solution; the raw material applicability of the technical solution of this invention is wide and has important promotion and application value. Attached Figure Description

[0039] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0040] Figure 1 This invention illustrates a process for preparing soda ash by defluorinating natural alkali according to the present invention. Detailed Implementation

[0041] To facilitate understanding of the present invention, a more comprehensive description will be provided below, along with preferred embodiments. However, it should be understood that these embodiments are merely for more detailed explanation and should not be construed as limiting the invention in any way, i.e., not intended to limit the scope of protection of the invention.

[0042] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0043] Example 1

[0044] A process for preparing soda ash by defluorination of natural alkali includes the following steps:

[0045] Pretreatment: The obtained natural alkali ore is calcined at 180–200℃ for 0.5–1 h; the crude alkali obtained from calcination is dissolved in a solution at 30–70℃ to obtain a nearly saturated sodium carbonate solution, wherein the concentration of Na₂CO₃ is 25 wt.%–35 wt.% and the concentration of NaF is 0.3 wt.%–0.5 wt.%. The resulting alkali solution is sent to a clarification tank for clarification, and the overflow yields the first brine. The main components of this first brine are: Na₂CO₃: 22 wt.%–30 wt.%, NaCl: 2 wt.%–2.5 wt.%, Na₂SO₄: 0.1 wt.%–0.15 wt.%, and NaF: 0.3 wt.%–0.5 wt.%.

[0046] Crystallization and separation of alkali decahydrate: Calcium decahydrate crystals were obtained by low-temperature crystallization, simultaneously generating a first effluent. The main components of the first effluent were Na₂CO₃: 10 wt.%–15 wt%, NaCl: 5 wt.%–15 wt.%, and NaF: 0.55 wt.%.

[0047] Decahydrate alkali dissolution: Decahydrate alkali dissolution is performed using process condensate to obtain a second brine with a total alkalinity of approximately 27 wt.% to 30 wt.% as of Na2CO3.

[0048] Monohydrate alkali evaporation, crystallization, and separation: The second brine is crystallized at 104℃ in a crystallizer to obtain monohydrate alkali crystals. Simultaneously, effluent 2 is discharged to reduce the sodium chloride and / or sodium sulfate content in the mother liquor. The main components are Na₂CO₃: 26wt.%–30wt%, NaCl: 2wt.%–3wt.%, and Na₂SO₄: 0.15wt.%. Monohydrate alkali crystals and the mother liquor are obtained after separation.

[0049] Soda ash monohydrate drying: Soda ash monohydrate crystals are dried in a fluidized bed, then crushed and sieved to obtain the final product, soda ash. The sodium fluoride content of this product is less than 0.05 wt.%.

[0050] In addition, the first effluent is treated in the ash slurry machine as follows:

[0051] First-stage causticization process: Approximately 20% of the first effluent and industrial water are added to the slaking machine to react with quicklime, yielding slurry. The quicklime used in the reaction includes calcium fluoride quicklime for subsequent calcination and some purchased quicklime. The main components of the resulting slurry are: Na₂CO₃: 2wt.%–5wt%, NaCl: 5wt.%–8wt.%, NaOH: 3wt.%–5wt.%, Ca(OH)₂: 13wt.%–18wt.%, CaF: 0.2wt.%–0.5wt.%. Solid particles with a diameter ≥1cm (including CaF, CaCO₃, and SiO₂) discharged from the tail end of the slaking machine are ground and then calcined along with the causticized mud. The slurry undergoes further screening to remove ash residue (containing CaF, CaO, and Ca(OH)₂) with a diameter ≥3mm. This ash residue is then ground and returned to the slaking machine.

[0052] Secondary causticizing process: Add about 80% of the first discharge liquid to the slurry obtained from the primary causticizing process, continue causticizing and then clarify to obtain a dilute caustic soda solution with a concentration of 5wt.% to 13wt.% and causticized mud.

[0053] Calcination process: After washing, the causticized mud and solid particles are calcined in a rotary kiln at 1100℃ to obtain quicklime containing calcium fluoride, which is then used in the first-stage causticization process.

[0054] In this embodiment, during the process operation, the mass ratio of solid particles and ash residue to ash slurry is monitored in the first-stage causticization process. When the sum of their masses is more than 5 wt.% of the mass of the first discharge liquid, for example, solid particles with a diameter ≥1 cm account for 1.5 wt.% of the ash slurry, and ash residue with a diameter ≥3 mm obtained by further screening accounts for 8 wt.% of the ash slurry discharged from the ash slurry machine, the causticized mud obtained from the second causticization is acid washed. Subsequently, the precipitated CaF is separated, washed, and filtered and sold as a by-product. The mother liquor and calcium chloride in the wash water are returned to the first-stage causticization process in the ash slurry machine for use.

[0055] Example 2

[0056] A process for preparing soda ash by defluorination of natural alkali includes the following steps:

[0057] Solution extraction: Brine 1 is obtained by injecting injection fluid at 60-80℃ into natural alkali mine. Its main components are Na2CO3: 10.27wt.%, NaHCO3: 7.5wt.%, NaCl: 0.5wt.%, Na2SO4: 0.1wt.%, NaF: 0.01-0.02wt.%, and total suspended solids (TSS): 50ppm.

[0058] Pretreatment: Filtering brine 1 reduces the TTS content to 10 ppm.

[0059] Steam stripping concentration: The filtered brine 1 is concentrated into concentrated brine 2 through two-stage gas concentration, the main components of which are Na2CO3: 23-26 wt.%, NaHCO3: 2.5-4 wt.%, NaCl: 0.8-1 wt.%, Na2SO4: 0.1-0.2 wt.%, and NaF: 0.02-0.05 wt.%.

[0060] Crystallization and separation of alkali decahydrate: Alkali decahydrate crystals were obtained through low-temperature crystallization, simultaneously generating a first effluent. The main components of the first effluent were Na₂CO₃: 14–15 wt%, NaCl: 8–10 wt.%, and NaF: 0.08–0.2 wt.%.

[0061] Decahydrate alkali dissolution: Decahydrate alkali dissolution is performed using process condensate to obtain a second brine with a total alkalinity of approximately 27-30% (based on Na2CO3).

[0062] Monohydrate alkali evaporation, crystallization, and separation: The second brine is crystallized at 104℃ in a crystallizer to obtain monohydrate alkali crystals. Simultaneously, effluent 2 is discharged to reduce the sodium chloride and / or sodium sulfate content in the mother liquor. The main components are Na₂CO₃: 26–30 wt%, NaCl: 2–3 wt.%, and Na₂SO₄: 0.15 wt.%. Monohydrate alkali crystals and the mother liquor are obtained after separation.

[0063] Soda ash drying: Soda ash crystals are dried in a fluidized bed, then crushed and sieved to obtain the soda ash product. The sodium fluoride content of this product is <50ppm.

[0064] Furthermore, the first effluent is processed in the slaking machine as follows: A portion of the effluent is contacted with quicklime to obtain a lime slurry; solid particles with a diameter ≥1 cm (including CaF, CaCO3, and SiO2) are discharged from the tail end of the slaking machine, followed by slag with a diameter ≥3 mm (containing CaF, CaO, and Ca(OH)2) which is then ground and returned to the lime slurry; subsequently, another portion of the first effluent is used to perform a secondary causticizing process on the lime slurry; after causticizing, the slurry is clarified, and a dilute alkali solution is separated to obtain causticized mud; this causticized mud and solid particles are calcined in a rotary kiln at a temperature of 800–1100°C to obtain quicklime containing calcium fluoride for primary causticizing. After multiple cycles, calcium fluoride is enriched in the quicklime.

[0065] First-stage causticization process: Approximately 20% of the first effluent and industrial water are added to the slaking machine to react with quicklime, yielding slurry. The quicklime used in the reaction includes calcium fluoride quicklime for subsequent calcination and some purchased quicklime. The main components of the resulting slurry are: Na₂CO₃: 2wt.%–5wt%, NaCl: 5wt.%–8wt.%, NaOH: 3wt.%–5wt.%, Ca(OH)₂: 13wt.%–18wt.%, CaF: 0.05wt.%–0.1wt.%. Solid particles with a diameter ≥1cm (including CaF, CaCO₃, and SiO₂) discharged from the tail end of the slaking machine are ground and then calcined along with the causticized mud. The slurry undergoes further screening to remove ash residue (containing CaF, CaO, and Ca(OH)₂) with a diameter ≥3mm. This ash residue is then ground and returned to the slaking machine.

[0066] Secondary causticizing process: Add about 80% of the first discharge liquid to the slurry obtained from the primary causticizing process, continue causticizing and then clarify to obtain a dilute caustic soda solution with a concentration of 5wt.% to 13wt.% and causticized mud.

[0067] Calcination process: After washing, the causticized mud and solid particles are calcined in a rotary kiln at 1100℃ to obtain calcium fluoride-containing quicklime, which is then used in the first-stage causticization process.

[0068] In this embodiment, during the process operation, the fluorine element in the brine 1 obtained by dissolving natural alkali is continuously enriched in quicklime. During the primary causticization process, the mass ratio of the solid particles and ash residue to the slurry (the slurry obtained by primary causticization of the first discharge liquid and quicklime) is monitored. When the sum of the mass of the solid particles and ash residue exceeds 5 wt.% of the mass of the slurry, for example, when the solid particles with a particle size ≥1 cm account for 2 wt.% of the slurry and the ash residue with a particle size ≥3 mm obtained by further screening accounts for 8.5 wt.% of the slurry discharged from the slaking machine, the causticized mud obtained by the secondary causticization is acid washed. Subsequently, the precipitated CaF is separated, washed, filtered, and sold as a by-product. The calcium chloride in the mother liquor and wash water is returned to the primary causticization process in the slaking machine for use.

[0069] It should be noted that the above description is a further detailed explanation of the present invention in conjunction with specific embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple improvements can be made without departing from the concept of the present invention, and all such improvements should be considered to fall within the scope of protection of the present invention.

Claims

1. A process for preparing soda ash by defluorination of natural alkali, characterized in that, include: The raw brine obtained by leaching or the natural alkali ore obtained by mechanical mining is pretreated to obtain concentrated first brine; the first brine is discharged as a first effluent containing fluoride after a decahydrate alkali crystallization and separation process. The first effluent is then subjected to the following operations: primary causticization process: a portion of the first effluent is contacted with quicklime to obtain lime slurry; the lime slurry is screened to discharge solid particles with a particle size ≥1cm; ash residue with a particle size ≥3mm is screened out, and the ash residue is crushed and returned to the lime slurry; Secondary causticizing process: The ash slurry is contacted with another portion of the first discharge liquid, and after clarification and separation, a dilute caustic soda solution and causticized mud are obtained; Calcination process: The solid particles and the causticized mud are calcined to obtain quicklime containing calcium fluoride, which is then fed into the first-stage causticization process; The process further includes: when the sum of the mass of the solid particles and the ash exceeds 5 wt.% of the mass of the ash slurry, the causticized mud is acid-washed and separated to output calcium fluoride product.

2. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, The mass ratio of the first effluent used in the first-stage causticizing process and the second-stage causticizing process is 1:(2-6), preferably 1:(3-4); Preferably, the crystallization and separation of the decahydrate alkali further includes dissolving the decahydrate alkali, evaporating and crystallizing the monohydrate alkali and separating it, and drying the monohydrate alkali to obtain the soda ash product.

3. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, This also includes adding a diluent in the first-stage causticizing process; Preferably, the diluent is derived from process condensate and / or industrial water; And / or, the primary causticizing process further includes pre-washing the ash residue before crushing it; Preferably, the wash water after washing is input into the first-stage causticizing process; Preferably, the wash water comes from process condensate and / or industrial water.

4. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, The concentration of the dilute sodium hydroxide solution is 5 wt.% to 13 wt.%. And / or, the dilute caustic soda solution is evaporated and concentrated to remove sodium chloride and / or sodium sulfate, resulting in a concentrated caustic soda solution; Preferably, the concentration of the concentrated caustic soda is 20 wt.% to 30 wt.%. Preferably, when soda ash is prepared from the extracted brine, the concentrated caustic soda is used to neutralize the first brine.

5. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, The calcination process also includes pre-washing the causticized mud; Preferably, the wash water is derived from process condensate and / or industrial water; Preferably, the wash water after washing is input into the first-stage causticizing process; Preferably, when soda ash is prepared from natural soda ash ore, the wash water after washing can be used as a dissolving solution in the pretreatment process.

6. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, The temperature of the calcination process is 800–1100℃; And / or, the calcination process is carried out in a rotary kiln.

7. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, In the pickling operation, hydrochloric acid is used to pickle the causticized mud. And / or, it also includes washing the separated calcium fluoride, the wash water being derived from process condensate and / or industrial water; Preferably, the wash water after washing is input into the first-stage causticizing process.

8. The process for preparing soda ash from natural alkali by defluorination according to claim 1 or 2, characterized in that, The concentration of Na2CO3 in the first brine is 23 wt.% to 33 wt.%. And / or, the decahydrate alkali is dissolved to obtain a second brine, the total alkalinity of the second brine being 23 wt.% to 30 wt.% as Na2CO3; And / or, the sodium fluoride content in the soda ash product is not greater than 0.05 wt.%.

9. The process for preparing soda ash from natural alkali by defluorination according to claim 1, characterized in that, When soda ash is prepared using the extracted raw brine as raw material, the pretreatment operation includes: filtering the raw brine and performing at least two stages of stripping to obtain the first brine. When soda ash is prepared using natural soda ash ore as raw material, the pretreatment operation includes: crushing and calcining the natural soda ash ore, then dissolving and clarifying it to obtain the first brine; Preferably, the calcination temperature of the natural soda ash ore is 180–200°C, and the calcination time is 0.5–1 hour; Preferably, the calcined product is dissolved in a solution at 30–70°C; Preferably, the solvent used to dissolve the calcined product is at least one of process condensate, industrial water, or first discharge liquid.

10. The process for preparing soda ash from natural alkali by defluorination according to claim 2, characterized in that, The second discharge liquid is discharged during the evaporation and crystallization of the aqueous alkali. Preferably, the second effluent is fed into the decahydrate alkali crystallization separation process; Preferably, when soda ash is prepared from natural soda ash ore, the second effluent is input into the pretreatment process as a dissolving liquid; And / or, the dissolving solution used in the decahydrate alkali dissolution process is at least one of process condensate and monohydrate alkali crystallization mother liquor.

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