Method for co-production of sulfuric acid and high-purity white calcium oxide based on desilicated phosphogypsum

Abstract

This application provides a method for the co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum, comprising the following steps: S1: combining phosphogypsum with water to form a slurry, and adding a reverse flotation desilication agent for desilication; S2: mixing the desilication phosphogypsum from S1 with a reducing agent; S3: adding the mixed desilication phosphogypsum to a fluidized bed reactor to generate calcium oxide solid, water vapor, carbon dioxide, and sulfur dioxide gas; S4: removing the generated calcium oxide solid; S5: extracting the water vapor, carbon dioxide, and sulfur dioxide gas, and drying and separating them; S6: sending the dried carbon dioxide and sulfur dioxide gas into a sulfuric acid generating device. This invention helps reduce the introduction of exogenous alkali metal ions, alleviates the risks of slagging and product contamination during traditional gypsum calcination, improves the purity and whiteness of the solid products, and facilitates the resource-based transformation of industrial solid waste.

Description

Technical Field

[0001] This application relates to the field of phosphogypsum co-production of sulfuric acid and calcium oxide, specifically to a method for the co-production of sulfuric acid and high-purity white calcium oxide based on desilicationized phosphogypsum. Background Technology

[0002] Phosphogypsum is an industrial byproduct of the wet-process phosphoric acid industry, typically producing 5 tons of phosphogypsum for every ton of phosphoric acid produced. Currently, this phosphogypsum cannot be properly disposed of and can only be stored, which not only occupies land but also causes serious damage to soil and water resources due to acid leaching. Long-term open-air storage also exacerbates air dust pollution. Although phosphogypsum is rich in sulfur and calcium, its CaSO4·2H2O content is only 75%–90%, while its SiO2 content is 5%–15%, and it also contains impurities such as phosphorus, fluorine, and organic matter.

[0003] Phosphogypsum is mainly used in low-end building materials, such as building gypsum powder, roadbed materials, and ecological restoration agents. However, due to weak market competitiveness and severe homogenization, the comprehensive utilization of phosphogypsum faces numerous difficulties. Phosphogypsum can undergo a thermal decomposition reaction to obtain important chemical raw materials such as quicklime (calcium oxide) and sulfur dioxide. The soaring price of sulfur has led to persistently high sulfuric acid prices, and phosphogypsum has recycling value. Therefore, the production of cementitious materials such as sulfuric acid-binding cement from phosphogypsum has become a current overall demand in the phosphorus chemical industry.

[0004] Currently, there are some problems with the direct heating and decomposition of phosphogypsum to produce sulfuric acid and co-producing cementitious materials. The CaSO4·2H2O content does not exceed 90%, resulting in insufficient sulfur dioxide concentration in the flue gas. The calcium oxide in the waste residue after high-temperature decomposition is not pure and has a dark color. It is also necessary to add siliceous raw materials (such as kaolin, silica, and fly ash) as cementitious materials.

[0005] The silicon-containing impurities in phosphogypsum are mainly poorly active silica, which not only cannot be used as a binder but also results in low-purity calcium oxide and a dark color in the waste residue after high-temperature decomposition of phosphogypsum, and makes it difficult to meet the sulfur dioxide concentration requirements for acid production in the flue gas. If the silicon-containing impurities in phosphogypsum are removed, the CaSO4·2H2O content can be increased to over 98%. After high-temperature decomposition, not only sulfuric acid can be produced, but also high-purity white calcium oxide can be co-produced, greatly improving its economic value. Flotation, as a commonly used impurity removal process, is relatively mature in China and has been widely used in the decolorization of phosphogypsum. However, because a reverse flotation desilication reagent suitable for phosphogypsum has not yet been successfully developed, flotation has not been applied in the desilication of phosphogypsum.

[0006] Therefore, there is an urgent need to develop a reverse flotation desilication agent for phosphogypsum to achieve efficient removal of silicon impurities from phosphogypsum, thereby enabling desilicationized phosphogypsum to be used for co-production of sulfuric acid (as a raw material for wet-process phosphoric acid) and high-purity white calcium oxide. Summary of the Invention

[0007] This application provides a method for the co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum. This method treats phosphogypsum, a solid waste from wet-process phosphoric acid production, to achieve the co-production of sulfuric acid and high-purity white calcium oxide products. It also realizes the high-value utilization of solid waste and solves the problems of pollution from traditional phosphogypsum stockpiling and low added value of products.

[0008] In a first aspect, this application provides a method for the co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum, comprising the following steps: S1: Phosphogypsum is combined with water to form a slurry, and then a reverse flotation desilication agent is added to mix and desilicate to obtain desilication phosphogypsum. The reverse flotation desilication agent includes 100 parts of organosilicon quaternary ammonium salt collector, 20-40 parts of composite frother and 60-80 parts of modifier. The composite frother is obtained by compounding tributyl phosphate and diethyl phthalate in a mass ratio of (1-5):1. The modifier is obtained by nucleophilic addition reaction of n-propanol and ethylene oxide under the action of a catalyst. S2: Mix the desilicationized phosphogypsum described in S1 with a reducing agent; S3: The mixed desilication phosphogypsum is added to a fluidized bed reactor and decomposes at high temperature to generate calcium oxide solid, water vapor, carbon dioxide and sulfur dioxide gas; S4: Take out the generated calcium oxide solid and cool it to obtain high-purity white calcium oxide; S5: The water vapor, carbon dioxide and sulfur dioxide gas are drawn out by a fan, and then absorbed and dried by concentrated sulfuric acid to obtain dry carbon dioxide and sulfur dioxide gas. S6: Dry carbon dioxide and sulfur dioxide gas are fed into the sulfuric acid generating unit.

[0009] According to this application, the above scheme achieves reverse flotation desilication of phosphogypsum ore by combining front-end reverse flotation impurity removal with back-end thermal reduction calcination and gas phase washing purification, without the need to add external acid and alkali adjusters. This improves the grade of gypsum entering the furnace and ultimately achieves efficient and high-value comprehensive utilization of phosphogypsum while realizing the efficient and green co-production of high-purity ultra-white calcium oxide solid and high-concentration sulfur dioxide (in-situ preparation of sulfuric acid).

[0010] Step S1 involves removing impurities from the phosphogypsum ore using a reverse flotation desilication process. Without adding exogenous acid or alkali adjusters, the reverse flotation desilication reagent chemically anchors and removes fine colloidal silica, improving the purity of the phosphogypsum. However, due to the strong collecting power and surface activity of the organosilicon quaternary ammonium salt collector for fine colloidal silica, it may form a layer of microparticle-stabilized dead foam with distorted mechanical strength and viscoelasticity after carrying silica mud to the gas-liquid interface. In this excessively tough, non-draining dead foam layer, a large number of heavy dihydrate gypsum particles in the slurry are mechanically entrained and trapped in the gaps between bubbles, leading to significant gypsum loss and a decreased recovery rate. Therefore, this application introduces TBP molecules with low surface tension and a high spreading coefficient to reduce the surface viscoelasticity of the interfacial film. In macroscopic flotation performance, this interface trimming effect tends to reduce the aggregation of malignant viscous large bubbles and promotes the evolution of the system towards finer, more fluid microbubbles, thus reserving a relatively smooth fluid channel for the gravity-driven desorption of mechanically entrained gypsum. Secondly, considering the technical tendency of TBP itself having extremely low surface tension and being prone to premature critical rupture of bubbles in the depths of the slurry due to shear forces when used alone, thereby reducing desilication efficiency, this application combines DEP molecules with a rigid benzene ring skeleton and sterically hindered ester groups in the above-mentioned proportion range. In the microemulsion dispersion state deep in the slurry, there may be a certain spatial self-assembly and steric hindrance complementarity effect between the rigid structure of DEP and the flexible butyl segments of TBP, which helps to improve the initial mechanical strength and compressive elasticity of the composite bubble film. This synergistic mechanism helps maintain the dynamic lifespan and lifting force of microbubbles deep within the slurry and during their ascent. This allows the silica fines anchored by the collector to be collected and transported to the surface of the slurry. The introduction of short-chain alcohol ether modifiers, whose molecular structures contain ether bonds with specific spans and terminal hydroxyl groups, enables them to form hydrogen bonds with the silanol groups (-Si-OH) on the surface of silica particles. This weakens the directional hydration layer on the silica surface, aiding in the selective adsorption of the collector in an acidic slurry environment. Furthermore, these short-chain alcohol ether molecules possess high interfacial activity, reducing the local fluid viscosity and drainage resistance of the slurry and foam layer. This promotes the rapid settling of heavy dihydrate gypsum particles, which have not adsorbed the collector and are only mechanically entrained, back to the bottom of the slurry under their own gravity, thereby reducing the gypsum loss rate.

[0011] In step S2, the desilication phosphogypsum and reducing agent are mixed before entering the furnace, so that the reducing agent is evenly coated on the surface of the high-grade gypsum particles. From the perspective of reaction kinetics, this reduces the activation energy of calcium sulfate pyrolysis, which is conducive to promoting the subsequent decomposition reaction to proceed more quickly and thoroughly.

[0012] In step S3, the mixture is fed into a fluidized bed reactor for pyrolysis. This causes the gypsum to decompose under a high-temperature reducing atmosphere, transforming it into solid calcium oxide and a mixed gas phase composed of water vapor, carbon dioxide, and sulfur dioxide. This process also reduces the problems of slagging, bed collapse, and dead bed formation in the fluidized bed at high temperatures caused by the introduction of exogenous alkali metal ions.

[0013] In step S4, the solid product at the bottom of the fluidized bed is removed and cooled to directly obtain high-purity white calcium oxide. Since the preceding process achieves desilication and there is no external alkali contamination, the solid product is not affected by the reaction of external colored silicates in the solid phase, thus maintaining the high purity and high whiteness of the calcium oxide product.

[0014] In step S5, the mixed gas phase is extracted and concentrated sulfuric acid produced by the system's downstream unit is used as the absorption, drying, and separation medium. This removes pyrolysis water vapor without introducing external chemical impurities, preventing severe corrosion of fluid pipelines and fans caused by gas phase condensation into dilute sulfuric acid, and also purifying the initial relative abundance of the target gas components. In step S6, the dried carbon dioxide and high-concentration sulfur dioxide gas are input into the sulfuric acid generation unit, enabling high conversion and efficient sulfuric acid production in the downstream acid production unit.

[0015] In some embodiments, the collector is an organosilicon quaternary ammonium salt collector having the following structural formula: , Where m is 8, 10, 12, 14 or 16; X is F, Cl, Br or I.

[0016] In some of the above embodiments, traditional desilication collectors lose their selectivity for silicon under strong acidity due to intense protonation, necessitating the addition of large amounts of alkali. In contrast, this application selects a collector with the aforementioned structure. The quaternary ammonium salt at the molecular center carries a positive charge, enabling strong electrostatic adsorption and collection with negatively charged fine-particle silica. The other end of the molecule contains a trimethoxysilane group (-Si(OCH3)3). Under strongly acidic hydration conditions, these siloxane groups can hydrolyze and undergo dehydration condensation with the silanol groups (-Si-OH) on the surface of amorphous SiO2, forming Si-O-Si covalent bonds, thereby achieving selective collection of silica. This compound also features a flexible long carbon chain at one end. When its polar end is anchored to the SiO2 surface, the long carbon chain can form a hydrophobic protective film on the surface of fine mud, providing a strong hydrophobic driving force for it to adhere to the air bubbles, and then float out with the foam. If m<8, the carbon chain is too short, resulting in insufficient hydrophobicity and difficulty in supporting colloidal silica mud; if m>16, the steric hindrance and water solubility of the molecules become poor, and they are prone to spontaneously agglomerate into inactive micelles in acidic slurry.

[0017] In some embodiments, the modifier is prepared by: n-Propanol and ethylene oxide are mixed in a molar ratio of 1:(1.5-2.5) and reacted at 60-120°C for 1-5 hours in the presence of an alkaline catalyst to obtain a modifier.

[0018] In some of the above embodiments, the modifier molecules synthesized within the above molar ratio range can effectively strip the silica mud water film deep inside to assist the main collector in capturing silica to the extreme, and can also rapidly migrate and intercalate on the surface to trigger industrial-grade rapid water release, allowing the entrained heavy dihydrate gypsum particles to desorb under their own gravity and slide back to the bottom of the tank, thereby balancing the contradiction between "desiliconization" and "high gypsum recovery rate".

[0019] In some embodiments, the reverse flotation desilication agent comprises: 100 parts of organosilicon quaternary ammonium salt compound, 20-40 parts of composite foaming agent, and 60-80 parts of modifier.

[0020] In some of the above embodiments, selecting the reverse flotation desilication reagent obtained by compounding in the above proportions can enable the entire reagent system to achieve efficient silica capture in a strongly acidic raw ore environment without the need for external acid-base adjusters, while simultaneously increasing the phosphogypsum recovery rate.

[0021] In some embodiments, in step S1, the slurry concentration is 30wt%–40wt%, and the dosage of the phosphogypsum reverse flotation desilication reagent is 100–500 g / t phosphogypsum. Based on the above embodiments, controlling the slurry concentration at a high solid phase of 30wt%–40wt% allows the strong shear force generated between solid particles during aeration and high-frequency scrubbing collisions to assist the modifier in quickly penetrating and expelling the directional hydration layer on the surface of the silica ultrafine mud, thereby increasing the probability of Si-O-Si chemical covalent bonding between the main collector and the silicon framework. Simultaneously, controlling the total reagent dosage within the concentration range of 100–500 g / t ensures that the reagent residue can undergo chain breakage and gasification with zero carbon residue in the medium-low temperature range, reducing the risk of solid products being contaminated by carbon black or gypsum loss due to excessively high fluid viscosity, and synergistically improving the gypsum recovery rate.

[0022] In some embodiments, the reducing agent is carbon powder, and the amount of reducing agent added is 3% to 5% of the mass of the desilication phosphogypsum. Based on the above embodiments, this specific amount of solid carbon powder can cause the material fed into the furnace to undergo instantaneous, violent, and desulfurization-reducing disintegration in the high-temperature section, completely catalytically converting the solid phase into high-purity CaO, while simultaneously converting all sulfur elements in situ into a high-abundance SO2 gas phase.

[0023] In some embodiments, in step S3, the upper temperature of the fluidized bed reactor is 300℃~600℃, the lower temperature is 1000℃~1200℃, and the residence time is 2h~4h. Based on the above embodiments, the upper temperature zone of 300℃~600℃ is used to preheat the material and promote the complete pyrolysis of residual organic agents in phosphogypsum in this zone, generating CO2 and H2O, which are discharged with the gas flow, preventing organic matter from entering the high-temperature zone and affecting the whiteness of the product. The material then sinks to the lower high-temperature reduction zone of 1000℃~1200℃ and stays for 2h~4h. Under these conditions, calcium sulfate undergoes a solid-solid reaction with the reducing agent, completing the reduction decomposition and generating solid calcium oxide and SO2-containing flue gas.

[0024] In some embodiments, the phosphogypsum is dihydrate phosphogypsum. Based on the above embodiments, the entire process of treatment directly uses virgin, unconverted dihydrate phosphogypsum, which eliminates the energy-intensive pretreatment step of converting solid waste into hemihydrate or anhydrous gypsum in traditional processes.

[0025] Compared with the prior art, the beneficial effects of this application are at least as follows: This application utilizes the acidic slurry environment of the raw phosphogypsum ore in the front-end reverse flotation desilication step to achieve efficient desilication without the need for additional acid-base adjusters. More importantly, the entire flotation process reduces the introduction of alkali metal ions, thereby reducing the problem of alkali metals reacting with residual silicon to form a low-melting-point eutectic glass phase at high temperatures. This allows the downstream fluidized bed reactor to operate stably for a long time in a high-temperature reducing atmosphere of 1000℃~1200℃, reducing the risk of adhesion, slagging, and dead bed formation. In traditional reverse flotation treatment of high-mud phosphogypsum, the foam layer easily mechanically entrains heavy gypsum particles and scrapes them out with the foam, leading to a decrease in gypsum recovery rate. This application, through the synergistic effect of collectors, composite foaming agents, and adjusters, maintains bubble stability during the foam rising stage to carry the desorbed silicon impurities. When the foam reaches the scraping area at the tank opening, the liquid is rapidly discharged, causing the gypsum particles that have not adsorbed the reagents to fall back to the bottom of the tank due to the loss of foam support. Thus, while ensuring desilication efficiency, the gypsum recovery rate is improved. The reagents used in this application are all short-chain ester / ether organic compounds, the dosage of which is controllable. Their poor thermal stability, combined with front-end desilication, can produce high-purity white calcium oxide, which can be directly used in high-end chemical applications. Attached Figure Description

[0026] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0027] Figure 1 This is a process flow diagram of the present invention.

[0028] Figure 2This is the flotation process diagram used in the process flow chart. Detailed Implementation

[0029] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0030] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0031] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0032] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0033] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0034] Organosilicon quaternary ammonium salt compound, dodecyl dimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, CAS No.: 27668-53-7; Carbon powder, with fixed carbon ≥85% and particle size of -200 mesh accounting for more than 90%.

[0035] Preparation Example 1 Preparation of the modifier: 100 parts by mass of dry anhydrous n-propanol and 0.3 parts by mass of solid sodium hydroxide catalyst were added to a stainless steel high-pressure reactor. Stirring was started and the temperature was raised to 65°C to completely dissolve the catalyst. Then, a vacuum was applied to -0.09 MPa for dehydration for 15 minutes. After the vacuum was removed, the gas inside the reactor was replaced four times with high-purity nitrogen, and a nitrogen protective positive pressure of approximately 0.05 MPa was maintained inside the reactor. Next, the jacket heating was activated to raise the temperature inside the reactor and stabilize it at 90°C. 147 parts by mass of [amount missing] ... A portion of dry ethylene oxide was introduced, and the feeding rate was adjusted or the jacket cooling water was turned on during the process. The reaction temperature was 90℃, and the reaction pressure inside the reactor was controlled at 0.3MPa. After the feeding was completed, the feed valve was closed, and the reaction was continued at 90℃ with stirring for 1.5h. After the reaction was completed, the reactor was cooled to 45℃, and the exhaust valve was opened to use nitrogen gas flow to remove the unreacted ethylene oxide residue in the reactor. Finally, dilute phosphoric acid was slowly added dropwise to the reactor to neutralize the residual catalyst until the pH value was 6.8. After filtration, the distillation product was collected by vacuum distillation to obtain the adjusting agent.

[0036] Example 1 Co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum: The detailed process flow diagram is as follows: Figure 1 and Figure 2 As shown.

[0037] S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound), 30 parts of the composite foaming agent (tributyl phosphate and diethyl phthalate compounded in a mass ratio of 3:1) and 70 parts of the modifier, and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0038] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water to the flotation cell, and stir to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, and after adjusting the slurry for 3 minutes, start aeration (0.25 m³ / t). 3 ( / h), continuously scrape and bubble for 4 minutes. Filter, wash, and dehydrate the gypsum slurry at the bottom of the tank to obtain desilication phosphogypsum.

[0039] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0040] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0041] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0042] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0043] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0044] Example 2 Co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum: Similar to Example 1, except that in step S1, 30 parts of the composite foaming agent are compounded from tributyl phosphate and diethyl phthalate in a mass ratio of 5:1.

[0045] Example 3 Co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum: Similar to Example 1, except that in step S1, 30 parts of the composite foaming agent are compounded by tributyl phosphate and diethyl phthalate in a mass ratio of 1:1.

[0046] Comparative Example 1 Co-production of sulfuric acid and calcium oxide based on desilication phosphogypsum: S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound) and 30 parts of the composite foaming agent (tributyl phosphate and diethyl phthalate compounded in a mass ratio of 3:1) and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0047] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water and stir in a flotation cell to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, adjust the slurry for 3 minutes, then start aeration (0.25 m3 / h), and continuously skim the bubbles for 4 minutes. Filter, wash, and dewater the gypsum slurry at the bottom of the cell to obtain desilicationized phosphogypsum.

[0048] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0049] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0050] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0051] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0052] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0053] Comparative Example 2 Co-production of sulfuric acid and calcium oxide based on desilication phosphogypsum: S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound), 30 parts of the foaming agent tributyl phosphate and 70 parts of the modifier, and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0054] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water and stir in a flotation cell to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, adjust the slurry for 3 minutes, then start aeration (0.25 m3 / h), and continuously skim the bubbles for 4 minutes. Filter, wash, and dewater the gypsum slurry at the bottom of the cell to obtain desilicationized phosphogypsum.

[0055] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0056] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0057] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0058] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0059] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0060] Comparative Example 3 Co-production of sulfuric acid and calcium oxide based on desilication phosphogypsum: S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound), 30 parts of the foaming agent (diethyl phthalate), and 70 parts of the modifier, and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0061] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water and stir in a flotation cell to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, adjust the slurry for 3 minutes, then start aeration (0.25 m3 / h), and continuously skim the bubbles for 4 minutes. Filter, wash, and dewater the gypsum slurry at the bottom of the cell to obtain desilicationized phosphogypsum.

[0062] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0063] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0064] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0065] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0066] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0067] Comparative Example 4 Co-production of sulfuric acid and calcium oxide based on desilication phosphogypsum: S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound), 30 parts of the composite foaming agent (tributyl phosphate and diethyl phthalate compounded in a mass ratio of 6:1) and 70 parts of the modifier, and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0068] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water to the flotation cell, and stir to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, and after adjusting the slurry for 3 minutes, start aeration (0.25 m³ / t). 3 ( / h), continuously scrape and bubble for 4 minutes. Filter, wash, and dehydrate the gypsum slurry at the bottom of the tank to obtain desilication phosphogypsum.

[0069] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0070] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0071] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0072] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0073] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0074] Comparative Example 5 Co-production of sulfuric acid and calcium oxide based on desilication phosphogypsum: S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound), 30 parts of the composite foaming agent (tributyl phosphate and diethyl phthalate compounded in a mass ratio of 1:2) and 70 parts of the modifier, and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0075] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water to the flotation cell, and stir to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, and after adjusting the slurry for 3 minutes, start aeration (0.25 m³ / t). 3 ( / h), continuously scrape and bubble for 4 minutes. Filter, wash, and dehydrate the gypsum slurry at the bottom of the tank to obtain desilication phosphogypsum.

[0076] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0077] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0078] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0079] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0080] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0081] Comparative Example 6 Co-production of sulfuric acid and calcium oxide based on desilication phosphogypsum: S1: Mix 100 parts of the main collector (dodecyl organosilicon quaternary ammonium salt compound), 30 parts of the composite foaming agent (tributyl phosphate and dimethyl phthalate compounded in a mass ratio of 3:1) and 70 parts of the modifier, and stir at room temperature for 30 min to obtain the reverse flotation desilication agent.

[0082] Take 10,000 parts of dihydrate phosphogypsum ore (dry basis), add water to the flotation cell, and stir to adjust the slurry concentration to 35%. Start stirring (1800 rpm), add 3.0 parts (300 g / t) of the above reagent, and after adjusting the slurry for 3 minutes, start aeration (0.25 m³ / t). 3 ( / h), continuously scrape and bubble for 4 minutes. Filter, wash, and dehydrate the gypsum slurry at the bottom of the tank to obtain desilication phosphogypsum.

[0083] S2: Take 5000 parts of the desilication phosphogypsum dry product obtained in step S1, add 200 parts of carbon powder reducing agent (i.e., 4% of the mass of desilication phosphogypsum), put it into a double helix conical mixer and mix for 15 minutes, then put it into the feeding hopper for later use.

[0084] S3: Start the fluidized bed reactor, controlling the upper layer temperature at 450℃ and the lower layer temperature at 1100℃. Continuously feed the mixture from step S2 into the reactor, with a total residence time of 3 hours. Calcium sulfate undergoes a reduction decomposition reaction with the carbon powder, generating solid calcium oxide and flue gas containing SO2 and CO2. Observation showed that no slagging, sticking, or dead bed phenomena occurred during reactor operation.

[0085] S4: The gas-solid mixture after pyrolysis enters a cyclone separator for separation. The solid product is discharged from the bottom of the separator and cooled to obtain calcium oxide product.

[0086] S5: The high-temperature mixed gas containing SO2 and CO2 discharged from the top of the cyclone separator is washed by countercurrent spraying with 98% concentrated sulfuric acid in a cooling scrubbing tower to remove water vapor and fluorine-phosphorus particles, and then dry carbon dioxide and sulfur dioxide gas is obtained.

[0087] S6: Dry carbon dioxide and sulfur dioxide gas are fed into the conversion absorption tower for catalytic oxidation to produce acid, resulting in industrial concentrated sulfuric acid. Part of the sulfuric acid is returned to the washing tower as a drying medium for recycling.

[0088] Test section Dihydrate gypsum recovery rate test: Weigh the mass of the phosphogypsum ore and record it as M0, the content of dihydrate gypsum as W0. After flotation, weigh the mass of the desilicationized phosphogypsum and record it as M1, the content of dihydrate gypsum after desilication as W1. Calculate the dihydrate gypsum recovery rate as (M1×W1) / (M0×W0)×100%; Calcium oxide chemical purity test: The sucrose complexometric titration method in GB / T 5762-2012 "Chemical Analysis Methods for Lime in Building Materials" was used for determination. Weigh 0.5 g (accurate to 0.0001 g) of the pulverized calcium oxide sample, record the specific mass as m, and retain four decimal places. Wet the sample with anhydrous ethanol, then add an appropriate amount of sucrose and water, and stir thoroughly to allow a complexation reaction to occur. Using phenolphthalein as an indicator, titrate with a standardized hydrochloric acid solution (C = 0.5015 mol / L). Calculate the effective CaO mass percentage X based on the volume V of hydrochloric acid consumed: X (%) = (C × V × 28.04) / m × 100%. C — The concentration of the standard hydrochloric acid titration solution, in moles per liter (mol / L). V — The volume of standard hydrochloric acid solution consumed in the titration, in liters (L); m — Mass of calcium oxide sample, in grams (g); 28.04 — the molar mass of 1 / 2 CaO, in grams per mole (g / mol).

[0089] Calcium oxide whiteness test: The whiteness was measured according to GB / T 5950-2008 "Methods for Measurement of Whiteness of Building Materials and Non-metallic Mineral Products". A fully automatic whiteness meter was used, with an absolute standard white plate as the reference, a D65 standard light source, and a 10° field of view to measure the blue light diffuse reflectance (i.e., R457 whiteness value) of the calcium oxide solid powder surface. The test results are shown in Table 1 below.

[0090] Table 1

[0091]

[0092] According to Table 1, Example 1 shows better performance than Comparative Examples 1, 2, and 3 in terms of gypsum dihydrate recovery rate, calcium oxide purity, and whiteness. This indicates that the reverse flotation desilication reagent system and its ternary compound formulation provided in this application can achieve a high gypsum recovery rate and improve the chemical purity and whiteness of the downstream co-produced calcium oxide solid product. The possible reason for this is that no modifier was added to the reagent system in Comparative Example 1. On the one hand, due to the lack of physical processes that allow short-chain modifiers to dynamically intercalate on the surface of the flotation cell, disperse the rigid network of the frother, and release the TBP's inherent ability to drain, the foam layer on the surface of the flotation cell degenerates into a viscous dead foam that cannot drain. This causes a large number of heavy dihydrate gypsum particles to be mechanically entrained and trapped in the foam gaps, and then scraped out with the gangue, ultimately resulting in a drop in the dihydrate gypsum recovery rate to 74.2%. On the other hand, due to the deterioration of foam fluidity during the impurity removal stage, the impurities encased in the gypsum inclusions were not fully separated, which indirectly led to a dilution of the purity and whiteness of the subsequent conversion product, calcium oxide. In Comparative Example 2, diethyl phthalate (DEP) was removed from the composite frother, and only tributyl phosphate (TBP) monomer was used. Because the rigid, high-steric-resistance locking network provided by DEP during the pulp rise stage is lost, the strong defoaming and drainage of TBP cannot be restrained. This leads to large-scale premature collapse and critical rupture of bubbles deep within the pulp, preventing the formation of an effective and stable froth layer during the flotation stage. This also affects the reverse flotation desilication process, resulting in a large amount of silica mud and gangue impurities remaining in the gypsum system and entering subsequent processes, thus lowering the final chemical purity and whiteness of the calcium oxide. In Comparative Example 3, only diethyl phthalate (DEP) monomer was used as the foaming agent. Although the foam-stabilizing effect of DEP caused the gangue fine mud to float, the lack of a low-surface-tension defoaming active component in the reagent system, which could trigger strong drainage in situ, resulted in the foam layer at the tank opening degenerating into a rigid "dead foam sponge" that could not be unlocked for drainage. The pumping and drainage function was rendered zero, and the fluid lifting force prevented the heavy dihydrate gypsum particles from smoothly sliding down and settling due to gravity, causing mechanical entrainment losses and reducing the dihydrate gypsum recovery rate to 68.5%. In Comparative Example 4, the mass ratio of tributyl phosphate to diethyl phthalate was increased to 6:1. Compared with Example 1, the recovery rate of gypsum dihydrate, the purity of calcium oxide, and the whiteness all decreased after the proportion of tributyl phosphate was further increased in Comparative Example 4. This indicates that when the proportion of tributyl phosphate exceeds the range of compounding ratios defined in this application, the effect of the composite foaming agent on maintaining foam stability is further weakened, leading to a decrease in the flotation and carrying effect of siliceous fine mud. At the same time, some siliceous impurities that were not fully removed enter the subsequent high-temperature decomposition process, thus adversely affecting the purity and whiteness of the calcium oxide product. Therefore, simply increasing the proportion of tributyl phosphate cannot further improve the flotation effect. Comparative Example 5 used a mass ratio of tributyl phosphate to diethyl phthalate of 1:2, further increasing the proportion of diethyl phthalate.Compared to Example 3, the recovery rate of gypsum dihydrate, the purity of calcium oxide, and the whiteness were all further reduced, indicating that when the proportion of diethyl phthalate is too high, the foam stability formed by the composite foaming agent is too strong, while the drainage and foam rupture processes are inhibited, making gypsum particles more susceptible to mechanical entrainment by the foam. Simultaneously, the effective separation between siliceous fine mud and gypsum particles is affected. Therefore, the amount of diethyl phthalate added also needs to be controlled within an appropriate range. In Comparative Example 6, diethyl phthalate was replaced with dimethyl phthalate. Compared to Example 1, under the same mass ratio of tributyl phosphate to ester foaming agent, the recovery rate of gypsum dihydrate, the purity of calcium oxide, and the whiteness of Comparative Example 6 all decreased, indicating that a foaming agent does not necessarily need to have an ester group structure to achieve the same effect as diethyl phthalate. In the composite system of this application, diethyl phthalate and tributyl phosphate can effectively balance foam stability and drainage characteristics. However, when they are replaced with dimethyl phthalate, the molecular structure and interfacial behavior of the foaming agent change, which disrupts the balance between foam stability and drainage. This affects the flotation and separation of siliceous fine mud and the recovery of heavy gypsum particles, ultimately leading to a decrease in the purity and whiteness of the subsequent calcium oxide product.

[0093] As shown in Examples 1-3, changing the mass ratio of tributyl phosphate to diethyl phthalate has a certain impact on the recovery rate of gypsum dihydrate, the purity of calcium oxide, and its whiteness. In Example 2, increasing the ratio to 5:1 slightly reduced the recovery rate of gypsum dihydrate, the purity of calcium oxide, and the whiteness, indicating that while a higher proportion of tributyl phosphate enhances the foam drainage effect, it weakens the foam stability, hindering the effective flotation and separation of siliceous fine mud in the slurry. In Example 3, reducing the mass ratio to 1:1 also resulted in a decrease in all indicators, indicating that a higher proportion of diethyl phthalate enhances foam stability but reduces drainage capacity, leading to some heavy gypsum particles being mechanically entrained and lost with the foam. Therefore, a suitable mixing ratio of tributyl phosphate and diethyl phthalate is needed to balance foam stability and drainage capacity.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for the co-production of sulfuric acid and high-purity white calcium oxide based on desilication phosphogypsum, characterized in that, Includes the following steps: S1: Phosphogypsum is combined with water to form a slurry, and then a reverse flotation desilication agent is added to mix and desilicate to obtain desilication phosphogypsum. The reverse flotation desilication agent includes 100 parts of organosilicon quaternary ammonium salt collector, 20-40 parts of composite frother and 60-80 parts of modifier. The composite frother is obtained by compounding tributyl phosphate and diethyl phthalate in a mass ratio of (1-5):

1. The modifier is obtained by nucleophilic addition reaction of n-propanol and ethylene oxide under the action of a catalyst. S2: Mix the desilicationized phosphogypsum described in S1 with a reducing agent; S3: The mixed desilication phosphogypsum is added to a fluidized bed reactor and decomposes at high temperature to generate calcium oxide solid, water vapor, carbon dioxide and sulfur dioxide gas; S4: Take out the generated calcium oxide solid and cool it to obtain high-purity white calcium oxide; S5: The water vapor, carbon dioxide and sulfur dioxide gas are drawn out by a fan, and the moisture is absorbed by concentrated sulfuric acid for drying and separation to obtain a dry mixture of carbon dioxide and sulfur dioxide. S6: A dry mixture of carbon dioxide and sulfur dioxide is fed into a sulfuric acid generating unit to obtain sulfuric acid and carbon dioxide.

2. The method according to claim 1, characterized in that, The collector is an organosilicon quaternary ammonium salt collector, having the following structural formula: ， Where m is 8, 10, 12, 14 or 16; X is F, Cl, Br or I.

3. The method according to claim 1, characterized in that, The modifier is prepared by the following method: n-Propanol and ethylene oxide are mixed in a molar ratio of 1:(1.5-2.5) and reacted at 60-120°C for 1-5 hours in the presence of an alkaline catalyst to obtain a modifier.

4. The method according to claim 1, characterized in that, In step S1, the slurry concentration is 30wt% to 40wt%, and the dosage of desilication reagent for phosphogypsum reverse flotation is 100 to 500 g / t phosphogypsum.

5. The method according to claim 1, characterized in that, In step S2, the reducing agent is carbon powder, and the amount of reducing agent added is 3% to 5% of the mass of the desilication phosphogypsum.

6. The method according to claim 1, characterized in that, In step S3, the upper temperature of the fluidized bed reactor is 300℃~600℃, the lower temperature is 1000℃~1200℃, and the residence time is 2h~4h.

7. The method according to claim 1, characterized in that, The phosphogypsum is dihydrate phosphogypsum.

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