Process and system for producing sulphuric acid from phosphogypsum

Abstract

The present application relates to a kind of phosphogypsum method and system of sulfuric acid.The method includes the following steps: (a) phosphogypsum and reducing agent are mixed granulation, and raw material particle is obtained;(b) raw material particle is treated by reduction, and after reduction material is obtained, after reduction material is treated by oxidation, and calcium oxide and flue gas are obtained;(c) flue gas is carried out gas-solid separation, after heat exchange of gas phase and oxygen-containing gas, preheated oxygen-containing gas and heat exchanged flue gas are obtained;(d) after heat exchanged flue gas is treated by impurity removal, sulfur dioxide in converted impurity-removed flue gas is converted into sulfur trioxide, and converted flue gas is obtained;(e) converted flue gas is absorbed by sulfuric acid with mass fraction of more than 98%, and sulfuric acid product and absorption tail gas are obtained;Wherein, preheated oxygen-containing gas is divided into two, and is respectively introduced into reduction process and oxidation process.The method of the present application can greatly reduce production cost, improve calcium sulfate decomposition rate, improve flue gas sulfur dioxide concentration and product quality.

Description

Technical Field

[0001] This invention relates to the field of chemical production technology, specifically to a method and system for producing sulfuric acid from phosphogypsum. Background Technology

[0002] Phosphogypsum is a major byproduct of the phosphate fertilizer industry. Its composition is complex, with over 90% being calcium sulfate dihydrate, containing impurities such as phosphorus, fluorine, organic matter, oxides, and small amounts of heavy metals and radioactive substances. The water of crystallization comprises 20%-25% by mass. Phosphogypsum has become an environmental problem, requiring large-scale land storage and easily causing secondary pollution. Currently, the resource-based treatment of phosphogypsum is urgently needed, and the development of a low-cost, high-efficiency phosphogypsum decomposition and utilization technology is imperative.

[0003] There are two main types of existing technologies for the decomposition and utilization of phosphogypsum. One type is the high-temperature decomposition of phosphogypsum to produce sulfuric acid and co-produce cement. This technology uses coke and pulverized coal as reducing agents and fuels in a cement rotary kiln to decompose phosphogypsum into cement clinker, while simultaneously producing sulfur dioxide flue gas for use as a raw material for sulfuric acid production. This technology has three main problems: First, the rotary kiln has low thermal efficiency and low system heat recovery efficiency, resulting in a complex and long production process and high production costs. Second, it is difficult to control the reducing-oxidation atmosphere in the rotary kiln, leading to low sulfur dioxide content and high carbon dioxide content in the flue gas, which is not conducive to subsequent sulfuric acid production and does not comply with current carbon dioxide emission reduction policies. Third, impurities in the phosphogypsum enter the cement product, resulting in poor cement quality. An improved process involves partially or completely replacing coke and pulverized coal with sulfur in the rotary kiln for high-temperature pyrolysis. However, due to the difficulty in controlling the reducing-oxidation atmosphere in the rotary kiln, the sublimated sulfur content in the flue gas is too high, thus preventing its industrialization. Another type is the high-temperature decomposition of phosphogypsum to produce sulfuric acid and co-produce calcium oxide or calcium sulfide. This technology uses coke and pulverized coal or sulfur as reducing agents and fuels in a rotary kiln or vertical furnace to decompose phosphogypsum into calcium oxide, while simultaneously producing sulfur dioxide flue gas that can be used as a feedstock for acid production. This technology reduces the cement clinker calcination process, making the process relatively simple and easy to operate. This technology mainly has two problems: first, the reducing-oxidizing atmosphere in the decomposition furnace is difficult to control, requiring solutions to the problem of sublimed sulfur in the flue gas; second, the problem of phosphoric acid dew point corrosion in the flue gas needs to be addressed to facilitate high-temperature heat recovery and subsequent flue gas-based acid production. Summary of the Invention

[0004] Through extensive research, the inventors discovered that existing phosphogypsum decomposition and utilization technologies all suffer from problems such as difficulty in controlling the furnace atmosphere leading to low calcium sulfate decomposition rates, complex flue gas composition making recovery difficult, low heat recovery rates, complex process flows, and low product quality. Therefore, it is essential to develop a low-cost, low-energy-consumption technology for producing sulfuric acid and co-producing calcium oxide from phosphogypsum. This technology can solve the pollution problem caused by phosphogypsum waste and maintain the sustainable development of the phosphate chemical industry.

[0005] The purpose of this invention is to overcome the problems of low calcium sulfate decomposition rate, complex flue gas composition making recovery and utilization difficult, low heat recovery rate, complex process flow, and low product quality in existing phosphogypsum decomposition and utilization technologies. This invention provides a method and system for producing sulfuric acid from phosphogypsum, which has the advantages of high calcium sulfate decomposition rate, easy flue gas recovery and utilization, high heat recovery rate, and good product quality.

[0006] To achieve the above objectives, the first aspect of the present invention provides a method for producing sulfuric acid from phosphogypsum, the method comprising the following steps: (a) mixing phosphogypsum and a reducing agent and granulating to obtain raw material granules; (b) subjecting the raw material granules to reduction treatment to obtain reduced material, and subjecting the reduced material to oxidation treatment to obtain calcium oxide and flue gas; (c) subjecting the flue gas to gas-solid separation, and after heat exchange between the gas phase and oxygen-containing gas, obtaining preheated oxygen-containing gas and heat-exchanged flue gas; (d) subjecting the heat-exchanged flue gas to impurity removal, and converting the sulfur dioxide in the impurity-removed flue gas into sulfur trioxide to obtain converted flue gas; (e) absorbing the converted flue gas with sulfuric acid of 98% or higher by mass to obtain sulfuric acid product and absorption tail gas; wherein, the preheated oxygen-containing gas is divided into two streams and introduced into the reduction treatment process and the oxidation treatment process, respectively.

[0007] A second aspect of this invention provides a system for producing sulfuric acid from phosphogypsum. The system includes: a drying unit, a granulation unit, a reduction-oxidation unit, a flue gas treatment unit, a calcium oxide treatment unit, an acid production unit, and a tail gas treatment unit. The drying unit sequentially dries the phosphogypsum using tail gas from the acid production unit and calcium oxide from the calcium oxide treatment unit, resulting in a free water mass fraction of 5%-10% in the phosphogypsum. The granulation unit receives the phosphogypsum from the drying unit and mixes it with a reducing agent to granulate, obtaining raw material granules. The reduction-oxidation unit reduces the raw material granules from the granulation unit to obtain reduced material, which is then oxidized to obtain calcium oxide and flue gas. The flue gas treatment unit purifies the flue gas from the reducing unit. The flue gas from the reduction-oxidation unit is sequentially connected in series with a gas-solid separation unit, a waste heat recovery unit, an oxygen-containing gas preheating unit, a dust removal unit, an acid washing unit, a drying unit, an optional heating unit, and a denitrification unit to obtain purified flue gas. The calcium oxide treatment unit is used to cool the calcium oxide from the reduction-oxidation unit and the solid phase separated by the gas-solid separation unit in the flue gas treatment unit to obtain hot water and cooled calcium oxide. The hot water is passed into the waste heat recovery unit to generate saturated steam. The acid production unit first converts sulfur dioxide in the purified flue gas from the flue gas treatment unit into sulfur trioxide, and then absorbs it with sulfuric acid with a mass fraction of more than 98% to obtain sulfuric acid product and tail gas. The tail gas treatment unit receives the tail gas from the drying unit, and discharges it after sequentially performing desulfurization and filtration.

[0008] Through the above technical solution, the present invention has the following advantages: The method for producing sulfuric acid from phosphogypsum according to the present invention can greatly improve the heat recovery rate, significantly reduce fuel consumption, lower production costs, increase the decomposition rate of calcium sulfate, and improve the concentration of sulfur dioxide in flue gas and product quality.

[0009] The device of the present invention is equipped with heat recovery devices at each stage to achieve thermal coupling within the system, fully recover and utilize heat energy, and significantly improve product quality while increasing the heat energy recovery rate. Attached Figure Description

[0010] Figure 1 A flowchart illustrating a preferred embodiment of the present invention; Figure 1 In the middle section: 1-Phosphogypsum dryer; 2-Mixer; 3-Granulator; 4-Reduction section of gypsum decomposition furnace; 5-Oxidation section of gypsum decomposition furnace; 6-Rotary calcium oxide cooler; 7-Cyclone separator; 8-Heat pipe waste heat boiler; 9-Air preheater; 10-Electrostatic precipitator; 11-Dilute acid washing and purification process; 12-SO2 fan; 13-Drying tower; 14-Denitrification reactor; 15-Converter; 16-Absorption tower; 17-Desulfurization tower; 18-Flue gas filter; 19-Chimney.

[0011] Figure 2 This is a schematic diagram of the structure of a reduction-oxidation unit decomposition furnace according to a preferred embodiment of the present invention; Figure 2 In the middle section: 21-Preheated air inlet for reduction section; 22-Air cap distribution plate for reduction section; 23-Raw material particle inlet; 24-Fuel inlet for reduction section; 25-Secondary air inlet for reduction section; 26-Reduced material outlet; 27-Fluorescence outlet for decomposition furnace; 28-Secondary air inlet for oxidation section; 29-Fuel inlet for oxidation section; 210-Calcium oxide outlet; 211-Air cap distribution plate for oxidation section; 212-Preheated air inlet for oxidation section. Detailed Implementation

[0012] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0013] In this invention, unless otherwise stated, directional terms such as "upper," "lower," "top," and "bottom" are generally used to describe the relative positional relationships of components in relation to the directions shown in the accompanying drawings or in relation to vertical, perpendicular, or gravitational directions. "Inner" and "outer" refer to the inner and outer contours of each component itself.

[0014] In this invention, unless otherwise specified, the term "bottle" or "bottom" or other approximate description of a container refers to 90%-100% of the container from top to bottom; the term "top" or "tower" or other approximate description of a container refers to 0-10% of the container from top to bottom; the term "upper part" or other approximate description of a container refers to 0-30% of the container from top to bottom; the term "middle part" or other approximate description of a container refers to 30%-70% of the container from top to bottom; and the term "lower part" or other approximate description of a container refers to 70%-100% of the container from top to bottom.

[0015] This invention provides a method for producing sulfuric acid from phosphogypsum, the method comprising the following steps: (a) Phosphogypsum and reducing agent are mixed and granulated to obtain raw material granules; (b) The raw material particles are reduced to obtain the reduced material, and the reduced material is oxidized to obtain calcium oxide and flue gas; (c) The flue gas undergoes gas-solid separation. After the gas phase exchanges heat with the oxygen-containing gas, preheated oxygen-containing gas and heat-exchanged flue gas are obtained. (d) After heat exchange, the flue gas is purified to convert sulfur dioxide into sulfur trioxide, thus obtaining the converted flue gas. (e) The converted flue gas is absorbed by sulfuric acid with a mass fraction of 98% or more to obtain sulfuric acid products and absorption tail gas; The preheated oxygen-containing gas is divided into two streams, which are introduced into the reduction process and the oxidation process, respectively.

[0016] The method for producing sulfuric acid from phosphogypsum according to the present invention can greatly improve the heat recovery rate, significantly reduce fuel consumption, lower production costs, increase the decomposition rate of calcium sulfate, and improve the concentration of sulfur dioxide in flue gas and product quality.

[0017] According to a preferred embodiment of the present invention, in step (a), the phosphogypsum is first dehydrated and / or its acidity adjusted, and then mixed with a reducing agent and granulated. By adopting the aforementioned preferred scheme, the decomposition rate of calcium sulfate and the product quality can be further improved.

[0018] In this invention, there are no particular requirements for the method of dehydration and / or acidity adjustment of the phosphogypsum in step (a). The dehydrating agent and acidity adjuster used can be conventional choices in the art. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the dehydration and / or acidity adjustment method includes first performing a pretreatment by purging the absorbent tail gas in step (e), and then mixing it with calcium oxide in step (b) for a secondary treatment. After the secondary treatment, the free water mass fraction in the phosphogypsum is 5-10%. The purging pretreatment is performed by counter-current convection heat exchange purging. By adopting the aforementioned preferred scheme, the calcium sulfate decomposition rate and product quality can be further improved while reducing costs.

[0019] According to a preferred embodiment of the present invention, the exhaust gas after purging pretreatment is sent to a desulfurization process to remove particles before being discharged into the atmosphere. Preferably, the desulfurization is performed using dry desulfurization with calcium oxide as described in step (b). By adopting the aforementioned preferred scheme, costs can be further reduced.

[0020] In this invention, there is no particular limitation on the equivalent diameter of the raw material particles. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the equivalent diameter of the raw material particles is 0.5-2 cm, preferably 0.8-1.2 cm. By adopting the aforementioned preferred scheme, the decomposition rate of calcium sulfate and the product quality can be further improved.

[0021] In this invention, the reducing agent can be a conventional choice in the art. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the reducing agent is selected from at least one of sulfur, coke powder, biomass pellets, and high-sulfur coal powder, preferably sulfur and / or biomass pellets. By adopting the aforementioned preferred scheme, the decomposition rate of calcium sulfate and product quality can be further improved.

[0022] In this invention, as long as sufficient reduction can be achieved, there are no special requirements on the amount of reducing agent in the raw material particles. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the mass ratio of the reducing agent in the raw material particles to phosphogypsum, calculated as calcium sulfate, is 0.2-0.7.

[0023] When the reducing agent is sulfur, the mass ratio of the reducing agent to phosphogypsum (calculated as calcium sulfate) is 0.5-0.7; when the reducing agent is coke powder, the mass ratio of the reducing agent to phosphogypsum (calculated as calcium sulfate) is 0.2-0.4; other reducing agents containing sulfur and carbon can be converted according to the above addition ratios.

[0024] In this invention, there are no special requirements for the conditions of the reduction treatment. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the conditions of the reduction treatment include: a temperature of 800-1000°C and an oxygen volume fraction of 0.5-2%.

[0025] To further improve the reduction effect, according to a preferred embodiment of the present invention, the conditions for the reduction treatment include: a temperature of 800-1000℃, an oxygen volume fraction of no more than 2%, and a residence time of 30-50s. The temperature can be any value between two endpoints or a range of values, including but not limited to integer values ​​such as 800℃, 850℃, 900℃, 950℃, and 1000℃. Similarly, the oxygen volume fraction and residence time can be, for example, 0.5%, 1%, 1.5%, and 2% oxygen volume fraction, and 30s, 35s, 40s, and 50s residence time.

[0026] In this invention, there are no special requirements for the conditions of the reduction treatment. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the conditions of the oxidation treatment include: a temperature of 900-1100°C and an oxygen volume fraction of 8%-12%.

[0027] To further improve the oxidation effect, according to a preferred embodiment of the present invention, the oxidation treatment conditions include: a temperature of 900-1100℃, an oxygen volume fraction of 8%-12%, and a residence time of 30-50s. The temperature can be any value between two endpoints or a range of values, including but not limited to integer values ​​such as 900℃, 950℃, 1000℃, 1050℃, and 1100℃; similarly, the oxygen volume fraction and residence time can be, for example, 8%, 9%, 10%, 11%, and 12%; and the residence time can be 30s, 35s, 40s, and 50s.

[0028] According to a preferred embodiment of the present invention, the oxidation treatment temperature is 100-200°C higher than the reduction treatment temperature. By adopting the aforementioned preferred scheme, the calcium sulfate decomposition rate and product quality can be further improved.

[0029] According to a preferred embodiment of the present invention, in step (b), the calcium oxide is recovered after cooling. To further reduce costs and improve heat recovery rate, water cooling is used for heat exchange cooling. The water after heat exchange is then exchanged with the gas phase before heat exchange with oxygen-containing gas in step (c) to generate saturated steam. The water is demineralized water, and the outlet temperature is controlled above 600°C to prevent dew point corrosion.

[0030] In this invention, the solid phase of the flue gas after gas-solid separation is optionally passed into the calcium oxide in step (b).

[0031] In this invention, in order to further improve the decomposition rate of calcium sulfate and the product quality, according to a preferred embodiment of the invention, the temperature of the preheated oxygen-containing gas is 300-400℃, for example, 300℃, 320℃, 340℃, 360℃, 380℃, or 400℃.

[0032] In this invention, the object to be removed in step (d) is a conventional choice in the art. The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, in step (d), the removal of impurities includes at least one of dust removal, dehydration and denitrification.

[0033] In this invention, there are no special requirements for the order of impurity removal in step (d). The following is an illustrative description, but it does not limit the scope of the invention. According to a preferred embodiment of the invention, the impurity removal sequentially includes: dust removal, dehydration, and denitrification. By adopting the aforementioned preferred solution, product quality can be further improved.

[0034] In this invention, the process prior to denitrification may optionally include defluorination, demisting, and re-dehydration. After demisting, the flue gas temperature drops below 50°C, and the water content in the flue gas is controlled at 100 mg / m³. 3 Next, dehydration is carried out by removing moisture from the flue gas with concentrated sulfuric acid of 92%-95% by mass. The dried flue gas is then heated to 400-430℃ through a cold gas heat exchanger before entering the denitrification process.

[0035] In this invention, after dust removal, the product is washed with dilute acid and cooled. The dilute acid is dilute sulfuric acid with a mass fraction of 5%-10%.

[0036] In this invention, denitrification includes using a reducing agent to reduce nitrogen oxides in flue gas into nitrogen gas. The reducing agent can be at least one of liquid ammonia, ammonia water, urea and ammonium carbonate, preferably liquid ammonia.

[0037] This invention provides a system for producing sulfuric acid from phosphogypsum. The system includes: a drying unit, a granulation unit, a reduction-oxidation unit, a flue gas treatment unit, a calcium oxide treatment unit, an acid production unit, and a tail gas treatment unit. The drying unit sequentially uses tail gas from the acid production unit and calcium oxide from the calcium oxide treatment unit to dry the phosphogypsum, resulting in a free water content of 5%-10% in the phosphogypsum. The granulation unit receives the phosphogypsum from the drying unit and mixes it with a reducing agent to granulate it, obtaining raw material granules. The reduction-oxidation unit reduces the raw material granules from the granulation unit to obtain reduced material, which is then oxidized to obtain calcium oxide and flue gas. The flue gas treatment unit purifies the flue gas from the reduction-oxidation unit. The flue gas from the purification unit is sequentially connected in series with a gas-solid separation unit, a waste heat recovery unit, an oxygen-containing gas preheating unit, a dust removal unit, an acid washing unit, a drying unit, an optional heating unit, and a denitrification unit to obtain purified flue gas. The calcium oxide treatment unit is used to cool the calcium oxide from the reduction-oxidation unit and the solid phase separated by the gas-solid separation unit in the flue gas treatment unit to obtain hot water and cooled calcium oxide. The hot water is passed into the waste heat recovery unit to generate saturated steam. The acid production unit first converts sulfur dioxide in the purified flue gas from the flue gas treatment unit into sulfur trioxide, and then absorbs it with sulfuric acid with a mass fraction of more than 98% to obtain sulfuric acid product and tail gas. The tail gas treatment unit receives the tail gas from the drying unit, and discharges it after sequentially performing desulfurization and filtration.

[0038] The device of the present invention is equipped with heat recovery devices at each stage to achieve thermal coupling within the system, fully recover and utilize heat energy, and significantly improve product quality while increasing the heat energy recovery rate.

[0039] In this invention, the devices constituting each unit can be conventional choices in the art. The following examples illustrate preferred unit device compositions discovered by the inventors through research, but do not limit the scope of the invention. Figure 1 As shown, the drying unit includes a phosphogypsum dryer 1 and a mixer 2 connected in series; the granulation unit includes a granulator 3; the reduction-oxidation unit includes a gypsum decomposition furnace reduction section 4 and an gypsum decomposition furnace oxidation section 5 connected in series; the flue gas treatment unit includes a cyclone separator 7, a heat pipe waste heat boiler 8, an air preheater 9, an electrostatic precipitator 10, a dilute acid washing and purification process 11 (including a dynamic wave scrubber using 5-10% dilute sulfuric acid by mass fraction), an SO2 fan 12, a drying tower 13, an optional cold gas heat exchanger (not shown in the figure), and a denitrification reactor 14 connected in series; the calcium oxide treatment unit includes a rotary calcium oxide cooler 6; the acid production unit includes a converter 15 and an absorption tower 16 connected in series; and the tail gas treatment unit includes a desulfurization tower 17, a flue gas filter 18, and a chimney 19. If necessary, a defluorination tower, a demister, and a re-dehydration tower are installed before the denitrification reactor according to the material flow direction.

[0040] According to a preferred embodiment of the present invention, the reduction-oxidation unit includes a reduction section and an oxidation section. The reduction section, from top to bottom, is provided with a reducing material outlet, a secondary air inlet, a fuel inlet, a raw material inlet, and a preheated air inlet. A wind cap distribution plate is provided between the preheated air inlet and the raw material inlet. The oxidation section, from top to bottom, is provided with a flue gas outlet, a secondary air inlet, a fuel inlet, an oxide feed inlet, a calcium oxide outlet, and a preheated air inlet. A wind cap distribution plate is provided between the preheated air inlet and the calcium oxide outlet. Specifically... Figure 2 As shown, the reduction section, from top to bottom, is provided with a reducing material outlet 26, a secondary air inlet 25, a fuel inlet 24, a raw material granule inlet 23, and a preheated air inlet 21. A reduction section air cap distribution plate 22 is installed between the preheated air inlet 21 and the raw material granule inlet 23. The oxidation section, from top to bottom, is provided with a decomposition furnace flue gas outlet 27, a secondary air inlet 28, a fuel inlet 29, an oxide material feed inlet, a calcium oxide outlet 210, and a preheated air inlet 212. An oxidation section air cap distribution plate 211 is installed between the preheated air inlet 212 and the calcium oxide outlet 210. The oxide material feed inlet of the oxidation section is connected to the reducing material outlet 26 of the reduction section.

[0041] The present invention will be described in detail below through examples. In the following examples, parameters such as water content, temperature, and oxygen volume fraction were measured by chemical analysis and instrument measurement methods; the phosphogypsum raw material was phosphogypsum by-product of Anhui Liuguo Chemical Co., Ltd.

[0042] Example 1 exist Figure 1-2 The process and apparatus shown are carried out in the following way: Phosphogypsum with a free water content of 12% from the phosphogypsum stockpile is fed into phosphogypsum dryer 1, where it is heat-exchanged by counter-current convection using the drying tail gas from absorption tower 16 to remove moisture from the phosphogypsum. The phosphogypsum is then fed into mixer 2 and mixed with a portion of calcium oxide from rotary calcium oxide cooler 6. After mixing, the free water content of the material is reduced to 8%, and the mixture is fed into granulator 3 to granulate into phosphogypsum balls with a diameter of 1 cm (the mass ratio of reducing agent sulfur to phosphogypsum, calculated as calcium sulfate, is 0.5).

[0043] The phosphogypsum balls are fed into the reduction section 4 of the gypsum decomposition furnace from the raw material particle inlet 23 by a belt conveyor. Under the action of 400℃ preheated air from the air preheater 9 and sulfur fuel, the phosphogypsum undergoes boiling decomposition at a high temperature of 900℃. The flow rate of the preheated air and the amount of secondary air introduced are adjusted to control the oxygen volume fraction in the furnace to 2% and the material residence time to 40s.

[0044] After reduction, the material and flue gas enter the oxidation section 5 of the gypsum decomposition furnace. Under the action of preheated air from the air preheater 9 and sulfur fuel, the phosphogypsum undergoes boiling decomposition at a high temperature of 1000℃. The preheated air flow rate and secondary air intake are adjusted to control the oxygen volume fraction in the furnace to 10% and the residence time to 50s. Sampling and testing show that the calcium sulfate decomposition rate reaches over 99%.

[0045] After the oxidation reaction, the high-temperature flue gas comes out from the upper part of the oxidation section 5 of the gypsum decomposition furnace and passes through the cyclone separator 7 to separate calcium oxide dust. Then the flue gas enters the heat pipe waste heat boiler 8 to recover heat, and uses the boiler feedwater preheated by the rotary calcium oxide cooler 6 to generate 3.82MPa medium-pressure saturated steam.

[0046] The high-temperature calcium oxide from the lower part of the oxidation section 5 of the gypsum decomposition furnace and the cyclone separator 7 is fed into the rotary calcium oxide cooler 6 for cooling before being sent to the calcium oxide storage silo. The quality of the calcium oxide meets the requirements of HG / T 4205-2011 "Industrial Calcium Oxide", with a calcium oxide mass fraction of 92%.

[0047] The flue gas from the heat pipe waste heat boiler 8, at an outlet temperature of approximately 600°C, enters the air preheater 9 and the electrostatic precipitator 10 for cooling and dust removal. After cooling, it then enters the dilute acid washing and purification process 11, where 10% dilute sulfuric acid is used to remove impurities and moisture from the flue gas.

[0048] After treatment, the flue gas is sent to the drying tower 13 by SO2 blower 12. After being dried with 93% sulfuric acid, the flue gas is heated to 400°C by cold gas heat exchanger and then enters the denitrification reactor 14, where liquid ammonia is injected to remove nitrogen oxides from the flue gas.

[0049] After denitrification, the flue gas enters the converter 15, where sulfur dioxide in the flue gas is converted into sulfur trioxide with a conversion rate of 99.8%. The flue gas containing sulfur trioxide is sent to the absorption tower 16 and absorbed with 98% sulfuric acid with a mass fraction of 99%.

[0050] The dried exhaust gas from the outlet of absorber tower 16 is sent to phosphogypsum dryer 1 via an exhaust gas booster fan. The flue gas from the outlet of phosphogypsum dryer 1 enters desulfurization tower 17, where calcium oxide from the calcium oxide storage silo is used as the desulfurizing agent for dry desulfurization. After desulfurization, the flue gas enters flue gas filter 18 to remove particulate matter before being discharged from chimney 19. The SO2 concentration in the discharged exhaust gas is less than 50 mg / m³. 3 Sulfuric acid mist less than 5 mg / m³ 3 Particulate matter less than 30 mg / m³ 3 NO x Less than 50mg / m 3 .

[0051] In this embodiment, the control of the atmosphere for the reduction and oxidation reactions is as shown in the attached figure. Figure 2 As shown: 400℃ preheated air enters from the preheated air inlet 21 of the reduction section, and burns with the fuel sulfur entering from the fuel inlet 24 of the reduction section to generate high temperature, causing the phosphogypsum balls entering from the raw material particle inlet 23 to boil and decompose at 900℃. Secondary air is appropriately supplemented from the secondary air inlet 25 of the reduction section according to the furnace working conditions. Preheated air at 400℃ enters from the preheated air inlet 212 of the oxidation section. It combines with fuel sulfur entering from the fuel inlet 29 of the oxidation section to generate high-temperature combustion, further boiling and decomposing the material from the reducing material outlet 26 at 1000℃. Secondary air is supplemented appropriately from the secondary air inlet 28 of the oxidation section according to the furnace conditions. The calcium oxide after high-temperature decomposition is discharged from the calcium oxide outlet 210, and the high-temperature flue gas after the reaction is discharged from the decomposition furnace flue gas outlet 27.

[0052] The volume ratio of 400℃ preheated air introduced into the reduction section and the oxidation section is 0.20.

[0053] Example 2 exist Figure 1-2 The process and apparatus shown are carried out in the following way: Phosphogypsum with a free water content of 12% from the phosphogypsum stockpile is fed into phosphogypsum dryer 1, where it is heat-exchanged by counter-current convection using the drying tail gas from absorption tower 16 to remove moisture from the phosphogypsum. The phosphogypsum is then fed into mixer 2 and mixed with a portion of calcium oxide from rotary calcium oxide cooler 6. After mixing, the free water content of the material is reduced to 9%, and the mixture is fed into granulator 3 to granulate into phosphogypsum balls with a diameter of 1.2 cm (the mass ratio of reducing agent sulfur to phosphogypsum, calculated as calcium sulfate, is 0.4).

[0054] The phosphogypsum balls are fed into the reduction section 4 of the gypsum decomposition furnace from the raw material granule inlet 23 by a belt conveyor. Under the action of 340°C preheated air from the air preheater 9 and sulfur fuel, the phosphogypsum undergoes boiling decomposition at a high temperature of 850°C. The preheated air flow rate and secondary air intake are adjusted to control the oxygen volume fraction in the furnace to 1.5% and the material residence time to 35s.

[0055] After reduction, the material and flue gas enter the oxidation section 5 of the gypsum decomposition furnace. Under the action of preheated air from the air preheater 9 and sulfur fuel, the phosphogypsum undergoes boiling decomposition at a high temperature of 950℃. The preheated air flow rate and secondary air intake are adjusted to control the oxygen volume fraction in the furnace to 10% and the residence time to 45s. Sampling and testing show that the calcium sulfate decomposition rate reaches over 99%.

[0056] After the oxidation reaction, the high-temperature flue gas comes out from the upper part of the oxidation section 5 of the gypsum decomposition furnace and passes through the cyclone separator 7 to separate calcium oxide dust. Then the flue gas enters the heat pipe waste heat boiler 8 to recover heat, and uses the boiler feedwater preheated by the rotary calcium oxide cooler 6 to generate 3.80MPa medium-pressure saturated steam.

[0057] The high-temperature calcium oxide from the lower part of the oxidation section 5 of the gypsum decomposition furnace and the cyclone separator 7 is fed into the rotary calcium oxide cooler 6 for cooling before being sent to the calcium oxide storage silo. The quality of the calcium oxide meets the requirements of HG / T 4205-2011 "Industrial Calcium Oxide", with a calcium oxide mass fraction of 91%.

[0058] The flue gas from the heat pipe waste heat boiler 8, at an outlet temperature of approximately 600°C, enters the air preheater 9 and the electrostatic precipitator 10 for cooling and dust removal. After cooling, it then enters the dilute acid washing and purification process 11, where 10% dilute sulfuric acid is used to remove impurities and moisture from the flue gas.

[0059] After treatment, the flue gas is sent to the drying tower 13 by SO2 blower 12. After being dried with 93% sulfuric acid, the flue gas is heated to 400°C by cold gas heat exchanger and then enters the denitrification reactor 14, where liquid ammonia is injected to remove nitrogen oxides from the flue gas.

[0060] After denitrification, the flue gas enters the converter 15, where sulfur dioxide in the flue gas is converted into sulfur trioxide with a conversion rate of 99.8%. The flue gas containing sulfur trioxide is sent to the absorption tower 16 and absorbed with 98% sulfuric acid with a mass fraction of 99%.

[0061] The dried exhaust gas from the outlet of absorber tower 16 is sent to phosphogypsum dryer 1 via an exhaust gas booster fan. The flue gas from the outlet of phosphogypsum dryer 1 enters desulfurization tower 17, where calcium oxide from the calcium oxide storage silo is used as the desulfurizing agent for dry desulfurization. After desulfurization, the flue gas enters flue gas filter 18 to remove particulate matter before being discharged from chimney 19. The SO2 concentration in the discharged exhaust gas is less than 50 mg / m³. 3 Sulfuric acid mist less than 5 mg / m³ 3 Particulate matter less than 30 mg / m³ 3 NO x Less than 50mg / m 3 .

[0062] In this embodiment, the control of the atmosphere for the reduction and oxidation reactions is as shown in the attached figure. Figure 2 As shown: Preheated air at 340℃ enters from the preheated air inlet 21 of the reduction section. It burns with the fuel sulfur entering from the fuel inlet 24 of the reduction section to generate high temperature, causing the phosphogypsum balls entering from the raw material particle inlet 23 to boil and decompose at 850℃. Secondary air is appropriately supplemented from the secondary air inlet 25 of the reduction section according to the furnace working conditions. Preheated air at 340℃ enters from the preheated air inlet 212 of the oxidation section. It combines with fuel sulfur entering from the fuel inlet 29 of the oxidation section to generate high-temperature combustion, further boiling and decomposing the material from the reducing material outlet 26 at 950℃. Secondary air is supplemented appropriately from the secondary air inlet 28 of the oxidation section according to the furnace conditions. The calcium oxide after high-temperature decomposition is discharged from the calcium oxide outlet 210, and the high-temperature flue gas after the reaction is discharged from the decomposition furnace flue gas outlet 27.

[0063] The volume ratio of the 340℃ preheated air introduced into the reduction section and the oxidation section is 0.15.

[0064] Example 3 exist Figure 1-2 The process and apparatus shown are carried out in the following way: Phosphogypsum with a free water content of 12% from the phosphogypsum stockpile is fed into phosphogypsum dryer 1, where it is heat-exchanged by counter-current convection using the drying tail gas from absorption tower 16 to remove moisture from the phosphogypsum. The phosphogypsum is then fed into mixer 2 and mixed with a portion of calcium oxide from rotary calcium oxide cooler 6. After mixing, the free water content of the material is reduced to 6%, and the mixture is fed into granulator 3 to granulate into phosphogypsum balls with a diameter of 0.8 cm (the mass ratio of reducing agent sulfur to phosphogypsum, calculated as calcium sulfate, is 0.6).

[0065] The phosphogypsum balls are fed into the reduction section 4 of the gypsum decomposition furnace from the raw material granule inlet 23 by a belt conveyor. Under the action of 360°C preheated air from the air preheater 9 and sulfur fuel, the phosphogypsum undergoes boiling decomposition at a high temperature of 950°C. The preheated air flow rate and secondary air intake are adjusted to control the oxygen volume fraction in the furnace to 1.0% and the material residence time to 45s.

[0066] After reduction, the material and flue gas enter the oxidation section 5 of the gypsum decomposition furnace. Under the action of preheated air from the air preheater 9 and sulfur fuel, the phosphogypsum undergoes boiling decomposition at a high temperature of 1050℃. The flow rate of preheated air and the amount of secondary air introduced are adjusted to control the oxygen volume fraction in the furnace to 10% and the residence time to 40s. Sampling and testing show that the calcium sulfate decomposition rate reaches more than 99%, and the SO3 concentration in the decomposition gas does not exceed 2%.

[0067] After the oxidation reaction, the high-temperature flue gas comes out from the upper part of the oxidation section 5 of the gypsum decomposition furnace and passes through the cyclone separator 7 to separate calcium oxide dust. Then the flue gas enters the heat pipe waste heat boiler 8 to recover heat, and uses the boiler feedwater preheated by the rotary calcium oxide cooler 6 to generate 3.85MPa medium-pressure saturated steam.

[0068] The high-temperature calcium oxide from the lower part of the oxidation section 5 of the gypsum decomposition furnace and the cyclone separator 7 is fed into the rotary calcium oxide cooler 6 for cooling before being sent to the calcium oxide storage silo. The quality of the calcium oxide meets the requirements of HG / T 4205-2011 "Industrial Calcium Oxide", with a calcium oxide mass fraction of 91.5%.

[0069] The flue gas from the heat pipe waste heat boiler 8, at an outlet temperature of approximately 600°C, enters the air preheater 9 and the electrostatic precipitator 10 for cooling and dust removal. Subsequently, it enters the dilute acid washing and purification process 11, where 8% dilute sulfuric acid is used to remove impurities and moisture from the flue gas.

[0070] After treatment, the flue gas is sent to the drying tower 13 by SO2 blower 12. After being dried with 93% sulfuric acid, the flue gas is heated to 400°C by cold gas heat exchanger and then enters the denitrification reactor 14, where liquid ammonia is injected to remove nitrogen oxides from the flue gas.

[0071] After denitrification, the flue gas enters the converter 15, where sulfur dioxide in the flue gas is converted into sulfur trioxide with a conversion rate of 99.8%. The flue gas containing sulfur trioxide is sent to the absorption tower 16 and absorbed with 98% sulfuric acid with a mass fraction of 99%.

[0072] The dried exhaust gas from the outlet of absorber tower 16 is sent to phosphogypsum dryer 1 via an exhaust gas booster fan. The flue gas from the outlet of phosphogypsum dryer 1 enters desulfurization tower 17, where calcium oxide from the calcium oxide storage silo is used as the desulfurizing agent for dry desulfurization. After desulfurization, the flue gas enters flue gas filter 18 to remove particulate matter before being discharged from chimney 19. The SO2 concentration in the discharged exhaust gas is less than 50 mg / m³. 3 Sulfuric acid mist less than 5 mg / m³ 3 Particulate matter less than 30 mg / m³ 3 NO x Less than 50mg / m 3 .

[0073] In this embodiment, the control of the atmosphere for the reduction and oxidation reactions is as shown in the attached figure. Figure 2 As shown: 360°C preheated air enters from the preheated air inlet 21 of the reduction section, and burns with the fuel sulfur entering from the fuel inlet 24 of the reduction section to generate high temperature, causing the phosphogypsum balls entering from the raw material particle inlet 23 to boil and decompose at 950°C. Secondary air is appropriately supplemented from the secondary air inlet 25 of the reduction section according to the furnace working conditions. Preheated air at 360°C enters from the preheated air inlet 212 of the oxidation section. It combines with sulfur fuel entering from the fuel inlet 29 of the oxidation section to generate high temperatures, further boiling and decomposing the material from the reducing material outlet 26 at 1050°C. Secondary air is supplemented as needed from the secondary air inlet 28 of the oxidation section according to the furnace conditions. The calcium oxide after high-temperature decomposition is discharged from the calcium oxide outlet 210, and the high-temperature flue gas after the reaction is discharged from the decomposition furnace flue gas outlet 27.

[0074] The volume ratio of the 360°C preheated air introduced into the reduction section and the oxidation section is 0.1.

[0075] Example 4 Same as Example 1, except that drying and dehydration are not performed, and the reducing agent is directly granulated in granulator 3.

[0076] Results: The calcium sulfate decomposition rate was 97%, fuel consumption increased by 5%, and the calcium oxide mass fraction was 88%.

[0077] Example 5 Similar to Example 1, except that during granulation, phosphogypsum balls with a diameter of 2.5 cm are formed.

[0078] Results: The calcium sulfate decomposition rate was 96%, and the calcium oxide mass fraction was 87.3%.

[0079] Example 6 Same as Example 1, except that the mass ratio of reducing agent sulfur to phosphogypsum (calculated as calcium sulfate) in the raw material granules is 0.1.

[0080] Results: The calcium sulfate decomposition rate was 94%, and the calcium oxide mass fraction was 86.5%.

[0081] Example 7 Same as Example 1, except that the temperature of the reduction section is 750°C.

[0082] Results: The decomposition rate of calcium sulfate was 93.5%, and the mass fraction of calcium oxide was 86.4%.

[0083] Example 8 Same as Example 1, except that the temperature of the oxidation section is 900°C.

[0084] Results: The mass fraction of calcium oxide was 89%.

[0085] Example 9 Same as Example 1, except that the temperature of the oxygen-containing gas after preheating is 500°C.

[0086] Effect: The temperature of the oxygen-containing gas after preheating is too high, which leads to a decrease in steam recovery rate and subsequent dew point corrosion.

[0087] Comparative Example 1 Same as Example 1, except that the preheated air is not introduced into the reduction section.

[0088] Effects: If the preheated air is not introduced into the reduction section, the fuel consumption of the reduction section will increase significantly and sublimation sulfur will be produced, which will affect subsequent equipment. At the same time, it will not be able to effectively decompose phosphogypsum. The decomposition rate of calcium sulfate in phosphogypsum is 88%, and the mass fraction of calcium oxide is 80%.

[0089] Comparative Example 2 Similar to Example 1, except that the phosphogypsum and reducing agent are not mixed and granulated, but are directly fed into the reduction section.

[0090] Results: In the reduction section, phosphogypsum and reducing agent separate into layers, making it impossible to effectively decompose phosphogypsum. The decomposition rate of calcium sulfate in phosphogypsum is 80%, and the mass fraction of calcium oxide is 76%.

[0091] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for producing sulfuric acid from phosphogypsum, characterized in that, The method includes the following steps: (a) Phosphogypsum and reducing agent are mixed and granulated to obtain raw material granules; (b) The raw material particles are reduced to obtain the reduced material, and the reduced material is oxidized to obtain calcium oxide and flue gas; (c) The flue gas undergoes gas-solid separation. After the gas phase exchanges heat with the oxygen-containing gas, preheated oxygen-containing gas and heat-exchanged flue gas are obtained. (d) After heat exchange, the flue gas is purified to convert sulfur dioxide into sulfur trioxide, thus obtaining the converted flue gas. (e) The converted flue gas is absorbed by sulfuric acid with a mass fraction of 98% or more to obtain sulfuric acid products and absorption tail gas; The preheated oxygen-containing gas is divided into two streams, which are introduced into the reduction process and the oxidation process, respectively.

2. The method according to claim 1, wherein, In step (a), the phosphogypsum is first dehydrated and / or its acidity is adjusted, and then it is mixed with a reducing agent and granulated. Preferably, the method for dehydration and / or acidity adjustment includes first performing a pretreatment by purging the exhaust gas in step (e), and then mixing it with calcium oxide in step (b) for a secondary treatment. After the secondary treatment, the free water mass fraction in the phosphogypsum is 5%-10%. More preferably, the pretreated exhaust gas is sent to a desulfurization plant to remove particles before being discharged into the atmosphere. More preferably, the desulfurization is carried out using dry desulfurization with calcium oxide as described in step (b).

3. The method according to claim 1 or 2, wherein, In step (a), the equivalent diameter of the raw material particles is 0.5-2 cm, preferably 0.8-1.2 cm.

4. The method according to any one of claims 1-3, wherein, In step (a), the mass ratio of reducing agent to phosphogypsum (calculated as calcium sulfate) in the raw material granules is 0.2-0.7; preferably, The reducing agent is selected from at least one of sulfur, coke powder, biomass pellets, and high-sulfur coal powder, preferably sulfur and / or biomass pellets.

5. The method according to any one of claims 1-4, wherein, In step (b), The conditions for the reduction treatment include: a temperature of 800-1000℃ and an oxygen volume fraction of 0.5%-2%; preferably, the conditions for the reduction treatment include: a temperature of 800-1000℃, an oxygen volume fraction of no more than 2%, and a residence time of 30-50 seconds. and / or The oxidation treatment conditions include: a temperature of 900-1100℃ and an oxygen volume fraction of 8%-12%; preferably, the oxidation treatment conditions include: a temperature of 900-1100℃ and an oxygen volume fraction of 8%-12%, and a residence time of 30-50s. Preferably, The oxidation treatment temperature is 100-200℃ higher than the reduction treatment temperature.

6. The method according to any one of claims 1-5, wherein, In step (b), the calcium oxide is recovered after cooling, preferably by water cooling for heat exchange. The water after heat exchange then exchanges heat with the gas phase before heat exchange with the oxygen-containing gas in step (c) to generate saturated steam.

7. The method according to any one of claims 1-6, wherein, In step (c), The solid phase of the flue gas after gas-solid separation is passed into the calcium oxide in step (b); and / or The temperature of the preheated oxygen-containing gas is 300-400℃.

8. The method according to any one of claims 1-7, wherein, In step (d), the impurity removal includes at least one of dust removal, dehydration, and denitrification; preferably, The impurity removal process includes, in sequence, dust removal, dehydration, and denitrification.

9. A system for producing sulfuric acid from phosphogypsum, characterized in that, The system includes: a drying unit, a granulation unit, a reduction-oxidation unit, a flue gas treatment unit, a calcium oxide treatment unit, an acid production unit, and a tail gas treatment unit; among which, The drying unit sequentially uses the tail gas from the acid production unit and the calcium oxide from the calcium oxide treatment unit to dry the phosphogypsum, so that the free water mass fraction in the phosphogypsum is 5%-10%. The granulation unit is used to receive phosphogypsum from the drying unit and mix the phosphogypsum with a reducing agent to granulate, thereby obtaining raw material granules; The reduction-oxidation unit is used to reduce the raw material particles from the granulation unit to obtain the reduced material, which is then oxidized to obtain calcium oxide and flue gas. The flue gas treatment unit is used to purify the flue gas from the reduction-oxidation unit, and includes, in series, a gas-solid separation component, a waste heat recovery component, an oxygen-containing gas preheating component, a dust removal component, an acid washing component, a drying component, an optional heating component, and a denitrification component to obtain purified flue gas. The calcium oxide treatment unit is used to cool the calcium oxide from the reduction-oxidation unit and the solid phase separated by the gas-solid separation component in the flue gas treatment unit to obtain hot water and cooled calcium oxide; wherein, the hot water is passed into the waste heat recovery component to generate saturated steam. The acid production unit first converts sulfur dioxide in the purified flue gas from the flue gas treatment unit into sulfur trioxide, and then absorbs it with sulfuric acid with a mass fraction of more than 98% to obtain sulfuric acid product and tail gas. The exhaust gas treatment unit receives exhaust gas from the drying unit, and discharges it after desulfurization and filtration.

10. The system according to claim 9, wherein, The reduction-oxidation unit includes a reduction section and an oxidation section, wherein, The reduction section is provided with a reducing material outlet, a secondary air inlet, a fuel inlet, a raw material inlet, and a preheated air inlet from top to bottom. A wind cap distribution plate is provided between the preheated air inlet and the raw material inlet. The oxidation section is provided with a flue gas outlet, a secondary air inlet, a fuel inlet, an oxide feed inlet, a calcium oxide outlet, and a preheated air inlet from top to bottom. A wind cap distribution plate is provided between the preheated air inlet and the calcium oxide outlet. The oxide feed inlet of the oxidation section is connected to the reducing material outlet of the reduction section.

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