Method for preparing SO2 and CaO by reducing and decomposing phosphogypsum

Abstract

The invention discloses a method for preparing SO2 and CaO through reduction decomposition of ardealite, and belongs to the field of resource application of ardealite. The method comprises the following steps: pretreating phosphogypsum to remove SiO2 and other impurities in the phosphogypsum, and drying to constant weight to obtain semi-hydrated phosphogypsum: CaSO4. 2H2O-> CaSO4. 1 / 2H2O + 3 / 2H2O; the method comprises the following steps: by taking coke powder as a reducing agent, carrying out thermal reduction decomposition in a flue gas atmosphere, and calcining to prepare SO2 and CaO: 2CaSO4 + C-> 2CaO + 2SO2 + CO2. The method for preparing SO2 and CaO through reduction decomposition of phosphogypsum provided by the invention can achieve the effects of high decomposition rate, high product purity and simple process.

Description

Technical Field

[0001] The invention relates to resource application of phosphogypsum, in particular to a method for reducing and decomposing phosphogypsum to produce SO2 and CaO. Background Art

[0002] Phosphogypsum is the main by-product of wet-process phosphoric acid production. Every ton of wet-process phosphoric acid produces about 4.5-5 tons of phosphogypsum slag. The comprehensive utilization of phosphogypsum has always been a global problem, and improving the comprehensive utilization level of industrial by-product phosphogypsum will promote the sustainable development of industries such as phosphorus chemical industry. CaO is a very important inorganic material. It can be used as a filler, desiccant, and in the manufacture of calcium carbide, soda ash, etc. It is also widely used in many industries such as building materials, refractory materials and cement products. In addition, it can also be used as a catalyst and can also be used to remove H2S and COS from coal gas. SO2 is an important product of gypsum decomposition, which provides an effective way for the comprehensive utilization of phosphogypsum. Although the production process of sulfur is developing rapidly, sulfur is still a scarce commodity. The production of chemical industry, tire industry, non-ferrous and ferrous metallurgical industry, building materials, etc. requires a large amount of sulfur. The reduction and decomposition of phosphogypsum to produce SO2 plays a positive role in solving these problems.

[0003] At present, there are relatively few research reports on the preparation of calcium oxide by decomposing phosphogypsum at home and abroad. The main methods include solution replacement and reduction decomposition. In solution replacement, ammonia is used as a medium, and ammonia decarbonization and phosphogypsum double decomposition are combined to generate ammonium sulfate and calcium carbonate. The calcium carbonate slurry is filtered to separate calcium carbonate from ammonium sulfate solution and then decompose calcium carbonate into CaO. It is necessary to construct a multi-reaction system such as solid-liquid, and the process involved is relatively complex; there is a competitive relationship in the reduction and decomposition reaction of phosphogypsum, which causes problems such as impure reaction products and low decomposition rate. Setting up multi-stage reduction can improve the purity of the product, but it increases the complexity of the process and has certain limitations in application. At present, there are few industrial applications of solution replacement. Existing production lines also have problems such as low decomposition rate, poor product purity, and low SO2 concentration in flue gas. Therefore, improving the reduction decomposition rate and product purity of phosphogypsum is the focus of current research.

[0004] The reduction decomposition method uses blast furnace gas, coke oven gas, etc. as reducing gas to reduce and roast phosphogypsum, and then prepare SO2 and CaO products. This method mainly uses fluid roasting technology to calcine phosphogypsum, involving two stages of fluid roasting process, which increases the complexity of the process. Among them, industrial by-product gas is difficult to obtain and transport, and its application has certain limitations. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a method for reducing and decomposing phosphogypsum to produce SO2 and CaO with high decomposition rate, high product purity and simple process.

[0006] Technical solution: The method for reducing and decomposing phosphogypsum to produce SO2 and CaO according to the present invention comprises the following steps:

[0007] (1) Pre-treating phosphogypsum and drying to constant weight to obtain hemihydrate phosphogypsum:

[0008] CaSO4·2H2O→CaSO4·1 / 2H2O+3 / 2H2O;

[0009] (2) adding a reducing agent to the hemihydrate phosphogypsum obtained in step (1), and performing thermal reduction decomposition in a flue gas atmosphere to obtain calcium oxide and sulfur dioxide:

[0010] 2CaSO4+C→2CaO+2SO2+CO2.

[0011] Preferably, the drying temperature in step (1) is 120° C. and the drying time is 24 h.

[0012] Preferably, the pretreatment method of the phosphogypsum in step (1) is: adding a flotation agent and a diluent to carry out flotation during the beneficiation process, scraping the froth to the overflow weir with a froth scraper, and removing impurities such as SiO2 in the phosphogypsum to obtain pretreated phosphogypsum.

[0013] Further preferably, the flotation agent is C 12 -C 18 At least one of alkyl quaternary ammonium salts, n-propanol, n-butanol, n-pentanol, kerosene, diesel, cyclohexane, vegetable oil, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, Span, Tween, and isopropanol, and the amount of flotation agent added is 1-2‰ of the mass of phosphogypsum.

[0014] Further preferably, the diluent is one of a mixture of diesel and No. 2 oil in a mass ratio of 2:4 and methyl isobutyl carbinol.

[0015] Preferably, the pretreatment method of the phosphogypsum in step (1) is: adding an agent to dissolve the calcium sulfate dihydrate component, removing impurities such as SiO2 and then recrystallizing to obtain pretreated phosphogypsum.

[0016] Further preferably, the agent is a chlorine-containing oxidant, and the added amount is 0.5-1 times the mass of the phosphogypsum; further preferably, it is chlorine water, hypochlorous acid, hypochlorite, or a mixture of hydrochloric acid and hypochlorous acid.

[0017] Preferably, in step (2), the reducing agent is coke powder, and the molar ratio of C to CaSO4 (abbreviated as C / S) is 0.7-1.1.

[0018] Preferably, in the reduction decomposition reaction of step (2), the heating rate is 10°C / min, the reaction temperature is 1000-1300°C, and the reaction time is 1-3h.

[0019] Preferably, in step (2), after the reaction is completed, the sample is taken out and cooled to room temperature at a cooling rate of 60° C. / min.

[0020] Preferably, in the flue gas components described in step (2), the molar percentages of CO2 and O2 are 30-40% and 0%-4% respectively, and the rest is N2.

[0021] Specifically, the gas produced in the reaction in step (2) is scrubbed by a scrubbing device to obtain pure SO2 gas.

[0022] Invention principle: The reaction mechanism of gypsum and coke is as follows:

[0023] 2CaSO4+C→2CaO+2SO2+CO2

[0024] CaSO4+2C→CaS+2CO2

[0025] There is a competitive relationship in this reaction. Studies have shown that the reducibility of coke has a great influence on the reaction, which is reflected in the reaction rate and efficiency. The products of the reaction between coke and gypsum are CaS and CaO. The smaller the C / S ratio, the greater the proportion of CaO in the product; when the C / S ratio is greater than 2, it is all converted to CaS. In order to increase the reaction rate, a certain amount of O2 is usually added, and the decomposition reaction is expanded to CO reduction gypsum. At the same time, CaS may be oxidized to CaO and CaSO4. When the temperature exceeds 900°C, phosphogypsum can generate CaO under the action of the reducing agent. When the temperature exceeds 1100°C, the main product after oxidation is CaO.

[0026] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The method pre-treats phosphogypsum to remove impurities such as SiO2 in the phosphogypsum, uses coke powder as a reducing agent, and calcines to obtain SO2 and CaO, thereby realizing the resource utilization of phosphogypsum; (2) The method adopts a rotary heating method to reduce and decompose phosphogypsum into SO2 and CaO in a one-step process. The phosphogypsum decomposition rate is high and the calcination temperature is low. The purity of calcium oxide in the product is as high as more than 90%, and the reaction time is short. The process is relatively simple and has few application limitations; (3) The method achieves precise phase transformation of phosphogypsum during high-temperature decomposition by precisely controlling C / S and flue gas atmosphere, and avoids excessive oxygen participating in the reaction, resulting in incomplete reaction. The overall reaction temperature is low, the reaction time is generally short, the thermal efficiency is improved, and energy consumption is saved. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below in conjunction with embodiments.

[0028] Example 1

[0029] During the beneficiation process, isopropanol (the amount added is 1.0-2.0‰ of the mass of the phosphogypsum) and methyl isobutyl carbinol (the amount added is 3-5 times the mass of the phosphogypsum) are added to the phosphogypsum for flotation. The froth is scraped to the overflow weir with a scraper plate to remove impurities such as SiO2 in the phosphogypsum, thereby obtaining pretreated phosphogypsum, i.e., flotation phosphogypsum. The SiO2 concentration in the flotation phosphogypsum is reduced to 1%-3%, and the CaO purity is 35%-40%.

[0030] Using flotation phosphogypsum as raw material, flotation phosphogypsum and coke are crushed, ground, and passed through a 100-mesh sieve. The specific steps are as follows:

[0031] (1) The flotation phosphogypsum is dried in an oven at 120°C to a constant weight, and the flotation phosphogypsum is converted from dihydrate phosphogypsum to hemihydrate phosphogypsum for use, and the coke powder is dried in an oven at 45°C.

[0032] (2) Flue gas with a composition of 67% N2, 30% CO2 and 3% O2 ​​is introduced into an electric rotary tube furnace in advance. Dry coke powder and hemihydrate phosphogypsum are mixed in a carbon-sulfur mass ratio of 0.75, and the mixed materials are placed in an electric rotary tube furnace for heating. After being kept at 1200°C for 3 hours, the sample is taken out and cooled to room temperature at a cooling rate of 60°C / min. The CaO purity (f) of the product and the decomposition rate (η) of phosphogypsum are determined by the glycerol-ethanol method.

[0033] The decomposition rate of phosphogypsum (η) is calculated by the following formula:

[0034] η=(w0-w1 / w0-w0w1)*100%

[0035] W0: Determination of SO3 content in the calcined base by calcining phosphogypsum at 850℃

[0036] W1: SO3 content in phosphogypsum after decomposition

[0037] Examples 2-6 and Comparative Examples 1-5

[0038] Using flotation phosphogypsum as raw material, according to the preparation method and reaction conditions disclosed in Example 1, only some reaction conditions were changed to construct Examples 2-6 and Comparative Examples 1-5, as shown in Table 1.

[0039] Table 1 Reaction conditions and experimental results of flotation of phosphogypsum as raw material

[0040]

[0041] Example 7

[0042] The similarities between this embodiment and embodiment 4 are not repeated here. The difference is that synthetic calcium sulfate is used as the reaction raw material instead of the flotation phosphogypsum in embodiment 4.

[0043] The preparation method of synthetic calcium sulfate is as follows:

[0044] Hypochlorous acid (0.5-1 times the mass of phosphogypsum) is added to phosphogypsum to dissolve calcium sulfate dihydrate, and impurities such as SiO2 are removed and then recrystallized to obtain pretreated phosphogypsum, i.e. synthetic calcium sulfate, in which the SiO2 concentration is reduced to 0%-1% and the CaO purity is 40%-45%.

[0045] Examples 8-10 and Comparative Examples 6-7

[0046] Using flotation phosphogypsum as raw material, according to the preparation method and reaction conditions disclosed in Example 1, only some reaction conditions were changed to construct Examples 8-10 and Comparative Examples 6-7, as shown in Table 2.

[0047] Table 2 Reaction conditions and experimental results of synthesizing calcium sulfate as raw material

[0048]

[0049]

Claims

1. A method for preparing SO2 and CaO by reducing and decomposing phosphogypsum, characterized in that: The following steps are involved: (1) Pre-treating phosphogypsum and drying to constant weight to obtain hemihydrate phosphogypsum: CaSO4·2H2O→CaSO4·1 / 2H2O+3 / 2H2O; (2) adding a reducing agent to the hemihydrate phosphogypsum obtained in step (1), and performing thermal reduction decomposition in a flue gas atmosphere to obtain calcium oxide and sulfur dioxide: 2CaSO4+C→2CaO+2SO2+CO2.

2. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: The reducing agent described in step (2) is coke powder, and the molar ratio of C to CaSO4 is 0.7-1.

1.

3. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: In the step (2), the reaction temperature of the reduction decomposition is 1000-1300° C., and the reaction time is 1-3 hours.

4. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: In the flue gas composition described in step (2), the molar percentage of O2 is 0%-4%.

5. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 4, characterized in that: In the flue gas composition described in step (2), the molar percentage of O2 is 3%.

6. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: In the reduction decomposition reaction described in step (2), the heating rate is 5-15°C / min.

7. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: In the step (2), after the reaction is completed, the sample is cooled to room temperature at a rate of 50-65°C / min.

8. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: The pretreatment method of phosphogypsum in step (1) is as follows: flotation agent and diluent are added during the mineral processing to carry out flotation, and froth is scraped to the overflow weir by a froth scraper.

9. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 1, characterized in that: The pretreatment method of the phosphogypsum described in step (1) is: adding a reagent to the phosphogypsum to dissolve calcium sulfate dihydrate, removing impurities such as SiO2 and then recrystallizing.

10. The method for preparing SO2 and CaO by reducing and decomposing phosphogypsum according to claim 9, characterized in that: The agent is a chlorine-containing oxidant, and the amount added is 0.5-1 times the mass of the phosphogypsum.

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