Method for preparing calcium oxide or calcium sulfide by decomposing phosphogypsum

Abstract

The invention relates to the technical field of ardealite, and discloses a method for preparing calcium oxide or calcium sulfide by decomposing ardealite, which comprises the following steps: S1, carrying out reverse flotation desiliconization treatment on ardealite, and grinding the obtained desiliconized gypsum into powder; s2, adding the gypsum powder obtained in S1 into a ferric salt solution for dipping treatment, and then filtering and drying to obtain pretreated gypsum; s3, mixing the pretreated gypsum and graphite powder according to a molar ratio of 1: (0.6-1), and calcining in a nitrogen-air circulation alternate atmosphere to obtain calcium oxide; mixing the gypsum pretreated by calcium sulfide and graphite powder according to a molar ratio of 1: (6-10), and calcining under the protective atmosphere to obtain calcium sulfide. According to the method, desiliconization treatment is carried out on the ardealite raw material through a reverse flotation method, then ferric chloride is used as a fluxing additive, a molten state is formed in the calcination reaction process of the ardealite and the graphite, the ardealite is promoted to be decomposed to generate CaS or CaO, and the ardealite can be promoted to be reduced more effectively.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of phosphogypsum, and particularly relates to a method for preparing calcium oxide or calcium sulfide by decomposing phosphogypsum. BACKGROUND

[0002] Phosphogypsum (PG) is a solid waste produced in the process of wet-process phosphoric acid production, and the main component is calcium sulfate dihydrate (CaSO4·2H2O), and it also contains a small amount of phosphorus, fluorine, organic matter, heavy metals and radioactive substances and other impurities. China is the largest phosphogypsum producer in the world, but the comprehensive utilization rate is only 49.9%, and a large amount of phosphogypsum is stored, with a cumulative storage amount of 800 million tons, which has caused serious threat to the ecological environment.

[0003] In recent years, new technologies such as fluidized calcination process have been developed in China, which has improved the application efficiency of phosphogypsum in building materials. However, phosphogypsum contains a large amount of silicon impurities and other organic impurities, which seriously affects its purity and subsequent application. It is very difficult to directly utilize phosphogypsum due to the presence of impurities, so it needs to be treated by reduction and decomposition and other technical means. The traditional reduction method of phosphogypsum is high-temperature decomposition, but the decomposition temperature of CaSO4 is as high as 1600℃ or above, and the temperature required for CaSO4 to reach a molten state is 1450℃. The addition of a reducing agent can effectively reduce the temperature of phosphogypsum reduction and decomposition. Phosphogypsum can be decomposed into calcium oxide (CaO) and sulfur dioxide (SO2) at high temperature, which is used for co-production of sulfuric acid and cement. This technology was first industrialized by Shandong Lubai Chemical Group, but it has problems such as high energy consumption, serious equipment corrosion and complex process, and the economic benefit is not good.

[0004] At present, the reduction and pyrolysis of phosphogypsum by using a reducing agent is an important way to utilize phosphogypsum resources. CN105036170A discloses a method for preparing calcium oxide by decomposing phosphogypsum, which sets up a reducing atmosphere and an oxidizing atmosphere, first decomposes phosphogypsum into calcium sulfide in the reducing atmosphere, and then switches to the oxidizing atmosphere to prepare calcium oxide. The decomposition rate of phosphogypsum can reach 98%, and the content of calcium oxide can reach 87.8%. The method uses gas phase as a reducing agent, which has high cost, and the effective calcium content in the obtained calcium oxide is low. SUMMARY

[0005] The present application provides a method for preparing calcium oxide or calcium sulfide by decomposing phosphogypsum, which removes silicon by first performing reverse flotation on phosphogypsum, and then mixing the phosphogypsum coated by impregnation with iron chloride with graphite for calcination after adding graphite, to obtain calcium oxide or calcium sulfide by controlling the amount of added graphite and the sintering atmosphere. The decomposition rate of calcium sulfate is high, and the directional conversion can be realized. The effective calcium content in the prepared calcium oxide product is high, which is convenient for utilization.

[0006] The technical scheme of the present application is to provide a method for preparing calcium oxide or calcium sulfide from phosphogypsum, comprising the following steps: S1, the phosphogypsum is subjected to reverse flotation desilication treatment, and the obtained desilicated gypsum is ground into powder; S2, the gypsum powder obtained in S1 is immersed in an iron salt solution for impregnation treatment, and then filtered and dried to obtain pretreated gypsum; S3, the pretreated gypsum and graphite powder are mixed in a molar ratio of 1:(0.6-1), and calcined under a nitrogen-air cyclic alternating atmosphere to obtain calcium oxide; the pretreated gypsum and graphite powder are mixed in a molar ratio of 1:(6-10) and calcined under a protective atmosphere to obtain calcium sulfide.

[0007] Optionally, in S1, when the reverse flotation desilication is performed, a decolorizing collector, a frother, an activator and a desilication collector are added, wherein the desilication collector is polyquaternary ammonium salt-39, amide gemini quaternary ammonium salt, dodecyl dimethyl benzyl ammonium chloride and / or imidazoline quaternary ammonium salt.

[0008] Optionally, when the reverse flotation is performed, at least one of the following conditions is met: The pH of the reverse flotation system is controlled to be below 6; the amount of the decolorizing collector is 200-500 g / t; the amount of the frother is 50-300 g / t, the amount of the activator is 50-200 g / t, and the amount of the desilication collector is 100-400 g / t.

[0009] Optionally, in S1, when the reverse flotation is performed, screening is first performed, and then roughing, cleaning and two reverse flotation operations are sequentially performed, and the desilicated gypsum obtained by cleaning is ground into powder.

[0010] Optionally, in S2, the iron salt solution is a salt solution of divalent iron or trivalent iron, and the concentration is 0.01-1 mol / L, and the impregnation time is 1-24 h. The iron salt can be FeCl3, Fe2(SO4)3 or Fe(NO3)3.

[0011] Optionally, after the impregnation in S2 is completed, the gypsum is filtered, dried at 100-130 DEG C, and finally sieved into powder.

[0012] Optionally, in S3, when the calcium oxide is prepared, nitrogen is introduced to heat to 1000-1100 DEG C, and the nitrogen-air-nitrogen-air atmosphere is cyclically and alternately circulated for 2-4 h.

[0013] Optionally, in S3, when the calcium sulfide is prepared, nitrogen is introduced to heat to 900-1000 DEG C, and the reaction is performed for 2-4 h.

[0014] Optionally, in S3, when the calcium oxide is prepared, the molar ratio of the gypsum to the graphite powder is 1:0.8; and when the calcium sulfide is prepared, the molar ratio of the gypsum to the graphite powder is 1:10.

[0015] Optionally, the tail gas generated during the calcination process in S3 is absorbed by an alkaline solution.

[0016] The present invention has the following beneficial effects: The present invention performs reverse flotation on phosphogypsum, adds a collector to the phosphogypsum raw material, makes the surface of the silicon mineral hydrophobic, thereby causing the silicon mineral to adhere to bubbles and float on the surface of the slurry, removes silicon impurities in the phosphogypsum, obtains relatively pure phosphogypsum, separates the silicon mineral from the phosphogypsum, and enables the desiliconized phosphogypsum to be reduced more effectively, especially when preparing calcium oxide, thereby increasing the effective calcium content of the reduced calcium oxide.

[0017] The present invention uses graphite as a reducing agent during high-temperature calcination of phosphogypsum. The reaction between graphite and phosphogypsum is mainly solid-phase diffusion, supplemented by gas-phase diffusion, resulting in low reaction efficiency. Before calcination, an iron salt solution is added for impregnation treatment, and the iron salt adheres to the surface of the phosphogypsum. During the subsequent calcination treatment, the iron salt can be used as a fluxing additive and melted on the surface of the phosphogypsum to form a liquid phase interface, causing liquid-phase diffusion between the phosphogypsum and the graphite, thereby increasing the contact area of ​​the reaction, promoting the generation of CaS or CaO, and promoting directional conversion, thereby achieving both improved decomposition rate and selectivity of the phosphogypsum. On the other hand, the iron element, as an electron transfer medium, can also accelerate the reducing ability of carbon, and its surface active sites can enhance the contact efficiency between the phosphogypsum and the reducing agent. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the flow chart of the reverse flotation desiliconization agent test; Figure 2 This is the flotation flow chart for the pH condition test; Figure 3 This is a flow chart of the decolorization collector dosage test; Figure 4 This is the flow chart of the foaming agent dosage test; Figure 5 This is the flow chart of the activator dosage test; Figure 6 This is the flow chart of the activator dosage test; Figure 7 This is the flow chart of the segmented flotation test; Figure 8 This is the flow chart of the whole desiliconization process test; Figure 9 This is a scanning electron microscope image of phosphogypsum raw material; Figure 10 This is a diagram of the experimental device for thermal reduction of phosphogypsum; Figure 11 The CaSO4 - C - CaS ternary phase diagram obtained by calculating the CaS production using Factsage software; Figure 12XRD pattern of CaS prepared for Example 1 and Comparative Example 5; Figure 13 Scanning electron microscope image of the product of Example 4; Figure 14 CaSO4-C-CaS ternary phase diagram determined by calculation using Factsage software for CaO production; Figure 15 XRD pattern of CaO prepared for Example 2 and Comparative Example 10; Figure 16 Scanning electron microscope image of the product of Example 2. DETAILED DESCRIPTION

[0019] The experimental methods in the following examples are all conventional methods unless otherwise specified. The materials used in the following examples are all commercially available products unless otherwise specified.

[0020] The phosphogypsum in the following examples and comparative examples is from a certain phosphorus chemical enterprise in Hubei. The elemental composition of the phosphogypsum is analyzed by X-ray fluorescence (XRF), and the test results are shown in Table 1. The scanning electron microscope (SEM) image of the phosphogypsum sample is shown in Figure 9 The crystals of the phosphogypsum sample exhibit a typical granular / plate-like structure, and the surface exhibits a regular geometric morphology.

[0021] Table 1. Chemical composition of phosphogypsum

[0022] Example 1: Evaluation test of reverse flotation desilication agent 300 g of phosphogypsum was weighed into a 0.75 L flotation tank, and under the conditions of a flotation machine speed of 1608 r / min and an air charge of 0.1 m 3 / h, the amounts of decoloring collector (kerosene), frother (methyl isobutyl carbinol MIBC), and activator (sodium dodecyl benzene sulfonate) were 400 g / t, 200 g / t, and 100 g / t, respectively, and polyquaternary ammonium salt-39, amide gemini quaternary ammonium salt, dodecyl dimethyl benzyl ammonium chloride, and imidazoline quaternary ammonium salt were used as desilication collectors at an amount of 400 g / t, and a once roughing flotation test was carried out. The specific flotation process is shown in Figure 1 Table 2. The test results are shown in Table 2.

[0023] Table 2. The test results are shown in Table 2.

[0024] As can be seen from Table 2, the CaSO4·2H2O content and whiteness of the concentrate obtained by using amide gemini quaternary ammonium salt as the desilication collector are higher than those of the other quaternary ammonium salts, which are 94.42% and 59.35%, respectively.

[0025] Process condition optimization test of Example 2 2.1 pH condition optimization test In order to explore the best pH of the action of the reagent and the mineral in the flotation, different pulp pH condition tests were carried out. The roughing was carried out under the conditions of the flotation machine speed of 1608 r / min, the aeration amount of 0.1 m 3 / h, the dosage of the decolorizing collector of 400 g / t, the dosage of the frother of 200 g / t, the dosage of the activator of 100 g / t, and the dosage of the amide geminal quaternary ammonium salt of 400 g / t. The concentrate and the tailings were obtained. In the test, Ca(OH)2 was used to adjust the pH. The test process is shown in Figure 2 . The test results are shown in Table 3.

[0026] Table 3 Test results of pH condition

[0027] As can be seen from Table 3, with the increase of the pH, the CaSO4·2H2O content of the concentrate presents a downward trend, and the whiteness of the concentrate presents a downward trend first and then an upward trend. At the natural pH (pH = 0.97), better whiteness and CaSO4·2H2O content indexes of 59.88% and 93.87% can be obtained.

[0028] 2.2 Test of the dosage of the decolorizing collector The roughing was carried out under the conditions of the flotation machine speed of 1608 r / min, the aeration amount of 0.1 m 3 / h, the dosage of the frother of 200 g / t, the dosage of the activator of 100 g / t, and the dosage of the amide geminal quaternary ammonium salt of 400 g / t, the speed of 2000 r / min, and the aeration amount of 0.1 m 3 / h. The best dosage of the decolorizing collector was explored. The specific process is shown in Figure 3 , and the test results are shown in Table 4.

[0029] Table 4 Test results of the dosage of the decolorizing collector

[0030] As can be seen from Table 4, with the increase of the dosage of the decolorizing collector, the yield of the concentrate presents a gradually upward trend. The CaSO4·2H2O content of the concentrate presents a first upward trend and then a downward trend. When the dosage is 400 g / t, better indexes of the CaSO4·2H2O content of 93.87%, the recovery rate of 80.61%, and the whiteness of 59.88% can be obtained.

[0031] 2.3 Test of the dosage of the frother The roughing was carried out under the conditions of the flotation machine speed of 1608 r / min, the aeration amount of 0.1 m 3Under the conditions of / h, the dosage of decoloring collector was set to 400 g / t, the dosage of activator was set to 100 g / t, and the dosage of desilication collector was set to 400 g / t, the influence of the dosage of foaming agent on the flotation grade and whiteness of phosphogypsum was explored, and the specific process was as shown in Figure 4 The test results are shown in Table 5.

[0032] Table 5 Test results of the dosage of foaming agent

[0033] As shown in Table 5, with the increase of the dosage of foaming agent, the yield of concentrate showed a downward trend, and the CaSO4·2H2O content of concentrate showed a trend of first increasing and then decreasing. When the dosage of foaming agent was 200 g / t, the CaSO4·2H2O content of concentrate reached a peak value of 93.87%, and the whiteness reached 59.88%. Therefore, the optimal dosage of foaming agent was 200 g / t.

[0034] 2.4 Activator dosage test Once roughing, under the conditions of a flotation machine rotating speed of 1608 r / min and air charge of 0.1 m 3 / h, the dosage of decoloring collector was set to 400 g / t, the dosage of foaming agent was set to 200 g / t, and the dosage of amide double-headed quaternary ammonium salt was set to 400 g / t, the optimal dosage of activator was explored, and the specific process was as shown in Figure 5 The test results are shown in Table 6.

[0035] Table 6 Test results of the dosage of activator

[0036] As shown in Table 6, with the increase of the dosage of activator, the yield of concentrate showed a downward trend, the CaSO4·2H2O content showed a trend of first decreasing and then leveling off, and the whiteness showed a gradually increasing trend. Considering comprehensively, the optimal dosage of activator was selected to be 100 g / t, at which the CaSO4·2H2O content of concentrate was 93.83% and the whiteness was 59.93%.

[0037] 2.5 Desilication collector dosage test 300 g of raw ore sample was weighed into a 0.75 L flotation tank, once roughing was carried out under the conditions of a flotation machine rotating speed of 1608 r / min and air charge of 0.1 m 3 / h, the dosage of decoloring collector was set to 400 g / t, the dosage of foaming agent was set to 200 g / t, and the dosage of activator was set to 100 g / t. The influence of the dosage of desilication collector amide double-headed quaternary ammonium salt on phosphogypsum flotation was explored, and the flotation test was carried out in one-stage roughing mode, and the specific flotation process was as shown in Figure 6 The test results are shown in Table 7.

[0038] Table 7 Test results of the dosage of desilication collector

[0039] As can be seen from Table 7, with the increase of the dosage of the desilication collector amide gemini quaternary ammonium salt, the CaSO4·2H2O content of the concentrate obtained presents a trend of first increasing and then tending to be stable, and the whiteness of the concentrate fluctuates above 59%. Therefore, the optimal dosage of the amide gemini quaternary ammonium salt is 200 g / t. At this time, the CaSO4·2H2O content of the concentrate is 93.83%, and the whiteness is 59.86%.

[0040] 2.6 Subsection flotation test 300 g of the raw ore sample was weighed into a 0.75 L flotation tank, and the total dosage of the decolorizing collector was 400 g / t, the total dosage of the frother was 200 g / t, the total dosage of the activator was 100 g / t, and the total dosage of the amide gemini quaternary ammonium salt was 200 g / t under the conditions of a flotation machine speed of 1608 r / min and an aeration amount of 0.1 m 3 / h. The effect of the 1:1 subsection dosing mode on the flotation effect was explored, and the specific flotation process is shown in Figure 7 . The test results are shown in Table 8.

[0041] Table 8. Subsection flotation test results

[0042] As can be seen by comparing the results in Table 7 and Table 8, after the subsection dosing mode treatment, the CaSO4·2H2O content of the concentrate obtained by flotation is increased from 93.94% to 97.27%, and the whiteness is increased from 59.86% to 62.08%, and the flotation effect is obviously improved.

[0043] Example 3: Desilication test in the whole process After being pre-screened through an 80-mesh ore screen, the total dosage of the decolorizing collector was set to 400 g / t, the total dosage of the frother was 200 g / t, the total dosage of the activator was 100 g / t, and the total dosage of the amide gemini quaternary ammonium salt was 200 g / t under the conditions of a flotation machine speed of 1608 r / min and an aeration amount of 0.1 m 3 / h, and the flotation was carried out in the subsection dosing mode, and the specific process is shown in Figure 8 , and the results are shown in Table 9.

[0044] Table 9. Desilication test results in the whole process

[0045] As can be seen by comparing the results in Table 8 and Table 9, after the pre-screening treatment, the CaSO4·2H2O content of the concentrate obtained by flotation is increased from 97.27% to 98.60%, and the whiteness is increased from 62.08% to 65.12%, and the flotation effect is obviously improved.

[0046] In the following decomposition research of phosphogypsum, the CaO content refers to the effective calcium oxide content, which is determined according to GB / T 5762-2012 calcium sucrose-hydrochloric acid titration method.

[0047] Example 4: Preparation of CaS from desilicated phosphogypsum The process for preparing CaS by reduction and decomposition of desilicated phosphogypsum (PG) or desilicated phosphogypsum (PG-IM) with added fluxing additive iron chloride and graphite powder (C) in Example 3 is as follows: (1) First, the desilicated phosphogypsum (PG) is ground and sieved to a uniform powder of 100 mesh or less. The sieved powder is added to a 0.1 mol / L iron chloride solution and stirred for 24 hours to allow the phosphogypsum powder to be coated with a layer of iron chloride on the surface. The obtained filter cake is then dried at 120°C for 5 hours and sieved to a uniform powder of 100 mesh or less. The sample is named PG-IM.

[0048] (2) The PG-IM powder obtained in step (1) is uniformly mixed with graphite powder at a molar ratio of CaSO4:C = 1:10. This ratio is configured according to point 1 in the ternary phase diagram of the CaSO4-CaS-C system simulated by Factsage thermodynamic software, as shown in Figure 11 .

[0049] (3) The mixture obtained in step (2) is placed in a hot reduction tube furnace. The experimental device is shown in Figure 10 . Nitrogen gas is introduced into the tube furnace at a flow rate of 25 mL / min, and the temperature is programmed to rise to 1000°C for a reaction time of 240 min. After the reaction, the CaSO4 content in the sample is 0.0%, i.e., the phosphogypsum decomposition rate is 100.0%, and the obtained CaS content is 98.69%, and the effective calcium content (CaO content) is 1.95%. The specific values are shown in Table 10, and the XRD pattern of the product is shown in Figure 12 . In the figure, (a) PG 1000°C 10:1 (b) PG(IM) 1000°C 10:1. The scanning electron microscope image of the product is shown in Figure 13 . It is observed that CaSO4 collapses and shrinks, and the surface of the product CaS exhibits a molten state characteristic.

[0050] (4) In step (3), 0.1 mol / L sodium hydroxide solution is used for absorption, and the absorption amounts of SO2 and CO2 in the tail gas are calculated using the tail gas absorption solution after the reaction. The absorption values are shown in Table 10.

[0051] Comparative Example 1: The PG-IM sample in Example 4 is ground and sieved to a uniform powder of 100 mesh or less, and is uniformly mixed with graphite powder at a molar ratio of CaSO4:C = 1:8. The mixture is placed in a hot reduction tube furnace, and the experimental device is shown in Figure 11The CaSO4-CaS-C system was placed in a tubular furnace and introduced with nitrogen at a rate of 25 mL / min. The temperature was programmed to 1000°C and the reaction was carried out for 240 minutes. A 0.1 mol / L sodium hydroxide solution was used for tail gas absorption. The absorption of tail gas SO2 and CO2 was calculated using the tail gas absorption liquid after the reaction. The absorption values ​​are shown in Table 10. After the reaction, the decomposition rate of phosphogypsum was 100%, and a mass content of 87.35% CaS and 11.06% CaO was obtained. Compared with Example 4, the CaS content was reduced and the effective calcium content of CaO was increased. The specific values ​​are shown in Table 10.

[0052] Comparative Example 2: The PG-IM sample in Example 4 was ground and sieved to a uniform powder below 100 mesh, and the powder was mixed with graphite powder at a molar ratio of CaSO4:C=1:6. Figure 11 The CaSO4-CaS-C system was placed in a tubular furnace and introduced with 25 mL / min of nitrogen. The temperature was programmed to 1000°C and the reaction was carried out for 240 min. A 0.1 mol / L sodium hydroxide solution was used for tail gas absorption. The absorption of tail gas SO2 and CO2 was calculated using the tail gas absorption liquid after the reaction. The absorption values ​​are shown in Table 10. After the reaction, the decomposition rate of phosphogypsum was 100%, and 78.09% CaS and 19.15% effective calcium were obtained. Compared with Example 4 and Comparative Example 1, the CaS content was further reduced, and the effective calcium content of CaO was further increased, indicating that reducing the CaSO4:C molar ratio is not conducive to the formation of CaS. The specific values ​​are shown in Table 10.

[0053] Comparative Example 3: The PG-IM sample in Example 4 was ground and sieved to a uniform powder below 100 mesh, and the powder was mixed with graphite powder at a molar ratio of CaSO4:C=1:10. Figure 11 The configuration ratio of point 1 in the ternary phase diagram of the CaSO4-CaS-C system was placed in a tube furnace, nitrogen was introduced at a rate of 25 mL / min, and the temperature was programmed to 1000°C for 120 minutes. A 0.1 mol / L sodium hydroxide solution was used for tail gas absorption. The absorption of tail gas SO2 and CO2 was calculated using the tail gas absorption liquid after the reaction. The absorption values ​​are shown in Table 10. After the reaction, the decomposition rate of phosphogypsum was 86.90%, and 70.21% CaS and 6.75% effective calcium content were obtained. Compared with Example 4, the decomposition rate of phosphogypsum was reduced, the CaS content decreased, and the effective calcium content of CaO decreased, indicating that a short reaction time is not conducive to the complete decomposition of phosphogypsum. The specific values ​​are shown in Table 10.

[0054] Comparative Example 4: The PG-IM sample in Example 4 was ground and sieved to a uniform powder of 100 mesh or less, which was mixed uniformly with graphite powder at a molar ratio of CaSO4:C = 1:10, and placed in a tube furnace with a nitrogen flow of 25 mL / min, and programmed to 900°C for 240 min. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution, and the absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction. The absorption values are shown in Table 10, and the phosphogypsum decomposition rate after the reaction was 62.88%, with 20.44% CaS and 3.25% effective calcium content. Compared with Example 4, the phosphogypsum decomposition rate decreased, the CaS content decreased, and the effective calcium content of CaO decreased, indicating that a too low reaction temperature is not conducive to the decomposition of phosphogypsum. The specific values are shown in Table 10. Figure 11 The point 1 in the ternary phase diagram of the CaSO4-CaS-C system was configured in a ratio, placed in a tube furnace with a nitrogen flow of 25 mL / min, and programmed to 900°C for 240 min. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution, and the absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction. The absorption values are shown in Table 10, and the phosphogypsum decomposition rate after the reaction was 62.88%, with 20.44% CaS and 3.25% effective calcium content. Compared with Example 4, the phosphogypsum decomposition rate decreased, the CaS content decreased, and the effective calcium content of CaO decreased, indicating that a too low reaction temperature is not conducive to the decomposition of phosphogypsum. The specific values are shown in Table 10.

[0055] Comparative Example 5: The PG sample described in Example 4 was ground and sieved to a uniform powder of 100 mesh or less, which was mixed uniformly with graphite powder at a molar ratio of CaSO4:C = 1:10, and placed in a tube furnace with a nitrogen flow of 25 mL / min, and programmed to 900°C for 240 min. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution, and the absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction. The absorption values are shown in Table 10, and the phosphogypsum decomposition rate after the reaction was 62.88%, with 20.44% CaS and 3.25% effective calcium content. Compared with Example 4, the phosphogypsum decomposition rate decreased, the CaS content decreased, and the effective calcium content of CaO decreased, indicating that a too low reaction temperature is not conducive to the decomposition of phosphogypsum. The specific values are shown in Table 10. Figure 11 The point 1 in the ternary phase diagram of the CaSO4-CaS-C system was configured in a ratio, placed in a tube furnace with a nitrogen flow of 25 mL / min, and programmed to 900°C for 240 min. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution, and the absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction. The absorption values are shown in Table 10, and the phosphogypsum decomposition rate after the reaction was 62.88%, with 20.44% CaS and 3.25% effective calcium content. Compared with Example 4, the phosphogypsum decomposition rate decreased, the CaS content decreased, and the effective calcium content of CaO decreased, indicating that a too low reaction temperature is not conducive to the decomposition of phosphogypsum. The specific values are shown in Table 10. Figure 12 .

[0056] Comparative Example 1: The PG sample without flotation was ground and sieved to a uniform powder of 100 mesh or less, which was mixed uniformly with graphite powder at a molar ratio of CaSO4:C = 1:10, and placed in a tube furnace with a nitrogen flow of 25 mL / min, and programmed to 900°C for 240 min. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution, and the absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction. The absorption values are shown in Table 10, and the phosphogypsum decomposition rate after the reaction was 62.88%, with 20.44% CaS and 3.25% effective calcium content. Compared with Example 4, the phosphogypsum decomposition rate decreased, the CaS content decreased, and the effective calcium content of CaO decreased, indicating that a too low reaction temperature is not conducive to the decomposition of phosphogypsum. The specific values are shown in Table 10. Figure 11The CaSO4-CaS-C system was configured at point 1 in the ternary phase diagram. The mixture was placed in a tube furnace, nitrogen was introduced at a rate of 25 mL / min, and the temperature was programmed to 1000°C for 240 minutes. Tail gas absorption was performed using a 0.1 mol / L sodium hydroxide solution. The absorption of tail gas SO2 and CO2 was calculated using the tail gas absorption liquid after the reaction. The absorption values ​​are shown in Table 10. After the reaction, the phosphogypsum decomposition rate was 98%, yielding 85.51% CaS and 3.12% effective calcium. Compared to Example 4, the CaS content decreased significantly, while the effective calcium content increased, indicating that flotation can effectively remove impurities and promote the formation of CaS. Specific values ​​are shown in Table 10.

[0057] Comparative Example 2: Reagent-grade CaSO4 (99%) was directly used and calcined under the same conditions to measure the effective calcium content of the generated CaS.

[0058] The reagent grade CaSO4 (99%) sample was ground and sieved to a uniform powder below 100 mesh, and mixed with graphite powder at a molar ratio of CaSO4:C=1:10. Figure 11 The reaction was carried out at point 1 in the ternary phase diagram for the CaSO₄-CaS₄-C system. The reaction mixture was placed in a tube furnace, nitrogen was introduced at a rate of 25 mL / min, and the temperature was programmed to 1000°C for 240 minutes. Tail gas absorption was performed using a 0.1 mol / L sodium hydroxide solution. The absorption of SO₂ and CO₂ in the tail gas was calculated using the tail gas absorption liquid after the reaction. The absorption values ​​are shown in Table 10. After the reaction, the phosphogypsum decomposition rate was 100%, yielding 98.57% CaS and 1.43% available calcium. These results are close to those of Example 4, indicating that the phosphogypsum reduction rate and CaS product purity obtained by reducing phosphogypsum under the conditions of Example 4 are close to those of reagent-grade CaSO₄.

[0059] Table 10 Data results of graphite powder thermal reduction of phosphogypsum to prepare calcium sulfide

[0060] It can be seen from Table 10 that the phosphogypsum after flotation desiliconization is loaded with iron salts, which can effectively improve the reduction rate of phosphogypsum and obtain high-purity CaS.

[0061] Example 5: A process for preparing CaO by reduction and decomposition of desiliconized phosphogypsum (PG) or desiliconized phosphogypsum with ferric chloride added as a fluxing additive (PG-IM) and graphite powder (C) in Example 3.

[0062] (1) The desilicated phosphogypsum was ground and sieved to a uniform powder of 100 mesh or less. The sieved phosphogypsum powder was added to a ferric chloride solution and stirred for 24 hours to allow the phosphogypsum powder to be coated with a layer of ferric chloride. The resulting filter cake was dried at 120°C for 5 hours and then sieved to a uniform powder of 100 mesh or less. The sample was designated PG-IM.

[0063] (2) The PG-IM powder obtained in step (1) was uniformly mixed with graphite powder at a molar ratio of CaSO4:C = 1:0.8. This ratio was configured according to point 1 in the ternary phase diagram of the CaSO4-CaS-C system simulated by the Factsage thermodynamic software, as shown in Figure 14 .

[0064] (3) The mixture obtained in step (2) was placed in a hot reduction tube furnace. The experimental device is shown in Figure 10 . Nitrogen gas was introduced into the tube furnace at a flow rate of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled and alternated in the order of nitrogen-air-nitrogen-air for 240 minutes (with an interval of 60 minutes). After the reaction, the phosphogypsum decomposition rate was 98.60%, and CaO with an effective calcium content of 76.52% was obtained. The specific values are shown in Table 11, the XRD pattern of the product is shown in Figure 15 , and the scanning electron microscope image of the product is shown in Figure 16 . It was observed that the CaSO4 collapsed and shrank into smaller CaO, and the surface showed characteristics of a molten state.

[0065] (4) In step (3), 0.1 mol / L sodium hydroxide solution was used for absorption. The absorption amounts of SO2 and CO2 in the tail gas were calculated using the tail gas absorption solution after the reaction. The absorption values are shown in Table 11.

[0066] Comparative Example 6: The PG-IM sample obtained in Example 5 was ground and sieved to a uniform powder of 100 mesh or less. It was uniformly mixed with graphite powder at a molar ratio of CaSO4:C = 1:1, which was configured according to point 1 in the ternary phase diagram of the CaSO4-CaS-C system in Figure 14 . The mixture was placed in a tube furnace and nitrogen gas was introduced at a flow rate of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled and alternated in the order of nitrogen-air-nitrogen-air for 240 minutes (with an interval of 60 minutes). 0.1 mol / L sodium hydroxide solution was used for tail gas absorption. The absorption amounts of SO2 and CO2 in the tail gas were calculated using the tail gas absorption solution after the reaction. The absorption values are shown in Table 11. The phosphogypsum decomposition rate was 90.35%, and the effective calcium content in CaO was 67.88%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that increasing the carbon ratio did not achieve the desired reduction effect. The specific values are shown in Table 11.

[0067] Comparative Example 7: The PG-IM sample obtained in Example 5 was ground and sieved to a uniform powder of 100 mesh or less, which was mixed uniformly with graphite powder at a molar ratio of CaSO4:C = 1:0.6, and was placed in a tube furnace with a nitrogen flow of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled alternately for 240 min (with an interval of 60 min) in the order of nitrogen-air-nitrogen-air. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution. The absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction, and the absorption values are shown in Table 11. The phosphogypsum decomposition rate was 77.92%, and the effective calcium content in CaO was 64.89%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that reducing the proportion of carbon cannot achieve the desired reduction effect. The specific values are shown in Table 11. Figure 14 The sample was placed in a tube furnace with a nitrogen flow of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled alternately for 240 min (with an interval of 60 min) in the order of nitrogen-air-nitrogen-air. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution. The absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction, and the absorption values are shown in Table 11. The phosphogypsum decomposition rate was 77.92%, and the effective calcium content in CaO was 64.89%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that reducing the proportion of carbon cannot achieve the desired reduction effect. The specific values are shown in Table 11.

[0068] Comparative Example 8: The PG-IM sample obtained in Example 5 was ground and sieved to a uniform powder of 100 mesh or less, which was mixed uniformly with graphite powder at a molar ratio of CaSO4:C = 1:0.8, and was placed in a tube furnace with a nitrogen flow of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled alternately for 240 min (with an interval of 60 min) in the order of nitrogen-air-nitrogen-air. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution. The absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction, and the absorption values are shown in Table 11. The phosphogypsum decomposition rate was 77.92%, and the effective calcium content in CaO was 64.89%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that reducing the proportion of carbon cannot achieve the desired reduction effect. The specific values are shown in Table 11. Figure 14 The sample was placed in a tube furnace with a nitrogen flow of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled alternately for 240 min (with an interval of 60 min) in the order of nitrogen-air-nitrogen-air. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution. The absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction, and the absorption values are shown in Table 11. The phosphogypsum decomposition rate was 77.92%, and the effective calcium content in CaO was 64.89%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that reducing the proportion of carbon cannot achieve the desired reduction effect. The specific values are shown in Table 11.

[0069] Comparative Example 9: The PG-IM sample obtained in Example 5 was ground and sieved to a uniform powder of 100 mesh or less, which was mixed uniformly with graphite powder at a molar ratio of CaSO4:C = 1:0.8, and was placed in a tube furnace with a nitrogen flow of 25 mL / min. The temperature was programmed to rise to 1100°C, and then the atmosphere was cycled alternately for 240 min (with an interval of 60 min) in the order of nitrogen-air-nitrogen-air. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution. The absorption amounts of SO2 and CO2 in the tail gas were calculated from the tail gas absorption solution after the reaction, and the absorption values are shown in Table 11. The phosphogypsum decomposition rate was 77.92%, and the effective calcium content in CaO was 64.89%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that reducing the proportion of carbon cannot achieve the desired reduction effect. The specific values are shown in Table 11. Figure 14The point 1 configuration ratio in the ternary phase diagram of the CaSO4-CaS-C system was placed in a tubular furnace and introduced with 25mL / min of nitrogen. The temperature was programmed to 1100°C, and then the atmosphere was cycled alternately in the order of nitrogen-air-nitrogen-air for 120min (alternating interval of 60min). A 0.1mol / L sodium hydroxide solution was used for tail gas absorption. The absorption amount of tail gas SO2 and CO2 was calculated using the tail gas absorption liquid after the reaction. The absorption value is shown in Table 11. The decomposition rate of phosphogypsum was 71.52%, and the effective calcium content in CaO was 21.26%. Compared with Example 5, the decomposition rate of phosphogypsum decreased, and the effective calcium content in CaO decreased, indicating that the reaction time was low and the decomposition of phosphogypsum was incomplete. The specific values ​​are shown in Table 11.

[0070] Comparing Example 5 with Comparative Example 9, the reaction in Comparative Example 9 involved only one nitrogen-air cycle, while the reaction in Example 5 involved two nitrogen-air cycles. The PG decomposition rate increased from 71.52% to 98.60%, and the effective calcium content increased from 21.26% to 73.52%. This indicates that one nitrogen-air cycle alone cannot completely decompose PG, while two N2-Air cycles allow for sufficient decomposition. This process effectively increases the reaction rate of CaS generated in the first step of phosphogypsum reduction with CaSO4 to produce CaO.

[0071] Comparative Example 10: The PG sample obtained in Example 5 was ground and sieved to a uniform powder below 100 mesh, and the powder was mixed with graphite powder at a molar ratio of CaSO4:C=1:0.8. Figure 14 The configuration ratio of point 1 in the ternary phase diagram of the CaSO4-CaS-C system was placed in a tubular furnace and introduced with 25mL / min of nitrogen. The temperature was programmed to 1100°C, and then the atmosphere was cycled alternately in the order of nitrogen-air-nitrogen-air for 240min (alternating interval of 60min). A 0.1mol / L sodium hydroxide solution was used for tail gas absorption. The absorption amount of tail gas SO2 and CO2 was calculated using the tail gas absorption liquid after the reaction. The absorption value is shown in Table 11. The decomposition rate of phosphogypsum is 96.76%, and the effective calcium content in CaO is 72.72%. Compared with Example 5, the decomposition rate of phosphogypsum decreases, and the effective calcium content in CaO decreases, indicating that ferric chloride fluxing can effectively improve the decomposition rate of phosphogypsum and the yield of calcium oxide. The specific values ​​are shown in Table 11, and the product XRD pattern is shown in Figure 13 .

[0072] Comparative Example 11: Grind and sieve the unfloated PG sample to a uniform powder below 100 mesh, mix it with graphite powder at a molar ratio of CaSO4:C=1:0.8, and Figure 14The point 1 in the ternary phase diagram of the CaSO4-CaS-C system was configured in proportion, placed in a tube furnace, and nitrogen was introduced at 25 mL / min. The temperature was programmed to rise to 1100 DEG C, and then the atmosphere was cycled alternately for 240 min (intermittent interval 60 min) in the order of nitrogen-air-nitrogen-air. Tail gas absorption was performed using 0.1 mol / L sodium hydroxide solution. The absorption amounts of tail gas SO2 and CO2 were calculated using the tail gas absorption solution after the reaction, and the absorption values are shown in Table 11. The phosphogypsum decomposition rate was 78.53%, and the effective calcium content in CaO was 49.37%. Compared with Example 5, the phosphogypsum decomposition rate decreased, and the effective calcium content in CaO decreased, indicating that flotation can effectively remove impurities in phosphogypsum to promote the generation of effective calcium. The specific values are shown in Table 11.

[0073] Comparative Example 3: Reagent-grade Ca(OH)2(99.8%) was directly calcined under the same conditions to measure the effective calcium content of CaO generated Reagent-grade Ca(OH)2(99.8%) was ground and sieved to a uniform powder below 100 mesh, placed in a tube furnace, and nitrogen was introduced at 25 mL / min. The temperature was programmed to rise to 1100 DEG C and react for 240 min. The effective calcium content of CaO was 76%, which was very close to that of Example 5. This scheme has achieved an increase in the reduction rate of phosphogypsum and an increase in the effective calcium content of calcium oxide generated from phosphogypsum to an optimal value.

[0074] Table 11 Data results of graphite powder thermal reduction of phosphogypsum to prepare calcium oxide

[0075] In the decomposition of phosphogypsum, the present application is based on the nitrogen-air double atmosphere circulation path. The reaction of phosphogypsum and graphite is mainly solid-phase diffusion, with gas-phase diffusion as an auxiliary. Iron chloride acts as a fluxing additive and melts on the surface of phosphogypsum to form a liquid phase interface, which promotes liquid-phase diffusion of phosphogypsum and graphite, increases the contact area of the reaction, and promotes the generation of CaS and the directional conversion of CaO. This scheme realizes the selective increase of CaO and the decomposition rate of PG, and confirms that iron-based additives as a promoter for the decomposition of phosphogypsum show significant advantages. Iron-based additives and graphite powder cooperatively construct an efficient reduction system. Iron elements act as electron transfer media to accelerate the reduction capacity of carbon, and their surface active sites can enhance the contact efficiency of phosphogypsum and reducing agents. The original solid-phase-solid-phase reaction of phosphogypsum and C reduction is changed into liquid-phase and solid-phase reaction by adding iron chloride, iron sulfate, or iron nitrate as a fluxing agent, which greatly improves the reduction efficiency of phosphogypsum.

[0076] The application greatly improves the purity of CaS and the effective calcium content of CaO by first improving the purity of phosphogypsum through processes such as flotation desilication and decolorization, and then preparing CaS from the phosphogypsum.

[0077] The above examples describe the preferred embodiments of the application, but the application is not limited thereto. Within the technical concept of the application, various simple modifications can be made to the technical solutions of the application, including the combination of various technical features in any other manner, and these simple modifications and combinations should also be considered as disclosed by the application and belong to the protection scope of the application. Therefore, the protection scope of the application patent should be subject to the appended claims.

Claims

1. A method for preparing calcium oxide or calcium sulfide by decomposing phosphogypsum, characterized in that: The following steps are involved: S1, phosphogypsum is subjected to reverse flotation desiliconization treatment, and the obtained desiliconized gypsum is ground into powder; The gypsum powder obtained in S2 and S1 is added to an iron salt solution for impregnation treatment, and then filtered and dried to obtain pretreated gypsum; S3. The pretreated gypsum and graphite powder are mixed in a molar ratio of 1: (0.6~1), and calcined under a nitrogen-air circulating alternating atmosphere to obtain calcium oxide; calcined under a protective atmosphere to obtain calcium sulfide. The pretreated gypsum and graphite powder are mixed in a molar ratio of 1: (6~10), and calcined under a protective atmosphere to obtain calcium sulfide.

2. The method according to claim 1, wherein: In S1, during reverse flotation desiliconization, a decolorizing collector, a frother, an activator and a desiliconizing collector are added, wherein the desiliconizing collector is polyquaternium-39, amide gemini quaternary ammonium salt, dodecyldimethylbenzyl ammonium chloride and / or imidazoline quaternary ammonium salt.

3. The method according to claim 1, characterized in that During reverse flotation, at least one of the following conditions must be met: The pH of the reverse flotation system is controlled below 6; the dosage of decolorizing collector is 200~500g / t; frother 50~300g / t, activator 50~200g / t and desiliconizing collector 100~400g / t.

4. The method according to any one of claims 1 to 3, characterized in that: During reverse flotation in S1, screening is performed first, followed by two reverse flotation operations of roughing and cleaning. The desiliconized gypsum obtained by cleaning is ground into powder.

5. The method according to claim 1, wherein: The iron salt solution described in S2 is a salt solution of divalent iron or trivalent iron, with a concentration of 0.01-1 mol / L, and the immersion time is 1-24 hours.

6. The method according to claim 5, characterized in that: After impregnation in S2, it is filtered, dried at 100-130°C, and finally sieved into powder.

7. The method according to claim 1, wherein: When preparing calcium oxide in S3, nitrogen is first introduced and the temperature is raised to 1000-1100°C, and the atmosphere is circulated alternately in the order of nitrogen-air-nitrogen-air for 2-4 hours.

8. The method according to claim 1, wherein: When preparing calcium sulfide in S3, nitrogen is introduced and the temperature is raised to 900~1000℃, and the reaction is carried out for 2~4h.

9. The method according to claim 1, wherein: When preparing calcium oxide in S3, the molar ratio of gypsum to graphite powder is 1:0.8; when preparing calcium sulfide, the molar ratio of gypsum to graphite powder is 1:

10.

10. The method according to claim 1, wherein: The tail gas generated during the calcination in S3 is absorbed by an alkaline solution.

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