A method for preparing calcium sulfide by decomposing phosphogypsum without carbon source and additive

Abstract

This invention discloses a low-temperature decomposition method for phosphogypsum that requires no carbon reducing agent or additives and produces pure products. This method eliminates the need for pretreatment of the phosphogypsum. By introducing a hydrogen-containing gas at 700-800°C with a molar ratio of 0.45-10.69:1 to calcium sulfate dihydrate in the phosphogypsum for 45-60 minutes, 100% decomposition of calcium sulfate in the phosphogypsum can be achieved without mechanical vibration. The solid product, except for silica impurities, consists mainly of high-purity calcium sulfide. This method enables highly efficient decomposition of phosphogypsum without the introduction of carbon reducing agents or the addition of accelerators. The pure products are easy to separate and produce zero carbon emissions. This method has positive significance for the high-value conversion of bulk phosphogypsum solid waste and the coupled development of hydrogen metallurgy technology.

Description

Technical Field

[0001] This invention relates to a method for decomposing phosphogypsum using hydrogen-containing gas without carbon source or additives, belonging to the fields of environmental protection and governance, chemical production technology and resource utilization. Background Technology

[0002] In recent years, driven by continuous environmental protection policies, industrial by-product gypsum has received much attention in the field of comprehensive solid waste utilization due to its cost advantage as a substitute for natural resources. However, the traditional phosphogypsum reduction and decomposition process has two major bottlenecks: carbon emissions and energy efficiency constraints. Coke reduction results in 0.8 to 1.2 tons of CO2 being generated from the decomposition of 1 ton of phosphogypsum; the decomposition temperature needs to be 1000-1200℃ (energy consumption ≥1200 kWh / ton), which exacerbates climate change and drives up treatment costs, seriously restricting large-scale application.

[0003] In current phosphogypsum decomposition processes, carbonaceous substances are commonly used reducing agents. Patent CN102556978A discloses a hydrogen reduction method for phosphogypsum decomposition using additives. This method promotes phosphogypsum decomposition by adding two mixed metal oxide additives. However, this process not only uses high-temperature pretreatment (drying at 600-650℃ for 120-150 min) for phosphogypsum, but also has an excessively high decomposition temperature (950-1050℃), significantly increasing energy consumption costs. Furthermore, the use of additives introduces difficulties in impurity separation, complicating subsequent product processing and easily causing secondary pollution.

[0004] Patent CN119284842A discloses a method for decomposing phosphogypsum using lignite reduction. In this method, lignite is used as a reducing agent, and hydrogen is added as an auxiliary reducing agent. The reaction is carried out at 700-800℃ for 1.5 hours to obtain calcium sulfide product. However, the use of carbon-containing lignite inevitably produces carbonaceous impurities that are difficult to separate, increasing carbon emissions. Furthermore, controlling the hydrogen concentration also increases the process cost.

[0005] Currently, publicly disclosed patents for the decomposition of phosphogypsum using hydrogen as a reducing agent suffer from problems such as complex processes, long reaction times, increased carbon emissions, and high energy consumption and costs. This invention, by directly reducing hydrogen, completely eliminates the carbon emission problems of traditional processes, greatly simplifies the process steps, and significantly reduces costs, energy consumption, and negative environmental impacts, all while improving the efficiency of phosphogypsum decomposition to calcium sulfide and reducing environmental pollution. It achieves the complete low-temperature decomposition of phosphogypsum into high-purity calcium sulfide and hydrogen sulfide under conditions without mechanical reinforcement (vibration / fluidization) and without the need for carbon sources or other additives. Summary of the Invention

[0006] The purpose of this invention is to provide a method for the cooling and decomposition of phosphogypsum without the participation of a carbon source. This method has zero carbon emissions and is simple in process. It can ensure that the decomposition products of phosphogypsum are free of residual carbon, while reducing energy consumption in the process. Under low temperature conditions, it can achieve complete decomposition of phosphogypsum and conversion into calcium sulfide.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for producing calcium sulfide from phosphogypsum without carbon source or additives includes the following steps:

[0009] Step S1, Filling: Fill the phosphogypsum into the quartz tube of the tube furnace with quartz wool, ensuring that there are no gaps between the phosphogypsum and the tube wall; the quartz wool forms a porous and stable "reaction bed", creating a stable and uniform "quasi-isothermal" reaction field, with uniform heat distribution, avoiding local overheating or overcooling, so that each phosphogypsum particle can be in the effective reaction temperature range at a relatively low overall temperature.

[0010] Step S2, reaction: Under the protection of inert gas, the filled phosphogypsum is heated to 700~800℃, then switched to hydrogen-containing gas, and reacted at a constant temperature of 700~800℃ for 15~60min.

[0011] Step S3, Cooling and Collection: After the reaction is complete, stop the flow of hydrogen gas, switch back to inert gas for purging, and cool to room temperature. Collect the solid and gaseous products. The solid product mainly consists of calcium sulfide, and the gaseous product includes hydrogen sulfide. The solid product is analyzed for its content using the national standard method, and its phase composition is analyzed using X-ray diffraction. The gaseous exhaust gas generated during the reaction is dried and can be recycled as raw material for reducing phosphogypsum.

[0012] Furthermore, the phosphogypsum is an industrial by-product of phosphogypsum.

[0013] Furthermore, the inert gas is nitrogen or argon.

[0014] Further, the hydrogen-containing gas is high-purity hydrogen with a volume fraction of 90% to 99.999% or hydrogen-rich tail gas with a volume fraction of 10% to 90%; the molar ratio of hydrogen in the hydrogen-containing gas to calcium sulfate dihydrate in phosphogypsum, by volume under standard conditions, is 0.45 to 10.69:1.

[0015] The mechanism of this invention includes:

[0016] The pretreatment of phosphogypsum includes the removal of crystal water and some soluble impurities. However, this invention, through the process of heating to the reaction temperature under inert gas protection, can simultaneously complete the pretreatment of phosphogypsum to remove crystal water and some volatile impurities, thus eliminating the need for further drying and washing. Furthermore, the impurities in phosphogypsum typically include silica, iron oxide, aluminum oxide, and magnesium oxide. These small amounts of impurities can promote the decomposition of phosphogypsum during hydrogen reduction.

[0017] The calcium-sulfur migration reaction process of hydrogen reduction of phosphogypsum includes:

[0018] <![CDATA[CaSO4(s)+H2(g)=CaO(s)+SO2(g)+H2O(g)]]> R1 <![CDATA[CaSO4(s)+4H2(g)=CaO(s)+H2S(g)+3H2O(g)]]> R2 <![CDATA[CaSO4(s)+4H2(g)=CaS(s)+4H2O(g)]]> R3 <![CDATA[3CaSO4(s)+CaS(s)=4CaO(s)+4SO2(g)]]> R4 <![CDATA[CaSO4(s)+3CaS(s)=4CaO(s)+4S(g)]]> R5 <![CDATA[CaS(s)+2SO2=CaSO4(s)+2S(g)]]> R6 <![CDATA[2CaS(s)+SO2=2CaO(s)+3S(g)]]> R7 <![CDATA[SO2(g)+2H2(g)=2H2O(g)+S(g)]]> R8 <![CDATA[SO2(g)+3H2(g)=H2S(g)+2H2O(g)]]> R9 <![CDATA[1.5CaSO4(s)+2H2S(g)=1.5CaS(s)+2H2O(g)+2SO2(g)]]> R10

[0019] from Figure 4 The relationship between temperature changes and ΔG and lnK for reactions R1-R10 shows that, based on the heat of reaction and spontaneity of H2 and CaSO4, reactions (R2-3) are predominant. The ΔG of reaction R(1) is less than 0 only above 1200℃, and the equilibrium constant LnK(R1) remains less than 0 with increasing temperature, indicating that reaction R(1) cannot occur at 800℃. Conversely, the ΔG of reactions R2 and R3 is less than 0 in the range of 500~1500℃, indicating spontaneous occurrence, and LnK remains greater than 0, suggesting that reactions R2 and R3 can occur within this temperature range. As the ambient temperature increases, lnK of R2 increases slightly, while that of R3 decreases slightly. This indicates that increasing the temperature favors reaction R(2), and higher temperatures promote the reaction of H2 with CaSO4 to produce CaS. However, at the same temperature, LnK(R3) > LnK(R2). Therefore, the reaction of H2 with calcium sulfate to produce CaS is thermodynamically more stable and the process is more favorable. This is consistent with the phenomenon observed in the tube furnace experiment where the CaS yield increases with increasing temperature and CaS is converted to CaO. Simultaneously, in this process, calcium in the products mainly exists in the form of CaS and CaO, while sulfur mainly exists in the form of CaS (solid) and H2S (gas). Reaction R(4) only occurs above 1100℃. Furthermore, ∆G of R5-8 is always greater than 0 and lnK is always negative, indicating that elemental S is not directly produced. Before 800℃, ∆G of R10 is less than 0 and LnK is greater than 0, which means that calcium sulfate can react with H2S produced by the reaction of R2 to generate SO2. However, ∆G of reaction R9 is less than 0 and LnK is greater than 0, so SO2 produced by R10 will react with H2 again to generate H2S. Therefore, according to thermodynamic calculations, the reactions that can proceed spontaneously and have a competitive advantage at 800℃ are R2 and R9. Combined with the results of the tube furnace experiment, the solid product of the theoretical decomposition of phosphogypsum is calcium sulfide and the gaseous product is hydrogen sulfide.

[0020] The beneficial effects of this invention include:

[0021] (1) This invention is the first to use a single reducing agent without additives for the decomposition of phosphogypsum, ensuring that no other impurities are introduced into the decomposition products, which is beneficial to the subsequent separation and utilization of the products.

[0022] (2) Simplified process: There is no need to pre-treat phosphogypsum separately. Dehydration and partial removal of impurities of phosphogypsum can be completed during the nitrogen heating process, which greatly reduces the process and energy consumption.

[0023] (3) Cost savings and targeted recovery of sulfur resources: By controlling the ratio of hydrogen-containing gas to phosphogypsum and the reaction time, the selective conversion of sulfur element into high-purity calcium sulfide is achieved. The hydrogen-containing gas mentioned in this invention can be selected from 10% to 90% of the hydrogen-rich tail gas generated in the comprehensive recycling of secondary aluminum ash from solid hazardous waste. This greatly reduces the cost of hydrogen reduction of phosphogypsum and provides a new way for the comprehensive utilization of phosphogypsum. The solid product of the reaction, calcium sulfide, can be used to prepare for further conversion and utilization. The mixture of hydrogen sulfide and hydrogen after drying can be recycled for the decomposition of phosphogypsum. Attached Figure Description

[0024] Figure 1 This is a flowchart of the experiment.

[0025] Figure 2 The images show the XRD patterns of the products from Examples 1, 2, and 3.

[0026] Figure 3 The graph shows the decomposition rate and product distribution of phosphogypsum at different reaction times under a hydrogen atmosphere at 800℃.

[0027] Figure 4 The graph shows the relationship between the temperature change of reactions R1-R10 and the changes in ΔG and lnK. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and comparative examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0029] Example 1:

[0030] The specific implementation process in this embodiment is as follows:

[0031] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 700℃ under a nitrogen atmosphere, and then 99.95% hydrogen gas was introduced. The hydrogen flow rate was controlled to maintain a molar ratio of hydrogen to calcium sulfate dihydrate in the phosphogypsum of 10.69:1 (calcium sulfate dihydrate content was 86.3%). The reaction was maintained at 700℃ for 60 min. After the reaction, heating was stopped, the hydrogen gas was turned off, and nitrogen was introduced for purging. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The results showed that the decomposition rate of calcium sulfate was 35.48%, and the calcium sulfide content was 22.37%. XRD analysis of the reactants is shown below. Figure 2 As can be seen from the figure, at 700℃, apart from the silicon dioxide originally contained in phosphogypsum, calcium sulfate is not completely converted into calcium sulfide, and the main component is still calcium sulfate.

[0032] The decomposition rate (φ) of phosphogypsum is calculated based on the difference in CaSO4 content before and after the reaction.

[0033]

[0034] Wherein, φ is the decomposition rate of phosphogypsum (%); CaSO4 (%) is the CaSO4 content in the product (%); and ω is the CaSO4 content in phosphogypsum (%).

[0035] Example 2:

[0036] The specific implementation process in this embodiment is as follows:

[0037] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 750℃ under a nitrogen atmosphere, and then 99.95% hydrogen gas was introduced. The hydrogen flow rate was controlled to maintain a molar ratio of hydrogen to calcium sulfate dihydrate in the phosphogypsum of 10.69:1. The reaction was maintained at 750℃ for 60 min. After the reaction, heating was stopped, the hydrogen gas was turned off, and nitrogen was introduced for purging. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The results showed that the decomposition rate of calcium sulfate was 79.97%, and the calcium sulfide content was 52.08%. XRD analysis of the reactants is shown below. Figure 2 As can be seen from the figure, at 750 °C, calcium sulfate is largely converted into calcium sulfide, but it is not completely decomposed and some calcium sulfate still exists.

[0038] Example 3:

[0039] The specific implementation process in this embodiment is as follows:

[0040] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 800℃ under a nitrogen atmosphere, and then 99.95% hydrogen gas was introduced. The hydrogen flow rate was controlled to maintain a molar ratio of hydrogen to calcium sulfate dihydrate in the phosphogypsum of 1.78:1. The reaction was maintained at 800℃ for 60 min. After the reaction was complete, heating was stopped, the hydrogen gas was turned off, and nitrogen was introduced for purging. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The results showed that the decomposition rate of calcium sulfate was 100%, and the calcium sulfide content was 69.32%. XRD analysis of the reactants is shown below. Figure 2 As can be seen from the figure, at 800 ℃, calcium sulfate completely decomposes and is converted into calcium sulfide, except for the silicon dioxide originally contained in phosphogypsum.

[0041] Example 4:

[0042] The specific implementation process in this embodiment is as follows:

[0043] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 800℃ under a nitrogen atmosphere, and then 99.95% hydrogen gas was introduced. The hydrogen flow rate was controlled to maintain a molar ratio of hydrogen to calcium sulfate dihydrate in the phosphogypsum of 0.45:1. The reaction was maintained at 800℃ for 15 minutes. After the reaction, heating was stopped, the hydrogen gas was turned off, and nitrogen was introduced for purging. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The product content results are shown below. Figure 3 The calcium sulfate decomposition rate was 61.7%, and the calcium sulfide content was 42.8%.

[0044] Example 5:

[0045] The specific implementation process in this embodiment is as follows:

[0046] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 800℃ under a nitrogen atmosphere, and then 99.95% hydrogen gas was introduced. The hydrogen flow rate was controlled to maintain a molar ratio of hydrogen to calcium sulfate dihydrate in the phosphogypsum of 0.89:1. The reaction was maintained at 800℃ for 30 minutes. After the reaction, heating was stopped, the hydrogen gas was turned off, and nitrogen was introduced for purging. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The product content results are shown below. Figure 3 The calcium sulfate decomposition rate was 93.8%, and the calcium sulfide content was 73.8%.

[0047] Example 6:

[0048] The specific implementation process in this embodiment is as follows:

[0049] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 800℃ under a nitrogen atmosphere, and then 99.95% hydrogen gas (by volume) was introduced. The hydrogen flow rate was controlled to maintain a molar ratio of hydrogen to calcium sulfate dihydrate in the phosphogypsum of 1.34:1. The reaction was maintained at 800℃ for 45 minutes. After the reaction, heating was stopped, the hydrogen flow was turned off, and nitrogen was introduced for purging. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The product content results are shown below. Figure 3 The calcium sulfate decomposition rate is 100%, and the calcium sulfide content is 75.3%.

[0050] Comparative Example 1:

[0051] The specific implementation process in this comparative example:

[0052] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 800℃ under a nitrogen atmosphere, and then anhydrous air (20% O2-80% N2) was introduced. The temperature inside the fixed bed was maintained at 800℃ for 60 minutes. After the reaction, heating was stopped, the anhydrous air supply was shut off, and nitrogen was introduced to purge and cool the furnace. The solid product was then analyzed. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The results showed a decomposition rate of 0.00% and a calcium sulfide content of 0.

[0053] Comparative Example 2:

[0054] The specific implementation process in this comparative example:

[0055] 3g of phosphogypsum was placed in a quartz tube of a tube furnace using quartz wool. The furnace was heated to 800℃ under a nitrogen atmosphere and reacted for 60 minutes. After the reaction was complete, heating was stopped, and the solid product was collected for analysis after the instrument had cooled down. Calcium sulfate was determined using the barium sulfate gravimetric method, and calcium sulfide was determined using the iodometric method. The decomposition rate was 0.00%, and the calcium sulfide content was 0.

[0056] Table 1. Decomposition rate and calcium sulfide content of Example 3 and Comparative Examples 1-2

[0057] Serial Number Reaction conditions Decomposition rate (%) CaS (%) Example 3 3g phosphogypsum, 99.95% hydrogen, 800℃, 60min 100.00 69.32 Comparative Example 1 <![CDATA[3g phosphogypsum, 20% O2 - 80% N2, 800 °C, 60 min]]> 0.00 0.00 Comparative Example 2 <![CDATA[3 g of phosphogypsum, 99.999% N2, 3 g, 800 °C, 60 min]]> 0.00 0.00

[0058] Analysis of the data from Examples 1-6 and Comparative Examples 1-2 shows that:

[0059] (1) As shown in Examples 1-2, phosphogypsum can decompose at 700°C in a hydrogen atmosphere, indicating that the calcium sulfate decomposition reaction in a hydrogen atmosphere has a low activation temperature threshold, and the decomposition rate increases significantly with increasing temperature.

[0060] (2) Example 3 shows that when a small amount of hydrogen is introduced at 800°C and reacted for 60 minutes, the calcium sulfate in phosphogypsum can be completely decomposed and the calcium sulfide content in the product is as high as 69.32%. This shows that hydrogen not only participates in the reaction as a reducing agent, but also significantly promotes lattice reconstruction. The hydrogen reduction environment is the key driving force for the high-temperature conversion of phosphogypsum.

[0061] (3) Examples 4-6 show that phosphogypsum can begin to decompose when reacted at 800°C for 15 minutes in a hydrogen atmosphere. When the reaction time is extended to 45 minutes, phosphogypsum is completely decomposed. The increase in reaction rate can further reduce energy consumption.

[0062] (4) In Comparative Example 1, under air atmosphere, the reaction was carried out at 800℃ for 60 min. The calcium sulfate in the phosphogypsum did not decompose, the main components were stable and did not change, indicating that it is difficult to decompose calcium sulfate without a reducing atmosphere.

[0063] (5) Comparative Example 2 also did not detect the formation of calcium sulfide under the protection of high-purity nitrogen, further proving that the inert atmosphere cannot promote the reduction reaction of calcium sulfate, and there is no obvious transformation even under high temperature conditions.

[0064] Hydrogen gas exhibits a significant reducing effect on phosphogypsum at high temperatures, effectively promoting the decomposition of calcium sulfate to form calcium sulfide. With optimized process parameters, the decomposition rate can reach 100%, demonstrating potential for industrial application. Comparative experiments further verified that the reaction is virtually non-reactive under inert or oxidizing conditions. Hydrogen, acting as a reducing agent at high temperatures, effectively breaks down the crystal structure of calcium sulfate, selectively reducing the sulfur element within it to form calcium sulfide, while the calcium element remains in a stable form within the product. No byproducts are generated during the reaction, and the product purity is high, verifying the cleanliness and efficiency of this process route.

Claims

1. A method for preparing calcium sulphide from phosphogypsum without a carbon source and without additives, characterized in that, Includes the following steps: Step S1, Filling: Fill the phosphogypsum into the quartz tube of the tube furnace with quartz wool, ensuring that there are no gaps between the phosphogypsum and the tube wall. Step S2, reaction: Under the protection of inert gas, the filled phosphogypsum is heated to 700~800℃, then switched to hydrogen-containing gas, and reacted at a constant temperature of 700~800℃ for 15~60min. Step S3, Cooling and Collection: After the reaction is completed, stop the flow of hydrogen gas, switch back to inert gas for purging and cool to room temperature, and collect the solid product and gaseous product; wherein the solid product is mainly composed of calcium sulfide and the gaseous product includes hydrogen sulfide.

2. The method of claim 1, wherein the phosphogypsum is decomposed without a carbon source and additives to produce calcium sulfide. In step S1, the phosphogypsum is an industrial by-product of phosphogypsum.

3. The method of claim 1, wherein the phosphogypsum is decomposed without a carbon source and additives to produce calcium sulfide. In step S2, the inert gas is nitrogen or argon.

4. The method of claim 1, wherein the phosphogypsum is decomposed without a carbon source and additives to produce calcium sulfide. In step S2, the hydrogen-containing gas is high-purity hydrogen with a volume fraction of 90% to 99.999% or hydrogen-rich tail gas with a volume fraction of 10% to 90%; the molar ratio of hydrogen in the hydrogen-containing gas to calcium sulfate dihydrate in phosphogypsum is 0.45 to 10.69:1 by volume under standard conditions.

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