A method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge

Abstract

This invention relates to a method for recovering phosphorus and sulfur from phosphoric acid sludge through carbothermic reduction, comprising the following steps: uniformly mixing raw materials phosphoric acid sludge, phosphate rock, flotation silica slag, coke, and additives to obtain mixture A; granulating mixture A by extrusion and then heating it in an electric furnace for a reduction reaction; the flue gas generated by the reduction reaction is cooled and separated after dust removal to obtain liquid yellow phosphorus and SO2-containing flue gas. This invention recovers phosphorus by generating phosphorus vapor through the carbothermic reduction of P2O5 in phosphoric acid sludge, and the generated CO gas can reduce CaSO4 in the phosphoric acid sludge, causing the sulfur in it to escape as SO2, which can be recycled, thus achieving simultaneous recovery of phosphorus and sulfur from phosphoric acid sludge; it also effectively reduces the amount of reducing agent coke used, lowers costs, improves reduction efficiency, and reduces reaction energy consumption.

Description

Technical Field

[0001] The invention relates to the field of phosphoric acid sludge, and in particular to a method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge. Background Art

[0002] Phosphoric acid sludge is a solid compound that accumulates in the phosphoric acid tank or clarification tank after a period of sedimentation due to its instability. The impurities in the phosphate ore, the fine crystals produced in the phosphoric acid extraction process, and the precipitation of a large number of ionic crystals in the dilute phosphoric acid storage and concentration process during the aging, concentration and storage of wet phosphoric acid form sludge. Sludge is a by-product that will inevitably exist in the wet phosphoric acid production process. With the decrease of the grade of phosphate ore, the amount of slag acid generated increases, and the composition is complex, so it is difficult to fully utilize the separated phosphoric acid sludge.

[0003] Patent CN104310364A discloses a comprehensive utilization method of phosphoric acid sludge, which uses hydrochloric acid to leaching sludge, titanium tetrachloride precipitation and separation to obtain titanium phosphate products to recover phosphorus, and then uses concentrated sulfuric acid to precipitate calcium to obtain calcium sulfate products and ferric chloride solution; the process is long and the process is complicated; Patent CN201310069651.1 discloses a method for producing multi-component nitro compound fertilizer by purifying sludge with wet phosphoric acid, which uses nitric acid decomposition and ammonia neutralization to prepare nitrophosphate fertilizer. Patent CN202111667837.8 discloses a method for preparing high-magnesium calcium sulfate from phosphoric acid sludge and high-magnesium tailings, adding high-magnesium tailings and concentrated sulfuric acid to the phosphoric acid sludge, and obtaining calcium sulfate products after reaction and maturation. Patent CN103241720 B discloses a method for preparing amorphous iron phosphate from phosphoric acid sludge, which uses sulfuric acid solution to leaching and decomposing the phosphoric acid sludge, and then adds sodium hydroxide or ammonia alkaline solution to precipitate and separate to obtain an amorphous iron phosphate product.

[0004] Therefore, conventional phosphoric acid sludge is mainly utilized by acid leaching to recover phosphorus and / or sulfur therein, which usually has problems such as long process, complicated process or inability to simultaneously recover phosphorus and sulfur. Summary of the invention

[0005] The purpose of the present invention is to overcome the above technical deficiencies and provide a method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge, thereby solving the technical problem that phosphorus and sulfur in phosphoric acid sludge cannot be recovered and utilized simultaneously in the prior art.

[0006] In order to achieve the above technical objectives, the technical solution of the present invention provides a method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge, comprising the following steps:

[0007] The raw materials of phosphoric acid sludge, phosphate rock, flotation silicon slag, coke and additives are uniformly mixed to obtain a mixture A;

[0008] Mixture A is heated to carry out a reduction reaction;

[0009] The flue gas produced by the reduction reaction is dedusted and then cooled and separated to obtain liquid yellow phosphorus and SO2 flue gas.

[0010] Furthermore, the raw materials include 50-80 parts of phosphoric acid sludge, 10-30 parts of phosphate rock, 15-25 parts of flotation silicon slag, 3-8 parts of coke, and 0.5-5.0 parts of additives, calculated by weight.

[0011] Furthermore, the phosphoric acid sludge is obtained by standing and stratifying the residue acid of wet-process phosphoric acid and then filtering it; in the phosphoric acid sludge, the P2O5 content is ≥5%; and the SO3 content is 10-35%.

[0012] Furthermore, the P2O5 content in the phosphate rock is ≥25%.

[0013] Furthermore, the flotation silicon slag is the residue after extracting CaSO4·2H2O from phosphogypsum; the SiO2 content in the flotation silicon slag is 55-90%.

[0014] Furthermore, the additive is one or a mixture of any proportion of two or more of sodium hydroxide, borax, boraxite, sodium carbonate, potassium sulfate and sodium sulfate.

[0015] Further, the mixture A is mixed uniformly by ball milling until the raw material particle size is 50-80 mesh; the mixture A is first extruded and granulated, and then subjected to reduction reaction; the particle size of the granulated particles is 20-50 mm.

[0016] Furthermore, the reduction reaction temperature is 1100-1550° C., and the time is 20-60 min.

[0017] Furthermore, the dust removal adopts a bag dust collector or a cyclone separator; the cooling method is condensation by spraying water in a condensation tower.

[0018] Furthermore, during the cooling separation, the flue gas temperature is cooled to below 160°C.

[0019] Compared with the prior art, the beneficial effects of the present invention include:

[0020] The present invention adopts a thermal reduction method, which can not only recover P in phosphoric acid sludge, but also recover S therein; phosphorus vapor is generated by carbon reduction of P2O5 in phosphoric acid sludge for recovery and utilization, and the generated CO gas can be directly used to reduce CaSO4 in phosphoric acid sludge, so that sulfur therein escapes in the form of SO2 and can be recovered, thereby achieving simultaneous recovery of phosphorus and sulfur in phosphoric acid sludge, and the reduction rate of phosphorus in the raw material reaches 92% to 98.5%, and the reduction rate of sulfur reaches 92.5% to 99.5%; and the amount of reducing agent coke is effectively reduced, the cost is reduced, the reduction efficiency is improved, the reaction energy consumption is reduced, and carbon emissions are reduced at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the preliminary separation roadmap of slag acid;

[0022] Figure 2 It is a schematic diagram of the process of recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge according to the present invention;

[0023] Figure 3 It is the XRD pattern of phosphoric acid extraction sludge. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0025] See also Figure 1 The phosphoric acid sludge used in the present invention comes from slag acid, which is separated by standing and stratifying, and then filtered to obtain the upper layer liquid and phosphoric acid sludge.

[0026] See also Figure 2 The method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge of the present invention comprises the following steps:

[0027] The raw materials of phosphoric acid sludge, phosphate rock, flotation silicon slag, coke and additives are ball-milled and mixed evenly in a certain proportion; extrusion granulation is carried out, and the mixture is put into an electric furnace for reduction reaction at a temperature of 1100-1550°C for 20-60 minutes, so that the coke reduces P2O5 and calcium sulfate, and CO reduces calcium sulfate at the same time, so as to respectively produce phosphorus vapor P2 and flue gas containing SO2; the flue gas is dedusted and passed into a cooling tower for condensation by spraying water, and the phosphorus vapor is separated to obtain liquid yellow phosphorus P4. After separation, the SO2 flue gas can be recycled for the preparation of sulfuric acid.

[0028] Furthermore, the raw materials of the present invention are composed of 50-80 parts of phosphoric acid sludge, 10-30 parts of phosphate rock, 15-25 parts of flotation silicon slag, 3-8 parts of coke, 0.5-5.0 parts of additives and the like.

[0029] Furthermore, in the phosphoric acid sludge, the P2O5 content is ≥5%, generally 5-28%; the SO3 content is 10-35%.

[0030] Furthermore, the P2O5 content of the phosphate rock is ≥25%.

[0031] Furthermore, the flotation silicon slag is the residue obtained by extracting CaSO4·2H2O from phosphogypsum using super dissolution technology, and its main component is SiO2 with a content of 55-90%.

[0032] Furthermore, the additive is one or a mixture of any proportion of two or more of sodium hydroxide, borax, boraxite, sodium carbonate, potassium sulfate and sodium sulfate.

[0033] Furthermore, the raw material is ball-milled to a particle size of 50 to 80 meshes. If the particle size is too large, it is not conducive to granulation, and if the particle size is too small, high requirements are placed on the equipment, and energy consumption and costs increase.

[0034] Furthermore, the particle size of the granules obtained by granulation is 20 to 50 mm.

[0035] Furthermore, the flue gas dust removal adopts bag dust collector or cyclone separator.

[0036] Furthermore, during condensation separation, the flue gas temperature is cooled to below 160°C, preferably to 140-155°C, to avoid SO2 loss and save energy while ensuring sufficient condensation and recovery of phosphorus.

[0037] The main mechanism of action of the present invention:

[0038] (1) The present invention produces phosphorus P2 and sulfur SO2 by reducing P2O5 and calcium sulfate with coke. The amount of coke added can be determined by carbon thermal reduction of phosphorus and sulfur. The specific reaction formula is as follows:

[0039] Phosphorus production: P2O5+5C=P2(g)+5CO(g)

[0040] Sulfur production: CaSO4+CO=CaO+SO2(g)+CO2(g)

[0041] 2CaSO4+C=2CaO+2SO2(g)+CO2(g)

[0042] The present invention solves the problem of utilizing phosphoric acid sludge. Phosphorus is reduced by carbon heat to obtain phosphorus steam. The generated CO gas can reduce calcium sulfate and recover sulfur resources, thereby realizing the simultaneous recovery of phosphorus and sulfur resources with high reduction efficiency.

[0043] (2) In order to increase the concentration of phosphorus vapor and make phosphorus production more economical, higher-grade phosphate ore is added to the raw materials, and flotation silicon slag is added. The addition of silicon can promote the escape of phosphorus and sulfur on the one hand, and realize the recycling of waste slag on the other hand. The addition of additives is mainly to promote the escape of phosphorus and sulfur.

[0044] (3) In the present invention, all raw materials are granulated together, which is convenient for feeding into the furnace. On the other hand, the granular feeding can ensure the air permeability of the furnace body, so that the reduction reaction proceeds smoothly, and at the same time, it is convenient for the gas generated during the reaction to rise and escape. The present invention adopts a closed electric furnace for reduction. The gas generated during the reaction keeps the entire furnace body in a slightly positive pressure state, preventing external gas from entering the furnace body, and ensuring the smooth progress of the reaction.

[0045] (4) The present invention uses spray water condensation to separate phosphorus vapor, wherein the dew point temperature of phosphorus vapor is 160-170°C, and at this temperature SO2 is still in a gaseous state. The furnace gas is condensed to reduce the flue gas temperature, so that the phosphorus vapor becomes a liquid phase and enters the phosphorus receiving tank together with water to obtain yellow phosphorus (exists in the form of P2 above 1100°C and yellow phosphorus P4 below 1100°C), thereby separating P4 and SO2.

[0046] The present invention is further described in detail below through specific examples and comparative examples.

[0047] The slag acid used in the present invention comes from Yunnan Phosphate Group Co., Ltd., and the pH value of the supernatant is 0.15. + )=0.7079mol / L. The upper layer liquid and phosphoric acid sludge are obtained by static stratification and pressure filtration and washing; the main components of the phosphoric acid sludge after pressure filtration are calcium, sulfur and fluorine.

[0048] See also Figure 3 From the XRD diagram, it was found that the main component of the phosphoric acid sludge was CaSO4·2H2O dihydrate gypsum, which contained a certain amount of citric acid-soluble phosphorus, which mainly existed in the form of calcium hydrogen phosphate.

[0049] Calculated in terms of oxides, the phosphoric acid sludge used in the present invention also contains certain amounts of P2O5, Al2O3, SiO2, MgO, and Fe2O3, and the contents of other impurities are relatively small; the main components are shown in Table 1 below.

[0050] Table 1 Main components of phosphoric acid sludge

[0051] Components CaO <![CDATA[SO3]]> F <![CDATA[P2O5]]> <![CDATA[Al2O3]]> MgO <![CDATA[SiO2]]> <![CDATA[Fe2O3]]> <![CDATA[Na2O]]> <![CDATA[K2O]]> total content / % 35.16 32.07 7.4 5.8 5.75 4.94 5.33 1.71 0.49 0.328 98.789

[0052] To avoid redundancy, the flotation silicon slag used in the following embodiments is the residue after extracting CaSO4·2H2O from phosphogypsum by super dissolution technology, and its main component is SiO2 with a content of 78%.

[0053] The fixed carbon content of the coke used is about 78.21%, and generally the fixed carbon content is required to be above 75%.

[0054] The raw materials here are only examples of specific raw materials used in the following embodiments, and are not limitations of the present invention. It can be understood that when the composition of phosphoric acid sludge and phosphate rock changes, the amount of flotation silicon slag and coke can be adaptively increased or decreased to ensure that phosphorus and sulfur are completely reduced.

[0055] Example 1

[0056] 60 parts of phosphoric acid sludge, 15 parts of phosphate rock, 20 parts of flotation silicon slag, 4 parts of coke and 2 parts of boron-magnesium ore as an additive are ball-milled and mixed evenly; extruded and granulated, the pellet size is 30mm, and then put into an electric furnace for reduction reaction at a temperature of 1200℃ for 60 minutes. The obtained mixed furnace gas is dust-removed and water-cooled to become liquid yellow phosphorus, which falls into the phosphorus receiving tank together with the spray water, and the sulfur-containing (SO2) furnace gas enters the acid-making system to produce sulfuric acid. The P2O5 content of the phosphate rock grade is 29.7%, and the additive is boron-magnesium ore. The residual phosphorus and sulfur contents in the slag are 1.29% and 2.25% respectively. The calculated phosphorus reduction rate is 92% and the sulfur reduction rate is 92.5%.

[0057] Example 2

[0058] 70 parts of phosphoric acid sludge, 10 parts of phosphate rock, 20 parts of flotation silicon slag, 3.5 parts of coke and 0.5 parts of additives are ball milled and mixed evenly. After extrusion granulation, they are put into an electric furnace for reduction reaction at a temperature of 1400℃ for 50 minutes. The mixed furnace gas is dust-removed and water-cooled to become liquid yellow phosphorus, which falls into the phosphorus receiving tank together with the spray water. The sulfur-containing (SO2) furnace gas enters the acid-making system to produce sulfuric acid. The P2O5 content of the phosphate rock grade is 28.5%, and the additive is a mixture of boron magnesium ore and sodium sulfate in a ratio of 1:1. The residual phosphorus and sulfur contents in the slag are 0.76% and 0.52% respectively. The calculated phosphorus reduction rate is 95% and the sulfur reduction rate is 98.5%.

[0059] Example 3

[0060] 80 parts of phosphoric acid sludge, 25 parts of phosphate rock, 25 parts of flotation silicon slag, 4.5 parts of coke, and 1.0 part of additives are ball milled and mixed evenly. After extrusion granulation, they are put into an electric furnace for reduction reaction at a temperature of 1500℃ for 30 minutes. The mixed furnace gas is dust-removed and water-cooled to become liquid yellow phosphorus, which falls into the phosphorus receiving tank together with the spray water. The sulfur-containing (SO2) furnace gas enters the acid-making system to produce sulfuric acid. The P2O5 content of the phosphate rock grade is 30.5%, and the additive is potassium sulfate and sodium carbonate mixed in a ratio of 1:1. The contents of residual phosphorus and sulfur in the slag are 0.44% and 0.30% respectively. The calculated phosphorus reduction rate is 97.5% and the sulfur reduction rate is 99.0%.

[0061] Example 4

[0062] 50 parts of phosphoric acid sludge, 20 parts of phosphate rock, 17 parts of flotation silicon slag, 3 parts of coke, and 0.8 parts of additives are ball milled and mixed evenly. After extrusion granulation, they are put into an electric furnace for reduction reaction at a temperature of 1550°C for 20 minutes. The obtained mixed furnace gas is dust-removed and water-cooled to become liquid yellow phosphorus, which falls into the phosphorus receiving tank together with the spray water. The sulfur-containing (SO2) furnace gas enters the acid-making system to produce sulfuric acid. The P2O5 content of the phosphate rock grade is 29%, and the additive is sodium hydroxide. The contents of residual phosphorus and sulfur in the slag are 0.27% and 0.14% respectively. The calculated reduction rates of phosphorus and sulfur are 98.5% and 99.5%.

[0063] Example 5

[0064] 75 parts of phosphoric acid sludge, 25 parts of phosphate rock, 23 parts of flotation silicon slag, 4 parts of coke, and 3.0 parts of additives are ball milled and mixed evenly, extruded and granulated, and then put into an electric furnace for reduction reaction at a temperature of 1300°C for 40 minutes. The obtained mixed furnace gas is dust-removed and water-cooled to become liquid yellow phosphorus, which falls into the phosphorus receiving tank together with the spray water. The sulfur-containing (SO2) furnace gas enters the acid-making system to produce sulfuric acid. The P2O5 content of the phosphate rock grade is 29%, and the additive is borax. The contents of residual phosphorus and sulfur in the slag are 0.77% and 0.48% respectively. The calculated phosphorus reduction rate is 95.5% and the sulfur reduction rate is 98.3%.

[0065] Comparative Example 1

[0066] The only difference from Example 2 is that no flotation silicon slag is added, and the other steps and conditions are the same as Example 1.

[0067] If the flotation silicon slag is not added, under the same conditions in other steps, the phosphorus reduction rate will be reduced by about 8% and the sulfur reduction rate will be reduced by about 3%.

[0068] Comparative Example 2

[0069] The only difference from Example 2 is that the additive boron magnesium ore is removed, and the other steps and conditions are the same as Example 2.

[0070] If the boron magnesium ore is not added, under the same conditions in other steps, the reduction rates of phosphorus and sulfur will be reduced by about 5% each.

[0071] It can be seen from Example 1, Comparative Example 1 and Comparative Example 2 that the present invention is beneficial to the escape of phosphorus and sulfur by adding flotation silicon slag and additives. When the two are present at the same time, a synergistic effect can be produced to effectively improve the reduction rate of phosphorus and sulfur.

[0072] Comparative Example 3

[0073] The only difference from Example 1 is that the reduction time is adjusted to 70 min, and the other steps and conditions are the same as Example 1.

[0074] If the reduction time is adjusted to 70 minutes, the reduction rate of phosphorus and sulfur will not increase, but will increase the production cost.

[0075] Comparative Example 4

[0076] The only difference from Example 1 is that the reduction temperature is adjusted to 1000° C., and the other steps and conditions are the same as Example 1.

[0077] If the reduction temperature is lowered under the same conditions, the reduction rates of phosphorus and sulfur will be significantly reduced, with the reduction rates of phosphorus and sulfur decreasing by about 10% and 8% respectively; this shows that the reduction temperature is also an important condition for regulating the reduction rates of phosphorus and sulfur.

[0078] When the temperature is further increased to 1550°C, the reduction rate of phosphorus and sulfur is not significantly improved. Considering the energy consumption, the preferred reduction temperature of the present invention is 1100°C to 1550°C.

[0079] In summary, Embodiments 1-5, the best solution of the present invention is Embodiment 4, by adjusting the raw material ratio, selecting the appropriate raw material type, and adjusting the process conditions such as reduction temperature and time accordingly, the reduction rate of phosphorus can reach 98.5%, and the reduction rate of sulfur can reach 99.5%.

[0080] Compared with the prior art, the advantages of the present invention are as follows:

[0081] 1. The conventional utilization of sludge acid basically does not use the carbon thermal reduction method. The present invention innovatively uses the thermal reduction method to simultaneously recover the phosphorus and sulfur in the phosphoric acid sludge, and collects them by classification, thereby realizing the resource utilization of sludge solid waste;

[0082] 2. The CO produced by reducing phosphorus can be directly used as a reducing agent for CaSO4, reducing the amount of reducing agent coke, reducing costs, and reducing carbon emissions.

[0083] The specific implementation of the present invention described above does not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge, characterized in that: The following steps are involved: The raw materials of phosphoric acid sludge, phosphate rock, flotation silicon slag, coke and additives are uniformly mixed to obtain a mixture A; Mixture A is heated to carry out a reduction reaction; The flue gas produced by the reduction reaction is dedusted and then cooled and separated to obtain liquid yellow phosphorus and SO2 flue gas; The raw materials include 50-80 parts of phosphoric acid sludge, 10-30 parts of phosphate rock, 15-25 parts of flotation silicon slag, 3-8 parts of coke, and 0.5-5.0 parts of additives by weight; Phosphoric acid sludge is obtained by standing and stratifying the residue acid of wet-process phosphoric acid and then filtering it; in the phosphoric acid sludge, the content of P2O5 is ≥5%; the content of SO3 is 10-35%; The mixture A is mixed uniformly by ball milling until the raw material particle size is 50-80 mesh; the mixture A is first extruded into granules, and then subjected to reduction reaction; the particle size of the granules obtained by granulation is 20-50 mm; The temperature of the reduction reaction is 1100-1550°C, and the time is 20-60 minutes.

2. The method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge according to claim 1, characterized in that: The P2O5 content in phosphate rock is ≥25%.

3. The method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge according to claim 1, characterized in that: Flotation silicon slag is the residue after extracting CaSO4·2H2O from phosphogypsum; the SiO2 content in flotation silicon slag is 55~90%.

4. The method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge according to claim 1, characterized in that: The additive is one of sodium hydroxide, borax, boraxite, sodium carbonate, potassium sulfate and sodium sulfate, or a mixture of two or more of them in any proportion.

5. The method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge according to claim 1, characterized in that: The dust is removed by bag dust collector or cyclone separator; the cooling method is condensation by spraying water in the condensation tower.

6. The method for recovering phosphorus and sulfur by carbon thermal reduction of phosphoric acid sludge according to claim 1, characterized in that: During cooling separation, the flue gas temperature is cooled to below 160°C.

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