A method for recovering phosphorus from sludge

Abstract

The present invention belongs to the field of phosphorus recovery technology, and in particular to a method for recovering phosphorus in sludge. The present invention provides a method for recovering phosphorus in sludge, comprising the following steps: adding dilute hydrochloric acid to the sludge after incineration, stirring and filtering, and obtaining filter residue and filtrate; adding ammonia water to the liquid after the filtrate is subjected to reduced pressure distillation, and then adding a chelating agent, stirring and filtering at 160-250°C to obtain crude iron phosphate and a second filtrate; adding phosphoric acid and hydrogen peroxide to the obtained second filtrate for reaction, controlling the pH of the reaction system to 3-4.5, filtering after the reaction is completed, and obtaining crude iron phosphate and a third filtrate; washing and drying the obtained crude iron phosphate to obtain finished iron phosphate. The sludge recovery method provided by the present invention can convert iron and phosphorus in the sludge into iron phosphate. In addition, by adding a chelating agent, the yield of crude iron phosphate can be further improved, and the method has a high recovery rate of phosphorus in the sludge, which greatly avoids the loss of phosphorus.

Description

Technical Field

[0001] The invention belongs to the technical field of phosphorus recovery, and in particular relates to a method for recovering phosphorus in sludge. Background Art

[0002] The proportion of domestic sewage in municipal sewage is relatively high, and its phosphate content is usually in the range of 2-10 mg / L. Phosphorus removal of municipal sewage is often completed in the biochemical treatment and physicochemical treatment stages, reducing the phosphate content to below 0.5 mg / L. Some important water source areas require it to be reduced to 0.2 mg / L, and the phosphate is solidified in the sludge in the form of sediment. In economically developed areas, such as Shanghai, sludge landfill treatment is no longer allowed, and sludge incineration is required to achieve the maximum reduction. Sludge incineration actually achieves an enrichment and concentration purpose for many elements. For example, the phosphate content in sludge with a water content of 80% is generally between 1-2%, and after incineration, it generally reaches 8-12% (calculated as P2O5). In 2020, the national production of sludge with a water content of 80% exceeded 60 million tons, and the phosphorus oxide content was calculated to be as high as 500,000 to 1 million tons. The national phosphate rock production in 2021 was 102.899 million tons. If the phosphorus in sludge is recovered, the recovery rate can be as high as 1.5-3%, so sludge is an important raw material for recovering phosphorus.

[0003] The prior art discloses a method for recovering phosphorus from sludge in a municipal sewage treatment plant, comprising a step of gravity concentrating a sludge mixed liquor, a step of releasing phosphorus from the sludge after gravity concentration, a step of solid-liquid separation from the sludge after phosphorus release, and a step of recovering phosphorus from the phosphorus-rich supernatant after solid-liquid separation. The method is characterized in that: the step of releasing phosphorus from the sludge after gravity concentration is to treat the sludge by a combination of acidification and normal pressure microwave radiation; the step of recovering phosphorus from the phosphorus-rich supernatant after solid-liquid separation is to treat the phosphorus-rich supernatant by a calcium phosphate crystallization method to obtain a precipitate containing calcium phosphate crystals, and then filtering the precipitate containing calcium phosphate crystals to recover phosphorus from the sludge.

[0004] However, the phosphorus recovery method in the prior art recovers calcium phosphate, and does not recover the iron in the sludge. Summary of the invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defect that the phosphorus recovery method in the prior art recovers calcium phosphate and does not recover the iron in the sludge, thereby providing a method for recovering phosphorus in sludge.

[0006] To this end, the present invention provides the following technical solutions:

[0007] The present invention provides a method for recovering phosphorus from sludge, comprising the following steps:

[0008] S1: Add dilute hydrochloric acid to the incinerated sludge, stir and filter to obtain filter residue and filtrate;

[0009] S2: adding ammonia water to the liquid obtained by vacuum distillation of the filtrate, and then adding a complexing agent, stirring and filtering at 160-250° C. to obtain crude iron phosphate and a second filtrate;

[0010] S3: adding phosphoric acid and hydrogen peroxide to the obtained second filtrate to react, controlling the pH of the reaction system to be 3-4.5, filtering after the reaction is completed to obtain crude iron phosphate and a third filtrate;

[0011] S4: Wash and dry the crude ferric phosphate obtained in step S2 and step S3 to obtain finished ferric phosphate.

[0012] Optionally, the complexing agent is a phosphate complexing agent;

[0013] And / or, the phosphate complexing agent is one or more of sodium tripolyphosphate, sodium pyrophosphate, and sodium hexametaphosphate.

[0014] Optionally, in step S3, the mass ratio of the phosphoric acid to hydrogen peroxide is (1-2): (0.01-0.1).

[0015] Optionally, the washing in step S4 is performed using dilute phosphoric acid;

[0016] The mass fraction of the dilute phosphoric acid is 10-20%;

[0017] The mass fraction of the dilute hydrochloric acid is 5-15%.

[0018] The mass fraction of the ammonia water is 15-20%.

[0019] Optionally, the mass ratio of the incinerated sludge, dilute hydrochloric acid, ammonia water, and complexing agent is (1-5): (30-100): (10-25): (0.3-0.8);

[0020] And / or, the mass ratio of the incinerated sludge to dilute phosphoric acid is (1-5):(25-45).

[0021] Optionally, the stirring time in step S1 is 1-5h;

[0022] And / or, the filtration in step S1 uses a filter screen of 300-400 mesh.

[0023] Optionally, the pressure of the reduced pressure distillation is 30-50 kPa;

[0024] And / or, the temperature of the reduced pressure distillation is 50-80°C.

[0025] And / or, the duration of the reduced pressure distillation is 1-2h

[0026] Optionally, the stirring time in step S2 is 20-60 min;

[0027] And / or, the filtration in step S2 uses a 300-mesh filter.

[0028] Optionally, the drying in step S4 is carried out by microwave drying;

[0029] And / or, the microwave drying temperature is 80-100°C.

[0030] The technical solution provided by the present invention has the following advantages:

[0031] 1. The present invention provides a method for recovering phosphorus in sludge, comprising the following steps: S1: adding dilute hydrochloric acid to the incinerated sludge, stirring and filtering to obtain filter residue and filtrate; S2: adding ammonia water to the liquid after vacuum distillation of the filtrate, and then adding a complexing agent, stirring and filtering at 160-250°C to obtain crude iron phosphate and a second filtrate; S3: adding phosphoric acid and hydrogen peroxide to the obtained second filtrate for reaction, controlling the pH of the reaction system to 3-4.5, filtering after the reaction, and obtaining crude iron phosphate and a third filtrate; S4: washing and drying the crude iron phosphate obtained in steps S2 and S3 to obtain finished iron phosphate. The sludge recovery method provided by the present invention can convert iron and phosphorus in the sludge into iron phosphate. In addition, by adding a complexing agent, the yield of crude iron phosphate can be further improved. The method has a high recovery rate of phosphorus in the sludge and greatly avoids the loss of phosphorus. After adding the complexing agent, stirring under high temperature conditions can further improve the yield of crude iron phosphate. Phosphoric acid and hydrogen peroxide are added to the second filtrate to control the pH value of the reaction system, so that the residual iron ions in the second filtrate can be further converted into iron phosphate, thereby increasing the yield of crude iron phosphate.

[0032] 2. The method for recovering phosphorus from sludge provided by the present invention finally obtains a filtrate which is a solution of aluminum chloride, ammonium chloride and ferric chloride, and can also effectively recover metal elements in the sludge.

[0033] 3. In step S2 of the present invention, concentrated hydrochloric acid is obtained after reduced pressure distillation. After adding water for dilution, it can also be used in step S1, which greatly reduces the cost and is more conducive to large-scale promotion and use. The second filtrate generated in the process has ammonium chloride and aluminum chloride as its main components. Calcium hydroxide is subsequently added, stirred and filtered. Ammonia is recovered by reduced pressure distillation during the reaction. The obtained filtrate is calcium chloride recovered as a snow-melting agent, and the obtained filter residue is aluminum hydroxide as a raw material for water treatment agents. DETAILED DESCRIPTION

[0034] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0035] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0036] Example 1

[0037] This embodiment provides a method for recovering phosphorus from sludge, comprising the following steps:

[0038] S1: Add 3 g of fly ash sample after sludge incineration, 5 g of ferric chloride hexahydrate and 90 mL of dilute hydrochloric acid (4 mol / L) into a round-bottom flask and heat under condensation reflux at 80°C for 2 hours. The obtained mixture is filtered to obtain a supernatant, which is marked as acid extract.

[0039] S2: The obtained filtrate is subjected to reduced pressure distillation at 30 kPa pressure and 70° C. for 2 h, 20% ammonia water is added to the obtained liquid to adjust the pH value to 1.6, the obtained mixture is filtered, and then 0.5 g of sodium tripolyphosphate is added and stirred at 160° C. for 30 min, and then filtered through a 500-mesh filter to obtain crude iron phosphate and the second filtrate, and the precipitate is collected and placed in an oven for drying;

[0040] S3: adding phosphoric acid and hydrogen peroxide to the obtained second filtrate to react, controlling the pH of the reaction system to 3.5, filtering after the reaction is completed to obtain crude iron phosphate and a third filtrate;

[0041] S4: After the obtained crude iron phosphate is fully washed, it is dried in an oven at 80° C. to obtain 0.43 g of finished iron phosphate.

[0042] S5: Phosphate content during determination

[0043] ①Prepare standard solution of potassium dihydrogen phosphate (100ppm)

[0044] ②Prepare H2SO4 solution (1+1), ascorbic acid solution (100g / L), resorcinol solution (5g / L), ammonium molybdate solution (25g / L), Na2SO3 solution (200g / L)

[0045] ③ Take 6 10mL centrifuge tubes and add 0, 0.1, 0.2, 0.4, 0.8, and 1.6mL of phosphate standard working solution, which is equivalent to phosphate (PO4 3- ) concentrations are 0, 0.7, 1.4, 2.8, 5.6, and 11.2 ppm, and 0.25 mL sulfuric acid (1+1), 0.4 mL ammonium molybdate solution, 0.5 mL Na2SO3 solution, and 0.5 mL resorcinol solution are added in sequence, and then 0.2 mL ascorbic acid solution is added, and the volume is made up to 10 mL with deionized water, shaken, and allowed to stand for 30 minutes. The first group is used as a blank sample, and the absorbance is measured at 700 nm. The standard working curve is drawn with mass concentration as the horizontal axis and the corresponding absorbance as the vertical axis.

[0046] ④ Sample treatment: After the sample is filtered through the filter head, take a certain amount of sample or diluent, add each solution (same standard solution), and then add deionized water to make up to 10mL. After 30 minutes, measure the absorbance with a spectrophotometer.

[0047] S6: Iron content during the determination process

[0048] Take the solution before and after pH adjustment, dilute it 1000 times, and measure Fe by icp. 3+ .

[0049] Example 2

[0050] This embodiment provides a method for recovering phosphorus from sludge, comprising the following steps:

[0051] S1: Add 10.0g of fly ash sample after sludge incineration, 1.6g of ferric chloride hexahydrate and 90mL of dilute hydrochloric acid (4mol / L) into a round-bottom flask, and heat under condensation reflux at 80°C for 2 hours. The obtained mixture is filtered to obtain the supernatant, which is marked as acid extract. ;

[0052] S2: The obtained filtrate is subjected to reduced pressure distillation at 30 kPa pressure and 70° C. for 2 h, 20% ammonia water is added to the obtained liquid to adjust the pH value to 1.6, the obtained mixture is filtered, and then 0.5 g of sodium tripolyphosphate is added and stirred at 250° C. for 30 min, and then filtered through a 500-mesh filter to obtain crude iron phosphate and the second filtrate, and the precipitate is collected and placed in an oven for drying;

[0053] S3: adding phosphoric acid and hydrogen peroxide to the obtained second filtrate to react, controlling the pH of the reaction system to 4, and filtering after the reaction to obtain crude iron phosphate and a third filtrate;

[0054] S4: After the obtained crude iron phosphate is fully washed, it is dried in an oven at 80° C. to obtain 2.8 g of finished iron phosphate.

[0055] S5: Phosphate content during determination

[0056] ⑤Prepare standard solution of potassium dihydrogen phosphate (100ppm)

[0057] ⑥Prepare H2SO4 solution (1+1), ascorbic acid solution (100g / L), resorcinol solution (5g / L), ammonium molybdate solution (25g / L), Na2SO3 solution (200g / L)

[0058] ⑦ Take 6 10mL centrifuge tubes and add 0, 0.1, 0.2, 0.4, 0.8, and 1.6mL of phosphate standard working solution, which is equivalent to phosphate (PO4 3- ) concentrations are 0, 0.7, 1.4, 2.8, 5.6, and 11.2 ppm, and 0.25 mL sulfuric acid (1+1), 0.4 mL ammonium molybdate solution, 0.5 mL Na2SO3 solution, and 0.5 mL resorcinol solution are added in sequence, and then 0.2 mL ascorbic acid solution is added, and the volume is made up to 10 mL with deionized water, shaken, and allowed to stand for 30 minutes. The first group is used as a blank sample, and the absorbance is measured at 700 nm. The standard working curve is drawn with mass concentration as the horizontal axis and the corresponding absorbance as the vertical axis.

[0059] ⑧ Sample treatment: After the sample is filtered through the filter head, take a certain amount of sample or diluent, add each solution (same standard solution), and then add deionized water to make up to 10mL. After 30 minutes, measure the absorbance with a spectrophotometer.

[0060] S6: Iron content during the determination process

[0061] Take the solution before and after pH adjustment, dilute it 1000 times, and measure Fe by icp. 3+ .

[0062] Experimental results:

[0063] Fe(ppm)[Recovery rate] <![CDATA[PO4 (ppm) [Recovery Rate]]]> Original sample 35000 134800 Acid leaching solution 15723[45％] 118447[88％]

[0064] Example 3

[0065] This embodiment provides a method for recovering phosphorus from sludge, comprising the following steps:

[0066] S1: 3 kg of incinerated sludge was added to 50 kg of 10% diluted hydrochloric acid and stirred for 2 hours, and then filtered through a 500-mesh filter to obtain a residue and a filtrate;

[0067] S2: The obtained filtrate is subjected to reduced pressure distillation at 30 kPa pressure and 70° C. for 2 h, 20 kg of 20% ammonia water is added to the obtained liquid, and then 0.5 kg of sodium tripolyphosphate is added and stirred at 200° C. for 30 min, and then filtered through a 500-mesh filter to obtain crude iron phosphate and a second filtrate;

[0068] S3: adding phosphoric acid and hydrogen peroxide to the obtained second filtrate to react, controlling the pH of the reaction system to 3, and filtering after the reaction to obtain crude iron phosphate and a third filtrate;

[0069] S4: The crude iron phosphate obtained in step S2 and step S3 is washed with 30 kg of dilute phosphoric acid with a mass fraction of 15%, and then dried at 100° C. by microwave drying to obtain 0.43 kg of finished iron phosphate.

[0070] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A method for recovering phosphorus from sludge, characterized in that: The following steps are included: S1: Add dilute hydrochloric acid to the incinerated sludge, stir and filter to obtain filter residue and filtrate; S2: adding ammonia water to the liquid obtained by vacuum distillation of the filtrate, and then adding a phosphate complexing agent, stirring and filtering at 160-250° C. to obtain crude iron phosphate and a second filtrate; the phosphate complexing agent is one or more of sodium tripolyphosphate, sodium pyrophosphate, and sodium hexametaphosphate; S3: adding phosphoric acid and hydrogen peroxide to the obtained second filtrate to react, controlling the pH of the reaction system to be 3-4.5, filtering after the reaction is completed to obtain crude iron phosphate and a third filtrate; S4: Wash and dry the crude ferric phosphate obtained in step S2 and step S3 to obtain finished ferric phosphate.

2. The method for recovering phosphorus from sludge according to claim 1, characterized in that: In step S3, the mass ratio of phosphoric acid to hydrogen peroxide is (1-2): (0.01-0.1).

3. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The washing in step S3 is performed using dilute phosphoric acid; The mass fraction of the dilute phosphoric acid is 10-20%; The mass fraction of the dilute hydrochloric acid is 5-15%; The mass fraction of the ammonia water is 15-20%.

4. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The mass ratio of the incinerated sludge, dilute hydrochloric acid, ammonia water and complexing agent is (1-5): (30-100): (10-25): (0.3-0.8); And / or, the mass ratio of the incinerated sludge to the dilute phosphoric acid is (1-5): (25-45).

5. The method for recovering phosphorus from sludge according to any one of claims 1 to 4, characterized in that: The stirring time in step S1 is 1-5h; And / or, the filtration in step S1 uses a filter screen of 300-800 mesh.

6. The method for recovering phosphorus from sludge according to any one of claim 1, characterized in that: The pressure of the reduced pressure distillation is 30-50 kPa; And / or, the temperature of the reduced pressure distillation is 50-80°C, And / or, the duration of the reduced pressure distillation is 1-2h.

7. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The stirring time in step S2 is 20-60 min; And / or, the filtration in step S2 uses a 300-mesh filter.

8. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The drying in step S3 is carried out by microwave drying; And / or, the microwave drying temperature is 80-100°C.