Method for safely treating hazardous waste containing phosphorus and arsenic

Abstract

The invention relates to the technical field of hazardous waste treatment, in particular to a method for safely treating hazardous waste containing phosphorus and arsenic, simple substance phosphorus and arsenite are oxidized in two stages through sodium hypochlorite, then phosphate and arsenate are converted into calcium phosphate, calcium arsenate and iron arsenate precipitates, stabilization of phosphorus and arsenic is achieved, and the hazardous waste containing phosphorus and arsenic is obtained. Finally, flocculation, solid-liquid separation and solidification landfill are conducted, and safe treatment of phosphorus and arsenic is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of hazardous waste treatment, and in particular to a method for safely disposing hazardous waste containing phosphorus and arsenic. Background Art

[0002] Semiconductor device doping technology is to introduce the required impurities into the specified area of ​​the semiconductor substrate in a certain manner, and to achieve the specified quantity and distribution that meets the requirements, so as to achieve the purpose of changing the electrical properties of the material.

[0003] Currently commonly used doping methods include diffusion and ion implantation. The diffusion method produces waste gas containing phosphorus and arsenic, which enters the waste gas treatment system after being filtered through a filter. The filter needs to be cleaned after a period of use (the cleaning liquid is generally a mixture of ammonia water + hydrogen peroxide + water). The waste liquid generated during the cleaning process contains a large amount of phosphorus and arsenic (although it is called waste liquid, it contains insoluble impurities such as elemental phosphorus that is insoluble in water). Specifically, it contains trivalent arsenic, pentavalent arsenic, elemental phosphorus, and phosphide, which are hazardous wastes and need to be treated before discharge.

[0004] The existing technology only has precedents for separately treating arsenic- or phosphorus-containing wastewater. Inorganic arsenic is highly toxic, and trivalent arsenic precipitates are highly soluble. During treatment, it must first be oxidized to pentavalent arsenic, and then calcium salts and iron salts are used to react to generate calcium arsenate and ferric arsenate precipitates, which are then solidified and landfilled. Elemental phosphorus is flammable and highly toxic. If it is discharged directly without treatment, it will cause serious harm to the environment. Since elemental phosphorus is insoluble in water, it is difficult to degrade after entering the water body. During treatment, potassium permanganate and calcium hypochlorite are used to oxidize elemental phosphorus, followed by chemical coagulation and precipitation.

[0005] As for the phosphorus- and arsenic-containing waste liquid produced by the above-mentioned semiconductor device doping process, the two existing impurity removal technologies are simply superimposed, and the steps are complicated and tedious. Therefore, the problem of safe disposal of the phosphorus- and arsenic-containing waste liquid needs to be solved urgently. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for the safe disposal of phosphorus- and arsenic-containing hazardous waste, which can be used to simply, quickly and safely dispose of waste liquid containing both phosphorus and arsenic.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for safely disposing of phosphorus- and arsenic-containing hazardous waste, comprising the following steps:

[0008] (1) One-stage oxidation: add an alkaline reagent to the waste liquid to adjust the pH to 11-13, add sodium hypochlorite, and react to obtain a first reaction solution;

[0009] (2) Second stage oxidation: adjust the pH of the first reaction solution to 7.5-8.5, add sodium hypochlorite again, and react to obtain a second reaction solution;

[0010] (3) Neutralization: Adjust the pH of the second reaction solution to 8.5-9.5, then add calcium chloride and ferric chloride, add a flocculant after the reaction, and separate the solid and liquid to obtain a filter residue and a filtrate;

[0011] (4) Solidification and landfill: Mix the obtained filter residue with cement and sand and then solidify and landfill.

[0012] In the above scheme, the present invention utilizes sodium hypochlorite to oxidize elemental phosphorus and arsenite in two stages, and then converts phosphate and arsenate into calcium phosphate, calcium arsenate, and ferric arsenate precipitation to achieve the stabilization of phosphorus and arsenic, and finally performs flocculation, solid-liquid separation, solidification and landfill to achieve the safe disposal of phosphorus and arsenic. DETAILED DESCRIPTION

[0013] The technical solution of the present invention is further described in detail below with reference to the embodiments.

[0014] A method for safely disposing of phosphorus- and arsenic-containing hazardous wastes comprises the following steps:

[0015] (1) One-stage oxidation: first add liquid alkali (sodium hydroxide solution with a mass concentration of 25-35%) to control the solution pH at about 12, then add sodium hypochlorite solution as an oxidant to the phosphorus- and arsenic-containing hazardous waste, and the added amount is 1.5-2.0 times the theoretical amount (i.e., the theoretical amount is calculated according to the following chemical equation based on the elemental phosphorus content in the waste liquid), oxidizing the elemental phosphorus in the waste liquid to phosphate to obtain a first reaction liquid, at which time the pH of the first reaction liquid is about 5-6.

[0016] The reaction equation for this process is as follows:

[0017] P4+10NaClO+6H2O=4H3PO4+10NaCl

[0018] P2+5NaClO+3H2O=2H3PO4+5NaCl

[0019] 2P+5NaClO+3H2O=2H3PO4+5NaCl

[0020] (2) Secondary oxidation: Liquid alkali (sodium hydroxide solution with a mass concentration of 25-35%) is added again to adjust the first reaction solution to a weak alkaline state (pH of about 8). Then, sodium hypochlorite solution is added as an oxidant in an amount of 1.0-1.2 times the theoretical amount (i.e., the theoretical amount is calculated according to the following chemical equation based on the trivalent arsenic content in the waste liquid). Arsenite is oxidized to arsenate to obtain a second reaction solution. At this time, the pH of the second reaction solution is about 6. The reaction equation for this process is as follows:

[0021] 2OH - +AsO2 - +ClO - =AsO43- +Cl - +H2O

[0022] (3) Neutralization reaction: Add lime milk to the second reaction solution to adjust the solution pH to about 9, then add calcium chloride and ferric chloride to react with phosphate and arsenate to form calcium phosphate, calcium arsenate, and ferric arsenate precipitates; the amount of calcium chloride added is the sum of the amount of phosphate and arsenate in the second reaction solution; the amount of ferric chloride added is 1 / 20 to 1 / 10 of the amount of arsenate in the second reaction solution. After the reaction, add flocculant PAC, and the solution flocculates to obtain a mixed solution. The reaction equation for this process is as follows:

[0023] 3Ca 2+ +2AsO4 3- =Ca3(AsO4)2

[0024] 3Fe 3+ +2AsO4 3- =FeAsO4

[0025] 3Ca 2+ +2PO4 3- =Ca3(PO4)2

[0026] (4) Solidification and landfill: Filter the mixed liquid obtained above to obtain filter residue, mix and stir evenly according to the mass ratio of filter residue, cement, sand and water of 0.1:1:1.2:0.5, let it stand for 24 hours, cure for 7 days, and carry out leaching test to meet the standards before safe landfill in hazardous waste landfill.

[0027] The above scheme uses sodium hypochlorite under different alkaline conditions (different pH values) to sequentially oxidize elemental phosphorus and trivalent arsenic in hazardous waste into more stable phosphates and pentavalent arsenic. The phosphates and pentavalent arsenic are then simultaneously precipitated, transforming the hazardous substances into a more stable state and improving the safety and stability of the solidified landfill. The scheme of the present invention features a simple process, convenient and rapid disposal, and uses a limited number of readily available reagents, significantly simplifying the safe disposal process for phosphorus- and arsenic-containing wastewater.

[0028] Example 1

[0029] Hazardous waste to be treated: elemental phosphorus content: 13823.84 mg / L, arsenic content: 10015 mg / L

[0030] Processing steps:

[0031] (1) One-stage oxidation: add 305 kg of 32% sodium hydroxide solution to the reaction tank, slowly add 43 kg of phosphorus- and arsenic-containing hazardous waste while stirring, and then slowly pump 1.5 times the theoretical amount of oxidant (10% sodium hypochlorite) into the phosphorus- and arsenic-containing hazardous waste to oxidize elemental phosphorus to phosphate, thereby obtaining a first reaction solution;

[0032] (2) Secondary oxidation: 20 kg of 32% sodium hydroxide solution was pumped into the above solution to adjust the pH of the solution to alkaline (pH of about 8), and then 1.0 times the theoretical amount of oxidant (10% sodium hypochlorite) was pumped into the solution to completely oxidize arsenite to arsenate, thereby obtaining a second reaction solution;

[0033] (3) Neutralization reaction: 25 kg of 10% lime milk was added to the second reaction solution to adjust the pH of the solution to about 9, and then 617.36 kg of 30% calcium chloride and 42 kg of 10% ferric chloride were added to react with phosphate and arsenate to form calcium phosphate, calcium arsenate, and ferric arsenate precipitates; PAC solution was then added to flocculate to obtain a mixed solution;

[0034] (4) Solidification and landfill: The mixed liquid is filtered by filter pressing to obtain a filter cake. The filter residue obtained after filtration is mixed and stirred evenly according to the mass ratio of the filter residue, cement, sand and water of 0.1:1:1.2:0.5, and then allowed to stand for 24 hours and cured for 7 days. After the leaching test is carried out and the filter cake meets the standards, it is safely landfilled in a hazardous waste landfill.

[0035] Example 2

[0036] The difference between this embodiment and embodiment 1 is that in step (2), the amount of sodium hypochlorite added is 1.5 times the theoretical amount.

[0037] Example 3

[0038] The difference between this embodiment and embodiment 1 is that in step (2), the amount of sodium hypochlorite added is 2.0 times the theoretical amount.

[0039] The comparison of trivalent arsenic and pentavalent arsenic contents in the liquid after dissolving the phosphorus- and arsenic-containing hazardous waste in Examples 1-3 of the present invention is shown in Table 1:

[0040] Table 1 Monitoring data of arsenic content in the treatment process of Examples 1-3

[0041]

[0042]

[0043] In summary, the present invention offers a simple process flow and safe operation, solving the difficult problem of elemental phosphorus disposal and reducing the risk of spontaneous combustion of elemental phosphorus, thereby achieving the safety of the present invention. Furthermore, the present invention utilizes sodium hypochlorite as the oxidant for arsenite oxidation after the oxidation of elemental phosphorus, thus minimizing process changes and reducing the difficulty and risk of disposing of phosphorus- and arsenic-containing wastewater.

Claims

1. A method for safely disposing of hazardous waste containing phosphorus and arsenic, comprising the following steps: (1) One-stage oxidation: add an alkaline reagent to the waste liquid to adjust the pH to 11-13, add sodium hypochlorite, and react to obtain a first reaction solution; (2) Second stage oxidation: adjust the pH of the first reaction solution to 7.5-8.5, add sodium hypochlorite again, and react to obtain a second reaction solution; (3) Neutralization: Adjust the pH of the second reaction solution to 8.5-9.5, then add calcium chloride and ferric chloride, add a flocculant after the reaction, and separate the solid and liquid to obtain a filter residue and a filtrate; (4) Solidification and landfill: Mix the obtained filter residue with cement and sand and then solidify and landfill.

2. The method for safe disposal of phosphorus- and arsenic-containing hazardous waste according to claim 1, characterized in that: In the step (1), the alkaline reagent added is liquid alkali.

3. The method for safe disposal of phosphorus- and arsenic-containing hazardous waste according to claim 1, characterized in that: In the step (1), the amount of sodium hypochlorite added is 1.5-2.0 times the amount of phosphorus in the waste liquid to be treated.

4. The method for safe disposal of phosphorus- and arsenic-containing hazardous waste according to claim 1, characterized in that: In the step (2), liquid alkali is added to adjust the pH value of the first reaction solution.

5. The method for safe disposal of phosphorus- and arsenic-containing hazardous waste according to claim 1, characterized in that: In the step (2), the amount of sodium hypochlorite added is 1.0-1.2 times the amount of trivalent arsenic in the waste liquid to be treated.

6. The method for safe disposal of phosphorus- and arsenic-containing hazardous waste according to claim 1, characterized in that: In the step (3), lime milk is added to adjust the pH value of the second reaction solution.

7. The method for safe disposal of phosphorus- and arsenic-containing hazardous waste according to claim 1, characterized in that: In step (3), the amount of calcium chloride added is the sum of the amounts of phosphate and arsenate in the second reaction solution; the amount of ferric chloride added is 1 / 20 to 1 / 10 of the amount of arsenate in the second reaction solution.

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