CN106746016A - A kind of method and apparatus for processing waste water - Google Patents

Abstract

The invention provides a method for treating wastewater. The wastewater to be treated is introduced into a pre-sedimentation tank, and the suspended matter and part of SO ₄ ^2- precipitation in the wastewater are removed by natural precipitation or adding lime; the pre-sedimentation tank The supernatant in the water is introduced into a magnesium and sulfur removal tank, and sodium hydroxide and aluminum salts are added to the magnesium and sulfur removal tanks. The sodium hydroxide and aluminum salts form hydrotalcite-like ions with magnesium ions and calcium ions in the wastewater. Precipitation; the above-mentioned treated wastewater is introduced into a magnesium and sulfur removal sedimentation tank, and the supernatant after solid-liquid separation is introduced into a decalcification reaction tank, and a decalcification agent is added to the decalcification reaction tank to generate calcium carbonate ; The above-mentioned treated wastewater is introduced into a decalcification sedimentation tank, and the supernatant after solid-liquid separation is introduced into a neutralization tank, and then enters the subsequent treatment unit after adding acid for neutralization. Also provided is a device for realizing the above wastewater treatment method. The invention can efficiently remove sulfate radicals and realize hardness softening of waste water.

Description

A method and device for treating waste water

technical field

The invention belongs to the field of chemical industry, and relates to a technology for environmental protection and rational utilization of water resources, in particular to a method and device for treating waste water.

Background technique

With the increasingly serious problems caused by water pollution, more and more attention has been paid to wastewater treatment and resource utilization. The "Water Pollution Prevention and Control Action Plan" issued in 2015 clearly stated that "strengthen the recycling of industrial water. Promote the comprehensive utilization of mine water, give priority to the use of mine water for supplementary water in coal mining areas, production and ecological water in surrounding areas, and strengthen the recycling of coal washing wastewater. Encourage Advanced treatment and reuse of wastewater from high-water consumption enterprises such as iron and steel, textile printing and dyeing, papermaking, petroleum and petrochemical, chemical industry, leather making, etc., and require that "steel, thermal power, chemical industry, pulp and papermaking, printing and dyeing, etc. New water intake permits shall not be approved for projects.” Therefore, the utilization of waste water resources and the development of zero-emission technology have become hot issues in the research and development of thermal power, petroleum and petrochemical, chemical, steel and other industries. Membrane technology is commonly used for deep resource utilization of wastewater, while zero discharge requires the combination of membrane technology and evaporation crystallization technology. However, there are high concentrations of hardness ions and sulfates in many industrial wastewaters, which are easy to form scale and precipitate during the process of membrane concentration or evaporation crystallization, causing serious membrane fouling problems and affecting the heat transfer efficiency of the evaporator. For a zero-discharge process aimed at industrial salt recovery, excessive hardness and sulfate ions will affect the purity of the salt. Therefore, the removal of hardness ions and sulfate radicals is the key to the development of zero-emission process technology.

Calcium and magnesium ions are less difficult to remove and can be removed by forming precipitation with carbonate and hydroxide respectively. For sulfate, due to the high solubility product of calcium sulfate, the concentration of sulfate in wastewater after calcium salt precipitation is usually high (>1000mg/L). Therefore, the development of efficient and stable sulfate radical removal technology is both a focus and a difficult issue in the removal of hardness ions. The invention patent "An integrated pretreatment process for desulfurization wastewater resources" (application number 201610304236.3) discloses a process for the removal of sulfate radicals in desulfurization wastewater and sludge recycling. The process uses sodium hydroxide to pre-precipitate magnesium ions, and then Using ettringite to remove sulfate radicals. Although this method is effective, the process flow is long and the operation management is relatively complicated.

Contents of the invention

Aiming at the above technical problems in the prior art, the present invention provides a method and device for treating waste water. The method and device for treating waste water will solve the complex process of removing sulfate ions and softening waste water in the prior art. , a technical problem with poor results.

The invention provides a method for treating waste water, comprising the steps of:

1) Import the wastewater to be treated into a pre-sedimentation tank, and remove the suspended matter and part of SO ₄ ^2- precipitation in the wastewater by natural precipitation or adding lime;

2) import the supernatant in the pre-sedimentation tank after step 1) into a magnesium and sulfur removal tank, add sodium hydroxide and aluminum salt, sodium hydroxide and Aluminum salts form hydrotalcite-like precipitates with magnesium ions and calcium ions in wastewater;

3) The waste water treated in step 2) is introduced into a magnesium and sulfur removal sedimentation tank, the supernatant after solid-liquid separation is introduced into a decalcification reaction tank, and decalcification is added in the decalcification reaction tank The agent reacts to generate calcium carbonate;

4) The wastewater treated in step 3) is introduced into a decalcification sedimentation tank, and the supernatant after solid-liquid separation is introduced into a neutralization tank, and then enters a subsequent treatment unit after neutralization by adding acid.

Further, in step 1), the pH in the pre-sedimentation tank is controlled to be 6.5-9.5 by adding lime into the pre-sedimentation tank;

Further, in step 2), the pH in the magnesium and sulfur removal tank is controlled to be 8.0-12.5 by adding sodium hydroxide to the magnesium and sulfur removal tank.

Further, in step 2), the molar ratio of the added aluminum salt to the sulfate radical in the magnesium and sulfur removal pool is 1.0-4.0:1.

Further, in step 3), the calcium remover is sodium carbonate or sodium bicarbonate, and the molar ratio of sodium carbonate or sodium bicarbonate to calcium ions in wastewater is 0.8-2.0:1.

Further, in step 3), the exhaust flue gas is directly introduced, and the carbon dioxide in the flue gas is used to remove calcium. The molar ratio of carbon dioxide to calcium ions in the waste water is 0.8-2.0:1.

The present invention also provides a device for realizing the above wastewater treatment method, including a pre-sedimentation tank, the pre-sedimentation tank is connected with a magnesium and sulfur removal tank, and the magnesium and sulfur removal tank is connected with a magnesium and sulfur removal sedimentation tank Connected, the magnesium removal and sulfur removal sedimentation tank is connected with a calcium removal reaction tank, the calcium removal reaction tank is connected with a calcium removal sedimentation tank, the calcium removal sedimentation tank is connected with a neutralization tank, the The pre-sedimentation tank is also connected with a first chemical dosing tank, the magnesium and sulfur removal tank is also connected with a second chemical dosing tank and a third chemical dosing tank respectively, and the decalcification reaction tank is also connected with a The fourth medicine box is connected.

Further, the magnesium and sulfur removal tank is also provided with a pH meter and a first stirring device.

Further, the decalcification reaction tank is also provided with a second stirring device.

In the pre-sedimentation tank, calcium oxide is added to adjust the pH to above 6.5. While removing suspended solids, calcium sulfate can also be formed to remove part of the sulfate radical. The reaction equation is as follows:

Ca ²⁺ +SO ₄ ^2- ＝CaSO ₄ ↓ (1)

In the magnesium and sulfur removal tank, aluminum salt is added and the pH is adjusted to above 8.0 with sodium hydroxide, so that the magnesium ions and calcium ions in the wastewater form hydrotalcite-like precipitates. Its reaction equation:

Al ³⁺ +4OH ^- ＝AlO ₂ ^- +2H ₂ O＝Al(OH) ₄ ^- (2)

Al(OH) ₄ ^- +2OH ^- ＝Al(OH) ₆ ^3- (3)

2Al(OH) ₆ ^3- +6Ca ²⁺ +24H ₂ O＝{Ca[Al(OH) ₆ ] ₂ ·24H ₂ O} ⁶⁺ (4)

{Ca ₆ [Al(OH) ₆ ] ₂ ·24H ₂ O} ⁶⁺ +3SO ₄ ^2- +2H ₂ O＝Ca ₆ [Al(OH) ₆ ] ₂ (SO ₄ ) ₃ ·26H ₂ O↓ (5 )

Mg ²⁺ +4OH ^- ＝Mg(OH) ₄ ^2- (6)

6Mg(OH) ₄ ^2- +2Al(OH) ₄ ^- +SO ₄ ^2- +mH ₂ O=[Mg ₆ Al ₂ (OH) ₁₆ ](SO ₄ )·mH ₂ O↓+16OH ^- (7)

In the magnesium and sulfur removal pool, hydrotalcites will preferentially form precipitates over ettringite, so controlling the reasonable pH range of the reaction pool can realize the efficient removal of magnesium ions. At the same time, maintaining an excess of calcium ions in the influent can achieve efficient sulfate removal. Due to the high solubility product of calcium hydroxide, it is difficult for calcium ions to form calcium hydroxide. Therefore, the sediment composition of the magnesium and sulfur removal tank is mainly hydrotalcite-like, and some industrial wastewater also contains some heavy metal co-precipitates.

Since the magnesium and sulfur removal tanks cannot completely remove calcium ions, a carbonate decalcifier is added to the strongly alkaline wastewater in the decalcification reaction tank to convert calcium ions into calcium carbonate for removal. Its reaction equation is

Ca ²⁺ +CO ₃ ^2- ＝CaCO ₃ ↓(K _sp ＝8.7×10 ^-9 ) (9)

If the molar ratio of carbonate to calcium ions is 1:1, the theoretical concentration of calcium ions is

Therefore, under this condition, the calcium ion concentration in the wastewater is 3.7mg/L. A large amount of calcium ions will enter the sediment of the decalcification reaction tank in the form of calcium carbonate.

According to the above analysis, it can be seen that the main component of the bottom flow sediment in the pre-sedimentation tank is gypsum (in power plants, it can be returned to the cyclone of the flue gas desulfurization system for use); the composition of the bottom flow sediment in the magnesium and sulfur removal sedimentation tank is hydrotalcite-like and ettringite, which can be recycled. After purification, hydrotalcite-like can be applied in agricultural film, which has the advantages of anti-reflection, slow release, heat preservation, synergy with light stabilizer and thermal stability. The main component of the bottom flow sediment in the decalcification sedimentation tank is calcium carbonate (in enterprises with wet flue gas desulfurization systems, it can be returned to the desulfurization slurry tank and used as a desulfurizer for flue gas desulfurization).

Beneficial effects of the present invention:

(1) As the hardness softening treatment unit of the current wastewater zero discharge system, it can effectively prevent the fouling of desalination units such as membrane concentration and evaporation;

(2) Set up a pre-sedimentation tank before softening, and form a slightly soluble calcium sulfate precipitate by adding cheap lime to reduce the cost of subsequent hydrotalcite-like or ettringite-removing chemicals;

(3) Preferentially remove magnesium and sulfate radicals in the form of hydrotalcite-like precipitates, use ettringite to remove residual sulfate radicals, and finally achieve efficient removal of sulfate radicals and magnesium ions in the magnesium and sulfur removal pool;

(4) Efficient removal of calcium ions is achieved through two precipitates of ettringite and calcium carbonate in the magnesium and sulfur removal tank and the calcium removal tank;

(5) According to the chemical resources around different waste water zero discharge systems, a new idea of using sodium bicarbonate or flue gas (carbon dioxide) to remove calcium is proposed, which can reduce the alkalinity of the effluent while removing calcium ions, and reduce the amount of hydrochloric acid in the neutralization pool ;

(6) Realize fewer reaction tanks and sedimentation tanks through process optimization, realize high-purity recovery of calcium and magnesium and high-purity independent recovery of desulfurizer (calcium carbonate) in the process of wastewater pretreatment, which has significant economic benefits.

Compared with the prior art, the technical progress of the present invention is remarkable. The present invention aims at the defects of the existing sulfate radical removal technology, and the present invention proposes a process flow for synchronously removing calcium, magnesium and sulfate radicals by using hydrotalcite-like mixed precipitates, so as to ensure the operation stability of the wastewater membrane concentration and evaporation unit, and meet the requirements A zero discharge system for recovering industrial salt can improve the quality of industrial salt. The process technology has the advantages of reliable operation, high efficiency and low cost, and is of great significance for water management and "zero discharge". The process principle of the present invention is significantly different from that of the patent 201610304236.3. The process is shorter, the operation and management are simple, and it can efficiently remove sulfate radicals and achieve hardness softening.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. For those skilled in the art, other drawings can also be obtained according to these drawings without any creative effort.

Fig. 1 is a structural schematic diagram of a device for treating wastewater according to the present invention. 1 is the pre-sedimentation tank, 2 is the first dosing tank, 3 is the magnesium and sulfur removal tank, 4 is the second dosing tank, 5 is the third dosing tank, 6 is the magnesium and sulfur removal sedimentation tank, and 7 is the calcium removal Reaction tank, 8 is the fourth dosing tank, 9 is the decalcification sedimentation tank and 10 is the neutralization tank.

detailed description

The present invention will be described in detail below in conjunction with accompanying drawing 1 and specific embodiments.

Example 1

The invention provides a method for treating waste water, specifically comprising the steps of:

After the waste water is collected, add excess lime calculated according to the sulfate radical removal requirement, control the pH in the pre-sedimentation tank to 6.5-9.5, mix well and then enter the pre-sedimentation tank 1 to realize solid-liquid separation. After solid-liquid separation, the supernatant flows into the magnesium and sulfur removal tank 3 .

Add sodium hydroxide and aluminum salt with a molar ratio of 1.0 to 4.0:1 to sulfuric acid to the magnesium and sulfur removal tank 3, and after fully mixing with the effluent from the pre-sedimentation tank, adjust the pH to 8.0 to 12.5 to make the waste water Magnesium ions and calcium ions are converted into hydrotalcite-like and ettringite precipitates, and solid-liquid separation is realized in the magnesium and sulfur removal sedimentation tank 6 , and the supernatant flows to the calcium removal reaction tank 7 . The sediment with hydrotalcite-like as the main component is recycled after being dehydrated by plate and frame.

Dosing a certain proportion of descaling agent (sodium carbonate, sodium bicarbonate or passing through carbon dioxide, the molar ratio of carbonate to calcium ion added is 0.8-2.0:1) in the descaling reaction tank 7 . It is fully mixed with the effluent from the magnesium and sulfur removal tanks to convert calcium ions into calcium carbonate precipitates. After the reaction, it enters the calcium removal sedimentation tank 9 to achieve solid-liquid separation. The main component of the precipitates is calcium carbonate. The supernatant flows into the neutralization tank 10, the water quality is adjusted to be neutral and then the water is discharged.

The device for realizing the above method includes a pre-sedimentation tank 1, the pre-sedimentation tank 1 is connected with a magnesium and sulfur removal tank 3, and the magnesium and sulfur removal tank 3 is connected with a magnesium and sulfur removal sedimentation tank 6, The magnesium and sulfur removal sedimentation tank 6 is connected to a calcium removal reaction tank 7, the calcium removal reaction tank 7 is connected to a calcium removal sedimentation tank 9, and the calcium removal sedimentation tank 9 is connected to a neutralization tank 10 connected, the pre-sedimentation tank 1 is also connected with a first dosing tank, and the magnesium and sulfur removal tank 3 is also connected with a second dosing tank 4 and a third dosing tank 5 respectively, and the The decalcification reaction tank 7 is also connected with a fourth medicine adding box.

Further, the magnesium and sulfur removal pool 3 is also provided with a pH meter and a first stirring device.

Further, the decalcification reaction tank 7 is also provided with a second stirring device.

Example 2

For flue gas desulfurization wastewater from thermal power plants, the raw water quality is Mg ²⁺ concentration 800mg/L, Ca ²⁺ concentration 2000mg/L, SO ₄ ^2- concentration 3000mg/L. Using the patented technology of the present invention, 2.5g/L lime is added to the pre-sedimentation tank 1, 15g/L of aluminum salt is added to the magnesium and sulfur removal tank 3 and the pH is adjusted to 11.0, and the lime is added to the calcium removal tank 7 After the reaction of 0.2g/L sodium carbonate, the concentration of Mg ²⁺ in the effluent was 0.1mg/L, the concentration of Ca ²⁺ was 1.12mg/L, and the concentration of SO ₄ ^2- was 276.77mg/L. The removal rate of Mg ²⁺ and Ca ²⁺ is higher than 99.9%, and the removal rate of SO ₄ ^2- is 92.3%.

Example 3

The nanofiltration concentrate of the waste incineration plant, the raw water quality is Mg ²⁺ concentration 432mg/L, Ca ²⁺ concentration 104mg/L, SO ₄ ^2- concentration 5492mg/L. Because it contains a large amount of organic matter, it is not conducive to the removal of sulfate radicals, so the method of coupling coagulation and oxidant is used to remove part of the organic matter, and the effluent enters the pre-sedimentation tank of the technology of the present invention. Using the patented technology of the present invention, 7.9g/L lime is added to pre-sedimentation tank 1, 34.5g/L aluminum salt is added to magnesium and sulfur removal tank 3 and the pH is adjusted to 11.0, and calcium removal tank 7 is added After adding 0.1g/L of sodium carbonate for reaction, the quality of the effluent is Mg ²⁺ concentration 2.1mg/L, Ca ²⁺ concentration 0.3mg/L, SO ₄ ^2- concentration 1191.44mg/L. The removal rates of Mg ²⁺ and Ca ²⁺ were 99.99% and 99.71%, respectively, and the removal rate of SO ₄ ^2- was 78.3%.

Example 4

The concentration of reverse osmosis in the waste incineration plant, the raw water quality Mg ²⁺ concentration is 302.4mg/L, Ca ²⁺ concentration is 108mg/L, SO ₄ ^2- concentration is 954.45mg/L. Using the patented technology of the present invention, 1.2g/L lime is added to the pre-sedimentation tank 1, 4.8g/L aluminum salt is added to the magnesium and sulfur removal tank 3 and the pH is adjusted to 11.0, and the lime is added to the calcium removal tank 7. After adding 0.1g/L sodium carbonate for reaction, the effluent quality is free of Mg ²⁺ and Ca ²⁺ , the concentration of SO ₄ ^2- is 162.51mg/L, calcium and magnesium are completely removed, and the removal rate of SO ₄ ^2- is 83.0%.

Example 5

Coal chemical wastewater, raw water quality Mg ²⁺ concentration 302mg/L, Ca ²⁺ concentration 460mg/L, HCO ₃ -concentration 600mg/L, SO ₄ ^{2- concentration} 2184mg/L. Using the patented technology of the present invention, 3.4g/L lime is added to the pre-sedimentation tank 1, 11.0g/L aluminum salt is added to the magnesium and sulfur removal tank 3 and the pH is adjusted to 11.0, and the lime is added to the calcium removal tank 7 After adding 0.9g/L of sodium carbonate for reaction, the concentration of Mg ²⁺ in the effluent is 0.4mg/L, the concentration of Ca ²⁺ is 24mg/L, there is no HCO ₃ ^- , and the concentration of SO ₄ ^2- is 302.64mg/L. Mg ²⁺ and Ca ²⁺ removal rates were 99.87% and 94.78%, HCO ₃ ^- was completely removed, SO ₄ ^2- removal rate was 85.14%.

In addition, it should be noted that the specific embodiments described in this specification may be different in parts, shapes and names of components. All equivalent or simple changes made according to the structure, features and principles described in the patent concept of the present invention are included in the protection scope of the patent of the present invention. Those skilled in the art to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, as long as they do not deviate from the structure of the present invention or exceed the scope defined in the claims. All should belong to the protection scope of the present invention.

Claims (9)

1. A method for treating waste water, characterized in that it may further comprise the steps: 1) Lead the wastewater to be treated into a pre-sedimentation tank, and remove the suspended matter and part of SO ₄ ^2- precipitation in the wastewater by natural precipitation or adding lime; 2) Import the supernatant in the pre-sedimentation tank treated in step 1) into a magnesium and sulfur removal tank, and add sodium hydroxide and aluminum salts, sodium hydroxide and Aluminum salts form hydrotalcite-like precipitates with magnesium ions and calcium ions in wastewater; 3) The wastewater treated in step 2) is introduced into a magnesium and sulfur removal sedimentation tank, and the supernatant after solid-liquid separation is introduced into a decalcification reaction tank, and decalcification is added to the decalcification reaction tank. The agent reacts to generate calcium carbonate; 4) The wastewater treated in step 3) is introduced into a decalcification sedimentation tank, and the supernatant after solid-liquid separation is introduced into a neutralization tank, and then enters the subsequent treatment unit after neutralization with acid. 2. A method for treating wastewater according to claim 1, characterized in that: in step 1), the pH in the pre-sedimentation tank is controlled to be 6.5-9.5 by adding lime to the pre-sedimentation tank. 3. A method for treating wastewater according to claim 1, characterized in that: in step 2), the pH in the magnesium and sulfur removal pool is controlled by adding sodium hydroxide to the magnesium and sulfur removal pool 8.0~12.5. 4. A method for treating wastewater according to claim 1, characterized in that: in step 2), the molar ratio of the added aluminum salt to the sulfate radical in the magnesium and sulfur removal tank is 1.0~4.0:1. 5. A method for treating wastewater according to claim 1, characterized in that: in step 3), the calcium remover is sodium carbonate or sodium bicarbonate, sodium carbonate or sodium bicarbonate and calcium in wastewater The molar ratio of ions is 0.8~2.0:1. 6. A method for treating wastewater according to claim 1, characterized in that: in step 3), directly introduce exhaust flue gas, use carbon dioxide in the flue gas to remove calcium, and the molar ratio of carbon dioxide to calcium ions in wastewater 0.8~2.0:1. 7. The device for realizing the wastewater treatment method according to claim 1 is characterized in that: it comprises a pre-sedimentation tank, and the said pre-sedimentation tank is connected with a magnesium removal and sulfur removal tank, and said magnesium removal and sulfur removal tank and A magnesium removal and sulfur removal sedimentation tank is connected, the magnesium removal and sulfur removal sedimentation tank is connected with a calcium removal reaction tank, the calcium removal reaction tank is connected with a calcium removal sedimentation tank, and the calcium removal sedimentation tank is connected with a The neutralization tank is connected, the pre-sedimentation tank is also connected with a first chemical dosing tank, and the magnesium and sulfur removal tank is also connected with a second chemical dosing tank and a third chemical dosing tank respectively, and the described The decalcification reaction pool is also connected with a fourth medicine adding box. 8. The device according to claim 7, characterized in that: said magnesium and sulfur removal tank is also provided with a pH meter and a first stirring device. 9. The device according to claim 7, characterized in that: said decalcification reaction tank is also provided with a second stirring device.