CN117299759A - A method for recycling heavy metals and soluble chlorine salts in domestic waste incineration fly ash - Google Patents

Abstract

The patent discloses a method for recycling fly ash from incineration of household garbage. The method comprises a series of process steps including acid leaching, heavy metal recovery, heavy metal and impurity removal, arsenic and sulfur removal and the like. In this process, hydrochloric acid is used to leach heavy metals, such as zinc, copper, lead, etc., from fly ash. The leachate obtained is used in a heavy metal recovery step in which the pH of the leachate is monitored to determine the end point of the precipitation reaction. And then, the heavy metal and impurities are removed, and the residual heavy metal, aluminum, iron, magnesium and other impurities in the leaching solution are further removed by increasing the pH value. The next step is arsenic and sulfur removal by adding an oxidizing agent and ferric flocculant to remove sulfur and arsenic from the leachate. The purified leaching solution is used for recovering the raw materials of the soluble chloride salt. By the method, heavy metals and soluble chloride salts can be recovered from the household garbage incineration fly ash, the reduction of the fly ash is realized, and the treated fly ash can be used as a cement raw material or a building material.

Description

A method for recycling heavy metals and soluble chlorine salts in domestic waste incineration fly ash

Technical field

The invention belongs to the technical fields of chemical industry and environmental protection, and specifically relates to a method for recovering heavy metals and soluble chlorine salts from domestic waste incineration fly ash.

Background technique

With the continuous development of social economy and the acceleration of urbanization, the production of garbage is also increasing, resulting in the phenomenon of "garbage siege" on a global scale. This situation has triggered a series of environmental pollution problems, including serious waste of land resources, groundwater pollution, and the spread of germs. In order to deal with these problems, incineration treatment technology has gradually emerged. Incineration treatment has become a widely used waste treatment method in many countries around the world due to its advantages such as small footprint, effective reduction of waste volume, and ability to achieve cogeneration of heat and power. In our country, this technology has also been widely used. However, after waste incineration treatment, domestic waste incineration fly ash (hereinafter referred to as fly ash) equivalent to 2 to 5% of the weight of the waste will be produced. These fly ash contain toxic heavy metals such as Cd, Cr, Pb, Zn, etc., and are easily leached under various natural conditions. But heavy metals such as Zn, Cu, Pb and soluble chloride salts such as calcium chloride, potassium chloride, and sodium chloride in fly ash also have high recovery value.

The main treatment method for fly ash in my country is to solidify and stabilize it before entering the landfill. However, this method has a large volume increase ratio, requires a large number of landfill sites, and cannot recover heavy metals and soluble chlorine salts in fly ash. Therefore, it is of extremely important significance to develop a new and medium-sized fly ash treatment method to realize the resource utilization of fly ash.

The acid leaching method can effectively leach heavy metals from fly ash, and the purified leachate can be used as raw material to recover soluble chlorine salts. In addition, the content of heavy metals and chlorine in fly ash after acid leaching is reduced, and it can be used as cement raw material or other building materials. At the same time, my country's chemical industry produces a large amount of by-product hydrochloric acid every year, which keeps the market price of by-product hydrochloric acid relatively low. This economic advantage provides the feasibility of using hydrochloric acid to leach heavy metals from fly ash. However, the current acid leaching method for treating fly ash still has the following technical difficulties:

1. Because the properties of fly ash are significantly different from those of ores, traditional hydrometallurgical methods are not suitable for leaching heavy metals in fly ash, resulting in low leaching efficiency.

2. Because the fly ash leachate contains calcium chloride, potassium chloride, sodium chloride and other soluble salts with recovery value, this limits the recovery of heavy metals. It is necessary to efficiently recover heavy metals in the leachate and reduce the impact on the recovery of soluble chlorine salts.

Contents of the invention

The purpose of the present invention is to develop a method for simultaneously recovering heavy metals and soluble chlorine salts in fly ash.

The technical solutions provided by the present invention are as follows.

A. Add fly ash and water to the acid leaching reactor and mix them so that the mixed mortar has good fluidity. Subsequently, hydrochloric acid was injected into the reactor to control the pH value of the mortar below 2.5. By using air stirring, the mortar is stirred and the required oxygen is provided to the reaction system. The reaction temperature should be determined based on actual operating conditions. Increasing the temperature can accelerate the leaching process of copper and lead, and can also increase the amount of zinc leached, but this will also be accompanied by an increase in energy consumption.

B. Perform solid-liquid separation on the acid-leached mortar, separate the liquid part to form leachate 1, and the solid part to form leach residue.

C. Wash the leaching residue obtained in step B once with water or multiple times with countercurrent water, and perform solid-liquid separation. The solid part obtained after separation is the treated fly ash, and the liquid part obtained after separation is mixed with the fly ash in step A to prepare new mortar.

D. Adjust the pH value of leach solution 1 to the range of 2.7 to 2.9. Subsequently, sodium sulfide solution is added to the leachate and stirred to promote the precipitation of sulfide from heavy metals. At the same time, monitor the changes in the pH value of the leach solution. When the pH value of the leach solution rises to the range of 3.1 to 3.25 and remains stable, stop adding sodium sulfide solution to it.

E. Perform solid-liquid separation on the leachate 1 to which sodium sulfide has been added. After solid-liquid separation, the solid part obtained is the sulfide precipitate of heavy metals, while the liquid part becomes leachate 2.

F. Adjust the pH value of leach solution 2 to a range of 8.5 to 11 to remove residual heavy metals and impurities such as silicon, aluminum, iron, and magnesium.

G. Perform solid-liquid separation on the pH-adjusted leachate 2, and the liquid part obtained after separation is called leachate 3.

H. Add oxidant and flocculant containing ferric iron to leachate 3, and adjust the pH value according to the operating conditions to remove divalent sulfide ions and arsenic in leachate 3.

1. Perform solid-liquid separation on the leachate 3 from which divalent sulfide ions and arsenic have been removed. The liquid part obtained after separation is the leachate 4. This leachate 4 can be used as a raw material for recovering soluble chlorine salts.

The theoretical basis of the present invention is:

1. Fly ash from domestic waste incineration contains a large amount of zero-valent aluminum. During the leaching process, zero-valent aluminum may undergo displacement reactions with metals such as zinc, copper, and lead, producing products such as zero-valent zinc, zero-valent copper, and zero-valent lead. Use gentle stirring in step A to oxygenate the reaction system. The presence of oxygen helps promote oxygen-absorbing corrosion of zero-valent copper and zero-valent lead, causing them to dissolve into the solution.

2. In step D, determine the end point of the heavy metal precipitation reaction by monitoring the pH change of the leach solution. This is because the solubility product constant of heavy metal sulfides is very low. When there is an excess of heavy metal ions in the leach solution, the concentration of sulfur ions in the leach solution is low, thus maintaining a stable pH value of the leach solution. However, when divalent sulfide ions are excessive, these ions will react with hydrogen ions in the leach solution to form hydrogen sulfide ions and hydrogen sulfide, causing the pH value of the leach solution to rise. As the pH rises above 3.25, the aluminum ions in the leach solution begin to react with hydroxide ions to form aluminum hydroxide precipitate, which will stabilize the pH value of the leach solution again. Therefore, in order to obtain a higher concentration of heavy metal precipitate, it is a reasonable decision to control the pH value between 3.1 and 3.25 as the end point of the precipitation reaction.

3. In step F, by increasing the pH value of the leach solution, the concentration of divalent sulfide ions in the leach solution can be increased, thereby promoting the precipitation of residual heavy metals. At the same time, at a higher pH value, silicon, aluminum, iron, magnesium and other ions in the leachate will also precipitate, thereby achieving the purpose of purifying the leachate.

4. In step I, by adding an oxidizing agent to the leachate, the remaining sulfide ions, hydrogen sulfide ions and hydrogen sulfide can be oxidized to form elemental sulfur precipitation. At the same time, the oxidant can also oxidize arsenite ions into arsenate ions. The use of flocculants containing ferric iron will produce a large amount of iron hydroxide, and arsenate ions react with the iron hydroxide to form a precipitate with very low solubility.

The invention has the following beneficial effects:

1. The treated fly ash has obvious reduction effect.

2. The chlorine content and heavy metal content in the treated fly ash are significantly reduced, making the treated fly ash more suitable as cement raw material or other building materials.

3. By recovering heavy metals in the form of sulfide precipitation, subsequent mineral processing and smelting processing can be facilitated.

4. By purifying heavy metals and arsenic in the leachate, the leachate becomes a raw material that can be used to recover soluble chlorine salts.

Description of drawings

Figure 1 is a process flow diagram of the present invention:

Among them, the leaching residue is washed with countercurrent water twice. During the implementation process, the number of water washings can be increased or decreased according to the actual operating conditions.

Figure 2 is a picture of the heavy metal precipitate in Example 1.

Specific implementation methods

The following is a further description of the present invention, rather than a limitation of the present invention.

Example 1:

A method for recycling heavy metals and soluble chlorine salts in domestic waste incineration fly ash. The process flow chart is shown in Figure 1. The leaching and recovery of heavy metals includes the following steps:

A. Add 150 grams of fly ash into a 1-liter beaker, then add 750 ml of deionized water, and mix thoroughly to form a mortar. Subsequently, the temperature was raised to 80°C in a water bath and vigorous stirring was performed using a mechanical stirrer at 600 rpm to ensure that the mortar was fully mixed and oxygenated. Subsequently, 31% hydrochloric acid was added to the mortar to control the pH of the mortar to approximately 2. Carry out leaching treatment for 1.5 hours.

B. Centrifuge the mortar at 3000 rpm for 5 minutes. After separation, the liquid part is filtered through filter paper to obtain the leachate.

C. Wash the separated solid part twice with 150 ml of deionized water, and then dry it at 105°C for 12 hours to obtain treated fly ash.

D. Use a NaOH solution with a concentration of 1 mol/L to adjust the pH value of the leach solution to 2.8.

E. Add 5 mol/L sodium sulfide solution dropwise into the leachate until the pH value rises to 3.2 and remains stable, then stop adding sodium sulfide.

F. Centrifuge the leachate with sodium sulfide dropped at a speed of 3,000 rpm for 5 minutes. The separated solid part was washed twice with 100 ml of deionized water and then dried at 105°C for 12 hours to obtain heavy metal precipitates.

XRF was used to determine the chemical composition of fly ash, post-treatment fly ash and heavy metal precipitates. The results are shown in Table 1.

Table 1.

The mass of the treated fly ash accounts for 19.69% of the original fly ash. Calculated, the leaching rates of Zn, Cu and Pb are 86.11%, 97.32% and 94.01% respectively.

Using ICP to measure the concentration of heavy metals in the leachate before and after recovering heavy metals, the recovery rates of Zn, Cu and Pb from the leachate were 99.88%, 99.99% and 99.89% respectively.

Then using this method to recover Zn, Cu and Pb from fly ash, the total recovery rates are 86.01%, 97.31% and 93.91% respectively.

Claims (3)

1. The method for recycling the fly ash of the household garbage incineration comprises the following steps: a. leaching heavy metals from fly ash using hydrochloric acid; b. performing solid-liquid separation to obtain leaching liquid 1 and leaching residues; c. washing the leached slag for 1 time or countercurrent washing for multiple times and separating solid from liquid; d. the separated solid phase is treated fly ash, and the separated liquid phase is sent to step a and mixed with fly ash to prepare mortar; e. adding sodium sulfide solution into the leaching solution 1 to precipitate heavy metals, and controlling the adding amount of sodium sulfide by adjusting and monitoring the pH value of the leaching solution; f. carrying out solid-liquid separation to obtain heavy metal precipitate and leaching liquid 2; g. adding alkali into the leaching solution 2 to precipitate aluminum, silicon, iron and magnesium, and further removing residual heavy metals in the leaching solution; h. performing solid-liquid separation to remove solid phase and obtain leaching solution 3; I. adding an oxidant and a flocculant containing ferric iron into the leaching solution 3 to remove arsenic and sulfur in the leaching solution; J. and (3) performing solid-liquid separation to remove solid phase to obtain a leaching solution 4, wherein the leaching solution 4 is a raw material for recovering soluble chloride salt.

2. According to claim 1, in the method for recycling fly ash from incineration of household garbage, the reactor is stirred with air in step a.

3. According to claim 1, in the method for recycling the fly ash of the incineration of the household garbage, in the step e, the pH value of the leaching solution is firstly adjusted to be 2.7 to 2.9, and then sodium sulfide solution is added into the leaching solution to precipitate heavy metals; after the pH of the leachate had risen to 3.1 to 3.25 and stabilized, the addition of sodium sulphide was stopped.