Harmless treatment method and equipment for heavy metals in household garbage incineration fly ash

Abstract

The invention relates to a harmless treatment method and equipment for heavy metal in household garbage incineration fly ash, and the method comprises the following steps: S1, washing fly ash with water, and mixing the obtained washed fly ash with silicon dioxide to obtain a washed fly ash-silicon dioxide mixture; or mixing the fly ash and silicon dioxide, and washing with water to obtain a washed fly ash-silicon dioxide mixture; s2, heating the obtained washed fly ash-silicon dioxide mixture to 600-1200 DEG C by a preferable program, and maintaining the temperature of the mixture; wherein the addition amount of the silicon dioxide is 0.1-18 wt% of the fly ash, preferably 0.5-15 wt%, based on the dry weight. According to the method, harmlessness of the heavy metal in the household garbage incineration fly ash is achieved by using the single silicon dioxide with the low dosage, and the treatment efficiency of the fly ash is remarkably improved. In addition, the thermal curing temperature used in the method is relatively low, the energy consumption is remarkably reduced, and the cost and secondary pollution are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of waste treatment, and in particular to a method and equipment for harmlessly treating heavy metals in fly ash from the incineration of domestic waste. Background Art

[0002] With the advancement of my country's urbanization process and the improvement of people's living standards, the amount of urban domestic waste generated has increased sharply year by year. Waste incineration technology is widely used due to its high reduction, low space requirements and high treatment efficiency. As of 2020, about 62.3% of urban domestic waste in my country is treated by incineration. This method can not only achieve high-temperature sterilization and power supply, but also reduce the country's dependence on fossil fuels and effectively alleviate energy pressure. However, fly ash is produced during the incineration of domestic waste, which is classified as hazardous waste (HW18). Fly ash is usually enriched with high concentrations of soluble salts and heavy metals such as lead (Pb), zinc (Zn) and cadmium (Cd), and its content is usually 10 to 100 times that in the soil, posing a great potential threat to the ecological environment and human health. These soluble salts and heavy metals pollute groundwater and soil through leaching, and can also affect human health through the atmosphere, water sources and food chains. Therefore, direct landfill of fly ash is extremely unfavorable to the sustainable development of the ecological environment. In addition, the differences in garbage composition in different regions lead to different soluble salt and heavy metal components in fly ash produced by different treatment plants, which further hinders the industrial use of fly ash as a recycled raw material. In view of this, it is necessary to reduce and harmlessly treat fly ash to ensure that it can be safely landfilled or reused.

[0003] In order to achieve the harmlessness of heavy metals in fly ash from the incineration of domestic waste, one idea is to leach heavy metals from fly ash. Leaching reagents usually use acidic or alkaline reagents. However, since fly ash itself is strongly alkaline, the consumption of acidic reagents is large, which increases the processing cost. Alkaline solvent treatment is not only costly, but the leaching effect is often unsatisfactory during the fly ash leaching process. In addition, the chemical waste liquid generated during the acid-base leaching process requires additional treatment, which further increases the disposal cost.

[0004] Heavy metal stabilization technology has become a research hotspot due to its low cost, high solidification efficiency and ease of operation. According to the HJ1124-2020 standard, the treated fly ash can be sent to a municipal solid waste landfill for safe landfill after meeting the GB16889-2008 standard. However, existing studies have shown that existing chelating agent stabilization, cement stabilization, asphalt stabilization and geopolymer stabilization technologies have obvious limitations in treating multiple heavy metals in fly ash, and there is a risk of incomplete stabilization and secondary leaching. Although cement stabilization is widely used and easy to implement, the process significantly increases the weight of fly ash, resulting in increased treatment costs. More importantly, cement-stabilized fly ash has larger pores and poor durability, and heavy metals may leach into the leachate under acidic conditions (such as acid rain), causing environmental pollution. Geopolymer stabilization is greatly affected by multiple heavy metals in fly ash, resulting in reduced strength and high dissolution risk. Although a large number of studies have been devoted to the development of chelating agents that can fix heavy metals in fly ash, the cost of chemical reagents and their long-term effectiveness remain the main challenges facing this technology. In addition, chelating agents are usually highly selective and can only stabilize some heavy metals in fly ash, making it difficult to achieve comprehensive harmless treatment of multiple heavy metals. Thermal sintering provides a versatile fly ash treatment method that can effectively decompose organic pollutants, significantly reduce the volume and mass of fly ash, and stabilize a variety of heavy metals. However, conventional sintering temperatures require high temperatures of 1200-1500°C, which not only consumes a lot of energy, but may also cause secondary pollution due to the volatilization of chlorides. Therefore, the industry has been looking for better methods to harmlessly treat fly ash from the incineration of domestic waste. Summary of the invention

[0005] In view of the problems existing in the prior art, on the one hand, the present invention provides a method for harmlessly treating heavy metals in fly ash from the incineration of domestic waste, which comprises the following steps:

[0006] S1. Washing fly ash with water, mixing the obtained washed fly ash with silica to obtain a washed fly ash-silicon dioxide mixture; or,

[0007] mixing fly ash and silica and washing with water to obtain a water-washed fly ash-silica mixture;

[0008] S2. The obtained washed fly ash - silica mixture is heated - preferably programmed - to 600-1200 ° C, and the mixture is maintained at this temperature for 0.3-30 hours, preferably 0.5-24 hours;

[0009] The silicon dioxide is added in an amount of 0.1-18 wt % of the fly ash, preferably 0.5-15 wt % based on dry weight.

[0010] By controlling the amount of silicon dioxide added within the scope of the present invention, the leaching of heavy metals in the stabilized fly ash can be effectively controlled to meet the standard requirements of GB16889-2008.

[0011] In a preferred embodiment of the present invention, the particle size Dv50 of the silicon dioxide is ≤20 μm; preferably, the particle size Dv50 is 1-10 μm.

[0012] The appropriate silica particle size helps promote the full mixing of silica and fly ash, thereby improving the solidification efficiency of heavy metals and ensuring that the treated fly ash meets environmental protection standards and safety requirements.

[0013] In a preferred embodiment of the present invention, in step S1, the liquid-to-solid ratio used in water washing is ≥2:1, preferably (3-10):1.

[0014] The appropriate liquid-to-solid ratio can ensure that most of the water-soluble components in the fly ash can be removed, which is beneficial for subsequent stabilization.

[0015] In a preferred embodiment of the present invention, in step S2, the washed fly ash-silicon dioxide mixture may be dried before heating, and the drying temperature is 40-120°C.

[0016] In a preferred embodiment of the present invention, in step S2, the washed fly ash-silicon dioxide mixture is maintained at 600-1200°C for a time of ≥0.5 h, preferably 2-24 h.

[0017] In a preferred embodiment of the present invention, the silicon dioxide comes from silica fume, or silica fume is directly used.

[0018] In addition, the present invention also provides a device for harmlessly treating heavy metals in fly ash from the incineration of domestic waste, comprising:

[0019] --Fly ash collection device, used to collect fly ash from the incineration of domestic waste;

[0020] - a washing device for washing fly ash or a mixture of fly ash and silica; and

[0021] --Thermal treatment device for heating the mixture of washed fly ash and silica.

[0022] In a preferred embodiment of the present invention, the apparatus further comprises a mixing device, which is located between the washing device and the thermal treatment device and is used to mix the silicon dioxide and the washed fly ash.

[0023] In a preferred embodiment of the present invention, the washing device and / or the mixing device and / or the heat treatment device further comprises a metering component and / or a screening component for silicon dioxide. By using the above-mentioned metering component and / or the screening component, the addition amount of silicon dioxide can be better controlled and its particle size can be controlled within the scope of the present invention.

[0024] In a preferred embodiment of the present invention, the apparatus further comprises a drying device, which is located between the washing device and the heat treatment device, or between the mixing device and the heat treatment device.

[0025] By means of the drying device, the water content of the mixture of fly ash and silicon dioxide can be controlled within a suitable range before the mixture is thermally treated.

[0026] The method of the present invention achieves the harmlessness of heavy metals in fly ash from the incineration of domestic waste by using a relatively low amount of single silicon dioxide, which significantly improves the treatment efficiency of fly ash. In addition, the thermal curing temperature used in the method of the present invention (i.e., heating the obtained washed fly ash-silicon dioxide mixture to 600-1200°C and maintaining the mixture at a temperature within this range) is relatively low, which significantly reduces energy consumption, costs and secondary pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The process flow of a specific embodiment of the present invention is shown;

[0028] Figure 2 The test results of fly ash obtained by treating at different temperatures in Example 1 are shown. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention and the comparative examples. Based on the embodiments and comparative examples in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] In the present invention, the features mentioned in the description of the method and the device can be combined with each other when necessary to jointly constitute the description of the present invention and form a corresponding technical solution.

[0031] Herein, general, preferred, more preferred, particularly preferred and most preferred embodiments may be combined with each other.

[0032] In the present invention, unless otherwise specified, all operations are carried out at room temperature (25° C.) and normal pressure (101 kPa).

[0033] The inventors of the present invention have found in their research that when fly ash is harmlessly treated, only washing with water to remove water-soluble components does not have any stabilizing effect on heavy metals in fly ash; when fly ash is directly mixed with silica but not subjected to heat curing treatment, it does not have any stabilizing effect on heavy metals in fly ash; if the washed fly ash-silicon dioxide mixture in which silica is mixed in the aqueous phase and the water-soluble components are removed by liquid-solid separation is not subjected to temperature curing, it does not have any stabilizing effect on heavy metals and cannot achieve the harmlessness of heavy metals. Directly heat curing the fly ash from the incineration of domestic waste reduces the leaching activity of heavy metals to a certain extent, but the stabilization is not thorough and the harmlessness is not achieved. Without adding silica, only washing the fly ash with water and removing the water-soluble components by liquid-solid separation and then heat curing, it is also impossible to effectively achieve the harmlessness of heavy metals. When the original fly ash is mixed with silica but the water-soluble components are not removed, not only can the stabilization of heavy metals not be achieved after heat curing, but the leaching activity of heavy metals will be increased. Unexpectedly, the inventors found that mixing washed municipal solid waste incineration fly ash with silica and then heat curing the resulting mixture at 600-1200°C can significantly improve the leaching of heavy metals compared to untreated fly ash, and the heavy metal leaching concentration of the resulting final product can be significantly lower than the GB 16889-2008 standard.

[0034] Therefore, on the one hand, the present invention provides a method for harmlessly treating heavy metals in fly ash from the incineration of domestic waste, which comprises the following steps:

[0035] S1. Washing fly ash with water, mixing the obtained washed fly ash with silica to obtain a washed fly ash-silicon dioxide mixture; or,

[0036] mixing fly ash and silica and washing with water to obtain a water-washed fly ash-silica mixture;

[0037] S2. The obtained washed fly ash - silica mixture is heated - preferably programmed - to 600-1200 ° C, preferably 700-1200 ° C, and the mixture is maintained at this temperature for 0.3-30 hours, preferably 0.5-24 hours;

[0038] The amount of silicon dioxide added is 0.1-18 wt %, preferably 0.5-15 wt %, preferably 0.8-15 wt %, preferably 1-15 wt %, preferably 2-14 wt %, further preferably 3-12 wt %, and further preferably 4-11 wt % of the fly ash based on dry weight.

[0039] In a preferred embodiment, the method of the present invention comprises the following steps: removing soluble components in fly ash from incineration of domestic waste by water washing, adding silicon dioxide and mixing them evenly at room temperature and pressure, and then heating the mixture (preferably programmed heating) to 600-1200°C for heat curing for 0.3-30 hours (preferably 0.5-24 hours) to achieve the harmlessness of heavy metals in the fly ash, and then safely landfilling; the soluble components in the fly ash include but are not limited to soluble calcium salts, potassium salts, sodium salts and chloride salts in fly ash from incineration of domestic waste; the fly ash from incineration of domestic waste is fly ash produced by grate furnace incineration; the silicon dioxide can be added during the water washing and stirring process at room temperature and pressure, or it can be added and mixed evenly after the fly ash is washed and liquid-solid separated.

[0040] In a preferred embodiment, the method of the present invention comprises the following steps: S11. mixing fly ash and silicon dioxide uniformly in a water phase stirring process at normal temperature and pressure; S12. removing soluble components in fly ash by liquid-solid separation to obtain a fly ash-silicon dioxide wet mixture; S21. subsequently heating the fly ash-silicon dioxide wet mixture (preferably programmed heating) to 600-1200°C for thermal curing for 0.3-30 hours (preferably 0.5-24 hours), thereby achieving the harmlessness of heavy metals in fly ash and safe landfill. The saline solution can be partially refluxed to S11, and the remaining tail liquid can be treated with water and discharged after reaching the standard, or the soluble components can be concentrated and evaporated for recovery (recovering water and soluble components).

[0041] In a preferred embodiment, the method of the present invention comprises the following steps: S11. placing fly ash in an aqueous phase, stirring and washing it, removing soluble components by liquid-solid separation, and obtaining desalted fly ash; S12. adding silicon dioxide to the desalted fly ash and mixing them evenly; S21. subsequently heating the mixture obtained in S12 (preferably programmed heating) to 600-1200°C for thermal curing for 0.3-30 hours (preferably 0.5-24 hours), thereby achieving the harmlessness of heavy metals in the fly ash, and then safely landfilling it.

[0042] In one embodiment of the present invention, the term "temperature programming" means that the temperature increase is generally linear, for example, the temperature increase rate may be 2°C / min to 200°C / min, preferably 10°C / min to 100°C / min.

[0043] In a preferred embodiment of the present invention, the water-soluble components in the fly ash include, but are not limited to, soluble calcium salts, potassium salts, sodium salts and / or chloride salts in fly ash from the incineration of domestic waste.

[0044] In a preferred embodiment of the present invention, the fly ash from the incineration of domestic waste is produced by deacidification of domestic waste incineration flue gas by lime method, especially fly ash produced by grate furnace incineration. However, those skilled in the art will appreciate that as long as the fly ash contains one or more of soluble calcium salts, potassium salts, sodium salts and / or chloride salts, the fly ash can be treated by the method of the present invention and falls within the scope of the "fly ash" of the present invention.

[0045] The composition of fly ash from the incineration of domestic waste is very complex. Its main components are water-soluble mixed salts and insoluble mixed minerals of various metal ions such as Ca, Mg, Zn, Pb, Fe, Al, etc., and its acidic anion ligands are mainly hydroxide, carbonate, sulfate, silicate, fluoride, chlorosilicate, etc. The above-mentioned various metal ions and various anion ligands are randomly combined or coupled together, cross-linked and contained with each other, thus forming a highly complex and mixed mixture of salts, amorphous minerals or crystalline minerals, which are present in the fly ash in the form of powder or colloid.

[0046] In the present invention, the fly ash obtained after the treatment of the present invention can be directly subjected to harmless treatment, such as direct landfill treatment.

[0047] In a preferred embodiment, the aqueous phase (ie, the saline solution) obtained from step S1 is at least partially (eg, 10 to 80 wt %, preferably 30 to 60 wt %, further preferably 45 to 55 wt %) returned to step S1.

[0048] In a preferred embodiment, the aqueous phase (i.e., the brine solution) obtained in step S1 is at least partially (e.g., 20 to 90 wt %, preferably 40 to 70 wt %, and also preferably 45 to 55 wt %) subjected to water treatment, and directly discharged after the water quality meets the standard, and / or concentrated and evaporated before recovery, for example, recovery of the salt component therein.

[0049] In a preferred embodiment, step S1 can be performed in the following manner: the fly ash and silicon dioxide are stirred and mixed in water, and the water-soluble components in the fly ash are removed by liquid-solid separation to obtain a water-washed fly ash-silicon dioxide mixture.

[0050] In a preferred embodiment, step S1 can be performed in the following manner: placing the fly ash in an aqueous phase and stirring and washing it, removing the water-soluble components by liquid-solid separation to obtain washed fly ash; then adding silicon dioxide to the washed fly ash and mixing them evenly to obtain a washed fly ash-silicon dioxide mixture.

[0051] In a preferred embodiment, the silicon dioxide is derived from silica fume, which is also called microsilica, and is an industrial byproduct. Therefore, in a preferred embodiment, the silicon dioxide used can also be replaced by silica fume. In this case, the definition of silicon dioxide herein also applies to silica fume.

[0052] In the present invention, it should be understood that the domestic waste incineration fly ash and silicon dioxide may also contain a small amount of moisture, preferably, the moisture content is less than 5% by weight, preferably less than 3% by weight, and more preferably less than 2% by weight. However, their moisture content may also be higher than the above ratio. In the case where the fly ash or silicon dioxide contains water, it can be converted after measuring its moisture content. The method for measuring the moisture content is a conventional method used in the art, such as a drying method.

[0053] In the present invention, the amount of silicon dioxide added is 1-18 wt %, preferably 2-15 wt %, further preferably 3-12 wt %, and further preferably 4-11 wt % of the fly ash, based on dry weight. The above amount should be understood as being based on dry weight after removing moisture.

[0054] In the present invention, unless otherwise specified, the amount of silicon dioxide added is based on the dry weight of domestic waste incineration fly ash (ie, unwashed fly ash).

[0055] In a preferred embodiment, the silica is finely dispersed silica, for example, its particle size Dv50 is less than 20 μm; for example, Dv50≤18 μm, preferably Dv50≤15 μm, more preferably the particle size Dv50 is 1-10 μm, further preferably Dv50 is 1-7 μm, and particularly preferably Dv50 is 1-6 μm.

[0056] In the present invention, the particle size Dv50 is the particle size corresponding to the cumulative volume percentage of the sample reaching 50%. The method for determining the particle size Dv50 is also well known to those skilled in the art. For example, the particle size Dv50 of the particles can be measured with reference to the standard GB / T 19077-2016 using a laser particle size analyzer (such as Malvern Master Size 3000). In the present invention, silica having a particle size greater than the above range can also be used, and it can be sieved to a particle size that meets the present invention before use.

[0057] In a preferred embodiment, the specific surface area of ​​the silicon dioxide may be 10-200 m 2 / g, preferably 15-200m 2 / g, more preferably 20-200m 2 / g.

[0058] In the present invention, the specific surface area is tested with reference to GB / T 19587-2017, and the nitrogen adsorption specific surface area analysis test method is tested by the Tri-Star 3020 specific surface area pore size analyzer of Micromeritics, USA, and the specific surface area of ​​the material is calculated by the BET (Brunauer Emmett Teller) method.

[0059] In a preferred embodiment, in step S1, a water-washed fly ash-silicon dioxide mixture is obtained by washing water and fly ash or a mixture thereof with silicon dioxide in water and then performing solid-liquid separation, wherein the liquid-solid ratio used (i.e., water:fly ash, based on weight) is ≥2:1, preferably (3-10):1.

[0060] In a preferred embodiment, in step S1, at the same liquid-to-solid ratio, the fly ash or its mixture with silicon dioxide may be washed one or more times (e.g., 1 to 5 times, preferably 2 to 3 times) to better remove the soluble components therein.

[0061] In the present invention, the liquid-solid ratio in step S1 can also be changed, as long as most of the water-soluble components in the fly ash can be removed. Here, the water-soluble components in the fly ash may also remain after water washing, but not more than 10% by weight, preferably not more than 5% by weight, more preferably not more than 3% by weight, based on the total dry weight of the fly ash after water washing.

[0062] In the present invention, the washing fly ash and silicon dioxide can be mixed in a variety of ways, for example, mixing in water or dry mixing, as long as they can be mixed uniformly.

[0063] In a preferred embodiment, to achieve liquid-solid separation, methods such as filtration separation, centrifugal separation and gravity separation can be used. There is no particular limitation here, as long as liquid-solid separation can be achieved.

[0064] In a preferred embodiment, in step S1, the water content of the obtained washed fly ash-silicon dioxide mixture is 20 to 80 wt%, preferably 30 to 60 wt%, preferably 40 to 50 wt%.

[0065] In a preferred embodiment, in step S1, the fly ash or the mixture of fly ash and silicon dioxide can be washed by mixing the fly ash or the mixture of fly ash and silicon dioxide in the water phase once or multiple times (for example, the fly ash or the mixture of fly ash and silicon dioxide is mixed with water multiple times (such as 2-5 times)), so that the fly ash can be fully mixed with water, and then liquid-solid separation is performed to remove the water-soluble components. It should be understood here that when only the fly ash is washed, the washed fly ash, which is optionally dried after washing, can be uniformly mixed with silicon dioxide.

[0066] It should be understood here that in the present invention, due to the properties of silica ash (its main component is silicon dioxide, for example, it contains more than 90 weight %, preferably more than 95 weight %, for example more than 98 weight % of silicon dioxide, and the remaining impurities are for example chloride salts, metals and their oxides), it is basically insoluble in water, for example, less than 0.5 weight %, preferably less than 0.3 weight %, more preferably less than 0.1 weight % of the components in the silica ash are soluble in water.

[0067] In a preferred embodiment, in step S2, when the water content in the water-washed fly ash-silicon dioxide mixture is too high, the water content in the water-washed fly ash-silicon dioxide mixture can also be reduced in advance before heating. To this end, for example, the water-washed fly ash-silicon dioxide mixture can be dried at a temperature of, for example, 40-120° C. for a drying time of, for example, 0.5 to 48 hours (preferably 1-16 hours) until the desired water content is reached.

[0068] In a preferred embodiment, in step S2, the water content of the washed fly ash-silicon dioxide mixture before heating can be 0 to 20% by weight, preferably 1 to 15% by weight, and more preferably 3 to 10% by weight. By controlling the above water content, the subsequent heating time can be reduced and the treatment efficiency can be improved.

[0069] In the present invention, in step S2, when the temperature rises to more than 900°C, the volatilization rate of heavy metals in the fly ash may increase significantly, which means that under this temperature condition, valuable metals can be recovered by equipping valuable metal recovery equipment such as bag dust collectors, thereby improving resource utilization efficiency; at the same time, the total content of heavy metals in stabilized fly ash is significantly reduced, thereby effectively reducing its potential environmental load. Therefore, in a preferred embodiment, in step S2, when the obtained water-washed fly ash-silicon dioxide mixture is heated to 900-1200°C for thermal curing, it is optionally included to recover metals from the tail gas generated by heating the water-washed fly ash-silicon dioxide mixture, and here, preferably, the metals are recovered by bag dust collectors.

[0070] In a preferred embodiment, in step S2, the obtained water-washed fly ash-silicon dioxide mixture is heated to 600-900° C., and the mixture is maintained at this temperature for a time of ≥0.5 h, such as 0.5-27 hours, preferably 2-24 h. In step S2, by controlling the final temperature, more metals can also be prevented from being discharged through the tail gas, which not only reduces energy consumption, but also eliminates the step of recovering metals from the tail gas generated by heating the water-washed fly ash-silicon dioxide mixture.

[0071] In a preferred embodiment, in step S2, the washed fly ash-silicon dioxide mixture is maintained at 600-1200°C for a time of ≥0.5 h, such as 0.5-27 hours, preferably 2-24 h.

[0072] Through the method of the present invention, the fly ash after thermal curing for a short time and at a lower temperature meets the pollution control standard GB16889-2008 for domestic waste landfills, and can be directly sent to domestic waste landfills for safe landfilling. The specific reasons and mechanisms include the following aspects: first, in the liquid-solid separation process, soluble salts and unstable water-soluble heavy metals (such as cadmium, lead, etc.) are basically removed, avoiding the thermal activation of heavy metals by the above-mentioned low-boiling soluble salts in the subsequent thermal curing process; secondly, silicon dioxide has a larger specific surface area and finer particle size, and when it is mixed with fly ash particles in the presence of water, it can fully act to generate a variety of silicate hydration products; heavy metal ions are adsorbed, coated or homogenously replaced with anion groups of hydration products to be bound, and heavy metals are converted into stable silicate phases under appropriate thermal curing conditions; finally, silicate substances react with silicon dioxide that wraps fly ash particles to generate calcium silicate substances, forming an encapsulation layer to further inhibit heavy metal leaching. Therefore, the method of the present invention can efficiently obtain harmless fly ash under low energy consumption conditions, which can then be safely landfilled.

[0073] A preferred embodiment of the present invention is described below in conjunction with the accompanying drawings.

[0074] like Figure 1 As shown, in a preferred embodiment, first, the fly ash (6) and part of the silicon dioxide (7) are stirred and mixed with water in a stirred reactor (1); the obtained mixture is transported to a liquid-solid separator (2), a part (212) of the liquid phase (21) obtained by liquid-solid separation is refluxed to the stirred reactor (1), and the remaining part (211) is sent to tail liquid treatment; the solid phase (22) obtained by liquid-solid separation is sent to a dryer (3) for drying, and the water vapor (31) can be sent to treatment, and part of the silicon dioxide (7) can also be added to the dryer (3) and mixed; the dried mixture is sent to a heating device (4) for solid phase thermal curing (at this time, part of the silicon dioxide (7) can also be added and mixed), and the tail gas (41) is sent to tail gas treatment; the final product (i.e., harmless fly ash) (5) is landfilled in a domestic waste plant. Here, it should be understood that the total amount of silicon dioxide used in each step meets the definition of silicon dioxide in the present invention.

[0075] Therefore, in a preferred embodiment, the method of the present invention comprises the following steps: first, in a stirred reactor, fly ash and silicon dioxide are fully mixed in an aqueous phase; then, solid-liquid separation is performed through a liquid-solid separator, the liquid phase is partially refluxed for reuse and / or enters a tail liquid treatment system, and the solid phase is transported to a dryer for drying, and the generated water vapor is purified by a tail gas treatment system; the dried solid phase material is subjected to thermal curing treatment in a heating device, and the volatilized valuable metals in the preferred process are centrally recovered by a bag filter; after ensuring that harmful heavy metals in the thermal curing solid phase are stabilized, the resulting harmless fly ash can be safely sent to a domestic waste landfill for treatment. In a preferred embodiment, the silicon dioxide can be added in three processing links: a washing device (i.e., step S1), a drying device, and a heating device (i.e., step S2).

[0076] In the present invention, the term "heat curing" should be understood as a process of continuous heating at a certain temperature (eg, 600-1200° C.).

[0077] In addition, the present invention also provides a device for harmlessly treating heavy metals in fly ash from the incineration of domestic waste, comprising:

[0078] --Fly ash collection device, used to collect fly ash from the incineration of domestic waste;

[0079] - a washing device for washing fly ash or a mixture of fly ash and silica; and

[0080] --Thermal treatment device for heating the mixture of washed fly ash and silica.

[0081] In a preferred embodiment, the apparatus further comprises a mixing device, which is located between the washing device and the thermal treatment device and is used to mix the silicon dioxide and the washed fly ash.

[0082] In a preferred embodiment, the washing device and / or the mixing device and / or the thermal treatment device further comprises a metering element and / or a screening element for the silicon dioxide.

[0083] By using the metering component and / or the screening component, the amount of silicon dioxide added can be better controlled and the particle size thereof can be controlled within the range of the present invention.

[0084] In a preferred embodiment, the apparatus further comprises a drying device, which is located between the washing device and the heat treatment device, or between the mixing device and the heat treatment device.

[0085] In a preferred embodiment, the apparatus further comprises a liquid-solid separation device, which is located between the washing device and the drying device and is used to separate the washed fly ash or the mixture of fly ash and silicon dioxide.

[0086] By means of the drying device, the water content of the mixture of water-washed fly ash and silicon dioxide can be controlled within a suitable range before the mixture is thermally treated.

[0087] The method of the present invention achieves the harmlessness of heavy metals in fly ash from the incineration of domestic waste by using a relatively low amount of single silicon dioxide, significantly improving the treatment efficiency of fly ash. In contrast, most existing technologies require the compounding of multiple additives and the amount of additives is as high as 15-90% by weight. In addition, the method of the present invention uses a relatively low thermal curing temperature, which significantly reduces energy consumption, costs and secondary pollution.

[0088] The present invention will be further described below in conjunction with embodiments, but they should not be construed as limiting the present invention.

[0089] Example 1

[0090] Silica ash with a particle size Dv50 of 10 μm (purchased from Henan Borun Casting Materials Co., Ltd., with a silica content of 98% by weight) was added in an amount of 10% by weight (based on dry weight, the same below) and grate furnace incineration fly ash (from a municipal waste incineration power plant, the same below) was mixed in water at a liquid-solid ratio of 10:1 for 2 hours, and liquid-solid separation was performed by filtration to obtain a water-washed fly ash-silica ash mixture and a salt water washing liquid. 50% of the salt water washing liquid was refluxed as an aqueous phase solution used when mixing silica ash and fly ash, and the remaining liquid phase was treated with tail liquid; the obtained water-washed fly ash-silica ash mixture was dried at 40°C (reduced to a water content of 10% by weight) and heated to 700°C for thermal curing for 24 hours to obtain stabilized fly ash. When the leaching solution prepared according to HJ / T 300-2007 was leached, the leaching concentration of heavy metal Cd was 0.0442 mg / L, while Pb and Zn were not detected in the leaching solution, which was significantly lower than the leaching limit of Cd (0.15 mg / L), Pb (0.25 mg / L), and Zn (100 mg / L) in the GB16889-2008 standard, and can be safely landfilled. Pb, Cd, and Zn in the original fly ash showed extremely strong leaching properties, reaching 12.4, 6.22, and 108.6 mg / L, respectively.

[0091] In addition, multiple tests were conducted according to the method described in Example 1, except that the temperature was different. The test results of the fly ash obtained are shown in Figure 2 As Figure 2As shown, it shows the results obtained when the fly ash obtained by thermal curing at temperatures of 0, 200°C, 400°C and 600°C is leached with the leaching solution prepared according to HJ / T 300-2007. The results show that the threshold value of lead, cadmium and zinc leaching cannot be obtained when the temperature is raised and cured below 600°C. This is attributed to the fact that the curing at a lower temperature cannot completely transform the form of lead, cadmium and zinc in the fly ash, and the lattice rearrangement and heavy metal stabilization are not achieved. In addition, the decomposition of organic matter and crystal water in the fly ash caused by the lower temperature will promote the leaching of lead, cadmium and zinc to a certain extent. Thermal curing within the temperature range of the present invention can achieve the harmless treatment of heavy metals lead, cadmium and zinc.

[0092] Example 2

[0093] The fly ash from the grate furnace incineration was mixed in the aqueous phase at a liquid-solid ratio of 10:1 for 2 hours, and then the liquid-solid separation was performed by filtration to obtain a first liquid phase and a first solid phase; the first liquid phase was subjected to tail liquid treatment; the first solid phase was fully mixed in the aqueous phase solution with 10% by weight of silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silicon dioxide content of 98% and a particle size Dv50 of 5 μm) at a liquid-solid ratio of 10:1, and the mixture was subjected to liquid-solid separation to obtain a second liquid phase and a second solid phase; the second liquid phase was subjected to tail liquid treatment; the second solid phase was dried at 40°C (reduced to a water content of 10% by weight) and heated to 800°C for heat curing for 2 hours to obtain stabilized fly ash; according to HJ / T The leaching solution prepared by 300-2007 was leached, and the leaching concentrations of heavy metals Cd, Pb, and Zn were 0.007, 0.002, and 0.012 mg / L, respectively, which are lower than the GB16889-2008 standard and can be safely landfilled.

[0094] It can be seen from Example 2 that the fly ash is first washed with water to remove the water-soluble components, and then silica fume is added and mixed evenly in the water phase to generate a washed fly ash-silica fume mixture, which can then be dried and heat-cured to achieve the stabilization of heavy metals.

[0095] Example 3

[0096] The fly ash from the grate furnace incineration was mixed in water at a liquid-solid ratio of 10:1 for 2 hours, and then the liquid-solid separation was performed by filtration to obtain a first liquid phase and a first solid phase; the obtained first liquid phase was subjected to tail liquid treatment; 10% by weight of silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silicon dioxide content of 98% and a particle size Dv50 of 5 μm) was directly added to the obtained first solid phase for thorough mixing, the mixture was dried at 40°C (reduced to a water content of 10% by weight) and heated to 800°C for heat curing for 2 hours to obtain stabilized fly ash; leaching was performed according to the leaching solution prepared in accordance with HJ / T 300-2007, and the leaching concentrations of heavy metals Cd, Pb, and Zn were 0.008 mg / L, 0.009 mg / L, and undetectable, respectively, which were lower than the GB16889-2008 standard and could be safely landfilled.

[0097] As can be seen from Example 3, heavy metal stabilization can also be achieved by first washing the fly ash with water to remove water-soluble components, then adding silica fume to directly mix to obtain a mixture, and performing drying and heat curing treatment.

[0098] Example 4

[0099] Silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silica content of 98% and a particle size Dv50 of 1 μm) was added in an amount of 1% by weight (based on dry weight) and mixed with grate furnace incineration fly ash in water at a liquid-solid ratio of 5:1 for 1 hour, and liquid-solid separation was performed by filtration to obtain a water-washed fly ash-silica ash mixture. The obtained water-washed fly ash-silica ash mixture was dried at 105°C (reduced to a water content of 10% by weight) and divided into six equal parts and heated to 700, 800, 900, 1000, 1100, and 1200°C for heat curing for 2 hours to obtain stabilized fly ash; when leached with the leaching solution prepared according to HJ / T 300-2007, the leaching of heavy metals Cd, Pb, and Zn was lower than the GB16889-2008 standard and successfully achieved harmlessness, and can be safely landfilled, as shown in Table 1.

[0100] It is worth noting that even at a relatively low temperature of 700°C, the harmless treatment of fly ash can be achieved, and within the temperature range of 700-1200°C, the fly ash after thermal curing can meet the requirements of GB16889-2008 standard and meet the conditions for landfill disposal. In addition, when the thermal curing temperature exceeds 900°C, the volatilization rate of heavy metals in fly ash increases significantly. This means that under this temperature condition, valuable metals can be recovered by equipping equipment such as bag dust collectors, while significantly reducing the total content of heavy metals in stabilized fly ash.

[0101] Table 1 Effect of thermal curing temperature on the harmlessness of heavy metals in fly ash

[0102]

[0103] ND means not detected

[0104] Example 5

[0105] Silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silica content of 98% and a particle size Dv50 of 5 μm) was mixed with grate furnace incineration fly ash in water at a liquid-solid ratio of 2:1 for 2 hours in the amounts shown in Table 2, and liquid-solid separation was performed by filtration to obtain a water-washed fly ash-silica ash mixture. The obtained water-washed fly ash-silica ash mixture was dried at 90°C (reduced to a water content of 10% by weight) and heated to 800°C for thermal curing for 0.5h to obtain stabilized fly ash; when leached with the leaching solution prepared according to HJ / T 300-2007, the leaching results of heavy metals Cd, Pb, and Zn are shown in Table 2.

[0106] Table 2 Effect of silica fume addition on the harmlessness of heavy metals in fly ash

[0107]

[0108] As can be seen from the above table, within the range of 1-15 wt% of silica fume addition, the leaching of heavy metals in stabilized fly ash after thermal curing can be effectively controlled to meet the standard requirements of GB16889-2008. However, when the addition amount of silica fume reaches 20%, heavy metals such as lead (Pb) and cadmium (Cd) will be activated, resulting in failure of the stabilization process. This phenomenon may be mainly due to the reaction of excess silica fume with residual chloride salts in fly ash to generate byproducts such as hydrogen chloride (HCl) or chlorine (Cl2), which will promote the conversion of heavy metal silicates and heavy metal oxides into heavy metal chlorides, thereby significantly increasing the leaching potential of heavy metals. Excess silica fume may also change the physical structure of fly ash and its buffering capacity for acid, thereby weakening the solidification effect on heavy metals and increasing the chance of the leachate contacting the inside of the fly ash, resulting in more heavy metals being leached. Therefore, in order to ensure the stabilization effect of fly ash, the addition amount of silica fume is preferably not more than 15 wt%.

[0109] Example 6

[0110] Silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silicon dioxide content of 98% and a particle size Dv50 as shown in Table 3 below) was added in an amount of 14% by weight and mixed with grate furnace incineration fly ash in water at a liquid-solid ratio of 10:1 for 3 hours, and liquid-solid separation was performed by filtration to obtain a water-washed fly ash-silica ash mixture. The obtained water-washed fly ash-silica ash mixture was dried at 120°C (reduced to a water content of 10% by weight) and heated to 800°C for thermal curing for 10 hours to obtain stabilized fly ash; when leached with the leaching solution prepared according to HJ / T 300-2007, the leaching results of heavy metals Cd, Pb, and Zn are shown in Table 3.

[0111] Table 3 Effect of silica fume particle size on the harmlessness of heavy metals in fly ash

[0112]

[0113] It can be seen from the above table that when silica fume with a particle size (Dv50) range of 1-10μm is selected, the leaching of heavy metals in stabilized fly ash can be effectively suppressed, making it meet the standard requirements of GB16889-2008. However, when the particle size of silica fume is greater than 20μm, its contact efficiency with fly ash in the aqueous phase is significantly reduced. This insufficient contact will lead to incomplete conversion of the water-washed fly ash-silicon fume mixture, thereby affecting the effective stabilization of heavy metals during thermal curing, and further resulting in poor harmless treatment effect. Therefore, the appropriate silica fume particle size helps to promote the full mixing of silica fume and fly ash, thereby improving the solidification efficiency of heavy metals and ensuring that the treated fly ash meets environmental protection standards and safety requirements.

[0114] Comparative Example 1

[0115] Comparative Example 1 provides 8 types of comparative combinations (1)-(8) (see Table 4 below), all of which are carried out in a similar manner to that described in Example 1, except that the conditions mentioned in Table 4 are different, see Table 4 for details.

[0116] Table 4 Effects of different treatment conditions on the harmlessness of heavy metals in fly ash

[0117]

[0118] “+” represents the existence of corresponding treatment; “-” represents the absence of corresponding treatment; “O” represents ineffectiveness; “×” represents that the heavy metals are activated; “☆” represents a stabilization effect but does not meet the standard of harmlessness; “★” represents a stabilization effect that can meet the standard of harmlessness.

[0119] The comparative combination (1) in Table 4 represents the original fly ash without any treatment, and the leaching concentrations of heavy metals lead, cadmium and zinc are 12.4, 6.22 and 108.6 mg / L respectively, all exceeding the GB 16889-2008 standard.

[0120] From the comparison combination (2), it can be seen that water washing to remove soluble components has no effect on the stabilization of lead, cadmium and zinc. The leaching concentrations of lead, cadmium and zinc are 2.49, 6.44 and 105.7 mg / L respectively, far exceeding the standard values. This is because the water washing and liquid-solid separation process can only remove a small part of the water-soluble lead and cadmium, while in the standard leaching process of HJ / T 300-2007, acetic acid can form a lead acetate complex with lead to increase the dissolution of lead; at the same time, cadmium is more soluble under acidic conditions, resulting in failure to meet the standard for harmlessness.

[0121] From the comparison combination (3), it can be seen that the direct mixing of raw fly ash and silica fume has basically no effect on the stabilization of heavy metals, and the leaching concentrations of lead, cadmium and zinc are 12.51, 6.37 and 108.2 mg / L respectively.

[0122] From the comparison combination (4), it can be seen that thermal curing of the original fly ash alone can play a certain stabilizing role, but it is still unable to make the cadmium meet the standard. The leaching concentrations of lead, cadmium and zinc are 1.19, 0.01 and 8.13 mg / L respectively.

[0123] From the comparison combination (5), it can be seen that the mixing of silica fume and fly ash in water can generate a water-washed fly ash-silicon fume mixture. The formed mixture has only a weak stabilization effect on lead and zinc. Without coupling thermal curing treatment, it is still impossible to achieve harmlessness. The leaching concentrations of lead, cadmium and zinc are 10.6, 6.37 and 101.1 mg / L respectively. This is because water promotes the contact and wrapping effect of silica fume on fly ash, thereby inhibiting the leaching of heavy metals to a certain extent, but cannot make the fly ash harmless to the standard.

[0124] From the comparison combination (6), it can be seen that the solid phase wet material of the fly ash obtained after liquid-solid separation cannot meet the harmlessness standard of heavy metal lead after thermal curing. The leaching concentrations of lead, cadmium and zinc are 0.38, 0.01 and 1.27 mg / L respectively.

[0125] From the comparison combination (7), it can be seen that direct mixing of the original fly ash and silica fume followed by thermal curing not only fails to stabilize the fly ash, but also activates the heavy metals in the fly ash, resulting in increased leaching, with the leaching concentrations of lead, cadmium and zinc being 2.46, 7.39 and 113.58 mg / L, respectively. Because there are a large number of soluble salts in fly ash that can promote the dissolution of heavy metals, and most of the heavy metals in fly ash exist in an oxidized state, the silicon dioxide in the silica fume may react with the soluble chloride salts at high temperatures to generate sodium silicate and byproduct hydrogen chloride, which promotes the conversion of oxidized heavy metals into soluble heavy metal chloride salts, thereby aggravating the leaching of heavy metals in the fly ash after thermal curing, making the treatment effect run counter to the goal of harmlessness.

[0126] It can be seen from the comparative combination (8) that by removing the soluble components in the original fly ash, adding silica fume of appropriate concentration and particle size range and uniformly mixing, and then subjecting the obtained mixture to thermal curing at a certain temperature, the leaching concentration of heavy metals in the final fly ash product in the leaching solution prepared according to HJ / T 300-2007 can be lower than that of GB 16889-2008 standard, and the leaching concentrations of lead and cadmium are 0.01 and 0.03 mg / L respectively, and no zinc leaching can be detected, thereby achieving the harmlessness of heavy metals.

[0127] Comparative Example 2

[0128] Silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silicon dioxide content of 98% and a particle size Dv50 of 10 μm) was directly mixed with grate furnace incineration fly ash in an addition amount of 10 wt % to obtain a mixture; the obtained mixture was subjected to a heat curing treatment at 700° C. for 24 hours to obtain a treated mixed solid phase; the obtained mixed solid phase was washed in water at a liquid-to-solid ratio of 10:1 for 2 hours, liquid-solid separation was performed by filtration separation, and then the filtered solid phase was dried at 40° C. (reduced to a water content of 10 wt %) to obtain treated fly ash; when leached with the leaching solution prepared according to HJ / T 300-2007, the leaching concentrations of heavy metals Cd, Pb, and Zn were 3.628, 2.628, and 64.42 mg / L, respectively, the leaching of Cd and Pb was higher than the GB16889-2008 standard, and the obtained treated fly ash could not be safely landfilled.

[0129] It can be seen from Comparative Example 2 and Example 1 that the operation sequence plays a decisive role in the treatment effect. The treatment sequence of the method of the present invention must be to first wash and desalinate the fly ash, and then heat and cure the mixture with silica fume. If the mixture is directly heated and cured first, hard scale will form on the fly ash, which will not only seriously damage the equipment in actual production, reduce the thermal efficiency of the equipment, and increase the operation and maintenance cost, but more importantly, the direct thermal curing of the mixture cannot stabilize the heavy metals, resulting in the heavy metal leaching concentration in the final fly ash solid phase far exceeding the GB 16889-2008 standard.

[0130] Comparative Example 3

[0131] Silica ash (purchased from Henan Borun Casting Materials Co., Ltd., with a silicon dioxide content of 98% and a particle size Dv50 of 10 μm) was added in an amount of 14 wt% and mixed with grate furnace incineration fly ash at a liquid-solid ratio of 10:1 for 3 hours to obtain a washed fly ash-silica ash mixture; the obtained mixture was dried at 120°C (reduced to a water content of 10 wt%) and heated to 800°C for heat curing for 10 hours to obtain treated fly ash; when leached with the leaching solution prepared according to HJ / T 300-2007, the leaching concentrations of heavy metals Cd, Pb, and Zn were 5.49, 0.76, and 176.13 mg / L, respectively, and the obtained treated fly ash was not harmless and could not be safely landfilled.

[0132] It can be seen from Comparative Example 3 and Example 6 that even if silica fume and fly ash are fully mixed in the aqueous solution, if the water-soluble components are not removed by liquid-solid separation, the heavy metals cannot be rendered harmless during the thermal curing process. This shows that the addition of silica fume and the removal of its water-soluble components are essential key steps. Because when thermal curing is performed without liquid-solid separation to remove water-soluble components, even if a fly ash-silicon fume mixture that can promote the stabilization effect is generated in the aqueous solution, during the thermal curing process, the above-mentioned low-boiling point soluble salts will have a thermal activation effect on the heavy metals, so that the degree of harmlessness does not meet the standard.

[0133] The above embodiments and comparative examples are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it should be understood by those skilled in the art that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for harmlessly treating heavy metals in fly ash from the incineration of domestic waste, comprising the following steps: S1. Washing fly ash with water, mixing the obtained washed fly ash with silica to obtain a washed fly ash-silicon dioxide mixture; or, mixing fly ash and silica and washing with water to obtain a water-washed fly ash-silica mixture; S2. The obtained washed fly ash - silica mixture is heated - preferably programmed - to 600-1200 ° C, and the mixture is maintained at this temperature; The silicon dioxide is added in an amount of 0.1-18 wt % of the fly ash, preferably 0.5-15 wt % based on dry weight.

2. The method according to claim 1, wherein the particle size Dv50 of the silicon dioxide is less than 20 μm; preferably the particle size Dv50 is 1-10 μm.

3. The method according to claim 1 or 2, wherein in step S1, the liquid-to-solid ratio used in the water washing is ≥2:1, preferably (3-10):

1.

4. The method according to any one of claims 1 to 3, wherein in step S2, before heating, the washed fly ash-silicon dioxide mixture is dried to have a water content of 0 to 20 wt%, preferably 1 to 15 wt%, more preferably 3 to 10 wt%; the preferred drying temperature is 40-120°C.

5. The method according to any one of claims 1 to 4, wherein the silicon dioxide is derived from silica fume.

6. The method according to any one of claims 1 to 5, wherein in step S2, the washed fly ash-silicon dioxide mixture is maintained at 600-1200°C for a time of ≥0.5 h, preferably 2-24 h.

7. Equipment for harmless treatment of heavy metals in fly ash from incineration of domestic waste, including: --Fly ash collection device, used to collect fly ash from the incineration of domestic waste; - a washing device for washing fly ash or a mixture of fly ash and silica; and --Thermal treatment device for heating the mixture of washed fly ash and silica.

8. The apparatus according to claim 7, further comprising a mixing device, which is located between the washing device and the thermal treatment device and is used to mix the silica and the washed fly ash.

9. The apparatus according to claim 7 or 8, wherein the washing device and / or the mixing device and / or the heat treatment device further comprises a metering unit and / or a screening unit for silicon dioxide.

10. The apparatus according to any one of claims 7 to 9, further comprising a drying device located between the washing device and the heat treatment device, or between the mixing device and the heat treatment device.

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