A semi-volatile heavy metal adsorbent suitable for cement kiln co-processing and a preparation method thereof

Abstract

The present application relates to the technical field of adsorbent, in particular to a semi-volatile heavy metal adsorbent suitable for cement kiln co-processing and a preparation method thereof.The preparation method comprises the following steps: step 1, mixing kaolin, calcium oxide and water to form a slurry, preparing a foaming agent into a foam, adding the foam into the slurry to stir, and obtaining a semi-dry high-calcium material; step 2, mixing kaolin, aluminum hydroxide and water to form a slurry, preparing a foaming agent into a foam, adding the foam into the slurry to stir, and obtaining a high-aluminum slurry; step 3, mixing and stirring the semi-dry high-calcium material and the high-aluminum slurry, and then performing die pressing forming and curing to obtain the semi-volatile heavy metal adsorbent.The present application successfully prepares an adsorbent material capable of efficiently adsorbing semi-volatile heavy metals in cement kiln co-processing, and can improve the environmental safety of waste co-processing in a cement kiln.

Description

Technical Field

[0001] This invention relates to the field of adsorbent technology, and in particular to a semi-volatile heavy metal adsorbent suitable for co-processing in cement kilns and its preparation method. Background Technology

[0002] Cement kiln co-processing of waste refers to the process of feeding waste that meets or has undergone pretreatment to meet the requirements for entering the cement kiln into the cement kiln, achieving the harmless disposal of waste while producing cement clinker. The types of waste co-processed in cement kilns mainly include hazardous waste, municipal solid waste, sludge from urban and industrial wastewater treatment, animal and plant processing waste, contaminated soil, and emergency event waste. Compared with traditional incineration methods, cement kiln co-processing of waste has significant advantages in terms of construction process, technology, and efficiency. It can also reduce the dependence of cement clinker production on resources, becoming a new growth point for cement companies.

[0003] However, most waste contains a certain amount of semi-volatile heavy metals such as lead (Pb), cadmium (Cd), and zinc (Zn). During the co-processing of waste in cement kilns, these metals undergo a volatilization-condensation process. In the high-temperature furnace, some volatilize into the gas phase. When the flue gas temperature decreases, the heavy metal vapors undergo homogeneous nucleation or heterogeneous condensation, accumulating on the fly ash surface. The particles generated by homogeneous nucleation are extremely fine, even reaching the nanometer scale. Some fly ash particles are PM1 in size, with a large specific surface area, leading to heterogeneous condensation and accumulation of heavy metals on their surface. These heavy metals, existing in submicron particle form, easily penetrate dust removal equipment and are emitted into the atmosphere. Therefore, the solidification rate of semi-volatile heavy metals in cement clinker (e.g., lead approximately 20%) and their content in kiln tail gas are very low. Most lead is captured and deposited into the kiln ash. However, cement kiln ash is not managed as waste; instead, it is returned to the kiln for recalcination or directly mixed with cement clinker, becoming part of the cement product. This essentially concentrates lead first and then disperses and dilutes it into the cement product. This will inevitably affect the quality of cement, including its mechanical properties and environmental safety. If lead is only physically enriched in cement rather than chemically solidified, it is easy for it to volatilize and condense again when exposed to high temperatures. Such cement products are not suitable for use in high-temperature engineering environments. On the other hand, it is not conducive to the sustainable development of cement kiln co-processing technology and is prone to pollution transfer, pollution extension or pollution diffusion.

[0004] Using mineral adsorbents such as kaolin to capture heavy metals in situ is a promising technology for controlling heavy metal emissions. However, metakaolin eventually transforms into crystalline mullite above 1100℃, accompanied by the collapse of its layered structure, causing it to almost completely lose its adsorption capacity. Given the high temperature range (up to approximately 1450℃) of cement kiln co-processing processes, there are currently no effective heavy metal adsorbents. Improving the solidification rate of semi-volatile heavy metals during cement kiln co-processing is crucial for enhancing its harmlessness and promoting the high-quality and sustainable development of cement kiln co-processing processes. Summary of the Invention

[0005] The purpose of this invention is to provide a semi-volatile heavy metal adsorbent suitable for co-processing in cement kilns and its preparation method, thereby improving the high-temperature adaptability of the adsorbent to the co-processing environment in cement kilns.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] One of the technical solutions of this invention is a method for preparing a semi-volatile heavy metal adsorbent, comprising the following steps:

[0008] Step 1: Mix kaolin, calcium oxide and water to form a slurry, prepare foam with a foaming agent, add the foam to the slurry and stir to obtain a semi-dry high-calcium material;

[0009] Step 2: Mix kaolin, aluminum hydroxide and water to form a slurry; prepare foam with a foaming agent, add the foam to the slurry and stir to obtain a high-alumina slurry;

[0010] Step 3: Mix the semi-dry high-calcium material with the high-alumina slurry, then mold and cure it to obtain the semi-volatile heavy metal adsorbent.

[0011] Further, in step 1, the mass ratio of kaolin, calcium oxide, water and foaming agent is 40-45:5-10:10-15:1-2.

[0012] In step 1, the stirring time is 3 minutes.

[0013] Further, in step 2, the mass ratio of kaolin, aluminum hydroxide, water and foaming agent is 40-45:3-5:15-20:1-2.

[0014] In step 2, the stirring time is 3 minutes.

[0015] The mass ratio of calcium oxide to aluminum hydroxide is 5-10:3-5.

[0016] If the ratio of calcium oxide to aluminum hydroxide is lower than the range mentioned above, the effect of solidifying heavy metals will be insignificant; if it is higher than the range, it will compromise the chemical stability of the adsorbent under high-temperature conditions and increase the burning rate. Too little foaming agent will result in insufficient bubble introduction, while too much will lead to waste.

[0017] Furthermore, in step 3, the material is molded into a sphere with a diameter of 0.8-1.2 cm.

[0018] In step 3, the stirring time is 5 minutes.

[0019] Furthermore, in step 3, the curing specifically refers to curing for 28 days under conditions of a temperature of 20±2℃ and a relative humidity of over 90%.

[0020] Furthermore, the foaming agent mentioned in steps 1 and 2 is specifically an emulsion prepared by mixing animal protein foaming agent and epoxy resin emulsion at a mass ratio of 15-20:3-5.

[0021] The animal protein foaming agent, by mass percentage, comprises 30-35% animal protein, 45-50% sodium bicarbonate, 4-6% edible alkali, 3-5% salt, and 12-15% vegetable oil; the epoxy resin emulsion comprises 35-40% epoxy resin, 1-5% lactic acid, <1% ethylene glycol butyl ether, <1% methyl isobutyl ketone, and 55-62% water.

[0022] The second technical solution of the present invention is a semi-volatile heavy metal adsorbent prepared by the above-mentioned preparation method.

[0023] The third technical solution of this invention is the application of the above-mentioned semi-volatile heavy metal adsorbent in the co-processing of waste in cement kilns.

[0024] This invention uses kaolin as the basic raw material and prepares a high-temperature resistant, highly adsorbent semi-volatile heavy metal adsorbent through composition adjustment and structural reconstruction. Specifically, foam is introduced to increase the internal porosity and physical adsorption specific surface area of ​​the adsorbent; the high reactivity of the high-calcium component is used to improve the chemical solidification effect of the adsorbent on semi-volatile heavy metals; and a high-aluminum component with excellent high-temperature stability is used as the outer layer of the adsorbent to improve the high-temperature adaptability of the adsorbent to the co-processing environment of cement kilns.

[0025] The present invention discloses the following technical effects:

[0026] The adsorbent of this invention contains numerous pores (porosity greater than 60%), exhibiting highly efficient physical adsorption of semi-volatile heavy metals. The inner layer is a high-calcium component, whose high pozzolanic activity facilitates the chemical bonding of the silica-alumina-calcium phase with the semi-volatile heavy metals. The outer layer is a high-alumina component, whose unique high-temperature stability ensures that the adsorbent does not undergo structural damage in the high-temperature environment of cement kiln co-processing. This adsorbent fully utilizes its high-temperature adsorption characteristics to effectively solidify semi-volatile heavy metals in cement kiln co-processing. Through scientific research and practical testing, this invention has successfully prepared an adsorbent material that can efficiently adsorb semi-volatile heavy metals in cement kiln co-processing, thereby improving the environmental safety of cement kiln co-processing waste.

[0027] The raw materials for this invention are readily available, the process steps are simple, and it is easy to promote and apply on a large scale. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 The mineral phase composition of kaolin used in the embodiments and comparative examples of this invention.

[0030] Figure 2 This is the calcination test apparatus of the present invention; in the figure, 1-gas cylinder; 2-float flowmeter; 3-thermocouple; 4-tube furnace; 5-corundum crucible; 6-tail gas treatment device.

[0031] Figure 3 This is a schematic diagram of the preparation process of the semi-volatile heavy metal adsorbent of the present invention.

[0032] Figure 4 The elemental distribution diagram is shown for the No. 4 semi-volatile heavy metal adsorbent prepared in Example 4 of this invention. Detailed Implementation

[0033] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0034] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0035] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0036] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0037] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0038] Unless otherwise specified, all raw materials used in the embodiments and comparative examples of this invention can be obtained through commercial channels.

[0039] The raw materials and properties used in the following embodiments and comparative examples of this invention are as follows:

[0040] Kaolin: Median particle size (d) 50 The particle size is 3.79 μm, and its chemical composition and mineral phases are shown in Table 1 and Table 2, respectively. Figure 1 As shown.

[0041] Table 1. Chemical composition of kaolin (wt.%)

[0042]

[0043] Both calcium oxide and aluminum hydroxide are chemically pure reagents.

[0044] Water: tap water.

[0045] The animal protein foaming agent described in this invention comprises 30-35% animal protein, 45-50% sodium bicarbonate, 4-6% edible alkali, 3-5% salt, and 12-15% vegetable oil. The epoxy resin emulsion specifically comprises 35-40% epoxy resin, 1-5% lactic acid, <1% ethylene glycol butyl ether, <1% methyl isobutyl ketone, and 55-62% water. In a specific embodiment, the animal protein foaming agent used comprises 30% animal protein, 50% sodium bicarbonate, 4% edible alkali, 4% salt, and 12% vegetable oil. The epoxy resin emulsion used specifically comprises 36% epoxy resin, 3% lactic acid, 0.5% ethylene glycol butyl ether, 0.5% methyl isobutyl ketone, and 60% water.

[0046] A schematic diagram of the preparation process of the semi-volatile heavy metal adsorbent of the present invention is shown below. Figure 3 As shown.

[0047] In this embodiment of the invention, "curing" specifically refers to curing in a standard curing room with a temperature of 20±2℃ and a relative humidity of 90% or higher.

[0048] Example 1

[0049] Step 1: Weigh 40g of kaolin and 6g of calcium oxide and add them to 12g of water to make a slurry; prepare foam by using a compressed air foaming machine with 1.2g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a semi-dry high-calcium material;

[0050] Step 2: Weigh 40g of kaolin and 3.5g of aluminum hydroxide and add them to 15g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.2g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a high-alumina slurry;

[0051] Step 3: Add the above semi-dry high-calcium material to the high-alumina slurry, stir for 5 minutes, and then mold it into spherical adsorbents with a diameter of about 1 cm. After curing for 28 days, the No. 1 semi-volatile heavy metal adsorbent is obtained.

[0052] In both steps 1 and 2, the foaming agent is an emulsion prepared by mixing 1g of animal protein foaming agent and 0.2g of epoxy resin emulsion.

[0053] Example 2

[0054] Step 1: Weigh 42g of kaolin and 8g of calcium oxide and add them to 13g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.4g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a semi-dry high-calcium material;

[0055] Step 2: Weigh 42g of kaolin and 4g of aluminum hydroxide and add them to 16g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.4g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a high-alumina slurry.

[0056] (3) Add the above semi-dry high-calcium material to the high-alumina slurry, stir for 5 minutes, and mold it to obtain a spherical adsorbent with a diameter of about 1 cm. After curing for 28 days, the No. 2 semi-volatile heavy metal adsorbent is obtained.

[0057] In both steps 1 and 2, the foaming agent is an emulsion prepared by mixing 1.1g of animal protein foaming agent and 0.3g of epoxy resin emulsion.

[0058] Example 3

[0059] Step 1: Weigh 45g of kaolin and 10g of calcium oxide and add them to 14g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.5g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a semi-dry high-calcium material;

[0060] Step 2: Weigh 45g of kaolin and 5g of aluminum hydroxide and add them to 17g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.5g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a high-alumina slurry.

[0061] Step 3: Add the above semi-dry high-calcium material to the high-alumina slurry, stir for 5 minutes, and then mold it into spherical adsorbents with a diameter of about 1 cm. After curing for 28 days, the No. 3 semi-volatile heavy metal adsorbent is obtained.

[0062] In both steps 1 and 2, the foaming agent is an emulsion prepared by mixing 1.2g of animal protein foaming agent and 0.3g of epoxy resin emulsion.

[0063] Example 4

[0064] Step 1: Weigh 42g of kaolin and 9g of calcium oxide and add them to 13g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.4g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a semi-dry high-calcium material;

[0065] Step 2: Weigh 42g of kaolin and 5g of aluminum hydroxide, add 16g of water and stir to make a slurry; prepare foam by using a compressed air foaming machine with 1.4g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes to obtain a high-alumina slurry.

[0066] Step 3: Add the above semi-dry high-calcium material to the high-alumina slurry, stir for 5 minutes, and then mold it into spherical adsorbents with a diameter of about 1 cm. After curing for 28 days, the No. 4 semi-volatile heavy metal adsorbent is obtained.

[0067] In both steps 1 and 2, the foaming agent is an emulsion prepared by mixing 1.2g of animal protein foaming agent and 0.2g of epoxy resin emulsion.

[0068] The solidification rate of semi-volatile heavy metals by the semi-volatile heavy metal adsorbents prepared in Examples 1-4 above was tested using the following methods:

[0069] use Figure 2 The calcination experimental setup shown was configured with a heating program of 800℃-1200℃-1450℃ at a heating rate of 20℃ / min. A corundum crucible containing adsorbent and 0.5% (by mass of adsorbent) heavy metals (PbO / CdO / ZnO components were tested separately) was slowly pushed into the tubular furnace tube from one end for calcination. The furnace door was sealed while the gas cylinder valve was opened to allow airflow within the furnace tube, with the flow rate controlled at 1L / min using a float flowmeter. Three tail gas absorption bottles were connected to the end of the furnace tube via a high-temperature resistant silica gel hose. Two of the bottles contained 250mL of a mixed absorbent solution of 5% HNO3 and 20% H2O2, while the other contained silica gel desiccant for absorbing heavy metals from the tail gas. After the calcination experiment, the crucible containing the sample was removed and rapidly cooled to room temperature using a fan. The calcined material was digested using the HNO3-HCl-HF-H2O2 method, and the heavy metal content was determined by ICP-MS. The results are shown in Table 2.

[0070] The formula for calculating the solidification rate of heavy metals by the adsorbent is as follows:

[0071]

[0072] In the formula: R: the solidification rate of heavy metals in the adsorbent (100%);

[0073] K: The content of heavy metals in the sample after calcination (mg / kg);

[0074] S: The content of heavy metals in the sample before calcination (mg / kg);

[0075] LOI: Loss on ignition (%) of the sample.

[0076] Table 2. Solidification rate (%) of adsorbent for semi-volatile heavy metals

[0077] project PbO CdO ZnO Example 1 56 62 75 Example 2 60 63 72 Example 3 55 60 68 Example 4 62 64 76

[0078] Table 2 shows that the semi-volatile heavy metal adsorbents prepared in Examples 1-4 of this invention can achieve efficient solidification of semi-volatile heavy metals in cement kiln co-processing. Without the adsorbent, the solidification rates of PbO, CdO, and ZnO in cement kiln co-processing were 15%, 17%, and 21%, respectively.

[0079] The elemental distribution diagram of the adsorbent prepared in Example 4 is shown below. Figure 4 As shown, by Figure 4 It can be seen that the inner layer of the adsorbent prepared in Example 4 is a high-calcium component, and the outer layer is a high-aluminum component.

[0080] Comparative Example 1

[0081] The only difference from Example 4 is that the chemical composition adjustment and structural reconstruction steps are omitted, and only kaolin is used as the adsorbent. The specific steps are as follows: Kaolin is molded into spherical adsorbents with a diameter of about 1 cm, and after curing for 28 days, a semi-volatile heavy metal adsorbent is obtained.

[0082] The semi-volatile heavy metal adsorbent prepared in this comparative example was subjected to the same semi-volatile heavy metal curing rate test as in Example 2. The results showed that the semi-volatile heavy metal adsorbent prepared in this comparative example had a curing rate of 21% for PbO, a curing rate of 28% for CdO, and a curing rate of 30% for ZnO.

[0083] Comparative Example 2

[0084] The only difference from Example 4 is that the chemical component adjustment step is omitted, and the adsorbent is prepared by mixing kaolin and a foaming agent. The specific steps are as follows:

[0085] Weigh 84g of kaolin and add it to 29g of water to make a slurry; prepare foam by using a compressed air foaming machine with 2.8g of foaming agent; add the foam to the above slurry and stir evenly for 3 minutes, then mold it to obtain spherical adsorbents with a diameter of about 1cm. After curing for 28 days, the No. 1 semi-volatile heavy metal adsorbent is obtained.

[0086] The semi-volatile heavy metal adsorbent prepared in this comparative example was subjected to the same semi-volatile heavy metal curing rate test as in Example 2. The results showed that the semi-volatile heavy metal adsorbent prepared in this comparative example had a curing rate of 35% for PbO, a curing rate of 39% for CdO, and a curing rate of 41% for ZnO.

[0087] Comparative Example 3

[0088] The only difference from Example 2 is that the structural reconstruction step is omitted, no foaming agent is added, and the adsorbent is prepared by mixing high-calcium materials and high-aluminum slurry. The specific preparation steps are as follows:

[0089] Step 1: Weigh 42g of kaolin and 8g of calcium oxide and add them to 13g of water and stir to obtain a semi-dry high-calcium material;

[0090] Step 2: Weigh 42g of kaolin and 4g of aluminum hydroxide and add them to 16g of water and stir to obtain a high-alumina slurry;

[0091] Step 3: Add the above semi-dry high-calcium material to the high-alumina slurry, stir for 5 minutes, and then mold it into spherical adsorbents with a diameter of about 1 cm. After curing for 28 days, a semi-volatile heavy metal adsorbent is obtained.

[0092] The semi-volatile heavy metal adsorbent prepared in this comparative example was subjected to the same semi-volatile heavy metal curing rate test as in Example 2. The results showed that the semi-volatile heavy metal adsorbent prepared in this comparative example had a curing rate of 41% for PbO, a curing rate of 45% for CdO, and a curing rate of 49% for ZnO.

[0093] This invention proposes a semi-volatile heavy metal adsorbent for cement kiln co-processing and its preparation method. Through chemical composition adjustment and structural reconstruction, it achieves efficient solidification of semi-volatile heavy metals in cement kiln co-processing. The heavy metal adsorbent of this invention has the same composition as cement clinker, does not interfere with clinker performance, maintains structural stability in the high-temperature environment of cement kiln co-processing, and exhibits strong physical adsorption and excellent chemical solidification effects. It is low in cost, widely applicable, and has good economic and social benefits.

[0094] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for producing a semi-volatile heavy metal adsorbent, characterized by, The method comprises the following steps: Step 1, mixing kaolin, calcium oxide and water to form a slurry, preparing a foaming agent into a foam, adding the foam into the slurry to obtain a semi-dry high calcium material; Step 2, mixing kaolin, aluminum hydroxide and water to form a slurry, preparing a foaming agent into a foam, adding the foam into the slurry to obtain a high aluminum slurry; Step 3, mixing the semi-dry high calcium material and the high aluminum slurry, then molding and curing to obtain the semi-volatile heavy metal adsorbent; The mass ratio of the calcium oxide to the aluminum hydroxide is 5-10:3-5.

2. The production method according to claim 1, characterized by, In step 1, the mass ratio of the kaolin, calcium oxide, water and foaming agent is 40-45:5-10:10-15:1-2.

3. The preparation method according to claim 1, characterized in that, In step 2, the mass ratio of the kaolin, aluminum hydroxide, water and foaming agent is 40-45:3-5:15-20:1-2.

4. The method of claim 1, wherein, In step 3, the molding is a round ball with a diameter of 0.8-1.2 cm.

5. The preparation method according to claim 1, characterized in that, In step 3, the curing is specifically curing for 28 days under the condition of temperature 20±2℃ and relative humidity above 90%.

6. The method of claim 1, wherein, In step 1 and step 2, the foaming agent is specifically an emulsion prepared by mixing animal protein foaming agent and epoxy resin emulsion with a mass ratio of 15-20:3-5.

7. The semi-volatile heavy metal adsorbent prepared by the method according to any one of claims 1-6.

8. The application of the semi-volatile heavy metal adsorbent according to claim 7 in cement kiln co-processing waste.

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