A method for increasing the slow-release phosphorus content of phosphorus-containing sludge-based biochar by double alkali modification, sludge-based biochar prepared by the method and its application

Abstract

The present invention belongs to the technical field of environmental protection and resource recovery, and relates to a method for increasing the slow-release phosphorus content of phosphorus-containing sludge-based biochar by double alkali modification, sludge-based biochar prepared by the method, and applications thereof. The present invention obtains a precursor mixture by mixing sludge, calcium oxide, and potassium hydroxide, and grinding the mixture; the precursor mixture is pyrolyzed under low-oxygen conditions to obtain sludge-based biochar, thereby achieving directional conversion of phosphorus in sludge-based biochar and inertization of heavy metals, thereby achieving the purpose of preparing green and harmless sludge-based biochar that can be directly reused in soil by pyrolysis of sludge.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental protection and resource recovery, and in particular to a method for increasing the slow-release phosphorus content of phosphorus-containing sludge-based biochar by double alkali modification, the sludge-based biochar prepared by the method and applications thereof. Background Art

[0002] Phosphorus is a non-renewable resource that is being consumed rapidly with the development of economy and industry. Sludge is rich in phosphorus, and its resource utilization can not only address the problem of weak sustainable guarantee capacity of phosphate resources, but also realize the harmlessness and resource utilization of sludge.

[0003] In addition to nutrient mineral elements and trace elements such as nitrogen, phosphorus, and potassium, sludge also contains pathogenic microorganisms, viruses, bacteria, parasite eggs, metals, and heavy metals. It is highly uneven, has a high water content, contains unstable organic compounds, and is prone to produce irritating gases such as ammonia, hydrogen sulfide, and mercaptans when stacked. Pyrolysis heats the sludge to a certain high temperature and maintains it for a certain period of time. This process can kill bacteria and parasites, break the chain and aromatize organic matter, crystallize metals and heavy metals, convert organic phosphorus into a slow-release state, and generate stable sludge biochar. The biochar generated by pyrolysis of sludge alone has the disadvantages of not having a significant enrichment effect on effective phosphorus and having strong heavy metal activity.

[0004] How to reduce the heavy metal activity and the content of slow-release available phosphorus in sludge pyrolysis biochar, so as to achieve the purpose of harmless and resourceful treatment, is an urgent problem to be solved by technicians in this field. Summary of the invention

[0005] The purpose of the present invention is to provide a method for increasing the slow-release phosphorus content of phosphorus-containing sludge-based biochar by double alkali modification, the sludge-based biochar prepared by the method and the application thereof, and to provide a technical method for directional catalytic pyrolysis of sludge that can be widely used for different sludges through a stable double alkali catalytic system, so as to solve the problems existing in the above-mentioned prior art, realize the directional conversion of phosphorus in sludge-based biochar and the inertization development of heavy metals, and further realize the purpose of preparing green and harmless sludge-based biochar that can be directly reused in soil by pyrolysis of sludge.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] One of the technical solutions of the present invention is to provide a method for increasing the slow-release phosphorus content of phosphorus-containing sludge-based biochar by double alkali modification, the steps comprising:

[0008] The phosphorus-containing sludge, calcium oxide (CaO) and potassium hydroxide (KOH) are mixed and ground to obtain a precursor mixture;

[0009] The precursor mixture is pyrolyzed under low-oxygen conditions to obtain sludge-based biochar.

[0010] Furthermore, the phosphorus-containing sludge is dried before being mixed with the calcium oxide and potassium hydroxide.

[0011] Preferably, the drying treatment is carried out at 80° C. for 48 hours.

[0012] Furthermore, the amount of calcium oxide used is 7.5% of the dry weight of the precursor mixture.

[0013] Furthermore, the amount of potassium hydroxide used is 1.5% of the dry weight of the precursor mixture.

[0014] Furthermore, the grinding speed is 200-300 rpm, and the grinding time is 10-30 min.

[0015] The ball mill used for grinding is made of corundum (Al2O3) and the grinding balls are made of zirconium oxide (ZrO2).

[0016] Furthermore, the oxygen-poor condition is provided by an inert atmosphere or a vacuum condition.

[0017] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0018] Furthermore, the pyrolysis temperature is 600-900°C, the heating rate is 10°C / min, and the time is 60-240min.

[0019] The second technical solution of the present invention is to provide a sludge-based biochar prepared by the above method.

[0020] The third technical solution of the present invention is to provide an application of the above-mentioned sludge-based biochar in the preparation of fertilizers.

[0021] Furthermore, the application is to provide slow-release phosphorus in fertilizer preparation.

[0022] The fourth technical solution of the present invention is to provide an application of the above-mentioned sludge-based biochar as a soil conditioner in improving acidic soil.

[0023] The fifth technical solution of the present invention is to provide a method for inertizing heavy metals in sludge-based biochar, comprising the following steps:

[0024] The sludge, calcium oxide (CaO) and potassium hydroxide (KOH) are mixed and ground to obtain a precursor mixture;

[0025] The precursor mixture is pyrolyzed under low-oxygen conditions to obtain sludge-based biochar.

[0026] Furthermore, the sludge is dried sludge.

[0027] Preferably, the drying treatment is carried out at 80° C. for 48 hours.

[0028] Furthermore, the content of calcium oxide is 7.5% of the dry weight of the precursor mixture.

[0029] Furthermore, the content of potassium hydroxide is 1.5% of the dry weight of the precursor mixture.

[0030] Furthermore, the grinding speed is 200-300 rpm, and the grinding time is 10-30 min.

[0031] The ball mill used for grinding is made of corundum (Al2O3) and the grinding balls are made of zirconium oxide (ZrO2).

[0032] Furthermore, the oxygen-poor condition is provided by an inert atmosphere or a vacuum condition.

[0033] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0034] Furthermore, the pyrolysis temperature is 600-900°C, the heating rate is 10°C / min, and the time is 60-240min.

[0035] Technical solution 6 of the present invention: Provide a method for harmless and resourceful treatment of sludge, comprising the following steps:

[0036] The sludge, calcium oxide (CaO) and potassium hydroxide (KOH) are mixed and ground to obtain a precursor mixture;

[0037] The precursor mixture is pyrolyzed under low-oxygen conditions to obtain sludge-based biochar.

[0038] Furthermore, the sludge is dried sludge.

[0039] Preferably, the drying treatment is carried out at 80° C. for 48 hours.

[0040] Furthermore, the content of calcium oxide is 7.5% of the dry weight of the precursor mixture.

[0041] Furthermore, the content of potassium hydroxide is 1.5% of the dry weight of the precursor mixture.

[0042] Furthermore, the grinding speed is 200-300 rpm, and the grinding time is 10-30 min.

[0043] The ball mill used for grinding is made of corundum (Al2O3) and the grinding balls are made of zirconium oxide (ZrO2).

[0044] Furthermore, the oxygen-poor condition is provided by an inert atmosphere or a vacuum condition.

[0045] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0046] Furthermore, the pyrolysis temperature is 600-900°C, the heating rate is 10°C / min, and the time is 60-240min.

[0047] The present invention discloses the following technical effects:

[0048] The present invention uses sludge, CaO and KOH as raw materials, which not only realizes the harmlessness and resource utilization of sludge, but also synthesizes sludge-based biochar with high soil recycling value. It is a green, circular and low-carbon sludge disposal method.

[0049] The two alkalis added in the pyrolysis of the present invention are low in price, simple in process and easy to operate, and can be used for small-scale directional pyrolysis treatment and on-site reuse of relatively scattered sludge.

[0050] The sludge-based biochar prepared by the present invention has a perfect pore structure and is characterized by phosphorus transforming into a hydroxycalcium phosphate crystal state and heavy metals transforming into an inert state. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0052] Figure 1 Crystal comparison diagram of SCaK-700, SCa3K-700, SSB-700, SCa-700 and SK-700;

[0053] Figure 2 is the adsorption-desorption isotherm of SSB-700;

[0054] Figure 3 is the pore size distribution curve of SSB-700;

[0055] Figure 4 is the adsorption-desorption isotherm of SCaK-700;

[0056] Figure 5 is the pore size distribution curve of SCaK-700;

[0057] Figure 6 This is the microscopic morphology of SSB-700;

[0058] Figure 7 This is the microscopic morphology of SCaK-700;

[0059] Figure 8 This is the heavy metal activity analysis diagram of SSB-700 and SCaK-700. DETAILED DESCRIPTION

[0060] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0061] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0062] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0063] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0064] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0065] The sludge used in the specific implementation scheme of the present invention is provided by a sewage treatment plant in Beijing, and its main elements and trace elements are shown in Table 1.

[0066] Table 1

[0067]

[0068] The grinding in the specific embodiment of the present invention is ball milling, and the ball mill used is made of corundum (Al2O3) and the grinding balls are made of zirconium oxide (ZrO2).

[0069] In the specific implementation manner of the present invention, the low-oxygen condition is provided by a nitrogen atmosphere, and the argon atmosphere and the vacuum condition can be replaced equally without affecting the realization of the technical effect.

[0070] Example 1

[0071] The preparation steps of sludge-based biochar are as follows:

[0072] S1. Dry the sludge at 80°C for 48 hours, grind it and pass it through a 200-mesh sieve for later use;

[0073] S2, mixing CaO, KOH and the dried sludge in step S1 uniformly, and ball milling for 30 minutes at 200 rpm to obtain a precursor mixture; wherein CaO accounts for 7.5% of the weight of the precursor mixture, and KOH accounts for 1.5% of the weight of the precursor mixture;

[0074] S3. The precursor mixture was placed in a tubular furnace and heated to 700°C at a heating rate of 10°C / min under a nitrogen atmosphere, and kept warm for 120 min. After the pyrolysis was completed, the product was cooled to room temperature and then taken out, which was sludge-based biochar, recorded as SCaK-700.

[0075] Example 2

[0076] Compared with Example 1, the only difference is that the pyrolysis temperature is 800°C, and the obtained sludge-based biochar is recorded as SCaK-800.

[0077] Example 3

[0078] Compared with Example 1, the only difference is that the pyrolysis temperature is 900°C, and the obtained sludge-based biochar is recorded as SCaK-900.

[0079] Comparative Example 1

[0080] The preparation steps of sludge-based biochar are as follows:

[0081] S1. Dry the sludge at 80°C for 48 hours, grind it and pass it through a 200-mesh sieve for later use;

[0082] S2. The dried sludge obtained in step S1 is placed in a tubular furnace, heated to 700°C at a heating rate of 10°C / min under a nitrogen atmosphere, kept warm for 120 minutes, and the product is taken out after cooling to room temperature, which is sludge-based biochar, recorded as SSB-700.

[0083] Comparative Example 2

[0084] Compared with Comparative Example 1, the only difference is that the pyrolysis temperature is 800°C, and the obtained sludge-based biochar is recorded as SSB-800.

[0085] Comparative Example 3

[0086] Compared with Comparative Example 1, the only difference is that the pyrolysis temperature is 900°C, and the obtained sludge-based biochar is recorded as SSB-900.

[0087] Comparative Example 4

[0088] Compared with Example 1, the only difference is that potassium hydroxide is not added (the mass of calcium oxide and sludge is the same as that of Example 1), and the obtained sludge-based biochar is recorded as SCa-700.

[0089] Comparative Example 5

[0090] Compared with Example 1, the only difference is that calcium oxide is not added (the masses of potassium hydroxide and sludge are the same as those in Example 1), and the obtained sludge-based biochar is recorded as SK-700.

[0091] Comparative Example 6

[0092] Compared with Example 1, the only difference is that the mass of KOH is three times the mass of KOH used in Example 1 (the masses of sludge and calcium oxide remain unchanged), and the obtained sludge-based biochar is recorded as SCa3K-700.

[0093] Comparative Example 7

[0094] Compared with Comparative Example 6, the only difference is that the pyrolysis temperature is 800°C, and the resulting sludge-based biochar is recorded as SCa3K-800.

[0095] Comparative Example 8

[0096] Compared with Comparative Example 6, the only difference is that the pyrolysis temperature is 900°C, and the resulting sludge-based biochar is recorded as SCa3K-900.

[0097] Test example

[0098] Figure 1 This is a crystal comparison chart of SCaK-700, SCa3K-700, SSB-700, SCa-700 and SK-700. Figure 1It can be seen that compared with sludge pyrolysis alone (SSB-700), some hydroxy calcium phosphate crystals have been generated in the product obtained by adding CaO alone (SCa-700) during sludge pyrolysis, while adding KOH alone (SK-700) has little effect on the phase change of the product crystals. When the two participate in the pyrolysis reaction together (SCaK-700), the addition of KOH not only promotes the formation of SiO2 crystals in the product, but also makes the peak shape of the target product hydroxy calcium phosphate crystals more obvious (indicating an increase in the content of slow-release phosphorus), and also shows the peak shape of heavy metals and phosphate-bound crystals, such as Ca / Al bound state. The XRD spectrum of SCa3K-700 has no obvious advantages over SCaK-700, but promotes the sequestration of Fe and S, and the cost of adding raw materials is high. Therefore, KOH should not be added excessively. The above shows that CaO is a necessary condition for the formation of hydroxy calcium phosphate, and the addition of KOH can not only further promote the formation of hydroxy calcium phosphate, but also promote the sequestration of some heavy metals in sludge-based biochar. The addition of CaO and KOH converts part of the P element in the sludge into calcium hydroxyphosphate, which, as an effective phosphorus, can usually be used as a phosphate fertilizer. It can be seen that compared with the pyrolysis of sludge alone, the addition of double alkali promotes the effective conversion of P in the sludge, which can provide a new idea for the recovery of P resources in the sludge. In addition, a large amount of SiO2 crystals are formed in both pyrolysis systems, and the loss of Si element is one of the main problems facing acidic soils. Therefore, the sludge-based biochar obtained from these two pyrolysis systems can be considered as an acidic soil conditioner.

[0099] Figure 2 is the adsorption-desorption isotherm of SSB-700; Figure 3 is the pore size distribution curve of SSB-700; Figure 4 is the adsorption-desorption isotherm of SCaK-700; Figure 5 is the pore size distribution curve of SCaK-700. Figure 2 and Figure 4 It can be seen that the sludge-based biochar obtained under the two pyrolysis system conditions formed a hysteresis loop during the adsorption and desorption process. Under the double-alkali system, most of the pores of SCaK-700 were mesopores (2-50 nm) and the pore size distribution was relatively uniform ( Figure 5 ); The sludge-based biochar SSB-700 obtained under the sludge pyrolysis system alone has both macropores (>50 nm) and mesopores (2~50 nm), and the pore size distribution is relatively dispersed ( Figure 3). In comparison, the small-diameter mesopores of SSB-700 are densely distributed, which may cause the specific surface area of ​​SCaK-700 (39.6171 m² / g) to be smaller than that of SSB-700 (48.0788 m² / g). Therefore, the addition of double alkali can improve the pore structure of sludge pyrolysis: on the one hand, the proportion of macropores in sludge-based biochar is significantly reduced; on the other hand, the addition of double alkali and co-pyrolysis of sludge can even out the pore size of sludge-based biochar.

[0100] Figure 6 This is the microscopic morphology of SSB-700; Figure 7 The microscopic morphology of SCaK-700. Figure 6~Figure 7 It can be seen that SCaK-700 has a more complete pore structure than SSB-700.

[0101] The BCR continuous extraction method was used to extract heavy metals in SSB-700 and SCaK-700 step by step, and ICP-OES was used to measure the contents of As, Zn, Cd, Pb, Ni, Cr and Cu. F1, F2, F3 and F4 represent the exchangeable, reducible, oxidizable and residual states of metals, respectively.

[0102] Figure 8 The heavy metal activity analysis diagram of SSB-700 and SCaK-700. Figure 8 It can be seen that the residual state of Zn, Cd, Pb, Cr, and Cu accounts for a high proportion (>70%), which indicates that sludge pyrolysis has a good fixation effect on the above five metals. Among them, the existence form of Cd in SCaK-700 is basically all in the residual state, indicating that the double alkali system has a better fixation effect on Cd in particular. Compared with the pyrolysis of sludge alone, the addition of double alkali causes Zn in the sludge to partially transform from an exchangeable state to a reducible state Zn, and all exchangeable Cd is converted into a residual state Cd, while also promoting Cu from an oxidizable state to a residual state. In general, the co-pyrolysis of double alkali and sludge promotes the transformation of heavy metals in the sludge from an unstable state to a stable state, especially for Zn, Cd, Cr, and Cu. The fixation effects on the four heavy metals are Cd> Zn> Cu> Cr.

[0103] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for increasing the slow-release phosphorus content of phosphorus-containing sludge-based biochar and inertizing heavy metals Zn and Cd by double alkali modification, characterized in that the steps include: The phosphorus-containing sludge, calcium oxide and potassium hydroxide are mixed and ground to obtain a precursor mixture; The precursor mixture is pyrolyzed under low-oxygen conditions to obtain sludge-based biochar; The amount of calcium oxide used is 7.5% of the dry weight of the precursor mixture; the amount of potassium hydroxide used is 1.5% of the dry weight of the precursor mixture; The phosphorus-containing sludge is dried before being mixed with the calcium oxide and potassium hydroxide; The grinding speed is 200-300 rpm, and the time is 10-30 min; the pyrolysis temperature is 600-900° C., the heating rate is 10° C. / min, and the time is 60-240 min.

2. The method according to claim 1, characterized in that The oxygen-poor condition is provided by an inert atmosphere or a vacuum condition.

3. A sludge-based biochar with inert heavy metals Zn and Cd and high content of slow-release phosphorus, obtained by the method according to claim 1 or 2.

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