Biochar-alkali-activated slag sludge curing agent and application thereof

Abstract

The invention discloses a charcoal-alkali activated slag sludge curing agent and application thereof, and belongs to the technical field of urban sludge curing and industrial solid waste resource utilization. The sludge curing agent is prepared from the following components in percentage by mass: 40 to 60 percent of slag, 20 to 40 percent of carbide slag and 10 to 20 percent of biochar. Urban sludge and a curing agent are mixed according to a certain proportion and uniformly stirred to prepare a sample, and moisture in the sludge is absorbed by biochar and alkali-activated slag and subjected to a curing reaction by controlling the mixing proportion and curing conditions, so that a firm cured body is formed. Through the synergistic effect of the adsorption / pore regulation and control function of the biochar and the hydration and gelling characteristics of the alkali-activated slag, the environment-friendly treatment of stabilizing the sludge and strengthening the mechanical property is realized, and moreover, the prepared solidified sludge can meet the requirement of the compressive strength of the base course of the extremely-heavy and extra-heavy traffic pavements of the second-grade and below roads, and has a good application prospect. Therefore, the method has application value in the field of road engineering.

Description

Technical Field

[0001] The present invention belongs to the technical field of urban sludge solidification and industrial solid waste resource utilization, and particularly relates to a biochar-alkali-activated slag sludge solidifying agent and its application. Background Art

[0002] With the acceleration of the urbanization process, the output of urban sludge has increased sharply, and its effective treatment and resource utilization have become urgent problems to be solved. Traditional sludge treatment methods such as landfilling and incineration not only occupy a large amount of land resources but also may cause secondary pollution to the environment. The treatment methods of sludge mainly include dewatering and landfilling, ocean dumping, and brick-making by incineration. However, these methods have obvious defects, such as high treatment energy consumption, damage to the ecosystem, increased carbon emissions, and low resource utilization rate. At the same time, traditional disposal methods such as landfilling also face the dilemmas of land resource occupation and secondary pollution diffusion.

[0003] As an important part of infrastructure, road construction often brings many challenges to construction during the construction process, such as insufficient foundation bearing capacity, significant settlement and deformation, and low treatment efficiency. In the construction of highway engineering, in-situ solidification technology is generally used. The advantage of this technology is that the treated sludge remains relatively stable, and the treatment time is short, and it will not cause significant impact on the environment in the short term after solidification. However, most traditional sludge treatment methods rely on high-energy-consuming materials such as cement and lime to shorten the solidification cycle and improve the mechanical strength. A large amount of greenhouse gases such as carbon dioxide are released during the production and transportation of portland cement and lime, resulting in increasingly serious problems of resource consumption and environmental pollution. Therefore, it is of great significance to provide a sludge solidifying agent with low energy consumption and excellent solidification effect. Summary of the Invention

[0004] Aiming at the deficiencies of the existing urban sludge treatment methods in the prior art, the present invention aims to provide a method for jointly solidifying the urban sludge base with biochar and alkali-activated slag to improve the mechanical strength of the sludge solidified body, improve its microstructure, and promote the resource utilization of sludge.

[0005] The present invention provides a sludge solidifying agent, which is composed of the following components by mass fraction: 40-60% of slag, 20-40% of carbide slag, and 10-20% of biochar.

[0006] In a specific embodiment, the sludge solidifying agent is composed of the following components by mass fraction: 51.38% of slag, 31.96% of carbide slag, and 16.66% of biochar.

[0007] In the above sludge solidifying agent, the biochar is prepared by the following method: crushing corn straw, then pyrolyzing it, and after the pyrolysis is completed, grinding the product to obtain biochar.

[0008] In the above method for preparing biochar, the pyrolysis conditions are selected from: pyrolyzing at 500-700 °C for 1-3 h; preferably: pyrolyzing at 500 °C for 2 h.

[0009] The present invention provides the application of the above curing agent in the solidification of urban sludge.

[0010] The present invention provides a method for solidifying the base layer of urban sludge, comprising the following steps: adding the above sludge curing agent to urban sludge and carrying out a solidification reaction to achieve the solidification of the base layer of urban sludge.

[0011] In the above method for solidifying the base layer of urban sludge, the addition amount of the sludge curing agent is 5-15%.

[0012] In the above method for solidifying the base layer of urban sludge, the conditions of the solidification reaction are selected from: solidifying at 20-30 °C for 7-28 d.

[0013] The present invention provides a subgrade material, which is composed of the following components by mass fraction: 64-80% of sludge, 5-15% of curing agent, and 15-21% of water.

[0014] In a specific embodiment, the subgrade material is composed of the following components by mass fraction: 67% of sludge, 15% of curing agent, and 18% of water.

[0015] The beneficial effects of the present invention are:

[0016] In the present invention, biochar is conventionally derived from the high-temperature pyrolysis of agricultural and forestry organic waste such as straw and rice husk. It has rich functional groups and a high specific surface area, has the characteristics of carbon sequestration and pollutant adsorption, and has excellent adsorption performance and physicochemical properties. The microporous structure of biochar itself provides a reaction site for the continuous hydration reaction, and the alkaline environment is also conducive to the self-activation modification of biochar. At the same time, the production of biochar does not cause more carbon emissions. Combustible oil and gas are generated during the preparation process of biochar, and these combustible biomasses provide energy for the production of biochar. Alkali-activated slag is a new type of cementitious material prepared from industrial waste slag, which has high strength and good durability. Combining the two for the solidification treatment of urban sludge can achieve the harmless, reduction and resource utilization of sludge.

[0017] In the present invention, the pore structure of biochar (pore diameter 5-50 nm) is loaded with C-S-H gel generated by the hydration of slag, forming a through-type reinforcement network; its surface oxygen-containing functional groups (-COOH, -OH) in the alkaline environment contribute to the formation of functional groups such as -COOH and -OH on the biochar surface, and react with Ca 2Complex formation of “Ca-biochar” composite promotes C-S-H nucleation (R-COOH + Ca 2+ →

[0018] R-COO-Ca + +H + ). The alkali activator (calcium carbide slag) can increase the reaction rate of slag, thus promoting the continuous hydration of slag and the stable combination with biochar during curing.

[0019] In summary, the porous structure and abundant functional groups of biochar can promote the cement hydration reaction, increase the formation of hydration products, and thus improve the mechanical strength of the solidified body. A dense network structure is formed inside the solidified body, enhancing the durability and impermeability of the solidified body and improving the microstructure. Converting urban sludge into stable solidified body materials can be used in road subgrades, dike projects and other fields, realizing the resource utilization of sludge. It reduces the landfill and incineration requirements of urban sludge, reduces environmental pollution and damage, and has significant environmental benefits.

[0020] The present invention realizes the environmental protection treatment of sludge stabilization and mechanical property strengthening through the synergistic effect of the adsorption / pore regulation function of straw biochar and the hydration and gelling characteristics of alkali-activated slag. The present invention mixes urban sludge with a curing agent in a certain proportion, stirs evenly and then makes specimens. By controlling the mixing ratio and curing conditions, the water in the sludge is absorbed by biochar and alkali-activated slag and undergoes a curing reaction to form a solidified body that can meet the compressive strength requirements of the subgrade of extremely heavy and super heavy traffic road surfaces below the second level and below, thus having application value in the field of road engineering.

[0021] The present invention provides a multi-source solid waste curing agent applicable to sludge. The full utilization of solid waste can solve the problem of difficult treatment of some solid wastes and provide a solution idea for the “waste-free city”. By using biochar-alkali-activated slag (BAS) to solidify urban sludge and applying the solidified sludge of biochar-alkali-activated slag to the road pavement subgrade, it has certain value and practical significance for improving road construction quality and efficiency, reducing engineering costs, and improving resource utilization rate. Biochar-alkali-activated slag replacing cement and lime can reduce carbon emissions and bring good environmental benefits.

[0022] The present invention solves the problems of high cost and poor stability of traditional curing agents, has the advantage of a 7-day compressive strength of more than 3.56 MPa, and is suitable for green restoration of scenarios such as river dredging sludge and sludge landfills. The solidified soil body has high strength, durability and good environmental benefits, and can be used in road subgrades, dike projects and other fields, realizing the resource utilization and environmental protection treatment of urban sludge. Description of the Drawings

[0023] Figure 1 Analysis of the interaction between the slag content and biochar; among them, (a) is the 7-day response surface diagram, and (b) is the 7-day contour diagram;

[0024] Figure 2 Analysis of the interaction between the carbide slag content and biochar; among them, (a) is the 7-day response surface diagram, and (b) is the 7-day contour diagram;

[0025] Figure 3 SEM images of the solidified silt at the age of 7 days (magnified 10,000 times); among them, (a) is BAS with a content of 5%, (b) is BAS with a content of 15%, (c) is cement with a content of 5%, and (d) is cement with a content of 15%. Specific implementation manner

[0026] In the present invention, the slag is provided by Gongyi Longze Water Purification Materials Co., Ltd., and its chemical composition is as follows: CaO (35.30%), SiO2 (34.50%), Al2O3 (16.70%), MgO (5.01%), Fe2O3 (1.50%), SO3 (1.24%), Na2O (0.52%), K2O (0.33%).

[0027] In the present invention, the carbide slag is provided by Henan Wuhu Environmental Protection Technology Co., Ltd., and its chemical composition is as follows: CaO (67.87%), SiO2 (1.55%), Al2O3 (1.96%), MgO (0.33%), SO3 (0.31%), Fe2O3 (0.26%), Na2O (0.03%). The carbide slag is an alkali activator and can react with the slag to form an alkali-activated slag gel material.

[0028] In the present invention, the cement is provided by Zhucheng Jiuqi Building Materials Co., Ltd., and its chemical composition is as follows: CaO (51.42%), SiO2 (24.99%), Al2O3 (8.26%), Fe2O3 (4.03%), MgO (3.71%), SO3 (2.51%).

[0029] In the present invention, the lime is provided by Zhucheng Jiuqi Building Materials Co., Ltd., and its chemical composition is as follows: CaO (87.3%), SiO2 (0.09%), Al2O3 (0.13%), Fe2O3 (0.05%), MgO (2.35%), SO3 (0.11%).

[0030] Other materials used in the present invention can be obtained through commercial channels without special statements. Other terms used in the present invention generally have the meanings commonly understood by those of ordinary skill in the art unless otherwise specified. The present invention will be further described in detail below with reference to specific examples and data. The following examples are only for illustrating the present invention and do not limit the scope of the present invention in any way.

[0031] Example 1

[0032] This example provides a sludge solidifying agent, which is composed of the following components by mass fraction: 51.38% of slag, 31.96% of carbide slag, and 16.66% of biochar.

[0033] The biochar is prepared by the following method:

[0034] The corn straw is dried at 60 °C for 5 h, and after drying, it is crushed into particles with a size of 2 - 4 mm. Then it is put into a muffle furnace, the heating rate is 10 °C / min, the pyrolysis temperature is 500 °C, and the pyrolysis time is 2 h. After pyrolysis, the product is ground and passed through a 40-mesh sieve to obtain biochar.

[0035] I. Optimal proportion of sludge solidifying agent components

[0036] The dosage of each component in the sludge solidifying agent described in Example 1 above is the best ratio calculated through response surface experiments. The specific experimental process is as follows:

[0037] (1) Solidified sludge test block

[0038] The treated sludge dry soil is watered and left to stand for 24 h. Weigh the corresponding masses of slag, carbide slag, and biochar, and mix them with the sludge after standing for 10 min. The solidified sludge uses a cylindrical mold with a size of . Before the test, wipe the mold clean, evenly apply a layer of vaseline on the inner wall of the mold, and pour the calculated mass of a single specimen into the mold at one time. To ensure uniformity during the pressing process, the upper and lower cushions after filling are both exposed 2 cm. After mixing evenly, compact it into shape and cure it at a humidity of 95% and 20 °C for 7 d and 28 d.

[0039] (2) Response surface experimental design

[0040] The Box-Behnken Design (BBD) method in the response surface was selected for experimental design. The slag content, carbide slag content, and biochar content were selected as independent variables, and the unconfined compressive strength of the dredged soil after 7 days and 28 days of curing (test blocks) was used as the response value to obtain the maximum compressive strength response value under limited dosages. The experimental design was carried out with three factors and three levels. As shown in Table 1, the slag content is represented by X1, the carbide slag content is represented by X2, and the biochar content is represented by X3. There were a total of 17 groups in this experimental design, and groups 4, 8, 10, 11, and 14 were the central point experimental groups.

[0041] Table 1 Response surface design parameters

[0042]

[0043] The unconfined compressive strength tests were carried out in sequence according to the experimental design table obtained from the response surface. The measured 7-day and 28-day compressive strength results are shown in Table 2. The difference between the maximum and minimum compressive strengths is relatively large at the same age, and the influence of the curing agent under different mix ratios on the compressive strength is obvious. Among them, the minimum 7-day unconfined compressive strength is 2.39 MPa in group 13, and the maximum is 3.81 MPa in group 4; the minimum 28-day unconfined compressive strength is 3.73 MPa in group 16, and the maximum is 5.28 MPa in group 14.

[0044] Table 2 Unconfined compressive strength test results

[0045]

[0046] Table 3 shows the model fitting results of this experimental design. For the models of 7 days and 28 days, the fitted P values are both less than 0.0001, and the models are extremely significant; the adjusted R 2 are 0.9842 and 0.9914 respectively, and the predicted R 2 are 0.9424 and 0.9591 respectively. The values are between 0 and 1 and close to 1, indicating that the error influence is small and the model fitting effect is good. The second-order polynomial model can be used as the model for this experimental design, and the fitting formulas are shown in the following two formulas.

[0047] Y7 = 3.75 + 0.34X1 + 0.19X2 + 0.1375X3 - 0.095X1X2 - 0.305X1X3 - 0.2275X1 2 - 0.5025X2 2 - 0.3825X3 2

[0048] Y 28= 5.22 + 0.3963X1 + 0.3213X2 + 0.165X3 - 0.11X1X2 - 0.1125X1X3 - 0.0875X2X3 - 0.317X1 2 - 0.377X2 2 - 0.3845X3 2

[0049] Table 3 Model fitting results

[0050]

[0051] Figure 1 and Figure 2 is the analysis diagram of the interaction of two factors at the age of 7d.

[0052] As Figure 1 shown, the slag is mainly composed of an unstable glass phase structure. After the carbide slag is dissolved in water, it provides a large amount of Ca 2+ and OH - , enhancing the alkaline environment of the soil. The generation of a large amount of hydration products such as C-S-H gel and C-A-H gel under the alkaline environment of the slag is the main reason for the increase in the unconfined compressive strength. The hydration products are cemented together between soil particles to form a network structure, forming a dense structure with a certain strength. However, when the dosages of the slag and the carbide slag are too high, too many hydration products are formed in the soil, resulting in an expansion phenomenon inside, destroying the adhesion between soil particles, and the unconfined compressive strength of the soil shows a decreasing trend with the further increase of the dosages of the slag and the carbide slag.

[0053] As Figure 2 shown, when the dosage of the main curing agent slag is fixed, the carbide slag and the biochar show a mutually promoting trend. The carbide slag provides an alkaline environment, increasing the specific surface area of the biochar, and the pore structure of the biochar also provides a reaction site for the full play of the alkali activation effect of the carbide slag. Although the carbide slag and the biochar promote each other, with the further increase of the dosage, the raw materials of the cementitious material are insufficient, resulting in a decrease in the compressive strength.

[0054] Through the Design expert software, the maximum value of the 28d unconfined compressive strength of the soft soil is used as the target value to optimize the mix ratio of the curing agent. After optimization, the optimal mix ratio of the biochar-alkali-activated slag (BAS) for solidifying the soft soil is slag: carbide slag: biochar = 13.26%: 8.25%: 4.30%. Converted into the weight percentage of the soft soil curing agent itself, it is slag: carbide slag: biochar = 51.38%: 31.96%: 16.66%.

[0055] II. Performance test of the soft soil curing agent

[0056] Unconfined compressive strength mechanical and water stability durability tests were carried out in accordance with the standards "Highway Geotechnical Test Procedures JTG 3430—2020" and "Test Procedures for Inorganic Binder Stabilized Materials of Highway Engineering JTG E51-2009".

[0057] The following method was used to prepare the solidified silt specimens: Add a solidifying agent to urban silt (from the river channels of the construction sites on both sides of Jingshi East Road in Jinan. The initial water content of the silt is 58.5%. The silt is air-dried and then its water content can be adjusted by adding water), mix evenly by turning over, compact and form, and cure at a humidity of 95% and 20°C. The silt solidifying agent used is the silt solidifying agent (BAS) prepared in the above-mentioned Example 1.

[0058] (1) Unconfined compressive strength

[0059] The following test groups were designed: The dosages of the solidifying agent are 5%, 10%, and 15% respectively, and the water contents of the silt are 15%, 18%, and 21% respectively (different water contents have different solidifying effects. Being too dry or too wet is not conducive to the specimens reaching the maximum compressive strength). The curing ages are 3d, 7d, 14d, and 28d. The compressive strength of the solidified silt was tested using a pressure testing machine. At the same time, the solidified silt with single admixture of cement, single admixture of lime, single admixture of slag, and pure silt were used as control tests. The specific test plan is shown in Table 4.

[0060] Table 4 Unconfined compressive strength of solidified silt

[0061]

[0062] As can be seen from Table 4, when the dosage of the BAS solidifying agent is 5%, the compressive strengths of the solidified silt at 3d, 7d, 14d, and 28d are 2.35MPa, 3.56MPa, 3.95MPa, and 4.23MPa respectively; when the dosage is 10%, the compressive strengths are 2.56MPa, 3.72MPa, 5.24MPa, and 5.46MPa respectively, which are 9%, 4%, 33%, and 29% higher than those when the dosage is 5%; when the dosage is 15%, the compressive strengths are 2.94MPa, 4.36MPa, 5.43MPa, and 5.97MPa respectively, which are 15%, 17%, 4%, and 9% higher than those when the dosage is 10%. As the dosage of the solidifying agent increases, the compressive strength of the solidified silt also increases, and the solidifying effect is enhanced.

[0063] In addition, the activity of slag without alkali activation is very low, and its hydration reaction is limited when used alone, so its ability to solidify silt is limited and the compressive strength is low.

[0064] Among different types of curing agents, under the conditions of the same dosage and water content, the curing effect of other curing agents on silt is significantly lower than that of the BAS curing agent prepared by the present invention.

[0065] (2) Water stability test

[0066] Design the following test groups: the dosages of the curing agent are 10% and 15% respectively, the water contents of the silt are 15%, 18% and 21% respectively, the curing age is 28 days, use a pressure testing machine to test the unconfined compressive strength of the cured silt, and the number of cycles is 0, 2, 4, 6, 8 and 10 times respectively. At the same time, the silt solidified with single cement and the silt solidified with single lime are used as control tests, and the specific test scheme is shown in Table 5.

[0067] Table 5 Dry-wet cycle test of cured silt

[0068]

[0069] As can be seen from Table 5, when the water content of the specimen is 18%, for the BAS-cured silt with a dosage of 10%, from 0 to 10 cycles, the unconfined compressive strength of the specimen decreases from 5.52 MPa to 4.12 MPa, and the compressive strength loss rate increases from 0.00% to 25.36%; when the dosage is 15%, the unconfined compressive strength decreases from 5.95 MPa to 4.55 MPa, and the compressive strength loss rate increases from 0.00% to 23.53%; for the specimen with a cement dosage of 15% from 0 to 10 cycles, the unconfined compressive strength decreases from 5.72 MPa to 4.25 MPa, and the compressive strength loss rate increases from 0.00% to 25.70%; for the specimen with a lime dosage of 15%, the unconfined compressive strength decreases from 1.78 MPa to 1.30 MPa, and the compressive strength loss rate increases from 0.00% to 26.97%.

[0070] It can be seen that with the increase of the number of cycles, the compressive strength of different types of cured silt decreases. However, in comparison, the cured silt incorporated with the BAS curing agent of the present invention still has a relatively high compressive strength and a relatively low strength loss rate after 10 cycles, which is significantly better than the cement silt and lime silt.

[0071] In addition, compared with a water content of 15%, the compressive strength loss rates of specimens with different types of curing agents are lower at a water content of 18%. The reason is that a reasonable water content is conducive to the progress of the hydration reaction. A low water content cannot provide sufficient water supply for the hydration reaction, the hydration reaction is insufficient, and the hydrated dense structure does not play an overall connection role.

[0072] (3) SEM test

[0073] For the SEM microscopic test, specimens were prepared under the condition of the optimal water content of 18%. With the water content variable fixed, the dosages of BAS curing agent were 5%, 10% and 15% respectively. The standard curing ages were 7 days and 28 days. Cement curing agent was used as the control group. The specific test design is shown in Table 6.

[0074] Table 6 Design table of SEM microscopic test for solidified silt

[0075]

[0076]

[0077] The test results are as Figure 3 shown:

[0078] For the silt soil with a curing agent dosage of 15%, the hydration reaction is stronger. As shown in Figure 3 (b) Under the image magnified 10,000 times, there are more needle-like Aft in the solidified soil with a 15% dosage than that with a 5% dosage, indicating that the curing agent dosage is an important factor affecting the soil strength. For the BAS solidified soil at the age of 7 days, the hydrated calcium silicate and ettringite on the surface increase with the increase of the dosage, indicating that the increase of the curing agent dosage is more conducive to the progress of the hydration reaction. Compared with the cement-solidified silt soil at 7 days, the BAS-solidified silt soil has a more sufficient hydration reaction at the same dosage, and the hydrated calcium silicate and ettringite further fill the pores of the biochar.

[0079] In summary, for the solidified silt formed by BAS curing of the present invention, its 7-day compressive strength ≥ 3.56 MPa and reaches 5.97 MPa at 28 days; the compressive strength loss rate under the dry-wet cycling condition with a 15% BAS curing agent dosage ≤ 25.34%; the 7-day unconfined compressive strengths of the BAS-solidified silt soil with dosages of 5% - 15% are 3.56 MPa, 3.72 MPa and 4.36 MPa respectively, all greater than 3.5 MPa. Therefore, the BAS-solidified silt soil can meet the requirements of the compressive strength of the subgrade base for extremely heavy and especially heavy traffic roads at or below the second level.

[0080] Example 2

[0081] This example provides a subgrade material, which is composed of the following components by mass fraction: 67% silt, 15% curing agent, and 18% water. The curing agent is the silt curing agent described in Example 1.

[0082] As described above, it is only the preferred embodiment of the present invention, and it is not intended to limit the present invention in other forms. Any person skilled in the art may use the technical content disclosed above to make changes or modifications into equivalent embodiments with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A sludge solidifying agent, characterized in that, The sludge solidifying agent is composed of the following components by mass fraction: 40-60% of slag, 20-40% of carbide slag, and 10-20% of biochar.

2. The sludge solidifying agent according to claim 1, wherein The sludge solidifying agent is composed of the following components by mass fraction Composition: 51.38% of slag, 31.96% of carbide slag, and 16.66% of biochar.

3. The sludge solidifying agent according to claim 1, characterized in that, The biochar is prepared by the following method: crushing corn straw, then pyrolyzing it, and after the pyrolysis is completed, grinding the product to obtain biochar.

4. The sludge solidifying agent according to claim 3, characterized in that, The conditions of the pyrolysis are selected from: pyrolyzing at 500-700 °C for 1-3 h.

5. Application of the solidifying agent according to any one of claims 1-4 in the solidification of urban sludge.

6. A method for solidifying the base layer of urban sludge, characterized in that, It includes the following steps: adding the sludge solidifying agent according to claim 1 to urban sludge and carrying out a solidification reaction to achieve the solidification of the urban sludge base layer.

7. The method for solidifying the urban sludge base layer according to claim 6, characterized in that, The addition amount of the sludge solidifying agent is 5-15%.

8. The method for solidifying the urban sludge base layer according to claim 6, characterized in that, The conditions of the solidification reaction are selected from: solidifying at 20-30 °C for 7-28 d.

9. A subgrade material, characterized in that, It is composed of the following components by mass fraction: 64-80% of sludge, 5-15% of solidifying agent, and 15-21% of water; the solidifying agent is the sludge solidifying agent according to claim 1.

10. The subgrade material according to claim 9, characterized in that, The roadbed material is composed of the following components by mass fraction: 67% of sludge, 15% of solidifying agent, and 18% of water.

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