A sludge-based saline-alkali soil conditioner, a preparation method and application thereof

Abstract

This invention provides a sludge-based saline-alkali land conditioner, its preparation method, and its application. The conditioner uses municipal wastewater treatment plant sludge and industrial calcium silicate slag as raw materials, prepared through a wet cross-linking reaction. The method includes raw material pretreatment, slurry mixing, hydrothermal cross-linking reaction, and solid-liquid separation and aging. During the reaction, the organic matter of the sludge and the calcium silicate slag undergo coordination cross-linking in a weakly alkaline environment, forming a three-dimensional network structure of "humic acid-calcium ion-aluminosilicate," which encapsulates unreacted calcium silicate slag particles and contains organic matter and nutrients. The resulting conditioner has a high cation exchange capacity, can efficiently replace sodium ions in the soil, improve soil structure, inhibit salt accumulation, and simultaneously release nutrients slowly, promoting vegetation growth. Its preparation process is drying-free and low-energy-consumption, achieving synergistic resource utilization of sludge and calcium silicate slag. The conditioner also has functions of salt reduction, soil improvement, fertilization, and heavy metal passivation. It is safe and environmentally friendly, suitable for the ecological restoration of saline-alkali land, and has significant economic and environmental benefits.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and soil remediation technology, specifically relating to a method and application for preparing a saline-alkali land conditioner using sludge from urban sewage treatment plants and industrial calcium silicate slag, as well as the conditioner prepared by this method. Background Technology

[0002] Coastal saline soils and inland secondary saline soils, due to their high salt content, poor structure, and low fertility, severely restrict the growth of landscaping and agricultural vegetation. Traditional methods for improving saline-alkali land mainly include salt leaching through water conservancy projects, application of chemical amendments, and soil replacement. Among these, chemical amendments mainly consist of gypsum and desulfurization byproducts. However, these physical and chemical methods are often large-scale and costly, and fail to fundamentally improve the soil's water and fertilizer retention capacity and biodiversity.

[0003] Sludge from urban wastewater treatment plants typically contains 15%–50% organic matter (on a dry basis) and essential nutrients for plants such as nitrogen, phosphorus, and potassium, possessing potential value for soil improvement and fertility enhancement. However, urban wastewater treatment plant sludge also exhibits characteristics such as high water content, large volume, and susceptibility to putrefaction and foul odors, and may carry risks of heavy metals and pathogens. Direct land application without proper treatment can easily cause secondary pollution, limiting its resource utilization. Existing technologies for preparing soil conditioners from sludge mainly include biocomposting, physical mixing, and thermochemical treatment. In traditional physical mixing processes, sludge is mixed with other auxiliary materials (such as fly ash and straw) only through mechanical stirring. The lack of chemical bonds between components results in a loose product structure, which easily disintegrates and separates after being applied to the soil, leading to nutrient ion loss and hindering the formation of a long-term soil improvement mechanism. Furthermore, the cation exchange capacity (CEC) of traditional physical mixing-type conditioners is typically low, only around 60 mmol / kg to 80 mmol / kg, which is insufficient to meet the requirements for efficient adsorption and replacement of sodium ions. To improve product performance, some technologies employ heat treatment or chemical solidification methods. However, high pH environments and high-temperature processes often lead to the burning or denaturation of organic matter in sludge, destroying its agricultural value as a carbon source and energy source.

[0004] In the metallurgical, power, and chemical industries, a large amount of alkaline silicon- and calcium-containing solid waste is generated annually, such as fly ash tailings for aluminum extraction (silicon-calcium slag) and dealkalized steel slag. The main mineral components of this type of solid waste are β-dicalcium silicate (β-C2S) and free calcium oxide, exhibiting potential cementing activity and weak alkalinity. Silicon-calcium slag can be used as a soil conditioner or silicon fertilizer; the calcium ions it releases can replace sodium ions in soil colloids, reducing soil alkalinity. However, existing silicon-calcium slag utilization technologies mostly remain at the level of simple crushing and mixing or direct application. Silicon-calcium slag itself has a dense structure and low activity; when applied alone, the release rate of its effective components is difficult to control, and it lacks the synergistic effect of organic matter, making it difficult to fundamentally improve the barren soil ecological environment of saline-alkali land. How to stimulate the activity of silicon-calcium slag and achieve functional complementarity with organic waste to form a structurally stable composite amendment material is a current technical challenge in the field of solid waste resource utilization.

[0005] There are few reports in the existing technology on the construction of sludge-based saline-alkali land conditioners through organic-inorganic reactions under normal pressure. Although hydrothermal treatment technology has been applied in material synthesis, there is no technical solution in the existing patent literature that specifically uses it to regulate the directional cross-linking reaction between sludge organic matter and calcium silicate slag to prepare saline-alkali land conditioners with specific pore structures and active sites.

[0006] In summary, the existing technology has the following main drawbacks: 1. Limited functionality and lack of synergy: Conventional soil conditioners are mostly physical mixtures with a lack of chemical bonding between components, making it difficult to achieve the synergistic effects of reducing salinity, improving soil, and enriching soil simultaneously.

[0007] 2. Low performance indicators: Modifiers prepared by traditional physical mixing methods have low cation exchange capacity (CEC) and limited adsorption and replacement capacity for sodium ions.

[0008] 3. High energy consumption and significant loss of organic matter: The preparation process, which relies on high-temperature drying or sintering, is energy-intensive and destroys valuable organic nutrients in the sludge.

[0009] 4. Unstable structure: Lacking microscopic structural design, the amendment is easily lost after entering the soil, and cannot play a long-term physical barrier and aggregate reconstruction role. Summary of the Invention

[0010] In view of this, the purpose of the present invention is to provide a sludge-based saline-alkali land conditioner, its preparation method and application, so as to overcome the shortcomings of the prior art.

[0011] To achieve the above objectives, the present invention is implemented through the following technical solution: On the one hand, a sludge-based saline-alkali land conditioner is provided, which is prepared by wet cross-linking reaction of sludge from urban sewage treatment plants and calcium silicate slag followed by dehydration and aging; it contains a three-dimensional network cross-linked structure composed of humic acid-calcium ion-aluminosilicate as the continuous phase, and encapsulates unreacted calcium silicate slag particles as the dispersed phase; it retains unconverted inert organic matter and the nitrogen, phosphorus and potassium nutrients inherent in sludge; it is rich in carboxyl and phenolic hydroxyl active sites, and the cation exchange capacity is greater than 150 mmol / kg.

[0012] On the other hand, a method for preparing a sludge-based saline-alkali land conditioner is provided, characterized by comprising the following steps: S1. Raw material pretreatment: Dewater the sludge from the municipal wastewater treatment plant to a moisture content of 75%~85%; grind the calcium silicate slag to a particle size of less than 200 mesh; S2. Slurry mixing: The pretreated sludge and calcium silicate slag are added to the reactor at a dry basis mass ratio of 1:(0.8~1.2), and water is added to adjust the solid content of the system to 20%~30%; S3. Hydrothermal crosslinking reaction: The reactor is sealed and heated to 65℃~80℃. At this temperature, the reaction is carried out with constant mechanical stirring for 4 to 10 hours. The weakly alkaline environment generated by the hydration of calcium silicate slag promotes the dissolution of organic matter in the sludge and crosslinks with the active calcium silicate to form a three-dimensional network gel. S4. Solid-liquid separation and aging: Dehydrate the reactants to a solid moisture content of 35%~45%, then age them under sealed conditions for more than 12 hours, and crush them into particles with a particle size of 1mm~10mm to obtain the final product.

[0013] Furthermore, in the method for preparing the sludge-based saline-alkali land conditioner, the calcium silicate slag is selected from one or more of fly ash alumina extraction tailings or dealkali-removing steel slag; wherein the mass fraction of β-dicalcium silicate (β-C2S) in the fly ash alumina extraction tailings is greater than 40%.

[0014] Furthermore, in the method for preparing the sludge-based saline-alkali land conditioner, the hydrothermal crosslinking reaction in step S3 is carried out in a closed reactor.

[0015] Furthermore, in the method for preparing the sludge-based saline-alkali land conditioner, the rotation speed of the continuous mechanical stirring in step S3 is 50~150 rpm.

[0016] Furthermore, in the method for preparing the sludge-based saline-alkali land conditioner, in step S3, the hydration of the calcium silicate slag releases OH... - Maintain the pH of the system between 9.0 and 11.0.

[0017] Furthermore, in the method for preparing the sludge-based saline-alkali land conditioner, the dewatering in step S4 is carried out mechanically using a plate and frame filter press, and the moisture content of the sludge cake after dewatering is less than 50%.

[0018] On the other hand, the present invention provides an application of the sludge-based saline-alkali land conditioner in the remediation of saline soil or acidic mine wasteland.

[0019] The beneficial effects of the technical solution of this invention are: 1. Unique composite structure The modifier prepared by this invention is not a homogeneous pure substance, but a complex organic-inorganic composite material. In this material, a three-dimensional cross-linked network acts as a "flexible binder" to provide toughness and active sites, while unreacted calcium silicate slag acts as a "micro-aggregate" to provide a rigid skeleton. This structure not only solves the problem of easy disintegration of traditional physical mixtures, but also forms a slow-release nutrient reservoir by encapsulating unreacted organic matter and nutrients.

[0020] 2. Synergistic function of salinization reduction, soil improvement, structural reshaping, and ecological restoration (1) Ion exchange and replacement: The high-density carboxyl and phenolic hydroxyl groups in the three-dimensional network skeleton serve as strong cation exchange sites, which can quickly capture and replace harmful sodium ions on soil colloids and leach them with water; at the same time, the unreacted silica-calcium slag wrapped inside and exposed on the surface continuously hydrolyzes to release calcium ions, replacing and replenishing soil colloids, thereby reducing salinity and pH value from the source.

[0021] (2) Physical barrier and aggregate construction: The material forms a porous cross-linked network similar to a "miniature sponge" in the soil, physically cutting off the capillary upwelling channels of groundwater and inhibiting the accumulation of salt on the surface; at the same time, its cementing effect can promote the formation of soil aggregate structure, break up compaction, and greatly improve the aeration and permeability of the soil.

[0022] (3) The unreacted inert organic matter retained in the soil conditioner is a high-quality carbon and energy source for soil microorganisms. After being applied to the soil, it can stimulate the proliferation of native microorganisms and rebuild the damaged soil micro-ecology. The nitrogen, phosphorus, potassium and other nutrients inherent in the sludge are encased in the three-dimensional network structure. Unlike the explosive release of chemical fertilizers, these nutrients are released slowly as the network degrades, achieving simultaneous "soil improvement" and "fertilization", which significantly promotes the establishment and growth of vegetation on saline-alkali land.

[0023] 3. No drying required, in-situ molding, significantly reducing energy consumption: Traditional sludge-based modifiers usually rely on energy-intensive thermal drying or sintering. This invention, through sludge hydrolysis and cross-linking reaction with calcium silicate slag, greatly improves the filtration performance of the slurry, making mechanical dewatering extremely smooth; after removing free water, the gel network generated in the material spontaneously achieves a certain degree of micro-solidification, without the need for external reagents and drying, realizing a low-carbon, low-energy green preparation.

[0024] 4. The effect of salinity reduction and soil improvement is non-linearly multiplied: Due to the formation of abundant organic-inorganic composite active sites, the cation exchange capacity (CEC) of the soil conditioner of this invention is as high as 150 mmol / kg~200 mmol / kg, while that of traditional physical mixtures is only 60 mmol / kg~80 mmol / kg. In addition, the bioactivation and plant nutrient supply effects of organic matter and nutrients in the soil conditioner are combined, and after being applied to saline-alkali land, its salinity reduction efficiency, long-term stability and ecological restoration ability far exceed those of similar products.

[0025] 5. Dual passivation effect, safe and environmentally friendly: The alkaline environment and calcium ions provided by the calcium silicate slag not only promote the humification of the sludge itself, but also effectively passivate the heavy metals in the sludge and soil, transforming them into a stable residue state.

[0026] 6. Convenient application and slow-release effect: By breaking down the aged material into coarse particles of 1-10mm, the problem of easy dust and loss of powder is solved, and the defect of insufficient contact area of ​​blocky materials is overcome. This achieves a long-term desalination mechanism of rapid ion exchange on the surface and slow release in the inner layer, and is suitable for mechanized application in modern agriculture.

[0027] In summary, this invention induces an inorganic-organic hybrid cross-linking reaction between sludge organic matter and calcium silicate slag by precisely controlling reaction conditions. While achieving sludge humification and harmlessness, it also produces a composite saline-alkali land improvement material containing a three-dimensional cross-linking network, calcium silicate aggregate, organic matter, and nutrients, which can achieve efficient and long-term improvement and ecological restoration of saline-alkali land. Attached Figure Description

[0028] To further illustrate the above-mentioned objectives, structural features, and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

[0029] Figure 1 This is a microscopic schematic diagram of a sludge-based saline-alkali land conditioner according to a preferred embodiment of the present invention.

[0030] In the diagram: 1. Humic acid molecular chain; 2. Ca 2+ 3. Crosslinking points; 4. Aluminosilicate network; 5. Calcium silicate slag micro-aggregate. Detailed Implementation

[0031] The terms “invention” and “the present invention” used in this specification are intended to broadly refer to all subject matter of this specification and any of the following patent claims. Statements containing these terms should not be construed as limiting the subject matter described herein or limiting the meaning or scope of any of the following patent claims. Furthermore, this specification does not attempt to describe or limit the subject matter covered by any claim of any particular component, paragraph, statement, or drawing of this application. The subject matter should be understood with reference to the entire specification, all drawings, and any of the following claims. The invention may have other embodiments and be practiced or implemented in other ways. Moreover, it should be understood that the wording and terminology used herein are for illustrative purposes and should not be considered limiting.

[0032] Details of the invention will now be discussed with reference to the accompanying drawings, which are illustrated by way of example only. In the drawings, similar features or components may be labeled using the same reference numerals.

[0033] The use of the terms "comprising," "having," and "including," and variations thereof, herein means to include the items listed herein and their equivalents and additional items. While reference may be made in the description of the drawings to directions such as above, below, upward, downward, backward, bottom, top, front, rear, etc., for convenience, reference is made relative to the drawings. These directions are not intended to literally accept or limit the invention in any form. Furthermore, terms such as "first," "second," "third," etc., are used herein for illustrative purposes and are not intended to indicate or imply importance or significance.

[0034] See Figure 1 As shown, this invention provides a sludge-based saline-alkali land conditioner, prepared by wet crosslinking reaction and subsequent dehydration and aging of sludge from urban wastewater treatment plants and calcium silicate slag. This conditioner contains a three-dimensional network crosslinked structure composed of "humic acid-calcium ions-aluminosilicate" as a continuous phase (in the figure, label 1 represents the humic acid molecular chain, label 2 represents Ca...). 2+ The cross-linking point (labeled 3 represents the aluminosilicate network) is coated with unreacted calcium silicate slag particles as the dispersed phase (labeled 4 in the figure represents calcium silicate slag micro-aggregate). This amendment retains unconverted inert organic matter and the inherent nitrogen, phosphorus, and potassium nutrients of the sludge, while being rich in active sites such as carboxyl and phenolic hydroxyl groups, with a cation exchange capacity greater than 150 mmol / kg.

[0035] This invention provides a method for preparing a sludge-based saline-alkali land conditioner, which includes the following steps: S1. Raw material pretreatment: Dewater the sludge from the municipal wastewater treatment plant to a moisture content of 75%~85%; grind the calcium silicate slag to a particle size of less than 200 mesh; S2. Slurry mixing: The pretreated sludge and calcium silicate slag are added to the reactor at a dry basis mass ratio of 1:(0.8~1.2), and water is added to adjust the solid content of the system to 20%~30%; S3. Hydrothermal Crosslinking Reaction: The reactor is sealed and heated to 65℃~80℃. At this temperature, the reaction is maintained with continuous mechanical stirring for 4~10 hours. The weakly alkaline environment generated by the hydration of the calcium silicate slag promotes the dissolution of organic matter in the sludge and its coordination and crosslinking with the activated calcium silicate, forming a three-dimensional network gel. Further, the hydrothermal crosslinking reaction is carried out in a sealed reactor with continuous mechanical stirring at a speed of 50~150 rpm. The hydration of the calcium silicate slag releases OH-... - Maintain the pH of the system between 9.0 and 11.0.

[0036] S4. Solid-liquid separation and aging: The reactants are dehydrated to a solid moisture content of 35%~45%, then aged under sealed conditions for more than 12 hours, and crushed into particles with a particle size of 1mm~10mm. Further, mechanical dewatering is performed using a plate and frame filter press, resulting in a filter cake with a moisture content of less than 50%.

[0037] The calcium silicate slag in the above preparation method is selected from one or more of fly ash aluminum extraction tailings or dealkali steel slag, wherein the mass fraction of β-dicalcium silicate (β-C2S) in fly ash aluminum extraction tailings is greater than 40%.

[0038] The present invention also provides an application of the sludge-based saline-alkali land conditioner in the remediation of saline soil or acidic mine wasteland.

[0039] Example 1 a. Raw materials: Dewatered sludge from a municipal sewage treatment plant, with a moisture content of 80% and an organic matter content of 55%; siliceous calcium slag is fly ash tailings for aluminum extraction, with a β-C2S content of 45%, ground to 200 mesh.

[0040] b. Mixing: Mix the sludge and calcium silicate slag at a dry weight ratio of 1:1, and add water to adjust the solid content to 25%.

[0041] c. Reaction: The mixture was transferred to a sealed reactor equipped with a reflux condenser and mechanical stirrer, heated to 70°C, and stirred continuously at a constant temperature (80 rpm) for 6 hours. During this period, the pH of the system naturally rose to 10.2.

[0042] d. Dehydration and aging: After cooling, the reactants are pumped into a plate and frame filter press and dehydrated to a moisture content of 40% under a pressure of 1.0 MPa. After the filter cake is sealed and aged for 18 hours, it is broken into 5mm particles to obtain sludge-based saline-alkali land conditioner A.

[0043] Example 2 a. Raw materials: Sludge from the urban wastewater treatment plant, the same as in Example 1; Calcium silicate slag is dealkali-treated steel slag, ground to 200 mesh.

[0044] b. Mixing: Mix the sludge and calcium silicate slag at a dry weight ratio of 1:0.8, and add water to adjust the solid content to 20%.

[0045] c. Reaction: Transfer to a sealed reactor equipped with a reflux condenser and mechanical stirrer, heat to 75°C, and maintain the temperature with continuous stirring (100 rpm) for 5 hours. During this period, the pH of the system naturally rises to 10.5.

[0046] d. Dehydration and aging: The reactants are then pumped into a plate and frame filter press and dehydrated to a moisture content of 45% under a pressure of 0.8 MPa. After the filter cake is sealed and aged for 12 hours, it is crushed into 10mm particles to obtain sludge-based saline-alkali land conditioner B.

[0047] The soil conditioners prepared in Examples 1 and 2 were incorporated into a severely saline-alkali coastal soil at a mass ratio of 10%. The initial electrical conductivity of the soil was EC = 4.5 mS / cm, and the pH was 8.7. A 90-day indoor incubation experiment was conducted. After 90 days, the soil's physicochemical properties were measured, and the percentage of original soil aggregate structure disintegration was tested after simulated rainwater runoff. The results are shown in Table 1.

[0048] Table 1. Comparison of the effects of saline-alkali soil conditioner on saline-alkali soil (after 90 days)

[0049] As can be seen from Table 1, Examples 1 and 2 have a good effect on increasing cation exchange capacity, reducing soil EC value and pH value, and can also improve soil structural stability, anti-compactment ability and vegetation restoration ability.

[0050] This invention enables the simultaneous harmlessness and resource utilization of sludge through the induction of inorganic-organic composite reactions, and constructs a sludge-based saline-alkali land conditioner with high cation exchange capacity, excellent physical structure and slow-release fertilizer effect, as well as its preparation method. This invention has important practical significance and application value for realizing the resource utilization of sludge and other solid wastes and promoting the ecological restoration of saline-alkali land.

[0051] The above are merely preferred embodiments of the present invention and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A sludge-based saline-alkali land conditioner, characterized in that, It is prepared by wet cross-linking reaction of sludge from urban wastewater treatment plants and calcium silicate slag followed by dehydration and aging; it contains a three-dimensional network cross-linked structure composed of humic acid-calcium ions-aluminosilicate as the continuous phase, and encapsulates unreacted calcium silicate slag particles as the dispersed phase; it retains unconverted inert organic matter and the nitrogen, phosphorus and potassium nutrients inherent in sludge; it is rich in carboxyl and phenolic hydroxyl active sites, and the cation exchange capacity is greater than 150 mmol / kg.

2. A method for preparing a sludge-based saline-alkali land conditioner, used to prepare the sludge-based saline-alkali land conditioner as described in claim 1, characterized in that, Includes the following steps: S1. Raw material pretreatment: Dewater the sludge from the municipal wastewater treatment plant to a moisture content of 75%~85%; grind the calcium silicate slag to a particle size of less than 200 mesh; S2. Slurry mixing: The pretreated sludge and calcium silicate slag are added to the reactor at a dry basis mass ratio of 1:(0.8~1.2), and water is added to adjust the solid content of the system to 20%~30%; S3. Hydrothermal crosslinking reaction: The reactor is sealed and heated to 65℃~80℃. At this temperature, the reaction is carried out with constant mechanical stirring for 4 to 10 hours. The weakly alkaline environment generated by the hydration of calcium silicate slag promotes the dissolution of organic matter in the sludge and crosslinks with the active calcium silicate to form a three-dimensional network gel. S4. Solid-liquid separation and aging: Dehydrate the reactants to a solid moisture content of 35%~45%, then age them under sealed conditions for more than 12 hours, and crush them into particles with a particle size of 1mm~10mm to obtain the final product.

3. The method for preparing the sludge-based saline-alkali land conditioner according to claim 2, characterized in that, The calcium silicate slag is selected from one or more of fly ash alumina extraction tailings or dealkali-removed steel slag; wherein the mass fraction of β-dicalcium silicate (β-C2S) in the fly ash alumina extraction tailings is greater than 40%.

4. The method for preparing the sludge-based saline-alkali land conditioner according to claim 2, characterized in that, The hydrothermal crosslinking reaction described in step S3 is carried out in a closed reactor.

5. The method for preparing the sludge-based saline-alkali land conditioner according to claim 2, characterized in that, The rotation speed of the continuous mechanical stirring in step S3 is 50~150 rpm.

6. The method for preparing the sludge-based saline-alkali land conditioner according to claim 2, characterized in that, The release of OH- during the hydration of the calcium silicate slag in step S3 - Maintain the pH of the system between 9.0 and 11.

0.

7. The method for preparing the sludge-based saline-alkali land conditioner according to claim 2, characterized in that, The dewatering described in step S4 is carried out mechanically using a plate and frame filter press, and the moisture content of the dewatered cake is less than 50%.

8. The application of the sludge-based saline-alkali land conditioner of claim 1 in the remediation of saline soil or acidic mine wasteland.

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