Preparation method of engineering soil

Abstract

The invention discloses a preparation method of engineering soil, and belongs to the field of comprehensive utilization of industrial wastes. The method comprises the following steps: sending alkali residue waste liquid and fly ash slurry into a barrel-shaped mixer, uniformly mixing to obtain a mixed material liquid, sending the mixed material liquid into a clarification device, clarifying and precipitating to obtain a supernatant liquid and lower concentrated slurry, adding citric acid into the clarification device, adjusting the pH value of the lower concentrated slurry to alkalescence, and filtering to obtain a filtrate; and finally, carrying out filter pressing on the lower-layer concentrated slurry to obtain the engineering soil. The engineering soil prepared through the method is high in compressive strength and good in stability, the plasticity index of the engineering soil is close to that of silty soil, and the engineering soil can be used for dredger fill or backfill.

Description

Technical Field

[0001] The present invention belongs to the field of comprehensive utilization of industrial waste, and particularly relates to a production process for engineering soil made from waste alkali residue generated in the production of soda ash. Background Art

[0002] During the production process of many soda ash plants, a large amount of alkali residue waste liquid is generated. The alkali residue waste liquid is discharged to the slag yard, and after natural sedimentation, the waste liquid is deposited as solid alkali residue. The waste clear liquid that seeps out during the deposition process mainly contains calcium chloride, abbreviated as "calcium liquid", which is concentrated and used for the production of calcium chloride. The stored alkali residue is a white paste with a high water content. It has fine particles, strong alkalinity, viscosity, and a high chloride ion content. After natural dehydration, it forms a "white mountain". Since leakage and piping phenomena may occur during the operation of the slag yard, which are likely to cause dam breakage, the safe and stable operation of the slag yard directly affects the safety and environmental protection of the surrounding areas.

[0003] During the local economic construction process, a large amount of engineering soil is required for the subgrade of roads or the foundation filling of yards. At the same time, since the main components of alkali residue are CaCO 3 , CaCl 2 , CaO, SiO 2 , CaSO 4 , NaCl, H 2 O and other substances, which are similar to the main components of engineering soil and have the basic conditions for being prepared into engineering soil. If low-cost mixed engineering soil made from alkali residue can be used as fill soil or backfill soil, it can not only make use of waste but also solve the environmental protection problem.

[0004] For example, the Chinese patent document with the publication number CN111606635A discloses a method for preparing engineering soil using ammonia-soda process alkali residue. This method is to sequentially perform sediment treatment, concentration treatment, homogenization modification, dehydration treatment, and stacking treatment on the alkali residue to obtain engineering soil. The specific steps of homogenization modification are: adding calcareous materials and pozzolanic active materials to the concentrated alkali residue and stirring evenly. The defects of such methods are as follows: ① Solid-solid mixing, with poor stirring effect, it is difficult to achieve true uniform mixing. The uneven mixing causes the non-uniform distribution of the complex formed by the reaction, and the effect of improving the skeleton structure is not good, and the problem of low strength caused by large pores in the alkali residue is not fundamentally solved, and the increased compressive resistance is limited; ② After adding calcareous materials and pozzolanic active materials to the alkali residue, due to the temperature and other environmental conditions not being in the complex reaction range, the reaction effect is poor, the hydrophilicity is reduced little, and the viscosity changes little; ③ Due to the poor complex reaction effect, the consumption of anions such as chloride ions is small, and the chloride ions continue to form pollution factors. Summary of the Invention

[0005] The object of the present invention is to provide a method for preparing engineering soil, which has simple operation, short process flow, and the prepared engineering soil has strong compressive strength and good stability, and can meet the requirements of large-scale engineering treatment.

[0006] To achieve the above object, the present invention adopts the following technical solutions.

[0007] A method for preparing engineering soil includes the following steps: (1) Mix fly ash and water by stirring to obtain a fly ash slurry, and control the solid content of the fly ash slurry at 15-20 wt%. (2) Feed the alkali residue waste liquid with a solid content of 15-20 wt% and the fly ash slurry into a circular mixer, and after mixing evenly, obtain a mixed liquid. (3) Send the mixed liquid to a clarification device with a temperature control system. After clarification and sedimentation, obtain the upper clear liquid and the lower concentrated slurry; and add citric acid to the clarification device to adjust the pH value of the lower concentrated slurry to weakly alkaline. (4) Feed the lower concentrated slurry into a pressure filtration device. After pressure filtration, obtain engineering soil, and control the water content of the engineering soil at 30-50%.

[0008] Preferably, in the step (2), the solid contents of the fly ash slurry and the alkali residue waste liquid are the same; the volume ratio of the alkali residue waste liquid to the fly ash slurry is 1.5-4:1; more preferably, the volume ratio of the alkali residue waste liquid to the fly ash slurry is 7 / 3:1.

[0009] Preferably, in the step (3), the addition amount of citric acid is 0.001-0.003% of the mass of the mixed liquid.

[0010] Preferably, in the step (3), the temperature of the mixed liquid is 30-50 °C.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The technical solution of the present invention mixes the alkali residue waste liquid and the fly ash slurry in a liquid phase manner. Compared with the traditional solid-solid mixing, the fly ash particles are mixed more evenly with the alkali residue, improving the complexation reaction effect, reducing the water content of the engineering soil, and increasing the compressive strength of the engineering soil.

[0012] 2. The present invention forms a complex by the complexation reaction of the active substances in the fly ash and anions such as chloride ions in the alkali residue to reduce the salt content and viscosity of the alkali residue, improve the dehydration efficiency of the alkali residue, and further reduce the salt content and water content of the engineering soil, and increase the compressive strength and stability of the engineering soil.

[0013] 3. By adding citric acid to the mixture liquid, the present invention adjusts the pH value of the lower-layer concentrated slurry to weakly alkaline to reduce the alkalinity of the engineering soil and mitigate environmental pollution; citric acid can also increase the solidification degree of the gel product formed by the complexation reaction, further improving the compressive strength of the engineering soil.

[0014] 4. The present invention controls the temperature of the mixture liquid at 30 - 50 °C to further improve the effect of the complexation reaction.

[0015] 5. The present invention simplifies the process flow. The alkali residue does not need to be dehydrated and is directly mixed with the coal ash water prepared from fly ash, reducing intermediate links and lowering the operation complexity and cost.

[0016] 6. The present invention adopts a cylindrical structure during the mixing process, which can make full use of the eddy current effect and does not require an external stirrer. Description of the Drawings

[0017] Figure 1 The cross-sectional view of the engineering soil prepared in Example 1 after low-temperature drying; Figure 2 The cross-sectional view of the engineering soil prepared in Comparative Example 1 after low-temperature drying; Figure 3 The cross-sectional view of the engineering soil prepared in Comparative Example 2 after low-temperature drying. Detailed Embodiments

[0018] The following further elaborates on the technical solutions of the present invention in conjunction with the embodiments, but it does not limit the present invention. It should be noted that the reagents and raw materials used in the embodiments of the present invention are all commercially available conventional products unless otherwise specified.

[0019] Situation of the main raw materials used: (1) Characteristics of the alkali residue waste liquid: The alkali residue waste liquid generated during the local ammonia-alkali soda production process is used, and the main components are as shown in the following table.

[0020]

[0021] (2) Characteristics of fly ash: The fine particle waste residue generated after burning coal in the local thermal power plant is used.

[0022] (3) The treatment agent citric acid, an industrial-grade product. Example 1

[0023] (1) Fly ash and water are mixed in a spiral ash mixer to obtain a fly ash slurry with a solid content of 18 wt%; (2) The alkali residue waste liquid with a solid content of 18 wt% and the fly ash slurry are fed into a cylindrical mixer according to a volume ratio of 7:3 and mixed evenly to obtain a mixture liquid; (3) Feed the mixed material liquid into a clarification tank equipped with a temperature control system, adjust the temperature of the mixed material liquid to 40 °C, and clarify and settle for 2.5 h to obtain the upper clear liquid and the lower concentrated slurry. (4) Add citric acid into the clarification tank to adjust the pH value of the lower concentrated slurry to weakly alkaline; the addition amount of citric acid is 0.002% of the mass of the mixed material liquid. (5) Feed the lower concentrated slurry into a filter press, and after filtration, obtain engineering soil with a moisture content of 40%. It can be directly applied after being stacked and naturally dried. Example 2

[0024] (1) Mix fly ash and water in a spiral ash blending device to prepare a fly ash slurry with a solid content of 20 wt%. (2) Feed the alkali residue waste liquid with a solid content of 20 wt% and the fly ash slurry into a cylindrical mixer according to a volume ratio of 6:4, and mix evenly to obtain a mixed material liquid. (3) Feed the mixed material liquid into a clarification tank equipped with a temperature control system, adjust the temperature of the mixed material liquid to 30 °C, and clarify and settle for 2.5 h to obtain the upper clear liquid and the lower concentrated slurry. (4) Add citric acid into the clarification tank to adjust the pH value of the lower concentrated slurry to weakly alkaline; the addition amount of citric acid is 0.001% of the mass of the mixed material liquid. (5) Feed the lower concentrated slurry into a filter press, and after filtration, obtain engineering soil with a moisture content of 50%. Example 3

[0025] (1) Mix fly ash and water in a spiral ash blending device to prepare a fly ash slurry with a solid content of 15 wt%. (2) Feed the alkali residue waste liquid with a solid content of 15 wt% and the fly ash slurry into a cylindrical mixer according to a volume ratio of 8:2, and mix evenly to obtain a mixed material liquid. (3) Feed the mixed material liquid into a clarification tank equipped with a temperature control system, adjust the temperature of the mixed material liquid to 50 °C, and clarify and settle for 2.5 h to obtain the upper clear liquid and the lower concentrated slurry. (4) Add citric acid into the clarification tank to adjust the pH value of the lower concentrated slurry to weakly alkaline; the addition amount of citric acid is 0.003% of the mass of the mixed material liquid. (5) Feed the lower concentrated slurry into a filter press, and after filtration, obtain engineering soil with a moisture content of 30%. Comparative Example 1

[0026] The difference between Comparative Example 1 and Example 1 is that step (1) is not included, and in step (2), solid fly ash and alkali residue waste liquid are directly added into the cylindrical mixer, and the other steps are the same as those in Example 1. Comparative Example 2

[0027] The difference between Comparative Example 2 and Example 1 lies in that in step (2), the alkali residue waste liquid and fly ash slurry are fed into a square mixer with stirring according to a volume ratio of 7:3, and mixed evenly to obtain a mixed material liquid. Comparative Example 3

[0028] The difference between Comparative Example 3 and Example 1 lies in that in step (4), the addition amount of citric acid is 0.0005% of the mass of the mixed material liquid. Comparative Example 4

[0029] The difference between Comparative Example 4 and Example 1 lies in that in step (4), the addition amount of citric acid is 0.005% of the mass of the mixed material liquid. Comparative Example 5

[0030] The difference between Comparative Example 5 and Example 1 lies in that in step (3), the temperature of the mixed material liquid is controlled at 20°C. Comparative Example 6

[0031] The difference between Comparative Example 6 and Example 1 lies in that in step (3), the temperature of the mixed material liquid is controlled at 60°C. Test Example 1

[0032] The engineering soil prepared in Example 1, Comparative Example 1, and Comparative Example 2 was sampled, dried at low temperature, and then compared. For the cross-sectional view, see the appendix Figures 1-3 : As can be seen from the three pictures, there is no stratification phenomenon after precipitation in Example 1, while obvious stratification phenomena can be observed in Comparative Example 1 and Comparative Example 2. This shows that in the liquid-phase mixing and circular mixing tank, the two substances of alkali residue and fly ash can be fully mixed evenly; the mixing methods in Comparative Example 1 and Comparative Example 2 cannot fully mix the two substances of alkali residue and fly ash, resulting in the complexing reactants not being fully filled in the alkali residue skeleton, and thus the alkali residue skeleton structure cannot be completely improved. Test Example 2

[0033] The engineering soil samples of Examples 1-3 and Comparative Examples 1-6 were taken for engineering soil index tests, and the test methods were carried out in accordance with the Standard for Geotechnical Test Methods GB / T 50123-2019.

[0034]

[0035] When the engineering soil is used as hydraulic fill or backfill soil, the closer the plasticity index of the engineering soil is to the plasticity index range of silty clay, which is 7-17, the better the use effect. At the same time, the requirements for the engineering soil in the Industry Standard JTG / T F20 Technical Rules for Construction of Highway Pavement Bases are that the unconfined compressive strength for 7 days exceeds 0.5 MPa and the bearing ratio exceeds 8%.

[0036] Analysis of the above data shows that: In Examples 1 to 3, the plasticity index is close to 17, and the data of Example 1 is the best. Except for Comparative Example 4, the plasticity indices of other comparative examples are much higher than 17, belonging to the range of highly cohesive soil and not being suitable for directly used as hydraulic fill or backfill soil. The main reason is that the mixing method of Examples 1 to 3 and Comparative Example 4 can evenly mix alkali residue and fly ash. Controlling the temperature at 30 - 50 °C is to ensure the occurrence of complexation reaction. Through the complexation reaction, the salt content of alkali residue is reduced, and at the same time, the complexation reactants can evenly fill in the alkali residue skeleton, improving the skeleton structure and making the plasticity index reach the range of the plasticity index of silty soil.

[0037] The data in Examples 1 to 3 and Comparative Example 4 show that the unconfined compressive strength at 7 days exceeds 0.5 MPa and the bearing ratio exceeds 8%, both meeting the requirements, and Example 1 has the best effect. In Comparative Examples 1 and 2, due to uneven mixing, there is no complexation reaction in some parts of the mixture, and the improvement of alkali residue is imperfect, not meeting the industry standard requirements. In Comparative Example 3, due to the small addition amount of citric acid, which is 0.0005% of the mixture liquid, the formation of strengthening complexes and the increase in strength are insufficient. Therefore, the data of the unconfined compressive strength at 7 days and the bearing ratio are not within the required range. In Comparative Example 4, after the addition amount of citric acid is increased to 0.005% of the mixture liquid, the formation of strengthening complexes and the increase in strength are basically unchanged, only the pH value continues to decrease, indicating that the addition amount is appropriate within the range of 0.001% - 0.003%. In Comparative Example 5, it shows that when the temperature of the clarification tank is controlled at 20 °C, the complexation reaction effect is not good, resulting in the data not being within the required range. In Comparative Example 6, it shows that when the temperature of the clarification tank is controlled at 60 °C, the complexation reaction effect is also not good, resulting in the data not being within the required range.

[0038] From the data of soluble salts and chloride ion content, it can be seen that both the salt content and the chloride ion content decrease significantly. The main reasons are that part of it is carried away by the dewatered clear liquid, and in addition, the complexes formed by chloride ions, sulfate ions, etc. in fly ash and alkali residue play a good wrapping effect, achieving solidification stability and reducing the salt content and hydrophilicity. From the data, it can be seen that the complexation reaction can greatly reduce the salt content and chloride ion content in alkali residue. Among Examples 1 - 3 and Comparative Example 4, due to the liquid-phase mixing method, the mixing is the most uniform and the complexation reaction effect is the best. Therefore, the reduction degree of the data of soluble salts and chloride ion content is the best. After being applied as engineering soil, chloride ions basically will not cause secondary pollution. Test Example 3

[0039] Samples of Examples 1 to 3 and Comparative Examples 3 to 4 were taken for strength testing. According to the Standard for Geotechnical Test Methods (GB / T50123 - 1999), the light compaction test was adopted, and the test data are as follows in the table:

[0040] Analyzing the above data, it can be known that: The maximum dry density data of Comparative Examples 3-4 and Example 1 were compared, indicating that during the continuous increase of the addition amount of citric acid from 0.0005% to 0.003%, it has the effect of increasing the strength of the engineered soil produced. However, after the addition amount exceeds 0.003% to 0.005%, the strength of the engineered soil no longer increases, and it only plays the role of reducing the pH value. It can be determined that the addition amount of 0.2% is the optimal amount. Compared with general engineered soil, the dry density of alkali residue soil is smaller (lighter self-weight). Therefore, if it is used as backfill soil, the additional stress generated on the original foundation is smaller, and the corresponding additional deformation is also smaller, which is beneficial to the project.

Claims

1. A method for preparing engineering soil, characterized in that: The following steps are involved: (1) mixing fly ash and water to obtain fly ash slurry, wherein the solid content of the fly ash slurry is controlled at 15 to 20 wt %; (2) sending the alkali residue waste liquid with a solid content of 15-20wt% and the fly ash slurry into a drum mixer and mixing them evenly to obtain a mixed liquid; (3) The mixed liquid is sent to a clarification device with a temperature control system, and after clarification and sedimentation, an upper clear liquid and a lower concentrated slurry are obtained; and citric acid is added to the clarification device to adjust the pH value of the lower concentrated slurry to a weak alkaline state; (4) The lower layer of concentrated slurry is filtered to obtain engineering soil, wherein the moisture content of the engineering soil is controlled at 30-50%.

2. The preparation method according to claim 1, characterized in that: In the step (2), the solid content of the fly ash slurry and the alkali slag waste liquid is the same; the volume ratio of the alkali slag waste liquid to the fly ash slurry is 1.5 to 4:

1.

3. The preparation method according to claim 2, characterized in that: In the step (2), the volume ratio of the alkali residue waste liquid to the fly ash slurry is 7 / 3:

1.

4. The preparation method according to claim 1, characterized in that: In the step (3), the amount of citric acid added is 0.001 to 0.003% of the mass of the mixed liquid.

5. The preparation method according to claim 1, characterized in that: In the step (3), the temperature of the mixed liquid is 30 to 50°C.

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