Composite cementitious material and method for immobilizing heavy metals

Abstract

The present application relates to the technical field of wastewater treatment, and particularly relates to a composite cementing material and a method for fixing heavy metals. The method for fixing heavy metals comprises the following steps: S1: adjusting the pH value of a heavy metal-containing solution to 1-3 to obtain mixed water A; S2: mixing inorganic cementing material, mixed water A and mixed water B to obtain mixed slurry, and then curing; wherein the inorganic cementing material comprises silicate cement clinker, sulphoaluminate cement clinker and gypsum; and the mixed water B is a calcium silicate hydrate aqueous solution. By optimizing the pH value of the heavy metal-containing solution during the fixing of heavy metals, the present application can effectively enhance the anti-leaching effect of the cement-based cementing material after fixing heavy metals, reduce the leaching concentration of heavy metals, and improve the mechanical properties of the cementing material after fixing heavy metals.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a composite cementitious material and a method for fixing heavy metals thereon. Background Technology

[0002] Cadmium wastewater is characterized by its high toxicity, difficulty in treatment, and high treatment costs. It mainly comes from mining, smelting, electrolysis, electroplating and other fields. Its concentration fluctuates greatly and it is highly polluting. If it is not strictly treated before discharge, it will pose a serious threat to the environment and human health.

[0003] Currently, the treatment of industrial wastewater containing cadmium heavy metals mainly includes chemical precipitation, ion exchange, electrolysis, adsorption, and cementitious material solidification. Among these, using cement-based cementitious materials as the solidification matrix for heavy metal-containing wastewater solidification has the advantages of readily available materials, simple processes, low cost, and significant solidification effect. Cadmium ions in cadmium-containing wastewater can achieve solidification through ion exchange, electrostatic adsorption, and complexation with the main hydration products such as CSH gel and ettringite produced by cement hydration. The alkaline hydration environment inside the cement can cause cadmium ions to form hydroxide precipitates, which are then physically encapsulated by the hardened cement blocks generated by the hydration reaction, thus preventing the leaching of heavy metal ions. However, the solidified blocks formed after solidification of heavy metals with cement-based cementitious materials have disadvantages such as easy leaching of heavy metals under the influence of the external environment and a significant decrease in the mechanical strength of the solidified cementitious material. The solidification effect still has room for further improvement. Due to its poor performance, the solidified cementitious material is usually only treated as solid waste and simply piled up or landfilled, which to some extent leads to resource waste.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a composite cementitious material and a method for fixing heavy metals therein, which enhances the anti-leaching effect of heavy metals in cement-based cementitious materials after curing, while also improving the mechanical strength of the cured body and increasing the resource utilization potential of the cementitious material after curing heavy metals.

[0006] This invention first provides a method for solidifying heavy metals, comprising:

[0007] S1: Adjust the pH value of the heavy metal-containing solution to 1-3 to obtain mixing water A;

[0008] S2: Mix the inorganic cementitious material, mixing water A and mixing water B to obtain a mixed slurry, and then cure it.

[0009] The inorganic cementitious material includes silicate cement clinker, sulfoaluminate cement clinker, and gypsum; the mixing water B is an aqueous solution of hydrated calcium silicate.

[0010] This invention unexpectedly discovered that by optimizing the pH value of the heavy metal-containing solution during the curing of heavy metals, the leaching prevention effect of heavy metals in cement-based cementitious materials after curing can be effectively enhanced, reducing the leaching concentration of heavy metals, while also improving the mechanical properties of the cementitious materials after curing heavy metals. Furthermore, this invention found that by introducing calcium silicate hydrate into the cementitious material system and adjusting the generation ratio of hydration products, the curing effect of the cementitious material on heavy metals and the strength of the cured body after curing heavy metals can be further enhanced.

[0011] Preferably, nitric acid is used to adjust the pH of the heavy metal-containing solution to 1-3.

[0012] This invention discovers that nitrate ions can form the AFm phase before sulfate ions during the hydration process of cementitious materials, inhibiting the transformation of ettringite to the sulfate-type AFm phase, improving the stability of ettringite, and thus improving performance. Simultaneously, this invention also finds that nitrate ions have a low impact on the hydration of cementitious materials, have high solubility, and have no adverse effect on the solidification of heavy metals in cementitious materials.

[0013] Preferably, when the mass ratio of sulfoaluminate cement clinker to gypsum in the inorganic cementitious material is 1:1 to 2, the cementitious material exhibits superior mechanical properties and volume stability.

[0014] Preferably, the inorganic cementitious material comprises 79-91 parts of silicate cement clinker, 3-7 parts of sulfoaluminate cement clinker, and 6-14 parts of gypsum.

[0015] In this invention, the gypsum is preferably anhydrite.

[0016] Preferably, during the curing process, spraying the surface with a saturated calcium hydroxide solution 5 to 10 times within a curing period of 1 to 7 days until the surface is wetted can help form a dense calcium carbonate carbonation protective layer on the surface of the cured body, prevent the leaching of cadmium ions from the slurry during the curing process, and at the same time improve the durability of the material and the resource utilization potential of the cured body.

[0017] Preferably, the ratio of the total mass of mixing water A and mixing water B in the mixed slurry to the mass of the inorganic cementitious material is (0.3~0.45):1.

[0018] Preferably, the concentration of heavy metals in the heavy metal-containing solution is 600–30000 mg / L.

[0019] More preferably, when the concentration of heavy metals in the heavy metal-containing solution is 20,000 to 30,000 mg / L, the mechanical properties of the solidified body can be further improved.

[0020] In this invention, the heavy metal-containing solution can be industrial wastewater containing heavy metals; preferably, the heavy metal solution is cadmium-containing wastewater.

[0021] This invention discovers that cadmium ions can replace Ca in the hydration products of cement-based materials. 2+ This causes Ca in the pore fluid to 2+ Increased concentration promotes early hydration of cement; at the same time, the addition of cadmium ions does not affect the later hydration process of cement materials.

[0022] Preferably, the mixing water B is a 4-6 wt% aqueous solution of hydrated calcium silicate.

[0023] In this invention, when mixing the inorganic cementitious material, mixing water A, and mixing water B, conventional stirring equipment in the art can be selected. Preferably, in the mixing process of S2, a stirring scheme of first stirring at low speed, then letting stand, and then stirring at high speed is adopted. More preferably, in the mixing process of S2, the stirring is first carried out at 30-60 r / min for 100-150 s, then let stand for 10-20 s, and finally stirred at 100-150 r / min for 100-150 s.

[0024] As a preferred embodiment of the present invention, the method for solidifying heavy metals includes:

[0025] a. Adjust the pH of the cadmium-containing wastewater to 0.8–1.5 using nitric acid to obtain mixing water A, and prepare a 4–6 wt% hydrated calcium silicate aqueous solution to obtain mixing water B;

[0026] b. Mix 79-91 parts of silicate cement clinker, 3-7 parts of sulfoaluminate cement clinker and 6-14 parts of gypsum with the mixing water A and mixing water B. First, stir at 30-60 r / min for 100-150 s, then let stand for 10-20 s, and finally stir at 100-150 r / min for 100-150 s to obtain a mixed slurry.

[0027] d. Inject the mixed slurry into the mold, cure for 12-24 hours and then demold. Spray with saturated calcium hydroxide solution 5-10 times during the 1-7 day curing period, with each spray lasting 30-60 seconds until the surface is wetted, and finally obtain the hardened solidified body.

[0028] In this invention, when mixing the inorganic cementitious material, mixing water A, and mixing water B, different mixing methods can be selected. Preferably, the inorganic cementitious material is uniformly mixed, divided into two equal portions, and mixing water A and mixing water B are added to each portion to obtain two slurry mixtures. Then, the two slurry mixtures are mixed and injected into a mold.

[0029] The present invention further provides a composite cementitious material, which is prepared by the above-mentioned method for curing heavy metals.

[0030] Based on the above technical solution, the beneficial effects of the present invention are as follows:

[0031] This invention optimizes the pH value of the heavy metal-containing solution during the curing of heavy metals, which can effectively enhance the anti-leaching effect of heavy metals in cement-based cementitious materials after curing, reduce the leaching concentration of heavy metals, and improve the mechanical properties of cementitious materials after curing heavy metals. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0033] Unless otherwise specified, all raw materials used in the embodiments are commercially available conventional raw materials, and the technical means used are conventional means well known to those skilled in the art.

[0034] The main chemical compositions of silicate clinker, sulfoaluminate clinker, and gypsum in the following examples and comparative examples are shown in Table 1 below:

[0035] Table 1

[0036]

[0037]

[0038] Example 1

[0039] This embodiment provides a method for fixing heavy metals, which specifically includes the following steps:

[0040] The pH of the cadmium-containing wastewater with a concentration of 30,000 mg / L was adjusted to 1.00 with pure nitric acid to obtain mixing water A. The following raw materials were weighed: 352 g silicate clinker, 16 g sulfoaluminate clinker, 32 g gypsum, 66 g water, and 70 g mixing water A. 4 g of synthetic hydrated calcium silicate was added to the water, and the mixture was ultrasonically vibrated for 10 min to obtain mixing water B. The uniformly mixed powder was divided into two equal portions, and added to mixing water A and mixing water B respectively. The mixtures were stirred at low speed for 120 s in a mixing pot, allowed to stand for 15 s, and then stirred at high speed for 120 s. The two cement paste mixtures were then combined and poured into the mixing pot, stirred at low speed for 60 s to obtain a mixed slurry.

[0041] The mixed slurry was then placed into a mold and cured for 24 hours before being demolded. It was then cured for 1, 3, and 7 days respectively. During the 1, 3, and 7-day curing periods, a saturated calcium hydroxide solution was sprayed 5, 7, and 10 times respectively, with each spraying lasting 30 seconds, to obtain composite cementitious material test blocks.

[0042] The preparation process of the cementitious material used to determine setting time is as follows: The pH of the cadmium-containing wastewater with a concentration of 30000 mg / L was adjusted to 1.00 with pure nitric acid to obtain mixing water A. The following raw materials were weighed: 440 g of silicate clinker, 20 g of sulfoaluminate clinker, 40 g of gypsum, 82.5 g of water, and 87.5 g of mixing water A. 5 g of synthetic hydrated calcium silicate was added to the water, and the mixture was ultrasonically vibrated for 10 min to obtain mixing water B. The uniformly mixed powder was divided into two equal portions and added to mixing water A and mixing water B respectively. The mixture was stirred at low speed for 120 s in a mixing pot, allowed to stand for 15 s, and then stirred at high speed for 120 s. The two slurry mixtures were then combined and poured into a mixing pot. After stirring at low speed for 60 s, the setting time was determined according to GB / T1346-2011 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement".

[0043] Example 2

[0044] This embodiment provides a method for fixing heavy metals, which specifically includes the following steps:

[0045] The pH of the cadmium-containing wastewater with a concentration of 30,000 mg / L was adjusted to 3.00 with pure nitric acid to obtain mixing water A. The following raw materials were weighed: 340 g of silicate clinker, 20 g of sulfoaluminate clinker, 40 g of gypsum, 66 g of water, and 70 g of mixing water A. 4 g of synthetic hydrated calcium silicate was added to the water, and the mixture was ultrasonically vibrated for 10 min to obtain mixing water B. The uniformly mixed powder was divided into two equal portions, and added to mixing water A and mixing water B respectively. The mixtures were stirred at low speed for 120 s in a mixing pot, allowed to stand for 15 s, and then stirred at high speed for 120 s. The two slurry mixtures were then combined and poured into a mixing pot, stirred at low speed for 60 s to obtain a mixed slurry.

[0046] The mixed slurry was then placed into a mold and cured for 24 hours before being demolded. It was then cured for 1, 3, and 7 days respectively. During the 1, 3, and 7-day curing periods, a saturated calcium hydroxide solution was sprayed 5, 7, and 10 times respectively, with each spraying lasting 30 seconds, to obtain composite cementitious material test blocks.

[0047] The preparation process of the cementitious material used to determine setting time is as follows: The pH of the cadmium-containing wastewater with a concentration of 30000 mg / L was adjusted to 3.00 with pure nitric acid to obtain mixing water A. The following raw materials were weighed: 425 g of silicate clinker, 25 g of sulfoaluminate clinker, 50 g of gypsum, 82.5 g of water, and 87.5 g of mixing water A. 5 g of synthetic hydrated calcium silicate was added to the water, and the mixture was ultrasonically vibrated for 10 min to obtain mixing water B. The uniformly mixed powder was divided into two equal portions and added to mixing water A and mixing water B respectively. The mixture was stirred at low speed for 120 s in a mixing pot, allowed to stand for 15 s, and then stirred at high speed for 120 s. The two slurry mixtures were then combined and poured into a mixing pot and stirred at low speed for 60 s. The setting time was then determined according to GB / T1346-2011 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement".

[0048] Example 3

[0049] This embodiment provides a method for fixing heavy metals, which specifically includes the following steps:

[0050] The pH of the cadmium-containing wastewater with a concentration of 600 mg / L was adjusted to 1.00 with pure nitric acid to obtain mixing water A. The following raw materials were weighed: 352 g of silicate clinker, 16 g of sulfoaluminate clinker, 32 g of gypsum, 66 g of water, and 70 g of mixing water A. 4 g of synthetic hydrated calcium silicate was added to the water, and the mixture was ultrasonically vibrated for 10 min to obtain mixing water B. The uniformly mixed powder was divided into two equal portions, and added to mixing water A and mixing water B respectively. The mixtures were stirred at low speed for 120 s in a mixing pot, allowed to stand for 15 s, and then stirred at high speed for 120 s. The two slurry mixtures were then combined and poured into a mixing pot, stirred at low speed for 60 s to obtain a mixed slurry.

[0051] The mixed slurry was then placed into a mold and cured for 24 hours before being demolded. It was then cured for 1, 3, and 7 days respectively. During the 1, 3, and 7-day curing periods, a saturated calcium hydroxide solution was sprayed 5, 7, and 10 times respectively, with each spraying lasting 30 seconds, to obtain composite cementitious material test blocks.

[0052] The preparation process of the cementitious material used to determine setting time is as follows: The pH of the cadmium-containing wastewater with a concentration of 600 mg / L was adjusted to 1.00 with pure nitric acid to obtain mixing water A. The following raw materials were weighed: 440 g of silicate clinker, 20 g of sulfoaluminate clinker, 40 g of gypsum, 82.5 g of water, and 87.5 g of mixing water A. 5 g of synthetic hydrated calcium silicate was added to the water, and the mixture was ultrasonically vibrated for 10 min to obtain mixing water B. The uniformly mixed powder was divided into two equal portions and added to mixing water A and mixing water B respectively. The mixture was stirred at low speed for 120 s in a mixing pot, allowed to stand for 15 s, and then stirred at high speed for 120 s. The two slurry mixtures were then combined and poured into a mixing pot. After stirring at low speed for 60 s, the setting time was determined according to GB / T1346-2011 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement".

[0053] Example 4

[0054] This embodiment provides a method for fixing heavy metals, which differs from Embodiment 1 only in that: the inorganic cementitious material used for curing includes 320g of silicate cement clinker, 48g of sulfoaluminate clinker, and 32g of gypsum; the inorganic cementitious material used for setting time determination includes 400g of silicate cement clinker, 60g of sulfoaluminate clinker, and 40g of gypsum.

[0055] Comparative Example 1

[0056] This comparative example provides a method for fixing heavy metals, which differs from Example 1 only in that the pH of the cadmium-containing wastewater is adjusted to 4.00 with pure nitric acid.

[0057] Comparative Example 2

[0058] This comparative example provides a method for fixing heavy metals, which differs from Example 1 only in that: the inorganic cementitious material used for curing includes 368g of silicate clinker and 32g of gypsum; the inorganic cementitious material used for setting time determination includes 460g of silicate cement clinker and 40g of gypsum.

[0059] Test case

[0060] The present invention further tests the curing stability, compressive strength, and setting time of the composite cementitious material specimens obtained in the examples and comparative examples. Specifically:

[0061] 1. Curing stability

[0062] The composite cementitious material test blocks obtained in the examples and comparative examples were tested. Test blocks with a curing age of 7 days were crushed, hydration was terminated with isopropanol, and they were vacuum dried in a 40℃ vacuum drying oven. Leachate was prepared according to HJ / T300-2007 "Leaching Toxicity Method for Solid Waste - Acetic Acid Buffer Solution Method," and the cadmium ion concentration in the leachate was tested. The cadmium ion leaching concentration is shown in the table below:

[0063] Table 2

[0064]

[0065] The results showed that the cadmium ion leaching concentration of the cementitious materials in both the examples and the comparative examples met the requirements of GB5085.3-2007 "Identification Standard for Hazardous Waste - Leaching Toxicity Identification". The cadmium ion leaching concentration of the cementitious material in Example 3 was lower than that in GB5749-2006 "Standards for Drinking Water Quality". Comparing Examples 1 and 2 with Comparative Example 1, when the pH was greater than 3, the cadmium ion leaching concentration increased with increasing pH; there was no significant change in leaching concentration between Example 1 and Example 4; and the leaching concentration of Comparative Example 2 was significantly increased compared to Example 1.

[0066] 2. Compressive strength and setting time

[0067] Referring to GB / T17671-1999 "Test Method for Strength of Cement Mortar", the compressive strength of the composite cementitious material specimens obtained in the examples and comparative examples was tested; the setting time of the mixed slurry in the examples and comparative examples was determined according to GB / T1346-2011 "Test Method for Standard Consistency Water Requirement, Setting Time and Soundness of Cement". The results of their strength and setting time are shown in the table below:

[0068] Table 3

[0069]

[0070]

[0071] Table 4

[0072]

[0073] The results showed that Example 1 had the highest early strength, with a compressive strength of 58 MPa after 1 day. Examples 1 and 2 had higher compressive strengths than Comparative Examples 1 and 2 within 7 days, and their setting times were shorter than those of Comparative Examples 1 and 2, which helps to quickly fix heavy metals in the early stages of hydration and reduce the risk of heavy metal leaching in the early stages. Example 3 had lower compressive strength than Examples 1 and 2, and a longer setting time than Examples 1, 2, and 4, but shorter than Comparative Example 2. However, its strength after 7 days was higher than that of Comparative Example 1. The setting time of Example 4 was slightly longer than that of Examples 1 and 2, but shorter than that of Comparative Examples 1 and 2.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for solidifying heavy metals, characterized in that, include: S1: Adjust the pH value of the heavy metal-containing solution to 1-3 to obtain mixing water A; S2: Mix the inorganic cementitious material, mixing water A and mixing water B to obtain a mixed slurry, and then cure it. The inorganic cementitious material includes silicate cement clinker, sulfoaluminate cement clinker, and gypsum; the mixing water B is an aqueous solution of hydrated calcium silicate. The heavy metal solution is wastewater containing cadmium ions.

2. The method for solidifying heavy metals according to claim 1, characterized in that, The pH of the heavy metal-containing solution was adjusted to 1–3 using nitric acid.

3. The method for solidifying heavy metals according to claim 1 or 2, characterized in that, In the inorganic cementitious material, the mass ratio of sulfoaluminate cement clinker to gypsum is 1:1 to 2.

4. The method for solidifying heavy metals according to any one of claims 1 to 3, characterized in that, The inorganic cementitious material comprises 79-91 parts of silicate cement clinker, 3-7 parts of sulfoaluminate cement clinker, and 6-14 parts of gypsum.

5. The method for solidifying heavy metals according to any one of claims 1 to 4, characterized in that, During the curing process, the surface is sprayed with a saturated calcium hydroxide solution 5 to 10 times until it is wetted within a curing period of 1 to 7 days.

6. The method for solidifying heavy metals according to any one of claims 1 to 5, characterized in that, The total mass ratio of mixing water A and mixing water B in the mixed slurry to the mass ratio of the inorganic cementitious material is (0.3~0.45):

1.

7. The method for solidifying heavy metals according to any one of claims 1 to 6, characterized in that, The concentration of heavy metals in the heavy metal-containing solution is 600–30000 mg / L.

8. The method for solidifying heavy metals according to any one of claims 1 to 7, characterized in that, The mixing water B is a 4-6 wt% aqueous solution of hydrated calcium silicate.

9. A composite cementitious material, characterized in that, It is prepared by the method of solidifying heavy metals according to any one of claims 1 to 8.

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