Method for curing / stabilizing chromium slag based on modified charcoal adsorption and alkali-activated cementing material

Abstract

The invention discloses a method for curing / stabilizing chromium slag on the basis of cooperation of modified charcoal adsorption and an alkali-activated cementing material, and belongs to the technical field of solid waste treatment. Biomass materials are carbonized to prepare the biochar, ultrasonic physical modification and phosphoric acid chemical modification are combined, the pore structure of the biochar is optimized, more surface functional groups are introduced, and the adsorption performance on hexavalent chromium is enhanced. Under the optimal adsorption condition, the maximum unit adsorption capacity of the self-made modified charcoal can reach 150.8 mg / g. The preparation process conditions of the composite alkali-activated cementing material are optimized by adopting a response surface method, and the uniaxial compressive strength of a solidified body in 28 days is 138.2 MPa. Through pre-adsorption treatment of the modified charcoal on hexavalent chromium in the chromium slag, the removal efficiency (adsorption rate) of hexavalent chromium is further improved, the environmental stability of a solidified body is enhanced, the solidified body subjected to adsorption-solidification co-treatment is ensured to meet the environmental protection standard, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of solid waste disposal, and in particular to a method for solidifying / stabilizing chromite slag by preparing a composite alkali-activated cementitious material through the synergistic response surface optimization of modified biochar adsorption. Background Art

[0002] Heavy metal pollution is one of the major challenges faced by the global environment and poses a serious threat to the ecosystem and human health. Chromium salts are an important class of chemical products and are widely used in industrial fields such as leather tanning, pigment manufacturing, electroplating, and anti-corrosion. Currently, the global chromium salt production capacity is approximately 1.1 million tons, and the annual output of the chromium salt industry in China is around 0.4 million tons, accounting for approximately 40% of the global output. However, for every 1 ton of chromium salt produced, 2.5 - 3 tons of chromite slag are generated. Due to the high solubility (acid-soluble and water-soluble) and oxidizing property of Cr(VI), it is difficult to remove and will persist in the environment for a long time, posing a great threat to the ecosystem and human health. Therefore, the rational utilization and standardized disposal of chromite slag have become the focus of current research.

[0003] In recent years, alkali-activated cementitious materials have been widely used as an environmentally friendly and efficient solidifying / stabilizing material. At the same time, modified biochar also exhibits excellent performance in heavy metal adsorption. Therefore, on this basis, pre-adsorption treatment of chromite slag is carried out to reduce the migration ability of Cr(VI), and then the adsorbed pollutants (modified biochar and chromite slag) are solidified / stabilized by alkali-activated cementitious materials to prevent them from being released into the environment again. Combining the adsorption ability of modified biochar with the solidifying / stabilizing ability of alkali-activated cementitious materials forms a synergistic effect, thereby achieving better results. Summary of the Invention

[0004] Aiming at the defects of the existing technology, the present invention provides a method for treating hexavalent chromium in chromite slag through the synergistic solidification / stabilization technology of modified biochar adsorption, including the following steps:

[0005] (1) Pyrolyzing and carbonizing biomass raw materials in a tubular furnace to prepare biochar;

[0006] (2) Conducting ultrasonic physical modification and phosphoric acid chemical modification on the biochar, and repeating pyrolysis and carbonization to obtain modified biochar;

[0007] (3) Mixing and stirring chromite slag with deionized water, and under the optimal adsorption conditions, pre-adsorbing hexavalent chromium in the chromite slag with modified biochar; after adsorption, the leachate is mixed evenly with the original chromite slag, dried, and sieved to obtain the chromite slag after adsorption treatment;

[0008] (4) Drying and sieving blast furnace slag and fly ash, and mixing them in proportion to obtain a composite alkali-activation raw material;

[0009] (5) Mix deionized water, sodium hydroxide, and water glass, and stir evenly to obtain a composite alkali activator solution;

[0010] (6) Mix the composite alkali-activated raw materials obtained in step (4) with the composite alkali activator solution obtained in step (5) to prepare an alkali-activated cementitious material, and optimize the preparation process conditions of the alkali-activated cementitious material using the response surface method;

[0011] (7) Mix the treated chromium slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold, cure, demold, and solidify to obtain a chromium slag solidified body;

[0012] Further, the biomass material in step (1) includes, but is not limited to, one or more of willow branches, coconut shells, and bamboo. The carbonization temperature is 300-700 °C, and the carbonization time is 1-3 h.

[0013] Further, in step (2), the physical modification time is 10-40 s, the mass fraction of phosphoric acid is 20%-40%, the addition ratio of biochar to phosphoric acid is 1:2-1:10, the chemical modification time is 10-12 h, and the modification temperature is 20-25 °C.

[0014] Further, in step (2), the repeated pyrolysis temperature is 300 °C, and the pyrolysis time is 1-2 h.

[0015] Further, in step (3), the mass ratio of deionized water to chromium slag is 1:1-2:1.

[0016] Further, the optimal adsorption conditions in step (3) include: the adsorption temperature is 25-55 °C, the adsorption time is 10-1440 min, the solution pH value is 2-10, and the biochar dosage ratio is 1-10 times the mass of Cr(VI).

[0017] Further, in step (4), the drying temperature is 100-105 °C, the drying time is 3-5 h, the particle sizes of blast furnace slag and fly ash are both less than 200 mesh, and the fly ash accounts for 5%-10% of the total amount.

[0018] Further, in step (6), for the corresponding optimized ratio of the surface, the water glass modulus is 0.33-1.67, the alkali solid content is 4.64%-11.36%, the curing temperature is 36.6-53.4 °C, and the liquid-solid ratio is determined to be 0.28 according to the previous experiment.

[0019] Further, in step (7), by mass, the chromium slag is 10-60 parts, the blast furnace slag is 34.8-78.3 parts, the fly ash is 1.8-4.1 parts, and the composite alkali activator is 3.4-7.6 parts.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] The present invention combines ultrasonic physical modification and phosphoric acid chemical modification to prepare modified biochar with excellent performance, significantly improving the adsorption capacity of biochar for hexavalent chromium (the maximum unit adsorption capacity of unmodified biochar is 95.1 mg / g). This method uses cheap and widely available biomass waste as raw materials, and the preparation process is simple and low-cost. The chromium slag after biochar adsorption is further fixed with hexavalent chromium through the solidification / stabilization effect of composite alkali-activated cementitious materials. While achieving double chromium solidification, it avoids secondary pollution after the adsorbent adsorbs heavy metals, ensuring that the entire treatment process is green, low-carbon, and environmentally friendly. Finally, the chromium slag solidified body meets environmental protection standards and can be used as building materials or landfill materials, having both economic benefits and resource utilization value, being applicable to various heavy metal waste treatment scenarios, and having broad prospects for popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a flow chart of the present invention;

[0023] Figure 2 is a scanning electron microscope image of biochar;

[0024] Figure 3 is the influence of pH on the unit adsorption capacity of Cr(Ⅵ);

[0025] Figure 4 is the influence of adsorption temperature on the unit adsorption capacity of Cr(Ⅵ);

[0026] Figure 5 is the influence of biochar dosage on the unit adsorption capacity of Cr(Ⅵ);

[0027] Figure 6 is the influence of adsorption time on the unit adsorption capacity of Cr(Ⅵ);

[0028] Figure 7 is the compressive strength diagram of the chromium slag solidified body;

[0029] Figure 8 is the leaching toxicity curve diagram of the chromium slag solidified body (sulfuric acid-nitric acid method);

[0030] Figure 9 is the leaching toxicity curve diagram of the chromium slag solidified body (TCLP method). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The following elaborates on the preferred embodiments of the present invention in conjunction with the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0032] Example 1

[0033] The method for solidifying / stabilizing chromite slag based on the adsorption of modified biochar and alkali-activated cementitious materials is as follows:

[0034] (1) Wash, dry, and crush willow branches, pyrolyze and carbonize them in a tube furnace at a temperature of 300 °C for 1 h to obtain biochar;

[0035] (2) Ultrasonically modify the biochar for 30 min, and soak it in a 40% phosphoric acid solution for 12 h for chemical modification. At this time, the ratio of biochar to phosphoric acid addition is 1:8; repeat step (1) for pyrolysis and carbonization, wash it to neutral after cooling, dry it at 105 °C for 5 h, and pass it through a 200-mesh sieve to obtain modified biochar;

[0036] (3) Mix chromite slag and deionized water at a solid-liquid ratio of 1:2, stir for 20 min, let it stand for 10 min, and filter it through a filter paper with a size of 0.45 μm to obtain a chromite slag leachate; adjust the pH of the leachate to 2, and adsorb it with the modified biochar for 1080 min at a temperature of 35 °C and a biochar dosage ratio of 5:1; after adsorption, mix the leachate with the original chromite slag evenly, dry it at 105 °C for 12 h, and pass it through a 200-mesh sieve to obtain the adsorbed chromite slag;

[0037] (4) Dry blast furnace slag and fly ash at 105 °C for 5 h, pass them through a 200-mesh sieve, and mix them according to the proportion of fly ash accounting for 5% to obtain a composite alkali-activated raw material;

[0038] (5) Mix deionized water, sodium hydroxide, and water glass, and stir evenly to obtain a composite alkali-activated solution;

[0039] (6) Mix the composite alkali-activated raw material obtained in step (4) with the composite alkali-activated solution obtained in step (5) to prepare a composite alkali-activated cementitious material; and conduct a three-factor and five-level experimental design according to the response surface method (as shown in Table 1). Finally, it is determined that under the conditions of a water glass modulus of 1.1, an alkali solid content of 8.4%, and an initial 24-h curing temperature of 44.8 °C, the prepared composite alkali-activated cementitious material has the highest compressive strength, and the actually measured compressive strength is 138.2 MPa;

[0040] Table 1 Response surface design table

[0041]

[0042] (7) Mix the treated chromite slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold it, cure it, demold it, and solidify it to obtain a chromite slag solidified body; by mass, weigh 10 parts of chromite slag, 78.3 parts of blast furnace slag, 4.1 parts of fly ash, and 7.6 parts of composite alkali activator respectively to obtain a chromite slag solidified body with a chromite slag addition of 10%.

[0043] Example 2

[0044] The method for solidifying / stabilizing chromium slag based on the adsorption of modified biochar and alkali-activated cementitious material is as follows:

[0045] (1) Wash, dry, and crush willow branches, and pyrolyze and carbonize them in a tubular furnace at a temperature of 300 °C for 1 h to obtain biochar;

[0046] (2) Ultrasonically modify the biochar for 30 min, and soak it in a 40% phosphoric acid solution for 12 h for chemical modification. At this time, the ratio of biochar to phosphoric acid addition is 1:8; repeat step (1) for pyrolysis and carbonization, wash it to neutral after cooling, dry it at 105 °C for 5 h, and pass it through a 200-mesh sieve to obtain modified biochar;

[0047] (3) Mix chromium slag and deionized water at a solid-liquid ratio of 1:2, stir for 20 min, let it stand for 10 min, and filter it through a 0.45-μm filter paper to obtain chromium slag leachate; adjust the pH of the leachate to 2, and adsorb it with modified biochar for 1080 min at a temperature of 35 °C and a biochar dosage ratio of 5:1; after adsorption, mix the leachate with the original chromium slag evenly, dry it at 105 °C for 12 h, and pass it through a 200-mesh sieve to obtain the adsorbed chromium slag;

[0048] (4) Dry blast furnace slag and fly ash at 105 °C for about 5 h, pass them through a 200-mesh sieve, and mix them according to the fly ash proportion of 5% to obtain composite alkali-activated raw materials;

[0049] (5) Mix deionized water, sodium hydroxide, and water glass, and stir evenly to obtain a composite alkali-activated solution;

[0050] (6) Mix the composite alkali-activated raw materials obtained in step (4) with the composite alkali-activated solution obtained in step (5) to prepare a composite alkali-activated cementitious material; and conduct a three-factor and five-level experimental design according to the response surface method (as shown in Table 1). Finally, it is determined that under the conditions of a water glass modulus of 1.1, an alkali solid content of 8.4%, and an initial 24-h curing temperature of 44.8 °C, the prepared composite alkali-activated cementitious material has the highest compressive strength, and the actually measured compressive strength is 138.2 MPa;

[0051] (7) Mix the treated chromium slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold it, cure it, demold it, and solidify it to obtain a chromium slag solidified body; by mass, weigh 20 parts of chromium slag, 69.6 parts of blast furnace slag, 3.7 parts of fly ash, and 6.7 parts of composite alkali activator respectively to obtain a chromium slag solidified body with a chromium slag addition of 20%.

[0052] Example 3

[0053] The method for solidifying / stabilizing chromium slag by adsorption of modified biochar in cooperation with alkali-activated cementitious materials comprises the following specific steps:

[0054] (1) Wash, dry, and crush willow branches, and pyrolyze and carbonize them in a tube furnace at 300 °C for 1 h to obtain biochar;

[0055] (2) Ultrasonically modify the biochar for 30 min, and soak it in a 40% phosphoric acid solution for 12 h for chemical modification. At this time, the ratio of biochar to phosphoric acid addition is 1:8; repeat step (1) for pyrolysis and carbonization, wash it to neutrality after cooling, dry it at 105 °C for 5 h, and pass it through a 200-mesh sieve to obtain modified biochar;

[0056] (3) Mix chromium slag and deionized water at a solid-liquid ratio of 1:2, stir for 20 min, let it stand for 10 min, and filter it through a filter paper with a size of 0.45 μm to obtain a chromium slag leachate; adjust the pH of the leachate to 2, and adsorb it with modified biochar for 1080 min at a temperature of 35 °C and a biochar dosage ratio of 5:1; after adsorption, mix the leachate and the original chromium slag evenly, dry it at 105 °C for 12 h, and pass it through a 200-mesh sieve to obtain the chromium slag after adsorption treatment;

[0057] (4) Dry blast furnace slag and fly ash at 105 °C for about 5 h, pass them through a 200-mesh sieve, and mix them according to the proportion of fly ash being 5% to obtain a composite alkali-activated raw material;

[0058] (5) Mix deionized water, sodium hydroxide, and water glass, and stir evenly to obtain a composite alkali-activated solution;

[0059] (6) Mix the composite alkali-activated raw material obtained in step (4) with the composite alkali-activated solution obtained in step (5) to prepare a composite alkali-activated cementitious material; and conduct a three-factor and five-level experimental design according to the response surface method (as shown in Table 1). Finally, it is determined that under the conditions of a water glass modulus of 1.1, an alkali solid content of 8.4%, and an initial 24-h curing temperature of 44.8 °C, the prepared composite alkali-activated cementitious material has the highest compressive strength, and the actually measured compressive strength is 138.2 MPa;

[0060] (7) Mix the treated chromium slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold it, cure it, demold it, and solidify it to obtain a chromium slag solidified body; by mass, weigh 30 parts of chromium slag, 60.9 parts of blast furnace slag, 3.2 parts of fly ash, and 5.9 parts of composite alkali activator respectively to obtain a chromium slag solidified body with a chromium slag addition of 30%.

[0061] Example 4

[0062] The method for solidifying / stabilizing chromium slag by adsorption of modified biochar in cooperation with alkali-activated cementitious materials comprises the following specific steps:

[0063] (1) Wash the willow branches, dry them and crush them. Pyrolyze and carbonize them in a tubular furnace at a temperature of 300 °C for 1 h to obtain biochar;

[0064] (2) Ultrasonically modify the biochar for 30 min, and soak it in a 40% phosphoric acid solution for 12 h for chemical modification. At this time, the ratio of biochar to phosphoric acid addition is 1:8; repeat step (1) for pyrolysis and carbonization, wash it to neutral after cooling, dry it at 105 °C for 5 h, and pass through a 200-mesh sieve to obtain modified biochar;

[0065] (3) Mix the chromium slag and deionized water at a solid-liquid ratio of 1:2, stir for 20 min, let it stand for 10 min, and filter it through a filter paper with a size of 0.45 μm to obtain chromium slag leachate; adjust the pH of the leachate to 2, and at a temperature of 35 °C and a biochar dosage ratio of 5:1, adsorb it with the modified biochar for 1080 min; after adsorption, mix the leachate with the original chromium slag evenly, dry it at 105 °C for 12 h, and pass through a 200-mesh sieve to obtain the chromium slag after adsorption treatment;

[0066] (4) Dry the blast furnace slag and fly ash at 105 °C for about 5 h, pass through a 200-mesh sieve, and mix them according to the fly ash proportion of 5% to obtain composite alkali-activated raw materials;

[0067] (5) Mix deionized water, sodium hydroxide and water glass, and stir evenly to obtain a composite alkali-activation solution;

[0068] (6) Mix the composite alkali-activated raw materials obtained in step (4) with the composite alkali-activation solution obtained in step (5) to prepare a composite alkali-activated cementitious material; and conduct a three-factor and five-level experimental design according to the response surface method (as shown in Table 1). Finally, it is determined that under the conditions of a water glass modulus of 1.1, an alkali solid content of 8.4%, and an initial 24-h curing temperature of 44.8 °C, the prepared composite alkali-activated cementitious material has the highest compressive strength, and the actually measured compressive strength is 138.2 MPa;

[0069] (7) Mix the treated chromium slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold, cure, demold, and solidify to obtain a chromium slag solidified body; by mass, weigh 40 parts of chromium slag, 52.2 parts of blast furnace slag, 2.8 parts of fly ash, and 5 parts of composite alkali activator respectively to obtain a chromium slag solidified body with a chromium slag addition of 40%; among them, the chromium slag solidified body without being adsorbed and treated by modified biochar is denoted as M40, and the chromium slag solidified body adsorbed and treated by modified biochar is denoted as T40.

[0070] Example 5

[0071] The method for solidifying / stabilizing chromium slag based on the adsorption of modified biochar and the synergistic alkali-activated cementitious material is specifically as follows:

[0072] (1) Wash the willow branches, dry them, and crush them. Pyrolyze and carbonize them in a tubular furnace at a temperature of 300 °C for 1 h to obtain biochar.

[0073] (2) Ultrasonically modify the biochar for 30 min, and soak it in a 40% phosphoric acid solution for 12 h for chemical modification. At this time, the ratio of biochar to phosphoric acid addition is 1:8; repeat step (1) for pyrolysis and carbonization, wash it to neutral after cooling, dry it at 105 °C for 5 h, and pass it through a 200-mesh sieve to obtain modified biochar.

[0074] (3) Mix the chromium slag and deionized water at a solid-liquid ratio of 1:2, stir for 20 min, let it stand for 10 min, and filter it through a filter paper with a size of 0.45 μm to obtain chromium slag leachate; adjust the pH of the leachate to 2, and use the modified biochar to adsorb it for 1080 min at a temperature of 35 °C and a biochar dosage ratio of 5:1; after adsorption, mix the leachate with the original chromium slag evenly, dry it at 105 °C for 12 h, and pass it through a 200-mesh sieve to obtain the chromium slag after adsorption treatment.

[0075] (4) Dry the blast furnace slag and fly ash at 105 °C for about 5 h, pass them through a 200-mesh sieve, and mix them according to the fly ash proportion of 5% to obtain composite alkali-activated raw materials.

[0076] (5) Mix deionized water, sodium hydroxide, and water glass, and stir evenly to obtain a composite alkali-activation solution.

[0077] (6) Mix the composite alkali-activated raw materials obtained in step (4) with the composite alkali-activation solution obtained in step (5) to prepare a composite alkali-activated cementitious material; and conduct a three-factor and five-level experimental design according to the response surface method (as shown in Table 1). Finally, it is determined that under the conditions of a water glass modulus of 1.1, an alkali solid content of 8.4%, and an initial 24-h curing temperature of 44.8 °C, the prepared composite alkali-activated cementitious material has the highest compressive strength, and the actually measured compressive strength is 138.2 MPa.

[0078] (7) Mix the treated chromium slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold it, cure it, demold it, and solidify it to obtain a chromium slag solidified body; by mass, weigh 50 parts of chromium slag, 43.5 parts of blast furnace slag, 2.3 parts of fly ash, and 4.2 parts of composite alkali-activator respectively to obtain a chromium slag solidified body with a chromium slag addition of 50%; among them, the chromium slag solidified body without being adsorbed and treated by modified biochar is denoted as M50, and the chromium slag solidified body adsorbed and treated by modified biochar is denoted as T50.

[0079] Example 6

[0080] The method for solidifying / stabilizing chromium slag based on the adsorption of modified biochar and the synergistic alkali-activated cementitious material is as follows:

[0081] (1) Wash the willow branches, dry them and crush them. Pyrolyze and carbonize them in a tube furnace at 300 °C for 1 h to obtain biochar;

[0082] (2) Ultrasonically modify the biochar for 30 min and soak it in 40% phosphoric acid solution for 12 h for chemical modification. At this time, the ratio of biochar to phosphoric acid addition is 1:8; Repeat step (1) for pyrolysis and carbonization, wash it to neutral after cooling, dry it at 105 °C for 5 h, and pass through a 200-mesh sieve to obtain modified biochar;

[0083] (3) Mix the chromium slag and deionized water according to a solid-liquid ratio of 1:2, stir for 20 min, let it stand for 10 min, and filter it through a filter paper with a size of 0.45 μm to obtain chromium slag leachate; Adjust the pH of the leachate to 2, and use the modified biochar to adsorb for 1080 min at a temperature of 35 °C and a biochar dosage ratio of 5:1; After adsorption, mix the leachate with the original chromium slag evenly, dry it at 105 °C for 12 h, and pass through a 200-mesh sieve to obtain the chromium slag after adsorption treatment;

[0084] (4) Dry the blast furnace slag and fly ash at 105 °C for about 5 h, pass through a 200-mesh sieve, and mix them according to the fly ash proportion of 5% to obtain composite alkali-activated raw materials;

[0085] (5) Mix deionized water, sodium hydroxide and water glass, and stir evenly to obtain a composite alkali-activated solution;

[0086] (6) Mix the composite alkali-activated raw materials obtained in step (4) with the composite alkali-activated solution obtained in step (5) to prepare a composite alkali-activated cementitious material; And conduct a three-factor and five-level experimental design according to the response surface method (as shown in Table 1). Finally, it is determined that under the conditions of a water glass modulus of 1.1, an alkali solid content of 8.4%, and an initial 24 h curing temperature of 44.8 °C, the prepared composite alkali-activated cementitious material has the highest compressive strength, and the actually measured compressive strength is 138.2 MPa;

[0087] (7) Mix the treated chromium slag obtained in step (3) with the composite alkali-activated cementitious material obtained in step (6), stir evenly, mold, cure, demold, and solidify to obtain a chromium slag solidified body; By mass, weigh 60 parts of chromium slag, 34.8 parts of blast furnace slag, 1.8 parts of fly ash, and 3.4 parts of composite alkali activator respectively to obtain a chromium slag solidified body with a chromium slag addition of 60%; Among them, the chromium slag solidified body without being adsorbed and treated by modified biochar is denoted as M60, and the chromium slag solidified body adsorbed and treated by modified biochar is denoted as T60.

[0088] Examples 1 to 6, the specific experimental process is shown in Figure 1 ; The scanning electron microscope image of the modified biochar is shown in Figure 2; The effects of adsorption conditions (pH, adsorption temperature, dosage ratio, and adsorption time) on the unit adsorption capacity of Cr(VI) are shown in Figures 3 - 6 as follows; as Figure 2 shown, the surface of the original biochar is relatively rough, with irregular shapes and fewer pores. The overall appearance is relatively dense, and the pore structure is not significantly developed. After physical modification, there are obvious cracks and holes on the surface of the biochar, the dispersibility between particles is improved, and the pore structure is optimized to a certain extent. In the scanning electron micrograph of the finally modified biochar, larger flaky structures and more obvious pores appear, and the pore distribution is more uniform. Its structure also corresponds to the change in the adsorption capacity during the modification of the biochar. Under the conditions of 25°C and 24-hour adsorption, the unit adsorption capacity of the finally modified biochar can reach 138.9 mg / g. Observation Figures 3 - 6 shows that when the modified biochar adsorbs for 1080 min at pH = 2, adsorption temperature of 35°C, and dosage ratio of biochar of 5:1, the maximum unit adsorption capacity can reach 150.8 mg / g, which is significantly higher than that of the original biochar (90.1 mg / g).

[0089] Study on the compressive strength of chromium slag solidified body: The compressive strength of the solidified body was tested according to GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)", and the compressive strength results are as Figure 6 shown. With the increase in the chromium slag addition amount, the compressive strengths of both groups of solidified bodies show a downward trend. When the chromium slag addition amount is less than 40 parts, the compressive strength of group T is higher than that of group M. When the addition amount exceeds 40 parts, the compressive strength of group T decreases significantly and is lower than that of group M. This may be because the modified biochar can adsorb and remove Cr(VI) or other pollutants in the chromium slag, reducing the adverse effects of harmful substances on the solidification of the solidified body. However, with the addition of chromium slag content, the acidic environment in the adsorption process interferes with the solidification process of alkali-activated cementitious materials to a certain extent, and the cumulative effect of acidic components in high-proportion chromium slag begins to appear, which has a more significant inhibitory effect on the hydration reaction and weakens the mechanical properties of the solidified body. When the chromium slag addition amount is 60 parts, T60 and M60 are 29.7 MPa and 36.1 MPa respectively. According to the concrete strength inspection and evaluation standard GB / T50107-2010, the lowest strength grade for building use is about 15 MPa. Therefore, even the solidified body with 60% chromium slag addition still has good mechanical properties.

[0090] Study on the leaching toxicity of chromium slag solidified body: The leaching toxicity of the solidified body was tested according to HJ / T 299-2007 "Solid Waste - Leaching Toxicity Leaching Method - Sulfuric Acid and Nitric Acid Method" and Toxicity Characteristic Leaching Procedure (TCLP method, USEPA Method1311). The results are shown in Figure 7, as shown in Figure 8, the leaching of both groups of solidified bodies did not exceed the safety limit under the TCLP leaching conditions and was less than 1 mg / L. Under the sulfuric acid-nitric acid leaching conditions, only the leaching concentration of Cr(VI) in M60 was 5.05 mg / L, exceeding the safety limit (5 mg / L). However, the leaching concentrations of Cr(VI) and total chromium in the T group were lower than those in the W group. When the addition amount of chromium slag ≥ 40 parts, although the compressive strength of the T group was lower than that of the M group, its leaching toxicity was also lower than that of the M group. This may be because the modified biochar adsorbed most of the hexavalent chromium and fixed it in the solidified body in a more stable form. In contrast, the hexavalent chromium in the M group was not adsorbed and directly participated in the solidification reaction, resulting in the easier release of hexavalent chromium under acidic leaching conditions. According to the "Pollution Control Standard for Landfill of Hazardous Wastes" GB18598-2019, the solidified bodies all meet the requirements of the landfill site. When the addition amount of chromium slag is 30 parts, the toxicity leaching of the solidified bodies in the T group is lower than the pollution control index limit of the domestic waste landfill site (total chromium < 4.5 mg / L, hexavalent chromium concentration < 1.5 mg / L), meeting the domestic waste landfill standard. This also confirms the effectiveness of the adsorption of the modified biochar.

[0091] This patent proposes an innovative method for treating chromium slag. By combining the adsorption of modified biochar and the solidification / stabilization technology of composite alkali-activated cementitious materials, the efficient adsorption and stabilization of hexavalent chromium in chromium slag are achieved. Through the combination of physical ultrasonic modification and phosphoric acid modification, the pore structure and surface chemical properties of biochar are significantly enhanced, and its adsorption capacity for hexavalent chromium is enhanced. The response surface method is used to optimize the solidification conditions, and the composite alkali-activated cementitious materials are prepared by a more scientific single-factor ratio. The adsorption process of the present invention not only effectively reduces the concentration of hexavalent chromium, but also weakens the adverse effects of other impurities on the solidification performance, improves the long-term stability of the solidified body, and reduces the potential hazards to the ecological environment and human health. This technology also realizes the recycling of waste by the resource utilization of biomass raw materials. At the same time, it reduces the environmental pressure caused by the stacking of solid wastes such as blast furnace slag and fly ash. While reducing the leaching toxicity of chromium slag, it further improves the comprehensive performance and economic value of the solidified body, having good environmental protection benefits and promotion prospects.

[0092] The above embodiments only represent the implementation modes of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can be made, and these all belong to the protection scope of the present invention.

Claims

1. A method for curing / stabilizing chromium slag based on modified biochar adsorption and alkali-activated cementitious materials, characterized in that: The specific steps are as follows: (1) preparing biochar by pyrolyzing and carbonizing biomass raw materials in a tubular furnace; (2) subjecting the biochar to ultrasonic physical modification and phosphoric acid chemical modification, and repeated pyrolysis and carbonization to obtain modified biochar; (3) mixing and stirring the chromium slag with deionized water, and pre-adsorbing the hexavalent chromium in the chromium slag using modified biochar under optimal adsorption conditions; mixing the adsorbed leaching liquid with the original chromium slag, drying, and sieving to obtain the chromium slag after adsorption; (4) drying, sieving, and mixing blast furnace slag and fly ash in proportion to obtain an alkali-activated raw material; (5) mixing deionized water, sodium hydroxide and water glass, and stirring evenly to obtain an alkali-exciting solution; (6) mixing the alkali-activated raw material obtained in step (4) with the alkali-activated solution obtained in step (5) to prepare an alkali-activated gelling material; and optimizing the preparation process conditions of the alkali-activated gelling material by using response surface methodology; (7) Mixing the treated chromium slag obtained in step (3) with the alkali-activated gelling material obtained in step (6), stirring evenly, molding, curing, demolding, and solidifying to obtain a chromium slag solidified body.

2. The method for solidifying / stabilizing chromium slag by modified biochar adsorption and alkali-activated cementitious materials according to claim 1, characterized in that: The biomass material in step (1) includes but is not limited to one or more of willow branches, coconut shells, and bamboos. The carbonization temperature is 300 to 700° C. and the carbonization time is 1 to 3 hours.

3. The method for solidifying / stabilizing chromium slag by modified biochar adsorption and alkali-activated cementitious materials according to claim 1, characterized in that: The physical modification time in step (2) is 10 to 40 seconds, the mass fraction of phosphoric acid is 20% to 40%, the addition amount of biochar and phosphoric acid is 1:2 to 1:10, the chemical modification time is 10 to 12 hours, and the modification temperature is 20 to 25°C.

4. The method for solidifying / stabilizing chromium slag by modified biochar adsorption and alkali-activated cementitious materials according to claim 1, characterized in that: The repeated pyrolysis temperature in step (2) is 300° C. and the pyrolysis time is 1-2 h.

5. The method according to claim 1, characterized in that: In step (3), the mass ratio of deionized water to chromium slag is 1:1 to 2:

1.

6. The method according to claim 1, characterized in that The optimal adsorption conditions in step (3) include: adsorption temperature of 25-55°C, adsorption time of 10-1440 min, solution pH of 2-10, and biochar addition ratio of 1-10 times the mass of Cr(VI).

7. The method for solidifying / stabilizing chromium slag by modified biochar adsorption and alkali-activated cementitious materials according to claim 1, characterized in that: The drying temperature in step (4) is 100-105° C., the drying time is 3-5 hours, the particle sizes of blast furnace slag and fly ash are both less than 200 meshes, and the fly ash accounts for 5%-10% of the total amount.

8. The method for solidifying / stabilizing chromium slag by modified biochar adsorption and alkali-activated cementitious materials according to claim 1, characterized in that: The curved surface in step (6) is optimized in proportion, the water glass modulus is 0.33-1.67, the alkali solid content is 4.64%-11.36%, the curing temperature is 36.6-53.4°C, and the liquid-solid ratio is determined to be 0.28 according to the previous experiments.

9. The method for solidifying / stabilizing chromium slag by modified biochar adsorption and alkali-activated cementitious materials according to claim 1, characterized in that: In step (7), the chromium slag is 10 to 60 parts by weight, the blast furnace slag is 34.8 to 78.3 parts by weight, the fly ash is 1.8 to 4.1 parts by weight, and the composite alkali activator is 3.4 to 7.6 parts by weight.

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