Low-temperature roasting-leaching chromium-reducing treatment method for chromium-containing steel slag and application of low-temperature roasting-leaching chromium-reducing treatment method

Abstract

The invention discloses a low-temperature roasting-leaching chromium-reducing treatment method for chromium-containing steel slag and application, and belongs to the technical field of metallurgical solid waste treatment and soil ecological restoration. The method comprises the following steps: (1) roasting the steel slag through an alkaline reagent and an oxidizing agent at a low temperature of 170-200 DEG C to promote oxidation of chromium; (2) the roasted product is subjected to weak acid leaching, and the leaching rate of chromium exceeds 80% under the condition that the pH ranges from 3 to 6; (3) reducing Cr (VI) in the leachate into Cr (III) by using vitamin C as a reducing agent, and carrying out precipitation separation to obtain clear liquid which reaches the discharge standard; and (4) mixing the leached solid residues with calcium sulfate and citric acid to serve as an exciting agent, and applying the exciting agent to the saline-alkali soil. Through field trials, chromium in the steel slag can be effectively removed (the content of hexavalent chromium is reduced to lt; the pH value of soil is reduced, and the emergence rate of crops is increased. The method is simple in process, low in energy consumption and environmentally friendly, and a new method is provided for resource utilization of metallurgical solid waste and ecological restoration of saline-alkali soil.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgical solid waste treatment and soil ecological restoration, and particularly relates to a method for reducing chromium content in chromium-containing steel slag by low-temperature roasting-leaching and its application in saline-alkali soil improvement. Background Art

[0002] Steel slag is the main solid by-product in the steelmaking process. Due to its huge output, it faces the dilemma of difficult disposal and urgently needs to find economically viable resource utilization ways. It contains a large amount of oxides such as calcium, iron, silicon, and aluminum, and also contains a certain amount of heavy metal elements such as chromium. Chromium elements in steel slag generally exist in low-valence forms such as Cr(II) and Cr(III), but under environmental exposure conditions, they may be oxidized to hexavalent chromium (Cr(VI)) with higher toxicity and mobility.

[0003] Saline-alkali soil usually faces problems such as too high pH, lack of nutrients, and loose structure, and it is necessary to reduce alkalinity and improve soil structure. However, directly adding untreated chromium-containing steel slag to the soil poses a potential risk of heavy metal pollution. In the prior art, in order to reduce the content of Cr(VI) in steel slag or prevent Cr(III) from being further oxidized, mostly high-temperature roasting at 600°C to 800°C or chemical reduction methods are used, which have high energy consumption, complex processes, and produce a large amount of by-products.

[0004] Therefore, how to carry out low-temperature leaching to reduce the chromium content in chromium-containing steel slag at a relatively low temperature, and at the same time neutralize the alkalinity of the steel slag to make it weakly acidic, and retain the characteristics rich in calcium, magnesium, silicon, etc. for saline-alkali soil improvement has become a technical problem urgently to be solved in this field.

[0005] Based on the above problems, designing a method for reducing chromium content in chromium-containing steel slag by low-temperature roasting-leaching and its application in saline-alkali soil improvement has important scientific significance and practical significance. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for reducing chromium content in chromium-containing steel slag by low-temperature roasting-leaching and its application to solve the problems raised in the background art. Through key steps such as low-temperature oxidation-roasting combined with weak acid leaching and reduction treatment, the chromium in the steel slag is removed to a safe level, so that the treated steel slag can play the functions of pH adjustment, improving soil structure and nutrient supply in saline-alkali soil. Through the present invention, the resource utilization of chromium-containing steel slag can be realized, and the soil characteristics of vast saline-alkali lands in our country can be greatly improved, with significant economic and social benefits.

[0007] To achieve the above purpose, the present invention adopts the following technical solutions:

[0008] A method for treating chromium-containing steel slag by low-temperature roasting-leaching to reduce chromium, characterized by comprising the following steps:

[0009] S1. Pretreatment:

[0010] Crush, grind and screen the chromium-containing steel slag, and control its particle size between 50 and 800 μm through screening; further divide the raw steel slag into two parts, coarse particles and fine particles, for separate treatment, wherein the particle size of the coarse particle part is greater than 300 μm, and the particle size of the fine particle part is less than 100 μm; detect and analyze the chromium content and main mineral phases of the raw steel slag and record relevant data;

[0011] S2. Low-temperature oxidation-roasting:

[0012] Mix the pretreated chromium-containing steel slag in S1 with an alkaline reagent, an oxidant and a flux and conduct roasting, so that chromium in the divalent or trivalent state in the steel slag is converted into soluble chromate, and the formation of insoluble calcium chromate is reduced under the action of the flux;

[0013] S3. Weak acid leaching:

[0014] After the roasting product obtained in S2 is cooled to room temperature, carry out stirred grinding treatment, transfer the ground product to a weakly acidic solution, and carry out leaching reaction under stirring conditions, so that chromium compounds such as chromate are dissolved and enriched in the leaching solution to obtain a chromium-containing leaching solution;

[0015] S4. Reduction treatment of the leaching solution:

[0016] Add a reducing agent to the chromium-containing leaching solution, so that chromium compounds such as chromate are converted into trivalent chromium, and then carry out chemical precipitation or other separation treatments to obtain chromium-containing steel slag that meets the discharge standards or can be further utilized for resource recovery.

[0017] Preferably, the roasting temperature in S2 is 170-300 °C, more preferably 200-250 °C; the roasting time is 1-4 h, more preferably 2-4 h.

[0018] Preferably, the alkaline reagent is one or a combination of NaOH, Na2CO3 or NaHCO3;

[0019] The oxidant is persulfate, nitrate or a mixture thereof.

[0020] Preferably, the flux is sodium metasilicate (Na2SiO3), and the dosage is 0.5-5% of the mass of the steel slag, which is used to form complex silicate with calcium and iron in the steel slag during roasting, inhibit the formation of insoluble calcium chromate (CaCrO4) and promote the formation of soluble sodium chromate (Na2CrO4).

[0021] Preferably, the weak acid solution in S3 is oxalic acid, citric acid, acetic acid, dilute hydrochloric acid or its buffer solution, with a pH of 3 - 6, and more preferably 3 - 5;

[0022] The liquid - solid ratio of the weak acid solution to the grinding product is 10:1 - 20:1 (L / kg);

[0023] The leaching reaction temperature in S3 is 25 - 50 °C, and the reaction time is 1 - 4 h.

[0024] Preferably, the reducing agent in S4 is vitamin, sodium sulfite or sodium bisulfite; the reduction reaction time is 0.5 - 2 h.

[0025] An application of chromium - containing steel slag, the chromium - containing steel slag is obtained by the above - mentioned low - temperature roasting - leaching chromium reduction treatment method. For the treated chromium - containing steel slag, heavy metal leaching detection is carried out. If it meets the local soil or solid waste leaching standard (the heavy metal leaching toxicity meets GB5085.3—2007 or its alternative discharge standard), it is used as a saline - alkali soil improvement material to improve the soil pH regulation ability and soil structure.

[0026] Preferably, the application amount of the treated chromium - containing steel slag as a saline - alkali soil improvement material is 2 - 10% of the dry weight of the soil, and more preferably 5 - 8%.

[0027] Preferably, when using the treated chromium - containing steel slag for saline - alkali soil improvement, it specifically includes the following steps:

[0028] In the pretreatment stage, the steel slag is divided into two parts, coarse particles and fine particles, and the oxidation - roasting and weak acid leaching processes are carried out respectively;

[0029] Optimize the roasting time, leaching time and reagent dosage according to the particle size difference to ensure the thoroughness of chromium conversion and removal;

[0030] The coarse - grained steel slag powder can fill the large pores of the saline - alkali soil and provide a framework support to improve the soil structure;

[0031] The fine - grained steel slag powder can fill the micro - cracks, enhance the compactness of the soil, and synergistically improve the soil anti - disintegration performance.

[0032] Preferably, an appropriate amount of activator is further added to adjust the pH value, reduce the soil permeability, reduce the salt accumulation on the ground surface, improve the compactness of the soil mass and reduce the porosity.

[0033] Compared with the prior art, the present invention provides a wide - temperature aqueous zinc - ion battery electrolyte, its preparation method and application, and has the following beneficial effects:

[0034] Through steps such as low-temperature oxidation-roasting combined with weak acid leaching and reduction treatment, the present invention realizes the effective removal of heavy metal chromium in chromium-containing steel slag or the reduction of chromium content, and utilizes the treated steel slag to play the roles of pH adjustment, improvement of soil structure and nutrient regulation in saline-alkali soil; through the above process, most of the soluble or potentially oxidizable chromium in the steel slag can be removed, and the obtained solid residue has characteristics such as high calcium and high silicon, which can be applied to the conditioning of saline-alkali soil; the chromium in the leachate is separated by reduction and precipitation, realizing the safe utilization of solid waste resources. Through the present invention, the resource utilization of chromium-containing steel slag can be realized, and the soil characteristics of a large number of saline-alkali lands in China can be greatly improved, with significant economic and social benefits. Brief Description of the Drawings

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings involved in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only schematic illustrations of some embodiments of the present invention, and those skilled in the art can also construct other forms of drawings based on these drawings without creative labor.

[0036] Figure 1 It is the overall flow chart of a low-temperature roasting-leaching chromium reduction treatment method for chromium-containing steel slag proposed by the present invention. Detailed Embodiments

[0037] The present invention will be further described below in conjunction with specific embodiments, and the advantages and characteristics of the present invention will become clearer with the description. However, these embodiments are only exemplary and do not constitute any limitation to the scope of the present invention. Those skilled in the art should understand that the details and forms of the technical solutions of the present invention can be modified or replaced without departing from the spirit and scope of the present invention, but these modifications and replacements all fall within the protection scope of the present invention.

[0038] Example 1:

[0039] Steel slag raw material

[0040] Source of raw material: The by-product steel slag from a steel mill, with a chromium content of about 2.0% (mass fraction), and the main mineral phases include RO phase, dicalcium silicate, free CaO, etc.

[0041] Please refer to Figure 1 , a low-temperature roasting-leaching chromium reduction treatment method for chromium-containing steel slag proposed by the present invention, includes the following content:

[0042] Pretreatment

[0043] Combined with XRD and SEM analysis, the morphology and mineral phase composition of steel slag minerals were analyzed. 1 kg of steel slag was taken, crushed and ground to <0.8 mm, and then divided into two parts, coarse grains and fine grains, by screening. The coarse grain part had a particle size >0.5 mm, and the fine grain part had a particle size <0.1 mm. The basis for this particle size classification was that the coarse grain particles provided a framework support, while the fine grain particles, due to their large specific surface area, filled the microcracks and enhanced the compactness of the soil. A handheld spectrometer was used to perform XRF detection on the total chromium content and uniformity of the raw materials to confirm the suitable range for treatment.

[0044] Low-temperature oxidation-roasting

[0045] The coarse and fine grains were treated separately, and the roasting formula, temperature and time were optimized respectively.

[0046] (1) Coarse grain part (>0.5 mm) formula: 50 g of NaOH, 20 g of NaNO3, 20 g of sodium metasilicate (Na2SiO3); roasted at 200 °C for 2 h; gradually heated up in the first 30 min and then kept at a constant temperature with stirring;

[0047] After roasting, it was cooled and sampled for testing. XRD was used to analyze some of the coarse grain samples, and the results showed that some low-valent chromium was converted into soluble chromate, and at the same time, the insoluble calcium chromate (CaCrO4) decreased significantly.

[0048] (2) Fine grain part (<0.1 mm)

[0049] Due to the fine particles and large specific surface area, roasting at 170 °C for 1 - 2 h could achieve the ideal effect; the dosage of alkaline reagent / oxidant could be according to the original formula or slightly reduced to avoid over-oxidation or side reactions.

[0050] After roasting, it was cooled and sampled for testing. Similarly, XRD was used to analyze the fine grain samples, showing that most of the Cr(II) and Cr(III) were oxidized to soluble CrO4 2- .

[0051] Weak acid leaching

[0052] The roasted products of the coarse grains and the roasted products of the fine grains were cooled, crushed and leached with weak acid respectively.

[0053] (1) Treatment of the roasted products of the coarse grains

[0054] After cooling, the coarse grain part could be first moderately crushed to <500 μm.

[0055] Take a certain amount of coarse-grained roasted product (at a liquid-solid ratio of 10:1 to 20:1 (L / kg)), and leach it with a weakly acidic solution at pH 3 - 6 (such as citric acid, acetic acid or dilute hydrochloric acid) under stirring at 25 - 50 °C for 1 - 4 h.

[0056] Detection: After the leaching process, take the leachate sample for detection. Determine the total chromium content by ICP-OES and detect the Cr(VI) concentration by UV-Vis to verify the leaching efficiency. The results show that the chromium leaching rate exceeds 80%, meeting the expectation.

[0057] (2) Treatment of fine-grained roasted product

[0058] After cooling, the fine-grained part is also crushed to <100 μm.

[0059] Take a certain amount of fine-grained roasted product (at a liquid-solid ratio of 10:1 to 20:1 (L / kg)), and leach it with a weakly acidic solution at pH 3 - 6 (such as citric acid, acetic acid or dilute hydrochloric acid) under stirring at 25 - 50 °C for 1 - 4 h.

[0060] Detection: Similarly, after the leaching process, take the leachate sample to detect the chromium content. Through the combined analysis of ICP-OES and UV-Vis, the results show that the chromium leaching rate exceeds 80%, meeting the expectation.

[0061] Reduction treatment of leachate

[0062] Adjust the pH of the leachate to 2.5, add an appropriate amount of vitamin C, and stir for 1 h;

[0063] The Cr(VI) content drops from 200 mg / L to <0.5 mg / L; then add NaOH to adjust the pH to 8.5 to form Cr(OH)3 precipitate for separation, and the chromium-containing sludge and clear liquid meet the discharge standards.

[0064] Post-treatment and soil application

[0065] Analyze the morphology and mineral phase composition of the remaining solid residue by combining XRD and SEM respectively. After the leaching toxicity test according to GB 5085.3 - 2007 and the hexavalent chromium content is <0.1 mg / L, mix them; Incorporate 5% (mass ratio) calcium sulfate (gypsum) and 5% (mass ratio) organic acid (such as citric acid) as activators into the mixed residue, which can improve the chemical and biological improvement effects simultaneously. And according to the soil pH value and salt content, adjust the dosage of the acidic activator to avoid secondary pollution caused by excessive acidification. Use a handheld spectrometer to detect the total chromium content of the mixture, and the total chromium content is <0.05%.

[0066] Apply the residue to a saline-alkali land at 5% (mass ratio). After 2 months of field tests, it is found that the soil pH drops from 9.0 to 8.5, the soil salt stress is relieved, and the crop emergence rate increases by about 15%.

[0067] It can be seen that this process can effectively remove chromium from steel slag under low-temperature conditions and achieve the effect of soil improvement.

[0068] The above is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A method for reducing chromium content in chromium-containing steel slag by low-temperature roasting-leaching, characterized in that, It includes the following steps: S1. Pretreatment: The chromium-containing steel slag is crushed, ground and screened, and its particle size is controlled between 50 and 800 μm through screening; the chromium content and main mineral phases of the raw material steel slag are detected, analyzed and relevant data are recorded; S2. Low-temperature oxidation-roasting: The chromium-containing steel slag pretreated in S1 is mixed with an alkaline reagent, an oxidant and a flux and roasted, so that chromium in the divalent or trivalent state in the steel slag is converted into soluble chromate, and the formation of insoluble calcium chromate is reduced under the action of the flux; S3. Weak acid leaching: After the roasted product obtained in S2 is cooled to room temperature, it is subjected to stirred milling treatment, the milled product is transferred to a weak acid solution, and leaching reaction is carried out under stirring conditions, so that chromium compounds are dissolved and enriched in the leaching solution to obtain a chromium-containing leaching solution; S4. Reduction treatment of the leaching solution: A reducing agent is added to the chromium-containing leaching solution, so that the chromium compound is converted into trivalent chromium, and then chemical precipitation or other separation treatments are carried out to obtain chromium-containing steel slag that meets the discharge standards or can be further recycled.

2. The low-temperature roasting-leaching chromium reduction treatment method for chromium-containing steel slag according to claim 1, characterized in that, The roasting temperature in S2 is 170-300 °C, and the roasting time is 1-4 h.

3. A low-temperature roasting-leaching chromium reduction treatment method for chromium-containing steel slag according to claim 2, characterized in that, The alkaline reagent is one or a combination of more of NaOH, Na2CO3 or NaHCO3; The oxidant is persulfate, nitrate or a mixture thereof.

4. A low-temperature roasting-leaching chromium reduction treatment method for chromium-containing steel slag according to claim 3, characterized in that, The flux is sodium metasilicate, and the dosage is 0.5-5% of the mass of the steel slag, which is used to form complex silicate with calcium and iron in the steel slag during roasting, inhibit the formation of insoluble calcium chromate and promote the formation of soluble sodium chromate.

5. A method for reducing chromium content in chromium-containing steel slag by low-temperature roasting-leaching, according to claim 1, characterized in that The weak acid solution in S3 is oxalic acid, citric acid, acetic acid, dilute hydrochloric acid or its buffer solution, and its pH is 3-6; The liquid-solid ratio of the weak acid solution to the milled product is 10:1-20:1 (L / kg); The leaching reaction temperature in S3 is 25-50 °C, and the reaction time is 1-4 h.

6. A method for treating chromium-containing steel slag by low-temperature roasting-leaching to reduce chromium, according to claim 1, characterized in that, The reducing agent in S4 is vitamin, sodium sulfite or sodium bisulfite; the reduction reaction time is 0.5-2 h.

7. Application of chromium-containing steel slag, wherein the chromium-containing steel slag is obtained by the low-temperature roasting-leaching chromium reduction treatment method described in any one of claims 1 to 6, and is characterized in that The heavy metal leaching of the treated chromium-containing steel slag is detected. If it meets the local soil or solid waste leaching standard, it is used as a saline-alkali soil improvement material to improve the soil pH regulation ability and improve the soil structure.

8. The application of a chromium-containing steel slag according to claim 7, characterized in that, The application amount of the treated chromium-containing steel slag as a saline-alkali soil improvement material is 2-10% of the dry weight of the soil.

9. The application of a chromium-containing steel slag according to claim 8, characterized in that, When using the treated chromium-containing steel slag for saline-alkali soil improvement, the steel slag is divided into two parts: coarse particles and fine particles. Among them, the coarse-grained steel slag powder is used to fill the large pores of the saline-alkali soil to provide a skeleton support to improve the soil structure; the fine-grained steel slag powder is used to fill the micro-cracks to enhance the compactness of the soil and synergistically improve the anti-disintegration performance of the soil.

10. The application of a chromium-containing steel slag according to claim 9, wherein, An appropriate amount of activator is further added to adjust the pH value, reduce the soil permeability, reduce the salt accumulation on the ground surface, improve the compactness of the soil body and reduce the porosity.

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