Method for comprehensively recovering calcium and arsenic from calcium-arsenic slag

Abstract

The invention provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag. The method comprises the following steps: S1, mixing the calcium-arsenic slag and a sodium carbonate solution for reaction, and filtering to obtain filtrate and filter residues containing calcium carbonate; washing filter residues, and collecting a washing solution A; the pH value of the mixed reaction is 10-11; the temperature of the mixed reaction is 25-60 DEG C; and S2, mixing the filtrate and the washing liquid to obtain a sodium arsenate solution, adding ferrous sulfate, and controlling the oxidation-reduction potential to be 400-550mv with hydrogen peroxide for arsenic precipitation. The invention provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag, which is characterized in that the sodium-calcium replacement efficiency is improved and the purity of a sodium arsenate solution is improved through combined regulation and control of pH, temperature and liquid-solid ratio, then scorodite with a wheat grain shape is generated under the conditions of relatively high pH, high potential and high iron-arsenic molar ratio by adopting combined regulation and control of pH-potential-iron-arsenic molar ratio, and the calcium and arsenic are comprehensively recovered from the calcium-arsenic slag. And the leaching stability is very good.

Description

Technical Field

[0001] The invention belongs to the field of resource utilization and harmless disposal of arsenic-containing hazardous waste, and specifically relates to a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag. Background Art

[0002] The resource utilization and harmless disposal of arsenic-containing hazardous wastes have always been an environmental concern. Arsenic-containing hazardous wastes include arsenic-containing waste liquids and arsenic-containing waste residues. Calcium arsenic slag is arsenic-containing sludge produced by the traditional lime-iron salt method to treat waste acid, and accounts for a large proportion of arsenic-containing waste residues. Calcium arsenic slag has a low arsenic content and a large amount of slag. The only way to dispose of it is safe landfill, which occupies a large amount of land resources. Most of the research on calcium arsenic slag focuses on harmless stabilization. How to achieve resource reduction of calcium arsenic slag is a meaningful environmental protection topic, and there are few related studies. At present, the main direction of calcium arsenic slag resource recovery is the preparation of arsenic products, such as copper arsenate, arsenic trioxide, elemental arsenic, etc. However, due to the high production cost of arsenic products and the saturated demand market, arsenic products can only be temporarily stored, which cannot bring economic benefits and is a huge safety hazard to the surrounding ecological environment. In existing research, a comprehensive treatment method for calcium arsenic slag is provided, in which the calcium arsenic slag is slurried, and arsenate in calcium arsenate is replaced by carbon dioxide to form calcium carbonate and arsenic acid solution, and the arsenic acid solution is concentrated and crystallized to obtain arsenic oxide, and the arsenic oxide is reduced with hot carbon to obtain arsenic element, thereby realizing the separation and recovery of calcium arsenic. The above method combines the wet method with the pyrolysis method, and finally realizes the separation and recovery of calcium arsenic and the reduction of arsenic slag, but the efficiency of carbon dioxide gas dissolving in water to obtain carbonate to replace arsenate is low, and the energy consumption of high-temperature carbon reduction is high, and the arsenic element needs to be sealed and stored, which is difficult to manage and has a high risk of secondary pollution.

[0003] In summary, there is an urgent need to provide a method for comprehensively recovering calcium and arsenic from calcium arsenic slag with low energy consumption. Summary of the invention

[0004] The present invention aims to solve at least one of the above-mentioned technical problems existing in the prior art. To this end, the present invention provides a method for comprehensively recovering calcium and arsenic from calcium arsenic slag, using sodium carbonate to provide high-concentration carbonate ions, and improving the sodium-calcium replacement efficiency and the purity of the sodium arsenate solution by jointly regulating pH, temperature and liquid-solid ratio, and then adopting pH-potential-iron-arsenic molar ratio to jointly regulate, under relatively high pH, ​​high potential and high iron-arsenic molar ratio conditions, generate scorodite with a grain-like morphology, which has good leaching stability, and at the same time ensures that the liquid arsenic concentration after arsenic precipitation is at a low level, ensuring the purity of the subsequent evaporation, concentration and crystallization product.

[0005] The first aspect of the present invention provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag, comprising the following steps:

[0006] S1: mixing calcium arsenic slag and sodium carbonate solution and filtering to obtain a filtrate and a filter residue containing calcium carbonate;

[0007] After washing the filter residue, washing liquid A is collected;

[0008] The pH value of the mixed reaction is 10-11;

[0009] The temperature of the mixed reaction is 25°C-60°C;

[0010] S2: the filtrate and the washing liquid are mixed to obtain a sodium arsenate solution, and then ferrous sulfate is added, and the oxidation-reduction potential is controlled to 400-550mv with hydrogen peroxide to precipitate arsenic.

[0011] The mechanism of the method is as follows:

[0012] The sodium carbonate solution is prepared and stirred evenly with the calcium arsenic slag to obtain a mixed slurry, the liquid-solid ratio, pH value and temperature are adjusted, the replacement reaction is carried out for a period of time, and the filter is filtered, the filter residue is washed and dried to obtain calcium carbonate; the filtrate in step S2 is mixed with the washing liquid to obtain a sodium arsenate solution, the pH value is adjusted with sulfuric acid, ferrous sulfate is added according to the molar ratio of iron to arsenic, the potential is adjusted with hydrogen peroxide, arsenic precipitation reaction is carried out at a certain temperature, and then it is filtered and washed to obtain scorodite with long-term stability, and the filtrate is a mixed solution of acidic sodium sulfate and ferric sulfate.

[0013] According to the embodiments of the first aspect of the present invention, there are at least the following beneficial effects:

[0014] In step S1, the present invention uses sodium carbonate to provide a high concentration of carbonate ions, and through the combined regulation of pH and temperature, the sodium-calcium replacement efficiency is improved, the purity of the sodium arsenate solution is improved, and the influence of excessive calcium ions on the subsequent arsenic precipitation effect is avoided. The sodium-calcium replacement efficiency is guaranteed by regulating the appropriate liquid-solid ratio, pH value and temperature of the mixed slurry. When the temperature is between 25°C and 60°C, the solubility of sodium carbonate is at a relatively high value, and the carbonate concentration is high, providing a higher driving force for sodium-calcium replacement. The temperature should not be too high. Excessive temperature causes arsenic volatilization to cause secondary pollution. The pH value is controlled between 10-11, which not only ensures that the arsenic alkali dissolution generates sodium arsenate and improves the replacement efficiency, but also facilitates the subsequent reduction of the pH value. The liquid-solid ratio is greater than 1:1, which avoids the influence of excessive arsenic concentration on the sodium-calcium replacement efficiency.

[0015] In step S2, the present invention adopts pH-potential-iron-arsenic molar ratio joint regulation, and generates a scorodite with a grain-like morphology under relatively high pH, ​​high potential and high iron-arsenic molar ratio conditions, which has good leaching stability, and at the same time ensures that the arsenic concentration of the liquid after arsenic precipitation is at a low level, thereby ensuring the purity of the subsequent evaporation, concentration and crystallization product. A mixed solution of acidic sodium sulfate and ferric sulfate can be used to adjust the initial pH value of the sodium arsenate solution, and the sodium sulfate can be recycled after enrichment.

[0016] According to some embodiments of the present invention, in step S1, the liquid-to-solid ratio in the mixing reaction is not less than 1:1.

[0017] Under the above liquid-to-solid ratio, the reaction efficiency is improved, ensuring that the calcium arsenic slag is in full contact with the sodium carbonate solution, thereby accelerating the chemical reaction and improving the reaction conversion rate; improving the filtration performance helps to obtain a better separation effect between the filtrate and the residue, reducing residual impurities in the residue and improving the efficiency of subsequent treatment; during the washing process, the soluble impurities in the residue can be more effectively removed to ensure that the collected washing liquid has a higher purity; maintaining the stability of the reaction environment, thereby improving the controllability of the reaction.

[0018] According to some embodiments of the present invention, the calcium arsenic slag is an arsenic-containing slag mainly composed of calcium arsenate produced by a lime iron salt neutralization method, and the arsenic content is 2-10%.

[0019] According to some embodiments of the present invention, in step S1, the molar ratio of the sodium carbonate to the arsenic is 1.1-1.3:1.

[0020] The above molar ratio can ensure that there is enough sodium carbonate in the reaction system to undergo double decomposition reaction with calcium arsenate, which helps to improve the dissolution efficiency of arsenic.

[0021] According to some embodiments of the present invention, in step S2, the molar ratio of calcium to arsenic in the sodium arsenate solution is not higher than 0.05.

[0022] During the precipitation of arsenic, a high calcium ion concentration will affect the nucleation and growth of scorodite crystals, resulting in incomplete precipitation or deformed crystal morphology, thereby reducing the arsenic precipitation efficiency and producing irregular or unstable solid forms. The above molar ratio is conducive to the effective precipitation of arsenic and ensures a high arsenic precipitation and solidification rate.

[0023] According to some embodiments of the present invention, in step S2, the pH value of the sodium arsenate solution is 1-3.

[0024] Under the above conditions, it helps to dissolve the metal ions in the liquid phase and maintain their activity, avoiding the formation of precipitation or other insoluble substances. It can prevent other metals (such as calcium, magnesium, etc.) from forming insoluble metal salts in the arsenic precipitation reaction. These byproducts may interfere with the arsenic precipitation reaction. It helps to form iron ions with better solubility, and then the iron ions and arsenate form insoluble arsenic iron compounds (such as scorodite), thereby effectively precipitating arsenic. When the pH is close to 1.5, the oxidation state of iron ions (such as Fe 3+ ) is more stable and can react with arsenate to form a precipitate, thereby improving the removal efficiency of arsenic.

[0025] According to some embodiments of the present invention, in step S2, the temperature of precipitating arsenic is 75-95°C.

[0026] In the above temperature range, it can more effectively combine with arsenate to form insoluble arsenic iron precipitate (such as scorodite), thereby improving the removal efficiency of arsenic. At the same time, this helps to generate a more dense and stable crystal structure, reducing the risk of arsenic release during subsequent treatment or long-term storage.

[0027] According to some embodiments of the present invention, in step S2, the molar ratio of iron to arsenic is 1.1 to 1.5:1.

[0028] According to some embodiments of the present invention, in step S2, the reduction potential is 500-550 mv.

[0029] Under the above conditions, the excess iron ion supply ensures that the combination reaction with arsenate can proceed fully. At the same time, during the arsenic precipitation reaction, some iron ions react with other substances or participate in side reactions, resulting in a decrease in effective iron ions. Adjusting the potential to 500-550mv helps to reduce the divalent iron (Fe 2+ ) is oxidized to ferric iron (Fe 3+ ), making Fe 3+ Continuously generated, so that arsenate (AsO 4 3- ) is more likely to interact with Fe 3+ Combine to form a stable insoluble arsenic-iron compound.

[0030] Under the premise that the above pH, iron-arsenic molar ratio and temperature meet the basic requirements, the most critical parameter is the potential. At a higher potential, the solution precipitates arsenic more thoroughly, the generation efficiency of scorodite is higher, and the morphology of scorodite crystals is wheat-grain-shaped. In the preferred scheme, the pH value of the sodium arsenate solution is 1-3, the potential is regulated by hydrogen peroxide at 400-550mv, and the temperature is controlled at 75-95°C. The scorodite obtained by arsenic precipitation is a wheat-shaped crystal with an arsenic leaching toxicity of less than 1mg / L, and the arsenic concentration of the liquid after arsenic precipitation is 1-2mg / L. The wheat-shaped scorodite crystals have a stable structure, low arsenic leaching toxicity, and long-term environmental safety during the arsenic precipitation reaction.

[0031] According to some embodiments of the present invention, in step S2, the arsenic precipitation further includes washing the arsenic residue obtained after the arsenic precipitation to obtain a washing solution B.

[0032] According to some embodiments of the present invention, step S2 further includes concentrating the washing solution B to obtain a concentrated solution B.

[0033] The concentrated liquid B and the washing liquid B are returned to prepare the sodium carbonate solution, and no secondary wastewater is generated.

[0034] The washing liquid B is concentrated and crystallized to obtain Glauber's salt, and sodium and sulfur are recovered. The concentrated liquid and washing liquid can realize the resource utilization and reduction of calcium arsenic slag and the long-term stable treatment of arsenic. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is the morphology of the scorodite recovered and prepared in Example 1;

[0036] Figure 2 This is the morphology of the scorodite recovered and prepared in Comparative Example 2. DETAILED DESCRIPTION

[0037] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0038] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0039] Unless otherwise specified, "room temperature" in the present invention means 25°C±5°C.

[0040] Unless otherwise specified, "about" in the present invention means that the allowable error is within ±2%.

[0041] If the specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If the manufacturers of the reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0042] The first aspect of the present invention provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag, comprising the following steps:

[0043] S1: mixing calcium arsenic slag and sodium carbonate solution and filtering to obtain a filtrate and a filter residue containing calcium carbonate;

[0044] After washing the filter residue, washing liquid A is collected;

[0045] The pH value of the mixed reaction is 10-11;

[0046] The temperature of the mixed reaction is 25°C-60°C;

[0047] S2: the filtrate and the washing liquid are mixed to obtain a sodium arsenate solution, and then ferrous sulfate is added, and the oxidation-reduction potential is controlled to 400-550mv with hydrogen peroxide to precipitate arsenic.

[0048] It should be noted that, in view of the problems in the existing process that high-temperature carbon reduction has high energy consumption, arsenic needs to be sealed and preserved, is difficult to manage, and has a high risk of secondary pollution, the present invention prepares sodium carbonate into a solution, stirs it evenly with calcium arsenic slag to obtain a mixed slurry, adjusts the liquid-to-solid ratio, pH value and temperature, filters after a period of replacement reaction, washes and dries the filter residue to obtain calcium carbonate; mixes the filtrate and the washing liquid in step S2 to obtain a sodium arsenate solution, adjusts the pH value with sulfuric acid, adds ferrous sulfate according to the molar ratio of iron to arsenic, adjusts the potential with hydrogen peroxide, performs arsenic precipitation reaction at a certain temperature, and then filters and washes to obtain scorodite with long-term stability, and the filtrate is a mixed solution of acidic sodium sulfate and ferric sulfate.

[0049] Example 1

[0050] This embodiment provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag, specifically:

[0051] S1: Weigh 500g of calcium arsenic slag produced by a certain enterprise by wet weight, containing 53.5% water, 3.5% arsenic and 15% calcium, put it into a 2L container, prepare sodium carbonate into a solution with a mass concentration of 5%, add the sodium carbonate solution into the container according to a carbon-arsenic molar ratio of 1.2:1, place the container in a water bath, heat it to 55°C, stir and mix continuously, adjust the pH value to 10.5 with 2% sulfuric acid, carry out the replacement reaction for 4h, then filter and wash the mixed slurry, dry the filter residue and measure the arsenic content and arsenic leaching toxicity;

[0052] S2: The filtrate was diluted to 1000 mL, and the pH value was adjusted to 1.5 with 10% sulfuric acid. Ferrous sulfate was added according to the molar ratio of iron to arsenic of 1.5. The potential was adjusted to 500 mv with hydrogen peroxide. The water bath temperature was controlled at 75 °C. The arsenic was precipitated for 4 h, filtered and washed to obtain scorodite with long-term stability. Figure 1 shown.

[0053] The arsenic content and arsenic leaching toxicity of scorodite were measured, and the arsenic precipitation filtrate and washing liquid were mixed to 1000mL to measure the arsenic concentration and calculate the arsenic precipitation rate. After testing, on a dry weight basis, the arsenic content in calcium carbonate was 0.006%, the arsenic leaching toxicity was 0.05mg / L, and the carbon arsenic replacement rate was 99.91% (in terms of arsenic, the same below). The arsenic content of scorodite was 32.12%, the arsenic leaching toxicity was 0.23mg / L, the arsenic content of the liquid after arsenic precipitation was 1.5mg / L, and the arsenic precipitation rate was 99.96%.

[0054] Example 2

[0055] This embodiment provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag, specifically:

[0056] S1: Weigh 500g of calcium arsenic slag produced by another enterprise by wet weight, containing 48.2% water, 5.2% arsenic and 16.8% calcium, put it into a 2L container, prepare sodium carbonate into a solution with a mass concentration of 5%, add the sodium carbonate solution into the container according to a carbon-arsenic molar ratio of 1.25:1, place the container in a water bath, heat it to 55°C, stir and mix continuously, adjust the pH value to 11 with 2% sulfuric acid, carry out the replacement reaction for 4h, then filter and wash the mixed slurry to obtain a filtrate and a filter residue, dry the filter residue and measure the arsenic content and arsenic leaching toxicity;

[0057] S2: The filtrate was diluted to 1000mL, and the pH value was adjusted to 1.5 with 10% sulfuric acid. Ferrous sulfate was added according to the iron-arsenic molar ratio of 1.5. The potential was adjusted to 550mv with hydrogen peroxide. The water bath heating temperature was controlled at 95°C. The arsenic was precipitated for 2h. The residue was filtered and washed. The arsenic content and arsenic leaching toxicity were measured. The arsenic precipitation filtrate and washing liquid were mixed and diluted to 1000mL to measure the arsenic concentration and calculate the arsenic precipitation rate.

[0058] After testing, based on dry weight, the calcium carbonate contains 0.0052% arsenic, the arsenic leaching toxicity is 0.055 mg / L, the carbon-arsenic replacement rate is 99.93% (calculated as arsenic, the same below), the scorodite contains 33.09% arsenic, the arsenic leaching toxicity is 0.11 mg / L, the arsenic content of the liquid after arsenic precipitation is 0.8 mg / L, and the arsenic precipitation rate is 99.97%.

[0059] Comparative Example 1

[0060] This comparative example provides a method for comprehensively recovering calcium and arsenic from calcium-arsenic slag. The difference between this comparative example and Example 1 is that the pH value of the sodium carbonate replacement reaction is different. In step S1, only the pH value is adjusted to 8.5, and other conditions are the same as in Example 1.

[0061] After testing, the dry weight of calcium carbonate contained 0.5% arsenic, the arsenic leaching toxicity was 5.5mg / L, the carbon arsenic replacement rate was 92.86%, and the arsenic content of scorodite was 27.84%, and the arsenic leaching toxicity was 1.13mg / L. The arsenic concentration of the liquid after arsenic precipitation was 55.8mg / L, and the arsenic precipitation rate was 98.32%. After the pH of the replacement reaction was reduced, the efficiency of the calcium-sodium replacement reaction decreased, and the calcium carbonate still contained a lot of arsenic. In addition, due to the incomplete calcium replacement, the calcium flowed into the sodium arsenate solution, resulting in a decrease in the arsenic precipitation efficiency of scorodite.

[0062] Comparative Example 2

[0063] This comparative example provides a method for comprehensively recovering calcium and arsenic from calcium arsenic slag. The difference between this comparative example and Example 1 is that the arsenic precipitation potential is different, which is 600mv. Other conditions are the same as those in Example 1.

[0064] On a dry weight basis, calcium carbonate contains 0.0048% arsenic, arsenic leaching toxicity is 0.053mg / L, carbon arsenic replacement rate is 99.93%, scorodite contains 32.18% arsenic, arsenic leaching toxicity is 2.68mg / L, the arsenic concentration of the liquid after arsenic precipitation is 0.82mg / L, and the arsenic precipitation rate is 99.97%. When the iron potential of the arsenic precipitation reaction increases, the arsenic-containing compounds produced by arsenic precipitation have increased leaching toxicity, and the product morphology also changes, such as Figure 2 shown.

[0065] Comparative Example 3

[0066] This comparative example provides a method for comprehensively recovering calcium and arsenic from calcium arsenic slag. The difference between this comparative example and Example 2 is that the molar ratio of iron to arsenic in precipitated arsenic is different, and the molar ratio of iron to arsenic is 1.1. Other conditions are the same as those in Example 2.

[0067] On a dry weight basis, calcium carbonate contains 0.0048% arsenic, arsenic leaching toxicity is 0.053mg / L, carbon arsenic replacement rate is 99.93%, scorodite contains 32.18% arsenic, arsenic leaching toxicity is 1.13mg / L. After arsenic precipitation, the liquid arsenic concentration is 48.8mg / L, and the arsenic precipitation rate is 98.60%. When the iron-arsenic molar ratio of the arsenic precipitation reaction decreases, the arsenic precipitation efficiency also decreases.

[0068] Comparative Example 4

[0069] This comparative example provides a method for comprehensively recovering calcium and arsenic from calcium arsenic slag. The difference between this comparative example and Example 2 is that the arsenic precipitation potential is different, which is 220mv. Other conditions are the same as those in Example 2.

[0070] Calcium carbonate contains 0.0048% arsenic on a dry weight basis, with an arsenic leaching toxicity of 0.053 mg / L and a carbon-arsenic replacement rate of 99.93%. Scorodite contains 32.18% arsenic, with an arsenic leaching toxicity of 0.68 mg / L. The arsenic concentration in the solution after arsenic precipitation is 1500 mg / L, and the arsenic precipitation rate is 57.11%. When the arsenic precipitation reaction potential is reduced, the arsenic precipitation efficiency is significantly reduced.

Claims

1. A method for comprehensively recovering calcium and arsenic from calcium-arsenic slag, characterized in that: The following steps are involved: S1: mixing calcium arsenic slag and sodium carbonate solution and filtering to obtain a filtrate and a filter residue containing calcium carbonate; After washing the filter residue, washing liquid A is collected; The pH value of the mixed reaction is 10-11; The temperature of the mixed reaction is 25°C-60°C; S2: the filtrate is mixed with the washing solution A to obtain a sodium arsenate solution, and then ferrous sulfate is added, and the oxidation-reduction potential is controlled at 400-550 mv with hydrogen peroxide to precipitate arsenic.

2. The method according to claim 1, characterized in that: In step S1, the liquid-to-solid ratio in the mixing reaction is not less than 1:

1.

3. The method according to claim 1, characterized in that In step S1, the molar ratio of the sodium carbonate to the arsenic is 1.1-1.3:

1.

4. The method according to claim 1, characterized in that: In step S2, the molar ratio of calcium to arsenic in the sodium arsenate solution is not higher than 0.

05.

5. The method according to claim 1, characterized in that In step S2, the pH value of the sodium arsenate solution is 1-3.

6. The method according to claim 1, characterized in that In step S2, the temperature of arsenic precipitation is 75-95°C.

7. The method according to claim 1, characterized in that In step S2, the molar ratio of iron to arsenic is 1.1-1.5:

1.

8. The method according to claim 1, characterized in that: In step S2, the reduction potential is 500-550 mv.

9. The method according to claim 1, characterized in that: In step S2, the arsenic precipitation step further includes washing the arsenic residue obtained after the arsenic precipitation to obtain a washing solution B.

10. The method according to claim 9, characterized in that Step S2 also includes concentrating the washing liquid B to obtain a concentrated liquid B.

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