A method for separating iron, silicon, gallium, aluminum and calcium from fly ash

Abstract

The present invention provides a method for separating iron, silicon, gallium, aluminum and calcium from fly ash, comprising: step S1: subjecting fly ash to a two-stage leaching reaction under nitric acid pressure, subjecting the first filtrate to a second-stage leaching, and processing the first filter residue into a silicon- and iron-containing product; step S2: adding a reducing agent to the second filtrate to react, and obtaining a reduced liquid; step S3: extracting the reduced liquid, and obtaining an organic phase and a raffinate, and the organic phase is stripped to obtain a gallium product; step S4: adding an alkaline substance to the raffinate to adjust the pH to perform an aluminum precipitation reaction, and obtaining aluminum hydroxide and a liquid after aluminum precipitation, and the aluminum hydroxide is washed and roasted to obtain an aluminum oxide product; step S5: adding an oxidant to the liquid after aluminum precipitation to react, and filtering to obtain a liquid after iron removal; step S6: adding sodium sulfate to the liquid after iron removal, filtering after sufficient reaction, and obtaining a calcium sulfate product and a liquid after calcium precipitation; step S7: the liquid after calcium precipitation is deeply decalcified and concentrated and crystallized to obtain a sodium nitrate product. The method of the present invention has low equipment requirements and simple process.

Description

Technical Field

[0001] The invention relates to the technical field of chemical metallurgy, and in particular to a method for separating iron, silicon, gallium, aluminum and calcium from fly ash. Background Art

[0002] At present, my country's thermal power generation and northern heating plants still use coal, producing a large amount of fly ash. Fly ash is widely used in industrial fields such as construction, building materials, metallurgy, chemical industry, environmental protection, backfill, high-performance ceramic materials, and can also be used in agricultural fields such as soil improvement and land reclamation. Fly ash contains a large amount of aluminum oxide, silicon dioxide, iron, and rare metals such as gallium and germanium. However, my country's aluminum resources are very scarce, so the resource utilization of fly ash has important research significance and broad market prospects.

[0003] The methods of extracting alumina from fly ash can be divided into acid method and alkaline method. The alkaline recovery process is mature, but the process flow is long and the recovery cost is high. The acid method currently has hydrochloric acid method and sulfuric acid method. The iron content in fly ash is relatively high. In the existing acid recovery process, the acid directly leaches the fly ash, and almost all the metal elements in the fly ash enter the leachate. The acid consumption is high, the leaching cost is high, the impurity content in the leachate is high, and the impurity removal cost is high.

[0004] At present, there are many methods for removing iron in the process of producing alumina by acid leaching, for example, the iron removal method is used industrially. Patent document CN103805779A proposes a method for removing iron in an acid-based aluminum extraction process, controlling the temperature of the reaction tank to 60-100°C, the aging time to 30-120min, and adjusting the pH value to <3.0 with Na2CO3. This method can effectively solve the problem of separating gallium from iron. However, this method requires the preparation of goethite seeds so that the concentration of goethite seeds reaches 0.9-3.0g / L, and Na2CO3 needs to be added to control the pH value of the solution during the reaction, and the process is complicated. Qi Dong et al. reported a method for removing iron by precipitation, using lime neutralization precipitation to control the pH of the solution to 2, removing part of the iron in the acid leaching solution in advance, and then extracting vanadium and iron by reducing solvent. This method can also effectively remove iron, but it is necessary to use a multi-stage washing method to reduce the loss of vanadium during the neutralization process, which makes the process longer and the process more complicated, and is not suitable for the separation of iron and aluminum in alumina. In addition, iron removal processes also include magnetic roasting, extraction, recrystallization, etc. Although the iron removal effect is good, the process flow is relatively complicated and the production cost is high, making it difficult to achieve industrialization.

[0005] In summary, it is of great research value to develop a method for separating iron, silicon, gallium, aluminum and calcium from fly ash with low equipment requirements, simple process, low energy consumption and high product added value. Summary of the invention

[0006] The method of the invention can recover useful elements in fly ash in steps and obtain industrial alumina, calcium sulfate and sodium nitrate products.

[0007] Specifically, the present invention provides the following technical solution: a method for separating iron, silicon, gallium, aluminum and calcium from fly ash, comprising:

[0008] Step S1: subjecting fly ash to a two-stage leaching reaction under nitric acid pressure, adding excess acid to the fly ash or the second filter residue in the first stage of leaching to increase the leaching rate of aluminum, filtering to obtain a first filtrate and a first filter residue after the reaction is completed, and processing the first filter residue into a silicon- and iron-containing product; mixing the first filtrate with fly ash for a second stage of leaching, neutralizing the acid in the first filtrate, and increasing the pH value of the solution, filtering to obtain a second filtrate and a second filter residue after the reaction is completed, and returning the second filter residue to the first stage of leaching;

[0009] Step S2: Add a reducing agent to the second filtrate to react and obtain a reduced solution; in this step, Fe 3+ Reduction to Fe 2+ , Fe 2+ with Ga 3+ The large difference in distribution coefficients in the extractant is conducive to the separation of iron and gallium. 2+ The higher pH of hydrolysis is conducive to the separation of iron and aluminum;

[0010] Step S3: extracting the reduced liquid to obtain an organic phase and a raffinate, and stripping the organic phase to obtain a gallium product;

[0011] Step S4: adding alkaline substances to the raffinate to adjust the pH to carry out aluminum precipitation reaction, filtering after sufficient reaction to obtain aluminum hydroxide and aluminum precipitation liquid, washing and roasting the aluminum hydroxide to obtain an alumina product;

[0012] Step S5: adding an oxidant to the above aluminum precipitation solution for reaction, filtering to obtain an iron removal solution; in this step, Fe 2+ Oxidized to Fe 3+ ;

[0013] Step S6: adding sodium sulfate to the iron removal solution, filtering after sufficient reaction, and obtaining a high-purity calcium sulfate product and calcium precipitation solution;

[0014] Step S7: adding sodium hydroxide and sodium carbonate to the post-calcification solution, removing the trace amount of calcium and magnesium remaining in the post-calcification solution in the form of carbonates under a high pH condition, obtaining a sodium nitrate solution with a high purity, and then performing negative pressure evaporation, concentration and crystallization to obtain a sodium nitrate product.

[0015] Preferably, in step S1, the first stage leaching conditions include: the solid-liquid ratio of fly ash or the second filter residue to nitric acid is 1:2-1:5 (g / mL), the concentration of nitric acid is 300-500g / L, the leaching temperature is 160-220°C, the stirring rate is 55-85rpm, and the leaching time is 0.5-5h.

[0016] The second stage leaching conditions include: the solid-liquid ratio of fly ash to the first filtrate is 1:2-1:5 (g / mL), the leaching temperature is 160-220°C, the stirring rate is 55-85rpm, and the leaching time is 0.5-3h. Preferably, the mass percentage of the main chemical elements of the fly ash includes: Fe is 1-5%; Al is 16-30%; Si is 18-25%; Ca is 2-8%; Mg is 0.2-0.8%.

[0017] Preferably, the concentration of nitric acid in the first stage leaching reaction is 300-500 g / L, and the leaching temperature is 160-200°C.

[0018] Preferably, in the second stage leaching reaction, the leaching temperature is 180-220° C. and the leaching time is 1-3 h.

[0019] In the above preferred nitric acid pressure two-stage leaching scheme, a large amount of iron in the fly ash can be slaged through this step, and more than 90% of the iron can be removed.

[0020] Preferably, in step S2, the reducing agent is at least one of iron powder and sodium sulfite.

[0021] Preferably, in step S4, the alkaline substance is one of sodium carbonate and sodium hydroxide.

[0022] Preferably, in step S4, the precipitation reaction pH value is 4-6, the reaction temperature is 60-100°C, the reaction time is 0.5-3.5h, and the feeding method is homogeneous precipitation. Under this preferred scheme, the aluminum precipitation rate through this step is greater than 99%, and the filterability is good.

[0023] Preferably, in step S5, the oxidant is one of hydrogen peroxide or oxygen. In this preferred embodiment, the iron content in the solution after this step is less than 0.01 g / L.

[0024] Preferably, in step S6, the amount of sodium sulfate added is 0.8-1.0 times the theoretical amount of calcium sulfate generated. Under this preferred embodiment, the calcium precipitation rate through this step is greater than 95%, and the filterability is good.

[0025] Preferably, in step S7, the amount of sodium hydroxide added is to adjust the pH value of the solution to 11-14, and the amount of sodium carbonate added is 1-1.5 times the theoretical amount of carbonate generated by calcium and magnesium in the solution after calcium precipitation; preferably, in step S7, sodium hydroxide is first added to adjust the pH value of the solution, and then sodium carbonate is added. Through this step, the calcium content of the solution is less than 0.005g / L, and the magnesium content is less than 0.0005g / L.

[0026] Preferably, in step S7, the negative pressure evaporation concentration temperature is 60-100°C, the concentration endpoint specific gravity is 1.500-1.600, and the crystallization temperature is 20-40°C. Under this preferred embodiment, the single crystallization rate of sodium nitrate can be as high as 50% or more, and the crystals are uniform and granular. The beneficial effects brought by the technical solution provided by the present invention include at least:

[0027] The method of the present invention uses fly ash as raw material, adopts nitric acid pressure two-stage leaching, silicon and iron enter slag, and the leached slag can be further processed into iron and silicon products; a small amount of iron in the leaching solution is reduced to Fe 2+ , first extract gallium, adjust pH to precipitate aluminum, and then oxidize to remove iron; precipitate aluminum hydroxide and wash and roast to obtain an alumina product; add sodium sulfate to the iron removal solution to obtain a calcium sulfate product, and use double alkali to deeply remove calcium and magnesium from the decalcified solution, evaporate, concentrate and crystallize to obtain a sodium nitrate product. The present invention can recover iron, silicon, gallium, aluminum, calcium and other elements in fly ash in steps, and leaches a large amount of iron into slag in two stages by pressurizing nitric acid to initially separate iron and aluminum; then reduce a small amount of iron in the solution and then oxidize it to achieve a thorough separation of iron and gallium, and iron and aluminum; the entire process does not consume acid for iron, has the advantages of low acid consumption, low impurity content in the leaching solution, low impurity removal cost, low equipment requirements, and simple process. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solution and advantages of the present invention more clear, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] The analysis results of the various components of the fly ash raw materials used in the following examples are shown in Table 1.

[0032] Table 1 Analysis results of main components of fly ash

[0033]

[0034] Example 1

[0035] like Figure 1 As shown, a method for separating iron, silicon, gallium, aluminum and calcium from fly ash specifically comprises the following steps:

[0036] S1: The first stage leaching conditions include: the solid-liquid ratio of fly ash to nitric acid is 1:4 (g / mL), the concentration of nitric acid is 400g / L, the leaching temperature is 180°C, the stirring rate is 60rpm, and the leaching time is 2h; the second stage leaching conditions include: the solid-liquid ratio of fly ash to the first filtrate is 1:3 (g / mL), the leaching temperature is 190°C, the stirring rate is 60rpm, and the leaching time is 1h.

[0037] The iron content in the obtained second filtrate was 0.75 g / L, the leaching rate of aluminum in the secondary leaching under nitric acid pressure was 94%, and the leaching rate of iron was 8.22%.

[0038] S2: Add sodium sulfite, Fe 3+ Reduction to Fe 2+ , and obtain the reduced liquid.

[0039] S3: extracting gallium from the reduced liquid to obtain an organic phase and a raffinate, stripping the organic phase to obtain a stripping solution, and further processing the stripping solution into a gallium product.

[0040] S4: adding sodium carbonate to the above raffinate to adjust the pH to 5, the reaction temperature is 60°C, the reaction time is 1 hour to precipitate aluminum, and after sufficient reaction, filtering to obtain aluminum hydroxide and aluminum precipitation liquid, and the aluminum hydroxide is washed and roasted to obtain an alumina product.

[0041] The aluminum content in the obtained aluminum precipitation liquid is 0.0015g / L, and the aluminum precipitation rate is 99.99%.

[0042] S5: Add hydrogen peroxide to the above aluminum precipitation solution, Fe 2+ Oxidized to Fe 3+ , filter to obtain the iron-removed liquid.

[0043] The iron content in the liquid after iron removal is 0.0020 g / L, and the iron removal rate is 99.56%.

[0044] S6: Add sodium sulfate to the above iron removal solution in an amount of 0.9 times the theoretical amount of calcium sulfate, stir and react at 70°C for 1 hour, filter after sufficient reaction, and obtain calcium sulfate product and calcium precipitation solution;

[0045] The calcium content in the obtained calcium precipitation solution was 1.76 g / L, and the calcium precipitation rate was 95.06%.

[0046] S7: First, sodium hydroxide is added to the above calcium precipitation solution to adjust the solution pH to 13, and then calcium and magnesium are added to generate sodium carbonate 1 times the theoretical amount of carbonate. The reaction temperature is 70°C and the reaction time is 1h. After sufficient reaction, the solution is filtered and evaporated and concentrated at 100°C for crystallization. The concentration endpoint specific gravity is 1.550 to obtain sodium nitrate product.

[0047] The purity of the aluminum oxide obtained in this embodiment is 98.65%, the purity of the calcium sulfate is 90%, and the purity of the sodium nitrate is 99.5%.

[0048] Example 2

[0049] A method for separating iron, silicon, gallium, aluminum and calcium from fly ash is implemented by the method described in Example 1, except that:

[0050] In step S1, the second stage leaching temperature is 200°C.

[0051] In step S1, the iron content in the obtained second filtrate is 0.45 g / L, the leaching rate of aluminum in the secondary leaching with nitric acid pressure is 94.5%, and the leaching rate of iron is 4.93%.

[0052] In step S4, the aluminum content in the obtained aluminum precipitation liquid is 0.0020 g / L, and the aluminum precipitation rate is 99.99%.

[0053] In step S5, the iron content in the liquid after iron removal is 0.0015 g / L, and the iron removal rate is 99.40%.

[0054] In step S6, the calcium content in the obtained calcium precipitation solution is 1.81 g / L, and the calcium precipitation rate is 94.91%.

[0055] The purity of the aluminum oxide obtained in this embodiment is 98.72%, the purity of the calcium sulfate is 90.04%, and the purity of the sodium nitrate is 99.49%.

[0056] Example 3

[0057] A method for separating iron, silicon, gallium, aluminum and calcium from fly ash is implemented by the method described in Example 1, except that:

[0058] In step S4, sodium hydroxide is added to the raffinate to adjust the pH to 4.5.

[0059] In step S1, the iron content in the obtained second filtrate is 0.72 g / L, the leaching rate of aluminum in the secondary leaching with nitric acid pressure is 94.1%, and the leaching rate of iron is 7.89%.

[0060] In step S4, the aluminum content in the obtained aluminum precipitation liquid is 0.12 g / L, and the aluminum precipitation rate is 99.52%.

[0061] In step S5, the iron content in the liquid after iron removal is 0.0022 g / L, and the iron removal rate is 99.69%.

[0062] In step S6, the calcium content in the obtained calcium precipitation solution is 1.75 g / L, and the calcium precipitation rate is 95.07%.

[0063] The purity of the aluminum oxide obtained in this embodiment is 98.62%, the purity of the calcium sulfate is 88.37%, and the purity of the sodium nitrate is 99.46%.

[0064] Comparative Example 1

[0065] A method for separating iron, silicon, gallium, aluminum and calcium from fly ash is implemented by the method described in Example 1, except that:

[0066] In step S1, the second stage leaching temperature is 150°C.

[0067] In step S1, since the leaching temperature of the second stage is lower than that of the embodiment, the iron content in the obtained second filtrate is 3.24 g / L, which is higher than the result in the embodiment, resulting in an increase in the subsequent iron removal cost. The leaching rate of aluminum in the secondary leaching with nitric acid pressure is 91.2%, and the leaching rate of iron is 35.51%.

[0068] In step S4, the aluminum content in the obtained aluminum precipitation liquid is 0.0014 g / L, and the aluminum precipitation rate is 99.99%.

[0069] In step S5, the iron content in the liquid after iron removal is 0.015 g / L, and the iron removal rate is 99.23%.

[0070] In step S6, the calcium content in the obtained calcium precipitation solution is 1.85 g / L, and the calcium precipitation rate is 94.79%.

[0071] The purity of the aluminum oxide obtained in this embodiment is 98.42%, the purity of the calcium sulfate is 89.75%, and the purity of the sodium nitrate is 99.34%.

[0072] Comparative Example 2

[0073] A method for separating iron, silicon, gallium, aluminum and calcium from fly ash is implemented by the method described in Example 1, except that:

[0074] In step S4, sodium hydroxide is added to the raffinate to adjust the pH to 3.5.

[0075] In step S1, the iron content in the obtained second filtrate is 0.73 g / L, the leaching rate of aluminum in the secondary leaching with nitric acid pressure is 94.05%, and the leaching rate of iron is 8%.

[0076] In step S4, due to the low pH value, the aluminum content in the obtained aluminum precipitation liquid is 12.51 g / L, which is higher than that in the embodiment, and the aluminum precipitation rate is 49.96%, resulting in an excessively low aluminum precipitation rate.

[0077] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for separating iron, silicon, gallium, aluminum and calcium from fly ash, characterized in that: include: Step S1: subjecting the fly ash to a two-stage leaching reaction under nitric acid pressure, filtering the first stage leaching to obtain a first filtrate and a first filter residue, processing the first filter residue into a silicon- and iron-containing product, subjecting the first filtrate to a second stage leaching to obtain a second filtrate and a second filter residue, and returning the second filter residue to the first stage leaching; Step S2: adding a reducing agent to the second filtrate to react and obtain a reduced liquid; Step S3: extracting the reduced liquid to obtain an organic phase and a raffinate, and stripping the organic phase to obtain a gallium product; Step S4: adding alkaline substances to the raffinate to adjust the pH to carry out aluminum precipitation reaction, filtering after sufficient reaction to obtain aluminum hydroxide and aluminum precipitation liquid, washing and roasting the aluminum hydroxide to obtain an alumina product; Step S5: adding an oxidant to the aluminum precipitation liquid to react, and filtering to obtain an iron removal liquid; Step S6: adding sodium sulfate to the iron removal solution, filtering after sufficient reaction to obtain a calcium sulfate product and a calcium precipitation solution; Step S7: after using sodium hydroxide and sodium carbonate to deeply remove calcium and magnesium from the solution after calcium precipitation, vacuum evaporation, concentration and crystallization are performed to obtain a sodium nitrate product; In step S1, the first stage leaching conditions include: the solid-liquid ratio of fly ash or the second filter residue to nitric acid is 1:2-1:5, the concentration of nitric acid is 300-500g / L, the leaching temperature is 160-220°C, the stirring rate is 55-85rpm, and the leaching time is 0.5-5h; The second stage leaching conditions include: the solid-liquid ratio of fly ash to the first filtrate is 1:2-1:5, the leaching temperature is 160-220°C, the stirring rate is 55-85rpm, and the leaching time is 0.5-3h; In step S4, the alkaline substance is at least one of sodium carbonate and sodium hydroxide; the precipitation reaction pH value is 4-6, the reaction temperature is 60-100° C., the reaction time is 0.5-3.5 h, and the feeding method is homogeneous precipitation.

2. The method according to claim 1, characterized in that: In step S1, the mass percentages of the main chemical elements of the fly ash include: Fe is 1-5%; Al is 16-30%; Si is 18-25%; Ca is 2-8%; and Mg is 0.2-0.8%.

3. The method according to claim 1, characterized in that In step S2, the reducing agent is at least one of iron powder and sodium sulfite.

4. The method according to claim 3, characterized in that In step S2, the reducing agent is sodium sulfite.

5. The method according to claim 1, characterized in that In step S5, the oxidant is at least one of hydrogen peroxide or oxygen.

6. The method according to claim 1, characterized in that In step S6, the amount of sodium sulfate added is 0.8-1.0 times the theoretical amount of calcium sulfate generated.

7. The method according to claim 1, characterized in that In step S7, the amount of sodium hydroxide added is to adjust the pH value of the solution to 11-14, and the amount of sodium carbonate added is 1-1.5 times the theoretical amount of carbonate generated by calcium and magnesium in the solution after calcium precipitation.

8. The method according to claim 7, characterized in that In step S7, sodium hydroxide is first added to adjust the pH of the solution, and then sodium carbonate is added.

9. The method according to claim 1, characterized in that: In step S7, the negative pressure evaporation concentration temperature is 60-100°C, the concentration endpoint specific gravity is 1.500-1.600, and the crystallization temperature is 20-40°C.

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