Method for producing aluminum hydroxide from fly ash and co-producing silicon-calcium-sulfur-magnesium fertilizer, silicon-calcium-sulfur-magnesium fertilizer, aluminum hydroxide

Abstract

The present invention provides a method for producing aluminum hydroxide from fly ash with co-production of calcium-sulfur-magnesium fertilizer, a calcium-sulfur-magnesium fertilizer, and aluminum hydroxide. The preparation method of the calcium-sulfur-magnesium fertilizer includes: performing a first reaction on fly ash and a sulfuric acid solution, and then separating the solid and liquid to obtain a first solid and a first liquid; under the action of an alkali, performing a second reaction on the first solid and a calcium source, and then separating the solid and liquid to obtain a second solid and a second liquid; performing a third reaction on the first liquid and a magnesium source, and then separating the solid and liquid to obtain a third solid and a third liquid; performing post-treatment on the third solid to obtain aluminum hydroxide; crystallizing the third liquid, and then separating the solid and liquid to obtain a fourth solid and a fourth liquid; mixing the second solid and the fourth liquid, and granulating to obtain the calcium-sulfur-magnesium fertilizer.

Description

Technical Field

[0001] The present invention relates to the technical field of solid waste recycling and fertilizer manufacturing. Specifically, it relates to a method for producing aluminum hydroxide from fly ash and co-producing calcium-sulfur-magnesium fertilizer, a calcium-silicon-sulfur-magnesium fertilizer, and aluminum hydroxide. Background Art

[0002] Fly ash is a large amount of solid waste in China, and its comprehensive utilization rate needs to be improved. High-aluminum fly ash is a unique "artificial ore deposit" resource in China. In addition to containing 40-50 wt% of alumina, it is also associated with rare and precious metal resources such as lithium, gallium, and rare earths, showing great development and utilization prospects. Aluminum hydroxide is a kind of basic chemical product and has important applications in industries such as materials, catalysis, medicine, and metallurgy. The sulfuric acid method for aluminum extraction can achieve high-temperature aluminum dissolution under normal pressure, with relatively low requirements for dissolution equipment, and the dissolution rates of aluminum and other rare metals are relatively high. The main component of the liquid phase product obtained by sulfuric acid aluminum dissolution is aluminum sulfate, and aluminum hydroxide products can be obtained by adding alkali for hydrolysis and precipitation.

[0003] The main component of the residual solid waste after aluminum extraction from fly ash is silicon dioxide. The secondary utilization of silicon components has become the key to the large-scale consumption and high-value utilization of high-aluminum fly ash. Silicon in fly ash itself has good activity. After being treated by the sulfuric acid method for aluminum extraction, the reaction activity of silicon is further improved, its content is further enriched, and most of the heavy metal elements such as lead, cadmium, and chromium that are sensitive to agriculture are removed, making it an ideal raw material for preparing silicon fertilizer.

[0004] Silicon is an essential medium nutrient element in the process of crop growth and is listed as the fourth major element after nitrogen, phosphorus, and potassium by the international soil community. Silicon fertilizer can not only provide active silicon nutrient elements to the soil but also has functions such as regulating the soil pH value, improving the soil structure, and solidifying heavy metals for soil improvement and remediation. The domestic potential demand for silicon fertilizer is about 40 million tons per year, while the production capacity is only 4 million tons, resulting in a huge market gap. Silicon fertilizers are divided into water-soluble silicon fertilizers and citrate-soluble silicon fertilizers (solid silicon fertilizers that can dissolve in weak acids). Citrate-soluble silicon fertilizers are usually made by roasting or hydrothermal activation of silicon-containing ores with alkaline earth metal activators. The products contain oxides of elements such as Ca, Mg, K, and Na in varying proportions, so the products are generally alkaline. Although the industry standard NY797 "Silicon Fertilizer" does not clearly limit the pH range of the products, a relatively high alkalinity will greatly limit the applicable regions and applicable crop ranges of the products. The most ideal state is to control the pH of the silicon fertilizer within a nearly neutral range (pH = 6.5 - 7.5), but it is technically difficult to achieve. In the national standard GBT36207-2018 "Calcium-Silicon-Potassium-Magnesium Fertilizer", the pH of the product is limited between 8 and 11.

[0005] Currently, a method for adjusting the pH of silicon fertilizer is required. The easily conceived scheme of acid flushing is not feasible. For example, when flushing with weak acid, the weak acid will dissolve citrate-soluble silicon, resulting in a decrease in the core index of available silicon; while the scheme of adjusting the alkalinity with concentrated acid is even more infeasible. As is well known, silicon elements will undergo excessive polycondensation to form non-citrate-soluble silica gel when encountering concentrated acid, which will also cause a significant decrease in the available silicon index.

[0006] Meanwhile, for the convenience of application, powdered fertilizers are usually granulated into granular fertilizers after being granulated. The requirements for the binder are as follows: under the premise of being able to generate sufficient compressive strength, agricultural harmful components cannot be introduced; the harmless components introduced cannot react with the original fertilizer powder to cause deterioration of performance indicators; the addition amount should not be too large to avoid dilution and decrease of key indicators; the price is low; the raw materials are easily available. Among the commonly used granulation binders, sodium silicate (water glass) can provide relatively high strength and can supplement some available silicon, but its alkalinity is too high, which will cause an increase in the pH value of the product; the costs of organic binders such as polyvinyl formal and hydroxymethylated polyacrylamide are relatively high; although inorganic binders such as attapulgite and desulfurized gypsum are low in price, their binding effects are relatively poor, and higher addition ratios are required, which will cause dilution of the available silicon index.

[0007] Therefore, there is an urgent need in the art for a low-cost method that can both reduce the pH value of silicon fertilizer and not significantly affect the content of available silicon. At the same time, there is also an urgent need in the art for a granulation binder and a corresponding granulation method that can both ensure the forming strength, introduce beneficial components, and have an acceptable cost. Summary of the Invention

[0008] The main object of the present invention is to provide a method for producing aluminum hydroxide from fly ash and co-producing silicon-calcium-sulfur-magnesium fertilizer, silicon-calcium-sulfur-magnesium fertilizer, and aluminum hydroxide, so as to solve the problems in the prior art that it is difficult to find a suitable method for reducing the pH of silicon fertilizer and the lack of a suitable granulation binder.

[0009] To achieve the above object, according to one aspect of the present invention, a method for producing aluminum hydroxide from fly ash and co-producing silicon-calcium-sulfur-magnesium fertilizer is provided. Calculated by weight percentage of fly ash, the fly ash contains 30-60% of SiO2, 10-50% of Al2O3, and 5-10% of C, and it includes: Step S1, subjecting fly ash to a first reaction with a sulfuric acid solution, and then performing solid-liquid separation to obtain a first solid and a first liquid; Step S2, under the action of an alkali, subjecting the first solid and a calcium source to a second reaction, and then performing solid-liquid separation to obtain a second solid and a second liquid; Step S3, subjecting the first liquid to a third reaction with a magnesium source, and then performing solid-liquid separation to obtain a third solid and a third liquid; performing post-treatment on the third solid to obtain aluminum hydroxide; Step S4, causing the third liquid to crystallize, and then performing solid-liquid separation to obtain a fourth solid and a fourth liquid; Step S5, mixing the second solid and the fourth liquid, and granulating to obtain silicon-calcium-sulfur-magnesium fertilizer.

[0010] Furthermore, the molar ratio of sulfuric acid in the sulfuric acid solution to Al2O3 in the fly ash is (2.2 - 2.4):1, preferably (2.3 - 2.4):1; preferably, the weight concentration of the sulfuric acid solution is 70 - 98%.

[0011] Furthermore, the conditions for the first reaction include: temperature 170 - 280 °C, time 90 - 180 min; preferably, the conditions for the first reaction include: temperature 180 - 220 °C, time 90 - 120 min.

[0012] Furthermore, the second reaction is a hydrothermal reaction. Preferably, the temperature of the hydrothermal reaction is 90 - 110 °C and the time is 3 - 6 h.

[0013] Furthermore, the molar ratio of calcium in the calcium source to SiO2 in the fly ash is (0.7 - 1.2):1, preferably (0.8 - 1.1):1; more preferably, the calcium source is calcium oxide and / or calcium hydroxide.

[0014] Furthermore, the base is added in the form of an alkali solution, and the weight ratio of the alkali solution to the first solid is (3 - 6):1, preferably (4 - 5):1; preferably, the weight concentration of the base in the alkali solution is 8 - 50 wt%, preferably 25 - 33 wt%; more preferably, the base in the alkali solution is selected from at least one of NaOH and KOH.

[0015] Furthermore, the SO4 in the first liquid 2- and the molar ratio of magnesium in the magnesium source is 1:(0.95 - 1.0); preferably, the magnesium source is one or more of magnesium hydroxide, magnesium oxide, magnesium carbonate, and basic magnesium carbonate.

[0016] Furthermore, the conditions for the third reaction are: temperature 60 - 80 °C, time 40 - 60 min; preferably, the temperature of the third reaction is 70 - 75 °C and the time is 45 - 55 min.

[0017] Furthermore, the weight ratio of the second solid to the fourth liquid on a dry basis is 1:(1.5 - 1.7).

[0018] Furthermore, the granulation method uses at least one of disk granulation, roll extrusion granulation, and high-speed granulator granulation; after granulation, the method further includes: standing and drying. Preferably, the standing time is not less than 7 days.

[0019] To achieve the above object, according to one aspect of the present invention, there is provided a calcium silicate sulfur magnesium fertilizer obtained by the above method. By weight, the effective silicon content calculated as SiO2 is 12 - 16 wt%, preferably 13 - 16 wt%; the effective calcium content calculated as CaO is 10 - 16 wt%, preferably 12 - 15 wt%; the effective magnesium content calculated as MgO is 7 - 10 wt%, preferably 7 - 8.5 wt%; the sulfur content calculated as S is 10 - 13 wt%, preferably 10.5 - 12.5 wt%; and the pH of the calcium silicate sulfur magnesium fertilizer is 8.2 - 8.6, and the compressive strength ≥ 15 N.

[0020] According to another aspect of the present invention, there is provided aluminum hydroxide obtained by the above method, and its dry - based Al2O3 content ≥ 63.5%.

[0021] Applying the technical solution of the present invention, there is provided a method for resource utilization of fly ash, and co - producing aluminum hydroxide and calcium silicate sulfur magnesium fertilizer. According to the method of the present invention, first, acid leaching is carried out to obtain a first liquid with the main component being aluminum sulfate, then a magnesium source is introduced to convert the liquid - phase components into magnesium sulfate (magnesium sulfate is the main component in the fourth liquid), thus realizing the extraction of aluminum and preparing aluminum hydroxide. On the other hand, the main component of the sulfuric acid aluminum - extraction residue (the first solid) is silicon dioxide, which reacts with a calcium source under the action of an alkali to obtain a powdery calcium silicate fertilizer (the second solid); the magnesium sulfate solution obtained in the previous step is further used for mixing granulation with the powdery calcium silicate fertilizer (the second solid), thus realizing the co - production of calcium silicate sulfur magnesium fertilizer.

[0022] Compared with the prior art, the present invention has at least the following beneficial effects:

[0023] First, the calcium source and magnesium source introduced in the present invention are valuable components in fertilizer products, and other by - products generated in the process, such as the fourth solid (mainly magnesium sulfate heptahydrate), also have certain economic value; basically no waste is discharged in the whole process of the present invention, achieving the goal of efficient consumption, full utilization and high - value conversion of solid waste and auxiliary materials.

[0024] Second, the addition of magnesium sulfate, on the one hand, acts as a granulation binder, solving the problem of the lack of a suitable binder in the prior art; on the other hand, it also plays a role in adjusting the pH value of the powdery calcium silicate fertilizer (the second solid); in addition, magnesium sulfate and the powdery calcium silicate fertilizer (the second solid) can undergo a chemical reaction without additional conditions to obtain new components such as CaSO2·2H2O, amorphous Mg(OH)2, and amorphous Si(OH)4. The above - mentioned newly formed components together provide the particle strength, that is, the generation of strength is the result of a chemical reaction, rather than a conventional physical effect. And the introduction of magnesium sulfate also simultaneously increases the effective magnesium and sulfur nutrients, and forms a calcium silicate sulfur magnesium quaternary compound fertilizer in combination with the powdery calcium silicate fertilizer, and its nutrition is more balanced and the use effect is better. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not unduly limit the present invention. In the drawings:

[0026] Figure 1 An XRD picture according to Embodiment 1 of the present invention is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments may be combined with each other. The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0028] In order to solve the problems in the prior art as described above, according to one aspect of the present invention, a method for producing aluminum hydroxide from fly ash and co-producing calcium-silicon-sulfur-magnesium fertilizer is provided. By weight percentage of fly ash, the fly ash contains 30-60% of SiO2, 10-50% of Al2O3, and 5-10% of C. It includes: Step S1, subjecting fly ash to a first reaction with a sulfuric acid solution, and then performing solid-liquid separation to obtain a first solid and a first liquid; Step S2, under the action of an alkali, subjecting the first solid and a calcium source to a second reaction, and then performing solid-liquid separation to obtain a second solid and a second liquid; Step S3, subjecting the first liquid to a third reaction with a magnesium source, and then performing solid-liquid separation to obtain a third solid and a third liquid; performing post-treatment on the third solid to obtain aluminum hydroxide; Step S4, causing the third liquid to crystallize, and then performing solid-liquid separation to obtain a fourth solid and a fourth liquid; Step S5, mixing the second solid with the fourth liquid and granulating to obtain calcium-silicon-sulfur-magnesium fertilizer.

[0029] According to the method of the present invention, first, acid leaching is carried out to obtain a first liquid mainly composed of aluminum sulfate, and then a magnesium source is introduced to convert the liquid-phase components into magnesium sulfate (magnesium sulfate is the main component in the fourth liquid), thus realizing the extraction of aluminum and preparing aluminum hydroxide. On the other hand, the main component of the residue after aluminum extraction with sulfuric acid (the first solid) is silicon dioxide, which reacts with a calcium source under the action of an alkali to obtain powdery calcium-silicon fertilizer (the second solid); the magnesium sulfate solution obtained in the foregoing steps is further used for mixing and granulating with the powdery calcium-silicon fertilizer (the second solid), thus realizing the co-production of calcium-silicon-sulfur-magnesium fertilizer.

[0030] Compared with the prior art, the present invention has at least the following beneficial effects:

[0031] First, the calcium source and magnesium source introduced in the present invention are both valuable components in the fertilizer product, and other by-products produced in the process, such as the fourth solid (mainly magnesium sulfate heptahydrate), also have certain economic value; the whole process of the present invention basically has no waste discharge, and achieves the goal of efficient disposal, full utilization and high-value conversion of solid waste and auxiliary materials.

[0032] Secondly, the addition of magnesium sulfate, as a granulation binder, solves the problem of lack of suitable binders in the prior art; on the other hand, it also plays a role in adjusting the pH value of the powdered calcium-silicon fertilizer (the second solid); in addition, magnesium sulfate and the powdered calcium-silicon fertilizer (the second solid) can react chemically without additional conditions to obtain new components such as CaSO2·2H2O and amorphous Mg(OH)2, amorphous Si(OH)4, etc. The above newly generated components jointly provide particle strength, that is, the generation of strength is the result of chemical reaction, not conventional physical action. In addition, the introduction of magnesium sulfate also increases the effective magnesium and sulfur nutrition, and forms a silicon-calcium-sulfur-magnesium quaternary compound fertilizer with powdered calcium-silicon fertilizer, which has a more balanced nutrition and better use effect.

[0033] In actual operation, the process of post-processing the third solid to obtain aluminum hydroxide includes: washing and drying.

[0034] In order to further improve the effect of acid leaching aluminum, in a preferred embodiment, the molar ratio of sulfuric acid in the sulfuric acid solution to Al2O3 in the fly ash is (2.2-2.4):1, preferably (2.3-2.4):1; preferably, the weight concentration of the sulfuric acid solution is 70-98%. The above preferred conditions are determined based on the characteristics of sulfuric acid and fly ash, and can better improve the leaching efficiency of aluminum. When the amount of sulfuric acid is higher than the above preferred range, more excess acid will remain, which will have a negative effect on the performance of the fertilizer. When the amount of sulfuric acid is lower than the above preferred range, the dissolution rate of aluminum decreases. At this time, it is usually difficult to make up for the loss of dissolution effect by further increasing the reaction temperature, which will result in a high aluminum content and a low silicon content in the aluminum extraction residue (first solid), which will lead to an imbalance in the silicon-calcium ratio.

[0035] In a preferred embodiment, the conditions of the first reaction include: temperature 170-280°C, time 90-180 min; preferably, the conditions of the first reaction include: temperature 180-220°C, time 90-120 min. Such preferred conditions are more conducive to the occurrence of the first reaction and improve the efficiency of aluminum extraction. The first reaction of the present invention is preferably carried out at normal pressure, which has the advantages of saving energy and simplifying the process. When the temperature of the first reaction is higher than 280°C, it may lead to aggravated decomposition of sulfuric acid, even sulfuric acid escape loss, corrosion of equipment, and air pollution, which will be detrimental to industrial production and environmental protection.

[0036] In a typical embodiment of the present invention, after the first reaction is completed, the method of the present invention further includes: adding water to wash the solid phase (aluminum sulfate). In such a step of adding water for washing, the weight ratio of water to sulfuric acid is (1.5 - 2):1, and the washing time is 30 - 60 min. After washing, solid-liquid separation is carried out again.

[0037] In order to better promote the production of the second solid (powdered calcium silicate fertilizer), in a preferred embodiment, the second reaction is a hydrothermal reaction. Preferably, the temperature of the hydrothermal reaction is 90 - 110 °C, and the time is 3 - 6 h. The above preferred conditions are more conducive to the occurrence of the hydrothermal reaction.

[0038] In actual operation, the obtained second solid can also be used as a kind of fertilizer, but because it is in powder form and has a relatively high pH, its application value is limited.

[0039] In actual operation, the second solid also undergoes water washing and moderate dehydration to obtain powdered calcium silicate fertilizer.

[0040] In a typical embodiment of the present invention, by weight, the water content of the powdered calcium silicate fertilizer is 20 - 40%, preferably 25 - 35%; the content of available silicon on a dry basis calculated as SiO2 is 32 - 38%, preferably 35 - 38%; the content of available calcium on a dry basis calculated as CaO is 20 - 40%, preferably 25 - 32%; pH = 10.6 - 11.4.

[0041] For better regulating the ratio of silicon and calcium elements in the calcium-silicon-sulfur-magnesium fertilizer and for the purpose of promoting the second reaction, in a preferred embodiment, the molar ratio of calcium in the calcium source to SiO2 in the fly ash is (0.7 - 1.2):1, preferably (0.8 - 1.1):1; more preferably, the calcium source is calcium oxide and / or calcium hydroxide. When the addition amount of the calcium source is insufficient, the activation of silicon will be affected to a certain extent, thus affecting the available silicon in the calcium-silicon-sulfur-magnesium fertilizer. In the case of insufficient calcium source, it is often difficult to avoid a decrease in the available silicon content even by increasing the amount of the alkali solution, and the residual alkali is not easy to wash, which will also cause the pH to increase. In actual applications, if for the purpose of further cost saving, solid waste containing calcium oxide and calcium hydroxide as the active components can be used as the calcium source.

[0042] In a preferred embodiment, the alkali is added in the form of an alkali solution, and the weight ratio of the alkali solution to the first solid is (3 - 6):1, preferably (4 - 5):1; preferably, the weight concentration of the alkali in the alkali solution is 8 - 50 wt%, preferably 25 - 33 wt%; more preferably, the alkali in the alkali solution is selected from at least one of NaOH and KOH. The above conditions are more conducive to promoting the occurrence of the second reaction.

[0043] For the purpose of further reducing costs, in actual operation, the washing water of the second liquid and the second solid can be combined and concentrated, and then recycled as recovered lye.

[0044] In a preferred embodiment, the molar ratio of SO4 in the first liquid 2- to magnesium in the magnesium source is 1:(0.95 - 1.0); preferably, the magnesium source is one or more of magnesium hydroxide, magnesium oxide, magnesium carbonate, and basic magnesium carbonate. The above preferred molar ratio is more conducive to introducing an appropriate concentration of magnesium for the reaction and to making the calcium-silicon-sulfur-magnesium fertilizer contain an appropriate amount of available magnesium.

[0045] To better promote the occurrence of the third reaction, in a preferred embodiment, the conditions for the third reaction are: temperature 60 - 80 °C, time 40 - 60 min; preferably, the temperature of the third reaction is 70 - 75 °C, and the time is 45 - 55 min. The above conditions can better ensure the sufficiency of the reaction.

[0046] In a typical embodiment of the present invention, in step S4, crystallization is carried out under natural cooling conditions, and the end temperature of crystallization is 10 - 48 °C, preferably room temperature (15 - 35 °C). The fourth solid obtained after solid-liquid separation by crystallization is the magnesium sulfate heptahydrate product, and the obtained liquid is a saturated magnesium sulfate solution.

[0047] In a preferred embodiment, the weight ratio of the second solid to the fourth liquid on a dry basis is 1:(1.5 - 1.7). The above conditions are more conducive to introducing an appropriate percentage of magnesium and more conducive to making the fourth liquid (containing magnesium sulfate) play several functions as detailed above, and finally obtaining a calcium-silicon-sulfur-magnesium fertilizer with excellent available magnesium, strength, and pH.

[0048] To enable the second solid and the fourth liquid to fully undergo a chemical reaction, in a preferred embodiment, the granulation method adopts at least one of disk granulation, roll extrusion granulation, and high-speed granulator granulation; after granulation, the method further includes: standing and drying, preferably, the standing time is not less than 7 days. Such a preferred time can enable the second solid and the fourth liquid to react more fully, obtaining new components such as CaSO2·2H2O, amorphous Mg(OH)2, and amorphous Si(OH)4, and further improving the strength of the fertilizer particles.

[0049] It should be noted that in the present invention, the dosage of magnesium sulfate and the standing time are both key parameters for the strength performance of the calcium-silicon-sulfur-magnesium fertilizer. During the standing time, the second solid and the fourth liquid react, and such a reaction requires a certain time and generally cannot be compensated by increasing the temperature.

[0050] In a typical embodiment of the present invention, the shapes of the granulated particles include spherical, cylindrical or sheet-like shapes.

[0051] According to another aspect of the present invention, there is provided a calcium-silicon-magnesium-sulfur fertilizer prepared by the above method. By weight, the effective silicon content calculated as SiO2 is 12-16 wt%, preferably 13-16 wt%; the effective calcium content calculated as CaO is 10-16 wt%, preferably 12-15 wt%; the effective magnesium content calculated as MgO is 7-10 wt%, preferably 7-8.5 wt%; the sulfur content calculated as S is 10-13 wt%, preferably 10.5-12.5 wt%; and the pH = 8.2-8.6, and the compressive strength ≥ 15 N. The calcium-silicon-magnesium-sulfur fertilizer provided by the present invention has multiple effective components, and at the same time has a suitable pH value and a suitable compressive strength, and has broad application prospects.

[0052] According to another aspect of the present invention, aluminum hydroxide has a dry basis Al2O3 content ≥ 63.5%.

[0053] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0054] In the following examples and comparative examples, unless otherwise stated, the composition (wt%) of the fly ash used is measured by XRD, as shown in Table 1. Other raw materials mentioned in this article are all conventional commercially available raw materials without special instructions.

[0055] Table 1

[0056] C <![CDATA[Na2O]]> MgO <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[P2O5]]> <![CDATA[SO3]]> <![CDATA[Fe2O3]]> <![CDATA[K2O]]> CaO <![CDATA[TiO2]]> <![CDATA[Cr2O3]]> MnO PbO 6.7 0.06 0.18 49.02 35.53 0.29 0.60 1.96 0.51 1.97 2.62 0.009 0.028 0.014

[0057] Testing methods:

[0058] 1. Testing of effective silicon, effective calcium and effective magnesium contents:

[0059] It is carried out in accordance with the standard NYT2272-2012 "Determination of Calcium, Magnesium and Silicon Contents in Soil Conditioners". In the stage of preparing the sample solution, the preferred ratio of 0.5 mol / L dilute hydrochloric acid solution to the solid sample is 150 ml:0.2 g; preferably, the plasma emission spectrometry (ICP-AES) method is used to measure the contents of Si, Ca, and Mg (μg / mL) in the solution, and then the effective silicon, effective calcium, and effective magnesium contents (wt%) in the sample are respectively converted.

[0060] An effective silicon content ≥ 12.86% (equivalent to Si ≥ 6%, referring to GB / T36207-2018 "Calcium-Silicon-Potassium-Magnesium Fertilizer") is considered qualified.

[0061] 2. Sulfur content determination

[0062] The test method complies with the standard GB / T 19203-2003 "Determination of Calcium, Magnesium and Sulfur Contents in Compound Fertilizers".

[0063] 3. Average Crushing Strength Test

[0064] The test method complies with the standard GB / T 20412-2021 "Calcium Magnesium Phosphate Fertilizer". An average crushing strength ≥ 15 N is considered qualified.

[0065] 4. Determination of pH Value

[0066] The test method complies with the standard NY / T 1973-2021 "Determination of Water-insoluble Matter Content and pH in Water-soluble Fertilizers". A pH range of 8 - 8.6 for the finished granular silicon calcium sulfur magnesium fertilizer is considered qualified.

[0067] 5. Phase Composition Analysis

[0068] X-ray diffraction (XRD) technology is used for phase analysis. The analysis software uses JADE 6.0 version combined with the PDF2-2004 version mineral card database.

[0069] Example 1

[0070] (1) Take 100 g of fly ash and add 115 g of concentrated sulfuric acid with a mass concentration of 98.6% (the molar ratio of sulfuric acid to aluminum oxide is about 2.4:1). After stirring evenly, place it in a glass reaction kettle at 180 °C and heat for 120 min; turn off the heating, add 300 g of water to the reaction kettle, start stirring, and use the residual heat to react for 60 min to complete the dissolution and washing of aluminum sulfate; the filtrate obtained by solid-liquid separation of the obtained solid-liquid mixture is the aluminum sulfate solution.

[0071] (2) Take the filter cake in step (1) and mix it thoroughly with 250 g of NaOH solution with a mass concentration of 30% and 32.7 g of CaO (the molar ratio of calcium source to silicon dioxide is about 0.99:1). Stir and react at 100 °C for 4 h, then carry out hydrothermal reaction. After the reaction is completed, carry out solid-liquid separation;

[0072] (3) Take the filter cake in step (2), add 500 g of water for washing, press filtration, and drying to obtain a water content of 30%, that is, obtain 83.3 g of powdery silicon calcium fertilizer with a dry basis mass; after testing, based on the dry basis, the effective silicon content of the powdery silicon calcium fertilizer is 35%, the effective calcium content is 32%, and pH = 11.2;

[0073] (4) Take the filtrate in step (2) and the filtrate in step (3), combine and concentrate them, and return them as the alkali solution to step (2) for recycling;

[0074] (5) Take all of the aluminum sulfate solution described in step (1) and react it with 46.1 g of powdered magnesium hydroxide at 75 °C with stirring for 40 min to fully precipitate aluminum hydroxide. Then, filter while it is still hot to obtain an aluminum hydroxide filter cake and a supersaturated magnesium sulfate solution.

[0075] (6) Take all of the aluminum hydroxide filter cake from step (5), wash it with water, and then dry it to obtain an aluminum hydroxide product. After testing, the purity of the aluminum hydroxide product is Al2O3 = 63.9%.

[0076] (7) Take all of the supersaturated magnesium sulfate solution from step (5) and carry out cooling crystallization. The crystallization end temperature is 20 °C at room temperature. Then, filter it. The resulting liquid is a saturated magnesium sulfate solution. The filter cake is dried at a low temperature to obtain a magnesium sulfate heptahydrate product.

[0077] (8) Take all of the powdered calcium silicate fertilizer from step (3) and 133.3 g of the saturated magnesium sulfate solution from step (5), mix and granulate them in a high-speed granulator. The resulting granules are allowed to stand for reaction and naturally dried for 7 days to obtain a granular calcium silicate sulfur magnesium fertilizer.

[0078] After testing, the effective silicon content of the resulting finished granular calcium silicate sulfur magnesium fertilizer is 15.9%, the effective calcium is 13.9%, the effective magnesium is 7.2%, and the sulfur content is 10.8%. The average crushing resistance is 32 N; pH = 8.55.

[0079] From Figure 1 the XRD analysis results, it can be seen that gypsum (CaSO4·2H2O) is formed in the granular calcium silicate sulfur magnesium fertilizer obtained after granulation with the addition of magnesium sulfate. The diffraction peaks of the original calcium silicate hydrate (Ca 1.5 Si 3.5 O 3.5 ·xH2O) basically completely disappear. The bulging spectral background represents the existence of amorphous silicon and magnesium substances. This proves that the added magnesium sulfate and the powdered calcium silicate fertilizer have undergone a chemical reaction during the standing reaction and natural drying process. The reaction products are calcium sulfate dihydrate, amorphous silicon dioxide, and amorphous magnesium hydroxide. The above reaction products provide the compressive strength for the granular calcium silicate high sulfur magnesium fertilizer. The weak acidity of the magnesium sulfate solution and the mild reaction conditions at room temperature for a long time ensure that the effective silicon index does not deteriorate significantly, and only a dilution factor caused by the addition of magnesium sulfate occurs.

[0080] Example 2

[0081] The difference from Example 1 is that in step (1), the concentration of the concentrated sulfuric acid added is 70 wt%, and the mass is 148 g (the molar ratio of sulfuric acid to aluminum oxide is about 2.2:1). The first reaction temperature is 220 °C, and the reaction time is 90 min. In step (2), the mass of CaO is 36 g (the molar ratio of the calcium source to silicon dioxide is about 1.09:1).

[0082] After detection, the pH of the powdery calcium silicate fertilizer is 11.25; the pH of the finished granular calcium silicate sulfur magnesium fertilizer is 8.37; the available silicon content of the obtained finished granular calcium silicate sulfur magnesium fertilizer is 15.7%, the available calcium is 14.1%, the available magnesium is 7.2%, and the sulfur content is 10.7%; the average crushing resistance is 30.5 N; the purity of aluminum hydroxide is Al2O3 = 64.7%.

[0083] Example 3

[0084] The difference from Example 1 is that in step (1), the concentration of concentrated sulfuric acid added is 80 wt%, and the mass is 135 g (the molar ratio of sulfuric acid to aluminum oxide is about 2.3:1), the first reaction temperature is 200 °C, and the reaction time is 90 min; in step (2), the mass concentration of the NaOH solution is 33%, and the mass is 200 g; the CaO is replaced by 34.5 g of Ca(OH)2 (the molar ratio of the calcium source to silicon dioxide is about 0.79:1); the hydrothermal reaction temperature is 110 °C, and the reaction time is 4.5 h; in step (3), the mass of the washing water is 600 g.

[0085] After detection, the mass of the powdery calcium silicate fertilizer is 74.7 g, the available silicon is 37.8%, the available calcium is 28%, and the pH is 11.16; the pH of the finished granular calcium silicate sulfur magnesium fertilizer is 8.25; the available silicon content of the obtained finished granular calcium silicate sulfur magnesium fertilizer is 15.2%, the available calcium is 18.0%, the available magnesium is 6.0%, and the sulfur content is 9.0%; the average crushing resistance is 29.5 N; the purity of aluminum hydroxide is Al2O3 = 63.9%.

[0086] Example 4

[0087] The difference from Example 1 is that in step (8), the amount of the saturated magnesium sulfate solution used is 142 g; the standing and natural drying time is 14 days.

[0088] After detection, the pH of the powdery calcium silicate fertilizer is 11.16; the pH of the finished granular calcium silicate sulfur magnesium fertilizer is 8.25; the available silicon content of the obtained finished granular calcium silicate sulfur magnesium fertilizer is 13.2%, the available calcium is 14.4%, the available magnesium is 6.9%, and the sulfur content is 10.0%; the average crushing resistance is 45.3 N; the purity of aluminum hydroxide is Al2O3 = 64.0%.

[0089] Example 5

[0090] The difference from Example 1 is that the temperature of the first reaction is 280 °C.

[0091] After detection, the available silicon content of the finished granular calcium silicate sulfur magnesium fertilizer is 16.0%; the pH is 8.40; the available calcium is 14.0%, the available magnesium is 7.0%, and the sulfur content is 9.8%; the average crushing resistance is 30.5 N; the purity of aluminum hydroxide is Al2O3 = 64.7%.

[0092] During the process, it was found that acid mist escaped, indicating that after the first reaction temperature reached 280°C, the decomposition of sulfuric acid showed an increasing trend; if the first reaction temperature was further increased on this basis, there would be a possibility of sulfuric acid escaping and loss, acid corrosion of equipment, and air pollution, which was not conducive to industrial production and environmental protection.

[0093] Example 6

[0094] The difference from Example 1 was that magnesium oxide in equimolar amounts was used instead of magnesium hydroxide.

[0095] After testing, the pH of the finished product of granular calcium silicate sulfur magnesium fertilizer was 8.35; the effective silicon content of the obtained finished product of granular calcium silicate sulfur magnesium fertilizer was 15.6%, the effective calcium was 14.1%, the effective magnesium was 7.3%, and the sulfur content was 10.9%; the average crushing resistance was 32.2 N; the purity of aluminum hydroxide was Al2O3 = 64.1%.

[0096] Comparative Example 1

[0097] The difference from Example 1 was that 2-fold molar hydrochloric acid was used instead of sulfuric acid. In a non-closed reaction vessel, acid gas escaped severely; after heating to 108°C, a constant-boiling solution of hydrochloric acid was formed, and the concentration was maintained at about 20.2%. At this temperature and acid concentration, the aluminum element in fly ash could not be effectively dissolved out, and the acid loss was serious. The operation was aborted.

[0098] Comparative Example 2

[0099] The difference from Example 1 was that attapulgite clay with a mass fraction of 10% was added to the powdery calcium silicate fertilizer in step (3) as a binder. After mixing, granulating, and drying in a high-speed granulator, the obtained granules were tested, and the pH was still as high as 10.8. The alkalinity of the product was too high and unqualified; the average crushing resistance of the granules was 12.2 N, which was unqualified.

[0100] Comparative Example 3

[0101] The difference from Example 1 was that a sodium metasilicate solution (1.0 mol / L) with a mass fraction of 40% was added to the powdery calcium silicate fertilizer in step (3). After mixing, granulating, and drying in a high-speed granulator, the obtained granules were tested, and the pH was as high as 11.2. The alkalinity of the product was too high and unqualified.

[0102] From the above description, it can be seen that the above embodiments of the present invention achieved the following technical effects:

[0103] According to the method of the present invention, the problems in the prior art of difficultly finding a suitable method to reduce the pH of silicon fertilizer and lacking a suitable granulation binder are solved. The prepared calcium sulfur magnesium fertilizer has balanced effective components and high strength, and has good market application prospects.

[0104] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for producing aluminum hydroxide from fly ash and co-producing silicon-calcium-sulfur-magnesium fertilizer. By weight percentage of fly ash, the fly ash contains 30-60% of SiO2, 10-50% of Al2O3, and 5-10% of C. It is characterized in that, Including: Step S1: React the fly ash with a sulfuric acid solution in a first reaction, and then perform solid-liquid separation to obtain a first solid and a first liquid; Step S2: Under the action of an alkali, react the first solid with a calcium source in a second reaction, and then perform solid-liquid separation to obtain a second solid and a second liquid; Step S3: React the first liquid with a magnesium source in a third reaction, and then perform solid-liquid separation to obtain a third solid and a third liquid; Perform post-treatment on the third solid to obtain the aluminum hydroxide; Step S4: Crystallize the third liquid, and then perform solid-liquid separation to obtain a fourth solid and a fourth liquid; Step S5: Mix the second solid with the fourth liquid and granulate to obtain the calcium-silicon-sulfur-magnesium fertilizer; The molar ratio of sulfuric acid in the sulfuric acid solution to Al2O3 in the fly ash is (2.2~2.4):1; The molar ratio of calcium in the calcium source to SiO2 in the fly ash is (0.7~1.2):1; The weight ratio of the second solid to the fourth liquid on a dry basis is 1:(1.5~1.7); The conditions of the first reaction include: temperature 170~280°C, time 90~180 min; The second reaction is a hydrothermal reaction; the temperature of the hydrothermal reaction is 90~110°C, and the time is 3~6 h; The conditions of the third reaction are: temperature 60~80°C, time 40~60 min.

2. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 1, wherein The molar ratio of sulfuric acid in the sulfuric acid solution to Al2O3 in the fly ash is (2.3~2.4):

1.

3. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 1, characterized in that, The weight concentration of the sulfuric acid solution is 70~98%.

4. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 1, characterized in that, The conditions of the first reaction include: temperature 180~220°C, time 90~120 min.

5. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 1, characterized in that, The temperature of the third reaction is 70~75°C, and the time is 45~55 min.

6. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to any one of claims 1 to 5, characterized in that, The molar ratio of calcium in the calcium source to SiO2 in the fly ash is (0.8~1.1):

1.

7. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to any one of claims 1 to 5, characterized in that, The calcium source is calcium oxide and / or calcium hydroxide.

8. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate-sulfur-magnesium fertilizer according to any one of claims 1 to 5, characterized in that, The alkali is added in the form of an alkali solution, and the weight ratio of the alkali solution to the first solid is (3~6):

1.

9. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 8, characterized in that, The weight ratio of the alkali solution to the first solid is (4~5):

1.

10. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur and magnesium fertilizer according to claim 8, wherein, The weight concentration of the alkali in the alkali solution is 8~50 wt%.

11. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 8, characterized in that, The weight concentration of the alkali in the alkali solution is 25~33 wt%.

12. The method for preparing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 8, characterized in that, The alkali in the alkali solution is selected from at least one of NaOH and KOH.

13. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur and magnesium fertilizer according to any one of claims 1 to 5, characterized in that, SO4 in the first liquid 2- The molar ratio of SO4 to magnesium atoms in the magnesium source is 1:(0.95 - 1.0).

14. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur and magnesium fertilizer according to claim 13, characterized in that, The magnesium source is one or more of magnesium hydroxide, magnesium oxide, magnesium carbonate, and basic magnesium carbonate.

15. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to any one of claims 1 to 5, characterized in that, The granulation method adopts at least one of disk granulation, roll extrusion granulation, and high-speed granulator granulation; after the granulation, the method further includes: standing and drying.

16. The method for producing aluminum hydroxide from fly ash and co-producing calcium silicate sulfur magnesium fertilizer according to claim 15, characterized in that, The standing time is not less than 7 days.

17. A silicon-calcium-sulfur-magnesium fertilizer obtained by the method for producing aluminum hydroxide from fly ash and co-producing silicon-calcium-sulfur-magnesium fertilizer according to any one of claims 1 to 16, characterized in that, By weight, the effective silicon content calculated as SiO2 is 12~16 wt%; the effective calcium content calculated as CaO is 10~16 wt%; the effective magnesium content calculated as MgO is 7~10 wt%; the sulfur content calculated as S is 10~13 wt%; and the pH of the calcium-silicon-sulfur-magnesium fertilizer is 8.2~8.6, and the compressive strength ≥15 N.

18. The silicon-calcium-sulfur-magnesium fertilizer according to claim 17, wherein, By weight, the effective silicon content calculated as SiO2 is 13~16 wt%.

19. The calcium-silicon-sulfur-magnesium fertilizer according to claim 17, characterized in that, The effective calcium content calculated as CaO is 12-15 wt% by weight.

20. The calcium-silicon-magnesium-sulfur fertilizer according to claim 17, characterized in that, The effective magnesium content calculated as MgO is 7-8.5 wt% by weight.

21. The calcium-silicon-sulfur-magnesium fertilizer according to claim 17, characterized in that, The sulfur content calculated as S is 10.5-12.5 wt% by weight.

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