CN114477257B - Method for preparing low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash - Google Patents

Method for preparing low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash Download PDF

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CN114477257B
CN114477257B CN202210254639.7A CN202210254639A CN114477257B CN 114477257 B CN114477257 B CN 114477257B CN 202210254639 A CN202210254639 A CN 202210254639A CN 114477257 B CN114477257 B CN 114477257B
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aluminum hydroxide
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CN114477257A (en
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魏存弟
邹吉远
雒锋
李楠
程妍
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Jilin University
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/34—Preparation of aluminium hydroxide by precipitation from solutions containing aluminium salts
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B25/00—Phosphorus; Compounds thereof
    • C01B25/16—Oxyacids of phosphorus; Salts thereof
    • C01B25/26—Phosphates
    • C01B25/36—Aluminium phosphates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01C—AMMONIA; CYANOGEN; COMPOUNDS THEREOF
    • C01C1/00—Ammonia; Compounds thereof
    • C01C1/16—Halides of ammonium
    • C01C1/164—Ammonium chloride
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/18—Oxygen-containing compounds, e.g. metal carbonyls
    • C08K3/20—Oxides; Hydroxides
    • C08K3/22—Oxides; Hydroxides of metals
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/32—Phosphorus-containing compounds
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/18—Oxygen-containing compounds, e.g. metal carbonyls
    • C08K3/20—Oxides; Hydroxides
    • C08K3/22—Oxides; Hydroxides of metals
    • C08K2003/2227—Oxides; Hydroxides of metals of aluminium
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/32—Phosphorus-containing compounds
    • C08K2003/321—Phosphates
    • C08K2003/327—Aluminium phosphate

Abstract

The invention relates to a method for preparing a low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash, which comprises the steps of leaching, purifying and removing iron, dispersing, neutralizing, modifying, re-neutralizing, and finally drying to obtain an aluminum hydroxide flame retardant product, wherein the content of ferric oxide in the product is not higher than 0.015wt% and the content of sodium oxide in the product is not higher than 0.02wt%. Compared with the prior art, the method selects the circulating fluidized bed fly ash with high activity as the raw material, adopts the method of direct acid dissolution and neutralization by adding ammonium-containing alkali liquor to prepare the aluminum hydroxide, has low raw material cost and lower iron content and sodium content in the product; phosphoric acid is introduced into the acid system in the early stage of aluminum hydroxide formation, so that the process flow is simplified, and the aluminum hydroxide flame retardant with uniform aluminum phosphate coating and good stability is obtained, wherein the initial water loss temperature is higher than 210 ℃.

Description

Method for preparing low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash
Technical Field
The invention relates to a method for preparing an aluminum hydroxide flame retardant, in particular to a method for preparing a low-sodium low-iron aluminum hydroxide flame retardant by using circulating fluidized bed fly ash and combining with ammonium chloride.
Background
The aluminum hydroxide flame retardant is an important inorganic flame retardant, is widely applied to various plastics, coatings, polyurethane, elastomers and rubber products, and has the consumption of more than 80 percent of the inorganic flame retardant in the world.
The stability of aluminum hydroxide flame retardant is one of the important parameters determining its flame retardant effect. The initial water loss temperature of the common aluminum hydroxide is low, and the stability is poor. One of the methods for improving the stability of aluminum hydroxide is to coat the surface of aluminum hydroxide with phosphoric acid or phosphate in an acidic system to form a heat-resistant aluminum phosphate composite film on the surface of aluminum hydroxide, thereby improving the stability of aluminum hydroxide. Sodium oxide (Na) 2 O) is the most hazardous impurity in aluminum hydroxide flame retardantA kind of electronic device is disclosed. Sodium oxide impurities can affect the color tone of the polymeric article and adversely affect the electrical insulation properties of the article. In addition, the sodium oxide content also affects the heat stability of the aluminum hydroxide, so that the dehydration temperature of the aluminum hydroxide is obviously reduced, and the flame retardant effect of the aluminum hydroxide is greatly reduced.
The fly ash is waste discharged from coal-fired power plants, the combustion temperature of the circulating fluidized bed is greatly reduced compared with that of a traditional pulverized coal furnace, and alumina in ash has good activity and can be used as a raw material for producing aluminum hydroxide. In the method for preparing aluminum hydroxide or aluminum oxide by taking fly ash as a raw material, which is disclosed at present, an alkaline process is mainly adopted. The alkaline process introduces excessive alkali in the production process, so that the washing process is complex, the production cost is increased, and the content of sodium oxide impurities can be reduced only to a limited extent by the enhanced washing. In addition, when the aluminum hydroxide is modified with phosphoric acid, phosphoric acid cannot be introduced in the early stage, and only after the formation of aluminum hydroxide, phosphoric acid can be mixed with the aluminum hydroxide, so that the process route is prolonged, and the dispersibility of phosphoric acid on the surface of aluminum hydroxide is affected. Another method for preparing aluminum hydroxide by using fly ash is an acid method, and the acid method process can avoid the introduction of sodium oxide impurities, but can bring higher iron impurities, thereby having adverse effects on the color and performance of the polymer product. To remove iron impurities from the acid solution, further treatment is generally carried out by alkaline methods. The existing method mostly adopts a process of firstly acid and then alkali, the flow is long, and sodium ions are introduced in the process of adding alkali for removing iron.
In the prior art, a method for preparing aluminum phosphate coated aluminum hydroxide flame retardant by using fly ash as a raw material does not exist, so that the problem to be solved by the invention is how to prepare high-quality aluminum hydroxide flame retardant by using solid waste of fly ash as a raw material through a simple process, and the aluminum hydroxide can be modified in the preparation process of the aluminum hydroxide, thereby improving the flame retardant effect.
Disclosure of Invention
In order to solve the technical problems, the invention provides a method for preparing a low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash, which comprises the following steps:
a) Leaching: under the airtight condition, leaching the circulating fluidized bed fly ash by hydrochloric acid with the concentration of 20-37 wt%, and reacting alumina with hydrochloric acid: al (Al) 2 O 3 +6HCl→2AlCl 3 +3H 2 The amount of added hydrochloric acid is 50-80% of theoretical value according to the concentration of hydrochloric acid and the content of alumina in the circulating fluidized bed fly ash, so that the obtained leaching solution only contains a small amount of free acid, has no volatility, and has less environmental pollution and less corrosiveness to equipment; controlling the pH value of the leaching solution to be 2.0-3.0, preferably 2.0-2.5, leaching at 150-250 ℃ by using autogenous pressure for 1.5-5 hours, wherein the leaching rate of alumina in the fly ash is higher than 85%; settling, filtering and washing the slurry after the reaction is finished to obtain leaching solution; the leaching solution contains iron impurities, and most of the leaching solution contains Fe 2+ Exist in the form of (2) and are small in part of Fe 3+ In the form of Fe in leaching solution 3+ When the concentration is higher than 1g/L, adding a reducing agent into the solution to enable Fe 3+ The concentration is lower than 1g/L;
b) Purifying and removing iron: maintaining the temperature of the leaching solution at 60-90 ℃, introducing oxygen-enriched gas into the leaching solution under stirring, and controlling the gas flow to maintain Fe in the leaching solution 3+ The concentration is always lower than 1g/L, fe 2+ Stepwise oxidation to Fe 3+ ，Fe 3+ FeO (OH) is formed according to the following reaction: fe (Fe) 3+ +2H 2 O→FeO(OH)+3H + FeO (OH) forms a precipitate at a pH value of more than 2.0, the pH value of which is 3.7, the leaching solution is purified according to the difference of the pH values of the precipitate formed by the FeO (OH) and the aluminium hydroxide, and the formed precipitate is aged for 5 to 15 hours at 50 to 85 ℃ and then filtered to obtain a refined aluminium chloride solution;
c) Dispersing: adding a dispersing agent solution with the concentration of 0.02-5% into the refined aluminum chloride solution, uniformly mixing, and dispersing the solution, wherein the adding amount of the dispersing agent solution is 1-5% of the volume of the refined aluminum chloride solution;
d) And (3) neutralization: adding ammonium-containing alkali liquor into the dispersion liquid until the pH value is 5-6 to obtain colloidal solution;
e) Modification: adding phosphoric acid into the colloidal solution, mixing, and adding phosphoric acid (H 3 PO 4 Calculated) and alumina in a molar ratio of 0.5-5:100;
f) And (3) neutralization: continuously adding ammonium-containing alkali liquor until the pH value is 10-12 to obtain modified aluminum hydroxide precipitate;
g) And (3) drying: filtering, washing and drying the precipitate to obtain an aluminum phosphate coated aluminum hydroxide flame retardant product, wherein the iron oxide content in the obtained aluminum hydroxide flame retardant product is not higher than 0.015wt%, the sodium oxide content is not higher than 0.02wt%, and the initial water loss temperature is higher than 210 ℃;
h) Crystallizing and purifying the filtrate obtained by filtering to obtain an ammonium chloride product, returning the crystallization mother liquor to the step a), and adding hydrochloric acid for leaching the circulating fluidized bed fly ash.
Further, in the step a), the circulating fluidized bed fly ash is pre-deironing by adopting a magnetic separation method before being leached by hydrochloric acid, so that the iron content in the fly ash is reduced to be less than 1.0 wt%; preferably, wet magnetic separation is used to remove iron: crushing the fly ash to below 100 meshes, adding water to prepare slurry with the solid content of 20-40wt%, removing iron by wet magnetic separation to reduce the iron content in the fly ash to below 1.0wt%, and filtering to obtain a filter cake.
Further, in the step a), the reducing agent is one of hydroxylamine hydrochloride and ascorbic acid; when in use, the reducer is dissolved in water to prepare aqueous solution with the concentration of 0.5-5 mol/L, and the aqueous solution is added into the leaching solution.
Further, in the step b), the oxygen-enriched gas is one of air, oxygen-enriched air or oxygen, and the ventilation speed is 3-5 m 3 /h。
Further, in the step c), the dispersing agent is one or more of polyvinyl alcohol, polyethylene glycol, polypropylene alcohol and polyethylammonium.
Further, in step d) and step f), the ammonium-containing lye may be added to the dispersion during the neutralization with the ammonium-containing lye, or the two may be mixed in parallel. In order to keep the pH constant, it is preferable to mix the dispersion and the ammonium-containing lye in cocurrent.
Further, in the step d) and the step f), the ammonium-containing alkali solution is one of an ammonium carbonate aqueous solution, an ammonium bicarbonate aqueous solution or ammonia water, and the concentration is 5% -28%.
Further, in the step h), the crystallization method is a cold precipitation, salting out or evaporation concentration method, and in order to prevent the introduction of new impurities, the crystallization is preferably performed by using a cold precipitation and evaporation concentration method.
The invention has the beneficial effects that:
compared with the prior art, the method selects the circulating fluidized bed fly ash with high activity as the raw material, adopts the method of direct acid dissolution and neutralization by adding ammonium-containing alkali liquor to prepare the aluminum hydroxide, has low raw material cost and lower iron content and sodium content in the product. In addition, as no sodium is added, the introduction of sodium oxide impurities is avoided, and the washing process is simplified; compared with the prior alkaline iron removal process, the method has the advantages of simple operation steps, low production cost and good iron removal effect. The invention relates to an aluminum hydroxide flame retardant product obtained by the method, wherein the iron oxide content is not higher than 0.015wt% and the sodium oxide content is not higher than 0.02wt%. The invention has the other beneficial effects that the phosphoric acid is introduced in the early stage of aluminum hydroxide formation by utilizing an acid system, so that the process flow is simplified, and the aluminum hydroxide flame retardant with uniform aluminum phosphate coating and good stability is obtained, and the initial water loss temperature is higher than 210 ℃.
Drawings
FIG. 1 is a schematic overall flow chart of the present invention.
Detailed Description
The invention is further illustrated with reference to the following examples, but the invention is not limited to the ones referred to in the examples.
The raw materials adopt a certain thermal power plant to produce circulating fluidized bed fly ash, and the chemical compositions are shown in table 1.
TABLE 1 chemical composition of fly ash in weight percent of circulating fluidized bed
Figure BDA0003548083820000041
Figure BDA0003548083820000051
Example 1
A method for preparing a low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash comprises the following steps:
a) Leaching: crushing the circulating fluidized bed fly ash to 200 meshes, adding water to prepare slurry with the solid content of 20-40wt%, carrying out wet magnetic separation to remove iron by using a vertical ring magnetic separator, reducing the iron content in the fly ash to 0.8wt%, and filtering to obtain a filter cake; the fly ash filter cake is put into an acid-resistant reaction kettle, and under the airtight condition, the solid-liquid ratio of the circulating fluidized bed fly ash to hydrochloric acid is 1Kg:3.16Kg (corresponding to 70% of theory) of industrial hydrochloric acid with a concentration of 22wt% is added, leaching is carried out at 200 ℃ by using autogenous pressure for 2 hours; the slurry after the reaction is subjected to sedimentation and filter pressing by a plate and basket filter press, and is washed to obtain leaching solution with the pH value of 2.3; when Fe is contained in leaching solution 3+ When the concentration is higher than 1g/L, adding a reducing agent into the solution to enable Fe 3+ The concentration is lower than 1g/L; the reducing agent is one of hydroxylamine hydrochloride and ascorbic acid;
b) Purifying and removing iron: cooling the leaching solution to 90 ℃ by heat exchange, and introducing oxygen-enriched gas with 70% of oxygen content into the leaching solution under stirring at a ventilation speed of 5m 3 And/h, maintaining Fe in leaching solution 3+ The concentration is always lower than 1g/L, and the precipitate formed after 1.5 hours is aged for 5 hours at 82 ℃ and filtered to obtain refined aluminum chloride solution;
c) Dispersing: adding polyethylene glycol 4000 solution with the concentration of 2wt% into the refined aluminum chloride solution, uniformly mixing, and dispersing the solution, wherein the addition amount is 1% of the volume of the refined aluminum chloride solution;
d) And (3) neutralization: adding ammonia water with the concentration of 20% into the dispersion liquid under strong stirring until the pH value is 5.6, so as to obtain a colloidal solution;
e) Modification: adding phosphoric acid into the colloidal solution and uniformly mixing, wherein the molar ratio of phosphoric acid to alumina is 5:100;
f) And (3) neutralization: continuously adding ammonia water with the concentration of 20% into the colloidal solution until the pH value of the solution is 11.2, so as to obtain modified aluminum hydroxide precipitate;
g) And (3) drying: filtering, washing and heating the precipitate to 180deg.CSpray drying to obtain aluminum phosphate coated aluminum hydroxide fire retardant product, and determining Fe in the product 2 O 3 The content is 0.014wt%, na 2 The O content is 0.017wt%, and the initial water loss temperature is 212 ℃;
h) Crystallizing and purifying the filtrate obtained by filtering to obtain an ammonium chloride product, returning the crystallization mother liquor to the step a), and adding hydrochloric acid for leaching the circulating fluidized bed fly ash.
Example 2
A method for preparing a low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash comprises the following steps:
a) Leaching: crushing the circulating fluidized bed fly ash to 200 meshes, adding water to prepare slurry with the solid content of 20-40wt%, carrying out wet magnetic separation to remove iron by using a vertical ring magnetic separator, reducing the iron content in the fly ash to 0.8wt%, and filtering to obtain a filter cake; the fly ash filter cake is put into an acid-resistant reaction kettle, and under the airtight condition, the solid-liquid ratio of the circulating fluidized bed fly ash to hydrochloric acid is 1Kg:3.18Kg (80% of theory) of industrial hydrochloric acid with a concentration of 25% by weight is added, leaching is carried out at 250℃for 1.5 hours by using autogenous pressure; the slurry after the reaction is subjected to sedimentation and filter pressing by a plate and basket filter press, and is washed to obtain leaching solution with the pH value of 2.5; when Fe is contained in leaching solution 3+ When the concentration is higher than 1g/L, adding a reducing agent into the solution to enable Fe 3+ The concentration is lower than 1g/L;
b) Purifying and removing iron: cooling the leaching solution to 80deg.C, and introducing oxygen into the leaching solution under stirring at a ventilation speed of 3m 3 And/h, maintaining Fe in leaching solution 3+ The concentration is always lower than 1g/L, and the precipitate formed after 2 hours is aged for 10 hours at 76 ℃ and filtered to obtain refined aluminum chloride solution;
c) Dispersing: adding a polyvinyl alcohol solution with the concentration of 1% into the refined aluminum chloride solution, uniformly mixing, and dispersing the solution, wherein the adding amount is 2% of the volume of the refined aluminum chloride solution;
d) And (3) neutralization: mixing the dispersion liquid and ammonium bicarbonate solution with the concentration of 25% in parallel flow until the pH value is 5.8, so as to obtain colloidal solution;
e) Modification: adding phosphoric acid into the colloidal solution and uniformly mixing, wherein the molar ratio of the phosphoric acid to the alumina is 1:100;
f) And (3) neutralization: continuously adding ammonium bicarbonate solution with the concentration of 25% into the colloidal solution until the pH value of the solution is 10.7, so as to obtain modified aluminum hydroxide precipitate;
g) And (3) drying: filtering and washing the precipitate by a plate-and-basket filter press, and spray drying at 180 ℃ to obtain an aluminum phosphate coated aluminum hydroxide flame retardant product, wherein Fe in the product is determined 2 O 3 The content is 0.013wt%, na 2 The O content is 0.018wt% and the initial water loss temperature is 211 ℃;
h) Crystallizing and purifying the filtrate obtained by filtering to obtain an ammonium chloride product, returning the crystallization mother liquor to the step a), and adding hydrochloric acid for leaching the circulating fluidized bed fly ash.
Example 3
A method for preparing a low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash comprises the following steps:
a) Leaching: crushing the circulating fluidized bed fly ash to 200 meshes, adding water to prepare slurry with the solid content of 20-40wt%, carrying out wet magnetic separation to remove iron by using a vertical ring magnetic separator, reducing the iron content in the fly ash to 0.8wt%, and filtering to obtain a filter cake; the fly ash filter cake is put into an acid-resistant reaction kettle, and under the airtight condition, the solid-liquid ratio of the circulating fluidized bed fly ash to hydrochloric acid is 1Kg:3.16Kg (corresponding to 70% of theory) of industrial hydrochloric acid with a concentration of 22wt% is added, leaching is carried out at 200 ℃ by using autogenous pressure for 2 hours; the slurry after the reaction is subjected to sedimentation and filter pressing by a plate and basket filter press, and is washed to obtain leaching solution with the pH value of 2.3; when Fe is contained in leaching solution 3+ When the concentration is higher than 1g/L, adding a reducing agent into the solution to enable Fe 3+ The concentration is lower than 1g/L;
b) Purifying and removing iron: cooling the leaching solution to 90 ℃ by heat exchange, and introducing oxygen-enriched gas with 70% of oxygen content into the leaching solution under stirring at a ventilation speed of 5m 3 And/h, maintaining Fe in leaching solution 3+ The concentration is always lower than 1g/L, and the precipitate formed after 1.5 hours is aged for 5 hours at 82 ℃ and filtered to obtain refined aluminum chloride solution;
c) Dispersing: adding a polypropylene alcohol solution with the concentration of 0.5% into the refined aluminum chloride solution, uniformly mixing, and dispersing the solution, wherein the addition amount is 3% of the volume of the refined aluminum chloride solution;
d) And (3) neutralization: the obtained dispersion liquid and ammonia water with the concentration of 25% are mixed in parallel flow until the pH value is 6.0, and a colloidal solution is obtained;
e) Modification: phosphoric acid was added to the gum solution and mixed well with a molar ratio of phosphoric acid to alumina of 0.5:100;
f) And (3) neutralization: continuously adding ammonia water with the concentration of 25% into the colloidal solution until the pH value of the solution is 11.6, so as to obtain modified aluminum hydroxide precipitate;
g) And (3) drying: filtering and washing the precipitate by a plate-and-basket filter press, and spray drying at 180 ℃ to obtain an aluminum phosphate coated aluminum hydroxide flame retardant product, wherein Fe in the product is determined 2 O 3 The content is 0.011wt%, na 2 The O content is 0.017wt%, and the initial water loss temperature is 213 ℃;
h) Inputting the filtrate obtained by filtration into a cold precipitation crystallizer, carrying out cold precipitation at 10 ℃, centrifugally separating out ammonium chloride, and enabling the mother liquor after cold precipitation to enter a triple effect evaporator, and carrying out evaporation concentration crystallization to obtain ammonium chloride; the crystallization mother liquor returns to the step a) and is used for leaching the circulating fluidized bed fly ash after being matched with hydrochloric acid.

Claims (4)

1. A method for preparing a low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash is characterized by comprising the following steps: the method comprises the following steps:
a) Leaching: the circulating fluidized bed fly ash is subjected to wet magnetic separation to remove iron before being leached by hydrochloric acid: crushing the fly ash to below 100 meshes, adding water to prepare slurry with the solid content of 20-40wt%, removing iron by wet magnetic separation to reduce the iron content in the fly ash to below 1.0wt%, and filtering to obtain a filter cake; leaching the circulating fluidized bed fly ash with hydrochloric acid with the concentration of 20-37 wt% under a closed condition, adding 50-80% of the theoretical value of hydrochloric acid according to the concentration of hydrochloric acid and the content of alumina in the circulating fluidized bed fly ash, controlling the pH value of leaching solution to be 2.0-3.0, leaching at 150-250 ℃ by using autogenous pressure for 1.5-5 hours, and finishing reactionSettling, filtering and washing the slurry to obtain leaching solution; when Fe is contained in leaching solution 3+ When the concentration is higher than 1g/L, adding a reducing agent into the solution to enable Fe 3+ The concentration is lower than 1g/L; the reducing agent is one of hydroxylamine hydrochloride and ascorbic acid; when in use, the reducer is dissolved in water to prepare an aqueous solution with the concentration of 0.5-5 mol/L, and the aqueous solution is added into the leaching solution;
b) Purifying and removing iron: maintaining the temperature of the leaching solution at 60-90 ℃, introducing oxygen-enriched gas into the leaching solution under stirring, and controlling the gas flow to maintain Fe in the leaching solution 3+ The concentration is always lower than 1g/L, the leaching solution is purified according to the difference of pH values of precipitates formed by FeO (OH) and aluminum hydroxide, and the formed precipitates are aged for 5 to 15 hours at 50 to 85 ℃ and then filtered to obtain refined aluminum chloride solution; the oxygen-enriched gas is one of air, oxygen-enriched air or oxygen, and the ventilation speed is 3-5 m 3 /h；
c) Dispersing: adding a dispersing agent solution with the concentration of 0.02-5% into the refined aluminum chloride solution, uniformly mixing, and dispersing the solution, wherein the adding amount of the dispersing agent solution is 1-5% of the volume of the refined aluminum chloride solution; the dispersing agent is one or more of polyvinyl alcohol, polyethylene glycol, polypropylene alcohol and polyethylammonium;
d) And (3) neutralization: adding ammonium-containing alkali liquor into the dispersion liquid until the pH value is 5-6 to obtain colloidal solution;
e) Modification: adding phosphoric acid into the colloidal solution and uniformly mixing, adding phosphoric acid into the colloidal solution to obtain H 3 PO 4 The molar ratio of the alumina to the alumina is 0.5-5:100;
f) And (3) neutralization: continuously adding ammonium-containing alkali liquor until the pH value is 10-12 to obtain modified aluminum hydroxide precipitate;
g) And (3) drying: filtering, washing and drying the precipitate to obtain an aluminum phosphate coated aluminum hydroxide flame retardant product, wherein the iron oxide content in the obtained aluminum hydroxide flame retardant product is not higher than 0.015wt%, the sodium oxide content is not higher than 0.02wt%, and the initial water loss temperature is higher than 210 ℃;
h) Crystallizing and purifying the filtrate obtained by filtering to obtain an ammonium chloride product.
2. The method for preparing the low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash according to claim 1, wherein the method comprises the following steps of: in step d) and step f), the ammonium-containing lye is added to the dispersion during the neutralization with the ammonium-containing lye or the two are mixed in parallel.
3. The method for preparing the low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash according to claim 1, wherein the method comprises the following steps of: in the step d) and the step f), the ammonium-containing alkali liquor is one of ammonium carbonate aqueous solution, ammonium bicarbonate aqueous solution or ammonia water, and the concentration is 5% -28%.
4. The method for preparing the low-sodium low-iron aluminum hydroxide flame retardant and co-producing ammonium chloride by using circulating fluidized bed fly ash according to claim 1, wherein the method comprises the following steps of: in the step h), the crystallization method is a cold precipitation, salting-out or evaporation concentration method.
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