RU2480413C2 - Method of purifying acidic salt solutions containing aluminium nitrate from iron - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to chemistry and can be used in processing alumina-containing material using acid-based techniques. Acidic salt solutions containing aluminium nitrate are mixed with a base until neutralisation of free nitric acid and hydrolysis of iron nitrate to iron hydroxide. The obtained solution is held for 30-90 minutes at temperature of 90-160°C. Once the obtained pulp reaches pH 1.5-2.3, it is removed from the reactor, cooled and mixed with a cationic polyelectrolyte. The iron-containing precipitate is thickened and separated from the purified salt solution. Control of the salt solution and base ratio and adjustment of the pH of the pulp are carried out by further addition of the salt solution or base directly into the pulp. The thickened iron-containing precipitate is separated from the salt solution by settling. The bases used are sodium or potassium hydroxides, or aqueous ammonia solution or aluminium hydroxide or aluminium oxide monohydrate - boehmite, AlOH. In the temperature range of 90-109°C, the reactor equipment operates at atmospheric pressure and at high pressure in the temperature range of 110-160°C.

EFFECT: invention enables to extract 75-95% iron from acidic salt solutions and remove iron from the process cycle.

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Description

The invention relates to the field of processing alumina-containing raw materials by acidic methods, and in particular to methods for cleaning acid solutions of salts of aluminum containing nitrates of aluminum, sodium, potassium, iron and other elements from iron, and can be used in the technology for producing aluminum compounds.

Acidic solutions of nitrate salts containing aluminum nitrate are intermediates in the technology for producing alumina, hydroxide and monohydrate of aluminum oxide (boehmite), coagulants and other products based on aluminum nitrate.

The concentration of iron nitrate in salt solutions in terms of Fe ₂ O ₃ oxide is 0.1-0.4%, while the glandular modulus, Al ₂ O ₃ / Fe ₂ O ₃ , does not exceed 20. The high iron content in solutions makes them difficult further processing and does not allow to obtain the target products based on aluminum nitrate. In order to ensure the required quality of the target products, it is necessary to purify acid solutions of nitrate salts from iron, including the step of extracting the bulk of iron from salt solutions and removing iron from the technological cycle as an intermediate suitable for further processing.

Due to the similarity of the properties of aluminum and iron compounds, the separation of aluminum and iron nitrates in acidic solutions is a difficult task. Based on the properties of the Al (NO ₃ ) ₃ -Fe (NO ₃ ) ₃ -HNO ₃ -H ₂ O system, the cleaning of acid solutions of nitrate salts from iron is carried out by crystallization of aluminum nitrate from evaporated aluminum-containing solutions in strong solutions of nitric acid with an acid to Al ₂ ratio O ₃ from 10: 1 to 18: 1 [Complex processing of aluminum-containing raw materials by acid methods / Yu.A. Liner, M .: Nauka, 1982, p.128].

The disadvantages of this method:

- low process efficiency due to isomorphism of aluminum and iron compounds;

- the use of a large amount of concentrated nitric acid for crystallization and washing of crystals of aluminum nitrate;

- the difficulty of processing the mother liquor of nitric acid and the removal of iron from the technological cycle.

More effective is the purification of acidic solutions of nitrate salts from iron by hydrolysis of iron nitrate. In the absence of free nitric acid in the salt solution and lack of acid for the formation of aluminum nitrate Al (NO ₃ ) ₃ , basic aluminum salts of the type Al (OH) _m (NO ₃ ) _{3-m are formed} . Under these conditions, iron nitrate is hydrolyzed to form iron hydroxide, which is a weaker base than aluminum hydroxide. In an aqueous solution, iron hydroxide is in the form of aquacomplexes, the subsequent destruction of which leads to coagulation and precipitation of iron compounds [Complex processing of aluminum-containing raw materials by acid methods / Yu.A. Liner, M .: Nauka, 1982, pp. 129-130].

According to the invention SU No. 272182 [published: 05.26.1970] leaching of alumina ore containing iron impurities is carried out with nitric acid, taken in an amount from 50 to 90% of the stoichiometrically necessary, at 140-220 ° C and a pressure of 5.6- 10.5 kg / cm ² in an autoclave for 1-6 hours, the reaction mass from the autoclave is diluted, filtered, the undissolved material is washed, which is then discarded, and the filtrates are used to produce alumina.

A known method of deferrization of solutions of aluminum nitrate [SU No. 489717, published: 10.30.1975], including the autoclave treatment of high ferrous silicon raw materials with 30% nitric acid at a temperature of 180 ° C for 2 hours, mixing the pulp after autoclaving for 1 -10 hours at a temperature of 18-100 ° C followed by filtration of the pulp, while the flow rate of nitric acid is 90% of the stoichiometric norm for the formation of normal aluminum nitrate, Al (NO ₃ ) ₃ .

A known method of cleaning pulp containing aluminum nitrate from iron compounds [SU No. 532571, published: 10.25.1976] by diluting the pulp with water and stirring the pulp at 18-100 ° C for 10-24 hours, while bauxite is opened 30% nitric acid at a temperature of 180 ° C for 2 hours and an acid consumption of 80% stoichiometric norm for the formation of aluminum nitrate, dilution of the pulp with water is carried out in a ratio of 1: 1.

The disadvantages of these known methods:

- leaching of alumina feedstock with nitric acid in autoclaves at high temperature complicates the hardware-technological scheme of the process;

- dilution of the autoclave pulp with water and prolonged mixing of the pulp lead to an increase in equipment volumes and an increase in energy consumption for evaporation of diluted salt solutions;

- decomposition of alumina raw materials with nitric acid with a flow rate of less than 100%) from the stoichiometrically necessary amount leads to the fact that undissolved material (silica precipitate) contaminated with iron compounds is discarded;

- when silica sludge is dumped into the dump, the processing of high-silica alumina raw materials becomes ineffective, since silicon dioxide is the main component of the raw material.

According to the patent of the Russian Federation No. 2202516 [published: 04/20/2003] the method of producing aluminum oxide from aluminosilicate raw materials involves cleaning the solution of aluminum nitrate from iron by adding dibasic aluminum nitrate, Al (OH) ₂ (NO ₃ ) to the solution, to neutralize the excess acid and precipitation of iron hydroxide precipitate, the resulting precipitate is filtered.

The disadvantages of this method are:

- use to neutralize excess acid and precipitate iron hydroxide precipitate of a hard-to-reach reagent - dibasic aluminum nitrate, Al (OH) ₂ (NO ₃ ), which is obtained by incomplete thermal hydrolysis of aluminum nitrate from pre-evaporated solutions;

- the need to prepare a suspension of dibasic aluminum nitrate, Al (OH) ₂ (NO ₃ ), and fractional dosing of this suspension in a solution of nitrate salts, which does not allow the process of deposition of iron in a dynamic mode.

Analogs of the invention to the authors are unknown.

The task of the invention is to extract 75-95% iron from acidic solutions of nitrate salts containing aluminum nitrate, using available reagents and removing iron from the process cycle without releasing aluminum into the solid phase.

The problem is achieved in that the salt solution is mixed with the base in a ratio that ensures the neutralization of free nitric acid and the hydrolysis of iron nitrate to iron hydroxide, then the solution is kept for 30-90 minutes at a temperature of 90-160 ° C to coagulate the iron hydroxide aqua complexes and precipitate iron compounds, upon reaching a pH of 1.5-2.3, the pulp is discharged from the reactor, cooled, mixed with a cationic polyelectrolyte, the iron-containing precipitate is concentrated and separated from the purified salt solution.

Upon receipt of the pulp with a pH of less than 1.5 or greater than 2.3, the ratio of the salt solution to the base is adjusted and the pH adjusted by additionally introducing the salt solution or base into the pulp.

The thickened iron-containing precipitate is separated by settling.

Saline solutions containing 0.5-5.0% of free nitric acid, 15-35% of nitrate salts of Al, Na, K, Ca, Mg, Fe, including 12-25%) of aluminum nitrate, are supplied to iron removal and 0.1-0.4% iron in terms of oxide Fe ₂ O ₃ .

During the purification of iron, acidic solutions of salts are mixed with bases, which ensure the neutralization of free nitric acid in solution and the hydrolysis of iron nitrate with the formation of iron hydroxide according to the following schemes:

Fe ³⁺ + 3OH ^- ---- → Fe (OH) ₃

Fe ³⁺ + 3NH ₄ OH ^- ---- → Fe (OH) ₃ + 3NH ₄ ⁺

2Fe ³⁺ + 3Al (OH) ₃ ---- → 2Fe (OH) ₃ + 3Al (OH) _m (NO ₃ ) _3-m , m = 1;

2Fe ³⁺ + 6AlOOH + 3H ₂ O ---- → 2Fe (OH) ₃ + 3Al (OH) _m (NO ₃ ) _3-m , m = 1.

The following reagents are used as bases:

- hydroxides of sodium or potassium, or ammonia in the form of aqueous solutions;

- aluminum hydroxide precipitated with sodium hydroxide or ammonia water from a solution of aluminum nitrate, in the form of a pulp or precipitate Al (OH) ₃ from the filter;

- aluminum oxide monohydrate (boehmite, AlOOH) obtained in the process of complete thermal hydrolysis of aluminum nitrate, in the form of pulp or filter cake.

The salts that are formed when neutralizing free nitric acid with these reagents do not adversely affect the further processing of solutions, since sodium and potassium nitrates are contained in the initial salt solution, ammonium nitrate is decomposed at subsequent stages of the process or used in fertilizer production technology, aluminum hydroxonitrates are processed in the general scheme for processing aluminum nitrate.

The main factor affecting the degree of removal of iron from the salt solution is the hydrogen index (or pH) of the pulp, which is formed during the exposure of the salt solution after neutralization of free nitric acid and hydrolysis of iron nitrate.

The pH of the beginning of the formation of iron hydroxide is 2, therefore, with an increase in the pH of the salt solution to 1.5-2.3, iron nitrate undergoes hydrolysis with the formation of iron hydroxide, Fe (OH) ₃ or Fe ₂ O ₃ * nH ₂ O, which is in an aqueous solution exists in the form of aquacomplexes.

The pH of the onset of aluminum hydroxide formation is 4, therefore, at pH 1.5-2.3, aluminum nitrate is hydrolyzed only partially to form the basic aluminum salt, Al (OH) (NO ₃ ) ₂ , which is a water-soluble compound. At a pH of 1.5-2.3, the precipitation of aluminum into the solid phase in the form of hydroxide does not occur.

When iron is precipitated under the indicated conditions, the solid phase is condensed and the precipitate is separated by the gravimetric method, the degree of iron extraction from the solution is 75-95%. The iron content in terms of Fe ₂ O ₃ oxide in salt solutions decreases from 0.1-0.4% to 0.005-0.07%.

At pH less than 1.5, the degree of hydrolysis of iron nitrate decreases, the completeness of deposition and the degree of extraction of iron from the solution are reduced. At a pH of more than 2.3, the iron compounds in the salt solution are in the form of colloidal particles that do not interact with the cationic polyelectrolyte and do not precipitate upon settling of the pulp. In addition, at a pH of more than 2.3, there is not only hydrolysis of iron nitrate to form iron hydroxide, but also the first stage of hydrolysis of aluminum nitrate, which leads to an increased consumption of reagents without changing the degree of extraction of iron from the solution.

Iron hydroxide easily gives colloidal solutions. The solid phase in them is in suspension, does not interact with the polyelectrolyte, does not precipitate due to the small particle size and high surface charge. To obtain easily precipitated particles of the solid phase, the salt solution is held at a temperature of 90-160 ° C for 30-90 minutes. Under these conditions, iron hydroxide aquacomplexes coagulate and iron compounds precipitate: Fe ₂ O ₃ , δ-FeOOH, Fe ₅ O ₇ (OH) * 4H ₂ O, which easily undergo aggregation when a cationic polyelectrolyte is added to the resulting pulp.

The choice of the indicated temperature range is due to the fact that, at temperatures below 90 ° C, the reaction rate decreases, coagulation of colloidal particles of iron hydroxide and their interaction with polyelectrolyte deteriorate, and the completeness of iron removal from the solution decreases. An increase in temperature above 160 ° C during high-temperature hydrolysis of iron nitrate under autoclave conditions does not affect the completeness of iron removal from the salt solution.

In the temperature range of 90-109 ° C, the reactor equipment operates at atmospheric pressure, in the range of 110-160 ° C at elevated pressure.

To carry out the purification of the salt solution from iron, one of the indicated bases is mixed, the solution is kept for 30-90 minutes at a temperature of 90-160 ° C until the pH of the pulp reaches 1.5-2.3, and pH is periodically monitored. When the pH deviates, the pH of the pulp and the ratio of reagents: salt solution and base are adjusted. To remove iron from the technological cycle, pulp with a pH in the range of 1.5-2.3 is discharged from the reactor, cooled and mixed with a cationic polyelectrolyte.

The choice of cationic polyelectrolyte is due to the fact that the surface of the particles of the solid phase in the pulp has a negative charge. As a cationic polyelectrolyte, in particular, a copolymer of acrylamide and methyl chloride in the form of a solution with a mass fraction of the copolymer of 0.1-0.2% is used. This polyelectrolyte is an affordable and economical reagent.

The thickened iron-containing precipitate is removed from the thickener settler for processing. The volume fraction of condensed sediment averages 4–7% and does not exceed 10% of the pulp volume.

The iron-containing precipitate is processed by known methods: filtered and / or centrifuged; if necessary, the precipitate is washed from water-soluble compounds. The washed iron precipitate is used, in particular, as a pigment additive.

The technology provides for the mixing of the liquid phase: the filtrate and the centrate with the purified salt solution or the return of the filtrate and the centrate, as well as the wash water from the washing of the precipitate to the initial acidic salt solution, which allows a waste-free process to be organized and salt losses with the precipitate to be eliminated.

The purified salt solution from the thickener settler is sent to the subsequent stages of the process to obtain aluminum, sodium, potassium compounds or mixed products.

New and significant in the proposed technical solution is the selection of available reagents and determination of the conditions for the deposition of iron from acidic solutions of salts containing aluminum nitrate, without converting aluminum to precipitate, the extraction of 75-95% of iron from the technological cycle using a simple hardware-technological scheme.

Tests of the proposed method were carried out using acidic solutions of salts containing aluminum nitrate obtained during the nitric acid processing of alkaline aluminosilicates, in particular nepheline concentrate. The method of purification of salt solutions from iron in accordance with the invention is illustrated by the following examples. The results of the experiments in accordance with the invention are shown in table 1.

Example 1

An acidic solution of salts of the following composition: the sum of salts - 24%, including aluminum nitrate - 17.6%, free nitric acid - 1.65%, iron in terms of iron oxide (III), Fe ₂ O ₃ , - 0 , 23% are mixed in a reactor with a 42% sodium hydroxide solution in a ratio of 100: 3 by weight and incubated for 60 minutes at a temperature of 97 ± 2 ° C, the resulting slurry having a pH of 1.4 is discharged from the reactor, cooled, polyelectrolyte is added Nalco 9914, an iron-containing precipitate, is concentrated and separated from the purified solution by settling.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.0730%), the degree of iron extraction is 70%.

Examples 2-11

Experiments 2-11 are carried out similarly to the experiment in example 1. An acidic salt solution is mixed in a certain ratio with a 42% sodium hydroxide solution and the solution is kept for 60 minutes at a temperature of 97 ± 2 ° C until the pulp pH is in the range of 1.5-2, 3. In the purified salt solution after separation of the iron-containing precipitate, the residual iron content is determined in terms of Fe ₂ O ₃ oxide and the degree of iron extraction from the solution is calculated.

Example 12

An acidic solution of salts of the following composition: the amount of salts - 30%, including aluminum nitrate - 22.64%, free nitric acid - 1.97%, iron in terms of iron oxide (III), Fe ₂ O ₃ , - 0 , 24% are mixed in a reactor with a 42% solution of sodium hydroxide in a ratio of 100: 4 by weight and incubated for 60 minutes at a temperature of 90 ± 2 ° C, the resulting slurry with a pH of 1.92 is discharged from the reactor, cooled, and Nalco 9914 polyelectrolyte is added , the iron-containing precipitate is concentrated and separated from the purified solution by sedimentation.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.0312%, the degree of iron extraction is 87%.

Example 13

The experiment is carried out similarly to the experiment in example 12. The solution is aged for 60 minutes at the boil of the solution at a temperature of 108 ° C, the resulting pulp with a pH of 2.02 is discharged from the reactor.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.0238%, the degree of iron recovery is 90%.

Example 14

The experiment is carried out analogously to the experiment of example 12. The solution is aged for 60 minutes at a temperature of 155 ± 5 ° C, the resulting pulp with a pH of 1.8 is discharged from the reactor.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.02%, the degree of iron extraction is 92%.

Example 15

An acidic solution of salts of the following composition: the sum of salts - 24%, including aluminum nitrate - 17.6%, free nitric acid - 1.65%, iron in terms of iron oxide (III), Fe ₂ O ₃ - 0, 23% is mixed in a reactor with a 42% sodium hydroxide solution in a ratio of 100: 3.7 by weight and kept for 30 minutes at the boil of the solution at a temperature of 106 ° C, the resulting slurry with a pH of 1.67 is discharged from the reactor, cooled, polyelectrolyte is added Nalco 9914, an iron-containing precipitate, is concentrated and separated from the purified solution by settling.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.035%, the degree of iron extraction is 85%.

Example 16

The experiment is carried out similarly to the experiment of example 15. The solution is kept for 60 minutes at the boil of the solution at a temperature of 106 ° C.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.026%, the degree of iron recovery is 92%.

Example 17

The experiment is carried out similarly to the experiment of example 15. The solution is incubated for 90 minutes at the boil of the solution at a temperature of 106 ° C.

The mass fraction of iron compounds in terms of Fe ₂ O ₃ oxide in the purified salt solution is 0.025%, the degree of iron extraction is 89%.

Table 1 The results of the purification of acid solutions of salts of iron Example pH Holding temperature Holding time Mass fraction of Fe ₂ O ₃ in the purified solution The degree of extraction of iron from a solution of salts ° C min % % one 1.4 97 ± 2 60 0,0730 70 2 1,5 97 ± 2 60 0,0580 75 3 1.66 97 ± 2 60 0,0496 78 four 1.74 97 ± 2 60 0,0308 86 5 1.8 97 ± 2 60 0.0290 87 6 1.84 97 ± 2 60 0.0210 91 7 1.91 97 ± 2 60 0.0184 92 8 2.06 97 ± 2 60 0.0114 95 9 2.10 97 ± 2 60 0.0140 94 10 2.13 97 ± 2 60 0.0084 96 eleven 2,31 97 ± 2 60 0.0087 96 12 1.92 90 ± 2 60 0,0312 87 13 2.02 108 60 0,0238 90 fourteen 1.8 155 ± 2 60 0.0190 92 fifteen 1,67 106 thirty 0,0350 85 16 1.7 106 60 0.0260 89 17 1.68 106 90 0.0250 89

Claims (5)

1. The method of purification from iron of acidic solutions of salts containing aluminum nitrate, which consists in the fact that the salt solution is mixed with a base in a ratio that ensures the neutralization of free nitric acid and the hydrolysis of iron nitrate to iron hydroxide, then the solution is kept for 30-90 minutes at at a temperature of 90-160 ° C for coagulation of aquatic complexes of iron hydroxide and precipitation of iron compounds, upon reaching a pH of 1.5-2.3, the pulp is discharged from the reactor, cooled, mixed with a cationic polyelectrolyte, the iron-containing precipitate is concentrated and separated from the purified salt solution. 2. The method according to claim 1, characterized in that upon receipt of the pulp with a pH of less than 1.5 or more than 2.3, the ratio of the salt solution to the base is adjusted and the pH adjusted by additionally introducing the salt or base solution into the pulp. 3. The method according to claim 1, characterized in that in the temperature range of 90-109 ° C, the reactor equipment operates at atmospheric pressure, in the range of 110-160 ° C - at elevated pressure. 4. The method according to claim 1, characterized in that the base used is sodium hydroxide, or potassium hydroxide, or aqueous ammonia, or aluminum hydroxide, or aluminum oxide monohydrate (boehmite, AlOOH). 5. The method according to claim 1, characterized in that the iron-containing precipitate is separated from the salt solution by sedimentation.