RU2522343C1 - Method of processing nitrate salts - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: aluminium and alkali metal nitrate salts are processed by evaporating the nitrate salt solution to salt concentration of 45-55 wt %, thermal hydrolysis of aluminium nitrate with conversion thereof to aluminium hydroxide with a boehmite structure in an autoclave to degree of deposit of aluminium of 98% in an equimolar mixture of sodium nitrate and potassium nitrate at temperature of 220-250°C and pressure of 0.6 MPa while feeding jet steam into the autoclave. Sodium and potassium nitrate salts are leached from the obtained pulp and then separated from the aluminium hydroxide residue.

EFFECT: invention enables to obtain aluminium hydroxide with a boehmite structure of high purity and alkaline nitrates in form of desired products.

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Description

The present invention relates to chemical technology, in particular to methods for the complex processing of nitrate salts of aluminum and alkali metals to produce aluminum hydroxide boehmite structure, as well as potassium nitrate and sodium nitrate.

The invention also relates to a method for the extraction of nitric acid formed at the stage of thermal hydrolysis of aluminum nitrate in the processing of a mixture of nitrate salts.

Aluminum hydroxide boehmite (pseudoboehmite) structure is a valuable product and is widely used in various industries: it is used as an adsorbent, the main material for the manufacture of catalysts and carriers for catalysts, filler, in the electronic and optical industries, in the production of flame retardants, paints and varnishes and decorative materials , ceramics, in construction, pharmaceuticals, in the manufacture of plastics, etc.

Boehmite (pseudoboehmite) is the initial crystalline hydrate in the production of γ-Al ₂ O ₃ active alumina, used in industry as an adsorbent for drying gases and liquids, in chromatography, a catalyst for cracking oil, dehydration of alcohols, decomposition of H ₂ S, a carrier for catalysts .

The term "boehmite" is used to denote alumina hydrates, including mineral boehmite, which corresponds to the formula Al ₂ O ₃ · H ₂ O and has a water content of about 15%, as well as pseudoboehmite having a water content of over 15%.

The structure of pseudoboehmite is identical to the structure of boehmite composition AlOOH or Al ₂ O ₃ · H ₂ O.

Modern methods of producing alumina include the use of raw materials obtained by precipitation from a solution of aluminum salt under the action of an alkaline agent, such as ammonia, followed by separation of the precipitate (for example, in the form of a gel) and its decomposition to obtain the target product (for example, by heating). In this case, sulfates, chlorides, sodium or ammonium nitrates are obtained as by-products of production. This makes it necessary to introduce significant volumes of acid into the cycle. Precipitates formed often represent colloids, which are difficult to wash off from the mother liquor and are slowly filtered or thickened even in the presence of special additives.

Known methods for producing aluminum hydroxide pseudoboehmite structure using solutions of sodium aluminate and aluminum salts. The process is time-consuming, leads to the use of expensive reagents, which leads to higher cost of the product.

Sodium and potassium nitrates are used as effective fertilizers, as a preservative in the food industry, in the glass and metal processing industries, and are used in pyrotechnics.

The methods for producing sodium nitrate and potassium nitrate are based on the absorption of nitrogen oxides by a solution of soda or alkali (sodium nitrate), solutions of caustic potassium or potash (potassium nitrate); on the exchange decomposition of other nitrates with sodium or potassium compounds, on cation exchange. The methods require the use of expensive reagents that are complex technologically (M.E. Pozin. Technology of mineral salts. Part II. L., "Chemistry", 1974, pp. 1216-1228).

More attractive economically, technologically and promisingly is a method of complex processing of nitrate salts obtained as a result of nitric acid decomposition of aluminum-containing raw materials, such as nepheline, to produce aluminum hydroxide oxides and related products, nitric acid regeneration. This technology and expanding the range of products obtained are important for improving technical, economic and environmental indicators.

In this case, one of the significant problems in the processing of nitrate salts is the complexity of the regeneration of nitric acid.

A known method of producing a metal oxide hydrate (RU, No. 2375306, 04/02/2008), comprising treating a metal salt with gaseous ammonia, isolating a hydrated precipitate from the suspension to form a solution containing an ammonium salt, washing the hydrated precipitate and drying, characterized in that it is used a metal salt taken in the form of a crystalline hydrate with a particle size of 0.1-3.0 mm, an aluminum, titanium or zirconium salt is taken as a metal salt, metal salts are treated with ammonia gas by passing it through a layer of crystalline hydrate particles to bespechenii pH of the aqueous extract of the reaction mixture of not less than 7, and the resulting reaction mixture is leached with water or rinsing solution from the hydrate precipitate to form a slurry from which the precipitate is isolated hydrate. As the aluminum salt, its sulfate, chloride, nitrate, potassium or ammonium alum are taken. Leaching of the reaction mass is carried out at a temperature of at least 40 ° C for at least 10 minutes The washing of the hydrated precipitate is carried out using the counterflow mode. The solution containing the ammonium salt is dehydrated to obtain ammonium fertilizer.

The main disadvantages of this method:

- the need for preliminary preparation of raw materials to obtain a specific fraction of the salt used;

- the difficulty of separating the gel-containing hydrated precipitate from a solution of ammonium salt with a large flow of water for washing it;

- the use of gaseous ammonia, which does not ensure the environmental safety of the process;

- the formation of dilute solutions and energy consumption for their evaporation.

A known method of producing aluminum hydroxide (SU, No. 1787941, 03/07/1991), comprising neutralizing an aluminum-containing solution with ammonia, heating the reaction mixture in an autoclave in the presence of seeding of aluminum hydroxide, filtering the resulting suspension and washing the precipitate of aluminum hydroxide, characterized in that for the purpose accelerating the process, the solution is neutralized directly in an autoclave at 130-180 ° C and a seed consumption of 25-150% of dissolved alumina, and ammonia is fed at a rate of 0.075-0.30 g / h per 1 g of nitric acid, freely and related to nitrates of aluminum and iron.

The disadvantages of the method:

- carrying out the process in the presence of a seed using a significant amount of production aluminum hydroxide as a seed;

- the method does not allow to obtain a product of high purity;

- the method does not provide for the rational use of reagents - ammonia and nitric acid, which leads to the need to introduce significant amounts into the cycle;

- large material flows requiring high energy costs for their processing.

A known method of producing aluminum hydroxide of a given pseudoboehmite structure of a high degree of phase and chemical purity (RU, No. 2144478, 07/01/1996) by decomposing an aqueous aluminate solution with hydrogen peroxide, followed by isolation of the resulting aluminum hydroxide, characterized in that the aluminate solution is taken with a molar ratio Al ₂ O ₃ : Na ₂ O, equal to 0.5-2.0, and a concentration of Al ₂ O ₃ 1-50 wt.%, Decomposition is carried out at a temperature of 0-30 ° C and a molar ratio of sodium aluminate to hydrogen peroxide in the calculation Al ₂ O ₃ equal to 0,02-0,15 formed guide alumina in the form of pseudoboehmite structure is recovered and washed with distilled water at a temperature of 0-30 ° C until pH = 7-8. The disadvantages of this method:

- the use of expensive reagents (hydrogen peroxide);

- preparation of sodium aluminate at boiling points of solutions;

- the need to use a large amount of water for washing the precipitate formed from alkali;

- the difficulty of separating the resulting pseudoboehmite from the mother liquor;

- the need for disposal of wash water.

A known method of regeneration of nitric acid and its reuse in the method of producing individual and complex metal oxides from nitrates (RU, No. 2047556, 03/13/1992) by their thermal decomposition in an atmosphere of water vapor with the formation of gaseous reaction products containing nitrogen oxides, characterized in that water vapor is supplied in an amount of at least 1.2 times higher than theoretically necessary, and the gaseous reaction products are removed and cooled to form nitric acid.

The disadvantages of this method:

- the need for high temperatures for thermal hydrolysis, which leads to high energy costs of the process;

- decomposition at high temperatures of nitric acid formed during thermal hydrolysis to nitrogen oxides of different oxidation states, which, when cooling gaseous products, leads to acid loss due to incomplete absorption of nitrogen oxides.

In the known method of producing alumina (RU, No. 2202516, 04/29/2002) from aluminosilicate raw materials by calcining, grinding, subsequent leaching with nitric acid and purifying the solution of aluminum nitrate from iron by introducing nitrate crocus, characterized in that the leaching with nitric acid is at the boiling temperature of the solution until the solution reaches a pH of not higher than 1.5, before the introduction of nitrate crocus, dibasic aluminum nitrate is added to the obtained solution of aluminum nitrate to neutralize the excess acid and dividing the precipitate of iron hydroxide, the resulting product is filtered, nitrate crocus is added to the resulting solution in an amount of at least twenty-fold excess in terms of the residual amount of iron oxide, after the nitrate crocus is introduced, the solution is kept for at least one hour at the boiling point, the resulting product is filtered, subjected thermal hydrolysis at a temperature not exceeding 330 ° C, the compounds of alkali and alkaline earth metals are separated from the obtained product, and the solid residue is calcined to obtain ksida aluminum, separating the alkali and alkaline earth metals after the thermal hydrolysis is carried out in distilled water at a temperature of 70-80 ° C, followed by stirring and filtration of the resulting slurry, and the thermal hydrolysis is carried out with nitric acid return to leaching.

The disadvantages of the known method of processing nitrate salts:

- carrying out the process of thermal hydrolysis at temperatures of 300-330 ° C increases its energy intensity;

- decomposition of nitric acid under conditions of thermal hydrolysis with the formation of nitrogen oxides, which leads to losses of nitrate nitrogen and necessitates the stage of absorption of nitrogen oxides;

- the conditions for the separation of the hydrated precipitate from the solution of compounds of alkali and alkaline earth metals after thermal hydrolysis do not allow for the effective filterability of the suspension and to obtain products of high purity.

A known method of producing pseudoboehmite (RU No. 2284297, 02.02.2001), comprising dissolving gibbsite (Al ₂ O ₃ · 3H ₂ O) in nitric acid, giving an acidic solution of aluminum nitrate; decomposition at 300-700 ° C with free air supply of this acidic solution to pseudoboehmite and NOx; regeneration of the resulting NOx in concentrated nitric acid and return of the obtained nitric acid to the Gibbsite dissolution step; recovering the obtained pseudoboehmite by methods selected from spraying on the inner surface of one or more rotary evaporators, drying in a fluidized bed, and drying in a conveyor furnace with a steel belt. The decomposition of the acidic solution is enhanced by the addition of an organic compound that is active in the reduction of nitrates.

The main disadvantages of the method:

- high energy intensity of the process due to the use of temperatures of 300-700 ° C;

- the complexity and complexity of the process in the processing of nitrogen oxides into concentrated nitric acid;

- the complexity of the hardware design at the stage of thermal decomposition of an acidic solution of aluminum nitrate;

- the use of organic compounds at the stage of decomposition of an acidic solution of aluminum nitrate, which limits the safety of the process - the method does not provide for the allocation of pseudoboehmite from a mixture of salts.

The closest in essence to our proposed method is a method of processing a nitric acid solution used in the well-known technical solution "Method for processing nepheline-feldspar raw materials" (RU No. 2372290, 02/15/2008), characterized in that the raw material is mixed with the alkaline activation product of feldspar concentrate, the resulting mixture is treated with nitric acid with the conversion of aluminum and alkaline elements into a nitric acid solution, and silica and feldspar into an insoluble residue, the nitric acid solution is separated from non-soluble soluble residue, silica and feldspar concentrates are isolated from the insoluble residue, the latter is mixed with potash at a mass ratio of the sum of K ₂ O and Na ₂ O in concentrate and K ₂ O in potash, respectively, equal to 1: 1-2, sintered at a temperature of 800-950 ° C to obtain an alkaline activation product fed to a mixture with nepheline-feldspar raw materials, the nitric acid solution is evaporated to obtain a mixture of aluminum nitrate salts and alkaline elements and heat treated, the resulting cake is processed on gly nozem and nitrates of alkaline elements or alumina, soda and potash. Evaporation of nitric acid solution lead to the formation of melt. Heat treatment of the melt is carried out at a temperature of 300-400 ° C to obtain a cake, which is processed into alumina and nitrates of alkaline elements. Heat treatment of the melt is carried out at a temperature of 800-950 ° C to obtain a cake, which is processed into alumina, soda and potash.

This method involves the integrated extraction of useful components.

In the method, nitric acid is regenerated from the gas phase formed during the heat treatment of a mixture of aluminum nitrate salts and alkaline elements.

The disadvantages of the proposed method of processing a mixture of nitrate salts:

the conditions for the thermal treatment of salts do not allow the regeneration of nitric acid without decomposition to nitrogen oxides;

- the decomposition of nitrate salts to nitrogen oxides at the stage of heat treatment requires a separate technological scheme and hardware to obtain nitric acid;

- preliminary evaporation of nitrate salts to a state of melt leads to a partial loss of nitrate nitrogen;

- the heat treatment conditions do not allow to obtain a product that does not contain sodium and potassium nitrates, due to their partial thermal decomposition;

- the obtained nitrate salts are contaminated with aluminum-containing compounds with an aluminum oxide concentration of up to 0.5%.

The objective of the invention is to develop a method for processing solutions of a mixture of nitrate salts, ensuring the achievement of a technical result consisting in the processing of salts to produce high-purity aluminum hydroxide and alkali nitrates as target products, in the absence of aluminum losses during processing, in reducing the energy intensity of the process, in nitrogen recovery acid without decomposing it to nitrogen oxides and achieving a high suspension filtration efficiency.

To achieve the technical result in the proposed method for processing nitrate salts of aluminum and alkali metals, including preliminary evaporation of a solution of nitrate salts, thermal hydrolysis of the evaporated mixture with the conversion of aluminum nitrate to aluminum hydroxide of high purity boehmite structure, leaching of water-soluble nitrate salts of sodium and potassium from the resulting pulp, separating them from the precipitate of aluminum hydroxide for further processing, preliminary evaporation of the solution is carried out to contain salts of 45-55 wt.%, the process of thermal hydrolysis of aluminum nitrate is carried out in an autoclave to a degree of precipitation of aluminum in the form of aluminum hydroxide 98% in an equimolar mixture of sodium nitrate and potassium nitrate at a temperature of 220-250 ° C, a pressure of 0.6 MPa with a feed in a steam autoclave.

Acute steam is fed to the hydrolysis stage in an amount that provides an acid concentration in the condensate of at least 31%, which allows it to be reused in technological processes without additional concentration.

At the leaching stage, to complete the process of hydrolysis of aluminum nitrate, the residual amount of aluminum nitrate salts is neutralized with a NaOH solution, which eliminates aluminum loss, improves the filterability of the pulp and ensures the purity of sodium and potassium nitrates.

When using the combination of the claimed features, the degree of distillation of nitric acid from nitrate solutions is at least 98% without decomposition to nitrogen oxides; there are no aluminum losses during processing; suspension filtration performance increases at least two times, and the amount of impurity salts in the form of potassium and sodium nitrates in the resulting aluminum hydroxide does not exceed 0.45% in terms of Na ₂ O, aluminum-containing compounds in the nitrate salts of potassium and sodium are absent.

Evaporation of solutions of nitrate salts is carried out in order to reduce mass flow at the stage of thermal hydrolysis. The degree of evaporation corresponding to the salt concentration after the evaporation stage of 45-55% is determined by the concentration of nitric acid obtained at the stage of thermal hydrolysis of at least 31 wt.%. The technological parameters of the process are determined, in addition, by the need to obtain a condensate with a low acid content at this stage.

Under the conditions of the autoclave process at temperatures of 220-250 ° C, a pressure of not more than 0.6 MPa, an acceleration of the hydrolysis reaction and an increase in the yield of the product — aluminum hydroxide — are achieved by increasing the degree of hydrolysis of aluminum nitrate.

The process temperature of not more than 250 ° C with the supply of sharp water vapor to the autoclave ensures the absence of nitrogen oxides in the vapor-gas phase; all nitrate nitrogen associated with aluminum is removed in the form of nitric acid vapor. With a lack of water vapor or the use of higher temperatures, the process will occur with the formation of nitrogen oxides.

Lowering the temperature to 200 ° C significantly affects the filterability of the final pulp and leads to a decrease in the degree of hydrolysis of aluminum nitrate.

The use of an equimolar mixture of sodium nitrate and potassium nitrate, having a melting point of 219 ° C, provides a thermal hydrolysis process with high efficiency at the indicated temperature and pressure parameters without decomposition of the nitrates of these metals.

In addition, this ensures highly efficient extraction of nitrate salts of potassium and sodium from aluminum hydroxide at the stage of leaching the reaction mixture, as well as the purity of aluminum hydroxide due to the absence of thermal decomposition and sintering with aluminum-containing products.

The degree of hydrolysis of aluminum nitrate 98% ensures the completeness of the process without thermal decomposition with the formation of nitrogen oxides.

The addition of alkali to the reaction mass at the leaching stage eliminates the loss of aluminum with nitrate salts of potassium and sodium by neutralizing the residual amount of nitrate salts of aluminum, thereby ensuring high purity of the resulting products and increasing the filtration rate of the suspension by improving the structure of the obtained boehmite (pseudoboehmite) aluminum hydroxide structure.

EFFECT: invention makes it possible to obtain aluminum hydroxide of boehmite or pseudoboehmite structure, as well as high purity nitrate salts of potassium and sodium, completely regenerate nitrate nitrogen bound to aluminum in the form of nitric acid, reduce energy consumption, eliminate aluminum losses, increase suspension filtration performance during separation of aluminum hydroxide.

The essence of the claimed invention and its advantages can be illustrated by the following examples.

Example 1

To 928 g of an initial solution of nitrate salts containing 158 g of Al (NO ₃ ) ₃ , 44 g of NaNO ₃ and 23 g of KNO ₃ with a water concentration of 75%, 72 g of an aqueous solution of potassium nitrate containing 29 g of KNO _{3 are} added to obtain an equimolar mixtures of nitrate salts of sodium and potassium. The solution is evaporated at a temperature of 91 ° C and an absolute pressure of 0.02 MPa to a water content of 46% and then through a heat exchanger, where it is heated to a temperature of 200 ° C, enters the autoclave.

The process of high-temperature hydrolysis of aluminum nitrate is carried out at a temperature of 220 ° C and a pressure of 0.6 MPa to a degree of hydrolysis of aluminum nitrate of 98%. The necessary heating of the solution in the autoclave is carried out by supplying high pressure steam to the jacket of the apparatus.

As the hydrolysis process is completed, sharp steam is supplied in an amount of 25 g (31 dm ³ ). In this case, the formation of a solid phase - aluminum hydroxide boehmite structure, and with the gaseous phase, nitric acid is removed along with the vapor. The vapor-gas phase leaving the apparatus is cooled in a heat exchanger and, in the form of acid condensate with a nitric acid content of 38.6%, is collected in a tank and used in production. There are no nitrogen oxides in the gas phase.

The completeness of the reaction of thermal hydrolysis with the formation of boehmite and distillation of nitric acid is ensured by the required temperature and the supply of sharp steam to the reaction zone of the autoclave.

At the end of the high-temperature hydrolysis operation, the reaction mass in the amount of 148 g, which is a melt of potassium, sodium, and aluminum nitrates in the amounts of 52 g, 44 g, and 3.16 g, respectively, containing 43.6 g of hydrated alumina AlOOН, is fed to the stage leaching of nitrate salts of Na, K. A NaOH solution is added to the repulper to neutralize the residual amount of aluminum nitrate, which eliminates the loss of aluminum with Na and K. The completeness of neutralization of the reaction mass is determined by the absence of aluminum compounds in the obtained filtrate. The resulting suspension is fed to a filter, where the precipitate of aluminum hydroxide is separated and washed with steam condensate at a ratio of T: W = 1: 2. The performance of the filtration of the suspension 55 l / m ² · h

The filtrate containing 47.8 g of NaNO ₃ , 52 g of KNO ₃ and not more than 0.01% insoluble residue is sent for further processing, and the wash water is used to leach the reaction mass after thermal hydrolysis.

The resulting precipitate in an amount of 73 g, containing aluminum hydroxide in the form of boehmite AlOOH 43.6 g, containing impurities of sodium and potassium nitrates in total 0.39% in terms of oxides, is sent for processing. The purity of the obtained aluminum hydroxide 98.5%.

A quantitative analysis of all process flows was carried out after their selection at all stages by known analytical methods.

The process of thermal hydrolysis under the selected conditions allows not only the regeneration of nitric acid, but also the washing of the product from nitrate salts of sodium and potassium, since there is no thermal decomposition and sintering with aluminum-containing products.

Example 2

The process is carried out analogously to example 1 and with the same quantities of stock solutions. The hydrolysis temperature is 250 ° C. Hot steam is fed into the autoclave in an amount of 27 g (33.6 dm ³ ). There are no nitrogen oxides in the gas phase. The degree of hydrolysis of aluminum nitrate is 98.4%. The concentration of nitric acid in the condensate is 33.8%. The purity of the obtained aluminum hydroxide 98.5%.

The performance of the filtration of the suspension of 60.8 l / m ² · h

Example 3

The process is carried out analogously to example 1 and with the same quantities of stock solutions. But at the same time, the hydrolysis temperature is maintained at 260 ° C, and at the stage of leaching of sodium and potassium nitrates, a neutralizing agent (NaOH solution) is not added. With a degree of hydrolysis of aluminum nitrate of 98.3%, the degree of decomposition of nitric acid to nitrogen oxides is 0.5%. The concentration of nitric acid in the condensate is 30.8%. The filtration rate at the stage of separation of aluminum hydroxide is reduced to 14 l / m ² · h, the content of aluminum compounds in a solution of sodium and potassium nitrates was found in an amount of 2.7% in terms of Al ₂ O ₃ .

Example 4

The process of high-temperature hydrolysis of aluminum nitrate is carried out with the same quantities and parameters of the initial solutions as in example 1, to a degree of hydrolysis of aluminum nitrate of 98%. But the amount of hot steam in the autoclave is reduced to 10 g (12.4 dm ³ ). The degree of decomposition of nitric acid to nitrogen oxides is 2.5%. The concentration of nitric acid in the condensate is 32%.

Example 5

The process is carried out analogously to example 1 and with the same quantities of stock solutions. The hydrolysis temperature is 200 ° C. The degree of hydrolysis of aluminum nitrate decreased to 81.0%. The concentration of nitric acid in the condensate is 27.7%. The precipitate obtained in this mode is X-ray amorphous, as a result of which the filtration rate of the final pulp decreased to 10.7 l / m ² · h. The content of aluminum compounds in a solution of sodium and potassium nitrates was found in an amount of 0.2% in terms of Al ₂ O ₃ .

The results of the experiments are summarized in table.

In all experiments, the content of nitrate salts of sodium and potassium was in equimolar amounts.

Claims (3)

1. A method of processing nitrate salts of aluminum and alkali metals, including pre-evaporation of the solution to a salt content of 45-55 wt.%, Thermal hydrolysis of aluminum nitrate with its transformation into aluminum hydroxide boehmite structure in an autoclave to a degree of aluminum deposition of 98% in an equimolar mixture of sodium nitrate and potassium nitrate at a temperature of 220-250 ° C, a pressure of 0.6 MPa with the supply of sharp steam to the autoclave, leaching nitrate salts of sodium and potassium from the resulting pulp, separating them from the precipitate of aluminum hydroxide. 2. The method according to claim 1, characterized in that the sharp steam is fed to the hydrolysis stage in an amount providing an acid concentration in the condensate of at least 31%. 3. The method according to claim 1, characterized in that at the stage of leaching to complete the process of hydrolysis of aluminum nitrate, the residual amount of aluminum nitrate is neutralized with a NaOH solution.