RU2816485C1 - Method of producing synthetic fluorite and caustic soda solution - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to production of synthetic fluorite and caustic soda solution as a result of treatment of sodium-fluorine-carbon-containing wastes and fluorine-containing industrial products of electrolytic production of aluminium with a lime-containing reagent in aqueous solution. Amount of active lime Ca(OH)₂, wt.%, added to 100% of wastes and industrial products, is calculated by formula Ca(OH)₂=(1±0.02)×[1.95×F+4.11×Al].

EFFECT: method enables to obtain a solution of caustic soda without impurities of an aluminate solution with a minimum sufficient consumption of active lime, expanding the raw material base of the fluorine-containing raw material, increasing output of mineralizer when processing wastes and industrial products of aluminium production.

4 cl, 1 dwg, 15 tbl, 3 ex

Description

The invention relates to the processing of sodium-fluorine-carbon-containing waste and fluorine-containing industrial products of electrolytic aluminum production and can be used to produce synthetic fluorite and caustic soda solution. The essence of the method is the causticization of waste and industrial products - their treatment with lime milk. As a result, synthetic fluorite is formed from sodium fluoroaluminates contained in waste and industrial products, and sodium goes into solution in the form of caustic soda. Waste and middling products from the electrolytic production of aluminum processed according to the proposed solution can be used in the cement industry as a fluorine-containing mineralizer (firing intensifier) when firing Portland cement clinker, in ferrous metallurgy as a substitute for fluorite concentrate, and a caustic soda solution - as an absorbent for “wet” gas purification of aluminum smelters and other metallurgical industries.

There is a known method for processing fine sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum, which includes mixing the waste used as a fluorine-containing mineralizer with a mixture containing calcium, aluminosilicate and ferrous components, and subsequent heat treatment of the resulting mixture, characterized in that the mixing is carried out when feeding the specified waste into the mixture to obtain Portland cement clinker in an amount of 0.10-0.25 wt. % in terms of fluorine and with a weight ratio of sodium to fluorine of no more than 0.8. In this case, dust from electric precipitators, gas cleaning sludge, coal foam flotation tailings, and a mixture of fine sodium-fluorine-carbon-containing waste from electrolytic aluminum production can be used as fine sodium-fluorine-carbon-containing waste from electrolytic aluminum production.

In addition, calcium fluoride can be additionally introduced into the fluorine-containing mineralizer in the following ratio of components, weight. %:

- finely dispersed sodium-fluorine-carbon-containing waste from electrolytic aluminum production 30-90

- calcium fluoride rest [RF Patent No. 2393241, IPC S22V 7/00, S04V 7/42, publ. 06/27/2010].

When sodium-fluorine-carbon-containing waste is introduced into the raw sludge of cement production, fluorite synthesis occurs during the firing of clinker at 800-1000°C [Kulikov B.P., Nikolaev M.D., Barinov V.V., Kuznetsov A.A. ., Pyrkova I.V. Cement and its use. - 2010 - No. 2 - P. 102-105].

The main disadvantage of the known solution is the significant content of the alkaline element sodium in the waste (up to 13 wt.% in electrostatic precipitator dust, up to 23 wt.% in gas purification sludge, up to 12 wt.% in a mixture of electrostatic precipitator dust, gas purification sludge, coal foam flotation tailings) . An increased sodium content in a fluorine-containing mineralizer based on aluminum production waste leads to an increase in the alkali content in the clinker, which in some cases is unacceptable due to the high alkali content in the main raw material. These restrictions worsen the consumer properties of the fluorine-containing additive and limit the volume of processing of sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum.

In the application for the invention [Application No. 2013122923/02, 05/17/2013, IPC S22V 7/00, S04V 7/02, publ. 11/27/2014] describes a method for processing sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum as a fluorine-containing mineralizer to produce Portland cement clinker. The method eliminates the disadvantage of the analogue [RF Patent No. 2393241, IPC S22V 7/00, S04V 7/42, publ. 06/27/2010], due to the removal of sodium from waste into a caustic soda solution. The method involves mixing waste with calcium, aluminosilicate and ferrous components in an amount of 0.10-0.25 wt. % in terms of fluorine with a weight ratio of sodium to fluorine in the mineralizer of no more than 0.8 and subsequent heat treatment of the resulting mixture, sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum is pre-treated in an aqueous solution with a lime-containing reagent with stirring, while maintaining the weight ratio of the amount of fluorine in waste to the amount of active calcium oxide in the lime-containing reagent 1: 1.40÷1.65. Pre-treatment of sodium-fluorine-carbon-containing waste from aluminum production is carried out in an aqueous solution with a lime-containing reagent with stirring, maintaining the weight ratio of liquid to solid 3.0÷6.5: 1, at a temperature of 65÷90C, for 30÷70 minutes. Fluff lime and/or carbide lime, a waste product from the production of acetylene from calcium carbide, can be used as a lime-containing reagent.

As sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum, crushed spent coal lining of electrolytic cells for aluminum production, dust from electrostatic precipitators, gas cleaning sludge, coal foam flotation tailings, a mixture of fine sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum, as well as a mixture of finely dispersed waste from aluminum electrolytic production can be used. sodium-fluorine-carbon-containing waste from electrolytic aluminum production with crushed spent coal lining of electrolyzers for aluminum production.

As a result of treating waste with lime containing a reagent, synthetic fluorite (CaF ₂ ) and a solution containing caustic soda (NaOH) are obtained.

The disadvantage of the known solution is associated with the restriction on the use of solid products from processing waste from aluminum production only in the cement industry. Another drawback is associated with the presence of sodium aluminate in the caustic solution, which is formed by the interaction of NaOH with aluminum hydroxide Al(OH) ₃ - a product of causticization of aluminum fluoride. A solution of caustic soda with an admixture of sodium aluminate is of limited use for the absorption of gaseous fluorine compounds, because in this case, cryolite will be synthesized at gas treatment plants. This cryolite will be partially lost with gas cleaning sludge during clarification of gas cleaning solutions, and will also partially settle in the solution pipelines, which will complicate the operation of gas cleaning plants.

The technical solution described in [RF Patent No. 2624570, IPC S22B 7/00, C01F 11/22, publ. 07/04/2017]. This method was chosen as the closest analogue. The method involves treating sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum with a lime-containing reagent at elevated temperatures, with stirring and the weight ratio of the amount of fluorine in the waste to the amount of active calcium oxide in the lime-containing reagent is 1:(1.40÷1.65), in this case, the treatment is carried out in a saline solution from a sludge field or in a solution from a wet gas purification system of an aluminum production, and the amount of lime-containing reagent supplied to the treatment of 1 ton of waste is additionally increased in proportion to the volume of the saline solution supplied to the waste treatment and the concentration in the saline solution NaF, Na ₂ CO ₃ , NaHCO ₃ : CaO _active. = (1.00÷1.05)x[0.88×Q _p-pa ×C _NaF +0.70×Q _p-pa ×C _Na2CO3 +0.44×Q _p-pa ×C _NaHCO3 ], where : CaO _act . - additional amount of active calcium oxide in the lime-containing reagent for processing 1 ton of waste in saline solution, kg, Q _p-ra - volume of saline solution supplied for processing per 1 ton of waste, m ³ , C _NaF , C _Na2CO3 , C _NaHCO3 - respectively , concentrations in saline solution NaF, Na ₂ CO ₃ , NaHCO ₃ , kg/m ³ , (1.00÷1.05) - coefficient of excess of active calcium oxide - interval in which maximum efficiency of treatment of sodium-fluorine-carbon-containing waste is achieved electrolytic production of aluminum with lime-containing reagent. In this case, the treatment of sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum with a lime-containing reagent is carried out with the addition of technical water to the saline solution, the processing temperature is reduced to 40÷65°C, a solid lime-containing reagent is used, which is loaded into a suspension of sodium-fluorine-carbon-containing waste in the saline solution. solution, and the caustic soda solution obtained as a result of processing waste with a lime-containing reagent is treated with gases containing carbon dioxide before being fed into the wet gas cleaning system. Disadvantages of the closest analogue:

- the presence in the caustic solution of sodium aluminate, formed by the interaction of NaOH with aluminum hydroxide Al(OH) ₃ - a product of causticization of aluminum fluoride;

- the raw material base of fluorine-containing raw materials is limited only by sodium-fluorine-carbon-containing waste from aluminum production.

- the weight ratio of the amount of fluorine in the waste to the amount of active calcium oxide in the lime-containing reagent varies widely: 1: (1.40÷4.65). At the same time, there is no justification for the necessary and sufficient excess of CaO _act .

Objectives of the proposed technical solution:

- obtaining a caustic soda solution without admixture of aluminate solution with a minimum sufficient consumption of active lime;

- expansion of the raw material base of fluorine-containing raw materials for processing by the claimed method;

- increasing the yield of the mineralizer when processing waste and industrial products from aluminum production.

Technical results:

- expansion of the raw material base for the preparation of gas cleaning solutions, thanks to the replacement of soda ash solutions with caustic soda;

- obtaining solid causticization products with a high content of synthetic fluorite, thanks to the involvement in the processing of fluorine-containing industrial products of aluminum production;

- the possibility of using synthetic fluorite in other industries, in particular, in ferrous metallurgy to increase the fluidity of metallurgical slags;

- reduction of costs for flushing solution pipelines of gas treatment plants;

- obtaining a concentrated solution of caustic soda for the needs of the chemical industry when evaporating caustic solutions.

Technical results are achieved by the fact that in the method for producing synthetic fluorite and caustic soda solution, which includes processing sodium-fluorine-carbon-containing waste from the electrolytic production of aluminum in a solution with a lime-containing reagent, waste and fluorine-containing industrial products from the electrolytic production of aluminum are used as sodium-fluorine-carbon-containing waste, and the amount of active lime Ca(OH) ₂ (% weight) added to 100% of waste and industrial products is calculated using the formula:

where F is the concentration of fluorine in waste and industrial products bound in NaF and AlF ₃ ,% weight;

Al is the concentration of aluminum in waste and industrial products bound in AlF ₃ ,% weight;

(1±0.02) is a confidence interval within which the experimental results fit with 95% reliability, and electrolyte carbon foam, recycled electrolyte, and secondary cryolite are used as industrial products for aluminum production.

The presence in the proposed technical solution of features different from those of the closest analogue allows us to conclude that the proposed solution complies with the patentability condition of the invention “novelty”.

The technical essence of the proposed invention is as follows.

During caustification, waste and middling products from aluminum production containing sodium fluoroaluminates (cryolite Na ₃ AlF ₆ , chiolite Na ₅ Al ₃ F ₁₄ ) are treated with a lime-containing reagent, the basis of which is milk of lime. Lime milk is an aqueous suspension of calcium hydroxide Ca(OH) ₂ with a small amount of dissolved lime. A small amount of Ca(OH) ₂ , which is currently in dissolved form, interacts with sodium fluoroaluminates:

As the Ca(OH) ₂ in solution is consumed, new portions of Ca(OH) ₂ dissolve and interact with sodium fluoroaluminates. Thus, caustification of fluorides proceeds through the gradual dissolution of solid Ca(OH) ₂ and the transition to reaction products: CaF ₂ , Al(OH) ₃ and NaOH.

When treating fluorine-containing waste and industrial products with a lime-containing reagent, it was found that in the solution separated from the solid products of causticization, the concentration of Na ₂ O _total. , as a rule, more than Na ₂ O _caustic.

Figure 1 shows the dynamics of the change in the concentrations of CaF ₂ in the solid product, Na ₂ O _total. and Na ₂ O _caustic. in solution when treating cryolite (Na ₃ AlF ₆ ) with lime milk.

The experimental data obtained indicate that part of the Na ₂ O in solution is associated with Al ₂ O ₃ to form sodium aluminate NaAlO ₂ . This circumstance suggests that the highly active aluminum hydroxide Al(OH) ₃ formed by reactions (1, 2) interacts with sodium hydroxide (NaOH) to form an aluminate solution (reaction 3).

Thus, the resulting solution, along with NaOH, contains a certain amount of aluminate solution. The presence of NaAlO ₂ in the solution limits its use in “wet” gas cleaning of aluminum smelters. Absorption of electrolysis gases containing hydrogen fluoride by a mixture of caustic and aluminate solutions will lead to crystallization of cryolite in “wet” gas cleaning installations (reaction 4). The consequence of this will be the loss of fluoride salts with gas cleaning sludge and the overgrowing of solution pipelines with cryolite sediments.

To decompose the aluminate solution and remove aluminum hydroxide Al(OH) ₃ from it, the proposed technical solution proposes using an additional amount (excess) of lime. Along with lime dosed for the formation of CaF ₂ , an additional amount of lime is introduced into the reaction mixture to decompose the aluminate solution. Excess lime decomposes the aluminate solution, binding aluminum hydroxide into the insoluble compound cathoite 3Ca(OH) ₂ ⋅2Al(OH) ₃ :

As a result, we obtain a caustic alkali solution without sodium aluminate, and the solid products of causticization are enriched with calcium and aluminum hydroxides.

During the experiments, an empirical dependence was obtained on the optimal dosage of active lime Ca(OH) ₂ for the treatment of sodium-fluorine-carbon-containing waste and fluorine-containing middlings of electrolytic aluminum production:

where Ca(OH) ₂ is the amount of active lime (% weight) added to 100% of waste and industrial products;

F - concentration of fluorine in waste and industrial products bound in NaF and AlF ₃ ,% weight;

A1 - concentration of aluminum in waste and industrial products bound in AlF ₃ ,% weight;

(1±0.02) is a confidence interval within which the experimental results fit with 95% reliability.

Empirical relationship (6) is valid not only for waste, but also for the treatment of industrial products of aluminum production with lime milk. Industrial products, in particular, include electrolyte carbon foam, recycled electrolyte, and secondary cryolite.

Electrolytic carbon foam contains 30-35% fluorine and about the same amount of carbon. Currently, coal foam is processed by flotation enrichment to produce flotation cryolite and waste - flotation tailings. Processing of coal foam using the causticization method according to the proposed solution is waste-free. Solid caustification products (CaF ₂ , carbon, cathoite) can be used by cementitious agents as a complex: fluorine-containing mineralizer + burn-out additive. A caustic soda solution is used in gas purification of aluminum smelters instead of soda ash.

The circulating electrolyte is produced in electrolyzers to produce aluminum and is periodically drained. The reason for the formation of a recycled electrolyte is associated with the excess sodium entering the electrolyte with alumina and the need to maintain a low cryolite ratio of the electrolyte (molar ratio NaF:AlF = 2.2-2.4). Sodium supplied with alumina increases the cryolite ratio of the electrolyte. Therefore, part of the electrolyte from the electrolyzer must be periodically drained and the cryolite ratio of the electrolyte must be adjusted by adding aluminum fluoride. The circulating electrolyte contains 52-55% fluorine. When treated with lime milk, a solid product with a high concentration of CaF ₂ is obtained.

Secondary cryolite is called flotation, regeneration cryolite or their mechanical mixture. The fluorine content in secondary cryolite is 43-48% wt. A decrease in the cryolite ratio of the electrolyte in aluminum electrolyzers led to a change in the balance in the structure of consumption of fluorine and sodium compounds. As a result, secondary cryolite turned out to be in limited demand. The return of secondary cryolite to electrolysis under new conditions is rather a necessary measure, since its formation is caused by the presence of the electrolysis process itself. Processing of secondary cryolite according to the proposed technical solution will reduce the alkaline load on the electrolyte and provide synthetic fluorite for the ferrous metallurgy and cement industry.

A comparative analysis of the proposed technical solution with other known solutions in this area revealed the following.

There is a known method for producing calcium fluoride from waste from the production of extraction phosphoric acid: phosphogypsum and fluorosilicate solutions. Calcium fluoride is obtained by mixing in an aqueous medium an insoluble calcium salt and a fluorine-containing compound; wastes of extraction phosphoric acid such as phosphogypsum and fluorosilicate solutions are used as interacting components, a mixture of which in a stoichiometric ratio in terms of calcium sulfate and fluorosilicate ion is treated with a solution sodium hydroxide to obtain a pH of 9-10. Product yield 98% [RF Patent No. 2029731, IPC C01F 11/22, publ. 02/27/1995].

There is a known method for extracting fluorine in the form of calcium fluoride from fluorine-containing solutions, including treatment with a solution of calcium hydroxide followed by separation of the product, in which the initial solutions are used in an amount ensuring a ratio of calcium ions to fluorine ions equal to 6÷8, while the content of fluorine ion in the original solution can be maintained at 0.015÷3.0 g/l, and processing is carried out at room temperature with a fluorine ion content of 0.15÷3.0 g/l, and with a fluorine ion content of 0.015÷0.15 g/l l - at 60÷90°C [A.s. USSR No. 1498711, IPC C01F 11/22, publ. 08/07/1989].

There is a known method for producing calcium fluoride primarily from solutions of cryolite production, including the treatment of fluorine-containing solutions with calcium hydroxide, in which the treatment of fluorine-containing solutions with calcium hydroxide is carried out at a mass ratio of Ca : F = (2+4) : 1 [A.s. USSR No. 1747385, IPC C01F 11/22, C01F 7/54, publ. 07/15/1992].

There is a known method for producing calcium fluoride from fluorine-containing gases of superphosphate plants or hydrofluorosilicic acid, in which fluorine-containing gases or hydrofluorosilicic acid are treated with a solution of ammonia or caustic or carbonate alkali, then the resulting alkaline fluoride, after separating the silicic acid, is brought into interaction with lime or lime milk or carbonate calcium, while for the treatment of fluorine-containing gases or hydrofluorosilicic acid, a circulating solution of alkali obtained during the treatment of alkaline fluoride can be used after filtering the calcium fluoride [A.s. USSR No. 101115, IPC C01F 11/22, declared 03/02/1951, publ. 1955].

As a result of a comparative analysis of the proposed solution with known solutions in this area, no technical solutions were identified that are characterized by a set of features similar to the proposed solution:

- waste and fluorine-containing industrial products of electrolytic aluminum production are treated with a lime-containing reagent;

- the amount of active lime Ca(OH) ₂ (wt.%) added to 100% of waste and industrial products is calculated using the formula:

where F is the concentration of fluorine in waste and industrial products bound in NaF and AlF ₃ ,% weight;

A1 - concentration of aluminum in waste and industrial products bound in AlF ₃ ,% weight;

(1±0.02) is a confidence interval within which the experimental results fit with 95% reliability.

- electrolytic carbon foam, recycled electrolyte, and secondary cryolite are used as fluorine-containing industrial products for the electrolytic production of aluminum.

Thus, the proposed technical solution meets the patentability condition of the invention “inventive step”.

The proposed method is implemented as follows.

Example 1

Samples of flotation tailings of coal foam, electrolyte coal foam and circulating electrolyte, 100.0 g each, are treated with a lime-containing reagent (milk of lime). The content of main compounds and elements in waste and industrial products is given in tables 1 and 2.

The conditions for processing waste and industrial products are given in Table 3. The weight ratio L:S for different types of waste and industrial products was chosen in order to obtain a caustic soda solution with a similar NaOH concentration. The amount of active Ca(OH) ₂ (wt.%) is calculated by the formula:

The content of the main compounds in solid and liquid products of causticization of waste and industrial products of aluminum production is given in tables 4, 5.

Conclusions from the experiments:

1. Synthetic fluorite and caustic soda solution are obtained by treating both waste and industrial products of electrolytic aluminum production with a lime-containing reagent.

2. The amount of active lime Ca(OH) ₂ added to 100% of waste and middlings and calculated by the formula:

Ca(OH) ₂ = (1±0.02) × [l.95×F + 4.11×Al], ensures complete conversion of fluorine into CaF ₂ , and also almost completely binds the resulting aluminum hydroxide into insoluble cathoite Ca ₃ Al ₂ (OH) ₁₂ .

3. The resulting solution contains caustic soda and does not contain sodium aluminate.

Example 2

Dust from electrostatic precipitators for electrolytic aluminum production is treated with a lime-containing reagent (milk of lime). Processing is carried out according to the proposed technical solution in comparison with the closest analogue. The composition of electrostatic precipitator dust is given in Tables 6, 7. The sample of electric precipitator dust in each experiment is 100.0 g.

In the closest analogue, the weight ratio of the amount of fluorine in the waste to the amount of active calcium oxide in the lime-containing reagent is regulated within the limits of F: Ca ₂ O _act. = 1: (1.40÷1.65). In terms of active calcium hydroxide, the ratio F : Ca(OH) ₂ will be 1 : (1.85÷2.18). Table 8 shows the conditions for processing electrostatic precipitator dust with a lime-containing reagent according to the proposed method and the closest analogue. Amount of Ca(OH) _{2 act.} calculated according to the closest analogue based on the lower and upper limits of the weight ratio to fluorine in waste (1.85 and 2.18).

Table 9 shows the molecular composition of solid products of causticization of electrostatic precipitator dust using the proposed method in comparison with the closest analogue.

Table 10 shows the composition of solutions obtained after causticization of electrostatic precipitator dust using the proposed method in comparison with the closest analogue.

Example 3

A mechanical mixture of electrostatic precipitator dust 50 g and crushed electrolyte carbon foam 50 g is treated with a lime-containing reagent (milk of lime). The content of the main compounds and elements in the mixture of waste and middling product is given in tables 11, 12.

The processing conditions for a mixture of electrostatic precipitator dust and carbon foam are given in Table 13. The amount of active Ca(OH) ₂ (wt.%) is calculated by the formula: Ca(OH) ₂ - (1±0.02) × [1.95×F + 4.11×Al].

The content of the main compounds in solid and liquid causticization products of a mixture of electrostatic precipitator dust and coal foam is given in Tables 14, 15.

As a result of the implementation of the proposed technical solution, the following positive results are achieved:

1. The raw material base for the production of synthetic fluorite and gas cleaning solutions based on caustic soda is expanding, thanks to the involvement in the processing of industrial products from aluminum production.

2. The yield of solid causticization products and the content of calcium fluoride in them increases.

3. The resulting solution contains caustic soda without sodium aluminate and can be used to absorb gaseous fluorine and sulfur compounds in aluminum smelters and other enterprises, as well as to obtain a concentrated NaOH solution.

Claims (8)

1. A method for producing synthetic fluorite and caustic soda solution, including processing sodium-fluorine-carbon-containing waste from electrolytic aluminum production in solution with a lime-containing reagent, characterized in that waste and fluorine-containing industrial products from electrolytic aluminum production are used as sodium-fluorine-carbon-containing waste, and the quantity active lime Ca(OH) ₂ , wt.%, added to 100% of waste and industrial products, is calculated by the formula Ca(OH) ₂ = (1±0.02) × [l.95 × F + 4.11 × Al], where F is the concentration of fluorine in waste and industrial products bound in NaF and AlF ₃ , wt.%, Al is the concentration of aluminum in waste and industrial products bound in AlF ₃ , wt.%, (1±0.02) is a confidence interval within which the experimental results fit with 95% reliability. 2. The method according to claim 1, characterized in that electrolyte carbon foam is used as fluorine-containing industrial products. 3. The method according to claim 1, characterized in that a recycled electrolyte is used as fluorine-containing industrial products. 4. The method according to claim 1, characterized in that secondary cryolite is used as fluorine-containing industrial products.