RU2184704C2 - Method of preparing lithium-containing fluoride salts for electrolytic aluminum production - Google Patents

Abstract

FIELD: nonferrous metallurgy. SUBSTANCE: invention, in particular, relates to those salts that can be used as complex additives in molten salt electrolysis-mediated production of aluminum and involves mixing - and fluoride-containing components. The former is lithium chloride solution obtained by selective sorption of lithium from naturally-occurring brine on inorganic sorbent. Sorbent is washed to remove brine, after which lithium chloride is desorbed by water. The latter operation is continued until residual content of brine in sorbent bed attains a value ensuring percentage of lithium chloride in desorbed phase with respect to the total chloride content therein to be 50 to 86%. Desorbed phase is concentrated to concentration of LiCl 40-50 kg/cu.m and then, for 40-60 min, solution containing stoichiometrically required amount of fluoride-containing reagent is added to precipitate lithium, calcium, and magnesium fluorides. Resulting sludge containing insoluble fluorides is thickened during 60 to 90 min, chloride- containing mother liqueur is discarded and resulting precipitate is dehydrated and dried on heating. As fluoride- containing reagent, sodium fluoride is used. EFFECT: enhanced process efficiency due to reduced cost price pf lithium- containing raw material. 3 cl, 1 dwg, 1 tbl

Description

 Currently, one of the most important tasks in the electrolytic production of aluminum is new technical solutions based on the introduction of various additives into the electrolyte that increase the efficiency of the electrolysis process and reduce the emissions of highly toxic fluorine into the environment.

 A known method of producing lithium fluoride by reacting lithium carbonate with concentrated hydrofluoric acid (Plyushchev V.E., Stepin BD Chemistry and technology of compounds of lithium, rubidium, cesium. - M .: Chemistry, 1970).

 Due to the use of expensive reagents, the use of the method is limited and is mainly used to obtain highly qualified lithium fluoride.

 A known method of producing lithium fluoride by fluorination of lithium hydroxide or carbonate with gaseous hydrogen fluoride (RF patent 2104932, C 01 D 3/02, 1996). The disadvantages of the method are the use of highly toxic gaseous hydrogen fluoride and the limited application of the method.

 A known method for the electrolytic production of aluminum and a method for producing granular sorbents of lithium-containing fluoride salts for the electrolytic production of aluminum, in which lithium carbonate or hydroxide is used as the initial lithium-containing components, and aluminum fluoride or a salt of sodium and aluminum fluorides are used as the fluorine-containing component. A mixture of these components is crushed, moistened and granulated. The mass ratio of aluminum fluoride to lithium is accepted at least four (RF patent 2087595, C 25 C 3/06, C 01 F 7/50, 1993).

 This method is the closest in technical essence and the achieved result to the proposed solution and we have chosen as a prototype.

 The disadvantages of the method are the use of expensive lithium reagents, as well as the low content of lithium component in the mixture.

 The proposed method is devoid of these disadvantages. The technical result is achieved by the fact that to obtain lithium-containing fluoride salts, lithium chloride contained in natural brine is used.

The technical result is achieved by the fact that the method of producing lithium-containing fluoride salts for the electrolytic production of aluminum involves mixing lithium and fluorine-containing reagents. To obtain lithium-containing fluoride salts, a solution of lithium chloride contained in natural brine is used. For this, the brine after the bromine and / or iodine is isolated is sent to the selective extraction of lithium using an inorganic sorbent. Then, the sorbent is washed from the brine and LiCl is desorbed with water, moreover, the sorbent is washed from the brine to the residual content of the sorbent in the sorbent layer, which ensures the content of lithium chloride in the desorbate with respect to the sum of all chlorides of 50-86 rel.%. The resulting desorbate is concentrated by any known method until a LiCl concentration of 40-50 kg / m ³ is reached in it and, with constant stirring, it is introduced into a solution containing the stoichiometrically necessary amount of a fluorine-containing reagent to precipitate lithium, calcium, magnesium fluorides for 40-60 minutes. The resulting insoluble fluoride pulp is concentrated in 60-90 minutes, the mother liquor is drained, and the precipitate formed is dehydrated, dried by heating and granulated using one of the known methods. As a fluorine-containing reagent, a solution of sodium fluoride or its aqueous suspension with a content of 40-80 kg / m ³ NaF is used, which are preliminarily obtained by mixing sodium hydroxide solution in a predetermined ratio with concentrated hydrofluoric acid. A mother chloride solution formed after thickening the pulp of insoluble fluorides, containing mainly sodium chloride in an amount of 35-50 kg / m ³ , is subjected to membrane electrolysis at a current density of 3-5 A / dm ² in a flow-circulation mode. In the process of electrolysis, the anolyte is fed with sodium chloride. In this case, a sodium hydroxide solution is used in the cathode space used to prepare NaF, and gaseous chlorine formed on the anode is used as an oxidizing agent for the initial brine at the stage of bromine and / or iodine extraction.

The method is as follows. According to the technological scheme of the process, a natural lithium bromine-containing brine or technological solution of a similar composition is subjected to treatment with an oxidizing agent, for example, gaseous chlorine, while the bromide ion is oxidized to elemental bromine, which is extracted from the brine by steam or air desorption. After bromine separation, the brine is sent to the selective extraction of lithium using the DGAL-C1 inorganic sorbent. After complete saturation, the sorbent is partially washed from the brine and lithium chloride is desorbed from it with water, obtaining as a desorbate an aqueous solution of LiCl (concentration of 7-10 kg / m ³ ) mixed with calcium, magnesium, sodium, and potassium chlorides. Moreover, there are optimal parameters for conducting the process of washing the sorbent from brine: the LiCl content in the obtained desorbate should be at least 50 rel.% (50-85 rel.%) Of the sum of all chlorides, only in this case it is possible to obtain a mixture of fluorides of the required composition with LiF content in the target product, not less than 50 wt.% (examples 2,3,4). Obtaining a desorbate with a LiCl content above 86 rel.% Is impractical for economic reasons, because requires a deeper washing of the sorbent from brine and leads to significant losses of lithium. The spent brine after bromine and lithium extraction and mineralized water from the stage of washing the sorbent from the brine are sent for injection into the reservoir, and the resulting desorbate is concentrated by any of the known methods, for example, by electrodialysis or evaporation. Moreover, from the point of view of ensuring the necessary LiF content in the complex additive, the costs of concentrating and achieving a sufficiently high lithium yield are optimal for concentrates with LiCl contents in the range of 40-50 kg / m ³ (total salinity of 80-100 kg / m ³ ).

Для осаждения фторидов используется насыщенный раствор фторида натрия или водную его суспензию с содержанием NaF 40-80 кг/м³, получаемые путем контакта 40% плавиковой кислоты с раствором NaOH концентрацией 40-80 кг/м³ по реакции
NaOH + HF --> NaF + H₂O
При этом содержании NaF в осадительном растворе нецелесообразно задавать менее 40 кг/м³, т.е. ниже предела растворимости фторида натрия, во избежание получения очень разбавленных пульп при осаждении LiF, CaF₂ и MgF₂. В то же время содержание NaF в осадительном растворе нецелесообразно устанавливать выше значения 80 кг/м³, т.к. для его получения используют раствор гидроксида натрия с концентрацией NaOH не выше 80 кг/м³, полученный методом мембранного электролиза раствора NaCl с применением катионообменной мембраны для разделения катодного и анодного пространств. Поскольку получение практически свободного от примеси хлор-ионов гидроксида натрия возможно только с использованием катионообменной мембраны, химическая устойчивость которой резко снижается в растворах гидроксида натрия при концентрации NaOH более 2,0 N (80 кг/м³).The LiCl concentrate thus obtained with impurities is introduced with stirring into a saturated solution or aqueous suspension of sodium fluoride, containing the amount of fluoride ions stoichiometrically necessary for the precipitation of lithium, calcium and magnesium fluorides. The process is carried out for 40-60 minutes. The chemical reactions occurring in this case are described by the following controls:

For the deposition of fluorides, a saturated sodium fluoride solution or its aqueous suspension with a NaF content of 40-80 kg / m ³ , obtained by contacting 40% hydrofluoric acid with a NaOH solution of a concentration of 40-80 kg / m ³ by reaction
NaOH + HF -> NaF + H ₂ O
At this NaF content in the precipitation solution, it is impractical to set less than 40 kg / m ³ , i.e. below the solubility limit of sodium fluoride, in order to avoid obtaining very dilute pulps during precipitation of LiF, CaF ₂ and MgF ₂ . At the same time, the NaF content in the precipitation solution is impractical to set above the value of 80 kg / m ³ , because to obtain it, a sodium hydroxide solution with a NaOH concentration of not higher than 80 kg / m ³ obtained by membrane electrolysis of a NaCl solution using a cation exchange membrane to separate the cathode and anode spaces is used. Since the production of sodium hydroxide chlorine ions practically free from impurities is possible only using a cation exchange membrane, the chemical stability of which decreases sharply in sodium hydroxide solutions at a NaOH concentration of more than 2.0 N (80 kg / m ³ ).

 The pulp of insoluble fluorides formed during the deposition is thickened within 60-90 minutes, the thickened phase is filtered off and dried by heating.

The filtrate is mixed with the clarified solution formed by thickening the pulp, obtaining a mother liquor containing mainly NaC1 with a small admixture of KC1, which is subjected to membrane electrolysis at a current density of 3-5 A / dm ² in a flow-circulation mode. As accumulation in the mother liquor KC1 above the allowable limit, the mother liquor can be removed from the process for injection into the reservoir. In this case, as a catholyte, a NaOH solution of a concentration of 40-80 kg / m ³ (~ 4-8%) is obtained, which is directed to the operation of preparing NaF by mixing with concentrated HF. To strengthen the anolyte, the introduction of sodium chloride into it can be used. Chlorine gas released at the anode is used as an oxidizing agent of the bromide ion contained in the starting brine at the bromine extraction stage.

 Thus, the hallmark of the method is the use of lithium chloride contained in natural brine to obtain lithium-containing fluoride salts of salt, which is achieved by selective extraction of lithium chloride from the brine on the inorganic sorbent DGAL-C1, washing the sorbent from the brine and desorption of LiCl with water, followed by concentration of the desorbate and by reacting it with a saturated solution or an aqueous suspension of sodium fluoride, prepared by mixing concentrated HF with a NaOH solution obtained by membrane eleme by the hydrolysis of the mother liquor formed after thickening the pulp of insoluble fluorides. This feature allows you to increase the technical and economic indicators of the method through the use of natural brine as a cheaper and more affordable lithium-containing raw materials.

 Feasibility studies show that the price of the complex lithium-fluorine-containing additive obtained by the proposed method does not exceed $ 1,500 US per 1 ton, and with the complex processing of natural brines (extraction of bromine, calcium, magnesium salts) it can be $ 1,000 per 1 ton and lower , while upon receipt of a similar complex additive from pure fluoride salts, its price is $ 2500 US and higher per 1 ton of product.

 List of drawings and tables.

 Drawing. Technological scheme for producing lithium-containing fluoride salts from brine.

 A table of comparative results for a mixture of fluoride salts from lithium chloride solutions of various compositions.

 Information confirming the possibility of implementing the method described in the examples.

Example 1. The brine after extraction of bromine and / or iodine, containing 2.2 g / l LiCl, is passed through a layer of selective inorganic sorbent DGAL-C1 with respect to LiCl until it is completely saturated. The sorbent is washed from the brine and desorbed from it with LiCl water, obtaining as a desorbate a chloride solution of the following composition (kg / m ³ ): LiCl - 10.1, CaCl ₂ - 4.9, MgCl ₂ - 2.1, NaCl - 3, 2, while the LiCl content in relation to the sum of all chlorides is 49.8 rel.%. To obtain a mixture of chloride salts, 200 ml of desorbate is added for 60 minutes with stirring to a solution of 15.6 kg / m ³ NaF, which is previously prepared by mixing 75 ml of a ~ 4% NaOH solution with 3.5 ml of concentrated (40%) HF in a fluoroplastic reactor. The resulting slurry of insoluble fluorides is settled for 90 minutes, the precipitate is separated from the mother liquor and dried by heating. The LiF content in the resulting product is 50 wt.%, The lithium yield of 69.7%.

Example 2. A mixture of fluoride salts is obtained in the same manner as described in example 1, with the difference that the desorbate is pre-concentrated by evaporation to a salt content (kg / m ³ ): LiCl - 25.0, CaCl ₂ - 12.3 MgCl ₂ - 5.2, NaCl - 8.0, and then introduced into a solution of NaF with a concentration of 40.6 kg / m ³ . To obtain the indicated NaF solution in a fluoroplastic reactor, 90 ml of an 8% NaOH solution and 8.2 ml of concentrated HF are mixed. The LiF content in the resulting product is 56 wt.%, The output on lithium is 84.8%.

Example 3. Selective sorption of lithium from brine is carried out as described in example 1, with the difference that the resulting desorbate is evaporated about 5 times and used to obtain a mixture of fluoride salts. For this purpose, 200 ml of one stripped off chloride solution with a concentration (kg / m ³ ): LiCl - 50.2, CaCl ₂ - 24.5, MgCl ₂ - 10.5, NaCl - 16.0 are added over 40 minutes to the aqueous suspension sodium fluoride containing stoichiometrically the required amount of fluoride ions. To prepare a suspension in a fluoroplastic reactor, 200 ml of ~ 8% NaOH solution are mixed with 17.5 ml of concentrated HF. The NaF content in the resulting suspension is 80.0 kg / m ³ . The resulting slurry of insoluble fluorides is settled for 60 minutes, the precipitate is separated from the mother liquor and dried by heating. The LiF content in the resulting product is 57.2 wt.%, The lithium yield of 91.3%.

Example 4. Prepare a model solution of the composition of salts, (kg / m ³ ): LiCl - 39.5, CaCl ₂ - 4.0, MgCl ₂ - 2.4 and used to synthesize a mixture of fluoride salts according to the method of example 2. In the resulting product contains 86.4 wt.% LiF, the lithium yield is 90.2%.

Example 5. Prepare a model solution of the composition of the salts, (kg / m ³ ): LiCl-80.0, CaCl ₂ - 8.0, MgCl ₂ - 1.8, which is used to synthesize a mixture of fluoride salts. The synthesis is carried out as described in example 4, while the resulting product contains 81.6 wt.% LiF, the lithium yield is 90.0%.

 The table shows the results of obtaining a mixture of fluoride salts of chloride solutions of various compositions.

Industrial applicability
The proposed method for producing lithium-containing fluoride salts for the electrolytic production of aluminum from brines in comparison with the prototype method allows to reduce the cost of the process by eliminating the need to use expensive lithium salts. In addition, the proposed technology allows for a closed production cycle with the rational use of reagents and intermediates: a mother liquor formed after thickening fluoride pulp (to obtain a NaOH solution by membrane electrolysis), gaseous chlorine released during electrolysis (for the oxidation of bromide ion in the initial brine )

The method has been tested on a laboratory scale on real natural highly mineralized brines of the Znamenskoye deposit in the Irkutsk region, containing 1.8-2.2 kg / m ³ of lithium chloride and 9-10 kg / m ^{3 of} bromine.

Claims (3)

1. A method of producing lithium-containing fluoride salts for the electrolytic production of aluminum, comprising mixing lithium and fluorine-containing reagents, characterized in that a lithium chloride solution obtained by selective sorption of lithium from natural brine on an inorganic sorbent, washing the sorbent from brine and desorption is used as a lithium-containing reagent lithium chloride with water, moreover, the washing of the inorganic sorbent from brine is carried out to a residual content of the latter in the sorbent layer, which ensures the content of lithium chloride in desorbate in relation to the sum of all chlorides 50-86 rel. %, the desorbate is concentrated until 40–50 kg / m ^{3 of} LiCl is reached in it and, for 40–60 min, with constant stirring, it is introduced into a solution containing the stoichiometrically necessary amount of a fluorine-containing reagent, which provides the precipitation of lithium, calcium, magnesium fluorides, and the insoluble pulp formed fluorides are concentrated over 60-90 minutes, the mother liquor is drained, and the precipitate formed is dehydrated and dried by heating. 2. The method according to p. 1, characterized in that as a fluorine-containing reagent use a solution of sodium fluoride or its aqueous suspension with a content of 40-80 kg / m ³ NaF, obtained by direct contact of a solution of sodium hydroxide with concentrated hydrofluoric acid. 3. The method according to p. 1 or 2, characterized in that to obtain sodium fluoride use sodium hydroxide obtained by membrane electrolysis of the mother liquor after separation of fluoride salts with sodium chloride in the amount of 35-50 kg / m ³ .

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