Method for transforming a crystal form of an electrolyte containing lithium for aluminum electrolysis

Abstract

A method for transforming a crystal form of an electrolyte containing lithium for aluminum electrolysis includes the following steps: S1, pulverizing the electrolyte containing lithium; S2, uniformly mixing an additive with the electrolyte powder to obtain a mixture, wherein the additive is one or more selected from the group consisting of an oxide of an alkali metal other than lithium, an oxo acid salt of an alkali metal other than lithium, and a halide of an alkali metal other than lithium; a molar ratio of a sum of alkali metal fluoride contained in the electrolyte, alkali metal fluoride directly added from the additive, and alkali metal fluoride to which the additive is converted under the high-temperature calcination condition in the mixture to aluminum fluoride is greater than 3; S3, calcining the mixture at a high temperature.

Claims

1. A method for transforming a crystal form of an electrolyte containing lithium for an aluminum electrolysis, comprising the following steps: S1, pulverizing the electrolyte containing lithium to obtain an electrolyte powder; S2, uniformly mixing an additive with the electrolyte powder to obtain a mixture, wherein the additive comprises an oxo acid salt of an alkali metal excluding lithium; the oxo acid salt of the alkali metal excluding lithium is converted into an alkali metal oxide under a high-temperature calcination condition; the additive is mixed with the electrolyte powder according to a type of the additive, a first molar ratio of NaF to AlF.sub.3 in the electrolyte powder, and a lithium salt content in the electrolyte powder, a second molar ratio of a sum of a first alkali metal fluoride, a second alkali metal fluoride, and a third alkali metal fluoride in the mixture to aluminum fluoride is greater than 3; wherein the first alkali metal fluoride is contained in the electrolyte powder, the second alkali metal fluoride is added by the additive, and the additive is converted to the third alkali metal fluoride under the high-temperature calcination condition; and S3, briquetting the mixture, and calcining the mixture at 300° C.-1200° C. for 1 hour-5 hours; wherein during a calcination process, an insoluble lithium salt in the electrolyte powder is converted into a soluble lithium salt; wherein, in step S2, the oxo acid salt of the alkali metal excluding lithium is at least one selected from the group consisting of Na.sub.2SO.sub.4, Na.sub.2CO.sub.3, Na.sub.2C.sub.2O.sub.4, NaNO.sub.3, CH.sub.3COONa, K.sub.2SO.sub.4, K.sub.2CO.sub.3, K.sub.2C.sub.2O.sub.4, KNO.sub.3, and CH.sub.3COOK.

2. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 1, wherein, in step S2, the additive further comprises an oxide of an alkali metal excluding lithium.

3. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 1, wherein, in step S2, the additive further comprises a halide of an alkali metal excluding lithium.

4. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 1, wherein, the second molar ratio is 3-8:1.

5. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 4, wherein, the second molar ratio is 3.5-6:1.

6. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 1, wherein, the additive is pulverized before being mixed with the electrolyte powder.

7. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 2, wherein, the second molar ratio is 3-8:1.

8. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 1, wherein, the second molar ratio is 3-8:1.

9. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 3, wherein, the second molar ratio is 3-8:1.

10. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 7, wherein, the second molar ratio is 3.5-6:1.

11. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 8, wherein, the second molar ratio is 3.5-6:1.

12. The method for transforming the crystal form of the electrolyte containing lithium for the aluminum electrolysis according to claim 9, wherein, the second molar ratio is 3.5-6:1.

13. The method for transforming the crystal form of an electrolyte containing lithium for the aluminum electrolysis according to claim 2, wherein the oxide of the alkali metal excluding lithium is selected from the group consisting of sodium oxide, potassium oxide, and a mixture of sodium oxide and potassium oxide.

14. The method for transforming the crystal form of an electrolyte containing lithium for the aluminum electrolysis according to claim 3, wherein the halide of the alkali metal excluding lithium is selected from the group consisting of NaF, NaCl, NaBr, KF, KCl, and KBr.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) In order to expressly describe the present disclosure to facilitate an understanding, hereinafter, the present disclosure is described in detail with reference to specific embodiments.

(2) The present disclosure provides a method for transforming a crystal form of an electrolyte containing lithium for aluminum electrolysis, including the following steps.

(3) S1, the electrolyte containing lithium is pulverized.

(4) S2, an additive is uniformly mixed with the electrolyte powder to obtain a mixture, wherein the additive is one or more selected from the group consisting of an oxide of an alkali metal other than lithium, an oxo acid salt of an alkali metal other than lithium, and a halide of an alkali metal other than lithium; the oxo acid salt of the alkali metal can be converted into an alkali metal oxide under a high-temperature calcination condition; the additive is mixed with the electrolyte powder according to a type of the additive, a molar ratio of the electrolyte, and a lithium salt content in the electrolyte by satisfying the following conditions: ensuring that a molar ratio of a sum of an alkali metal fluoride contained in the electrolyte, an alkali metal fluoride directly added from the additive, and an alkali metal fluoride to which the additive is converted under the high-temperature calcination condition in the mixture to aluminum fluoride is greater than 3.

(5) S3, the mixture is compacted or briquetted, and calcined at 300° C.-1200° C. for 1-5 hours. During the calcination process, an insoluble lithium salt in the electrolyte is converted into a soluble lithium salt.

(6) In the present disclosure, the extracted samples of electrolytes for aluminum electrolysis are respectively derived from a 300 kA electrolytic cell, a 400 kA electrolytic cell and a 200 kA electrolytic cell in some aluminum electrolysis plants. The samples are directly crushed and pulverized for analysis. The elemental compositions and content of the electrolyte are expressed by a molar ratio, an alumina concentration, a calcium fluoride concentration, and a lithium fluoride concentration. The calcinating device can be a universal muffle furnace or a belt calciner to meet the requirements for large-scale production and reduce heat consumption. Optionally, the calcinating device can also employ other heating devices or thermal insulation devices that provide a high temperature.

(7) The electrolyte containing lithium is pulverized, and/or the additive is pulverized before being mixed with the electrolyte. In either way, the additive and the electrolyte can be fully mixed uniformly, so that insoluble lithium salts in the electrolyte are fully converted into soluble lithium salts during the high-temperature calcination process, which increases the conversion rate of lithium salts to leach more lithium salts in the acid solution, thereby improving the leaching rate of the lithium salts, and effectively recovering the lithium salts.

(8) In step S2, the oxide of the alkali metal other than lithium can be one selected from the group consisting of sodium oxide, potassium oxide, and a mixture of sodium oxide and potassium oxide.

(9) The addition of the oxide of the alkali metal other than lithium results in the reactions expressed by the following chemical equations:
3Na.sub.2O+2AlF.sub.3=6NaF+Al.sub.2O.sub.3
3K.sub.2O+2AlF.sub.3=6KF+Al.sub.2O.sub.3

(10) In step S2, the oxo acid salt of the alkali metal other than lithium is one or more selected from the group consisting of Na.sub.2SO.sub.4, Na.sub.2CO.sub.3, Na.sub.2C.sub.2O.sub.4, NaNO.sub.3, CH.sub.3COONa, K.sub.2SO.sub.4, K.sub.2CO.sub.3, K.sub.2C.sub.2O.sub.4, KNO.sub.3, and CH.sub.3COOK; wherein the oxo acid salt of the alkali metal other than lithium is converted into an alkali metal oxide under the high-temperature calcination condition.

(11) When the oxo acid salt of the alkali metal other than lithium is added, the oxo acid salt of the alkali metal can be heated and decomposed into the alkali metal oxide by the following chemical equations, and the decomposed alkali metal oxide reacts with aluminum fluoride in the same way as described above.
K.sub.2CO.sub.3=K.sub.2O+CO.sub.2
Na.sub.2NO.sub.3=Na.sub.2O+NO.sub.2
K.sub.2C.sub.2O.sub.4=K.sub.2O+CO.sub.2+CO
2CH.sub.3COONa+3O.sub.2=Na.sub.2O+2CO.sub.2+3H.sub.2O

(12) According to the present disclosure, in step S2, the halide of the alkali metal other than lithium is one or more selected from the group consisting of NaF, NaCl, NaBr, KF, KCl, and KBr.

(13) According to the reactions of the various additives mentioned above, when a molar ratio of a sum of alkali metal fluoride contained in the electrolyte, alkali metal fluoride directly added from the additive, and alkali metal fluoride to which the additive is converted under the high-temperature calcination condition in the mixture to aluminum fluoride is greater than 3, (i.e., LiF+NaF+KF)/AlF.sub.3>3), fluorides of alkali metals other than lithium and a mixture of oxides of alkali metals other than lithium and/or oxo acid salts of alkali metals other than lithium can be employed to dramatically reduce the addition of fluorides (e.g., NaF, KF) of alkali metals other than lithium in comparison with the single addition of fluorides of alkali metals other than lithium. In other words, the use of alkali metal oxides and/or alkali metal oxo acid salts can significantly cut down the production cost compared with the single addition of fluorides (e.g., NaF) of alkali metals other than lithium, and the conversion rate of lithium salts can be increased by 1%-3%. In the present disclosure, various additives can be mixed, e.g., sodium carbonate and potassium carbonate.

(14) The typical and non-restrictive embodiments of the present disclosure are as follows.

EMBODIMENT 1

(15) 1 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 1%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.5:1. The sodium sulfate powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the sodium sulfate powder are uniformly mixed to prepare a mixture of sodium fluoride, lithium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 3.8:1, and the mixture is calcined in a muffle furnace at 800° C. for 4 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 97.8%.

EMBODIMENT 2

(16) 20 kg of electrolyte aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 4%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.4:1. The potassium carbonate powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the potassium carbonate powder are uniformly mixed to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 3.5:1, and the mixture is calcined in a belt calciner at 500° C. for 3 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 98.2%.

EMBODIMENT 3

(17) 10 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 7%, the KF content of 3%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.6:1. The sodium oxalate powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the sodium oxalate powder are uniformly mixed to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 3:1, and the mixture is calcined in a belt calciner at 300° C. for 5 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 95.4%.

EMBODIMENT 4

(18) 1 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 1%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.5:1. The sodium carbonate powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the sodium carbonate powder are uniformly mixed to prepare a mixture of sodium fluoride, lithium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 4:1, and the mixture is calcined in a muffle furnace at 900° C. for 3.5 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 98.5%.

EMBODIMENT 5

(19) 20 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 4%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.4:1. The potassium oxide powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the potassium oxide powder are uniformly mixed to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 6:1, and the mixture is calcined in a belt calciner at 1000° C. for 1 hour to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 98.7%.

EMBODIMENT 6

(20) 10 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 7%, the KF content of 3%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.6:1. The sodium oxide powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the sodium oxide powder are uniformly mixed to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 5:1, and the mixture is calcined in a belt calciner at 1200° C. for 2 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 99.1%.

EMBODIMENT 7

(21) 20 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 4%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.4:1. The potassium acetate powder after being crushed and pulverized is selected as an additive, after a calculation, the electrolyte powder and the potassium acetate powder are uniformly mixed to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 8:1, and the mixture is calcined in a belt calciner at 800° C. for 4 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 97.6%.

EMBODIMENT 8

(22) 10 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 1%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.5:1. The sodium sulfate powder and the potassium carbonate powder after being crushed and pulverized are selected as the additives, after a calculation, the electrolyte powder is uniformly mixed with the sodium sulfate powder and the potassium carbonate powder to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 5:1, and the mixture is calcined in a belt calciner at 1000° C. for 5 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 99.4%.

EMBODIMENT 9

(23) 10 kg of electrolyte for aluminum electrolysis is crushed and pulverized to obtain electrolyte powder. The electrolyte has the LiF content of 5%, the KF content of 4%, and the molar ratio (molar ratio of NaF to AlF.sub.3) of 2.4:1. The potassium sulfate powder and the sodium oxide powder after being crushed and pulverized are selected as the additives, after a calculation, the electrolyte powder is uniformly mixed with the potassium sulfate powder and the sodium oxide powder to prepare a mixture of lithium fluoride, sodium fluoride, potassium fluoride, and aluminum fluoride, wherein the molar ratio of a sum of lithium fluoride, sodium fluoride, and potassium fluoride to aluminum fluoride, i.e., (LiF+NaF+KF)/AlF.sub.3, in the mixture is 7:1, and the mixture is calcined in a belt calciner at 600° C. for 3 hours to obtain calcined products. Lithium salts in the calcined products are all converted from Na.sub.2LiAlF.sub.6, K.sub.2LiAlF.sub.6, KLi.sub.2AlF.sub.6, NaLi.sub.2AlF.sub.6 into soluble lithium salts including LiF, Li.sub.2O, Li.sub.3AlF.sub.6 and others, and the conversion rate of the obtained lithium salts is 98.5%.

(24) It can be seen from embodiments 1-9 that, the alkali metal oxides and/or the alkali metal salts mentioned above are added and calcined at a high temperature to transform the crystal form of the phase of the lithium salt in the electrolyte for aluminum electrolysis, so that the insoluble lithium salts in the electrolyte are converted into soluble lithium salts, which facilitates the subsequent extraction and separation by acid leaching, and significantly improves the leaching rate of lithium salts. Meanwhile, high purity industrial electrolyte is obtained and can be returned to the aluminum electrolysis cell, which significantly diminishes the energy consumption in aluminum production via electrolysis. For example, the concentration of LiF in the electrolyte is reduced from 5% to 1.5%, the liquidus temperature of the electrolyte can be increased by approximately 20° C., the cell temperature for aluminum electrolysis can be controlled at 940° C.-950° C., the current efficiency can be increased from 92% to more than 93%, and the current efficiency can be increased by 1%. For an aluminum electrolysis plant with an annual output of 1 million tons aluminium, the aluminum output can be increased by 10,000 tons, and the profit can be increased by 140 million CNY (calculated on 14,000 CNY/per ton of aluminum).

(25) In the present disclosure, a molar ratio of a sum of alkali metal fluoride contained in the electrolyte, alkali metal fluoride directly added from the additive, and alkali metal fluoride to which the additive is converted under the high-temperature calcination condition in the mixture to aluminum fluoride is 3-8:1, and is preferably 3.5-6:1. The conversion rate of lithium salts has been increased to reach more than 97% or even more than 99%. Due to the phase conversion, the leaching rate of lithium salts in the electrolyte has been increased from less than 5% to more than 98%. A high conversion rate of lithium salts can be realized by selecting a mixture of various additives.

(26) It should be understood that the foregoing description of the specific embodiments of the present disclosure is only intended to describe the technical route and features of the present disclosure and to facilitate an understanding and implementation of the present disclosure for those skilled in the art. The present disclosure is not limited to the specific embodiments described above. Various changes or modifications made within the scope of the claims of the present disclosure shall fall within the scope of protection of the present disclosure.