Abstract:
The disclosure provides an electrolyte supplement system in an aluminum electrolysis process, which includes low-molecular-ratio cryolite, wherein the low-molecular-ratio cryolite is selected from mKF.AlF 3 , nNaF.AlF 3  or mixture thereof, where m=1˜1.5 and n=1˜1.5. When the electrolyte supplement system provided by the disclosure is applied to the aluminum electrolytic industry, electrolytic temperature can be reduced obviously in the aluminum electrolysis process without changing the existing electrolytic process; thus, power consumption is reduced, volatilization loss of fluoride is reduced and the comprehensive cost of production is reduced.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The disclosure relates to an electrolyte, and in particular to an electrolyte supplement system in an aluminium electrolysis process and a method for preparing the same. 
     BACKGROUND OF THE INVENTION 
     At present, aluminium electrolytic industry still employs a conventional Hall-Heroult process; electrolyte always takes cryolite-aluminium oxide as a basic system, and the cryolite generally adopts sodium fluoroaluminate. As the aluminium electrolysis process progresses, the sodium fluoroaluminate and aluminium oxide are continuously consumed; therefore, in order to ensure the continuous progressing of aluminium electrolysis, it is necessary to supplement electrolyte and aluminium oxide to the electrolyte basic system respectively. The existing electrolyte supplement system mainly includes aluminium fluoride and sodium fluoroaluminate; the electrolytic temperature needs to be maintained at about 960 DEG C. in the entire aluminium electrolysis process and thus power consumption is high, this is mainly because the liquidus temperature of the electrolyte is high and it is necessary to keep a certain temperature of superheat degree to make the aluminium oxide have a better solubility. 
     The method for preparing cryolite in industry generally adopts a synthesis method, in which anhydrous hydrofluoric acid reacts with aluminium hydroxide to form fluoaluminic acid; then the fluoaluminic acid reacts with sodium hydroxide or potassium hydroxide at a high temperature; after processes of filtering, drying, melting and crushing, the cryolite is prepared, wherein the cryolite synthesized by this method has a molecular ratio of m=3.0, with a relatively high melting point. The cryolite synthesized by the existing industrial synthesis method has a molecular ratio of m=2.0˜3.0, and it is difficult to obtain the relatively pure low-molecular-ratio cryolite with a molecular ratio of m=1.0˜1.5. 
     Therefore, the conventional art has disadvantages that the electrolytic power consumption is high and the electrolyte supplement system is not ideal. 
     SUMMARY OF THE INVENTION 
     In order to solve the technical problem existing in the conventional art, the inventor has done a great deal of research in the selection and preparation of electrolyte supplement system and unexpectedly finds that replacing the existing electrolyte supplement system by the electrolyte supplement system of low-molecular-ratio cryolite to perform aluminium electrolysis can cause an obvious reduction of electrolytic temperature in the aluminium electrolysis process without changing the existing electrolytic process, thus, power consumption is reduced, volatilization loss of fluoride is reduced and comprehensive cost of production is reduced. 
     The disclosure provides an electrolyte supplement system in an aluminium electrolysis process, which includes low-molecular-ratio cryolite, wherein the low-molecular-ratio cryolite is selected from mKF.AlF 3 , nNaF.AlF 3  or mixture thereof, where m=1˜1.5 and n=1˜1.5. 
     With the technical scheme above, when the electrolyte supplement system provided by the disclosure is applied to the aluminium electrolytic industry, the solubility property of aluminium oxide is improved, thus, the electrolytic temperature is reduced, the power consumption is reduced, the volatilization loss of fluoride is reduced, the electrolytic efficiency is improved and the comprehensive cost of production is reduced. 
     As a further improvement of the disclosure, the low-molecular-ratio cryolite is selected from mKF.AlF 3 , where m=1, 1.2 or 1.5. Aluminum oxide has a solubility of about 7 g/l in a 3NaF.AlF 3  melt liquid, and compared with the 3NaF.AlF 3  the aluminum oxide has a solubility of 15˜20 g/l in a melt liquid including 3/2KF.AlF 3 , KF.AlF 3 , 6/5KF.AlF 3 , thus, the solubility of the latter is greatly improved; the addition of the electrolyte supplement system consisting of low-molecular-ratio potassium cryolite (3/2KF.AlF 3 , KF.AlF 3 , 6/5KF.AlF 3 ) can cause an obvious reduction of electrolytic temperature and finally to a temperature of between 800 and 850 DEG C. 
     As a further improvement of the disclosure, the low-molecular-ratio cryolite is selected from the mixture of mKF.AlF 3  and nNaF.AlF 3 , and the mole ratio of the mKF.AlF 3  to the nNaF.AlF is 1:1˜1:3, where m=1, 1.2 or 1.5 and n=1, 1.2 or 1.5. Aluminum oxide has a solubility of about 7 g/l in a 3NaF.AlF 3  melt liquid, a solubility of 12 g/l in a system consisting of 3/2KF.AlF 3  and 3/2NaF.AlF 3  with a mole ratio of 1:1, and a solubility of 15 g/l in a system consisting of KF.AlF 3  and NaF.AlF 3  with a mole ratio of 1:1; the addition of the electrolyte supplement system consisting of low-molecular-ratio potassium cryolite mixture (consisting of mKF.AlF 3  and nKF.AlF with a mole ratio of 1:1˜1:3) can cause a reduction of electrolytic temperature and finally to a temperature of between 850 and 900 DEG C. 
     As a further improvement of the disclosure, the low-molecular-ratio cryolite is selected from nNaF.AlF 3 , where n=1, 1.2 or 1.5. Aluminum oxide has a solubility of about 7 g/l in a 3NaF.AlF 3  melt liquid, and compared with the 3NaF.AlF 3  the aluminum oxide has a solubility of 7˜10 g/l in a melt liquid including 3/2NaF.AlF 3 , NaF.AlF 3 , 6/5NaF.AlF 3 , thus, the solubility of the latter is slightly improved; the addition of the electrolyte supplement system consisting of low-molecular-ratio sodium cryolite (3/2NaF.AlF 3 , NaF.AlF 3 , 6/5NaF.AlF 3 ) can cause a reduction of electrolytic temperature and finally to a temperature of between 900 and 960 DEG C. 
     As a further improvement of the disclosure, the electrolyte supplement system in the aluminium electrolysis process provided by the disclosure further includes 3KF.AlF 3 , 3NaF.AlF 3  or mixture thereof. The low-molecular-ratio cryolite provided by the disclosure is mixed with 3KF.AlF 3 , 3NaF.AlF 3  or mixture thereof and then the mixture is added in an electrolytic cell as the electrolyte supplement system in the aluminium electrolysis process, when the weight percentage of the low-molecular-ratio cryolite is over 20% of the entire electrolyte supplement system, the electrolytic temperature can be reduced; moreover, the electrolytic temperature decreases as the weight percentage of the low-molecular-ratio cryolite increases. 
     Correspondingly, the disclosure also provides a method for preparing the electrolyte supplement system in the aluminium electrolysis process, which includes the following steps: 
     A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C., adding one or more of potassium fluotitanate, potassium fluoborate, sodium fluotitanate and sodium fluoborate in the reactor; 
     B) stirring for 4 to 6 hours, then pumping the superstratum melt liquid to obtain the electrolyte supplement system in the aluminium electrolysis process, wherein the reaction formula involved includes: 3/4K 2 TiF 6 +Al=3/4Ti+3/2KF.AlF 3 ; 3/4Na 2 TiF 6 +Al=3/4Ti+3/2NaF.AlF 3 ; KBF 4 +Al=B+KF.AlF 3 ; NaBF 4 +Al=B+NaF.AlF 3 ; K 2 TiF 6 +2KBF 4 10/3Al=TiB 2 +10/3[6/5KF.AlF 3 ]; Na 2 TiF 6 +2NaBF 4 +10/3Al=TiB 2 +10/3[6/5NaF.AlF 3 ]; 
     when aluminium is excessive, 
                   K   2     ⁢     TiF   6       +     KBF   4     +       Al   ⟶   Al     ·   Ti   ·   B     +           3   ⁢   y     +     6   ⁢   x           3   ⁢   y     +     4   ⁢   x         ⁢     KF   ·     AlF   3           ;         
when aluminium is excessive,
 
     
       
         
           
             
               
                 Na 
                 2 
               
               ⁢ 
               
                 TiF 
                 6 
               
             
             + 
             
               NaBF 
               4 
             
             + 
             
               
                 Al 
                 ⟶ 
                 Al 
               
               · 
               Ti 
               · 
               B 
             
             + 
             
               
                 
                   
                     3 
                     ⁢ 
                     y 
                   
                   + 
                   
                     6 
                     ⁢ 
                     x 
                   
                 
                 
                   
                     3 
                     ⁢ 
                     y 
                   
                   + 
                   
                     4 
                     ⁢ 
                     x 
                   
                 
               
               ⁢ 
               
                 NaF 
                 · 
                 
                   
                     AlF 
                     3 
                   
                   . 
                 
               
             
           
         
       
     
     The preparation method provided by the disclosure has advantages of mild reaction conditions, easy control, simple process, full reaction and high quality of reaction product. 
     As a further improvement of the disclosure, the method for preparing the electrolyte supplement system in the aluminium electrolysis process includes the following steps: 
     A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C., adding potassium fluoborate in the reactor; 
     B) stirring for 4 to 6 hours, then pumping the superstratum melt liquid to obtain the electrolyte supplement system KF.AlF 3  in the aluminium electrolysis process, wherein the reaction formula involved is: KBF 4 +Al=B+KF.AlF 3 . 
     As a further improvement of the disclosure, the method for preparing the electrolyte supplement system in the aluminium electrolysis process includes the following steps: 
     A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C., adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of 2:1 in the reactor; 
     B) stirring for 4 to 6 hours, then pumping the superstratum melt liquid to obtain the electrolyte supplement system 6/5NaF.AlF 3  in the aluminium electrolysis process, wherein the reaction formula involved is:
 
Na 2 TiF 6 +2NaBF 4 +10/3Al=TiB 2 +10/3[6/5NaF.AlF 3 ].
 
     As a further improvement of the disclosure, the method for preparing the electrolyte supplement system in the aluminium electrolysis process includes the following steps: 
     A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C., adding potassium fluoborate; stirring for 4 to 6 hours, then pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is KF.AlF 3 ; adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of 2:1 in the reactor; stirring for 4 to 6 hours, then pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is 6/5NaF.AlF 3 ; 
     B) mixing the obtained potassium cryolite KF.AlF 3  with the obtained sodium cryolite 6/5NaF.AlF 3  in a mole ratio cryolite of 1:1˜1:3 to obtain the electrolyte supplement system cryolite mixture in the aluminium electrolysis process, 
     wherein the reaction formula involved is: KBF 4 +Al=B+KF.AlF 3 ;
 
Na 2 TiF 6 +2NaBF 4 +10/3Al=TiB 2 +10/3[6/5NaF.AlF 3 ].
 
     As a further improvement of the disclosure, the method for preparing the electrolyte supplement system in the aluminium electrolysis process includes the following steps: 
     A) putting excessive aluminium into a reactor, heating the reactor to a temperature of between 700 and 850 DEG C., adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is 
                     3   ⁢   y     +     6   ⁢   x           3   ⁢   y     +     4   ⁢   x         ⁢     KF   ·     AlF   3         ;         
putting excessive aluminium into another reactor, heating the reactor to a temperature of between 700 and 850 DEG C., adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is
 
     
       
         
           
             
               
                 
                   
                     3 
                     ⁢ 
                     y 
                   
                   + 
                   
                     6 
                     ⁢ 
                     x 
                   
                 
                 
                   
                     3 
                     ⁢ 
                     y 
                   
                   + 
                   
                     4 
                     ⁢ 
                     x 
                   
                 
               
               ⁢ 
               
                 NaF 
                 · 
                 
                   AlF 
                   3 
                 
               
             
             ; 
           
         
       
     
     B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3, wherein the reaction formula involved is: 
     
       
         
           
             
               
                 
                   K 
                   2 
                 
                 ⁢ 
                 
                   TiF 
                   6 
                 
               
               + 
               
                 KBF 
                 4 
               
               + 
               
                 
                   Al 
                   ⟶ 
                   Al 
                 
                 · 
                 Ti 
                 · 
                 B 
               
               + 
               
                 
                   
                     
                       3 
                       ⁢ 
                       y 
                     
                     + 
                     
                       6 
                       ⁢ 
                       x 
                     
                   
                   
                     
                       3 
                       ⁢ 
                       y 
                     
                     + 
                     
                       4 
                       ⁢ 
                       x 
                     
                   
                 
                 ⁢ 
                 
                   KF 
                   · 
                   
                     AlF 
                     3 
                   
                 
               
             
             ; 
           
         
       
       
         
           
             
               
                 K 
                 2 
               
               ⁢ 
               
                 TiF 
                 6 
               
             
             + 
             
               NaBF 
               4 
             
             + 
             
               
                 Al 
                 ⟶ 
                 Al 
               
               · 
               Ti 
               · 
               B 
             
             + 
             
               
                 
                   
                     3 
                     ⁢ 
                     y 
                   
                   + 
                   
                     6 
                     ⁢ 
                     x 
                   
                 
                 
                   
                     3 
                     ⁢ 
                     y 
                   
                   + 
                   
                     4 
                     ⁢ 
                     x 
                   
                 
               
               ⁢ 
               
                 NaF 
                 · 
                 
                   
                     AlF 
                     3 
                   
                   . 
                 
               
             
           
         
       
     
     Compared with the conventional art, the disclosure has advantages as follows: when the electrolyte supplement system provided by the disclosure is applied to the aluminium electrolytic industry, the electrolytic temperature in the aluminium electrolysis process can be obviously reduced without changing the existing electrolytic process, thus, power consumption is reduced, volatilization loss of fluoride is reduced and comprehensive cost of production is reduced; the method for preparing cryolite provided by the disclosure has advantages of mild reaction conditions, easy control, simple process, full reaction and high quality of reaction product. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The disclosure is described below in further detail through specific embodiments. 
     Embodiment 1 
     Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 780 DEG C., adding dried potassium fluoborate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form boron and potassium cryolite KF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite KF.AlF 3  through a siphon-pump. 
     Adding the electrolyte supplement system KF.AlF 3  in the aluminium electrolysis process to the electrolyte basic system consumed continuously, which can cause an obvious reduction of electrolytic temperature and finally to a temperature of between 800 and 820 DEG C. Since the corrosivity of the potassium cryolite KF.AlF 3  is stronger than that of the sodium fluoroaluminate, in order to prolong the service life of the electrolytic cell, the electrolytic cell needs to adopt an inert anode or a cathode subjected to inert surface treatment. 
     Embodiment 2 
     Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 800 DEG C., adding dried potassium fluotitanate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form titanium sponge and potassium cryolite 3/2KF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite 3/2KF.AlF 3  through a siphon-pump. 
     Adding the electrolyte supplement system 3/2KF.AlF 3  in the aluminium electrolysis process to the electrolyte basic system consumed continuously, which can cause an obvious reduction of electrolytic temperature and finally to a temperature of between 820 and 850 DEG C. Since the corrosivity of the potassium cryolite KF.AlF 3  is stronger than that of the sodium fluoroaluminate, in order to prolong the service life of the electrolytic cell, the electrolytic cell needs to adopt an inert anode or a cathode subjected to inert surface treatment. 
     Embodiment 3 
     Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 750 DEG C., adding the mixture of dried sodium fluoborate and sodium fluotitanate in the reactor slowly in accordance with a reaction ratio, wherein the mole ratio of the sodium fluoborate to the sodium fluotitanate is 2:1; stirring quickly for 5 hours to form titanium boride and sodium cryolite 6/5NaF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite 6/5NaF.AlF 3  through a siphon-pump. 
     Adding the electrolyte supplement system 6/5NaF.AlF 3  in the aluminium electrolysis process to the electrolyte basic system consumed continuously, which can cause a reduction of electrolytic temperature and finally to a temperature of between 900 and 930 DEG C. 
     Embodiment 4 
     Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 780 DEG C., adding dried potassium fluoborate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form boron and potassium cryolite KF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite KF.AlF 3  through a siphon-pump. Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 750 DEG C., adding the mixture of dried sodium fluoborate and sodium fluotitanate in the reactor slowly in accordance with a reaction ratio, wherein the mole ratio of the sodium fluoborate to the sodium fluotitanate is 2:1; stirring quickly for 5 hours to form titanium boride and sodium cryolite 6/5NaF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite 6/5NaF.AlF 3  through a siphon-pump 
     Mixing the obtained potassium cryolite KF.AlF 3  with the obtained sodium cryolite 6/5NaF.AlF 3  in a mole ratio of 1:1 and adding the mixture to the electrolyte basic system consumed continuously as the electrolyte supplement system in the aluminium electrolysis process, which can cause an obvious reduction of electrolytic temperature and finally to a temperature of between 860 and 880 DEG C. Since the corrosivity of the potassium cryolite KF.AlF 3  is stronger than that of the sodium fluoroaluminate, in order to prolong the service life of the electrolytic cell, the anode and cathode of the electrolytic cell had better subject to inert surface treatment. 
     Embodiment 5 
     Weighing 5 tons of aluminium and putting it into a reactor, heating the reactor to a temperature of 750 DEG C., adding 2 tons of mixture of dried potassium fluoborate and potassium fluotitanate in the reactor slowly, wherein the mole ratio of the potassium fluoborate to the potassium fluotitanate is 1:1; stirring quickly for 4 hours to form aluminium-titanium-boron alloy and potassium cryolite 9/7KF.AlF 3  due to excessive aluminium, opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite 9/7KF.AlF 3  through a siphon-pump. Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 780 DEG C., adding dried sodium fluoborate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form boron and sodium cryolite NaF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite NaF.AlF 3  through a siphon-pump. 
     Mixing the obtained potassium cryolite 9/7KF.AlF 3  with the obtained sodium cryolite NaF.AlF 3  in a mole ratio of 1:1 and adding the mixture to the electrolyte basic system consumed continuously as the electrolyte supplement system in the aluminium electrolysis process, which can cause an obvious reduction of electrolytic temperature and finally to a temperature of between 850 and 880 DEG C. Since the corrosivity of the potassium cryolite 9/7KF.AlF 3  is stronger than that of the sodium fluoroaluminate, in order to prolong the service life of the electrolytic cell, the anode and cathode of the electrolytic cell had better subject to inert surface treatment. 
     Embodiment 6 
     Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 750 DEG C., adding the mixture of dried potassium fluoborate and potassium fluotitanate in the reactor slowly in accordance with a reaction ratio, wherein the mole ratio of the potassium fluoborate to the potassium fluotitanate is 2:1; stirring quickly for 5 hours to form titanium boride and potassium cryolite 6/5KF.AlF 3 , opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite 6/5KF.AlF 3  through a siphon-pump. Weighing 5 tons of aluminium and putting it into a reactor, heating the reactor to a temperature of 750 DEG C., adding 2 tons of mixture of dried sodium fluoborate and sodium fluotitanate in the reactor slowly, wherein the mole ratio of the sodium fluoborate to the sodium fluotitanate is 1:1; stirring quickly for 4 hours to form aluminium-titanium-boron alloy and sodium cryolite 9/7NaF.AlF 3  due to excessive aluminium, opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite 9/7NaF.AlF 3  through a siphon-pump. 
     Mixing the obtained potassium cryolite 6/5KF.AlF 3  with the obtained sodium cryolite 9/7NaF.AlF 3  in a mole ratio of 1:3 and adding the mixture to the electrolyte basic system consumed continuously as the electrolyte supplement system in the aluminium electrolysis process, which can cause an obvious reduction of electrolytic temperature and finally to a temperature of between 870 and 890 DEG C. Since the corrosivity of the potassium cryolite 6/5KF.AlF 3  is stronger than that of the sodium fluoroaluminate, in order to prolong the service life of the electrolytic cell, the anode and cathode of the electrolytic cell had better subject to inert surface treatment. 
     The above are the further detailed description of the disclosure made in conjunction with specific preferred embodiments; it can not be considered that the specific embodiment of the disclosure is only limited to the description above. For the common technicians in the technical field of the disclosure, umpty simple deductions or substitutes can be made without departing from the concept of the disclosure and they are deemed to be included within the scope of protection of the disclosure.