Reactions in Molten Salt System(Chemistry of molten salts)

INTRODUCTION 
Molten  salt  refers  to  a  salt  that  is  in  the liquid phase that is normally a solid at standard temperature  and  pressure  (STP).  A  salt  that  is normally a liquid at STP is usually called a room temperature  ionic  liquid,  although  technically molten salts are a class of ionic liquids (ILs). Molten salts have a variety of uses. Molten chloride  salt  mixtures  are  commonly  used  as baths for  various alloy heat  treatments, such as annealing and  mar  tempering  of  steel.  Cyanide and chloride  salt  mixtures  are  used  for  surface modification  of  alloys  such  as  carburizing  and nitro carburizing of  steel. Cryolite is  used as a solvent for aluminum oxide in the production of aluminum Fluoride, chloride, and hydroxide salts can be  used  as  solvents. Molten  salts  can  also  be  used  as  heat transfer fluids as well as for thermal storage New  electrolytes  for  lithium  battery  consisting of  room  temperature  molten  salts  have  been tested. 
 
IMPORTANCE 
▪ The chemistry of solution of molten salts,their equilibria and reactions is of very great technical importance e.g glass is a super cooloed complex. 
▪ Most of the industrial metallurgical processess for the extraction of metals involves the use of high melts to decrease m.p of ore,high m.p solid salts are used. 
▪ The production of volatile products from molten salts system is particularly easy and often extremely conveniently. 
▪ Most salts show sharp increase in conductivity in melting .the high conductivities ,m.p and large temp. Range and solubility of liquids show strong bonding in them.Exception of HgCl2 which which has realtively low equilibrium conductance and a liquid range of only 25° indicating that the melt consists of uncharged particle.   
Properties of molten salts 
The physicochemical and electrochemical properties of various types of real molten salt electrolytes are comprehensively studied, and they are classified according to the temperature and concentration dependences of their physical and electrochemical properties, which are determined by the type of chemical bond between particles, complexing, and the contribution of electron charge transfer. properties of these systems, including (a) the ability to dissolve many inorganic and organic compounds, attaining high concentrations of reagents, (b) the possibility to support oxidation states of ions not readily obtained. Molten salt technology is a catch-all phrase that  includes  some  very  diverse  technologies: electrochemistry,  heat  transfer,  chemical oxidation/reduction  baths,  and  nuclear  reactors. All  of  these  technologies  are  linked  by  the characteristics of molten salts. 
 
 
 

 
General characteristics of molten salts are following: 
1) Can function as solvents    
2) Have good heat transfer characteristics (heat capacity)    
3) Function like a fluid (like water)    
4) Can attain very high temperatures    
5) Can conduct electricity    
6) Some molten salts have chemical catalytic properties. 
   
Some  simple  physical  properties  of  the ionic  liquids  that  make  them  important  as potential solvents for synthesis are the following:  
1)  Unusual combinations of reagents that can be brought into the same phase. 
2)  Composed of poorly coordinating ions and have the potential to be highly polar yet non coordinating solvents.    
3)  Immiscible with a number of organic solvents  and  provide  a  non  aqueous,  polar alternative for two-phase systems. 
4)  Nonvolatile  and  can  be  used  in high-vacuum  systems  and  eliminate  many containment problems.   One  of  the  important  features  of  molten salts is their ability to conduct electricity. 
 
 For example,  solid sodium chloride does not conduct electricity;  it  is  an  insulator.  If  NaCl  is  placed into water, the mutual attraction both sodium and chlorine  have  for  water  molecules  cause  their bonds to break and form ions within the water These  electrically  charged  ions  can  conduct electricity if there is a voltage potential. Molten salts  conduct  electricity  the  same  way  they do when they are  dissolved  in  water;  some of the salt  molecules  are  dissociated  into  ions,  which allows the ions to conduct electricity . 
 
Uses Molten salts have a variety of uses. 
➢ Molten chloride salt mixtures are commonly used as baths for various alloy heat treatments, such as annealing and martempering of steel. Cyanide and chloride salt mixtures are used for surface modification of alloys such as carburizing and nitrocarburizing of steel 
➢  Cyanide and chloride salt mixtures are used for surface modification of alloys such as carburizing nitrocarburizing ofsteel. Cryolite (a fluorid e salt) is used as a solvent for aluminium oxide in the production of aluminium in the Hall-Héroult process.

➢ Fluoride, chloride, and hydroxide salts can be used as solvents in pyroprocessing of nuclear fuel. Molten salts (fluoride, chloride, and nitrate) can also be used as heat transfer fluids as well as for thermal storage. This thermal storage is commonly used in molten salt power plants .

➢ . A commonly used thermal salt is the eutectic mixture of 60% sodium nitrate and 40% potassium nitrate, which can be used as liquid between 260550 °C. 
Methods for study of Molten salt solutions:

1. Spectroscopic Meaaurnents:

Both Vibration IR and Electronics UV .Spectra of molten salt solution, the suggest that complex ions are produced in the Molten salts system .Audition of KCL to Molten zncl2 to produces2 new vibrational frequencies and at 50 moles% KCl the original spectrum of zncl2 Has disappered.

2. Cryoscopic Method( depression in freezing point method):

The dissociation state of the solutes M2MoO4, M2Mo3O10, M2Mo4O13, M2Mo5O16 (MRb or Cs), Na2CrO4·MoO3, K2CrO4·2 MoO3, Cr2Mo3O12 and V2MoO8 was studied cryoscopically in molten K2 Cr2O7 and KNO3 solvents. The freezing point depression, ΔT, of the solvents was obtained by measuring the cooling curves of the binary salt mixtures over unlimited range of solute concentration. 
 
 The number of foreign ions obtained ν, showed that the solutes were either simply dissociated in the melt into the probable stable species (MoO4)2−, (Mo3O10)2−, (Mo4O13)2− and (Mo5O16)2− or, in some cases after reactions and rearrangements, into heteropolyions The solute V2MoO8, on the other hand, was found to dissolve without any apparent dissociation. An agreement between the experimental and calculated values of activity, a, based on the Temkin and Random Mixing models and that of Van't Hoff's equation support the proposed simple dissocia- tion scheme for K2Cr2O7.Cs2MoO4 system. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 
3.Phase Diagram:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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4.Electrical conductivity and transport properties: 
Molten salts (or ionic liquids) represent a unique class of electrochemical systems. These materials may be corrosive, moisture- or oxygen-sensitive, and/or with a high melting temperature. All these factors must be taken into account in the design of electrochemical cells for molten salt research. Figure 2.21 depicts two gas-tight Pyrex glass electrochemical cells for use with moderatemelting molten salts. Figure2.22 demonstrates a setup for high-melting molten salts. Typically, in these cell setups, additional provision must be made for temperature measurements and control (through a thermocouple or thermistor). Gas-tight Pyrex glass electrochemical cell for use with (A) moderate-melting molten salts (35) and (B) gas-tight cell with a magnetically coupled drive for rotating-disk electrode voltammetry in moderate-melting salts. Molten salts are present in many technical processes.