Phase Equilibria and Phase Rule (Part - 2) - Thermodynamics, Physical Chemistry, CSIR-NET - Government Jobs PDF Download

Desilverisation of Argentiferrous Lead (Pattinson’s Process)

The process, which is used for the recovery of silver from argentiferrous lead is called Pattinson s process and involves the desilverisation of lead in accordance to the phase diagram of lead-silver system.

The argentiferrous lead contains a small percentage of silver (less than 0.1%). For its recovery, the argentiferrous lead is heated above its melting point when a liquid melt consisting of silver-lead solution is obtained. Now if the silver lead solution is cooled, then Pb continues to separate out and is regularly removed. In the end, a eutectic solution containing 2.6% Ag (corresponding to point C) is obtained. Thus, the above process increases the percentage of silver in the argentiferrous lead. Therefore, the eutectic mixture containing 2.6% silver can be treated for the recovery of silver profitably.

(B) Systems having Congruent Melting Point

A binary system is said to possess a congruent melting point when it melts at a sharp temperature to give a liquid of the same composition as that of the solid.

The components of a binary mixture at a certain stage enter into chemical combination and form one or more compounds (inter-metallic compounds) in stochiometric proportions. These compounds melt sharply at a constant temperature into a liquid having the same composition as that of the solid. The temperature at which such a compound melts is called the congruent melting point.

Some common examples of this type of system are zinc-magnesium system, mercury-thallium system, gold-tin system and ferric chloride-water system etc.

1 Zn-Mg system

Zn-Mg System is a two-component system and possess a congruent melting point. The phase diagram of Zn-Mg system is shown in Fig. 2.3. In this system, the two components are zinc and magnesium, which melt at 419°C and 650°C respectively which are represented as point B and A in the phase diagram of the system. Both metals enter into chemical combination and form an intermetallic compound MgZn₂ and melts at 590°C to give a liquid of the same composition. Hence, 590°C is the congruent melting point of the system.

In the reduced form, the system has the following four phases:

Solid magnesium, solid zinc, solid MgZn2 and liquid solution of Zn and Mg.

Fig. 2.3 Phase diagram of Zn-Mg system [Congruent melting point system]

On applying the reduced phase rule

F' = C - P + l = 1 - 2 + 1 = 0

Therefore, at point D constitutes a non-variant system. 
(ii) Point E (Eutectic point): Point E represents the eutectic point of the system at a temperature of 345°C which is the least melting point of Mg-MgZn₂ system. Here, also three phases existing together in equilibrium at point E are solid Mg, solid MgZn₂ and liquid MgZn₂.

Hence, C = 2 and P = 3,

F' = C - P + 1 = 2 - 3 + 1 = 0 The system is non-variant.

(iii) Point C (Eutectic point): This point also represents the eutectic point (380°C) which is the least melting point of Zn-MgZn₂ system. At this point, the three phases—solid Zn, solid MgZn₂ and liquid MgZn₂ exist together in equilibrium. Therefore,

C = 2 and P = 3

F' = C - P + l = 2 - 3 + 1 = 0

Hence, point C represents a non-variant system.

(c) Areas The phase diagram of zinc-magnesium system consists of many areas. The area above the curve BCDEA constitutes a single phase system. The phase present in this area is a liquid melt consisting of a liquid solution of zinc and magnesium.

Hence C = 2 and P = 1,

F' = C - P + 1 = 2 - 1 + 1 = 2 that the system is bivariant.

Most of the other areas of the Zn-Mg system consists of two phases and they are univariant systems as represented in the phase diagram. These areas are explained in datail in table 2.3.

Table 2.3: Some salient features of the Zn-Mg system

(C) Incongruent Melting Point System

There are several systems in which components combine together to form one or more compounds which are unstable and do not possess congruent melting points.

A system (compound) is said to possess incongruent melting point, if on heating it decomposes much below its melting point and forms a new solid phase and a solution having different composition from the corresponding solid state. It has no sharp melting point. The decomposition at this temperature is known as transition or meritectic or peritectic rection and the temperature (the incongruent melting point) is known as transition or meritectic or peritectic temperature. 

Original solid new solid + solution (melt)

Examples: Following are some examples of the binary systems which possess
incongruent melting point:

1. Gold-antimony system
2. Sodium-bismuth system
3. Sodium-potassium system
4. Sodium sulphate-water system
5. Potassium chloride-copper chloride system

1 Na-K system This is a two-component system having incongruent melting point. The melting points of sodium and potassium are 97.8°C and 63.8°C respectively which are shown in the phase diagram in Fig. 2.4. Both elements chemically combine together in the ratio of 2:1 to form a compound Na₂K. But this compound is unstable and decomposes into solid Na and melt at a temperature of 70°C. It is the incongruent melting point or peritectic temperature of this system. This system consists of four phases i.e. Solid K, Solid Na, Solid Na2K and Liquid of Na and K. 

Fig. 2-4 Phase diagram of Na-K [Incongruent melting point system]

As the pressure does not have any effect on this type of equilibria hence the degree of freedom for such a system is reduced by one, So, reduced phase rule is applicable on the Na-K system. (F' = C - P + l)

The phase diagram contains the following curves, points and areas.

1 Curves

(i) Curve AC. (Freezing point curve of potassium):    This curve shows the
lowering in freezing point of potassium by addition of sodium and continues till the point ‘C’ is reached. Along this curve, potassium (K) separates out as solid phase. A new phase Na₂K separates out at point C. The two phases exist in equilibrium along this curve.

P = 2, [K(solid) and liquid (Na-K melt)] and C = 2

On applying the reduced phase rule

F' = C - P + 1 = 2 - 2 + 1 = 1 Hence, the system is univariant along this curve.

(ii) Curve CD. (Fusion curve of Na₂K):    Along this curve the two phases exists in
equilibrium. The Na₂K is stable along this curve. [If further the compound would be stable as having congruent melting point then the curve may be plotted up to the stable melting point E of the compound, which is shown in the phase diagram of the Na-K system].

Na₂K(s) Liquid

P = 2 [Na₂K(s) and liquid] and C = 2

Hence    P = 3, C = 2

So, according to the phase rule

F' = C - P + 1 = 2 - 3 + 1 = 0

Thus, the system is invariant at point D.

3 Areas

(i) Area above ACDB: The area above the ACDB contains only liquid phase i.e. melt of Na, K and Na₂K.

Hence    P = 1, C = 2

On applying the reduced phase rule

F' = C - P + 1 = 2 - 1 + 1 = 2

Thus, the system is bivariant in the area above ACDB.

(ii) Area ACF: It consists of two phases solid K and liquid.

(iii) Area ECG: This area consists of two phases solid Na₂K and liquid.

(iv) Area BDH: It consists of the two phases, which exist in equilibrium i.e. solid Na and liquid.

(v) Area below FCG: It consists of solid K and solid Na₂K.
(vi) Area IHLK: This area consists of two phases solid Na and solid Na₂K.
All areas from (ii) to (vi) are having two phases and two components

Hence    P = 2, C = 2

On applying the reduced phase rule

F' = C - P + 1 = 2 - 2 + 1 = 1 F' = 1

Therefore, all the above areas represent univariant systems.

Table 2.4: Some salient features of the Na-K system.

6 Applications of The Phase Rule

Phase rule has wide applications in electronic industries, pharmaceutical science, medical science, etc. Some major applications of phase rule are as follows:

[1] Solders

Solder is an alloy, which is homogenous mixture having melting point lower than that of the corresponding metal pieces, which have to be joined together. Solders have compositions somewhat different from the eutectics so that the freezing occurs over a range of temperatures. The quality of solder depends upon the formation of a surface alloy between the solder and parts of metals being used. The selection of solder alloy is based upon the melting point desired and the pieces of metals to be joined. Some essential qualities of the solder are as follows:

1. Melting point of the solder should be less than the material to be soldered.
2. Solder should spread in liquid form and also form homogeneous mixture with the metals.

Some common solders which are available in the market are:

1. ‘Soft solder’ alloy of Pb and Sn.
2. ‘Plumber alloy’ contains Pb = 67% and Sn = 33%.
3. ‘Half-half alloy’ contains Pb = 50% and Sn = 50%

Half-half alloy is commonly used for soldering the pipes with a bright surface finishing after soldering but very expensive. Also due to high contents of tin, it is not widely applicable for several electric appliances. The solder which contains about 60% Pb is used in the electrical wires.

[2] Safety Plug

Safety plugs are also known as the safety fuses. It is an alloy having low melting point, used to ensure the safe working and avoid accidents. Safety fuses are used in buildings to protect them against fires. One alloy is woods metal, which is used in the safety fuses. This alloy melts at 65°C and consists of the composition woods metal Bi = 50%, Pb = 25%, Sn = 12.5% and Cd = 12.5%. and the frozen liquid (ice) is directly converted into a gaseous state. The removed water is stored again through the condensers. The material or tissue is left almost as a skeleton and original matter can be obtained by adding water. By this process shrinkage of flowers is eliminated or minimized.

All botanical samples, fruits and vegetables can be freeze-dried. This technique is also used in pharmaceutical industry, museums, taxidermy, floral industry and camping/hiking food processors.