metallurgy

1. Design of metal separation using high temperature
distillation
By:
Ravi Roshan Law Kumar Roy Pushkar Kumar

2. Introduction
The discovery of distillation is
generally attributed to Arab
alchemists in VIII century Spain. The
actual process, however, was kept
secret until 1286, when Montpelier
University professor Arnold de
Villeneuve described the first distiller.
In 1500, German alchemist
Hieronymus Braunschweig published
Liber de arte destillandi (The Book of
the Art of Distillation) the first book
solely dedicated to the subject of
distillation, followed in 1512 by a
much expanded version. In 1651,
John French published The Art of
Distillation the first major English
book on the subject.
Distillation
Principle: In heating a mixture of
substances, the most volatile or the
lowest boiling distils first, and the others
subsequently or not at all.
Apparatus: Distillation
apparatus comprises of three
major, firstly
distillation flask which is
used for heating the mixture
and volatizing the
components, secondly
condenser; used for cooling
the vapors back to liquid
state, and lastly collection
vessel.
Working: This basic
operation requires the
use of a still or retort in
which a liquid is heated,
a condenser to cool the
vapour, and a receiver
to collect the distillate.

3. Simple Distillation Fractional Distillation Steam Distillation

4. Distillation in metal refining:
Type equation here.
 The activity of an element in a liquid solution, relative to the pure substance standard state, is defined by the
relation:
𝑎𝑚 = 𝑝𝑚 𝑝𝑚
°
where, 𝑝𝑚
°
is the vapor pressure of the pure element at the temperature considered.
 From the kinetic theory of gases, Langmuir derived an expression for the mass of vapor molecules, 𝜔𝑖, of a
species, i, striking unit area of surface in unit time:
𝜔𝑖 = 𝑝𝑖 (𝑀𝑖/2𝜋𝑅𝑇)
where 𝑀𝑖 is the atomic or molecular weight of the species in the gas phase and 𝑝𝑖 is the vapor pressure.

5. Distillation in metal refining (contd.):
Type equation here.
 Substituting the first equation into second we get:
𝜔𝑖 = 𝑎𝑖𝑝𝑖
°
(𝑀𝑖/2𝜋𝑅𝑇)
 At equilibrium, the rate of condensation is equal to the rate of evaporation, so that this equation also gives
the rate at which atoms or molecules are transferred from the melt to the gas.
 Hence, if two species, A and B, can volatilize at a given temperature, the relative rates of volatilization are
given by:
𝜔𝐴
𝜔𝐵
=
𝑎𝐴𝑝𝐴
°
(𝑀𝐴/2𝜋𝑅𝑇)
𝑎𝐵𝑝𝐵
°
(𝑀𝐵/2𝜋𝑅𝑇)

6. Distillation in metal refining (contd.):
Type equation here.
 If A and B represent the solute and the solvent, respectively, the extent to which the solute can be removed
without excessive loss of the solvent increases as the ratio
𝜔𝐴
𝜔𝐵
is increased. This is the basis of refining by
distillation.

7. Why pressure lower than atmospheric pressure is used?:
Type equation here.
 The column operating pressure is the most important distillation design parameter; it affects the
temperatures at which heating and cooling are required, the type of heating and cooling utility required, as
well as heating and cooling duties.
 Furthermore, it impacts on the number of theoretical stages needed for the separation and the diameter of
the column.
 With a higher operating pressure, the capital cost of the column will increase. A taller column will be
needed, to accommodate a greater number of theoretical stages and a thicker shell may be needed
to withstand the pressure.
 It is possible to carry out distillation processes at atmospheric pressure or at pressures that are higher or
lower than atmospheric.
 Vacuum distillation is distillation performed under reduced pressure, which allows the purification of
compounds not readily distilled at ambient pressures or simply to save time or energy.

8. Why pressure lower than atmospheric pressure is used?:
Type equation here.
 This technique separates compounds based on differences in boiling points. This technique is used when the
boiling point of the desired compound is difficult to achieve or will cause the compound to decompose.
 A reduced pressure decreases the boiling point of compounds. The reduction in boiling point can be calculated
using a temperature-pressure nomograph using the Clausius–Clapeyron relation

9. Vacuum Distillation:
Type equation here.
 Boiling commences when the vapor pressure of a liquid or
solution equals the external or applied pressure (often
atmospheric pressure).
 Thus, if the applied pressure is reduced, the boiling point
of the liquid decreases (see the graph for cinnamyl alcohol
in Figure a).
 This behavior occurs because a lower vapor pressure is
necessary for boiling, which can be achieved at a lower
temperature. For example, if water was subjected to a
reduced pressure of 100 mmHg, it would boil at 50°C (see
the graph figure b).

10. Vacuum Distillation:
Type equation here.
 The dependence of boiling point on applied pressure can
be exploited in the distillation of very high boiling
compounds (normal B.P. > 150 °C ). which may decompose
if heated to their normal boiling point. A vacuum
distillation is performed by applying a vacuum source for
reducing the pressure.
 A vacuum distillation apparatus is shown in Figure a, using
a simple distillation setup. But in this setup before
heating, we turn on the vacuum source to begin reducing
pressure inside the apparatus.

11. Understanding Vacuum Distillation through
Thermodynamics:
Type equation here.
 The difference in vapor pressure of each metal at different temperatures is the basic principle of crude metal
vacuum distillation.
 The thermodynamic performance of components of crude lead in the vacuum distillation process was
investigated systemically in order to provide a simple, clean, efficient and referential way for the removal of
Cu, Sn, Ag, Zn, As and Sb from crude lead.
 the relationship between saturated vapor pressure of the main components and temperature is shown in
equation, and the evaporation constants A, B, C and D for different components are shown in Table (next
slide).
log 𝑝 ∗ = 𝐴𝑇−1 + 𝐵 log 𝑇 + 𝐶𝑇 + 𝐷
where, p* is the saturated vapor pressure; T is the temperature.

12. Understanding Vacuum Distillation through
Thermodynamics:
 According to the previous equation and the given table,
the saturated vapor pressure can be calculated.

13. Understanding Vacuum Distillation through
Thermodynamics:
 This graph shows the trend of the saturated vapor
pressure.
 It shows that the saturated vapor pressure of As or Zn
are much higher than that of Pb at 873-1073 K. At 823 K,
As begins to sublimate, which indicates that As and Zn
are easier to volatilize into the vapor phase and can be
removed from crude lead completely.
 The saturated vapor pressure of Sb is also high in
comparison with Pb, which can be partially removed at
an appropriate temperature.

14. Understanding Vacuum Distillation through
Thermodynamics:
 The saturated vapor pressure of Cu, Sn or Ag is much
lower than that of Pb at 1273-1523 K, which shows that
Cu, Sn and Ag are difficult to volatilize into a vapor phase
and were concentrated in the residual phase.
 It also can be seen that the saturated vapor pressure of
Bi is close to that of Pb, which indicates that Bi cannot be
separated from lead by vacuum distillation.

15. Understanding Vacuum Distillation through
Thermodynamics:
Based on the above results we can conclude that, we can see:
 The impurities of Cu, Sn, Ag, Zn, As and Sb in crude lead can be easily removed by vacuum distillation in
thermodynamics, but Bi cannot be removed.
 The vacuum distillation should be taken to obtain lead from crude lead. Zn, As and Sb are removed at
lower temperature of 923-1023 K. Lead is distilled from the residual liquid containing Cu, Sn, Ag and Bi at
higher temperature of 1323-1423 K, and Cu, Sn and Ag are concentrated and remain in the residual liquid.
 The sufficient thermodynamic calculations are helpful to choose the conditions of operation and acquire
reliable results in vacuum distillation refining process for crude lead.

16. Distillation of Mercury:
Type equation here.
 Most of the world's mercury is obtained from its main ore, cinnabar or vermillion with the chemical structure
mercury sulphide (HgS).
 The most common refining method for mercury is triple distillation, in which the temperature of the liquid
mercury is carefully raised until the impurities either evaporate or the mercury itself evaporates, leaving the
impurities behind. This distillation process is performed three times, with the purity increasing each time.
 Distillation of impure mercury constitutes the best method of removing foreign metals, and distillation in a
vacuum is the only feasible plan of conducting the operation in laboratory.
 Mercury can be purified by distillation because it is very volatile for a metal. But the boiling point at normal
pressure is ~360 °C which is inconveniently hot. Using reduced pressure (say around 1/1000th of an
atmosphere) can make this practical at closer to 100 °C which is far more convenient.

17. Zn Distillation:
Type equation here.
 Another example of use of distillation in metal refining is in Zn refining.
 Lead and cadmium are the major impurities present in Zn produced in the blast furnace. 𝑝𝑃𝑏
°
is much lower
and 𝑝𝐶𝑑
°
is much larger than 𝑝𝑍𝑛
°
at any given temperature.
 Thus, when the impure Zn is completely distilled in a refluxing unit, Cd is also distilled, but, by controlling the
maximum temperature, Pb can be retained as a liquid that can be drained off.
 Most of the Cd remains in the vapor phase if the vapor is then cooled just sufficiently to condense the Zn.
 The method has the advantages of little device investment, high metal recovering rate, small occupation area
of a workshop and little construction investment, and the environmental pollution is basically eliminated.

18. Other Applications:
Type equation here.
 Vacuum distillation is one of the techniques used for removal of major impurities at ppm level in cadmium
from 3N+ to 5N+. Although the zone refining and allied techniques are used to remove the impurities from 5N
and above, the vacuum distillation is used as a preceding supportive to remove the high melting point
impurities.
 Pb−Sn alloys were separated successfully by the vacuum distillation in small- scale and continuous
industrialized experiments, and lead content in refined tin decreased to less than 0.01%.
 An interesting alternative for recycling all types of scrap magnesium is vacuum distillation. This method aims
at refining magnesium scrap into very high-purity magnesium (99.999%) to be used in the semiconductor
industry.
 The impurities in crude tin were effectively removed at 1473 K for 35 min and material weight of 80 g under 5
Pa. Under this condition, 98.67 mass% of tin in the residue can be recovered, and 84 mass% of arsenic in
crude tin was removed by vacuum distillation. Arsenic can be removed effectively from crude tin by using
vacuum distillation.

19. Advantages:
Type equation here.
 Simple process and easy to operate.
 Low loss of valuable metals and high metal recovery metals.
 Low processing cost and investment & good economic returns.
 Environmentally friendly.
 Various choice for end products: alloy, pure metal ingot, and metal powder.

20. Disadvantages:
Type equation here.
 While operating under vacuum makes the separation easier, the low vapor density requires a greater cross-
sectional area in the column to accommodate the high volumetric flow of material.
 More importantly, operating under vacuum increases the complexity of the flowsheet, requiring additional,
costly equipment to draw the vacuum and recover material drawn into the vacuum system.
 Vacuum also introduces safety hazards, related to risk of air ingress and resulting fire or explosion of
flammable process fluids.
 The principal disadvantages are that the equipment is more complicated to build and that low vapor pressure
tends to limit the flow rate of the vapor and hence reduces the capacity for equipment of any given size.

21. References:
Type equation here.
 Distillation: Operation and Applications by Andrzej Górak and Hartmut Schoenmakers.
 Distillation: Fundamentals and Principles by Andrzej Górak and Hartmut Schoenmakers.
 The Extraction and Refining of Metals by Colin Bodsworth.
 https://www.totalmateria.com/
 https://chem.libretexts.org/