Separation of Mixtures using Sublimation and Magnets

Sublimation is the separation technique where a substance makes a transition from the solid to the gas state directly. The substance does not undergo the liquid stage. Therefore, it is used to illustrate a solid-to-gas transition between the states of a substance. This process is endothermic in nature and takes place at the lowest pressure at which any compound can exist in the liquid state. The conversion of a substance from the gaseous state to the solid-state is termed deposition or de-sublimation. It is the reverse process of sublimation. 

Sublimation is a physical change in the state of the substance. Sublimation is not a chemical reaction with oxygen. For instance, if we consider the chemical compound solid ammonium chloride, upon heating in the hydrogen chloride solution, the compound dissociates. For instance, the combustion of candles, into the separate components of carbon dioxide and water vapour is a chemical reaction taking place in the presence of oxygen. 

The transition of the substance from the solid to the gas state requires passing through the intermediate liquid state. This requires the usage of external pressure, called the partial pressure of the substance. Some of the solids easily sublime, like carbon and arsenic since the pressure of their triple point is very high. This makes it difficult to retain the liquid state. To summarise, certain substances, like camphor easily turn into a gaseous state and convert to a solid state upon cooling. 

How sublimation occurs? 

The molecules of a substance are bound together by strong intramolecular forces of attraction. When we heat the substance, the heat energy is supplied which increases the energy possessed by the molecules. As a result of the increase in energy, some of the molecules break from the forces of attraction existing with the surrounding particles so that they can escape into the vapour phase. Since the process of sublimation incurs the usage of external energy, it is considered to be an endothermic change. The enthalpy of sublimation is equivalent to the summation of the enthalpy of fusion and the enthalpy of vaporization.

Importance of Sublimation

Sublimation can be important in the recovery of compounds that are suspended or dissolved in a liquid or a solid like dry ice. The compounds can be recovered, at least in crude form, by allowing the suspending matrix to sublimate away. This method of recovery is usually gentle, which is advantageous in preserving the chemical structure or even activity of the target drug (i.e., cocaine) or enzyme. Many compounds will sublimate when heated. The effective temperature can be characteristic of the compound and can be measured in a forensic laboratory inexpensively, using a common, not plate.

Sublimation has practical applications in forensic science. Forensic analysis of a crime or accident scene often relies on the examination of photographic evidence after the scene has been cleaned. A dye-sublimation printer enables digital pictures to be rendered in print form in a very realistic and detailed fashion, which helps investigators in their analysis.

The basis of a dye-sublimation printer is the vaporization of various coloured dyes housed in the printer. The vaporized dyes penetrate the glossy surface of the photographic paper before returning to their solid form. 

Applications of sublimation 

• Water
  Snow tends to sublime in case of extremely low temperatures and dry winds. Extreme temperatures cause the snow to vaporise before it undergoes the melting procedure.
• Dry Ice
  Dry ice is the solid form of carbon dioxide, which is most commonly used in ice cream parlours. The substance is safe to handle and creates a smoky effect. When we submerge this compound in water as a solvent, the bubbles emerge out due to the emission of heat to create smoke. This causes the sublimation of dry ice.
• Printers
  Dry sublimation can be used in order to print high-quality images using a special film. These are more convenient and better when compared to ink printers.
• Air fresheners
  The solid air fresheners are considered to be sublime in nature. When we turn on an air freshener in the restroom, the solid air fresheners directly turning to gas.
• Mothballs
  The mothballs are composed of a material known as Naphthalene. The naphthalene balls directly convert to vapour state, that is, sublime. It eliminates the moths.
• Freeze Drying
  Frozen food items leave behind the ice crystals upon undergoing sublimation. Items are frozen to preserve the perishable materials. The frozen good is subjected to reduced pressure and more heat.
• Separation of chemical compounds
  Sublimation can be easily used to separate a volatile solid from a non-volatile solid. The following steps are followed: 
  The powdered substance, in this case, the ammonium chloride crystals are taken in a China dish.
  • The system is covered with a perforated filter paper and an inverted funnel.
  • The dish is then carefully heated on a sand bath.
  • The vapours that pass through the holes of the filter paper cool down and condense on the inner sides of the funnel.
  • The non-volatile impurities will remain as a residue in the china dish.

Separation by a Magnet

The technique of separation by a magnet depends upon the process of separating components belonging to mixtures by the use of a magnet. It is used to attract magnetic materials. The basic principle behind this technique is that it detaches non-magnetic material apart from the magnetic ones. This technique is extensively in use for ferromagnetic minerals, that is the substances that are strongly affected by magnetic fields or paramagnetic minerals, that is the substances that are less affected in the presence of magnetic fields. Magnetic Separation is one of the most common and important physical separation techniques. 

How this technique works?

The magnetic materials are basically separated by mechanical means. During the execution of this process, the magnets are located inside two separator drums bearing liquids. The magnetic particles are being drifted by the movement of the drums due to the magnets. This can lead to the creation of a magnetic concentrate. For instance, an ore concentrate.

Where to use magnetism?

1. Removal of iron in coarse and intermediate-crushing circuits, to simulate protection to the crushing machine.
2. Concentration of iron ore other than magnetite, followed by the preliminary conversion of iron minerals to artificial magnetite. This process is performed by the process of Roasting.
3. Segregation of small quantities of iron ore minerals from the Ceramic raw materials.
4. Concentration of Paramagnetic minerals of less intensity such as those of Manganese, chromium, tin, titanium, etc.

Most of the materials tend to behave in a certain way in the presence of a magnetic field. However, with most of these materials, the effect is too slight to be detected. There are basically three categories depending on how substances behave in the presence of a magnetic field: 

• Diamagnetic
• Paramagnetic
• Ferromagnetic

Comparison of Dia, Para and Ferro Magnetic materials

Diamagnetic	Paramagnetic	Ferromagnetic
Substances that are feebly repelled by a magnet.	Substances that are feebly attracted by a magnet.	Substances that are strongly attracted by a magnet.
Example: Antimony, Gold, Silver,  Alcohol, water, Hydrogen, Air etc.	Example: Aluminium, Alkali and Alkaline earth metals, Oxygen, etc.	Example: Iron, Nickel, Gadolinium, etc.
The lines of force tend to avoid the substance in the presence of the magnetic field.	The lines of force prefer to pass through the substance in place of air in the presence of the magnetic field.	The lines of force tend to crowd into the specimen in the presence of the magnetic field.

Classification of Magnetic Separators

Magnetic source	Strength of magnetic field	Materials	Type of magnetic field	Body shape
Permanent magnetic separator Electromagnetic separator	Weak magnetic separator Intermediate magnetic separator Strong magnetic separator materials	Wet type magnetic separator Dry type magnetic separator	Constant Pulsating Alternating	Belt-type magnetic separator Drum-type magnetic separator Roll-type magnetic separator Disc-type magnetic separator Ring-type magnetic separator Cage-type magnetic separator Pulley-type magnetic separator

Applications of Separation by a Magnet

• Drum Magnetic Separator
  • This type of separator is primarily used to remove fine irons from a large amount of material separation.
  • These type of separators is best used where it is necessary to supply big working surface, thereby increasing the throughput rate of materials.
  • Extensively used in sugar mills, food and rubber processing, fertilizer and glass industries.
• Permanent Suspended Magnetic Separator
  • This type of separator is primarily used to remove undesired metal from heavy burden depths of material. In order to simulate separation, this is further conveyed on the fast-moving cross belt. These permanent suspended magnets are equally effective on both dry or wet as well as fine or coarse material.
• Pipeline Magnetic Separator
  • This type of separator is primarily used to purify products in liquid lines by extracting out the irons contaminants.
  • It is also used to offer protection to pipelines, screens from tramp irons damaging.
• Magnetic Pulleys
  • Magnetic pulleys are primarily used for the product purification of foodstuffs and other materials, chemicals, glass, sugar, and pharmaceuticals etc.
  • Magnetic Pulleys are convenient to install and remove.
• Plate Magnetic Separator
  • This type of separator is primarily used to remove tramp irons from items found in various kinds of industries, like, glass, food, minerals and the textile Industry.
• Crossbelt Magnetic Separators
  • This type of separator is primarily used to concentrate moderately magnetic ores.
  • The primary advantage is that various types of magnetic products can be recovered in one pass through these separators.
• Over Band Magnetic Separator
  • This type of separator is primarily used to remove the iron from a non-magnetic material, which is present on the belt conveyor as well as an electric magnetic vibrating feeding machine.
• Hump Magnetic Separator
  • This type of separator is primarily used to ferrous iron particles from pneumatic conveyor systems.
  • It is extensively used where heavy and loose materials such as textiles and chemical industries.

Sample Questions

Question 1: Explain briefly the process of sublimation of naphthalene balls. 

Answer: 

Naphthalene is a solid substance which undergoes sublimation at a standard atmospheric temperature. The sublimation point of the naphthalene balls is at around 80 °C or 176 °F.  At low temperature, its vapour pressure becomes high enough, that is, at 1 mmHg at 53 °C the naphthalene evaporate from solid form into gaseous state.

Question 2: What is the requirement of carrying out purification of organic compounds before performing sublimation?

Answer:

The organic compounds obtained from natural sources are not entirely pure. The organic compounds derived from the laboratory reactions are also not pure and may contain other compounds with them. However, in order to understand and analyse the structure and properties of an organic compound, it should be in the purest form. Therefore, the purification of organic compounds is mandatory before carrying out sublimation.

Question 3: How can we separate the compound containing iron and sulphur undergoing heating? 

Answer: 

The mixture of iron and sulphur can be separated using the process of magnetic separation. Upon heating this mixture, sulphur undergoes melting before iron since the melting point of sulphur is very low in comparison to iron. and At a temperature of about 445^oC, the mixture ignites and eventually leads to the formation of sulphur dioxide and sulphur trioxide. Therefore, iron gets extracted from the mixture whereas sulphur can be extracted from its oxide.

Question 4: Give some real-life examples of magnetic separation.

Answer:

Some of the real-life examples of magnetic separation are : 

• Removing iron bearing minerals from the chemical compound silica sand which is used in the production of glass;
• Recovering aluminium cans from the household waste;

Question 5: Difference between the processes of sublimation and diffusion.

Answer: 

Sublimation	Diffusion
The process where a substance transforms directly from a solid to a gas. Energy is required for a solid to sublime into a gas. The solid substance undergoing the process of sublimation is known as sublime. The solid substance obtained by cooling the vapours of the solid is called as sublimate. Example:- The solid iodine turning into a gas	The movement of a fluid from an area of higher concentration to an area of lower concentration. It occurs as a result of the kinetic properties of particles of matter. Diffusion essentially means to “to spread out.” Example:- Perfume diffuses throughout an entire room.

Question 6: Give a brief listing of the conditions where the ice would be most likely to sublimate?

Answer: 

At high temperature and pressure – Water is more likely to melt followed by its evaporation. 

At low temperature and low pressure and At low temperature and high pressure – Water is more likely to stay solid. 

.At high temperature and low pressure – It is most likely that ice will sublimate. The presence of high pressure makes it energetically favourable for water to melt before it undergoes evaporation. On keeping the atmospheric pressure low, it becomes more favourable to directly reach into the gaseous phase.

Question 7: Explain the similarities and differences between sublimation and deposition.

Answer: 

Sublimation is a separation technique where there is a phase transition from the solid to gaseous state, without passing through the liquid state. 

Deposition is a separation technique where there is a phase transition from the gas to solid, without passing through the liquid state. It is the reverse of sublimate. 

Both of these phases skip passing through the liquid state, but they serve as an indicator to the physical changes occurring in different directions. For instance, Carbon dioxide at standard pressure exerts both properties, that is sublimation as well as deposition, 

CO₂(s) ⇢ CO₂(g) —Sublimation

CO₂(g) ⇢ CO₂(s) —Deposition