Methods of Refining of Metals

Minerals and ores abound in the earth’s crust. Some ores have proven to be a valuable resource for humanity. Iron, for example, derived from iron ore (Hematite), laid the groundwork for the industrial revolution. Aluminium, on the other hand, was a critical strategic resource for aviation during World Wars I and II. Despite this, aluminium metal continues to dominate the market due to its unique features and fast and cost-effective extraction. Metals account for 70% of all naturally occurring elements.

Metals can be found in nature in both free and combined forms. In general, reactive metals exist in mixed states as oxides, sulphides, carbonates, and so on. Metals in the center of the reactivity series, such as zinc, iron, lead, and so on, are moderately reactive and can be found in the earth’s crust as oxides, sulphides, carbonates, and so on. Some of these metal complexes can be inexpensively removed to produce pure metal. Since ancient times, metallurgy techniques have been employed in India. The first traces of metallurgy in India were discovered in Mehrgarh, Baluchistan, around 6000 BCE. Gold, copper, silver, lead, tin, iron, and mercury are the seven metals of antiquity.  

What are Ores and Minerals?

Minerals are naturally occurring, homogeneous inorganic solids with distinct chemical composition and crystalline structure, colour, and hardness. 

Minerals include copper pyrite, calamine, and others. An ore is a mineral from which a maximum amount of metal may be cheaply mined.

Extraction of Metals from Ores  

• The process of getting pure metals from their ores is known as metallurgy. Metals are extracted from their ores using methods based on their reactivity.
• Electrolysis is used to extract highly reactive metals from their ores, while calcination, roasting, and reduction techniques are used to remove metals of medium reactivity.
• While roasting and refining are used to recover metals with low reactivity from their ores. Although most low reactive metals, such as gold, are found in a free state or in combination with silver and mercury (As amalgam).
• The concentration of ores is done before using all of these methods of extraction of metals based on their reactivity. Ores are found in nature with a variety of impurities, including rock particles, sand, and other impurities.
• These impurities in an ore are referred to as matrix or gauge impurities. Concentration ore refers to the removal of a matrix or gauge from the ore.

Refining  

Some impurities persist in a metal recovered by any process, such as calcination/roasting or electrolysis. Depending on the differences in the characteristics of metals and impurities, many different processes are utilised to achieve a very pure metal. Depending on the metal and the impurity present, very distinct procedures are applied. 

• Distillation

This process is used to purify metals that have a low boiling point, such as mercury and zinc. The impure metal is heated over its boiling point in this procedure, allowing it to generate vapours. Because the impurities do not evaporate, they are separated. The impurities are left behind as the pure metal vapours condense.

• Liquation

The melting points of the metals are taken into account in this procedure. Metals with low melting points are purified using this process. Impurities have a greater melting point than metal. By delivering heat at a temperature just over their melting point, the metals are transformed into liquids. Impurities are left behind as pure metal melts and flow down from the furnace.

• Zone Refining and Electrolytic Refining

Electrolytic refining and zone refining are two more common refining technologies. Impure metal is employed as an anode in electrolytic refining, while pure metal is utilised as a cathode. During electrolysis, pure metal moves from the anode to the cathode, while impurities accumulate as anode mud.

Zone refining works on the premise that impurities are more soluble in molten metal than in solid metal. In this technology, mobile induction coil warmers are constructed to move over impure metal rods. On the rod that moves with the heater, a molten zone forms. Pure metal crystallises and leaves behind as the heater moves forward, whereas impurities travel along with the molten zone due to their more soluble nature in a molten state.

• Principle of Zone Refining

The premise of zone refining is that contaminants in a metal ingot or ore are more soluble in the melt stage than in the corresponding solid-state.

Impurities are focused at one end of the metal block during the zone refining process, allowing the rest of the block to be cleansed. It is worth noting that the segregation coefficient (defined as the ratio of impurity in the solid-state to impurity in the liquid or melt state) is usually less than one.

This means that when the circumstances are set at the solid-liquid boundary, the impurity atoms tend to diffuse into the liquid zone.

• Chromatography

Aside from these procedures, chromatography is utilised in element purification when there is a very small variation in the chemical characteristics of contaminants and metals. The principle of chromatography is based on the observation that different components of a mixture have varied affinities for the stationary phase. 

By spreading components throughout the stationary and mobile phases, components are physically separated. Weak forces and nonionic forces, such as hydrogen bonds or Van der Waals forces, are used to adsorb components by the stationary phase. The constituents of a mixture travel at different rates and become separated. 

The separation is accomplished through differential partitioning of the mobile and stationary phases. The differing partitioning coefficients of the components cause differential retention on the stationary phase, resulting in their separation on the stationary phase.

Sample Questions

Question 1: What is the principle of vapour phase refining of metals?

Answer:

Metal vapour phase refinement is one of the refining processes. In the refining process, impure metal is transformed into its volatile component. This substance is gathered and degraded. The complex is decomposed by heating it at a high temperature to extract the pure metal.

Question 2: What is the meaning of zone refining of metals?

Answer:

Metal zone refinement is a method of refining in which a circular mobile heater is mounted to one end of an impure metal rod. When the heater moves, the molten zone of the rod moves as well. This motion causes the creation of pure metal crystals from impurities, which are then transmitted to the opposite side of the molten zone. Impurities are gathered at one end of the rod as a result of the process being repeated several times. The end at which the contaminants are collected is subsequently severed. The concept behind this approach is that metal impurities are more soluble in a molten state than in a solid form.

Question 3: What is benefaction?

Answer:

The process of eliminating undesirable components (such as sand, clays, and other minerals) from ore is referred to as benefaction. 

Question 4: What are ores?

Answer:

To obtain a certain metal, we must first search for minerals, which are chemical components found naturally in the earth’s crust and obtained through mining. Only a few minerals that contain a metal are acceptable for use as a source of that metal. These minerals are referred to as ores.

Question 5: What is magnetic separation?

Answer:

Magnetic properties of either the ore or the gangue are utilised to separate the ore and gangue. The ore is finely ground before being delivered on a conveyor belt that passes through a magnetic roller. The magnetic mineral remains on the belt, while the gangue falls away.

Question 6: What is leaching?

Answer:

It is a chemical process that involves treating the ore with the proper reagent to make it soluble. It is feasible to separate the soluble ore or mineral from the insoluble gangue. Once the ore has been separated, several chemical techniques can be utilised to recover it.