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What is Refining of Metals?

  • Last Updated : 29 Oct, 2021

In metallurgy, refining is extremely important. In nature, any metal that has been taken from its ore is usually impure. Crude metal refers to the impure metal that is removed. Refining is the process of eliminating impurities from metals in order to achieve high-purity metals. Impurities are removed from unrefined metal using a variety of processes based on the metal’s and impurities’ qualities.

Metal can be found in ore in the form of a compound with other elements. Metal extraction is the process of obtaining metal from its ore. Refining is one of the processes involved in the extraction of metals from their ores. Metal refining is discussed further below.

What is the Refining of Metals?

Impurities are common in metals produced by various reduction methods. As a result, most metals are impure. Refining metals is a term used to describe the process of purifying metals. As a result, metal refining implies metal purification. 

The procedure for refining impure metals is determined by the nature of the metals as well as the nature of the impurities present. For different approaches, different refining procedures are used. Impurities are removed from crude metal using a variety of methods based on the metals and the properties of impurities. The following are some of the processes used in the purification of crude metals:

  • Liquation – The melting points of the metals are taken into account in this method. This method is used to purify metals having low melting points. The impurities have a greater melting point than the metal. Heat is applied at a temperature slightly over the melting point of the metals to convert them to a liquid condition. Impurities are left behind as pure metal melts and flow down from the furnace.
  • Distillation – This is a method for purifying metals with a low boiling point, such as mercury and zinc. The impure metal is heated above its boiling point to create vapours in this process. Since the impurities do not evaporate, they must be separated. The pure metal vapours are condensed, leaving the impurities behind.
  • Vapour phase refining- In this sort of purification, the metal should produce a volatile compound in the presence of a reagent, which should easily break down to recover the metal. The metal is converted into a gaseous compound. This volatile chemical is subsequently decomposed to yield pure metal. Nickel, for example, is purified in this way.
  • Chromatographic methods – The crude mixture is placed in a liquid or gaseous media in this process. An adsorbent is used to transport this medium. Different components of the mixture are adsorbed at different levels of the column. Using appropriate solvents, these components of the mixture are removed.
  • Zone refining – This procedure is used to remove impurities from metals such as germanium, silicon, gallium, indium, and boron. The impure metal is connected to a circular mobile heater at one end. The pure metal crystallises out as the heater is moved, while the impurities transfer to the adjacent part of the metal. Impurities accumulate at the other end of the rod, which is cut to yield pure metal.

The most important and most widely used method for refining impure metals is electrolytic refining.

What is Electrolytic Refining?

Electrolytic refining is the process of purifying using electrolysis. Copper, zinc, tin, lead, chromium, nickel, silver, and gold are just a few of the metals that can be processed electrolytically. A thick block of impure metals is used as an anode for electrolysis refining of impure metals, and it is connected to the positive terminal of the battery. 

As it is connected to the negative terminal of the battery, a thin strip of pure metal is used as the cathode. As an electrolyte, a water-soluble salt of the metal to be refined is used. When an electric current passes through the anode, the impure metal dissolves and enters the electrolyte solution, while pure metal from the electrolyte deposits on the cathode. The soluble impurities in impure metal dissolve in solution, whereas insoluble impurities settle at the bottom of the anode as anode mud. Consider an example to better understand electrolytic metal refining. Let’s look at how this method works for refining copper metal.

Electrolytic refining of Copper

An electrolytic tank containing an acidified copper sulphate solution as an electrolyte is used in the electrolytic refinement of copper. As the anode, a thick block of impure copper metal is attached to the positive terminal of the battery. Since it is connected to the negative terminal of the battery, a thin strip of pure copper metal is used as the cathode. 

Impure copper from the anode dissolves and enters the copper sulphate solution when an electric current passes through it, whereas pure copper from the copper sulphate solution deposits on the cathode. Thus, on the cathode, pure copper metal is produced. The soluble impurities dissolve in the solution, while the insoluble impurities gather as anode mud below the anode.

Explanation of Electrolytic refining of Copper: Copper ions and sulphate ions are present in a copper sulphate solution. The following reaction occurs at the two electrodes when an electric current is passed through the copper sulphate solution.

  • The positively charged copper ions, Cu2+, from the copper sulphate solution go to the negative electrode (cathode) and are reduced to copper atoms by taking electrons from the cathode. Pure copper metals are formed when these copper atoms are placed on the cathode.

At cathode:                                                               Cu2+        +          2e                          Cu

                                                                         (Copper ions)          (Electrons)                      (Copper atom)

  • The impure anode’s copper atoms each lose two electrons to the anode, forming copper ions Cu2+ in the electrolytic solution.

At anode:                                                                 Cu          –          2e                →              Cu2+

                                                                                (Copper atom)            (Electrons)                   (Copper ion)

The copper ions are removed from the copper sulphate solution at the cathode and added to the solution at the anode in this way. The impure anode gets thinner and thinner as the process progresses, whereas the pure cathode gets thicker and thicker. At the cathode, pure copper is thus obtained.

What happens to the impurities in impure copper that are metallic?

Impure copper contains metallic impurities that are either more reactive or less reactive. The more reactive metals in impure copper, such as iron, now flow into the electrolyte solution and stay there. The less reactive metals in impure copper, such as gold and silver, settle in the bottom of the electrolytic cell below the anode in the form of anode mud. The anode mud can be recovered for gold and silver metals. As a result, electrolytic metal refining serves two objectives.

  • It allows other valuable metals, such as gold and silver, to be recovered from impurities in the metal being refined.
  • It purifies the metal in consideration.

Solved Questions

Question 1: Define anode mud.

Answer:

During electrolytic refining, the soluble impurities in impure metal dissolve in solution, whereas insoluble impurities settle at the bottom of the anode and are known as anode mud. 

Question 2: What is the cathode made up of in the electrorefining of copper?

Answer:

The cathode is the negative electrode in the electrorefining of copper. It is made entirely of copper. As a result, the cathode in copper electrorefining is made up of pure copper.

Question 3: What is the range of purity for copper obtained by electrolytic refining?

Answer:

The transfer of pure metal from the anode to the cathode is the result of electrolytic refining. More basic metal impurities remain in the electrolytic solution as ions, while less basic metal impurities settle down as anode mud. The copper that has been purified by electrolytic refining is 99.95-99.99% pure.

Question 4: What is the anode made up of in the electrorefining of copper?

Answer:

The anode is the positive electrode in the electrorefining of copper and it is made from impure copper blocks. As a result, the anode in copper electrorefining is made up of impure copper blocks.

Question 5: During the electrolytic refining of copper, which electrolyte is used?

Answer:

The electrolyte in the electrolytic refining of copper is a solution of copper sulphate acidified by sulphuric acid.


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