Outline the principles of refining of metals by the following methods:
(i) Zone refining
(ii) Electrolytic refining
(iii) Vapour phase refining
(i) Zone refining: This method is used for production of semiconductors and other metals of very high purity, e.g., Ge, Si, B, Ca and In.
It is,based on the principle that the impurities are more soluble in the molten state (melt) than in the solid state of the metal.
The impure metal in the form of bar is heated at one end with a moving circular heater. As the heater is slowly moved along the length of the rod, the pure metal crystallises out of the melt whereas the impurities pass into the adjacent molten zone. This process is repeated several times till the impurities are completely driven to one end of the rod which is then cut off and discarded.
(ii)Electrolytic refining: Many metals, such as Cu, Ag, Au, Al, Pb, etc., are purified by this method. The impure metals is made the anode while a thin sheet of pure metal acts as a cathode. The electrolytic solution consists of a salt or a complex salt solution of the metal. On passing the current, the pure metal is deposited on the cathode while the impurities fall down as anode mud.
(iii)Vapour-phase refining: The crude metal is freed from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at a lower temperature and then decomposing the volatile compound at some higher temperature to give the pure metal.
(a)Mond's process: When impure nickel is heated is a current of CO at 330-350 K, it forms volatile nickel tetracarbonyl complex leaving behind the impurities. The complex then heated to a higher temperature (450-470K) when it undergoes thermal decomposition giving pure nickel.
(b)Van Arkel method: This method is Used for preparing ultra-pure metals by removing all the oxygen and nitrogen present as impurities in metals like zirconium and titanium (which are used in space technology).Crude Zr is heated in an evacuated vessel with iodine at 870 K. Zirconium tetraiodide thus formed is separated. It is then decomposed by heating over a tungsten filament at 1800 – 2075 K to give pure Zr.
Assertion (A): Zirconium can be purified by van Arkel method.
Reason (R): ZrI4 is volatile and decomposes at 1800 K.
The main reactions occurring in blast furnace during extraction of iron from haematite are
t the temperature corresponding to which of the points in figure of Q. 11-13, FeO will be reduced to Fe by coupling the reaction 2FeO —> 2Fe + 02 with all of the following reactions?
Which of the following statements are correct about the role of substances added in the froth floatation process?
(a) Collectors enhance the non-wettability of the mineral particles.
(b) Collectors enhance the wettability of gangue particles.
(c) By using depressants in the process two sulphide ores can be separated.
(d) Froth stabilizers decrease wettability of gangue.
The value of ΔfG ° for formation of Cr2O3 is – 540 kJ mol-1 and that of Al203 is – 827 kJ mol-1 . Is the reduction of Cr2O3 possible with Al?
Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?
Extraction of gold and silver involves leaching the metal with CN+ ion. The metal is recovered by
(a) displacement of metal by some other metal from the complex ion
(b) roasting of metal complex
(c) calcinations followed by roasting
(d) thermal decomposition of metal complex
In the extraction of aluminium by Hall-Heroult process, purified A1203 is mixed with CaF2 to
(a) lower the melting point of A1203.
(b) increase the conductivity of molten mixture.
(c) reduce Al+3 into Al(s).
(d) acts as catalyst.
Name the processes from which chlorine is obtained as a by-product What will happen if an aqueous solution of NaCl is subjected to electrolysis?
Why is sulphide ore of copper heated in a furnace after mixing with silica? Iron oxide present as impurity in sulphide ore of copper forms a slag of iron silicate and copper is produced in the form of copper matte.
Assertion (A): Sulphide ores are concentrated by froth floatation method. Reason (R): Cresols stabilize the froth in froth floatation method.
Assertion (A): Hydrometallurgy involves dissolving the ore in a suitable reagent followed by precipitation by a more electropositive metal.
Reason (R): Copper is extracted by hydrometallurgy.
Is it true that under certain conditions, Mg can reduce Al203 and Al can reduce MgO? What are those conditions?
The choice of a reducing agent in a particular case depends on thermodynamic factor. How far do you agree with this statement? Support your opinion with two examples.
Wrought iron is the purest form of iron. Write a reaction used for the preparation of wrought iron from cast iron. How can the impurities of sulphur, silicon and phosphorus be removed from cast iron?
How are metals used as semiconductors refined? What is the principle of the method used?
Name the common elements present in the anode mud in electrolytic refining of copper. Why are they so present?
What criterion is followed for the selection of the stationary phase in chromatography?
Which of the following statements are correct?
(a) A depressant prevents certain type of particle to come to the froth.
(b) Copper matte contains Cu2S and ZnS.
(c) The solidified copper obtained from reverberatory furnace has blistered appearance due to evolution of SO2 during the extraction.
(d) Zinc can be extracted by self-reduction.
Why is an external emf of the more than 2.2 V required for the extraction of Cl2 from brine?
For the reduction of FeO at the temperature corresponding to point D, which of the following statement is correct?
For the metallurgical process of which of the ores, calcined ore can be reduced by carbon?
(a) Haematite (b) Calamine (c) Iron pyrites (d) Sphalerite
In which of the following method of purification, metal is converted to its volatile compound which is decomposed to give pure metal?
(a) Heating with stream of carbon monoxide
(b) Heating with iodine
(c) Liquation
(d) Distillation
Choose the correct option of temperature at which carbon reduces FeO to iron and produces CO.
(a) Below temperature at point A
(b) Approximately at the temperature corresponding to point A
(c) Above temperature at point A but below temperature at point D
(d) Above temperature at point A
