Engineering Chemistry S Datta

320 ENGINEERING CHEMISTRY
reach the other end. The rod is allowed to cool and the impure zone is cut off and
removed. The remaining portion of the rod consists of the pure metal.
Molten zone
Resolidified
portion
Source of heat
Fig. 14.12 Zone refining.
Most impure
zone to be
cut out
2. Electro-refining. This method is widely used. Usually an electrolytic cell is set up using
thin sheets of pure metal as the cathode and comparatively thick impure metal slag as anode.
The electrolyte used is an aqueous solution of a suitable salt. During electrolysis, pure metal
from the electrolyte gets deposited on the cathode and the equivalent amount of metal from the
anode passes into the solution. The impurities either remain in solution or are deposited down
the anode. This deposit is known as anode mud. Sometimes some valuable metals are recovered
from anode mud. A typical example of electrolytic refining is of copper (Fig. 14.13).
15% CuSO4
solution in dil.
HSO 2 4 as
electrolyte
Impure Cu anode
Pure Cu cathode
Fig. 14.13 Electrolytic refining of copper.
POWDER METALLURGY
Powder metallurgy (Fig. 14.13) is a technique or operation or process to produce metallic
or alloy articles. In this technique, the metal powders or alloy constituents are compacted in a
die with melting or no melting followed by sintering. The simultaneous method of compacting
and sintering is called pressure sintering.
The article is made into the desired shape by powder metallurgy in the following steps:
(i) Pulverisation of metallic powders
(ii) Mixing and blending
(iii) Compacting
(iv) Pre-sintering, and
(v) Sintering
(vi) Finishing operation.