What happens to insoluble impurities during electrolytic refining?

To understand what happens to insoluble impurities during electrolytic refining, let us break down the process step-by-step:

1. What is Electrolytic Refining?
Electrolytic refining is a process used to purify impure metals (such as copper, zinc, tin, lead, nickel, etc.) using electricity. In this setup:
- A block of the impure metal is made the anode (the positive electrode).
- A thin strip of pure metal is made the cathode (the negative electrode).
- An aqueous solution of a soluble salt of the same metal is used as the electrolyte.

2. Action at the Electrodes during Electrolysis:
When an electric current is passed through the electrolyte:
- Metal ions from the electrolyte are deposited at the cathode as pure metal.
- An equivalent amount of metal from the impure anode dissolves into the electrolyte as metal ions.
- Soluble impurities present in the anode go into the electrolyte solution.

3. Behavior of Insoluble Impurities:
- Impurities that are insoluble do not dissolve into the electrolyte solution.
- Because they are not ionized and do not dissolve, they fall off the decomposing anode and settle down at the bottom of the electrolytic cell, directly below the anode.
- These collected impurities are known as anode mud (or anode slime).

Therefore, the insoluble impurities settle at the bottom below the anode as anode mud.

The correct option is: They settle as anode mud.