Question

Q-2) write down the process of iodine manufacture in detail ?

Answer 1

Iodine is obtained from
a) brine which contains iodide ions
b) nitrate ores

(a) From brine

Brine containing 100-150 ppm iodine as iodide ions (the amount depending on the source) is purified and acidified with sulfuric acid. It is then chlorinated to liberate the iodine:

Liberated iodine is purified by two methods
i) the blowing out method
ii) the ion-exchange method

(i) The blowing out method

The liberated iodine is removed in an extraction tower by a counter-current stream of air, a process known as anti-blowout. The iodine-rich air passes into an absorbing tower containing hydroiodic and sulphuric acids and is treated with a solution of sulphur dioxide, which reduces the iodine to hydroiodic acid

Most of the solution is reacted again with chlorine and the iodine produced allowed to settle, remelted and then flaked.

The remainder of the solution is recycled to the absorption tower and 99.5% pure is obtained

The ion-exchange method

The iodine which is liberated reacts with iodide ions to form, principally, I3- ions:

The I3- ions are adsorbed on an anionic exchange resin. The resin is transferred to another column, where a solution of sulfur dioxide drips through to regenerate the iodide ions

The reaction can be represented thus (because the equilibrium above is displaced to the left as the iodine reacts):

As in the blowing out process, this solution is heated again with chlorine and iodine is liberated and purified.

(b) From nitrate ores

In Chile, iodine occurs in nitrate deposits as iodate ions (IO3-). The ore contains approximately 95% sodium nitrate and 5% sodium iodate, NaIO3.
After the removal of sodium nitrate by crystallization, the crushed ore is leached with hot water. The clear solution, in which the iodate ions accumulate, is run off and cooled. A spray of the solution is treated with sulfur dioxide in an absorption tower to release the iodine and convert it to iodide ions:

The overall reaction is thus:

The absorption tower solution is mixed with a small quantity of the solution containing iodate ions to liberate free iodine:

The solid iodine is separated from the solution in flotation cells. The iodine is extracted using a hydrocarbon solvent (kerosene). The iodine/kerosene suspension passes into a reactor and is heated at 400 K and at a pressure above atmospheric. The molten iodine is removed and cooled to give the solid product, known as prill.