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Sodium sulphate is recovered from <strong>the</strong> solution called “mo<strong>the</strong>r liquor” by concentration and
water evaporation in a multi-effects evaporator. With temperature increase and concentration,
sodium sulphate solubility decreases and leads to <strong>the</strong> precipitation of <strong>the</strong> sodium salt while
methionine remains in solution.
The solid phase is separated from <strong>the</strong> solution by decantation <strong>the</strong>n filtration on a centrifuge in
which <strong>the</strong> cake is washed by water.
At <strong>the</strong> outlet of <strong>the</strong> centrifuge, sodium sulphate is dried in a hot air stream heated by steam.
Then <strong>the</strong> solid is separated from <strong>the</strong> gaseous phase by cyclone and sent by pneumatic transport
to <strong>the</strong> bulk storage silos from which it is loaded in bulk onto trucks to deliver to customers.
Downstream <strong>the</strong> dryer, airflow is cleaned in a scrubber supplied with process water to remove
<strong>the</strong>fine particles. During <strong>the</strong> drying, <strong>the</strong> humidity of sodium sulphate and <strong>the</strong> organic sulphur
compounds dissolved in <strong>the</strong> water, are transferred to <strong>the</strong> air. Before being released to <strong>the</strong>
atmosphere, <strong>the</strong> airflow is deodorised by contact with bleach in air-mix equipment in which a
chemical oxidation of <strong>the</strong> organic compounds is carried out.
Part of <strong>the</strong> production is purified in order to obtain a white coloured solid without odours.
The process treatment is performed in three steps:
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humidification and spraying with sodium chlorate (NaClO3)
oxidation by calcination in a rotary kiln
cooling by air in a rotary mixer.
The off-gases from <strong>the</strong> calcination have to be purified before <strong>the</strong>y can be released to <strong>the</strong>
atmosphere. In <strong>the</strong> first step, <strong>the</strong> solid particles are kept by a cyclone and recycled into <strong>the</strong>
process. The gaseous effluent is sent to a scrubber fed with water in order to eliminate residual
dust. The liquid waste only contains sulphate and chloride in such a low concentration that it
can be released to <strong>the</strong> environment.
A flow scheme of <strong>the</strong> production of sodium sulphate from <strong>the</strong> methionine process is given in
Figure 1.14.