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To change <strong>the</strong> sodium sulphate into <strong>the</strong> anhydrous <strong>the</strong>nardite form, <strong>the</strong> separated crystals must
be suspended and heated with steam in a remelting vessel. Thus, <strong>the</strong> water leaves <strong>the</strong> sodium
sulphate crystals. The crystals are separated from <strong>the</strong> mo<strong>the</strong>r liquid using centrifuges. The
separated crystals are dried in a flash dryer and stored in different silos.
For reasons of efficiency, <strong>the</strong> mo<strong>the</strong>r liquid from <strong>the</strong> sodium sulphate centrifuges is cooled and
<strong>the</strong> additional crystallised Glauber’s salt is also brought to <strong>the</strong> remelting vessel. The primary
mo<strong>the</strong>r liquid and part of <strong>the</strong> secondary mo<strong>the</strong>r liquid are purged to effluent. Steam is produced
on <strong>the</strong> site using natural gas. The dryers are heated directly with natural gas.
A flow scheme of sodium sulphate production by <strong>the</strong> Messo process is shown in Figure 1.11.
Salt purge
Cooling Remelting Drying
Purge
Steam
Natural gas
Recovery
Purge
Figure 1.11: Flow scheme of sodium sulphate production by <strong>the</strong> Messo process
[64, CEFIC-SSPA, 2004]
1.6.2.3 Chromium process
Sodium sulphate
Crude sodium sulphate, a by-product from <strong>the</strong> sodium dichromate plant containing sexivalent
chromium compounds (Cr +6 ), is dissolved and transferred into treatment tanks. At this stage, <strong>the</strong>
solution is acidified in order to facilitate <strong>the</strong> chemical reduction of <strong>the</strong> solution using sulphur
dioxide. After reduction, <strong>the</strong> pH is raised leaving an insoluble species of chromium suspended
in a solution of sodium sulphate. This is filtered and <strong>the</strong> resulting solids are recycled back to <strong>the</strong>
primary process of <strong>the</strong> production of sodium dichromate. The purified sodium sulphate solution
goes forward subsequently into a crystalliser, centrifuge, dryer and classifier to form pure
sodium sulphate crystals. A flow scheme of <strong>the</strong> production of sodium sulphate from <strong>the</strong>
chromium process is given in Figure 1.12.
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