Two liquor flows are withdrawn from the salt wash tank:
1. An overflow
of clarified wash liquor (the clarification is achieved by sedimentation of
most crystals in the gravity settling zone of the tank). This overflow will,
however, still have particles of very small size suspended in it. Due to the
fact that a large fraction of the Gypsum crystals and part of the other
particulates are smaller than sodium chloride crystals, this overflow actually
removes from the process these solids. The overflow is then fed to a thickening
step (this can be a gravity settler or a bank of hydrocyclones), where fines
are separated and discharged as sludge, and the clear wash liquor is pumped
back to the process as a solids-free wash medium to the washing thickener, if
one is used.
Multi cyclone for the preparation of Gypsum-free wash solution
2. The salt crystal suspension, which is pumped to the separation station.
This consists of another bank of hydrocyclones, the (optional)
washing-thickener and the centrifuge. The hydrocyclone thickens the suspension
before it is fed to the washing-thickener or the centrifuge, and returns a
significant part of the wash liquor to the salt wash tank. The cyclone overflow
is essentially, but not completely free of suspended solids. It still may
contain very small salt and gypsum crystals. The cyclone underflow, on the
other hand, contains mostly gypsum solids. The final separation occurs in the
washing-thickener that follows.
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