When you recirc or agitate chemical cargoes you basically do de-flocculation or de-agglomeration and de-sedimentation.
It is NOT about pulverizing sediments from the tank bottom with a cargo jet through a recirc diffuser as simple sailors often think . It is about building up an electric charge that will keep the cargo in suspension.
Some solid cargoes like clay slurry are carried at sea in a liquid form. Certain cargoes like phosphoric acid get sedimented .
Solid Kaolin clay is made into a slurry and carried at sea to be pumped through pipelines , as it is cheaper.
During the sea passage the solid and water will separate out-causing a sediment at the bottom.
This is NOT good, as the pump impeller is right at the bottom and will get cemented, and at discharge port, ship cant pump out cargo and then has to be pumped ashore using a portable pump.
So at sea, the same pump is used to push the cargo back into the tank through small orifices or diffuser nozzles in a tank to tank recirculation procedure . This produces a static electricity charge which keeps the solids in suspension.
Latex is stabilized by protective colloids that prevent agglomeration. Latex suspension is stabilized by electric charges. Colloids can be precipitated by adding electrolytes that neutralize the electric charges.
De-emulsification is done by adding polyvalent ions to neutralize electric charges. Physical means include heating and centrifuging or use of high potential alternating current.
Agglomeration is combination of colloids in a liquid to flocs and is achieved by neutralization of the electric charges which maintain the stability of the colloidal suspension.
Aggregation is the tendency of large molecules or colloidal particles to combine in clusters or clumps in a solution. This happens by adding an electrolyte to remove the electrical charges . Aggregation by coalescence or coagulation is irreversible.
The destruction of colloids called coagulation can be accomplished by heating or by adding an electrolyte. Colloids are much bigger than atoms and therefore can be observed through an optical microscopes
Flocculation is combination of colloids so that they form clumps or tufts. Flocculation can be reversed by agitation because the cohesive forces are relatively weak. Alum is an inorganic flocculant.
A stable colloidal system is one in which the particles resist flocculation or aggregation and exhibits a long shelf-life.
This will depend upon the balance of the repulsive and attractive forces that exist between particles as they approach one another.
If all the particles have a mutual repulsion then the dispersion will remain stable. However, if the particles have little or no repulsive force then some instability mechanism will eventually take place e.g. flocculation, aggregation etc.
In certain circumstances, the particles in a colloidal disperson may adhere to one another and form aggregates of successively increasing size that may settle out under the influence of gravity.
An initially formed aggregate is called a floc and the process of its formation flocculation. The floc may or may not separate out.
If the aggregate changes to a much denser form, it is said to undergo coagulation. An aggregate usually separates out either by sedimentation (if it is more dense than the medium) or by creaming (if it less dense than the medium).
The term’s flocculation and coagulation have often been used interchangeably. Usually coagulation is irreversible whereas flocculation can be reversed by the process of deflocculation.
Wet phosphoric acid must be constantly recirc during the voyage to prevents sediments from depositing. Phosphoric acid (from Richards Bay, Morroco, Tunisia etc ) require daily recirculation through diffuser installed on drop line.
Certain cargoes have to be recirculated at sea for successful discharge . It may not be a requirement of the shippers or charterers. A cemented deepwell pump impeller can cause delays in start of discharge or severe pump damage. Ensure that such cargoes are loaded in tanks whose drops can be fitted with diffuser nozzles.
CAPT AJIT VADAKAYIL
30 YEARS IN COMMAND