Wet carbon has a density of 1.4sg. If the slurry density is higher than this then you will need to reduce it. If lower it may indicate that the carbon is not fully washed before use.

The slurry density is within normal range. What is really puzzling is that this phenomenon is happening in the 4th tanks in a circuit of 6 tanks. All the other tanks are not exhibiting the same thing.

I would check the agitator in that problem tank. Keeping carbon in suspension requires right agitation pattern in the tank.

I have found that the carbon will sink if the agitator is not working properly as the slurry begins to settle in the tank.

It could also be an issue of high platelet content in the carbon. Did you recently add it to your system?

Yes we recently added carbon into the circuit, but it is introduced through the last Tank 6, and then migrates upstream. The tank that is giving us the problem is Tank 4.

Could the Carbon be fouled by a leaking gearbox on the tank that is giving you problems?

Checked agitator rotational direction? Depending on design it could be that you are inadvertently concentrating the carbon in the upper half of the vessel due to the agitator running in the wrong direction thus appearing to be floating up.

Is the gear box on no.4 tank you are using similar to the other gear boxes you are using on the other tanks?

Check pulp density in that specific tank to compete to other tanks.

Maybe some hydrocarbon leaking in the tanks! (Grease, oil, etc.) This would make carbon floating.

The tanks have the same type of gearboxes and that particular tank's gear box is not leaking oil. The carbon is still floating even at lower pulp density which is very much surprising.

Do you inject air in this tank? Maybe unbalance on air distribution can drag carbon to the surface. I suggest checking the draft tube to certify that the slurry is being fed in the right position. I suggest to check the turbulence in the tank (check the baffles) and problems in the stator.

Do you inject air in this tank? Maybe unbalance on air distribution can drag carbon to the surface. I suggest checking the draft tube to certify that the slurry is being fed in the right position. I suggest to check the turbulence in the tank (check the baffles) and problems in the stator.

What is the density in your tank if it is too high it will cause your carbon to float as well!

Have you left fresh carbon in water to conditioning for long enough before it gets sent to circuit? And if it is regened carbon, has it been fully submerged in water in quench tank? My point is that with agitation and constant air pump drawing carbon together with slurry to upstream, you may not see significance of 'floating carbon' in active tanks, but it might become problem in the tank with no carbon movement to upstream. I am wonder if your tank4 air lift has been closed off and no carbon/slurry getting sent to tank3?

Reading all comments and your answers it is tank specific and most likely caused by specific hydraulics. Your suggestion is worth following - this may easily happen if electricians were not well trained. Check what is different in the tank when compared to the others. Perhaps empty the tank and see what is in the tank. It needs to be explained.

Considering all the contributions given, as rightly said, the problem is with that particular tank (Tk 04). First check and parameters like the current drawn by that agitator motor, agitator gearbox speed and the DO level of that tank to see if defers from the rest. If these values defers from the rest the agitation in that tank is questionable. Secondly I will suggest you isolate that tank, lower it and inspect the agitator shaft and blades and baffle plates to see if we can get a clue.