There is a solid-liquid separation system in the factory where I work
and I am supposed to make some modification to it to make it work
properly. There is a gas stream containing CO2 and solid particles of
SiO2. The gas stream passes through two scrubbers in which it is
brought into contact with water. The wetted SiO2 that is in a cottony
form (something like wool) goes down the scrubbers to enter a basin in
which the wetted cottony SiO2 has to be separated from the water with
the clarified water being recycled to the scrubbers. The pool is
divided into three pools. The fluid leaving the scrubbers enters the
first pool and flows into the second pool through a few 15 centimeter
holes at the bottom of the wall. A sludge pump has to pump the settled
sludge to a filter press and the water overflows to the third pool to
be recycled.
The problem is that about half the wetted cottony SiO2 does not settle
in the first pool of the basin. The fact is that about 40% of it
settles and about the same amount remains on the surface with about 20
percent floating in the middle of the fluid in the first pool. So it
seems beside the intendancy of the whole sludge to settle the settled
fraction doesn't flow easily into the second pool. The pump also seems
not to be able to pump the sludge properly. This was proved by my
taking a sample of the discharge of the pump while it was working. The
sample was a thin suspension containing not very much solid, but when I
closed the second suction line, a thick sludge emerged. Another problem
is that the recycled water is not clarified enough.
I have myself thought of some modifications such as merging the three
pools and sending all the fluid to the filter press by using stronger
pumps and recycling the clear water coming out of the filter. Or a
chain and flights system may solve the problem. Or the whole system can
be replaced by for example a bag filter or something else. But, it
seems the present system is quite good and just needs some changes.
I would appreciate it if anybody could help me with this matter.

it seems your settling problems are due to gas trapped inside the SiO2
fiber network (besides never heard about cottony silica - do you have
some reference information about it or web site?).

Therefore a solution could be to agitate the fiber flocs or to use
flotation for removal.
As the silica is in a form of a kind of sticky fiber network  'normal'
pumps will have problems. So you should inquire about special pumps for
handling of fibrous material like pulp and paper.

Kind regards
T. Sobisch