See Stoke's law for a small sphere moving through a viscous fluid.

For a quicker sedimentation rate then you need the particles to be in agglomerated form and that is favored by a zeta potential close to 0 mV.  You don't say what your material is so there's no ability to judge (What pH? Is it + or - charged? etc) what you need to do - it may be a change of pH (in line with the chemistry) or the addition of polyvalent ions such as PO42- or Al3+ .  See slides 4 onwards in :

Nov 11th, 2008. Dispersion and nanotechnology http://tinyurl.com/hpywsge

Stokes' law for gravity settling applies over a limited range (say 2 - 50 um according to Allen Particle Size Measurement).  Below that size we have competition with Brownian motion and the particle des not settle in a straight line.  Above that level then we're into a turbulent flow region.

Thank you both, for your quick responses!

I have 90% WC and 10 % Co (wt.%) particles (below 20 um). When I use isohexane, the sedimentation is extremely fast. For acetone, the sedimentation is slower and a harder cake is obtained, as compared to isohexane. In this case, in my opinion, the sedimentation speed is causing a harder cake

However, when I use water, the agglomerates formed after drying are very hard (I need a mortar and a pestle to disintegrate them). In my opinion, here, there are two mechanisms which form the agglomerates: 1) the oxidation and dissolution of constituents in aqueous media (I have read some papers on silica powder agglomeration during drying of aqueous solutions) and 2) the very slow sedimentation rate. I am using a commercially available corrosion inhibitor to limit the oxidation, but if I could enhance the sedimentation rate I could limit the oxidation and dissolution processes also by removing the excess water instead of waiting for all the water to dry and thus develop agglomerates similar to the organic processed powders. 

Although in water, after 5 minutes, the majority of the particles have sedimented, there are numerous submicronic particles floating, so I cannot remove the excess water without influencing significantly the particle size distribution.

P.S. Mr. Alan Rawle, I cannot access the provided link.

Try the longer form instead:

Nov 11th, 2008. Dispersion and nanotechnology

http://www.brainshark.com/brainshark/vu/view.asp?pi=106756756&text=M021507 

How were the particles made?  There is an incompatibility between polar solvents (such as water or acetone) and non-polar such as hexane. Dependent on the origin of the particles and their hydrophobicity or hydrophilicity plus the fact that like prefer like will influence how the materials behave in water.  For materials like hexane the concept of zeta potential is ill-defined.

"there are numerous submicronic particles floating".  Do you mean floating or in suspension?  If floating then this tells you that you have a wetting problem (incompatibility between particle and medium).  This is counteracted with a surfactant.

 Hi Alan, thank you for you answers I found very useful to my work as well! I can not access the link though. 

Manahil Kadhim try internet explorer!