@Talat Zakeri has asked me this question by PM.
In order that she can benefit from input from many experts I post it below:
Hi,
I have two powders from Sasol company. one of them is alumina and the other is Boehmite. their both d50 is around 35 micron. I solve the boehmite in water and adjust the PH on 3 , the result is a stable suspension. But about Alumina I do the same but the suspension is very very unstable. Can you please help me to understand this different behavior?
thank you in advance
@Talat Zakeri
A little more information is required and you have some work to do:
- What do you mean by stability and instability?
- What do you mean by PH?
- Have you carried out a Stokes' law calculation for settling? With an x50 of ~ 35 microns, both powders will settle quickly and will not remain in suspension
- Have you measured the zeta potential of the materials in suspension over a reasonable pH range?
Perhaps the term "stability" refers to the tendency of powders in suspension to settle? I assume the following.
Bemite (AlO (OH)) is a crystalline layered structure, based on the alternation of packets consisting of two oxygen (internal) and two hydroxyl (external) layers with aluminum atoms in the cavities. Water enters interpacket space, particles swell and no longer settle (like clay)
Alumina (Al2O3) has a non-layered crystalline structure, so the particles immediately settle in water.
O.s. Khomenko At a median size of 35 microns both will settle quickly in water, and at this size zeta potential is practically irrelevant.
So my guess is wrong. I thought that there is an analogy with clay.
Then what kind of stability are we talking about?
O.s. Khomenko You are correct in that claylike particles will settle more slowly than their spherical counterparts. However, this effect only becomes important in the sub-2 micron region where claylike particles do not settle in a straight line as required by Stokes’ Law. Brownian motion also becomes important in relation to gravitational settling. These effects are negligible at 35 microns. We have not heard from the originator of the question. My only thoughts are that the pH of the continuous phase has caused agglomeration or (better) dispersion. The limited facts are not enough to evaluate this information at this stage.
As you say, we need to understand what the poster means by ‘stability’. Do both materials wet in the liquid? Is one surface hydrophobic and one surface hydrophilic?
Yes, I also thought that it was about the size of the particles. Thank you for the interesting information.
See the section on sedimentation in Terry Allen’s book ‘Particle Size Measurement‘ for more details.
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