3 to 4 hours are enough 

Dear Dr. Tahir,

Alright. Thank you for the clarification. 

Do you start with a well-dispersed suspension of ceramic particles and want to mix a binder in it or you start with a dry powder, which has to be disagglomerated first? After mixing the powder with water (usually it also requires a suitable dispersant and/or adjustment of pH), the milling jar has to be left on the rotating bank at least overnight, or better for 24 h or so. The speed of rotation has to be chosen in the range where milling media experiences "cascade" action, that is, the balls should gain enough energy to get up to a certain hight and fall down hitting other balls and the particles of powder. If the speed is too slow, the slurry has a flat horizontal surface, the ball almost don't move in vertical direction and disagglomeration/mixing is not efficient. If the speed is too high, you centrifugate the slurry without mixing it too. However, in many rolling banks you can use the maximum rotation speed - they are unable of bring it in "centrifugation" regime. The binder should be added after the particles have been dispersed properly.

small balls of dia 13-14 mm are best and its better to have a speed of 100 rpm

Dear Dr. Roman,

Thanks a lot for the detailed explanation. I really appreciate your gesture. 

Actually, I'd thought to mix my ceramic particles of micron-sized with water, a binder (PVA) and a surfactant (PEG)  at first using a magnetic mixer. Then to obtain a well-dispersed slurry, I'm opting ball-milling.

According to you, it should be ball-milled for around 24 hours. Please advise me if there's any further suggestion that could be said based on the above information.

Dear Dr. Tahir,

I haven't received the balls yet but I've taken note of your suggestion.

Thank you once again. 

Based on your additional information, I would suggest to omit the magnetic stirring step - if you do ball milling anyway, preliminary mixing is not a big help. To my knowledge, PEG acts as a lubricant or plastisizer in spray-drying slurries, not as a surfactant or you meant "lubricant" too? You haven't mentioned any dispersants. In certain situations, you don't need a dispersant at all, but you better make sure you have this situation. The test is simple - after a day of ball milling let you slurry to rest unmoved for a while - if it sediments fast, say on the scale of an hour or faster, then you definitely need a dispesant too. As already stated, you first properly disperse the slurry and add PEG/PVA only later on.

Dear Dr. Roman,

Many thanks for your prompt response. It's kind of you for having taken time to share your knowledge here.

I'm sorry, the PEG is meant to act as a plastisizer. For your information, I intend to mix a hydrophobic silica powder with another ceramic, such as borosilicate or zirconia, in order to spray-dry the resulting slurry.

I can anticipate that the hydrophobic silica will not mix readily with the water. Do you have any suggestions that could guide me, please? And would you mind telling me a suitable dispersant?

Thank you very much.

Best Regards.

I don't have experience with silica slurries but still would recommend that you consider buying/producing a hydrophilic grade. Hydrophobic is made non-dispersible in water on intention. You run into unnecessary troubles by using it. You can of course try to find a suitable dispersing agent, apply ultrasonic bath or wet silica with some organic solvent before mixing it with water but I cannot guarantee it is going to be simple. Hope that someone else here can give you better advice on this issue.

Dear Dr. Roman,

Many thanks for the answer and guidance in this matter.

Best Regards.

https://www.researchgate.net/post/Is_it_possible_to_prepare_an_aqueous-based_slurry_consisting_of_a_ceramic_which_is_hydrophobic

Thank you for sharing the link, Dr. Roman. It does confirm your suggestions you provided. 

Hi 

I would like to confirm with you guys, as my doubt falls under this topic, whether I can keep my ball-milled slurries for some days, say 10 days, and then spray-dry them? The spray-drying facility is quite far and not possible to get a slurry prepared and spray-dry on the same day. I plan to prepare a batch, then carry out the spray-drying.

Of course, I would be able to mix the ball-milled slurries using a magnetic mixer prior to spray-drying.

Your suggestions will really help.

Thanks.