Dear Dr. Bheekhun:

Typically, the solid fraction used in the preparation of ceramic slurry is giving in vol%. Due the difference of molecular weight between fluid and solid, the number of solid fraction take in wt% are always highest.

Conventional maximum fraction of solid only can be reached after further optimization of direct rheological parameters, which are connected with viscosity of slurry and its zeta potential and pH, as commented before by the Dr. Kubrin. In this sense, 25 a 35 vol% or more (40 vol%) of ultrafine alumina can be disperse after correct addition of proper dispersant agent. Actually, there is some kind of polymeric monomers that exhibits a double functionality providing steric and a kind of electrostatic contributions to stability of the slurry.

As function of solid charge in the slurry, as in high solid concentration slurry, it is possible consider that should occur an increasing in the time of processing. The homogeneitization of dispersant and its adsorption are slow, can be reaching in 24 hours in a ball milling. Further pH correction, another 24 hours of ball milling. Prior to viscosity measurement (rheometer or viscosimeter with concentric-cylinders as peripheric) half hour of mechanical agitation to removing of macro bubble of air is necessary, removing of micro bubble of air other half hours is necessary. Application of routine of measurement of rheometer or viscosimeter (concentric-cylinders) 15 to 30 min. See, each time that optimization is tried are necessary 24 or 48, more 1 hours more 15 min, a minimum of 25, 15 hours.

As a whole, also, as commented by Dr. Vecstaudza, there is some difficulty in the manipulation of very concentrated slurry in a Spray-dry system. Therefore, ultrafine particles can be give incredible slurry in terms of solid concentration but none exactly, functional slurries,to  further processing via Spray-dry, further consult to Dr. Vecstaudza would be interesting.

Kind Regards

Marcos Nobre

Weight fraction is rather a secondary measure, for adjustment of rheological properties volume fraction should be more important. That is, for a powder with higher gravimetric density, you need more mass of particles to obtain the same volume fraction as with lighter material, and, what is actually the aim, comparable viscosity, etc. On one hand, the volume fraction usually is maximized in order to increase production rate of the granulate. on the other hand, you need a stable defloculated slury with minimum amount of organics because you will have to remove it from ceramics during sintering. This is easier in the range of moderate particle concentrations in the slurry. That is why for zirconia something like 70 wt% loading is used (approx. 25 vol%). For lighter materials it can be somewhat less. For even denser ceramics, it probably can go above 80 wt.%.

Concerning the spray drying technology. The best results will be achieved by experimenting  with different concentrations. Because spray drying equipments are different, spray nozzles are different, you can choose different spraying parameters and so on... Also particle nature (size, charge etc.) will have an effect. Of course you might be lucky to find a paper with the same equipment.

But if you have a limited amount of precious material I would suggest to start with not so concentrated slurry not to block the spraying system. Also the initial slurry concentration affects the size of dried material particles. If you does not succeed at obtaining the right size, you may redisperse the dried material and start over with different concentration. But if you don't need a specific size, than it is much more easier.

Also I agree with Roman Kubrin who mentioned the importance of volume fraction.

Dear Dr. Bheekhun:

Typically, the solid fraction used in the preparation of ceramic slurry is giving in vol%. Due the difference of molecular weight between fluid and solid, the number of solid fraction take in wt% are always highest.

Conventional maximum fraction of solid only can be reached after further optimization of direct rheological parameters, which are connected with viscosity of slurry and its zeta potential and pH, as commented before by the Dr. Kubrin. In this sense, 25 a 35 vol% or more (40 vol%) of ultrafine alumina can be disperse after correct addition of proper dispersant agent. Actually, there is some kind of polymeric monomers that exhibits a double functionality providing steric and a kind of electrostatic contributions to stability of the slurry.

As function of solid charge in the slurry, as in high solid concentration slurry, it is possible consider that should occur an increasing in the time of processing. The homogeneitization of dispersant and its adsorption are slow, can be reaching in 24 hours in a ball milling. Further pH correction, another 24 hours of ball milling. Prior to viscosity measurement (rheometer or viscosimeter with concentric-cylinders as peripheric) half hour of mechanical agitation to removing of macro bubble of air is necessary, removing of micro bubble of air other half hours is necessary. Application of routine of measurement of rheometer or viscosimeter (concentric-cylinders) 15 to 30 min. See, each time that optimization is tried are necessary 24 or 48, more 1 hours more 15 min, a minimum of 25, 15 hours.

As a whole, also, as commented by Dr. Vecstaudza, there is some difficulty in the manipulation of very concentrated slurry in a Spray-dry system. Therefore, ultrafine particles can be give incredible slurry in terms of solid concentration but none exactly, functional slurries,to  further processing via Spray-dry, further consult to Dr. Vecstaudza would be interesting.

Kind Regards

Marcos Nobre

Dear all,

Thank you so much for your valuable contributions. In my case, the density of the powder is around 0.2 g/cm3 which is relatively low. I do have intention to use a DOE as well.

If I set the weight of my slurry in all runs constant as 500g (powder+water only), and range the vol% of the powder from 15 vol% to 40 vol%, would it make sense?

That would on the other hand means that the wt% of my powder varies between 3 wt% and 8 wt%.

Please note that I haven't included the binder and surfactant in the calculation just to simplify things.

But as far as PVA is concerned and if my memory serves me well, they use 2-4 wt%.

It would be a pleasure to hear your opinions in this matter, dear fellows.

Many Thanks.

Nadiir,

are you sure the density is 0.2 and not 2.0? 2.0 is the typical density of colloidal silica, I am actually not aware of any ceramics with 5 times less density than that of water.

I don't know what the final goal of study is but if you want to undertake a thorough optimization of spray drying, I would first start with 25 vol.%, then increase to 40% in several steps. You can see later on if you need to go down to 15% or not - usually it's not an advantage.

You probably know that the amount of a binder is in wt.% per weight of dry powder, not per weight of the slurry, so it scales with the concentration of ceramics.

Dear Dr. Bheekhun:

Really, very low particle concentration can be critical in this option of processing. The option of a experimental design based on a gradual decreasing of solid concentration in the slurry make sense. I agree with Dr. Kubrin.

Kind regards

Marcos Nobre

Dear Dr. Roman,

Thank you for the enlightenment.

Indeed, the primary powder that I intend to produce could achieve that low density. I am however agglomerating it with a secondary powder in order to improve the flowability of the resulting spray-dried powder. This is my goal.

As for the binder, I got your point. It's noted.

Dear Dr. Marcos,

I appreciate your prompt response as well. The discussion over here has been fruitful. I'm considering your advice and will implement it while formulating the slurry.

Best Regards.

What type of ceramic powder you have? Oxides, Nitrides or carbides. In order to achieve higher solid loading and properly spray dry slurry, you should disperse the powder. For doing this you have to have a proper dispersant and also find a proper pH using Zeta potential.  Regards

Dear Dr. Mohammad,

Different Oxides. Can you propose me a dispersant as I have been informed before that PVA and PEG might be enough for an initial study. What is your opinion?

Thank you!

What is the particle size of your powder? You have to do DOE with these variables:

Dispersant type,

amount of dispersant

solid content

pH of the slurry

milling time.

Try these dispersant s for your DOE:  Darvan C, PVA, PEG and Darvan 821A

Dear Dr. Mohammad,

My particle size would be around 20 um. I am keeping the milling time constant for 24 hous.

Alright noted, I'll check out these dispersants.

Lots of Thanks.

Hi, it will depend from functionality you like to get. Generally concentration vatration may be defined if some properties of a suspension start to change in a non-linear way (apparent viscosity, multiple scattering ect. or start to appear (e.g. network formation, may start in some cases already at a few vol. %). Generally one wound expect vol. concentrations to be higher than 10 - 20 %.

dl

Thank you for your explanation, dear Prof. Lerche. I'll keep it in mind.

I can recommend this paper:

Millán A., Moreno. R. and Nieto M. I., "Aqueous injection moulding of alumina using agarose", Materials Letters, Volume 47, Issue 6, 324–328 (2001).

The rheology studies (vary the particle size, concentrations, pH, type of dispersant, among others) can lead you to obtain a low viscosity and highly stable slurry (due the selection of the optimal parameters). Also don't forget the aging effect.

Dear Yuk,

Thank you for the above paper and clarifications.

Best Regards.

For a derivation of Brongniart’s formula ─ often used for ceramic slurries ─ and of other formulas relating slurry density with concentration, you may check my answers to another question at this forum:

https://www.researchgate.net/post/Can_anyone_suggest_a_smart_way_to_calculate_volume_fraction_of_a_solid_ferrofluid_in_a_solvent_or_solids_fraction

Dear Dr. Carlos,

Many thanks for your suggestion. I'll look into the Brongniart’s formula and the answers you provided.

Kind Regards.