Preparation of Slurries: Powder Wettability and Other Factors

In this article I write about process technologies for preparation of slurries, AKA “solids in suspension,” and the considerations that must be made in equipment design. Mainly this includes powder wettability, but I will also briefly outline reactions, rheology, and several other considerations.

For slurry preparation, our initial state is one in which the dry powder’s intersticial gas is the continuous phase. We aim to achieve a state where the wetting liquid is the continuous phase. The rate of wetting is dictated by the ease of movement between these states. Thermodynamics is captured in the hydrophobicity or hydrophilicity of the solid surface with respect to the solvent and gas. Together with capillary resistance to flow in the interstices of the powder voids, these are captured in the concept of wettability.

The terms “hydrophobic” and “hydrophilic” imply contacting with water; however, this concept applies to any solvent whether aqueous or not. Furthermore, hydrophobic does not mean “impossible to wet;” it just means they are more difficult than hydrophylic particles with respect power and time, all else equal.

Slurries are prepared on purpose when a dry powder by itself can’t meet a specified process objective. Sometimes, safety alone dictates the need to make a slurry—for instance, if the powder is pyrophoric (ignites in air) or moisture-sensitive. Submerged in a hydrocarbon, the powder is much less dangerous to handle. Certain applications, such as precise metering of catalyst particles into a process, are more easily done as a slurry than as a dry powder. In other cases, the sole objective is to dissolve or melt a solid so the slurry is a transient problem. Some slurries are prepared in situ (including crystallizations and precipitations) and are not in scope for this blog.

Solids Wettability

Degree of hydrophilicity is described in terms of the contact angle θ between a solid surface and a liquid as demonstrated in Figure 1. Complete hydrophilicity is characterized by a θ value that approaches zero, and thermodynamics becomes less cooperative as θ increases. A value of θ above 90° is annotated as hydrophobic and is less favorable to wet.

Figure 1: Concept of wetting as characterized by the contact angle, θ.

This seems straightforward; however, closer inspection shows there is a major problem with this concept when applied to small particulate. Namely, there is no flat surface on which to do the θ measurement for particulate.

A shake test is an easy method that tells you much of what you need to know. It is recommended for any system for which it can be done safely in the lab. For systems at high temperature and pressure, specialized visualization equipment is required such as a Parr reactor with a sapphire window [1].

A more complete description is provided by the rate of liquid uptake into the powder.  This can be captured quantitatively with the Washburn Capillary Rise (WCR) Method [2, 3, 4 ,5, 6] in which rate of liquid uptake (wicking) into a column of powder is measured and related to a contact angle via Washburn’s equation:

where

For very hydrophobic talcum powder, WCR measures a contact angle of 82 to 89 degrees [5], depending on compaction. The 90-degree rule for large solid surfaces does not apply so directly to powders. Sometimes the comparison of several solvents is required to fully characterize wettability, such as is shown in [6].

For a hydrophilic solid with particle size above ~100 µm, the solid immediately incorporates through the liquid surface, such as pouring sugar into tea. For smaller particles, even hydrophilic ones, air displacement rate starts to play a role; powdered sugar (median diameter 50 µm) requires more effort to wet than a larger grain sugar (400 µm).

Particles can float even if much more dense than the liquid.  For instance, the aformentioned hydrophobic talcum powder (Mg₃Si₄O₁₀(OH)₂, particle specfic gravity 2.7) floats on water and is challenging to wet. See Figure 2 (a). The gravity force cannot overcome the resistance to wetting.  Particle density is not a factor.

Figure 2. Talcum powder (sp. gr. 2.7, median particle size ca. 20 µm), (a.) floating on water after dumping onto surface, and (b.) after application of power to disperse the particles.

Again, hydrophobic does not mean “impossible to wet,” as talcum particles will become dispersed if power is applied in such a way as to displace air. See Figure 2(b).

One sign of wetting resistance is clumping. Clumps of powder are dry inside, with air not yet displaced. Clumps are visible in Figure 2(a). Even hydrophilic powdered sugar forms some clumps. They are broken up with application of power, such as multiple passes through a tank impeller. In a process such as making a formulation, it is important to give sufficient time to the powder incorporation step to ensure that any clumps are fully broken before proceeding to the next step.

Solids Incorporation

If we know a powder is difficult to wet, the next question is how to design to account for it.  Slurries are generally prepared via stirred tanks or inline rotor-stator type devices.

Stirred Tank

The initial contacting is done by dropping the powder through the headspace of a stirred tank, preferably uncompacted. Figure 3 shows an example of a proper design in which corn flour is being dropped onto a liquid water surface with a vortex. The system is turbulent and baffled so that there is good vertical flow in the tank. The best drawdown occurs when the impeller is able to create a moving vortex that sweeps floating solids into it. The rate of powder addition should be low enough that new powder is not added on top of floating powder that is yet to be wetted.

Figure 3. Side and top views of corn flour being incorporated into a stirred tank containing water.

Figure 3 shows a single impeller near the tank bottom. If a powder is very difficult to wet, it is important to place an impeller near the surface of the liquid, in addition to one near the bottom. A submerged distance of 0.5 impeller diameters is recommended to ensure high turbulence at the interface.  For cases with significant level change during the preparation, more than two impellers may be required. A high-flow impeller with diameter of 0.4 – 0.5 tank diameter is recommended.

To determine the stirring rate, Froude number (Fr) scaling is recommended. For stirred tanks, Fr is the ratio of the flow inertia to gravity as per this equation:

where N = rotation frequency, 1/time, D = impeller diameter, and g = acceleration due to gravity.

Fr characterizes the liquid surface in a stirred tank. It can be thought of as capturing the ability of the liquid to rise at the edges of the tank against the force of gravity. Aim for a Fr value of > 0.6 if no data is available. For scaling up or down from a known good drawdown condition,

N₁² D₁ = N₂² D₂

Strictly speaking, the Froude number applies to unbaffled vessels, but I have found it to be trustworthy for baffled vessels as well.  (Note that in some references, the Froude number is defined to be the square of the equation shown above, i.e., N²D/g, so it is important to distinguish. In addition, some sources may use radians/second instead of rotations/second.)

Inline

The initial contacting is accomplished with an inline option such as powder induction into a rotostator. In my previous blog in inline mixing [7], I cover rotor-stator devices is some detail. They can be configured as a pump, as shown in reference [8] for a Silverson FlashMix, with powder induced into the suction of a continuous liquid flow.  These can be used on the recycle line of a stirred tank to ensure complete uniformity of the slurry after the powder introduction is completed.

Further Considerations

In my experience, there are several other considerations that should also be made to avoid problems in preparing slurries.

• Ensure uniformity when necessary. For catalyst injections and drumming out product, slurry uniformity is critical. For purposes of dissolving the solid, it is unimportant as along as all solids are moving. (The liquid is well-mixed even if the solid phase is not uniform.) It requires considerably more power to make a suspension uniform that to simply keep particles moving [9]. Use of a tickler impeller such as the KT-3 [10] may be required to ensure uniformity in the final stages of draining a tank and to prevent large heel deposits. In pipeline transport, there are similar considerations;  absolute uniformity of the solids isn’t necessarily required and some heterogeneity is acceptable as long as all particles are moving. This is analogous to opperating above saltation velocity in pneumatic conveying of powders. Stratified flow (settled solids) is generally not advised. Becht has experts available for further discussion.
• Operate above just-suspended speed (N_js) for heavier particles. This will ensure that a heel is not formed during the normal operation. See [11, 12] for more information. Similarly, for wetted particles that are buoyant, it is necessary to operated above the just-drawn-down speed, N_jd, as outlined in [13, 14].
• Account for rheology changes. A shear thinning fluid (pseudoplastic) is the most common non-Newtonian behavior for slurries. Shear thickening fluids (dilatant) are uncommon but can only occur with solids in suspension; corn starch in water is the most well-known example. Thixotropic (time-dependent) behavior is known in drilling muds, for instance; time is required for particles to re-organize to a steady rheology before and after shear is applied.
• Consider use of a surfactant to aid powder wetting. Surfactants work great in the laboratory for sample preparation and can also work in production, provided the impact of the surfactant elsewhere in the process is fully accounted for.
• Manage heat transfer within reacting particles. A 10 °C temperature rise in the bulk is generally considered a mild heat of reaction; however, the adiabatic temperature rise inside the particle can easily be ten times this value. If convective heat transfer from the particle is lacking, the original particle form can be destroyed.
• Be mindful of particle size reduction and attrition. There are several mechanisms to bear in mind, such as particle-wall and particle-particle aspects. Becht has experts available for further discussion.
• Prevent fouling from particle agglomeration and settling. Fouling has many mechanisms and can occur when particles agglomerate or deposit onto a heat transfer surface. Splashing of a slurry onto a tank wall, wherein the solvent drains and leaves a rind behind, is a common problem to watch for.
• Nano-particle dispersion requires special considerations. Because nanoparticles are smaller than the smallest turbulent eddies, their behavior differs from larger particles. Becht has experts available for further discussion.

Summary and Conclusion

The wettability of a powder is governed by the rate of interstitial air displacement from the powder and liquid incorporation into the powder. Wetting solids are easier to disperse or dissolve. Non-wetting solids may clump/agglomerate and act as large particles.

A shake test is a good method to use for wettability studies. Clumping is a sign of a wettability problem. The powder contact angle measured via WCR is a useful parameter because it incorporates thermodynamics via degree of hydrophobicity and capillary flow resistances; an angle above 90 degrees is not measured even for very hydrophobic powders such as talcum powder.

For stirred tanks, there are design methods available to ensure solids incorporation and that wetted solids don’t collect at the tank bottom or at the top liquid surface. The Froude number should be used for design, with Fr > 0.6 adequate.

Rotor-stator (high-energy, high-shear) mixers are required for some challenges and are recommended for inline mixing processes with introduction of solids.

Feel free to contact Becht for further discussions on this topic or others related to mixing—whether inline, stirred tank, jetted tank, or other options.

Further Reading

• Paul, E.L., Atiemo-Obeng, V.A., Kresta, S.M., eds., Handbook of Industrial Mixing: Science and Practice, John Wiley & Sons, 2003.
• Kresta, S.M., Etchells, III, A.W., Dickey, D.S., Atiemo-Obeng, V.A., eds., Advances in Industrial Mixing: A Companion to the Handbook of Industrial Mixing, John Wiley & Sons, 2016.

References

1. https://www.parrinst.com/products/stirred-reactors/options-accessories/see-more-with-parr-reactors/
2. Washburn E.W. (1921) “The dynamics of capillary flow,” Rev., 17: 273–283.
3. Liu, Z., Yu, X., & Wan, L. (2016) “Capillary rise method for the measurement of the contact angle of soils,” Acta Geotechnica, 11, 21-35.
4. Kirdponpattara, S., Phisalaphong, M., Newby, B.Z. (2013) “Applicability of Washburn capillary rise for determining contact angles of powders/porous materials,” Journal of Colloid and Interface Science, 397, 169-176.
5. Galet, L., Patry, S., & Dodds, J. (2010). Determination of the wettability of powders by the Washburn capillary rise method with bed preparation by a centrifugal packing technique. Journal of Colloid and Interface Science, 346(2), 470-475.
6. Dang-Vu, T., Hupka, J. (2005) “Characterization of Porous materials by Capillary Rise Method,” Physicochemical Problems of Mineral Processing, 39, 47-65.
7. Cloeter, M.D., “Inline Mixers for Chemical Processes” published on Becht Engineering web site blog, Inline Mixers for Chemical Processes – Becht, August 27, 2024.
8. High Shear Flashmix Powder/Liquid Mixers – Fast, Agglomerate-Free
9. Oldshue, J.Y., Herbst, N.R., Post, T.A., “A Guide To Fluid Mixing,” (2014) Chapter 3 Flow-Controlled Operations, 8^th, Lightnin, Rochester NY.
10. https://www.spxflow.com/lightnin/products/kt-3-low-level-mixing-impeller/
11. Zwietering, T. N., “Suspending of Solid Particles in Liquids by Agitators, Chemical Engineering Science, Vol. 8, pp. 244-253, 1958.
12. Grenville, R.K., Mak, A.T.C., Brown D.A.R. (2015) “Suspension of solid particles in vessels agitated by axial flow impellers,” Chemical Engineering Research and Design, 100: 282-291.
13. Joosten, G.E.H., Schilder, J.G.M., Broere, A.M. (1977) “The suspension of floating solids in stirred vessels.” Trans IChemE, Part A, Chem Eng Res Des. 55(3): 220.
14. Khazam, O. and Kresta, S.M. (2008), “Mechanisms of solids drawdown in stirred tanks,” J. Chem. Eng., 86: 622-634.