Welcome to

---

Solid-Liquid Mixers (suspension)

Solid-Liquid mixing equipment

How to efficiently mix a solid and a liquid ?

Question or remark ? Please contact us at admin@powderprocess.net

---

Section summary
1. Introduction to solid-liquid suspension
2. Liquid-Solid suspension in agitated tanks
3. Solid-Liquid suspension with an inline high shear mixer

---

The page is focusing on solid-liquid mixing equipment. The creation of a suspension of solids within the liquid, also called slurry, is done thanks to solid-liquid mixing equipment such as agitated tanks and high shear pumps. key concepts are explained here.

1. Introduction to solid-liquid suspension

What is solid-liquid mixing ?

Solid-liquid mixing is a very widespread operations within process industries and the objectives of such a process step can be varied : simple suspension, floculation, dissolution, crystallization...

Among those different tasks achieved by solid-liquid mixers, the present page is focusing on the creation of a suspension of solids within the liquid. Such a solid and mixing suspension is also often called a slurry.

Mixing of solids and liquid to generate a suspension are often done in the following equipment :

• Agitated tank
• High shear pumps

Both concepts are explained in this page.

Such an operation of suspension is often used to convey the mixture to another unit operation, for example milling. It is then important to have a good homogeneity and a suspension sufficiently stable so that solids do not sediment and start clogging pipes.

How is designed the mixer will depend on :

• The fraction of solids in the suspension
• The rheology of the suspension (mixing solids particles to liquid, especially fine particles, can lead to strong non Newtonian behavior)
• The time available to make the suspension, if the suspension needs to stay in the tank for a long time
• How the particles can be wetted, also if there is a tendency that those particles float

Such mixers are found all over industries :

• Agriculture / pet food
• Food processing / baking
• Dairy industries and infant nutrition
• Pharmaceuticals
• Cosmetics

2. Liquid - solid suspension in agitated tanks

How to mix solids and liquids ?

The simplest form of liquid-solid mixer is to have an agitated tank following the same logic as for the mixing of liquids. The dispersion of solids in a liquid however present some very specific challenges that must be addressed by choosing special agitators, or even combining agitators.

The choice of the agitator will depend on how difficult is the mixture :

• Easy suspension, no modification of viscosity of the mixture : no need of high shear, the impeller is mainly axial and can be designed to have a good pumping.
• Difficult suspension / tendency to aggregate : the impeller is radial and must supply a lot of shear to the mixture.
• Turbines
• Rotor / Stator agitators
• Suspension leading to a strong change in viscosity : it can be necessary to have 2 agitators in the vessel fitting 2 purposes. 1 radial agitator providing shear and 1 tangential agitator designed for viscous liquid allowing to avoid dead areas in the tank and keeping the particles from sedimenting

Typical mixer profiles used for liquid solids suspension are the following :

Impeller	Characteristics
High speed dispersers (turbines)	>1000 rpm Creates a Vortex dragging particles to the impeller for dispersion
High speed dispersers (rotor stator)	The association of a rotor and a stator allows to suck, centrifugate then apply a strong shear
Tangential mixers (Anchor)	Only be used to keep a suspension from settling, not to create the suspension
Propeller / Helix	Axial flow, allow to "lift" solids from the bottom of the tank if not too much viscosity
Combination of mobiles	Typically the combination of a tangential mixer and a disperser. The tangential mixers allows to avoid dead areas in the mixer, prevents sedimentation, increase heat transfer at the wall, and brings back particles to the disperser. The disperser ensures the creation of the suspension. In some cases, it may be turned off after creating the suspension.
Planetary mixers	Mixing in small tanks, easily accessible. Interesting especially if the mixture creates a very viscous suspension

It can be interesting to install as well a recirculation pump that will help preventing the settling of the suspension.

The addition of solids to liquids in an agitated tank may be challenging. It is especially the case for non dissolving particles that can also be wetting well with the liquid involved. One of the risk is to have the solids staying on top of the liquid and taking a lot of time to be wetted and then introduced within the liquid. [Dickey] gives some indications in how to manage this issue :

• Adapting the particle size of the solids if possible (not too fine)
• Using a surfactant to improve the wetting if possible
• Adapting the rate of addition of the powder, quite slowly and regularly, in order to avoid to have a thick amount of product on top of the liquid, very difficult to mix in afterwards. The addition can also be done by step, add, mix, then add again...
• The agitation must be pulling the solids within the liquid, which can be achieved by a small Vortex

An inline shear mixer, as proposed below, can also be a solution in some cases, to have a complete and quick wetting of particles. The mix may then be send to an agitated tank to finalize or just keep the mixture away from segregation.

3. Solid-Liquid suspension with an inline high shear mixer

It is also possible to create a liquid-solid suspension by using high shear agitators that have a function similar to a pump. Those systems have different variations but can be typically used in the recirculation line of a tank in which solids have been loaded in the liquid. However some executions exist where the pump can be positionned below a tank full of solids allowing to admit powder directly in the pump without having the need to wet it previously in a tank.

Source

[Dickey] Tackling Difficult Mixing Problems, AICHE, 2015