The advantages of High Shear Mixers

At Silverson we have been providing mixing solutions to customers for over 75 years. With our extensive knowledge and one of the most comprehensive product ranges from a single manufacturer, whatever scale you are mixing at, your process could benefit from the advantages of high shear mixers.

At Silverson we have been providing mixing solutions to customers for over 75 years. With our extensive knowledge and one of the most comprehensive product ranges from a single manufacturer, whatever scale you are mixing at, your process could benefit from the advantages of high shear mixers.

But why are high shear mixers advantageous when compared to conventional mixing equipment such as agitators and stirrers? Agitators or stirrers come in a vast array of shapes and sizes, including marine propellers, foils, axial and radial flow turbines, gate and anchor stirrer/scrapers, etc. Despite the wide range of capacities and viscosities this type of mixer can handle, the low shear nature of agitators limits their effectiveness to performing simple tasks such as blending liquids of like viscosities, maintaining in-tank uniformity, and producing flow in a vessel to promote heat transfer from jacketed vessel walls. Where the product has to be processed in some way – liquids of differing viscosities are being blended, powders dispersed and hydrated, an emulsion or suspension formed, solids dissolved or disintegrated and so on, an agitator can take a long time to achieve the required results or, more often than not, is simply not capable.

That's where high shear mixers come in. High shear mixers are ideal for these types of mixing tasks and can cut mixing times by up to 90%, offering improved yield, product quality and greater batch-to-batch consistency. To understand why and how high shear mixers can achieve these results, it is helpful to know how they work.

The high-speed rotation of rotor blades within the mixing workhead exerts a powerful suction, drawing liquid and solid materials into the centre of the workhead. Centrifugal force then drives materials towards the periphery of the workhead where they are subjected to a milling action in the precision-machined clearance between the ends of the rotor blades and the inner wall of the stator. This is followed by intense hydraulic shear as the materials are forced, at high velocity, out through the perforations in the stator and circulated into the main body of the mix. At the same time, fresh material is continually drawn into the workhead, maintaining the mixing cycle.

But how does this translate into improvements to mixing processes? Shear energy is often required to obtain the best results for the following tasks:

Blending liquids of differing viscosities

When blending easily miscible liquids of similar viscosities, an agitator will produce satisfactory results. But where there is a significant difference in viscosity between the two liquids, an agitator tends to move the two liquids around without actually blending them together, and it can take a long time to achieve a uniform blend. For example, when manufacturing honey-based products, which can be extremely difficult to process due to the high viscosity and ‘sticky’ characteristics of honey.

With a Silverson high shear mixer, the positive mixing action of the rotor/stator assembly draws the liquids into the workhead where they are rapidly combined before being forced out through the stator and projected back into the vessel; the liquids are blended almost instantaneously.

Deagglomerating and Powder/Liquid mixing

When adding powders to liquids they have a tendency to form agglomerates which conventional agitators cannot readily break down. Instead, powders remain in the mix as 'fish eyes' - globules of powders with a 'wetted out' outer layer and a dry powder centre. Agitators lack the energy required to break down the fish eyes or agglomerates and hydrate the trapped powder inside.

The high shear action of the Silverson rotor/stator workhead rapidly disperses agglomerates, exposing increasing areas of the solid to the surrounding liquid. The result is an agglomerate-free solution within minutes.

Forming stable emulsions

The key to creating a stable emulsion is obtaining the finest possible droplet size. The more shear energy introduced into the mix, the smaller the suspended droplets will become, creating a uniform, stable emulsion.

Due to their low shear nature conventional agitators simply move the two liquid phases around the vessel, so although it may appear as though the phases have blended, the droplet size hasn’t been significantly reduced, so once the agitation stops the emulsion will soon separate into the two distinct phases once more.

High shear mixing subjects the liquids to intense high energy which reduces the droplet size sufficiently and allows the two phases to combine, resulting in a stable emulsion which will not separate once the shear energy has been removed. The high shear rotor/stator design of the Silverson mixer is ideally suited to this application and can easily achieve emulsions with a droplet size of 2 to 5 microns.

High Shear mixers can also be beneficial for many other mixing tasks including dissolving, disintegration of solids, dissolving and reclamation to name a few. To find out more contact Silverson at sales@silverson.co.uk or visit the Silverson website: www.silverson.com.