Knowledge of the right tools is vital for effective mixing and blending

Choosing the optimum batch mixer for dispersing solids into liquids will depend on several factors, but the viscosity of the resulting product is among the most basic considerations. Tradition has it that the diversity of batch mixers falls into three broad categories when it comes to the viscosity ranges they serve.

For relatively runny, low-to-medium viscosity products, high speed dispersing units have a mixing element mounted on a central shaft. As well as applying shear to disperse the powder into the liquid in the working zone immediately around the shaft, the mixing element also sets up a bulk flow regime that sees the entire contents of the vessel circulating in and out of the working zone.

In contrast, at the top end of the viscosity spectrum, very high-viscosity products need planetary mixers. These have the dispersing tools mounted on the arms of a central rotor that constantly rotates the working zones around the chamber, since the mixture itself will not circulate.

“You can have two low-speed mixing tools driven by gears off the planetary head and that gives you a kneading action,” says Andy James, sales manager with NETZSCH. “Alternatively, you can have one low-speed and one high-speed tool.” This second set up would normally be preferred for hard-to-wet powders that need a high-shear action to incorporate them successfully. A question of degree feature plant equipment

Horses for courses

Somewhere between the lower viscosity dispersions and the very high-viscosity pastes and doughs are the medium-to-high viscosity products that traditionally demand the use of twin shaft mixers.

These jobs can’t be tackled with conventional, centrally mounted mixing elements alone because the higher viscosity involved makes it difficult to keep the bulk flow circulating properly. Instead, the high-shear tool at the centre of the mixer throws the contents out towards the vessel wall and the resulting fluid is too thick to be able to spontaneously flow back towards the working zone. This leaves the mixing tool spinning uselessly in a void.

“You effectively drill a hole through the middle,” explains James. “You need something else to send the mixture back towards the middle of the chamber, and this is where twin-shaft mixing systems come in.”

Twin-shaft mixers include a separate, centrally mounted stirrer, such as a butterfly or trifoil mixer, to keep pumping the contents of the vessel around so that all the material circulates through the working zone.

The working zone itself is created by a high-shear mixing tool mounted on a second shaft that is offset to one side. The dispersing tool in a twin-shaft unit is often smaller than a single-shaft mixer because the offset positions it closer to the wall of the vessel. This in turn limits the rate at which a twin-shaft mixer can deliver energy to the mix.

New kid on the block

This established, three-tier hierarchy of mixing solutions was an accepted template for matching mixers to different viscosities. This has been disrupted by the arrival of high-shear mixing elements that effectively eliminate the need for twin shafts.

These newer systems combine powder dispersing with a pumping action that maintains the bulk flow around the vessel at medium-to-high viscosities and prevents the formation of a void. They provide a single-shaft solution to suit applications ranging up to the viscosity levels where the need for planetary mixers kicks in.

The solution from NETZSCH is a patented double suction disc mixing tool, although other companies have devised their own alternatives to achieve a similar result.

Results have been game changing, says James: “The double suction disc combined with a side scraper [to prevent a build-up of product at the vessel wall] allows you to use a high-speed dispersing machine right up to the level of viscosity where you’d need to deploy a planetary mixer. It achieves high-shear dispersion and delivers a pumping action to maintain flow around the vessel.”

A mixer based on centrally mounted, double suction disc technology can reduce the time needed to mix each batch to a third of that needed to achieve the same with a twin-shaft mixer, says James and substantially reduces overall energy consumption.

He explains that this is in part thanks to the newer mixer’s ability to deliver energy at a much higher rate, creating a vortex that helps to disperse powders rapidly before agglomerations can form. This contrasts with systems that offer a slower rate of dispersal, which can sometimes allow lumps to form that then need to be broken down, using more time and energy.

There are no downsides to the newer technology

Andy James, sales manager, NETZSCH

“When you add solids ... you want them to enter a vortex, not lie on the surface... [they] are sucked down and the mixer wets the powder out immediately, without giving lumps the chance to form.”

One customer had previously been forced to reduce mixing speeds and increase batch times to prevent its twin-shaft mixer from overheating as it struggled to disperse lumps of pigment uniformly through the mix. “The new mixer worked much faster... without overheating,” says James.

Reducing batch times by two thirds could increase processing capacity compared to twin-shaft mixers, enabling one new machine to do the work of three. Each new machine is also simpler, leading to lower upfront costs and simplified maintenance. For customers wanting twin-shaft machines the firm suggests running tests with the double suction disc mixer to demonstrate the advantages. “There are no downsides to the newer technology,” he says.