Five myths about mixing demystified

Feb. 1, 2015

Mixing is both art and science. Its process-, material- and machine-related variables determine finished-product quality.

Mixing is both art and science. Its process-, material- and machine-related variables determine finished-product quality.

The road to reliable mixing isn’t straightforward. This is in part because there are often at least several different ways to arrive at any formulation. Process-engineering managers explore strategies and make decisions based on common sense and dollars & cents, as well as available technologies.

Further complexities arise when myths and misconceptions hinder proper mixer selection, leave equipment under-utilized or prevent crucial upgrades from going forward.

It’s therefore important to clarify five myths about mixing.

1. When dispersing powders that tend to form lumps, add dry ingredients slowly and in small amounts, into a vigorously agitated liquid, to produce a lump-free finished product.

Fumed silica, carboxymethylcellulose (CMC), carbomers, alginates, carbon black, xanthan gum, clays and starches are among the many powders often difficult to disperse into liquid.

In conventional batching tanks, agitated by propeller or turbine blade, portions of these powders persist on the surface while others form clumps with tough outer layers that prevent complete inner-particle wetting.

Even after prolonged and vigorous mixing, finished product can still suffer from defects such as grainy texture, reduced viscosity or "fish eyes" (stubborn lumps).

Some operators resort to slow, scoop-by-scoop powder additions, sometimes even sifting for good measure. Others add an inline rotor-stator mixer that re-circulates the dispersion until most agglomerates disintegrate.

In certain thickened solutions, however, high-shear agitation applied over extended time periods can back-fire when already hydrated particles are over-mixed or exposed to high temperatures, resulting in permanent viscosity loss.

Done properly, though, slowing powder addition or increasing mixing, along with appropriate chemical dispersants use, produces uniform and smooth dispersions. But, while the resulting long processing times may be acceptable for small batches, if large powder amounts are involved, ingredient addition can take several hours.

Extreme overdosed solids

In extreme cases, to shorten cycle times, solids are intentionally overdosed and any un-dispersed agglomerates filtered out. The waste generated and added filtration steps are an inefficient, unnecessary cost.

High-speed sub-surface powder injection is a better way to mix powders into liquid. It eliminates sifting, slow material addition, prolonged mixing, over-dosing and filtration.

One such technology is the solids/liquid injection manifold (SLIM). In a SLIM mixer, a rotor-stator generates a vacuum that draws powders into the mix chamber’s high-shear zone.

Injected sub-surface, solids are instantaneously dispersed. With solids and liquids combined and mixed simultaneously, agglomerates and fish eyes don’t form. Shorter mixing times mean product is less likely to be overworked. Operation is also simple, with no eductor or vacuum pump involved.

For example, a manufacturer of specialty chemicals had used a propeller agitator and conventional inline rotor-stator mixer to disperse fumed silica into an emulsifier solution. Polysorbate 80 was heated in a 4,000-gallon tank into which fumed silica was charged at a rate of about one 25-lb. bag every five minutes. Even with supplemental high-shear agitation, delivered as product re-circulated through the rotor/stator mixer, the batch still took several hours to complete.

Replacing the rotor-stator mixer with a SLIM mixer to inject fumed silica powders into the re-circulating liquid stream — instead of dumping powders right into the open tank — cut cycle time more than 60 percent. In addition, mixing-area dust is reduced, operator safety improved and material handling made more convenient. Installing the SLIM mixer, no changes to the propeller agitator or piping were necessary.

2. Submicron emulsions and dispersions require milling or high pressure homogenization.

Manufacturers rely on colloid mills, media mills and high-pressure homogenizers for droplet or particle-size reduction to the submicron level.

In reality, basic single-stage rotor-stator mixers can produce submicron emulsions and dispersions — depending on the product. Formulation chemistry plays a huge role, quite apart from mixing intensity. Raw-material inherent properties, their molecular interactions, the presence, type and amount of surfactant, operating temperature, etc. — all these factors affect size reduction when product is exposed to mechanical or hydraulic shear.

That said, submicron emulsions and dispersions do require, more often than not, intense shear levels not attained by conventional rotor-stator mixers.

Homogenizers use high pressure to break down droplets by forcing the product through a narrow-gap valve into a lower pressure environment. The pressure gradient across the valve and resulting turbulence and cavitation reduce dispersed droplet and solid particle size. The drawbacks are high capital cost and energy consumption, low throughput and tendency to clog. In addition, thoroughly cleaning homogenizers often requires disassembly and significant time-out-of-service.

Ball-mill operation

A ball mill is a common media-milling machine consisting of a rotating cylindrical shell partially filled with grinding media, such as ceramic or metal balls. Product to be milled is loaded into the shell. As the cylinder rotates along the horizontal axis, grinding media pieces collide against each other and vessel surfaces. Impact collisions impart the grinding action that reduces solid-particle size.

While a ball mill is easy to operate and relatively versatile — most designs handle dry and wet feeds — it too suffers from long cycle times as well as product loss, high energy consumption, clogging and labor-intensive clean-up.

Colloid mills, on the other hand, are essentially rotor-stator type mixing devices. A typical configuration includes a conical or disk rotor and a stator; each piece has complementary grooves that serve as channels for fluid flow. Fluid is pumped between the rotor-stator surfaces. Hydraulic shear forces are generated within this gap. Like homogenizers and media mills, colloid mills are limited to low flow rates and can be difficult to clean in place.

Anyone looking to improve productivity will benefit from evaluating other methods, besides milling and high-pressure homogenization.

One such is the PreMax Ultra-High Shear Mixer, a patented batch-style rotor-stator which runs at tip speeds up to 5,000 ft/min and is uniquely contoured for high pumping capacity. It is also available in an inline, or continuous, configuration. These advanced rotor-stator designs turn at tip speeds of more than 11,000 ft/min and are proven for many submicron applications. While achieving much higher production rates, the intensive clean-up that mills and homogenizers require is avoided. Shorter cleaning times mean faster changeovers, and longer production runs between cleaning cycles.

3. Running a mixer at maximum speed overworks the machine and reduces its service life.

Especially with high-intensity mixers, a machine often operated at maximum speed is seen as "overworked."

However, taking viscosity, density, product level, flow and other relevant factors into consideration, as long as the amperage power draw is within the mixer’s range, running at the maximum speed delivers the highest tip speed available.

Subjected to shear and agitation at a particular tip speed, material eventually approaches an equilibrium state in terms of particle-size distribution, grind fineness and other product physical or chemical properties.

Past the equilibrium point, processing longer at the same speed delivers diminishing returns. If the product was mixed at a lower speed, say 50 percent of maximum, its equilibrium state could be different. Mixing longer at half of the maximum rpm will not produce better results, but running at a higher speed will.

Variable speeds essential

In reality, running at variable speeds is essential for almost any mixer since some mixing procedure stages — including raw material addition, heating/cooling, deaeration and discharge — don’t require vigorous agitation.

Well-designed mixers work just as well at maximum as at lower speeds.

It is noted, however, that high-speed shafts and shafts with mechanical seals or packing glands must operate within .002" total indicated runout (TIR). High-speed shafts will vibrate at their critical speeds — defined as the speeds at which deflection produces vibrations in resonance with the natural frequency of the rotating shaft — but will severely vibrate at almost any speed when the TIR is above .002". The chance of the shaft bending or twisting during operation then increases dramatically.

Operating requirements of low-speed shafts are not stringent compared to high-speed shafts. It is highly recommended that low-speed shafts also operate within the same .002" TIR. The least amount of TIR is desirable in all conditions and allow a shaft system and bearing set to operate for years longer than one that is not as straight.

Motors and shaft bearings should be greased regularly, at least once every 500 hours. Of course, sealed bearings require no lubrication. It is good practice to periodically inspect mixer drives for signs of wear and tightness. If a bearing makes an unusual noise or runs unusually hot, it must be replaced.

4. Customized mixing systems are a luxury that only large businesses can afford.

Off-the-shelf mixers and blenders fulfill basic mixing requirements and enable manufacturers to address very urgent needs or unexpected demand. However, when time and capital funding allow, it is normally a good idea to consider customization because it extends the capability and versatility of any new equipment which in turn provides a greater return on investment. Far from being an unnecessary luxury, customizing certain features of the mixer helps reduce operator errors, maximize yield, improve batch-to-batch consistency, increase cleanability and ensure a more seamless integration with the rest of the processing line.

Just a few questions to consider:

  • How are raw materials going to be delivered to the mixer?
  • Should the mixer be rated for vacuum so as to eliminate downstream de-aeration?
  • Can temperature be accurately measured from the sidewall or should the thermowell be in contact with the center of the batch?
  • How much time can be allotted for product discharge? If the material is viscous, does it need a hydraulic press or will positive pressure be sufficient to make it flow?
  • Apart from mixer speed and cycle time, what other functions and readouts should be on the operator control panel? Should batch reports and recordkeeping be automated?
  • What accessories are process desirable: CIP spray nozzles, interchangeable agitators, built-in vacuum pump, dry running seals, level sensors, viscosity probe, alarms or some other?

Answers to these and other questions often lead to customization. When processing objectives are defined in detail, small entrepreneurial companies and well-established manufacturers alike benefit from exploring customization options to confidently select a mixer that is neither underspecified nor over-engineered.

5. Evaluating a mixing technology entails significant amounts of time, money and energy.

When process or product upgrades prompt a review of mixing technologies, innovative equipment evaluation can seem a daunting task. But it can be done methodically and without great expense, especially through partnerships with mixer suppliers and makers.

Whenever practical, perform mixer testing using your own raw materials and simulate actual operating conditions as close as possible. Manufacturers offer testing services supervised by mixing experts. Witness testing in person as your expertise ensures a comprehensive evaluation of mixer capabilities applied to your product.

Additional thoughts

Renting a mixer for in-house trials at your facility is another option. To obtain reliable data for scale-up, a good rule of thumb is to test on a machine no smaller than 10 percent of the target capacity.

Staying open to new ideas and investing reasonable effort in confirming a true match between product and equipment poses high returns to any business, big and small. Sticking to the seemingly risk-averse mentality of "That’s how we have always done it" can actually hurt competitive advantage when frequent reworks, batch-to-batch inconsistencies, low throughput or long cycle times are simply accepted as the norm.

Ross is an equipment supplier serving the process industries for more than 170 years. Ross produces a wide range of mixers and blenders used around the world in the manufacture of pharmaceuticals, food, cosmetics, personal care products, adhesives, sealants, ceramics, paints, inks, coatings, plastics, composites, chemicals, energy materials and many other products.

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