Common Mixing Problems and How to Solve Them

Incorporating powders into liquids may sound like an easy task, but as many manufacturers know, depending on the type of powder, a range of different processing problems may be encountered. The type of equipment used for mixing powders into liquids can have a huge effect on the quality of the end product, mixing times, and energy requirements, and even the amount of raw materials included in the formulation. With so many different types of powder/liquid mixers available on the market, it can be difficult to decide which type will be best for each individual’s processing requirements.

In this article, we explore some common problems that arise when mixing powders into liquids, as well as the different types of mixers available, noting their advantages and disadvantages to help manufacturers gain the knowledge they need to make informed decisions when purchasing their next powder/liquid mixer.

Common Problems

The problems highlighted here do not apply to all powders; some can be processed easily, while others may need more complex mixing and additional equipment such as high-pressure homogenization or a filtration stage for additional processing after mixing. We are covering the most common problems that we encounter when speaking to customers having issues with incorporating powders into liquids.

Some problems arise before the powder has come into contact with the liquid. With hydrocolloids that have a strong tendency to form agglomerates when added to water, it is sometimes recommended to dry premix them with other ingredients, such as sugar, to separate the particles and reduce the possibility of agglomeration. This adds time to the process before the mixing has even begun. Controlled powder addition, which is another way to aid wetting out of the powder and reduce the formation of agglomerates, further increases the process time and in some cases causes dust emissions and associated vessel sanitation issues, where partially hydrated material sticks to the mixer shaft and the vessel walls. Powders with poor flow qualities make this stage more time consuming and batch-to-batch consistency is hard to obtain.

When the powder comes into contact with the liquid, some raw materials, such as gums and thickeners, begin to increase in viscosity. This makes it difficult for mixing equipment to incorporate further powder as the mixture gains viscosity and localized mixing can occur when the mixer is unable to continue moving product around the vessel. As noted above, some ingredients also have a tendency to form agglomerates, sometimes also called “fish eyes” because they consist of a gelled outer layer, with dry powder remaining unhydrated in the center. Long mixing times are required to try and dissolve or hydrate these lumps, increasing costs and energy requirements. Vigorous agitation may be used to speed up the process, but this can lead to aeration problems. If agglomerates remain in the mix, they can cause undesirable viscosity changes in subsequent storage and affect the quality of the mix. As such, a filtration stage may be needed to remove the agglomerates. This results in some formulations requiring excessive percentages of powder to achieve the desired end product to compensate for poor yield of the raw ingredients. This further adds to time and costs.

Low-density powders can cause issues when added to liquids as they float on the liquid surface, other powders need a degree of shear to activate the gelling or thickening effect, and some processes require colloid mills or other grinding equipment to reduce particle sizes down, depending on the type of raw materials.

Powder/Liquid Mixing Solutions

There are many factors to consider, and the list of problems above highlights the need for the correct processing equipment to mitigate these factors. But incorporating powders into liquids does not need to be a troublesome, time-consuming process. Manufacturers can improve quality, and in some cases profits (where excessive raw material is being used), by getting the mixing stage right.

Here we explore the benefits and disadvantages of four common types of powder/liquid mixers with a general overview of each below:

Pump/Powder Blenders

These mixers are based on a centrifugal pump mounted vertically. Liquid is fed through a tangential inlet while the powdered ingredients are loaded into a hopper directly above the pump. A diffuser tube keeps the liquid and powdered ingredients separated until they enter the mixing chamber, where they blend on contact before being passed through a special filtering screen on discharge from the pump. These devices are usually compact and easily movable and designed to be easily and quickly installed. They are relatively effective on simple powder/liquid mixing applications, although they may struggle to properly incorporate powders which increase in viscosity on contact with liquid, such as gums and thickeners, which can block the system and raise hygiene issues with repeated batches. The typically small capacity of this type of system makes it unsuitable for bulk powder dispersion. An additional feed pump may be required, especially on higher viscosity mixes.

Shear/Feed Pump Combination

These mixers consist of a self-priming feed pump that works in combination with a shear pump, which is basically a modified centrifugal pump with a number of rotor/stator teeth incorporated into the impeller design. The powder is added to the liquid stream via a hopper, and then homogenized with the help of the shear pump. The suction pump creates a constant vacuum within the flow stream, which minimizes powder bridging or ratholing and is beneficial for powders with difficult flow properties. Powder feed rates are adjusted through regulating valves, which can be automated, which is useful for controlled powder addition. However, if the valves are adjusted manually, they need to be correctly set and repeatedly adjusted as the viscosity alters to ensure that the proper concentration of dry materials is added, which can increase the possibility of operator error. As these mixers combine two pumps, they have a high-power requirement.

Induction Mixers

The powder is inducted from the container through a suction hose and introduced into the vessel under the liquid level. This immediately wets out the powder and prevents it from rafting on the surface. This also helps to prevent build-up on the container and vessel walls. As well as powders, including more challenging materials like gums and thickeners, this type of mixer is also suitable for incorporating liquids and gases. Inductor systems can be based on an in-tank mixer or an external in-line device. This type of mixer can be beneficial to those using powders prone to dusting as the powder is incorporated directly from the bag, without the need for loading into a hopper or conveying into a vessel. However, cleaning can be an issue with some ingredients if the powder tends to form a gel on wetted surfaces such as the inlet to the vessel and feed hose.

High-Shear Powder/Liquid Mixers

High-shear mixers have the advantage of being able to efficiently mix ingredients that require shear to activate the gelling and thickening effect. This can be done with both an in-tank or immersion type mixer or with an in-line device. These mixers feature interchangeable workheads and stator screens that allow the mixer to be used on a variety of materials. Recent developments in in-line rotor/stator systems offer high-capacity liquid flow and powder incorporation rates without the need for an additional feed pump. They can also be used on higher viscosity mixes and at higher temperatures than vacuum-fed systems. 

As noted earlier, these are just a few of the many powder/liquid mixers available. The type of mixer most suitable for each application will depend on a variety of factors, which will be different for each customer, such as budget, type of ingredients, the viscosity of the mix and the end product, and even the amount of space on the factory floor. These factors and more must be considered before deciding which mixer is the best fit. Many equipment manufacturers offer testing facilities or trial machines so clients can ensure they can get the results they need before purchasing. This also enables clients who want to improve their existing mixing systems to review their process with expert guidance and reconsider these factors to determine if another type of mixer would be beneficial to them.

Matt Smith is sales director, Silverson Machines (East Longmeadow, MA). For more information, call 413-525-4825 or visit www.silverson.com.