One of the most difficult challenges in mixing is in the use of wetting powders. They can float on top of a batch and not ever mix in properly. Even using forceful agitation can be unsuccessful. Even when forced into the mixture they can still form lumps and resist separation and dispersion. Reviewing equipment can help find which method will work best for your lab.
The easiest way of adding powders is to pour them directly into a vessel. A turbine will rotate and pull the powder into the liquid. This is one of the most inexpensive ways to achieve mixing. However, it can be a very long process. The main trouble with adding powders into an open vessel is dusting. It allows small particles to become airborne and can be hazardous. Since the vessel remains open it ha a tendency to pull air into the mix. This means additional steps must be added to the process to remove it.
Some prefer a high speed disperser for mixing. It has a much shorter cycle and mixes a little better. The agitation process is more forceful that a turbine and it uses a high level of shear to mix the ingredients. This mixers disadvantage is that it captures too much air in the batch and also has a lot of dust escape into the air. Another disadvantage of this type of mixer is that it tends to foam as it is being agitated.
A finer dispersion can be achieved by using a rotor/stator mixer. It break materials down quicker than the disperser. It creates an intense shear and achieves a finer dispersion. However the flow of materials through the vessel can limit efficiency. The actual wetting process is very slow especially if the powders being mixed are light weight.
The newest system used to wet out powders is a SLIM, or a solid-liquid-injection system. This process combines solids with liquid where mixing takes place. It uses a spiral port which creates a strong vacuum the pulls the powder into the high shear where it is dispersed into the stream of liquid. The mixing is so fast that there is not time for agglomerates to form. As it continues to pass the substance through the generator dispersion is improved and has virtually no chance of clogging. The SLIM mixer is clearly the best choice for most labs. There is little dusting and no clogging. This save more time and expense which means it is more efficient and ultimately saves time and money.
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