Mixing of powders is probably one of the most widely performed unit operation in pharmaceutical manufacturing. This fact is still reflected in the design of most of the industrial mixers nowadays. Their basic design has sometimes been developed 50 or more years ago. And although most mixing operations still can be performed with relative simple equipment a trend is developing towards more complex mixing requirements.
Besides blending of components, modern mixers also have to coat and granulate, and also more stringent mixing quality requirements are demanded in the market.
This article gives an overview of equipment used in the mixing of powders.
The mortar and pestle represent one of the most commonly used small scale mixing equipment. The mortar and pestle method combines comminution and mixing in single operation. Hence, it is very useful when some degree of particle size reduction, as well as mixing, is required.
Mortars can be made of different materials and shapes, and pestles are made of the same materials as the mortars. Conventional mortars include glass mortars, Wedgwood mortars, and porcelain mortars, and there is an increasing use of a newly developed mortar, called the
electronic mortar and pestle (EMP), among pharmacists.
Different types of mortars have specific utility in compounding different materials. For example, glass mortars are suitable for preparing solutions, suspensions, and ointments; Wedgwood mortars are designed primarily for size reduction for most of the materials in modern pharmacy
practice; porcelain mortars are similar to Wedgwood mortars, but they are more preferable for comminution of soft aggregates, crystals, and mixing of powders with uniform
particle size; EMP is specifcally designed for blending of creams, ointments, and oral liquids. The EMP uses a spinning blade and a moving arm to mix products. The largest benefit of EMP is that the materials can be weighed, mixed, and dispensed all in the same container.
Tumbling mixers/blenders are most commonly used for powders with similar densities. There are different types of tumbling mixers, including V-shaped, rotating cube, cylindrical, double-cone, oblique, and Y-cone mixers.
Tumbling mixers are likely to mix powders with good ﬂowability and granules, rather than cohesive or powders with poor ﬂowability, because shear force provided by these mixers is not enough to separate the individual particles from agglomerates.
The regular loading ability of tumbling mixers ranges from approximately 50 g to 100 kg. For this equipment, segregation tends to occur, if particle size differs significantly.
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In the ribbon mixer, spiral ribbons, paddles, or blades are mounted on a shaft, which is located in the center of the mixing equipment. Rotating of the shaft leads to the circular movement of the blades and subsequent mixing of the powder.
The ribbon mixer can mix powders with poor ﬂowability and avoid segregation as compared to tumbling mixers.
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Planetary mixer, commonly called beater consists of an anchor type of paddle/agitator revolving in a cylindrical pot with a hemispherical base. The paddle rotates on its axis and the axis equally rotates around. The bowl (cylindrical pot with a hemispherical base) is used for feeding and discharging of the product.
The bowl can be raised to the position of the mixing blades for the actual mixing process. The mixing blades or paddles are located off the center of the mixer. They rotate around their center and simultaneously rotate around the center of the mixer. This double rotating process can cause a more complete mixing of the entire equipment, so “dead spots” are avoided.
Planetary mixers can break agglomerates easily.
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The Nauta mixer, also called orbiting screw blender or vertical cone screw blender is composed of a bottom discharger with a rotating screw-fastened to the upper end of the rotating arm. The screw conveys the product to the top, where it can ﬂow back to the powder feed. Hence, for this equipment, the vertical impeller and horizontal rotating arm are combined together to induce a combination of convection, shear, and diffusive mixing.
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The ﬂuid-bed mixer is usually used for powder mixing, prior to granulation in the same bowl. Therefore, the ﬂuid-bed mixer is part of a ﬂuid-bed granulator system.
Fluidization of the powder particles can be achieved by blowing heated and filtered air into the equipment. Efficient mixing is achieved by circulation of the ﬂuidized powder.
After mixing, granulation liquid is pumped from the liquid receiver/holder through a spray nozzle and onto the ﬂuidized powder bed to facilitate granulation. When sufficient liquid has been sprayed to achieve appropriate granule size, the nozzle is turned off and the wet granules are dried by the ﬂuidizing heated air.
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Equipment used in the mixing of powders