Diffusion mixers, also known as tumbling mixers/blender is a powder/solid mixing equipment that consists of a closed metallic vessel (usually stainless steel) that rotates about an axis either manually or with the help of a motor at an optimum speed. Powder particles, unlike fluids, must first be set in motion by some external action to achieve a proper mix.
Diffusion is the main mechanism of mixing in tumbling mixer. The powdered materials to be blended are loaded into the blender container and the movement of the powdered particles occurs by tilting the material beyond the angle of repose using gravity to impel flow. Different shapes of the diffusion mixer result in the movement of the material in various planes, which is necessary for rapid mixing.
The degree of mixing/blending achieved by using tumbling mixer in carrying out a mixing operation is dependent on
There are several sub-classes of tumbler but the popular and commonly used ones are the v-blenders and the double cone blenders. Others include Slant cone blenders, cube blenders, Bin Blender, Horizontal/Vertical/Drum Blenders, Static Continuous Blenders, and Dynamic Continuous Blenders. They are primarily distinguished by geometric shape and the posited of the axis of rotation.
The V-blender, also known as twin-shell mixer consists of two hollow cylindrical vessels/containers that are joined at an angle of 750 to 900 which is mounted on trunnion to allow it tumble. The free fall of the material within the vessel, and the repetitive converging and diverging motion, coupled with increased frictional contact between the material and the vessel’s long, straight sides as the mixer tumbles, split the material and recombines them continuously results in a homogenized blend. Removal of the blended material from the V-blender is normally through the apex port which is fitted with a discharge tube.
A V-blender can be modified by providing it with a high-speed intensifier bar also known as lump breaker running through trunnion into the vessel along with spray pipes for liquid addition. This modified v-blender is called V-Blender with intensifier bar.
This modification makes V-blender broaden its application in the pharmaceutical industries and they include;
Provision of intensifier bar to a V-blender is not devoid of some shortcomings. These disadvantaged includes;
The double cone blender is an efficient and multipurpose tumbler blender for mixing dry powders and granules homogeneously. It is made of two conical shaped stainless vessels (available in different capacity ranging from 5 – 200 kg or even more) that are separated by a cylindrical section. It is mounted at the center of the container between two trunnions that allow the blender to turn end over end.
Double cone blender has no dead spots mixing and it is easily cleanly after use. Double cone blender is not a suitable choice of blender for very fine particles and particles with greater particle size difference due to less shear.
A cubic mixer is a tumbling mixer suitable for gentle blending of dense powders and granules. As the name implies, it consists of a cube-shaped stainless container mounted in a titled manner at the center of the container or hanged from the corner between two trunnions that allows it to rotate about an axis. Mixing occurs by sliding of powders on its wall. Cubic blender gives mixing in three dimensions (3D) when hanged from the corner.
Despite the fact that corner blender gives mixing in three dimensions, the presence of different corners makes it difficult to clean after use sliding action causes abrasion of particles. The above can be said to be the advantages and disadvantages of using cube-shaped blenders as blending equipment.
Drum blender which can also be referred to as vertical blender, horizontal blender, or tumbling drum consists of a horizontal cylinder (or vertical cylinder) rotating about its axis. Its mixing action is essentially diffusive and gravity impels flow.
The powder to be mixed is placed inside the drum and as the drum rotates, the powder is lifted up until the angle of repose is exceeded. This leads to the falling back of the powder on the rest of the bulk.
Diffusive mix takes place as the cycle of lifting and falling keeps on repeating itself. The efficiency of the drum blender is increased if the blender is attached in an inclined manner. This allows light movement and so it makes it suitable for friable particles. This suitability of drum blender for friable particle is an advantage in its application as a blender. Drum blenders also have low shear as its main disadvantage.