Ball mixer
Ball mixers are mixing machines for homogenising/mixing dry, moist or wet powders. Ball mixers can also mix liquids. Ball mixers are generally used for batch mixing processes. The inside of the ball is the mixing chamber. A mixing tool rotates in the mixing chamber and stratifies the mix in three dimensions. This flow process results in a very high mixing efficiency if the axis of rotation of the mixing tool is neither horizontal nor vertical. The best mixing effect is achieved with an inclined arrangement. Ball mixers can also be used as synthesis reactors or vacuum mixing dryers. In this case, the ball is double-walled. A heat transfer medium flows through the double jacket. In this way, a mass and heat exchange takes place.
Ball mixers are equipped with at least one rotating mixing tool. This is used to completely mix the entire volume of the ball. Additional mixing tools can be provided as an option. These are usually fast rotating choppers or knives. These have a smaller effective diameter and cause deagglomeration during the mixing process.
The manufacturers of ball mixers have established various designs of ball mixers on the market. Some manufacturers mount the mixer shaft at an angle on top of the ball. Others mount the mixer shaft at an angle below the ball. Still others mount the mixer shaft at an angle on top and the additional disagglomerating tool below the ball. Still others mount both the main mixing tool and the deagglomerating tool above the ball.
Main mixing tools in ball mixers
The manufacturers of ball mixers have established different tool geometries. Some have designed the main mixing tool as a spatially curved armature. Others have helical spirals. Still others use shovel-like tools. The decisive factor is that the mixing tools rotating in the sphere always have the same distance from the sphere, regardless of the position of the mixing tool. The same distance between the ball wall and the mixing tool must also be ensured when mixing heavy or sticky materials. In this respect, the mixing tools must be designed to be particularly dimensionally stable.
Filling with mix
The components to be mixed are filled into the mixing chamber from above. At least one filler neck must be provided for this purpose. Some ball mixers also have several filler openings. The shut-off valve can be designed as a rotary valve, flat slide valve or segmented ball valve. In contrast to the drain valve, the filling valve does not have to operate without dead space. There is also a venting nozzle on the top of the ball.
Emptying the mix
A shut-off valve is located at the lower end of the spherical mixing chamber. When closed, this should have the shape of a ball cut-out. Only such a design is free of dead space. This means that every volume fraction in the sphere is mixed - even directly above the shut-off valve.
Cleaning and inspecting the mixing chamber
Some ball mixers have two hemispheres that can be opened for inspection. Other manufacturers install round or oval inspection doors in the mixing chamber. These must also be free of dead space. Rotating tank washing nozzles are installed in the mixing chamber for wet cleaning.
Question: What advantages does the sphere offer as a mixing tank?
The volume of a sphere with the radius r can be calculated using the following formula:
V = 4/3 * r³ * PI
The surface area of a sphere with radius r can be calculated using the following formula:
O = 4 * r² * PI
If you compare different rotationally symmetrical bodies with each other, the sphere has the smallest specific surface area. This suggests that a spherical mixing chamber is easier to clean than a cylindrical or conical mixing chamber. On the other hand, it can be assumed that a conical mixing chamber favours the emptying of residues more than a ball. Manufacturers are endeavouring to produce their mixes as contamination-free as possible. The complete emptying of a mixing system plays an increasingly important role in this.
Cleaning and inspecting the mixing tool
A far more important aspect concerns the cleaning of the dynamically moving mixing tool. This part of the ball mixer is much more complex than the mixing chamber. Special requirements therefore apply here:
- The mixing tool must have a simple geometry
- The mixing tool must have the smallest possible surface area
- The mixing tool must be designed in such a way that all areas are easily and ergonomically accessible.
- The mixing tool must be designed in such a way that optimum mixing results can still be achieved efficiently and quickly.
The optimum design can be found for every category of mix. We recommend visiting amixon® and trying out the available test mixers (cylindrical, spherical, conical). amixon® offers first-class options for mixing, moistening, drying and discharging your various mixes.
In addition to the mixing tests, amixon recommends cleaning the mixer after the test.
Different mixing materials require different cleaning programmes. This concerns
1. dry cleaning
2. cleaning with damp cloths
3. manual wet cleaning
4. mechanical wet cleaning.
(2), (3) and (4) also include the important issue of residual water drying.
It is not unusual for all four cleaning regimes mentioned above to be practised alternately in one and the same mixed operation. Depending on the degree of soiling and contamination tolerance
Liquid addition and distribution in the ball mixer
In practice, in addition to mixing a wide variety of powders, it is also a matter of wetting them homogeneously. So-called liquid feed lances flow into the effective range of the rapidly rotating grinding tool. In this way, an agglomerate-free, homogeneous end product is quickly achieved.
Note on the term ball mixer.
The term ‘hollow ball mixer with dynamically moving mixing tools’ would actually be more correct than ‘ball mixer’.
The term ‘ball mixer’ is also used in the sanitary sector. This is a modified tap fitting. A ball with multiple holes is connected to a lever and can mix and regulate the flow of cold and hot water.