Mixing equipment: blade-paddle mixers with horizontal shafts
To mix clay in semi-dry and plastic mold pressing of ceramic products, and to prepare batch in glass, silicate and other industries, continuous and cyclic operation single- and double-shaft blade-paddle mixers are used.
Mixers of this group are used for preparing the batch based on several components, and for preparing the uniform homogeneous dry or moistened mass. Water or low-pressure steam can be used for moistening.
In the latter case a higher product quality is achieved, as the steam heats the mass and then moistens it during condensation. The main parameter of the blade-paddle mixers is their capacity. The industry manufactures mixers of the following capacity (based on clay): 3, 5, 7, 18 and 35 m3/h with the paddle blade diameter of 350, 600 and 750 mm respectively.
Continuous operation double-shaft paddle blade mixer. It consists of a trough-shaped housing closed with a cover. Horizontal shafts with paddle blades installed on them are arranged inside the housing. The shafts are driven by a motor via a friction coupling, gearbox and a gear set to rotate in opposite directions.
The paddle blades are installed at angles at which the optimum ratio of circumferential and axial particle movement speeds is achieved resulting in the proper time of passage of the components from the window to the discharging hatch and, consequently, the proper mixing quality.
The mixture is moistened by the steam delivered via a pipe through distributors through gaps in the scale-like bottom. To reduce the heat losses. the lower part of the housing is closed with a casing filled with mineral wool. The mass can also be moistened by water fed through the manifold.
To ensure high-quality mixing, double-shaft counter-current mixers are used. Their design is identical to the mixer described above, but the the angles at which their paddle blade are installed are opposite in sign to each other. Such a paddle blade arrangement pattern creates some counter-current flows with the general mixture movement towards discharging aperture, since the angular sped of the first shaft is greater than that of the second shaft.
The paddle blade installation angles and ratio of angular speeds of the shafts for particular conditions are determined experimentally. Single-shaft paddle blade mixers are used for preliminary mixing of dry mixtures. They carry out two functions most often: mixing and transporting materials, for example, from bins to other units. Design of such mixers is the same as that of the discussed above, but they are equipped with one shaft.
To ensure thorough mixing (of hard-homogenised mixtures) cyclic action mixers are used, for example, double-shaft mixers with Z-shaped paddle blades. Depending on the required homogeneity degree, duration of mixing in such mixers can amount 20 to 30 min.
Continuous operation double-shaft paddle blade mixers can also operate in cyclic mode, if they are equipped with a gate, and the paddle blade installation pattern is changed.
The cyclic mixer calculation basis:
N = V·z,
where V is the mixer volume
z is the number of cycles per hour.
The continuous operation mixer capacity is generally calculated as:
N = 3600·F·vос,
where F is the cross-section area of the material flow in the mixer, m2;
voc is axial speed of the material, m/s.
With some assumption, the paddle blade operating members can be treated as a worm auger with interrupted screw. The axial material movement speed (m/s) depends on the circumferential speed of the [addle blades, their shape and installation pattern.