Vertical Screw Mixers

Fig. 37. Various mixer types for dry soflds (a) the V-mixer, (b) ribbon blender, (c) vertical screw mixer, and (d) Nautamix.

Fig. 1. Schematic of continuous smelter for porcelain enamel glass production where A is a raw batch hopper B, batch hopper mixer C, screw feeder D, refractory furnace E, burner for gas and air (oxygen) F, spout G, water-cooled rollers for quenching glass H, finger breakers to break glass ribbon I, horizontal vibratory conveyer J, hammer mill K, vertical vibratory conveyer L, surge bin M, rotary magnetic separator N, product container O, exhaust...

In vertical screw mixers a rotating vertical screw is located in a cylindrical or cone-shaped vessel. The screw may be mounted centrally in the vessel or may rotate or orbit around the central axis of the vessel near the wall. Such mixers are schematically shown in Figure 6.4a and b, respectively. The latter arrangement is more effective and stagnant layers near the wall are eliminated. Vertical screw mixers are quick, efficient, and particularly useful for mixing small quantities of additives into large masses of material. 

Vertical screw mixers (a) central screw and (b) orbiting screw. 

A vertical mill was developed in Japan for grinding fine minerals. The tower mill is a combination of vertical tanks, a screw mixer, a mill, and a classifier. From a chute on the side of the mill, the rocks, steel balls, and liquid are introduced. The vertical screw rotates around a vertical shaft and creates an upward vertical counterflow. The finer materials float to the top and are led to a side chamber for classification, while the heavier and coarser solids sink with the steel balls (Figure 7-22). 

Screw mixers (vertical and horizontal and orbiting types)... 

The use in rubber mixing lines of the VSS (Vertical Screw Sheeter) developed by Pomini is described. The machine consists of a hopper, a vertical extrader with two conical, counter-rotating non-intermeshing screws arranged in V formation, and an open mixer, all of which are placed beneath an internal mixer. 

The material flow in an internal mixer is oriented vertically. On top of the internal mixer a more or less sophisticated feeding system can be found. Underneath the mixer a downstream device, such as a sheeting extruder with one or two tangential screws, or two roll mills are placed. 

Figure 10.29 Photograph of screw in a vertical milling machine a) the mill is cutting the flow channel for a Maddock-style mixer, and b) the keyway is being cut into the shank of the screw (courtesy of Jeffrey A. Kuhman of Glycon Corporation)...

Convection mixers use a different principle for blending. These mixers have an impeller. This class includes ribbon blenders, orbiting screw blenders, vertical and horizontal high-intensity mixers, as well as diffusion blenders with an intensifier bar. Scale-up considerations are similar to those for the tumble blenders. 

The source material is supplied from tank (2) to the feed hopper (3) of mixer (4). The feed hopper serving to continuously feed a homogenizing screw of mixer (4) comprises a vertical agitator with blades in its upper portion to prevent sticking of the product on the hopper walls. Arranged in the lower portion of the agitator is a separable screw... 

Conical Mixer Dryer The conical mixer dryer is a batch-wise operating unit commonly used in the pharmaceutical and specialty chemical industries for the drying of solvent or water wet, free-flowing powders. The process area is a vertically oriented conical vessel with an internally mounted screw. Figure 12-80 shows a schematic of the bottom drive conical mixer dryer. The dryer utilizes the heatable, internal rotating screw to provide agitation of the batch of material and thus improve the heat and mass transfer of the process. Because it rotates around the full circumference of the vessel, the screw provides a self-cleaning effect for the heated vessel walls. 

A vertical helical ribbon blender can be combined with an axial screw of smaller diameter (Fig. 18-25). Such mixers are used in polymerization reactions in which uniform blending is required but in which high-shear dispersion is not a factor. Addition of the inner flight contributes little more turnover in mixing newtonian fluids but significantly shortens the mixing time in nonnewtonian systems and adds negligibly to the impeller power [Coyle etal.. Am. Inst Chem. Eng.J., 15, 903 (1970)]. 

Vertical impeller mixers employ a screw-type impeller, which constantly overturns the batch. 

Coal fines stored in bins (2) are discharged through several pantlegs (3) into a metering hopper (4). Variable speed discharge belts (8) under the bin (7) and hopper (4) meter the correct amount of each feed into the screw conveyor (9) the binder is pulverized in the hammer mill (10). Both components are premixed in a double screw mixer (11) and elevated (12) into another screw feeder (13). The latter discharges into two vertical pug mills (14) excess feed overflows at the end of the conveyor through a chute (30). 

Other batch or continuous mixers with an almost vertical axis of the mixing tool are the orbiting type screw blenders (Fig. 7.48) often also called Nauta mixers. The total contents of the conical silo is mixed by the orbiting screw which rotates such that it transports material from the depth of the bin to the surface. Although not often... 

Fig. 8.76a is a schematic cross section through an extruder with vacuum degassing. It shows first the end of the mixing/conditioning equipment (in this case an open pug mill but more commonly a closed paddle or screw mixer/ conditioner [see, for example, Fig. 8.70, Section 8.4.2]) in which the feed is prepared for extrusion. The mass is then densified in a pug sealer in which a compacted plug of material is formed that provides a dynamic seal against the vacuum that is created in the feed chamber of the extruder. While dropping into the extruder screw by gravity, gas is removed from the particulate mass. Alternatively, there is a vertical feeder/sealer (Fig. 8.76b) for smaller capacities which mounts directly to the vacuum chamber of the extruder and accomplishes a similar degassing effect.

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