Material predensified

In certain cases the limited densification effect of force feeders does not suffice to attain the necessary precompaction required for a product with acceptable quality. Then separate machines are used for this task or predensified material is recirculated in a closed-loop system. 

Raw materials and recycle (undersized, somewhat predensified material) are made available in day bins. It is essential that at all times enough material is available (controlled by level indicators) for continuous discharge from the silos. Activators and/or mass flow bin designs may be necessary to ensure constant flow. In most cases, the individual components of a mixture will be metered for correct analysis. It is important to take into consideration the recirculating material as it may influence the final analysis and, on the other hand, may be necessary to ensure acceptable densification. 

In medium-pressure agglomeration (see Section 8.4.2), the extrusion through perforated die plates (II.2), higher densification can be obtained by two effects. First, the feed, that must have binding characteristics and be somewhat plastic, is predensified prior to extrusion by the screw(s) or between the press roUer(s) and the die. Secondly, the material is passed through holes in the die where pressure builds up due to wall friction. The length over diameter ratio of the hole (= extrusion channel) and its shape as well as the properties of the feed determine final densification. Because of the wall friction, the surface of the extrudate is always denser than the center (see also Section... 

In Kahl flat die pellet presses, a unique feature is the hydraulic roller adjustment device. As depicted in Fig. 8.61, the roller assembly can slide up and down the main shaft and is supported by strong springs. A so called hydraulic nut , a hydraulic cylinder arrangement, with which the rollers can be pushed down or lifted, is installed at the end of the shaft. This allows to adjust the clearance or gap between the rollers and the die to optimize densification and extrusion characteristics for different materials and to modify the thickness of the predensified layer after the extrusion step. Since adjustments can be made anytime, even during operation, it is possible to optimize press performance as needed. 

The most obvious difference is the application of a vertical feed screw which is required to stuff the typically loose, voluminous feed material into the press chamber. Providing predensified material allows to produce briquettes with sufficient size (thickness) without excessive stroke length of the ram. 

The specific gravity of microsilica is around 2.20. Because of its very small particle size and high specific surface area, the bulk unit weight of the loose material is very low, and ranges typically between 240 and 300 kg/m. To lower the transportation costs, it is common to ship the material in a predensified form, by which the unit weight may be increased to 540-600 kg/m. It is even more convenient to transport and handle the material in the form of a slurry with a high solid content. Blended cements, in which microsihca is combined with Portland clinker and calcium sulfate, are also commercially available. A comparison of different forms of microsilica indicates that densification of this material decreases its chemical reactivity (Sanches de Rojas et al, 1999). 

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