Snowballing

When you squeeze snow to make a snowball, you are hot-pressing a ceramic. Hot-pressing of powders is one of several standard sintering methods used to form ceramics which require methods appropriate to their special properties. 

It is clear from the records of ice ages (see, e.g., Fig. 1-2) that Earth can have and has had climates that are different from our current state. Other, more extreme possibilities have been suggested, each of which could be stable for considerable periods of fime. The frozen "snowball Earth" already mentioned is analogous to present-day Mars, where its CO2 green-... 

Fig. 2. Disk operating in the snowballing mode of pellet growth.

Fig. 10. Four elementary pellet growth mechanisms (a) snowballing (b) crushing and layering (c) coalescence (d) abrasion transfer. [From Sastry and Fuerstenau (S5).]...

The experimental results of Capes and Danckwerts (C6) and Kanetkar (K1), using a balling drum and a disk granulator, respectively, have shown that the thickness of the snowballed layer is approximately independent of the initial size of the seed pellet. It then follows that the rate of pick up of the loose particulate material is proportional to the surface area of the granule hence,... 

Inspection of this equation shows that in the course of the snowballing growth the size distribution curves at various times are simply shifted toward the right on the pellet size scale without any change in their shape, as demonstrated by Capes (C2) for sand pellets snowballed in a pan granulator (Fig. 13). 

Fig. 13. Size spectra of pellets grown by snowballing. [From Capes (C2).]...

For modeling a continuous pelletizer, it is advantageous to formulate the snowballing kinetics in the well-known continuity equation for the pellet species... 

The liquid-phase requirement for the snowballing mechanism may be computed in the following manner (S10). It is assumed that the liquid withdrawal is negligible (i.e., g = 0) and the fractional saturation in the deposited... 

Equations (106) and (109) give expressions for the liquid content and granule size in a snowballing granulation system. 

Sherrington (S10) has presented a static model of the granulation loop operating in the snowballing mode. The model combines the solution phase theory in Eq. (106) with the material balance across the granulator. The recycle ratio defined as recycle/raw solid feed, is given by... 

Fig. 30. Simulated behavior of fertilizer loop operating in the snowballing mode. [From Sherrington (S10).]...

Han (H3) and Han and Wilenitz (H4) have also presented steady-state models of fertilizer granulators based on population balance on the granules in the process loop operating in the snowballing mode. From the viewpoint of process control some interesting interrelationships between various recycle ratios, crusher speed, crusher product size, and the granule growth rate have been established. 

Typical results of the dry pelletizing process when a charge of carbon black and its plus 52-mesh seeds is tumbled in a drum (12) are shown in Figs. 31 and 32. Initially, the seeds grow in size rapidly by snowballing. The dip in the bulk density of the seeds (Fig. 32) suggests that the deposited layers are quite porous in the beginning but get compacted in due course to... 

Kc,K t Empirical pellet growth rate T Thickness of snowballed layer... 

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

Mission Deep Impact In July 2005, NASA steered a projectile, about 370 kg in weight, at the comet 9F/Tempel (dimensions 4x4x14km), in order to obtain more exact information on its structure and composition. The impact was visible from Earth the Rosetta spacecraft discussed above also sent pictures to Earth. The dust/ice ratio determined after the impact is very probably greater than unity, so that comets are probably icy dustballs rather than (as had previously been surmised) dirty snowballs . The density of the cometary nucleus, which seems to consist of porous material, is roughly equal to that of ice. The impact set free around 19 GJ of... 

---