Drying Procedure of Individual Balls

This drying c o m m e n c e s when heated air or c o m b u s t i o n gases flow over wet green balls. In this connection, the gas t e m p e r a t u r e , dew point, quantity and drying velocity play an i m p o r t a n t role. T h e moisture contained in the green pellets starts to evaporate evenly on the entire surface. A f t e r this evaporation, water f r o m the pellet interior is emitted by capillary forces to the surface. As long as this p r o c e d u r e continues, the drying velocity remains constant. However, if the drying velocity on the surface is greater t h a n the water emission f r o m the pellet interior, the drying front travels d o w n into the pellet interior. T h e water v a p o u r thus arising has to cover an ever increasing distance t h r o u g h the capillaries which are already dried out until it reaches the pellet surface f r o m where it can escape together with the air flow. D u r i n g this drying phase, the drying velocity is no longer constant b u t decreases. As soon as the capillary water evaporates, the drying p r o c e d u r e is terminated. However, if the pellet contains hygroscopic or chemically-combined water, the drying p r o c e d u r e only continues w h e n the t e m p e r a t u r e of the drying m e d i u m is high enough to dissociate the c o m p o u n d s . T h e velocity of this third drying phase is again lower. T h e drying p r o c e d u r e of such a green ball, which also contains hygroscopic water, was described by Krischer a n d Jaeschke 27) in a curve shown in Fig. 15.

surface of material to be dried x unit of time

O n the abscissa, the time is plotted which is n e e d e d for the various drying phases.

T h e r e are three different drying velocity ranges which are indicated by three breaks of the curve:

Range I: T h e water evaporates at constant velocity on the entire pellet surface. This p r o c e d u r e continues as long as h u m i d i t y f r o m the pellet interior is emitted at a sufficient speed by capillary forces or d i f f u s i o n to the ball surface. T h e constant drying velocity is influenced by the following factors:

— T e m p e r a t u r e and moisture content (dew point) of t h e drying gas,

— V o l u m e and speed of the drying gas w h i c h flow, p e r unit of time, over the ball surface,

— Surface of ball.

This range I is thus characterized by the evaporation of surface water.

Range II: If the moisture f r o m the interior of the pellet is emitted to the surface m o r e slowly t h a n water evaporates there, the evaporation level

Fig. 15. Drying stages of moist material

(drying level) moves f u r t h e r towards the pellet core. T h e water evaporates inside the pellet, the d i f f u s i o n distance to b e covered by w a t e r v a p o u r becomes longer and the drying velocity diminishes. It is now d e t e r m i n e d by the :

- D i f f u s i o n resistance

- D e p t h of drying level below the ball surface - D i f f u s i o n index expressed in m / h .

Range II is characterized by the evaporation of capillary water f r o m the pellet interior. A f t e r this stage, the pellets are n o r m a l l y dry.

Range III: If the pellet contains m o i s t u r e other t h a n surface a n d capillary water, the drying p r o c e d u r e continues u n d e r o t h e r conditions,

In this case, the t e m p e r a t u r e of drying gas is i m p o r t a n t for the dissocia-tion of the water c o m p o u n d s . Besides the longer d i f f u s i o n distance, this dissociation has to b e considered a n d consequently the drying velocity decreases even more. It is virtually d e t e r m i n e d by the operations pro-ceeding in the pellet core a n d by the a m b i a n c e of the pellet.

In R a n g e III, the evaporation of the hygroscopic or chemically combined water mainly takes place.

T h e term " m o i s t u r e " is not u n i f o r m l y d e f i n e d in literature. O n e version refers to the water a m o u n t of the dry substance. In practice, the wet substance is frequently used as reference value on which the d a t a given below are also based. Moisture is thus d e f i n e d according to the following formula:

T h e drying p r o c e d u r e determined with one individual pellet on a thermoscale is shown in Fig. 16. T h e ore, a h e m a t i t e with 67.5% F e , 1.2% Al2O3

and 0.7% SiO2, as well as 1.2% loss on ignition, was pelletized with 0.8%

W y o m i n g bentonite and 8.2% water and slowly dried in a laboratory-scale drying oven at a t e m p e r a t u r e of 400 ° C u n d e r air flow.

Fig. 16. Drying of an individual green pellet

T h e left o r d i n a t e shows the r e m a i n i n g water content of the pellet as well as the moisture decrease in percent while the right ordinate indicates the loss of weight. U p to a moisture loss of a b o u t 67% during a drying period of 4.5 mins. — plotted on the abscissa — the drying curve proceeds linearly.

T h e drying velocity u p to this moisture content is constant. A b o u t % of the water evaporates f r o m the pellet surface. F r o m now on, the drying proceeds m o r e slowly and asymptotically until the residual moisture content of the pellet core has evaporated.

By volume a n d t e m p e r a t u r e of the drying gas the heat supply for water evaporation has to be adapted to the relevant heat c o n s u m p t i o n in such a way that d u r i n g the capillary m o v e m e n t of water and d i f f u s i o n of water vapour, the relative position of the individual ore grains to each o t h e r is not changed.

If the heat supply a n d thus the f o r m a t i o n of water v a p o u r proceed too quickly, cracking or even decrepitation m a y occur (shock t e m p e r a t u r e ) .

T h e material-dependent properties of the ore, t h e grain size, size distribution, dense or loose packing of grains play such an i m p o r t a n t role that it is advisable to find out the drying conditions by tests with the ore to be pelletized. A l t h o u g h the drying p r o c e d u r e of the i n d i v i d u a l pellet can still b e surveyed the drying conditions of green balls in layers are m o r e complicated.

In industial plants, green pellets are always thermally treated in layers of a different d e p t h , either in shaft furnaces, on travelling grates or in grate-kiln plants.

T h e b e h a v i o u r of green balls in layers of d i f f e r e n t thickness is thus of interest for all t h r e e processes.

In view of the great n u m b e r of factors affecting the drying technology, it is difficult to describe precisely each of these factors. O n the other hand, it is possible to ascertain qualitative dependencies. F o r this purpose, ex-periments furnish, for the present, m o r e reliable results t h a n m a t h e m a t i c a l pellet bed. In this case, an up-draught drying is involved.

A c o m b i n a t i o n of the above m e t h o d s is also possible a n d is, in practice, pellet layer is condensed out and this condensation heat is transferred to the second pellet layer which is thus also heated to e v a p o r a t i o n t e m -perature.

T h e energy required for the evaporation of the water f r o m pellets of the directly a d j a c e n t layers is supplied f r o m the i n c o m i n g hot drying gases. In this case the gas again cools d o w n to evaporation t e m p e r a t u r e as long as water is to be evaporated.

However, in the course of this interplay between heat and material exchange, a n over-wetting m a y occur so t h a t a w e a k e n i n g of the pellet structure a n d hence a decrease of pellet strength m a y t e m p o r a r i l y take