Bonding Mechanisms for Green Ball Formation

At an early stage, both experimental and theoretical considerations were m a d e with regard to the bonding m e c h a n i s m s underlying the green ball formation. S o m e of the theories developed in this connection were already known f r o m o t h e r processes, e.g. those of the fertilizer industry, food-stuffs industry, p h a r m a c e u t i c a l and refractory industries. Similar b o n d i n g mech-anisms also play a great part in the granulation and c r u m b l i n g of the sinter mix. T h e rapid progress of pelletizing technology was achieved particu-larly by almost simultaneous experimental a n d theoretical progress in contrast to o t h e r process developments, e.g. in the blast f u r n a c e tech-nology.

2 . 1 . 1 I m p o r t a n t B o n d i n g F a c t o r s

T h e decisive factors for the green ball f o r m a t i o n a n d green ball properties can b e divided into the following groups 23):

(a) physical forces, such as van der Waals', m a g n e t i c or electrostatic forces (b) surface-dependent factors, such as particle size, particle size

distribu-tion, particle s h a p e a n d crystalline structure

(c) material-dependent factors, such as wettability, absorptive capacity d u e to p o r o u s structure, availability of swelling components, chemical properties in p r i m a r y ores or by-products a f t e r previous treatment (d) capillary forces and surface tension during the addition of liquid

binders, such as water or others.

Some of these factors, mainly the raw m a t e r i a l - d e p e n d e n t factors are not variable. H o w e v e r , they influence the green ball f o r m a t i o n to a great extent. O t h e r forces also acting on the green ball properties are variable.

By utilizing such forces, the raw materials to be pelletized can be a d a p t e d to the relevant requirements.

Variable factors are, for example the quantity of wetting agent a d d e d , the particle fineness and shape, the balling e q u i p m e n t used for green ball formation, the forces arising in such e q u i p m e n t as well as the m o v e m e n t of raw materials in these units.

2 . 1 . 2 B a l l F o r m a t i o n A l t e r n a t i v e s

G r e e n balls can be f o r m e d in different ways. In each particular case, the various b o n d i n g m e c h a n i s m s act at different intensity.

2.1.2.1 Compacting Method

T h e solids are pressed into briquettes only by the application of high mechanical forces. Sharp-edged briquettes m a y be r o u n d e d off by the solid particles, liquid and air. T h e s e interface forces consist, on the one hand, of the surface tension of the b i n d e r , usually water, and on the other, of capillary forces developing in the liquid bridges between the in-dividual ore particle faces, the surfaces of w h i c h are of concave shape.

U n d e r these conditions, a certain tensile strength occurs. T h e forces resulting f r o m the surface tension f o r m a concave liquid surface w h e r e b y tight packing. Capillary and surface forces co-act with adhesive forces2 4).

T h e liquid bridges and surfaces between the individual ore particles f o r m

automatically. In most cases the relevant balling facilities o p e r a t e intermittently. T h e y are less suitable for the t h r o u g h p u t of big ore quantities

2.1.2.3 Mechanism of Ball Formation

T h e feed material m a y consist, according to its p r e p a r a t i o n , of either a conglomerate of dry grains or a wet filter cake. In the one case, t h e ores were subjected to dry grinding, in the other, to wet grinding. M o s t con-centrates are available as filter cake. If dry solid particles c o m e into contact with water, the ore surface is wetted. T h e ore particle is coated with a water film, as is schematically shown in Fig. 12, p h a s e A. In m a n y places, the wet particles touch each other. D u e to the surface tension of the water film, liquid bridges are f o r m e d , p h a s e B. As a result of the m o v e m e n t of the particles inside the balling unit and of the c o m b i n a t i o n of individual water droplets, each containing one or several ore grains, the first agglomerates are f o r m e d , p h a s e C. In the interior of the loose agglomerate, the first liquid bridges a p p e a r a m o n g a large n u m b e r of voids still existing. T h e s e liquid bridges hold the particles together as in a network. L o o s e balls are f o r m e d . W i t h the f u r t h e r supply of water the agglomerates condense. M o r e a n d m o r e water is layered in the interior a n d the agglomerates b e c o m e m o r e dense, p h a s e D. A t this stage of green ball f o r m a t i o n the capillary forces of the individual liquid bridges are essentially active. T h e o p t i m u m of this b a l l - f o r m a t i o n phase is attained w h e n all pores inside the balls are filled with l i q u i d but the latter does not yet u n i f o r m l y coat the whole agglomerate, p h a s e E. T h e effect of the

capil-lary forces is clearly shown by Ilmoni and Tigerschiold in Fig. 13 24sobrescrito). Concave liquid surfaces f o r m on the outer pores a n d capilcapil-lary suction is hold-ing the ore particles together.

T h e final stage is exceeded, w h e n the solid particles are fully coated with a water film. N o w , the surface tension of the water droplets

Fig. 12. Influence of water addition on green ball formation

containing the solid particles becomes fully active, Fig. 12, p h a s e F, a n d the effect of the capillary forces drops sharply, see Fig. 135.

Besides this effect, the rolling m o v e m e n t of the grains and the m o v e m e n t or shifting of the particles relative to each other plays an important role too. T h e y increase a d h e s i o n by the great n u m b e r of contact points at a s i m u l t a n e o u s compression strength d u e to the load of the rolling material.

Fig. 13. Influence of capillary forces on bonding mechanism

A p a r t f r o m the c o m p a c t i n g a n d solidification of agglomerates d u e to pressure and m o v e m e n t , these factors m a y also h a v e a negative influence.

They d a m a g e mechanically weak granules w h i c h have not yet attained a sufficient adhesive strength. T h e d a m a g e resulting is that the weak green balls m a y either c r u m b l e into m i n o r f r a g m e n t s or disintegrate into even finer particles. D u r i n g green ball f o r m a t i o n , these fractions m a y b e layered onto moist stable green balls and b e i n c o r p o r a t e d into the latter.

Besides the ideal ball f o r m a t i o n f r o m fine-grained individual solid particles initially described, various o t h e r possibilities exist m o r e or less simultaneously in practical operation. T h i s also applies to loose agglomer-ates as in the case of wet filter cake. According to Sastry and Fuerstenau^{2 5}) the following methods, as shown in Fig. 14, can b e adopted:

A) Layering of very fine particles to others a n d thus f o r m a t i o n of an agglomerate.

B) C o n g l o m e r a t i n g of smaller balls already existing resulting f r o m relative m o t i o n and a certain pressure.

C) Layering o n and incorporation of m i n o r f r a g m e n t s f r o m d a m a g e d green balls into existing s o u n d ones.

D) Incorporation of fine-grained a b r a d e d m a t e r i a l f r o m weak pellets into the surface of stronger pellets.

Fig. 14. Alternatives for green pellet formation

D u r i n g green ball production, their f o r m a t i o n proceeds parallel to the disintegration of a certain n u m b e r of balls. Only such balls which can withstand the dividing or destructive forces d u r i n g rolling survive. A selection of the best balls takes place. T h e contest of the constructive a n d destructive forces favours the f o r m a t i o n of astonishingly uniform, dense and stable green balls2 3).

As set forth u n d e r item 2.1.1, various factors m a y co-act during green ball f o r m a t i o n , either dependently on raw materials or i n d e p e n d e n t l y thereof. M u c h d e v e l o p m e n t a n d research w o r k has b e e n p e r f o r m e d to in-vestigate the raw material i n d e p e n d e n t factors. In this connection, s o m e of the researchers who have played an i m p o r t a n t role are m e n t i o n e d^{2 6}) .

T h e theory a n d the m a t h e m a t i c a l f o r m u l a resulting t h e r e f r o m are dealt with under item 13.1. These investigations are based, for the most part, on the use of exactly defined particle shapes, such as balls or on specific raw materials, such as quartz, limestone or glass. T h e laws discovered can thus only serve as a guide since each ore — even of the s a m e size — has its typical size distribution or particle shape. T h i s again requires an individual investigation of each ore or mixture.