Intensive drying and the related microstructure features in agglomerate spheres

Abstract

More metal ore concentrates are fine particulates with a wide particle-size distribution. Industrially they are pelletized by tumbling in balling discs or drums into spheres, an operation which requires the addition of typically up o 10% by weight of water. Further processing of these agglomerates involves first drying and then induration by heating up to 1250*C.

The main objective of this thesis was the study of the interrelationship between the microstructure of the agglomerates with, on the one hand, the mechanical and physical properties of the pellets and their behaviour during intensive drying, on the other.

The previously developed model of the drying process identified the loss of capillarity, resulting from the vapour lock, to be a critical component of the mechanism of intense as opposed to 'classical' drying. It was shown that the absence of the constant-rate drying period is a natural consequence of this effect.

Several significant shortcomings of the previous model have been identified. This model treats the period of transition between surface- and shrinking-core drying as an instantaneous event. The new extended model, which overcomes the original model limitations, was developed in this project. In its formalism, the new model includes the poe-size distribution and thus simulates a gradual surface/shrinking-core transition.

It was shown that the nature of the transition between the surface- and shrinking-core drying regimes during intensive drying is fundamentally different from that of classical drying, i.e. carried out at mild temperatures. In the latter case, liquid is being delivered to the surface through the network of interconnected small pores reaching the surface. The transition occurs when the larger pores, also reaching the surface, are being drained. On the other hand, under intense-drying conditions, the rate-limiting factor is the vapour lock. The latter phenomenon will occur in the smaller pores first, as they have smaller liquid pressure. Hence, they will be the first to become dry, while surface drying continues through the system of interconnected larger pores reaching the surface.

Experimental research to be described validates the extended model for the drying of agglomerates that have a wide range of particle size and have been dried under wide range of drying conditions.

New insights have been gained by applying this new drying model. Critical aspects of microstructure of agglomerates were investigated more specifically in the light of these new insights. They include pore-matrix expansion during drying due to the engulfment of fine particles into the contacts between the larger, structure-creating ones. Experimental results validate the matrix-expansion hypothesis developed in this study.

Although this study focused on a specific industrial process, the pelletizing of iron-ore concentrates, the interrelationship between microstructure and drying behaviour has important implications in understanding the nature of soils, rocks, ceramics and processed foods.