(120d) Stability of Granules Formed by Solid Bridging

Introduction In a number of industries granules and agglomerates are used. Often the binding mechanism is solid bridging. Primarily these are the food industry, detergents and pharmaceuticals but also fertilisers and fine chemicals manufacturing. In general, the properties of granulated material are determined by its composition, size distribution shape and internal structure. The composition is often fixed by chemical requirements to achieve the final function of the powder. The desired size distribution is chosen due to appearance, dedusting and bulk density targets. Hence particle morphology, shape and structure are determining the instant and mechanical properties of the granule. For practical purposes mechanical stability of granulates and agglomerates is of key interest. In different production and handling stages the material is subjected to stresses that cause attrition, wear and breakage resulting in fines generation. Enhanced fines fractions are critical to product behaviour, i.e. powder flow, bulk density, dissolution and dispersion behaviour. Increased dust formation may be critical with respect to sealing efficiency in the packaging process of powders. Typically granulated products have to fulfil at least powder flow, dissolution and dispersibility requirements. Making the granules as strong as possible by maximising the bridges would be an easy solution to the above described issue. However this would mean to maximise the binder content, which is limited by process constraints. In the granulation process, water is mixed into or sprayed onto a powder mixture of soluble solids. The water dissolves part of the solids and binds the particles by capillary forces. When the water is dried off the dissolved material solidifies and forms a solid bridge between the primary particles. The bridge may consist of crystalline or glassy material. This bridge is orders of magnitude stronger than a liquid bridge. Its strength is determined by the volume of binder material in relation to the particle volume and the strength of the binder material. The bridge volume is given by the amount of water, hence spray on rate and spraying time. The intermediate water amount can not be raised above a process specific limit. The strength of granulates or agglomerates is determined by the internal structures and the strength of each binding point. The different binding mechanism results in different principle strength. According to Rumpf /1/ one distinguishes the main binding mechanism in van der Waals forces, capillary binding, adhesion and cohesion forces i.e. of highly viscous binder and solid bridging. The structure considerations can be reduced in the simplest version to a porosity ?contact point relation as given by Molerus /2/. Granule strength is typically handled according to a covering breakage behaviour, ductile, semi- brittle or brittle. For a solid bridging it seems save to assume brittle breakage, as the bridge is formed by crystalline or glassy material. A brittle breakage behaviour means that the crack propagation may be assumed and fatigue plays a less important role. Good progress has been made in the case of impact breakage by use of air gun type of equipment, single particle milling and use of DEM (Peukert, Vogel /3/, Ghadiri et al. /4, 5/). In this study shear and compression forces are considered rather than impact as stressing case.

Structure consideration Granulated material has an internal structure and a given porosity. This is immanent to the process of binding primary particles together forming one bigger particle. It is obvious that a lower porosity will lead to an increase in stability of a porous system. A lower porosity leads at the same time to slower dissolution. The optimum granule has the shortest possible dissolution time at the required mechanical strength. The mechanically ideal case would be a completely homogenous system. The crack will form at the weakest point, which will be the weakest binding point i.e. solid bridge. The tensile strength of such a system can be calculated by the following equation: , where denotes the tensile strength of the agglomerate, the volume ratio of the solid bridge (SB) and the agglomerate,  is the agglomerates porosity and the tensile strength of the bridging material. However this relation is only valid for a homogeneous system of monosized spheres. A more complex version takes care of particle size distribution and particle shape effects. In industrial practice, both the granule structure as well as the stress situation are different. Fig. 1 shows two cross-sections of particles bound by solid bridging. It is apparent that simple geometric models will fail to predict the strength of these systems accurately. An experimental approach has been chosen to investigate how the strength of the granule depends on stress conditions as well as granule structure. The experimental data support a probabilistic approach to explain the fines generation of granules bound by solid bridging. Three typical stress conditions can be distinguished. In handling e.g. mixing shear stresses in rapid powder flow occurs, in intermediate storage compression takes places resulting in positive, compressive normal forces and in transport cases vibrations stresses causes fines generation. These three cases have been mimicked to asses the stress specific granule strength.

A physical interpretation of the data is based on population balance considerations as well as quantitative strength measurements using a ring shear tester.

Summary Granules that are bound by solid bridging have been made with different volume fractions of bridging material and different primary particle size distributions. The material has been tested by different methods. The results have been interpreted with respect to stress parameter i.e. stress intensity and stress duration or frequency and granule structure.

Literature /1/ Rumpf, H.: Strength of granules and agglomerates. In: Agglomeration (ed. Knepper), Proc. First Int. Symposium Agglomeration, Philadelphia, USA, (1962) /2/ Molerus, O.: Schüttgutmechanik, Springer Verlag Berlin Heidelberg New York Tokyo (1985) /3/ Vogel,L. Peukert, W.: Characterisation of Grinding relvant Particle Properties by Inverting a Population Balance Model, Part,. Part. Syst. Charact. 19 (2002) /4/ Samimi, A.; Boerefijn, R.; Kohlus, R.; Ghadiri, M. Breakage of Soft Granules Under Uniaxial Bulk Compression and Impact Conditions World Congress on Particle Technology 4, 21. ? 25. July 2002 Sydney, Australia, ISBN 085 825 7947 /5/ Golchert, D.; Moreno, R.; Ghadiri,M.; Litster, J.; Effect of morphology on breakage behaviour during compression, Powder Technology 143-144 (2004)

Fig.1: Cross-sections through granulated particles, left pure sugar system, right mixed system; micrographs made by x-ray tomography