Simon M. Iveson

Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review

Wet agglomeration processes have traditionally been considered an empirical art, with great difficulties in predicting and explaining observed behaviour. Industry has faced a range of problems including large recycle ratios, poor product quality control, surging and even the total failure of scale up from laboratory to full scale production. However, in recent years there has been a rapid advancement in our understanding of the fundamental processes that control granulation behaviour and product properties. This review critically evaluates the current understanding of the three key areas of wet granulation processes: wetting and nucleation, consolidation and growth, and breakage and attrition. Particular emphasis is placed on the fact that there now exist theoretical models which predict or explain the majority of experimentally observed behaviour. Provided that the correct material properties and operating parameters are known, it is now possible to make useful predictions about how a material will granulate. The challenge that now faces us is to transfer these theoretical developments into industrial practice. Standard, reliable methods need to be developed to measure the formulation properties that control granulation behaviour, such as contact angle and dynamic yield strength. There also needs to be a better understanding of the flow patterns, mixing behaviour and impact velocities in different types of granulation equipment

Growth regime map for liquid-bound granules: further development and experimental validation

An attempt was made to quantify the boundaries and validate the granule growth regime map for liquid-bound granules recently proposed by Iveson and Litster (AlChE J. 44 (1998) 1510). This regime map postulates that the type of granule growth behaviour is a function of only two dimensionless groups: the amount of granule deformation during collision (characterised by a Stokes deformation number, St(def)) and the maximum granule pore saturation, s(max). The results of experiments performed with a range of materials (glass ballotini, iron ore fines, copper chalcopyrite powder and a sodium sulphate and cellulose mixture) using both drum and high shear mixer granulators were examined. The drum granulation results gave good agreement with the proposed regime map. The boundary between crumb and steady growth occurs at St(def) of order 0.1 and the boundary between steady and induction growth occurs at St(def) of order 0.001. The nucleation only boundary occurs at pore saturations that increase from 70% to 80% with decreasing St(def). However, the high shear mixer results all had St(def) numbers which were too large. This is most likely to be because the chopper tip-speed is an over-estimate of the average impact velocity granules experience and possibly also due to the dynamic yield strength of the materials being significantly greater than the yield strengths measured at low strain rates. Hence, the map is only a useful tool for comparing the granulation behaviour of different materials in the same device. Until we have a better understanding of the flow patterns and impact velocities in granulators, it cannot be used to compare different types of equipment. Theoretical considerations also revealed that several of the regime boundaries are also functions of additional parameters not explicitly contained on the map, such as binder viscosity

Limitations of one-dimensional population balance models of wet granulation processes

Most traditional models of wet granulation processes use one-dimensional population balances, which assume that granule size is the only independent granule property that significantly affects granule growth behaviour. However, several other independent granule properties have been identified, which can also strongly influence granulation behaviour. These include granule binder content, porosity, and primary particle size distribution and composition. Most population balance models also implicitly assume that conditions are spacially uniform throughout the granulator. However, segregation has the potential to occur in many commercial granulators. This will alter the frequency and velocity of collisions between different sizes of granule. Hence, current one-dimensional models of wet granulation are inadequate. These models need to be expanded to three or four dimensions to include size, porosity, binder content, and composition (where applicable) as independent granule properties and also need to include the effects of mixing and segregation within the granulator.

The dynamic strength of partially saturated powder compacts: the effect of liquid properties

Abstract The flow stress of partially saturated pellets was measured at deformation velocities varying from 0.01 to 150 mm/s. The pellets, 20 mm in diameter and 25 mm high, were made with glass ballotini of a surface mean particle size, dp, of 35 μm. Water, glycerol and a series of silicone oils were used as the liquid binder, covering viscosities, μ, and surface tensions, γ, ranging from 0.001 to 60 Pa·s and 0.025 to 0.072 N/m, respectively. It was found that there was a critical strain rate (which was binder dependent) below which the peak flow stress, σpk, was independent of the strain rate, e. Above this critical strain rate, the flow stress increased with increasing strain rate. When plotted in terms of two dimensionless groups, the results collapsed onto one curve of the form: Str *=k 1 +k 2 Ca n where Str*=σpkdp/γcosθ is the dimensionless peak flow stress and Ca=μedp/γcosθ is the dimensionless capillary number, the ratio of viscous to capillary forces. θ is the contact angle (assumed zero in this work). The best fit values of the parameters were: k1=5.3±0.4, k2=280±40 and n=0.58±0.04. This result suggests that viscous forces are negligible at low strain rates, but become dominant at high strain rates. k1 is related to the static peak compressive strength of the pellets, k2 determines the critical Ca at which viscous effects become significant and n gives the power law dependence of viscous forces on the strain rate. This work is significant for granulation research since it highlights the fact that strengths measured under pseudo-static conditions may not be representative, even qualitatively, of how materials behave at the higher strain rates encountered in commercial granulators.

The importance of wet-powder dynamic mechanical properties in understanding granulation

Granule impact deformation has long been recognised as important in determining whether or not two colliding granules will coalesce. Work in the last 10 years has highlighted the fact that viscous effects are significant in granulation. The relative strengths of different formulations can vary with strain rate. Therefore, traditional strength measurements made at pseudo-static conditions give no indication, even qualitatively, of how materials will behave at high strain rates, and hence are actually misleading when used to model granule coalescence. This means that new standard methods need to be developed for determining the strain rates encountered by granules inside industrial equipment and also for measuring the mechanical properties of granules at these strain rates. The constitutive equations used in theoretical models of granule coalescence also need to be extended to include strain-rate dependent components.

Contact angle measurements of iron ore powders

Abstract The wettability of three iron ore powders from different sources were measured by comparing the air pressures required to halt the capillary flow of water and cyclohexane up through packed beds of the powders. The calculated contact angles of the ores ranged from 30 to 70°. The contact angles were compared to the chemical composition of the ores. The contact angle was seen to increase as the ratio of oxide/oxy-hydroxide (haematite/goethite) was increased. During these measurements, a previously unnoted problem was encountered. As liquid rises up the packed bed, air was forced down the wall of the column where the packing fraction is lower than in the bulk. Unless prevented, when this air reached the base of the column it rapidly expanded and flowed into the capillary through which liquid was supplied. This cut off liquid flow into the powder bed, making it impossible to determine the contact angle. A simple method to overcome this problem is to support the bed on a medium that can generate a higher capillary pressure than the powder of interest. In this case, a thin precoat layer of fine material (diatomaceous earth) was successfully used.

Tensile bond strength development between liquid-bound pellets during compression

Novel experiments have been performed to measure the tensile bond strength developed between two liquid-bound pellets when they are compressed together at low strain rates. Pellets 20 mm in diameter were made from 75-μm mass-mean sized glass ballotini with water and three different viscosity silicone oils (0.01, 1 and 60 Pa s). The water-bound pellets formed bonds which were brittle and ruptured quickly when strained in tension. The silicone oil-bound pellets were plastic and stretched back a significant fraction of their original length before the bond ruptured. The peak tensile strengths and rupture energies of the bonds were proportional to the radial strain in the bond zone. A model was developed based on a cold-welding analogy to predict the peak tensile strength of the bond as a function of the strength of the bulk pellets and the extent of radial strain in the bond region. There was good agreement between the model predictions and the experimental results for radial strains less than 7%. At higher radial strains, the bond strength appeared to level off, probably because the bonds began to fail by gradually peeling apart rather than by simultaneous rupture across the whole failure plane. This simple model should help in predicting the bond strength formed when two liquid-bound agglomerates collide, which will be important in understanding and modelling granule coalescence growth behaviour. It was also observed that the compressive strength of the pellets decreased with increasing liquid viscosity, probably due to a lubrication effect reducing inter-particle friction. This contrasts with the effect of liquid viscosity seen by other workers at high strain rates, and suggests that the strength ranking of formulations with viscous binders may be strain-rate dependent.

Granule coalescence modelling: including the effects of bond strengthening and distributed impact separation forces

Many existing models of granule coalescence assume that the two granules have successfully coalesced provided that they stick during their initial impact. However, although non-rebound is a necessary condition for coalescence, it is not sufficient. It is also required that the bond formed between the two granules be strong enough to resist being broken by subsequent impacts within the granulator. A theoretical framework is proposed in which both the rate of bond strengthening and the distribution of impact separation forces are included in order to calculate the probability that the collided dumbbell will survive and hence permanently coalesce. This work highlights the need for further research into the mechanisms and the rate at which a granule dumbbell becomes moulded into a single larger granule, and also the distribution of impact forces inside a granulator.

Brittle to Plastic Transition in the Dynamic Mechanical Behavior of Partially Saturated Granular Materials

The effect of liquid viscosity, surface tension and strain rate on the deformation behavior of partially saturated granular material was studied over a ten order of magnitude range of capillary number (the ratio of viscous to capillary forces). Glass spheres of average size 35 microns were used to make pellets of 35% porosity and 70% liquid saturation. As the capillary number increased, the failure mode changed from brittle cracking to ductile plastic flow. This change coincided will the transition from strain-rate independent flow stress to strain-rate dependent flow stress noted previously [Iveson, S. M. Beathe, J. A., and Page, N. W., 2002, The Dynamic Strength of Partially Saturated Powder Compacts: The Effect of Liquid Properties, Powder Technol., 127, pp. 149-161]. This change in failure mode is somewhat counter-intuitive, because it is the opposite of that observed for fully saturated slurries and pastes, which usually change from plastic to brittle with increasing strain rate. A model is proposed which predicts the functional dependence of flow stress on capillary number and also explains why the flow behavior changes. When capillary forces dominate, the material behaves like a dry powder: Strain occurs in localised shear planes resulting in brittle failure. However, when viscous forces dominate, the material behaves like a liquid: Shear strain becomes distributed over a finite shear zone, the size of which increases with strain rate. This results in less strain in each individual layer of material, which promotes plastic deformation without the formation of cracks. This model also explains why the power-law dependency of stress on strain rate was significantly less than the value of 1.0 that might have been expected given that the interstitial liquids used were Newtonian.

Gravity Separation and Desliming of Fine Coal: Pilot-Plant Study Using Reflux Classifiers in Series

Two pilot-scale Reflux Classifiers (600 mm × 600 mm cross-section) arranged in a cascading sequence were used to beneficiate fine -2 mm coal. The first Reflux Classifier performed a density separation that produced a coal product contaminated with fine high-ash slimes. This was then washed in the second Reflux Classifier to remove the fine clays and mineral matter. This combination reliably produced a clean coal product and allowed gravity separation performance to be extended from the usual eight-fold limit of upper to lower size to a much broader size range. Performance was similar to previous laboratory-scale results units with cross-sectional areas of only 100 mm × 80 mm each. Hence, full-scale desliming units can be confidently designed based on laboratory trials. The cut size varied linearly from 0.04 to 0.24 mm with increases in the overflow channel velocity from 25 to 55 m3/(m2 h). The Ep values increased from 0.02 to 0.07 mm (Whitten factor α from 2 to 6) over the same range. The linear dependence of the cut size on velocity in the Reflux Classifier was consistent with the theory and with the significant throughput advantage of the technology.