Freeze granulation and spray drying of mixed granules of Al2O3
Graphical abstract
Introduction
Granulation techniques are widely used in many industrial sectors, and for various purposes. The pharmaceutical industry uses granulation for coating or encapsulation of active ingredients [1]. In the food industry, granulation provides additional properties such as aesthetic, shelf life, odor, taste, UV or moisture protection etc. In the ceramic industry, granulation is used to facilitate the flow of powders, and therefore their handling by dosers and conveyors. It also improves the pressability of the materials, and thus green density and densification rate for sintering. Another application is for the preparation of intimate ceramic-organic mixtures for specific processes such as Selective Laser Sintering [[2], [3], [4]]. Granulation techniques can also be used to improve bulk density and fluidity of nanoscale powders to increase their workability while maintaining their other properties as required to build nanostructured coating by plasma spraying [5,6]. In all these application, a good control of the granule size and morphology is required, as it impacts the green mechanical properties of the granules [7].
The spray drying process consists of pumping a suspension or a solution to a spray nozzle, then drying the resulting droplets in a chamber containing a stream of hot gas (usually air but could be nitrogen for oxidation-sensitive materials) and finally collecting with a cyclone (or other technique) the dry granules formed [1]. Depending on the way in which the suspension is atomized into fine droplets, a distinction is made between pressure nozzle atomizers, rotary atomizers and pneumatic atomizers [8]. The gas flow can be in the same direction as the spray (so-called co-current atomization) or in the opposite direction (so-called counter-current atomization). The shear rates experienced by the suspension during spraying can reach values as high as 106 s−1 [9]. To encapsulate a ceramic powder in a polymer, spray drying can be used as it has been the case with alumina, glass or silicon carbide powders [[2], [3], [4],10]. The granule size increases with the suspension viscosity [9], solid content [11,12], surface tension [1], particle size [13], feed rate [1] and nozzle diameter [1]. It decreases with the density of spraying gas, atomization pressure [1] and, for rotary atomizers, with rotational speed and diameter of the rotating disc [8]. During the drying of the granules (after spraying), the species in solution tend to migrate with the liquid to the surface. This phenomenon leads to segregation of the species in solution, most of which are then found on the surface. Typically, this occurs when using suspensions with organic additives, such as binders. Although the binders have a certain affinity with the oxide surfaces, a large part of them is found in solution. For example, Baklouti et al. determined that the adsorption limit of polyvinyl alcohol (PVA) on the alumina surface is around 0.6 mg/m2 [14]. Therefore, when the amount is higher, the binder remains in solution and migrates to the surface of the droplets during drying. Binder segregation is promoted by a high binder content, a large amount of liquid, large granule and drop sizes, and a high solvent evaporation rate [15].
Freeze-granulation consists in spraying a suspension above a bath at very low temperature (usually liquid nitrogen) to freeze the droplets before they dry [16]. The extremely fast cooling rate allows the solvent (usually water) to solidify without crystallization. The frozen granules are then recovered and dried by lyophilization, i.e. liquid sublimation. Since the spraying method of the freeze granulation is the same as these by atomization, all spraying parameters affecting the granules size are identical [17]. Granule density increases with solid content. Granule surface is smoother when the solid content of the suspension is higher. Freeze granulation can overpass the quality of spray drying when the product is sensitive to high temperature. In addition, due to fast freezing and lyophilisation, the granules are mainly spherical, contrary to spray drying [18]. Although more complex to implement, freeze granulation may be preferred to spray drying when the product to be treated is very sensitive to the high temperatures required for drying. In addition, it can be complex to obtain full and spherical granules by spray drying because of the migration phenomenon mentioned above, whereas freeze granulation ensures spherical, homogeneous and low density granules that is useful for dry pressing operation to achieve high green density (better pressability thanks to low granule density), high green strength (strength of a non-sintered ceramic) and better homogeneity [19]. When high homogeneity granules (multi-material granules) are required, as for nuclear applications [20] and transparent ceramics [21], freeze granulation is recommended. Freeze granulation could be useful for 3D printing techniques which used powder bed like binder jetting [22,23]. In addition, for hazardous powders, freeze granulation is a better option than spray drying because it is a dust-free process [24].
These previous studies have revealed the importance of some experimental parameters and their influence on the final granule size and their morphology. However, only trends are derived from these studies (size increases or decreases with a given parameter) and it is not possible to determine in advance the size of a granule using the characteristics of the suspension and the granulation process. In this paper we propose a new parameter called Pu (for pulverization number), able to predict the granule size through a multi-parameter approach, taking into account both slurry and process parameters. This study was conducted in the context of the formulation of mixed granules of alumina with a high content of organic binders to improve their plasticity at a medium temperature. Thus in a preliminary study the rheological behaviour of the suspensions was conducted to optimize their formulations and better predict the influence of the relative concentrations of dispersant and binder to the alumina content.
Section snippets
Materials
Alumina powder is supplied by Alteo, France, under the reference P172LSB. Specific surface area is measured at 8.6 m2/g (BET method) and the mean diameter at 0.4 μm (laser diffraction method). According to the supplier, the alumina content is 99.8%.
The dispersant used is an ammonium polymethacrylate (Darvan C-N, Vanderbilt) with molar mass of 10–16 kg/mol.
Three organic additives of different molecular weight are used: polyethylene glycol (from 4 to 100 kg/mol, named PEG-1 to PEG-3),
Rheological behaviour of the suspensions for the granulation process
Ceramic suspensions with organic additives exhibit generally a non-Newtonian behaviour which can be described by the Herschel-Bulkley law (Eq. 1).Where τ is the shear stress (Pa), the shear rate (s−1), τ0 the yield stress (Pa), n the flow index and K a constant. The yield stress is the minimal shear stress required to initiate flow. The flow index represents the deviation from a Newtonian behavior: if n < 1, the suspension is shear thinning and if n > 1, the suspension is shear
Conclusions
This comparative study of spray drying and freeze granulation was conducted for the preparation of mixed granules of submicronic alumina with a high content of polymeric binder. First, the variation of the viscosity of the ceramic suspensions filled with a high molecular weight polymer or with a dispersed rosin ester was studied. The polymer content has a very large impact on the viscosity but not on the flow index. In comparison, the use of rosin ester leads to very low viscosities, even at
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank Safran Ceramics for the financial support (and in particular Gautier Mécuson) and Jérôme Kiennemann from Alteo (France) for the kindly supplying of alumina powder used in this research. We would also like to thank François Louvet, Associate Professor at the University of Limoges, for the technical expertise in construction the experimental matrices for the rheology study and for the subsequent data processing.
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