Solar Thermal Treatment of Non-Metallic Minerals: Powder Flowability As Critical Application Parameter

SOLPART intends to decompose non-metallic minerals by concentrated solar irradiance of a fluidized bed reactor. Two different types of particles will be treated, i.e. free-flowing and cohesive particles. This is already evident from the particle size, when the Sauter mean particle size is considered, being ≤ 10 µm for cement raw meal (CRM), 50-130 µm for limestone and dolomite, and < 350 µm for milled phosphate and other ores. The combination of mean particle size and density of most of the minerals and their products (e.g. CaO from CaCO₃) classifies them as Geldart A or B-type powders, easily handled, stored, discharged and conveyed. CRM is even visually seen as highly cohesive. This cohesiveness was hence examined in detail at temperatures between 20 and 850 °C according to various criteria, being the Geldart classification: C/A; the Hausner ratio = ρ_tapped/ρ_loose; the Yield limit: consolidation vs. shear; the Angle of Repose (AoR); and the balance of forces. A Hausner ratio in excess of 1.25 is characteristic for a cohesive powder. Results clearly illustrate the difference between CRM and 58 μm limestone, with values for CRM ranging from1.25 to 1.38, against 1.05 for limestone. Bulk densities are moreover slightly higher at higher temperature, meaning that the powder is more consolidated, with typical values of 1191 kg/m³ (850°C) and 1078 kg/m³ (20°C) for CRM, and 1560 kg/m³ (850°C) against 1492 kg/m³ (20°C) for 58 µm limestone.The Angle of Repose (Geldart et al.) provides additional information and was determined in an electrical furnace by pouring the powders on a flat plate and measuring the angles of the slopes. Criterion of assessment are: AoR < 30 (flowable); AoR 30 - 45 (some cohesiveness); AoR 45 - 55 (true cohesiveness) and AoR > 55 (high cohesiveness, limited flowability). Results are again confirming the cohesiveness of CRM*,with AoR increasing from62-65° at 20°C, to 70-72° at ~850°C. Limestone values vary from 27° at 20°C to 31° at ~800°C.Initially developed by Smolders and Baeyens¹, a total force balance for the powder behaviour can be established. Whereas the CRM powder will be characterized by very strong Van der Waals cohesive forces, coarse particles such as limestone-lime will show no cohesiveness. The experimental work on the Yield limit (Jenike criteria²) assesses the effect of the consolidation on the incipient flow. The flow factor ratio ff_c = σ₁/σ_c characterizes the flowability of bulk solids, where greater values of the ratio represent a greater flowability of the powder. σ₁ is the normal consolidation stress, whereas σ_c is the stress at failure. The numerical classification is the following [Schulze³]: ff_c < 1 (non-flowing); 1 < ff_c < 2 (very cohesive); 2 < ff_c < 4 (cohesive); 4 < ff_c < 10 (easy-flowing); and 10 > ff_c (free-flowing). Results obtained in a shear cell at different temperatures will be illustrated. With a ff_c ≤ 2, CRM is considered very cohesive. Limestone and dolomite have a ff_c > 10, and are freely flowable. It can hence be concluded that Limestone/dolomite and other A/B-type minerals pose no problem, since perfectly flowable, whereas Cement raw meal is cohesive to very cohesive according to all criteria. Shear is needed (air flow) to reduce the cohesive Van der Waals forces, and this explains the use of a lean phase pre-calciner and aerated hopper design in CRM storage silos.

Acknowledgements: “This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654663, SOLPART project."

References

(1) K. Smolders K, J. Baeyens. A charaterisation of cohesive (C) and free-flowing (A) powders. Powder Handling and Processing 2005， 17(4), 196-199.

(2) W. Jenike, Storage and flow of solids, Bulletin n° 123 Utah Engineering Experiment Station, 11th edition, 1986.

(3) D. Schulze, in Powder technology and pharmaceutical processes, Handbook of powder technology, volume 9, Elsevier, 1994