COMPRESSIBILITY AND FLOW PROPERTIES OF A COHESIVE LIMESTONE POWDER IN A MEDIUM PRESSURE RANGE

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1 COMPRESSIBILITY AND FLOW PROPERTIES OF A COHESIVE LIMESTONE POWDER IN A MEDIUM PRESSURE RANGE L. Grossmann 1, J. Tomas 1 and B. Csőke 2 1. Otto-von-Guericke-University Department of Mechanical Process Engineering Universitätsplatz 2, D-3916 Magdeurg, Germany 2. University of Miskolc Department of Mechanical Process Engineering Egyetem út 17. H-3515 Miskolc, Hungary Astract The most important design parameters for roller presses can e referred to flow characteristic of ulk materials. Usually the flow properties are measured in the low stress range 1-5 kpa at the shear rate aout 1 mm/min. But this does not fit the stressing conditions in the roller press. Press shear cell was used for shear tests with cohesive limestone powder from Eger in the so-called medium pressure range 5-1 kpa. 1. INTRODUCTION An important agglomeration process is the press agglomeration y roller press. The result of the process is fundamentally influenced y the flow properties of the powder feed [1, 2]. Roller presses can e designed using Johanson s [3] theory. The most essential design parameters of roller presses can e referred to characteristic powder properties, like stationary angle of internal friction, compressiility index and angle of wall friction. The flow ehaviour is usually tested using Jenike shear cell in the low stress range 1-5 kpa at shear rate of aout 1 mm/min and shear displacement only up to 6 mm. However the most roller presses operate with circumferential roller speeds from.1 m/s to 1 m/s at high pressure range of p > 1 MPa. Therefore, the compression and flow ehaviour of the powder has to e investigated for higher pressures, shear rates and shear displacements. Such studies are presented in this work. Equivalent to the stressing in a press agglomeration test, the powder is pre-consolidated and presheared in a shear cell. Then the resulting shear strength is measured as a function of normal stress or pressure. Consequently this happens also during compression and the agglomeration strength test for the proof of staility of granulated materials. In the chair of Mechanical Process Engineering in Magdeurg a press shear cell was uilt in order to test the flow ehaviour of cohesive powders under similar conditions like those of the roller presses. With this press shear cell the shear rate up to.42 m/s can e reached. This machine operates in the medium pressure range which is suitale to avoid too high power consumption y dissipation. 2. COMPRESSIBILITY OF COHESIVE POWDERS Compressiility and compactaility of a powder are influenced y the flow properties, and in the microscale, y the adhesion forces etween particles. Compressiility is the aility to reduce the volume under pressure and compactaility is the aility to uild a solid agglomerate under pressure with sufficient strength and staility. These properties of ulk materials can e analysed using a shear cell. And the strength of the agglomerate here the compressed powder can e determined y shear test. The compression can e descried y compression rate, compression function or specific compression work [4]. For the correlation etween ulk density or agglomerate

2 density ρ and the characteristic stress during steady-state flow or average pressure σ M,st only three material parameters from powder mechanics are used: the ulk density ρ, for a loose packing without any compaction, the isostatic tensile strength σ for the loose packing and the compressiility index n. The so-called compression rate descries a compression increment [5, 6], which includes the compressiility index n as the characteristic for volume reduction of a cohesive powder: dρ ρ = n (1) dσ σ + M, st σ M, st The physical asis of this comfortale expression was shown in previous paper [4]. The compression function descries the relationship etween the applied pressure and the agglomerate density. The compression function can e otained y integrating the compression rate Eq. (1) n ρ σ M, st = 1 + (2) ρ, σ and the mass related or specific compression work is otained y an additional integration of the reciprocal compression function Eq. (2). 1 n n σ M, st W = 1 + m, 1 (3) n 1 σ The compressiility index n lies etween n =, i.e. incompressile stiff ulk material and n = 1, i.e. gas compressiility index [4], see Fig. 1. ρ Bulk density ρ, n = 1 ideal gas compressiility index < n < 1 compressile n = incompressile σ Centre stress Fig. 1: Compression of a cohesive powder [4] The compressiility indexes of powders are summarised in the Tale 1 which are referred to a semiempirical estimation [4] for low pressure. The extension of this classification for the medium pressure range is recommendale. Tale 1.: Compressiility index of powders Index n Evaluation Examples Flowaility.1 incompressile gravel free flowing.1.5 low compressiility fine sand free flowing.5.1 compressile dry powder cohesive.1-1 very compressile moist powder very cohesive σ M,st

3 3. PRESS SHEAR CELL The tests of the flow ehaviour were carried out y means of the press shear cell, Fig. 2, uilt y Reichmann [7]. The measuring instrument consists of ring piston and the ring cell. During the test the gap is filled with powder. The ring piston is installed under the hydraulic cylinder. Using that the load or stress levels can e set. The pressure up to 5 MPa can e created in the shear cell. During the measuring process the ring cell is rotated, while the ring piston is kept from rotating using a transverse ar and the force sensors fixed on the frameworks. The drive consists of the electric motor and the transmission to reach low revolutions numer per minute, which is necessary for the measurements. Numer of revolutions of the engine and pressure of the piston are controlled directly y the computer. The input pressure and numer of revolutions are transformed y the measuring card into the appropriate signals and passed on to the hydraulic unit or the electric motor. Also the electrical signal from force sensor is processed over the measuring card and then analysed y the computer. Pressure Shear strength Piston head Ring piston Ring cell with ulk material Shear force Fs Preshear Shear Shear stress Preshear, steady-state flow End point Axial earing τc Revolution Shear displacement s σ σ2 σc σ1 Normal stress Fig. 2: Press shear cell and shear testing method The sample in the ring cell is sheared with normal force F N and with the shear force F S. Using the ring piston normal force can e set and the shear force can e measured with force sensor. Friction arises etween the wall of the ring cell and the ring piston. For this reason friction should e measured efore and after each test and the friction is to e taken off from the measured shear forces. A defined density must e reached efore each measurement of one yield locus (steady-state flow). The sample is sheared with determined vertical load F N,d until the horizontal force F S,d reaches steady-state flow and therefore constant ulk material density is achieved. This procedure is called preshear. Then the normal force is reduced to F N (F N < F N,d ), and the shear stress is measured under this new load, which is necessary for the incipient flow. This process is called shear. The procedure should e repeated with the same powder sample several times under the same preshear conditions ut different shear pressure values and in this way all other values σ = F N / A and τ = F S / A can e otained, see Fig. 2. The value of normal stress during steady-state flow σ st = F N,d / A and the shear stress during steady-state flow τ st = F S,d / A results the initial shear point, see Fig THE TEST RESULTS Tests with cohesive limestone powder from Eger (d 5 = 22 µm) were carried out with shear rates from 25 to 25 mm/min and preshear displacement from.1 to 2 m. Five yield loci were

4 measured. The preshear stress τ An as a function of the shear rate is shown in Fig. 3. Increasing normal stresses of yield loci (YL 1 to YL 5) were used as curve parameter. One should notice, that the averaged preshear stresses are hardly influenced y the shear rate in the given range. No shearthickening effect was oserved for the preshear stresses of the dry limestone powder. Preshear stress τ An in kpa YL 1 YL 2 YL 3 YL 4 YL Shear rate v in mm/min Fig. 3: Preshear stress versus shear rate 1,5 Compression rate in g/j 1,25 1,,75,5 v = 252 mm/min v = 252 mm/min,25, Centre stress during steady-state flow σ M,st in kp a Fig. 4: Compression rate versus centre stress during steady-state flow The compression rate in dependence of the centre stress σ M,st for steady-state flow are demonstrated in Fig. 4 for three different values of shear rate. All three degressive curves fit completely the model approach which was predicted from Eq. (2). The compression rate is infinite when σ M,st approaches the isostatic tensile stress in the negative stress range. This stress σ characterises the average adhesion level etween the particles. Here one may consider the largest slope of increasing ulk density to create the random packing of particles. The typical compression function of ulk density

5 is shown in Fig. 5. The compressiility index lies etween.62 and.93. Using Tale 1 the limestone powder can e classified as a compressile powder. The ulk densities of the yield loci increase slightly with the shear rate. All three nearly linear curves (exponent 1 n 1) of specific compression work fit accurately the model which was predicted from Eq. (3), Fig. 6. Oviously, the consequence for the largest shear rate is a higher ulk density and a larger compression work. 16 Bulk density in kg/m v = 252 mm/min v = 252 mm/min Centre stress during steady-state flow σ M,St in kpa Fig. 5: Compression function as ulk density versus centre stress during steady-state flow Specific compression work W m, in J/kg v = 252 mm/min v = 252 mm/min Centre stress during steady-state flow σ M,St in kpa Fig. 6: Specific compression work versus centre stress during steady-state flow

6 The frequency distriution of particle size efore and after shear tests is illustrated in the Fig. 7. Shear rates are used as the curve parameters. Comparatively high shear rates and high pressures during the shear test causes a certain grinding effect. Particles smaller then 1µm in diameter were not located due to the applied test conditions, ut after the shear test sumicron particles d < 1 µm were found. That indicates surface arasion effects y shear stressing with a specific energy input up to 7 J/g.,5 Particle size frequency distriution,4,3,2,1 efore shearing v = 252 mm/min,,1, Particle size in µm Fig. 7: Particle size frequency distriution of the limestone powder efore and after shear tests 5. CONCLUSIONS Compression and flow properties of the cohesive limestone powder in similar condition like at the roller presses were tested y means of press shear cell. The compression of limestone was descried using compression rate, compression function and specific compression work. These functions are ased on a physical approach and can e descried only y three material parameters, the ulk density ρ, for a loose packing without any compaction, the isostatic tensile strength σ for the loose packing and the compressiility index n which were otained from powder mechanics. 6. REFERENCES 1. A. W. Jenike, Storage and flow of solids, Bull. 123 University of Utah, O. Molerus, Schüttguttechnik Grundlagen und Anwendungen in der Verfahrenstechnik, Springer Verlag, Berlin, J.R. Johanson, A rolling theory for granular solids, Transactions of the ASME, pp , J. Tomas, Zur Mechanik trockener kohäsiver Schüttgüter, Schüttgut 8, pp , K. Kawakita, K-H. Lüdde, Some considerations on powder compression equations, Powder Technology 4, pp , P. R. Mort, R. Saia, D. E. Niesz, R. E. Riman, Automated generation an analysis of powder compaction diagrams, Powder Technology 72, pp , B. Reichmann, Modellierung der Filtrations- und Konsolidierungsdynamik eim Anpressen feindisperser Partikelsysteme, Diss. Universität Magdeurg, 1999