Slovak Society of Chemical Engineering Institute of Chemical and Environmental Engineering Slovak University of Technology in Bratislava PROCEEDINGS 41 st International Conference of Slovak Society of Chemical Engineering Hotel Hutník Tatranské Matliare, Slovakia May 26 3, 214 Editor: prof. Jozef Markoš ISBN: 978-8-89475-13-1, EAN: 978889475131 Stankovič, M., Peciar, M., Peciar, P.: Extrusion of powder materials in a paste form, Editor: Markoš, J., In Proceedings of the 41st International Conference of Slovak Society of Chemical Engineering, Tatranské Matliare, Slovakia, 587 594, 214.
EXTRUSION OF POWDER MATERIALS IN A PASTE FORM M.Stankovič, M. Peciar, P. Peciar Faculty of Mechanical Engineering STU,Námestieslobody 17, 812 31Bratislava 1, Slovakia, matej.stankovic@stuba.sk KEY WORDS: extruder, shear rate, pressure ABSTRACT Pastes are matters formed from granular materials and liquid phase alternatively gas phase aswell.their processing is often made by extrusion, which is sort of agglomeration. This article deals with the pressure distribution under the blade of the radial extrusion, with the aim to discover its influence and shear rate influence on the paste flow through the dies.visualization of paste flow under the blade was made aswell. INTRODUCTION In Industrial practice we can often see extruders, which find their use not only in the powder processing in form of paste, which are all kinds of pasta but also in metallurgical,cosmetic and ceramic industry.as an example could be placed pipe fabrication or pencil graphite.extrusion is also an integral part of many Chemical and Food Technologies aswell as Environmental Technologies.Extrusion is by powder processing a sort of agglomeration, whose function is reduce the loss of the powder share into surroundings and thus last but not least increase the cost savings on the material..paste is than a form in which is the powder material processed.it s multiphase system consisting from solid phase, liquid phase and(or) gas phase. Skeleton of paste forms particulate matter, which interparticle area is filled with liquid.it is basically the mixture of liquid and solid phase, where their relative amount is in such ratio that the material could be easily formed and in the same time could retain its shape for next processing which can be drying. Typical representative of paste is mixture of clay and water or flour and water. Amount of water markedly influence the rheological behavior of paste.these are describing the behavior of paste which is many times unpredictable.paste preparation is carried out by mixing and homogenization in different types of mixing devices. The device so called extruder is used for processing the paste by means of extrusion. The main working element of extruder is the blade, piston or the screw, which generates the stress in the paste and it is subsequently pushed through the dies. Product so called extrudate, is getting its shape same as the holes in the matrix have. There are many different extruders with different construction variations. We will deal with the processing of the paste in the radial extruder. THEORY In this case the radial extruder is the device consisting from cylindrical matrix and rotational head to which is conected the blade with the arch shape. The space between the blade surface and the matrix has the shape of wedge gap. Relative motion of the blade and the matrix creates the paste flow through the dies near the blade. Paste flow pattern in such device is described on the Fig.1, where the wedge gap area could be divided into two parts. In area I paste is being compressed, particles closer together and eventual gas phase is pushed away from this interparticle area. Paste is being compressed until interparticle area is full filled with 587
liquid phase. In area II is the paste already consolidated, ieinterparticle area is full filled with water, which performs the role of lubricant. The stress ratio in this area is insufficient for the further compression of the interparticle area. The forces in area II caused by relative motion of the blade and the matrix are forcing the paste to flow through the dies. In comparison with other extrusion technology, radial extruders producing granulate of intermediate stiffness and bulk density, which can be awarded to the mechanism such the pressure is generated as well the paste flow. Comparing to piston flow where are generated much higher pressures by uniaxial compression, is in radial extruder into paste brought also shear deformation. This is intended to server for processing of unsaturated powder materials, which, when subjected to pressure are decreasing their volume and extrude the liquid from the interparticle area and thus worsen the flow properties. Fig.1 Schematic representation of paste flow in radial extruder Relations between strains and deformations in the substance study the science field rheology. By the real liquids we deal with relations between shear stress and shear rate. Similar issue is used by experiments listed in this paper. PASTE PREPARATION It is necessary to pay attention to the paste preparation because as said before liquid modifies the rheological attributes of such system. Its highly necessary to achieve the homogenous water distribution in the skeleton of particulate matter. This is achieved by a sufficiently long period of mixing particulate matter with water. In this case it was very fine milled limestone (Calmit, Baumits.r.o - Rohožník) mixed with water in the mixing device with cylindrical chamber and plough blades and as additive was used sand. Into 4kg dry limestone(more than 8% CaCo 3 ) was blended 1kg of sand(2% mass concentration) and,98822kg of water (H 2 O) which represent 16,5% of the absolute moisture. Paste properties are mostly influenced by the size and the shape of the particles as well as their amount in given matter. The parameters as shape can be classified by microscope (for very tiny particles order of micrometers Fig.2). 588
a.) b.) Fig.2Particleshape and size of milled limestone a.)limestone particle with visible outside pores, b.)limestone particles and their shapes Particle size and their amount is classified by fraction analysis, which gives us the exact vision about what are the particle size in given matter and their amount. On the Fig.3 is fraction analysis of limestone used in experiments. It is visible that this limestone contains higher number of larger particles than those smaller ones. This may result in creation of capillary channels due to a lack of gap filling of smaller particles. This fact too leads to problems with water migration in this pastes, therefore in our experiments we add to limestone a sand, therebyassuming that this will fills a gap and limits the compressibility of such systems. Fig.3 Fraction analysis milled limestone CALMIT, Baumit.s.r.o.-Rohožník TableI. Physical properties of limestone Specificavarage: d 5 = 7,5 µm Hausner ratio: HR = 1,41 Particle density: ρ S = 2762,5 kg.m -3 Geldart group: C-cohesive Bulk density: ρ N = 12 kg.m -3 External friction angle: φ w = 21,9 Angle of repose: α s = 26,7 Angle of internal friction: φ i = 28 589
EXPERIMENTAL DEVICE Measurements were made in experimental station which consists from the matrix, rotational head of extruder on which is the blade attached with hinge, from ballast tube with the revolution change possibility and electromotor. Matrix itself is manufactured from perforated sheet of 3 mm thickness and rolled into cylinder.holediameters in the matrix are 8 mm with the spacing of 12 mm with enforcement of holes as shown on the Fig.4. Fig.4Perforated sheet die geometrywith the hole enforcement. d- die diameter, t- enforcement On the outside diameter of matrix is welded the agent which serves for the pressure sensor placement. The sensor with ceramic membrane of diameter 18 mm with marktsz 13G S 4 XKQSQ was used to pressure measurement. On the Fig.5 is displayed the construction of experimental station used for measurements, which is radial extruder whilst his parameters are in the table II. Fig.5 Radial extruder. 1- pressure sensor agent, 2- matrix, 3- rotor, 4- blade d- die diameter, d r - rotor diameter, d - outside diameter of matrix, h m - matrix thickness, n- rotor revolutions, β- tilt angle of the blade. 59
Table II. Extruder parameters: Matrix outside diameter: d o = 267 mm Matrix height: h = 87 mm Matrix thickness: h m = 3 mm Rotor diameter: d r = 158 mm Die diameter : d = 5 mm Rotor revolutions: n = -25 min -1 Spacing of holes: t= 8 mm Tilt angle of blade: β= 2 EXPERIMENTAL MEASUREMENTS Paste prepared in the blender with the moister of 16,5% has been manually transported into extruder. The space between rotor and the matrix was filled up to 75% of the volume. The power drive with the rotor revolutions of 1 min -1 was activated.by the one revolution was the signal from pressure sensor recorded with the SPIDER8 measure card at the frequency of 1 Hz. It means that during 1 second, was recorded 1 pressure values. In the next measurements the experiment was repeated with the same moisture but with different rotor revolutions of 16 min -1. The same has been done with measurements with the rotor revolutions of 22 min -1. This we consider as one measurement series. For the statistics we made up to 5 series of such measurements. Visualization of paste flow has been done with same procedure, during which each of color have been arranged into layers. The placement methods of colored paste layers were different, once the layersturns around the perimeter of the matrix, another time along the radial direction. These layers have been done through the spacing the area between matrix and rotor with the very thin sheets(around,3mm), between which the paste was laid down. As the sheets were carefully removed the only thing left were colored paste layers near each other. RESULTS In the next graphs (Fig.6)are shown pressure profiles over time in wedged gap during extrusion as well as recalculated pressures under the blade on the extended length of the blade. This is due to the allocation of extrudates mass flow at a given pressure and exploring how this pressure affects the amount of extruded pasted which will be further examined. 1 8 Pressure profile over time at rotor revolution 1 min -1 1 8 Pressure profile under the blade at rotor revolutions 1 min -1 -,5 1 1,5 2 2,5 3 time[sec] 2 4 6 8 1 12 14 16 18 Extended lenght of blade[mm] 591
8 - Pressure profile over time at rotor revolutions 16 min -1,5 1 1,5 2 time[sec] Pressure profile under the blade at rotor revolutions 16 min -1 8 2 4 6 8 1 12 14 16 18 Extended lenght of blade[mm] Pressure profile over time at rotor revolutions 22min -1 Pressure profile under the blade at rotor revolutions 22min -1 8 8 -,5 1 1,5 2 time[sec] 2 4 6 8 1 12 14 16 18 Extended lenght of blade[mm] Fig.6Pressure profiles over time and under the blade at different revolutions namely in graphs. From these graphs we can plot how the change of revolution, which represents the shear rate, affects the pressure necessary for extrusion, which is the driving force of process. This dependence is shown on Fig.7.,9 Tlak p [Mpa],85,8,75,7,65,6,55 y =,65x +,596 R² =,975 priemer min max,5 1 16 22 frekvencia otáčania [min -1 ] Fig.7Dependence of extrusion pressure on the revolutions 592
As already mentioned the paste flow visualization in the device with a given geometry was carried out by colored pastes. The results of tracking the paste flow by the visual method at the one turn of rotor are on Fig.8-9. The results of experiment, before which the layers of paste were arranged around the circumference of the matrix are shown in Fig.8, at which is the wedged cropping of the paste from the extruder. At the top of Figure was the cropping bordered by the blade and at the bottom by the matrix. We can see as if under the blade (on the figure as area 1) were formed pseudo-static zone, where the paste wasn t flowing much. But in the rest area (2) it seems as if the layers of paste were sliding on each other. This assumption we tried to confirm with the next experiment, where the colored paste layers were arranged as shown in Fig.9a. The result of this was underflow of the red colored paste under the pseudo-static zone, which his visible on fig.9b. Fig.8 Visualization of paste flow 1-static zone, 2-layers flow area a.) b.) Fig.9 Visualization of paste flow a.) arrangement of colored paste layers, b.)formed underflow 593
Conclusion During measurements could occur some deviations (as for example during measurement number 2-represented by red color on the graphs).paste may not be arranged always the same and could be situations when paste was more oppressed or may arise fissure in the paste during arrangement.during experiments was observed how water is runing out through the dies. This tiny water loss in paste is responsible for worsened rheological properties of the paste (shear rate increase) and thus worsens its flow properties, which leads to increase of generated pressure. In practice it means that the longer manipulation with paste is the bigger water loss in paste occurs. Despite little uncertainties were on the measurement basis observed tendency, where we could say that with the increasing revolutions the extrusion pressure rises as well. Higher pressures are not good for such processing of pastes cause it lead to even intense water migration.from the current research it is not clearly understandable if the sand as additive increased the flow properties of such matter as the very milled limestone is. Therefore it s necessary in the next examination to track the influence of parameters such as tilt angle of blade or different mass concentrations of sand on the extrusion process. This article was created with the supportmšvvaš SR within OP Research and Development for the project: UniversityScience Park STU Bratislava (UVP STU Bratislava), ITMS 26242284, co-financed by the European Regional Development Fund and the Technical University in Bratislava under the young researcher program. References: 1.Martin, P.J.:Mechanics of Paste Flow in RadialScreenExtruders :dizertačnápráca. Cambridge :St John`s College, University of Cambridge, UK 22. 2.Peciar, M. - Fekete, R. Gužela, Š.:Procesné strojníctvo.1.vydanie.bratislava Vydavateľstvo STU, 27.197 s. ISBN 8-227-2766-2. 3.Bridgwater, J. Bengow, J.:Paste flow and extrusion. Birmingham: Calderon Press Oxford, 1993, 153 s. ISBN -19-856338-8. 594