Specific energy consumption of cake dewatering with vacuum filters Manu Huttunen a,⇑ , Lauri Nygren a , Teemu Kinnarinen b , Antti Häkkinen b , Tuomo Lindh a , Jero Ahola a , Vesa Karvonen c a LUT School of Energy Systems, Lappeenranta University of Technology, 53850 Lappeenranta, Finland b LUT School of Engineering Science, Lappeenranta University of Technology, 53850 Lappeenranta, Finland c LUT School of Business and Management, Lappeenranta University of Technology, 53850 Lappeenranta, Finland article info Article history: Received 15 July 2016 Revised 27 October 2016 Accepted 29 October 2016 Keywords: Specific energy consumption Specific power demand Cake dewatering Cake drying Vacuum filtration abstract The energy consumption of vacuum filtration operations in cake filtration depends on the properties of the cake, the filtration conditions applied, and the progress of the cake dewatering process. Operating a vacuum filter at a high pressure difference requires a high air flow rate and thus has high energy con- sumption. By taking the filtered solids content into consideration together with the power demand and energy consumed at a certain pressure difference level, it is possible to investigate the specific power demand and energy consumption relative to the filtered cake solids content. When the mother liquor in the void space of the filter cake is replaced by air, the flow rate of air through the cake increases, which has a dramatic influence on the specific energy consumption. In this study, dewatering of calcite mine tailings is investigated with respect to the specific power demand and energy consumption of vacuum generation calculated using the assumption of an ideal isentropic process. The results of this study demonstrate clearly that both the air flow rate and the specific energy consumption in dewatering increase sharply after a certain solid content of the cake is reached. The results suggest that pumping costs in vacuum filtration can be reduced substantially by allowing a slight increase in the residual mois- ture content of the filter cake. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Vacuum filtration is a unit operation used in the dewatering of various mineral slurries such as calcium carbonates, phosphates and sulphates. Continuous vacuum belt filters are typically applied when the solids to be separated do not contain much fines, settle rapidly, and form a permeable cake that can be dewatered at a moderate pressure difference, or when the cake has to be (counter- current) washed in the filter unit (Sparks, 2012a; Svarovsky, 2000; Tarleton and Wakeman, 2007). Vacuum filtration with belt filters is an energy intensive operation, partially due to the large volumes of air flowing through the pores and cracks of the cake, as described by Ripperger et al. (2012), and because of the leakage of air into the vacuum box, e.g., near the edges of the filter medium. A simple way to simulate the operation of horizontal vacuum belt filters in a laboratory is to use a Büchner test apparatus that consists of a cylindrical slurry basin, on the bottom of which the fil- ter medium is installed. The test unit is connected to a vacuum source and experiments are then performed batchwise under various conditions. In cases when the filter cake is not washed, the total filtration time is divided into two periods, namely, sepa- ration and drying, the latter of which is often referred to as dewa- tering. During the separation period, the filter cake is formed and liquid is filtered through the cake until the solid particles form a rigid structure. The height of the cake remains constant, provided that the material is incompressible – a condition that is often assumed in dewatering calculations and modelling (Condie et al., 2000), although not exactly true for real mineral slurries. The sep- aration period can be described mathematically using cake filtra- tion equations derived from Darcy’s law (Darcy, 1856). Several properties of the slurry and solids, such as particle size distribu- tion, particle shape, solid concentration and surface charge of the suspended solids, have an effect on the cake formation, and thus also on the flow of liquid through the cake (Besra et al., 2000; Fan et al., 2015; Sorrentino, 2002; Wakeman, 2007). Darcy’s law and filtration equations derived from it don’t take into account all these properties, thus the results of these equations can only be considered as indicative. The purpose of the drying period is to remove liquid from the pores of the cake until the target moisture content of the cake is achieved. Removal of pore liquid opens flow channels for air to flow through the cake, which increases the air flow rate (as http://dx.doi.org/10.1016/j.mineng.2016.10.025 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: manu.huttunen@lut.fi (M. Huttunen). Minerals Engineering 100 (2017) 144–154 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng