The effect of diameter and height of the inserted rod in a dense medium cyclone to suppress air core R. Sripriya a,⇑ , Nikkam Suresh b , Sanjay Chandra a , Debashish Bhattacharjee a a R&D Tata Steel, Jamshedpur, Jharkhand, India b Fuels and Minerals Engineering Department, Indian School of Mines University, Dhanbad, Jharkhand, India article info Article history: Received 31 May 2011 Accepted 2 November 2012 Available online 29 December 2012 Keywords: Dense medium separation Air core Hydrocyclones coal Ep abstract Studies were carried out by suppressing the air core in a 100 mm DMC using a metal rod inserted through the spigot. The effect of the diameter of the inserted rod and its height of insertion on the Ep of the cyclone was studied. A spigot blocking ratio of 0.4 with 60% of free VF clearance gave the best separation efficiency. Multiphase CFD studies showed that the improved performance of the cyclone was because of (i) higher tangential velocity and hence higher centrifugal force acting on the particles (ii) lower radial and axial velocities at the VF reducing misplacement. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The dense medium cyclone (DMC) is commonly used in coal washing where it effects a sharper separation than other technol- ogies, particularly for hard to clean coal in the size range of 50– 0.5 mm. This work aims to improve the efficiency of the DMC by intro- ducing a small structural modification, i.e., introducing a solid core in the centre of the cyclone to suppress air core. Earlier studies in a 100 mm Perspex cyclone with water as the medium and tracer par- ticles had shown that the recovery of tracer particles were higher when the air core was suppressed (Sripriya et al., 2007). The paper describes the tests carried out to study the effect of the diameter and the height of the rod through a 100 mm stainless steel cyclone and its effect on the separation efficiency. Multiphase CFD (compu- tational fluid dynamics) studies were carried out to understand the reasons for the improved performance of the cyclone when the air core was suppressed. 2. Experimental 2.1. Characterisation of magnetite The magnetite used for test work was commercial magnetite used in the coal washeries. The specific gravity of magnetite was 5.0 and its magnetic content 97%. The viscosity of magnetite sus- pension at a specific gravity of 1.4 as determined from the rota- tional viscometer was .003 Pa s. 2.2. Coal characterisation The coal used for the test work was 3 + 0.5 mm in size. The proximate analysis of the coal sample along with is caking index is given in Table 1a. The analysis shows that the coal was a coking coal. The size wise ash analysis of coal (Table 1b) showed that the coarser fractions had higher ash than the finer fractions. The wash- ability curves for the coal are given in Fig. 1. It can be seen that, to get a clean coal ash of 15%, the specific gravity of separation would be 1.48 and the yield 39%.The NGM at the specific gravity of sepa- ration was 32.5%. 2.3. Cyclone tests Before the start of the experiment, a spigot of 28 mm and VF of 45 mm diameter were inserted in the cyclone. Rods with different ratios of rod to spigot diameter, varying be- tween 0.4 and 0.6 were fabricated. Provisions were made to move the rod so that the rod could be placed at different heights inside 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.11.003 Abbreviations: Saq , source term; CFD, computational fluid dynamics; DMC, dense medium cyclone; Ep, Ecart probable; k, turbulent kinetic energy; m pq , mass transfer from phase p to phase q; m qp , mass transfer from phase q to phase p; n,c,k, constants; NGM, Near Gravity Material; p, q, phases; r, radius; R 2 , goodness of fit; RANS, Reynolds Averaged Navier Stokes Equation; RSM, Reynolds stress model; S 2 , co-relation co-efficient; Sg 50 , specific gravity of separation; Sg f , specific gravity of feed; u i , liquid velocity; u 0 i , velocity fluctuation; VF, vortex finder; VOF, Volume of Fluid; v t , tangential velocity of particle; a, volume fraction; e, turbulent dissipation rate; q, density. ⇑ Corresponding author. Tel.: +91 422 2456635. E-mail addresses: sripriyarajendran@yahoo.com, sripriyarajendran@gmail.com (R. Sripriya). Minerals Engineering 42 (2013) 1–8 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng