Study on buildup of fine weakly magnetic minerals on matrices in high gradient magnetic separation

Physicochem. Probl. Miner. Process. 53(1), 2017, 94−109 Physicochemical Problems of Mineral Processing www.minproc.pwr.wroc.pl/journal/ ISSN 1643-1049 (print) ISSN 2084-4735 (online) Received February 29, 2016; reviewed; accepted April 22, 2016 STUDY ON BUILDUP OF FINE WEAKLY MAGNETIC MINERALS ON MATRICES IN HIGH GRADIENT MAGNETIC SEPARATION Xiayu ZHENG, Yuhua WANG, Dongfang LU School of Minerals Processing & Bioengineering, Central South University, Changsha 410083,China, (X. Zheng), (Y. Wang) Abstract: Buildup of magnetic mineral particles on matrices determines the saturated deposit volume of minerals, which is of great importance in the high gradient magnetic separation (HGMS) systems. In this paper buildup of fine weakly magnetic minerals on the matrix is studied with a force equilibrium model. Elaborate rules of particle buildup on the matrix are presented. An imaginary sector ring is used to approximately quantify the volume of saturated particle buildup. The influence of the particle size, magnetic induction, fluid viscosity and velocity as well as matrix size on saturated particle buildup is investigated and discussed. With the same matrix size, the saturated buildup volume decreases with the decrease of the particle size, applied magnetic induction and increase of the fluid viscosity and velocity. The saturated buildup volume normalized by the matrix volume, and the ratio of particle deposit volume to the matrix volume (Vd/Vm) decreases with the increase of the matrix size. Under the same matrices packing fraction, the total mineral deposit volume, when adopting small size matrices, is larger than that when adopting large size matrices. Only small size matrices can be used for recovery of minerals in size of several micrometers. Based on performed analyses, the ore feeding time in a cycle for a cyclic HGMS system and the rotation speed of the swivel for a continuous HGMS system under different circumstances are also discussed. Keywords: particle buildup, force equilibrium, magnetic matrices, magnetic separation Introduction High gradient magnetic separation (HGMS) is an effective method to recover fine weakly magnetic particles from a slurry. Due to its high efficiency and eco-friendly characteristic, HGMS has been widely used in many industrial and scientific fields such as mineral processing (Zeng and Xiong, 2003; Gao and Chen, 2010; Li et al., 2011), environmental engineering (Wu et al., 2011; Merino-Martos, 2011; Nomura et al., 2012), medical engineering (Chimma et al., 2010; Ueda et al., 2014) and bioengineering (Inglis et al., 2004). The most significant and successful application of http://dx.doi.org/10.5277/ppmp170108 Study on buildup of fine weakly magnetic minerals on matrices in high gradient magnetic… 95 HGMS is the recovery of fine weakly magnetic minerals. A series of high gradient magnetic separators have been developed (Jones, 1960; Iannicelli et al., 1969; Marston and Nolan, 1975; Cibulka et al., 1985; Xiong, 1997; Ning et al., 2012; Chen et al., 2015; Zeng et al., 2015). In high gradient magnetic separation, high susceptibility magnetic matrices dehomogenize the uniform magnetic field, inducing high magnetic field gradient. The magnetic particles are captured onto the matrices surface due to large magnetic force, and then they are trapped there. The capture of magnetic particles by the matrices and their buildup on the matrices are the main theoretical mechanisms in high gradient magnetic separation and have been extensively investigated by many researchers (Watson, 1973; Luborsky and Drummond, 1976; Briss et al., 1980; Ciesla, 1996; 2004; 2006; 2007; Badescu et al., 1996; Natenapit and Sanflek, 1999; Chen et al., 2012). For the fine weakly magnetic minerals, their capture efficiency determines the recovery rate. The capture efficiency of a specific magnetic particle is calculated from the so-called capture radius (Watson, 1973). The capture radius of magnetic particle is influenced by many configuration and operation parameters and it was systematically investigated in our previous study (Zheng et al., 2015). Besides the capture of magnetic particles by the matrix, accumulation of magnetic particles on the matrix is also very important. The particle capture efficiency provides prediction of the recovery rate, but it is based on the assumption that all captured particles can be recovered. However, high gradient magnetic separation is a time-dependent process. When buildup of the particles on the matrix reaches saturation, no particle can be accumulated on the matrix. Under this circumstance, the magnetic particles will not be further recovered if there isn’t any operation performed. Therefore, the investigation of saturated buildup of the magnetic particles on the matrix is also important. There are many configuration and operation parameters which influence the capture efficiency of magnetic particles in HGMS. These factors include the particle radius b and density ρ, applied magnetic field strength H0, magnetic field gradient gradH, velocity of the slurry V0, dynamic viscosity of the fluid η and size of the magnetic matrix (Zheng et al., 2015). These factors also have significant influence on buildup of the magnetic particles on the matrix. Generally, the high gradient magnetic separators can be categorized into two types: cyclic and continuous types (Svoboda and Fujita, 2003). A continuous HGMS system usually includes a swivel on which the matrices are installed. For the cyclic type, saturated particle buildup determines the ore feeding time in a cycle. For the continuous type, saturated particle buildup determines the rotation speed of swivel. Therefore, it is of great significance to clarify the influence of those factors on saturated particle buildup on the matrix. Moreover, the elaborate rules of particle buildup on the matrix can provide us a better understanding of the basic principles of HGMS, which can be of great importance for either optimization of the HGMS system or development of a novel high gradient magnetic separator. In this paper, buildup of the field-dependent susceptibility magnetic 96 X. Zheng, Y. Wang, D. Lu minerals on the matrix is investigated with a force equilibrium model. Saturated buildup of the magnetic particles under different circumstances are depicted and compared quantitatively. The influence of particle size, applied magnetic field strength, viscosity of the fluid and matrix size on particle buildup is investigated, and the effects on the performance of HGMS are discussed. The force equilibrium model In high gradient magnetic separation (HGMS), there are three configurations, that is longitudinal, transversal and axial. In the longitudinal configuration, the magnetic field is parallel to the direction of the flow and both are perpendicular to the matrix axis. In the transversal configuration, the magnetic field, direction of the flow and matrix axis are mutually perpendicular. In the a (...truncated)