David R. Kelland

Magnetic separation of nanoparticles

Magnetic particles in the nanometer size range can be captured by sufficiently large magnetic forces in competition with thermal diffusion. This paper reports on the results of applying two types of magnetic separation, matrix and continuous, to particles in the nanometer size range. Rather than capture particles on a matrix, it is advantageous for some applications to employ a continuous method of separation. We used a dilute ferrofluid in both matrix and continuous flow through an axial HGMS separator in a transverse magnetic field of 0.01 T. The initial particle size distribution was designed to be 5-20 nm. but had some larger clusters. The measured size distribution of the feed material had two peaks, at 12-14 nm. and at 81-84 nm. Particle size distributions of separated fractions were obtained by dynamic laser scattering and show peaks at 12 and 47 nm. for 0.01 T and 5 and 63 nm. for 0.02 T for matrix separations and at 74, 78, and 115 nm. respectively for the continuous. The size distribution of the largest particles was extremely narrow.

High gradient magnetic separation in coal desulfurization

High Gradient Magnetic Separation (HGMS) has been successfully applied to the removal of pyrite from water slurries of raw coal in bench scale tests in several laboratories. Dry processing is more difficult but some advances have been reported. Coal-methanol slurries offer promise. HGMS has also been successful in filtering out particulate inorganic sulfur from liquefied solvent refined coal. The separation process is critically dependent on the thermomagnetic properties of the iron sulfides.

Theory and experimental verification of a model for high gradient magnetic separation.

A mathematical model of high gradient magnetic separation (HGMS) is presented, along with data on a characterized and sized feed material. The data are fitted to the model which uses elliptical co-ordinates to approximate the ribbon-like nature of the fibers. Magnetic force terms are developed for both paramagnetic and ferromagnetic particles in the vicinity of idealized matrix fibers which can either be magnetically saturated or unsaturated. The fluid flow is simulated by superimposing a boundary layer upon the solution for potential flow thus extending the range of validity to low Reynolds Numbers. Single particle trajectories are calculated in a piecewise linear manner by considering the force balance of magnetic, hydrodynamic, gravitational and inertial forces over each increment of the trajectory. By taking orientation of the fiber with respect to the field and flow direction into account, loading can be allowed for by assuming elliptical deposits. Experimental data were generated using high grade hematite prepared in ten separate size fractions. Correlation with the model is generally fairly good except for large particles where mechanical entrapment dominates the process. Considerable discussion of the results is included by analysing the physical concepts upon which the model is based. The validity of various assumptions pertinent to HGMS modeling is tested.

Diamagnetic particle capture and mineral separation

A method has been developed to collect diamagnetic particles and to separate diamagnetic components from a critical mineral resource, in this case, aluminum minerals from alunite ore. A study of matrix design for this purpose and for enhancement of the magnetic force by the use of a paramagnetic liquid has resulted in particle collection from model diamagnetic particle slurries and separation of diamagnetic ore components.

Continuous selective HGMS in the repulsive force mode

Single and double wire HGMS separators with multiple outlets were studied. The method allows continuous separation of micron-sized and even submicron particulates. It was found that a selective separation can be performed best in the repulsive force mode. Separation profiles both in the diamagnetic and paramagnetic capture modes are shown theoretically and are confirmed experimentally.

Magnetic separation and thermo-magneto-chemical properties of coal liquefaction mineral participates

Abstract Sulphide solid particulates have been successfully separated from solvent-refined coal (SRC) liquid streams by high-gradient magnetic separation (HGMS) preceded by a sulphidation pretreatment in an H2S-rich gaseous atmosphere. This pretreatment is derived from studies of the properties of dry sulphide residual liquefaction solids, treated in an atmosphere of various H2/H2S compositions. Procedures have been devised for promoting rapid conversion of the weakly magnetic hexagonal pyrrhotites to the more strongly magnetic monoclinic form at a temperature intermediate to those of the SRC dissolver and the magnetic separation stage.

HGMS coal desulfurization with microwave magnetization enhancement

The performance of high-gradient magnetic separation (HGMS) in removing mineral pyrite from coal has been improved by increasing the pyrites magnetization through selective microwave heating. Separations were carried out on conventional 2-T iron magnet separators and using a superconducting magnet capable of 15 T. A critical temperature for conversion of pyrite to ferrimagnetic monoclinic pyrrhotite was determined by vibrating-sample magnetometer measurements, also used to assess the conversion of pyrite in irradiated coal samples. Thermal analysis demonstrated that temperatures reached after heating were higher in the pyrite than in the coal. Pyrite-coal samples were separated using continuous axial separation and conventional wire-matrix HGMS. >

A review of HGMS methods of coal cleaning

High Gradient Magnetic Separation has been applied to the problem of cleaning coal in several ways. One is by direct mineral removal from coal slurried in water or oil. Similarly, attempts have been made to desulfurize dry coal. Coal liquids, or solvent refined coal, can have their sulfur and ash content reduced by the filtration or separation of undissolved well liberated minerals. The results of such efforts depend on the magnetic properties of the coal minerals. These have been studied and even altered to improve magnetic separation performance. HGMS has also been employed for indirect use in coal cleaning to recover heavy medium (magnetite) used in fine coal cleaning circuits. This paper is a review of our work on these applications and that of others over several years.

Efficient HGMS for highly magnetic materials

The range of applicability of high gradient magnetic separation has been extended to recovery of ferrimagnetic material from fluids containing an equal weight of nonmagnetic particles at solids concentrations of 20 to 35%. A newly designed matrix permits unimpeded passage of nonmagnetic particles as large as 3 mm and at the same time, recovers the ferrimagnetic particles in thin sheets. In this manner entrainment of nonmagnetic materials within flocs of ferrimagnetic particles is minimal. It is demonstrated that efficient separation of magnetite having a mean particle diameter of 12 μm from fine coal having a mean particle size of 850 μm can be attained at a throughput of 30 tons of magnetite/hr/ft2(300 metric ton/hr/m2).

Matrices for selective diamagnetic HGMS

Both the diamagnetic and paramagnetic capture forces in HGMS are calculated for infinite arrays of elliptical and circular wires, and sheet matrices. The forces for elliptical wire arrays are obtained by summation of magnetic potentials due to magnetic surface charges. Matrices for selective diamagnetic capture are studied and evidence of capture presented.