The process of magnetic separation is based on the differences among magnetic susceptibilities of various mineral species. Performed either wet or dry, the separation has application in both the recovery and concentration of value minerals, and in removing deleterious mineral constituents from a product stream. There are two major categories to be considered when addressing magnetic separation of minerals.

Particle Characterization

A magnetic field can be described as lines of magnetic flux generated by a circuit configuration. When subjected to a magnetic field, all minerals will respond in a particular manner and can be classified as one of the three groups as follows: Diamagnetic or non-magnetic, Paramagnetic or weakly magnetic and Ferromagnetic or strongly magnetic. Although diamagnetic minerals have a negative magnetic susceptibiliy and are actually repelled from the lines of magnetic flux, the force is very weak and is considered negligible for any practical industrial application. Diamagnetic minerals are commonly termed non-magnetic. Two common diamagnetic cations are Si and Al and correspond to a host of diamagnetic minerals such as quartz, feldspar and kyanite to name a few. Conversely, paramagnetic minerals have a positive magnetic susceptiblity and are attracted to the lines of magnetic flux. T

Separator Design

In the design of the magnetic separator, the strength and the gradient of the magnetic field are two first order variables that affect mineral response. The strength of the magnetic field or the field intensity refers to the number of lines of flux passing through a unit area. Lines of flux are measured in Gauss (1 line/square cm) or Tesla (10,000 Gauss). Regions of high magnetic field strength have a greater number of flux lines. The second variable is the magnetic field gradient or the rate of change of the magnetic field intensity.

The most common method of producing a high gradient is to drive the lines of flux from a flat pole piece of relatively large surface area to a narrowed opposing pole piece. This necessitates the convergence of the flux lines to the closest conducting surface. An alternative method of producing a high gradient is to laminate the conducting surface with alternating magnetic and non-magnetic material.

In very simplified terms, the field intensity holds the particle while the field gradient moves the particle. From the earlier equation for magnetization, the magnetic attractive force acting on a particle is the product of the particle magnetization and the magnetic field gradient and can be expressed as follows:

Fm = mxH(dH/dx) or M(dH/dx)

where Fm is magnetic attractive force, and (dH/dx), is the magnetic field gradient.

Types of Separators

High Intensity Magnetic Separators are designed for either wet or dry applications. Dry separators typically consist of a magnetized rotor in which the magnetic force is opposed by centrifugal and gravitational forces. The magnetized rotor is usually grooved or laminated to produce a high gradient. Separations occur when the paramagnetic particles are deflected from the diamagnetic particle stream.

Wet High Intensity Magnetic Separators incorporate a matrix which is magnetized by an externally produced magnetic field. The matrix amplifies the applied field and produces very high gradients. The feed slurry flows through the matrix which captures the paramagnetic particles while the diamagnetic particles are washed through. Two basic types of Wet High Intensity Magnetic Separators are available for commercial use.

The Wet High Intensity Magnetic Separator (WHIMS) consists of an annular ring or carrousel which contains the matrix and rotates through the open magnetized gap of the “C” frame circuit. Feed is continuously delivered to the matrix while it is positioned in the magnetized field. The paramagnetic particles are captured while the diamagnetic particles pass through. As this particular section of the matrix is rotated out of the magnetic field, the paramagnetics are rinsed out and separately collected. This type of separator is capable of producing up to 15,000 gauss in the open air gap and substantially higher fields within the matrix. The irregular shape of the matrix material generates a very high gradient.

Detailed Description and Mineral Response

The Induced Magnetic Roll (IMR) has been in existence for many years and has become a staple in industrial mineral purification. The unit derives its name from the magnetism that is induced on a roll positioned between two poles of a magnetic circuit. These rolls are energized by induction from a stationary electromagnet. The poles of the magnet are in close proximity to the rolls and magnetic flux converges on the edges of the magnetic laminations producing a high gradient field.

Material to be treated is fed in a thin layer across the surface of the first roll. As the roll revolves, the material passes through the narrow gap between the pole of the magnet and the roll. The non-magnetic particles are discharged from the roll in their natural trajectory while the magnetic particles are attracted to the roll and are deflected from the non-magnetic stream. A splitter arrangement is used to separate the streams. The non-magnetic particles from the first stage separation pass to a lower roll where the process is repeated in a cleaner stage.

The magnetically induced rolls are 5 inches in diameter and up to a meter long. Roll speed is variable with approximately 100 rpm being average for glass sand operations. The gap between the pole and the roll dictates the strength of the applied magnetic field. The narrower the gap, the stronger the magnetic field. A gap setting of 3/16 of an inch typically provides a field strength of 18,000 gauss.

Applications of Roll Type Separator

When producing a magnetic concentrate of value, it may be desirable to operate the initial stage of separation at a relatively low speed and the second stage of separation at a considerably faster speed. At a low roll speed, a clean apatite product will be thrown from the roll with all ilmenite containing particles reporting to the magnetic fraction. Retreating the ilmenite concentrate at a high roll speed will result in the locked particles being thrown from the roll and the liberated ilmenite reporting to the magnetic fraction. This process would result in a high grade apatite and a high grade ilmenite concentrate along with a recycled middling product.

Another variation of a high intensity dry magnetic separator is the “lift type” separator. The most common type is the Cross Belt separator. The Cross Belt separator has application in treating a mineral stream of high value at relatively low capacity. Typical applications are the concentration of monazite, columbite or tantalite ores where these values represent only a minor constituent of the feed material. An advantageous feature of this separator is that it provides very high extraction while producing a clean magnetic concentrate.

By employing multiple magnets in sequence, the feed material can be subjected to several successive magnetic fields of progressively increased intensity. In this manner, feed materials such as complex ores can be selectively separated. The first pole of a multi-pole separator would initially remove the most highly magnetic fraction.

High Intensity Wet Magnetic Separation

Wet High Intensity Magnetic Separation (WHIMS) and High Gradient Magnetic Separation (HGMS) are essentially fine particle separation techniques utilizing high intensity electromagnets and a flux converging matrix to generate regions of high field strengths and magnetic gradient. Technically WHIMS is distinquished from HGMS by the direction of slurry flow being perpendicular to the lines of magnetic flux (Ex. a “C” circuit) rather than parallel (Ex. canister filter).

For a typical application, the characteristic matrix size, (in this case ball diameter), and magnetization are established by matrix selection leaving flow velocity and applied field as major independent variables. In most WHIMS designs, flow is by gravity and cannot be directly controlled while HGMS typically operates with the feed slurry pumped to the canister at a predetermined rate which can be advantageous.

The largest particle size also effects the choice of matrix since particles must be able to physically pass through the matrix without becoming trapped. Slurry viscosity is influenced both by pulp density and surface chemistry and is particularly important for WHIMS as the volumetric flow is often a prime factor effecting the size of separator required. The importance of surface chemistry on magnetic separation of U3O8 using WHIMS is discussed in a recent paper by Svoboda.

The most important feature of the carrousel-type WHIMS is the ability to process large quantities of magnetics in a continuous operating mode. Typical feed rates range from less than 1 up to 120 tons per hour with magnetic concentrations from parts per million levels to over 99%. Compared to dry methods, WHIMS has the advantage of higher efficiency of separation for fine particles and typically produces low percentages of misplaced non-magnetics (i.e. high grade magnetic concentrate).

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