Paul Fears details the latest technology for the purification of dry non-metallic minerals
High-intensity magnetic separators remove ferro and para magnetic particles from dry non-metallic minerals. However, the design of the separation system dictates the final product purity and product losses. The two most common designs of dry high intensity magnetic separator are the induced roll (IMR) and the rare earth (RE) roll.
Bunting Magnetics is a leading designer and manufacturer of magnetic separators and metal detectors for the recycling, quarrying and mining industries. Its wide range of metal separation and detection equipment is manufactured at its Master Magnets facility just outside Birmingham, UK. Over several decades, the Master Magnets technical team has developed an extensive portfolio of high-intensity magnetic separators for purifying and concentrating minerals. Laboratory-sized versions of many of the designs are found in the firm’s Technical Test Facility in the UK.
High-intensity magnetic separators are used widely in the mineral processing industry to purify non-metallic minerals such as silica sand or feldspar, and in the ceramics sector to cleanse spray-dried powders and raw materials.
Selecting the optimum separation system is often determined through controlled laboratory tests. Representative samples are tested on laboratory-scale versions of the IMR and RE Roll, which confirm capacities and separation performance.
The induced Roll Magnetic Separator (IMR)
The IMR uses electromagnetically generated high-intensity magnetic fields to continuously separate small paramagnetic particles from materials with a particle size range between -2mm to 45 microns. The IMR comprises an electro-magnetically induced steel roll positioned between a bridge bar and pole piece. The peak magnetic field generated on the induced roll is 2.2 Tesla (22,000 Gauss).
In operation, a controlled rate of material is fed from a hopper or vibratory feeder onto the rotating induced magnetic roll (roll speeds vary between 80 to 120rpm). Weakly magnetic material is attracted and either deflected or held to the roll surface. The trajectory of the non-magnetic material is unhindered, discharging away from the separated material. Captured magnetic material discharges from the roll at a point of lower magnetic intensity, often aided by a brush. A splitter plate is interposed between the two product streams to enable a clean separation.
The IMR is often set up to produce a ‘middlings’ stream (i.e. very weak magnetic material mixed with non-magnetics) by the addition of a second splitter plate. It is common practice to have two induced rolls mounted in series on the same unit to enable a double pass for improved separation efficiency and process performance.
The flexibility of the IMR is popular with many mineral-processing engineers. The magnetic field strength is variable due to the adjustable electro-magnetic coil and the roll speed is adjustable. In addition, the gap between the induced roll and pole is adjustable to accommodate different feed size ranges.
One key advantage of the IMR is the ability to process hot mineral feeds (up to 80-100°C) without compromising the separation efficiency. This is unlike the RE, which uses permanent magnets. The IMR also tends to generate very little static charge on the surface, resulting in minimal carryover of fine particles into the magnetics fraction. This aids higher grades and improved recoveries.
Typical capacities for a 1m-wide unit vary based on mineral type, density and particle size distribution and are ideally determined by laboratory test trials.
Typical capacities per metre width are:
- Concentration of ilmenite sands: 4tph
- Chromite concentration: 3-5tph
- Silica sand upgrading: 2-3tph
The Rare Earth (RE) Roll Magnetic Separator
An RE separator features a high-intensity permanent neodymium iron boron magnet head pulley on a short conveyor. The magnets and interspacing steel poles are aligned to produce intense magnetic forces at points across the whole width of the pulley. The system is suited to handle materials with a particle size range between 15mm and 75 microns, although optimum separation is achieved by having a material with a narrow particle size distribution.
In operation, a thin and evenly spread layer of material is fed from a vibratory feeder onto the short centred thin conveyor belt. As the material moves towards and onto the magnetic head roll, magnetically susceptible material is attached to the high-intensity magnetic poles. The attracted material is either deflected or held by the magnetic field and falls into a collection chute underneath the head roll. Non-magnetic material is discharged forward of the roll in a normal trajectory. An adjustable splitter divides both fractions.
The magnetic strength of the permanent rare earth roll is adjusted by using different thicknesses of belt. Material trajectories are set by altering the conveyor speed using an inverter control and adjusting the splitter chutes. The spacing between the poles on the magnetic roll is set to suit different particle size ranges.
RE separators have roll diameters of 75mm, 150mm and 200mm and widths up to 1.5m. Multiple configurations of rolls enable enhanced separation. Typical applications include the removal of iron mineral contamination from silica sands, feldspar and other industrial minerals, as well as processing of granulated slag, ilmenite upgrading, beach sand processing, and recycling applications such as crushed glass. Typical capacities range from 2 and 4tph, depending on the specific application.
Magnetic separator selection
Both the IMR and RE separator produce high-intensity magnetic fields to separate ferro and para magnetic minerals. The electromagnetic IMR offers greater magnetic field flexibility and is able to handle high temperature materials. The RE separator has a permanent magnetic roll and therefore has a low power consumption. The design is also more compact, which reduces the amount of space used in a processing plant.
Ultimately, the separation performance will dictate which magnetic separator is selected for a specific application.
Paul Fears is with Bunting Magnetics