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Permanent Magnetic Separators
The science of magnetic separation has experienced extraordinary technological advancements over the past decade. As a consequence, new applications and design concepts in magnetic separation have evolved. This has resulted in a wide variety of highly effective and efficient magnetic separator designs.
In the past, a process engineer faced with a magnetic separation project had few alternatives. Magnetic separation was typically limited and only moderately effective. Magnetic separators that utilized permanent ferrite magnets, such as drum-type separators, generated relatively low magnetic field strengths. These separators worked well collecting ferrous material but were ineffective on fine paramagnetic particles. High intensity magnetic separators that were effective in collecting fine paramagnetic particles utilized electromagnetic circuits. These separators were large, heavy, low capacity machines that typically consumed an inordinate amount of power and required frequent maintenance. New developments in permanent magnetic separation technology now provide an efficient alternative for separation of paramagnetic materials.
Technological advances in the field of magnetic separation are the result of several recent developments. First, and perhaps most important, is the ability to precisely model magnetic circuits using sophisticated multi-dimensional finite element analysis (FEA). Although FEA is not a new tool, developments in computing speed over the last decade have made this tool readily accessible to the design engineer. In this technique, a scaled design of the magnetic circuit is created and the magnetic characteristics of the individual components quantified. The FEA model is then executed to determine the magnetic field intensity and gradient. Using this procedure, changes to the magnetic circuit design can be quickly evaluated to determine the optimum separator configuration. This technique can be applied to the design of both permanent and electromagnetic circuits. As a consequence, any type of magnetic separator can be developed (or redesigned) with a high level of confidence and predictability.
Equally important has been the recent development of rare-earth permanent magnets. Advances in rare-earth magnet materials have revolutionized the field of magnetic separation. The advent of rare-earth permanent magnets in the 1980’s provided a magnetic energy product an order of magnitude greater than that of conventional ferrite magnets. Rare-earth magnetic circuits commonly exhibit a magnetic attractive force 20 to 30 times greater than that of conventional ferrite magnets. This development has provided for the design of high-intensity magnetic circuits that operate energy-free and surpass the strength and effectiveness of electromagnets.
Finally, the materials of construction used in the fabrication of magnetic separators have advanced to a point that significantly extends service life while decreasing maintenance. Advanced materials, such as fiber composites, kevlar, ultra high molecular weight polyester, and specialty steel alloys are now commonly used in contact areas of the separator. These materials are lightweight, abrasion resistant, and comparatively inexpensive resulting in significant design advantages as compared to previous construction materials.
The evolution of high strength permanent rare-earth magnets has led to the development of high-intensity separators that operate virtually energy free. The use of rare-earth magnetic separators for beneficiation of industrial minerals has become the industry standard with literally hundreds of separators placed in recent years. The following sections present an overview of the most widely used permanent magnetic separators: rare-earth drum and rare-earth roll-type separators.
Rare-Earth Magnetic Separator Types
There are three distinct types of magnetic separators using rare earth magnets, in addition to simple magnetic traps in the form of rods and grids:
1. Roll separators, see Figure 2, usually with an enveloping belt, supported by an idler roll.
2. Drum separators for dry and wet processing, see Figures 3-6, essentially similar to well-known low-intensity drum separators using ferrite magnets.
3. Matrix-type separators, such as the narrow-width vertical wheel.
Of the roll separators, there are at least fourteen manufacturers. Most of the different makes are based on the original Permroll design concept originated by this author. Various enhancements have been mainly focused on the belt tracking methods. New magnetic roll configurations and optimization of roll designs are relatively recent innovations. Additional optimization efforts are in progress.
At last count, seven manufacturers have commercially available drum separators, most based on magnet circuits derived from the use of conventional ferrite magnet. Two unique designs have been developed with one clearly offering advantages over older configurations.
Rare-earth elements have some unique properties that are used in many common applications, such as TV screens and lighters. In the 1970’s, rare-earths began to be used in a new generation of magnetic materials, that have very unique characteristics. Not only were these stronger in the sense of attraction force between a magnet and mild steel (high induction, B), the coercivity (Hc) is extremely high. This property makes the magnetization of the magnet body composed of a rare-earth element alloy very stable, i.e., it cannot easily be demagnetized.
It was a well known fact that permanent magnets positioned on both sides of a flat steel body can magnetize the steel to a high level, if the magnet poles were the same on each side, i.e., the magnets would repel each other. However, in the past, large magnet volumes were required to achieve any substantial magnetization. With the new powerful magnets, the magnet volume could be relatively small to generate high steel magnetization. In 1981 this author determined the optimum ring size for samarium-cobalt magnets. Maximum steel magnetization (near saturation) could be obtained if the rings were stacked to make a roll using a 4:1 ratio of magnet to steel thickness, see Figure 1. Since magnetized particles are attracted to the magnetized steel surface on the roll periphery, this means that 20% of the exposed roll surface would collect such material. This collection area is an order of magnitude greater than what could be achieved with prior art magnets, making the magnetic roll useful for mineral separation.
Although one of the first prototype rare-earth magnetic rolls was calculated to have about 14,000 gauss steel magnetization, it was found in comparative testing with electromagnetic induced roll (IMR) separators operating at about 21,000 gauss, that similar performance was obtained in fine particle processing (smaller than 1 mm). When processing coarser particles an improved performance was established (e.g., less weakly magnetic contaminants remaining in the upgraded product — and fewer separation passes to achieve high quality). The improvement results because the magnetic force acting on the particles is high, due to a high flux gradient. An electromagnetic induced magnetic roll separator has an air gap, which must be increased to accommodate the processing of larger particles. The rare-earth magnetic roll (REMR) magnetic separator has no such air gap. Consequently, the magnetic force does not decline in the manner of an IMR set with a large air gap.
Magnetic Separators Application in Mineral Industry
One of the oldest tools used in mineral concentrating is the magnetic separator. Virtually every mineral handling system has need for some type of magnetic equipment in two types of basic usage.
- Equipment protection through tramp iron removal.
- Mineral concentration based on.magnetic properties of the minerals contained in the ore.
Magnetic Protection Equipment
Mineral processing plants utilize a variety of material handling systems. The type of magnetic protection applied is normally matched to the system at the point that protection is required.
The types of equipment most commonly protected are:
- Size Reduction Equipment
(crushers, pulverizers, rolls and mills)
- Sizing Equipment
(screens and classifying equipment)
- Material Handling Equipment
(conveyors, bucket elevators, pneumatic systems, etc.)
As the name implies, suspended magnets are installed over conveyors to lift tramp iron out of the burden. Suspended magnets have been more frequently applied as conveyor speeds have increased. Suspended type magnets are capable of developing very deep magnetic fields and magnet suspension heights as high as 36″ are possible.
Suspended magnets are of two basic types – (1) circular and (2) rectangular. Because of cost considerations, the rectangular suspended magnet is nearly always used. Magnet selection requires careful analysis of the individual system to insure adequate tramp iron removal. Factors that must be considered include:
- Conveyor speed
- Conveyor width and loading
- Maximum lump size
- Minimum size of tramp iron to be removed
- Shape of tramp iron to be removed
- Mounting position
The position in which the magnet must be mounted will also influence the size of magnet required. The preferred position is at an angle over the head pulley of the conveyor where the load breaks open and the tramp iron is free to move easily to the magnet face. When the suspended magnet must be mounted back from the head pulley parallel to the conveyor, tramp iron removal is more difficult and a stronger magnet is required.
Magnetic Drum Separators
Magnetic drum separators come in many different styles. Tramp iron drum separators usually use a magnet design referred to as a radial type. In such a unit the magnet poles alternate across the width of the drum and are of the same polarity at any point along the drum’s circumference. The magnet assembly is held stationary by clamp bearings and the drum shell is driven around this magnet assembly.
Drum-separators lend themselves to installation in chutes or at the discharge point of bucket elevators or screen conveyors. The capacity and type of tramp iron to be removed will determine the size selection of a drum separator. They are available in both permanent and electro magnetic types.
Wet Type Magnetic Separators
Wet drum magnetic separators have been applied in several types of service including:
- Media recovery in HMS plants
- Magnetite ore concentration
- Removal of grinding iron from ball and rod mill pulps.
- Recovery of precipitated cobalt and nickel
Standard drum diameters are 30″ and 36″. General guide lines, in diameter selection, are based on (1) feed volume (2) magnetic loadings and (3) particle size. The 30″ diameter drum guide lines are roughly maximum of 75 GPM per foot feed volume, 8 TPH per foot magnetic loading and 10 mesh particle size. The 36″ guide lines are 125 GPM per foot feed volume, 15 TPH per foot magnetic loading and 3/8 inch particle size.
For many years, wet magnetic drum separator magnet rating has been on the basis of a specified gauss reading at 2″ from the drum face. The gauss reading is an average of readings taken at the centerline of each pole and the center of the magnet gap measured 2 inches from the drum surface. This rating tends to ignore edge of pole readings and readings inside of the 2 inch distance, particularly surface readings which are highly important in effective magnetic performance.
Dry Type Magnetic Separators Alternating Polarity Drum Separators
We have previously discussed dry drum separators as used for tramp iron removal. A second variety of drum separator is the alternating polarity drum separator. This separator is designed to handle feeds having a high percentage of magnetics and to obtain a clean, high grade, magnetic concentrate product. The magnet assembly is made up of a series of poles that are uniform in polarity around the drum circumference. The magnet arc conventionally covers 210 degrees. The magnet assembly is held in fixed operating position by means of clamp bearings and the cylinder is driven around this assembly.
Two styles of magnet assemblies are made up in alternating polarity design. The old Ball-Norton type design has from 8 to 10 poles in the 210° arc and develops a relatively deep magnetic field. This design can effectively handle material as coarse as 1 inch while at the same time imparting enough agitation in traversing the magnetic arc to effectively reject non-magnetic material and produce a clean magnetic concentrate product. The 30″ diameter alternating polarity drum is usually run in the 25 to 35 RPM speed range.
Application of the high intensity cross-belt is limited to material finer than 1/8 inch size with a minimum amount of minus 200 mesh material. The cost of this separator is relatively high per unit of capacity approaching $1000 per inch of feed width as compared to $200 per inch of feed width on the induced roll separator.