Recently, magnetic separation has experienced major technological advancements in the area of magnetic particle filtration for the metalworking industry. Increases in the strength of permanent magnets has been extraordinary, and the advent of rare-earth permanent magnets has allowed the design of high intensity magnetic circuits operating without electrical energy. Magnetic circuits designed with rare earth magnets now generate a magnetic force of an order of magnitude greater than the conventional ferrite magnets used only a few years ago.
The significance of the technological advances is that they focus specifically on the magnetic collection of fine particles, resulting in highly effective magnetic filters for the collection of micron-sized ferromagnetic and paramagnetic particles. These filters collect not only metal chips, but also harmful contaminants such as rust and fine iron of abrasion continually eroding from machine components, pipe lines, chutes, bins and process equipment.
The magnetic collection of fine ferrous particles requires a high intensity, high gradient magnetic field. Some basic information about magnets is necessary in order to fully understand magnetic particle filtration.
There are three basic types of magnets used in industrial applications: Alnico, standard ceramic and rare earth.
- Alnico. Alnico is one of the earliest magnet materials used in the magnetic separator industry. Most commonly recognized by the horseshoe shape that was often used in the early years, this material is a casting of aluminum, nickel, cobalt and iron. Once made, it cannot be easily machined or drilled because of its hardness. Today, it is used in applications with high temperature atmospheres (more than 250° Fahrenheit). Alnico is comparable in strength to ceramic materials and is used to remove relatively large pieces of ferrous metals, such as nuts, bolts and staples.
- Ceramic. In the late 1960s, the price of cobalt began to rise and manufacturers began looking for a substitute for Alnico. At this time, manufacturers considered using a ceramic material in their magnetic separators, which became the standard until the early 1980s. Ceramic was easy to work with because it could be cut in any direction, assembled into a circuit and then charged as a complete unit. Its strength in the tube magnets is comparable to Alnico castings, and its only basic limitation was that it had a greater reversible loss at high temperatures. Ceramic-type magnet material is best when the goal is to remove relatively large pieces of ferrous metals because it has minimal holding strength on small particles, which, even if captured, would be washed off by the product flow in a short period of time.
- Rare Earth. Originally thought to be rare, the metallic elements with an atomic number between 57 and 71 are classified as "rare earth." Because samarium cobalt (number 62 on the periodic table) was the first material used to make these magnets, they were called "rare earth." Samarium cobalt could produce more than 4100 surface gauss in a tube magnet circuit compared to 1,000 and 2,000 gauss for standard Alnico and ceramic. (Gauss is a unit measuring electromagnetic force.)
The newest generation of rare earth magnets consists of neodymium iron boron. (Neodymium is number 60 on the periodic table.) The first neodymium iron boron magnet that came to market developed a surface gauss of approximately 4800 in a tube-type circuit. Strength levels have been increasing over the past ten years and are now producing surface gauss of more than 10,000 in a tube-type circuit. While magnet manufacturers make many different strengths of rare earth separators, they are generally seven to ten times stronger than ceramic magnets.
The required background magnetic field for effective particle collection is typically determined through an identification of the magnetic contaminant or by quantitative testing. Some general guidelines for magnetic field requirements are shown in Table I.
|1500 Gauss||Relatively coarse (+50 micron) ferromagnetic iron of abrasion.|
|2500 Gauss||Fine (-50 micron) ferromagnetic iron of abrasion or scale.|
|5,000 Gauss||Very fine (submicron) ferromagnetic iron of abrasion or scale, or paramagnetic contaminants such as iron-bearing minerals or nickel and cobalt compounds.|
|10,000 Gauss||Fine paramagnetic contaminants.|
The evolution of permanent magnets provides a cost effective alternative for generating high intensity magnetic fields. Specifically, in recent years, the strength of permanent magnets has increased several fold with neodymium-boron-iron rare earth magnets now leading the way.
The next factor to be considered when discussing magnetic particle filtration for the metalworking industry is coolant. Coolant manufacturers continue to increase coolant life and improve coolant performance. There are now many types to choose from, including synthetics or semi-synthetics and water- or oil-based.
Disposal of used machining coolant has become its own concern. When machining heavy metal alloys with lead, chromium, nickel or other toxic heavy metals as an ingredient, the coolant may be considered hazardous waste depending on local EPA regulations. That is why the less material there is to dispose of, the better. And one way to reduce disposal of machining coolant is to clean and recycle it.
Recycling means that the coolant has to be cleaned and reused. Cleaning includes removing any ferrous materials that get into the coolant.
When subjected to a magnetic field, all particles will respond in a particular manner and can be classified as one of three groups: ferromagnetic, paramagnetic or diamagnetic. Materials that have a very high magnetic susceptibility and are strongly induced by a magnetic field, such as iron, are termed ferromagnetic. Iron, nickel and cobalt are all ferromagnetic elements. Materials that have a low magnetic susceptibility and a weak response to a magnetic field are termed paramagnetic. Many ferrous alloys like stainless steel or several varieties of iron-bearing minerals are classified paramagnetic. Materials with a negative magnetic susceptibility are termed diamagnetic and for all practical purposes are non-magnetic.
Ferromagnetic, and to a lesser extent paramagnetic, materials will become magnetized when placed in a magnetic field. The amount of magnetization induced on the particle depends on the mass and magnetic susceptibility of the particle and the intensity of the applied magnetic field.
Different Methods Of Filtration
Effective magnetic particle filtration can be achieved in several ways. In order to choose the appropriate method for a specific application, several factors should be taken into account including the level of filtration, types of materials that have to be removed, and the type of coolant being cleaned.
Traps. Magnetic pipeline traps use high energy, rare-earth permanent magnets to collect ferrous contaminants in fluid lines. The "in-line" traps contain a series of magnetic tubes that are generally one inch in diameter and extend down through the entire diameter of the pipe to provide complete contact with the fluid. The tubes house a circuit of rare-earth permanent magnets alternating with steel slugs. The steel slugs concentrate the magnetic flux, producing regions of very high magnetic gradient. Because of this high gradient, ferrous contaminant will collect at the point of the magnetic tube where a steel slug is situated. The thickness of the rare earth magnets and steel slugs are just at the point of magnetic saturation, utilizing the full potential of the rare-earth permanent magnets.
Traditionally, magnetic traps have been used in industries other than metalworking. Installed directly in front of the coolant pump, magnetic traps can be used in the central coolant system as a final clean-up. One major benefit of the magnetic traps is that they work well with both water-based and oil-based coolant.
Housed inside the traps, magnetic tubes generate a peak magnetic field in excess of 9,000 gauss. Rare earth traps are effective in removing both fine ferrous contaminants such as iron of abrasion as well as tramp metal such as nuts, bolts or wire. The effective capacity through a magnetic trap is dependent on the viscosity of the fluid.
Chip And Parts Conveyors. Magnetic chip and parts conveyors can provide a low-maintenance method of moving and elevating ferrous chips, turnings, small parts and stampings. Powerful permanent magnetics inside the conveyor housing slide the material along the surface, and there are no external moving parts to jam or break. A wide variety of conveyor configurations, in many sizes, widths and magnetic strengths, are available for hard-to-reach areas under presses, milling machines and other machine tools.
Specific to the metalworking industry, chip and parts conveyors can provide assistance to those who have problems separating chips and coolant. Typically, a hinged steel belt conveyor allows coolant to flow through with the chips staying on top of the conveyor and then being carried out of the machine.
Ceramic Coolant Cleaners. Permanent magnetic coolant cleaners remove grinding swarf and turning fines, helping machine tools run longer and maintain accuracy, with lower costs per unit produced. Indexing units, with either a smooth-faced or extended-pole roll, are used when both ferrous and nonferrous contaminants are present.
Coolant cleaners are designed for use with surface grinders, gear grinders, honing and lapping machines, broaches, milling and drilling machines, face grinders, oil-reclaiming machines and wherever clean coolant is needed.
- Rare-Earth Coolant Cleaners. A coolant cleaner, whose power source is a high quality rare-earth permanent magnetic material, develops magnetic fields many times stronger than conventional ceramic or Alnico magnets. There is no increase in size and the additional strength improves removal of very fine iron particles from a wide range of coolants and other liquids.
The rare-earth coolant cleaner manufactured by Eriez, for example, is a drum-type magnetic separator of this kind. It works the same way as a ceramic coolant cleaner except that it utilizes a rare earth magnet that maximizes magnetic field strength. The resulting high field strength removes particles down to 3 microns.
In operation, liquid contaminated with fine ferrous particles enters the sump area and flows past a counter-rotating magnetic drum. Particles attracted to the drum are held tight and lifted to the top, where a mechanical discharge mechanism moves them to a discharge chute. Cleaner liquid is discharged from the bottom of the separator.
Fine magnetic particles suspended in the dirty liquid tend to flocculate when introduced to the strong magnetic field, thus increasing separation efficiency. Accordingly, this coolant cleaner is effective even on moderate, or paramagnetic particles.
The efficiency of magnetic separators, such as this, depends on the magnetic susceptibility and concentration of the contaminants, as well as the viscosity of the liquid. The rare-earth coolant cleaner helps machine tools run longer and maintain accuracy by removing grinding swarf.
Magnetic-Cyclone Filters. When machining both ferrous and nonferrous materials on the same grinding machine, an additional level of filtration is necessary. For example, with a water-based coolant, the Eriez magnetic-cyclone filter removes particles down to 5 microns. This kind of filter is especially effective at handling the high stock removal rates of through-feed, centerless, surface and small double disc-type grinders. It also removes steel wool from the coolant used by ID, abrasive belt and free abrasive grinders.
Dirty coolant from the machine tool enters a reservoir and flows through a magnetic coolant cleaner. Ferrous particles, the bulk of the contaminant, are collected on the magnetic roll and discharged into a swarf bucket. The coolant and nonferrous contaminants fall into a tank below.
The filter then pumps the almost-clean coolant to a manifold, which evenly distributes it to the second stage filter, the hydrocyclone. The filtered particles, along with a small amount of coolant, are discharged from the nozzle and fall into a portable swarf tank. The clean coolant leaves the hydrocyclone through the top and is sprayed into the clean coolant tank, where the clean coolant pump returns it to the machine tool, completing the cycle.
Properly cleaning and filtering coolant will lead to longer lasting machine tools, reduced production time and lower coolant disposal costs. In addition, removing metal chips dramatically reduces expensive damage to machine tools while it increases productivity. The right method of magnetic particle filtration will make an impact on any metalworking facility.
About the authors: Dan Norrgran is manager, Minerals & Materials Processing, and John Mackowski is manager, Magnamation, at Eriez Magnetics, Erie, Pennsylvania.