Super-strong magnets are making this approach to keeping coolant clean a very attractive option.
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.
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.
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.
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.
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.
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.blog comments powered by Disqus