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Magnetic particle separation: a look at the basics.

In many applications, magnetic coolant cleaners offer more filtration per dollar than other types of equipment. The most common type of magnetic coolant cleaner uses a roll that rotates continuously. These units can be furnished with either smooth-faced or extended-pole rolls and are designed for use where little or no nonmagnetic material is contaminating the coolant.

During operation, coolant is gravity fed to the reservoir where it flows under the magnetic roll. The roll's powerful magnetic field reaches out and captures particles of ferrous contamination in the fluid. The roll is rotating against the current flow, so large and small ferrous particles, clinging tightly to its surface, are carried upward and out of the liquid. Coolant drains back into the reservoir as particles are removed by a scraper blade.

Chips, grindings, and other contaminants are scraped off the drum and deposited onto a discharge tray that slopes down to a waiting receptacle. Clean coolant is drawn from the bottom of the unit and reused.

Neither a smooth-faced nor a stepped roll works best in every application. When operating at maximum capacity, cleaners with smooth poles will provide higher recovery of fines under 15 microns than models with extended poles. Models with smooth-faced rolls also provide higher throughout per unit length. But, at greatly reduced flow rates (for any given model), extended pole units will yield higher recoveries.

Indexing cleaners

When applications vary from, perhaps, finish grinding of mild steel to, say, rotary surface grinding of allow steel (which will probably be less magnetic), magnetic coolant cleaners may still be appropriate. In fact, indexing models often are used when both ferrous (magnetic) and nonferrous contaminants are present. With this type of unit, as the name implies, the magnetic roll, under which the coolant flows, indexes or rotates intermittently.

Magnetic material builds up between the magnetic roll and the housing in a mass of fine hair-like fibers. This buildup serves as filter media to trap fine, nonmagnetic particles such as grinding wheel scale. A mass of magnetically-held material builds up until it restricts the passageway between the roll and housing. This causes coolant to back up into the sump where the rising liquid level triggers a pressure-actuated switch to automatically index the magnetic roll forward. The packed filter plug moves away from the restricted passageway, permitting coolant to once again flow through the magnetic field.

The amount of incremental roll movement is adjustable within a range of 1/4" to 1 1-4" (6 to 32 mm). Frequency of movement depends on flow and feed conditions. Therefore, the cleaner operates automatically at maximum efficiency under varying conditions. As with the continuous motion unit, a scraper blade removes accumulated fines from the roll at the discharge point.

Indexing units are not recommended for coolants containing a high percentage of fines under 15 microns. Such small particles will be continuously washed off the roll and it will not index properly.

Indexing units also are not recommended for oils with a rating of 100 SSU or greater. Even in continuously rotating coolant cleaners, recovery of magnetic particles will be only poor with 100 + SSU oils.

High-volume operation

Permanent magnet drum cleaners often are used when coolant volume exceeds the practical limit of magnetic coolant cleaners. These high-volume coolant cleaners use stationary magnetic elements housed inside a rotating stainless-steel (nonmagnetic) cylindrical shell.

Most high-capacity coolant drum cleaners are used to remove fine iron solids in rolling mill wastewater. Removing these solids can reduce operating costs of filters downstream, improve quality, reduce roll damage, speed up settling pond time, and allow the user to recycle more water. In some cases, it also can provide a separate ferrous product.

Some steel-cutting users collect coolant from a number of machine tools magnetic coolant drum that removes fine iron contaminants and allows reuse of coolant. Magnetic drums also can be used to help purify effluent. One major steel company uses the drum to reduce traces of iron from wastewater, which after another stage of processing, is discharged into a nearby river, almost pure enough to drink.

Efficiency

The article "Recycling metalworking fluids--the afterthrough that pays big bucks," part five of Tooling & Production's seven part series on metalworking fluids and compounds (Oct '82, Pg 97), reported: "Magnetic separators can be effective for particles as small as 5 microns, but to have effective 5-micron removal, the coolant must pass by more slowly than for, say, 30- to 5-micron particles. Residence time, therefore, is critical to efficient operation. With proper flow rates, removal efficiencies can approximate 90 percent."

A low operating-efficiency percentage doesn't necessarily mean that a coolant cleaner isn't working well. Depending on particle size, type of material, distance from the magnetic roll, etc, a certain amount of contaminant will always get past the roll with the clean coolant.

For example, if a magnetic cleaner is fed a quantity of coolant containing five pounds of solid contaminants and the units removes all but two ounces of the solids, efficiency is 97.5 percent. If the same coolant cleaner is then fed the same quantity of coolant containing only five ounces of solids and it misses the same two ounces, efficiency drops to 60 percent. In the latter instance, if greater than 60-percent efficiency is required, a different type of separator, called a magnetic filter, might be required.

In general, the larger the contaminant particle size, the higher the efficiency of the magnetic separator. Even so, the largest particle size usually extracted with a magnetic coolant cleaner is about 1/8" square X 1/16" thick. And, although magnetic separators are capable of removing larger particles, other types of filters are usually more economical.

Also, the slower that coolant is passed through a magnetic cleaner, the greater will be the percentage of contaminant removed. High coolant velocities reduce the time particles are in proximity to the magnets. And, if the particles do reach the roll, they can be washed off its surface.

Particles under 5 microns cannot be removed with magnetic separation equipment. Recovery of fines in the 5-to 20-micron range are improved in coolant slurries containing a significant amount of coarse (20- to 75-micron) particles. Coarse sludge buildup assists infiltration of the fines.

To determine the exact efficiency for a magnetic coolant cleaner in a particular application, it is common to either purchase a unit on trial or submit about 10 gal of dirty coolant to the magnetic cleaner manufacturer for lab testing.

Most coolants contain additives that prevent oxidation or rust. If rusting is a problem, the coolant should be checked to be sure it has the correct concentrate-to-water ratio. Many times fine ferrous particles are present that could be removed by a magnetic coolant cleaner.

The pH of most coolants should be maintained between 8.0 and 9.5 for good rust protection. If rusting persists, it may indicate a decrease in pH value. This can be caused by either bacterial activity or dissolved-iron buildup. Dissolved iron oxidizes on exposure to air to form precipitates. If this is the problem, addition of a magnetic coolant cleaner will be a definite plus.

Some bacteria can be found in all water, except that which has been distilled or sterilized. The harmful level in potable water is low; however, when a miscible fluid is diluted with potable water, bacteria are introduced into the system. This microbial contamination, left uncontrolled, will cause foul odors. It also may be a factor in operator dermatitis. The natura action of a hydrocyclone will aerate coolant, thereby inhibiting bacterial growth and extending coolant life up to 20 times longer than with typical gravity settling tanks.

Magnetic-cyclone filtration

One way of accomplishing both aeration and separation is to use a magnetic coolant cleaner in conjunction with one or more hydrocyclones. A magnetic-cyclone filtration system is an efficient, economical, trouble-free way to remove particles (both magnetic and nonmagnetic down to 5 microns) from water-soluble coolants or other water-based liquids. Its operation is quite simple. Dirty coolant from the machine tool flows through a magnetic coolant cleaner (width dependent on required flow rate). Ferrous contaminants are collected on the magnetic roll and are discharged into a swarf bucket, while the semi-cleaned coolant falls into the tank below.

A filter pump then pumps the semi-cleaned coolant to a manifold that distributes it evenly to a second stage filter(s), the hydrocyclone(s). Filtered-out particles, along with a small amount of coolant, are discharged from the nozzle(s) and fall into portable swarf buckets. Clean coolant exists the hydrocyclone(s) through the top and is sprayed into a clean coolant tank. A clean coolant pump returns the clean coolant to the machine tool, completing the cycle.

A hydrocyclone filter has no moving parts and works on a simple principle. Liquid is pumped through the inlet at high velocity, causing it to whirl rapidly in the whirl chamber. From there, it flows in a continuously whirling stream down along the conical wall of the lower section. Its path has an ever-decreasing radius causing a corresponding ever-increasing velocity.

Centrifugal forces cause particles to move outward at a rate determined by their specific gravity, shape, and size. Impurities susceptible to this force are thrown outward to the wall of the conical section. They collect in the bottom and are discharged through an apex opening. As the whriling coolant stream approaches the bottom of the conical section, it is constrained to flow inward toward the center of the device, and then up and out through the outlet tube. Only a very small fraction of the total liquid exits through the apex opening with the particulate matter.

The most common applications for this type of filtration are with all types of grinders, particularly centerless, through feed, surface, and small double-disc models. In addition, it will also remove steel wool from the coolant used by ID, abrasive belt, and free abrasive grinders.

A magnetic-cyclone filtration system will remove up to 98 percent of all solids, including grinding wheel and abrasive grit, 5 microns and larger, with a specific gravity of 1.5 or higher. With the removal of particulate matter from the coolant, scratches on work surfaces caused by entrained solids are eliminated, thus lowering scrap rates and reducing the need for grinding wheel dressing. Also, the need for filter paper, which is costly to purchase and replace, is eliminated.

In choosing a magnetic coolant cleaner it's important to remember that there are a number of different types, any one of which can be best suited to a particular application.

For more information, circle 542.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Jones, Keith W.
Publication:Tooling & Production
Date:Oct 1, 1985
Words:1755
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