Separation

Management of the Metal Removal Fluid Environment

Separation

Table of Contents	Separation is one of the two ways in which metal removal fluid is clarified. Often it is the first of two steps, the second step being filtration. Separation is the process of removing particles through settling out, magnetic attraction, centrifugal force, etc., rather than a mechanical barrier.
Getting Started
Glossary
Search
Information&Links

How is separation of chips and swarf performed?

There are four major methods of separation:

settling

magnetic separation

cyclonic separation

dissolved air separation.

Each of these has its advantages and disadvantages, as summarized in the table below.

What is settling?

A settling tank is a very effective method of allowing the larger, heavier chips to settle out through gravity. Systems are usually sized so that chips can settle for 3-5 minutes. The chips that accumulate in the settling tank are carried up a ramp by a drag conveyor for either automatic or manual chip handling. A settling tank is usually used prior to the primary filter to reduce the chip load and medium usage.

The advantage of this method is that there are no media to be replaced or disposed of.

The disadvantages are that

the method is not true filtration, since there is no septum to trap and remove particles—clarity depends on retention time and the weight of the chips.

it is not effective on aluminum chips or grinding swarf, which tend to float or stay in suspension.

What is magnetic separation?

A number of types of magnetic separators are available. The larger units usually have magnetic plates or bars that collect iron-containing particles as the MRF flows over. The fines are then scraped off and removed by a drag or screw conveyor.

The advantage of this method is the same as in settling: there are no media to replace or dispose of.

The disadvantages are

the method will not remove non-magnetic materials or grinding swarf

it requires low flow rate, which increases floor area required.

What is cyclonic separation?

In this method, dirty MRF is pumped into a cone-shaped, vertically mounted vessel. The liquid is directed into the cyclone so it spins at high velocity around the cone wall. The solids (contaminant) are thrown outward by centrifugal force and downward by back pressure. The contaminant is discharged through an underflow opening back into the dirty tank, while the clean MRF follows a vortex column in the center and is discharged through an overflow opening into the clean tank.

The advantages of the method are

there are no moving parts in the unit

there are no media to be replaced.

The disadvantages are that

the flow rate is limited, requiring many cyclones that require extensive piping and valving

high maintenance is required to keep underflow openings unplugged

the method will not produce as consistent quality MRF as most filtration methods

it requires 2 sets of pumps; one pumps dirty MRF through the cyclones to the clean tank, while the second pumps clean MRF from the clean tank to the system.

What is dissolved air separation?

Plant compressed air is injected into the MRF, forming bubbles. The bubbles provide a large surface area in which the surface tension of the working fluid attracts the contaminants. The contaminants and bubbles form a foam on the MRF surface. A wiper blade then skims the foam from the surface into a disposal container.

The advantages of the method are

it removes tramp oil without stripping MRF from water

there are no media or pre-coat to dispose of

it works on fine particles.

The disadvantages are

the large footprint-to-capacity ratio (6.5' x 6.5' of floor space for 25 GPM)

it increases the usage of plant compressed air.

The following table sums up the advantages and disadvantages of each method:

Method Advantages Disadvantages

Settling - no media to replace or dispose of
       - not true filtration
       - not effective on aluminum

Magnetic separation - no media to replace or dispose of
- will not remove non-magnetic
     materials
- requires low flow rate, so more
     floor area needed

Cyclonic separation
   - no moving parts in unit
    - no media to replace or dispose of
- limited flow rate means much
     piping and valving
   - high maintenance
   - quality not as consistent as
      other methods
   - 2 sets of pumps needed

Dissolved air separation
    - removes tramp oil without
        stripping MRF from water
     - no media to dispose of
     - works on fine particles

    - large footprint-to-capacity ratio
    - increases usage of plant
      compressed air

Prepared by AWARE Services