[0001] This invention relates to a process for separating finely divided magnetic particles
from a liquid in which they are dispersed.
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[0002] In the preparation of magnetic particles to be used for magnetic recording, for example
on disks or tapes, the particles at one time are normally dispersed in a large amount
of a liquid, such as water. For example, see U.S.-A-4,280,918. It is necessary to
separate the magnetic particles from the liquid in order to go on with the next step
of the manufacturing process. In the past, this separation step has been a very difficult
one. Filtration cannot be used, since the particles go through filters. Centrifuging
is not desirable, since it causes the particles to form lumps. Heating to evaporate
the liquid is not desirable, since it can cause chemical change in the particles.
For the most part, in the past, separation has been accomplished by allowing the particles
to settle in the liquid in the presence of a magnetic field and by then decanting
the liquid. This is a very slow and unsatisfactory process.
[0003] The invention seeks to provide an improved process for separating magnetic particles
from a liquid in which they are dispersed.
[0004] A process for separating finely divided magnetic particles from a liquid in which
the particles are dispersed, is characterised, according to the invention, by magnetically
attracting the particles to a transfer member moving through the liquid, magnetically
maintaining the particles on the transfer member while the particles are removed from
the liquid to a transfer station, and removing the particles from the transfer member
at the transfer station.
[0005] In one preferred embodiment of the present invention, the process is carried out
using a magnetic drum positioned to rotate above and below the surface of the liquid
in which the particles are dispersed. The drum rotates on an axis roughly in the same
plane as the surface of the liquid. The process may then be summarized as comprising
the steps of rotating a magnetic drum positioned so that the lower portion of the
drum is below the surface of the liquid and the upper portion of the drum is above
the surface, the drum driving a continuous transfer belt first below the surface of
the liquid where the magnetic particles stick to the belt, then above the surface
of the liquid, and then removing the solid particles from the transfer belt.
[0006] How the invention can be carried out will now be described, by way of example, with
reference to the accompanying drawing which is a schematic representation, not to
scale, of one type of apparatus which may be used to perform the process of the present
invention.
[0007] Referring to the drawing, a magnetic drum 1 is used to carry a continuous transfer
belt 2. The bottom of the drum is below the surface of a liquid containing dispersed
magnetic particles, and the top is above the surface. As the belt goes through the
liquid 3 the magnetic particles are magnetically attracted to the drum and stick to
the belt, which raises them out of the liquid, where they may be washed at a wash
station 4 and then deposited in a chamber 5 to be held in readiness for additional
processing. During normal operation the dispersion may be continuously pumped between
the apparatus and a storage tank, through an outlet not shown on the drawing.
[0008] The drum is conveniently made of stainless steel. Typically, the drum is a right
circular cylinder containing bar magnets arranged in parallel rows on its surface,
with each bar being 1 to 2 cm shorter than the cylinder. The magnets are so arranged
that the north pole end of each magnet is between the south poles of the magnets on
either side of it. The magnets may conveniently be, for example, ceramic magnets such
as cobalt rare earth magnets. The bars are usually about 0.5 to 1 cm in thickness,
and separated from each other by about 0.5 to 2 cm. When so desired, the removal of
the particles from the belt may be facilitated by using a doctor blade or roller.
[0009] Vacuum or pressurized air may be applied to help removal of filtrate or cake as desired.
Furthermore, the drum immersion depth should be adjustable to various types of slurry.
Wash liquid can be sprayed on to the drum or belt to improve the efficiency.
[0010] A preferred material for the belt is polyethylene terephthalate.
[0011] Typical magnetic particles separable by the present invention are silica coated magnetic
iron oxide particles which are needle shaped, having a length of 0.5 microns or less.
At the beginning of the process they are dispersed in water along with non-magnetic
material such as colloidal silica. The magnetic particles are usually piesent in the
liquid at about 5% by weight concentration before the process of the invention is
performed. Following the process, their concentration is up to 30% on the belt. The
particles have also been separated trom non-magnetic materials and are ready for use
in the making of disks or tapes for magnetic recording media.
1. A process for separating finely divided magnetic particles trom a liquid in which
the particles are dispersed, characterized by magnetically attracting the particles
to a transfer member (2) moving through the liquid, magnetically maintaining the particles
on the transter member (2) while the particles are removed trom the liquid to a transfer
station, and removing the particles from the transfer member at the transfer station.
2. A process as claimed in claim 1, in which the transfer member is a belt (2) driven
by a rotatable magnetic drum (1) positioned so that the lower portion of the drum
is below the surface of the liquid and the upper portion of the drum is above the
surface, the arrangement being such that the drum first drives the transfer belt below
the surface of the liquid where the magnetic particles are attracted to the belt and
then drives the belt above the surface of the liquid to the transfer station.
3. A process as claimed in claim 1 or claim 2, in which the magnetic particles are
silica coated iron oxide particles.
4. A process as claimed in any preceding claim, in which the particles are needle-shaped
and less than 0.5 microns in length.
5. A process as claimed in any preceding claim, in which the transfer member is made
of polyethylene terephthalate.
