Ferrohydrostatic separation method and apparatus

The invention concerns a ferrohydrostatic separation method in which the apparent density of a ferrofluid used to separate materials according to density is controlled by a magnetic field generated by a solenoid.

BACKGROUND TO THE INVENTION 
This invention relates to a ferrohydrostatic (FHS) separation method and 
apparatus. 
As defined in the specification of U.S. Pat. No. 3,483,969, a ferrofluid is 
a material comprising a permanent, stable suspension of ferromagnetic 
material in a suitable liquid carrier. A common ferrofluid comprises fine 
particles (typically 10.sup.-9 m or less in size) of magnetite in a 
liquid. In this case, the extremely fine nature of the particles maintains 
them indefinitely in suspension without sinking or agglomerating. 
The use of a ferrofluid to separate materials of different densities, 
referred to in the art as ferrohydrostatic separation, is also known and 
is, for instance, described in the specification of U.S. Pat. No. 
3,483,969. The materials which are to be separated can be solid 
particulate materials or liquids which are immiscible with the carrier 
liquid of the ferrofluid. In essence, the separation process involves 
applying a magnetic field to the ferrofluid with a view to controlling the 
apparent density of the ferrofluid within close limits. The materials 
which are to be separated are then deposited in the ferrofluid, with the 
result that those materials which have a density exceeding the controlled 
apparent density of the ferrofluid will sink in the ferrofluid while those 
which have a density less than that of the ferrofluid will float in the 
ferrofluid. The sink and float fractions can then be recovered separately. 
In all known prior art FHS separators using ferrofluids, the required 
magnetic field is generated by means of electromagnets or permanent 
magnets with an iron yoke, with the ferrofluid situated between the pole 
tips of the magnet. This has a number of significant disadvantages which 
may be summarised as follows: 
1. In order to ensure that the FHS process operates with a well-defined cut 
point it is essential that the pole tips of the magnet be carefully 
designed to produce a constant magnetic field gradient in the working 
space between the pole tips. This can be difficult to achieve even with 
complicated mathematical models, because of the non-linear magnetic 
behaviour of iron. As a result it is generally only possible to achieve an 
approximately constant magnetic field gradient in the ferrofluid. 
2. In order to achieve a magnetic field across a suitably large volume to 
enable the FHS technique to be used for large throughputs, it is necessary 
to increase the gap between the pole tips of the magnet. This in turn 
results in an enormous and uneconomical increase in the volumes of iron 
and copper required to construct the magnet and, in general, in the 
overall size and mass of the separation apparatus. 
3. In the conventional iron yoke magnets the magnetic field strength across 
the air gap between the yoke tips is non-homogeneous. This means that only 
a central region of the air gap can usefully be employed in the FHS 
technique. 
SUMMARY OF THE INVENTION 
According to the present invention the apparent density of the ferrofluid 
used in an FHS technique is controlled by a magnetic field generated by a 
solenoid. The required constant magnetic field gradient, in a vertical 
direction, is achieved by a non-uniform solenoid winding, multiple 
windings or by varying the current density at different positions in the 
winding. 
The solenoid may, if required, be clad with an iron return frame. 
The use of a solenoid has many advantages compared to the use of an iron 
yoke electromagnet or permanent magnet, as follows: 
1. With a solenoid, it is possible to generate an equivalent magnetic field 
to that generated by an iron yoke magnet, in the same space, with a far 
more compact design which requires less iron and copper material. A 
particularly compact solenoid design is possible if the solenoid is clad 
with an iron return frame, as mentioned above. 
2. Whereas it is necessary with an iron yoke magnet to increase the air gap 
in order to achieve an increase in throughput of material which is to be 
separated, with the attendant disadvantages mentioned above, with a 
solenoid it is possible to increase the throughput merely by increasing 
the relevant transverse dimension of the solenoid, the axial length of the 
air gap remaining constant. Because the number of ampereturns required to 
generate a given magnetic field is dependent on the length of the air gap 
a solenoid can be scaled up to any required, practical size and still have 
the number of ampereturns constant. 
3. With a solenoid it is possible to design the magnetic field pattern in a 
simple and highly accurate manner. This facilitates the provision of a 
magnetic field gradient which is constant, thereby enabling close control 
to be maintained over the apparent density of the ferrofluid and 
accordingly over the cut point which is achieved in the FHS separator. As 
mentioned above, this can, for instance, be achieved by precisely 
designing the winding of the solenoid, by varying the current density at 
different positions in the winding or by using a multiple winding 
arrangement. 
4. The magnetic field across the transverse dimension of a solenoid is 
homogeneous, which means that the same, constant apparent density of 
ferrofluid can be achieved across the full transverse dimension. Thus the 
entire transverse dimension can be used for separation and the overall 
design is accordingly more efficient and compact. 
5. Because of the relatively small mass and size of a solenoid compared to 
an iron-yoke magnet capable of generating an equivalent magnetic field, it 
is possible to arrange two or more FHS separation units in to provide for 
multi-stage separation, as described below in more detail. 
Further according to the invention there is provided a method of separating 
materials of different density, the method comprising introducing the 
materials into a ferrofluid, using a solenoid about the ferrofluid to 
generate a magnetic field which controls the apparent density of the 
ferrofluid to a value between the densities of the materials, and 
separately recovering from the ferrofluid materials which sink and float 
therein. 
Still further according to the invention there is provided a 
ferrohydrostatic separation apparatus for separating materials having 
different densities, the apparatus including a separation chamber for 
accommodating a ferrofluid into which the materials can be introduced, and 
a solenoid about the chamber for generating a magnetic field to control 
the apparent density of the ferrofluid. 
Other features of the invention are set forth in the appended claims.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
FIG. 1 shows an electromagnet 10 which includes windings 12 arranged about 
the limbs 14 of an iron yoke 16 having pole tips 18. A working space 20 is 
defined between the pole tips 16. 
In a conventional FHS separation system employing a magnet 10 of this type 
a ferrofluid, typically a suspension of fine magnetite particles in stable 
suspension in a suitable liquid will be located in the working space 20 
between the pole tips and is held in place by the magnetic field generated 
by the magnet. The apparent density of the ferrofluid is controlled, to a 
desired value, by ensuring that the magnetic field gradient, in the 
vertical direction, is kept at least approximately constant. The surfaces 
22 of the pole tips must be carefully designed to ensure that a magnetic 
field gradient which is as constant as possible is generated in the 
ferrofluid. 
Materials which are to be separated into fractions of different density 
respectively greater and less than the controlled apparent density of the 
ferrofluid are introduced into the ferrofluid, with the result that the 
denser particles sink while the less dense particles float. 
The present invention proposes that the conventional iron yoke magnet be 
replaced by a solenoid. FIG. 2 diagrammatically illustrates a typical size 
comparison between the conventional magnet 10 and a solenoid 24 which is 
capable of generating an equivalent magnetic field and the windings of 
which are designated with the numeral 26. 
For purposes of comparison, the solenoid is shown in FIG. 2 with a 
horizontal axis, but it will be understood that in practice, the axis of 
the solenoid will be vertical. 
It will also be noted that in FIG. 2 the solenoid 24 is illustrated with an 
iron return frame 28 located about the windings 26. From the comparison 
diagrammatically represented in FIG. 2 it will readily be appreciated that 
an FHS separator making use of a solenoid, in accordance with the 
invention, is far less bulky and uses far less material than an FHS 
separator making use of an equivalent iron yoke magnet. It will also be 
apparent from the comparison in FIG. 2 that scaling up a solenoid based 
FHS separator, to allow for material separation in a larger working space 
20, can be achieved far more readily than in the case of the iron yoke 
magnet. 
FIGS. 3 and 4 diagrammatically illustrate an embodiment of FHS separator, 
according to the invention, which is capable of continuously separating 
materials at a high throughput rate. In this embodiment, a ferrofluid 30, 
once again typically a stable suspension of very fine magnetite particles 
in a suitable liquid, is accommodated in a separation chamber 32. 
The numeral 34 indicates a non-uniform solenoid winding which surrounds the 
chamber 32 and which is carefully designed to produce the constant 
magnetic field gradient in the ferrofluid which is required to maintain 
the apparent density of the ferrofluid at a selected value between the 
densities of the materials which are to be separated. 
The iron return frame referred to previously is omitted from FIGS. 3 and 4 
in the interests of clarity. 
As shown in FIG. 4, the separation chamber 32 is inclined relative to the 
horizontal, and the body of ferrofluid 30, held in position by the applied 
magnetic field, has a similar inclination. Feed material 35, composed of 
solid particulate materials which are to be separated from one another, is 
introduced into the ferrofluid 30 by means of a feeder 36, in this case a 
vibratory feeder. 
The particles in this embodiment will typically have a size of 
+100.times.10.sup.-6 m. Those particles which have a density less than the 
apparent density of the ferrofluid will float in the ferrofluid and report 
to an elevated float outlet 38, from which they can be removed. Those 
particles which have a density greater than the apparent density of the 
ferrofluid will sink through the ferrofluid and report to a sink 
collecting chute 40 which removes them. It will be recognized that the 
outlet 38 is created by an appropriate gap in the solenoid winding 34. The 
FHS separation process accordingly operates continuously with the sink and 
float fractions being removed separately from the separation chamber. 
The separation chamber 32 and the solenoid winding may have a circular or 
other shape. The chamber and winding preferably have an oblong shape which 
is, in the illustrated case, elliptical. The major axis 42 of the ellipse 
is substantially longer than the minor axis 44 thereof. For a given rate 
of transverse movement of the particles, only a certain distance, i.e. the 
length of the minor axis, is required to ensure thorough separation of the 
float and sink fractions. The major axis may be made as long as 
practically feasible to give the required throughput. Also, the vertical 
dimension 46 of the separation chamber, i.e. the vertical dimension of the 
body of ferrofluid, can be kept as low as is necessary for proper 
separation of the sink and float fractions. Thus the dimensions 44 and 46 
determine the residence time of the particles in the ferrofluid and hence 
the efficiency with which the sink and float fractions are separated while 
the dimension 42 determines the throughput. In a typical example, the 
dimension 44 may be 400 mm, the dimension 46 200 mm and the dimension 42 
one metre or more. 
Although FIGS. 3 and 4 show an FHS separator operating with a single cut 
point, i.e. a single apparent density of the ferrofluid, which enables a 
single separation to be made between particles of greater and lesser 
density, it is believed that it will be possible, with appropriate design 
of the solenoid winding, to achieve several cut points. This could for 
instance be achieved with multiple solenoid windings and/or by varying the 
current supplied to the winding(s) at different vertical positions. With 
such arrangements, it is envisaged that it will be possible to separate a 
feed material simultaneously into three or more fractions consisting of 
float, middlings and sink fractions. The float and middlings fraction(s) 
will each be withdrawn through separate outlets at different elevations. 
Separation into a greater number of fractions can also be achieved with a 
multi-stage arrangement, an example of which is illustrated 
diagrammatically in FIG. 5. In this case, a first FHS separator 50, 
operating in the manner described above for FIG. 3, separates feed 
material 52 into a float fraction which is withdrawn through an elevated 
outlet 54 and a sink fraction which forms the feed for a second FHS 
separator 56. The second separator also operates in the same manner, but 
in this case the cut point is controlled, by the design of the solenoid 
winding, between less dense and more dense particles contained in the feed 
supplied as the sink fraction from the first separator. Thus in this case, 
the densest particles are recovered as the sink 58 from the second 
separator and particles of intermediate density are recovered as middlings 
through an outlet 60. It will be appreciated that a multi-stage 
arrangement as exemplified in FIG. 5 could have three or even more FHS 
separators arranged in series to separate the initial feed material into a 
greater number of fractions. 
In each case, the accuracy with which the solenoid windings can be designed 
to produce a desired magnetic field gradient, and hence the close control 
which can be maintained over the apparent density of the ferrofluid, will 
enable separation to be achieved between particles which have densities 
that are very close to one another. 
Kerosene will most commonly be used as the liquid carrier of a ferrofluid 
which has magnetite particles in suspension, but water may be preferred in 
some cases.