Process for operating a short-belt type magnetic separator

The invention is of a process for increasing the productivity and separation efficiency of a short-belt type dry magnetic separator such as is used for separation of magnetic iron ore from gangue. The process includes providing magnets extending in an arc within the pulley head which begins at a location spaced beyond the point where substantially all of the particles are thrown off of the belt surface in the direction of belt travel.

BACKGROUND OF THE INVENTION 
This invention relates to "short" belt magnetic separators having a pulley 
head with magnets located therein that do not rotate with the pulley head, 
and particularly to a process for increasing the productivity of such 
separators. 
Two basic types of endless belt magnetic separators are known in the art. 
In one type, called disc-type separators, the pulley head has magnets that 
rotate with the head. Previous testing of this type of separator has 
demonstrated that a capacity of approximately 30 to 35 long tons per hour 
per foot of magnet width (ltphpfmw) can be achieved with minus 1-inch 
magnetic taconite ore from USS's Minntac mine near Mountain Iron, Minn. 
With this type of magnetic separator, the strongly magnetic particles form 
magnetic floccules as they are "grabbed and held" until they are carried 
out of the magnetic field by the movement of the belt. To minimize the 
entrapment of non-magnetic particles within magnetic floccules, the layer 
of particles must be thin and, therefore, the capacity is low. In the 
other, "short" belt separator, the magnets are stationary during separator 
operation. I refer to this type as an "ore-agitating" type of separator 
because the magnetic particles (which are little magnets themselves) 
reorient and flip 180.degree. as they pass by the fixed internal magnets 
of alternating polarity. The magnets in this latter type of separator may 
be adjustable to various positions within the head but do not rotate with 
it. The term, "short" belt, arises from the restrictions on belt length 
due to the type of bearings required for the pulley of this type 
separator. The productivity or throughput of material on short belt 
separators has been somewhat limited due to the manner in which such 
separators are conventionally used. Under the theory of operation used in 
the past, it was thought to be necessary to maintain the magnetic ore 
particles in the magnetic field on the belt for at least a portion of the 
time in which the belt passes over the pulley head. Thus, conventional 
practice has been to operate the belt at a sufficiently slow speed so that 
the magnetic particles are not released from the magnetic field until they 
are turned downwardly in a shorter trajectory than the non-magnetic gangue 
particles to assure effective separation by splitters or baffles located 
outwardly of and beneath the upper level of the pulley head. Manufacturers 
of the short-belt, "ore-agitating" type of head pulley show in their 
catalogs a capacity on the order of 9 to 21 ltphpfmw at belt speeds of 250 
to 490 feet per minute (fpm) for magnetic iron ores similar to the 
taconite ore mentioned above. Conventional magnetic head pulley designs 
have typically 5 to 9 pole pieces of alternating polarity magnets in a 
magnetic arc of about 120 to 180 degrees. The interpole design has (in 
addition) small bucking magnets between each pole piece. When a typical 
short-belt, dry magnetic head pulley separator is operated at a low speed, 
i.e. 250 to 400 fpm for a 48-inch diameter drum, none of the particles 
(magnetic or non-magnetic) leave the belt adjacent to the drum surface 
until they have undergone significant agitation by passing over several 
magnet poles of alternating polarity. At these conditions, the magnetic 
particles form magnetic floccules that entrap non-magnetic particles. In 
addition, the band of ore particles thrown from the belt is narrow and the 
exact splitter-position between magnetics and non-magnetics is very 
critical and gives variable results when the belt speed is low. 
It is a primary object of this invention to increase the rate of 
productivity and separation efficiency of short-belt type magnetic 
separators having a pulley head with magnets located therein that do not 
rotate with the pulley head. 
SUMMARY OF THE INVENTION 
This invention is of a process for separating magnetic ore particles from 
non-magnetic particles using a short belt type magnetic separator having a 
pulley head with magnets located at fixed positions therein during 
operation of the separator. The process includes feeding a mixture of 
magnetic ore and non-magnetic particulate materials onto a feed end 
portion of an endless belt of said short belt type magnetic separator. The 
method includes providing magnets within the pulley head extending along 
an arc beginning at a location slightly beyond the point of tangency where 
substantially all of the particles are thrown off of the belt surface so 
that the magnetic particles are subsequently drawn back toward the belt 
surface and projected in a shorter trajectory than the non-magnetic 
particles in order to obtain a more effective separation from said 
non-magnetic particles than when the particles are not projected at the 
point of tangency.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a typical short-belt type ore-agitating magnetic 
separator includes an endless conveyor belt 10 mounted on a head pulley 12 
and a tail pulley 14. Magnetic ore particles including gangue are fed from 
a hopper 15 onto a feed end of the belt. A baffle or splitter 20 is 
provided at spaced positions below the midpoint of pulley head 12 to 
separate magnetic ore particles from the gangue. A plurality of axial-pole 
magnets 16 of alternating polarity are mounted within the drum of head 
pulley 12. Interpole magnets 18 are located between the axial-pole magnets 
for a purpose described below. The magnets are typically adjustable so 
that the position of the "magnetic arc" 22 may be selected according to 
various factors, such as the type of ore, particle size and belt speed 
used. The magnets are left in the fixed adjusted position during separator 
operation. In prior art, the magnets are adjusted to the position shown in 
FIG. 4. FIG. 3 shows the effect of belt speed on the trajectory of 
one-inch diameter non-magnetic particles from a nominal 48-inch diameter 
head pulley having a one-half inch thick belt thereon with the belt 
inclined upwardly at an angle of 12 degrees with respect to a horizontal 
direction. The various baffle or splitter positions which can be used to 
obtain an effective separation of magnetic and non-magnetic particles are 
illustrated for the various belt speeds. With the magnets in the position 
according to the prior art as shown in FIG. 4, the point at which 
non-magnetic particles leave the belt can be found from the following: 
First, a factor F is calculated from the relation F=v.sup.2 /(g R cos b) 
where R is the radius in feet as measured from the axis of the head pulley 
to the middle of the particle on the belt (i.e. the radius of discharge 
circle), v=belt speed in feet per second, g is the acceleration due to 
gravity 32.2 feet per second, and b is the belt incline angle in degrees. 
If the factor F is calculated to be equal to or greater than 1.0, then the 
particle leaves the belt at point T where the belt becomes tangent to the 
head pulley. If the factor F is less than 1.0, then the particle leaves 
the belt at a point C which will vary with the belt speed. For example, 
the point C.sub.2 (for a belt speed of 250 fpm) may be found by 
calculating the angle X (FIG. 3) between a radius at point C.sub.2 and a 
horizontal direction. The angle X is found from the relation sine 
X=v.sup.2 /(g R). The aforementioned calculations determine where a 
non-magnetic particle leaves the belt and begins its trajectory therefrom 
with the magnets in the position shown in FIG. 4, or in the absence of any 
substantial influence of a magnetic field where, based on belt speed and 
angle of inclination of the belt, substantially all of the particles leave 
the belt. However, with the magnets adjusted to the position shown in FIG. 
4, magnetic particles will remain on the belt slightly longer and for 
greater arc distances than the arc for non-magnetic particles. In the 
prior art relatively slow belt speeds have been used (250 to 490 fpm) with 
the magnets in the adjusted position of FIG. 4 so that magnetic particles 
entered the magnetic field before leaving the belt surface. Thus, in the 
prior art, magnetic particles were intentionally retained on the belt past 
the point where non-magnetic particles left the belt surface. Also by 
arranging the magnets with poles of alternating polarity adjacent to the 
belt, the magnet particles were caused to "flip" and reorient themselves 
while they travelled on the belt to a point slightly beyond point C 
described above. The purpose of causing the particles to flip as just 
mentioned was to permit smaller non-magnetic particles which tended to 
become entrapped within floccules of the magnetic particles to separate 
and fall out during reorientation of the particles so that a better 
separation could be obtained. However, in actual practice it is believed 
that magnetic floccules made up of many magnetic and non-magnetic 
particles flip as a floccule rather than an individual particle so that 
the non-magnetic particles are not separated as expected. An alternate 
magnet design illustrated in FIG. 2 includes "interpole" magnets 18 
positioned between each axial pole magnet 16. Interpole magnets have the 
same polarity on their side surfaces (in the radial direction) as their 
adjacent axial poles. This design results in a magnetic field that extends 
further out from the pulley than other magnet designs. This tends to 
accentuate the degree of flip and reorientation of the magnetic particle 
floccules. 
According to this invention, a process is provided for more effectively 
separating magnetic ore particles from non-magnetic particles using a 
short-belt type, ore-agitating magnetic separator. The process also 
provides significantly increased production rates (my experience showed a 
400 to 500% increase in capacity compared to the prior art) for the 
magnetic separation of ores using such separators. An essential feature of 
the invention is that the magnets within the pulley head must be located 
in a position such that the magnetic arc begins beyond the point where, 
based on belt speed and angle of inclination, substantially all of the 
particles are thrown off of the belt surface. Specifically, the magnets 
should be located such that a radius extending from the axis of the pulley 
head through the closest effective point P (FIG. 2) of the magnet adjacent 
to the point where the particles leave the belt should make an angle of at 
least 1.degree. in a downstream direction with respect to a radius through 
said point where the particles leave the belt. Desirably, the angle should 
be within a range of about 1.degree. to 10.degree. and preferably within a 
range of 2.degree. to 6.degree.. 
To increase the rate of production, it is desirable to operate the 
separator at higher than normal belt speeds. Desirably, the belt travel 
speed should be as high as possible to increase production capacity. 
However, the maximum travel speed of the belt should be less than that at 
which the centrifugal force imparted to the magnetic particles becomes 
equal to or higher than the magnetic attractive force that pulls the 
magnetic particles back toward the belt surface. Preferably, the minimum 
belt speed should be high enough so that substantially all of the 
particles are thrown off of the surface of the belt at the point of 
tangency T. Also, the angle of inclination b of the belt for a particular 
travel speed should not exceed that at which substantially all of the 
particles come to rest on the belt (i.e. get up to belt speed) prior to 
arriving at the point of tangency T so that the degree of separation 
obtained is not significantly decreased. By substantially, I mean all of 
the particles except those which contain more than 4 or 5 percent 
moisture. It is desirable that the particles should not contain more than 
1 or 2 percent moisture. Desirably for treatment of magnetic taconite ore, 
the belt should be inclined at an angle within the range of 0 to 16 
degrees, preferably 12 to 14 degrees. It is also desirable that at least 
one and preferably two changes in polarity should be provided within the 
magnetic arc. Preferably, the first change in polarity with respect to the 
direction of travel should be located on a radius making an angle within 
the range of 10.degree. to 25.degree., more preferably 12.degree. to 
18.degree. with respect to the point where the particles leave the belt. 
Finally, a smaller magnetic arc may be used than was used in the prior 
art. Preferably, for a more economic design of separator, the magnetic arc 
provided should be within the range of 50.degree. to 90.degree.. 
Finally, it is desirable to feed the particles on the belt at a rate and in 
a manner so that a relatively uniformly spread thin "bed" thickness is 
obtained. Preferably, the belt is agitated, e.g. by bouncing idler rolls 
24 as claimed in U.S. Pat. No. 4,370,225, the specification of which is 
incorporated herein by reference. Such agitation permits finer particles 
to settle through the bed onto the belt so that fine magnetic particles 
are not lost with the large non-magnetic particles because they are lying 
on top of them when they reach the point of discharge. In the most 
preferred form, the belt consists essentially of a monolayer, one particle 
in depth. A significant advantage of the invention is that higher 
production rates can be achieved with a broader range of particle sizes 
than was possible previously. For example, it is not necessary to perform 
the separation in more than one step by segregating the particles into 
more than one size range to accomplish an efficient separation. A high 
degree of efficiency can now be obtained by a single pass through the 
separator on feed containing particles over a broad size range.