Dense medium separation

In a dense medium separation process a product of a separation step containing separated material and magnetic dense medium particles is passed through a sieve bend and the undersize only is treated for the magnetic recovery of the dense medium particles.

BACKGROUND TO THE INVENTION 
This invention relates to dense medium separation processes of the kind in 
which material to be beneficiated is passed with a dense medium suspension 
to a gravity separation step, e.g. in a cyclone, to give two fractions. 
One fraction is a suspension containing light particles from the material 
and some of the dense medium particles, while the other fraction is a 
suspension containing dense particles from the material and some medium 
particles. 
The present invention is particularly concerned with methods of treating 
these two product fractions to separate medium particles from the 
constituent originating from the original material. The invention is also 
concerned with the overall treatment process. 
The applicant has already proposed that each of the abovementioned two 
fractions be subjected to at least a single stage high relative density 
separation to form a secondary first fraction containing substantially 
only dense medium particles and a secondary second fraction containing the 
bulk of the other particles. The secondary second fraction may then be 
subjected to a recovery process, such as magnetic recovery, for the 
recovery of the remaining dense medium particles. 
SUMMARY OF THE INVENTION 
According to the present invention the secondary second fraction is passed 
through a screening step, e.g. through a sieve bend, to screen out coarse 
particles from the original feed, and the undersize is subjected to a 
recovery process, such as magnetic recovery, for the recovery of the 
remaining dense medium particles. 
The invention is predicated by the fact that in the formation of the 
secondary fractions not only density separation is effected but there is 
also a classification process tending to cause dense medium particles of 
an average particle size less than those in the secondary first fraction 
to pass into the secondary second fraction. In addition much of the water 
in the feed passes into the secondary second fraction and thus there is a 
saving on the amount of water required for spraying purposes in the 
screening step. In the result the load on the magnetic separator is 
considerably reduced so that a smaller separator may be used for a given 
throughput.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The invention is further discussed with reference to the accompanying flow 
sheet of a coal beneficiating process. 
In the drawing a raw coal feed which is sized, say, to plus 1000.mu. is 
first passed through a cleaning cyclone 10 with water only. The underflow 
from this cyclone is the discard and the overflow is thickened in a 
thickening cyclone 11. The overflow from this cyclone 11 is used as spray 
water and so on. The underflow is subjected to a dense medium separation 
process. 
In this process the coal mixed with a dense medium suspension of, e.g. 
magnetite, is fed firstly to a conventional dense medium cyclone 12 to 
give an overflow as a primary first fraction and an underflow as a primary 
second fraction. Each of these fractions is fed to a cyclone 13 or 14, as 
the case may be. 
The underflow from the cyclone 13 and 14 rejoins the dense medium circuit. 
The overflow from the cyclone 13 is passed to a magnetic separator 15 to 
provide clean middlings and overdense medium for return to the dense 
medium circuit. 
The overflow from the cyclones 13 and 14 contain, in addition to a portion 
of the magnetic dense medium particles, the separated fractions of the raw 
coal feed. In conventional practice they would be separated by passing the 
overflows to magnetic separators. According to the present invention the 
burden on the magnetic separators are reduced by taking advantage of a 
property discovered in the products of the cyclones 13 and 14. 
The dense medium particles used are nominally minus 75.mu.. However, in a 
test it was found that this resulted in a mean particle size of 21.9.mu.. 
In the underflow of the cyclone 14 the mean particle size increased to 
32.1.mu. while in the overflow it became 11.5.mu.. 
Consequently the difference in the particle size between the clean coal and 
the discard on the one hand and the magnetite on the other hand is 
accentuated. In the result a screening step would remove much of the coal 
or discard which would otherwise load the magnetic separators. 
The overflow from the cyclone 14 or the secondary second fraction is now 
passed to a sieve bend 16 where reasonably easy separation of the 100.mu. 
and over coal from the dense medium particles is effected. The amount of 
spray used is minimal due to this and the dilution of the feed to the 
sieve bend as a result of the density separation of the medium particles 
occurring in the cyclone 14. The coarse product from the sieve bend 16 is 
high quality coal. 
The fine product is passed to a magnetic separator 17 via a sieve bend 18 
also to produce good coal and a return feed of dense medium particles. 
The process thus produces a discard which goes to waste, good quality coal 
which may be used to make form coke and middlings which may be used for 
steam raising.