FIELD OF THE INVENTION
The present invention relates to a process for increasing the dewatering
efficiency of iron fines of size less than 6mm.
BACKGROUND OF THE INVENTION
Iron ore cannot be used directly in metallurgical plants and needs to be
upgraded to increase the iron content and reduce the gangue content. A process
adopted to upgrade ore is called beneficiation. Iron ore is upgraded to higher
iron content through concentration. Iron ore is beneficiated to meet the quality
requirement of iron and steel industries. However, each source of Iron ore has
its own peculiar mineralogical characteristics and requires the specific
beneficiation and metallurgical treatment to get the best product out of it. The
choice of the beneficiation treatment depends on the nature of the gangue
present and its association with the ore structure. Several techniques such as
washing, jigging, magnetic separation, advanced gravity separation and flotation
are being employed to enhance the quality of the iron ore.
Iron ore before being used in metallurgical operations is mined, crushed and
screened to particular sizes as required. As iron ore is crushed from larger to
smaller size fractions, lot of fines are generated in the process. Here fines are
broadly classified as any iron ore of size less than 6 mm. There are also gangue
mineral associated with the iron ore. So after crushing the iron ore is subjected
to washing for the removal of the gangue minerals and in this washing process,

the fines gain moisture. The moisture content in the iron ore fines ranges from
11 to 14 % depending upon a variety of factors such as feed quality of ore,
amount of fines generated in process. The moisture content goes up to 16-18%
in the fines during the monsoon season. The high moisture content of fines
poses problems in downstream operations of conveying, stockpiling and stacking.
The higher spillages, higher freight charges have a draining effect on bottom-
line. The consistently higher moisture content in classifier fines has been a
persistent problem in wet processing plant. Use of dewatering screen alone is not
able to dewater the fines to tolerable limits.
US Patent No.4,410,431 describes a method of dewatering a wet, particulate
mineral mass which comprises treatment of the wet mineral with composition
comprised of a surfactant having a hydrophile-lipophile balance number of from
about 6.0 to 12.0, and a quantity ("surfactant absorption inhibitory amount") of a
fatty acid substance such as tall oil fatty acid. Preferred surfactants are
composed of the reaction product of one mole of primary aliphatic alcohol
containing from 6 to 13 carbon atoms with two or seven moles of ethylene oxide.
Tridecyl alcohol is specifically mentioned as a useful alcohol for ethoxylation.
However, this prior art does not discuss dewatering alumina trihydrate. Moisture
reduction by hot air is further known in the art which however is energy-
intensive process.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for increasing
dewatering efficiency of iron ore fines and size less than 6 mm.
Another object of the invention is to propose a method for increasing dewatering
efficiency of iron ore fines and size less than 6 mm which decreases the moisture
content in iron ore fines.
Another object of the invention is to propose a method for increasing
dewatering efficiency of iron ore fines and size less than 6 mm which is easily
implementable without making many changes in the known processes.
A further object of the invention is to propose a method for increasing
dewatering efficiency of iron ore fines and size less than 6 mm which is easily
implementable without making many changes in the known processes.
SUMMARY OF THE INVENTION
Accordingly, there is provided a dewatering process that produces less than 10%
moisture in iron ore fines of size less than 6mm. The process comprises
developing a dosing system and making use of appropriate chemical reagents
with the proper identification of the dosing points. The process further results in
reduction in alumina content by 0.2 to 0.4 % (absolute). The designing of dosing
system involves arrangement of pipes, pumps, tanks, stirrers, water line
connections.

The first step of the process involves mixing of surface active reagent
(surfactant) with water in a tank. The solution is provided with a stirrer to make
a homogeneous aqueous solution. An optimum concentration of surfactant
solution is maintained with respect to iron ore. In an embodiment of the
invention, the reagent concentration was maintained at 1-15 ppm with respect to
the Iron ore. Excess quantity of surfactant results in foam generation and less
effect on moisture reduction. However, excess quantity of surfactant addition
results in positive effect on alumina reduction. Further addition of less quantity of
surfactant result in lesser moisture reduction and hence, it is very important
aspect to optimize the surfactant concentration with respect to iron ore.
Iron ore is treated with surfactant in a drum scrubber. The drum scrubber is
cylindrical equipment which rotates at a particular rpm. Water and iron ore is
mixed in the drum scrubber and the iron ore surface is cleaned by attrition
process. In the drum scrubber the surfactant interacts with iron ore particles.
After the iron ore particles are treated in the drum scrubber with the surfactant,
they are passed through the vibrating dewatering screen where the fine ore
(-6mm) particles are separated from the coarser ones (-40mm+6mm). This is
necessary as the finer particles have much more surface area and retains much
more moisture than the coarser particles. Then the ore fines are fed into a
classifying equipment which again classifies the fines (-6mm+0.5mm) from the
ultra-fines (-0.5mm). The ultra-fines are washed away with water and the fines
are dewatered by passing over a vibrating high frequency dewatering screen.

More the amount of ultra-fines associated with the fines, more is the moisture
content as the ultra-fine ores have extremely high surface area which adsorbs a
lot of moisture and they tend to stick to the fines and do not pass through the
dewatering screen. Also the ultra fines tend to block the pores of the dewatering
screen as they are agglomerated. The surfactant also gets adsorbed to the
surface of the mineral particles rendering them hydrophobic to some extent
which reduces the adsorption of moisture by the ore fines. The surfactant also
reduces the surface tension of the slurry as a result of which the ultra-fines do
not tend to stick with each other and with the fines thus reducing the moisture
content in the iron fines. The ultra-fines are generally the gangue minerals and
effectively separated from the fines. This is because on crushing the points of
associations of different minerals are more susceptible to breakage. Also the
gangue minerals like silica, alumina, and kaolinite are softer than iron oxide and
are crushed to finer sizes compared to iron oxide. The effective separation of
fines and ultrafine gangue from the iron ore fines result in a lower alumina level
in the iron ore fines. So the surfactant reduces moisture content in the iron ore
fines by a dual action: 1) By rendering the surface of the mineral hydrophobic,
2) by not allowing the ultrafine particles to stick with the fines.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 : shows a process flow chart according to the present invention.
Figure 2 : a graphical representation of reduction in percentage moisture using
different surfactants at different concentration levels.

DETAIL DESCRIPTION OF THE INVENTION
The Process as per the invention is illustrated in Figure 1. After crushing and
screening, ores with size less than 40 mm pass through a drum scrubber. In the
drum scrubber, the ore is washed with water to wash off the gangue material
associated with it. After the drum scrubber, the ore passes through a dewatering
screen. Washing of iron ore is practiced to clean the clay coatings and the
associated liberated gangue minerals. To remove the clay coatings, effective
scrubbing action is necessary which is provided in the drum scrubber. Also the
fine clay minerals sticking to the iron ore lumps adsorb much moisture. The
scrubbing action in the drum scrubber is therefore essential to remove the clay
coatings as far as possible.
The said iron ore with size less than 40 mm is mixed with a surfactant solution.
In an embodiment of the invention the surfactant solution can be mixed directly
with iron ore run of mine in a screw classifier. In another embodiment, the
surfactant solution and iron ore can be mixed simultaneously both in the
scrubber and in the classifier. The process is tested and evaluated with
surfactants such as Cetyl Tri methyl ammonium bromide (CTAB), Sodium
Dodecyl Sulphate (SDS), Poly ethylene glycol 4000 (PEG). All of these surfactants
when used as per the inventive process gave positive results as depicted in
Table 1. Out of the given surfactants, SDS gave best results with respect to
moisture reduction. Further, concentration of the surfactant solution played an
important role in the process. Surfactant solution with a concentration of up to

100ppm (w.r.t. dry solids) showed good results as depicted in the table 1 with
respect to moisture reduction. There was also observed reduction in Alumina
level by 0.2-0.4% as shown in Table 1.
According to the invention, the surfactant solution solid is mixed with water in a.
tank provided with a stirring arrangement. SDS solid is added in such a quantity
that the concentration of the solution is about 40-50 ppm with respect to solids.
After mixing the iron ore with the surfactant solution, the lumps of size 6 mm to
40 mm are screened and the fines of size less than 6 mm go to the screw
classifier. At the screw classifier, the ultra-fines are separated from the fines ore.
Better the separation, better is the fines product with respect to gangue
(Alumina, silica) and moisture content. The ultra-fines tend to stick to the fines
and can adsorb high amount of water owing to their large surface area and high
hydration energy. This separation or classification step is mandatory in the
dewatering process. Finally the ore fines are passed through a vibrating
dewatering screen to shed away as much water as possible.
Table 1 illustrates the benefits of current process in terms of reduction of
moisture and alumina when tested using different surfactants at different
concentration levels (the tests were performed using one kilogram of iron ore).


Figure 2 illustrates reduction in percentage moisture using different surfactants
at different concentration levels.

A known SDS was used to conduct the plant trial.
SDS was mixed with water (40kg in 5000 litres of water) and pumped to the
mixing points that are drum scrubber and screw classifier, The surfactant
solution needs proper interaction time with the mineral particles to adsorb on
their surface and render their outer surface hydrophobic and also reduce the
surface tension of the system in the process. The addition point at classifier was
provided so that the surfactant solution had a chance to interact with the fines
and ultra-fines again.
Initially the dosing was started with a concentration of 100 ppm w.r.t. solids at
the drum scrubber. The advantage of injecting the reagent into the drum
scrubber was that it had a good mixing scope with the ore and also it got a
retention time of about two minutes in the drum scrubber. But this generated a
huge quantity of froth due to the attrition action inside the scrubber along with
the water flow rate which created downstream problems. So the dosing was
optimized at a concentration of 10-15 ppm w.r.t. solids that is 10-15 gm. Per ton
of ore treated. Due to the frothing action it was decided to add the reagent at
one point only that is either at the classifier or the scrubber. For study we found
that the reagent when added to the drum scrubber was more effective. Due to
the reduction in surface tension the tendency of the ultra-fines to stick to the
fines ore diminished to a great extent which was found from the misplacement
study carried out form the overflow and underflow products of the dewatering
screen. The amount of undersized ore reporting to the oversized fraction
reduced. In another way the dewatering efficiency of the screen improved.
Sample was collected from the dewateriser screen output of the screw classifier.

There was an average moisture reduction of about 2.7% (absolute) from around
12-13% to 9%-10%.
Table 2. Illustrates the plan trial results as per the current invention in terms of
moisture% reduction during the month of June 2012.


Table 3. Illustrates the plan trial results as per the current invention in terms of
moisture% reduction during the month of July 2012


Table 4. Moisture and alumina %comparison with and without application of the
process of current invention.


WE CLAIM:
1. A method for increasing dewatering efficiency of iron ore fines of size less
than 6 mm, the method comprising:
- crushing iron ore to a size less than 40 mm;
- mixing a surface active agent with water to prepare a homogeneous
surfactant solution;
- pumping the homogeneous surfactant solution in a drum scrubber to mix
with the iron ore;
- passing the iron ore from the drum scrubber through a vibrating
dewatering screen and separating the iron ore fines of size less than 6
mm from coarser iron ore of size 6 mm to 40 mm, coarser iron ore of size
6 mm to 40 mm being separated for use in a blast furnace;
- classifying the iron ore fines of size less than 6 mm through a screw
classifer in to iron ore fines of size less than 0.2 mm and size 0.2 mm to
6 mm;
passing the iron ore fines of size 0.2 mm to 6 mm through a high
frequency vibrating dewatering screen of aperture of about 0.5 mm;
passing the iron ore fines of size less than 0.2 mm obtained from overflow
of the screw classifier and size less than 0.5 mm obtained from underflow
of the high frequency vibrating dewatering screen through a
hydrocylcone; and
collecting the underflow of the hydrocylcone and mixing with the high
frequency vibrating dewatering screen overflow of size 0.5 mm to 6 mm.

2. The method as claimed in claim 1, wherein the surface active agent is
selected from a group consisting of anionic, non-ionic and cationic
surfactants.
3. The method as claimed in claim 1, wherein the surface active agent is
selected from a group consisting of Cetyl Tri methyl ammonium bromide
(CTAB), Sodium Dodecyl Sulphate (SDS) and Poly ethylene glycol 4000
(PEG).
4. The method as claimed in claim 1, wherein the concentration of the
surfactant is in the range of 10-20 ppm with respect to iron ore.

ABSTRACT

The present invention provides a dewatering process that produces less than
10% moisture in iron ore fines of size less than 6 mm. The process comprises
developing a dosing system and making use chemical surface active reagents
with the identification of dosing points. The process further results in reduction in
alumina content by 0.2 to 0.4 %. The process involves steps of crushing the iron
ore, screening at different stages, scrubbing the ore and treating the iron ore
with surface active agents.