FIELD OF THE INVENTION
The present invention generally relates to a dewatering process for ultrafines
iron ore slimes. In particular, the invention relates to dewatering of alumina-
silicate rich iron ore of size below 10 micron using chemically assisted thickening
process followed by filtration for reducing the final cake moisture to 20% (by
weight). More particularly, the invention relates to an improved dewatering
process for ultrafine iron ore slime of size less than 10 micron to achieve a
moisture content in ultrafine iron ore slime cakes, to 20% (by weight).
BACKGROUND OF THE INVENTION
In a beneficiation process of iron ore, around 15-20% of run of mine (ROM) is
wasted as the iron ore slime dam as byproduct. The proportion of iron ore slime
to ROM is likely to further increase in future with higher proportion of lean grade
ores in the ROM. Accordingly, with the increasing demand of iron concentrate,
the amount of this waste material in the form of iron ore slime shall also increase
with increasing demand of iron concentrate. Presently, this slime is stored in the
dams in absence of an effective beneficiation technology in the art to treat.
Indian iron ore slime containing high alumina ultrafine.
The ultrafine fraction of slime in particular can not be utilized directly in
commercial application and perforce stored as waste in slime dam due to
absence of effective dewatering process.

Conventional dewatering process consists of several steps. In the first step,
slurry is thickened from 35 to 75% solids in a large thickener, while free water is
removed from the top and recycled back to the plant. In the second step, the
thickened pulp is subjected to a mechanical dewatering process, such as filtration
or centrifugation, to further remove the water. However, this process is highly
inefficient, particularly when the mineral/coal particles are fine. In general, the
moisture content in the dewatered product increases with decreasing particle
size. Very often, these products need to be further dewatered in a third and the
most costly step, i.e., thermal drying, which may be an option for high-priced
materials. However, it is difficult to justify employing the costly thermal drying
step for low-priced commodities such as coal. Even for the high-priced materials,
eliminating thermal drying has significant economic and environmental
advantages.
Admittedly, several developments in dewatering process has been reported in
prior art, in terms flocculant application and improved filter design for treating
fine particles of minerals/coal. However, development of effective dewatering
process for ultrafine particles is still a big challenge.
Dewatering of ultrafine particles is essential in order to reduce the make-up
water by recycling of the process water. Dewatering also reduces impoundment
in dam rehabilitation and land cost, minimises environmental pollution.
Therefore, an improved dewatering process for ultrafine slime particles is
necessary to reduce the final moisture content of the iron ore slime to make the
byproduct cable of commercial application.

US patent 4563285 describes a method for dewatering phosphate slimes and
recovering a liquid solution which involves adding a calcium sulphate
hemihydrate to the slimes, mixing the resulting admixture to effect formation of
calcium sulfate dehydrate crystals in the admixture, and recovering a liquid
solution from the resulting crystal-containing admixture.
US patent 3932275A describes a method of dewatering mineral slimes by adding
fly ash and then blending with a polyelectrolyte slowly and thoroughly. The clear
water supernatant is be drawn off from the sedimented coagulated solids. The
coagulated solids is further dewatered or treated in ecologically beneficial ways.
US patent 4303532A discusses a dewateing of phosphate slimes using a
flocculation agent followed by a rotating cylinder screen. Final moisture content
of solid product was found to be 23%.
US patent 20040144731 discusses a method for separating suspended clay fines
from water in mining clay slurry. Settling of fine particles of size of less than 100
micron is carried out using polymeric flocculation agent.
With reference to the above mentioned patents, patent applications and
technology known in the art, there is a need to develop an improved dewatering
process for ultrafine iron ore slime of size less than 10 micron to achieve a
moisture content in ultrafine iron ore slime cakes, to 20% (by weight).
OBJECT OF THE INVENTION
In view of the foregoing limitations inherent in the prior-art, it is an object of the
invention to propose an improved dewatering process for ultrafine iron ore slime

of size less than 10 micron to achieve a moisture content in ultrafine iron ore
slime cakes, to 20% (by weight), which overcomes the drawbacks inherent in
the prior art while offering some additional advantages.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided an improved dewatering
process for ultrafine iron ore slime of size less than 10 micron to achieve a
moisture content in ultrafine iron ore slime cakes to 20% (by weight), the
process comprising the steps of conducting a thickening process where optimized
dose of cationic coagulant followed by anionic clocculant is added and mixed to
form a slurry (5 wt% of solid) inside a hi-rate thickener and allowing the slurry to
settle for a specific time for producing a thick slurry in the underflow and a clear
water in overflow; subjecting the underflow from the hi-rate thickener to a
filtration unit for producing a cake containing 80% moisture content;
conditioning the slurry in an agitator for 10 minutes before transferring the
conditioned slurry to said hi rate thickener; operating the hi-rate thickner under
optimum operating parameters consisting of coagulant dosing at the rate of 200
gm/ton, flocculant dosing at the rate of 30 gm/ton and a lime dosing maintaining
7.0 pH value of the slurry, settling the slurry initially @ 24.66 m/hr; setting the
slurry further for a period of 10 minutes for determining the moisture content of
thickened slurry; subjecting the thickened slurry if the moisture content of the
slurry by wt% is about 41:7 to a filtaration unit for achieving 20% moisture
content of the tinal product; wherein during the filtration operation the
parameters of filtration time, pressing time, drying time and technical time are
maintained at 5 minutes, 17 minutes, 3 minutes and 5 minutes respectively to
achieve the desired moisture content of the cakes.

These together with the other aspects of the invention, along with the various
features of novelty that characterize the invention, are pointed out with
particularity in the description, along with the abovementioned summary,
annexed hereto and form a part of the invention. For a better understanding of
the invention, its operating advantages and the specified object attained by its
uses, reference should be made to the accompanying drawings and descriptive
matter in which there are illustrated embodiments of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :
FIG. 1 shows XRD pattern of ultra fine iron ore slime in accordance with an
embodiment of the invention.
FIG. 2 shows a schematic flow sheet illustrating various steps performed in
accordance with an embodiment of the invention.
FIG. 3 shows a mechanism of micro-floc formation
FIG. 4 shows zeta potential of iron ore slurry at different pH.
FIG. 5 shows a mechanism of floc formation.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a benefication process of iron ore slimes of size
less than 45 micron. According to process, a feed material of slimes in slurry
form is firstly introduced into a hydrocyclone to classify the particles coarser than
10 micron from the rest. Thus, iron bearing minerals and alumina-silica bearing

minerals become seperated in coarser fraction (-45+10 micron) and finer
fractions (-10 micron) respectively. Coarser fraction possess excellent dewatering
property due to high density of iron bearing minerals. Finer fraction (-10 micron)
of slime is also known as ultrafine slime, consists of majority of alumina-silica
bearing minerals and accumulates in form of an unsettable very fine suspension
in water because of high surface charge.
In the present invention, an improved dewatering process has been developed
for ultrafine iron ore slimes (<10 micron size) to achieve a target of 20% (by
weight) moisture in the final product.
As mentioned hereinbefore, ultrafine iron ore slime of size less than 10 micron is
collected from hydrocyclone overflow in from of slurry and used as a feed
material for the slurry. This sample is characterized using XRD methodology
(shown in FIG. 1) and it revealed that ore comprises majority of clay particles
such as kaolinite and quartz with iron bearing minerals such as hematite and
goethite.The size-wise chemical analysis of feed sample is given in Table 1 which
shows that % of the material is of size below which is very fine in nature.
Referring now to FIG. 2 a schematic flow sheet with illustrating various steps
involved in dewatering process respectively, for ultra fine iron ore slimes of size
less than 10 micron and more than 95% moisture, in accordance with an
embodiment of the invention.
Mixing of slurry is carried out using mechanical agitator for making uniformity of
the particles of ultrafine iron ore slime in water before carring out settling tests
in hi-rate thickener.

Settling test on ultrafine iron slime is performed inside a thickener which
separates thickened mass in underflow and water in overflow. Before carrying
out settling test, coagulant and flocculant are being added into thickener feeding
line for enhancing the settling rates and thereby moisture in thickener underflow
reduces.
Addition of coagulant causes extremely fine colloidal particles to adhere directly
to each other. All particles exert mutual attraction forces, known as London Van
der Waals' forces, which are effective only at very close range. Normally, the
adhesion due to these forces is prevented by the presence around each particle
of an electrically charged atmosphere, which generates repulsion forces between
particles approaching each other. The repulsion forces not only prevent
coagulation of the particles, but also retard their settlement by keeping them in
constant motion, this effect being more pronounced the smaller the particle.
Coagulants are electrolytes having an opposite charge to the particles, thus
causing charge neutralization when dispersed in the system, allowing the
particles to come into contact and adhere as a result of molecular forces.
Coagulants with opposite charges to the surface charge of suspended solids are
added. It neutralizes the charge on them and the repulsion between the particles
is reduced. They become capable of sticking together. Slightly larger particles,
also known as microflocs, are formed through it. Mechanism of microfloc
formation is shown in FIG 3.
In present invention, pH value of the iron ore slurry used for settling was 7.0.
From zeta potential study as shown in FIG 4. It was found that point of zero
charge of iron ore slurry was 5.73 and zeta potential value of the particle surface
at pH 7.0 was negative value which reveals that addition of cationic coagulant
can neutralize negatively charged surface for better settling of colloidal particles.
Therefore cationic coagulant 8103+ supplied by M/S Nalco with 1% strength was

selected for settling of colloidal particles. Dosing range of coagulant was in
between 200-400 gm/ton.
Flocculants is another chemical which is added in the slurry after coagulant, acts
as a bridge and forms bigger size of flocs as compared to coagulant. The main
process of formation is through bridging of solid particles through polymer chains
of flocculants. The microflocs formed via coagulants and other coarse particles
act as adsorbent for the flocculants and they get attached to each other via
polymeric chains of it. By increasing the effective particle size of the solid phase,
the stability of the suspension is broken and the liquid phase is released.
Mechanism of flocculation process is shown in FIG 5.
During flocculation process, anionic flocculant was found suitable for faster
settling as it contains negative charge which forms bridge due to its increased
hydraulic radius on neutralize surface of the particle. Anionic polymeric flocculant
83376 supplied by M/S Nalco was selected due to above explained reason.
Dosing range of flocculant was in the range of 15-40 gm/ton.
Optimum dosage of coagulant at 200 gm/ton and flocculant at 30 gm/ton
increases the settling rate of the particles thereby underflow product moisture
was reduced to 61% which is shown in Table 1.


Thickener underflow product moisture is further reduced by using pressure filter
model FP 0.3 supplied by M/S Outotec having filteration area 0.27 m2. Filteration
tests were performed by varying filteration parameters like feeding time,
pressing time and airblowing time to achieve the minimum moisture of cake.
Filter cloth used in the filter was ASKO 511 made of poly-propylene having air
permeability 6.0 m3/m2min at 200 Pa.
Value of feeding time was in the range of 3-5 minutes, pressing time was in the
range of 7-17 minutes while airblowing time was in the range of 2-3 minutes.
Technical time was 5 minutes which was constant for all the tests. Minimum
moisture of cake (20.6%) was achieved with feeding time at 5 minutes, pressing
time at 17 minutes and airblowing time at 3 minutes. Total cycle time including
technical time was 30 minutes. The values of air pressure at the time of feeding,
pressing and airblowing were 5 bar, 12 bar and 6 bar respectively for all the
experiments. Results of filteration tests are shown in Table 2.


WE CLAIM :
1. An improved dewatering process for ultrafine iron ore slime of size less
than 10 micron to achieve a moisture content in ultrafine iron ore slime
cakes to 20% (by weight), the process comprising the steps of :
- conducting a thickening process where optimized dose of cationic
coagulant followed by anionic clocculant is added and mixed to
form a slurry (5 wt% of solid) inside a hi-rate thickener and
allowing the slurry to settle for a specific time for producing a thick
slurry in the underflow and a clear water in overflow;
- subjecting the underflow from the hi-rate thickener to a filtration
unit for producing a cake containing 80% moisture content;
- conditioning the slurry in an agitator for 10 minutes before
transferring the conditioned slurry to said hi rate thickener;
- operating the hi-rate thickner under optimum operating
parameters consisting of coagulant dosing at the rate of 200
gm/ton, flocculant dosing at the rate of 30 gm/ton and a lime
dosing maintaining 7.0 pH value of the slurry, settling the slurry
initially @ 24.66 m/hr;
- setting the slurry further for a period of 10 minutes for determining
the moisture content of thickened slurry;
- subjecting the thickened slurry if the moisture content of the slurry
by wt% is about 41:7 to a filtaration unit for achieving 20%
moisture content of the tinal product;

2. The process as claimed in claim 1, wherein the coagulant added into the
hi-rate thickener is a cationic coagulant of 1% strength.
3. The process as claimed in claim 1, wherein the flocculant added into the
hi-rate thickener is an anionic polymere flocculant of 0.05% strength.
4. The process as claimed in claim 1, wherein the cationic flocculant is added
first into the thickener before adding the anionic flocculant into the hi-rate
thickener.
5. The process as claimed in claim 1, wherein the filtration unit is a pressure
filter having filteration area of 0.27 m2.
6. The process as claimed in claim 1, wherein the pressure filter is operated
with the following operating parameters :-
Feeding time: 3-5 minutes
Pressing time: 7-17 minutes
Air blowing time: 2-3 minutes

7. The process as claimed in claim 1, wherein the pressure filter is operated
with following pressures :-
Feeding pressure: 5 bar
Pressing pressure: 12 bar
Air blowing pressure: 6 bar
8. The process as claimed in claim 1, wherein pressure the filter is having
filter cloth made of poly-propylene having air permeability 6.0 m3/m2min
at 200 Pa.
9. The process as claimed in claim 1, wherein the pressure filter is having
double sided pressing arrangement.