FIELD OF THE INVENTION
The present invention relates to beneficiation of the ultrafine chromite particles
(<37μm) present in the tailings to produce marketable grade (>40%) Cr2O3
concentrate. More particularly, the present invention relates to a process of
enrichment of ultrafine particles by selective flocculation process followed by
desliming of the as received tailing sample.
BACKGROUND OF THE INVENTION
During beneficiation process, a huge amount of tailings are generated from
chromite ore beneficiation plants which not only consumes a huge space for
storage, but causes environmental hazardous also. The better way to handle the
issue is to further utilize the tailings, so that it can overcome the environmental
issues as well it can fulfill the future market demand also.
Many conventional attempts were made for beneficiating the ultrafine particles
present in the tailings (Aslan.2009; Guney et al., 1999; Smith and Allard, 1983,
Murthy etal.2011). But due to particle size limitations the conventional methods
(gravity separation, magnetic separation, physicochemical method i.e. flotation
etc.) could not work much effectively for enriching the ultrafine particles present
in the tailings. Selective flocculation is one of the alternative physicochemical
methods which can be applied for beneficiating the ultrafine particles present in
the tailings. Very few non-patent literature for beneficiating the ultrafine particles
present in the tailings by using selective flocculation method is available in the
prior art. US Patent No.3473656 (1969) shows selective flocculation for
enrichment of low grade chromite ore of 18% Cr2O3 assay values present in the

tailings. The low grade ore was grounded to generate < 74 μm of particles and
furthermore the generated particle size < 74 μm was processed by selective
flocculation technique. Selective flocculation process was carried out at pH 11.5,
sodium silicate solution was used as the dispersant. Carboxyl methylcellulose was
used as the flocculant. Further, after selective flocculation process, the chromite
value of the product was further upgraded by using flotation techniques by
addition of tar oil as anionic collector.
Akdemir and Hieylmaz, 1998 has studied a selective shear flocculation using
synthetic mixtures of chromite - serpentine of less than < 10 μm of particles.
The test was carried out by using the artificial mixtures of 2g in slurry but they
could not successfully establish the process because, the used reagents Na2SiO3
and Na2SiF6 shows detrimental effect to selective flocculation method. Becklioglu
and Aral, 2004 have attempted to establish selective flocculation by using the
synthetic mixtures of chromite and serpentine minerals of particle size less than
20 micron. They used corn starch as a flocculant and sodium silicate solution as
dispersant. But the process shows some limitations i.e. their findings shows
selective separation of chromite from synthetic mixture and serpentine synthetic
mixtures by selective flocculation method is possible, if the solid concentration
(%) of the serpentine should be kept low in the slurry. Panda, 2010 have
attempted selective flocculation of chromite tailings where the chromite value
was upgraded from 19.7% to 26%. The sample (<37 μm ) which was used in
the study, was generated by wet sieving of the received feed sample. Sodium
silicate solution was used as the dispersants in their process.

Indian Patent No. (291/KOL/2013 of 14.3.2013) shows establishing process for
beneficiating the low grade chromite tailings by using selective flocculation
followed by magnetic separation process. In the process, chromite tailings
(< 37 μm) of particle size was used as the feed for selective flocculation. For
further up gradation of the chromite values, the concentrate generated after
selective flocculation was subjected for magnetic separation process. The
findings shows it was possible to obtain Cr2 O3 (%) upto 28.11% with 31.89% of
Fe (T)% from 14% Cr2O3 of feed assay.
Moreover except Panda, 2010 and Patent No. 291/KOL/2013, in all other cases
the researchers have used either low grade chromite ore or synthetic mixtures of
chromite and serpentine minerals but not chromite tailings. Panda 2010 achieved
approximately 26% of Cr2O3(%) in the concentrate.
The present inventors through extensive study and experimentation , recognized
that selective flocculation is a better method for enrichment of the ultrafine
chromite tailing (< 37μm ). A higher grade (>40% of Cr2O3%) could be obtained
if the as received tailing sample can be grounded and deslimed prior to
selective flocculation process.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process or enriching
ultrafine chromite particles (< 37 μm ) present in chromite tailings.

Another object of the invention is to propose a process for enhancing the
liberation of the chromite particles by further grinding in the ball mill to generate
particle size <100 mesh (150 μm).
A still another object of the invention is to increase liberation by grinding in the
ball mill followed by desliming using a 2 inch hydrocyclone before using the
particles in the selective flocculation process.
A further object of the invention is to generate particle size (< 37 μm) by
further grinding the hydrocyclone underflow product and used as a feed for
selective flocculation process whereas the hydrocyclone overflow is rejected as
tailing.
A still further object of the invention is to enrich the ultrafine chromite values by
selective flocculation process to achieve O2O3 (%) grade upto 40.44% with
31.7% yield (%).
SUMMARY OF THE INVENTION
According to the invention, the as received tailing was subjected for grinding in
a ball mill to generate particle size less than (< 150 μm) for improving the
liberation of the particles. Furthermore, the generated particle size was subjected
for desliming by 2" hydrocyclone. The hydrocyclone overflow products was
rejected and the underflow product was subjected for further grinding for
generating particles size of (< 37 μm). Subsequently the grounded hydrocyclone
underflow product (<37μm) was used as a feed for selective flocculation
process.

For selective flocculation process, 12% of solid concentration was mentioned.
The mechanical stirrer speed, after addition of the dispersant (sodium
hexameata phosphate solution) was maintained around 2000 RPM for proper
mixing of the dispersant dose with feed slurry particle. High mechanical stirrer
mixing rate was maintained for 1 hour for proper mixing. Subsequently after
addition of flocculant dose (degraded wheat starch) the slurry speed was
reduced to 100 RPM, to prevent breakage of the floe.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - shows a flow sheet describing the process of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Chrome Ore beneficiation Plant produces tailings which contain ultrafine chromite
particles and was stored in the tailing pond. From the size analysis, it was
observed that around 40 to 50% of the particles present in the chromite tailings
are (<37 μm). Along with size analysis, chemical analysis, mineralogical analysis
(XRD and SEM) of the received tailing sample was carried out. From the
mineralogical analysis, it was observed that particularly at 100 mesh, (less than
(< 150 μm) particles shows better liberation. For better liberation, the as
received tailing sample was grounded by the ball mill and further subjected for
wet sieving to generate particles less than 150 micron (chemical analysis of the
particle size (< 150 μm) was shown in Table l).These chromite
particles(<150μm) present in the tailing subjected for desliming process by using
2" hydrocyclone. All the experiments were conducted by varying different

process parameters of hydrocyclone, ie bed pressure (.5, 1,1 5 and 2) kg/cm2,
vortex finder (14.3,11.1 and 8.3 ) mm and apex diameter ( 9.4,6.4,5 and 3.2 )
mm etc. Chemical analysis of the optimized results for hydrocyclone products are
presented in Table - 2. From the analysis, it was observed that, around 7-8% of
Cr2O3 (%) with 30-34% of Fe (T) % was discarded in the overflow whereas the
hydrocyclone under flow product contains 24% Cr2O3 (%), 21.9% of Fe (T) %
with 78% of yield.By desliming the hydrocyclone underflow product becomes
preconcentrated with 24% Cr2O3 (%) and 78% of yield.Subsequently the
underflow product was further grounded in the ball mill to generate particle size
of less than (37 μm), which was used as feed for selective flocculation process.
In the first stage of selective flocculation process, suitable dispersant solution
(sodium hexameta phosphate solution) was added to keep all the particles
dispersed phase present in the slurry. After addition of the proper dispersants,
stirrer speed should be maintained at a higher rate around 2000 RRM. Then the
required pH 7 was maintained. Subsequently after addition of the flocculant dose
(degraded wheat starch) stirrer speed was reduced to a lower speed and 100
RPM was maintained. After 120 second of addition of the flocculant dose, formed
floe was separated from the supernant liquid. Both the products were subjected
for detail analysis. The detail analysis of both the products was given in Table 3.
Table 1. Chemical analysis of grounded sample (< 150 urn)


Table 2. Chemical analysis of underflow and overflow products Pressure 1.5
kg/cm2, Apex diameter 6.4 mm, Vortex finder 11.1 mm.


WE CLAIM :
1. A process for enriching chromite values of ultrafine particles in chromite
tailings (< 37 μm) by selective flocculation and desliming technique to
produce higher grade of Cr2O3(%) concentrate, the process comprising
the steps of:
a) grinding as received chromite tailing samples by a ball mill to generate
particles of size to below 150 μm size for better liberation;
b) subjecting the grounded particles for desliming in a hydrocyclone;
c) subjecting the underflow product from the hydrocyclone to grinding below
37 micron size particles and the grounded product (below 37 μm) using as
a feed for selective flocculation process.
2. The process as claimed in claim 1, where desliming of the grounded
particles in hydrocyclone is done with operating parameters i.e., pressure
drop, vortex finder, apex diameter were maintained at 1.5kg/cm2 , 11.1
mm, 6.4 mm respectively and the pulp density was maintained 10% by
weight during this process.

3. The process as claimed in claim 1, wherein the underflow product
contains 24.9% Cr2O3 (%) with 21.9% Fe(T)% assay values and 78% of
yield, and wherein the overflow assay values contain 7.1% Cr2O3,34.1%
Fe(T)with 22% yield.
4. The process as claimed in claim 1, wherein the grounded particle is added
to water to maintain the pulp density between 9% to 12% by weight.
5. The process as claimed in claim 1, wherein a pH value of 7 is maintained
during the selective flocculation process.
6. The process as claimed in claim 5, wherein degraded wheat starch is
used as the flocculant and sodium hexametaphosphate solution is used as
the dispersant during the selective flocculation process.
7. The process as claimed in claim 1, wherein the chromite tailings of
ultrafine size (< 37 μm ) after desliming and selective process is upgraded
up to 44.04 % Cr2O3
8. The process as claimed in claim 6, wherein the degraded wheat starch is
used in the range of 200g/t to 250 g/t, and the sodium
hexametaphosphate is used in the range of 350 g/t to 400 g/t for the
effective recovery.

9. The process as claimed in claim 1, wherein after selective flocculation
process concentrate grade comprises approximately Cr2O3 (%) 44.04 %,
12.01% Fe(T) with 31.7 yield (%).

ABSTRACT

The invention relates to a process for enriching chromite values of ultrafine
particles in chromite tailings (< 37 μm) by selective flocculation and desliming
technique to produce higher grade of Cr2O3 concentrate, the process
comprising the steps of grinding as received chromite tailing samples by a
ball mill to generate particles of size to below (150 μm) size for better
liberation; subjecting the grounded particles for desliming by a hydrocyclone;
subjecting the underflow product from the deslimed hydrocyclone to grinding
below 37 micron size particles and using as a feed for selective flocculation
process.