FIELD OF THE INVENTION
This invention relates to a process for selective fragmentation and liberation to
recover iron values from low grade iron ores with complex liberation
characteristics. More particularly, the invention relates to a method of selective
fragmentation of low grade iron ores exhibiting an interlock relationship of iron
minerals with gangue minerals to liberate at a particle size less than 45microns.
BACKGROUND OF THE INVENTION
The high depletion of superior grade (>55 Fe%) iron ore reserves rendered
imperative search for alternate resources such as sub grade or low grade or
inferior grade iron ores (<55 Fe%). These low grade iron ores have not been
optimally exploited because of challenges in beneficiation due to poor liberation
and complex relationship of minerals. Therefore, these consolidated iron ores are
associated with the requirement of fine grinding to achieve necessary liberation
of valuable minerals. But, the traditional mechanical size reduction of ores is an
energy-intensive and highly inefficient process due to indiscriminate nature of
breakage. Most of the energy is absorbed due to impact and dissipated as heat
or noise in the prior art process of mechanical size reduction, excepting a small a
part of energy is utilized to generate new surfaces. Based on the energy
requirement to create new surfaces, conventional grinding efficiency is found to
be less than 1%. Additionally, the ultra-fine slime generation is inevitable and
thereby increases the loss of valuable minerals due to the inefficiencies of prior
art towards mineral recovery at finer sizes. Unfortunately, conventional grinding
technologies are driven to reduce size by producing transgranular fractures
(random) in ore particles.
Therefore, no control can be exercised for intergranular or phase boundary
fracture compared to transgranular fracture. Inadequate mineral liberation leads

to improper beneficiation and low recovery of mineral with high operating costs.
Hence, phase boundary fracture is highly desirable which interalia allows
recovery of mineral particles, thus making the separation process more efficient.
The accumulated intergranular fractures in fact lowers the strength of mineral
and increase the grinding efficiency. The promoted improvement in comminution
is significant only to reduce its high operating costs. The important role of
comminution is not merely to reduce the particle size but to liberate minerals from
each other, which interalia improves the efficiency of size reduction processes.
Therefore, the selective fragmentation and liberation by pre-treatment processes
can significantly enhance the grinding and beneficiation to recover iron values.
The available low grade or inferior grade iron ores (40-45% Fe) are in the form
of banded iron formations (BIF) composed of alternate bands of iron oxide
minerals and gangue minerals. BIF ores are considered as mother rock of earth
and highly available in the form of hematite-jasper or hematite-quartz or
magnetite-jasper or magnetite-quartz deposits at various zones of India, South
Africa etc. These ores are generally rejected at mine site due to presence of high
gangue composition with iron ore. Besides, conventional grinding of banded iron
formations ores cannot produce unlocked particles due to its banded structure
and dissemination of minerals. Therefore, it is necessary to carry out fine
grinding (<10um) to liberate the iron minerals from gangue minerals which is
difficult to beneficiate by conventional flotation or magnetic separation
concentration methods. And also, separation efficiency considerably falls down in
the size range of <10um. Hence it is significant to develop a new method of crack
formation and selective fragmentation to liberate iron minerals from banded iron
formations by pre-treatment processes.
According to the technique known from prior-published non-patent literatures the
low grade iron ores of hematitic/ magnetitic banded formations with quartz/jasper
are subjected to pre-treatment processes such as high pressure compression,

electric and ultrasonic treatment, thermal treatment or microwave treatment to
improve the liberation, to reduce to grinding energy and to enhance the
separation through downstream processes. During high pressure compression,
the material get nipped between the counter rotating rolls under pressure and
interalia generate micro cracks. The ores are broken down along the boundaries
of minerals because the molecular structures in the boundary are much weaker.
However, high pressure compression method has disadvantages over the known
processes of thermal or microwave treatment because of simultaneous formation
of fractures including closing of existed fractures. Further, the technique of
electroacoustic comminution to generate hydraulic Shockwave of explosive
intensity is unsafe and not feasible at process plant. Unfortunately, these
processes require high energy for breakage and selective only for coarser sizes.
It is known that during the thermal heating, the energy is transferred through
convection, conduction and radiation of heat from the surface to the center of ore
particles due to thermal gradients. The method of thermal heating relies on cyclic
heating and cooling of an ore, and promotes intergranular fracture by exploitation
of different responses (resistance, liberation and size distribution) of ore
constituents. Thermal shock fragmentation and its severity to nucleate fracture in
the ore particles can be estimated. The relationship of critical stress time with the
fracture length as per critical stress theory have been developed (Lehnhoff T. F.,
1975; Geller L. B., 1972). It is known that application of thermal treatment on
quartz (>650°C) followed by quenching, reduce specific fracture energy (Slavomir
H. 2005; Pocock J., 1998).
In contrast to thermal heating, the microwave heating rapidly penetrate and
deposit energy directly to the center of ore particles through molecular interaction
with the electromagnetic field. This heating is due to the transfer of
electromagnetic energy to thermal energy which does not rely on diffusion of
heat from the surfaces. Microwave is part of nonionizing electromagnetic

spectrum with frequencies of 300MHz-300GHz and wavelengths of 1m-1mm.
These include bands of ultra-high frequency, super high frequency and extremely
high frequency. However, 2.45GHz is the accepted frequency for commercial
applications. The energy can be transferred from the source through a hollow
nonmagnetic metal tube, known as a waveguide. The generation of
electromagnetic radiation results from the acceleration of charge. The microwave
sources are vacuum tubes such as magnetrons, traveling wave tubes, and
klystrons. Magnetron tubes are efficient and most reliable. These tubes use
resonant structures and capable of generating a fixed frequency electromagnetic
field. TWT are used to generate variable frequency electromagnetic field.
Microwave heating is unique and have number of advantages over conventional
heating such as:
♦ Non-contact heating;
♦ Energy transfer rather than heat transfer (radiation);
♦ Rapid heating;
♦ Material selective heating - responsive phases;
♦ Volumetric heating reduce processing times and save energy (No bulk
heating);
♦ Fast start-up and stop;
♦ Interior heating of the material;
♦ Safe and ease of automation
The interaction of microwaves with material depends on magnetic and dielectric
properties described in terms of permeability and permittivity respectively. The
rate of microwave energy absorption occur at molecular or atomic level is
controlled either by migration of electronic or ionic species (conductivity) and/or
rotation of free or bound charges such as electrons or ions or permanent dipolar
molecules (number per unit volume) which interalia induces the transient motions

as shown in Figure 1. These dipoles may be natural or induced feature of
dielectrics. Distortion of the electron cloud around the non-polar molecules or
atoms through the presence of an external electric field can induce a temporary
dipole moment. The resistance to these motions causes losses, which result in
attenuation of the electric field and increased dissipation of energy in the
material.
Therefore, the efficiency of microwave heating depends on its heat dissipation
factor which is the ratio of dielectric loss factor to dielectric constant of the
material. The dielectric constant is a measure of the ability of the material to
absorb and retard microwave electrostatic energy as it passes through; the
dielectric loss factor is a measure of the ability of the material to dissipate the
energy as heat due to the friction during vibration of dipolar species. Therefore,
the material with high dielectric loss factor is easily heated by microwave energy.
Thus, the resulting heating is directly proportional to the product of these factors.
The polarizability of mineral in electric field can be expressed as the sum of four
dominant mechanisms:
a) Electronic contribution: Displacement of electron cloud relative to the
nucleus
b) Ionic effect: Movement of ion with respect to other ions in the presence of
electric field
c) Orientational polarization (dipole movement): Presence of permanent
dipoles in the host lattice
d) Interfacial polarization: Random or layered heterogeneities.

Microwaves can be reflected from metallic surface or refracted at a dielectric
interface or focused by parabolic reflectors or horn antennas. Materials which
reflect microwaves produce no heat and have high conductivity (conductors) are
used as conduits waveguide for microwaves. Materials which are transparent to
microwaves (insulators) used to support the material to be heated. Materials with
excellent absorption abilities are rapidly heated (dielectrics) as shown in the
Figure 2.
Most of dielectrics exhibit a difference in phase, in which the polarization
movement lags behind the alternating electric field frequency and indicates a loss
in conductivity within the material filling the cavity.
Researchers noted that the frequency of applied field, temperature, physical
properties influence pre-treatment of a material. These parameters are
compositionally dependent and has significant effect on heating rate. Different
minerals attribute to the differences in conductivities or dielectric loss factors and
bonding properties. In respect of bonding, minerals with metallic bonds can more
effectively absorb microwaves than those with purely ionic or covalent bonds.
Prior art infers that pretreatment of coals show significant crack formation,
reduced Bond work index and increase in specific rate of breakage due to
expansion of responsive inherent moisture and phase changes under differential
expansion of gangue phases (Lester E., 2004; Marland S., 1998, 2000, 2001;
Sahoo B. K., 2011; Ed Lester, 2005). It has been established that preferential
heating of pyrite and ash fraction of coal enhances the magnetic susceptibility
and improves desulphurization of coal (Fanslow G. E., 1980; Rowspn, 1990).
The phenomenon of a phase silica to p phase silica transformation in coal
slurries further improves the rheological flow characteristics and hence the
separation (Meikap B. C. et al, 2005). The microwave treatment of metallurgical
coke attenuates the grinding energy up to 40% (Ruisanchez E. et al, 2012).

The pre-treatment along with quenching of different copper ores at high electric
field strength with short exposure time showed a significant reduction in ore
strength and Bond work index including an increase in specific rate of breakage
with improved liberation due to differences in thermal expansion coefficient of
minerals (Vorster W., 2001; Kingman S. W., 2004; Sahyoun, 2004; Scott G.,
2008; Vladimir, 2011). The influence of pre-treatment on flotation of copper ore
exhibited an improvement in recovery and the cumulative grade with increasing
treatment time and power (Sahyoun C, 2005). The composition of ore is found to
be the most influential factor in the microwave treatment. The pretreatment on
Zinc and Cu-Cobalt, Lead-zinc ores was elucidated (Antoine F. M., 2005,
Kingman, S. W., 2004). This process was used to augment the grinding of gold
ore and beneficiation by gravity concentration (Amankwah R. K., 2005, 2011).
US 2003/0029944 A1 described the process of facilitating recovery of copper
from its ore by microwave treatment to form a plasma and oxidation of sulfur to
enhance comminution and recovery by downstream processes such as leaching,
smelting.
During thermal or microwave treatment of magnetite-quartz/jasper magnetic
taconite ores, cracks are formed due to difference in thermal expansion which
interalia enhanced the magnetic separation (Tevfik A., 2011). Pre-treatment of
hematite-quartz/jasper nonmagnetic taconite ores at high temperatures at
reducing atmosphere converted the hematite to magnetite (reduction roasting)
and enhanced magnetic separation (John W. W., 1995). It was found that first
order grinding kinetics with faster specific rate of breakage was followed for the
treated ore (Javad, 2012).
U.S. 3202502 described a process of conversion of hematite to magnetite of
hematite-quartz ore at high temperatures in the presence of reducing gases

(chemically active atmosphere) to improve the concentrate yield by magnetic
separation.
U.S. 7476829 B2 describes a process of microwave pretreatment of multi-phase
materials to weaken the bond between the phases of ores due to differential
expansion rates. The influence of pre-treatment on ilmenite ore showed that the
factors such as microwave exposure time, power density and sample mass had
effect on heating and the recovery and grade during magnetic separation was
improved (Kingman S.W., 1999, Guo Sheng-hui, 2011). The oolitic type iron ore
with high phosphorus was subjected to microwave treatment to induce internal
cracks and reduced the energy of grinding (Shaoxian Song, 2013).
US 20100263483 A1 discloses a process of recovering iron ore composed of
hematite, magnetite and silica by microwave treatment and spiral classification.
Because the constituents of ores typically have very different thermal and
dielectric properties with difference in thermal expansion coefficients so that
stresses of sufficient magnitude to create fractures can be developed differently
for different composition ores.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process of selective
fragmentation of low grade iron ores exhibiting an interlock relationship of iron
minerals with gangue minerals to liberate at a particle size less than 45microns,
which can form intergranular fracture at the phase boundaries of mineral-gangue
phases in the iron ore.
Another object of the invention is to propose a process of selective fragmentation
of low grade iron ores exhibiting an interlock relationship of iron minerals with

gangue minerals to liberate at a particle size less than 45microns, which
enhances the existing cracks of iron ores by pre-treatment process.
A still another object of the invention is to propose a process of selective
fragmentation of low grade iron ores exhibiting an interlock relationship of iron
minerals with gangue minerals to liberate at a particle size less than 45microns,
which improves liberation of iron minerals from iron ores
A further object of the invention is to propose a process of selective
fragmentation of low grade iron ores exhibiting an interlock relationship of iron
minerals with gangue minerals to liberate at a particle size less than 45microns,
which constitutes a process of thermal or microwave pretreatment of iron ores for
crack formation due to phase transformation and, differential heating.
A still further object of the invention is to propose a process of selective
fragmentation of low grade iron ores exhibiting an interlock relationship of iron
minerals with gangue minerals to liberate at a particle size less than 45microns,
which enhances both the grade and recovery of treated iron ore by downstream
process such as flotation.
Yet further object of the invention is to propose a process of selective
fragmentation of low grade iron ores exhibiting an interlock relationship of iron
minerals with gangue minerals to liberate at a particle size less than 45microns,
which is enabled to recover iron values from low grade banded iron formations.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method of selective fragmentation to improve
liberation and beneficiation of different types of low grade iron ores banded iron
formations where quartz/jasper can be phase transitioned from a-quartz to p-
quartz during pre-treatment processes such as microwave heating or thermal

heating followed by quenching. This method enhances the liberation
characteristics and amenable to beneficiation through intergranular fractures by
pre-treatment processes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows dipole rotation of free or bound charges induce transient
motion
Figure 2 - shows microwave interaction with different materials.
Figure 3 - illustrates the disintegration of ore rock (pre-treated) and original
rock (untreated) of 20-30mm size
Figure 4 - exhibits the fracture formation along the phase boundaries during
pre-treatment process of 1min exposure time.
Figure 5 - shows Schematic representation of selective fragmentation of
Banded Iron Formations
Figure 6 - shows Alternate bands of Quartz and Hematite of original rock.
Figure 7 - shows Microwave heating device
Figure 8 - shows High temperature XRD analysis of iron ore sample
Figure 9-(a),(b) represents respectively the untreated and treated iron ore sample
Figure 10 - (a), (b), (c) represents the selective crack formation along phase
(hematite-quartz) boundaries

Figure 11- shows composition of respective elements in the treated particle of
Figure 10 (c).
Figure 12- shows Random cracks formation during thermal shock heating
Figure 13- shows Enhancement of random crack formation due to thermal
shock heating followed by quenching.
Figure 14- shows Crack formation at phase boundaries of hematite and quartz
bands
Figure 15- shows Random crack generation at fine particles during thermal
heating
Figure 16 - shows Macro and micro cracks formation at mineral-gangue phase
boundaries during thermal heating and quenching
Figure 17- shows Particle size distribution of treated, untreated and feed
sample of grinding
Figure 18- shows Appearance of bands of hematite-quartz for untreated iron
ore sample
Figure 19- shows Massive bodies of liberated hematite, quartz and sporadic
hematite with quartz for treated iron ore sample
Figure 20 a, b, c & d: shows Liberation analysis of treated iron ore sample
DETAILED DESCRIPTION OF THE INVENTION
The low grade iron ores of banded iron formations of hematitic - quartz iron ores

was used in the present invention. These iron ores were subjected to thermal or
microwave pre-treatment to recover and liberate iron oxide minerals from gangue
minerals such as quartz or jasper.
The present invention is to liberate iron oxide minerals from banded iron
formation ores along the boundaries of the minerals as represented
schematically in Fig 5. Banded Iron formation particularly, Banded Hematite
Jasper (BHJ) ore is collected from iron ore mines, crushed to 10mm size and
screened to prepare the feed material. Before using any of the screened
fractions, sampling was done using coning-quartering method, to ensure
homogeneity in the sample. The mineralogical data showed that BHJ consists of
hematite and jasper as mineral and gangue phases respectively. The
characterization studies revealed the alternate band formation of hematite and
quartz of variable thickness; massive bodies of hematite and quartz; sporadic
presence of quartz in hematite band and vice-versa (Fig.6). The gangue mineral,
quartz or jasper relatively disseminated over bands of hematite and hematite
relatively disseminated over bands of quartz. This type of dissemination is highly
responsive to microwave heating.
The liberation size analysis of BHJ showed that free hematite can be obtained
below 45u size. The chemical analysis of the sample is shown in Table 1.
e
Table 1: Chemical analysis of Banded Iron Formations (as received)

The BHJ sample is pre-concentrated in a duplex jig concentrator to separate
massive bodies of quartz to reduce the grinding load and improve the process.

The ore is crushed to -10mm size and subjected to jig operation at 200-260 of
strokes/min with frequency, 0.25-0.75" stroke length. The chemical analysis of
concentrate of jig unit is as shown in Table 2.
Table 2: Chemical analysis of pre-concentrated Banded Iron Formations

The concentrate from Jig unit operation is pre-treated through microwave
treatment or thermal treatment.
Microwave treatment
The microwave heating device is a multimode household microwave oven with
dimensions with an input rating of 230V and 900W (Output power). The
microwave heating device comprises a source for power supply, a magnetron, an
applicator i.e., oven for the heating of the target material and a waveguide for
transporting microwaves from the generator to the applicator.
The microwave source and wave guide is present at the right-hand side wall of
the oven cabinet, insulation is provided to the stainless steel cabinet which is
formed with alumina-silicate refractory bricks based on glass wool protection.
The oven consists of a radiation proof, transparent door, and an electronic
programmable panel with options for controlling the microwave power and
heating duration as schematically shown in Figure 7.

The jig concentrate was taken in silica crucibles and heated in the microwave
oven at the desired temperature for a limited time duration. A portion of the BHJ
was taken for chemical analysis. The samples of BHJ that were treated with
microwave mounted on a resin base for SEM analysis to verify the micro crack
propagation along grain boundaries. The effect of duration of heating, power of
the microwave irradiated and quenching after heat treatment on the crack
propagation along the phase boundaries to form fractures and obtained free
hematite particles during conventional grinding. The presence of quartz in the
iron ore increases the cost of production due to increase in grinding energy
because of hardness of the ore. Quartz undergoes a reversible change in crystal
structure at 573°C from a-quartz to p-quartz. This phenomenon is termed as
quartz inversion or phase transformation which is accompanied by changes in
properties. Six-sided rings of tetrahedral crystal structure have 3-fold symmetry in
a-quartz and the angle between these rings changes with the increase in the
temperature and reaches to P quartz structure. These changes in the structure
increase its crystal volume by 2%.
This phenomenon along with the anisotropic expansion of quartz crystals builds
internal strains and high thermal stresses of sufficient magnitude throughout the
structure (stress concentrations, inclusions, and microscopic cracks) and at grain
boundaries. Due to linear thermal expansion coefficient and Young modulus,
micro cracks and intergranular fractures along mineral boundaries are produced .
These developed stresses due to volumetric expansion of quartz are responsible
for fractures at quartz-mineral interface and enhance the liberation and
grindability of ores. Besides, quenching of treated iron ore with water or acid or
alkali or salt solution can release the internal stored energy and append the
process of cracking. The rapid quenching of high temperature treated iron ore
particles produce internal stresses with intense micro cracking result in ease of
comminution and reduction of grinding energy. This process further weakens the
ore due to enhanced stresses and reduces the boundary cohesion. This kind of

micro cracking and intergranular fractures have the potential to improve grinding
efficiency, reduce Bond work index, increase liberation of individual mineral
phases and amenable to beneficiation. The mechanisms of microwave heating of
iron ore are differential heating of the two different mineral phases (hematite and
quartz); transient heat conduction during pre-treatment between minerals; phase
transformation of quartz from a-quartz to P-quartz; volumetric expansion during
phase transformation, heating and quenching; thermal damage associated with
material failure and strain softening.
In the present invention, Banded iron formations such as banded hematite jasper
has two types of textures such as bands of hematite & quartz and sporadic
distribution of phases in these bands. In the bands of these type of iron ore,
hematite is an active material to microwave heating, while quartz is inactive. It is
reported that the microwave heating rates of hematite and quartz are 170 and
2°C/s, respectively. At the same exposure of time and power of microwave
radiation, hematite would be heated to a much higher temperature than quartz,
leading the hematite to expand much more than that of the quartz. This
difference on the expansion results in formation of the fractures along the
boundaries (differential heating). The presence of sporadic distribution of iron
oxide phases in quartz phases increases the heating rates of the quartz due to
transient heat conduction between the hematite and quartz which results in
formation of micro-cracks even at finer sizes. The pre-treatment time required for
fracture formation is decreased due to this phenomena. Further fracture
formation enhances after reaching to 573°C due to phase transformation of the
quartz.
Thermal Treatment
The jig concentrate of BHJ iron ore of particle size of below 10mm was subjected
to thermal treatment in a muffle furnace. Two factors which are controlled during

the thermal treatment are heating rate and temperature. The sample was tested
for continuous thermal treatment and thermal shock treatment During continuous
thermal treatment, the sample was inserted inside the furnace at room
temperature and subjected to heating up to a set temperature of 573°C. The
sample was held in the furnace for 15min at this temperature.
During thermal shock treatment, the furnace was preheated to 573°C and the
sample was inserted at 573°C without lowering the temperature. The heating
continued for 15 - 45 min at heating rate of 10°C/ min. After this treatment, the
sample was removed from the furnace and immediately quenched. Rapid
quenching with water and other coolants was performed by dropping the iron ore
particles immediately in a bucket full of water or other coolants after taking the
sample out from the furnace. No measurable rise in temperature of the water or
coolants was noticed. Excess water or coolant was decanted and the samples
were dried for further analysis. Quenching of the treated iron ore with water or
acid or alkali or salt solution allows release of the internal stored energy and
append the process of cracking. This kind of micro cracking and intergranular
fractures have the potential to improve grinding efficiency, reduce Bond work
index, increase liberation of individual mineral phases and amenable to
beneficiation.
Changes in the resistance to breakage of the treated and untreated ore was
quantified in terms of comparative uniaxial compressive strength, hardness of
ore, work bond index and size distribution. Mineralogical investigations and
phase transformation was confirmed using X ray Diffraction analysis. The
selective breakage along the grain boundaries was analyzed using a SEM
(scanning electron microscope) for microwave and/or thermal treatment
processes. The present invention showed the effect of microwave and/or thermal
treatment on recovery of high siliceous iron ore. Mechanical properties and
flotation tests were included to evaluate the performance of the pretreatment
processes.

XRD Analysis:
High temperature XRD analysis of BHJ iron ore sample showed a shift in the d-
spacing value from 3.365 to 3.41 and confirmed the phase transformation of a-
quartz to (3 quartz structure during microwave or thermal treatment of the iron ore
(Figure 8).
SEM Analysis:
SEM analysis is most reliable tool to study the fracture pattern and mineral
liberation. After the microwave and/or thermal treatment of BHJ iron ore sample,
the sample was mounted into epoxy resin and polished for scanning electron
microscope (SEM) analysis. Each of the polished samples are systematically
examined and the minerals are identified on the basis of chemistry. SEM images
of the samples are captured for changes of fractures on the surfaces before and
after the thermal or microwave treatment and to compare the pre-existing and
newly developed fractures on mineral interface (Figure 9 a,b). The formation of
fractures on the ore surfaces is determined. Three kinds of results could be
obtained from the observation, namely no change (there was no difference
before and after physical treatment), new fracture (new fractures were formed
after the pretreatment) and widening (the old fractures before the treatment are
widened after the treatment). The numbers of the images for the three results are
recorded with which the percentages of the each case (new fracture, widening
and no change) in the total 50 images are reported. Bright phase present in
sample is hematite and grey phase is quartz. No fracture pattern is observed for
untreated particles. Thermal and microwave treated particles are appeared with
fractures of different intensities and in different directions. These fractures are
near or at the mineral-gangue phase boundaries.

This process enhanced the liberation of finely disseminated ore and gangue
minerals which cannot be recovered by beneficiation without chemical leaching.
Fracture pattern looks very affirmative to prove that thermal or microwave
treatment initiates and propagates the fractures at mineral-gangue phase
interfaces.
Microwave Treatment
The SEM analysis of treated particles showed the selective formation along the
bands (Figure 10 a, b, c) and their chemical analysis as shown in the Figure 11.
Thermal Treatment
1. Thermal Shock Treatment: Thermal shock heating induced heat stresses at a
very rapid rate without much volumetric heating and phase transformation
lead to cracks formation at random directions throughout the BHJ sample
(Figure 12). No selective cracks are observed along the mineral-gangue
- phase boundaries. Thermal shock heating followed by quenching of ore is
also responded as thermal shock heating. But random crack formation is
enhanced during quenching as shown in Figure 13.
2. Continuous thermal treatment: Continuous thermal heating of ore induced
cracks along the mineral-gangue phase boundaries of bands of hematite and
quartz (Figure 14). Besides, random crack formation is observed at fine size
due to the presence of weak zones where intensity of fracture formation along
the phase boundaries is difficult at low power densities (Figure 15).
Therefore, at fine size particles (< 20pm) of ore indicated the random crack
formation without any selective direction during microwave or thermal heating.
This may be attributed to the fact that the rate of heating is insufficient to induce I

ocalized thermal gradients of a magnitude that would generate thermal stresses
that exceed the strength of the ore material. Therefore the magnitude of fracture
in ores during thermal or microwave treatment will increase with increase in
power density at minimum exposure time.
Continuous thermal heating along with quenching enhanced the crack formation
and propagation along the mineral-gangue phase boundaries. Macro cracks and
micro cracks are observed along the mineral-phase boundaries for coarse and
ultra-fine particles respectively (Figure 16).
Grinding Tests
The untreated and treated iron ore sample is subjected to conventional grinding
and sieving at different size fractions using standard sieves. The particle size
distribution of treated, untreated and feed sample (before grinding) is analyzed
as shown in the Figure 17. It clearly indicated the improvement in liberation and
generation of fine size particles at same input energy. The feed sample
comprises D80 passing 9mm size, whereas untreated sample comprises D80
passing 90um and treated sample comprises 80um size. This reveals that the
fineness of product is increased by thermal treatment and showed ease of
grinding.
Liberation Study
The treated and untreated samples after grinding are analyzed to determine the
liberation degree of hematite and main gangue minerals by using SEM image
analysis. SEM analysis (Figure 18) showed the presence of hematite and quartz
bands of ore even at 80-100pm size for untreated sample due to poor liberation
characteristics.

SEM analysis for the treated sample (Figure 19) showed the improved liberation
in terms of phase existence in the particle. No band formation is observed for this
sample. Pure hematite and quartz particles are observed along with sporadically
distributed particles.
The liberation of treated iron ore samples is shown in the Figure 20. Figure 20 a
& b represents the presence of hematite rich and quartz rich sporadic particles.
Figure 20 c & d represents the presence of liberated massive bodies of hematite
and quartz for treated iron ore samples.
Physical Properties
Hardness Test
BHJ iron ore sample of 20-30mm size is taken for hardness test. The sample is
prepared by surface grinding technique before treatment. The treated and
untreated sample hardness is measured using Vickers micro hardness testing
instrument as shown in Table 5. The hardness test for untreated samples
showed the points comprised quartz has very high hardness value of 1500-1660
HVI, and the points comprised hematite has comparatively less hardness value
of 865-910HVI.
The points comprised sporadic quartz with hematite and vice-versa has an
intermediate values of 1150-1200 HVI. The same sample of ore is subjected to
thermal or microwave treatment and the hardness is measured. The differences
in the hardness values of different phases of untreated and treated iron ore
sample is tabulated as shown in the Table 5. The bulk hardness of the BHJ
material is greatly reduced, that is evident from the values.

Table 3: Hardness test for untreated and treated iron ore samples at different
phases

Compressive Strength
The compressive strength of iron ore is obtained for untreated and treated ore as
150-200 MPa and 75-120MPa. The reduction of strength under compressive
forces of iron ore based on the a to P phase transition temperature and
subsequent crack formation.
Bond Work Index
The purpose of the Bond grindability test is to determine a parameter which
expresses the resistance of a material to grinding. This parameter is known as
the Bond work index. The Bond work index of a material is an indicative of
energy required to reduce one ton of material from theoretically infinite size to
80% passing 100um. It is determined by the grindability test and is expressed in
kWh/t.
The Bond work index can be used in the calculation of the energy consumed
during grinding in a ball mill of 30.5cm diameter operating with 250% recirculation
load in a circuit closed by classifier.

Standard Test Procedure
The standard Bond grindability test is a closed cycle dry grinding and screening
process which is carried out until steady state conditions are obtained. The
standard ball mill of size 30.5 by 30.5cm diameter and length is used at 70rpm
with a charge of 285 steel balls ranging in the size from ZA inch to 3/2 inch in
diameter and weighing approximately 20.12kg. The ball media distribution used
in bond work index test with surface area of 842sq in. is as follows:
Media distribution:
43 Balls - 3.7cm (9.094kg)
67 Balls - 3cm (7.444kg)
10 Balls-2.5cm (0.694kg)
71 Balls -1.9cm(2.078kg)
94 Balls-1.55cm (0.815kg)
Total 285 Balls - 20.12kg
The dry feed material is stage crushed to 3.3mm size and the circulating load is
maintained constant at 250% by adjusting the number of revolutions for each
grinding cycle. The ore of 3.3mm particle size is packed to 700cc using vibration
table. The weight of material is noted using weighing balance. The feed material
size distribution is
obtained through series of sieving at different mesh sizes. The feed material is
subjected to grinding at specified number of revolutions. At the end of the first
cycle, the product is discharged from the mill and screened on a test sieve (Pi:
100um). The oversize fraction is returned to the mill for the second run together
with fresh feed to make up the original weight. The weight of product produced
per unit of mill revolution defined as grindability of ore is calculated (Gi) and used
to estimate the corresponding number of revolutions required for the second run,

equivalent to a circulating load of 250%. The process is continued until a
constant value of grindability is achieved which is the equilibrium condition. The
average value of the last three cycles is taken as the standard Bond grindability
(G) which is the net grams of undersize produced per mill revolution. Bond work
index is defined by the following empirical equation:

The energy required for grinding is calculated as

Where, W is the energy input to ball mill (kWh/t)
Wi is the Bond work index (kWh/t)
P80 is 80% passing size of mill product (urn)
F80 is 80% passing size of mill feed (urn)
The Wi value obtained from the above equation is conformed with the motor
output power to an average overflow ball mill grinding wet in closed circuit. For
dry grinding, the work input should normally be multiplied by 1.3. However ball
coating and packing can increase the work input in dry grinding.
The bond work index of untreated and treated iron ore samples is determined.
The Bond work index of untreated iron ore is 14-16 and treated iron ore is 8-11.
The Bond work index of material is decreased 25-40% by the pre-treatment
processes.
Beneficiation

Flotation Test
Flotation test is performed for untreated and treated iron ore sample. Liberation
of iron ore is improved during pre-treatment processes showed the effect on
flotation of iron ore.
The untreated and treated iron ore sample of 0.1mm size produced during
conventional grinding is taken at 20-30% solids in a Denver flotation cell. The
cationic reverse flotation of iron ore at pH of 9 is considered. Different reagents
are used to control the solution chemistry and to modify the surface properties of
iron ore. Sodium poly meta phosphate is used as dispersant at a dosage of 0.5-
2g/kg of ore and polysaccharides solution as depressant at a dosage of 2-4g/kg.
The slurry is conditioned with cationic surfactants at a dosage of 0.5-2g/kg of ore
to collect selectively silica at the top of the flotation machine. Further is added to
the conditioned slurry before flotation. The air flow rate is kept at constant value
and impeller speed is consistently maintained. The bed of froth is formed at the
top surface of flotation for about 1min, after which it is skimmed off at regular
time intervals and collected in pans. The froth and concentrate samples are
analyzed for chemical analysis. The feed, untreated and treated flotation results
at same yield are as shown in Table 4.
Table 4: Flotation results for untreated and treated ore


The enhancement of grade is achieved with pre-treatment processes than
conventional grinding which based on probability. The iron values can be
recovered easily from low grade iron ores using pre-treatment processes due to
occurrence of phase boundary fractures and improved liberation. This process
may also increase magnetic properties of the material during pre-treatment
processes due to alignment of dipoles in the iron ore.

WE CLAIM
1. A method of selective intergranular fragmenting and liberating iron oxide
minerals from low grade iron ores, the method comprising:
- heat treating the low grade iron ore to a temperature where a-quartz is
phase transition to (3-quartz; and quenching the heat treated low grade
iron ores, wherein the iron oxide minerals are liberated at a particle size of
less than or equal to 45 microns.
2. The method as claimed in claim 1, wherein the heat treatment involves
thermal heat treatment.
3. The method as claimed in claim 1, wherein the heat treatment involves
microwave heat treatment.
4. The method as claimed in claim 1, wherein the quenching is done using
water or brine solution.
5. The method as claimed in claim 1, wherein the low grade iron ores
comprises banded iron formations of hematite/magnetite bands and
alternate quartz/jasper gangue mineral bands.
6. The method as claimed in claim 5, wherein banded iron formations iron
ore undergo phase transformation from a-quartz to 3-quartz at least at
573oC temperature.
7. The method as claimed in claim 2, wherein the thermal treatment is
conducted at a heating rate of 5 to 10°C/min for at least 30 minutes.
8. The method as claimed in claim 3, wherein the microwave treatment is
conducted at 0.5-1kW output power with frequency of 2.45GHz for 3 to 7
minutes.

9. The method as claimed in claim 1, wherein heat treatment reduces the
grinding Bond work index of the treated iron ore sample by 25-40%.
10.The method as claimed in claim 1, comprising the step of cationic reverse
flotation at pH 8-10 with ether diamine collector.
11.The method as claimed in claim 10, wherein the grade of flotation
concentrate increases by at least 3-6 units.
12.The method as claimed in claim 10, wherein the recovery of iron from the
iron ore increases by at least 3%.
13. The method as claimed in claim 11, wherein the flotation concentrate of
60-63 Fe% is used for blend of the ore for sintering or pellet making.
14.The method as claimed in claim 1, wherein the phase transformation of
the sample from α-quartz to β-quartz during the heat treatment takes
place with the shift in d-spacing value of the iron ore sample from 3.365 to
3.41.