FIELD OF INVENTION:
This invention relates to the process for the evaluation of magnetite in
ore deposit and coal washeries.
BACKGROUND OF THE INVENTION:
In coal washeries, coal of low ash fractions is separated from the rest
through different washing methods. Primarily two broad physical
beneficiation methods are used in coal washing process. Gravity
separation methods are used for washing coarser size coal that is coal of
size more than 0.5 mm. Flotation is the other method used for washing
the finer size coal (i.e. coal of size fraction finer than 0.5 mm).
Approximately, in India 75% of the feed coal treated in a coal washery is
coarser in size hence; it is washed through Gravity separation methods.
Gravity separation methods cover wide spectra of unit operations which
includes heavy media separation, spiral operation, and cross-flow
separation etc. Among these three, heavy media separation is most
popular and commercial plants used machines such as Dense Media
Cyclone (DM Cyclone) and Heavy Media Bath (HM Bath). In both these
machines a dense media is prepared by mixing fine magnetite powder in
water. In these media the heavier ash containing particles are allowed to
settle and thus the clean coal with low ash gets separated (US Patent
5794791, 5277368, 4802976). The role of media is very important in this
separation. In this process of separation, specific gravity of cut
corresponds to an ash level in the clean coal. An increase in specific
gravity of cut would deteriorate the clean coal quality while decrease in
specific gravity of cut would cause loss of clean coal yield. Specific gravity

of cut which is the specific gravity of the media has to be maintained at
the desired level so that separation is sharp minimizing misplacement
and it is possible when segregation of magnetite is restricted in the
separating vessel. Property of magnetite powder such as purity of the
magnetite grains and its fineness are important in this context and hence
these parameters are closely monitored by the operator to ensure high
separation efficiency (US Patent 5314124 and 5280836). Pure magnetite
particles will have consistent specific gravity and their breakage property
will also be of uniform. Therefore, purity of magnetite particles is the
single most important media property which would ensure best
separation.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a process for the evaluation of
magnetite in ore deposit and coal washeries;
Another object of this invention is to propose a process for the evaluation
of magnetite in ore deposit and coal washeries by using high separation
efficiency;
Further object of this invention is to propose a process for identifying a
good grade magnetite resource.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a process for the evaluation
of magnetite in ore deposit and coal washeries comprising:

collecting magnetite samples from geological sites and marking then
crushing the magnetite samples to below 45 micron size;
subjecting the crushed magnetite to the step of sink float tests;
analyzing the 3 float fractions by various techniques and the sink
fraction is tested in a Davis tube.
BRIEF DESCRIPTION OF THE ACCOMPANY DRAWINGS:
Figure 1: This is a float type lean magnetite deposit of Jharkhand.
Figure 2: This is also moderately lean deposit with very good grade of
magnetite present as lenses. The mine is owned by Jharkhand State
Mines and Development Corporation.
Figure 3: A simple representation of the magnetic spins in a
ferrimagnetic oxide.
Figure 4: XRD pattern of an Indian magnetite sample collected and
prepared as per the suggested method. [Note: J- Jacobsite, H-Hematite;
M- Magnetite; TM-Titanomagnetite; Hy - Hydrohematite; R-Ringwoodite;
Te- Tenorite; Chc-Chalcocite; Rt-Rutile; Ch-Chalcopyrite.]
Figure 5: XRD pattern of the imported magnetite sample with identified
phases (Hm-hematite; Mg-magnetite; Mgf-Magnesioferrite; Hd-
Hedenbergite).

Figure 6: Micrograph of an Indian magnetite sample collected and
prepared as per the suggested methods. [Note: The sample contains
mostly magnetite grains. Only along the weak plane magnetite is partially
altered to hematite. There are voids (black in colour) in the sample.]
Figure 7: Photomicrograph of the imported magnetite. It shows different
sizes of magnetite grains and few grains of hematite.
DETAILED DESCRIPTION OF THE INVENTION:
This invention is to evaluate a magnetite deposit that can be a resource
for washery grade heavy media. This is possible when the deposit
contains good quality ferri-magnetic particles and they are sufficiently
liberated. The invention suggests the method to find out the ferri-
magnetite particles in a deposit and their liberation status. X-Ray
Diffraction (XRD) method shows crystal structure of mineral and this
information coupled with the chemical composition of the mineral would
suggests minerals present in the given sample. The same sample is then
analysed microscopically to find out the mineralogical texture and grain
boundary to establish the liberation status. Selection of samples from the
ore body is vital for carrying out the studies as mentioned above. This
selection is based on field observation and other macroscopical study.
This invention is to find out good grade magnetite deposits which can be
exploited to produce washery grade magnetite. The very first step in this
direction should be a site visit in which the details of the deposit have to
be collected. This includes deposit location, reserve, structural history,
mining history if any and, geological formation. In the second step,
relevant samples are to be collected from the deposit. In Indian context,

two different types of deposits are found and they are insitu vein type
deposit and drifted type deposit which is locally known as float. Best
quality magnetite are usually found in insitu type deposit wherein along
with these best quality materials patches and veins of other non-
magnetic materials are also found. The float grade material are magnetite
which are dislodged from the main deposits and then re-accumulated in
a near by place as detritus material. In this type of deposit the material
flow itself helps in removing the trash or gangue particles to distant
places as these gangue particles are lighter in nature. Thus, the nature
helps in up-grading the magnetite quality and this is why this magnetite
is also marketed by the traders. However this magnetite is also exposed
to atmosphere resulting to oxidation; and it leads to partial conversion to
hematite. Figure 1 and 2 shows photographs of float grade and insitu
type magnetite deposit. Typically magnetite deposit contains three
categories of minerals and they are; (i) Magnetite, (ii) Magnetite & altered
magnetite and, (iii) Non-magnetic minerals. The required quality of
washery grade magnetite is very stringent and this can only be fulfilled
by the high-grade magnetite. Based on macroscopic observation high
grade magnetite samples are collected from the field. These samples are
ground to the required size and then subjected sink - float tests followed
by Davis tube test to recover magnetite particles from the rest of the
gangue particles. The magnetite particles are then subjected to three
different tests namely; chemical, XRD and microscopic analysis for its
quality evaluation.
Chemical analysis is carried out to find the FeO content in the sample.
Magnetite is a special class of magnetic materials and this is known as
Ferrimagnetic material as they show ferrimagnetism. In ionic
compounds, such as oxides, more complex forms of magnetic ordering
can occur as a result of the crystal structure. A simple representation of

the magnetic spins in a ferromagnetic oxide is shown in Fig. 3. One type
of magnetic ordering is called ferrimagnetism. The magnetic structure is
composed of two magnetic sub lattices (called A and B) separated by
oxygen. The exchange interactions are mediated by the oxygen anions.
When this happens, the interactions are called indirect or super
exchange interactions. The strongest super exchange interactions results
in an anti parallel alignment of spins between the A and B sub lattice. In
ferromagnetic material, the magnetic moments of the A and B sub
lattices are not equal and result in a net magnetic moment. Magnetite is
a well known ferrimagnetic material. The tetrahedral and octahedral sites
form the two magnetic sub lattices, A and B respectively. The spins on
the A sub lattice are anti parallel to those on the B sub lattice. The two
crystal sites are very different and result in complex forms of exchange
interactions of the iron ions between and within the two types of sites.
The structural formula for magnetite is
[Fe3+] A [Fe3+, Fe2+] B O4
This particular arrangement of cations on the A and B sub lattice is
called an inverse spinel structure. With negative AB exchange
interactions, the net magnetic moment of magnetic is due to the B-sites
Fe2+. The FeO content in the sample indirectly measures the proportion
of Fe2+ in the B-sites. Table 2 provides the list of minerals that shows
ferromagnetic properties. Presence of these minerals in the ore would
make it suitable for further beneficiation so as to produce the washery
grade magnetite. X-Ray Diffraction (XRD) analysis is carried out to know
the mineralogical composition of the ore. Figures 4 and 5 shows XRD
analysis report of a magnetite sample prepared from ore and the
imported magnetite; which is being currently used in the washery to
produce coarse clean coal. The analysis report clearly shows that the
washery grade magnetite contains many other ferrimagnetic materials

other than magnetite. The ore contains primarily magnetite as
ferrimagnetic material and altered product such as hematite. Therefore,
beneficiation processes are required to separate washery grade magnetite
from this ore. The prospect of up gradation through beneficiation
primarily depends on the textural association of the ferrimagnetic
material and other gangue minerals. This is studied through
microscopical analysis. Figures 6 and 7 shows microscopical analysis of
a magnetite sample prepared from an Indian ore deposit and the
imported magnetite powder currently being used in the washery. It is
evident from the microscopical analysis that the ore contains quality
grade magnetite with marginal alteration along the grain boundary.
These magnetite grains are free from any non-magnetic inclusions.
Therefore, these ore are good feed for beneficiation and would produce
washery grade magnetite.
Testing methods
Magnetite samples are collected from the field based on the information
of local geology and on site macroscopic analysis. These samples are
numbered as it helps to trace back the deposit location. These samples
are then brought to laboratories for detail analysis. A part of the sample
is then ground to below 45 microns and then subjected to series of sink -
float tests. Sink-float tests were carried out in three different stages
using three different heavy media liquids. Among these three liquids the
lightest one is used in the first stage of sink - float tests and the heaviest
liquid is used at the final/third stage of sink-float tests. The sink from
the first stage of sink-float tests was used for the second stage of sink -
float tests similarly, the sink of the second stage of sink - float tests is
tested in the final stage of sink - float tests. At the end of the sink - float

tests four products are generated; float - 1, float -2, float- 3 and sink -3.
This process enriches the ferromagnetic proportion in the Sink- 3
fraction. Sink-3 is then subjected to Davis tube test wherein the
magnetic fraction is separated from the rest. The magnetic fraction of the
Davis tube test product mostly contains ferromagnetic material and
hence this fraction is used for further evaluation study. This fraction is
divided into three fractions for chemical, XRD and, microscopical
analysis. Chemical analysis shows FeO content in the enriched fraction.
FeO content of 23% and around would indicate the ore contains washery
grade magnetite powder. Chemical analysis also indicates the other
chemical components present in the ore. This information is used in XRD
analysis to calculate the semi-quantitative mineralogical composition of
the sample. This test shows the types of ferromagnetic minerals and
other type of minerals present in the ore. With this information the
microscopic analysis is carried out. In this analysis at the first step, the
major minerals are identified. In the next step, the association of
ferromagnetic minerals and rest of the minerals (that is impurities) are
studied. Finally, the liberation status of ferromagnetic minerals is
established. In case the impurities are coarser and accumulated in
places then beneficiation would be easy and washery grade magnetite
can be produced from such ore deposits. On the contrary, if impurities
are finer, disseminated and occurring as inclusion within the grains of
ferromagnetic minerals then the ore is difficult to wash and may not yield
the washery grade magnetite. Such ore deposits are not good grade
deposits.

Table 1: Quality comparisons for magnetite from indigenous source and
imported.

†This is an indicator of magnetic content in the magnetite.
Table 2: Magnetic properties of iron minerals.


WE CLAIM:
1. A process for the evaluation of magnetite in ore deposit and coal
washeries comprising:
collecting magnetite samples from geological sites and marking then
crushing and grinding the magnetite samples to below 45 micron size;
subjecting the ground magnetite to the step of sink float tests;
analyzing the 3 float fractions by various techniques and the sink
fraction is tested in a Davis tube. The magnetic fraction obtained from
the Davis tube was further tested through chemical, XRD and,
microscopical analysis.
2. The process as claimed in claim 1, wherein the sink and float fractions
are carried out in the following 3 steps:
first step lowest density liquid is used for the test and at the end the
highest density liquid is used, at each stage the float fraction is kept
aside and the sink is used for the next stage of sink-float tests.
3. The method as claimed in claim 1, wherein the magnetite evaluation
involves a series of sink-float test, Davis tube test, Chemical, XRD &
microscopical analysis.
4. The method as claimed in claim 1, wherein the heavy media liquids of
2.96, 3.30 and, 4.03 gm/cc density were used.

5. Chemical and XRD analysis together indicates the mineralogical
composition of the ore. Presence of minerals such as magnetite and/or
other ferromagnetic minerals makes the ore suitable for its use in
washery.
6. Microscopical study shows the textural analysis of these minerals and
thus indicates liberation characteristics of the ore. This study helps in
further in selecting a magnetite ore for its use in the washery.
7. The method as claimed in claim 1, wherein the washery graded
magnetite as well as any new magnetite deposit is evaluated with clear
understanding of the subject. This method is less cumbersome and
robust than any arbitrary methods being followed by the traders.

A process for the evaluation of magnetite from ore deposit and magnetite used in coal washeries comprising: collecting magnetite samples from
geological sites or from washery and marking then crushing an grinding the magnetite samples (if required) to below 45 micron size; subjecting
these magnetite to the step of sink float tests; analyzing the 3 float fractions by various techniques and the sink fraction is tested in a Davis tube. The magnetic fraction is then further analysed through chemical, XRD and microscopical methods.