FIELD OF INVENTION
The present invention relates to a method to produce cold bonded chromite-coal
composite pellets for smelting in arc furnace for the production of ferrochrome.
BACKGROUND OF INVENTION
Ferrochrome is produced through carbothermic reduction of chromite burden in
submerged arc furnace (SAF). The SAF smelting process is found to be highly
effective due to use of permeable burden for the uniform flow of reduction gases
including smooth furnace operation. Chromite burden for SAF generally
comprises a combination of sintered chromite pellets, chromite briquettes and
lumpy chromite ore. However, sintered pellets are found to be a more suitable
burden when compared to lumpy ore and briquettes due to their higher porosity,
uniform size and shape. Sintered pellets are produced from the chromite fines. In
the pellet plant the fines are ground in the wet grinding ball mill along with the
coke fines to achieve the required fineness for pelletisation. The suitable grain
size after grinding is 60% minus 74 microns. The ground chromite-coke slurry is
filtered by a drum or disc filter to get a filter cake with moisture contents in
the range of 10-15%. The filter cake is mixed with the bentonite binder in a

belt mixer to prepare the pelletising mix for the production of green pellets.
Green pellets of 10-20 mm diameter are prepared on a pelletising disc or drum.
After screening on a roller screen they are fired in a shaft furnce or steel belt
furnace maintained at a temperature range of 1300 - 1400 °C to attain the
required strength. The pellets need 60-150 kg/pellet strength to form a suitable
feed for the SAF. Low strength pellets (<60 kg/pellet) degrade quickly during
handling, and further reduction in SAF generate more fines. This affects the
permeability of the 'burden' ultimately resulting in improper furnace operation.
The existing method of making sintered chromite agglomerates for ferrochrome
production in submerged arc furnace (SAF) or electric arc furnace (EAF) has
certain drawbacks. Production of sintered agglomerates consumes a huge
amount of energy during their firing in the furnace maintained at 1300-1400 °C.
The fired chromite agglomerates are incapable to contain any solid reductant.
Reduction of these sintered agglomerates in the arc furnace warrants a higher
quantum of electrical energy.
Many attempts have been made to improve the method of making chromite
agglomerates thereby to reduce the overall energy consumption during
ferrochrome production. Some of the prior art on record known to the inventors
are :

• Dolbear et al.'s, Patent (US 2279033) dated 7 April 1942, describes
a process of briquetting of fine chromite fine with bentonite or clay
like material and to use these briquettes in arc furnace.
• Brennan et al.'s, Patent (US 2915378) dated 1 December 1959,
describes a method to produce synthetic agglomerates substantially
free of carbon by combining chromium ore with one of the
inorganic materials like clay, ferrochrome slag, manganese leach
residue, aluminate salt and silicate salt and sintering at 1500 °C to
achieve required strength.
• Kusama et al.'s, Patent (US 3661555) dated 9 May 1972, describes
a method of producing internally porous and uniformly hard oxide
coated chromium pellets containing substantially chromium
carbides, iron carbides and free carbon, unreacted oxides of
chromium, iron, magnesium, aluminum and calcium. The required
strength of chromite agglomerates was achieved by firing in the
oxidizing atmosphere furnace at a temperature range of 1290 to
1510°C.

• Krogerus et al.'s Patent (2004/0071583 A1) dated 15 April 2004
describes a method to reduce the amount of carbon bearing
material during the production of sintered chromite pellets in
continuously operated sintering furnace by adding carbon bearing
material on the surface of the pellets.
The drawbacks of the prior art methods/processes are basically a decrepitation
and weathering of the briquettes, a huge energy requirement for firing the
agglomerates, and adding unwanted gangue materials which again increases the
energy requirement. Thus, there exists a need to provide a process or method
that overcomes the aforesaid limitations of prior art.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method to produce cold
bonded chromite pellets by using a composite binder and fluxing agents which
eliminates the need of firing the pellets at high temperature.

Another object of the invention is to propose a method to produce cold bonded
chromite pellets by using a composite binder and fluxing agents, which
possesses sufficient strength to permit handling, transportation and use of the
pellets as firerd pellets.
A still another object of the invention is to propose a method to produce cold
bonded chromite pellets by using a composite binder and fluxing agents, which
is enabled to incorporate a solid reducing agent in the pellets for reducing the
chromium and iron oxides during processing the pellets in the smelting furnace.
Yet another object of the invention is to propose a method to produce cold
bonded chromite pellets by using a composite binder and fluxing agents, which is
enabled to incorporate fluxing agents in the pellets that provides contributes
building-up high temperature strength in the pellets and enhances the reduction
of chromium and iron oxides in the pellets by forming a diffusive medium.
A further object of the invention is to propose a method to produce cold bonded
chromite pellets by using a composite binder and fluxing agents, which adapts
the binder to prevent contamination of the pellets.

A still further object of the invention is to propose a method to produce cold
bonded chromite pellets by using a composite binder and fluxing agents, which
reduces the curing time of the pellets and yet provide the required strength to
the pellets.
Yet further object of the invention is to propose a method to produce cold
bonded chromite pellets by using a composite binder and fluxing agents, which
reduces the CO2 emissions during the production of chromite agglomerates.
Another object of the invention is to propose a method to produce cold bonded
chromite pellets by using a composite binder and fluxing agents, which are
capable of being used for carbothermic reduction of chromite burden in Electric
arc furnace (EAF), submerged arc furnace (SAF) and Rotary health furnace
(RHF).

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS.
Fig-1 - Shows an optical microstructure of a reduced cold bonded chromite
pellet according to the invention.
Fig-2 - Shows an SEM image of a reduced chromite pellet in accordance of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure - 1 shows a pictorial view of the microstructure of a reduced cold bonded
chromite pellet indicating various phases for example, white, grey, and dark
which respectively represents ferrochrome, slag, and pore.
Table-4 shows the result of an EDS - analysis of the SEM image of the reduced
chromite pellet of Fig-2.
The cold bonded pelletizing of the chromite fines involves the following steps;
1. Mixing the chromite fines with composite binder and fluxing agents to
prepare a palletizing mix.
2. Preparing green pellets.
3. Curing the green pellets.

To prepare the green pellets, the chromite fines require to possess the required
fineness and size distribution. Accordingly, the chromite fines are ground to the
required fineness (60% to 80% less than 45 microns) in a ball mill. If the fines to
be pelletized already possess the requirerd fineness and size distribution, then
there is no need of grinding.
Ground chromite fines are mixed with a solid reductant, a composite binder and
fluxing agents to prepare the pelletizing mix. The composite binder, in the
present invention, is a combination of Dextrin and Bentonite in the desired
proportions. Dextrin is a polysaccharide intermediate between Starch and Sugars
and is produced by the hydrolysis of starch. They have same general formula as
starch but having a smaller and less complex molecule. Bentonite is naturally
occurring plastic clay of commercial importance and it falls under montmorillonite
group. Bentonite, (Na Ca) (A1 Mg) Si4O10(OH)2 is basically an alumino-silicate
clay. Chemical composition of bentonite is shown in Table.1. Dextrin and
bentonite are well known binders and are used individually in variety of
applicantions.
Table - 1 shows the chemical composition of the bentonite.


The inventors of the present invention surprisingly found that the mixing of
dextrin and bentonite together in a certain proportion results in a superior
composite binder that exhibits a higher binding performance compared to either
dextrin or bentonite alone. Alternatively, starch can be used in place of the
dextrin to prepare the composite binder. The composite binder is made by
mixing dextrin and bentonite in a ratio of 60:40 or 80:20, more preferably in a
ratio of 75:25. Alternatively, addition of alkaline metal compounds like NaOH,
sodium carbonate or like can be used to improve the green and dried pellets
properties by adjusting the pH of the pelletizing mix. Addition of alkaline
compounds improves the binding characteristics of dextrin and bentonite.
The fluxing agents added to the pellets, according to the present invention,
contains mill scale and one or more of the following: lime, limestone, hydrate
lime or like, quartz powder, banded hematite quartzite, micro silica, silica fume.
These fluxing agents, during the reduction, react with each other to form a low
melting point eutectics, and flux the impurities including the accompanying raw
materials within the pellets. During the reduction step, these flux agents form

FeO.SiO2 type of slag and this slag assimilates the other impurities like A12O3
and MgO. The slag thus formed allows the pellet to be a strong mass and
prevents any escape of the slag in the form of dust particles alongwith the
outgoing gases. Furthermore, the slag formed also enhances the reduction of
chromium and iron oxides by forming a diffusive medium for chromium and iron
ions and carbon, which allows the chromite spinal to be broken down to
accelerate its reduction. The slag further decreases the initial reduction start
temperature within the pellets.
The solid reductant added to the pelleitzing mix is anthracite coal or coke breeze
or any like material. The amount of solid reductant ranges from 15-30% based
on the weight of chromite fines/concentrate, preferably in the range of 18-22%.
The amount of fluxing agents ranges from 20-40 wt.% based on the weight of
the chromite fines/concentrate, preferably in the range of 28-32%. The fluxing
agents consist of 40-60% lime stone, 30-50% silica and 5-15% mill scale. The
pelletizing mix comprises 60-70% chromite concentrate/fines, 10-15% coal fines,
and 15-20% fluxing agents.

The binder requirement is selected considering a compromise between cost of
the binder, and strength of the final pellet. To get a satisfactory quality, 3-5 wt%
of the composite binder depending on the weight of the palletizing mix is
required. To adjust the pH of the palletizing mix, alkaline compounds like NaOH,
sodium carbonate or like is added in the range of 0.01 to 0.05 wt% based on the
weight of the palletizing mix. Prewetting of the palletizing mixture is done by
adding adequate quantity of water, normally in the range of 4-6% by weight of
the mixture. After prewetting the mixture is called green pelleitzing mixture.
Pelletizing is carried out in a known manner by adding water with the mineral.
The amount of water to be mixed is not quite important, and thus decided to be
an amount required to achieve a satisfactory pelletizing, normally in the range of
9-11% by weight of the pelletizizng mix, which however, changes according to
the type of the ore or mineral for example, hydrophilic or hydrophobic in nature.
The pellets, in the size range of 8-16 mm are then dried, either by allowing them
to dry in air at ambient temperature for one to two days or in a hot air oven at
around 75-250 °C, preferably in the range of 120-160 oC.

EXAMPLE : Cold bonded chromite-coal composite pellets for submerged arc
furnace (Preparation process).
(A) Ground chromite fines, solid reductant fines, flux powder, and composite
binder (75% dextrin and 25% bentonite) are premixed in a dry state to
form a homogeneous mixture as shown in Table.2. Prewetting of the dry
pelletizing mixture is done by adding 8-9% water. Pelletizing are
conducted on a balling disc having a diameter of 600 mm the angle of
inclination and rpm of the disc being set at 45° and 25 respectively
during the balling. During pelletizing 1-2% water is added on the disc.
Time taken for pelletizing 2 kg batch is approximately 20 minutes.
Table.2. Ingredients of pelletizing mix for cold bonded pellets


(B) Drying of green pellets :
Green pellets were dried in a hot air dryer at 150 °C for 2 hours.
(C) Testing of cold bonded pellets :
The green pellets are tested for drop strength from 45 cm and 100 cms
height. The drop strength is measured by means of dropping the pellets
on a steel plate from a specified height. The number of drops the pellet
withstands without damages is measured as the drop strength of the
pellets. Pellets of 10-12.5 mm diameter can be used for the drop test and
an average value of 20 pellets is used to determine the drop strength.
(D) Reduction of cold bonded pellets
Cold bonded pellets are reduced at 140 °C for 30 minutes in nitrogen
atmosphere. Reduced pellets were characterized by an electron
microscope (SEM) with EDS to record the chemistry of different phases.

Results :
Table.3 Drop strength of cold bonded pellets as a function of curing time.

The cold bonded chromite pellets thus produced offer more advantages
compared to sintered chromite pellets produced through conventional heat
hardening palletizing route.


The composite binder (mixture of dextrin and bentonite) and fluxing agents
described in the text is not limited to pelletizing process and can also be used
for conventional briquetting process or any other like process. It is to be
understood that the invention is not limited by the specific examples and
embodiments described hereinabove, but includes such changes and
modifications as may be apparent to one skilled in the art upon reading the
appended claims.

WE CLAIM
1. A method for agglomerating ore or mineral fines to produce cold bonded
chromite-coal composite pellets comprising the steps of:
(a) mixing chromite ore fines or concentrate with solid reductant
fines, fluxing agents, a composite binder, a pH adjusting
compound and an amount of water ranging between 2-8% to
prepare a green pelletizing/briquetting mixture ;
(b) pelletizing/briquetting the green mixture to form green
pellets/briquettes;
(c) curing the pellets/briquettes either at ambient temperature or at
temperature substantially below a firing temperature of a
conventional heat hardened pelletizing process.

2. The method as claimed in claim 1 wherein the composite binder is a
combination of organic and inorganic binders.
3. The method as claimed in claim 2 wherein the organic binder comprises
dextrin, starch and their derivatives for example, white dextrin, yellow
dextrin or British gum; or lignin, lignosulfonate and their derivatives for
example Sodium lignosulfonate or calcium lignosulfonate.

4. The method as claimed in claim 2 wherein the inorganic binder comprises
bentonite, montmorillonite, colloidal silica, silica fume/micro silica and
other water expanding clays.
5. The method as claimed in claim 2 wherein the composite binder comprises
dextrin and bentonite mixed in a ration 60:40 or 80:20, more preferably in
the ration of 75:25.
6. The method as claimed in claim 1 wherein said composite binder dosage
in green mixture ranges from 1-5% based on the weight of the pelletizing
mix., preferably in the range of 3-5%.
7. The method as claimed in claim 1 wherein said composite binder is added
to the mineral fines prior to pelletizing or briquetting.
8. The method as claimed in claim 1 wherein said pH adjusting compound is
an alkaline compound for example, NaOH, sodium carbonate added in a
range of 0.01 to 0.05 wt% based on the weight of the palletizing mix.

9. The method as claimed in claim 1 wherein the palletizing mix comprises of
60-70% chromite concentrate/fines, 10-20% solid reductant fines, 15-
20% fluxing agents.
10.The method as claimed in claim 1 wherein said solid reductant fines are
coal or coke breeze preferably anthracite coal, the amount of solid
reductant ranging from 15-30% based on the weight of chromite
fines/concentrate, preferably in the range of 18-22%.
11.The method as claimed in claim 1 wherein said fluxing agents is selected
from a group of materials comprising lime, limestone, hydrate lime, quartz
powder, banded hematite quartzite, micro silica, silica fume, and a mill-
scale, the amount of fluxing agents ranging from 20-40 wt.% based on
the weight of the chromite fines/concentrate, preferably in the range of
28-32%.
12. The method as claimed in claim 11, wherein said fluxing agents
comprises 5-15% mill scale, 40-60% one of lime, limestone, and hydrate
lime and one of quartz powder, banded hematite quartzite, micro silica
and silica fume in a range of 30-50%.

13.The method as claimed in claim 1 wherein said curing is conducted at a
temperature in a range of 75° C. to 150°C.
14.The method as claimed in claim 1 wherein said curing is conducted at
ambient temperature in air for 1-3 days.
15. The method as claimed in claim 1, wherein said chromite or mineral fines
can be substituted by manganese ore fines, ilmenite or other titanium
bearing minerals fines, nickel bearing lateritic fines or nickeloferrous fines
or concentrates.
16.The method as claimed in claim 1 wherein said agglomeration comprises
any of disc pelletizing, drum pelletizing, briquetting, cylindrical blocks
making, and granulation.
17. A method for agglomerating ore or mineral fines to produce cold bonded
chromite-coal composite pellets as substantially described and a illustrated
hereinwith reference to the accompany drawings.

The invention relates to a method for agglomerating ore or mineral fines to
produce cold bonded chromite-coal composite pellets comprising the steps of:
mixing chromite ore fines or concentrate with solid reductant fines, fluxing
agents, a composite binder, a pH adjusting compound and an amount of water
ranging between 2-8% to prepare a green pelletizing/briquetting mixture ;
pelletizing/briquetting the green mixture to form green pellets/briquettes; curing
the pellets/briquettes either at ambient temperature or ast temperature
substantially below a firing temperature of a conventional heat hardened
pelletizing process.