FIELD OF THE INVENTION
The invention is related to a process for the production of ultra-low carbon
ferrochrome alloys via aluminothermic reduction route. The invention describes
utilization of high grade chromium, iron bearing ores, using prior oxidation
treatment of the raw material.
Background of the Invention
Metals and alloys like ferromanganese, ferrochrome, ferrovanadium,
ferromolybdenum and other similar products are produced in Sub-merged Arc
Furnace (SAF) by reduction of respective oxides of the minerals. In the process
of production of chromium and iron containing alloys by the above mentioned
route, 6-7% of carbon gets dissolved in the produced alloy. This content of
carbon needs to be controlled in ferrochrome alloys as it has detrimental effect in
quality steel making.
Various types of raw materials have been used for the production of low carbon
ferrochrome such as chromium-iron bearing ore, chromite sludge, chromium
bearing slags where chromium varies between 20-50% [1-7]. Higher content of
chromium requires more energy for reduction.
Since chromite spinel is stable structure and thus it requires much higher energy
to break the bonds of the stable structure to release iron and chromium from the
structure. Mineral chromite has a spinel structure with formula as AB2O4 in which
A is a divalent cation (Fe, Mg), and B is a trivalent cation (Fe, Al, Cr). In oxide
spinel, the oxygen anions from the face centered cubic structure, which has 8

tetrahedral and 4 octahedral sites per unit cell. Eight such unit cells combine to
form one unit cell of spinel. In a normal spinel, all the divalent, A, cations
occupies tetrahedral sites and the trivalent cation occupies octahedral sites of the
unit cell. With increase of chromium content of the ore, the iron content
decreases and hence lead to lessen the smelting characteristic.
In India, with growth of steel sector, the demand for chromium bearing alloys as
raw materials is also increasing; which leads to processing of chromium-iron
bearing raw materials with chromium content varying between 50-60%. This
kind of chromium bearing raw materials are classified as high grade or refractory
grade chromite ores, which are difficult to reduce under normal process
conditions as specified. Direct utilization of these ores for alloys production
demands very high amount of energy.
Most common known method for production of medium and low carbon
ferrochrome is through thermit process, invented by Hassan Goldsctin [8]. In the
process, fine metal oxide is mixed with aluminum or silicon powder, chips or
turnings, and ignited to produce alloys and slag in the process; without heating
the mixture externally. This process has been patented in 1839 and later mostly
used for rail track repairing.
Many inventors have used similar techniques by either varying raw material or
reducing agent to produce alloys of desired composition. Invention IN 184274 [9]
describes a process for production of chrome metal by charging the ore into the
furnace for about 90-120 minutes at temperature of 250°C to remove the
retained moisture of the ore. The heated ore is mixed with chosen reductant and
energizer in a furnace and later transferred to crucible where reaction is initiated
using magnesium wire resulting in formation of chrome metal of desired

composition. A similar kind of invention has been performed to utilize the
chromium based sludge containing about 20% chromium for recovering chrome
value [10]. In the process, the chrome sludge is heated in the temperature range
of 300-600°C and mixed with non- carbon based reducing agent, flux material to
recover chrome value from chrome sludge.
US1609970 patent [11] have given a general process to utilize two reductants
one after another on the basis of its reactivity, to reduce the raw materials to
valuable product. In the process, the reductants are added one after another to
control the heat content of the reaction and for better slag metal separation of
the process. Another category of invention works in similar lines but start with
high-carbon ferrochrome as a source for chrome to produce low carbon
ferrochrome as end product. In the process, the high carbon ferrochrome is
roasted with calcium oxides or its derivatives to form a compound which is mixed
with reducing agents like aluminum or silicon and ignited using magnesium wire
to desired grade of alloy [12].
The main drawback lies with utilization of high carbon ferrochrome as raw
material, which contains 6-8% carbon as chromium carbide. Due to presence of
carbon in the system, this will result in higher consumption of calcium treatment.
In the process, the total carbon doesn't get removed and its can lead to
contamination of the alloy in case for production of low carbon ferroalloys. Other
than this inventions, few other inventors have used various techniques like
layering of the reaction mixture in the order of calorific values for better metal
yield and control of reaction [1]; pellets of reaction mixture are synthesized
(even in vacuum) and charged into induction furnace, which leads to better
handling of raw materials [2, 4, 6-7] for better control of the reaction.

However, none of the above specified inventions discuss about optimization of
energy consumption to utilize higher percentage based chromium raw material
for medium or low carbon ferrochrome production. Hence, there is a scope to
utilize high grade chromite ores to produce ultra-low carbon ferrochrome using
thermit process route.
Objects of the Invention
An object of the invention is to produce ultra-low carbon ferrochrome alloy from
iron-chromium based ores.
Another object of the invention is to develop an energy conserving process for
the production of ultra-low carbon ferrochrome alloy from iron-chromium based
ores.
Still another object of the invention is to develop a process that can enable use
of high chromium containing ores for the production of ultra-low carbon
ferrochrome alloy.
SUMMARY OF THE INVENTION
In the process of producing ultra-low carbon ferrochrome, refractory grade
chromium-iron raw material is reduced using thermic based reduction route in
presence of lime as flux, calcium fluoride as viscosity modifier for the slag and an
oxidizer like potassium nitrate. Chromite ores are structurally spinel and, thus its
reactivity decreases with increase of chromium content of the ore. To smelt such
high chromium bearing raw materials using thermite process, there is need to

pretreat the ore to increase its reactivity; which results in breakage of chromite
spinel. Hence the treatment provides better exothermic nature of the reaction
which leads to higher metal recovery, when reduced by non-carbon based
reducing agent. In this invention, a two-step modification in the process has
been proposed: a) oxidative roasting; b) Reduction of roasted ore by
alumonothermic reduction to produce metal and slag. Chromite ore is subjected
to oxidative treatment of the ore in air with flow rates of oxygen varying
between 5-20 Ipm at a temperature range of 600-1100°C for 3-5 hrs. The heat
treated ore in heated or cooled condition can be mixed with aluminum powder
along with flux, slag modifiers and oxidizing agent for aluminothermic reduction.
Additional step of using oxidation roasting will allow increasing the reactivity of
the chromite ore by formation of iron sequioxide and leads to destabilization of
chromite spinel. With increase of Cr:Fe ratio of the chromite ore, stability of the
spinel increases. Oxidation treatment will allow the iron to diffuse from the spinel
structure towards the grain boundary of the chromite, this results in increase of
reactivity of the chromite grain.
DETAILED DESCRIPTION OF THE INVENTION
An ultra-low carbon ferrochrome is defined based upon percentage of chromium
and carbon. A typical specification for ultra-low carbon is: Cr: 63% (min), Si: 1.0-
1.5 %( max), Fe: 23-27%, C: 0.05-0.1% (max), N: 3-4%, P: 0.03% (max) and
S: 0.02% (max).
In industrial operation, low carbon ferrochrome is very essential alloy for
production of special grade of steel. Though in present practice, high carbon
ferrochrome (with carbon varying between 6-8%) is added as alloying element

for special grade steel by various steel makers and in the steel making process,
carbon of the steel is controlled. The control of steel during the steel making
process involves time and cost. Hence, there is need for a robust and economical
process for production of ultra-low carbon ferrochrome, which can save the
consumption of oxygen during steel refining operations and leads to higher
productivity. The present invention provides a method to produce ultra-low
carbon ferrochrome from iron-chromium bearing raw materials.
Chromite ores are classified as metallurgical grade, chemical grade, and
refractory grade, according to its end usage. According to geographical
occurrence of the chromite ores, these ores are broadly classified as stratiform
and podiform. With change in class of the ore, their physical and chemical
characteristics also changes which affect reduction behavior of these ores. Table
1 provides the major characteristics of chromite ores. India is the world's fourth
largest chromite deposit which has disrupted stratiform complexes. Podiform
deposits are also available at few pockets in chromite deposits at Orissa. Table 2
provides the grades of the ores which has been used for reduction studies.



From the above table it can be seen that, the ores collected for the study is
combination of both category of chromites. Magnesia to wusite ratio in the
chromite ore gives an indication that higher the ratio more difficult is the
reduction of these ores.
Ores with higher chromium to iron ratio doesn't have better reduction and
smelting property. Further, with increase of magnesia in the chromite spinel, the
amount of energy required to smelt the ores further increases. Chromium and
iron content of the chromite ore also contributes towards smelting behavior of
the ore. With increase of chromium in the spinel, subsequently magnesium as
well as aluminum will increase to maintain the charge balance in the spinel
structure. But, an opposite relationship can be observed for iron in the chromite
spinel with increase of chromium.
The present invention focus on oxidation process to enhance the reducibility of
the chromite ore. To facilitate a proper oxidation treatment, the ore needs to be
grounded into smaller size which leads to increase the surface area to unit
volume. To aid proper reaction and intimate contact with reducing agent, the
suitable size fractions should be less than 150 mesh. During the oxidative
roasting process, the particles tend to form a lose agglomerate by sticking of
smaller particles but can easily disintegrate during the handling, thus no further
energy is required for grinding.

In an embodiment of the invention, air with flow rates of oxygen varying
between 5-20 Ipm at a temperature range of 600-1100°C for 3-5 hrs. was used
for oxidative roasting. During the oxidation treatment, Fe3+ presence as iron
seqoxide can be determined by X-Ray Diffraction analysis, where a peak shift can
be observed due to oxidation treatment.
In another embodiment of the invention, oxidative roasting can also be
performed at lower temperature and oxidation can be enhanced at lower
temperature with the aid of oxidizing agents like sodium per chlorate, potassium
nitrate, sodium nitrate etc. Addition of these chemicals during oxidation leads to
release of oxygen which results in in-situ oxidation of the ferrous to ferric
conversion in the chromite ore. This practice helps in reducing time, however
there is a chance of inclusion in the ore which might affect further process. This
kind of roasting operation can be performed in open hearth or reverberator
furnace with arrangement for mechanical mixing or similar furnaces like rotary
kilns.
During the experimentation, the color of the ore changes from reddish brown to
blackish brown, this is due to conversion of ferrous to ferric oxide. With aid to
oxidative treatment, the divalent iron at tetrahedral site diffuses along the plane
to create a vacancy in the spinel structure. Similar behavior can be observed
across all the Fe2+ occupied sites. With further oxidation, the formation of Fe2+
will lead to form Fe3+ in the process. Diffusion of Ferrous ions from the structure
of chromite will make the structure unstable and chromium can be easily
reduced in the process. Thus with oxidation of chromite ore, the formation of
iron seqoxide will form which can be estimated by chemical analysis and even
through XRD as well microscopic technique. In the process of oxidation, a bright
pattern lines can be observed over the chromite grain and this pattern is known

as Widmanstatten Pattern. Oxidation helps in increasing the reactivity during the
reaction, as more amount of oxygen is available per gram mole of cation of the
spinel [13].
After the oxidation of iron-chromium bearing raw material, the grounded iron-
chromium bearing raw material is mixed with a non-carbonaceous reducing
agent, an oxidizer, a flux and a viscosity modifier for further reduction. The non-
carbonaceous reducing agent such as aluminum, silicon or alloy of aluminum or
silicon along with other elements can be used as per the current invention. In
the process, stoichiometric requirement of reducing agent is employed, other
than that extra percentage of reducing agent can be added in the system for
reduction of other elements required in the alloy. But, if in excess reducing agent
is used; this will result in energy cost and even it will contaminate the grade of
the respective alloy. With enchantment of oxidation, the exothermicity of the
reaction increases per gram of the reactant involved and results in lowering of
the reductant consumption. In an embodiment of the current invention, the
oxidizer like NaCIO3/ KNO3/ NaNO3 is used to promote oxidation during the
startup of the reaction. Further, fluxing agents like CaO is used and CaF2 is
added as viscosity modifier.
To establish such intimate contact, there is requirement of controlled grinding of
the oxide bearing raw material as well as reducing agent. This invention also
focus on degree of comminution such that a large proportion of the oxide
bearing material and reducing agent consists of particles sufficiently small to
pass through a 150-mesh screen (Tyler series) and grinding of the materials in
contact, or together. For proper oxidation, the ore and reducing agents need to
be grounded to the required size. Such products can be grounded in ordinary ball
mills or any similar kind of setup to control the degree of finesse.

Mixing of the roasted ore with reducing agent is an important step for the proper
reduction of the ore. Various methods can be used for example oxidized product,
may be added to the furnace in admixture with the reducing agent when
reduction has to be carried out in electric arc furnace. When the reduction is to
be carried out in crucibles, then a proper mixer like V-shaped drum mixer should
be employed. Depending on the mixing requirements, the mixer should be
chosen to handle the powder characteristics, flowablity of the constituents,
quality requirements in the final product and finally process requirements and
associated limitations. The mixture may be employed in a lose condition, or it
may be employed in a compact form as briquettes.
In an embodiment of the invention, reaction mixture is arranged in ascending
order, according to heat of reaction. A segregated mixing of the mixture in the
reaction vessel helps in improving the process efficiency in view with slag metal
separation. Maximum heat is at the bottom of the reaction vessel as metal
trickles from top to bottom. Thus by using the layering phenomenon, more heat
is available in the reaction vessel to drive a better slag metal separation. Charge
mixture which is prepared by blending all the ingredients is poured in the
reaction vessel or in crucible with alumina or magnesia based refractory lining.
The reaction mixture is initiated with magnesium wire and after that reaction is
self-sustaining. In the process we get an ultra-low carbon ferrochrome and slag,
which is cooled in the crucible under natural cooling conditions. During the
reaction, temperature of the reaction is almost reaching in the range of 1800-
2100°C. Reaction during reduction is given as:
Cr2O3(s) + 2AI(s) = 2Cr(l) + Al2O3(l)
Fe2O3(s) + 2AI(s) = 2Fe(l) + AI2O3(I)

Even mixture can be arranged by layering the charge mixture on the basis of %
Al in the charge. Mixture with higher aluminum is added at the bottom of vessel
followed by lesser aluminum charges.
During the course of reaction, the flux will combine with alumina of the ore.
Alumina is formed from reaction of aluminum (acted as reducing agent) with
chromite ore to form molten slag of non-refectory grade with the temperature
same or equivalent that of the metal. A mixture of aluminum and silicon can also
be used as reducing agent. With increase of silicon in the blend, the heat of
reaction decreases which doesn't produce enough heat to raise the liquidus
temperature of the mixture.
The following examples show the specific embodiments of the present invention.
EXAMPLE I
Chromite ore with Cr: Fe ~ 3.5 was collected from mines site and grounded in
ball mill to a fineness of less than 150 mesh. The grinded ore particles are
oxidized using a muffle furnace by placing the ore in layers for providing
maximum surface area for oxidation. A temperature 1100°C, with air flow varies
between 5-10 Ipm was maintained in the furnace during oxidation for
approximately 5-6 hours. A higher flow rate of air was maintained in the furnace
for forced diffusion of oxygen from the air to the layered chromite ore without
avoiding the condition for fluidization. The oxidized product was mixed with
aluminum with respect to ore taken and additional amount of aluminum is added
in the mixture to reduce silicon which is required in the alloy. Aluminum in the
charge is varied according to ore: aluminum ratio, between 3.0-3.6| 1-5% CaO
and 2-4% CaF2 are added in the reaction mixture along with 8-12% of KNO3 as
booster. In the process of making of charge, the charge is prepared in such a
manner that exothermic behavior of the reaction can be controlled. In order

carry out the practice, the percentage of aluminum is sub-divided into parts and
mixed with equal percentage of the ore. Similarly, the flux and oxidizing agent is
also added in the mixture as explained above. Once the reaction mixture is filled,
approximately 4-10g of KNO3 added on the top layer of the reaction mixture,
depending on the batch size; to initiate the reaction quickly. The reaction mixture
is initiated with magnesium wire and after that reaction is self-sustaining. In the
process we get an ultra-low carbon ferrochrome and slag, which is cooled in the
crucible under natural cooling conditions. During the reaction, temperature of the
reaction is almost reaching in the range of 1800-2100°C. Reaction during
reduction is given as:
Cr2O3(s) + 2AI(s) = 2Cr(l) + Al2O3(l)
Fe2O3(s) + 2AI(s) = 2Fe(l) + AI2O3(I)
Once the reaction starts, due to exothermic behavior of the reaction; reaction
propagates like a Shockwave. The flame moves from ignition point to towards
periphery and subsequently moves like this until whole charges gets reacted.
Afterwards slag and alloy is extracted from the reaction vessel by breaking the
slag layer, which releases the metallic alloy. Some of the metal which solidifies
within the slag layer is afterwards extracted by crushing and screening. This type
of reactions can be carried out in crucible or any other reaction vessel, with
alumina or magnesia based refractory lining.


For making a grade above 70% Cr in alloy, the obtained roasted product is
mixed with 20% extra than stoichiometric amount of aluminum powder
(industrial grade), 1% of lime as flux, 2% calcium fluoride as slag modifier and
7.5 % of potassium nitrate as oxidizing agent. The charge mixture is charged
into reaction vessel in parts as mentioned above. Alloy obtained from the process
have following composition: Cr: 71%, Fe: 26%, Si: 1.1%, P: 0.0%, C: 0.1%. The
chromium recovery in the alloy was about 75%. The slag contained 15% Cr.
EXAMPLE II (without oxidation)
A set of experiment was performed in similar way with same charge recipe as
provided in example I. Primary difference is to analyze the effect of oxidation on
the metal yield and metal grade. Chemical analysis of the chromite ore and metal
obtained is provided below:

Recovery of chromium in the metal obtained with utilization of ore without pre-
oxidation treatment is 30%. Basicity of the slag (MgO+CaO/AI203+SiO2) is
approximately 0.22 and chromium content of the slag is 20%.

EXAMPLE III
Segregated layering of the charge (as explained above) can help in controlling
the reaction and even enhance the slag-metal behavior. To estimate the
fundamental understanding about such system, experimentation has done with
oxygen roasted chromite ore with 20% excess of stoichiometric aluminum
required. In the process, total ore quantity is divided into three parts: 60%, 20%
and 20%. Respective ore percentages are mixed with equal percentages of
aluminum, potassium nitrate and flux. Charge obtained after rigorous mixing is
charged into the crucible with higher exothermic reaction mixture at the bottom
followed by reduced reactive material. At the top, some amount of KNO3 is
added to initiate the reaction smoothly and subsequently initiated by magnesium
wire and after that reaction is self-sustaining. Chemical analysis of the chromite
ore and metal obtained is provided below:

Recovery of chromium in the metal obtained with utilization of ore without pre-
oxidation treatment is 56%. Basicity of the slag (MgO+CaO/AI2O3+SiO2) is
approximately 0.23 and chromium content of the slag is 18%.

From the examples, it can be proved that oxidation treatment of chromite ores
helps in improving its performance during smelting and reduction to produce
ultra-low carbon ferrochrome. With gradual increase of MgO: FeO ratio in
chromite ore, reactivity of the ore will gradually go down; but oxidation
treatment will help in improving the reduction potential of the ore. So there is a
need to perform optimization between energy required for heating the ore to
higher temperature for oxidation treatment or utilization of higher reducing agent
to achieve same chrome recovery in the produced alloy.
REFERENCE
1. Thermit production of metal and alloys. JP4318127
2. Aluminothermic Process. GB1531152.
3. Aluminothermic reduction process for the production of chromium metal
using top priming. IN184274
4. Iron-Chromium alloy suitable for alloying steel and alloys. SU310948.
5. Reductive method for production of metallic elements such as chrome
using a crucible with a perforated wall. US7513930.
6. Method for production of metallic elements of high purity such as
chromes.US7361205.
7. Method to manufacture of aluminothermic nitrogen poor chromium nickel
alloys. DE3129563.
8. Goldschmidt, H. Verfahren zur herstellung von metallen Oder metalloiden
oder legierungen derselben. German Pat, no. 96317. 13th Mar., 1895.
9. Sen, P.K., Mohanty, O.N.,Choudary, M.K., Sharma, R., Alumonothermic
reduction process for the production of chromium metal using top
priming. Indian Pat, no. IN184274.
10. Eddie, CJ.C, Leo, W. B., Charles, J.K. Jr., Chromium recovery process.
US Pat, no. 4917726. 26th Oct 1988.

11. Schroeder, C.R., Method of affecting exothermic reaction. US Pat., no.
1609970. 7th Dec 1926.
12. Marvin, I., Udy, J., Improvements in or relating to materials for use in the
production of alloys containing chromium, and to the production of alloys
therefrom. UK Pat., no. 520331. 22nd April 1940.
13. Tathavadkar,V.D., Antony, M.P. and Jha, A., The physical chemistry of
thermal decomposition of south African chromite minerals. Metallurgical
and materials transactions B, 2005, Vol. 36B, no.2, pp.75.
14. Tathavadkar,V.D., Antony, M.P. and Jha, A., An investigation of the
mineralogical properties of chemical grade chromite minerals.
Scandinavian Journal of Metallurgy, 2004, Vol. 33, pp. 65-75.

WE CLAIM:
1. A process for production of ultra-low carbon ferrochrome from an iron-
chromium bearing raw material, the process comprising:
grounding the iron- chromium bearing raw material to a size less
than 150 mesh;
subjecting the grounded iron- chromium bearing raw material to an
oxidative roasting at oxygen gas flow rate in the range of 5-20
ml/min and temperature in the range of 600-1000°C;
mixing the oxidized iron-chromium bearing raw material with a
non-carbonaceous reducing agent, an oxidizer, a viscosity and a
fluxing agent in a reaction vessel; and
initiating the reaction with a magnesium wire.
2. The process as claimed in claim 1, wherein the ultra-low carbon
ferrochrome has carbon amount less than 0.1 wt. %.
3. The process as claimed in claim 1, wherein Cr:Fe ratio of the iron-
chromium bearing raw materials is in the range of 1.5 to 4.0.
4. The process as claimed in claim 1, wherein the non-carbonaceous
reducing agent is selected from a group consisting of aluminum, silicon,
an alloy of aluminum, an alloy of silicon and a combination thereof.

5. The process as claimed in claim 1 further comprising the step of optionally
adding an oxidizing agent to the grounded iron- chromium bearing raw
material before the oxidative roasting step, the oxidizing agent being
selected from a group consisting of sodium per chlorate, potassium nitrate
and sodium nitrate.
6. The process as claimed in claim 1, wherein the oxidizer is selected from a
group consisting of NaCIO3/ KNO3, NaNO3 and a combination thereof.
7. The process as claimed in claim 6, wherein the oxidizer is added in the
reaction mixture in the range of 5-15 wt. %.
8. The process as claimed in claim 1, wherein the viscosity modifier is CaF2.
9. The process as claimed in claim 1, wherein the flux is CaO.

ABSTRACT

In this invention, ultra-low carbon ferrochrome is produced from refractory grade
chromite ores with prior oxidation treatment and then subjected to thermit
process using aluminum as prime reductant. Pre-oxidation of chromite ore helps
in improving the reactivity of chromite ores and results in achieving better
metallic yield with desired metallic grade of the alloy.