FIELD OF THE INVENTION
This invention generally relates to a method for oxidation of chromite ore fines in
the size range of 0 to 4.0 mm in tunnel kiln for use of oxidized chrome ore fines in
direct reduction processes of ferrochrome production. More particularly, the
invention relates to a method of oxidation of non-agglomerated chromite ores
fines in one of a tunnel klin and rotary kiln to obtain oxidized chrome ore fines for
use in production of ferrochrome/charge chrome.
BACKGROUND OF THE INVENTION
Chromite ore is the prime raw material for production of ferrochrome using a
submerged arc furnace (SAF) process. Globally, most of the ferrochrome is
produced by submerged arc furnace route except a small fraction, by a DC arc
furnaces route. The smelting reduction of chromite ores is carried out in
submerged arc furnaces using the coke as a reductant and the quartzite as a flux.
Chromite ores in the form of lump are smelted in SAF directly whereas chromite
ore fines are agglomerated to pellets or briquettes or any other form of
agglomerate. Chromite ore fines are conventionally agglomerated to pellets
without any prior heat treatment of ore fines. These agglomerates are then
subjected to sintering process in shaft furnace prior to smelting in submerged arc
furnace. According to the present invention, a process is developed for heat
treatment of chromite ore fines in tunnel kiln or rotary kiln prior to its
agglomeration to composite agglomerates. The heat treatment up to temperatures
1100 °C with residence time of 30 to 300 minutes is carried out in the tunnel kiln
or rotary klin in presence of air. According to the invention, the FeO present in
chrome spinel is oxidized. Oxidation process results in breakage of the chrome
spinel and formation of Fe-sequioxide phase lamellae on the surface of chromite
ore particles. Oxidation process also generates ion vacancies in the
chrome spinel lattice which makes the lattice unstable. The Fe-sequioxide phase
together with presence of ion vacancies in the chrome spinel lattice helps in
improvement of the ore reactivity during its subsequent reduction process. The
selection of chromite ore fines in the size range of 0 to 4.0 mm for oxidation in
tunnel klin or rotary klin ensures the uniform oxidation reaction across the surface
of the ore particles since oxidation process for higher size particles (greater than
4.0 m) results in inefficient or incomplete oxidation of ore particles. Therefore in
the present invention method for oxidation of chromite ore fines in the size of 0 to
4.0 mm in tunnel klin or rotary klin is proposed of use of the oxidized chrome ore
fines in direct reduction processes or in ferrochrome/charge chrome production.
A plurality of primary processes for example, at least five processes are known to
be in use for production of ferrochrome. These include conventional process with
open or semiclosed submerged arc furnaces, the conventional process with closed
submerged arc furnace, the Outokumpu process, the DC Arc route and the Premus
process. Each of these processes are discussed briefly below:
Conventional Process with Open or Semiclosed Submerged Arc Furnaces:
In the conventional process, a mixture of chrome ore, reductants and flux is fed
cold with minimum pre-processing directly into open/semiclosed type submerged
arc furnaces. The furnace off-gases are cleaned in a bag plant before being vented
into the atmosphere. The metal and slag are then tapped from the furnace for
further processing. The primary advantage of this process is that it requires lowest
capital investment and is very flexible in terms of raw materials that can be used in
the process. The main disadvantage of this process is that it is increasingly being
perceived as being less environmentally friendly than other available processes and
it has the lowest efficiencies.
Conventional process with closed submerged arc furnace: In conventional
process with closed submerged arc furnaces, a mixture of sintered pellets, lumpy
chromite ores, coke as reductant and flux are fed to closed submerged arc
furnaces. The furnace off-gas is cleaned using wet scrubbers in the gas cleaning
plant. The metal and slag are then tapped for further processing. The primary
advantage of closed submerged arc furnace is that it is energy efficient process as
compared to open or semiclosed furnaces. The main disadvantages of the process
are that, the energy consumptions are still higher up to 4500 kWh/t of metal,
requires costly metallurgical coke and high losses of metal to slag upto 10% by
weight.
Outokumpu process: Fine chrome ore is wet milled and then pelletized using a
binder such as bentonite. The pellets are then sintered and then air cooled and
stockpiled. The pellets together with fluxes may be heated in a pre-heater located
above the furnace bins. Reductants consisting of coke and char, are added to the
(preheated) raw materials and fed into closed submerged arc furnaces. The
furnace off-gas is cleaned in wet scrubbers and used as an energy source in the
sintering and preheating processes. The main advantage of this process is that the
sintered pellets and preheating of the charge to the submerged arc furnaces
results in reduced specific energy consumptions and improved chromium
recoveries.
DC arc furnace: The furnace uses a single solid carbon electrode and produces a
DC arc to an anode in the bottom of the furnace. The arc is normally an open or
semi-submerged one. Raw materials can be charged either directly into the
furnace, or by using a hollow electrode. The primary advantage of this process is
that the process utilizes any of the available raw materials including 100%
chromite fines with minimum or no pre-processing, thus eliminating the need for
an expensive agglomeration plant. Inferior grades of reductants like coal and
anthracite can be used in this process, which is also considered an additional
advantage of the process.
Premus Technology: Fine chrome ore, bentonite and a reductant such as
anthracite fines are dry milled, pelletized and preheated before being fed into
rotary kilns where partial pre-reduction of the chromium oxidie and iron oxides
take place. The metallized pellets are then hot charged into closed submerged arc
furnaces. The furnace off-gas is cleaned in ventury scrubber and used throughout
the plant as an energy source. Initial capital costs for this process are high and the
level of operational control required to ensure smooth operation of the process is
also very high.
The prior art processes as discussed hereinabove essentially involve pre-reduction
or preheating of agglomerates in rotary kilns. The advantage can be seen in
reducing the cost of electrical energy. Chromium recoveries are improved and cost
of smelting is reduced by these pretreatment methods. The steel belt sintering
furnace process developed by Outokurhpu offered reduced energy consumption
and emissions and advantageous to harden pellets in order to withstand handling.
The preheating of smelting charge by burning carbon monoxide in shaft furnace up
to a temperature of 700 °C, removes moisture and volatiles, which gave major
savings in electricity and increased the capacity of the smelting furnaces by 20%.
The best metal recoveries were reached with agglomerated chromites from fine
materials. Optimal smelting furnace operation was achieved by the right charge
mix, depending on the raw materials. The preheating and pre-reduction methods
have improved the performance of SAF process; however none of it could replace
the SAF process completely. It is worthwhile to mention that, most of these
methods for ferrochrome production included the pre-treatment such as preheating
or pre-reduction of the agglomerates obtained either by agglomeration of chromite
ores such as for sintering or composite agglomerates obtained from agglomeration
of chromite ores, reductant, flux and binder. However it should be noted that,
none of these processes (pre-heating, pre-reduction) have used the tunnel kiln
reduction process or process based on oxidized chromite ore fines which are
obtained typically by pretreatment of chromite ore fines such as oxidation of
chromite ore fines prior to its agglomeration. The oxidation process of chromite ore
fines ensures a uniform degree of oxidation across the oxidized chromite ore
particles which help in enhancing the reduction reactions during subsequent
reduction process of oxidized chromite ore fines.
The prior art technologies are designed for processing of FeO-rich South African
chromite ores for producing the charge chrome with relatively lower chromium
content, varying between 50 to 60 percent. On the other hand, the MgO-rich
Indian chromite ores are highly refractory in nature and therefore needs a different
approach. The key problems with SAF process used for production of high carbon
ferrochrome are (a) Highly energy intensive process, and consuming about 2600-
4500 kWh/t of FeCr (b) High Met coke consumption of 500-700 kg/t c) Requires
high quality feed material with Cr2O3 greater than 48 percent (d) about 10 percent
(wt) Cr2O3 loss in slag e) large volumes of slag (Metal : Slag ratio of 1.1) f)
environmental problems with respect to CO2 emissions and slag. Therefore, a need
exists to propose an innovative technology, which is enabled to overcome the prior
art disadvantages and capable of using low cost materials; using secondary energy
produced, increasing the process efficiency, minimizing the operating costs and
improving the working environment.
Tunnel kilns are traditionally used for heat treatment of refractory bricks or
calcinations of materials. For example, a calcination process of chrome ore and
lime mixture bricks in the tunnel kiln at 1400 °C is disclosed in patent number
GB496890 to produce chromium chemicals such as calcium chromate and calcium
bichromate. The calcined bricks of chrome ore-lime mixture obtained from the
tunnel kiln were ground further and boiled with water in presence of air to remove
CO2 and produce calcium carbonate precipitate and calcium chromate solution. In
patent number US3295954, a process for alkaline roasting of chromium ore with
Na2O is disclosed wherein molding of ore and soda mixture is heated in tunnel kiln
at 1100 °C to obtain improved chromium yield. A method for reduction of
chromium ore in tunnel kiln is disclosed in patent number JP62256938, wherein the
lump (molding) of mixture of ore, carbonaceous reducing agent and binder is
provided with a protective film of Cr2O2, SiC and fly ash on the surface for
preventing intrusion of oxidizable combustion gas into the molding during
reduction between 1200 to 1500 °C to obtain high chromium reduction. In patent
application number JP62243723, a method for reduction of chromite was disclosed
wherein the mixture of chrome ore, coke reductant and binder (carboxymethyl
cellulose) is filled in ceramic (aluminous) vessel to form green compact of about 10
to 200mm diameter inside vessel. The ceramic vessels were then placed into
tunnel kiln of about 30 to 100 m length for atleast 30 minutes in temperature
range of 1400 to 1500°C for obtaining about 90% reduction. A new smoke
pollution device for tunnel kiln is disclosed in patent number US5603615, which
can prevent the discharge of pollutants such as sulphides, steam and ashes to
atmosphere. Tunnel kiln without exterior combustion chamber for producing coal
based direct reduced iron is disclosed in patent number CN1804049, the invention
claims to enlarge the width of the tunnel kiln upto more than 10 m and single kiln
production capacity up to 0.1 mtpa and more than 0.5 mtpa using suitable product
quality and 1050 to 1300°C baking temperature. In patent number CN101497933,
a method for rapidly and directly reducing iron/hematite ore or limonites into
ferrous powder is disclosed. The hematite ore is crushed washed and subjected to
magnetic separation. The ore is mixed with coal dust, lime and calcium chloride
(CaCl2) and binder. The mixture is pressed into blanks, dried and stacked on cars
to send in tunnel kiln. The blanks are subjected to water quenching to separate
iron powder and slag automatically. Patent number CIM101113488 discloses a
method for comprehensive utilization of V-Ti-bearing iron ore concentrate to
produce iron powder and vanadium pentaoxide compounds by using tunnel kiln
reduction-grinding-separation. High temperature metal recovery process is
disclosed in patent number US2003047035, for particulate metal containing dust
such as electric arc furnace dust wherein metallic values are recovered from the
dust using tunnel kiln. The mixture of metal containing dust alongwith
carbonaceous fines is heated in moving bed horizontal tunnel furnace to release
volatile and alkali metals in gaseous product principally zinc and iron. The zinc and
iron are separated from gases as process product. A new process and apparatus
for producing direct reduced iron is disclosed in patent number CN1147018,
wherein oxide of iron as raw material is mixed with bituminite, lime, coal and
nutshell as additive. The mixture is fed in to a material column reactor which is
passed through externally heated tunnel kiln for 30 to 38 hours at about 950°C.
The process was used for direct reduction of iron-containing raw material in solid
state into metal iron. In patent number CN1142541, a coal based direct reduction
process of iron containing material is disclosed to produce sponge iron using tunnel
kiln. The mixture of iron ore, coal is fed to metallic reactor and calcined in slope-
type tunnel kiln to produce sponge iron in metallic reactors.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a method for oxidation
of non-agglomerated chromite ore fines (0 to 4.0mm) in one of a tunnel kiln and a
rotary klin to obtain oxidized chromite ore fines for production of ferrochrome or
charge chrome.
Another object of the invention is to propose a method for oxidation of non-
agglomerated chromite ore fines (0 to 4.0mm) in one of a tunnel kiln and a rotary
klin to obtain oxidized chromite ore fines for production of ferrochrome or charge
chrome, which is enabled to chrome spinel in chromite ore fine particles due to
oxidation and convert of FeO present in chrome spinel to Fe-sequioxide lamellae on
the surface of chromite ore particles.
A still another object of the invention is to propose a method for oxidation of non-
agglomerated chromite ore fines (0 to 4.0mm) in one of a tunnel kiln and a rotary
klin to obtain oxidized chromite ore fines for production of ferrochrome or charge
chrome, which improves the reactivity of the chromite ore fines due to oxidation.
A further object of the invention is to propose a method for oxidation of pre-
agglomerated chromite ore fines (0 to 4.0mm) in one of a tunnel kiln and a rotary
klin to obtain oxidized chromite ore fines for production of ferrochrome or charge
chrome, in which the chromite ore fines are oxidized in the tunnel kiln or rotary
klin upto a maximum temperature of 1100°C with residence time between 30 to
300 minutes.
A still further object of the invention is to propose a method for oxidation of pre-
non-agglomerated chromite ore fines (0 to 4.0mm) in one of a tunnel kiln and a
rotary klin to obtain oxidized chromite ore fines for production of ferrochrome or
charge chrome, in which a heating cycle across the tunnel kiln length and a
process of charging the chromite ore fines on car surface arc predesigned for
obtaining the desired oxidation effect on the chromite ore fines.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method for oxidation of non-agglomerated
chromite ore fines in one of tunnel kiln and rotary kiln process to obtain oxidized
chromite ore fines for production of ferrochrome/charge chrome.
According to the present invention, a process is developed for heat treatment of
chromite ore fines prior to its agglomeration. The heat treatment up to
temperatures 1100 °C with residence time of 30 to 300 minutes is carried out in
the tunnel kiln or rotary klin in presence of air. According to the invention, the FeO
present in chrome spinel is oxidized. Oxidation process results in breakage of the
chrome spinel and formation of Fe-sequioxide phase lamellae on the surface of
chromite ore particles. Oxidation process also generates ion vacancies in the
chrome spinel lattice which makes the lattice unstable. The Fe-sequioxide phase
together with presence of ion vacancies in the chrome spinel lattice helps in
improvement of the ore reactivity during its subsequent reduction process. The
selection of chromite ore fines in the size range of 0 to 4.0 mm for oxidation in
tunnel klin or rotary klin ensures the uniform oxidation reaction across the surface
of the ore particles since oxidation process for higher size particles (greater than
4.0 m) results in inefficient or incomplete oxidation of ore particles. Therefore in
the present invention method for oxidation of chromite ore fines in the size of 0 to
4.0 mm in tunnel klin or rotary klin is proposed for use of the oxidized chrome ore
fines in direct reduction processes or in ferrochrome/charge chrome production.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention is explained in greater details with the accompanying drawings :
Figure 1 shows a typical size analysis of Indian chromite ore fines obtained by
grinding run of mine chromite ore or a product of beneficiation of run of mine
chromite ore.
Figure 2 presents a typical heating and cooling cycle along the length of the tunnel
kiln for oxidation of chromite ore fines, indicating temperatures required for
oxidation process in three major zones of the tunnel kiln.
Figure 3 shows the chromite ore fines before and after oxidation in the tunnel kiln
including a technique of placing the chromite ore fines on the tunnel kiln cars for
oxidation process.
Figure 4 presents a comparison of optical micrograph of Mgo-rich chromite ore
particles before and after oxidation in the tunnel klin, the microstructure of the
chromite ore particles after oxidation showing an exolved lamellae of Fe-sequioxide
on the surface developed due to oxidation process.
Figure 5 shows a typical size analysis of FeO-rich chromite ore used as an
additional example for oxidation of the chromite ore in the tunnel klin.
Figure 6 shows an optical micrograph of FeO-rich chromite ore particle oxidized in
the tunnel klin oxidation process.
DETAILED DESCRIPTION OF THE INVENTION
Chromite ore fines are typically obtained by grinding the run of mine chromite ore
to fine size fractions (less than 4.0mm). The typical particle size analysis of Mgo-
rich chromite ore fines is shown in Fig. 1. It can be seen that, the d80 (80% of
particles having average diameter) of the chromite particles is in the range of 80 to
90 µm. The typical chemical composition of chromite ore fines is shown in Table 1.
The tunnel kiln or rotary kiln used in the present invention having an externally
heated combustion chamber. The air is blown into the tunnel kiln at entry, exit and
immediately after the firing zone for rapid cooling. The air blown inside the kiln
allows maintaining an oxygen-rich environment inside the kiln apart from
maintaining a desired gas flow of the combustion gases. Schematic of the tunnel
kiln showing the broad operating temperatures in different zones of tunnel kiln is
shown in figure 2. The tunnel kiln, starting from the entry has preheating,
oxidation (firing) and cooling zone as the major zones across the total length of the
kiln. Indicative lengthwise distribution of temperature in the tunnel kiln zones is
also shown in figure 2. In order to achieve oxidation of the chromite ore fines, the
firing zone temperature is typically maintained to less than 1100°C. The process
can also be carried out in a rotating hearth/cylinder type of furnaces such as rotary
hearth or rotary kiln as long as the chromite ore is exposed to temperatures upto
1100°C for 30 to 300 minutes of duration in presence of air.
Chromite ore fines are placed on the tunnel klin cars, typically such that the
thickness of the ore fines bed on the refractory surface of the car is about 30 to 60
mn as shown in figure 3. One of the important features of the inventive method is
to oxidize the chromite ore fines tunnel kiln or rotary kin prior to chromite ore fines
agglomeration for achieving effective oxidation and heat treatment of ore fines.
The ore may also be charged in metal trays and in a stacked manner so that
multiple trays can be accommodated on each of the cars for increasing the tunnel
kiln throughput. The residence time for the cars with chromite ore fines or
chromite ore fines such that each car or the chromite ore fines has total residence
time (in to out) of 30 to 300 minutes in tunnel or rotary kiln. The tunnel kiln cars
are pushed out of the kiln after experiencing the heating and cooling cycle inside
kiln. The chromite ore particles are uniformly oxidized during the heating and
cooling process in tunnel kiln or rotary kiln. Figure 4a shows the optical micrograph
of Indian chromite ore particles before and after (Figure 4b) oxidation process.
During oxidation reaction, FeO present in chrome spinel gets oxidized to Fe2O3 as
exsoluted Fe-sequioxide phase and in turn cation vacancies are generated in spinel
lattice. In an oxidizing atmosphere, in addition to the intrinsic driving force for
phase transformation, the imposed oxygen chemical potential promotes the
diffusion of Fe2 + ions from the core of chromite grain towards the solid-gas
interface on the surface of the grain. This outward diffusion of Fe2 cations and
oxidation to Fe3+ cations takes place via following reactions :

Where O2-o represents oxygen anions on the cubic closed packed lattice, Vncat is
cation vacancy, h* is hole, and e' is electron. Oxidation of chromite ore also results
in opening up of the spinel structure. The optical micrograph of chromite ore
particles after oxidation process is shown in Fig. 4b, which shows the
Widmanstatten structure of bright lines on chromite mineral. It can be seen that,
the Fe-sequioxide phase has exolved on the surface on the chromite particles. The
EDX analysis of oxidized chromite particles has indicated that the bright lines are
iron rich phases and the matrix is magnesium rich phase. It can be concluded
from above that iron is precipitating from the matrix on {111} crystallographic
planes. The oxidation of chromite resulting in Widmanstatten structure is in
accordance with established theory of oxidation and precipitation of sequioxide
phase which states that preferential crystallographic orientation of Widmanstatten
lamellae was along the {111} plane of spinel matrix phase. The {111} planes of
spinel and the {001} planes of sesquioxide have a similar close packing
arrangement of oxygen ions, which account for the common orientation of
sequioxide lamellae along the {111} plane of the spinel matrix. Tapered terminals
develop at the intersection of two or more lamellae, which are indicative of a
diffusion-controlled process. This newly formed Fe-sequioxide phase on chromite
particles along with the vacancies generated during oxidation improves the
reactivity of chromite ore during reduction. The typical view of chromite ore fines
after oxidation in tunnel kiln is shown in figure 3. The oxidized chromite ores
obtained from tunnel kiln are then removed from the cars and used in direct
reduction process or in ferrochrome production. For additional example, South
African chromite ore (0 to 4.0mm) having typical size analysis as shown in figure 5
and chemical composition as given in Table 2 was also subjected to oxidation
process in tunnel kiln as described. The oxidation process was highly effective, and
high degree of oxidation is achieved for South African Chromite Ores. The optical
micrograph of oxidized South African Chromite Ore is shown in Figure 6. It can be
seen that, the FeO present in chrome spinel was oxidized to Fe2O3 as exolved Fe-
sequioxide phase which improves the reactivity of ore in subsequent reduction
processes. The oxidized South African Chromite ore can be used further for
agglomeration such as pellets or briquettes to use in direct reduction processes to
produce pre-reduced chromite ore or in charge chrome production.
WE CLAIM
1. A method for oxidation of non-agglomerated chromite ore fines in one of
tunnel kiln and rotary kiln process to obtain oxidized chromite ore fines for
production of ferrochrome/charge chrome.
2. The method as claimed in claim 1, wherein the chromite ore fines comprise
particle size range from 0 to 4.0 mm, and wherein 80% of the particles are
having average diameter less than 90 µm.
3. The method as claimed in claim 1, wherein the chromite ore fines comprises
Cr2O3 in the range of 25 to 55% (by weight) and Fe(t) in the range of 10 to
25% (by weight).
4. The method as claimed in claim 1, wherein the starting material is obtained
by grinding one of mine chromite ore/lumps and a product of beneficiation
of mine chromite ore.
5. The method as claimed in claim 1, wherein the chromite ore fines are
oxidized in tunnel kiln one of a rotary kiln having a maximum firing
temperature 1100°C in presence of air with a total residence time of
chromite ore fines in the tunnel kiln or rotary kiln for about 300 minutes.
6. The method as claimed in claim 5, wherein the air is injected to the tunnel
kiln or rotary kiln atleast at one of an entry point, an exit point, and a
location immediately after the firing zone for cooling purpose.
7. The method as claimed in claim 5, wherein the tunnel kiln comprises
preheating, firing and cooling zone, and wherein the rotary kiln and/or the
tunnel kiln is maintained at a maximum temperature 1100°C.
8. The method as claimed in claim 6, wherein the preheating zone, firing zone,
and cooling zone in respectively maintained at a temperature less than
800°C, about 1100°C, and less than 600°C.
9. The method as claimed in claim 5, wherein the chromite ore fines are placed
on the tunnel kiln car or fed to the rotary kiln in such a manner that the bed
thickness of chromite ore fines is about 30 to 60 mm on the tunnel kiln car.
10. The method as claimed in claim 5, wherein the tunnel kiln or rotary kiln has
approximate length up to 45m.
11. The method as claimed in claim 8, wherein the oxidized chromite ore fines
obtained from tunnel kiln or rotary kiln comprises Fe-sequioxide (Fe2O3)
phase or lamellae on atleast one of the chromite particles which is iron rich
phase in the microstructure of oxidized chromite particle.
12. The method as claimed in claim 1, wherein the agglomerates process
product is one of spherical pellet, briquettes, and lump or any other form of
agglomerates.
13. The method as claimed in claim 12, wherein the agglomeration product
made from oxidized chromite ore fines is useable in ferrochrome or charge
chrome production or in direct reduction processes of chromite ores to
produce pre-reduced chromite ore.
14. The method as claimed in claim 13, wherein the direct reduction processes
or ferrochrome production or charge chrome production is characterized by
use of atleast one carbonaceous product such as coal, coke or charcoal.

The invention relates to a method for oxidation of non-agglomerated chromite
ore fines in one of tunnel kiln and rotary kiln process to obtain oxidized
chromite ore fines for production of ferrochrome/charge chrome.