FIELD OF INVENTION
This invention provides a charge mixture for making a crucible to be used in high
temperature heat treatment processes. The invention further relates to the
development of high melting point charge mixture comprising of self-oxidizing, self-
baking material which transfers heat by conduction and radiation means to the
material inside the crucible required for high temperature processes (up to 1700 °C)
such as in direct and or indirect reduction of oxide ores in tunnel kiln or rotary
hearth furnace.
BACKGROUND OF INVENTION
Tunnel kiln based ore reduction (solid state reduction) processes (such as sponge
iron or direct reduced iron (DRI) production processes) achieve high degree of
reduction and obtains highly metallized reduced product when carried out in silicon
carbide containers or saggers. The crucibles or containers are subjected to repeated
heating and cooling cycles inside tunnel kiln while the oxide ore or agglomerates
made of oxide ore are reduced inside crucible. The crucible material usually
comprises silicon carbide as the main constituent which is chemically neutral or non-
reactive to the surrounding oxidizing gases and ore, reductant etc.
The crucibles or saggars are used in tunnel kiln direct or indirect reduction processes
mainly due to two reasons. Firstly, it protects or avoids the direct contact of the
material inside crucible, to the oxidizing atmosphere inside kiln. Secondly, the
crucible gets heated and transfers heat to the materials inside crucible by known
heat transfer mechanisms such as conduction, convection and radiation. The said
crucibles or containers or saggars are made of material comprising silicon carbide as
primary constituent and may contain ceramic high aluminous material.

Ceramic crucibles made of silicon carbide have high melting point and thermal
conductivity and are chemically neutral. These ceramic crucibles transfers heat to
the material inside crucible. The crucibles are costly since they are typically obtained
by hot moulding process. The crucibles or saggars made of silicon carbide are also
brittle and difficult to manufacture in close proximity near to tunnel kilns.
PRIOR ART
Tunnel kilns are primarily used for heat treatment of refractory bricks and calcination
of materials. Also, tunnel kiln based processes have been developed and in use for
reduction of oxide ores. For example, 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 3P62243723,
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.
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.

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.
A method for recovery of valuable metals from metal complex using tunnel kiln is
disclosed in patent application number JP2013064177A wherein the metal
complex is roasted in ceramic container followed by melting and separation of slag
from molten complex. The method is useful for recovering metal from metal complex
i.e lithium ion battery which contains valuable metal such as cobalt. In patent
application number IN20110141412 and IN20110141312, a system for
continuous production of shaped sponge iron from iron ore pellets/briquettes is
disclosed wherein iron ore in the form of composite pellets and adapted for
reduction along with reductant such as coal or coke fines and limestone are packed
in silicon carbide crucibles/containers/saggars and heated in the kiln so as to ensure
compact and atmospheric oxygen/air free contact of the iron ore and reductant in
the containers in the kiln at temperature range of 850-1150 0C, sufficient to thereby
effect the reduction of iron ore to sponge iron in the form of pellets/briquettes,
without induration/heating of pellets of iron ore, saving energy and reducing the
cost of production.
As can be seen from the prior art references, the direct (composite agglomerates) or
indirect reduction (mixture of reductant and oxide ore) of oxide ores is carried out
using ceramic (aluminous) containers or silicon carbide crucibles/saggers. These
crucibles or saggers are mostly made up of silicon carbide material which is costly.

Therefore, in the present invention a charge mixture is disclosed for making
crucibles or saggers and the crucibles for use in high temperature heat treatment
processes in tunnel kiln or rotary hearth furnaces.
OBJECTS OF THE INVENTION
It is therefore object of the present invention to provide a charge mixture for making
a crucible for use in high temperature heat treatment process of upto 1700 °C.
Another object of the present invention is to provide a high melting point charge
mixture for making a crucible comprising of self-oxidizing and self-baking material.
Further object of the present invention is to provide a crucible of desired shape
made from a charge mixture material, by cold compression (punching) or from
combination of individually cold compressed (punched) shapes.
Yet further object of the present invention is to provide a high strength crucible or
container or saggar for carrying out direct or indirect reduction of oxide ores in
tunnel kiln or rotary hearth furnace.
Yet further object of the present invention is to provide a crucible or saggar made of
a charge mixture which can be reused for repeated heat treatment cycles (heating,
soaking and cooling) in tunnel kiln or rotary hearth furnace.
A still further object of the present invention is to propose a crucible that can be
reused as reactive metallurgical feedstock in high temperature smelting process such
as in electric submerged arc furnace of ferrochrome production after completion of
desired repeated use in tunnel kiln or rotary hearth furnace.
Summary of the Invention:
The invention provides a charge mixture for preparation of a crucible. These
crucibles are used for reducing metal oxide ores in high temperature furnaces such
as in tunnel kiln or rotary hearth furnace. The charge mixture of the present
invention comprises of 50 to 95 weight % of a chrome ore, 1 to 25 weight % of a

carbon source such as coal, coke, carbon paste and graphite, 1 to 25 weight % of
quartz or quartzite, 1 to 10 weight % of bentonite and 1 to 10 weight % of a
binding agent such as cement. The ingredients of the charge mixture are mixed in
water and then cold compressed to form the container or the crucible of desired
shape and size. The containers or crucible of the present invention gains further
strength when used in the high temperature furnaces as the oxidizing gases in
tunnel kiln or rotary hearth furnace reacts with charge mixture on outer side of
crucible/container to oxidize FeO to Fe203 in chrome ore and bake the charge
mixture.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1: Crucible obtained by assembly of individually cold compressed parts.
1a: Hollow cylindrical vessel obtained by cold compression of a charge mixture as
per the current invention.
1b: Cover or lid obtained by cold compression of said charge mixture.
1c: Cover or lid of different shape obtained by cold compression of said charge
mixture.
1d: Assembly of individually compressed shapes to obtain a closed crucible.
1e: Sealant applied at interface between hollow cylindrical vessel and bottom lid.
1f: Agglomerates comprising of metal oxide ore to be reduced filled in crucibles
made of said charge mixture.
1g: Sealant applied at interface between cylinder and top lid to obtain a closed
crucible loaded with metal oxide or agglomerates of metal oxide inside the crucible.
1h: Sealed crucible or saggar or container having special ceramic material or sealant
applied at interface or on outer and or inner surface of crucible for subjecting to high
temperature process in tunnel kiln or rotary hearth furnace.
1i: Crucible obtained after subjecting to a typical high temperature process
comprising of heating, soaking and cooling cycle in tunnel kiln.

Figure 2: Crucible and reduced metallized products obtained in typical high
temperature process in tunnel kiln or rotary hearth furnace.
2a: Image of crucible with reduced products obtained after subjecting to first use or
first heat treatment process inside tunnel kiln or rotary hearth furnace.
2b: Image of empty crucible and products removed without any sticking to crucible
so that the crucible can be reused.
Figure 3: Empty crucibles after removal of top lid.
3a: Crucible after first use or obtained after subjecting to one typical heating,
soaking and cooling cycle in tunnel kiln or rotary hearth furnace.
3b: Crucible after second use or obtained after subjecting to two typical heating,
soaking and cooling cycles in tunnel kiln or rotary hearth furnace.
DETAILED DESCRIPTION OF THE INVENTION
Direct reduction of metal oxide ore such as iron ore, chrome ore etc. is carried out in
a rotary hearth furnace or a tunnel furnace. For achieving high degree of reduction
and to obtain a highly metallized product, reducing atmosphere is required in the
furnace. This reducing atmosphere is enabled by reducing gases generated from the
feed agglomerates (comprising metal oxide ore, reducing agents and flux) during the
heating. Also, in order to achieve higher productivity (kg product/hr/m2 hearth area),
it is preferred to load higher amount of feed composite agglomerates on the hearth
area. However, loading of feed composite agglomerates greater than few inch
heights for reduction purpose is difficult in tunnel kiln as well as in rotary hearth
furnace due to limited heat transfer to feed agglomerates in the bottom layer of the
material charged on the hearth area.

Tunnel kilns are traditionally used for refractory oxidizing heat treatment applications
and therefore, not widely used for reduction of oxide ores since achieving reducing
atmosphere is difficult in conventional designs of tunnel kiln except a few processes
mentioned in the prior art which use crucibles or saggars or containers for carrying
out direct and or indirect reduction of metal oxides placed inside crucibles or
containers. Saggars or crucibles made of silicon carbide material are commonly used
in these processes to achieve high degree of metallization in product and also to
obtain high productivity of the process. However, the silicon carbide crucibles are
costly and acts as only heat transfer agent. In the present invention, a charge
mixture comprising of chromite ore is disclosed which is less costly, self-oxidizing
and self-baking.
The charge mixture is used for preparing the crucibles or containers which act as
heat transfer agent to the materials inside the crucible/container. Further, these
containers/crucibles also provides added advantage of self-utilization of heat for
phase changes and thereby value addition to convert into a material which on
crushing can be suitably used as feedstock in smelt reduction process in submerged
arc furnace of ferrochrome production. The said charge mixture for making crucibles
or saggars or containers is obtained by mixing chromite ore containing Cr2O3 in the
range of 25 to 62% by weight with a carbon source, bentonite and/or cement binder
and quartz or their combinations. The carbon source is selected from the group
consisting of coal, pure carbon, coke, carbon paste and graphite.
The silica (SiO2) in the charge mixture is either comprised from the chrome ore or
may also be added externally in the charge mixture. All the raw materials are in fine
powdered form obtained by grinding of the respective material. The typical chemical
composition of various raw materials mixed to obtain the charge mixture for
preparing crucible is given in Table 1. The weight percent or quantity of individual

raw material in the charge mixture may vary, however a typical range for amount of
various raw materials in the charge mixture by weight is provided in Table 2.
Table 1 Chemical composition in percent by weight for raw materials in the charge
mixture


Required quantity of water in the range of 1 to 10% by weight is added to the said
mixture. The mixture is thoroughly mixed and placed in die or mould of desired
shape and load in the range of 2 to 5 ton is applied to obtain a crucible or individual
part such as hollow cylindrical vessel or lid. The cold compression process by
applying load is carried out at room temperature. It should be noted that a load of 2
to 5 ton is necessary in the present case to obtain a crucible made of said charge
mixture having thickness of 10mm, internal diameter of 40 mm and height of 50mm,
however to form a large size crucible or saggar higher amount of load will be
required and suitable arrangement or compression enabler can be used to compress
the said charge mixture to obtain the desired shape crucible.
Also, the thickness of the compressed parts may vary as per the requirement of
shape or to increase or decrease the amount of heat transfer to the metal oxides or
agglomerates placed inside the crucible. In order to obtain the cold compressed
parts of crucible, suitable die or mould made of mild steel is made. In the present
case, two different die assemblies were used to obtain two different shapes namely
hollow cylindrical vessel and cover or lid. The hollow cylindrical vessel after
compression is dried. Similarly, the cover or lid is compressed separately using
different die assembly.
The bottom and top cover can be pasted using a sealant or mortar commonly used
in the high temperature applications. The sealed crucible (containing metal oxide ore
agglomerates inside) thus obtained is then placed inside a tunnel kiln or rotary
hearth furnace subjected to the conventional or desired heating, soaking and cooling
cycle of the kiln.
During high temperature process in tunnel kiln or rotary hearth furnace, the FeO in
chrome ore of the charge mixture is oxidized to Fe203. The oxidizing gases in tunnel
kiln or rotary hearth furnace reacts with charge mixture on outer side of
crucible/container to oxidize FeO to Fe203 in chrome ore and bake the charge
mixture. The fusion of phases in charge mixture takes place with carbon and binding

agents during high temperature process in tunnel kiln or rotary hearth furnace which
bakes the charge mixture and increases strength of the crucible further.
The crucible on subjecting to high temperature process in tunnel kiln or rotary
hearth furnace acts as reactive material favorably wherein the reducing gases
generated from materials placed inside crucible reacts with charge mixture on inner
side of crucible to reduce metal oxides. Figure 1 shows the images of hollow
cylindrical crucible, lid of two different shapes with arrangement to obtain the
crucible by applying a sealant at the interfaces between crucible and lid. Figure lh
shows the crucible or saggar to be subjected to heating, soaking and cooling cycle in
the furnace with the metal oxide comprising agglomerates placed inside it.
Figure li shows the image of crucible obtained after experiencing first typical
heating, soaking and cooling cycle in tunnel kiln furnace or in rotary hearth furnace.
Figure 2a shows the images of crucible wherein the top cover or lid is removed after
first use or after experiencing the first heating, soaking and cooling cycle in tunnel
kiln showing the reduced product inside the crucible. Figure 2b shows the reduced
product removed from the crucible and the crucible. Figure 3a shows the typical
crucible made with said charge mixture obtained after experiencing first heating,
soaking and cooling cycle or after first use. Figure 3b shows the typical crucible
made with said charge mixture obtained after experiencing two rounds of heating,
soaking and cooling cycle in tunnel kiln or after repeated twice use of the same
crucible.
The crucibles/containers obtained after first heating can be reused again for multiple
usages in the tunnel kiln and on completion of desired repeated use in the tunnel
kiln, the crucible material can be crushed and used as reactive feedstock for
submerged arc furnace which will provide a reactive metallurgical feedstock for the
smelting process and help in minimizing the energy and coke consumption and
improvement in recovery of valuable metallic in the smelt reduction process.
The crucibles obtained from the said charge mixture also results in effective
utilization of heat in tunnel kiln since the heat is also utilized to effect favorable
phase change in the crucible charge mixture which is beneficial for the downstream
use of the crucibles on completion of its desired use in tunnel kiln or rotary hearth
furnace.

We Claim:
1. A charge mixture for preparing a crucible, the crucible being used for
reduction of metal oxide ores in a high temperature furnace, the charge
mixture comprising:
50 to 95 weight % of a chrome ore;
1 to 25 weight % of a carbon source;
1 to 25 weight % of quartz or quartzite;
1 to 10 weight % of bentonite; and
1 to 10 weight % of a binding agent.
2. The charge mixture as claimed in claim 1, wherein the high temperature
furnace is a tunnel kiln or a rotary hearth furnace.
3. The charge mixture as claimed in claim 1, wherein water is used to prepare
the charge mixture.
4. The charge mixture as claimed in claim 1, wherein the chrome ore comprises
of chromium oxide (Cr2O3) in the range of 25 weight % to 62 weight %.
5. The charge mixture as claimed in claim 1, wherein the carbon source is
selected from the group consisting of coal, pure carbon, coke, carbon paste
and graphite.
6. The charge mixture as claimed in claim 1, wherein the binding agent is
cement.
7. The charge mixture as claimed in claim 1, wherein the charge mixture is self-
oxidizing and self-baking.
8. The crucible prepared from the charge mixture claimed in claim 1 to claim 7.
9. A method of reducing metal oxide ores in a high temperature furnace, the
method comprising:
preparing a crucible from a charge mixture comprising 50 to 95 weight % of a
chrome ore, 1 to 25 weight % of a carbon source, 1 to 25 weight % of quartz
or quartzite, 1 to 10 weight % of bentonite and 1 to 10 weight % of a binding
agent; and

heating the metal oxides agglomerates placed inside the crucible in the high
temperature furnace to a temperature of up to 1700 °C, the crucible being
sealed from top and bottom using a sealant.
10.The method as claimed in claim 9, wherein the sealant is a refractory
material.
11.The method as claimed in claim 9, wherein the sealant is mortar.
12.The method as claimed in claim 9, wherein the crucible is used for one or
more heating cycles to reduce metal oxides in the high temperature furnace.
13.The method as claimed in claim 9, wherein the charge mixture is self-
oxidizing and self-baking.
14.The method as claimed in claim 9, wherein the crucible is crushed to a
reactive feedstock for submerged arc furnace in ferrochrome production after
being used in one or more heating cycles.
15.The method as claimed in claim 9, wherein the crucible is prepared by cold
compression moulding process.
16.The crucible prepared as per the charge mixture claimed in claim 9 and claim
15, wherein the cold compression molding process comprises filling the
charge mixture in a metal mould and applying a compression load to obtain
desired shape of the crucible.

ABSTRACT

This invention provides a charge mixture for making a crucible or a container for use
in high temperature heat treatment processes. The crucible or the container can be
used for oxide ores reduction process in tunnel kiln or rotary hearth furnace. A high
melting point charge mixture comprising of self-oxidizing, self-baking material is
developed which transfers heat by conduction and radiation means to the material
inside the crucible. The crucible charge mixture is comprised of chrome ore,
sintering flux, binders which bake and fuse during heat treatment due to sintering
and oxidation of chrome ore and gains strength. The crucible has sufficient strength
to sustain severe heat from burner flames and oxidizing atmosphere generated due
to fuel combustion. On exposure to typical heat treatment cycle in tunnel kiln, the
strength of crucible increases further due to self-baking and oxidizing characteristic
of chrome ore in the charge mixture. The crucible or the container, after desired
repeated use in tunnel kiln or rotary hearth furnace, can be crushed and used as
reactive metallurgical feedstock in high temperature smelting process such as in
ferrochrome production.