The invention relates to the utilisation of water granulated ferrochrome slag
generated from ferrochrome smelting process to make value added high
temperature refractory material. It is also related to the engineering of water
granulated ferrochrome slag to produce periclase-forsterite-spinel refractory. This
invention further relates to the improved chemical composition of the final refractory
material there by converting the water granulated solid slag into valuable high
temperature refractory material for use as hot face lining of metallurgical process
equipment’s with operating temperatures greater than 1650 0C.
BACKGROUND OF THE INVENTION
Stainless steel producers are the largest consumer of ferrochrome. Most of the
world’s Ferro-Chrome is produced in South Africa, Kazakhstan and India. High
carbon ferrochrome is produced by carbothermic reduction process. Most commonly
submerged arc furnaces are employed for smelting of chromite ores by
carbonaceous reductants.
Chromite mineral found in spinel form having general formula of (Fe2+,
Mg+2)O.(AI3+, Cr3+, Fe3+)2O3. During the smelting process slag is produced and
molten metal is produced. Heavier metallic ferrochrome coalesces into droplets,
settled through the slag layer. Thus the Ferrochrome metal is separated out of the
slag and collected at the bottom of furnace. The slag composition is an important
part in ferrochrome smelting technology. The ferrochrome slag composition, its
melting point mainly depends on the chrome ore gangue and fluxing material used
during smelting. The Ferrochrome slag consists of SiO2, MgO, Al2O3, in different
proportion along with few minor amount of CaO, chromium and iron oxides.
Chromium is generally present in form of partial altered chromite (PAC) and
entrapped alloy. The chromium content of slag is influenced by slag composition,
temperature and tapping arrangement. The temperature of the slag during tapping is
1700-1750 °C and that of the ferrochrome 1550-1600 °C. The optimum smelting
point has been practically noted between 1680-1720 °C. The proportion of quartzite
in the charge mix controls the right slag composition. The typical ferrochrome slag
composition is 30 % SiO2, 26 % Al2O3, 23 % MgO and 2 % CaO. The chrome
(Cr2O3) content in the slag is about 8-15 % and the iron (FeO) content 2-4 %

respectively. Ferrochrome slag is acidic in nature. Its basicity is 0.8 when it is
calculated by Formula given below:

Slag and metal are tapped through a same tapping hole. Lower density slag
collected on the top of the metal is separated by pouring the top layer. The
ferrochrome slag is directly granulated during tapping where ferrochrome is tapped
into ladles. The overflow from the ladle flows along the slag runner to the granulation
pond, where high-pressure water breaks slag into small fractions and efficiently it
cools down. Granulated slag is a very homogenous product. A granule is tight,
spongy and partly crystalline.
Rate of ferrochrome slag production is 1.1 to 1.5 times that of ferrochrome
metal produced. Safely disposal of huge quantity of slag is a major challenge to the
ferrochrome industry. Worldwide ferroalloy producers are striving hard for effective
utilisation of ferrochrome slag through value addition which will solve the
environment issue and will reduce the overall cost of their production.
Ferrochrome slag is classified as harmless in terms of the IARC (International
Agency for Research on Cancer) classification, as the chromium exists in
ferrochrome slags as Cr (Ill). In South Africa Ferrochrome slag is safely used in
landfills. Ferrochrome producers from Finland, South Africa, and Russia have
successfully utilized these waste materials in road making and building material
applications. Many researchers addressed the oxidation of Cr(+3) to Cr(+6) in
presence of strong oxidants in laboratory conditions and there exists a controversy
regarding the possibility of slowly releasing of Cr(+6) to environment in long run term
practice. Therefore safe and profitable utilisation of these wastes is still a challenge
for the ferrochrome producers. Utlization of ferrochrome slag in refractory application
is one of the most promising environment friendly applications which may reduce the
production cost of ferrochrome production process.
The prior art processes developed in the past mainly included production of
refractory type of material by changing the chemistry of ferrochrome slag or direct
utilisation of ferrochrome slag for low temperature refractory application without

changing its chemical composition. The chemistry of the ferrochrome slag was
changed by adding SiO2 (max 40% by wt.), Al2O3 (max 36% by wt.) and MgO (max
40% by wt.) and chromite ore (max 30% by wt.) in molten condition. For example a
method of manufacturing heat resistant, fire resistant and alkali resistant mineral
wool composition from ferrochrome slag is disclosed in patent application number
US4818290. According to this method aluminium oxide and/or silicon oxide is added
into the molten slag to adjust the defibration temperature of the slag by changing its
viscosity. The final composition can be adjusted by adding aluminium oxide in the
range of 20-30% by weight for making fibers for high temperature application. Silicon
oxide to aluminium oxide in the ratio (2.5 -5.0: 1) can be added to ferrochrome slag
to get low temperature application fibers. Patent application number US4946811
claimed a method for producing fire resistant and chemical resistant fibers. Their
approach was based on mixing molten iron silicate slags with ferroalloy slag to adjust
the molar alkalinity (Feo+CaO+MgO/SiO2+Al2O3 within 0.5 -0.7) in the suitable range
for defibering into fibers. Patent application number US4561885 disclosed a method
of producing economical refractory material from high carbon ferrochrome slag.
According to this method a solidified slag layer on the surface of inner wall of a slag
receiving vessel can act as a refractory lining for high temperature application.
Patent application number US4751208 claimed a method of producing a spinel type
ceramic sintered body from chromiferous slag which is a waste discharge from
sodium chromate production. According them the sintered ceramic body can be
reproducible by selecting R2O/MgO to 0.9-2.0 and SiO2/MgO to 1-6 where R
represents Al, Fe, and Cr. Patent application no RU2182140 disclosed that
ferrochrome slag can be used for making magnesia-silica refractory for lining of
heating and roasting furnaces and other thermal units. The used refractory material
comprises mineral phases at the following ratio, in mass %: forsterite 43%,
aluminomagnesium spinel 14 to 22%, magnesia pedalferic spinellide 12 to 20%,
periclase 4 to 11 % and motecellite 1 to 4 %. Making of refractory materials from
molten carboniferous ferrochrome conversion slag and calcined magnesite is
disclosed in patent application no SU672184-A. According to this invention 60 to 90
wt% molten slag and 10-40% calcined magnesite of preferred granularity in the
range of 3-15 mm fraction yields a better refractory material. Attempt to increase the
refractoriness of ferrochrome slag (of compsn. (in wt. %): SiO2 28-35; MgO 35; 45;
Al2O3 15-20; FeO 1-3; CaO to 100) 70-90) by enriching chrome-picotite phase with

addition of chromium ore (of composition. (in wt. %): Cr2O3 38-59; FeO 7-12; MgO
10-19; Al2O3 9-18; SiO2 1-8; CaO to 100) 10-30) to it is disclosed by patent
application no SU775092-B.
Some prior art technologies used the air cooled dense solidified ferrochrome
slag and binder for making refractory using appropriate binder/cement with
appropriate size fraction of the crushed slag. In patent application no CN1144786-A ,
a method of producing refractory material composition having Magnesium (Mg) 45-
50%, Alumina (Al2O3) 15 to 20%, chromium oxide (Cr2O3) 0.5 to 3%, silica (SiO2) 25
to 30% and calcium oxide (CaO) 2 to 6% from high carbon ferrochrome slag is
disclosed wherein 20-30% magnesium sand is used as high temperature binder with
or without sintering. Patent application no US 3798043-A revealed that a cemented
bonded refractory blocks for graphitising carbon production can be made from
ferrochrome slag of suitable composition: 25-40% MgO, 20-50% SiO2 and 10-40%
Al2O3. The suitable ratio of cement to slag (1:4) and water to cement (0.6) is
necessary to develop strength of refractory after firing at 1000 0C in air. Patent
application number US 4222786 disclosed that ferrochrome slag can be used as
refractory material having good mechanical strength. According to this invention
ferrochrome slag containing MgO less than 25% by weight have good resistance to
mechanical compression and wear. The ferrochrome slag used in this process
consist of calcium oxide 1 to 12% by weight, silicon oxide 20 to 40% by weight and
aluminium oxide 20 to 36% by weight. The ferrochrome slag is finely grounded below
5 mm and suitably blended with calcium aluminate cement to make bricks, concrete
etc. The prior art technologies were designed for molten slag and natural cooled
dense solidified slag. On the other hand water granulated slag is porous/ spongy,
less dense and glassy in nature and also slag composition of Indian ferrochrome
industries is typically different from others due to MgO-rich Indian chromite ores.
None of the above prior art technology describes the use of spongy and glassy type
of ferrochrome slag granules in refractory making process and therefore need
different approach.
Considering all the above prior art to produce refractory from ferrochrome slag
a novel process approach based on altering the slag composition in ball milling route
of cold water granulated slag without melting condition to produce high temperature
refractory material having refractoriness above 1650 0C.

OBJECT OF THE INVENTION
It is therefore an object of the present invention to propose a method for production
of refractories having refractoriness above 1650 0C from water granulated
ferrochrome slag. The other objective of the invention is to establish a method to
change the final stoichiometry of ferrochrome slag by MgO enrichment for useful
refractory application.
SUMMARY OF THE INVENTION
In the process of development of refractory material, the chemical composition of the
granulated ferrochrome slag is altered by MgO enrichment. The process flow chart in
fig 1 summarizes the process of development of refractory material from ferrochrome
slag. In this process the ferrochrome slag granules are grounded in presence of
magnesite raw material in a ball mill to produce fine composite powders having
intimate mixture of slag and magnesite followed by compaction and sintering of
powders to develop necessary refractory phases.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Process flow chart for development of refractory material from ferrochrome
slag
Figure 2 - MgO-Al2O3-SiO2-CrO pseudo ternary phase diagram at 10% CrO
Fig 3: microstructure of the water granulated ferrochrome slag
Fig 4: Schematic drawing of forsterite phase development during sintering of
Ferrochrome slag and Magnesite composite powders
Fig 5: The optical micrograph of the final refractory material developed from
ferrochrome slag
Fig. 6: DSC curve of ferrochrome slag and magnesite composite powders
DETAILED DESCRIPTION OF THE INVENTION
Both ferrochrome slag and metal are collected in a ladle through same tap hole from
submerged electric arc furnace during slag metal tapping operation and the molten
ferrochrome slag being lighter in density compared to molten metal, gets separated

on the top of the metal layer. The top slag layer present above the molten metal is
removed by careful pouring of it into a slag quenching area. In the slag quenching
area the molten ferrochrome slag is granulated by applying stream of water jet.
The size of water granulated slag varies from 150 micron to 5mm. The growth of all
expected mineral crystalline phases are arrested due to water quenching as
compared to natural cooled ferrochrome slag. The water quenched granulated
ferrochrome slag is also porous and glassy in nature. So it is weak in strength and
can be easily grounded to very fine size with less grinding energy compared to the
naturally cooled slag. The chemical composition of the water granulated slag is close
to the refractory composition and is given in Table 1. The MgO-Al2O3-SiO2 ternary
phase diagram in fig 2 is used to explain ferrochrome slag system. It contains high
refractory phases such as spinel, forsterite and low melting phases such as enstatite
and siliceous glass enriched with oxides of calcium, aluminium, chromium and iron.
Fig 3 shows the microstructure of the ferrochrome slag with all possible phases
present in it. Presence of low melting phases decreases the melting temperature of
the slag. In the present invention attempts were made to change the slag chemistry
of water quenched granulated slag to remove the low melting phases. To increase its
melting point, it is necessary to lock the low melting silica phases in a high
temperature refractory phase such as Forsterite with addition of MgO.
The main principle of this technology consists of enriching the Magnesia component
in the ferrochrome slag (water quenched and granulated). In the present invention
the production of refractory material from water granulated ferrochrome slag follows
important steps like: (1) Milling and mixing of ferrochrome slag with magnesite of
chemical composition given in (2) Compaction and shaping to make green compact
(3) Sintering to develop necessary refractory phases.
In the present invention dry ball milling of water granulated ferrochrome slag and
sintered magnesite are employed to achieve very fine slag powder enriched with
magnesite component with very intimate mixture of both these components in it. The
ball milling of ferrochrome slag with magnesite is performed in a ball mill in dry
condition with hardened steel ball as grinding media. The weight of the grinding
media is taken as three times that of the grinding material (ferrochrome slag and
magnesite). The sizes of the grinding materials are selected in the range of 0.5 to 2

mm. During milling of the ferrochrome slag with magnesite, the amount of magnesite
added in such a manner to allow sufficient intimate contact of both materials.
Generally excess amount of MgO than the stoichiometry requirement is selected to
allow intimate mixture of slag and magnesite during milling operation. Approximately
95% of the grinding materials are milled to powder of size below 150 micron, when
the milling is carried out with weight of grinding media (Hardened steel ball) as three
times that of grinding material (ferrochrome slag with magnesite) at 100 revolutions
per minute for 8 hours. The ratio in the
final composition ensures the refractoriness above 1650 0C.
The chemical composition in the final refractory materials is present in Table 3.
The very fine magnesite enriched slag powders obtained from grinding operation are
densified by application of pressure. Green compact of brick or briquettes are made
under pressure of 25 to 40 Mpa in a press. Approximately 2% by weight of molasses
and 3-5% by weight of moisture are added to enhance the green strength of the
compact.
The compacted bricks or briquettes are allowed for sintering in a furnace/kiln at 1450
to 1500 0C for 3 hour. In the initial stage of sintering the necking formation starts and
the growth of neck proceeds as the temperature and time of sintering increases.
Several independent necks on each particle grow during sintering process. The
green compact contain both ferrochrome slag and magnesite powder in intimate
contact with each other. During sintering there is probability of getting magnesite
powder particle in the vicinity of ferrochrome slag powder particle and magensite
powder particle and vice versa. During sintering the possibility of formation of
necking between magnesite-magnesite particles, magnesite-ferrochrome slag
particles, ferrochrome slag-ferrochrome slag particles. Fig 4 describes the schematic
of development of forsterite phases during sintering of FeCr slag and Magnesite
composite powders. Because of interaction of MgO phase with low melting silicate
phases, the forsterite (Mg2SiO4) crystal phase development occurs between
magnesite-ferrochrome slag particles during sintering. During sintering the
necessary crystal phases like forsterite (Mg2SiO4), spinel (AB2O4 where A=Mg, B=Al,
Cr) and periclase (pure MgO) are developed which increase the refractoriness of the

refractory. The optical micrograph in fig 5 shows the necessary refractory phases of
the final refractory material developed from ferrochrome slag. The exothermic peak
in the DSC (Differential scanning calorimetry) thermograph in fig 6 indicates the
crystallograhic chage at around 9000C. At this temperature transformation of the
glassy enstatite (MgSiO3) phase to forsterite crystal phase starts according to the
reaction given below:
MgO + MgSi03(Enstatite) -> Mg2Si04(Forsterite)
Example-1
Equal proportion of water granulated Ferrochrome slag (size range 0.5 to 2mm) of
chemical composition mentioned in table 1 and magnesite (size range 0.5 to 2 mm)
of chemical composition mentioned in table 2 are milled together for 8 hour in a ball
mill to produces composite powder of size less than 150 micron having intimate
contact of slag and magnesite. The MgO enriched composite powders are mixed
with 2% molasses and 3% moisture to make a green compact under pressure of
35Mpa by using a press. The green compact is allowed for sintering at 1450 0C for
3hr to develop necessary refractory phases in the sintered body. The stoichiometry
of this composition contains ratios of in
the final refractory material. The refractoriness of this sintered product is more than
1743 0C.
Example-2
water granulated Ferrochrome slag (size range 0.5 to 2mm) of chemical composition
mentioned in table 1 and magnesite (size range 0.5 to 2 mm) of chemical
composition mentioned in table 2 are mixed in the ratio of 3:2 respectively and milled
together for 8 hours in a ball mill to produce composite powders of size less than 150
micron. The MgO enriched composite powders are mixed with 2% molasses and 3%
moisture to make a green compact under pressure of 35Mpa by using a press. The
green compact is allowed for sintering at 1450 0C for 3hr to develop necessary
refractory phases. The stoichiometry of this composition contains ratios of
in the final refractory material. The
refractoriness of this sintered product is 1683 0C.

Cylindrical samples of size 50mm height and 50mm diameter were made from the
refractory materials and process as explained in example-1. The refractoriness
under load for these samples are tested under constant compressive load of 2kg/cm2
and heated at a rate of 5ºC/min unit a particular percentage of deformation (0.6%) is
reached. The value of the refractoriness under the load is 1480ºC.
Example – 3
Cylindrical samples of size 50mm height and 50mm diameter were made from the
refractory materials and process as explained in example-2. The refractoriness
under load for these samples are tested under constant compressive load of 2kg/cm2
and heated at a rate of 5ºC/min until a particular of deformation (0.6%) is reached.
The value of the refractoriness under the load is 1550ºC.


WE CLAIM
1. A method for producing refractory material with refractoriness above1650 ºC
from water granulated ferrochrome slag comprising:
(i) grinding of ferrochrome slag in presence of magnesite to produce
fine composite powder having intimate mixture of both;
(ii) Compaction and shaping of the powder to make a green compact;
and
(iii) Sintering of the compact to develop necessary refractory phases in
the final refractory.
2. A method for producing refractory material with refractoriness above 1650 ºC
from water granulated ferrochrome slag by a process illustrated in figure 1.
3. The method as claimed in claim 1, comprising a dry grinding operation of
water granulated ferrochrome slag in presence of a raw material of MgO
source such as magnesite in a ball mill to produce composite powders having
intimate contact of slag and magnesite.
4. The method as claimed in claim 3, wherein the size of the slag granules and
magnesite grains are selected in the range of 0.5mm to 2.mm.
5. The method as claimed in claim 1, comprising shaping of the composite
powders to a green compact under pressure of 25 to 40 Mpa in a press from
the powders.
6. The method as claimed in claim 5, wherein the green strength of the compact
is enhanced by using a binder such as molasses (2%) and moisture (3-5%).
7. The method as claimed in claim 1, wherein the sintering of the green compact
is carried-out in a furnace/kiln at temperature in the range of 1450 to 1500 ºC
for 2 to 4 hours to develop the desired refractory phase such as forsterite
(Mg2SiO4), periclase (MgO) and spinel (AB2O4 where A=Mg, B=A1, Cr).

8. The method as claimed in claim or claim 8, wherein the development of
forsterite phase during sintering of Ferrochrome slag and Magnesite
composite powders is exhibited in figure – 4.
9. The method as claimed in claim 1, wherein the ratio and
in the final composition ensures the refractoriness
above 1650 0C.
10. The method as claimed in claim 1, wherein the chemical composition of the
final refractory material comprises, in weight%, MgO: 53 to 81, Al2O3:7 to 19,
SiO2:7 to 16, Cr2O3:3 to 10, CaO: <1, FeO: <1.
11. The method as claimed in claim 1, wherein the microstructure of the final
refractory material developed from ferrochrome slag is shown in figure – 5.
12. The method as claimed in claim 1, wherein the water granulated ferrochrome
slag comprises, in weight%, MgO: 24 to 29, Al2O3:24 to 26, SiO2:29 to 33,
Cr2O3:8 to 12, CaO: 2.5 to 3.5, FeO: 2 to 4.
13. The method as claimed in claim 1, wherein the water granulated ferrochrome
slag consists of microstructure as in fig 3.
14. The method as claimed in claim 1, wherein the refractoriness under load of
the material is 1450-15500C.