FIELD OF INVENTION
The present invention relates to the field of sintering of manganese ore fines. In particular, the invention relates to a method for sintering of MnO2 rich manganese ore fines by addition of a composition of siliceous fluxes containing SiO2, MgO or CaO, and a binder to a sinter mix containing manganese ore fines and solid fuel during the granulation stage.
BACKGROUND OF THE INVENTION
Ferro-Manganese (FeMn) and Silico-Manganese (SiMn) are presently produced by smelting reduction in a Submerged Arc Furnace (SAF) by using lumpy manganese ores. Typical manganese ores that are MnO2 rich have an adverse Al2O3/SiO2 ratio, which necessitates addition of silica through in the SAF to achieve the desired slag composition and viscosity. Fluxes such as dolomite and/or limestone are also used to adjust the slag chemistry. While quartz is a potential source of silica, the high melting point of quartz makes it difficult to use
quartz lumps as a flux in the SAF, unlike the way it is used in the production of Ferro-Chrome (FeCr) which employs temperatures higher than those used for production of FeMn or SiMn. Using fine sand/quartz has an adverse effect on the permeability of the furnace. Formation of slag would also be non-homogeneous as it has to form from two materials with very different melting characteristics -ore fines and sand/quartz. Therefore siliceous manganese ores are used as a source of silica to adjust slag chemistry and properties.
An alternative known method of adding silica to the SAF is by way of manganese ore sinter. The use of manganese ore sinter has an additional advantage of utilizing the manganese ore fines (<10 mm) that are generated due to mechanized mining and cannot be charged directly into the SAF as the permeability of the furnace would be adversely affected. In the process of sintering, a mixture of ore fines, solid fuel, fluxes and other materials such as plant wastes and return fines are granulated in a mixing and granulation drum by addition of optimum amount of moisture to obtain 'green mix'. Ores fines that are less than 10 mm in size are typically accepted for sintering. All other materials are crushed to less than 5 mm. The green mix is then charged onto a
sinter strand (or a sinter pot in batch experiments) upto a pre-determined height of 300-600 mm. Before charging the green mix, cold sinter typically in the size range 10-20 mm termed as 'hearth layer' is charged on the sinter strand (or sinter pot) upto a height of 50-60 mm. This is done in order to prevent the grate bars on the strand (or pot) from overheating and to prevent green mix from falling through the grate bars.
The top surface of the green mix is then ignited using a burner for a fixed amount of time while applying a negative pressure across the bed to initiate the process of sintering. The flame temperature for ignition is typically in the range 1100-1150 °C. The negative pressure is applied using a blower or a suction fan and forces air to pass from the top to the bottom of the bed. Combustion of fuel produces a high temperature in the range 1100-1400 °C that fuses the fine particles together. As air is drawn through the bed, the high temperature zone moves down the bed. Sintering is complete when the high temperature zone reaches the bottom of the bed and this is indicated by a maximum in the temperature of the waste gas that flows from underneath the grate bars. The hot sinter cake produced at the end of sintering typically reduced to a size less
than 200 mm and convectively cooled to room temperature. Cooled sinter is further crushed to less than 40 mm and typically screened at three levels - 5 mm, 10 mm and 20 mm. Sinter in the lowest size 0-5 mm is termed as 'return fines' and is recycled in the sintering process. Sinter in the intermediate size range, 10-20 mm is used as hearth layer while the rest is designated as 'Product Sinter' and sent to the Blast Furnace or SAF. The strength of sinter in the cold condition is measured by means of the standard, ISO 3271-1987 and is known as Tumbler Index.
However, Indian manganese ores are not amenable to sintering. The high Al2O-3/SiO2 ratio does not form sufficient melt of appropriate viscosity and this demands a higher solid fuel rate in sintering. The presence of high AI2O3 in ore fines also results in weaker sinter. Further Indian manganese ores are subject to decrepitation. Decrepitation of manganese ore depends on its phase composition and phase association [1]. MnO2 rich manganese ores are known to decrepitate more due to the stress induced by the volumetric contractions and expansions caused by the thermal decomposition of the oxide in the temperature range 600-700 °C. Presence of moisture increases the decrepitation of ]
manganese ore [2]. Therefore, manganese ore decrepitates and forms fines during sintering which lower the permeability of the bed. Indian manganese ores having a lower Mn/Fe ratio in the 1.5-2 results in a lower than required Mn/Fe ratio in the sinter.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a method for sintering of MnO2 rich manganese ore fines by addition of a composition of siliceous fluxes and a binder to increase the melt volume including modification of chemistry of the melt.
Another object of the invention which produces a sinter with high strength.
A further object of the invention is to propose a method for sintering of MnO2 rich manganese ore fines by addition of a composition of siliceous fluxes, which
reduces the fuel rate in sintering and minimize the adverse effect of descrepitation of MnO2. A still further object of the invention is to propose a method for sintering of MnO2 rich manganese ore fines by addition of plant wastes having a high Mn/Fe ratio.
SUMMAARY OF THE INVENTION
Accordingly, the present invention provides a flux-binder composition applicable during sintering of manganese ore fines to increase the melt volume and modify the chemistry of the melt, thereby produce a sinter with higher strength. The invention allows to reduce the fuel rate in sintering and minimize the adverse effect of decrepitation of MnO2. Such flux-binder composition also eliminates the need to add siliceous manganese ore and fluxes into the SAF. According to the invention, plant wastes having a high Mn/Fe ratio may be alternatively used to the sinter bed in order to increase the Mn content and Mn/Fe ratio of sinter.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1 - graphically shows the amount of slag/melt formed at various
temperature with or without addition of siliceous flux in the range of oxygen partial pressure that occur during sintering.
DETAIL DESCRIPTION OF THE INVENTION
According to the present invention, flux-binder composition such as siliceous dolomite and calcined lime or pyroxenite and calcined lime are added to the sinter mix during the granulation stage along with manganese ore fines and solid fuel. The flux should be in the size range 0-3.15 mm while the binder should be in the size range 0-0.074 mm. The addition of the above mentioned flux-binder compositions to the sinter mix minimizes the adverse effect of decrepitation of MnO2, increases the melt volume for the same solid fuel rate in sintering, and produce melt of desirable composition thereby increasing the cold strength of sinter, including reduction in the solid fuel input to sintering. Use of fluxed manganese ore sinter also eliminates the addition of siliceous manganese ore
and fluxes directly into the SAF thereby reducing the fuel rate in the SAF. According to the invention, plant wastes such as dried sludge from the Gas Cleaning Plant (GCP) attached to a SAF producing ferromanganese having a high Mn/Fe ratio of 17-20 may also be added to the sinter mix in order to improve the manganese content and Mn/Fe ratio of sinter. Screened coke fines, ore fines and flux fines may also be added to the sinter mix.
During granulation, calcined lime reacts with the added moisture to form calcium hydroxide which binds the fines together by crystalline bonds even when the moisture in the green mix evaporates during sintering. Therefore the formation of loose fines due to decrepitation of MnO2 is minimized. Also the presence of CaO in the vicinity of the ore fines results in the formation of binding phases of the type Ca2MnO4 and (Ca,Mn)SiO4 that form stronger sinter.
The addition of fluxes containing MgO, CaO and SiO2 to the ore fines decreases the fusion temperature of the charge as a result of which more melt is formed for the same heat input in sintering. Equilibrium calculations were performed using FACTSAGE (v5.4.1) for the amount of slag/melt formed from sinter mix
containing manganese ore fines with and without the addition of siliceous dolomite/pyroxenite in the temperature range of sintering 1100-1400 °C and for the range of oxygen partial pressures (10-8 atm to 10-3 atm) that typically occur during sintering and are shown in Fig. 1. It can be seen from the figure that any temperature and oxygen partial pressure, the amount of melt formed is always higher in the case where flux was added.
Therefore, the addition of siliceous dolomite or pyroxenite to the sinter mix increases the volume of melt formed during sintering. The melt formed from the sinter mix containing siliceous fluxes contains CaO and MgO along with AI2O3 and SiO2 which on solidification forms stronger binding phases compared to those formed from melt containing only Al2O3 and SiO2- As the volume of melt formed at a certain temperature due to the addition of siliceous flux and binder is higher, there is a scope for reduction of the solid fuel input to sintering while achieving the required sinter quality.
WE CLAIM
1. A method for sintering of MnO2 rich manganese ore fines by addition of a composition of siliceous fluxes containing SiO2, MgO, or CaO, and a binder to a sinter mix containing manganese ore fines and solid fuel during the granulation stage to minimize the adverse effect of decrepitation of MnO2 during sintering, produce sinter with higher strength through modification of the melt chemistry, and reduce the solid fuel input to sintering while maintaining the quality of sinter, wherein the siliceous flux is siliceous dolomite or pyroxenite, having a silica content between 40-55 wt% and is in the size range 0-3.15 mm, and wherein the binder is calcined lime, having a CaO content of atleast 80% and is in the size range 0-0.074 mm.
2. A method as claimed in claim 1, wherein plant wastes such as dried sludge from the Gas Cleaning Plant attached to a SAF producing ferromanganese with a high Mn/Fe ratio of 17-20 can be added to the
sinter mix to improve the Mn content and the Mn/Fe ratio of manganese ore sinter.
3. A method as claimed in claim 1, wherein fines obtained due to screening of coke, manganese ores and fluxes can be added to the sinter mix.

ABSTRACT

A method for sintering of MnO2 rich manganese ore fines by addition of a composition of siliceous fluxes containing SiO2, MgO, or CaO, and a binder to a sinter mix containing manganese ore fines and solid fuel during the granulation stage to minimize the adverse effect of decrepitation of MnO2 during sintering, produce sinter with higher strength through modification of the melt chemistry, and reduce the solid fuel input to sintering while maintaining the quality of sinter, wherein the siliceous flux is siliceous dolomite or pyroxenite, having a silica content between 40-55 wt% and is in the size range 0-3.15 mm, and wherein the binder is calcined lime, having a CaO content of atleast 80% and is in the size range 0-0.074 mm.