FIELD OF THE INVENTION
The present invention relates generally to development of process and system for lowering the alumina content of iron ore fines and specifically, it relates to physico-chemical treatment of iron ore fines of size less than 6mm to produce a concentrate with substantially lower alumina content than the feed material.
BACKGROUND OF THE INVENTION
The blast furnace process of iron making requires about 1.5 tonnes of iron ore (Fe about 64%) per ton of pig iron produced. The iron ore is supplied in the form of lump (which is also known as calibrated lump ore), sinter or pellet. The iron ore sinter is produced through metallurgical treatment of iron ore fines that results in iron ore sinter with superior physical and metallurgical properties than the feed iron ore fines. The iron ore fines required for the sintering process should have specifications of AI2O3 preferably less than 2.5%. The iron ore fines are produced from iron ore mines after suitable processing/beneficiation so as to enrich Fe and lower AI2O3 content.
The process of iron ore beneficiation involves crushing of 1000mm size of iron ore boulders to a size of less than 40 mm. The crushing is carried out in primary, secondary and tertiary stages. After crushing, the -40mm material is treated in a drum scrubber to scrub away the clay coatings from the surface of iron ore. The drum scrubbing process is a wet physical process, where the material is scrubbed along with water in a cylindrical drum. The scrubbing is usually carried out at water/ore ratio of 1:1.

The iron ore beneficiation is typically carried out to remove the gangue minerals AlzO3 and SiO2. Few of the techniques like Jigging and Heavy media separation are also practiced across iron ore industries to fractionate gangue minerals and iron ore minerals separately. The US patent US2309275A describes the use of Jigging for beneficiating low grade iron ore. This method consists of jig concentration and is particularly adapted for use on low grade iron ore or other ore where it is desirable to make a more perfect separation of the waste material from the ore. In carrying out this method any standard multiple cell jig can be used. The difference between this process and standard jig processes is that some of the waste rock is drawn of the top of each cell and the remainder of the ore is retreated in the next jig cell.
Iron ore fines are also beneficiated through heavy media separation involving use of heavy media baths. These heavy media baths are prepared with a heavy media which is mostly ferrosilicon. The density of the ferrosilicon media is maintained at about 3.2-3.5 by suitably mixing the ferrosilicon media in water. The ferrosilicon suspension is thus used to separate the iron ore from gangue minerals. The US patent US2428777A describes the use of heavy media separator for beneficiating a low grade iron ore to effectively separate lighter gangue minerals from the heavier ore minerals.

In yet another method involving chemical beneficiation of iron ore, the US patent US3163518A describes the process of separation of Si02 minerals from iron ore minerals. The method of liberating silica from siliceous iron ore containing granular silica comprising the steps in combination of: comminuting said ore to particle sizes exposing surfaces of the silica grains therein and to about minus mesh screen size; mixing said comminuted ore with sodium hydroxide in an amount between about 1 and about 14 percent by weight of said ore subjecting said mixture to a temperature between about 500 Fahrenheit and about 750 Fahrenheit at a low pressure between respectively about pounds per square inch and normal atmospheric pressure; simultaneously adding water to take said sodium hydroxide into solution to cause thereby weakening of the grain boundaries of silica by chemical attack and to cause weakening of said grains and grain boundaries by thermal shattering due to physical forces arising from a difference in thermal coefficient of expansion of silica grains and other ore particles; thereafter cooling said mixture; agitating said cooled mixture to liberate the silica grains weakened at their grain boundaries from other particles of the ore; and separating said ore particles from said liberated silica grains.
The physical beneficiation process like drum scrubbing though is effective in removing clay coatings but during the subsequent stage of washing with water, the clay coatings get adhered to the coarse and fine iron ore particles thereby reducing the efficiency of the drum scrubbing process. The limitation of the drum

scrubbing and washing process is that the ultrafine clay minerals are carried away with the iron ore fines due to their high surface energy. The other benefication processes like jigging and heavy media separation processes do not scrub away the day coatings. These processes just fractionate the iron ore minerals based on their specific gravity. Thus the particles rich in clay minerals get separated from particles rich in iron ore minerals, since their specific gravity difference is large. The iron ore particles that are just weathered on the surface (clay coated) get reported in the heavier concentrate fraction even though their clay content is higher.
In view of the above limitations there is a need to develop processes which increase the efficiency of drum scrubbing and washing, so that the ultrafine clay coatings that are removed in scrubbing stage get separated with the tailings stream.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a process for lowering the alumina content of iron ore fines of size -6+0.15mm.
An object of this invention is to improve the process of drum scrubbing of iron ore fines such that there is an improvement in alumina reduction.
An object of this invention is to propose a suitable chemical that would improve the process of drum scrubbing

Another object of this invention is to recommend the dosage of chemical, its
addition point in the process and retention time of the suspension in drum
scrubber
SUMMARY OF THE INVENTION
The iron ore scrubbing process comprises of preparing slurry of iron ore solids along with water. The suspension is maintained at about 50% solids and fed into a cylindrical drum in which the inter particle attrition and the particle-drum attrition provides the required scrubbing action. During this process the adhering clay coatings are removed and get suspended in water. However because of the very high surface energies of these ultrafine clay coatings, they have a tendency to stick to iron ore particles thereby diluting the iron ore concentrate grade. The present invention relates to the development of a method to effectively scrub and wash away the clay mineral coatings. The method essentially comprises of addition of a dispersant chemical Sodium hexa meta phosphate in drum scrubber to disperse the ultrafine particles. This is caused due to the reduction of surface energy of the ultrafine clay particles. The lower surface energy enables a better stability of the suspension which consequently results in easy washing of the adhering clay coatings. The dispersant sodium hexa meta phosphate decreases the zeta potential of the iron ore suspension. The decrease in zeta potential induces strong similar charge on the ultra-fine clay particles.

This similar charge on clay particles creates a repulsive force that results in de-agglomeration of the clay particles. The de-agglomerated ultra-fine clay particles lose their tendency to agglomerate among themselves or with the coarser iron ore particles. Thus when iron ore suspension is washed with water, the coarser iron ore particles do not have any adhered clay coating. The effective washing of the ultra-fine clay coatings results in reduction of total alumina content of iron
ore fines.
The invented process is capable of treating all types of iron ore fines which
contain clay coatings. The iron ore fines typically assaying 61.7% Fe and 3.65%
Al203 was treated in a drum scrubber in presence of chemical sodium hexa meta
phosphate (SHMP). The retention time of the suspension in drum scrubber was
kept between 0.5 - 10 minutes. The dosage of the chemical SHMP was kept
between 30-3000 ppm to produce a final product assaying 63.2% Fe and 2.45%
Al203 after chemical aided drum scrubbing.
Overall with the invented process of chemical aided drum scrubbing, it is possible
to decrease alumina by a maximum 1.2% while without chemical scrubbing the
reduction in alumina is about maximum 0.3%.
The process of drum scrubbing was therefore improved through the aid of
chemical SHMP. The chemical aided drum scrubbing process has lot of merits
over the conventional drum scrubbing process, the most important being the
effective removal of alumina and silica and a lower retention time in drum
scrubber.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Schematic of iron ore processing with Chemical addition
DETAILED DESCRIPTION OF THE INVENTION
The beneficiation of low grade iron ore is required to reduce the gangue minerals alumina and silica and improve the iron values. The process of beneficiation comprises of crushing, screening, drum scrubbing, washing and classification operations. The drum scrubbing operation is a physical separation process where the iron ore particles (both lump and fines) are scrubbed in wet condition in a cylindrical drum. The objective of drum scrubbing is to scrub and remove the clay coatings that are weakly adhered to the iron ore particles. The effectiveness of this process depends on the washing of iron ore which is carried out to wash away the clay coatings that are removed during the process of drum scrubbing. The clay coatings being ultrafine in size have very high surface energy which results in the agglomeration of these coatings. These agglomerated clay particles ultimately report in the iron ore concentrate thereby diluting the iron ore grade. The present invention relates to the development of a method to effectively scrub and wash away the clay mineral coatings. The method essentially comprises of primary crushing of 1000-1500 mm boulders of iron ore to a size of less than 200mm. The crushing is carried out with a Gyratory crusher which crushes the material to a size of about 200mm. The crushed material is then dry

screened at a size of 40mm to screen out two different fractions. Fraction 1 is oversize material that is retained above the 40mm screen. This material is termed as -200+40mm material. Fraction 2 is the undersize material which passes through the 40mm screen is termed as -40mm material. The -200+40mm material is again crushed in a secondary crusher so as to reduce the size of material from a top size of about 200mm to a top size of about 40mm. The material crushed in secondary crusher is dry screened at 40mm to fractionate oversize and undersize material.
The oversize material from screen which is termed as Fraction 3 (+40mm) is again crushed in a tertiary crusher to size of less than 40mm. The undersize material (-40mm) from primary crusher (Fraction 2) and the undersize material from secondary crusher (Fraction 4) are then processed in drum scrubber. Before processing in drum scrubber the undersize material -40mm is mixed with water in a ratio that varies from 2:3 to 3:2. Simultaneously a dispersant solution namely Sodium Hexa meta phosphate is prepared by mixing 1 part of Sodium hexa meta phosphate with 20 parts of water. This solution is stored in a tank for further dosing into the drum scrubber. The iron ore-water suspension is then passed through a drum scrubber which is a type of cylindrical drum. Along with the iron ore suspension a separate stream of sodium hexa meta phosphate solution is also added to the iron ore-water suspension. The dosing rate of this chemical is kept in the range of 30-3000 ppm or 0.03-3.0 kg/tonne of dry solids fed into the drum scrubber. The feed rate of iron ore-water suspension is controlled so that the retention time of the suspension in drum scrubber varies from 0.5 - 10.0 minutes.

The suspension from drum scrubber is then screened and washed at a screen size of 6mm to separate -40+6mm and -6mm fractions. The dispersant (SHMP) addition causes the ultrafine clay particles to disperse in water. This is caused due to the reduction of surface energy of the ultrafine clay particles. The lower surface energy enables a better stability of the suspension which consequently results in easy washing of the adhering clay coatings. The dispersant sodium hexa meta phosphate decreases the zeta potential of the iron ore suspension. The decrease in zeta potential induces strong similar charge on the ultra-fine clay particles. This similar charge on clay particles creates a repulsive force that results in de-agglomeration of the clay particles. The de-agglomerated ultra-fine clay particles lose their tendency to agglomerate among themselves or with the coarser iron ore particles. Thus when iron ore suspension is washed with water, the coarser iron ore particles do not have any adhered clay coating. The effective washing of the ultra-fine clay coatings results in reduction of total alumina content of iron ore fines.
The -40+6mm material which also known as lump ore is separately stacked while the -6mm material which is in suspension form is processed in a screw classifier. The Screw classifier is an inclined helical screw which separates coarser material from finer material. The -6mm material fed to the screw classifier then gets separated into fractions -6+0.15mm and -0.15mm. The -6+0.15mm material is then allowed to pass through a dewatering screen which dewaters the water that is carried along with this material. The dewatering screen of size 0.5mm is used to dewater the -6+0.15mm material. The undersize suspension coming out from dewatering screen and the -0.15mm suspension coming out from screw classifier are both collected in a single launder from where they are ultimately transferred to a sump for further processing in Hydrocyclone.

The invented process is capable of treating all types of iron ore fines which contain clay coatings. In one of the example of iron ore chemical scrubbing (Table 2), the iron ore fines typically assaying 61.7% Fe and 3.65% Al203 was treated in a drum scrubber in presence of chemical sodium hexa meta phosphate (SHMP). The retention time of the suspension in drum scrubber was kept between 0.5 -10 minutes. The dosage of the chemical SHMP was kept between 30-3000 ppm. The chemical aided scrubbing resulted in a product assaying 63.2% Fe and 2.45% Al203.
Overall with the invented process of chemical aided drum scrubbing, it is possible to decrease alumina by a maximum 1.2% while without chemical scrubbing the reduction in alumina is about maximum 0.3%.
The process of drum scrubbing was therefore improved through the aid of chemical SHMP. The chemical aided drum scrubbing process has lot of merits over the conventional drum scrubbing process, the most important being the effective removal of alumina and silica and a lower retention time in drum scrubber.

WE CLAIM:
1. A process for lowering alumina content of iron ore fines of size -6+0.15mm the
process comprising:
Crushing run of mine iron ore to less than 40mm;
subjecting the crushed -40mm iron ore sample to step of drum scrubbing;
Simultaneous dosing of sodium hexa meta phosphate during drum scrubbing;
Subjecting the drum scrubber output to washing and screening at 6mm screen size to separate -40+6mm and -6mm fractions; and
Subjecting the -6mm material to screw classification to separate -6+0.15mm and -0.15mm fractions.
2. The process as claimed in claim 1, wherein the dosage of Sodium hexa meta phosphate is in the range of 30 to 3000 ppm.
3. The process as claimed in claim 1, wherein the solids to water ratio in drum scrubber is maintained in the range of 2:3 to 3:2.
4. The process as claimed in claim 1, wherein the retention time of iron ore-water suspension is drum scrubber in the range of 0.5 to 10 minutes.

5. The process as claimed in claim 1, wherein the iron ore suspension is washed and screened after drum scrubbing at a screen size of 6mm to separate -6mm material.
6. The process as claimed in claim 1 and 5, wherein the -6mm iron ore suspension is further classified in a screw classifier to separate -6+0.15mm and -0.15mm material.