Field of the Invention
This invention relates to a process for the production of low iron content chromite ore from ferruginous chromite ore deposits. The low iron content chromite concentrates can be used in ferro-chrome making processes. This invention describes the dry magnetic separation of low grade ferruginous chromite ore by pre-treating in microwave energy for the change in the magnetic susceptibility of associated minerals.
Background of the Invention
Chromite ore is the only source for chromium metal. Generally chromite ores are used in the industry based on the physical and chemical properties. Major portion of the chromite is utilized for ferro-alloy making process which can be used for making alloy steels of different type and grade. In world major portion of the deposits are ferruginous in nature with high iron content in the chromite crystal lattice as well as associa,ted gangue minerals in the form of hematite, goethite and laterite, For user perspective chromium-to-iron ratio should be more than 1.8 to use in ferro-alloy making processes. The ores of such nature is difficult to beneficiate by using conventional gravity separation process. So, it is necessary to explore the alternative processes for beneficiating such ferruginous low grade chromite ore.
Patent 2006/0123951A1 discusses about a method of improving the chromium-to-iron ratio of chromite ores. The process involves testing the ore for improving the chromium-to-iron ratio of chromite ore and products by selective chlorination method. The process comprises mixing the chromite ore or product with a salt and thereafter selective chlorination of iron in the presence of CO at a temperature in between 250° C to 720° C. The chromium-to iron ratio of the product is enriched to 10 from a feed assaying 1.8. However the process is found to be very complex and the ore is rich in nature which is evident from the high chromium-to-iron ratio.

Patent application No. PCT/CA97/00158, PCT/AU2005/00'499 discusses about the microwave treatment of metal bearing ores and concentrates to pre-treat prior to conventional processes. The process outlines treatment of different minerals excluding the chromite ore and in particular ferruginous chromite ore. These patents are described about the refractory gold ore and its pre-treatment and its change in the metallurgical properties. However there is no such explanation for the treatment of minerals of near magnetic susceptibility in nature.
Patent application No. PCT/US2005/030741 describes the process of expanding the perlite particle under microwave radiation pre-treatment. This innovation explains about the method of rapid heating of expandable ore which resulted in lowering the particle density as well as product quality. Patent application No. PCT/US2004/014609 describes the method of enhancing the segregation roast of metallic sulphides, slags, metallic oxides, iron oxides, silicates and carbonates. This invention discloses the reduction of metals and recovery by adding carbon and chloride ions with complex ore assemblages and slags. These patents are application of microwave radiation for different minerals other that chromite ore.
Patent application No. 651/DEU2007 discusses about a method of beneficiating such low grade ferruginous chromite s containing high iron content. The process involves testing of the ore for removing high iron content impurities, the use of conventional floatation steps to remove iron bearing impurities, crushing, scrubbing, grinding, classifying or desliming the grounded ore particles to ground below 200 urn and thereafter conditioning the slurry at 10% solids by wt., pH of at 11, gum acacia as depressant, sodium hexa meta phosphate as dispersant and sodium olate as collector to float the chromite particles.

Patent application No. PCT/IB2005/052605, from Sishen Iron Ore Company Ltd, discusses about a method of enhancement of liberation pattern of low grade iron ore by exposing to microwave energy. This invention discloses the change in the aspect ratio of different mineral grains in the iron ore. In the literature, there is no such comprehensive process to beneficiate the ferruginous chromite ore by exploiting the magnetic susceptibilities by pre-treating in microwave radiation.
Objects of the Invention
An object of the present invention is to propose a beneficiation process for the production of high chromium-to-iron ratio of chromite concentrate from low grade ferruginous chromite ores.
Another object of the invention is to develop an alternative separation process of higher chromium-to-iron ratio of chromite concentrate from low grade ferruginous chromite ores.
Another object of this invention is to reduce the iron content in the chromite ore.
Further object of this invention is to propose a dry separation process for producing higher chromium-to-iron ratio of chromite concentrate from low grade ferruginous chromite ores.
Summary of the Invention
Accordingly, there is provided a process for beneficiation of low grade ferruginous chromite ore, to produce higher chromium-to-iron ratio concentrate, comprising the steps: crushing the low grade ferruginous chrotnite ore to produce the particle size below 5 mm with lower chromium-to-iron ratio, subjecting the crushed product to a microwave radiation zone with carbon, further subjecting the pre-treated crushed products to dry low intensity magnetic separation process, and collecting the non¬magnetic fraction as a concentrate.

Brief Description of the Accompanying Drawings
The process which is object of the present invention, allows to lower the iron content from the low grade ferruginous chromite ore in a feasible, efficient, and economical way which can be better understood from figure 1. The description of the figure allows for a complete and comprehensive understanding of the process. The Figure shows a flow diagram of an alternative separation process for achieving high chromium-to-iron ratio product from ferruginous low grade manganese ore.
Detailed Description of the Invention
As shown in figure 1, the inventive beneficiation process of minimizing the iron content from low grade ferruginous manganese ore or fine consists the steps of crushing, pre-treatment by microwave radiation and low intensity dry magnetic separation. The low grade ferruginous ore fed to crushing unit (2) for reducing the particle size to below 5 mm, from which the crushed product can be collected and fed to a mixer for mixing (3) along with carbon particles (4). The mixed product (3) is than fed to microwave furnace (5). The product generates from microwave furnace is fed to a low intensity dry magnetic separator (6) for the separation of high iron content fraction as a magnetic product (8) which can be discarded as a tailing or can be used as a mine filling. The non-magnetic fraction of the process is of low iron product along with chromite collected as a concentrate (7). This concentrate (7) can be used as burden for ferro-chrome production process.
As depicted in Figure 1, a method is provided to produce high chromium-to-iron ratio chromite concentrate from low grade ferruginous manganese ore or fines. The process handles the coarser particles of maximum particle size having 5 mm. Further, 80% (by weight) of the fines have a particle diameter in the range of 0.5-3 mm. The low grade ferruginous chromite ore or fines contain chromite, goethite, hematite, kaolinite, gibbsite and quartz as the primary minerals along with minimum amount of olivine, pyroxene and others. To understand the mineral composition properly, samples were

processed in QEMSCAN and the result is given in Table 1. Further mineral analysis of the sample envisaged that the low grade chromite ore contains 38-43% chromite content whereas rest is gangue content. The low grade chromite ore contains Cr203 in the range of 30-35% with Fe(T)(20-25%), silica (5-9%), Al203 (8-13), MgO (6-7%)and loss on ignition (LOI) of 5 to 7%.

The low grade ferruginous chromite ore or fines of above mentioned composition are fed in to the crushing unit (2) i.e. jaw crusher; roll crusher, etc. to reduce the feed particle size to below 5 mm for better liberation of chromite particles. The product from the crusher i.e. crushed product is targeted to get a coarser particle size distribution 0.5-3 mm (D80 size). During the crushing, it was also tried to produce minimum finer particle size i.e. below 75 m is in the range of 10 to 20% (by wt). The crushed product is subjected to mixing (3) by means of a rotary mixer or other type of mixer along with carbon (4) particle of size range (-1 mm), The carbon content is varied in between 3 to 15% (by wt.) of the total mix. The carbon used in the mixing is solid fuels such as coal, charcoal, coke etc. with ash content varied from 5-15% (as on dry basis).

The mixed product from the mixing unit (3) is subjected to microwave furnace or oven (5). The furnace is of microwave frequency of 2.4 GHz with microwave power varied in between 750 W to 1200 W. The retention time of the mixed material inside the furnace is in between 45 sec. to 150 sec. In the furnace, during the microwave heating, the carbon was interacted with the iron bearing minerals and reduced to higher phases of iron. It is ensured that the reduction of the iron should be in between wustite and magnetite.
The product generated from microwave furnace (5) is cooled in the air in closed atmosphere or can be quenched with water for the stabilization of the reduced phases. The product generated from the microwave furnace (5) is subjected for measuring the weight loss and it is found that the weight loss after the pre-treatment of the sample is 11.3, 13.2 and 11.8% (by wt.) respectively for test 1, 2 and 3. These cooled products collected and fed to a low intensity dry magnetic separator (6). The reduced product is subjected to phase analysis and found that about 30 -35% of the total mass has converted to magnetite phase. The complete phase analysis of the reduced product is given in Table 2. The magnetic field intensity of the separator should be varied in between 700 to 1200 gauss. The separator may of electro-magnetic type or permanent magnetic type. This magnetic separator (6) may be of roll or drum or belt type in design. This magnetic separator (6) separates the particles into two different streams such as magnetic (8) and non-magnetic (7). The magnetic fraction (8) generated from the magnetic separator mainly consist of iron compounds which can be rejected as tailing whereas non-magnetic fraction (7) is mainly consists of chromite particles with minimum gangue content. This non-magnetic fraction (7) can be considered as a concentrate and can be further used for ferro-chrome process. The yield of the non¬magnetic fraction (7) of the magnetic separation is varied in between 16% to 69% (by wt.). The Cr203 content of the non-magnetic fraction (7) is varied from 42% to 45% whereas iron content (Fern) varied from 15 to 18%. The chromium-to-iron ratio of the concentrate is varied from 1.6 to 2.0. The overall recovery of Cr203(%) of the process is

varied in between 21 to 89%. Similar experiments on the tow grade ferruginous manganese ore was carried out as mentioned in the flow sheet.

As mentioned in Fig.l, the feed material is subjected to jaw crusher to crush below 5 mm. The crushed ore is fed to a mixer along with carbon at different levels for different tests. The mixed product from the mixer fed to a microwave furnace at different microwave power levels and retention time. After this pre-treatment of the mixture in microwave furnace for a given conditions, subjected to cooling (which can be air cooled or water quenched) and fed to dry low intensity magnetic separator of drum type. The process conditions maintained during the tests for mixing, microwave furnace and magnetic separation are given in Table 3. These mentioned three tests are carried out by varying the carbon content in the mixture, microwave power, retention time in the furnace, type of cooling, type of magnetic separation and magnetic field intensity of the separator. The results obtained from the mentioned test conditions are given in Table 4.


The products generated after the crushing in three different batches are subjected for the particle size distribution. It is found that D80 particle size of the crushed product of the test 1, 2 and 3 are 0,8, 1.5 and 1.8 mm respectively. The crushed sample contains 32.8% Cr203r 22.4% of Fe(T) 12.7% AI203 and 9% of Si02, 6.7% MgO and 6% LOI (loss on ignition). The crushed product of chromite contains chromite, hematite, goethite, gibbsite, kaolinite and quartz as major mineral phases which are evident from XRD pattern of the sample (Figure 2). Further to understand and the magnetic property (magnetization) of the sample is determined by vibratory sample magnetometer and found that magnetization achieved for the sample in normal temperature is 0,3, 0.48 and 0.62 emu/g. at magnetic field intensity of 0.1, 0.5 and 1.0 Tesla respectively (Figure 3).

The crushed product from the crusher was subjected to a mixer along with charcoal of known quantity i.e. 7.5, 15 and 15% (by wt.) for test 1, 2 and 3 respectively. The particle size of the mixed charcoal is below 100 urn and the ash content is 7% (by wt.). After proper mixing, the burden is fed in a microwave oven (for test 1) laboratory microwave furnace (for test 2 and 3). The microwave oven is domestic model of LG make, 2.4 GHz and 900 W. whereas microwave furnace used in the present investigations is GN-Tech model, operating frequency: 2.45 GHz, WR 340 Waveguide, infrared pyrometer ranging from 350 - 1800° C, microwave output 6 kW, magnetron power variable from 0.3-6.0 kW. Based on the test conditions mentioned in Table 1, the sample was placed inside the furnace (5) for the pre-determined time and microwave power. After the pre-treatment sample taken out from the furnace and quenched with water or cooled air (as mentioned in Table 1). These pre-treated samples were subjected for vibratory sample magnetometer and the hysteresis curve is given in Figure 4. It is found from the Figure 4 that the magnetization of the sample is higher than the feed value. Also, it is found that the magnetization value of Test 1 is higher than Test 2. This increase in the magnetic property is basically due to change in the phase i.e. goethite and hematite to magnetite. For better understanding, comparison of phase analysis of test 2 and feed is carried out and is given in Table 5. There after the cooled sample was subjected for characterisation and magnetic separation in a low intensity magnetic separator (6). The magnetic field intensity maintained during the separation is 800, 1000 and 1000 gauss for test 1, 2 and 3 respectively. Magnetic separation is carried out repeatedly to collect all the magnetized material at that particular pre-treatment as well as magnetic field intensity. After the separation, magnetic (8) fraction which is rich in iron can be discarded as tailing or can be recirculate in the magnetic separation circuit by reducing the particle size further. Similarly, non-magnetic (7) fraction of the separator is rich in chromite and with minimum iron content is considered as a product. This non-magnetic (7) fraction of the

separator contains higher chromium-to-iron ratio which is 1.7, 1.8 and 1.9 for test 1, 2 and 3 respectively. Further the Cr203 recovery of the process is found to be 21.9%, 88.2% and29.4% respectively for test 1, 2 and 3.

The current method of the invention reduces the iron content of the low grade ferruginous chromite ore for effective utilization. Further, the invention opens up possibility of utilization of low grade ferruginous chromite ore in the ferrochrome

making process which is ingredient for several metallurgical applications. The invention of course is useful from environment perspective as it ensures effective utilization of low grade ferruginous chromite ore as well as production of higher chromium-to-iron product from the chromite deposits. This results in avoiding the environmental hazard such as storage of these low grade dumps in mine site. The process further envisages dry processes which decreases the water consumption for the processing.

WE CLAIM:
1. A process of reducing iron content in low grade ferruginous chromite ore or fines, the process comprising:
- Crushing the chromite ore to achieve the product size below 5 mm;
- Crushing the chromite ore to achieve the product size in the range of 0.5 to 3 mm as a D80 size;
- Mixing the crushed chromite ore product with carbon in the range of 5 to 15 weight %;
- Heating the mixed chromite product in a microwave oven or a furnace at microwave power varied in the range of 750 W to 1200 W for a retention time of 45 sec. to 150 second; and
- Exposing the heated chromite product to a dry low Intensity magnetic separation process.

2. The process as claimed in claim 1, wherein the low grade ferruginous chromite ore or fines, have a particle size of less than 25 mm.
3. The process as claimed in claim 1, wherein the low chromite grade ore comprises in weigh% ,Cr203 in the range of 30-35% with Fe(T)(20-25%), silica (5-9%), AI2O3 (8-13), MgO (6-7%)and loss on ignition (LOI) of 5 to 7% (wt).
4. The process as claimed in claim 1, wherein the crushed product is mixed with carbon of particle size below 1 mm.

5. The process as claimed in claim 4, wherein carbon used in the mixing is selected from a group consisting of coal, charcoal, coke and a mixture thereof, with ash content varied from 5 to 15 weight % (as on dry basis).
6. The process as claimed in claim 1, wherein the microwave power is varied in the range of 750 W to 1200 W in a furnace of microwave frequency of 2.4 GHz
7. The process as claimed in claiml, wherein the retention time of the mixed material inside the furnace is in the range of 45 seconds to 150 seconds
8. The process as claimed in claim 1, wherein the carbon interacts with iron bearing material during the microwave heating and is reduced to phases of iron such as magnetite.

9. The process as claimed in claim 1 further comprising the step of cooling the product generated from furnace in the closed atmosphere for the stabilization of the reduced phases.
10. The process as claimed in claim 1 further comprising the step of quenching the product generated from furnace with water for the stabilization of the reduced phases.
11.The process as claimed in claim 1, wherein the magnetic field intensity of the separator is varied in the range of 700 to 1200 gauss.
12.The process as claimed in claim 1, wherein the non-magnetic fraction of the magnetic separation varies in the range of 16% to 69% (by wt.).

13. The process as claimed in claim 1, wherein the Cr203 content of the non-magnetic fraction is varies in the range of 42% to 45% (by wt.).
14.The process as claimed in claim 1, wherein the iron content (Fe(T)) of the non-magnetic fraction varies in the range of 15 to 18% (by wt.).
15.The process as claimed in claim 1, wherein the chromium-to-iron ratio of the concentrate varies in the range of 1.6 to 2.0.
16.The process as claimed in claim 1, wherein the overall recovery of Cr203(%) is in the range of 21 to 89% (by wt.)-