METHOD FOR SMELT REDUCTION OF CHROMITE ORE

TECHNICAL FIELD
[0001] The present disclosure, in general, relates to of complete utilization of chromite ore tailings with zero discharge. In particular, the present disclosure related to a submerged arc based method for smelt reduction of chromite ore.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Chromium has diverse applications and is widely used in refractory, chemical, metallurgical and casting industries. Chromite ore is the only source of producing chromium. Of all the chromium produced, almost 90% is being used in the metallurgical field. Like most of the heavy metals, chromite ores are treated by gravity separation method for its upgradation, which in turn produces a huge number of fine-sized tailings (<25 µm) having higher silica content. These tailings cause huge environmental hazards and have a considerable amount of metallic content which goes untapped in most of the ferrochrome production plant as of now. As of now, these tailings generated in steel manufacturing plants are dumped in dump yards, which is further raising concern related to space as well as environment.
[0004] Various approaches have been proposed in the state of the art to address the issues related to the tailings. One of the approaches mentioned in Patent Publication bearing no. US20040086438A1 discusses the simultaneous recovery of chromium and iron from chromite ore processing residue (COPR) as a chromate salt and iron salt and avoids landfilling of toxic metals. The process involves treating the COPR with metal hydroxide followed by leaching in the temperature range of 30-100 °C and other hydrometallurgical routes. This approach of US20040086438A1 proposes the recovery of Chromium and iron as salt, not as metal, which may have very restricted uses.
[0005] Another approach mentioned in Patent bearing no. CN101380510 discusses the comprehensive utilization of chromium slag via leaching route by converting hexavalent chromium to trivalent chromium. The process further after detoxifying chromium slag also prepares nine other high-quality products namely, chromium, Magnesium and aluminum hydroxides, silicon dioxide, magnetic tri-iron tetroxide, calcium sulfate dihydrate, corundum intermediate, sodium chloride, tailings- chromite ore.
[0006] Similarly, patent publications CN108580021 and CN108246491 discuss recovering chromium concentrate from tailings of high carbon ferrochrome dry and wet slag respectively. Other Indian patent publications like IN20110068112, IN20140031612, IN201631008680A, and IN201731008862 talk about the beneficiation of chromite tailings mostly for the enrichment purpose.
[0007] However, none of the above-specified approaches and/or references discusses the complete utilization of chromite tailing to produce value-added product. These fines are wastes for the industry and create environmental concerns. Till now, there is no such comprehensive process or approach to utilize these fines from waste dump ore low-grade deposit.

OBJECTS OF THE DISCLOSURE
[0008] In view of the foregoing limitations inherent in the state of the art, some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0009] It is a general object of the present disclosure to develop an agglomeration process of chromite tailings.
[0010] It is an object of the present disclosure to produce ferrochrome from chromite tailings.
[0011] It is another object of the present disclosure to produce slag with high glassy phase content.
[0012] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

SUMMARY
[0013] This summary is provided to introduce concepts related to a submerged arc based method for smelt reduction of chromite tailing. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0014] In an embodiment, the present disclosure relates to a submerged arc based method for smelt reduction of chromite trailing. The method includes mixing fine chromite tailings of size ranging from 100 to 25 µm with coke of size less than 3 mm and lime of size less than 75 µm forming a mixture; adding bentonite powder to the mixture; briquetting the mixture in an extruder to form briquette; smelting the briquette in a submerged arc furnace; and generating a ferrochrome metal with slag obtained from the submerged arc furnace.
[0015] In an aspect, the ferrochrome metal obtained has at least 0.9% nickel.
[0016] In an aspect, the slag obtained in the submerged arch furnace is glassy in nature.
[0017] In an aspect, the mixture comprises chromite tailings, coke, and lime in the proportion of 68.56 - 71.8, 8.4 - 9.2, and 18.95 - 25.97 respectively in weight percentage.
[0018] In an aspect, the bentonite is 1% by weight of the mixture.
[0019] In an aspect, the chromite tailing contains Chromium/Iron ratio of 0.54 -0.64.
[0020] In an aspect, the chemical composition of fine chromite tailing, by weight, is Chromium 12-18%, Iron 20-30 %, Silicon dioxide 20-26 %, and Aluminum Oxide 8-14 % Nickel 0.2-0.8%.
[0021] In an aspect, moisture is 14-18% of the mixture with the bentonite.
[0022] In an aspect, the input basicity (B) of the mixture with the bentonite is (CaO+MgO/SiO2) = 0.8 to 1.2.
[0023] In an aspect, the input burden (R) of the mixture with the bentonite is (CaO+MgO/Al2O3) = 0.9-1.0.
[0024] In an aspect, the extruder is stiff vacuum extruder with die pressure of 2.5-4.5 Mpa.
[0025] In an aspect, the briquette has a cold compressive strength in the range of 350 to 450 kgf.
[0026] In an aspect, the power required wherein the power required for per ton of the chromite tailing is 2600-3500KWh/ton.
[0027] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
[0028] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
[0029] FIG. 1 illustrates a process flow diagram for the utilization of chromite tailing in accordance with an embodiment of the present disclosure;
[0030] FIG. 2 illustrates an optical microscope image of a slag particle, transparent particles, representing the glassy phase;
[0031] FIG. 3 illustrates Metal and Slag XRD showing amorphous phase in slag; and
[0032] FIG. 5 illustrates a method for smelt reduction of chromite ore in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0033] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0034] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0035] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, “consisting” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0036] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0037] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0038] Embodiments explained herein pertain to a method of complete utilization of chromite ore tailings with zero discharge. The process is based on smelting of chromite tailing to produce ferrochrome metal and slag which has high glassy phase content. The high glassy phase slag can be used as the replacement in slag cement. These tailings have been dumped for years, which has not only caused space concern for the plant but also poses environmental concerns directly or indirectly. The major concern of the present disclosure is value generation from the Chromite ore tailings.
[0039] The process, which is the object of the present disclosure, allows preparing ferrochrome and high glassy slag from chromite tailing fines in a feasible, efficient, and economical way which can be better understood from FIG. 1.
[0040] As per FIG. 1, the inventive smelting and agglomeration process of utilizing the chromite tailing fines consists the steps of sizing, mixing, briquetting, and smelting. The chromite tailings (1), nut coke (2), and lime (3) are reduced to respective size by initial grinding and mixed (5) with bentonite (4) powder and briquetted (6). The briquettes after drying in the oven at 150 °C are smelted (7) in an electric arc furnace. Resulting metal (8) and slag (9) are collected. The air-cooled slag is further ground (10) to below 9075?m and glassy phase was analyzed.
[0041] In FIG. 1, a method is depicted to produce ferrochrome and clinker material from low-grade chromite ore tailings or fines. The process handles the coarser particles of maximum particle size having 3 mm. Further, 80% (by weight) of the fines have a particle diameter in the range of 100 to 25 ?m. The low-grade chromite ore tailing fines contain chromite, alumina, quartz, iron oxide as the primary minerals along with a minimum amount of goethite and others. The chromite ore tailing (1) fines contains iron in the range of 20-30% with Silicon Dioxide (SiO2) (20-26%), Chromium Oxide (Cr2O3) (12-18%), Aluminium Oxide (Al2O3) (8-14%), Magnesium Oxide (MgO) (1-5%), Nickel 0.2-0.8% and loss on ignition (LOI) of 8 to 10%.
[0042] Carbon for the reduction purpose can be added either from the coal or coke or other carbonaceous material. The coke (2) used for the reduction process contains fixed carbon in the range of 82-88% with volatile matter (0.5-1%) and ash (10-14%). The particle size of the coke can be varied from anywhere to less than 3mm. To form the required slag flux was added. Flux can be either lime, dolomite, hydrated lime or calcined lime. The hydrated lime used for the slag formation and to maintain the proper basicity (0.8-1.2) had Calcium Oxide (CaO) in the range of 55-68% with MgO (1-8%), SiO2 (1-8%). The low-grade chromite tailing fines (1) of the above mentioned composition is mixed (5) with the coke (2) and limestone (3) and formed into briquettes (6) using the binder (4). In an aspect, the binder (4) can be either bentonite, dextrin, cement, some cementitious material or some slags. The composition of the raw material is shown in Table 1:
Constituents Cr-Tailings Coke Lime
Cr2O3 12-18
Fe(T) 20-30
SiO2 20-26 6-10 1-8
MgO 1-5% <1 0-1.5
CaO 0-2 <1.5 55-68
Al2O3 8-14 <1
MnO 0-1
FC 80-87
Table 1: Chemical composition of raw material (wt%)

[0043] The mixed material was briquetted, with moisture varying from 12-20%. The briquetted material was then oven dried and smelted (7) in the electric arc furnace. The smelting process resulted in the formation of metal (8) whose chemical composition can vary in the range of Fe:65-82 wt%, Cr: 12-20 wt%, Si: 0.2-1.5 wt%, Al:0.2-0.9 wt%, Ni: 0.9-2 wt% the slag (9) generated from smelting were air cooled which contains CaO: 33.25-35.86 wt%, MgO: 1-4 wt%, SiO2: 30-38.4 wt%, Al2O3: 13.5-19 wt%, MnO: 0.2-0.83 wt%, FeO: 2-5.2 wt%. the obtained slag’s composition was almost like the Blast Furnace Slag. The slag (10) was further ground to below 90µm, and glassy phase was observed. The observed glassy phase was sufficiently high (88-94%).
[0044] FIG. 2 is the optical image of the slag showing the glassy phase. The presence of the glassy phase is further supported by the amorphous peaks in the XRD image FIG. 3. It can be clearly seen from the XRD image that metal doesn’t show any peak below 40 degrees whereas there is the presence of a diffused peak in the slag phase.
Example: 1
[0045] In the first example, 7 kg of chromite tailings, 1848 gm of lime, 900 gm of coke and 97.50 gm of bentonite are blended to make the charge mix. Sufficient water was added to make the moisture 14% followed by briquetting. The formed briquettes were oven dried at 150 °C for 4 hours post which they were charged in the submerged arc furnace. During smelting experiments the operation time consists of (a) heating the crucible for 30 min (b) charging the raw material for 20-25 min, and (c) holding or soaking for 15-25 min. The heating of the crucible was done by arcing between electrode and coke at the bottom. Charging of raw materials into the crucible was started after it attains the desired temperature. The raw materials were added slowly to the crucible in small quantities and the charging operation completed in 20-25 minutes. During the experiment the operation temperature was maintained at 1600±500C and monitored by an infrared pyrometer. A holding time or soaking time of 15-25 min was allowed to the molten mass. Molten slag and metal are allowed to cool inside the furnace after completion of the reduction experiment. The metallic and non-metallic parts are physically separable and can be recovered by breaking the cakes. The metallic part is of ferrochrome alloy type and its composition Si- 0.27 wt%, Fe-79.23 wt%, Cr2O3-12.94 wt%, Al-0.55 wt%, Ni-1.2 wt%, C-5.6 wt%. The Slag composition consists of: CaO-33.86, MgO-5.66 wt%, Al2O3-15.2 wt%, SiO2-32.77 wt%, MnO-0.38 wt%, FeO- 4.9 wt%.

Example: 2
In the second example, 7 kg of chromite tailings, 2310 gm of lime, 900 gm of coke and 102 gm of bentonite are blended to make the charge mix. Sufficient water was added to make the moisture 16% followed by briquetting. The formed briquettes were oven dried at 150oC for 4 hrs post which they were charged in the submerged arc furnace. The charging of the materials and smelting experiment in an arc furnace and slag metal separation were conducted in a similar manner as explained in Example 1. The ferrochrome alloy obtained after experiment consists of Si-19 wt%, Fe-69.86 wt%, Cr2O3-19.96 wt%, Al-0.68 wt%, Ni-1.1 wt%, C-6.2 wt%. The Slag chemical composition consists of: CaO-25.32 wt%, MgO-2.87 wt%, Al2O3-12.55 wt%, SiO2-31.14 wt%, MnO-0.83 wt%, FeO- 11.62 wt%.

Table 2: Experimental results for the mixing process.
Test no. Material Size Wt%

1. Chromite Tailings -3mm 71.8
Coke -3mm 9.2
Lime -75?m 18.95
Bentonite -25?m 1 percent of the mix
2. Chromite Tailings -3mm 68.56
Coke -3mm 8.81
Lime -75?m 22.62
Bentonite -25?m 1
3 Chromite Tailings -3mm 65.59
Coke -3mm 8.4
Lime -75?m 25.97
Bentonite -25?m 1

[0046] FIG. 4 illustrates example submerged arc based method 400 for smelt reduction of chromite ore in accordance with an embodiment of the present disclosure. The order in which the method 400 is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method 400, or an alternative method.
[0047] At block 402, the method 400 includes mixing fine chromite tailings of size ranging from 100 to 25 µm with coke of size less than 3 mm and lime of size less than 75 µm forming a mixture.
[0048] At block 404, the method 400 includes adding bentonite powder to the mixture.
[0049] At block 406, the method 400 includes briquetting the mixture in an extruder to form a briquette.
[0050] At block 408, the method 400 includes smelting the briquette in a submerged arc furnace.
[0051] At block 410, the method 400 includes generating a ferrochrome metal with slag obtained from the submerged arc furnace.
[0052] In an aspect, the ferrochrome metal obtained has at least 0.9% nickel.
[0053] In an aspect, the slag obtained in the submerged arch furnace is glassy in nature.
[0054] In an aspect, the mixture comprises chromite tailings, coke, and lime in the proportion of 68.56 - 71.8, 8.4 - 9.2, and 18.95 - 25.97 respectively in weight percentage.
[0055] In an aspect, the bentonite is 1% by weight of the mixture.
[0056] In an aspect, the chromite tailing contains Chromium/Iron ratio of 0.54 -0.64.
[0057] In an aspect, the chemical composition of fine chromite tailing, by weight, is Chromium 12-18%, Iron 20-30 %, Silicon dioxide 20-26 %, and Aluminum Oxide 8-14 %, Ni-0.2-0.8 %.
[0058] In an aspect, moisture is 14-18% of the mixture with the bentonite.
[0059] In an aspect, the input basicity (B) of the mixture with the bentonite is (CaO+MgO/SiO2) = 0.8 to 1.2.
[0060] In an aspect, the input burden (R) of the mixture with the bentonite is (CaO+MgO/Al2O3) = 0.9-1.0.
[0061] In an aspect, the extruder is stiff vacuum extruder with die pressure of 2.5-4.5 Mpa.
[0062] In an aspect, the briquette has a cold compressive strength in the range of 350 to 450 kgf.
[0063] In an aspect, the power required wherein the power required for per ton of the chromite tailing is 2600-3500KWh/ton.
[0064] Furthermore, those skilled in the art can appreciate that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0065] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0066] While the foregoing describes various embodiments of the present disclosure, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof. The scope of the present disclosure is determined by the claims that follow. The present disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

WE CLAIM:

1. A submerged arc based method for smelt reduction of chromite ore, the method comprising:
mixing fine chromite tailings of size ranging from 100 to 25 µm with coke of size less than 3 mm and lime of size less than 75 µm forming a mixture;
adding bentonite powder to the mixture;
briquetting the mixture in an extruder to form briquette;
smelting the briquette in a submerged arc furnace; and
generating a ferrochrome metal with slag obtained from the submerged arc furnace.
2. The method as claimed in claim 1, wherein ferrochrome metal obtained has at least 0.9% nickel.
3. The method as claimed in claim 1, wherein the slag obtained in the submerged arch furnace is glassy in nature.
4. The method as claimed in claim 1, wherein the mixture comprises chromite tailings, coke, and lime in the proportion of 68.56 - 71.8, 8.4 - 9.2, and 18.95 - 25.97 respectively in weight percentage.
5. The method as claimed in claim 1, wherein the bentonite is 1% by weight of the mixture.
6. The method as claimed in claim 1, wherein the chromite tailing contains Chromium/Iron ratio of 0.54 -0.64.
7. The method as claimed in claim 1, wherein the chemical composition of fine chromite tailing, by weight, is: Chromium 12-18%, Iron 20-30 %, Silicon dioxide 20-26 %, and Aluminum Oxide 8-14 %, Nickel 0.2-0.8 %.
8. The method as claimed in claim 1, wherein moisture is 14-18% of the mixture with the bentonite.
9. The method as claimed in claim 1, wherein the input basicity (B) of the mixture with the bentonite is (CaO+MgO/SiO2) = 0.8 to 1.2.
10. The method as claimed in claim 1, wherein the input burden (R) of the mixture with the bentonite is (CaO+MgO/Al2O3) = 0.9-1.0.
11. The method as claimed in claim 1, wherein the extruder is stiff vacuum extruder with die pressure of 2.5-4.5 Mpa.
12. The method as claimed in claim 1, wherein the briquette has a cold compressive strength in the range of 350 to 450 kgf.
13. The method as claimed in claim 1, wherein the power required wherein the power required for per ton of the chromite tailing is 2600-3500KWh/