USING TWO STAGE REACTORS

CROSS-REFERENCES TO RELATED APPLICATION [0001] This application claims priority to Indian patent application no. 202041017475 entitled SYSTEM AND METHOD FOR PRODUCING MAGNETITE FROM RED MUD USING TWO STAGE REACTORS fried on 23 April, 2020.

FIELD OF THE INVENTION

[0002] The disclosure generally relates to mineral extraction and, in particular, to a method of producing magnetite from red mud.

DESCRIPTION OF THE RELATED ART

[0003] Solid waste produced during the alumina extraction process from bauxite ore is known as red mud. Depending on the quality and composition of bauxite, about 55- 65% of the ore is usually generated as red mud during metallurgical processing. In many parts of the world, red mud is disposed of either on land or in water bodies like sea or ocean. Such practices are a major concern because red mud is highly alkaline and causes a harmful effect on air, water, and the land of the surrounding area. Based on several reports millions of tons of red mud is generated per annum in the world. Considering the expected growth of the aluminum industries and the associated disposal problems, the environmental health is at stake and therefore remedial actions are required.

[0004] In addition to the alkaline properties, red mud also possesses some valuable elements, such as iron. Therefore, red mud could be processed to produce magnetite instead of large scale dumping in the environment. The increase in iron ore price in the current markets could be reduced by recovering valuable elements from red mud. Such a process would also have significant environmental and economic importance.

[0005] Existing solutions as disclosed in the publication “Reduction Behavior of Hematite to Magnetite under Fluidized Bed Conditions” (Feilmayr et al.) propose fluidized bed as an option for solid-state reduction of hematite to magnetite using gaseous or carbon-based solid reductant followed by magnetic separation. However, the conversion of magnetite from hematite in a single-stage fluidized bed reactor using coal as a reductant is not efficient.

[0006] Further, some existing patent applications US20090175782A1 and US20150203362A1 disclose producing magnetite from red mud in a fluidized bed reactor and a rotary kiln furnace using carbon and coke as reductant, respectively, using single-stage processes. Other patent publications have attempted to address some of the challenges. IN201711032401 discloses a process for the recovery of Fe304 (magnetite), FeO (wustite) & metallic iron from bauxite residue (red mud) by segregation roasting and magnetic separation. US20140356263A1 discloses a method for producing high- purity synthetic magnetite by oxidation from metal waste and appliance for producing the same. US20170320751A1 relates to particle separation in a method for recovering magnetite from bauxite residue. There is a requirement of processes for processing red mud with high energy efficiency, and yield.

SUMMARY OF THE INVENTION

[0007] Systems, devices and methods for continuous extraction of magnetite from red mud are disclosed. A system for continuous extraction of magnetite from red mud, may comprise a plurality of two-stage reactors attached in a serial connection, each two- stage reactor comprising a lower section having an inlet to receive input gas and an upper section having an electric furnace with thermal insulation forming a first stage and a second stage. Each two-stage reactor is configured to operate in a charging mode, a preheating mode, a first reaction mode, a second reaction mode or a discharging mode. [0008] The charging mode charges the first stage with coal particles and the second stage with red mud particles, wherein the first stage is heated to a first temperature and the second stage to a second predetermined temperature. In the pre-heating mode a supply of hot gas is received at the first stage, from the lower section. In the first reaction mode in the first stage, a mixture of reducing gases is produced by gasification of coal. In the second reaction mode magnetite is produced in the second stage, by fluidization of the red mud particles and reduction of hematite using the mixture of reducing gases. In the discharging mode the magnetite is discharged into a slurry tank having water therein to form a slurry thereof. Each two-stage reactor may be configured to operate in charging mode, pre-heating mode, first and second reaction modes, and discharging mode sequentially with a predetermined time lag At. The system may further comprise a wet magnetic separator configured to separate magnetite from the non-magnetic particles in the slurry.

[0009] In some embodiments, the system may comprise 4 or more two-stage reactors, wherein the reactors are configured to operate in charging, preheating, reaction and discharging modes with a time lag At. Each two-stage reactor operating in the reaction modes produces hot gas, that is supplied to another two-stage reactor operating in pre-heating mode, and each two-stage reactor operating in the discharging mode receives cold gas that is heated and supplied as hot input gas to another two-stage reactor operating in pre-heating mode. [0010] In some embodiments of the system the coal is low grade coal of particle size 4-8 mm and the red mud is of particle size between 50-90 pm. In some embodiments the system may comprise a control unit configured to control gas flow and heating of the electric furnace in each of the reactors to ensure that the second predetermined temperature is in the range 700-800 °C and to divert hot or cold gas to the reactors in sequence with the predetermined time lag At.

[0011] In various embodiments, a method of producing magnetite from red mud using a plurality of two-stage reactors attached in serial connection is disclosed. The method may comprise charging coal particles in a first stage and red mud fines comprising hematite in a second stage of each two stage reactor sequentially with a predetermined time lag At, heating the first stage to a first temperature and the second stage to a second predetermined temperature in each two-stage reactor with the predetermined time lag At, receiving input gas for gasification of the coal particles; producing a mixture of reducing gases in the first stage in each two-stage reactor with the predetermined time lag At, fluidizing the red mud particles and reducing the hematite using the mixture of reducing gases in the second stage to produce magnetite in each of the two-stage reactors with the predetermined time lag At; and discharging the magnetite from each two-stage reactor into a slurry tank containing water with the predetermined time-lag At to obtain a magnetite slurry. The magnetite slurry is processed using magnetic separation to obtain magnetite of concentration 95% or better.

[0012] The method may in some embodiments comprise supplying hot air during the magnetite production in a first two-stage reactor to a second two-stage reactor as input gas. The method may in some embodiments comprise supplying cold gases to a two- stage reactor before discharge of the magnetite, wherein the cold gases are heated and circulated to another two-stage reactor as input gas. In some embodiments the second predetermined temperature is in the range 700-800°C. In some embodiments the coal is low grade coal of particle size 4-8 mm and the red mud is of particle size between 50-90 pm. [0013] In various embodiments, a two-stage reactor apparatus for producing magnetite from red mud is disclosed. The apparatus comprising a lower section having an inlet to receive input gas and an upper section having an electric furnace with thermal insulation. The electric furnace comprising a first stage loaded with coal particles and a second stage loaded with red mud particles. The apparatus is configured to heat the first stage to a first temperature to gasify the coal and the second stage to a second predetermined temperature. Hot gas is received, at the first stage, from the lower section to produce a mixture of reducing gases obtained from gasification of the coal. At the second stage magnetite is produced by fluidization of the red mud particles and reduction of hematite using the mixture of reducing gases.

[0014] In some embodiments, the apparatus comprises a first perforated gas distributor plate separating the lower section and the upper section, and a second perforated gas distributor plate separating the first stage and the second stage. The apparatus also comprises a thermocouple configured to measure temperature of the red mud particles in the second stage and a rotameter at the inlet to measure gas flow rate. Further, the apparatus comprises a control unit configured to control gas flow and heating of the electric furnace to ensure that the second predetermined temperature is in the range 700-800 °C.

[0015] This and other aspects are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention has other advantages and features, which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0018] FIG. 1 illustrates a system for producing magnetite from red mud in a continuous process, according to one embodiment of the present subject matter.

[0019] FIG. 2 illustrates a two-stage reactor for producing magnetite from red mud, according to one embodiment of the present subject matter.

[0020] FIG. 3 illustrates flow diagram for a method of producing magnetite from red mud using a plurality of two-stage reactors attached in serial connection, according to one embodiment of the present subject matter.

[0021] FIG. 4 A and FIG. 4B illustrate XRD spectrum and phase fraction of red mud sample before reduction, according to one example of the present subject matter.

[0022] FIG. 5A illustrates XRD spectrum after magnetite production process, according to one example of the present subject matter.

[0023] FIG. 5B illustrates phase fraction before and after reduction and magnetic separation, according to one example of the present subject matter.

DETAILED DESCRIPTION

[0024] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0025] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0026] The present subject matter describes methods, systems, and apparatuses to produce magnetite from red mud using two-stage reactors.

[0027] A system for continuous extraction of magnetite from red mud is illustrated in FIG. 1, according to one embodiment of the present subject matter. The system 100 may include a plurality of two-stage reactors 102-N attached in a serial connection. In some embodiments, four different two-stage tubular reactors 102-1, 102-2, 102-3, and 102-4 may be used. The two-stage reactors may be loaded with red mud samples and low-grade coal and processed in different chambers to produce magnetite. In the serial configuration of the system illustrated in the figure the reactors may be connected for several advantages.

[0028] A detailed view of the two-stage reactor 102 is also illustrated in FIG. 2, according to one embodiment of the present subject matter. The two-stage reactor 102 may include a lower section 104 having an inlet 108 to receive input gas and an upper section 106 having an electric furnace 110 with thermal insulation. In various embodiments, the reactor may be tubular in shape. Alternatively, the reactor may be in a non-cylindrical shape, such as spherical, cuboidal, etc. In various embodiments, the input gas may be air, a mixture of air+N ₂ or a mixture of air + producer gas, or combustion product gas carrying waste heat from a separate unit.

[0029] The lower section 104 may be configured to operate as a wind-box. The electric furnace 110 may include a first stage 112 and a second stage 114, each having independent temperature control. The furnace may be a hinge type openable electric furnace. The thermal insulation may be provided by a thick line of ceramic refractory inside the furnace. In various embodiments, each two-stage reactor 102 may be configured to operate in a plurality of modes, such as a charging mode, pre-heating mode, first and second reaction modes, and discharging mode sequentially with a predetermined time lag At.

[0030] The two-stage reactor 102 in charging mode may be configured to charge the first stage 112 with coal particles and the second stage 114 with red mud particles. The coal in some embodiments may be low grade coal. In various embodiments the coal as well as red mud may require preheating to a certain temperature for reaction to start. The first stage 112 may be heated to a first temperature and the second stage 114 may be heated to a second predetermined temperature. In some embodiments, the second predetermined temperature may be in the range 700-800°C. Similarly, the red mud sample and coal may be charged to another reactor, say, in reactor 102-2 with the time delay. In other reactors R ₃ 102-3 to R ₄ 102-4 charging is done with a time delay.

[0031] The two-stage reactor 102 may also operate in a pre-heating mode to receive a supply of hot gas at the first stage 112 from the lower section 104. The two-stage reactor may operate in first reaction mode to produce mixture of reducing gases from gasification of coal in the first stage. Next, the two-stage reactor 102 may operate in a second reaction mode to produce magnetite in the second stage 114 by fluidization of the red mud particles and reduction of hematite using the mixture of reducing gases. Further, the reactor 102 may operate in a discharging mode to discharge the reduced product comprising the magnetite into a slurry tank 120. The slurry tank 120 may include water to mix with the magnetite to form slurry suitable for further wet magnetic separation.

[0032] A sequence of the operating modes for each reactor at time interval At is depicted below.

Table 1: Sequence of Operation of Different Reactors at Different Times

[0033] As depicted in the table 1, when raw materials are charged in R ₂ 102-2, the reactor Ri 102-1 may be on preheating mode. When reactor R3 102-3 is charged, reactor Ri 102-1, R ₂ 102-2 may be in reaction mode and preheating mode respectively. The term “reaction mode” or “reaction” may include both the first reaction mode and the second reaction mode.When reactor R4 102-4 is in charging mode, reactor Ri 102-1, R ₂ 102-2 and R3 102-3 may be in discharging, reaction and preheating modes, respectively.

[0034] In some embodiments, each two-stage reactor 102 after operating in the second reaction mode may produce hot gas, which may be supplied as input gas to another two-stage reactor 102 operating in pre-heating mode. For instance, initially, when Ri is charged with red mud and coal particles, an external supply of hot gas may be provided to the lower section of Ri and R ₂ . Hot gas produced after the reaction mode in Ri may be transferred to another reactor operating in pre-heating mode, for example R3. Similarly, hot gas produced in the reaction mode in reactor R ₂ may be supplied to another reactor, such R ₄ , operating in pre-heating mode. In other embodiments, each two-stage reactor 102 may operate in the discharging mode receives cold gas, which may be heated and supplied as hot input gas to another two-stage reactor 102 operating in pre heating mode.

[0035] In some embodiments, the two-stage reactor apparatus may further include a first perforated gas distributor plate 116 separating the lower section 104 and the upper section 106. The reactor 102 may also include a second perforated gas distributor plate 118 separating the first stage 112 and the second stage 114. In some embodiments, the reactor may further include a thermocouple 122 configured to measure temperature of the red mud particles in the second stage 114. In some embodiments, the reactor 102 may further include an acrylic rotameter configured to measure gas flow rate.

[0036] In various embodiments, the reactors 102-N may include a control unit coupled to one or more sensors, such as rotameter, thermocouple, and the like. The second stage 114 of the reactor may be affixed with one or more thermocouples to monitor temperatures within the fluidized bed. The control unit may be configured to receive sensor data from the one or more sensors and control the operation of the reactor 102-N. For example, the control unit may receive the flow rate data from the rotameter and control the input gas flow. Similarly, the control unit may control heating of the electric furnace to ensure that the second predetermined temperature within the second stage 114 is in the range 700-800 °C based on the temperature data. In some embodiments, the control unit may be configured to control the transfer of hot gas from one reactor to another based on one or more sensor data. For example, the control unit may receive sensor data associated with hot gas produced in reaction mode and accordingly transfer to a reactor in pre-heating mode. In some embodiments, change in operating mode and timing of each operating mode may be configurable in the controller. [0037] Alternatively, the control unit may receive sensor data associated with a cold input gas supplied before discharging mode and monitor the gas temperature. Once the cold gas is heated sufficiently up to threshold, the control unit may transfer the hot gas to a reactor in pre-heating mode. The transfer of hot gas between the reactors may be performed in sequence with the predetermined time lag At. In some embodiments, the control unit may control the flow rate or volume of the hot gas to be transferred from one reactor to another. In various embodiments, the system 100 may be configured to be operable with the assistance of electric heating of furnace 110 or by supply of preheated gas, or a combination of the two, as may be considered expedient. In some embodiments, the system may be operable entirely using preheated gas.

[0038] Referring back to FIG. 1, the system 100 may further include a magnetic separator 124 configured to separate magnetite from the non-magnetic particles from the slurry collected in the slurry tank 120. A pump 126 may be configured to transfer the slurry from the slurry tank 120 to the magnetic separator 124. The magnetic separator 124 may be configured to separate magnetite from the non-magnetic particles from the slurry. The magnetic particles were stored in the magnetic tank 128 and the non magnetic particles were stored in the non-magnetic tank 130.

[0039] A flow diagram of the method 300 of producing magnetite from red mud using a plurality of two-stage reactors 102-N attached in serial connection is illustrated in FIG. 3, according to an embodiment of the present subject matter. The method 300 may include charging low grade coal particles in a first stage 112 of the reactor at block 302. The particle size of low-grade coal may be 4-8 mm. Red mud samples may be charged in a second stage 114 of each two stage reactor 102-N sequentially with a predetermined time lag at block 304. The particle size range of red mud fines may be 50-90 pm. In some embodiments, the weight of red mud sample and coal samples may be 40 g and 100 g, respectively. [0040] Next, the method may include heating the first stage 112 to a first temperature and the second stage 114 to a second predetermined temperature in each two-stage reactor 102-N with the predetermined time lag at block 306. The method also involves receiving the hot gas for gasification of the coal particles at block 308. The hot gas may be provided by an external supply or from another reactor. In some embodiments, the hot gas may be received at a flow rate of 20 LPM through the bottom of the reactor. The gas triggers incomplete combustion of coarse coal particles in first stage 112. A mixture of reducing gases is produced in the first stage in each two-stage reactor with the predetermined time lag at block 310. Specifically, once the desired temperature is reached in first stage 112, de-volatilization, combustion, and gasification of coal happens in the presence of hot gas, which produces reducing gases in presence of excess carbon as given in equation (1) and (2) and forms a mixture of reducing gases of CO and CO2.

C O _' - ( O _' . (1)

C CO. - 2CO . (2)

[0041] Further, the method includes producing magnetite by fluidizing the red mud particles and reducing hematite using the mixture of reducing gases at the first stage in each of the two-stage reactor with the predetermined time lag at block 312. Consequently, reduction of the red mud fines takes place in second stage 114. In the second stage, the hot gaseous mixture fluidizes the red mud fines as well as reduces the hematite phase present in red mud fines to the magnetite phase following the reaction

(3).

3Fe ₂ 0 ₃ + CO - 2Fe ₃ 0 ₄ + C0 ₂ . (3)

[0042] In one embodiment, the reduction to magnetite may be at a temperature of 650-800° C for a time interval of 1-15 minutes. Complete reduction of hematite to magnetite may be obtained at 700° C for 5-15 min and at 800° C for 1-15 minutes. At 800 °C, the red mud may be completely reduced for a fluidization time of 5 minutes and at a flow rate of 20 liters/minute. The mass ratio of coal to red mud may be maintained at 2.5:1.

[0043] In a further step, in various embodiments the method may involve supplying cold gas through the first and second stages of the reactor to cool the obtained product in the second stage at block 314. The cooling may in some embodiments be performed using a mixture of CO and N2 gases. The cold gases absorb heat inside the reactor and this flow may be reused as pre-heating input gas in another reactor 102-N. Further, the method includes discharging the reaction product from the second stage composed substantially of magnetite from each two-stage reactor to aqueous tank 120 with the predetermined time-lag at block 316. The magnetite slurry from aqueous tank 120 may be further processed using wet magnetic separation to purify the magnetite and remove non-magnetic particles.

[0044] In various embodiments, the method 300 may also include supplying hot gas produced during the magnetite production in a first two-stage reactor to a second two- stage reactor as input gas. In some embodiments, the method may also include supplying cold gases to a two-stage reactor during discharging of the magnetite. The cold gases may be heated and circulated to another two-stage reactor as hot input gas for pre heating. The capture and recirculation of heat from the reactor may be configured to improve energy economy of the entire process.

[0045] In various embodiments, the systems, devices and methods may produce magnetite from red mud that is more than 95% magnetite (Fe304). In some embodiments the invention may produce an end product that is more than 96% magnetite. The systems, devices and methods of the invention provide higher magnetite concentrations than other thermal roasting and combustion methods in the literature. The maximum magnetite concentration from red mud reported ranges from 30-90% depending on the source red mud and its composition. Some methods such as hydrothermal process also require large quantities of water and time. The corresponding magnetite purity levels obtained using other methods are provided in Table 2.

Table 2: Magnetite Purity Obtained In Other Methods

EXAMPLES

[0046] Example 1: Chemical composition and phase analysis before processing

[0047] The chemical composition and phase analysis of red mud sample are described in Table 3 and FIG. 4A and FIG. 4B, respectively. Compressed air was supplied through the rotameter from the bottom of the reactor 102. In the first stage 112, incomplete combustion of coarse coal particles takes place in a packed bed, followed by a reduction of red mud fines in a fluidized bed at the second stage 114 of the reactor 102.

Table 3: Chemical Analysis of Red Mud

[0048] The XRD spectrum of the red mud sample is shown in FIG 4A. The phase fraction of the red mud sample was observed and around 70 wt. % of the sample was hematite along with silica (zeolite and cristobalite) as a minor phase (after Rietveld refinement) as shown in FIG. 4B.

[0049] Example 2: Analysis after processing

[0050] The XRD analysis as illustrated in FIG. 5A shows almost complete conversion of hematite to magnetite. The mass fraction of magnetite was found to be around 90 wt. % after reduction as shown in FIG. 5B. The particle size of silica and rutile phases were smaller compared to that of the hematite phase in the red mud. A significant fraction of silica and rutile phases in the red mud was elutriated with the gas during fluidization leading to an enrichment of magnetite concentrate. The mass fraction of silica was found to be reduced from 9.7 wt. % to 4.1 wt. % while that of magnetite increased to about 96 wt. % after magnetic separation FIG. 5B.

[0051] The disclosed two-stage reactor system includes redirection of output gases to neighboring reactors for pre-heating without requiring additional external source of heat, and is therefore fast and energy efficient. The magnetite concentrate produced from the above methods can act as a raw material for the production of pig iron in the iron and steel industry and can be used to make dense media suspension for separation of coal from gangue in the mining industry. The magnetite can also be used as a synthetic abrasive by mixing with corundum (aluminium oxide), as a pigment in paints, and as an aggregate in high-density concrete. With increased purity, the magnetite can be used as a catalyst in the chemical industry, as nanoparticle for targeted drug delivery and magnetic resonance tomography in the biomedical industry and waste-water treatment for removal of heavy metals in the environmental sector.

[0052] Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed herein. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the system and method of the present invention disclosed herein without departing from the spirit and scope of the invention as described here.

[0053] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope.