System and method for collecting and pre-treating process gases produced by electrolytic cells

Abstract

An apparatus and method for collecting and pretreating process gases generated in an electrolytic cell during aluminum production are disclosed. The apparatus includes a collection unit configured to draw primary process gas from the electrolytic cell, for example by drawing the primary process gas from a specially made orifice above the electrolytic bath; and a pretreatment unit fluidly connected to the collection unit and configured to receive a fluorinated alumina fluidized bed for pretreating the primary process gas. In the electrolytic plant, the collection and pretreatment unit is located in the electrolytic cell or in close proximity to the electrolytic cell. The apparatus can be used in conjunction with a gas treatment center (GTC) located outside the electrolytic plant. Among other advantages, the technology allows for the collection of primary process gas directly at the level of the electrolytic bath, separation of primary process gas and hood space process gas to pretreat the primary process gas with alumina before the GTC, and the use of a fluidized bed reactor without filter bags.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims the benefit of priority to U.S. Provisional Patent Application No. 62 / 820,917, entitled “SYSTEM AND METHOD FOR COLLECTING AND PRE-TREATING PROCESS GASES GENERATED BY AN ELECTROLYSIS CELL,” filed with the U.S. Patent and Trademark Office on March 20, 2019, the contents of which are incorporated herein by reference. Technical Field

[0003] The present invention generally relates to systems, apparatus and methods for treating process gases, and more particularly to systems, apparatus and methods for pre-treating primary process gases produced by electrolytic cells, such as during the production of metals such as, but not limited to, aluminum (also referred to as aluminum). Background Art

[0004] Aluminium metal is produced by electrolysis of aluminium oxide, also known as alumina (IUPAC), in a molten electrolyte contained in a number of electrolytic cells. The effluent of the electrolytic cells consists of a number of gaseous and particulate components, known as process gases, such as gaseous fluorides (Fg or HF) and particulate fluorides (Fp).

[0005] The process gases are treated in post-treatment systems at production plants to recover valuable components from them before release into the atmosphere. The capacity of the post-treatment system in any given plant is set by the number of electrolytic cells based on the Hall–Héroult aluminium smelting process.

[0006] As new or improved smelting processes are developed, there is a need to increase the capacity of the reprocessing system. However, a complete overhaul of the reprocessing system of an existing plant to accommodate the new or improved process would be costly and could hinder its adoption.

[0007] The present invention addresses the need for increased capacity of aftertreatment systems without requiring a complete overhaul. Summary of the invention

[0008] This Summary is provided to introduce a selection of concepts in a simplified form that 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 as an aid in determining the scope of the claimed subject matter.

[0009] According to a first aspect, the present invention relates to an apparatus for collecting and pre-treating a primary process gas generated in an electrolytic cell during the production of aluminium. The apparatus comprises: a collecting unit configured to draw the primary process gas from an electrolytic bath of the electrolytic cell; a pre-treatment unit fluidly connected to the collecting unit and configured to receive fluorinated aluminium oxide for pre-treating the primary process gas, wherein the collecting and pre-treatment unit is within the electrolytic cell or in close proximity to the electrolytic cell.

[0010] According to a preferred embodiment, the collecting unit is configured to lead the primary process gas from a purposely made orifice above the electrolytic bath of the electrolytic cell.

[0011] According to a preferred embodiment, the pretreatment unit is configured to receive and contain a fluidized bed of fluorided alumina. Preferably, the pretreatment unit is further configured to drive the primary process gas through the fluidized bed of fluorided alumina. Preferably, the apparatus further comprises a discharge unit connected to the pretreatment unit and configured to discharge the pretreated primary process gas from an upper portion of the fluidized bed and drive the pretreated primary process gas out of the pretreatment unit.

[0012] According to a preferred embodiment, the apparatus further comprises an alumina flow control system located upstream of the pretreatment unit for controlling the alumina flow, preferably by changing the operation time of the discontinuously operated pneumatic conveying system.

[0013] According to a preferred embodiment, the apparatus further comprises a buffer unit which is fluidically connected to the pre-treatment unit and is configured to buffer the fluorinated aluminum oxide before the fluorinated aluminum oxide is injected into the pre-treatment unit.

[0014] According to a preferred embodiment, the pretreatment unit is fluidically connected upstream of the electrolytic cell and is further configured to inject at least a portion of the fluorided aluminum oxide contained in the pretreatment unit into the electrolytic cell as a raw material for producing aluminum.

[0015] According to a preferred embodiment, the pretreatment unit is located within and fluidly connected to the electrolytic cell and is further configured to inject at least a portion of the fluorided aluminum oxide contained in the pretreatment unit into the electrolytic cell as a feedstock for producing aluminum.

[0016] According to a second aspect, the invention relates to a method for collecting and pre-treating primary process gas produced in an electrolytic cell during the production of aluminium, the method comprising:

[0017] a) collecting a primary process gas from an electrolytic bath of an electrolytic cell; and

[0018] b) pre-treating the primary process gas obtained in step a) with fluorinated alumina, the pre-treatment step being carried out in or in the immediate vicinity of the electrolytic cell.

[0019] According to a preferred embodiment, the collecting step comprises withdrawing the primary process gas from a purposely made orifice above the electrolytic bath of the electrolytic cell.

[0020] According to a preferred embodiment, the pretreatment step b) comprises driving the primary process gas through a fluidized bed of fluorided alumina. Preferably, the method then further comprises the step of discharging the pretreated primary process gas from the upper part of the fluidized bed. More preferably, the method then further comprises driving the pretreated primary process gas out of the pretreatment unit.

[0021] According to a preferred embodiment, the method further comprises controlling the alumina flow rate, for example by using an alumina flow control system located upstream of the pretreatment unit. Preferably, controlling the alumina flow rate comprises changing the operation time of the discontinuously operated pneumatic conveying system.

[0022] According to a preferred embodiment, the method further comprises buffering the fluorided alumina before said fluorided alumina is used in a pre-treatment unit.

[0023] According to a preferred embodiment, the method further comprises injecting at least a portion of the fluorided alumina contained in the pretreatment unit into the electrolytic cell as a raw material for producing aluminum.

[0024] According to a third aspect, the present invention also relates to a method for treating a process gas produced by an electrolytic cell, the method comprising the following steps:

[0025] a) collecting primary process gas from the electrolytic bath of the electrolytic cell;

[0026] b) collecting hood space process gas from the hood space of the electrolytic cell;

[0027] c) pre-treating the primary process gas collected in step a) with partially fluorinated alumina, the pre-treatment step being carried out in or in close proximity to the electrolytic cell;

[0028] d) combining the primary process gas obtained from step c) with the gas obtained from step b);

[0029] e) contacting the combined process gas obtained from step d) with fresh alumina to form semi-washed process gas and fluorided alumina;

[0030] f) separating the semi-washed process gas from the fluorided alumina to obtain recycled fluorided alumina, and

[0031] g) using at least part of the recycled fluorided alumina obtained from step f) for pre-treating the primary process gas in step c).

[0032] According to a preferred embodiment, steps a) and c) are carried out in an electrolytic cell.

[0033] According to a preferred embodiment, step a) of the method comprises the step of leading the primary process gas from a specially made orifice above the electrolytic bath of the electrolytic cell.

[0034] According to a preferred embodiment, the pre-treatment step c) comprises the step of driving the primary process gas through a fluidized bed of fluorided alumina.

[0035] According to a preferred embodiment, the method further comprises controlling the alumina flow rate, for example by using an alumina flow control system located upstream of the pretreatment unit. Preferably, controlling the alumina flow rate comprises changing the operation time of the discontinuously operated pneumatic conveying system.

[0036] According to a preferred embodiment, the method further comprises the step of exhausting the pre-treated primary process gas in the upper portion of the fluidized bed prior to merging with the hood space process gas.

[0037] According to a preferred embodiment, the method further comprises the step of buffering at least a portion of the fluorided aluminium oxide obtained from step f) before being used in step c) for pre-treating the primary process gas.

[0038] According to a preferred embodiment, the method further comprises the step of injecting at least a portion of the fluorided alumina used in step c) into the electrolytic cell as a raw material for producing aluminum.

[0039] According to a preferred embodiment, the method further comprises the step of buffering at least part of the fluorided alumina produced in step f) before injection into the electrolytic cell as feedstock for producing aluminium.

[0040] According to a preferred embodiment, the method further comprises the step of contacting the process gas in step e) with a portion of the fluorided aluminium oxide obtained from step f). Preferably, a portion of the fluorided aluminium oxide is combined with fresh aluminium oxide.

[0041] According to a preferred embodiment, the method further comprises the step of discharging the scrubbed process gas and residual fluoride obtained from step f) into the environment.

[0042] According to a third aspect, the invention also relates to a module assembly for treating a gas produced by an electrolytic cell, the module assembly comprising:

[0043] a first collection module fluidly connected to the electrolytic bath of the electrolytic cell and configured to collect a primary process gas generated in the electrolytic cell;

[0044] a second collection module fluidly connected to a hood space of the electrolytic cell and configured to collect hood space process gas generated in the electrolytic cell;

[0045] a pretreatment module fluidly connected to the first collection module and configured to pretreat the primary process gas with partially fluorinated alumina, the pretreatment module being located within or in close proximity to the electrolytic cell;

[0046] a merging module fluidly connected to the pre-treatment module and the second collection module and configured to merge the hood space process gas with the primary process gas pre-treated in the pre-treatment module;

[0047] a scrubber module fluidly connected to the combining module and configured to contact the combined process gas with fresh alumina to form a semi-washed process gas and fluorided alumina; and

[0048] A filter module is fluidly connected to the scrubber module and configured to separate the semi-scrubbed process gas from the fluorinated alumina to obtain the fluorinated alumina, the filter module is further fluidly connected to the pre-treatment module and configured to at least partially reinject the fluorinated alumina into the pre-treatment module.

[0049] According to a preferred embodiment, the first collection module and the pretreatment module are located within the electrolytic cell, the pretreatment module is also fluidly connected to the electrolytic cell and is further configured to inject at least a portion of the fluorinated aluminum oxide contained in the pretreatment unit into the electrolytic cell as a raw material for producing aluminum.

[0050] According to a preferred embodiment, the first collecting module is configured to lead the primary process gas from a specially made orifice above the electrolytic bath of the electrolytic cell.

[0051] According to a preferred embodiment, the pre-treatment module is configured to house a fluidized bed of fluorided alumina and to drive the primary process gas through the fluidized bed of fluorided alumina.

[0052] According to a preferred embodiment, the module assembly further comprises a discharge module located downstream of the pretreatment module and upstream of the merging module and configured to discharge the pretreated primary process gas from the upper part of the fluidized bed and drive the pretreated primary process gas toward the merging module.

[0053] According to a preferred embodiment, the module assembly further comprises a first buffer module located downstream of the filtration module and upstream of the pre-treatment module and configured to buffer at least a portion of the fluorinated alumina before being injected into the pre-treatment module.

[0054] According to a preferred embodiment, the module assembly further comprises a second buffer module located downstream of the filtration module and upstream of the electrolytic cell and configured to buffer at least a portion of the fluorinated alumina before being injected into the electrolytic cell as a raw material for producing aluminum. Preferably, the first buffer module and the second buffer module may form a unique buffer module fluidly connected to the pretreatment module and the electrolytic cell.

[0055] According to a preferred embodiment, the pretreatment module is also fluidly connected to the electrolytic cell and is configured to inject at least a portion of the fluorided alumina contained in the pretreatment module into the electrolytic cell as a feedstock for producing aluminum.

[0056] According to a preferred embodiment, the scrubber module is also fluidly connected upstream of the filtration module and is configured to receive a portion of the fluorided alumina from the filtration module.

[0057] According to a preferred embodiment, the scrubber module is further configured to combine a portion of the fluorided alumina with fresh alumina.

[0058] According to a preferred embodiment, the module assembly further comprises an exhaust module fluidly connected to the filter module and configured to exhaust the scrubbed process gas and residual fluoride separated from the fluorided aluminum oxide in the filter module to the environment.

[0059] According to a preferred embodiment, the module assembly further comprises an alumina flow control module located upstream of the pretreatment module for controlling the flow of fluorinated alumina. Preferably, the alumina flow control module comprises a discontinuously operated pneumatic conveying system, and the fluorinated alumina flow is controlled by changing the operating time of the discontinuously operated pneumatic conveying system.

[0060] According to another aspect, the present invention relates to an electrolytic cell assembly for producing aluminum, comprising: at least one electrolytic cell configured for producing aluminum; and for each of the at least one electrolytic cell, an apparatus for collecting and pre-treating primary process gas generated in the electrolytic cell during aluminum production, the apparatus being as defined herein.

[0061] According to a further aspect, the present invention relates to an electrolytic cell assembly for producing aluminium, comprising: at least one electrolytic cell configured for producing aluminium; and at least one module assembly as defined herein for treating gases produced by the electrolytic cell.

[0062] According to a further aspect, the present invention relates to the use of an electrolytic cell assembly as defined herein for producing aluminium. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Other features and exemplary advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0064] Figure 1 is a schematic diagram of an apparatus for collecting and pre-treating primary process gas produced in an electrolytic cell during aluminum production according to a preferred embodiment of the present invention;

[0065] Figure 2 is a flow chart of a method for collecting and pre-treating primary process gas produced in an electrolytic cell during the production of aluminum according to a preferred embodiment of the present invention;

[0066] Figure 3 is a schematic diagram of a module assembly for treating a process gas produced by an electrolytic cell according to a preferred embodiment of the present invention; and

[0067] Figure 4 is a flow chart of a method for treating a process gas produced by an electrolytic cell according to a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0068] A new method will be described below. Although the present invention is described in terms of specific illustrative embodiments, it should be understood that the embodiments described herein are by way of example only and the scope of the present invention is not intended to be limited thereby.

[0069] The terms used herein are consistent with the definitions listed below.

[0070] Unless otherwise indicated, % or wt.% as used herein refers to weight%. When used herein, % refers to weight % compared to the total weight percentage of the phase or composition in question.

[0071] "About" means that the value of weight % (wt.%), time, pH, volume or temperature can vary within a certain range depending on the error margin of the method or device used to evaluate such weight %, time, pH, volume or temperature. A 10% error margin is generally accepted.

[0072] The following description and the embodiments described therein are provided by way of examples of specific embodiments illustrating the principles of the present invention and various aspects. These embodiments are provided to explain, but not to limit, those principles of the present invention. In the following description, throughout the specification and the accompanying drawings, the same parts and / or steps are marked with the same corresponding reference numerals.

[0073] Combined with the attached drawings, Figure 1 , Figure 2 , Figure 3 and Figure 4 , embodiments of the present invention will be more clearly understood with reference to the following detailed description.

[0074] In the production plant, the electrolytic cell is covered by a removable cover and most of the process gas is exhausted from the cell by multiple exhaust fans through multiple gas-solid reactors and filters (called scrubbers). Most of the gaseous fluorides and particulates (including particulate fluorides) are captured on the alumina and ultimately returned to the electrolytic cell as feed, called reactive alumina or fluorided alumina.

[0075] There are two types of dry scrubbers: injection-type scrubbers, in which the alumina is injected through a high velocity stream of process gas; and fluidized bed scrubbers, in which the process gas and alumina form a high residence time fluidized bed, thereby enhancing the adsorption kinetics of gaseous fluorides on the alumina surface. Fluidized beds are described, for example, in U.S. Pat. No. 3,503,184 (Knapp et al.).

[0076] Centralized gas treatment centers (GTCs) usually allow the washing of hydrogen fluoride with fresh alumina injected into the process gas and fluorinated alumina (for injection-type GTCs) to form semi-washed process gas and fluorinated alumina, which are subsequently separated by filtration. Fluorinated alumina is known to improve the contact quality between process gas and alumina. The washed process gas and residual fluoride are vented or discharged to the environment, while the fluorinated alumina is usually stored before being fed back to the electrolytic cells as a raw material for the production of aluminum.

[0077] Aluminium producers aim to continuously increase the amperage of the potline to improve cell productivity. This is called a potline revamp. In many plant revamps, the concentration of the process gas increases and can exceed the handling capacity, especially when the scrubber is an injection type scrubber.

[0078] As previously mentioned, according to a first aspect, the present invention relates to an apparatus 100 for collecting and pre-treating a primary process gas generated in an electrolytic cell 300 during the production of a metal such as, but not limited to, aluminum. Figure 1 The apparatus 100 includes a collecting unit 110 configured to draw a primary process gas from an electrolytic bath 310 of an electrolytic cell 300. The collecting unit 110 is generally configured to draw a primary process gas from an orifice 320 intentionally made above the electrolytic bath 310 of the electrolytic cell 300. "Primary process gas" refers to the hot and concentrated gas collected above the electrolytic bath before any dilution with external air.

[0079] The apparatus 100 further includes a pretreatment unit 120 fluidly connected to the collection unit 110 and configured to receive the fluorinated alumina to pretreat the primary process gas. Typically, the pretreatment unit 120 may consist of a fluidized bed reactor, without filter bags, and installed near the electrolytic cell, typically as Figure 3One per electrolytic cell, or one for multiple electrolytic cells as shown. The reactor 120 is configured to receive and contain a fluidized bed 130 of fluorided alumina and to drive the primary process gas through the fluidized bed 130. The collection and pretreatment units 110, 120 are located directly inside, beside, near or adjacent to the electrolytic cell 300. The fluidized bed reactor is preferably equipped with a discharge unit 140, which is composed of one or more exhaust fans, preferably connected to the upper part 132 of the reactor to drive the pretreated primary process gas out of the fluidized bed reactor 120. Alternatively, there may be one or more exhaust fans 140, thereby serving several fluidized bed reactors 120. The discharge unit 140 may be located inside or outside the electrolysis plant building 400. For each electrolytic cell, a portion of the process gas, which is considered as the primary process gas, is collected at an orifice 320 deliberately made in the cover separating the electrolytic bath 310 from the outside, driven through the relevant fluidized bed and discharged at the upper part of the fluidized bed.

[0080] Also like Figure 1 As shown, the apparatus 100 may further include a buffer unit 150, such as a buffer hopper, which is fluidly connected to the pretreatment unit 120 and configured to buffer the fluorinated aluminum oxide before it is injected into the pretreatment unit 120. For example, Figure 3 As shown, the buffer hopper 150 may be located below the alumina distribution system 510 from the central gas treatment center or GTC 500. The alumina recovered from the hopper 150 may then be raised to a suitable level in the required amount to feed the fluidized bed 130 of the pretreatment unit 120 and ultimately to the electrolytic cell 300 or electrolytic cell group. In practice, the pretreatment unit or fluidized bed reactor 120 may be fluidly connected to the electrolytic cell 300 upstream 160, such as with an air slide or air conveyor, to inject at least a portion of the fluorided alumina contained in the pretreatment unit 120 into the electrolytic cell 300 as a raw material for the production of aluminum. Known electrolytic cell feed systems are used, such as those disclosed in WO 2017 / 173169 A1 (Lucey et al.), the contents of which are incorporated herein by reference.

[0081] like Figure 1 As shown, the apparatus 100 may further include an alumina flow control system 170, which is located upstream of the pretreatment unit 120 and is used to control the alumina flow, preferably by changing the operation time of the discontinuously operated pneumatic conveying system. Since the electrolytic cell is supplied with alumina from the fluidized bed reactor 120, the alumina demand rate of the electrolytic cell actually determines the alumina supply rate to the fluidized bed reactor 120.

[0082] Figure 2 A preferred method for collecting and pre-treating primary process gases produced in electrolytic cells during the production of aluminium is shown.

[0083] The method 1000 includes a first step 1100 of collecting a primary process gas from an electrolytic bath of an electrolytic cell. Preferably, the first step may include drawing the primary process gas from a specially made orifice above the electrolytic bath 310 of the electrolytic cell 300, such as Figure 1 As described above, before the electrolytic bath 310 is driven through the fluidized bed reactor and discharged from the upper part of the fluidized bed of the reactor, an orifice or hole may be made in the cover that separates the electrolytic bath 310 from the outside of the electrolytic cell. Other possible ways of collecting the primary gas from the electrolytic bath of the electrolytic cell are contemplated within the scope of the present invention.

[0084] The method 1000 comprises a second step 1200 of pre-treating the primary process gas collected in the first step 1100 with fluorided alumina. Advantageously, the pre-treatment step 1200 is carried out inside the potroom building 400, in close proximity to or adjacent to the electrolytic cells. Preferably, the pre-treatment step 1200 may comprise driving the primary process gas through a fluidized bed of fluorided alumina. Preferably, the method 1000 then further comprises a step 1300 of discharging the pre-treated primary process gas from the upper part of the fluidized bed, thereby driving the pre-treated primary process gas out of the pre-treatment unit as such.

[0085] The method 1000 may further include a step 1300 of buffering the fluorided alumina before using it in the fluidized bed reactor of the pretreatment unit 120. A buffer hopper such as disclosed above may be used.

[0086] At least a portion of the fluorided alumina contained in the fluidized bed reactor 120 is then injected into the electrolytic cell 300 as a feedstock for producing aluminum in the fourth step of the method 1000. As described above, the recovered alumina from the hopper 150 can then be raised in the required amount to a suitable level to feed the fluidized bed 130 of the pretreatment unit 120 and ultimately the electrolytic cell 300 or a battery of electrolytic cells.

[0087] A preferred embodiment of the module assembly according to the invention is Figure 3 , and generally includes one or more pre-treatment devices 100 located in a potroom building 400 via an electrolytic cell 300 as disclosed above, and operably fluidly connected to a central gas treatment center or GTC 500.

[0088] The module assembly 200 includes at least one first collection module 110, each fluidically connected to the electrolytic bath 310 of the electrolytic cell 300, and configured to collect the primary process gas generated in the electrolytic cell. For the collection unit of the apparatus 100 disclosed above, the first collection module 120 is configured to lead the primary process gas from the orifice 320 specially made above the electrolytic bath 310 of the electrolytic cell 300.

[0089] For each electrolytic cell 300, the module assembly 200 also includes a second collection module 210, which is fluidically connected to the hood space 320 of each electrolytic cell 300 and is configured to collect the hood space process gas generated in the electrolytic cell. "Hood space process gas" refers to the gas collected under the electrolytic cell hood and includes dilution with external air. Their concentration and temperature are lower than the concentration and temperature of the primary process gas.

[0090] The module assembly 200 comprises, for each electrolytic cell 300, at least one pre-treatment module 120, also preferably designated herein as a fluidized bed reactor 120, which is fluidly connected to the first collection module 110 and configured to pre-treat the primary process gas with partially fluorinated alumina.

[0091] For each electrolytic cell 300 , the module assembly 200 may further include a merging module 220 fluidly connected to the pre-treatment module 120 and the second collection module 210 for merging the hood space process gas delivered from the hood space 320 of the electrolytic cell 300 with the primary process gas delivered from the pre-treatment module 120 .

[0092] like Figure 3 As shown, the module assembly 200 may further include an alumina flow control system 170, which is located upstream of the pretreatment unit 120 and is used to control the alumina flow, preferably by changing the operation time of the discontinuously operated pneumatic conveying system. Since the electrolytic cell is supplied with alumina from the fluidized bed reactor 120, the alumina demand rate of the electrolytic cell actually determines the alumina supply rate to the fluidized bed reactor 120.

[0093] like Figure 3 The illustrated module assembly 200 further comprises a scrubber module 230 fluidly connected to the merging module, for example using a central gas conduit 240. Scrubbing of the gas mixture comprising hydrogen fluoride from the cap space process gas occurs in a reactor 232 of the scrubber module 230, wherein fresh alumina 234 and fluorided alumina 236 are contacted with the cap space process gas delivered by the central gas conduit 240. The treated mixture then leaves the reactor 232 as a semi-scrubbed process gas through the semi-scrubbed process gas conduit 242.

[0094] like Figure 3The illustrated module assembly 200 further includes a filtration module 250 having one or more filters 252 fluidly connected to the one or more reactors 232 via a semi-scrubbed process gas conduit 242. The scrubbed process gas 260, once separated from the fluorided alumina by the filters, is conveyed toward an exhaust module 280, which is fluidly connected to the filtration module 250 and includes an exhaust fan 282 and a chimney 284 to exhaust the scrubbed process gas 260 and residual fluoride 286 to the environment.

[0095] In the filtration module 250, a portion 236 of the fluorinated alumina 270, referred to as recycled alumina, may be reinjected into the scrubber reactor 232 to contact the gas, while another portion of the fluorinated alumina 270 is conveyed toward an alumina distribution system 510 including a bin 512. The fluorinated alumina, also referred to as semi-fluorinated alumina, may then be injected into each pre-treatment module 120 of the module assembly 200, and / or into each electrolytic cell as an alumina feedstock.

[0096] As for the pretreatment unit disclosed above, the pretreatment module 120 comprises a fluidized bed 130 of partially fluorinated alumina through which the primary process gas collected from the electrolytic bath is driven before leaving the upper portion 132 of the fluidized bed, thanks to a discharge module 140 located downstream of the pretreatment module 120. The discharge module 140 is also located upstream of the merging module 220, where the pretreated primary process gas is conveyed before merging with the hood space process gas in the merging module 120.

[0097] Figure 3 The illustrated module assembly 200 further includes at least one first buffer module 150 located downstream of the alumina distribution system 510 of the GTC 500 and upstream of the pretreatment module 120. Each buffer module 150 buffers at least a portion of the semifluorinated alumina before it is injected into the pretreatment module 120.

[0098] According to a preferred embodiment, not shown in the figure, the module assembly may further include a second buffer module located downstream of the GTC 500 to buffer at least a portion of the semi-fluorinated alumina before being directly injected into the electrolytic cell as a raw material for producing aluminum. Preferably, the first and second buffer modules may form a unique buffer module fluidly connected to both the pretreatment module and the electrolytic cell. A portion of the partially fluorinated or semi-fluorinated alumina 160 contained in the pretreatment module 120 may be injected into the electrolytic cell 300 as a raw material for producing aluminum.

[0099] Figure 4 A method for treating a process gas produced by an electrolytic cell is shown. The method 2000 comprises the following steps:

[0100] a) collecting primary process gas (2100) from an electrolytic bath of an electrolytic cell;

[0101] b) collecting hood space process gas (2200) from the hood space of the electrolytic cell;

[0102] c) pre-treating the primary process gas collected in step a) with partially fluorinated alumina (2300);

[0103] d) combining the primary process gas obtained in step c) with the hood space process gas obtained in step b) (2400);

[0104] e) contacting the combined process gas obtained in step d) with fresh alumina to form semi-washed process gas and fluorided alumina (2500);

[0105] f) separating the semi-washed process gas from the fluorinated alumina to obtain washed gas and fluorinated alumina (2600), and

[0106] g) using at least part of the recycled fluorided alumina obtained from step f) to pre-treat the primary process gas in step c) (2700).

[0107] Step a) 2100 of the method 2000 may include the step of directing 2110 the primary process gas from a purposely made orifice above the electrolytic bath of the electrolytic cell. Pretreatment step c) 2300 may include the step of driving 2310 the primary process gas through a fluidized bed of fluorided alumina.

[0108] The method 2000 may further include the step of exhausting 2320 the pre-treated primary process gas in the upper portion of the fluidized bed prior to combining 2400 the pre-treated gas with the hood space process gas.

[0109] The method 2000 may further comprise the step of buffering 2800 at least a portion of the fluorided alumina obtained from step f) 2600 prior to use in step c) 2300 to pretreat the primary process gas and / or prior to injection into the electrolytic cell as feedstock for producing aluminium.

[0110] The method 2000 may further comprise the step of contacting 2520 the process gas in step e) 2500 with a portion of the fluorided alumina obtained from step f) 2600. Preferably, a portion of the fluorided alumina is combined with fresh alumina.

[0111] The method 2000 may further comprise a step 2900 of discharging the semi-washed process gas and residual fluoride obtained from step f) 2600 into the environment.

[0112] In other words, the new method disclosed herein includes collecting the vast majority of the process gas drawn from the electrolytic bath, such as using, for example, an orifice deliberately made above the electrolytic bath, and then pre-treating this portion in a pre-treatment unit or module, such as a fluidized bed reactor (one for each electrolytic cell or multiple electrolytic cells) using alumina (e.g., partially fluorinated alumina from a GTC). Once pre-treated, this gas portion is returned to the central process electrolytic cell pipeline for further treatment and gas-solid separation from the remaining process gas in the central GTC. The fluorinated alumina leaving the fluidized bed is used as a raw material for the corresponding electrolytic cell. The new safe alumina raw material is located immediately downstream of the feed point from the ordinary alumina distribution system to the electrolytic cell. Alumina is then recovered from the safe raw material and raised to an appropriate level to supply the fluidized bed reactor.

[0113] GTCs known in the art have provided effective gas handling, but at the expense of a much larger footprint since the entire process gas is handled. In addition, in some retrofit plants, GTCs alone are not sufficient to overcome the increase in process gas concentration.

[0114] Integrated gas treatment (IGT) known in the art has several disadvantages. For example, the collection point of the process gas is not modified; the fluidized bed is housed in the superstructure; the entire process gas is treated in the fluidized bed (i.e., the GTC is no longer used); the fluidized bed is fed with fresh alumina; filters are used to separate the scrubbed process gas from the alumina; and the scrubber process gas is discharged into the open environment.

[0115] Existing solutions allow to retrofit existing injection type GTCs, but can only cope with negligible over-concentration of process gases. Central fluidized bed scrubbers as an alternative to injection type GTCs in the context of plant retrofits are a very costly solution, difficult to connect to existing gas pipes and alumina silos, and difficult to position in the already crowded existing GTCs. IGT solutions as an alternative to injection type allow to handle concentrated process gases, but require filters to be installed inside the electrolytic cell superstructure. Moreover, the safety feedstock usually provided by the superstructure alumina silos disappears in case of failure of the upstream alumina circuit.

[0116] The systems, apparatus and methods disclosed herein do not require complicated connections to existing potline piping and alumina silos: only connections are located at each pot, upstream of the existing fluorinated alumina delivery system and downstream of the existing potline gas piping and potline fluorinated alumina delivery system.

[0117] Furthermore, the systems and apparatus disclosed herein may be located immediately adjacent to or adjacent to each electrolytic cell, or outside of the potroom building, thereby not requiring any area above the electrolytic cells.

[0118] Furthermore, the pretreatment equipment disclosed herein does not require an inline filter bag, which is a difficult and time-consuming task for machine maintenance.

[0119] The technology disclosed herein does not affect the safe alumina feedstock required for each electrolytic cell.

[0120] The technology disclosed in this article provides the following solutions:

[0121] Collect primary process gases directly at the electrolytic bath level;

[0122] Separation of primary process gas and hood space process gas;

[0123] Fluidized bed reactor using filter bags;

[0124] Integration of existing GTCs with fluidized bed reactors in a countercurrent scrubbing process; and

[0125] • High contact time between the primary process gas and the fluorided alumina, which preheats the raw alumina at a temperature close to that of the primary process gas.

[0126] Advantageously, the technology disclosed herein allows:

[0127] Recover some components of the primary process gas because of their high concentrations;

[0128] Recover waste heat from primary process gases;

[0129] Reduced HF release due to less contact with moisture from the outside air entering between the hoods;

[0130] Reduce the release of particulate fluoride in the hood space;

[0131] Reduce the total area of ​​the fluidized bed;

[0132] Eliminate the need to change bags within the electrolytic cell superstructure;

[0133] Eliminate connections to existing facilities (such as silos);

[0134] Reduce the velocity and concentration of the gas in the GTC, thereby providing spare capacity from increased extraction of process gas from the hood space;

[0135] Eliminates the requirement to completely remove gaseous and particulate fluorides in a fluidized bed;

[0136] Reduced HF release due to fewer residual hydroxyl groups (-OH) at the surface of fluorided alumina; and

[0137] Improve the energy efficiency of electrolytic cells.

[0138] While illustrative and presently preferred embodiments of the invention have been described above in detail, it should be understood that the concepts of the invention may be otherwise variously embodied and used, and that the appended claims are intended to be interpreted to include such variations unless limited by prior art.

Claims

1. An apparatus for collecting and pre-treating primary process gases produced in an electrolytic cell during aluminum production, the apparatus comprising: a collection unit configured to draw the primary process gas from an electrolytic bath of the electrolytic cell before dilution with external air; and a pretreatment unit fluidly connected to the collection unit and configured to receive fluorinated alumina for pretreating the primary process gas; wherein the collecting unit and the pretreatment unit are adjacent to the electrolytic cell, and The collecting unit is configured to lead the primary process gas from an orifice specially made above the electrolytic bath of the electrolytic cell before diluting it with external air.

2. The apparatus of claim 1, wherein the pretreatment unit is configured to receive and contain a fluidized bed of fluorided alumina.

3. The apparatus of claim 2, wherein the pre-treatment unit is further configured to drive the primary process gas through the fluidized bed of fluorided alumina.

4. The apparatus according to claim 2 or 3, further comprising a discharge unit connected to the pretreatment unit and configured to discharge the pretreated primary process gas from an upper portion of the fluidized bed and drive the pretreated primary process gas out of the pretreatment unit.

5. The apparatus of claim 2, further comprising an alumina flow control system located upstream of the pretreatment unit for controlling a flow of fluorinated alumina.

6. The apparatus according to claim 5, wherein the alumina flow control system comprises a discontinuously operated pneumatic conveying system, and the fluorinated alumina flow is controlled by changing the operation time of the discontinuously operated pneumatic conveying system. 7 . The apparatus according to claim 1 , further comprising a buffer unit fluidly connected to the pretreatment unit and configured to buffer the fluorided aluminum oxide before the fluorided aluminum oxide is injected into the pretreatment unit.

8. The apparatus according to claim 1 or 2, wherein the pretreatment unit is fluidically connected to the electrolytic cell upstream and is further configured to inject at least a portion of the fluorided alumina contained in the pretreatment unit into the electrolytic cell as a raw material for producing aluminum.

9. The apparatus according to claim 1 or 2, wherein the pretreatment unit is located within the electrolytic cell and is fluidically connected to the electrolytic cell, and is further configured to inject at least a portion of the fluorided alumina contained in the pretreatment unit into the electrolytic cell as a raw material for producing aluminum.

10. A method for collecting and pre-treating primary process gas produced in an electrolytic cell during aluminium production, the method comprising: a. collecting the primary process gas from the electrolytic bath of the electrolytic cell before dilution with external air; and b. pretreating the primary process gas obtained in step a) with fluorinated alumina, the pretreatment step being carried out adjacent to the electrolytic cell, The collecting step includes drawing the primary process gas from an orifice intentionally made above the electrolytic bath of the electrolytic cell.

11. A method according to claim 10, wherein the pre-treatment step b) comprises driving the primary process gas through a fluidised bed of fluorided alumina.

12. The method according to claim 11, further comprising the step of discharging the pretreated primary process gas from an upper portion of the fluidized bed.

13. The method of claim 12, further comprising expelling the pretreated primary process gas out of a pretreatment unit.

14. The method of claim 10 or 11, further comprising controlling the flow rate of the fluorided alumina during the pretreatment step.

15. The method of claim 14, wherein controlling the flow rate comprises varying an operating time of a discontinuously operated pneumatic conveying system.

16. The method of claim 10 or 11, further comprising buffering the fluorided alumina before it is used in a pretreatment unit.

17. The method according to claim 10 or 11, further comprising injecting at least a portion of the fluorided alumina contained in a pretreatment unit into the electrolytic cell as a raw material for producing aluminum.

18. A method for treating a process gas produced by an electrolytic cell, the method comprising the steps of: a. collecting the primary process gas from the electrolytic bath of the electrolytic cell by diluting the primary process gas from a specially made orifice above the electrolytic bath of the electrolytic cell before diluting the primary process gas with external air; b. Collecting process gas from the hood space of the electrolytic cell; c. pretreating the primary process gas collected in step a) with partially fluorinated alumina, the pretreatment step being carried out adjacent to the electrolytic cell; d. combining the primary process gas obtained in step c) with the hood space process gas obtained in step b); e. contacting the combined process gas obtained from step d) with fresh alumina to form a semi-washed process gas and fluorinated alumina; f. separating the semi-washed process gas from the fluorinated alumina to obtain fluorinated alumina, and g. Using at least part of the fluorided alumina obtained from step f) for pre-treating the primary process gas in step c).

19. The method of claim 18, wherein steps a) and c) are performed in the electrolytic cell.

20. A method according to claim 18 or 19, wherein the pre-treatment step c) comprises driving the primary process gas through a fluidised bed of fluorided alumina.

21. The method of claim 20, further comprising the step of exhausting the pre-treated primary process gas in an upper portion of the fluidized bed before the primary process gas is combined with the hood space process gas.

22. The method of claim 18 or 19, further comprising controlling at least a portion of the flow of recycled fluorided alumina obtained from step f) before recycling the fluorided alumina for use in step c) to pretreat the primary process gas.

23. The method of claim 22, wherein controlling comprises varying the operating time of the discontinuously operated pneumatic conveying system.

24. A method according to claim 18 or 19, further comprising the step of buffering at least a portion of the recycled fluorided alumina obtained from step f) before being used in step c) to pre-treat the primary process gas.

25. The method according to claim 24, further comprising the step of injecting at least a portion of the recycled fluorided alumina used in step c) into the electrolytic cell as a feedstock for producing aluminium.

26. A process according to claim 18 or 19, further comprising the step of buffering at least a portion of the recycled fluorided alumina produced in step f) prior to injection into the electrolytic cell as a feedstock for producing aluminium.

27. The method according to claim 18 or 19, further comprising the step of contacting the process gas in step e) with a portion of the fluorided aluminum oxide obtained from step f).

28. The method of claim 27, wherein a portion of the fluorided alumina is mixed with fresh alumina.

29. The method according to claim 18 or 19, further comprising the step of discharging the scrubbed process gas and residual fluoride obtained from step f) into the environment.

30. A module assembly for treating gas produced by an electrolytic cell, the module assembly comprising: a first collection module fluidly connected to an electrolytic bath of the electrolytic cell and configured to collect a primary process gas produced in the electrolytic cell by drawing the primary process gas from an orifice intentionally made above the electrolytic bath of the electrolytic cell before diluting it with external air; a second collection module fluidly connected to a hood space of the electrolytic cell and configured to collect hood space process gas generated in the electrolytic cell; a pretreatment module fluidly connected to the first collection module and configured to pretreat the primary process gas with partially fluorinated alumina, the pretreatment module being proximate to the electrolytic cell; a merging module fluidly connected to the pre-processing module and the second collection module and configured to merge the hood space process gas with the primary process gas pre-processed in the pre-processing module; a scrubber module fluidly connected to the combining module and configured to contact the combined process gas with fresh alumina to form a scrubbed process gas and fluorided alumina; and A filter module is fluidly connected to the scrubber module and configured to separate the semi-scrubbed process gas from the fluorinated alumina to obtain fluorinated alumina, and the filter module is further fluidly connected to the pretreatment module and configured to at least partially re-inject the fluorinated alumina into the pretreatment module.

31. The module assembly of claim 30, the pretreatment module also being fluidly connected to the electrolytic cell and further configured to inject at least a portion of the fluorided alumina contained in the pretreatment unit into the electrolytic cell as a feedstock for producing aluminum.

32. A modular assembly according to claim 30 or 31 , wherein the pre-treatment module is configured to contain a fluidised bed of fluorided alumina and to drive the primary process gas through the fluidised bed of fluorided alumina.

33. The module assembly according to claim 32 further comprises an exhaust module, which is located downstream of the pretreatment module and upstream of the merging module, and is configured to discharge the pretreated primary process gas from the upper part of the fluidized bed and drive the pretreated primary process gas toward the merging module.

34. The module assembly of claim 33, further comprising a first buffer module located downstream of the filtration module and upstream of the pretreatment module and configured to buffer at least a portion of the fluorided alumina before the fluorided alumina is injected into the pretreatment module.

35. The module assembly of claim 34, further comprising a second buffer module located downstream of the filtration module and upstream of the electrolytic cell and configured to buffer at least a portion of the fluorided alumina before it is injected into the electrolytic cell as a feedstock for producing aluminum.

36. The module assembly of claim 35, wherein the first buffer module and the second buffer module form a single buffer module fluidly connected to the pre-treatment module and the electrolysis cell.

37. The module assembly of claim 30 or 31, wherein the pretreatment module is also fluidly connected to the electrolytic cell and is configured to inject at least a portion of the fluorided alumina contained in the pretreatment module into the electrolytic cell as a feedstock for producing aluminum.

38. The module assembly of claim 30 or 31, wherein the scrubber module is also fluidly connected upstream of the filtration module and is configured to receive a portion of the fluorided alumina from the filtration module.

39. The modular assembly of claim 38, wherein the scrubber module is further configured to combine a portion of the fluorided alumina with the fresh alumina.

40. The module assembly of claim 30 or 31, further comprising an exhaust module fluidly connected to the filtration module and configured to exhaust the scrubbed process gas and residual fluoride separated from the fluorided alumina in the filtration module to the environment.

41. The module assembly of claim 30 or 31, further comprising an alumina flow control module located upstream of the pretreatment module for controlling the flow of fluorinated alumina.

42. The modular assembly of claim 41, wherein the alumina flow control module comprises a discontinuously operated pneumatic delivery system, and the fluorinated alumina flow is controlled by varying the operation time of the discontinuously operated pneumatic delivery system.

43. An electrolytic cell assembly for producing aluminum, comprising: - at least one electrolytic cell configured to produce aluminium; and - for each of said at least one electrolytic cell, an apparatus for collecting and pre-treating primary process gas produced in said electrolytic cell during aluminium production, said apparatus being as defined in any one of claims 1 to 9.

44. An electrolytic cell assembly for producing aluminum, comprising: - at least one electrolytic cell configured to produce aluminium; and - At least one module assembly as defined in any one of claims 30 to 42 for treating gases produced by an electrolytic cell.

45. Use of an electrolytic cell assembly as defined in claim 43 or 44 for the production of aluminium.

Images