DESC:*** Complete Specification ***

“Method for recovery of pure Alumina from Aluminium dross”

Cross references to related applications: This complete specification is filed further to application for patent No. 202231074119 filed on 21/12/2022 with provisional specification, the contents of which are incorporated herein in their entirety, by reference.

Field of the invention
This invention relates generally to pyrometallurgical techniques for recycling of residue of Aluminium dross residue. A yet-preferred embodiment of the present invention disclosed hereunder particularly outlines an inventive method, and its implementing system, for recovery of Alumina by the oxidation, at elevated temperatures, of Aluminium Nitride component of Aluminium dross residue.

Definitions and interpretations
Before undertaking the detailed description of the invention below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect, with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. The following definitions shall hold for the instant document-
(a) “Dross” typically refers to a byproduct formed during the production of non-ferrous metals, particularly Aluminium. It is generated when Aluminium or other non-ferrous metals are melted, and impurities or oxides separate from the molten metal. The composition of dross can vary depending on the specific metal production process and the raw materials used. The term "dross" is commonly associated with Aluminium production, but similar byproducts exist in the production of other non-ferrous metals.
(b) “Dross residue” refers to dross plus impurities.
(c) “NMP” refers to Non-metallic particles.
(d) "SiC heaters" refers to heaters that use silicon carbide (SiC) as the heating element.
(e) “MS” refers to Mild Steel.
(f) “PID” refers to Proportional–integral–derivative.

Background of the invention
Aluminium makes up about 8% by weight of the earth’s solid surface, and never occurs as a free element in nature (P.E. Tsakiridis, J. Hazard. Mater. 217, 1. (2012)). The world over, Aluminium is extracted from its ore, bauxite, typically by either Bayer process followed by the Hall–Heroult electrolysis, or recycling Aluminium from process scrap and used Aluminium products.

During the manufacturing processes of Aluminium, especially during the processes of melting and alloying, oxidation occurs at the surface where molten Aluminium meets the oxidizing atmosphere, leading to the formation of a semisolid mass, known as dross. This dross comprises Aluminium oxide, metallic Aluminium, magnesium spinel, periclase, quartz and other salts with small traces of Aluminium carbides and nitrides (S.O. Adeosun, O.I. Sekunowo, O.O. Taiwo, W.A. Ayoola, and A. Machado, Adv. Mater. 3, 6. (2014)).

Dross formed as a by-product during production of Aluminium and its alloys contains residual Aluminium which, if unrecovered, would be a valuable loss. White dross / primary dross, formed during the primary production of Aluminium, contains a high percentage of elemental Aluminium and Aluminium oxides (S.K. Verma, V.K. Dwivedi and S.P. Dwivedi, Mater. Today Proc. (2021)). Black dross on the other hand, which is formed at the time of the secondary Aluminium refining processes, is a mix of Aluminium oxides and slag and, in comparison to White dross, contains a smaller amount of elemental Aluminium (O. Manfredi, W. Wuth, and I. Bohlinger, JOM 49, 48. (1997); D.B. Masson, and M.M. Taghiei, Mater. Trans. 30, 411. (1989); B. Lucheva, T. Tsonev, and R. Petkov, J. Univ. Chem. Technol. Metallurgy 40, 335. (2005)).

Non-metallic particles of dross residue mainly consist of Aluminium micro fines, Alumina, Aluminium Nitride and some carryover fluxes like bath material. The non-metallic particles cannot be charged back to the smelter because Aluminium Nitride breaks in oxidizing condition but not in reducing conditions and hence will become an inclusion in the Aluminium smelted, as smelter works in highly reducing conditions.

Aluminium dross residue is also toxic and poses a grave environmental hazard if untreated as it contaminates surface and ground water, which further leads to production of hazardous gases such as phosphine and ammonia which pollute the atmosphere. Aluminium Nitride present in dross is hazardous in nature. On contact with moisture, it turns to Aluminium hydrate and releases ammonia. The carryover materials (fluxes for smelting alumina) make it inconsistent for use in refractories. Thus, the safe disposal of Aluminium dross residue is extremely burdening in financial and procedural aspects. Aluminium dross residue arising from the Aluminium smelting industry has been in fact categorized in India under the hazardous category as per the Hazardous and Other Wastes (Management and Transboundary) Movement Rules 2016.

Aluminium dross residue, which in principle can be assigned to new scrap, consists of up to 80% of metallic Al, Aluminium oxides in various forms (a and ?), and Aluminium Nitride (Aluminium Nitride). In addition, silicon, iron, calcium, and magnesium oxides (as well as other Al compounds such as carbides, chlorides, and fluorides).

Hence, reutilization of Aluminium dross residue is important for environmental protection as well as an economic point of view and, as the reader shall appreciate, the recovery of Aluminium content in the process would be a much desired advantage.

Description of related art
Prior art lists some scattered attempts to address the issues mentioned hereinabove. Processes for recovery of Aluminium from Aluminium dross residue are known in the art. Common methods for recycling Aluminium dross residue include-
(a) Salt Recovery Process: This involves treating the dross with salt to recover the metallic Aluminium. The salt reacts with the Aluminium oxide, forming salt cake, which is then processed to extract the Aluminium.
(b) Mechanical Processing: This involves crushing and screening the dross to separate the metal from the non-metallic components. This process can recover a significant amount of Aluminium, and the remaining material can be used in construction or other applications.
(c) Pyrometallurgical Methods: These involve heating the dross to high temperatures to separate the metal from the oxide components. This can include processes like rotary salt fluxing and plasma processing. Dross can be processed to recover Aluminium metal through various methods such as rotary furnaces or reverberatory furnaces. The recovered metal can then be reintroduced into the Aluminium production process
(d) Leaching Process: This involves treating the dross with a solution that dissolves the Aluminium oxide, leaving behind the metallic Aluminium. This can be done using acids or alkaline solutions.
(e) Thermal Reduction: This involves dross being heated in the presence of reducing agents to extract the metal.
(f) Bayer Process: The Bayer process is a well-known method for extracting alumina from bauxite ore, but it can also be adapted for recovering alumina from certain types of dross. The process involves digestion of the dross with sodium hydroxide (NaOH), followed by precipitation of alumina and further purification steps.
(g) Hydrothermal Treatment: This involves subjecting the dross to high-temperature and high-pressure conditions in the presence of a solvent. This process can selectively dissolve alumina, allowing for its recovery.
(h) Smelting: This can be employed to recover alumina from dross. The dross is melted, and impurities are removed, leaving behind molten alumina. This requires careful control of temperature and other process parameters.

Among patent prior art, examples of approaches for recycling Aluminium dross residue include JP6828036B2 (filed by Byung Doo) which teaches recovery of alumina from black dross. Another reference is KR20190112527A (filed by Ahn Byung-doo) teaches a Aluminium black dross recycling system for recycling black dross generated when dissolving Aluminium scrap in a flux-treated Aluminium melt.

However, all the approaches in prior art, without exception, are not able to find true applicability in the art due to various lacunae and process inefficiencies that persist in them.

Technical issues to be resolved
As stated above, the state of art, as it exists today, is inundated with lacunae and process inefficiencies, particularly being-
(a) Low efficiency in recovery of Aluminium
(b) Stringency for reagents / chemicals.
(c) Inability to effectively adjust to varying dross compositions.
(d) Ineffective Separation of Metal and Oxides, leading to lower recovery rates.
(e) Dependency on pre-treatment of dross before the recovery process, leading to lower recovery rates if pretreatment is not effective.
(f) High stringency of temperature and other process parameter control.
(g) High costs associated with apparatuses, chemicals, process implementation.
(h) Indispensability of skilled labor.
(i) Indispensability of high fuel requirements and consequent burden on operational budgets.

Prior art therefore, does not list a single effective solution embracing all considerations mentioned hereinabove, thus preserving an acute necessity-to-invent for the present inventor/s who, as result of focused research, has come up with novel solutions for resolving all needs once and for all. Work of the presently named inventor/s, specifically directed against the technical problems recited hereinabove and currently part of the public domain including earlier filed patent applications, is neither expressly nor impliedly admitted as prior art against the present disclosures.

As the aforesaid lacunae and process inefficiencies remain to be addressed, the need for further research in this domain is preserved. The Applicant has therefore undertaken targeted research in this direction, and consequently come up with novel solutions for resolving all needs of the art once and for all.

A better understanding of the objects, advantages, features, properties and relationships of the present invention will be obtained from the following detailed description which sets forth an illustrative yet-preferred embodiment.

Objectives of the present invention
The present invention is identified in addressing at least all major deficiencies of art discussed in the foregoing section by effectively addressing the objectives stated under, of which:

It is a primary objective to provide an efficient and sustainable process for the recovery of Alumina from Aluminium dross residue.

It is another objective further to the aforesaid objective(s) that the process so provisioned is cost-effective to implement.

It is another objective further to the aforesaid objective(s) that the process so provisioned does not mandate the involvement of skilled personnel.

It is another objective further to the aforesaid objective(s) that the process so provisioned has high throughput of recovered Alumina.

It is another objective further to the aforesaid objective(s) that the process so provisioned is unaffected by the technical issues outlined earlier in this document.

The manner in which the above objectives are achieved, together with other objects and advantages which will become subsequently apparent, reside in the detailed description set forth below in reference to the accompanying drawings and furthermore specifically outlined in the independent claims. Other advantageous embodiments of the invention are specified in the dependent claims.

Brief description of drawings
The present invention is explained herein under with reference to the following drawings, in which:

FIGURE 1 is a schematic drawing showing the plan view of the electrically-heated continuous tunnel kiln used in the present invention.

FIGURE 2 is a schematic drawing showing the elevation view of the electrically-heated continuous tunnel kiln used in the present invention.

FIGURE 3 is a schematic drawing showing the side view of the electrically-heated continuous tunnel kiln used in the present invention.

FIGURE 4 is a schematic drawing showing the isometric view of the electrically-heated continuous tunnel kiln used in the present invention.

FIGURE 5 is a schematic drawing showing the plan view of the Ceramic Sagger Tray
used in the present invention.

FIGURE 6 is a schematic drawing showing the elevation view of the Ceramic Sagger Tray used in the present invention.
FIGURE 7 is a schematic drawing showing the elevation view of the Ceramic Sagger Tray used in the present invention.

The above drawings are illustrative of particular examples of the present invention but are not intended to limit the scope thereof. The drawings are not to scale (unless so stated) and are intended for use solely in conjunction with their explanations in the following detailed description. In above drawings, wherever possible, the same references and symbols have been used throughout to refer to the same or similar parts, as under-
(01) - Insulated Chamber (MS painted)
(02) - Roller (Ceramic)
(03) - Support structure (MS painted)
(04) - SiC heaters
(05) - Coil Heaters
(06) - Sagger Tray (Ceramic)
(07) - ISMC Track (MS painted)
(08) - Exhaust Damper (MS painted)
(09) - Exhaust Blower
(10) - Primary Blower


Though numbering has been introduced to demarcate reference to specific components in relation to such references being made in different sections of this specification, all components are not shown or numbered in each drawing to avoid obscuring the invention proposed.

Attention of the reader is now requested to the detailed description to follow which narrates a preferred embodiment of the present invention and such other ways in which principles of the invention may be employed without parting from the essence of the invention claimed herein.

Statement of the invention
The present invention is identified in recovery of Alumina by the oxidation, at elevated temperatures, of Aluminium Nitride component of Aluminium dross residue. The recovered metal can then be reintroduced into the Aluminium production process, and the few impurities that remain can be used to manufacture calcium aluminate cement and synthetic slag for use in refractory and steel.

Detailed description
The present invention is directed at absorbing all advantages of prior art while overcoming, and not imbibing, any of its shortfalls, to thereby establish an inventive method, and its implementing system, for recovery of Alumina by the oxidation, at elevated temperatures, of Aluminium Nitride component of Aluminium dross residue.

Accordingly, the present invention focusses on improving the efficiency of Aluminium recovery from dross holistically considering the specific characteristics of the dross, process optimization, economic factors, and environmental considerations.

Process:
Dross residue is the input material, which is preconditioned, that is it is first crushed using Jaw Crusher and pulverizers. This material is then screened using a 1mm screen / sieve. All material that is above 1mm size has high Aluminium content as Aluminium metal cannot be crushed beyond a certain size. Greater than 1mm material high in Aluminium content is melted and casted into Aluminium ingots for sales. The less-than 1mm material is called Non-metallic particles (NMP) which is the input for heat treatment as will be described in the continuing part of this document.

The NMP of dross residue mainly consist of Aluminium micro fines, Alumina, Aluminium Nitride and some carryover fluxes like bath material.

Accordingly, the method of this invention is intended to be practiced by the heat treatment of Aluminium Nitride, in oxidizing conditions at a temperature range of 700°C to 1300°C. Alumina and Nitrogen are formed in this process as per reaction (1).

4AlN + 3O2 ? 2Al2O3 + 2N2 ……………………………..……. (1)

The high grade alumina (Al203) having 75% to 99% Aluminium content so produced is smelter grade alumina that can be again used as raw material input for refractory and / or smelters. The alloy-grade alumina will be used by the refractory industry. The poor grade residue is granulated in small sizes and fired along with lime to make calcium aluminate cement.
The heating step described hereinabove is intended to be carried out in equipment selected among suspended calciners, rotary kilns, fluidized bed boilers, tunnel kilns, vertical shaft kilns and the like. The temperature range required for the above-mentioned reaction to occur is in the range of 700°C to 1300 °C.

Experimental validation
The present invention has been reduced to practice by the applicant named herein, and found to be successful in meeting all of the objectives set forth in the foregoing part of this document. Reference is now made to one experimental trial conducted, with details as under-

This experiment has been performed on Grey Alumina Powder to speed up the heating rate of the product. The sample was taken in a crucible vessel for this experimental run and then placed in a Muffle furnace. The appearance of the sample was observed after treatment.

Input material was Alumina Grey powder. After processing by methodology in equipment of the present invention, Alumina White powder was obtained as an output with characteristics as under-
Description Content (%)
Al AlN Al2O3 SiO2 MgO
Grey Powder (Input) 9.2 28.1 57.2 3.3 2.1
White Powder (Output) --- --- 89.3 6.9 3.6

Industrial scale processing equipment (Electrically-heated continuous tunnel kiln)
In a related aspect, the specific equipment for the heat treatment of Aluminium dross residue is a continuous tunnel kiln, design of which, as seen in FIGURE 1 to 4, read with the reference numerals, is electrically-heated (Convection + Radiation heating process), the construction and operability of which can be understood from the following context-
1) Layout -
As can be seen in the accompanying FIGURES 1 to 4, the electrically-heated continuous tunnel kiln hereof is a conveyorized insulated oven in a straight line, having a trolley that passes through the oven. Electric coil and ceramic heaters are provided on the top as well as bottom for heating purpose.

The heating system is designed in modular sections for ease of assembly and extension if required in future. The chamber is essentially designed as a chamber within a chamber, with outer chamber made of insulated enclosure and inner chamber of reflective walls. The design is suitable for heating the object from top side.

Lengthwise, the electrically-heated continuous tunnel kiln hereof is divided into six zones. Zones 1 and 2 are provided with electric coil heaters. Zones 3, 4, 5 and 6 are provided with SiC heaters.

The ID, exhaust centrifugal indirect fan is provided at the top of the chamber for cooling products in trays. The FD, impingement centrifugal direct fan is provided which helps for cooling products in trays.

Temperature sensors are provided for sensing air temperature in the chamber at multiple, at least 8 locations. Control scheme comprises closed loop automatic control through air temperature sensors and PID controllers in 6 zones.

Power supply is 415V 3ph 4 wire system, properly earthed.

2) Support Structure -
As to the support structure, the electrically-heated continuous tunnel kiln hereof is self-supporting, and modular section of system has its own supports.

3) Roller Conveyors
Concept Continuous (Stop-n-Flow).
Type of conveyors Roller Floor Conveyors.1. Load Conveyor.2. Drying/Curing Conveyor.3. Unload Conveyor.4. Return Transfer Conveyor.
Dimensions & other details(floor conveyor) 1. Widths of conveyor: 425 mm2. Length of conveyors:Load 30000mm/Curing 19500mm/Unload 3000mm/Return 19500mm C-to-C.3. Max object size handled in mm: width 400mm.4. Working Height of conveyor: 850 + 50 mm.
Roller MOC Curing/Cooling Conveyors: Ceramic coated Steel with water cooled & rotary union provided for water circulation and piping. Load/Unload/Return Conveyor: MS Plated.
Speed variation 1. Motor Power each: 1 HP (Load), 5HP (Curing), 1HP (Cooling), 1HP (Unload), &5HP (Return).2. Motor type: TEFC.3. Type of drive: Variable Speed, AC.
Conveyor speed Variable: 0.05 MPM to 0.3 MPM. Designed speed: 0.08MPM.
Saggar Trays. Ceramic trays are provided.
MOC Corundum Mullite Ceramic

4) Insulation panels –
The insulation panels are mounted on an outer structure and are hinged for facilitating opening the same for maintenance purposes. The inner insulation is lined with 300mm thick ceramic refractories. Painted MS is the material of construction for outer side of panels.

5) PLC Control Panel
Type MS powder coated self-standing cabinet.
Mains Switch Through MCCB.
Interlock Through zone contactors.
Distribution No. of zones: 6 nos. Zone switch ON through individual contactors provided for each zone.
Fan Switch on &protection Overload relay & single phasing protection provided for fans/ blowers
PID Controller Make: Reputed. Features: Universal input / process output / Hi-lo alarm
Safety features • Interlock provided of Heaters with conveyor motors.• Interlock provided of Heaters with fan motors.• Synchronizing of all conveyor motors for start-stop and speeds.• Excess temperature interlocks in all zones.• Trays sensing at various locations for transfer.• Emergency switch off.
Cabinet cooling Through an axial fan
Cable entry From bottom

6) Air Handling Unit.
Purpose Air impingement, proper exhaust and cooling.
Primary Exhaust Blower 1. The exhaust blower is provided at the entry of the curing chamber which help toextract out the moisture/volatiles from zone-1.2. Type: Centrifugal Blower.3. Motor: 1HP/1440 RPM.4. Qty: 01No.
Exhaust Blower 1. This is provided to extract thermal heat from the trolleys.2. Type: Centrifugal Blower indirect.3. Motor: 1HP/1440 RPM.4. Qty: 01No.
Cooling Blower 1. For rapid cooling, centrifugal blower is provided in the cooling chamber.2. Qty: 01No.
Actuated dampers Provided.

The electrically-heated continuous tunnel kiln used in the implementation of this invention operates continuously, being designed for the continuous firing / heat treatment of preconditioned Aluminium dross residue input. Basic Features of this electrically-heated continuous tunnel kiln are as listed in Table below.

Installed Heating Load 180 KW.
Heating Source Electric coil heaters / SiC heaters.
Supply voltage 415V, 50Hz, 3Phase 4 wire supply with earthed.
Overall dimensions in mm Length Width Height
Zone-1 2500 1100 1100
Zone-2 2000 1100 1100
Zone-3 2000 1100 1100
Zone-4 2000 1100 1100
Zone-5 2000 1100 1100
Zone-6 2000 1100 1100
Cooling zone 3000 1100 1100
Overall space requirement 21500 4000 2500

Process parameters for operation of the electrically-heated continuous tunnel kiln are as listed in Table below.

Object description Alumina Clay Powder.
Powder Size Course & Fine Powder.
Density Estimated 1500-1750 kg/m3.
Specific Heat Capacity Not known.
Initial Moisture Estimated 3-7%.
Inlet Temperature 30-35oC.
Design Temperature 1100°C.
Temperature Profile Settable in six zones independently.
Outlet Temperature AT + 15oC (Approx. 50-55oC).
Throughput 2-3 TPH.
Residence Time Variable.
Working Hours 24hr

The above kiln offers advantages in terms of efficiency, uniformity, and productivity and is particularly suitable for large-scale production where a constant and controlled heat treatment process is essential.

Industrial applicability
From the foregoing narration, an able methodology for recovery of smelter grade or alloy-grade Alumina by the oxidation, at elevated temperatures, of Aluminium Nitride component of Aluminium dross residue is thus provided with marked novelty, inventive contribution, and industrial applicability than any background and / or prior art. The recovered metal can then be reintroduced into the Aluminium production process and / or used in refractories.

The present invention is directed not only to conservation of natural resources (Aluminium in particular) but also reduces the environmental impact associated with the disposal of industrial waste.

Furthermore, the residue from the aforementioned process (Al2O3 & few impurities) can be used to manufacture calcium aluminate cement and synthetic slag for use in refractory and steel.

As the reader shall appreciate, the parameters of any recycling method depends on factors such as the composition of the dross, the desired end product, and economic considerations. Additionally, environmental regulations and sustainability goals may influence the optimization of the method propounded herein. Hence all alternations by equivalents and localizations for use-case presented are all intended to be covered by the ambit of this invention.

As will be realized further, the present invention is capable of various other embodiments and that its several components and related details are capable of various alterations, all without departing from the basic concept of the present invention. Accordingly, the foregoing description will be regarded as illustrative in nature and not as restrictive in any form whatsoever. Modifications and variations of the system and apparatus described herein will be obvious to those skilled in the art. Such modifications and variations are intended to come within ambit of the present invention, which is limited only by the appended claims. ,CLAIMS:We claim,
1] A process for recovery of Alumina from Aluminium dross residue, comprising: -

(a) Preconditioning, using Jaw Crusher and pulverizers, the Aluminium dross residue input to a non-metallic particle format having an average particle size of less than 1mm consisting mainly of Aluminium micro fines, Alumina, Aluminium Nitride and carryover fluxes, bath material in particular.

(b) Subjecting the non-metallic particle format obtained in step (a) to heat treatment under oxidizing conditions to obtain high grade Alumina and Nitrogen via the underlying reaction:
4AlN + 3O2 ? 2Al2O3 + 2N2

(c) Charging the high grade Alumina obtained in step (b) to Aluminium smelters to thus achieve recovery of pure Aluminium from input Aluminium dross residue, with poor grade residue as a byproduct;

(d) Finely granulating and firing the poor grade residue obtained in step (c) along with lime to obtain calcium aluminate cement.

2] The process for recovery of Alumina from Aluminium dross residue as claimed in claim 1, wherein the step of preconditioning consists of crushing the Aluminium dross residue input using Jaw Crusher and pulverizers and finally sieving the crushed material using a 1mm sieve to result in a preconditioned non-metallic particle format characterized in having an average particle size of less than 1mm.

3] The process for recovery of Alumina from Aluminium dross residue as claimed in claim 1, wherein the temperature for heat treatment is provisioned in the range of 700°C to 1300 °C.

4] The process for recovery of Alumina from Aluminium dross residue as claimed in claim 1, wherein the step of heat treatment under oxidizing conditions is carried out in apparatuses selected among suspended calciners, rotary kilns, fluidized bed boilers, tunnel kilns, vertical shaft kilns, and their equivalents.

5] The process for recovery of Alumina from Aluminium dross residue as claimed in claim 1, wherein the high grade Alumina outputted has from 75% to 99% of Aluminium content.

6] The process for recovery of Alumina from Aluminium dross residue as claimed in claim 1, wherein the step of heat treatment is undertaken in an electrically-heated continuous tunnel kiln.