Modern Trends in Aluminium Industry

1) Use of Metallic Inert Anodes: In the past decade, a lot of development has been made in developing metallic inert anodes. The researchers have given much attention to binary or trinary alloys with Cu being a common element in most of the alloys. Some researchers have stated that pre-oxidizing the anode materials can improve corrosion resistance. The use of low-temperature electrolytes can improve the stability of the anode oxide scale. A clean energy source is recommended to reduce greenhouse gas emissions. Considerable steps should be taken to reduce cell energy consumption. No industry has successfully launched an inert anode for aluminium electrolysis yet. Future will show whether the inert anodes can ever replace the consumable carbon anode used for aluminium reduction. The inert anodes that have been studied are:

• Fe – Ni Alloy (68% Fe – 32% Ni)
• Fe – Ni – Al alloys (57.9% Fe – 38.2% Ni – 3.9% Al)
• Cu – Ni – Fe Alloys (52% Cu – 30% Ni – 18% Fe)
• Ni – Fe – Co alloys (50% Ni – 20% Fe – 30% Co)
• Cu – Al alloys (90% Cu – 10% Al) with W cathode.

2) Use of Cermet Inert Anodes:- It was observed during the experiments on Metallic Inert Anodes that if the Metallic Anodes are pre-oxidized at 600 to 8000C they give better performance in terms of anode dissolution as well as current efficiency. This has encouraged the use of Cermets (a class of heat-resistant materials made of ceramic and sintered metal). e.g.

• The (52Cu – 30Ni – 18Fe) – xNiFe2O4 cermet showed excellent corrosion resistance.
• Use of Fe – Ni Cermet Inert Anodes with 10 to 30% NiO
• Thermal spraying of Metallic Inert Anodes with (Co – Ni)O powders.

3) Elysis Technology – Rio Tinto & ALCOA have done a lot of research on Inert Anodes & are ready with a commercial level Aluminium Smelting plant.

4) CO2 evolved can be successfully captured and electrochemically converted to carbon and oxygen using Li – Na – K carbonates, a Ni cathode and a SnO2 inert anode at 500 0C.

5) Control of Fluorine Emissions by Dry Scrubbing Method – Alumina can play a major role & a very important one. Alumina is used to capture fluoride emissions from the cells by anode gas cleaning, by use of the Dry Scrubbing method. Alumina powder adsorbs the hydrogen fluoride (HF) gas evolved, and it also entraps fluoride condensates, mainly particulate sodium tetrafluoroaluminate (NaAlF4). The resulting alumina is called secondary alumina and is then used as feed material to the cells. The cleaned exhaust gas, containing CO2 and smaller amounts of perfluorocarbon gases, is then sent for CO2 capture.

6) Measures to control power consumption:

• Larger anodes and/or larger and modified anode stubs and yoke
• Slotted anodes for better gas bubble drainage, reducing the anode effect
• Better anode rodding procedures to minimize external voltage drops
• Changes in current collector bar design and larger dimension (use of copper in the bars)
• The casting of cathode bus bars instead of ramming to obtain better contact resistance
• Modification of sidelining from carbon to SiC
• Better sidelining and steel shell ventilation
• Improved magnetic field compensation
• Conductor redesign and making a trade-off between voltage reduction and heat dissipation decrease. (Courtesy Blog of AlCircle)
• Reducing cell voltage & increasing current efficiency
• Vedanta Ltd. has proposed to reduce power consumption by adopting the following measures:

i. Change in Geometry of collector bar (Energy Saving Kwh/MT 76.8)
ii. Change in % of graphite in cathode block (Energy Saving Kwh/MT 60.8)
iii. Change in % of graphite in ramming paste Energy Saving Kwh/MT 32.0)
iv. Reduction of the stub to carbon mV drop from 135 to 79 mV.
v. Magnetic Field Compensation (Energy Saving MWh/Annum 324000 Mwh)

7) Utilization of Red Mud & other waste materials:

The key to solving red mud stockpiling is to develop a comprehensive utilization technology that consumes red mud or converts it into a secondary resource. Since the 1950s, scientists have carried out research projects that explore the disposal and utilization of red mud, according to the unique physical and chemical properties of red mud.

(i) Recovery of Components in Red Mud – Red mud primarily contains elemental components such as Fe2O3, Al2O3, SiO2, CaO, Na2O and K2O. Besides, it also contains other components, such as Li2O, V2O5, TiO2 and ZrO2. For instance, the content of TiO in red mud produced in India can be as much as 24%. Because of the huge amount of red mud, value elements like Ga, Sc, Nb, Li, V, Rb, Ti and Zr are valuable and abundant secondary resources. Therefore, it is of great significance to recover metals, especially rare earth elements & TiO2 from red mud.
(ii) Production of Construction Materials from Red Mud such as cement, bricks, roofing tiles and glass ceramics. The bulk production of building materials could eliminate the disposal problem.
(iii) Production of geo-polymers & clay-based products,
(iv) Utilization of Red Mud as filling material: High-grade road base material using red mud from the sintering process is promising, which may lead to large-scale consumption of red mud.
(v) Utilization of Red Mud as filling material in Mining:- A new technology named “pumped red mud paste cemented filling mining” has been developed by the Institute of Changsha Mining Research in cooperation with the Shandong Aluminum Company. They mixed red mud, fly ash, lime and water in a ratio of 2:1:0.5:2.43, and then pumped the mixture into the mine to prevent ground subsidence during bauxite mining. The tested 28-day strength can reach 3.24 MPa. This technology is a new way not only for the use of red mud but also for non-cement cemented filling, successfully resolving the problem of mining methods and can effectively reduce the filling costs, increase the safety factor of the stop and increase the comprehensive benefits of mining.
(vi) Utilization of Red Mud in Plastics:- For PVC (polyvinyl chloride), red mud is not only a filler that has a reinforcing effect but is also an efficient and cheap thermal stabilizer, providing the filled PVC products with the excellent anti-ageing property. Its lifetime is 2 to 3 times that of ordinary PVC products. At the same time, the fluidity of red mud is better than other fillers, which makes it plastic with good processing properties. And the red mud + PVC composite plastics have fire retardant properties and can be made into red mud plastic; solar water heaters and plastic construction profiles
(vii) Utilization of Red Mud in Wastewater Treatment:- Red mud presents a promising application in water treatment for removal of toxic heavy metal and metalloid ions, inorganic anions such as nitrate, fluoride and phosphate, as well as organics including dyes, phenolic compounds and bacteria.
(viii) Utilization of Red Mud in Soil Improvement:- Red mud has a favourable environmental repair effect on the soil that has been contaminated by heavy metal elements [105]. One of the explanations for the mechanism is that red mud can absorb heavy metal ions such as Cu2+, Ni2+, Zn2+, Pb2+, Cd2+, Cr6+, Mn4+, Co3+ and Hg2+ in the soil; the form of heavy metal ions changes from exchangeable ions into bonding oxides. Another mechanism is the precipitation reaction of carbonate in red mud with the heavy metal ions, and that causes these ions to deposit. In turn, the activity and reactivity of heavy metal ions in the soil are reduced, microbial activity and plant growth are promoted.
(ix) Utilization of Red Mud for Treatment of Waste Gas Containing Sulphur:- Activated red mud produced by drying and roasting, can be used for the absorption of SO2 gas. The desulfurization rate is initially 100% and is still as high as 94% after 10 cycles.
(x) Utilization of Red Mud as a Coagulant, Adsorbent & Catalyst: Red mud can also be employed as catalysts for hydrogenation, hydrodechlorination and hydrocarbon oxidation. It has also been studied as support in catalytic wet oxidation of organic substances present in industrial wastewaters. The use of red mud as a catalyst can be a good alternative to the existing commercial catalyst. Its properties such as iron content in form of ferric oxide (Fe2O3), high surface area, sintering resistance, resistance to poisoning and low cost makes it an attractive potential catalyst for many reactions. The red mud is to be used as an adsorbent, catalyst, ion-exchanging substance and clarifying substance particularly concerning the catalytic cracking, decolorization of hydrocarbon, clarification of waste gas and adsorption processes.
(xi) Separation of TiO2 from high TiO2 containing Red Mud:- Some of the Indian Bauxites (especially the Bauxite Deposits of HINDALCO) have 10 to 12% TiO2. Therefore the Red Mud produced from these Bauxites contains 18 to 20% TiO2. The author of this article has developed a process for the separation of TiO2 from such Red Muds. This new technology named “Modified Bayer’s Process” is free from Carbon Emissions as it does not involve any drying, roasting or carbo-thermic reduction of Red Mud. The residue generated after primary leaching of Red Mud contains as high as 60 to 65% TiO2, making it the richest resource of Titanium. The other oxides such as Fe2O3 & Al2O3 are also recovered in this process.
(xii) Some other Uses of Red Mud:- A novel process for making radiation-shielding materials utilizing red mud has been developed by adopting a ceramic-chemical processing route using phosphate bonding. Efforts were made to utilize red mud for developing plasma spray coatings (ceramic and cermets) on metal substrates, stainless steel, mild steel, Cu and Al employing thermal plasma. Building Material and Technology Promotion Council of India (BMPTC) has produced composite from red mud, polymer and natural fibres, called Red Mud Jute Fibre Polymer composite (RFPC), to replace wood in the wood-based panel products in the building industry.

Futuristic Trends in Aluminium Industry

1) Electrolysis of anhydrous fused Aluminium Chloride: The electrolysis of the chloride= and the chlorination of the low-grade bauxite ores’ offer the greatest promise. Given the availability of cheap chlorine, as a byproduct of the alkali industry, it was considered worthwhile to carry out studies on bauxite ores of high alumina content but also of high iron and titanium & silica contents.
The chloride process was developed by Alcoa from the 1960s and into the 1980s. Aluminium chloride is dissolved in a molten salt bath of Alkali & Alkaline Earth Metal and subjected to electrolysis using graphite electrodes, which in this process are inert.
Of the various processes thought of, the following are considered to be the most significant and leading producers of aluminium metal like the ALCOA, ALCAN, Nippon Light Metal etc. have shown considerable interest. (i)Pechiney-Alcoa process (ii) Alcan process or sub-halide process (iii) Toth process and (iv) chloride electrolysis.
Out of the four processes mentioned above, the last mentioned one is considered to be the best and attracted the eyes of aluminium producers very much.
Although the process has suffered initial setbacks & met with failures, likely, it would again attract the attention of Aluminium Manufacturers in near future, due to the following reasons:

• Substantially lower working temperature (700°C) compared to Hall Heroult cell (980°C).
• Relatively higher current densities could be applied since the critical current density for the anode effect is fairly high. By this, the throughputs per cell can be considerably increased thereby reducing the capital costs.
• It does not require a consumable carbonaceous anode which for Hall- Heroult’s process accounts for more than 7 % of the total cost.
• Added freedom one can get in the choice of raw materials. Low-cost non-metallurgical-bauxite ores can also be beneficially used.
• No environmental pollution is involved.
• The chloride process would be the most energy-efficient because it is operated at the lowest temperature.
• Near theoretical power, efficiencies are possible in the advanced chloride bipolar cells thereby providing a direct saving in energy costs.
• The chloride cells can survive power interruptions more easily than the Hall-Heroult cells. This is possible because the chloride cell with its high efficiency causes little excess heat generation and hence the cell is well insulated. In addition, the chloride cell has a much lower temperature liquid range for the electrolyte than the fluoride cells.
• Chloride electrolysis provides metal of superior purity. Undesirable contamination of sodium as found in Hall-Heroult metal are greatly reduced in this system.
• The decomposition potential of aluminium chloride with an inert anode is 1.8 Vat 7W°C compared to 1.2 Vat 970°C for the aluminium oxide with a consumable carbon anode. However, the operating cell voltage for the chloride electrolysis is much lower (around 3 V) compared to the Hall-Heroult cell (about 4.5 V). This is possible because of the lower polarization voltage, iR and electronic voltage drops in the case of chloride electrolysis. The chloride electrolysis claims less energy consumption to the extent of at least 30% especially when the bipolar cells are operated.

However, the scientists will have to find answers to some of these problems before the process becomes commercially successful:

1. The chlorination process is an extra step in the process, while Bayer alumina is the starting material in Hall’s cell.
2. Manufacturing anhydrous Alcl3 is a very costly process.
3. The aluminium chloride and its compounds are highly corrosive to many construction materials.
4. The high volatility of the electrolyte also poses problems for the recovery of aluminium chloride from the fumes.
5. Possibilities are there for the formation of phosgene and other poisonous gases during the chlorination of alumina.

2) Use of Hydrogen for Reduction of Al2O3:- Theoretical considerations based on published thermodynamic data show that condensed aluminium cannot be formed by direct reaction between hydrogen and alumina. Nevertheless, laboratory experiments and observations reported in the literature have led to the hypothesis that hydrogen dissolved in molten aluminium can reduce alumina to aluminium at high temperatures (700–1,700°C). The use of Hydrogen plasma has also been tried at laboratory levels.

3) Reduction of Aluminium Chloride using Hydrogen & Anhydrous Ammonia:- This author has proposed & patented a similar process, where it has been hypothesized that if hydrogen & ammonia are dissolved in molten aluminium at 10000 K & pressure of 2 Kg/cm2 & Aluminium Chloride is injected in it, the reduction of AlCl3 by Hydrogen can be achieved, with anhydrous NH3 neutralizing the resultant HCl to produce sublimable NH4Cl. The overall reaction can be written as:-
2AlCl3 + 3H2 + 6NH3 ——— 2Al + 6NH4Cl

4) Magnesio-thermic Reduction of AlCl3 &/or Al2O3:- This author has also proposed & patented a process for Magnesio-thermic reduction of AlCl3 by conducting the electrolysis of a molten bath consisting of Alkali Metal (NaCl & KCl) chlorides & Alkaline Earth Metal Chlorides (CaCl2 & MgCl2) in a Twin Chamber Electrolytic Cell, where the Anode Chamber holds the electrolyte with Graphite Anode & Molten Aluminum encased in a graphite lined Cathode Chamber acting as Cathode. Both the chambers are separated by a suitable semi-permeable membrane. The composition of the bath of molten electrolyte is maintained to produce predominantly Mg++ ions which migrate to the cathode chamber through the semi-permeable membrane & dissolve in the molten Aluminium to form Al-Mg alloy. The operating voltage of the cell is 2 to 2.5 V. AlCl3 is injected in the Cathode chamber, to achieve its magnesio-thermic Reduction. The reaction can be summarized as:-
2AlCl3 + 3Mg++—— 2Al + 3MgCl2
The MgCl2 is used again as electrolyte & Cl2 produced at the Anode is used again for carbo-chlorination & only the Al formed in the reaction is tapped periodically, keeping the critical volume of molten Al in Cathode Chamber constant.

5) As Aluminium Smelting contributes greatly towards GHG emission, there is a thought which is being put forward is that Al should be produced only from the Hydro-Electric Power or renewable energy resources & the production of Aluminium using coal, oil or natural gas-based thermal power should be banned.
In this connection, the suggestion is that the three major Primary Aluminium Manufacturers in India like NALCO, Vedanta & HINDALCO which are mainly located in Odisha, should jointly enter into Hydro Power Generation using the waters of Mahanadi & Hirakud dam, instead of expanding their thermal power generation capacities.

(Note: This content is a concise summary of various papers published so far on the subject matter as well as the own research of the author)

Mr Shreekant Kulkarni, CEO of Sejal Techno Services, offers Metallurgical Consultancy. He is an alumnus of VNIT, Nagpur (India) 1982 and also an MBA pass out from KSBM, Mumbai. He got 3 patents for Green Aluminium Smelting with Green Hydrogen and two patents for Modified Bayer’s Process for extraction of TiO2 from Titaniferous Red Mud.