a brief study of energy efficiency in alternative presentation

A BRIEF STUDY OF ENERGY EFFICIENCY IN ALTERNATIVE ROUTES OF EXTRACTION OF

ALUMINIUM AND MAGNESIUMA P R E S E N T A T I O N B Y• M A I N A K S A H AD E P T T O F M E T A L L U R G I C A L A N D M A T E R I A L S E N G I N E E R I N GN I T D U R G A P U R

DIRECT CARBOTHERMAL REDUCTION PROCESS FOR ALUMINIUM

• I N T H I S P R O C E S S , A C C O R D I N G T O C O C H R A N A N D F I T Z G E R A L D , A T F I R S T A L U M I N A I S R E A C T E D W I T H C A R B O N T O O B T A I N L I Q U I D M I X T U R E O F A L U M I N I U M O X I D E A N D A L U M I N I U M C A R B I D E A N D T H E R E A C T I O N I S P E R F O R M E D I N A S T A C K T Y P E R E A C T O R A T 2 0 0 0 D E G C A T U P P E R R E A C T I O N Z O N E .

• S E C O N D LY , L I Q U I D M I X T U R E O F A L U M I N I U M O X I D E A N D A L U M I N I U M C A R B I D E A R E F O R M E D W H I C H A R E T R A N S F E R R E D T O L O W E R R E A C T I O N Z O N E F O R A L E X T R A C T I O N A T 2 2 0 0 D E G C . B Y T H E I N V E S T I G A T I O N M A D E B Y T H E A L C O A A N D E L K E M , I N T H E F I R S T S T E P 5 0 % O F B O T H A L U M I N I U M O X I D E A N D A L U M I N I U M C A R B I D E W A S P R O D U C E D U S I N G T H E A D V A N C E D R E A C T O R P R O C E S S ( A R P ) R E A C T O R

• B O T H S T E P S I N V O LV E T H E G E N E R A T I O N O F C O W H I C H M A Y B E U S E D T O P R E H E A T T H E R E A C T A N T S B U T H O W E V E R , R E A C T A N T S A R E N O T H E A T E D B Y P A R T I A L C O M B U S T I O N O F C A R B O N . S O , H E A T I N G T H E R E A C T A N T S R E Q U I R E S A L A R G E A M O U N T O F E L E C T R I C I T Y.

SCHEMATIC OF HALL-HEROULT PROCESS FOR ALUMINIUM

PRIMARY ALUMINIUM ELECTRIC CONSUMPTION IN THE US

HALL-HEROULT CELL CURRENT EFFICIENCY

SCHEMATIC OF CARBOTHERMIC REACTOR FOR ALUMINIUM EXTRACTION

DIRECT CARBOTHERMAL PROCESS FOR ALUMINIUM

• The ARP includes a method for vapour recovery for recycling the aluminium and aluminium sub-oxide vapours generated.

• The rate of formation of Al4C3 from Al2O3 and Al gases in CO is controlled by diffusion of the reactant gases through the Al4C3 product layer.

• Reactions occurring at ARP (Advanced reactor process) are


• So , the first step of the reaction requires large amount of energy due to use of stack- type reactor which is electrically powered.

• However , the reactor has very less efficiency as only a minor amount of input energy is converted to electrical energy.

• So , the energy efficient technique is to go for the first step being performed at low temperature zone and the second step at higher temperature zone with two zones at different locations but at same levels in an Electric Arc Furnace with integrated shaft attachment and vacuum conditions.


• The EAF is the main reactor where liquid aluminium is produced and the shaft acts as condenser for the Al and Al2O vapours. By carrying out the process under vacuum, the formation of aluminium rich vapour will be favoured over Liquid aluminium.

• The process can also be carried out at lower temperatures, e.g at 1500oC at 10 Pa pressure.

• Experimental study may be carried out using a solar furnace to demonstrate the process. At temperatures 1027 to1727oC and pressures 350 to 1200Pa, Al (up to 19%) along with Al4C3and Al4O4.

MINTEK THERMAL MAGNESIUM PROCESS(MTMP)

• This process may be used as an alternative for Magnetherm process which requires a temperature of 1550degC in vacuum atmosphere in AC submerged arc surface.

• Magnetherm process was developed in 1950s and served as an advanced version of Pidgeon’s process(the Pidgeon process is carried out in retorts and at 1100-1200°C, where the pressure is kept near vacuum).

• In this process , magnesium extraction is carried out in an AC submerged arc furnace. The magnesium vapour is condensed in a surface condenser, largely to a solid phase.

• When the condenser is full, the operation is stopped to remove the condenser and to replace it with an empty one.

• In 1980s , the developments in DC arc furnace technology led to the development of Mintek process.

MINTEK THERMAL MG PROCESS PROCESS (MTMP)

The MINTEK process is a silicothermic process operating at atmospheric pressure and at a temperature of about 1600-1800degC.

Thus , there is a fairly high chance of more energy consumption by this process as compared to that in case of Magnetherm process

In order to minimise the energy requirements for the process , while charging magnesia or dolime into smelter, they may be kept at high temperature and also that the smelters may be integrated.

The energy efficiency of DC electric arc furnace is about 51.5% while the efficiency of coal fired furnace is only about 12%.

MINTEK THERMAL MAGNESIUM PROCESS• In this method, Condensation to a liquid phase for volatile Mg

in condenser vessel reduces the energy requirements in the cleaning and refining stages, as re-melting of the crude magnesium is not required . Besides, high condensation efficiency may also be achieved by increasing the partial pressure of Mg vapour, Improving the sealing facility of equipment to keep the air ingressed to a minimum and steady operation of plant for long

• As metal condensation is controlled by energy transfer, bath agitation contributes to the improved condenser efficiency by quickly dissipating the energy of condensation and thus maintaining a uniform temperature throughout the condenser crucible, particularly the condensing surface temperatures.

SCHEMATIC OF MINTEK PILOT PLANT

MULTI-STEP SOLID OXIDE MEMBRANE(SOM) PROCESS FOR MAGNESIUM

• In this process,reduction of magnesium oxide dissolved in fluoride-based electrolytes (MgF2-CaF2-MgO) is carried out by passing electric current at 1150 to 1300 degC. When electric field is applied, magnesium oxide dissociates into magnesium and oxygen. Ions . Oxygen ions are pumped out through Yttrium Stabilised Zirconia (YSZ) membrane to the anode. The magnesium vapour evolves at the cathode and condenses in a separate chamber. The reported energy demand is 10 kWh/kg Mg.

• The energy efficiency of the present process is attributed, in part, to the combination of two steps, one in which a metal oxide is reduced to form highly reactive metal and another in which a metal compound of interest is reduced while the highly reactive metal is oxidized. In one or more embodiments, it takes significantly less electrical energy to reduce the highly reactive metal ion and use it to produce the desired metal from the metal oxide of interest than it does to reduce the metal compound of interest to produce the desired metal in an alternate, single-step process.


Schematic of SOM process for Mg


• In the second step of the multi-step process, no substantial amount of energy is needed to reduce the metal ion of interest because the highly reactive metal in the metal compound facilitates a chemical reaction.

• In sum, less energy is used to reduce a selected amount of the metal oxide of interest in multi-step process than would be used in a comparable single-step electrolysis process.

CHEMICAL STABILITY OF ALUMINIUM,MAGNESIUM AND IRON

OXIDES THROUGH GIBB’S FREE ENERGY AND ENTHALPY OF FORMATION

CONCLUSIONS•As compared to Bayer’s and Hall-Heroult’s process , direct carbothermic production of Al using EAF with integrated shaft attachment as condensor (for Al and aluminium sub-oxide vapours) , thereby allowing only Al vapour instead of liquid Al and high vacuum atmosphere can reduce energy requirements 35% below those of the best Bayer-Hall technology.Implementation of carbothermic reduction processes in aluminium production may lead to energy savings of up to 21 %, GHG emissions reductions of up to 52 % and energy efficiency increase of up to 10%.•The energy consumption for producing metal from ore is strongly linked to the chemical stability of mineral being processed, availability and the richness of ore, and process route. The chemical stability of an oxide compound can be reflected by the Gibbs free energy while the enthalpy of formation determines the minimum energy requirement of a process. So , as Gibbs free energy of magnesium oxide is low , it indicates that magnesium oxide is highly stable and requires high energy to extract the metal. The difference between the theoretical energy requirements and the actual energy usage reflects the different aspects of the process route.

CONCLUSIONS•The Pidgeon process urgently needs improvement in order to reduce energy consumption and greenhouse gas emissions.Improvement that has been proposed includes utilisation of cleaner energy source (e.g. natural gas) for reducing GHG emissions and integrating small smelters to improve energy efficiency.•The solid oxide membrane electrolytic process has a lower Global Warming Potential compared to the Pidgeon process (47.3 kg CO2/kg Mg to 62.7 kg CO2/kg Mg), but suffers from low productivity and high capital costs.•In case of Mintek process (for Mg) charging magnesite or dolomite as hot as possible into the smelters and even integrating the smelters may lead to significant energy reduction . At the same time , condenser efficiency may be improved by increasing the partial pressure of Mg vapour ,improving the sealing facility of condenser, maintaining a uniform temperature of condenser and proper bath agitation to quickly dissipate energy of condensation.

REFERENCES• S. Namboothiri, M. P. Taylor, J. J. J. Chen, M. M. Hyland, M. Cooksey,

“Aluminium Production Options with a Focus on the Use of a Hydrogen Anode: A Review”, Asia-Pacific Journal of Chemical Engineering, Vol.2, (2007), pp. 442-7.

• I.M. Morsi, K.A. El Barawy, M.B. Morsi, S.R. Abdel-Gawad, Silicothermic reduction of Dolomite Ore under Inert Atmosphere, Canadian Metallurgical Quarterly, Vol. 14, No. 1,(2002),p.15.

• Brooks, G., Trang, S., Witt, P., Khan, M.N.H , Nagle, M, 'Carbothermic Route to Magnesium', Journal of Minerals, Metals and Materials Society, Vol.58, no. 5, p.51-55.

• R.T. Jones, Computer Simulation of Pyrometallurgical Processes, Proceedings of the Twentieth International Symposium on the Application of Mathematics and Computers in the Minerals,Industries, Vol. 2,( 1987),p.265.

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