The beginning and the end of the aluminium value chain

Metallic aluminium does not naturally occur in nature, and it was largely unknown, virtually a mystery, until 200 years ago. The modern aluminium production using a hydrometallurgical refining process for making alumina followed by electrolysis of this mineral was first developed in 1886 and, in principle, the same technology is still used to this day. About 90% of alumina refineries in the world use the Bayer process for refining Bauxite ore. It is very efficient, but it can only be used on high quality bauxite with low content of admixtures, especially silicon. The Bayer process also generates a Bauxite Residue (BR), maybe better known as Red Mud (RM) which is a thick red-brown, high-basicity paste consisting of silicon, iron, aluminium, titanium and others. The International Institute of Aluminium estimates that since 1886 almost a billion tonnes of aluminium were produced around the world with three fourths of this amount still being in use today, of which about 35% is located in buildings and structures, 30% in electric cables and equipment and 30% in transport. Aluminium scrap is collected all over the world. In the home, it mostly consists of aluminium beverage cans. It is claimed that 1 ton of recycled empty beverage cans save 8 tons of bauxite, 4 kg of various fluorides and 14 kWh of electricity

The beginning and the end of the aluminium value chain

Metallic aluminium does not naturally occur in nature, and it was largely unknown, virtually a mystery, until 200 years ago. The modern aluminium production using a hydrometallurgical refining process for making alumina followed by electrolysis of this mineral was first developed in 1886 and, in principle, the same technology is still used to this day. About 90% of alumina refineries in the world use the Bayer process for refining Bauxite ore. It is very efficient, but it can only be used on high quality bauxite with low content of admixtures, especially silicon.The Bayer process also generates a BauxiteResidue (BR),maybe better known asRedMud (RM) which is a thick red-brown, high-basicity paste consisting of silicon, iron, aluminium, titanium and others. The International Institute of Aluminium estimates that since 1886 almost a billion tonnes of aluminium were produced around the world with three fourths of this amount still being in use today, of which about 35% is located in buildings and structures, 30% in electric cables and equipment and 30% in transport.Aluminium scrap is collected all over the world. In the home, it mostly consists of aluminium beverage cans. It is claimed that 1 ton of recycled empty beverage cans save 8 tons of bauxite, 4 kg of various fluorides and 14 kWh of electricity1. Additionally, recycling aluminium significantly reduces the negative environmental impact of ever-expanding RM landfills. As the idea of environmental responsibility is gaining more and more traction, separate household scrap recycling is becoming more and more popular around the world. How challenges related to such activity can be met will be the main topic of this paper alongside discussing new developments for alumina production without RM generation

Iron Ore Reduction with CO and H2 Gas Mixtures – Thermodynamic and Kinetic Modelling

The reduction of iron ore pellets has been studied using different techniques. Thermodynamic studies, experi-mental investigations and mathematical modelling have all been undertaken to better understand the behaviour of different pellet types in the new direct reduction process. The mathematical pellet model gives a good fit to most of the experimental conditions used in this work. There are some discrepancies between the experimental and calculated results under certain conditions, which are thought to be due to limitations in the experimental set up rather than fundamental issues in the model. The micromodel indicates that the hematite within the pellets is reduced to magnetite quickly, which in turn is reduced fairly quickly to wüstite. The reduction of wüstite to metallic iron seems to be the limiting stage in the reduction of the pellets, which is in line with what would be expected

Microscopic Study of Carbon Surfaces Interacting with High Carbon Ferromanganese Slag

The interaction of carbon materials with molten slags occurs in many pyro-metallurgical processes. In the production of high carbon ferromanganese in submerged arc furnace, the carbothermic reduction of MnO-containing silicate slags yields the metal product. In order to study the interaction of carbon with MnO-containing slags, sessile drop wettability technique is employed in this study to reduce MnO from a molten slag drop by carbon substrates. The interfacial area on the carbon substrate before and after reaction with slag is studied by scanning electron microscope. It is indicated that no Mn metal particles are found at the interface through the reduction of the MnO slag. Moreover, the reduction of MnO occurs through the contribution of Boudouard reaction and it causes carbon consumption in particular active sites at the interface, which generate carbon degradation and open pore growth at the interface. It is shown that the slag is fragmented to many micro-droplets at the reaction interface, potentially due to the effect on the interfacial energies of a provisional liquid Mn thin film. The rapid reduction of these slag micro-droplets affects the carbon surface with making deep micro-pores. A mechanism for the formation of slag micro-droplets is proposed, which is based on the formation of provisional micro thin films of liquid Mn at the interface

Behavior of Slag in Ferromanganese and Silicomanganese Smelting Process

A review of studies by Safarian et al. and Kim show that the smelting reaction at equilibrium for ferromanganese and silicomanganese alloys is defined by the coupled reaction in the carbon-saturated condition 2Mn––––+(SiO2)=2(MnO)+Si––. The behavior of slag at equilibrium is described by MnO and SiO2 as dependent variables and by non-reacting species, CaO, MgO, and Al2O3, as independent variables. Its characteristic behaviors are assessed in the pseudobinary system of MnO and SiO2 fixed by non-reacting components with analyses of ferromanganese and silicomanganese slag from one-month smelting operations. The behavior of fluid slag is defined by their melting temperature provided by phase equilibria of slag system. Liquidus of manganese slag systems by Kang et al., Zhao et al., and Roghani et al. is reconstructed in coordinates of MnO and SiO2 at fixed contents of CaO, MgO, and Al2O3. Conditions for fluid smelting slag are examined by referencing characteristic behaviors of smelting slag to liquidus of manganese slag systems to assess the effect of MgO and Al2O3. MgO facilitates fluid silicomanganese slag but would make ferromanganese slag viscous. Al2O3 makes silicomanganese slag fluid at Al2O3 content with 0.41 by weight ratio to SiO2. At higher contents of Al2O3, silicomanganese slag would be viscous with low MnO contents in slag. Al2O3 facilitates the development of fluid ferromanganese slag

The (love & hate) role of entropy in process metallurgy

Process metallurgy is the basis for the production, refining and recycling of metals and is based on knowledge of transport phenomena, thermodynamics and reaction kinetics, and of their interaction in high-temperature, heterogeneous metallurgical processes. The entropy concept is crucial in describing such systems, but, because entropy is not directly observable, some effort is required to grasp the role of entropy in process metallurgy. In this paper, we will give some examples of how entropy has a positive effect on efforts to reach the process objectives in some cases, while in other cases, entropy acts in contradiction to the desired results. In order to do this, it is necessary to have a closer look at both the entropy concept itself as well as at other functions like free energy and exergy since they encompass entropy. The chosen case is the production of silicon. It is the huge entropy change in the process that is utilized. The case is not chosen arbitrary. Indeed, it is the authors’ strong belief that silicon will be one of the foundations for the environmental and energy future planned for in the “Paris-agreement”. We will also explore relatively recent research in physics and thermodynamics that led to the description of the concepts like “dissipative systems and structures”. Dissipative systems are thermodynamically open systems, operating out of, and often far from thermodynamic equilibrium and exhibit dynamical regimes that are in some sense in a reproducible self-organized steady state. Such structures can arise almost everywhere provided this structure, feeding on low entropy resources, dissipates entropy generated in the form of heat and waste material in parallel with the wanted products/results. Examples range from metallurgical processes to the emergence of industrial symbiosis

Road-map for gas in the Norwegian metallurgical industry: greater value creation and reduced emissions

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Evaluation of Calcium Aluminate Slags and Pig Irons Produced from the Smelting-Reduction of Diasporic Bauxite

This work evaluates the characteristics of calcium aluminate slag and pig iron samples obtained from the smelting of calcined and reduced diasporic bauxite ore. The study is conducted in the Pedersen process framework, which is a method to produce alumina from low-grade resources. Parameters such as the effect of crucible type, lime addition, and atmospheric conditions are studied considering the characteristics of the product pig irons and calcium aluminate slags for further uses. The behavior of the bauxite and distribution of the species between slag and metal was assessed based on the applied analytical techniques and thermodynamic calculations. Iron was reduced and separated from the slags in the presence of carbon (graphite crucible) for both the reduced and calcined bauxite. Si and Ti were mainly concentrated in the slags. Iron was separated from the slag in the absence of carbon (alumina crucible) for the H2-reduced bauxite. The results show that slags with increased lime additions are composed mainly of 5CaO.Al2O3 and CaO.Al2O3, that are considered highly leachable compounds. An optimum CaO/Al2O3 mass ratio of 1.12 was suggested. The presence of O2 and/or OH- in the furnace atmosphere will result in the formation of 12CaO.7Al2O3

The Leachability of Calcium Aluminate Phases in Slags for the Extraction of Alumina

Alumina is used primarily as feedstock for aluminum production. It occurs naturally in bauxite and clay and other minerals, and can be concentrated in industrial by-products such as coal gangue, fly ash, blast furnace slag, etc. The hydrometallurgical treatment of bauxite to recover alumina has been widely adopted industrially since the Bayer process was first employed commercially. However, the sustainability of alumina production by this means is less than ideal, due to the high production rate of poorly utilized and highly alkaline by-product that the process yields; bauxite residue or red mud. On the other hand, digestion of alumina-containing slags produced by reduction of bauxite results in no red mud production. In this work, the leachability of binary phases of CaO and Al2O3 in slags is studied under given conditions of temperature and time. Advanced characterization techniques are used to study the chemical composition, phases and microstructure of the slags and the digestion products. It is apparent that the leachability of a phase affects that of other phases. A less leachable phase could hinder the leachability of a more leachable phase. The experimental data shows that the leaching rate of slag from highest to the lowest is CaO.Al2O3, 3CaO.Al2O3, and CaO.2Al2O3 respectively