Dimitrios Panias

Mechanisms of dissolution of iron oxides in aqueous oxalic acid solutions

The dissolution of pure iron oxides by organic acids has been extensively reviewed. The mechanism of dissolution comprises three distinct steps: (1) adsorption of organic ligands on the iron oxide surface; (2) non-reductive dissolution; and (3) reductive dissolution. Reductive dissolution involves two stages: an induction period and an autocatalytic period. The overall dissolution process is affected by the pH of the initial solution, temperature, the exposure of solution to UV radiation and the addition of bivalent iron in the initial solution.

Solubility of boehmite in concentrated sodium hydroxide solutions: model development and assessment

Abstract The Bayer process for the production of alumina consists of three main stages: bauxite digestion with sodium hydroxide solution at high temperatures and pressures, precipitation of crystalline gibbsite from the aluminate liquor under atmospheric conditions, and finally, calcination of the precipitated gibbsite. During the last decade, a new variation of the precipitation stage is under development in the Laboratory of Metallurgy, NTUA, whereby boehmite (monohydrate alumina, Al2O3·H2O) is precipitated instead of gibbsite under atmospheric conditions. Accurate knowledge of the solubility of boehmite in concentrated sodium hydroxide solutions under mild temperature conditions is necessary in order to study the boehmite precipitation conditions. So far, the solubility of boehmite in sodium hydroxide solutions has been studied experimentally in diluted sodium hydroxide solutions and at temperatures higher than 100°C. These conditions are completely different from those applied in the new boehmite precipitation process. Therefore, the aim of this work is on the one hand, the development of a theoretical model predicting the solubility of boehmite in 2–4.5 M sodium hydroxide solutions and in the temperature region of 30–150°C, and on the other hand, the experimental determination of boehmite solubility in order to assess the model validity. The speciation diagram of aluminium ions in sodium hydroxide solution has been developed taking into consideration all mononuclear and polynuclear hydroxoaluminate ions. The mathematical model comprises 21 complex equations and its development was based on the hypothesis that the only existing aluminium-bearing species in sodium hydroxide solutions at pH higher than 10 is the tetrahydroxoaluminate ion. This was deduced from the speciation diagram of aluminium ions in sodium hydroxide solutions which shows the stability regions of all the mononuclear and polynuclear hydroxoaluminate ions as a function of the pH of the solution. Although this observation significantly simplified the mathematical model, it was not possible to obtain an analytical solution and for this reason the mathematical model was solved using “Mathcad” software. Regression analysis performed on the data obtained from the solution of the mathematical model resulted in an easy-to-use regression equation relating the solubility of boehmite to the initial concentration of sodium hydroxide and the temperature. A very good agreement exists between the theoretical values of solubility of boehmite and the experimental ones. The observed deviation between the theoretical and the experimental values varies generally within the margins of repeatability of experiments ±2 g/l. As a conclusion, the theoretical model can predict with accuracy ±2 g/l the solubility of boehmite in concentrated 2–4.5 M sodium hydroxide solutions and in the temperature region 30–150°C.

UTILIZATION OF ALUMINA RED MUD FOR SYNTHESIS OF INORGANIC POLYMERIC MATERIALS

Red mud is a residue coming from the metallurgical treatment of bauxite with the Bayer process. Million of tons of red mud are produced annually worldwide and disposed of on land, degrading vast areas. Therefore, red mud utilization is a first-priority issue for any alumina plant. In the present work, the potential use of red mud for synthesis of inorganic polymeric materials through geopolymerization process was studied. The main focus was the production of inorganic polymeric materials that could be used in the construction sector as artificial structural elements such as massive bricks. The geopolymerization process involves a chemical reaction between red mud and alkali metal silicate solution under highly alkaline conditions. The product of this reaction is an amorphous to semi-crystalline polymeric structure, which binds the individual particles of red mud transforming the initial granular material to a compact and strong one. The effect of main synthesis parameters—like solid-to-liquid ratio, caustic soda as well as soluble silica concentrations, and metakaolin addition—on the properties of red mud-based inorganic polymeric materials was investigated. The results showed that the produced materials have high compressive strength, very low water absorption, satisfactory apparent density, and excellent fire resistance. Therefore, this work proved that the red mud-based inorganic polymeric materials have promising properties and have the potential to be used as artificial structural elements in the construction sector.

Removal of iron from silica sand by leaching with oxalic acid

Abstract The removal of iron from silica sand with oxalic acid has been studied under various experimental conditions in order to optimise the process parameters and reach a high degree of iron removal at minimum operating cost. The parameters studied were: temperature, pH of the solution, oxalate concentration, Ar purging, and ferrous ions addition to the solution. For the specific silica sand sample used, at temperatures varying between 90–100°C the maximum iron extraction that can be achieved is approximately 40%. At temperatures lower than 80°C this extraction is decreased to 30%. At these temperatures purging of the oxalate solution with Ar and ferrous ions addition has no effect on the iron extraction, while at temperatures lower than 25°C iron dissolution is accelerated with the addition of ferrous ions. Iron dissolution is significantly affected by the pH, while it is practically independent of the oxalate concentration and the pulp density. Without the addition of bivalent iron, iron extraction is optimised in high acid solutions; when ferrous ions are added in the oxalate solution, best results are achieved at pH 3.

Kinetics of boehmite precipitation from supersaturated sodium aluminate solutions

Abstract This work presents the effect of the most important parameters, the precipitation temperature, the sodium hydroxide concentration and the initial seed ratio (SR) in the solution, on the boehmite precipitation from supersaturated sodium aluminate solutions. A kinetic model that describes the experimental data was developed. According to that model, boehmite precipitation follows second order reaction kinetics and has activation energy of 89 kJ/mol. The orders of the precipitation reaction with respect to the initial sodium hydroxide concentration and the initial seed ratio are estimated to be −1.8 and 0.54, respectively. The most important result is that the boehmite precipitation reaches an apparent equilibrium stage at which the alumina concentration is much higher compared to the value of the boehmite solubility under the same experimental conditions. The results reveal that the boehmite precipitation is a self-decelerated process and the observed kinetic inhibitions are related to the sodium hydroxide concentration in the supersaturated solution. Finally, a mechanistic interpretation of the experimental data is presented based on the concept of the surface precipitation of boehmite on the active sites of the boehmite seed.

Dissolution of hematite in acidic oxalate solutions

Abstract The dissolution of hematite in acidic oxalate solutions has been studied under various experimental conditions. The effect of temperature, oxalate concentration and pH on hematite dissolution were studied. In order to study the effect of atmospheric oxygen and light on the dissolution reaction, experiments were carried out in an inert atmosphere (purging with argon), in an ‘oxidising atmosphere’ (without purging), in the presence of visible light and in darkness. It was found that the dissolution process is much faster in an inert atmosphere under visible light. The dissolution process in all other cases was very slow, including a characteristic induction period, attributed to ferrous ion generation in solution through a heterogeneous, time-consuming reductive pathway. In an oxidising atmosphere the dissolution process is seriously retarded due to the oxidation of ferrous to ferric ions by the dissolved atmospheric oxygen. Iron dissolution is highly dependent on temperature and pH of the solution, while it is practically independent of the total oxalate concentration.

Energy and Exergy Analysis of the Primary Aluminum Production Processes: A Review on Current and Future Sustainability

The common industrial practice for primary aluminum production consists of the Bayer process for the production of alumina followed by the Hall–Héroult process for the production of aluminum. Both processes were developed at the end of the 19th century and despite continuous optimization, their basic thermodynamic inefficiencies and environmental issues remain till today unchanged. As a result, primary aluminum production industry is the worlds larger industrial consumer of energy, is ranked among the most CO2 intensive industries, and is associated with the generation of enormous quantities of solid wastes. In this paper a detail energy and exergy analysis of the primary production of aluminum is presented and alternative sustainable processes are reviewed.

Dissolution of hematite in acidic oxalate solutions: the effect of ferrous ions addition

Abstract Studies on the dissolution of hematite in acidic oxalate solutions have shown that there is a strong relationship between dissolution rates and ferrous ion generation in the solution. The effects of temperature, pH of the solution, oxalate and ferrous ions concentration on the dissolution of hematite in acidic oxalate solutions containing ferrous ions were studied. It was found that the pH of the solution is very important for the dissolution as it affects: (1) the activation of hematite surface with adsorbed hydrogen ions; (2) the adsorption of [Fe(C 2 0 4 ) 2 ] 2- complex ions on surface active sites and (3) the speciation in oxalate solutions containing ferrous ions. Iron dissolution is highly dependent on temperature and ferrous ion concentration, while it is practically independent of the total oxalate concentration. The dissolution of hematite in oxalate solutions comprises three stages: (1) activation of the solid surface; (2) generation of ferrous ions in the solution (induction period); and (3) an autocatalytic dissolution period. When ferrous ions are added, the induction period is eliminated and dissolution proceeds through the autocatalytic pathway.

MODELING CHEMICAL EQUILIBRIUM OF ELECTROLYTE SOLUTIONS

ABSTRACT Modeling the chemical equilibrium in ionic solutions encountered in industrial applications, especially in the field of hydrometallurgy, still remains an unresolved issue. The complicated speciations, as well as the high ionic strengths encountered in these solutions, render the theories of analytical chemistry practically useless. The Debye–Hückel theory is examined in depth, so as to reveal the reasons of its failure. To remedy this problem, over the years, a large number of semi-empirical models have been proposed that are here reviewed, in order to help the reader find a model best suited for a system of interest. The models are classified into three main categories, based on their fundamental logic: ion interaction models describe the system through the physical interactions of the ions; ion association models describe the system through chemical equilibriums; and finally hybrid models use concepts from both previous categories. Focus is given in presenting the idea upon which each model is based rather than simply presenting the equations required for its implementation.

Use of Ionic Liquids as Innovative Solvents in Primary Aluminum Production

Today, the primary aluminum production is based on two processes: (a) the Bayer process and (b) the Hall–Heroult process. Both processes deal with several economic and environmental drawbacks. The production of aluminum is an energy intensive process, consuming 53–61 GJ/t of aluminum, while huge amount of red mud and gaseous emissions are inevitably produced through the whole process. The utilization of a new family of solvents called ionic liquids (ILs) in the primary aluminum production is the subject of this paper, which examines the possibility of dissolving metallurgical alumina, hydrated alumina, and bauxites in 1-ethyl-3-methyl-imidazolium hydrogen sulfate ([Emim]HSO4). The results show that hydrated alumina can be dissolved relatively easily at 210°C, forming a melt that contains 9% w/w of dissolved alumina, which is higher than the alumina content in Hall–Heroult melts. Bauxites can also be directly dissolved in this IL with iron presenting higher dissolution than aluminum, while silicon dissolution is negligible.