Electrokinetic remediation of copper mine tailings: Implementing bipolar electrodes

Important process parameters to optimize in electrokinetic soil remediation are those influencing remediation time and power consumption since these directly affect the cost of a remediation action. This work shows how the electrokinetic remediation (EKR) process could be improved by implementing bipolar electrodes in the porous material. The bipolar electrodes in EKR meant two improvements: (1) a shorter migration pathway for the contaminant, and (2) an increased electrical conductivity in the remediation system. All together the remediation proceeded faster with lower electrical resistance than in similar experiments but without the bipolar electrodes.The new electrokinetic remediation design was tested on copper mine tailings with different applied electric fields, remediation times and pre-treatment. The results showed that the copper removal was increased from 8% (applying 20 V for 8 days in sulphuric acidified tailings) without bipolar electrodes to 42% when bipolar electrodes were implemented.Furthermore, the results showed that in this system sulphuric acid addition prior to remediation was better than citric acid addition. In addition, applying a too strong electric field (even with bipolar electrodes) could cause a severe polarization (e.g. a high electrical resistance) in the remediation system.     

Alkali-silica reaction: A method to quantify the reaction degree

We propose a new chemical method for quantitative measurement of the reaction degrees of the alkali-silica reaction (ASR). We apply this method to a crushed natural reactive aggregate kept in contact with an alkaline solution, lime saturated by an appropriate amount of portlandite. This chemical system is designed to model the concrete capillary pores alkaline solution, in contact with reactive aggregates. Two reaction steps are taken into account in the mechanism: formation of Q₃ sites made by breaking up siloxane bonds of the reactive silica and dissolution of these Q₃ sites. The dissolution degree is measured by a selective acid treatment, and the nature of silica into solution is characterised by liquid NMR spectroscopy. The remaining silica is composed of Q₄ tetrahedrons and Q₃ protonated sites identified by solid NMR spectroscopy. These Q₃ protonated sites are measured by thermogravimetry analysis. We show that the formation Q₃ sites prevails on dissolution as the reaction progresses and contributes to an internal silica gel generation. The limiting step is the siloxane breaking up. Petrographic observations show that the reaction front penetrates in the aggregate through its porosity.     

Nitrogen adsorption on carbonaceous materials: A comparison between static and dynamic methods

The purpose of this study was to investigate the influence of the method of adsorption of N₂ at − 196 °C on the isotherm obtained for, and hence derived textural parameters of, a wide series of carbonaceous materials (CM). Two pyrolyzed products, six activated carbons and two carbon blacks were used. The carbonized products were prepared by pyrolysis of cherry stones at 600 or 900 °C in nitrogen atmosphere (P-600, P-900). Three activated carbons were made by activation of P-600 at 275 °C in air and of P-900 at 850 °C in carbon dioxide or steam, whereas the remaining CM were commercial products. The adsorption isotherms for N₂ at − 196 °C were determined by static and dynamic methods in Quantachrome equipments. The CM were further characterized texturally by means of mercury porosimetry and helium and mercury density measurements. Because of the presence of helium in the adsorptive gas stream, the adsorption of nitrogen noticeably decreases for the CM containing micropores obstructed with tarry products (i.e. P-600 and the activated carbon prepared from it by air activation). For the rest of the activated carbons the adsorption increases, as they must possess narrow micropores having easier access to N₂ at − 196 °C. Helium causes a decrease in the degree of interaction between the nitrogen molecules in the gas stream and as a result the diffusion of nitrogen in pores of the adsorbent increases. For the carbon blacks, however, helium hardly affects the adsorption of nitrogen, except for at high relative pressures of this gas. Helium also influences the capillary condensation phenomenon occurring in mesopores. The variation percentages in the micro- and mesopore volumes are as high as 20 and 50, respectively. Such percentages as a rule are higher for the activated carbons.     

Rapoport-Samoilenko test for cathode carbon materials: I. Experimental results and constitutive modelling

The sodium expansion of semigraphitic commercial cathode material used for aluminium production has been measured on solid cylinder samples. Experimental results have been obtained for three current density values using the Rapoport-Samoilenko-type apparatus. A constitutive model for cathode carbon materials which is able to reproduce the relationship between the sodium expansion and time during the Rapoport-Samoilenko-type test has been proposed. Parameters required in the proposed model such as the sodium expansion after infinite time and the diffusion coefficient have been calculated from experimental data by a least-square minimization process. The distribution of sodium concentration, radial stress, tangential stress, axial stress, first principal stress and von Mises equivalent stress in the solid semigraphitic cylinder with time have been obtained.     

Study of direct type ethanol fuel cells: Analysis of anode products and effect of acetaldehyde

The anode products are observed when ethanol fuel is circulated in the direct ethanol fuel cell system using Nafion^® as an electrolyte. The main products are CO₂ and acetaldehyde. I-V characteristics of a direct type fuel cell using ethanol and acetaldehyde as fuels are investigated. Anode and cathode overpotentials are also measured to analyze the characters of the polarization curves obtained for both fuels. The MEA consisted of PtRu anode catalyst. The voltage drops as the concentration of acetaldehyde solution increases. In the case of ethanol solution, the voltage increases as the concentration increases. The anode overpotential increases as the concentration of acetaldehyde increases although the increase of cathode overpotential is smaller than that of anode overpotential. The opposite result is observed for ethanol solutions, i.e., the anode overpotential increases as the concentration of ethanol decreases. This result shows that the voltage drop observed for acetaldehyde solution results from the anode overpotential. Rotating disc electrode (RDE) measurements and polarization curve measurements were also performed to confirm the relation between acetaldehyde concentration and overpotentials. It is supposed that the electrocatalytic oxidation mechanism of acetaldehyde on PtRu catalyst is different from that of ethanol.     

Fly ash effects: II. The active effect of fly ash

This paper examines the method for determining the hydration degree of cement clinker and the pozzolanic reaction degree of fly ash in the system of cement and fly ash. In the base, the active effect of fly ash is studied. The studied results show that the active effect includes two aspects: (1) Fly ash has stronger pozzolanic activity and can react with Ca(OH)₂, and (2) it can promote the hydration of cement. When the content of fly ash is less, its pozzolanic activity can exert well, but its promoting role to the hydration of cement is weaker. When the content of fly ash is more, it is less than its pozzolanic activity can be used, but its promoting role to the hydration of cement is stronger.     

Modelling and simulations of the chemo-mechanical behaviour of leached cement-based materials: Leaching process and induced loss of stiffness

The present paper aims at modelling the decalcification process in cement-based materials and its impact on the material stiffness, which represents a serious matter in terms of long-term durability. The resistance of cementitious materials to this chemical alteration is strongly conditioned by their mineral composition and porosity. For this purpose, a multi-scale homogenization approach (Stora et al., Trans. Por. Med. 73, 3, 2008) is implemented in the numerical platform ALLIANCES (P. Montarnal, C. Mügler, J. Colin, M. Descotes, A. Dimier, E. Jacquot, Presentation and use of a reactive transport code in porous media, Phys. Chem. Earth 32, 2007) to estimate from these data the elastic and diffusive properties of cement-based materials. The association of this homogenization model and of the integration platform, which can couple different numerical codes, then allows for evaluating the evolution of the mineral composition and of the diffusive and mechanical properties of a concrete material during chemical deterioration processes. Simulations of pure water leaching of hydrated cement pastes are performed and the consequences of this decalcification on the material's residual elastic behaviour are estimated. The numerical results are confronted with available experimental data and analyzed. The simulations of the non-linear mechanical behaviour of leached cementitious materials taking into account interactions between damage and leaching is not reported here for conciseness but can be found in another document (Stora, Modelling and simulations of the chemo-mechanical behaviour of leached cement-based materials. PhD Dissertation, univ. of Paris-Est 2008).     

Cement composition and sulfate attack: Part I

Four cements were used to address the effect of tricalcium silicate content of cement on external sulfate attack in sodium sulfate solution. The selected cements had similar fineness and Bogue-calculated tricalcium aluminate content but variable tricalcium silicates. Durability was assessed using linear expansion and compressive strength. Phases associated with deterioration were examined using scanning electron microscopy and X-ray diffraction. Mineralogical phase content of the as-received cements was studied by X-ray diffraction using two methods: internal standard and Rietveld analysis.The results indicate that phase content of cements determined by X-ray mineralogical analysis correlates better with the mortar performance in sulfate environment than Bogue content. Additionally, it was found that in cements containing triclacium aluminate only in the cubic form, the observed deterioration is affected by tricalcium silicate content. Morphological similarities between hydration products of high tricalcium aluminate and high tricalcium silicate cements exposed to sodium sulfate environment were also observed.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Electrochemical investigations of the oxidation-reduction of furfural in aqueous medium: Application to electrosynthesis

The present study concerns the electrochemical properties of furfural in aqueous medium on noble (Au and Pt) and non-noble (Pb, Cu and Ni) metal electrodes. The anodic and cathodic reactions are investigated by cyclic voltammetry on Au, Pt and Ni electrodes and during prolonged electrolyses on Pt, Pb and Cu in order to find the optimum conditions for a paired electrosynthesis. Anodic reactions are controlled by diffusion in the range of the stability of the solvent (water). Beside these limits, the gas evolution competes with the conversion of furfural. The best conditions for preparative electrooxidation (Ni anode, 0.5 M NaOH, j=0.8 mA cm⁻¹) gave furoic acid in a 80% yield and furfuryl alcohol was obtained by electroreduction in a 55% yield on Cu cathodes at pH 10 and 30 mA cm⁻².     

Water vapour sorption experiments on hardened cementitious materials: Part I: Essential tool for analysis of hygral behaviour and its relation to pore structure

This paper forms the first part of a series. In this first part, a broad range of normal and high-performance (HP) hardened cement pastes and concretes is studied under both laboratory and in-situ conditions.Water vapour desorption-adsorption experiments are carried out by means of the saturated salt solution method on very thin specimens. The effect of various parameters and in particular of the mix-composition is studied on the so-called water vapour sorption isotherms (WVSIs). It is found that the presence of aggregates does not influence the curves. Likewise, the peculiarities of HP materials are highlighted: within the high relative humidity (RH) range, significant lower water contents are measured than for normal materials, and large RH changes induce only slight variations of water content. More generally, desorption isotherms can be partitioned in several ranges, which are influenced or not by W/C: a unique partition is pointed out, valid for every material tested. Moreover, the hysteretic behaviour of the materials is investigated by plotting scanning isotherms within various RH ranges and by performing first and second desorption-adsorption cycles. Furthermore, a pore structure analysis is carried out from WVSIs. The bulk porosity accessible to water, the C-S-H ‘‘gel’’ porosity, the BET specific surface area, and the BJH pore size distribution are thus assessed, along with the C-S-H ‘‘gel’’ amount. In addition, a master curve is exhibited as regards the average adsorbed water layer thickness vs. RH, for RH ≤ 63.2%.Moisture profiles in structural elements exposed to various drying conditions in laboratory and in natural environments (RC test specimens and bridge deck) are assessed by means of gamma-ray attenuation measurements. A perfect consistence is pointed out between the water contents measured by this technique on 70- to 150-mm thick cylinders and those provided by the desorption experiments previously mentioned (on 1- to 3-mm thick specimens). The very similar moisture profiles and the high degrees of liquid water saturation recorded in HP materials, whatever the mixture, the age and the environmental conditions, confirm that these materials are weakly sensitive to environmental hygral changes within a broad RH range.     

Pyrrole nanoscaled electropolymerization: Effect of the proton

Polypyrrole (Ppy) nanowires are electrochemically synthesized using the nanochannels of a proton-modified natural zeolite clinoptilolite (HNZ). The synthetic inorganic structures Y zeolite and clay montmorillonite (M) in acid form (HY and HM) are also employed for pyrrole polymerization. The generation of Ppy with electrochemical activity is favored in a strongly acidic nanoscaled environment, in comparison with the polymer synthesized in the hosts without previous proton modification. It is proposed that the reduction/oxidation responses are a consequence of the polymer protonation/deprotonation where the H⁺ ions possibly act as charge transfer promoters. The well defined redox signals of Ppy included in HNZ, compared to the HY and HM, suggest that the polymeric units are held more tightly to the channels walls due to its complex framework. The TEM image of HNZ-Ppy reveals the presence of the polymer nanowires inside the natural zeolite framework. It is also found that the one electron oxidation process do not depend on first order monomer concentration but lower (0.38) for Ppy growth in HNZ, which is related to the spatial restriction of the host.     

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet.The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions).A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.     

Experimental study of cement grout: Rheological behavior and sedimentation

Three basic elements (cement, water and admixture) usually make up injectable cement grouts used for prestressed cable coating, repair and consolidation of masonry, soil grouting, etc. The present study was divided into two parts. First, in order to characterize rheologically fresh cement paste with water/cement ratios (W/C) varying between 0.35 and 1, an experimental study was carried out and has revealed that the cement past behaves like a shear-thinning material, whatever is the W/C ratio. Second, to study the time evolution of their density, a γ-densitometer bench was used. Relying on the water content and the density measured, we demonstrate that the computation of the degree of hydration of cement is possible.The cement/geotechnics interdisciplinary approach proposed here has made it possible to obtain a large range of original results useful to improve our understanding of the sedimentation processes for cement pastes with different W/C ratios.     

Physico-chemical properties of low-rank coals: Thermal and demineralisation effects

Five low-rank coals, including leonardites and lignites, have been physico-chemically characterised. The effects of the carbonisation and demineralisation of some coals on mass and physical properties have also been studied. The characterisation was carried out by the chemical analyses of the coals and also by Fourier transform infrared (FT-IR) spectroscopy, gas adsorption (N₂, −196 °C; CO₂, 0 °C), mercury porosimetry, and density measurements. The contents of moisture, volatile matter, oxygen, carboxylic acids, and carbon-carbon double bond-containing structures are higher in leonardites than in lignites. The degree of development of surface area and microporosity is small for all coals. All samples appear to possess constrictions in micropores. The mesopore volume is significantly higher for a number of coals. The carbonisation and demineralisation of the coals produce a marked increase in the open porosity.     

Characterization of the ASR rim: Application to the Potsdam sandstone

Chemical properties of the reaction rim associated with alkali-silica reaction (ASR) were investigated using microprobe and scanning electron microscope (SEM). The studied aggregate is the Potsdam sandstone, a Cambrian siliceous sandstone well known for its reactivity. This particular rock is composed of well-crystallized quartz grains surrounded by a poorly crystallized siliceous cement that is considered to be the reactive constituent. Research was conducted on laboratory concrete specimens having reached various expansion levels and on some samples taken from an ASR-affected dam. Results indicate that the dark rim surrounding reactive particles is mainly composed of silica. This suggests that the reaction rim is formed by the precipitation of dissolved silica. Some alkalis and calcium were detected inside the thin intergranular joints in concentration ranging from 1% to 10%. These ions come from the cement paste and play a major role in dissolving original reactive silica.     

CFD analysis of turbulence non-homogeneity in mixing vessels: A two-compartment model

A two-compartment model has been developed for calculating the droplet/particle size distribution in suspension polymerization reactors by taking into account the large spatial variations of the turbulent kinetic energy and its dissipation rate in the vessel. The two-compartment model comprised two mixing zones, namely an impeller zone of high local energy dissipation rates and a circulation zone of low kinetic energy. Computational fluid dynamics (CFD) was employed for generating the spatial distribution of energy dissipation rates within an unbaffled mixing vessel agitated by a flat two-blade impeller. A general methodology was developed for extracting, from the results of the CFD simulations, the volume ratio of the impeller over the circulation zone, the ratio of the average turbulent dissipation rates in the two zones, and the exchange flow rate between the two compartments. The effect of agitation rate, continuous phase viscosity, impeller diameter, and mixing vessel scale on the two-compartment model parameters was elucidated. The two-compartment model was then applied to a non-homogeneous liquid-liquid dispersion process to calculate the time evolution of the droplet size distribution in the mixing vessel. An excellent agreement was obtained between theoretical and experimental results on droplet size distributions obtained from a laboratory-scale reactor operated over a wide range of experimental conditions.     

Water-entrained cement-based materials : II. Experimental observations

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].