G. Valentin

Direct electrochemical preparation of solid potassium ferrate

Abstract Mixed KOH–NaOH solutions were shown to allow the production of solid potassium ferrate in an electrochemical cell, due to the low solubility of the key-salt. Potassium ferrate was produced at 12 g l −1 with current yields of approximately 20% by dissolution of a cast iron anode with 2.8% Si, immersed in a 400 g l −1 KOH, 400 g l −1 NaOH solution. The ferrate produced can be directly separated by conventional solid–liquid techniques, and the recovered solution with a low ferrate content can be reused for further electrochemical runs.

Electrodeposition of zinc-nickel alloys from ammonia-containing baths

This paper describes the use of ammonia-containing baths for Zn–Ni alloy electrodeposition. Buffering properties of the ammonia/ammonium couple limit the local change in pH in the vicinity of the electrode surface caused by simultaneous hydrogen evolution. In addition, it is shown that the divalent zinc and nickel species exist in the form of Zn(NH3)42+ and Ni(NH3)62+ complexes over a large pH range. The electrochemistry of the deposition at pH 10 was investigated by galvanostatic experiments and cyclic voltammetry, and compared with deposition from ammonium chloride baths at pH 5. The Ni content in the alloys were found to be 40–60% higher from the ammonia-containing bath than from the acidic baths. Reduction of divalent ions and hydrogen evolution were shown to occur at potentials 250 mV more cathodic than with baths at pH 5; the deposition mechanism may be affected by complexation of the metal cations by ammonia.

Electrochemical ferrate generation for waste water treatment using cast irons with high silicon contents

This paper deals with the electrochemical preparation of ferrate in 15 M NaOH media, with a view to treatment of waste waters. Grey cast irons with high silicon contents were shown to allow current yields in the range 20–40% depending on the applied current density, up to 34 mA cm−2. Ferrate solutions with contents up to 0.08 M could be produced in a divided cell of simple design, and provided with flat or packed-bed electrodes. Deactivation of the anode surface was shown to be of moderate significance for hour-long runs. The ferrate produced was tested for treatment of industrial liquid wastes: coagulation efficiency of the suspended matter was comparable with that of an electrocoagulation process with sacrificial Al anodes. The potential of waste water treatment by addition of ferrate is discussed.

R-curve behavior and roughness development of fracture surfaces

We investigate the idea that the fractal geometry of fracture surfaces in quasibrittle materials such as concrete, rock, wood and various composites can be linked to the toughening mechanisms. Recently, the complete scaling analysis of fracture surfaces in quasibrittle materials has shown the anisotropy of the crack developments in longitudinal and transverse directions. The anomalous scaling law needed to describe accurately these particular crack developments emphasizes the insufficiency of the fractal dimension, usually used to characterize the morphology of fracture surfaces. It is shown that a fracture surface initiating from a straight notch, exhibits a first region where the amplitude of roughness increases as a function of the distance to the notch, and a second one where the roughness saturates at a value depending on the specimen size. Such a morphology is shown to be related to an R-curve behavior in the zone where the roughness develops. The post R-curve regime, associated with the saturation of the roughness, is characterized by a propagation at constant fracture resistance. Moreover, we show that the main consequence of this connection between anomalous roughening at the microscale and fracture characteristics at the macroscale is a material-dependent scaling law relative to the critical energy release rate. These results are confirmed by fracture experiments in Wood (Spruce and Pine).

Anomalous roughening of wood fractured surfaces

Scaling properties of wood fractured surfaces are obtained from samples of three different sizes. Two different woods are studied: Norway spruce and Maritime pine. Fracture surfaces are shown to display an anomalous dynamic scaling of the crack roughness. This anomalous scaling behavior involves the existence of two different and independent roughness exponents. We determine the local roughness exponents z loc to be 0.87 for spruce and 0.88 for pine. These results are consistent with the conjecture of a universal local roughness exponent. The global roughness exponent is different for both woods, z51.60 for spruce and z51.35 for pine. We argue that the global roughness exponent z is a good index for material characterization. @S1063-651X~98!00512-1#

Scaling of crack surfaces and implications for fracture mechanics

The scaling laws describing the roughness development of crack surfaces are incorporated into the Griffith criterion. We show that, in the case of a Family-Vicsek scaling, the energy balance leads to a purely elastic brittle behavior. On the contrary, it appears that an anomalous scaling reflects an R-curve behavior associated with a size effect of the critical resistance to crack growth in agreement with the fracture process of heterogeneous brittle materials exhibiting a microcracking damage.

Electrodeposition of Metal Iron from Dissolved Species in Alkaline Media

The electrodeposition of metal iron from iron dissolved species in alkaline media has been investigated. Dissolved ferric species in equilibrium with hematite (α-Fe 2 O 3 ) have been electrochemically identified and their reduction to iron was demonstrated. The reduction efficiency was poor, however, because of the low concentration of dissolved matter (2.6 X 10 -3 M). In order to determine more precisely the electrochemical features of the deposition reaction from iron ions, more concentrated solutions at 1.9 X 10 -2 M have been obtained using an iron anode as the ion source. Voltammetric and chronoamperometric investigations using a rotating disk electrode revealed that such concentrated solutions contain ferric and ferrous species, with higher concentration of the trivalent form. Metal can be deposited with higher current efficiency in these concentrated solutions with less than 30% of the current spent in hydrogen evolution.

Recovery of zinc and nickel from electrogalvanisation sludges using glycine solutions

Abstract The paper deals with a novel technique for metal recovery from electrogalvanisation sludges using glycine. Zinc and nickel contained in these sludges can be recovered by leaching with glycine and upon addition of sodium hydroxide: high efficiency and selectitivity were obtained from other metal species e.g. iron or calcium. ZnNi alloy deposition was investigated with both synthetic media and solutions prepared by leaching samples of industrial sludges at pH 8. The electrolytic treatment of the solutions prepared was carried out in a batch reactor operated in recycle mode. Zinc was found to deposit with current efficiencies of approx. 50%, hydrogen evolution competing with the deposition. More than 90% of the zinc contained was recovered at the cathode. Besides, nickel deposition appeared to be inhibited by glycine and the efficiency of the overall technique was quite poorer for nickel than for zinc.

Size effect in fracture: Roughening of crack surfaces and asymptotic analysis

Recently the scaling laws describing the roughness development of fracture surfaces were proposed to be related to the macroscopic elastic energy released during crack propagation. On this basis, an energy-based asymptotic analysis allows us to extend the link to the nominal strength of structures. We show that a Family-Vicsek scaling leads to the classical size effect of linear elastic fracture mechanics. On the contrary, in the case of an anomalous scaling, there is a smooth transition from the case of no size effect, for small structure sizes. to a power-law size effect which appears weaker than the linear elastic fracture mechanics one, in the case of large sizes. This prediction is confirmed by fracture experiments on wood.

Mass and momentum transfer enhancement due to electrogenerated gas bubbles

The effect of electrogenerated gas bubbles with simultaneous bulk liquid flow on the mass and momentum transfer at a wall of an electrolytic cell is experimentally determined. The local mass transfer coefficient and electrolyte shear stress are obtained using two types of microelectrodes imbedded in the channel wall. The influence of the most important parameters (electrolyte velocity, position along the wall, gas electrogeneration rate) on the transfer enhancement is studied and an analogy between mass and momentum transfer in the presence of bubbles is clearly demonstrated from the experimental results. The comparison with classical correlations, valid for systems involving natural turbulence, shows the higher energetic efficiency of devices where the turbulence is artificially generated by electrolytic gas bubbles.