Leached Surface Research Articles

The leaching of chalcopyrite with cupric chloride

A comparative study of electrochemical leaching and chemical leaching of chalcopyrite was performed mainly at 343 K to elucidate the leaching mechanism of chalcopyrite with CuCl2. Also, the morphology of the leached chalcopyrite surface was studied by using a single chalcopyrite crystal. The leaching with CuCl2 produced a porous elemental sulfur layer on the chalcopyrite surface, showing a similar morphology to that produced during leaching with FeCl3. The leaching kinetics were found to be linear over an extended period, followed by an acceleration stage, as a result of an increase in the reaction surface area. The leaching rate of chalcopyrite was proportional to C(CuCl2)0.5, whereas it was inversely proportional to C(CuCl)0.5. The mixed potential of chalcopyrite exhibited a 66 mV decade−1 dependency upon C(CuCl2), and—69 mV decade−1 upon C(CuCl). Based on these observations together with other findings, an electrochemical mechanism involving the oxidation of chalcopyrite and CuCl − 2 and the reduction of CuCl+ was proposed. The Tafel plot between the mixed potential and the current density obtained by converting the rate of chemical leaching gave a straight line whose slope was in good agreement with that of the electrochemical leaching. These findings strongly support the electrochemical mechanism of chalcopyrite leaching with cupric chloride.

The use of leached gradient index anti-reflection surfaces on borosilicate glass solar collector cover tubes

When alkali-borosilicate glasses are heat treated, they can be caused to phase separate. The silica and non-silica constituents are separated on a sub-microscopic scale. This enables small amounts of the non-silica phase to be chemically leached from the surface, which results in a graded reduction in the index of refraction near the surface. The index gradient can be controlled so that it leads to the most effective broad band, antireflection (A.R.) films known. At normal incidence, average reflection per surface can be kept well below 1% in the spectral region from 0.35 to 2.5 μm. Further, films produced by this method maintain their effectiveness even at incident angles up to 70°. All these aspects; i.e. manufacturing method, spectral response and angular response, combine to make the leached gradient index A.R. treatment ideally suited to enhance the efficiency of linear trough solar collector systems. Such systems typically use borosilicate glass cover tubes of the absorber tubes to cut heat losses, but in absence of A.R. treatment, reflective losses from the cover tubes can cut system output by about 7–8%. The leached A.R. surface eliminates reflective losses almost entirely. Exposure to the elements in an industrial environment for five (5) years has resulted in no degradation of the effectiveness of the leache A.R. surfaces. While the surface is somewhat sensitive to contamination by body oils because of its porous nature, once installed, the inherent chemical durability of the nearly pure silica leads to excellent long-term performance.

Nuclear Glass Corrosion Mechanisms Applied to the French LWR Reference Glass

AbstractThe French LWR reference glass was leached under different experimental conditions. Analysis of the leachate together with electron microscopic examination of the leached surfaces led us to the following conclusions. The corrosion of the nuclear glasses is congruent at the hydration front, but only a fraction of the elements enters the solution ; the other elements contribute to the formation of the surface layer resulting in incongruent dissolution from the standpoint of the entire sample. The leaching rate in static experiments was observed to decrease sharply in time. Specific experiments were performed to assess the protective role of the hydrolyzed layer with respect to the silica solubility limit, and the predominant effect of the solubility limit was clearly demonstrated.

Control of Oxidation Potential for Basalt Repository Simulation Tests

AbstractBorosilicate waste glass durability in simulated repository environments can be assessed by use of static tests in leach vessels fabricated of the representative geomedia. Control of the oxidation potential during the test simulates a basalt repository environment. Under very anoxic conditions (i.e. at negative Eh values), the interactions between basalt and SRP waste glass in silica-saturated basaltic groundwaters are the same as those of basalt and groundwater when no waste glass is present. The lack of significant leaching of ions from the waste glass and the lack of any significant changes in either the leached surfaces of glass or basalt under anoxic conditions suggests that the components of this system are at equilibrium when oxygen is absent.

Early Stages of Film Formation During the Leaching of Radioactive Waste Glasses

AbstractThe early stages of surface alteration-layer formation were examined during the leaching of some candidate radioactive waste glasses. The composition-depth profiles of the leached surfaces were obtained with sputter-induced photon spectroscopy (SIPS) and secondary-ion mass spectroscopy (SIMS). It was observed that the formation of multiple surface layers is a general feature of the glass dissolution process. The sequence of the layer compositions depends upon the order in which solubility limits are attained in solution, the adsorption or precipitation of insoluble hydrates at the glass/water interface, and subsequent incorporation of these insoluble hydrates behind the advancing reaction front. The boundary between the surface layer and bulk glass is sharp.

Dynamic leaching of sodium‐glass capillary columns

AbstractIt is well known that removal of metal ions from the glass surface greatly improves both the thermal stability and the adsorption characteristics of glass capillary columns. Usually metal ions are removed from a glass surface by acid leaching; in particular, the static leaching procedure with hydrochloric acid is used very often for the preparation of glass capillary columns. This paper discusses the results of an investigation into dynamic leaching. Special attention is paid to those factors which affect the adsorption properties of the leached surface. The importance of the non‐specific adsorptivity of the capillary surface due to changes of surface properties induced by the leaching process is pointed out.

New Data on Ion-Induced Modifications of Aqueous Dissolution of Silicates

ABSTRACTThe ion-induced modifications of aqueous dissolution have been thoroughly investigated in several silicates (amorphous silica, quartz, soda-lime glass and albite) by applying depth profiling techniques (RBS and resonant nuclear reactions) for the characterization of leached surfaces. We show in particular that the H penetration after implantation and leaching is markedly increased when the ion dose exceeds a critical value comprised between 1013 and 1014 Pb.cm-2. This feature supports the radiation damage origin of the observed effects. Such experiments provide interesting clues about the aqueous dissolution of silicates and show that no important effect of α-decay is expected on the hydration of HLW glasses during disposal.

Leach Behavior of SRL TDS-131 Defense Waste Glass in Water at High&Low Flow Rates

ABSTRACTLeach tests carried out on SRL TDS-131 Defense Waste Class indicate that at high flow rates the controlling mechanism is simple corrosion. The matrix elements (Si, Al) are leached out at rates similar to those of the leaching of the alkalis and of boron, and the leaching process is nearly linear with time. At slow flow rates (below 1 m/yr) leaching becomes controlled by the build-up of a protective layer. Al and most of the Si remain in the leached surface layer. The leach rates decrease in the course of the test before leveling off at constant values which are almost inversely proportional to the contact time, indicating that leachate concentrations have become solubility-limited. The low concentrations observed at this stage indicate the formation of alteration products.

Surface studies on a leached sphene glass

Titanates and titanate- or titanosilicate-based ceramics are being considered as possible nuclear waste hosts in several national waste management programmes1–4, largely because of their excellent resistance to leaching. The leach resistance of titanates has been attributed to the formation of largely insoluble, titanium dioxide rich layers, formed at the surface by selective leaching of univalent and divalent cations5. We have examined the leaching of the titanosilicate sphene (CaTiSiO5) in the form of natural samples, sintered ceramics and glasses of stoichiometric sphene composition. We report ESCA (electron spin for chemical analysis) or XPS (X-ray photoelectron spectroscopy) and SIMS (secondary ion mass spectrometry) studies of leached CaTiSiO5 glasses, demonstrating the nature of leached surface layers on a titanosilicate. These results also illustrate the parallels between the leaching of sphene glasses in deionized water and the leaching mechanism proposed for titanates5.

Aqueous corrosion studies of a fluorozirconate glass

The effects of aqueous corrosion on a fluorozirconate glass containing ZrF 4, BaF 2, AlF 3, LaF 3 and LiF were investigated using soaking solution analysis, infrared transmission and reflectance spectroscopy and scanning electron microscopy. The dissolution of the glass was nearly congruent. The leached surface of the glass was covered with hydroxide crystals of Zr and Ba. The outer leached layer was thick, highly hydrated and cracked while the glass below was also hydrated and cracked indicating that no protective surface layer was formed. Drying studies of leached samples showed the appearance of an infrared absorptance peak at 7 μm (1440 cm −1) caused by bridging oxygen bonds formed during dehydration.

Surface Layer Formation on a Nuclear Waste Glass

Surface layers are a common feature of leached surfaces of borosilicate waste glasses. Layers are also observed upon weathering of volcanic glasses[l] and of silicate minerals[2]. The question of whether these layers can protect the glass against further attack by decreasing the leach rate is stïll a subject of controversy[3]. Both in geochemical work[4] and in work on waste forms [5,6], surface layers are attributed a protective function, and the stability of leached, million years old volcanic glasses[1] may be due to the presence of palagonite, a thin (≤100 μm) alteration layer, which forms in a few years but does not seem to increase in thickness after this time. The present study investigates the effects of layer formation on leaching kinetics of a borosilicate waste glass containing 20 wt.% LWR-type simulated waste oxides.

An overview of the physical characterization of leached surfaces

A summary of some of the most important parameters affecting glass corrosion is presented. Four of them are examined in greater detail. These are: solution pH, solid surface area to solution volume ratio (SA/V), solution composition, and crystallization (devitrification).

Reaction of Dehydrated Surface of Partially Leached Glass with Water

Glasses containing sodium and potassium were leached with water and the alkali was extracted by a linear relation with square root of time. The leached surfaces were then dried by different techniques, heat‐treated at various temperatures, and again attacked by water. For glasses containing Na2O, the heat‐treated preleached grains showed neither extraction behavior having a linear relation with time nor extraction by a square root‐time relation. For glasses containing K2O, the heat‐treated preleached glass grains showed a linear relation of extraction. The results were explained by a structural transformation for the sodium glasses, in which an S‐shaped sodium ion distribution at the surface layer was greatly affected by the heat treatment. For the potassium glasses no such transformation could occur since the alkali ion distribution was exponential and unaffected by temperature.

Photoelectron spectroscopy analysis of asbestos dissolution in acidic media of biological interest

Many authors have studied asbestos dissolution, in relation with physical and chemical properties and/or biological effects. Most often, the experiments have been carried out under conditions poorly related to health problems. A possible conclusion is a correlation between the reactivity of leached mineral surfaces and the cancerigenic power. In earlier works (Thomassin et al. 1977) we have studied the dissolution kinetics of chrysotile using XPS analysis of the solids, after leaching in non-stirred media by strong reagents (0.1 n oxalic acid and 6 n HCl). It appears that the two possible rate-limiting steps are: • -diffusion of Mg ++ through an external silica gel, • -chemical reaction. Complementary experiments carried out with 0.01 n HCl favor a rate mechanism involving a chemical reaction, followed by diffusion through a silica layer and a chemical reaction after some collapse or corrosion of the ordered silica gel. Using XPS, we have studied three aspects of asbestos dissolution related to biological problems: 1. 1.comparative leaching study of various asbestos minerals (chrysotile, crocidolite, amosite and anthophyllite) by 0.1 n chemical reagents from the Krebs cycle (pyruvic, citric and oxalo-acetic acids); 2. 2.study of the dissolution mechanism of chrysotile at very low concentrations of oxalic acid (10 −4 and 10 −6 n); 3. 3.study of the effects of grinding: The most soluble asbestos is chrysotile, the best of the investigated reagents being pyruvic acid, which is not so active as oxalic acid at the same concentration. At low concentrations, the dissolution of chrysotile in oxalic acid is limited to the first brucite layer; grinding of crocidolite favors significantly its dissolution. These few experiments enable us to suggest that in the human body, except in the stomach, asbestos dissolution is very low, favored by grinding and leaves a surface layer having ordered silanols groups, the high reactivity of which has been demonstrated in the case of chrysotile (pacco et al., 1976).

Investigation of electrode glass membranes: Proposal of a dissociation mechanism for pH-glass electrodes

Leached surface layers of pH-glass electrode membranes formed in aqueous solutions were investigated by an ion-sputtering method. Reversible formation of steady-state lithium concentration profiles was observed on changing temperature, pH and hydrodynamic conditions of the leaching solution. While new steady-state profiles were reached within periods comparable with times necessary for generating the corresponding layers on fresh membranes, temperature decreases were followed by extremely slow profile changes depending on the temperature difference. The formation of reproducible, thermodynamically defined, pH-dependent Galvani tensions between solutions and pH-electrode membranes, despite the non-equilibrium and non-steady state of their leached surface layers, is explained by the dissociation of ⩾ SiOH groups at the interface between layer and solution which should be an equilibrium independent of layer structure and state.

Investigation of surface layers, formed on glass electrode membranes in aqueous solutions, by means of an ion sputtering method

A method of measuring concentration profiles in glass surface layers by photon emission during ion sputtering, that offers a depth resolution of 30 to 50 Å, is applied to leached surface layers of pH-electrode membranes. Two steady state layer regions different with respect to ion diffusion are found at low pH. Within the outer layer the interdiffusion coefficient is constant. Within the transition layer interdiffusion coefficients are observed with a minimum that is two orders of magnitude lower than the lithium diffusion coefficient within the glass and is explained by a low diffusion coefficient of interdiffusing protons within a highly glass-like structure. The explanation is supported by the similar magnitude of the diffusion coefficient obtained here and that of protons which have replaced lithium ions within this glass in an electric field. The corresponding maximum resistivity agrees with measurements by Wikby. Surface layer properties and additional experimental results suggest that the equilibrium determining the pH-membrane potential is the dissociation of SiOH-groups at the immediate layer surface rather than an ion exchange of glass alkali and solution hydrogen ions.

Variables affecting sugar removal in freeze-leached potato strips

Strips cut from cold storage (5 C), potatoes were immersed in difluorodichloromethane (R-12) from 5 to 13 sec. This rapid freezing was found to selectively damage only the frozen cells and to increase their permeability with respect to intracellular soluble materials. Subsequent immersion in water (17 to 52 C) leached the soluble materials from the frozen cells. This method of leaching in 20 min removed less than 15% of the starch released by potato strips leached at 65 C for 15 min (simulated commercial conditions). The depth of freeze determined by physical methods and by analysis of the leached surface layers showed reducing sugar removal 0.1 to 0.2 mm below the calculated average depth of freeze. In experimental lots the weight of leached materials increased with increased freezing time. Manipulating freezing depth and leaching time made it possible to achieve the required sugar removal from different lots of tubers.

PHYSIOGRAPHICAL SIGNIFICANCE OF THE OCCURRENCE OF RED EARTHS IN NANNING BASIN KWANGSI

SUMMARYIn this paper, the author has brought out three points in connection with the occurrence of red earths in Nanning Basin.First of all, the occurrence of red earth has a remarkable physiographical significance: they occur exclusively on the terrace topography where drainage is perfect. Much of the red earth was undoubtedly developed before the present topography was shaped. It is supposed that by the time of late Pliocene when the basin was nearly filled up to a peneplain and when activities of erosion and deposition were reduced to their minimum, the red earth was in full development. The same condition existed in other basins in Hunan, Kiangsi and other provinces in South China.Secondly, the development of red earths is under definite pedological process which is now known as lateritization. The process beside its climatic factors is favored by a gentle mature topography. The soils as occurring in the basin have undergone more than one cycle. Generally the red earths have all been podzolized—forming a highly leached or podzolic surface soil and a lateritic subsoil.Thirdly, the author holds that lateritization is still going on, and therefore the separation of Pliocene from Pleistocene cannot be based on lateritization.