The study presented is about the leachingkinetics of ulexite ore in aqueous media under various CO 2 partial pressures. The results show that the leaching rate of ulexite increased with the increase in the pressure of CO 2 according to the studies conducted under atmospheric conditions (the other conditions are kept stable). Therefore, application of overpressures of CO 2 is more favoured. However, optimization study is required to make a decision (Elçiçek, 2012). The study conducted by Ata et al. (2000) was about optimization of boric acid extraction from colemanite in CO 2 -saturated water. Colemanite dissolution in CO 2 -saturated water is slower than that of ulexite. The dissolution kinetics of inderite [MgB 3 O 3 (OH) 5 •5(H 2 O)] and inyoite [Ca 2 B 6 O 6 (OH) 10 •8(H 2 O)], which have no economic value, has less importance in CO 2 saturated water. Ulexite ore used in the present study contains nearly 75% ulexite, and is processed industrially (Alkan et al., 1991).
43 natural hemimorphite, as well as natural and synthetic willemite. The authors have observed that the acidic dissolution was diffusion-controlled for hemimorphite and chemically-controlled for willemite leaching. Abdel-Aal (2000), studying the leachingkinetics of low grade zinc silicate, proposed that the process was controlled by diffusion on an “ash” layer with an associated activation energy of 13.4 kJ/mol (3.2 kcal/mol). As shown in equations (3.2) and (3.3), there is no solid reaction product formed during leaching, as only zinc ions and silica gel are produced, although the latter can affect the transport properties of the solution. It should be pointed out that Abdel-Aal (2000) did not present an explanation for the proposed diffusion control in the product layer. It is also possible that diffusion through the solid’s pores would explain the controls observed during silicate leaching. It has been shown that if the transport through a solid’s pores is the rate-determining step, an expression similar to the shrinking core model (SCM) with diffusion control (Georgiou and Papangelakis, 1998) is achieved.
Experiments with 5.0 g/L ferrous iron lead to a marked increase in copper leachingkinetics after 48 h, reaching 75% extraction in the following three days (day 2–5), accompanied by a concomitant raise in ferric iron concentration inside the rotating-drum reactor – Fig. 9(b). In such a way the results pictured in Fig. 9(b) are confirm- ing the hypothesis of fast bacterial growth already suggested by the Eh trend seen in Fig. 8(a). Nevertheless, it should be mentioned that the relatively low Eh values shown in Fig. 8(a) (<550 mV) could be explained by the fast consumption of ferric iron due to its reaction with metallic copper (Eq. (2)). Consequently, Eh values were low and similar to those recorded at non-inoculated systems (Petersen and Dixon, 2007b). Moreover Fig. 9(a) and (b) suggest that copper leaching was low at the beginning of the experiment (until day 2). During this period the process was limited by the Fe 3+ concentration (lag phase) and when ferric iron started to
kinetics of covellite produced from the reaction between chalcopyrite and gaseous sulphur (Padilla et al., 2013). Nevertheless, some inconsis- tencies can be found in the scientiﬁc literature, particularly in terms of the rate-determining step (Nicol et al., 2010). Some researchers pro- posed that chalcopyrite leaching is a chemical controlled process (Córdoba et al., 2008b; Hirato et al., 1987; Kaplun et al., 2011; Ruiz et al., 2011); whereas others reported a diffusion-controlled mechanism (Bonan et al., 1981; Carneiro and Leão, 2007; Córdoba et al., 2008c). Therefore, the activation energy vary broadly with values between 38 kJ/mol and 130 kJ/mol proposed for ferric sulphate leaching in the 35 °C–100 °C temperature range (Dutrizac, 1981). Another source of in- consistencies is the type of experiment selected. Some works were per- formed with particulate systems (Al-Harahsheh et al., 2008; Bonan et al., 1981; Córdoba et al., 2008b; Dutrizac, 1981; Havlík and Kammel, 1995; Lu et al., 2000; Yoo et al., 2010) whereas other investigated the leachingkinetics of massive chalcopyrite samples (Cai et al., 2012; Palmer et al., 1981) or applied rotating disc techniques to synthetic and natural chalcopyrite (Dutrizac, 1978; Hirato et al., 1987). Also, ki- netics analysis using electrochemical data can be usually found (Lundström et al., 2005; Majima et al., 1985; Majuste et al., 2012). In ad- dition, some works did not have a detailed description of the mineral features (Watling, 2013).
The change in the oxidation mechanism is likely the reason for the faster reduction on the Fe(III) concentra- tion observed during leaching with NaCl in the present study. This can be inferred by comparing Figs. 2 and 3. Although part of ferric iron is precipitated as natrojar- osite, the concentration of ferrous iron is higher in the presence of NaCl than in its absence. This effect is higher for 1.0 mol/L NaCl and higher concentrations compared to the experiment carried out with 0.5 mol/L NaCl, matching the behaviour observed for copper dissolution (Fig. 1). The increase in Fe(II) concentration would be ascribed to the fast Cu(I) oxidation (Eq. (7)) instead of the direct mineral oxidation which is slow due to chalcopyrite passivation. The fast leachingkinetics observed with the Cu(II)/Cu(I) couple is attributed to a good overlap between the energy levels of the Cu(II)/Cu (I) couple and the conduction band of chalcopyrite while the energy levels of the Fe(III)/Fe(II) couple are within the chalcopyrite band gap, and the leachingkinetics is therefore slower (Venkatachalam, 1991).
When the SCM model was unsuccessful in describing the leachingkinetics, the grain pore model, GPM, (Szekely et al., 1976) was applied (Souza et al., 2007; Souza et al., 2009). It has been shown that if the transport through the pores of the solid is the rate-determining step, the GPM model predicts an expression similar to the shrinking core model (SCM) with diffusion control (Georgiou and Papangelakis, 1998). This approach was applied by Souza et al. (2007) to the acid leaching of zinc silicates and an activation energy of 59.5 ± 2.9 kJ/mol was obtained for the dissolution kinetics of a calcined zinc silicate. The same model was applied afterwards to the leaching of non-calcined zinc silicates containing either high (8–11%) or low (3%) iron content (Souza et al., 2009). The authors suggested that the iron content did not affect the leachingkinetics of the silicate concentrate as the activation energy was statistically similar for both materials (78.2 ± 12.1 kJ/mol and 66.8 ± 9.2 kJ/mol, for the high- and low-iron silicate, respectively). As the leachingkinetics in alkaline medium is far less studied than in acid solutions, the purpose of the present work is to assess the leachingkinetics parameters of a zinc silicate ore in sodium hydroxide.
of various methods, such as precipitation, crystalli- zation, and evaporation. Gaining of copper from various synthetic pure solutions or actual leach solu- tions containing copper ions by the cementation re- action has been studied by using iron, aluminum, and zinc as the reductant metal (Djokic, 1996; Dib and Makhloufi, 2004; Karavasteva, 2005; Demirkıran et al., 2007; Ahmed et al., 2011; Demirkıran and Künkül, 2011; Ekmekyapar et al., 2012b; Demirkıran, 2013b). In this work, the leachingkinetics of malachite in ammonium sulfate solutions was investigated, and metallic copper was recovered by the cementation method from the resulting actual leach solution. Copper (II) oxide was prepared by an isothermal oxidation method from the cement copper. In the leaching experiments, the effects of reaction tem- perature, particle size, and stirring speed on copper leaching from malachite ore were studied. In the cementation experiment, metallic zinc was used as the reductant metal to recover the copper from the solution. Due to the above-mentioned advantages of ammoniacal solutions, ammonium sulfate was cho- sen as solvent in this study. In addition, the solution containing sulfate is more suitable for electrolytic recovery of copper from the solution.
In the rural communities of the Amazon, ‘babassu’ flour is not a valued product due to the lack of standardization in the manufacturing process. In general, flour production is made by family activities in rudimentary molds with quality evaluation through color and taste of the product. Drying is performed on concrete floors using natural ventilation, and the water content is controlled empirically (Mendonça et al., 2015). There is little information in the literature on the drying kinetics or even about the effect of the management of ‘babassu’ mesocarp on the quality of the produced flour.
mammalian organisms . Our method can easily include variable velocities, including fork blocks due to DNA damage. (Our approach allows both I and v to be space-time dependent but the results of our test case are easier to interpret when there is only one inhomogeneous contribution to the replication kinetics.) However, experimental results indicate that the effects of the inhomogeneity of I (x,t) are much more important than the effects of the inhomogeneity of v(x,t) (see below and Demczuk et al., unpublished). For simplicity, we used periodic boundary condi- tions (PBCs) for the fork propagation in our simulations (forks reaching a boundary are re-inserted at the other boundary). Therefore, it is formally equivalent to a circular chromosome (e.g., as in bacteria). Of course, whole-chromosome simulations of eukaryotic chromosomes would not use periodic boundary conditions and would take into account the specific (low) initiation rates found in telomeres .
For the thickness of 0.5 cm, among the tested models, Midilli showed the best fit to the experimental data, with the highest coefficients of determination and the lowest mean quadratic deviations. Furtado et al. (2010) studied the drying kinetics of ‘ceriguela’ (Spondias purpurea) pulp, at temperatures of 60, 70 and 80 ºC, and observed that the Midilli model was satisfactory. According to Azzouz et al. (2002), the parameter n in the Page model depends on the drying air speed and initial water content of the product and k depends on temperature and initial water content. The obtained k values decreased with the increase in drying temperature, while the parameter n increased as temperature increased.
The thermogravimetric analyses of PEEK in nitrogen atmosphere showed that its thermal decomposition occurred in two steps. It was observed by the onset temperatures that the PEEK starts its decomposition in about 526 °C and generated a great amount of residue (about 45%). The decomposition kinetic study showed that the material has high thermal stability. The necessary time for the material decomposes in 5% is of approximately 216 years if it is submitted to temperatures of 350 °C. The kinetics study by Coats Redfern showed that the D3 mechanism (Three-dimensional diffusion (Jander equation)) had better adjustment to the decomposition kinetics of the material.
These presences may have led to the difference in the behavior of current generation kinetics of the MFC in this study in relation to the values obtained for pure cultures. In addition, as previously dis- cussed, the metabolisms present in anode biofilms are also influenced by external resistance. An MFC operating at 1 kΩ and with a microbial consortium has a quite different portion of dominant exoelectro- genic microorganism and behavior compared to a pure culture or an MFC operated with very low re- sistances, as found by Rismani-Yazdi et al. (2011). In fact, the characteristic kinetic parameters ( j max ,
TEBUCONAZOLE PHOTOCATALYTIC DEGRADATION KINETICS. The tebuconazole photocatalytic degradation kinetics was studied in a batch reactor using TiO 2 (P25-Degussa) as catalyst and a high pressure mercury lamp. The photolysis, adsorption and irradiation effects in the reaction rate were evaluated. Afterward, the suspension catalyst concentration and initial pH to the maximum reaction rate was determined. It was observed that the reaction rate can be approached by a pseudo-irst order, with a maximum kinetics constant at 260 mg L -1 catalyst concentration and pH 7.7.
The mechanism of leaching of semiconducting minerals such as CuS, ZnS, UO 2 , etc., has been the subject of intensive speculation by hydrometallurgy researchers in the early 1950s who assumed the formation of intermediate surface complexes that could be neither separated nor identified by physico-chemical techniques. The electrochemical theory of leaching introduced in the late 1960s resolved this problem by comparing the leaching process to a corrosion phenomenon similar to the corrosion of metals. A historical summary of these proposals is presented.
Table 5 shows that the effective diffusion coefficients increased as the temperature increased. A similar behavior was observed by Goneli et al. (2009) for the drying kinetics of hulled coffee. The authors highlighted that as the temperature increases, the water viscosity decreases. Viscosity is a measure of a fluid ’s resistance to flow. Variations in this property imply changes in water diffusion in the capillaries within the seed that favor the movement of this fluid inside the product.
Copper containing silicates can be prepared by a simple acid-catalyzed sol-gel process. From several analysis techniques one can conclude that the metal is attached to the surface of the silicate and not homogeneously dispersed into its framework. Using MeSi(OEt) 3 as silicon precursor hydrophobic materials with methylated surfaces can be obtained. The metal can easily be removed from the catalysts, especially the methylated ones, by soxhlet extraction with acetonitrile, indicating weakly bound Cu species. Cu-SiO 2 is an active and selective catalyst for the oxidation of cyclohexane to cyclohexanol and cyclo- hexanone. Cyclohexanol is further oxidized to cyclo- hexanone. Catalyst recycling experiments show that the catalyst can be used repeatedly but a significant decrease in cyclohexane conversion is observed. Leaching tests show an extensive leaching. However, dissolved copper species exhibit no catalytic activity in the homogeneous phase, showing that the catalytic activity is due to heterogeneous copper species.
The temperature and time effect on the anthocyanin stability in açai drinks was evaluated at three different temperatures: 0, 25, and 40 °C. At all temperatures tested, the decrease of anthocyanins against time was linear (Figure 1), and it exhibited zero-order kinetics. When stored at 40 °C, the degradation of anthocyanins was 1.8 times faster than at 25 °C and 15 times faster than at 0 °C. The half-life time (t 1/2 ) is very sensitive to higher temperatures: at 40 °C the t 1/2 is 23.9 hours, at 25 °C t 1/2 is 42.9 hours, and at 0 °C t 1/2 is 372.7 hours (Table 1). These values are lower than those found in the literature, studies in which low pH samples were used.
fertilizer, for example. Nevertheless, Rosolem & Silva (2010) found that increased ammonium nitrate rates affected Mg most of all cations, in terms of mobility in soil depth. In this study, N rates commonly used for grain crops had a major effect on base cation leaching through the soil profile (Figures 5, 6, 7 and Table 2). Subsurface liming effects may depend on the release of organic acids (Miyazawa et al., 2002; Franchini et al., 2003) but a vertical movement of bases in soil was observed both with or without crop residues on the soil surface (Rosolem & Silva, 2010). For these authors, this mobility was related to N– NO 3 - released by N fertilizer, rather than to organic acids release. Besides, Moraes et al. (2007) observed no influence of different crop residues on liming effects, even with an amount of 20 t ha -1 of residues on the