Apparent Dissolution Rate Constant Research Articles

Isothermal Kinetic Mechanism of Coke Dissolving in Hot Metal

The carburization of molten iron is close to saturation in the blast furnace process, while that in the flash ironmaking process is uncertain because there is no pressure from solid charge and no carburization reactions occurring between the deadman and hot metal. Some experiments were conducted to reveal the kinetic mechanism of coke dissolving in carbon-iron melts. Reduced iron powder, electrolytic iron as well as chemical pure graphite were used as experiment materials. With high-purity argon injected as the protective gas, the specimens were heated up to 1873 K in a tubular resistance furnace to study the isothermal mechanism. The results show that the composition of the ferrous sample affects the dissolution rate. When the FeO content of the iron-bearing material rises from 0% to 4.76%, the apparent dissolution rate constant, kt, falls from 7.98 × 10−6 m/s to 5.48 × 10−6 m/s. There are some differences amongst the dissolution rate coefficients of different cokes despite interacting with similar carbon-iron melts, with coke 1 of 7.98 × 10−6 m/s, coke 2 of 5.17 × 10−6 m/s, and coke 3 of 3.77 × 10−6 m/s. Besides, this index decreases with the increase of the dissolution time and solely depends on the procedure of the mass transfer. A negative correlation is demonstrated between kt and the sulfur content in the iron bath as well. The content of silicon dioxide in the coke has a significant influence on kt. Additionally, the dissolution rate coefficient increases with the increase of the graphitization degree of coke.

Dissolution of nontronite in chloride brines and implications for the aqueous history of Mars

Increasing evidence suggests the presence of recent liquid water, including brines, on Mars. Brines have therefore likely impacted clay minerals such as the Fe-rich mineral nontronite found in martian ancient terrains. To interpret these interactions, we conducted batch experiments to measure the apparent dissolution rate constant of nontronite at 25.0°C at activities of water (aH2O) of 1.00 (0.01M CaCl2 or NaCl), 0.75 (saturated NaCl or 3.00molkg−1 CaCl2), and 0.50 (5.00molkg−1 CaCl2). Experiments at aH2O=1.00 (0.01M CaCl2) were also conducted at 4.0°C, 25.0°C, and 45.0°C to measure an apparent activation energy for the dissolution of nontronite.Apparent dissolution rate constants at 25.0°C in CaCl2-containing solutions decrease with decreasing activity of water as follows: 1.18×10−12±9×10−14molmineralm−2s−1 (aH2O=1.00)>2.36×10−13±3.1×10−14molmineralm−2s−1 (aH2O=0.75)>2.05×10−14±2.9×10−15molmineralm−2s−1 (aH2O=0.50). Similar results were observed at 25.0°C in NaCl-containing solutions: 1.89×10−12±1×10−13molmineralm−2s−1 (aH2O=1.00)>1.98×10−13±2.3×10−14molmineral m−2s−1 (aH2O=0.75). This decrease in apparent dissolution rate constants with decreasing activity of water follows a relationship of the form: logkdiss=3.70±0.20×aH2O−15.49, where kdiss is the apparent dissolution rate constant, and aH2O is the activity of water. The slope of this relationship (3.70±0.20) is within uncertainty of that of other minerals where the relationship between dissolution rates and activity of water has been tested, including forsteritic olivine (logR=3.27±0.91×aH2O−11.00) (Olsen et al., 2015) and jarosite (logR=3.85±0.43×aH2O−12.84) (Dixon et al., 2015), where R is the mineral dissolution rate. This result allows prediction of mineral dissolution as a function of activity of water and suggests that with decreasing activity of water, mineral dissolution will decrease due to the role of water as a ligand in the reaction.Apparent dissolution rate constants in the dilute NaCl solution (1.89×10−12±1×10−13molmineralm−2s−1) are slightly greater than those in the dilute CaCl2 solutions (1.18×10−12±9×10−14molmineralm−2s−1). We attribute this effect to the exchange of Na with Ca in the nontronite interlayer. An apparent activation energy of 54.6±1.0kJ/mol was calculated from apparent dissolution rate constants in dilute CaCl2-containing solutions at temperatures of 4.0°C, 25.0°C, and 45.0°C: 2.33×10−13±1.3×10−14molmineralm−2s−1 (4.0°C), 1.18×10−12±9×10−14molmineralm−2s−1 (25.0°C), and 4.98×10−12±3.8×10−13molmineralm−2s−1 (45.0°C).The greatly decreased dissolution of nontronite in brines and at low temperatures suggests that any martian nontronite found to be perceptibly weathered may have experienced very long periods of water–rock interaction with brines at the low temperatures prevalent on Mars, with important implications for the paleoclimate and long-term potential habitability of Mars.

Basalt and olivine dissolution under cold, salty, and acidic conditions: What can we learn about recent aqueous weathering on Mars?

To test which variables may be important for weathering on Mars, the effects of temperature (22°C, 6°C, and −19°C), high ionic strength, and oxygen concentrations were investigated in batch dissolution experiments containing forsterite, fayalite, and basalt glass. CaCl2‐NaCl‐H2O brine can remain liquid to temperatures of −55°C and thus may be liquid in the cold, dry climate that currently characterizes Mars. To understand weathering under such conditions, dissolution rates were measured in experiments in distilled water with and without CaCl2 and NaCl. As observed by others, dissolution rates increased with temperature, and only fayalite dissolution was significantly affected by the presence or absence of oxygen. Enhanced fayalite dissolution under anoxic conditions suggests that Fe‐rich olivine would dissolve more rapidly than Mg‐rich olivine on Mars. Dissolution in the two most dilute experimental solutions (deionized water and CaCl2‐NaCl‐H2O solution of ionic strength = 0.7 m) were the same within uncertainty, but apparent dissolution rate constants in CaCl2‐NaCl‐H2O brines were significantly slower. Steady silica concentrations are decreased in the brines, consistent with other work, and precipitation rates of silica decrease with decreasing temperatures. These results suggest that enhanced silica precipitation could be an indicator of high ionic strength solutions on Mars. Consistent with these observations, weathering of basalt has been observed to sometimes be accompanied by precipitated layers of silica in cold, dry environments on Earth. If dissolution on Mars occurs or occurred under conditions similar to our experiments, cation leaching would be expected to be accompanied by silica precipitates on weathering surfaces.

Physicochemical Characterization and Dissolution Properties of CS-891 with Different Crystallinity

Ground CS-891 (N-[1-(4-methoxyphenyl)-1-methylethyl]-3-oxo-4-aza-5a-androst-1-ene--carboxamide) of poorly water soluble drug was obtained using a Heiko Seisakusho model TI-100 vibration mill, and samples with different crystallinity were prepared at mixture ratios of 10:0, 7:3, 5:5, 3:7 and 0:10 (intact;ground CS-891). Physicochemical characterizations were obtained using qualitative and quantitative X-ray diffractometry, different scanning calorimetry (DSC), scanning electron microscopy (SEM), Quantasorb surface area analyzer, and controlled atmosphere microbalance. With increase of amorphous CS-891 in mixture ratios, the intensities of X-ray diffraction peaks of crystalline CS-891 were decreased, whereas surface area, water absorption, and exothermic peaks in DSC were increased. The apparent solubility of ground CS-891 was and the solubility of intact CS-891 was at . The apparent precipitation rates of CS-891 in a supersaturated solution during the solubility test were increased with an increase of amorphous CS-891, and a crystalline form of CS-891 transformed from amorphous CS-891 after the solubility test was found by X-ray diffraction analysis, DSC and SEM. The dissolution profiles of CS-891 with different crystallinity at by the USP paddle method were investigated, and the apparent dissolution rate constant of ground CS-891 was about 5.9-fold higher than that of intact CS-891. A linear relationships between the crystallinity of CS-891 and the apparent dissolution rate constant (r>0.96) were obtained.

Surface alteration of dolomite in dedolomitization reaction in alkaline media

The phases precipitated on the dolomite surface and the influence of alkalis on the properties of this surface were characterized. Experiments consisted of the immersion of single crystals of dolomite in saturated portlandite solutions with different alkalinity, temperature, and silica content. Most calcite forms in the solution as {104} rhombohedra. Brucite crystallizes as platelets, piles of platelets, sponge-like and {001} truncated ditrigonal pyramids attached to the dolomite surface. The morphology of brucite crystals depends on the pH of the solution. Apparent dissolution rate constants for dolomite at 75 °C are one order of magnitude higher than at room temperature. Initially, calcite grows on the dolomite surface with the same structural orientation {104}. In a second stage, when calcite has reached equilibrium with the solution, the new-formed {104} calcite surface starts playing an important role on the precipitation of brucite, while calcite continues crystallizing in the bulk solution. The calcite surface, negatively charged, can adsorb specifically metal cations. Mg 2+, liberated by the dissolution of dolomite, would move attached to the surface, until they are trapped on these negatively charged surfaces, while OH − supplied by the alkaline solution favors the nucleation and growth of brucite. The attachment of Mg 2+ to the surface controls the place where brucite grows.

Kinetic Studies on Dissolution of UO2 Powders in Acid Solutions by Using Cerium(IV) or Chlorine Dioxide as Oxidants.

The UO2 powders of 100–150, 150–212, 212–250, and 250–300μm were dissolved in HNO3 or HCl solutions containing strong oxidants to examine their effect on the dissolution rates. Cerium(IV) species and ClO2 were used as oxidants in HNO3 and HCl solutions, respectively. The Ce(IV) species were prepared by bubbling O3 gas into HNO3 solutions containing Ce(NO3)3. All dissolution experiments were carried out under rapid stirring conditions which make it possible to neglect the diffusion effect. Dissolution reactions were analyzed on the assumptions that the UO2 powders are spherical particles and homogeneously dissolved from their external surface. Dissolution rate constants (Φ) in mol·cm−2·min−1 were measured at various concentrations of oxidants and temperatures. Furthermore, the effect of acid concentrations on the dissolution rate was also examined. As a result, the dissolution rate constants Φ were found to be expressed as Φ=kN[Ce(IV)][H+]0.6 for HNO3 solution([HNO3]=1-5 M, Temp.≦30°C) and Φ=k H, [ClO2][H+]−0.3 for HCL solution([HCl]=1-5 M, Temp.≦30°C), respectively. Activation energies(kJ·mol−1) for the apparent dissolution rate constants are 44.1±1.9 for the HNO3 solution system and 33.5±2.8 for the HCl solution system, respectively.

Preparation and Evaluation of Eudragit Gels. II: In Vitro Release of Salicylic Acid, Sodium Salicylate, and Ketoprofen from Eudragit L and S Organogels

The in vitro dissolution characteristic of salicylic acid, sodium salicylate, and ketoprofen from Eudragit L and S organogels was investigated by the rotation disk method. The dissolution pattern of salicylic acid and erosion of Eudragit L polymer from the organogels followed apparent zero-order kinetics, providing strong evidence for a surface erosion mechanism and negligible diffusional release of salicylic acid. On the other hand, the dissolution of salicylic acid from Eudragit S organogels was a linear function of the square root of time. The apparent dissolution rate of salicylic acid from Eudragit S organogels increased with increasing temperature from 32 to 42 °C and agitation rate from 50 to 200 rpm. A linear relationship was obtained between the logarithm of apparent dissolution rate constants and the reciprocal of absolute temperatures. The activation energy for release of salicylic acid from Eudragit S organogels was in the range of 2.99 to 5.57 kcal/mol. From various experimental results, it was concluded that the release process of salicylic acid from Eudragit S organogels was diffusion controlled through the organogels matrix.