Solid Product Layer Research Articles

High-temperature reactive melt spreading: Experimental modeling of SHS reactions

Initial stages of high-temperature spreading and inreaction of molten droplets with solid substrates were experimentally investigated in the systems Ti–C and Nb–B. Thin layer of solid product (carbide, boride) crystalline grains forms immediately in the spreading frontier, while the melt infiltrates through nano-scale gaps between grains. The formation of a continuous layer of oxidation- and wear-resistant solid refractory (∼ 3000°C) product can be utilized for coating protection of refractory metals and carbon materials, as well as in high-temperature joining of carbon materials via a thin layer of titanium carbide.

A Study of the Occurrence of Selected Rare-Earth Elements in Neutralized–Leached Bauxite Residue and Comparison with Untreated Bauxite Residue

This study investigates the chemical associations of the selected rare-earth elements (Sc, Y, Ce, La, and Nd) with major element phases in the postprocessed bauxite residues and compares them with the untreated bauxite residue. The treatment of bauxite residue considers our previous published process which involved the neutralization with CO2, followed by leaching with H2SO4. Neutralized bauxite residue resulted with larger aggregates than the untreated bauxite residue after making contact with CO2 as the consequence of additional CaCO3 formation. Neutralization with CO2, however, has a negligible effect on the distribution of the rare-earth elements (REEs) with respect to the untreated bauxite residue, but a large amount of rare earths remained unreacted after acid leaching. Electron probe microanalysis (EPMA) confirmed the chemical associations of Sc(III) and Ce(IV) with Fe(III)- and Al(III)-containing minerals in the postprocessed bauxite residues, i.e., bauxite residues subjected to CO2-neutralization and neutralization–acid leaching processes. The occurrence of Nd(III) is positively correlated to that of La(III) in the untreated bauxite residue, but both of them may be associated with the same mineralogical phase as Ce(IV) after processing. Y(III) may remain associated with the Al/Si-minerals, cancrinite and chamosite. Ergo, the extractability of Sc, Y, Ce, La, and Nd from neutralized bauxite residue is more difficult in H2SO4 media due to the presence of coarser particles compared to those of the untreated bauxite residue, but also due to the formation of a solid product layer (i.e., CaSO4) that is presumably adsorbed on the surface of Fe(III)-rich phases (hematite and goethite) and Al(III)-containing minerals (diaspore, gibbsite, boehmite, and chamosite).

Mechanochemical activation of chalcopyrite: Relationship between activation mechanism and leaching enhancement

This work addressed the mechanochemical activation of chalcopyrite by vertically stirred ball milling (VSBM) to enhance copper dissolution in leaching. Grinding with Zr balls and quartz sand as dispersing agent produced a significant increase of copper extraction from +10% to +40%, depending on grinding conditions. X-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) results highlighted that the mechanochemical activation of chalcopyrite occurred through partial oxidation and rupture of crystal lattice. The contribution of each activation mechanism to leaching enhancement was assessed by two-stage leaching. The 10% increase of copper extraction in first stage leaching by H2SO4 confirmed the increase of soluble copper due to mechanochemical oxidation. Second stage leaching by H2SO4-Fe2(SO4)3 highlighted a larger dissolution of copper from chalcopyrite and enhanced kinetics upon activation. Whether grinding by VSBM or not, the kinetics of second stage leaching was found to be controlled by ions diffusion through the solid product layer. The partial amorphization of chalcopyrite resulted into additional increase of copper extraction ranging from +6 to +11% and produced a decrease of leaching activation energy from 91.0 to 79.4 kJ/mol.

Rate of MgO Pickup in Alumina Inclusions in Aluminum-Killed Steel

This work aims to clarify the rate and mechanism of MgO pickup by alumina inclusions and the effect of oxide impurities in MgO crucibles on this transformation. Two MgO crucibles from different batches from the same supplier were used in laboratory experiments with Al-killed steel. A kinetic model was developed, based on mass-transfer control in the liquid steel. Rate constants were fitted using inclusion analysis. The rate of magnesium transfer from the two types of crucibles was found to differ by a factor of 20; faster magnesium transfer was associated with formation of a slag layer on the inner surface of the crucible wall (rather than a solid spinel product layer). The kinetic model was also used to simulate industrial scale ladle refining (1) to illustrate the effects of total oxygen concentration and (2) to evaluate the contribution of steel-refractory reaction (in addition to steel-slag reaction) on the rate of MgO pickup in alumina inclusion. The rate of MgO pickup was higher with a lower inclusion concentration. For ladle desulfurization, the extent of MgO pickup in inclusions is directly linked to the extent of desulfurization; both reactions are controlled by the oxygen potential at the steel-slag interface.

Leaching Kinetics of Vanadium and Chromium During Sulfuric Acid Leaching With Microwave and Conventional Calcification-Roasted High Chromium Vanadium Slag

ABSTRACTA comparative method via microwave and conventional calcification-roasting to extract vanadium and chromium from high chromium vanadium slag has been proposed. When leaching temperature and leaching time are 100 ºC and 120 min, the optimic leaching ratio of vanadium and chromium are 95.92%, 4.27% after microwave roasting and 84.96%, 2.15% after conventional roasting. The kinetics of vanadium in the first stage controlled by solid product layer diffusion and the second stage controlled by solid product layer diffusion and chemical reaction. The kinetic of chromium is controlled by solid product layer diffusion and chemical reaction.

Insights into the Role of H2O in the Carbonation of CaO Nanoparticle with CO2

The enhancement of H2O on the carbonation reaction of CaO with CO2 is now widely recognized in the calcium-looping systems. However, the microscopic origins of steam-enhanced reactions remain unclear. A new insight into this issue from the atomic level is provided. We performed molecular dynamics (MD) simulations using a recently developed ReaxFF reactive force field to study the role of H2O on the carbonation reaction of CaO for enhancing CO2 capture. First, the effects of H2O on the carbonation reaction of CaO with CO2 were investigated by MD simulations combined with thermogravimetric analysis (TGA) experiments. Our calculation results well-supported by the TGA experiments showed that H2O just enhances CaO carbonation at the diffusion-controlled stage, whereas there is little influence on the kinetic stage. Then, we analyzed the properties of ion/gas diffusion and the solid product layer to deeply understand the role of H2O in the diffusion-controlled stage. It was found that the ion/gas diffusion coul...

Bioleaching and Kinetic Investigation of WPCBs by A. Ferrooxidans, A. Thiooxidans and their Mixtures

Bioleaching was used to mobilize Cu, Zn and Ni from waste printed circuit boards (WPCBs) and eliminate hazardous metal species from these wastes. Pulp density (PD) and medium culture are two effective factors which have been optimized in this paper. The bacteria Acidithiobacillus ferrooxidans (A. ferrooxidans) and Acidithiobacillus thiooxidans (A. thiooxidans) and their mixture were grown and adapted in the presence of WPCBs and then used as bioleaching bacteria to solubilize metals from PCBs. The experimental results demonstrated that 15 g/L WPCB is the best solid concentration which can be tolerated by the bacteria. Comparing different inoculation ratios, Cu (86%), Zn (100%) and Ni (100%) were recovered after 25 days of bioleaching, which suggests that the rate of metal recovery is significantly influenced by PD. Kinetics of bioleaching reactions was investigated in this work and the shrinking core model (SCM) was used to describe the kinetics of the process of no pretreated WPCBs. A constrained multi-linear regression analysis using the least square technique was employed to determine the rate controlling mechanism in each operating condition. Based on the results, diffusion through solid product layer was the major controlling mechanism.

Dephosphorization of high phosphorus oolitic hematite by acid leaching and the leaching kinetics

It is highly difficult to remove phosphorus from high phosphorus oolitic hematite by the usual dressing process. Acid leaching is an effective method for the dephosphorization of high phosphorus oolitic hematite. The acid leaching experiments were conducted to study the effect of acid concentration, temperature, leaching time, solid-liquid (S/L) ratio and the stirring speed on the dephosphorization of the high phosphorus oolitic hematite. The results demonstrate that hydrochloric acid is the best selection for leaching acid for the dephosphorization, and treatment of the sample in 0.2mol/L hydrochloric acid at 298K for 10min with the S/L ratio of 0.03g/mL and a stirring speed of 300rpm is optimum. Thus, the dephosphorization can reach 90% with <0.18% iron loss. We also investigated the hydrochloric acid leaching kinetics. There were two distinct stages in the leaching process for dephosphorization, and the kinetics of both stages followed a shrinking core model. The apparent activation energy for leaching in leaching stage one (initial 10min) and stage two (10–60min) was estimated to be 2.51kJ/mol and 5.59kJ/mol, respectively. The results demonstrated that leaching of the two stages was controlled by acid diffusion through the solid product layer. The leaching with iron dissolution was mostly controlled by chemical reaction between Fe2O3 and acid.

Direct extraction of perovskite CaTiO3 via efficient dissociation of silicates from synthetic Ti-bearing blast furnace slag

A new method of extract perovskite CaTiO3 from synthetic Ti-bearing blast furnace slag (Ti-slag) is proposed. After thermal modification of Ti-slag with 30% NaNO3, the silicon containing components has been reconstructed, which can be more easily dissolved in the leaching process. The effect of composition, morphology and mineral phases of the silicon containing components on the direct migration into acid soluble silicates in minerals and solutions is illuminated. Furthermore, the leaching kinetics of the modified slag in a 10% hydrochloric acid solution was described by the shrinking core model (SCM). The apparent activation energy of the dissociation of “Si” was calculated to be about 17.23kJ/mol, which indicates that the perovskite extraction is controlled both by interface transfer and solid product layer diffusion. The content of perovskite in the residue is 88.3% with 1.4% SiO2. The results of this study can provide an important method for direct extraction perovskite CaTiO3 in the Ti-slag and also establish a new strategy for resources of complex high silicate minerals.

A single particle model of lime sulphation with a fractal formulation of product layer diffusion

A fractal-like formulation of the random pore model (RPM) is proposed for the reaction of a gas with a solid, with the rate controlled by both diffusion in the layer of solid product and the kinetics of reaction. This approach gives a time-dependent diffusivity in the product. The model’s predictions are compared with measurements of the removal of SO2 with limestone, carried out in a lab-scale fluidized bed reactor. The fractal-like RPM described the production of CaSO4 better than the standard RPM, which overestimated the uptake of SO2 in the first 40min. The decrease of diffusivity in the product layer with time was ascribed to the degree of crystallization of the product, CaSO4, increasing with time and resulting in a lower ionic mobility in its lattice. The equation proposed is a simple and general one for a gas reacting with a solid (alternative example: the carbonation of lime), whose microstructural properties change significantly with the extent of reaction.

Optimization and dissolution kinetics of vanadium recovery from LD converter slag in alkaline media

Alkaline roasting-alkaline leaching process was used to recover vanadium from LD (Linz Donawitz) converter slag. The independent leaching parameters investigated were liquid to solid ratio (L/S) (10–20 mL/g), temperature (40–60°C), NaOH concentration (1.0–3.0 M), and time (60–120 minutes). Response surface methodology (RSM) was utilized to optimize the leaching parameters and as a result, the most influencing parameter was found to be liquid to solid ratio. Based on the results, the optimum recovery condition (approx. 99%) was obtained with L/S ratio of 20, temperature of 40°C, NaOH concentration of 3.0M, and leaching time of 100 minutes, respectively. Furthermore, the kinetics of alkaline leaching process was investigated using shrinking core model (SCM) equations. It was found that the rate of vanadium leaching is controlled by a mixed controlling mechanism which is comprised of chemical reaction and diffusion through the solid product layer.

Pyrargyrite (Ag3SbS3): Silver and antimony dissolution by ozone oxidation in acid media

Silver and antimony leaching from pyrargyrite (Ag3SbS3) by ozone in acid media was experimentally studied in order to evaluate this method as an alternative to the conventional cyanidation. When 1g pyrargyrite was leached in 800mL of 0.18M sulfuric and 0.079g O3/L gas was bubbled, it was observed that after 80% of metals dissolution the silver and antimony dissolution rates decreased until completely stopping. These results suggest the presence of a solid product layer onto the unreacted pyrargyrite cores. The SEM-EDS and XRD analysis showed that antimony oxides were formed as a layer covering the surface of pyrargyrite particles, hindering reactants transferring/diffusion to the unreacted surface. The chemical composition of the layer was identified as a silver/antimony oxide AgxSb2Oy (x=0.51±0.04 and y=6.08±0.43). For low solid/liquid ratios the pyrargyrite dissolution reached about 98%, however, for high solid/liquid ratios a maximum value of antimony concentration was found to be around 0.0008mol/L. An activation energy of 27.88kJ/mol was calculated, which is a typical value for mixed-control systems. The kinetic study showed that the controlling step changed as the reaction proceeds. The oxidation pretreatment with ozone is a promising alternative for pyrargyrite dissolution, currently under development.

New Perspectives on the Gas–Solid Reaction of α‐Si3N4 Powder in Wet Air at High Temperature

The reaction behavior of chemically vapor‐deposited silicon nitride (α‐Si3N4) powder in wet air at 1673–1873 K for 10 h has been investigated. The oxidation and volatilization reaction coexist initially while volatilization reaction progressively becomes the limiting step with extended reaction time. Based on the experimental curves, a new kinetic model regarding both solid product layer and hypothetically volatile product layer has been developed to interpret its short‐term reaction and predict long‐term corrosion behavior. The calculated results agree well with the experimental data. Predictably, this new kinetic model can not only be used to treat the reaction of Si3N4 powder in wet air but also can be applied to deal with the gas–solid reaction of other ceramic powders.

Enhanced modelling of heterogeneous gas–solid reactions in acid gas removal dry processes

Acid gases as hydrogen halides and sulphur oxides are typical pollutants of combustion processes. Their removal from flue gas can be performed via the injection of dry powdered sorbents, as calcium hydroxide. However, the efficiency of dry treatment methods is hindered by the limited final conversion of the solid reactant, due to an abrupt decline of its reactivity during the reaction process. Fundamental gas–solid reaction models such as the shrinking core model and the grain model are able to reproduce this phenomenon only introducing an arbitrary value of the final conversion or an adjustable value of the solid-state diffusivity of the gaseous reactant. In the present study, the conventional grain model approach was integrated with a crystallisation and fracture (CF) submodel, which links the chemical potential of nucleation to the work needed to displace the layer of solid product formed on the reaction interface. The decline in reactivity of the sorbent was accounted by a twofold effect of the product layer growth: (i) the increase of the characteristic length for solid-state diffusion, accounted for in the grain model, and (ii) the increase of the mechanical work required for nucleation as a function of product layer thickness, accounted for in the CF submodel. This approach, validated against literature data on the Ca(OH)2/HCl system, allowed reproducing the conversion of the solid reactant at different operating temperatures.

Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores

Saprolitic ores from Sulawesi Island (SS ore) contain serpentine and goethite as major minerals, whereas the main minerals in saprolitic ore from Halmahera Island (SH ore) are talc and goethite. Most of the nickel was incorporated in a magnesium–silica-containing mineral. The effects on nickel extraction of leaching temperature, citric acid concentration, and ore particle size were determined to investigate the leaching performances and leaching kinetics of the two saprolitic ores. Nickel leaching efficiency from SS ore is always higher than that from SH ore under the same leaching conditions. The mineral contents of the ores significantly affected the leaching performances and mineral dissolution behaviors of the samples. The results of nickel leaching efficiency and analysis of the solid residues suggest that all dissolved nickel originated from serpentine, which is more easily leached than goethite and talc. The rate of nickel extraction for SS ore was faster than that for SH ore. Nickel leaching from SS ore and SH ore followed the shrinking-core model and was controlled by diffusion of a reactant or product through the solid product layer.

Kinetics for gas–solid reactions involving dual-reaction interfaces

Abstract A series of new equations have been developed, based on the pseudo-steady state approximation, to describe the kinetics of gas–solid reactions involving two reaction interfaces. These formulae can be applied to cases where two solid product layers (stoichiometric compounds or solid solution products) are involved, or to one solid product layer and one volatile species (formed by the reaction of solid product with gas). For the latter case, when either the solid product layer is dense, or the diffusion process is the controlling step, the relationship is equivalent to the widely-used Tedmon model.

The leaching of uranium from betafite

Betafite is a uranium–titanium–tantalum–niobium complex oxide mineral generally described as (Ca,U)2 (Ti,Nb,Ta)2 O6 (OH). Significantly lower uranium extractions have been observed when betafite containing ores has been treated via conventional process flow sheets. Therefore it is necessary to understand the leaching behaviour of this mineral in order to open the potential for it to become an economic source of uranium in the future.The leaching kinetics and reaction mechanism of uranium from a natural, metamict betafite sample have been investigated using varied temperature under oxidative acidic conditions. The practically complete extraction of uranium was observed by leaching the natural betafite in a solution containing 214.5g/L H2SO4 and 2.0g/L Fe3+ at 89°C for a period of 48h. Alternative leaching conditions using 57.1g/L H2SO4 and 36.7g/L Fe3+ at the same temperature also resulted in practically complete extraction of uranium but improved the selectivity for uranium over titanium, tantalum and niobium. Kinetic modelling has indicated that the rate of the leaching process is controlled by the rate of diffusion of products or reactants involved in the dissolution reaction to the reacting surface through a solid product layer containing niobium.A portion of the betafite mineral sample used in the leaching testwork was recrystallised by heating in air at 1100°C. The crystalline form of betafite was then leached under conditions similar to those applied to the metamict mineral to determine how crystal structure may affect the extraction of uranium. Less than 12% uranium was extracted from the recrystallised betafite under the conditions that gave practically complete extraction of uranium from the natural betafite sample which gives strong evidence that heat treatment prior to leaching should be avoided. The thermal recrystallisation of betafite in the present study appears to have resulted in tantalum enrichment on the surface of the betafite particles, which may be related to the lower extraction of uranium observed.

Kinetics of microwave roasting of zinc slag oxidation dust with concentrated sulfuric acid and water leaching

The microwave irradiation is used to roast zinc slag oxidation dust in the presence of concentrated sulfuric acid with the aim of accelerating roasting process. The roasted material is leached with tap water for the recovery of zinc and indium. The influence of roasting temperature, particle size, and the ratio of acid to ZSOD mass on the rate of roasting reactions were investigated. The results show that the extraction rate of zinc and indium increases with increasing the roasting temperature and the ratio of acid to ZSOD mass, while decreases with increase in the particle size. The samples were characterized using XRD and SEM. Under the conditions employed in the present work a maximum recovery of 94% and 90% of zinc and indium could be achieved. Shrinking core model was used to describe the kinetic parameters and to identify the rate controlling step. Both zinc and indium extraction were controlled by the internal diffusion in the solid product layer, with the apparent activation energy of the roasting reaction were 15.94kJ/mol and 12.68kJ/mol respectively. The reaction order with respect to concentrated sulphuric acid was 1.75 and 1.59 for zinc and indium, respectively.

Kinetics of nickel extraction from Indonesian saprolitic ore by citric acid leaching under atmospheric pressure

The kinetics of leaching of a saprolitic ore from Indonesia by citric acid solution under atmospheric pressure was investigated. An examination of the effects of leaching temperature, citric acid concentration, pulp density and ore particle size on the dissolution rate ofnickel found that they all had significant influence on the rate. The highest nickel recovery (95.6 percent) was achieved under the leaching conditions of ore particle size of 212–355 microns, citric acid concentration of 1 M, leaching time of 15 days, pulp density of 20 weight/volume percent, leaching temperature of 40°C and shaker speed of 200 rpm. The shrinking core model was found to be appropriate for describing the leaching kinetics of this ore in citric acid solutions at atmospheric pressure. The experimental data were well interpreted by this model with rate ofreaction controlled by diffusion through the solid product layer. Using the Arrhenius expression, the apparent activation energy for nickel dissolution was evaluated as 12.38 kJ/mol. Finally, on the basis of the shrinking core model, a proposed empirical kinetic model for the leaching of nickel from this Indonesian saprolitic ore was expressed as a mathematical model, which was verified as consistent with the obsenved experimental results.

Leaching kinetics of calcification roasting calcinate from multimetallic sulfide copper concentrate containing high content of lead and iron

The multimetallic sulfide copper ore containing large amounts of lead and iron was roasted in air atmosphere in the presence of calcium oxide, and the calcinate obtained was leached in sulfuric acid solution to extract copper. Characterization of the raw materials, calcinates and leach residues was conducted by using XRD and SEM/EDS analysis. The calcification mechanism of the complex ore was studied. The effects of temperature, stirring speed, liquid/solid ratio and sulfuric acid concentration on the kinetics and mechanism of copper dissolution from the calcinate were also investigated. Results of experiments show that sulfur retention efficiency in the calcinate achieves 97.77%, and that increasing both reaction temperature and acid concentration are capable of resulting in the increase of dissolution rate of copper. Leaching kinetics follows the un-reacted shrinking core model with a rate controlling step by diffusion through the solid product layer and the corresponding apparent activation energy is calculated as 19.21kJ/mol, and consequently the rate of the dissolution of copper on aspect of H2SO4 concentration, liquid/solid ratio, and stirring speed can be expressed as 1+2(1-x)-31-x2/3=6.44×10-4×CA1.72×L/S1.25×ω0.94exp(-2310/T)t.