Dissolution Of MgO Research Articles

Foaming Behavior of CaO-SiO2-FeO-MgOsatd-X (X=Al2O3,MnO, P2O5, and CaF2) Slags at High Temperatures.

The foaming index was measured for CaO-SiO 2 -FeO-Al 2 O 3 and CaO-SiO 2 -FeO-MgO satd -X (X=Al 2 O 3 , MnO, P 2 O 5 , and CaF 2 ) slags to understand the foaming behavior. The foaming index of the CaO-SiO 2 -FeO-10Al 2 O 3 slags (C/S=0.93 and 1.2) decreases with increasing content of FeO up to 20 % and is almost constant for FeO content through 20 to 40 %. The viscosity of slags could be considered as the major contributor to foaming behavior. The addition of Al 2 O 3 into the silicate slags results in an increase of foaming index due to an increase of slag viscosity; this could be explained by the structural role of Al 2 O 3 in aluminosilicate slags. In the MgO-saturated and Al 2 O 3 -containing slags, FeO behaves as an acidic oxide, because slag melts would be more basic than MgO-saturated and non-Al 2 O 3 slags, where FeO behaves as a basic oxide, due to Al 2 O 3 enhances the dissolution of MgO into the slags. The addition of MnO into the MgO-saturated slags decreases foaming index, simply due to a decrease of slag viscosity. However, the addition of CaF 2 and P 2 O 5 into the slags results in the complex foaming behavior of slags; this is probably due to the Marangoni effect. The relationships between foaming index and the physical properties of slags can be obtained from the dimensional analysis as follows: Σ=214 μ/√ pO (for the CaO-based slags) Σ=999 μ/ √ O (for the MgO-saturated slags) The foam height is predicted as a function of decarburization rate from the molten iron and the contribution of slag foaming in EAF process was discussed as a function of decarburization rate.

Degradation mechanisms of magnesia-chromite refractories in vacumm-oxygen decarburisation ladles during production of stainless steel

Magnesia–chromite bricks are used as refractories for the refining of stainless steel in vacuum–oxygen decarburisation (VOD) ladles. Refractory wear is not uniform. In the present work, worn bricks from different zones in the ladle have been analysed, and a set of interdependent degradation mechanisms is proposed. Refractory wear as a function of position in the ladle is discussed. Slag infiltration and MgO dissolution from the refractory were observed in all samples, whereas FeOx decomposition was seen at two levels in the high wear samples. First, partial decomposition of primary chromite crystals ( (Mg) [Fe3+, Cr, Al]2 O4 ) occurred at the hot face of the brick. Three layers were distinguished in the reacted chromite crystals and a reaction mechanism is presented. Second, a decrease in the FeOx content of the magnesia phase occurred at the hot face of the brick. The negative effect of the presence of FeOx in magnesia–chromite refractories is discussed and the influence of the ferrostatic pressure is demonstrated. Finally, the consequences of the following phenomena are discussed: increase in brick porosity, slag infiltration, corrosion, erosion of the partially liquid bonded refractory system, and spalling and cracking.

Reactions Between MgO-C Refractory, Molten Slag and Metal

The behavior of MgO-C refractory-slag-metal system, which is caused by the reactions such as the dissolution of MgO and graphite in the refractory into slag and metal respectively and the generation of gas bubbles, was observed directly by using a high temperature X-ray radiographic apparatus and analysed theoretically. The local corrosion is regarded as due to the cyclic dissolution of MgO and graphite in the refractory into slag and metal phase respectively. The local corrosion is greatly influenced by the generation position of gas bubbles and the number of gas bubbles generated. Bubbles generated in the refractory-slag-metal three-phase boundary restrain the local corrosion, while bubbles generated at the refractory-metal interface enhance the local corrosion. Gas bubbles form mainly according to the reaction between (FeO) in the slag film and C(s) in the refractory contacting with the slag film: (FeO)+C(s)=Fe(l)+CO(g). Some fundamental principles to improve MgO-C refractory were put foward.

The kinetics and mechanism of MgO dissolution

Reactions and atomic rearrangements at fluid–crystal interfaces play an important role in catalysis and in controlling the kinetics and mechanisms of dissolution. We have studied the attachment and reactions of water molecules at the MgO–water interface by combining measurements of 1 H and 2 D surface penetration and etch pit morphology with ab initio calculations. These studies show that the most common MgO cleavage surface, (001), is thermodynamically unstable when hydrated. Proton rearrangement on such surfaces precedes proton–cation exchange and provides a general mechanism for the detachment of ions during dissolution. The kinetics of dissolution are strongly influenced by the concentration of surface defects and a simple model based on the ab initio results predicts a dissolution rate of 10 −10 mol cm −2 s −1 for a typical surface defect concentration of 0.1.

Kinetics of MgO Dissolution and Buffering of Fluids in the Waste Isolation Pilot Plant (Wipp) Repository

AbstractDOE has proposed to add MgO to backfill in the WIPP repository to stabilize pH and reduce gas pressures and spallation. Experiments show that MgO dissolution rates are proportional to surface area, nearly independent of pH, and decrease with increasing dissolved NaCl. The probable repository MgO dissolution rate will be ≤ 10 to 20 μmol m2 min−1. Simulations indicate that MgO dissolution will buffer pH and provide stabilizing cement; however, under highly reducing conditions MgO will not control total gas pressures. Adding oxidizing agents or materials containing oxidized species, for example anhydrite, to the backfill, might control gas pressures.

5626659 Means and method of recycling asphalt composition shingles: Chivers Morgan Fremont, CA, United States

Silica scaling is one of the main bottlenecks in the reuse of papermaking effluents by reverse osmosis. The low hardness of deinking paper mill effluents makes necessary the addition of magnesium compounds to increase silica removal at high pH. Based on the results obtained in Part I, MgO was selected as the most efficient magnesium source. Its efficiency was tested at different dosages (150–10,000 mg/L), pH values (8.2–9.5) and temperatures (25–50 °C) and the optimization of the reaction time was also carried out. Silica removals over 95% were obtained at the 4 pHs and 3 temperatures with MgO dosages over 500 mg/L; however, MgO can only be applied if water temperature is higher than 35 °C, as the dissolution of MgO is limited. Moreover, the analysis of the solids obtained (SEM–EDX and FTIR) showed that the main mechanism for silica removal was co-precipitation of magnesium silicates (forsterite and antigorite) while adsorption was less significant.

Microstructural analysis of corroded alumina-spinel castable refractories

Microstructural analysis indicates that the corrosion of alumina-spinel castables by a liquid steel ladle slag involves initial reaction of CaO in the slag with alumina fines in the refractory bond. The slag becomes saturated in CaO and Al 2O 3 and CA 2 is precipitated from it along with other calcium aluminates. The spinel fines and grains in the refractory take up the MnO/FeO/Fe 2O 3 from the slag, which becomes silica-rich as CA 2 is precipitated and highly viscous. The thickness of the remnant slag layer on the corroded samples indicates the viscosity of the liquid slag under the test conditions. Further into the refractory CA 6 is formed, firstly crystallized with a tabular morphology in dense regions and then with a similar morphology in more porous regions. Deeper still in the refractory CA 6 takes a whisker/needle habit suggesting some vapour-phase reaction. MgO-rich spinel-alumina castables undergo less corrosion under these conditions as dissolution of MgO leads to a viscous MgO-rich slag which is less corrosive to the Al 2O 3 components of the refractory.

Phase Relation in the System Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-MgSiO<sub>3</sub>

Phase relations in the system tricalcium phosphate (3CaO⋅P2O5=C3P)-enstatite (MgO⋅SiO2=MS) have been investigated in the temperature range 1100 to 1600°C by means of the quenching method. C3P takes β-type (whitrockite) in a wide region by dissolution of MgO, and diopside (d) appeared in the composition range more than 40mass% C3P by a reach in between MgSiO3 and liberated CaO. The enstatite transfers to iron-free pigeonite (pi) by taking CaO in an MS-rich region, where pi coexists with protoenstatite, MgSiO3 (pr). Primary crystals which appeared in a wide region are β-C3P and pi, and primary diopside appears in a narrow region at a minimum temperature (1265-1305°C). In the subsolidus region below 1245°C, β-C3P coexists with d, pi or pi-pr mixture. The system is not binary and it should be discussed from the ternary system C3P-M-S and the quaternary system C3P-CMS2-MS-S.

Dissolution of MgO in CaO–“FeO”–CaF2–SiO2 Slags under Static Conditions

The dissolution of MgO in CaO–“FeO”–CaF2–SiO2 slags has been studied in the temperature range 1573°–1673° K under static conditions. The concentration profiles of Mg in the product Mg1‐xFexO solid‐solution layer as well as slag were determined by EDS analysis. From this, the diffusivities of MgO in the slag and the. interdiffusivities in solid solution were estimated. The dissolution of MgO in CaO–“FeO”–CaF2–SiO2 slags increased with CaF2 content.

Kinetics of dissolution of ZnO, MgO and their solid solutions in aqueous sulphuric acid solutions

The dissolution kinetics of polycrystalline preparations of ZnO, MgO and their solid solutions was studied in H 2SO 4 aqueous solutions using the rotating disc technique. For each of these oxides, ranges of experimental parameters were established within which the rate of dissolution was diffusional, activation- diffusional or activation-controlled. For ZnO the limits of these ranges were shifted towards higher acid concentrations and lower temperatures than for MgO. In the diffusional regime the dissolution rate was controlled by the protons transporting ions (H 3O + and HSO 4 −). At concentrations of 0.01–0.1 M H 2SO 4, MgO dissolution occurred under activation-diffusional and activation-control; the reaction order with respect to the concentration of H 3O + ions was 0.5. At those H 2SO 4 concentrations, the dissolution rate of ZnO remained diffusionally controlled. In solutions of CH 2SO 4 ≥ 0.1 M H 2SO 4, the reaction order with respect to H 3O + concentration was close to 0.1 for each preparation. The rate of chemical reaction found for ZnO and Zn 0.93Mg 0.07O was about ten times higher than for MgO and Mg yZn 1−yO, the reduction of y within the range 1 ≥ y≥0.65 led to an increase of the dissolution rate. An attempt has been made to give an explanation for the results obtained.

Preparation of Aluminum Nitride/Aluminum Composites by Nitridation of Aluminum

AlN/Al composites were prepared by the nitridation of Al-MgO (0-7.5wt%) powder compacts at 1100-1400°C. The dense composite layer grew outward from the bottom surface of the powder compact at 1250-1400°C. The conversion of Al to AlN and the thickness of the composite increased with increasing content of MgO, reaction temperature and time. The nitridation of the powder compact proceeded in the following four steps: (i) nitridation of the surface of Al particle, (ii) destruction of the surface AlN film and the eruption of molten Al by thermal mismatch between Al and AlN, (iii) dissolution of MgO into erupted Al, (iv) growth of AlN/Al composite by the nitridation of MgO-dissolved Al. The microstructure of the resulting composite from which Al metal was removed by etching shows the channels which molten Al might wick through during nitridation. According to the outward growth and the microstructure of the composite, the growth mechanism of AlN/Al composite may be the same as that of so-called LanxideTM process.

Effect of Ultrasonic Irradiation on Hydration of MgO Powder.

High-power ultrasound was applied during the hydration of MgO fine powders in order to see the effect of ultrasound in a heterogeneous fluid where the dissolution of MgO particles and the precipitation of Mg(OH)2 are included. The results obtained showed that the irradiation of ultrasound fairly promotes the dissolution of MgO, which is the rate-determining step in the reaction, and that it promotes the nucleation of Mg(OH)2, which has resulted in the reduction of crystallite sizes of the Mg(OH)2 precipitated.

Enthalpy change of the reaction [formula omitted] enstatite + corundum = pyrope and the enthalpy of formation of pyrope

The enthalpy of formation ( ΔH f 0) of pyrope has been redetermined by solution calorimetry in lead borate solvent at 700°C (973 K) on synthetic pyrope and on mixtures of synthetic enstatite + corundum on pyrope composition. Attempts to obtain ΔH f 0 more directly by dissolving oxide mixtures on pyrope composition failed because of incomplete dissolution of MgO. ΔH f 0 (pyrope) is obtained from the reaction 3 2 Mg 2Si 2O 6 + Al 2O 3 = Mg 3Al 2Si 30 12 enstatite corrundum pyrope, using measured solution values for each side of the reaction combined with an enthalpy of formation of enstatite. Measured solution values for reaction (A), at 973 K, are (kJ/mole) as follows: ΔH sol( 3 2 En + Cor) = 139.22 ± 3.74, n = 14 ΔH sol ( Py) = 111.49 ± 2.18, n = 19. Uncertainties are twice the standard deviations on the means (δ). Using these values and the ΔH f,1073 0 (enstatite) from alkali borate solution calorimetry of Brousse et al. (1984) on synthetic enstatite, the calculated enthalpy of formation of pyrope at 298 K, corrected with measured heat content data, is ΔH f,298 0 ( Py) = −72.01 ± 6.98 kJ/mole. This value agrees well with the −70.96 kJ/mole value from the internally consistent dataset of Holland and Powell (1990) and with the value of Berman(1988) of −74.25 kJ/mole. This agreement indicates that major discrepancies no longer exist between thermodynamic measurements and experimental phase equilibrium data for pyrope. Anomalous solution behavior of MgO in Pb 2B 2O 5, most likely incomplete dissolution, is the most probable reason for the substantially more negative ΔH f 0 of pyrope found by Charlu et al. (1975).

Al<sub>2</sub>O<sub>3</sub> Coating on MgO Particles

MgO particles were coated with a chelate compound of aluminum and heated in air to prepare Al2O3-coated MgO particles for modifying surface properties. The dissolution of MgO in water was suppressed by the Al2O3 coating. The amount of dissolved Mg ion was minimum after heating at 500°C. The Al2O3-MgO layer 10-30nm thick was observed on the surface of the MgO particles heated at 500°C. Fine particles 10nm in diameter were seen in the coating layer and they aggregated into clusters 300-500nm in diameter after heating at 1000°C. It was also found that the adherence of coating layer to MgO particles decreased after heating at 1000°C compared with those heated at 500°C.

Formation Mechanism of Mg(OH)<sub>2</sub> Prepared by Hydration of Magnesia Clinker in Magnesium Salt Solution (Part 1)

Mg(OH)2 particles of hexagonal plate and prism are prepared by hydration of magnesia clinker in magnesium salt solution. The hydration mechanism of magnesia was investigated by measuring reaction rates and SEM observations. The hydration rate was proportional to the surface area of MgO particles, suggesting that the reaction proceeds by the dissolution of MgO particles followed by the precipitation of Mg(OH)2 and that the dissolution was rate-controlling. In Mg(CH3COO)2 system, the reaction was divided roughly into two stages. The rate of the first stage was about half that of the second stage. MgO surface was covered with dense Mg(OH)2 layer during the first stage. At the second stage, however, Mg(OH)2 layer peeled off from MgO surface. These results suggested that the rate-controlling mechanism in the dissolution of MgO was diffusion through the product layer of Mg(OH)2 at the first stage, and surface reaction at the second stage. In MgCl2 system, the rate remained constant only in a short period and decreased gradually with reaction time. Mg(OH)2 films on MgO were observed as in Mg(CH3COO)2 system, though it did not peel off completely. Hence, this product layer prevented MgO from dissolution and limited the rate in MgCl2 system.

Dissolution kinetics of MgO in aqueous, acidic media

The method of the rotating disc was used to study the kinetics of MgO dissolution. Single crystals of MgO with the orientation {100}, {101}, and {111} were dissolved in N2-saturated HClO4-NaClO4 solutions of constant ionic strength (I=1.0 mol kg−1). Chemical, mixed and diffusional control of the reaction leading to dissolution has been found between 25 to 90°C and 0.5<pH<3. The dissolution at 90°C andpH⩾3 occurred diffusionally controlled with respect to H+ ions. Independent measurements of the limiting current densities for the cathodic reduction of H+ ions were carried out in solutions of the same composition and confirmed this kind of control. In the chemical control region the dissolution rates were proportional to the Mg2+ ion densities of the single crystal surfaces investigated. The dissolution rates relative to {111}, at 40°C andpH=1, were found to be: 1.0±0.1 (1.0) {111}; 1.3±0.2 (1.2) {101} and 1.7±0.2 (1.7) {100}. Figures in parentheses refer to the relative Mg2+ ion densities of the surface. Reproducible results of high quality can only be obtained when “fresh” dislocations are removed by chemical polishing.

Governing equations for heat and mass transfer in heat-generating porous beds—II. Particulate melting and substrate penetration by dissolution

Upon dryout of the bed, the dominant modes of heat transfer are conduction and radiation. Radiation is modeled through the Rosseland approximation. The melting of stainless-steel particulate imbedded in the fuel is modeled by assuming the bed to be a continuum with conduction and radiation as the dominant modes of heat transfer. The molten steel, after it drains to the bottom of the bed, is assumed to disappear into cracks and mortar joints of the MgO bricks. The melting of fuel in the interior of the bed is modeled identically to the steel particulate, except for the bed settling which is more pronounced in the case of fuel melting and is assumed to be instantaneous owing to the significant weight of overlying bed and sodium pool. The molten layer of fuel, as it collects at the bottom of the bed, causes the heatup of the MgO lining to the eutectic temperature (2280°C), and the MgO lining begins to dissolve. The density gradient caused by the dissolution of MgO leads to natural convection and mixing in the molten layer. The submerged fuel particulate also begins to dissolve in the molten solution and ultimately leads to the conversion of debris to a molten pool of fuel and MgO. The process of penetration of the MgO lining continues until the mixing process lowers the concentration of fuel in the volume of the pool to the level where the internal heat rate per unit volume is not enough to keep the body of the pool molten and leads to freezing in the cooler part of the pool. As the molten pool reaches a frozen or a quiescent state, the MgO brick lining thickness provided is deemed ‘safe’ for a given bed loading and the external rate of cooling.

Dissolution and hydration kinetics of MgO

The dissolution and hydration kinetics of MgO single crystals and powder samples were investigated with regard to the H + and Mg 2+ concentrations and the temperature. The rate of dissolution of rotating MgO discs in buffered solutions was determined from measurements of [Mg 2+] and those of the crystals and powder fractions were determined by pH and conductivity analysis. The degree of hydration was analysed by means of a thermogravimetric method. Several rate-controlling processes depending on pH were present at room temperature. (1) At pH < 5 the rate-controlling step was proton attack followed by desorption of Mg 2+ of OH - depending on the value of [Mg 2+]. The rate was proportional to either -pH or pMg-pH. These processes are part of the overall neutralization reaction. MgO + 2H +→Mg 2+ + H 2 O (2) At pH ≈ 5 the rate-controlling step was a diffusion-limitation process due to protons. The rate was proportional to the proton concentration. (3) At pH > 7 the rate-controlling step was OH - adsorption followed by Mg 2+ and OH - desorption leading to a rate maximum. These processes are part of the overall dissolution reaction. MgO + H 2 O→Mg 2+ + 2OH - The neutralization processes are interpreted in terms of irreversible thermodynamics yielding a linear dependence of the rate on pH or pMg-pH. It is concluded from conductivity and scanning electron microscopy measurements during and after hydration experiments that the hydration rate is controlled by the dissolution rate under given conditions. After a supersaturation period Mg(OH) 2 precipitates preferentially at the MgO surface, so that an MgO lattice reaction can be excluded. All processes undergo an Arrhenius acceleration with increasing temperature (activation energy, 70 kJ mol -1) and the overall reactions are then limited by proton and OH - diffusion.

Dissolution of MgO during Sintering of Alumina

Bayer's alumina of submicron order was doped with MgO either as Mg(N03)2 or magnesio-aluminate hydrate. The interaction of the additives with alumina powder was studied al 1450° and 1500°C in the form of thin pellets. Structural changes have been observed for the products sintered at 1500°C. Expansion of the unit cell of alumina was observed when MgO was added as Mg(NO3)2 and there was contraction in the unit cell when magnesio-aluminate hydrate was used. It was observed that MgO dissolved in the corundum structure to the extent of around 0.28 wt%. An explanation has been suggested for the difference in behaviour in the case of the two additives.

Specific chemical rate constants for the acid dissolution of oxides and silicates

Correlations relating the specific chemical rate for the acid dissolution of MgO, Mg 2SiO 4, ZnO, Zn 2SiO 4 and ZnFe 2O 4 to temperature and hydrogen ion concentration/activity have been produced from a critical analysis of published dissolution data. In determining specific chemical rates it is important that consideration is given to the means of determination of the surface area of the solid and to the likelihood of a diffusional resistance contributing to the dissolution rate. It is demonstrated that at 25°C, the chemical dissolution rates of the silicates are significantly lower than those of the corresponding oxides. This finding is discussed in terms of current theories of the oxide dissolution process.