Showing papers on "Calcium aluminates published in 2015"

Corrosion behavior of calcium zirconate refractories in contact with titanium aluminide melts

Abstract: This contribution investigated the corrosion mechanisms of calcium zirconate crucibles during vacuum induction melting of titanium aluminides. The crucibles withstood several melts and exhibited no cracking due to thermal shock. However, a significant amount of zirconium dissolved in the melt, whereas the oxygen content of the melt increased on a much lower level. In contrast to melting Ti6Al4V in the same crucibles an increased calcium zirconate content of the crucibles did not reduce the melt contamination of the titanium aluminide melts. The investigation of the corrosion front of the crucibles revealed the formation of calcium aluminates, whereas titanium was almost absent. The melting of titanium aluminides with subsequent refining by electro slag remelting could be a viable production route to produce titanium zirconium aluminum alloys. Furthermore, the relation between the corrosion of calcium zirconate refractories in contact with titanium based alloys depending on their composition should be further evaluated.

High Temperature Reaction of MCrAlY Coating Compositions with CaO Deposits

Abstract: The reactivity of β-NiAl + γ-Ni-based NiCoCrAlY alloys with and without CaO deposits was studied by means of isothermal exposures in air Reaction with CaO at 1100 °C produced multi-layer scales of Al2O3 and calcium aluminates, and a mixture of liquid calcium chromate and nickel–cobalt oxide particles Calcium chromate formation was a rapid, transient process, and the transition to a steady-state of slower Al2O3 growth was favored by increasing the alloy β fraction The thermally-growing Al2O3 reacted with the deposit to form calcium aluminates in a solid-state diffusion process, which led to an increased oxidation rate The analysis of Al2O3 growth kinetics in the production-destruction regime was used to account for the increased flux of aluminum entering the multi-layer scale The effect of temperature on the ability to kinetically sustain an Al2O3 scale was then evaluated on the basis of Wagner’s criterion Predicted results were consistent with the experimentally observed absence of passivation at 900 °C

Calcium hexaaluminate synthesis and its influence on the properties of CA2–Al2O3-based refractories

Abstract: The investigations of low thermal expansion coefficient refractories, made of calcium dialuminate (CA2) and corundum, have been presented. Classifying them into a group of reactive materials means that there is a chemical reaction at the contact area of CA2 and Al2O3 grains, accompanied by a formation another compound CaAl12O19 (CA6) during firing. The conversion degree of the reaction as a function of temperature and time has been determined, including the formulation of kinetic equations, the assessment of apparent activation energy and a presumable mechanism of CA6 formation has been proposed. It has been proved that the formation of plate-shaped CA6 crystals at the contact area of Al2O3 grains with the surrounding CA2 matrix changes pore size distribution and total porosity of the materials. It is also related with local changes of thermal expansion coefficients, which in turn influences total expansion of the tested samples.

Characteristics of Copper-based Oxygen Carriers Supported on Calcium Aluminates for Chemical-Looping Combustion with Oxygen Uncoupling (CLOU)

Abstract: Eight different oxygen carriers (OC) containing CuO (60 wt %) and different mass ratios of CaO to Al2O3 as the support were synthesized by wet-mixing followed by calcination at 1000 °C. The method of synthesis used involved the formation of calcium aluminum hydrate phases and ensured homogeneous mixing of the Ca2+ and Al3+ ions in the support at the molecular level. The performance of the OCs for up to 100 cycles of reduction and oxidation was evaluated in both a thermogravimetric analyzer (TGA) and a fluidized bed reactor, covering a temperature range of 800 to 950 °C. In these cycling experiments, complete conversion of the OC, from CuO to Cu and vice versa, was always achieved for all OCs. The reactivity of the materials was so high that no deactivation could be observed in the TGA, owing to mass transfer limitations. It was found that OCs prepared with a mass ratio of CaO to Al2O3 in the support >0.55 agglomerated in the fluidized bed, resulting in an apparent deactivation over 25 cycles for all tempe...

Study of the hydration of calcium zirconium aluminate (Ca7ZrAl6O18) blended with reactive alumina by calorimetry, thermogravimetry and other methods

Abstract: The reactivities of hydratable alumina, calcium zirconium aluminate and their binary mixtures were investigated by calorimetric method, X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), differential thermal/thermogravimetric analysis (DTA–TG) and scanning electron microscope observations. The rate of heat evolution illustrates only one distinct exothermic peak in the mixtures containing calcium zirconium aluminate which corresponds the subsequent precipitation of hydrated material mainly in the form of amorphous CaO–Al2O3–H2O phases and crystalline C4AH19 occurs simultaneously after wetting of Ca7ZrAl6O18 grains. Nevertheless, coexistence of both unstable calcium aluminate hydrates (CAH10, C2AH8, C4AH19) and thermodynamically more stable C3AH6 detected by FT-IR and DTA–TG in the paste cured at 20 °C may have originated in the thermally induced partial conversion reaction due to the considerable amount of heat generated by the Ca7ZrAl6O18 hydration. It is also found that curing temperature affected the hydration products formed in the hydration products of both Ca7ZrAl6O18 and Ca7ZrAl6O18/Al2O3 blends. Reactive alumina influences the hydration behavior of Ca7ZrAl6O18 facilitating the nucleation and growth of hydration products, causes characteristic changes in the microstructure of hardened blended pastes and favors recrystallization of dehydrated calcium aluminates.

Effect of Ca on the Behavior of Double Oxide Film Defects in Commercially Pure Aluminum Melt

Abstract: The change in the composition of oxide layers and the possibility of the formation of bonding between the two layers of a double oxide film defect when held in Al-0.01 and 0.05 wt pct Ca melt was investigated, and the thermodynamics of the transformations occurring in the Al-Ca-Al2O3 system was studied. The defect was modeled experimentally by maintaining two aluminum oxide layers in contact with one another in the liquid metal at 1023 K (750 °C) for times ranging from 5 seconds to 50 hours. Any changes in the composition and morphology of these layers were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the initial Al2O3 layer started to transform to a mixture of calcium aluminates in both alloys when the Al bars were still in the solid state. This transformation caused the two oxide layers to bond with each other gradually after the bars melted. The degree of bonding, however, was not consistent between the repetitions of the experiments, which was attributed to the difference between the initial H content of the melt on different experimental days.

Quick hardening grout composition

Abstract: PROBLEM TO BE SOLVED: To provide a quick hardening grout composition capable of being given a sufficient initial expansion property by material age of 24 hours.SOLUTION: There is provided a quick hardening grout composition containing cement, calcium aluminates, gypsum, sodium percarbonate, sodium carbonate and setting retarder, with the content of sodium carbonate of 0.3 to 1.2 pts.mass and the content of sodium percarbonate of 0.03 to 0.2 pts.mass based on 100 pts.mass of the total content of cement, calcium aluminates and gypsum.

Ternary binder system

Abstract: A mineral binder system that includes a mineral binder B1 of calcium aluminates having a ratio C/A of calcium oxide C to aluminum oxide A of 0.7 to 1.1, a mineral binder B2 of calcium aluminates having a ratio C/A of calcium oxide C to aluminum oxide A of 1.2 to 2.7, and at least one sulfate carrier is provided. Also provided are a dry mortar which includes this mineral binder system, a wet mortar based on the disclosed dry mortar, and the use of the dry mortar as tile adhesive mortar, levelling compound, waterproofing slurry, grout mortar, screed binder, screed mortar, repair adhesive, and/or damp proof membrane.

Phase transformation and leaching performance of Na2O-doping calcium aluminate compounds

Abstract: Calcium aluminate clinkers doped with Na2O were synthesized using analytically pure reagents CaCO3, Al2O3, and Na2CO3 at 1350℃ for 1 h. XRD, SEM and EDS were used to study the phase transformation of calcium aluminates and the leaching performance of clinkers from CaO-Al2O3 system doped with Na2O. The results show that the main phases of clinkers from CaO-Al2O3 system are CaO·Al2O3 and 12CaO·7Al2O3, while those of clinkers from CaO-Al2O3 system doped with Na2O are CaO·Al2O3, 12CaO·7Al2O3, Na2O·Al2O3 and Na4Ca3(AlO2)10, when the molar ratio of CaO to Al2O3 is 1.0. Some of the Na2O forms compounds containing Na2O, the rest is doped in 12CaO·7Al2O3, meanwhile nearly none of Na2O is doped in CaO·Al2O3. With the increasing of the Na2O content in clinkers, the amounts of 12CaO·7Al2O3 and Na4Ca3(AlO2)10 increase while the amount of CaO·Al2O3 decreases, and the crystallinity of 12CaO·7Al2O3 and Na4Ca3(AlO2)10 decrease while that of CaO·Al2O3 increases. Due to the doping of Na2O, the leaching performance of alumina in the clinkers using the sodium carbonate solution is improved and the space groups of CaCO3 in the leached residues are changed from R-3CH and P63/MMC to only R-3CH.

Effect of the conditions of REM microalloying of steel on the corrosion activity of nonmetallic inclusions

Abstract: Experimental heats of low-alloy steel are performed under various conditions of rare-earth metal microalloying and aluminum and calcium deoxidation. Electron-probe microanalysis of nonmetallic inclusions and a metallographic investigation of a metal are used to show that, when interacting with water, nonmetallic cerium oxide inclusions do not form hydrates and, correspondingly, are not aggressive. When aluminum, calcium, and cerium additions are sequentially introduced into a melt, a continuous cerium oxide shell forms on calcium aluminates, protects corrosive nonmetallic inclusions against interaction with water, and weakens local metal corrosion.

Reactivity of Coke Ash on Alumina-Carbon Blast Furnace Hearth Refractories

Abstract: Iron making blast furnaces may be described as continuously operated counter current shaft reactors that are lined with carbon and oxide based refractory materials. Minimising refractory degradation in the blast furnace hearth is necessary to achieve the typical operational furnace life of 20 years or longer. The hearth refractories are principally carbon or carbon based and may also be lined with aluminosilicate and alumina–carbon refractories at the hot face.1) Refractory degradation mechanisms include, thermal cycling, erosion via the flow of iron and slag, abrasion with coke, reactions with the liquid phases (iron or slag) and reactions with the coke ash remaining after coke dissolution in liquid iron. Coke is a key reagent in a blast furnace and may contain up to 15 mass% ash. This ash component often contains significant amounts of silica.1) This coke ash may react and degrade the furnace hearth refractories if it persists in the coke until it is lower in the blast furnace. While the coke added to the blast furnace may contain significant amounts of silica there is evidence that suggest that much of this silica is reacted from the coke prior to it arriving in the hearth Reactivity of Coke Ash on Alumina-Carbon Blast Furnace Hearth Refractories

Formation and evolution of Spinel in aluminum killed calcium treated linepipe steels

Abstract: Magnesium aluminate (Spinel) inclusions have been observed to cause hook crack in line pipe steels. These inclusions are typically formed during liquid steel processing at the ladle metallurgy furnace (LMF) and subsequently modified by calcium to form less harmful liquid inclusions. Unfortunately, spinel inclusions are still sometimes observed in samples taken from the tundish and coil after calcium treatment. The formation and evolution of spinel inclusions during liquid steel processing at two industrial mini mills was investigated by analysis of lollipop samples taken at different stages of the steelmaking process. The effect of reoxidation on the formation of spinels after calcium treatment was also investigated by thermodynamic calculations and laboratory experiments. The results showed that spinel inclusions are first formed in liquid steel after desulfurization and by the reaction between dissolved aluminum in liquid steel and MgO in the ladle slag. These inclusions were modified by calcium to both liquid and solid calcium aluminates. Also observed after calcium treatment, was the formation of MgO rich and CaS inclusions. During liquid steel transfer to the tundish, evidence of reoxidation and ability of CaS to act as a source of calcium to modify inclusions formed during reoxidation was observed. Analysis of the results from the laboratory experiment showed that with sufficient magnesium in steel (about 4ppm) spinel inclusions can be formed during liquid steel reoxidation. The results also showed that spinel inclusions observed after reoxidation of calcium treated steel are larger in size than those observed prior to calcium treatment. The modification of spinels by calcium and reformation after liquid steel reoxidation was verified by thermodynamic calculations.

ВПЛИВ МіКРОКРЕМНЕЗЕМУ НА ФАЗОВИЙ СКЛАД і ВЛАСТИВОСТі ШПіНЕЛЕУТВОРЮЮЧОї КОМПОЗИЦії

Abstract: Experience in using low cement alumina-magnesia castables, containing spinel-forming reagents, in monolithic ladle linings identified their significant advantages in terms of thermomechanical properties, corrosion resistance and slag resistance. A particular feature of these castables is the synthesis of «in situ» magnesium aluminate spinel at high operating temperatures, which provides increased operational lifetime of the lining.The microsilica influence on the phase constitution of the composite mixture, containing calcium aluminate cement and spinel-forming reagents - calcined alumina and sintered periclase after firing at 17000C was investigated in the paper. It was found that as a result of the interaction of cement calcium aluminates with microsilica, fusible compounds - anorthite and helenite that, at increased microsilica content, impair physical and technical properties of the composite are formed. It was determined that the ratio of spinel-forming reagents and content of the silica-containing material in a matrix component of alumina-magnesia castables is a more important factor of the synthesis of high-melting-point crystalline phases than the microsilica content. Directed regulation of the phase constitution of the matrix component of alumina-magnesia castables by adjusting the grain-size composition of spinel-forming reagents and their optimal ratio will ensure achieving a set of the given physical and technical characteristics of concrete and increasing the operational lifetime of monolithic ladle linings.

Study on the Evolution of Nonmetallic Inclusions in N510L Beam Plate During Production Process

Abstract: The results showed the microinclusions were mainly Al2O3 and sulfide inclusions from tapping to the beginning of LF refining. Al2O3 inclusions were modificated into calcium aluminates after calcium treatment. Lots of spherical calcium aluminates composite inclusions containing MgO, TiO2 and MnS were found after soft blowing and during tundish metallurgy and left in the slab which caused the defects of beam plates. Pure Al2O3 inclusions were observed again after soft blowing 8min. Average sulfur content in inclusions decreased because of the high desulfurization ability of refining slag. Average Ca/Al ratio value in inclusions increased during LF refinging because of the reduction of CaO in slag by carbon and it increased obviously during the process of soft blowing 8min because of the dissolusion of calcium wire.