Showing papers on "Calcium aluminates published in 2010"

Carbon Transfer in the Lower Zone of a Blast Furnace

Abstract: In this paper results from an investigation focused on coke/iron reactions occurring in the hearth of a blast furnace, below the slag layer, are presented. In this region, as coke dissolves in the iron there is the potential for a mineral layer to form and persist at the coke/iron interface and inhibit the rate of coke dissolution. Key findings of the investigation include that the composition of the mineral layer was principally composed of oxides of aluminium and calcium, present as various calcium aluminates and that the morphology of the layer profoundly affected the rate of carbon transfer. Follow-up up studies on the kinetics of carbon transfer focussed on how the wetting and interfacial characteristics of iron on the calcium aluminates would affect iron penetration of the mineral layer are also reported and discussed as was a new material, called coke analogue, that could be used in the study of real industrial coke. Analysis of the wetting behaviour of liquid iron on alumina and the calcium aluminates investigated indicated that iron wetting of the mineral layer was not a key parameter in coke dissolution in iron for the coke composition studied. The brief details given for the coke analogue indicated that it did replicate coke dissolution behaviour in iron, though more experimental work is required to fully understand and exploit the potential of this material.

Neutron diffraction study of molten calcium aluminates

Abstract: We use the aerodynamic levitation technique combined with CO2 laser heating to study the structure of liquid calcium aluminates using neutron diffraction. We determine the structure factors and corresponding pair correlation functions describing the short-range order in the liquids above the melting points. The combination of the experiments with ab initio molecular dynamics simulations makes possible to get a detailed description of the liquid structure. We find that ordered tetrahedral sites around the Al atoms become more prevalent with increasing CaO concentration, while the Ca atoms present highly distorted environments at all compositions studied.

Microstructure and properties of silicon nitride ceramics with calcium aluminate additions

Abstract: Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength, and air oxidation resistance has been studied. The results demonstrate that the Si3N4 ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and is stable to air oxidation up to 1300°C.

Modification of Spinel Inclusions by Calcium in Liquid Steel

Abstract: In the production of Al-killed steels, calcium treatment is a common practice for modification of solid alumina inclusions to liquid/partially liquid calcium aluminates. Alumina inclusions may contain some MgO (5-40%). Because the MgO is not soluble in A1 2 O 3 , the presence of any non-zero amount of MgO causes spinel inclusion to form; the resulting Al-Mg spinels are more stable than alumina and may be more difficult to modify. Recently, some authors have shown successful modification of spinels by calcium and have also proposed that MgO in inclusions may help in the liquefaction of inclusions. The present work studies the modification of spinels by calcium. A vacuum induction furnace is used for melting iron in a MgO crucible, followed by addition of aluminum (Al or Al-Mg alloys) and calcium. Reduction of the MgO crucible by aluminum was an ineffective source of MgO in inclusions in the melt, so Al-Mg alloys were used for deoxidation, to produce Al 2 O 3 -MgO inclusions. The effectiveness of calcium treatments was evaluated by tracking the compositions of inclusions in steel samples taken from the melt. Just after calcium treatment, inclusions were found to be dual-phased, with pure CaS in contact with unmodified oxide inclusion. Equilibrium calculations showed that this phase combination is not stable and CaS can react with the oxide inclusion to yield modified (CaO-Al 2 O 3 ) inclusions. At longer times after calcium treatment, inclusions were found to be inhomogeneous in composition, with areas richer in Ca being depleted of Mg and vice versa. This two-phase structure is as expected for equilibrium at steelmaking temperature: the MgO-rich region is unmodified spinel, and the CaO-rich region is liquid oxide (modified spinel), containing some of the MgO from the spinel.

Liquid Iron Wetting of Calcium Aluminates

Abstract: An investigation has been carried out to assess the wetting behaviour of liquid iron carbon alloys on alumina, CA6 (CaO · 6Al2O3), CA2 (CaO · 2Al2O3) and CA (CaO · Al2O3). The melt compositions studied were 2 and 5 mass% [C] over a temperature range of 1 450 to 1 550°C. It was found that the systems studied were in general non-wetting, and that the contact angle dropped from approximately 140° to 110° as the calcium content of the substrates increased. The data for alumina were in good agreement with the literature. These data have been used to assess whether capillary phenomena play a significant role in coke dissolution in liquid iron. In previous studies it was found that as coke dissolved, a mineral layer consisting of alumina and calcium aluminates formed at the coke iron interface and that as time passed the mineralogy of the layer changed from CA6 to CA2 to CA. The rate of coke dissolution slowed considerably with the occurrence of the CA phase. It was not clear whether this was solely a densification effect, or if a capillary wetting issue was contributing to a reduction of the contact area. The contact angle measurements of iron on alumina, CA6, CA2 and CA have been discussed in light of this coke dissolution study, and a simple capillary penetration model used to assess the wetting effects. It was found that the change of wetting associated with the calcium enrichment of the mineral layer did not have a significant effect on the rate of coke dissolution.

Catalyst for high temperature decomposition of nitrous oxide

Abstract: The present invention provides the catalyst for high temperature decomposition of nitrous oxide in process gas mixture, for nitric acid plants, which is based on calcium aluminates. The catalyst according to the invention is composed of CaO from 25 to 49 % by weight and Al 2 O 3 from 26 to 51% by weight and SiO 2 up to 26% by weight, MgO up to 10% by weight, SrO - up to 32% by weight and BaO up to 12% by weight and exists in the form of active catalytically main phase in the minimum amount of 80% of the Mayenite structure CIF 62040-ICSD, described by the chemical formula as: Ca 12 - k X k Al 14-n Si n O 33+(n/2) , where k has numerical value from O to 6, and n has numerical value from O to 7, and X stands for the following chemical elements: magnesium or strontium or barium or the mixture of them, and the remaining phase having at least one of the auxiliaries components: Ca 3 AI 2 O 6 , CaAI 2 O 4 and Ca 2 Al 4 O 7 , with the catalyst grains size 0.5 - 10 μm.

Modeling the coagulation and removal of nonmetallic inclusions during the injection of an inert gas into a melt

Abstract: A mathematical model is constructed to describe the coagulation of nonmetallic inclusions and their removal from steel in a large pouring ladle. It is established that the mechanism of flotation by bubbles of inert gas effectively removes only the relatively large inclusions. The feasibility of removing nonmetallic inclusions in steel by treating it with calcium depends on the state of aggregation of the inclusions. In order to efficiently remove calcium aluminates by coagulation and flotation with bubbles of argon or vaporous calcium, the inclusions must be liquid and cannot be covered by a sulfide sheath.

Ettringite binder for dense mortar, comprising calcium sulphates and a mineral calcium aluminate compound

Abstract: The invention relates to an ettringite binder for dense mortar, containing calcium sulphates and a mineral compound of calcium aluminates, said mineral compound of calcium aluminates comprising calcium C and aluminium A oxides which are soluble and combined in one or more crystallised and/or amorphous mineralogical phases, in proportions such that the useful C/A molar ratio of the mineral compound of calcium aluminates is between 12 and 27 and the weight sum of the useful phases (C+A) represents at least 30 % of the total weight of the mineral compound

Method for manufacturing steel-making flux

Abstract: PURPOSE: A method of manufacturing steel-making flux composition, using a calcium oxide source and an alumina source, is provided to reduce manufacturing costs for steel-making flux composition by using industrial byproduct scavenged as waste. CONSTITUTION: Alumina supply source and calcium oxide supply source, industrial byproduct, are prepared. The alumina supply source and the calcium oxide supply source are mixed at a weight ratio, 1:0.4-0.5. The calcium aluminates based compound is cooled after forming calcium-aluminates compound by plasticizing and melting the mixture at 1400-1600°C. The frozen calcium aluminates based compound has a 5-50mm grain size.

The mechanism of sulphate attack on portland cement

Abstract: It seemed appropriate that a paper contributed to the present number in honor of Professor Thorvaldson should deal with a subject to which he has made major contributions The opportunity has, therefore, been taken to present some results of work carried out intermittently before the war which, though incomplete, throrv some further light on the mechanism of sulphate attack on Portland cement' The action of sulphate solutions on Portland cement is determined by the nature of the chemical reaction and by physical factors such as the permeability of the mortar or concrete concerned and the degree to which the more reactive constituents of the set cement may be protected by surface films of less reactive components The chemical reactions possible are (i) the conversion of calcium hyclroxide in the set cement to calcium sulphate, (ii) the conversion of hydrated calcium aluminates and ferrites to calcium sulphoaluminate or sulphoferrite or their solid solutions, and (iii) the decomposition of hydrated calcium silicates In calcium sulphate solution only (ii) can occur, but in sodium sulphate solution (i) may occur in addition The

Method of combined production of aluminates of calcium and phosphorus

Abstract: FIELD: chemistry. ^ SUBSTANCE: invention can be used in production of aluminate cement, portland cement and synthetic slag, fertilisers, detergents and thermal phosphoric acid. A mixture is prepared from calcium phosphate powder as an oxidising agent and aluminium metal as fuel. The mixture is loaded into a closed reactor, heated until initialisation of the self-propagating high-temperature synthesis (SHS) reaction and combustion conditions are maintained until the end of the reaction. The solid product is a mixture of oxides of calcium and aluminium. ^ EFFECT: invention enables production of calcium aluminates and phosphorus gas from calcium phosphate in a single step, where the phosphorus gas can be used to obtain end products without additional purification. ^ 2 tbl, 2 ex

Ação sonoquímica e influência das condições de tratamento térmico na preparação de cimentos do sistema binário CaO-Al 2 O 3 (Sonochemical action and the influence of heat treatment conditions on the preparation of cements of the CaO-Al 2 O 3 binary system)

Abstract: Cements of the CaO-Al 2 O 3 binary system were prepared through a sonochemical process at room temperature followed by heat treatment. The conventional production consists of a reaction in which a stoichiometric mixture of lime and bauxite or alumina is melted or sintered. High temperatures and energy consumption is an inconvenience associated to this type of synthesis. In the sonochemical process, the initial reagents, calcia and alumina, are put in an aqueous suspension under the action an ultrasonic bath during some time. After that, the material is heat treated, after the evaporation of water. Ultrasonic waves can induce changes in the surface morphology of the particles, including size reduction of them. Consequently, the reagent particles become more reactive and this facilitates the final synthesis of the calcium aluminates during the heat treatment. The action of ultrasonic waves and the influence of thermal treatment conditions were studied on two initials molar compositions calcia:alumina of 1:1 and 1:2. Temperatures of 1000, 1200 and 1300 °C for 1 and 6 h were employed. SEM and X-ray diffraction were used to characterize the obtained materials and the phases were semi-quantified through Rietveld method. In addition, mechanical strength of the products was evaluated through splitting tensile tests. Pastes consisting of cement, alumina and water were prepared using the calcium