Calcium Oxide Refractory

Calcium oxide has excellent refractory properties and many efforts have been made to commercialise its use (e.g., [32.48]). However, hydration by atmospheric water vapour has proved to be a major problem. Despite this, the nuclear industry is reported to use dead-burned calcium oxide crucibles, with an apparent density of 3.15 g/cm [32.4, 32.49]. 

Relatively little lime is used in glass manufacture as limestone is generally more cost-effective (see section 12.2). However, dolomitic lime and occasionally high calcium lime are used in finely ground forms under specific circumstances. 

Burned lime imparts greater brilliance and transparency to the glass than limestone on account of  

This reduces the requirement for costly decolouriser additives. 

For glass fibre production, the number and size of acid insoluble refractory particles (mainly grains of silica sand) are of particular importance as they can cause breakages of the fibres as they are drawn out to the required diameter. 

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Low Cement, Ultra-Low Cement, and No-Cement Castables are classified on the basis of calcium oxide content. These are 1—2.5, 0.2—1.0, and 0.2% CaO maximum, respectively. In the latter case the lime content is not a result of a hydrauHc setting cement constituent but comes from aggregate impurities. The insulating class is also subdivided. This division is shown in Table 14. Refractories used in steel-pouring pits are classified under ASTM C435 (Table 15). 

Acrylic acid, Initiator, Water, 1148 Aluminium chloride, Water, 0062 Barium peroxide, Propane, 0216 1,3-Benzodithiolium perchlorate, 2677 1,1 -Bis(fluorooxy)tetrafluoroethane, 0641 Borane-tetrahydrofuran, 0138 Boron tribromide, Water, 0122 Bromine, Aluminium, Dichloromethane, 0261 Bromine, Tungsten, Tungsten trioxide, 0261 f 1,3-Butadiene, 1480 Calcium oxide, Water, 3937 Chlorine trifluoride, Refractory materials, 3981 Chromium trioxide, Acetic acid, 4242 Copper(II) oxide, Boron, 4281 Diazoacetonitrile, 0675 Dihydroxymaleic acid, 1447 Ethyl azide, 0872... 

Calcium oxide is commercially obtained from limestone. The carbonate is roasted in a shaft or rotary khn at temperatures below 1,200°C untd aU CO2 is driven off. The compound is obtained as either technical, refractory or agricultural grade product. The commercial product usually contains 90 to 95% free CaO. The impurities are mostly calcium carbonate, magnesium carbonate, magnesium oxide, iron oxide and aluminum oxide. 

Refractory bricks composed of oxides of magnesium, chromium, aluminum and iron and trace amounts of silica and calcium oxide are used in roofs of open hearths, sidewalls of electric furnaces and vacuum apparatus and copper converters. Such refractories are made in an arc furnace by fusing mixtures of magnesite and chrome ore. 

Calcium oxide and magnesium oxide are particularly important in the manufacture of cements and refractories. Natural carbonates are their main sources (refer to Chapter 3.1). In pure form, they can be prepared by decomposition of pure carbonates or hydroxides the oxides are then stable up to their respective melting points. They are hydrated under the effect of atmospheric humidity, CaO in particular. 

The chief contaminant is 0.3-0.5% sodium oxide, which fortunately does not affect electrolysis, with <0.05% calcium oxide, <0.025% of silica or iron oxide, and <0.02% of any other metallic oxide [4]. Apart from metal production, some of this high temperature alumina is used for the manufacture of synthetic abrasives and refractory materials. Activated alumina destined for adsorptive uses is produced in the same way, except that more moderate calcining temperatures of about 500°C are employed, which produces a highly porous product with excellent surface activity. The volume of alumina from the world s major producers is listed in Table 12.3. Australia has been the largest producer for many years (Table 12.3). 

Zirconium dioxide, zirconia, is the only oxide of zirconium stable chemically at temperatures below 2000 K. At higher temperatures some dissociation into ZrO and oxygen takes place. The phases of ZrOj, their densities, and phase-transition temperatures are listed in Table 7.5. Zirconia stabilized in the high-temperature cubic phase by addition of 3 to S percent calcium oxide is used as a refractory at temperatures up to 2200° C. ZrOj has been used to dilute UOj in fuel elements. 

E. Marino, The Use of Calcium Oxide as Refractory Material in Steelmaking Processes . In Refractories for the Steel Industry , Elsevier Applied Science, London, 1990, pp. 59-68. 

Stable compound formation will always cause a depressive effect. Typical examples are the lowering of alkaline earth metal absorbances in the presence of phosphate, aluminate, silicate and some other oxo anions, the low sensitivity of metals which form thermally stable oxides (refractory oxide elements), and the depression of the calcium signal in the presence of proteins. In addition, some refractory oxide elements may also form stable carbides, especially in rich hydrocarbon flames. 

The Chemical Abstract Service has defined these materials under the CAS number 142844-00-6 as Refractories, fibers, aluminosilicates. Amorphous man-made fibers produced from melting, blowing or spinning of calcinated kaolin clay or a combination of alumina (AI2O3) and silica (SiOa). Oxides such as zirconia, ferric oxide, magnesium oxide, calcium oxide and alkalines may also be added. 

The X-ray phase analysis confirmed that the synthesis products are TiC, ZrC, NbC and TaC carbides, which do not contain free carbon. Fineness of the carbides is determined by the grain size of the metal powder. The synthesis time depends on the diffusion rate of carbon into the volume of the grain or the crystallite. We used oxides of refractory metals as the starting materials for production of fine (1-5 pm) carbides. In this case the mass of the metallic calcium was increased taking into account its consumption for the calcium thermal reduction of the oxides. The presence of a higher amount of the calcium oxide in the salt melt did not incur large difficulties in the synthesis of the carbides and their washing with water to remove salts, because the calcium oxide easily dissolves in acidified water. This method of the carbide synthesis has been covered by a Russian Federation patent [6]. 

Fireclay bricks (alumina silica bricks, alumina calcium oxide silica bricks, and other silicate bricks) are not optimal barrier materials for A1 reductirMi cells. As we have mentioned, cryolite-based electrolyte for A1 reduction is a substance that dissolves alumina better than anything else. Certainly, it will dissolve all alumina-based refractory compositions and almost all other oxides similar in chemical structure to alumina. From a chemical point of view, the effective refractory barriers against the penetration of cryolite might be tin oxide, nickel oxide, compounds of nickel oxide, iron oxide, or zinc oxide (such as spinel Fe NiOs). These oxides almost do not react with NaF and aluminium fluoride [175]. Yet the cost of these materials, which is 50-100 times higher than that of firebrick, provides the impetus to find less costly variants of alumina silica materials. 

The surface of the honeycomb is covered by the alumina washcoat, which can be stabilized against sintering at high temperature by the addition of more refractory materials such as barium oxide, calcium oxide, magnesium oxide or lanthanum oxide. A very thin surface layer of the washcoat, up to about 10-30 xm, is applied although the thickness increases to about 150pm at the comers of the square channels. 

Minimizing Chemical Interferences The quantitative analysis of some elements is complicated by chemical interferences occurring during atomization. The two most common chemical interferences are the formation of nonvolatile compounds containing the analyte and ionization of the analyte. One example of a chemical interference due to the formation of nonvolatile compounds is observed when P04 or AP+ is added to solutions of Ca +. In one study, for example, adding 100 ppm AP+ to a solution of 5 ppm Ca + decreased the calcium ion s absorbance from 0.50 to 0.14, whereas adding 500 ppm POp to a similar solution of Ca + decreased the absorbance from 0.50 to 0.38. These interferences were attributed to the formation of refractory particles of Ca3(P04)2 and an Al-Ca-O oxide. 

At elevated temperatures, CaH2 reacts with halogens, sulfur, phosphoms, alcohols, and ammonia. At high temperatures, it reacts with refractory metal oxides and haUdes. Calcium hydride is substantially inert to organic compounds that do not contain acidic hydrogens. 

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