Chrome-magnesite Refractories Research Articles

Novel heat-transfer mechanisms affecting the thermal conductivity of porous ceramics

Thermal conductivity, λ, and thermal diffusivity, a, of porous refractory oxide ceramic materials vary in a complicated manner with temperature, T, and gas pressure, p, which cannot be explained on the basis of known classical heat-transfer mechanisms in porous materials, which are conduction in solid and gas phases, gas convection, and heat radiation. This abnormal behaviour includes: (i) different T dependence of λ at atmospheric and at low pressure; (ii) at high T, thermal diffusivity measured in vacuum can exceed this property measured at atmospheric pressure; (iii) λ and a can depend on the measurement method. In order to explain the peculiarities of behaviour of λ and a, two additional groups of heat-transfer mechanisms are reviewed, including heterogeneous heat and mass transfer processes occurring in pores and in cracks, and microstructural changes due to nonuniform thermal expansion of particles and grains. A physicomathematical model for calculation of λ and a of porous ceramic materials and their dependence on T and p is described, which incorporates the above heat-transfer mechanisms. This model is used to explain and correlate the extensive experimental data for λ and a collected over wide ranges of Tand p by steady-state and transient methods. It is shown that λ may increase with Tat low and atmospheric pressure as a result of the microstructural changes due to the thermal expansion mismatch between the different grains. The increase of a with T in vacuum, and the dependence of λ and a on the measurement method, observed for chrome-magnesite and other refractories, can be attributed to heterogeneous reactions on the pore surfaces, accompanied by gas emission.

Effect of Grain Thermal Expansion Mismatch on Thermal Conductivity of Porous Ceramics

Many ceramics contain microcracks, which are often situated between sintered grains. These microcracks constitute thermal resistances, which may affect heat transfer through the material and its effective thermophysical properties. The thicknesses and the contact areas of the microcracks change with temperature as a result of the thermal expansion mismatch between the grains on opposite sides of the microcracks. This physical mechanism affects changes of the material's thermal conductivity, k, with temperature. The above mechanism usually plays a minor role at atmospheric pressure, where heat may flow via the gas filling the cracks. Hence, the temperature‐induced changes of the crack geometry have little effect on heat transfer. However, at low gas pressures, where the heat flow between the grains occurs mainly via the contact areas, the grains' thermal expansion mismatch causes unusual temperature behavior of the material's thermal conductivity observed for several industrial refractories. In this paper, the influence of the above physical mechanism is discussed relative to other heat transfer mechanisms described in the literature. A simple physical model of the thermal expansion of grains bonded by an agent, having different thermal expansion coefficients, is developed. This model allows calculation of the contact area and the average microcrack opening between the grains as functions of the temperature, the characteristic grains sizes and their thermal expansion coefficients, and the permanent crack area. These parameters are evaluated and used to calculate the effective thermal conductivity of ceramic materials containing microcracks that appear as a result of thermal contraction of grains. The calculated thermal conductivity satisfactorily correlates with the experimental data collected for several chrome‐magnesite refractories over a wide range of temperatures and gas pressures.

Mechanism of refractory wear in the reaction shaft of furnaces for melting of raw material in the suspended state

The mechanism of failure of chrome-magnesite refractories under conditions of autogenous melting of sulfide ore was studied.

Effect of Iron Oxide Additions on the Direct Bonding in Chrome-Magnesite Refractories

The effect of iron oxide on direct bonding in chrome magnesite refractories was studied in two series of experiments. In one series purified chromite and pure magnesium carbonate sintered with different iron oxide additions were used; and in the other a high grade chrome ore and natural magnesite sintered with different iron oxide additions. It was observed that additions of iron promote direct bond in purified chromite and magnesia compositions. But with natural chrome ore and magnesite, even with high iron oxide concentration some amount of silicate phase is necessary to develop direct chromite-periclase bond.

Microstructure and Properties of Chrome-Magnesite Refractories made from Indigenous Raw Materials

Using Orissa chrome ore, Almora magnesite and Salem magnesite several chrome-magnesite and magnesite-chrome compositions were prepared and fired to temperatures between 1400°C-1700°C. Physical and refractory properties of the fired specimens were studied. The microstructure was examined in detail. Good direct bonding between chromite and periclase grains developed at 1700°C. The extent of direct bonding was more pronounced in compositions containing larger amount of monticellite in the matrix.

Use of sonic control methods for refractories

We developed and introduced a sonic method for determining the apparent density, porosity, and compressive strength of magnesite-chromite, chromemagnesite, and chamotte (fireclay) refractories.

Reaction between refractories and slag and oxidizing steel

Scale, removed from the lining of a heating furnace reacts weakly at 1200–1450°C with corundum, high-alumina, chromite, chrome-magnesite, forsterite, and carborundum refractories.

Mineral formation in chrome-magnesite refractories in rotary furnaces used to produce defluorinated phosphates

Silicates of chrome-magnesite refractories in a large or small degree react with the components of the batch and the decomposition products of apatite. With rise in temperature this reaction sharply increases.

Development of Chrome-Magnesite Refractories Utilising Indian Chrome Ores

The suitability of five Indian chrome ores from Singhbum, Orissa (Boula Mines), Bhandara (Bombay State), Byrapur (Mysore State) and Enuconda (West Godavary, Andhra Pradesh), for the manufacture of chrome-magnesite refractories was evaluated. Physical and refractory properties of the fired composition have been given and mineralogical composition of the fired bodies has been studied in relation to their firing temperature. Direct bond formation was extensively developed in all the specimens fired to 1700°C. The results showed that ores from Singhbhum, Orissa, Bhandara and Byrapur are suitable for the production of conventional chrome-magnesite refractories.

The Impact of Some Recent Technological Developments on the Production of Chrome Magnesite Refractories in India

(1967). The Impact of Some Recent Technological Developments on the Production of Chrome Magnesite Refractories in India. Transactions of the Indian Ceramic Society: Vol. 26, No. 1, pp. 41N-43N.

Unfired brick from magnesite-chromite concrete for open-hearth furnace building

Unfired brick made of magnesite-chromite concrete with periclase cement, tested in several parts of open-hearth furnaces, showed the same resistance as fired magnesite or chrome-magnesite refractories. Unfired magnesite-chromite brick made of concrete can be used for building structures with air-setting, water-containing cement instead of iron plates. It is necessary to organize the industrial production of unfired goods made of magnesite-chromite concrete using the method developed by UNIIO for extensive testing and subsequent use in open-hearth furnaces.

A Comprehensive Study of Indian Magnesites as Refractory Material

The investigation includes six bulk samples of magnesite from Salem, Mysore, Ajmer and Almora to represent a comprehensive account of the indigenous resources. They constitute two distinct types: cryptocrystalline and crystalline. The different deposits have been outlined comparatively in their modes of occurrence and origin. Mineralogical data include petrography, dta and x-ray analysis; they are in close correspondence with the chemical compositions of the different samples. Studies on sintering at temperatures between 1500° and 1700°C indicate that the Indian magnesites can be properly dead-burnt around 1650°C. Girechchina magnesite containing about 35% dolomite may be calcined inert at about 1550°C. The segregation of matrix, and the role of iron and other impurities during calcination have been discussed. Magnesite and chrome-magnesite refractories made from each type have been found to develop satisfactory properties, in general, and particularly with respect to refractorieness underload, and volume...

The phase composition of chrome-magnesite refractories

Study of the phase composition and structure of chrome-magnesite refractories of different composition and degrees of firing led to the development of a range of practical proposals to improve the production technology.

The influence of physicochemical factors of basic refractories on the life of linings of oxygen converters

Work on increasing the slag resistance and strength of PShK brick at high temperatures should be continued. The high Cr2O3 content in converter goods and an adequate spalling resistance may be obtained by adding waste magnesite-chromite or chrome-magnesite refractories to the batch. To increase the life of converter goods it is necessary that the deviation in the properties from the mean should be at a minimum. In connection with the high cost of periclase-spinel goods it is necessary that they be replaced by dolomite and magnesia-dolomite refractories. Considering that the erection of large converters needs high strength in the linings at high temperatures further study directed at increasing the strength of the brick at high temperatures should be made.

Determination of ferrous oxide in chromite and chrome-magnesite refractories

In exact analyses of chromite and chrome-magnesite refractories for the ferrous oxide content, current-flow methods of titration using either Mohr's salt or permanganate can be used. The results are better in the first case.

Composition and properties of chromite ores from new deposits in Kazakhstan

The chromite ores from the four deposits studied show a varying composition and a variety of physical properties due to their modification by secondary processes — metamorphosis and ancient weathering. In chemical composition, most of the chromite ore samples after averaging come up to the required standard for the production of normal types of chrome magnesite parts. As far as this property is concerned, it is only some of the samples from deposit B which are unsatisfactory. For the production of important types of chrome-magnesite refractories, the ores need to be enriched or screened so as to separate the high-grade varieties. All the mineralogical compositions of the chromite ores can be combined into six types with the following cementing binders: 1) serpentine; 2) serpentine-carbonate; 3) serpentine-carbonate-brucite; 4) serpentine-carbonate-chlorite; 5) serpentine-carbonate-amphibole; 6) serpentine-carbonate-quartz with the possible development in each one of iron oxides and hydroxides. The centralized preparation of raw material and the organization of the enrichment of low-grade varieties will help to supply the refractory industry with chromite powders with identical composition and physical properties.

Elastic and thermal properties of magnesite-chrome brick which has been used in open-hearth furnace roofs and artificially saturated with iron oxide

The elastic and thermal properties of roof basic refractories which have been artifically saturated with iron oxide and have served in a roof do not reveal any great changes, compared with the initial brick; hence it should be considered the importance attributed to the differences in the properties of zones of chrome-magnesite refractories, forming through the action of iron oxides as the dominating factor in cracking bricks during temperature variations is an exaggeration.

Model investigation of firing conditions of chrome-magnesite refractories in gas-fired chamber kilns

The results of investigations of a model and of an industrial kiln show the favorable effect of a baffle plate arrangement above the gas outlets of tunnel kilns which enhances uniform firing temperatures throughout the chamber.

Use of tagged atoms for studying ferric iron diffusion in chrome-magnesite refractories

We checked and recalculated measurements of the diffusion factor by the layer removal and absorption methods. Both methods gave results which tallied.

Use of tagged atoms for studying ferric iron diffusion in chrome-magnesite refractories

Use of tagged atoms for studying ferric iron diffusion in chrome-magnesite refractories