THE REVERSIBLE THERMAL EXPANSION OF REFRACTORY MATERIALS

The reversible thermal expansion from 15–1000°C was measured for kaolin, siliceous and aluminous fire clays, quartzite, alumina, magnesia, and carborundum, after preliminary burnings at cones 06, 9, 14 and 20, and as well as for English commercial silica bricks before and after use in a coke oven and the roof of a steel furnace. Kaolin and bauxitic fire clay after calcination have a regular reversible thermal expansion which does not vary much with the temperature of calcination. Siliceous fire clays, after calcination at cone 06 (980°C) or cone 9 (1280°C) display irregularities (departures from uniformity) in their expansion. Between 500° and 600°C they show a large expansion due to contained quartz and on cooling the contraction in that region is larger than the corresponding expansion. Moreover, the expansion between 100° and 250°C after being fired to cone 9 (1280°C) exceeds the average. After calcination at higher temperatures, cone 14 (1410°C) or cone 20 (1530°C). these materials gradually lose these peculiarities until on incipient vitrification a linear expansion similar to that of kaolin is attained. This change is due to the destruction of quartz by its interaction with the clay material and fluxes; it takes place most easily in a fine-grained, rather friable clay such as ball clay. The previous thermal treatment necessary for a particular clay in order to obtain regular expansion in use can only be determined by trial. It can be stated with confidence that in such a piece of apparatus as a glass pot or crucible, a distinct gain will result from maintenance at a high temperature for some time before use, but that the red heat of an ordinary pot arch is useless for the purpose. An increase in the porosity of a fire clay was accompanied by a corresponding decrease in expansion between 15° and 1000°C until a porosity of 50% was attained. Further increase in porosity produced very little change in the expansion. No irregularities in expansion were shown by magnesia brick, carborundum, or alumina bonded with 10% of ball clay. Welsh quartzite with lime bond, either unfired or after burning at cone 06, had a large expansion to 550 °C and a much larger expansion from 550–600 °C due to the inversion of α to β quartz while from 600–1000°C a slight contraction took place. Firing to cone 9 converted part of the quartz into cristobalite, thus increasing the expansion from 200–250°C. This conversion was considerably increased on burning for two hours at cone 14, which greatly reduced the expansion from 550–600°C with a corresponding increase of that from 200–250°C. The conversion of the quartz into cristobalite was completed by a further heating for two hours at cone 20. Determinations of refractive indices and specific gravities confirmed these results. Flint inverted to cristobalite with greater ease than quartz. Commercial silica brick consisted chiefly of cristobalite and unconverted quartz and showed a large expansion up to 300°C, followed by a considerably smaller but regular expansion to 550°C. From 550° to 600°C the rate of expansion was considerably increased, but above 600°C the change in dimensions was small. The innermost exposed layer of a silica brick after use in a coke oven was an impure glass with a steady expansion, but only half as large as that of the layers of brick behind, which was made for shelling away. A silica brick after use in a steel furnace was divided into four layers. The layer exposed to the furnace heat was practically all cristobalite and silicates, the next layer the same, the third layer showed some α to β quartz expansion as well as the α to β cristobalite expansion, while the fourth (outermost) layer exposed to air was similar to the brick before use. In these bricks exposure to high temperature had evidently completed the change from quartz to cristobalite which had been largely effected in the kiln during manufacture. Little or no tridymite had formed. The reversible thermal expansion from 15–1000°C of the commercial silica brick examined was 1.1 to 1.3%, about double that of fire clay brick.     

X-RAY INVESTIGATION OF THE EFFECT OF HEAT ON CHINA CLAYS1

X-ray diffraction patterns were obtained from an English china clay and a Georgia sedimentary kaolin, both raw and fired to various temperatures. The chief crystalline constituent of the raw clays was found to be kaolinite. Upon dehydration the kaolinite lattice was destroyed and the clays gave no diffraction pattern. Mullite was formed in both clays at 950°C and the amount increased with increase of firing temperature. In addition to the mullite, free alumina was present in the Georgia clay from 950 to 1100°C and cristobalite at temperatures above 1200°C.     

PROPERTIES OF SANDSTONES AS A REFRACTORY MATERIAL*

Sandstories from Chungking were tested for their properties as a refractory material. Chemical, petrographical, rational, and sieve analyses were made; properties, such as porosity, refractoriness, specific gravity, thermal expansion, and resistance to spalling. were determined; and studies were made on changes in firing up to 1550°C. No inversion of the quartz grains in the rock into tridymite or cristohalite could be detected. The interstitial clay substances fused first, and the quartz grains then dissolved gradually into the glassy matrix on firing at high temperatures. Being free from mineralogical and structural changes, unusually low in thermal expansion and porosity, and good in spalling resistance after firing at high temperatures, sandstones are shown to be an excellent refractory if properly employed. Of the two kinds of rocks, the dense and the porous, the former was found much better in test for refractoriness under load. Prefiring of the material before lining in a furnace structure is desirable to eliminate the permanent expansion and to improve the resistance to spalling in the raw state.     

EFFECT OF FIRING AT 1500°C ON THE POROSITY AND SPECIFIC GRAVITY OF QUARTZITES1

Quartzites from the various deposits being commercially utilized for the manufacture of silica brick in the United States, Canada, and Europe were tested for porosity and apparent specific gravity before and after firing at 1500 °C for 2 hours. Petrographic examination was made of several of the raw quartzites, and the differences in crystalline structure are shown by photomicrographs. The rate of conversion of the quartz. was not constant for the quartzite? reported. The porosity after firing varied from 2 to 30% and the apparent specific gravity varied from 230 to 244. A fine-grained quartzite tends to give lower porosity after firing.     

EFFECT OF VARIOUS CATALYSTS ON CONVERSION OF QUARTZ TO CRISTOBALITE AND TRlDYMlTE AT HIGH TEMPERATURES*

The action of a considerable number of pure oxides, fluorides, chlorides, borates, phosphates, carbonates, and other compounds and of certain mixtures, as well as the effect of several minerals in promoting or accelerating the inversion of quartz to cristobalite and tridymite at temperatures between 1000 and 1500°C. has been studied. Alkali oxides are found to be superior to all other substances, and they are effective in amounts of less than 0.1%. Acid oxides, such as BIOs or Plot., seem to have little or no value as catalysts for this inversion. Possible mechanisms for the inversion process are presented, and some applications of controlled inversion in the heat treatment of ceramic bodies are mentioned. A few data are also given on the temperature of the “low-high’ change in cristobalite.     

A STUDY OF THE HEATING AND COOLING CURVES OF JAPANESE KAOLINITE

Effect of heating Japanese kaolinite at 100° to 1400° C for 3 to 4 hours.—Ignition loss of weight Was found to occur chiefly between 400° and 600°C, the rate of increase per degree reaching a maximum at about 460 °C. Changes of microstructure were observed at 600°, 900–1000°, 1250–1300° and at 1400 °C, when sillimanite began to develop. Heating and cooling curves for Japanese kaolinite, to 1400°C.—A differential method was used with quartz sand as the comparison substance. In addition to the known reactions: (1) an endothermic from 450° to 700°, and (2) an exothermic near 950°, (3) an exothermic change between 1200° and 1300° was discovered, and it was observed that the endothermic reaction seems to include two periods of heat absorption, (1a) 450° to 650° and (1b) 650° to 700°. In explanation , the author suggests that 1a is due to dehydration, 1b to dissociation of kaolinite into free alumina and free silica, 2 to a polymerization of the alumia and 3 to the formation of amorphous sillimanite. In the discussion, E. W. Washburn calls attention to the fact that the author has neglected the endothermic reaction of quartz at 575 °C and suggests that some of his conclusions are therefore erroneous. Heating and cooling curves for alumina obtained from the nitrate, hydroxide and sulfate by calcination are given in figure 6. Exothermic reactions which are ascribed by the author to polymerization of alumia occur at 800° to 900° and at 1100° to 1200° instead of at 950° and at 1250° as in the case of kaolinite.     

THE SOLUBILITY OF QUARTZ AND CLAY IN FELDSPAR1

The literature on the subject is completely reviewed. The method used involved the addition of increasing amounts of kaolin to fixed amounts of feldspar and the addition of increasing amounts of quartz to fixed amounts of feldspar. All compositions were examined microscopically and the presence of undissolved kaolin was shown by the appearance of mullite needles, while the presence of undissolved quartz was shown by the presence of cristobalite. Soda feldspar is a better solvent for both quartz and clay than potash feldspar. The solution of quartz in soda feldspar starts at about 1350°C and increases with the temperature until at 1425°C. 32 parts of quartz per 100 of feldspar are dissolved. The solution of quartz in high potash feldspar does not start until 1400°C and only 4 parts are soluble at 1425°C to every 100 parts of feldspar. The solution of clay in soda feldspar starts at 1225°C and increases with the temperature until at 1425°C thirty-six parts are soluble. The solution of clay in high potash feldspar starts at 1250°C and at 1425°C the solubility is 20.5 parts per 100 of feldspar. The solution of quartz in porcelain bodies starts at the same temperature as it does in the pure feldspar which the body contains. The amount of solution per unit of feldspar is much greater. The general trend of one of the boundary lines in the ternary system K₂O-A1₂O₃-SiO₂ has been found.     

Sealing Glass‐Ceramics with Near‐Linear Thermal Strain,Part II: Sequence of Crystallization and Phase Stability

The sequence of crystallization in a recrystallizable lithium silicate sealing glass‐ceramic Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO was analyzed by in situ high‐temperature X‐ray diffraction (HTXRD). Glass‐ceramic specimens have been subjected to a two‐stage heat‐treatment schedule, including rapid cooling from sealing temperature to a first hold temperature 650°C, followed by heating to a second hold temperature of 810°C. Notable growth and saturation of Quartz was observed at 650°C (first hold). Cristobalite crystallized at the second hold temperature of 810°C, growing from the residual glass rather than converting from the Quartz. The coexistence of quartz and cristobalite resulted in a glass‐ceramic having a near‐linear thermal strain, as opposed to the highly nonlinear glass‐ceramic where the cristobalite is the dominant silica crystalline phase. HTXRD was also performed to analyze the inversion and phase stability of the two types of fully crystallized glass‐ceramics. While the inversion in cristobalite resembles the character of a first‐order displacive phase transformation, i.e., step changes in lattice parameters and thermal hysteresis in the transition temperature, the inversion in quartz appears more diffuse and occurs over a much broader temperature range. Localized tensile stresses on quartz and possible solid‐solution effects have been attributed to the transition behavior of quartz crystals embedded in the glass‐ceramics.     

THE TESTING OF SILICA BRICK

The Steger Load Test Apparatus. —Pressure was applied, in a granular-carbon resistance furnace, to the small test cylinders by means of a hollow carborundum rod. The temperature was read with an optical pyrometer sighted through this hollow pressure red. The volume changes were indicated by a pointer and scale and recorded by means of a rotating drum. (See Fig. 1.) Estimation of Quartz and Cristobalite in the Finished Br —The areas occupied by each type of crystal on the photomicrograph were determined with the aid of a transparent cross-section grating. Suggested Specifications for A1 Silica Brick. —Trust specific gravity not greater than 2.38; not more than 2% permanent Linear expansion after heating to 1600°C in one and one-half hours, and holding at that temperature for one-half hour; the amount of quartz land silicates) as determined by the grating method applied to photomicrograph should not exceed 15%.     

Copper Silicates in the Timna' ORE Deposit

Timna' chrysocolla was divided into several types based on two end members differing in water content, specific gravity, CuO:SiO₂ ratio and apparently also in their DTA behaviour. The chrysocolla replaces quartz. In places an intergrowth of chrysocolla and wilkeite was observed by electron probe scanning. The X-ray diffraction pattern of chrysocolla is rather poor; however diffraction patterns were obtained after controlled heating (DTA). These indicated the crystallization of tenorite, cuprite, quartz and cristobalite. Dioptase, plancheite and bisbeeite were also identified by X-ray diffraction. Several possibilities of chrysocolla genesis are discussed.     

Study of Heat Effect on the Composition of Dunes Sand of Ouargla (Algeria) Using XRD and FTIR

The spectra of x-ray diffraction (XRD) and infrared spectroscopy (FTIR) shows that the dunes sand of Ouargla’s region consists naturally of crystalline structures of α-quartz and gypsum, as well as other uncrystallized compounds with low concentrations like kaolinite and hematite, in addition to some organic compounds. The sand heating process at temperatures between 200 and 1200 °C affects its composition. By heating at 200 °C crystalline phases of anhydrite and bassanite appear due to the continuing loss of water from the gypsum. All the gypsum transforms into anhydrite, and the kaolinite transforms into metakaolin because of the breaking of the OH bond, producing water vapor by heating in the range of 400–800 °C. The heating at 1000 °C disassembles the kaolinite into aluminium-silicon and cristobalite, and leads to the emergence of a new crystalline phase related to wollastonite resulting from the start of a reaction between the anhydride and the quartz. Heating at 1200 °C leads to the disappearance of all the anhydrite because of its interaction with the quartz, producing the wollastonite and the release of sulfur dioxide SO₂ and oxygen O₂, in addition to the increase of the cristobalite proportion because of the disintegration of all the kaolinite into mullite and cristobalite, or the transformation of quartz phase into cristobalite. Also occuring is an interaction between the hematite and the quartz producing the ferrosilite characterized by its green color.     

Oxygen Absorption in Free-Standing Porous Silicon: A Structural, Optical and Kinetic Analysis

Porous silicon (PSi) is a nanostructured material possessing a huge surface area per unit volume. In consequence, the adsorption and diffusion of oxygen in PSi are particularly important phenomena and frequently cause significant changes in its properties. In this paper, we study the thermal oxidation of p ⁺-type free-standing PSi fabricated by anodic electrochemical etching. These free-standing samples were characterized by nitrogen adsorption, thermogravimetry, atomic force microscopy and powder X-ray diffraction. The results show a structural phase transition from crystalline silicon to a combination of cristobalite and quartz, passing through amorphous silicon and amorphous silicon-oxide structures, when the thermal oxidation temperature increases from 400 to 900 °C. Moreover, we observe some evidence of a sinterization at 400 °C and an optimal oxygen-absorption temperature about 700 °C. Finally, the UV/Visible spectrophotometry reveals a red and a blue shift of the optical transmittance spectra for samples with oxidation temperatures lower and higher than 700 °C, respectively.     

Effect of alkali roasting and sulfatization on the extraction of different metal oxides from waste alum sludge

Water treatment plants (WTP) generate a significant amount of sludge as byproducts with environmentally harmful elements. Thus, this work focused on the recycling of alum sludge through the extraction of different metal oxides, i.e., Al₂O₃, Fe₂O₃ and SiO₂, for use in different applications, such as ceramics, cement, and agriculture. The extraction of Al₂O₃, Fe₂O₃, and SiO₂ from alum sludge was performed using sulfatization and roasting to compare which of the two processes could produce the metal oxides of the highest purity. Precipitated powders were calcined at 700°, 900° and 1100 °C. Moreover, the obtained prepared and calcined powders were characterized by studying their phase compositions, microstructure, particle size, and surface area. Results indicated that roasting achieved the highest yield of alumina. Iron oxide was extracted mostly in maghemite form through roasting after calcination at 1100 °C. Further, silica was obtained in cristobalite and quartz phases after calcination at 1100 °C for the samples prepared through sulfatization. However, these phases of silica were combined with albite and obtained after calcination at 1100 °C for the samples prepared through roasting method.     

Firing transformations of Chilean clays for the manufacture of ceramic tile bodies

This contribution is focused on the study of the mineralogical changes occurring in the ceramic body after heating ceramic clays. Chile has an important local ceramic industry. Five deposits of clays with industrial applications were studied. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L), Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP). The samples were heated to 830, 975, 1080 and 1160 °C keeping at the maximum temperature for 35 min. The bending strength of each ceramic body was determined at 1100 °C. Mineralogical analysis of the fired samples was carried out by X-ray diffraction. The SVTT contained quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite; the LC clays quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/muscovite and talc; the LH clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite; the MP clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite and wollastonite and the L clays quartz, microcline and mullite. The persistence of illite at at least 900 °C was observed for LC and LH. SVTT and LH showed the required specifications for earthenware. The L clays were refractory clays with very low bending strength.     

Silica Structure Studies: V, The Variable Inversion in Cristobalite

It is demonstrated experimentally that even in the purest laboratory silicas available the temperature of the α-β inversion in cristobalite is variable and depends on the structure of the starting material and on the temperature and length of heat-treatment. It is shown that this variability is an index of the order achieved in the cristobalite structure quite independent of impurities. There is no specific order characteristic of a particular temperature; the completely ordered 3 C stacking of cristobalite is the most stable cristobalite throughout the temperature range and has an α-β inversion temperature of 267°± 2°C. All disordered cristobalites will tend toward the 3C 267° cristobalite with time. The conversion of cristobalite to tridymite involving the 3 C → 2 H stacking change does not alter the cristobalite inversion temperature, nor have any regular polytypes other than those recognized as tridymite been encountered.     

SOME CHARACTERISTICS OF ILLINOIS POTTERY CLAY*

Four Illinois pottery clays, representing two extreme types and two intermediate types, are described. Porosity and thermal expansion tests were made on the raw and fired clays. The temperature of the inversion of low to high cristobalite in these clays is much lower than is usually accepted for the lower limit of this inversion because cristobalite is formed at low temperatures. A limited amount of data was obtained on the volume changes of raw clays during firing, the organic matter content, and thermal dissociation.     

THE DEVITRIFICATION OF “PYREX” GLASSES1

A study has been made of the devitrification phenomena of three “Pyrex” glasses, which were found to have liquidus temperatures lower than any other known mixtures containing 50 high a silica content. “Pyrex” glass 774 (702EJ), refractive index 1.471, has a liquidus temperature of 1077°, 772 (702P), index 1.486, 1042° and 776 (720GO), index 1.473, 1036°. In all cases tridymite is the primary phase and cristobalite was never observed.     

Kinetics of Cristobalite Formation in Sintered Silica

The kinetics of the cristobalite transformation are reported for sintered silica glass from 1200°C to 1650°C and plotted as a time–temperature–transformation diagram. The 1200°C–1350°C transformation data were fit to the Johnson–Mehl–Avrami–Kolmogorov expression with an time exponent of 3.0 ± 0.6 and an apparent activation energy of 555 ± 24 kJ/mol for the kinetic constant. The temperature of maximum transformation rate was found to fall between 1500°C and 1600°C. Seeding amorphous silica powder with cristobalite resulted in accelerated transformation kinetics. Silica glass powder containing residual quartz had faster transformation kinetics than fully amorphous powder seeded with cristobalite.     

SUBSTITUTION OF TOPAZ,DOMESTIC KYANITE,AND SYNTHETIC MULLITE-CORUNDUM FOR INDIA KYANITE: IV,RAW TOPAZ AS HIGH-TEMPERATURE BOND FOR DOMESTIC KYANITE*

Raw topaz has been found to act as a high-temperature bond for both raw and calcined kyanite. It is thought that this bonding action is due in part to volatiles which emerge from the raw topaz between 1000° and 1200°C. Some of the topaz decomposition products are absorbed and retained by the calcined kyanite. Within the limits of this investigation, all refractories which contained either calcined or raw kyanite, or mixtures of the two, with a minimum of 10% raw topaz had a good structure and were well bonded. The results of the load tests indicated that increased firing temperatures in the preheat improved the load resistance, but excellent high-temperature resistance was obtained by preheating as low as 1300°C. in laboratory kilns.     

Characterization of transparent glaze for single-crystalline anorthite porcelain

The objective of this work was to design a transparent glaze for matching single-crystalline anorthite porcelain. Excessive amounts of quartz were used in glaze to improve surface hardness. Technological properties including hardness and thermal shock resistance were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies were also carried out to analyze the microstructure. The phases found in glaze were aluminosilicate glass, quartz and cristobalite crystals. The Vickers hardness of the transparent glaze was about 2.48 GPa, which was much higher than that of commercial soft glaze and was close to hard porcelain glaze due to forming dispersed crystal particles (quartz and cristobalite) in the glass matrix. Moreover, the thermal expansion coefficient of the glaze was slightly lower than that of porcelain body which was easy to produce compressive stress in glaze surface to increase the strength of porcelain. And no cracks were observed on glaze surface after heat exchange three times from 220 °C to 25 °C, presenting excellent thermal shock resistance.