Corundum Products Research Articles

Solid-state exchange reactions during sintering of dispersed alumina

In this article, the mechanism of sintering of Al2O3 in the presence of small amounts of Na2O and CaO was investigated. Based on the results of the electron microscopy, the granulometry and morphological features of the particles of the studied alumina were established. The uniform nature of the distribution of sodium-containing phases was revealed, in contrast to silicon-containing ones, and the dislocation of submicron particles from calcium-containing phases was determined mainly on the basal planes of relatively large corundum particles. It was shown that such an arrangement of calcium-containing phases promotes the formation of a dense layered microstructure during sintering, especially in the presence of -alumina. The general pattern of the branched mechanism of the reaction phase formation during the sintering of the compositions in the Na2O–СаО–Al2O3 system was illustrated by a diagram explaining the trend of physicochemical processes and the feasibility of using specific types of dispersed alumina for technologies of corundum products and refractory concretes with different contents of aluminous cements.

Синтез сложно-оксидной керамики в пучке быстрых электронов

The synthesis of ceramic materials in a fast electrons beam for the production of complex oxide ceramics is considered. Powder reagents, in addition to irradiation, are exposed to air currents that prevent gases and particles from entering the accelerator. To keep powder mixtures of ultrafine powders in the irradiation zone, they were granulated. Two methods of granulation of an ultrafine powder of the composition (in mass %) 80 % Al2O3 + 20 % (ZrO2 – 3 Y2O3) were used. The first method involves moistening, drying and then sifting through a coarse sieve. In the second method, a binding additive was introduced into the powder mixture, which gave the sample a stable volumetric shape. For the granulation methods used, the features of short-term heating of oxide powders in air with a powerful beam of fast electrons with an energy of 2 MeV and the synthesis of zirconia corundum under these conditions were studied. Granulation of the ultrafine powder made it possible to minimize the loss of its mass during irradiation. During irradiation of the powder mass, its local melting took place, which was accompanied by intense gas release processes leading to the formation of hollow ceramic droplets. X-ray phase analysis has shown that mutual dissolution of oxides in their walls does not occur, and recrystallization processes are accompanied by the formation of cubic aluminum oxide microcrystallites and the transition of aluminum oxide in them from the monoclinic to the corundum phase. The presence of microcrystallites of evenly distributed small particles of zirconia dioxide in the interboundary space indicates the production of zirconia corundum under irradiation. At the same time, the phase composition of zirconium dioxide after irradiation does not change in comparison with the initial powder.

Recovery of K by NH4HSO4 low-temperature roasting from brown corundum Fly Ash

Brown corundum dust ash(BCFA) is an industrial solid waste from the brown corundum production process. The alkali metal is a key factor limiting its comprehensive use. Currently, BCFA is mainly stockpiled and occupies a large amount of land. Its fine particles are easy to cause air pollution. The addition of NH4HSO4 allows for the effective extraction of K from BCFA and the efficient use of BCFA. Under optimum conditions: NH4HSO4 to BCFA mass ratio of 1.2:1, the roasting temperature of 240 °C, roasting time of 2 h, water leaching time of 90 min, water leaching temperature of 65 °C water leaching liquid to solid ratio of 3:1, the leaching rate of K from BCFA reached 97%. The leachate was crystallised by evaporation to obtain K, N compound fertiliser for agricultural use. The leaching residue is mainly Al and Si, which can be used for the preparation of refractory materials, aluminium and silicon molecular sieves, construction materials and other raw materials.

Production of corundum armored ceramics

The purpose of this study was to develop the compositions and technology of corundum armored ceramics with high ballistic characteristics based on aluminum oxide with an α-Al2O3 content of more than 99 wt.%. The results of the study of the physicochemical processes of obtaining corundum ceramics modified with complex additives consisting of magnesium - aluminosilicate eutectic mixture and oxides of magnesium, yttrium, titanium, zirconium are presented. The introduction of complex additives into the composition of corundum ceramics provides a significant reduction (100-150°C) of the sintering temperature of the product. In this case, additives of yttrium and zirconium oxides contribute to the formation of a uniform-grained microstructure of ceramics, and in the case of adding titanium oxide, a collective recrystallization of corundum grains is noted. The use of small additives (0.3-0.5 wt.%) of magnesium and yttrium oxides together with a eutectic mixture in the compositions of corundum ceramics based on high-quality alumina contributes to the formation of a uniform-grained, dense structure of the material and giving it a high level of physical and mechanical properties as a result of directional the action of each component of the additive on the physicochemical processes of phase formation of the crystalline matrix. A model idea of the mechanism of action of additives on the formation of the microstructure of ceramics during stage-by-stage heating from 1350 to 1650°C in the sintering mode of products is presented. The developed compositions and technologies of corundum armored ceramics have a production focus and implementation in practice.

Development of 3D Printing Technology with Ceramic Paste and Study of Properties of Printed Corundum Products

Today 3D printing with ceramics is a promising direction in the development of additive technologies. In this work, we have developed a technology for printing with ceramic pastes based on aluminum oxide and wax, namely: an extruder for printing with ceramic pastes was modeled and manufactured, the composition of the slip was selected and the paste for printing was made. After choosing the print parameters, test samples were printed: a disk and a box. Since 3D printing with ceramics is just one of the stages of manufacturing ceramic products, then we selected the parameters for drying and sintering the raw material. Drying of products is necessary to burn off an excess amount of a binder (paraffin), and due to sintering; the raw material acquires final strength and mechanical characteristics. After sintering, the sintering parameters and physical and mechanical properties of the products were measured. The microstructure of the printed products was studied using scanning electron microscopy. The phase change during sintering was studied by X-ray analysis. All obtained properties were compared with a reference sample (corundum tile made of aluminum oxide of the same grade, but using traditional ceramic technology, including pressing, drying and sintering of the product). In terms of all properties, the printed ceramics are not significantly inferior to the reference sample; however, in general, the additive technology has more advantages, such as a wide variety of shapes, shorter manufacturing time for parts, and lower energy costs.

Briquetting of porous alumina-containing materials with organic binders

Waste from corundum production in the form of porous alumina sludge is a promising material for providing ferrous metallurgy with cheap alumina-containing slag-formers. However, the direct feeding of the pulverized materials to the steelmaking furnace generally results in a significant carryover of such materials with waste gases. This paper considers the possibility of making briquettes from porous sludge of corundum production by cold briquetting using various common binders (molasses, cement, powder based on polyacrylamide, emulsion based on polyvinyl acetate). A comparison of the features of cold briquetting of powdered porous materials (slimes from the production of electrocorundum) and dispersed crystalline materials (fines of chrome ore) was made. Experiments were carried out to determine the impact strength of briquettes on different binder (“cold” strength) and tests to determine the “hot” strength (by the “thermal shock” test method). The authors have determined the consumption of the binder required to obtain satisfactory characteristics of briquettes from corundum slimes and from chrome ore fines. A technique has been developed and a mechanism for the binding of particles of loose and crystalline materials has been determined during briquetting using polyacrylamide powder. The destruction of a briquette of loose materials occurs mainly along the grains of the most porous material, and briquettes of crystalline materials are destroyed along the boundaries of the grains glued with a binder. For porous materials, the binder consumption increases more than twice as compared to briquetting on the same binder crystalline bodies of a fine fraction, and the binder must necessarily impregnate the entire volume of the porous material.

Development of a new plasma technology for producing pure white corundum

The paper presents the results of mining and primary approbation of plasma method for producing pure white corundum. Upgrading ways of pure corundum production is an important task for industry as part of reducing the energy consumption and environmental contamination. The purposes of the research at this stage are as follows: the selection of raw materials, formative evaluation for characteristics of the technology, conducting an experiment on melting and assessment of the sample. The corundum melting is conducted in the reactor using high-voltage plasmatron. Mixture of argon and 25–30 percent of nitrogen is used as the working fluid. The authors suggest using a four-layered protection of a melting reactor in order to ensure both thermal insulation properties and strength characteristics. This is especially relevant under temperature difference of the order of 2000 K and elimination of defective crystallization of the melt from the walls of the reactor. As a result of an experiment on melting alumina marked G-00 using high-voltage air powered plasmatron, the sample with alumina oxide in the amount of 99.79 percent and with absolute hardness equal to 500 was obtained. Further experiments make it possible to determine the prospects of using the proposed technology to obtain samples with an increased content of aluminum oxide. The paper discusses the application of the described technology for industrial production of pure corundum single crystals. The technology will make it possible to obtain samples to be used as abrasives for optical systems and for the production of sapphire glasses and scalpels.

Issues of resource and energy saving in corundum production

The purpose of this article is study and identification of the most promising trends and engineering solutions in order to improve resource saving and energy efficiency in the production of corundum on the basis of the conducted patent review on melting improvement and optimization. The ways to optimize the corundum production are considered in three directions from the point of view of energy saving. The first direction relates to the development of promising engineering developments. The latter are studied to select rational operating modes and determine the main factors affecting voltage surges during the technological process and useful product yield. Consideration is given to the conditions for reducing specific energy consumption and improvement of production environmental friendliness at all stages beginning from isothermal sintering of corundum, production of electrocorundum, fine corundum to single corundum crystals. The second direction is the optimization of corundum production at all stages for the development of an optimal control algorithm for the technological process. In this case the electricity consumption might be reduced by 10-12% as compared to current standards. The third direction is the development of engineering solutions involving the change of individual structural units of furnaces, namely, the use of modern components and new heat insulating materials, as well as the application of spent heat carriers as the sources of secondary energy resources and the introduction of additional controllers of the automated control system of the process. The analytical study has shown that the result of optimization should be upgraded designs of plants and electrical equipment, which can provide maximum electrochemical efficiency, and corresponding furnace tightness. Criteria for energy supply and energy quality making possible to stabilize furnace material balance and solve resource saving issues have been developed. These measures allow to reduce the loss of raw materials up to 20-25%, the specific energy consumption under the production of corundum by 2-3 thousand kWh per 1 t.

Recovery of K2SO4 and Separation of SiO2/Al2O3 from Brown Corundum Fly Ash

Brown corundum fly ash (BCFA), which is the collected ash in brown corundum production, has received lots of environmental concerns due to its ultra-fine particle size and complex composition. Aiming to recycle the major elements including K, Al and Si from BCFA environmentally, this work proposed a simple and non-pollution process to utilize BCFA by water leaching, size screening and solution evaporation. The influences of water leaching conditions including leaching temperature, time and liquid-to-solid ratio was considered to optimize the K2SO4 recovery efficiency. Results show that the potassium sulfate content of the product is 75.7% after water leaching at 60 °C for 15 min with the liquid–solid ratio of 20:1. The wet screening and size separation with a 1 μm sieve can separate and enrich aluminum and silicon significantly. The aluminum-rich product is composed of 54.65% of alumina and 11.04% silica, with the alumina and silica ratio (A/S) of 4.95. The silicon-rich product with a particle size of less than 1 μm has a silica content of 57.57% can be used as high-value micro-sphere silica. The research results revealed in this work offers a potential and environmentally industrial treatment technique for the BCFA.

Response to Comment on "High-surface-area corundum by mechanochemically induced phase transformation of boehmite".

Li et al commented that our report claims that methods reported thus far cannot enable the production of high-purity corundum with surface areas greater than 100 m2 g-1, and that our obtained material could be porous aggregates rather than nanoparticles. We disagree with both of these suggestions.

Influence of vibration time on the formed structure strength in a vibrocast coarse­grained corundum­chromia mass

An influence of vibration time on the formed structure strength in a vibrocast coarse­grained corundum with a chromium oxide addition mass was studied. Along with scientific, these studies are primarily of practical interest, since they allow to establish the possibility of proper transportation of moulded green product without destroying its structure formed during keeping of this product in the form.
 Under the influence of vibration on the formed structure in the above mentioned coarse­grained mass, it was revealed that the effect of vibration on this mass for 60 minutes of its continuous application practically does not lead to the destruction of the formed structure, which indicates its high stability and strength. The results obtained in this article confirmed the results of previous work on the structure formation in the vibrocast coarse­grained corundum­chromia mass containing a mixture of dispersing alumina grades ADS­1 and ADW­1.
 The studies results were used to improve the manufacturing technology of fired shaped corundum products with the Cr2O3 ­ addition, characterized by high properties and designed to perform a lining working layer of production reactors of carbon black. The indicated advanced technology has been mastered at the pilot production of institute.

Comparative Analysis of Triply Periodic Minimal Surface Corundum Products Obtained by 3D Printing

The paper studies the properties of triply periodic minimal surface corundum products of the Schwarz primitive topology printed on different 3D printers. The possibility of an alternative technology, 3D plastic mold slip casting, is considered.

Study of Refractory Degradation Under Action of Cyclic Thermal Loads

Results are provided for thermal cycling tests of a number of highly refractory products. It is shown that periclase products from fuzed powders coated with ZrO2 are the most resistant to cyclic variable thermal loads. The structure of corundum products in the process of thermal cycling tests deteriorates significantly due to recrystallization and the formation of new phases. In order to organize the lining and test unit heat exchange fittings operating under conditions of variable loads in pulsed piston units objects made from fuzed periclase powders modified with ZrO2 are recommended.

The study of the degradation of refractories under the action of cyclic thermal loads

The results of thermocyclic tests of a number of products of higher refractoriness are given. It is shown that the most resistant to cyclic variable thermal loads are periclase products from fused powders coated with ZrO2. The structure of corundum products in the process of thermocyclic testing significantly degrades due to recrystallization and the formation of new phases. Products made of fused periclase powders modified with ZrO2 are recommended for the organization of the lining and heat exchange tips of test benches that are operated under conditions of variable thermal loads in pulsed piston systems. Ill. 5. Tab. 4.

Novel Oxide Coatings on Single-Crystal Sapphire and Sintered Ceramics

The formation of the protective and hardening oxide coatings on the surface of the single-crystal sapphire and sintered corundum (ceramics) is studied. Corundum systems with oxide coatings are as yet unknown and our studies are original. The compositions of the coating based on the mixture of Al2O3 and B2O3 oxides and a technique of their deposition on the samples are worked out. The strength of the samples during the bending test is studied. It is shown that the hardening of several samples achieves ~30% and microhardness (HV) reaches 30 GPa. Basing on the results of an investigation of the significant number of the samples obtained under different conditions, it is concluded that the developed method of the protection and hardening of the corundum products is promising.

Leaching of gallium from corundum flue dust using mixed acid solution

Abstract In order to extract gallium from a high-silica-content flue dust generated in corundum production, a mixed acid solution of H2SO4 and HF was used for leaching, and test parameters of the leaching process were optimized. Experimental results show that the leaching rate of gallium was only 38% when H2SO4 was used as leaching agent. Composition analysis results of micro areas in this corundum flue dust indicate that the content of gallium in silica-enriched phases was high; this portion of gallium was insoluble in H2SO4 solution. The leaching rate of gallium increased significantly with addition of HF due to corrosion of silica. Effects of reaction time, temperature, and concentrations of HF and H2SO4 on leaching rates of gallium were investigated. The leaching rate of gallium reached 91% when this corundum flue dust was leached in a mixed acid solution of H2SO4 and HF for 4 h, at a temperature of 80 °C, with a liquid-to-solid ratio of 5:1 (mL/g). The optimal concentrations of H2SO4 and HF in the mixed acid solution were 1.5 and 6.4 mol/L, respectively.

Research of the rheological and casting properties of alumina slips and samples from them containing a new dispersing addition and its combination with a hardening addition

The corundum ceramics technology has been developed and implemented at JSC “The URIR named after A. S. Berezhnoy”, which involves the production of high refractory corundum high density products using different methods of forming. A slip casting in gypsum form is one of the often used for forming a corundum ceramics production. The advantage of this method is the simplicity of the equipment, the relatively low cost of forms, as well as the possibility of manufacturing products of various shapes, sizes and weights.
 In depending of the wall thickness of the product using two methods of slip casting: bulk method with using a slip with a density of 2.5 g/cm3 of alumina MARTOXID brand with a dispersing addition (based on an aqueous solution of an ammonium salt of an acrylic copolymer) in an amount of 0.4 % for casting products of complex configuration, insulators, rods, plates, mortars, pestles, and for casting thick-walled products (4—5 mm and more) and a drain method with using a slip with a density of 2.1 g/cm3 of alumina MARTOXID brand with a dispersing addition in an amount of 0.3 % and a hardening addition in an amount of 0.2 % for casting hollow articles in the form of crucibles, covers, pipes, a ditch, boats, etc. for casting thin-walled products (1—3 mm).
 The researches of rheological properties of alumina slips from alumina MARTOXID brand which contain a new dispersing addition based on carboxylic acid and its combination with a hardening addition were carried out in this work.
 The optimum values of rheological and casting properties of the slips, ensuring the production of castings with a maximum density, are achieved with the introduction of 0.3 and 0.2 % dispersing addition into the slips with a density of 2.5 and 2.1 g/cm3 respectively were established.
 It was established that, at use of the dispersing addition based on carboxylic acid the apparent density of green body corundum ceramics lower than with the dispersing addition based on an aqueous solution of an ammonium salt of an acrylic copolymer, but it is sufficient to produce the same high-quality products after firing at 1580 оC (open porosity is not > 0.8 %) with a slightly higher shrinkage during firing, i. e. the use of this addition, if necessary, is possible.

Новые оксидные покрытия на монокристаллическом сапфире и спеченной керамике

AbstractThe formation of the protective and hardening oxide coatings on the surface of the single-crystal sapphire and sintered corundum (ceramics) is studied. Corundum systems with oxide coatings are as yet unknown and our studies are original. The compositions of the coating based on the mixture of Al_2O_3 and B_2O_3 oxides and a technique of their deposition on the samples are worked out. The strength of the samples during the bending test is studied. It is shown that the hardening of several samples achieves ~30% and microhardness (HV) reaches 30 GPa. Basing on the results of an investigation of the significant number of the samples obtained under different conditions, it is concluded that the developed method of the protection and hardening of the corundum products is promising.

Refractory Cellular Concrete Based on Phosphate Binder from Waste of Production and Recycling of Aluminum

The article is considering the results of development of refractory cellular concrete based on aluminum-phosphate binder. It was prepared from aluminum production slag, aluminum recycling slag and aluminum etching sludges. The selection of aluminum phosphate is based on the fact that the aluminum-phosphate binders and materials are the most refractory. Fire clay, wastes of corundum production and used aluminum-chromium catalyst IM-2201 were used as aggregates. These are dispersed alumina and high-alumina industrial waste that do not require additional costs for milling. The basic properties of raw materials have been tested.The heat-resistant phosphate cellular concrete was developed based on fire clay and the corundum production waste, it has working temperature 1400-1600°C. Cellular concrete is setting due to the exothermic reaction fine powder of aluminum and phosphate binder. The porosity is formed by the reaction of metal - phosphate binder. The phase composition of the phosphate binders was investigated after setting and heating. The scheme of the phase composition changes is presented. The main heat-resistant properties are investigated for the resulting cellular concrete. It is found that the obtained material is not inferior to cellular concrete with pure aluminum-phosphate binder.

Gas-Transport Reactions in the Processing of Kyanite Concentrate from Keivy Ore in Corundum Production

Gas-transport reactors are widely used in the chemistry of solids because the direct interaction of solid phases with one another is impeded. The process developed for enriching kyanite concentrate, which maintains a rational direction of the gas-transport reactions, gives a product with a high content of aluminum oxide (99%) and low content of silicon, iron, and titanium oxides.