Hot press compaction of magnesium oxide obtained by decomposing certain compounds

Conclusions A study was made of the densification during hot pressing at 1300–1600°C of magnesium oxide activated by decomposing the hydroxide and the basic magnesium hydrocarbonate. During decomposition of these compounds at 500–700°C with a soak of 15 min, magnesium oxide forms that is actively compacted almost to the theoretical density (98.5–99.5%) at relatively low temperatures (1500–1600°C) and pressures of 150 kg/cm².We investigated the influence of the time and temperature of heat processing of the hydroxide and the basic hydrocarbonate of magnesium on the fineness of the grains and the defects of the crystalline lattices of periclase thus formed, and also on the capacity for subsequent compaction during hot pressing.The reduction in the degree of compaction during hot pressing of the materials, heat processed at temperatures below 500°C, is due to the increase in the content in them of undecomposed residue, which hinders the diffusion sintering in subsequent stages of pressing.A reduction in the degree of compaction with rise in temperature of heat processing above 500°C or with an increase in the heat-processing time with the optimum temperature, is connected not with a reduction in the defects of the crystalline lattice of the periclase formed, but with the sizes and physical state of its particles.We also studied the effect of additions of magnesium oxide obtained by heat processing the hydroxide or the basic hydrocarbonate of magnesium on the compaction during hot pressing of industrial magnesia. The introduction of 10–20% of this additive ensures a reduction in the optimum pressing temperature of 100–300°C and an increase in the density of the specimens almost to theoretical.Translated from Ogneupory, No.2, pp.46–53, February, 1967.     

The sintering kinetics of well-brine magnesia

Conclusions The sintering of well-brine magnesium hydroxide proceeds in accordance with the laws governing solid-phase sintering, regardless of its degree of purity (within an MgO content of 96–98%) and of the type of precipitant used.The contraction kinetics of the magnesium hydroxide under isothermal conditions is characterized by the fact that l/lis approximately proportional to t^1/2. The contraction and compaction rates are at a maximum at 1000–1300°C and decrease significantly at higher temperatures.The contraction rate of specimens from calcined magnesium hydroxide was found to be 2–3 times lower than that of dried magnesium hydroxide.The specimens are compacted while contracting; in the elimination of the open pores P/P is approximately proportional to t^1/2 and in that of the closed pores to t^1/3.With an increase in the temperature from 1000 to 1700°C the compaction of the material is accompanied by periclase recrystallization. The periclase grains begin to grow rapidly after 1500C at an open porosity of 10–12%.At a temperature above 1500°C the recrystallization rate is so high that some open pores are entrapped in the growing crystals, resulting in closed porosity, the elimination of which is difficult. The sintering rate increases sharply at the same time.Given these general regularities, which support earlier findings, it is possible that the contraction will vary slightly with the chemical composition and heat treatment conditions of the specimens of well-brine magnesium hydroxide.Translated from Ogneupory, No. 6, pp. 39–44, June, 1974.     

Sintering magnesium oxide made from magnesium hydroxycarbonate

Conclusions The studies have shown that the fine structure of activated MgO depends on whether the temperature regime under which the magnesium hydroxycarbonate is calcined is fast or slow. In the rapid activation regime, subdispersed particles of MgO with maximum defects of the crystal lattice are formed and this help the diffusion sintering of the MgO.The use of nonaqueous organic binding agents which burn out rapidly in the 300–460°C interval and (in the case of the slow activation regimes) the addition of chrome-spinel and ZrO₂ also improve the sintering of MgO.The high initial disperseness of MgO and the presence of a surface-active binder make it possible by firing at 1500–1600°C to produce a high-density material which is suitable for the manufacture of dense and thermal shock resistant periclase articles.Deceased.Translated from Ogneupory, No. 5, pp. 47–51, May, 1979.     

Obtaining magnesia and periclase refractories from bischofite from the volgograd deposits

Conclusions Experimental work was carried out to obtain from bromine liquors of bischofite of the Volgograd deposits magnesium oxide, using low capacity industrial equipment for chemical purification of magnesite, including a recuperator, a tank-collector, a spray-reactor, a cyclone, an absorption column, spray traps, a vacuum pump, and a filter press.We worked out the parameters for the processes of boiling off the liquor, thermohydrolysis of the magnesium chloride, absorption of the hydrogen chloride with the production of hydrochloric acid, washing and leaching the hydrolytic magnesium oxide, and filtration of the resulting magnesium hydroxide. During the reprocessing of the liquor containing the bischofite, using this technology, without preliminary purification to remove boron and sulfates, we obtained magnesium hydroxide with a high purity level (98.5–99.5% MgO on the calcined weight).It is shown that the magnesium hydroxide thus obtained is a high-quality raw material for the production of fused and sintered periclase with a purity of 98–99.5% MgO. The technology of sintering, including carbonization of the magnesium hydroxide, hot palletizing without bond, and firing at moderate temperatures, ensures the production of densely sintered periclase. Tiles with inserts made from fused periclase of high purity with a clear anisotropic structure facilitate the casting of two heats through the gate valves.The results obtained can serve as the starting data for the development of specifications (TLZ) and planning the production of magnesium oxide from the debrominized bischofite liquors at the Volgograd site using the thermohydrolysis method. We need to solve the problem of the use of the 20% HCl that is obtained.Translated from Ogneupory, No. 1, pp. 27–30, January, 1988.     

The influence of some technological factors on the properties of ceramic spinel based on MgO-Al2O3

Conclusions An analysis of the influence of certain technological factors on the synthesis and sintering processes of spinel with excess periclase showed that spinel formation from various starting materials is largely complete at 1500°C. Firing at 1650–1750°C results in complete synthesis and approximately equal densities with various magnesite components. It was shown that waste materials from the production of abrasives can be used in place of commercial alumina.The addition of chromite ensures complete synthesis primarily at temperatures up to 1500°C but the effect on the properties of the end-product is slight. The density of the moldings increases with the molding pressure but in high-temperature firing this effect diminishes.Translated from Ogneupory, No. 1, pp. 36–43, January, 1975.     

Sintering and minerogenesis in mixtures of dunite and dolomite

Conclusions The firing of mixes of dunite and dolomite produces a chemical reaction between the components giving periclase, forsterite, and lesser amounts of other orthosilicates of magnesium and calcium. The composition and amount of the newly formed minerals depend on the quantitative ratio of the mix components and on the firing temperature.Mixes containing less than 60% dolomite sinter at temperatures not below 1500–1600°C. The lime and the calcium and magnesium orthosilicates undergo complete combining which results in the formation of a high-quality magnesite-dolomite refractory.Translated from Ogneupory, No. 6, pp. 36–39, June, 1976.     

Sintering magnesium oxide obtained by the chemical beneficiation of magnesite

Conclusions A quantitative relationship was established for sintering chemically beneficiated magnesite and a method of obtaining the MgO; we also established the parameters of hydrolysis of the magnesium chloride, the quantity of residual volatiles, the temperature and rate of thermal activation of residual volatiles, the temperature and rate of thermal activation of the magnesium oxide, not washed, and washed to get rid of the calcium impurities, and also the completeness of the hydration of the magnesium oxide during leaching.A production flow line is proposed for obtaining from the enriched material sintered periclase clinkers differing in density and degree of purity.Translated from Ogneupory, No. 7, pp. 43–50, July, 1982.     

Sintering high purity,magnesia with additions of hafnium dioxide

Conclusions The introduction of hafnium dioxide into high-purity active magnesia accelerates its sintering and with 0.1–0.3 mol % HfO₂ a ceramic with an apparent density of 99–100% of the theoretical (3.56–3.63 g/cm³) can be obtained after firing at 1300–1400°C. The optimum quantity of HfO₂ additive is close to 0.25 mol %.The calcination temperature of the mixture of magnesium hydroxide and additive obtained by precipitation from a solution of MgCl₂ and Hf(SO₄)₂ with ammonia, and the fabrication pressure do not greatly affect the final density of the ceramics.Sintering of spectrally pure MgO containing 0.25 mol. % HfO₂ begins at 950°C, then the apparent density grows rapidly with rise in firing temperature, approaching 3.40 g/cm² at 1100°C. Selective recrystallization at these low temperatures is slow and sintering is not accompanied by a substantial grain growth. At 1300°C and higher firing temperatures densification of the ceramics approaches the limit in several minutes.The mechanism of the transfer of substance during the sintering of these specimens is volume self-diffusion from the grain boundaries to the surfaces of the bridges formed between them. The energy of activation of this process [3.9 eV (6.2 × 10^–16 J) in the 1000–1300°C range] and the coefficient of self-diffusion for MgO calculated in accordance with this (6 × 10^–14 cm²/sec at 1100°C) correspond to existing data on the diffusion of magnesium into MgO.With the incorporation of 0.25 mol. % HfO₂ in less pure magnesia obtained from chemically pure MgCl₂, sintering is little different from the sintering of spectrally pure MgO, but the limiting apparent density of the ceramics in this case is somewhat lower-of the order of 99% of the theoretical.     

Refractories from fused periclase

Conclusions High-grade refractories can be produced from periclase containing at least 97% MgO by preparing a suitable grain size distribution with a vibro-ground component in the powder, molding in stages at high specific pressures (1500–2000 kg/cm²), and firing for 8 h or more at a high temperature (1750°C or higher).Translated from Ogneupory, No. 6, pp. 5–8, June, 1974.     

Service of rammed crucibles for melting noble metals in induction furnaces

Conclusions We prepared and tested in service crucibles of fused magnesium oxide with special additions, having a considerably higher resistance in induction-furnace conditions at temperatures of about 2000°C. The additives guaranteed the necessary sintering of the working zone in the crucible, and helped to form a direct bond between the crystals of periclase.The structure of the used crucibles is described. The formation of the zone structure is connected with the temperature gradient, and in the hot zones is accompanied by intensive recrystallization and growth of periclase crystals.Translated from Ogneupory, No. 5, pp. 19–23, May, 1969.     

Properties of refractories produced from synthetic magnesia

Conclusions The sinterability, open porosity, and cold-crushing strength of magnesite and magnesite-chromite specimens processed from magnesia powders (96–97% MgO) are improved with a decrease in the size of the periclase grains in the powder and with an increase in the firing temperature and do not depend on the percent and composition of the silicates and on the B₂O₃ content of the magnesia powder.The high-temperature bending strength of both types of refractories increases with a decrease in the B₂O₃ content of the magnesia powder. The creep resistance of the magnesite specimens increases with the ratio CaO/SiO₂ in the magnesia powder while the creep resistance of the magnesite — chromite specimens does not depend on this index.The indices of the open porosity and strength of the magnesite and magnesite — chromite specimens were optimal when they were produced with magnesia obtained by the bicarbonate method from dolomite.To produce dense and strong magnesite refractories from magnesia, they should be fired at a temperature not below 1700°C. The firing temperature of magnesite — chromite refractories should not be below 1750°C.Translated from Ogneupory, No. 6, pp. 53–57, June, 1978.     

Sintering fine grained sillimanite and kyanite concentrates

Conclusions It is necessary to add 3% sulfite lye (on the dry residue) to the batch used to prepare fine-grained (finer than 0.3 mm) kyanite and sillimanite concentrates for calcination in rotary furnaces in order to obtain dense, volume-constant material for the production of high-grade refractories. The pressing force for the briquette should be not less than 500 kg/cm² (about 50 Mn/m²) and the firing temperature 1630–1650°C.It is possible to recommend from 1 to 3% calcium and magnesium fluorides or magnesium oxide as sintering additives for articles that need not have outstanding thermomechanical properties.Translated from Ogneupory, No. 6, pp.43–49, June, 1967.     

Reaction between phosphate bonds and magnesium oxide

Conclusions During the reaction of phosphate bonds with magnesium oxide, magnesium phosphates are formed confering hardness on the magnesia concretes.In the specimens based on aluminophosphate bond the magnesia reacts with monosubstituted aluminum phosphate, and under these conditions di- and trisubstituted magnesium phosphates and aluminum orthophosphate are formed, which in turn react with magnesia at temperatures above 1000°C, with the formation of magnesium orthophosphate and magnesia-alumina spinel. Spinel develops at 600°C in the specimens based on chrome-phosphate bond.The phosphate bonds in contrast to orthophosphoric acid reduce the quantity of magnesia-phosphate glass in the specimens by a factor of 1.5–3.The aluminophosphate bond to a greater degree contributes to recrystallization of the periclase compared with H₃PO₄, magnesium- and chromium-phosphate bonds.Translated from Ogneupory, No.6, pp.53–57, June, 1972.     

Production technology of high-density periclase-spinel roof materials

Conclusions High-density periclase-spinel refractories can be produced from a body of magnesite powder containing 94–96% magnesium oxide and Kempirsai chromite by high-pressure molding and high-temperature firing in a tunnel kiln. The product is strong with good thermal stability and high onset temperature of deformation under a load. The structure of the refractory is improved as a result of the formation of a large proportion of direct intergranular bonds between the periclase and spinel.It is planned to produce experimental industrial-scale batches of periclase-spinel roof refractories and to subject them to trials in the roof of open-hearth furnaces in continuous operation.Translated from Ogneupory, No. 8, pp. 39–44, August, 1973.     

Vibro-cast periclase refractories of a grainy structure and some of their properties

Conclusions We have studied the process of obtaining vibro-cast periclase refractories based on fine- and coarse-grained molding systems using a low-concentration solution of HCl as the binder.Using a combined experimental and calculation method based on the use of a cone, we have carried out studies to optimize the production parameters of the vibro-casting process.The materials obtained in the initial (unfired) state are characterized by fairly high strength (_comp=12–30 MPa) and can be used as refractory concretes.The process of sintering the materials was studied and it is shown that their properties are comparable with the normal periclase refractories of a grainy structure. After firing at 1550–1600°C the refractories show virtually no additional shrinkage and their true porosity is 19–23%, ultimate compressive strength 30–60 MPa, and ultimate bend strength 12–26 MPa.The results were proved industrially. Large details of the smelting unit of induction furnaces were manufactured and successfully tested.Translated from Ogneupory, No. 8, pp. 9–15, August, 1986.     

High-temperature abrasion of refractories

Conclusions The most rapid abrasion in dinas refractories occurs in the 700–900°C range; periclase 800–1000°C; mullite-corundum articles exhibit a smooth increase in abrasion with rise in temperature. The abrasion of refractory concretes is much higher than for bricks.It is possible by determining the abrasion resistance to carry out quality control of the firing process, and also to check the quality of linings made from concretes.Translated from Ogneupory, No. 1, pp. 13–14, January, 1991.     

Sintering of caustic magnesite dust over the length of a rotary kiln

Conclusions The intensive formation of the pellets (granules) of caustic magnesite dust commences from the moment of completion of the decarbonization of the particles of raw magnesite. The development of pellets is established at a distance of 29 m from the charging end of the kiln.The formation of grains and their densification and reinforcement occur in the section 29–45 m of the length of the kiln as a result of the compaction of the particles of caustic dust.The recrystallization of the periclase, sintering, and agglomeration of the powder occur mainly in the section 45–75 m of the furnace length in the presence of melt.During the burning in a 90-m rotary kiln of unground caustic magnesite dust, the apparent porosity of the powder resulting from its grains diminishes from 50–60 to 24%.Translated from Ogneupory, No. 8, pp. 52–55, August, 1971.     

Investigating the sintering of seawater magnesium hydroxide

Conclusions To obtain densely sintered seawater magnesia powders it is necessary to ensure a molar ratio of CaO/SiO₂ of not more than 1 in the slurry.Incorporating B₂O₃ in amounts of not less than 0.5% on the calcined weight improves the sintering of the seawater magnesium hydroxide: the porosity of the fired (rotary furnace) powder varies from 8 to 12%.Preliminary drying or heating of the slurry, followed by briquetting of the material, produces a seawater magnesia with a porosity of 7–11% and a content of 95–97% MgO, and 2.3-0.5% CaO. Preference should be given to predrying of the slurry, since in this case no fuel is needed for removing the chemically bonded water.Translated from Ogneupory, No. 2, pp. 16–21, February, 1969.     

The sintering of flotation-concentrated magnesite

Conclusions An investigation was carried out of the sintering of flotation-concentrated magnesite containing (in terms of the calcined substance) 95.3% MgO, 0.52% SiO₂, and 2.0% CaO.The process parameters for the production of a high-density (open porosity less than 11%) sintered powder from concentrated magnesite proved to be as follows: 15% caustic magnesite added as binder to the concentrated magnesite, grinding the mix of concentrated and caustic magnesites in tube mills to a powder of a specific surface greater than 6000 cm²/g (i.e., containing 98% particles smaller than 60), wetting with water to a moisture content of 6–8%, forming 10–12-mm-thick briquets on a smooth-roller press at a pressure of 1000–1200 kgf/cm², and firing the briquets in a rotary kiln at a flame cone temperature of 1760°C.This technology for the mass production of a high-density, good-quality powder from concentrated magnesite precludes the use of a sintering additive and thermal activation both of which lower the refractoriness and increase the net cost of the finished product.Translated from Ogneupory, No. 2, pp. 3–7, February, 1977.     

Effect of firing temperature of aluminum hydroxide on the possibility of preparing ceramics with the use of cip and on the properties of corundum ceramics

It is established that structural corundum ceramics (bioceramics) can be fabricated from aluminum hydroxide powder. The effect of the firing temperature and the pressure on the physicomechanical properties of the sintered material is established.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 11, pp. 34–36, November, 1996.