Sintering Of MgO Research Articles

A Radioisotope Study of the Sintering Process of Magnesia Studies on Dead-burned Sea Water Magnesia, III

A radioisotope technique was used for the determination of the rate of movement of Ca2+ and Fe3+ ions in MgO added with or without a small amount of SiO2, Al2O3, etc. As the tracers RI45Ca and RI59Fe were used. The relationship between the rate of movement of these ions and the sintering behaviors of the mixtures were also studied.The results obtained were summarized as follows:(1) The movement of these ions is caused by thermal diffusion and draining. In MgO added with CaO and SiO2, the rate of movement of Ca2+ ion increased with increasing the molar ratio CaO/SiO2, reached to a maximum when CaO/SiO2≅1, and then decreared; no movement was observed when CaO/SiO2_??_2. Additions of Al2O3 and Fe2O3 to MgO together with CaO and SiO2, however, produced different results, i.e., the rate of movement of Ca2+ in this mixture increased with increasing the ratio (CaO/SiO2) without having a maximum.(2) Additions of oxides such as TiO2, Cr2O3, MnO2, B2O3, etc., which were found, by the author's previous works, to be effective in promoting sintering of MgO and the growth of periclase crystals, always increased the rate of movement of Ca2+.(3) Comparing with Ca2+ ions, Fe3+ ions in MgO had a low rate of diffusion. Draining occured only when Ca2+ coexisted in MgO.(4) X-ray analysis showed that the compounds, CaO⋅MgO⋅SiO2, 2CaO⋅Fe2O3, 4CaO⋅Al2O3⋅Fe2O3, etc., were produced in MgO by the movement of Ca2+ and Fe3+ ions.(5) It was suggested that when the above compounds melt at high temperatures producing the liquid phase, the loard-bearing capacity of MgO is lowered.

Factors Affecting Sinterability of Oxide Powders:BeO and MgO

Data on the sintering of BeO and MgO were reviewed with reference to the diffusion sintering model devised by Kuczynski and Herring. Sintering rates for compacts of the dead‐burned oxide powders were in agreement with the bulk diffusion model with grain‐boundary sinks, but those of the active low‐calcined powders were not. During isothermal sintering the low‐calcined powders had very high initial shrinkages, after which the shrinkage rate was low, corresponding to a rate index of about 10. The initial high rate was ascribed to crystallization and coalescence of the material comprising individual particles. The activation energy associated with the sintering of active BeO was estimated as 249 kcal. per mole.

Effect of Minor Additions on Sintering of MgO

Fourteen selected metal ions were added to magnesium basic carbonate, and the MgO obtained after calcining was fired at various temperatures to determine the effect of the additives on the sintering behavior. The magnesia without additives showed rapid increase in density with firing temperature near 1200° C. The majority of the additives caused increased density, at a given firing treatment, but some were without effect and one (Cr) dramatically inhibited sintering in certain percentages. It is believed that most of the additives which aided sintering did so by entering into the magnesia lattice and creating defects, although at least one (V) clearly promoted sintering by liquid formation.

Effect of Some Lithium Compounds on Sintering of MgO

The addition of 0.25% or more of certain lithium compounds, particularly the halides, facilitates the pressing and sintering of reactive forms of high‐purity magnesium oxide. Densities up to 3.46 gm. per cm.3 have been obtained at 1400°C. with reagent‐grade magnesium oxide containing 0.5% lithium chloride as an additive. The effectiveness of lithium salts as sintering catalysts for magnesium oxide depends on the original surface area and particle size of the MgO. After autoclaving at 150 lb. per sq. in. gauge for 2 hours, samples of MgO sintered at 1400°C. with 0.5%of LiCl showed a water absorption comparable to that of fused magnesium oxide. Similar samples of lithium‐treated periclase had about the same resistance to attack by a NaOH‐FeO melt as pure MgO fired at 1800° to 1850°C.