| 摘要 |
PURPOSE:To obtain magnesian powder for melt-spraying easy to sinter and melt in a flame by calcining magnesite at a specified temp. CONSTITUTION:Magnesite is calcined at 700-1,650 deg.C. When the calcining temp. is below 700 deg.C, thermal decomposition represented by MgCO3 MgO+CO2 is caused during melt-spraying to drop the temp. of a flame, or the decomposed gas is tightly sealed in a melt-sprayed layer to induce a foaming phenomenon, thereby resulting in a lower melt-spraying yield and deteriorated density, strengh and corrosion resistance of the sprayed layer. When magnesite is calcined at >1,650 deg.C, periclase crystals are grown to 20-100mum to reduce the specific surface area and the open pores, so the propagation of heat is made difficult to prevent the instantaneous melting of the powder in a flame, thereby resulting in a lower spraying yield and deteriorated density and corrosion resistance of the sprayed layer. In addition, the cost of fuel is increased because of high temp. calcination. Magnesia thus obtd. is pulverized to about 30-100mumwt. average diameter in consideration of feedability during spraying. |
