In the end, refining is divided into two types: "high temperature method" and "low temperature method".
The final method is the high-temperature refining temperature controlled at 680 ~ 720 ℃, feed was added 0.5% to 1 weight% NaOH, and 1 to 2 kg KNO 3, after stirring for about 2 hours after the slag was bubbled chlorine removal bismuth air , sampling and analysis of lead and silver , to lead less than 0.001%, silver below 0.003% is qualified. Then add a small amount of solid alkali to melt and cover the liquid surface, and cool down to 300-400 ° C ingot. The fine mold is cast in a vertical mold, and the inner dimensions of the mold are 370 × 140 × 30 mm, each weighing about 15.6 kg.
The low temperature method controls the refining temperature to about 550 ° C. Other operations and technical conditions are similar to the high temperature method. Practice has proved that the use of low temperature method does not affect the quality, and can reduce fuel consumption and extend the service life of the refining pot.
The final refining slag composition of a factory is shown in Table 1.
Table 1 Final refining slag composition (%)
Rose Bengal refined bismuth metal surface was glossy, no slag, no porosity, no Trichoptera. In order to make the surface of the fine enamel silver-white, a factory increases the “surface-making†process. After passing the qualified sputum, the temperature is lowered to 320-340 ° C, and 0.2-0.5 kg of ammonium chloride is added. After 2 to 5 minutes, the surface of the sample is observed to be silver-white, and the pan-cast type can be obtained.
Refined No. 1 or No. 2 can be produced by direct fire refining. The finest national standards are shown in Table 2.
Table 2 Fine national standard GB915-84
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