Simple Solutions That Work! Issue 15

CASE STUDIES 25 Continued on next page metal is exposed to the oxygen and this results in higher metal melt loss. Some furnace companies think designing the burner to have flame impingement on the bath helps the melting process, but it actually increases dross as you severely superheat the metal surface. Metal and atmospheric temperatures are extremely relevant to dross formations. For every 50 degree rise in metal temperature above 1400 degrees F, you increase dross by 100% Dross formation increases as thermal head in the furnace increases. Holding furnaces with lower thermal head temperatures produce much less dross. Holding metal in general produces far less doss than melting metal. But when melting using oxy fuel or oxygen enriched burners, be careful where the flame envelope ends. These burners produce super hot flames that can drastically overheat your aluminum and cause an increase in dross formation. Several aspects of the furnace itself can lend to increased dross. Charge rates, hold to melt ratios and: • The hourly-rated capacity every 15 minutes • 8-1 hold to melt ratio • The temperate of the charge • The type of charge • Sludge factor Dross can be managed if you follow some simple industry best practices: Example #1 12,000,000 BTUs 100,000 SCFH air 12,000 SCFH natural gas Chamber temperature = 1900 Degrees F out of ratio Results: lower production, temperature, efficiency, increases maintenance and more dross. Example #2 12,000,000 BTUs 120,000 SCFH air 12,000 SCFH natural gas Chamber temperature = 2100 Degrees F stochiometric Results: higher melt rate, temperature, and efficiency with less Maintenance and less dross. If aluminum oxide formations are left on the hearth and walls after each cleaning, this will drastically reduce refractory service life. Good furnace maintenance is something only you can control. Oxide-latent furnaces require more BTUs to melt. Oxide is more dense than refractories and absorbs more heat. The furnace must work harder to do the same job so dross levels increase. Things to remember when fluxing for metallurgical purposes – cleaning the molten metal: • Follow the recommendations of the flux manufacturer, but to the conservative side of quantities. • Metal drossing and wall cleaning • Premature wall erosion from flux attack • Flux raises the temperature of what it touches to 3,100 degrees Splashing metal is the #1 cause of excessive oxide growth. Keeping metal and walls clean reduces oxides growth and dross formations.