Simple Solutions That Work! Issue 17

films are droplets of non-oxidized aluminum. Flux is applied to the dross layer and mixed in using a rake or skimmer. The flux reacts to create thermite and generates heat, promoting coalescence of the Al droplets and flow back into the aluminum bath. Thermite is a reaction between the flux, available oxygen, and aluminum. When ignited or heated, it gives off an enormous amount of heat as a result of the chemical combination of the aluminum with the oxygen of the oxide. The reaction temperature is estimated to be about 2,400° C (4,400° F). If cleaning a reverberatory furnace the thermite will soften oxide buildup on refractory walls at the metal line and removes the metal from the oxide to facilitate easy separation for the aluminum to drop into the melt. A rake or scraper should be used to remove the oxide. DROSSING FLUXES / ALUMINUM RECOVERY The most economical aspect recognized by a caster is metal recovery. When properly used, a good drossing flux can reduce the metallic content in the dross by over 50%. If no flux is used the dross is considered wet as shown in figure 1. The proper flux addition will result in a dry dross. Excessive flux addition will lead to metal loss. A small percentage of aluminum is consumed by the exothermic reaction. In figure 2, the dross has a light gray appearance with some thermite. This dross should be allowed to cool before placing into a dross hopper. If excessive thermite occurs while following the documented process, a review should take place and any changes documented. EXCESSIVE THERMITE Excessive thermite will consume aluminum in the furnace and dross pan. Dross will glow white hot and generate fumes when agitated, once cooled a white powder dross will be evident and virtually all recoverable metal consumed. COOLING / EXTINGUISHING THERMITE • Appropriate Personal Protective Equipment (PPE) is required while handling dross. • Keep dross away from air movement, fans, open doors, or windows. • Spread the dross over a large area, this will allow the thermite to cool quickly. • Hot thermite dross should not be mixed with other dross containing a high percentage of aluminum. • Cover the thermite dross with a layer of dry rock salt, thus smothering the thermite. • Reclaim dross pans can remove molten aluminum and reduce thermite. • Incorrect flux selection or excessive flux addition are the two most common causes of excessive thermite. DROSSING FLUXES / GAS FIRED REVERB FURNACE CLEANING Flux addition rates will vary depending on how often the furnace is cleaned, charge material, and throughput. When developing a cleaning procedure for any furnace, it's best to start on the low end of the flux addition range and increase as needed to achieve desired results. Multiple cleanings will be required to zero in on the addition that works best for the furnace. Once again, develop, document, and follow the procedure. Weighing the dross and a visual inspection after each cleaning is a common practice during the procedure development. Furnace cleaning is a physically demanding task, even more so for large reverb furnaces. Furnace tenders must be given adequate time, proper tools in good condition, PPE, and training to perform the task. The furnace Recommended Flux Addition Range Reverberatory furnace, crucible, and holding furnaces .05% to .20% = 1/2lb. to 2.0lbs. Flux Per 1000lbs. Molten Aluminum Fig. 1 – Wet Dross Metal content 80% to 95% No Flux/Excessive Flux - 20% Consumed Fig. 2 – Dry Dross Metal content 30% to 40% Correct Flux addition – 5% Al Consumed 12