With energy prices forecast to rise 30% in the next two years foundries all over the world are looking for ways to reduce wasted energy and materials in their processes. More and more foundry managers see the melt room as a place to recover costs. While controlling metal melt loss is a generally accepted approach, there is considerable potential for cost savings by controlling the aluminum content in dross.
Controlling aluminum content in the dross can be accomplished several ways. First the furnace type is directly related to the amount of dross created:
- Electric wet-bath reverberatory melters lose less than 1% in metal melt loss.
- Natural gas-fired, wet-bath/low-headroom reverbs lose 2-4%.
- Gas-fired, wet-bath/high-headroom high bath reverbs lose 4-5%
- Gas-fired, dry-hearth furnaces lose 7-12%.
- Tower-jet or stack melters lose 5-7%.
The driving factor for metal melt loss is the weight-to-density ratio of what is charged. Lightweight scrap oxidizes faster and over a greater area then does a dense 1,000# sow. If you melt aluminum in a combustion rich atmosphere it will oxidize and you will lose more metal.
By melting metal under the surface of the bath you greatly reduce the oxidation process and limit it to only the surface metal in the bath. Batch melting creates more dross than does continuous charging because it usually means the metal is out of the aluminum during the batch charge, so more metal is exposed to the oxygen, and this results in higher metal melt loss. Some furnace companies design burners to create flame impingement on the bath, to help the melting process, but actually this increases dross creation as it super heats the metal surface.
Metal and atmospheric temperatures are extremely relevant to dross formations. For every 50°F rise in metal temperature above 1400°F, dross formation increases by 100%.
Dross formation increases as thermal head in the furnace increases. This is proven by the fact that holding furnaces with lower thermal head temperatures produce much less dross.
Of course, holding metal produces far less doss than melting metal. But, when melting with oxy-fuel or oxygen-enriched burners, be careful where the flame envelope ends. These burners produce super-hot flames that can drastically overheat the aluminum and cause an increase in dross formation.
Several other melting practices can increase dross formation: Charge rates, hold-to-melt ratios and others (hold-to-melt ratio, charge temperature, charge type, sludge factor). Much of the dross creation can be managed by following some simple best-practices: Keep the burner in ratio; Too much air can cause excessive oxide growth, and not enough can cause a cold flame and make the furnace work harder to do the same job.
If aluminum oxide formations are left on the hearth and walls after each cleaning, this will significantly reduce refractory service life. Good furnace maintenance is something only you can control.
Oxide latent furnaces require more BTUs to melt. Oxide is denser than refractories and absorbs more heat. The furnace must work harder to do the same job so dross levels increase.
Fluxing for metallurgical purposes — Follow the recommendations of the flux manufacturer, but to the “lean” side of quantities. Use metal drossing fluxes, and wall-cleaning fluxes as needed. Wall fluxes cause premature wall erosion from flux attack. Wall flux raises the temperature of what it touches to 3,100°F. Most hot-face refractories are good to 3,000°F.
Splashing metal and lack of cleaning frequently are the causes of excessive oxide growth. Keeping metal and walls clean reduces oxide growth and dross formation.
Recommended fluxing procedures — Here are 10 steps for flux application:
1. Apply wall cleaning flux to walls by using a tool that will allow you to place the flux directly on the oxide growing from the wall. Close door and heat for approximately 15 min. (Step 1 is used only if wall flux is needed to remove buildup, if not go to Step 3.)
2. Open door, preheat scraping tool, scrape walls to remove buildup. If buildup is heavy, it will need to be taken off in layers with several applications.
3. After scraping, broadcast drossing flux over the bath surface (approximately 0.5 lb./1000 lb. of metal) depending on depth of dross layer.
4. Preheat, rake, and rabble (stir) the flux thoroughly.
5. Dross layer should turn cherry red; if so go to Step 7. If dross is black add a small amount of flux and repeat Step 4 or turn on high flame for 5-10 min., and no longer.
6. Rabble flux again. It should turn cherry red dross (if dross is white hot, high flame was on too long; if black, turn high flame back on for 5-10 min.)
7. Pull dross to charge well. (Rabble dross in the well to recover more metal.)
8. Skim well, tapping skimmer to release additional metal while removing.
9. Use proper tools for the job.
10. Clean quickly and carefully, but thoroughly.
Circulation’s effect on dross — The alloy remains constant throughout the furnace when properly circulated. Six to eight volume changes per hour is recommended. This will keep a uniform temperature in the furnace, which will result in less dross and oxides formed. The reason for this is that there is now no superheating of the metal surface, and the BTUs absorbed into the aluminum are distributed into the flow from the circulation, resulting in much lower surface temperatures (80-190°F.) It conserves metal and reduces dross, and saves maintenance and refractory costs, as well as energy loss.
Every metalcasters that produces dross should look at all the methods of recovering metal from that dross. There are rotary furnaces for large volumes, and dross presses that squeeze aluminum from hot dross for medium volumes of dross. For smaller dross producers, dross stirrers work well. These allow the furnace tender to place hot dross in a bowl and apply additional flux, and allow the unit to stir the flux into the dross, further reacting with the aluminum to remove it from the dross.
In this economy it just does not make sense to pay someone else to get the metal out of your dross when you can recover it yourself and pay for the investment in less than 18 months.
In conclusion:
• Keep the burners stoichiometric
• Keep the furnace clean
• Watch what and how you charge the furnace
• Flux properly
• Have the right tools
• Recover your own aluminum
Everyone talks about saving energy when there is much more money to be saved by recovering your own aluminum from the process, reducing scrap and having cleaner furnaces. This will amount to hundreds of thousands of dollars a year for some foundries, secondaries and large diecasters. Appoint a metal saving guru in your organization to manage the metal you melt and send off for reclamation. The personnel cost will be returned many times over because metal melt loss is still more expensive than energy!
David W. White is the national sales manager for The Schaefer Group Inc. Contact him at [email protected]