The process of dross management (in my experience) is rather a neglected, overlooked or forgotten subject and not a lot of companies pay the due diligence to this topic and as a consequence fail to retain the hidden costs associated. The misconception is that that dross is unavoidable and part of the process , its not a problem, its has relatively a low cost impact and its a messy gaseous job that no one like to do.
This blog is aimed to stimulate and will add a little detail into what to look out for, how to minimise the dross formation at source, recover the aluminium from the dross and process it for maximum recovery with minimum environmental impact.
While dross, or Aluminium Oxide, to be more specific, is an unavoidable 'problem' in aluminium smelting and casting the shrewd companies know how to minimise this to their commercial advantage. I say "problem" in speech marks as while its a problem in molten metal when disturbed. In the solidified form its a great benefit as aluminium oxide (Al2O3) has many fantastic characteristics. The oxide is formed literally in hundreds of picoseconds and stops any further oxidation of the host (aluminium) unlike many other metals thus protecting it and adding a very hard, thermally insulating coating impervious to further oxidation and corrosion.
We know that dross is a by-product of aluminium production but what actually is it and how is it formed in our industry?
There are two types of dross, WHITE dross and BLACK dross. White dross is formed in primary smelting and remelting primary 'clean' metal.
It has a high metal (aluminium) content (sometimes up to 70% net weight). Little to no salt content (if any coming from from the RFI / TAC treatments, but for sake of argument lets consider there is none). The remainder of the composition of white dross is made up from NMP (Non Metallic Particulate), oxides and various other aluminium and alloying compounds.
Black dross is formed from the scrap recycling process, its has a lower metal content than white dross and contains up to 50% salt which is used in the aluminium separation process during recovery.
In both white and black dross there can be other impurities such as electrolytic bath, carbides and refractories.
If we consider the dross value versus the aluminium content its literally a linear relationship with the lower the aluminium content the higher the dross cost.
Dross processing costs are around $400/mt and remains constant its not linked to aluminium content, however processing losses increase with aluminium content so its very important to minimise the aluminium content in the dross and we will discuss this in more detail later in the blog.
Calculating Dross generation (gross and net) :
As dross processing comes at a cost the mot effective way to reduce cost is to avoid the occurrence from the start.
Minimising the dross can come in a number of forms. Melting Process:
Scrap selection and staging. (scrap thickness is important)).
Charging process and sequence.
Furnace parameters (temperature, design, burner configuration, stirring etc).
Surface Turbulence:
Crucible treatment.
Crucible transfer to furnace.
Furnace preparation.
Fluxing.
Alloying.
Skimming / drossing.
Temperature.
Casting:
Spout design.
Fall heights.
In terms of dross generation, moving or disturbing the metal (surface turbulence) either through crucible to furnace transfer, pouring or casting contributes to the highest impact on dross formation (around 65%).
The remaining 35% comes from furnace preparation and skimming.
Turbulence can be caused within the furnace from several processing factors such as crucible transfer, stirring, fluxing, alloying and skimming of the dross itself.
There are a few ways to lower the dross formation which could be lowering the transfer fall height, stirring the metal slowly and steadily and using good alloying additions that go into solution quickly. Skimming the furnace we will discuss later.
One way to reduce the oxidation on crucible - furnace metal transfer is to use a syphon. From the image above you can see how this system operates, using the The Venturi effect to allow the molten aluminium to flow through the constricted section of a pipe from the raised crucible and into the furnace. The flow is regulated and below the metal surface in the furnace as not to create turbulence. Results on melt loss can be immediate and as low as 0.6%.
Scrap staging inside the furnace and gauge selection are critical to maximise the aluminium recovery and minimise the dross formation during the melting and transfer process. From the charts below you can clearly see that the gauge of the scrap influences the dross formation, this is directly linked to the overall surface area of the scrap and the oxide which resides (protects) upon it. All coatings on the metal such as paints, lacquers and contamination (oils, inks, sealants) should be removed prior to melting. This does not only effect the metal recovery but the volatiles can act as a combustible fuel and cause safety concerns.
"Remember the melting process is the dominant operation for dross generation"
The next stage after melting the scrap and filling the furnace is to remove the excess dross, this acts as a thermal barrier either restricting heat to be absorbed by the melt or released from it if the temperatures are too high to cast. This is known as skimming. Below you can see an image of the skimming process. In this process the dross is manually moved to the side of the furnace and then with the use of a fork lift and tool removed into dross pans.
A good skimming practice removes the dross without removing the aluminium and contaminating or enriching the dross. Skimming should be conducted following the SWI as high temperatures particularly on Mg alloys can lead to thermiting and melt further losses. Thermiting dross is also a hazard to the plant and personnel and can damage dross pans and cooling equipment, like the dross presses or STAS inert gas coolers.
Below are some images of thermiting dross and the consequences not only to melt losses.
After the skimming process and assuming that the dross is not thermiting the next stage is to cool it as quickly as possible. We do this to capture the aluminium value within the dross.
Thermiting dross can lose between 1 and 2% of its metallic content per minute when exposed to air If we look at the burned out dross pan in the images above the thermiting dross was allowed to cool overnight and burn itself out. All the residual aluminium was lost and what resulted was only aluminium oxide, the aluminium reverted back to its previous form before all that energy was input at the smelter to break the oxygen - aluminium chemical bonds... What a waste!
Cooling The Dross
There are several tried and tested methods to cool the dross so it can be loaded and sent to your dross processor, or if your lucky enough, processed in house.
For the sake of discussion I wont give my opinion or any lengthy detail on which method or equipment is best, but share some insights into what methods are available and how they differ.
Air Cooling:
Highly wasteful, inefficient and lowers recoveries significantly. AVOID!
Closed, constrained air excluded:
A very good method which uses either Argon or nitrogen to 'cool' the dross. The process rather than cool the dross evacuates the air so thermiting can not take place. Simple method with low maintenance costs. Particular attention should be made to cooler seal condition so expensive inert gasses are not wasted.
Dross Compression:
Another method to cool the dross is by compression. The dross and pan are squeezed and the molten metal collected in the base of the pan for future charging directly back to the furnace. The remaining dross 'puck' is sent to the processor for recovery. The maintenance of these units can be quite significant.
Rotary Dross Cooler:
No longer such a common method due to the safety aspects of this method. The dross is cooled in a rotating drum which has water sprayed onto the outer surface. The maintenance costs are high and the capital investment significant when compared to other methods.
When working with your dross processors its always a good idea to know the content of aluminium in your dross, obviously this can change from alloy to alloy, furnace to furnace and shift to shift, BUT knowing this value ensures that your processor also knows how serious you take this 'recycling' and what you do with this data will help your operations team understand how to improve.
Spend time working with production on skimming don't pull off too much aluminium into the dross pans.
Use the data to justify melt losses and any process capital you may require. The payback just may surprise you.
A fire essay test can be used to determine the aluminium content in the dross. Your dross processor can also help perform this test.
Once you have this data you can work on the mass balance of your systems and the actual metal recoveries. All good information for your casthouse management team.
Concluding:
Review your methods, practices and standard work instructions.
Minimise metal transfers and cascading fall heights to avoid unnecessary turbulence.
The best method for crucible transfer is by Siphon.
Prepare the scrap and stage correctly, avoid contaminated scrap and balance the gauge to aid recoveries.
Adding the scrap by vortex, submergence with the aid of stirring (automatic or manual) will help dissolution rates but care should be taken to avoid excess turbulence.
Don't add more than 10% scrap to a melt charge.
Consider fluxing with an RFI to clean up the melt and remove alkali metals. This will also aid homogeneity.
Skim with clean tools, coated and with a steady pace, again avoid undue turbulence. Pause at the cill before pulling of the dross to allow metal to fall back into the furnace. Follow the SWI for the frequency.
Use the dross press, inert gas cooler or rotary dross cooler. Don't leave the material to thermite and cool by air. Justification for CAPEX can be made quickly to invest in any of these methods. Remember 1-2% losses of Aluminium in the dross for every minute it thermites!!!
Many thanks for taking the time to read this blog, please feel free to share with your friends and colleagues.
Should you still require further help on this particular subject reach out and please contact : albergtech@gmail.com
George