Giovanni Campice, T.T. Tomorrow Technology Spa, gcampice@tomorrowtechnology.it

Process description and advantages by dross press

In the current overview of systems and technologies for treatment of aluminium dross removed from melting and holding furnaces, the dross press is by far the most widespread technology which ensures the best economic results, as well as environmental and operational benefits.

Dross press size
The total maximum volume of dross charged into the skim box of the press ranges from 50 to 2.600 kg depending on press models (total of 13 press models are available).  As densities of various types of dross differ depending on types of melting process, solid charges composition and furnaces management, it is difficult to accurately project the weight of dross removed from furnaces that the skim box set will hold for the pressing process. It is easier to estimate the in house and overall Aluminum recovery by dross pressing thanks to the data collected from more than 400 dross presses in operation worldwide. As a guide, it should be noted for white dross 15%-35% metal recovery, with dross density of about 1.00-1.20 ton/cubic meter. For a rich dross 45%-50% metal recovery can be reached, while the density of the dross will be about 1.40-1.60 ton/cubic meter.

Dross press operation
The skim box, with freshly skimmed dross (together with the sow mold), is transported by forklift truck from the furnace to the press. The skim box and sow mold posare placed into the pressing unit. Once the skim set is placed into the pressing chamber and optical barriers are free, all functions automatically start. If the skim box is not set properly against the back wall of the press, the system will not start. Once the box is set in place, the safety shield door will come down and the cycle will begin. All automatic and manual functions are controlled and performed by the Programmable Logic Controller (PLC). The safety shield door eliminates the need of the operator opening and closing a door prior to the PRESS operation, either manually or hydraulically. The pressing cooling operation may be programmed into the Programmable Controller, presetting the cycles for each group or type of dross. Hotter dross may require longer cycle times. The pressing program includes multiple strokes for maximum pressing efficiency and minimizing the possibility of sticking. When the cycle starts the hydraulic pump starts running, the pressing cylinders begin compressing the dross into the skim box. The press compresses the dross for a preset period of time. This cools the dross and drives out the molten aluminum into the sow mold. When the pressing cycle is completed, the cylinder will automatically stop the compression and cooling cycle, raises the press head, and illuminate the cycle complete light. The press safety shield door is now fully up and the skim set can be removed from the pressing chamber by forklift. The next skim box may then be placed in the pressing chamber immediately thereafter. The same procedure described above is repeated. The operator can then proceed to dump the skim box that has just been removed. These pieces of pressed dross may be collected according to segregated alloys. Once cooled, the pieces may be loaded directly into shipping containers. The sow molds containing the pressed drained metal may be emptied and the sows returned directly to the generating furnace (it still contains the alloying elements formerly added to the bath in the furnaces).

Dross press physics and metallurgy
During this pressing operation several things occur. Much of the molten metal in the hot dross is drained and pressed and collected in the sow mold. The compression of the dross compacts the dross and drives out the oxygen and eliminates any future access of oxygen to the dross. This stops the oxidation process quickly. The compression of the dross allows for fast cooling due to the efficient heat transfer to the cast steel skim box and press head. Finally, the molten metal droplets remaining in the pressed dross due to the pressing forces increase size and migrate to the surface of the skull under the pressure of the press head. This process in fact consolidates the small particles into large plates or sheets of metal improving the metal recovery of the pressed dross skulls at the secondary processor as well as forming a shield (skin) which stops entrance of oxygen (thus avoiding further metal oxidation). The oxides are encapsulated in the center of the skulls containing the dust. The pressed dross is easier to handle and reduced in volume to save room. It is recognized that in the melting of aluminum operating practices vary, and the types of dross generated vary too. To accommodate such variances and to satisfy customers’ preferences and options, the system ultimately supplied is aimed to meet all these customer conditions; easy setting of main operating parameters of the dross press allows to maximize in situ and overall metal
recovery. Dross treatment must be anyway just the down stream process, after correct furnaces management to reduce dross generation and effective as well as careful cleaning operation, executed with proper equipment in order to ensure the removal of the dross without removing metallic aluminum from the furnaces. The Italian Company T.T. Tomorrow Technology designs and manufactures dedicated vehicles and rail mounted or fixed equipment for de-drossing and furnaces cleaning (as well as loading solid material into the furnaces); all these equipment are well used and proved to be ideal solution for furnaces tending operations. Use of above mentioned equipment replaces the forklift trucks in front of the furnaces, increasing operators’ safety and comfort level while ensuring important economic and operative advantages.

The two major dross pressing systems (Altek and Tardis) are from July of this year the same entity, under Altek management. They will converge into a single product lead by Altek technology. T.T. Tomorrow Technology SpA, based in Due Carrare - Padova, in the north east of Italy, exclusively produces and distributes Aluminium Dross Presses under license of Altek-MDY for the Italian market and for all the Countries in the Mediterranean region.