Recikliranje gnetnih aluminijevih zlitin iz nizkocenovnih vrst metalurško čistega odpadnega aluminija

In the recycling of wrought aluminium alloys from lower grades of scrap (metallurgically clean but highly contaminated with non-metallic impurities) the following two tasks were identified as the most demanding: (i) achieving the required final chemical composition of an alloy with a minimal addition of primary aluminium and alloying elementsand (ii) keeping the level of impurities (inclusions, hydrogen, trace elements and alkali metals) in the molten metal below the critical level. Because of the lack of chemically based refining processes for reducing the concentration of alloying and trace elements in the molten aluminium, once the concentrations of these constituents in the melt exceed the corresponding concentration limits, the only practical solution for their reduction would be an appropriate dilution with primary metal. To avoid such a costly correction, carefully predicting and ensuring the chemical composition of the batch in the pre-melting stage of casting should be applied. Fortunately, some of the impurities, like hydrogen and alkali metals, as well as various (mostly exogeneous) inclusions, could be successfully reduced by employing existing refining procedures. In this work, (i) the state-of-the-art technologies, including some emerging technical topics such as the evolution of wrought alloys toward scrap-intensive compositions, monitoring of the content of organics in the incoming scrap and the quality of molten metal achieved by different smelting and refining technologies, and (ii) the relevant economic advantages of the recycling of wrought aluminium alloys from the lower grades of scrap are reported. By analyzing the market prices of various grades of scrap and the total cost of their recycling, the cost of aluminium ingots made from recycled aluminium was modelled as a function of aluminium and the alloying-element content in the incoming scrap. Furthermore, scrap mixtures for producing aluminium wrought alloys of standard quality from lower grades of scrap and with a significant new added value were illustrated.Pridobivanje recikliranega aluminija standardne kakovosti iz nizkocenovnih virov bo v prihodnje odločilno vplivalo na konkurenčnost in uspešnost evropske aluminijske industrije. Pri recikliranju gnetnih aluminijevih zlitin iz nizkocenovnih vrst odpadnega aluminija (metalurško čistih, vendar onesnaženih z nekovinskimi nečistočami) sta posebej zahtevni naslednji dve tehnološki nalogi: (i) zagotavljanje želene kemijske sestave aluminijeve zlitine ob minimalnem dodatku primarnega aluminija in legirnih elementov in (ii) ohranjanje nivoja nečistoč (vključkov, vodika, elementov v sledovih in alkalijskih kovin) v talini v mejah dovoljenega. Ker koncentracijo legirnih elementov in elementov v sledovih v talini tehnološko ni mogoče spreminjati s kemijskimi postopki rafinacije, je, brž ko njihova koncentracija preseže dovoljeno mejo, edina rešitev redčenje z dodatkom primarnega aluminija. Tovrstnemu dragemu načinu zagotavljanja predpisane kemijske sestave taline se lahko izognemo le z doseganjem želene kemijske sestave pred taljenjem, tj. na stopnji načrtovanja vhodne zmesi. Preostale nečistoče, kot so npr. vodik in alkalijske kovine ter nekateri vključki (predvsem primarni), lahko uspešno obvladujemo že s sedanjimi postopki rafinacije taline, ki jih tu opisujemo. V tem delu tudi opisujemo (i) sodobne tehnološke postopke načrtovanja recikliranju prijaznih sestav gnetnih aluminijevih zlitin s povečanim deležem odpadnega aluminija, spremljanja koncentracije organskih nečistoč v vhodnem odpadnem aluminiju ter določanja kakovosti taline, pridobljene z različnimi postopki taljenja in rafinacije, in ugotavljamo (ii) ekonomske prednosti proizvodnje gnetnih aluminijevih zlitin z recikliranjem metalurško čistega odpadnega aluminija nižjega cenovnega razreda. Izhajajoč iz sestave in cen različnih vrst odpadnega aluminija ter stroškov njihovega recikliranja nam je uspelo razviti funkcionalni model, ki določa ekonomičnost proizvodnje ingotov iz recikliranega aluminija na osnovi vsebnosti aluminija in legirnih elementov v vhodnem materialu. Model smo aplicirali na različne vrste nizkocenovnega odpadnega aluminija in pokazali, da proizvodnja gnetnih zlitin standardne kakovosti iz tovrstnih virov zagotavlja najvišjo mogočo dodano vrednost