Recyclability of aluminum
High reusability of aluminum after the end of the life of the aluminum product, in which he was kept, follows from the following properties :
- High resistance to corrosion under most environmental conditions. Due to this, a large proportion of metal that can be recycled and reused is retained in an old aluminum product..
- Energy, which is required to melt this "old" aluminum into "new" is only 5 % from energy, which was required for its primary smelting.
- The flexibility of aluminum alloying makes it possible to use a large number of industrial aluminum alloys, which are specially created taking into account the possibility of the presence in them of an increased level of contaminants.
The goal of any secondary aluminum production is to produce affordable and quality industrial aluminum alloys. Currently, more than 300 chemical compositions for wrought and cast alloys are internationally registered. Many of these alloys have been developed, to allow for deviations in the chemical composition and the level of impurities, which can occur during the processing of aluminum scrap.
Role of Secondary Aluminum
Recycling aluminum scrap is a complex interactive process, which includes centers for the collection and processing of scrap, primary aluminum producers, secondary aluminum producers, metal processing industries and consumers of aluminum products. The figure 1 shows material flows in the aluminum industry, starting from primary aluminum producers and through various production facilities for the processing and remelting of aluminum scrap. The figure 2 presents a model of the structure of the global secondary aluminum production, which shows the material flows of various types of aluminum scrap and the main types of equipment used .
Secondary aluminum producers: remelters and refiners
Secondary aluminum from aluminum scrap is recovered by two types of industrial enterprises, which we will call below for brevity “remelters” and refiners ”:
– melted down a relatively large, pure and slightly oxidized aluminum scrap (figure 3);
– produce wrought alloys (content of alloying elements up to 10 %);
– remelted painted (figure 4), contaminated and highly oxidized aluminum scrap, including crushed (figure 5), shavings (figure 6) and slags (figure 7);
– produce casting alloys and aluminum for steel deoxidation; casting alloys have an alloying element content of up to 20 %.
Remelters must select the right amount and quality of aluminum scrap, to "get" into the chemical composition of a given deformable aluminum alloy. So they take action, to carefully separate different aluminum alloys.
Refiners work in less severe conditions in relation to various aluminum alloys. They specialize in the remelting of mixed casting and wrought alloys into standardized aluminum alloys.. It is common practice for refiners to proportionally mix different alloys in a batch., which is intended to be loaded into the oven.
Some of the aluminum scrap is also consumed by producers, so called, primary shaped casting.
Secondary aluminum is produced from purchased, give-and-take and internal aluminum scrap. Contract scrap is scrap, which remains the property of the customer and is melted down for a fee. Scrap, which is formed and melted within the same company or group of companies is called internal scrap. With a few exceptions, refiners are separate small and medium-sized enterprises., and remelters are subdivisions of an enterprise or a group of enterprises, who produce any aluminum products. Therefore, only remelters produce secondary aluminum from internal scrap..
New scrap is generated at the stages of aluminum production and the manufacture of semi-finished or finished products from it. Old scrap is formed, when an aluminum-containing product, reaches the end of its service life, goes to scrap, sorted and remelted.
Remelting aluminum scrap
It is known, that aluminum cannot be reduced from its oxide to metal with carbon or hydrogen near its melting point, how does this happen, for example, with iron. In this regard, in the production of primary aluminum, an energy-intensive electrolysis process has to be used., to recover metallic aluminum from aluminum oxide (Al2O3). Recycling of aluminum is limited to the remelting (smelting) of aluminum from aluminum-containing materials, which make up the aluminum scrap. This provides a simple method for calculating the mass balance in aluminum recycling (without taking into account the influence of alloying elements) :
(metallic aluminium input) =
= (metallic aluminium output) + (oxidized metallic aluminium) /1,89
All oxides or other inorganic non-metallic components of the scrap remain unchanged during melting and, when calculating the mass balance, simply pass from input to output. Salts (fluxes) remain salts, which are needed mainly as an enveloping material for inorganic non-metallic components. Oxides either enter the furnace as part of the charge or are formed during the melting process. Volatile organic compounds (e.g., oils, grease) and moisture leave with flue gases, but may carry some components, such as salts or oxides .
Melting furnaces for remelters
- mostly traditional melting furnaces bath type without the use of melting fluxes. These are usually reverberatory deck ovens of various types: front loading furnaces, shaft furnaces, ovens with inclined (dry) hearth, etc.. (drawings 8-10)
Refiner melting furnaces
- bath-type furnaces without the use of a layer of melting fluxes with additional electromagnetic stirring, as well as with a side loading well (figure 11);
- rotary furnaces with a stationary (fixed) axis of rotation (figure 12);
- rotary furnaces with a tilting axis of rotation (tiltable rotary ovens) (figure 13);
- furnaces for processing salt slag, which give out recovered (secondary) fluxes and granular aluminum;
- equipment for processing wet shavings;
- other types of furnaces (in small quantities).
Remelters: features of work
For remelters, the main characteristic is the quality of the charge:
- clean scrap, unoxidized and unpainted
- in most cases - scrap with one type of aluminum alloy.
The metal yield is very high, that is, the metal content is from 96 % to 100 %. Such scrap does not require melting fluxes, which are designed to envelop inorganic non-metallic components. A small amount of coating flux is required to protect against oxidation during slag formation. Remelters can use different types of furnaces, however, the most typical is a bath oven, gas and inclined hearth. Near 0,15 % the total amount of metal in the charge is converted into oxides. Hot slag, removed from the surface of the melt, contains about 70 % metal, mostly absorbed by slag. This slag is collected in a steel box., where it is covered with a layer of flux (up to 5 % from the volume of removed slag) to prevent oxidation of most of the slag. The final metal content in the cooled slag will be about 60 %. This slag is sent to refiners for processing., which remelt it under a layer of fluxes.
Refiners: features of work
The main types of ovens, who apply refiners, are:
- Rotary furnaces of various types for melting under a layer of oxidized flux, painted or contaminated scrap, as well as slag. Rotary kilns with fixed axis of rotation form ordinary liquid salt slag, which contains on average 50 % salt. Average oxidation of aluminum is 3 %. The metal content in the slag is 8,5 % at the time of its unloading. Tilting rotary kilns are more suitable for remelting slag..
Some refiners use hearth furnaces (often multi-chamber) for scrap with a high content of organic contaminants - for continuous melting of a homogeneous type of scrap. These furnace work without the layer of flux unlike rotary furnaces. They are usually equipped with an electromagnetic pump., which provides fast immersion in the melt, for example, dry aluminum shavings. This ensures the level of oxidation of the chips during melting no more than 0,2 %.
A common practice for refiners is to drain the metal from the smelting furnace using a chute into a special holding furnace., which is designed to process the melt and transfer the metal to casting machines or to a ladle for further transportation. In this furnace, the melt is degassed, adjusting the chemical composition of the alloy, settling and other treatments. The slag formed in this furnace is returned to the rotary furnace., with fixed axis of rotation or tilted.
- Aluminum Recycling and Processing for Energy Conservation and Sustainability/ed. John A.S. Green – ASM International – 2007
- Melting Standardized Aluminum Scrap: A Mass Balance Model for Europe /U.M.J. Boin and M. Bertram - JOM, August 2005
- Aluminium Dross and Scrap Recycling – The Hybrid Tilting Rotary Furnace MASTERmax – www.kmf.at – 2021
- Handbook of Aluminium Recycling / Сh. Schmitz, 2014
- TALAT Lecture 3202 – The Liquid Metal / John Campbell and Richard A. Harding – 1994
- TALAT Lecture 1102 – Environmental Factors / Geoff Budd – 1994.