Benefit from scrap aluminum
Recycling of aluminum scrap has a number of key environmental and economic benefits. Compared with other materials, which are produced in large volumes, производство алюминия имеет самую большую разницу в потреблении энергии между первичным и вторичным производством:
- 186 megajoules per kilogram for primary aluminum and
- 10-20 megajoules for secondary aluminum.
Therefore, many manufacturers are looking to increase the use of recycled materials.. However, accumulation of harmful impurities in these streams of secondary aluminum is a significant barrier and limitation to advance these goals.
Impurities and impurities in aluminum scrap
The accumulation of harmful impurities is a common problem in the processing of all secondary materials.. In the case of aluminum, the list of unwanted elements is quite long., including - and not only - silicon, magnesium,, nickel, zinc, lead, chromium, iron, copper, vanadium and manganese.
Recycling of secondary metals is a metallurgical process and therefore is governed by the laws of thermodynamics. Removal of unwanted elements from the scrap metal stream is associated with energy considerations during the melting process. Compared to other metals, aluminum exhibits a high degree of difficulty in removing impurity elements simply due to thermodynamic barriers.. Therefore, in the absence of thermodynamic solutions to this problem, manufacturers must solve them in other ways..
Dilution with primary aluminum
There are many solutions used in the aluminum industry, to overcome the negative impact of scrap recycling on the growth of unwanted elements in the finished metal. Dilution with primary aluminum is the most common solution, which is currently used in industry. Besides economic losses, this method also reduces the productivity of remelting secondary alloys, as it requires an additional technological operation.
Another common method is the processing of aluminum scrap and waste with a decrease in their original quality.. In English it is called "down-cycling". This allows the use of lower categories of aluminum scrap, but negatively affects the profitability of aluminum processing. An example of such "downsizing" is the use of wrought aluminum scrap for the production of aluminum cast alloys due to their ability to absorb higher silicon content..
In recent years, new technologies have begun to develop for the preliminary - before remelting - sorting of aluminum scrap. One of the directions of such technologies is based on the physical separation of streams of solid scrap to prevent mixing of metals and elements..
Physical sorting technologies
Physical sorting of aluminum scrap used for scrap, which is already crushed. Most car scrap, for example, undergoes a certain grinding operation before being sold for remelting by secondary aluminum producers. It is on this car scrap that most physical sorting technologies have been developed., so this scrap accounts for the majority of the total turnover of aluminum scrap.
for instance, more than 200 of enterprises, who are engaged in shredding of aluminum scrap. They mainly use large hammers to flatten and break aluminum parts into pieces no larger than 100 mm. Physical sorting methods for aluminum scrap are often applied after some pre-sorting., and also after heat treatment to remove paint from aluminum, paper and plastic stickers and other coatings.
Physical sorting methods
The methods of physical sorting of aluminum scrap include the following.
- Magnetic method. Separates the iron and non-iron components of the scrap.
- Air method. Отделяет легкие компоненты (пластик, rubber, пена) от тяжелых (металлы);
- Current heat method. Separates various metals.
- Dive-ascent method. Separates metals of different density by immersing them in liquids of high and different density.
- Sort by color, Produced by hand or with a spectrometer.
- Hot destruction method. Separates eutectic silumins from wrought aluminum alloys. Heating to a temperature just below the eutectic strongly weakens the eutectic alloys along the grain boundaries - in contrast to alloys with a low silicon content.
See. also Aluminum scrap: sorting of alloys
G. Gaustad et al / Resourses, Conservation and recycling 58 (2012) 79-87