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WEEE
WEEE (Waste from Electrical and Electronic Equipment) or WEEE (Waste Electric and Electronic Equipment) are waste equipment, electronic products at the end of life or in any case no longer used by consumers. This category includes PCs, displays, telephones, integrated circuits, etc.The production of WEEE is associated with technological innovation and the growth of demand in the electronic sector. The decrease in the life time of the equipment, combined with inadequate awareness on the part of consumers, play a fundamental role in increasing the production of WEEE. The problem is particularly acute in developing countries, as the amount of WEEE produced is positively correlated with economic development. Expect rapid growth in WEEE production in these countries over the coming decades.For these reasons, the best solution would be to upgrade the WEEE and reintegrate it into the economic cycle by recovering metals and the plastic fraction they contain. In particular, given their critical metal content, the recovery of the latter is a primary goal. Critical metals are a group of metals indicated by the EU as decisive for technological development, for which there is a risk associated with their supply for geopolitical reasons and for their scarcity. Given the scarcity of these raw materials, it is necessary to increase the efficiency of the recycling of secondary resources, reducing the pressure on the extraction of virgin materials. WEEE could therefore constitute an important metal resource in the transition to a circular economy.Several technical innovations have been developed to try to manage WEEE in a sustainable manner, using an approach aimed at understanding the entire WEEE management chain (collection,pre-treatment, recovery and final disposal). The main problem is due to the fact that WEEEthey are rich in complex mixtures of metals, multi-element alloys and polymetallic structures that make extraction and separation difficult. The complexity of WEEE also tends to increase, going hand in hand with technological development: modern devices can also consist of 60 elements present as metal mixtures.
Potential environmental hazards
WEEE poses a series of environmental problems, since it can contain lead glass, brominated flame retardants, brominated and chlorinated polydiphenyls (PBBs and PCBs) and polybrominated diphenylethers (PBDEs). The presence of different classes of pollutants involves a series of environmental risks in the case of inappropriate disposal. Inappropriate treatment activities of WEEE, such as acid stripping or combustion in open spaces, involve the release of the pollutants contained therein. These activities are unfortunately widespread in countries such as China, India and Africa, particularly in Ghana. In fact, these countries constitute the largest global WEEE receptacles where landfill disposal is the most common method of managing this waste.As for Europe and the USA, WEEE is often transported to developing countries as used or waste devices. The export of such materials to developing economies often compromises public health, ecosystem services and primary resources.As for Italy, it is estimated that around 14 kg / inhabitant of WEEE are produced (for a total of about 800,000 tons) of which only 20-30% seem to be managed correctly.
Critical metals in WEEE
The content of critical metals in electrical and electronic equipment is of high economic importance in parallel with the high risk associated with availability. For example, REE (Rare Earth Elements), indium, platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, platinum), lithium and copper are critical metals. These elements are used to producemicroprocessors, printed circuits, cathode ray tubes, liquid crystal displays, LEDs and permanent magnets.
The availability of REE, however, is also essential for the innovative green economy technologies: in fact they play a fundamental role in the production of electric cars, wind turbines and photovoltaic panels. Therefore the separation between economic growth and the dependence on hydrocarbons as the objective of the circular economy is threatened by the shortage of metals for technological uses. A significant problem is the fact that these metals are present with low concentrations in WEEE and only as a small percentage within a complex matrix. Sometimes, therefore, it is precisely the economic unsustainability that makes recovery impossible.
Indio
Indium is a basic component for display production. On the basis of the quantities of metal extracted and the high consumption in LCD screens, the reserves of indium in the earth's crust could already be exhausted by 2020. Indium is also critical for geopolitical reasons as global production is dominated by some Asian countries including the China that since 2005 has reduced the export of this metal. With the advancement of display technology, the demand for Indian is expected to be enormous.
REEs
REEs are a relatively abundant group of 17 elements: the 15 lanthanides, scandium and yttrium. REEs are key components in many technological products such as smartphones, flat screens, batteries and special metal alloys. Although they are well distributed geographically, they are mostly extracted, concentrated and separated in China. Since 2010, China has reduced the distribution of REE through quotas, licenses, and taxes to maintain its reserves and for concerns about the environmental effects caused by thetheir extraction. Generally the elements become rarer as the atomic number increases and the elements with even atomic number are more abundant than those with odd atomic numbers (Oddo-Harkins rule). This imbalance is further accentuated by the higher demand for the rarer elements.Platinum group metals
The metals of the platinum group are a group of metals with similar chemical-physical characteristics such as good electrical conductivity, high melting point and resistance to corrosion which make them indispensable for many industrial applications and electronic equipment. They are extracted in South Africa, Russia, Zimbabwe, Canada and the USA.Lithium
Lithium is found only in some places like Chile, Bolivia, Argentina, Australia, Congo and China, therefore posing problems from a geopolitical point of view. The demand for lithium is linked to its extensive use in batteries.Copper
Copper is the prevalent metal in integrated circuits with a variable concentration between 15 and 35% by weight and constitutes the conductive layer for electrical connections. Vine extracted mainly in Chile, China and Peru.Main metals recovery technologies
Most industrial WEEE metal recovery processes involve physical pre-treatment followed by pyrometallurgical processes or, more rarely, hydrometallurgical processes. Physical separation is a common technique for processing all types of WEEE. However, a common approach to all types of WEEE is actually inadequate given their high variety. In addition, high energy consumption, relatively low recovery efficiency and potential metal contamination pose important obstacles in the management of this waste.
Pretreatment
The mechanical processes normally constitute the first step in the recovery of metals from WEEE and allow to separate the metal elements contained in them. Several physical treatments have been developed based on the chemical and physical differences of the materials contained in the WEEE. In fact, there are several processes currently available: manual and semiautomatic disassembly, dimensional reduction, gravimetric separation, magnetic selection and optical separation. Manual systems are currently the most widely used as they have a higher recovery efficiency.Pyrometallurgical treatment
Pyrometallurgical treatments, ie melting and pyrolysis, require the heating of WEEE at very high temperatures (up to 1500 ° C). The merger is currently the best available technique (BAT) and there are already operating facilities. At Boliden Rönnskär (Skelleftehamn, Sweden) the discarded integrated circuits are inserted in a converter to recover the metals. In Umicore, Belgium, integrated circuits are first treated in a furnace to recover precious metals which are then refined by hydrometallurgical and electrolytic processes.Pyrolysis is an endothermic process in the absence of oxygen that allows, depending on the conditions, to obtain solid liquid and gaseous products. The high temperature pyrolysis of waste integrated circuits, in the presence of inert gases, generates oil, gas, and a residue rich in metals. It is however a method with a high energy cost and high consumption of reagents.These are processes that present some disadvantages such as high energy consumption, potential negative effects on the environment and low selectivity for different metals. Furthermore, many WEEE are not suitable for a direct fusion process due to the low calorific value.Hydrometallurgical treatment
The hydrometallurgical treatments include an oxidative leaching for the extraction of metals followed by separation and purification processes. In hydrometallurgical treatments leaching agents in aqueous solution are used as strong acids (sulfuric acid, nitric acid, hydrochloric acid) and / or bases (sodium hydroxide, sodium hypochlorite) often coupled with oxidizing agents (hydrogen peroxide andtrivalent iron) and complexing agents (cyanide and thiosulphate) in order to extract metals. It has some advantages over pyrometallurgy such as lower residues and toxic emissions and greater energy efficiency. However, these processes have disadvantages due to the use of large quantities of corrosive and flammable toxic reagents and the generation of high volumes of solid waste.Bioidrometallurgia
The use of microorganisms to process metals is a technology that allows metals to be extracted from waste. In this process acidophilic bacteria, cyanogenic heterotrophs, and / or heterotrophs are used which produce acids which solubilize the metals. The presence of high amounts of metals in solution is toxic therefore the microorganisms used must be able to adapt to these conditions. These processes have the great advantage of being environmentally sustainable. Despite the relatively slower kinetics compared to those of traditional processes, biolisciation is now a mature technology to be engineered.Recently, hybrid technologies have also been applied that combine chemical (more efficient) and biological (less environmentally impacting) processes. In these processes, the biogenic excretion of the leaching agent is increased, catalyzed or assisted by chemical processes. In the event that the biological leaching agent is insufficient, the chemical leacher compensates for the oxidative capacity required to solubilize the metals.Conclusions
The production of electrical and electronic equipment (and therefore of WEEE) strongly depends on the availability of raw materials. Many of these are critical because of their scarcity, economic or political importance: the primary minerals of these metals are in fact distributed heterogeneously in the world causing a monopoly control of these resources. For these reasons it is essential to proceed by trying to recover these metals from electrical and electronic equipment at the end of their life. However, several treatments are required to do this. Initially thepre-treatment frees the metals from the matrix in which they are contained, then a pyrometallurgical or hydrometallurgical treatment is carried out, the latter being less expensive and less energy-intensive. A biohydrometallurgical treatment has also been developed which, although not yet widespread, is less impactful for the environment. Finally, hybrid treatments between hydrometallurgy and biohydrometallurgy are possible, combining greater recovery efficiency with environmental sustainability.Bibliografia
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[3] S. Zhang, Y. Ding, B. Liu , C. Chang, 2017. Supply and demand of some critical metals and present status of their recycling in WEEE. Waste Management 65, 113–127.
