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aluminum scrap - an overview | sciencedirect topics

Aluminum scrap is generated in many steps of the manufacturing process and use by the end user. During manufacturing, aluminum material is lost from the process either during the melting step to form dross or in one of the many machining operations applied to the aluminum piece. Based on Figure 2.5.25, it makes the most sense to return a scrap material to the same alloy or at least the same family whenever possible. In a manufacturing operation, this is normally a simple operation. When the machining operations take place within the aluminum mill, the scrap is always returned to the cast house and remelted. When the scrap is generated at a subsequent manufacturing operation, it is highly likely that the scrap will return to a recycling operation and will normally be returned to the original alloy since the pedigree is known. This type of scrap has the highest value due to its known pedigree. Manufacturing scraps could include stamping skeletons, machine turnings, scrapped pieces due to imperfections, or any other form due to a multitude of reasons. Once the part is installed in a larger assemblage or is delivered to a customer, the traceability of the part decreases. Typically, postconsumer scrap is a mixture of alloys and sometimes even a mixture of metals. Some scrap types, like Used Beverage Containers (UBCs), are easily identified, but most postconsumer scrap is at best identified between one of several broad categories.

At one time, the use of recycled or secondary scrap was a very small portion of the entire aluminum metal stream. However over the past 20 years in the United States, the contribution of aluminum recycling has remained steady, as domestic primary aluminum capacity has been replaced by imported metal. As an example, US usage of secondary Al in the past 20 years has accounted for 1520% of the total metal input. The US domestic consumption for aluminum by source is presented in Figure 2.5.26 [32].

The material flow in the global aluminum market in 2010 is presented in Figure 2.5.27 [33]. The red lines and circles show the various recycle paths that occur within the larger aluminum metal flow. Note that the large size of the total aluminum products is still in an active role. As this old scrap comes out of service, it becomes old scrap or postconsumer scrap. It has been said that two-thirds of the aluminum ever created is still in use today [32].

In recent years, the processing of aluminum scrap (mainly chips and used drink cans) has attracted increasing attention. Aluminum scrap has a complex chemical composition based on aluminum (>90%). Magnesium, zinc, silicon, iron, etc., are the main impurities. Sources of impurities are the composition of the alloy for producing drink cans (mainly, AlMg alloy containing up to 3%4% Mg) and mechanical impurities that enter into the scrap due to inefficient sorting, classification, and storage of the scrap.

Nowadays, all over the world, the main method of processing aluminum scrap is remelting in units of various types. The melt is usually cast into ingots. In some cases, the melt is granulated or gas atomized to produce powder of recycled aluminum (for example, APV powders in Russia). These remelting-casting technologies are rather power consuming.

Processing of aluminum scrap to powders and granules is an alternative to remelting scrap to produce ingots. The advantage of this technology is that it is a simpler and cheaper process of producing granules from recycled aluminum. The size of granules is easily regulated by the adjustment of grinders and varies from 1 to 10mm. Such granules are used more effectively than ingots in ferrous metallurgy for steel deoxidation.

The advantage of new production off-cuts is that they can easily be segregated into their various alloys, they generally dont contain attachments which may be a contamination and they require minimal labour to pre-process prior to melting. In many cases the scrap is uncoated and therefore will have a higher possible metal yield. Likely pre-processing will include baling, shearing and in the more high-tech plants de-coating if lacquered or painted. Also included is this category of scrap is casting trim or rejects. A couple of typical ISRI specifications for new production off-cuts are:

Shall consist of new, clean, uncoated and unpainted aluminium scrap of two or more alloys with a minimum thickness of .015(.38mm) and to be free of hair wire, wire screen, dirt and other non-metallic items. Oil and grease are not to total more than 1%. Also free from punchings less than (1.27mm) in size.

Shall consist of one alloy (typically 6063). Material may contain butt ends from the extrusion process but must be free of any foreign contamination. Anodized material is acceptable. Painted material or alloys other than 6063 must be agreed upon by the buyer and seller. An example of baled Tata is shown in Fig.4.3.

Depending on the type of scrap and the desired product quality, different types of furnaces for melting aluminum scrap are used. Scrap for the production of casting alloys is commonly melted in rotary furnaces under a layer of liquid melting salt. A company producing casting alloys from old and new scrap is commonly called a refiner. Producers of wrought alloys prefer open hearth furnaces in varying designs. These furnaces are normally used without salt. Wrought aluminum from mainly clean and sorted wrought alloy scrap is produced in a remelter.

After the melting process, the liquid melting salt used in rotary furnaces is removed as salt slag. In the past, the salt slag was land filled. Today, the salt slag is prepared as a rule. The aluminum and the salt within the salt slag are recovered.

One of the application areas of the new water-atomization process may be in the recycling of secondary aluminum. Contamination of aluminum scrap by iron and silicon is the important problem of recycling of secondary aluminum [82]. Both iron and silicon have limited solubility in the aluminum matrix. As a result, cast ingots of secondary aluminum contain coarse Fe and Si base precipitates with diameters of a few microns. Such hardly deformable inclusions are often located at grain boundaries or triple junctions and they usually have weak cohesion with the matrix [83]. Under load, cracks first of all nucleate at the interfaces of the matrix and the coarse precipitates. Therefore, such precipitates serve as principal fracture sources. As is known, if nucleation and propagation of the crack are connected with a single fracture source, then characteristics of the tensile strength and especially parameters of the fatigue strength decrease as compared with the opposite case when many cracks simultaneously arise from several sources. Coarse particles can in a similar manner influence corrosion resistance. Therefore, the use of cast secondary aluminum for production of high-quality alloys for crucial structural applications is hindered by the presence of coarse Fe and Si base precipitates in the microstructure [18].

In the work [84], the possible effects of iron and silicon on the structure and properties of PM produced on the basis of water-atomized powders were studied. The high strength Al-Zn-Mg-Cu system alloys were selected for investigation (Table 16.15). The total bulk concentration of all alloying elements lies between 13.4 and 14.55mas%. Various combinations of Fe and Si with a maximum total concentration of 1.8mas% were added to furnace charges during powder atomization in order to study their effect on the structure and mechanical properties.

The particle size distributions of the water-atomized Al-Zn-Mg-Cu alloys obey a lognormal law (Fig. 16.25). The water-atomization conditions were as follows: water pressure 10MPa, superheat of the melt temperature equal to 150200K, and diameter of a gravity melt stream 7.07.5mm. In these conditions, the median diameters of the mass distribution are in the range 7283m and of the surface one equal to 3347m. The geometric standard deviation represented the slope of the curve, and the spread of particle size about the median value, respectively, is in the range 2.02.3.

The cooling rate of melts is estimated with the use of the dendrite parameter, which is equal to the average size of dendrite cells [18,27]. The correlation between particle size and cooling rate is shown in Fig. 16.26.

PM Al-Zn-Mg-Cu system alloys based on WA powders generally show a high level of room temperature tensile strength (~700MPa) with sufficient plasticity (elongation ~7%9%). A correlation between the characteristics of mechanical properties and the common bulk concentration of impurities is shown in Fig. 16.27. The strength and hardness curves clearly demonstrate two regions. At lower impurity concentrations (up to 1%), both strength and hardness increase with impurity concentration without any evident loss of plasticity. A further increase of the impurity concentration, however, reduces the level of strength: If the common concentration of Fe and Si rises from 1% to 1.8%, then the strength goes down from approximately 700 to 600MPa while plasticity is less sensitive to the impurity concentration. The hardness behavior generally correlates with the strength.

The use of RS allows one to dissolve impure atoms in the matrix. The atomization regimes used provide dissolution of at least 1.1mas% Fe and Si. At higher impurity concentrations, coarse Fe and Si base particles precipitate and serve as fracture sources. As a result, the tensile strength of PM alloys decreases. Higher cooling rates (or finer powder fractions) should be used to restrain precipitation of the above precipitates [84].

The Aluminum Association reports that in 1998 approximately one-third of the aluminum supply in the USA was recycled aluminum. Recycling of aluminum scrap is a critical component of the aluminum industry because it is economically favorable. Recycling saves almost 95% of the energy needed to produce aluminum metal from bauxite ore and aluminum can be repeatedly recycled without a decline in materials performance or quality.

In the automotive sector, >85% of post-consumer aluminum scrap and essentially all manufacturing aluminum scrap is recycled. Sixty to seventy percent of all automotive aluminum is sourced from recycled metal. While aluminum accounts for <10% of a vehicles weight, it can represent 3550% of the total material scrap value at the end of a vehicles useful life.

Closed-loop recycling is a major factor in the success of the aluminum beverage can. Figure 5 shows the aluminum can market share and recycling rates in the countries of Western Europe and the USA. Market share for the aluminum can ranges from a low of 10% in Germany to 100% in Finland, Greece, Norway, Ireland, Sweden, Switzerland, and the USA. The recycling rate is 63% in the USA, where over 100 billion cans are produced, and is as high as 91% in Sweden.

Montero et al. [15] evaluated the use of a leaching column for precious metal dissolution from computer processors. First, they performed a manual separation in order to remove all aluminum scrap from PCBs. Then, the material was crushed and sieved with a hammer mill (CONDUX D6431) and a screener. Material particles ranging from 3.35 to 0.43mm were employed for the cyanidation process since it contained more than 80% of the precious metal concentration. A 10L sodium cyanide solution (4g/L) was used to treat 500g of ground PCBs. The column leaching technique was used for cyanidation with a flow of 20L/d.kg PCBs, at pH 11 over 15 days.

Gold recovery from the pregnant solution was performed every day by using adsorption onto activated carbon in a column where the mass ratio (active carbon/PCBs) was 3:1. Each day after the cyanidation and adsorption process, the concentrations of precious metals (Au and Ag) and copper were determined by atomic absorption spectroscopy (AAS). PCB composition was measured at the beginning and at end of the 15-day cyanidation and adsorption process. Fire assay was employed to determine gold and silver concentrations in treated and untreated PCBs. For other metals, like copper, PCBs were digested in nitric acid and aqua regia before AAS analysis. During the cyanidation process, the pH and cyanide concentration were controlled periodically and regulated by adding lime and sodium cyanide, respectively. In Fig. 5.3 can be seen the evolution of gold, silver, and copper dissolution during the 15 days of the cyanidation process.

At the beginning of the experiment it was observed that gold dissolution exceeds greatly silver and copper dissolution, which is expected since gold dissolution has the fastest kinetics [1]. However, on day 11, copper dissolution increases, with a consequent reduction in the gold and silver dissolution rates. After day 11 it is clear that copper dissolution dominates the cyanidation process. This phenomenon occurs since copper cations can readily react with cyanide which results in a cyanide concentration decrease and therefore not enough free cyanide ions exist to react with gold and silver. This experiment demonstrates the importance of removing as much copper as possible in order to avoid poor performance in cyanidation.

At the end of the 15 days a total gold recovery of 53.7% was obtained, and 74.7% of copper recovery was also achieved. In addition, 319g/ton of gold and 733g/ton of silver were still in the PCBs after the cyanidation and adsorption process. Those concentrations are still very high, which indicates that precious metal dissolution was incomplete.

The results showed that 48.1% of Nb contained in WPCBs is dissolved in the pregnant solution after 15 days of cyanidation column besides Cu, Au, and Ag. Then, recovery of these dissolved metals is straightforward via activated carbon adsorption. However, since copper is in present in a high concentration it can contaminate the final product in the subsequent refining stages.

For the effective use of cyanidation it is necessary to decrease the copper concentration of PCBs a priori. In the literature, there are certain examples that mention several pretreatments for copper dissolution. For instance, Lee et al. [25] conducted an experiment that consisted of selective leaching of copper and gold using chlorine as the leaching reagent.

Waste characteristics differ between the three die casting processes. In the case of aluminum die casting, trim pieces are knocked off mechanically, as well as by hand. The solid waste stream that results is recycled back to the scrap aluminum crusher at the head of the process, and associated oils, greases, and dirt thus become a portion of the waste discussed earlier. In the case of magnesium and zinc die casting, trimming is done by grinding, which creates dust. The wastes (dust) from this grinding operation are captured by scrubbers. The scrubber blow-down then becomes the principal waste stream from the magnesium die casting finishing process. This waste stream can be treated by chemical precipitation, coagulation, sedimentation, and filtration. Recycle and reuse of the clarified effluent as quench water, cooling water, and/or plant wash-down water can significantly reduce overall waste discharges from the plant. Since lead and zinc are listed as toxic pollutants, all steps in the waste treatment and recycle system must be carefully managed.

Aluminium is the most common metal used in civil and military aircraft and, therefore, the ability to recycle this material using an economically viable and environmentally friendly process is essential to sustainable aerospace engineering. Aluminium is the second most recycled metal (after steel), and about one-third of all aluminium is extracted from scrap products. The most common source of scrap aluminium is general purpose items, such as beverage cans and household products. Aluminium reclaimed from aircraft is a small but growing source of scrap material.

There are several important benefits in the recycling of aluminium. Firstly, the quality of aluminium is not impaired by recycling; the metal can be recycled repeatedly without any adverse affect on the properties. Secondly, aluminium recycling is less expensive than the production of new aluminium from ore. Recycling of aluminium generally results in significant cost savings over the production of new aluminium, even when the costs of collection, separation and recycling are taken into account. As a result, it is financially viable to recycle scrap aluminium from aircraft, and based on current price the estimated value for reclaimed aluminium from the skin and airframe of a Boeing 747 is in the range $200 000 $250 000. The process of aluminium recycling simply involves re-melting the metal, which is much cheaper and less energy intensive than producing new aluminium by electrolytic extraction (via the Bayer process) from bauxite ore. Recycling scrap aluminium requires about 5% of the energy needed to produce new aluminium. One of the challenges with recycling aluminium or any other metal is the unstable price of the recycled material. For instance, Fig.24.3 shows the fluctuations in the price of recycled aluminium from 1990 to 2005, with the price adjusted for inflation. The price can vary greatly over relatively short periods of time, and this affects the financial profitability of the recycling process. Although the price of new metal also changes, it is almost always more expensive than recycled metal.

The third key benefit of aluminium recycling is that it is less polluting than producing new material. The environmental benefits of recycling aluminium are enormous. The electrolytic extraction of aluminium from bauxite is an energy-intensive process requiring large amounts of electricity. For example, the production of new aluminium in the United States consumes about 3% of the national energy requirements. The amount of nonrenewable resources (e.g. coal, liquefied gas) needed to generate the electricity for the melting and refinement of scrap aluminium is much less than the production of new aluminium. Therefore, scrap aluminium has less environmental impact because less energy is needed to power the recycling process and therefore fewer greenhouse gases and other pollutants are generated. It is estimated that recycling 1kg of aluminium saves up to 6kg of bauxite, 4kg of chemical products used in the electrolytic refinement process, and 14 kWh of electricity. Therefore, in the recycling of a mid-sized passenger aircraft, which contains about 20 tonne of aluminium, over 130 tonne of bauxite is saved along with 300 MWh of electricity needed to extract the metal from the ore. If brown coal is used to generate the electricity, then recycling also saves over 100 tonne of CO2 and other pollutants.

Recycling of scrap aluminium from aircraft is a simple process. The aluminium components are removed from the aircraft, cut and shredded into small pieces, and then chemically treated to remove paint, oils, fuels and other contaminants. The aluminium pieces are compressed into blocks and then melted inside a furnace at 750100 C. Refining chemicals (e.g. hexachloroethane, ammonium perchlorate) are added to the molten aluminium. The furnace is tapped to cast the refined aluminium into ingots, billets, rods or other product forms. The cast aluminium can be reused for any application, including the production of new aircraft parts.

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a systematic review of paste technology in metal mines for cleaner production in china - sciencedirect

A systematic review of the paste technology for cleaner production in metal mines.The academic and application gains of paste technology in China were introduced.The representative engineering cases and core technologies were listed.Current challenges and future directions of paste backfill technology are presented.

Cleaner production is an integrated preventive environmental strategy intended to minimize waste and emissions and maximize product output. The mining industry continually supplies raw materials to other industries, which inevitably leads to enormous emissions. In recent decades, mining engineers have examined many pathways to advance the implementation of cleaner production practices, of which paste backfill is one of the representative technologies. Moreover, this technology is an effective practical approach for the management of process tailings, the largest source of mining waste, since this technology offers significant environmental, technical, and economic benefits. However, due to the more stringent requirements of clean production, the greater the depth of excavation is, the lower the grades of ore (or the more abundant and finer the tailings are) and the longer the distances of paste transport are, and the application of paste backfill technology encounters increasing challenges. For this purpose, the paper presents a systematic review of tailings paste applications in mines for cleaner production. First, the basic concepts and characteristics of paste technology, typical compositions of the paste system, and development progress of the paste practices in China are comprehensively introduced. Second, the latest research on core paste technology is elaborated, such as tailings thickening, pipeline transportation, multiphysics mechanisms, and new paste materials. Third, a few typical engineering cases of underground paste filling, open pit or ground collapse backfilling, and tailings paste stacking are described, and their engineering backgrounds, key equipment, and operating parameters are studied. Finally, the challenges and future research direction of paste technology, such as paste backfill in deep mining and paste systems with a large capacity, low cost requirements, intelligent control, and multiphase paste materials, are identified to promote cleaner production practices in mining.