is made from an aluminium alloy containing 92% to 99% aluminium. Typically between 0.00017 and 0.0059 inches in thickness, foils are available in a variety of widths and strengths for hundreds of applications. It is used in the manufacture of thermal insulation for the construction industry, fins for air conditioners, electrical coils for transformers, capacitors for radios and televisions, insulation for storage tanks, decorative products, and containers and packaging. The popularity of aluminum foil in so many applications is due to several major advantages, the most important of which is the abundance of raw materials required for its manufacture. Aluminum foil is inexpensive, durable, non-toxic, and oil-resistant.
Also, shipments of aluminum foil (1991) totaled 913 million pounds, and packaging accounted for 75% of the foil market. The popularity of aluminum foil as a packaging material is due to its excellent water vapor and gas impermeability. It also extends shelf life, uses less storage space, and generates less waste than many other packaging materials. As a result, the preference for aluminum in flexible packaging has become a global phenomenon. In Japan, aluminum foil is used as a barrier component for flexible cans. In Europe, aluminum flexible packaging dominates the market for pharmaceutical blister packs and confectionery wrappers. Aseptic beverage boxes, use a thin layer of aluminum foil as a barrier to oxygen, light, and odors, aluminum is a recently discovered metal that is heavily used in modern industry.
Aluminum is one of the most abundant elements: after oxygen and silicon, it is the most abundant element found on Earth's surface, making up more than 8 percent of the Earth's crust at a depth of ten miles and occurring in nearly every common rock. However, aluminum does not exist in its pure metal form, but in the form of hydrated alumina (a mixture of water and alumina) combined with silica, iron oxide, and titania.
The most important aluminum ore is bauxite, named after the French town of Les Baux, which was discovered in 1821. Bauxite contains iron and hydrated alumina, the latter being its largest constituent material. Bauxite is now abundant enough that only deposits with 45% alumina or more can be mined to make aluminum. Concentrates are found in both the northern and southern hemispheres, with most of the ore used in the United States from the West Indies, North America, and Australia.
Since bauxite is very close to the earth's surface, the mining procedure is relatively simple. Explosives are used to open large pits in bauxite deposits and then remove the top layer of dirt and rock. The exposed ore is then taken out by front-end loaders, stacked in trucks or railcars, and transported to processing plants. Bauxite is heavy (typically four to six tons of ore can produce one ton of aluminum), so these plants are usually located as close to the bauxite as possible due to transportation costs.
Extracting pure aluminum from bauxite requires two processes. First, the ore is refined to remove impurities such as iron oxide, silicon dioxide, titanium dioxide, and water. Then, the resulting alumina is smelted to produce pure aluminum. After that, the aluminum is rolled into the foil.
Refining - Bayer process
1. The Bayer process for refining bauxite consists of four steps: digestion, clarification, precipitation, and calcination. During the digestion stage, bauxite is ground and mixed with sodium hydroxide, which is then pumped into large pressurized tanks. In these tanks, called digesters, a combination of sodium hydroxide, heat, and pressure break down the ore into a saturated solution of sodium aluminate and insoluble contaminants, which then settle to the bottom.
2. The next stage of the process, clarification, requires passing the solution and contaminants through a set of tanks and presses. During this stage, the cloth filter captures the contamination and then disposes of it. After filtering again, the remaining solution is sent to a cooling tower.
3. In the next stage, precipitation, the alumina solution enters a large silo, and in the case of the Deville method, hydrated aluminum crystals are added to this fluid to promote the formation of aluminum particles. As the seed crystals attract other crystals in the solution, chunks of aluminum hydrate begin to form. These are first filtered and then rinsed.
4. Calcination is the last step in the Bayer refining process, which requires exposing the hydrated aluminum to high temperatures. This extreme heat dehydrates the material, leaving a fine white powdery residue: alumina.
5. Smelting is the separation of aluminum oxide compounds (alumina) produced by the Bayer process, and is the next step in extracting pure metal aluminum from bauxite. Although the current procedure is derived from the electrolysis method invented at the same time by Charles Hall and Paul-Louis-Toussaint Héroult in the late 19th century, it has been modernized. First, alumina is dissolved in a smelting bath, a deep steel mold lined with carbon, and filled with a heated liquid conductor consisting mainly of the aluminum compound cryolite.
6. Next, an electric current is passed through the cryolite, causing a crust to form on top of the alumina melt. This crust also breaks down and stirs in when additional alumina is regularly stirred into the mixture. As the alumina dissolves, it breaks down electrolytically, creating a layer of pure molten aluminum at the bottom of the smelting bath. Oxygen fuses with the carbon used to line the cells and escapes as carbon dioxide.
7. The purified aluminum still in the molten state is taken out of the melting tank, transferred to the crucible, and then poured into the furnace. At this stage, other elements can be added to produce aluminium alloys with properties suitable for the final product, although foils are usually made from 99.8% or 99.9% pure aluminium. The liquid is then poured into direct chill casting equipment, where it is cooled into large slabs called "ingots" or "rerolled stock." After annealing (heat treatment to improve machinability), the ingot is suitable for rolling into the foil.
Another method of melting and casting aluminum is called "continuous casting." The process involves a production line consisting of a smelting furnace, a holding furnace containing the molten metal, a conveyor system, a casting unit, a combined unit consisting of pinch rolls, shears, and strips, and a winding and coiling car. Both methods can produce stock in thicknesses ranging from 0.125 to 0.250 inches (0.317 to 0.635 cm) and various widths. The advantage of continuous casting is that it does not require an annealing step prior to rolling the foil, just like the melting and casting process, since annealing is done automatically during the casting process.
8. After the raw material of aluminum foil is made, it must be thinned to make aluminum foil. This is done in a rolling mill, where the material passes multiple times through metal rolls called work rolls. As sheets (or coils) of aluminum pass through the rolls, they are squeezed thinner and extruded through the gap between the rolls. The work rolls are paired with heavier rolls called backup rolls, which apply pressure to help keep the work rolls stable. This helps keep product dimensions within tolerances. The work rolls and backup rolls rotate in opposite directions. Add lubricant to facilitate the rolling process. During this rolling process,
Foil reduction is controlled by adjusting the rotational speed of the rolls and the viscosity (flow resistance), amount, and temperature of the rolling lubricant. The roll gap determines the thickness and length of the foil leaving the mill. This gap can be adjusted by raising or lowering the upper work rolls. Rolling on foil produces two natural finishes, bright and matte. When the foil is in contact with the work roll surface, a shiny finish is produced. To create a matte finish, two sheets must be wrapped together and rolled up at the same time; when this is done, the sides that touch each other will end up in a matte finish. Other methods of mechanical finishing, usually produced in converting operations,
9. As the foils pass through the rolls, they are trimmed and cut by circular or razor-like knives mounted on the rolling mill. Trimming refers to the edges of the foil, while slitting involves cutting the foil into sheets. These steps are used to produce narrow web widths, trim the edges of coatings or laminates, and produce rectangular pieces. For some manufacturing and converting operations, coils that break during rolling must be rejoined or spliced. Common types of joints used to join flat and/or foil-backed webs include ultrasonics, heat seal tape, pressure seal tape, and electric welding.
10. For many applications, aluminum foil is used for IV/combination with other materials. It can be coated with a variety of materials, such as polymers and resins, for decorative, protective, or heat-sealing purposes. It can be laminated to paper, cardboard, and plastic films. It can also be cut, formed into any shape, printed, embossed, cut into strips, pressed, etched, and anodized. Once the foil is in its final state, it is packaged accordingly and shipped to the customer.
In addition to the process control of parameters such as temperature and time, the finished foil product must also meet certain requirements. For example, it has been found that different processing techniques and end uses require different degrees of drying of the foil surface for satisfactory performance. The wettability test is used to determine dryness. In this test, different ethanol solutions in distilled water were poured uniformly onto the surface of the foil in 10% volume increments. If no droplets are formed, the wettability is zero. The process continues until it is determined that the minimum percentage of alcohol solution will fully wet the foil surface.
Other important properties are thickness and tensile strength. The American Society for Testing and Materials (ASTM) has developed standard test methods. The thickness is determined by weighing the sample and measuring its area, then dividing the weight by the product of the area times the alloy density. Tension testing of foils must be carefully controlled because test results can be affected by the presence of rough edges and small defects, among other variables. The sample is placed in the grips and a pulling or pulling force is applied until the sample breaks. Measures the force or strength required to break a sample.
The popularity of aluminum foil, especially flexible packaging, will continue to grow. Four-sided seal bags have been widely used in military, medical, and retail food applications, as well as larger-sized institutional food service bags. Also introduced are sachets for packaging 1.06 to 4.75 gallons (4-18 liters) of wine for the retail and food service markets and other food service markets. In addition, other products are continuing to be developed for other applications. The growing popularity of microwave ovens and ovens has led to the development of several forms of aluminum-based semi-rigid containers designed specifically for these ovens. More recently, special cooking foils for grilling have been developed.
However, even aluminum foil has come under scrutiny for its environmental "friendliness". As a result, manufacturers are stepping up their efforts in the recycling arena; in fact, all U.S. foil producers have started recycling programs, even though the gross tonnage and catch rate of foil is much lower than that of easy-to-recycle cans. Aluminum foil already has the advantage of being lightweight and small, which helps reduce its contribution to the solid waste stream. In fact, laminated foil packaging accounts for only 17/10% of 1% of solid waste in the United States.
For packaging waste, the most promising solution may be to reduce the source. For example, packing 65 lbs (29.51 kg) of coffee in a steel can requires 20 lbs (9.08 kg) of steel, but only 3 lbs (4.08 kg) of a laminate including aluminum foil. The packaging also takes up less space in landfills. The Aluminum Association's Foil Division is even developing a foil education program for universities and professional packaging designers to help these designers understand the benefits of switching to flexible packaging.
Aluminium foil also uses less energy to manufacture and distribute, and waste within the factory is recycled. In fact, recycled aluminium, including cans and foil, accounts for more than 30% of the industry's annual metal supply. This number has been increasing for several years and is expected to continue. Additionally, the processes used in foil manufacturing are improving to reduce air pollution and hazardous waste.
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