The pure form of aluminium does not naturally occur in nature, so remained largely unknown until as recently as 200 years ago. Creating aluminium using electricity was first developed in 1886 and is still used to this day.
The aluminium production process starts with the mining of bauxites, an aluminium rich mineral in in the form of aluminium hydroxide. About 90% of global bauxite supply is found in tropical areas.
Bauxite is crushed, dried and ground in special mills where it is mixed with a small amount of water. This process produces a thick paste that is collected in special containers and heated with steam to remove most of the silicon present in bauxites.
The ore is loaded into autoclaves and treated with limecaustic soda. Aluminium oxide appears in the resulting slurry while all the admixtures settle to the bottom as red mud.
The sodium aluminate solution is stirred in precipitators for several days, eventually pure alumina or Al2O3 settles at the bottom.
At an aluminium smelter, alumina is poured into special reduction cells with molten cryolite at 950. Electric currents are then induced in the mixture at 400 kA or above; this current breaks the bond between the aluminium and oxygen atoms resulting in liquid aluminium settling at the bottom of the reduction cell.
Primary aluminium is cast into ingots and shipped to customers or used in the production of aluminium alloys for various purposes.
The process where the aluminium is shaped to its required form. This process is used for making the vast majority of aluminium products from spectacle frames, telephone bodies, aeroplane fuselages or spaceship bodies.
The malleability of aluminium means it can be easily rolled into thin sheets. To that end, aluminium alloys are cast into rectangular beams up to 9 metres in length, these are then rolled into sheets from which aluminium foil and beverage cans are made, as well as parts of automobile bodies and a vast array of other products.
The process where the aluminium is shaped to its required form. This process is used for making the vast majority of aluminium products from spectacle frames, telephone bodies, aeroplane fuselages or spaceship bodies.
Unlike iron, aluminium is corrosion resistant so it can be remelted and reused an infinite number of times. The added benefit is that recycling aluminium requires only 5% of the energy needed to make the same amount of primary aluminium.
Molecular sieve technology is widely used for the simultaneous removal of water and mercaptans from both gas and liquid feed streams. However, a better understanding of the design principles and the operation of molecular sieve units is needed. For economic reasons, it is important not to overdesign the molecular sieve unit. At the same time, it is essential to ensure that the unit does not become the bottleneck of the gas processing plant at the sieves end-of-run condition.
Details of the design and operation of molecular sieve units used for natural gas dehydration are discussed here. The application of expert know-how and practical recommendations can provide an effective way to maximize molecular sieve lifetime and performance. Part 1 focuses on the design principles and practices of molecular sieve units.
Molecular sieves background. When treating a gas or liquid stream so that it can be processed by a specific unit, one of the commonly used treating units is an adsorption unit. These units are commonly used to remove water from a feed stream, but they can also remove additional contaminants (e.g., mercaptans). Specifically, when deep removal is required (below 1 ppmv), molecular sievesan adsorbent composed of a zeolite and, typically, a clay binderare the preferred adsorbent.
Adsorption units are capable of reaching extremely low specifications, which makes them viable pieces of equipment for incorporation into a process lineup. A major advantage of molecular sieves is that they can be regenerated, which reduces the required amount of molecular sieve to economically feasible quantities.
This article examines the use of molecular sieve units for natural gas dehydration, as they are critical components in the operation of an LNG or gas processing plant (typically combined with NGL extraction by cryogenic separation), and any limitation or loss in capacity of this unit can have a significant effect on overall plant economics.1,2
An adsorption unit used for water removal is called a dehydration unit (DHU). A DHU often consists of two or more vessels, filled with molecular sieves, that adsorb water during an adsorption period and are subsequently regenerated using a heated stream of treated gas. A sketch of a typical molecular sieve DHU is shown in Fig. 1.
The high temperature during regeneration causes water to desorb from the molecular sieve, a process called temperature swing adsorption (TSA). Although TSA is a discontinuous process, the overall DHU behaves like a continuous process because one or more vessels are always in adsorption mode, while another vessel(s) is in regeneration mode.
In a typical DHU, the regeneration gas used is a side (slip) stream of the product stream (typically approximately 10%). Downstream of the adsorber vessel, the wet regeneration gas is cooled, and water is condensed and subsequently removed in a knockout (KO) drum. When a molecular sieve unit is used for dehydration, it is possible to send the regeneration gas back to the feed after compression is performed to negate the pressure drop. Sending the regeneration gas back to the feed minimizes valuable product losses. Note: This type of lineup is possible only for DHUs designed for water removal, as knocking out the water in the regeneration KO drum provides a drain from the system. Components that cannot be removed from the loop in such a manner (e.g., sulfur species, such as mercaptans) will build up in the regeneration gas loop. Consequently, in such a lineup, the regeneration gas must first be treated by an appropriate absorption unit before it can be reinjected in the feed gas.
Basic design rules and operational requirements. As mentioned previously, an adsorption unit is a discontinuous process, since adsorption is essentially a batch process. One of the most important design parameters of an adsorption vessel is the available water uptake capacity of the molecular sieve bed. However, a few factors make this important design parameter less straightforward.
In adsorption, gas usually flows from top to bottom. During the adsorption period, the amount of water that can be adsorbed on the molecular sieve is dictated by both the capacity that can be reached in the limit of reaching equilibrium, and the capacity that results from the competition between adsorption kinetics and the flow of gas past a molecular sieve pellet.
In the adsorbing bed, two zones can be identified. The capacity-limited portion is situated at the top. It is saturated with water under the feed conditions of temperature, pressure and water concentration, and is referred to as the saturation zone (SZ). The part of the bed below the SZ that is engaged in dehydrating the gas from feed water concentration (wet) to effluent water concentration (dry) is called the mass-transfer zone (MTZ) (Fig. 2). During adsorption, the MTZ migrates from the top of the bed to the bottom of the bed, thereby lengthening the SZ. Once the MTZ leaves the bed, breakthrough occurs and the bed must be taken offline for regeneration.3,4,5
Another important parameter to take into account is that the capacity of the adsorbent decreases over time as a function of the number of regeneration cycles; therefore, end-of-run (EOR) capacity must be used when considering the required amount of adsorbent. When the capacity of the adsorbent falls below the level where all water in the feed can be adsorbed during the minimum adsorption time, then the adsorbent must be replaced (Fig. 3).
Another important design parameter is the ability to predict the rate of deactivation. It is important to understand the main factors contributing to the deactivation of the molecular sieve. In general, these contributing factors are consequences of the thermal regeneration process.6
During regeneration of the water-saturated molecular sieve, hot dry gas is passed upward through the molecular sieve bed, and water desorbs. If the beginning of the regeneration cycle were started with dry gas at the maximum temperature, then water would first desorb from the bottom portion of the bed, using much of the heat of the gas. Subsequently, water that desorbed from the bottom portion of the bed would be carried to the top portion, which would not yet have been heated by the regeneration gas. The top portion of the bed would still be cold and saturated with water, so the desorbed water from the bottom portion of the bed could condense to form liquid water. The formation of hot liquid water in the bed could cause the clay binder of the molecular sieve to dissolve, subsequently forming cake during the regeneration step.2,512
Caking due to hot liquid water formation during regeneration can be avoided by using a well-designed heating profile during the regeneration cycle. If the temperature of the regeneration gas entering the bed is slowly increased, then the top portion of the bed can be preheated before much water desorbs from the bottom portion of the bed. Preheating the top portion of the bed prevents condensation of liquid water when the water from the bottom portion of the bed is finally desorbed. If a non-ideal temperature profile is used during regeneration, then degradation of the molecular sieve might still occur without the formation of a large solid cake.
The characteristics of the temperature profile required to avoid liquid water formation depend on a number of factors. The flowrate and composition of the regeneration gas dictate how much heat is delivered to the molecular sieve bed. The diameter and materials of the vessel determine how much heat is transferred to the vessel, and at what rate, as opposed to the amount of heat that is transferred to the molecular sieve. Furthermore, the amount of water adsorbed on the molecular sieve, taking into account the MTZ, dictates the gas-phase concentration profile of water moving upward through the bed.
Another form of molecular sieve deactivation is through coke deposition on the molecular sieve. If the feed to the unit is heavy, then the heavier hydrocarbons can be adsorbed onto the molecular sieve binder during adsorption. A portion of the hydrocarbons will be desorbed during regeneration, but the residual will remain on the binder, decompose and irreversibly form coke under hot regeneration conditions, which can lead to pore blocking that results in a lower water uptake capacity.
The least contributing factor is the effect that continual thermal cycling has on the molecular sieve. Some thermal degradation, and the subsequent loss in water removal capacity of the molecular sieve, will occur. The extent of this degradation and loss is dependent on the thermal stability of the molecular sieve. The regeneration process also causes degradation of the binder material. The expansion and contraction experienced during thermal swings lead to dust production and an increase in pressure drop that is detrimental to LNG production.
As the capacity of an adsorbent is limited and the vessels become more expensive as the diameter (i.e., the required wall thickness) increases, a significant economic driver exists to keep the vessels as small as possible. One way to achieve this is to operate at high pressures (typically 60 bara), as the mass flow to be treated is then compressed in a smaller volume. Another way is to reduce the amount of water deposited on the bed by cooling the feed stream and knocking out excess water in the feed KO drum. The temperature reduction is limited by the hydrate formation temperature, and the operating temperature is chosen for safe operation above that temperature.1 Another reason to operate at a low temperature is that the uptake capacity of the molecular sieve is higher.
Not yet discussed is the type of molecular sieve to be used.2,5,7,13 For dehydration, and especially for molecular sieve units used in LNG application, type 4A is used. As previously mentioned, a molecular sieve is composed of a zeolite and a binder, typically clay. The binder is used as a glue to strengthen the particles. The type of zeolite typically used for dehydration is the Linde Type A (LTA). A refers to Angstrom, indicating the diameter of the zeolite channels where the water is adsorbed; i.e., 4A refers to a zeolite with a channel diameter of 4 Angstrom. Apart from 4A, 3A LTA and 5A LTA sieves are also used. In these sieves, the type of cation determines the channel diameter (K+ for 3A, Na+ for 4A and Ca2+ for 5A).
Water is the molecule that is adsorbed the strongest; consequently, water will displace all other adsorbed molecules. For LNG applications, a 4A sieve is more stable and has higher water uptake capacity than a 3A sieve, and less coadsorption will occur compared to a 5A sieve. However, in gas processing plants that process pipeline gas, a 3A sieve is typically used. The reason is that if methanol is added to the feed gas to prevent hydrate formation, it will pass the 3A sieve bed, since methanol does not fit into the pores. The methanol can be recovered downstream of the beds in the part of the lineup where heavier hydrocarbons (e.g., propane and butane) are separated from the natural gas. Furthermore, coadsorption of methanol on larger-pore sieves will adversely affect the effectiveness of the molecular sieve with regard to water removal.
The question of why molecular sieves are best suited for deep dehydration is illustrated in Fig. 4, where typical isotherms of adsorbents used for dehydration are shown.2,13,14 Although molecular sieves have a lower saturation capacity than alumina or silica gel, that capacity is much less dependent on the partial water pressure, which enables deeper specifications to be reached (< 0.1 ppmv H2O).
This characteristic also results in a shorter MTZ, enabling more efficient use of the adsorbent bed. In this context, it should be noted that the length of the MTZ is also determined by the particle size. For that reason, molecular sieve beds are typically layered, with the larger particles (e.g., 18 in.) used in the top layers to minimize the overall bed pressure drop and the smaller particles (e.g., 116 in.) in the bottom layer.
Another reason to select molecular sieves for dehydration is that the water uptake capacity is less dependent on feed temperature in comparison to silica gel and alumina. Furthermore, in contrast to molecular sieves, alumina and silica gel are large-pore adsorbents and, therefore, more co-adsorption of hydrocarbons will occur. Similar to molecular sieves, water will displace the coadsorbed components since it adsorbs stronger to the adsorbent; however, co-adsorption will also influence the length of the MTZ. More importantly, when regenerating the adsorbents, these co-adsorbed species will leave the adsorbent as peaks. If the regeneration gas must be treated, then the treating unit must be sized such that it can process these peaks. This factor significantly increases the cost of such a unit.
The majority of topics discussed to this point are related to the chemistry involved in the process. However, other factors are equally important from a design point of viewmore specifically, those determined by process engineering and economics.
In gas processing, a plant is typically designed and operated with a single purpose: to process natural gas so that the maximum amount of condensates can be recovered, and so that produced gas adheres to certain specifications. In general, the gas is produced for use in the natural gas grid or for liquefaction and transport to markets. The economic aim for such a lineup is to minimize the number and size of vessels, process steps and plot space. From an engineers point of view, that means safely pushing as much mass flow as possible through the smallest possible vessels. The total amount of gas and liquids to be processed is the economic basis of such a project. The design of the vessel must adhere to maximum flow and minimum flow criteria.
In gas phase systems, gas flow during adsorption moves downward to prevent fluidization of the bed under abnormal flow conditions. The diameter of the bed can be calculated once the superficial velocity is determined.1 The superficial velocity must be chosen so that the following criteria are met:
Some of these criteria are of importance to the main process flow, but they should also apply to the regeneration gas flow (in each stage of the temperature profile), which, for gas processing vessels, is typically upflow. Apart from these criteria, the regeneration gas flow must carry enough heat into the system to ensure that the beds are properly regenerated. Sufficient flow will ensure the stripping efficiency of the desorbed molecules, and the beds should be cooled within the time frame available.
Within these constraints, the objective is to secure a regeneration gas flow that is as small as possibletypically around 10% of the feed flow. Particularly when the regeneration gas is sent back to the feed, a minimum regeneration flow ensures minimum-size regeneration loop equipment and, even more importantly, helps reduce the size of the adsorber vessels. For lineups such as those shown in Fig. 1, the flow passing through the main process line units is the feed flow combined with the regeneration flow. As the regeneration flow is upflow, another important design constraint is to avoid fluidization of the bed.
The length (L) and diameter (D) of the beds are limited mainly for practical and economic reasons. One important limitation is the strength of the adsorbent particles, especially those at the bottom of the bed. These should be sufficiently strong to withstand the sum of the pressure drop over the bed, the weight of the bed above them when saturated with water, and the weight of the pressurized gas.
A more practical observation is that bed heights of more than 10 m are rarely observed. In such cases, the adsorber vessels tend to become the highest units at the site. Minimizing the diameter is an important economic constraint. A larger diameter is associated with greater wall thickness, which will increase the cost of the vessel. A rule of thumb that is often applied in vessel sizing is the L/D > 2 criterion. With a lower ratio, the result is often a so-called pancake reactor. In this type of reactor construction, most of the steel (i.e., cost) used for the vessel ends up in the top and bottom domes, where the curvature is more or less fixed.
A sketch of a typical adsorber vessel is shown in Fig. 5.7 A certain distance between the bottom of the inlet distributor and the top of the bed is required to ensure pressure and flow equalization. The flow is reversed during regeneration, and the same requirement applies to the bottom portion of the bed. A careful observer will notice that the difference in size between the ceramic ball layers never exceeds a factor of 2. This ratio is preserved to lower the probability that smaller particles will migrate between larger particles.
In a worst-case scenario, such migration could lead to a small depression in the top of the bed, thereby creating a preferential flow path due to the lower pressure difference in the part of the bed referred to as a channel. If a channel is created in a bed, then the mole sieve in and around that path will be depleted much more quickly than the surroundings, since a larger part of the flow moves through it. This depletion will ultimately result in an early breakthrough of the bed.
As the capacity of a molecular sieve bed is limited and declines as a function of the number of regeneration cycles, an important design criterion is the required lifetime (i.e., how long the bed must last before a changeout of the inventory is required). That depends somewhat on the application; in general, molecular sieve beds are changed out during a major shutdown of the plant. Major turnarounds are planned every 2 yr3 yr, as various pieces of kit require maintenance.
For an LNG site, turbine and compressor maintenance needs determine when a major shutdown occurs. For most sites, the timing is every 4 yr. Although this timing provides the process engineer with a time frame for designing a molecular sieve bed, a translation to the number of cycles that fits into that period is required because the beds must retain sufficient water uptake capacity at the end of the 4-yr period.
An adsorption cycle comprises several steps. The major steps are adsorption and regeneration, whereby the regeneration step can be subdivided in heating, cooling and standby steps.2,5,9,12,15 The highest temperature to which the molecular sieve can be exposed during regeneration is determined by the thermal stability of the molecular sieve. For a 4A sieve, the temperature limit is 320C.
Although regeneration can take place at lower temperatures, the consequence is that more water will remain on the molecular sieve, thereby reducing the effective water uptake capacity. A typical regeneration profile is shown in Fig. 6. The step during ramp-up is inserted to prevent an overly rapid heating, which can result in hot water formation.
A typical adsorption time is 16 hr. Although TSA is a discontinuous process, the overall DHU behaves like a continuous process because one or more vessels are in adsorption mode, while another vessel(s) is in regeneration mode. Consequently, there is a limit to the time available for regeneration. With an adsorption time of 16 hr, in a 1+1 lineup, 16 hr are available for regeneration (cycle time is 32 hr), while in a 2+1 lineup (Fig. 1), only 8 hr are available for regeneration (cycle time is 24 hr). A cycle time sequence for a 2+1 lineup is depicted in Fig. 7.
The timing in a cycle time sequence is critical, as the transfer of beds from adsorption to regeneration must happen flawlessly. A mistake in the timing could result where one bed is coming out of the adsorption step while the bed being regenerated is still in the heating step, thereby creating a forced shutdown. Most sites work with fixed cycle times, which means that when the beds are fresh, excess capacity is available as the bed heights are designed for end-of-run conditions.
It is possible to minimize this effect by applying a variable cycling design, where the cycle time is adjusted at regular intervals in such a manner that these changes roughly follow the deactivation profile shown in Fig. 8. In this way, smaller beds can be designed, or the lifetime of the beds can be somewhat extended by reducing the overall number of cycles during troubleshooting or debottlenecking, thereby slowing the deactivation rate of the adsorbent.
As a consequence of this mode of operation, the minimum required capacity illustrated in Fig. 8 is essentially determined by the minimum time needed for regenerationi.e., the fastest time heating and cooling can be achieved while ensuring that the beds are fully regenerated.
Additional comments. Only the main design elements were discussed for a fairly typical molecular sieve unit. Not discussed are the various other permutations possiblefor example, the number of vessels deployed (e.g., 1+1, 3+1, 3+2, 4+2 lineups), the choice and the routing of the regeneration gas, regeneration gas heating options, regeneration at lower pressures, external or internal insulation of the vessels, etc. Although these options will influence vessel sizing and the total cost of the unit, the basic design elements discussed here will be the same.
Takeaway. The main design elements required for designing and operating a molecular sieve DHU are well understood. When taking these elements into account, it is possible to design molecular sieve units that are reliable, that deliver on specification and with required lifetime, and that require relatively little operational attention. GP
Ruud H. M. Herold was formerly a Senior Process Engineer at Shell Global Solutions International BV in Amsterdam, The Netherlands. He joined the company in 1986 and began working in the gas processing group in 2001, where he specialized in adsorption and catalytic processes used in gas and liquids treating. Mr. Herold holds an MSc degree in chemical engineering from the University of Amsterdam.
Saeid Mokhatab is one of the most recognizable names in the natural gas processing industry. He has been actively involved in the design and operation of several gas processing plants around the world, and has contributed to gas processing technology improvements through 300 technical papers and two well-known handbooks (published by Elsevier in the US). He founded Elseviers Journal of Natural Gas Science & Engineering, and has given invited lectures on gas processing technologies worldwide. As a result of his work, Mr. Mokhatab has received a number of international awards and medals, and has been listed in prestigious biographical directories.
Many processes within oil and gas pipelines and processing plants depend on maintaining specific temperatures and pressures at which the process fluids are liquids or gases. In addition, anytime water is a component in the process fluid hydrates can form and plug piping and vessels. Learn how Sensias Throughput optimization solution allows operators, and control systems to see inside the process in real time to understand where the facility is operating with respect to critical physical constants, including the phase envelope and hydrate temperature. This insight allows for more stable operation, reduced energy expenditure and associated emissions, and greater facility throughput. Case studies will include controlling methanol injection, managing heaters, virtual sensors for sulfur recovery units and more.
For composite applications, these hollow microstructures displace a lot of volume at low weight and add an abundance of processing and product enhancements. #marketing #cuttingtools #discontinuousfiber
Of the many fillers now available to composites manufacturers, microspheres, also called microballoons, are the most versatile. To the naked eye, the small, hollow spheres appear like fine powder. Ranging from 12 to 300 m in diameter (by comparison, a human hair is approximately 75 m in diameter), microspheres pack a lot of functionality into a very small package. Integrated into composite parts, they provide a variety of product enhancements and process improvements including low density, improved dimensional stability, increased impact strength, smoother surface finish, greater thermal insulation, easier machinability, faster cycle times, and cost savings. Composite manufacturers, already adept at making the most of their materials, regularly exploit these benefits sometimes all at once.
The composites industry is unique when it comes to hollow microspheres, says Chris Rosenbusch, marketing manager for microsphere manufacturer Expancel Inc. (Duluth, Ga.), part of Sweden-based Akzo-Nobel. Most users focus on one or two attributes of the spheres, but in the composites industry, manufacturers are taking advantage of six or seven attributes of the spheres.
Moreover, microspheres can be used in all standard processing methods for thermoset and thermoplastic composites, including extrusion and injection molding, and have found a variety of applications across all industries. Microspheres find end uses in applications as diverse as simulated wood furniture and lumber, fiberglass-reinforced core materials, automotive brake components and engineered syntactic foams.
There are a number of producers of hollow microspheres, and the performance of their products varies greatly across product lines. Microspheres are not all interchangeable, Rosenbusch warns, explaining that each manufacturer has developed proprietary processes to control a wide range of microsphere variables that include chemical composition, wall thickness, and particle size and shape. Each of these variables makes a contribution to one or, more typically, several desirable properties that have made microspheres an effective delivery system for a number of notable benefits.
Microspheres are produced for a variety of applications using a fairly broad range of materials (see Microspheres: Material Alternatives on p. 3). However, most of the microspheres commonly used in composites manufacturing are hollow and are made of either glass or plastic.
Glass microspheres. In general, a multistep process is used to produce high-temperature glass microspheres. Glass is initially produced at high temperatures from soda-lime-borosilicate, after which it is milled to a fine particle size. Trace amounts of a sulfur-containing compound, such as sodium sulfate, are then mixed with the glass powder. The particles are run through a high-temperature heat transfer process, during which the viscosity of the glass drops and surface tension causes the particles to form perfect spheres. Continued heating activates the blowing agent, which releases minute amounts of sulfur gas that form bubbles within the molten glass droplets. The result is a rigid, hollow sphere manufactured with an eye to increasing crush resistance (that is, the ability to withstand external pressure and avoid fracture of the bubbles) without sacrificing low density.
Glass microspheres also can be produced by processing perlite common volcanic glass. Noble International SA (La Pin, France) produces its trademarked Noblite microspheres by chemically processing perlites. Typically, the process involves an acid-leaching treatment, using hydrochloric or sulfuric acid at temperatures from 150C to 200C (302F to 392F), which is followed by a heat treatment process for finishing. Unlike engineered glass microspheres, which consist of a single closed cell, those produced from perlites are multicellular.
Plastic microspheres. Although they have less compressive strength, plastic microspheres offer many of the same advantages as rigid glass microspheres and are among the lightest fillers available. Standard specific gravities are as low as 0.025, providing large volume displacement at a very low weight. Plastic microspheres are primarily used in spray-up fiber-reinforced thermosetting composites and extrusion applications, according to Rosenbusch. Heat limitations (damage can occur at temperatures above 191C/375F) make injection molding challenging but not impossible. You would be limited to a small part with low pressures and low heat, says plastics consultant Paul A. Tres, president of ETS Inc. (Bloomfield Hills, Mich.).
Originally founded as a joint venture with Union Carbide Chemicals and Plastics of USA (Danbury, Conn.), APM produces phenolic and amino-based spheres. Phenolic microsphere production is based on a process originally developed by Sohio (Cleveland, Ohio), an American oil company that was acquired by British Petroleum (BP, Chicago, Ill.). The technology was eventually sold to Emerson & Cuming Inc. (now Trelleborg Emerson & Cuming) and then licensed to Union Carbide. In the process, water-miscible phenolic resole resins are dissolved in water, after which a blowing agent, ammonium carbonate, is added. Spray drying produces discrete, uniform hollow spheres in sizes ranging from 5 m to 50 m in diameter. Significantly, the use of phenolic resin, which is naturally fire resistant, provides fabricators a nonhalogenated flame-resistant filler with far less mass than other flame-retardant fillers, such as alumina trihydrate (ATH).
Expancel recently added an ultralightweight microsphere with a density of 0.015 g/cc to its line of expandable thermoplastic microspheres. Expancel-brand microspheres consist of a very thin thermoplastic shell (a copolymer, such as vinylidene chloride, acrylo-nitrile or methyl methacrylate) that encapsulates a hydrocarbon blowing agent (typically isobutene or isopentane). When heated, the polymeric shell gradually softens, and the liquid hydrocarbon begins to gasify and expand. When the heat is removed, the shell stiffens and the microsphere remains in its expanded form. Expansion temperatures range from 80C to 190C (176F to 374F), depending on the grade. The particle size for expanded microspheres ranges from 20 m to 150 m, depending on the grade. When fully expanded, the volume of the microspheres increases more than 40 times.
Unlike glass microspheres, plastic micro-spheres are much less susceptible to breakage. Excessive pressure will cause the plastic sphere to flatten but not burst, says Rosenbusch. When the pressure is released, the microspheres tend to recover. In a spray-up application, for instance, the microspheres will deform when the resin is pressurized prior to spraying, explains Rosen-busch. However, once the material hits the mold and returns to ambient pressure, the microspheres will rebound to their spherical shape.
This compressive capability can provide some control over thermal expansion as well, says Rosenbusch. The heat of exotherm during cure can be problematic in composite manufacture, he explains. By incorporating plastic microspheres, as the part heats up, the resin is able to expand inward, causing the microspheres to compress. Once the heat dissipates, the spheres rebound. The microspheres retain this flexibility even after cure. Therefore, if you have a part that is subjected to thermal stress, such as a windmill blade that gets hot in the summer and cold in the winter, the microsphere will help absorb some of the expansion/contraction force, says Rosenbusch.
Expancel microspheres can be supplied in either expanded or unexpanded form. Unexpanded microspheres, which can be expanded in-situ, have been effectively used as foaming agents in wood plastic composites (WPCs). Foaming can remove from 5 percent to more than 30 percent of a WPC boards weight, and the internal pressures generated during the foaming process reportedly result in a texture and appearance that is more like wood. The presence of the thin-walled, hollow spheres in the finished board also decrease the boards resistance to cutting and drilling. According to Maf Ahmad, technical and business manager at Expancel, density reductions of 38 percent can be achieved with the optimal concentration of 3 percent thermoplastic microspheres (by weight) and between 20 and 30 percent wood content (see last photo, this page).
Rosenbusch points out, however, that while plastic microspheres do not burst, and are, therefore, well suited for high shear mixing and spray-up applications, they are more susceptible to heat damage and chemical interaction than glass spheres. Therefore, the choice of material could be dictated, to some extent, by the molding process and the product end use.
The most obvious benefit of the hollow microsphere is its potential to reduce part weight, which is a function of density. Compared to traditional min-eral-based additives, such as calcium carbonate, gypsum, mica, silica and talc, hollow microspheres have much lower densities. For example, at a density of 0.6 g/cc, Sphericel hollow glass microspheres from Potters Industries (Valley Forge, Pa.), an affiliate of PQ Corp., can displace the same volume as talc at one-quarter the weight. Densities and crush ratings, however, vary dramatically across product lines.
The density of the sphere will have a huge impact on the formulation of the part, says Rosenbusch. Typical loadings are 1 to 5 percent by weight, which can equate to 25 percent or more by volume. For example, Potters lightweight Q-Cell hollow glass microspheres have a density (from 0.14 to 0.20 g/cc) approximately one-fifth that of most thermosetting resins. Therefore, on an equal weight basis, Q-Cell spheres occupy about five times more volume than the resin, which can reduce compound weight, VOC content and cost.
Historically, crush strength for hollow glass microspheres has been directly linked to density i.e., a glass sphere with a density of 0.125 g/cc would be rated at 250 psi (1.8 MPa), while one with a density of 0.60 g/cc would be rated at 18,000 psi (124 MPa). To some degree, there remains a correlation.
Wall thickness. This variable is primarily, but not exclusively, responsible for the spheres density and its crush strength (also see particle size, below). Generally speaking, the thicker the wall, the stronger the material, says Gladysz, whose company readily tailors its trademarked sodium borosilicate glass Eccospheres for clients in the aerospace, military, electronics and oil and gas industries. However, he adds, there are many factors that affect strength and density, including glass chemistries and manufacturing processes. Both factors are important in the companys range of aerospace-grade, high-purity glass and ceramic microballoons, for which strength and density (the latter ranging from 0.16 g/cc to 0.380 g/cc) are dependent on the companys ability to produce spheres with uniform wall thicknesses and consistent size distribution.
Particle size. Along with wall thickness, particle size plays a critical role in the microspheres relative density and its survival rate, because smaller microspheres are better able to withstand the processing conditions of higher shear rates and faster screws.
In pursuit of increased strength in its hollow glass microspheres, 3M Energy and Advanced Materials Div. (St. Paul, Minn.) recently introduced iM30K, which it touts as the first 30,000 psi (~200 MPa) isostatic compressive strength hollow glass microsphere with a density of 0.6 g/cc. At 16 m in diameter, iM30K microspheres are half the size of 3Ms S60HS grade rated at 18,000 psi (124 MPa). According to ETS Inc.s Tres, loadings of 3Ms iM30K can be as high as 20 percent with very little change in the molded parts impact strength, which is due, in part, to its small particle size.
Automotive parts manufacturer Hyundai Mobis (Seoul, South Korea) tested iM30K for use in an automotive instrument panel (IP) core, with positive results (see photo, p. 31). Glass-filled polypropylene that contained iM30K reportedly achieved a 16.8 percent weight reduction and a 50 percent cost reduction compared to a similar polycarbonate/acrylonitrile butylene styrene (PC/ABS) IP core.
Improved dimensional stability was very important where the IP core met the windshield, adds 3M Energys business manager, Lou Lundberg, who notes that parts produced using iM30K reportedly had better dimensional stability than the current talc-filled polypropylene (PP) part and improved material flow compared to the PC/ABS core.
In the same way, small balloons perform better during molding processes as well. Potters Sphericel-brand glass microspheres, for example, range in size from 11 m to 18 m and have crush strengths ranging from 10,000 psi to 8,000 psi (69 MPa to 55 MPa) depending on the grade. They have been compounded successfully with a 25.4-mm/1-inch diameter Killion 2-stage single screw ex-truder and injection molded on a 75-ton Newbury machine without significant sphere breakage, reports the company.
As important as these previously discussed properties are, one of the microspheres greatest assets is the contribution it makes to part processability, which, in a filler, is a direct function of particle shape.
Arguably, the microspheres small, spherical structure is the perfect shape for a filler. Without exception, the mineral fillers available to composites manufacturers are irregularly shaped. That irregularity results in a relatively large surface area, which increases the viscosity of the resin into which the filler is added. By contrast, the microspheres regularity minimizes its surface area. The low surface area allows for higher solids loading with less of an impact on the viscosity and flow characteristics of the composite, explains Rosenbusch.
Additionally, the microsphere has a nominal 1:1 aspect ratio, giving it inherently isotropic properties that composites manufacturers can use to great advantage. For example, in parts fabricated by a resin injection process, chopped glass fiber, with a high aspect ratio, results in ~60 percent less stiffness in the cross-flow direction than in the flow direction because the fibers become oriented in the direction of flow. This alignment of the fibers can contribute to warpage, especially when introduced to crystalline matrices, such as nylon or polypropylene, which have molecular chains that also tend to align along flow lines. Microspheres, on the other hand, do not orient and, in fact, tend to obstruct directional orientation of reinforcing fibers and matrix. The result is that stresses are more evenly distributed, enhancing both reinforcement and dimensional stability.
Microspheres can act as mini ball bearings, adds 3Ms Lundberg. The ball bearing effect enables the resin to more easily infiltrate complex mold geometries, resulting in faster cycle times. Further, successful infiltration can occur at lower mold temperatures and injection pressures than are possible when mineral fillers are used.
The microspheres regular shape can contribute to product surface quality as well. To illustrate, ETS Inc.s Tres notes the example of a major automotive parts supplier that was having difficulty achieving the required stiffness and Class A finish on an exterior automotive panel. The company was using a formula with 20 to 25 percent, by weight, chopped fiber, but was unable to achieve a Class A surface, says Tres. Unlike chopped fiber, which tends to migrate to the part surface during processing, microspheres tend to remain more evenly dispersed throughout the part. By adding 10 to 15 percent, by weight, of high-strength glass microspheres and reducing the glass fiber to 5 percent, they were able to achieve the desired mirror finish without sacrificing the stiffness required by the part.
No less important is the fact that the spheres help shorten mold heating and cooling cycles. Because the spheres are hollow, there is less mass to heat or cool, which leads to faster overall throughput, says Lundberg.
The cost of microspheres varies considerably depending on a variety of factors, including material, density, strength and volume. Expancel microspheres, for example, range in price from $5/lb to $30/lb, depending on grade and volume. The manufacturing process employed in the production of the microsphere also affects cost. In general, high-strength, glass microspheres cost two to three times more than chopped glass fiber, says Tres. However, according to Tres, the cost of 3Ms iM30K is significantly reduced because of the proprietary manufacturing process the company developed.
When comparing the cost of microspheres to resins and competing mineral fillers, its critical to think in terms of cost per unit of volume rather than cost per pound because microspheres can displace a large volume of higher-density material at a very low weight. Although most microspheres are sold by weight, users need to be aware of volume costs, stresses Rosenbusch. By comparing the cost of a gallon of resin (or other material) with a gallon of microspheres, you can see what the cost effect will be.
For example, even if the specified hollow microspheres cost twice as much per pound as the filler or resin that it will displace, significant density reduction (for example, a microsphere with a density of 0.6 g/cc displacing nylon 6/6 with a density of 1.15 g/cc) means that for a given volume, the price is nearly the same. Volume cost can be calculated by multiplying true (not bulk) specific gravity times 8.34 to give pounds per gallon, then multiplying that by the cost per pound to give cost per gallon.
Specialized surface treatments also can drive up cost; however, such coatings add properties beyond those inherent in the microspheres materials and construction, allowing manufacturers to tailor their products for specific applications. Coating the microsphere adds new levels of functionality, such as dielectric or thermal imaging properties, says Trelleborgs Gladysz. For example, coatings such as titanium dioxide (TiO2) or silver can provide signature management that is, control over the way in which objects are viewed when imaged using technologies such as radar or infrared (thermal) imaging. One obvious application is to help reduce the radar detectability of aircraft.
Surface treatments can be added to make microspheres magnetic, fluorescent and/or conductive or simply to improve bonding between the microsphere and the matrix. Microsphere Technology Ltd. (MTL; Edinburgh, Scotland, U.K.) specializes in coating hollow glass microspheres with a variety of pigments and metals for specific applications. Typically, to coat a microsphere with a metal, such as aluminum, silver, copper, stainless steel, platinum, gold or zinc, microspheres are mixed with an adhesive until coated, after which metal flakes are slowly added to coat the spheres. Curing permanently bonds the metal flakes onto the microsphere. Coating thicknesses can range from a few nanometers to several microns to manipulate final part characteristics.
Although glass and plastic microspheres are the most widely used in composite manufacture, they are not the only materials used to manufacture microspheres. Wall materials can be chosen to suit varying application requirements. Here are some of the other options, many of which are heavily used in paints and coatings:
Cenospheres. These low-density, hollow, free-flowing alumino-silicate microspheres are extracted from pulverized fuel ash that is produced by coal-fired electric power plants. Cenospheres are available from a number of suppliers, including Trelleborg Emerson & Cuming (Mansfield, Mass.), which supplies trademarked Fillite cenospheres in sizes ranging from 5 to 500 m. E-Spheres, produced by Envirospheres Ltd. (Lindfield, Australia), are used in spray-up, hand lay-up, resin transfer molding and syntactic foam applications.
Ceramic. Engineered ceramic microspheres are available from a wide range of manufacturers, including 3M Energy and Advanced Materials Div. (St. Paul, Minn.). Although they are used in syntactic foams, paints and coatings remain the primary market for ceramic microspheres.
Carbon. Phenolic microspheres can be carbonized or pitch can be treated and carbonized to produce carbon spheres, which can be used in composites and syntactic foam for a variety of applications. Due to their smooth surface, good mobility and thin walls, which permit deformation in response to sound pressure, carbon microspheres have been effectively used in the production of carbon microphones. Also, specially processed pitch carbon microsphere composites are suitable for use as honeycomb fillers for high-temperature or ionizing radiation fields.
Composite and Metal. Aluminum and copper/silver microspheres are currently available. Companies are continuously manipulating materials to create a seemingly endless array of composite microspheres to suit very specific applications. Present developments include polymer-metal spheres that combine a polymer core with a metal shell as well as a product in which multiwalled carbon nanotubes are bonded to the surfaces of polystyrene microspheres.
Solid. Solid glass microspheres, commonly called glass beads, are widely used as resin extenders. Though they do not offer low density like hollow spheres, they can enhance physical properties. Potters Industries (Valley Forge, Pa.) produces Spheriglass solid microspheres, which it says can increase the strength of plastic. Shrinkage of glass-filled nylon 6/6 reportedly can be cut by 70 to 80 percent when 30 percent solid glass spheres are used, and warpage is said to be reduced by 95 to 97 percent.
Microspheres of all materials (see Microshperes: Material Alternatives, above) are well suited for foaming processes. In fact, the makers of low-density composite syntactic foam systems, such as those used in deepwater buoyancy applications, require the controlled, closed cells that only microspheres can deliver.
Syntactic foams are highly functional materials that can be optimized for very specific applications, says Gary Gladysz, VP of technology at Trelleborg Emerson & Cuming (Mansfield, Mass.). If you want strength plus thermal insulation plus acoustic properties plus energy absorption, then youre in the realm of syntactic foam, he explains. And the basis for all of that functionality is the wide selection and targeted properties that can be achieved through microballoon selection.
By definition, syntactic foams are a combination of microspheres and a polymeric resin. A variety of different wall materials and resins can be used to create optimized foam. For applications ranging from acoustic panels to aerospace structural cores, Cornerstone Research Group (CRG; Dayton, Ohio) builds its engineered foams (trade named Advantic) using glass, polymer or ceramic microspheres embedded in a resin matrix such as cyanate ester, silicone or epoxy. Using a proprietary low-stress resin removal system, excess resin and fractured microspheres are extracted from the syntactic material prior to curing, reportedly resulting in a low density, void-free content. Densities for Advantic, which can be fabricated in a variety of shapes blocks, cylinders or sheets can range from 0.30 g/cc to 0.55 g/cc.
Trelleborg Emerson & Cuming typically relies on its Eccosphere microspheres to build its syntactic foam. If you want functionality beyond the basic glass microsphere, explains Gladysz, You can tailor a foam by using a different wall material, such as carbon for higher conductivity, add a coating, combine different densities of microballoons, or even add interstitial void. Syntactic foam offers a high compressive-strength-to-weight ratio. Compressive properties are driven by the properties of the microsphere, while tensile properties depend on the matrix material.
Trelleborg supplies syntactic foams for a variety of end uses, including lightweight panels for deepsea submarines and marine barriers, acoustic damping and insulation panels as well as a variety of aerospace applications. Notably, the company also is developing a syntactic foam product for blast mitigation. There are millions of microballoons in one square inch of syntactic foam, and at each interface there is an event that dissipates the energy of a blast, explains Gladysz. He believes this makes syntactic foams ideal for production of moldable protective panels that absorb and dissipate blast energy rather than just transmit and reflect blast effects.
Key applications for antiblast products include lightweight armor plating for military vehicles, lightweight helmet covers, flak jacket inserts and protective composite walls, doors and other structures.
There are numerous methods for fabricating composite components. Selection of a method for a particular part, therefore, will depend on the materials, the part design and end-use or application. Here's a guide to selection.
The matrix binds the fiber reinforcement, gives the composite component its shape and determines its surface quality. A composite matrix may be a polymer, ceramic, metal or carbon. Heres a guide to selection.
Alumina: Alumina also known as Aluminium Oxide is an amphoteric oxide comprising of aluminium and oxygen. Alumina is symbolised as Al2O3. Alumina has the appearance of white solid like table salt. Alumina has a boiling point of 2980 degree Celsius and melting point of 2040 degree Celsius. Its molar mass is 101.96 g/mol. The Bayer Process: Alumina is the main components of bauxite, so bauxite is refined in order to produce Alumina. The Bayer process is the principal way for producing alumina by refining Bauxite. Bauxite other than with 30-60percentage of aluminium oxide contains mixture of silica, iron oxides and titanium dioxide. Bayer process of producing alumina can be divided into following four steps: i.Digestion: In this step of Bayer process, bauxite ore is crushed, milled and then heated with sodium hydroxide at the temperature of around 150-200 degree Celsius. In the Bauxites, aluminium compounds are present as gibbsite (Al (OH) 3), boehmite (AlOOH) and diaspore (-AlO (OH)). ii. Filtration: In this step the mixture is clarified to remove impurities. Other than alumina and silica, all other components present in Bauxite do not get dissolved. The solids which are not dissolved get settled down at the bottom forming red mud. This red mud is then discarded from the solution commonly by using rotary sand trap. The filtration process converts the aluminium oxide to soluble sodium aluminates, 2NaAlO2, as per the equation: Al2O3 + 2NaOH 2NaAlO2 + H2O. iii. Precipitation: The clear remaining mixture is added to precipitators by using heat exchangers, which turns the mixture from heat to cool liquor. Silica is precipitated from the mixture because heating. Crystals of aluminium hydroxide are discovered in this step. Some amount of aluminium hydroxide discovered in this step is used as a water treatment chemical. iv. Calcination: 90 percentage of the gibbsite manufactured is converted into alumina by washing, drying, and then heating aluminium hydroxide in a rotary klins or fluid flash calciners at temperature 1010-1260 degree Celsius. 2Al (OH) 3Al2O3+ 3H2O. More than 90 percent of alumina manufactured is used to produce aluminium by Hall-Heroult Process. History of the Bayer Process: Bayer process was invented by Austrian chemist, Carl Josef Bayer in 1887, while working in Saint Petersburg, Russia for developing a method to supply alumina to the textile industry. The Bayer process started gaining importance after the invention of Hall-Heroult aluminium process. Till today the process is unchanged and is used to produce nearly all the worlds alumina supply. Applications of Alumina: Along with use of chemical alumina in the production of aluminium it has other key uses. Being chemically inert and white in appearance, aluminium is preferred as filler in plastics. In sunscreen it is a common ingredient and is sometimes also present in cosmetics like lipsticks, blush and nail paints. In many glass formulations, alumina is used as an ingredient. Alumina is used in purification, to remove water from gas streams. In Claus process alumina is the catalyst. It is also used for dehydrating alcohols to alkenes. Aluminium oxide is widely uses as an abrasive because of its strength and hardness. Paint: Flakes of alumina are used for reflective decorations in paints, like in automotive or cosmetics industry.
Alumina is the main components of bauxite, so bauxite is refined in order to produce Alumina. The Bayer process is the principal way for producing alumina by refining Bauxite. Bauxite other than with 30-60percentage of aluminium oxide contains mixture of silica, iron oxides and titanium dioxide.
i.Digestion: In this step of Bayer process, bauxite ore is crushed, milled and then heated with sodium hydroxide at the temperature of around 150-200 degree Celsius. In the Bauxites, aluminium compounds are present as gibbsite (Al (OH) 3), boehmite (AlOOH) and diaspore (-AlO (OH)).
ii. Filtration: In this step the mixture is clarified to remove impurities. Other than alumina and silica, all other components present in Bauxite do not get dissolved. The solids which are not dissolved get settled down at the bottom forming red mud. This red mud is then discarded from the solution commonly by using rotary sand trap.
iii. Precipitation: The clear remaining mixture is added to precipitators by using heat exchangers, which turns the mixture from heat to cool liquor. Silica is precipitated from the mixture because heating. Crystals of aluminium hydroxide are discovered in this step. Some amount of aluminium hydroxide discovered in this step is used as a water treatment chemical.
iv. Calcination: 90 percentage of the gibbsite manufactured is converted into alumina by washing, drying, and then heating aluminium hydroxide in a rotary klins or fluid flash calciners at temperature 1010-1260 degree Celsius.
Examination of the general form of the production route for alumina ceramics from ore to finished shape provides an insight into some of the important factors and working principles which guide the ceramics technologist and an indication of the specialized shaping methods that are available for ceramics. As mentioned earlier, each stage of the production sequence makes its own individual and vital contribution to the final quality of the product and must be carefully controlled.
The principal raw material for alumina production is bauxite Al2O(OH)4, an abundant hydrated rock occurring as large deposits in various parts of the world.2 In the Bayer process, prepared bauxitic ore is digested under pressure in a hot aqueous solution of sodium hydroxide and then seeded to induce precipitation of Al(OH)3 crystals, usually referred to by the mineral term gibbsite. (The conditions of time, temperature, agitation, etc. during this stage greatly influence the quality of the Bayer product.) Gibbsite is chemically decomposed by heating (calcined) at a temperature of 1200C. Bayer calcine, which consists of -alumina (>99% Al2O3), is graded according to the nature and amount of impurities. Sodium oxide, Na2O, ranges up to 0.6% and is of special significance because it affects sintering behaviour and electrical resistance. The calcine consists of agglomerates of a-alumina crystallites which can be varied in average size from 0.5 to 100 m by careful selection of calcining conditions.
Bayer calcine is commonly used by manufacturers to produce high-purity alumina components as well as numerous varieties of lower-grade components containing 8595% Al2O3. For the latter group, the composition of the calcine is debased by additions of oxides such as SiO2, CaO and MgO which act as fluxes, forming a fluid glassy phase between the grains of -alumina during sintering.
The chosen grade of alumina, together with any necessary additives, is ground in wet ball-mills to a specified size range. Water is removed by spraying the aqueous suspension into a flow of hot gas (spray-drying) and separating the alumina in a cyclone unit. The free-flowing powder can be shaped by a variety of methods (e.g. dry, isostatic-or hot-pressing, slip- or tape-casting, roll-forming, extrusion, injection-moulding). Extremely high production rates are often possible; for instance, a machine using air pressure to compress dry powder isostatically in flexible rubber moulds (bags) can produce 300400 spark plug bodies per hour. In some processes, binders are incorporated with the powder; for instance, a thermoplastic can be hot-mixed with alumina powder to facilitate injection-moulding and later burned off. In tape-casting, which produces thin substrates for micro-electronic circuits, alumina powder is suspended in an organic liquid.
Although around 93% of alumina production is subsequently used as the feedstock in the smelting of the metal, there is a significant market for specialty aluminas. These markets lie in ceramics, particularly insulators and refractories, abrasives, catalysts, catalyst supports and absorbents. Some 4538 kT of such materials was produced for such markets in 2008 (International Aluminium Association, 2009).
Pure alpha alumina (corundum) has a Mohs hardness of 9.0, second only to diamond. The melting point of 2040C for the pure material makes this useful for high temperature, high strength ceramics (Morrell 1987). Although an electrical insulator, it has a relatively high thermal conductivity (40 Wm1K1) for a ceramic material, making it particularly useful in electrical insulators. The fabrication of such ceramics requires extended calcination of the green body at temperatures above 1300C to complete conversion from the transition aluminas. Alpha alumina also occurs naturally as the gemstones ruby and sapphire, the colour depending on the nature of impurity substitutions in the lattice.
Outside of natural gemstones and the use of synthetic single crystal sapphire as a semiconductor substrate, the highest value uses, and the widest reported literature amongst the aluminas, are involved with the transition aluminas. The defect structure, surface acidity and high surface areas make these materials, and particularly gamma alumina, widely useful as absorbents, catalysts and catalyst supports. The surfaces exhibit both electron acceptor (Lewis acid acidity) and some proton donor (Bronsted acidity) behaviour. Peri (1960, 1965) derived the structural configurations of five possible surface hydroxyl groups in -alumina, explaining the Lewis acid acidity in relation to the range of sites observed in infrared spectroscopic studies. The model was further developed by Knoezinger and Ratnasamy (1978) to account for the range of coordination of the aluminiums (octahedral and tetrahedral) to which the hydroxyl groups can be bound.
The critical stage in the customising of -alumina as a catalyst lies in the control of surface area (Paglia et al. 2004), and the distribution and activity of these hydroxyl groups, several of which are frequently involved in the catalytic steps in a given reaction (Ghorbel et al., 1973, 1974). Activity is thus dictated by a combination of calcination conditions and subsequent treatments of the catalysts. For example in the Claus process for the desulphurisation of natural gas in gas plants and refineries, sulphur is removed from the process stream by reaction over an activated alumina catalyst (Eq.2.7).
Alpha alumina is fully dehydroxylated, has only geometrical surface area, and is not active in this sense, although its thermal stability makes it an ideal catalyst support in applications such as reforming (Moreno et al. 2009; Pompeo et al. 2009).
The particular properties of -alumina have also prompted a range of approaches to its synthesis in increasingly exotic morphologies. Nanorods (Shen et al., 2007), nanobelts (Peng et al., 2002; Gao et al., 2005), nanotubes (Pu et al., 2001; Zhang et al., 2002) and nanoflowers (Ma and Zhu, 2009) are amongst many reported morphologies, synthesised by electrochemical, sol-gel, etching and evaporation techniques.
The RM as the bulky waste of the Bayer alumina production process, is formed as the result of the reaction between sodium hydroxide and bauxite ore. The bauxite ore is usually a combination of minerals rich in aluminum oxide and hydroxide. However, bauxite also contains minerals of iron, silicon, titanium and rare earth elements. For each ton of alumina produced, about 12 tons (dry mass) of RM are produced (Wang et al., 2008). Fig. 3 shows a schema of the traditional Bayer alumina manufacturing process.
This process involves the separation of alumina from unwanted components such as iron, titanium, silica, calcium, vanadium, manganese, etc., in bauxite. In this process, bauxite is heated along with the caustic soda at high pressure (~30 atm), resulting in the formation of sodium aluminate (reactions (1)(4)) (Gil, 2005; Ma et al., 2009).
Aluminate is hydrolyzed and converted to aluminum hydroxide (reaction (5)), followed by the production of aluminum oxide or alumina after calcining the produced aluminum hydroxide at high temperature (1200C) (reaction (6)) (Gil, 2005). After extraction, the insoluble residue is known as red mud or bauxite residue. Its color and name are due to its high iron oxide content. For each tone of alumina produced, 23 tons of bauxite ore should be used. Depending on the quality of bauxite ore, 12 tons of RM waste are also produced as a byproduct (Rai et al., 2017).
Aluminium is the most widely used nonferrous metal, the production process of aluminium includes bauxite mining, alumina production, electrolysis aluminium, castings, rollings, the production of consumer products and recycling. In the electrolysis aluminium production line, more than 99% pure molten aluminium is formed at the cathode deposited at the bottom of the electrolytic cell, and is tapped from the cell into a crucible by a vacuum siphoning system. The molten metal is transported and poured into a holding furnace and caster to be cast into ingots, extrusion or rolling ingots.
In a typical aluminium smelter, there may be hundreds of electrolysis cells. Each crucible taps the metal from up to three cells that contain different purity of the molten aluminium. The capacity of the holding furnace is more than three times the crucible, so there can be several different crucibles feeding one holding furnace. The molten aluminium which is mixed and stabilized synchronously in one holding furnace is called a charge. Therefore, both crucible and holding furnace are batch production mode. Therefore, in the aluminium electrolysis process, the production schedule is to form the batch tapping of the cells into crucibles and arrange batch feeding of the crucibles into holding furnaces, considering the constraints of the electrolysis and cast.
Ryan (1998) formulated the tapping of the cells problem as a set-partitioning model and solved it by the LP relaxation. Piehl (2000) used the revised simplex method and branch and bound with a constraint branching approach to solve the similar cell batching problem. Prasad et al. (2006) provided a MILP model for the scheduling of aluminium casts of different alloys with respect to the actual number of batches to be processed in multistage. For the formulation of the scheduling problem, we refer to Floudas and Lin (2004) and Harjunkoski et al. (2014).
The remainder of this paper is organized as follows. In Section 2, a precedence-based model is presented for this problem. In Section 3, a novel unit-specific event based continuous-time mixed integer programming model are proposed to describe the problem. Section 4 reports the experimental results of the two models solved by CPLEX. Finally, Section 5 presents the conclusions.
The Bayer process was invented 130years ago and remains the global method of choice for converting bauxite to alumina for aluminum and industrial alumina production. In general, the three largest problems in the Bayer process are as follows:
Given the pivotal importance of the Bayer process for both alumina and aluminum production, both in terms of production cost in monetary and manpower terms (given the global scale of the industry), and the quality of the end product, particularly in terms of alumina ceramics thereby produced, it is important to review these issues here.
To utilize the industrial waste and to avail the advantageous properties of the industrial waste, researchers uses it as the secondary reinforcement in the fabrication of composite material. Insoluble waste residue called red mud from the alumina production is an industrial waste which is easily available from the aluminum manufacturer [19,21]. Fig.11.1 shows the different industrial waste used as the filler in composite material to enhance the mechanical property as well as to improve the wear resistance of the fabricated composite. Local marine litter such as kelp brown algae (Eklonia spp.) and bivalve mollusk shells (Veneridae spp.) were used as secondary biofillers for fabricating wood fiber-reinforced polypropylene hybrid composite (Fig.11.2). From the results it was inferred that biofillers enhance the mechanical and moisture resistance of the fabricated composites . This overcomes the disadvantage of the natural fiber composite by improving the moisture resistance by adding bio-fillers to the composite.
Solid waste from the iron ore during separation of iron from it is called blast furnace slag. Blast furnace slag is the industrial waste which was used as the filler material for fabricating short glass fiber-reinforced polypropylene hybrid composites . Fly ashfilled woven jute/glass fabric hybrid composite shows better erosion wear resistance, tensile, and flexural strength compared with the hybrid composite without filler.
The chemical, electrochemical, and thermal processes for the production of aluminum from alumina, including alumina refining, electrolysis, and recycling, are discussed. The Bayer process is used to produce over 90% of the world alumina production. The key steps used to produce metallurgical grade alumina from bauxite ore are described including the extraction of alumina trihydrate, removal of iron oxide and silicon dioxide impurities, generation of red mud residue, precipitation of alumina trihydrate crystals, and calcination of gibbsite.
Aluminum is a relatively young material since it was discovered and named in 1808 and yet today its production volume exceeds that of all other nonferrous metals combined. It is malleable and ductile, and its melting point at 660C is among the lowest of the metals. The HallHroult electrochemical process developed in 1886 is still the only industrial process used to produce over 40 million tons of pure aluminum metal annually. The basic principles of electrochemistry are explained for electrolyzing alumina, Al2O3, dissolved in a molten fluoride solvent called cryolite to produce molten aluminum, and the carbon anodes are being continuously consumed by reacting with alumina dissolved in the electrolyte. The typical features of a commercial aluminum electrolysis cell are described. Modern aluminum cells operate with large current intensities, between 300 and 500kA, and are highly automated with sophisticated computer control systems. Waste fluoride cell gases are collected and thoroughly cleaned, or scrubbed before being exhausted to the atmosphere, though greenhouse gases, CO2 and CF4, are emitted to the atmosphere.
Due to the inherent value of aluminum metal, a highly developed collection and processing system for aluminum by-products and aluminum scrap has been developed to recover the metal content for reuse into new aluminum products. The ease with which aluminum can oxidize at elevated temperatures and its reactivity in comparison with other elements make the processing of aluminum significantly different from other nonferrous metals. Thermodynamics and kinetics associated with processing aluminum will be examined, and various types of equipment used in the processing will be discussed.
A life cycle analysis of aluminum shows distinct advantages to recycling the material. The recycling rate of aluminum cans in the United States was 65% in 2011. The primary benefit of recycling aluminum is reduced energy consumption. Aluminum recovery from scrap requires only 5% of the energy required to extract it from alumina. Therefore, secondary aluminum production from recycling scrap has the potential to significantly reduce greenhouse gas emissions. The most common source of aluminum scrap is aluminum cans, but automobiles, building materials, and appliances are also viable sources. Repeated recycling of aluminum does not affect its quality. Today, aluminum is the most commonly recycled metal in the world.
The raw material for aluminium and alumina is bauxite (named after Les Baux-en-Provence in France), a mixture of the oxide hydrates and clays (aluminosilicates) with impurity oxides such as SiO2 and TiO2 and small amounts (ppm) of the strategic compound Ga2O3 and iron oxides that occur as a weathering product of low iron and silica bedrock in tropical climatic conditions. The most common mineral constituent of bauxite is gibbsite. A description of bauxite mineralogy can be found in a number of textbooks (e.g., Wells, 1984), and summaries are found in several revisions (Doremus, 1984). Evolution of gibbsite with temperature has been recently studied by neutron thermodiffractometry (Rivas Mercury, Pena, de Aza, Sheptyakov, & Turrillas, 2006). Deposits of bauxite exist around the world, the largest bauxite deposits being found in Guinea, Australia, Brazil, and Jamaica.
Purification of bauxite to fabricate aluminium, and to a lesser extent, alumina, is done by the Bayer process. Two to three tonnes of bauxite are required to produce a tonne of alumina and 4-6tonnes of bauxite for the production of 1tonne of aluminium metal (International Aluminium Institute, 2011). Figure 1(a) shows the geographical share of alumina production by weight in 2011 recognized by the International Aluminium Institute. Most of the alumina is used for the production of aluminium and a small part goes to the ceramic industry (Figure 1(b)).
Figure 1. Alumina production by weight in 2011, recognized by the International Aluminium Institute (2011). (a) Geographical share. WE, West Europe; ECE, East and Central Europe; NA and SA, North and South America. (b) Metallurgical (Al production) and chemical (Al2O3 production) share.
The Bayer process starts by dissolving crushed bauxite in sodium hydroxide under pressure at 300C to form a supersaturated solution of sodium aluminate at normal conditions of pressure and temperature. The insoluble oxides are then removed and the hydrated aluminium oxide is precipitated as gibbsite by seeding, more frequently, or as metastable bayerite by reduction of pH by carbon dioxide. The precipitated low-temperature forms, -alumina, are then washed and subsequently dehydrated at 1000-1200C to fully convert into the stable -alumina phase. This material is named "calcined alumina" and typically contains 0.1-0.5wt% of sodium oxide and calcium oxide. Calcinations at intermediate temperatures give mixtures of -Al2O3 and transition aluminas; these powders are usually called "reactive aluminas". The coarse aggregates made of large alumina single crystals for the refractory industry (fused alumina) are obtained by fusing this alumina powder and crushing the obtained material. Also, it can be graded to be used for grinding and abrasives.
The calcined agglomerates have sizes up to 100m, even though the sizes of the primary crystals can be smaller than 1m. The powders required for the fabrication of high-performing ceramics are much smaller (m); thus, a major objective of the calcination step is to obtain soft agglomerates in order to avoid intensive milling as much as possible. Then, the calcined agglomerates can be milled down to get uniform sized and small particles. The other main characteristic of the calcined aluminas is the presence of up to 0.5wt% NaO2 as mentioned above. Low soda-calcined aluminas are considered when the NaO2 is lower than 0.05wt%. Typical specifications of calcined aluminas can be found elsewhere (Riley, 2009).
Bauxite production has increased from 144Mt worldwide in 2002 to 178Mt in 2006. Most of this is mined from open cast mines in Australia (62Mt), Brazil and China (both 20Mt) followed by Guinea (15Mt), Jamaica (15Mt), and India (13Mt). In 2006, Alcoa, Chinalco, Alcan, Rusal and BHP Biliton accounted for or controlled almost 60% of the 69Mt of worldwide alumina production from the 178Mt of bauxite.9 The largest alumina producers in 2006 were Australia (18Mt) and China (14Mt). Like many commodities, alumina is sold both on spot prices and contract terms, for which it is typically priced between 11.5% and 13.5% of the aluminum price quoted on the LME. The recent rapid growth in Chinese demand for aluminum has led to a predicted increase in annual demand from 69Mt per year in 2006 to 88Mt per year by 2011. The largest producers of primary aluminum in 2003 were China (5.5Mt), Russia (3.5Mt), Canada (2.8Mt), United States (2.7Mt), Australia (1.8Mt) and Norway (1.1Mt). By 2007, Chinese production had increased to 12.6Mt out of the total world primary production of 34Mt.9 The amount of electrical energy to produce aluminum has been reduced from more than 50kWhkg1 in 1890 to 16.1kWhkg1 in 1990 and to 15.2kWhkg1 in 2006.
Four tons of bauxite is used to produce 2 tons of alumina, which then produces 1 ton of aluminum. The industry average emissions associated with primary aluminum production is 9.73kg CO2e per kilogram, 55% of this from electricity generation, so this varies considerably depending on how the electricity is generated.3 Historically, over 50% of the electricity used to produce aluminum has been hydroelectrically generated, and although it is expected that this trend will continue, recently significant smelter capacity has been installed, particularly in the Middle East, using gas. Aluminum production consumes 3% of the world's electricity and about 10% of its hydropower.
The aluminum industry maintains a close watch on the composition of primary aluminum by regular chemical analysis of samples from each individual electrolysis cell. Purity is calculated by subtracting all the trace element concentrations from 100%. Trends are noted with respect to age of the cell, anode technology and alumina source, as well as the use of recycled alumina from the environmental control systems (scrubbers) used in smelters that are used to trap elements that volatilize from reduction cells. Use of this scrubber alumina results in higher levels of nickel, lead, gallium and vanadium. From the corrosion perspective, the most significant impurities are iron (typically 0.030.2wt %) and silicon (0.030.1wt%) and the lower levels of elements such as copper, manganese, nickel, titanium, zinc, vanadium and gallium. Volatile elements such as sodium, calcium and phosphorous are removed by flux treatments prior to ingot casting.
Aluminum and its alloys are readily recyclable, with recycled scrap providing an increasingly important and growing contribution of 23Mt per year to the more than 60Mt total annual metal supply.3,10 The ever-growing environmental concerns over raw material processing and primary aluminum production as well as the favorable economics of recycling have led to a strong secondary aluminum production industry based on reclaimed scrap accounting for about 3035% of total aluminum production since the early 1990s. The recycling of aluminum requires 95% less energy than that is required for primary aluminum production, and recycling of used aluminum products generates only 0.5kg of CO2e per kilogram of aluminum produced.
However, in order to meet the mechanical and corrosion performance requirements of many alloy and product specifications, much of the recycled metal must be sweetened with primary metal to reduce impurity levels. The result is that in many cases (except beverage cans) recycled metal tends to be used primarily for lower grade casting alloys and products. While a certain amount of this is acceptable, the recycle-friendly world will be truly optimized only when the recycle loop is closer to a closed loop within a number of product lines. Elements that increase in the recycled metal are mainly iron and silicon, and other elements such as magnesium, nickel and vanadium.11 It is generally advisable to separate wrought alloys from cast alloys.
The total weight of aluminum products in use in 2006 was estimated to be 584Mt, of which 32% is in building products, 28% in transport applications, 28% in engineering and cable, 1% in packaging and 11% in other products. Since the 1880s, close to 800Mt of aluminum have been produced, and about three-quarters of this metal, more than 580Mt, is still in productive use. This is a testament to the excellent corrosion resistance and recyclability of aluminum in almost all its applications. Recycling the metal currently in use would equal 17years' primary aluminum output.3
The basis for this overview was a classification of the wastes used in the ceramic industry according to two criteria: (1) the European Waste Catalogue (EWC) that categorizses them, taking into account what they are and how they were produced (EWC,2002) and (2) their behavior or role in the ceramic process, as well as the main effects caused to the ceramic product.
The EWC (2002) is a hierarchical list of 20 codes of waste descriptions established by Commission Decision 2000/532/EC which classifies and categorizes waste materials (Table 7.1). Figure7.1 shows the classification of the studied wastes into these 20 different codes. The studies analyzed in this review usually mix clay with one or more wastes from different EWC codes to obtain ceramic products. However, the incorporation of some wastes corresponding to EWC codes of wastes 08, 09, 12, 14 and 15 into ceramic products is not usually studied. Ceramic products containing wastes that belong to different wastes have been classified according to the EWC code of the waste introduced in higher proportion or to the waste that affects most properties of the product.
Fifty percent of the studied papers introduce residues classified under the EWC codes 01, 10 and 19. Wastes classified in the EWC code 01 are wastes resulting from exploration, mining, quarrying and physical and chemical treatment of minerals. The main source processes generating this type of waste are physical and chemical processing of ornamental rocks, mining and quarrying works and alumina production. Wastes belonging to the EWC code 10 are wastes from thermal processes: power stations and other combustion plants (except those classified as EWC 19), iron and steel industry and aluminum, lead and zinc thermal metallurgy. In this category are included, among others, coal or biomass fly ash, metallic sludge and slag and foundry sand. Wastes classified in the EWC code 19 are wastes from waste management facilities, off-site wastewater treatment plants and the preparation of water intended for human consumption and water for industrial use such as sewage sludge or incinerated sewage sludge ash. In addition, a high percentage of the wastes introduced into ceramics comes from agriculture (EWC 02) and from construction and demolition waste including excavated soil from polluted sites such as waste bricks or river and marine sediments (EWC 17).
Another more general classification based not only on the specific nature and origin of the waste but also on the different roles that the alternative raw materials can play in the brick-making process is proposed (Petavratzi & Barton, 2007).
Figure7.2 shows seven different categories of roles (A=fluxing agents, B=fillers, C=clay substitutes, D=body fuels, E=pore formers, F=property affecting wastes) that the waste could play in the ceramic matrix during the firing process. A single alternative material may have different roles in the firing step. For instance, fly ash with high content of carbonaceous matter and alkaline compounds may act as both body fuel and fluxing agent, forming liquid phases at relative low temperatures, contributing to the sintering consolidation. On the other hand, fly ash with low carbon content and negligible content of alkaline compounds may act as clay substitute or as a filler when these ashes are predominantly composed of quartz (Vieira & Monteiro, 2009).
The role A of fluxing agents is composed of materials with a relatively high amount of alkaline oxides, mainly K2O and Na2O, which in reaction with silica and alumina promote liquid phase formation at relatively low firing temperatures and, thus, contribute to the sintering consolidation and densification of the ceramic structure. Among the wastes included in this category are glassy wastes, boron-containing residues, Waelz slag, steel slag, bone ash, ash from the gasification of coal and sludge from the ornamental stone industry. For instance, granite is considered a flux material due to its large amount of alkaline oxides. These oxides derive from feldspars and micaceous minerals that are common constituents of granite rock.
The role B of fillers includes wastes which can partially substitute the effect caused by the sand in the vitrification process; this means the dissolution of the inorganic waste material in the glassy phase that occurs during the sintering of clay or the reaction with clay minerals to form new mineral phases in the fired clay structure. Among the wastes possibly found in this category are sewage sludge, ash generated in the incineration of sewage sludge, steel dust, steel refining sludge, foundry sand and spent catalyst rejects.
The role C of clay substitutes is made up of waste with a certain amount of clay minerals that confer the plastic behavior to the ceramic matrix. Among the wastes included in this category are municipal solid waste incineration slag, grog or chamotte, water treatment residues and fly ash from the paper industry. The role D of body fuel agents contains wastes including combustible carbon-containing matter, which has a relative heating power, and is therefore desirable for saving energy. Among the wastes included in this category are oily residues, blast furnace sludge which still has a significant amount of coke, sludge from the paper industry, ash with high carbon content, sewage sludge and sawdust.
The role E of pore formers includes wastes that usually possess a high content of organic matter that burn out to form pores. It is important to observe that the inclusion of alternative materials in the ceramic process normally changes the properties of the fired product with substances that modify the ceramic behaviour and cannot be included in the previous role categories. Therefore, an additional role F of property affecting wastes has been included that contains all the summarized wastes.
The most common roles played by wastes in the ceramic matrix are role E of pore formers, role A of fluxing agents and role D of body fuels. Porosity is a desirable property to obtain acoustic and thermal isolating products, while fluxing agents and body fuels promote energy savings by reducing considerably the firing temperature of the bricks. This leads not only to a much lower heat requirement as compared to the traditional process but also to higher economic profits.
extracted from bauxite through the Bayer process, which was developed for the aluminum industry in 1888. In the Bayer process bauxite is crushed, mixed in a solution of sodium hydroxide, and seeded with crystals to precipitate aluminum hydroxide. The hydroxide is heated in a kiln in order to drive off