During many industrial processes, a filtering step may be required to remove impurities and improve quality of the final product. Depending on the process, the filter may be subjected to high temperatures and a corrosive environment. A filter material with good temperature and chemical resistance is therefore needed.
Ceramic filters meet these requirements and are finding use in a wide range of applications. One major application is filtration of molten metal during casting of various components. Another is diesel engine exhaust filters. The world market for molten metal filters exceeds $200 million per year.
The metal casting industry is the sixth largest in North America, contributing over $20 billion to the U.S. economy. About 13 million tons of metal castings are shipped every year, with 85% made from ferrous (iron) metals. Castings are used in over 80% of all durable goods.
In the casting process, a solid metal is melted, heated to proper temperature (and sometimes treated to modify its chemical composition), and is then poured into a cavity or mold, which contains it in the proper shape during solidification. Thus, in a single step, simple or complex shapes can be made from any metal that can be melted. Cast parts range in size from a fraction of an inch and a fraction of an ounce (such as the individual teeth on a zipper), to over 30 ft (9.14 m) and many tons (such as the huge propellers and stem frames of ocean liners).
Though there are a number of different casting processes, die casting is used for over one-third of all metal castings and contributes over $7.3 billion to the U.S. economy every year. This process involves injecting molten metal into a steel die under high pressure. The metal—either aluminum, zinc, magnesium, and sometimes copper—is held under pressure until it solidifies into the desired shape. Parts range from automobile engine and transmission parts; to intricate components for computers and medical devices; or to simple desk staplers.
The various casting processes differ primarily in the mold material (whether sand, metal, or other material) and the pouring method (gravity, vacuum, low pressure, or high pressure). All of the processes share the requirement that the materials solidify in a manner that would maximize the properties, while simultaneously preventing potential defects, such as shrinkage voids, gas porosity, and trapped inclusions.
These inclusions can be removed by placing ceramic filters in the gating system leading to the mold. Such filters must resist attack at high temperature by a variety of molten metals. These metals can contain such reactive elements as aluminum, titanium, hafnium, and carbon. Using these filters can reduce scrap rates by 40% and increase yields by 10% for manufacturing a wide range of parts made out of iron alloys, stainless steel, super alloys, aluminum, or other nonferrous alloys.
Molten metal filters generally come in two forms: a porous foam-like structure with interconnected pores that vary in direction or cross section, or an extruded porous cellular or honeycomb structure with cells of various shapes (square or triangular) and constant cross section. Though globally the most popular type of filter is foam, cellular filters are used in 75% of applications in North America.
Filters can have either open cells or closed cells. Open cell (reticulate) filters consist of a network of interconnected voids surrounded by a web of ceramic and are widely used for molten metal filtration. Closed cell filters (foams) consist of a similar network but the beams are bridged by thin faces which isolate the individual cell. The open porosity in an open cell structure is critical in filter applications. The properties of a filter depend on both the cellular geometry (density, cell size) and the properties of the material. Advantages include high temperature stability and low weight.
The pore size of these filters are defined as cells or pores per linear inch (ppi). For honeycomb filters, this ranges from 64-121 ppi or 240 ppi. For foam filters, pore size is much more difficult to measure but generally ranges from 10-30 ppi.
Foam filters, which were first introduced over 20 years ago for nonferrous casting, are also used in direct pour units for casting steel. Inclusions that range from 0.125-2 in (0.3175-5.1 cm). or more in length and up to 0.25 in (0.635 cm) in depth can be removed. These inclusions come from molding materials, ladle refractories, and reoxidation during the pouring process.
Filtration occurs by mechanical interference, with large inclusions separated at the filter face and smaller inclusions trapped within the filter. Foam filters are able to trap inclusions significantly smaller than their open pore areas and can also remove liquid inclusions.
Thermal shock behavior (the resistance to sudden changes in temperature) for foam filters is dependent on their cell size, increasing with larger cells. Strength is initially retained after thermal shock and then gradually decreases with increasing quench temperature. A higher density may also improve thermal shock resistance.
The filter material is usually a metal oxide powder of various compositions. These include aluminum oxide, zirconium oxide, spinel (a combination of magnesium and aluminum oxides), mullite (a combination of aluminum and silicon oxides), silicon carbide, and combinations thereof. Ceramic fibers of various compositions may also be added to improve certain properties. Other additives include binders (alumina hydrate, sodium silicate), antifoaming agents (silicone or alcohol), and other chemicals to improve slurry properties. Water is usually used to make the ceramic slurry.
For optimal filter performance, a filter must be designed with the proper composition, pore size, and properties that match the specific application. Size and shape must be tailored to fit the mold system of the part being cast. Sufficient port area must be allowed so the filter does not choke the gating system during filtration. Filter area should be between three to five times the total choke area that the filter is feeding.
The major performance criteria when designing a filter are flow rate, filtering efficiency, hot/cold strength, slag resistance, thermal shock resistance, quality level, and cost. Each design is better at some than others, with significant design tradeoffs required in many cases.
There are several methods used to make ceramic filters. The polymeric-sponge method, which will be described in more detail here, produces open-cell structures by impregnating a polymeric sponge with a ceramic slurry, which is then burned out to leave a porous ceramic. The direct foaming method can produce both open-cell and closed-cell structures, with the foam structure more common. In this method, a chemical mixture containing the desired ceramic component and organic materials is treated to evolve a gas. Bubbles are then produced in the material, causing it to foam. The resulting porous ceramic material is then dried and fired. For the honeycomb or cellular structure, a plastic-forming method called extrusion is used, where a mixture of ceramic powder plus additives is forced through a shaped die (like play dough). The cellular structure can also be produced using a pressing method.
Selecting the sponge
- 1 First, a polymeric sponge must be selected with suitable properties. The pore size of the sponge determines the pore size of the final ceramic after firing. It must also be able to recover its original shape and convert into a gas at a temperature below that required to fire the ceramic. Polymers that can satisfy these requirements include polyurethane, cellulose, polyvinyl chloride, polystyrene, and latex. Typical polymeric sponges range in size from 3.94-39.4 in (10-100 cm) in width and 0.394-3.94 in (1-10 cm) in thickness.
Preparing the slurry
- 2 After the sponge is selected, the slurry is made by mixing the ceramic powder and additives in water. The ceramic powder usually consists of particles less than 45 microns in size. The amount of water can range from 10-40% of the total slurry weight.
Immersing the sponge
- 3 Before immersion, the sponge is usually compressed to remove air, sometimes using a mechanical plunger several times. Once it is immersed in the slurry, the sponge is allowed to expand and the slurry fills the open cells. The compression/expansion step may be repeated to achieve the desired density.
Removing excess slurry
- 4 After infiltration, between 25-75% of the slurry must be removed from the sponge. This is done by compressing the sponge between wooden boards, centrifuging, or passing through preset rollers. The gap between rollers determines the amount removed. Sometimes the impregnated foam goes through another shaping step since it is still flexible.
- 5 The infiltrated sponge is then dried using one of several methods—air drying, oven drying, or microwave heating. Air drying takes from eight to 24 hours. Oven drying takes place between 212-1,292° F (100-700° C) and is completed in 15 minutes to six hours.
Burning out the sponge
- 6 Another heating step is required to drive off the organics from the slurry and burn out the sponge. This takes place in air or inert atmosphere between 662-1,472° F (350-800° C) for 15 minutes to six hours at a slow and controlled heating rate to avoid blowing apart the ceramic structure. The temperature depends on the temperature at which the sponge material decomposes.
Firing the ceramic
- 7 The ceramic structure must be heated to temperatures between 1,832-3,092° F (1,000-1,700° C) to densify the material at a controlled rate to avoid damage. The firing cycle depends on the specific ceramic composition and the desired final properties. For instance, an aluminum oxide material may require firing at 2,462° F (1,350° C) for five hours.
Raw materials usually must meet requirements regarding composition, purity, particle size, and other properties. Properties monitored and controlled during manufacturing are usually dimensional and then design specific. For foam filters, the weight of the filter must be measured to determine coating efficiency. Extruded filters are measured for density. Both parameters relate to strength properties.
The manufacturing process is carefully controlled to minimize waste. In general, excess slurry cannot be recycled since it could change the purity and solid loadings of the original slurry, thereby affecting final properties.
The metal casting market is expected to decline by 2.7% in 1999, mainly because of the weakening global economy, with total shipments expected to reach 14.5 million tons. Sales will increase slightly to $28.8 billion. Though casting shipments will continue to decline slightly in 2000 and 2001, over the long term, shipments are expected to reach almost 18 million tons in 2008, with sales of $45 billion. Shipments and sales will see 10-year growth rates of 1.7% and 4.75%, respectively.
The increased use of lighter-weight metal components, such as aluminum die castings, has spurred growth in the automotive sector. Today, there is an average of 150 lb (68.1kg) of aluminum castings per vehicle, an amount projected to grow to 200 lb (90.8 kg) per year by the year 2000.
Ceramic filters will continue to play an important role in producing quality castings and will follow the growth of the casting market. Dollar volume may decrease due to continued price reductions. Quality and productivity demands for metal castings are increasing the need for filters since they provide a fast and reliable way to obtain good castings. Thus, casting buyers are specifying "filtered" more and more often.
Where to Learn More
Ishizaki, Kozo et al., ed. Porous Materials, Ceramic Transactions. The American Ceramic Society, 1993.
"Metal pouring/filtering." Foundry Management & Technology (January 1996): C2-C6.
Outen, John. "Reduce defects with direct pour technology." Foundry Management & Technology (August 1996): 108-111.
Saggio-Woyansky, J., C. Scott, and W. Minnear. "Processing of Porous Ceramics." American Ceramic Society Bulletin (November 1992): 1674-1682.
Hamilton Porcelains Ltd. Hamilton Technical Ceramics. 25 Campbell Street, Box 594, Brantford, Ontario, Canada N3t 5N9. (519) 753-8454. Fax: (519) 753-5014. http://www.hamiltonporcelains.com/.
Kirgin, Kenneth. "1999 Contraction to cause demand to dip to 14.5 million tons." Modern Casting Online (January 1999). Http://www.moderncasting.com/archive/feature-026_01.html/.
Sand is a loose, fragmented, naturally-occurring material consisting of very small particles of decomposed rocks, corals, or shells. Sand is used to provide bulk, strength, and other properties to construction materials like asphalt and concrete. It is also used as a decorative material in landscaping. Specific types of sand are used in the manufacture of glass and as a molding material for metal casting. Other sand is used as an abrasive in sandblasting and to make sandpaper.
Sand was used as early as 6000 b.c. to grind and polish stones to make sharpened tools and other objects. The stones were rubbed on a piece of wetted sandstone to hone the cutting edge. In some cases, loose sand was scattered on a flat rock, and objects were rubbed against the sandy surface to smooth them. The first beads with a glass glaze appeared in Egypt in about 3,500-3,000 b.c. The glass was made by melting sand, although naturally-occurring glass formed by volcanic activity was probably known long before that time.
In the United States, sand was used to produce glass as early as 1607 with the founding of the short-lived Jamestown colony in Virginia. The first sustained glass-making venture was formed in 1739 in Wistarburgh, New Jersey, by Caspar Wistar. The production of sand for construction purposes grew significantly with the push for paved roads during World War I and through the 1920s. The housing boom of the late 1940s and early 1950s, coupled with the increased use of concrete for building construction, provided another boost in production.
Today, the processing of sand is a multi-billion dollar business with operations ranging from very small plants supplying sand and gravel to a few local building contractors to very large, highly automated plants supplying hundreds of truckloads of sand per day to a wide variety of customers over a large area.
The most common sand is composed of particles of quartz and feldspar. Quartz sand particles are colorless or slightly pink, while feldspar sand has a pink or amber color. Black sands, such as those found in Hawaii, are composed of particles of obsidian formed by volcanic activity. Other black sands include materials such as magnetite and homblende. Coral sands are white or gray, and sands composed of broken shell fragments are usually light brown. The white sands on the Gulf of Mexico are made of smooth particles of limestone known as oolite, derived from the Greek word meaning egg stone. The white sands of White Sands, New Mexico, are made of gypsum crystals. Ordinarily, gypsum is dissolved by rain water, but the area around White Sands is so arid that the crystals survive to form undulating dunes.
Quartz sands, which are high in silica content, are used to make glass. When quartz sands are crushed they produce particles with sharp, angular edges that are sometimes used to make sandpaper for smoothing wood. Some quartz sand is found in the form of sandstone. Sandstone is a sedimentary, rock-like material formed under pressure and composed of sand particles held together by a cementing material such as calcium carbonate. A few sandstones are composed of almost pure quartz particles and are the source of the silicon used to make semiconductor silicon chips for microprocessors.
Molding sands, or foundry sands, are used for metal casting. They are composed of about 80%-92% silica, up to 15% alumina, and2% iron oxide. The alumina content gives the molding sand the proper binding properties required to hold the shape of the mold cavity.
Sand that is scooped up from the bank of a river and is not washed or sorted in any way is known as bank-run sand. It is used in general construction and landscaping.
The definition of the size of sand particles varies, but in general sand contains particles measuring about 0.0025-0.08 in (0.063-2.0 mm) in diameter. Particles smaller than this are classified as silt. Larger particles are either granules or gravel, depending on their size. In the construction business, all aggregate materials with particles smaller than 0.25 in (6.4 mm) are classified as fine aggregates. This includes sand. Materials with particles from 0.25 in (6.4 mm) up to about 6.0 in (15.2 cm) are classified as coarse aggregates.
Sand has a density of 2,600-3,100 lb per cubic yard (1,538-1,842 kg per cubic meter). The trapped water content between the sand particles can cause the density to vary substantially.
The preparation of sand consists of five basic processes: natural decomposition, extraction, sorting, washing, and in some cases crushing. The first process, natural decomposition, usually takes millions of years. The other processes take considerably less time.
The processing plant is located in the immediate vicinity of the natural deposit of material to minimize the costs of transportation. If the plant is located next to a sand dune or beach, the plant may process only sand. If it is located next to a riverbed, it will usually process both sand and gravel because the two materials are often intermixed. Most plants are stationary and may operate in the same location for decades. Some plants are mobile and can be broken into separate components to be towed to the quarry site. Mobile plants are used for remote construction projects, where there are not any stationary plants nearby.
The capacity of the processing plant is measured in tons per hour output of finished product. Stationary plants can produce several thousand tons per hour. Mobile plants are smaller and their output is usually in the range of 50-500 tons (50.8-508 metric tons) per hour.
In many locations, an asphalt production plant or a ready mixed concrete plant operates on the same site as the sand and gravel plant. In those cases, much of the sand and gravel output is conveyed directly into stockpiles for the asphalt and concrete plants.
The following steps are commonly used to process sand and gravel for construction purposes.
- 1 Solid rock is broken down into chunks by natural mechanical forces such as the movement of glaciers, the expansion of water in cracks during freezing, and the impacts of rocks falling on each other.
- 2 The chunks of rock are further broken down into grains by the chemical action of vegetation and rain combined with mechanical impacts as the progressively smaller particles are carried and worn by wind and water.
- 3 As the grains of rock are carried into waterways, some are deposited along the bank, while others eventually reach the sea, where they may join with fragments of coral or shells to form beaches. Wind-borne sand may form dunes.
- 4 Extraction of sand can be as simple as scooping it up from the riverbank with a rubber-tired vehicle called a front loader. Some sand is excavated from under water using floating dredges. These dredges have a long boom with a rotating cutter head to loosen the sand deposits and a suction pipe to suck up the sand.
- 5 If the sand is extracted with a front loader, it is then dumped into a truck or train, or placed onto a conveyor belt for transportation to the nearby processing plant. If the sand is extracted from underwater with a dredge, the slurry of sand and water is pumped through a pipeline to the plant.
- 6 In the processing plant, the incoming material is first mixed with water, if it is not already mixed as part of a slurry, and is discharged through a large perforated screen in the feeder to separate out rocks, lumps of clay, sticks, and other foreign material. If the material is heavily bound together with clay or soil, it may then pass through a blade mill which breaks it up into smaller chunks.
- 7 The material then pass through several / perforated screens or plates with different hole diameters or openings to separate the particles according to size. The screens or plates measure up to 10 ft (3.1 m) wide by up to 28 ft (8.5 m) long and are tilted at an angle of about 20-45 degrees from the horizontal. They are vibrated to allow the trapped material on each level to work its way off the end of the screen and onto separate conveyor belts. The coarsest screen, with the largest holes, is on top, and the screens underneath have progressively smaller holes.
- 8 The material that comes off the coarsest screen is washed in a log washer before it is further screened. The name for this piece of equipment comes from the early practice of putting short lengths of wood logs inside a rotating drum filled with sand and gravel to add to the scrubbing action. A modern log washer consists of a slightly inclined horizontal trough with slowly rotating blades attached to a shaft that runs down the axis of the trough. The blades churn through the material as it passes through the trough to strip away any remaining clay or soft soil. The larger gravel particles are separated out and screened into different sizes, while any smaller sand particles that had been attached to the gravel may be carried back and added to the flow of incoming material.
- 9 The material that comes off the intermediate screen(s) may be stored and blended with either the coarser gravel or the finer sand to make various aggregate mixes.
- 10 The water and material that pass through the finest screen is pumped into a horizontal sand classifying tank. As the mixture flows from one end of the tank to the other, the sand sinks to the bottom where it is trapped in a series of bins. The larger, heavier sand particles drop out first, followed by the progressively smaller sand particles, while the lighter silt particles are carried off in the flow of water. The water and silt are then pumped out of the classifying tank and through a clarifier where the silt settles to the bottom and is removed. The clear water is recirculated to the feeder to be used again.
- 11 The sand is removed from the bins in the bottom of the classifying tank with rotating dewatering screws that slowly move the sand up the inside of an inclined cylinder. The differently sized sands are then washed again to remove any remaining silt and are transported by conveyor belts to stockpiles for storage.
- 12 Some sand is crushed to produce a specific size or shape that is not available naturally. The crusher may be a rotating cone type in which the sand falls between an upper rotating cone and a lower fixed cone that are separated by a very small distance. Any particles larger than this separation distance are crushed between the heavy metal cones, and the resulting particles fall out the bottom.
Most large aggregate processing plants use a computer to control the flow of materials. The feed rate of incoming material, the vibration rate of the sorting screens, and the flow rate of the water through the sand classifying tank all determine the proportions of the finished products and must be monitored and controlled. Many specifications for asphalt and concrete mixes require a certain distribution of aggregate sizes and shapes, and the aggregate producer must ensure that the sand and gravel meets those specifications.
The production of sand and gravel in many areas has come under increasingly stringent restrictions. The United States Army Corps of Engineers, operating under the Federal Clean Water Act, has required permits for sand extraction from rivers, streams, and other waterways. The cost of the special studies required to obtain these permits is often too expensive to allow smaller companies to continue operation. In other cases, residential development in the vicinity of existing aggregate processing plants has led to restrictions regarding noise, dust, and truck traffic. The overall result of these restrictions in certain areas is that sand and gravel used for construction will have to be transported from outside the area at a significantly increased cost in the future.
Where to Learn More
Brady, George S. and Henry R. Clauser. Materials Handbook, 12th Edition. McGraw-Hill, 1986.
Hornbostel, Caleb. Construction Materials, 2nd Edition. John Wiley and Sons, Inc., 1991.
Siever, Raymond. Sand. W.H. Freeman and Company, 1988.
Grover, Jennifer E., Bob Drake, and Steven Prokopy. "100 Years of Rock Products, History of an Industry: 1896-1996." Rock Products, July 1996, pp. 29+.
Mack, Walter N. and Elizabeth A. Leistikow. "Sands of the World." Scientific American, August 1996, pp. 62-67.
Miller, Russell V. "Changes in Construction Aggregate Availability in Major Urban Areas of California Between the Early 1980s and the Early 1990s." California Geology, January/February 1997, pp. 3-17.