PCA R&D Serial No. 2868

Beneficial Reuse of Materials in the Cement Manufacturing Process by Schreiber, Yonley & Associates

©Portland Cement Association 2007 All rights reserved

KEYWORDS Alternative, cement manufacturing, emissions, energy, environment, fuels, raw materials.

ABSTRACT Cement manufacturers are increasingly evaluating and using alternative materials for raw materials and fuel in the cement manufacturing process. This common practice is growing throughout the world and in the United States. The reuse of alternative materials in the manufacturing process provides a proven method for the recovery of energy and material values from materials that would otherwise be considered wastes. The beneficial use program results in decreased reliance on naturally occurring raw materials and fuels for the process, and at the same time provides a safe and effective use for by-products. The industry’s goal for the future is to continue to emphasize the beneficial reuse of materials, meeting EPA’s objectives for resource recovery and contributing to the reduction of greenhouse gases and other criteria pollutants. This document is intended to describe the advances made by the cement manufacturing industry by describing the manufacturing process, the factors for evaluating alternative materials, and the types of fuel and raw material alternatives that can be used beneficially.

REFERENCE Schreiber, Yonley & Associates, Beneficial Reuse of Materials in the Cement Manufacturing Process, SN2868, Portland Cement Association, Skokie, Illinois, USA, 2007, 11 pages.

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Beneficial Reuse of Materials in the Cement Manufacturing Process by Schreiber, Yonley & Associates* INTRODUCTION Increasingly, cement companies are using alternative materials as a replacement for traditional materials used in their manufacturing process. As part of the growing worldwide practice of sustainable development, this proven technology conserves virgin non-renewable raw materials and fuels and provides great benefits to the environment as well as the cement manufacturers. One of the benefits is that it provides an opportunity to reuse materials that would otherwise be discarded. This type of recycling not only reduces the pollution and energy necessary to extract virgin materials, but it can also reduce air pollution such as greenhouse gases from the cement manufacturing process. Because this recycling is conducted under stringent safety and environmental standards, it meets EPA's objectives for resource recovery and pollution prevention. There are several critical factors that cement manufacturers must evaluate and control in order to successfully reuse alternative materials. The primary considerations are: 1) to accept only materials that can be processed without negatively affecting product quality; 2) to maintain an evaluation and acceptance program that ensures that environmental, health, and safety standards are maintained; 3) to effectively communicate the program to employees, neighbors, regulators, and other stakeholders. This document is intended to describe the advances made by the cement manufacturing industry by describing the manufacturing process, the factors for evaluating alternative materials, and the types of fuel and raw material alternatives that can be used beneficially.

THE CEMENT MANUFACTURING PROCESS The goal in any industrial process is to produce a quality product more efficiently and at a lower unit cost. Evaluating and reusing new alternative sources of raw materials and fuels are important factors in accomplishing these goals. The cement manufacturing industry is very energy intensive, and it is a raw material driven industry. In general, alternative fuel and raw materials that can lower operational cost without negatively affecting cement quality have been, and are being, evaluated for use in the manufacturing process. For the last 20 years, the industry has evaluated and used a broad variety of alternative raw materials and fuels while continuing to make quality product and meet environmental standards. In order to better understand the beneficial use concept for alternative materials in the cement manufacturing process, it is important to first understand the basic manufacturing process steps and feed requirements. The manufacturing of cement is a large-scale mass production process using large volumes of raw materials. The raw materials to manufacture cement include *

Schreiber, Yonley & Associates, 271 Wolfner Drive, Fenton, MO 63026-2801 USA, (636) 349-8399 www.perma-fix.com/sya.

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sources of calcium carbonate, silica, alumina, and iron. The raw mix typically includes about 80% calcium carbonate, about 10% to 15% silica, and small amounts of alumina and iron. Traditional sources of these ingredients include limestone, marl, or chalk for the calcareous component; clay, shale, slate, or sand to provide the silica and alumina; and iron, mill scale, or similar material to provide the iron components. The raw materials are interground and blended to a uniform chemical composition known as raw feed or kiln feed. The kiln feed is fed to a rotary kiln for pyroprocessing. Modern rotary cement kilns are cylindrical, refractory-lined, rotating steel ovens ranging in length from 200 to 760 feet, with a diameter of 10 to 25 feet. The kiln feed (or raw mix) is introduced to the “cold end” of the kiln system, with the rotary kiln slightly elevated at the feed end. As the raw feed tumbles down the length of the kiln toward the flame in the lower end of the kiln, the raw materials are transformed physically and chemically by being exposed to extremely high temperatures. There are essentially four stages to the pyroprocessing that occurs in the kiln system: evaporation and preheating, calcining, clinkering, and cooling. Fuel normally is fired at the front of the kiln or in the preheater tower, although in some kiln designs some of the fuel may be fired at more than one location as described below. The hot kiln gases are drawn countercurrent to the material flow by an induced draft fan, providing for heat exchange between the process gases and the kiln feed. Evaporation and preheating remove moisture from the kiln feed and raise the feed temperature to about 1650°F. At this temperature, calcining takes place and breaks the calcium carbonate down into calcium oxide and carbon dioxide. Clinkering completes the pyroprocessing and fuses the calcined material into hard nodules called clinker. This clinkering process occurs in the hottest zone of the kiln, called the burning zone, where the material temperature reaches about 2700°F. Clinkering is critical to the manufacturing process and requires accurate control of energy input. Insufficient energy input will keep the necessary chemical reactions from occurring in the clinker, which may result in unreacted lime in the clinker, thereby producing poor quality cement. Too much energy input will shorten the life of the refractory lining of the kiln and may damage the kiln shell. Large amounts of fuel are necessary to drive the pyroprocessing that occurs in the kiln system. Typical fuels used are coal, petroleum coke, natural gas, oil, and/or alternative materials burned for energy recovery. In a typical kiln, 3 to 6 million Btus are required to produce a ton of clinker. This can be between 400-500 tons of coal equivalents per day. In the kiln, the kiln feed is converted to calcium silicates and calcium aluminates that give cement the hardening characteristics on reaction with water. The production of 1 ton of clinker requires about 1.55 tons of kiln feed because about 35% of the kiln feed is released as carbon dioxide. Modern kilns produce 4 to 5 thousand tons of clinker per day. This requires processing between 6 to 8 thousand tons of raw materials daily. Due to the large amounts of fuel and raw materials required to produce cement clinker, there is a production incentive in the cement industry to find alternate fuels and materials compatible with the manufacturing process, where cost-effectiveness of the operations can be improved. Two main pyroprocessing system types are used in the industry – wet process and dry process. In a wet process, the raw materials are mixed and ground with water (approximately 30% - 35%) to produce slurry. The slurry simplifies the blending of the kiln feed to produce a uniform chemical composition. This process requires more thermal energy since the water must be evaporated from the feed. The kiln must also be lengthy, since all stages of the pryoprocessing occur inside the kiln.

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Dry process kilns are further divided into three types – long dry, preheater, and precalciner. A long dry kiln is similar to a wet kiln with the exception that the raw materials are fed to the kiln as a dry powder, therefore requiring less energy to produce clinker. The preheater kiln system also uses dry raw materials, but the feed passes through a tower containing one or more cyclone-type vessels before entering the rotary kiln. The kiln feed is suspended by the hot countercurrent kiln gases prior to entering the kiln, and the preheating phase of the process is accomplished in the tower. This allows a rapid and more efficient energy transfer between the hot kiln gases and kiln feed. A precalciner kiln system has a tower with preheater vessels but also contains a secondary combustion vessel, called a calciner, near the lower stages of the preheater. Fuel is added to the kiln feed at this location, and the feed is about 90% calcined in that vessel. In this process, evaporation, preheating, and most of the calcining take place before the feed enters the kiln. Preheater and precalciner systems require shorter kilns, and the process is the most energy-efficient from a fuel standpoint. Approximately 60% of the overall fuel requirements for a precalciner system are at the calciner versus the main burner. The final step in clinker manufacturing for each process type is cooling the clinker from approximately 2700°F to 200°F by discharging the clinker from the kiln into a clinker cooler where ambient air is forced through a slowly moving bed of clinker for cooling to freeze its molecular structure. Typically, the air used to cool the clinker is recovered and used for combustion air in the kiln. The cooled clinker is conveyed to a ball mill for finish grinding. In the finish mill system, the clinker is interground with a small amount of gypsum (CaSO4 • 2H2O, an agent that controls the setting time of the concrete) into the fine gray powder that is known as portland cement. At this point, the product is ready for distribution in commerce. For the cement industry, as with other industries, it is critical to control the emissions and by-products from manufacturing. The exhaust gas from a kiln system comprises excess air, combustion products, alkali salts, carbon dioxide, and water vapor. Cement kilns employ air pollution control devices including electrostatic precipitators (ESPs) and baghouses to “clean” exhaust gases prior to their exiting the stack. Emissions are monitored and controlled to comply with the prevailing environmental standards. An inherent by-product of the cement manufacturing process is cement kiln dust (CKD). CKD consists of finely ground and partially calcined raw feed that becomes entrained in the countercurrent gas flow through the kiln system. CKD is collected in the air pollution control devices and is either returned to the process as feed, reused beneficially offsite, or in some cases, removed for disposal.

CRITICAL PROGRAM EVALUATION FACTORS When a cement manufacturer evaluates a new alternative raw material or fuel stream, there is a range of site-specific issues that deserve consideration. The challenge is to identify and secure sources of fuels and raw materials that can be economically handled through existing or add on material handling systems, without causing product quality problems, operational upsets, or environmental, health, and safety issues. A careful evaluation of candidate materials is necessary, and can be accomplished between the industry generating the by-product and the cement manufacturer to determine if there is a good fit for the material. Several key evaluation questions for materials being considered are: • Will the use of the material improve the cost-effectiveness of operations? • Does the material fit the mix design of the plant or negatively affect cement quality? • Are there constituents in the material that will negatively affect kiln operations?

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• • •

Is the material compatible with regulatory requirements? Are there health and safety issues? Is there any public/employee communication needed?

Often a cement manufacturing facility has a good history with identifying and accepting alternate materials and has an established set of criteria that a material must meet. However, each material must be separately evaluated to ensure that each criterion can be met and that plans are in place for effective handling and use of the material. The generator knows its material the best and can work with the cement manufacturing facility to fully characterize the material.

Cost Effectiveness The first question always asked is whether the cost-effectiveness of the operation will be improved by using an alternate raw material or fuel. If the material causes an increase in the unit cost of the manufacturing process, there is little likelihood that the material will be considered for use. However, there are many economic factors to consider besides the actual cost (positive or negative) of the material. Some of these factors are: How will the new material affect the use of other raw materials? There may be a source of a raw material that is less expensive than one of the raw materials presently being used. However, when this raw material is evaluated there may be an increase in the use of another expensive raw material component. For example, substituting a less expensive iron ore for fly ash might cause an increase use of an even more expensive source of aluminum and may result in an overall increase in the cost per ton of the raw mix. Each new raw material must be evaluated in terms of the overall cost of all raw materials, and not as a stand-alone ingredient. Are there material handling challenges with the material? Many potential raw materials and fuels are available at a lower cost than those presently used but may present very difficult handling problems. These problems can result in the new material increasing costs to the process. Will add-on material handling systems be required? Most alternative materials are subject to their generation, and a plant cannot rely on their availability. This obligates the plant to maintain duplicative material handling systems. A system suitable to handling typical feedstocks and fuels that is readily available may not be suitable for an alternative material. Operation and maintenance of duplicative systems must be off set by the cost saving achieved by the proposed beneficial use. Even when existing material handling systems can be used the material may not be available in a form suitable for direct use with that system. For instance, some sources of potential fuels are only available in a bulk bag or container. Transferring the material into a silo can be a very expensive handling process, resulting in an overall cost increase. Some raw materials and fuels that are available contain high moisture content. These materials may not present problems during warm weather; however, during periods of cold weather, handling may be almost impossible. Another material handling problem concerns the grindability of the material. If a fuel or raw material increases the energy requirements for grinding the material or causes excessive wear to the mill components, the material may cause an actual increase in cost. Some of the handling problems can be overcome with capital expenditures; however, there must be significant return on investment before the expenditure is warranted. These projects will also involve environmental permitting considerations.

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Is there sufficient quantity of the new material for consideration of its use? Many times a supplier of a new raw material or fuel will approach a cement manufacturer stating the availability of this new material in large quantities. On further investigation, it is determined that the supplier produces 10 tons per month. This might appear to be a large quantity to the supplier, but to the cement plant that uses 10 tons of the material per hour, this amount is trivial. If there are handling costs associated with the material, there is less chance the material will be accepted.

Mix Designs Another question to be answered is whether the alternative material affects the mix design of the plant. The mix design is the specific combination of raw materials necessary to produce kiln feed with the desired chemical specifications. The first requirement for consideration of a new material is a detailed analysis of the chemical constituents of the material that are pertinent to the mix design for the particular plant. Each plant has specific chemical requirements for the cement being manufactured at the plant, and if the new material causes the product to fall outside the plant’s specifications, it will not be possible to use the material. For instance, if the plant’s market calls for low aluminum content, a raw material with very high aluminum content may not be acceptable. For raw materials, the necessary analysis includes: calcium oxide, silica, alumina, iron oxide, magnesium, and alkali content (i.e., potassium, sodium). Two very important factors in a raw material for many plants are the magnesium and alkali content. Although the levels of these two constituents in cement are site-specific, if the plant is already at the concentration limit for these constituents, the addition of additional magnesium or alkali content can be detrimental to the cement quality. Both of these constituents can cause the failure of concrete above certain limits. Quality impacts at one facility may result in quality enhancements at another facility. Different raw material characteristics at one site may increase the cement setting time outside the acceptable range, while the same increased setting time at another plant may result in enhanced cement performance. Fuels also can have a considerable effect on the plant’s mix design. If an alternative fuel is used that has less ash content than the coal that it replaces, this must be accounted for by altering the raw mix design for the change. Although the ash change may appear small, there can be a significant effect on the quality of the cement and the burnability of the mix in the kiln. Therefore, the timing and quantity of fuel receipts must also be considered, since raw material mix batches are dependent on the type of manufacturing equipment used by the facility.

Factors Affecting Kiln Operations There are several constituents in alternate materials that can have an effect on the kiln operations. These include: (1) sulfur content, (2) alkali content, (3) chlorine content, and (4) other minor constituents. Changes in the amount of sulfur entering the kiln system for raw materials or fuels can have operational effects on the kiln. Either increases or decreases in the sulfur content can upset the sulfur balance and cause the buildup of rings in long wet kilns, and plugging of the lower stage vessels of preheater and precalciner kilns. Normally, there is an alkali-to-sulfur balance in a kiln system under which kiln operational problems are minimized. When the balance is outside these limits, there can be severe operational ramifications. The amount of chlorine entering the system also can have an adverse effect on buildups in kiln systems.

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Chlorine introduced into the kiln system combines with alkali and sulfates to form low melting mixtures, and the effect upon the operation of the kiln can be so serious that a limitation of the chlorine input is warranted. If the kiln clinker production is reduced from buildups in the kiln due to adding chlorine to the system, the potential cost saving from the lower cost raw material will quickly be nullified. Additional chlorine to the system also can affect the operation of an electrostatic precipitator. The chlorine can result in changes to the CKD, causing it to be more difficult to collect in the pollution control device. Some minor constituents also can cause kiln operational problems. For instance, excess fluorine entering the kiln acts as a flux, causing the clinkering process to be completed at a lower temperature. If care is not taken by removing fuel from the kiln on the addition of a flux, the kiln materials can liquefy, causing significant operational problems.

Compatibility with Regulations and Permits One benefit inherent to the cement industry is the kiln’s capability to utilize a wide variety of raw materials and fuels in the manufacturing process. Many alternative materials contain either heat value or raw material ingredients that meet the needs of the cement manufacturing process, and many materials have both. There are several items needed by the cement manufacturer to determine the need for obtaining regulatory approvals before acceptance of any new material. These items include: (1) a Material Safety Data Sheet (MSDS), (2) a Toxicity Characteristic Leaching Procedure (TCLP) analysis, (3) additional analyses for some materials, and (4) a description of the process generating the material. The MSDS contains several pieces of information important to the plant. For example, it notes if there are reportable quantities of Toxic Release Inventory (TRI) constituents in the material. Federal regulations dictate reporting quantities of these constituents. Also, the MSDS identifies any hazardous constituents in the material that can affect the health and safety of the employees, which is critical information as described below. Lastly, the MSDS will show if the material has any hazardous constituents that may be regulated under the Resource Conservation and Recovery Act (RCRA). The acceptance of listed hazardous waste is highly regulated, and in no case should a raw material be accepted that is a listed hazardous waste unless the company has clear approvals to do so. Another item needed for the acceptance of any raw material is a TCLP analysis of the material. The TCLP results along with other available data assist in identifying any additional constituents that may prohibit the material’s use. NSR and PSD reviews may impact permit modifications. NSR requires examination of recent actual emissions. These actual emissions can become the new allowable levels, even if the recent emissions are impacted by a short term fuel or raw material use. An example of this scenario would be if a facility has recently changed their permit to use a low sulfur coal, thus lowering their sulfur dioxide emissions. If the facility decides to return to the original fossil fuel supply, PSD/NSR permitting may be triggered due to the potential increase in sulfur dioxide emissions. The use of alternative materials or fuels must justify such long term changes.

Health and Safety From a facility and employee health and safety perspective, knowing the character and constituents of the material is also critical. Just as with an environmental evaluation, the ability

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of the facility to safely handle materials and have the appropriate training and personal protective equipment for employees is a very high priority. Because the generator of the material typically knows its material the best, it is important that the generator work with the receiving facility to ensure that any health and safety concerns are addressed. As described above, an MSDS, along with other material information, is critical for communicating health and safety, environmental, and product quality information. Any precautions identified must be addressed.

Communication Also critical to large manufacturing facilities is the ability to effectively communicate to the employees, neighbors, regulators, and other stakeholders any new programs that use alternative materials. Public perception and being a good corporate neighbor go a long way toward the acceptance of any new program, particularly the use of alternative materials and fuels.

RAW MATERIAL ALTERNATIVES Although the items discussed above concentrated on potential obstacles to the use of alternative materials, there are many sources of the materials that can be found that would otherwise be discarded and that have the necessary constituents to be beneficial to the cement manufacturing process. These sources are varied, and many manufacturers have the types of by-products that can be used in the process. By-products or wastes that are suitable for raw material alternatives are fairly consistent in composition, large volume materials for which the cement chemistry can be adjusted to maintain a consistent quality product. Large volume, low toxicity materials can provide the necessary ingredients for the process. There are many examples of alternative raw materials that have been used in the cement manufacturing process. The following presents a few examples with the elements that each material contributes to the raw mix: • • • • •

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Mill scale (Fe, Si, Al) Filter cake (element varies) Cracking catalysts (Si, Al) Blast furnace slag (Si, Fe, Al) Foundry sand (Si)

Petroleum contaminated soil (Si, Al) Bottom ash (Fe, Si, Al) Water treatment sludges (Al, Si) Fly ash (Fe, Si, Al) Refractory brick (Al, Si)

There are also materials that may not fit well in the process. Several examples of materials or constituents that may not be beneficial for use are described in the section above on factors that affect kiln operations. The key is to know the constituents of the material and whether they substantially satisfy the chemical needs of the process. Additionally, depending on the type of minor constituents, the material may need to be fed at a particular location in the kiln (i.e., materials with organic constituents need to be subjected to combustion temperatures for complete destruction). Each of the critical criteria needs to be evaluated on a site-specific basis.

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FUEL ALTERNATIVES Cement kilns require large quantities of fuel to drive the process, and the source of fuel traditionally has been coal or petroleum coke. With the continued increase in coal and coke pricing, cement manufacturers are searching continually for alternative fuel sources. Traditional fossil fuels such as coal can be replaced readily by a wide range of materials containing fuel value to reduce a kiln’s dependence on non-renewable fossil fuels. Many by-products and waste materials contain fuel value that can be used by the cement manufacturing process while destroying the organic constituents inherent in the material. In addition to reducing the use of non renewable virgin materials, this provides an opportunity to keep the material out of the disposal system and provides a safe and effective use of the material. A key criterion in matching up alternative fuels at a cement manufacturing facility is the physical consistency of the fuel versus the type of fuel feed systems already in place. Alternative fuels vary from liquids to solids. Viscous liquids or sludges are examples of fuel compositions that have been used successfully. The most important analytical information the supplier needs to provide to evaluate alternate fuels are:1) heating value of the fuel in Btu/lb, 2) ash content of the fuel, 3) moisture content of the fuel, and 4) halogen content of the fuel. Fuels generally can be utilized at various locations in the kiln system: (1) at the hot end of the kiln and (2) in the calcining zone of the kiln (3) in the feed shelf of a dry process kiln, and (4) mid kiln. Fuels fired in the hot end generally need to have a relatively high heat value as fired to maintain the high flame temperature required at the front of the kiln. Fuel fired at the hot end of the kiln also must be finely divided enough to promote rapid combustion, which is important to clinker quality. However, a variety of fuels with lower heat values and larger sizing can be accommodated in other firing zones of the kiln. This zone is in the secondary furnace for precalciner kilns and generally at the mid-kiln for long kilns. One important factor for fuel is the consistency. If each load has a different consistency, maintaining stable kiln operations, maximum kiln production, and product quality using the fuel will be difficult. There are many types of alternative fuels that can be used in the cement manufacturing process. The following presents a few examples with the approximate range of the heat values for these fuels: • • • • • • • •

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Scrap tires (13000-15000 Btu/lb) Plastics (10000-16000 Btu/lb) Municipal refuse (4000-8000 Btu/lb) Coal tar sludge (7000-10000 Btu/lb) Meat & bone meal (4000-8000 Btu/lb) Carbon black residue (12000-14000 Btu/lb) Off-spec consumer products (Btu/lb varies) Spent water treatment resins (6000-12000 Btu/lb)

• • • • • • •

Oils (including used and waste) or solvents (Btu/lb varies) Wood products (5000-8000Btu/lb) Rice hulls (6000-8000 Btu/lb) Carbon fly ash (900-1500 Btu/lb) Spent activated carbon (10000-12000 Btu/lb) Spent toner (12000-15000 Btu/lb) Spent aluminum potliner (4000-8000 Btu/lb) Hazardous Waste (8000-13000 Btu/lb)

During the past several years many plants have successfully used scrap tires in the process, and the use of alternate materials such as plastics, carbon black, and waste oils has been 8

increasing. There are other potential fuel candidates that have not had as much acceptance in the cement industry. One example is the use of waste carpets. There is an abundance of waste in the carpet industry that is presently being landfilled, and the carpet manufacturing industry has approached cement manufacturers about using this material as an alternate fuel. At the present time, no kiln is using this material. The major determents to its use involve physically getting the material into the manufacturing process and the transportation cost. Other materials, such as spent toner, have only had marginal success as a fuel replacement. Spent toner is a good candidate for an alternate fuel in that it has high heat content and is very finely ground. However, most toner manufacturers store the toner in super sacks, and transferring the toner from these containers to silos has proven to be difficult. While all kiln systems are capable of destroying the organic constituents while using the fuel value in these materials, there are some materials that the industry historically has not accepted typically due to public perception issues. The following presents some examples of materials that the industry has not promoted as alternative fuels, although from a product quality and environmental standpoint, they potentially could be used if the perception issue were overcome: • •

• •

Dioxin-containing waste PCB-containing waste

Radioactive materials Medical wastes

Additionally, some fuel sources may have levels of metal or other inorganic constituents that may cause environmental or product issues and therefore can not be used in the process. Many times the fuel and raw material feedrate can be adjusted to eliminate or mitigate these concerns.

SUMMARY The cement manufacturing industry is working to increase the practice of using alternative fuels and raw materials in the process to promote resource recovery and economic and environmental benefits. There are a variety of noncombustible by-products that are suitable as alternative raw materials. However, each material must be evaluated in terms of its effect on the entire manufacturing process and not considered in isolation. The major requirements for these materials are: cost effectiveness, material handling concerns, product quality issues, environmental consideration, health and safety issues, and public acceptance. To be used in the cement process, the material must contain constituents that are useful in the manufacturing process, there must be a practical cost effective method for handling and managing the material, and the material must be in sufficient quantity to make its use practical. The search for alternate fuels in the cement manufacturing process to replace more costly fossil fuels continues. During the past several years, tires have been used successfully in the process and have provided an alternate energy source to the cement manufacturer, as well as an environmental benefit. Recently, there has been an increase in the use of nonchlorine plastic as an alternative fuel, and other materials that contain heat value. There are many materials that are now being landfilled that have sufficient fuel value for use in manufacturing cement; however, at the present the economics for the use of these materials has not been attractive. Nonetheless, the cement industry will continue to explore alternative fuels that fit the demands of the manufacturing process.

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