Stone, Clay, Glass, and Concrete :
SIC Code 32
In 1994, 16 facilities with SIC Code 32 ("Stone, Clay, Glass, and Concrete")
submitted a New Jersey Release and Pollution Prevention Report to the NJDEP describing their use of hazardous chemicals in 1993. Since plants with SIC Code 32 are considered Group Two facilities by the NJDEP, they must prepare a Pollution Prevention Plan and submit a Pollution Prevention Plan Summary by July 1, 1996. As part of the preparation of a Pollution Prevention Plan, facilities are asked to find and analyze pollution prevention options and set reduction goals for hazardous chemical nonproduct output (NPO) and use.
The following pages list some pollution prevention techniques compiled by the United States Environmental Protection Agency (USEPA) that SIC Code 32 facilities may find useful to help set (and eventually meet) hazardous chemical NPO and use reduction goals.
Pollution Prevention and Waste Minimization Techniques
In the glass manufacturing industry, one opportunity for pollution prevention is increasing the use of waste glass, or cullet, as a feedstock, therby reducing the amount of cullet requiring disposal. Currently, about 67 percent of all cullet is landfilled or stockpiled. Glass manufacturers typically use 30 percent cullet along with raw materials to make new glass. Increasing the use of cullet reduces energy consumption, since it requires less energy to melt cullet than to melt other raw materials. One problem with using cullet is that the composition of the cullet may vary widely from the virgin batch, leading to product quality problems. Waste glass which is not reused on site can be used in the production of road materials (known as glasphalt).
Refractory scrap from glass facilities can also be recycled. Spent refractory brick can be used as a feedstock by brick manufacturers without affecting the quality of the final product. Since refractory bricks only have to be replaced approximately every ten years, recycling of this material is a relatively minor pollution prevention opportunity.
Glass container recycling has been increasing, from over 20 percent in 1988 to 37 percent in 1994. This recycling rate reflects the percentage of containers actually recycled by manufacturers, not just the percentage collected. Recycled container glass is used in the production of new bottles and jars as well as in secondary markets such as fiberglass and glasphalt.
A major quantity of hazardous waste generated from glass making is generated in the receiving and delivery areas. Improvements such as clean-up and maintenance in receiving areas can minimize this waste. Keeping the receiving areas clean would allow material spills to be collected and added to the raw materials. Also, by paving receiving areas, collection and clean-up becomes much more efficient and effective and allows spilled material to be identified and separated for recycling back into the process.
Air pollution control technologies used in the glass industry commonly transfer contaminants from one media (air) to another (water or hazardous waste). Process improvements can help reduce total waste generation and improve manufacturing efficiency. One available process improvement is called "Rapid Melting Systems," which involves preheating the batch prior to melting. This practice reduces process time, energy consumption, and air emissions. Another process improvement, which can reduce nitrogen oxide and particulate emissions, is the substitution of oxygen for combustion air. The drawbacks of using pure oxygen rather than air include oxygen's high cost and localized hot spots during combustion.
Souce reduction in the concrete industry can be achieved through raw material substitution. For example, many concrete product manufacturers have moved from volatile organic compound (VOC)-mold release agents to trichloroethane (TCA)-based agents due to air quality restrictions on VOC material. However, TCA has been added to the list of ozone depleting substances and will be phased out by 2002. Concrete product manufacturers that use TCA as a mold release are working with mold release manufacturers to develop alternatives, such as water-based mold-releases.
Alternative cement finishing processes, including the use of water-based and powder coatings, can reduce the amount of paint-related wastes generated by manufacturers of cement products. Water-based coatings can be applied by conventional spray, airless, or air assisted airless guns. Since water has a higher density than organic solvents, overspray is reduced and transfer efficiency is improved. Powder coatings, made by mixing resins with a hardener, pigments, and other additives, are 100 percent solids that are applied to parts of various shapes, sizes, and materials of construction. Transfer efficiencies in powder coating application are high, and no solvents are used in manufacturing or applying the coatings. Paint that does not adhere to the workpiece is collected and reused. Consequently, there are virtually no emissions and very little waste from powder coating systems. Powder coating systems require new application equipment, which can be a major capital cost for some companies.
Cement kiln dust is the largest waste stream produced by cement manufacturers. There are three primary means to decrease the amount of dust generated by a kiln. Dust can be minimized by reducing gas turbulence in the kiln and avoiding excessive flow velocities. The use of chains near the cool end of the kiln can also minimize dust by trapping the dust before it is released in the kiln exhaust. Most kilns are already equipped with such cool-end chain sections. The use of fuels with a low ash content, such as liquid hazardous wastes, can also reduce the amount of cement kiln dust generated.
Cement kiln dust collected from the baghouse dust collectors can be reused both on-site and off-site. Direct return of dust to the kiln is a common recycling practice. The dust may be returned to the hot end, to the middle of the kill, or to the feed material. However, cement kiln dust can only be reused if contaminant concentrations fall within specified limits, because clinker quality can be affected by the presence of certain constituents. Alkali metals, such as lithium, sodium, and potassium, are of primary concern. The raw materials used to produce clinker and the kiln fuel influence the chemical composition of the dust generated, and thus may affect recycling rates.
Cement kiln dust that contains alkalis or possesses other undesirable characteristics may be treated so that it can be returned to the kiln system. Treatment techniques include pelletizing, leaching with water or a potassium chloride solution to remove alkali salts, alkali volatilization, recovery scrubbing (also known as flue gas desulfurization), and fluid bed dust recovery.
In addition to being reintroduced to the kiln, cement kiln dust can be reused beneficially in a variety of ways. Cement kiln dust has been sold by some plants for sewage sludge solidification. It has also been reused as an adsorbent for desulfurization, particularly in the cement plant's air pollution control equipment; as a neutralization agent for acidic materials; as a soil stabilizer; and as an ingredient in various agricultural and construction products. Material accumulated from desulfurization can be ground and reused as an additive and/or retarding additive to the clinker to make cement.
Wastes generated from other industries can be recycled at cement kilns as fuels and raw material substitutes. The recycling of wastes in cement kilns as fuel offers a cost-effective, safe, and environmentally sound method of resource recovery for some hazardous and non-hazardous waste materials. Currently used hazardous wastes are waste oils and spent organic solvents, sludges, and solids from the paint and coatings, auto and truck assembly, and petroleum industries. Some nonhazardous wastes, including foundry sand and contaminated soils, have high concentrations of the conventional components of cement, such as silicon, aluminum, and iron. These wastes, therefore, can be used in place of the conventional raw materials.
Cement manufacturers who have laboratories in-house to conduct product testing and research often generate hazardous wastes as a result of laboratory testing and research. Approximately 40 percent of the hazardous wastes generated in a lab are due to unused and off-spec reagent chemicals. Traditionally, reagents are purchased in large quantities, but laboratory technicians prefer to use fresh reagents for experiments, and therefore tend not to use reagents in previously opened containers. This leads to large quantities of unused reagents. Implementing a purchasing and inventory control, surplus chemicals exchange, and experiment modification system at laboratories would reduce the amount of unused reagents that need to be disposed of as wastes. Purchasing only the required amounts or smaller container sizes of reagents will also reduce reagent waste and disposal costs.
Gaseous emissions from cement manufacturing plants are mainly nitrogen oxides and sulfur dioxide. Process controls, including balancing the alkali content in raw materials and fuels, increasing oxygen partial pressure, increasing dust load, and reducing kiln volume load, can reduce sulfur emissions in the process. Process controls to reduce nitrogen oxide emissions include avoiding excessive sintering temperatures and staged combustion in the calciner. Other measures may reduce emissions, including the use of ammonia to control nitrogen oxide emissions.
Reuse of wastes generated by air pollution control equipment is one pollution prevention opportunity available to facilities which produce structural clay products. Clay product manufacturers commonly use wet scrubbing to treat particulate emissions. The waste generated by wet scrubbers can often be returned to the production process as a raw material substitute to replace clay or other alkaline additives. Waste generated during raw materials receiving can be eliminated by modifying the equipment and operating practices. For example, paved receiving areas prevent spilled raw materials from contaminating soil, allowing spilled materials to be recaptured for use.
Product substitution is one means of reducing paint waste generated by plants engaged in finishing of pottery products. Water-based finishes, including paints and enamels, can be substituted for solvent-based finishes, reducing the amount of volatile emissions from finishing processes. The use of water-based finishes may, however, result in hazardous waste generation and waste water discharges.
Pottery manufacturers can recycle wastes recovered from pollution control devices. The dry powder waste recovered from air pollution control equipment is virtually identical in composition to the tile/ceramic product itself, and therefore may be recycled as raw materials into the body preparation process. The overspray dust gathered in dust collectors can also be recovered. Enamel overspray from finishing operations can also be reused if not contaminated. Enamel overspray is often washed down and collected in settling pits, where it can be reclaimed and re-introduced as a raw material.
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