1 Introduction

The Natural Stone Council (NSC) is a collaboration of businesses and trade associations that have come together to promote the use of Genuine Stone in commercial and residential applications. By pooling resources, their goal is to increase the understanding of, preference for, and consumption of these natural products. Trade associations affiliated with the NSC include Allied Stone Industries, Elberton Granite Association, Indiana Limestone Institute, Natural Stone Institute, National Building Granite Quarries Association, and the National Slate Association.

Recognizing that green building was becoming a permanent element of the marketplace, the NSC established a Sustainability Committee made up of key industry members to elevate the issue of sustainability within the industry and provide a body responsible for planning and implementing relevant initiatives. In 2007, the NSC Sustainability Committee engaged in a partnership with the Center for Clean Products (CCP) at the University of Tennessee to assess current industry operations relating to dimensional stone production. Prior to this evaluation, the environmental implications of stone extraction and fabrication processes had received little attention compared to other industries. In particular, life-cycle inventory (LCI) data on natural stone products was limited, not well documented, and out-of-date. This information gap was partially due to the size and varying scale of industry members, the vast diversity of products and materials produced, and the global distribution of stone quarrying activities. As such, this work presents the most comprehensive survey to-date of the natural stone industry’s practices.

Provided in the following text are the results of the first phase of a three-year project launched by the NSC to benchmark and improve the environmental profile of the natural stone industry. Specifically, the information that follows is an initial LCI characterizing sandstone extraction and production operations in North America. These data will serve as a baseline from which industry best practices can be identified, comparisons to competing products can be made with regard to environmental considerations, and future research can be prioritized.

2 Slate Quarrying and Processing Operations

2.1 Slate

Slate is a metamorphic rock consisting of numerous minerals. While slate is primarily comprised of quartz and either muscovite or illite, quantities of biotite, chlorite, hematite, and pyrite are also commonly present. Less frequently, apatite, graphite, kaolin, magnetite, tourmaline, and zircon can be constituents, as well.

Slate is formed when sedimentary deposits—particularly those containing clay, such as shale— are subjected to extreme pressure. During metamorphosis, the molecules align such that the resulting rock exhibits perfectly cleaved layers that are both broad and thin, a characteristic known as slaty cleavage. This attribute of slate is what allows it to split so readily and cleanly. Slate is naturally found in an array of colors. The most common include black, gray, blue-gray, and mottled varieties. When iron compounds are present in the formation, slate can take on hues of brick red, deep purple, or one of many shades of green. Some slate quickly fades to softer tones once exposed to the atmosphere, while others—classified as “unfading”—will retain their original coloration for many years.

Slate is most commonly employed as roofing and flooring tile (Dolley 2006) but is also frequently used for countertops, hearths, risers and treads, and landscaping. According to annual studies conducted by the US Geological Survey (USGS), slate accounted for 1% of total U.S. dimensional stone production (by tonnage) from 2003-2008 (Dolley 2004, 2005, 2006, 2007, 2008, 2009). In the US, the majority of this stone is extracted from extensive slate belts in the northeast, particularly New York, Pennsylvania, and Vermont. Additional quarries are located intermittently throughout the country.

Two general phases of slate production exist: quarrying and processing. Each of these phases is described in detail below.

2.2 Slate Quarrying Operations

Extraction (more commonly referred to as quarrying) consists of removing layers or large slabs of stone from an identified and unearthed geologic deposit. Differences in the particular quarrying techniques used stem from variations in the physical properties of the deposit itself—such as density, fracturing/bedding planes, and depth—financial considerations, and the site owner’s preference. Nevertheless, the process is relatively simple: locate or create (minimal) breaks in the stone, remove the stone using heavy machinery, secure the stone on a vehicle for transport, and move the material to storage. A flow diagram of typical quarrying operations is shown in Figure 1.

As shown in Figure 1, the first step in quarrying is to gain access to the slate deposit. This is achieved by removing the layer of earth, vegetation, and rock unsuitable for product—collectively referred to as overburden—with heavy equipment and transferring to onsite storage for potential use in later reclamation of the site. Additionally, a “plug” of poor-quality stone may sit atop the material that has commercial value; this plug must also be removed with the overburden.

After the face of the deposit is exposed, the stone is removed from the quarry in layers or slabs. These slabs range in size from a few square feet to 60 square feet or more with thicknesses from 4-24 inches or greater. Extraction is accomplished by introducing a small explosive charge to the deposit or cutting through the stone with a diamond belt saw. Since slate has such prominent cleavage, steel wedges can be driven between the strata to pry apart the slabs, as well. Loose pieces are scooped up with front-end loaders, dump trucks, or other equipment. Once the slabs are secured on the heavy machinery, they are transferred to an inspection area for grading, temporary storage, and eventual shipment from the site. Slate of insufficient quality or size for current demand is stored on-site for future use, such as for site reclamation activities, or sent to a crushing facility to be used in other applications.

2.3 Slate Processing Operations

Processing operations include much more variation than extraction. Nevertheless, the general procedures begin with initial cutting, followed by splitting, application of a finish, and a second cutting or shaping step. Based on the specific slate product being fabricated, the second and/or third steps may be eliminated, particularly when the product is to have a “natural” appearance. Figure 2 depicts the fabrication process.

The first step in slate processing is a primary cutting or shaping of the material. This is often accomplished using a circular blade saw, but a diamond wire saw, splitter, or steel-shot-blade system can also be implemented. When operating a circular saw, a continuous stream of water over the saw is required in order to dissipate heat generated by the process; sufficiently-elevated temperature can cause major machine and material damage.

Once the stone has been initially shaped, splitting may be required. This process can be accomplished manually with chisels and hammers or with automatic and semi-automatic splitting machines. Natural-faced products, such as roofing, may be completed with this step, while other products require a finishing application, secondary cutting, or both.

An array of finishing applications exists, and each uses specific types of equipment to accomplish the resulting appearance. Polished or honed finishes as well as a thermal flame treatment are most frequently given to slate products, but others are also possible, including sandblasted, water blasted, and machine gauged. The former is applied manually and/or mechanically through the use of polishing pads, while thermal finishes are applied with a flame or blow torch apparatus.

A secondary shaping step may be necessary if the product includes any features or custom size or shape. For this step, power drills, punchers (manual and automatic), and trimmers (barrel, inline, rotary blade) are most frequently employed in slate production.

Once a product is completed, it is packaged and stored for shipment or direct sale. Slate of insufficient quality or size for current demand is stocked on-site for future use, crushed for use in paving and construction applications, or stored for site reclamation activities.

3 LCI Methodology

3.1 LCI Data Collection

Information for this study was acquired through the distribution of a technical data collection tool. This survey was developed by the Center for Clean Products after touring approximately 15 stone quarries and processing facilities located throughout the United States, and through extensive consultation with industry experts and quarry operators. Choosing a diverse array of facilities was key to this process as a broad understanding of stone industry operations was needed to fashion questions that apply to all members. As such, facilities of diverse magnitudes, locations, and products were toured during the beginning half of 2007.

The survey was distributed to slate quarries and processing facilities throughout North America in January of 2009. Responses were received, follow-up conducted, and the resulting data aggregated and analyzed in the period from April to July 2009.

3.2 Quality of LCI Data Set

The dataset presented in this report represents nearly 540,000 ft³ of quarried dimensional slate and over 260,000 ft³ of dimensional slate products generated in North America. Respondents indicated net annual quarry production ranging from approximately 87,000 ft³ to nearly 210,000 ft³, while processors reported a range of roughly 67-140,000 net ft³/year. Quarry data were submitted from companies located in three U.S. states, and reporting processing facilities are located in five states.

3.3 LCI Boundaries

3.3.1 Slate Quarry Operations

The LCI for quarry operations includes the inputs and outputs for each of the processes depicted in Figure 1. Specifically, processes and operations represented in the inventory presented in this report include the following:

• Removal of overburden using heavy equipment
• Transfer of overburden to on-site storage
• Quarry operations required to remove stone from deposit including excavating, prying, and use of explosive charges.
• On-site transport of stone using heavy equipment.
• Transport of scrap stone to on-site storage
• Onsite generation of energy and compressed air
• Capture and treatment of wastewater
• Upstream production of energy and fuels
• Manufacture of ancillary materials and equipment (e.g., diamond belt)

Equipment and ancillary materials (e.g. drill bits, maintenance items) are listed in Tables 5 and 6 but have not been included in this inventory.

3.3.2 Slate Processing Operations

The LCI for slate processing operations includes the inputs and outputs for each of the processes depicted in Figure 2. This inventory has been created by allocating the entire stone industry’s data set by stone type. In other words, each fabrication plant’s data was divided according to the distribution of its production; if slate, for instance, comprised 30% of a facility’s net production, 30% of the inputs was given to the slate dataset.

Processes and operations represented in this portion of the inventory include the following:

• Primary shaping of stone into large, less-refined pieces
• Splitting of slate into thinner products
• Application of a surface finish or texture
• Secondary shaping, including hand detailing, of stone into specific products
• Packaging of finished slate products for shipment
• On-site transport of stone using heavy equipment, such as dump trucks
• Transport of scrap stone to on-site storage or reclamation
• Onsite generation of energy and compressed air
• Capture and treatment of wastewater and other waste materials, such as dust
• Upstream production of energy and fuels
• Manufacture of ancillary materials and equipment (e.g., diamond belt)

References

Dolley, T.P. 2004. Mineral Commodity Summaries 2003. U.S. Geological Survey. US
Government Printing Office, Washington, DC: 158-159.
Dolley, T.P. 2005. Mineral Commodity Summaries 2004. U.S. Geological Survey. US
Government Printing Office, Washington, DC: 158-159.
Dolley, T.P. 2006. Mineral Commodity Summaries 2005. U.S. Geological Survey. US
Government Printing Office, Washington, DC: 160-161.
Dolley, T.P. 2007. Mineral Commodity Summaries 2006. U.S. Geological Survey. US
Government Printing Office, Washington, DC: 156-157.
Dolley, T.P. 2008. Mineral Commodity Summaries 2007. U.S. Geological Survey. US
Government Printing Office, Washington, DC: 160-161.
Dolley, T.P. 2009. Mineral Commodity Summaries 2008. U.S. Geological Survey. US
Government Printing Office, Washington, DC: 156-157