Limestone Quarrying and Processing: A Life-Cycle Inventory

A Report Prepared for: The Natural Stone Council

Prepared by: University of Tennessee Center for Clean Products

October 2008

(c) COPYRIGHT 2008 NATURAL STONE COUNCIL

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 In particular, life-cycle inventory (LCI) data on natural stone products was limited, not well documented, and out-of-date.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 limestone 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 Limestone Quarrying and Processing Operations

2.1 Limestone

Limestone is a sedimentary rock composed primarily of calcium carbonate with the occasional presence of magnesium. Most limestone is biochemical in origin meaning the calcium carbonate in the stone originated from shelled oceanic creatures. Limestone can also be chemical in origin as is the case with travertine. Chemical limestone forms when calcium and carbonate ions suspended in water chemically bond and precipitate from their aquatic sources.

Because of its high calcium content, limestone is usually light in color, although many variations exist. Commercially, the term limestone includes dolomite, dolomitic limestone, oolitic limestone, and travertine (Dolley 2007), a porous calcitic rock that is commonly formed near hot springs.

The leading stone produced in the US, limestone accounts for 42% of total domestic production. It is quarried in nine states with Wisconsin and Indiana producing over 87% of US tonnage. Limestone is most commonly employed as rough block for building and construction. Additionally, it is used as dressed stone in various applications including curbing, paneling, veneer, and tile (Dolley 2007). Two general phases of limestone production exist: quarrying and processing. Each of these phases is described below.

2.2 Limestone Quarrying Operations

Extraction (more commonly referred to as quarrying) consists of removing blocks or pieces of stone from an identified and unearthed geologic deposit. Differences in the particular quarrying techniques used often stems 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.

Figure 1. Process flow diagram for quarrying operations.

As shown in Figure 1, the first step in quarrying is to gain access to the limestone deposit. This is achieved by removing the layer of earth, vegetation, and rock unsuitable for product—collectively referred to as overburden—with heavy equipment that is sometimes coupled with small explosive charges. The overburden is then transferred to onsite storage for potential use in later reclamation of the site. After the face of the limestone is exposed, the stone is removed from the quarry in benches, usually 8 to 12 feet square extending 20 feet or more using a variety of
techniques suitable to the geology and characteristics of the limestone deposit. Quarrying operations typically include drilling holes along the perimeter of the bench followed by cutting the stone out of the deposit using saws equipped with diamond wire, or by splitting the stone using hydraulic splitters. If bedding planes are visible, forklifts can be used to pry up the blocks. Once the bench is cut or split loose from the deposit, heavy equipment is used to lift the limestone bench and transfer it to an inspection area for grading, temporary storage, occasional preprocessing into slabs, and eventual shipment from the site. Limestone of insufficient quality or size for current demand is stored on-site for future use, crushed for use in paving and construction applications, or stored for future site reclamation activities.

2.3 Limestone Processing Operations

Processing operations include much more variation than extraction. Nevertheless, the general procedures begin with initial cutting, followed by application of a finish, and conclude with a second cutting or shaping step. Due to the array of stone products, the second and/or third steps may be eliminated, specifically when the product will have a “natural” appearance. Figure 2 depicts the fabrication process.

Figure 2. Process flow diagram for processing operations.

Processing commences with transportation of the (raw) stone from the quarry to the processing facility, as depicted by Figure 2. It should be noted that this step may consist of multiple transportation steps; prior to reaching the doors of the facility, the stone may be transferred to a number of vendors or distribution locations worldwide. Additionally, some limestone (blocks) may have been cut into slabs before reaching the main fabrication plant. These are most commonly sliced to a thickness of 3/4 in (2 cm), 1-1/4 in (3 cm), or more in lengths of approximately 10-12 ft and widths around 3-5 ft. The route that the stone takes through the plant therefore depends on its physical state upon arrival, as well as the product to be produced.

The first step of the process is a primary cutting or shaping of the material. This is typically accomplished for limestone using a circular blade saw, diamond wire saw, or a splitter. When operating a circular or diamond wire 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. Natural-faced products, such as veneer or flooring, may be completed with this step, while other products require a finishing application, secondary cutting, or both.

Limestone is often produced with a natural surface, but finishes can be applied. In such cases, often a polished or honed finishing is given to limestone products, but a variety of other finishes are also common. Polishing and honing are manually and/or mechanically accomplished through the use of polishing pads or bricks.

A secondary shaping step may be necessary if the product includes any features or custom size or shape. For this procedure, a circular blade saw is frequently implemented for limestone, but a variety of hand tools are also common. Cooling water is again necessary for large circular saws. Once a product is completed, it is packaged and stored for shipment or direct sale. Limestone 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 limestone quarries and processing facilities throughout North America in January of 2008. Responses were received, follow-up conducted, and the resulting data aggregated and analyzed in the period from March to July 2008.

3.2 Quality of LCI Data Set

The dataset presented in this report represents approximately 570,000 tons of quarried dimensional limestone and nearly 250,000 tons of dimensional limestone products generated in North America. Data also reflects a diversity of operations with respect to size and location. Respondents indicated net annual quarry production ranging from approximately 301 tons to 103,000 tons, while processors reported a range of roughly 600-45,000 net tons/year. Quarry data were submitted from companies located in 44% of the nine states containing active quarries in 2006 (Dolley 2007), as well as one Canadian province. Reporting processing facilities are located in three states and one Canadian province.

3.3 LCI Boundaries

3.3.1 Limestone 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:

  • Removal of overburden using heavy equipment
  • Transfer of overburden to on-site storage
  • Quarry operations required to remove stone from deposit including drilling, cutting, 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

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 Limestone Processing Operations

The LCI for limestone processing operations includes the inputs and outputs for each of the processes depicted in Figure 2. Specifically, processes and operations represented in this portion of the inventory include:

  • Primary shaping of stone into large, less-refined pieces, such as tiles or flagstone
  • Application of a surface finish or texture
  • Secondary shaping, including hand detailing, of stone into specific products
  • Packaging of finished limestone products or slabs for shipment
  • On-site transport of stone using heavy equipment, such as forklifts
  • 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

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.

Since a fabrication facility often processes more than one stone type, each facility was categorized as a “limestone” facility if the majority of their production was indicated to be limestone. Under this condition, all respondents who are labeled “limestone” processors indicate that at least 97% of their production is limestone.

 

References

Dolley, T.P. 2007. Stone, Dimension. USGS 2006 Minerals Yearbook. Vol. 1, Metals & Minerals.
http://minerals.usgs.gov/minerals/pubs/myb.html

4 LCI Results

Data have been obtained for the quarrying and processing of 570,000 tons and 250,000 tons of
limestone, respectively. The average gross energy required to produce one ton of limestone is
0.808 million BTUs. Table 1 shows the breakdown of this gross energy per ton of limestone
product produced. Table 2 displays the water required for the same production. Table 3 and 4
display the life-cycle inputs and outputs for both the quarrying and stone processing operations,
as well as their accumulated totals. Table 5 gives the additional ancillary inputs required for the
quarrying and stone processing operations, and Table 6 gives the ancillary outputs for these
same processes. (Note that Tables 5 and 6 may be incomplete as level of detail reported for
ancillary materials was quite varied.) Each of these tables are available in an excel spreadsheet
for your convenience on the Natural Stone Council website.

Note that the abbreviations found in Tables 1-4 imply the following:

  • W = Withheld to avoid disclosure of company proprietary information
  • N/A = Not applicable due to a lack of data
  • NR = Not reported by any facility (i.e., all surveys left this survey question blank)

Table 1

Table 2

Table 3

Table 4