Granite
Granite is an intrusive igneous rock which is widely distributed throughout Earth’s crust at a range of depths up to 31 mi (50 km). Granite’s characteristic grainy structure and strength is the result of many individual crystalline structures which form tightly together as magma slowly cools within large, deeply buried rock bodies known as plutons. True granite contains 20-60% quartz as well as both plagioclase and alkali feldspars of which the former may not exceed general balance. Other minerals such as hornblende and biotite may also occur in granite, accounting for its variety of appearances (Alden 2004).
Commercially, the term ‘granite’ includes a range of other types of non-granite dimension stone including any feldspathic crystalline rocks or other igneous or metamorphic rocks which possess qualities similar to granite’s grainy, interlocking texture. Many variations of granite appear on the commercial market with white, gray, pink, and red being the most common primary colors. Greens and browns are also available as well as darker grays and black.
Commercial granite is the second largest category of dimensional stone after limestone. Granite is currently produced in 17 U.S. states with Massachusetts, Vermont, Georgia, South Dakota, and New Hampshire leading production (Dolley 2007). Its primary uses include curbing, monumental rough and dressed stone, as well as an array of applications throughout the building industry; this study focuses on dimensional granite production. Two general phases of granite production exist: quarrying and processing. Each of these phases is described below.
Granite Quarrying Operations
Process flow diagram for granite 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.
As shown in Figure 1, the first step in quarrying is to gain access to the granite 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. After the face of the granite 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 granite deposit. Quarrying operations typically include the drilling of holes along the perimeter of the bench, followed by either cutting the stone out of the deposit using saws equipped with diamond , or by splitting the stone using hydraulic splitters or small explosive charges. Once the bench is cut or split loose from the deposit, heavy equipment is used to lift the granite bench and transfer it to an inspection area for grading, temporary storage, occasional preprocessing into slabs, and eventual shipment from the site. Granite 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.
Granite 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.
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 granite (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) or 1-1/4 in (3 cm) in lengths of approximately10-12 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 granite using a circular blade saw, but a diamond saw, a gang saw with steel shot, or a splitter can also be implemented. When operating a circular or diamond 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.
An array of finishing applications exists, and each uses specific types of equipment to accomplish the resulting appearance. Polished or honed finishing is frequently given to granite products, but thermal finishes are also common. 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. As with primary cutting, a circular blade saw as well as a diamond saw are the most common tools implemented for granite. High-pressure water, a CNC (computer numerical control) machine, or a splitter also may perform the shaping. Cooling water is again necessary for large circular and diamond saws, as well as for cutting with high-pressure water and a CNC. Splitters are simply guillotine-like machines and are operated hydraulically.
Once a product is completed, it is packaged and stored for shipment or direct sale. Granite 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.