Quarrying Of Cement Manufacturing Process
Generally, cement raw materials consisting of mainly limestone (71%) and combinations of cement rock (16%), shale, clay, sand, or iron ore are extracted from a quarry near the cement plant. Limestone and cement rock are the most common source of CaCO3 for cement production, making up about 87% of the raw materials (Appendix A – Table A3). Typically, the limestone used in cement production contains 75% to 90% CaCO3 with the remainder predominately MgCO3 and minor impurities. Limestone is typically categorized as high-calcium (<5% MgCO3), magnesium-limestone (5% to 20% MgCO3), or dolomitic-limestone (>20% to <45.6% MgCO3). Cement rock is impure limestone possessing the ideal balance of silica, alumina, and CaCO3 for Portland cement.
A typical limestone/cement rock quarrying process for producing crushed and broken stone includes:
- • removal of the overburden (i.e., soil, clay, and loose rock overlaying the deposit)
- • blasting of the limestone deposit
- • loading and transporting the blasted limestone to the crushing plant
- • Crushing to reduce stone to about 5 inches (125 mm) in primary crushers, then to roughly 3/8 inch to 3/4 inch (10 mm to19 mm) in secondary crushers.
The total energy required to quarry and process limestone/cement rock is 29,932 Btu/tonne. The quarrying of the limestone/cement rock consumes approximately 88% of the total energy requirement and the crushing of the ore accounts for the remaining 12% of the energy requirement.18 Table 5 provides a detailed listing of the equipment used and the Btu per tonne consumed for each equipment operation.19 Appendix A Table A.5 lists quarrying fuels and consumption values. In the quarrying of the limestone/cement rock, diesel fuel is the main source of energy and provides about 80% of the quarrying operations energy. Diesel fuel is used to run the heavy machinery in the limestone quarry’s daily operation. Utilization of biodiesel provides an opportunity for quarrying operations to lower lifecycle emission profiles.
Primary crushing reduces quarried stone to about 5 inches. Primary crushed stone is fed into secondary crushers where it is broken down to a size (-0.375 to -0.75 inch) suitable for feed to the fine grinding machines located at the cement manufacturing plant. Crushing consumes an estimated 2,927 Btu/tonne of limestone.
Limestone/cement rock account for 87% of the raw materials used to produce cement. The energy and emissions associated with quarrying of the remaining 13% of raw materials has been estimated using the limestone/cement rock data (Appendix A Table A.6).
The environmental issues related to quarrying are mostly local and are common to most surface mines. These issues include dust, increased sediment loads to local streams, noise, and ground vibrations from blasting.
Last update: December 30, 2011
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