Dolomite is a rock-forming mineral recognized chemically as calcium magnesium carbonate (\(\text{CaMg}(\text{CO}_3)_2\)), which is the primary constituent of the sedimentary rock dolostone. It is a highly valued industrial commodity due to its unique chemical and physical properties. Dolomite is used extensively in construction as a crushed aggregate, as a fluxing agent in metallurgical processes like steel production, and as a soil conditioner in agriculture. The preparation of this material for market involves a precise, multi-stage process.
Site Identification and Preparation
Securing a dolomite deposit begins with extensive geological exploration and surveying to locate large, commercially viable reserves. Geologists create accurate subsurface profiles to determine the precise depth of the dolomite layer. This work also measures the thickness of the overlying, non-mineral material.
Test drilling involves drilling boreholes into the deposit to provide core samples. These samples confirm the quality, purity, and quantity of the dolomite. This step is essential for calculating the total available reserve and ensuring the rock meets the chemical specifications required for its intended use. Necessary permits and environmental clearances are secured before physical excavation begins.
The initial physical work involves stripping, which is the removal of all non-dolomite material, or overburden, including topsoil and weathered rock. Heavy machinery removes this layer, exposing the underlying, high-quality dolomite bedrock. The overburden must be moved to a designated area for later use in site reclamation.
Extraction Techniques: Drilling and Blasting
With the dolomite layer exposed, extraction typically proceeds through open-pit mining, also known as quarrying. This method involves creating horizontal working surfaces called benches, which establish the quarry face. Bench height, which can range between 15 and 21 meters, is engineered to ensure operational safety and efficient rock fragmentation during the blast.
The next step is drilling a precise pattern of blast holes into the bench face using specialized rotary or percussion drills. These holes are drilled to a specific depth, often including subdrilling below the final floor level to ensure the entire bench breaks cleanly. The hole diameter is selected based on the desired fragmentation and the type of explosive used.
The blast holes are loaded with explosive agents, commonly a mixture of Ammonium Nitrate and Fuel Oil (ANFO). The charge is concentrated at the base of the hole. The upper section is filled with inert material called stemming, which contains the explosive energy and directs it toward the rock face. Engineers calculate the spacing and burden—the distance between holes and the distance to the free rock face—to optimize fragmentation and control ground vibration.
The blast sequence uses millisecond delays, creating a staggered detonation pattern that breaks the rock into a manageable pile, known as the muck pile. Following the blast, large loaders or excavators scoop up the fragmented dolomite. The material is then loaded into haul trucks and transported out of the pit to the processing plant, completing the extraction phase.
Processing the Raw Dolomite
Once the haul trucks deliver the material to the processing facility, it begins its journey through size reduction and separation steps. The first stage is primary crushing, where large, irregularly shaped blocks are fed into a jaw or gyratory crusher. This powerful equipment breaks the run-of-mine material down to a more manageable size.
The partially crushed material is then transferred to secondary and sometimes tertiary crushers. These machines progressively reduce the particle size further. Dolomite is a hard mineral, requiring multiple crushing stages to achieve the fine fractions required by various industries.
Following the crushing circuit, the material moves to the screening operation. This mechanical process separates the stone into specific, marketable grades using a series of vibrating screens with different mesh sizes. The separated material is classified into various products, from large construction aggregates to fine sand-sized particles. Any oversized material is typically returned for reprocessing.
For specialized uses, some dolomite is sent for additional processing beyond simple crushing and screening. The final, sized products are then moved by conveyor to stockpiles, where they are stored, ready for loading and transport to customers.
Milling
Milling grinds the material into extremely fine powders. These powders are used for specialized applications such as agricultural lime or industrial fillers.
Calcination
Calcination involves heating the dolomite in a rotary kiln to very high temperatures. This process chemically removes carbon dioxide, producing calcined dolomite. Calcined dolomite is used to make refractory bricks or magnesium compounds.