Steps of a typical frac sand mining operation
This aerial photograph is numbered in the order steps occur, from start to finish. Each step is described below. While individual mine operations may handle various steps slightly differently, the basic order remains the same.
Image courtesy Superior Silica Sands.
1. Remove vegetation
Before mining can occur, all vegetation must be removed from an area.
2. Remove topsoil and unwanted rock layers
To reach the target sandstone, overlying soil and unwanted rock layers must be removed. These materials are often stockpiled separately on the site for use during mine reclamation (step 10).
3. Expose top of sandstone, place explosive charges
Unlike loose beach sand, the sandstone mined for frac sand is cemented together as rock. Depending on how hard the rock is, explosive charges may need to be placed in holes drilled near the edge of the mine wall to break the sandstone into smaller pieces.
4. Excavate sandstone
Excavation equipment such as backhoes and front-end loaders are used to scoop up sandstone from near the wall of the mine pit.
5. Crush sandstone chunks
Trucks haul loads of sandstone from the mine pit to a crusher which breaks the sandstone chunks into individual sand grains. Grains that are too large for use as frac sand are separated out and either sold as an alternative industrial sand product or added to the reclamation pile (step 10). The remaining sand is then mixed with water and piped to the washer (step 6).
6. Wash sand
Sand grains are washed to remove any unwanted silt and clay particles. Wash water is then pumped to a nearby settling tank where flocculants (polyacrylamides) are added to remove suspended particles. These particles, along with sand that’s too fine for hydraulic fracturing, make up about 20–30% of the total weight of the raw sandstone. While the fine silt and clay particles are commonly used for mine reclamation (step 10), sand that is too fine can be sold as industrial sand.
Due to the expense and effort of using settling tanks and ponds, some sites (including the mine shown here) now use belt presses to squeeze water out of the flocculated sediment. In addition to being faster, this system recycles more of the water and requires less water for washing and processing the sand.
7. Stockpile sand for drying, sorting, and final shipment
Clean sand is stockpiled until it can be loaded into trucks for final sorting and drying off-site. (Some mines sort and dry sand on-site rather than sending it out.) Because the sand still contains moisture when it is stockpiled, drain tiles beneath the piles channel water to a lined process-water pond. This water is reused to wash and transport sand (steps 5 and 6).
An important step in processing frac sand, which occurs off-site for this operation, involves drying and sorting the sand into grain-size ranges to meet industry needs. The sand is then shipped out of state by rail or barge.
8. Allow reject material to settle out of water
Reject material (silt and clay) from the washing process is moved to holding ponds to allow the material to settle out of the water. The water is reused to wash and transport sand (steps 5 and 6).
9. Dry reject material
Sludge from the settling ponds is moved here to allow the material to drain for a final time before adding it to the reclamation pile.
10. Backfill unwanted rock and fine material (mine reclamation)
During reclamation, the mined pit is backfilled with a mix of unwanted rock (from step 2) and reject materials (from steps 5, 6, and 9). As the mine wall is excavated, the area behind it is continuously being filled in. The new surface will eventually be graded, covered with stockpiled topsoil (from step 2), and replanted in accordance with the approved reclamation plan.
