Rapid Reclamation using a Sand-Clay-Overburden (SCO) Mixture
The HPS families are committed to rapidly returning mined land to beneficial use. Since the families will continue to ranch the land during and after mining, restoring the property is a high priority. To achieve this, the families have been working alongside researchers at the Florida Industrial and Phosphate Research Institute (FIPR) to develop reclamation strategies that avoid the need for large ponds of watery clay, known as clay settling areas (CSAs).
The byproducts of phosphate beneficiation are sand and a liquid mixture of clay and water. Traditionally, the sand is used as backfill for the mine cut, while the water-clay mixture is pumped into CSAs. The clay particles settle to the bottom, and the water is drawn off for re-use and/or dries by evaporation. CSAs are land intensive, covering up to 40% of a mine area. To minimize land needed for CSAs, the mining industry has developed techniques to extend their life. This means CSAs remain for decades until being reclaimed at the end of a mine’s life.
Since the 1970s, the mining industry has been seeking quicker and more efficient methods for dewatering the discarded clay. Dewatering reduces the volume and weight of the waste clay, facilitating easier transport and disposal. FIPR has published 38 reports on this subject; most recently, in 2017, a report detailed the successful use of a sand-clay-overburden mixture (SCO) for accelerated reclamation in a pilot project*.
The pilot project, led by Julian Hazen (HPS), demonstrated mining byproducts can be dewatered using centrifugal force, sieves, and mechanical separation methods. This new process can be accomplished in minutes instead of years. The sand-clay mixture is then returned to the mine site and combined with overburden, the soil that is stripped from atop the phosphate matrix and set aside during the mining process. Overburden (also comprised of sand and clay) facilitates further dewatering to achieve the ideal soil composition for agricultural use and habitat restoration.
Testing by the Institute of Food and Agricultural Sciences has shown that the SCO mixture is advantageous to crops and other vegetation due to its nutrient content and ability to retain water. Importantly, the SCO mixture is adjustable as well. The ratio of sand and clay can be modified by adjusting the overburden content in the mix, so the backfill in a mine cut mimics the soil type and upper profile removed from the mine cut. This adjustability will ensure proper hydraulic conductivity to allow post-mining landforms to have the same functionality as pre-mining landforms. HPS will monitor the functionality of SCO before and after placement.
HPS has worked hard to ensure the post-reclamation landscape will support the planned wetlands and streams following reclamation. The innovative SCO mixture—the culmination of decades of research—allows reclamation to occur concurrently with mining and eliminates clay settling areas. Rapid reclamation of the land, and its quick return to agricultural uses, will minimize disruption to the community and the families that have worked the land for generations.
*Publication No. 02-191-256