CSN Mineração committed, still in 2017, to eliminating tailing dams from its operations and started to implement a new dry tailing filtering and stacking technology in the Casa de Pedra Mine, in Congonhas (MG). We incorporated a magnetic concentration plant into our production process, enabling greater reuse of tailing that would be discharged, increasing our efficiency according to the circular economy scope.
The tailing piles will be revegetated, and we are reusing 94% of the water present in the tailing. This project was concluded in 2020, and the tailings filtering plant, which has an annual processing capacity of 9.5 million tons of tailings, now absorbs 100% of Casa de Pedra's production.
Historically, our operation is aligned with state-of-the-art mining processes, taking as an example the construction of the Casa de Pedra dam, which made the most of the best engineering practices. The dam’s massif was raised according to the downstream method because it was built outside the dam, and foundations were grounded on natural terrain.
The structure is inspected and monitored daily (including holidays and weekends) by 139 instruments. Henrile Meireles, general manager of Technical Services, enforces that CSN strictly complies with the current legislation and sends reports to the National Mining Agency (ANM) every fortnight. The manager also points out that the dams are audited by independent professionals from Brazil and abroad and inspected by regulatory officers. The Congonhas highest ever rainfall in one day was 126 mm on Dec, 16th, 2008. The dam was projected to withstand 342 mm in one day.
In the last five years, the Company made large investments to increase the dam’s reliability. These investments made it possible to reduce 94% of the volume of water present in the structure.
Henrile Meireles says that “in 2014 there were approximately 5.5 million m³ of water, equivalent to 2,200 Olympic swimming pools. In 2020, this volume dropped to about 353 thousand m³ of water or 141 Olympic swimming pools ”.
The next step will be the decharacterization of the Casa de Pedra dam, which means the structure stops receiving tailings, water and does not maintain the dam characteristics anymore. This step will be done gradually and allow the recovery of part of the iron ore present in the tailings.