The industrial process used by PRNC generates residue. This residue, once treated and neutralized, must be stored. The residue park is a large basin with the purpose of storing this residue. This basin is enclosed by the dam called KO2.

How else can one store the residue?

Different ways of managing mine tailings exist: disposal at sea, storage in a tailings pond, storage in the form of solid / dry tailings, etc. The most conventional and widespread storage is storage in a tailings pond. With technological developments and increasingly stringent directives, we are seeing more instances of storing residue in solid / dried form. Prony Resources has foreseen this transition by launching the Lucy project.

The basin contains neutralized wet residue. It is deposited via a pipe into the basin which is completely covered with a membrane in order to minimize infiltration.

Before being sent to the basin, the residue is treated at the plant by mixing it with lime. The lime neutralizes the sulfuric acid contained in the residue and thus eliminates all traces of acidity in it. The residue has a neutral pH: the residue contained in the KO2 residue pond therefore does not contain acid, and is mainly composed of iron, aluminum, and gypsum. The supernatant liquid part, which then only contains a few dissolved metals (magnesium, calcium, sodium, manganese), is returned to the plant for treatment before being discharged into the sea.

It is a large basin and therefore a large dam. We understand that the size of this element may cause concern, but this structure is reliable, highly supervised, in accordance with the law and subject to numerous independent controls.

The KO2 dam has been built and is monitored according to international best practices. The PRNC teams as well as external researchers, with extensive expertise, carry out continuous monitoring of the structure. The dam is also checked on a regular basis by recognized international experts.

A reliable structure

The design of the dam as well as the technical requirements for its construction received a favorable opinion from the Permanent Technical Committee for Dams and Hydraulic Works (CTPBOH) on two occasions, in 2005 and equally in 2008. For the record, the CTPBOH is the uppermost important French authority within this matter.

A supervised structure

• 230 measuring instruments enable the measurement of all the structure’s parameters.
• Various inspections are carried out on a daily, weekly and monthly basis.
• Technical inspections are organized regularly and entrusted to the best international experts, including:
• The independent Tailings Review Board, a technical committee consisting of 3 independent international experts who carry out an annual appraisal on the KO2 tailings site and related projects such as the Lucy project;
• An accredited French research team in charge of the annual in-depth technical visit;
• International researchers in charge of carrying out an annual inspection of the dam (Dam Safety Inspection);
• Many specialized research teams are also in charge of carrying out various studies.

Yes of course, the meteorological risk has been considered, in particular extremely rainy and windy weather. This is an imperative condition to obtain an official authorization to operate.

15 major events have been recorded over the past 18 years. Suffice to say that the dam has already demonstrated its resistance.

The most intense event is also very recent and dates to February 29, 2020. 386 mm of rain fell within 6 hours, equivalent to the amount of rain that we would normally experience during a full month in February! This episode occurs on average every 400 years and the dam showed absolutely no signs of disorder.

Yes, the dam was designed, built and approved with consideration of the seismic risk in the area. The stability of a structure under seismic stress is essentially evaluated based on three factors: the earthquake’s frequency, its intensity and the acceleration of the soil generated.

The safety margin will be further improved by the construction of a stopper. This is not due to a risk of failure, it is to follow evolution in knowledge and guidelines. This stopper will withstand earthquakes of exceptional magnitude.

The reports received from our network of instruments and international external experts indicate that the dam is functioning normally and does not present any faults. The dam is therefore not weakened. A toe stopper (downstream of the dam) should be installed, not because of any poor performance, but because of an evolution in international design guidelines (ANCOLD in particular).

Studies to design this toe stopper are in progress. Preparatory work in the area (earthworks and water management) should begin in 2021 to allow construction of the toe stop. The construction of this toe stop will begin as soon as the preparation work is finalized. This work is integrated into the Lucy project.

The dam passes through three phases during its lifespan:

• Construction,
• operation,
• and closing.

Once the operation is complete and closed, the residue park will continue to be physically present, but it will be inactive. Its closing is followed by a rehabilitation and revegetation phase. This type of closure plan is included in the Lucy project.

The commonalities between the Brumadinho dam and KO2 are the shareholder’s nationality and the fact that it is a dam. There are many fundamental differences between the two dams, including the method of construction:

• Brumadinho was erected as it was filled, by successive extensions, each resting on the previous deposited residue. It had no internal drainage system or sealed core. This is a so-called bottom-up construction.
• KO2 was built using a top-down method in 3 phases, which rely entirely on the foundation soils and not on the tailings. The dam has an internal drainage system and a sealed core, these two organs are intended to prevent the installation of a sheet of water in the body of the dam.

It's understandable that the general public compares one dam with another dam, but comparing Brumadinho and KO2 is a bit like comparing two cars because they have wheels and a steering wheel.