Remediations of ponds containing salt: an innovative method from COMINAK

Why does salt represent a challenge for remediation works?

In a climate that's fairly dry and very hot, liquid evaporates and salts present in the COMINAK waste ponds form a crust with a dry and crumbly surface. This crust, just a few centimeters thick, functions as a screen that slow and obstruct the evaporation of liquids still contained in sludge under the surface. 
Therefore, the drying process remains incomplete, and the ponds remain sludgy. That’s not to mention the salts which crystallize and redissolve on contact with the water. All these phenomena complicate the remediation of these ponds.

What did not work out the first time?

Before the mining site was closed, we made the first go at the remediation of one of the ponds. Materials, mostly argilite, were deposited progressively from the edges of the pond, but the process was very slow, the weight of the cover and the earth-moving machinery caused the advancing front to collapse, and boreholes revealed that several pockets of liquid remained. So, it was necessary to come up with a different solution, in a short time. 

Where did the idea of using sand come from? 

There was no existing method for remediation of the ponds containing salt. Several avenues of research were explored, Orano experts were brought in, and engineers were consulted. The existing practices concerned ponds different in nature. In some cases, the ponds were simply left without remediation.

Our objective was to secure and stabilize the ponds by drying them out. That meant that we needed to find the best filling material and the best filling method, following our first unsuccessful attempt. 

In November 2020 we decided to do some trials to better understand how salts behaved, but also to study the effects of the sludge on possible filling materials, and vice versa. 
Laboratory trials showed that sand yielded the best results thanks to its consistent and relatively low density, its rapid and substantial absorption abilities (up to 30% of its volume), its positive effect on capillary uptake and its resistance over time to the high acidity of the sludge. 

Small-scale trials on site, meanwhile, showed how easy it was to use wind-blown sand, local material,  the stability it can offer once it has been dampened for moving machinery, the absence of capillary uptake, the low penetration of sand into the sludge but also the possibility of creating pumping shafts in the sand to deal with residual pockets of liquid. 

Beyond finding the right material, have you also identified the right method?

At the same time, to avoid spillage of sludge, we have created a unique implantation method which consists of creating sand jetties, regularly spaced out from two opposing dykes. This method makes it possible to distribute the force exerted on the sludge in multiple directions. Once the jetties meet in the middle of the pond, the uncovered spaces between the jetties are divided into sectors by intermediate jetties, until they are completely covered. A layer of argilite is then deposited over the whole surface, and this can support the weight of the machinery. 

“After pond remediation, a 30-ton truck can safely drive over the stable surface.” 

Thanks to this reliable, efficient method that uses only locally available materials, 50% of the ponds will be filled by the end of 2024

The end of works is forecast for 2028/2030, and the site will then be completely secure and as close as possible to its original state. The area can then be returned to the public domain after remediation.