2021 prediction, page-22

There are two operating regimes for ISL, determined by the geology and groundwater. If there is significant calcium in the orebody (as limestone or gypsum, more than 2%), alkaline (carbonate) leaching must be used. Otherwise, acid (sulfate) leaching is generally better. In this case the leach solution is at a pH of 2.5-3.0, about the same as vinegar. Acid leaching gives higher uranium recovery – 70-90% – compared with 60-70% for alkaline leach, and operating costs are about half those of alkaline leach.

Using normal alkaline leaching PEN was not economically viable. Below links to some reading material:

https://www-pub.iaea.org/MTCD/Publications/PDF/te_1239_prn.pdf
https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/mining-of-uranium/in-situ-leach-mining-of-uranium.aspx