To further investigate the use of low pH lixiviates (ISR extraction solutions), in 2020 Strata collected fresh core samples from an area that had not been impacted by alkaline ISR production operations (now the MU1A area). Side by side columns were prepared to explore differences in “soft” (slow) and “hard” (rapid) start-up acidification procedures and the subsequent acid requirements. Care was taken to ensure core was not unnecessarily exposed to the atmosphere during preparation of the test columns.
While the “hard” acidification procedure, the standard process in all prior testing, delivered better results, neither column generated results that were comparable to previous column tests. A hypothesis was developed that the introduction of an oxidant may be necessary to effect rapid uranium dissolution when the mineral is not previously oxidised by alkaline mining.
Extending the duration of the test, a controlled amount of oxidant was introduced to the feed stream of the soft acidification column test. There was an immediate positive response as the uranium grade quadrupled. This is illustrated in the following chart, which presents the test results of uranium grade (mg/L U) against the processed volumes (pore volume). Oxidant was first introduced during test mining phase #4 (MP-4).
Oxidant was introduced in the later stages of the second column (which had higher acid concentrations) producing a similar effect. In both tests, the uranium grade reached its peak level within five pore volumes of the initial introduction of the oxidant. Both tests were operated for an extended period due to inefficient leach chemistries prior to introducing an oxidant.
The general conclusion derived from these tests was that in the absence of pre-oxidation of the core samples (which occurred in previously mined areas under the alkaline method), additional oxidant is required for effective low pH leaching, even though the system contains ample iron (ferric iron is an oxidant which is essential in the low pH leach system).
Additional column leach studies were designed to assess the impact of the addition of an oxidant from the commencement of leaching. The site team collected fresh core samples from test holes located within an undeveloped area of the planned Mine Units #3 and #4. The site team was diligent when collecting these samples to prevent any incidental pre-oxidation of the core samples. Two side by side column tests were prepared to again compare the effects of soft and hard start up acidification procedures, but in these tests, oxidant was introduced with the feed stream from commencement.
Uranium concentrations and recovery rates derived from these two tests provide meaningful and encouraging results. The hard acidification test with oxidant again outperformed the soft acidification test with respect to uranium grade and recovery. The hard acidification test with oxidant yielded recovery rates of 50% after 7 pore volumes, 88%, after 14 pore volumes, and 93% upon the test completion at 19 pore volumes. The test yielded a peak grade of 351 mg/l U3O8 and an average grade of 120 mg/l U3O8.
A chart of the uranium grade and recovery performance of the hard acidification column test follows (Figure 3 below):
These results have been compared to the 2018 FS assumptions as presented in Figure 4 and Figure 5 below. Most critically, the recovery curves for both are comparable within the bounds of reasonable test variation. Both the 2018 FS and the column test result show a recovery level of approximately 90% after 14 PV’s of flow.
The peak and average grade obtained in the column test are substantially higher than the 2018 FS levels, as expected, and reflect the higher-grade core sample used compared to the average grade modelled in the 2018 FS. Higher grade areas of ore bodies are expected to deliver higher grade uranium solutions, but recovery rates can vary, so having this particular test on a higher-grade sample of Lance ore confirming the 2018 FS model is encouraging.
The 2018 FS model assumptions with respect to acid consumption were also strongly validated by recent column tests. The acid requirement projection used in the 2018 FS was 58 pounds of sulfuric acid per pound of recovered U3O8. The hard acidification test result was 55 pounds per pound and the soft acidification test result was 50 pounds of acid per pound of recovered U3O8. Additional acid and processed PV’s would be required in the soft acidification test to achieve a comparable overall uranium recovery level.