Conference Agenda

Potential alternatives to acid mine drainage (AMD) treatment, in abandoned mines in Iberian Pyrite Belt were using industrial byproducts. Repurposing these waste materials, could decrease environmental impacts and promote a circular economy.

The paper industry produces alkaline residues by the kraft process. These solid residues are rich in lime and recalcitrant organic compounds are commonly disposed of in landfills. Nevertheless, high alkalinity and calcium concentrations suggest the residues could be used to neutralize AMD. Organic matter in the residue could facilitate metal attenuation and settling. Likewise, biochar has emerged as another organic material of interest for AMD remediation due to its adsorptive properties. Biochar is produced through the oxygen-limited thermal combustion of biomass residues from agriculture, (vineyard olive orchards), forestry and sewage sludge.

Bench-scale experiments were conducted simulating ponds to investigate the potential for paper sludge or biochar to neutralize and attenuate metals in AMD. Various material-to-AMD ratios (1:50, 1:100, and 1:200) were investigated over 10-day tests. For each ratio, oxic/anoxic and different stability conditions were tested. The pH and electrical conductivity of the effluent were measured daily, and on the 10^th day, the final effluent and solids were analyzed using ICP-OES.

The best results came from conditions simulating stable, open ponds. The paper sludge increased the pH from 1.64 to 6.23 at 1:50 proportion, 3.30 at 1:100, and 2.20 at 1:200. Attenuation of Fe was 100%, 99.3%, and 62.3% at the 1:50, 1:100, and 1:200 proportions, respectively. However, the biochar tests did not result in substantial Fe removal, as the pH remained low for the same AMD: materials ratios as above.

Kinetic tests were also conducted while maintaining a pH of 5 to evaluate the retention of metals by each material. Biochar had adsorption rates of 98% for Cu, 18% for Mn, 21% for Zn, and 18% for Hg over 24 hours.

Based on the bench-scale results, a geochemical model of potential treatment of the AMD was created using these waste materials. The simulation included a downflow pond, using paper sludge as the bottom layer material to raise the water pH and precipitate Fe, followed by a reactive barrier made of biochar for the retention of Cu and other metals. The modeling indicates sequential changes in the chemistry of the treated AMD as a function of the specified AMD: materials ratios and reaction times and may be useful to indicate potential size of a field-scale treatment.

Herein, acid mine drainage (AMD), a highly acid and biorecalcitrant wastewater matrix, was used as a valuable secondary source for valuable mineral recovery. Specifically, AMD is amongst the main issues of environmental concern in South Africa and countries with strong mining industries, being also responsible for water scarcity and socioeconomic pressures. As such, research has focused on its sustainable treatment, including its beneficiation (minerals recovery) and valorisation (water reclamation). For this reason, real AMD was collected from an active coal mine in South Africa and was used for the selective recovery of ferric oxide (Fe(III)). By doing so, the pH of the AMD was slightly corrected, and its dissolved metals loading was also reduced.

Along with Fe(III), aluminium (Al) and sulfate (SO^-2₄)were also recovered and this material was then used for the production of iron chloride (FeCl₃) an important coagulant that is currently used by water and wastewater industries throughout the world. The produced FeCl₃ also included Al and SO^-2₄ in its matrix, which could further improve its efficiency as a coagulant.

For this reason, batch experimental studies were carried out to identify the effectiveness of the AMD-synthesized FeCl₃ for water and wastewater treatment applications. The efficiency in contaminants removal from real river water as well as from wastewater in the South African setting was identified and compared to the one of commercially available FeCl₃ coagulant. Preliminary results suggest that the efficiency of the AMD-synthesized FeCl₃ in contaminants removal from those aqueous matrices is on par or even better than the one achieved when using commercially available FeCl₃ coagulant. As such, the beneficiation of AMD can present a novel and sustainable avenue for the production of ferric salt coagulant in South Africa and further afield.

The demand for critical raw materials (CRMs) is rising due to their essential role in industries like electronics and renewable energy. Current reliance on ore mining to source these materials raises concerns about sustainability. This study addresses these challenges by investigating CRM extraction from a notable waste stream – mining-influenced water (MIW) – as a sustainable alternative to traditional methods.

In this study, the recovery potential of 19 CRMs from MIW using ion exchange (IX) technology was assessed. The novelty of the study lies in the selection/evaluation of ten different IX resins, including aminomethylphosphonic, carboxylic, phosphonic, polyamine, sulfonic, and chelating acid resins. The focus was on extracting high-value CRMs while minimizing iron (Fe) extraction, as previous studies indicated that Fe in solution hindered other CRM extraction. This study aimed to better understand the differences in selectivity, affinity, breakthrough behaviour, and desorption characteristics between CRMs. Equilibrium experiments (spanning 72 hours) were conducted along with continuous-flow column studies at varying bed volumes (BV) (6 - 15 BV/hour). Modified competitive isotherm models were applied to evaluate the data obtained.

Key findings revealed that sulfonic acid resin exhibited the highest overall capacity for CRM recovery while minimizing Fe adsorption. Chelating resins, although lower in capacity, demonstrated greater affinity for specific lanthanides. The selectivity order for the chosen sulfonic resin was: Tm > Lu > Tb > Ho > Eu > Yb > Er > Pr > Sm > Dy > Gd > La > Nd > Y > Ce >> Mg >> Fe. The presence of magnesium (Mg), thorium (Th) and uranium (U) had minimal influence on other CRM adsorption, while Fe showed competitive behaviour. Breakthrough and desorption studies indicated that Fe formed an outer layer over an inner lanthanide layer, which was released first during elution. Selective desorption was possible based on the eluant used, with chelating agents offering better CRM separation.

Approximately $35 USD per 1000 L of MIW was extracted through selective CRM recovery in this study. These findings suggest that substantial value can be obtained from MIW. Prioritizing and implementing practices like this could reduce the environmental effect of CRM sourcing while supporting the development of a circular economy.