Treatment of mining waters by electrocoagulation

The mining industry is known to be one of the major contributors to the pollution of aquatic systems. The presence, in mining water, of various contaminants in relatively high concentrations (the most common cations being Fen+, Al3+, Si4+, Ca2+, Mg2+, Cu2+, Zn2+, Ni2+, Na+, K+ and anions Cl-, SO42-, NO3-, CO32-, HCO3-) can cause severe health problems, as well as hindering the reuse and recycling of process water. Currently, there are two approaches used in mining waters management. The first concept aims to reduce the concentrations of harmful dissolved contaminants to acceptable discharge levels. The basis for the second concept is the reuse and recycling of water within the process. Among treatment methods, electrocoagulation (EC) is receiving more and more attention as an alternative technology to treat mining waters. Eventually, electrocoagulation may replace conventional technologies that require an addition of chemicals and an even higher energy consumption. To ensure the suitability of electrocoagulation for the treatment of mining waters, the prime objective of this thesis was to investigate the removal mechanisms of sulfate and cyanide involved in EC-processes. Along with the removal mechanisms, parameters affecting the electrocoagulation process were investigated. 33-full factorial design and response surface methodology were implemented in order to systematically study the significance of process parameters (applied current, initial sulfate concentration and initial pH) and their combination on sulfate removal by electrocoagulation. In addition, EC performance using iron and aluminum electrodes was compared. Laboratory tests with both synthetic and real mining waters were performed. In terms of cyanide removal, the electrodes materials and electric charge were the main studied parameters. Another research question was devoted to the development of a novel treatment concept based on continuous EC operation and solids recirculation. Established process resulted in a more efficient sulfate and metals removal as well as allowed operation at desired initial pH conditions comparing to the batch operation. The performance of this novel continuous treatment concept was compared with batch electrocoagulation and conventional chemical coagulation. According to the results of this study, removal mechanisms of sulfate are different at neutral/base and acidic conditions, while the removal mechanisms of cyanide vary greatly, depending upon the material of the electrodes. Iron electrodes are more suitable for the EC-treatment of sulfate and cyanide rich waters. With iron electrodes, partial removal of sulfate and almost complete removal of cyanide were obtained. Under the studied conditions the iron species produced were positively charged favoring the removal of negatively charged ions. Most probably, the lower removal rates of studied contaminants with aluminum electrodes are due to the negatively charged species formed hindering the particle charge neutralization of anions. In batch operation, the applied current and initial concentration of sulfate are the most critical parameters affecting the removal of sulfate by electrocoagulation, while the effect of initial pH is insignificant and mainly affects the formation of metal species. No matter what the initial pH of the solution, the EC-treatment took place at base conditions with final pH over 11. On the contrary, the initial pH of the solution had a pronounced effect for continuous EC tests, the initial pH remained constant and operation at acidic conditions improved the removal of sulfate. In this study, received knowledge on sulfate and cyanide removal prove the suitability of electrocoagulation to treat mining waters. Awareness of the removal mechanisms makes the scale-up of the electrocoagulation process more robust and exact. For further process implementation, at industrial scale, there is a clear need to characterize the solids formed during the EC-process, to develop methods for sludge dewatering and recovery of valuable components from the sludge formed. In addition, there is a need to establish scale-up rules and validate the process at pilot and industrial scales. Having covered these considerations, it will be possible to conclude that electrocoagulation is a sustainable water treatment technology that meets all circular economy principles

Systematic study on sulfate removal from mining waters by electrocoagulation

The mining industry is known to be a major producer of sulfate-rich waters that are harmful to aquatic systems, accelerate acid mine drainage formation and hinder the reuse and recycling of process water. In recent years, many treatment techniques have been studied and developed to treat sulfate-rich streams. One such technique, electrocoagulation (EC), was proposed as a possible alternative to conventional treatment technologies. Electrocoagulation has been used for the removal of nitrate, cyanide and toxic metals from mining waters, but the information about sulfate removal is scarce. In this paper, the results from a systematic study on sulfate removal by EC with iron electrodes applying a 33-full factorial design are discussed. The results show the leading role of applied current on sulfate removal. In addition, the study concludes that the utilisation of iron electrodes was more efficient in terms of sulfate removal comparing to aluminium electrodes. The removal of sulfate was as high as 54% and 10% using iron and aluminium electrodes respectively. Under the studied experimental conditions, sulfate was proposed to be removed because of particle charge neutralisation and enmeshment of the studied anion in iron oxides and hydroxides.Post-print / Final draf

Pressure filtration properties of sludge generated in the electrochemical treatment of mining waters

In this study, batch electrocoagulation (EC) experiments were performed with synthetic mining water in various conditions in a laboratory-scale 1L reactor. The process was scaled up and the selected results were verified with both synthetic and real mining water in a 70 L reactor. The generated solids were characterized by XRD, SEM, and a laser diffraction particle size analyzer. After preconcentration by settling and decantation, the EC solids were separated by constant pressure filtration at 2–6 bar. In order to improve the separation, various filter aids were used in body-feed and precoat modes. The results show that the overall removal efficiency was the highest with consumable electrode pairs such as Fe/Fe, Al/Al and Fe/Al, and the highest treatment efficiency was achieved with Fe/Al electrodes where 100/100% of the nitrate and 96/87% of the sulfate were removed in small/large-scale experiments. Depending on the dissolved electrode material, different solid species were formed: crystalline primary particles with a minor degree of agglomeration were observed in Fe/Fe slurry, whereas aluminium-containing solids (Al/Al and Fe/Al) were mainly amorphous agglomerates. High values of average specific cake resistances (αav = 2·1012 - 4·1013), average porosities (>90%) and moisture contents (>68 wt%) of filter cakes were obtained for all filtered samples. The highest values of the above-mentioned cake characteristics were observed for aluminium-based solids, which might be explained by its highly amorphous structure. The application of filter aids improved the filterability of the sludges by reducing the average specific cake resistance by as much as 95–96% in the body-feed mode and by 84% in the precoat mode.Post-print / Final draf

Treatment of mining wastewater polluted with cyanide by coagulation processes: a mechanistic study

In this work, coagulation and electrocoagulation for the removal of cyanide ions contained in synthetic mining wastewater were evaluated paying particular attention to the elucidation of the coagulation mechanisms. Iron and aluminum salts with concentrations ranging from 0.01 to 10 000 mg dm-3 metal were used in chemical coagulation. Experimental data were properly fitted to Freundlich isotherm to elucidate that the main mechanism to remove cyanide during chemical coagulation was adsorption onto coagulant flocs although a maximum cyanide removal percentage of only 25% was attained. Then, electrochemical coagulation with iron and aluminum electrodes was evaluated at 1, 10 and 100 A m-2, obtaining completely different results. Iron electrochemical coagulation leads to the complete cyanide removal regardless of the current density applied, although the TOC removal was much lower than expected. On the contrary, only 60% of cyanide removal was reached by aluminum electrochemical coagulation and its efficiency was found to be highly dependent on the current density applied. Furthermore, no cyanate or hazardous inorganic chlorine species were detected during both electrocoagulation processes. However, chloride was oxidized to hypochlorite and then, it reacted with ammonium ions (contained in mining wastewater or produced by chemical reduction of nitrate by aluminum) to form chloramines. A proposal of coagulation mechanisms during the electrochemical process that explains experimental results was developed which involved the formation of iron-cyanide complexes, charge neutralization, adsorption on a superficially charged metal precipitate and/or enmeshment into a sweep metal floc.Post-print / Final draf

Treatment of mining wastewater polluted with cyanide by coagulation processes: A mechanistic study

In this work, coagulation and electrocoagulation for the removal of cyanide ions contained in synthetic mining wastewater were evaluated paying particular attention to the elucidation of the coagulation mechanisms. Iron and aluminum salts with concentrations ranging from 0.01 to 10 000 mg dm−3 metal were used in chemical coagulation. Experimental data were properly fitted to Freundlich isotherm to elucidate that the main mechanism to remove cyanide during chemical coagulation was adsorption onto coagulant flocs although a maximum cyanide removal percentage of only 25% was attained. Then, electrochemical coagulation with iron and aluminum electrodes was evaluated at 1, 10 and 100 A m−2, obtaining completely different results. Iron electrochemical coagulation leads to the complete cyanide removal regardless of the current density applied, although the TOC removal was much lower than expected. On the contrary, only 60% of cyanide removal was reached by aluminum electrochemical coagulation and its efficiency was found to be highly dependent on the current density applied. Furthermore, no cyanate or hazardous inorganic chlorine species were detected during both electrocoagulation processes. However, chloride was oxidized to hypochlorite and then, it reacted with ammonium ions (contained in mining wastewater or produced by chemical reduction of nitrate by aluminum) to form chloramines. A proposal of coagulation mechanisms during the electrochemical process that explains experimental results was developed which involved the formation of iron-cyanide complexes, charge neutralization, adsorption on a superficially charged metal precipitate and/or enmeshment into a sweep metal floc

Removal of sulfate from mining waters by electrocoagulation

This work focuses on the removal of sulfate from mining waters by using electrocoagulation with iron electrodes. A comparison of the results obtained by electrocoagulation with those obtained with the application of conventional chemical coagulation is provided. The results show that sulfate can be removed from synthetic mining waters by electrocoagulation, and that the pH and coagulant dosage play a very important role. During chemical coagulation under acidic conditions, it is possible to use a low dosage of iron and remove more than 80% of the sulfate present in water. However, chemical coagulation seems to behave as a kind of ion-exchange process (from the viewpoint of effluent quality). Thus, significant concentrations of accumulated chloride (counter ion of iron in the coagulant added) prevents the use of the technology. This problem is avoided by the application of electrocoagulation, which attains good efficiencies that can be even increased by using a continuous process with a flocculation tank. This technology also helps to remove other ionic pollutants contained in the wastewater

Water Conscious Mining (WASCIOUS)

The main objective of the NordMin WASCIOUS project was to develop a technology concept for water conscious mining, where innovative water and tailings treatment technologies provide good-quality water for recycling and discharge and enable safe disposal or utilization of tailings. The work included a survey on current practices and requirements in Nordic mines and laboratory and pilot scale development of several technologies. Computational simulations of water treatment and recycling practices were performed for a feasibility study of some technology alternatives and technologies for dewatering of tailings were evaluated. As an important outcome of the project, a future Nordic research platform was established related to environmental issues in mining for the Nordic region, enabling exchange of ideas and collaboration in future project calls, and facilitating ideas for future projects