11 Novel Materials and Processes for Pollution Control in the Mining Industry Alan Fuchs and Shuo Peng University of Nevada, Reno, Reno, Nevada Tremendous opportunities exist in the area of novel materials for environmental engineering applications. These opportunities have traditionally been in the areas of membranes, ion exchange, and adsorbents, but new areas relating to techno- logical advances in “nanomaterials” and “bio-applications” have spawned new generations of designed materials for many pollution control applications. The emphasis in this chapter will be on new technologies which have been or will be useful for pollution control in the mining industry. This will require a review of developments in the general areas of membranes, ion exchange, and adsorption, and discussion of how these materials are useful in mining applications. 1 MEMBRANES MATERIALS AND PROCESSES A great deal of work has been done on the use of membrane processes for treatment of mine waters. Some of the typical membrane configurations of membranes separators are shown in varied textbooks (e.g., Ref. 1). This text also has examples of hollow fiber membranes and typical flow arrangements of these systems. Recent examples of this include separation of rare earths using liquid membranes (2) and copper recovery from Chilean mine waters, also using Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. liquid membranes (3). Earlier studies in this area related to the removal of ammonium and nitrate ions from mine effluents using nanofiltration and reverse osmosis (4,5). A major focus in the area of membrane development during the past few years has been in the use of liquid membranes. Valenzuela et al. (3) describe the use of a hollow fiber-supported liquid membrane for the recovery and concentra- tion of copper from Chilean mine water. This system works by impregnating the porous structure of the membrane with an organic film which acts as a selective extraction medium. The film is salicylaldoximic extractant. Utilizing a low concentration of the extractant, a high degree of copper recovery is possible using this technique. An organic solution, containing 5-dodecylsalicylaldoxime was dissolved in a solvent containing 91% aliphatics and 9% aromatics. A feedstream containing 0.8–1.4 g/liter Cu(II) was used for testing, with a pH of 2.8–3.2 and a density of 1.05 g/ml at 20˚C. Concentrated sulfuric acid solutions were used as metal-acceptor stripping agents. The hollow fiber membranes were microporous PTFE fibers with a membrane pore size of 2.0 µm. Because of the hydrophobic nature of the membrane used, the pores are rapidly and easily filled with the solvent-containing “carrier” extractant. A similar system was also studied by Valenzuela et al. (3). After impregnation with the the solvent, the feed solution and acid strip solution was recirculated through the fiber bores and outside the fibers, respectively. Copper extraction in a membrane extractor occurs by diffusion of copper ions from the bulk feed into the solvent. The reaction which takes place in the membrane is as follows: Cu +2 (aq) + 2HR (org) = CuR 2(org) + 2H + (aq) where HR is the acidic extractant and CuR 2 is the metal complex extracted into the organic phase. The extractant diffuses through the membrane into the acid, where the stripping reaction takes place. In this way the carrier is regenerated and copper ions are free to be collected in the stripping liquor. The results of this work indicate that copper concentrations can be reduced from 1 g/liter to below 0.2 g/liter in 8 h. The effectiveness of copper removal is dependent on the sulfuric acid concentration in the stripping solution. Yang et al. (2) describe the use of a combined extraction/electrostatic psuedo liquid membrane (ESPLIM) for extraction and separation of rare earths in a simulated mine water. In this process, a continuous organic phase, consisting of 20% di-(2-ethylhexyl) phosphoric acid and 80% kerosene serves as the bulk liquid membrane. This stream contains a specific extractant for the metal ions to be extracted and separated. A discontinuous, aqueous stream, <0.2 mm in diam- eter, is used for phase settlement. The system consists of a grounded electrode coated with polyethylene film mounted at the sides of extraction and stripping Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. cells in a rectangular reaction tank. A high-voltage electrode coated with poly- ethylene is wound on the perforated baffle plate separating the extraction and stripping cell. When a high-voltage electrostatic field is applied to the reaction tank, the aqueous drops in the organic continuous phase disintegrate into numer- ous smaller droplets under the action of the electrostatic field. This provides a great deal of surface area for separation. The extractant dissolved in the continu- ous organic phase acts as a shuttle to transport metal ions from the extraction cell to the stripping cell. A summary of opportunities for membrane technologies in the treatment of mining and mineral process streams was presented by Awadalla and Kumar (4). This study indicated a variety of applications including acid mine drainage (AMD), treatment of flotation water, copper smelting and refining wastewater, mill wastewater, removal of ammonium and nitrate ions, membranes in the aluminum industry, treatment of groundwater, treatment of uranium wastewater, treatment of dilute gold cyanide solutions, recovery of zinc from pond water, rare earth (RE) concentration, and separation of selenium from barren solution. AMD contains pollutants such as iron, manganese, calcium, magnesium, and sulfate ions. Although lime neutralization is considered the “best available technology economically achievable,” it is no longer considered environmentally acceptable because of the low-level contamination of heavy metals which cannot be removed. Alternatively, almost complete removal of dissolved solids can be achieved by the use of ion exchange, distillation, and reverse osmosis (RO) to produce high-quality water which can be used by municipalities or industry. The use of RO is best implemented as a supplement to neutralization processes. The RO concentrate stream is neutralized and clarified prior to discharge or recycled. Coupled RO/ion exchange can be used when high concentration of calcium sulfate and/or iron fouling is a problem. For the case of water reuse in which completely demineralized water is not essential, a charged ultrafiltration process using negatively charged noncellulosic membranes was utilized. For the case of AMD for coal conversion processes, high-ultrafiltration recovery with high removal of calcium sulfate and iron and good flux are required. Recovery of up to 97% is achievable by introducing an interstage settling step. Commer- cially available charged ultrafiltration membranes by PSAL (millipore type of noncellulosic skin on cellulosic backing) were used in this study. Cost for treatment using UF with interstage settling are $1.33/1000 gal of AMD, including membrane replacement cost, pumping cost, and lime cost. In order to avoid problems with recycling wastewater from flotation mills which contain the breakdown products of collector-frother reagents, the water must be purified before recycling to the mining operation. The traditional method for treatment of flotation water involves lime precipitation, ozonation, adsorption on activated carbon, and biological treatment (4). Biological treatment requires excessive holdup and is dependent on the climate, the presence of toxic heavy Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. metals, and sensitive control of the microorganisms. Reverse osmosis has been used for the recovery of flotation reagents. Commercial RO membranes have been used to remove 95% of organic carbon, calcium, and magnesium from the flotation feed stream. Scrubber blowdown from a primary copper smelting plant and acid process- ing water from a selenium-tellurium plant have been treated using negatively charged noncellulosic ultrafiltration membranes (4) . Removal of over 85% of As and Se from the acid processing water was made possible when the pH was adjusted to 10 and the solids were settled prior to ultrafiltration. Scrubber blowdown was effectively treated without pH adjustment to a pH of 4.5. Arsenic- containing wastewater was also pretreated with UF and polished using RO. This method produced a permeate stream containing less than 50 ppb arsenic. Alkaline solutions of NaCN are used to leach gold-containing ores, produc- ing dilute gold cyanide solutions (4). The two conventional methods of recovering gold from these solutions include the Merill-Crowc process of cementation using zinc powder and adsorption using activated carbon. Concentrated gold solutions are formed by elution. Reverse osmosis has been investigated as a means to concentrate the dilute gold solutions. In the case of metal finishing operations using gold and cyanide solutions, FilmTec FT-30 membranes have been used to provide rejections in the range of 91–99% for free and combined cyanides (with copper and zinc). Membrane performance was strongly pH dependent. Reverse osmosis has also been used for silver and copper cyanide concentration (Os- monics, Inc). This study utilized a nitrogen-containing aromatic condensation polymer. Experiments indicated that the feed could be concentrated three times with 70% removal of permeate, resulting in low gold content in the permeate. Nanofiltration (NF) and RO have been used for removal of ammonium and nitrate ions from synthetic and actual mine effluents (5). In mine and mill water, ammonium and nitrate ions are generated from the degradation of cyanide from gold mill effluents and ammonium nitrate-fuel oil (ANFO) blasting agents in mines. Nitrogen-containing reagents are also used in ore processing and extrac- tive metallurgy. The results of experiments using NF and RO membranes were reported for testing and actual mill effluent. The results of the testing were that good removal of ammonium (>99%) and nitrate ions (>97%) were achieved using RO, while NF was less effective. Lower effectiveness of the NF membrane was believed to be caused by ammonium being present in the sulfate form and not the larger ammonium iron sulfate complex which does not form because there is no iron in the mining effluent. No scaling or fouling problems were observed in these studies. Cross-flow membrane technolgies have also been applied to mineral sus- pensions (6). In this study, using microporous filtration (0.1-µm membranes) suspensions of CaCO 3 were investigated using an intermittent cleaning approach in order to increase the permeate flux. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. A thorough review of membrane technology for applications to industrial wastewater treatment has been made by Caetano (7). In this review, E. Drioli provides a broad overview in the areas of desalination, gas separation, pervapora- tion, membrane bioreactors, enzyme membrane reactors, and hybrid systems based on pervaporation and distillation. In the more general area of environmental applications, significant work has been done on the treatment of streams containing metals. There has been a great deal of interest in the use of ion-exchange membranes in this area. Sengupta (8) has investigated electromembrane partitioning as a means for heavy metal decontamination. This is a unique and rather interesting new approach for the in-situ removal of metals from contaminated soils. A low-level direct current (DC), less than 1 V/cm, is applied to the soil while a composite ion-exchange membrane is wrapped around the cathode. Upon imposition of the DC potential, the cations move toward the cathode, where they are captured by the composite membrane. By the design of the ion-exchange membrane, the nonselective ions should pass freely through the membrane. The membrane utilized for this work is a thin sheet prepared by grinding a cross- linked polymer ion exchanger and suspending the ion exchanger in a PTFE porous matrix. These membranes are 90% ion exchanger, 10% PTFE, and are microporous with >40% voids with a pore size distribution below 0.5 µm. One potential problem with this process is that periodically these membranes must be removed and chemically regenerated with strong (3–5%) mineral acid solution. Electrodialytic decontamination of soil polluted with heavy metals has been investigated using ion-exchange membranes by Hansen et al. (9). The process for removal of metal ions from soils using electric current and passive membranes is known as electrokinetic soil remediation. This method involves the use of passive membranes to separate the polluted soil from the electrodes. There are several shortcomings to this approach, including addition of acid counterions into the soil, return of heavy metals back into the soil, and heavy-metal precipitation at the H + and OH – front. By introduction of ion-exchange membranes into the electro- kinetic soil remediation process, an electrodialytic soil remediation process results. The ion-exchange membranes are oriented in certain directions. This orientation, with pairs of anion- and cation-exchange membranes placed on both sides of the polluted soil, eliminates all three of the problems mentioned above. This configuration also provides two compartments containing liquid solutions and the heavy metals, which can be withdrawn as needed. In this situation, heavy-metal ions pass through the cation-exchange membrane in the direction of the cathode and are prevented from passing through the anion-exchange mem- brane and never reach the cathode. They end up in the compartment between the two ion-exchange membranes. Li et al. (10) have investigated the use of a cation-selective membrane for removal of heavy metals from soils. An improvement in the traditional elec- Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. troremediation approach is described. In this work a cation membrane is placed around the cathode to prevent hydroxyl species moving toward the anode. This prevents precipitation of metals in the soil, and the metals precipitate in the column of water around the cathode. Membranes are also used for removal of metals for industrial applications (11). Bulk liquid membranes are used for facilitated transport of silver using a rotating film pertraction device. In this process two aqueous solutions are separated by an organic liquid. The membrane liquid is in contact with the donor and acceptor liquids adhering to the surfaces of the rotating disks. Transport of the solvent involves extraction from one solution and stripping in the other. This paper describes the recovery of silver from nitrate solutions using the rotating film pertraction method using tri-isobutylphosphine sulfide (TIBPS) in n-octane as the liquid membrane. Aqueous silver nitrate was the donor phase and the acceptor phase was aqueous ammonia. The results of the study indicated that because of low rates of transport of other metals, including copper, zinc, and nickel, rotating film pertraction can be used effectively to separate silver from solution. Yang et al. (2) describe a unique metal extraction method using two sets of hydrophobic microporous hollow fiber membranes for separation of metals in solution. One set of hollow fibers carries an acidic organic extractant (LIX 84, anti-2-hydroxy-5-nonylacetophenone oxime) in a diluent. The other set of hollow fibers carries a basic organic extractant (TOA, tri-n-octylamine). The aqueous, metal-containing stream is carried on the shell side of the membrane system. Cations, including copper, zinc, and nickel, are transported into the acidic extractant. Anions, including chromium(VI), mercury, and cadmium, are ex- tracted into the basic stream. Palladium has also been separated from silver in a nitric acid solution using liquid surfactant membranes (12). The organic carrier used in these studies is LIX 860, which is a β-hydroxyoxime. The liquid surfactant membrane is Span 80, a commercially available surfactant, and the solvent is n-heptane. The aqueous donor phase contains silver and palladium and is acidified using nitric acid. The receiving phase contains thiourea and is tested in hydrochloric, perchloric, nitric, and sulfuric acids. Under optimal conditions, palladium was separated from silver recovered in entirety. Another liquid membrane, investigated by Fu et al. (13), is trioctylamine (TOA) as a mobile carrier in kerosene. Precious metals, including gold, palladium, platinum, iridium, and ruthenium in hydrochloric acid, were ex- tracted using this membrane system. The metals were extracted into perchlorate and nitric acid solutions. An inert PTFE polymer 80 µm thick, 74% porous, and 0.45 µm in average pore diameter was used as a support for the liquid membrane. Low-pressure reverse osmosis (RO) was used by Ujang and Anderson (14) for separation of mono- and divalent ions. Sulfonated polysulfone membranes are Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. used as a low-pressure reverse osmosis process for separation of mono- and divalent zinc ions. It was observed that the higher the operating pressure, the greater was the permeate flux for both species. At lower operating pressure, higher permeate fluxes were observed using divalent ions. Metal removal of divalent ions was greater for divalent ions than for monovalent ions for all concentrations. 2 ADSORPTION MATERIALS AND PROCESSES Recovery of gold from cyanide has been evaluated using many different ad- sorbent materials. Petersen and Van Deventer (15) investigated the competitive role of gold and organics on adsorption by a variety of adsorbents, including activated carbon, ion-exchange resin, ion-exchange fibers, and membranes. A variety of adsorbents were investigated, including coconut shell activated carbon, macroporous ion-exchange resin, ion-exchange membrane, and ion-exchange fibers (polypropylene-based strong-base and weak-base fibers). Adsorbents were evaluated after being exposed to the organic compound, sodium ethyl xanthate, for 6 h. The absorbents were challenged with a variety of organic compounds, including ethanol, sodium ethyl xanthate, potassium amyl xanthate, and phenol. The two mechanisms investigated to explain the reduced adsorption of gold in the presence of the organics were (a) blockage of the carbon pores by the organic, and (b) competition between gold cyanide and organics for the active sites on the carbon surface. The results of the study indicated that both the rate of adsorption and the equilibrium loading were affected by the organic on the adsorption of gold cyanide onto activated carbon. The resin particles were only effected by the rate of adsorption, while the membranes and fibers experienced both kinetic and equilibrium changes. The results of this study indicated that the long-chain organics (xanthates) have a higher degree of inhibition of mass transfer of gold cyanide compared to the low-molecular-weight substance (ethanol). The aromatic substances did not affect the performance of the fibers or membrane. This is because the small pore diameters did not permit the large aromatics to penetrate. The results indicated that the second mechanism, a competitive effect between gold cyanide and the organic compounds, was responsible for the results observed for the gold-equilibrated absorbents. Klein et al. (16) have investigated polymeric resins as adsorbents for industrial applications. The motivation for investigation of polymeric resins versus activated carbon is their ease of regeneration. Activated carbon systems are typically regenerated using steam or thermal methods, while polymeric resins can be regenerated using simple solvents such as aliphatic alcohols. The resins used were methylene-bridged styrene divinylbenzene-based co-polymer (Dow Chemical, Midland, MI). Some of the characteristics of these polymeric resins which may be controlled are hydrophobicity, pore size, and surface area. These Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. resins were challenged with benzoic acid and chlorobenzene, and adsorption isotherms and bed regeneration curves were generated. The results of this study indicated that with only few bed volumes (15–25), using methanol as regenerant, 90–95% of the adsorbed solute could be recovered. The polymeric resins main- tained good adsorptive capacity after repeated cycling. 3 ION-EXCHANGE MATERIALS AND PROCESSES Applications of ion exchange to leaching solutions of an Algerian gold ore have been investigated by Akretche et al. (17). In the cyanide medium, the gold and other metals such as silver, copper, and iron attach to the anion-exchange resin. These metals are later eluted with acid thiourea to yield a concentrated solution which is treated by cementation or an electrolytic method. This work describes the use of electrodialysis of copper(I), which is normally not feasible due to the presence of formamidine disulfide. This is accomplished when the solutions are obtained by elution of cuprocyanides by thiourea. 4 CONCLUSIONS There have been great strides in the development of new technologies for pollution control in the mining industry during the past five years, many in the development of new materials and processes. Many of these developments are in the areas of membranes, adsorbents, and ion exchange. In the area of membranes, a great of work has been done using liquid membranes. These are generally supported synthetic membrane systems with a variety of liquids to facilitate transport. Electroremediation and electrodialytic membrane approaches have also seen a great deal of attention. Activated carbon-based and other organic ab- sorbents have been used for treatment of mining wastes. Polymeric resins have also been used as adsorbents for industrial applications. Anionic ion-exchange resins have also been used for treatment of leaching solutions. REFERENCES 1. W. L. McCabe, J. C. Smith, and P. Harriott, Unit Operations of Chemical Engineer- ing, 5th Edition, New York: McGraw-Hill, 1993. 2. Z. F. Yang, A. K. Guha, and K. Sirkar, Ind. Eng. Chem. Res., vol. 35, pp. 1383–1394, 1996. 3. F. Valenzuela, C. Basualto, C. Tapia, and J. Sapag, J. Membrane Sci., vol. 155, pp. 163–168, 1999. 4. F. T. Awadalla and A. Kumar, Separation Sci. Technol., vol. 29, no. 10, pp. 1231– 1249, 1994. 5. F. T. Awadalla, C. Striez, and K. Lamb, Separation Sci. Technol., vol. 29, no. 4, pp. 483–495, 1994. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. 6. D. Si-Hassen, A. Ould-Dris, M. Y. Jaffrin, and Y. K. Benkahla, J. Membrane Sci.,vol. 118, pp. 185–188, 1996. 7. A. Caetano (ed.), Membrane Technology: Applications to Industrial Wastewater Treatment. Dordrecht, The Netherlands: Kluwer, 1995. 8. S. Sengupta and A. K. Sengupta, Hazardous and Industrial Wastes—Proceedings of the Mid-Atlantic Industrial Waste Conference Proceedings of the 1997 29th Mid- Atlantic Industrial and Hazardous Waste Conference, July 13–16, 1997, Blacksburg, VA, Lancaster, PA: Technomic Publishing Co. Inc., pp. 174–182. 9. H. K. Hansen, L. M. Ottosen, S. Laursen, and A. Villumsen, Separation Sci. Technol., vol. 32, no. 15, pp. 2425–2444, 1997. 10. Z. Li, J. Yu, and I. Neretnieks, Environ. Sci. 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Sengupta and A. K. Sengupta, Hazardous and Industrial Wastes—Proceedings of the Mid-Atlantic Industrial Waste Conference Proceedings of the 1997 29th Mid- Atlantic. methylene-bridged styrene divinylbenzene-based co-polymer (Dow Chemical, Midland, MI). Some of the characteristics of these polymeric resins which may be controlled are hydrophobicity, pore size, and. treatment of flotation water, copper smelting and refining wastewater, mill wastewater, removal of ammonium and nitrate ions, membranes in the aluminum industry, treatment of groundwater, treatment of