Classification of Conventional Active Treatment Methods
There are a number of methods that are considered as active treatment processes. The most common technique involves oxidation of AMD through aeration especially for solutions containing iron, dosing with an alkali to raise the pH for neutralisation followed by sedimentation for solid liquid separation. This process is similar to that of traditional wastewater treatment plants (Younger et al., 2002). Other traditional or active treatments common to wastewater treatment plants include sulphurisation, adsorption, ion exchange and membrane processes like filtration and reverse osmosis (RO) (Younger et al., 2002). Some of these processes will be considered in the subsequent sections.
Since the principal contaminant is often dissolved ferrous iron, a key aspect of treating AMD is aeration. Iron is generally present in mine water as Fe2+ although Fe3~ can also be detected. Since ferrous iron (Fe2+) is highly soluble over a larger pH range compared to ferric iron (Fe3+), a larger amount of neutralizing agent is generally required to reduce the amount of ferrous iron in solution through precipitation as a hydroxide as compared to that needed to precipitate the iron present in the ferric state. Even at lower Fe2+ concentrations, aeration increases the level of dissolved oxygen and promotes oxidation of iron and manganese, increases chemical treatment efficiency and decreases remediation costs (INAP, 2012). As a result, most processes involving neutralisation as a form of mine water treatment generally tend to incorporate a pre-treatment step involving ferrous iron oxidation to the less soluble ferric form, which can then be precipitated as ferric hydroxide at a much lower pH (Dinardo et al., 1991). For mine water solutions with a high prevalence of iron, it is important to either pre-treat the solutions for iron removal or to initiate and maintain conditions that facilitate the easy removal of iron during the acid mine treatment process. In most cases, however, aeration is commonly applied simultaneously with addition of lime and flocculant to increase pH, oxidise metals species and precipitate metal hydroxides that are then treated through settlement, filtering or other processes (INAP, 2009). The iron precipitate generated is suitable for disposal or for application in a number of other uses. Research into possible uses and/ or means of disposal indicates the potential for the use of by-products as coagulants, mine backfill, or in the production of concrete, bricks, pigments or magnetite (MEND, 1994) as discussed in Chapter 9.
18.104.22.168 Treatment of Acid Mine Drainage Using the Neutralisation Process
The use of alkali neutralisation to remove dissolved metals from industrial effluent, mine water and groundwater is a well-established technique. In the process, sufficient alkalinity is added to raise the pH of AMD so that insoluble metal hydroxides precipitate and settle out of the AMD. The conventional approach to the neutralisation process involves three main steps such as (1) neutralisation of acidity, (2) aeration which is a key component for iron treatment, and (3) a flocculation and clarification stage (Taylor et al.,
2005). The treatment process relies on the application of neutralizing agents such as lime, limestone or a combination of both to raise the solution pH, lower the solubility of dissolved metal ions and subsequently remove them as hydroxides (Skousen, 1988; Coulton et al., 2003). Flocculation agents are added to facilitate the formation of stable solids that can be easily filtered out and removed from the effluent.
For the actual treatment of AMD, the conventional neutralisation treatment process involves contacting an AMD solution with a controlled dosage of lime in a mixing tank to attain a desired pH set point (Aube, 2004). The slurry is then contacted with a flocculant and fed to a clarifier for solid/ liquid separation. The sludge is collected from the bottom of the clarifier and can either be pumped to a storage area or pressure filtered to increase its density prior to transportation. The sludge produced by a conventional plant tends to settle to between 2% and 5% and can be mechanically dewatered to between 25% and 40% solids (Coulton, 2003). However, the low-density sludge produced requires significant power for pumping and a large storage area which presents high operational and disposal costs (MEND, 1994). This remarkable disadvantage has led to a slight modification of the conventional process flowsheet to include a sludge recycle stream resulting in the production of sludge with higher density. The high-density sludge (HDS) process substantially reduces the sludge volume and is a now a standard procedure in the AMD treatment industry.
The HDS process is a proven, well-established and understood process that has been in operation for a number of years and has been applied widely in a number of acid mine water treatment plants. The process is an improvement of the conventional neutralisation processes which tend to generate a high volume of precipitation sludge as a final product. It has been widely adopted by the mining industry where the relatively high metal concentrations and large flows make sludge volume minimisation imperative.
The key difference associated with HDS plants from the conventional plants is that the neutralisation phase is undertaken in two stages and a proportion of alkaline treatment sludge from the thickener underflow is recycled back through the plant to the first phase of the neutralisation process (Aube, 2004). Basically, the recycled sludge is mixed with the lime slurry in a sludge/lime mixing tank and the resulting solution is mixed with acidic drainage in the chemical oxidation reactor (neutralisation tank). The recirculated sludge acts as seed for further metals precipitation, thus allowing for the generation of much higher density sludge than achieved by most conventional alkali neutralisation processes.
Figure 7.1 shows the typical flowsheet for a treatment plant utilizing the HDS method. Precipitation reactions come into completion in the lime reactor tank in which air is also added in order to help oxidise ferrous iron to ferric iron (Kuyucak, 2006). A typical HDS plant will produce a sludge that settles to between 35% and 50% solids and can be mechanically dewatered to between 50% and 70% solids (Coulton et al., 2003). The HDS process results in a substantial reduction in sludge volume leading to a reduction in sludge disposal costs. There is also an increase in sludge stability, both chemically and physically and a much higher quality effluent is produced (Aube, 2004; Taylor et al., 2005). The HDS process is, therefore, particularly advantageous where large amounts of sludge are generated or where sludge disposal costs are significant. When this process is implemented, the toxicity
The conventional HDS process. (From Aube, 2004.1
of the AMD solution is reduced to the extent where the treated water is, in most cases, regarded as non-toxic to animals and many organisms (Corbett, 2001).
The process has been implemented with a few variations made on the original flowsheet in order to meet the different needs of various plants, such as the differences in quality and quantity of the AMD (MEND, 1994; INAP, 2003). The major disadvantage associated with the process, however, is that the lime demand is relatively high, which has a significant impact on the operational costs. In addition, the process produces large volumes of metal hydroxide and gypsum sludge that have no direct value and require safe disposal (INAP, 2003). Lastly, although the quality of the water is much better than that of a conventional neutralisation process, it still does not meet the environmental or potable water quality specifications, and hence, further treatment would be required.
Beside the HDS process, there have also been a number of numerous technologies developed by researchers that focus on the acid-neutralisation step. There is the acid-barium-calcium (ABC) process developed by a team of researchers at the CSIR in South Africa (see Chapter 9 also). The ABC water treatment process is designed to achieve neutralisation as well as metal and sulphate removal (<100 mg/L) from AMD through the optimal and efficient use of readily available and affordable chemicals (De Beer et al., 2012; Maree et al., 2013). The process makes provision for three processing stages, pretreatment with lime and CaS to remove free acid and metals; BaCO, treatment to form barite; waste processing to recover alkali, barium and calcium in a coal-fired kiln (Merta, 2015). The original process has been further modified to minimise the gypsum generation, thereby reducing the quantity of gypsum crystallised during the water-treatment stage with all the sulphate removed as barite. This design increases the sludge load to the barium sludge processing stage, significantly reducing the gypsum sludge processing whose CAPEX alone is very high and was estimated at about R1 billion in 2004 (Maree et al., 2004). The major disadvantage of the process is the high energy requirement due to the thermal reduction in the barium sludge processing step which is required for BaCO, recycling (Mottay and Van Staden, 2018).
Another process developed in South Africa is the SAVMIN process which is indicated in Figure 7.2 (see Chapter 9 also). This process was developed by researchers at Mintek, South Africa and focuses on the removal of sulphate from the AMD solutions through the precipitation of Ettringite (Smit, 1999). The process has a number of sequential steps for the selective precipitation of insoluble complexes and the recycling of some of the reagents that are used in the process. The main treatment steps involve the following (Smit, 1999): (1) heavy metal precipitation, (2) gypsum crystallisation, (3) selective sulphate removal by ettringite precipitation using aluminium hydroxide, (4) aluminium recovery and (5) softening and pH adjustment by re-carbonation.
The end products are potable water and a number of potentially saleable by-products such as metal rich gypsum sludge, relatively pure gypsum sludge and calcium carbonate sludge. A major disadvantage of the process
The SAVMIN process flow diagram. (From INAP, 2003.)
is the amount of the sludge produced which adds to the disposal costs. A mine water treatment demonstration plant, employing the SAVMIN technology, was established in partnership with gold mining company Sibanye-Stillwater at Randfontein, west of Johannesburg in 2018 (Mining Review, 2018).
22.214.171.124 Treatment of Acid Mine Drainage Using Sulphide Precipitation
Sulphide precipitation has been demonstrated to be an effective alternative to neutralisation using alkaline reagents for removing various heavy metals from industrial wastewater (Bhattacharyya et al., 1979; Peters et al., 1984; Abdulkadir, 2009). However, these processes have not been applied significantly on a commercial scale in the past due to a number of disadvantages. The costs of sulphide reagents are relatively higher than those used in hydroxide precipitation. In addition, the sulphide reagents used tend to produce hydrogen sulphide fumes when contacted with acidic wastes. However, this can be prevented by maintaining the pH of the solution between 8.0 and
9.5 and may require ventilation of the treatment tanks (MEND, 1994). The other disadvantage is that excess sulphide ions must also be present to drive the precipitation reaction to completion. Since the sulphide ion itself is toxic, sulphide addition must be carefully controlled to maximise heavy metals precipitation with a minimum of excess sulphide to avoid the necessity of post-treatment (MEND, 1994). Lastly, the sulphide sludge generated is more prone to oxidation resulting in resolubilisation of the metals as sulphates and as a result the sludge should, therefore, be stored carefully or be recycled for metal recovery (Peters et al., 1984). However, recent advances in technology and process development have seen the minimisation of some of the aforementioned challenges leading to a significant shift and popularity of the sulphide precipitation processes. This is largely because the solubilities of the metal sulphide precipitates are dramatically lower than those of hydroxide precipitates and, thus considerably lower metal concentrations in the effluent can be achieved. Furthermore, the metal sulphide precipitates exhibit better thickening and dewatering characteristics (Peters et al., 1984). The valuable metals can also be recovered selectively as a compact metal sulphide precipitate, which can be reprocessed at an appropriate stage in the flowsheet of a smelter.
The basic mechanism of the sulphidisation process involves the addition of a soluble metal sulphide to form an insoluble metal sulphide. The sulphide ion used in the precipitation processes can be supplied through a chemical or a biological source. Chemicals such as calcium sulphide (CaS), sodium sulphide (Na2S), sodium hydrogen sulphide (NaHS) and iron sulphide (FeS) are commonly used in the process and these dissociate in solution to provide the sulphide ion necessary for metal sulphide precipitation (Kim, 1981; Peters and Ku, 1984; Lewis, 2010). The NaHS in particular is commonly used in waste water treatment following conventional lime treatment to reduce concentrations of residual metals, particularly cadmium.
The sulphide precipitation process can be represented by Equations 7.1 and 7.2, in which FeS, the precipitating reagent, dissociates to generate the sulphide ion that subsequently reacts with the metal ions (M2+) in solution to precipitate a metal sulphide, MS
The BQE Water firm (formerly BioteQ) in Canada has developed a sulphide precipitation technology that uses chemical (ChemSulphide® process) sources of sulphide to selectively precipitate dissolved metals from wastewater. Successful commercial operations have shown that high quality effluents that comply with stringent discharge limits can be produced and that metals can be recovered selectively into saleable high-grade concentrates from AMD or leach solutions (Nodwell and Kratochvil, 2012; Kratochvil et al., 2015).
The ChemSulphide process uses chemical sulphides such as NaHS for the removal of metals from contaminated wastewaters (Stedman, 2010). In the process, the chemical sulphide is added to a contactor tank where it mixes with the water to be treated under controlled conditions to selectively precipitate metals as a metal sulphides. The precipitated metals and treated water are then pumped to a clarifier tank where the clean water is separated from the metal solids and is either discharged to the local environment or recycled (Nodwell and Kratochvil, 2012). Thereafter, the metal solids are filtered to remove excess water, thus producing a high-grade metal product in the form of for example, copper sulphide that is suitable for refining. The rate of recovery for the metals is greater than 99%, and the recovered metal products are of a sufficiently high grade to be suitable for refining (Stedman,
2010). This technology has been applied since 2001 at multiple sites (Canada,
Wellington Oro ChemSulphide process flow diagram. (From Kratochvil et al., 2015.)
the United States, Australia and China) with different site conditions and requirements. The ChemSulphide process can be integrated with the HDS process, for the recovery of valuable metals, control of iron and sulphate and production of value-added construction materials from waste sludge, thus leading to a subsequent reduction or elimination of waste sludge (Lopez et al., 2009; Kratochvil et al., 2015).
The Wellington Oro Mine in the United States (Figure 7.3) is one example where the ChemSulphide technology has been successfully applied for the treatment of acid mine solutions (Lopez et al., 2009; Kratochvil et al., 2015). The mine, which is non-operational and treats post-closure solutions, utilises the ChemSulphide process to cost effectively remove dissolved metal ions of zinc and cadmium from the mine solution stream in order to meet strict effluent discharge targets (Nodwell and Kratochvil, 2012; Kratochvil et al., 2015). The dosing of NaHS is carefully controlled so that zinc and cadmium are removed to meet discharge limits and also to ensure that excess hydrogen sulphide gas is not produced (Smit, 1999). A small amount of soda ash is also added to the process to maintain the pH at the optimal range for sulphide precipitation. The precipitated metal sulphides can be sold to smelters for further processing thus helping to offset some of the process costs.
Another example is the Raglan Mine, an active nickel mine in Northern Quebec, which is owned and operated by Xstrata Nickel and was commissioned in 2004 with the first full year of operation in 2005 (Jones et al., 2006). The water treatment plant at Raglan employs the ChemSulphide process to selectively recover nickel from low-grade contaminated mine water. The treated water quality produced in the plant can be released directly into the local environment whilst the nickel sulphide precipitate is dewatered and shipped periodically to the Raglan concentrator where it is added to the flotation concentrate for shipment to the smelters (Lawrence and Fleming, 2007). Another mining operation that applies the ChemSulphide process is Jiangxi Copper in China. The company utilises this technology for a water treatment plant at its Dexing site in southeast China. Through this technology, the company is able to remove copper from mine wastewater to produce a high- grade marketable copper product, along with water that is clean enough to be safely discharged to the environment or recycled into the mining process.
Industrial effluents can also be treated using biogenic sulphide in the form of H2S generated through the reduction of elemental sulphur in a bioreactor (Kuyucak, 2001, Lawrence et al., 2007). In fact, in recent years, the development of biological technologies for process-related applications in the mining and metallurgical industries has made great progress. For these applications, the focus is on minimizing the use of chemicals used for precipitation of metals and caustic used for removing sulphur dioxide from dilute off-gas streams (Dijkman et al., 1999). This subsequently translates to large-scale savings of costs. In addition, with regard to AMD treatment, an environmental problem can be solved while the metals recovered as sulphides generate a revenue stream.
In a biological process, the biogenic sulphide gas is transferred to a gas/ liquid anaerobic agitated contactor in order to selectively precipitate the metals to be recovered as sulphides (Huisman et al., 2006; Lawrence and Fleming, 2007; Adams et al., 2008). In this set-up, there is no direct contact between the bacteria and the liquid stream treated, so issues with regard to possible toxic compounds in the liquid stream or temperature fluctuations are of no concern. Figure 7.4 shows a generic flowsheet for the metal recovery using biogenic H2S gas (Boonstra and Buisman, 2003).
The advantage of the biogenic sulphide approach is that it can be very profitable in very low metal concentration solutions as the costs are relatively lower compared to NaHS or Na2S (EPA, 2003). Compared to the addition of a NaHS or Na2S solution, another obvious advantage of the use of biogenic H2S is that no sodium ions are introduced to the precipitation circuit (Dijkman et al., 1999). In addition to the significant cost savings, biogenic sulphide produced on demand at site provides the additional benefit of improved safety due to the elimination of the transportation, handling and storage required for chemical sulphide reagents (Lawrence and Fleming, 2007).
Process diagram for metal recovery using biogenic H2S. (From Boonstra and Buisman, 2003.)
Sulphide can also be generated by the biological reduction of sulphate, although the generation of sulfide from this source is limited to passive water treatment applications in which low flows containing low concentrations of metals can be treated to precipitate metals in natural wetlands and sediments (Lawrence and Fleming, 2007). One example of the process that uses biological reduction of sulphate is the Thiopaq process. The Thiopaq process is a well-established process which uses sulphate reduction reactions to generate biogenic sulphide that can be used in the treatment of metal sulfate solutions such as AMD. The Thiopaq system utilises two distinct microbiological populations and processes: (1) conversion of sulphate to sulphide by sulphate-reducing bacteria (SRB) and precipitation of metal sulphides, and (2) conversion of any excess hydrogen sulphide produced to elemental sulphur, using sulphide-oxidizing bacteria (Johnson and Hallberg, 2005).
The bacterial reduction of sulphate to sulphide is an electrochemical process and a reductant (electron donor) is needed to supply electrons required for the conversion reaction. Examples of typical electron donors used in the Thiopaq process include hydrogen, methanol, ethanol or other organic material as electron donors for the production of biogenic H2S (Reinsel, 2015). The choice of the type of reductant is not only dependent on the sulphur load to be processed, but also on the geographical location of the installation, reagent availability and cost. In the use of ethanol as the electron donor, this is first converted to acetic acid and hydrogen by bacteria as given in reaction 7.3.
Both hydrogen and acetic acid formed are consumed in the sulphate-reducing reaction taking place in the anaerobic reactor resulting in the formation of biogenic hydrogen sulphide.
The hydrogen sulphide produced can then be employed for the precipitation of metals by contacting it with the solution to be treated. Careful control of pH can allow for selective precipitation of metal sulphides from a solution stream that contains a number of different metal ions.
The BioSulphide® process developed by BQE Water firm is another process that utilises biologically generated hydrogen sulphide gas for the removal of metals from contaminated water. The process is based on the Thiopaq process (Dijkman et al., 1999; Reinsel, 2015) and uses naturally occurring sulphur-reducing bacteria, to produce sulphide which is used to precipitate and recover metals selectively from contaminated industrial water. The BioSulphide process has two components - the biological and chemical stages which are fully integrated, but operate independently. The bioreactor contains a mixture of SRB that reduce the contained sulphate and produce the sulphide used in the chemical stage. Hydrogen or an organic electron donor is supplied to the bioreactor. Raw AMD enters the chemical circuit where it comes into contact with hydrogen sulphide generated in the biological circuit leading to metal sulfide precipitation. The characteristics of the BioSulphide process that make it different from other conventional sulfate reduction processes can be summarised as follows: (1) the biological component of the process is separated from the chemical precipitation/ neutralisation stage, (2) only a fraction of the stream volume, as determined by sulphide and/or alkalinity requirements, enters the bioreactors, (3) AMD treatment to discharge quality is achieved entirely with bacterially generated products and (4) metal concentrates, metal sludge and biomass can be produced separately for sale or disposal.
Figure 7.5 shows the flowsheet for the application of the BioSulphide process at Bisbee, Arizona. The plant recovers copper from the drainage of a large low-grade stockpile. Copper had previously been recovered by cementation with iron, but is now precipitated into a high-grade copper sulphide concentrate using H2S generated in a bioreactor in which elemental sulphur is reduced as shown in the flowsheet.
The BioSulphide process is generally used for high metals loading and higher sulphide demand due to a lower operating cost per tonne of sulphide required (Nodwell and Kratochvil, 2012). The BioSulphide process technology can be integrated with other water treatment technologies to improve overall water treatment. For example, it can be introduced upstream of an existing lime treatment plant to recover metals contained in the contaminated water. In such a case, lime plant economics are improved with lower lime consumption and the volume and toxicity of the sludge is reduced significantly.
Another process that utilises biologically produced sulphide is the Rhodes Biological Sulphate Reduction (Biosure) process. This is a cost-effective and proven option for the treatment of AMD so as to mitigate the effect of AMD on water quality. The Biosure process utilises SRB that reduce sulphate to sulphide under anaerobic conditions. The bacteria require an organic source of readily biodegradable carbon such as primary sewage sludge and organic waste from the dairy and abattoir industries. The sulphide produced can be reacted with the metals in the AMD to form metal sulphides, which then precipitate and can be removed from the water in a clarifier. However, the metal sulphide sludge needs further treatment to prevent pollution of the environment at the disposal site; otherwise it will revert back to acid and sulphate on exposure to moisture and oxygen in the atmosphere, which is the cause of AMD in the first instance (DWA, 2013).
The Biosure process has been tested on a large scale at a mine shaft in Grootvlei, South Africa by the East Rand Water Care Company (ERWAT) (Godongwana et al., 2015). The process has been shown to be able to potentially produce water that complies with the general standard of waste water. The major disadvantage of the Biosure process is its limitation with regard to the carbon source. Large volumes of primary sewage sludge and/or external readily biodegradable carbon sources are required to meet the sulphate reduction demand of the AMD (DWA, 2013). The availability of the carbon source, i.e., sewage sludge, determines the placement of the treatment works and, in most cases, there is a lack of adequate sewage facilities at the AMD point source. Therefore, the application of the process at commercial scale becomes feasible only in the presence of an adequate source of readily available and biodegradable carbon to augment the carbon from sewage or alternatively to form the main carbon source.
126.96.36.199 Treatment of Acid Mine Drainage Using Reverse Osmosis
Membrane filtration by RO is a well-established strategy for heavy metal removal as it is capable of achieving strict metal discharge criteria whilst providing high efficiency, easy operation and saving space (Fu and Wang, 2011). It is an increasingly popular wastewater treatment option in chemical and environmental engineering and accounts for more than 20% of the world's desalination capacity (Shahalam et al., 2002). Large-scale applications of this process exist for AMD treatment and South Africa in particular have developed multistage RO concentration and gypsum precipitation process that have been used on a commercial scale in mining communities (DWA, 2013).
In the RO process, cellophane-like membranes separate purified water from contaminated water. A pressure is applied to the concentrated side of the membrane forcing purified water into the dilute side and the rejected impurities from the concentrated side are washed away in the reject water. The RO can be used to recover trace metals from AMD, remove 90% to 99% of the total dissolved solids to produce water of high quality enough to be reused as process water (Slater et al., 1983; Mortazavi, 2008) (see Chapter 9 also). The advantages of RO include low cost, high selectivity and flux, low capital and operating cost, chemical resistance and resistance to fouling, but the disadvantages include the feed to the membrane being very specific.
As a result of the high scaling tendency of the membranes in the presence of metal precipitates, this desalination process requires that the feed to the membranes be very specific with the removal of metals present in solution undertaken prior to the RO process. This usually involves the application of an upstream treatment plant such as the HDS process to ensure that metals are removed to the standards required by the suppliers of RO membranes. Some of the proven processes involving the commercial application of the RO techniques include the high pressure reverse osmosis (HiPRO) process. Modified processes including seeded reverse osmosis (SPARRO) that uses a suspension of salt crystals to promote precipitation have also been developed to improve the efficiency of the RO applications (Pulles et al., 1992).
The HiPRO™ technology developed by Aveng Water makes use of multiple stages of ultrafiltration and RO membrane systems which produce supersaturated brine streams (less than 3% of the total feed), from which sparingly soluble salts may be released in precipitation reactors. However, depending on the feed water quality, a zero liquid discharge (zero brine) solution is possible, thus, eliminating the need for high-cost evaporation and crystallizer plants. Solid waste generation can be eliminated through the production of useful by-products such as calcium sulphate of saleable grade as well as metal sulphate products. This process has been successfully applied at a number of mining operations in South Africa such as the eMalahleni water reclamation plant (Gunther and Mey, 2008; Karakatsanis and Cogho, 2010). A modular mine water treatment plant has also been installed at the Anglo American Thermal Coal's Kromdraai opencast mine.
The advantages of the HiPRO process include the following: the process has been proven at large-scale commercial operations, produces highest quality drinking water which meets the South African National Standard (SANS), has a very high water recovery that is usually in excess of 98% and generates potentially useful by-products (Karakatsanis and Cogho, 2010).
188.8.131.52 Treatment of Acid Mine Drainage Using Ion Exchange Technology
Ion-exchange processes have been used for the extraction of several metals from AMD and for the conversion of AMD to potable water (Feng et al., 2000; INAP, 2003). The ion-exchange process involves an exchange of ions or molecules between solid and liquid with no substantial change to the solid structure. It is a reversible chemical reaction where an ion from the solution is exchanged for similarly charged ion (typically hydrogen or hydroxyl) attached to an immobile solid particle (resin) thus rendering the target ion immobile (Bowell, 2000). Various resin forms are available to remove either cations or anions and a range of silica-based and polymeric resins can be used for metal recovery or removal (Dinardo et al., 1991). Synthetic organic resins are the predominant type since their characteristics can be tailored to specific applications. In the treatment of sulphate containing water, a two-stage process that uses cationic and anionic resins to remove calcium or magnesium and sulphate, respectively, is usually employed (Robertson et al., 1993). The resins are then regenerated, which typically requires acidic and basic reagents.
The appropriate ion-exchange resin is selected based on the acid mine water chemistry and the specific parameters that need to be removed from solution. Large flows generally require a full-scale treatment plant, but for small to intermediate flows, standard tank sizes that allow systems to be set up quickly can be used (ITRC, 2015). Many ion-exchange technologies appear to be technically effective at achieving water quality targets, but few have proven to be commercially viable or are in widespread use for the treatment of acidic mine water. The GYP-CIX process (Figure 7.6) is a fluidised bed ion-exchange process developed in South Africa to remove sulphate from water that is close to gypsum saturation and, therefore, it can be used as a polishing step after lime precipitation (Reinsel, 2015). The GYP-CIX process can tolerate relatively high concentrations of calcium, however, the TDSs need to be less than 4000 mg/L (Mottay and Van Staden, 2018). The process requires pre-treatment to remove metals, which may interfere and decrease the efficiency of resins in the downstream ion-exchange process. The resins
are designed so as to target calcium and sulphate in order to reduce gypsum levels in the effluent thereby reducing the total dissolved solids and corrosion potential (Smit, 1999; Bowell, 2000).
The cation resin exchanges Ca2+, Mg2~ and other cations (i.e., metal ions) by the following reaction:
The anion resin exchanges S042-, CT and other anions by the following reaction:
Unlike conventional ion-exchange technology, the GYP-CIX process uses Ca(OH)2 and H2S04 (lowest cost industrial reagents) to regenerate the ion- exchange resins. The cationic and anionic exchange processes result in the generation of a large amount of gypsum that can be sold commercially thus offsetting treatment costs whilst the treated water meets standards for reuse. The continuous precipitation of gypsum in the regeneration of ion-exchange resins also allows the reuse of the regeneration solutions (INAP, 2003).
Another process that uses ion-exchange chemistry which is also largely based on the GYP-CIX process is the Sulf-IX™ process (Figure 7.7), developed by BQE Water (formerly BioteQ) (Lawrence, 2007; Lopez et al., 2009). It overcomes difficulties of the GYP-CIX process associated with limited process
The Sulf-IX™ process. (From Lawrence, 2007.) flexibility for varying feed chemistry, mechanical entrainment of gypsum in the regeneration stage and limitations on sulphate removal when magnesium is present in significant concentration in the feed water (Lopez et al., 2009). The process uses low-cost, off-the-shelf resins to remove calcium and sulphate ions from water in various concentrations. Removal of sulphate to meet new regulations worldwide is more efficient when using the Sulf-IX™ process compared with biological sulphate reduction and membrane technology. The process requires no pre-treatment and leaves no residual waste for special disposal (Nodwell and Kratochvil, 2012). The resins are regenerated using the low-cost reagents, sulphuric acid and lime, so that the only products of the process are clean water (water recoveries to up to 97%) that can be discharged or reused and solid gypsum product that can be used for the production of fertiliser and building materials (Nodwell and Kratochvil,
2012). The Sulf-IX process has been extensively piloted to remove sulphate from water at sites in Canada, the United States and Chile (Nodwell and Kratochvil, 2012).
Table 7.1 gives a summary of the active processes discussed in the sections above, highlighting the advantages and disadvantages of each technology.