Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
Description of the site
The case study area is located in the Central Block of the Witbanks Coalfields in South Africa. The coalfield contains sediments of the Dwyka and Vryheid Formations of the coal-bearing Ecca Group, Karoo Supergroup. Geographically the study area is located in the Mpumulanga Highveld region. Average minumum temperatures in the winter are around 0°C while in the summer the average minimum temperature is close to 13°C. The annual average rainfall is between 624 and 713 mm, most of which occur in the high rainfall months from October to March. Hydrologically, the area belongs to the Upper Olifants River catchment. (Günther and Mey 2006, Golder Associates 2010)
The region hosts extensive coal mining operations, both existing and historic dating back ca. 100 years. Today Anglo Termal Coal mines own several collieries in the Witbank area, including Greenside, Landau and Kleinkopje. The three mines form the South African Coal Estates Complex (SACE).
The Greenside colliery produces 3.1 million tonnes of saleable coal from four underground sections. The colliery covers an area of ca. 3,800 ha. It is estimated that ca. 30 Mm3 of mine water is stored underground. Mine closure is expected in 2026. (Golder Associates 2010, Golder Associates 2012, Anglo American 2014)
The open cast Landau colliery was established in 1992 and comprises four sections – the Kromdraai open-cast mine, Excelsior, the Schoongezicht mini-pit and the Umlalazi mini-pit. The current mining exploits the coal left behind in historic underground mining activities. This colliery produces a total of 4 million t/a coal and has expected closure date in 2027 (Excelsior and Kromdraai sections already in 2017). Approximately 52 Mm3 mine water is stored in underground workings. (Golder Associates 2010, Golder Associates 2012, Anglo American 2014)
The Kleinkopje colliery was opened in 1979 and nowadays operates as opencast mine extracting both new reserves and old reserves that were previously mined underground. The surface area of the mine is ca. 6,800 ha. The production rate of the Kleinkopje colliery is 4-4.5 million t/a and the expected mine closure is in 2020. The water volume stored underground is ca. 26 Mm3. (Golder Associates 2010, Golder Associates 2012, Anglo American 2014)
BHP Billiton Energy Coal South Africa (BECSA) (formerly Ingwe Collieries) owns the closed South Witbank coal mine in the area. In South Witbank mine the underground mining ended in 1975 and all activities in 1991 (period of inactivity between 1975-1989). The coal mines serve as large reservoirs of surface and underground water. (Mey et al. 2008, Hutton et al. 2009, Bhagwan 2012, WCA 2014)
Need for water reclamation
The coal mining activities have had various negative impacts on the water resources in the area, such as (van Niekerk et al. 2006):
- Interception of natural run-off and reduction in catchment yield
- Discharge of acidic and saline mine drainage to waterways
- Diversion of natural waterways
- Modification of ground water regime
It is estimated that ca. 140,000 Ml of water is stored in the coal workings in the eMalahleni area and the figure is rising by over 25 Ml per day (Fisher and Naidoo 2014). Over the years the removal of excess mine water was becoming more problematic threatening the utilization of the coal reserves (van Niekerk et al. 2006). Water treatment activities at Anglo collieries in the area date back to 1990’s. However, the lime neutralization plant at the Navigation Coal Processing Plant of Landau colliery was not successful as the inflow water quality was not sufficiently understood. Further optimization work was carried out but the problem of vast gypsum sludge formation prevailed. (Günther and Mey 2006)
In the BECSA owned South Witbank mine the underground workings as well as the opencast void had been filling with seepage and drainage water since the mine cessation in 1991. Over the years there had been attempts to reduce the amount of surface water entering the mine and on also to prevent the flow of mine water to natural waterways. Still, ca. 2.2 Ml/d of water was decanting from the mine workings. Water decanting to the void was neutralized in a liming plant which was meant as a temporary solution. (Mey et al. 2008)
The eMalahleni local municipality incorporates Witbank and the surrounding areas. The water demand of the municipality is growing fast and already in early 2000’s the municipality was heavily over-abstracting the Witbank dam which is its main source of raw water. (Günther and Mey 2006)
Thus, there was a clear need for water treatment for various reasons: to solve the operational and safety problems related to rising underground mine water, to minimize the negative environmental impact of the mine water decanting to the surroundings, as well as to assure the sufficiency of water resources for the use by the local community. Anglo Coal and BECSA formed a joint initiative in 2002 to develop a water treatment plant in eMalahleni. The plant was commissioned in 2007 and today, waters from the four mines mentioned in the previous chapter are treated in the same treatment plant, mine water from BECSA representing up to 15% of the total water flow to the plant. (Bhagwan 2012, Fisher and Naidoo 2014)
Mine water quality and treatment objectives
The mine water to be treated in the eMalahleni treatment plant is gathered from four different mines into feed ponds with a volume of 46 Ml, equal to 48 h storage capacity. The water from the South Witbank colliery is pumped from the underground workings at the southern most corner of the mine and piped to the treatment plant via ca. 2,400 m long pipeline. (Mey et al. 2006, Hutton et al. 2009)
The waters from Greenside and Kleinkopje collieries are close to neutral whereas waters from South Witbank and Landau (Navigation section) have high acidity. The variable water characteristics emphasize the need for adequate mixing and equalization prior to treatment. (Hutton et al. 2009)
Table 1 shows the typical quality of the feed water treated in the eMalahleni treatment plant. The given water quality (mostly divalent ions, low concentrations of monovalent ions) is typical especially for mine water in the northern Mpumalanga coalfields. In the southern parts of the coalfields monovalent ions become more prevalent. (Günther and Naidu 2008)
Table 1. Typical feed water characteristics (Günther and Naidu 2008)
The aim of the treatment is to produce potable quality water which can be directly conveyed to the water distribution system of eMalahleni community. The targeted quality is South African potable water standard SANS 241-1:2011. The main characteristics of mine water requiring treatment are pH, conductivity, TDS, Fe, Mn and Al.
Description of the technology
HiPRO (Hi recovery Precipitating Reverse Osmosis, RO) by Keyplan was chosen as the technology for the eMalahleni water treatment plant. The choice of technology and the vendor were based on extensive research phase of ten years, comprehensive technology evaluation and final bidding. The key consideration was the minimization of waste generated by maximising the water recovery rate. 13 different treatment technologies were evaluated in demonstration projects on different scales. Keyplan offered the first demonstration plant to Anglo Coal South Africa in 2003. The demonstration pilot plant with capacity 4.5 m3/h was commissioned in 2004-2005 and proven successful in over three months trial. Full-scale plant was commissioned in 2007. In addition to the plant itself, the infrastructure for delivering water to the drinking water system of eMalahleni community was put in place and the parties established a bulk drinking water supply agreement. (Hutton et al. 2009, Bhagwan 2012, Fisher and Naidoo 2014)
The full-scale treatment process has three stages all of which reclaim final product water as RO permeate (Figure 1). In stage 1 water is oxidized and neutralized by CSIR limestone neutralisation and the sludge is removed by clarification. Ultrafiltration (UF) acts as a pretreatment for reverse osmosis (RO) where the permeate recovery is 65%. The RO retentate is conveyed to stage 2 where precipitation is followed by hydrocyclones, clarification and membrane processes similar to stage 1 (permeate recovery 65%). The RO retentate is further treated in stage 3 which is similar to stage 2. Final brine is formed in the stage 3 RO process. The chemical gypsiferous sludge is removed from all three clarifiers for further treatment. (Günther and Mey 2006, Hutton et al. 2009)
Figure 1. eMalahleni water treatment plant (modified from Hutton et al. 2009)
The designed plant capacity of the first construction phase is 25 Ml/d and has water recovery rate > 99%. The next phase of the treatment plant is being constructed by Aveng Water which will increase the total treatment capacity to 50 Ml/d. The plant is to be commissioned by the end of 2014. After the expansions the plant will treat waters from six coal mines and Anglo American is continuing discussions with other mining companies in order to improve the regional water management. The water treatment plant is designed to operate long after the closure of contributing mines. (Bhagwan 2012, Aveng Water 2014, WCA 2014)
Treatment results and end-users for reclaimed water
The produced water is of potable water quality and complies with requirements the South African potable water standard SANS 241 sets for water for lifetime consumption. The reclaimed water constantly complies with the South African standard SANS 241 for potable water, except on turbidity (NTU ~ 1) due to the use of limestone saturator (Hutton et al. 2009). The standard sets turbidity limits at NTU <1 on operational basis and NTU < 5 on aesthetic basis, therefore slight turbidity does not compromise the safety of the reclaimed water as drinking water. (WCA 2014)
After treatment, the produced water is stored in two concrete basins before being pumped to the users. The eMalahleni municipality is the main user of the product water (20 Ml/d). The remaining 5 Ml/d is used in Anglo Coal mining activities in Greenside, Kleinkopje and Landau collieries, e.g. for dust suppression as well as domestic use. The second construction phase of the plant, adding a capacity of 8-10 Ml/d of industrial quality water, was completed in 2010. This water is supplied to nearby mines and a coal-washing plant owned by the two mining companies. (Bhagwan 2012, Fisher and Naidoo 2014, WCA 2014)
Waste generation and management
The wastes generated by the plant include brine (production 100 m3/d) and gypsum waste (200 t/d). The two waste streams were characterized already in the pre-feasibility phase of the project in order to plan an appropriate waste disposal scheme. According to chemical analysis, the brine was high in Na, S, Ca, K, Fe, Li, Mn and Sr. Due to high Mn content brine was classified as hazardous waste. Metal and gypsum sludge samples were classified as hazardous based on the amounts of Al and Mn present. (Günther and Naidu 2008, Bhagwan 2012, WCA 2014)
Brine and solid waste are disposed of separately in order to facilitate waste utilization projects. As a short-term solution, brine is stored and concentrated in plastic-lined evaporation ponds. The gypsiferous waste removed from the clarifiers is dewatered by filter presses and trucked to separate landfill sites for disposal. Already in the prefeasibility study phase of the water treatment plant it was concluded that even with high water recovery rate (99%) the safe waste disposal would represent up to 25% of the total life cycle costs over 20 years. Therefore, it was advisable to start developing viable utilization routes for both waste streams. (Günther and Naidu 2008)
Algal brine remediation has been investigated as a brine treatment technology. In this technology high rate algal ponds are used to grow certain species of algae on brine. The process is able to reduce the brine volume by up to 90% and there is also potential to produce valuable substances via algae metabolism, such as betacarotene and glycerol. The system is, however, sensitive to seasonal variations. (Günther and Naidu 2008, Anglo Coal 2009)
Another potential method that has been researched for brine treatment is eutectic freeze crystallization (EFC). Compared to evaporation technique EFC is beneficial by being more energy efficient and by providing a means of salt recovery. In conventional evaporation method the resulting solid product is mixed salt which still requires appropriate disposal. In EFC the temperature of a solution saturated with salts, such as brine, is lowered to the eutectic freezing point where both ice and salt crystallize. The precipitated salts sediment on the bottom of the reactor by gravitation whereas the ice floats on top of the liquid phase. As different salts have different solubilities and related eutectic freezing points, the process can be used to selectively recover desired salts by manipulating salt concentrations within the brine. The laboratory scale results obtained by Randall et al. (2011) showed that EFC technique is able to reduce the amount of brine by 97%. Salts, CaSO4 and Na2SO4, with > 96% purity can be produced along with potable water. The most prevalent ions found in the brine generated at eMalahleni treatment plant include sodium, sulphate, potassium, calsium and chloride. (Günther and Naidu 2008, Anglo Coal 2009, Randall et al. 2011, Bhagwan 2012, WCA 2014)
Anglo Coal has investigated the utilization options of gypsum waste in two projects: Gyp-SLiM and Gyp-BuMP. In Gyp-SLiM process waste gypsum is converted into valuable by-products sulphur, limestone/lime and magnesium carbonate (if waste contains magnesium) by a thermal method patented by CSIR (Council for Scientific and Industrial Research). Gypsum is dried and fed to a rotary kiln with fine coal. The produced limestone can be utilized in the neutralization phase of the water treatment process. In 2009 a pilot plant (capacity 1.2 t/d dry gypsum) was commissioned for the testing of the process. In Gyp-BuMP project the gypsum waste was characterized and usable building products (plasterboard, paint extenders, fire resistant doors, etc.) and mining products (e.g. materials for extinguishing fires) were developed using a patented process (Tower Technologies). After relevant testing and approval procedures, gypsum waste has been processed and utilized as a low-cost construction material to build housing for local Anglo American employees. (Günther and Naidu 2008, Anglo Coal 2009, Bhagwan 2012, Fisher and Naidoo 2014, WCA 2014)
Advantages and disadvantages
The water treatment plant has prevented serious environmental damage caused by excess mine water. In addition to this, for the eMalahleni community the main benefit of the plant is the assured supply of drinking water that requires no further treatment. (WCA 2014) The main advantages of the HiPRO technology are listed by Bhagwan (2012) as follows:
- High water recovery
- Simple configuration
- Easy operation
- Low operating and capital costs
- Minimum waste generation
The mining companies provided the technical expertise to the project and financed the investment. Anglo Coal’s investment on the treatment plant was 54 million US$ (Fisher and Naidoo 2014). The treatment cost is 1.50 US $ /m3 and the municipality pays 1.00 US $/m3 for the water it receives which made the project viable for the mining companies. The energy consumption of the plant is 2,500 kWh/m3. (Anglo Coal 2009, DWA 2011, Bhagwan 2012)
Plant reliability and maintenance needs
In the ramp-up phase the plant experienced a number of problems related to e.g. clarifier performance optimization, filter press availability, membrane blockage due to suspended solids break-through, and need for revising cleaning procedures membrane feed pump failure, limitations in sludge handling capacity and foam formation. Thus, in addition to the main process, the auxiliary systems may cause process downtimes, due to e.g. delays or unreliable operation. However, after the ramp-up and stabilization phases, the major problems had been solved by utilizing the know-how of design, construction and operation teams. In the full operational phase the utilization rate of the plant has been as high as > 98%. (Hutton et al. 2009, Anglo Coal 2009)
Monitoring and control needs
The eMalahleni water treatment plant is fully automated. The Supervisory Control and Data Acquisition system (SCADA) provides the operator an interface for controlling the operating modes of the plant, provides data on the operational status of process equipment and online measured variables, generates alarms and allows recording operational parameters. All process control procedures are programmed in the Programmable Logic Controller (PLC) by Siemens. Manual procedures are also available if the automated controlling system should fail. (Hutton et al. 2009)
Personnel are needed for chemical change-over, monitoring and routine maintenance. The staff numbers 35 which adequate to keep the plant running smoothly and consistently. A permanent training officer has been employed in order to ensure effective transfer of knowledge to new employees and to minimize any risks to safety and plant operation. (Hutton et al. 2009)
The core of the treatment plant is the membrane process and their performance is vital for the plant functionality. Therefore the risks to membrane performance are carefully addressed and guidelines are given on how to cope with these risks. All in all, according to the experience attained at eMalahleni, the treatment process is sensitive but very reliable if managed in a right manner. (Hutton et al. 2009)
The eMalahleni project can be considered a good example of public-private-partnership. The project provides a benchmark of a water treatment solution for the coal mining industry in water-stressed areas. Optimum Coal Holdings has established a water treatment plant (capacity 15 Ml/d) close to eMalahleni plant in 2010 based on the same model. Up to four other projects where the system would be replicated are in different phases of feasibility studies in the Witbank coalfields. (WCA 2014)
The project has shown that in order for the water treatment solution to be economically feasible in long term the water recovery rates should be as high as possible and the waste generation should be minimized. Effective separation of waste is important for subsequent waste utilization. Zero waste disposal should be the ultimate development target. (Anglo Coal 2009)
The eMalalahleni concept can be evaluated as potentially suitable for mine water treatment in northern conditions when the treated water quality requirement is set very high. The reactor based treatment is not subject to seasonal variations. The eutectic freeze crystallization technique for RO brine treatment could take advantage of the cold temperatures of the winter season.
The eMalahleni treatment plant is planned to partly address the post-closure liability of the mines in question. The production of sealable commodities is an important asset supporting the long-term plant operation. In the already closed South Witbank mine the water level in the underground workings will be maintained such that mine water is not decanted to the surface and the risk of spontaneous combustion in the workings is controlled. The liming will continue in the opencast void until completely dewatered after which the area will be rehabilitated. (van Niekerk et al. 2006, Mey at al. 2006)
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