Treatment/disposal of water treatment residues
Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
The treatment of mine waste water commonly generates new waste streams, such as sludge, brine, spent media, concentrates or gaseous emissions, requiring appropriate treatment and/or disposal. These wastes ultimately contain the pollutants removed from the treated water streams as well as the reagents (or residuals of them) needed in the treatment process. The management including possible further utilization of these treatment wastes, as well as the development of processes with minimized waste generation is currently getting more attention. In many cases the treatment of the individual water streams with different characteristics separately will help in minimizing the generation of hazardous waste. (Department of Water Affairs and Forestry 2007, INAP 2009, Simate & Ndlovu 2014)
The management plan for water treatment waste streams is based on the following aspects (Department of Water Affairs and Forestry 2007, INAP 2009):
- Individual waste streams generated and their volumes
- Typical characteristics; chemical composition and physical properties (e.g. moisture content, dewaterability, stability)
- Waste classification (whether classified as hazardous or not) and other regulatory requirements
- Potential environmental impacts and risks over the relevant lifetime of the water treatment plant
- Phase of mine life cycle; in operation or at post-closure phase
- Appropriate management/disposal options
Alkaline sludge characteristics
All alkaline treatment systems generate large quantities of sludge requiring appropriate disposal. The main sludge characteristics are defined in the water treatment process (Aube & Zinck 1999).
Sludge dewaterability is one of the key considerations regarding the treatment, disposal and possible recycling options. The dewatering properties of sludge depend on the particle size, morphology and surface charge. Non-spherical particles have increased surface area per unit volume compared to spherical particles and thus dewater more poorly. The above-mentioned sludge properties are dictated by the water treatment process and the water chemistry. (INAP 2009) Aube and Zinck (1999) concluded that high density sludge (HDS) particles present greater homogeneity and smaller particle size compared to low density sludge (LDS) from conventional alkaline treatment.
The long-term stability of sludges under various disposal conditions is by far not fully understood (INAP 2009). During storage the properties of sludge are altered. Usual transformations taking place in alkaline treatment sludges over time include the decrease in sludge alkalinity, increase in calcite and gypsum content and increase in density due to freeze-thaw cycles and natural dewatering. A single freeze-thaw cycle has been observed to reduce the sludge volume by up to 90% and therefore it is beneficial to optimize the depth of sludge bed at the disposal site. (Kuyucak 2006)
It has been shown that in standard alkaline treatment (LDS) of iron containing mine water amorphous ferrihydrite sludge is formed on which other metals such as Cu and Zn are adsorbed. These metals are rapidly leached out if sludge is placed in acidic conditions. High neutralizing potential of sludge may at least temporarily delay the pH reduction. Compared to LDS, HDS sludge tends to have higher metal content and improved iron oxide/hydroxide crystallinity resulting in lower potential for metal leaching during disposal due to the incorporation of metals within the mineral structure. However, the neutralization potential (buffering capacity) of HDS sludge is lower due to more efficient lime usage. In general, the sludges with high crystallinity are easier and cheaper to handle compared to amorphous materials as they have lower viscosity, higher density and lower tendency to release metals. Crystalline (iron-rich) sludge also provides a possibility to separate useful products from sludge e.g. by magnetic separation as the main mineral is magnetite (McDonald et al. 2006). In alkaline treatment process the crystallinity of the formed sludge can be controlled by redox conditions and especially by the Al concentrations. (Aube and Zinck 1999, Kalin et al. 2006, McDonald et al. 2006)
Several disposal scenarios have been realized for sludge produced in alkaline water treatment process. The choice of the disposal option is based on regulations, sludge characteristics, sludge volume, availability of disposal sites, economy and aesthetic factors. Sludges can be hauled off-site for disposal; in this case sludge is to be compacted as dry as possible to reduce transportation costs. (Aube 2004, INAP 2009)
More usual option is the storage at the mine area. In this case possible disposal locations/methods include the following:
- with tailings (co-disposal)
- on waste rock piles or tailings
- in engineered ponds
- under a water cover
- in natural hollows, in mine workings or in pit lakes
Co-disposal with tailings is usually not possible at closed mine sites as no fresh tailings are generated. Water treatment sludge as a cover on tailings ponds can help to reduce oxidation acting as a wet barrier. However, laboratory scale results by CANMET indicate that sludge is not effective in preventing oxidation of sulphidic tailings (Zinck 2006). If sludge is disposed of in waste rock piles it fills the void spaces and the excess alkalinity can neutralize some of the acid generated. (Aube 2004, INAP 2009)
Engineered ponds can be designed to drain water from the bottom and allow evaporation from the surface enabling sludge densification and volume reduction, thus acting as post-treatment phase for sludge. Engineered ponds can also be used as intermediate storages to compact the sludge before final disposal. Disposal above the water table requires a barrier from the groundwater by a natural geological liner or a synthetic liner and a cover or a cap to reduce the infiltration. Underwater storage in a pond does not have the benefit of sludge densifying but instead the water cover helps to keep sludge in a stable stage. (Robertson and Shaw 1997, Aube 2004, INAP 2009)
Disposal in underground inactive mine workings clearly reduces the land area needed for sludge disposal and minimizes the potential for surface water pollution. In practice sludge is pumped or transported by trucks and injected to boreholes in the workings. In some cases, the underground disposal of alkaline sludge can reduce the acidity of mine water. The acceptability of this disposal method depends on e.g. ratio between sludge / mine water and chemistries of sludge and mine water. Underground workings disposal is probably most viable for sludge with high iron content. Abandoned open pits can be utilized as a short or long-term disposal site for sludge. (Robertson & Shaw 1997, Aube 2004, Zinck 2006, INAP 2009)
Disposal of sludge in a paste backfill with tailings and other waste materials is widely utilized in the mining industry. Paste backfill is an engineered mixture of fine solid particles and water + optionally a binder with a solids content of 72-85%. The mixture can be placed in the mine workings. The integration of sludge into paste backfill may provide additional stabilization. (Zinck 2006)
In addition to alkaline treatment, also other precipitation processes (e.g. sulphide or barium precipitation, iron co-precipitation or ettringite process) generate sludge which requires appropriate management and the characteristics of which may differ from those of alkaline treatment sludge. Some sludges may present higher potential for the recovery of different products (e.g. sludge from sulphide precipitation process for the recovery of metals). The overall management of water treatment wastes must be integrated in the planning and implementation of water treatment of a mine.
In some cases, water treatment sludge may require stabilization prior to disposal. Stabilization methods may include chemical and/or physical methods in order to increase the physical stability, decrease the surface area and permeability, and immobilize the pollutants contained in the sludge. Stabilization may also involve the safe placement of the waste. (Robertson and Shaw 1997, Department of Water Affairs and Forestry 2007)
There are a number of different techniques applied for sludge stabilization. Addition of sorbents such as clay or zeolite that bind water or other polar ions results in improved sludge stability. Fixation includes the addition of noncrystalline silica (pozzolanic materials, such as fly ash or cement-kiln dust) or cement + hydrated lime. As a result the sludge is solidified and the possible radioactive substances and heavy metals are immobilized. In thermoplastic and polymeric methods sludge is mixed with thermoplastics (e.g. asphalt, polyethylene, polypropylene, wax or bitumen) or with monomers + catalyst. The resulting material is solidified due to cooling or polymerization. In encapsulation process sludge is covered by solid materials such as powdered polyethylene which is merged with the bulk of waste by heat and pressure in order to segregate sludge from water. Encapsulation is often used together with other stabilization methods. Vitrification is an energy requiring process where sludge is turned into a glassy matrix under a high temperature. Phosphate containing additives can also be used to bind metals contained in the sludge. (Robertson and Shaw 1997, Kuyucak 2006, Department of Water Affairs and Forestry 2007)
Management of other water treatment waste streams
Other waste streams generated in water treatment processes include brine from membrane processes and concentrates from ion exchange and adsorption processes as well as spent media (adsorbents, membranes, resins etc.). Brines and concentrates are highly concentrated and their disposal requires special attention. In membrane processes the formation of final brine requiring disposal is reduced by employing recycling within the process. The brine generated in membrane processes is typically highly saline and toxic. The ion exchange processes produce concentrated regenerant solutions or large amounts of gypsum waste (GYP-CIX). (Department of Water Affairs and Forestry 2007)
Brines or concentrates may be incorporated into mine waste or tailings. Other methods for the treatment/disposal of brines and concentrates include solar evaporation in ponds, mechanical evaporation and crystallization, biological treatment or discharge in a sanitary sewer when applicable. These waste streams can also be utilized in the cultivation of halophilic algal species with commercial value or in the irrigation of salt resistant plants. (INAP 2009)
In some cases there is a potential to use water treatment waste as a source of valuable by-products. As the environmental obligations and the operational costs are increasing this possibility is getting more attention. The recovery of saleable materials may (partly) offset the water treatment costs. Recovered products can be sold outside if there is a market value, as at the Wellington-Oro mine site in Colorado, or they can be reused in the mine process at operating sites. (Department of Water Affairs and Forestry 2007) By far the application of the recovery scenarios is still quite limited and heavy metals are in many cases the main concern limiting their wider adoption (Zinck 2006).
Possible products include metals, supplements for mine site rehabilitation, alkaline chemicals, construction materials (e.g. gypsum, supplements for cement, gravel), adsorbents or coagulants for industrial wastewater treatment, growth media for halophilic organisms (brine), S and Mg salts, fertilizers, inorganic pigments, magnetic particles or media for CO2 sequestration (Zinck 2006, INAP 2009, Simate & Ndlovu 2014). The recovery of by-products must be taken into account in the selection, design and operation of the water treatment process.
Metals recovery from mining wastes such as water treatment sludge can be carried out via either hydrometallurgical or pyrometallurgical routes. Hydrometallurgical recycling involves wet chemistry techniques to separate metals for subsequent recovery. In pyrometallurgical options sludge is treated in a smelter to recover metals. The viability of this option depends on the availability and the location of the smelter, transportation costs, sludge volumes and sludge characteristics. The addition of alkaline sludge to a smelter process may be beneficial also due to added lime. After metal recovery process the residues still require disposal but contain less metals and are therefore simpler to manage. (Zinck 2005 and 2006)
Aube, B. 2004. The Science of Treating Acid Mine Drainage and Smelter Effluents
Aube, B. & Zinck, J. 1999. Comparison of AMD treatment processes and their impact on sludge characteristics. Proceedings for Sudbury ’99, Mining and the Environment II. p 261-270.
Department of Water Affairs and Forestry 2007. Best Practice Guideline H4: Water Treatment.
INAP 2009. The International Network for Acid Prevention. Global Acid Rock Drainage Guide (GARD Guide). Available: http://www.gardguide.com
Kalin, M., Fyson, A. & Wheeler, W.N. 2005. The chemistry of conventional and alternative treatment systems for the neutralization of acid mine drainage. Science of the Total Environment 366:395–408.
Kuyucak, N. 2006. Selecting suitable methods for treating mining effluents. Golder Associates Ltd. PerCan Mine Closure Course, July 13-23, 2006, Lima, Peru.
McDonald, D.M., Webb, J.A. & Musgrave, R.J. 2006. The Effect of Neutralisation Method and Reagent on the Rate of Cu And Zn Release from Acid Rock Drainage Treatment Sludges. 7th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006.
Robertson, A. MacG. & Shaw, S.C. 1997. Option for the Stabilization of Sludges from Acid Mine Drainage Water Treatment Plants. Wismut 97 Workshop, September 23, 1997.
Simate, G.S. & Ndlovu, S. 2014. Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2:1785-1803.
Zinck, J. 2005. Review of Disposal, Reprocessing and Reuse Options for Acidic Drainage Treatment Sludge. MEND Report 3.42.3. Mine Environment Neutral Drainage program (MEND). January 2005.
Zinck, J. 2006. Disposal, Reprocessing and Reuse Options for Acidic Drainage Treatment Sludge. 7th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO