Lime Treatment of Acid Mine Drainage in Canada

Although many different biological and chemical technologies exist for treatment of acid mine drainage, lime neutralisation remains by far the most widely applied method. This is largely due to the high efficiency in removing dissolved metals through neutralisation, combined with the fact that lime costs are low in comparison to alternatives. Lime treatment essentially consists in bringing the pH of the AMD to a point where the metals of concern are insoluble. These metals therefore precipitate to form minuscule particles. A subsequent separation of these precipitates is then required to produce a clear effluent which meets regional discharge criteria. The solid/liquid separation forms a sludge which, depending on the applied process, can contain 1 to 30% solids by weight. The different types of lime neutralisation processes, including their respective strengths and weaknesses, are discussed in this paper. The evaluated processes are linked to concrete examples of existing treatment systems throughout Canada. The Basics of Lime Treatment The principle of lime neutralisation lies in the insolubility of heavy metals in alkaline conditions. By controlling the pH to a typical setpoint of about 9.5, metals such as iron (Fe), zinc (Zn), and copper (Cu) are precipitated. Other metals such as nickel (Ni) and cadmium (Cd) require a higher pH, in the range of 10.5 to 11 to effectively precipitate the hydroxides (Figure 1). The greatest difference between the lime treatment processes is the method of separating the solids and the sludge that is formed. The resulting effluent chemistry is very similar for all lime treatment processes. Lime dissolution is the first step of the neutralisation process. For large treatment systems, quicklime is used. This lime must first be hydrated (slaked) and is normally fed to the process as a slurry. The hydrated lime then dissolves to increase pH. The two following equations illustrate these reactions: CaO + H2O ⇒ Ca(OH)2 Ca(OH)2 ⇒ Ca + 2OH Presented and published at “Brazil-Canada Seminar on Mine Rehabilitation”, Florianópolis, Brazil, December 1-3, 2003. The increased pH then provides hydroxide ions which precipitate the metals. The following reaction shows the precipitation reaction with Zn as an example: Zn 2+ + 2OH ⇒ Zn(OH)2 Among the metals to precipitate as per the above equation is ferrous iron. Unfortunately, ferrous hydroxides are not as stable as ferric hydroxides when the sludge is exposed to acidic waters or natural precipitation. For this reason, aeration is often applied to oxidise the iron to the more stable form, as per the following equation: Fe(OH)2 + 1⁄2 H2O + 1⁄4 O2 ⇒ Fe(OH)3 Figure 1: Metal Hydrolysis A common by-product of lime neutralisation is gypsum. Gypsum precipitation occurs as the AMD is often rich is sulphate and the calcium added from lime will bring the solubility product well above saturation. This reaction is often responsible for scaling in treatment processes. Ca(OH)2 + H2SO4 ⇒ CaSO42H2O Another common by-product of lime neutralisation is calcium carbonate. The inorganic carbon for this reaction can either come from the AMD itself or be a result of carbon dioxide from air, which is dissolved during aeration. This carbon dioxide converts to bicarbonate and then partially to carbonate due to the high Presented and published at “Brazil-Canada Seminar on Mine Rehabilitation”, Florianópolis, Brazil, December 1-3, 2003. pH. The carbonate fraction will precipitate with the high calcium content of the slurry to form calcite (calcium carbonate). This calcite can play an important role in the stability of the final sludge product as it provides neutralising potential to the sludge as it is stored. It is also an indicator of the process lime efficiency: more efficient neutralising processes will produce less calcite. Ca(OH)2 + CO2 ⇒ CaCO3 + H2O Lime Treatment Processes The following sub-sections explain the concept and discuss advantages and disadvantages of different treatment processes. The processes are in order of the simplest to the most complex and recent treatment processes. The older methods often use lime less efficiently and do not allow for good control of the treatment system. The more recent processes require a greater capital investment but are considerably more efficient for lime usage and waste production. The metal precipitates created during all processes are wastes typically identified as sludge. This sludge must be disposed of in an environmentally acceptable manner. As sludge disposal costs can be important, the most advanced processes minimise the waste volumes by creating a higher-density sludge. The sludge disposal and lime costs over the long term usually justify a more important capital investment due to significant savings in operating costs. Pond Treatment This treatment system entails adding lime in a stream or mixing system and allowing the precipitates to settle in a pond. The pond is often divided into a primary and secondary section. The primary pond serves to accumulate the precipitated sludge and can quickly be filled. These often require yearly dredging of sludge which then requires a storage area. The secondary pond is larger and requires a long retention time with laminar conditions to allow for “polishing” of the effluent.