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Presented and published at & 8220;Brazil-Canada Seminar on Mine Rehabilitation& 8221;, Florianópolis, Brazil, December 1-3, 2003. Lime Treatment of Acid Mine Drainage in Canada Bernard Aubé, P & 8230; & 8230; Eng. EnvirAubé 361 Aumais, Ste-Anne-de-Bellevue Québec, Canada, H9X 4A9 [email protected] and Janice Zinck, M.Eng CANMET-MMSL Natural Resources Canada 555 Booth Street, Ottawa, Ontario, Canada K1A 0G1 [email protected]. ca Abstract Although many different biological and chemical technologies exist for treatment of acid mine drainage, lime In fact, pH control sometimes requires an alkali instead of an acid as ferric & 8230;. Process for Efficient and Low Cost Treatment of Acid Mine Drainage& 8221; & 8230;

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 + H 2 O ? Ca(OH) 2 Ca(OH) 2 ? Ca2+ + 2OH Presented and published at & 8220;Brazil-Canada Seminar on Mine Rehabilitation& 8221;, 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 + ½ H 2 O + ¼ O 2 ? Fe(OH) 3 Presented and published at & 8220;Brazil-Canada Seminar on Mine Rehabilitation& 8221;, 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 + ½ H 2 O + ¼ O 2 ? 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 + H 2 SO 4 ? CaSO 4 . 2H 2 O 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

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