The mineral processing and separation is usually made with the ore particles suspended in water, and at the end of the process this water is either reused or disposed of. The wastewater can contain compounds and metals from the ore or from chemicals used in the processing and cleaning is needed before the water can be disposed of. Different cleaning processes are used depending on the allowable limits.
There is a hierarchy of processes for cleaning mining-influenced wastewater. These have developed over time as a response to ever-tightening demands on discharge limits. Which approach is best depends on the specific situation of the site and nature of the contaminants. Where the recipient is particularly sensitive stricter demands are placed, and more extensive treatment is required. In general terms the following list provides increasing degree of cleanliness, but the actual performance will depend on the nature of the wastewater.
- Internal recirculation of process water. As far as possible the water used in mineral processing operations is reused, but a fully closed loop will result in a build up of contaminants to the point they affect the product quality or operation. Hence some water must be taken from the system and processed for release to the environment.
- Geochemical barriers to contain the wastewater contaminants. The wastewater is discharged first to artificially-constructed holding pools. This allows the fine particles to settle to the bottom. The construction of the holding pool uses special minerals and materials that ensure the contaminants to not pass through. As part of the SEESIMA project the use of mine waste as part of the geotechnical barriers was studied. Read more here.
- Neutralisation and precipitation. Typically the waste water is acidic, which results in metal ions being dissolved. If the pH is raised by adding a neutralising agent some of the metal ions will precipitate out as solid particles. There is a limit to how clean the wastewater can be made, due to the solubility of the metal compound. There is also a cost involved in terms of the neutralising agent used, such as lye (NaOH) or lime (CaCO3). As an example iron hydroxide, Fe(OH)2, has a solubility of 0,52 ppm (mg/kg) while iron carbonate, FeCO3 has a solubility of 6,7 ppm. These correspond to 0,34 and 3,4 ppm Fe respectively, while the limit for drinking water is 0,2 ppm. The solubility represents a theoretical limit, which in practice may be difficult to achieve.
- Precipitation with hydrogen sulphide. Many ores contain sulphide minerals and in the processing this is released as sulphate in the wastewater, contributing to the acidity, and resulting in the dissolution of metal ions. So the wastewater needs treatment to remove both the sulphate and the metal ions. Certain bacteria can convert sulphate back to sulphide, and what is interesting is that many metal ions form insoluble compounds with sulphide. For example iron sulphide is essentially insoluble in water. Drinking water has an allowable limit of 250 mg/litre of sulphate. In the SEESIMA project the conversion of sulphate to sulphide and removal of metal ions was studied in a PhD study, which is described in more detail here. A special focus was made on demonstrating that this process could also work in cold climate, with temperatures as low as 6 deg C.
- Use of sorbents. The precipitation methods can make it difficult to recover the ions for reuse. Another approach is to use sorbents, which are certain materials that have a strong affinity to capture ions and bind them to themselves. In practice the wastewater is pumped through a bed of the sorbent material until it becomes saturated, and then is replaced with fresh sorbent. The spent sorbent can then be treated to recover the ions, or disposed of as a stable, solid waste. The SEESIMA project involved collaboration between UO and KSC on developing and testing different sorbent products. Read more here.
- Ion exchange. While sorbents are often natural materials, there have also been developed special materials like ion exchange resins that have even greater affinity for binding ions. Because of the higher material costs an important aspect is the regeneration of the ion exchange resin. Doing this in a controlled manner simplifies the recovery of the adsorbed ions for reuse. In the SEESIMA project a study was made demonstrating the potential for using bacteria to regenerate ion exchange resin. Read more about ion exchange here.
- Membrane separations. As the discharge limits become stricter some mines have implemented membrane separation, and in come cases have ended up with cleaner water than the town drinking water. In the SEESIMA project a review was prepared of applications of membrane separations for treating minewater, with emphasis on full-scale implementations rather than lab-scale studies. Cost estimates were summarised at around US$1,3/m³, which lies at the higher end of costs for precipitation systems, but lower than ion exchange systems. Read more here.