Case study: Temporary remediation of calcium sulphite dams
Lonmin recently completed the temporary remediation of two calcium sulphite (CaSO3) dams (residue stockpiles), which are situated west of the company’s BMR and Smelter complex near Marikana in the North West Province. CaSO3 is a residue generated from the treatment of SO2 emissions from the smelting operation, through a wet/dry scrubbing process where lime is added to neutralise and capture the emissions. The remediation process cost in the region of R14 million (US$1.7 million). The process was designed to prevent environmental impacts by effectively encapsulating the dams for a minimum of 20 years, during which time we will be able to explore opportunities to rework the CaSO3 into saleable gypsum, which is chemically non-toxic and used in the manufacturing of cement.
The two emergency dams were constructed in 2003/2004 to store CaSO3 sludge emanating from the smelter’s scrubber system. The last of this sludge was deposited into these dams in 2006, after which we began sending it to a hazardous waste disposal facility for disposal.
During the operational period, the two dams were joined to form one large dam (residue stockpile). The dams consisted of waste mine rock and soil embankment walls, while the bottom was lined with a high-density polyethylene (HDPE) liner. Over time, the integrity of the liner was compromised and the dams began to leak. This resulted in groundwater, surface water and soil contamination. Furthermore, the dry surface of the dams resulted in windblown CaSO3 dust affecting the surrounding environment. Incidents of vandalism to the pumping system, fencing and electrical installation also took place, including theft of parts of the dam’s liner, compromising its integrity further.
- Groundwater: groundwater monitoring results indicated very high sulphate levels in the boreholes around the dams, suggesting contamination, mainly of the shallow weathered rock aquifer, as a result of seepage from the leaking dams.
- Surface water and soil: there was visible seepage of contaminated water from the dams along the toe of the dams, particularly along the eastern and southern sides. Fortunately, the rate of surface seepage was slow allowing evaporation to occur with only the remaining salts resulting in contamination of surface soils. The soil was removed and placed under the capping system to contain residual contamination. The nearest water course is approximately 2,8km away with extensive infrastructure obstructing any potential water flow in that direction.
- Dust: the surface of the dams dried out in winter and resulted in windblown dust from the surface onto the surrounding areas during the windy months of August and September.
Lonmin made a decision to temporarily close and cap the dams and to install a seepage drainage collection system, to capture residual seepage so as to prevent further environmental impacts. This is regarded as a temporary, medium- to long-term measure while the preferred long-term option is to re-work and convert the estimated 100,000 tonnes of stored CaSO3 into gypsum for the use in the production of cement and other building products. However, the technical and economic viability of this option still has to be confirmed. The capping design facilitates the possible subsequent removal of CaSO3 for re-use or disposal elsewhere. If an alternative use for the CaSO3 cannot be found, Lonmin is committed to remediate and rehabilitate the dams’ in-situ to the required departmental and regulatory standards. With this in mind, the temporary remediation measures have been carried out in such a manner that they can be incorporated into the permanent closure design at a later stage.
Technical and waste landfill engineers with geotechnical, geosynthetic and landfill qualifications and experience, were appointed to carry out the required remediation work, which began in August 2011 and was completed in April 2012.
The remediation design was based on the capping requirements for a hazardous waste disposal facility, as outlined in the Minimum Requirements for Waste Disposal by Landfill, published by the DWA, and on international best practice. The design approach was to isolate the contaminant body from the environment by effectively encapsulating it – and in this way containing the source of pollution for at least 20 years. The measures had to be sufficiently robust to resist theft and vandalism and to use low technology systems that would require minimal maintenance.