7.6 Arsenic remediation at Obuasi
In many parts of the world, gold and base metals are associated with naturally-occurring deposits of arsenic in what are commonly referred to as arsenopyritic orebodies. When this host rock is crushed and treated to release gold, arsenic may also be released into the tailings or waste residues.
Ghana is host to one of the world's largest arsenopyritic gold-bearing orebodies, and the orebody at Obuasi mine - where mining has been conducted in some form for hundreds of years - is no exception. In fact, because of its geology, the Obuasi region has a higher than normal natural arsenic background level.
In the early 1990s an arsenic precipitation plant was installed at the Pompora Treatment Plant (PTP) in Obuasi for the commercial recovery of arsenic from the roaster flue gases. The arsenic trioxide was precipitated, recovered, placed in bags, and initially sold into Europe for commercial applications. (arsenic trioxide is used for preserving wood and as a pesticide.) However, the market for arsenic declined and this ultimately led to the cessation of sales in the mid-1990s. With no markets, or alternative disposal methods, bags of arsenic were accumulated on site at Obuasi, creating a large stockpile of approximately 10,000 tons of unusable product. The PTP roaster facility was shut down in 2000 and arsenic generation ceased.
In the interim, the Biox treatment process was introduced at the Sansu Sulphide Treatment Plant (STP). During this process, arsenic trioxide is converted to arsenic pentoxide (and other more stable compounds) and deposited onto the tailings dams. It appears, however, that over the years, the Pompora stream has been polluted by arsenic from Obuasi as a result of inadequate storage of the stockpiled bags. This issue was identified as a significant concern during the AngloGold due diligence study, prior to the business combination with Ashanti. Subsequently, AngloGold Ashanti has set about addressing the problem.
As a short-term remediation step, a lined storage dam has been constructed at the old heap leach site, at the south end of the mine adjacent to the new STP plant. This is a secure area where, with all the appropriate personnel protective equipment and handling facilities, it will no longer pose an uncontrolled hazard. The bagged arsenic is being moved to this new facility where it will be stored and gradually disposed of by blending it into the Biox process circuit (at a pre-determined rate) where it will be chemically stabilised. From the STP it will be deposited as a component of the tailings residue onto the Sansu Tailings Storage Facility. At current production rates, it will take approximately six years to dispose of in this fashion. One of the alternatives to disposing of the arsenic trioxide into a lined hazardous waste land-fill facility, which will require indefinite management, is to convert the arsenic into a more stable complex and dispose of it onto the conventional tailings storage facility. This approach continues to be evaluated.
The Ghanaian Environmental Protection Agency (EPA) has given its permission for the arsenic to be moved to the new holding pond where it will be stored until the EPA is satisfied that the Biox treatment process is a reliable method of disposal. The Ghanaian EPA has recently indicated that effluent discharged into the Nyam River, should not exceed 0.2 parts per million (ppm). Following the incident in June 2004 (see
box below) sampling systems have been improved to monitor the process more rigorously and an improved alert system has been put in place in the event of higher-than-permitted arsenic levels being founded in tailings dam discharge.
Elevated arsenic levels in the Nyam River being addressed
The STP Biox plant at Obuasi was commissioned in 1993 with an arsenic stablisation plant, specifically to address the safe disposal of naturally-occurring arsenic. As part of the process, dissolved arsenic was initially fixed into a stabilised form by a reaction with limestone prior to discharge into the tailings stream. A cost-saving exercise initiated in 1996/1997 saw the replacement of limestone with carbon-in-leach (CIL) tailings for the stabilisaton of the arsenic. As the volumes of CIL tailings have increased, so the residence time in the arsenic stabilisation tanks has
reduced, resulting in reduced effectiveness of the stabilisation process.
The Ghanaian EPA has as a guideline that arsenic discharge should not exceed 0.2ppm. While arsenic levels in the tailings in the dam have exceeded EPA guidelines for some time, the levels of arsenic in the nearby Nyam River have been within EPA limits after dilution. However, an accidental spillage in June 2004, resulted in a discharge in excess of this limit.