Strategies for Treatment of Mine Impacted Waters with High Ammonia-N Feed Levels using Membrane Systems
Bonner, Ricky; Naidu, Jivesh; Germishuizen, Charne; Franzsen, Sebastian
Miwatek, South Africa
Extraction of minerals in mining operations requires an explosive typically containing ammonium nitrate. Incomplete reaction allows residual ammonium nitrate to be mobilized in drainages from the site. Environmental protection agencies (EPAs) have not standardized the ammonia-N limit in receiving surface waters. However, for example due to toxicity to aquatic life a limit of 1 ppm is recommended by the Ghanaian EPA. Mining companies typically deploy membrane systems to treat impacted waters due to the broad range of contaminants that can be separated. This enables the water balance over the tailings storage facility (TSF) and waste rock dumps to be managed as well as satisfy permitting requirements for environmental authorities. Ammonia-N RO rejection data for feed streams emanating from mine dewatering operations are scarce.
Ammonia-N rejection data was generated for 4-inch high rejection brackish and low energy sea water RO membranes on a typical neutral pit lake feed matrix synthesized in the Miwatek laboratory. RO machine design and rejection tests were performed in accordance with the necessary ASTM specifications. A preliminary scaling potential assessment of the feed indicated an 80% recovery is achievable in a full-scale plant with a two-stage RO. Flux and feed chemistry should vary considerably across the element string and hence the permeate ammonia-N loading from each element. Tests were performed at a permeate flux and feed chemistry resembling lead and tail elements in the design. Testing enabled computation of membrane-specific ammonia-N permeability coefficients (B-values) and pure water permeability coefficients (A-values) to predict overall permeate quality.
The ammonia-N B-value and A-value for the low energy sea water membrane were found to be 34% and 59% lower than those for the high rejection brackish water membranes, respectively. A case study was presented for a feed containing 10 ppm ammonia-N and TDS of 1800 ppm with an environmental discharge target of 0.8 ppm ammonia-N. The calibrated model projected that only the low energy sea water membrane could produce a permeate within specification whilst requiring considerably higher power draw. An opex optimization study was presented comparing (1) running sea water membranes only, (2) brackish water membranes with permeate recycle and (3) brackish water membranes with zeolite for ammonia-N polishing.
Conventional mine water treatment plants using brackish water RO membranes cannot produce treated water within acceptable environmental compliance targets with high ammonia-N feed levels (>10 ppm). A more sophisticated design is required incorporating either a more complex RO system or secondary treatment.