Acid Mine Water – Can it be a valuable resource?
Waterfall, Witwatersrand National Botanical Gardens
The farm where gold was first discovered in 1886 by that an Australian gold miner, George Harrison
SA mineral reserves and production (% of World)(from: DME, South Africa’s mineral industry 2006/2007)
Commodity Reserves Production
Platinum 87.7 (1) 59.3 (1)
Vanadium 31.0 (1) 48.0 (1)
Zirconium 19.4 (2) 41.6 (2)
Ferro-chromium - 40.5 (1)
Vermiculite 38.9 (2) 39.6 (1)
Chrome ore 72.4 (1) 38.7 (1)
Alumino-silicates - 36.4 (1)
Titanium minerals 18.3 (2) 19.8 (2)
Manganese ore 80.0 (1) 13.3 (2)
Gold 40.1 (1) 11.1 (1)
Ferro-Mn/Fe-Si-Mn 6.0 (4)
Fluorspar 16.7 (2) 5.0 (4)
Coal 6.1 (8) 4.5 (5)
Water quality of gold mines in Gauteng
ParameterWestern
basinEastern
basinCentral
basinRand
Uranium Grootvlei ERPMIRB
Flow (Ml/d) 20 108 60Flow (m3/h) 833 4500 2500pH 8 3.5Free acidity (mg/l as CaCO3) 700 0 300Iron(III) (mg/l as Fe) 100 0Aluminium (mg/l as Al) 6.4 0 3Total acidity (mg/l as CaCO3) 2 437 183 1 749 Iron(II) (mg/l as Fe) 800 102 800Total/Free acidity 0.29 0.00 0.17Alkalinity (mg/l as CaCO3) 0 350Sulphate (mg/l as SO4) 4800 1075 4096Calcium (mg/l as Ca) 528 216 582 Magnesium (mg/l as Mg) 147 128 250Manganese (mg/l as Mn) 228 2 15Zinc (mg/l as Zn) 11.9 4Cobalt (mg/l as Co) 4.55 1.5Nickel (mg/l as Ni) 18 5Copper (mg/l Cu) 21Uranium (mg/l U) 0.465Silicon (mg/l Si) 11Barium (mg/l Ba) 0.2Chloride (mg/l as Cl) 37.03 157 180Sodium (mg/l as Na) 50 202 104Potassium (mg/l as K) 14TDS (mg/l) 6 777.1 2 092.0 6 060.6
Decant water from gold mines in Gauteng
Basin Flow rate (Ml/day)
Far Western 65
Western 60
Central 100
Eastern 120+
Total for Gauteng gold mines
345+
Mpumalanga coal mines
80+
Limited area
Question
Can we deal with this problem in a cost-effective way?
Neutralized mine water
CONSEQUENCES OF OVERFLOWING
Pyrite (FeS2) or “Fool’s Gold” remains stable when deep underground ..
... but if exposed to 0² and H²0 (through decanting) it decomposes and forms Acid Mine Drainage (AMD) Environmental Critical Level is approximately 23m below water level – lowest natural spring
10
National Water Act of 1998
Key Principles• Decentralization• Equitable access• Efficiency• Sustainability
Key instruments:• Polluter pays principle• Mine closure certificate
Decant started March 2002 – Harmony emergency measures
Directives and apportionment (2006)
Randfontein Estates Gold Mine (Harmony 46%)
West Witwatersrand Gold Mine (DRD 44%)
Mogale Gold Mine (Mintails 0.8%)
Co-operation agreement between Mintails and DRD
Heads of Agreement between Harmony, DRD and Mintails
Apportionment
12
What is mine water?What is mine water not?
Components of AMD
4FeS2 + 14O2 + 4H2O 4FeSO4 + 4H2SO4
1. Free acid: H2SO4 already partially neutralized
2. Fe(II) acid: Cannot react with limestone/dolomite3. Low concentrations of heavy metals4. Salt (Ca, SO4)
How does AMD form?
1. Dissolution of limestone/dolomiteCaCO3 + H2O Ca2+ + HCO3
- + H+
2. Pyrites oxidation as a result of ingress water running through broken rock; 4FeS2 + 14O2 + 4H2O 4FeSO4 + 4H2SO4
3. Neutralization (natural attenuation)H2SO4 + CaCO3 CaSO4 + CO2 + H2O
4. Reciprocating contact of pyrites-rich ore with water and oxygen as the water level fluctuates from water being pumped out at a constant rate while the incoming water flow-rate fluctuates with seasonal rainfall.
Water quality of gold mines in Gauteng
ParameterWestern
basinEastern
basinCentral
basinRand
Uranium Grootvlei ERPMIRB
Flow (Ml/d) 20 108 60Flow (m3/h) 833 4500 2500pH 8 3.5Free acidity (mg/l as CaCO3) 700 0 300Iron(III) (mg/l as Fe) 100 0Aluminium (mg/l as Al) 6.4 0 3Total acidity (mg/l as CaCO3) 2 437 183 1 749 Iron(II) (mg/l as Fe) 800 102 800Total/Free acidity 0.29 0.00 0.17Alkalinity (mg/l as CaCO3) 0 350Sulphate (mg/l as SO4) 4800 1075 4096Calcium (mg/l as Ca) 528 216 582 Magnesium (mg/l as Mg) 147 128 250Manganese (mg/l as Mn) 228 2 15Zinc (mg/l as Zn) 11.9 4Cobalt (mg/l as Co) 4.55 1.5Nickel (mg/l as Ni) 18 5Copper (mg/l Cu) 21Uranium (mg/l U) 0.465Silicon (mg/l Si) 11Barium (mg/l Ba) 0.2Chloride (mg/l as Cl) 37.03 157 180Sodium (mg/l as Na) 50 202 104Potassium (mg/l as K) 14TDS (mg/l) 6 777.1 2 092.0 6 060.6
Volume of mine water under Johannesburg
• Kariba Lake = 185 km3 or 185 000 000 Ml
Parameter BasinEastern Central
Depth (m) 700 500Rise rate (m/d) 0.42 0.61 Time before decant (d) 1 667 821 Time before decant (years) 4.57 2.25 Void volume (Ml) 180 000 49 260 Mine depte (m) 3 000 3 000 Total void volume (Ml) 771 429 295 560 Total void volume (Ml) 1 066 989 Cariba lake (Ml) 185 000 000 AMD/Cariba Lake (%) 0.58
Battery acid
Mass fraction H2SO4
Density (kg/L)
Concentration (mol/L)
Common name
10% 1.07 ~1 dilute sulfuric acid
29–32% 1.25–1.28 4.2–5 battery acid (used in lead–acid batteries)
62–70% 1.52–1.60 9.6–11.5 chamber acid fertilizer acid
78–80% 1.70–1.73 13.5–14 tower acid Glover acid
95–98% 1.83 ~18 concentrated sulfuric acid
Acid Acidity (mg/l CaCO3)
Eastern Basin 50
Western Basin 1800
Central Basin 1500
Battery Acid 450 000
Gas cool drink 800
Game Reserve put at risk
Four point plan
1. Encourage mining activities to stimulate job creation (reduced labour cost, beneficiation of raw materials). The pumping and treatment cost should be offset by the value of the mined minerals, the treated water, and dissolved by-products reclaimed from the mine water.
2. Immediate implementation of:– limestone neutralization for removal of free acid, iron(II) and partial
desalination. – lime treatment for removal of toxic heavy metals and radioactivity,– In the case of Grootvlei Mine, water is already neutralized due to natural
attenuation - Passive treatment3. Implement desalination to meet water demand by 2014. Selected technology
based on capital and running costs, performance, process stability. 4. Pumping or not