Precipitation with barium salts
Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
Barium sulphate BaSO4 (known as barite) has very low water solubility and therefore the addition of suitable Ba salts to sulphate containing mine water is one option for sulphate removal.
Description of the technology
As the solubility of barium sulphate is low compared to calcium sulphate, soluble barium can be used to effectively precipitate sulphate from mine effluents. Alternative chemicals include BaCO3, Ba(OH)2 and BaS, all of which are capable to remove sulphate to low levels. The process is also able to remove transition metals, magnesium and ammonia (and limited amount of Na). (Bowell 2004)
Ba(OH)2 and BaS can be used over a broad pH range while BaCO3 requires neutralization as a pretreatment. However, some lime is needed with any reagent to reduce metal hydroxide precipitation on the surface of barium salt. The use of Ba(OH)2 causes significant formation of gypsum which enhances sulphate removal but simultaneously increases the sludge production. (Lorax Environmental 2003, Bowell 2004)
The quantity of soluble barium needed in the process exceeds the stoichiometric amount. To save barium it can be regenerated by roasting the produced BaSO4 to form BaS. Further purging BaS with CO2 generates BaCO3 which can be recycled to the process. Alternatively, BaS can be directly returned to the precipitation step. (Bowell 2000, Bowell 2004)
A commercial process utilizing BaCO3 is CSIR ABC (alkali-barium-calcium) which consists of three stages (Bowell 2004, Kauppila et al. 2011, Simate & Ndlovu 2014)
- pre-treatment with lime and CaS to remove free acid and metals
- BaCO3 treatment to form barite
- waste processing to recover alkali, barium and calcium in coal-fired kiln
TUT MBA utilizing Ba(OH)2 as a precipitating agent is a modification of the CSIR ABC process. In this process Ba(OH)2 is utilized in two functions: for sulphate removal and in magnesium removal (precipitation of Mg(OH)2). (Simate & Ndlovu 2014)
According to Bowell (2000) the use of BaS as a precipitating agent would be beneficial compared to BaCO3 or Ba(OH)2 as it reduces the amount of gypsum produced and eliminates the need of pretreatment step as acidic water can be treated directly. Furthermore, the reaction is rapid and the only precipitate formed is BaSO4. BaS is directly obtained in the thermal recovery process and it can be reused in the precipitation phase.
Precipitation with barium salts can be applied for the reduction of sulphate from mine waters to low concentrations.
Advantages of barium salt precipitation (INAP 2009, DWA 2013) include e.g.:
- The process effectively removes sulphate
- Some processes generate by-products (such as elemental sulphur or H2S) which can potentially be processed to usable products
Disadvantages of barium salt precipitation (INAP 2009, DWA 2013) are e.g.:
- Limited experiences in full scale
- High reagent cost; can be overcome by reagent regeneration
- High demand of thermal energy for the regeneration of the barium
- Produces sludge requiring appropriate treatment and/or disposal
- Long retention time needed in some process variants
- Presence of toxic (soluble barium, H2S) and flammable (H2S) substances
- Complex process (several process steps) in some commercial processes (ABC)
The commercial CSIR ABC process can produce water with less than 100 mg/l sulphate (Simate & Ndlovu 2014). For the process to function properly, the metals must be removed before the barium precipitation phase e.g. by high density sludge (HDS) process (DWA 2013).
So far, the CSIR ABC process has been tested in limited pilot plants (DWA 2013).
The commercial CSIR ABC operates at high temperatures. Together with corrosive gases present in the process this induces the need of using special materials in the equipment. The environmental and occupational health risks of the process are considerable and must be taken into account in the process design. (DWA 2013)
Bowell, R.J. 2000. Sulphate and salt minerals: the problem of treating mine waste. Mining Environmental Management, May 2000.
Bowell, R.J. 2004. A review of sulphate removal options for mine waters. – In: Jarvis, A.P., Dudgeon, B.A. & Younger, P.L.: Mine water 2004 – Proceedings International Mine Water Association Symposium 2. – p. 75-91, 6 Fig., 7 Tab.; Newcastle upon Tyne (University of Newcastle).
DWA 2013. Feasibility Study for a Long-term Solution to Address the Acid Mine Drainage Associated with the East, Central and West Rand Underground Mining Basins. Treatment Technology Options. Study Report No.5.4. Third draft
INAP 2009. The International Network for Acid Prevention. Global Acid Rock Drainage Guide (GARD Guide). Available: http://www.gardguide.com
Kauppila, P., Räisänen, M.L. & Myllyoja, S. 2011. Best Environmental Practices in Metal Ore Mining. Finnish Environment 29 en/2011.
Lorax Environmental 2003. Treatment of sulphate in mine effluents. INAP International Network for Acid Prevention. October 2003.
Simate, G.S. & Ndlovu, S. 2014. Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2:1785-1803.