Ettringite precipitation

Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi

Introduction

Ettringite precipitation (addition of lime and Al(OH)₃) can be used to remove sulphate and heavy metals. Ettringite (3CaO·3CaSO₄·Al₂O₃·31H₂O) has very low water solubility and therefore the resulting sulphate concentrations are low.

Description of the technology

Known processes for sulphate removal with ettringite precipitation contain the main unit operations shown below.

pH lift with lime to range of 11.5-12 to enable Al-dissolution
Al³⁺ -addition to remove sulphate as precipitated ettringite
pH-reduction of the treated water with CO₂ to meet effluent discharge criteria and precipitate CaCO₃

In addition, some of the known processes apply initial precipitation of some of the sulphate as gypsum and precipitation of metals as hydroxides in a gypsum matrix before the actual ettringite precipitation. (Bowell 2004) Calcium sulphate has a solubility of approximately 2,000 mg/l as sulphate. The addition of aluminium containing chemical allows the precipitation of ettringite and the removal of sulphate to low concentration. The process provides also simultaneous reduction of calcium. The ettringite sludge must be separated by gravitation separation and/or filtration. (Reinsel 1999, Lorax Environmental 2003, Outotec 2014)

Development stage

Known commercial processes include the SAVMIN Process, the CESR (Cost Effective Sulphur Removal) Process and Outotec Ettringite Process. All commercial processes have similar principles. The SAVMIN Process uses aluminium oxide (aluminiumtrihydroxide in amorphous or gibbsite form) to create the ettringite, with recovery, whereas the CESR process uses a proprietary Al-containing chemical obtained from cement production, without recovery of the aluminium source. In contrast to other commercial processes, Outotec process does not contain separate lime addition step before ettringite precipitation step. (Lorax Environmental 2003, Outotec 2014)

Appropriate applications

Non-ferrous concentrators
Gold plants
Coal mining
All mine sites with sulphidic ore

The processes have been developed for sulphate bearing mine waters with sulphate concentrations > 2,000 mg/l.

Advantages

Based on globally available aluminium and calcium chemicals
Low concentrations of sulphate in treated water
Removal of metals
No liquid waste
Enables water recycling

Disadvantages

Solid residues to be disposed of, stability of which may vary
Relatively high reagent costs

Performance

Ettringite precipitation can reduce the sulphate concentration to ca. 200 mg/l (concentrations < 100 mg/l have also been reported) and provides also simultaneous reduction of calcium. Ettringite formation can also provide a polishing effect, allowing precipitation of metals and metalloids such as Ni, Cd, Cu, Zn, Cr, As, Se, and Cd. Boron, fluoride and up to 30% of the chloride and nitrate in water have also been removed. (Reinsel 1999, Outotec 2014)

Design requirements

The SAVMIN process utilizes stirred tank reactors in ambient temperature and pressure for the precipitation reactions. Lime is added with a conventional lime slaker and slurry pumping. Constant water monitoring for sulphate and pH is needed to allow optimized chemical dosages. (Lorax Environmental 2003)

References

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).

Lorax Environmental 2003. Treatment of sulphate in mine effluents. INAP International Network of Acid Prevention. October 2003.

Reinsel, M.A. 1999. A New Process for Sulfate Removal from Industrial Waters. Proceedings 16th Annual National Meeting of the American Society for Surface Mining and Reclamation. August 13 – 19, 1999. Scottsdale, Arizona

Outotec 2014. Outotec Ettringite Process brochure.