TreatmentofSulphate inMineEffluents
EXECUTIVESUMMARY
In the treatment of Acid Rock Drainage (ARD) little attention has focused on the mitigation ofdissolved sulphate; this may be attributed to its lower environmental risks and regulatorystandards when pared to those for acidity and dissolved metals.However regulatoryagencies are being increasingly concerned over elevated sulphate concentrations in effluents owing largely to its impact to the salinity of receiving waters. This concern will like result innecessary. Although sulphate treatment is currently under investigation and in many cases inpractice paratively little information has been documented. In light of this the InternationalNetwork for Acid Prevention (INAP) has decided to investigate the current state of the art ofsulphate treatment.
The primary objective of this review is to present an overview of the current state of the arttreatment processes to reduce sulphate (with or without dissolved metals) in mine efluents. Thedevelopments in reducing dissolved sulphate in ARD. organization and sharing of knowledge on such processes will hopefully guide future
The consumption of drinking water containing sulphate concentrations in excess of 600 mg/L monly results in laxative effects. The taste threshold for the most prevalent sulphate saltsranges from 250 to 500 mg/L. While the World Health Organization (WHO) does not propose ahealth-based guideline for sulphate in drinking water it does remend that health authoritiesare notified if sulphate concentrations exceed 500 mg/L.Accordingly most countries in theworld remend a drinking water standard for sulphate between 250 and 500 mg/L.
After a review of the available information several treatment processes were selected for review.The selection criteria for each process were based on: (1) applicability to sulphate removal and(2) availability of data on sulphate removal and costs. The treatment processes meeting thesecriteria were organized into 4 categories:
(1) chemical treatment with mineral precipitation;(2)membranes;(3) ion-exchange; and (4) biological sulphate removal.
The selected sulphate treatment processes are wel-documented and often tested in (pilot) plantsor large field-trials. For many of the selected treatment processes at least some estimate of theincluded in this review because they cannot be used for the removal of sulphate from ARD. capital and/or operational costs are available. Several other ARD treatment processes are not
lime or limestone addition (2) addition of barium salts (3) the SAVMIN process and (4) the Treatment processes that represent chemical ammendments with mineral precipitation are: (1)
cost-effective sulphate removal (CESR) process.Membrane technology include: (1) ReverseOsmosis (RO) (2) the SPARRO process and (3) Electrical Dialysis Reversal (EDR) while ion-exchange technologies include: (1) the GYP-CIX process and (2) Metal Precipitation and Ion-removal include: (1) Bioreactors (2) Constructed Wetlands (3) Alkalinity Producing Systems and (4) Permeable Reactive Barriers.
The most important characteristics of the sulphate treatment processes used in the case studies aresummarized in Table I to Table III for the different types of treatment technology.
When paring costs of the different treatment processes it should be noted that: (1) estimatedcosts depend strongly on the specific process design local market prices and local labour costs;(2) cost estimates are time-sensitive and have not been normalized to present day cost.Accordingly the reported capital and operating costs should be used with caution.
Among the processes that use chemical treatment with mineral precipitation (Table I) themine water. Although the limestone/lime process can only reduce sulphate concentrations to 1200 mg/L it is inexpensive and therefore useful as a pretreatment process for other moreexpensive treatment systems. The more expensive SAVMIN process can reduce sulphateconcentrations to very low levels. Both processes also remove (trace) metals from mine water.The BaS and CESR processes are probably the most expensive and the CESR process producesthe largest amount of sludge. Compared to other types of treatment technology (Table II andTable Il) chemical treatment processes with mineral precipitation produce the largest amount ofsludge. Any development of new techniques which reduce or recycle these sludges would makethe chemical treatment processes much more attractive.
Among the treatment processes that use membranes or ion-exchange (Table I) only the GYP- CIX process and possibly also the SPARRO process are really suitable for the treatment ofscaling mine waters (high in SO and Ca as opposed to high in Na and CI). The major advantageof all treatment proceses is that they can produce high quality water that can be used (sold) asdrinking water. A major disadvantage of this class of treatment processes is the production ofbrine that requires disposal (and additional costs). Operating costs of the GYP-CIX process andpossibly also the SPARRO process could be greatly reduced if the water pretreatment involvedthe limestone/ime treatment process.
The bioreactor and the permeable reactive barrier appear to be the most eficient among thetreatment processes that use biological sulphate removal. Although the processes operate on different scales both show the greatest potential for sulphate removal from mine water. Operatingcosts of the bioreactor could be further reduced by developing alternative less-expensive carbonand energy sources.The latter could also contribute to the long-term performance of the
sulphate removal also remove trace metals by precipitation of metal sulphides.
Constructed wetlands and alkalinity producing systems are the least efficient sulphate removalsystems and the contribution to the sulphate removal in constructed wetlands appears to belimited. The contribution of mineral precipitation (gypsum) to the removal of sulphate from minewater in wetlands appears to be more important than the contribution of sulphate reduction. Thelimited extend of sulphate reduction in constructed wetlands may be related to their design whichwere originally based on the removal of other dissolved elements (e.g. Fe Mn). Hence newdesigns may have to be developed if constructed wetlands are to be used specifically for sulphateremoval by sulphate reduction.Despite the limited sulphate removal trace metals are veryeffectively removed in constructed wetlands.
Based on an extensive review of available treatment technologies that can be used for sulphate removal from mine drainage the following conclusions can be made:
Existing treatment technologies for mine drainage are generally poorly documented and
not readily available.A better organization (centralized) and exchange of informationcould greatly improve and guide future advances in the development of better and lessexpensive treatment technologies.●Although stringent guidelines for sulphate concentrations in mine water do not yet exist it is relatively easy to bine the removal of trace metals with the removal of sulphateusing existing treatment processes (e.g. SAVMIN GYP-CIX or Biological SulphateReduction).Chemical treatment processes with mineral precipitation are generally the least expensive but produce the largest amounts of waste (sludge).Improved methods to reduce orrecycle the voluminous sludge need to be developed.With the possible exception of the SPARRO process membrane treatment processes arenot well suited for the treatment of mine waters because of the high concentrations of Caand sulphate.Even with the production of high quality (drinking) water operating costs are presently very high. Thus unless high-purity water is required on site or can be soldto offset operational costs membrane treatment processes willikely have less applicationfor sulphate treatment.●Treatment processes that use ion-exchange (GYP-CIX) are a good alternative for membrane treatment processes if scaling mine waters require treatment. The frequency ofion-resin regeneration can be reduced by pretreatment of the mine water (e.g.limestone/lime process). Similar to the water produced in membrane treatment processes water produced by the GYP-CIX process can be sold as drinking water.●Among the treatment processes that use biological sulphate reduction the bioreactor andpermeable reactive barrier are the most efficient. An added benefit of biological sulphatereduction is the removal of trace metals from the mine water. For an improvement of thesystems additional research and development of their design is required.
