Effective acid and metals treatment in tailings dams

Mine sites are complex, open systems. Unlike industrial wastewater treatments, which are largely conducted in closed systems where inputs are known and outputs are controlled, mine sites are affected by a wide range of environmental factors, including rainfall, wind, complex ecologies, background geology and large-scale, man-made interventions.

When legacies from mining can last hundreds or even thousands of years (uranium mine tailings, for example, need monitoring and management for up to 10,000 years), it is sometimes hard to know how to sustainably manage and treat mining waste and remediate sites after mining has discontinued. When issues such as the large quantities of mine waste tailings and the remoteness of many mine locations are coupled with the complexity of mine chemistry and the open nature of mine site environments, the challenges for site management, treatment and remediation are compounded further.

There are three types of primary wastes from mining: waste rock, waste tailings, and wastewater. Waste rock is the mined rock not processed for ore extraction, and may include overburden, low-grade ore, and other rock and soil wastes. Mine tailings are the fine milled rock which is deposited into dams after ore extraction; and wastewater is the leftover process water and other liquid wastes which is used to pump tailings into holding dams or pumped into wastewater dams independently.

Most mine wastes are contaminated with heavy metals and acidity when sulphur is present in the mined rock; this cocktail of minerals and acidic processes often means that mine site wastes have significant polluting potential.

The formation of most acidic conditions on mine sites is due to the exposure of sulphidic minerals (often pyrite in tailings as FeS2), which are exposed to moisture and atmospheric oxygen, thereby producing acid, metal ions and sulphate ions. Additional oxidising pathways can be promoted when bacteria are present in waste tailings, causing microbial mediated oxidising reactions to occur. Examples of standard oxidising pathways for sulphidic materials are:

2FeS₂ + 7O₂ + 2H₂O = 2Fe²⁺ + 4SO₄^2- + 4H⁺

when pH > 4.0, hydrolysis can generate more acid as indicated by:

Fe³⁺ + 3H₂O = Fe(OH)₃ + 3H⁺ or 4Fe²⁺ + 6H₂O + O₂ = 4FeOOH + 8H⁺

when tailings pH is less than 3.0, reactions with ferric ions can produce more acid by:

4Fe²⁺ + O₂ + 4H⁺ = 4Fe³⁺ + 2H₂O and FeS₂ + 14Fe³+ + 8H₂O = 15Fe²⁺ + 2SO4^2- + 16H⁺

Other acid generating reactions for sulphidic ore include those generated by pyrrhotite (Fe_1-xS), sphalerite (ZnS), iron-rich sphalerite ((Zn,Fe)S), covellite (CuS), galena (PbS), chalcopyrite (CuFeS₂), bornite (Cu₃FeS₄), enargite (Cu₃AsS₄), arsenopyrite (FeAsS) and siderite (FeCO₃), when any of these replaces or exists in combination with FeS₂ in the above formulae.

Acid mine drainage (AMD), a typical input of tailings dams, commonly has pH values below 3.0 and is enriched in metals such as aluminium, arsenic, cadmium, cobalt, copper, iron, mercury, nickel, lead and zinc. Iron and sulphur oxidising bacteria, especially Thiobacillus ferrooxidans, can increase reaction rates at low pH by up to six orders of magnitude, thereby accelerating the release of metals from tailings.

The scale of problems associated with contaminated tailings at mine sites is highlighted by the number of documented sites throughout the world. There are 150,000 to 400,000 such sites in the European Union containing more than 100Mm³ of contaminated waste rock and tailings and some 110,000 contaminated sites in Australia. In 2008, the US Environmental Protection Agency estimated the number of contaminated sites in the US at 480,000, amounting to a total of 15M acres, many of them mine sites.

When tailings dams or open cut pits overflow, when dam walls collapse, or when tailings are disposed into creeks and rivers, the environmental and economic losses can be enormous. The potential penalties and liabilities that companies face from regulators can be high, and the social and environmental devastation caused can be inestimable.

A sustainable treatment for tailings dams

ViroMine Technology has been applied to treat all forms of mining waste, including AMD (which can have a pH as low as 1.0 and be produced at disused mines for hundreds of years after mining operations have ceased), tailings dam water, and sulphidic mine tailings. The technology neutralises acid, binds heavy metals in a non-leachable form, reduces on-site sludge volumes, and neutralises the long-term effects of sulphur. After treatment, the technology aids in the revegetation and remediation of mine sites. Measures such as total suspended solids (TSS), turbidity and electrolytic conductivity (EC) can also be positively affected by the technology.

Virotec has developed a range of unique ViroMine reagents to treat or prevent the damaging and sometimes devastating effects of tailings dam water and sulphidic mine tailings by permanently neutralising acid, trapping trace metals and preventing leaching, and promoting plant growth. These include Acid B, Acid B Extra, Neutra B and Terra B, each of which is safe to handle and easy to apply. ViroMine reagents settle through 10M of water within 48hrs, extracting metals in the process. Unlike lime, the treatment does not leave behind a toxic sludge but generates only a thin, non-toxic, stable sediment. The application of ViroMine reagents can prevent the formation of AMD by stabilising tailings, or it can be applied to contaminated wastewater in tailings dams to treat the problems caused by current or legacy mining.

Alternative methods

The traditional application of lime at mine sites to treat contaminated tailings dam wastewater has proven to be an inadequate, short-term treatment as it produces a toxic sludge that breaks down over time, allowing heavy metals to leach into groundwater and waterways. While generating a ‘shock’ pH adjustment, lime does not produce a viable, long-term solution to tailings dam treatment or other mine-related problems.

Given the unique chemical and physical properties of mine tailings, the quality of water used to either cover the tailings to keep them anaerobic or the waste process water that is mixed into the dam is often exotic and hard to treat. An alternative to ViroMine Technology in reducing acid and removing metal contamination in mine dam wastewater is the use of either calcium carbonate (CaCO₃) or calcium hydroxide (Ca(OH)₂). A comparison between the uses of lime and Virotec’s reagents can be summarised as follows:

• Lime dosing of tailings dam wastewater results in the production of large quantities of contaminated sludge that have to be managed and disposed of safely. In contrast, the use of Virotec’s reagents to treat the same water results in the production of comparatively small volumes of stable, readily consolidated, inert sediment.

• The sludge produced from lime dosing is unstable and may readily release heavy metals if geochemical conditions change slightly or when exposed to even mild leaching; in most cases the treatment of concentrated sludge poses a greater challenge to environmental managers than the treatment of the wastewater from when it was produced. The use of Virotec’s reagents will not release bound metals, and the longer the sludge is left to age after treatment the more tightly metals are bound.

• Under-liming can result in increased acidity because bicarbonate ions that form when lime dissolves can accelerate the decomposition of sulphides. Multiple projects show that the application of insufficient Virotec reagents, which contain very little carbonate or bicarbonate, does not accelerate the decomposition of sulphides.

• Over-liming can result in higher than desirable pH levels because lime is not adequately buffered. If the pH rises above about 9.0 the concentration of elements such as arsenic, antimony and selenium present as oxyanions and the concentrations of elements such as aluminium, copper and zinc that form anionic species (e.g., Al(OH)4-) at elevated pH can increase substantially. Virotec’s reagents are well buffered at around pH 8.5, and overdosing does not cause such problems.

• The use of Virotec’s reagents to treat metal contaminated wastewater results in much lower metal concentrations in the treated water than can be achieved by using lime or its equivalents.

Radio Hill tailings dam: a case study

Fox Resources operates the Radio Hill mine, 35km south of Karratha in the Pilbara region of Western Australia, with a plant that can produce up to 200,000 tonnes of nickel each year. Processing copper and nickel ore to produce a concentrate uses water, and after processing the metal contaminated wastewater is pumped into a 50ML tailings dam about 1km away from the plant. The two problems faced by Fox Resources in 2009 were: The tailings dam water is too contaminated to be reused in the processing plant; the dam was filling up.

Space restrictions precluded using another tailings dam and hence Fox Resources had to find a way to treat the wastewater in the existing dam in order for it to be pumped out and reused. The dam was in use at the time of treatment and both tailings sludge and contaminated water were constantly being pumped into the dam. The key objectives of the project were to remove heavy metals from the water and to achieve concentrations that would enable wastewater to be reused in the floatation process to extract copper and nickel sulphides.

The water conservation that would result from reuse of the wastewater following the application of ViroMine Technology was particularly important given the mine is located in the Pilbara region, an arid zone. The ViroMine Technology application, using Acid B reagent, converted the contaminated water in the tailings dam into a clean reservoir of water by extracting heavy metals. It also created an isolating blanket over the settled tailings in the dam to prevent escape of contaminated pore water that could interfere with treatment of the surface water. The Acid B reagent treated existing contaminants in the tailings dam as well as the constant inflow of contaminated water and tailings.

The ViroMine Technology components at Fox Resources included treatment design, engineering of plant and equipment, reagent application and treated water analyses at a certified laboratory. Application of the solution resulted in wastewater being treated to within limits that would allow it to be reused in the processing plant. This approach not only eliminated the need to find ways of disposing highly contaminated wastewater, but also substantially reduced water consumption by enabling treated wastewater to be reused as process water.

Acid B reagent was mixed into a slurry using water from the tailings dam and then sprayed onto the surface of the dam. The good dispersion properties of the reagent ensured that there was an even coverage of the entire dam. The treatment produced a thin cover of high density sediment on the floor of the dam, thus ensuring tailings could be reprocessed in the future if it became technically and economically viable to do so. The in-situ treatment was simple to apply and required no expensive capital infrastructure.

Samples taken during treatment were analysed on site and showed a steady decline in metal concentrations as the reagent was applied. Samples of the treated water were also collected for validation and analysed for copper, iron and nickel concentrations. The raw wastewater analyses, treatment targets and outcomes are shown in Table 1.

The results in the table show that despite the tailings dam continuing to receive metal-contaminated wastewater and tailings, Acid B reagent was able to produce treated water with metal concentrations well below the targets. Furthermore, Acid B reagent bound metals in such a way that they cannot be released back into solution under existing geochemical conditions or under conditions that are ever likely to exist in the future.

The high quality of the treated water made it possible to reuse the water in the processing plant, thereby greatly reducing the consumption of scarce water resources. ViroMine Technology thereby provided a cost-effective way to lower the concentrations of heavy metals in the tailings dam to well below target limits. The treatment was ecologically safe and produced a thin, stable sediment cover on the tailings and not the unstable, gelatinous sludge commonly associated with more conventional chemical treatments.

Dr Lee Fergusson is Virotec’s group chief executive officer. Email: lfergusson@virotec.com

Web: www.virotec.com