Marja Liisa Räisänen, Geological Survey of Finland, P.O.Box 1237, FI-7021 Finland, e-mail: marja-liisa.raisanen(at)gtk.fi

Passive water treatment in the former settling pond, the closed Vihanti Zn-Cu-Pb mine

Introduction of the mine site

The Vihanti closed mine, also known as the Lampinsaari closed mine, is located about 70 km South of Oulu in western Finland. The mine was opened in 1954 and shut down in 1992. At present, the area includes a tailings facility with former water treatment pools and mine village. Two mine towers, concentrator, repair workshop and heating plant were demolished when the mine was closed. The building area and its new diesel workshop is owned and in use by a private company.

Geology, ore mineralogy and chemistry

The bedrock of the Vihanti area consists mainly of gneisses and granitic, granodioritic and gabbroic rocks (Rouhunkoski 1968). The Lampinsaari ore formation is enveloped by intensely metamorphosed mica gneisses with amphibolite intercalations. The main rock type within the ore complex is a felsic metavolcanic rock which is composed of homogenous quartz porphyry, and tuff or tuffite. Other rocks include dolomite, calc-silicate rocks, cordierite gneiss and metamorphosed black shales (Rauhamäki et al. 1980, Loukola-Ruskeeniemi et al.1997).

The Lampinsaari ore complex consists of zinc and lead-zinc ores (including Au and Ag), pyrite ore, disseminated copper ore, and uraninite-apatite mineralization (Mikkola 1963, Rouhunkoski 1968, Rehtijärvi et al. 1979, Autere et al. 1991). The dominating sulphide minerals are sphalerite [(Zn,Fe)S], chalcopyrite (CuFeS₂), galena (PbS), pyrite (FeS₂) and pyrrhotite (FeS). Accessory minerals included cubanite (CuFe₂S₃), vallerite (Fe-Cu-sulphide-Mg-Al-hydroxide), gudmundite (FeSbS), tetrahedrite and tennantite (Fe-Cu-Ag-Sb-As-sulphide), and metallic gold (Rauhamäki et al. 1980, Loukola-Ruskeeniemi et al. 1997).

The zinc ore consisted of 3-20% Zn, 0.1-1.0% Cu, and 0.1-0.7% Pb (Autere et al. 1991). The trace element concentrations were on average: Au 0.4 g/t, Ag 26-30 g/t, Se 0.004%, Sb 0.02%, As 0.01-0.05%, Co 0.005%, Ni 0.01%, Mo 0.005-0.02%, and Cd 0.03% (Loukola-Ruskeeniemi et al. 1997). According to Loukola-Ruskeeniemi et al. (1997), the sulphur concentration of the pyrite ore ranged from 20-30% and average Cu and Zn concentrations from 0.2-0.4%. The average concentrations of the disseminated copper ore were 0.5% Cu, 0.1-0.5% Zn and 10-15% S (Rauhamäki et al. 1980). The mean concentrations of U and P₂O₅ in the uraninite-apatite mineralization were 0.03% and 3-5%, respectively (Rehtijärvi et al. 1979).

Mining and milling processes

The Lampinsaari ore was extracted through underground mining. In total 30.8 Mt of rocks and 27.9 Mt of ores were mined. The annual production of ore was 0.4 Mt in 1957 and about 0.75 Mt annually from 1967-1979. In 1980, the annual production increased to 0.93 Mt and reached a maximum of 1.15 Mt in 1987. A total of 1.41 Mt of zinc, 0.13 Mt of copper, 0.098 Mt of lead, 190.4 tons of silver, and 3.1 tons of gold were produced in the Vihanti concentrator. (Autere et al. 1991)

At the concentrator, zinc, copper and lead (including Ag) concentrates were produced by flotation from a combined feed of zinc ore, zinc-lead ore and disseminated copper ore. Pyrite concentrate was produced up to 1975 (Autere et al. 1991). The ore was crushed (cone crushing) and ground (rod and ball mills) for frothing with chemicals. The first step included the frothing of Cu-Pb bulk concentrate, and was followed by two parallel circuit stages of Cu-Pb separation and zinc separation from the bulk flotation tails (Autere et al. 1991). Lead minerals were separated from copper minerals by depressing. The chemicals used in the flotation were lime, K-ethyl and K-amyl xanthates, Na cyanide, Na bichromate, dextrin, copper sulphate and flotol (pine oil) (Hukki 1964).

Mine waste management

Tailings characterization

The main mine waste facility on the ground is the tailings impoundment covering 90 hectares (Fig. 1). Tailings from the flotation of different ore types were pumped as mixed layers in the impoundment. Based on the mineralogy of the ore rocks (Rauhamäki et al. 1980), tailings consist mainly of Ca and/or Mg bearing silicates (diopside, tremolite, hornblende, cordierite, garnet, and tourmaline), quartz, feldspars, micas, dolomite, barite, apatite, uraninite and sulphides (pyrite, pyrrhotite, sphalerite, chalcopyrite, and galena). According to unpublished data from the Outokumpu Company, the mean sulphide sulphur content is about 3.0% for the oldest tailings in Pond I, and 5.5% for tailings in Ponds II and III (Saari et al. 1993). According to researches of the Outokumpu Company, sulphidic metal concentrations for Cu ranged from 0.112% in the oldest tailings, to 0.08% in ponds II and III, for Zn from 0.54% to 0.22%, for Pb from 0.11% to 0.1%, and for Fe from 4.5% to 8.0%, respectively (Saari et al. 1993, Tuovinen 1999). According to static tests based on a Sobek test modified by the Outokumpu Company, tailings were classified as acid generating mine waste, of which NP/AP ratios ranged from 0.76 to 1.22 (Saari et al. 1993).

Design of the tailings facility

The tailings facility and two settling pools are locating on a bog (Fig. 1), which was drained before the facility was constructed. The tailings waste lies on peat that is compressed and forms a waterproof basal structure. Silt and till, respectively, underlie the peat layer (Antila & Hartikainen 1980). In the northwest (close to Hoikkasalo) and the northeast, water permeable sand overlies the till instead of silt. According to Antila & Hartikainen (1980), the water permeability coefficients of the silt and till ranged from a rate of 10^-7 m/s to 10^-8 m/s, and was 10^-6 m/s in the sandy layer.

The tailings facility was divided into three ponds separated by staged conventional dams. Altogether 13.7 Mt (8.1 Mm³) of tailings are stored in the facility. The oldest tailings impoundment (Pond I) in southwest was dammed with coarser tailings materials and was used from 1954 to 1967. It consists of 2.3 Mt of tailings waste. The initial starter dams of the second and third ponds (Pond II and III) were constructed with clayey till and raised with coarser tailings materials. In some localized areas, dry-sides of dam banks were propped up with waste rock boulders. Ponds II and III contain 11.4 Mt of tailings dumped from 1968 to 1992. About 10.7 Mt of tailings were backfilled in the underground mine. The grain size of backfilled tailings was coarser than the tailings pumped in the impoundment. There are no available data on the chemical composition of the backfilled tailings.

Water management of the tailings facility

Two shallow settling pools were dammed on the western side of the tailings facility (Fig. 1). During the operation, the pools settled the process waters discharged from the facility in addition to seepage and rain waters running in the inner ditch surrounding the facility. At present, the settling pools are naturally transformed wetland pools growing tall aquatic plants such as reeds, horsetails and cattails. The water level in the pools depends on annual precipitation and seeping from the tailings facility. Pool waters run off into the Alpuanoja creek, which discharges into the Vihantijoki River and finally into the Baltic Sea.

A ditch to the south of ponds I and II discharges seepage waters into three wetland pools in the southeastern corner of the facility (Fig.1). The pools were excavated in the bog and are lined by a peat layer on silt and/or till basement (similar to that of the facility). The pools are divided with boulder dams that are covered by road sand. Water flows from one pool to another by seeping through the boulders, which consists of sulphide bearing rocks, and are presumably waste rocks from the mine. The water from the final pools discharges into the outer ditch surrounding the tailings facility, and lastly into the Alpuanoja creek. The water level in the pools depends on annual precipitation and water seeping from the south side of tailings ponds 1 and II. Rims of the ditch and pools are growing reeds and their floors are covered by Fe precipitates (see rusty colour in Fig. 1).

Waste rocks management

There are no actual waste rock-heaps in the closed Vihanti mine area. Waste rocks were mainly backfilled in mine tunnels. A minor amount of waste rocks were used in road construction and to cover the facility dams. A shallow rusty waste-rock heap (<0.1 ha) was found east of the building area during the 2013 field work.

Figure 1. The closed Vihanti tailings facility, western Finland. The facility includes large and smaller settling pools collecting tailings seepage in the west, and excavated pools of three wetland segments for tailings seepage in the southeast. Arrows refer to surface water flow directions, discharging into the Alpuanoja creek. Airphoto © National Land Survey of Finland

State of the tailings facility closure

Ponds I and II of the tailings facility are covered by till and related glacial sediments, while Pond III is covered by peat overlain with wash- and peel waste from the local potato factory. Planted pines, birch and willow bushes, grass and different species of meadow flowers grow in the waste area. A free water pool exists in both tailings ponds II and III, which indicates the highest elevation of the water table in the tailings. Assumedly, the water table decreases from the pools towards the banks.

The settling pools have been allowed to transform to naturally flooded wetland pools to which seepage waters from the western sides of the facility flow. The overflow into the Alpuanoja creek depends on the annual water level in the pools. Seepage waters from the southern side of the facility flow via the excavated pools into the outer ditch of the bog surrounding the facility. The outer ditch collects the surface waters and perhaps seepage waters from the eastern banks of the facility. The water from the outer ditch runs directly into the creek on the western side of the facility.

Introduction of the Vihanti study case

The Vihanti case study introduces passive water treatment of tailings seepage in a settling pool that was originally constructed for tailings water clarification. The settling pool consists of two segments, a main settling pool and a smaller corner pool west of the tailings impoundment. The water level in the pools has fluctuated according to annual precipitation and seepage volume from the tailings. At present, the pools look like flooded mires surrounded by sandy road. Reeds and horsetails are growing in shallow water, and sedges on wet sites. Obviously, the pad consists of peat mixed with secondary inorganic solids.

This case study aims to examine water quality in the former settling pools (referred to here as wetland pools) and receiving waterways (the collection ditch and Alpuanoja creek). It is assumed that analysis of a few water samples will give a general overview of water purification in the wetland pools. Quantitative estimates of metal loads and salinity to receiving waterways are not included due to lack of monitoring data on annual water flow (volumes) and rainfall.

Materials and methods

Study materials consist of 5 water samples and their physical and chemical analysis (See sampling sites in Figure 2). Samples were collected from three sites in the wetland pools and two sites in downstream waterways (i.e. a collection ditch surrounding the pools in the north and the furthest downstream site in the Alpuanoja creek) (Fig. 2). The collection ditch receives waters from both the wetland pools and an upstream ditch surrounding tailings pond III. Electrical conductivity (EC), pH, dissolved oxygen concentration (O₂ mg/l), oxygen saturation (O%) and redox potential were measured in the field using a multi-parameter field meter (YSI Professional Plus). In addition, alkalinity was determined with a Hach digital titrator with 1.600 N H₂SO₄ to an end point of 4.5 in the field.

Unfiltered water samples of one litre volume were collected and divided into subsamples. Half litre subsample aliquots were analyzed for anions by IC. 100 ml subsample aliquots were analyzed for total organic content (TOC) with a carbon analyzer (pyrolytic method) and for total element content analysis by ICP-AES and ICP-MS. In addition, second 100 ml subsample aliquot was filtered using a multilayer 0.45 µm filter (GD/AP PVDF) and analyzed for dissolved major cation and trace metals by ICP-OES and ICP-MS, respectively, and for dissolved organic content (DOC) with the carbon analyzer. The unfiltered and filtered subsamples for the ICP measurements were acidified with suprapur HNO₃ acid and those for the TOC and DOC measurements with concentrated phosphorus acid in the field. All the laboratory analyses were carried out at the FINAS-accredited testing laboratory of Labtium Ltd.

Figure 2. Location of water sampling sites, the Vihanti mine area. Airphoto © National Land Survey of Finland.

Results and discussion

The surface water of the smaller wetland pool was neutral (7.0) and that of the main wetland pool basic (7.6-7.7, Table 1). The pH of the ditch water outside the wetland pools was 7.2, and pH of the Alpuanoja creek water somewhat lower, at pH 6.9. The pH values measured in situ in the field and after couple of days in the laboratory were almost the same (within precision and accuracy).

Redox potentials of the wetland pool waters were much higher (110-190 mV) than those of the receiving waters (<100 mV, Table 1). Electrical conductivity (at 25 °C) of the waters was slightly greater (180 mS/m) in the main pool than in the smaller pool (160 mS/m), but it almost doubled in the collection ditch (340 mS/m) outside the pools and decreased at the furthermost site of the Alpuanoja creek to the same level as it was in the smaller pool.

Table 1. Physical quality parameters, concentrations of total organic carbon (TOC), dissolved carbon (DOC) and anion concentrations of the wetland pool waters, collection ditch water and Alpuanoja creek water, Vihanti (See sampling sites in Figure 2).

The chemistry of the wetland pool waters was characterized by abnormally high concentrations of elemental remains of the flotation chemicals and minor concentrations of elements associated with sulphides (Mn, Zn, Ni, Co, Cu, Pb, As, Mo, Sb) and P, B, Ba and U (Tables 1 and 2). Typical remains of the chemicals are Ca, Mg (lime remains), Na and K (xanthates remains) and SO₄ (sulphur acid remains), and NO₃, Na and Cl (explosive chemical remains). The sum of base cation concentrations in the pool waters was on average less than half (400 mg/l) of that in the collection ditch water (930 mg/l). At the furthest downstream site total base cation concentration was 350 mg/l. A similar trend characterized the SO₄ distributions. In contrast, the NO₃ concentrations were below the lowest detection limit of NO₃ in the pools, but elevated to 3.9 mg/l in the collection ditch water and decreased to 2.7 mg/l in the water of the Alpuanoja creek at the furthermost sampling site. Concentrations of Cl were lowest in the water of the smaller pool (4.4 mg/l) and twice as much (8.0-8.2 mg/l) in the main pool. By contrast, the collection ditch water had the greatest Cl concentration (13 mg/l), which slightly decreased (11 mg/l) at the furthest downstream site.

In the wetland pool waters, total concentrations of Fe were very low, ≤0.3 mg/l, of which over 90% was soluble Fe. The collection ditch water had about 1.6 mg/l Fe (1.1 mg/l soluble Fe) and the Alpuanoja creek water considerably more at about 17 mg/l (15 mg/l soluble Fe, Table 2).

Expectedly, neutral and slightly basic pool waters had low total concentrations of Al (≤10 µg/l), of which 20 to 30% was water soluble. The concentration of Al doubled (20 µg/l) in the collection ditch water but slightly decreased (17 µg/l) in the Alpuanoja creek water. However, the solubility of Al in the ditch water was somewhat greater than in Alpuanoja creek (Table 2).

Since the zinc-lead ore deposit had disseminated U-apatite mineralization, some U and P was anticipated in the tailings seepage waters. The wetland pool waters had 6.9-8.7µg/l U, the collection ditch water 48 µg/l U, and the Alpuanoja creek water had 8.7µg/l U, of which 70-80% was water soluble (Table 2). The smaller pool had the greatest concentration of soluble P (54 µg/l) and the second greatest concentration was measured from the ditch water outside the pools (39 µg/l P). The main pool water had 30-36µg/l P and the Alpuanoja creek water 30µg/l P. Total concentrations of P (from unfiltered samples) were under the lowest detection limit (<300 µg/l) and therefore the proportions of solid bound and water soluble P could not be calculated.

Manganese and zinc were the most common sulphide metals (Table 2). Their soluble concentrations (Mn 3-15 µg/l, Zn 10-30 µg/l) were approximately a tenth of the concentrations in the collection ditch water (Mn 80 µg/l, Zn 210 µg/l). The Alpuanoja creek water had an abnormally high concentration of Mn (740 µg/l) but the Zn concentration was less than half (70 µg/l) of that of the collection ditch water. In the pool water, zinc occurred entirely in soluble form whereas manganese was more soluble in the collection ditch water (<80%) than in the pool waters (35-55%). Furthermore, the solubility of Mn (>90%) increased downstream in waterways which indicates another source for Mn than the collection ditch water. Solubility percentages were calculated by dividing concentration of the filtered sample with that of non-filtered sample (assumed to be a total concentration).

Concentrations of the other sulphide trace metals were pretty small. Concentrations of soluble Ni, Co and Mo were <1.5 µg/l and those of soluble Pb, Cu, Cd, Sb and As <0.7 µg/l. Overall, the ICP measurements from the filtered waters had greater accuracy than those from unfiltered waters. Therefore, the differences between the total and soluble concentrations could not be compared to evaluate the element concentrations bound by solid particles.

Table 2. Soluble element concentrations of wetland pool waters, collection ditch water and Alpuanoja creek water, Vihanti (See sampling site in Figure 2).

The physical characteristics and chemical content of the wetland pool waters showed no indicators for acid seepage from the tailings ponds. The sum of soluble element concentrations in the pool waters was about 40% from that of the collection ditch water, outside of the pools. This finding gives a rough estimation on the treatment rate (60%) of the wetland pools if it is assumed that the water of the collection ditch represents untreated surface water quality (contaminated with tailings seepage). Despite the fairly good function of the wetland pools, the majority of the tailings seepage waters are discharging without any controlled treatment as the results of the collection ditch and Alpuanoja creek waters showed. Furthermore, the water quality in the Alpuanoja creek indicates a natural metal source. Some of metals can be released via the drainage from the local peat being geogenically enriched with, for example, Mn and Fe. Overall, waters discharged from the ditch surrounding the tailings ponds have greater impact on the Alpuanoja creek water than the outflow waters from the wetland pools. Even though the pool waters had small concentrations of potentially toxic metals and metalloids (compared with those of the collection ditch water), they had fairly high concentrations of SO₄ as well as alkaline earth and alkaline metals, exceeding their geological background values (SO₄: 2.8 mg/l, Ca: 3.7 mg/l, Mg: 1.3 mg/l, Na: 3.7 mg/l, K: 1.5 mg/l, Lahermo et al. 1996). It can be concluded that at this site passive water purification in the wetland-type pools does not effectively prevent discharge of base cations and anions, which subsequently increase the salinity of receiving waters.

In wetland pools, the seepage waters are obviously purified by adsorption mechanisms (on organic matter and secondary precipitates) since the redox potentials were not low enough (<-150 mV) to reduce sulphates as metal sulphides and/or hydrogen sulphide (Connell & Patrick 1968, Walton-Day 2003). The slightly reducing state of the downstream waters in the bog ditch and Alpuanoja creek is obviously caused by oxygen consumption related to organic matter and precipitation of oxidized Fe (binding oxygen). Secondary Fe precipitates together with fine organic matter were observed at the furthest downstream water sampling site of the Alpuanoja creek. However, precipitation did not effectively remove Fe and Mn from solution, as they were observed to be abundantly soluble at the site.

Conclusions

Former settling pools which naturally transform to wetland pools can be suitable for passive treatment of tailings seepage. At the Vihanti study site the main water purification mechanism seems to be adsorption by solids in the pools. The water data did not reveal any indicators for sulphate reducing reactions. Compared to the water quality of the collection ditch which receives tailings seepage, the wetland pools trapped about 60% of the soluble elements (main cations, anions, trace elements). Nevertheless, the retention of SO₄ and base cations (Ca, Mg, Na, K) was not effective enough, since their concentrations in the pools were over a hundredfold compared to the background levels of Finnish surface waters.

The Vihanti case study shows that a few water samples and their chemical analysis give a very rough overview of passive water treatment in the former settling pools, i.e. present wetland pools. Furthermore, results show that the water quality of the recipient Alpuanoja creek is more dependent on the water quality of the seepage collection ditch surrounding the tailings ponds than the effluent waters from the wetland pools. The majority of tailings seepage waters are actually flowing into the Alpuanoja creek without any controlled passive or active treatment. The validation of the wetland performance cannot be based only on a few water samples and their analysis. The surface water drainage in the Vihanti tailings pond area turned out to be more complex than was originally assumed. It is necessary to examine not only the surface water quality of ditches surrounding the whole tailings pond area but also the chemical content of the wetland basal layer and stream sediments.

References

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