Geochemistry, Mineralogy and Environmental Impact of Precipitated Efﬂorescent Salts at the Kabwe Cu–Co Chemical Leaching Plant in Zambia ⇑ O

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Applied Geochemistry 25 (2010) 1815–1824

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Applied Geochemistry

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Geochemistry, mineralogy and environmental impact of precipitated efﬂorescent salts at the Kabwe Cu–Co chemical leaching plant in Zambia ⇑ O. Sracek a,b, , F. Veselovsky´ c,B.Krˇíbek c, J. Malec c, J. Jehlicˇka d a OPV s.r.o. (Protection of Groundwater Ltd.), Beˇlohorská 31, 169 00 Praha 6, Czech Republic b Department of Geology, Faculty of Science, Palacky´ University, 17. listopadu 12, 771 46 Olomouc, Czech Republic c Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic d Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic article info abstract

Article history: Precipitated efflorescent salts in Cu–Co chemical leaching plant wastes in Kabwe, Zambia, have been Received 28 January 2010 studied using XRF analysis, powder X-ray diffraction, scanning electron microscopy (SEM), Raman spec- Accepted 14 September 2010 troscopy, and evaporation experiments combined with geochemical modeling. Field samples of efflores- Available online 18 September 2010 cent salts contained up to 14.32 wt% Cu and 1.42 wt% Co. In the field, the principal minerals in the salts Editorial handling by B. Wang 2þ were gypsum (CaSO42H2O), moorhouseite (Co0.6Ni0.3Mn0:1(SO4)6H2O), bloedite (Na2Mg(SO4)24H2O), starkeyite (MgSO44H2O), chalcanthite (CuSO45H2O) and kroehnkite (Na2Cu(SO4)22H2O). In the evaporation experiment, gypsum precipitated in the first and second evaporation steps, chalcanthite and biebe-

rite (CoSO47H2O), started to precipitate in Step 3, after evaporation of 60% of water, and maximum amounts of Cu and Co sulfate salts accumulated in Step 4 after precipitation of 80% of water. Epsomite

(MgSO47H2O), and hexahydrite (MgSO46H2O), precipitated only after evaporation of 97.5% of water in Step 5. Presence of chalcanthite, bloedite, and kroehnkite in precipitates of Step 4 was also confirmed by Raman spectroscopy. Magnesium and Na content in Cu and Co-sulfate phases gradually increased with evaporation progress, e.g. bieberite was replaced by Co-bloedite and chalcanthite was replaced by kroehnkite in later stages of evaporation. The precipitation order was consistent with results of geochemical modeling. The principal difference between field data and data obtained in the evaporation experiment was the presence of less hydrated sulfate phases in the field, which can be explained by different time scales available for evaporation and consecutive dehydration. Dissolution experiments using efflorescent salts collected in the field indicated fast dissolution with an instantaneous drop in pH to about 4.0 and a very fast increase of dissolved species concentrations. Such behavior may have a serious environmental impact at the beginning of the rainy period in November and December, when seepage through the impoundment dam was recorded. The wastes at the Kabwe site represent a long-term source of contamination, which may be especially significant as a consequence of on-going climatic change with increasing intensity of precipitation. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction capacity of waste rock may exceed its acid generation potential 2 with resulting neutral pH and high concentrations of SO4 , but Acid mine drainage (AMD) is one of the most serious global low concentrations of Fe in the drainage waters. Under neutral environmental problems. It has been studied in many countries and oxidizing conditions, Fe precipitates as oxyhydroxides on the around the world such as Canada (McGregor et al., 1998; Johnson surface of sulfides such as pyrite, which limits further pyrite oxida- et al., 2000; Lefebvre et al., 2001; Sracek et al., 2004; Salzsauer tion (Nicholson et al., 1990), and results in adsorption of released et al., 2005), Sweden (Strömberg and Banwart, 1999; Salmon and metals (Blowes et al., 1998; Romero et al., 2007). Malmström, 2004), Peru (Smuda et al., 2007), Russia (Gieré et al., During periods of high evaporation, secondary sulfate minerals 2003) and Australia (Ritchie, 1994). Typical features of mine drain- may precipitate as efflorescent salts on the surface of mining 2 age waters are low pH and high concentrations of SO4 , Fe and wastes (Bayless and Olyphant, 1993; Seal and Hammarstrom, other metals (Blowes et al., 2003). In some cases the neutralization 2003). Common minerals in efflorescent salts are melanterite 2+ (FeSO47H2O), rozenite (FeSO44H2O), halotrichite (Fe Al2(SO4)4 2+ 3þ 22H2O), copiapite (Fe Fe4 (SO4)6(OH)220H2O), chalcanthite (Cu- ⇑ Corresponding author at: Department of Geology, Faculty of Science, Palacky´ SO 5H O), goslarite (ZnSO 7H O), among others (Jambor et al., University, 17. listopadu 12, 771 46 Olomouc, Czech Republic. Tel.: +420 4 2 4 2 585634538. 2000; Hammarstrom et al., 2005). The behavior of the principal E-mail address: [email protected] (O. Sracek). ions in the evaporation process is described by the Hardie–Eugster

1816 O. Sracek et al. / Applied Geochemistry 25 (2010) 1815–1824 model (Hardie and Eugster, 1970; Eugster et al., 1980). This model mite deposited in a continental rift environment. The ore grades interprets the chemistry of water undergoing evaporation in terms average 3 wt% Cu and 0.18 wt% Co in deposits where Co is recov- of a succession of geochemical divides. Successive water types de- ered. Trace amounts of Au, Pt and Ag have been recovered from pend on initial water chemistry and molar ratios of principal ions Cu slimes. It is estimated that 30 million tones of Cu metal have (Drever, 1997). Common efflorescent mineral salts like melanterite been produced in Zambia since mining began on a full scale in and rozenite may store significant amounts of metals such as Cu, 1930 (Kamona and Nyambe, 2002). Co, and Zn, which in some cases reach up to 50 mol%, but these In 2008, 601,505 t of Cu and Co concentrates from the Copper- metals may also precipitate in distinct mineral phases. Storage of belt were transported to the Sable Zinc Plc plant at Kabwe where metals in secondary minerals may also significantly affect water they were milled, leached and refined, producing 10,767 t Cu and chemistry. For example, at Iron Mountain, California, Alpers et al. 565 t Co. Large volumes of sludge produced during the chemical (1994) reported decreasing Zn/Cu ratios during the wet season leaching of ore concentrates were deposited in the local tailings and increasing ratios during the dry season. This is caused by a pond. preferential uptake of Cu over Zn into precipitated melanterite in The climate in northern and central Zambia is characterized by the dry season and release of Cu from dissolving melanterite in three principal seasons: a rainy season from November to April, a the wet season. dry-cold season from May to July, and a dry-hot season from Au- The efflorescent salts sequester acidity and metals temporarily gust to October. The total annual precipitation is about 1000 mm, and release them later during rain or snowmelt events (Keith which falls mostly during the rainy season (JICA, 1995; Petterson et al., 2001; Hammarstrom et al., 2005). These spikes of acidity and Ingri, 2001). This means that maximum amounts of efflores- and metal concentrations can pass through a watershed in a few cent salts can be expected in the late dry season (September– hours with disastrous consequences such as killing of aquatic spe- October). cies. The impact of such spikes will become more significant with The principal objectives of the study were: (1) to determine the expected climatic change, which will cause longer dry periods fol- succession of secondary mineral precipitation during continuous lowed by very intense rains and mine tailings flushing (Nordstrom, evaporation of pore water, (2) to determine the composition of 2009). This means that knowledge of efflorescent salt composition these secondary minerals, and (3) to determine the impact of min- and their solubility is necessary for prediction of environmental eral dissolution on the environment. Laboratory evaporation impact of mine wastes. experiment data were compared with field data collected from The study took place at Kabwe in central Zambia (Fig. 1) where the Kabwe chemical leaching plant wastes. Since Cu and Co are local Pb and Zn ores were mined and processed in the past (the the principal metals in the exploited ore and leaching fluids, large Kabwe mine; Kamona, 1993; Kamona and Friedrich, 2007). At amounts of precipitated Cu- and Co-sulfate phases were expected. present, ore flotation concentrate is treated in the local plant which is owned and operated by Sable Zinc Plc. located approximately 2. Materials and methods 2 km south of the Kabwe town centre. The plant comprises a sol- vent-extraction and electro-wining tank house. During 2005– 2.1. Field sampling, evaporation experiment and dissolution 2006, the Sable Zinc circuit, originally constructed for Zn treat- experiment ment, was converted to Cu and Co treatment. Now the facility treats Cu and Co flotation concentrates of sulfide and carbonate Sludge resulting from chemical leaching was sampled close to ores from the Copperbelt area. The Copperbelt of Zambia and the the discharge point at the Kabwe chemical leaching plant (Fig. 1). Democratic Republic of the Congo represents one of the world’s The sludge was decanted and filtered through a 1 lm filter in the largest Cu and Co ore districts. The Cu–Co mineralization in the field and the produced water was stored at 4 °C until the evapora- Copperbelt occurs within the lower part of the Neoproterozoic Ka- tion experiment. One liter of water was then gradually evaporated tanga Supergroup adjacent to its contact with underlying pre-Ka- at 25 ± 2 °C and samples of water and precipitated salts were taken tanga geological units (Porada and Berhorst, 2000). Copperbelt at the following remaining water fractions: 0.8, 0.6, 0.4, 0.2, and type ores are stratiform to stratabound, sediment-hosted deposits 0.025. In the dissolution experiment, 20 g of efflorescent salts were characterized by finely disseminated Cu–(Co)–Fe sulfides (predom- dissolved in 400 mL of deionized (DI) water, the same solids to inantly chalcopyrite, cobaltiferous pyrite, and bornite ± carrolite) water ratio as used by Hammarstrom et al. (2005). During dissolu- in host rocks that may include quartzite (arkose), shale and dolo- tion, pH and EC were monitored and samples were collected for

Fig. 1. Geographic location of Kabwe site and location of sampling points for: water (circle), efﬂorescent salt (crosses), bulk waste (inverted triangle), and soil proﬁle (). Author's personal copy

O. Sracek et al. / Applied Geochemistry 25 (2010) 1815–1824 1817 analyses after 1 h, 6 h and 24 h. Simultaneously with sludge sam- 3. Results pling, the efflorescent salts which had precipitated on the surface of the Kabwe leaching wastes were also sampled. Samples were 3.1. Macroscopic description and bulk composition of precipitates kept in plastic bags until analyzed. Samples were also collected from an excavation of a soil profile ca. 100 m south of the 3.1.1. Field impoundment dam (Fig. 1). However, the excavation was impossi- Typical efflorescent salts found at the surface of wastes from the ble below 20 cm due to the presence of a very hard ferruginous Kabwe chemical leaching plant are shown in Fig. 2. Efflorescent hardpan. mineral crusts form on the surface of the wastes up to several cm thick at elevated sites surrounded by water ponds. A crust of 2.2. Solid phase analyses efflorescent salts was also found on the plain south of the impoundment dam (Fig. 1), where seepage of contaminated water Bulk metal concentrations in field precipitate samples were occurs during the rainy period. The color of the crust varies from determined by X-ray fluorescence (type ALPHA, Innov-X, Woburn, white to blue (Cu-phases) and pink (Co- and Mn-phases), Fig. 2. USA). Selected solid phase samples were analyzed by X-ray diffrac- These precipitates contain up to 143,200 ppm (14.32 wt%) of Cu tion (XRD), using a Philips X’Pert System diffractometer with a sec- and 14,170 (1.42 wt%) of Co (Table 1). Contents of other metals ondary graphite monochromator. The analyses were performed in precipitates such as Zn are also high (up to 1926 ppm). The efflo- using Cu Ka radiation (40 kV, 40 mA), in the range 3–80° 2h (step rescent salt crust is enriched in several metals compared to the size 0.05° 2h, counting time 1.5 s per step for qualitative phase bulk wastes and the concentration factor may reach about 20 for analyses and for unit cell parameters step size 0.02° 2h, counting Cu and 30 for Co. However, there are some exceptions because time 15 s per step). The XRD patterns were interpreted using the the crust is depleted in Fe, As, Pb and Ba compared to the bulk ZDS-WX Search/Match X-ray diffraction software. Refined unit cell waste composition (Table 1). This surface layer is scraped off by lo- parameters were calculated with the Burnham (1962) program. cal people, packed into sacks and sold back to the mining company. Several solid phase samples were also studied using a scanning The waste water pH is between 2.7 and 2.9 and, thus, the salt col- electron microscope CamScan S 3200, equipped with microanalytic lectors are walking through H2SO4 to reach the elevated sites cov- system Link ISIS 300 with an energy-dispersive spectrometer (EDS, ered by efflorescent salts. manufacturer Oxford Instruments), accelerating voltage of 15 kV, Profiles of soil metal contents vs. depth at an excavation along and a probe current of electron beam of 3 nA. For micro-Raman the flood plain of the mining wastes (Fig. 1) are shown in Fig. 3. analyses of evaporation experiment precipitates, a multichannel There is a strong enrichment in the surface layer (reaching more Renishaw InVia Reflex Raman microspectrometer coupled with a Peltier-cooled CCD detector was used. Excitation was provided by the 514.5 nm line of a continuous-wave 10 mW Ar-ion laser. The samples were scanned from 100 to 1600 cm1 and from 2000 to 4000 cm1 at a spectral resolution of 2 cm1. The scanning parameter for each Raman spectrum was taken as 10 s and 20 scans were accumulated for each experimental run to optimize the signal-to- noise ratio. Multiple spot analyses on different areas of the same sample provided similar spectra and confirmed the spectral reproducibility.

2.3. Water analyses

In the laboratory, collected leachate was filtered through a 0.45 lm filter, then split into one subsample acidified with ultra- pure HCl for determination of cations and metals, and a second unacidified subsample. Samples of water collected during the evaporation–dissolution experiments were treated in the same way as the leachate samples. Due to very high concentrations, samples were diluted in 1:100 ratios and calculated concentrations were scaled back to mg/L. Cations and metals were determined by FAAS (Varian AA 280 FS) under standard analytical conditions. The analytical precision of the individual solution AAS analysis was below 5% for both Co and Cu determinations. The detection limits for the solution were 0.010 mg/L Co and 0.005 mg/L Cu, respectively. The accuracy (expressed as a percentage deviation from recom- mended values) did not exceed 10% for Cu. Anions were determined by HPLC (Dionex ICS 2000).

2.4. Geochemical modeling

Speciation calculations for field leachate samples and for each evaporation step were performed using the program PHREEQC (Parkhurst and Appelo, 1999) and using thermodynamic data com- piled from databases of minteq.dat and llnl.dat. The SIMUL program (Reardon et al., 1995) which uses Pitzer’s parameter approach was used to check applicability of PHREEQC at high ionic Fig. 2. Photographs of efflorescent salts at Kabwe: (a) efflorescent salt crust, (b) strength range. mud cracks with precipitated Cu- and Mg-sulfates. Author's personal copy

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Table 1 Bulk composition of samples, contents in ppm.

Element/sample Fe Mn Cu Co Ni Zn As Pb Ba Bulk material 73,450 317 7647 430 36 587 74 601 646 Surface precipitate 10,492 23,386 143,204 14,165 591 1926 10 200 325 Evaporation experiment 1947 14,372 106,644 146,894 1864 2209 8 10 1725

layer and had acquired a lighter color after dehydration. Finally, after evaporation of 97.5% of the water (Step 5, remaining water fraction 0.025) a layer of grey and pink precipitate without strati- ﬁcation could be seen at the bottom.

3.2. Water chemistry

The water chemistry of decanted sludge samples collected at the Kabwe site is provided in Table 2. The water is acidic, with

pH 2.89 and is of Na–Mg–SO4 type, roughly corresponding to the brine of Type VI in the Hardie–Eugster model, with low Cl concentration (Drever, 1997). Sulfate concentrations are very high, reaching 126,000 mg/L and the respective Na and Mg concentrations are 15,100 mg/L and 6470 mg/L. Carbonate alkalinity is zero due to low pH. Concentrations of Cu and Co are 34,400 mg/L and 17,700 mg/L, respectively, indicating relatively low efficiency and significant losses during the ore treatment process. The concentration of Mn is 3570 mg/L. Respective concentrations of Ca and Fe are much lower, reaching 778 mg/L and 196 mg/L. Among other metals, only Zn occurs in significant concentrations of 567 mg/L. Fig. 3. Metal contents vs. depth out of mining wastes lagoons. The evolution of selected species concentrations during the evaporation experiments is shown in Fig. 4. Concentrations of both than 10,000 ppm) of Ca, Zn and Co. Below this layer metal contents Na and Mg increased gradually from Step 1 to Step 4, but the in- are lower, about 1000 ppm, and are almost constant down to a crease in Mg concentration was much steeper. The concentration depth of 20 cm, where ferric hardpan did not allow further excava- of Mg only decreased in Step 5 (Fig. 4a). This behavior indicates tion. In contrast, the Cu content is almost constant along the entire no precipitation control until the very late stages of evaporation profile, about 900 ppm. Lead shows a similar depth profile to Ca, Zn for Mg, and a partial precipitation control for Na. and Co, but its content is about an order of magnitude lower. The concentration of K increased sharply, but Ca concentration decreased during evaporation and leveled-off between Steps 3 and 4(Fig. 4b). This behavior suggests a strong control by the precipi- 3.1.2. Evaporation experiment tation of Ca-phases such as gypsum. In the laboratory, 1 L of tailings water from Kabwe was gradu- Concentrations of Cu and Co generally increased until Step 3 ally evaporated at room temperature. After evaporation of 20% of and then started to decrease (Fig. 4c). This indicates precipitation the water (Step 1, remaining water fraction 0.8), gypsum precipi- control in this step and later. An unexpected decrease of Co con- tated and remained in suspension. White needles of gypsum were centration in Step 2 could be caused by analytical error. The con- retained on filters. After evaporation of 40% of the water (Step 2, centration of Mn increased until Step 4 and then slightly remaining water fraction 0.6) white needles of gypsum again ap- decreased (Fig. 4c). peared on the filters, but there were no precipitates at the bottom The concentration of SO4 increased gradually and only leveled of the flask. After evaporation of 60% of the water (Step 3, remain- off after Step 4 (Fig. 4d). However, the increase does not indicate ing water fraction 0.4) a massive precipitation of Cu- and Co-salts conservative behavior of SO4 because there is evidence of precipi- started. A layer of dark blue crystals more than 1 cm thick formed tation of SO4 salts even before Step 4. Also, the almost linear in- at the bottom of the beaker, overlain by about a 1 cm thick layer crease is different from the expected exponential increase in the composed of large violet to dark pink crystals on the top. After re- late stages of evaporation. The concentration of SO4 was much moval of water and consecutive drying, the dark pink crystal color higher than concentrations of co-precipitated cations such as Ca, turned to light pink. The dark blue layer at the bottom composed Cu and Co and, thus, the precipitation had only limited impact more than 50% of the precipitate. After evaporation of 80% of the on the SO4 concentration. By Step 5, Mg–SO4 precipitation had al- water (Step 4, remaining water fraction 0.2) a dark blue layer ready started to play a role. had formed at the bottom again and a dark pink layer at the top There was also a gradual increase of Al and Cl concentrations of the precipitate, but the pink crystals were smaller than in the (Fig. 4e). Both species seem to behave conservatively, without pre- previous step. In this case the pink layer was thicker than the blue cipitation control.

Table 2 Water chemistry of decanted sludge sample collected at Kabwe site, concentrations in mg/L.

Parameter pH Ca Na Mg K SO4 Cl Cu Co Fe Mn Zn Pb F Kabwe-water 2.89 778 15,100 6470 120 126,000 253 17,700 34,400 196 3240 567 1.3 12 Author's personal copy

O. Sracek et al. / Applied Geochemistry 25 (2010) 1815–1824 1819

Fig. 4. Evolution of water chemistry in evaporation experiments: (a) Mg, Na, (b) Ca, K, (c) Cu, Co, Mn, (d) SO4, (e) Cl, Al.

3.3. Mineralogy of precipitates and Co-SO4 phases were less abundant. The principal Cu phase was chalcanthite (CuSO45H2O), while kroehnkite (Na2Cu(SO4)2 3.3.1. Field 2H2O), was present as a minor Cu phase. The principal phases of Results of powder X-ray diffraction are shown in Table 3 and in Co were CoSO4H2O and moorhouseite with nominal composition 2þ Fig. 5a. In the ﬁeld, gypsum (CaSO42H2O), predominated and Cu (Co0.6Ni0.3Mn0:1(SO4)6H2O), but Co was also present as a substitu-

Table 3 Results of X-ray diffraction of precipitate samples.

Mineral Formula Field Steps 1 and 2 Step 3 Step 4 Step 5

Gypsum CaSO42H2O113

Chalcanthite CuSO45H2O4 11

Bieberite CoSO47H2O11

(Co-rich bloedite) Na2(Co,Mg)(SO4)24H2O1

Kroehnkite Na2Cu(SO4)22H2O5 34

(Co-retgersite) (Ni,Co)SO46H2O5

(Co-pentahydrite) (Mg,Co)SO45H2O5 5

Hexahydrite MgSO46H2O 3

Starkeyite MgSO44H2O3

Co-poor bloedite Na2(Mg,Co)(SO4)24H2O3 1 Moorhouseite 2þ 4 Co0.6Ni0.3Mn0:1(SO4)6H2O CoSO4H2O3

Higher number indicates a lower amount (1 = maximum, 5 = minimum). Minerals in parentheses are theoretical Co-phases, which have not been described in nature to date. Author's personal copy

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Fig. 5. Selected X-ray diffraction patterns of precipitates: (a) ﬁeld, Kabwe, sample FZ-25, 1 Bloedite, 2 Co-Pentahydrite, 3 Kroehnkite, 4 Gypsum, (b) evaporation, Step 4, sample FZ-38. 1 Bieberite, 2 Co-Regersite, 3 Co-Bloedite, 4 Kroehnkite. tion element in several other phases including bloedite (Na2Mg(- 3.3.2. Evaporation experiment SO4)24H2O), and starkeyite (MgSO44H2O). Microphoto images Results of X-ray diffraction of samples collected for the evapo- based on SEM in back-scattered electron (BSE) mode are shown ration experiment revealed only gypsum in Steps 1 and 2 (Table in Fig. 6. Crystals of chalcanthite precipitated in the upper crust 3 and Fig. 5b). Massive precipitation of Cu and Co-salts started in of mine wastes are shown in Fig. 6a. A mixture of Co-poor bloedite Step 3 with a 0.4 remaining fraction of water. The principal miner-

(average Co content of 2.28 wt% based on EDS), kroehnkite and als were chalcanthite, bieberite (CoSO47H2O), and gypsum. Strati- gypsum in pink surface crust (Fig. 2a) is shown in Fig. 6b. Other fication was observed with chalcanthite at the bottom of the minerals detected by SEM were gypsum, bloedite and starkeyite. beaker and bieberite on the top. Bieberite contained a significant Based on energy-dispersive spectrometry (EDS), bloedite included amount of Mg. In Step 4, stratification was preserved with Cu-salts small amounts of Co and Mn. at the bottom and Co-salts at the top, but the thickness of the Cu-

Fig. 6. (a) SEM microphotograph, Kabwe, chalcanthite crystals in surface crust, (b) SEM microphotograph, Kabwe mine tailings, mixture of Co-poor bloedite, kroehnkite and gypsum, (c) SEM microphotograph, large crystals of bieberite in Co-rich bloedite matrix, evaporation experiment, 0.4 fraction of initial water, (d) SEM microphotograph, Co- poor bloedite aggregates, evaporation experiment, 0.2 fraction of initial water. Author's personal copy

O. Sracek et al. / Applied Geochemistry 25 (2010) 1815–1824 1821 salts decreased compared to Step 3. Chalcanthite predominated as In the precipitates of Step 4, Raman spectra (Fig. 7) indicate the a Cu salt, but bieberite was replaced by Co-rich bloedite (average presence of chalcanthite with Raman bands at 986, 1098, 1124, 1 Co content of 12.47 wt% based on EDS) (Fig. 6b). Another Cu-phase 1147 cm for SO4 related vibrations, 210, 280, 333, 428 and was kroehnkite. Cobalt was also present in Co-retgersite ((Ni,Co)- 614 cm1 for partly or completely Cu–O related features as well 1 SO46H2O), and Co-pentahydrite ((Mg,Co)SO45H2O). The composi- as second order bands at 3205, 3350 and 3482 cm , confirming tion of the precipitates in Step 4 was similar to the composition of the presence of five molecules of water. This corresponds well to precipitates collected in the field. In the last Step 5, Co-poor bloe- the data obtained by Bouchard and Smith (2003). Bloedite phases dite predominated. Kroehnkite and hexahydrite (MgSO46H2O), display (S–O) stretching modes bands at 990–991 (s), 1071, 1123 were also found, but hexahydrite was probably formed by dehy- and 1164 cm1 which roughly correspond to values obtained by dration of epsomite (MgSO47H2O). One week after the experiment Stoilova et al. (2003). Raman spectra of another hydrated SO4 was finished bieberite was loosing water and was transformed to phase contain (S–O) stretching modes at 797, 1091, 1124, and Co-pentahydrite. Unit cell parameters of Co-rich bloedite are con- 1144 cm1 and lattice modes at 218, 253 and 465 cm1. A rela- sistent with those of other minerals of the group (Table 4). How- tively broad Raman band occurs with a maximum at 3400 cm1, ever, regarding similar unit cell parameters of several minerals, it which confirms the presence of H2O molecules in the structure. is difficult to use them for determination of chemical composition. Due to the content of Na and Cu in this phase, detected Raman Large crystals of bieberite in a Co-rich bloedite matrix which had spectroscopy provides a plausible confirmation of the presence of precipitated in evaporation Step 3 (0.4 fraction of remaining water) kroehnkite. are shown in Fig. 6c. The mutual position of crystals indicates ear- The Raman spectra clearly show structural differences between lier precipitation of bieberite followed by precipitation of Co-rich some of SO4 phases. On the other hand, minor variations in chem- bloedite filling the surrounding matrix. Aggregates of Co-rich bloe- istry of structurally similar minerals are reflected by small wave- dite which precipitated in evaporation Step 4 (0.2 fraction of number shifts in their Raman spectra. remaining water) are seen in Fig. 6d. In summary, bieberite precipitated in earlier evaporation steps and was replaced by Co-bloedite in later evaporation steps. In Co-bloedite, the content of Co de- 4. Geochemical modeling creased with evaporation progress while the Mg content increased. Based on EDS, the contents of Na and Mg in precipitated salts in- Saturation indices for selected minerals during the evaporation creased with evaporation progress. The content of Mn increased experiment are shown in Table 5. No redox parameters were mea- until Step 3 and then decreased. sured and water at each modeling run was assumed to be at equi-

librium with atmospheric O2 with a log PO2 of 0.68. The initial water, which corresponds to water discharged at the Kabwe site, is already supersaturated with respect to gypsum and Table 4 anhydrite and both minerals may precipitate due to their relatively Unit cell parameters of bloedite-like phases Na2Me(SO4)24H2O. fast precipitation kinetics. Water is also supersaturated with re- 3 Me a [Å] b [Å] c [Å] b [°] V [Å ] Ref. spect to barite, K-jarosite, and pyrolusite. In Step 1 of the evapora- Mg 11.128 8.246 5.543 100.80 499.62 1 tion, water becomes supersaturated with respect to chalcanthite Fe 11.156 8.256 5.557 100.20 503.73 1 and ﬂuorite. Supersaturation with respect to bieberite is reached Co 11.104 8.249 5.541 100.30 499.36 1 in evaporation Step 2. Supersaturation with respect to both miner- Ni 11.045 8.193 5.535 100.50 492.48 1 Zn 11.080 8.256 5.534 100.20 498.23 1 als is then maintained until Step 4. This is consistent with the (Co,Mg) 11.075(2) 8.283(1) 5.532(1) 100.01(1) 499.7(2) 2

1–ICDD (2002). Table 5 2 – This paper. Saturation indices (SI) for selected minerals in evaporation experiment calculated by PHREEQC.

Mineral/ Initial Step 1 Step 2 Step 3 Step 4 Step 5 step water

Anglesite 0.14 0.34 0.21 0.33 0.95 0.63 Anhydrite 0.31 0.25 0.46 0.60 0.90 0.61 Barite 1.44 1.75 1.84 2.11 1.87 1.38 Bianchite 3.02 2.72 2.50 2.43 2.41 2.67 Bieberite 0.05 0.04 0.16 0.27 0.18 0.16 Brochantite 6.00 6.82 7.45 6.89 8.51 11.70 Chalcanthite 0.45 0.13 0.21 0.23 0.09 0.15

CoSO4 5.24 5.17 4.91 4.69 4.68 4.98

CoSO46H2O 0.07 0.05 0.16 0.28 0.21 0.13 Epsomite 0.91 0.68 0.47 0.21 0.02 0.03

Fe(OH)3(a) 1.74 3.27 3.80 3.71 6.34 8.43 Fluorite 1.30 0.07 0.29 0.74 2.71 3.16 Goethite 4.17 2.49 2.12 2.23 0.39 2.72 Goslarite 2.79 2.50 2.30 2.23 2.23 2.49 Gypsum 0.49 0.57 0.61 0.72 0.99 0.72 Halite 4.41 4.27 3.82 3.36 2.62 2.38 Hausmanite 9.24 11.50 11.85 10.78 11.34 14.01 Hexahydrite 1.29 1.05 0.83 0.56 0.34 0.32 K-jarosite 4.47 1.49 0.88 1.44 5.36 10.04 Manganite 1.21 1.96 2.09 1.74 1.94 2.82 Melanterite 9.48 9.90 10.18 3.36 12.52 10.53 Pyrolusite 3.38 2.63 2.52 2.86 2.68 1.80 Tenorite 3.51 3.97 4.21 4.03 4.55 5.55

ZnSO4H2O 4.06 3.71 3.47 3.31 3.22 3.45 Fig. 7. Raman spectra of (a) chalcanthite and (b) kroehnkite in the 160–1225 cm1 region. Bold font – supersaturation. Author's personal copy

1822 O. Sracek et al. / Applied Geochemistry 25 (2010) 1815–1824 water chemistry evolution, where both minerals started to precip- negative than those calculated by PHREEQC. Water is slightly itate massively between Steps 2 and 3, with chalcanthite precipi- undersaturated with respect to gypsum and there is a higher de- tating somewhat earlier. In Step 2, supersaturation is also gree of undersaturation with respect to chalcanthite and especially reached with respect to CoSO46H2O, which corresponds roughly with respect to epsomite. It is probable that PHREEQC-based activ- to moorhouseite. Supersaturation with respect to gypsum, anhy- ity coefficients are overestimated and thus so are ion activity prod- drite and fluorite is maintained until the last evaporation Step 5. ucts (IAP) and SI values. However, based on water chemistry On the other hand, water becomes undersaturated again with re- evolution and identified precipitated phases, saturation with respect to Cu- and Co-sulfates after Step 4 because most of the Cu spect to gypsum and chalcanthite (not with respect to epsomite) and Co has already been precipitated. This is in good agreement is reached. It seems that SIMUL-based SI values are underestimated with the water chemistry evolution where a sudden drop in Cu and the real values are somewhere between values calculated by and Co concentrations is observed in this step (Fig. 3c). Supersatu- both programs. ration with respect to epsomite starts only in Step 5, when 97.5% of water has evaporated. Water becomes relatively close to saturation 5. Dissolution experiment with hexahydrite, but saturation is not reached. It is probable that the hexahydrite found by mineralogical analyses had formed by Values of pH and electrical conductivity (EC) measured during dehydration of epsomite. the dissolution experiment using efflorescent salts collected in However, SI values calculated using PHREEQC are only esti- the field are provided in Fig. 8. An instantaneous drop in pH from mates because the ionic strength of the water samples is beyond 7.2 down to 3.9 was followed by a stabilization at pH 4.1. The in- the range of the applicability of the extended Davies equation used crease of EC was also fast, but more gradual, with initial values for the calculation of activity coefficients (Zhu and Anderson, reaching 12.2 mS/cm after only 1 min. After 1 h, the EC increased 2002). The program SIMUL (Reardon et al., 1995) based on Pitzer’s to 19.1 mS/cm and kept increasing to 20.4 mS/cm after 6 h. How- equations and which is applicable for highly concentrated solu- ever, there was no more change after 24 h. The fast increase of tions was used to check the SI values calculated by PHREEQC. How- EC after 1 h, reaching 93.6% of its final value after 24 h, is consis- ever, the database for SIMUL is much more limited compared to tent with the very high solubility of sulfate minerals. PHREEQC and no data for Co were available. The SI values for some Concentrations of principal ions in dissolution experiment are minerals in evaporation Steps 2–4 are provided in Table 6. The given in Table 7. After only 1 h, respective concentrations of Cu, trends are consistent, but it is evident that these values are more Co, Mg and SO4 reached 4886 mg/L, 9606 mg/L, 1864 mg/L and 38,179 mg/L. This corresponds to 98.7%, 97.9%, 96.5% and 96.8% of concentrations reached after 24 h of dissolution. Somewhat Table 6 slower was the increase of Na and especially Ca concentrations, SI values for some minerals in the evaporation experiment as calculated by SIMUL. with respective values of 3528 mg/L and 297 mg/L after 1 h, corre- Mineral/step Step 2 Step 3 Step 4 sponding to 90.1% and 85.3% of final concentrations. Gypsum 0.15 0.24 0.20 Chalcanthite 0.38 0.40 0.60 6. Discussion Epsomite 1.19 1.04 0.96

Processes and mineral assemblages found in the field and during the evaporation experiment were similar, but there were also some differences, which can be explained by several factors. First, the time scales are very different. The climate in central Zambia is characterized by a prolonged dry period from May to October. Al- most all precipitation, about 1000 mm/a, falls in the period from November to April. Once the rainy period is over, efflorescent salts precipitate on the surface of the wastes. This means that several months are available for evaporation and crystallization in the field compared to only few days in the evaporation experiment. One consequence is the formation of dehydrated forms of initially hy-

drated minerals, in the ﬁeld. For example, the initial Mg–SO4 phase in the evaporation sequence is typically highly hydrated hexahy-

drite (MgSO46H2O). However, in the ﬁeld at the Kabwe site less hydrated starkeyite (MgSO44H2O) was found. This mineral phase was not observed in the evaporation experiment. This is consistent

with observations of fewer hydrated SO4 phases found at sheltered places during dry periods in mining regions around the world including southwestern Indiana, USA (Bayless and Olyphant, 1993), the Andean Altiplano in Peru (Smuda et al., 2007), and Fig. 8. Evolution of pH and EC values in the dissolution experiment. Oslavany site in the Czech Republic (Dokoupilová et al., 2007). Dehydration of hydrated mineral phases depends on temperature

Table 7 Concentrations in dissolution experiment, units are mg/L.

Time/ion Na Ca Mg Fe Mn Cu Co Zn Al SO4 1 h 3528 295 1864 72 929 9606 4886 123 106 38,179 6 h 3660 307 1900 69 1054 9628 4899 124 112 39,213 24 h 3916 346 1931 69 1055 9731 4986 124 115 39,438 Author's personal copy

O. Sracek et al. / Applied Geochemistry 25 (2010) 1815–1824 1823 and relative humidity conditions. Epsomite is transformed to hex- There is an on-going deposition of chemical leaching wastes at ahydrite at 25 °C and at a relative humidity of about 54% (Chou and the Kabwe site. Recently, the impoundment dam height has been Seal, 2003a). On the other hand, transformation of bieberite to increased, but seepage through the dam still occurs. Assuming in- moorhouseite at 25 °C occurs at a higher relatively humidity of creased intensity of precipitation linked to climatic change (Nord- about 59% (Chou and Seal, 2003b). However, some dehydration strom, 2009), the site may represent a long-term source of was also observed in the evaporation experiment, e.g., bieberite contamination for the surrounding environment. which formed in Step 3, was later transformed to Co- pentahydrite. Another factor contributing to the observed difference in min- 7. Conclusions eral assemblages between the field data and evaporation experiment is the impact of sulfide oxidation and neutralization A study of efflorescent salts precipitated at the Cu–Co chemical reactions in the underlying wastes after their deposition in the leaching plant, in Kabwe in central Zambia, revealed a strong field. Efflorescent salts originally precipitated from water disposed enrichment of Cu and Co salts. Field samples of efflorescent salts with the waste material. However, in the late dry period, the evap- contained up to 14.3 wt% of Cu and 1.42 wt% of Co. In the field, orated solution is fed mainly by unsaturated upward flow from the principal minerals in the salts were gypsum, moorhouseite, deeper zones, where several geochemical processes take place. In bloedite, starkeyite, chalcanthite and kroehnkite. In evaporation mining wastes, the process generating mine drainage is the oxida- experiments, gypsum started to precipitate in the first evaporation tion of sulfides like pyrite (Blowes et al., 2003), steps, followed by chalcanthite and bieberite precipitation in Step 2 þ 3, after evaporation of 60% of water. Maximum accumulation rates FeS2ðsÞþ3:75O2ðgÞþ3:5H2O ¼ FeðOHÞ3ðsÞþ2SO4 þ 4H ð1Þ of Cu- and Co-SO4 salts were observed in Step 4 after precipitation and chalcopyrite, of 80% of water. Mg–SO4 minerals such as epsomite and hexahy-

2þ drite started to precipitate only after evaporation of 97.5% of the CuFeS2ðsÞþ4:25O2ðgÞþ2:5H2O ¼ FeðOHÞ3ðsÞþCu water. The order of precipitation was consistent with results of 2 þ þ 2SO4 þ 2H ð2Þ geochemical modeling. Due to very different time scale of the evaporation experiment, less hydrated phases were found in the Both processes result in precipitation of Fe(OH)3(s) as indicated field. A dissolution experiment using efflorescent salts collected in the equations above (at pH > 3.0) or jarosite, KFe3(SO4)(OH)6 (at in the field indicated fast dissolution with a resulting instanta- pH < 3). Precipitation of Fe(III) phases is consistent with the rela- neous drop in pH to about 4.0 and a fast increase in dissolved con- tively low dissolved Fe concentrations observed at the Kabwe site. centrations. In Zambia this behavior may cause a serious Copper is released into pore water or is adsorbed on precipitated environmental impact at the beginning of the rainy period in Fe(III) hydroxides. Similarly, oxidation of carrolite, Cu(Co,Ni)2S4, November and December. Compared to temperate humid climate provides a source of Co and Ni into pore water. Also, secondary sites, there is only one principal flush-out event, which may result minerals like malachite, which form in the oxidation zone of the in extremely high dissolved concentrations at the beginning of the ore deposit and which are not removed during treatment, may rainy period, when increased seepage through the impoundment be an additional source of metals. dam occurs. The environmental impact may be enhanced by on- When fast-acting neutralization minerals like calcite are pres- going climatic change with increasing intensity of precipitation. ent in the solid phase, acidity produced by the oxidation of sulfides is neutralized and gypsum precipitates simultaneously, Acknowledgements þ 2 CaCO3ðsÞþ2H þ SO4 þ 2H2O ¼ CaSO4 2H2OðsÞþH2CO3 ð3Þ The funding for this study was provided by the Czech Science Magnesium can be supplied by the dissolution of dolomite, Foundation (GACR 205/08/0321/1) and by the Ministry of Educa- which is frequently present as a gangue mineral in Zambia (Sracek tion, Youth and Sports (MSM0021620855). We thank associate edi- et al., 2010). Products of reactions in the unsaturated zone of tor Bronwen Wang and an anonymous reviewer for comments, wastes such as Cu, Co, Mg, and SO4 are transported upward by cap- which helped to improve the manuscript. illary forces and contribute to the formation of efflorescent salts. 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