acid-forming capacity of lead–zinc mine tailings and its implications for mine rehabilitation

Acid-forming capacity of lead±zinc mine tailings

and its implications for mine rehabilitation

J.W.C. Wong, C.M. Ip and M.H. Wong

Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong

Acid mine drainage problemswere experienced in a Pb^Znmine operation at Lechang, in the northern partof Guangdong Province, People's Republic of China. Geochemical and acid generation evaluations weremade on fresh tailings including tailings fine, tailings sand and high sulphide tailings, and oxidised tailingswith the aim of providing information onmine rehabilitation. All fresh tailings had a pH higher than 7 while theoxidised tailings had a pH of 4.9 (range 1.6 to 7.4). Only samples with pH < 3 had an electrical conductivity(EC) > 4 dSmÿ1.Total S contents of all tailings sampleswere very highwith the high sulphide fraction havinga mean S content of 38%. All fresh tailings had a high acid neutralisation capacity (ANC) while half of theoxidised tailings had an ANC less than zero.The results from the acid^base account and the net acid genera-tion test indicated that all fresh and oxidised tailings samples were acid-forming except for the sand fractionsamples. All tailings samples contained high total concentrations of Cd,Cu, Fe,Pb and Zn but low concentra-tions of total nitrogen and phosphorus. The preliminary results demonstrated that the tailings were all acid-forming especially the high sulphide fraction which should be kept permanently unexposed under imperme-able cap or water.

Keywords: Lead-zincmine tailings, acid generation test

Introduction

Tailings produced from the mining industry consti-

tute about 23% of the 618 million tonnes of solid

wastes arising in China in 1992. Currently about 220

million m2 of land have been occupied by these tail-

ings from various mining operations in China and

about 5% of these areas were arable land (National

Environmental Protection Agency, 1992). One of the

major impacts of tailings on the environment is the

generation of acidic drainage from sulphide contain-

ing tailings (Ferguson and Erickson, 1988). Iron di-

sulphide is the major acid-forming component and is

associated with metal ores. The exposure of the sul-

phide-containing tailings to water and oxygen will

result in the formation of a series of soluble hydrous

iron sulphates which hydrolyse to produce highly

acidic, and ferrous- and sulphate-enriched drainage

(Caruccio et al., 1988).

Under highly acidic conditions, metals including Fe,

Mn, Cu, Al, Pb, Cd, Zn and As will be released from

tailings in levels toxic to plants and animals, espe-

cially those produced from metal mining. Acid drain-

age from these tailings' disposal areas may result in

significant contamination of surface and ground-

water (Ferguson and Erickson, 1988; Short et al.,

1990). Physical transportation of mill tailings may

also pose a significant health risk and cause serious

soil and plant contamination (Robbed and Robin-

son, 1995; Walder and Chavez, 1995). Restoring tail-

ings' disposal areas does not just aim to maintain a

self-sustaining ecosystem but also to stabilise the

tailings from acid-forming conditions, to reduce ero-

sion of tailings by vegetation cover, and to minimise

leaching via increasing evapo-transpiration (Norland

and Veith, 1995). The low pH, and elevated heavy

metal and salt contents are the major limitations

affecting the establishment of vegetation in these

tailings. Elevated sulphate contents are commonly

found in tailings' disposal areas where acid mine

drainage occurs (Robbed and Robinson, 1995; Fer-

guson and Erickson, 1988). Deficiencies in organic

matter, nitrogen and phosphorus are also found in

mine wastelands including tailing ponds, contribut-

ing to the poor physical condition and low nutrient

content in these areas.

An understanding of the geochemistry and acid-

forming ability of tailings is of the essence in man-

aging acid mine drainage. The aim of the present

study was therefore to evaluate the tailings geochem-

istry and the acid-generation capacity of a Pb±Zn

mine at Lechang in the northern border of Guang-

dong Province, People's Republic of China, to

understand the implications for developing a

revegetation programme for the tailings' disposal

areas (Figure 1). The Pb±Zn mine is about 4 km

east of Lechang City with an area of about 1.5 km2.

The climate is humid sub-tropical with an annual

rainfall of about 1500mm, mainly from summer

thunderstorms. The mine deposit is located in the

east of the mine site. The mineralisation mainly con-

sists of pyrite, sphalerite, galena, magnetite and to a

lesser proportion of calcite, muscovite, and quartz.

Mining is carried out in a conventional underground

0269±4042 # 1998 Chapman & Hall

Environmental Geochemistry and Health (1998), 20, 149±155

way. Approximately 25 000 tonnes of waste rocks

and 30 000 tailings are generated annually with a

dump area about 8 300m2 and 60 000m2, respect-

ively.

Materials and Methods

Eight composite surface oxidised tailings' samples

were collected from the 9-year-old tailings' disposal

area which showed a strong evidence of the oxidation

of sulphide minerals in the tailings. The 60m� 130m

tailings pond was divided into eight sampling re-

gions. Five individual samples were sampled from

the surface 10 cm layer of each region and mixed to

form one composite sample.

Another eight fresh tailings samples were collected

from the fresh tailings' disposal area receiving tail-

ings freshly produced from the mill. Froth flotation

was the main recovery method utilised during the ore

processing at the Lechang Pb±Zn Mine. Lime is used

in this process to separate pyrite from the other ore

sulphide minerals. Three composite high-sulphide

tailings samples were collected from the discharge

outlet of the high-sulphide tailings pond. The high-

sulphide tailings are sold for the production of sul-

phuric acid in nearby factories. After separating the

pyrite enriched high-sulphide tailings, the rest of

the tailings are disposed of as a slurry into the tail-

ings pond with the coarse fraction (tailings sand)

settling first at the point of addition, and the fine

fraction (tailings fine) being pushed towards the cen-

tre of the pond. Three composite tailings sand sam-

ples and two composite tailings fine samples were

collected from the surface 10 cm of the tailings dis-

posal area by mixing five individual samples into one

composite sample. All samples were stored immedi-

ately in tightly sealed plastic bags, and then trans-

ported back to the laboratory for subsequent

analysis.

Each sample was analysed for saturated paste pH

and electrical conductivity with a pH and a conduc-

tivity meter (Page et al., 1982). Soluble SO2ÿ4 and

alkalinity were determined on water extract at 1:5

solid:water ratio (w/v) by turbidimetric and titration

methods, respectively (American Public Health Asso-

ciation, 1985). Total organic carbon was determined

by the Walkley±Black method while total N and

total P were extracted by Kjeldhal digestion followed

respectively by the Berthelot reaction and the molyb-

denum blue method (Page et al., 1982). Soluble and

total metal contents were extracted by deionised

water at 1:5 solid:water ratio and concentrated

HNO3 acid digestion, respectively, and determined

by atomic absorption spectrometry (AAS) (Page

et al., 1982).

Acid neutralisation capacity (ANC) was determined

by mixing 1 g of bulk tailings with a fixed aliquot of

0.1N HCl in a 100mL beaker (Miller et al., 1991a).

Excess acid was determined by titrating against 0.1N

NaOH. The amount of neutralising material was

then calculated in terms of kg H2SO4 per tonne of

tailings. Total S contents of tailings samples were

determined using a Perkin Elmer 2400 CHNS Ana-

lyser. The NAPP was then calculated from the dif-

ference between total S and ANC results.

The net acid generation capacity (NAG) involved the

addition of 250mL of 15% H2O2 to 2.5 g of pul-

verised dry sample in a 500mL Pyrex conical beaker

(Miller et al., 1991a). The mixtures were left over-

night in a fume cupboard, and boiled for 1 hour to

drive away any residual H2O2. The pH of the mix-

ture was recorded after boiling and was regarded as

Figure 1 Location of the Lead±Zinc Mine in relation to Lechang City

150 J.W.C. Wong, C.M. Ip and M.H. Wong

the final pH of the NAG solution. The mixture was

then filtered through a 0:45�m filter paper and the

acidity of a subsample of the filtrate was determined

by titrating to pH 7 with standard NaOH solution.

The acidity of the filtrate represents the net acid

generated by the sample and is expressed as kg

H2SO4 per tonne of sample. Heavy metal contents

in the filtrate were determined by flame AAS and

graphite furnance AAS.

Results and Discussion

Geochemistry of tailings samples

Three out of the eight oxidised surface tailings were

found to be naturally acidic having pH values less

than 4 (Table 1). The pH of the other oxidised sur-

face tailings ranged from 7.0 to 7.35 and were con-

sidered as neutral. For the fresh tailings, tailings fine

had an alkaline pH of 10.2 while the tailings sand

and high-sulphide tailings were about neutral. The

high pH was due to the addition of lime to neutralise

the pH of mixture during the metal refinery process.

The paste EC reflects the soluble salt content in the

tailings. Only the naturally acidic tailings samples

had an EC > 4 dS mÿ1 which is the limit that plants

can tolerate without showing any harmful effects

(Richards, 1960). Obviously, the change of paste

EC is related to the change in pH, because under

acidic conditions, the tailings matrix will dissolve

more resulting in a higher salt content in the tailings.

All other tailings, including the rest of the oxidised

tailings and the fresh tailings, had an EC lower than

4 dS mÿ1.

Similar to EC, the soluble SO 2ÿ4 contents were

higher in all naturally acidic and slightly acidic oxi-

dised surface tailings while no significant difference

was found among all the fresh tailings. Overall, so-

luble SO 2ÿ4 contents of the oxidised surface tailings

were about an order of magnitude higher than that

of the fresh tailings. This explains the high salinity in

the oxidised tailings (Evangelou and Zhang, 1995).

All tailings, except tailings fine, had a total organic

carbon content lower than the 2±3% commonly oc-

curring in ordinary soil. This might be due to the

dissolution of organic matter under acidic condition

in these tailings. The total organic carbon contents of

tailings fine were significantly higher than that of the

other two fresh tailings. The total N contents of all

tailings samples were all below the concentration of

500±3000mgkgÿ1 required for satisfactory plant

growth (Gumming and Elliot, 1991). The total N

contents for most of the oxidised tailings were higher

than the fresh ones. The total P contents in all tail-

ings samples lay within the range of 272 to 473mg

kgÿ1, which were close to the lower limit of the range

of 200±1500mg kgÿ1 of total phosphorus required

for satisfactory plant growth (Gumming and Elliot,

1991). The low levels of total organic carbon, and

total N and P indicated that nutrient deficiency

would probably be a major problem that had to be

solved prior to revegetation.

Table 2 shows the total metal contents of the tailings.

All tailings samples had total Cd, Pb and Zn con-

tents that exceeded the respective trigger levels of

5,750 and 300mg kgÿ1 of the Dutch contaminated-

Table 1 Geochemistry of tailings collected from Lechang Lead±Zinc Mine.

Paste

pH

Paste EC

(dS mÿ1)

Soluble SO 2ÿ4

%

TOC

(%)

Total N

(mg kgÿ1)

Total P

(mg kgÿ1)

Oxidized surface tailings 4.93b* 3.37a 1.89a 2.02b 210a 343ab

(2.57) (2.35) (0.91) (1.10) (71.5) (66.5)

Tailings fine 10.18a 1.32a 0.21b 4.52a 74.4b 382a

(1.52) (0.21) (0.02) (0.30) (7.76) (31.8)

Tailings sand 7.10ab 0.71a 0.23b 1.29b 107b 276b

(0.18) (0.06) (0.05) (0.42) (20.5) (30.1)

High sulphur tailings 7.00b 1.53a 0.16b 0.82b 25.9b 272b

(0.44) (0.25) (0.08) (0.32) (3.35) (32.9)

* Values followed by the same letter within the same column do not differ significantly at the 5%

level according to the Duncan multiple range test.

Values in parentheses are standard deviation of means.

Table 2 Total metal contents (mg kgÿ1 of tailings collected

from Lechang Lead±Zinc Mine.

Cd Cu Pb Zn Fe

Oxidized surface

tailings

9.94a* 133b 2426a 2732ab 191985a

(8.64) (72.9) (595) (2687) (59896)

Tailings fine 18.7a 490a 2463a 67110a 250667a

(1.65) (15.6) (132) (30.3) (13650)

Tailings sand 15.8a 148b 1272b 4366ab 27056b

(6.47) (78.6) (712) (2410) (5514)

High sulphur

tailings

7.6b 78.3b 1551ab 2134b 24110b

(1.22) (8.52) (284) (655) (3080)

* Values followed by the same letter within the same

column do not differ significantly at the 5% level according

to the Duncan multiple range test.


Acid-forming capacity of lead±zinc mine tailings 151

soil investigation criteria (ANZECC/NHMRC,

1992). High concentrations of total Pb and Zn in

the tailings are possibly due to the residue Pb and

Zn left after the ore refining process. For Cu con-

tents, only one of the surface oxidised tailings and

the tailings fine had concentrations exceeding the

trigger level of 200mg kgÿ1. High concentrations of

Fe were recorded in all tailings samples especially the

surface-oxidised tailings with values ranging from

0.21 to 2.77%. High soluble-metal concentrations

were found in the acidic oxidised surface tailings

with the concentrations of Cd, Cu and Pb exceeding

the respective trigger levels of soil solution of 0.2, 0.5

and 3mg kgÿ1 (Table 3) (ANZECC/NHMRC, 1992).

This was due to the dissolution of metal compounds

under acidic condition, which implies that metal toxi-

city due to the high levels of Cd, Cu, Pb and Fe may

occur only in the acidic tailings. Water flowing over

exposed material can carry away the acid, salts and

associated metals released during exposure, and so

contaminate surface and groundwater. Moreover,

the reclamation of impoundments and dumps con-

taining these acid-forming materials is extremely dif-

ficult due to acid and metal phytotoxicity (Miller

et al., 1991).

Acid generation capacity of tailings samples

All the tailings samples contained very high total S

contents which ranged from 9.5 to 43%. As expected,

high S tailings contained the highest S content of

about 38% while tailings sand contained the lowest

S content of 12%. Among the fresh tailings, tailings

sand had the highest ANC of about 600 kg H2SO4

per tonne followed by the tailings fine and the high S

tailings. In general, oxidised surface tailings had a

lower ANC than the fresh tailings, which may be due

to the neutralization of alkalinity by the acid pro-

duced from the oxidation of sulphide minerals in the

tailings. Negative ANC were recorded in four of the

oxidised surface tailings, indicating the highly acidic

nature of the tailings once they were oxidised.

Figure 2 shows the acid±base account results on a

standard net acid producing potential (NAPP) dia-

gram with total S plotted against ANC for all the

tailings samples. The ANC � 0 line defines the

boundary between acid-forming and potentially

acid-forming while the NAPP � 0 line defines the

boundary between potentially acid-forming and

non-acid forming (Miller et al., 1991a). All tailings

sand samples were non-acid forming with NAPP < 0

Table 3 Soluble metal contents (mg kgÿ1) of tailings

collected from Lechang Lead±Zinc Mine.

Cd Cu Pb Zn Fe

Oxidized surface

tailings

0.37a* 1.59a 21.3a 1.44a 2454a

(0.35) (2.06) (23.4) (2.11) (3437)

Tailings fine 0.19a 0.04a 0.12a 0.26a 0.51a

(0.012) (0.002) (0.012) (0.012) (0.041)

Tailings sand 0.19a 0.06a 0.04b 1.46a 0.37a

(0.017) (0.010) (0.015) (0.67) (0.057)

High sulphur

tailings

0.19a 0.06a 0.13a 1.27a 0.34a

(0.0041) (0.012) (0.027) (0.62) (0.053)



to the Duncan multiple range test.


Figure 2 Acid-forming potential of tailings samples from Lechang Lead±Zinc Mine.


while all other fresh tailings were potentially acid-

forming. For the surface oxidised tailings, four sam-

ples were classified as acid-forming while all others

were potentially acid-forming. The acid generated

from these tailings materials is likely to be a limiting

factor affecting natural recolonisation. Other mines

of similar sulphide-bearing mineralisation also had

the potential to form sulphuric acid and to leach

heavy-metal sulphides (Gregorio and Massoli-

Novelli, 1992; Walder and Chavez, 1995)

The amount of net acid generated from the tailings

ranged from 0 to 542 kg H2SO4 per tonne (Table 4).

As expected, high S tailings had a higher NAG value

than that of the tailings fine and the tailings sand.

When pyrite oxidation occurs, production of acid

drainage could be indicated by the final pH in

NAG solution and samples with pH < 4 can be

classified as potentially acid-forming (Miller et al.,

1991a). Using this as a criteria, all tailings samples

were potentially acid-forming except for the tailings

sand and tailings oxidised on one surface. The NAG

prediction coincides well with those of the acid±base

account except tailings oxidised on one surface which

was classified as potentially acid-forming in the acid±

base account but turned out to be non-acid forming

in the NAG test. The discrepancy may be due to the

type of sulphide minerals that existed in the samples.

Caruccio et al. (1988) showed that high organic S or

sulphate S contents would not take part in the acid

generation process.

The relationship between NAG and NAPP can be

described by the following linear model.

NAG � 0:244NAPP� 62:8 �r2 � 0:47; p < 0:05�

On the whole, the NAPP value calculated was higher

than the NAG value predicted for each specific tail-

ings sample. This might be due to the fact that the

sulphide minerals in the tailings were not as reactive

as pyrite. Hence, the acid±base account which

assumes all sulphide minerals in the tailings will con-

tribute to the acid generation process might over-

estimate the net acid generation capacity of the tested

tailings (Miller et al., 1991b). A determination of

pyritic S in the tailings samples for the acid±base

account would give results with better agreement to

the NAG values (Caruccio et al., 1988). Despite this,

both the NAG test and the acid±base account can be

used to predict the acid generation capacity of the

tailings for the Lechang Pb±Zn Mine Operation.

Attempts were made to relate the final pH in NAG

solution and the acid-forming capacity of tailings

predicted by the NAG test (Figure 3). A logarithmic

relationship was established between the final pH

and the acid-forming capacity with r2 greater than

0.9. The tailings samples with a final NAG pH of < 3

were coincidentally those samples predicted to be

Table 4 Acid±base account and net acid generation analysis of tailings collected from Lechang Lead±Zinc

Mine.

Acid±base account NAG test

Total

sulphur (%)

ANC

(kg H2SO4tÿ1)

NAPP

(kg H2SO4tÿ1)

NAG

(kg H2SO4tÿ1)

Final pH

in NAG

Oxidized surface tailings 17.7c* 88c 454b 200ab 2.87b

(5.81) (129.48) (110.30) (163) (1.91)

Tailings fine 28.0b 365b 491b 106ab 2.50b

(0.76) (4.73) (27.88) (3.77) (0.04)

Tailings sand 12.1c 601a 230c 1.83b 8.32a

(2.28) (60.1) (19.69) (3.17) (1.93)

High sulphur tailings 38.3a 174c 999a 266a 2.01b

(4.70) (82.1) (208) (52.2) (0.045)

* Values followed by the same letter within the same column do not differ significantly at the 5% level

according to the Duncan multiple range test.


Figure 3 Plot of NAG pH versus net acid generation for

tailings samples from Lechang Lead±Zinc Mine


potentially acid-forming or acid-forming tailings by

both NAG and NAPP tests. These samples all had a

NAG value exceeding 100 kg H2SO4 per tonne. For

those having a final NAG pH of > 6 were samples

being classified as non-acid forming. No tailings

samples had a final NAG pH between pH 3 and 6.

Therefore, a final pH of 6 can be tentatively used to

delineate acid-forming and non-acid forming tailings

materials for the Lechang Pb±Zn mine tailings to

provide a conservative degree of safety. Since the

establishment of this relationship is based on only

16 samples and samples with a final NAG pH be-

tween 3 and 6 were not observed, this provides scope

for further research to test the validity and specificity

of this relationship.

The amounts of metals released following the oxida-

tion of sulphide minerals in the tailings could be

reflected by the solution of the NAG test (Table 5).

Cadmium and Cu contents of all tailings samples

were all within the B value of the Dutch Criteria

for groundwater indicating that no further investiga-

tion is required except for the tailings fine which was

about 5 times that of the B value (ANZECC/

NHMRC, 1992). All tailings samples contained

very high concentrations of Pb and Zn in the final

NAG solution and were higher than those of the

investigation levels except for the tailings sand and

one- surface oxidised samples. This can be simply

explained by the non-acid forming characteristics of

these samples resulting in a low dissolution of the

metals from the tailings matrix. The final pH of

the NAG solution for these tailings were neutral to

alkaline and metals are likely to precipitate in in-

soluble form.

Conclusions

According to acid±base account and the NAG test

results, all the tailings samples from this Pb±Zn mine

operation were potentially acid-forming except for

one-surface oxidised tailings and the fresh tailings

sand. Acid mine drainage is likely to occur once all

the tailings samples were exposed to oxidising condi-

tions such as those in the tailings disposal area. Pre-

diction of acid mine drainage in the field is necessary

for its prevention and treatment. Acid drainage had

occurred in some parts of the tailings pond and many

of the tailings were potentially acid-forming. This

indicates that the existing disposal strategy does not

address the acid generation problem, allowing the

tailings to be exposed to oxidising conditions. The

tailings sand which was non-acid forming should be

separated from the tailings fine for future use as

cover materials. However, the tailings fine should

be permanently kept under water or a low oxygen

diffusion layer. This not only reduces the risk of acid

generation but would also facilitate a future revege-

tation programme.

The geochemistry also revealed that the tailings con-

tained high levels of sulphate and heavy metals, but

low levels of nutrients and organic matter. The bar-

ren condition of the old tailings dams reveals the

extreme conditions experienced in the field for plant

growth. One uncertainty about revegetation of the

old tailings dam is the possibility of tailings becom-

ing acidic as indicated by the acid±base account and

the NAG test. It is likely that the decommissioned

tailings disposal areas will be used for crop produc-

tion because of the strong local demand for agricul-

tural land. It is recommended that sufficient liming

material should be applied to these tailings disposal

areas to buffer against any acid released through the

oxidation of sulphide minerals in the tailings. Or-

ganic ameliorants will be needed to improve the soil's

physical properties and in the same time to facilitate

the building up of the soil environment. This will

require further investigation to develop the vegeta-

tion strategy for the tailings disposal area and to

evaluate the potential uptake of heavy metals by

plants.

Acknowledgements

The authors would like to thank Messrs K.K. Ma

and M.S. Shu for their excellent technical support

throughout the experiment. Financial support from

Research Grant Council of University Grants Com-

mittee and Faculty Research Grant of Hong Kong

Baptist University is gratefully acknowledged.

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acid-forming capacity of lead–zinc mine tailings and its implications for mine rehabilitation

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