Journal of Scientific & Industri al Research Vol. 60, November 200 I, pp 883-889

Assessment of Effective Design of Tailing Pond for Safe Disposal of Iron Ore Tailings

M KGhose'

Centre of Mining Environment , Indi an School of Mines, Dhanbad 826 004 and

P K Sen

MECON Ltd, 50, Chowringhee Road, Calcutta 700 071 Received : 16 March 200 I; accepted : 26 June 200 I

The management of tailings upholds the principl e of economic development. Tai ling ponds for the containment of iron ore tai lings is an important issue, not only from the point of view of pollution control but also from the conservation of resources for the future. But due to huge cost of land and safety of the dam, it is worth designing an appropriate tailing pond size to a minimum level. The purpose of iron ore beneficiation and the cause of surface water pollution and ground water contamination are described. Physico-chemical characteristics of the tailings of an iron ore beneficiation plant and the effectiveness o f tailing pond for the removal of toxic pollutants are discussed. To carry out the e ffective design of tailing pond. settling characteristics of tailings were carried out. The results reveal that the optimum area required for the containment of the tailings should be much less, in comparison to the area utilized for the tailing pond .

Introduction

In iron ore beneficiation plants, ore is finely ground for liberation of the mineral. The fine tailings generated ore usually disposed in tailing ponds'. The tailing pond deposits can mostly be considered as permanent structures and always have impact on environment2. By 2000AD, in India the estimated iron ore production per year would be 85Mt, generating 27Mt of tailings per year for which safe disposal by way of containment in tailing ponds, are needed to be planned3

· 4

· Tailing di sposal planning not only depends on the nature and engineering behavior of tailings but also on the understanding of the various methods available for tailings disposal 5

.

The avai lability of land for di sposa l of tailings is a problem of concern . It is feas ible in the case of copper and lead zinc ore to di spose off the generated tailings to underground mines for backfilling6

. But thi s alternati ve is not feasib le in the case of iron ore mines. The creation of artificial tailing pond for containment of iron ore ta ilings creates defo res tation and ha s impact on agriculture and plantation. The management of tailings upholds the principles of sustainable development 7

• It is an important issue, not onl y from the point of view of

' Corresponding author

pollution control, but also for keeping conservation of resource in mind x. In future , when the present day rich resource will be used up, it may become economical ly viable to extract iron content from tailings Y_ But due to huge land cost and also for the safety of the dam, it is worth examining the feasibility to eva lu ate the appropriate tailing pond size to a minimum level 10

Purpose of Iron Ore Beneficiation

The iron ore ex tracted from the mtn es are benificiated to separate out the valuable mineral content. If iron content is nearly or more than 60 per cent the ore is only crushed and screened to size and can be sent directly to metallurgical plant without beneficiati on. In case ore containing low Fe content, it is beneficiated by crushing, screening, grinding, washi ng classifier, jigging, cycloning, and processing in the magnetic separator etc. 11

The rejected portion of the iron in the state of fine particulate mixed in water in a slurry form , known as tailings are needed to be disposed in tailing pone!.

Environmental Impact due to Disposal of Tailings

Huge quantity of water is required for iron ore beneficiati on. Before th e tailings are hydrauli cal ly transported to tai I i ng pond for impoundment, the water

------~-----------------------~~----- ..- .....~-..-.--.~.~.--..- ----------

884 J SCI IND RES VOL 60 NOVEMBER 2001

,,0

DUlKI

""-9 '~v'~G'

\.\

o KAl WAR Hl NGANGH·~ ..•..

t

JHILKAPARA

N

f

QKAMTA

,.ARIDOND .

..•.....••.•...•.<.

Figure I-Location map of Dalli mines

is recovered for recycling by dewatering process utilizingthickener". Since the incoming tailing feed to thethickener is very fine, coagulant and coagulant aids areused to get clarified water of relatively quality forcirculation". The underflow contacting thickened tailingslurry having pulp density (wt of solid per unit wt ofslurry) 15 to 55 per cent is produced by typical thickeneroperation 14. 15. The thickened tailings are generallyconveyed by pipeline and sometimes by gravity flowopen launders to the tailing pond"-". But the mereconstruction of tailing pond has no guarantee ofenvironmental compliance IX, The cause of pollution ofsurface water is due to the discharge of excesssupernatant effluent into a .watercourse 19. The risk of

contamination of aquifer by contaminants leaving thetailing deposits also exists. The physical nature of tailingscannot be separately considered in isolation withoutconsidering the associated liquid mill effluent from thebeneficiation plant 20. The design of the dam is similarto the conventional water retaining earth darns".

Applications

Description of the Study AreaDalli iron ore mine is located in the Durg district

of Madhya Pradesh having latitude 20° 35' Nandlongitude 810 2' E. The location of the mine is shown inFigure 1 The mine was originally designed in 1966 forthe output of 2,5 Mt/y ore, with a provision for expansion

GHOSE & SEN: EFFECTIVE DESIGN OF TAILING POND R85

to 5.0 Mt/y. To meet the demand of Bhilai Steel Plant (BSL) in the year 1989 it was decided to implement the second stage plant i.e. 5 Mt/y. The ore process in g envisaged only wet processing and generating tailings on continuous basis. The iron reserve of Dalli mines was estimated to be 180Mt 22

Disposal ofTailings

In order to conserve process water for washing of the iron ore, recycling of the process water with treatment facility was being practiced. The contaminated water after ore washing flows to the thickener for recovery of sinter grade fin es which can be utilized for steel making. Out of 1580 m3 /h of contaminated process water, about 935 m-1/h was being recyc led afte r c larification. The tailin gs coming out of underflow from thick e ne r amounting to 640m3/h was being di sposed off in the tai ling pond. The tailings from the underflow of thickener carried the entire milling waste which amounted to 40 per cent of total Run-of-Mine, It worked out to be 1500 tid having maximum of I OOt/h . Table I shows the s ieve analysis of the tailings. The major components of the tai lings were found to be: Fe (43.92 per cent), Si0

2 ( 12.39

per cent), Alp, ( I 0. 11 per cent).

Iron was present mainly in the form of Fe,01

. Traces ofFeO were also found. Negligible amount of oxides of Ca, Mg, Ti , and P are also present in tailings. Bulk density of settled tailin g s lime was found to be 2.8t/m3

.

Generation of tailings per year in 2.5 Mt stage was 420,000 t, which in the 5 Mt stage was estimated as 688,000 t. Thus, the total tailings to be generated during the life of the mine worked out to 7.4Mm3

. A suitable location was se lected at 2.5 to 3 km from the ore process ing plant toward s the NW direction . In thi s direction two valleys were located and dams of 34 m and 32m height were constructed to form two holding ponds of capacity 5Mm-1 and 4.3 M m3 respective ly. The salient features of the tailing pond are shown in Table 2. The tailing slurry was be ing conveyed to the tailing pond through two numbers of MS pipelines of 350-mm diam each. The pipe outlet was provided with shifting arrangement. The tailing slurry was being di scharged below the water.

Methods

In order to characterize tailing s lime, it is essentia l to conduct a sys tematic waste survey. No exc lusive procedure is available for sampling tailings of mines.

Table !-Sieve analysis of slime of Dalli iron ore washing plant

Size Percentage

Tyler mesh Microns (by weight )

+65 +208 2.8

-65 +100 -208 +147 5.0

-100 +150 -147 +104 8.2

- 150 +200 -1 04 +74 11.8

-200 -74 72.2

and ground water. Tailing sampl es were collected from two locations ofDalli iron ore mines. Tailing slurry from the beneficiation plant led to thickener. Recovery of water for recycling after separation of tailings was affected . The locations of the sampling are presented in Figure 2. The sampling technique, used for the survey mu st assure representative samples . Based on the constituents of iron ore, parameters to be monitored were chosen23

. Equal volumes of samples were collected hourly in a washed plastic container and mixed at the end of the sampling period at each of the sampling locations. Composite samples were collected, once in a month round the year 24

• Samples were preserved and analysed in the laboratory. All the water quality parameters were determ in ed, following the method s given in Standard Meth ods2:;.

Temperature , pH and di sso lved oxygen (DO) were measured at site2(' . Fe, Mn , AI , Cu and Zn were analysed by Atomic Absorption Spectrophotometer (AAS) make Shimadzu, Mode l-680. Cr, Mo, Ni, and Co were measured by Induced Coupled Plazma (ICP), make­Shimadzu, Mode l-l 003.Results of physico-chemical characteristics are given in Table 3.

Cylinder test was used where suspended solids exceeded 5000/mg/L 27

• It was carried out in the laboratory for the tailing sample, following the method developed by Talmande and Fitch2x. In thi s meth od, a column with a height of+ I is filled with suspension of tailing slime of uniform concentration of 'C'. As the suspension settles the position of the interface charges . The rate at which interface subsides is equal to the slope of the curve at that point. According to methodology, critical area for thickening is g iven by :

A= Q!V,

where A =Area in m,2

J SCIIND RES VOL 60 NOVEMBER 2001

Figure 2-Dalli iron arc mines- tailing pond and ore processing plant, sampling station locations

Table 2-Salicnt features of Dalli iron ore tailing pond

Storage capacitySubmersed areaCatchment areaRain fall (annual average)Dam no. 1Maximum heightLengthTop of the darnDam no. 2Maximum heightLengthTop of the damLength of waste weir

9.3 Mm'0.042 sq.km0.84 sq. krn1400mm

34m402m452m above the MSL

32m685m452m above MSL10m

Q and V are calculated with the data on input totailing pond in m ' h -I and cylinder test results. Samples(IOOmL) were poured in the cylinder after shaking (tomake suspension uniform throughout). The movementof the interface was noted at intervals as indicated inTable 4. TSS and iron were estimated in the supernatantliquid after 24., 48 and 72h respectively of settling. Theadvance of the interface of the water and the settled solidswas recorded with time and a graph was plotted betweenthe heights of the tailing slurry vs time (Figure 3).

Results and Discussion

Tailing samples were collected from under flow ofthe thickener CD 22) and overflow from the thickenerCD5 I) 2Y. The tailing samples mostly containhighsuspended solids and ranged from 102-308 glL withan average of 167.75 glL and SD of 61.5. The meanvalues of iron, manganese and aluminum observed were

33.56 glL. 039 g/L and 1.26 g/L respectively. Results ofthe clarified water samples (0 15). showed meanconcentration of Fe. Mn, and AI as 0.52 g/L, 0'()06 g/Land 0.018 glL respectively. Result of Cr. Ni, Cu, and Znwere found to be low.

In the proposed scheme, it is suggested to providean additional unit of sedimentation tank before thedisposal of tailings to the tailing pond. Measures likeuse of baffles are to be taken in the sedimentation tankto ensure uniform flow across the width of the tankperpendicular to the flow direction and to minimize deador non-flow areas. The overflow of the sedimentationtank can be recycled or safely discharged and theunderflow would he discharged to the tailing pond. Tocarry out the design of decant pond, cylinder test wascarried out. Based on the results given in Table 4 and thecurve.shown in Figure 3, it may he seen that the input tothe tailing pond of 640 Jll3/h of tailing slurry. Initiallythe height of the tailings in the cylinder was I ()O ern.

After 2X min of settling the tailings height was reducedby 56cJll. It Jllay be seen that had the initial rate ofsettlement was continued till the de sired level (Ifsettlement wax achieved, i.c. 40 min. But the rate ofsettlement continued to decline with the passage of time.A linear settlement could be observed after 40 nun, butafter the lapse of 60 min the rate of settlement furtherreduced and the ultimate of X2 ern could be achievedafter 180 min. As it may be noted from the curve thathas there been no reduction in the rate of settlement after40 min, ultimate settlement should have been achievedin 100 min. The cylinder test result also showed that the

GHOSE & SEN: EFFECTIVE DESIGN OF TAILING POND K87

Table 3-Physico-chemical characteristics of Dalli iron ore tailing slime

Parameters Unit Sample No. D-22 Sample No. D-51

(Under flow of the thickener) (Under flow of the thickener)

Range Average SD Range Average SD

pH 5.4-6.1 6.3-7.1

Temperature °C 19-38 20-32

TSS gIL 102-308 167.75 61.51 1.2-4. I 2.39 0.81TDS mgIL 76-92 82.58 4.52 82-95 86.9 3.75Chloride mgIL 25-32 28.67 2.01 28-36 32.10 2.78

Hardness mgIL 22-31 25.58 2.17 30-38 32.84 2.08Sodium(Na) mg/L 1.8-5.3 3.15 0.96 1.3-2.6 1.71 0.42

Potassium (K) mgIL 0.9-2.5 1.56 0.41 0.5-1.3 0.76 0.24Iron (Fe) gIL 18.4-61.6 33.56 12.53 0.24-0.90 0.52 0.18

Manganese (Mn) mgIL 0.24-0.71 0.39 0.14 0.003-0.009 0.006 0.002Chromium (Cr) mgIL 34.52-104.2 53.76 18.96 0.41-0.95 0.80 0.27Cobalt (Co) mgIL 1.56-4.74 2.58 0.95 0.018-0.047 0.036 0.012Nickel (Ni) mgIL 6.2-18.95 10.31 3.79 0.025-0.088 0.051 0.017Copper (Cu) mgIL 15.69-47.4 25.82 9.48 0.18-0.63 0.36 0.12Molybdenum (Mo) mgIL 0.78-2.37 1.29 0.47 0.014-0.050 0.029 0.009Aluminum (AI) gIL 0.77-2.30 1.26 0.46 0.009-0.030 0.018 0.006Zinc (Zn) mgIL 6.27-18.95 10.32 3.78 0.11-0.34 0.22 0.076

COD mg/I 30-62 41.58 10.06 12-24 14.1 3.26

120

~ETTL1NG ANALY~I SCAlCUl ATION100

o BMic Data

Input to r';bng pond= 640 m3/h

o Soubsidftlu!V~locity V fromhmdr~d settlingportion of CUN!!

V" • 1·0m - 0.440m J211/60h

• 1.2 filth

o OVIH" flow ra~

0.", 640· (1.0m.-:- O.ll1mJ1.0

= 524 II rrfl/h

o Arl!a reQuiri!d for clarification

A = ~V e 524.8 = 437.am21.2

100 200Time Minutes

300 1.00

Figure 3-Settling rate of iron ore tailings (Dalli mines)

888 J SCIIND RES VOL 60 NOVEMBER 2001

Tables 4 -Sedimentation of tailings of Dalli iron ore mine

Time, Movement Settling rate. mmlhmin of interface

(Volume in the cylinder, ml.)

0 100

10 80 1.2

20 60 1.2

35 35 1.0

60 25 0.24

90 22 0.06

120 20 0.04

180 18 0.02

360 17 0.003

settling velocity V was 1.2 mlh, overflow rate Q wasfound to be 524.8 m3/h,and the area required forclarification estimated to be 437.3m2.

In the absence of laboratory sedimentation text data,an empirical formula for tailing pond size was used. Itwas calculated for 5 d of retention time and surface pondarea of 10 t025 acres/1 OOOttailing discharge/d. The areawas worked out to be 43000 m', which was found bemuch larger in comparison to experimental value. In theexisting system the tailing volume was assumed to bethe same, as that of the tailing slime. But the actual areaof the sedimentation tank should be calculated on thebasis of settling characteristics of tailings and the requiredvolume of tailing pond should be estimated on basis ofsludge volume index. The sludge volume index wasfound to be 18 per cent. As the clarified water was foundto be more than 80 per cent, it is adequate to carryoverof finer portion of tailings with the decant water.Maximum dry density (MDD) of tailings sample wasfound to be 1.99 g/cc and optimum moisture waterCOMC) was 18.2 per cent. The height of the Jam wouldbe estimated on the basis of pondage volume of thetailings. However, a much larger area would be requiredto limit the height of the tailings to a reasonable valuefrom a practical as well as safety point of view. The actualheight of the dam depends the topography of the area,load bearing capacity and other safety aspects of thetailing dam. In the existi~g system, it was observed that

the clarified water could not limit the carryover of thefiner particles though decant tower/ waste weir wereprovided and causing water pollution in which they weredischarged. In the proposed scheme, it would be possibleto remove the finer fractions with the aid of coagulantsin the sedimentation tank and also it would be easier tohandle the settled sludge (tailings).

Conclusions

The constituents of iron ore tailing slime are mostlysilica, alumina and iron in the form of oxides. The otherconstituents are oxides of Mn, Cr, Co, Ni, Cu, and Zn.The tailings contain toxic element and find their wayinto the water environment. Provision of tailing pondfor the disposal of tailings is a kind of conservation ofresource in addition to pollution control. In future, whenthe present day rich resource will be used up, it may beeconomically viable to extract iron from tailings. Fe andTSS are the main source of water pollution, which canbe managed by down stream treatment for discharge.Ground water contamination can be prevented with theuse of liners. It is useful to recover the decanted waterfrom the settling tank and to recycle it in the plant.Sedimentation test incites a much less land area incomparison to area utilized for tailing pond. Thus, it isessential to conduct sedimentation test to predict therequired tailing pond size. The study has formedguidelines to evaluate the effective design of tailing pondand the process will work on industrial scale for varioussites.

Acknowledgement

The authors are grateful to the Steel Authority ofIndia Ltd., for providing financial support.

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