Disposicion de Relaves, El Costo Del Exceso de Agua en La Presa
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Transcript of Disposicion de Relaves, El Costo Del Exceso de Agua en La Presa
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7/24/2019 Disposicion de Relaves, El Costo Del Exceso de Agua en La Presa
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h e d is p o s a l o f m i n e t a i l i n g s t h e r e a l
c o s t o f e x c e s s w a t e r in r e s id u e s
by J. A . WATES*, P r.E ng., M .S e. (C iv. E ng.),
M.B.A . , M.S.A. I.C.E.
SYNOPSIS
T he p ape r ex am ines the co nseq ue nce s o f da m b uild in g w ith slim e o f hig h w ater co nte nt, an d th ere fo re Iow relativ e
density, such as the tailings usually associated w ith the extraction of uranium . It show s how a Iow relative density
reduces the practical m axim um rate of rise, and hence the establishm ent cost, of slim es dam s built by conventional
m ea ns . F urth er h id de n c os ts o fs uc h ta ilin gs d am s a re e xamin ed .
It is co nclu ded th at de waterin g o f tailin gs b efo re th eir disp osal is th e m ost co st-effectiv e m ean s of inc reasin g th e
rate at w hich a particular dam can be constructed, or of reducing operating or stability problem s on existing dam s.
In the longer term, underground disposal seem s to be a feasible alternative. However, it is pointed out that there
a re oth er fa ctors tha t ca n com pou nd the o peratin g d iffic ulties, an d th at th ese sh ould be inv estiga ted b efo re rem edia l
step s are tak en .
SAMEVATTING
Die referaat ondersoek die gevolge van die bou van dam m e met slyk met n hoe w aterinhoud en gevolglik
n
lae
relatiew e digtheid soos die uitskot w at gew oonlik m et die ekstraksie van uraan geassosieer is. D it toon hoe n lae
relatiew e dig the id d ie p rak tiese m ak sim um sty gtem po en gev olglik d ie vestig ing sko ste v an sly kd amme w at v olgen s
d ie k on ve nsio nele m eto des g ebo u w ord, v erlaag . V erd er w ord d ie v erb orge ko ste v an so da nig e u itsk otdamme o nder-
soek.
D ie gevolgtrekking w ord gem aak dat ontw atering van die uitskot voordat dit w eggedoen w ord, die m ees koste-
e ffe ktiew e m eto de is om d ie tem po w aarteen
n
bep aald e dam g eb ou kan w ord, te ve rh oo g, of d ie bed ry fs- o f stab ili.
teitsprobleme by bestaande damme te verm inder. Cor die langer termyn Iyk ondergrondse wegdoening na
n
uitvoerbare alternatief. D aar w ord egter op gew ys dat daar ander faktore is w at dIe bedryfsproblem e kan vererger
e n dat h ierd ie fak to re o ndersoek m oet w ord vo ord at herstelm aatreels g etre f, o f nu we d am m e ge bo u w ord.
Introduction
It is known, and generally accepted, that slime of low
relative density or high water content creates difficulties
in the building of dams and often leads to instability.
However, the reason for these difficulties is not often
understood.
This paper examines the consequences of dam building
with the slim e of high water content (low relative density)
that is usually associated w ith the extraction of uranium ,
particularly w here flotation plants are utilized to extract
pyrite (and the occluded residual gold) from uranium
ta iling s b efo re th eir disp osal.
Low Relative Density
Various processes for the extraction of uranium are in
use in South Africal. The fundamental difference
betw een the processes lies in the solid-liquid separation
and primary uranium-recovery circuits. The most com-
mon of these are illustrated in Fig. 1. The various
solid-liquid separation m ethods produce different ratios
between solids and water, and hence slime of different
re la tiv e d en sity .
The basic difference in quality between the water bled
from the process after the uranium has been extracted
from the filtrates that is relevant here can be found in the
prim ary extraction process. The Buffiex process, incor-
porating resin columns (continuous ion exchange) and a
sm all solvent-extraction plant, produces a large volum e of
*
Partner, Jones W agener Inc., P.O . Box 1434, Rivonia 2128,
Trans-vaal.
@ The South Mrican Institute of M ining and Metallurgy, 1983.
SA ISSN 0038-223X/S3.00 + 0.00.
solution (barren solution), which is free of entrained or-
ganic matter. The Purlex process, on the other hand,
incorporates only a large solvent-extraction plant and
produces a large volume of water (raffinate) , which
contains entrained organic m aterial.
W here flotation plants are coupled to uranium plants
so that the pyrite, and sometimes the occluded residual
gold, can be extracted, entrained organic m atter adverse-
ly affects the flotation performance. Raffinate (from
Purlex) can therefore not be used as filter wash or in the
repulping of feed to the plant. Barren solution, on the
other hand, can be used for both. Therefore, whereas all
the raffinate produced by the Purlex process must be
discarded, as much as ~ per cent of the barren solution
produced by the Buffiex process can be recycled.
In efficient plants, the raffinate, which is discarded
w ith the flotation-plant tailings, reduces the relative den-
sity to a typical value of about 1,25. Plant inefficiency,
spillage, and gland service water reduce the relative
density further, and in some cases values as low as 1,15
are reached. Compared with relative density values of
1,4, which can be achieved from a Buffiex process or
from conventional gold extraction, this difference is
significant.
The total volume of slurry pumped to the tailings dam
is significantly influenced by the relative density. For
example, about 1 t of water goes to the dam for each ton
of solids in a tailing with a relative density of 1,45, while
about 3 t goes with each ton of solids in a tailing with a
relative density of 1,18. Since the capital, m aintenance,
and running costs of pumps and piping are approxim ate-
ly proportional to the volumE> of slurry pum ped, the COfjt
of disposal per dry ton m illed can be significantly greater
for slim e of low relative density.
JO UR NA L O F TH E S OU TH A FR IC AN IN ST ITU TE O F M IN IN G A ND M ET ALLU RG Y
NO VEM BER DECEMBER 983
25 7
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S O L ID S T O R E S ID U E
O R F L O T T IO N
f i L T R T E
L E C H
F I L T R T I O N
F IL T R T IO N W IT H P U R L E X O R B U F F L E X
S O L ID S T O R E S ID U E
O R F L O T T IO N
L E C H
C C D
f i L T R T E
F I L T R
T
C C O W IT H P U R L E X O R B U F F L E X
Fig. I-C ircuits for solid-liquid separation and prim ary recovery in a uranium plant
C C D
=
co unte rcu rren t d ec an tatio n
C IX
=
C ontinuous ion exchange
SX
=
S ol ve nt e xt ra ct io n
Further to the obvious influence that tailings density
has on pum ping costs, the effect on the establishm ent
cost for the tailings dam can be significant. T he section
that follow s illustrates how the relative density influences
the m axim um practical rate of rise, and hence the cost
of d am s.
R ate o f R ise
In p ractical term s for g old and uranium tailing s, th e
m axim um rate of rise is the rate of d epo sition th at allow s
a lo w en oug h cycle tim e on the d am s d ay p add ock s to
fa cilita te d ry in g o ut d esic ca tio n) o f th e ta ilin gs. D esic ca -
tion of the slim e is essential in the first instan ce to brin g
abo ut co nsolidation a nd the resu lting strength g ain an d,
in th e seco nd instan ce, to crack the slim e and thereb y
reduce th e ratio o f h ori2:on tal to v ertical perm ea bility .
F ro m a p ractical p oint of v iew , slim e m ust be relativ ely
dry to allow access and to fac~ litate h and pack ing of a
25 8
N O V E M B E R /D E C E M B E R 1983 JO U R N A L O F T H E S O U TH A F R IC A N IN S T IT U TE O F M IN IN G A N D M E TA L L U R G Y
}
]
P U R L E X
B U F F L E X
}
P U R L E X
}
B U F F L
re la tiv ely im p erv io us b un d o n th e in ne r, a nd in p artic ula r
on th e o uter, sho uld ers of the d ay paddo ck. In tho se
cases w here m echanical p ackers are em ploy ed, the sur-
face o f the pad dock m ust b e dry en ou gh to p ro vide a
tra ve llin g su rfa ce fo r th e tra cto rs.
T he cycle tim e sh ould also be low enou gh to lim it
rech arg in g o f the ph reatic su rface w ater tab le) w ith in
the slim es d am . E xcessive recharging results in a rise in
the p hreatic surface, an d m ay cau se unstable con ditio ns
at the d am to e if the drain age cap acity o f the artificial
blan ket drain s w hen p resen t, an d of th e natural fou nda-
d ati on m a te ri als , a re e xc ee de d.
T h e m axim u m p ractical rate of rise, and hen ce the
m inim um cy cle tim e requ ired for effectiv e desicca tio n,
stable p hreatic su rface con dition s, an d access req uire-
m ents, is co m m o nly tho ugh t to b e a un iq ue value for a
giv en clim atic region. F or go ld and u ran ium dam s in the
Tran sv aal an d O ran ge Free S tate goldfields, a value o f
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1100
1000
900
800
en
en
70 0
::;:
>-
m
0
60 0
-
>..:
Z
UJ
I-
Z 50 0
0
U
IX :
UJ
I-
.00
~300
20 0
10 0
0
2,5
m per year has been adopted as a maximum rate of
rise2,4. T he argum ent that follow s is intended to illustrate
that the maximum practical rate of rise is not unique,
but that cycle time, which depends on the relative den-
sity of the tailings, should be lim ited.
Effect of Relative Density on Cycle Time
Conventional dam-building technique dictates the
limiting volume of a day paddock. Fig. 2 shows a typical
cross-section through a dam, and illustrates the width
and depth of a day paddock. For a given depth and
width, the length of paddock required to accommodate
8 daylight hours of slime increases as the total volume of
slime increases. Simplified assumptions can be made to
show that the cycle time therefore decreases as the
volume of slime increases. Since the total volume can be
increased by increasing water volume without a corres-
ponding increase in dry tonnage, it follows that cycle
tim e is a function of relative density.
D E P T H Idl
Fig.2-Conventional method of slime deposition with
paddocks
The relationship between water content m ass of water
over mass of solids and cycle time in Fig. 3 was derived
theoretically from the assumption of a typical paddock
depth of 200 mm and from the assumption that the
individual day paddocks are filled in a discrete manner.
For example, if the settled slime would occupy 20 per
cent of the total volume of slurry, then after the super-
natant water had been decanted, the paddock would have
risen by 40 mm. For an annual rate of rise of 2,5 m , the
paddock would have to be repacked and refilled about
62 times, or every 6 days. Similarly, if the settled slime
occupied 40 per cent, the cycle time would be 12 days.
The problems associated with slim ing at low relative
density were further compounded recently as a result of
a change in technique occasioned partially by labour
costs and shortages. Previously, the slim e was led to sm all
paddocks formed by crosswalls at about 60 m centres via
a furrow cut on the day wall. Crosswalls have now been
abandoned on some dams, and the day wall is divided
into longer units that are often more than 500 m long.
During slim ing, the entire surface of the large paddock
is wet, and this is sustained for a longer period than for
the smaller paddocks since the filling time is longer.
R O 1 3 5
1 0
1 2 1 3
1
C Y C L E T I M E D A Y S
F i g
3-Relationship between cycle time and water content
The consequence of this practice is illustrated in Figs. 4
and 5. Fig. 4 shows a dam with 14 paddocks, and Fig. 5
shows the same dam with only two paddocks. If one of
the small paddocks takes a day to fill, then the cycle time
would be 14 days, leaving 13 days for drying of the slime.
The large paddocks would, for a dam of the same size,
take 7 days each to fill, and the cycle time would be only
7 days, leaving the other 7 days for drying.
If it is accepted for the time being that cycle time,
rather than rate of rise, is unique for given tailings and
given climatic conditions, then the allowable rate of rise
can be calculated as a function of water content or
relative density. The relationship betw een allow able rate
of rise and relative density, which is shown in Fig. 6,
was derived on the assumption that the previously
acceptable rate of 2,5 m per year was fixed from ex-
perience w ith gold tm ilings, i.e. for tailings w ith a relative
density of 1,45. This, in effect, assumes that a cycle time
of about 2 weeks is acceptable and should be maintained
as a constant.
In practice, paddocks are not filled discretely but
rather over a longer period, while supernatant water and
some slim e are decanted from the low side of the paddock.
The argument presented above and the relationship
derived in Fig. 2 are therefore not strictly correct for
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
NO VEM BER DEC EM BER 98 259
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N I G H T
P A D DO C K
3
7
8
DA Y
PADDOCK
Fig 4 Fourteen day paddocks
actual practice. H ow ever since the paddocks cannot be
filled com pletely without loss of freeboard in the night
area the actual practice is a com prom ise betw een the
two alternatives. Current practice com bined w ith Iow
relative density therefore leads to a com bination of a
decrease in cycle tim e and an increase in the period
during which any particular paddock is exposed to w et
ting.
Recent experience of operational and stability prob
lem s on som e dam s has confirm ed the hypothesis that
cycle tim e rather than rate of rise should be considered
to be the controlling variable. Failures and severe
operating difficulties with uranium tailings dam s have
occurred at rates of rise w ell below 2 5 m per year. These
have been attributed to high water content rather than
to other factors com m only accepted. Failures of dam s
and operating difficulties are shown on Fig. 6 by a graph
for the rate of rise at w hich the problem s occurred
against the relative density of the tailings for the dam in
question. These cases can be seen to correlate relatively
w ell w ith the calculated upper lim it.
It is certain that skilful m anagem ent of dam operation
and tim eous rem edial m easures could obviate som e of the
problem s associated w ith slim ing at Iow relative densi
ties. H ow ever it is never certain that a dam will be
operated in the m ost skilful possible w ay throughout its
life.
M echanization is likely to becom e m ore popular ow ing
to labour shortages and costs and drier conditions w ill
th erefo re b e req uired .
If it is believed that skilful operation and m anagem ent
could increase the m axim um practical rate of rise then
a m odification such as the one show n for optim al dam
building procedure in Fig. 6 could be adopted.
Since the risk of foundation instability on dolom ites is
high a reduced m axim um allowable rate of rise has been
proposed3. This m axim um is also show n in Fig. 6.
If it is accepted that in practice Iow relative density
leads to operating and stability problem s then it w ould
be prudent to recognize these difficulties and to design
n ew d am s ac co rd in gly .
26 0
N OV EM BE R/D EC EM BE R 1 983
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
DAY
PADDOCK
NIGHT
PADDOCK
2
F ig 5 T w o d ay p ad do ck s
Costs
A n exam ple of the cost of the sam e dam pum ps
piping and w ater return system designed for an opera
tion involving 250 000 dry tons per m onth at relative
density values of 1 25 and 1 45 was prepared from Jan.
1983 rates for the construction and power. The dam w as
assum ed to be located at the sam e elevation as the resi
due disposal pachucas in the plant and at a distance of
5 km from the plant. The velocity of flow in the pipeline
w as taken as 1 7 m /s and the pow er consum ption was
based on operating experience. The life of the schem e
was taken as 30 years and the pum ping and m aintenance
costs w ere first inflated at 12 per cent per annum and
then discounted at 20 per cent to arrive at a present
value of costs.
The tailings dam was assum ed to be located on an
even ground slope of 1 :100 and the underdrainage
provided by the natural soils underlying the dam was
assum ed to be poor. The dam was taken as rectangular
with a length equal to tw ice the w idth and w as divided
into two com partm ents. Provision w as m ade for a lined
return water dam and an unlined storm w ater dam . The
capacity of the return water dam w as taken as four days
of m axim um average m onthly return from the slim es
dam . The storm w ater dam was sized to retain the run off
from a storm w ith a recurrence interval of 1 :100 years
and of 24 hours duration on the slim es dam and im m e
d ia te c atc hm en t.
Poor natural underdrainage w as assum ed to necessitate
substantial artificial underdrainage. The dam foundation
was taken to be relatively stable w ith no other geotech
nical problem s. N o special features other than statutory
requirem ents pertaining to pollution control w ere taken
in to a cc ou nt.
Fig. 7 which illustrates the relative costs of tailings
dam s of Iow relative density and of high relative density
shows that there can be a significant increase in the cost
of capital expenditure and pum ping as the relative den
sity decreases. The increased cost is thought to be signifi
cant enough to w arrant an exam ination of alternative
approaches.
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