analisis de la conservacion del cobre el pilas de lixiviacion

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.Hydrometallurgy 58 2000 2741

www.elsevier.nlrlocaterhydromet

Analysis of heat conservation during copper sulphideheap leaching

David G. Dixon

Department of Metals and Materials Engineering, Uniersity of British Columbia, 309-6350 Stores Road, Vancouer, BC, Canada V6T 1Z4

Received 1 November 1999; received in revised form 31 March 2000; accepted 24 May 2000

Abstract

An analysis of heat conservation during copper sulphide heap leaching has been carried out with the aid of a computer

model. As a result, a new mode of heap behaviour called Aevaporative autocatalysisB is postulated. In this mode, air is blown

upward through the heap at a rate sufficient to drive the net advection of heat upward through the heap, resulting in much

higher and more uniform internal heap temperatures than can be achieved in the absence of forced aeration. The effects of

flow rate, heat generation rate, solution and air heating, and application of an evaporation shield to the heap surface are also

investigated. q2000 Elsevier Science B.V. All rights reserved.

Keywords: Heat conservation; Copper sulphide; Heap leaching

1. Introduction

Bacterial heap leaching of secondary copper sul-

phides, both with and without prior agglomeration

and acid curing, is one of the most important emerg-

ing technologies for the treatment of secondary cop-

per sulphide and mixed sulphideroxide ores. Cur-rently, a host of companies operates such heaps,

primarily in Chile, and several more large operations

are in development. Unfortunately, the process has

been plagued with problems, often leading to lower

than anticipated copper extractions and low rates ofextraction, thereby necessitating heaps much larger

and more costly to operate than anticipated in order

to meet copper production quotas. Also, the basis for

design of these operations remains almost entirely

anecdotal. Hence, each new installation involves ex-

pensive testing and piloting campaigns, and scale-up

remains problematic.

A pressing need exists for a better understandingof the biological, chemical, and mass and heat trans-

port phenomena involved in bacterial heap leaching

processes, and how these factors interact. Key to this

understanding is the development of a robust mathe-

matical model of the process which can be validated

in the laboratory and in on-site pilot-scale tests.

As a first step in this process, the author has

developed a robust, 1D computer model of heat

transport in heaps. Heat transport is especially impor-

tant in copper sulphide heap leaching, in which one

or more highly exothermic sulphide oxidation reac-tions take place, for several reasons. First, the bacte-

ria which catalyze the oxidation reactions are sensi-

tive to temperature. For example, the most common

microbial species associated with bioleaching,

Thiobacillus ferrooxidans, is a mesophile which

thrives at temperatures in the neighbourhood of 308C,

but which becomes inactive at temperatures below

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. .P I I : S 0 3 0 4 - 3 8 6 X 0 0 0 0 1 1 9 - 5


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( ) D.G. Dixonr Hydrometallurgy 58 2000 274128

about 108C, or above about 408C. On the other hand,

moderate thermophiles such as Sulfolobus can thrive

at temperatures as high as 658C, but may have more

stringent toxicological requirements than Thiobacil-

lus.

Secondly, many oxidation reactions central to

copper recovery from secondary ores are strongly

temperature-dependent. For example, arguably the

most important reaction from an economic stand-

point is the oxidation of so-called Ablue remainingBcovellite, or blaubleibender, which is the intermedi-

ate product of chalcocite oxidation. Chalcocite typi-

cally comprises the bulk of the copper in secondary

copper sulphide ores, and is therefore the prime

target of copper sulphide heap leaching. In acid

ferric sulphate solutions, chalcocite is oxidized by

ferric ions in two major steps:

Cu Sq2Fe3q

Cu2q

q CuSq 2Fe2q

, A .2

CuSq2Fe3q Cu2qqS0 q2Fe2q, B .

where CuS denotes the blaubleibender, or blueremaining phase. It bears noting that in bioleaching

operations, the product elemental sulphur from Reac- .tion B is typically further oxidized to sulphate in

.the presence of sulphur-oxidizing bacteria. The in- .trinsic rate of Reaction A is very high; so high, in

fact, that the reaction rate is typically limited byw xdiffusion of ferric ions to the mineral surface 1 .

Hence, approximately one half of the copper fromchalcocite may be recovered very quickly from the

.heap. However, Reaction B is very slow, and there-fore largely determines the necessary heap leaching

times. These leaching times typically exceed 1 year,

and can be as long as 18 months in some cases.

However, the Arrhenius activation energy of Reac- .tion B has been reported to be on the order of 98

w xkJrmol 2 . Hence, at least within the temperaturerange of viability for most iron- and sulphur-oxidiz-

ing bacteria, every 108C increase in temperature

represents an almost threefold increase in the intrin- .sic rate of Reaction B .

Depending on the heat-generating potential of the

ore, which is largely a function of pyrite content, and

the ambient environmental conditions, which govern

the net rate of heat gain at the heap surface, two

similar heaps operating in identical fashion at differ-

ent locations may behave very differently. For in-

stance, in the high Chilean desert, heat loss from the

heap surface by evaporation during the hot, dry days,

and by radiation during the cold, clear nights can

have a dramatic effect on heap performance by

holding heap temperatures below the range for effec-

tive bioleaching. On the other hand, a heap in a more

moderate climate with a high pyrite content may be

subject to overheating.

In this paper, we will investigate those factors

which have the largest effect on heap temperature,

and discuss which parameters are best manipulated

to control heap temperature. To the authors knowl-

edge, although several investigators have developed

heat balances around dump leaching operations, usu-w xally as part of an overall dump leaching model 3 5 ,

none has looked specifically at copper sulphide heap

leaching operations, and none has accounted explic-

itly for either the effects of the environment on the

heap surface boundary condition, or the all-importanteffects of forced aeration and in situ evaporation on

the thermal profile within heaps.

2. The general enthalpy balance for sulphide heap

leaching

A general enthalpy balance is derived based on

the following assumptions:

1. The heap is comprised of three phases: a continu-

ous stagnant phase consisting of all solids and

stagnant solution; a liquid phase flowing down-

ward; and a gaseous phase flowing upward.

2. All three phases are in thermal equilibrium at any

depth within the heap.

3. The gaseous phase is saturated in water vapour at

all points within the heap.

4. All physical and thermal properties of the heap

remain constant and uniform throughout the

leaching cycle.

The general enthalpy balance within a heap iswritten as:

4Rate of heat accumulation

4s Net rate of heat input by conduction

4q Net rate of heat input by advection

4q Net rate of heat production .


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( ) D.G. Dixonr Hydrometallurgy 58 2000 2741 29

In symbols, across a shell of thickness Dx and

cross-section A, taking the net flow of liquid water

downward and of air and water vapour upward, and

assuming perfect thermal equilibrium between all

phases at any depth x, the enthalpy balance be-

comes:

Erh<


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then the final expression of the enthalpy balance is

written as:

ETrCp

Et

E2 T

sk y G yk r c TN . l g a xs02Ex

yv c T qG c T yc TN C . . .. .` ` a xs0 pl

ETXyG C qC c T qlc T qS, . . 4a pa pv

Ex

9 .

X .where c T is the first derivative of the saturation .humidity with respect to temperature. Eq. 9 has the

following boundary and initial conditions:

ETBC1: yk sq qG C T y TN , .l pl l xs0

Ex xs0

ETBC2: yk

Ex xsX

sG C qC v c T TN yT . . .a pa pv a a xsX a

ql c TN yv c T , . . .xsX a a

IC: TN s T ,x , ts0 0

where v and v are the relative humidities of the` a

ambient atmosphere and the feed air, respectively.

The boundary conditions BC1 and BC2 represent

enthalpy balances across the top and bottom heap

surfaces, respectively. The left-hand terms represent

heat conduction at the boundaries. The first term on

the right-hand side of BC1 represents the net en-

thalpy gain to the upper heap surface owing to

interaction with the environment, while the second

term represents the net enthalpy gain owing to the

application of process water. The right-hand terms ofBC2 represent the net enthalpy loss owing to the

application of process air to the lower heap surface,

and the in situ saturation of that air with water

vapour. The environmental heat flux components of

BC1 are:

q sq q q q q ,co nv ec ti on e va po ra ti on r ad ia ti on

where

q sh T yTN , .convection ` xs0

q syk r l c TN y v c T , . . .evaporation g a xs0 ` `

q s a G z yeF s T4 N yT4 . . .radiation s 1 2 xs0 sky

The convective and evaporative heat fluxes are

defined in the standard way according to ANewtons

law of cooling.B Once the heat transfer coefficient his known as a function of hydrodynamics at the

.heap surface, i.e., windspeed, etc. , the mass transfercoefficient k may be determined by the ChiltongColburn analogy:

1r31r3Sh Sc 1 ks s s ,

1r3 / /Nu Pr rC DLe p v a2r3k r C rC Dg a pa p v 2r3

[ s yLe , /h k awhere Le is the Lewis number. The radiative heat

flux includes a term for the absorption of solar

radiation, and for grey body radiative exchange with

the sky, which is only important at night as a net loss

of heat from the heap. The intensity of solar radia-

tion is a function of the zenith angle of the sun:

.sec zG t cos z cos z ) 0 . .0 aG z s . 0 cos z - 0 .where

cos z s sin l sin d qcos l cos d cos a , . . . . . .

and where G is the solar irradiation constant which0may be taken as 1390 Wrm2, t is the atmosphericaattenuation factor, and l, d, and a are the latitude,

declination, and local hour angle, respectively.

Both the ambient temperature T and the effective`

blackbody sky temperature T are taken as func-skytions of zenith angle, and both remain constant at

their minimum specified values after the sun goesdown. For ambient temperature:

cos z .min max minT q T yT cos z )0 . .` ` `~ /cos z N .T s as 0` minT cos z -0, .`


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and for effective blackbody sky temperature:1r4 cos z .

min max minT q T yT cos z )0 . .sky sky sky~ /cos z N .T s as 0sky minT cos z - 0. .skyRecasting the coefficients of the differential en-

thalpy balance in the form of timescales facilitatessolution and provides a means for comparing the

relative importance of model parameters. Defining

the following variables, functions, and parameters:

x S qzs , S s , q s ,

X rC rC Xp p

G k r C la g a pvhs , fs , bs , ks ,

G G C C l l pa pa

and the following timescales of conduction, liquid

phase advection, and gas phase advection, respec-

tively:

2rC X rC X rC Xp p pt s , t s , t s ,c l a

G C G C k l pl a pa

then the general enthalpy balance is written as:

2ET 1 E T f T f T ET . .l v s y y qS, 10 .2Et t t t EzEzc l a

where

f T s1 yf c TN y v c T . . .l zs0 ` `qh c T yc TN , . .zs0

and

f T s 1q bc T q kcX T , . . .v

with the boundary and initial conditions:

1 ET 1BC1: y sq q T y TN , l zs0

t Ez tzs0c l

1 ETBC2:

y t Ez zs1c

1s 1q bv c T TN yT . a a zs1 a

ta

qk c TN yv c T , . . 4zs1 a a

IC: TN sT .z,ts0 0

The physical significance of the two functions . .f T and f T is as follows: f represents both thel v l

loss of applied water owing to evaporation at the

heap surface and the degree of Aliquid film thicken-

ingB owing to the flow of water vapour in the

opposite direction within the heap, while f repre-vsents the degree of enhancement of the net flow of

enthalpy with the gas phase due to the presence of

water vapour.

The enthalpy balance is a second-order, non-lin-

ear partial differential equation with mixed non-lin-

ear boundary conditions. This equation is solved

numerically using a fully implicit finite difference

formulation, including central differences for the

conductive and advective terms and a forward differ-

ence for the accumulation term. As long as the value

taken for thermal conductivity is sufficiently highi.e., as long as the Peclet number is sufficiently low, bearing in mind the possibility of wide variation in

. .f T over time , no problems of instability arevencountered, and numerical solution may be consid-

ered strictly from the Eulerian viewpoint.

3. Results and discussion

3.1. Parameter selection

For the purposes of providing a consistent basis

for comparing the importance of various factors onheat conservation, a set of standard conditions has

been employed as a base case. These should conform

roughly to conditions during the vernal equinox in

the high Chilean desert, and are presented in Table 1.

In all of the simulations shown below, the rate of

heat generation S has been assumed uniform and

constant, and is therefore represented as the initial

rate S . While perhaps not very realistic, as the rate0of heat generation will necessarily fall with the rate

of oxidation as the sulphide minerals in the heap are

depleted, this assumption should suffice for the pur-pose of comparing the effects of various factors on

heat conservation. Furthermore, as will be shown, a

steady state is typically achieved within the first 2

months, over which time one would not typically

expect the rate of heat generation within most heaps

to fall to a significant degree. In any case, analysis of

change in the rate of heat generation would require


6/15


Table 1

Parameters for the base case

Parameter Value

3Average heap density rs1700 kgrmAverage heap heat capacity C s1000 Jrkg KpAverage heap thermal conductivity ks1 Wrm KHeap height Xs6 m

.Initial heap temperature T s293 K 208C03Rate of heat generation S s 20 Wrm0

2Solution irrigation rate G s 5 kgrm hlSolution heat capacity C s 4184 Jrkg Kpl

.Solution temperature T s283 K 108Cl .Aeration rate dry basis G s variablea

Dry air heat capacity C s1000 Jrkg Kpa .Air temperature T s 283 K 108Ca

Air relative humidity v s 30%aLatent heat of vaporization ls2360 kJrkgWater vapour heat capacity C s1840 Jrkg KpvAtmospheric pressure ps0.6 atm

min .Minimum ambient temperature T s273 K 08C`max .Maximum ambient temperature T

s293 K 208C`

Ambient atmospheric relative v s30%`humidity

2Surface heat transfer coefficient hs 20 Wrm KAir vapour Lewis number Les1.25Heap surface solar absorptivity a s 0.7sHeap surface grey body emissivity es0.9Heap-to-sky view factor F s 0.751 2Atmospheric attenuation factor t s 0.9aLatitude l sy238

.Solar declination ds08 equinoxmin . .Minimum night sky temperature T s213 K y608Cskymax . .Maximum noon sky temperature T s313 K 408Csky

knowledge concerning the rates of sulphide oxida-

tion within the heap as a function of temperature,

which is beyond the scope of this paper.

Another source of potential uncertainty is the

surface heat transfer coefficient, h. Owing to the low

atmospheric pressure and dry conditions in the high

desert, even small changes in h can have a dramatic

effect on the rate of evaporation, and hence, on the

rate of heat loss from the heap surface. Anticipating

this, most heap operators can and do make provi-

sions for minimizing evaporation. These include: .using drip emitters instead of sprayers or wobblers

for heap irrigation, which only wet the heap surface

directly below the drip, and thereby minimize the

available wetted surface for evaporation; burying

half of the drip lines below the heap surface; or, in

extreme cases, even applying some sort of physical

shield to the heap surface. In any case, h is difficult

to predict in the absence of hard data. The selected

value of 20 Wrm2 K, while somewhat lower thanwould be expected in the windy conditions prevalent

in the high desert, has been chosen to reflect the fact

that only a small proportion of the heap surface is

wetted. And, as will be shown, even this small value

of h results in a dramatic drop in temperature near

the heap surface.

3.2. Modeling the effect of changing flow rates

The results of changing the rates of solution

irrigation, forced aeration, and heat generation are

shown in Fig. 1. The standard conditions have been

assumed in all cases. The ordinate of each plot in

Fig. 1 represents the integrated average internal heap

temperature:

1 X 1T s T x d x s T z dz . .H Have

X 0 0

3It will be noted that a value of S s20 Wrm0corresponds roughly to a heap which contains 2%

sulphide sulphur, and leaches to 80% oxidation over

a period of 1 year. It should also be noted that even2 the lowest aeration rate of G s1.67 kgrm h ora

3 .approximately 0.14 N m dry airrt h taken in Fig. 1

is somewhat higher than standard industrial practice,and would result in only about 20% oxygen utiliza-

tion in the present scenario, assuming that all of the.sulphide sulphur is oxidized completely to sulphate.

Comparing the left and right plots shows that

increasing the solution irrigation rate is generally

detrimental to heat conservation. This is simply be-

cause more solution carries more heat away from the

heap. In fact, if one compares the top left and bottom

right plots, one sees that even when the rate of heat

generation is twice as high, the heap will be no

warmer if the solution irrigation rate is also twice as

high at the same G rG ratio. On the other hand,a lincreasing the aeration rate relative to the solution

.irrigation rate has a dramatic and beneficial effecton heat conservation, although, as shown in the

bottom left plot, the ultimate average temperature

goes through a maximum with increasing aeration rate note that the curve with open squares G rG sa l


7/15


Fig. 1. The effects of solution irrigation rate, aeration rate, and heat generation rate on the integrated average heap temperature vs. time.

.2r3 is higher than the curve with open triangles ..G rG s1 .a l

The effect of increasing the aeration rate is illus-

trated by comparing the time histories of the internal

heap temperature profile at two different aeration

rates, as shown in Fig. 2. At the lower rate of 2G rG s 1r6 or 0.83 kgrm h, which correspondsa l

3 .to roughly 0.07 N m dry airr

t h , the heap is

warmer towards the bottom as the initially cold

solution carries the heat of reaction downward. The

reason why the heap stays so cold near the top is

because the cold temperatures of the night are car-

ried downward by the solution, abetted by large

evaporative heat losses at the surface during daylight

hours, to create a kind of Astanding waveB which

culminates in a cyclic steady-state temperature mini-

mum located about 30 cm from the heap surface.Even at midday the hypothetical time of the plots in

Fig. 2; hence, the much higher temperatures at x s.0 , when the heap surface reaches its maximum

temperature, the region just below the surface re-

mains cold.

However, when the aeration rate is increased, by

a factor of 4 in this case, one sees an entirely

different sort of behaviour. Now, the direction of the

temperature profile is reversed as the gas phase

carries the heat of reaction upward. With the solution

carrying heat downward and the air carrying heat

upward, the net result is the accumulation of heat in

the centre. However, as the heap gradually heats up,


8/15


.Fig. 2. Time histories for temperature profiles carried out to 75 days at two different aeration rates each curve represents 1 day of leaching .

the gas phase carries more and more heat in the form

of latent heat of vaporization. As a result, the peak

temperature shifts towards the top of the heap. As

steady state is approached, even the temperaturetrough near the heap surface is virtually eliminated.

This behaviour is best understood by making .reference to the heat balance, Eq. 10 :

2ET 1 E T f T f T ET . .l vs y y qS. 10 .2Et t t t EzEzc l a

At the standard conditions, and at the flow and

heat generation rates specified in Fig. 2, the values .of the timescales in Eq. 10 are as follows:

Conduction timescale t s17,000 hcLiquid phase advection t s 488 hltimescale

Gas phase advection t s12,239 h atatimescale G rG s1r6a l

t s3060 h at G rGa a ls2r3


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Given the very large timescale for conduction, it

is safe to say that the major mode of heat transfer is

by advection. However, the direction in which the

heat will be carried is determined largely by the

relative values of the two terms in the square brack-

ets the Acombined advection coefficient.B By

simply comparing the values of the two advection

timescales, one would conclude that liquid phase

advection would always dominate, regardless of the

aeration rate. However, one must also account for

the presence of water vapour in the gas phase, and

the large enthalpy associated with its formation. In

other words, one must account for the advection . .functions, f T and f T .l v

The value of the liquid phase advection function .f T oscillates between roughly 1.0 during the nightl

.when evaporation from the heap surface is minimaland roughly 0.7 at midday, and is a very weak

function of temperature. However, the vapour advec- .tion function f T is a strong function of tempera-v

X . .ture primarily because of the kc T term . Forexample, at a total pressure of ps 0.6 atm from the

.standard conditions :

.T 8C 10 20 30 40 50 .f T 3.08 4.82 7.87 13.42 24.41v

.Hence, as heat accumulates in the heap, f Tvincreases, and the combined advection coefficient

tends towards more positive values. If the combined

advection coefficient can become sufficiently posi-

tive over a wide-enough region within the heap

before the cold front from the heap surface has

reached the bottom, then heat will begin to accumu-

late faster than it is removed, the temperature will

continue to increase, and the combined advection

coefficient will continue to grow. This concept is

illustrated in Fig. 3, where negative ordinate values .indicate downward liquid phase advection domi-

.nant, and positive values indicate upward gas phaseadvection dominant. As the coefficient becomes more

and more positive, upward advection becomes

stronger, and the temperature peak begins to shift

towards the top of the heap, as shown in Fig. 2. In

this sense, a sulphide heap should behave Aautocata-

lyticallyB with temperature at a sufficiently high

aeration rate. Only here, the autocatalytic AreactionB

is just the evaporation of water. Any increase in therate of sulphide oxidation with increasing tempera-

ture would only enhance the effect.

It would thus seem that increasing the aeration

rate should be just as effective as adding a fuel

source such as sulphide sulphur to increase the heat

generation rate at low aeration rates. However, suc-

cess of the technique hinges completely on blowing

air above a certain critical rate. At the conditions of

Fig. 2, the critical aeration rate lies somewhere be-

Fig. 3. Time histories of the combined advection coefficient profile at two different aeration rates.


10/15


Fig. 4. Temperature profiles after 75 days of leaching at six different aeration rates.

tween G rG s1r3 and G rG s1r2, as shown ina l a lFigs. 4 and 5. In fact, the critical rate in this case is

.very near G rG s0.35.a lAs shown in Fig. 5, operating too close to the

critical rate results in a slow temperature response,

while operating too far above the critical rate results

in a lower steady state temperature, as warm humid

air is simply blown out of the top of the heap. The

ideal aeration rate is therefore a compromise be-

tween response time and ultimate temperature.

Hence, for the purposes of heap start-up, it may be

beneficial to err on the side of overblowing at first,

but to decrease the aeration rate as the heap tempera-

ture increases. Finally, as shown in Fig. 6, starting

Fig. 5. Integrated average temperatures vs. time at six different aeration rates.


11/15


Fig. 6. The effect of initial heap temperature.

the heap at a higher or lower initial temperature

changes the time required to reach steady state, but

has no effect on the ultimate average temperature

reached.

3.3. Modeling the effect of an eaporation shield

Since the primary mode of heat loss from a heap

in the high desert would be evaporation from the

heap surface, it follows that the application of anevaporation shield would have a beneficial effect on

heat conservation. However, the effect that such a

shield would have on surface heat loss can only be

guessed at in the absence of hard data.

Any evaporation shield would probably consist of

some sort of plastic covering, most likely perforated

to facilitate air flow. Such a cover would have three . major effects: 1 it would decrease but probably

.not eliminate the effective heat transfer coefficient; .2 it would decrease the rate of radiative heat

.transfer both to and from the heap surface; and 3 itwould trap humid air at the heap surface. For thepurposes of analysis, the following parameter changes

have been assumed:

Surface heat transfer 20 5 Wrm2 Kcoefficient, h

Heap surface solar 0.7 0.5

absorptivity, asHeap surface grey 0.9 0.3

body emissivity, e

Heap-to-sky view 0.75

1.0

factor, F1

2

Ambient atmospheric 30 100%

relative humidity, v

Typical values of the surface heat fluxes over a

single day of leaching are shown in Fig. 7. The net

daily heat savings by virtue of applying the evapora-

tion shield is approximately 1.33 kW hrm2. Com-pare this with the total daily heat of reaction of

approximately 2.88 kW hrm2 at S s 20 Wrm3,0and the heat savings are substantial.

The effects of the rates of forced aeration and heat

generation in the presence and absence of an evapo-

ration shield are shown in Fig. 8. At G rG s1r6,a l

the evaporation shield increases the temperature ev-erywhere in the heap by a certain amount, but does

not change the basic shape of the temperature pro-

file, regardless of the heat generation rate. At G rGa ls1r3, the behaviour is different depending on thevalue of the heat generation rate. At S s 10 Wrm3,0the increased aeration rate has very little effect.

However, at S s20 Wrm3, the evaporation shield0


12/15


Fig. 7. Surface heat fluxes in the absence and presence of an evaporation shield at the standard conditions.

raises the heap temperature enough to push the heap

into the evaporative autocatalytic mode. If one com-

pares this temperature profile with the profile at the

slightly higher aeration rate of G rG s1r2 showna lin Fig. 4, one sees that the peak temperature in the

presence of the evaporation shield is slightly higher,

but that the profile is less uniform. At G rG s2r3,a lsomewhat similar behaviour is seen at the lower heat

generation rate of S s10 Wrm3. In this case, the0profile was just barely autocatalytic to start with, and

the shield has simply pushed the heap over the

critical point. At S s20 Wrm3, however, the shield0

has a negligible effect in addition to the high aera-

tion rate.

From the above, we may conclude the followingconcerning evaporation shields assuming that their

present mathematical interpretation is, at all, accu-.rate :

1. In the high desert, an evaporation shield will raise

the internal temperature of the heap by a uniform

increment of a few degrees Celsius perhaps by

as many as 10.


13/15


. .Fig. 8. The 75-day temperature profiles and integrated average temperature vs. time both with dashed curves and without solid curves an

evaporation shield, at two different heat generation rates.

2. In the absence of a significant aeration rate, that

is all the shield will do.

3. In the presence of an inadequate aeration rate, the

shield may raise the temperature of the heap

enough to initiate evaporative autocatalysis.

4. In the presence of an adequate aeration rate, the

shield has a negligible additional effect.

Hence, it seems fairly safe to say that providing

an adequate aeration rate to the heap should be a

more effective means of heat conservation than ap-plying an evaporation shield.

3.4. Modeling the effects of solution and air heating

The effects of raising the temperature of the

applied solution, both in the presence and absence of

an evaporation shield as defined above, are shown in

Fig. 9. In either case, at a low aeration rate, raising

the solution temperature T from 108C to 308C re-lsults in a net heap temperature increase of 38C or

48C. Again, the net beneficial effect is well below

that which would be achieved simply by increasing

the aeration rate to the heap by a factor of 4.

Moderate increases in air temperature have a neg-

ligible effect on heat conservation. However, feeding

warm, fully humidified air to the heap results in

significant thermal gains, as shown in Fig. 10. How-

ever, again, success of the technique hinges on pro-

viding an adequate aeration rate. At G rG s1r6,a lthe humid air has a negligible effect on the average

temperature, while at G rG s 2r3, the effect isa lfairly dramatic, although in the latter case, tempera-

tures are already high due to evaporative autocataly-

sis.


14/15


Fig. 9. The effect of increasing the solution feed temperature in the presence and absence of an evaporation shield.

In order to produce fully humidified air at a given

temperature, one must heat air to the temperature

which corresponds to the Aadiabatic saturation lineBon the psychrometric chart, and then pass the heated

air through a packed bed in countercurrent flow with

water which is recycled to the bed. In our case,

having assumed perfect thermal equilibrium at any

depth within the heap, identical results would be

achieved simply by blowing air heated to the adia-

batic saturation temperature directly into the heap.

Fig. 10. The effect of blowing fully humidified air into the heap at different temperatures and aeration rates.


15/15


To obtain fully humidified air at 308C, this tempera-

ture would have to be 1408C. To obtain fully humidi-

fied air at 408C, this temperature would have to be

2428C. The cost to achieve such high temperatures

would be prohibitive.

4. Conclusions

The major modes of heat loss from a sulphide

heap operating in the high desert are by evaporation

from the heap surface and by sensible heat loss with

the effluent solution. Several options have been con-

sidered for conserving this lost heat, and thereby

increasing the internal temperature of the heap, in-

cluding:

Decreasing the solution irrigation rate;

Increasing the aeration rate; Applying an evaporation shield to the heap sur-

face;

Heating the solution; and

Heating the air, with and without humidification.

From the foregoing analysis, it may be safely

concluded that the most effective means of achieving

a high degree of heat conservation within a sulphide

heap in the high desert is to blow air into the heap

above a certain critical rate while maintaining a low

rate of solution irrigation. Any other means of con-

serving heat is only effective when these two have

been achieved, and in most cases, the additional

effect is minimal and probably would not justify the

expense.

References

w x1 P.J. Marcantonio, Chalcocite dissolution in acidic ferric sul-phate solutions, PhD Thesis, University of Utah, 1976.

w x2 S.A. Bolorunduro, Kinetics of leaching of chalcocite in acidferric sulfate media: chemical and biological leaching. MSc

Thesis, University of British Columbia, 1999.w x3 L.M. Cathles, L.M. Apps, J.A. Apps, Metall. Trans. B 6B

.1975 617624.w x .4 G. Pantelis, A.I.M. Ritchie, Appl. Math. Modell. 16 1992

553560.w x5 J.M. Casas, J. Martinez, L. Moreno, T. Vargas, in: T. Vargas

.Ed. , Biohydrometallurgical Processing vol. I University of

Chile, Santiago, 1995, pp. 447457.

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