443

Model l ing of bioleaching copper sulphide ores in heaps or dumps

L. Moreno a, J. Martinez a and J. Casas b

a Department of Chemical Engineering and Technology, Royal Institute of Technology, 10044 Stockholm, Sweden

b Department of Chemical Engineering, University of Chile, Beauchef 861, Santiago, Chile.

The effects of variable permeability and number of bacteria on bioleaching of copper sulphide in dumps and heaps were studied. The calculations were performed using a two dimensional model for heaps or dumps containing mainly chalcocite and pyrite. The rate of mineral dissolution was related to the rate of oxidation by bacteria attached onto the ore surface. Oxygen transport through the ore bed is caused by natural convection. The results show that variations of bed permeability with depth may have a great influence on the aeration of the bed. In wide ore beds of uniform permeability, aeration of the central parts is deficient and in most of the cases, oxygen is totally depleted at the centre. In ore beds where permeability increases with the depth, aeration is substantially improved. It suggests that dumps and heaps should be built with coarse materials at the bottom to improve aeration. The impact of the number of bacteria, on the other hand, is small. This is explained by the control exerted by the availability of oxygen on the reaction rate. A high concentration of oxygen results in a large number of bacteria and in a high reaction rate. If the concentration of oxygen is small or oxygen is completely depleted the reaction rate is negligible; thus the number of bacteria is not crucial.

1. INTRODUCTION

Bed permeability is an essential process parameter since it controls aeration of the ore bed. Bacterial activity is also important owing to its roll in the oxidation of the ferrous ions to ferric. In earlier models for leaching of copper sulphide ores in heaps or dumps, a constant and uniform air permeability and number of bacteria were assumed. The assumption of a uniform permeability neglects the segregation of coarse particles at the bottom that usually takes place when dumps are built. Temperature and availability of oxygen and nutrients, on the other hand, determine the number of bacteria.

Bioleaching of ore-beds is applied at industrial scale to treat low-grade ores such as" secondary copper sulphide ores, uranium ores and as pre-treatment process for refractory gold ores. It is an aerobic process where acidophilic iron oxidizing bacteria are developed

444

within the bed dependent on the oxygen availability. Oxygen can be supplied by gas diffusion, natural air convection and forced air ventilation.

The reacting system for the bioleaching of sulphidic ore beds is heterogeneous, i.e., the oxidation rate changes with position and bacterial activity is determined by the bacterial population, the oxygen concentration, and the temperature. The bioleaching rate is controlled by the slowest of the following four steps: 1- oxygen supply, 2- bacterial oxidation of ferrous ions, 3- ferric ion diffusion through the ore particles, and 4- the intrinsic reaction of the sulphide minerals with the ferric ions. These controlling steps may coexist in different zones in the ore bed (Ritchie, 1994; Casas et. al, 1998).

Modelling of the leaching operation for sulphide ores-beds, has receive considerable attention in the last 25 years (See, for example, Cathles and Schlitt, 1980; Neuburg et al, 1991; Pantelis and Ritchie,1992; Ritchie, 1994; Bartlett, 1997; Casas et al.; 1998). The macroscopic models developed by Cathles (1994), Ritchie (1994) and Casas et al., (1998) showed that when the ore-bed permeabilities are too low, the oxygen supply occurs mainly through air diffusion inside the bed. Under these circumstances, bacterial leaching of the sulphide minerals is low since it is limited by the slow supply of oxygen. The improvement of bed aeration by natural air convection demands an adequate level of bacterial population and adequate bed permeability. Today the tendency is the use of forced ventilation in order to improve the air supply and to obtain high bioxidation rates. This operation mode has higher capital and operational costs, but they are compensated by the faster metal recovery (Bartlett, 1997).

Particle size, ore density, and porosity, ore loading and mode of heap construction, and ore-bed compaction determine the bed permeability distribution. The amount of fine particles in the ore is a key parameter and it is recommended that the fine fraction is less than 10%. Fines can be mobilised by the leaching solution and obstruct the void space between larger particles causing a reduction in the bed permeability. The finest fractions can be reduced by agglomeration prior to the leaching. Wetting and agglomeration of the ore prior to stacking can increase the average bed permeability in 1-2 orders of magnitude for ore crushed between-3/8" and-1" (Bartlett, 1996).

In the published models for bioleaching of copper sulphide in ore beds, a constant bed gas permeability and constant number of bacteria were assumed. The objective of this work is to study the influence of permeability variations on the aeration efficiency and bacterial development over the copper bioleaching process performed in ore beds.

2. MODEL FORMULATION

The model describes the bio-leaching process of an ore bed containing chalcocite and pyrite as the main sulphide minerals. Bacterial population was assumed to be only composed of Thiobacillus ferrooxidans. These bacteria live in mining environments at temperatures ranging from 10~ to 40~ and at pH values between 1.5 and 3.5 [Rossi, 1990].

445

The configuration analysed consists of an ore bed that is irrigated on the top and on the slope with a dilute solution of sulphuric acid. The solution percolates through the ore particles and dissolves the minerals. Oxygen transport takes place both by molecular diffusion due to concentration gradients and by natural air convection caused by pressure gradients. A two- dimensional geometry is considered. Since the system is symmetric a half of the cross section is modelled.

The model considers the processes occurring in an ore bed when the oxidation of the copper sulphide is controlled by the activity of the bacteria. This means that transport of ferric ions inside the particle is not rate limiting. This assumption is not valid for large particles (larger than several centimetres, Casas et al., 1998) since the reaction will then be mainly controlled by the diffusion inside the particle.

The model is based on a set of equations describing the physicochemical and transport mechanisms of a bio-leaching process controlled by the activity of bacteria. The bio- leaching rate is represented by the Michaelis-Menten equation where the dissolved oxygen is the limiting substrate. In this case the rate of copper dissolution can be expressed as:

d(x= (5"11 CL ]XV m ~ (1) dt oBG ~ K m + C L

where V m is the maximum oxidation rate of the bacteria. A relationship between V m and the temperature was obtained by fitting a model equation to experimental data for ferrous oxidation by T. Ferrooxidans. X is the number of bacteria attached onto the mineral ore, which is a function of the oxygen availability and the temperature. ~1 is the stoichiometric factor considering the following dissolution reactions:

Chalcocite:Cu2S + 2.502 + H2SO 4 "-) 2Cu SO4 + H20 (2)

Pyrite: FeS 2 + 3.502 + H20 --) FeSO 4 + H2SO 4 (3)

2.1. Air flow by natural convection

The velocity of air is described by Darcy's law. The relative gas permeability for the unsaturated ore bed is used in this case. The relative permeability may be a function of the location inside the pile.

k.krg /___ ~ ~gVg =qg = I, VP-pgg] (4)

gg

The movement of air inside the bed is by natural convection caused by the gradients of gas density due to heating, humidification and variation in gas composition when oxygen is depleted.

446

2.2. Mass balance for the gas phase

The mass balance for the gas is expressed as follows:

-- ~gV" (Vg[:)g)= [DBG~ dot _ V(GgHg ) (5) ~ dt

where the term on the left hand side represents the advective flow of gas. The first term on the right hand side considers oxygen consumption by bacteria and the last term the vaporisation of water.

The other model equations, the energy balance and the balance of gaseous oxygen, have been previously described by Casas et al. (1998). The oxygen balance includes the transport by molecular diffusion, bulk transport due to natural convection of air and the rate of oxygen consumption. The dissolved oxygen concentration is assumed to be in local equilibrium with the oxygen concentration in the gas phase. There are several mechanisms by which heat may be transferred in the bed. The most important are: the transport by liquid flow, the gas flow in the bed including the effect of water vaporisation and the heat generated by the leaching reactions. The model assumes a local thermal equilibrium between solid, liquid and gas phases.

2.3. Boundary conditions and numerical solution

Flow of air and heat across the central axis of the ore bed is zero by symmetry conditions. Air flows freely across the top and slope surface of the heap. Inlet liquid temperature, ambient air pressure and temperature as well as ambient oxygen concentration are the boundary conditions of the system at the sides.

The mathematical model described above was transformed into a set of dimensionless equations and then discretised using the integrated finite difference methodology. The resulting algebraic equation system is non-linear and it was solved by the successive over- relaxation iterative method, using a personal computer.

3. CALCULATED CASES

In the earlier paper [Casas et al., 1998] we have assumed a uniform and constant gas permeability and number of bacteria. A uniform permeability throughout a dump is difficult to achieve in practice since segregation of particles of different sizes usually occurs when the bed is built. Normally, the coarse material tends to accumulate at the bottom. The number of bacteria should also exhibit local variations since it depends on variables that are functions of the position in the dump as oxygen concentration, available nutrients and temperature.

In this paper, it was assumed that the gas permeability is larger for the material close to the bottom. A continuously variation of the permeability is considered, with a value 10 times larger at the material at the bottom compared with material at the top.

447

As mention before, the number of bacteria is controlled by several factors such as temperature and nutrients and oxygen availability. In this discussion, only the effect of temperature and oxygen concentration will be considered. To find a relationship between the number of bacteria and these factors is not trivial. At the initial period, the number of bacteria increases with time. If the conditions are maintained constant, the amount of bacteria reaches a constant value, where the rate of growth of the bacteria is compensated by the rate of bacteria that are lost from the system. Bacteria transported away by the solution are also included between the bacteria that disappear from the system. Due to the lack of data, the following general relationship is proposed:

Xactual = Xoptimum " Ftemperature " F oxygen ( 6 )

where Xoptimu m is the number of bacteria in the bed when the temperature is optimum and

the oxygen concentration in the gas is equal to the composition of the outside air. The two functions, shown in Figure 1, take the value one if optimum conditions prevail; otherwise the value is less than one. For temperature dependence, an optimum temperature of 30 C is chosen. For oxygen the function is zero when oxygen is totally depleted and one when the composition is equal to that of the outside air.

0.8 r.-

.o "6 0.6 E:

E ~ 0.4

o

0 . 2

~ o'.2 o4 Oxygen

o'1 o'3

1

80.8

~ 0 . 6

~0.4

I - 0 .2

~ 20 40 60 ..... 80 Temperature, C

Figure 1 Functions used to express the dependency of the number of bacteria with the oxygen concentration and temperature

4. RESULTS AND DISCUSSION

The impact of variable permeability and number of bacteria in the model was studied by observing the influence of these variables on the distribution of airflow, oxygen concentration, temperature and copper recovery in the dump. A pile with a height of 6 m and a width of 6 and 18 m on top and the base respectively was simulated. As reference

5

4 E

1

and for comparison proposes, calculations were also performed using a pile with uniform and constant permeability and constant number of bacteria. The results for this case are shown in Figure 2. The model parameters used in the calculations are the same reported in previous works [Casas, 1998]. See Table 1.

Stream Functions

448

5

4 E

, - 3 o -1-2

1

2 4 6 8 Half Width, m

Temperature, C . . . .

/ / + \'~o~

3O

2 4 6 8 Half Width, m

4 E

.E3 O3 ~2

5

4 E

1

O~gen Concentration, % 1

_0

�89 4 6 8 Half Width, m

Recovery, %(1 month) 0.5 1.5 . . ~.~ '

".5 2 4 6 8

Half Width, m

Figure 2 Results for a pile with constant permeability and constant number of bacteria.

Table 1. Values for the parameters used in the calculation

FPY, pyrite reaction factor 2.5 kg of pyrite per kg of chalcocite leached Chalcocite ore grade 0.63 wt % Effective permeability of the ore bed 5x10 -11 m 2 Km, Michaelis constant [3] 10 .3 kg/m 3 Bacterial population attached to the ore 10 '3 bacteria/m 3 of ore bed

449

4.1. Influence of a varying permeability

The permeability was continuously increased from top to bottom. At the bottom, the permeability was set to a value 3.3 times larger than the value used in the case with constant permeability (5x1041 mE). The permeability at the top was set to a value 3.3 times less than that constant permeability. Results of simulations are shown in Figure 3.

The results show that the flow through the bed and at the bottom of the pile increase. This has a great impact on the concentration of oxygen in the pile. The levels of 75 and 50 % (with respect to concentration of outside air) are closer to the centre of the pile compared to the case of uniform permeability. The centre of the pile presents relative oxygen concentrations above 10 % in a large extent whereas in the reference case the centre of the pile exhibits relative oxygen concentrations lower than 10%.

The temperature in the pile also increases. Temperatures above 30~ are found in the centre of the pile near the bottom. The higher temperature and oxygen concentration in the interior parts of the pile resulted in a copper recovery.

5

4 E

5

4 E , -3

3::2

1

Stream Functions

5

Half Width, m Temperature, C

. . . .

- ~ ~ x 2 25.5

7

2 4 6 8 Half Width, m

Oxygen Concentration, %

5

4 E

-r 2

. . "

Half Width, m Recovery, % (1 month)

5

4 E E:3

-r 2

1

~. " + ~ ' 1 . ' ' '

" ~ 5

3 0.5

2 4 6 8 Half Width, m

Figure 3 Results for a pile with varying permeability.

450

4 . 2 . I n f l u e n c e o f a v a r y i n g n u m b e r o f b a c t e r i a

The results of simulations with a variable number of bacteria are shown in Figure 5. A constant permeability was use in the calculations. No large differences were observed between these calculations and those performed with a constant number of bacteria (10 '3 bacteria/m 3 of ore bed). The flow distribution within the bed is similar in both cases. The location of relative oxygen concentration levels around 50 and 70 % is not changed. The levels of oxygen concentration in the centre of the pile are above 10 % whereas for a constant number of bacteria, these levels were below 10 %. This reveals that some impact of the number of bacteria is observed at locations where the activity of the bacteria is low. The copper recovery is slightly greater at the zone below the slope and somewhat lower at the centre of the pile. The relationship proposed to represent the dependence of the number of bacteria on temperature and oxygen concentration is only approximate. It represents only a qualitative description of the dependency. However, similar results were obtained with other possible functions revealing that the form of this function has a limited influence.

4 E ,--3

-r- 2

5

4 E ,--3

-r 2

Figure 4

Stream Functions

- - 5 " ' ~"

2 4 6 Half Width, m

Temperature, C . . . .

* / / ~ 2 5 5

, , , \ ,\ 2 4 6 8

Half Width, m

5

4 E ?_3 .g) -r 2

1

5

4 E

t:2 1

Oxygen Concentration, % ,~1 t q t

: \\:\

\~.. 75

2 4 6 8 Half Width, m

Recovery, % (1 month) - - ~ , , ~ , , , , . . . . ,.

115

+2 21 05

2 4 6 8 Half Width, m

Results for a pile with varying number of bacteria.

451

5. CONCLUSIONS

The impact on the leaching of copper ores caused by a variable permeability and number of bacteria was studied. The results were compared with a case of uniform and constant permeability and number of bacteria.

The permeability of the pile may vary for several reasons. In this study, a bed with a larger permeability at the bottom, caused by segregation of coarse particles when the dump is built was simulated. The results show that a larger permeability at the lower part of the pile has a great impact on the airflow through the pile resulting in a better aeration of internal parts of the bed.

It was assumed that the number of bacteria is determined by the temperature and oxygen availability. It was found that the impact of a variable number of bacteria is not crucial, if the total number of bacteria in the bed is maintained constant.

NOTATION

CL g Gg G ~ Hg k krg

P qg t

Vg Vm x

Oxygen concentration in liquid phase, kg/m 3

Gravity, m/s 2 Mass flux of dry gas, kg/(m2/s) Chalcocite ore grade, wt% Gas humidity, kg water/kg dry gas Bed permeability, m 2 Relative gas permeability, Michaelis constant, kg/m 3 Pressure, Pa Volumetrix gas flux, m3/m2/s) Time, s gas velocity, m/s Maximum specific respiration rate by bacteria, kg 02/bacterium/s Bacterial population attached onto the ore, bacteria/m 3 of ore bed

Greek letters

~g Pg ~g (Yl

Chalcocite conversion or copper recovery Porosity occupied by the gas, m3/m 3

Average gas density,, kg/m 3 Gas Viscosity, kg/(m.s) Stoichiometric factor

452

REFERENCES

- Bartlett R.W. and K.A. Prisbrey, Int. J. of Mineral Processing, 47 (1996) 75. - Bartlett R.W., Metallurgical and Materials Transactions B, 28B (1997) 529. - Casas J.M., J. Martinez, L. Moreno and T. Vargas, Metallurgical and Materials

Transactions B, 29B (1998) 899. - Cathles L.M., ACS Symposium Series, USA., Vol 550 (1994) 123. - Cathles, L.M. & Schlitt, W.J., Leaching and Recovery of Copper from as Mined

Materials. Las Vegas Symposium, Schlitt, W.J., (ed.), AIME., (1980) 9. - Neuburg H.J., J.A. Castillo, M.N. Herrera, J.V. Wiertz, T. Vargas and R. Badilla-

Ohlbaum, Int. J. Miner. Proc. 31 (1991) 247. - Pantelis, G. & Ritchie, A.I.M. Applied Mathematical Modelling, 16 (1992) 553. - Ritchie A.I.M., The Environmental Geochemistry of Sulfide in Mine-Waste, D.W.

Blowes and J.L. Jambor (eds.). Mineralogical Association of Canada, Nepean, Ontario, Canada. 22 (1994) 201.

- Rossi, G., Biohydrometallurgy. McGraw-Hill, Hamburg, 1990.