Environmental Geochemistry of Sulfide Oxidation - American

Chapter 9

Rates of Mechanisms That Govern Pollutant Generation from Pyritic Wastes

Downloaded by UNIV OF NEW ENGLAND on February 9, 2017 | http://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0550.ch009

Α. I. M. Ritchie Environmental Science Program, Australian Nuclear Science and Technology Organization, Private Mailbag 1, Menai 2234, Australia

The environmental impact of pollutants generated by the oxidation of pyrite in mine wastes involves a number of processes which have very different characteristic timescales. In the first instance water quality estimates require the convolution of pollutant generation in and water transport through the wastes followed by convolution with water transport through the aquifer underlying the wastes. As a significant fraction of the wastes may be unsaturated, transit times may be two orders of magnitude greater than those in the saturated aquifer. Such a difference requires care in interpreting the impact of rehabilitation measures. The pollution generation rate within the wastes also depends on a number of interacting processes with greatly differing timescales. For example oxidation rates of pyrite under optimized conditions are typically three orders of magnitude greater than 'high' oxidation rates measured in wastes. In this paper these various rates and the influence that they have on the overall environmental impact is discussed. Data on some important mechanisms is sparse and some indication is given as to how this situation may be rectified. The fact that the proceedings of the First International Conference on Control of Environmental Problems at Metal Mines (Roros, Norway, June 1988) were contained in one volume while the proceedings of the Second International Conference on the Abatement of Acidic Drainage (Montreal, Canada, September 1991) filled four volumes is indicative of the international focus on the environmental problem posed by oxidation of pyrite in mine wastes. The pollution appearing in surface water near a deposit of pyritic wastes results from a complex of many mechanisms. At the heart of this complex is the oxidation of pyrite which, even in the absence of bacterial catalysis, is a complex of mechanisms itself. The rate of oxidation of pyrite in a dump is governed by the transport of reactants to oxidation sites in the dump as well as the intrinsic oxidation rate. The

0097-6156/94/0550-0108$06.00/0 © 1994 American Chemical Society Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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oxidation products, which are usually acid and metal sulfates, are pollutants which are transported to the base of the dump and thence to surface water through an aquifer or aquifer system below the deposit The characteristic timescale of the transport processes will impact on the concentration of pollutants and the way this concentration changes with time in ground or surface waters. The object of rehabilitation measures is to reduce the concentration of pollutants to an acceptable level over the lifetime of oxidation in the mine wastes. In this paper, I look at various mechanisms and their timescales to note which of them is important in determining pollutant generation rates within the pyritic material and the rate of change of pollutant levels in ground and surface water near the deposits of pyritic waste. I will focus on pyritic waste dumps but many processes are transferable to some tailings dams. I will illustrate the processes with models which, although simple, contain the essential features of important mechanisms. These simple models assist in focusing on what measures to adopt to reduce the environmental impact of oxidation in pyritic wastes and on the parameters which need to be measured to be better able to predict the effectiveness of rehabilitation measures. The Intrinsic Oxidation Rate Figure 1 shows a set of equations which describes oxidation in a heap of pyritic material (1). The equations, as they stand, take account of the shape of the heap and the transport of heat, oxygen and water through the heap but not of the changing chemical or microbiological conditions within the heap. The "source" term on the right hand side of the first equation is just a function of the temperature and of the concentration of oxygen and pyritic material. If we added more equations we could take account of the changing chemistry. The source term, which is in fact the oxygen consumption rate, would then become dependent on the water flow rate and the concentration of whatever chemical species were believed to be important. It is convenient to describe this term as the intrinsic oxidation rate of the system. In any attempt to model the pollution generation rate in a heap of pyritic material it is necessary to have a model for the intrinsic oxidation rate. Both the chemistry and microbiology of pyritic oxidation have been the focus of much experimentation over many years (see reviews by Lowson (2) and Brierley (3)). We would expect such work to be a starting point for our model of intrinsic oxidation rate. Intercomparison Of Oxidation Rates Table I shows oxidation rates for pyrite measured by a number of workers with what can be considered as a chemical, microbiological and physical slant. I have deliberately given the rates as quoted to underscore the difficulty in comparing rates measured in one experiment with that in another with nominally similar conditions. The table is not intended to be exhaustive. The three types of experiment, physics, microbiology and chemistry, have been chosen either because they cover a range of conditions of some importance or they provide sufficient data that allow intercomparisons with the results from one or more of the other types of experiment.

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION

Oxygen Concentration z

a

at

+ ε ν * · Vc - V · (D Vc) = - Si(c c J) α

a

t

Solid Reactant Concentration i f = -5 (c,F,T) 2

Temperature ΣαΡ^α

+I

aPot

c v (air.water; respectively) and p is related to the intrinsic density p by p = ε ρ . a

a

α

a

The volume fractions Σ ε « = 1.

α

ε , α = {a ,w ,s } must satisfy α

α

Figure 1. Equations describing heap oxidation.



Measurements in waste rock dumps

Inferred from temperature profiles measured in a dump pH 2.0-4.0 Temperature 35-56°C

(uM S0 min ) Oxidation in DO saturated solutions and range 0.021-0.085 ferric solutions; average 0.057 the data quoted are for DO solutions pH 2.2-9.1 Temperature 22-25°C results normalized to 1 g pyrite in 300 mL

Chemical mechanisms

8

(kg-m-V) range 0.3-8.8 χ 10"

1

(mg Fe VW) range 7.8-17.8 average 12.4 abiotic rate average 0.36

Comparison of results from eight labs 1 g pyrite in 50 mL pH 1.3-3.4 Temperature 28°C

Microbiological mechanisms

4

Quoted rate

Conditions of experiment

Type of experiment

5

3

(10) 1.0 χ 10" for model dump

8

(5)

(4)

3.2 χ 10" from average rate

6

7

(kg-rnY at a dump pyrite density of 56.3 kg-m" ) 1.9 χ 10" from biotic average 5.6 χ 10" from abiotic average

1

Normalized rate

Table L Comparison of Oxidation Consumption Rates Derived from Different Types of Experiments


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It should be stressed that many of the chemically and microbiologically based experiments have as their objective the clarification of the mechanisms of pyrite oxidation rather than the provision of data to construct an intrinsic oxidation rate model It is noteworthy that the abiotic rate in microbiologically based experiments (4) is lower than the chemical rates (5). It is of considerable interest that the lowest laboratory-based oxidation rates are about two orders of magnitude greater than those measured in field experiments.


Modelling Using Simple Intrinsic Rate Models As discussed above, a heap model as exemplified by the equations in Figure 1 does not describe the effect of changing chemical or microbiological conditions within the heap. In particular, this means that it cannot predict adequately the "lag" phase where the microbial population builds up and acid reacts with alkaline minerals. Simple Constant Rate Model (SCRM). It is illustrative to consider an intrinsic oxidation rate model where the oxygen consumption rate is independent of pore-gas oxygen concentration and of the pyrite concentration except where these approach zero, where it is assumed that the oxygen consumption rate tends to zero in some way. Although this model appears very simplistic there is evidence (D. Gibson, written commun., 1992) that it applies to some pyritic material. Let us further assume that this intrinsic oxidation rate applies in a heap with the physical properties given in Table Π. Again for simplicity we will assume that the heap was built sufficiently quickly that little or no pore-space oxygen was consumed during the construction phase and further that the moisture content of the heap as built is the equilibrium one for the infiltration rate given. The first point to note is, that for the stated oxidation rate and assuming the stoichiometry of the first equation in Table ΙΠ, the initial pore-space oxygen, pyrite and pore-space water in the dump will be used up in about 3 months, 166 years and 2000 years respectively. It is clear that oxygen needs to be supplied to the system for pyrite oxidation to continue. It is easy to show that oxygen dissolved in water infiltrating the heap is about three orders of magnitude too small to sustain oxidation at the required rate. It has also been shown Q) that unless the air permeability, K, is larger than about 10" m , diffusion dominates over convection as the oxygen transport mechanism. It also follows that in most of the heap one-dimensional transport will be a good description of oxygen transport and the oxygen concentration in the pore space will decrease as indicated in Figure 2a. For the parameters chosen the concentration will fall to zero close to the base of the heap. For higher heaps, it follows that, once the initial pore-space oxygen is used up, the region below 15 m will not contribute to pollution generation until all of the pyrite in the top 15 m is oxidized. It also follows from our simple model that the concentration of oxidation products (pollutants) in the water infiltrating the heap increases linearly until it reaches the base (see Figure 3a). Again if the heap were higher the concentration of oxidation products in the pore water below 15 m would remain constant It should be 9

2


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113

Table Π. Physical Properties of Model Waste Rock Dump


Units

Value

Definition

Symbol S

The intrinsic oxidation rate for SCRM

1 χ ΙΟ"

kg oxygen m'Y

L

Dump height

15

m

A

Dump area

20

ha

Prs

Sulfur density as pyrite

30 (2%)

kgm"

Ρ

Bulk density of dump material

1500

kgm'

q

Infiltration rate

0.5

m-y"

Oxygen diffusion through dump pore space

4.1 χ 10'

nrV

Oxygen concentration in air

0.265

kgm"

Mass of oxygen consumed per unit mass of sulfur oxidized

1.75

Volume fraction of water phase

0.1

Co

ε

8

1

3

3

1

6

1

3

Table ΙΠ. Sulfide Oxidation Reactions

1

= 1440 kJ-mol

FeS + | θ + HjO - FeS0 + 2

2

2FeS0 + I^SO, + \θ

4

2

4

AH = 102 kJmol

- Fe^SO^ + HjO

FèS + Fe^SOJ, + 21^0 + 30 - 3FeS0 + 2H S0 2

MS + Fe^SO^ + | 0

2

2

4

2

4

+ I^O - MS0 + 2FeS0 + I^S0 4

4

4

where MS stands for any metal sulfide.


1


114


Figure 2. Oxidation concentration profiles for different models of the intrinsic oxidation rate: a) simple constant rate model (SCRM) at low rate; b) simple homogeneous model (SHM); c) shrinking core model (SCM).



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3a [c]=SL/q

3b

I

[c]=S(t-t)/q 1

[c]=S(t)/q

η

x = X(0

Top of dump

Bottom of dump

Figure 3. Pollutant concentration in the pore water for different models of the intrinsic oxidation rate: a) simple constant rate model (SCRM); b) simple homogeneous model (SHM) where t is the water transit time from X(t) to the bottom of the dump. x



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noted that, with this oxidation rate model, the pollutant load from the base of the dump is independent of the infiltration rate. In particular, decreasing the infiltration rate does not decrease the pollution load emanating from the base of the dump. I shall return to this point below. For the parameters chosen the acid concentration at the base of the heap is about 0.12 molar with a consequent pH of about unity. If the infiltration rate were decreased by about a factor ten the pH would decrease to about 0.2 if the oxidation rate remained unchanged. On the basis of data quoted in the literature, the microorganisms which catalyze the oxidation of pyrite cannot tolerate such a low pH. If the bacteria are important in maintaining this very low oxidation rate we would expect it to decrease at some point above the base, with a consequent decrease in total load. We could well expect intrinsic oxidation rates to be two to three orders of magnitude higher than the rates assumed for the SCRM on the basis of quoted microbiologically catalyzed and chemical oxidation rates. This is such a large factor that it is as well to consider an infinitely high intrinsic oxidation rate. We can encompass such high rates in another simple model. Simple Homogeneous Model (SHM). In this model it is assumed that the oxidizable material is uniformly distributed through the heap and the oxidation rate in the heap is limited by the rate at which oxygen can be supplied to an oxidation front which starts at the surface and moves into the heap. In mathematical terms it is a classical moving boundary problem (6) and its properties have been discussed elsewhere (7). The oxygen concentration in the dump takes the form shown in Figure 2b; the position of the oxidation front is given by,

and the oxygen consumption rate by, S = «(* - X(r)),

The pollution concentrations then take the form shown in Figure 3b. Table IV presents some concentrations and loads predicted by applying these simple models to a waste rock dump with the properties given in Table Π. The SHM predicts conditions early in the dump's history which are too acid to be consistent with the survival of microorganisms according to the acid tolerances quoted in the literature. Such a model also predicts temperatures in the dump (8) which thermophiles can tolerate but not Thiobacillus SPP. It is also clear, however , that oxidation rates high enough to pose a significant environmental problem can be encountered without invoking the catalytic properties of microorganisms.


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The Shrinking Core Model (SCM). The infinitely high pollution rates predicted by the SHM at early times are clearly not realistic. The shrinking core model allows such infinities to be circumvented and has the added attraction that the intrinsic oxidation rate predicted by this model decreases as the pore-gas oxygen concentration and the concentration of the oxidizable material decreases (9). Intuitively this is a property we seek in a realistic intrinsic oxidation rate. The SCM gives rise to two moving fronts in the dump; one, above which all the oxidizable material is oxidized; and one, below which there is no oxygen and no oxidation. The resulting oxygen concentrations have the form shown in Figure 2c. The pollution load and the pollutant concentrations predicted by this model at the base of the dump are substantially the same as those predicted by the SHM. Timescales These simple models indicate that pollution generation in a typical waste rock dump lasts on a timescale of tens to hundreds of years. It is of interest to examine the equations which describe oxidation in a waste dump (see Figure 1) to see if such timescales are typical of the system. Table V lists the timescales which arise in these equations and evaluates them for typical parameter values. The longest timescale is that associated with the diffusive transport of oxygen in the dump while the shortest is that for oxidation of a single particle when the shrinking core model is used to describe the intrinsic oxidation rate. Not surprisingly, it has been shown (1) that in a waste dump where diffusional transport dominates, a reduction in the time to oxidize a particle has little effect on the oxidation rate of the dump as a whole. It has also been shown (1) that, unless the air permeability of the dump is large, convection is not a significant gas transport mechanism even though the timescale associated with convection is so much shorter than that associated with diffusion. One reason is that, initially the only gas transport mechanism is diffusion. The temperature gradients which might potentially drive convection are established on the diffusive timescale and are established at the toe of the dump. If gas were supplied all over the base of the dump then oxidation would proceed at a timescale closer to that associated with convective transport. Doubling times for the microorganisms which catalyze pyritic oxidation are typically less than a day. This timescale is short compared to those above and it would take less than three weeks for the population to increase by six orders of magnitude and be at a level where the bacterial population was not rate-limiting. Pollutant Transit Times Waste rock dumps are generally unsaturated except possibly for the bottom meter or so and unless the climate is very wet or very dry a reasonable value for the infiltration rate is about 0.5 m-y . With a water-filled porosity of 0.1 the transit time of water from the top to the base of our 15 m high model dump would be about three years unless preferred paths transmit a significant quantity of water. Waterflowrates will be very much faster, say 500 my" in the saturated zone at or below the base of 1

1



118


Table IV. Some Indicative Pollutant Concentrations Predicted by SCRM and SHM Assuming No Further Interaction of Oxidation Products with Dump Material Pollutant

Units

Sulfate

gm"

SCRM (At base of dump)

SHM (At 20 years)

16,200

23,000

1.08

0.97

3

pH

460

3

Copper g-m/ 320 (assuming sulfate-to-copper ratio of 50:1)

Table V. Characteristic Timescales

Timescale

Expression

Process Oxidation of pyrite in a particle

P

3γΖ) ε 2

Convection of gas through a heap with competing chemical reaction Diffusion of gas through a heap with competing chemical reaction

Λ

0.18 years for 2 mm diameter particle (time to completely oxidize a spherical particle is tJT) 1.5 years

a

K ^T c 9

a

0

345 years , d

» J >

cJD

a


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119

the dump into which pollution discharges but the distances, at about 500 m, over which this water must travel to traverse the heap will, however, be much greater. If as discussed above, pollution generation is greatest near the surface of a dump then a toe drain at the base of a dump will not receive pollution for about three years after completion of the dump even if there is no lag time in the initiation of oxidation. The rate of change of pollution levels in the drain at these early times will also reflect a convolution of different travel times from the base of the dump to such a drain.


Implications for Rehabilitation It follows from the simple models above that measures adopted to reduce the rate of water infiltration into pyritic material may not reduce an existing environmental impact permanently. The reduced infiltration will certainly lead to lower pH within the dump but any consequent inhibition of bacterially catalyzed pyrite oxidation is still likely to leave a chemical rate which is much higher than can be supported by oxygen transport rates if gas transport is dominated by diffusion. The overall oxidation rate will then be determined by oxygen transport rates into the dump. Reduced infiltration at the top surface of the dump will certainly lead to decrease flow from the base of the dump into some underlying aquifer. When steady state has been established after imposition of the reduced infiltration rate, pollutant concentrations will, however, be proportionately increased and the load exiting the base of the dump be the same as before implementation of the rehabilitation measures. The environmental impact will be unchanged. In practice there will be a drop in the load from the dump immediately after infiltration is reduced as water exiting the dump will contain the "old" pollutant levels. This reduction is a transient. If we assume a tenfold reduction in water infiltration rates then the load exiting the dump will initially be reduced by the same factor but will steadily increase to the steady state value. The timescale for this increase will be of the order of the transit time for water through the waste dump. Using the parameters for our model dump this transit time will increase from 3 years to 30 years with a factor of ten decrease in infiltration rate. The implication is that in 30 years time the environmental impact of the dump will return to its prerehabilitation level. A permanent decrease will be obtained if the measures adopted lead to a reduction in the overall oxidation rate in the waste dump. This argument will hold unless chemical interactions within the wastes place an upper limit on pollutant concentrations in the water-filled pore space. In this case a reduction in the rate at which water infiltrates will lead to a reduction in load; a reduction in the overall oxidation rate in the dump may not lead to a reduction in the load. Bacterially catalyzed pyrite oxidation rates are very sensitive to pH in the range near neutral to near zero while chemical rates where oxygen rather than ferric ion is the oxidant are much less sensitive over the range 8 to close to zero. Both rates are much higher than the very low rates which can still lead to significant pollution generation from a waste rock dump. It therefore seems likely that pyritic oxidation rates in dumps will be insensitive to changes in pH over a wide range. The


120



implication is that blending of acid-producing material with acid-consuming material may well reduce acidity and some heavy metal production but possibly at the expense of exchanging an AMD problem for a salinity problem. It is our experience that, although pyritic waste dumps appear to be very heterogeneous, measurements of oxygen and temperature profiles indicate that they behave as if they were homogeneous on the scale of many meters. This implies that an intrinsic oxidation rate can be ascribed to a large volume of dump; it is a measurable quantity and a useful quantity in modelling dump behavior. Again it is our experience that an intrinsic oxidation rate can be measured in a suitably designed column. I believe that the relationship between oxidation rate and the pollution generation rate can also be determined in such experiments. Conclusions Intrinsic oxidation rates measured in pyritic waste dumps are much smaller than chemical or bacterially catalyzed pyrite oxidation rates measured in the laboratory. A simple model shows that at the low oxygen consumption rate of 1 χ 10" kg-m'V the pollution load from a waste dump of typical size and composition will be environmentally significant. If, as is usually the case in such dumps, gas transport into the dump is dominated by diffusion then for intrinsic rates more than ten times this low rate the overall oxidation rate will be largely independent of the intrinsic rate. This simple model also indicates that, unless the concentration of pollutants in the water-filled pore space is limited by interactions between the oxidation products and gangue minerals, the load exiting the base of the dump under (pseudo) steadystate conditions will be independent of the rate water infiltrates the dump. This means that rehabilitation measures aimed purely at reducing water infiltration rates will reduce the pollution load in the short term but the load will return to the value dictated by the overall oxidation rate in the long term. Depending on the detailed profile of oxidation within the dump, the timescale for pollution from a new dump to first appear or alternatively the timescale for pollution loads to reach a maximum is dictated by the transit time of water infiltrating the dumps. In these simple models the timescale ranges from the order of a few years for infiltration rates typically due to net precipitation, to tens of years where the infiltration rate has been reduced by rehabilitation measures. These are also the timescales appropriate to the establishment of new steady or pseudo-steady state conditions after a change to the infiltration conditions. It follows that field observations are required to determine whether or not water transit times in waste dumps are dominated by preferred paths or by diffusive water transport processes. It also follows that to improve predictive modelling in pyritic waste dumps we require measurements of the intrinsic oxidation rate. This is particularly so if it is low. If it is high then overall oxidation rates will be dictated by oxygen transport rates rather than intrinsic oxidation rates. It further follows that if the correct rehabilitation strategy is to be applied to a particular waste dump then data on the interrelation between oxidation rate and pore water chemistry is required. 8


1

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121

Finally more attention needs to be given to the provision of good field data on the effect of specific rehabilitation measures. Both concentration and load information are required and due allowance made for the quantity and timescale of water infiltrating the dump and passing through to the point of collection near the dump base. Nomenclature a: c: o c: c:


c

:

a

D: £> : D: g: K: L: p\ T: T: i: t: h

a

2

Q

p

c

t: d

a

v: β: γ: ε: ε: p: p: p*: p: p.: α

a

a

re

particle radius (m) oxygen concentration in the pore-space (kgm" ) oxygen concentration in air (kgm" ) density of the reactant in the solid phase (kgm ) specific heat of the alpha phase (J-kg^K ) (a=s, solid phase; oc=a, air phase; a=w, water phase) coefficient of heat diffusion (Jm^KY ) diffusion coefficient of oxygen in the air in the heap (m ^" ) diffusion coefficient of oxygen in the particle (nrV ) acceleration due to gravity (m -s ) air permeability of the porous material (m ) height of the dump (m) pressure in the air phase (kgm'Y ) temperature relative to T characteristic temperature, usually annual ambient average (K) characteristic time to oxidize a particle (s) characteristic time for gas convection in a dump where the gas is involved in a chemical reaction (s) characteristic time for gas diffusion in a dump where the gas is involved in a chemical reaction (s) macroscopic velocity of the α phase (m/s) coefficient of thermal expansion of a gas (°C ) a proportionality constant encompassing both Henry's Law and the gas law mass of oxygen used per mass of solid reactant in the oxidation reaction the volume fraction of the α phase density of the a phase (kgm ) intrinsic density of the a phase (kgm' ) density of air (kgm ) initial density of reactant in solid phase (kgm ) viscosity of air phase (kgm'Y ) 3

3

3

1

1

2

1

1

2

2

2

2

Q

_1

3

3

3

3

1

Acknowledgments Much of the above discussion and conclusions spring from access to good field data. I would like to pay tribute to the high level of field experiments which have been conducted by Dr. John Bennett, Dr. Yunhu Tan, Mr. Allan Boyd, Mr. Warren Hart and Mr. Viphakone Sisoutham and the understanding which has followed from discussions of these field results. At the sametimeI thank Dr. David Gibson and Dr. Garry Pantelis for the insight on mechanisms resulting from discussions with them on how to model the complex of processes involved in the pollution generation from pyritic wastes. Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10.

Pantelis, G.; Ritchie, A.I.M. Appl. Math. Model. 1991, 15, 136-143. Lowson, R.T. Chem. Rev. 1982, 82, 461-497. Brierley, C.L. CRC Crit. Rev. Microbiol. 1978, 6, 207-262. Olson, G.J. Appl. and Env. Micro. 1991, 57, 642-644. Moses, C.O.; Nordstrom, D.K.; Herman, J.S.; Mills, A.A. Geochim. Cosmochim. Acta 1987, 52, 1561-1571. Crank, J.C. The Mathematics of Diffusion, 2nd Edition, 1956, Clarenden Press: Oxford, England. Davis, G.B.; Ritchie, A.I.M. Appl. Math. Model. 1986, 10, 314-322. Ritchie, A.I.M. 1977, AAEC/E429. Davis, G.B.; Ritchie, A.I.M. Appl. Math. Model. 1986, 10, 323-330. Harries, J.R.; Ritchie, A.I.M. Water. Air, Soil Pol. 1981, 15, 405-423.

RECEIVED October 20,

1993


Environmental Geochemistry of Sulfide Oxidation - American

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