4. Results
4.2 Experiment B Results
4.2.1 Mass Balance-Speciation Modelling Results- Experiment B
The mass balance-speciation modelling was carried out using the assumptions mentioned in the methodology section (section 3.3.3). Since both reactors have the same initial conditions and feed, the model is representative of both. From the initial conditions, the successive feed-react-decant cycles were modelled until the dissolved and precipitate concentrations reached a pseudo-steady state. Thereafter, the metals washout experiment was modelled. Cycles below zero represent cycles modelled with a micronutrient cocktail in the feed. Cycle 0 and above represent those cycles modelled where no micronutrients were dosed to the system.
4.2.1.1 Precipitate formation
The following graph shows the types of precipitates predicted, their concentrations as well as their reduction due to micro-metal washout.
Figure 18: Model prediction of the concentrations of different precipitates formed and their changes with each cycle modelled.
The main feature of the graph above is that from the moment the metal washout is modelled (Cycle -1 to 0), the concentrations of the precipitates are predicted to decrease. By Cycle 2, the concentrations of the precipitates have reduced to micro-quantities in the nano and pico g/l ranges.
1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01
-4 -3 -2 -1 0 1 2 3 4 5 6
Concentration (mg/l)
Cycle
Covellite (CuS) NiS
CoS Sphalerite (ZnS)
MoS2 Pyrite (FeS2)
MnHPO4 Hydroxyapatite (Ca5(PO4)3OH)
83 The graph that follows displays the percentage of the total ions predicted to be within these precipitates, as well as the rate at which these are released from the precipitates due to the metal washout.
Figure 19: Percentages of the metal ions found within precipitates and their changes with each cycle modelled.
With the exception of calcium, hydrogen sulphide and magnesium, the metal ions are predicted to occur only within precipitates prior to the metal wash-out. This suggests that these ions, like copper, cobalt and hydrogen sulphide from experiment A (refer to figure 17, section 4.1.4.2) are locked within precipitates and are not in a bioavailable phase for micronutrient absorption.
It is apparent from the graph above that the metal ions found within the precipitates wash out at different stages. The first metal that is not present in precipitates is calcium which is depleted in cycle 0. Manganese is predicted to be released from precipitates by cycle 3 while zinc and cobalt are released by cycle 7 followed by nickel released by cycle 9. Iron, copper and molybdenum are released by cycles 11, 18 and 20 respectively.
0 20 40 60 80 100 120
-5 0 5 10 15 20
% of Ion in Precipitates
Cycle
Ca2+ Co+2 Cu+2 Fe+2 HS-1
Mg2+ Mn2+ MoO4 2- Ni2+ Zn2+
84 4.2.1.2 Soluble Concentration Changes
The changes in soluble ion concentrations for the model based on Experiment B will be presented in this section. These concentrations are also compared to the experimental values wherever these have been determined. Mg, Ca, and Fe were detectable during the experimental analysis and are included in the analysis that follows. However, Co, Cu, Mn, Zn and Ni were not detected.
The graph that follows displays the soluble concentrations for Mg and Ca determined from the speciation modelling together with the values determined experimentally from the supernatant ICP- AES analysis.
Figure 20: Concentration of ions in the dissolved phase for Mg and Ca, and their changes with each successive cycle modelled for Reactor II, including comparisons to experimental values.
From the start of the washout experiment, both Mg and Ca experience a decrease in the soluble concentrations. The experimental values for Mg are comparable with those determined from the model; however for Ca this is not the case. The predicted Ca concentrations are in the range of 0.82 to 0.05 mg/l while the experimental values range from 16 to 4.5 mg/l.
A similar graph was plotted for the concentrations for Mn and Zn with the exception that these metal ions were not experimentally detected in the supernatant. The following graph shows the predicted dissolved concentration values and their changes with the washout experiment.
0 2 4 6 8 10 12 14 16 18
-5 0 5 10 15 20 25
Dissolved Concentration (mg/l)
Cycle
Ca2+ Mg2+ Mg Exp Ca Exp
85
Figure 21: Concentration of ions in the dissolved phase for Mn (in mg/l x10-4) and Zn (in mg/l x10-8) and their changes with each successive cycle modelled for Reactor II.
The model predicts that these two ions occur in minute quatities with Mn in the approximate micro range and Zn in the approximate pico range. Nevertheless, the effect of a metal washout experiment may be observed, although the magnitude of the model predictions suggests that these values will not be measured accurately using any available techniques. The Mn soluble concentration initally increases with the washout experiment and thereafter from cycle 3 decreases. For Zn, the concentration starts decreasing from cycle 7 onwards.
The graph that follows displays the chages in soluble concentration as predicted by the mass balance-Speciation model for Co, Fe and Ni.
0.0 1.0 2.0 3.0 4.0 5.0 6.0
-5 0 5 10 15 20 25
Dissolved Concentration Mn: mg/l x10-4 Zn: mg/l x10-8
Cycle
Mn2+ Zn2+
86
Figure 22: Concentration of ions in the dissolved phase for Co, Fe and Ni as predicted by the model.
The predicted dissolved concentrations for Co, Fe and Ni are in minute quantities suggesting that the bulk of these ions are locked up within precipitates. Once the metal washout experiment was performed, Co was predicted to have started decreasing by cycle 7 and Ni by cycle 9. For Fe, a small decrease is observed from the washout until cycle 10. Thereafter, the concentration of Fe is reduced at the same rate as that of Ni. The graph that follows displays the model predicted concentration of Fe in the dissolved phase as compared to the experimentally determined values.
0.0E+00 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09
-5 0 5 10 15 20 25
Dissolved Concentration (mg/l)
Cycle
Co+2 Fe+2 Ni2+
87
Figure 23: Concentration of Fe ions in the dissolved phase as predicted by the model compared to the experimentally determined values
Although the experimental values determined for Fe follow a similar trend to those of the predicted concentrations (as seen in figure 31 above), the numbers are extremely different as the graph above suggests. This indicates that either there is experimental uncertainty for the measured values or that the precipitates are not in equilibrium, either due to mass transfer or redox effects.
Cu and Mo (shown as molybdate ion) were predicted to occur in x10-18 mg/l and x10-22 mg/l quantities respectively. The graph below shows the washout for these two ions, with Cu values in mg/l x10-18 and Mo values in mg/l x10-22.
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40
-5 0 5 10 15 20 25
Dissolved Concentration (mg/l)
Cycle
Fe Exp Fe+2
88
Figure 24: Washout pattern observed for the soluble concentrations of Cu and Mo (as MoO42-
) as predicted by the model
Besides the observation that these ions occur in extreme micro quantities, the graph above shows that these ions soluble concentrations reduce from the start of the washout experiment. However, this rate increases by cycle 18 for Cu and by cycle 21 for Mo due to the predicted depletion of the precipitates.