Validation Input Conditions

In order for the model to operate in the same operational ranges as the real plant did at the time of the gathering of the data, the inputs of the model need to be matched with that of the data. This has to be done by setting up mass balances, since much of the required flow rate and compositional information is missing in the data. This is first done for the second stage leach and thereafter the third stage. A number of assumptions are necessary to be able to do the balance, and these include the following:

 The solid and liquid phases mix in an ideal fashion in 400-TK-10

 The densities of formic filtrate and copper spent electrolyte are both 1.15 kg/L

 The solids phase has a density of 4.45 kg/L (Dorfling, Bradshaw, & Akdogan, Characterisation and dynamic modelling of the behaviour of platinum group metals in high pressure sulphuric acid/oxygen leaching systems, 2012)

Due to the fact that the composition of the contents of stream 1 is unknown, the first step is to determine it from the data. The balance set up around 400-TK-10 can be seen below:

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Table 19: Flow rates and densities for streams 1 to 5, and its phases.

Phase Stream 1 2 3 4 5 Liquid Mass Flow (kg/h) 947.5 2390 3841 0 7178 Density (kg/L) 1.1 1.15 1.15 - 1.143 Volume Flow (L/h) 861.3 2078 3340 0 6279 Solid Mass Flow (kg/h) 521 0 0 0 521 Density (kg/L) 4.45 - - - 4.45 Volume Flow (L/h) 117.0787 0 0 0 117.0787 Total Mass Flow (kg/h) 1468.5 2389.7 3841 0 8498.36 Density (kg/L) 1.5 1.321006 1.15 - 1.18 Volume Flow (L/h) 979 1809 3340 0 7202

The values used in this balance are means of the first range identified in section 3.3.4. The solids fraction of stream was chosen in such a way that the mass balance in stream 1 is satisfied under perfect mixing conditions. It can, however, be calculated that the total mass flow rates across the table (streams 1 to 4) add up to 7699.2 kg/h, which gives a 9.4% error. This error value can be caused by mistakes in either flow measurements or density readings, as mentioned in section 3.3.4.

With the phase fractions of stream 1 determined, and the element composition of the respective phases set – it is important to note that the compounds in which the element occur in the solids remains to be determined. Due to uncertainties about additional compounds that may be present in the first stage leach residue, a component balance on the solids compositional data is not sufficient to determine the exact component fractions. Instead, a mass balance is used, in which the following assumption is made:

The base metal reactions in the second stage leach can be approximated by the following four (Dorfling, Akdogan, Bradshaw, & Eksteen, Determination of the relative leaching kinetics of Cu, Rh, Ru and Ir during the sulphuric acid pressure leaching of leach residue derived from Ni-Cu converter matte enriched in platinum group metals, 2010):

1. NiS + 2O2 → Ni2+ + SO42-

2. 2Ni3S4 + 2H2O + 15O2 → 6Ni2+ + 4H+ + 8SO42-

3. Cu9S5 + 8H+ + 2O2 → 4Cu2+ + 5CuS + 4H2O

4. CuS + 2O2 → Cu2+ + SO42-

Using the entering flow rates and compositions as were specified for the previous table, the following table was set up:

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Table 20: Flow rates and densities for streams from the second stage leach, and its phases.

Phase Stream 1 2 3 4 5 14 Liquid Mass Flow (kg/h) 947.5 2389 3841 0 7177.5 7903.547 Density (kg/L) 1.070 1.150 1.150 1.840 1.139 1.200 Volume Flow (L/h) 890 2080 3340 0 6300 6586 Solid Mass Flow (kg/h) 521 0 0 0 521 6.54 Density (kg/L) 4450 - - - 4450 4450 Volume Flow (L/h) 0.1170787 0 0 0 0.117079 0.00147 Total Mass Flow (kg/h) 1468.5 2389 3841 0 7698.5 7910.089 Density (kg/L) 1500 1150 1150 - 1199.146 1200.725 Volume Flow (L/h) 0.98 2.08 3.34 0.00 6.42 6.59 Element Mass Flow Rates (kg/h) Cu 306.42 0.00 0.00 0.00 306.42 1.59 Ni 64.97 0.00 0.00 0.00 64.97 0.00 H2SO4 0.00 0.00 0.00 0.00 0.00 0.00 H2O 0.00 0.00 0.00 0.00 0.00 0.00 O2 0.00 0.00 0.00 0.00 0.00 0.00 Fe 1.21 0.00 0.00 0.00 1.21 1.21 Pt 1.71 0.00 0.00 0.00 1.71 1.71 Pd 0.84 0.00 0.00 0.00 0.84 0.84 Rh 0.30 0.00 0.00 0.00 0.30 0.30 Ru 0.69 0.00 0.00 0.00 0.69 0.69 Ir 0.19 0.00 0.00 0.00 0.19 0.19 S 127.85 0.00 0.00 0.00 127.85 0.00

The fraction of copper as Cu₉S₅ (rest is CuS) and of nickel as NiS (rest is Ni₃S₄) is assumed and the extent of each of the assumed reactions is set to 100%. The latter is done for the following reason: for the correct ratio‟s – disregarding the PGM compounds – such complete reactions would ensure that no sulphur is left in the solids phase. With this in mind, the chosen values are solved with Excel‟s solver function, setting the sulphur in the solids phase as the end of the reaction to zero. The following values result from this, with PGM values still being at the values provided by Dorfling (2012):

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Table 21: Compounds in which metallic elements occur in the first stage leach residue

Element Major Compound Fraction Rest Cu Cu9S5 0.914 CuS Ni NiS 0.903 Ni3S4 Rh Rh2S3 0.5 Rh Ru RuS2 0.7 Ru Ir Ir2S3 0.6 Ir

With stream 1 characterised, a number of variables need to be set from the data in order to have a completely specified system. Two such variables are the flow rate and ratio of oxygen into the autoclave, and the manner in which the thickener/centrifuge split the contents of stream 15 into 16 and 17. Dorfling (2012) recommends that oxygen is fed at a rate that is 20% more than what is required stoichiometrically. Data are only available for the oxygen flow rate into compartments 2 and 3. As mentioned, if the flow rate into compartment 4 would be the same as that of the other two, the total oxygen flow rate would be much too low, if compared with the amount calculated by the model. There are three possible explanations for this situation:

1. The model wrongly predicts the amount of oxygen that is necessary 2. The real plant does not sparge enough oxygen

3. The total oxygen flow rate is split in such a way that the majority of it (approximately 60%) goes to the last compartment

Scenario‟s 1 and 2 are outside the scope of this project, which leaves the assumption that scenario 3 is true as the only possibility for this project. The recommendation is that scenarios 1 and 2 should be investigated in future work.

Due to the shortage of data for the third leaching stage leach (more specifically, the lack of data for the effluent stream of 400-TK-050), the composition of the underflow of the thickener/centrifuge (stream 17) cannot be determined with a mass balance. Since the solid-liquid separation step is outside the scope of this project, the operation of this step is assumed to be perfect, and that the contents of stream 17 is the solids in stream 15. This same assumption is made by Dorfling (2012).

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