MT MORGAN OPEN CUT PIT WATER QUALITY MODELLING stage 2 ● 17
to hold and manipulate chemical analyses. The Geochemist's Workbench can mix two or more fluids in a variety of ways. It can mix samples in any volume or mass ratio. The Geochemist's Workbench program was used to predict the pH and other parameters in the OCP water after adding new tailings. The test results of the new tailings (Table 3-4) were not used directly as inputs data for geochemical mixing model because they are total concentrations of the filtered solid of new tailings. In order to mix two solutions, water soluble concentrations of the new tailings are required as inputs to the mixing model. To address this, we synthesized data based on the provided laboratory results to approximate mixing of new tailings with OCP water (Appendix B) and used this as the input new tailings data (Table 3-6). It was assumed that the results of OCP water after new tailings added as the results of mixing of two solutions. The fractions of metals and other elements released (or absorbed) from (to) the new tailings are not considered in the mixing modelling. The pH of OCP water and new tailings slurry are 3.7 and 9.6 respectively. Heritage Minerals proposed w/w mixing ratio is 1.84 tonnes of new tailings per m 3 of OCP water (ie; 250 t/hr new tailings / 136 m3/hr water from the OCP). Specific gravity of new tailings was assumed as 3.0. It is estimated that volume to volume ratio of new tailings to OCP water was calculated as 0.61:1. Five pH values and 4 volume ratios (new tailing: OCP water) were used as modelling scenarios to provide a range of options for consideration (Table 3-5). The geochemical modelling considered 20 mixing scenarios (Table 3-5) with two variables: pH and volume ratio of new tailings to OCP water ( n = 20 based on combination of 5 pH levels and 4 volume ratios). Table 3-6 summarises the measured OCP water data and estimated water-soluble new tailings that were input into the modelling process.
Table 3-5 Geochemical modelling scenarios
Variable
pH
7
8
9.57 (Final tailing)
11
12
Volume Ratio (New tailing: OCP water)
0.3 : 1
0.61 : 1 (Proposed Volume)
1 : 1
2 : 1
Table 3-6 Input parameters of the mixing model
Final Tailings (Solution)
Analyte
Unit
OCP Water
pH
3.77
9.57
EC
µS/cm
15962
4000
TDS
mg/l
11166
30
Mg ++
mg/l
2553
100
Na +
mg/l
554
70
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