Figure 2. Variability of P2O5 composition in the feedstock from four layers in one mine. Each layer has a different average composition. The curves a representative generated feedstock composition with respect to cumulative extracted tonnage, and the light shaded bands represent the range of values attained over 100 different realizations. Table 1 – Fixed mine and pile parameters. Parameter Value Units Mine production capacity 5000 tonnes/day Mine operating cost 5 $/tonne Number of piles 4 Pile capacity 100000 tonnes Pile initial inventory 0 tonnes Process pathways define possible processing pathways through which material can flow, rather than specific processes themselves. For example, process pathways for phosphate processing could include washing; washing and flotation; washing, grinding, and flotation; calcination; directly selling material with no mineral processing; or sending material to the waste pile. Each process pathway has a defined processing cost per ton, a tonnage reduction factor, and a set of process transformation factors (one for each tracked chemical compound, e.g., one for P2O5 and one for MgO) that describe the way material is transformed through processing. To represent stochasticity, these process factors are generated in a similar way to feedstock variability—they are drawn from a probability distribution defined by a mean and variance, but they vary over time, rather than in space. Process parameters are displayed in Table 2. Table 2 – Process parameters. Stochastic parameters display mean with standard deviation in parentheses. Parameter Washing Washing + Flotation Washing + Grinding + Flotation Sell/Wast e Capacity (tonnes/day) 2000 2000 2000 6000 Operating cost ($/tonne) 5 15 30 2 Tonnage reduction factor 0.82 (0.026) 0.65 (0.045) 0.60 (0.032) 1.0 P2O5 transformation factor 1.08 (0.017) 1.18 (0.026) 1.22 (0.022) 1.0 MgO transformation factor 0.9 (0.026) 0.35 (0.063) 0.25 (0.045) 1.0 Clients have strict quality constraints that dictate a minimum grade and maximum level of unwanted impurities. Clients can be set up to be market-based, with a set demand, or contractbased, with a strict tonnage requirement per day that must be filled. In either case, supply to a client cannot exceed demand, and clients each come with a fixed price/ton that they pay. The
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