A cost perspective for long distance ore pipeline water and energy utilization. Part I: Optimal base values

Abstract

Long distance ore pipelines are intensive in water and energy use. Although past efforts have been made to identify the best operational points in terms of energy efficiency, an approach to concurrently include water value, representing aspects such as price or scarcity, is lacking. In the present paper, an optimization scheme to look for better operational points considering energy and water utilization is proposed. A scalar function built upon the computation of energy and water consumption including restrictions inherent to hydraulic transport of solids through pipelines is defined. The relative importance of energy and water consumption is parameterized through the inclusion of water and energy unit costs, along with system variables such as throughput, solids concentration and system utilization fraction. The optimization problem is solved for different throughput and hydraulic conditions resembling a long distance copper concentrate pipeline and a range of water and energy costs, using a nonlinear, constrained optimization scheme. Results show the appearance of a low water cost regime for low throughput conditions, with a steep, quasi-linear change on optimal properties with water cost, followed by a nonlinear, high water cost regime, related to a weaker, monotonic change of concentration, flow rate, pipeline utilization and water consumption, respectively. For fixed water costs, increasing the energy cost causes an incentive for an additional use of water, thus appearing a double non-linear dependence of optimal results with energy and water costs.

For high enough throughputs, the low water cost regime disappears, and is replaced by constant, minimal optimal flow rate and solids concentrations, related to the maximum possible pipeline utilization constraint. Present results show that an equivalent pipeline oversizing at constant throughput would allow, in addition to the computed optimal conditions, for operation at lower specific energy consumption scenarios. Results, compared with some typical copper concentrate pipeline operational conditions, show that optimal values in the sense of the present hydraulic-cost analysis tend to require higher concentrations and lower pipeline utilization fractions than in typical systems, with differences in costs ranging from 16% to 28%.