Algorithms to control coal segregation under non-stationary conditions - Part I: Moving window and SPC-based updating methods

With highly heterogeneous run of mine coal streams, physical segregation. u sing on-line quality measurements, can serve as an economically important f irst stage of the mineral beneficiation process. Segregation enables high q uality fractions of the stream to bypass cleaning operations, thereby savin g cleaning costs and avoiding the yield Losses inherent in any downstream s eparation process. This paper develops control strategies that address the objectives of quality targeting of the no-wash coal to meet desired quality specifications while maximizing yield of the segregation process to the no -wash stock. It extends earlier work by the authors to deal with the case w here the stochastic nature of the quality levels is non-stationary over tim e, a situation any practical segregation system should accommodate. Two tec hniques, involving the use of moving windows and auto-correlated statistica l process control techniques, are employed to define intervals of time wher e the process might be considered as "locally" stationary. Subsequently, tw o different methods, involving the use of empirical and Gaussian distributi ons, are used to characterize the distribution of quality levels over these intervals. Given this characterization, a special decision criterion is ap plied to make segregation decisions that enables one to meet the control ob jectives stated above. Performance of different variants of these control s trategies are compared with each other and to an existing industrial segreg ation algorithm using data obtained at a representative mine. The methods s how dramatically improved capability to control quality targets and increas e yields. Moreover, different variants of the algorithms are more effective in different application circumstances. (C) 2001 Elsevier Science B.V. All rights reserved.