Optimal energy management of crushing processes in the mining industry

Abstract

It has been shown that mining processes, especially the comminution processes (grinding and crushing) are some of the biggest consumers of electricity in the world where energy management application would have a significant impact on the sustainability of both the energy supply and demand sides. However, very little research has been done in this area. Energy efficiency management in the mining industry can be done at four levels, namely the technology level, equipment level, operational level and performance level. In this work, operation energy management is considered for crushing circuits with the aim of minimizing the total energy cost by accounting for the time-of-use (TOU) electricity tariff. The work is limited to three types of crushing machines, namely the jaw crusher, vertical shaft impact (VSI) crusher and high-pressure grinding rolls (HPGR) crusher. Firstly, the energy model of each crusher is developed and expressed in terms of its control variables. Secondly, an optimal energy control model is formulated, where both physical and technical/operational constraints of the crushing process are taken into account. Thirdly, a case study is done in order to evaluate the effectiveness of the developed models. Lastly, experimental results of the performance model of the jaw crusher are presented. Simulation results show that when TOU electricity tariff is applied there is potential for achieving energy cost saving in all types of crushing processes, depending on the size of storage systems and plant production requirement. However, achieving energy saving is not always evident. When the throughput rate and product size distribution of the crusher are both controlled by a unique variable, as in the case of the jaw crusher, it is shown that no significant energy saving can be achieved. This is due to the trade-off between the throughput rate, product size distribution and specific energy consumption of the jaw crushing machine. Increasing the closed-side setting (unique control variable) of the jaw crusher with the aim to decrease the specific energy consumption will result in coarse particles in the product. However, there is a great opportunity for energy saving through optimal switching control due to the high no-load power consumption of the jaw crusher. On the other hand, when the throughput rate and product size distribution are controlled by more than one variable, as in the case of the VSI and HPGR crushers, more energy saving can be achieved in presence of varying feed size. In the VSI crushing process, for instance, the product size distribution is controlled by the rotor speed while the throughput rate is controlled by the rotor feed throttle. Hence, energy consumption reduction is achieved through any small decrease of the crusher rotor speed whenever the feed size decreases. The same applies to the HPGR crushing processes where the rolls operating pressure is used to control the product size distribution and the throughput rate is controlled by the rotor speed. The analysis of the performance model of the jaw crusher reveals that although the Bond energy model presents larger prediction error compared to the throughput and product quality index, this can still be used as performance indicator in jaw crushing energy optimization, as it shows a strong linear correlation with experimental results. However, for actual energy consumption and energy saving calculation, a field test should be conducted.

Daar is aangetoon dat mynbouprosesse, veral die vergruisingproses (maal en breek) een van die grootste verbruikers van energie in die wêreld is, waar die toepassing van energiebestuur’ n beduidende impak sal hê op die volhoubaarheid van sowel die engergievoorsiening- as aanvraagkant. Daar is egter nog min navorsing op hierdie gebied gedoen. Die bestuur van energie-effektiwiteit in die mynbou-industrie kan op vier vlakke plaasvind, naamlik die vlakke van tegniek, toerusting, werkverrigting en prestasie. In hierdie werk word werkverrigtingbestuur oorweeg vir vergruisingkringlope, met die doel om die totale energiekoste te minimeer deur die tyd-van-gebruik-elektrisiteitstarief in berekening te bring. Die werk word beperk tot drie tipes vergruisingsmasjiene, naamlik die bekvergruiser, vertikaleskagimpakvergruiser (VSI) en parallele hoëdruk- breekrollervergruiser (HDBR). Eerstens word die energiemodel van elke vergruiser ontwikkel en uitgedruk volgens die kontrole-veranderlikes daarvan. Tweedens word ’n optimale energiekontrolemodel geformuleer, waarin sowel die fisiese as tegniese/operasionele beperkings van die proses in ag geneem word. Derdens word ’n gevallestudie gedoen om die effektiwiteit van die modelle wat ontwikkel is, te evalueer. Laastens word die eksperimentele resultate van die werkverrigtingmodel van die bekvergruiser aangebied. Simulasies toon aan dat wannneer die tyd-van-gebruik-elektrisiteitstarief toegepas word, energiekostebesparings potensieel in alle tipes vergruisingsprosesse bereik kan word, afhangend van die grootte van die bergingstelsels en aanlegproduksievereiste. Die bereiking van energiebesparing is nogtans nie altyd duidelik nie. Wanneer die toevoertempo en produkgrootteverspreiding van die vergruiser albei deur een veranderlike beheer word, soos in die geval van die bekvergruiser, word aangetoon dat geen beduidende energiebesparing bereik kan word nie. Dit is die gevolg van die ruil tusen die toevoertempo, produkgrootteverspreiding en spesifieke energieverbruik van die bekvergruisermasjien. Om die sluitsystelling (kontrole-veranderlike) van die bekvergruiser te verhoog met die doel om die spesifieke engergieverbruik te verminder, sal lei tot growwe deeltjies in die produk. Daar is nogtans goeie geleentheid vir energiebesparing deur optimale skakelkontrole omrede die hoë geenlading-kragverbruik van die bekvergruiser. Aan die ander kant, as die toevoertempo en produkgrootteverspreiding deur meer as een veranderlike beheer word, soos by die VSI- en HDBR-vergruiser, kan groter energiebesparing bereik word saam met veranderlike toevoergrootte. In die VSI-vergruisingproses word die produkgrootteverspreiding byvoorbeeld deur die rotorspoed beheer, terwyl die toevoertempo deur die rotortoevoerversneller beheer word. ’n Afname in energieverbruik word gevolglik verkry deur enige klein afname in die vergruiserrotorspoed wanneer die toevoergrootte afneem. Dieselfde geld in die HDBR-vergruisingprosesse, waar die operasionele druk van die rollers gebruik word om die produkgrootteverspreiding te beheer en die toevoertempo deur die rotorspoed beheer word. Analise van die werkverrigtingmodel van die bekvergruiser toon dat alhoewel die Bondenergiemodel ’n groot voorspelfout lewer, dit steeds gebruik kan word as ’n aanduiding van werkverrigting in energie-optimering vir die bekvergruiser, aangesien dit sterk lineêre korrelasie toon met eksperimentele resultate. Nietemin sal veldwerktoetse uitgevoer moet word om die werklike energieverbruik en energiebesparing te bepaal.