On the energy and separation efficiency of dense medium cyclone coal beneficiation processes

Abstract

In light of the energy shortage and environmental pollution, improving the energy efficiency of energyintensive industrial processes is a research topic of increasing importance. The mining industry is one such heavy energy user. In particular, pumping systems are responsible for a noticeable portion of the total energy usage and they are identified as an area of potentially high energy efficiency improvement. This thesis studies one of the most important processes in coal mining industry, dense medium cyclone (DMC) coal beneficiation process, in terms of energy and separation efficiency improvement. The pumping system used in the DMC process is studied to reduce its energy consumption and associated costs. The DMC coal cleaning process is investigated to improve its separation efficiency with the aim of efficient coal utilization and therefore conservation of the non-renewable coal resource. Energy efficiency and separation efficiency are first studied individually before an integrated solution to both of them is designed. For energy efficiency improvement, a pumped-storage system (PSS) is introduced to the existing pumping system supplying dense medium to the DMC process. This PSS essentially introduces a structural change to the existing medium circulation in the DMC circuit to save energy. The main idea of introducing PSS to the DMC coal preparation process is to prevent energy wastage due to the industrial practice of constant medium over-pumping. The PSS adds additional medium circulation loops in the DMC coal washing plant to pump over-pumped medium back to the DMC process with a smaller pump. Optimal operation of the PSS requires dealing with multi-pumps and medium storage equipment, which is a challenging task. To achieve optimal operation, a mathematical model of the PSS is developed and the optimal operation of the pumps is formulated into a binary integer programming problem. The solution of the formulated operation problem is then used in economic analysis of the PSS with life cycle cost and payback period techniques to determine its financial effectiveness and viability. According to a case study based on a real-world coal beneficiation plant, it is concluded that the PSS is a financially favorable solution to reduce energy consumption and cost. In terms of separation efficiency, current industrial practice uses rule-of-thumb and empirically based controls to adjust the density of the medium used to enhance separation in the DMC. There is no modelbased controller that is able to improve separation efficiency of the DMC process. Two controllers based on advanced control technology are designed in this thesis to promote separation efficiency of the DMC coal cleaning process. The first one is a feed-forward controller, which optimizes the medium density according to sampled measurements from the ROM coal because many coal washing plants do not measure the composition of the fines yield from the DMC output because of its high cost. This feed-forward controller is essentially an open loop controller and it is designed according to a mass balance model of the DMC using a model predictive control method. To improve the performance of the DMC circuit achieved by the feed-forward controller further, the second controller is designed. This is done by taking advantage of feedback measurements from the output of the DMC to design a closed-loop controller. Some of the coal preparation plants are sending samples from the output of their DMC process to laboratories to analyze the fine coal composition and some of them are planning to invest in on-site measurement devices for this purpose. The closed-loop controller aims to make use of these measurements to improve the performance of the DMC process. In the design of this closed-loop controller, delays in the measurements of fine coal quality are considered. The closed-loop control introduces a change to the control obtained by the feed-forward control according to sampled and delayed measurements from the output of the DMC to improve the performance of the DMC process, making it robust against model plant mismatch (MPM) and external disturbances, which are inevitable in the DMC circuit. Case studies are also conducted to validate the effectiveness of the two controllers that are designed. It is then concluded that closed-loop control is superior to the open loop feed-forward controller in the presence of MPM and external disturbances, as expected. However, it is obvious that both of the controllers can find their best application in the coal mine according to the specific situation. Finally, the interplay between the energy and separation efficiency is studied and an integrated control system is designed. The new control system exhibits a dual closed-loop control architecture and features: simultaneous improvement of energy efficiency and separation efficiency; medium density profile optimization by the outer loop controller and direct control of actuators used for the medium density adjustment by the inner loop controller; incorporation of the PSS scheme in the controller design to improve energy efficiency; and use of a model predictive control method to control the DMC process optimally with the ability of looking ahead. In other words, the integrated control system designed is a complete solution to the DMC coal beneficiation process for both energy efficiency and separation efficiency improvement. It combines the benefits of the PSS for energy efficiency and the controllers designed for separation efficiency. In addition, it further extends the controller designed for separation efficiency by including the PSS and magnetite recovery circuit in the controller design, enabling direct manipulation of the actuators used to formulate the medium solutions by the control system that has been designed.