Intelligent Operational Control For A Whole Hematite Grinding Process

China is rich in hematite ore resources, but most of them are characteristics of low-grade, composition complexity, low specific susceptibility, non-homogenous distribution, and closely mineral intergrowth. As far as such ore is concerned, a two-stage closed-circuit grinding process, which includes ball mill-spiral classifier first circuit and ball mill-hydrocyclone circuit second circuit, is usually adopted. The most important tasks of such grinding process are to control the grinding particle size (usually defined as the fraction of particles passing a sieve of 200 mesh,%<200 mesh) inside its technological target range, thereby making valuable mineral constituent and gangue liberate sufficiently, while maintain the ball mill load at the vicinity of its optimum so as to increase the mill throughput and grinding efficiency. As a result, the operational control for the whole grinding process has a great practical significance and plays an important role in improving beneficiation production indexes as well as reducing energy consumption.Different from the abroad widely used two-stage grinding circuit that is make up of a rod mill in open circuit and a ball mill in closed circuit with a hydrocyclone classifier, the hematite grinding process has a comprehensive complexity, mainly in:1) The process is typical of several successive working procedures with a long production period and a lot of factors affecting the grinding product particle size index, such as fresh ore feed rete of ball mill, rate of the mill water, flurry density of the spiral classifier overflow, flurry density of hydrocyclone feed, etc. In addition, the magnetic agglomerate usuall appears in the strong magnetic particles, which makes the on-line particle size analyzer difficult measure actual particle size index accurately. It is thus hard to employ the conventional control method to realize the operational feedback control for such process. In most of actual production process, the manual control mode based on the laboratory assay of the grinding particle size is mainly adopted to adjust basic control loop setpoints according to operation experience. But the long period of the laboratory analysis cannot guarantee the real-time production guidance for the grinding process, which results in a large ore size fluctuation range of grinding. In this case, the improvement of pulp size qualified can not be obtianed; 2) Due to fact that the frequently fluctuations of hematite ore properties increase the uncertainty of the first stage grinding process, the operating point of mill load is easy to drift during grinding operation. This maybe lead to "under load" or "over load" phenomena of the ball mill, and even result in mill belly or empty tamping accidents, which seriously affect the production efficiency of the grinding process; 3) The grinding particle size and slurry flow in aboard first grinding procedure are relatively stable, which benefits from the quantitative feeding ore the rod mill and addition water in ratio. Then, the conventional setpoint control method of sump level cannot result in larger fluctuations of hydrocyclone feeding pressure. But in hematite first stage grinding procedure, the spiral overflow fluctuates in a large flow scope, and the sump level is also affected by other random disturbances such as sewage, wash water and underflow of the hydrocyclone. Such fluctuation would generally lead to large variations in the sump level and the pump speed would inevitably vary in a wide range when using existing setpoint control for the sump level, and the classification efficiency of hydrocyclone would reduced significantly in turn. Therefore, the conventional setpoint control method is unsuitable in hematite regrinding process. Consequently, it is necessary to develop the basic loop control methods and the operational feedback control method as a whole in accordance with domestic hematite grinding process, which would have great practical significance in improving grinding product quality, increasing grinding efficency and reducing production costs and energy consumptions.This dissertation is supported by the National Basic Research Program of China "the entirety control strategy and operation control method of integrated control system in plant-wide complex manufacture process" (No.2009CB320601). In this paper, the whole control strategy aimed to control the grinding particle size index within its specified target range and restrict the ball mill load in its maximum value neighborhood, is proposed. The basic loop control methods and the whole operation feedback control methods for hematite grinding process are also presented. Based on these control methods, the control software for hematite grinding process is designed and developed. On the background of the industry project, the integrated automation system for the concentrator of Jiuquan Steel (Group) Co., this paper has carried out control software structure design, equipments installation, system debugging, industrial experiments and system operation of the intelligent control system. The main contributions of the work are summarized as follows:1. According to the comprehensive complexity existing in the hematite grinding process and considering the two consecutive closed-circuit grinding procedures as a whole process, an operational feedback control (OFC) method for such whole process is proposed to realize the setting control and track control for the basic control loops, namely, ore feed rete of ball mill loop, rate of the mill water loop, flurry density of the spiral classifier overflow loop and flurry density of hydrocyclone feed loop. The developed approach empolyes two-layer structure, i.e., intelligent setting control layer (ISC) and the basic loop control layer (BLC). The ISC layer includes the BLC loop pre-setting model on the basic of case-based reasoning (CBR) method, the soft sensor for the particle size index with by using the radial basis function (RBF) neural networks, the particle size feedback regulator with rule based reasoning (RBR) method, and ball mill load fault diagnosis and self-healing controller. Furthermore, an internal control method with gain adaptive for ball mill feeding procedure, and a compound control strategy based on mathematical ore-balance models and the fuzzy-PID cascaded feedback control method are introduced in BLC layer. Based on these control methods, the setpoints of ISC can be modified and tracked online. Hence, the OFC can guarantee the particle size index within its target range, restrict the ball mill load in its maximum value neighborhood.2. In regrinding process of hematite beneficiation, the sump level fluctuates frequently influenced by some large disturbances. The pump speed inevitably changes in a wide scope by adopting the existing setpoint control method for sump level, and the oscillations of hydrocyclone feeding pressure can hardly be restricted in its desired range consequently. In this paper, a hierarchical control structure based on fuzzy switching control method is proposed, which includes a switching controller of sump level interval and a feedback controller of hydrocyclone feeding pressure. By switching between a retainer and a fuzzy compensator to hydrocyclone feeding pressure setpoint, the controller of sump level interval can guarantee the variations of hydrocyclone feeding pressure setpoint within its desired range. In addition, the PI controller of hydrocyclone feeding pressure can track its setpoint. Therefore, the sump level and the oscillations of hydrocyclone feeding pressure can be limited in their target ranges respectively. The safety operation of regrinding process has been realized, and the improved classification efficiency of hydrocyclone has been achieved.3. The intelligent operational control software and regrinding process control software based on above mentioned intelligent operational feedback control method and basic control loop method of regrinding process have been designed. The control system hardware includes instrumentations (belt weigher, flow meter, liquid meter, pressure sensor etc.), PLC control system, monitoring computers, computer networks and actuators (electric control valve, transducer, etc). The control software consists of process monitoring software, OFC software, and regrinding process control software. Technological parameters monitoring (ball mill feeding, water flow, pulp density, pulp pressure pulp level, etc.) and production equipment start and stop function are performed through the process monitoring software. The OFC realized the control of grinding particle size index and ball mill load operating conditions by adjusting and tracking the BLC setopints. In addition, the regrinding process control software establishs the fuzzy switching control between the sump level interval and the hydrocyclone feeding pressure.4. The proposed intelligent control system for hematite grinding process has been applied in the concentrator of Jiuquan Steel (Group) Co so as to realize the intelligent loop setting and tracking control for the particle size index and ball mill load condition. For a long-term operation, the particle size index has been evidently improved from 72.98% to 76.72%, increased by 3.76%. In addition, the hourly throughput of grinding process has been enhanced by 2.65%, and the energy consummation per ton of production has been reduced more than 2%. The ball mill "under load" and "over load" faults have also been avoided significantly. In conclusion, the successful application demonstrates the validity and effectiveness of the proposed approach.