Autogenous grinding mill process control practices

This paper considers pre-requisite conditions for developing grinding mill control systems and describes the main problems of control and existing constraints. The author selected a set of grinding equipment that consists of autogenous grinding mills and ball mills. The paper considers the selected mill types as control objects. It is shown that there exists a good correlation between the mill charge and its acoustic and vibration noise. The paper examines the relationship between the level of vibroacoustic noise and the mill motor power draw and defines what can be considered an extreme dependence, as well as the conditions in which a stable operation of the automatic system can be maintained excluding the mill overload mode. The paper specifies what hardware and software means would be necessary to implement such system and describes the mill charge analyzer VAZM-1M developed by Soyuztsvetmetavtomatika that was selected for this application. The author looks at certain downsides typical of the conventional control scheme when the head mill feed rate changes as the mill motor power draw changes without allowing for changing physical and mechanical properties of the ore material. The author also considers the capability of the VAZM-1M analyzer in terms of mill load estimation accuracy. This laid the basis for developing mill protection and optimization algorithms for the AG and Ball Mill comminution circuit. The paper features a block diagram of the control algorithm and its brief description. The algorithm consists of blocks, which are responsible for the following actions: they receive key process parameters from the process control system database, check them for validity, perform initial processing and filtering; after that they analyze the trends of the measured parameters and analyze if an overload condition is probable. As decided by the mill operator, the ore flow rate can be adjusted. The paper describes a case study of running an AG mill control system on the basis of the above described algorithm and using the VAZM-1M analyzer. It is noted that this algorithm can be implemented both as an adviser for the operator and for automatic control of the mill when running in overload mode.

Adaptive control of ore mill charge

The system of mining operations does not enable long-lasting mining of single-type ores that causes instability of mineral materials intended for concentration. To solve the problem of controlling these processes on the basis of operating data on characteristics of processed raw materials, the method of controlling mill charge is suggested considering measurements of its output product. This method is based on correlation of the output product quantity and the rate of the mill charge. The process of the object’s operation is modeled, while time characteristics of the output controlled value and the mill charge are approximated to assess accuracy of the method of determining the mill charge. The possibility to use the discrete Kalman filter is studied in order to indentify the object’s characteristics under noises by measurement results. It is shown that application of the Kalman filter to controlling the mill charge enables not only solving the problem of filtration aimed to obtain data on the object’s current state, but also predicting its state.

Comparative Analysis of Autogenous and Media Assisted Grindability of a Copper and Manganese Ores

This paper presents a comparative analysis of autogenous and media assisted grinding of a manganese ore and copper ore both obtained from Otjihase in Namibia. The objective was to understand the relationship between ore properties and Grindability and thus effectively select comminution circuit and equipment based on this relationship. Laboratory studies were carried out to determine the ease of grinding the ore samples relative to one another. The sample of known weight was crushed and the particles thoroughly homogenized for sieve size analysis and Grindability tests. The Grindability test on each ore was both autogenous and media assisted for dry grinding at -250 microns with constant mill charge of 200 g per run, media charge of 40% by weight and mill operations of 100 to 500 revolutions corresponding to mill speeds of 5 to 25 rpm. The Grindability index of each ore was calculated and compared based on the tests results and used to calculate work indexes for the ores based on the Bond’s model. The autogenous tests produced average Grindability values of 0.55 kg/ton/rev and 0.65 kg/ton/rev for the copper and manganese ores respectively; and media assisted tests gave average values of 0.8 kg/ton/rev and 1.45 kg/ton/rev respectively for the copper and manganese ores. The results show that although media charge grinding produced higher Grindability values, both ores are economically amenable to autogenous grinding which suggests that with proper circuit design, the use of autogenous grinding for these ores (especially the manganese) can save significant cost. The results also show that Grindability increases with mill speed up to an optimum value beyond which grindability decreases with increasing speed and may even drop to zero while mill is running. It is also observed that grindability has close relationship with ore properties especially hardness and compressive strength. It should be noted that the inefficiency factors for the grinding are not considered in these results.

EXPLORING THE DESIGN AND DEVELOPMENT OF AN "ON-THE-SHELL" TUMBLING MILL ACOUSTING MONITORING SYSTEM

In the mineral processing industry, tumbling mills are used in the size reduction or comminution of ore to a target size distribution. As the tumbling mill environment is quite violent it is difficult to monitor mill charge motion. One way is to develop an “on-the-shell” acoustic system. This system will be used in the determination of the charge toe angle of an operating tumbling mill. The instrument, microphone system with the appropriate data acquisition system, collects the acoustic data. An analysis of the signal determines the location of the toe. The results of this development along with some trial data are presented.

INSTRUMENTED BALL DESIGN AND DEVELOPMENT

In the mineral processing industry, tumbling mills are used in the size reduction or comminution of ore to a target size distribution. As the tumbling mill environment is quite violent it is difficult to monitor mill charge motion let alone single ball or particle motion. An effort was initiated to investigate the development of an instrumented ball that includes data collection capabilities. A variety of sensors, including accelerometers and gyros, form the instrument package. The data collected by the instruments allow the calculation of a number of dynamic and kinematic quantities as a function of time. Using instrumented ball data, the error model of the instrument can be found. Finally, the importance of the data with respect to ball mills is discussed along with the relationship between the instrumented ball data and charge motion simulation results is discussed.