Control of ball mill operation depending on ball load and ore properties

. Ball mills are used for the second stage of ore grinding in mining operations by proportioning ore piles. Controlling the operating conditions of ball mills is one of the key factors for optimal mill perform and high mill load turnover. One of the factors affecting the efficiency of the ball mill is the loading of the balls. Optimum distribution of ores according to the size of the mill, increasing the efficiency of the grinding process and reducing energy consumption are achieved. In addition, the ore properties are one of the important parameters for the optimal utilizing of the ball mill. Ore can have various properties, such as hardness, density, moisture content, etc., which can affect the ore's productivity and grinding efficiency.


Introduction
According to the operating mode, the rotation speed of the mill is divided into 4 types.Slow rotation (cascade), fast rotation (waterfall), medium rotation (mixed), very fast rotation (central rotation or critical).Reducing the granularity of the ready product (ground particles) coming out of the mill depends on the following main factors:  properties of ores in the mill (mass, size, hardness, density and granulometric composition of particles):  characteristics of grinding agents (mass, density, size and distribution of balls);  rotation speed of the mill;  density of straw during wet grinding [5].
The main purpose of this process is to increase the grinding productivity and reduce the production costs for the whole process.To achieve these goals, various mathematical models and management methods are developed and used in practice.In this article, we provide a brief overview of the basic principles of modeling the process of grinding and analyze several control strategies used in the implementation of a suitable method of process control.
The main idea of modeling all mining processes is to obtain a mathematical relationship between the volume of the ore and the volume of the ready product.The granularity of the ore is continuously reduced due to energy transfer by the grinding condition, which reduces their bond strength.Ore size reduction is the result of the following three main fragmentation mechanisms.
1. Abrasion 2. Cleavage 3. Fracture In practice, the three mechanisms never operate separately, and in the process of particle size reduction, they all work with possible advantage depending on the type of mill, operating conditions, and type of ore ground [1].

Experimental research
The relative consumption of electric energy is the ratio of the useful power consumed by the mill's electric drive to the efficiency of the grinding cycle without taking into account the circulating load.The specific power consumption with a given volume of ore and cycle is the main industrial parameter that describes the grinding of material in a given product preparation equipment.The specific energy consumption for the same material depends on the type of ore preparation device, which is associated with significant fluctuations in the efficiency of energy transfer in the workplaces of different equipment.The specific power consumption for the required product productivity provides a useful solution for mill size and electrical drive and is expressed as follows [2].
Here, P 80 and F 80 are the sieve mesh sizes, through which 80% of the soil and the starting material are separated in microns, respectively; W i is the working index of the material, kW/t; E -required power consumption, kW/t.The value obtained by E tr determines the maximum efficiency of the use of equipment for the preparation of ore and corresponds to the scheme, including medium and small stage ball grinding provides a comparative analysis of a wide range of ore processing steps and requires 5-15% energy in a self-grinding mill [3]. Figure 2 shows that the input and output signals depend mainly on the mill and the hydrocyclone.Some malfunctions occur as a result of system disturbances that occur during the processes that take place at the entrance and exit of ore to the mill (including the hydrocyclone).In order to eliminate these, it can be achieved through the following management system.In this case, we can obtain the following information through the input signals:  Control of the filling level of the mill by an ultrasonic sensor.
 Size, hardness and density of ore particles according to M. M. Protodiakonov scale.It is a system that can measure ore content on 4 inputs and includes realtime measurement of pulp density.
 The rate at which fresh ore from the belt conveyor enters the mill.
 From the electromagnetic measurement of the consumption water coming to the mill.
 Determining the density of the pulp entering the hydrocyclone.
 Determination of the speed of butane supplied to the classifier with an electromagnetic flow meter.
Through the flexible control system of the technological process, it is possible to reduce the waste of electrical energy, taking into account the load on the electric motor.We can control the speed of the electric motor depending on the load.We adjust the speed of the electric motor using frequency and thyristor converter devices.
Process control methods.The control of solution schemes is affected by the non-linear and uncertain nature of the process, inaccuracies in the mathematical model, the presence of interacting process variables with significantly different dynamics, the effects of non-dimensional disturbances and large time delays, is a difficult task due to many factors such as rough working conditions.On the other hand, effective process management is of great importance to increase the quality of the ready product, as well as to significantly reduce production costs [4].The input variables of the process in Figure 2 are: u 1water consumption required for the mill, u 2 -ore arrival speed, u 3 -percentage of the critical speed of the mill, u 4pulp water consumption and u 5 -mill incoming pulp consumption.Output parameters can be controlled by these variable values [3,5]: x 1 -the mass fraction of the product whose particle size is less than the specified value, x 2 -the concentration of solid particles of the product, x 3 -the consumption of the product, x 4 -the level of suspension in the product.x 5 -the concentration of solid particles in the mill.The most important violations of the technological process are changes in the hardness of the ore and the volume of raw materials.The input-output model vector of the process can be written in matrix form as follows: Here M ij (s) is the transfer function consisting of iinput and j-output signals i, j = 1.....5.Transfer functions are usually determined experimentally by applying initial variations to the input and measuring the output characteristics.A fairly large number of experiments are usually necessary to obtain accurate results.In addition, depending on the configuration of the circuit and equipment, different input and output signals can be used in the control process [5,6].
Control of the technological process in the grinding scheme of the ball mill has the following disadvantages:  the process is not linear, has many disturbances and unmodeled dynamics;  the time constants of the process have a wide range of values, and significant time delays are observed in some input-output pairs;  a system model has output signals corresponding to a series of input signals;  process parameters change over time;  controlled variables have technological limitations  measurements are unreliable and noisy.The main task of control is to ensure stable operation with specified values of output variables.As part of this goal, it is necessary to ensure a stable distribution of the volume of the ready product, the volume of ore is smaller than the specified value.Alternatively, the management objective can be formulated as a task containing a performance criterion that must be optimized given certain constraints.This leads to the maximization of the performance of the grinding process or the minimization of production costs [7,8].

Research results
Ball mills are filled with balls during operation.It is expressed in percentages  Ш , %.This ball is determined according to the size class that determines the maximum possible productivity of the drum mill and is determined using the following expression: The control system that takes into account the ball filling of the mill is based on the active (useful) power consumed by the electric motor of the mill (P, kW) and the ball filling of the mills.Existing mathematical models of the grinding process are developed based on mass or energy balance equations, which describe the reduction of the particle size of the ground material in terms of grinding time or specific energy consumed.In both cases, the main parameters of the model are the unknown selection and distribution functions, and their determination requires additional experimental studies.Decentralized and multivariable control methods prevail in the research results and practical applications reviewed in process control.Nevertheless, theoretical and simulation studies of systems using advanced control methods such as adaptive, unclear logic are attracting more and more interest.Important, unsolved problems for further research activities in this area are related to economic evaluation of ball mill energy efficiency and process control.

Conclusion
Different methods can be used to take into account ore properties in ball mill operation control, such as monitoring and adaptive control methods.Adaptive control is a system control method that allows the system to respond to changes in external and internal factors, such as changes in ore properties, to ensure optimal mill utilizing and quality.It is based on the principle of feedback and allows the control system to respond to changes in ore properties in real time.For example, if ore properties change during mill operation, an adaptive control system can automatically change mill operating parameters by generating signals and influencing the ball weight and synchronous motor drive current to achieve the desired throughput.
We offer a customized control system for ball mills, in which the ball loading (with required diameters) and the motor are regulated according to the excitation current, noise signals and the stator current of the synchronous motor.Thus, the system consists of two input signals and two output parameters.

Figure 1 .
Figure 1.Grinding of ores inside a ball mill

Figure 2 .
Figure 2. Ball mill process control system using ACS (Adaptable Control System).

Figure 3 .
Figure 3.A multi-value controlled grinding scheme