Measurement of Steel Ball Impacts Inside a SAG Mill

In large-scale mining, SAG (Semi Autogenous Grinding) mills are widely used. These mills use steel balls that go up to 6.5" in diameter and have a high risk of fracturing the internal linings of the mill.

Potential impacts of the steel balls against the mill lining will be generated if the rotational speed of the mill is greater than that required for the ball to fall on the ore rock and fracture it. 

In order to determine if the balls are impacting the liners, intelligent sensors called Impact meters are used.

How is the steel ball impact detection and counting done?  

The impact meters use microphones that are installed outside the mill in the charge toe zone. These microphones acquire the sound signal, which is processed in real time to detect the noise pattern generated by the metal-metal impact of a ball against the liners or against other balls.

Once the impacts have been detected, the next step is to classify them, based on their energy. In order to carry out this classification, the intensity thresholds must be previously defined, an impact is classified as a standard-impact when it exceeds a first threshold and if it exceeds a second high-intensity threshold, it will be classified as a critical impact.

Impact meters are not a new technology. What improvements have been made to this equipment or to its implementation process compared to what has been done in the past?

The first SAG mill impact meter was developed more than 20 years ago in Chile and became widely used in large mills. Several of these impact meter did not exceed their initial life cycle, that is, once they were damaged, customers did not replace them. In many cases, the initial purchase of the equipment was a mandatory requirement to extend the warranty period by the manufacturer of the mill, so once this warranty expired, the customer did not see a point in continuing to maintain the equipment.

Many of the impact meter installations were not integrated into advanced mill control, so the benefit of impact-based mill speed control was not realized.

Considering that they are specialized and high-cost equipment, customers expected high availability from this, which was not achieved if the microphone became dirty with mineral pulp or some other damage and they had to wait for a mill stoppage to repair it.

With all these factors against, many clients explored other technologies to be able to estimate the impact zones of the balls inside the mills, while other users who did have impact meters integrated into their control system continued to use this technology and only updated it. for more innovative alternatives that solve the problems listed above. 

This is a summary of improvements made to the impact meters:

1. Fault tolerance by implementing redundant microphones, that is, two microphones are in operation while two other microphones are in backup.

2. Mounting of microphones: Options have been developed for mounting the structure that supports the microphones that does not require tools to be removed, it can also be removed without accessing the lower part of the mill, this improvement, although it requires the mill to be stopped, It is an alternative to make quick changes of microphones. The other alternative to improve the installation is a slide-type conduit that allows the assembly and disassembly of microphones with a mill in operation.

3. Integration to the Process Control System (DCS) through an industrial network, as a base characteristic in the equipment.

4. Reproduction of the noise captured by the microphones in the control room.

5. Better Interoperability with the new Mills Digital Twin platforms.

Given the new wave of digital transformation, specifically the implementation of digital twins, mining users have searched for new and better smart sensors, including impact meters, which with the improvements that have been listed above are a mature option to determine the internal dynamics of the mill.

Sound versus vibration measurement systems: What is better in each technology?

Vibration-based systems are smart sensor alternatives to estimate the internal dynamics of the mill. Vibration sensors are installed in the mill cylinder. Although many mining users sought to implement them as a substitute for the impact meter, the reality is that these vibration-based systems provide complementary information, since they make a very good estimate of the load shape, but it is impossible for them to count the impacts.

Obtaining the load shape angles or detecting the point of greatest vibration in the mill are not determining factors to estimate the impacts of the steel balls, since for the same level of total filling (Jc) and the same level of ball (Jb) the mill can present different load shape depending on the granulometry of the mineral fed.

Vibration-based systems cannot count critical impacts (ball-liner impact) because these impacts are typically below 50 impacts per minute (less than one impact per second) and the vibration sensor passes through the impact zone in a fraction of a second, so it could not capture enough information to be able to infer the number of impacts using statistical tools.

Impact classification to define operational scenarios.

The impact meter provides the DCS with impact values ​​classified into two types of impacts, the first are critical impacts, which are unwanted impacts that can cause ball or lining fractures, and the second are standard impacts, which are impacts that generate the grinding, which is the impact of the ball against the rocks.

The time trends of these impacts per minute that are written to the DCS are used to define operating scenarios in conjunction with the weight and power variables of the mill.

In general, control strategies seek to maximize standard impacts, which are those generated by grinding, and minimize critical impacts.

Integration to digital twins for process autonomy

Integrating the ball impact variables to the Digital Twins allows validating with simulation that the values ​​collected with the smart sensors coincide with the particle simulators. This allows more knowledge of the process to be generated and enables the parameters of various phenomenological models available for SAG grinding to be calibrated with simulation.

Going further for an autocalibration platform 

The main challenge of implementing smart sensors is the large amount of time involved in collecting field data, interpreting sound, and calibrating models.

Autonomous mining will require smart sensors in each mill and to deal with the implementation of impact meters in large and medium mining mills implies that the time of specialized resources is minimized and that the work is carried out by cloud systems that can do the calibration automatically.

Remarks

The implementation of digital twins is revaluing the offer of smart sensors for SAG milling available on the market, where interoperability, self-calibration and self-diagnosis capabilities will be among the most valued capabilities by mining companies.

Both the smart sensors and the grinding models will go through a progressive process of updating and evolution until dynamic process models are obtained that will allow a better explanation of what happens inside a SAG mill.