High pressure grinding rolls are steadily becoming ubiquitous in the copper mining industry, replacing the SAG mill grinding stage.
The HPGR exposes feed material to very high pressure for a short amount of time. The compression typically causes the rock to crack and cleave along the grain boundaries, weakening the rock structure. This high pressure also creates a large amount of fines and causes the formation of microcracks in the larger particles, lowering the Bond work index of the ore which, in turn, reduces both ball mill loading and power consumption. The higher the pressure, the more fines (“final product”) are produced.
The main challenge for the customer, a South American copper miner, at the initial stage was to keep a healthy level of inventory in the feeders to ensure a consistent feed between the rolls. A layer of ore between the rolls helps the compression forces to break the particles more effectively and consequently a softer ore will be fed to the next stage.
To keep this layer of material at its desired level the operator must constantly monitor the level of the four discharge bins, making sure they don’t get too full (this can cause the feeder to get stuck between the discharge bins) or too empty (causing the rolls to stop resulting in a decrease in overall throughput). The process to fill the bins happens relatively quickly and the operator must monitor other equipment in the area at the same time, making the task of keeping everything at an optimal operation daunting.
Another challenge with the HPGRs is to maximise the comminution energy by increasing the power in the rolls. With hardness changes it is necessary to adjust the pressure dynamically to produce a softer ore for the ball mills.
Controlling the HPGRs with Expert Control
Our expert control software has the ability to capture the human knowledge with ease, distilling the expertise of metallurgists and operators and expressing this knowledge in rules that use fuzzy logic. By being a multivariable control system, it can monitor all of the components of a process and take the best decisions, in a matter of seconds.
The customer had great knowledge of this area, obtained from theory and practice. Expert control was the best tool to capture this mastery and automate the tertiary crushing 24/7/365.
Expert Control Strategy
The following objectives were pursued with the Expert Control for the HPGRs:
- Increase the operational continuity and reduce the variability in the production
- Manipulate and stabilise the inventory level in the bins
- Increase the power in the HPGR rolls
- Increase the tonnage downstream
- Protect the conveyor belt against high current, speed deviation and excessive tonnage
Three control blocks are configured in the Expert System for the HPGRs. These modules work in parallel for the four grinding rolls and take decisions every minute.
- Moving bin control: this control block deals with the movement of the bin to distribute the load evenly between the bins that feed the HPGRs.
- Feeder control: this control block receives a tonnage target from the conveyor that feeds the grinding circuit. To reach this target it controls the feeder speed that feeds the HPGRs, controlling bin level, hopper level, roll speed and conveyor tonnage. At all times these rules are looking not to exceed the current and the speed of the belt.
- Pressure control: these rules control the pressure target for the HPGRs with the objective of maximising power in the rolls. The controlled variable is temperature, which is closely monitored to not be exceeded.
Increase in tonnageThe following graph shows the benefits obtained when implementing Expert Control for the HPGRs. The analysis shows an increase of 1745 t/h in throughput when Expert Control in turned on, this accounts for an increase of around 21%.
The increase in tonnage for the HPGRs can be attributed to the tight control over the bin levels, by manipulating the moving bin and the feeders. These actions are taken every minute and have a significant impact on operational continuity and overall tonnage delivered to the grinding stage.
Comminution energy increase
Another objective achieved by implementing Expert Control in the HPGRs is the increase in comminution energy by increasing power in the rolls. Figure 3 shows an increase in power of 680 kWt with the Expert System turned on, an increase of around 17.5%.
By operating with higher power, you ensure that breakage occurs between the particles, alleviating the ball mills and increasing overall throughput.
Circulating load decreaseTo close the loop of benefits obtained with Expert Control we analysed the circulating load when the system is on. Figure 5 shows a decrease of 4.9% in circulating load for the crushing stage. This is because the expert system ensures a smaller ore when the system is on, by keeping an optimum layer of material between the rolls and by increasing power.
By implementing expert control in the HPGR’s we can achieve operational continuity by constantly monitoring the bin levels. Two benefits are immediately presented:
- higher throughput by reducing load disruptions, and
- maintaining an optimum layer of ore between the rolls to achieve the best rupture.
Once the inventory for each HPGR has been stabilised, the Expert System increases power in the rolls to enhance its breakage capabilities and provides a softer ore downstream. This in turn decreases the work done by the ball mills and increases overall throughput.
This strategy also influences the circulating load in crushing, lowering it by a noticeable margin, and increasing the fresh feed to the equipment.