|Big benefits can be won by controlling a crushing plant carefully. (Photo courtesy of Sandvik)|
Crushers are energy-hungry pieces of equipment, so optimizing drive and control systems makes good economic sense
By Simon Walker, European Editor
At the end of last year, the Australian provider of design, automation, control, instrumentation, electrical engineering and process optimization systems, MIPAC Engineering, announced the award of a significant contract from Germany’s Tenova Takraf. MIPAC has been tasked to design and configure the electrical, instrumentation and control for three semi-mobile crushers at a major project in Western Australia’s Pilbara region.
According to the company’s business development director, Alan Thorne, it had been liaising with Tenova Takraf about the project since 2011. “MIPAC’s experience in crusher control systems and other mining operations made us a natural choice to undertake the crusher control design and configuration,” he said. “The work will be completed by August.”
The crusher control system is PLC- and SCADA-based, with MIPAC developing the functional specification for the control philosophy in conjunction with Tenova Takraf. The design involves plans for one crusher that will then be replicated for the other two. This will significantly reduce engineering costs, according to MIPAC project manager, Ashish Mahajan, who said, “MIPAC will also be involved in HAZOP studies and machine safety studies conducted by Tenova Takraf as part of the development of the crusher operational philosophy.”
This example is just one of a growing body of practice founded on ensuring that crushers run at their most economical setting, while at the same time maintaining consistent throughput. All stages of comminution are energy-hungry, and while energy usage rises in line with the fineness of reduction, a lot can be gained from optimizing each stage of the crushing and grinding process. Hence ensuring that the drive and control systems installed on crushers and mills are doing the job for which they were designed makes strong economic sense.
The ‘Brain’ in Control
According to Sandvik, the intelligent drive and control systems in its CH series of cone crushers and CG series of primary gyratories enable real-time performance management, most tangibly equaling maximized crusher performance and productivity. The company explained that the “brain” in its crusher control system, the Automatic Setting Regulation system (commonly known as ASRi), protects the crusher from overload and constantly monitors the power draw, the hydroset pressure and the mainshaft position (and thus the closed-side setting—CSS) in real time. It adapts the crusher’s settings in real time to match feed-curve variations as well as variations in feed material hardness, thereby ensuring consistent maximum performance, Sandvik added.
By continuously measuring and automatically compensating for cone crusher liner wear, the ASRi allows customers to fully utilize crusher liners and schedule liner replacements to coincide with scheduled maintenance stops, Sandvik said, while noting that the ASRi also allows quick and easy remote calibration, even from a control room.
The company added that when the “Auto-CSS” regulation mode is selected, the ASRi aims to maintain the stipulated CSS. Alternatively, in “Auto-load” mode, the system focuses on keeping the selected set-points for the main motor power consumption and main shaft hydroset pressure, such that the crusher operates at the required load level. If the highest permitted load level is selected, maximum reduction will occur as the CSS will always be the smallest possible.
According to Sandvik, the software package supplied with the ASRi includes an OPC-server that allows seamless integration with superior control systems such as SCADA and DCS systems. This communication provides complete access to all the parameters in the ASRi as well as allowing adjustments to be made remotely during operation.
For customers who do not wish to build their own pictures in SCADA, Sandvik supplies its WINi graphics software option. This can allow a customer to control their ASRi remotely using the same graphical user interface as on the ASRi. Sandvik makes the analogy that it is like installing an extension of ASRi on a PC, so that a control-room operator can independently select pages on WINi without interfering with the operator in the field.
It is also possible to have different settings on WINi than on ASRi—such as language and units. Where an operation has more than one ASRi, the WINi software can be used with up to nine different ASRis, displaying them on an overview picture, so that the user can easily monitor and compare several machines at once.
Sandvik pointed out that automated systems are not just limited to overall crusher control. Proper lubrication will maximize the lifetime of vital crusher components, for instance. All of its CH800 series and CG series crushers are equipped with a Tank Instrumentation Monitoring System (TIMS), which monitors all auxiliaries in the crusher to ensure optimized lubrication. Meanwhile, the lubrication system for its gyratory crushers is designed to be controlled by variable-frequency drives (VFDs), using flow meters to deliver exactly the right flow into the crusher.
|An ASRi is being used to control a Sandvik CH800 series crusher.|
Sandvik gave some examples of the benefits that can be gained from using its ASRi control system. In a CG-type primary gyratory, if a big rock becomes jammed in the crusher intake, the ASRi system allows the operator to switch from “automatic” to “manual” mode so that the crusher can be opened. This might help to re-orientate the rock sufficiently to enable it to be crushed. The ASRi is then switched back to “automatic” and the crusher returns to normal operating conditions, with minimal disruption to production.
The company cites an installation at the Teberebie gold mine in Ghana, which used a size 54-74 primary gyratory crusher equipped with ASR+ (an older version of ASRi). Flexible crusher operation was essential as the mine produced three different types of ore—hard, soft and sandy—each with different crushing characteristics. Each ore type had its own individual operating programs with preset parameters in the ASR+, allowing the operator to select the correct program depending on the ore type. The crusher could then work at the optimal settings so as to best utilize its capabilities, without any risk of overloading.
In a second example, Sandvik describes a case where an operator has several CH870 crushers with ASRi and WINi installed, allowing crushing to be operated from a control room. The same operator also has some bowl-type crushers on site and, the company said, has commented on the difference when calibrating.
As this particular customer is dependent on a certain product size, they run their CH870s on Auto CSS, which automatically learns and compensates for wear. However, Sandvik said, they wanted to be completely sure about the setting. Taking just 30 seconds to complete, the ASRi allows them to calibrate the crusher several times a day to ensure that the product size is indeed correct.
With the bowl-type crushers, the situation is completely different, Sandvik pointed out. The customer needs to go out to the crusher, stop the feed, unclamp the bowl, manually measure the setting and reclamp the bowl, before restarting the feed. As this takes more than 30 minutes, it is only done once a day because of the crusher downtime involved, so the product size is not as consistent.
HPGR Presents Other Challenges
With high-pressure grinding rolls (HPGR) offering a viable alternative to conventional crushing in appropriate situations, acceptance of this technology is becoming increasingly widespread within mineral production. Yet, as Siemens pointed out, controlling an HPGR installation presents a different set of challenges, particularly in relation to keeping wear at an acceptable level.
The company noted that since HPGR operations have to be designed to perform at optimum efficiency to minimize excessive energy and maintenance costs, its optimized drive systems can help by providing accurate speed and operating torque control. Siemens claimed that by matching the best-suited components at each step from its extensive drive system, motor, gearbox and load-distribution control product portfolios into a completely integrated system, it can ensure that HPGR installations can handle large overloads for long periods.
Siemens noted that one particular area where its control systems are proving invaluable is in torque sharing in HPGR plants. Since there are separate drives to each of the rollers, it is essential to have equal torque sharing—if this is not the case, one roller will end up doing more work than the other.
Once this happens, there will be more wear on that particular roller and throughout its drive components, so maintenance and wear-part replacement will increase. However, the other roller will probably still be serviceable then, and will only need maintenance later on, meaning that overall, downtime will be higher and the crusher will be out of action for longer.
Siemens stated that as different sizes of material enter the HPGR rolls, fast torque-to-speed response is critical. Enabling the machine to compensate for load variations allows for more consistent quality, less strain on components, and lower energy consumption. Speed compensation also adjusts for the reduction in the roller diameters resulting from wear on the roller faces.
Siemens said that it uses a proprietary load-sharing technology to ensure that torque is shared accurately to within 1%—reducing wear on mechanical components and preventing system overloads. Continuous speed control also reduces process vibration by compensating for variability in the ore feed.
|Sandvik’s ASRi can be run in Auto-load or Auto-CSS modes, allowing easy collaboration.|
Electrical Drives Bring Benefits
Turning to conventional mining crushers, Siemens noted that the inclusion of an electrical drive at the crusher’s front end provides benefits such as improved energy efficiency during operation as well as a reduction in the inrush current at startup. Together with smoother control during startup, this in turn helps to reduce any negative impact of crusher operation on the stability of the power network, the company added.
Other benefits include the optimization of the relationship between torque and speed control, which allows the machine better to compensate for variations in the ore characteristics. Accurate speed control also helps maintain consistency in the size of the crushed material, while the use of an effective electrical drive gives the operator improved capabilities to avoid or clear system jams.
As well as this, crusher systems with multiple motors benefit significantly from drive control as mechanical wear caused by torque imbalance is reduced, Siemens said. There is also greater operational flexibility through the use of independent motor control, while maintenance requirements can also be simplified.
Writing in the most recent edition of the company’s customer magazine, Results, Metso’s Pasi Airikka explained that automation decreases variations due to raw material quality changes, process and machine loads, and external circumstances such as moisture contents and the weather. “Consequently, stabilized variations result in better production, quality and plant availability,” he said.
A product manager for Metso Automation, Airikka noted that while even the most intelligent process automation system for a crushing and screening process may not outperform the best operator at his best—in the long run, it will prove its worth. “Achieving maximum results requires integration of the process, machine, and automation expertise already in the plant engineering and design phase,” he stated.
In a paper presented at MEI’s Comminution 2014 conference, held in Cape Town earlier this year, Dr. Erik Hulthén, Magnus Evertsson and colleagues from Chalmers University of Technology and LKAB in Sweden described a study they had undertaken on optimizing cone crusher operation at the Malmberget iron-ore mine. As they pointed out, modern cone crushers are equipped to adjust the closed-side setting automatically to compensate for increasing wear over time. In addition, the use of a frequency converter to adjust the eccentric speed means that this can now be done in real time, without the need to stop the crusher and change drive-belt positions.
The study used a Sandvik CH680 cone crusher that is used to reduce 100-mm feed to minus-30 mm. In summary, the work showed that by selecting the optimum control points for the CSS and speed, the crusher efficiency could be improved significantly. There was also a marked increase in the output of minus-1-mm material, meaning that later, higher energy-use comminution stages would have less work to do. The authors noted that they are now developing a fully automated real-time algorithm, with the aim of consistently maximizing the circuit’s output.
The product and production development department at Chalmers has a long history of research into rock crushing, with a spin-off company, Roctim, now offering the “eYe” process optimization system that includes features such as Hulthén’s real time optimization algorithms.
Roctim has also developed its Crusher Control Unit (CCU) to supervise the crushing process and protect crushers from overload and fatigue damage. The company noted that, at around half the price of major OEM systems, the CCU offers an alternative solution for older crushers that have outlived their original control unit, or have never had one fitted. Various operating settings can be programmed into the unit’s PLC, and the CCU can be operated either via a touch screen or through a site-wide HMI/SCADA system.
Clearly, there are considerable incentives to operate crushers as efficiently as possible, while minimizing the risk of premature wear and failure. Modern drive and control systems have gone a long way to achieving these aims, bringing better energy efficiency and higher, more consistent output to the crushing process.