Optimised mill throughput is one of the most significant areas of operation on a working mine site. Ensuring that ore is being crushed and ground to size and passed through as efficiently as possible is as critical as minimising costly downtime required for relining.
It’s all about tonnes per hour, but there are some new smart ways to achieve more total throughput, monitor the condition of the mill’s wear lining to maintain steady flow and also increase safety when it comes time to reline.
Bradken has been manufacturing and installing consumable liners for SAG and Ball mills for many years, but that journey has seen the company constantly develop new technology to help its customers increase profitability and decrease overheads. One of its most recent innovations is the Millsafe liner and fastener system.
Mill diameters have almost doubled in the last 15 years and 12m (40ft) mills are now becoming much more prevalent. A mill of this size requires a large number of wear liners and replacing all these individual components takes time and is also potentially dangerous for reline specialists who typically work inside the mill to manually insert bolts through the liners and mill shell so their teams on the outside can install washers and nuts to secure the internal liner sections in place.
Large mill liner handlers hold each section in the correct location while it is being secured to the mill shell and this exposes workers to the risk of crush injuries if these huge pieces are accidentally dislodged or inadequately secured. By making each liner section larger and therefore having fewer pieces to install, precious time is saved during a shutdown but sections weighing over six tonnes pose a significant risk to personnel.
The Millsafe bolting system allows all personnel to remain outside the mill and stay safe, while it is being relined.
Bradken’s unique liners have captive head nuts that are pre-installed in each section and this means that the liner can be held in place by a remotely controlled liner handler and studs are inserted from outside the mill into the liner and pick up the captive nuts on the inside.
In this way, all workers are situated outside the mill and they are well away from any suspended components.
Tight tolerances between hole centres mean that alignment is perfect between the OEM outer mill shell and the Millsafe liner, so engaging the captive nuts and torquing the bolts to the correct preload is done quickly and easily.
Bradken has improved the safety aspect of the installation by also incorporating a robotically applied rubber strip on the liners to seal the design gaps between each section. This not only saves more time but, importantly, removes the manual process of inserting the rubber strip between liners and therefore eliminates another risk to installation personnel from a known pinch point.
Furthermore, as part of Millsafe’s total liner package, every part inside the mill is individually bolted into place. This removes yet another risk from conventional liner designs where some sections are only held in place by adjacent sections bolted to the shell. If those bolted sections become loose or fail, they can allow the unbolted pieces to shift or fall, creating a significant safety hazard.
Due to the clever design, a unique bolting system and larger reline sections, Bradken estimates it would take approximately 72 hours to reline a 12m (40ft) diameter SAG mill; this includes removal and replacement of all internal wear parts. Historically this may have taken upwards of 100 hours. This additional mill uptime represents significant revenue for the customer.
In this day and age it is no longer acceptable to simply bury scrap liners on site – especially when they have resale value.
Bradken’s mill liners are 100% recyclable and they buy back the worn liners from clients where possible in order to melt them down and utilise to cast brand new liners. This not only represents a cost saving to the client from the scrap value of the old liner, but it eliminates the need for the customer to identify alternative means of disposal.
As part of the ongoing mission to improve the efficiency of a mill, Bradken utilises highly advanced 3D software modelling to verify the effects of any design changes prior to adopting these changes.
This ensures that design changes deliver the outcomes that the customer is expecting and delivers quantifiable financial benefits.
One such feature can be incorporated into the pulp chamber of the mill. Exhaustive testing has resulted in a curved vane geometry being designed (as opposed to a conventional straight radial vane arrangement). Curved vanes allow the ground material to be more efficiently directed into the discharge cone. This is especially true if mill rotation speed is increased to achieve a specific ball and ore trajectory because the curved path physically directs the sized material into the discharge cone and does not allow it to be thrown outwards by centrifugal force like a straight vane does. Bradken’s pulp chamber resembles the inside of a centrifugal pump.
Ultimately, this software simulation also allows Bradken to develop a liner profile that delivers optimum efficiency inside the mill by ensuring the huge 14kg steel balls and ore impact at the toe of the charge at all times (given wear and other factors).
The simulation also allows Bradken’s engineers to predict the most efficient design to process different types of ore – anywhere in the world. This extends to predicting any necessary changes to mill rotational speed or ore levels required to compensate for liner wear and profile change. As the liners begin to wear and the trajectory of the balls and ore change, mill operation needs to be modified to compensate.
Using constant modelling iterations that incorporate actual historical data and new simulated data, mill design is constantly evolving and improving and the result is more tonnes per hour for the customer over a far longer period between partial or full shutdowns.
Even the chemistry of the metallurgy in the wear liner itself can be tailored to achieve the sweet spot for balancing impact resistance with wear resistance for any given application.
Bradken has offices around the world and part of its service model is to visit customers on a monthly basis and measure the wear inside the mills.
This is currently done with a static 3D laser scan conducted from inside the mill itself. A laser map, comprising millions of point dots, creates a 3D image of the interior and allows engineers to measure the exact wear at any point.
A wear prediction report is then supplied to the client, which allows them to more accurately plan the timing and extent of any required mill maintenance or liner replacement. This is particularly significant if the location of the customers site creates a significant logistical task due to remoteness or weather conditions. Personnel requirements to undertake a major mill reline and associated maintenance can run into the hundreds and when that site is remote or access is restricted by weather conditions it is critical that it is planned well in advance and wear predictions are reliable.
Avoiding unexpected stoppages is key and ongoing wear reports provide critical data about the health of the wear components and the efficiency of a mill.
In its ongoing quest to supply live data to their customers around the globe, Bradken has launched the first phase of its new Smart Liner technology.
This involves four wear sensors and three vibration sensors physically inserted into the wear liner and onto the mill body at strategic points. The sensors transmit through WiFi to a data collector, then to an API and finally to the mines onsite data platform. Key codes form part of a rigorous security system that safeguards the data at each stage of transmission.
This now enables Bradken to collect real time data from the mill – as it rotates – and eliminates the need to shut it down for a static scan.
However, the benefits of this system are far more powerful than just thickness measurement. By utilising a combination of wear data and impact data from this small array of sensors, engineers can calculate the impact force and the trajectory of the balls and ore at any given time.
As the profile of the liners change over time due to wear, the trajectory may not crush the new ore as efficiently.
By knowing exactly how much wear has taken place, it is possible for the Smart Liner system to compensate for this by either increasing the mill’s rotating speed or varying the level of ore inside.
This allows the mill operator to maintain optimal impact over a far longer period and leads to significant increases in mill throughput – and therefore mine profitability.
The end goal is to have a system which runs the mill optimally, in real time and completely autonomously.
Financial and Block Modelling
Trials of this revolutionary system are underway currently. Clients are supplied daily live updates on their mill wear rates. As mentioned, this data is then combined with historical data and simulation data to continuously refine the model for that particular mill and provides some exciting projection modelling for mine owners.
For example, if an operator can run a mill optimally – with total tonnes per hour as the metric – and perhaps regardless to trying to optimise liner wear, then what could that look like for total profitability?
Well, if a complete mill reline costs $1m but total throughput has increased by 3%, then the cost of the reline is insignificant in relation to the increase in production.
This ‘Optigrind’ platform will ultimately lead to a full digital twin being created for the mill and will allow this type of financial modelling to be carried out with very high degrees of accuracy – based on the iterative nature of the data collection.
The platform will also work seamlessly with onsite block modelling to calculate optimal mill performance to process specific blocks of material of differing grades or hardness.
Bradken’s multi-faceted system addresses all the key issues surrounding the safety of relining, increasing the speed of removal and replacement, improving the fundamental design of each component to improve overall efficiency and now enables live wear monitoring and even real-time optimisation of the mill itself.
A total mill solution from start-up to shutdown.
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