In the island state of Papua New Guinea, Ok Tedi Mining Limited (OTML) runs the copper and gold mine Ok Tedi. OTML’s drive train infrastructure at the Ok Tedi mine is in excess of 30 years old. In order to ensure long term profitability, OTML is using Siemens technology to supply and install complete new variable speed drive trains for the two Semi-Autogenous Grinding mills SAG 1 and SAG 2.
To ensure that the copper and gold mine Ok Tedi can continue to operate profitably, its operator Ok Tedi Mining Ltd (OTML) has decided to modernise the 30 years old mill drive train on its two Semi-Autogenous Grinding (SAG) mills and change the operational concept from fixed speed to variable speed. In the process, OTML has commissioned Siemens to supply and install new motors and couplings, variable speed drive converters and an online mill drive train condition monitoring system combined with remote access and a two year drive train analytics contract.
On SAG 1, the two existing motors will be replaced with 12 pole, low speed induction motors operated at 20 hertz (Hz). Due to the low speed nature of the drive train, a torque limiting coupling is fitted to protect pinion and girth gear from eventual transient torques. As a result, the air-clutch that previously transferred the drive power to the mill can be removed, eliminating all air-clutch related outages and maintenance activities. For SAG 2, Siemens supplies two 8 pole, high speed induction motors to replace the existing slip-ring motors, eliminating all brush or slip-ring related outages and maintenance activities. The four identical variable speed drives (VSD) operating all four mill motors are Sinamics Perfect Harmony GH180 units. The VSDs are fitted with cell bypass technology which enables them to internally bypass and isolate faulty components and continue to operate until the next planned shutdown.
In 2014, Siemens and OTML conducted a mill drive train audit and identified areas for potential improvements. Initially, OTML’s main goal was to change the operating concept from fixed to variable speed and to install new VSDs while keeping the existing motors. Over the next two years, Siemens developed several modernisation scenarios for the existing mill drive train, ranging from a partial modernization (keeping the existing motors) to a complete modernisation (installing new motors and VSDs). In 2016, OTML chose the latter. A change from fixed to variable speed will improve operational efficiency of the Grinding mills and increase throughput, while the change of the motors and the installation of the condition monitoring system will reduce maintenance and increase availability of the complete mill drive trains. With the increase of mill motor capacity to 4.2 MW (megawatt), momentary power limitations and throughput restrictions caused by changing ore hardness in the mill are eliminated. The online condition monitoring system will allow OTML to track mechanical wear and assist with proactive maintenance. Through remote access and the two year drive train analytics contract, Siemens will provide OTML with quarterly reports on equipment performance and host a cloud based, online platform where OTML can check on historical drive train data as well as actual process and equipment status information. This will enable OTML to better manage their new infrastructure and increase overall productivity. The new drive trains are scheduled to be commissioned in autumn 2018.
The new VSD systems also have related benefits. Reduced starting currents allow the mills to start smoother, which also leads to the elimination of mechanical stress on pinion and girth gear. Frozen Charge detection is automatically performed during every start-up, which helps save precious process time. From the start command through grinding operation up to the stop in a balanced position, Siemens Mill Technology Control guarantees optimal load sharing between the two mill motors, while variable speed allows compensating for liner wear and adapting to fluctuations in ore size and consistency.
The project involving the two SAG mills is the culmination of a longstanding relationship and successful cooperation between Siemens and OTML that started in 2005. Siemens has since been committed to improve operational performance in many areas on site – most recently through the supply of two Planurex P2KB20 planetary gear units in 2015. As part of a two-year service contract, Siemens will continue to support the drive technology implemented in the SAG mills 1 and 2.
