Using APC to Increase Output and Improve Operations at the Letšeng diamond mine

Customer Summary

Gem Diamonds is a leading producer of high-value diamonds. The group owns the Letšeng diamond mine located in the Maluti mountains of Lesotho, which is renowned for its recovery of large, high-quality diamonds. The Letšeng operations process ore from two kimberlite pipes through two concentrator plants. These concentrators are conventional diamond recovery plants, comprising primary, secondary and tertiary crushing circuits, along with coarse and fine Dense Media Separation (DMS) circuits.
Minopex is contracted to operate the Letšeng diamond mine and its processing plants.

Challenges

Letšeng aimed to increase the throughput of their plants, while maintaining crushing and DMS efficiencies, to achieve maximum diamond recovery with minimised losses. Operating the plants at a steady state while driving the processing equipment to their ultimate technical performance required specialised control of the crushing and DMS circuits to maintain process stability, for the following reasons:

• Variations in ore feed conditions (type and coarseness) cause throughput constraints to shifts within the process, between the crushing and DMS circuits.
• Long conveyors situated throughout the process introduce significant time delays between processing units as well as delays in control responses.
• Multiple operating targets are required to be achieved simultaneously
  (throughput targets, DMS medium to ore ratio targets, DMS tails flow rate targets).

These factors were difficult to handle in the PLC base layer using PID controllers since they link only a single input and output. Therefore, changes made by a controller in one section of the plant could have negative effects on downstream processes. Leading to the activation of process interlocks or missed opportunities that cannot be recovered due to capacity constraints.

Some of these challenges could be overcome somewhat by developing custom PLC logic to replace the base layer control. However, these schemes normally required hard coded rules to be implemented, which limits the flexibility of the process in the presence of feed disturbances. The result would be a control system that may work well under the ideal conditions it was tuned for but that became a hindrance to the process when conditions changed dynamically.

"The envisaged benefits are real. We now have faster evaluation of opportunities."

 - Siphiwe Buthelezi - Head of Treatment Operations 

Solutions

Building on the established partnership between Gem Diamonds, Letšeng, Minopex and 4Sight, the simulation study conducted on Letšeng’s Plant 1 by 4Sight’s Operational Technologies Asset Simulation team, highlighted the active plant constraints in the crushing and DMS circuits. The simulation study results indicated a potential throughput benefit of an APC implementation. Guided by these findings, 4Sight’s Operational Technologies, Asset Optimisation division was able to design an APC application spanning the primary, secondary and tertiary crushing circuits, as well as the DMS feed. Multivariable Advanced Process Control (APC) solutions, such as Model Predictive Control (MPC) are designed to simultaneously coordinate multiple process units to achieve synergy throughout a processing plant, resulting in stable operation and a steady platform for optimisation.

MPC utilises dynamic models of the interactions between the process inputs and outputs to calculate optimal control outputs which are sent to multiple plant areas, providing real-time control and coordination of these process units. Since the models are time-based, process information such as time delays introduced by material transport systems like conveyors are inherently built in to the model, allowing the MPC to effectively overcome these challenges. MPC allows the process to be operated against its active constraints, maximising throughput, and operating equipment within its limits for a given set of feed conditions.
The technical design and implementation of the APC application at Letšeng leveraged the deep domain expertise and collaboration between Gem Diamonds, Letšeng, Minopex & 4Sight, resulting in a control solution that was aligned with the process configuration and production objectives of Letšeng.

Benefits & Outcome

The implementation of APC at Letšeng by 4Sight provided several improvements in key process KPIs. Based on preliminary analysis, the improved coordination of the crushing and DMS circuits resulted in the following benefits compared to the quarter preceding the APC commissioning:

In addition, APC rendered a throughput benefit of up to 290 000 tons per annum while stabilising the operation of unit processes in the circuit, providing significant potential for improved diamond recovery. Furthermore, the improved plant operation under APC control has highlighted process areas where equipment maintenance efforts should be focused, allowing personnel to devote attention to other process optimisation initiatives. The APC installation at Letšeng is being expanded to include Plant 2, using the results of the simulation study conducted by 4Sight Asset Simulation to guide the functional design of the APC application.

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