DCS upgrade to PLC system is pure gold

Placer Dome North America’s Dome mine in South Porcupine, Ont., has been in continuous underground production since 1910. In 1988, an open pit operation was added. Between 1910 and 2000, approximately 14 million ounces of gold were extracted. Proven and probable ore reserves are estimated at 2 million ounces of gold. The mill, operating continuously, outputs an average of 14,500 tonnes of ore per day.

“All that glitters is not gold” … however, if you are Placer Dome North America Ltd., effectively separating the gold from the waste rock requires careful attention to process control, to ensure that maximum yield is achieved at minimal cost to productivity and the environment. The process runs as follows.

The mined ore is first crushed in the crushing plant and then conveyed to the 4,000-ton- capacity fine ore bins. The crushed ore is extracted from the bins and fed with water to the rod or ball mill for further crushing. The resulting slurry is pumped to a group of cyclones that separate the slurry into coarse and fine particles. The coarse particles, after being reground in ball mills #1 and #2 and reseparated in a second set of cyclones, pass through four 50-tonne-per-hour Knelson concentrators that capture any free gold particles not attached to quartz or other minerals. The fine particles are gravity-fed to a linear trash removal screen and pumped to a thickener.

The thickened slurry is pumped to a leach train (consisting of eight gravity-fed mixing tanks) where oxygen, cyanide and lime are added to leach the gold. The leach discharge is pumped to the carbon-in-pulp (CIP) circuit where grains of activated carbon electromagnetically absorb the dissolved gold from solution. The resulting loaded carbon is removed from the CIP circuit in batches and the gold is stripped using a caustic soda/cyanide solution.

The gold-bearing solution from the strip is pumped to two electrowinning cells where the gold is attracted to negatively-charged steel wool cathodes. The stripped carbon’s pores are opened by an acid wash procedure followed by a kilning before it is reused in CIP. The gold on the steel wool is removed using a high-pressure washer. The gold concentrate is pumped to a press, dried, combined with flux and loaded into an electric induction furnace. Impurities form a slag while the liquid gold settles to the bottom of the furnace and can be poured into a bar.

The clear effluent produced at the tailings dam is treated in order to destroy and reduce cyanide and heavy metal values, before discharge into the environment. Only effluent that meets with government standards is discharged back into the waterways.

Mill’s distributed control system upgraded to PLC/HMI system

Placer Dome’s global mandate is to design, operate and close its mines in a responsible and sustainable manner that meets the social, environmental and economic needs of present and future generations in the relevant local communities. Much importance is placed on protecting human health and safety, and returning mine sites to a state compatible with a healthy environment. “To achieve our mandate of maximizing mine safety and efficiency and environmental sustainability while producing high quality gold, our processes must be controlled with leading-edge technology,” states Victor Gignac, chief electrician, Dome Mines. “Constant system upgrades to new technology are therefore critical.”

Obsolete and not Y2K-compliant, the mill’s old distributed control system (DCS) was upgraded to a programmable logic control (PLC) system in the summer of 1999. The existing human machine interface (HMI) system was also difficult for the mill personnel to configure and maintain: mill programmers had to write custom drivers for the operating system to facilitate interfacing with the PLCs. Frequent driver failures combined with the single-server architecture, caused system crashes, resulting in process disruptions and unnecessary downtime.

To ensure system reliability, Dome Mines ideally required a control system solution provider that could provide connectivity to existing I/O and wiring; high PLC scan rates to maximize productivity; and user-friendly, easily configurable and easily networkable HMI software. Equally important was the ability to supply connectivity between HMI and PLCs, and to other third-party devices communicating over a plant-wide fibre optic and 10/100 BaseT network. Dome Mines also required a highly reliable control system, because the mill is a continuous operation with little internal surge capacity.

PLC/HMI system reliable and flexible

The Schneider Electric Automation Team suggested a solution based on Modicon PLCs, first and foremost because the Concept software used to program the Quantum is IEC-1131 compliant and can be programmed using DCS-like function blocks in addition to conventional ladder logic. This enabled the programmers to implement a system with a process controller that has a familiar look and feel to the mill personnel. Second, the new Quantum and existing Square D PLCs easily interface into the third-party HMI system, allowing a common HMI for all operations in the mill. Third, the Quantum PLCs interface with the existing Square D I/O. This would enable the mine to upgrade their CPUs with minimal shutdowns while keeping the existing I/O in place until it became necessary to replace it. “This was one of the most critical performance criteria of the upgrade,” observes George Fera, Dome Mines’ systems programmer. Equally important was the fact that the Quantum processor’s scan rates are much faster than the DCS, even when under heavy analogue loads. The final reason for suggesting a PLC-based solution was the fact that the Quantum PLC supports Internet web-based data monitoring and control, and the Quantum modules offer hot swap capabilities.

Hinz Automation Inc. was selected to design and install the PLC/HMI system because of its proven track record and ability to optimize the existing systems and develop the new ones. Headquartered in Saskatoon, Sask., with a network of offices extending throughout North America, Hinz Automation boasts extensive experience developing control systems for industrial applications in Canada, the United States and Latin America, with specific expertise in mining and mineral processing applications.

Metalec Sudbury Inc. was selected to provide the PLC panels, and Avad Industrial provided the hardware, PLC software and technical support because of their local presence and track record for advanced application support.

PLC system upgrade maximizes reliability

Using custom function blocks enabled Hinz Automation programmers to optimize the program and include functions unique to the Dome Mines mill operation–Cascade control, Totalizer Block Functions and autotune proportional integral and derived (PID) blocks. The Totalizer function blocks are used to calculate the quantity of stripping solution and rinse needed to strip gold from the carbon, and the tonnage of rock fed to the original grinding circuit. The Cascade control function blocks are used for the underground backfilling operations. Autotune PID blocks are used for pressure and temperature control, and for stripping solution, slurry and cement flow control.

The Concept software Function Block programming allowed programmers to include extensive documentation to explain the precise operation of the program and the various blocks. Most of the function blocks used in the program are standard to Concept. Several blocks, however, were custom-built for the application. These custom blocks were created using the Concept DFB Editor, then inserted into the Program Block Library.

The ease of migrating from the DCS structure to Concept facilitated reprogramming. “Because the Concept control library is similar to the DCS control library, reprogramming time was reduced by 60%,” explains Gignac.

“The Quantum PLC was favorable to this application owing to the high number of analogue points, which are traditionally dealt with in a DCS rather than a PLC system,” adds Vince Solari, systems designer with
Hinz Automation. “The Quantum’s use of Pentium technology allowed us to successfully execute the project while minimizing the processor load, thereby leaving ample room for future expansion.”

System controls 5,000 I/0 points

The mine’s control system consists of 17 PLCs that control a total of 5,000 input/output points and communicate to the HMI/Scada system via a 10/100 base T Ethernet network.

Four PLCs control the mine’s underground ore hoisting/conveying system; and three PLCs control the underground ventilation, air management (compressed air for underground drills and ore pass controls) and pumping system.

Five PLCs control the mill, which includes the tailings/backfill, leaching/cyanidation, grinding, secondary crushing, and CIP/electrowinning processes.

The crushing plant, primary crusher, compressor plant, effluent treatment plant and water reclaim system are each controlled by a single PLC.

Preconfigured software facilitates upgrade

Installing and commissioning the PLC/HMI system required only three days’ shutdown at Dome Mines. Virtually no system debugging was required. Hinz Automation did factory testing and I/O simulation on-site. “The upgrade ran smoothly because the software was preconfigured and because the Concept software facilitated the migration to the new system,” states Fera.

Adds Ross Libby, Dome Mines’ mill training supervisor: “Because operators can now view all the operating components of each mill process (for example, backfill, stripping) on a single screen–as opposed to several screens on the old system–their training time on the new system was reduced and daily troubleshooting has become more effective.”

PLC/HMI upgrade key to maximizing uptime/productivity

Dome Mines’ upgrade to a new Quantum PLC/Concept software/HMI SCADA system provides a viable alternative to the expensive and restrictive architecture of conventional DCS systems. “Upgrading to this system has generated savings in downtime for rewiring, hardware installation, and training costs,” affirms Gignac. “This system allows us–via the Quantum PLC–to adjust the PIDs to enhance productivity, and facilitates operator awareness of the process, as well as troubleshooting and process flow trending.” He concludes, “This system has helped us to increase capacity utilization and output, and will facilitate future upgrades of Dome Mines’ existing underground PLC network system to a Modicon system.”

Another glittering example of productivity increases through the application of technology!

For more information, contact Ron Nemcko at nemckor@squared.com or Tel. (905) 643-4707, ext. 12; or Greg Lee at greg@avadindustrial.com, Tel. (705) 897-2584.

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