Control system created for mine tailings reprocessing
By Benjamin Orchard
Mine tailings are the crushed and milled rock residue that remains after mineral extraction. These tailings must be deposited for storage in a cost effective way that also meets environmental guidelines and mandates. Dams, dumps, and other types of surface piling are some of the more common tailings deposition methods used today. However, these all pose serious environmental concerns, as tailings often contain trace and sometimes substantial quantities of the metals found in the host ore, as well as certain amounts of chemicals and compounds used in the mining processes. As a result, establishing and maintaining tailings dumps represent a significant cost for mining companies.
Historically, mineral separation processes have been somewhat inefficient because after the required crushing, grinding, and metallurgical treatment, some amount of valuable metal is left behind in the residue. But recently, improved mining technologies and the ever-increasing price of gold in the marketplace have opened the opportunity for mining companies to purchase old mining dumps and metallurgically treat the tailings in order to recover residual gold.
South African miner Mintails Limited is currently focused on exploring, evaluating, and processing hundreds of millions of tons of surface gold bearing tailings in the Witwatersrand ("Rand") area of South Africa-a region that, according to many experts, has produced more than 40% of all the gold ever mined from our planet. These types of gold reclamation efforts are never 100% efficient, and automation and control plays a significant role in improving the metallurgical processes.
Opto Controls (Pty) Ltd was contracted by Mintails to assemble and configure an automation system for Mintails' Mogale Sands Plant ("Mogale Gold"). The system connects to, monitors, controls, and acquires data from a large assortment of field devices, equipment, and instrumentation and also manages the many mechanical and chemical processes used to remove the gold from the tailings. Key components of the system are rack-mounted and standalone programmable automation controllers (PACs) and I/O housed in containers situated between the gold mine and the dump.
Initially, these controllers were to be networked via fiber optic cable, but Mintails management later decided to go with a wireless solution, so as to avoid having to dig trenches and lay conduit for the cabling. The wireless communications platform used features a Linux-based operating system, omnidirectional antennas, and wireless communication capabilities utilizing all four of the 802.11 standards (a/b/g/n), as well as frequency ranges within the industrial, scientific, and medical radio bands. It also has meshing capabilities and a range that easily bridges the average distance (about 150 meters) between the Mogale site controllers.
Tailings processing begins with the pumping of water from the mine to the dump site, where it is used to wash down the tailings and create slurry. The control system regulates the pumping of approximately 600 cubic meters of water per hour. The controller communicates to digital output modules-used to start and stop the water pumps and also to change the state of valve actuators within the pipeline-along with digital input modules that aggregate feedback from the flow and level switches and differential pressure sensors used to confirm that pumping is executing properly. There are also connections to magnetic flowmeters to detect and monitor flow rates, and analog input modules to get feedback from scores and scores of valves. These flowmeters have no moving parts, making them low maintenance and ideal for use in pumping dirty liquids.
Once the water arrives at the dump, large water jets disperse the water, and the resulting slurry is pumped back to the mine. The slurry then goes through a series of screens for removal of debris and other material.
Carbon-in-leach (CIL) and elution
Leaching, sometimes referred to as metallurgical extraction, is a chemical process for removing minerals by dissolving the mineral and moving it from a solid to liquid state. At Mogale Gold, this is accomplished by absorbing the dissolved gold onto activated carbon. This form of carbon is extremely porous (a single gram can have the same surface area as a small parking lot), making it well suited for adsorbing gold.
During elution, gold is stripped off the carbon by a hot chemical mix of caustic and cyanide. At an operator's command, the Opto Controls control system opens the necessary valves, starts a series of feed pumps, and begins transferring water, caustic (sodium hydroxide), and sodium cyanide into the elution make-up tank. A large diesel burner is started, and the cyanide solution is circulated through heat exchangers to bring the solution to a temperature of 80-90°C, after which, the solution is fed into the elution make-up tank.
There are several devices and many types of instrumentation used in this process, and control system connects to and monitors and/or regulates nearly all of them, including motors on the caustic and cyanide feed pumps, pressure indicators, temperature probes on storage tanks, air valves, and flowmeters. Of particular importance are the variable speed drives (VSDs) that regulate the pump motors. To ensure this equipment operates optimally and proper flow rates are maintained, several proportional integral derivative (PID) loops were created.
PID control in distributed architecture
With PID loop control, the difference between a process variable and the desired setpoint is measured, and the controller attempts to minimize any disparity by adjusting the process control outputs. In operations at Mogale Gold, the differences between the desired flow rates and the actual flow rates are measured, and the disparities are corrected to constantly adjust the speed of VSDs regulating the pumps.
Significantly, however, these PID loop control calculations are not performed by the controllers, but by remote processors local to the many racks of I/ O scattered across the Mogale site. The reasoning was that PID calculations are ongoing and processing intensive, so offloading these functions from the controller level to the I/O level would create a distributed architecture offering many benefits. Chief among these is a significant reduction in chances of a system-wide failure, because if the host PAC should malfunction, the independent I/O processors would continue to perform without interruption. At Mogale Gold, this means if one of the central PACs should somehow get knocked off line, all PID-dependent flow would continue to take place, and potential catastrophes like slurry overflow would be avoided.
PID control also plays a role in Mogale's cyanide-related processes. During the CIL and elution processes, it is critical to avoid the conversion and release of hydrogen cyanide liquid as cyanide gas, which is extremely poisonous. The cyanide may begin to convert to its gaseous state through the acquisition of free protons. Therefore, during Mogale's gold processing, the free proton concentration of the cyanide is kept low through the addition of an alkali such as quicklime (calcium oxide) to ensure the pH level is maintained above 10.5.
Cyanide analyzers and pH sensors interfacing to analog input modules provide continuous readings, and PID loop control is used to perform the calculations needed for chemical dosers to add the appropriate amount of lime. As a safeguard, if the pH reading is less than 10, the cyanide will not be added, and the elution process is automatically suspended until the problem is rectified.
The Opto Controls control system also manages burners and monitors temperatures as the caustic/cyanide solution is heated, along with the pumps that bring the carbon and slurry from CIL to a measuring hopper. A level probe communicates through an analog input module to the controller, and carbon transfer is automatically ceased when the hopper is full.
During the next step, the caustic/cyanide solution strips the gold, making it easy to extract. The solution is electrolyzed, resulting in separation of the gold, which is then plated onto cathodes. The solution is then tested to determine the volume of caustic and cyanide required for the following batch. Air is used to purge the elution column of solution and the residual liquid is transferred back to the elution tank. The solid carbon is then transferred to a measuring hopper via transport water. With the elution column drained, it is now ready for the next batch.
Alarming and HMI
The slurry process takes from 18 to 20 hours, and the total elution process approximately 20. Full recovery and smelting averages about two days from tailings to gold, so smooth execution of each phase is extremely important. To help ensure this, numerous alarms were incorporated into the developed control programs.
For example, the established flow rates for the water, slurry, and chemicals used during various stages of the process are carefully monitored, and operators receive visual alarms on human-machine interfaces (HMIs) viewable on PCs and operator interface terminals located throughout the site. Upon receiving these types of alarms, the operators can then adjust pumps, valves, other devices, and instrumentation appropriately.
There are also alarms established for when pH levels drop too low, and for equipment failures and safety spray stations. The Opto Controls programmers used many conditional statements to define the functionality and "next steps" of the controllers whenever alarms are triggered.
Historically, throughout South Africa (and the Rand in particular), mining has had an adverse effect on the environment. In this regard, the Mogale project has had the added benefit of correcting or offsetting this type of negative impact. Mintails' removal of the gold dumps will eliminate the dust that blows off the pilings and will significantly decrease surface and groundwater pollution. Also, by clearing the land, Mintails is effectively providing valuable new real estate for development all over the West Rand.
Future plans: Mogale Gold 2
Ultimately, Mintails' gold recovery activities on the Rand will process some 322 million tons of surface tailings. As stated in the company's annual report, Mogale Sands Plant (Mogale Gold 1) production for the 2009 financial year was 1.7 million tonnes, producing 571 kg (18,365 oz) of gold.
Construction of a second gold processing plant adjacent to the existing one is nearly complete, and this facility recently produced its first gold bar. Once the plant is fully operational, it is estimated Mogale Gold 2 will treat up to 350,000 tons per month of tailings for gold extraction.
ABOUT THE AUTHOR
Benjamin Orchard (firstname.lastname@example.org) is an Application Engineer at Opto 22 specializing in consulting, specifying, and implementing hardware and software for industrial control, monitoring and data acquisition projects. He holds a B.A. in building engineering management and has more than 20 years experience in the automation industry.