When we think of large pieces of land, we tend to put it in terms of football fields or acres. To the copper mining industry, these units are far too small when a heap leach extraction pad can be a mile long and half-a-mile wide. Heap leach extraction captures the copper from the ore by leaching it out with acid (called raffinate) in a chemical process, and it’s often used instead of smelting. The tricky part is covering between 10- and 14-million square feet of crushed copper ore evenly with the raffinate.
Alena Johnson talks about how one mining company solved the problem in her article in the October 2018 issue of Mining Engineering: Improved Flow Measurement and Control are Key to Efficiency. As she points out, cost-effective production is all about even raffinate distribution. The massive pile is divided into 144 sections, or modules, roughly 400 x 200 feet. Each module is covered by a network of drip irrigation tubes to distribute the raffinate.
The biggest challenge is getting the raffinate distributed uniformly over the entire ore bed, which can approach 10 million sq ft. Ideally, the flow should be a constant trickle of 0.002 gpm/sq ft, spread evenly edge-to-edge over the enormous surface. Theoretically, the drip irrigation system can provide the necessary uniformity, but only when the flow to each segment is nearly perfect.
Alena goes into much more detail, which makes for interesting reading if you try to imagine covering millions of square feet with the precision of 0.002 gpm. For the company the nightmare was trying to get that kind of precision without any instrumentation. Needless to say, it wasn’t going well.
Most sites try to balance the distribution manually, using operators walking the pile watching for flooded and dry spots. They move from module to module, tweaking flow-control valves and evaluating the flow visually. Since regenerated raffinate is fed to the entire pad from one enormous pipe, changes in one area affect the entire system, so by the time the operators reach one end, they have to start again, evening out all the earlier changes. Having operators walking around on an acid-soaked pile more than 100 ft above the ground presents serious safety issues since the edges are often prone to landslides.
Since the entire pile is divided into 144 modules and each module has its own feed pipe, any instrumentation solution would have to work at that level to be effective.
The strategy called for monitoring and controlling liquid flow at the point where it feeds the individual module using a portable instrumentation skid, but no existing design was available. Something new would have to be created, and the company brought the challenge to Emerson.
Again, read the whole story to get the details, but in a nutshell, working with Emerson, the company built 144 individual portable skids, each equipped with a Rosemount 3051SFC flow meter, a regulating valve and a Rosemount 3051S pressure transmitter, all equipped to send their data via WirelessHART. Because there’s no wiring, the skids can be moved anywhere on the pile as needed. Control room operators can now look at a screen showing all 144 individual modules across the entire pile, and know exactly how much raffinate is being fed to a given area. If a module is still too high or low in terms of raffinate level, an operator can adjust the regulator, which still has to be done manually, but any change can be seen immediately. The result:
The company created a quality score system based on an ideal pressure target using very strict performance expectations. A deviation of 0.05 psi from the ideal value at the skid causes a 1 percent score deduction. With so tight a measure, the plant thought it would be an accomplishment to run at better than 80 percent across the site. After using the new system for a few months, they routinely scored higher than 87 percent.
With 144 zones and millions of square feet, that’s an impressive improvement. You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Pressure Group and other specialty areas for suggestions and answers.
Maurizio De Francesco