The daily average? It had dropped to 1,150 tonnes per hour. But the shift tonnage—the real money—was actually up 5% because the mill never stopped.
“For the last six hours,” she said, pointing to a string of seven points all below the centerline, “we have been running fine. But this run of seven points all below the mean? That’s a Nelson Rule violation. It’s not out of control statistically, but the probability of this happening by chance is less than 1%. It’s a trend. The mill is grinding finer because the new media supplier’s ball hardness is different. We need to back off the feed rate now—not in two hours.” Statistical Methods For Mineral Engineers
“You’re chasing your tail,” she said. “The crusher power draw spikes, you back off. It drops, you tighten. But the lag in your feedback means you’re always reacting to what happened five minutes ago. By the time you fix it, the feed has already changed. You’re creating the instability you’re trying to solve.” The daily average
At the end of her shift, she walked back past the primary crusher. Gus had taped her run chart to his console. He wasn't touching the CSS. The belt scale’s one-minute readings were still noisy, but the variation had narrowed by half. “For the last six hours,” she said, pointing
“Here to fix what ain’t broke, Doc?” he grunted.
She drew a Shewhart control chart on a whiteboard in the control room. Upper control limit. Lower control limit. And in the center, the target P80 of 150 microns.