“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.”
“Yes,” Elara said. “Because if we don’t, the cyclones will blind off in three hours from the fines overload. Then we’ll spend four hours washing them out. Lower throughput now means higher availability later. That’s the trade-off statistics taught us.”
Elara calculated the correlation coefficient between feed rate and product fineness. It was -0.85. Strong, negative, and ignored. Statistical Methods For Mineral Engineers
The control room fell silent. A junior metallurgist raised a hand like a schoolboy. “So... we should intentionally lower throughput?”
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. “You’re chasing your tail,” she said
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.
Gus blinked. “Speak English.”
“Here to fix what ain’t broke, Doc?” he grunted.
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. But the lag in your feedback means you’re
There, the problem was different. The mill power wasn't erratic—it was stubbornly stable. And that was worse. Because the cyclone overflow particle size (the % passing 75 microns) was drifting downward, slowly but surely. The shift supervisor kept increasing the mill feed rate to compensate, chasing the tonnage target.
Elara was the site’s mineral processing engineer, but her secret weapon wasn't a froth flotation cell or a high-pressure grinding roll. It was a battered copy of Montgomery’s Introduction to Statistical Quality Control and a stubborn refusal to trust averages.