by steve fiscor, editor-in-chief
The only thing worse than losing magnetite is losing clean coal. Both are unnecessary expenses that can be prevented with regular inspection and maintenance. Drain-and-rinse screens play a pivotal role in magnetite recovery in heavy-media prep plants. They use a set of water sprays to wash the coarse coal as it passes across the screening media. A wet-drum magnetic separator is then used to recover the magnetite from the slurry that flows from the drain-and-rinse screens.
While magnetite losses are often blamed on the magnetic separator, they could be occurring in another area. Magnetite could be lost in multiple stages of the coal beneficiation process. The drain-and-rinse screens, for example, are trying to wash as much magnetite off the clean coal as possible. If they do not have enough water, however, the magnetite could be lost to the clean coal stream. Ideally, most prep plant operators make sure the sprays on drain-and-rinse screens operate correctly with lots of water to ensure all the magnetite finds its way to the slurry so the wet drums can do their jobs.
Wet-drum, magnetic separators haven’t changed much since the self-leveling tanks were introduced more than 10 years ago. They were matched with 950-gauss, interpole magnetic elements. “We have made some minor improvements to the separators, but in general, the wet drum that we had 10 years ago is pretty much the same device that is currently in use,” said Jose Marin, director-minerals and materials processing for Eriez. “We are always looking at future improvements as we discover new magnetic elements. If we feel stronger magnetic elements will be beneficial to the coal preparation process, we will consider adopting them.”
By matching the proper magnetic element to a particular tank, Eriez allowed prep plant operators to replace aging double wet-drum magnetic separators, which were typically concurrent to counter-rotation wet drums. “Many of those double drums were replaced by a single, wet-drum system and they provided the same performance or better.”
As a result, most modern prep plants today operate single-stage magnetic separators. “These units perform well as long as they are sized properly, meaning that they are designed for a certain hydraulic throughput, magnetic loading and percent solids,” Marin said. “There are other variables that could influence design, but many times, when the coal company is building the plant, they do not know those variables yet.”
One of those would be the coal-to-magnetite ratio in the slurry. Having more coal fines than magnetite in the slurry could create a great deal of interference at the magnetic separator and the efficiency of the separator would drop. Coal fines should not exceed 40% solids in the slurry. Anything higher could interfere with the separation process.
The engineers at Eriez size the separators using a set of guidelines that have been established over the years. “As long as the wet drums are operating under those conditions, the recovery of magnetite will average 99.98%,” Marin said. “Typically we will see less than 1 gram per gallon of magnetite in the effluent.” A lot of coal fines in the slurry usually signals another problem upstream and modern prep plants have the means to control that.
Steady Improvements Over the Years
The combination of the self-leveling tank and the 950-gauss interpole magnetic element gave prep plant designers the ability to eliminate one stage of separation. That was a major leap. Once the equipment is installed today, it operates reliably until a bearing replacement or wear wrap repair is scheduled. “More recently, we incorporated an easy adjustment for the operating gap and the discharge gap on the drum,” Marin said. “We have also added a window on the tank so the technicians can open it and easily locate the last pole of the magnetic element and measure the discharge gap for the magnetite. That should help the maintenance crews reset the wet drum to its location prior to maintenance.”
Wet drums have three drive configurations: the conventional chain-and-sprocket arrangement, a belt drive, and a direct drive. “The direct drive is the most desirable in terms of reliability,” Marin said. “There is no need to perform maintenance on the chain or the belt. It’s a n