APC conquers long flash furnace lag

Paul Studebaker is chief editor of Control. He earned a master's degree in metallurgical engineering and gathered 12 years experience in manufacturing before becoming an award-winning writer and editor for publications including Control and Plant Services.The Vale INCO Copper Cliff Smelter in Sudbury, Ontario, Canada, sports the western hemisphere’s tallest chimney (380 meters) as well as one of advanced process control’s more interesting challenges, control of slag silica by flash furnace input, with a four to 10 hour lag time. In the flash furnace, a dry solid charge (DSC) of concentrate, sand and revert slag is flashed with oxygen to form a layer of matte, a slag of iron and silica, and sulfur dioxide gas.

“Slag is the biggest smelter product—approximately 70% of the DSC becomes slag,” said Gerry Seguin, senior automation specialist, Vale INCO, to attendees of his session, "Application of Advanced Process Control (APC) to Flash Furnaces," at the Schneider Electric CONNECT 2016 user conference this week in New Orleans.

Slag silica is controlled by the amount of sand in the charge, and controlling the slag silica content is important because it affects the amount of metal lost to the slag, the furnace integrity, the potential for slag leaks and the potential for injuries while cleaning the slag chute. “At 36% SiO2, the slag runs well and we can clean the skimming chute with one person. At 47%, it takes three strong people, picks and a sledge hammer, which is hard on the operators and can damage the chute. It also takes time, so we may have to slow or stop the furnace,” Seguin said.

The flash furnaces have three main control variables—slag temperature, slag silica and matte grade—and they are interrelated. Slag temperature is controlled by modifying the oxygen-to-DSC ratio, which also impacts the matte grade. Swings in matte grade have an impact on slag silica, and slag temperature is critical to furnace operation.

“Slag silica is critical for slag quality and for slag metal losses, and it is the most complex control loop,” Seguin said. One controller is used for two furnaces, and each furnace has independent and significant disturbances (i.e. slag returns). Slag temperature and slag silica are both measured while skimming, so the dead time is very long—four to 10 hours depending on bin levels and feed rate.

APC offers the opportunity for better control, allowing the plant to operate closer to specification with fewer and smaller excursions for improved safety and equipment life. On the flash furnaces, model predictive control was applied to the slag silica composition to set the DSC. “The risks were that we would have larger swings in slag silica and furnace temperature, so we added limits,” Seguin said, “But it didn’t happen, there were no swings.”

Slag temperature, slag silica and matte grade are now all meeting setpoints, without significant increase in variability. “For slag silica, this is especially significant,” Seguin said. “Maintaining the setpoint of 37% minimizes slag viscosity and metal losses.”

As a result of running on setpoint, “Flash Furnace 1 average slag silica increased by 1%, equivalent to metal value of $8,000 per day. Flash Furnace 2 average slag silica increased by 1.5%, equivalent to metal value of $12,000 per day,” Seguin said. That’s at January 2016 metal prices. “We implemented them in January, which is the best time of the year for furnace operation because of the weather and cooling water temperature. Now we’re coming into a more challenging time of year,” he added.

The APC application continues to evolve. “We’re adding logic for additional situations as they’re encountered,” Seguin said. “Going forward, we’re improving the feed belt scale accuracy, working on the analysis lab service level, and adding a second temperature reading during each skimming.”