Zinc smelting at Nyrstar’s Hobart plant in Tasmania began in 1917. For generations it has involved hard, dirty and risky jobs.
One of the toughest, skimming the waste ‘dross’ off molten zinc just poured into ingot molds, was done by hand, with a rake, until four ABB industrial robots took it over in 2008.
Now the work of 16 men, who sat beside the 600ºC molten metal around-the-clock in 30-minute spells, over four shifts, is automated, and Nyrstar is producing cleaner, smoother, correct weight ingots with unprecedented consistency.
Nyrstar Senior Project Manager Michael Kupsch led 40 people, from Nyrstar and systems integrator Lewis Australia, who installed the robotic cells on four lines producing 25 kg zinc and 9 kg zinc alloy ingots.
“We make Special High Grade, 99.995% pure zinc and EZDA, a zinc alloy,” Kupsch said.
“It’s used in galvanizing, alloying and die-casting, in battery casings, car panels, and even zinc cream.”
In Australia, Nyrstar also operates a smelter at Port Pirie, South Australia, and is by far the biggest zinc producer.
The process
Molten metal from the furnace is first pumped into pouring bowls on the four casting lines.
Until robotization, a pneumatically controlled system then poured just enough metal into each mold on a conveyor, and operators raked off the waste ‘dross’, for re-processing. Pouring speed could be changed, manually, to improve consistency, but the process was complex.
“Four fulltime operators each shift just sat beside the conveyor, for 30 minutes at a time, in cocoons of safety clothing, (hard hats, face visors, hoods, gloves, coveralls) with a rake,” Kupsch said.
“We got quite a bit of reject-weight zinc. Imagine, pouring a 10 L bucket of water into a mold in six seconds, repeatedly, without splashing. That’s quite difficult.”
Robotizing the process was “like putting an SL500 Mercedes engine into a Model T Ford,” he said.
The project
Each robotic cell comprises: an automated servo-control system for the pouring bowl; an ABB ARB4400 robot, with 1.95 meter reach and 60 kg payload; a vibratory conveyor for the dross; and lasers which check the zinc level in the molds and adjust the pouring system and testing.
The project cost $3 million, the robotic component about $1 million, according to Kupsch, and an awful lot of development and testing.
“The new and existing equipment in the plant communicate seamlessly, through Devicenet,” Lewis Australia senior project engineer Graeme Little said.
“The existing Allen Bradley PLCs (Programmable Logic Controllers) and touch screens have been upgraded to run Contrologix Version 16.
“Each casting conveyor has a robot tracking system matched to the robot via an ABB IRC5 robot controller.
“This was a particularly complex robotics application. Typically, in the robotics industry you don’t have a moving target.”
The installation
Two ABB IRB 6600 six-axis robots, with 200 kg payloads and 2.75 m reach, were commissioned in Hobart in 2007, for stacking ingots.
“We completed full workshop set up and testing at our base in Melbourne, before we started bringing over the cells,” Kupsch said.
“The only thing we couldn’t test in our workshop was molten zinc.”
The first new cell went in at Nyrstar in February, 2008, the last in mid July.
“It was a staggered process,” Kupsch said.
“You can’t just walk into a hot zinc area whenever we like; there’s permits, risk assessments, job safety analysis, a lot to get through.”
Nyrstar will do most ongoing work on the robotic cells itself, Kupsch said.
“But for specific warranty on software and components, Lewis or ABB will be coming back in to do the work on those — depending on the component.
The benefits Eliminating manual skimming was a key benefit in itself, but Nyrstar also was looking for quality gains, Kupsch said.
“Overall, we’re seeing a 60% decline in reject-weight ingots and we’re aiming for the project deliverable target of 85%,” he said.
“People who mind the stacking end now look after the pouring end as well.
“The new pouring bowl system prevents ‘flash’ — splashed metal which cools on the sides of molds and interferes with the shape of the slabs. Now we have a clean, smooth consistent size product.”