Cleaning up one of Tasmania’s dirtiest jobs

Zinc smelting began in 1917 on the site of Nyrstar’s Hobart plant in Tasmania. 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 moulds — 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 600 °C 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 kilogram zinc and 9 kilogram zinc alloy ingots.

“We make special high-grade, 99.995% pure zinc and EZDA, a zinc alloy,” says Kupsch. “It’s used in galvanising, alloying and die-casting, in battery casings, car panels — even zinc sunscreen cream. Most now goes to China and India.”

In Australia, Nyrstar also operates a smelter at Port Pirie, South Australia, and is by far the biggest zinc producer.

The process of producing the ingots involves pumping the molten metal from the furnace into pouring bowls on the four casting lines. Until robots were installed, a pneumatically controlled system then poured just enough metal into each mould 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 full-time 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 and overalls) with a rake,” says Kupsch.

“We got quite a bit of reject-weight zinc,” says Kupsch. “Imagine, pouring a 10-litre bucket of water into a mould in six seconds, repeatedly, without splashing. That’s quite difficult.”

Each robotic cell comprises an automated servo-control system for the pouring bowl, an ABB ARB4400 robot with 1.95 m reach and 60 kilogram payload, a vibratory conveyor for the dross, and lasers which check the zinc level in the moulds to adjust the pouring system.

The project cost $3 million; the robotic component about $1 million, says Kupsch — and an awful lot of development and testing.

“The new and existing equipment in the plant communicate seamlessly, through DeviceNet” says Lewis Australia’s Senior Project Engineer Graeme Little.

“The existing Allen Bradley PLCs and touch screens have been upgraded to run ControlLogix Version 16,” says Little.

“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,” says Kupsch. “Typically, in the robotics industry you don’t have a moving target.”

Previously, two ABB IRB 6600 six-axis robots, with 200 kilogram payloads and 2.75 m reach, were commissioned in 2007 for stacking ingots. So familiarity was one driver for choosing ABB equipment.

The first new cell went in at Nyrstar in February 2008, the last in mid July.

“It was a staggered process”, said Kupsch. “You can’t just walk into a hot zinc area whenever you like — there are permits, risk assessments, job safety analysis, that need to be completed. In fact, the installation window was only four days for each robot.”

Eliminating manual skimming was a key benefit in itself — but Nyrstar also was looking for quality gains, says Kupsch.

“Overall, we’re seeing a 60% decline in reject-weight ingots and we’re aiming for the project deliverable target of 85%. 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 moulds and interferes with the shape of the slabs. Now we have a clean, smooth consistent size product. The robots will pay for themselves within two years.

“We’ve considered having the robots perform other functions, such as mould spraying and wire buffing, for example. We’ll discuss that in the second half of next year.”

ABB Australia Pty Ltd
www.au.abb.com