Turning muck into brass
An award-winning company has been quietly converting solid waste from iron and steel-making into aggregates used in road building, soil drainage and water treatment. BY GAVIN RILEY.
Though solid wastes have always been an inevitable part of iron and steel-making, staff at New Zealand Steel’s Glenbrook mill refer to them respectfully these days as “co-products” rather than “by-products”.
That’s because around 550,000 tonnes of the 700,000 or so tonnes of solid waste generated annually at Glenbrook have achieved value by being recycled, re-used or sold.
The main co-product of the iron and steel-making process is a non-metallic residue called slag. More than 300,000 tonnes are created at Glenbrook each year – 250,000 tonnes of iron-making or melter slag and 70,000 tonnes of steel-making slag.
For more than a quarter of a century it has been possible to process the majority of New Zealand Steel’s slag so it has a range of uses as aggregates. Given today’s demonstrated need to conserve natural resources, it is of significance that processed slag has reduced the need to quarry for alternative materials such as scoria and river gravel.
The company carrying out this important processing is SteelServ Ltd, owned jointly by New Zealand Steel and Multiserv, an international mill-services provider which handles more than 20 million tonnes of slag products a year.
SteelServe is the successor to the Slag Reduction Co (NZ) Ltd, formed in 1980 as a joint venture between the UK-based Slag Reduction Co and New Zealand Steel, and winner in 1993 of a government environmental award and a similar award the following year from the Aggregate and Quarry Association.
When Multiserv acquired the UK company’s international interests in 2002, the name of the New Zealand business was changed to SteelServ.
SteelServ, with a staff of about 100, is responsible for a full range of mill-servicing activities common to many Multiserv sites, as well as the processing and marketing of the slag aggregate products.
SteelServ inherited stockpiled iron-making slag at Glenbrook totalling some 650,000 tonnes. It has been reducing this stockpile steadily for four years in addition to receiving New Zealand Steel’s 250,000-tonne annual output of this slag. From this vast total SteelServ produces more than 300,000 tonnes of aggregate products a year.
The process of creating such products starts by air cooling and solidifying the hot molten slag in excavated pits, followed by water sprays just before dig-out. Once it has cooled, slag is broken up and transferred to a stockpile where it is left to weather from six to 18 months.
From there on the process is similar to normal quarry operation, with the exception that any residual metallics are removed during the crushing and screening process.
A broad range of standard aggregates is produced for a variety of traditional aggregate uses such as chip seals, asphalt manufacture, drainage and filter materials, and basecourse and sub-base products.
Iron slag sold as drainage material has been used by Auckland Regional Council throughout the area in the construction of football fields, in bowling greens at Howick, in an all-weather hockey field at Pukekohe, and in greens at the Whitford Park, Pakuranga Country Club and Formosa golf courses.
Steel-making slag or KOBM, which is a very different material, has been used for many years as a road stabilisation material, as well as a soil conditioner (particularly by maize growers) and more recently as an additive in cement manufacture.
SteelServ’s aggregate-production capability has been substantially upgraded in the past three years following the appointment as aggregates manager of Hendrik Wortman, formerly with Fletcher Building.
SteelServ now has state-of-the-art chip screening and washing operations, as well as custom-designed process routes to prepare the slag material for its variety of aggregate uses.
Slag aggregates, apart from offering sound engineering properties and excellent skid resistance, also have excellent attributes for water treatment. Blast-furnace slags have been used in Britain since the 1960s in wastewater-treatment plants to remove phosphorous, and New Zealand Steel’s melter slag exhibits similar performance levels, as well as the ability to reduce chemically a wide range of heavy metals.
SteelServ, in conjunction with a number of New Zealand-based scientific researchers, has been conducting extensive studies into melter-slag’s attributes, including its use to reduce phosphorous in dairy-farm runoff and from groundwater in creeks that feed into lakes, in addition to dosing degraded lakes themselves.
New Zealand Steel has installed two melter-aggregate stormwater filters to “polish” the site stormwater before discharge, chiefly targeting zinc and aluminium. Work is now being undertaken to trial the material in a wider range of stormwater applications and in specialised uses such as acid water and heavy-metals retention in the mining industry.
“SteelServ is leading the Multiserv organisation, and possibly the entire slag industry worldwide, in developing water-treatment end uses,” says SteelServ’s head of sales/marketing and business development, Bill Bourke.
“We’ve put a lot of work in over the past five years in exploring this market and taking part in tests and trials. As a result the slag industry internationally is taking note.
“The attributes of slag aggregates for this purpose have been understood for many years but not pursued with vigour. What the industry has done is take the easy option and produce roading aggregate, whereas we looked for sustainable competitive advantage – and water treatment has that.”
After 21 years with New Zealand Steel, Bourke joined the company in 2000 with a directive to find additional market outlets that would help reduce those growing stockpiles.
Today, aided by securing new markets and by New Zealand’s road-building boom, SteelServ is not only shrinking the stockpiles but is processing and selling all New Zealand Steel’s annual melter-slag output. Contracts have been signed with two major producers for the supply of asphaltic aggregates, with Transit New Zealand recognising them as a qualifying surface material and regional basecourse.
Aggregates manager Wortman is pleased with the progress made under his stewardship – but aware also that there can be no let-up.
“In essence, SteelServ’s operation is about the size of a small regional quarry,” he says. “But we have to be up with the best in the business in terms of aggregate manufacture and quality control if we are to realise our sales targets and consolidate the market niches we have developed over the years.
“Remember, our source rock arrives 24/7, 365 days of the year. It’s like having a continual lava flow in your back yard. It’s coming, ready or not. If the market slows down you can’t just leave it in the ground.”
Wortman says SteelServ’s programme is one of continuous improvement.
“Along with other aggregate producers we will be examining ways of enhancing our basecourse material’s rutting resistance, as well as refining the product further for skid resistance and other surfacings,” he says.
“We are also strongly pursuing water-treatment initiatives as it is a market where our aggregates can offer some advantages over traditional filter materials.”Q&M Vol.3 No.6 Dec-Jan 2006
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