Optimising iron and steel waste
Solutions for waste recovery have gained impetus in recent years. The Waste Recovery in Ironmaking and Steelmaking Processes event, held in London, UK, symbolised the importance of sharing knowledge in the sector. Ledetta Asfa-Wossen reports
Steadfast members of the steel and iron industry gathered to resolve the ongoing issue of waste under the flapping banner of Roosevelt’s mantra. ‘To waste, to destroy our natural resources, to skin and exhaust the land instead of using it so as to increase its usefulness, will result in undermining in the days of our children...’.
The two-day IOM3 conference on Waste Recovery in Ironmaking and Steelmaking Processes, held in London, UK, from 13-14 December 2010, presented a mixture of debate and tangible solutions. However, it was universally agreed that the steel and iron industries were in need of both a change of mindset and further education on how to tackle and use waste as both a sustainable resource and commodity.
Waste or asset?
Kevin Linsley of Tata Steel RD&T gave attendees some food for thought, stating, ‘In the 1850s ore mined from the Cleveland Hills contained ~30% FeT but was considered prime raw material. [Now] some gas cleaning slurries contain ~60% FeT, but in some instances are considered as a waste and are disposed of as such. Is it really a waste or simply an example of a wasted opportunity?’ he asked.
Dr Paul Brooks of Tata Steel Europe, explained that the psychology of dealing with waste needed to be tackled in Europe and that waste should be seen as a valuable substitute for raw materials.
According to the World Steel Association, iron ore based steelmaking accounts for approximately 70% of world steel production. The most common forms of byproduct generated are slag, sludges, dusts, scales and fines. Exploitation of these byproducts can create savings in the recovery of iron and other valuable units, reducing mineral costs and minimising landfill charges.
To highlight the incentives for the proper disposal and use of iron and steel waste, Brooks revealed an estimation of hazardous waste landfill costs for Tata Steel for 2010/2011. Costs were divided as transport/other £10/t, 8%; gate fee £30/t, 23%; lost value in use £40/t, 31%; and tax £48/t, 31%.
Yet delegates voiced concerns about the lack of information on processes and technology to further improve waste use by industry.
A ‘zero waste concept’ to avoid landfill and ‘optimise’ waste was put forward by a speaker from Harsco Metals group, Surrey, UK, as an achievable goal for companies. The concept is based on recycling byproducts internally or converting them into products that can be marketed, using a combination of technologies to recover the value in these resources.
Methods of exploiting waste that were discussed included briquetting. An example of this is at the US Steel Kosice Slovakia site, which processes over 55,000t of dusts, ores and scales each year to use as a basic oxygen steelmaking coolant.
Sludges, dusts and scales can all be briquetted and returned to iron and steelmaking furnaces. Sludges can also be processed to reduce the moisture content. Dusts and scales are then introduced to produce a base mix. Various binders are finally added to ensure product strength. The resulting product can be charged via a blast furnace (BF), blast oxygen furnace (BOF) or electric arc furnace (EAF), increasing the internal recycling rate.
Another technique for optimising iron and steelmaking is micro-pelletising byproducts, to produce a single homogeneous blend for improved sinter bed permeability.
The use of EAF dust in the cement industry and in ceramic mixtures for building applications was also explored and presented by researchers from Isfahan University in Iran.
The right terminology
A delegate highlighted that there are problems with the term ‘waste’ and if, on a regulatory level, waste should be classified as a ‘residue’ or ‘byproduct’.
She stated, that ‘if you are using a material again internally, it would seem to be a residue and not necessarily a commercial “byproduct”’.
Another delegate also added that there needed to be more stringent criteria assigned to byproducts, that stated them as ‘fit for regulations’, as having ‘a positive market value’ and demonstrate that ‘the material is inert and has no adverse effect on human health’.
Regarding scrap waste, Jonathan Aylen of Manchester Business School, UK, urged attendees to create ‘a reclaimed materials market and stated that the steel and iron industry needed to upgrade the value of its waste, particularly scrap processing, which he said is ‘still a traditional industry and needs to meet modern needs’.
Aylen noted the problems with using reclaimed materials, such as ‘changes in design principles and...building regulations, and the risk associated with the structural integrity of reclaimed steel’. He added that to make best use of scrap as a sustainable commodity, ‘the costs must not be out-of-line with using new material’. To develop the use of scrap, Aylen said there needed to be agreed standards and a ‘realised value’ of scrap, with regards to alloy content, freedom from tramp elements and low residuals.
A recovery process from Japan that uses a high temperature rotary hearth furnace (RHF) to reduce steel dust into direct iron pellets (DRI) created intrigue at the event. The furnace, combined with a hot briquette machine, is said to have an estimated treatment capacity of 220kt pa.
Daisaku Johbe of Nippon Steel Engineering explained the pellet ting technology offers low running and equipment costs with a zinc removal ratio of 80-95%.
He stated that the ‘zeroemission’ system operates using a waste heat recovery system, which uses a combination of a boiler and heat exchanger for exhaust gas.
The DRI pellets can be used as a supplement to scrap steel and iron ore as the main raw material. The removed zinc can also be used as a part substitute for pure zinc ore.
David Deegan of Tectronics, Swindon, UK, presented attendees with a treatment method for steel plant waste that uses plasma-assisted carbothermic reduction.
The technique involves a thermal plasma that emits intense UV light and cracks/ reforms the organic matter, and melts or vitrifies inorganic matter into an inert low leaching aggregate that can be used for building applications. A 95-98% recovery rate can be expected from the patented technology, claims Deegan.
‘For plain carbon steel, the primary objective is to recover zinc from the waste as zinc oxide, while upgrading the ironbearing material for internal recycling. For stainless steel dust treatment, the primary objective is the recovery of chromium, nickel and molybdenum values as a ferroalloy’, he noted. By applying this technique, said Deegan, ‘the heat input is independent of process chemistry and the system is readily retro-fittable for existing plants’.
A combination process for recycling EAF dust and steelmaking residues into nickel-chromium alloys and zinc was introduced by Both Ingo of Paul Wurth, Luxembourg.
The EAF, called i-Meltor, has been designed for reducing melting, settling and fuming of slag and residues. It operates with bottom gas stirring and central charging between three electrodes.
The technology, added Ingo, can be used as a part/or stand alone system. The main components include a central charging duct, graphite electrodes and water coolers – in the form of spray coolers that increase the lifetime of the refractory lining in the slag zone. It also uses an offgas cleaning system.
Steely solutions for de-oiling
Problems associated with using byproducts are common. One difficulty in treating sludge and scales in particular is the oil content. Further still, dumping highly oily sludge induces costs and liabilities.
A process for de-oiling steel mill sludge and recycling it back into iron ore for sinter plants could offer a viable solution.
‘Common practice is to recycle oily scales at sinter plants but due to severe dioxin emission limits, as well as the deflagration risk in the ESP filter, recycling is limited,’ added Ingo.
‘Some operators intend to recycle internally oily sludge in BOF’s in form of briquettes but this recycling disturbs the BOF process and impacts on the steel quality. The popular process is the pyrolysis, yet this is operated at high temperatures (above 500°C) and requires a high energy consumption’.
The Paul Wurth Lhoist (PLD) deoiling process, he says, uses an auto-thermal system operated in a multiple hearth furnace, removing organic compounds from oily residues at low temperature by using lime (see image).
He explains, ‘The process is based on a close mixing between lime and sludge and controlled without any additional energy supply. The three main parameters for controlling de-oiling efficiency are lime dosing, air injection and residence time.’
Post-combustion air is injected hearth by hearth in order to smoothly control the temperature profile and lime that quickly dries the sludge and enables hydrocarbon oxidation at low temperatures.
This technique can allegedly process products with up to 20% of hydrocarbon and achieve a final oil content below 0.1%.
Shedding light on its energy consumption benefits, he added, ‘the off-gas cleaning system is adapted in order to avoid emissions such as CO2, volatile organic compounds, sulphur oxide and dioxins’.
It is clear financial constraints placed on industry have pushed metal waste recovery further up the agenda, spurring on advancements in technology and treatments of byproducts.
Nonetheless, crucial discussions between steel and iron making operators and technology developers are needed in working towards a more sustainable and profitable supply chain.