‘Waste recovery in ironmaking and steelmaking processes’ 13 and 14 December 2010

Session Chairmen: Mike Copeland and Kevin Linsley

This session was opened by Dr P. Brooks, Group Director Environment, Tata Steel, with his keynote paper; ‘Is waste just a dirty word or an asset to be exploited?’ The paper gave an insight into some of the legal, financial and technical drivers and barriers for increasing waste utilisation. The paper also introduced the psychology of dealing with wastes. Dr Brooks stated that many manufacturers often see waste as exactly that; ‘something inevitable arising from the process route that can be tolerated and which is handled by someone else, either inside or outside of the organisation’. Also, and to make matters worse, he considered that the costs of handling waste are rarely transparent. To highlight disposal costs, Dr Brooks gave a breakdown in landfill charges for Tata Steel UK for 2010/2011 as 8% transport, 23% gate fee, 31% lost value and 31% government tax. With landfill charges of £80/t and upwards, depending on possible hazardous content, the 31% lost value can mean a significant saving to the organisation. Dr Brooks continued by arguing that there is a need to treat some wastes as a ‘secondary’ raw material as primary resources are depleted or competition for these resources increases and he gave an indication of some resource availability, for example coal at 150 to 200 years. In conclusion, he stated that in order to recover valuable iron units, reduce mineral and raw material costs and minimise landfill, there is a need to improve the usage of by-products from coke making, slags and flue dusts from ironmaking and steelmaking and sludge form casting and rolling.

The second paper in this first session, ‘Helping iron and steelmakers towards the zero waste goal’, was given by Daniel Devid of Harsco Metals Group Ltd. For the steelmaker, zero waste or resource recovery translates into greater efficiency and through recovery of by-products; significant tangible economic benefits can be achieved. This is the philosophy behind the recovery process developments and specific engineered solutions provided by the Harsco Metal Group. Mr Devid stated that according to the World Steel Organisation, every tonne of steel produced generates 200 kg of by-products via the electric arc furnace (EAF) route and approximately double this figure from the blast furnace (BF)/basic oxygen furnace (BOF) route. Environmental regulations, as well as the economics associated with the process are making land filling of such by-products an increasingly expensive option. The Harsco Metals Group has developed a variety of proprietary technologies to solve many of the problems created by the by-products of the steel industry. Slags, dusts, fines, slurries and various scales are all processed to recover the valuable iron units from the carbon steel route and Ni and Cr units from the stainless steel route and these include; micropelletising, plasma smelting and adding value (valorisation) to otherwise waste slag. Three specific examples were given: (i) the processing of all slag, dusts and sludges for Hadeed in Saudi Arabia, (ii) briquetting/pelletising of dusts and sludges for US Steel in Kosice, Slovakia. From this technology, Harsco have developed a specialist additive mixture that can be recycled into the cement making industry, and (iii) the smelting of stainless steel dusts in a DC plasma arc furnace (termed the Plasminox process) at TKS Terni, Italy. Mr Devid concluded by stating that ‘the positives of resource recovery of by-products extend beyond simple economics, recycling and recovery of steelmaking by-products greatly reduces the environmental impact of steelmaking in a number of ways by reduced land fill of materials, reduced usage of natural resources, reduced transport footprint and reduced CO₂ emissions’.

The third paper in session 1, entitled ‘By-products in iron & steelmaking – troublesome materials or secondary raw materials?’ was given by Dr Peter Drissen of the FEhS Building Materials Institute of Germany. Recycled BF and steelmaking slags do not always meet the end-user requirements and there is also considerable scope for improvement in metal recovery and properties of ceramic material produced from slags. This has been the scope of the work of the Institute for a number of years. Dr Drissen’s presentation gave four specific examples of the treatment of iron and steel slag aiming at improved building materials, on metal recovery from slag and the internal recycling of steelmaking slag:

In his paper, ‘Recycling of solid wastes, Tata Steel’s experience’, Tapas Chakraborty reported that in the last five years up to 2009/2010, solid waste utilisation increased from 80·2 to 91·1% at Tata Steel Jamshedpur. As with most integrated works the sinter plant bears the brunt of the recycling. 92% of BOF sludge, 7·9% of BOF slag, 68% of flue dusts, 100% mill scale and 97% of mill sludge are recycled via the sinter plant with little or no pre-treatment or enhancement, other than being incorporated into the piles ready for sintering. Currently, BF sludge is not recycled due to its alkali material content, but recent efforts to lower the alkali input to the BF means that it is now possible to recycle a proportion of this material. Millscale and sludge recycling to the sinter plant has an oil contamination limit of 5% due to the potential for glow fires to occur in the electrostatic precipitator dusts. Cost savings by utilisation of wastes through the sinter plant are estimated at $2·66/t of sinter, equivalent to $222M for an 8 Mtpa sinter plant. Energy recovery was also thought important and as such Tata Steel, Jamshedpur is investigating the possibilities of selective waste gas recovery and heat recovery from the sinter cooler.

In the paper, ‘The plasma processing of steel plant wastes’, Dr David Deegan of Tetronics limited described the use plasma-fired carbothermic reduction furnaces to recover metals from EAF and argon oxygen decarburisation (AOD) dusts. Although the technology is aimed primarily at the stainless steelmaking production route it is equally applicable to the carbon steel route. The combined fly ash by-product from EAF melting, the AOD process and vacuum refining operations is typically in the range of 1·5 to 2·5% of the total metal production. In addition, significant amounts of residue materials are generated in finishing processes, such as grinding and flame cutting, which may represent a further 1·0 to 1·5% of metal production. These stainless steel plant dusts contain a range of valuable, but leachable toxic metal species including chromium, nickel, lead and cadmium, which prevent them from being disposed of in open land-fill sites. Typically these dusts contain 32%Fe₂O₃, 13%CrO₂, 13·6%ZnO and 3·5%NiO. As well as recovering the high value metals, hazardous wastes are converted into inert, vitrified products, which avoids punitive landfill liabilities; these vitrified products also have an end-user application. The recovered ferroalloy can be returned to the melt shop or, in the case of zinc and lead recovery, sold on as oxide products. The net economic benefit of the technology was quoted as typically £100 s per tonne of waste, with payback within five years.

In April 2010, the first European waste treatment facility based on rotary hearth furnace (RHF) technology was started successfully at the integrated steel plant at Lucchini Piombino Italy. The plant and process were described in the paper ‘RedIron technology for the recycling of iron-bearing residues at the Lucchini Piombino iron and steel plant’ given by Piergiorgio Fontana. The aim of the facility is a target of zero iron and steel plant waste, recycling all of the BF sludges and BOF dusts arising at the plant; totalling about 60 000 metric tpa (dry basis). The recycling plant is based on the Paul Wurth RedIron technology and consists of three main steps: (i) sludge drying, mixing and pelletising, (ii) iron pre-reduction and zinc removal at the RHF, and (iii) hot briquetting of the pre-reduced pellets. The hot briquetted iron is then recycled to the BF. The off-gas and briquetting system were recycled from another operation and a balling disk, rather than briquetting, was chosen as it is a one-pass operation. The plant uses 630 Nm³ h⁻¹ natural gas and the hot air flow is 7500 Nm³ h⁻¹. The operation costs are approximately €100/t. At the time of the conference, the plant was experiencing operational problems due to fouling within the recuperator exchange surface in the heat exchanger, caused by the condensation of alkalis within the off-gas system.

The final paper in session 1 entitled ‘The DK process; a proven technology for the sustainable recovery of iron and zinc from BOF dusts and sludges’, was given by Dr Carston Hillman of DK Recycling und Roheisen GmbH, Germany. DK Recycling und Roheisen GmbH was formed in 1991 to recycle sinter plant, BF, BOF and EAF dusts and BF and BOF sludges and today treats some 400 300 tpa of wastes from integrated steel plants within Europe. The wastes are recycled via the sinter plant. As the sinter plant has no mixing beds, the raw mix is made using weigh hoppers rather than beds. There is a secondary gas cleaning system installed at the sinter plant, which includes lime and lignite injection to aid SO₂ and dioxin reduction respectively. The plant is allowed to by-pass temporally the bag filter on start-up due to the high temperatures. The finished sinter contains 2%Zn and 0·06%Pb. DK operate only one BF that has a Zn loading of 38 kg/t of hot metal and an alkali loading of 8·5 kg/t of hot metal. Of the BF Zn input, 40% is lost to the flue dust, 4% to bucket conveyor dust, 56% as Zn concentrate (BF gas cleaning sludges) and <1% to the slag and pig iron. The Zn concentrate contains 65 to 68 wt-%Zn and 1 to 2 wt-%Pb. High quality pig iron is produced by the BF and this is cast into 8 to 10 kg pigs. The process has several limitations: (i) low particle size, which causes a loss in productivity, especially sintering, (ii) residues connected to the high emissions in the sinter plant, and (iii) zinc in the BF, which increases coke consumption. Alkali content, rather than Zn, limits the DK feed raw mix due to the swelling effect on pellets in the BF. Less than 1% of DK’s output is classified as waste and subsequently disposed off-site, electrostatic precipitator dust being the main waste and this is sent for disposal at an underground mine.

Session Chairmen: Mike Copeland and Richard Reasbeck

The keynote paper for the second session, entitled ‘Overview of baosteel waste management and utilisation’, was given by Mr Exefu Hu from Baosteel Headquarters. In 2009, Baosteel produced a total of 14·605 million tons of solid wastes including dusts, sludges, slags, fly ash and refractories. Of these BF slag, steel slag and Fe-bearing dusts and sludges were the three major wastes and these accounted for 80% of the total. The total utilisation rate was 98·4% of which 3·55 million tons (24·3%) were recovered for internal processing. 20·1% of the waste processing was of high value added products. Baosteel use hydro cyclones to produce low/high zinc fractions of the BF slurry. The low zinc fraction is reused in sintering and the high zinc fraction sent with EAF and BOF dusts to smaller external plants with smaller BFs that can utilise these materials. BOF slag is processed through the Baosteel BSSF process to effect free lime reduction in the product. Of the 200 k tons of used refractories, some are reused after internal treatment, but most are sold to refractory plant manufacturers for recycling. Baosteel plans to build up a production base with capacity of 22 k tons for used refractories, all the used refractories will be processed and recovered by Baosteel itself. In addition, Baosteel have recently announced that they are planning to invest 2·76 billion Yuan on solid waste management systems and processes.

Nippon Steel Engineering Co. Ltd (NSEC) RHF process for steel dust and sludge recycling was described by Mr Daisaku Johbe of NSEC in his paper ‘RHF process for dust recycling system’. To date eight plants have been supplied to treat a wide range of sludges and dusts with the agglomerated product in pellet, briquette or extruded form. The process is based around a rotary hearth furnace that consists of four zones, feeding, heating, reduction, and discharging. The hearth floor rotates at a constant speed carrying the agglomerates, in the opposite direction to the gas flow. Burners are set onto the walls and inject air and fuel to control the atmosphere and the temperature. The agglomerates on the floor are at first heated, reduced and on completion of reduction discharged into a DRI cooler. Zinc oxide in the agglomerates reacts with the charge carbon to produce zinc vapour that leaves the furnace with the off-gasses (80 to 90% of the Zn in the charge is removed). The off-gas dusts and condensates consist mainly of Zn and BP oxides and alkaline halides. As the latter are very viscous there is patented NSC technology to ensure no off-gas problems. Total process time was quoted as 10 to 12 min. At Kimitsu, three RHF furnaces treat 1800 tons of dusts and sludges to produce 1000 tons of DRI per day.

In the paper ‘The adaption of three pyro-technologies for recycling iron- and steelmaking residues’, Mr Ingo Both of Paul Wurth described three thermal based technologies for recycling. The use of the rotary hearth furnace technology RedIron^TM was described earlier in the conference in paper by Piergiorgio Fontana. This is a stand-alone reduction tool that produces DRI, but the process can be combined with a smelter in the RedSmelt^TM process. The Paul Wurth Primus® process is based on a multiple hearth furnace. Based on a conventional EAF, and using submerged charging, the i-Meltor^TM process can be used to process dusts and sludges from carbon, high alloy and stainless steel manufacture, slag and sludge from the ZU industry and, under development, solid residues containing valuable metals from spent catalysts. One problem of the treatment of oily sludges and scales is the possible formation of dioxins and the risk of VOC deflagration (slow burning of volatile organic compounds) in the off-gas system. To enable the treatment of these wastes, Paul Wurth and Lhoist have developed the PLD process, which is similar in principle to the Primus process except that lime is added to the oily mill scale and sludge producing a quick temperature rise that is followed by complete oxidation of the organic compounds at a temperature of <450°C. The gaseous product of the oxidation is CO₂ and H₂O and the solid product is high in iron and contains CaO.

At Tata Steel Strip Products, UK, process residues and wastes, identified to have potential for increased utilisation included scales and sludges arising from the steel rolling mills. The main parameter limiting the use of these wastes in the process is their relatively high oil content, which restricts their recovery in onsite processes, due to the potential increase in the propensity for glow fires in the sinter plant process. In her paper ‘Application of thermal desorption on integrated steel plant process arisings’, Mrs Fiona Abbott reviewed the work done to identify suitable technologies, plant scale trials with the chosen technology and future applicability. Early pilot plant Worf using thermal desorption had shown promising results and therefore a commercial direct fired rotary kiln was selected for the plant trials. The initial plant trial processed successfully 1000 m³ of mill process arisings, albeit with some problems with respect to waste handling and moisture control. The trial is ongoing.

MIDREX Technologies’ FASTMET^TM process was described in the paper ‘FASTMET^TM process – steel mill by-product recycling’ given by Mr Larry Shields. The process uses the MIDREX rotary hearth furnace technology to process iron-bearing by-products and iron ore fines into highly metallised DRI, of which three outputs are available, DRI, hot DRI, or hot briquetted iron. For feed materials containing zinc, the process produces a high quality zinc oxide product suitable for sale.

Landfill taxing, the ‘China Effect’ and the requirement for price stability and long term supply are quoted as the reason for an increase in the recycling of iron and steel plant refractories in the paper ‘Recycling of steel plant refractories’, given by Melvin Bradley of Minelco. The presenter stated that for successful refractory recycling a structured system must be in place to ensure used refractories can be segregated, transported away from the steel plant, and sorted to ensure any contaminants have been removed. Once a clean pile has been created, the material can be tested to ensure consistency and then transported to a processing plant in order to get it to sizes that a refractory manufacturer can use. Typical applications for recycled refractories include medium range castables and precast shapes, tundish spray and furnace gunning repair products. Other possible uses of reclaimed refractories are slag conditioning in EAF and ladle slag chemistry.

Ms Cristina Lausin from Global ArcelorMittal R&D, presented the final paper of the session, ‘Win-win approach between steel and ceramics industries’. The presentation reported on a project on the valorisation of steelmaking by-products, namely LD slag and BF sludge, as raw materials with an added value for the ceramics industry, specifically, structural bricks. The project partners were Global ArcelorMittal R&D, ArcelorMittal Environmental Department, ArcelorMittal by-products department and University of Oviedo, with characterisation of the bricks done by Cerámica La Espina. LD sludge was investigated as a degreasing agent replacement, due to its drying capacity from the free lime content and BF sludge as an organic component and flux, due to the Fe and high C content. It was noted that under Spanish legislation BF sludge is classified as a non-hazardous material, though it does contain 3%Zn and 48%C. At pilot scale it was found that up to 10% of by-products could be added successfully to the bricks without any detriment the properties of the bricks. Test work, undertaken by a brick manufacturer, showed that the addition of LD slag can be up to 20% of the degreasing agent and BF sludge could substitute up 2·5–3%. ArcelorMittal consider that the recycling of BF sludge via the sinter strand is not viable due to its Zn content and its consequential detrimental effect on the BF process.

Session Chairmen: Mark Sexton and Richard Thackray

Session 3 started with a very thought provoking keynote paper from Dr Jean-Pierre Birat of ArcelorMittal, entitled ‘The sustainability footprint of steelmaking by-products’. To summarise this particular paper is very difficult and to it do full justice the best way is to quote verbatim Jean-Pierre’s conclusions (A full version of this paper will be published in Ironmaking and Steelmaking). Quote:

‘The Steel Industry is not only producing steel, it also delivers secondary raw materials to other sectors:

By- and co-products usually command low prices in their markets, because they retain the image of a waste, which they are not. Legislation and practice are slowly changing their image and the scarcity of raw materials adds to the trend, especially when this causes prices to increase. With higher prices, more preparation of the co-products is possible in order to turn them into true and sophisticated secondary raw materials.

Beyond their economic value, these co-products have a sustainability value related to the fact that they avoid using primary raw materials and often involve less preparation (less process steps) in the client industries and, thus, exhibit a smaller overall ecological footprint.

We have dealt with this issue in the particular case of BF slag, which is both a large volume example of existing commercial connections between the steel and the cement sectors and a clear case where this industrial ecology synergy can indeed bring significant energy savings and cuts in CO₂ emissions. This is intuitive and this is true. It shows particularly clearly when one compares the two sectors as separate entities or as synergistic ones: the coupling of a large integrated steel mill with a large cement kiln saves 0·24 t of CO₂ per ton of steel produced or 0·64 t per ton of cement produced.

It is however difficult to allocate CO₂ shared in this manner to any of the two sectors, because beyond the apparent win-win strategy, there are issues due to the fact that CO₂ will very soon command prices that may rise to very high levels, so high actually that the price of cement, and to a lesser extent of steel, may change radically. With this perspective, it is not so simple to choose the proper allocation method, especially since the method which seems most appropriate for doing this, life cycle analysis (LCA), is actually not capable of doing it in a fair manner: LCA has not been developed to make such strategic decisions.

This is a serious methodological difficulty, which leads to serious practical hurdles in areas where slag is discussed. There are several ways out of this conundrum:

Going back to the planet-wide picture from which this paper started, it is clear that the collision between the anthroposphere and the ecosphere is under way; the noise and the furore of the clash is all around us! Climate Change is the most obvious of the consequences of this ‘cosmic’ event, but one should also be wary of the loss of biodiversity or of the threat to water resources. Since economics mediates between the two worlds, the consequence of this shock is an eruption of environmental issues in the economic sphere, the econosphere. This is happening slowly, but irreversibly, like a catastrophe movie shown in slow motion; a price of externalities is being introduced in the system, except that the scenario is not yet fully written and its timeline is fuzzy. This introduces the kind of uncertainty, which the economy and business dislike most.

To make things more complex, not only environmental externalities, but social ones as well are entering the stage. In the middle or long terms, it means that more synergies will develop between sectors, that the amount of waste will dwindle and that by-products will be used more extensively as secondary raw materials.

The transition may be confusing as the set is not fully reorganised, in terms of tools and methodologies in particular, to yet let the new show run smoothly.

One may also wonder at the distinction between products, co-products, by-products, residues and waste, which has been introduced in the fields of law and regulation (the legislosphere or the regulatosphere?) to solve important international trade issues in an opportunistic way. Nature does not abide with these different concepts related to the anthroposphere and the technosphere; it might just touch upon the subject through the concept of ecotoxicity, with the important caveat that, of course, all waste is not toxic!! All outputs from an industrial process have, in the long-run, the same fate of going back eventually to the environment, directly or after being used by consumers or by an industry, at some end-of-life or after one or several steps of recycling. They may go back in their chemical form, or be transformed into other compounds. The anthroposphere is thus giving back these products to the ecosphere, after borrowing them for a while.

The concept of ‘bio-slag’ to restore seaweed beds in venerable coastal locations in Korea was introduced by Dr Sungkil Park of POSCO, Germany in the paper ‘Response to coastal climate change by steelmaking slag’. Coral ‘bleaching’ (whitening due to the death of algae living in symbiosis with the polyps) occurs when seawater temperature rises, especially due to global warming. This phenomenon is also known as coralline flat or desertification of seaweed bed and characterised by the disappearance of valuable algae such as brown seaweeds, sea tangles, and sargassos from the rock mass of the coastal water and the surface of rock turning white due to white crustose coralline algae. Eventually it will reduce the growth of fish and shellfish populations that use the seaweed bed as breeding grounds or for feeding. On Jeju Island located in the South Sea of Korea, about 31% of coastal fisheries round were found to be influenced by the coralline flat. Treated BOF slag has been used to create 179 artificial reefs in trials ranging from the application of between 5 to 25 000 tons. The slag is stabilised by cooling after the steelmaking process. The Fe content within the BOF slag promotes the growth algae/seaweed blooms and as a result there has been an increase in the mass of seaweed forests in the test areas than compared to other areas. This is a great result as the seaweed and algae act as a CO₂ sink, via photosynthesis; a sink estimated at 0·3 t CO₂/t slag. The bio-slag has proved more effective than any other materials due to the calcining of the CaO in the BOF slag.

The advent of hot dip galvanising or electrolytic galvanising of low carbon strip for packaging applications has had the consequence of an increased Zn load in recycled scrap. Zn evaporates during the steelmaking process and condenses in the off-gas, producing flue dusts and sludges rich in Zn. Mr Gerald Stubbe of VDEh-Betriebsforschungsinstitut GmbH, in a paper entitled ‘Zinc and iron recovery from filter dust by submerged injection in hot metal’ gave a summary of the sources of Zn in the recycling route, especially the recycling of Zn galvanised low carbon steel. In a relatively simple process being developed by VDEh-Betriebsforschungsinstitut GmbH and DK Recycling und Roheisen GmbH, Zn containing dusts and sludges are injected into a molten iron bath held in an induction furnace. The dried dusts and sludges, including BF sludge, BOF sludge and zinc leaching residues are injected at a rate of between 8 and 40 kg min⁻¹. The injectants must be conveyable pneumatically. By reduction with carbon, metallic iron and zinc is produced in the hot metal. The zinc evaporates and after reoxidation generates a high quality ZnO product dust, which is separated in the off-gas system. The results are a very good with zinc yield into a zinc oxide product >84% and a 66% Fe yield into the hot metal.

In the fourth paper of session 3, Professor A. Saidi from Isfahan University of Technology presented the paper ‘Investigation of possible usage of electric arc furnace dust in cement industry’. This work has been done using EAF dusts from the Mobarakeh Steel Company, which utilises a mainly DRI charge (70 to 80%) and very low scrap usage. Thus, EAF dusts are significantly lower in Zn and Pb than from many other EAF plants. However alkali content can be high in the dusts and because of this two separate trials were done; one using mixtures of the EAF dust and Portland cement and the other using mixtures of Portland cement and EAF dust treated to reduce the high alkali levels by soaking the dust in water. Tests done on the mixtures were to investigate the most important properties of cement, namely hydration heat, water coefficient, viscosity, initial and final setting time, strength development and leachability of heavy metals. The results revealed that the EAF dust alters characteristics of cement unfavourably. However, the addition of the water treated EAF dust was not detrimental and enhanced some of the cement’s characteristics due to the low alkaline compounds that are responsible for unfavourable alternation of cement characteristics. Leaching tests also proved that heavy metals can be effectively removed.

In 2009, a £60 m investment was undertaken at Tata Steel’s Port Talbot Plant to collect and use internally BOS gas (BOSG). Already collected and used on-site for electricity generation, were coke oven gas (COG) and BF gas (BFG). Following the BOSG recovery project electricity generation was planned using BFG and BOSG and the COG to be used for reheating in the Hot Mill, thus reducing the requirement for purchased natural gas. Projected saving are £30 m pa and an estimated reduction in CO₂ emissions of around 3%. One drawback of using mixed gasses for electricity generation is the variable calorific value, for example BFG can vary between 2·7 and 4 MJ Nm⁻³. In addition, with the BOS being a batch process there could be gas availability problems due to plant operation and schedules. In the paper ‘Dynamic support tool for optimised use of process gases’, Mrs Zoe Hughes of Tata Steel Strip Products, UK, described the further development of ‘Discrete Event Simulation’ modelling, in use prior to the BOSG recovery project for mixing COG and BFG, for application of the new gas mix for electricity generation. The dynamic model, which forecasts the volume of process gas produced, and then recommends the optimum method of use has been well received by the process operators. Future development of the model will include a modification to the assumed calorific value as in practice this has been found to be greater that assumed in the model. The main limitation of the model is that it is totally dependent on the BOS Plant schedule. The schedule must be always updated to reflect exactly what will be happening at the BOS Plant, so that the volume of gas into the holder can be calculated correctly. Problems that occur are changes to the schedule without the database being updated. This leads to errors in gas production calculations and in turn hourly flow to the Power Plant is incorrect and may result in too little or too much gas leaving the gasholder.

The penultimate paper in this session, ‘Iron and steel process wastes as resource’ was presented by Dr R Vasant Kumar of Cambridge University. The paper described three innovative applications to utilising waste products. In laboratory experiments, 1·5 kg of baled Sn coated steel scrap in a packed bed reactor was treated by chlorination in an air gas mixture (10∶1 air chlorine) rate of 330 cm³ min⁻¹. The amount of tin in the sample was reduced from an average value of 0·25to 0·04 wt-% after several hours. In a second example, the potential possibility of using zinc oxide bearing dust for desulphurising hot metal was explored; this being shown to be feasible thermodynamically. In the final example, the potential for recycling BF top gas (approximately 25%CO and 2%H₂) was described using mathematical modelling. The modelling illustrated the need for injection of oxygen in the BF rather than air, i.e. a nitrogen-free BF.

One of the main drivers for recycling wastes is to reduce costs. This was particularly the case when the former Tata Steel plant on Teesside [Teesside Cast Products (TCP)] was facing closure. One possible large source of iron units on site was slag skim arising scrap recovered from the hot metal desulphurisation plant. The use of such material was described by Dr S Millman in his paper, ‘Internal arisings and recovered scrap’. At TCP approximately 100 kt/a of iron scrap was recovered from hot metal desulphurisation skimmings. Historically, TCP had set a directive that the recovered smaller lump size fraction of hot metal desulphurisation skimmings were to be processed to a 55%Fe yield using a range of screening and magnetic separation techniques. This meant that approximately 45% of high sulphur bearing skimming slag remained attached to the recovered desulphurised BF iron. Therefore, addition of this material to the BOS charge when it was returned to the steelmaking process route inevitably led to high sulphur pick-up levels during converter processing. As a consequence of this, plant operations management were always reluctant to use these materials except on steel qualities with high sulphur or no sulphur specification. Using a combination of drop balling and magnetic recovery techniques, the larger lump size fractions could be processed to an iron scrap with Fe yield of >90%, but at an increasing process cost. However, it was still perceived by steel plant operators that these materials produced high amounts of sulphur pick-up when used in the steelmaking process. Due to these perceptions, very little of this material was used in the BOF. This meant that large quantities of recovered iron arisings were not returned to the steelmaking process and instead, they were left to accumulate on site. In a joint project, Tata Research Development and Technology, Tata TCP and Harsco Metals, who were contracted to handle all slag and scrap processing on the TCP site, investigated the effect of higher yield recovered iron scrap on the BOF process, resulting sulphur levels in the steel and whether the increased cost of recovery outweighed the benefits of its use. The trial work illustrated that the extra processing of the skimming slag iron led to a substantial increase in its value-in-use, together with a significant financial advantage when it was subsequently charged to the converter as a substitute for merchant scrap.

Session Chairmen: Louis Brimacombe and Kevin Linsley

The session was opened by Mr Jonathan Aylen, Manchester Institute of Innovation Research, with his keynote paper ‘Instituting New Markets for Recovered Waste’. An ever increasing recovery of metals and slags from steelmaking production route wastes means that new markets have to be found for these recovered products. As many of these new markets do not exist, they must be created and developed. For example, a market for the recycling of aluminium beverage cans was created and done with incentives for the cans’ recycling. Once a market is defined and before it can be developed, the recycled product must be shown to meet legislation, have value in use, be inert and not detrimental to human health. End of life legislation means increasing quantities of scrap and Jonathan questioned whether the current scrap market was capable of segregating scrap from different sources (white goods, cars and construction) in order to maintain their value. He also questioned the need to re-melt, why not reuse, for example sections? However, any recycled section would need to meet all standards and may not be possible to verify these. An alternative option is to lease the steel and an example of leased piling was given.

‘If steel slags can be fully utilised, there is a win-win situation for both the environment and the steel producers as it maximises utilisation of a resource and minimises a costly waste’. This was an opening quote of Nick Jones of Harsco Metals Group Limited on his paper ‘Utilisation of steelmaking slag – an environmental and sustainable solution’. Harsco currently handle and market between 8 and 10 million tonnes of iron and steel making slags per annum; of this just over 50% is steelmaking slag from integrated plants. These slags are generated from 50 sites in 16 countries and are processed and sold as products into more than 20 different applications. There have been concerns over slag derived products due to free lime with its expansion and instability. Processing of the slags has developed such that some slag products are now considered a better option to natural aggregates; more abrasive, more robust and no ‘polishing’ finish. Other uses of slag derived products have involved embankments (low value and high quantity slags), gabion boxes, percolating filters in reservoirs, landfill drainage layers (the shape helping this), cement, metallurgical additives (via bagging or briquetting), abrasives and agricultural additions. On the marketing side, Nick Jones stated that the potential for the development of markets for significant volumes of slag derived products depends primarily on the properties of the slag, location of the steel plant with respect to the location of both processing facilities and the customer and the slag volume.

In China the generation of stainless steel slag is in excess of 3 mt and there is an issue of how to use effectively, or dispose of the slag. In his paper, Dr Ruyi Wang of the Baosteel Research Institute, Iron and Steel Co Ltd, ‘Beneficial use of stainless steel AOD slag as composite cement admixture and its safety risks’, discussed some of the options for the use of this type of slag. Intensive laboratory testing was done on the raw and prepared AOD slags followed by trials where the treated slag was added to mixtures of Portland cement. It was shown that stainless steel AOD slag, mainly Ca₂.SiO₄ with some Cr containing minerals, has cement like activity so it can be used as composite cement mixture. In addition, the results also showed that most heavy metals in the stainless steel AOD slag existed as stable forms and consequently the leaching concentration is far lower than the limit values of the identification standards for hazardous waste and lower the limit values of national Chinese standards for chromium slag used as cement admixture. Therefore, the heavy metals in the stainless steel AOD slags posed little pollution risk and the stainless steel AOD slag can be safely used as admixture of Portland cement to produce composite cement.

A new and successful method for the recovery of Cr from the slag during the stainless steelmaking process was reported by Mr Gerald Stubbe, VDEh-Betriebsforschungsinstitut GmbH. The paper ‘New technology for recovery of chromium from EAF stainless steelmaking slag’, details the cooperative research programme between VDEh-Betriebsforschungsinstitut and BGH Edelstahl Siegen, (with industrial plant trials at the BGH stainless), in which Al is added (injected) into the slag to reduce the Cr. The addition of Al also releases large amounts of heat, which decreases significantly the electric energy consumption for steel melting and enables increased productivity. The most commonly employed method of reducing chromium loss via the EAF slag is through the addition of ferrosilicon (FeSi). The FeSi usually added as reducing agent in a balanced quantity when the furnace is charged, but a disadvantage of the slag reduction treatment by FeSi is the often difficult dosage and the influence of the reaction product SiO₂ on the basicity of the slag. This requires increased lime addition in order to maintain the slag basicity to the required level of approximately 1·3 to 1·7 and in this way prevent excessive refractory attack by the slag. An alternative reductant is carbon or calcium carbide, but the Cr recovery from the EAF stainless steel making slag is often poor. The aluminium may be added as secondary raw material by injection in the form of mechanically processed granulated metal and in this way the overall primary energy input and the raw material costs are minimised. Quoted improvements to the EAF process for stainless steelmaking were a chromium recovery yield from the slag of up to 87%, electric energy consumption reduced by about 10% and a decreased power-on time of about 17%.

In the paper ‘Long term stability of steel slags from EAF processes’, Dr Caisa Samuelsson of Lulea University presented interesting data on the stability of recycled steelmaking slag used for road construction. Over a 24 month period two different recycled EAF slags were monitored for leaching and change to its mineralogy using SEM, XRD and a standard test for leaching. The pH and the conductivity decreased with time for all samples. The leaching of calcium, chromium and aluminium decreased with time whilst the leaching of magnesium increased. CaCO₃ formed on slag surfaces as calcium rich minerals reacted with moisture and CO₂ from the air.

The final paper of the conference, ‘Dry slag granulation – the environmentally friendly way to making cement’, was presented by Mr Ian McDonald of Siemens VAI Metals Technologies Ltd. The traditional method for processing BF slag is wet granulation where the liquid slag is quenched quickly in a granulation plant using large quantities of water. The fine grained, amorphous and very wet product is known as ‘slag sand’. Due to the frozen crystallisation energy, the slag sand when ground to cement fines, form hydration products in conjunction with water that correspond to the hydration products of Portland cement clinker, the main component of Portland cement. Thus the key prerequisite for the use of slag sand as a binding agent in the building material industry is satisfied. Despite mechanical dewatering, 10 to 12% moisture remains in the slag sand, thus drying is required. The glass content of the slag sand (target >95%) is the key parameter for its reactivity and has a direct impact on the strength of the cements and concretes. However, the required glass content can only be achieved by sudden cooling below the transformation temperature of approximately 900°C. Due to the less efficient cooling mechanism of water-free quenching, the dry process is technically more challenging than conventional water based granulation. Dry granulation does produce a larger sieve size product, but grinding times are with industry standards. Prior to its mothballing, the BF at TCP utilised a dry granulation process. The substitution of cement clinker by BF slag sand is an attractive economic alternative for the cement industry, because it reduces high energy costs and reduces CO₂ emissions. Approximately 1 ton of CO₂ can be saved for each tonne of clinker substituted by slag sand because not only is primary energy saved, but also the release of the CO₂ bound chemically in the limestone is avoided. Dry granulation requires no subsequent drying of the slag sand and this leads to a CO₂ reduction of roughly 30 kg/t in comparison with wet process. Given global production of approximately 210 mt of slag sand (2007), this is equivalent to a potential CO₂ reduction of over 6·3 mta.