9th Global Slag Conference 2013
10 - 11 December, Dubai, UAE
Reviewed by Dr Robert McCaffrey, Global Cement Magazine
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Video of the Global Slag Conference, Exhibition and Awards 2013
Global Slag 2013 - First day
Charles Zeynel of ZAG International started the conference with a global overview of slag trends. He suggested that organisations around the world are coming to the realisation that slag is a useful material, for example as a highway construction material, and that this will bolster the demand for the material. Global cement production is currently at around 3.7bn t and will double in size in the next ten years: there are increasing pressures towards environmental regulation and continued pressure to decrease costs and an increasing recognition that the costs and benefits of the use of supplementary cementitious materials (SCMs) are not currently balanced. Slag availability varies around the world – it is in plentiful supply in Europe, and in short supply in Asia. Mill scale is in very short supply and is being hoarded by the steel industry instead of being made available to the cement industry. Charlie pointed out that the US is a large resilient economy with high productivity and innovation, steady immigration and population growth, increasing energy independence and decreasing feedstock costs and increasing incentives for 'green' buildings. US cement capacity utilisation has grown from a low of 62% in 2010 to the nearly 90% forecast in 2014 – essentially at full capacity. GBFS is globally traded, originating in blast furnaces around the world and economically-transportable half way around the world. Japan is currently producing around 28Mt of GBFS, but no Japanese slag is available on the open market, due to the merger and rationalisation of Sumitomo and NSC, post-tsunami reconstruction and the 2020 Olympics in Japan. Charles concluded with news of ZAG's recent deal with Steag for the marketing of flyash originating in northern Europe.
David Roth of GPS Global Solutions gave a paper on steel slag processing. David pointed out that recycling plants typically crush air-cooled slag down to only <50mm, meaning that a large proportion of metals is not recovered. Given that stainless steel might be valued at $2600/t and even carbon steel might be $650/t, when limestone may be worth only $15 and slag products used as construction materials typically have a price of less than $100, it makes sense to make full recovery of the metals a priority. Aggressive sustainability, in David's mind, involves maximising metal recovery, crushing with controlled dust generation and maximising by-product value. He pointed out that there is as much metal in <50mm product as there is in >50mm product. He sought to defend slag-derived materials as product, stating that 'To be considered as a product, a material has to be used for a specific purpose, there has to be a market or demand for it, it should meet technical requirements and legislative standards and it should not have any deleterious health or environmental effects.' Slag-based products, he said, fulfilled all of these conditions and should be classified as a product. David introduced the Didion rotary tumbler, which can crush large lumps down to 3mm to maximise metal recovery and which can classify and produce materials in up to eight different material sizes in one process step.
Els Nagels next spoke about the use of software to provide a better understanding of slag properties. Els suggested the use of 'slag engineering' to optimise slag product characteristics. The controlling factors for slag characteristics are composition, temperature, and the composition and temperature histories or 'trajectories' of the slag. These factors will control liquidus (when the slag becomes fully liquid) and solidus (when the slag is fully solid) temperatures, equilibrium mineral phases, solid fractions, viscosity, and fluxing behaviour. The Spark software from InsPyro allows the input of a variety of slag properties and compositions and will model/forecast the corresponding output properties. The software is based on thermodynamic calculations involving the minimisation of the Gibbs free energy of the slag system, and also presuming that once solid phases are made in the slag that they do not then change composition – the cooling trajectory of the slag in this case can be important for the final mineralogical composition of the solidified slag. Enthalpy is also taken into account in the calculations, as is the calculated viscosity of the slag and Els said that the combination of physics and thermodynamics gives a good basis for forecasting slag properties. This knowledge and understanding can be used in slag engineering, R&D, process optimisation and daily reporting on slag quality.
At the start of the next session, on molten slag granulation, Kazuma Hagiwara of JFE Steel, Japan, presented the first of two papers on JFE's new slag solidification and heat recovery process, part of a programme to reduce CO2 emissions from steel blast furnaces. Part of the programme is based on the use of hydrogen for iron ore reduction, with eventual CO2 capture and storage. The new slag solidification process uses a pair of water-cooled copper rolls rotating at 5 - 10rpm to cool molten slag from 1600°C to less than 300°C to produce 3-20mm-thick slag 'plates' on an apron conveyor, which are then fed into a hot rotary hammer crusher and then into a heat recovery chamber consisting of a packed-bed with forced-air transfer at 50,000Nm3/hr. A conventional Kalina cycle (steam-based) process is used for waste heat recovery. Following heat recovery, fines are collected in a cyclone while the coarser fraction of the solidified slag is sent for further processing.
Yasutaka Ta of JFE Steel gave the second part of the paper, outlining results from the pilot plant. He pointed out that to recover CO2 from a CO2 absorbent, a large amount of energy is required – energy that is currently wasted in the steel making process, albeit at temperatures too low to be easily used. Mr Ta pointed out that to maximise the heat recovery, the heat transfer area of particles should be maximised by reducing their particle size to less than 5mm in thickness, whereas on the other hand, the slag needs to be coarser if it is eventually to be used as an aggregate. Thinner slag plates can be formed with a faster roll speed and this thinner form can give a heat recovery rate of over 30%.
Horst Kappes of the Paul Wurth company, Luxembourg, next spoke on dry slag granulation at a rate of 2.4t/minute. Historically, slag can be dumped and air-cooled, including high handling costs, sulphurous emissions and no heat recovery. More recently, the INBA slag wet granulation plant can produce a more valuable fine sand, reduced sulphurous emissions but with no energy recovery. Dry granulation, on the other hand, can produce a valuable dry product with the possibility of energy recovery. The discontinuous flow of slag from a blast furnace has to be transformed into a continuous energy output to be useable in a waste heat recovery process, while at the same time dealing with the changeable properties of the molten slag. In the new Paul Wurth process, cold steel balls are mixed with molten slag to create a cooled slag cake at a temperature of around 600°C. The slag is thus cooled rapidly (critically at a rate faster than the 20°C per second required for glass formation), producing a glass content of around 95%. A buffer storage area is used to even-out the process, with the slag cake being broken up rior to heat recovery. The grindability of this slag was found to be within the range of wet-granulated blast furnace slag, and the strength of mortars prepared from wet and dry granulated slags are comparable. Recooling of the steel balls is undertaken separately and is a significant rate-limiting step in the process. A conventional heat exchanger is used to produce steam at 15 bar and 325°C from the process. When used to produce electricity, the payback time is just over four years, and when used to produce direct energy in the form of hot air, the payback is just less than four years. The process has been used for BF, BOF, EAF and FeNi slags on an experimental scale.
A final paper in the session from Ian McDonald of Siemens VAI gave details on the new slag dry granulation process, developed in a joint venture with Voestalpine Stahl, the University of Leoben and the FEhS institute in Germany. He pointed out that air-cooling of molten slag equates to 1.5GJ of wasted energy per tonne of slag, or Euro3.5m of wasted energy based on a 6000t/d hot metal furnace. His company's answer for dry slag granulation is to pour the molten slag onto a rapidly-rotating metal alloy dish: the slag is spun off at high speed forming molten droplets which impact the walls of the granulator and which then fall onto a cooling bed fed with pressurised air – which can then be used for heat exchange and recovery. The produced granulated slag has >98% glass content and a grain size of 1 - 3mm and crucially is delivered in a dry state, with no contaminated water produced and limited emissions to atmosphere. Air is drawn from the granulating chamber at 600°C, either for electricity generation or for thermal preheating of fuel or raw materials. The cost of granulating the slag is lower than with a wet process granulator and the system has a lower capital cost than an equivalent-capacity wet granulation process. The produced particles are rounded, with no open pores and low closed porosity. Ian gave details of a new pilot plant at the Voestalpine Stahl Linz BF plant which will prove the technology on an industrial scale and which will be in operation in late 2014.
Tim Stanfield of Tube City IMS finally revealed 'the true identity' of steel slag – and pointed out that it is no good just changing the name of 'slag' – we have to change the perception of slag as a material. Tim pointed out the differences between recycled materials, by-products, co-products and products and concluded that cooled steel furnace slag is a co-product. Indeed, the USGS considers steel slag as a commodity. On the other hand, in the US regulation of slag typically falls under the aegis of state Bureaus of Waste Management – with the regulatory status of steel slag varying greatly from state to state, ranging from co-product, product, excluded from waste-regulations, or decided on a case-by-case basis. The National Slag Association in the US and steel makers came together starting in 2011, to try to have slag defined country-wide as a product: the ASTM already has standards for slag as a product, and the so-called Slag Coalition has decided to work with the ASTM and the European REACH regulations to promote this standpoint – of basing the specifications for slag as a product on the basis of the combination of mineralogical components and the lengthy history of the successful use of steel slag as a useful industrial material around the world. He concluded that steel slag is a 'product that is a result of the steel manufacturing process and is managed as an item of value in a controlled manner and not as a discarded material.' "Steel and slag companies" stated Tim, "must come together all over the world to promote steel slag as a product."
Global Slag Awards Dinner
Delegates had the opportunity to visit the viewing deck of the world's tallest building, the Burj Khalifa, prior to the Global Slag Awards Dinner at the Armani Hotel. The Global Slag Awards are presented on the basis of online nominations and votes, and registered the greatest number of votes so far for the awards. Global Slag 'plant of the year' went to Harsco's Taiyuan project in China, 'equipment supplier of the year' was awarded to Loesche, 'slag user of the year' went to Holcim and the Global Slag 'personality of the year' was awarded to Nick Jones of Harsco, a long-time supporter of the Global Slag Conference. USG was awarded the 'slag-based product of the year' award for its slag-based insulation, while Calumite LLC was awarded the 'technical innovation of the year' award for its slag-based glass.
Second day
Nick Jones of Harsco started the second day of the conference, looking at metal recovery from slag, specifically at a case study with Tisco Taiyuan Iron and Steel, 250km west of Beijing, the largest stainless steel producer in the world. Nick mentioned the 'Harsco Zero Waste Vision,' aiming to reuse or recycle all by-products wherever commercially viable. At the Taiyuan plant, the preexisting steel scrap recovery system did not recover all of the scrap in slag, and the slag was difficult to market since it still contained stainless steel. In addition, Tisco's on-site landfill became full and there was no option for off-site landfill. To solve the problem, Tisco and Harsco have entered into a 25 year JV to process steel and carbon steel slags to recover Fe, Ni and Cr and to produce valuable by-product fertiliser, cement and aggregate. The new recovery plant will process 1Mt of stainless steel slag and 400,000t of carbon steel slag each year and will eliminate landfill at the site. The recovery plant is situated on a large plot where stainless steel slag is wet-ground in ball mills, down to <1mm for maximum steel recovery. The stainless steel slag fertiliser was originally introduced for soil remediation, but with the addition of micronutrients it can be sold as a higher value product. The fertiliser has a high level of soluble silica, providing plants with resistance to heat stress and disease and also conferring increased insect resistance. The plant also manufactures 'Superfines' which is a cement additive manufactured from BOS steel slag, enhanced by fine grinding. Superfines-enhanced cement can be used in bridge construction, underground and marine applications and can be used in both common and higher performance concretes. Fineness is around 4000 - 5000Blaine, with metallic steel magnetically separated from the materials at various stages in the process. Superfines is capable of replacing up to 50% of GGBFS in blended cements, with no loss of strength and has a lower heat of hydration than GGBFS with less microcracking. The Tisco plant also makes a fully-recycled base for concrete or asphalt roads, composed of a steel slag aggregate bound with granulated steel slag and GGBFS, to make a slow-curing roadbase.
Fernando Duenas of Cemengal then introduced the Plug'n'Grind modular grinding plant concept for slag grinding. The components of the grinding plant can be transported anywhere in the world in only eight 40-foot shipping containers, allowing producers to follow and supply cement demand wherever it occurs. The time between order and delivery can be as short as eight months, and the installed plant – with simple civil works – can be operated with as few as four employees per shift. The Plug'n'Grind plant can produce ground slag at 7.5t/hr at 4000Blaine (78kWh/t total power), using its 500kW two-chamber ball mill. The integrated manual packing unit can produce 200 x 50kg bags per hour. For slag grinding, moisture content must be less than 1.5% which can be achieved with outside drying in the Middle East or with an external dryer. Increased production can be achieved with a grinding aid, at a rate up to around 9t/hr. Fernando gave some details of a slag grinding Plug'n'Grind installation for slag in Saudi Arabia.
Caroline Woywadt of Gebr. Pfeiffer next spoke about Vertical Roller Mill (VRM) grinding of granulated blast furnace slag. The company will celebrate its 150th birthday in 2014, having been founded in 1864. In 2011 the company sold the MVR 6700 C-6 mill, the largest-capacity cement and slag grinding mill in the world, with six 1920kW MultiDrives. Gebr. Pfeiffer's MVR mills are an improved version of its MPS VRM mills, with an increased number of rollers, use of roller suspension, a flat table, cylindrical rollers and a parallel grinding gap, the use of the MultiDrive concept with variable speed drives and built-in active redundancy. The mill is capable of drying, grinding and separating in one machine and also features a metal removal system. Caroline usefully pointed out that Blaine fineness systematically varies by around 10% according to whether it has been measured using ASTM or EN standards. For example 4500Blaine 'European' equates to around 3900Blaine 'American.'
Stefan Baaken of Loesche went on to speak about slag powder valorisation, specifically the conversion of steel slag into a highly reactive cementitious product. Loesche takes a slightly different approach to vertical roller mill technology from its competitors, using a smaller support roller to precompact the grinding bed, to allow the main roller to apply its full grinding force to the material bed. The company has installed 119 mills worldwide for GBFS grinding, many in China where steelworks will typically grind their own slag. As revealed at the Global Slag Conference in Las Vegas in 2012, Loesche is also in the process of building a steel slag grinding and carbonisation plant based on the SlagRec process – with more details to come soon. Loesche's newly-announced process for conversion of steel slag into a reactive cementitious product involves taking slag from the Linz-Donawitz process, and placing it into a converter for heat treatment: a recovered metal product is tapped-off and the remaining low-metal calcareous slag with higher alite and reduced belite content carefully cooled to produce a modified steel slag that can be ground in a Loesche VRM to produce a high-quality metal fraction and a reactive cementitious material. The modified slag, at a proportion of 30% with OPC, has a lower early strength but at 28 days has a higher compressive strength than 100% OPC. The company has so far conducted tests on a laboratory scale, and has now entered into an agreement with Siemens VAI of Linz, Austria to proceed to a pilot-scale plant.
Christoph Pichler of the University of Leoben, Austria, spoke about the use of slags remaining from metals recovery processes, both ferrous and non-ferrous slags. Christoph pointed out that electric arc furnaces produce around 9Mt/y of EAF dust, the majority of which is currently landfilled. However, this dust has a zinc content of around 20%, alongside a lead content of 2 - 7% and an iron content of 18 - 35%. A Waelz kiln can be used to oxidise zinc in the dust, leaving behind a recycling slag with zinc content of 5-9% and FeO content of 35 - 40%. This recycling slag can then be further reacted in a rotary slag smelting furnace or a metal bath process, where a reducing agent such as coke or charcoal is added to reduce Fe and Zn to recoverable oxides. The process produces an iron scrap, a zinc-rich filter dust and a residual slag that could possibly be used as a clinker raw meal replacement material due to its high CaO content.
In the penultimate session of the conference, on the use of granulated slag as a cement additive, Manish Wani of UltraTech Cement India spoke on the use of quadruple blends (cement, flyash, slag and ultra-fine slag) to produce mass concrete for high rise buildings. In mass concrete, it is important to dissipate or reduce the exothermic heat of hydration, which may otherwise lead to thermal stresses and cracking, with reduced compressive strength. For high-strength concrete, this cracking must be avoided, potentially through careful mix design. At the same time, concrete flowability and pumpability must not be adversely affected, while maintaining high compressive strengths and sufficient early strengths. Knowing the characteristics of the individual components in the mix goes some way to allowing the forecast of the final mix performance, but components can also interact in the final mix in unexpected ways. Careful testing of final mixes must be undertaken to ensure performance in place.
Anil Parashar of the Binani Cement Factory, Dubai, next spoke about NanoFine, a slag-based high performance cementitious binder. Binani Cement in Dubai is based in the Jebel Ali free port and is the largest clinker grinding plant in the UEA, with a capacity of 2Mt/yr using three ball mills. NanoFine, despite its name, actually occupies a niche between the nano- and micro-scales, and is based on GBFS, with addition of around 10% of a mix of other materials. The material has a fineness of 8 - 9000Blaine, or 30% below 1micron, with a reactive phase of above 90%. Addition of NanoFine to OPC actually increases the compressive strength at all testing stages by around 60%, partly through the mechanism of efficient particle packing and densification of concrete.
David Ball of the David Ball Group plc spoke about 'the beauty' of no-cement concrete. David introduced 'Cemfree,' ultra-low carbon concrete and cementitious materials containing no OPC. Cemfree is a composite material composed of GGBFS and pulverised fly ash (PFA) activated by a patented proprietary activator. Use of Cemfree leads to low heat of hydration, acid and sulphate resistance, low water permeability, low water demand and a light-coloured concrete. As an example of high-slag cement use, David mentioned the continuous pour of the 5600m3 3m-thick foundation high-slag concrete slab of the UK's tallest building, the Shard, which had a temperature rise of only 7°C.
In the final session on slag product applications, Pouyan Naseri of the Pasargad Steel Complex in Iran spoke about the use of EAF slag to control acid mine drainage. Acid mine drainage forms where sulphide minerals such as pyrite and Marcasite are placed in oxidising surroundings. AMD is characterised by acidity, raised levels of sulphides and elevated concentrations of dissolved metals. The selection of the most effective AMD control system depends on the initial pH level, effluent water composition, available resources and budgets, and it is sometimes required to combine active and passive control systems. Steel slag yields hundreds of times more alkalinity than limestone, and the neutralisation potential of steel slags exceeds both lime and hydrated lime and for longer periods of time. Pouyan outlined an experiment to demonstrate the effectiveness of Pasargad steel slag for acid mine drainage remediation, using a down-flowing circulation of acid liquids through a column of slag with measurement of pH levels over 60 days. It was found that precipitation of iron, aluminium and manganese hydroxides can lead to the clogging of surface pores of the slag and reduction of slag activity. During the experiment, the ability of the slag to reduce acidity decreased as fine slag particles were dissolved: the ability of the slag to retain its activity depended on a continuing fresh supply of active surfaces and particles, which can be provided by having a wide range of particle sizes in the slag.
At the end of the presentation programme, delegates split into four discussion groups to further examine trading opportunities, slag optimisation, grinding technologies and slag applications. This 'Global Slag Exchange' session was greatly enjoyed and valued by the delegates.
Farewell and prize-giving
At the farewell party on the last evening of the conference, a number of awards were presented after delegates voted for their favourite presentations. Stefan Baaken of Loesche was in third place for his paper on steel slag processing to produce a cementitious product, while Ian McDonald's paper on Siemens VAI's dry slag granulation process was in second place. The winner of the best presentation award, however, was David Ball of the David Ball Group, for his eloquent paper on 'no-cement concrete.'
Delegates rated the conference very highly in all categories, and agreed that it had been one of the best-ever Global Slag Conferences:
• 'I loved the breakout at the end – very informative.'
• 'The discussion in small groups is very useful.'
• 'Good conference!'
• 'Good mix of steel producers, cement producers and suppliers.'
• 'The Global Slag Exchange was interesting and should be built upon at future events.'
It was announced at the event that the next Global Slag Conference – the 10th – will take place in Aachen, Germany, on 8 - 9 December 2014. We look forward to seeing you there!
