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STEP ONE Scrap steel is delivered by truck and tipped into the mill’s scrap pit which has a capacity of 18,000 tonnes – carefully divid­ed into four grades held in separate bays. That’s enough to feed the mill for two weeks. STEP TWO According to the demands of the furnace production schedule, the various grades are loaded into scrap buckets for transfer by rail trolley into the melt shop. These days, a significant proportion of the steel pro­duced in the USA and Australia comes from Mini Mills. These use an electric arc furnace to melt down scrap steel, thereby saving a considerable amount of energy which would otherwise be required to make steel from iron ore. In this article, we will look at what constitutes a mini mill and visit the mini mill at Rooty Hill in Sydney. STEP THREE In the melt shop, a crane lifts the scrap bucket and empties its 80 tonne load into the opened arc furnace. The melting and refin­ing stages are conducted at 1600°C. As the scrap is melted by the electric arcs, fluxes are added to form a slag on top of the molten steel. Oxygen is then injected by lance through the slag, into the molten steel. The resulting chemical reactions cause the slag to absorb most of the impuri­ties from the steel. Those impurities that are not transferred to the slag are ex­hausted as fumes for cooling and collecting via the mill’s fabric filter bag system. By BOB YOUNG Steel Mini Mills: a recycling success story T HERE ARE SEVERAL mini mills in Australia and quite a few in the USA and they produce a considerable amount of steel from what is essentially quite a small plant. So what is a mini mill? A mini mill is a steel production facility that uses an electric arc furnace to melt the scrap steel. In contrast, the 12  Silicon Chip traditional Integrated Steel mill has blast furnaces or basic oxygen furnaces using iron ore and coke as the basic ingredients with some scrap thrown in. Although some integrated mills have electric arc furnaces for specific purposes, the arc furnace is the key component of a mini mill. Over the past 20 to 30 years, there has been substantial growth in mini mills. In 1970, mini mills accounted for less than 10% of US steel production. These early mills typically produced between 100,000 and 300,000 tonnes per annum, with the number of grades of steel and product types kept to a minimum. In 2001, mini mills produced nearly half of the www.siliconchip.com.au STEP FOUR When the steel has reached the required temperature and chemistry it is tapped into a ladle for transfer to the ladle furnace area. In the ladle furnace there is more refining to be done – this time through selective addition of alloys. In the ladle, the temperature of the molten steel is increased and maintained by electric arcs mounted in the ladle furnace cover. steel shipped by United States mills. Nor is it stopping there. Mini mills are no longer mini, with production capacities now approaching 1,000,000 - 2,500,000 tonnes per annum while still using a single but now quite large arc furnace. In addition, the list of grades of steel produced and product types has increased considerably. North American carbon steel mini-mills continue to be among the most competitive steel makers in the world. Some idea of the growth in productivity can be obtained from the following fig­ures. In 1983, integrated mills were producing about 200 tonnes of steel per employee, rising to approximately 700 tonnes in 1996. The latest mini mills are claiming 4,000 tonnes per employee. Although arc furnaces use large amounts of electric energy to heat the steel scrap, there is a significant saving on raw mate­rials. For every tonne of steel recycled, there are 1.25 tonnes of www.siliconchip.com.au STEP FIVE When the required alloys have been added and exact specifications have been reached, the molten steel is taken to the cast shop where it is poured into a tundish from where it flows into a series of water-cooled moulds to form continuously cast billets. The 127mm square partially solidified billet strands are further cooled by direct water spray in a secondary cooling zone. While they are still hot, however, they are cut into 12-metre lengths by automatic hydraulic shears. The billets are then air-cooled before being carried by overhead cranes fitted with electromagnets into the outdoor billet stack­ing yard. iron ore, 0.5 tonne of coal and about 20kg of limestone saved. As the name would indicate, mini mills have a relatively low steel production capacity when compared with integrated steel mills but they can be accommodated comfortably on just a few hectares. Because they do not need to be located near a railway or water transportation facilities, mini mills have a much wider range of suitable geographic locations. The factors driving mini mill location are an adequate supply of electricity, scrap availability and a local market for products. While mini mills are more specialised in the types and quality of the steel produced, the wastes are similar to those from iron and steel making. The major difference in mini mill waste is increased concentrations of toxic metals in dust, sludge and slag, due to the scrap metal used as the input. Stainless steel scrap for exam- ple, is high in nickel and chromium, while other steel scrap may often have a coating of zinc, tin, nickel, lead or chromium. Certain scrap may need to be chemically or physically treated before entering the arc furnace to remove its coating (eg, de-galvanising) before being processed into new steel and it is here that the mini mill metallurgist has to exercise some of his magic. To take an unknown mix of scrap steel and finally produce a certifiable grade of new steel requires considerable expertise and yet it is all in a day’s work for the mini mill metallurgist. Finally, as most mini mills are located in large cities, great care is exercised in maintaining the required environmental controls. Indeed, environmental factors dominate the design of any modern mini mill. Electric arc furnaces The arc furnace has been in use for February 2002  13 STEP SIX Billets are fed into a natural gas-fired furnace where they are reheated to rolling temperature of about 1150°C. The billets are fed through a series of rolling mill stands where they are reduced and formed into various sections and sizes, such as angles, reinforcing bars and wire rod. The rod and bar products produced in the mill are cooled by water sprays and air before being sheared to customer specified lengths. nearly 100 years as a method of making steel. Originally confined to small (several tonne) furnaces for the production of highly specialised steels, recent developments have seen the arc furnace growing in size and popularity as technical problems have been overcome and reliable sources of cheap electricity have become widespread. In the last 15 years or so, the arc furnace has undergone something of a renaissance as technical innovations have led to very significant improvements in productivity, steel quality and operating cost. These developments have proceeded to the point where the arc furnace is now the preferred, low capital, flexible route for the production of a significant proportion of flat products and almost exclusively, long products. The modern electric arc furnace consists of a refractory lined steel shell or hearth that holds the scrap charge while it is being melted and retains the liquid steel until it is ready to tap. The walls above the liquid steel level are typically water-cooled, replaceable copper panels. The furnace has a water-cooled roof that can be swung aside to allow for scrap recharging. In most cases, loading of the furnace is carried out via overhead clamshell buckets or baskets. All types 14  Silicon Chip STEP SEVEN After shearing or coiling, products are transferred to the mill’s finishing area for straightening, bundling, strapping and dis­patch to customers. of scrap and scrap substitutes can be added in this manner. Sometimes charge carbon and fluxes (lime and dolomite) are also added in this manner. The preferred method of adding smaller input materials is via a conveyer belt, loading into the “fifth” hole in the furnace roof. This is known as continuous charging. The number of buckets required to reach the specified tap weight will be determined by the scrap charge density. Arc furnaces can be either AC (three electrodes, each with its own phases) or DC (single or twin electrode). In an AC furnace, the roof has at least five openings, one for each of the three electrodes, one for fume evacuation and the abovementioned “fifth” hole. Electric arc furnace In an AC furnace, the current path is from the electrode tip to the bath and back to the next electrode in the phase rotation. In a DC furnace, the current passes from the electrode through the bath to a return electrode in the furnace hearth. The electrodes are made of graphite manufactured to have special properties of conductivity combined with high strength at high temperatures. The electrodes are consumed in the process and need to be continually replaced. This is achieved by “slipping” or lowering the electrode through the holding arm into the bath and adding a new section to the top. This is done by screwing electrode sections together. Electrical power is supplied from a substation, then to a step-down transformer. The furnace also has its own transformer that serves to alter the furnace electrode voltage. These voltage “taps” are usually selected automatically but they may also be www.siliconchip.com.au adjusted manually by the furnace operator. The electrode voltage determines the arc length and therefore the power applied to melt the steel. As conditions inside the furnace are constantly changing, it is necessary to continually reposition the electrodes to maintain the desired arc current and power setting. This is normally achieved by constantly measuring the impedance (voltage divided by current) and feeding this back to a control system that will raise or lower the electrode arms as the conditions inside the furnace change. Oxygen is used to assist in refining the steel by burning off impurities such as phosphorus. A lime-rich slag is used to collect this and other unwanted elements and provide a base material to be foamed by the evolution of carbon monoxide and carbon dioxide gases. This foamy slag helps improve energy effi­ ciency by preventing unwanted radiation of the un­shielded arc to the furnace roof. The foaming action causes the slag to be con­tinuously flushed from the furnace up until tap time. The molten slag also helps in suppressing the considerable amount of noise produced by the electric arcs. When the appropriate steel chemistry and temperature have been achiev­ ed, the steel is tapped either through a spout or a submerged taphole, into a ladle and then on to a ladle furnace for secondary treatment. Furnaces manufactured by EMCI for example may be bottom tap design or conventional design with standard or current conducting electrode arms. EMCI’s electric arc furnaces also feature hydrau­lic circuitry that allows for rapid electrode travel and fast back tilt to minimise slag carry-over. Rooty Hill’s mini mill To illustrate the operation of a typical mini mill, we now look at the One Steel plant at Rooty Hill in Sydney, NSW. This is quite a small facility and yet the very first thing that stands out is the small size of the actual Melt Shop and electric arc furnace itself in comparison to the overall size of the whole facility. The 60-tonne electric arc furnace has been coaxed by an ingenious and industrious staff into taking 80 tonnes of scrap in a single feeding. It is this “small” furnace, working on a con­tinuous basis, that produces the www.siliconchip.com.au All the steel from the Rooty Hill mill is first produced as continuously cast billets such as these emerging from the tundish via water-cooled moulds. The extreme heat rising from these billets has to be experienced to be believed yet it is a pale shadow of that from the electric arc furnace. The billets are later reheated prior to being fed to the rolling mill to produce a wide variety of sections such as angles, rein­forcing bars (rebar) for concrete, round bars, flats, fencing wire and so on. vast stacks of steel in the Rooty Hill mill; some 500,000 tonnes per annum, 200,000 tonnes of which is sold as bar stock. Here then is a very efficient opera­tion by any standard. The furnace consumes large amounts of electricity, the actual rating being 62MVA (equivalent to 62 megawatts). The magnetic fields surrounding the furnace is so strong that the video monitors in the nearby control room flicker continuously in spite of serious efforts to shield them. How the staff manage to watch these flickering images continuously defies comprehension. We assume that the computer monitors will shortly be upgraded to LCD monitors which would cure the problem completely! To enter the melt shop and confront the electric arc fur­nace in full song is to finally come face to face with Dante’s Inferno. Housed in a towering, dark, windowless building, black­ened February 2002  15 in spite of careful sorting, can lead to eruptions that may spit molten steel across the melt-shop floor. Looking after the furnace is certainly not a job for the fainthearted! The main steps Fumes generated in the mill are treated in the bag filtration plant. This acts like a gigantic vacuum cleaner to remove dust and particles before release to the atmosphere. The righthand bag is operational while the other is on standby. internally by years of fumes and dust, the furnace presents an eerie sight. Crouching in one corner and tied to the Melt Shop building by a staggering array of cables of all sizes – cables that soar loftily up into the inky blackness of the dimly lit, almost invisible ceiling – the furnace resembles some prehistoric fire-breathing monster chained down to prevent its escape. The monster within To confront this monster, the visitor is given earplugs, glasses and a woollen jacket. Roaring and spitting and emitting an intense light so bright that it can only be viewed through special glasses, and heat that is fearsome to the extreme, one is immediately struck with the thought that those who care for this monster are special people. To office workers who confront noth­ ing more daunting in their working day than a hot cup of coffee, here is a different world indeed. Control of the furnace is a delicate operation. To begin, the scrap steel 16  Silicon Chip input is carefully prepared to stringent quality standards by suppliers. Composed largely of old car bodies, washing machines, fridges and the like, combined with structural steel from demolition sites, waste and offcuts left over from other steel mills, the input is a disparate mix. For example, car bodies and white goods (fridges, etc) will have been shredded to remove all paint and plating so that they are completely unrecog­nisable. From this raw material, the metallurgists at the mill will eventually produce certified grades of steel used in such indus­ tries as construction, farming and transport. The mix in the electric arc furnace must therefore be continuously monitored and adjusted by the addition of other raw materials such as burnt limestone and dolomite, carbon, ferro alloys, oxygen and nitro­gen. Feeding the electric arc furnace continuously with a stream of scrap that may on occasions still contain an unknown quantity of impurities, There are five major steps in the transformation of scrap into graded steel at the Rooty Hill steel mill. (1) The carefully graded scrap is delivered by truck to the mill’s scrap pit which measures 100 x 23 x 6 metres deep. It is capable of holding up to 18,000 tonnes, enough to feed the fur­nace for about three weeks. (2) Depending upon the demands of the furnace production sched­ule, the various grades are loaded by electromagnetic crane into scrap buckets for transfer into the melt shop. Interestingly, all the electromagnetic cranes in the plant have battery backup, for safe depositing of loads in case of power failure. (3) In the melt shop, a crane lifts the scrap bucket and empties its load into the opened arc furnace. The furnace capacity is 80 tonnes and the number of buckets required to load the furnace depends on the scrap density. The furnace is manufactured by Danieli, Italy. Bath diameter is 5.5 metres and the tap weight is 75 tonnes. By the way, the electrode diameter is 550mm. The melting and refining stages are conducted at 1600°C. As the scrap is melted by the electric arcs, fluxes are added to form a slag on top of the molten steel. Oxygen is then injected by a lance through the slag, into the molten steel. The resulting chemical reaction causes the slag to absorb most of the impurities from the steel. Those impurities that are not trapped in the slag are vented for cooling and collecting via the mill’s fabric filter bag system. The time from charging to finish of a batch is 44 minutes and tap-to-tap time is 56 minutes. (4) When the steel has reached the required temperature and chemistry, it is tapped into a ladle for transfer to the ladle furnace area. The ladle furnace is again manufactured by Danieli and is a 12MVA, bottom-stirring furnace featuring 350mm elec­trodes. Power consumption is 15kWh/liquid tonne. (5) In the ladle furnace there is www.siliconchip.com.au more refining to be done, through selective addition of alloys. When the required alloys have been added and exact specifications have been reached, the molten steel is taken to the cast shop where it is poured into a tundish from where it flows into a series of water-cooled moulds to form continuously cast billets. The continuous caster is a Danieli, four-strand billet caster with either 121mm or 127mm sections. The extrusion rate of the billets varies between 2.9 - 3.1 metres per second, the maximum sequence length being 36 hours. The square partially solidified billet strands are further cooled by direct water spray in a secondary cooling zone. While they are still hot however, they are cut into 12-metre lengths by automatic hydrau­lic shears. A fully laden furnace will produce approximately 88 billets. The billets are then air-cooled before being carried by overhead cranes fitted with electromagnets into the outdoor billet-stacking yard. It is interesting to note that steel loses its magnetic properties above 600°C so cooling must be well under way before the steel billets can be handled with electromagnets. Rolling into finished stock As stated earlier, of the approximately 500,000 tonnes of steel billets produced at the Rooty Hill mill, approximately 200,000 tonnes are sold as raw billets to other steel mills. The remainder is processed into lengths of various shapes and sizes. The rolling process begins with the billets being reheated in a natural gas-fired recuperative walking hearth furnace where they are raised to the rolling temperature of about 1150°C. Billets are then fed through a 16-stand 800kW rolling mill with a throughput of 270,000 tonnes per annum. Here they are reduced and formed into such shapes as rein­forcing bar for the building industry, angles and flats for construction and transport and wire rod for the fastener and wire industries. The rod and bar products produced in the rolling mill are cooled by water sprays and air before being sheared to cus­tomer specified lengths. Shearing is carried out on-the-fly by a swinging-arm guillotine, the finished stock moving through the shears at 13 metres per second. After shearing or coiling, products are then transferred to the mill’s finishing area for straightening, bundling, strapping, identification and dispatch to customers in the Sydney area. The plant’s warehouse area can accommodate up to 18,000 tonnes of finished product. Environmental considerations As the mill is located in the heart of a Sydney residential suburb, housing the electric arc furnace obviously required considerable care, as indeed did the whole mill. The seven hec­ tares of mill buildings are located in the centre of a 27-hectare landscaped area. Soundproofing the Melt Shop and all noise generating plant required the use of 320mm thick, sound-absorbing precast concrete wall panels. Soundproofing is further en­ hanced by surrounding the entire plant with a 5-metre high solid earth berm, created from some 63,000 cubic metres of topsoil material. An air-monitoring station near the site incorporates a high volume sampler and dust fallout gauge, as well as wind-direction and wind-velocity meters. During mill operation, fumes generated are removed from the building and filtered through thousands of filter bags housed in the mill’s bag­house. Designed to handle a total volume of 730,000 normal cubic metres of fumes per hour, the bags act like a gigantic vacuum cleaner to remove dust and particles to levels well below the limits set by the NSW Clean Air Act. The mini mill also recycles all of its processed water before it passes to the sewer, to standards set by the Water Board. Site runoff is strictly controlled via a separate drain­age system feeding into settling ponds where sediment collects before clear water runs into nearby Eastern Creek. So there it is. Rooty Hill is one of several mini steel mills in Australia and is relatively small by the standards of such mills overseas but it still manages to produce half a mil­lion tonnes SC of steel per annum. Acknowledgments: our thanks to John Prestidge and Vince Ivancic, One Steel, Rooty Hill, Sydney for their kind assist­ance in preparing this article and for a conducted tour of the One Steel mill. All photos and diagrams courtesy BHP Steel. This diagram shows the layout of the Sydney Mini Mill. The plant has been designed to minimise noise by enclosing all noise-generating machinery and by constructing a 5-metre high earth berm. www.siliconchip.com.au February 2002  17