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50 years of Outotec travelling grate technologyTravelling grate pelletising is a mature technology and the solution of choice for the challenges of the 21st century in raw material preparation for the steel industry. Through its scalability up to 9 Mt/yr, its flexibility regarding the feedstock, and its high energy efficiency, low environmental impact, travelling grate pelletising will provide the basis of the growth of the industry, whose capacity is projected to increase from currently approximately 360Mt/yr currently to about 550Mt/yr in 2026

Until the beginning of the 20th century, iron ores that were charged into blast furnaces were crushed and

partly classified either at the mine or at the iron and steel works. Fines that were screened out were dumped.

Since they were of no economic use, the possibility of agglomerating such fines by means of sintering and briquetting was examined. Sintering was developed to treat medium-sized fractions. The first patents for forming green balls and firing them were granted in 1912 to the Swede A.G. Anderson and in 1913 to the German C.A. Brackelsberg.

In 1935, a pilot plant was built in Germany. In the 1940s, intensive development of pelletising technology was carried out in the USA and Sweden, leading to the installation of several smaller plants.

The first industrial travelling grate pelletising plant was installed in 1955 for the Reserve Mining Company in Silver Bay, Minnesota, USA (today, Cliffs Natural Resources)

Authors: Thomas Schwalm and Karl-Heinz HofmannOutotec

r Fig 1 Grain size distribution of iron ore carriers

with a reaction area of 93.6m2. In the same year, the first pelletising shaft furnace was installed for LKAB in Malmberget, Norrbotten, Sweden. The first grate-kiln pelletising plant followed in 1960 for Humboldt Mining in Humboldt, Michigan, USA. Figure 1 illustrates the size range of various iron ore-based products.

In 1956, Lurgi, the predecessor of Outotec, built its first pelletising plants for artificial magnetite with a reaction area of 120m2 for the International Nickel Company of Canada in Copper Cliff, Ontario. In 1963, it built a plant for natural magnetite concentrate with a reaction area of 125m2²for Marcona Mining in San Nicolas, Ica, Peru (see Figure 2).

TECHNOLOGICAL MILESTONESOver the last 50 years, travelling grate technology has undergone substantial developments, mainly focused on the following improvements:` Broadening the raw material basis with the processing of: A Highly weathered iron ores with a loss on ignition

of up to 10%, as in the case of the Robe River plants, installed in 1972 for Cliffs Western Australia Mining at Cape Lambert

A Virtually iron-free harbour mud, as successfully demonstrated in pilot scale test work for the City of Hamburg, Germany, in 1994

A Iron ore concentrates with sulphur contents up to 1.5%, as in the case of the Chinese concentrates from Inner Mongolia in 2013

` Increasing the specific capacity for hematite processing plants by more than 20% by adding carbon carriers to the pellet feed as a standard operating procedure

` Reducing the thermal and electrical energy consumption of the plants by:

A The addition of carbon carriers to the pellet feed in hematite processing plants

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a

A The application of optimised gas flow schemes with up to four recuperation principles for the re-use of process gases

A The installation of variable speed drives and frequency inverters for all process fans

A The refinement of the screening operation up- and downstream of the indurating machine to narrow grain size distribution and minimise pressure drops

` Minimising the environmental impact of the plants by: A The substitution of mechanical dust cleaning

systems with electrostatic precipitators as standard equipment for all waste gas streams and, in recent projects, bag houses

A The installation of desulphurisation units, such as for LKAB’s mk3 pelletising plant at Malmberget, Sweden in 2006

A The development of primary NOx-minimisation concepts

` Broadening the fuel basis for the plants through: A The partial use of carbon carriers, such as coal, coke

or pet coke, for up to 50% of the overall thermal energy demand of the furnaces

A The use of steel plant gases, such as coke oven gas, mixed blast furnace and coke oven gas or Corex-off gas with minimum calorific values of 2.500kcal/Nm3³

A The use of coal tar as a substitute for heavy fuel oil A The partial use of producer gas from coal

gasification ` Increasing the plant availability from initially

approximately 300 days per year to currently up to 352 days per year through a variety of measures, such as:

A The development of improved wearing systems for rotating equipment

A The application of new refractory concepts A The installation of stand-by equipment that is easy

to change over to or exchange A The development of improved furnace components,

such as pallet cars, slide rails, lifting and lowering wheels and grate bars

` Increasing the size of the plants from initially around 1Mt/yr to currently above 9Mt/yr in single units. The trend in grate sizes is shown in Figure 3.

OUTLOOKIron ore pelletising, a technology with a history of 50 years of continuous evolution, is addressing today the future megatrends for the sustainable development of the iron and steel industry (see Figure 4).Leaner ores The beneficiation of lean iron ores, mostly magnetites, as practised for many years in countries like China, Russia, Sweden, the USA and Canada, is also being

r Fig 3 Development of travelling grate plant sizes

r Fig 2 Construction of Outotec’s first iron ore pellet plant for Marcona Mining at San Nicolas, Peru

r Fig 4 Mega trends for a sustainable technology development

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considered for current big scale development projects in North America and Africa. This results in the production of fine concentrates with grain sizes below 45µm that can be easily transported and used in downstream processes only after their conditioning by pelletising. Legal restrictions limiting the iron content in tailings (eg, in India to below 45%) require the further conditioning of the resulting super fines into a form in which they can be processed even more.

Energy efficiency and reduction of emissions Due to its various recuperation principles, the Outotec travelling grate pelletising process is the most energy efficient commercially available. With the introduction of the fourth recuperation principle, the application of advanced burner technology and the utilisation of plant expert systems for the efficient operation of such plants, the new generation of pelletising plants consumes substantially less energy and consequently emits less CO2 and other ecotoxic waste gases (see Figure 5).

Recycling In most cases, the pelletising process is configured to internally recycle 100% of the generated and separated fines, making it a process with a mass and concentrate recovery of >99%. After an adequate material preparation for moisture and fineness, a pelletising plant can also process a great variety of steel plant residues, either mixed with virgin iron units (such as in Mexico) or exclusively as a conditioning step for further processing (eg, on a sinter machine, as in Taiwan).

Capacity: Standard plant and tailor-made mega-solution Today, modularised pelletising plants with a capacity of around 1.2Mt/yr are available for markets

r Fig 5 Reduction of CO2-emission in travelling grate pelletising plants

r Fig 6 Standard pellet plant with 1.2Mt/yr capacity

RAW MATERIALS AND IRONMAKING

such as India, catering for the needs of smaller mining companies with regards to simple design, fuel diversity and fast project implementation (see Figure 6).

Tailor-made mega plants with a yearly capacity beyond 9Mt in single strands mark the high end of the capacity range. Such plants are equipped with features to maximise their operational availability through easy maintainability, and possess sophisticated solutions for the minimisation of their thermal and electrical energy consumption. An example is shown in Figure 7.

SUMMARYTravelling grate pelletising is a mature technology with a history of more than 50 years, and it is also the solution of choice for the challenges of the 21st century in raw material preparation for the steel industry. Through its scalability, covering a production range up to 9Mt/yr, its flexibility regarding the feedstock, and its high energy efficiency, with low environmental impact, travelling grate pelletising will provide the basis of the growth of the industry, whose capacity is projected to increase from approximately 360Mt/yr currently to about 550Mt/yr in 2026. MS

Thomas Schwalm is with Outotec, Oberursel, Germany. Karl-Heinz Hofmann is with Outotec Belo Horizonte, Brazil.

CONTACT thomas.schwalm@outotec.com

r Fig 7 816m² travelling grate pelletising plant under construction in late 2012

[Vision becomes reality

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