February 2022
Steel is manufactured by two main process routes. One is the integrated steelmaking process featuring the blast furnace/basic oxygen furnace (BF/BOF), where iron ore is the major source of iron units; the other method is electric steelmaking based on the electric arc furnace (EAF), where steel scrap or direct reduced iron (DRI) are the major iron feedstock materials.

The BF/BOF route is expected to remain the principal steelmaking route over the current decade, despite the growth of mini mills with EAFs. Exactly how long the BF/BOF route retains this role will depend on the pace of technological advances, especially in terms of direct reduction and smelting reduction.

Pellet usage is expected to be well supported in the near future by China's carbon emissions policies. The country’s pollution control measures have sparked a preference for iron ore lump and pellets and pushed up premiums as China’s mills try to maximise direct charge products in the blast furnace to avoid polluting sintering operations. In the short term, the government may impose increasing controls to further reduce emissions.

 

BF/BOF

Traditionally, ironmaking and steelmaking have been considered separate disciplines. The end product of ironmaking is either molten pig iron (generally called hot metal) produced through large capacity blast furnaces or sponge iron produced through smaller capacity direct or indirect reduction processes. The concentrations of dissolved impurities like silicon, phosphorus and sulphur in hot metal have to be kept within specified limits for the product to be suitable for steelmaking. To improve the performance of the blast furnace, ferrous feed is sometimes upgraded prior to being fed into the furnace.

Where the iron ore has low iron (Fe) content, it is processed in a concentration plant and the resultant concentrate is used to produce an agglomerated iron ore feed—either pellets or sinter. If the fine concentrate was not agglomerated into larger particles, it would impede the necessary upward flow of gases in the blast furnace or else be blown out of the furnace with the spent gases. Where the iron ore contains a large amount of moisture or carbon dioxide, it is subjected to a process of calcination in a shaft or rotary kiln.

In the calcination process, iron ore is fed into the upper end of a rotary kiln, which is then fired at the lower end by either blast furnace gas or oil or natural gas. The temperature is raised enough to expel the water and carbon dioxide from the ore. The calcined ore is ejected at the lower end of the kiln. Normally the calcination process takes place during sintering.

 

Ferrous Feed for Blast Furnaces

Iron ore is traded in three principal forms: fines, lumps and pellets. Lumps and pellets are most commonly used as charge straight to a blast furnace or in a direct reduction plant.

Screening the run of mine (ROM) ore separates the fines from the larger lump ore. Often it is necessary not only to crush, wash and screen the ore but also to use gravity or cyclone separation to create a consistent and more saleable fines product. Sometimes more expensive beneficiation processes must be used, such as grinding to a very fine particle size and then using magnetic separation or flotation to obtain iron ore of a suitable quality. As large quantities of fine ore cannot be added directly to the blast furnace without adversely affecting its productivity, fine ore is agglomerated into sinter or pellets to make an appropriate charge material.

 

Fine

Fines are used mainly as either sinter or pellet feed, the resulting agglomerated product being charged to the blast furnace. Fines comprise a wide range of products, which fall into three size categories. So-called natural fines, for example, consist of particles commonly less than 6.3mm in diameter, with less than 10–15% below 150 microns.

 

Concentrates

Concentrates have undergone a beneficiation process to produce a grain size less than 1mm in diameter. The finest-grained material, typically below 75mm with a high proportion below 45mm, is used for pellet feed.

 

Lump

Lump is essentially un-beneficiated, naturally occurring pellets or clumps of iron ore, above 5mm and generally below 30mm in diameter. The value of lump ore is greater if it does not break up during transport. An integrated steel mill will normally screen lump prior to its being fed into the blast furnace. Lump that does not quickly decrepitate (break down under thermal load) in the blast furnace is highly valued by steelmakers. There are relatively few deposits worldwide that produce lump ore with these properties, making it a more expensive product than fines. The grade, or iron content, of the product is also important. Sources of lump ore suitable for direct reduction plants (high Fe grade, low acid gangue content and very low friability) are even more limited.

 

Pellets

Pellets are produced by an agglomeration process that involves very fine, pure ore (pellet feed) being mixed with a binder (e.g. a slurry of bentonite), rolled into ‘green’ ball-like pellets and then fired on a grate or in a kiln at about 1,200°C to produce the final indurated product. The process was first developed in North America to convert low-grade taconite ores into an economic feed for integrated steel mills. Pellets are typically 8–20mm in diameter and may also contain fluxes to aid smelting.

 

Sinter

Sinter is created by firing a mixture of fine iron ore and flux (e.g. limestone or dolomite) and then breaking the resulting ‘cake’ into fistsized pieces.

 

Pellet vs. Sinter

The typical iron (Fe) content of sinter is around 55-58%, while BF pellets are normally 62-66% Fe. The higher Fe content of pellets increases blast furnace productivity. In addition, the total acid gangue content of pellets is significantly lower than that of sinter.

To separate silica and other impurities from the iron minerals in taconite ores, these ores have to be very finely ground—generally too fine to be used directly as sinter feed. Agglomeration of the ground ore is therefore necessary for it to be useful. The process is costly, making pellets the most expensive form of iron ore sold on the international market.

 

 

The decision to sinter or pelletise fines depends primarily on purity and granulometry. As noted above, very fine ore is usually pelletised; coarser grades are commonly sintered. Pelletising plants are generally integrated into the iron ore production process and are located adjacent to mine sites or loading terminals. However, sinter plants are almost always integrated into blast furnace operations at steel mills. This is because of the tendency of sinter to break down during transport and handling. For this reason, sinter is not normally a traded iron ore product.

 

Looking Forward

China’s government estimates that carbon emissions from the country’s steel industry will reach their peak by 2025 and achieve a 30% (~420Mt) reduction from the peak by 2030. There will be a substantial decline by 2035, and the steel industry in China will be significantly decarbonised by 2060. Decarbonisation is expected to boost demand for direct feed iron ore products—pellet and lump. BF Pellet demand in China is expected to significantly increase as China’s steel industry tries to lift the pellet ratio in blast furnaces to 30% by 2025 from 17% in 2020.