Sinter technology was developed for converting ore fines into lumps of material which are easier to put into blast furnaces. This can reduce the waste of ores which have broken down to fines and would be otherwise unusable.
The sinter process works by the formation of agglomerates of various types of ores, together with coke and flux fines. Lime (limestone or chalk) is one of the flux fines within the charge for the sinter process.
The constituent materials are fused to make a single porous mass with little change in the chemical properties of the ingredients.
This is important for two reasons;
The porosity of the sinter allows larger surface area for reaction in the vessel and the bound mass reduces the likelihood of the fine components escaping from the vessel during iron production.
The fact that the properties of the ore does not change during the sinter process is important as the sintered material is used as a feed for the blast furnace for iron and further steel making processes.
There the ores still exhibit the chemical characteristics required for an effective melt.
The flux properties of the lime are particularly important for their part in removing iron melt impurities in the blast furnace.
Iron ore is not only pure iron oxide, it also contains an assortment of other impurities within the material. These do not necessarily melt at the temperature of the blast furnace, and could eventually clog it up.
In addition, sulphur is an issue for higher grades of iron and steel, it can create embrittlement and reduce toughness of the further formed steels.
Lime can be added to the molten pig iron as part of a powdered mixture including other desulphurising agents. These aid in the removal of sulphur as a drawn off slag and improve the quality of the iron melt.
Typically the removal of sulphur in the blast furnace process from molten iron into slag is a partition process.
A ground quicklime may also be used in injection systems and can be used as a pre-treatment process for desulphurisation.
This is process has multiple names, BOS (basic oxygen steelmaking), BOP (basic oxygen process) and LD (after Linz Donawitz, a manufacturer of converters)
Addition of the correct level of lime is considered essential at most modern BOS plants.
For many years, the main production method for steel was the conversion of molten iron from the blast furnace using a basic oxygen furnace (BOS). A BOS furnace is charged with molten iron from the blast furnace and often with the addition of scrap additional iron ores. Carbon is removed from the melt by injecting oxygen at high speed. This addition of oxygen also oxidises some of the iron and many of the impurities present including silica and phosphorous.
Near the start of the blow quicklime is added. The lime and other components react with the oxidised impurities and iron oxide to form a slag. The slag is composed of complex silicates, ferrites and phosphates.
The quicklime acts as source of the CaO required in the process. This CaO allows the viscosity of slag to be modified as calcium silicates are formed. The viscosity affects the ability of the slag to mix with the molten metal and to absorb the impurities.
The formation of a ‘good’ basic slag is essential to ensure that a steel with the desired levels of impurities is achieved which will dictate its end properties without the need for further processing.
Where high strength steels (for automotive, construction tools, wires etc.) are produced, lime with low residual CO2 and no hydration are typically required.
Electric arc steelmaking has until recently involved the re-melting of scrap steel with relatively little refining. However this has now changed with large high productivity units producing steel from scrap of varying quality.
Due to the varying quality of the scrap there may be higher impurities in the melt, so once again quicklime is added to remove the impurities and refine the melt.
Modern furnaces melt using gas and oxygen burners as well as electric power to reduce tap-to-tap times. Quicklime is used by steelmakers as a flux agent to efficiently and effectively remove impurities such as phosphorus, silica and alumina from scrap melting in the EAF.
The smaller size of the furnace with the less extreme agitation compared to the BOS process and short power on times usually mean that the lime is added as a smaller size than the BOS process.
Secondary steel processing plays a key role in fine tuning steel chemistry and addition of complementary alloys.
Lime is used in the process to lower oxygen content and remove impurities such as sulphur and reduce inclusions trapped by the basic slag.
In stainless steel manufacture, lime is used in the Argon Oxygen Decarburization Furnace (AOD). Lime is also used in Ladle Metallurgical Furnace and (LMF) and Vacuum Degassing secondary processes.
These secondary processes can change the physical properties of the steel and ensure its suitability for more specialist steel applications.
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