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Question 1 Of 500 Stainless Steel Smelting: How To Evaluate The Quality Of Metallurgical Lime?

May 17, 2021

Lime is the main slagging material for alkaline slag. The pros and cons of lime quality are mainly manifested in the chemical activity, reaction capacity, slag-forming melting speed and impurity element content of lime.

The quality evaluation indexes of high-quality metallurgical lime have the following points.

1. Low density and high porosity

About 40% (mass fraction) of carbon dioxide gas is released during the calcination of limestone. High-quality lime has large pores and low density. Usually the density of active lime is 1.5-1.7g · cm-3, and the porosity is greater than 50%. This kind of lime is very light and is called high-quality light lime. This kind of lime has strong reaction capacity and quick slag melting speed.

2. High calcium oxide content, low ignition loss

The higher the Cao content in metallurgical lime, the better the quality. The Cao mass fraction of lime used in induction furnace smelting should be greater than 85%. Lime is white in appearance, light in texture, and has very little undecomposed matter.

3. The content of carbon and sulfur in lime should be low

Inferior lime contains 0.30%-1.90% of C mass fraction. When the C mass fraction ≤0.030% is used to smelt steel, it can increase the carbon of molten steel by about 0.008%. The mass fraction of sulfur in lime can reach 0.15%. When smelting precision alloys, the sulfur can be increased by about 0.005%-0.025%.

4. The moisture content in lime should be low

Lime easily absorbs moisture in the atmosphere and deliquescence, that is, deliquescence reaction occurs:

CaO(s)+H20(g)== Ca (OH)2(s)

The moisture in the lime is one of the reasons that lead to the increase of hydrogen in molten steel, and it is also a problem that is often overlooked by steelmakers. Therefore, the metallurgical lime used for slagging must be stored in a dry environment, and it should be baked at about 850°C before use to remove the crystal water. Bake below this temperature cannot remove the crystal water.