Converting (metallurgy)

Converting is a term used to describe a number of metallurgical smelting processes. The most commercially important use of the term is in the treatment of molten metal sulfides to produce crude metal and slag, as in the case of copper and nickel converting. Another, now uncommon, use of the term referred to batch treatment of pig iron to produce steel by the Bessemer process. The vessel used was called the Bessemer converter.

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Converting in copper metallurgy

A mixture of copper and iron sulfides referred to as matte is treated in converters to oxidize iron in the first stage, and oxidize copper in the second stage. In the first stage oxygen enriched air is blown through the tuyeres to partially convert metal sulfides to oxides:

FeS + O₂ = FeO + SO₂ CuS + O₂ = CuO + SO₂

Since iron has greater affinity to oxygen, the produced copper oxide reacts with the remaining iron sulfide:

CuO + FeS = CuS + FeO

The bulk of the copper oxide is turned back into the form of sulfide. In order to separate the obtained iron oxide, flux (mainly silica) is added into the converter. Silica reacts with iron sulfide to produce a light slag phase, which is poured off through the hood when the converter is tilted around the rotation axis:

2FeO + SiO₂ = Fe₂SiO₄ (sometimes denoted as 2FeO•SiO₂, fayalite)

After the first portion of slag is poured off the converter, a new portion of matte is added, and the converting operation is repeated many times until the converter is filled with the purified copper sulfide. The converter slag is usually recycled to the smelting stage due to the high content of copper in this by-product. Converter gas contains more than 10% of sulfur dioxide, which is usually captured and subjected to the production sulphuric acid.

The second stage of converting is aimed at oxidizing the copper sulfide phase (purified in the first stage), and produce blister copper. The following reaction takes place in the converter:

CuS + O₂ = Cu + SO₂

Copper content in the obtained blister copper is typically more than 95%. Blister copper is the final product of converting.

See also Smelting.