Discontinuous Matte Conversion

in the molten state by blowing with air; this stage of concentration is known as converting. Copper and iron sulfides, the main constituents of matte, are oxidized to a crude copper, ferrous silicate slag, and sulfur dioxide.

The batch converting process has been em-ployed for many decades in two operating steps at ca. 1200^◦C in the same vessel. Investigations and development of continuous methods are be-ing made [87], [88].

The conventional converting of matte is a batch process that yields in the first step an impure copper(I) sulfide containing ca.

75 – 80 wt % Cu, the so-called white metal, and in the second step the converter, or blister, cop-per averaging 98 – 99 wt % Cu. The name blister copper derives from the SO₂-containing blisters that are enclosed in the solidified metal.

First Step. The main reactions are oxidation of iron(II) sulfide and slagging of iron(II) oxide by added silica [7631-86-9] flux:

2 FeS + 3 O₂ −→ 2 FeO + 2 SO2

2 FeO + SiO₂−→ Fe2SiO₄

Formation of magnetite occurs near the tuyeres:

3 FeS + 5 O₂−→ Fe3O₄+ 3 SO₂

Copper(I) sulfide is partially oxidized, but it is also reformed

Cu₂S + 1.5 O₂−→ Cu2O + SO₂

Cu₂O + FeS −→ Cu2S + FeO

In Figure 6, the first step corresponds to moving along the pseudobinary Cu₂S – FeS line to form an impure copper(I) sulfide.

Second Step. Continuing oxidation occurs as in a typical roasting reaction process:

In Figure 6and Figure 20, the composition moves along the Cu₂S – Cu line from the cop-per(I) sulfide to crude metallic copper, the two phases being immiscible.

Figure 20. The Cu – Cu₂S system [89]

The blister copper contains< 0.1 wt % S, ca.

0.5 wt % O, and traces of other impurities.

Converter Slags. The slags from the first step are iron(II) silicates (40 – 50 wt % Fe) with high magnetite content (15 – 30 wt % Fe₃O₄).

The initial copper content of 3 – 8 wt % can in-crease up to 15 wt % at the end of the reaction by formation of copper(I) oxide. This slag can

be decopperized by returning it to the smelting unit or by froth flotation (cf.page 28).

The high-viscosity small-volume converter slags from the second step have a high copper content (20 – 40 wt %) in the form of copper(I) oxide or silicate. When enough slag has accumu-lated, it is returned to the first converting stage.

Temperature. Converting is a strongly exothermic process that can overheat during oxidation of iron-rich mattes. The temperature must be held ca. 1200^◦C by adding fluxes, cop-per scrap, precipitates from hydrometallurgical treatment (e.g., cement copper), or concentrates.

The off-gas (5 – 10 vol % SO₂) is transferred to a sulfuric acid plant.

The blowing time per batch is a few hours;

however, as the copper content of the matte in-creases, the converting time decreases. Occa-sionally, oxygen-enriched air is used to increase the throughput.

Impurities. The distribution of other ele-ments among the phases during converting is as follows:

1) Noble metals and most of the nickel, cobalt, selenium, and tellurium collect in white metal and then in blister copper.

2) The bulk of zinc and some nickel and cobalt collect in converter slag.

3) The oxides and sulfates of arsenic, antimony, bismuth, tin, and the basic sulfates of lead are found in flue dust.

Converter Types. The copper converter was invented in 1880 by Manh`es and David, based on the Bessemer converter, which had been used in the steel industry since 1855. This develop-ment led to the incorrect name “copper besse-merizing,” although the true Bessemer process is a refining step. Originally, the copper con-verter was upright, and such obsolete units were in operation until the early 1980s, e.g., the Great Falls converter developed by Anaconda Mining Co., United States.

The following types are in use currently (Fig. 21):

1) The Peirce – Smith converter has been the most important apparatus for converting for many decades, and the number in operation

may be in the range of a thousand. More than 80 % of the worldwide produced cop-per comes from this type of converter. It is a horizontal cylinder lined with basic bricks (magnesite, chrome – magnesite) that can be rotated about its long axis (Fig. 22); blast air is blown through a horizontal row of tuyeres.

In practice, the punching of tuyeres with spe-cial devices is necessary to maintain the flow of air. The largest vessels are 12.5 m long with a diameter of 4.6m.

2) Hoboken or syphon converter [91]. This vari-ation of the P – S type was developed years ago by Metallurgie Hoboken N.V., Belgium, but is now used by only a few smelters in Eu-rope and in North and South America; larger units are operated at Glogow smelter in Sile-sia, Poland; Cyprus Miami Smelter in Ari-zona; and Paipote smelter (ENAMI), Chile [92]. Its advantage is its freedom from suck-ing in air, so the off-gas can attain SO₂levels up to 12 %. Special features of the design are the small converter mouth and the syphon or goose neck that guides the off-gas and flue dust flow.

3) Inspiration converter [93]. The vessel has two mouths, the smaller for charging, the larger for the off-gas. The latter is well hooded in all operating positions. It is only in operation at Cyprus Miami Smelter.

4) Top-blown rotary converter [95]. The TBRC, which is known in the steel industry as the Kaldo converter, was adopted by the nonfer-rous industry (first by INCO, Canada) be-cause of its great flexibility. Air, oxygen-enriched air, or on occasion, commercial oxygen is blown through a suspended water-cooled lance onto the surface of a charge of copper-containing materials. In practice, the TBRC is used batchwise for special op-erations on a small scale, but generally not for converting copper matte. Tests of direct smelting of concentrates (clean, complex, or dirty) to white metal or blister copper were performed at R¨onnsk¨ar smelter (Boli-den Metall AB), Swe(Boli-den, but the process was not realized technically. Copper extraction from copper scrap and other secondary ma-terials is also carried out. The TBRC is also well suited for lead/precious metals metal-lurgy.

Figure 21. The evolution of the copper converter [90]

Figure 22. Schematic cross section and back view of a Peirce – Smith converter [96]

Developments for Increased Productivity and Environmental Protection To increase the productivity of a Peirce – Smith converter, usu-ally the oxygen enrichment in the blast is in-creased. Depending on process step (slag or cop-per blowing) up to 30 % O₂is used. The resulting increases in the temperature in the vessel must be controlled. Copper smelters mostly in Europe or Japan add copper scrap during the copper blow phase to adjust the temperature in the converter bath. Special systems (lift and conveyor) have been developed to add up to 70 % of the cooling material without stopping blowing [232], [233]

(Figure 23).

Figure 23. Machine for automatic charging of cooling ma-terial

Automatic temperature measurement (opti-cal spectrometer or thermocouple through the tuyere) is used for process control and to avoid high erosion of the brick lining of the con-verter. Also the chemistry of the converting process is monitored by optical spectrometry [234]. This allows the blowing time to be easily controlled without overblowing the charge and thus minimizes the copper loss to the converter slag and magnetite formation in the slag. High-velocity tuyeres have been developed [235], [236] which can be operated with high oxygen enrichment without extensive refractory ero-sion. At the front of the tuyere a tubular accre-tion is formed which protects the brick lining.

Another advantage is that these tuyeres need not be punched (saves labor and maintenance cost).

A disadvantage is the relatively high pressure of about 3 bar of compressed air (high investment).

These high-velocity tuyeres have been tested in Peirce – Smith and Hoboken converters. Today (ca. 2000) the first smelters (Hidalgo) are plan-ing to install the system.

One of the problems of Peirce – Smith con-verters is leakage SO₂-containing off-gas during charging and pouring operations in the working environment. To avoid these fugitive emissions, special secondary hooding systems have been developed and installed. For example, Figure 24 shows the hooding system used at Norddeutsche Affinerie. The collected fugitive gases are dilute in SO₂ (up to 0.2 %) and are treated by vari-ous techniques (scrubbing with basic solutions or dry absorption of the SO₂producing gypsum) [237], [238].

5.7. Continuous Matte Conversion