5. Production
5.7. Continuous Matte Conversion
pro-cesses have been in operation for many years, continuous converting of matte has come into use slowly. The potential benefits are mini-mization of materials handling (especially over-head crane transport of liquids), more efficient off-gas capture, and continuous SO2 produc-tion for the sulfuric acid plant. Two multiple-furnace processes are used today for continuous smelting and converting: the Mitsubishi process and the Kennecott/Outokumpu Flash converting process. Noranda has developed a continuously running converter which has been in operation since 1997 at Horne Smelter.
5.7.1. Noranda Process
In the 1970s Noranda started with the Noranda reactor for directly smelting blister. This was not useful and was therefore switched to smelt-ing high-grade matte. A second reactor similar to the smelting one is proposed for continuous conversion of matte (patented 1985 [239]). Since 1997 it has been operated at Horne smelter. It is a horizontal cylindrical vessel with two mouths (one for adding liquid matte by ladle, the other for the off-gas) and a row of submerged tuy-eres. There is also the possibility to feed solid matte or coal by slinger belt through one end wall. The converter is usually fed with liquid high-grade matte (70 % Cu) from the Noranda smelting reactor. It produces semi-blister cop-per with high sulfur content (1 – 1.5 %). The semi-blister is poured into a ladle which is trans-ported by special ladle car to the anode furnace.
The Noranda converter can be operated in four
Figure 24. Hooding system for the prevention of fugitive emissions at Norddeutsche Affinerie
modes: with molten matte feed, with any com-bination of solid and molten matte feed, up to 100 % solid feed, in smelting mode with concen-trate like the Noranda reactor and a conventional Peirce – Smith converter. The converter at Horne Smelter (Figure 25) [240] is 4.5 m in diameter and 19.8 m long (inside the brick lining). It has 42 tuyeres. The process off-gas is collected in a water-cooled hood and sent to the sulfuric acid plant. The converter has a secondary ventilation system, the gases of which are sent directly to a stack. This process operates continuously and has very high flexibility but not all of the poten-tial benefits have been achieved. There is still crane transport and handling with ladles which causes fugitive emissions.
5.7.2. Mitsubishi Process
Mitsubishi Metals Corp., Japan, tested the new concept during the 1960s and started the first commercial plant at Naoshima smelter in 1974. Today (ca. 2000) four companies are op-erating this process (Naoshima, Japan; Kidd Creek, Canada; LG Metals, Korea; Gresik, In-donesia). The principle of this process is the in-terconnection of three furnaces, as shown in Fig-ure 26[241], [242]:
1) Smelting or S-furnace: Dried concentrates, flux material, pulverized coal, return
con-verter slag, copper scrap, and flue dust are smelted in this furnace. The fine material (concentrate, coal) is fed through nine or ten vertical steel lances on the top of the molten bath. The blowing lances consist of two concentric pipes. Through the inner pipe, the dried concentrate is air blown, and through the outer one, oxygen-enriched blast (40 – 50 % O2). The lances are rotated to pre-vent them sticking in the roof. They extend to 0.5 – 1 m above the molten bath. About 0.3 – 0.5 m of the lance is consumed each day.
The off-gas contains 30 – 45 % SO2 and is sent to sulfuric acid production.
2) Slag-cleaning furnace: Slag and matte from the S-furnace flow continuously by grav-ity into the elliptical slag-cleaning furnace, which is an electric furnace with three or six submerged graphite electrodes. The tem-perature of the slag is kept at 1250 ◦C.
The slag is decopperized to 0.6– 0.9 % Cu and discarded. The matte flows continuously through a siphon into the converting furnace.
3) Converting or C-furnace: The matte (68 % Cu) is converted continuously by blowing enriched air (30 – 35 % O2) and CaCO3flux through six lances on the top of the bath. Also copper scrap is added through the sidewall of the furnace. Conversion only takes place where the oxygen comes into contact with the
Figure 25. Noranda converter at Horne Smelter
sulfides. This is achieved by running the con-verter with a special basic calcium ferrite slag (40 – 50 % Fe, 15 – 20 % CaO). The usual silica slag is not possible because when oxy-gen is blown onto the top, solid magnetite is formed and blocks the surface. In the calcium ferrite slag dissolves the magnetite but also a large amount of copper (15 – 18 %). The con-verter slag is returned to the S-furnace. The blister copper contains slightly more sulfur (0.7 %) than from a Peirce – Smith converter (0.02 %). It is sent to an anode furnace.
Some operating data for the Mitsubishi pro-cess (Naoshima Smelter, Japan) are summarized in the following:
Commisioning date 1991
S-furnace
Diameter inside brick, m 10
Number of lances 9
Concentrate throughput, t/d 2050
Copper scrap, t/d 20
Blast, m3/h (STP) 37 500
O2enrichment, % 49
Off-gas volume, m3/h (STP) 39 000
SO2in off-gas, % 29
Matte grade, % 68
Slag-cleaning furnace
Size, inside brick w× l × h, m 6× 12.5 × 2
Number of electrodes 6
Residence time ca. 2 h
Slag 0.6% Cu
Off-gas volume, m3/h (STP) 50
C-furnace
Diameter inside brick, m 8
Number of lances 10
oxygen enrichment, % 33
Blast flow rate, m3/h (STP) 25 500
Copper Scrap, t/d 190
Off-gas volume, m3/h (STP) 25 000
SO2in off-gas, % 26
The major advantage of the Mitsubishi pro-cess are the good SO2 capture and the lower handling expense of materials. A disadvantage during the initial operating time was that no scrap material could be added. This is has now been solved [243]. With a smelting capacity of about 2000 t/d of copper concentrate, Naoshima Smelter processes also about 40 000 t/a of cop-per scrap from the market and additional anode scrap from the refinery. Another problem is the impurity behavior. Especially lead is a problem because of the calcium ferrite slag. If too much lead is in the feed, the copper anodes are too rich in lead for electrorefining. A comparison of the impurity behavior is given in Table 14.
Table 14. Impurity contributions to anodes (wt % of input)
Process Reverbatory/Peirce – Smith Mitsubishi
Pb 8 – 13 15 – 19
As 9 – 11 4 – 6
Sb 28 – 34 15 – 20
Bi 14 – 26 15 – 25
Ni 45 – 50 6 7 – 75
Figure 26. Mitsubishi process
5.7.3. Kennecott/Outokumpu Flash Converting Process
In the 1980s Kennecott announced the develop-ment of their Solid Matte Converting (SMOC) process [244]. This process is based on solidify-ing the molten matte from the smelter, grindsolidify-ing, and feeding the solid to an high-oxygen blast converter. This process decouples the smelting and converting steps and gives great flexibility.
Later on, it was developed with Outokumpu, and for the converting step also a well-proven flash furnace was chosen. In 1995 the process went into operation at Garfield Smelter.
For matte smelting a flash furnace with a con-centrate feed rate of about 140 t/h was built (oxy-gen enrichment 65 – 75 %). The liquid matte contains 68 – 70 % copper and is granulated in water. In the second step the matte is milled and then fed together with lime as flux material to the flash converter, which operates with an oxy-gen enrichment of 65 – 75 % O2. The feed rate of matte is about 60 t/h (up to max. 80 t/h) [245].
This furnace is constructed like the smelting fur-nace. A blister copper and a calcium ferrite slag with about 18 % copper (like in the Mitsubishi process) is obtained. The slag is granulated and fed back to the flash smelter.
The off-gases contain> 30 % SO2 and are converted to sulfuric acid (input concentration of 14 % SO2in the sulfuric acid plant). This process has very low SO2emissions of only 3.5 kg SO2 per tonne of copper, which is until now the
low-est in the world. This is achieved because it has almost no fugitive emissions. The disadvantage is that no scrap can be added to the converter.