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The Kennecott/Outokumpu Flash Converter, which uses calcium ferrite slag, has reported a number of problems in containing the slag within the magnesia-chrome refractory lined furnace. During the furnace relining operation, Newman et al. (1998) reported that the wear of the refractory lining inside the converter was most severe at the sidewall area beneath the reaction shaft. The refractories in this area were in direct contact with the highly corrosive calcium ferrite slag. The containment of the ferrite slag is also a major issue in the Mitsubishi converter. As reported by Ajima et al. (1993), calcium ferrite slag is far more aggressive towards the magnesia-chrome refractories than iron silicate slag. Campaign lives of less than one year were experienced in the early development stages of the Mitsubishi converting process. Lee et al. (1999) documented that after 13 months of operation; the Mitsubishi converter at the Onsan Smelter required refractory relining. Similar to the Kennecott flash converter, inspection of the Mitsubishi C-furnace during programmed shutdown revealed that brick damage was most severe at the furnace sidewalls in contact with the slag as well as the slag outlet port. MacRae et al. (1998) and Tanaka et al. (1999) further reported that in addition to the refractory damage caused by slag penetration at the slag/brick interface, the attack was aggravated by the melt splash and waves generated by the lance blowing O2/air.

MacRae et al. (1998) found that the aggressive refractory corrosion within the slag zone and the outlet port is further exacerbated by the presence of copper oxide in the penetrated slag, which causes structural stresses and crack propagation in the brick. The nominal copper oxide content of the slag is 16-19%. Whilst the campaign lives of the refractory linings in the Mitsubishi and Kennecott Flash converters have been extend by various means, including moving to fused-cast magnesia-chrome refractories, water cooling jackets and the magnetite

protective layer, refractory wear caused by the calcium ferrite slag is still a major issue in smelter operations around the world.

Copper smelters around the world have implemented a number of strategies to reduce the rate of wear of refractory lining in the converter. The Mitsubishi Corporation initially implemented the replacement of direct bonded bricks with fused-cast magnesia-chrome refractories in the high wear region at the slag line to deal with the slag containment problem. Fused-cast bricks have a closed pore network, which consequently reduces slag infiltration. Whilst application of fused-grain refractories proved partially successful in limiting slag penetration, the bricks are considerably more expensive than the magnesia-chrome refractories as they are processed at temperature above 2000oC (Cherif et al., 1997).

Mitsubishi also installed water cooled copper jackets in the side walls of the C-furnace to further reduce refractory wear caused by calcium ferrite slag. The copper bars inserted into the brickwork extract heat by water-cooling the ends of the bars which project from the furnace. The water-cooling elements were specifically designed to handle the high heat flux developed by the converting process. The sidewall brick and jacket arrangement designed by Mitsubishi is shown in Figure 2.6.1. The design of the converting furnace’s copper cooling jacket arrangement consisted of placing the jackets directly behind and between the fused-cast bricks at the slag line. The extraction of heat by the cooling jacket arrangement from the brick lining lowers slag corrosion by increasing the temperature gradient within the brick, containing and limiting the slag penetration region of the brickwork close to the slag/brick interface. Water-cooling the furnace lining has lowered refractory consumption at the slag line.

Figure 2.6.1: Arrangement of refractory brick and cooling jackets in the C-furnace (Ajima,

This strategy proved to be effective, with the life of the sidewalls doubling from approximately two years to four years (Ajima et al., 1993). The implementation of the improved cooling system, although relatively successful, comes at a significant cost, since the heat removed as a result of the cooling system must be replaced to maintain smelting temperature. Thus in order in achieve this, increased fossil fuel usage and process air enrichment is necessary.

Whilst Kennecott Copper also has employed a sidewall cooling system, they also control slag composition in order to precipitate a protective layer of magnetite on the sidewall bricks (Newman et al., 1998). This is done by controlling the lime and copper levels in the slag. Any reduction in the thickness of the magnetite protective layer on the surface of the bricks can be readily detected by monitoring the heat losses in the settler sidewall cooling system. Whilst magnetite coating prolongs furnace campaign lives by protecting the bricks and jackets inside the furnace, if the magnetite content in the slag is too high, troubles such as accretion build-up and taphole blockages and increase in slag viscosity results.

The cost of maintaining and replacing refractory bricks as a result of slag attack is one of the significant cost components in the copper industry. All current converting processes use magnesia-chrome refractories in contact with slag because they have the best properties available at present. With the application of all the techniques mentioned to reduce refractory wear caused by calcium ferrite slag, the bricks are still only adequate in containing the ferrite slag. The alternative FCS slag for copper converting proposed by Yazawa et al. (1999) has been predicted to have the potential to avoid the refractory wear problems encountered by use of calcium ferrite slag in the converting furnace (Takeda, 2001). However experimental data to support such predictions is yet to be published.