Magnesia-chromite Bricks

Keywords Stillwater Mining Company, Ca-ferritic slag. Magnesia-chromite bricks. Rotary kiln. Corrosion resistance test. Refractories, RHl... 

All tested brick samples show crack formation due to thermal shock during kiln operation (Fig. 3). In case of the magnesia-chromite brick brands there is either minimal or no wear visible. The high wear observed for the alumina-chromia brick RESISTAL RK30SR can be explained by crack formation and loss of the brick parts due to high thermal shock from the beginning of testing. 

The main microstructural changes to the investigated magnesia-chromite bricks, after the rotary kiln test, are summarized below (exemplary see RADEX VFG, Fig. 4). 

From the different investigated magnesia-chromite bricks the RADEX FGLI shows the best result and the lowest solubility rate. But the difference of the various brands with exception of RADEX 0X6 COMPACT is low. 

All tested magnesia-chromite bricks show macroscopically either no or minimal wear after both rotary kiln tests. The possible explanation for the latter is the trial duration of 20 cycles which is standard testing method at the TC Leoben due to tightened testing methods. Nevertheless the following observations can be summarized comparing both tests ... 

For that purpose 120 kg of slag was received from Stillwater Mining Company, for a detailed investigation and for additional rotary kiln tests to be carried out on different magnesia-chromite- and alumina-chromia brick brands. Prior to testing a complete chemical and mineralogical characterization of the slag was earried out at RHl lab facilities. 

Figure 3. Selected exemplary magnesia-chromite RADEX VFG (a and b), RADEX 0X6 COMPACT (c and d) and alumina-chromia RESISTAL RK30SR (e and f) bricks after the rotary kiln test at 1500°-1550°C (left side a, c, e) and 1650°C (right side b, d, f). Slag coating (C). Crack formation (arrows). 

The slag amount in dependence of has been compared for the different refractory bricks (see Figure 7) at 1550°C and 1650°C. The alumina-chromia brick shows a considerably higher solubility compared to the magnesia-chromite brands that can be ranked with regard increasing solubility ... 

Refractory-grade chromite is used for manufacturing magnesia-chrome bricks used in the extractive metallurgy of platinum group metals. Foundry sands made of chromite fines are used for making mould used for casting nonferrous metals. 

Dead burned magnesia with chromite Fused magnesia Magnesia-carbon bricks... 

Cbrome bricks are manufactured in much the same way as magnesite bricks but are made from natural chromite ore. Commercial ores always contain magnesia and alumina. Unburned hydraulically pressed chrome bricks are also available. 

Chromite refractories are employed in furnaces as partings between the silica and magnesite bricks and heiice are considered neutral. They consist principally of crushed and ground chromite, and bricks are molded from this material which may be bonded together with a small amount of clay or burnt magnesia by pressing under a heavy pressure. The bricks are then dried and fired to a temperature of about cone 16. The composition of these refractories fluctuates between the following values ... 

The immediate brick hot face was covered with a 1-2 mm thin reaction zone. Within this area the magnesia component is completely dissolved, only some chromite relics are still visible. Below the reaction zone a deep reaching infiltration and corrosion of the brick microstructure can be observed. The lowest infiltration of the brick microstructure was observed for RADEX 0X6 COMPACT and the highest for RADEX FM5 (see also Table I). 

In the infiltrated and partly completely degenerated brick microstructure (0-5 mm from the hot face) the single brick components cannot be distinguished any more. The high supply of Fe-oxide resulted in formation of a low melting Ni-rich magnesia-wuestite ((Ni)-Fe-Mg-oxide). The second brick component chromite, as well as, chromite precipitations are corroded. Due to corrosion of chromite Ca-Mg-Fe-Al-Cr-oxide and Ca-Fe-Cr-Al-oxide formed. 

Figure 4. Exemplary microstructural overview detail. Immediate brick hot face. RADEX VFG. (a). Reaction layer (R). Infiltrated and corroded brick microstructure (I), (b) Approx. 2 mm fi-om hot face. Corroded magnesia (1) and chromite (2). Ca-Mg-Fe-Al-Cr-oxide (3). Ca-Fe-Cr-Al-oxide (4). Dicalcium silicate (5). Cu-Fe-sulphide (6).

Based on the mineralogical investigation the brick microstructure is highly degenerated especially within the area 0-20 mm from the hot face. Infiltration, corrosion of both brick components e.g. magnesia and chromite, formation of a low melting Cu-Ni-rich magnesia-... 

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