Pitch-impregnated refractory brick

Abstract

 The invention relates to an improvement of refractory materials for the lining of electrolytic reduction cells used for the winning of aluminum by molten salt electrolysis.
If the refractory brick consists of ceramic material having an open pore volume which is filled with pitch or resins of high carbon residue the resistance of the refractory against cryolite is improved. Further improvements can be made by impregnating the carbon materials with pitch or resins of high carbon residue.

Description

[0001] The invention is related to the improvement of refractory materials for the lining of electrolytic reduction cells as used for the winning of aluminium by molten salt electrolysis.

[0002] A typical example of a reduction cell lining is shown in Figure 1. The bottom of the electrolysis basin consists of carbon or graphite cathode blocks (1). Steel collector bars (2) are embedded in the cathode blocks and serve as electrical connection to the outside of the potshell (3). Carbon materials (4) are also used as a major part of the side wall lining of the cell, particularly in contact with the liquid metal pad on the cathode bottom as well as with the electrolytic melt above the metal. The main component of the electrolyte is molten cryolite. It contains a few percent of dissolved alumina and some other additives such as aluminium and calcium fluoride. The cell operating temperatures range from 950 to 970°C.

[0003] Various types of fireclay bricks (5) are commonly applied as back material of the carbon cathode bottom and carbon side wall lining. The carbon materials at the hot or front face of the electrolysis cell are pervious to the cryolitic melt. For this reason, the fireclay bricks are impregnated by cryolite, react with it and are partially converted to a magma-like slag. As time proceeds, electrolyte constituents penetrate deeper into the refractory lining, degrade its insulation capacity and cause swelling and softening of the fireclay bricks. The insulating and structural conditions become even worth when the light-weight insulation bricks (6) in the outer zone of the potshell lining are affected.

[0004] The objective of the invention is to effectively improve the resistance of the refractory, especially of the fireclay bricks against cryolite. This can be achieved by filling the open pore volume of the refractory bricks with impregnating pitch. The procedure of pitch impregnation is carried out as follows: The refractory bricks are placed in an impregnation autoclave where they are pre-heated to 150 - 250°C. Preheating of the bricks can also be done outside the autoclave prior to the impregnation process. The autoclave and the bricks are evacuated. Thereafter the bricks are flooded with liquid impregnating pitch. After pitch flooding pressure of 5 - 10 bar is applied (compressed air or nitrogen) to press the liquid pitch into the open pores. When thorough impregnation is completed, the pitch is withdrawn and the bricks are taken out from the autoclave.

[0005] The impregnated bricks may be directly used in the reduction cell lining if their number is limited. During heating up of the reduction cell, fumes and volatiles are released from the pitch due to its carbonization. Therefore, good ventilation and fume collection is required at the cell operating temperature. For environmental reasons it is more advantageous to subject the pitch-impregnated refractory bricks to a carbonization or coking process in dedicated furnaces up to 1000°C.

[0006] The cryolite resistance of normal, non-impregnated fireclay bricks was compared with pitch-impregnated heat-treated fireclay bricks by two testing methods, namely the "finger" and the "cup" test. Both well-established tests showed distinct differences in cryolite resistance between the two preceding types of bricks. The non-impregnated fireclay brick is strongly attacked and a substantial portion of the brick material is dissolved and scorified. The pitch-impregnated brick did practically not indicate any sign of surface erosion and slagging. Plant tests proved this result and the favourable behaviour of the pitch-impregnated fireclay bricks. In the reduction cells, the pitch-impregnated bricks are preferably laid adjacent to the carbon (4) or graphite lining materials (7), i.e. under the cathode blocks and below or behind the carbon material in the side lining.

[0007] Service life and thermal conditions of the electrolytic reduction cell is greatly enhanced by the pitch-impregnated refractory bricks according to the invention.
Furthermore, it could clearly be demonstrated that the pitch-impregnated bricks are also highly resistant against liquid aluminium in the presence of cryolite-based melts.

[0008] Also, other carbonaceous impregnating fluids such as phenolic and/or furane resins can successfully be applied for the impregnation of refractory lining bricks for aluminium reduction cells. Therefore the new refractory brick for the lining of aluminium reduction cells is characterized in that the open pore volume of fireclay bricks or other ceramic bricks is fully impregnated with pitch or resins of high carbon residue to provide a superior resistance of the impregnated bricks against components and reaction products of cryolite-based melts as well as against molten aluminium.

Claims

1. Aluminium reduction cell comprising a refractory brick lining, carbon materials used as part of the side wall and/or fireclay materials used as back materials of the cathode bottom and side wall lining,
characterized in
that the refractory brick lining consists of ceramic material having an open pore volume which is filled with pitch or resins of high carbon residue.

2. Aluminium reduction cell according to claim 1 wherein the carbon materials are impregnated with pitch or resins of high carbon residue.

3. Aluminium reduction cell according to claim 1 wherein the pores of the fireclay material are filled with impregnating pitch or resins of high carbon residue.

4. Aluminium reduction cell according to claim 1 wherein pitch impregnated bricks of fireclay or carbon materials are positioned under the cathode blocks and below or behind the carbon material in the side lining.

Drawing