The cathode carbon block and cathode steel bar are closely combined into one process. Cathode carbon block is an important part of aluminum reduction cell, which is directly used as cathode lining and conductive material. The cathode circuit of aluminum reduction cell is formed by connecting cathode steel bar and cathode bus. The assembly process of cathode carbon block and cathode steel bar should reduce the contact resistance between them as much as possible.
There are three ways to assemble cathode carbon block and cathode steel bar( 1) Cast iron casting method. The gap between them is cast with molten phosphorus pig iron, which is condensed to form a whole( 2) In the carbon paste tamping method, the gap on both sides of the steel rod is filled with cathode paste called steel rod paste, and tamped and compacted with pneumatic tools( 3) Bonding method. The cathode carbon block and steel bar are bonded together with carbon glue or adhesive. Graphite or metal powder (aluminum or iron) is added to the binder to reduce its contact resistance. Pig iron casting method and paste tamping method are commonly used in China.
The cross section of cathode steel bar is square or rectangular, and the cross section size is generally 115mm × 115mm、130mm × 130mm、130mm × 65 mm。 The cathode carbon block with square cross section is generally assembled with a single rod, that is, a cathode steel rod is assembled on a cathode carbon block. This kind of packing block is mostly used in electrolytic cells with capacity less than 135kA. The cross section of large pre baked electrolyzer with capacity above 140ka is generally 130 mm × 65 mm Rectangular Steel rod, and double rod assembly, that is, two cathode steel rods are assembled at the end of a cathode carbon block.
The cathode steel bar must be derusted before assembly, such as sand washing and polishing, so as to make it closely and firmly connected with the sealing medium and reduce the contact resistance.
There are two pouring methods of pig iron: horizontal pouring and vertical pouring.
(1) Horizontal pouring method. Place the carbon block horizontally with the dovetail groove upward, and place a 15 mm thick phosphorus pig iron cushion block in the center of the groove. 50% clay and 50% carbon powder are used to make mud grid strips with diameter of 15 ~ 20 mm, which are placed on both sides of dovetail groove. A number of mud grid strips are placed in the dovetail groove, and the spacing is 150 ~ 170 mm. After drying, the carbon block and the steel bar are preheated in the flat bottom coke oven, and then poured with molten phosphorus pig iron, and the mud grid bar is used as a unit for subsection. The surface of ferrophosphate is covered with yellow sand; Cool, clean, check.
(2) Vertical pouring method. The cathode carbon block is placed vertically in the pit type heating furnace, phosphorus pig iron block is placed at the end of dovetail groove, and the steel bar is placed and fixed. Seal dovetail notch with graphite plate; The carbon block and steel bar were preheated and poured in sections with phosphorus pig iron, each section was about 100 ~ 160 mm long, and each section was poured for 10 ~ 15 min intermittently; Cooling; Cleaning and inspection.
The cathode carbon block group made by carbon paste tamping method can produce plasticity due to the softening of carbon paste during the initial operation of electrolytic cell, which can buffer the stress caused by the thermal expansion of carbon block. The cathode voltage drop of tamping method is 400 ~ 450mv, which is higher than that of casting method (380 ~ 420mv); Half a year later, with the further sintering of cathode paste, the two are basically close.