What Is a Graphite Anode?

Graphite anodes are graphite plates, blocks or rods used as anodes in electrolytic cells in the electrolytic industry. There are two types of electrolytic processes using graphite anodes, one is aqueous solution electrolysis and the other is molten salt electrolysis. The chlor-alkali industry producing electrolytic caustic soda and chlorine gas is the largest user of graphite anodes. In addition, there are some electrolytic cells for preparing light metals and rare metals such as magnesium, sodium, and tantalum by molten salt electrolysis, and graphite anodes are also used.

The chlor-alkali industry is a basic chemical industry. Its caustic soda is widely used in many industrial sectors such as textile, papermaking, metallurgy, organic and inorganic chemical industry. Chlorine is also an important chemical raw material. Chlor-alkali products have been produced by electrolytic salt solution for more than 100 years. In the long years, there were two types of electrolytic cells, namely mercury electrolytic cells and diaphragm electrolytic cells, which had coexisted for a long time and kept pace with each other, which had played an important role in the development of the chlor-alkali industry. However, due to the serious environmental pollution caused by mercury, such electrolytic cells have been gradually phased out since the 1970s. Asbestos diaphragms used in diaphragm tanks are also harmful to the environment. The ion-exchange membrane method is an advanced chlor-alkali production process that has greatly improved the membrane technology.

For a long time, graphite anodes have been used in both mercury and diaphragm electrolytic cells (see Figure 1). During the operation of the electrolytic cell, the graphite anode will be consumed slowly. The mercury electrolytic cell consumes 4-6 kg of graphite anode per ton of caustic soda, and the diaphragm electrolytic cell consumes about 6 kg of graphite anode per ton of caustic soda. As the graphite anode becomes thinner and the distance between the cathode and anode increases, the cell voltage will gradually increase. Therefore, after a certain period of operation, the anode needs to be stopped and replaced. In the mid 1960s, the Dutchman H. Bill invented a metal anode coated with ruthenium on the surface of titanium. Because the metal anode is basically not consumed, it is also called a dimensionally stable anode (DSA). In addition to non-consumption and long service life, metal anodes also have the advantages of high current efficiency, good quality of chlor-alkali products, energy saving, no special pollution, and low maintenance workload, so they are favored and quickly applied. However, because metal anodes are expensive, metal ruthenium resources are scarce, and supply is difficult to guarantee. When metal anodes are used, the electrolyte brine must contain calcium and magnesium ions not greater than 5 × 10. Therefore, strict purification treatment of the electrolyte is required. It can be expected that metal anodes will not completely replace graphite anodes for quite some time.

During the operation of the electrolytic cell, the graphite anode is mainly consumed by the following forms of corrosion. First, because the electrolyte contains a large number of hydroxide ions and a small amount of sulfate and hypochlorite ions as impurities, these ions will be discharged on the anode to generate oxygen. The generated primary ecological oxygen oxidizes graphite into carbon dioxide or carbon monoxide, and this oxidative loss accounts for more than half of the total graphite anode loss. Secondly, with the deepening of oxidation, the bonding between the graphite anode body particles was gradually destroyed, and the graphite anode surface layer combined with the loose particles, and slag was generated under the saltwater scouring. Oxidation and slagging make the anode thin, and to a certain extent, it needs to be stopped and replaced. Such residual loss accounts for more than 1/4 of the total consumption of graphite anodes. Both oxidation and slag consumption are related to the porosity of the graphite anode. The porosity is large, and the consumption is also large. The most commonly used method to reduce the porosity of graphite anodes is to impregnate the whole or root with a dry oil such as linseed oil and tung oil before use. ^[1]

The production process and raw materials of graphite anodes are basically the same as graphite electrodes. Due to the high requirements for the density and strength of graphite anodes, the semi-finished products after baking must also be impregnated with asphalt, and then graphitized and machined. Table 1 lists the size specifications of graphite anodes in China. The physical and chemical properties of graphite anodes are shown in Table 2. The vanadium content is also specified for graphite anodes for mercury tanks.

Table 1.Dimensions of graphite anodes in China

use	specification
	Square thickness / mm × width / mm × length / mm		Round diameter / mm × length / mm
Aqueous solution electrolysis	51 × 51 × 970 40 × 180 × 760 40 × 180 × 960 50 × 180 × 635 50 × 180 × 940	50 × 250 × 640 50 × 250 × 1140 75 × 180 × 640 115 × 400 × 1050 115 × 400 × 1300	50 × 1000 65 × 1300 65 × 1000 65 × 650 65 × 330	75 × 1320 15 × 320 100 × 1360 100 × 252
Molten salt electrolysis to produce magnesium to produce sodium	170 × 840 × 1755 (100 150) × (400 500) × (1800 2000)		400, 800

Table 2 Physical and chemical properties of graphite anodes

category	index
	Resistivity / · m no greater than	Ash /% no greater than	Compressive strength / MPa not less than	Flexural strength / MPa not less than	Bulk density / g · cm-3 not less than	Vanadium content /% Mercury tank anode is not larger than
For electrolytic solution
Level 1 Level 2	8 9	0.2 0.4	29.4 24.5	16.7 14.7	1.62 1.65	10 × 10-5
For molten salt electrolysis
Magnesium production	10.12		17.6 19.6
Sodium production	17.1	0.3	17.4		1.55

The graphite anodes used for molten salt electrolysis are mostly some large-scale products. For example, electrolytic cells for the production of sodium metal by molten salt electrolysis use large graphite anodes with a diameter of 400 to 800 mm, and electrolytic cells for the production of magnesium metal by molten salt electrolysis. It is a large-sized graphite block. ^[2]