Anode carbon consumption

For this reason the consumable anodes must be replaced periodically. The cathode consists of a molten aluminum layer on the bottom of the cell, and the anode-cathode distance is 4-5 cm. Alumina is periodically added to the cell in the proportion that it is consumed by electrolysis. The electrode processes during aluminum electrolysis are very complex [141] and a proper understanding of these processes is important because of the economic implications energy and carbon consumption, cell control, pollution of the environment, etc. 

Reaction (89) requires twice as much carbon as reaction (88) for the same quantity of electricity. One might then expect that the problem of the primary anode product could be easily solved by determining the gas composition and the carbon consumption. 

Data from industrial cells show that the exit gas contains 90-60% C02 and 10-40% CO. The carbon consumption usually ranges from 400 to 550 kg/ton Al, while the theoretical amount at 95% current efficiency is 350 kg/ton A1 for reaction (88) and 700 kg/ton Al for reaction (89). The question is then whether the extra consumption, compared to the theoretical value for reaction (88), is due to simultaneous primary formation of C02 and CO or if it is due to the Boudouard reaction between gaseous C02 and the sides and the interior parts of the anode [141], or carbon dust [200] dispersed in the electrolyte, or air burning of the anode. In any case, it is known [200,201] that in the laboratory a high content of CO is formed at low cds (<0.05 A cnr2) and almost pure C02 is formed at 1 A cm-2. 

The aluminum industry consumes much more carbon, as baked anode composites, than the total of all other industrial uses for baked and graphitized carbon products. The free world s total annual aluminum production capacity is approximately 16 million short tons, about one-third being produced in the United States. World aluminum production involves the consumption (oxidation) of about eight million tons of anode carbon. Production occurs by electrolytic deposition from cryolite-alumina melts using a process patented simultaneously, but independently, in 1886 by Hall in America and Heroult in France. While minor process modifications have been made in the intervening years, and productivity greatly increased, substantially the same process is still used. The industrial electrolytic cell consists of a shallow carbon vessel about 10 ft. wide by 30 ft. long, and 1-2 ft. deep, which acts as the cathode and contains the fused salt bath and molten aluminum product. The carbon anodes are supported above the cathode and lowered into the cell at the rate of... 

It is the purpose of this paper to review the important factors which affect anode carbon usage in the aluminum industry, Consideration is given to the entire chain of events affecting carbon consumption, from the properties of the precursors for filler cokes and binder pitches, through production of these raw materials and their fabrication into anode carbon, and concluding with anode performance evaluation in full-size prebake and Soderberg cells of different designs. 

The third mechanism of carbon consumption is airburn of prebake anode tops and the bottom edges of Soderberg anodes during cell operation. This mechanism typically accounts for about 17% of total prebake carbon consumption, but can vary (for different cell designs) from less than 10% to about 40% during severe airburn problems. The following equation represents such airburn reactions ... 

In addition to oxidation losses-by the above four mechanisms, mechanical carbon loss (dusting) also occurs due to uneven oxidation of the anode surface. Furthermore, a variety of other factors, related to anode fabrication and use, can affect carbon consumption. The most important of these factors are (1) raw material quality,... 

Anode porosity is important because it affects the extent of oxidant-accessible surface. This surface is influenced both by coke microstructure and the fabrication process for converting the raw materials into baked carbon. The prime requirement for good anode carbon is minimum oxidant-accessible surface. It is also desirable that this surface have a low, uniform specific reactivity. Anode surface with pores having diameters in the 1-10 micron range are accessible to oxidation unless blocked in some manner. Submicron porosity, such as that produced by thermal desulfurization of coke, is oxidant diffusion-limited and will not affect carbon consumption significantly. Increasing anode carbon density will usually increase anode performance because the oxidant-accessible surface is reduced. 

In this case, anode carbon using such a filler-coke blend exhibited 15% higher carbon consumption than that carbon made with the anisotropic filler alone. 

Because of the highly aggressive conditions in the high-temperature, molten salt media, attempts to find alternative anode materials or cell chemistries have been unsuccessful and developments in anode technology have been limited to minimizing the carbon consumption toward the stoichiometric requirement. Indeed, carbons are one of the few materials to be stable to cryolite at the operating temperature and hence, for example, it is also employed as an internal lining to the steel cell bodies. 

The question whether CO2 or CO is the primary anode product has been studied extensively [2, 3] and it has been shown that, except at very low current densities, the primary product is CO2 [2]. This conclusion has been based on carbon consumption studies (Equation 1.1.2) requires twice as much carbon per Faraday as Equation 1.1.1) and careful gas analysis, by avoiding disturbing side reactions, for example by using a diaphragm to separate the anode and cathode compartments [4], Side reactions are reactions between CO2 and carbon, either within pores in the interior of the anode, with carbon particles dispersed in the electrolyte, or with metal dissolved in the melt. In all cases the reaction product of these side reactions is normally CO. Dissolved aluminium can even reduce CO2 all the way to carbon [5]. By bubbling CO2 underneath a graphite anode it was shown [5] that, while Equation 1.1.3 did occur at zero current, the reaction ceased when the electrode was anodically polarised, even at quite low current densities (0.05-0.1A cm ). 

Sulfur originates mainly from two sources. Petroleum coke used in the production of carbon anodes contains 0.7-3.5 wt% sulfur (cokes with higher sulfur contents are usually blended with low-sulfur cokes). Cryolite and aluminum fluoride also contain sulfur, mainly as sulfate (up to 1 wt%). The chemistry of sulfur in carbon anodes is not fully understood, especially its influence on the electrolysis parameters. Since the sulfur content in the crude oil used in the production of petroleum coke tends to increase with time, the effect of the sulfur content on the carbon consumption (CC) and the current efficiency (CE) was studied in the present work. 

Figure 1.5.3 Influence of the anode sulfur content on the carbon consumption (CC)...

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1),... 

The anode may be operated in the temperature range — 18 C to 65 C and at currents up to 0 05 A per linear metre in soil and O-Ol A per linear metre in water, which corresponds with an effective maximum current densities of 0-66 Am in soil and 0-13 Am in water. No precise details on the anode consumption rate have been provided by the manufacturer, but since the electroactive material is carbon the consumption rate would be expected to be of a similar order to that exhibited by graphite anodes. 

Preliminary electrochemical tests of materials obtained have been performed in two types of cells. Primary discharge measurements have been executed in standard 2325 coin-type cells (23 mm diameter and 2.5 mm height) with an electrolyte based on propylene carbonate - dimethoxyethane solution of LiC104. Cathode materials have been prepared from thermally treated amorphous manganese oxide in question (0.70 0.02g, 85wt%.) mixed with a conductive additive (10 % wt.) and a binder (5wt%). Lithium anodes of 0.45 mm thickness have been of slightly excess mass if compared to the stoichiometric amount, so as to ensure maximal possible capacity of a cell and full consumption of the cathode material. 

Originally, a Clark oxygen electrode was used to measure a reduction in current due to the consumption of oxygen. Anodic detection of the hydrogen peroxide by oxidation at a platinum or carbon electrode was then introduced but, owing to the high electrode potential required, suffered from interference from other electroactive compounds in the sample. 

---