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1027BAKED ANODE DENSITY IMPROVEMENT THROUGH OPTIMIZATION OF GREEN ANODE DRY AGGREGATE COMPOSITION


Light Metals 2010 Edited by: John A. Johnson TMS (The Minerals, Metals & Materials Society), 2010

BAKED ANODE DENSITY IMPROVEMENT THROUGH OPTIMIZATION OF GREEN ANODE DRY AGGREGATE COMPOSITION
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Khalil Khaji1 , Hameed Abbas1 Aluminium Bahrain, P.O.Box 570, Manama, Kingdom of Bahrain

Keywords: Baked Anode Density, Green Anode Dry Aggregate Composition, Dry Density Abstract At Alba, green anodes were manufactured as per the dry aggregate composition recipe given by the technology suppliers. For given set of raw materials, paste plant process and equipment parameters, the dry aggregate composition was giving baked anode density in the range of 1.565-1.570 g/cm3. Pot rooms gradually increased the line current to increase aluminium metal production. Therefore there was need to improve baked anode density, net carbon consumption so that the butts thickness at increased line current will be maintained. With in-house research, baked anode density of 1.600 g/cm3 was achieved. This was achieved by optimization of dry aggregate composition. The paper describes the work done on the optimization of the dry aggregate composition and the results achieved over a period of two and half years. Introduction Alba is an 870,000 mt/year aluminium smelter located at Bahrain, Middle East. Its pot lines 4 and 5 are based on AP 30 smelter technology and use prebake anodes. The green anodes are made using vibrocompactors, baked in open top horizontal flue furnaces and sealed with 6-stub brackets in rodding plants for use in pots. Green anodes are made by using the dry aggregate composition recipe from technology suppliers. The pitch addition to the dry aggregate is based on the well known “dry density test” method. The green paste is vibrocompacted. The baked density obtained from these vibrocompacted anodes was in the range of 1.565 1.570 g/cm3. Initially at lower line current, the butts thickness below the stubs was adequate, however as the pot lines began increasing the line current, there was a need to improve the baked anode density to maintain the butts thickness. One of the main objectives of quality improvement programmes in carbon plants is to produce baked anodes with higher density. The baked anode density is influenced by: ? ? ? ? ? Nature of raw materials (coke, butts, and pitch) Dry aggregate composition of green anode Pitch content in green anode Process and Equipment Parameters Baking parameters factors that could influence baked anode density for the present study. The main process and equipment parameters of green anode manufacture are: ? ? ? ? ? ? ? ? Coke temperature Pitch temperature Paste mixing temperature Anode forming temperature Mixing energy Paste cooler power Vibrocompactor RPM Vibrocompaction duration etc.

Over the years the above parameters have been optimized and are maintained during green anode production. Therefore, our attempts to improve the baked anode density were focused on the optimization of dry aggregate composition of the green anodes. Optimization of Green Anode Dry Aggregate Composition The dry aggregate is composed of very coarse fraction, coarse fraction, medium fraction and dust (ball mill product). The very coarse fraction is composed of butts material while the remaining fractions are from calcined petroleum coke. Each fraction is proportioned accurately and all the fractions are then blended together to get a dry aggregate composition as shown in the graph below:

At Alba, carbon plants use calcined petroleum coke manufactured in captive calciner located at the marine terminal, 16 km from main smelter plant. The coal tar pitch suppliers are also fixed on long term procurement basis. The consistency in the calcined petroleum coke quality and the liquid coal tar pitch quality is reasonably maintained; therefore they are not taken as variable

Figure 1. The original dry aggregate composition curve

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The dry aggregate composition is designed to give optimum quality to baked anodes, namely good apparent density, high thermal shock resistance etc. The pitch addition to the dry aggregate is based on the “dry density test” method. It is adjusted to obtain maximum dry density for the given dry aggregate composition, raw materials and process & equipment parameters. The dry density of green anodes is defined as: Dry Density = Apparent Density (1 - Pitch Content/100) (1)

It may be observed that the recipe became slightly less coarse at the upper end of the composition, while at the lower end of grain size distribution it has become slightly coarser than the original dry aggregate recipe. A comparison of the both dry aggregates is shown in the graph below.

The baked anode density is related to green anode dry density. Baked Anode Density



Green Anode Dry Density

(2)

With the optimum process and equipment parameters, a baked density of 1.565 -1.570 g/cm3 was being achieved with the original dry aggregate composition. However this density was not sufficient to maintain the butts thickness with increasing line current. Alba adopted an approach to improve the baked density by improving the dry density, as both of them are strongly correlated. In order to improve the dry density it was necessary: ? ? ? To increase the weight of dry aggregate To reduce the void space within dry aggregate To reduce the pitch addition to the dry aggregate Figure 3. Comparison of original and modified dry aggregate composition curves. Results The modifications in the dry aggregate composition resulted into following major changes: ? ? ? The dry density improved. The pitch content in the green anodes decreased. The baked anode density increased.

It may be seen that all of the three above are interrelated. In addition to the above there was another important requirement, namely to maintain high thermal shock resistance while improving the density. Keeping both the requirements of baked anode density and thermal shock resistance into consideration, modifications were made stepwise in the dry aggregate composition, both at the upper end of the composition as well as lower end of the composition. For every incremental change in dry aggregate composition, the baked anodes quality was monitored by analyzing the anode core samples. With encouraging results the modifications in the dry aggregate recipe continued till we have reached the composition as shown by the graph below:

The changes in the green anode dry density, pitch addition and baked anode density are shown in the graphs below.

Figure 4. Improvement in green anode dry density with modifications in the dry aggregate composition Figure 2. Modified dry aggregate composition curve

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small size particles enabled further filling of void space between the coarse particles. Both the above reduced the void space and increased the weight of the dry aggregate. As the voids space decreased, lesser quantity of liquid pitch was required for preparing green paste.. Since pitch addition dropped, the loss of weight of anode during baking also reduced. The combined effect of the above resulted into higher green anode dry density and higher baked anode apparent density. The impact of the modifications in the upper part and lower part of dry aggregate composition on the green anode dry density is shown in the graphs below.

Figure 5: Pitch content in green anodes reduced with modifications in the dry aggregate composition.

Figure 7: Improvement in the green anode density as the %age of very coarse decreased or as %age of the coarse increased

Figure 6: Increase in baked anode apparent density with modifications in dry aggregate composition.

Discussions The modifications in the green anode dry aggregate composition were done from two ends of the composition. The upper end of the dry aggregate composition was made slightly less coarse. Too coarse particles were creating too much of void space in the dry aggregate so it would not be filled completely with the available finer material. Therefore it was crushed to produce smaller particles however care was taken not to over crush particles. This reduced the void space without loosing much of very coarse particles. Very coarse and coarse particles give weight to the dry aggregate as well as flexibility to anodes under thermal stress. The lower end of the composition was made slightly coarser so that keeping the total surface area of the material constant more of small size material could be added. Addition of more quantity of

Figure 8: Increase in green anode dry density with increase in %age of lower end of dry aggregate composition

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Impact of Baked Anode Apparent Density The improvement in the baked anode density had impact on net carbon consumption, anode chemical reactivity and butts thickness below the stubs. With increase in the density, the porosity of the anode decreased as well as the surface area available for the chemical reactions between gases and the carbon decreased. This helped to reduce the reaction between CO2 and C, thereby reducing the consumption of carbon, which subsequently could be utilized for aluminium production. During the period when the density was being improved, there was also reduction in the baked anode desulphurization. Decrease in the anode chemical surface area due to increase in density, and increased availability of sulphur which acts as an inhibitor to the reaction between CO2 and C had a double knock- on effect on the net carbon consumption. There was reduction in the net carbon consumption as could be seen by the graph below.

During the period of these improvements in the baked anode density, the pot rooms increased line current by 5 kA. Not only the butts thickness below stubs was sustained, but it also showed improvement of about 8-10 mm. This is attributed not only for the density increase but also due to reduction in the net carbon consumption.

Figure 10: Increase in butts thickness below stubs due to increase in baked anode density

Conclusions It was possible to increase the baked anode density by modifying the green anode dry aggregate composition. The increase in density enabled pot rooms to creep the current by 5 kA as well as it was possible to maintain/improve the butts thickness. The increase in density reduced chemically reactive surface area, improved chemical reactivity of anodes and reduced net carbon consumption. It was also possible to maintain high thermal shock resistance in anodes, required in high kA pots.

Figure 9: Reduction in the net carbon consumption due to improvement in baked anode density The improvement in the CO2 reactivity residue is shown below:

References 1. J.F.Claver and B. Coste, Paste Plant Design and Control- A New Approach?(Light Metals 1993), 641646 David Belitskus, An Evaluation of Relative Effects of Coke, Formulation, and Baking Factors on Aluminium Reduction Cell Anode Performance(Light Metals 1993), 677-682 S. Wilkening and M.Beilstein, The Principles of VAW/KHD Paste Plant Technology (Light metals 1994), 711-718 A.S.S.Bin Brek and M.H.Vaz, Process Optimization in the Green Mill (Light Metals 1994), 611-616 K.M.Khaji, “ Proposal to Improve Baked Anode Density” (Internal Report, 49/KMK/aem, April 2007)

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Figure 10: Improvement in the CO2 reactivity residue with increase in baked anode density

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