COKE BLENDING AT ANGLESEY ALUMINIUM

Les Edwards1, Franz Vogt1, and John Wilson 2

1CII Carbon, L.L.C., P. O. Box 1306, Chalmette, Louisiana 70044 2Anglesey Aluminium Metal Limited, P. O. Box 4, Holyhead, Gwynedd LL65 2UJ, U.K.

Abstract

For commercial and logistical reasons, the Anglesey Aluminium smelter has operated with two different coke sources over the last 30 years. During the last two years, the smelter has switched regularly between high and low sulfur cokes. Whilst anode performance has been satisfactory over this period, Anglesey initiated a study to investigate the benefits of blending the high and low sulfur cokes to reduce overall process variation and improve anode performance. This paper presents the results of a laboratory anode study used to justify expenditure on coke blending equipment and compares the laboratory results to plant anode results. Details of the blending equipment and preliminary plant results are also discussed.

Introduction

Anglesey Aluminium Metal Limited (AAM) is a 140,000 tonnes per year aluminum smelter located near the town of Holyhead in North Wales. The smelter began producing aluminum metal in 1971 and comprises two potlines of Kaiser P69 cells operating at a nominal current of 165kA. Pre-bake anodes for the potline are produced in a carbon plant located on site at a rate of 220 anodes/day. The carbon plant uses an anode recipe comprising three aggregate fractions (butts, coarse and fines), green anode scrap and liquid pitch. The green anode paste is prepared in a series of eight batch mixers which in turn feed a non-vacuum vibroformer. Anodes are baked in a Kaiser designed open top baking furnace using a 28 hour fire cycle. Cast iron is used to make the electrical connection between the anode stubs and anodes and molten aluminum is sprayed around the top two-thirds of the anode sides to protect the anodes from air burn in the cell. This is common practice in P69 cell designs, which have a relatively shallow cavity making it difficult to uniformly cover newly set anodes.

Historically, AAM has sourced its calcined petroleum coke from two different suppliers. This policy has been maintained for a variety of reasons, including security of supply (in the event of problems with one supplier), cost competitiveness and logistical reasons. One of the suppliers is able to rail coke directly to the plant which enables lower inventories and more flexibility at the plant compared to coke shipped in bulk from overseas. Initially, the two cokes were quite similar in sulfur level but in 1998, AAM began experimenting with a higher sulfur level in one of the cokes. The low sulfur coke had a sulfur content of 1.0-1.2% and the high sulfur coke, a sulfur content of 2.5-2.8% The carbon plant was not designed to blend cokes, and as a result the two cokes have been run through the plant on a campaign basis. The average run time for each coke was six weeks and when the cokes were changed, adjustments were made in the green carbon plant (principally to pitch level) to optimize green anode production. In a drive to reduce process variation and improve anode quality and performance, AAM commenced a project in 1999 to examine the cost and potential benefits associated with blending their two coke sources. The following paper describes the results of laboratory work undertaken to justify the project and reports on the design and early results with the blending system installed. Initial Justification for Coke Blending Project Blending different petroleum coke qualities is common practice among calcined coke producers [1-3]. Cokes are most commonly blended in the green state to produce calcined cokes with required properties. Some calcined coke producers also blend cokes after calcining. This can be advantageous since it allows the calcining conditions and coke properties to be optimized for each coke in the blend [2,3].

Until recently, few carbon plants were built with the flexibility to blend different quality calcined cokes. In most cases, smelters adopt a single coke quality specification to minimize process variation. Using this approach, the smelter will either stay with a single coke supplier and a single coke quality or will purchase coke of the same nominal specification and quality from a range of different coke suppliers. Whilst this approach is more than capable of producing high quality anodes, it can be restrictive. Some calcined coke producers are only able to offer a single quality calcined coke and this quality may not always be appropriate for the smelters needs. Technology improvements at the smelter for example, may mean that a lower grade, lower cost coke could be used to either partially or fully replace a higher grade material. Conversely, a smelter wanting to increase potline amperage may require a higher density anode and higher density coke to avoid additional plant capital expenditures or operating costs. The ability to blend different coke qualities at the smelter can offer significant advantages. AAM is a good example of such a smelter. For many years, AAM used two low sulfur cokes, one available locally while the other was imported. The smelter alternated between the two cokes with production runs lasting around 6 weeks. The use of low sulfur coke (<1.5%) is not always desirable from an anode performance perspective. A significant number of papers have been published [4-8], which highlight the positive benefits of sulfur in coke. Sodium contamination from recycled butts catalyzes air and CO2 reactivity of anodes and can lead to significant problems with dusting during cell operation. The presence of sulfur in the coke has been shown to significantly reduce the sodium sensitivity of anodes by reacting with the sodium to render it non-catalytic. In 1998, AAM decided to experiment with a higher sulfur coke. If such a coke could be used successfully, then blending with the locally available low sulfur coke would be possible. The preliminary test with the higher sulfur coke was successful, so the use of this lower cost coke continued. Over the last two years, AAM has alternated between the two different coke qualities. The coke was stored in five silos with a useable capacity of 1500-1800 tonnes/silo. The low sulfur coke has typically been stored in one of the 1800 tonne silos and the higher sulfur coke in the remaining silos. The coke was transferred from the storage silos to two, 200 tonne day tanks located in the carbon plant. Previously there was no capability to blend the coke from these two tanks. Since recycled butts from the different anode qualities were not segregated in any way, there was in effect, some blending of the two cokes via the butts fraction. The cycling of sulfur content in the baked anodes is shown in Figure 1. This shows twelve months of data from mid-1999 to mid-2000. Blending of the two cokes at the plant would require a significant capital expenditure, so AAM started investigating the benefits of such a project in 1999. The principal benefits were expected to be: • Greater consistency in anode quality • More stable green carbon plant operation • Improved anode performance

Figure 1: Baked Anode Sulfur Levels at AAM The first two benefits were easy to quantify on the basis of existing plant data. AAM samples around 1% of its anode population and measures the following properties on anode cores: baked apparent density (BAD), air permeability, electrical resistivity, CO2 reactivity residue, air reactivity residue, flexural strength and impurity levels (S, Na, V, Fe, Si, Ca). These properties were known to vary according to the coke type used to produce the anodes. The pitch level in the green carbon plant also had to be increased by around 0.2% when the change from the low sulfur to high sulfur coke was made. The final benefit was considered a key to justifying the capital expenditure on the project, but it was not possible to quantify without a full scale coke blending trial. To minimize the risk that blending would produce a negative anode performance outcome, AAM agreed that a go/no-go decision on the project would be based on performance evaluations of laboratory scale anodes. AAM worked closely with one of its two coke suppliers to set up the laboratory test program at an independent laboratory. The following section details the results of the tests carried out from this work. Laboratory Scale Anode Property Tests As a first step, the properties of the two cokes (Coke A and Coke B) were measured. AAM requested that the sulfur content of the high sulfur coke be increased to 3% from its typical level of 2.5-2.8%. This was so that a 50/50 blend of the two cokes produced a sulfur content in a range AAM were comfortable with over the longer term (~2.0%). To minimize the sodium sensitivity of anodes, the aim was to keep the sulfur content as high as possible without compromising plant sulfur emission levels. The measured properties of the cokes are shown in Table 1. Coke A is characterized by a lower sulfur content and higher density than Coke B. Densities were measured using both a tapped bulk density measurement on discrete particle size fractions and a mercury intrusion apparent density procedure. Other properties to note are the real density and Lc, which indicate a higher level of calcination for Coke B. Despite significantly different vanadium, nickel and calcium contents, the air and CO2 reactivities of the two cokes were very similar. The air reactivity tests were performed using the RDC

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method at 10oC/min [9]; the CO2 reactivity tests also used the RDC method [9].

Table 1: Coke Property Analysis Property Unit Coke A Coke B

S ppm 1.08 2.99V ppm 82 270Ni ppm 61 197Na ppm 80 63Ca ppm 43 86Tapped Bulk Density 8-4 mm g/cm3 0.75 0.704-2 mm g/cm3 0.86 0.782-1 mm g/cm3 0.94 0.841-0.5 mm g/cm3 0.98 0.88Apparent Density g/cm3 1.78 1.70Real Density g/cm3 2.064 2.078Lc Å 27 32Specific Electrical Resistivity m-ohm-m 468 481Air Reactivity %/min 0.20 0.17CO2 Reactivity % 10.0 7.9Grain Stability % 94 84+4 mm % 28 39

Laboratory scale anodes were prepared at four pitch levels for each of the 100% cokes and a 50/50 blend of the two cokes using the method described in the literature [10]. AAM’s standard pitch was used but no recycled butts fraction was added. To simulate AAM’s typical anode sodium levels, the equivalent of 500ppm sodium in the form of crushed bath was added to each anode mix. Twenty anodes were produced at each pitch level giving a total of 240 laboratory anodes. The measured properties for the various blends and pitch levels are shown in Figures 2-5. In all cases, the optimum pitch levels were found to be between 15-16%. Additional tests were carried out on the anodes produced at 15 and 16% pitch and the results were averaged to produce the final results presented in Table 2. To help in the interpretation of these results, simple bar graphs are presented in Figures 6-13, comparing the relative values of the 100% cokes and the 50/50 blend.

Table 2: Lab Anode Properties at Optimum Pitch Level

Property Unit Coke A Anodes

Coke B Anodes

Blend Anodes

S % 1.03 2.66 1.87 V ppm 74 214 145 Na ppm 208 497 423 Baked Apparent Density g/cm3 1.57 1.52 1.56 Specific Electrical Resistivity µΩm 56 61 56

Compressive Strength Mpa 40.4 37.2 41.4 Elasticity Modulus Gpa 4.2 3.5 4.2 CTE 106/K 3.6 4.0 3.8 Thermal Conductivity W/mK 3.9 3.3 3.7 Real Density g/cm3 2.074 2.077 2.072 Air Permeability nPm 1.11 1.26 1.16 CO2 Reactivity Residue % 74.5 90.5 86.8 CO2 Reactivity Dust % 6.6 1.6 3.7 Air Reactivity Residue % 70.4 68.0 69.7 Air Reactivity Dust % 6.6 3.7 6.4

Figure 2: Green Apparent Density

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Figure 6: Baked Apparent Density

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Analysis of the results shows the following:

• The baked apparent density (BAD) of the 100% Coke B anodes is significantly lower than the BAD of the 100% Coke A anodes. This is in line with what would be expected on the basis of the coke bulk and apparent densities.

• The BAD of the 50/50 blend anodes is only slightly lower than the Coke A anodes, and better than would be expected on the basis of averaging the density of the Coke A and B anodes.

• The specific electrical resistivity, compressive strength and air permeability all show a similar trend to the above where the blended coke anodes are better than might be expected on the basis of the individual results.

• The CO2 reactivity ratio and CO2 dusting are significantly worse for the Coke A anodes due to the lower sulfur content and increased sodium sensitivity. The reactivities of the Coke B anodes and 50/50 blend anodes are similar.

• The sodium content of the Coke A anodes is significantly lower than the Coke B and blended coke anodes. This is due to the lower sulfur content of the Coke A anodes and the inability to “fix” the sodium through reaction to form sodium-sulfur compounds [8]. This results in higher sodium vapor losses during baking which can have a negative effect on flue-wall refractory life. For the higher sulfur cokes, the formation of sodium-sulfur compounds renders the sodium non-catalytic [6-8].

• The air reactivity residues of all the anodes are similar.

• The coefficient of thermal expansion (CTE) and thermal conductivity of the blended coke anodes are intermediate between the Coke A and B anodes. With its lower CTE and higher thermal conductivity, the more anisotropic Coke A would be expected to be more resistant to thermal shock [11].

• The elasticity modulus of anodes produced with the blended cokes is the same as the modulus of anodes produced with 100% Coke A. This demonstrates, that it is not always easy to predict the properties of anodes made with blended cokes on the basis of the individual coke and anode properties.

Comparison to Production Anode Results A commonly expressed concern about laboratory anode results is that they do not reflect production anode properties. Table 3 provides a comparison of the lab anode results for the blended coke with AAM production anodes having a sulfur content in the range of 1.7-2.1% (i.e. production anodes which best represent a blend of the two cokes). The measured properties compare remarkably well (despite the absence of a butts fraction in the lab anodes). This adds confidence to the relevance of the lab results. The second comparison of lab anode results to production results was made with CO2 reactivity ratios. CO2 reactivity ratios measured on production anode cores between May 1999 and May 2000 are shown in Figure 14. Plotted on the second y-axis is the measured sulfur content of the anodes. The trend of

increasing and decreasing CO2 reactivity as a function of anode sulfur levels is obvious and supports the results of the laboratory work. It also supports previous published results in this area.

Table 3: Comparison of Plant and Lab Anode Results

Property Unit Plant Lab (Blend) S % 1.92 1.87 V ppm 159 145 Baked Apparent Density g/cm3 1.556 1.558 Specific Electrical Resistivity µΩm 54 56

Compressive Strength Mpa 44.8 41.4 Elasticity Modulus Gpa 4.2 4.2 CTE 106/K 4.0 3.8 Thermal Conductivity W/mK 4.0 3.7 Real Density g/cm3 2.085 2.072 Air Permeability NPm 3.00 1.16 CO2 Reactivity Residue % 84.9 86.8 CO2 Reactivity Dust % 4.6 3.7 Air Reactivity Residue % 70.7 69.7 Na ppm 402 392

Figure 14: Plant Anode Sulfur and CO2 Reactivities Design and Installation of Blending Facilities The laboratory anode results provided the final justification for AAM to proceed with its coke blending project. A number of blending options were investigated including blending cokes from the 1800 tonne silos and blending cokes from the two, 200 tonne day tanks. Blending cokes from the individual storage silos provides the greatest flexibility since, in principle, it would be possible to blend cokes from any of the five silos in any proportion required. The capital cost of this option is high however, since it would require a larger capacity blending system. Coke is transferred at a significantly higher rate from the silos to the day tanks than it is from the day tanks to the fraction preparation plant. Installation of a metering system for each silo would also be considerably more expensive than installing a system capable of blending only two cokes. AAM chose to install a blending system on the 200 tonne day tanks. Several different technologies were investigated to meter

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the coke out of the bottom of the day tanks, including belt weigh scales, loss of weight feeders and rotary valve feeders. Based on the plant layout, equipment cost and installation costs, the most cost effective of these systems (by a significant margin) were the rotary valve feeders. It was accepted that they would not produce the same level of accuracy as the other two systems, but a blending tolerance of ±5% was considered good enough. A photograph of the rotary valve feeders is shown in Figure 15. The feeders were installed at the bottom of each day tank and the speed of the rotary valve determines the mass-flow rate of coke from the tank. Once calibrated, the motor speeds can be set to deliver the blend ratio required. AAM do not anticipate a need to vary the blend ratio, but if so, it would probably only be across a narrow range.

Figure 15: Rotary Valve Feeders Installed at AAM Installation of the rotary valve feeders was completed in April 2000 and they were commissioned in June 2000. The feeders were relatively easy to install and only minor modifications were required to the rest of the plant. Preliminary Results The feeders appear to do a good job in blending the coke and at the time of writing, anode sulfur levels were starting to reach a steady state concentration. It is too early to comment on anode performance improvements, but along with the trend in anode sulfur levels, anode properties have started to stabilize. Green anode production has also stabilized and green anode densities and pitch levels are now relatively constant. One problem, which has been experienced with the feeders, is a periodic jamming of the rotary vanes. The feeders were sized with an expected top coke particle size of 25mm. Although there are relatively few coke particles above this size (<0.1%), it only takes one or two particles to cause a problem. Both coke sources show the same problem. Although the job of clearing the coke particles is relatively simple, AAM would like to eliminate the problem completely by either redesigning the feeders slightly or installing a 25mm vibrating screen on the coke streams feeding the day tanks. Another issue is that control of coke inventories is now more critical. In the past, AAM were not as dependent on the timely arrival of imported coke. For blending to be successful, it is

important that AAM maintain an inventory of both cokes at all times. Conclusions AAM remain pleased with the results achieved to-date with the coke blending project. For a modest capital investment, green carbon and anode quality variation has been reduced significantly. Although it is too early to make projections about anode performance, the laboratory blending studies suggest that performance improvements, particularly in the area of excess consumption and dusting due to CO2 burn, are possible. The reduced sodium vapor loss during baking is also expected to have a positive impact on refractory life in the baking furnace Even in the absence of quantifiable improvements, some improvement in potroom operations would be expected through delivery of more consistent anode quality. The paper highlights the significant potential benefits for aluminum smelters to invest in coke blending facilities. It provides a smelter with greater flexibility to maximize performance and minimize costs by blending different quality cokes from different suppliers. The need for this sort of flexibility is expected to grow in the future as coke sources become more diverse and smelters come under increasing pressure to drive down costs and improve performance. References 1. M. F. Vogt, “A Strategic View of Calcined Coke for

Aluminum Smelting”, JOM, Nov 1993, 34 2. R. C. Perruchoud, M. Meier and W. K. Fischer, “Coke

Characteristics from the Refiners to the Smelters”, Light Metals 2000, 459-465

3. W.K. Fischer and R. Perruchoud, “Influence of Coke Calcining Parameters on Petroleum Coke Quality”, Light Metals 1985, 811-826

4. T. Müftütoğlu and H.A. Oye, “Reactivity and Electrolytic Consumption of Anode carbon with Various Additives”, Light Metals 1987, 471-476

5. S. M. Hume, W. K. Fischer and R. C. Perruchoud, “A Model For Petroleum Coke Reactivity”, Light Metals 1993, 525-531

6. S. M. Hume, W. K. Fischer and R. C. Perruchoud, “Influence of Petroleum Coke Sulphur Content on the Sodium Sensitivity of Carbon Anodes”, Light Metals 1993, 535-541

7. M. Sorlie, Z. Kuang and J. Thonstad, “ Effect of Sulphur on Anode Reactivity and Electrolytic Consumption”, Light Metals 1994, 659-665

8. P. Stokka, “The Effect of Petroleum Coke Sulphur Content on the Loss of Sodium During Baking”, Light Metals 1994, 695-699

9. W. K. Fischer and R. C. Perruchoud, “Test Methods for the Determination of Anode Grade Calcined Petroleum Coke Properties”, Anode for the Aluminum Industry, R&D Carbon Ltd, 1995, 119-132

10. W. K. Fischer and R. C. Perruchoud, “Bench Scale Evaluation and Chemical Behaviour of Coke in Anode Manufacturing”, Anodes for the Aluminum Industry, R&D Carbon Ltd, 1995, 93-101

11. M. W. Meier, W. K. Fischer, R. C. Perruchoud and L. J. Gauckler, “Thermal Shock of Anodes – a Solved Problem”, Light Metals 1994, 685-694