Aluminum Reduction Cell Life
During bullish aluminum pricing cycle, smelter management typically emphasizes increasing production, but during bearish cycles, it turns to cost reduction strategies. Unfortunately most of the smelter’s costs are determined by factors that are beyond the control of internal management. For example:
- Alumina, coke and power pricing are usually determined by long term contract
- Labor rates and staffing levels are often determined by collective bargaining agreements for 3-5 year periods
- Alumina utilization rate is determined by the chemistry of the process and cannot be reduced significantly
- Power and carbon utilization may have potential for improvement, but many plants are already operating at near world-class levels based on their originally installed technology
This leaves maintenance and cell relining costs as the main factors under the local manager’s control. Maintenance cost reduction in the short run usually means increased long term costs due to damage caused by low maintenance. Improvement of Aluminum Reduction Cell Life becomes a priority for cost reduction process. Reduction of cell relining cost by increasing service life has no such downside, and initiatives to increase cell life usually pay dividends for many years. Annual cell relining costs in a 250 kTPY smelter can run from $10 to $40 million per year. The potential for cost reduction at the high end of the range may be $25 million ranging to less than one million at the low end.
Cell Life Determinants
Increasing cell life begins with definition of “Ideal Cell Life”. This is the life that is achieved when failure occurs due to “wear out” of the cathode carbon blocks and subsequent exposure of the current collector bars. Thus it depends only on design factors such as the thickness of the cathode block above the collector bars and the rate of erosion of the cathode blocks. Any cell failure that occurs by another cause can and should be thought of as “premature” and is therefore preventable. These preventable causes may include poor quality raw materials, construction defects, poor thermal or structural design, and operational upsets.
A useful model for thinking about cell lining problems was popularized by Hale in Journal of Metals, Nov 1989. He proposed that there are five basic determinants of cell life which can be visualized as the pieces of a pie as in figure 1.
Figure 1. Determinants of Cell Life
The relative sizes of the pieces of the pie can be the subject of endless debate, but this goes beyond the classic case of the potroom manager blaming bad cathode blocks for poor potlife and the cathode block supplier pointing a finger at poor operating practices. It is an important role for technical management to understand the relative importance of each piece of the pie for the specific plant situation and direct the resources to work on the right problem. It does no good to change block suppliers if the relining crew does a sloppy job of ramming or if the thermal design is not suitable for the operating conditions. This is especially important given the cost of modern cell linings and the long time span of the cell relining cycle. Today’s cells may cost $100-300,000 and are expected to live 6, 7 or even 10 years. The effect of a wrong decision may not be seen for 2-4 years by which time hundreds of cells costing tens of millions of dollars could be affected.
A real example provides an illustration. Figure 2 shows a steep drop in potlife from 1997 to 1999 caused by a “mistake”. The specific causes and lessons learned from this example will be discussed in another article. The significant point here is that, even though the problem was recognized and corrected quickly, it took nine years for cell life to recover to the previous level. The cost of the lost service life was more than $18 million.
Figure 2. Illustration of cell life lost due to a “mistake”
In the subsequent articles, we will focus on the various determinants of potlife and how they can be optimized to result in the maximum potlife and minimum expenditure on pot relining.
(Richard Jeltsch is an MSc. In Chemistry from Cleveland OH, who worked for 22 years with Kaiser Aluminum in process engineering and environmental management, last 12 years spent as Cathode Technology Specialist. He has 13 years of consulting experience in the area of cathode technology, specializing in cell life improvement and reduction cell autopsies. He is a lecturer on cathode technology for TMS Aluminum Electrolysis course since 1999 and presented several papers at TMS.)