S. N. Bertel, Skamol A/S
Skamol has examined spent pot lining from some 15 melters, and is developing laboratory tests to study changes in its insulation materials during their lifetime.
Insulation materials are essential in the Hall-Héroult electrolysis cells to maintain the desired heat balance so as to produce aluminium cost-efficiently. The insulation materials must resist the thermo-mechanical stresses the cathodes and barrier layer start to swell and heave due to sodium diffusion and bath exposure. Furthermore, they must also resist the harsh chemical environment of the electrolysis process when e.g. bath components and molten aluminium react. Especially sodium and fluorine are critical reactant species when migrating as vapours into the insulation layers before the viscous magma/barrier layer forms.
Cost and production efficiency are key parameters when making a smelter profitable. Modern Hall-Héroult technology, i.e. state-of-art smelters, consumes 12,5-13,4 kWh (specific energy consumption) to produce 1 kg of aluminium, while theoretically the chemical reaction need only 6,5 kWh/kg of aluminium. This means that the overall electrical efficiency is 50% or less. Thus is room for improment, and smelters and cell designers are working hard to decrease the specifiv energy comsumption to produce aluminium at a lower cost.
When crisis struck the global economy in 2008, the aluminium industry changed forever. Even though the global economy recovered long ago, the aluminium industry is still far from 'the golden days' when, basically, all smelters globally were operating at 100% capacity and produced aluminium at a very high intensity and rate.
Since then the LME price has more than halved and has found an oscillating but semi-stable level around USD1500/t. Recently, the premiums have decreased significantly as well, and have turned profit into loss at many smelters. Globally, this often results in production curtailment and sometimes permanent smelter closures. Even the biggest smelter groups are influenced by this negative trend, reducing staff and laying off many employees. The smelters and cell designers have look somewhere to find specialized expertise, like to Skamol for insulation. However, the cut-back is not seen in China, where greenfiels and brownfiels projects are quite common, and smelters exceeding amperages 600 kA are being put into operation. Today, more than 50% of global aluminium production is in China. This figure is still growing, and it will keep on growing.
The reduction in staff at smelters and at cell designers has resulted in a mentality change, meaning that customers need a supplier like Skamol to serve alsp as an expert advisor. Our role in the industry has hence changed significantly: Skamol is now a partner as well as a supplier, who smelters and cell designers can rely on when designing or optimizing cells or technology. Today we are working on:
When designing a new cell or optimizing an existing cell, thermophusical properties like service temperature, thermal conductivity and crushing strenght are great importance. These figures are found in the data sheets and they give the designers their data input for the cells operating at 'beginning-of-life'. However, what about lining properties at 'middle-of-life' and at 'end-of-life'? Is it not interesting, or maybe even important, to predict how the cell will behave from beginning- to end-of-life? How will the different materials installed in the cell influence e.g. specific energy consumption as a function of time? Will the insulation materials, for instance, degrade due to the harsh chemical environment or creep/compression, causing the energy consumption to increase and/or cell life to decrease?
To answer these questions Skamol has initiated a project called: Skamol Factbook Project.
Together with approximately 15 different smelters globally we have collected samples of spent pot lining (just our own insulation products) of vermiculite, calcium silicate and moler bricks.
We have analyzed samples of all the materials Skamol supplies to the aluminium industry. Samples have been extracted from different pot technologies with amperages ranging from 120 to 440 kA. We analyse material properties like thermal conductivity, density and total thickness, and compare these with new samples. After testing, the smelter receives a confidential report about the materials in its own plant. However, we use normalized, anonymized data to compare changes in material properties across amperages and technologies. It is a very interesting project, and an increasing number of interested smelters would like to participate.
The initial results indicate that materials like Skamol vermiculite, calcium silicate and moler bricks (of verious densities) have not significantly changes their thermochemical properties as function of time. We found no creep under compression and no change in thermal conductivity. This should help smelter and cell designers to predict energy consumption.
Together with Skamol Factbook Project we have started developing new sodium and fluorine vapour tests.
Historically, the bath and cup tests have been used to evaluate both barrier and insulation materials. However, the bath and cup tests are not suitable for insulation materials because these are not meant to resist the extremely aggressive liquid bath components: if liqyid bath penetrates the barrier layers of a cell in operation and reaches the insulation layer, then it is only a matter of hours before the cell fails.
In our opinion, it is more appropriate to look at how bath components in the vapour phase influence the insulation material properties, such as thermal conductivity and physical appearance.
Skamol A/S was founded in Denmark in1912 ans has been the key supplier of insulation materials to smelters globally for decades - from Norway to New Zealand and the UAE. The insulation materials are produced in Denmark and in Russia. Vericulite boards, calcium silicate boards and diatomaceous earth (moler) bricks are produced at three different plants in Denmark. Diatomaceous (moler) bricks are produces i one plant in Russia. Skamol also supplies silicon carbide bricks from its OEM, Hexagon, a Chinese manufacturer.
Initial examinations preformed in Skamol's insulations materials (vermiculite, calcium silicate and moler bricks) indicate that they have resisted the mechanical, thermal and chemical stresses in service without significant change to their thetmomechanical properties. This result is of great importance then optimizing existing or developing new cell designs so as to reduce the specific energy consumption and/or to increase potlife.
Søren Nørgaard Bertel is application manager at the Aluminium department of Skamol A/S, which is based in Rødding, Denmark.