A wide variety of insulating materials are available on the market and it can be difficult to understand the benefits of one material compared to another. However, key factors to consider, when insulating electrolysis cells in the aluminium industry, are the products’ impact on heat loss, cell efficiency and installation process. These factors can among other things improve performance efficiencies, prolong the time lifetime of the cells and increase time between relining.
One of the challenges often faced in the aluminium industry is achieving a cost-efficient electrolysis cell. This involves being able to keep a stable heat balance in order to reduce energy costs and to minimise the risks and the effects of penetrating gasses, bath and molten aluminium. It also involves a more efficient relining process, where time is the key factor, but also a process that embraces the fact that the physical environment in many work places is becoming increasingly important. Insulating the electrolysis cells with highly efficient insulation materials can contribute to a solution to these challenges by bringing benefits like an energy-efficient production with focus on reducing heat loss, improving cell efficiency and also contributing to an efficient relining process.
There is a wide range of materials available on the market and among these are calcium silicate insulating boards and fibre-based boards. These products are among some of the world’s most efficient insulating materials when insulating electrolysis cells - their low thermal conductivity combined with their specific product characteristics provides highly insulating solutions that are easily installed thus ensuring an efficient production with a minimum of heat loss.
Calcium silicate and fibre-based boards are both highly insulating products and the use of either product can contribute to reduced heat loss, energy savings and to an improved control of the production process by contributing to maintaining a stable heat balance inside the cells. The calcium silicate insulating boards, SUPER-ISOL and SUPER-1100 E from Skamol A/S, both have a thermal conductivity of 0.14 W/(m x K) at 800° C, which is among the lowest in the industry. Recent independent tests conducted to ASTM C-201/182 in 2013 conclude that the thermal conductivity of these two calcium silicate insulating boards is 27 % lower than a leading bio soluble fibre-based board between the operating temperatures of 600-800° C. The test was conducted at several temperatures from 250° C to 800° C and the results vary from a difference of 18 % at 600° C to 27 % at 800° C – all in favor of calcium silicate. The reason for the low thermal conductivity of calcium silicate can be found in the structure of the product. To obtain a good insulation value at high temperatures, the insulating product has to have a well-defined pore size – the smaller the pores, the higher insulation value is obtained when exposed to high temperatures. The structure of small pores with relatively short distances from fix point to fix point makes the calcium silicate highly insulating compared to fibre-based boards that are structured by large pores and as a result, have a lower insulation value.
Besides finding a highly insulating solution, another aspect to consider is the strength and compressibility of the product in order to avoid damages to the insulation material from heaving cathode blocks. When infiltrating the cathode blocks, gas, bath and molten aluminium can cause the blocks to heave and the insulation material becomes in danger of compressing due to the increased pressure. When the insulation material is compressed, the insulation value of the product will be decreased and the direct result is a significant increase in heat loss (/cell voltage drop) and the electrolysis cell will be taken out of operation.
The SUPER-ISOL and SUPER-1100 E calcium silicate boards both have a compressive strength of 2.6 MPa at room temperature making them 700-800 % stronger than comparable bio-soluble fibre-based boards that have a compressive strength of app. 0.30 MPa. The reason for this difference is evident when looking at the structure of the products. To obtain strength to the fibre-based boards, an organic binder is added in the manufacturing process and the compressive strength reaches app. 0.30 MPa. However, this binder burns off at app. 400° C and so the strength disappears with the consequence of the fibre-based boards compressing and the insulating lining becoming unstable. This is not the case with calcium silicate due to its microstructure and the fact that the pores inside this product are intertwined, which makes the use of binder unnecessary. Calcium silicate, therefore, is able to maintain strength better than fibre-based boards.
Efficient insulation not only means a highly insulating and strong product, it also refers to a product that is, to a certain extent, able to resist the influences from destructive gasses etc. In the aluminium industry and especially in electrolysis cells, the risk of fluoride and sodium gasses, bath and molten aluminium penetrating the cathodes, graphite blocks, ramming paste and lining material is always present. These facts make it important to have a solid layer of barrier bricks that is able to withstand this kind of aggressive environment and underneath it, a strong insulation material that is able to withstand these substances if these destructive forces should reach them, thus avoiding damage to the entire cell.
The calcium silicate boards from Skamol A/S contain a lot of small pores, which provide them with low gas permeability. These small pores make it hard for the gasses to penetrate the product and hence, the insulation value remains intact. Combined with the low degree of compressibility and the fact that they retain significant strength up to app. 900° C with only a 5 % creep in compression, the calcium silicate insulating boards will provide a stable lining that is protected from gas attacks. The fibre-based boards, however, consist of large pores, making them vulnerable to e.g. gas attacks, because of the fact that the gasses condense and freeze inside the boards at 700-720° C thus diminishing the thermal conductivity. Combined with the risk of creating gaps inside the cell, due to the risk of the material compressing, gas etc. can penetrate the lining and a risk of damage to the lining and increasing heat loss is created.
Although carefully lined with a strong insulating product and highly resistant barrier bricks, gas, bath or molten aluminium will always, to some extent, be able to penetrate the carbon/graphite. Therefore, many smelters are trying to figure out ways of increasing the protective layer of barrier bricks while maintaining a highly efficient insulation of their electrolysis cells in order to protect the lining material. Even at small thicknesses, the low thermal conductivity of the calcium silicate insulation board opens up for the possibility of decreasing the thickness of the insulating lining – making more room for an increased layer of barrier bricks. The calcium silicate insulating board, SUPER-ISOL and SUPER-1100 E from Skamol A/S, can be delivered with a single layer up to 100 mm thickness while maintaining a low thermal conductivity of 0.14 W/(m x K). Besides being able to increase the layer of barrier bricks, reducing the thickness of the insulating material also makes it possible to install bigger cathodes. This can, other things being equal, prolong the lifetime of the cell due to the fact that most cells are taken out of operation, when the cathode block is worn down. A combination of a reduced refractory layer and bigger cathodes is also possible – making the cell even more efficient.
Using calcium silicate can not only provide more space and protection of the electrolysis cells, it can also be installed in combination with castable material. In cases where castable material is applied directly onto the backup lining material in order to e.g. even out gaps between the materials, calcium silicate boards, like the SUPER-ISOL and SUPER-1100 E, can be surface-treated to be water-repellent. The water repellent surface makes them unable to absorb water from the castable material and the risk of using this product, whether installing barrier bricks or applying castable material directly onto the boards, is non-existent.
Relining electrolysis cells lasts app. 5-7 days from when the cell is turned off to restarting it again including the cooling and relining time, depending on the size of the cell and the complexity of the process at the specific aluminium smelter. This is done manually and a contractor is often hired to do the work and it is a process that is connected with many costs. Combined with the pursuit of minimising cost due to the decrease in aluminium prices, these costs are often sought reduced.
Insulation materials like the calcium silicate insulating boards and fibre-based boards are mostly delivered as whole boards in standard sizes making it necessary to cut the products before installation as the electrolysis cells are often shaped with round corners and conical walls. Although both products are easily handled and cut, more and more smelters in the aluminium industry see the advantages of installing pre-cut boards ready for installation without further adjustments. This opens up for the possibility of reducing installation time significantly as no further measurements and machining of the product is necessary and the product can easily and quickly be installed. The calcium silicate potsets from Skamol A/S can be placed on the pallets according to the sequence that they will be used on site, reducing the inventory and shortening the installation process significantly.
Although relining can be done more efficiently and a long lifetime of the cell can be achieved using the right combination of production process and insulating material, relining will continue to be a recurrent event for aluminium smelters and so, protecting the health of the people working with and around the cells during relining is still an important factor. The calcium silicate board, SUPER-ISOL and SUPER-1100 E, has been classified as non-hazardous by government bodies worldwide, so the risk of handling the product is therefore non-existent. This is due to the fact that the product is manufactured as 100 % fibre- and shot-free and so the product emits no hazardous dust. This is also the case during dismantling of the product opposed to fibre-based boards – though easily managed and cut - the silica inside the fibre boards crystallizes over time and they thereby pose a health risk especially during dismantling which makes it important to wear a lot of personal protective equipment. Calcium silicate is therefore a healthier solution.
Calcium silicate insulating boards from Skamol were developed and introduced to the market in the 1980’s. Since then, the material has been used extensively throughout many industries, including the aluminium industry, where it is primarily used as sub-cathodic and sidewall insulation to improve the thermal efficiency of energy-intensive processes. Skamol is one of the leading suppliers of insulation solutions to the aluminium industry and the Skamol calcium silicate board is used by aluminium smelters all over the world from Norway to New Zealand, Canada and Abu Dhabi etc.