Technical Paper such as operating temperature;
www.ireng.org duration of exposure; compression;
environment; installation method; single or multiple use; amount of handling; and airborne fibre exposure. Recent tests carried out at the most common operating temperatures for furnace back-up board – between 600ºC (1112ºF) and 800ºC (1472ºF) – revealed that in the key area of thermal conductivity, the latest low biopersistent fibre-based board outperformed calcium silicate alternatives by an average of 20% at 600ºC (1112ºF) and 15% at 800ºC (1472ºF).
Block products are also available for use as insulation layers in aluminium reduction cells where they offer low thermal conductivity – no higher than 0.16W/m.K at a mean temperature of 900°C (1652°F), high dimensional stability and hot compressive strength, and high cryolite resistance. Thickness shrinkage reaches a maximum of 2.8% at 1,100°C after 24 hours’ soaking, with linear shrinkage under the same conditions no higher than 1.8%.
The latest low biopersistent fibre systems also available in paper, felt, modules and custom shapes. Specialised materials are even available for caster tips while furnace cones, seals, gaskets, thermal covers and flexible launders are also on offer.
Lining developments
In the area of melt-hold furnace lining, continued investment in the optimisation of monolithic materials is delivering enhanced productivity and quality. These furnaces present a variety of challenges as each area of the furnace has varied requirements in terms of factors such as temperature, metal contact, flux contact and thermal shock, meaning suppliers must offer a variety of products with differing performance attributes. Products used on ramps, for example, must offer strong resistance to abrasion and thermal shock, as well as to aluminium and alkalis. Some of the latest products boast abrasion loss as low as 2.8cm³ at 815°C (1499°F), significantly lower than that of competing products. Their pick-up of at 0.011% at 1,000°C (1832°F) over 100 hours is also more than 10 times lower than that of the nearest competing product.
Ultimately current consumable innovation and supply is driven by the need to transfer energy better and to optimise finished product quality.
Optimising furnace insulation
Given the high energy usage of aluminium furnaces and the need to maintain consistent temperatures to optimise quality, any action which can be taken to reduce energy loss during the melting process is to be welcomed. Alongside this sits the requirement to meet increasingly stringent local and global safety regulation in the area of insulation materials. For many years, refractory ceramic fibreboard was the industry standard but concerns about its carcinogenic properties – meaning it is being outlawed completely in some regions – led to the development of the first low biopersistent fibre- based alternatives. These were originally launched to the market in the late 1990s, and recent innovations have delivered higher melting points and improved insulation to meet ever more demanding process requirements. Well-suited to the aluminium industry because of their ability to withstand temperatures of up to 1,200°C (2192ºF), these products are available in both blanket and board forms, making them suitable for applications in anode bake ovens, casthouses and potlines, and boast key properties such as low shrinkage – less than 1% at 700°C (1292°F) - and compression. A suitable solution can be developed based on individual application requirements
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It is a similar story on belly bands, where the highly aggressive metal-to-air interface makes resistance to salts and alloys crucial, as well as resistance to abrasion, aluminium and thermal shock. The lower walls, superstructure, door, jambs and lintels, back-up lining and burner blocks all have their own requirements too – and the issue of testing is complicated by the fact that many industry standard test conditions, based on lower temperatures and operating times, do not truly reflect how operators use their furnaces. The only real way to ensure the product is appropriate is to test it under real operating conditions in the application in question.
Modern products are improving all the time and the right combination is not just easily achievable but integral to optimising performance and productivity while reducing energy usage.
Supporting the journey towards enhanced quality
Quality in the secondary aluminium processing sector is inextricably linked to purity, especially in high-specification applications in sectors such as electronics. One of the key sources of impurity and physical imperfections – and therefore strength and performance issues - in cast aluminium components is the presence of gas, in particular dissolved hydrogen. This makes effective degassing technologies vital to production.
However, their role in removing gas from the process area must be married to a long service life and an inertness to the presence of molten aluminium, as any reaction with the aluminium will itself cause impurities and potentially
ENGINEER THE REFRACTORIES May 2016 Issue
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