This article was originally published on July 31, 2014.
The twin-chamber melting furnace process offers an efficient and economical way to melt aluminum scrap. For the design of the high-performance twin-chamber melting furnace (TCF), computational fluid dynamics (CFD) calculations were carried out to optimize scrap melting performance. The results of the calculations have since been confirmed by process measurements. Thermographic analyses are also used for a well-founded analysis of the plant.
Over the past several years, aluminum scrap recycling has become increasingly important in aluminum melting. Aluminum recycling with the twin-chamber melting furnace (TCF) is applicable to end-of-life and production return scrap – both loose and packed – with or without adhering contamination.
In addition to good metal yields and low energy consumption, the present technology allows the processing of different types of scrap at the same time.
LOI developed the TCF and continuously improved the furnace design and the process to meet the requests of recycling companies regarding efficiency and economics of melting.
Twin-Chamber Melting Furnace
The TCF consists of two chambers installed in one furnace casing. In the heating chamber, the liquid metal is heated via the bath surface. Scrap is charged into the scrap chamber using a charging machine. An electromagnetic pump installed beside the furnace circulates liquid metal from the heating chamber into the scrap chamber. The pump feeds the liquid metal to the charge well, which is connected to the scrap chamber with its return pipe. Chips and thin-wall scrap are charged directly to a charge well using a continuously working feeding system. In the charge well, a vortex is generated by the special shape of the refractories, ensuring that the scrap is rapidly drawn under the surface of the molten metal.
Figure 1 shows the TCF with the heating chamber door on the right-hand side, the chips charging system on the left and the charge well with metal pump and tapping system in the center.
Charge Material and Charging
The scrap mix used can be varied with virtually no limits:
• 100% cuttings or thin-wall scrap such as preconditioned used beverage containers into the charge well via the chips charging system
• 100% scrap with up to 10% contamination via the scrap chamber
• 50% scrap and 50% chips or thin-wall scrap via the scrap chamber and the charge well
• 0-100% ingot material via the heating chamber
• Any combination of scrap, chips, thin-wall scrap and ingots
Figure 2 shows a dosing and charging system for chips and thin-wall scrap. The metal to be melted is loaded into the hopper and then fed to the conveyor belt in a controlled way via a vibrating chute. The charge material is lifted by the conveyor belt to a second vibrating chute and fed to the charge well.
Large scrap items are charged into the scrap chamber using a charging machine (Fig. 3). While contaminations are burned off the previous charge in the scrap chamber, the charging machine can be carefully loaded with scrap. Scrap from storage can be selected to the alloy required without using ingot material or additional alloying elements. In addition, contaminated and less-contaminated scrap can be batched in a way to allow easier removal of contamination in the furnace.
Process and Aluminum Yield
When contaminated scrap is melted, the pyrolysis and melting stages of the process should be separated in order to increase the metal yield. Preheating and pyrolysis of the contaminants take place on the scrap-chamber bridge. Recirculation fans support the process by increasing the convective heating of the scrap. It is important to ensure that scrap batches are made up in order to optimize the pyrolysis process. When the new charge is loaded, the previous charge is pushed into the bath. The smoke that develops from the pyrolysis process is removed safely and reliably into the heating chamber. Contact between contaminated materials and the liquid metal should be avoided because it results in higher burn-off losses.
The twin-chamber melting process is primarily designed for high metal yields with a view to ensuring efficiency and the conservation of resources. Pyrolysis time and charge weights are carefully adapted to each other, taking the charge material into account in such a way that the holding time on the bridge is not too short and not too long.
For scrap that needs a more careful pyrolysis process, the holding time on the bridge must be extended or the charge weight must be reduced. The result is a reduction in melting performance, which can be compensated for by charging chips or other thin-wall material via the charge well at the same time. The charging rate via the charge well should be sufficiently high to keep the melting bath temperature constant between 730-745?C (1346-1373?F), for example.
Energy Consumption and Emissions
The central regenerator system (central cycling regenerator – CCR) was optimized to ensure a further reduction in energy consumption. Even in full-load operation, the flue-gas temperature downstream from the regenerator does not exceed 200?C (392?F). Figure 4 shows the CCR. The regenerators are equipped with honeycomb systems designed to ensure that particulate matter entrained in the flue gas does not block the ducts. Two regenerator packages, operated alternately, reach combustion-air preheating temperatures in excess of 900?C (1652?F).
The pyrolysis gas produced from the contaminants is completely combusted in the heating chamber, where the energy released from the combustibles in the pyrolysis gas can be used for heating the molten metal. Combustion of the pyrolysis gas in the heating chamber ensures a safe and reliable afterburn, releasing completely burned flue gas to the environment. The oxygen concentration in the heating chamber is constantly maintained at a low level.
This system ensures extremely low energy consumption – in some cases even lower than 500 kWh/t. In addition, a low oxygen concentration also minimizes dross formation in the heating chamber. Dross on the surface of the molten metal forms an insulating layer, preventing effective heat transfer to the metal. The chosen mode of operation maximizes the scrap melting performance of the plant.
The atmosphere in the scrap chamber is kept free of oxygen. This is a prerequisite for low metal loss caused by burn-off and, therefore, a high metal yield.
The latest generation of regenerators is also designed to ensure that pollutants in the waste gas are rapidly quenched. This is necessary in order to prevent the recombination of dioxins and furans during cooling after thermal cracking in the heating chamber.
The special feature of the TCF system is that all the flue gas leaves the furnace via a flue-gas duct and passes through the regenerator. This prevents hot and cold waste gases from being mixed, which may lead to aforementioned recombination. Regarding NOx emissions, the firing system and low-NOx burners used reflect state-of-the-art aluminum melting, and future developments on the burners can be applied when needed.
Oxygen concentration control is used to reduce carbon monoxide and hydrocarbon emissions to the lowest possible level. This plant technology ensures that emissions are kept below the limits in the applicable standards.
The use of fossil fuels naturally means that CO2 is also produced. Carbon dioxide emissions are reduced by minimizing energy consumption.
Pyrolysis gas is fed from the scrap chamber to the heating chamber using part of the flow from one of the recirculation fans. The arrow in Fig. 5 indicates the direction of flow from the scrap chamber to the heating chamber.
The trend in aluminum recycling is charging 100% scrap and chips. The advantage of this procedure for the melting process is that very little dross, which prevents the heat transfer to the liquid metal, is formed on the surface of the metal bath in the heating chamber of the TCF. On the scrap side of the furnace, an efficient liquid-metal injection system is used to improve scrap melting performance. Following the burn-off of contamination on the bridge, the scrap is pushed into the bath when loading the following charge.
At this point, the scrap has only absorbed about two-thirds of the energy required for melting. An optimized recirculation of the molten metal from the heating into the scrap chamber is required to prevent a critical temperature drop as the scrap is pushed into the scrap-chamber bath. Experience in TCF operations has shown that scrap melting performance is largely determined by the flow path of the molten metal into the scrap chamber.
In order to optimize the conditions, LOI Thermprocess carried out CFD studies to calculate the metal flow with respect to flow rate and vector. The study for two pumps showed that the vectors of the injection should not simply be set up in a mirror-image configuration but with a view to ensuring a secondary flow within the bath. This requires different arrangements for the pumps and the charge wells (Fig. 6).
Figure 7 shows a comparison of the scrap chamber inlet and outlet temperatures as well as the even temperature distribution in the scrap chamber. A very homogenous temperature distribution can be achieved by adjusting the pump leg angles. A homogeneous bath temperature not only improves melting performance but also ensures a higher metal yield, as the dwell time in the furnace and the enrichment of heavy-metal alloying elements at the solid-liquid transition temperature are reduced.
The metal pump is also used for tapping the molten metal from the charge well. The tap hole, closed by a conventional plug, can be opened without any hazards with the pump in reverse operation. During reverse operation, the tap hole is above the level of the molten metal in the bath. Figure 8 shows the release of molten metal from the charge well. The mass flow may be set by controlling the pump.
Normally, tests to determine melting performance and energy consumption are required under the plant’s contract. It is also necessary to adopt a holistic view of the plant. Temperatures are measured at all points in the plant for continuous improvement. To identify any weaknesses on the surfaces of plant components, it is also recommended to take thermograms of the plant. Figure 9 shows a thermogram of a TCF together with a photograph of the same plant section to give a clearer view.
Photos and thermograms of this type are taken of all furnace details. Apart from optimization analyses, thermograms of this type can be used to identify operating phenomena such as fatigue, shrinkage, erosion or vibration.
The LOI TCF (twin-chamber melting furnace) has set new standards in terms of melting performance, flexibility, energy consumption and emissions reduction. The flexible possibilities of melting different types of scrap with very low energy consumption provides a firm foundation for economically viable melting-furnace operation. IH
For more information: Contact Jared Kaufman, VP Light Metals Sales and Technology, Tenova Core Inc., 100 Corporate Center Drive, Coraopolis, PA; tel: 412-262-2240; e-mail: Jared.Kaufman@tenova.com; web: www.tenovacore.com