Aluminum Scrap Recycling with Twin-Chamber Melting Furnace
Scrap recycling with the twin-chamber melting furnace (TCF®) has become state-of-the-art worldwide. Nearly any type of scrap can be treated in the TCF. The latest installations show that the TCF fulfills the complex requirements of scrap processing regarding environment, safety and metal yield.
The twin-chamber melting furnace (TCF®) has been Tenova LOI Thermprocess’ unit for melting contaminated and thin-gauge aluminum scrap for about 30 years. Initially developed for the recycling of extrusion scrap, the system is currently being used for any type of contaminated aluminum scrap. Numerous installations (Fig. 1) at different sites and under different pre-conditions – regarding final product and scrap source – show the flexibility of the system. This can be adapted to a wide range of scraps, production rates and target alloys.
The contamination adhering to the scrap can be of rubber, plastics, insulation material, lacquer or oil. The complete processing of scrap is done in the furnace without prior treatment – mechanical or thermal. The contamination level can be practically unlimited because the processing and its automatic control intelligence can handle the conditions inside the furnace in a safe manner. Compared to a conventional furnace, energy consumption is decreased. Even combustibles, which are contained in the contaminants, contribute to the heating and replace typical energy resources.
The salt-free recycling results in environmentally friendly processing and avoids cost for disposal.
Twin-Chamber Melting Furnace (TCF®)
The TCF consists of a furnace casing containing two independent chambers with one common liquid-metal bath. The chambers are the heating chamber (HC) and the scrap chamber (SC). The HC takes care of heating the entire furnace and providing the heat for melting, while the scrap is loaded into the SC and is completely treated in it (Fig. 2).
The scrap is loaded into the charging machine and then placed onto the dry hearth of the SC. It is here where the drying and pyrolysis process takes place. After a period of time, the duration of which is dependent on the type of scrap and the specific contamination, the preheated scrap is pushed onto the liquid-metal bath while the next load is placed on the dry hearth for preheating. After closing the furnace door, the burner of the SC, which is working under-stoichiometrically, takes care of burning off the oxygen that entered the furnace chamber during charging. This creates an oxygen-free condition that is prepared for treating the scrap without additional oxidation, which increases the metal yield.
Charging of scrap to the furnace is done by the charging machine, which can be filled by forklift truck, wheel loader or overhead crane. The charging machine is equipped with a hood, which seals to the hood of the SC door. This arrangement prevents fumes from escaping into the building during charging.
The furnace is equipped with liquid-metal circulation in order to provide heat for melting the scrap. Liquid metal is taken from the HC bath and pushed into the SC via the charge well. The circulation is activated by an induction pump system. The charge well is for melting chips and alloying components.
Heating of the Furnace
Both furnace chambers are equipped with hot-air burners, which are supplied with hot air from the central regenerator system (Fig. 3). This system provides the reliable quenching of all flue gas leaving the furnace via one outlet on the HC. The flue gas is completely burned and can be transferred to the fume purification plant without further treatment or cooling.
The central regenerator system allows for a large heat-exchanger package, which supports reliable quenching of the flue gas and efficient preheating of the combustion air. The continuous firing of the burners provides stable conditions in the furnace chambers. Switching the burners would result in unstable mixing conditions, which leads to higher emission of incompletely burned off-gases and less-optimum conditions for low-NOx flames.
The burning conditions in the furnace are controlled via sensors. For information about the state of the process in the furnace, sensors record data from the furnace that is combined and analyzed by the LOI Thermprocess algorithms of the automatic-control system. This provides optimum and safe process conditions. The control system will accept access to the SC for charging only if the conditions inside the furnace are acceptable.
The furnace atmosphere of both chambers of the TCF is controlled in order to have an oxygen-free atmosphere in the SC and a controlled-oxygen atmosphere in the HC. The oxygen level in the HC has to be maintained at about 3% in order to ensure complete burning of fuel and pyrolysis gas. The oxygen level of the flue gas leaving the furnace is measured continuously, which enables the control system to adjust the combustion air supply accordingly.
After the scrap is placed on the SC dry hearth, the pyrolysis process takes place as soon as the applicable temperature of the material is reached. This happens in two steps. First, the scrap is heated to the temperature sufficient for drying the scrap. Second, by further heating, the process continues with pyrolysis of the adhering contaminants.
Drying of scrap is important because wet scrap should not contact liquid metal or even water that might be entrapped in the scrap bulks. Therefore, the charging machine of the TCF places the scrap on the dry hearth of the SC in a manner that prevents scrap from falling into the liquid-metal bath.
The pyrolysis takes place on the dry hearth and continues on the surface of the liquid-metal bath. This ensures that the pyrolysis is finished before the scrap dips into the liquid metal. The products of the pyrolysis are transferred to the HC for complete burning. The dwell time is much longer than required by official standards and process requirements. Due to the control of the furnace atmosphere, enough oxygen is available for reaction with the combustibles in the HC. The combustibles of the pyrolysis gas contribute directly to heating the furnace. The energy contained in the contaminants doesn’t go to waste because it is used for the melting process.
Metal loss would typically be generated by excess oxygen in the furnace and by reaction of aluminum with carbon. The possible metal yield from scrap is dependent on the scrap quality and influenced by the metal loss inside the furnace resulting from oxidation of aluminum. During cleaning, the resulting oxides are taken from the furnace with the dross.
In the LOI Thermprocess TCF, the access of oxygen to aluminum is avoided as much as possible. This is achieved by the oxygen-free atmosphere and by the burner incinerating all of the oxygen resulting from the charging process in the SC. This happens within seconds after closing the furnace door. The oxygen level in the HC is controlled via oxygen probes and maintained by the continuously firing burners in addition to the improved mixing of fuel and combustion air. The arrangement of the burners ensures that the flames are not hitting the liquid-metal bath, which would increase dross generation.
Residual carbon is attached to the scrap after pyrolysis of the contaminants. The access of carbon to the liquid metal is reduced in the TCF by the special melting process inside the SC. The preheated scrap is not dropped into the liquid metal. It is pushed onto the bath, which is covered with dross. By slowly dipping the scrap into the liquid metal, the oxide surface layer mostly stays within the dross. The carbon attached to the surface and the oxides accumulated in the dross layer on the bath surface are thereby taken out of the furnace.
A different process with active mixing of liquid metal into the scrap would blend the residual carbon into the liquid-metal bath. This would result in more aluminum-carbide generation, increased metal loss and contamination of the liquid metal.
The portion of metal loss due to the furnace and its processing is the minor loss of material from the scrap. The scrap contains metal, oxides and contaminants. Only the metal can be recovered from the scrap. The oxides and contaminants become content of the dross, which is taken from the furnace during cleaning. The amount of dross taken from the furnace depends mainly on the scrap quality.
It is mandatory to check the composition of delivered scrap in order to achieve the target alloy and operate safely. Incoming scrap sampling should be done before mixing it with other scrap deliveries. This provides the chance to find any impurity and to check the alloy of the specific scrap delivery.
Knowing the content of a scrap pile helps avoid uncontrolled events during processing and also assists in combining scrap to achieve the target alloy during charging. The latter reduces the demand for pure-metal additions by adjusting the alloy of the liquid metal, which reduces costs by increasing the efficiency of the production process.
The target alloy can be achieved by controlled composition of different known scraps. An automatic charge-management system is available that collects information about incoming scrap and provides information about the alloys available. It automatically calculates the optimum scrap composition for each load considering the target alloy and scrap price. It provides information to the operator about the amount of scrap to be taken from specific piles to load the charging machine.
In addition to scrap, chips can be melted in the TCF. Chips are fed to the charge well. The installed strong vortex immediately sucks the chips into the liquid-metal bath, where heating and melting take place under liquid metal. This avoids oxidation of the chips and increases the metal yield accordingly.
Pretreatment is recommended for wet and oily chips. This can be done mechanically by centrifuge, followed by thermal treatment in a drum dryer. The first step would be the centrifuge, which will leave about 3% moisture on the chips. The second step is the thermal process, which completely removes the residual moisture from the chips.
The process uses the furnace atmosphere to heat the chips. The moisture is vaporized and absorbed by the circulating gas flow in the drum. A part of the circulating flow is fed directly into the furnace. The combustibles contained in the vapor will be incinerated and contribute to heating the furnace. Due to the optimum condition in the HC for complete burning, the flue gas leaves the furnace in an environmentally friendly state.
The drum is continuously supplied with crushed chips. They pass the drum and are fed directly into the charge well with a typical throughput of about 5 tons/hour. The heat for melting the chips is provided from the furnace, where other scrap can be processed in parallel.
The TCF® allows efficient and resource-conserving processing of scrap. The wide range of scrap treated and the possibility of chips melting in parallel offer a very flexible system for efficient processing. Energy savings, high metal yield and environmentally friendly processing are significant features of the LOI Thermprocess TCF.