Environmentally Friendly Aluminum Scrap Dryer and Melter Promise Lower Energy Use, Higher Yields
A problem faced by the primary metal and glass industries is their intense energy usage. For example, the secondary aluminum processing industry uses about 1,000 Btu/lb of aluminum to remove oil, paint, plastics and other organics from the scrap. Another 3,000 Btu/lb is used in melting the scrap, for a total energy use of 4,000 Btu/lb. In addition, emissions and generation of waste materials are also industry problems.
An innovative melter and dryer developed by Energy Research Co. (ERCo) demonstrated a capability to reduce energy use for the secondary aluminum industry. Scrap is first dried and/or decoated in a controlled-atmosphere, indirect-heated rotating kiln (IDEX) that completely removes organics. The heat content of the organics drives the process with little or no supplementary fuel required. Preheated scrap from the IDEX is then fed to the top of a vertical flotation melter (VFM) where it falls through a melting cone and into a holding furnace. The products of combustion (POC) flow up through the cone, countercurrent and in direct contact with the scrap. The gases impose a variable drag force on the scrap, which impedes its descent. For most scrap pieces, equilibrium is achieved in which the scrap weight equals the gas drag force and the scrap is "hung-up" and does not fall for 15 to 30 seconds. This greatly increases the scrap residence time, allowing the scrap to be melted in the cone. As the scrap reaches a liquid state, it takes on a more aerodynamic drop shape, which reduces the drag force and allows it to fall into the holding furnace.
ERCo demonstrated the feasibility of the VFM concept, determined VFM performance, devised a marketing strategy and identified target markets. The VFM and IDEX will substantially reduce energy; that is, when combined with the IDEX, the VFM has a thermal efficiency of 75%, compared with a conventional furnace at 19%. This results in an energy savings of 2,250 Btu/lbm translating to a possible savings of 15.3-trillion Btu per year using this technology. In addition, a reduction in NOx, SOx, CO and VOC emissions with IDEX results in measured emission levels to be below 20% of the regulated allowable limits. This technology could lead to an increase in revenue for the secondary aluminum industry by over $400 million.
Aluminum alloys can be made by primary production from bauxite, or by secondary production from scrap aluminum. The major attraction of secondary aluminum production is that it uses only 5% of the energy of that for primary production because the energy intensive reaction of converting Al2O3 to aluminum has been done previously.
Secondary aluminum production consists of shredding scrap, decoating in a rotary kiln, melting in a reverbertory furnace and cast into ingots and sows. This production process is not only energy intensive, but it also experiences significant melt loss and high emissions.
A new approach
Two innovative technologies were developed to solve these problems: a vertical floatation melter (VFM), an advanced furnace with a high thermal efficiency and low melt loss; and an advanced dryer, called IDEX, which removes organics from the scrap aluminum prior to melting in the VFM.
Three major sections of the VFM are the melting zone, recirculation fan and holding furnace (Fig. 1). Scrap decoated and preheated by the IDEX is passed to the top of the melting cone where it falls towards the holding furnace. The products of combustion (POC) flowing upward in the cone impose a drag force on the scrap which impedes its descent. Different sizes and shapes of scrap will hang up in different elevations of the cone. A gas- or oil-fired burner is positioned in the end wall of the holding furnace to provide the high temperature POC, and a fan blows the recirculating POC into the holding furnace and then up through the cone.
The major advantages of the VFM are reduced energy consumption and reduced dross loss compared with conventional furnaces. The VFM has a thermal efficiency of about 57% compared with 19% for a conventional furnace. In addition, the VFM meets the three needs for minimum dross production; low temperature, low residence time, and low oxygen content.
The IDEX™ dryer (Fig. 2) consists of three major components: A rotating kiln to process the scrap, an incinerator to destroy the organics and control system and associated hardware. The purpose of the kiln is to completely remove coatings from the scrap without oxidizing the metal. The scrap is indirectly heated, so dust formation from direct flame impingement does not occur. Advantages of the IDEX over conventional equipment include energy reduction, emissions reduction, production increase and baghouse dust reduction.
Energy use for a conventional furnace and the VFM combined with an IDEX is shown in Table 1. Most of the energy use for a conventional furnace is lost in the flue gases, and the second largest loss is for the wall and holding losses. The amount of energy actually going into the scrap is 560 Btu/lbm for a furnace thermal efficiency of only 19%.
The VFM receives preheated scrap from the IDEX, so only an additional 320 Btu/lbm is required to melt the scrap. Because the flue gas temperature drops to 1400°F (760°C) and the firing rate decreases, flue losses are only 256 Btu/lbm. Wall and holding losses are also significantly reduced. The thermal efficiency of the VFM, based on a theoretical energy use for scrap melting of 560 Btu/lbm, is 75%.
The VFM not only can be used as a melter for clean aluminum scrap, but also it can be used as a decoater to remove organics from aluminum scrap (VFD) and a combined decoater and melter to simultaneously remove organics and then melt the scrap (CFM).
Decoating tests were conducted in ERCo's pilot-scale VFD (see lead photo) using both used beverage cans (UBC) and turnings. Figure 3 shows the oily turnings and Figure 4 shows the turnings after being decoated in the VFD. The oil has been completed removed and no oxidation is evident. Figure 5 shows the UBC before decoating and Figure 6 after decoating. The decoating process has completely removed the organics with no visible sign of oxidation. Scrap aluminum organics are removed in a minute or two, compared with 10 to 20 minutes using conventional equipment. Other advantages include rapid scrap type change out, small footprint, ability to both preheat and decoat, little fuel requirement, emissions regulations compliant, process wide variety of scrap, no moving parts, comparatively inexpensive and simple control system.
Both melting of clean aluminum scrap and simultaneous decoating and melting of coated and oily scrap were done in the pilot-scale VFM. Figure 7 shows the specific energy use vs. throughput. Energy use decreases with increasing throughput increases. At 453 kg/hr (1,000 pph), energy use varies between 1973 to 3196 J/g (849 and 1,375 Btu/lbm).The low measured specific energy use is a result of low flue gas temperature due to gas recirculation. Additionally, scrap is floated in the flue gases, which also adds to VFM efficiency since the flue gas energy is being used to heat the scrap. Essentially, VFM flue gas losses are only 7% of that of the conventional furnace.
A major advantage of the VFM is rapid melting, which leads to low melt loss. In essence, the VFM does not allow enough time for complete oxidation to take place. As the scrap aluminum is oxidized, it forms a protective layer that prevents further oxidation. The time to melt in the VFM is only about 20 seconds, so it is possible the VFM will lead to improved metal yield since the scrap aluminum is only.
Waste-gas operated. Low gas temperatures can be used and still provide relatively fast melting. Temperatures as low as 1250°F (675°C) can melt aluminum scrap. This leads to the possibility of using waste heat from a source within the plant to drive the VFM process. Two sources of waste heat are flue gas (2200°F, or 1200°C) from a conventional furnace and exhaust gas (1500°F, or 815°C) from the IDEX with the use of auxiliary burners. Advantages of using waste heat include nearly complete elimination of fuel use and elimination of the recirculation duct and fan thus minimizing the VFM size and installed cost.
Mag-melter. The addition of magnetic enhancement to the VFM is a recent development (Fig. 8). Aluminum is fed from the bottom and entrained by the high velocity gases. Permanent magnets (electromagnets can also be used for improved performance) placed around the melter impart a magnetic field on the aluminum inside. As the aluminum melts, it falls into the holding chamber. Operating at a very high inlet gas velocity causes the aluminum to hang up in the cylinder, and pulsing the gas periodically (30 sec or less cycle) allows withdrawing Al from the Mag-Melter. IH