Casting Aluminum in China
The Nanshan Phase I facility comprises two melters and two holders and has been in full production for over one year. This facility consistently produces between 12,000 and 13,000 MT of aluminum ingots per month. Ingots up to 640 mm x 2,130 mm x 9,150 mm long weighing 34 MT can be cast from a variety of alloys, the most common being 1050, 1235, 3004, 5182 and 8011. These furnaces feed a single caster. As one set of furnaces melt and prepare the aluminum for casting, the second set is casting. The total cycle time for each set of furnaces is less than eight hours, allowing the installation to produce six 75-MT casts per day.
Melting Furnace OverviewAt the heart of this process are the melting furnaces. They have removable domed roofs for loading scrap aluminum. Each has a maximum bath capacity of 82.5 MT and a 75-MT pouring capacity (Fig. 1). For maximum efficiency, the actual bath geometry in a round-top furnace is a critical balance between diameter and capacity.
The furnaces operating at Nanshan employ Bricmont Generation III combustion technology. Four regenerative ultra-low-NOx natural gas burners, operating in two pairs, provide energy to the process (Fig. 2). These burners operate in a cycling mode with only one of each pair firing at any one time with 80% of the total flue products being evacuated from the furnace chamber through the inactive burner port. In an abnormal situation, all four burners can fire simultaneously in the conventional mode to prevent the loss of the cast.
The placement of burners in a round-top application where only two of the four burners are firing at any one time is not intuitive. The current fuel rates have only been possible by the skillful use of computational fluid-dynamics techniques by experienced designers (Fig. 3).
During the development stage, various burner angles and positions were rigorously tested before designs were finalized. The result is that Generation III furnaces use less than half of the energy required for the First Generation furnaces designed in the mid-1950s.
The lid is removed and repositioned by a special cover lift carriage that is shared between the two melting furnaces (Fig. 6).
An electromagnetic stirrer (EMS) adds to melting efficiency. Situated beneath the melting furnaces, the EMS provides a stirring action that reduces temperature variations, mixes the aluminum for alloying, provides a means to melt the aluminum more rapidly by convection between the molten aluminum and the solid aluminum, and reduces dross (Fig. 7). It also reduces the amount of time that the dross door would need to remain open to manually stir the aluminum.
The two melters share a common EMS, which is shuttled between the furnaces by means of a transfer carriage. Since the EMS must be within 10 mm of the bottom of the melter to operate effectively, this car must also raise the EMS into position. Precise positioning is required, which calls for the use of reliable electronic sensing mechanisms linked to the main computer control system.
When ready, aluminum is transferred to the tilting/holding furnace by opening the tap plugs. Transfer of aluminum from the melter to the tilting/holding furnace takes approximately 20 minutes. A special remote-actuated tap-plug machine has been developed to provide safe operation.
Tilting/Holding Furnace OverviewThe holding furnaces are of rectangular configuration, and their purpose is to condition the molten aluminum prior to use and then to discharge it, by tilting, to the DC casting station. Each of the two furnaces have a 90-MT capacity, although the maximum casting capacity is normally limited to 75 MT.
For a successful cast, molten metal of the correct composition is required to be delivered at very precise conditions of flow rate and temperature to the outlet launder.
Conditioning involves the adding of alloying elements, skimming dross and degassing and the removal of tramp elements such as calcium and hydrogen. Porous plugs, through which chlorine/nitrogen flux gases are metered at precise volumes, are fitted to the hearth of each holder for this purpose. The required metering devices are part of the supplied system.
The correct molten-metal temperature is critical for proper casting, and different alloys may require different casting temperatures. The burners and computer combustion-control system – using Bricmont proprietary software – can maintain aluminum launder temperature within 3°C. During the casting operation, molten-metal control is switched from the furnace bath to the launder itself to ensure precise control of temperature at the critical point of usage.
Tilting of the furnace is accomplished with two large-bore hydraulic cylinders fitted to each side of the furnace and connected to a fabricated box beam, through which all loads are transmitted (Fig. 8).
Early aluminum tilting furnaces used mathematical algorithms that predicted the discharge rate, but today lasers are used at the outlet launder to precisely monitor metal levels. Using feedback from the laser device, Bricmont uses a computer-controlled anticipatory micro-pulse system to accurately regulate the flow of hydraulic oil to the cylinders. Using specially developed valves, furnace tilting rate is micro-adjusted as determined by the launder laser. This control produces an almost indiscernible movement in the cylinders, and with this system, launder levels are routinely kept within 1.5 mm.
The aluminum is transferred to the launder through a multiple-piece cast-steel spout. Aluminum is not poured into the launder from above since this would introduce air. Instead, the spout is designed to maintain a liquid-tight seal while tilting. Aluminum at the outlet of the furnace is at the same elevation as the aluminum in the launder. The design is such that aluminum is drawn out of the furnace from under the surface to minimize the risk of furnace dross entering the launder system.
RefractoryAll refractory materials used in the construction were procured and manufactured in the PRC. This required a thorough review of the materials offered. This culminated in the testing of potential materials to ASTM standards to gauge their effectiveness against standards that have been proven to be successful in the past. After selection, stringent quality-control testing ensured that all refractory selected met the furnace designers strict requirements.
Automatic ControlsThe control-system hardware includes a PC-based HMI (Human Machine Interface) with an expandable software package and PLC (Programmable Logic Controller) for control of the equipment modules. All process functions are monitored and controlled with the following amenities.
Proportional Integral Derivative (PID) Control
PIDs are used for the following functions:
- Cascade roof and bath temperature control (holder and melter furnace)
- Air – fuel flow control (holder and melter furnace)
- Combustion air pressure (holder and melter furnace)
- Exhaust temperature control (melter furnace)
- Furnace pressure control (holder and melter furnace)
Preheating of the regenerative burner media beds to temperatures of 350-400°F results in high thermal efficiency.
Comprehensive Alarm System
This system provides an historical logging of all system alarms available for viewing via panning back and forth in a given time period to aid maintenance in diagnosing system problems.
In addition to the system alarms, historical data logging of all key system variables is maintained. With this feature, real-time and historical trending of variables can be performed.
Communication Interface with Caster and Auxiliary Equipment
The following process data is monitored for communication from the melter furnace:
- Stirrer car traverse
- Electromagnetic stirrer (EMS system)
- Cover carriage used for furnace dome removal
- Hydraulic system control
- Motor control center (MCC)
- Trough level control
- Hydraulic system control
- Fluxing/purging system
- Fume removal system
SummaryTo meet the rapidly growing needs of the Chinese market, Nanshan is currently completing Phase II cast house featuring similar Bricmont round-top furnaces. At the time of publication, ground has been broken to install a third cast house, which will triple the size of Nanshan I and II installations. By the end of the decade more than 1 million metric tons per annum will be processed in Bricmont high-efficiency top-charge melting installations at Nanshan and other locations in China.
The high-production round-top melting furnaces and tilting/holding furnaces have been proven to deliver dependable performance. When asked about the furnace operation, Mr. Liu Shaoyu, who heads the casting operations at Nanshan, noted, “Nanshan considers that the performance of Bricmont furnace equipment is good, with high production efficiency. It is easy for operation and usage and is able to meet the production requirement of the ingot.” IH
For more information: Contact Eric Blake, P.E., vice president of process technology, Bricmont, Inc., 500 Technology Drive, Canonsburg, PA 15317; Tel: 724-746-2300; fax: 724-746-9420; e-mail: email@example.com; web: www.bricmont.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: aluminum melting, ultra-low NOx burner, round-top furnace, top-charge furnace, electromagnetic stirrer, regenerative burner