Shaft furnace – quality and economy
If the enterprise for the production of aluminum castings - cast products and parts made of cast aluminum alloys are high-quality metal task, high melting rate and a low specific energy consumption, a shaft smelting furnace is a good choice for him (Figure 1).
melting speed of the shaft furnace
Melting speed shaft furnaces begin approximately 300 aluminum kilograms per hour and can reach as much 7000 kilograms per hour. This range of melting rates (from 300 to 7000 kg / h) corresponds to the ability of the same furnace to hold aluminum melt - the volume of the piggy bank - from 500 to 20000 kg.
Generally speaking, depending on the technological characteristics of the production can be various combinations of the melting capacity of the furnace volume and its forehearth, which is intended for storing and holding the melt prior to casting. In practice, the following "rule" is usually adhered to: the size of the bathtub of the piggy bank (in kilograms of aluminum) should be at least twice as large, than the melting furnace rate, expressed in kilograms per hour.
The design of the shaft furnace for aluminum
In figures 2.1 и 2.2 shows the construction scheme of a shaft melting furnace. Thanks to the special design of the furnace shaft and the well-thought-out burner system, the various technological stages of the aluminum melting process occur spontaneously and sequentially:
- preheating and drying the charge,
- heating the batch to melting temperature and
- directly melt blend.
The heating and aluminum melting in a shaft furnace
Cold aluminum blend is loaded into the upper part of the shaft and gradually heated in the course of its descent down the shaft. This occurs due to the heat of the hot gases, which rise up the shaft of the melting zone, which occurs in the lower part of the shaft. Thus, shaft furnace operates on the principle of energy efficient counterflow batch and hot gases.
Heat from the flue gas is transferred by convection and the charge continues even at relatively low temperatures.
When the charge reaches the hollow aluminum is fed to the bottom of the shaft, she already warmed up and therefore melts at high speed. Therefore, the molten metal only for a short time is in the zone of the furnace with a high temperature and directly exposed to the hot gases flow from burners. That is why the shaft kilns produce minimal fumes, and the aluminum melt has a low content of non-metallic inclusions and gases.
Preheating the charge in the shaft furnace ensures its effective drying and, thereby, It eliminates the possibility of explosion due to the wet batch. Molten metal flows with minimum turbulence from the oblique hearth forehearth, where it maintained at a predetermined temperature casting.
Shaft furnaces have a high degree of automation. The only manual operation is to load the charge (ingots and casting waste) into the loading device.
Two cameras - two temperatures
The design of the shaft furnace comprises a burner system for isolated smelting shaft furnace and forehearth. This enables continuous production of the molten aluminum with tolerances of ± temperature 5 oC. This very important property of the shaft furnace allows it to supply the foundry with metal with maximum equipment productivity..
The shaft furnace has a two-chamber design: a melting hearth and a hoarder, where the melt is accumulated and held. This ensures the high quality of the poured metal.. Aluminum melt from the shaft furnace is characterized by a low concentration of suspended, insoluble contaminants, and the hydrogen content is lower than the concentration of its equilibrium solubility. Therefore, shaft kilns are in many cases the best choice among other types of furnaces for foundries for the production of aluminum castings, special, serial production for casting in a metal mold and molding under pressure.
Do not melt the shavings!
Shaft furnaces are not adapted for melting aluminum scrap excessively small, such as, for example, shavings.
The efficiency of two-compartment shaft furnace becomes limited when dealing with excessively low casting speed - lower 150 kilograms per hour, and when frequent changes of aluminum alloys, especially from copper-bearing aluminum alloys, aluminum alloys, which do not contain copper, and vice versa.
For restrictions on shaft furnaces, see. also here.
The main advantages of shaft furnaces
Thus, The most important advantages of shaft furnaces for smelting aluminum over other types of furnaces are:
- High quality metal, including, low content of gases and non-metallic inclusions;
- low waste - small losses of metal from oxidation in the furnace atmosphere;
- low energy consumption per ton of molten metal;
- high uniformity of the melt temperature during casting;
- ease of access to the inside of the furnace for slag removal and lining cleaning;
- minimal maintenance and long lining life;
- batch loading automation;
- minimum occupational noise, isolation of products of combustion and other contaminants in the working zone of the furnace.
Reflective and shaft furnaces // ASM Handbook- Volume 15: Casting