Melting and Holding Furnaces

In stationary melting and holding furnaces, the distance between anchor bolts should not exceed 500 mm in the walls in the vertical direction and should not be less than 1,200 mm in the horizontal direction. In the roof, the distance between anchor bolts should be within 450 mm. For tilting furnaces, these distances should be smaller. 

In the A1 industry, the temperatures are not high. Steel anchor bolts can be in service up to 1,100-1,200 °C. Above these temperatures only anchor bricks should be used. 

The typical flux (there are the variations) is KCl MgCl2 (melting point 488 °C) it interacts with impurities and with A1 as well, giving A1 and sodium chlorides. At holding, these chlorides (dross) appear on the surface and are taken off. The temperature of molten aluminium alloys at casting is 690-710 °C (Fig. 3.15). 

The requirements to refractories were mentioned earlier porosity 15 % average pore size 5 pm and low wetting by Al. 

The service hfe of melting furnaces is lower than that of holding furnaces because of the mechanical action of ingots at loading. Consequently, the requirements for refractories for melting furnaces are tougher. 

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Fig. 3.31. Sectional view of a tilting aluminum melting and holding furnace in Hungary that tips either left or right to fully drain its liquid load. This avoids the problem of the bottom portion of the next charged load of solids being shielded from furnace gas convection and radiation. Two burners in diagonally opposite corners are tilted downward 3.5 degrees from horizontal. (See also fig. 5.28.)...

Aluminium is a rather reactive metal, and it can react with oxygen at temperatures of 700-800 °C (which are common for an A1 foundry) to form alumina. In the melting and holding furnaces of an A1 foundry, an atypical process takes place in ferrous metallurgy oxidation of A1 melt on the triple border refractory-melt-air. Oxidation takes place with the positive volume effect, and the resulting alumina takes more space, so the thickness of the refractory wall increases (Fig. 1.29). 

As we mentioned earlier in this chapter, fireclay, muUite, and alumina refractories have been used for the layer that is in contact with molten Al. There were trials of magnesia, zirconium, aluminium titanate, and silicmi carbide refractories. The results of these trials cannot be called tmsuccessful. Most likely, these trials were sustainable for the entire range of actions necessary at implementation of a new material. In a limited amount, these materials are used today (crucibles for secondary Al for low-pressure dye casting, runners between melting and holding furnaces for special alloys, burner blocks, tap hole blocks). Yet the existing technical decision is to use alumina silica refractories with a relatively sophisticated structure and antiwetting additives. 

There are no strict criteria when choosing refractory materials for A1 melting and holding furnaces. The action of fluxes is not taken into account, but their influence on corrosion resistance in the metallurgy of A1 is not much. If the material is not corrosion-resistant, it is possible to estimate the depth of penetration The deeper the penetration, the less resistant the refractory. 

It is possible to use various heat insulation materials for the melting and holding furnaces. Fiber materials are usually not used in the bottom, but they may be used in the walls and in the roof. Materials that may withstand relatively high pressures at relatively high temperatures (diatomaceous bricks, vermiculite slabs, calcium... 

Approximately 20-30 years ago, in the mid-1980s and 1990s, it was possible to see a two-layer liming of melting and holding furnaces with a total thickness up to 1,000 mm in the bottom (mullite or fireclay brick plus diatomaceous brick or vermiculite slab), as seen in Fig. 3.10a. The paradox is that having additional heat insulation layers - whose purpose was to decrease the heat flow through the... 

A modem concept is lining melting and holding furnaces by castables, yet a brick lining is more resistant to thermal shock and mechanical stresses. One of the decisions is the application of preshaped blocks (Fig. 3.16). Multilayer blocks are cast and heat-treated in a refractory plant and shipped to an Al plant for installation in the steel shell. 

The roof in contemporary melting and holding furnaces is flat and made from preshaped castable blocks. The precast blocks consist of two or three layers with heat insulation and are hung on anchor bolts. The pads between the preshaped blocks are fiber materials. In tilting furnaces, the roofs are usually not from preshaped blocks and are made by gunning. If there is a chance that a temperature at the roof will exceed 1,200 °C, the refractory anchor bricks are used. 

The requirements for refractories for the vanes of the degassing units do not differ from the requirements for the melting and holding furnaces (Table 3.6). They are high thermal shock resistance, low wetting by Al, and high erosion resistance. 

Bricks in the 80% and 85% AI2O3 class were originally developed for use in aluminum melting and holding furnaces. It is rare that they find application in... 

Induction furnaces are excellent melting units, but in general they are less efficient holders. When they are used for melting only, the molten metal is mostly moved to an efficient holding furnace as soon as it has reached the desired temperature. Many types of coreless induction furnaces are available, with fixed or removable cmcibles. For aluminium, both channel and coreless induction furnaces are available for melting and holding. However, the charmel type is seldom used, due to difficulties in keeping the channel open and due to the need to maintain a molten heel at all times. 

This type of furnace is mainly used for holding purposes, though it may also be used as a combined melting- and holding aggregate. 

Crucible melting and holding of aluminium, copper, lead and zinc For the operation of crucible furnaces, BAT is to ... 

The spout on a holding furnace broke during casting and molten aluminum fell into a pool of water. An explosion resulted An aluminum-manganese alloy was being melted in a refractory crucible. Due to an accident to the crucible, molten metal spilled into the furnace pit which contained water. An explosion resulted... 

Place the dry salt in a platinum boat in a silica combustion tube in an electric-tube furnace. Flush the air out of the cold tube by the rapid passage of hydrogen for a few minutes. Then over a period of 30 minutes bring the temperature of the furnace to 700 to 725°C., and hold for 10 minutes to allow time for the salt to melt (m. 680°C.). Allow the fused salt to cool to below 200°C. in hydrogen, remove to a desiccator, and store there until needed. 

At about daily intervals, as sufficient aluminum accumulates in each cell, a stout pipe attached to an evacuating vessel is broken through the crust on the cell and dipped into the pool of molten aluminum. The pressure in the receiving vessel is decreased, which pulls the melt into the container, a process, which is repeated for each cell down the potline, to collect the molten aluminum in a central holding furnace ready for fabrication. 

Three basic schemes for the ultrasonic cleaning of the melt can be proposed (1) in a liquid bath of an ingot (the melt surface is in a contact with a waveguide-radiating ultrasonic system) (2) in a mold (oscillations transmit to the melt through mold walls) and (3) in an intermediate vessel placed in the flow of melt from a holding furnace to a mold (oscillations transmits to the melt according to the first scheme). 

The studies on the efficiency of the operating schemes mentioned show that the treatment of the melt in the liquid bath of an ingot at metal temperatures close to the liquidus (when the viscosity of the metal is sufficiently high and hydrogen bubbles are evolved in the opposite direction to the acoustic energy flow) is less efficient than the treatment performed on the melt flow from a holding furnace to a mold. The degassing process is incidental with the ultrasonic treatment of the melt... 

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