Direct-chill casting 

Melting furnaces for DC aluminium casting

Aluminium melt preparation

The productivity of a aluminium DC casting plant often depends on the melt batch preparation cycle time. Most plants are constrained by the melt preparation cycle time rather than the DC casting pit cycle time. The melt preparation operation must deliver molten aluminium to the DC caster at the correct temperature, on time, and within the specified alloy composition.

The type of furnace technology used and its management is crucial to these goals. Additionally, operators seek to achieve this with low melt loss, minimum capital cost, long furnace life, safely, and with a minimum energy consumption to reduce fuel costs and greenhouse gas emissions [1].

Scrap remelting and DC casting

Almost every aluminium extruder has its own shop for remelting internal technological aluminum scrap. These shops can be large, medium and large depending on the number of extrusion presses for a given aluminum extruder.

These furnaces have special features as they must be suitable for aluminum extrusion scrap remelting and direct-сhill casting using vertical DC casting machines (VDC casting machines).

Below are recommendations for the selection of melting furnaces for such a remelt shop. They can be useful for remelt operation independent from an extrusion plant. These recommendations are based on the excellent materials of Ashford Engineering Services [2].

Melting furnace selection

There are several options of furnace selection and layout which depend on the total volume of production planned [2]:

  • With production volumes of up to 8000 tonnes per year a single furnace can be used. Melting and holding take place in the same furnace (Fig. 1).
  • Where production requirements exceed 10000 tonnes/year it is possible to use two parallel melting/holding furnaces (Fig. 2).
  • For higher production rates a three furnace melting furnace system can be used. This is especially true in cases where frequent changes of alloy are intended. Such an arrangement with two melting furnaces feeding a single holding furnace give flexibility with production volumes of up to 60000 tonnes per year.

Fig. 1 – A single melting/holding furnace as part of the shop equipment
for vertical DC casting of extrusion ingots [2]

Figure 1 – The general layout of two parallel furnaces and melt treatment units
in relation to verical DC casting machine [1]

Furnace types

A melting/holding furnace can be

  • fixed hearth furnace
  • tilting furnace.

Fixed hearth furnace

The fixed hearth furnace has the lower capital cost and is the most simple construction. Molten metal is fed from this type of furnace through a tap hole. The special tap hole block is plugged by a cone fitted to a handle (Fig. 3).

Fig. 3 — Fixed hearth melting furnace [2]

Tilting furnace

The tilting furnace has a higher capital cost, because the furnace is rised on hydraulic rams to pour the metal onto the launder run and then onto the casting machine (Fig. 4). Figure 5 shows a 15 tonne tilting melting furnace.

Figure 4 – Tilting melting furnace [2]

Figure 5 – A 15 tonne capacity tilting/holding furnace [2]

The tilting furnace’s higher cost are justifed by reason of ease and accuracy of metal feed control. This type of furnace also provides improved safety to plant and operators. if there is a problem, the tilting furnace returns to its rest position. Molten metal is cotained within the furnace and there is an immediate stop to the flow of metal [1].

The fixed hearth furnace operates with its tap hole has to be pluggen manually. It is can be difficult if there are problems in the casting area. Therefore there is a great risk that the contents of the furnace dischargeing into the casting pit and shop floor.

Furnace heating

Furnaces are usually heated by gas or oil. Holding furnaces are occasionally heated with electricity.

There are three types of burner systems [2]:

  • cold combustion air burner
  • recuperative burner
  • regenerative burner.

Cold combustion air burners

Cold combustion air burner gives maximum fuel consumption. Suitable for heavy oils, kerosine and gas [2].

Recuperative burners

Recuperative burners are suitable for gas and oil. Waste gases pass through a heat changer to provide preheat to the combustion air to burner [1]:

  • They reduce fuel consumption by about 20% compared with cold air system.
  • Capital costs are approxmately 1,7 times the cost of cold air system.
  • Not really practical for painted or oily scrap.

Regenerative burners

Key features [2]:

  • They reduce fuel consumption by about 45% compared with cold air system.
  • Regenerative burners used mainly on gas.
  • They are not suitable for use with paitned or oily scrap.
  • Capital cost are approximately 3 times for of cold air system.

This system uses two burners fired alternatively with waste gasses from one passing back through the other and then into a ceramic medium. The ceramic medium absorbs the heat, storing it, so that when the firing is reversed this heat is added to the combustion air. This heat storage medium can be removed for periodic cleaning. Regenerative ceramic burner’s system require complex control system using micro-processor and have higher maintenance skill levels and costs [2].

Figure 6 – Schematic of regenerative burner arrangement
for reverberatory melting furnace.
(Bowers, J.D.) [3]

Furnace temperature control

In all furnace systems requirements the requirements for efficent fuel consuption demand good temperature control.

Control of the bath temperature should be the overriding factor so that fuel is not wasted by adding exess temperature to the metal. Excessive bath temperature cause loss of the alloyin elements and requireextra alloying addition to makeuo this loss thus adding to running costs [2].

Melting control can be either on roof temperature or flue gas temperature with burners modulating as the temperature is reached. Hower the required metal temperature should override the flue or roof temperature. The maximum allowable metal temperature is usually considered to be 750 degrees Celsius [2].

Sources:
1. Direct-Chill Casting of Light Alloys /John Grandfield, D. G. Eskin, John Grandfield, 2013
2. D.C. Casting Remelt Shop Handbook – Ashford Engineering Services, 1997
3. Aluminum Recycling / Mark E. Schlesinger, 2007