製品情報

アメリカ21st.Heat Treating Society Conferenceにて発表

ASM-HTC 21th
Nov. 2001

High Efficiency Crucible Melting Furnace for Non-Ferrous Metals

A. Tanaka, M. Tsuri, H. Ando, M. Kikura,
Hokuriku Techno Company Ltd., Imizu-gun, Toyama-Pref., Japan
and
M. Ueki,
Tokyo Akasaka Laboratory
Kanazawa Institute of Technology, Minato-ku, Tokyo, Japan


Abstract:

A spiral guide way of the burning flame surrounding the outer-wall of the crucible installed in order to raise the heat efficiency and minimized the energy consumption in the crucible furnace. These modifications of the furnace shorten the melting duration by about 30%, suppressed energy consumption to 55-60% and increased the life of crucible by 50% for aluminum melting and holding operations using cast iron and graphite crucibles.


Introduction

The crucible furnaces have been frequently utilized for small and medium scale melting, casting and holding operations of various non-ferrous metals like aluminum and copper. In the furnace, a crucible is installed in the combustion chamber surrounded by the furnace wall made of refractory. Usually, a fuel for the furnace is gas, and the gas burner is faced to the bottom side outer wall of the crucible. In the condition, the burner flame heated-up the metal inside the crucible quite locally and the most of the heat generated was blown quickly out from the furnace, resulting in the melts with inferior quality and low heat efficiency.

In order to raise heat efficiency and to minimize the energy consumption in the circumstance, a spiral guide way of the burning flame surrounding the outer-wall of the crucible, was newly constructed. New furnace named "spiral crucible melting furnace1" was investigated their performance in comparison with that of the conventional furnace in special emphasis on fuel consumption and heat efficiency.


Experimental

Preliminary Experiments

Before construction of the refractory flame guide, the flame guide made by steel(cast iron) was tried preliminary, for melting 200kg aluminum. The results exhibited an apparent effectiveness of the flame guide in both time to melt and fuel(LP gas) consumption, they were 2h45min and 23.57m3 for the furnace with flame guide against 4h20min and 33.3m3 for the conventional furnace.


Construction of the Furnace

The newly developed furnace has a spiral guide way of the burning flame surrounding the outer-wall of the crucible to improve the heat efficiency and to minimize the fuel consumption. The guide way was constructed through stacking-up the pre-cast bricks with trapezoidal cross section as shown in Figure 1. As the material for the pre-cast brick, a castable alumina-based refractory, "PHLOX 1560SR(Lafarge Refractories)", was used with incorporation of 3wt% stainless steel needles in size of φ0.4×30, since the flame guide is required to have refractoriness against direct burning flame, thermal shock durability for instant cooling down in the off-time, and high temperature strength for over-hanging guide way from the furnace wall. The high temperature durability of the bricks was improved by incorporating2 the stainless steel needles with size of φ0.4×30 into the alumina based matrix of the brick by amount of 3wt%. Such an incorporation raised significantly the strength of the brick at 1300℃.

Wood-made mold for pre-cast brick Formed pre-cast brick.
(a) Wood-made mold for pre-cast brick (b) Formed pre-cast brick.
Figure1: Preparation of the spiral guide way.

Experimental Procedure

The experimental melting operations were conducted on the developed spiral furnace and conventional crucible furnaces for the comparison. The typical drawings of the cross section are shown in Figure 2(a) and (b). Both of the furnace are essentially same melting capacity with crucible size of #500.

installed spiral guide way conventional type
(a) (b)
Figure 2 Typical drawings of the cross section of both crucible furnaces; (a) installed spiral guide way and (b) conventional type.

Results and Discussion

Appearance of Crucible after Melting

The outer-wall of the crucible for the spiral furnace has monotone color in comparison with three-colors for the conventional furnace as shown in Figure 3(a) and (b). As a matter of fact, the monotone colored outer-wall was created by homogeneous heat-effect for the furnace, whereas the colorful wall yielded by heterogeneous heat-effect to the crucible. Actually, in the conventional furnace, the burning flame reached evacuation duct with only one-turn of the crucible wall. Therefore, the burning is localized to the limited area of the crucible wall. Such un-homogeneous thermal distribution caused to cracking of the crucible and occurrence of the temperature gradient in the molten metal interior the crucible.

In the spiral furnace, the homogeneous thermal distribution materialized by slower turning of the burning flame due to the spiral guide way, resulting in both prolonged crucible life and elimination of the temperature gradient in the molten metal which deteriorate the quality of the products.


spiral crucible furnace conventional type crucible furnace
(a) (b)

Figure3: Appearance of crucible after melting operations.
(a) spiral crucible furnace and (b) conventional type crucible furnace.

Fuel Efficiency

The fuel consumption for the spiral furnace was compared with those for the different type furnaces. They are so-called "re-generation burner furnace" and conventional type of crucible furnace. The comparison was made by total and daily average amount of fuel consumption in Friday to Friday 6 days operation including weekend two-off days, under the following conditions; crucible used was K227 with capacity of 450kg aluminum; melting temperature was 640℃ and burner performance of 105kcal/h: measurements of the fuel consumption was made at 5 o'clock every day; operation continued 21h every day by hot-charging to die-casting machine.

The results of the comparison are shown in Table 1. As apparent from the table, in the spiral furnace, day average consumption and Tuesday to Friday average decreased by 8.07m3(17.1%) and 5.75m3(14.1%), respectively. The fuel efficiency of the spiral furnace was comparable to the re-generation burner furnace equipped with thermal exchange system of the waste heat. As seen in the Table 1, total and average consumption of the both spiral and re-generation furnaces were almost equivalent.


Table1: Comparison of the fuel consumption(m3)

Spiral furnace Re-generation
burner furnace
Conventional
crucible furnace
Total consumption (6-day) 194.82 192.00 235.15
6-day average 38.96 38.40 47.03
4-day (Tue.-Fri.) average 34.84 34.25 50.59

Efficiency of the Spiral Crucible Furnace

The effectiveness of the spiral furnace was demonstrated in the following several operations for aluminum melting. Comparison was made before and after installation of the spiral guide way.


Graphite crucible for sand-mold casting

Three cases were compared in graphite crucible one type #500(pot-type) and two of type #1100(shallow type). Results are shown in Table 2. In the first case of #500 crucible, Al(AC4C) melting operation was conducted by 600kg/day with 3 charges of each 200kg. By installation of the spiral guide way, the time to melt was shortened 0.5h and daily gas consumption decreased by as much as 11m3, which corresponded to 19.6% energy saving with certain extension of crucible life.

In the first case of #1100 crucible, Al(AC4C) melting operation was performed by 550kg/day with 1 charge of 250kg and 4 charges of 75kg. The time to melt was shortened as much as 1h and daily gas consumption decreased by 12m3, which corresponded to 22.6% energy saving with certain extended crucible life by installation of the spiral guide way. For the second case of #1100 crucible, Al(AC4C and AC2B) melting operations were performed by 800kg/day with 2 charges of each 400kg. By installation of the spiral guide way, the time to melt was also shortened by 0.5h and the daily gas consumption decreased by as much as 15m3 which corresponded to the 19.2% energy saving with certain extended crucible life.


Cast-iron crucible for die-casting

In the retaining furnace to be used in hot-charge for die-casting, Al(ADC12) melt kept at temperature of 640℃, poured-into die-casting machine by totally 3000kg/day with 20 charges of each 150kg. As described in the Table 2, the performance of the furnace was improved by installation of the spiral guide way. Namely, the time to melt was shortened by 0.5h and daily gas consumption decreased by as much as 13m3 which corresponded to 28.9% energy saving with expected life extension of the crucible.


Table2:Comparison of fuel consumption in various operations using crucible furnaces before and after installation of the spiral guide way.(1 m3 =24,000kcal)
Gas consumption Amount of
melt/day
(kg)
Installation of the
spiral guide way
Time to melt
(h)
Gas consumption
m3/day
(kcal/day)
Energy
/unit weight
m3/kg
(kcal/kg)
Crucible (type) & Operation
Graphite crucible &
sand-mold casting
#500
740℃
8h/day
600
(200kg×3ch.)
before 3.0 56
(1,344,000)
0.093
(2,240)
after 2.5 45
(1,080,000)
0.075
(1,800)
#1100
760℃
8h/day
550
(250kg×1 and
75kg×4ch.)
before 3.5 53
(1,272,000)
0.096
(2,313)
after 2.5 41
(984,000)
0.075
(1,789)
#1100
750℃
8h/day
800
(400kg×2ch)
before 4.0 78
(1,872,000)
0.098
(2,340)
after 3.5 63
(1,512,000)
0.079
(1,890)
Cast iron &
die-casting
450kg
640℃
20h/day
3000
(retaining furnace)
before 4.0 45
(1,080,000)
0.015
(360)
after 3.5 32
(768,000)
0.011
(256)

Summary

A spiral guide way of the burning flame surrounding the outer-wall of the crucible exhibited significant improvements in the performance of the crucible furnace in their heat efficiency and minimization of the energy consumption. These modifications of the furnace energy consumption to 55-60% of its original state and increased the life of crucible by 50% for aluminum shorten the melting duration by about 30%, suppressed melting and holding operations using cast iron and graphite crucibles.


References

  1. A. Tanaka and M. Kikura, U.S. Patent application(in processing)
  2. H.W. Hayden, W.G. Moffatt and J. Wulff, The Structure and Properties of Materials, Vol.III Mechanical Behavior, p.189, John Wiley & Sons, Inc., New York(1965)