Improved cupola melting with silicon carbide and ferrosilicon.

Inside This Story

* Silicon carbide and ferrosilicon each have positive and negative attributes in cupola operations.

* Investigations clarified differences in their performance and suggest optimal use depending on application.

Alloys are the most costly materials used in the production of cast iron. Cupolas consume large amounts of alloy, particularly silicon (Si), due to oxidation losses and the need to alloy large amounts of steel. Traditionally, only ferrosilicon (FeSi) was used in this capacity; however, in recent years silicon carbide (SiC) has come into increasing use. The popularity of SiC is due to economics because SiC introduces alloy carbon (C) as well as Si to the iron. Some grades of SiC contain large amounts of additional uncombined C, known as free-C. These free-C sources are supplied at a very low cost, which facilitates reductions in the more expensive coke. This is particularly attractive today with metalcasting facilities scrambling to find ways to reduce production costs.

Unfortunately, nothing is free. Whereas FeSi melts predictably like any metallic charge material, SiC does not melt. To incorporate SiC into molten iron, it must be dissolved, like coke dissolving in iron. This is a slow and more complex procedure that introduces variability to the alloying process. Because of this unpredictability, many cupola operators add FeSi and SiC together in order to hedge their bet.

A study was undertaken to develop an understanding of the differences in the performances of SiC and FeSi in hopes to open the door for their utilization in a more effective manner. The study was conducted in two lined cupolas with diameters of 60 and 72 in. Tests of different Si alloys were conducted under identical conditions. The level of alloy Si added to the charges was in the 1-2% range. Coke levels were not adjusted to compensate for the extra carbon added with the SiC materials.

Taking the Test

The performances of four Si-bearing material were studied. Their compositions are given in Table 1. Briquetted 35% SiC is one of the most commonly used alloying materials due to the large content of free-C, which reduces the coke requirement of the cupola. Ninety-seven percent SiC was studied in order to separate the contributions of SiC from those of free-C and cement in briquetted SiC materials. Two FeSi materials were examined; both contained 50% Si. One was a briquetted material bonded with cement and the other was a pure, lump material.

Alloy Recovery--Figure 1 compares the performances of three of the Si bearing alloy materials in the recovery of Si. Very similar patterns (not shown) were obtained for C, manganese (Mn), sulfur (S) and iron temperature.

[FIGURE 1 OMITTED]

In the first period lump, FeSi was melted, producing a relatively constant level of Si because of its ability to melt and easily combine with melting iron and steel. In the second period with briquetted 35% SiC in the charge, the Si concentration increased steadily for at least 2.5 hrs before reaching what appeared as a steady level that was considerably higher than that attained with FeSi. In another test, concentrations increased for about 5 hrs before ultimately achieving higher levels of Si than with FeSi. In the final period, briquetted 50% FeSi was charged. Initially, Si rose and then decayed to the same level as in the first period.

The performance of briquetted 35% SiC is consistent with slow dissolution of SiC. In order to achieve high rates of dissolution, large amounts of SiC must accumulate in the melt zone. The presence of accumulated SiC dissolution occurred well into the third period, which accounted for the initial increase in Si at the start of the period and the decay that followed. Steady Si concentrations were restored when all the SiC had dissolved.

As a result of a parallel study using 97% SiC, it became evident that the gradual increase in Si observed with 35% SiC material was not a property of SiC, but rather due to other factors involved with the briquette. Figure 2 shows data for Si during the charging of 50% FeSi lump in the first period and 97% SiC in the second, and it can be seen there wits no discontinuity between the two periods. The three high Si points at the end of the FeSi period was due to a purposeful addition of higher Si to mark the transition.

[FIGURE 2 OMITTED]

Table 2 compares the recovery of C, Si and Mn for each of the four conducted studies. The subscripts refer to lump (L) and briquetted (B) alloys. Because of carbon pickup from coke, the recovery of C is greater than 100%. SiC shows lower C pickup because the C combined with SiC was assumed to be part of the initial metallic charge. It indicates that with SiC, less coke-carbon dissolved, allowing more coke to serve as fuel. In three of the four studies, the Si recoveries were higher with SiC after the Si reached its steady, maximum level. Interestingly, Mn recoveries were greater in periods utilizing SiC even though SiC contained no Mn. It suggests the C associated with SiC reduced the manganese oxide (MnO) formed by the oxidizing conditions in the cupola.

Finding More

In addition to the alloy recoveries, other significant conclusions were made.

Sulfur Levels in Iron--For ductile iron castings, the S in cupola iron is commonly reduced by a chemical process performed in a ladle outside the cupola. The cost of desulfurization is proportional to the amount of S that must be removed. Figure 3 shows that charging 35% SiC significantly increases sulfur levels in the iron, but no increase in sulfur levels was observed with the use of 97% SiC. It indicates the source of the sulfur was the free-C.

[FIGURE 3 OMITTED]

Iron Temperature--Despite the considerably higher levels of fuel available with the use of 35% SiC, iron temperatures were lower in two of three studies and about equal in one study (Table 3). This is attributed to two causes: higher Si losses with FeSi generate large amounts of heat; and large quantities of heat are released when FeSi dissolves in iron (not SIC).

Slag Basicity--Table 3 also provides data for slag basicity. Slags produced with SiC in the charge have higher basicity, which reflects the lower production of silcon dioxide (Si[O.sub.2]) resulting from higher Si oxidation losses. Because acid slags cannot dissolve significant amounts of S, the higher basicity produced in studies A and B was not reflected in lower S levels. However, there was a benefit shown for the neutral slag produced in Study D.

Slag Composition--Maintaining low levels of ferrous oxide (FeO) and MnO in a slag is important to produce minimum S levels in the iron by creating neutral or basic slags. Figure 4 compares the amount of FeO and MnO in the slags produced when charging FeSi and SiC. It is clear that there is a large reduction in both FeO and MnO during the period in which 35% SiC was charged. This cannot be ascribed to the presence of free-C as the same behavior was obtained with the use of 97% SiC. The conclusion is that the C combined as SiC is responsible for the reduction.

[FIGURE 4 OMITTED]

Melt Rate--Melt rate is another important consideration in cupola operation. Cupola melt rates decrease in proportion to an increase in available fuel. This would lead to the suspicion that melt rates would be lower with the use of briquetted 35% SiC because of the higher levels of available carbon. The cupola charging (melt) rates were 15-18% lower with 35%, SiC than with FeSi (Table 4, third column). This correlates closely with the 14-22% in crease in free C available in the briquettes (see Table 4, fourth column). The conclusion is that free-C performs as a cupola fuel.

Alloy Combination with Steel--One of the cupolas used in this study had the ability to remove iron drops at different levels inside the cupola. The analysis of the drops indicated that both SiC and FeSi dissolve primarily in steel. This is in accord with chemical principles that materials will dissolve fastest in liquids that contain the least amount of the given material.

The overall conclusions in this study are referenced in Tables 5 and 6.

There's Room To Discover

Laboratory studies of SiC performance in electric furnaces have indicated that the rate of dissolution will improve with increasing temperature, intensity of mixing and the addition of fluxes that dissolve the Si coating that surrounds SiC particles. The rate of dissolution also can increase with increasing SiC particle size. This has not been fully researched, but it has been suggested that there is a larger portion of Si on smaller SiC particles, thus causing difficulties for them to break down and dissolve. On the contrary, the rate of dissolution will worsen with increasing levels of Si and C in the molten iron.

Although the characteristic performances of SiC and FeSi have been rather well defined and causes for their observed behavior have been suggested, it will require further study to completely capitalize on their benefits.

This article was adapted from "04-153" presented at the 2004 Metalcasting Congress.

Table 1. Compositions of Typical SIC Briquettes

Material                     35% SiC       60% SiC         97% SiC
                            Briquette     Briquette         Lump
                                %             %

Silicon                        24.5          42.0            67.9
Carbon (combined with SiC)     10.5          18.0            29.1
Carbon (free)                  27.5          13.0            2.0
Silica                         15.0          12.0            1.0
Cement                         10.0          10.0            0.0
Other                          12.5           5.0            0.0

Table 2. Percentage of Alloy Recovery Under Steady Performance
Conditions

Study                         Carbon           Silicon       Manganese
                             Recovery         Recovery        Recovery
                                %                 %              %

A-Fe[Si.sub.L]                 165               46              24
A-Si[C.sub.B]                  139               64              68
A-Fe[Si.sub.B]                 162               42              44
B-Fe[Si.sub.L]                 171               47              18
B-Si[C.sub.B]                  136               47              41
B-Fe[Si.sub.B]                 171               42              36
C-Fe[Si.sub.L]                 158               69              68
C-Si[C.sub.B]                  136               80              89
D-Fe[Si.sub.L]                 152               72              --
D-Si[C.sub.L]                  137               83              --

Table 3. Data for Iron Temperature and Slag Basicity

Study            Iron Temp (F)       Slag Basicity

A-Fe[Si.sub.L]       2,804               0.47
A-Si[C.sub.B]        2,777               0.59
A-Fe[Si.sub.B]       2,795               0.45
B-Fe[Si.sub.L]       2,844               0.44
B-Si[C.sub.B]        2,802               0.54
B-Fe[Si.sub.B]       2,836               0.46
C-Fe[Si.sub.L]       2,782                --
C-Si[C.sub.B]        2,790                --
D-Fe[Si.sub.L]         --                0.95
D-Si[C.sub.L]          --                1.05

Table 4. Relationship Between Levels of Coke per Charge and Charge Rates

Study         Charge Rate    Fuel        Chg[R.sub.Si]   %[C.sub.Sic/
              (charges/hr)  Carbon (%)   /Chg[R.sub.Sic] %[C.sub.FeSi]

[A.sub.FeSi]     14.6         9.3          1.18              1.22
[A.sub.SiC]      12.4        11.3
[B.sub.FeSi]     13.9         9.4          1.18              1.21
[B.sub.SiC]      11.8        11.4
[C.sub.FeSi]     12.6         9.0          1.15              1.14
[C.sub.SiC]      11.0        10.3

Table 5. Summary of Performance of 35% SiC

* Long times are required to reach
steady operation.

* Free-carbon acts as a fuel.

* Compared to FeSi:

   * Under conditions of steady
     operation--higher C, Si
     and Mn recovery;

   * Higher S levels;

   * Lower iron temperatures,
     accentuated if coke is reduced;

   * Lower melt rates if coke is
     not reduced;

   * Lower FeO and MnO levels
     in slag.

Table 6. Summary of Performance of 97% SiC

* Rapid attainment of steady operation.

* Compared to FeSi:

  * higher C, Si and Mn;

  * lower S levels;

  * no reduction in melt rate;

  * lower FeO and MnO levels in slag.

For More Information

"Cupola vs. Electric: A Battle for Melting Efficiency," M. Boehm, MODERN CASTING, July 2002, p. 36-38.

Visit www.moderncasting.com to view the entre report, "Performance of Briquetted and Lump Silicon Carbide in Cupola Operations."

Sy Katz is the president of S. Katz Associates Inc., West Bloomfield, Mich. Mark E. Bauer is a staff project engineer for General Motors Powertrain Saginaw Metalcasting Operations, Saginaw, Mich. Thomas J. Mutton is the Technical Director for Exolon Co., Lawrence, Mich.