Increasing Si and C During Melting Using Graphite Electrode Byproducts

By Wang Qiang, Li Shugui, and Ju Qiming

Silicon carbide is a compound containing 70% silicon (by weight) and 30% carbon. It is commercially produced by the reduction of silica sand with carbon, in the form of petroleum coke, in cylindrical electric resistance furnaces. The outer layer of the furnace product, containing approximately 90% SiC is known as foundry melting silicon carbide and can be used as an alloying additive for introducing silicon and carbon into gray cast iron, ductile iron and steel melts.

Several researchers have reported the beneficial effects of SiC additions to gray iron melts. These include the slow fading of its inoculation effect, its effectiveness in improving the microstructure (especially, graphite type and distribution), enhancing machinability and impoving mechanical properties. It has also been reported that the ability of SiC for reduction of FeO and MnO in the slag leads to increased refractory lifetime.^[1]

In this experiment, the byproducts of graphite electrode production (composed mainly of SiC) are used to increase the content of silicon. The material used for the study was obtained from a graphite electrode manufacturer, and is composed of the byproduct of graphiteelectrode sinter (illustrated in Figure 1.)

The insulating materials are composed of SiO₂ and petroleum coke, and arranged as filler around the electrode. During the production of a graphite electrode, the insulating materials are heated to form a mass of SiC by the reaction: SiO₂(l)+3C(s) = SiC(s)+2CO(l)Δ+ (Q). This byproduct cannot be recycled.

The advantages of using the byproduct of graphite electrode production in the foundry melting process are its low price, its ready availability, and that fact that it is comprised mainly of two components, carbon and silicon, that can be used to adjust these two elements simultaneously.

Experimental details
The experimental research was conducted in two stages, namely, an analysis of the elements of the byproduct of graphite electrode production and a study of the effects of the byproduct.

Materials — The materials used for the studies were obtained from a graphite electrode manufacturing operation. The sample elements for the byproduct of graphite electrode production that are diagrammed in Fig. 1 were subjected to X-ray analysis. Fig. 2 shows that the sample consists mainly of SiC, SiO₂, and C. To analyze the detail element of the SiC, the samples are separated into four parts. All the powders are dissolved in hydrogen fluoride (HF) and all the powders were calcined at 900°C for 10 hours. Then, the powders were cooled to room temperature in a desiccator before being finely ground in an agate mortar. The content of SiC used for the studies is the same as for the byproduct of graphite electrode production — 82.55% SiC.

Apparatus and procedure — During the experiment the sample of the graphite electrode byproduct must have some form. For this purpose a mixture of 98% graphite-electrode byproduct and 2% adhesive was mixed on a porcelain enamel plate. Then, this mixture was packed in pure iron cylinders, 20 mm in inner diameter and 103 mm in height. In effect, the mixture was made into rods 20 mm in diameter, 103 mm long, and 110 g in weight. Finally, the cylinders were heated up to a temperature of 100°C, then held at that temperature for 10 hours to ensure complete dryness. The experiments were carried out using an induction furnace. The raw materials are composed of 1.5 kg cast iron and 1.5 kg scrap steel. Both raw materials and the sample of the graphite electrode byproduct were added into the induction furnace and melted. The results of increasing the silicon using the graphite-electrode production byproducts are shown in Table 1.

Results and discussion — The prepared samples were subjected to spectral analysis. The carbon and silcon contents are shown in Table 1. The first sample is the raw sample (no addition). The others include the graphite electrode byproduct. Table 1 shows that the raw material contains 1.9756% of carbon and 0.8454% of silicon, and in the other samples the average of carbon and silicon are 2.8760% and 3.1296%, respectively. The content of carbon and silicon respectively increased 45.58% and 270.19% owing to the addition of the graphite-electrode production by-products.

The graphite-electrode byproduct is composed mainly of SiC that acts as silicoferrite during foundry melting. Many research groups have discussed the cardinal principle in many different ways. Research indicates that SiC shall be decomposed as silicon and carbon at some temperature. The silicon diffuses into hot metal and forms silicoferrite solid solution.^[2] The carbon gathers around the silicoferrite and forms graphite. The correlative reactions are:

SiC₂[Si] + C (s) [1]
SiC(s) + Fe(s) = Fe3Si(s) + C(s) [2]
ΔG⁰_Τ = -38220+5.04T
ΔG⁰1300_Τ = -3.03KJ*mol^-1

It is obvious that SiC/Fe is a thermally unstable composition. The negative ΔG⁰_T provides the drive for the reaction and promotes it.

SiC does not dissolve immediately when added to the melt and its dissolution is slow. The behavior of SiC in iron melts is not well understood, but it has been claimed that during the dissolution of SiC in the melt, graphite clusters are formed around the SiC particles.^[1].

The SiC additions could result in the higher graphite formation observed in the SiC-treated irons.

Conclusions

1. The graphite electrode byproduct came from the waste of the production process. It is obvious that using this graphite-electrode byproduct is economical and effective. In addition, such use can save energy costs, and experimental results indicate that the technique can have metallurgical benefits.
2. The graphite-electrode production byproduct is composed mainly of silicon carbide (SiC). Not only the content of the silicon of the melt, but also its carbon content, were greatly improved owing to the addition of byproducts of graphite electrode products.

Fig. 2. The chemical composition of the byproduct of graphite electrode illustrated by X-ray analysis

The authors are faculty members at the Dept. Materials Science and Engineering, and the Key Laboratory of Automobile Materials, both at Jilin University in Changchun, China. Contact Ju Qiming at [email protected]

References

1. K. Edalati, F. Akhlaghi and M. Nili-Ahmadabadi: J. Mater. Proc. Technol., 2005, vol. 160, PP183-187.
2. T. Benecke: Giesserei, 1981, vol. 68, PP344–349.