Increasing Si and C Using Graphite-Electrode Byproducts

Silicon carbide is a compound containing 70% silicon (by weight) and 30% carbon. It is commercially produced by reducing 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 to introduce silicon and carbon into gray 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 improving mechanical properties. And, it has been reported that the ability of SiC for reduction of FeO and MnO in the slag leads to increased refractory lifetime.

In this experiment, the byproducts of graphite electrode production (mainly SiC) were used to increase the content of silicon. Material used for the study was the byproduct of graphite-electrode sinter.

The insulating materials are composed of SiO2 and petroleum coke, and arranged as filler around the electrode. The advantages of using these byproducts in foundry melting 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.

The experimental research was conducted in two stages: an analysis of the elements of the byproduct of graphite electrode production; and a study of the effects of the byproduct.

Materials — The sample elements for the graphite-electrode byproduct were subjected to X-ray analysis, revealing that it 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, and then cooled to room temperature in a desiccator before being ground fine. The SiC content used for the studies is the same as for the graphite-electrode byproduct — 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 iron cylinders, 20 mm ID 103 mm long, and the cylinders were heated to 100°C and held at that temperature for 10 hours to ensure complete dryness. Both raw materials (1.5 kg cast iron and 1.5 kg scrap steel) and the graphite-electrode byproduct sample were melted in an induction furnace.

Results and discussion — The samples were subjected to spectral analysis. The carbon and silicon 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 byproducts.

The graphite-electrode byproduct is composed mainly of SiC that acts as silicoferrite during melting. 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. The carbon gathers around the silicoferrite and forms graphite.

Conclusions — Using graphite-electrode byproducts is economical and effective, and can save energy costs. And, experimental results indicate that the technique can have metallurgical benefits. Because the graphite-electrode production byproduct is composed mainly of silicon carbide (SiC), the content of the silicon and carbon in the melt were greatly improved.

Table 1. Chemical Composition of the Samples
C(%) Si(%) Mn(%) S(%) P(%)
Sam. 1 1.9756 0.8454 0.4097 0.0205 0.0560
Sam. 2 2.8541 3.0388 0.4205 0.0213 0.0574
Sam. 3 2.8603 3.0134 0.4498 0.0150 0.0555
Sam. 4 2.9135 3.0466 0.4177 0.0166 0.0563

Read the complete presentation by Wang Quiang, Lu Shugui, and Ju Qiming

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