Abstract
Silicon carbide is a versatile, useful, manmade material.  It is made by blending coke and sand and applying heat.  The North American market has grown significantly while a large part of it is now imported.  The major users of silicon carbide grain are the abrasive, refractory, and alloy industry.  The iron foundry industry uses a significant amount of silicon carbide in the form of refractories and alloy additions.  Trends in the market point to stable to slightly decreasing pricing for SiC in the near future.

Silicon Carbide Production
Silicon carbide is a black to green material that is a combination of 70% silicon and 30% carbon. In nature, only a small amount exists, where coal and sand have been exposed to each other in the presence of a significant amount of hear. No commercially viable deposits exist, so for practical use, the material is manufactured.

To make silicon carbide, usually low sulfur petroleum coke and high purity sand are selected as the raw materials. In some cases, coal is used in place of petroleum coke, though this tends to have a negative affect on properties as well as pollution problems. The standard method for producing SiC is in a electrical resistance furnace. The coke and sand are mixed together and an electric current is run through the material. The resistance of the blend creates a significant amount of heat that drives the reaction:

2 SiO₂ + 3C + heat = 2SiC + CO (Gas)

The core of the arc will generate the highest purity silicon carbide, with the further from the arc, the lower the grade becomes.

Properties
The properties of silicon carbide make it a useful material for several different manufacturing processes. It is very hard, a 9.1 on the Moh’s hardness scale. It has a very high resistance to heat, in reducing conditions subliming at approximately 2700^oC. Though very hard, it is also very brittle, making it difficult to keep large pieces together.

For a ceramic material, SiC also has a couple of unique properties. It has a very high degree of volume stability through a wide temperature range. It also has a very high thermal conductivity. A combination of these two properties will make a body composing of SiC very resistant to thermal shock stresses.

Chemically, SiC is a very non-wetting material, giving it high resistance to many molten slags and metals. Since the slags do not coat the SiC well, it inhibits any reactin.

Abrasive Applications
The initial use for silicon carbide is in the abrasive industry. With a hardness between that of corundum and diamond, SiC makes an excellent abrasive material. Many sandpaper and abrasive wheels take advantage of this property.

For the abrasive industry, the most important property is purity. The higher the SiC content the harder the material. Bulk density of the SiC, which can be affected by the manufacturing process, is also important.

SiC in Refractories
SiC is useful in refractories because of its high resistance to heat, its low thermal expansion, its high thermal conductivity, and its low wetability to many types of metals and slags. The properties that a refractory design engineer will look at in a particular SiC material vary somewhat for different applications. A general list of important properties for refractories are listed here:

1. SiC Content
2. Moisture Level
3. Magnetic Level
4. PH
5. Acid Compatibility
6. Particle Size
7. Accessory Oxide Type and Content

In the foundry industry, there are several important uses for refractory grade silicon carbide. SiC is often added to iron runner and cupola repair materials, as well as to cupola refractories and occasionally in induction furnace linings.

Outside the iron industry, silicon carbide containing refractories are often found in red metal applications, kiln furniture, blast furnace runners, tapholes and boshes, and in heat exchange units.

SiC as an Alloy Addition
On a volume basis, the largest use of silicon carbide is as an alloy addition. On a silicon unit basis, SiC is a very competitive source of this important element. Compared to many of its competing sources of silicon such as ferrosilicon or silvery pig, it is very low in accessory oxides. It also has other advantages compared to these materials such as a deoxidation affect and a pre-inoculant affect. In the steel industry, SiC is a more efficient fuel source than ferrosilicon.

In the iron industry, SiC is used in two ways. One is as an additive to induction furnaces and the other is as an additive to cupolas. In induction furnaces, the material is usually added as crude grain of 90% SiC purity level. In cupola applications, grain would tend to fly into the dust collector; so briquetted silicon carbide is used.

For induction furnaces, several factors should be considered before choosing the grain used. The first is the actual SiC content of the grain. Obviously, the higher the SiC content, the more available Si for the melt. The moisture content is important, with lower being better. Besides the safety issue of adding moisture to a molten metal, the operator should not expect to pay for water since it is not needed in this case. The particle size is also important, with excess fines tending to lead to lower recoveries.

Another chemical factor besides the SiC content is the amount and type of accessory oxides that are present. The preferred elements present in the non-SiC part of the material are carbon and silica. The three most common other elements that could cause problems are nitrogen, sulfur, and alumina content.

Nitrogen, of course, can cause casting problems due to nitrogen porosity if found in excessive amounts. Earlier shipments from China were reported to contain very high amounts. Three shipments from China tested by Leco Laboratories in 1999 averaged 0.14% nitrogen. This is still about twice as high as reported numbers for domestic material but probably below the threshold where nitrogen porosity is considered a factor. Nitrogen levels of Russian and South American materials were tested in the 0.97% area, about the same as reported domestic numbers.

Sulfur contents of all materials tested have consistently run below 0.1% for 1999. Past Chinese samples, again, were significantly higher in the past. Today, traders are doing a better job of screening for nitrogen and sulfur and both of these values have fallen significantly in the Chinese material.

Alumina may or not be a problem depending on the particular alloying application. In one gray iron induction furnace shop, where melt temperatures were as low as 1400^oC, silicon recoveries were reduced with silicon carbide that was high in alumina compared to an equivalent grade of SiC with lower alumina. In ductile iron shops, where melt temperatures are higher, no difference between low and high alumina materials has been observed. Alumina is a strong refractory material and it may be possible that at lower melt temperatures it thickens the slag to the point that interfere with recoveries.

Again, high alumina problems are a Chinese situation. No domestic numbers were available for this paper, but Russian material was tested at 0.5% and South American at 0.2% alumina. It is assumed that Canadian and U.S. materials, with the abundant low alumina silica available, are at or below the South American levels. Chinese materials have been tested as high as 2.5% alumina. Further testing of new shipments is being conducted. It is likely that the alumina is an impurity from either the sand used or from the coal ash that is left in the burn. Since there are a significant amount of different Chinese producers, it is likely that the alumina contents vary significantly from region to region.

A critical factor when choosing a silicon carbide grain is cost. A 90% silicon carbide gives the melt operator about 63% available silicon. If we compare the price of 90% silicon carbide with the November 1, 1999 price of 75% ferrosilicon at 36.65 cents per pound of silicon, then a price of $462.00 a short ton would be an equivalent silicon price. This is not including the benefit of the carbon. Chinese materials are currently selling close to this price in the market, making them a competitive decision. Domestic, Russian, and South American products tend to be higher than this, making a buying decision dependent on the other advantages of silicon carbide besides silicon price.

Cupola Foundries
In cupola foundries, SiC grain cannot be readily used due to its particle size. A significant amount of the grain would readily leave the system via the dust collector.

Briquettes are made on a block or paver machine in a variety of SiC and C levels. A picture of a paver machine used to make briquettes can be seen in figure 1. Lower levels of SiC, such as 36 to 40%, are used in conjunction with high carbon contents to lower the coke rate of the cupola furnace. Higher SiC levels, 65 to 70%, are made as more or less straight silicon replacements. Briquettes are made in either slab or bulk form. Slabs are made to a given weight and usually manually thrown into the charge bucket. Since their weights and chemistries are known, a given number of slabs will be put into the charge to give the desired percentage of silicon.

Bulk briquettes are designed to flow through bulk handling systems that weigh a certain amount for each charge. Again, since the chemistry is known, a given weight is selected to obtain the desired silicon content. The briquettes are designed to handle well and fines are kept to a minimum to prevent plugging of gas flow in the furnace.

Other alloys, such as manganese, can also be added to the briquette mix. Typical chemistries for two common industry standards can be seen in figure 2.

Because of the perceived advantages and a competitive pricing situation with SiC, its use has increased significantly over the last 10 years as an alloy addition. For 1998, the estimated total market for SiC in the United States was 300,000 metric tons, with approximately 200,000 metric tons being earmarked as alloy additions.

As pollution control requirements have increased in North America, the number of active producers has fallen from 8 to 2 over the last 10 years. In 1998, the last year the USGS reported domestic production, output in the U.S. was estimated at 68,000 tons. Thus, the increase in SiC as an alloy addition has largely been covered by imports. Figure 4 shows total SiC crude imports as reported by the USGS. The four largest importing countries over the last 5 years have been China, Canada, Brazil, and Russia. Chinese imports have dwarfed those of other countries as can be seen in the comparison with Canadian imports in figure 5. Ten years ago, imports from Canada made up the majority of the imported material. Today, approximately 80% of imported material is from China.

As silicon carbide has become more important as an alloy addition, Ryan’s Notes started tracking the price of crude Chinese 88% material. This started in March of 1998. This price at first dropped steadily. In February 1999, the Chinese government imposed a $50 per ton tax on silicon carbide. Looking at figure 6, this started to affect the price of material reaching the shore of the U.S. in July. Shipments between July and October, according to the USGS, were very low. Shipments started to pick up in November and some softening of the price was observed. It is expected that some continued softening from current pricing would continue as competitive materials, such as ferrosilicon, are also dropping in price.

Conclusions Silicon Carbide is and will continue to be an important commodity for the abrasives, refractory, and ferrous-metals industry. As an alloy addition, SiC has increased in importance substantially over the last 10 years. A lot of this increase is due to low cost imports from China. Recent increases in SiC price are under pressure due to falling prices of some competitive materials such as ferrosilicon. Thus, it is expected that crude silicon carbide pricing will be stable or fall slightly through 2000.