Understanding the Process of Obtaining Silicon from Silicon Dioxide

Silicon is a vital element used in a wide range of industries, particularly in the semiconductor and solar sectors. Its utilization in these fields hinges on its ability to be obtained from silicon dioxide (SiO?) through a well-defined process, primarily known as carbothermic reduction. This article will provide an in-depth understanding of how silicon is extracted from silicon dioxide, detailing the entire procedure step-by-step.

Raw Materials and Initial Steps

The process begins with the procurement of raw materials. The primary component is silicon dioxide, which is commonly sourced from silica sand. Carbon, in the form of coke or coal, is used as a reducing agent. These materials are then mixed in specific proportions, typically with a ratio of about 1 part silica to 2 parts carbon.

Mixing and Heating

The next step involves the mixing of the silica and carbon in an electric arc furnace. The mixture is heated to extremely high temperatures, around 1500 to 2000 degrees Celsius (2732 to 3632 degrees Fahrenheit), which is well above the melting point of silicon (1410°C).

The Reduction Reaction

At these high temperatures, a reduction reaction takes place:

SiO? 2C → Si 2CO

In this reaction, silicon dioxide reacts with carbon, resulting in the production of silicon and carbon monoxide gas. This core step is critical in the conversion of silicon dioxide into elemental silicon.

Collection and Purification

The molten silicon gradually settles at the bottom of the furnace and can be tapped off as a liquid. However, the process does not yield pure silicon, as the impurities in the source material need to be addressed.

For applications requiring higher purity silicon, such as in electronics or solar cells, additional purification steps are necessary. These include the Siemens process or the use of hydrochloric acid. These methods further refine the silicon, achieving a level of purity often referred to as “nine nines,” which means 99.9999999% pure silicon.

Description of the Simplified Process

A simplified version of the silicon production process describes the use of coal (C) and quartz (SiO?) or sand or silica. The mixture undergoes a reduction in an electric arc furnace, achieving temperatures well above the silicon's melting point (1410°C).

The oxygen in the silicon dioxide separates, resulting in the formation of carbon monoxide. The intense heat in the furnace melts the separated silicon, which is then drawn off and solidified into large polycrystalline chunks of varying sizes.

The process also produces carbon monoxide, which is separated and further treated. Additionally, there is a fine silica dust that is light enough to be pulled from the exhaust gas stream, representing about 10-20% of the initial silicon dioxide used. The energy requirement for this process is significant, consuming approximately 12 kWh of energy per kg of silicon produced, equivalent to about 1/3 of a gallon of gasoline.

The silicon produced from this process is not pure enough for the production of silicon wafers for integrated circuits. It must undergo further refinement to achieve a level of purity often referred to as “nine nines,” which involves the fractional distillation of trichlorosilane (SiHCl?).

Understanding these detailed steps in the process of obtaining silicon from silicon dioxide is crucial for anyone involved in the semiconductor or solar industries, as it underscores the importance of purity and the critical role silicon plays in these modern technologies.