The objective of this step is to reduce the raw quartz into a size suitable for the fusion method and machinery to be utilized. Natural quartz undergoes a series of size reduction steps such as crushing and milling (ball milling or roll milling). Quartz is very brittle in nature, which makes comminution quite easy. Afterwards, the particle size is analyzed and larger grains are separated.
In this stage, thermal energy is used to break the strong silicon-oxygen bond. With increasing temperature, more bonds are broken and result in the less viscous flow of quartz. After shaping and cooling to its final form, the ordered crystalline structure of SiO2 molecules is converted into a vitreous, amorphous structure and metastable form of quartz.
Depending on the desired purity level and end use application, the natural quartz may be homogenized and formed through the following fusion methods:
This method produces an industrially known Type I quartz glass. Electric fusion method is used if a high level of purity and low hydroxyl (OH) content (> 1 ppm – 30 ppm) is to be obtained. The quartz glass with low OH content produced from this method has high infrared transmission, but aesthetically pleasing bubbles and drawing lines are present in the glass surface. The starting material is natural quartz grains, and may be subject to the following production modes:
Continuous Mode: The quartz sand is continuously fed on top of a refractory metal crucible column which contains an electric heating device. The internal chamber of the crucible is maintained at a dry and vacuum-sealed atmosphere to keep the melted quartz from reacting with the refractory material. After passing through the hot crucible column, melted quartz is collected in an orifice located at the bottom of the column in which it is shaped and cut into plates, tubes and rods. This method is suitable for high volume manufacturing.
Batch or Boule Mode: Large quantity of quartz is placed inside a refractory-lined vacuum chamber which also contains an electric heating device. After the quartz is fused, the viscous melt is collected and shaped into its final form. This method is used to create quartz glass with more sophisticated shapes and details.
In this method, a natural quartz or a synthetic precursor can be a starting material. Natural quartz passes through a chamber with a high temperature hydrogen/oxygen (H2/O2) flame until the starting material is fused. If silicon tetrachloride (SiCl4), a gaseous synthetic precursor, is to be used, it is made to react with the H2/O2 flame. The viscous melt is deposited in a refractory-lined vacuum chamber, collected slowly by a die at the bottom of the container, and shaped to its final form. Due to its direct contact with H2/O2 flame, this method produces quartz crucible with 150-200 ppm OH content from natural quartz and up to 1000 ppm for synthetic silica.
Glass produced from crystal quartz through flame fusion is classified as Type II, and from synthetic precursor as Type III. Type III synthetic silica glass is a product of a chemical reaction. The combustion of silicon tetrachloride gives synthetic quartz and leaves environmentally toxic byproducts, chlorine, and hydrochloric acid.
Natural quartz or a synthetic precursor may be the starting material for this method. Quartz glass produced from the combustion of a synthetic precursor in plasma flame is known as Type IV.
Electric Arc Fusion
The quartz sand is melted in an electric arc furnace. The resulting glass ingots are crushed and molded; the formed parts are dried and sintered. In this method, the quartz glass produced is white and opaque and does not generally belong to any types of quartz glass. However, it is comparable to transparent quartz in terms of purity level.
Shaping and Finishing Processes
A manufacturer can process quartz glass just like any other kinds of glass.