Under the spotlight: high-temperature lamp tubing,
such as mercury and halogen lamps, uses a
high purity quartz envelope to contain the lighting
components
When is quartz ‘high purity’? Many players in the quartz and silica sand industries define the grade as starting at the 99.95% SiO2 level (see table- High purity quartz at a glance). This equates to 500 ppm of impurities- including boron, alkalis, and transition metals- which all affect the performance of the final quartz product.
At its Spruce Pine operation in North Carolina, Sibelco subsidiary Unimin Corp. produces the material that has set the purity benchmark for the rest of the market; a high purity quartz, marketed as Iota, which contains 20ppm of impurities as standard- equating to 99.998% SiO2. The group’s purest grade, Iota 8, is 99.9992% SiO2 in composition, containing only 80 parts per billion of impurities (see panel).