How pure can metals become after zone-melting purification?

Zone melting is a highly effective technique for the purification of metals, often achieving purity levels as high as 99.999 percent.1

Through the process of zone refining, even greater levels of purity can be reached. For instance, in the early development of germanium for transistors, the method achieved a purity of less than one part of detectable impurity in 10,000,000,000 parts of the material. By utilizing multistage operations—such as recycling the heating ring or using multiple rings in succession—ultrahigh purities can be consistently obtained.21

The final level of purity depends on factors such as the distribution coefficient of the impurities and the number of zone passes performed. While the process is exceptionally effective for creating high-purity materials, it is generally considered a slow method, making it more suitable for producing limited quantities of high-purity substances rather than large-scale industrial tonnage.2