The sorting mentioned here refers to the process of grading the particle size, selecting the shape and performing magnetic separation of purified diamonds.

3. Magnetic separation

Magnetic separation, along with sieving and shape selection, is listed as the three main methods for sorting synthetic diamonds. The so-called magnetic separation is a sorting method based on the difference in magnetic strength of diamond particles.

Research results show that there is the following relationship between the magnetic separation of diamond and other properties:

• Ⅰ. There is a linear relationship between magnetism and impurity content. The more impurities there are, the stronger the magnetic separation of diamond will be, and vice versa. The impurities in diamond mainly come from the catalyst used in synthesis, and these catalysts are generally metals such as Fe, Co, and Ni, which are all ferromagnetic substances.
• Ⅱ. Magnetism is related to strength and particle form. The stronger the magnetism, the better the strength and particle form. But the relationship between them is not a strictly linear relationship.
• Ⅲ. Magnetism is also related to thermal stability. Experiments have found that the strength of strongly magnetic diamonds decreases by 60% after being roasted at 700°C, while weakly magnetic diamonds only begin to show a tendency of strength decrease when sintered at 1100°C.

Since magnetism has an adverse effect on the performance of diamond, especially the strength decreases after being subjected to high temperature. Therefore, diamond products used in metal-bonded abrasives, especially those used in stone saw blades and geological drill bits, need to be subjected to magnetic separation so that they can withstand the test of high-temperature sintering during manufacturing. Magnetic separation is also required for manufacturing electroplated products. Otherwise, nickel nodules will appear on the coating, resulting in waste products.

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──Siya Miao

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E-mail: siya@microndiamond.com