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Magnetic Separation And Magnetic Susceptibility Of Diamond

June 03,2023

Pure natural diamond is a non-magnetic substance. When synthesizing synthetic diamond using high-pressure method, metal alloys such as Fe, Co, Ni, Mn, etc. are commonly used as catalysts to promote the nucleation and growth of diamond. A small amount of catalyst, as an impurity, is wrapped in diamond particles during the growth process of diamond, causing internal defects in the diamond and making it magnetic to a certain extent. At the same time, under high temperature conditions of manufacturing and use, diamond abrasive particles are prone to cracking, resulting in a sharp decrease in strength. So in the process of diamond production, it is necessary to separate diamond abrasive particles with different impurity contents, and detect the impurity content of different grades of diamond products to make them meet the standards. In recent years, with the increasing development of diamond research and production in China, people have increasingly realized that magnetic sorting of diamond products can produce diamonds with different qualities and suitable for different occasions. This classification makes the application of diamonds more reasonable.

1. Basic principles of magnetic separation

Magnetic separation is a method of separating diamonds by utilizing the effects of magnetic and mechanical forces on diamonds. Due to the difference in magnetic susceptibility between magnetic particles and non magnetic particles, their motion trajectories are also different. Stronger magnetic particles are subjected to magnetic force greater than mechanical force, and are therefore attracted to the magnetic roller and rotate with it. When the magnetic roller rotates to an area beyond the action of the magnetic field, the magnetic particles fall off the roller or are swept down by a brush and enter the magnetic product hopper; Non magnetic particles are brought out of the magnetic field by the material slot and enter the non magnetic product hopper due to the magnetic force being less than mechanical force. In this way, diamonds with different magnetic properties can be classified into different grades of magnetic, medium magnetic, non magnetic, or weak magnetic products by changing the magnetic field strength. The magnetic separation process is shown in the following figure.

2. magnetic susceptibility of synthetic diamond abrasive

(1) Definition of magnetic susceptibility in electromagnetismAccording to its magnetism, substances can be divided into diamagnetism, paramagnetism, ferromagnetism, etc. When a substance is placed in a magnetic field with magnetic field strength H, it will be magnetized, and the degree of magnetization is expressed by magnetization M. For substances other than ferromagnetic substances, experiments have shown a proportional relationship between M and H, namely:M=XmHXm, called magnetic susceptibility, is a dimensionless constant. Xm of diamagnetism material is negative, Xm of paramagnetism material is positive, and Xm of vacuum is zero.Magnetic susceptibility is an inherent property of a substance, independent of its shape and size.

(2) Magnetic susceptibility of diamond abrasiveThe magnetic size of artificial diamond is not only directly related to its internal impurities and inclusion content, composition, but also closely related to other physical and chemical properties and quality indicators (such as toughness, strength, specific gravity, thermal stability, color, transparency, etc.), as well as its use and classification. For example, it is reflected in another important test index of diamond, thermal shock toughness (TTI), that is, the higher the magnetic susceptibility is, the lower the thermal shock toughness is, the easier it is to break at high temperature, and thus the lower the strength. Therefore, the magnetic separation of diamond is of great significance to the separation of diamonds with different quality. The diamond abrasive sample in production is a large number of unrelated particles, so its measurement result is the average magnetic susceptibility of all measured particles. The sorted products must pass the magnetic susceptibility test to meet the requirements of the control standard, ensure the reliability and consistency of the product performance, and keep the product performance unchanged for years. For manufacturers, it is an essential testing method. The magnetic susceptibility of each metal is different, so the magnetic susceptibility of each manufacturer is not comparable due to the different catalyst systems, but for diamonds synthesized with the same catalyst, the magnetic susceptibility classification is still effective.

(3) Test method of magnetic susceptibilityThe physical and chemical properties of artificial diamonds largely depend on their impurity content, especially the different forms of inclusions present inside the crystal, which have a significant impact on the quality of diamonds. The commonly used detection technique for determining impurity content is ashing, which involves using the burning method to oxidize and volatilize the C element in diamonds, and then conducting quantitative physicochemical analysis of the residue to determine its element and weight. These methods not only consume a large amount of diamond, but also have high detection costs and low detection efficiency, which cannot meet the needs of production. These harmful impurities are basically ferromagnetic substances, so magnetic testing can be used to indirectly determine the amount of impurities, which is a non-destructive testing method. In foreign countries, magnetic susceptibility is used to characterize the content of impurities. There are many determination methods, such as magnetic balance method, equivalent magnetic balance method, magnetic conductivity meter impact method, etc. The detection instruments used are complex in structure, large in volume, and inconvenient in operation; In addition, due to the limited sample size required for testing, the representativeness of the samples is poor. Referring to the principle of the latest testing instrument abroad, China has independently developed a diamond magnetic susceptibility tester. It has strong anti-interference ability, small size, light weight, high precision, good stability, and can adapt to the detection of various diamond magnetic susceptibility. It has been widely used in the industry. The following figure shows the JCC-B diamond magnetic susceptibility analyzer developed by Zhengzhou Sanmo Institute. Put the specified amount of diamond particles into the feed port of the magnetic susceptibility analyzer, and the digital display meter will automatically display the readings.