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Refinement Of Diamond Micropowder

June 14,2023

With the development of modern advanced electronic manufacturing technology, it is required to maintain high purity in the processing of electronic products such as single crystal silicon wafers, integrated circuit boards, and computer hard disk discs, to avoid introducing trace impurities that may cause slight changes in the electrical properties of the products. Therefore, the grinding and polishing products for manufacturing high purity products require high-purity diamond powder as the raw material.

A few days ago, we introduced the rough purification process of diamond micropowder, and after sorting, it was found that due to the incomplete rough purification in the early stage, the addition of dispersants during the grading process, and the presence of pollution in other processes, it is obvious that the requirements for high-purity micropowder cannot be met. Therefore, the micropowder products that have completed the grading still need to undergo another fine purification, and the specific principles and operating methods are the same as those of rough purification, The determination of purification process mainly depends on the condition of the product itself and the market requirements for the product.

Diamond micro powder products that have completed rough purification and grading will still contain small amounts of impurities such as metal elements, silicates, and graphite. According to the types of impurities, different processes can be selected to remove them separately. The conventional method is to use perchloric acid to remove metal impurities and residual graphite, and the hydrofluoric acid method to remove silicates. Products that have undergone refined purification treatment can generally meet the needs of the vast majority of customers.

General operating steps

The rough purified diamond powder raw materials obtained from static pressure synthesis or detonation synthesis are sequentially placed in nitric acid, perchloric acid, hydrofluoric acid, and other acids, heated until the acid boils, and kept for several hours to dissolve various impurities adsorbed on the diamond surface. Then, they are rinsed with high-purity water to remove residual acids and impurities. After drying, they become high-purity diamond powder.

During impurity removal, it should be noted that perchloric acid is a strong oxidant, which can slowly oxidize graphite after heating (it should be noted that perchloric acid can react with graphite only at high temperature, and 72% perchloric acid is not enough to oxidize graphite oxide at normal temperature). Put the materials that have removed metal catalyst and a large amount of graphite in a beaker, pour perchloric acid, sometimes add a small amount of catalyst, heat the solution, and then start the reaction. As the reaction proceeds, The color of the solution ranges from black gray to green to brown to orange red. When the solution turns orange red, the graphite has been completely removed. It can be removed from the electric heating plate, cooled to room temperature, poured out the upper liquid, washed with high-purity water until neutral, and then dried.

The process of removing graphite from perchloric acid involves a violent reaction, and attention should be paid not to overflow at all times. Sometimes, in order to accelerate the reaction speed, an appropriate amount of sulfuric acid can be added to perchloric acid.

However, in practice, the cost of removing graphite with perchloric acid is high and it also pollutes the air. Therefore, the process of purifying diamond with perchloric acid to treat graphite is mostly replaced by shaking tables, washing plates, or mixed acid treatment of sulfuric acid and nitric acid. Perchlorate treated diamonds are only used in high-purity diamonds, fine-grained diamonds, and as raw materials for sintered polycrystalline diamonds after treatment.

The nitric acid, perchloric acid, and hydrofluoric acid used in the acid treatment process are all analytical grade or above reagents. The reaction vessels used in the treatment process have the characteristics of strong acid corrosion resistance, high temperature resistance, and do not dissolve relevant metal or non-metallic impurities. For the special requirements of some customers, it is also possible to choose to remove metal elements through aqua regia treatment again, or to use high-temperature alkali treatment for deep treatment of silicates.

To ensure the purity of the processed product, various purification processes should pay attention to the sequence of process arrangements. Due to the fact that high-temperature resistant glass containers are commonly used in the post-processing process of removing metals, it is easy for silicon elements to exceed the standard due to continuous stirring and wear. At the same time, the chemical reagents used in the process of removing silicates can corrode the glass containers. Therefore, it is generally arranged to first remove metals and then remove silicates to avoid possible cross contamination between processes; At the same time, it is required to strictly control the quality of rinsing water in this stage to avoid secondary pollution.

The entire process of purification has certain risks, so the operating essentials must be standardized and require precise operation by professional personnel.

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