Preparation of Aluminum Hydroxide During the Synthesis of High Purity Alumina via Aluminum Anodic Oxidation
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INTRODUCTION
HIGH purity alumina (99, 99 wt pct aluminum oxide) or HPA is a key product on the non-metallurgical alumina market. High purity alumina is widely used in the production of special ceramics, LEDs, and semiconductors.[1–3] Recently high purity alumina became a material of interest for 3D printing.[4,5] However, the main purpose of HPA is the production of monocrystalline corundum (a-Al2O3), which is used in many areas of civil and defence equipment.[6–8] According to its chemical composition, high purity alumina is classified into three categories: 6 N, 5 N, and 4 N with an impurity content from 1 ppm to 100 ppm, respectively.[9–11] The most required product on the market is 4 N HPA. It has a relatively low cost (compared to 5 N and 6 N HPA), and satisfactory ANTON YU NALIVAIKO, DMITRIY YU OZHERELKOV, and ALEXANDER A. GROMOV are with the KINETICA Engineering Center, National University of Science and Technology MISIS (NUST MISIS), Leninsky ave 4, Moscow, Russian Federation 119991. Contact e-mail: [email protected] VYACHESLAV I. PAK and SERGEY S. KIROV are with the Department of Non-ferrous Metals and Gold, National University of Science and Technology MISIS, Moscow, Russian Federation 119991. ALEXEY N. ARNAUTOV is with the New Projects Directorate, United Company RUSAL Plc, Moscow, Russian Federation 121096. Manuscript submitted October 22, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
characteristics for the majority of applications, including the production of monocrystalline corundum, additive technologies, semiconductors production, and chemical industry.[12] In industrial production, alumina is obtained by the processing of raw minerals. The methods of alumina production for the needs of the metallurgical industry are alkaline methods,[13] acid methods,[14,15] and thermal methods.[16] Production of HPA using the above-listed technologies is associated with the modernization of existing technological stages and the introduction of additional purifying operations of the product.[17] This requires significant operating expenses and has an impact on the final cost of HPA. Currently, hydrothermal, alcohol, and electrochemical methods are used to produce HPA. These methods are based on the oxidation of metallic aluminum. Alcohol technology consists of aluminum powder dissolution in alcohol (e.g., isopropyl alcohol) to produce organometallic compounds, which are then subjected to distillation and hydrolysis to form aluminum hydroxide. The aluminum hydroxide then undergoes further heat treatment to obtain aluminum oxide.[18–21] Due to the presence of several purification stages in the technological scheme, this method allows for obtaining 5 N HPA. However, this method uses highly toxic chemical reagents that poison the environment and pose a danger to human health and life.
The hydrothermal method consists of the aluminum metal powder oxidation by water under the heat treatment (200 C to 400 C) in a high-pressure reactor (20 to 40 MPa) followed by thermal dehydration.[22–24] The electrochemical met
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