Optimizing oxidation process to ensure metal powder quality. Ensuring process safety by inert gas monitoring.
Metal powders of defined size and pureness are an important intermediate for further use in the production of minerals based industrial products such as paints, pyrotechnics, silicon nitride ceramics or semiconductor wafers. In general these powders are produced from raw materials in a crushing or grinding process in ball mills or classifier mills under controlled inert gas or oxygen concentrations.
Two important examples for technical metal powders are:
• Aluminum flakes
• Silicon powder
For both, the accurate measurement of oxygen for many hours is of great importance for the production process.
Production of aluminum flakes
Aluminum flakes have a typical diameter from 20 to 100 μm and are used as feedstock in different industries and applications. In the minerals industry, aluminum flakes act as pore former and propellant during the production of aerated concrete. They serve for the thermal insulating properties of aerated concrete.
The paint industry uses Aluminum pigments in many applications either fulfilling functional tasks or enhancing the end products optically (metallic effect). Finally, Aluminum flakes act as a source of
energy in many pyrotechnical products.
Measuring task – Process control of oxidation process
Starting material for the production of metal pigments is aluminum powder with a grain size of several um. To enlarge the surface, the powder is crushed in a ball mill, processduration being several hours. The specific surface area, the grain distribution and the surface coverage of the particles essentially determine the reaction behavior of the products in later application processes. For example
aluminum flakes with improved wetting in water are required for aerated concrete blocks. In order to obtain the resulting flakes in a stable form and chemically inert and optimized for the
later production of the end product, a defined oxidation process is required, which is controlled in the mill during crushing at O2 concentrations of 6 to 14 %. Typical requirement for processes that run for several hours are stable gas analyzers with low drift. Analyzer adjustments are performed typically only every few weeks.
Production of pure silicon powder
Pure silicon powder is the raw product for the semiconductor industry. It is also used in the ceramics industry in the production of reaction-bound silicon nitride powder (and other materials. Silicon Nitride for example is needed for the production of photovoltaic modules.
Measuring task – Inert gas monitoring
The grinding chamber of a classifier mill is filled with inert shield gas N2 or Ar in order to prevent oxidation or explosive reactions that could occur with ambient air inside the
mill. The process conditions are dry O2 in N2 and the typical process lasts for more than 24 hours during which a threshold of max. 4 % O2 is monitored. Several subsequent process runs are
performed during a typical production period, during which no calibration of the instrument is posssible. Therefore, drift stability is a key requirement for the O2 measurement. Adjustments are only made typically once a month. Diagram 01 shows a typical process run for silcon powder production. In certain phases of the process, for instance when the O2 concentration in the mill approaches the
threshold value, inert gas, regulated by the compressor, “reconditions” the atmosphere in the mill. To avoid overpressure a valve to environmental atmosphere opens, which explains the short periods of elevated O2 concentrations.