Monitoring Oxygen in the Production of Technical Metal Powders

Metals powders of defined size and pureness are a vital intermediate for additional use in the manufacture of mineral-based industrial goods such as pyrotechnics, paints, semiconductor wafers or silicon nitride ceramics. Generally, these powders are manufactured from raw materials in a grinding or crushing process in classifier mills or ball mills under regulated oxygen or inert gas concentrations.

Ball mill for processing metals powder.

The following are two significant examples of technical metal powders:

• Silicon powder
• Aluminum flakes

The accurate measurement of oxygen for a number of hours is of great significance for the production process of both these metal powders.

Production of Aluminum Flakes

Aluminum flakes have a conventional diameter from 20 to 100 μm. These flakes are used as feedstock in different applications and industries. Aluminum flakes function as pore former and propellant during the production of aerated concrete in the minerals industry. They contribute to the thermal insulating features of aerated concrete. Aluminum pigments are used in many applications in the paint industry either enhancing the end products optically (metallic effect) or fulfilling functional tasks. Finally, aluminum flakes serve as a source of energy in numerous pyrotechnical products.

Measuring Task – Process Control of Oxidation Process

The starting material for the production of metal pigments is aluminum powder with a grain size of several μm. To inflate the surface, the powder is pulverized in a ball mill with process duration of several hours. The grain distribution, the specific surface area and the surface coverage of the particles substantially influence the reaction behavior of the products in subsequent application processes. For example, aluminum flakes with enhanced wetting in water are imperative for aerated concrete blocks.

To acquire the emerging flakes in a chemically inert and stable form and optimized for the later production of the end product, a specific oxidation process is needed, which is controlled in the mill during crushing at O₂ concentrations of 6% to 14%. Stable gas analyzers with low drift are conventional requirements for processes that run for several hours. Analyzer adjustments are conducted typically only every few weeks.

Production of Pure Silicon Powder

Pure silicon powder is the unprocessed product for the semiconductor industry. In the ceramics industry, it is used in the manufacturing of reaction-bound silicon nitride powder and other materials. For example, silicon nitride is essential 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 to prevent explosive reactions or oxidation that could occur with ambient air inside the mill. The process conditions are dry O₂ in N₂ and the conventional process lasts for more than 24 hours during which a threshold of maximum 4% O₂ is monitored. Numerous sequential process runs are carried out during a typical production period, during which calibration of the instrument is not possible. As a result, a crucial requirement for the measurement of O₂ is drift stability. Usually, adjustments are made only once a month. A typical process run for silicon powder production is shown in the following figure. In specific phases of the process, such as when the concentration of O₂ in the mill approaches the threshold value, inert gas, which the compressor regulates, “reconditions” the mill’s atmosphere. To prevent overpressure, a valve to environmental atmosphere opens explaining the short periods of inflated O₂ concentrations.

Inert gas monitoring in the silicon powder process.

ABB Solution: Magnos28

The future of paramagnetic oxygen measurement is exemplified by Magnos28, leveraging ABB's pioneering technology leadership and more than 75 years of innovation in the domain of continuous gas analysis. This impressive product thoroughly re-evaluates paramagnetic oxygen analysis, replacing the glass dumbbell with an innovative new silicon sensor, the microwing, and automating historically manual manufacturing processes leading to levels of quality and reproducibility beyond anything that is presently available on the market.

Magnos28. Sensor with microwing.

Revolutionary New Microwing Technology

The Magnos28 introduces a basic revision of the sensor design. The patent-pending microwing replaces the glass dumbbell with its mirror, circuit path, mounting and taring weights as an all-in-one device without any extra attachments. Applying the latest semiconductor-based production technologies, multiple sensors are produced on a wafer slice. This is a completely new approach to magneto-mechanical oxygen measurement. Absolutely reproducible silicon sensor elements, the microwing, are the basis for a product which promises greatly enhanced repeatability and precision. The microwing sensor reacts very accurately to changes in oxygen concentration. This is because of its high width-to- thickness ratio, very low mass, and optimized magnetic field distribution in the measurement position.

Refined for Challenging Applications

Magnos28 is best suited for these measurements. Special coatings shield internal sensor parts that are sensitive such as the pole shoes. In the sensor production, no adhesives are used, which could interact with the sample gas and influence the measurement. Accordingly, Magnos28 offers the necessary qualities for low zero drifts and long-term stability of span.

Fast Results When Every Second Counts

The internal chamber volume is reduced by a factor of three when compared to its predecessor. A rapid gas exchange is the result of completely redesigned gas paths and optimized drillings. The new Magnos28 facilitates more than 15% improvement in response time with its optimized design. This feature makes Magnos28 a foolproof fit for threshold monitoring, when process conditions change very quickly.

Gas flow scheme at a classifier mill.

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.