(244a) Development and Design of Handling Technology for Highly Explosible Powders

Many powders/dusts can explode under the right combination of dust concentration, particle size, oxidant (usually air) and ignition source. Many agricultural dusts produce relatively weak to moderate explosions (e.g. Kst = 100 to 200 bar m/s, Pmax = 9 bar g and minimum ignition energy = 50 to 100 mJ). In contrast, metal powders can be extremely explosible (e.g. Kst = 1000 to 1500 bar m/s, Pmax = 15 barg and minimum ignition energy = 10 mJ). Hence, when designing and operating metal powder processes, it is essential to ensure: (a) explosion prevention philosophy is adopted wherever possible; (b) powder does not come into contact with air; (c) any dust clouds are generated in an inert atmosphere; (d) fabric filters (even with conductive filter elements) are NOT used; (e) all possible ignition sources (e.g. electrical faults, electrostatic and thermite sparking, even lightning strikes, etc) are prevented; (f) product velocities are kept to a minimum; (g) dust or product concentrations are kept as often as possible below the LEL (e.g. 20 to 50 g/m3) or above the UEL (e.g. 2 to 6 kg/m3), (h) appropriate methods of dust explosion control are implemented and maintained correctly. As a part of increasing the capacity of a metal powder production plant, it was required to produce, handle, process and transport much greater quantities of highly explosible powder. The plant considered two main options: unit handling or pneumatic conveying. The former option was considered too awkward and dangerous (e.g. loading and unloading various grades of metal powder into and out of process equipment and in contact with air). Hence, the safer pneumatic conveying option was pursued. This paper describes the research that was undertaken to develop new pneumatic conveying technology for this project (e.g. dense-phase pneumatic transportation of different types and grades of fine/dusty metal powder without high explosion-risk components, such as cyclones or filters) and some of the interesting design features that were implemented to maximum safety and reliability during plant operation. The fluidised dense-phase mode of transport was selected for the processed powders and internal-bypass technology had to be developed for the coarser atomised powders. Operating conditions were optimised to maximise solids loading and minimise gas consumption for the full range of metal powders. The subsequent results were very impressive (e.g. very high m*, very low gas consumption, gas cleaning devices not needed). In fact, the gas consumption was so low, it was not considered necessary to recycle the nitrogen gas. Using the data generated from the various test programs and after developing appropriate pressure drop and minimum transport models, hardware requirements and reliable operating conditions were predicted for the two sections of plant (viz. atomised and processed metal powders). Based on these design parameters, existing sections of plant were modified and new conveying systems designed and installed. The plant was commissioned and has been operating successfully ever since.