Using abrasive grains to blast-clean material has been an established practice in industrial manufacturing and construction for years. The technique mechanically removes oxide layers, protective coatings and contamination. It is also used to finish and deburr surfaces. The problem with abrasive cleaning using hard grains is that fine dust is emitted and, depending on the material being cleaned, this is not just dangerous because it is so small, it can even be highly toxic. A research project carried out by the Steinbeis Transfer Center for Process Development together with mps Strahltechnik GmbH aimed to develop a process that would use a screen of water droplets to absorb fine dust particles and prevent them escaping into the surrounding area. The project was partially funded through innovation vouchers from the State of Baden-Württemberg.
Dust is a natural byproduct of abrasive cleaning processes. As such, the grains used for the cleaning process are not actually a problem as they are typically 50 to 200 μ in diameter and are easy to bind or remove. The real difficulty lies in the particles that shoot off the material as it is being blasted. The fine particles that are discharged can measure anywhere between 1 and 20 μ. These particulates spread out from the cleaning area due to the pressure of the jets. Engineers soon reach the boundaries of technical possibility trying to filter out these particulates because of the large volumes of air. And despite protection, these processes are still hazardous to workers.
The process developed by the Steinbeis Transfer Center for Process Development involved free-form testing of the impact on material surfaces of jet-blasting using hard grains such as pumice, glass beads, aluminum oxide, steel shot, sand or crushed nutshells. The grains are blasted onto work surfaces at high velocity under high pressure. Important process parameters include not just the kind of grains used, but also the speed of delivery, the volume of grains and, in particular, exposure time.
The development project enabled the Steinbeis experts to solve the key dust problem by positioning a water screen around the blasting nozzle. This was formed out of water droplets to provide a protective shield around the dust source. It involved six or eight jets supplied by the very latest sprinkler technology as well as hollow-cone nozzles used in standard household pressure cleaners. The water nozzles were positioned in a coaxial arrangement inside a circular housing around the blasting nozzle. The device can be adapted to any kind of conventional nozzle found in trade.
Blasting tests carried out by the Steinbeis experts showed that the process absorbed more dust than expected. Also, there was a marked rise in the amount of dust removed from the material being cleaned. This had not been expected since particle binding is only possible when fast grains and slower water droplets are travelling at exactly the right velocity.
The particle movements in such a process were so complex that a flow model was developed along with a computer simulation to explain what was happening. This was followed by verification testing. The results were as remarkable as they were logical. Abrasive blasting without a water screen creates a pressure cushion on the material surface formed by lots of tiny discharged particles. This cushion brakes particles approaching the material. With a water screen, a major proportion of the miniscule particles that slow down movement are absorbed – thus allowing abrasive grains to do their job and erode surface layers on the material. As a result, not only are fewer pollutants emitted, the process actually becomes more effectives.