(First published in JOT, July 2001)
At the works of a company that produces stainless steel sections, a combined blast cleaning/spray pickling system is used – for the first time in Europe - to remove scale from rolled metal products. Compared with traditional surface treatment by blast cleaning and dipping, this plant configuration offers some major advantages.
The German company Walzwerke Einsal GmbH being domiciliated at Nachrodt-Wiblingwerde manufactures among other products a large variety of stainless steel sections in different shapes and designs. Within the wide range of materials processed by this company, austenitic steel is the one that is most frequently used. Other materials are, for example: steel 1.4301, 1.4571, 1.4305 and other austenitic parts. These products are both hot rolled and cold-drawn. Most of the production are bars. For these products a new pickling system has been installed.
For the first time in Europe a manufacturer operates a combined blast wheel cleaning and tunnel spray-pickling system that is equipped with loading, unloading and packing units. Compared with the traditional surface treatment methods by blast cleaning and cold-drawing, this plant configuration offers some essential advantages: Combined to one single line, the plant components need a relatively small space. This ensures a continuous operation of the system regardless of whatever shape and length the products to be treated may have.
Warm hydrochloric acid used as pickling solution
The pickling solution - a warm hydrochloric acid - deserves some special attention. Some technical improvements in the spray process and the use of additional addivites (inhibitors) have made this application possible. Use of this acid reduces environmental impact and allows cost-saving for final disposal. The entire plant runs in continuous operation with a thoroughly designed acid regeneration and neutralization system.
The entire production line comprises the following plant components:
- Workpiece loading unit with spacing system
- Blast wheel cleaning system with roller conveyor
- Tunnel spray pickling system
- Spraying system
- Drying system
- Workpiece unloading unit with transport to the stacker
- Packing unit with integrated weighing system
The blast cleaning system delivered by DISA features a roller conveyor of 900 mm width and is equipped with six heavy-duty blast wheels with a drive power of 15 kW each. The number and arrangement of the blast wheels ensures a uniform blast cleaning result and perfect scale removal from the various rolled steel sections.
A blow-off device removes the remaining abrasive from the rolled steel sections to ensure transport of cleaned material into the tunnel pickling system.
The roller conveyor speed is infinitely variable from 0,5 up to 5 m/min, with the average speed of passage being 2 m/min for a surface quality of 2,5 A SA. Furthermore the blast cleaning system is equipped with a mechanism for adjusting the quantity of abrasive being thrown onto the workpieces. Adjusting the speed of passage and the quantity of abrasive permits setting of ideal operating parameters for the different rolled steel sections and material thickness.
In the exit area of the blast cleaning system a blow-off device is installed to remove remaining abrasive from the blast cleaned rolled steel sections and to feed this abrasive back to the abrasive circulation system. Due to this design only clean material enters the tunnel pickling system.
Furthermore, the blast cleaning system can be provided with a sensor technique which ensures that the abrasive feed system will not be open unless there is material to be treated in the blast zone, i.e. the blast chamber.
Considerable decrease of wear
Due to the possible settings and functions, wear on the blast cleaning system and the consumption of abrasive is optimized and thus considerably reduced, and the levelled material remains straight.
Bus technology is used for integrating the control system (CNC, PLC) of the blast cleaning system into the overall control concept. Due to this it has been possible to include the blast cleaning system and its function in the visualizing unit of the overall control system. All components of the plant can be controlled from a central control room.
Working principle of the roller conveyor blast cleaning system. The blast wheels are arranged in such a way that all sides of the parts are blast cleaned without shadows in one single operation.
The advantage of integrating the control system into the overall concept lies in the fact that supervision of all plant components is possible from one place. Whenever a malfunction occurs in the plant, the component or part of the plant having caused the fault is immediately displayed. The uptime (main time) of the blast cleaning system can thus be considerably increased. As it is not necessary to have personnel at the control system all the time, a light visualization system and an acoustic signalizing system have been installed. Whenever a malfunction occurs, the cause of the fault can be located immediately.
The entire production line is placed in a newly designed factory building where many other work stations are located. The maximum noise level of the system is 80 dB (A). In order to meet the applicable noise standards, the blast cleaning system is enclosed by a noise abatement cabin. Due to these measures the noise level on the different workplaces has been considerably reduced and cleanliness around the workplaces in the production hall has been improved for the benefit of the operators.