The demand for steel is unbroken. The worldwide production of raw steel has increased by approximately 5percent per year over the past 10years. This growth trend is expected to continue at this level until 2015. China has increased its world market demand from approximately 13percent in 1996 to about 34percent today.
The main motors are the building industry, infrastructure measures and ship building which will also grow strongly in other Asian countries. Existing plants are being modernised and expanded and new steel works built to meet this demand.
The materials and process technology for manufacturing high quality steels is complex. The finished semi, section or steel band is produced from iron ore in several steps. In addition to coke, iron ore, alloy metals and various additives such as limestone, gravel and dolomite, large amounts of auxiliary and operating materials are required.
Most importantly, water and steam for heating, cooling, cleaning, rinsing, etc. But chemicals and oils are also required for pickling and other refining processes for example. The recycling and return of these media into the process or their post-treatment for safe disposal are a matter of course today. A modern steel works is a veritable city. Miles and miles of metal and plastic pipeline systems are required to supply and dispose of the different media. For this reason you will also find all sorts of valve types. The process of the pickling plant is described below and the valves used there described in more detail. There may be deviations depending on the plant manufacturer and the process technology implemented.
The raw iron is refined into steel in further process steps. To do this, it is mixed with steel scrap and alloy components and melted again in special furnaces of different types. The liquid steel is drawn from these furnaces in a continuous casting process.
From there it reaches the hot rolling mill directly where it is rolled into steel section or steel band depending on the plant setting.
An oxide layer forms on the surface of the steel during rolling. This consists of different percentages of Fe2O3, Fe3O4 and FeO depending on the temperature. The oxides are partly broken and dissolved by the mechanical stress in the rolling mill. This simplifies the downstream chemical pickling. There, the oxides are removed by hydrochloric acid HCL or sulphuric acid H2SO4 as well as caustic soda NaOH. The processes take place at operating pressures up to 6bar and temperatures between 20°C and 90°C.
Pickling is followed by a rinsing station in which the chemicals and the dissolved scale coating are washed off by desalinated water. Pickling is followed – depending on the desired end product – by the different further processing steps such as rolling, cutting and surface coating by galvanic or other application processes.
For distribution of the clean acids, in this application case hydrochloric acid (HCL) in a concentration of about 20percent, centric butterfly valves of the Bianca type are used. The valve body is made of GGG 40.3, the liner of PTFE and the shaft seal of PFA. The acid contaminated by hard scale particles is usually disposed of via diaphragm valves of the GEMÜ675 and GEMÜ620 series.
The valve bodies are also made of GGG 40.3 lined with PFA, the shut-off diaphragm is PTFE. The diaphragm valves are relatively insensitive to the hard oxide particles. Clean rinsing water and concentrated NaOH for neutralisation are distributed via butterfly valves with EPDM seal and Halar coating. The rinsing water is again disposed of via diaphragm valves because of the particle contamination.
At temperatures up to a maximum 80°C, plastic diaphragm valves GEMÜ690 and GEMÜ677 with valve bodies made of polypropylene (PP) and shut off diaphragms made of PTFE are also used for caustic soda. The advantage of the plastic lined S.G. iron body is the higher pressure and temperature resistance to solid plastic valves. At GEMÜ, the metal valve bodies are lined in injection moulding machines. This produces a very high quality, long lasting lining with a defined and constant material strength. Glass lined bodies are rarely used in steel works today because of their impact sensitivity.
A part of the hydrochloric acid reacts with the iron oxide during pickling. The chemical reaction is as follows:
Fe2O3+Fe+6HCl →3FeCl2+3H2O
Fe3O4+Fe+8HCl →4FeCl2+4H2O
FeO+6HCl →FeCl2+H2O
Pickling is followed by regeneration of the acid. In the process, FeCl2 is oxidised to Fe2O3 and HCI in a spray roaster. The hydrochloric acid is fed back into the pickling process, the acid is agglomerated, pelletised and fed back into the blast furnace. The chemical reaction is as follows:
4FeCl2+4H2O+O2→2Fe2O3+8HCl
Wherever the pneumatic valves have to perform control tasks, they are fitted with GEMÜ1435 and GEMÜ1436 positioners and process controllers. Many customers prefer the GEMÜ1435 for the rugged environment of a steel works because of its robust metal housing. The steam required for heating in the processes is controlled by GEMÜ512 and GEMÜ520 globe valves. The valves are also equipped with the above positioners.l
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Thomas Schulz is with GEMÜ Gebr. Müller Apparatebau GmbH & Co KG, Ingelfingen, Germany. www.gemue.de
