Fluidized bed design suitable for reduction with hydrogen and metallic iron production from steel pickling waste

Demand for flat steel is continue to increasly in parallel with the automotive, white goods and machinery industries that continue to develop worldwide. Due to the long life of flat steel products and the necessity of painting on them (automotive, white goods) flat steel products are generally used by hot dip galvanizing and zinc plating process. Residues such as dirt, oil and rust on the steel coating material, are not allow to a standard and successful coating. For this reason, as in every coating process, steel surfaces are cleaned in galvanizing process. It is an important issue on the regeneration of the waste acidic solution after the cleaning process. Various methods can be chosen and applied according to different waste acid concentrations for regenaration process. Within the scope of this thesis, it is aimed to obtain metallic iron with a high metallization rate by direct reduction method of FexOy which is the result of pyrometallurgical decomposition of FeCl2 obtained from steel pickling waste of Borçelik Çelik Sanayi Ticaret A.Ş. Experimental studies were carried out using two different systems, rotary furnace and fluidized bed. Since there is no fluidized bed suitable for working with hydrogen in our country, fluidizied bed design and manufacturing have been performed primarily for use in experimental works. Cold system studies and modeling studies have been carried out in order to assist in design and manufacturing. In the first cold system operation, a 140 mm diameter plexiglass fluidized bed was designed different nozzle systems, continuous channel gas distributor and mesh gas distributor systems used as a gas distributor to control the fluidization conditions and experiments were made by with 8 kg, 5 kg, 3 kg and 1 kg samples. Then a second system with 80 mm diameter was designed and tested with continuous channel and mesh gas distributor systems. Experiments were carried out using 1.5 kg and 1 kg of sample and the flow conditions were monitored. As a result of the work done, a bed of 1 kg sample was successfully fluidized. It has been determined that the most ideal condition is obtained with 16m3 / h, 1 kg sample in the work carried out with mesh system. The fluidized bed simulation for reduction is prepared by using numerical analysis methods to realize the system which will be manufactered within the scope of the thesis. The simulation was carried out in reactors with a diameter of 70-100 mm and a length of 1000-4500 mm. First, in order to test the reliability of the model, about 20 different reactor sizes have been tried in extreme conditions, and no problems have been identified. In the created model, first the initial conditions were checked, then particle velocities and behaviors were examined. The calculations have been continued until the system has entered the regime. Euler-Euler model was established with optimized parameters according to the work done, and fluidization stages at room temperature and 650oC temperature were followed. The design and manufacture of hot fluidized bed furnaces have been carried out in the information obtained from cold system and modeling. Because the temperature is the most critical parameter in the fluidized bed operation during the manufacturing process, a three-zone, open-door construction oven has been designed to accommodate the thermocouples and to provide rapid cooling of the fluidized bed after the reduction. In addition, a preheating oven has been added to eliminate the negative effect of the cold gases being supplied to the system. Before to doing experiments, 100% impermeability checks were made due to the necessity of operating in high gas turbines and the explosive nature of hydrogen. In order to observe the reduction behavior of iron oxide particles, experiment were carried out in a rotary kiln at 850oC for 60 minutes, with mixed(original) and -850 + 500, -1400 +850 μm grain sizes and ratio of 2/1, 3/1, l/min used for reductant/carrier gases. and it was observed that the metallization was not stable as a result of the experiments. For this reason it is thought that the material has maghematite (magnetite + hematite) structure. Bed formation in the fluidized bed system used in acid regeneration and the inoculant loaded into the system to grow the reaction product FexOy structure are transformed into the magnetite structure in the waiting process and / or during the grinding stages. Since the material is based on magnetite + hematite and the magnetite can not move to the grain due to the spinel structure, it causes different reduction characteristics to be observed in different grain sizes. Reduction was performed by applying pre-oxidation treatment (to make the hematite completely out of the sample) to ensure that all materials have similarity and the same reduction characteristics on the basis of this data. The pre-oxidation process is provided in a standard electric oven, atmospheric conditions in which samples are kept at 600°C for 24 hours. After the oxidation process was applied, metallization was achieved over 95% of the samples reduced in the rotary kiln. In the first work on the designed and builded fluid bed, a 20-minute trial was carried out at 500oC with 16m3/h flow rate 40% H2 content in order to test systems operating status . Subsequently, 120 minutes of experiments were carried out with 16 m3/h of 40% H2 at 500, 575, 650 and 725°C, respectively. It has been observed that reductions at low temperatures are possible as a result of the experiments carried out. Cooling to room temperature was carried out at 650°C. However, it was observed that the sample removed from the reactor was oxidized back very quickly and the material was pyrophoric and therefore a longer cooling process was required. After the experiment performed at 725oC, it was observed that the granules started to stick to each other and to the furnace walls. Last experiment done at 725oC because the progress of adherent adversely affect the fluidized bed parameters. It has been observed that the reduction progresses topochemically in the experiments performed. Experiments with 16 m3/h, 120 min., 40% H2 at 725oC achieved 86% metallisation and reduction was observed in all grains.