The background art described in this document focuses specifically on ferrous alloys; however, the apparatus and the process in accordance with the invention applies to all metal materials.
Compared to iron oxidation, steel oxidation is also affected by the behaviour of the elements found in the steel alloy. Although the oxidation phenomena are more complex, surface scale found on steel products is typically formed by iron oxides and always contains FeO (also called wustite), Fe3O4 (also called magnetite), Fe2O3 (also called haematite), and Fe(OH)3 or FeOOH (also called rust or limonite). Following exposure to pure air or oxygen, the scale formed on pure iron consists of several layers. Below 570° C., FeO is unstable and only Fe3O4 and Fe2O3 are present; while, above said temperature, an internal layer of FeO is formed along with the two oxides. Often, the presence of other elements leads to structural changes in the scale and affects the growth kinetics of the scale. Furthermore, the underlying metal is modified due to the phenomenon of selective oxidation of this binding additional material.
Most scale formed during steel production develops at much higher temperatures than 570° C.; consequently, all three aforementioned oxides are present. It is generally believed that the diffusion of vacancies in FeO and in Fe3O4 and the diffusion of oxygen in Fe2O3 contributes to the growth of said oxides in pure iron. Nevertheless, the diffusion of ferrous gaps or vacancies can also occur in Fe2O3; while, both in Fe2O3 and in Fe3O4 the diffusion of oxygen along the distribution channels, the edges of the grains, and microcracks can significantly promote the formation of the phenomenon. The kinetics of oxidation can be controlled by the reactions that occur at the different interfaces between the following: Fe and FeO, FeO and Fe3O4, Fe2O3 and Fe3O4.
Sometimes, oxidized products are exposed for prolonged periods of time to industrial and/or sea air. This, leads to considerable rusting (thick layers of complex iron hydroxides (millimetres). Therefore the products to be pickled can appear like material coated by a dark grey layer, e.g. black strip, made of mixed oxides, whose thickness is comprised between fractions of μm and 10 μm maximum. Generally this kind of scale is the easiest to be removed. It is more difficult to remove the scale from materials having been subject to corrosion so as to produce a thick layer of oxides or very deep cavities, even in the range from 50 to 100 μm.
The most widely used process for removing scale from metal products is pickling with acid; this process involves treating the metal products with H2SO4 or HCl at a temperature of approximately 80° C. for a period of time ranging from 10 to 30 minutes. The thicker the scale layer, the longer the required pickling time; while, is the temperature remains constant.
For example, before drawing metal products, the metal is normally cleaned by immersing the coils in a container filled with hot hydrochloric or sulphuric acid. Sulphuric acid mainly eliminates scale by means of a mechanical, rather than chemical, action. The acid is able to penetrate into the metal under the scale layer where it reacts with the iron forming water-soluble iron sulphate and releasing a gas mixture consisting mainly of H2.
This action detaches the scale from the iron; then, at the end of the pickling process with acid, the surfaces of the metal product are cleaned with high-pressure jets of water.
Temperature control plays an important role in this type of pickling since the speed of the acid-metal reaction is highly affected by temperature; for example, the reaction is 100 times faster at 88° C. than at ambient temperature. At the other end of the scale, overheating the acid wastes energy, consumes an excessive amount of acid very quickly, and creates unnecessary fumes that are highly corrosive to the structure of the plant. Not only, acid at high temperatures is also damaging to the surface of the metal: it produces pitting. To help prevent pitting or the excessive decomposition of the metal surface, inhibitors are commonly used. Said inhibitors are products based on nitrogenous hydrocarbons. The time required to clean the metal product varies depending on the type of scale to be eliminated and the type of metal to be treated. This can range from 10 minutes for bars with a high-carbon content to 35 minutes for bars with a low-carbon content and a considerable amount of scale. For this reason, pickling with acid is most suited for metal surfaces covered with a thin scale layer.
After cleaning with water jets, the metal product pickled with acid is rinsed and covered with a protective coating.
The main drawback of using the acid pickling method is the significant negative environmental impact and the reduced kinetics of the reaction. The acid residues found in the acid baths are potentially dangerous; handling, disposing of, and storing these products is complex and costly. Furthermore, depending on the type of scale to be eliminated, efficiency can fall to below 33%.
Another commonly used method is mechanical descaling; this, can be done through bending, shot peening, sand blasting, brushing, or using ultrasounds. Once again, the purpose of these methods is to detach, remove, or break off mechanically the scale layer. Mechanical descaling is more effective on fragile scale with low adherence to the metal product; thus, mechanical descaling is more appropriate for thick layers of the scale since, the thicker the scale layer, the lower its bond to the metal.
Another pickling method involves the use of a salt in liquid form. K2O (Na2O, SiO2) based salts are able to dissolve iron oxides and produce two immiscible liquids. The liquid with the highest content of FeO can be regenerated. The regenerated salt will be reutilized for pickling. Thus, the scale is washed with a liquid and the acid is replaced by a bath of dissolved salts.
Several known descaling processes—for example, those described in U.S. Pat. No. 2,197,622 and U.S. Pat. No. 2,625,495—feature, at a specific stage of the descaling process, the injection of a condensed reagent, liquid or solid, combined with some form of intermediate gaseous oxidizing reaction.
Document WO 00/03815 describes a dry descaling process where the scale is removed from the strip in a chamber; here, the surface of the strip is heated, exclusively through induction winding, and H2 flows only in a counter current manner. The solution described in the WO '815 document involves the use of an amount of H2 greater than the stoichiometric one; however, the efficiency of the process is not satisfactory neither from the technical nor financial point of view. Other known descaling processes use hydrogen and other reducing gases, such as carbon monoxide, to reduce oxides in minerals where they are substantially consumed in reducing furnaces or in containers or tanks. However, hydrogen burns easily and can be an explosion hazard; while, carbon monoxide is a toxic gas and is generally considered dangerous if not confined and made react in a tank of the type generally used for reducing minerals. Thus, although the basic chemical principles for reducing oxides with gases are known, the state-of-the-art technology does not include technical solutions that make it possible to perform fast, homogenous, and compact processes for removing the scale from metal surfaces in a continuous manner.
In the process described in U.S. Pat. No. 6,217,666 and in U.S. Pat. No. 6,402,852, hereinafter also referred to as acid-free pickling, or AFP, surface oxides are reduced by using a reducing gas, for example H2 or CO, at the right temperature. The plant described in the aforementioned patents comprises a reactor, where the metal product is descaled, that features three main functional areas, specifically:
a first heating area where the metal is raised from ambient temperature to the reaction temperature in a non-oxidizing atmosphere,
a second reaction area where the metal is reduced in a reducing atmosphere and fans constantly renew the gas mixture,
a third cooling area where the metal is cooled to 120° C., or lower, in a non-oxidizing atmosphere.
Depending on whether the type of furnace used in said first area is of the electric type only or also has CH4 burners, the main inputs are, respectively, electricity only or electricity, N2, H2, and air-CH4, the last item is used when the furnace is also equipped with natural gas burners. The products leaving the plant are water vapor and H2 and, in the case of a furnace equipped with gas burners, also the combustion products of natural gas.
Acid-free pickling has many advantages over pickling with acid including the absence of dangerous toxic waste, the absence of corrosion on the metal surface, and the use of mildly aggressive cleaning means.
The main phases of this process, disclosed in U.S. Pat. No. 6,217,666 e U.S. Pat. No. 6,402,852, are the heating of the metal product, the reduction of the oxides, and the cooling of the metal product. The scale-reducing stage in the reaction area is carried out ensuring a turbulent and/or vigorous injection of the reducing gas, preferably in the presence of elementary carbon. A disadvantage of these types of processes is that gas flows in a disorderly manner inside the reactor, and hydrogen is supplied taking for granted that it will react with the scale found on the metal product. The presence of the chaotic gas flow inside the reactor limits the speed of reaction and significantly lengthens the descaling process. Furthermore the use of fans to recycle the reducing gas inside the reactor can cause accumulation of gas products issued from the reduction, e.g. H2O, thus slowing down oxides reduction reactions in the same parts and causing a general reaction slow down and also product non-uniformity.
As a result, the efficiency of the AFP process is greatly reduced; alternatively, to offset this problem and obtain a level of productivity comparable to the one of traditional acid-pickling plants, the process must take place in very long plants. Apart from the inconveniences related to constructing a large plant, the large amount of reducing-gases required for the reactor present a great hazard in the event of an emergency. Furthermore, in very long plants, it is also necessary to take into account the significant amount of time required to fill the plant with the reducing gas, the significant duration of the thermal transient, and the high thermal losses; these factors make the AFP process financially less appealing compared to acid-based pickling processes.
Another problem that generally arises with acid-free pickling processes of the known type is the poorer quality results obtained when treating metal products totally covered with thick and/or highly adhesive scale. In this case, when a piece of metal covered with a uniform, or not, scale layer is fed through an AFP reducing plant, the top scale layer is reduced and the surface looks shiny. However, in many cases, the reduction does not occur throughout the thickness of the scale. In other cases, the reduction does not occur uniformly making the resulting metal surface not very suitable for further machining. Another drawback is that, the gaseous stages of the pickling process use heating and reducing techniques that have not specifically been designed for acid-free pickling; consequently, the efficiency of the entire process is reduced.
To date, there are no known AFP-type processes featuring the thermo-fluid dynamic control of the boundary layer of the reducing gas on the surface to be treated and a chemical control of the reducing mixture that achieve high reduction rates of the scale and a homogeneous reduction of all the points covered with scaling.