Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
43

Development of i Box Pickling Tank for Hot Rolled Strip
Enabling Energy-Saving, High-Speed Pickling

KOSEI TSUJI*1

RYUSUKE NAKATSUKA*2

HIDEAKI SUEMORI*1 MASAO TAMBARA*3

KEIJI MIZUTA*4

NAOHIKO MATSUDA*5

Recently the production volumes of high tensile strength steel strip has increased. This typeof steel requires longer pickling times compared to normal low carbon steel. This prolonged
pickling time becomes the bottle-neck in the steel production process. At the same time, a decreasein energy consumption and production costs is desired, calling for greater importance ofhigh-speed pickling.
To solve this problem, conventional electric-pumped forced pickling systems, calledcirculation type tanks, are applicable. Unfortunately this system’s electric power consumption will
not satisfy the steel maker’s operation cost reduction and energy saving requirements.
To satisfy such requirements, Primetals Technologies Ltd. has developed an energy-efficient,easily-maintainable pickling system called i Box. This system has a pickling performanceequivalent to that of circulation type pickling, while only using the strip driving force to move theacid in the tanks.

|1. Introduction
The purpose of the pickling tank is to remove the oxidized surface created during hot rollingfrom the steel strip. Pickling typically uses acids like hydrochloric, sulfuric, nitric or hydrofluoric
at an elevated temperature. This paper covers hydrochloric acid type pickling to remove theoxidized surface on low carbon steel strip, including high tensile strength steel.
In recent years, responding to the needs to reduce the weight of car bodies and consequently
CO2 emissions from automobiles and save energy, client steel mills are increasing the productionof high-formability, high-tensile strength steel strip (TRIP and/or DP steel)(1) These kinds of steel
contain more silicon than normal steel, making them harder to pickle. To increase the production ofhigh tensile strength steel, it is necessary to improve pickling performance.
It is generally known that a boundary layer of acid is formed on the strip surface travelingthrough the pickling tank. In this boundary layer, the acid concentration decreases as the acid isconsumed by the reaction with the oxide and descaling time is prolonged. A thicker boundary layer
results in longer pickling time. Pickling tank suppliers have tried to reduce this boundary layer
thickness through forced circulation by pumping acid to the strip surface through nozzles poweredby electric pumps. But this method requires electric energy and agitates the silicon sludge in thetank. The agitated sludge clogs pipes and makes maintenance more difficult and time consuming.
To solve these problems, Primetals Technologies Ltd (herein after Primetals) has developed anew pickling technology called i Box (Immersed Box Pickling Tank), which has the same pickling
performance as circulation type pickling tanks without the need of electric pumps.
*1

Senior Engineer, Project Management Department, Primetals Technologies Japan, Ltd.

*2

Project Management Department, Primetals Technologies Japan, Ltd.

*3

General Manager, Project Management Department, Primetals Technologies Japan, Ltd.

*4

Chief Staff Manager, Heat Transfer Research Department, Research & Innovation Center*5

Chief Staff Manager, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)44|2. Structure of i Box pickling tank
Generally, one pickling line has 3 or 4 acid tanks. Figure 1 shows the structure of i Box in a
pickling tank. Several low-height boxes are immersed in the acid tank. The strip travels throughthese immersed boxes. Each box has one granite block called a skid and one roll called a divider
roll at the exit side of the box. The divider roll and skid create a shearing effect in the acid andreduce the thickness of the boundary layer on the surface of running strip. The re-growth of theboundary layer is restricted by the top and bottom walls of the i Box, which are close to the surfaceof the strip. Through this effect, it is possible to maintain a high mass transfer ratio of acid to thestrip surface and promote the reaction with the oxidized surface.
In addition to that, i Box has two characteristic features. The first is that the heat exchangers
required to heat up the acid are installed between the immersed boxes and the acid tank outer wall,in the space through which the acid self circulates inside the tank. The second is that during line
stops, the acid is drained from the i Box pickling tank to storage tanks, lowering the acid level inthe tank and preventing overpickling of the strip. Just before operation is started, the acid ispumped back to the pickling tank by electrical actuated pumps.

Figure 1

Schematic diagram of i Box pickling tank

Figure 2 shows the acid circulation type called jet pickling within Primetals. This type ofpickling requires acid circulation from a circulation tank to the jet nozzles through pipes by electricpumps. The acid heat exchangers are installed in the middle of this pipe line to heat up acid. Thissystem is prone to certain problems which the i Box pickling tank can solve, as described in thefollowing chapters.

Figure 2

Example of circulation-type pickling (Jet pickling)|3. High-speed pickling performance
3.1

Descaling process in pickling tankThis chapter explains the chemical reaction which takes place during pickling. The main
composition of the oxidized strip surface after hot rolling is FeO (Wustite), Fe3O4 (Magnetite) andFe2O3 (Hematite) as shown in Figure 3. Each composition is dissolved by acid in the reactions
shown in the following chemical formulas from (a) to (d).

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)45

Generally Fe3O4 (Magnetite) and Fe2O3 (Hematite) create FeCl3 in acid. But in real
production lines, since the amount of FeCl3 in whole pickling tanks is detected less thantheoretically estimated. This means base material (Fe) is reacted in whole pickling tank with acidsolution to change FeCl3 to FeCl2. So chemical formulas (a) (b) mainly occurs in pickling.
Fe3O4 + Fe + 8HCl → 4FeCl2 + 4H2O----(a)Fe2O3 + Fe + 6HCl → 3FeCl2 + 3H2O----(b)FeO + 2HCl → FeCl2 + H2O---------------(c)Fe + 2HCl → FeCl2 + H2↑----------------(d)Note) The chemical formulas in this paper have been corrected from a previous paper(3)Figure 3

Scale composition and crack

Figure 4 Acid coating pickling dissolution process and appearance of pickling solution in
the experiment

There are three steps in the descaling process as shown in Figure 4 (on left side), which wasexplained in the previous paper(3). In the first step, acid is absorbed into cracks created in theoxidized surface by scale breakers. In the second step, acid dissolves the base metal and FeO.
Lastly, in the third step, the oxidized surface is removed from the strip. Primetals has proven thisphenomena through experiments.
Figure 4 (on right side) shows the results of the experiments: two magnified photos show themoments of acid dissolving the base metal and FeO, which were taken 2 or 5 seconds afterpickling. This indicates that the chemical reactions in formulas (c) and (d) are faster than thereactions in formulas (a) and (b).

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)46

3.2

Prediction of acid concentration distribution in pickling tankAccording to the mechanism of pickling, achieving high speed pickling requires to maintaina high concentration of acid near the strip surface. There have been very few examples how to
predict the concentration of acid in pickling through calculations. Primetals have tried to simulate
the flow and the concentration dispersion of the acid in the tank by numerical simulations.
Figure 5 shows one example of this simulation. When starting to design the i Box, because
the moving strip was the only method to move the acid, Primetals was concerned about the possiblelack of agitation of the acid inside and outside the immersed box. However, the simulation results
showed that the acid concentration distribution of i Box is similar to that of jet pickling tanks.

Figure 5

Acid concentration distribution in picking tank

Figure 6 Mass transfer ratio distribution in pickling tank

Next, Primetals focused on the acid concentration distribution on the strip surface. The area
after the divider rolls has a high acid concentration because of the thin boundary layer created bythe rolls. After the rolls, the thickness of the boundary layer grows, leading to a decreased acidconcentration on the strip surface. Figure 6 shows this phenomena: the horizontal line indicates thedistance from the entrance of acid tanks and the vertical line indicates mass transfer ratio. This
figure also shows the predicted data for jet pickling and conventional deep tank pickling. Based on
this analysis, there is no difference between i Box and jet pickling. In the future, this numericalanalysis shall be further enhanced for continued improvements of i Box pickling tank technology.

|4. Energy efficiency and maintainability of i Box
4.1

Energy saving by means of circulation-free system

In general, the temperature of the acid is kept at approximately 85-90 degree C for highchemical reaction of the acid. The heated acid also has the function of heating the strip, which
enters the pickling tanks at room temperature. The required heating energy for the strip must besupplied by steam fed heat exchangers immersed in the tank. Conventional forced circulation type
pickling systems have external heat exchangers installed in the middle of the acid circulation pipes.
The forced circulation systems incur the following two additional energy losses:

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
47(1)The first is electrical energy of electric circulation pump.
(2)The second is heating loss from the circulation pipes and tank surface.
i Box is different on these points, as i Box has self-circulation flow of acid inside the tank.
The driving force for the circulation of the acid is supplied by the movement of the strip. Thiscauses the acid to flow along the tank towards the outlet and increase the acid level near the outlet
compared to that by the inlet. The space between the immersed boxes and the tank outer wall
allows the acid to flow back towards the inlet side of the tank. Locating the heat exchangers in thispath, makes it possible to heat up the acid without the need of electric pumps or pipes, thus
eliminating heat losses from the pipes. Table 1 shows the a comparison of heat loss data for two
pickling systems.
Table 1Comparison of Energy Consumption (Ratio with Jet Pickling Tank as 100 Given)
Circulation type(jet pickling)i Box10018

Pickling tank100162Circulation system

1000Total

10076

Description
Electrical energy
Radiation from pickling tank and circulation system(excluding heat from temperature increases in steelstrip)

4.2

AverageEnhance maintenance performance

Recent mass production of high tensile strength steel has increased the amount of siliconsludge created and remaining in the acid tank. In forced circulation type tanks, this sludge
accumulates in the circulation pipes and gradually crystallizes, eventually blocking the acid flow inthe pipes, resulting in pipe breaks and acid leaks. Periodical cleaning is necessary, but is very
difficult because of sludge crystallization. In contrast, i Box does not require pipes for acidcirculation. The silicon sludge simply sinks to the bottom of the tank. By pulling the easily
removable heat exchangers, the sludge accumulated at the bottom of the tank circulation path canbe removed. In addition, the heat exchangers are easily cleaned offline.

|5. Flexible designing of i Box
Pickling lines installed in developed countries during the 1950’s have reached the end of
their useful life and require tank replacements. These lines must also be able to produce hard to
pickle materials like high tensile strength steel and satisfy reduced energy requirements. i Box isone of best options for the replacement of aged pickling tanks.
Primetals have developed several types of i Box designs to revamp conventional deep
pickling tanks. Deep pickling tank lines have no sub-tank used in i Box for acid storage during tankmaintenance. By installing the i Box in the upper part of the space occupied by the deep tank andthe sub-tank in lower part, as shown in Figure 7, the cost of revamping can be minimized. Primetals
has continued to improve the design of the i Box technology to suit the specific requirements of allcustomers.

Figure 7 Reformed-type i Box pickling tank

|6. Conclusion
This paper explained the advantages and high performance of i Box, using the results ofnumerical analysis, among others. Further optimization of the i Box pickling tank flow state bymeans of the herein described numerical analysis will contribute to design improvements. The
client’s growing demands for energy saving and maintenance-free characteristics are driving theseMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
48improvements.
Despite being an indispensable process in pickled/cold-rolled steel strip manufacturing, thepickling process is being researched and developed less than other production processes. Primetals
has supplied a large worldwide share of continuous picklers. By closely working with this installedbase and through continued R&D efforts, Primetals will help improve productivity and cost savingsof clients.

References
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Tatsuya NAKAGAITO et al., Influences of Si and Al on Mechanical Properties of Low Alloy
TRIP-assisted Multiphase Steel Sheets,Tetsu-to-HaganéVol. 97 No. 5 (2011) pp. 35-43
Shibatomi, N. et al., Development of New Type Jet Pickling，Mitsubishi Heavy Industries TechnicalReview Vol. 29 No. 3 (1992) pp.177-183
Hirai, E. et al., Present State and Future Trend of Pickling Technologies, Mitsubishi Juko Giho Vol. 36
No. 6 (1999) pp. 308-311