WO1986005523A1

WO1986005523A1 - Method for pickling iron or steel objects

Info

Publication number: WO1986005523A1
Application number: PCT/SE1986/000116
Authority: WO
Inventors: Harry Ericson
Original assignee: Korrosionsforskning Ab
Priority date: 1985-03-19
Filing date: 1986-03-17
Publication date: 1986-09-25
Also published as: DE3671042D1; SE8501329L; SE8501329D0; EP0244417A1; CA1256357A; EP0244417B1; JPS62502268A; SE452481B

Abstract

A method for pickling iron or steel objects before a subsequent surface treatment or mechanical processing. The objects (11) are transported into a pickling bath (10) having a temperature above 25oC and containing a solution of ferrous chloride and a low proportion of free hydrochloric acid. The pickling bath is continuously regenerated in a circulation system with electrolytic cells (14). These are preferably electrically connected in series and in parallel with the flow of the pickling bath, in order to deposit iron on the cathode (19) and recover free acid.

Description

Method for pickling iron or steel objects

This invention relates to methods for pickling iron or steel objects, before a subsequent surface treatment or a mechanical processing.

Before the mechanical processing of objects of iron or steel, e.g. by drawing, milling or pressing, or before application of any anti-corrosive film, the surface coating usually must be removed. This coating can comprise various oxides, e.g. rust, being formed by chemical corrosion. Other coatings can consist of scale from a preliminary heat treatment or rolling skin from a preliminary rolling. These different coatings are normally removed by pickling in acid baths.

Normally, either sulphuric acid or hydrochloric acid is used for pickling carbon steel or cast iron. The former is less expensive to buy, but the hydrochloric acid presents several technical advantages, often making the total economy for this acid the most profitable.

A pickling bath intended for pickling with hydrochloric acid normally comprise about 200 g HC1 per liter. During pickling, normally performed at a temperature of about 20°C, iron is solved as Fe*-**^"*". The proportion of iron in the pickling bath rises gradually until it, after some use, reaches about 80-85 g per liter pickling solution. The proportion of acid in the pickling bath is now about 80-100 g per liter pickling solution. The pickling continues very slowly during these circumstances making it necessary to exchange the pickling solution for regenerating or dumping. In total, 7-8 kg of hydrochloric acid is used per kg of dissolved iron.

The depositing or regenerating of used pickling solution is very important from various points of view. Partly because these pickling baths constitute a dangerous waste, which has to be destructed before deposition. Partly because the pickling baths has a significant value with respect to the content of remaining free acid, solved iron and a corresponding amount of negative ions.

The hydrochloric solution can be regenerated through a roasting process within an oil-burning oven. Thereby the solved iron forms iron oxide and hydrogen chlorine. The latter is absorbed while the pickling acid is recovered. One disadvantage with this process is that a certain amount of hydrogen chlorine is lost because also the remaining free acid goes into the oven.

It is also possible to regenerate the pickling solution through electrolysis. The iron in the solution will be deposited at the cathode of the electrolytic cell. The electrolyte (the pickling solution) must form an almost neutral solution of ferrous chloride in order to enable the electrolysis. Since the pickling solution, however, contains a comparatively large proportion of reamining acid, there will mainly be development of hydrogen gas at the cathode, i.e. the deposition of metal will be small in relation to used electric energy.

The solution can be neutralized by evaporation and crystallization of iron chloride and separation of remaining surplus acid, which can be returned to the pickling process.

Because of the solubility of iron chloride, the evaporation of the bath must be complete to enable crystallization. Furthermore, the subsequent electrolysis will consume a lot of electric energy, since a great deal of the chloride is used up by the development of gas, while oxidation of ferrous chloride to ferric chloride takes place at the anode. The gradual increase of Fe III changes the polarity leading to the forming of basic salts, while the electrolytic efficiency declines steeply.

From this, an electrolytic cell for recovery of pickling acid must have a ion-exchange membranes as a partition between anode and cathode, so that the acid formed at the anode will not prevent the deposition of metal at the cathode.

An useful pickling solution may comprise between 100-200 g free hydrochloric acid per liter solution with between 0-80 g Fe²⁺ and usually attacks the base metal furiously. When objects of iron or steel with scales, normally comprising Fe2θ3"FeO-oxides, are pickled, pittings occur. This happens because the surface of the base metal usually is rather small compared to the oxide surface, i.e. the cathode, wherein the oxide has the high electric potential in relation to the iron within said solution and is cathodic in relation to the base metal, which becomes ihe dissolving electrode, because of its anodic potential. Therefore, the scales are not dissolved in the acid, but rather ^■•explodes" away from its surface, because the acid penetrates below the scale and lifts it away. The consequence is severe pittings on the base metal, because of the anodic current density (corrosive current) is very high. While the proportion of acid in the solution declines, the difference of potential also declines and thereby the pickling effect of the solution. The Fe3θ_|- oxide is deposited as a sludge on the bottom of the pickling bath. The proportion of iron rises during conventional pickling, while the proportion of acid declines. At the beginning of the process the difference of potential between the iron oxides is at least 1000 mV. The base metal Fe then acts as anode, meaning that the iron oxides from Fe^—^Fe**-*^"*". The surface of the metal is therefore pitted when it is exposed by fractures and pores in the oxide coating. It is normal to use an inhibitor or restrainer in the pickling bath to reduce the pitting damage on the base metal.

The object of the present invention is to provide a method for pickling objects of iron and steel, which enables a continuous regenerating of the pickling solution, at a high level of electrolytic efficiency, wherein the pickling can be performed in a closed process having as residual products pickled objects of iron and steel and pure electrolytic iron corresponding to the amount of dissolved iron.

The method according to the present invention is characterized in that the objects are transported into a pickling solution having a temperature above 25°C and containing ferrous chloride and a low proportion of free hydrochloride acid, and that said solution is continuously regenerated in a circulation system with electrolytic cells, which are connected to a source of direct-current, in order to deposite iron electrolytically and recover free acid.

Preferably, the pickling fluid contains at least 300 g FeCl2 per liter and no more than 50 g of free acid reckoned as HC1 per.liter.

Preferably the electrolytic cells are electrically connected in series with the source of direct-current and in parallel with the flow of pickling solution in said circulation system.

Preferably, the proportion of ferrous chloride sinks no more than 10 g per liter solution during its passage through any of the electrolytic cells, and the current intensity at each of their respective cathode is especially between 0.2-10 A per dm² of cathodic surface.

A solution for pickling objects of iron or steel, before a subsequent surface treatment or mechanical processing, comprising: FeCl2, Fe2 and HC1, preferably contains the following proportions: FeCl2 between 250 and 0 g/1, Fe₂ between 110 and 200 g/1 and HC1 between 5 and 50 g/1.

Since the pickling is carried through at a high proportion of iron and therefore at a low proportion of free acid and also at rised temperature, the oxides are dissolved efficiently, without any pitting of the base metal. This very favourable pickling effect, in spite of the high proportion of iron in the solution, is a result of the ability of chloride to form large complexes. That is, a large proportion of the iron in the pickling solution is bound in anion form as:

(FeCl4)2- This complex ion is in balance with other ions in the solution tanks to the rised temperature, which results in an increased proportion of free hydrogen ions.

Since the ionization energy is comparatively low in a solution of ferrous chloride with a low proportion of free acid it is possible to electrolytically deposit the iron with a high electrolytic efficiency, and cathodically within a simple electrolytic cell, without any partitioning ion-exchanging membrane between anode and cathode.

The invention will now be described in further detail, with reference to the accompanying drawing, which represents a schematic elevation of a pickling plant for using the method according to the invention with continuous regenerating of the pickling solution in a closed circuit.

The figure shows a pilot plant for pickling of cast iron and hot rolled objects before hot zinc coating. A tank containing pickling solution is generally denoted 10. Objects represented by the arrow 11 are transported down into the solution after a preliminary degreasing and rinsing. Pickled objects are represented by the arrow 12.

The pickling solution is continuously pumped, by means of a pump 13, from the tank 10, into five electrolytic cells 14, connected in parallel with the flow from the pump 13, and returns to the tank 10 by means of a return pump 15- The electrolytic cells are electrically connected in series to the positive and negative terminals 16 and 17_. respectively, of a direct-current source. Each electrolytic cell 14 conventionally comprises one anode 18 and one cathode 19«

These conditions prevail:

In the pickling solution

Volume: 10 m3

Flow of objects: 2 tons/h

Dissolved iron: 10 kg/h

Theoretical amount of acid used per hour .=- 15 kg HC1 at 100 . « 50 kg acid at 30 . Concentrations: FeCl 340 g/1

Fe 150 g/1 free Hcl 20 g/1 Temperature: 4θ°C Among others, the following chemical reactions take place in the solution: Fe° _Fe²⁺, Fe3+ ->Fe²⁺

In the regenerating section

Dimensions: 1500x1000x1200 mm

Electrode system: 5 cells with bipolar electrodes anode: graphite cathode: stainless, acid resistant

Voltage drop per cell: 2.5 V

Current density: 2 A/dm²

Static current changer: 16 V, 1000 A

Amperage per cell: 900 A

Total quantity of current: 4500 Ah

Theoretical amount of dissolved Fe: 4680 g

Real amount of dissolved Fe: 4300 g

Estimated electrolytic efficiency: 1 %

Amount of recovered acid: 22 kg at 30 % The following reactions take place in the electrolytic cells: Cathode: Fe²⁺ —»Fe°, Anode: Fe^2"1" *>Fe3+

The cathode plates 19 are made of 0.1 mm thin stainless, acid resistant sheet. The cathodes are changed regularily in the cells 14. By bending the sheets, the electrolytic iron can easily be removed from said sheets.

The pickling solution circulates through the cell system during the electrolysis at a rate of about 30 liter per minute. This implies that the difference in concentration of iron in the solution leaving vs. entering the tank is between 2.0-2.5 g/1. In the present case, when the dissolving rate of iron the pickling solution is 10 kg/h, 40 . of the acid is recovered.

In order to get a full recovery of acid, a cell system with a total current of 10000 A is therefore required. This means that the static current changer must be rated at, e.g. 16 V and 200 A or 24 V and 1000 - 1500 A. The latter alternative is preferable, since the cost of a static current exchanger mainly depends on current capacity and not on output or voltage.

The above described method radically alters the properties of the pickling solution. The iron oxides Fe3θ_j becomes the dissolving electrode and the base metal is not attacked by pitting. The sludge of iron oxides is completely dissolved leaving no remains at the bottom of the tank. At the same time the objects are cleaned with no trace of dirt film. The surrounding pickling solution is strongly reducing at the conditions according to the invention.

The very high electrolytic efficiency during the process according to the invention results from the relation between the overpotential of the hydrogen towards the cathode and the dissolving potential of the iron. The overpotential of hydrogen is preferably as high as possible, since the electrolytic efficiency declines with a rising proportion of free acid in the solution.

The regenerating cells can be built as a modular system, which can be adapted to most existing pickling plants, while changing from a conventional process to the pickling process according to the invention.

The above mentioned dimensions, rates and values can obviously be varied within the scope of the following claims. The method according to the invention can be used in combination with conventional pickling methods. An activator can be added to the above described pickling solution, comprising a surface active agent with a high affinity to the base metal surface, to facilitate the penetration of the pickling fluid beneath the scales. Further, the electrolytic current between terminals 16, 17 can be pulsed at low frequency, giving a lower electrolytic polarization and therefore a higher electrolytic efficiency.

The pickling effect of the solution according to the invention can be even more improved by addition of any of the salts: magnesium chloride, calcium chloride or aluminium chloride. This increases the overpotential at any given proportion of free acid and therefore the electrolytic efficiency. An additive of 50 g/1 I_gCl2 reduces tensile stress at the iron deposited at the cathode, so that a ductile film of iron is formed. The iron can be plated to form any desired thickness. The growth is about 1 /jm/mln. at a current density of 5 A/dm² and 90 . electrolytic efficiency. This addition of magnesium chloride also improves the pickling process.

Claims

1. A method for pickling iron or steel objects before a subsequent surface treatment or a mechanical processing, c h a r a c t e r i z e d in that the objects (11) are transported into a pickling solution having a temperature above 25°C and containing ferrous chloride and a low proportion of free hydrochloric acid, and that said solution is continuously regenerated in a circulation system with cells (14), which are connected to a source of direct-current (16, 17), in order to deposite iron electrolytically and recover free acid.

2. A method according to claim 1, c h a r a c t ¬ e r i z e d in, that the pickling solution contains at least 300 g ferrous chloride per liter.

3. A method according to claim 1 or 2, c h a r a c t e r i z e d in, that the pickling solution (10) contains no more than 50 g free acid, reckoned as HC1, per liter.

4. A method according to any of claims 1 - 3, c h a r a c t e r i z e d in, that the electrolytic cells (14) are electrically connected in series with the source of direct-current (16, 17).

5. A method according to any of claims 1 - 4, c h a r a c t e r i z e d In, that the electrolytic cells (14) are connected in parallel with the flow of pickling solution in said circulation system.

6. A method according to any of claims 1 - 5_. c h a r a c t e r i z e d in, that the proportion of ferrous chloride is brought down no more than 10 g per liter solution, during its passing through any of the electrolytic cells.

7. A method according to any of the preceding claims, c h a r a c t e r i z e d in, that the amperage at each cathode (19) in the electrolytic cells (14) is between

0.2 - 10 A per dm² cathodic surface.

8. A solution for pickling of iron or steel objects before a subsequent surface treatment or a mechanical processing, containing: FeCl2, e2 and HC1, c h a r a c t e r i z e d in, that the proportion of FeCl2 in the solution is between

250 and 450 g/1, that the proportion of Fe2 in the solution is between

110 and 200 g/1 and that the proportion of free acid, HC1 is between

5 and 50 g/1.

9. A solution according to claim 8, c h a r a c t e r i z e d in, that it contains a magnesium or calcium chloride, in order to improve the pickling process, increase the electrolytic efficiency and reduce the tensile stress at the iron deposited on the cathode.