EP0654546A1

EP0654546A1 - Method for pickling and an installation therefor

Info

Publication number: EP0654546A1
Application number: EP94203432A
Authority: EP
Inventors: Piet De Vries
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-11-24
Filing date: 1994-11-24
Publication date: 1995-05-24
Also published as: NL9302035A

Abstract

The present invention relates to a pickling method comprising of

i) pickling in a pickling bath with a pickling medium with an iron concentration of more than 150 g iron/l;
ii) cooling the pickling medium in order to crystallize iron chloride;
iii) separating formed iron chloride crystals; and
iv) returning pickling medium with iron removed to the pickling bath,

and to a pickling installation therefor.

Description

The present invention relates to a method for pickling iron and steel products in an acid solution, wherein rust and possible mill scale are removed. The iron content of the pickling medium hereby increases gradually. At an iron content of 80 to 90 g iron/l the pickling action of the pickling medium has practically ceased. At that moment the pickling medium must be refreshed.
A number of problems occur during regeneration. Firstly, the pickling medium itself has an environmental impact even after neutralization. Secondly, the pickling medium contains valuable components which are lost as a result of neutralization. These are reasons enough to look for an adapted pickling method with improved properties, particularly with respect to regeneration.
The present invention is based on the insight that a pickling medium regains its pickling action when the iron content of the pickling medium is further increased to generally above 150 g iron/l, whereby it also becomes worthwhile to separate iron salts, such as iron chloride, formed by means of crystallization.
Thus is achieved that the pickling action is maintained or even improved when compared to the use of a pickling medium with an iron content lower than 75 g/l, and a valuable crystallization product results with a comparatively small content of contaminants, while the treated pickling medium can be re-used. This results in a pickling method wherein due to continuous regeneration the properties of the pickling medium, particularly the pickling action of the pickling bath, remain substantially constant.
This is achieved with a pickling method according to the invention which comprises of

i) pickling in a pickling bath with a pickling medium with an iron concentration of more than 150 g iron/l;
ii) cooling the pickling medium in order to crystallize iron chloride;
iii) separating formed iron chloride crystals; and
iv) returning pickling medium with iron removed to the pickling bath.

Although the iron content of the pickling medium generally lies above 150 g iron/l, it is further recommended to work with a pickling medium with an iron content lying above 175 g/l. Under practical conditions the iron concentration will generally lie in the range of 180 to 250 g/l, more preferably between 190 and 210 g/l. A pickling medium is thus provided with a pickling action equal to or even greater than that of a pickling medium which works with an iron content smaller than 75 g/l.
The pickling medium is subjected to crystallization conditions by cooling generally to a temperature below 2°C, more preferably to below 0°C. This results in iron chloride crystals and the iron content falls to about 100 g/l.
When, however, concentrated acid is added prior to or during crystallization, then crystallization will already occur at a temperature below 10°C, generally within the range of 2 to -5°C. Herein is further achieved that after crystallization the iron content in the pickling medium with the iron removed falls below 100 g iron/l. It is therefore recommended to add acid to the pickling medium for crystallizing, more preferably by first adding the acid after the pickling medium has been cooled. The concentrated acid can be chosen from hydrochloric acid, sulphuric acid and phosphoric acid. The use of sulphuric acid and phosphoric acid is herein preferred because re-dissolution of the iron chloride crystals is further inhibited.
Crystallization can be further enhanced by performing crystallization under the influence of ultrasonic sound (more than 20,000 Hz, in general between 0.1 - 100 MHz depending on the crystal structure).
The present invention further relates to a pickling installation for performing the method according to the invention, which pickling installation comprises

a pickling bath,
at least one crystallization unit which is connected via a feed line to the pickling bath,
a solid/liquid separator for separating crystallized iron chloride which is connected to the crystallization unit and is provided with a recirculation line for feeding liquid with iron removed to the pickling bath.

Because the pickling is performed at a temperature generally in the order of 20-50°C and crystallization is performed after cooling, it is recommended for reasons of heat economy that the feed line and the recirculation line are in heat-exchanging contact.
Mentioned and other features will be further elucidated hereinbelow on the basis of a number of embodiments of the pickling installation according to the invention, which are only given by way of example and with reference to the annexed drawing.
In the drawing:
figures 1-3 each show a schematic view of a pickling installation according to the invention.
Figure 1 shows an installation 1 for pickling steel and iron products, wherein for instance 2-3 kg iron/ton product is removed.
The installation 1 comprises a pickling bath 4 with a content of for instance 60,000 l. The pickling bath contains a pickling medium with an iron content of roughly 175-210 g/l, a hydrochloric acid content of about 30-60 g/l (pH ≦ 1), while the temperature amounts to around 40°C. Pickling medium is continuously drained via a feed line 5 to a crystallization unit 6, wherein an intermediate tank 7 is arranged in feed line 5.
The crystallization unit 6 is manufactured from stainless steel and protected against corrosion by means of the circuit 8, an anode 9 of which is immersed in the pickling medium 10 for treating.
The crystallization unit is provided with cooling means 11 with which the pickling medium 10 is cooled to below 0°C. By means of a stirring gear 12 a circulation is maintained in crystallization unit 6. After crystallization is completed the medium is guided via a line 13 through a solid/liquid separator 14 in which formed iron chloride crystals are separated and removed via discharge line 15. Pickling medium with iron removed is returned via recirculation line 16 to pickling bath 4. This pickling medium contains about 160 g/l iron.
In the pickling installation 2 shown in figure 2 the same construction elements are designated with the same reference numerals. However, the crystallization unit 6 is also provided with a line 17 with which concentrated acid (for instance hydrochloric acid) is supplied from a tank 18 to crystallization unit 6. Cooling thereby only need take place for crystallization to a temperature of around 0°C, while the iron content of the pickling medium returned via the recirculation line 16 to pickling bath 4 has an iron content of only 100 g/l.
In the pickling installation 3 shown in figure 3 the crystallization unit 6 is likewise embodied with means 19 for externally radiating ultrasonic sound, whereby germing is greatly enhanced and therewith crystallization.
In addition, the liquid/solid separator now takes the form of a centrifuge 20, the rotating drum 21 of which is scraped with a knife 22 and the solids removed via discharge line 15. In this case the iron chloride has a moisture content far below 60%, such as 10-30%, and in a practical embodiment 18%. This further contains iron chloride (50%), zinc (60 ppm) and other metals (less than 25 ppm). This material is very suitable for use as flocculant in water treatment plants.
The recirculation line 16 is further provided with a heat-exchanging part 23, whereby heat-exchanging contact is possible in the intermediate tank 7, whereby pickling medium for treatment is cooled and treated pickling medium is heated and the heat economy thereby greatly improves.
Although not discussed, it will be apparent that in the installations 1-3 a plurality of crystallization units 6 can be connected to the intermediate tank 7, so that in continuous batch-wise manner crystallization can take place, crystals can be separated, and treated pickling medium can be returned to pickling bath 4. Thus remains ensured that after crystallization centrifuging can take place at low temperature, thus avoiding as far as possible that the unstable iron crystals dissolve again.
In a variant of the pickling method of figure 3 a belt filter is used instead of a centrifuge, wherein the iron chloride crystals are washed with concentrated hydrochloric acid (33%).
The washed clean iron chloride crystals can be discharged as such or further refined. The washing liquid is returned to the pickling bath (optionally after prior heat exchanging in heat exchanger 7).
The washed clean iron chloride crystals can be further refined to for instance iron(III)chloride of commercial quality. For this purpose the washed clean crystals are placed into a dissolving tank to which water is added. The heat of solution is provided by heat exchanging with the solution leaving the pickling bath.
The iron chloride solution is fed to a membrane oxidation unit to which hydrochloric acid is supplied. Iron(III)chloride of commercial quality is formed in a concentration of 140 g iron/l.
It will further be apparent that zinc, lead, chromium, manganese, magnesium and copper formed during pickling remain substantially in the pickling medium and thus do not enter the environment.

Claims

Pickling method comprising of
i) pickling in a pickling bath with a pickling medium with an iron concentration of more than 150 g iron/l;

ii) cooling the pickling medium in order to crystallize iron chloride;

iii) separating formed iron chloride crystals; and

iv) returning pickling medium with iron removed to the pickling bath.
Pickling method as claimed in claim 1, wherein pickling is performed with a pickling medium with an iron concentration of more than 175 g iron/l.
Pickling method as claimed in claim 1 or 2, wherein the iron concentration lies in the range of 180-250 g iron/l, more preferably 190-210 g iron/l.
Pickling method as claimed in claims 1-3, wherein crystallization is performed while adding concentrated acid.
Pickling method as claimed in claim 4, wherein concentrated acid is added in an amount of 1-10% by weight, preferably 2-5% by weight.
Pickling method as claimed in claim 5, wherein the concentrated acid is chosen from hydrochloric acid, sulphuric acid and phosphoric acid.
Pickling method as claimed in claims 1-6, wherein crystallization is performed under the influence of ultrasonic sound.
Pickling installation for performing the method as claimed in claims 1-7, comprising
- a pickling bath,

- at least one crystallization unit which is provided with cooling means and connected via a feed line to the pickling bath,

- a solid/liquid separator for separating crystallized iron chloride which is connected to the crystallization unit and is provided with a recirculation line for feeding liquid with iron removed to the pickling bath.
Pickling installation as claimed in claim 8, wherein the crystallization unit is provided with means for supplying concentrated acid.
Pickling installation as claimed in claim 8 or 9, wherein the crystallization unit is provided with means for supplying ultrasonic sound.
Pickling installation as claimed in claims 8-10, wherein at least two crystallization units connected in parallel are connected between the pickling bath and the liquid/solid separator.
Pickling installation as claimed in claims 8-11, wherein the feed line and the recirculation line are in heat-exchanging contact.