EP0463905B1 - Process for separating silicon compounds from pickling baths and apparatus therefor - Google Patents

Abstract

Process for the separation of silicon compounds from baths for pickling steel substrates containing iron ions and silicon compounds, consisting of the following stages: a) a concentrated spent pickling bath is cooled in a controlled manner to a temperature below 60 DEG C so as to precipitate the silicon compounds, b) the pickling bath is left to cool for at least 2 hours, c) the pickling bath is reheated, d) the precipitated silicon compounds are separated from the pickling bath.

Description

The present invention relates to a process for the separation of silicic compounds from pickling baths containing iron ions and silicic compounds, as well as to a device for carrying out the process.

EP-0 141 034 discloses a process for separating a silicic compound from a spent pickling bath of steels.

In this process, the spent pickling bath passes through a filter made of a material adsorbing the silica compound.

Such a method has the drawback of requiring an operation for regenerating the adsorbent materials constituting the filter. In fact, during its use, the adsorbent filter becomes clogged and the flow and volume of the treated pickling bath gradually decreases.

Application JP-A-59162139 describes a process for treating spent pickling baths, comprising a concentration of the bath, followed by filtration to remove the insolubilized silicic compounds.

The pickling baths currently used for pickling steel substrates contain iron ions, essentially in the form of ferrous ions, as well as a certain proportion of silicic compounds formed from the silicon element coming from the substrate.

These pickling baths after having been previously concentrated are thermally decomposed by known methods and are in particular calcined in an oven of the RUTHNER type, which makes it possible to recover iron oxides, essentially in Fe₂O₃ form.

Iron oxides, which are sufficiently pure, can be used as colorants, for example in paints and cosmetic products, or as excipients in the pharmaceutical industry. They are also used in the manufacture of magnetic components or still in the manufacture of cements to improve their mechanical characteristics.

The silica content of the iron oxides currently recovered from the pickling baths is at least 350 ppm, while it would be desirable to lower it below 100 ppm for the applications mentioned above.

The object of the present invention is to provide a process which makes it possible to increase the purity of the iron oxides by lowering their content of residual silicic compounds.

• a) on concentre un bain de décapage usé,
• b) on refroidit le bain de décapage usé concentré à une vitesse de refroidissement comprise entre 0,2 et 4°C/min, jusqu'à une température inférieure à 60°C, de manière à précipiter les composés siliciques,
• c) on stabilise la température du bain de décapage pendant au moins 2 heures,
• d) on réchauffe le bain de décapage,
• e) on sépare les composés siliciques précipités du bain de décapage.

The subject of the invention is therefore a method of separating silicic compounds from the pickling baths of steel substrates containing iron ions and silicon compounds, consisting of the following stages:

• a) concentrating a spent pickling bath,
• b) the concentrated spent pickling bath is cooled at a cooling rate of between 0.2 and 4 ° C / min, to a temperature below 60 ° C, so as to precipitate the silicic compounds,
• c) the temperature of the pickling bath is stabilized for at least 2 hours,
• d) the pickling bath is heated,
• e) the precipitated silicic compounds are separated from the pickling bath.

• la vitesse de refroidissement est de préférence d'environ 2°C/mn,
• la température de refroidissement du bain de décapage est de préférence comprise entre 20 et 40°C,
• on réchauffe le bain de décapage à une température d'environ 80°C,
• les composés siliciques précipités sont séparés du bain de décapage par tout moyen approprié notamment par décantation, centrifugation et/ou filtration. Dans le cas d'une filtration, celle-ci est effectuée avantageusement sur des filtres ayant une porosité inférieure ou égale à 20 µm.

The other advantageous features of the invention are:

• the cooling rate is preferably around 2 ° C / min,
• the cooling temperature of the pickling bath is preferably between 20 and 40 ° C,
• the pickling bath is heated to a temperature of around 80 ° C.,
• the precipitated silicic compounds are separated from the pickling bath by any suitable means, in particular by decantation, centrifugation and / or filtration. In the case of filtration, this is advantageously carried out on filters having a porosity less than or equal to 20 μm.

The invention also relates to an installation comprising means for the regeneration of the pickling bath containing iron ions and silicic compounds, characterized in that the regeneration means include means for concentrating the spent pickling bath, at least one tank. for cooling the pickling bath and reheating it, at least one device for separating the precipitated silica compound (s) from the pickling bath and a device for thermally decomposing the pickling bath into iron oxides and vapor water and acid. The separation device consists advantageously in a centrifuge device and / or a filtration device comprising filters with a porosity less than or equal to 20 μm.

The method and the installation of the invention are described in more detail below with reference to FIG. 1 which represents a diagram of an installation for pickling steel sheets.

The installation comprises a pickling unit 1 at the outlet of which the pickling bath containing water, hydrochloric acid, iron chlorides and one or more silicic compounds is depleted in acid and enriched in iron. The bath temperature is around 90 ° C, it contains 30 to 60 g / l of hydrochloric acid and 90 to 140 g / l of iron ions, and optionally a compound preventing attack of the substrate. The spent pickling bath is then directed to an evaporator 2 to be concentrated there. The evaporator 2 consists of a liquid-gas exchanger supplied by hot gases leaving a device thermal 3 via a cyclone 4.

The evaporator 2 makes it possible to preheat the bath while concentrating it by evaporation of the water contained in it.

The concentrated spent bath recovered at the outlet of the evaporator 2 is directed to cooling and reheating tanks 5 and 6, where it is cooled to a temperature equal to or less than 60 ° C. preferably with a cooling rate about 2 ° C / min. The pickling bath is then allowed to stabilize for a period of at least 2 hours, which makes it possible to create favorable conditions for the silicic compounds to crystallize in the form of relatively large particles.

Then, the bath is heated so that the dissolution, in particular of the iron chloride, is sufficient and to obtain a viscosity of the bath allowing separation, for example by filtration of the silica compound (s).

The heating rate is not critical and can vary from a few minutes to several hours.

At the outlet of the cooling and reheating tanks 5 and 6, the pickling bath is advantageously centrifuged and / or filtered on the centrifugation and / or automatic filtration unit 7 which makes it possible to collect the precipitated silicic compound (s) and not redissolved.

The pickling bath depleted in silicic compounds is then directed towards the upper part of the thermal device 3 where it is treated to obtain iron oxide. Iron oxide is collected at the base of the thermal device 3.

In the thermal device 3 known for the implementation for example of the RUTHNER process, the water and acid vapors produced containing a residual quantity of iron oxide are sent into a cyclone 4 which separates the iron oxide. The residual iron oxide thus separated is reintroduced into the thermal device 3.

The hot gases taken off at the head of the cyclone 4 are used for the operation of the evaporator 2.

At the outlet of the evaporator 2, the hot gases essentially comprising water vapor and hydrochloric acid pass through an absorber 8 supplied with water in its upper part by water coming from the rinsing unit 9 used for rinsing the substrates leaving the pickling unit 1.

The acid-depleted vapors exiting at the upper part of the absorber 8 are directed to a smoke washing device 10 supplied with water by an additional water 11. The smoke washing water is added to the supplying water the absorber 8 from the rinsing unit 9.

At the lower part of the absorber 8, a regenerated bath is collected containing a hydrochloric acid solution which can again be used for a second pickling and regeneration cycle.

The results of tests carried out on several pickling baths using the method according to the invention which will consist of cooling pickling baths containing iron ions and silicic compounds at a temperature of about 80 will be given below. ° C at a temperature of approximately 40 ° C with a cooling rate of approximately 2 ° C / min, to allow to stabilize pickling baths for a period of approximately 3 hours, heating the baths to 80 ° C., then separating the precipitated silica compound (s) from the pickling baths by filtration on filters of variable porosity.

• méthode gravimétrique pour mesurer la quantité de silice retenue par les filtres
• méthode d'absorption atomique pour mesurer la quantité de silicium contenue dans le filtrat
• par ICP (Plasma à couplage inductif) pour mesurer la quantité de silicium dans les oxydes de fer obtenus par évaporation du bain de décapage.

The efficiency of the process of the invention is checked by dosing the silicon or the silica according to the following methods:

• gravimetric method to measure the quantity of silica retained by the filters
• atomic absorption method to measure the amount of silicon in the filtrate
• by ICP (inductively coupled plasma) to measure the quantity of silicon in the iron oxides obtained by evaporation of the pickling bath.

The results obtained expressed in silica content are summarized in the table below:

The process and the device according to the invention for the substantial elimination of silicic compounds from pickling baths are easily transposable in an industrial environment and make it possible to obtain iron oxide, the silica content of which is less than 100 ppm.

Claims (8)

1. Process for separating silicic compounds from baths for cleaning steel substrates containing iron ions and silicic compounds, consisting of the following stages:

   a) a spent cleaning bath is concentrated,

   b) the spent, concentrated cleaning bath is cooled at a cooling rate between 0.2 and 4°C/min to a temperature below 60°C so as to precipitate the silicic compounds,

   c) the cleaning bath is reheated,

   d) the silicic compounds precipitated from the cleaning bath are separated.
2. Process according to claim 1, characterized in that the rate of cooling is approximately 2°C/min.
3. Process according to claim 1, characterized in that the concentrated spent cleaning bath is concentrated at a temperature between 20 and 40°C.
4. Process according to any one of the preceding claims, characterized in that the reheating temperature is approximately 80°C.
5. Process according to any one of the preceding claims, characterized in that the precipitated silicic compounds are separated from the cleaning bath by settling, centrifuging and/or filtration.
6. Process according to claim 5, characterized in that the precipitated silicic compounds are separated from the cleaning bath by filtration on filters which have a porosity of equal to or below 20µm.
7. Plant comprising means for regenerating a spent cleaning bath containing iron ions and silicic compounds, said means comprising means (2) for concentrating the spent cleaning bath, at least one cooling and reheating tank (5, 6), at least one device (7) for separating the silicic compounds precipitated from the cleaning bath and a device (3) for thermally decomposing the cleaning bath into iron oxides and vapours of water and acid.
8. Plant according to claim 7, characterized in that the device (7) for separating the silicic compounds consists of a centrifuging device and/or a filtration device comprising filters with a porosity of equal to or below 20 µm.

Images