La formation de tartre : suivi en continu sur électrode métallique

Scaling: follow-up in situ on a metallic electrode

¹ Laboratoire de Corrosion et Traitement de Surface, UFR Sciences et Techniques F-25030 Besançon Cedex
² Laboratoire d'Electrochimie et des Systèmes Microdispersés, UFR Sciences et Techniques F-25030 Besançon Cedex

Résumé

De multiples techniques expérimentales ont été développées ces dernières années afin de quantifier le pouvoir entartrant des eaux ainsi que les effets de traitements inhibiteurs. Toutefois aucune d'entre elles ne permet de visualiser en continu la cristallisation du sel précipitant ; ceci étant pourtant essentiel à la compréhension des mécanismes d'entartrage. Fondée sur l'électrocristallisation du carbonate de calcium, nous avons donc récemment développé une technique analytique permettant de suivre in situ le développement des cristaux des que leur taille est de 1 µm.

Le montage expérimental est composé d'une cellule électrochimique, ainsi que d'un ensemble optique et vidéo; il est ainsi possible de quantifier les particules à la surface de l'électrode de travail en déterminant leurs nombres ainsi que leurs caractéristiques morphométriques moyennes.

Dans cet article, nous présentons les résultats obtenus avec des eaux minérales et synthétiques.

Suivant la qualité des eaux étudiées, l'importance des phases de germination et de croissance des cristaux est évaluée et le développement des cristaux à la surface de l'électrode quantifié.

Cet outil analytique s'avère donc intéressant pour l'étude des mécanismes d'entartrage.

Abstract

Scale which is a hard adherent mineral is generally composed of calcium carbonate. The precipitation of this salt may cause a lot of technical and economical problems.

Several electrochemical tests were developed to study scaling power of water; however they don't allow to make visible in real time the crystallisation of the salt. That's why we developed a new method in order to follow-up the electro-crystallisation of calcium carbonate in situ.

The experimental assembly is composed of an electrochemical cell and a video set-up. The electrochemical cell in which water flows is made up of three electrodes :

The working electrode is a steel circular piece of 1 cm²

The counter electrode is a glass coated with a tin oxide film which is electrically conductive and optically transparent

A silver wire serves as pseudo reference.

The steel surface is polarized at a negative potential (- IV/Ag). So, there is a reduction in the oxygen dissolved and the hydroxide ions bringing about a sudden increase in pH, inducing the transformation of bicarbonate ions into carbonates which in turn react with calcium ions forming a precipitation of calcium carbonate on the surface of the working electrode.

A video recording of the surface is carried out during the experiment. So by using an image analysing System, it is possible to quantify the mean characteristics of crystals for different time. The mean features are number of crystals, the mean morphometric characteristics of crystals (size, diameter, shape), rate of covering of the metallic electrode.

In this article, we present results obtained with waters of different quality: synthetic water and mineral water. The results showed that the method brought about qualitative as well as quantitative piece of information on the crystallisation of calcium carbonate. It was especially possible to establish the importance of nucleation and growth and relating to nucleation if it was instantaneous or progressive.

Moreover the quantification of the mean characteristics of crystals for different periods of time allowed to evaluate the kinetic growth of crystals. A statistical analysis gave especially complementary information: for a particular time, size classes were determined and then the percentage of crystals corresponding to each size was given; so, it was possible to establish if the distribution of crystals was homogenous or not.

So, it is an interesting analytical test to study in situ scaling mechanisms.