L'inhibition de l'entartrage par les eaux géothermales du sud tunisien. Étude sur site Inhibition of scale formation by southern Tunisia geothermal waters

Résumé

Une nappe d'eaux fossiles à grande profondeur (800 à 2 700 mètres) a été mise en exploitation dans le Sud-Tunisien pour alimenter une usine d'osmose inverse située à Gabès ayant une production de 15 000 m³ /jour, afin de lutter contre la désertification par irrigation et d'assurer le chauffage de serres pour la production de primeurs. La grande dureté (TH de l'ordre de 100 à 140 °F) de ces eaux géothermales a pour conséquence le colmatage rapide des conduites de distribution : 40 à 50 tonnes de tartre par forage, constitué essentiellement de carbonate de calcium, précipitent chaque année. Ce tartre est constitué d'aragonite comme le montrent la microscopie électronique à balayage et la diffraction des rayons X. Une technique électrochimique, la chronoélectrogravimétrie, permet d'étudier l'inhibition de l'entartrage par des composés de la famille des phosphates inorganiques, des phosphonates organiques et des polycarboxylates. La concentration efficace de chacun de ces inhibiteurs agissant par effet de seuil a été déterminée : elle est de l'ordre de 1,1 à 1,5 mg.l^-1 pour l'eau du forage de EL HAMMA. Un essai sur le site de EL MANSOURA a été effectué en privilégiant un inhibiteur produit industriellement dans le Sud-Tunisien, le triphosphate de sodium. A la concentration de 1 mg.l^-1 il évite l'entartrage du système de refroidissement de type cascade - piscines et des conduites de distribution.

Mots-clés:

• Eaux géothermales,
• entartrage,
• chronoélectrogravimétrie,
• inhibiteurs,
• phosphonates,
• polycarboxylates,
• triphosphate

Abstract

Deep fossil waters are used in Southern Tunisia (Gabès, Kébili, Tozeur) for the Gabès reverse osmosis plant, which delivers a flow rate of 15 000 m³ /day for irrigation and for heating greenhouses used for the production of early fruits and vegetables. Drilling depths vary between 800 and 2 700 meters. Water emerges under a pressure of ca. 20 bars and has a temperature between 50 and 73 °C. Mean flow rate is 7 800 m³ /day.

Intake water at the Gabès plant has a salinity of 3.3 g.l^-1 ; after reverse osmosis the salinity is less than 0.1 g.l^-1. Water used for irrigation has to be cooled. Geothermal waters are characterized by high concentrations of calcium, magnesium, sulphate and chloride. Bicarbonate anions are present at limited concentrations (approx. 2.10^-3 mol.l^-1) that are, however, sufficient for the formation of large quantities of scale - 40 to 50 tons per year for each drilling. At the outlet of the drill hole, pressure decreases strongly, liberating carbon dioxide to the atmosphere. Water pH increases and the following equilibrium is displaced to the right, with scale precipitation : Ca²⁺ + 2HCO₃^-

CO₂ (g) + CaCO₃(s) + H₂O

Scale precipitation has two consequences :

- the plugging of distribution pipes: a 85% reduction of the pipe has been observed, after four years, for an initial diameter of 15 cm;

- water cooling installations such as cooling towers or pool systems are blocked by large quantities of scale, which have to be removed regularly.

Scales have been analysed through inductively coupled plasma spectroscopy and thermogravimetry: calcium carbonate may represent, depending on the origin of the drilling water, 60 to 95% by weight of the solid. Iron oxides, silica, calcium phosphate and aluminum are present. Scanning electron microscopy and X-ray diffraction show that calcium carbonate precipitates in the form of aragonite. This is due to two reasons: the temperature at the drilling outlet is greater than 60 °C and the high magnesium concentration favours aragonite formation. Scale inhibition is possible through the use of certain chemicals such as phosphates, organic phosphonates and polycarboxylates.

Chronoelectrogravimetry was used as the experimental method to determine the inhibitor concentration able to suppress scale precipitation. Dissolved oxygen is electrochemically reduced on a gold electrode; hydroxide anions are produced in the vicinity of the electrode and calcium carbonate precipitates according to:

4Ca²⁺ + 4HCO₃^- + O₂ + 4e -> 4CaCO₃(s) + 2H₂O

The gold electrode is deposited on the quartz disk of a recording microbalance. The electrode is polarized at - 1 V/SCE with a three-electrode potentiostatic device and the weight of CaCO₃ deposited is recorded versus time.

Four inhibitors have been studied :

- PERMATREAT 191, which is the sodium salt of aminotris(methylenephosphonic) acid N(CH₂COONa)₃;

- a proprietary organic phosphonate with high resistance to chlorine oxidation (DEQUEST 6004) ;

- phosphonobutanetricarboxylic acid (DEQUEST 7000, BAYHIBIT-AM) ;

- a copolymer of acrylic acid and acrylamidopropanesulfonic acid (FERROPHOS 5248).

Breakthrough effects are obtained in the case of EL HAMMA water for the following concentrations: PERMATREAT 191: 1.1 mg.l^-1 ; DEQUEST 6004: 1.5 mg.l^-1 ; DEQUEST 7000: 1.3 mg.l^-1 ; FERROPHOS 5248: 1.4 mg.l^-1. These concentrations are low and, as a consequence, these inhibitors can be used for antiscale action even with high water flow rates.

A field experiment was carried out on the EL MANSOURA drilling where water is cooled in three pools (input water: 5 184 m³ /day, temperature: 60 °C). For economical reasons the chosen inhibitor was sodium triphosphate Na₅ P₃ O₁₀, which is produced industrially in Southern Tunisia. By chronoelectrogravimetry it has been shown that, with EL MANSOURA water, a breakthrough effect is obtained with a sodium triphosphate concentration of 0.75 mg.l^-1.

A dosing pump was used to inject sodium triphosphate in such a way that the inhibitor concentration would be 1 mg.l^-1 in one of three pools, the two others not being treated. After four months a scale deposit of 23 cm was obtained in the untreated pools and the pipe diameter was reduced by 39%. In the treated pool scale deposit was not observed and the pipe diameter remained unchanged. In the untreated basins, examination of scale with electron scanning microscopy revealed that it was aragonite; in the treated basin, the precipitate was amorphous and an X-ray diffraction pattern with no characteristic bands was obtained.

Some algal development was observed in the pool due to phosphate addition but this development was not a nuisance after the four month period. However, it could be suppressed by the use of an organic phosphonate or polycarboxylate as scale inhibitor.

Keywords:

• Geothermal waters,
• scaling,
• chronoelectrogravimetry,
• inhibitors,
• phosphonates,
• polycarboxylates,
• triphosphate