La domanda del lettore fa evidentemente riferimento alla possibilità di utilizzare un sistema fisico per evitare incrostazioni di calcare nelle tubazioni. Il carbonato di calcio, responsabile delle incrostazioni, può infatti precipitare principalmente in due diverse forme: calcite e aragonite. Secondo alcuni esisterebbe anche una forma amorfa: in realtà sembra però trattarsi di una forma criptocristallina formata cioè da cristalli microscopici (esiste infine un'ulteriore fase chiamata laterite, metastabile).

La calcite e il carbonato “amorfo” possono facilmente formare depositi aderenti alle tubature a causa della forma e delle dimensioni delle loro particelle. L'aragonite invece (che forma cristalli cosiddetti aciculari) aderirebbe meno e non darebbe quindi origine a incrostazioni. Da qui la necessità di far precipitare il carbonato di calcio in questa forma cristallina con qualche opportuno sistema fisico.

Da diversi anni esistono in commercio dispositivi anticalcare magnetici che, in base alle promesse dei costruttori, dovrebbero proprio garantire questo risultato. Mi ero già occupato di questo genere di dispositivi in una risposta di qualche anno fa. In tale risposta avevo sottolineato come la questione sia piuttosto controversa dal punto di vista scientifico. Diversi lavori pubblicati sull'argomento fornivano infatti dati contrastanti. Dal momento in cui scrissi la precedente risposta, sono stati pubblicati altri articoli sull'argomento. In questi studi sembra che effettivamente l'applicazione di un campo magnetico (statico o variabile) possa influenzare il tipo di cristallizzazione del carbonato di calcio. Tuttavia non si è ancora arrivati a conclusioni univoche. Il problema principale resta la scarsa riproducibilità del fenomeno, dovuta ai molti fattori che influenzano il processo di cristallizzazione. A livello di laboratorio sembra oramai che la riproducibilità del fenomeno sia stata abbastanza dimostrata. Non è però detto che il fenomeno sia immediatamente trasferibile a livello applicativo, come i dispositivi commerciali affermano invece con certezza.

Per maggiori informazione, riporto di seguito l'abstract di alcuni articoli pubblicati sull'argomento:

Masden H. (1995): Influence of magnetic-field on the precipitation of some inorganic salts. "JOURNAL OF CRYSTAL GROWTH" 152 (1-2): p. 94-100.

Abstract: Calcium and manganese(II) carbonate, phosphates of magnesium, calcium, iron(II), cobalt(II) and zinc, and calcium oxalate and sulphate have been precipitated at 25 degrees C in a magnetic field of 0.27 T. An effect of the field was found only for carbonates and phosphates with diamagnetic metal ions. Rates of nucleation and crystal growth are increased in magnetic field. The results are explained in terms of faster proton transfer from hydrogen carbonate and hydrogen phosphate ions to water, due to proton spin inversion in the external field or in the field of a paramagnetic ion.

Knez S, Pohar C (2005): The magnetic field influence on the polymorph composition of CaCO3 precipitated from carbonized aqueous solutions. "JOURNAL OF COLLOID AND INTERFACE SCIENCE" 281 (2): p. 377-388.

Abstract: One of the most debated effects the magnetic fields exert on aqueous solutions and dispersions is their influence on the crystal structure of the main scale component, CaCO3. This study presents the results of an experimental program performed to quantitatively evaluate influence of the key magnetic treatment parameters-magnetic induction, exposure time, and fluid velocity-on the polymorph composition of CaCO3, precipitated from carbonized aqueous solutions. The results show that magnetic treatment favored the precipitation of aragonite. The key treatment parameters affecting the aragonite content were the magnetic induction and the exposure time, while the fluid velocity exerted no significant influence. The magnetic field has no significant influence oil the zeta potential of the precipitated particles in any stage of the treatment. These experimental findings indicate that the magnetic field influence on the crystal structure of CaCO3 cannot be attributed to the magnetohydrodynamic influence on the charge distribution within the electrical double layer of the forming crystallites. The results rather suggest that the magnetic fields influence the CaCO3 polymorph phase equilibrium either by influencing the CO2/water interface or through the hydration of CO32- ions prior to the formation of stable crystal nuclei in the solution.

Kobe S, Drazic G, McGuinness PJ, Meden T, Sarantopoulou E, Kollia Z, Cefalas AC (2003): Control over nanocrystalization in turbulent flow in the presence of magnetic fields. "MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS" 23 (6-8): p. 811-815.

Abstract: The influence of the magnetic field and the water flow on the crystal form of calcium carbonate precipitated from low-concentration water solution was followed systematically. By changing the strength of the field, the calcite/aragonite/vaterite ratio varied. The crystal form and the particle size distribution of the precipitated calcium carbonate were determined by using X-ray analyses and transmission electron microscopy (TEM). A simple hydrodynamical model, using the Navier-Stoke's and Maxwell's equations, predicts that there is a strong energy coupling and transfer between turbulent flow and the magnetic fields, which can be amplified to high values. Since the formation of aragonite is enhanced in the presence of magnetic field, scaling is prevented in turbulent flow.

Kobe S, Drazic G, Cefalas AC, Sarantopoulou E, Strazisar J (2002): Nucleation and crystallization of CaCO3 in applied magnetic fields. "CRYSTAL ENGINEERING" 5 (3-4): p. 243-253.

Abstract: The formation of calcium carbonate is not only a common ionic reaction that takes place in natural processes, but also creates a problem known as scaling, which is present in our every day life and in various industrial processes and technologies. In spite of the simplicity of the reaction there is considerable variability in the properties of the solid product, such as: crystal form, particle size distribution, electro-kinetics potential, etc. The influence of the magnetic field on calcium carbonate precipitation has been known for a long time but despite a lot of effort, which has been made to explain this effect, researchers still disagree on the mechanism(s) responsible for it. The focus of our research work was to follow systematically the influence of the magnetic field on the crystal form of calcium carbonate precipitated from low concentration water solutions. By changing the strength of the field and the flow rate of the water through the system the calcite/aragonite/vaterite ratio varied. The crystal form and the particle-size distribution of the precipitated calcium carbonate were determined by using X-ray analyses and TEM. The theoretical part of the work was to study the mechanism of the influence of the magnetic field on the nucleation and further crystallization of calcium carbonate. Starting from ab initio calculations the fundamental physics knowledge was used to propose a mechanism for a better understanding of the phenomena.

Kobe S, Drazic G, McGuiness PJ, Strazisar J (2001): The influence of the magnetic field on the crystallisation form of calcium carbonate and the testing of a magnetic water-treatment device. "JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS" 236 (1-2): p. 71-76.

Abstract: By using X-ray analysis and a TEM equipped with a link AN-10000 EDXS analysing system and an ultra-thin-window Si(Li) detector, different crystal forms of CaCO3 crystals were characterised. These crystals were grown from tap water and model water both with and without a magnetic field. Separate aragonite crystals were formed in the treated water and clusters of calcite in the untreated water. We observed that under the influence of a magnetic field higher than 500 mT, the nucleation and subsequent growth of aragonite could be successfully used as a way of preventing scale. The prototype of a magnetic water-treatment device (MWTD) was constructed for testing in a pilot plant that treats tap water. It has been in use for more than 2 years and the results look very promising for reducing the need for chemically treated water. The weight gains of the heat exchangers, which were used in the three parallel pipelines equipped with three different devices against scaling, were followed. The MWTD designed and built in the US laboratory, showed only a slightly higher weight gain than that achieved with the use of chemicals.

Al di là di quelli magnetici o elettromagnetici non mi risulta che esistano altri sistemi fisici in grado di indurre la cristallizzazione del carbonato di calcio in aragonite. Un fattore fisico che può influenzare il rapporto tra le varie forme cristalline è sicuramente la temperatura, ma non sembra che tale fattore possa trovare applicazioni pratiche nelle normali tubature casalinghe.

Nota: ringrazio il Dott. Mauro Prencipe e il Dott. Giorgio Temporelli per le utili informazioni fornitemi relativamente all'argomento trattato.