In the United States of America, Kraft digesters and evaporators are typically acid cleaned to remove scales that precipitate from chemical species present in liquors. Though the presence of such scales is not harmful from the corrosion point of view, their removal may affect the base metal where they grow on. Scale removal may in turn affect the condition and integrity of the equipment by causing general or localized corrosion, which can lead to unexpected failures. The objective of this work was to determine optimal cleaning practices for Kraft digesters, evaporators and other equipment during scheduled shut downs to minimize cleaning costs, maximize its effectiveness during cleaning and to ultimately minimize corrosion (metal loss) during scale removal. This article presents results from a laboratory study dedicated to study the variables obtained in a nation wide survey.
Most Kraft digesters and evaporators in North American paper mills are periodically acid cleaned to remove insoluble scales like burkiete (Na2CO3.2N2SO4) and carbonate scales on heat exchanger surfaces ~. Despite the importance of such maintenance procedures, many mistakes have occurred causing significant damage (i.e., corrosion) of digester and evaporators vessel walls, screens and piping 24.
Scale characteristics (e.g., composition) may be different in evaporators compared to those observed in digesters or liquor heater surfaces, but have to be regularly removed for efficient heat exchange. Scales are considered generally non-corrosive because a protective carbonate film (carbonaceous) is formed on the surface of the metal. The presence of carbon dioxide in waters favors the formation of bicarbonate ions as pH is increased S. The bicarbonate ion forms insoluble calcium carbonate that may also precipitate on the surface of the equipment. Carbonate compounds have reverse solubility and precipitate out as temperature rises. In terms of corrosion the presence of carbonate layers (some times referred as scaling or fouling) are considered beneficial. However, throughputs are reduced, while pumping costs and heating or cooling requirements are increased because of the lowering flow rates and heat transfer coefficients and by the increased pressure drops 6, hence, the need for acid cleaning to remove those scales.
Although most cleaning for pulping equipment is performed with inhibited muriatic acid (HCI), other acids such as sulfamic, formic and nitric have been studied 2. Acid concentration, inhibitor, temperature, time and flow are important parameters that will establish well-defined procedures to minimize damage to the equipment. Despite the relevance of such procedures to the pulp and paper industry, very little published work has addressed the problems faced during acid cleaning. In the past, only the work conducted by Crowe in the early nineties 2 and Henrikson in the early eighties 7 looked at the importance of acid cleaning procedures on the integrity of structural equipment. The topic has been long forgotten though of utmost importance.
In general, corrosion rates increase with acid concentration, and temperature. In the case of carbon steel, most acids will dissolve base metal at corrosion rates beyond 2.5 mm/y (100 mpy) 2. Despite the use of corrosion inhibitors, the metallurgical damage associated with acid cleaning is reflected in the form of pitting corrosion regardless of steel type, end-grain corrosion of stainless steels (SS) and ferrite attack in welds 2"4.