Metal Finishing Guide Book


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Page 715 of 903

Silicate Amines In this treatment, the hydrophilic portion is a polyamine and the hydrophobe is a silicate (usually sodium metasilicate). These materials are fed separately to the booth in a fixed ratio based on the paint overspray rate. (Most typically 4 to 13 parts of silicate to 1 part of polyamine and both at 5% to 15% based upon overspray.) The pH level is critical, since too high a pH can cause the complex to redissolve. The main drawback in silicate amine treatments is that they do not disperse paints very well, nor do they provide instantaneous detackification. Because of this, it is not uncommon to find sticky deposits in the back sections of spray booths where good mixing does not occur. Better detackification usually takes place as the system runs longer. Silica Amines The silica amine program is very similar to the silicate amine treatment. The primary difference is that it utilizes an aqueous colloidal silica sol as the hydrophobe. Colloidal silica can be thought of somewhat as a nonswelling clay. The silica sol is fed at a ratio of 1 to 3 parts to each part of polyamine. Since the pH of these materials is essentially neutral, an alkalinity source (usually potassium hydroxide) is fed to bring the system pH to 8.0 to 9.0. The primary drawback of this program is that under conditions of high shear, such as might take place with a centrifuge separator, the small size of the silica might not allow itself to fully embed onto the paint, resulting in partially killed sludge. Melamine Formaldehyde This copolymer was originally developed by Du Pont in the early 1940s. It makes use of its unique organic structure to act to detackify paint. The alternating melamine and formaldehyde in the polymer chain form a two dimensional netlike structure, the melamine portion acting as the hydrophobe and the formaldehyde functioning as the hydrophile. Under alkaline conditions, the compound forms an associated complex.The melamines orient on the surface of the paint while the formaldehyde groups attract the water layer that prevents the paint from sticking. Because both of these groups are on the same molecule, the effect of detackification is nearly instantaneous. Also, because the size of the groups is small relative to that of silicate or silica amine, the melamine formaldehyde coats the paint particle much more effectively. One of the drawbacks of melamine formaldehyde treatment is the relative fragility of the coating. Because of this, it is necessary to disperse the paint well. Under conditions of high shear the coating can be ruptured, releasing sticky paint. The other fact to consider is that because of the sensitivity of this treatment to waterborne particulates, the cleaner the system, the more effective the melamine formaldehyde is in killing the paint. As the solids loading increases, the level of detackification decreases and the ability to form a good floc is affected. SLUDGE REMOVAL METHODS AND EQUIPMENT Once the paint sludge has been detackified or otherwise concentrated, it is necessary to use some mechanical means to remove it from the water. The methods used to remove the captured paint overspray from paint booths vary widely in type, effectiveness, and cost. A great deal of the choice as to which method is selected is dependent upon the type of booth, the amount of paint sprayed, the desired end results of the sludge removal, and the money available for equipment. Options available for side- draft and downdraft systems will be examined separately, in terms of both manual and automatic methods. 704

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