Feasibility study on utilization of IRON (III) OXIDE (RUST) for paint production.
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During hot rolling/ heat treatment of steels, oxygen from the atmosphere reacts with iron in the surface of steel to form rust (iron oxide). These oxides are undesirable when steel is to be subsequently shaped or cold rolled. They are therefore removed with the help of several methods including abrasive blasting, tabling, brushing, salt bath descaling, alkaline descaling and acid pickling(Robson & Robson, 2012). The pickling process is the most preferred method of oxide removal. It is done by dissolution of steel in an acid. Sulphiric acid and hydrochloric acid are used for carbon steels while a mixture of nitric acid and hydrofluoric acid is used for stainless steel. Acid pickling is used because it eliminates irregularities during cold reduction and permits alloying/adherence of metallic coatings. Hydrochloric acid is the most preferred acid for the pickling process. It uses hydrochloric acid solution to dissolve iron oxide from the metal surface without any significant attack on the steel itself(Robson & Robson, 2012). It offers a faster and cleaner pickling process, there is low acid consumption and reduced quantities of waste pickle liquor (WPL), more uniform product quality than sulphiric acid. Its only disadvantage is that it is volatile. When iron oxide dissolves in HCL, ferrous salts and water are formed according to the equation 1 Fe2O3 (s) + 6 HCl (l) → 2FeCl3 +3H2O… ............ equation 1 HCl also reacts with the base steel by the mechanism in equation 2. Fe(s) + 2HCl (l) → FeCl2 + H2O… .................... equation 2 An inhibitor is usually added to the acid solution to inhibit acid attack on the steel itself while permitting preferential attack on the iron oxide. Inhibitors are natural or synthetic organic compounds added to acid solutions to reduce metal surface corrosion. Rodine acid inhibitor is the most commonly used. The rate of the pickling process is affected by several variations. These include base steel constituents, acid concentration and temperature of the solution. The rate of pickling increases with increase in concentration of the acid and increase in temperature. In the pickling process, free HCl depletes and ferrous chloride builds up in the pickle liquor to an extent that pickling cannot be accomplished and the quality of the treated steel deteriorates. At this point, the WPL is discharged to the storage tank and the pickling tank replenished with fresh acid solution.