Corrosion Protection of Grinding Mills in the Phosphate Industry Using Impressed Current Technology
Grinding mills are commonly used in the Florida phosphate industry to reduce particle size. The corrosion of metallic grinding media and mill liner is a very serious problem, particularly in acidic conditions as encountered in the Florida phosphate fertilizer industry. Approximately 50% of the total wear of grinding mills can be attributed to metal corrosion. It is known that the corrosive wear of grinding mill leads to an increase in operating cost and plant downtime, a loss of process efficiency, and product contamination. An effective protection of grinding mills from corrosion will improve the process performance and economics, enhance product quality, and increase the lifetime of mills and grinding media.
The proposed project is aimed at developing a practical and effective technique for minimizing corrosive wear of the ball mill and its grinding media. Metal corrosion reactions are electrochemical in nature and their reaction rates are controlled by the electrochemical potential at the surface. A reduction in potential with excess electrons will depress the anodic dissolution reaction of metal (e.g., M = M2+ + 2e–). This can be accomplished by supplying an impressed current to the object to create a negative potential change called cathodic polarization which reduces the rate of metal reaction. The impressed current was supplied by an EG&G PARC, Model 273 potentiostat for a specially designed ball mill. The results from this study more closely resemble those to be expected from industrial ball mills.
Cathodic protection using impressed current was employed to reduce mill wall wear during ball milling. Wear rates were determined from weight-loss measurements made on three 3/8-inch-diameter mild carbon steel and high chromium alloy coupons that were flush with the interior surface of the mill wall. Experimental results indicate that the corrosive wear rate was reduced by 92.9% to 94.5% and total wear was reduced by 47.8% to 49.6% for 1018 carbon steel when a potential of -1.0 V was applied. Similar results were obtained with the high chromium alloy, except that the required polarization potential was -0.7 V.
The polarization diagram gives a fundamental quantitative assessment of the decrease in corrosion rate caused by cathodic polarization. The effects of solution pH, electrode material, gaseous environment, and solution composition on polarization curves have been performed. Polarization diagrams indicate that the current density is higher in nitrogenated solution than in oxygenated solution at the same pH value. The polarization diagrams also suggest that the current density of 1018 carbon steel was higher than that of high-chromium alloy under the same condition. Experimental results also indicate that the current density is higher in buffer solution than that in pond water solution under the same operating conditions.
A statistical Box-Behnken Design (BBD) of experiments was performed to evaluate effects of individual variables and their interactions on wear rate of grinding ball mill used in phosphate industry. The variables examined in this study included grinding time, solution pH, rotation speed, mill crop load, and solids percentage. The most significant variables and optimum conditions were identified from statistical analysis of the experimental results using response surface methodology. Solution pH had the most significant effect on the wear rate for both 1018 carbon steel and high chromium alloy. The optimum process parameters for minimum wear rate were solution pH at 7.36, rotation speed at 70.31 RPM, solid percentage at 75.50, and crop load at 71.94% for 1018 carbon steel; solution pH at 8.69, rotation speed at 61.13 RPM, solid percentage at 64.86, and crop load at 57.63% for high-chromium alloy.
To understand the cathodic protection process, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) methods were used to investigate the corrosion products, surface morphology, and composition when grinding phosphate rock with and without cathodic protection in different solutions. The main corrosion type and corrosion products for 1018 carbon steel and high chromium alloy under different conditions have been determined.